An Investigation of the Sound Absorption Properties of Flax/Epoxy Composites Compared with Glass/Epoxy Composites

ABSTRACT

Natural fiber based composites are becoming attractive candidates for use in various applications owing to their mechanical and sound absorption properties. It has been proposed that they could potentially replace glass fiber composites owing to their minimized impact on human health and the environment. Though studies have been dedicated to understanding their mechanical properties, few focus on quantifying their sound attenuation behavior. We investigated the sound absorption properties of flax/epoxy composites and found them to be superior to those of glass/epoxy composites. A noteworthy result was that the noise reduction coefficient increased from an average value of 0.095–0.11 for unidirectional flax/epoxy composite and to 0.10 for cross-ply flax/epoxy system. Results suggest that flax/epoxy composites could be less expensive, viable and ecologically superior substitutes for glass-fiber based composites, particularly in applications where sound absorption is important.

基于天然纤维的复合材料具有出色的机械性能和吸声性能，因而在各种应用中日益受到青睐。研究指出它们对人类健康和环境的影响很小，可能会取代玻璃纤维复合材料。各项研究关注了其机械性能，但很少关注和量化其声音衰减行为。本文研究亚麻/环氧树脂复合材料的吸声性能，发现它们优于玻璃/环氧复合材料。一个值得注意的结果是，单向亚麻/环氧树脂复合材料的降噪系数从平均值0.095–0.11增加至亚麻/环氧交叉层体系的平均值0.10。结果表明，亚麻/环氧复合材料可能是玻璃纤维基复合材料的更便宜、更可行且更具生态优势的优良替代品，特别是在吸声应用中更为重要。

KEYWORDS:

• Acoustic properties
• epoxy resin
• flax fiber
• natural fiber

关键词:

• 声学特性
• 环氧树脂
• 亚麻纤维
• 天然纤维

Introduction

Natural fiber based composites are finding increasing use in products like the interior parts of automotives, electronics casings, as reinforcements in the building and construction industry, etc. They are now gaining attention as a viable and environmentally friendly substitute for mineral and synthetic fiber composites. Though their mechanical properties are frequently found to be somewhat poorer than their mineral and synthetic fiber counterparts, it is becoming increasingly obvious that natural fiber based composites make up for these shortcomings in other aspects. For example, natural fiber composites are CO₂ neutral (and thus environmentally friendly), less expensive to produce, recyclable, biodegradable, non-abrasive to machinery, and pose little health risk upon inhalation (Wambua et al. 2003). In the light of these merits, considerable research efforts are being directed at investigating the material properties of natural fibers and their composites, so that they can be better applied in designing and manufacturing products that are both ecologically benign and of good quality (Biagiotti et al. 2004; Kicińska-Jakubowska et al. 2012; Puglia et al. 2005).

Among the numerous natural fiber species, flax in particular has enjoyed considerable interest, perhaps owing to its ready availability and relatively low price. Several studies are dedicated to quantifying the mechanical properties of flax fibers both as raw fibers as well as in the form of reinforcements for polymer resin composites. For example, Le Duigou et al. (2014) immersed a flax/polylactic acid-based composite in seawater and discovered that its mechanical integrity was compromised by fiber degradation (Le Duigou et al. 2015); Yan et al (2015) studied the combined effect of ultraviolet radiation and water spraying on flax/epoxy composite systems and concluded that though there was a decrease in their strength and stiffness, they could still be considered for civil engineering application, especially after treatment. The damping properties of hybrid flax--carbon epoxy composites have been investigated by Assarar et al. (2015), who remarked that the exact position of the flax layers in the overall composite layup had a substantial effect on the mechanical and damping behavior. Still within the context of the dynamic behavior of flax composites, Yan (2012) added alkali solution to a flax/epoxy composite and reported that though its compressive and shear strength was improved, its impact strength and damping ratios were attenuated, perhaps due to the improved fiber-to-matrix adhesion. Recently, Khelifi et al. (2016) added flax fibers to cements and discovered that cast-made composites showed little improvement in mechanical properties, while extruded composites showed substantially better mechanical characteristics. The possibility of using flax/epoxy composites in energy absorbing structures is being rigorously studied by Yan and Chouw (2013, 2014), Yan et al. 2014a, 2014b), who are working on cylindrical flax/epoxy specimens, both with and without foam cores, under axial and transverse crushing. Cylinders with foam cores absorbed more impact energy than hollow ones, and the predominant failure mechanism was found to be progressive crushing. They concluded that, generally, there is significant potential for the use of flax/epoxy composite systems in impact mitigating structures.

In developed countries, excessive noise has had a significant negative impact on people’s health and well-being. In the vicinity of major highways, airports, and construction sites, there are numerous complaints from residents and passers-by pertaining to the inconvenience and health effects arising from the sound of moving vehicles and heavy machineries (Zhu et al. 2013). Studies are being undertaken to quantify the pernicious effects of noise on human circadian rhythms (Freedman et al. 2001). Noise can be attenuated through two major means: active control involves the reduction of noise production at the source, while passive control is achieved by the use of sound absorbing material at the location of the recipient (Bies and Hansen 2009; Yang and Li 2012). Since it is frequently very expensive or not practical to implement active noise control mechanisms, it is imperative to develop passive noise mitigation technologies like sound absorption panels. In this context, researchers have recently begun investigating natural fiber composites as potential materials for the manufacture of sound absorption structures. For example, Ersoy and Küçük (2009) tested tea leaf fibers with a single layer of cotton cloth and reported that it absorbed sound better than polyester and polypropylene based nonwoven fibers; while Zulkifli et al. (2008) quantified the acoustic properties of multi-layer coir fibers and concluded that their sound absorption coefficients were comparable to commercially available rock wool and synthetic fibers. Also, Yang, Kim, and Kim (2003) reported that the acoustic properties of rice straw wood particle composite boards is superior to other wood-based materials. A noteworthy contribution to the use of analytical models in acoustic property estimation was by Fouladi et al. (2011), who compared experimental results of coir fiber with the Delaney--Bazley and the Allard models. Finally, Fatima and Mohanty (2011) noted that jute-based fibers are similar in acoustic behavior to glass-fiber based materials, while Glé et al. (2011) studied hemp fibers using equivalent-fluid models like the Biot-Allard model, and Oldham et al. (2011) undertook a large-scale study on several natural fibers (e.g., jute, hemp, cotton, flax, etc.) to assess their acoustic performance. Most of these works have focused on natural fibers in their raw form (i.e., unimpregnated with any polymer resin). In our present work, we have tested flax and glass fibers used as fillers in epoxy matrix which may affect their properties considerably.

A specific question that has been frequently posed is whether natural fiber composites can replace glass composites (Wambua et al. 2003) in all or some of the abovementioned applications. In attempting to address this question, several studies have focused on investigating the mechanical properties (e.g., strength, fatigue life, etc.) of natural fiber based composites (Bos et al. 2002). Also, from the environmental point of view, natural fiber composites have already been shown to be superior to glass fiber composites, primarily because the former are less dependent on non-renewable energy, have lower pollutant emissions, lower greenhouse gas emissions, greater energy recovery, and excellent biodegradability (Joshi et al. 2004). Nevertheless, more work still needs to be done with regard to the sound attenuation capabilities of natural fiber composites vis-à-vis their glass fiber counterparts. In this work, we have characterized and compared the sound absorption behavior of flax/epoxy composites versus glass/epoxy composites.

Materials and methods

The vacuum assisted resin infusion technique (VARI) was employed to manufacture the flax/epoxy specimens. The Epoxy Epolam 5015 Resin was supplied by Axson Technologies, while the unidirectional (UD) flax fiber tape with area density of 200 g/m² was purchased from Lineo (Belgium). Layers of UD flax fibers with dimensions of 270 mm by 270 mm were cut out and dried at 80°C for 24 h in an oven, before cooling down to room temperature. The layout sequence of the flax fiber layers will determine the overall UD or cross-ply (CP) orientation of the composite sample. Next, they were placed on an aluminum plate used as a mold and then sealed in a vacuum bag. The epoxy resin was mixed with hardener and subsequently degassed in a vacuum oven at room temperature for 15 min to remove air bubbles, after which it was drawn through an inlet tube into the bag to infuse and wet the fibers. Once the fibers were completely impregnated by the resin, the sample was cured at room temperature for 24 h before demolding. Finally, the sample was subjected to a post-curing process at 80 °C for 16 h to obtain a complete cured matrix. Specimens of glass/epoxy composites were also similarly prepared. Table 1 shows the dimensions and volume fractions of all the specimens used in the current study. We note that the UD flax composite (UF) sample is thicker than the sample of CP flax composite (CF) in spite of the fact that both have 20 layers, due to the nature of the manufacturing process, whereby CP fibers were able to align alternately better and permit greater compaction than in the UD sample. Typical fiber material properties are: 800–1500 MPa and 2400 MPa for the tensile strengths, and 60–80 GPa and 73 GPa for the Young’s moduli, of flax and glass fibers, respectively (Wambua et al. 2003). The tensile strength and elastic modulus of epoxy resin are 35–100 MPa and 3–6 GPa, respectively (Yan et al. 2014c).

Table 1. Physical properties of studied samples: Glass fiber epoxy composites (G1 and G2), UD flax epoxy composite (UF) and cross-ply flax epoxy composite (CF).

Sample	Material	Number of Layers	Average Thickness (mm)	Fiber Volume Fraction
G1	UD Glass/Epoxy	16	5.56	0.53
G2	UD Glass/Epoxy	24	8.51	0.55
UF	UD Flax/Epoxy	20	9.32	0.26
CF	CP Flax/Epoxy	20	7.87	0.32

The sound absorption coefficients of the samples were measured using the standing wave apparatus type 4002. The workings of the apparatus are based on the standing wave principle. Briefly, a loudspeaker is situated at one end of an acoustically rigid tube and the sample material to be tested is placed at the other end. A plane sound wave is then generated by the loudspeaker in the tube toward the sample. The wave will be partially reflected by the sample, resulting in a standing wave due to the superposition of the incident and reflected waves. The microphone probe, connected to the microphone carriage, can be moved inside the tube to detect the alternating maximum amplitude and minimum amplitude of the sound pressure. The ratio of the maximum sound pressure to minimum sound pressure is known as the standing wave ratio (SWR). The sound absorption coefficient can be calculated using the following relationship:

This absorption coefficient can be read off directly from the calibrated scale of the measuring amplifier of the standing wave apparatus on a scale of 0–1.

The frequency range of the standing wave apparatus is limited at the lower frequencies by the length of the tube, which must be at least one-quarter of the wavelength under consideration, and at the higher frequencies by the diameter of the tube, which theoretically should be less than 0.586 of the wavelength under consideration in order to exclude the possibility of transverse resonances with the tube. Therefore, a large tube is used for measurements in the frequency range from 100 to 1600 Hz for circular sample size of diameter 102.5 mm, while a small tube is used for measurements in the frequency range from 800 to 6300 Hz for circular sample size of diameter 32.5 mm. The two standing wave apparatus setups are shown in Figure 1 below.

Figure 1. Standing wave apparatus with (a) large tube for low frequency range and (b) small tube for high frequency range.

Results and discussion

Figure 2 shows the sound absorption coefficient values for all samples across the measured range of frequencies. The trend of the graphs is similar for all the samples and their values are also quite close to each other. There is little to no absorption at low frequencies below 500 Hz, while absorption gradually increases as the frequency increases beyond 500 Hz, before becoming relatively constant from 3150 Hz to 6300 Hz. This is because the lower the frequency, the longer the wavelength of sound and the shorter the propagation path of sound wave. Therefore, there will be less dissipation of sound energy at lower frequencies and more dissipation at higher frequencies.

Figure 2. Sound absorption coefficients of samples at different frequencies.

Yang and Li (2012) have found the sound absorption coefficient of flax/epoxy composite to be over 0.8 at 10,000 Hz. They employed the impedance tube, based on the transfer function method, to measure the sound absorption coefficients up to a frequency of 10,000 Hz. Unfortunately, the standing wave apparatus we have used can only measure up to 6300 Hz and thus cannot verify the sound absorption coefficients of our samples at higher frequencies. On the other hand, while they studied plain woven samples, we investigated specimens with both unidirectional and cross-ply fiber orientations.

It can also be observed that G2 has higher absorption coefficients than G1 for all frequencies. This is expected because increasing the thickness of the sample means that the sound will have to pass through more material, resulting in greater frictional losses that dampen the sound energy.

For an easier visual comparison between the samples, the noise reduction coefficients (NRC), calculated by taking the arithmetic mean of all the sound absorption coefficients across the frequency range, are summarized in Figure 3.

Figure 3. Noise reduction coefficients (NRC) for all samples.

The NRC of UF is marginally higher than that of CF. Although UF is thicker than CF, its fiber volume fraction is lower than that of CF. The NRCs of both G1 and G2 are in general lower than those of UF and CF. Comparing between G2 and CF, G2 is thicker and has a higher fiber volume fraction but their NRCs are almost identical. This suggests that flax fibers are indeed better sound absorbers than glass fibers, which agrees with the findings of the reported study done by Yang and Li (2012). They compared the microscopic cross-sections of natural fibers and synthetic fibers and found a single sisal fiber to be made up of a bundle of hollow sub- fibers with lumen inside while a glass fiber has a regular and solid construction throughout. This unique lumen structure means that there would be greater frictional and thermal losses as the sound wave tries to propagate through the air spaces and inside the lumen, making natural fibers better sound absorbers than synthetic fibers.

Nevertheless, the NRC values of UF and CF are still relatively low compared to that of plain woven flax fibers, which is around 0.65 as found by Yang and Li (2012). The composite samples are compacted during manufacturing, which may have resulted in the diminishing of free spaces within and between the flax fibers. The epoxy resin may also have occupied some effective volume of air flow and the air cavities between the fibers and inside the lumens. Moreover, the sound absorption performance of epoxy is generally poor. These explain why flax/epoxy composites are poorer sound absorbers than plain flax fibers. However, there are still possible ways to improve their overall sound absorption capabilities. Additives like precipitated calcium carbonate can increase the stiffness of the composite to provide better absorption of sound waves. Having a sandwich structure can help to increase the sound insulation between the adjoining composite panels with a lightweight core inside. Similarly, a honeycomb core structure may also enhance sound insulation (Zhu et al. 2013).

Conclusion

In this study, we quantified the sound absorption properties of flax/epoxy composites and subsequently compared them with glass-fiber/epoxy composites. The present results showed that flax/epoxy composites have very good acoustic properties and show promise as environmentally safe and sustainable replacements for glass/epoxy systems.

Funding

The authors would like to acknowledge the financial support from the Agency for Science, Technology and Research (A*STAR) under the Science and Engineering Research Council (SERC) grant number 1426400041.

Additional information

Funding

The authors would like to acknowledge the financial support from the Agency for Science, Technology and Research (A*STAR) under the Science and Engineering Research Council (SERC) grant number 1426400041.