Biodegradable Hybrid Polymer Film for Packaging: A Review

ABSTRACT

Biodegradable film-based packaging is a current concern to replace packaging that depends on petroleum. This is because petroleum-based packaging is very unfriendly to the environment. The bioresource-based biodegradable film has the disadvantage that its mechanical properties are still low, that it requires two or three bioresource materials to be used as polymer hybrids. As a film hybrid polymer that has superior properties such as flexibility, strength, heat resistance, and environmental resistance, it can be applied to various packaging materials and can be developed for various purposes. In this review, the authors review biomass sources as a matrix, as a filler in hybrid polymer-based packaging, the properties of the biodegradable file, and the development for the future. In addition to the raw materials as matrices and fillers, the synthesis method is also described. The properties of biodegradable hybrid polymer film include physicochemical, mechanical, and thermal. This review also summarizes the emerging aspects of bioresources for the development of sustainable and biodegradable hybrid polymer film friendly to the environment.

摘 要

可生物降解薄膜包装是替代依赖石油的包装的当前关注点. 这是因为石油包装对环境非常不友好. 基于生物源的生物降解膜的缺点是其机械性能仍然较低，需要两种或三种生物源材料用作聚合物混合物. 作为一种具有优异性能如柔韧性、强度、耐热性和耐环境性）的薄膜混合聚合物，它可应用于各种包装材料，并可用于各种用途. 在这篇综述中，作者回顾了生物质源作为基质，作为混合聚合物包装中的填料，生物可降解文件的特性，以及未来的发展. 除了作为基质和填料的原料外，还描述了合成方法. 生物可降解杂化聚合物膜的性质包括物理化学、机械和热. 该综述还总结了生物资源在开发对环境友好的可持续和可生物降解的混合聚合物膜方面的新兴方面.

KEYWORDS:

• Biodegradable hybrid polymer
• packaging
• biopolymer
• filler, plasticizer, film properties

关 键词:

• 生物降解杂化聚合物
• 包装材料
• 生物聚合物
• 填料，增塑剂，薄膜特性

Introduction

In the last decade, non-degradable polymer materials have been widely used, increasing the utilization rate. The non-biodegradable polymers have been used mostly as packaging materials because of their good physical and mechanical properties such as tensile strength and elongation, water and gas barrier, also heat and UV resistance (Siracusa et al. 2008). However, the use of these polymers as materials packaging has been limited recently, mostly because they are produced from nonrenewable petroleum, low biodegradability, and non-totally recyclable (Siracusa et al. 2008). These packaging materials have caused many ecological and environmental problems, often contaminated by foodstuffs and biological substances, increasing waste disposal in nature (Avérous and Pollet 2012; Robertson 2004).

Biodegradable (hybrid) polymers are one alternative solution to substitute non-degradable wastes and limit their detrimental impact on the environment. Biodegradable hybrid polymers are a combination of bio-based polymer matrices and fillers used as reinforcing agents in a polymer matrix to enhance the packaging properties. Then, combinations lead to desirable properties that allow the product to compete with the existing petroleum-based polymers. Hybrid leads to new material that can exhibit new properties that cannot be found in individual components and the mixture of two materials as a composite. The materials of the polymeric matrices and the fillers for the hybrid polymer films are classified into natural and synthetic. The natural materials of biopolymer typically come from agricultural wastes and living organisms, while the synthetic materials biopolymer is from nonrenewable sources and materials produced from the fermentation process using bacterial to produce aliphatic polyesters (Marrazzo, Di Maio, and Iannace 2007; Shariatinia and Fasihozaman-Langroodi 2019). Many reports are available on the combination of natural and synthetic polymers. The research focuses on the preparation method to modify the packaging film based on polysaccharide, protein, PLA, PVA, PCL, and PHB, which provide good compatibility with other polymers or fillers (Calva-Estrada, Jiménez-Fernández, and Lugo-Cervantes 2019; Cazón et al. 2017; C. H. Lee et al. 2020). The filler also improved the films physicochemical, mechanical, and thermal depending on factors such as an equilibrium between the degree of cross-linking of the polymer matrices and the addition of additive (Nandhavathy et al. 2017; Rogovina 2016). Combining bio-based polymers with other polymers also offers great potential for packaging applications.

Many reviews on biofilm materials have tried to address the addition of different types of biopolymers and additives. However, adding a combination of polymer and filler into hybrid packaging films still needs to be reviewed. This review provides a systematic discussion on recent developments of biodegradable hybrid polymer materials for packaging. The type of polymeric matrices, fillers, plasticizer and characteristics of component in hybrid films are detailed. Lastly, perspectives on the future development and direction for biodegradable hybrid polymers film for packaging are also included.

Biopolymeric matrices

The biodegradable polymer is derived from renewable organic substances and compounds to make bio-based packaging that is biodegradable at the end of its life cycle. Based on specific criteria, a biodegradable polymer can be classified into numerous categories based on the presence of its elements. However, the most used criteria to classify biodegradable are the source of raw materials and the synthesized process (Yuvaraj et al. 2021). Biopolymers are classified into two important groups, namely biologically derived or natural biopolymers and synthetics biopolymers. Natural biopolymers come from agricultural wastes and living organisms. On the other hand, synthetic biopolymers are mostly produced from nonrenewable sources like petroleum, natural gas, and polymers obtained by microbial production (Vroman and Tighzert 2009). As illustrated in Figure 1, the natural biopolymers mainly include polysaccharide and protein-based, while the synthetic biopolymers mainly include polyvinyl alcohol (PVA), polylactic acid (PLA), polyhydroxyalkanoates (PHA), and polycaprolactone (PCL). All biopolymer matrices are summarized in Table 1 and discussed in detail in the following sections.

Figure 1. Classification of biopolymer matrices.

Table 1. Summary of natural and synthetic polymeric matrices of selected materials.

Type Polymer	Polymer Matrix	Chemical structure	Advantage	Disadvantage	Application	Reference
Polysaccharide	Starch		Excellent barriers against oxygen transmission, good elongation	Poor mechanical, and high vapor permeability	When combined with PVA, PLA, and PHA used as a replacement of polystyrene foam and LDPE films, packaging, biomaterial, biomedical and clinic field	(Edhirej et al. 2017; Mose and Moffat Maranga 2011; Zafar et al. 2016)
	Cellulose		Low cost, Non-toxicity, biodegradability, good compatibility, and chemically stable	Poor resistance to water vapor transport limited soluble with solvent	Packaging, biomaterial, composite film, biodegradable film	(Cazón et al. 2017; Brinchi et al. 2013Wang, Lu, and Zhang 2016)
	Chitosan		Nontoxic, biodegradable, good compatibility, and good film-forming	Poor barrier properties, insoluble in water and common organic solvent	Antimicrobial agent, packaging, biomaterial, hybrid composite film, and drug delivery	(Aider 2010; Choo et al. 2016)
Protein	Zein		Low oxygen permeability, good mechanical properties	Structural brittleness, and limited due to high sensitivity to humidity	A good replacement of petroleum-based films in the application for barrier, packaging, and biomaterial	(Calva-Estrada, Jiménez-Fernández, and Lugo-Cervantes 2019; Kashiri et al. 2016)
	WPI		Good film-forming, an excellent barrier against oxygen	Poor mechanical properties and limited due to high sensitivity to humidity	In combination with synthetic polymers as edible film and coating, packaging, biomaterial	(Alizadeh-Sani, Khezerlou, and Ehsani 2018; Wakai and Almenar 2015)
Protein	SPI		Good film-forming capacity, good compatibility, natural active agent, biodegradable	Poor mechanical properties and limited due to high sensitivity to humidity	Edible film and coating, packaging, biomaterial	(Calva-Estrada, Jiménez-Fernández, and Lugo-Cervantes 2019)
	PVA		Low-cost, good thermal and mechanical properties, and biocompatibility	Low resistance to water and humid environment	In combination with chitosan, starch, and other natural polymers as packaging, biomaterial, hybrid composite film, and drug delivery	(Ciannamea, Stefani, and Ruseckaite 2016; Jun Feng 2017)
	PLA		Biodegradability, compatibility, and easier processability	Low thermal and mechanical properties	In combination with natural polymers as a packaging and biomaterial	(Monika et al. 2018; Nagarajan, Mohanty, and Misra 2016)
	PHA		Good physical and mechanical properties, excellent biodegradability	High crystallinity cause brittle and rigid film and poor resistance to high temperature	In combination with natural fillers and other polymers as replacement petroleum-based packaging, biomaterial	(Anjum et al. 2016; Cinelli et al. 2019)

Note : *PVA=polyvinyl Alcohol, PLA= Polylactic Acid, PHA=Polyhydroxy Alkanoate, LDPE=Low Density Polyethylene, WPI=Whey Protein Isolate, SPI=Soy Protein Isolate.

Natural biopolymers

Polysaccharides

A polysaccharide is one of the types of carbohydrates that can take different forms with varying numbers of carbon atoms. They have been used as polymer-based to form film for food packaging products. The applications of polysaccharide-based film in the packaging industry have offered the possibility to develop a new packaging material (Cazón et al. 2017). Several polysaccharide substances have been considered as packaging matrices. They include starch, cellulose, chitosan, and various other substances. It has been reported in many studies that several types of polysaccharide-based such as starch, cellulose, chitosan, and pectin have excellent film-forming ability (Choo et al. 2016; Edhirej et al. 2017; Espitia et al. 2014; Mose and Moffat Maranga 2011; Xu et al. 2016; Zafar et al. 2016).

There are a number of polysaccharide-based and low-cost biopolymer materials available on the market, including starch. There are a wide variety of plants that produce starch, including wheat, corn, beans, and potatoes. It consists of linear chain amylose and branched amylopectin (Pawar and Purwar 2013). In its native form, starch is hardly usable because of its brittleness and poor mechanical properties, which lead to poor film-forming capacity. These weaknesses are overcome through plasticization, blending with other polymers, and the addition of fillers (Bonilla et al. 2013). Cellulose is one of the most abundant organic molecules and has been widely studied and used as one of the biodegradable materials because of its properties such as renewability, low cost, non-toxicity, biocompatibility, and physical-chemical stability (S. Wang, Lu, and Zhang 2016). Chitosan is another abundant natural polysaccharide that has been explored as polymer matrices in packaging material. It is obtained by the alkaline N-deacetylation of chitin and is mainly extracted from the waste fishery industry (Haghighi et al. 2020). Chitosan is insoluble in water and most organic solvents and can be combined with other synthetic polymers to improve the thermal properties of the resulting composite film. However, chitosan films are highly permeable to water vapor, which limits their use as food packaging material (Aider 2010; Cazón et al. 2017).

Protein-based

Protein-based biopolymer films recently have received attention and are increasingly being utilized in food packaging applications because of their consumer demand to get a natural preservatives in food packaging (Calva-Estrada, Jiménez-Fernández, and Lugo-Cervantes 2019). Many studies on protein-based biopolymers in food packaging focus on animal and vegetable proteins because of their relative abundance, renewability, excellent film formation ability, biodegradability, and high nutritive value (Calva-Estrada, Jiménez-Fernández, and Lugo-Cervantes 2019; Wakai and Almenar 2015). Protein-based films are mostly made from collagen, gelatin, soy protein, whey protein, and zein (Kumar, Kaur, and Bhatia 2017). Applications of this material are diverse from emulsifiers to biodegradable composite films, driven by the requirement to change synthetic material with natural and biodegradable material. Collagen and gelatin-based films have shown excellent barriers against oxygen and aroma compounds at low relative humidity with good mechanical properties (Garavand et al. 2017; Liu et al. 2017; Nuanmano, Prodpran, and Benjakul 2015). On the other hand, whey protein and zein are a byproduct in soybean, cheese, and corn starch industries that form a good film-forming capacity with an excellent barrier against oxygen at low relative humidity (Ciannamea, Stefani, and Ruseckaite 2016; Hadidi et al. 2022; Kashiri et al. 2016; Pellicer et al. 2017).

Properties of polysaccharide-based and protein-based film

The properties of polysaccharide-based and protein-based materials are one major literature study in considerable research to give insight into improving the characteristic of hybrid packaging film. The physical and mechanical properties of these materials are summarized in Table 2. Biodegradable films are based on polysaccharides and proteins, such as starch, chitosan, cellulose, pectin, and whey protein. When starch with high amylose was used for starch-based films, the obtained film gave higher tensile strength and a lower percentage of elongation than films obtained from starch with low amylose. Both types of starch also showed a decrease in the tensile strength and an increase in the elongation percentage when glycerol as a plasticizer was added into films with a concentration above 15%. The addition of a plasticizer reduces the intra-molecular attraction forces between the starch molecules and plasticizer, resulting in greater flexibility for films (Muscat et al. 2012). The plasticizer is the most effective in reducing the rigid structure and brittleness in biodegradable films and commonly decreases the tensile strength and increases the elongation of the film.

Table 2. Physical and mechanical properties of the selected natural and synthetic biopolymer (polysaccharide, protein, PLA, PVA, PHA, and PCL-based film).

Film composition	Tensile strength (MPa)	Elongation (%)	Relative Humidity (%)	Reference
Natural biopolymer-based film
Starch mixture with plasticizer	5.06–44.3	2.4–70.7	53	(Muscat et al. 2012)
Chitosan	22.2–39.6	13–73.6	30-60	(Abugoch et al. 2011; Shen and Pascal Kamdem 2015)
Cellulose mixture with filler	51–28.3	8.1–4.8	71.9–70.5	(De Moura, Mattoso, and Zucolotto 2012)
Cellulose mixture with filler cellulose nanowhisker	43-44	14-20	-	(Yang et al. 2013)
Potato starch mixture with glycerol	30-68	3-5	-	(Sessini et al. 2016)
Pectin mixture with chitosan nanoparticles	58.5–26.07	0.94–2.91	50	(Lorevice et al. 2016)
Whey protein mixture with filler	12.10–15.85	78.83–62.14	-	(Alizadeh-Sani, Khezerlou, and Ehsani 2018)
Synthetic biopolymer-based film
PLA/cellulose nanocrystal	23-294	1.6–5.2	-	(Z. Wang et al. 2019)
PVA/whey protein isolate/nanosilica	3.73–10.2	78.2-127	-	(Lara et al. 2021)
PHA/cellulose microfiber	20.2.23.5	0.54–0.93	-	(Khalid et al. 2018)
PCL/starch/pomegranate rind	8.18–8.41	20.14–69.50	-	(Zeng et al. 2020)
PCL/quaternized chitosan	18.8–15.9	190.5–195.5	-	(Mármol, Gauss, and Fangueiro 2020)
PLA/cellulose microfiber	52.3–55.2	1.95–2.39	-

Another study on starch-based films shows that adding plasticizers affects the mechanical properties of starch-based films. Figure 2 shows that the tensile strength decreased while the elongation percentage increased with increasing plasticizer in all starch-based films (Sessini et al. 2016). The mechanical properties of films only with starch polymer matrix demonstrated higher tensile strength than films with plasticizer addition because of their influence in the botanical origin of starch, precisely the proportion of amylose and amylopectin (Hulleman, Janssen, and Feil 1998).

Figure 2. Mechanical properties (A) tensile strength and (B) elongation at break of a starch-based film with different glycerol, catechin and starch nanocrystal concentrations (Sessini et al. 2016).

For chitosan-based films with the addition of protein from quinoa seed give a lower tensile strength and elongation than the neat chitosan films. These indicate that adding another biopolymer, such as protein, decreased the mechanical properties of chitosan film. The presence of protein in films also increased the extensibility up to four times compared to films made with only chitosan-based (Abugoch et al. 2011). In another study, the chitosan-based films also had the same result: a higher tensile strength value compared to film with the addition of additives (Shen and Pascal Kamdem 2015).

In cellulose-based films, the effect of adding fillers on the physical and mechanical properties of cellulose film has been increased. The incorporation of fillers into polymer matrix formulation resulted in a significant increase in tensile strength and elongation. In films of cellulose acetate containing cellulose nanowhisker fillers, the film’s tensile strength prepared using 4.5 wt % of fillers solution increased by 9% and the elongation percentage by 44% (Yang et al. 2013). Figure 3 shows that the increase in filler concentrations also increases the tensile strength and elongation.

Figure 3. The mechanical properties of the cellulose-based film with different cellulose nanowhisker filler content (Yang et al. 2013).

In pectin-based films, the mechanical properties analysis showed that low-methyl pectin content led to stronger and stiffer films than high-methyl pectin because of the higher occurrence of a polar group in low-methyl pectin and possibly leading to higher content of hydrogen bonds and more compact pectin network (Lorevice et al. 2016). This study also used chitosan nanoparticles as reinforcement to improve the tensile strength and elongation of both types of pectin. The addition of chitosan nanoparticles enhances more than 50% of the mechanical properties for high-methyl pectin and low-methyl pectin (Lorevice et al. 2016).

For protein-based films, incorporating cellulose nanofibers as fillers with different concentrations increased the tensile strength but decreased the elongation. Results show that the highest tensile strength value was observed in the composition of whey protein isolate and 7.5% cellulose nanofibers (15.85 MPa), which increased by 54.8% compared to pure whey protein isolate-based films without fillers (Alizadeh-Sani, Khezerlou, and Ehsani 2018). On the other hand, elongation decreased because the biopolymer such as a protein with rigid fillers causes a higher hardness and brittleness effect on films than pure protein films. However, the rigidness and hardness in the incorporation of filler can be improved by adding a plasticizer. The smaller the plasticizer’s molecular weight, the more effective for film, and plasticizers with low polarity are less effective due to their low competition with hydrogen bonding sites (Wihodo and Moraru 2013).

Synthetic biopolymers

Biopolymer produces by chemical fusion from biomass monomer

Polylactic acid (PLA)

Polylactic Acid (PLA) is a synthetic thermoplastic biopolymer derived from the fermentation of agricultural products such as corn, wheat, sugarcane, and tapioca which make it renewable and compostable. PLA has been explored intensely in biofilm studies because of its remarkable properties like biodegradability, compatibility, and easier processability compared with other synthetic biopolymers (Kamal and Khoshkava 2015). However, PLA as a packaging material poses major weaknesses, including low thermal and mechanical properties as well as high water vapor and gas permeabilities. The combination of biopolymers and fillers with PLA matrices is known as one of the most efficient and cost-effective strategies to enhance the resulting composite properties (Mahmoodi, Ghodrati, and Khorasani 2019; Monika et al. 2018; Nagarajan, Mohanty, and Misra 2016).

Polyvinyl alcohol (PVA)

Polyvinyl alcohol (PVA) is one of the synthetic biopolymers easily obtained from renewable bio-based monomers or mixed sources of biomaterial and petroleum (Aslam, Ali Kalyar, and Ali Raza 2018; Ciannamea, Stefani, and Ruseckaite 2016; Jun Feng 2017). PVA also possesses good biocompatible, biodegradable, nontoxic, odorless, water-soluble, and has a higher crystalline biopolymer (Srivastava et al. 2019). It has interesting mechanical properties and excellent film-forming ability because of the abundance of hydroxyl groups and intermolecular hydrogen bonding (Choo et al. 2016). PVA has been attracted great attention as a potential packaging material. However, PVA-based film has low resistance to water and humid environments because of its hydrophilic nature that leads to swelling, poor oxygen barrier, and mechanical properties (C. H. Lee et al. 2020). To overcome those drawbacks, it can be blending with other biopolymers and reinforcement.

Biopolymer produces by microbial production

Polyhydroxyalkanoate (PHA)

Polyhydroxy Alkanoate (PHA) is a biobased polymer and a family of polyesters of several R-hydroxy alkanoic acids. It can be processed thermally and synthesized by several microorganisms in the presence of excess carbon and when essential nutrients like oxygen, nitrogen, or phosphorus are limited (Anjum et al. 2016; Cinelli et al. 2019). PHA is biodegradable, compatible, and can be obtained from renewable resources. These polymers have similar thermoplastic properties, good mechanical properties, and possess excellent biodegradability in various applications with melting points ranging from 40 to 180°C (Yalcin et al. 2006). Despite their good properties and excellent biodegradability, PHA film has some drawbacks due to its high crystallinity, leading to brittle and rigid film, low toughness, and poor resistance to high temperatures. The properties of PHA also depend on the monomer composition and microbial fermentation technique (Modi, Koelling, and Vodovotz 2011; Reichert et al. 2020).

Polycaprolactone (PCL)

Polycaprolactone (PCL) is an aliphatic polyester and biodegradable polymer produced by chemical synthesis from crude oil via the ring-opening polymerization of caprolactone (Sharmin et al. 2012). This biopolymer, with a low melting (approximately 60°C) and glass transition (around −60°C) temperature, offers an ideal biodegradable matrix for natural antimicrobial agents as it can be processed at low temperature and have the potential for use in commercial applications. PCL is widely used in food packaging because of its degradability, biocompatibility, and superior rheology properties. However, there is some drawback to this biopolymer which has lower modulus and rigidity that limits their applications (Khalid et al. 2018).

Properties of PLA, PVA, PHA, and PCL based film

The PLA, PVA, PHA, and PCL-based film materials are shown in their mechanical properties and summarized in Table 2. The PLA-based film incorporating cellulose nanocrystals gives a higher tensile strength than the pure PLA film. The highest tensile strength value of 294 ± 7.2 MPa was obtained using PLA-based film incorporating cellulose nanocrystal 0.1% (Z. Wang et al. 2019). However, the elongation of the film decreased with increasing the concentration of cellulose nanocrystal. This reduction can be attributed to the mobility of the polymer matrix caused by the increased stiffness in the PLA films. Another study of PLA-based film with cellulose microfibers also has the same result and gives a higher tensile value than film without filler (Mármol, Gauss, and Fangueiro 2020). In PVA and whey protein isolate film, an increase of nanosilica filler also slightly increases the elongation. Figure 4 shows the difference in tensile strength and elongation of PVA and whey protein-based film with and without fillers. The tensile strength value of 10.2 MPa was obtained using nanosilica 4% (Lara et al. 2021). This increase can occur because of the mobility restriction that provided a more rigid film.

Figure 4. Mechanical properties of a PVA/WPI-based film with different nanosilica (NS) concentrations (Lara et al. 2021).

In PHA-based film, the mechanical properties showed the improvement of tensile strength and elongation value with the addition of cellulose microfibers filler. Results show that the addition of cellulose microfiber gives higher tensile strength and elongation value with 23.5 MPa and 0.93%, respectively (Mármol, Gauss, and Fangueiro 2020). The improvement in this study showed affinity between cellulose microfiber and the polymer matrix of PHA without the addition of a coupling agent to improve the reinforcing effect of the fibers in PHA-based film. For PCL-based film, incorporating starch and pomegranate rind as fillers with different concentrations increased the tensile strength but decreased the elongation. Results show that the highest tensile strength value was observed in addition to 25% starch and pomegranate rind 20% (8.41 MPa) (Khalid et al. 2018). On the other hand, elongation percentage decreased in addition to the filler by 20.14%. In another study, PCL-based film with chitosan added higher mechanical properties than PCL film with starch with 18.8 MPa and 195.5% for tensile strength and elongation, respectively (Zeng et al. 2020).

Fillers

Filler or reinforcement is added to the polymer matrix to improve the barrier, physical, and mechanical properties while also filling voids in the polymer matrix with a relatively inexpensive molecule. Various materials can be added to achieve these criteria, including natural and synthetic polymers. The addition of filler into hybrid material also offers numerous environmental advantages for the packaging industry, such as lower density, lower cost, reduced pollutant emission, and enhanced mechanical strength of hybrid films (Faruk et al. 2012).

Natural fillers

Natural-based fillers are very popular because of their renewability and the volatility of other petroleum-based fillers. Natural fillers from natural fiber have been extensively used to fabricate hybrid and composite films. Their advantages, such as low cost, low density, non-abrasive to processing equipment, low production energy, and their renewable and degradable features are some of the important properties of natural filler resources, which make them more suitable fibers to use as a filler in hybrid polymer film (Saba, Md Tahir, and Jawaid 2014). Many reports are available on using lignocellulose as reinforcement for polymer matrix. Lignocellulose fibers are mainly composed of cellulose, hemicellulose, lignin, and other extractives in smaller amounts (Pandey et al. 2012; Prado and Spinacé 2019). The origin and type of material greatly influence the composition of natural filler resources. Therefore, detailed information about the building block of lignocellulose is very important to explore its potential as reinforcement for hybrid polymer in packaging applications. The chemical composition of some natural filler resources is listed in Table 3.

Table 3. Chemical compositions of selected natural materials (Pandey et al. 2012; Prado and Spinacé 2019; Wang et al. 2019).

Natural filler resource	Cellulose (%)	Lignin (%)	Hemicellulose (%)
Abaca	53-63	7-9	20-25
Bamboo	26-43	21-31	30
Banana	63-83	5	-
Coconut Coir	36-43	0.15–1.25	41-45
Cotton	83-91	-	3
Kenaf	84.7–80.9	-	-
Pineapple	70-82	5-12	-
Pineapple crown leaf	51-78	13.4	13.3
Baggase	55.2	25.3	16.8
Ramie	68.6–76.2	0.6–0.7	13.1–16.7
Rice Husk	35-45	20	19-25
Hemp	70.74.4	3.7–5.7	17.9–22.4
Flax	71	2.2	18.6–0.6
Hardwood	43-47	25-35	-
Softwood	40-44	25-29	-

Natural-based filler is also susceptible to degradation, which is affected by the compositions (Gurunathan, Mohanty, and Nayak 2015). A component-like hemicellulose is mainly responsible for biological and thermal degradation and hygroscopic property. In contrast, lignin is responsible for UV degradation and is a thermally very stable component (Mochane et al. 2019). The physical and mechanical properties of selected natural-based filler resources are presented in Table 4. The high tensile strength of flax as one of the natural resources for filler compared to other listed fibers can be attributed to flax’s unidirectional cellulose microfibrils and tubular structure (Mochane et al. 2019; Sanjay et al. 2018).

Table 4. Physical and mechanical properties of selected natural (Gurunathan, Mohanty, and Nayak 2015).

Natural filler resource	Density (g/cm^3)	Tensile strength (MPa)	Elongation (%)
Abaca	1.5	430-813	2.9-10
Bamboo	0.6–1.25	290	-
Banana	1.35	529-914	2.6–5.9
Coconut Coir	1.15–1.25	131-220	15-40
Cotton	1.51	400	3-10
Kenaf	−1.4	284-930	1.6
Pineapple	1.44	13-1627	14.5
Baggase	0.89	350	5.5
Ramie	1.5	500	2
Hemp	1.48	550-900	1.6-4
Flax	1.4	800-1500	1.2–1.6
Hardwood	0.3–0.88	51-121	-
Softwood	0.3–0.59	45.5-111	-

Synthetic fillers

Some synthetic fillers have been introduced because of their unique value and functional properties. Inorganic fiber, such as glass and carbon fibers, are the most used filler in the hybrid composite film (Mochane et al. 2019). Most of these materials have been experienced their success in different industrial sectors such as large-scale production, construction, and many others (Haldorai, Jin Shim, and Taek Lim 2012). Synthetic fillers possess high mechanical properties, good stability, and have a long-lasting life span. These properties advance the synthetic fillers over the natural fillers, in which the latter suffer from inconsistency because of the variation in growth conditions as well as their environmental conditions (Gangil et al. 2020). In a previous study, adding Nano clay filler achieved a higher impact and tensile strength, whereas glass fiber has optimum flexural strength value (Mutalikdesai et al. 2018). By lowering the reinforcing addition, this filler lowers the cost without sacrificing the characteristics of the composite. Particularly these fillers improved the cost of carbon, kevlar, and glass fiber composites. The cost to performance ratio is an important consideration for the production of these fillers in order to come up with a low-cost manufacturing process. The game-changing properties there are their high tensile strength (1.9 to 3.5 GPa for glass fibers) and tensile modulus (between 50 and 90 GPa) that overshadow other disadvantages, resulting in the successful applications in industrial scale (Salman 2019).

Plasticizers

Plasticizer additive is used to strengthen the flexibility, elasticity, and toughness of polymer films and at the same time also decrease hardness and stiffness (Malathi, Santhosh, and Udaykumar 2014). The plasticizer can be used in polymers that are brittle and fragile at room temperature and have a lower glass transition temperature, and can be used to produce film or plastic for packaging. Currently, many plasticizer products have been applied in polymers as packaging. Plasticizers used in the film-forming solution are summarized in Table 5.

Table 5. Plasticizer used in biodegradable film-forming solutions (Malathi, Santhosh, and Udaykumar 2014).

Type of polymer-based	Plasticizer	Application system
Polysaccharides-based	Glycerol, Ethylene glycol, xylitol, and sorbitol	Potato starch films
	Glycerol and sorbitol	Cassava starch films
	Glycerol and Ethylene glycol	Chitosan films
	Glycerol and sorbitol	Maize starch films
	Xylitol and glycerol	Cellulose and cellulose acetate films
	Glycerol	Carrageenan edible films
	Glycerol	Alginate/pectin films
Protein-based	Oleic and linoleic acids	Zein-based films
	Glycerol and sorbitol	Whey protein-based films
	Glycerin	Wheat gluten films
	Saturated fatty acid	Sunflower protein films
	Glycerol and sorbitol	Gelatin-based films
	sorbitol	Caseinate-pullulan films

Plasticizers generally have an average molecular weight between 300 and 600 kDa, a high boiling point, and linear or cyclic carbon chains (14–40 carbons) (Vieira et al. 2011). The small molecular size of plasticizers allows occupying of intermolecular spaces between polymer matrix chains and decreases secondary forces among them. As a result, there is an increase in free volume and molecular mobility. The polymer–plasticizer interactions are dominant at low plasticizer concentrations, while at high plasticizer concentrations, plasticizer-plasticizer interaction can become more significant. This also explains the observation of non-plasticization, wherein very low concentrations of plasticizer can increase the rigidity of the polymer (Höfer and Ecosiris 2012). Different type of elasticity and flexibility of biodegradable polymers mostly depends on the chemical structure, molecular weight, chemical composition, and functional group of the plasticizer (Hourston 2010; Rungsinee and Krochta 2005). The variety of plasticizers also affects the flexibility of the final product materials (Cao, Yang, and Fu 2009).

Formulation of a natural biopolymer-based film can alter some properties like microstructure, water vapor, and gas permeability. The starch film with two different plasticizers (sorbitol and glycerol) was tested in different concentrations (1–8 g/L) (García, Martino, and Zaritzky 2000). Results show that the addition of plasticizer improved starch-based film performance by increasing barrier properties to water vapor and maintaining the selective gaseous permeability. In other studies, adding a blend plasticizer (Sorbitol/glycerol) also improves protein-based films’ flexibility and barrier properties (Jafarzadeh et al. 2018).

Properties hybrid polymer film

Physicochemical properties

Biodegradability, gas barrier, and moisture barrier are essential physicochemical properties of hybrid polymer films. The biodegradability of hybrid film is used to determine its lifespan. Environmental conditions such as temperature, moisture, pH, and nutrient availability that the materials are exposed to in the presence of microorganisms affect biodegradation (Obasi et al. 2013). The film should have a particular lifespan that allows for ample time for use while also allowing for rapid degradation when disposed of. Table 6 shows the standard methods for determining the biodegradability of packaging film.

Table 6. Standard methods to measure biodegradation of packaging film (Obasi et al. 2013).

Standard Test	Type of environment	Characteristic of properties
ASTM D5209-92	Aerobic sewage sludge	CO2
ASTM D5210-92	Anaerobic sewage sludge	CO2/CH4
ASTM D5247-92	Aerobic specific microorganism	Molecular weight
ASTM D5271-93	Active sewage sludge	O2/CO2
ASTM D5338-92	Controlled composting	CO2
ASTM D5509-94	Simulated composting	Physical properties
ASTM D5511-94	High solids anaerobic digestion	CO2/CH4
ASTM D5512-94	Simulated compost using externally heated reactor	Physical properties
ASTM D5525-94	Simulated landfill	Physical properties
ASTM D5526-94	Accelerated landfill	CO2/CH4

Biodegradability consists of two phases. The first phase is characterized by the defragmentation of film material initiated by the presence of moisture, heat, and enzymes excreted by a microorganism like bacteria, fungi, or algae (Naveena and Sharma 2020). The second phase is biodegradation, initiated by naturally occurring enzyme and acids, which transforms long into short compounds (Naveena and Sharma 2020; Zhai et al. 2004). Subsequently, the molecules are reduced to a smaller size and absorbed/fixated through the cell walls of the microorganisms, where they are then metabolized for energy. The metabolisms eventually produce carbon dioxide or methane depending on the pathways (Prathipa, Sivakumar, and Shanmugasundaram 2018).

Different standard methods or principles for evaluating biodegradation for packaging film are available. To determine the biodegradation of hybrid polymer films there are mainly three methods, which include laboratory test, simulation test, and field test, as illustrated in Figure 5. The laboratory test is the most reproducible biodegradation test, where specific media are used, such as synthetic media, and inoculated with one of the mixed microbial populations from wastewater or individual microbial strains, which may have been specially chosen for particular polymers (R. Müller 2002).

Figure 5. Illustration of the biodegradation test (Müller 2002).

The method for field test situations is by burying film or plastic materials in soil and placing them in an open landscape or performing a full-scale composting process with films that can represent the ideal practical with environmental conditions. This test also has several disadvantages like environmental conditions such as temperature, humidity, and pH content that cannot be controlled when the test occurs (R. Ciencia and Andes 2018; Müller 2002). This type of test also limits the analytical opportunities to monitor the degradation process.

As alternatives to this type of test, different simulations have been used to measure the biodegradation of films. The degradation test might take place in the soil, compost, or seawater placed in a controlled reactor in a laboratory (R. Müller 2002). Despite this, the environment is still very close to the field test situation. The parameters such as temperature, pH content, and humidity can be controlled and adjusted when the test occurs. Occasionally, to reduce the time taken to process the tests, nutrients are added to the reactor to increase the microbial activity and stimulate the degradation process (Naveena and Sharma 2020).

Gas and moisture barriers are also important physicochemical properties of the hybrid polymer film. To withstand shelf life and maintain the quality of food during storage, specific gas pressure conditions are required for the food. The gas mixture consists of oxygen, carbon dioxide, and nitrogen, or a combination of these three gases in most packaging applications. To achieve the gas pressure condition and certain gas compositions, the material in the film needs certain gas barrier properties. These film properties are closely related to the permeation capacity of the material to exchange low molecular weight substances through film. The use of multilayers could give better barrier properties for packaging materials (Cazón et al. 2017).

Moisture barrier properties of the film refer to the ability of film to resist the entry of undesired vapor, such as vapor by permeability, diffusivity, solubility across the barrier, and the affinity of the packaging film materials toward water (Cazón et al. 2017). The term permeability describes the overall mass transport of the gas or liquid across the film, whereas the term of diffusivity describes the movement of the molecules inside the bulk of polymer (Siracusa 2012). There are studies about the hydrophobic type of bio-based materials that reduce natural polymers water vapor permeation. These properties depend upon the crystallinity and chain conformation of material resources in the film. Generally, hybrid films from natural biopolymers have a higher value of gas and moisture barrier than synthetic materials. Because of the higher value for barrier properties, many studies have been done to decrease the value by adding fillers and layers to natural biopolymer-based films. The barrier properties of the selected hybrid film are shown in Table 7.

Table 7. Barrier Properties of selected hybrid film.

Hybrid film composition	Water vapor permeability/WVP (g/m.s.Pa).10^−11	Temperature (℃)	Relative Humidity (%)	Reference
Cassava starch/glycerol/clay nanoparticle	3.88–8.19	25	75	(Souza et al. 2012)
Sodium caseinate/starch/glycerol	0.25–15.1	25	84.3	(Arvanitoyannis and Biliaderis 1999)
Methyl cellulose/starch/glycerol	0.65–25.9	25	84.3	(Arvanitoyannis and Biliaderis 1999)
Cassava starch/glycerol/ nanoclay	8.33–2.16	25	75	(Müller, Borges Laurindo, and Yamashita 2011)
Chitosan/starch/silver nanoparticle	0.58–0.64	25	50	(Yoksan and Chirachanchai 2010)
Chitosan/tapioca starch/glycerol	2.8–12.1	32	65	(Vásconez et al. 2009)
Methylcellulose/nanocellulose	4.86–6.82	25	60	(Khan et al. 2010)
Hydroxypropyl cellulose/silver nanoparticles	1.33–2.22	25	0	(De Moura, Mattoso, and Zucolotto 2012)
Tara-gum/chitosan nanoparticle/glycerol	9.91–1.28	25	53	(Antoniou et al. 2015)

Biopolymer films based on cellulose materials are very efficient in preserving oxygen and aroma compounds. Among the available cellulose derivatives, methylcellulose is one of the most studied in materials biopolymer films. Studies about the effect of methylcellulose content with the addition of starch on the barrier properties resulted in significant variation in the water vapor permeability from 0.65 × 10⁻¹¹ to 25.9 × 10⁻¹¹ g/m. s. Pa for concentrations ranging from 45% to 30% (Arvanitoyannis and Biliaderis 1999). These values are explained by a hydrophilic characteristic of the film resulting from the hydroxyl groups in the polymer chain of methylcellulose and starch. When a combination of methylcellulose and starch was used as a polymer matrix and fillers, the water vapor permeability increased significantly as the polymer concentrations decreased (Arvanitoyannis and Biliaderis 1999).

In starch-based films, the addition of chitosan decreased the water vapor permeability value from 12.1 × 10⁻¹¹ to 2.8 × 10⁻¹¹ g/m. s. Pa (Vásconez et al. 2009). This decreasing trend can be attributed to the higher hydrophobicity when compared to starch. Additionally, hydrogen bond interaction between chitosan and starch reduces the availability of the hydrophilic groups and decreases the interaction between materials and water molecules, affecting the value of barrier properties.

Recently, the addition of nanofiller for improving the barrier properties of films has been the main focus in several studies. The materials that are commonly used for nanoparticle materials in biodegradable hybrid films are chitosan, cellulose, and starch. Chitosan nanoparticle materials have been widely used as a filler to improve barrier properties and thermo-stability (Antoniou et al. 2015).

The studies of chitosan nanoparticles and bulk chitosan with tara gum films showed that the chitosan nanoparticle affected the water vapor permeability values. The permeability decreased from 9.91 × 10⁻¹¹ to 1.28 g/m. s. Pa, when 10% of chitosan nanoparticle was incorporated (Antoniou et al. 2015). The presence of chitosan nanoparticles in tara-gum films could increase the tortuosity of the two biopolymers, leading to a slower diffusion process and lower permeability (Akter et al. 2014).

Cellulose and derivative nanoparticles were also studied in this topic. In methylcellulose films, the addition of nanocellulose resulted in the value of water vapor permeability decreasing sharply as the concentration of nanocellulose increased. Thus, a 26% reduction of water vapor permeability value in films was obtained at 1% nanocellulose addition (Khan et al. 2010). The effect of nanocellulose content on the water vapor permeability is shown in Figure 6.

Figure 6. Effect of different nanocellulose concentrations on the water vapor permeability of methylcellulose-based films (Khan et al. 2010).

Other materials for enhancing barrier properties are clay nanoparticles and silver nanoparticles. In starch-based films, it was observed that the clay nanoparticles addition caused a significant decrease in the water vapor permeability of 8.33 × 10⁻¹¹ g/m. s. Pa (Müller, Borges Laurindo, and Yamashita 2011). In chitosan/starch-based films, the addition of silver nanoparticles increased the water vapor permeability value (Yoksan and Chirachanchai 2010). The growth in this permeability might be a result of the obstruction of intermolecular hydrogen bond formation between chitosan and starch molecules, causing the incompatibility of the film polymer matrix and adsorption of water vapor at the hydrophilic section of chitosan and starch molecules (Yoksan and Chirachanchai 2010). In another study with cellulose-based films, a decrease in the water vapor permeability value of the cellulose-based film was observed. The addition of silver nanoparticles with smaller sizes give varied result in different particle sizes from cellulose-based films containing from 41 nm to 10 nm (De Moura, Mattoso, and Zucolotto 2012).

Mechanical properties

The mechanical properties of hybrid polymer film are the most crucial in packaging applications. They include tensile strength (TS), elongation (E), and Young modulus (YM). They depend on the hybrid film blend and the nature of every component in film. Mechanical properties are very important in packaging hybrid film products. They are considered because the material can withstand external stress while maintaining the film’s consistency. Table 8 shows the mechanical property values of some selected hybrid films. In whey protein-based films, the reinforcement of cellulose nanofibers and TiO₂ nanoparticles offers good mechanical properties. The results show that the addition of 7.5% cellulose nanofibers and 1.5% TiO₂ nanoparticle into whey protein-based films reached a higher value in tensile strength (17.03 MPa) and reduction in elongation (59.04%) (Alizadeh-Sani, Khezerlou, and Ehsani 2018). The improved tensile strength of hybrid films can be ascribed by the suitable compatibility between two biopolymers and nanoparticles through a unique interaction (El-Wakil et al. 2015). In another study, whey protein-based films were also blended with synthetic biopolymer such as polyvinyl alcohol. The addition of synthetic biopolymer increased elongation and decreased the tensile strength and Young modulus of the whey protein films (Lara et al. 2019). Despite the lower mechanical properties of these blend films, the result can be considered close to the low-density polyethylene (LDPE) films, which are materials with large applications in the food packaging sector (Doak and James 1986).

Table 8. Mechanical properties of selected hybrid film.

Hybrid film composition	Tensile Strength (MPa)	Elongation (%)	Young modulus (MPa)	Reference
Whey protein/cellulose nanofiber/TiO2 nanoparticle	17.03	62.08	311.52	(Alizadeh-Sani, Khezerlou, and Ehsani 2018)
Whey protein/polyvinyl alcohol/glycerol	9.49	126.8	175	(Lara et al. 2019)
Polyvinyl alcohol/cellulose nanocrystal/chitosan	51.48	30	1917.62	(El Miri et al. 2015)
Polyvinyl alcohol/cellulose nanofiber and nanocrystal	53	120	1200	(Lee et al. 2020)
microcrystalline cellulose/corn starch/glycerol	90	2.3	158.9	(Psomiadou, Arvanitoyannis, and Yamamoto 1996)
Corn starch/nano-clay/chitosan	12.46	5.35	805	(Chung et al. 2010)
Cassava starch/glycerol/nano-clay	16.47	2.60	789	(Müller, Borges Laurindo, and Yamashita 2011)
Starch/cellulose nanocrystalline cellulose/glycerol	17.4	9.1	520	(Slavutsky and Bertuzzi 2014)
Chitosan/Polyvinyl alcohol/polylactic acid/ZnO	11.54	25.56	-	(Shariatinia and Fasihozaman-Langroodi 2019)

In Figure 7, mechanical properties show that whey protein films behaved as a fragile materials, and polyvinyl alcohol gives an elastic structure in protein-based films. The addition of polyvinyl alcohol caused an increase in elongation for all ratios of whey protein and polyvinyl alcohol blends, but on the other hand, the tensile strength and Young modulus decreased by increasing the loading of polyvinyl alcohol (Lara et al. 2019). This reduction in tensile value strength and Young modulus was caused by the chain characteristic in polyvinyl alcohol that provided high film flexibility. It was shown that the whey protein and polyvinyl alcohol films with a ratio of 70/30 resulted in a lower value of tensile strength and higher value in elongation than the blend film of whey protein and polyvinyl alcohol with the ratio of 90/10 (Lara et al. 2019).

Figure 7. Value of Tensile strength (a), Young modulus (b), and Elongation (c) of hybrid films with different ratios of whey protein and polyvinyl alcohol (Lara et al. 2019).

Apart from increasing the flexibility in films, polyvinyl alcohol is also used to combine a polymeric matrix with chitosan and cellulose nanocrystal as a filler. It showed that the tensile properties of polyvinyl alcohol and chitosan films are affected by their blending and the incorporation of different concentrations of nanocrystals cellulose (El Miri et al. 2015). After adding fillers into the polyvinyl alcohol and chitosan blend films, an increase in tensile strength and Young modulus was visible. This could be present in the form of more hydrogen bonds interaction that occurred between the fillers and polymer matrix blend. When 5% nanocrystal cellulose was added in blend films, the tensile strength and Young modulus increased by 77% and 105%, respectively (El Miri et al. 2015). On the other hand, the elongation decreased with a higher concentration of nanocrystalline cellulose. The same result also appears in the starch-based film, in which the addition of nanofillers such as nano-clay and nanocrystalline cellulose increased the tensile strength and Young modulus but decreased the elongation (Chung et al. 2010; Müller, Borges Laurindo, and Yamashita 2011; Slavutsky and Bertuzzi 2014). In chitosan films, the use of polyvinyl alcohol and polylactic acid with a 1:1 ratio and ZnO nanoparticles as fillers improved the mechanical properties of the resulting hybrid films. Results of all samples in the study showed that the addition of fillers with 6% imposed the highest tensile strength of 11.54 MPa, which increased from 2.92 MPa without the addition of fillers (Shariatinia and Fasihozaman-Langroodi 2019). The ZnO nanoparticle also reduced the elongation of films from 50.56% to 25.56%, which can be attributed to the origin characteristic of these materials with low elasticity.

Thermal properties

Excellent thermal properties ensure the protection of the product against thermal damage during a period of storage in the application. Thermal properties are also the most critical function for the hybrid biodegradable film in which they determine the suitability of biofilm for a certain application. Various techniques are applied in the processing of bio-film. They include plasticization, reinforcement of filler, blending with other polymer matrices, and converting biofilm into the thermoplastic phase to improve the hybrid films thermal properties (Prathipa, Sivakumar, and Shanmugasundaram 2018). Figure 8 shows a general thermogravimetric analysis (TGA) graph for the decomposition process of biodegradable polymers and fillers as material components. In thermogravimetric curve for hybrid biodegradable film shows a process of thermal degradability in the sample. The model of the thermogravimetric curve in the sample is determined by various kinetic parameters of the pyrolysis process, such as reaction order, frequency factor, and activation energy. The values in these parameters depend upon conditions, weight, shape, flow rate, heating rate, and the treatment used before evaluating the data from the sample film (Azwa et al. 2013). Theoretically, when a reaction in the process develops in a differential thermal analysis (DTA), the change in thermal properties of the sample film is indicated by deflection or a peak in the curve. If the reaction rate in the process varies with different temperatures, the position of the peak will also vary from the heating rate if different experiment conditions are stable. This difference in peak temperature could determine the activation energy for the reaction of different reaction orders in the process (Di Franco et al. 2004; Lee and Wang 2006; Suardana, Ku, and Kyoo Lim 2011; S. H.). Shortly, thermogravimetric (TG) and derivative thermogravimetric (DTG) curves are mostly used to determine the weight loss in materials and to determine the decomposition of material at a certain temperature for development in the study of thermal in materials hybrid.

Figure 8. The general thermogravimetric decomposition process of biodegradable polymers and fillers from natural resources (Azwa et al. 2013; Di Franco et al. 2004; Lee and Wang 2006; Suardana, Ku, and Kyoo Lim 2011).

Concluding remarks and perspectives

Green materials are an alternative packaging for the future food industry. Many materials from natural and synthetic biopolymers can be a good prospect for changing the properties of packaging. Hybrid packaging with more than two sources of materials that combine and be degradable packaging is a challenging material for the future and hopes to be sustainable for many applications, including as a replacement for plastic. Materials made from bioresources, such as reinforcing matrix and filler, will greatly impact the environment. Where currently, various countries are competing to reduce petroleum-based plastics. Of course, these bioresources can reduce environmental hazards and pollution, so this plastic hybrid is an alternative to that problem. With superior properties such as good mechanical, thermal, and environmental, hybrids film should be looked at as a source of future packaging. In addition, the method of making hybrids films can also be done in a big way and is an advantage for the industry to be able to produce on a large scale, and this also supports the world economy as well as solving environmental problems.

Highlights

• This paper reviews recent developments in biodegradable film-based packaging.

• Biodegradable film-based packaging is seen as viable alternative to replace the ones from petroleum derivatives.

• The weak mechanical properties of bioresource-based film can be enhanced by forming composite.

• Biomass-based material can be used as a matrix and a filler in hybrid polymer-based packaging.

• Hybrid material enhanced physicochemical, mechanical, and thermal properties of the resulting films.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.