Biopolymer composites: a review

ABSTRACT

In spite of the fact that a prodigious portion of petroleum covers multitudinous products in the commercial world, its non-biodegradable characteristic is an unenviable factor. The utilization of biodegradable polymers or biopolymers is a prominent alternative to petroleum-based plastic products. Reinforcing natural fibers to biopolymer matrices significantly improves the properties of prepared plastic products. Such biopolymer composites have been developed by researchers to provide environmentally responsible materials and thereby reduce carbon footprint. Modification or functionalization of natural fibers is important to ameliorate the interfacial bonding with biopolymers, and to successfully obtain high-performance composite materials that can compete with conventional petrochemical-based polymer composite counterparts. Another widely accepted composite development technique is by combining different types of fibers into a single matrix to develop highly valued hybrid composites. The review focuses on the extraction, processing and characterization of bio-based materials; natural fibers and biopolymers, and their synergistic application and future scope as biopolymer composites in automotive and other growing sectors.

KEYWORDS:

• Biodegradable
• biopolymer
• natural fiber
• interfacial bonding
• hybrid composite
• automotive sector

Introduction

Petroleum-based plastics have found their way into our routine livelihood as it showed the potential to replace various traditional materials in varied applications. Fiber-reinforced polymer composites are progressively equipped in high-end applications, such as automobiles, aerospace, and medical science. Apart from this monetary domain, most of the petroleum-driven materials give rise to an untenable environment. So an alternative way to develop environmentally safe products is to use bio-based materials. Bio-based materials are the ones whose building blocks constitute the substances procured from living matter. Natural fibers reinforced biopolymers are known as “green composites”. It is an environmentally degradable biopolymer composite that can be degraded by the action of environmental factors such as air, light, heat, or microorganisms. Despite low stiffness and strength, natural fibers are more attractive than synthetic fibers. The use of natural fibers offers some supremacy such as easy accessibility, combustibility, biodegradability, and non-toxicity. Yet overall quality variance, low processing temperature, and high moisture absorption of natural fibers adversely affect their application. To achieve high-performance products from natural fiber-reinforced biopolymer composites, myriad studies have been reported on functionalizing natural fibers. In a biopolymer composite, mostly the biopolymer matrix governs the structure, environmental tolerance, and durability, and the reinforcement fiber determines the stiffness and strength of the composite. Value-added novel applications of biopolymer composites ensure potential growth in international markets. The efforts to develop environmental friendly composite products with improved performance have achieved some major global applications and are still continuing. In this review, attention will be focused on production, processing, and application of biodegradable composites prepared from polymers like cellulose, starch, polylactic acid (PLA), polyhydroxyalkanoate (PHA), etc. that are currently commercialized and available in the market and the ones that are more promising as matrices for natural fiber composites in future. The review will not cover the non-biodegradable (bio-based and partially bio-based) or fossil-fuel-based biopolymers.

Natural fibers

Since early civilization natural fibers have been employed to make ropes, twines, and fabrics and played a significant role in society. Natural fibers do not damage the environment as they are sustainable, carbon neutral, renewable, and biodegradable resources [1]. Due to their versatility and local availability, natural fibers are potential alternative materials to synthetic fibers in composite industry [2]. Unlike synthetic fibers, natural fibers are biodegradable and eco-friendly in nature. They are renewable resources which possess remarkable properties such as lightweight, low density, easy production and processability, and cost effectiveness [3,4]. Natural fibers are currently preferred over synthetic fibers, as the former produce lesser wear and tear to the machine and its parts during processing, and also do not contaminate the landmass or water bodies [3]. These natural fibers are widely used for reinforcements in to polymer matrices, where polymers are used as the binding materials to hold fibers and provide dimensional stability [5]. The exceptional strength per unit mass of natural fibers makes them as an excellent and attractive choice as reinforcing materials [6]. Around 30,000,000 tonnes of natural fibers are produced annually and have been used as suitable raw material in sectors like clothing, paper making, packaging, sports equipments, automobiles, and building materials [7].

Classification of natural fibers

On the basis of biological origin, natural fibers are classified as animal and plant fibers. Mostly, animal fibers are composed of proteins while plant fibers are composed of cellulose. Plant fibers are abundantly available and cost effective; on the other hand animal fibers have recently gained attention as reinforcing material for biopolymer composites. Animal fibers have received appreciable interest because of their flexibility, high surface toughness, high aspect ratio, and low hydrophilic nature when compared to plant fibers [8]. Plant fibers like hemp, kenaf, flax, cotton, ramie and animal fibers like wool and silk are usually used in textile sectors [3].

Animal fibers

On the basis of sources, animal fibers are mainly animal hair, bird feather and silk. The exceptional chemical, physical and mechanical properties of animal fibers make them a potential reinforcement material in polymer composites. Wool is the animal hair from hairy mammals like sheep, alpaca, bison, angora rabbit, cashmere/pashmina goats, etc. It is a commonly used textile fiber, and the major producers are China, New Zealand and Australia [9]. The physico-chemical properties of wool differ according to its origin. For example, the diameter of angora, alpaca, qiviut, and cashmere wool fibers ranges between 12 to 16 μm, 12 to 29 μm, 15 to 20 μm, and <18.5 μm, respectively [9]. Wool fibers are hydrophilic in nature and absorb water to one-third of their weight. The rate of flame spread, the rate of heat release, and heat of combustion is low for wool fibers [9]. Bodily hairs of goat, horse and camel are also commonly used animal fibers. Feathers and feather fibers of chicken and other birds can be regarded as avian fibers. Chicken feather that is mainly composed of keratin are the by-product waste from slaughter houses [3]. The chicken feathers fiber comprises of 91% keratin, 8% water, and 1% lipids [9]. Chicken feather fibers are used in microbial corrosion-resistant application and in production of absorbent sponges and composite sponges for cleaning lenses and highly polished surfaces because of the absorbance nature of keratin [10]. Studies have revealed that chicken feather fiber-reinforced composites showed good mechanical, acoustic, and thermal properties because of their chemical composition, low density, and morphological structure [9].

Silk is usually procured from approx. 14,000 species of butterfly larvae and also from approx. 4000 species of spiders [3]. Silk is the fiber secreted by glands near the mouth of insects during cocoon development. Chitin, a highly structured protein is present in silk which gives the fiber good chemical resistance and mechanical strength. The most widely utilized mulberry silk (Bombyx mori) is composed of two components, fibroin, and sericin. The fibroin consists of 5263 heavy chain and 262 light chain amino acids. It has a tensile strength of about 600 MPa which is relatively more than most of the plant fibers. Silk is used in biomedical applications like tissue engineering and for the production of medical scaffolds by fiber reinforcements in polymers [9]. The silk produced by giant wood spiders or banana spiders (Nephilia) is known as dragline silk. Dragline silk fiber is composed of two highly repetitive proteins, spidroin I and spidroin II. The fiber is semi-crystalline and shows a lower degree of crystallinity than mulberry silk [9]. Amongst the silk fiber from various species, dragline silk is one of the strongest silks with a tensile strength of 1.1 GPa that can be compared to high tensile steel (1.5 GPa) [10].

Plant fibers

Over years, the plant fibers are used to produce clothing materials, automobile interiors, partition boards, packaging materials, and other accessories, thus served as a good source of raw material for various uses [11]. The fibers obtained from plants are mostly composed of cellulose, hemicelluloses, lignin, pectin and wax where their proportion varies from one plant to another and also differs with various parts of the same plant [5]. The chemical composition and physical properties of plant fibers depend on the geography and soil condition, type of plant, age and growth of the plant, part of the plant where fiber is obtained and the mode of extraction of fibers [12]. Plant fibers are of remarkable demand to replace synthetic fibers to use it as environmental friendly renewable reinforcing materials to develop biopolymer composites. Kenaf and sisal are two most commonly used plant fibers instead of glass fibers for producing polymeric composites [13].

Depending on the utility, the plant fibers are classified as primary and secondary fibers. Primary fibers are kenaf, hemp, jute, sisal, cotton, etc. which are from the plants that are mainly grown for fibers. Coir, oil palm, pineapple, and banana leaf fibers that are obtained from their respective plants as by-products are called secondary fibers [3]. For the commercial uses, the plant fibers are widely classified on the basis of botanical origin as bast, leaf, seed, fruit, stalk, grass, and wood fibers. Bast fibers are the ones obtained from the stem of a plant (kenaf, hemp, ramie, flax, jute, banana, etc.), leaf fibers (sisal, agave, abaca, PALF (pineapple leaf fibers), curaua, raphia, fique, etc.), seed fibers (coir, cotton, kapok, soya, rice hulls, etc.), fruit fibers (luffa, coir, oil palm, etc.), stalk fibers (rice, wheat, maize, barley, rye, oats, etc.), grass fibers (bamboo, baggase, esparto, elephant grass, canary grass, switchgrass, phragmites, etc.) and wood fibers are mainly softwood and hardwood (rosewood, teak, etc.) [4]. Besides, certain plants possess more than one type of fiber. For instance, kenaf, hemp, flax, and jute have bast as well as core fibers, while coconut, oil palm and agave have both stem and fruit fibers and cereal grains have hull and bast fibers [11]. Among the plant fibers, the bast fibers show excellent modulus of elasticity and flexural strength whereas the leaf fibers show great impact properties [13].

Kenaf (Hibiscus cannabinus L.) from the family of Malvaceae is a warm season annual herbaceous fiber plant native to Central Africa [14]. Kenaf fiber is a widely accepted bast fiber comprising around 65.7% cellulose and 21.6% pectin and lignin [11]. The fiber possesses excellent tensile and flexural strength, and can substitute fiber glass and other synthetic fibers [13]. Kenaf fiber is suitable for textiles, carpet padding, automobile dashboards and corrugated medium and other extruded or injection molded polymer applications [11]. Flax (Linum usitatissimum) also from the family of Malvaceae is one of the oldest grown fiber crops in temperate regions [15]. Flax fiber consists of 70% cellulose, 20% of hemicelluloses, and 10% lignin and pectin. The fiber shows tensile strength and modulus between glass and aramid fibers [11]. Flax fiber is employed as an alternative to glass fibers for varied applications like reinforcements in concrete and polymers, asbestos replacement, packaging materials, lining materials, panel boards and to manufacture other lighter parts in automotive industries [11,14]. Hemp fiber from an annual herbaceous flowering plant, Cannabis sativa, from the family of Cannabaceae is a widely used bast fiber [16] native to Asia and largely cultivated in Europe [13]. Raw hemp is usually thin, coarser, shiny, and light colored [14] and exhibit excellent strength and water resistance [16]. The tenacity of hemp fiber is 20% higher than flax fiber. Hemp fibers are used in varied applications like textiles, paper, medical supplies, construction materials, automobiles, etc [14].

Jute fiber from annual plant Corchorus capsularis, Tiliaceae family is a versatile strong lignocellulosic bast fiber grown in monsoon climate in India, Bangladesh, and China [15]. For retting process, the plant is cut and soaked in water throughout all seasons. The jute fiber has low gravimetric thickness, low scraped spot, low thermal conductivity, moderate water retention and good mechanical properties. It also possesses excellent isolate and antistatic properties [11]. Jute fibers are used in shopping bags, ropes, and as polymer reinforcement materials. Ramie (Boehmeria nivea) from Urticaceae family is a flowering plant producing one of the easily accessible bast fibers [16]. Ramie fibers are obtained underneath a thin bark layer of the stem. It consists of 73–74% cellulose, 13–15% hemicelluloses, 1.0–5.5% pectin, and 0.6–1.5% lignin. Ramie fibers are finer and strong in dry state and become stronger once wet [11]. It has cellulose content and longer durability when compared to other natural fibers [16]. Besides, the characteristics of ramie fibers match other bast fibers like jute and flax. Out of all bast fibers, ramie, flax and hemp are more resistant to insects and pests and do not require special soil conditions for growth [11].

Sisal (Agave sisalana) from the family of Asparagaceae is native to Mexico, yields a stiff fiber on decortication which is traditionally used for twine and rope making [13,16]. It is commercially grown in India, Indonesia, Brazil, Haiti, and East Africa. Sisal fiber is a strong, coarse and hard fiber. Its annual production is half of the total textile fiber production. The cultivation of sisal plants is comparatively easy as it can withstand hot climates and have potential to grow in arid regions. It is resistant to any insect or pest diseases and can grow in any soil type except clay [17]. Sisal fiber is composed of 65% cellulose, 12% hemicellulose, 10% lignin, 0.8% pectin, and 0.3% wax [16]. It has specific strength and modulus comparable to glass fibers. Sisal fibers are used in polymer composites in automotive applications such as internal engine covers, seat backs, sun visors, package trays, hat racks, door panels, and exterior or under-floor paneling. It is also used in interior paneling in aircraft industries [17]. The abaca fiber obtained from banana plant (Musa textilis) of Musaceae family, is durable and resistant to seawater. It is one of the strongest commercially available cellulose fibers in Ecuador and Philippines [15]. Abaca is used in making cordages and it is a good replacement to glass fibers in automobiles [3]. Pineapple leaf fiber (PALF) which is rich in cellulose is isolated from Ananas comosus, a tropical plant from Bromeliaceae family native to Brazil. PALF is an abundantly available waste product obtained after the pineapple cultivation, which is currently used for varied polymer reinforcement applications [15]. Curaua fibers are leaf fibers obtained from an Amazon forest plant, Ananas erectifolius that is similar to pineapple plant. Compared to other natural fibers, curaua fibers exhibit low density, good tensile strength and elongation at break suitable for practical applications [18].

Coconut fiber or coir is a lignocellulosic fiber located between the outer shell and the husk of coconut (Cocus nucifera), a tropical palm from the family of Arecaceae [13]. It is an abundantly available by-product after coconut processing and mostly used in textiles, ropes, bags, baskets, mats, etc. The durability and hard-wearing quality of coir make it an acceptable reinforcement material in polymers [16]. Coir fiber-reinforced composites are employed in the manufacture of cushions for car and bike seats [3]. Oil palm fiber is also a leading lignocellulosic fiber obtained from trunk, frond, fruit mesocarp, and empty fruit bunch of oil palm (Elaeis guineensis) from Arecaceae family. Oil palm fiber has gained growing acceptance as reinforcement for polymer matrices due to its characteristic properties [16]. Bamboo (Bambusoideae) is an evergreen perennial flowering plant from the grass family Poaceae, which grows up to 40 m height in monsoon climate [15]. Bamboo fibers are obtained from the hard trunk, branches, and leaves of the plant mostly through steam explosion and mechanical treatment [16]. Apart from all the natural fiber properties, bamboo fibers exhibit good ultraviolet (UV) light absorbency and are thus employed in hand-made paper, carpentry and construction applications [11,15]. From studies, it is noticed that bundles of bamboo fibers showed sufficient specific strength comparable to conventional glass fibers [15]. Bagasse is the fibrous residue from sugarcane juice milling process. After harvesting, the sugarcane plant (Saccharum officinarum) is crushed between the serrated rollers in the mill. The cellulose and hemicellulose comprise around 70 % of the sugarcane bagasse [19]. Bagasse fibers are gaining acceptance from many researchers as they can be used as a reinforcement material in composite preparation [13]. Wood flour or wood fiber is one of the mostly used natural fillers for numerous applications [2]. Wood fibers, commonly obtained from the wood waste from different processing (like pulp and paper, carpentry, building and construction, etc.) are employed in reinforcement purposes after a simple sieving process [4]. Recently, many novel plant fibers, namely, Parthenium hysterophorus [20], Impomea pescaprae [21], Catharanthus roseus [22], Coccinia grandis stem [23], Carica papaya bark [24], Cardiospermum halicababum [25], Saccharum bengalense grass [26], Tridax procumbens [27], Cereus hildmannianus [28], Ficus racemosa [29], and Dracaena reflexa [30] were observed as a potential reinforcement for polymer matrices than the existing natural fibers.

Extraction and processing techniques of natural fibers

Fiber extraction process is a critical step governing the qualitative and quantitative nature of fiber. Type of fiber and required application decide the method of fiber extraction [3]. The animal fibers are extracted by different methods according to their origin. For instance, the wool is extracted by manual labor and further washed to remove the impurities [9]. On the other hand, the silk from cocoon is obtained by slightly boiling the cocoon in mild soap solution. The spider silks are obtained by sedating the spider and flipping them upside down to expose the spinnerets. Later the fibers are removed and separated with the help of a brush and microscope [3]. Water vapor exposure, sun exposure, and dry heat exposure are the other silk cocoon stifling processes reported by the researchers [31].

Like animal fibers, there are varied ways of retting to extract fibers from plant sources. Retting is the process of separating and extracting the fibers from non-fibrous tissues of plants by detachment, disintegration, and deterioration of pectin, gums and other adhesive substances [11]. The method of retting determines the yield, quality, structure, chemical composition, and properties of the fibers. Two major points to consider while performing retting process are to completely separate fibers from pectin materials and to cease the retting process at the correct time to prevent over-retting [11]. Thus, continuous screening retting procedure is crucial to guarantee the quality of fibers. Various types of retting are biological retting, mechanical or green retting, physical retting, chemical retting, protein retting, and enzyme retting. Earlier, the most commonly utilized retting methods were water retting and dew retting process. In these methods, the part of the plant takes around 14–28 days to degrade into individual fiber strands [3]. Since, it is a time-consuming process, alternate techniques like mechanical, chemical, and enzymatic methods have been currently encouraged.

Biological retting

Biological retting is of two types; natural and artificial retting [11]. Dew or field retting and water retting are two types of natural retting. Dew retting is usually performed in optimum humidity and temperature ranges. After the plant is mown, the harvest is laid over the fields for the microbes to isolate the fibers from xylem and cortex. Later, the stalks are dried and baled [32]. In water retting, the water penetrates to the central part of the stem and ruptures the outer layer of the harvested plants. Microorganisms present inside the stem of the plant are used to break down large cellular tissue, pectin and various gums around the fibers which are submerged in lakes, ponds, small streams, or artificial water tanks or vats [3] (Figure 1). This method relies on the bacterial inoculums, type and temperature of water, and usually takes about 7–14 days to complete the process [11]. Dew retting is widely used to produce bast fibers. However, it is not a highly preferred method, as it is time-consuming, contaminates water bodies and produces low-quality fibers [3]. Another type of water retting is simulated retting, which is waterway or warm water retting that produces high quality homogenous and clean fiber within 3–5 days [11].

Figure 1. Water retting process

Mechanical or green retting

Mechanical retting is also mostly performed in bast fibers. Unlike water retting, mechanical retting produces better quality fibers within short duration. The mechanical decorticators are made of beaters and rollers through which the plant stems are fed. On repeated feeding of plant stem between the rollers, the gums and skin are removed [3]. The machine finally ribbons the bark, packs the strips and ties up the individual ribbon into a set (Figure 2). The process of ribboning separates the bark from core material and the single portion of bark is called ribbon. Decortication is also a similar procedure to ribboning [11]. Finally, all the retted fibers are collected, washed, and sun dried.

Figure 2. Mechanical retting process

Physical retting

Steam blast strategy and ultrasound are two mainly used physical retting techniques. In steam blast strategy, pressure and expanded temperature are applied along with other added substances to enter into the interspaces of bast fiber packs. The abrupt relaxation of steam causes powerful separation and degradation of fiber composites to fine fibers [11]. In ultrasound retting, the plant stems are cut and washed after reaping. Partially squashed stems are then immersed into water containing alkalis and surfactants as pre-treatment to extreme ultrasound. This continuous process separates the fibers and hurds [11].

Chemical and surfactant retting

In chemical and surfactant retting, the plant parts or straws are immersed into warm water tanks which contain sodium hydroxide (NaOH) or potassium hydroxide (KOH), chlorinated lime, soda ash and corrosive sulfuric acid (H₂SO₄) to break and separate the pectin at high temperature. Oxygen is also supplied in controlled condition for dying and shining of fibers. The fibers obtained from these physical-chemical retting techniques are usually uneven in vibe, coarser and solid. So it is mandatory to modify the fiber strands by manual rubbing to reduce their stiffness [11].

Protein and enzyme retting

Protein retting employs pectin-corrupting pectins to extract fibers from woody tissue [33]. It uses enzymes to control the process by specific biodegradation of pectinaceous substances. By extending the temperature to an ideal temperature, the enzyme action augments and denatures the fibers [11]. In enzyme retting, the fibers are first subjected to pre-treatment with aqueous solution of Triton X-100 (0.85%) and further treated with aqueous solution of pectinase (1.0 %) at 50 °C for 24 hours. The diameters of the fibers are reduced in this retting procedure [3]. Several factors like equipment accessibility, assembly location, availability of industrial markets, and economic variables determine the processing methods and equipments. All the fiber separation techniques have distinctive economic ramifications which aid in harvesting, processing, integrated production and utilization systems [34]. If the innate properties of fiber are not disturbed by the extraction process, that particular process may be considered as its standard fiber extraction process.

Chemical composition of natural fibers

The chemical composition of natural fibers varies with the size of the fibers [2]. Biochemical composition of plant fibers differ from one plant to another with type, age, and maturity within different tissues of the same plant, and also with soil type, climatic condition, fiber extraction and degradation process [4]. Animal fibers are generally composed of proteins such as keratin, fibroin and collagen and chitosan, which govern the strength of the fibers. The protein structure varies with species [3]. Whereas the plant fibers are mainly composed of cellulose, hemicellulose, pectin, lignin and wax that are joined by hydrogen bonds to provide inflexibility and strength [5].

The cellulose forms the main skeleton component of plant fibers. Cellulose is a linear polysaccharide of β-1,4-linked glucose units that impart good strength, stiffness, and structural stability to the fiber [11]. The tighter cell wall in natural fibers is obtained when the celluloses are clubbed together by pectin and this accounts for strength and resistance to lysis in the presence of water [1]. It is a thermally stable polysaccharide having decomposition temperature range between 315 and 400°C. While the hemicelluloses are accountable for the moisture absorption, thermal and biological degradation of plant fibers, as they exhibit least resistance when compared to celluloses [1]. Lignin is an amorphous, cross-linked polymer network composed of irregular array of bonded hydroxy- and methoxy-substituted phenylpropane units [11]. In plant fibers, even though lignin is responsible for UV degradation, it is thermally stable as the decomposition temperature is between 165 and 900°C [1]. Pectin is a structural acidic heteropolysaccharides that is composed of modified glucuronic acid and rhamnose residues. The structural integrity of plant is improved by the pectin chains that are often cross-linked with calcium ions [11]. The cellulosic framework structure in plant fiber composites on a whole is held together by collective function of hemicelluloses, lignin and pectin as a matrix. On an average, plant fibers are mostly composed of 60–80% cellulose, 5–20% lignin, and around 20% moisture [11]. The percentage of chemical composition of each constituent varies with type and parts of plants from where fiber is extracted [1]. The chemical constituents play a key role in predicting both thermal and mechanical performance of fiber and composite [3].

The chemical constituents of fibers are analyzed by several methods. With chemical analysis, the weight percentages of these constituents are determined [3]. To analyze different chemical components, the Technical Association of the Pulp and Paper Industry (TAPPI) standards like T203 cm-99 (α-cellulose) and T222 cm-06 (lignin) are used [3]. To determine total cellulose content, the calorific method apparatus is usually employed [35]. There are numerous American Society for Testing and Materials (ASTM) International standards to analyze the chemical composition of a material. Lignin and cellulose content are determined according to ASTM D1106–96 and ASTM D110360 standard, respectively [36]. The holocellulose is determined according to ASTM D1104–56 by delignification and mercerization methods [3]. The moisture content of fibers can be determined by an electronic moisture analyzer. The ash content of fibers is analyzed by ASTM E1755–01 and is expressed in mass residue percentage obtained after dry oxidation [3]. The percentage of wax and other fatty substances are determined by Conrad method with Soxhlet extraction [37].

Physical properties and characterization techniques of natural fibers

Unlike chemical composition, the physical properties of natural fibers shows great differences among each other [38]. Animal as well as plant fibers do soften, shrink or expand on heating or become brittle and ductile on cooling and are prone to microbial degradation [39]. Physical properties of natural fibers are dependent on various factors like species, age and maturity of plant, geographical location, ecotype (sun, rain and soil conditions) and fiber extraction methods [5,38]. The natural fiber properties, especially mechanical properties depend on the amount of cellulose, microfibril angles, and the degree of polymerization of cellulose in the fibers [40].

Knowing the characteristics of fiber is essential to expand its utilization and application in composites preparation with improved performance. The structure and morphology of natural fibers are greatly controlled by the amount of lignin present in the fiber [4]. Major structural differences in cell wall length, thickness, diameter and density develop huge differences in physical properties [15]. The surface roughness, cell wall structure, diameter, and other morphological characteristics of fibers are determined by scanning electron microscope (SEM) and atomic force microscope (AFM). SEM analysis gives a detailed high-resolution image of the fiber by scanning through an electron beam, as it is focused over the desired surface by detecting the secondary and backscattered electronic signal [3]. AFM is a very high-resolution type microscope of scanning probe microscope (SPM). It shows resolution on the order of fractions of nanometer (nm), which is 1000 times better than optical diffraction limit. Surface roughness parameters like average surface roughness (Ra), maximum peak to valley height (Rt), and root mean square roughness (Rrms), etc., are determined by AFM [3]. The surface roughness of fibers is also determined by vertical scanning methods like confocal microscopy, confocal chromatic aberration and coherence scanning interferometry (CSI), and by horizontal scanning methods like structured light scanning and scanning laser microscope. The non-scanning methods like digital holography microscopy are also used to determine the surface roughness [3]. The diameter and surface roughness of the fibers play a significant role in deciding the wettability nature of fiber with matrix and mechanical strength of the composites. The wettability of natural fibers by water is determined by contact angle measurement, and it depends on the nature and amount of waxy substance in the fibers [4]. The density of the fiber is studied with the help of pycnometer experimentation using toluene and real density analyzer (helium pycnometer). It is also determined by ASTM D3800 M based on the Archimedes method using hexane [41].

Fourier-transform infrared (FTIR) spectroscopy is an experimental technique to qualitatively and quantitatively analyze the organic compounds, and thereby provide necessary information on molecular structure and chemical bonding in natural fibers [1]. Out of the two FTIR spectroscopy methods; absorbance and transmittance methods, the FTIR transmittance method is widely employed for natural fibers [3]. The spectrograms are determined as the specific peaks which appears in wave number versus transmittance graph that depends on the stretching, bending and vibration of the functional groups in the fiber [30]. X-ray diffraction (XRD) is a nondestructive analytical method used to examine the crystallographic structure, crystal size and crystallinity index of natural fibers. The scattered intensity of an X-ray beam hitting a fiber sample is observed as the function of wavelength or energy, polarization, incident and scattered angle [1]. The two distinct peaks of diffractogram are detected in plant fibers. The first peak is observed around at 2θ = 18°, indicating the presence of amorphous constituents like, hemicellulose, pectin lignin, wax, etc. The second peak is observed around 2θ = 22° corresponding to cellulose I [3]. The crystal size and crystallinity index play a major role in enhancing the mechanical properties of fiber-reinforced composites, and by surface modification or chemical treatment, the same can be modified. As the crystal size increases the hydrophobicity of fiber and composite are also improved [3].

Natural fibers possess good acoustic insulation and thermal properties as these fibers are hollow and lignocellulosic in nature [2]. The chemical composition and thermal performance are determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). TGA is also used to find out the thermal stability and weight loss percent of natural fiber on degradation. Amongst natural fibers, the plant fibers show good thermal insulation and are used to develop products, which can be used for thermal applications [3]. Fiber strength is a major criterion for selecting natural fibers for specific applications [15]. Even though the mechanical properties of natural fibers are comparatively lower than synthetic fibers, they can be made equal or even greater in mechanical performance by appropriate surface modification techniques. Unlike synthetic fibers, the low density, high-specific modulus and elongation at break attract many industries to employ natural fibers for various polymer engineering composite applications [2,5]. Usually in plant fibers, the cellulose content affects the mechanical properties [4]. The hydrogen bonds and other linkages in the cellulose determines the strength and stiffness of fibers [40]. As the cellulose content increases the tensile strength and Young’s modulus were found to increase [4]. Thus the overall performance of a fiber for specific application depends on major factors like structure, cell dimension, density, microfibrillar angle, chemical composition, mechanical properties, and fiber–matrix interaction under specific environmental conditions [42].

Drawbacks of natural fibers

The natural fibers have some innate drawbacks such as irregularity in the size, shape, and diameter of the fiber and their effect on the density, hardness, and strength of the composite [3]. Most of the existing fiber extraction methods are a time-consuming process, especially for fibers like silk. Thus, better extraction techniques need to be identified to produce fibers in an environmentally friendly way. Other major problems of natural fibers are the poor resistance to microbes and pests, high moisture sensitivity, and moderate thermal stability [4]. The low microbial resistance and high moisture sensitivity increase the chance of getting rotten by spoilage, and subsequently releasing unpleasant smell [43]. Plant fibers are susceptible to decomposition in humid and dark environment, while animal fibers like silk and wool are susceptible to pest and insect infestations. One way to get rid of this problem is to develop natural fibers with anti-microbial and anti-pest coupling agents to prevent microbial degradation and pest infestation, respectively [3]. The high hydrophilicity characteristic of natural fibers, obstructs in widening their reinforcement in composites in automotive applications. The function of natural fibers as compliance products is affected by the hydroxyl (OH) and other polar groups and various physical impurities found in the fibers [2]. Another key resistance of natural fiber composite application is the innate hydrophilicity of fibers and hydrophobicity of polymer matrices, resulting in poor coupling at the interface and weak interfacial bonding of the composite that leads to poor performance [2,40]. The unpredictable durability that affects both processing and application of natural fibers is one major obstacle to use them in composites [38]. To overcome these shortcomings, the natural fibers are subjected to biological, physical and chemical treatments to remove impurities, improve water resistance, mechanical properties, and interfacial bonding with the composites.

Modification methods of natural fibers

Since natural fibers contain several OH groups within cellulose and lignin compounds they are very much flexible to modifications [11]. The chemical modifications activate these groups or introduce new functional groups which make the fibers interact effectively with the matrix and improve the compatibility between fibers and biopolymers [4]. By reacting with the OH groups, chemical treatment reduces the hydrophilicity, improves the moisture resistance property and ameliorate the mechanical properties of fibers [43]. Overall the natural fiber modification brings about alteration in the physical, chemical, thermal and mechanical properties of natural fibers and it depends on various factors like method, duration, temperature and chemical concentration. The surface modification of natural fibers can be classified into physical and chemical methods.

Physical methods

The physical treatments are mainly done to separate fiber bundles into individual fibers and to modify fibers for composite applications. The physical process in which bundles of natural fibers are separated to discrete filaments is one of the oldest fiber modification methods. The natural fibers are physically modified by stretching, calendaring, thermo treatment, or by electrical conduction [11]. By these treatments, the structural and surface properties of natural fibers are altered to augment their mechanical bonding to polymers. Other physical methods employed in treatment of fibers are plasma treatment, corona treatment, steam explosion treatment, dielectric barrier technique, laser, and γ-ray treatment [44]. Treatments like plasma and corona aid in surface cleaning of fibers by removing the unwanted materials and functionalizing them [4].

In plasma treatment, the surface of the fiber is exposed to very high voltage, which is supplied for ionizing the gas and to use it as plasma in vacuum conditions [4]. The treatment improved the surface strength of the fiber by crosslinking the fiber surface by introducing free radicals [11]. Oxygen plasma refers to plasma treatment by introducing oxygen to the plasma chamber. Oxygen plasma treatment removes the organic contaminants by chemical reaction with highly reactive oxygen radicals [4]. With this method, the surface of the fiber is cleaned by surface etching and improved the surface roughness for enhancing the adhesion and interfacial bonding by creating interlocks with the matrix [45]. Oxygen plasma treatment also promotes surface hydroxylation, oxidation, and increases the wetting property of the natural fiber surfaces [46]. Corona treatment carried out in a corona discharge reactor modifies the fiber surface oxidation to improve the interface between hydrophilic fiber and hydrophobic matrix [47]. The treatment improves the mechanical strength by changing the surface energy [42], and acidity of the fiber surface [48]. Steam explosion process is a high-pressure steaming, which requires heating of lignocellulosic fibers at high pressures and temperatures, followed by mechanical disruption of pre-treated fibers to a collecting tank by violent discharge (explosion). Steam explosion process resulted in reduced stiffness, smoother surface, better fineness distribution, and improved bending properties of the fibers. This process is used in lignocellulosic fibers to improve the dispersibility and adhesion with polymeric matrices [44]. The vacuum ultraviolet (VUV) irradiation is a widely accepted method for removing impurities from fiber surface. The fibers were placed in a stainless steel chamber and high energy radiations <200 nm and pressure of 2.5 Torr is applied at room temperature [49]. The treatment enhances the wettability, insulation, tribological properties, and biocompatibility of fibers with the matrix [3]. γ-rays treatment enhances the inter-crosslinking of cellulose molecules and improves the mechanical strength of the fibers [50]. Laser treatment is a way to remove the lignin content in plant fibers and to ameliorate the structural and tensile strength of the fibers [49]. Ozone or oxygen-fluorine gas is a successful method to enhance the surface of the natural fibers. The mechanical properties of natural fibers can be maintained for a long period by ozone treatment [51].

Chemical methods

Chemical treatments that are frequently used are bleaching, mercerization, and acetylation. Other chemical treatments are benzoylation, silanization, acrylation, grafting and use of peroxide, permanganate, triazine, isocyanate, maleated coupling agents, and fatty acid derivates [11]. Bleaching is the process of soaking natural fibers in bleaching solution like sodium hydroxide (NaOH) and hydrogen peroxide (H₂O₂). By alkalization the fine structure of native cellulose I changes to cellulose II [52]. Alkali treatment removes the OH bond by dissolving the amorphous constituents, improves the surface roughness and interfacial bonding between the fibers and matrix [49]. It also enhanced the thermal stability and tensile strength of the fibers [53,54]. Research has shown varied types of alkali treatments. For instance, the alkaline treatment with 6% NaOH increased the crystallinity index by removing the amorphous compounds from the fiber [55]. In another study, Borassus fruit fine fibers were treated with 5% NaOH solution for varied time interval (1, 4, 8, and 12 h). It was noted that after 8 h of alkali treatment, the tensile properties such as strength, modulus, and percentage of elongation of fibers increased by 41%, 69%, and 40%, respectively [56]. It was also observed that alkali-treated natural fiber composites exhibited increased tensile strength when compared to untreated fiber composites [11]. Mercerization is also an alkali treatment of fibers that depends on the type and concentration of the solution, temperature, duration, and tension on the fiber and additives [42]. Mercerization improves the surface roughness of fiber by dissolving lignin and waxes and thereby increases the cellulose crystal exposure [4].

Acetylation or esterification is another surface modification process for plasticizing of natural fibers in which an acetyl group is introduced into the organic compound in natural fibers. This method modifies the OH groups and makes the fiber more hydrophobic in nature [42] and enhances the dimensional stability of the composites. The treatment imparts rough surface topography with a lower number of voids, and that provides strong mechanical interlocking with the matrix [35]. Acetylation with acetic acid to improve initial thermal degradation and tensile strength involves submerging the fibers in 10% acetic acid along with concentrated sulfuric acid solution. Further, a solution of acetic anhydride with necessary concentration is also added [57]. Benzoylation treatment uses benzoyl chloride to reduce the hydrophilic nature of fiber, enhance interfacial adhesion, and thus improving the composite strength. For benzoylation process, alkali pre-treatment is done to remove the extractable materials such as lignin, waxes, and oil covering materials, and OH groups are thus exposed to the surface of the fibers. Later, the fibers are treated with benzoyl chloride, where the OH groups are replaced by benzoyl groups and get attached on the cellulose backbone [35]. The peroxide-induced grafting of biopolymer adheres over the fiber surface. The peroxide initiated free radicals react with the OH group of fiber and matrix. Thus, peroxide treatment improves the interface properties of fiber and matrix. It also slows down the moisture absorption and enhances thermal stability [35]. Benzoyl peroxide treatment enhances the interfacial bonding of fiber and matrix by removing the OH bond. It involves pre-treatment of fibers with NaOH solution followed by treating the fibers with required amount of benzoyl peroxide with acetone [58].

Acrylation uses acrylic acid to improve the interfacial bonding between the fiber and matrix. Acrylic acid reacts with the cellulosic OH groups of the fiber and provides more access of reactive cellulose macro-radicals to the polymerization medium. The carboxylic acids from coupling agents form ester linkages with cellulosic OH groups, and reduces hydrophilic OH groups from the fiber and promote moisture resistance properties [35]. For fiber surface modification, isocyanate works as a coupling agent. The functional group of isocyanate reacts with OH groups of cellulose and lignin in fiber, and forms a urethane linkage. The linkage imparts strong covalent bonds between fiber and matrix [35]. Stearic acid in ethyl alcohol solution is used for the modification of fiber surface. The carboxyl group of stearic acid reacts with the hydrophilic OH groups of the fiber and enhances moisture resistance. The treatment removes non-crystalline constituents from fiber, which leads to the breakdown of fiber bundles and fibrillation occurs. Fiber dispersion into the matrix facilitates better bonding at the interfacial region and improves composite properties [35]. Permanganate treatment on natural fibers is done by using potassium permanganate (KMnO₄) in acetone solution. The treatment produces highly reactive permanganate ions that react with the cellulose OH groups and forms cellulose-manganate for initiating graft copolymerization. This improves the chemical interlocking at the interfacial region and increase the adhesion with matrix [35]. The treatment of fibers with potassium permanganate also removed impurities and waxes from fiber surface and improved its physico-chemical properties [59].

Silane is a multi-functional molecule which is used as a coupling agent to modify fiber surfaces. The composition of silane forms a chemical link between the fiber surface and the matrix through a siloxane bridge [35]. Silane treatment improves the physico-chemical properties and tribological performance of the natural fibers [60]. Grafting by applying compatibilizers (maleic anhydride modified polymers) into natural fiber composites is a popular method. Maleated coupling agents helps in efficient interaction of functional surface of fiber and matrix. During grafting, maleic anhydride reacts with the OH groups in the amorphous region of cellulose structure and removes OH groups from the fiber cells. It creates a brush-like long chain polymer coating on the surface of the fiber and reduces hydrophilic characteristics. Thus, maleated coupler forms carbon-carbon bond with the polymer chain of matrix. The covalent bond created between OH groups of the fiber and anhydride groups of the maleic anhydride helps in bridging interface for efficient interlocking [35]. Numerous studies have reported the improvement of mechanical properties of natural fibers by maleic anhydride grafting method. In a study with PALF, it was found that graft copolymerization technique enhanced the chemical constituents and thermal stability of the fiber [61]. Another possible way to modify fiber surface is by coating them. Coating imparts more homogeneous fibers by smoothing the surface by filling the imperfections [56]. It is also found that the compatibility of fiber and the matrix is enhanced by polymer coating over natural fibers [53].

Biopolymer

Biopolymers are the polymers that are derived from plants, animals, and microbes. They are abundantly available renewable materials usually employed to produce eco-friendly bioplastics [1]. Biopolymers are produced commercially on a large scale for varied applications. Despite the fact that biopolymers make up only a small percentage of the polymer market, it has been predicted that in future they might replace petroleum based polymers about 30–90 %. Mostly biopolymers are biodegradable except few, that is, they have the ability to microbiologically decompose into carbon dioxide (CO₂), water (H₂O), methane (CH₄), and inorganic compounds [38]. The degradation capacity of biopolymers is dependent on many factors like type of polymer, chemical composition and environment conditions [3]. Recently produced biodegradable polymers possess a wider range of properties which are very much comparable with traditional polymers used in the market. The biopolymers are employed in specific fields depending upon their cost, availability, moisture absorption, thermal stability, mechanical behavior, degradation stability, and biocompatibility [38]. The chemical constituents, molecular weight, morphological characteristics, mechanical attributes, and processing technique of a biocomposite are governed by the biopolymer part of the composite [62]. In spite of wider applicability, few shortcomings of biopolymers are their hydrophilic nature, low mechanical properties and low durable degradation ratio in moist environment [11].

Types of biopolymers

One easiest way of classification of biopolymers on the basis of their origin is natural and synthetic biopolymers. Natural biopolymers are from polysaccharides, proteins and microbes whereas synthetic biopolymers are the polymers developed by microbial fermentation, biotechnological and petrochemical production [63]. With regard to biopolymers’ response to thermal conditions, they can be divided into elastomers, thermosets, and thermoplastics [62]. PLA and PHA are two most commonly used biopolymers with regard to production and application [38].

Polysaccharides made up of monosaccharides are linked by glycosidic linkages. Polysaccharide-based biopolymers are cellulose, chitin and chitosan, starch, hyaluronic acid (HA), dextrin, alginate, carrageenan and various gums. Cellulose, the most abundant carbohydrate is a promising biodegradable polymer seen in almost all plant materials [64]. Cellulose is eco-friendly, has a low energy requirement during manufacturing, possesses remarkable film-forming properties and is easy to recycle by combustion [65]. Cellulose is a structural polysaccharide made up of microfibrils linked together to make cellulose fibers [66]. Chitin comprises 2-(acetylamino)-2-deoxy-D-glucose residues linked by covalent β-1,4 linkages. Chitin is a polysaccharide that is acidic or neutral in nature which is hydrophobic and insoluble in water and many organic solvents [67]. The second most abundant natural amino polysaccharide is chitosan. It is obtained by deacetylation of native chitin isolated from shrimp and other crustaceans [63]. Various functionalities of chitosan like oxygen permeability, water sorptivity, cytokine induction, and biodegradability make it suitable for preparing scaffolding materials [67]. Starch is a storage polysaccharide that consists of linear amylose and branched amylopectin chains of size 0.1–1 nm. The major sources of starches are roots, tubers and cereals. Thermoplastic starches (TPS) are the type of starch that is thermally and/or mechanically plasticized to substitute the use of synthetic polymers [68]. TPS is a promising material in biopolymer composites due to its availability, renewability, recyclability, and thermoplastic behavior [69].

Proteins are large biological molecules made up of one or more long chain amino acid residues. Protein can be classified on the basis of origin such as animal proteins (e.g. collagen, gelatin, silk, keratin, casein, and whey) and plant proteins (e.g., gluten, zein, pea protein, and soy protein). Collagen found in cartilages, tendons, bones, skin, and connective tissues is the most abundant protein on earth [63]. The properties like biodegradability, water absorption, high mechanical strength, biocompatibility, cross-linking ability, and low antigenicity make it a suitable material for bioactive molecule delivery and tissue engineering [67]. Gelatin is a water-soluble denatured fibrous protein prepared by the partial or controlled hydrolysis of insoluble protein, collagen [70]. Gelatin is a preferred biopolymer with OH, NH₂, COOH, etc. functional groups which aid in the production of biocomposites. Gelatin is a widely accepted protein biopolymer due to its abundance, notable biodegradability, biocompatibility, film-forming property, gelling ability, superior gas barrier properties and cost effectiveness [70]. Silk fibroin is a natural protein isolated from the silkworm cocoon. It is primarily comprised of alanine, glycine, and serine [67]. The core is made up of fibroid and is covered with sericin. Sericin is used in wound dressing and other medical materials [71]. Soy protein isolate (SPI) is a commercially graded by-product of soy bean oil which comprises over 90 % protein. SPI is a biopolymer that can be utilized as a potential alternative to petroleum-based polymers. Soy-based biopolymers possess superior mechanical properties because of the polar and functional groups of amino acids that help in inter- and intra- molecular interactions [64]. Wheat gluten is a biopolymeric material containing approx. 50% gliadin and approx. 50% glutenin proteins obtained as a co-product from food processing industries. Gluten contains cysteine that determines the functional and mechanical properties because of its inter and intra-molecular disulfide bonds [72]. The heat treatment helps in improving the intermolecular disulfide bonds, which results in polymerization and thermosetting of gluten [73]. Thus, polymer processing techniques like extrusion, compression, or injection molding are used to develop gluten-based bioplastics. Biodegradable composites possessing specific strength and stiffness are prepared by reinforcing natural fibers such as hemp, flax, banana, coconut and wood fibers to gluten matrix, and are used in packaging, medical, and automotive sectors [72].

Polylactide or PLA is a promising synthetic biopolymer which has unique characteristics such as, strong clarity and high rigidity to substitute conventional polymers [2]. It is a biodegradable, renewable and recyclable hydrophobic aliphatic polyester showing excellent processing ability [40]. The PLA synthesis is a multi-step process starting from lactic acid production to its polymerization [74]. Lactic acid (LA), which is the basic constitutional unit and main precursor in PLA synthesis, is largely produced from lactic acid bacteria (LAB) fermentation of a carbon source [67]. The outstanding mechanical and barrier properties of PLA make it suitable for producing various biomaterials for several industrial applications in textiles, packaging, biomedical and automotive manufacturing industries. The research work on natural fiber-reinforced PLA composites has revealed noticeable benefits like renewability, recyclability, high durability, strong processability, high-specific resilience, and compostability [2]. PHAs are the only biopolyesters family derived from microbial fermentation of sugars or lipids. They are prepared by exposing bacteria to carbon source and limiting all other necessary nutrients [1]. It is an attractive biopolymer used in pharmaceutical, tissue engineering, and conventional medical devices because of its hydrophobicity, biocompatibility, biodegradability, and thermoplastic properties [67,74]. Polyhydroxy valerate (PHV) or polyhydroxybutyrate (PHB) and their copolymers poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) with different molar ratios are the representatives of PHAs which are all biodegradable and biocompatible [14]. The mechanical properties of PHBs are similar to the properties of petroleum-based plastics such as polyethylene (PE) and polypropylene (PP) [67]. The PHAs do not need any arable land for the production and can be even produced from CO₂ in a marine environment [75,76].

The biodegradable polymers derived from petrochemical resources are polybutylene succinate (PBS), poly(butylene adipate-co-terephthalate) (PBAT), polyvinyl alcohol (PVA), polyglycolic acid (PGA), and polycaprolactone (PCL). PBS are easily processed semi-crystalline thermoplastic polyesters with acceptable thermo-mechanical properties comparable to PP [77], and thus widely used in bottles and films production. PBS is made up of 1,4-butanediol and succinic acid from renewable resources [14]. The tensile strength (30–35 MPa) of PBS is similar to that of PP and the Young’s modulus (300–800 MPa) is between low-density polyethylene (LDPE) and high-density polyethylene (HDPE) [74]. PBAT belonging to the family of aliphatic-aromatic copolyesters is a biodegradable copolymer of terephthalate and butylene adipate produced by melt polycondensation process [74]. PVA is a hydrophilic semi-crystalline biopolymer exhibiting good mechanical strength and biocompatibility. The properties of PVA make it a promising material for biological as well as industrial applications [63]. PCL is a hydrophobic semi-crystalline aliphatic polyester prepared by ring-opening polymerization of ε-caprolactone. PCL is tough and ductile (elongation at break ~600–800%) and has low glass transition temperature (~ −60°C) and melting temperature (~60°C) [74]. It is mainly used in packaging and biomedical applications.

Biopolymer composites

Composites are heterogeneous structural material mixtures prepared by mixing two or more constituent materials exhibiting remarkably varied properties [38]. The term “biopolymer composites” can be referred to as the biodegradable composites made by reinforcing various natural fibers from animal and plant sources to natural and/or synthetic biopolymers [1]. In biopolymer composites, natural fiber (reinforcement agent) in discontinuous phase is added to the biopolymer matrix in continuous phase to improve the stiffness and tensile strength of the prepared composite. The purpose of preparing this kind of composite is to obtain a product with good mechanical behavior and durability performance imparted by natural fiber and biopolymer, respectively. Usually, maximum stiffness and tensile strength of biocomposites ranges between 1 to 4 GPa and 20 to 200 MPa, respectively [38]. The natural fiber reinforcements also affect the thermal properties, electrical conductivity, morphological characteristics, crystallinity, degradability, and production cost of biopolymer composites. The primary applications of natural fiber reinforced polymer composites are in the fields of automobiles, aerospace, and structural constructions [4]. Some noticeable benefits of biopolymer composite applications are sustainability, cost effectiveness, lightweight characteristics, appreciable specific strength, biodegradability, environmental friendliness of renewable materials and health and safety of manufacturer and consumers [11].

The fibers and matrices can be made by two methods, bulk and laminate composites. The fibers are randomly oriented in three dimensions structure, almost exhibiting isotropic behavior in bulk composites. On the other hand, laminate composites are orthotropic, where fibers are oriented in multiple layers and bound together in the matrix. In this composite, each fiber layer shows a two-dimensional orientation [38]. Adhesion of natural fiber to polymer matrix can be by inter-diffusion, adsorption, chemical bonding, reaction bonding, electrostatic attraction, and mechanical bonding. The mechanisms of adhesion depend on many factors such as chemical composition and molecular conformation, morphological property of natural fiber, diffusivity of element materials and also the atomic arrangement of fiber and matrix [1]. The biopolymer composites properties are affected by the type of fiber, percentage of fiber content, moisture absorption of fiber, surface modification method of fiber, structure, and design of composite, interfacial adhesion between fiber and matrix, presence of voids and incorporation of additives like plasticizers, compatibilizer, nanofiller and binding agent [2,35]. Various reinforcement materials and plasticizers affect the density, water sensitivity, gas permeability, degradability, and shelf life of biopolymer composites. Thus, the hydrophilicity of natural fiber and hydrophobicity of biopolymer drives the researchers to surface modify the fibers to improve the adhesion between them in a composite. The performance of biopolymer composites is improved by chemical modification depending on the processing type, processing requirements, and environmental conditions [3]. Biocompatibility and durability of biopolymer composite are of major concern as there is no proper solution adapted till now to completely control these two factors.

Processing techniques of biopolymer composites

Many processing techniques have been developed and employed for biopolymer composites according to their application. Commonly used processing methods of biopolymer composite preparations are compression molding, injection molding, and extrusion. Thermoforming, compounding and long fiber thermoplastic-direct (LFT-D) methods are also used for natural fiber-reinforced thermoplastics. In compression molding, the flat semi-finished products or hybrid fleeces are mostly used that are either cut exactly to the size of the required part or are larger than the form [78]. Ramie fiber incorporated PLA/PCL matrix-based thermoplastic biodegradable composites along with silane coupling agent for increasing interfacial adhesion was prepared by compression molding process using in situ polymerization method. The effect of fiber content and fiber length on the tensile and impact strength was determined and found that the same were at the highest when 45 wt. % fiber content with 5–6 mm length were added [79]. For injection molding process, the raw materials are incorporated as fiber-reinforced granules to the injection molding machine and melted to liquid state. Further, under high pressure, the plasticized thermoplastic material is added into the form [78]. The mechanical, thermal, and other properties of biopolymer composites prepared by different processing techniques are summarized in Table 1.

Table 1. Improved properties of various biopolymer composites prepared by different processing techniques

Matrix system	Reinforcement	Processing technique	Remarkable characteristics of biopolymer composites	References
PLA	Silkworm silk fiber	Extrusion and injection molding	The modulus of elasticity, ductility, glass transition temperature, and thermal stability increased. The composites can be used to produce polymeric scaffolds for tissue engineering applications	[80]
PLA	Chicken feather fiber	Extrusion and injection molding	Tensile moduli and thermal stability increased	[81]
Wheat starch	Alkali treated wheat straw fiber	Extrusion	The mechanical properties improved	[82]
PBS	Kenaf fiber	Melt mixing	The crystallization rate, tensile modulus, and storage modulus improved	[83]
Maleic anhydride grafted PCL	Bamboo fiber	Hot press molding	Tensile strength and elongation at break increased, and rate of biodegradation decreased	[84]
SPC	Mercerized sisal fiber	Soaking and squeezing	Young’s modulus and fracture stress increased	[85]
PLA	Silane treated ramie fiber	Two-roll milling and hot pressing	Tensile, flexural, and impact strength increased. Dynamic mechanical properties and thermal degradation resistance improved	[86]
PBS	Kenaf fiber	Compression molding	Flexural strength and modulus improved	[87]
PHB	Kenaf and lyocell fibers	Compression molding	The mechanical properties except elongation at break increased	[88]
PLA	Kenaf and lyocell fibers	Compression molding	Tensile and impact strength, Young’s modulus, and flexural modulus increased	[88]
PLA	Alkali treated hemp	Extrusion and injection molding	Crystallinity and crack propagation improved	[89]
PLA	Poly(butyl acrylate) treated rice straw fiber	Melt-compounded in an HAAKE rheometer and hot-press molded	Hydrophobicity, tensile strength, and thermal stability increased	[90]
PBS	Alkali treated coir fiber	Hot press molding	Tensile strength and modulus, and flexural strength and modulus increased	[91]
PHBV	Detergent washing, dewaxing, alkalization, and acetylation on jute fibers	Compression molding	Flexural strength, tensile modulus, and dynamic modulus increased	[92]
PLA	Hemp	Film stacking method	The coefficient of thermal expansion decreased	[93]
Pehuen starch	Pehuen husk fiber	Melt blending	Mechanical properties and thermal stability improved	[94]
PLA	Alkali, permanganate, peroxide, and silane treated jute fibers	Hot pressing of solvent impregnated prepregs	Tensile and flexural properties improved	[95]
PCL/PLA blend	Flax fiber	Hot press molding	The mechanical properties improved	[96]
PLA	Jute fiber	Injection molding	Tensile modulus and impact strength increased	[97]
PLA/PCL	Silane coupled jute fiber	Hot press molding	Tensile strength and modulus increased	[98]
PLA	Alkali, silane and maleinized polybutadiene rubber treated chicken feather fiber	Twin-screw extrusion	The mechanical properties improved	[99]
PHBV	Kenaf fiber	Melt blending	Impact strength and flexural moduli improved	[100]
PCL	Acylated cork fiber	Melt blending	Thermal stability improved. The composite can be used for composite panels in automotive industries and flooring and insulation panels in construction industries	[101]
PLA	Hemp fiber	Blending-spun, twin-screw extrusion and injection molding	The mechanical properties improved	[102]
PLA	Oil palm fiber	Mixer and injection molding	Tensile strength, flexural strength, and heat deflection temperature increased	[103]
PLA	Curaua leaf fiber	Twin-screw extrusion	Impact strength, Young’s modulus and thermal stability increased	[104]
Wax/ethylene acrylate copolymer/butyl acrylate modified PLA	Flax fiber	Hot press molding	Stiffness and impact resistance increased	[105]
PLA	Silane treated bamboo fibers	Injection molding	Impact strength and elongation at break increased	[106]
PLA/PCL blend	Silanized oil palm fiber	Melt blending	The mechanical properties and thermal stability improved	[107]
PLA	Chicken feather fiber	Extrusion and injection molding	Compressive strength, Young’s modulus, flexural modulus, and hardness improved	[108]
Wheat starch	Alkali and silane treated hemp fiber	Mixer blending	Stiffness increased	[109]
PLA	Alkali, corona discharge, maleic anhydride grafted and aminopropyltriethoxysilane treated flax fiber	Twin-screw extrusion	Impact strength and elongation at break increased.	[110]
PHBV	Silk fiber	Matrix assisted pulsed laser evaporation technique	Resistance improved and degradation rate decreased. The composites can be used for drug release schemes and other biomedical applications	[111]
PBS	Eugenol grafted coconut fiber	Hot press molding	The mechanical properties improved	[112]
PLA	alkali and silane treated sisal fiber	Melt blending	Tensile strength increased	[113]
PLA	Titanium oxide grafted flax fiber	Hot press molding	Tensile strength, impact strength, thermal stability and hydrophobicity improved	[114]
Chitosan	Emu feather fiber	mixture was dropped into an alkaline coagulant solution	The composite can be used as an absorbent for treating industrial waste water containing lead	[115]
Sugar palm starch	Seaweed blended sugar palm fiber	Melt blending	The thermal stability, tensile and flexural properties improved. Composites exhibited good water and biodegradation resistance	[116]
PLA	Untreated sisal fiber and alkali treated sisal fiber blend	Compression molding	The impact properties improved	[117]
Corn starch	Curaua leaf fiber	Injection molding	The hardness, tensile modulus, tensile and impact strength increased	[118]
PVA	Acylated and chitosan grafted silk fiber	Film casting technique	Thermal stability increased. The composite films exhibit good anti-bacterial and anti-fungal properties which is suitable for wound healing applications	[119]
PVA	Acylated and chitosan grafted silk fiber	Film casting technique	Surface roughness increased. The haemocompatibility of composite films can be used for tissue engineering applications	[120]
Tapioca starch	Water hyacinth fiber	Casting method	Tensile strength, thermal stability, and hydrophobicity improved	[121]
Tapioca starch	PALF	Compression molding and machine processing	The mechanical properties improved	[122]
Potato starch	Sugar beet pulp fiber	Piston and wooden molding	The mechanical properties and thermal conductivity improved. The composites can be used as building materials because of its good acoustical and thermal performance	[123]
PLA	Maleated PLA treated agave fiber	Rotational molding	Thermal stability, tensile strength and modulus increased and hydrophilicity, diffusion coefficient and damping behavior decreased	[124]
Corn starch	Hemp fiber	Dry incorporation method	The mechanical properties improved. The composite boards can be used in building industries	[125]
Wheat gluten	Wool fiber	Compression molding	Tensile strength and flame retardancy improved	[72]
PLA	Silane treated flax fiber	Manual hot plate compression molding	The mechanical properties improved	[126]

Characterization of biopolymer composites

The most commonly employed characterization techniques for determining the interface of biopolymer composites are FTIR spectroscopy, laser Raman spectroscopy (LRS), solid-state nuclear magnetic resonance (ssNMR) spectroscopy, ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), wide-angle X-ray scattering (WAXS), and also contact angle measurement [1]. For examining the particle distribution, reinforcements in the matrix, surface interaction between the fiber and biopolymer and voids, microscopic visualization techniques like polarized optical microscope (POM), scanning tunneling microscope (STM), AFM, SEM, and field emission scanning electron microscope (FESEM) are employed. The images are captured at required magnifications for easy visualization analysis. AFM uses high resolution nondestructive analysis for determining the surface roughness of fibers or biopolymer composites [3]. These morphological techniques also give information of surface morphology, surface depth profile, physico-chemical changes and fiber–matrix interaction [1]. Saxena et al. [127] in their study reinforced acacia fibers, bleached softwood kraft fibers and nanocrystalline cellulose to xylan/sorbitol films, and examined the impact of reinforcement on water transmission. From the experiment, it was revealed that the control film has a more open structure than nanocrystalline cellulose composite films. The FTIR analysis exhibited a strong interaction at the OH band indicating good interfacial bonding between nanocellulose and matrix surface, and the intensity of band depends on the nanocellulose concentration. The effect of natural fiber on crystallinity of biopolymer composites was studied with the help of XRD. The results of XRD are mostly correlated to the FTIR analysis, thermal, mechanical and barrier properties of the composites. It is widely noted that, by the incorporation of natural fibers to the biopolymer composite the interfacial bonding and crystallinity of the composite is improved which thereby lead to an enhancement of the mechanical properties [1].

Mechanical properties of biopolymer composites

Natural fiber-reinforced biopolymer composites exhibit appreciable mechanical properties needed for different applications. There are many aspects which affect the mechanical properties of biopolymer composites, like chemical composition and physical properties of natural fiber and biopolymer, surface modification of fiber, composite processing techniques (temperature and force applied), processing environment, fiber loading concentration, orientation of fiber in the matrix, copolymerization and plasticization [2,3]. The mechanical properties are also influenced by the presence of waxy substances in fiber which affects the adhesion and wettability characteristics of the composites [128]. The mechanical properties of biopolymer composites, mainly, tensile, flexure, impact, dynamic mechanic thermal property, toughness, hardness, ductility, brittleness, creep and fatigue are been discussed.

Tensile properties

One of the most popularly employed tests for determining the mechanical properties and knowing the structural design of biopolymer composites are tensile tests. Young’s modulus is the ratio between stress and strain (in short known as stiffness) of a material at the elastic stage of a tensile test [1]. Since fibers have higher strength and stiffness than biopolymers, the latter is reinforced to the biopolymer matrix to enhance the tensile properties of the composite. Depending on the type of fiber and biopolymer, the tensile strength, Young’s modulus and elongation at break varies [128]. Out of all natural fibers, bast fibers exhibit highest tensile strength and Young’s modulus, and are most commonly used in biopolymer composites because of their availability. Flax fibers also show similar properties because of low microfibrillar angle and high amount of cellulose. Ramie and curaua fibers exhibit excellent modulus of rupture and hemp fiber possess stiffness much greater than E-glass fibers. It is reported that unidirectionally oriented ramie fiber-reinforced soy protein concentrate (SPC) composites showed tensile properties similar to steel [129]. One the other hand, coir possesses the lowest tensile strength among all natural fibers, due to high microfibrillar angle and low amount of cellulose [128]. The concentration of reinforced fiber greatly determines the tensile properties of biopolymer composites. PHB composites were prepared by reinforcing 0–30 wt. % coconut fiber. From the experiment, the best results were acquired for the composite prepared with 10 wt. % coconut fiber, which improved tensile strength and elongation at break by 35 and 25%, respectively, when compared to pure PHB composite [130]. However, Petinakis et al. [131] in their study stated that tensile strength of wood flour fiber/PLA composite is not much dependent on wood flour content. They improved the tensile strength and tensile modulus of the biopolymer composites by 10 and 135%, respectively, with the addition of a coupling agent, methylenediphenyl-diisocyanate (MDI). Besides, many studies showed that surface modification of fibers by chemical treatment also improved the tensile properties of the biopolymer composites by minimizing the hydrophilic nature of the fiber and improving the fiber–matrix interaction.

Flexural properties

Flexural stiffness is a factor for determining the deformability, and flexural characterization test is the second most preferred mechanical test. Flexural property is dependent on Young’s modulus and moment of inertia (a function of the cross-sectional geometry) of a material [42]. The wood fiber-reinforced composites showed highest Young’s modulus whereas the hemp, ramie, sisal, and caraua fiber-reinforced composites showed better flexural modulus of rupture [128]. Usually, by increasing the fiber content, the Young’s modulus and flexural strength along the longitudinal direction increased in biopolymer composites, while the Young’s modulus and flexural strength exhibited minimal changes along the transverse direction [128]. Sawpan et al. [132] in their study investigated the flexural properties of 30 wt. % hemp fiber-reinforced thermoplastic (PLA) and thermosetting (unsaturated polyester resin) composites prepared by compression molding method. It was revealed that, by the increase in fiber loading, the Young’s modulus increased but the flexural strength decreased. The decreased flexural strength is due to the weak spots (also known as “kinking”) in the hemp fiber which induces stress concentration in the polymer matrix. So, by increasing the fiber content, the number of kinks increases, which thereby reduces the flexural stress of the composites. In another study with melt blending technique, biodegradable composites were made by reinforcing oil palm empty fruit bunch fiber in PCL matrix. Due to the incompatibility of oil palm fiber with PCL, polyvinylpyrrolidone (PVP) was used as a binder to enhance the fiber–matrix interaction, and the prepared composites were subjected to irradiation to augment the mechanical properties. The study indicated that, with 10 kGy dosage and addition of 1 wt. % PVP, the flexural strength, and modulus of biodegradable composites were increased [133].

Impact properties

The capacity of a material to withstand fracture under applied speed is the impact strength of that material [42]. Since natural fibers innately possess low impact strength, it is required to perform impact tests on biopolymer composites. Several factors like fiber and biopolymer type, particle size, interfacial adhesion and test, and specimen condition (unnotched or notched) affect the impact resistance of the composites [128]. In a study, it was found that, by reinforcing PLA composites with bamboo fiber the impact strength decreases, and for increasing the same, the fibers were modified by silane treatment. After treatment, there was 33% increment in impact strength of the composite [134]. The impact strength of the biopolymer composites can also be enhanced by hybridization technique [128].

Dynamic mechanical thermal properties

Dynamic mechanical thermal analysis (DMTA) determines the heat deflection temperature (HDT) of biopolymer composites. The test compliments both mechanical test methods as well as thermal techniques (DSC and TGA) [128]. The test is usually performed with ASTM D648, ASTM D5023–15 and measured in Thermal Analysis Instruments. The elastic response (storage modulus), viscous response (loss modulus) and damping (tan δ) values of a material are the function of temperatures at 3 °C/min [3]. Incorporation of natural fibers to a biopolymer matrix increases the stiffness and imparts good stress transfer to the matrix, which further augment the storage modulus of the composite [128]. The overall dynamic properties are governed by the type of polymer, plasticizer and reinforcement technique [3]. Oksman et al. [135] prepared flax fibers (30 and 40 wt. %) reinforced PLA composites in a twin-screw extruder by adding triacetin as plasticizer. The thermal properties were decreased by the addition of plasticizer, but from the DMTA analysis, it was found that the storage modulus increased and the material softening enhanced from 50 to 60°C. PLA composites were prepared by reinforcing kenaf fibers (0 to 40 wt. %) and polyethylene glycol (PEG) (as plasticizer) in a mixer, followed by compression molding. The incorporation of PEG created plasticizing effect and decreased the glass transition (Tg) from 63 to 40°C [136]. The qualitative information of fiber matrix interfacial characteristics can also be obtained by DMTA analysis [128].

Toughness and hardness properties

The toughness and hardness properties depend upon the strength and ductility of a biopolymer composite. The ability to absorb energy and plastically deform without fracturing is regarded as toughness of a material. Whereas, the fracture toughness is the property of a material to resist fracturing by getting a crack [128]. The toughness of a biopolymer composite can be measured by impact tests, while the fracture toughness is analyzed by varied techniques like Kahn test and plane strain fracture toughness. Hardness of a biopolymer composite is its ability to resist the permanent plastic deformation when a compression force is applied [128]. It is mostly determined by Rockwell hardness tester, besides others like indentation, rebound and scratch. The usual indentation hardness scales are Vickers, Rockwell, Brinell and Shore. Several factors like stiffness, toughness, ductility, plasticity and visco-elasticity affect the hardness of a biopolymer composite [128].

Creep and fatigue properties

Creep in a biopolymer composite is the tendency to deform slowly but permanently due to mechanical stress [128]. Even though creep occurs at high-stress levels, because of long-term stress exposure it will be below the yield strength of material. Unlike brittle fractures, creep fracture is not instantaneous and is a time-dependent deformation, where the accumulated strain is due to long-term stress [128]. The creep properties like creep strength and creep modulus are determined as time and temperature function, and most of the creep behavior is dependent on time, temperature, relative humidity (RH) and load [137]. Fatigue is the structural damage due to cyclic loading and unloading of a material. Similarly like creep failure, fatigue failure takes place within the yield stress limit. The structure or shape of the biopolymer composite influences the fatigue properties [128].

Brittleness and ductility properties

If a biopolymer composite under stress, breaks with or without noticeable deformation then the material could be considered as brittle or ductile. Unlike glass fibers, natural fibers deform before fracture, and thus have a significant relation with impact properties. Under stress, if the biopolymer composites are ductile, they tend to deform before complete failure and if the biopolymer composites are brittle, they break. The ductile biopolymer composites gain more energy before fracture while brittle ones gain relatively less energy [128]. The brittleness and ductility behavior of the biopolymer composites are examined along impact tests and usually not determined by any separate methods. The ductile-brittle transition PLA composites reinforced with kenaf fibers were investigated by Kaiser et al. [138]. The team explained the sensitivity of the composite toward mechanical properties to temperature with respect to brittle and ductile fracture. Since, many studies have revealed that the mechanical properties of fiber-reinforced biopolymer composites have relatively higher mechanical properties than unreinforced biopolymer composites. The enhancement in these properties is due to the use of small-sized fiber particle, surface area and elevated aspect ratio of the fiber, homogeneous distribution of fiber in the matrix, and strong interaction between fiber and matrix [1].

Tribological properties

Due to wider production and application of natural fiber-reinforced biopolymer composites it is necessary to understand their tribological properties. Tribology is the study of friction and wear of two mating surfaces [3]. Many research works have revealed that by the addition of natural fibers the friction and wear performance of biopolymer composites can be improved [60,139]. A commonly used wear test is “Pinon disc” test procedure, which includes sliding wear, with constant contact area. Another test for determining the friction and wear performance is “Dry sand rubber wheel”, performed according to ASTM G65 and “Block on ring tribo test”, performed according to ASTM G77, G137–95 [3]. It is necessary to understand the environmental conditions on performing tribological tests as each practical application has a different environment.

Application of biopolymer composites

The detrimental increase in the use of conventional plastics has led to the invention of fully biodegradable alternatives to reduce the environmental pollutions. The natural fibers have shown their excellence in replacing expensive carbon fibers and glass fibers over decades. When compared to these synthetic fibers, natural fibers possess lesser density, and hence are light weight and help in enhancing fuel efficiency [43]. During late twentieth century, the practical applications of biopolymer composites started [3]. World-wide the natural fibers are commonly used in varied applications such as automobiles and aerospace, sports, medical implant and drug deliveries, textiles, packaging, infrastructure and building, upholstery and furniture, and many household products [140]. In the near future, wider applications of biopolymer composites are likely to increase considerably in industrial markets.

In automobile industries, natural fiber-reinforced biopolymer composites can impart several advantages over conventional composites, like improved stability, reduced cost and weight, enhanced acoustic insulation and mechanical properties, reduced wear of machinery and tools, lesser or no release of toxic/noxious gases, along with recyclability, renewability, and eco-efficiency [12,34]. The biopolymer composites prepared from PLA reinforced with natural fibers like kenaf and jute with good strength and mechanical properties are found to be an excellent application in interior automotive parts, where strength of composite is a necessity for performances. Kenaf fibers have also been explored to improve the mechanical properties of structural elements in automobiles. Cotton and coconut fibers are used to make insulation materials in automotive sectors. In Germany, the renowned automotive companies like Bayerische Motoren Werke AG (BMW), Mercedes, Volkswagen, Audi, Opel, Daimler Chrysler and Ford employ natural fiber reinforced polymer composites for numerous exterior and interior applications [141]. PROTON Holdings Berhad (PHB) or Proton, a Malaysian automotive company also engages in natural fiber-reinforced composites for various applications [140]. Natural fiber-reinforced composites are used to manufacture components like door panels, cushioned seats, cabin lining, dashboards, package trays and headliners [11]. FIAT, an Italian automobile manufacturer employed wood flour filled door panels in Brava, Punto, Marea, and Alfa Romeo models [142]. From studies it is seen that Daimler-Benz incorporated coconut fibers in the body parts and Audi A3 used hemp fiber-reinforced composite in the side panels. Toyota, a Japanese multi-national automotive manufacturer in their first commercial vehicle employed composites manufactured by reinforcing kenaf fibers in PLA matrix developed from sugar cane and sweet potatoes in spare tire cover of RAUM 2003 model [3]. Other automotive applications of biopolymer composites are seen in bamboo and kenaf fibers reinforced composites in interior components (package shelves, floor mats and luggage compartment) in Toyota’s Lexus models (CT200h and ES 300, HS), ramie and kenaf fiber-reinforced PLA composites in translucent roof in Toyota 1/X plug-in hybrid concept vehicle and bamboo fiber-reinforced PBS composites in headliner/ceiling material and bamboo fiber-reinforced PLA and PBS composites in preparing car floor mats in Mitsubishi Motors Corporation [43,74].

As an alternative to aluminum alloys and E-glass/epoxy composites, completely biodegradable bamboo fiber reinforced PLA composite panels and boards were prepared possessing excellent mechanical strength [143]. The PLA/PBAT composites developed by reinforcing surface modified kenaf fiber with 2 wt. % (3-aminopropyl)trimethoxysilane (APTMS) as compatibilizer in 90:10 wt. % ratio exhibited strong improvement in toughness, tensile and flexural strength because of good interfacial adhesion between natural fiber and biopolymers [144]. Dorez et al. [145] prepared PBS based composites by adding natural fibers along with ammonium polyphosphate (APP) as fire retardant for designing interior body parts of vehicles. On fire exposure, the APP improved the fire retardancy property by imparting phosphorylation of natural fibers and hot hydrolysis of PBS, thus forming as a barrier layer over the composite. Improved mechanical strength of biopolymer composites fulfills the requirement in designing and engineering applications in automotive industries.

The noticeable characteristics of plant fibers like lightweight, stiffness, good acoustic insulation, and thermal properties have made them suitable for use in the construction industry [11]. Plant fibers are usually employed in manufacturing construction materials such as internal/secondary structural elements, light-weight structures, partitioning, surface panels, facade panels, doorframes, and other architectural uses [11,38]. Biopolymer composites have shown their ability to substitute furniture and wood fittings, fixtures, noise-insulating panels, marine piers, and flower pots [38]. The wood fiber-reinforced polymer composites are employed in outdoor decking, windows, and furnitures. The wood-filled polymer composites are composed of about 30–70 % wood content [11]. The natural fibers have also proved their functionality in wound healing and drug delivery. The nanofibers prepared from natural fibers were used for release of vitamins/drugs, as they are compatible with any hydrophobic polymer and extend the durability and release of vitamins/drugs in a sustained manner [146]. Natural fibers are widely accepted in transdermal drug delivery (TDD) applications, because of their ease of addition in biological compounds, controlled drug release and permeation characteristics [146]. In biomedical and electronic application, the nanofiber structured silk/chitosan composite films prepared by facile solution casting technique exhibited improved hydrogen bonding interaction, mechanical performance, and biocompatibility with living cells. The biopolymer composite films can be potentially employed in biomedical and electronic display screens because of their strong nanofibril structure [147].

Durability of biopolymer composites

Natural weathering conditions like humidity, temperature, rain, and UV radiation negatively affect the characteristics and properties of biopolymer composites [38]. Thus determining durability of biopolymer composites is an essential factor to understand the application and performance of the materials for a longer run. The durability of biopolymer composites is governed by many factors like moisture absorption, thermal stability, flame retardancy, UV resistance and biodegradability of natural fiber, biopolymer as well its composites.

Moisture absorption

Moisture exposure of biopolymer composites always negatively affects the performance of the composites. Water has the tendency to plasticize the natural fiber and biopolymer and thereby reduces the stiffness of the composite. Differential expansion between natural fiber and biopolymer lead to the permanent damage of the fiber–matrix interface [38]. When the natural fibers absorb moisture and get saturated, they swell, shrink and lose their physical characteristics, which leads to the entry of moisture to the fiber-matrix interfacial region [6]. The swelling of fibers leads to poor dimensional stability, and as the moisture enters to the fiber–matrix interface delamination occurs. The fiber-matrix debonding is actuated by osmotic pressure development at the surface of the fiber due to leaching of water-soluble substances from the fiber [148]. A pictorial representation of moisture absorption of biopolymer composites is summarized in Figure 3. Research has also stated that moisture absorption increases with increase in fiber volume fraction [38]. The biopolymer composites manufactured with hydrophilic biopolymers (like cellulose and starch) show greater moisture absorption when compared to composites prepared with hydrophobic biopolymers (like PHB and PHBV) [38]. From studies, when hemp/cellulose acetate and sisal/MaterBi-Y (derived from cellulose and starch) biopolymer composites exposed to water immersion and high humidity, there was 70% and 30–50% loss in tension modulus and flexure modulus, respectively [149–151]. It is stated that even though hydrophobic matrices absorb less moisture, they retain more permanent damage than their hydrophilic counterparts due to moisture absorption. This is because of the high internal stress in stiffer materials leading to cracking, caused by swelling of fibers in the biopolymer composites. Christian and Billington [149] demonstrated that there was remarkable cracking and less recovery of mechanical properties for hemp/PHB composites when compared to hemp/cellulose acetate composites on drying.

Figure 3. Effect of moisture absorption on biopolymer composites

Thermal stability

Even though, each material has its own innate decomposition temperature, the physical characteristics and decomposition temperature of the material change if it is blended or combined with another material [1]. To ensure that there is no change in chemical and physical properties of biopolymer composite, it is necessary to know and monitor the thermal stability of materials. Thus the thermal stability of natural fiber and biopolymer determines the thermal activity of the composite. The thermal stability of biopolymer composites is usually determined by DSC, TGA and DMTA. By TGA, the degradation of composite material at high temperatures is analyzed and it is used for manufacturing and fire exposure of composites. While the degradation at lower temperatures is useful for the comparison of thermal stability in specific service ranges [148]. The analysis help to determine the maximum heat required to degrade the biopolymer composite and to evaluate the residue left behind after degradation. Besides, it is possible to determine the weight loss of the biopolymer composites with increase in temperature [1].

Many studies reported that by fiber incorporation the thermal insulation properties or thermal stability of biopolymer composites can be improved. Recently, the thermal stability of PBAT/PLA composites reinforced with varied natural fibers, wood fibers, wheat husk, rice husk, and textile waste fibers was analyzed by TGA. From the experiment, it was observed that by the addition of natural fibers, the initial degradation temperature decreased [152]. The effect of reactive compatibilization on thermal stability of miscanthus/PHBV biopolymer composites prepared by extrusion and injection molding was determined by Muthuraj et al. [153]. It was found that, by the incorporation of natural fiber and dicumyl peroxide (DCP) there was significant reduction in the onset degradation temperature, along with improvement in tensile and flexural strength of the composites. Similar results on thermal stability was observed in miscanthus fiber-reinforced PBAT composites [154]. Hence, natural fiber-reinforced biopolymer composites can be employed in manufacturing interior parts of aircraft and automobiles. Modification of natural fibers by acrylonitrile grafting and acetylation has demonstrated improvement in thermal stability [38].

Flame/fire retardancy

When biopolymer composite materials are chosen for thermal insulation applications or as building materials, it is mandatory to determine the flammability and flame retardancy property of composites by flame retardancy tests. Since natural fibers alone are highly flammable, it is more likely to ignite and combust exothermically and burn itself vigorously on combustion. Several factors influence the flammability of biopolymer composites, like type of natural fiber and biopolymer, type of reinforcement, adhesion between reinforcement and matrix, and the structure of the composite [155]. As the chemical composition and microstructure differs from one fiber to another, the flammability also varies accordingly. At elevated temperature, the major fiber components, cellulose, hemicelluloses, and lignin markedly behave in different ways and thus the composition of a natural fiber greatly influences the composite performance [38]. The flammability of natural fibers increases with increase in cellulose content, and its decomposition emits flammable volatiles, noncombustible gases and tars leading to char formation. Lignin content is also responsible for char formation and it acts as a protection of the underlying biopolymer composite integrity by serving as an insulation layer. The natural fibers having high crystallinity and low polymerization are less flammable [38]. Usually, the flame retardancy test for natural fiber-reinforced biopolymer composites is performed in a vertical Bunsen burner test, according to Federal Aviation Regulation (FAR). Other horizontal and vertical fire tests are also employed to evaluate the flame retardancy. By using tests with UL-94 horizontal and UL-94 vertical standards in accordance with DIN EN 60,695–11–10, the ignitability and flame spreading rates could be determined [3]. The fire testing instrumentation methods like Ohio State University (OSU) rate of heat release apparatus technique, limiting oxygen index, pyrolysis combustion flow calorimetry and cone calorimetry are also used [155]. All these analyses correlate each other with the results of Bunsen burner test, horizontal and vertical fire tests, thermo-mechanical analysis methods (DSC and TGA), spectroscopic methods (FTIR and XRD), TGA-FTIR coupled systems and microscopic methods (SEM and TEM) [155].

Jang et al. [2012, 156] by commingled yarn method prepared plasma-treated coconut fibers reinforced PLA composites and investigated the flammability performance via loss on ignition (LOI) method. They reported that even though no remarkable difference between the LOI values for the coconut fibers/PLA composites was found, it was above 20 vol. % which indicates a non-flammability characteristic. The tensile strength and Young’s modulus of the reinforced composite also improved. The flame retardancy of ammonium polyphosphate (APP) added thermoplastic starch (TPS) composites were analyzed and found that APP enhanced the flame retardancy of the biopolymer composites. Also, a similar trend was noticed when keratin fibers replaced 20 of 30 % of APP [157]. The fire resistance of biopolymer composites can be increased by decreasing the flammability of the components of composites [158]. To improve the flame resistance, various agents/additives like dispersion and fire retardant agents and coupling are used in biopolymer composites. The addition of flame retardants reduce or avoid the combustibility in composites. The flame retardants by addition of filler and endothermic process, cool and thins the flammable materials in gaseous or solid phase, and isolate the material from fire source by forming a protective film coating over biopolymer composites [42]. The composites can also be protected by intumescent systems painted on its surface. On fire exposure, the systems char and expand, and create a surface layer for limiting the access and permeation of oxygen and heat [158].

In a study with commercial biodegradable blend kenaf/E-PHBV composites the flame retardancy performance was assessed. The E-PHBV was composed of PBAT and PHBV. To this composite, metal oxide (Sb₂O₃) and a phosphate-based additive were added as flame retardants. The cone calorimetry test demonstrated that heat release rate (HRR), peak/maximum heat release rate (PHRR) and fire spread indices decreased by the addition of flame retardants. From the SEM analysis of the char obtained from cone calorimetry tests, it was noted that the dense and porous structure in the residue is due to the OH group rich kenaf fibers which stood as a carbonizing agent that formed as a network of cavities and holes by pyrolysis gases release [159]. The researchers at the Council of Scientific and Industrial Research (CSIR) engaged in a project by collaborating with AIRBUS, a European multi-national aerospace corporation for developing natural fiber-reinforced thermoset panels for aircraft applications. They developed aqueous-based flame retardant treatments for flax fibers to confirm that the composite panels follow the regulations of Federal Aviation Airworthiness (FAA). Non fibrous natural silicate was also incorporated as a fire-resistant material along with primary fire retardants in the composites. Finally, the manufactured composites exhibited superior toxicity, smoke and flammability properties for the aforementioned purpose [160]. Since gluten-based composites are susceptible to fire, the fire retardancy can be improved by adding lasonol [161]. In a study by Zainuddin et al. [162] on bacterial grade PHBV, the fire retardancy is increased by incorporation of halloysite nanotube as a fire-retardant agent. But by halloysite nanotube addition the mechanical properties of composite reduced. There are numerous fire retarding agents employed in biopolymer composites, but most of them possess some limitation or implement some concerns like hindering the mechanical properties of the composites. So there is a need to discover newer fire retarding agents that are compatible with composites without any effect in properties and performance.

Ultraviolet resistance

Photodegradation occurs when the biopolymer composites are exposed to UV radiation. In natural fibers, lignin is more susceptible to photodegradation and this results in imparting a yellow color discoloration to the composites [38]. And as the lignin degrades gradually, the cellulose which is less prone to UV degradation develops on the surface of the composites [42]. When the biopolymer composites are exposed to solar radiation, the radiation cleaves the covalent bonds in organic polymers, and further causes, discoloration, surface roughening, degradation of mechanical properties, and embrittlement [148]. Campos et al. [163] in their experiment found that thermoplastic like sisal/starch and sisal/PCL composites on UV exposure led to the decrease in mechanical properties, holes in the surface, embrittlement and discolouration of the composites. In the laboratory setup, the biopolymer composites are not just alone exposed to UV radiation, but they are exposed to a combination of moisture, temperature and UV source intended to simulate actual conditions at an accelerated rate. To an extent, the UV degradation can be overcome by bonding chemicals to the cell wall polymers that decreases the lignin degradation and also by adding polymers to the cell matrix to keep the degraded fibers structure together [42].

Biodegradability

The microbial growth on biopolymer composite depicts the biodegradation ability of the material. The presence of lignin in natural fibers makes them resistant to microbial degradation. While, less lignified natural fibers are prone to microbial degradation of cellulose and hemicelluloses [6]. Also, moisture exposure and absorption promote bacterial growth [148]. The mechanical strength of biopolymer composites are reduced by cellulose degradation via dehydration, hydrolyzes and/or oxidation [42]. From laboratory studies, it is found that natural fiber-reinforced biopolymer composites from PHB, PBS and PLA, biodegrade by anaerobic digestion and/or enzymatic degradation [164,165]. The soil burial tests of sisal/Mater Bi-Y composites shows that the microbial load in soil biodegrades the biopolymer composites by creating holes and cracking over it and thereby reducing its mechanical properties [166]. Barkoula et al. [167] in their work stated that flax fiber-reinforced PHB matrix composite showed noticeable reduction in tensile strength in the initial stage of biodegradation study, but later it was more gradual in the next stages of degradation. In a study, a series of composite boards were prepared from varied mixtures of bamboo/PLA and chemical, physical, mechanical and biological properties were analyzed. The samples were exposed to three different wood-decaying fungi for 60 days and observed that all the species degraded all the composites expect pure PLA composite. After 1500 h of biological exposure, the biopolymer composite samples showed nearly zero water absorption and swelling thickness. The study showed that PLA composites has the ability to remain water resistant, but more research is needed to confirm the durability of these composites [168].

The natural fibers can be protected from biodegradation by making them less hygroscopic and less susceptible to enzymes [6]. Other way is the surface modification of natural fibers by chemical treatment and thereby reduces the moisture content below microbial requirement for growth. Biodegradability of biopolymer composites can also be delayed by harsh chemical against microbes and to slow down biological attack [42]. One main issue governing the durability of biopolymer composites is that how easily and rapidly the resistance of the composite materials could be evaluated for biodegradation study [6]. Several groups like European Standardization Committee (CEN), American Society for Testing and Materials (ASTM), International Organization for Standardization (ISO), Italian Standardization Agency (UNI), German Institute for Standardization (DIN), Institute for Standards Research (ISR), and Organic Reclamation and Composting Association (ORCA) made different techniques for biodegradability assessment under varied environment [6]. However, a single standard method for determining the biodegradability of biopolymer composites has not been stated. The lack of standardization leads to testing of different polymers and conditions suitable for optimizing the composite materials. Ultimately, the methods to control the biodegradability of biopolymer composites should be developed to use these eco-friendly materials as an alternative to petrochemical-derived counterparts.

One of the feasible ways to conduct durability management of biopolymer composite is to wisely choose the function and application of each raw material, so that the required performance is met by controlling the weakness and taking the advantages of the strengths of these materials [38]. So while designing a biopolymer composite, the most essential things to consider are the thermal stability, dispersion, and surface attachment qualities of fibers in biopolymer matrices. Due to thermal decomposition/degradation of natural fibers, the compounding temperature is confined to max. 200 °C [169], as natural fiber mostly starts to degrade around 240°C [35]. The fiber volume fraction also should be considered while designing a biopolymer composite, as increase in the fraction can have both merit and demerit, as it give rise to improved mechanical performance and moisture-related damages, respectively [38].

Hybrid composites

Hybrid composites are the composites developed by reinforcing two or more natural fibers to a matrix. Researchers in their studies have selected the best combination of natural fibers to obtain the best possible result to utilize the combined benefits of fibers and reduce the negative aspects [5]. Mixing different fibers enhances the tensile properties of the composites [128]. Hybridization of fibers with different diameter increases the fibers matrix adhesion effective area for uniformly transferring the stress. For instance, even though a fiber having low elongation is used, the mechanical properties can be maintained by transferring the stress to another fiber with high elongation in the composite [4]. Usually, reinforcement methods in hybrid composites are by mixing two types of short fibers prior the addition of matrix, or by adding each fiber alternatively by layering in the matrix [5]. The strength of the hybrid composites depends on the length of individual fiber, fiber properties, aspect ratio of fiber content, orientation of fiber, extent of intermingling of fibers, fiber-matrix interfacial bonding and arrangement of both the fibers and failure strain of individual fibers [7].

Bax and Müssig [170] in their study found that the rigidity and strength of the PLA composite was remarkably improved by reinforcing the composite with man-made cellulose, abaca, and jute fibers. Depending on the fiber type, the mechanical properties of the composites varied. By solution casting procedure, a blend of 90% PVA and 10% cellulose nanofiber (from varied sources like hemp, flax, kraft pulp or rutabaga) was prepared to make nanofiber-reinforced composite film. It was observed that the Young’s modulus increased four to five times greater than pure PVA film [171]. PLA composites were prepared by reinforcing sisal and banana fiber by twin-screw extrusion and injection molding. Prior to composite processing the sisal fibers were subjected to alkali treatment with 6% benzoyl peroxide and acetone solution for 30 min at 70°C. The study reported that by chemical modification of natural fibers, the tensile, flexural, and impact strength of the hybrid composites improved [172]. The properties of hemp, kenaf, cotton, and lyocell PLA composites was observed and the study revealed that kenaf and hemp/PLA composite exhibited great tensile strength and Young’s modulus and the cotton/PLA composite showed good impact strength [173].

Recyclability of biopolymer composites

The end of life scenarios of polymers and polymer matrix composites receive increasing attention, since a huge amount of plastics is simply being mismanaged. The “circular economy” vision for plastics is far from being within reach, with the industry talking about “thermal recycling”, a gross misnomer for “incineration”. Ngaowthong et al. [174] studied the recycling of sisal fiber-reinforced PP and PLA composites and found that 4 rounds of recycling did not affect the stability of the PP composite, whereas the PLA-based material showed strong degradation. Increasing the bio-based carbon content of products can reduce their carbon footprint. In any case, a circular economy approach where plastics composites and plastics in general are used more than once is to be targeted. Feedstock recycling is one option, another one cascaded use, apart from established collection and remolding in less demanding products as for thermoplastics. Out of all polymers, thermoset recycling is more challenging. The huge varieties of composites express great difficulties to meaningfully recycle the materials. So standardization could help increase recycling rates.

Limitations and future trends of biopolymer composites

Besides advantages, there are some noticeable limitations in biopolymer composites. The poor moisture resistance, dimensional stability, thermal decomposition temperature, fire, UV, and biological resistance of biopolymer composites when compared to conventional plastics limit the growth of its potential applications [38,42]. The use of natural fibers improves the impact strength of the biopolymer composite, and it increases with increase in fiber content. But, as the amount of fiber increases the ductility, water absorption tendency and undesirable odor is increased in the composite [15]. Another notable limiting factor for utilization of biopolymer composites in heavy load-bearing, interior, and non-structural applications is the limited compatibility of natural fibers and biopolymers [3]. The extent of compatibility between natural fibers and biopolymers influence the mechanical properties, durability, and performance of the composites. The long-term characteristics like creep and fatigue behaviors, durability and lifetime prediction of biopolymer composites is precisely unknown and is a major concern [175]. So some new methods and experimental techniques should be adopted to study the performance of biopolymer composites over its lifetime. It is also noted that for commercial production, the biopolymer composites are engineered through fiber treatment, biopolymer blending, plasticizer, and other filler addition, coating and other advanced processing methods. In spite of the fact that natural fibers and biopolymers are cost-effective than synthetic counterparts, some processing methods demand a good amount of capital investment and energy requirement [38]. So to combat these limiting factors, some critical aspects and recommendations to be noted in future researches on biopolymer composites are:

• Exploration, identification, and characterization of new natural fibers are essential for the wider application of natural fibers to replace hazardous synthetic fibers. Studies on animal fibers reinforced biopolymer composites are still minimal, even though animal fibers are known to possess better mechanical properties than plant fibers.

• Among currently used fiber extraction methods, no extraction technique has been identified as standard method for a specific type of fiber. It is also very well observed that there is no specific standard method for manufacturing biopolymer composites. Establishing standard procedures for composite preparation saves optimization time, energy and raw material utilization.

• For application of biopolymer composites in various fields like automotive and construction sectors, it is necessary to perform quality assurance and develop optimized natural fibers due to its widely dispersed resources and variable properties.

• It is also important to study more about the effect of reinforcing modified natural fibers to biopolymer composite to predict the suitable application and also to foresee its potential performance. And also, innovations in surface modification techniques need to be explored to adopt methods which are more cost-effective and eco-friendly.

• Even though natural fiber-reinforced biopolymer composites have shown its applicability in automotive, aerospace, electronic and biomedical applications, more research is needed in medical scaffolds for wound healing process and tissue engineering. In automotive sectors, biopolymer composites have made substantial commercial markets for value-added products. Likewise, value addition in other fields need to be focused and promoted.

• One major drawback of bio-based materials is the difficulty in understanding its durability. The unpredictable durability along with limited mechanical strength is a major limiting factor for the use of biopolymer composites in automotive sectors. Most of the knowledge about the durability of biopolymer composites is based on laboratory-scale experiments. The in situ behavior of composites has not been greatly analyzed to validate the laboratory results and findings. New experimental techniques should be explored to monitor, evaluate and control the durability and biodegradability of biopolymer composites.

• The research should also focus one major area, that is, load-bearing characteristics of biopolymer composites. Developing load-bearing biopolymer composites widens the use of biopolymer composites for various structural applications.

• The long-term performance of biopolymer composites need to be improved and monitored to its extensive use in construction sectors. New techniques to conduct performance evaluation in biopolymer composites are necessary to understand the effect of environmental conditions in thermal, mechanical, and durability of composites.

• Hybridization is an effective technique that needs more attention to develop novel and efficient biopolymer composite products. It is an advantageous method to combine natural fibers of varied characteristics to produce composites with excellent strength and properties.

• Recyclability of composites is a promising area to explore. Future studies on the recyclability aspects of biopolymer composites is needed to reuse the composites in way or the other, rather than subjecting it for biodegradation after its specified use.

• Since biopolymer composite is a product of synergism of properties of natural fibers and biopolymers, the invention and development of component-dependent and non-dependent blending system is a major area need to be focused.

• The use of nanotechnology has shown numerous opportunities for enhancing characteristics of biopolymer composite products. Studies have reported that the nanocrystalline cellulose is stiffer than aluminum and stronger than steel. So development of nanofibers from natural fibers is a promising field for structural applications of biopolymer composites.

Conclusion

Since last few decades, there is gradual increase in the industrial interest in biopolymer composites. Natural fibers on modification have shown its ability to overrule synthetic fibers with its abundant availability, low cost and eco-friendly characteristics. Out of all natural fibers, kenaf fiber has shown its sturdy mechanical properties and jute fiber has exhibited maximum strength and excellent compatibility with biopolymers. Since natural fibers are used for composite preparation and other purposes, it promotes farming and sericulture, thereby reduces air pollution by reducing greenhouse effect and also helps in improving soil fertility. Biopolymer composites are renewable, non-corrosive, eco-friendly, biodegradable, lightweight, easily manufactured, and also possess high specific strength and stiffness that is amenable to be tailored to satisfy varied performance requirements. The performance of biopolymer composites depends upon numerous factors such as chemical composition, physical properties of fiber and biopolymer, fiber modification techniques, fiber volume fraction, amount of additives and plasticizers, method of reinforcements, geometry and orientation of fiber in matrix, structural defects, interaction between fiber and biopolymer, and environmental condition of the composites. Another widely accepted composite development technique is by combining different types of fibers in a single matrix to develop highly valued hybrid biopolymer composites. The studies revealed that the biopolymer composites are compatible with automotive, aerospace, construction, biomedical, and food industries, and proved to have versatile applications. In near future, further researches on biopolymer composites have the potential to substitute the conventional petroleum based plastics. There is possibility in the development of new markets when these biopolymer composites become more durable with improved performance. Few characteristics need to be noted whenever a natural fiber is reinforced to a biopolymer matrix, is to know the purpose, final design, and environmental condition that the material is exposed to. Continuous exploration on performance and life-cycle assessment is necessary to find out more applicability of the biopolymer composites.

Acknowledgments

The authors wish to thank Dr. Shijin A, veterinary surgeon, Akhila P P and Zahle Khan for their constant support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Basheer Aaliya

Basheer Aaliya is a researcher scholar in Food Science and Nutrition, Department of Food Science and Technology, Pondicherry University. Her research interest is the isolation, modification and characterization of non-conventional starch.

Kappat Valiyapeediyekkal Sunooj

Kappat Valiyapeediyekkal Sunooj is an assistant professor in Department of Food Science and Technology, Pondicherry University. His doctoral research was on meat science from Defence Food Research Laboratory (DFRL), DRDO. Currently Dr. Sunooj is widening his research on starch modification, plasma technology and biopolymer science.

Maximilian Lackner

Maximilian Lackner earned his Ph.D. from Vienna University of Technology. From 2004 to 2011, he worked in the polymer industry in Austria and China in several senior leadership positions. In 2013, he founded a company actively involving bioplastics trading, development and consulting. Dr. Lackner is a lecturer at The University of Applied Sciences FH Technikum Wien.