Effect of fiber modified by alkali/polyvinyl alcohol coating treatment on properties of sisal fiber plastic composites

Abstract

Demand for natural fibers reinforced composites is growing as an alternative to synthetic fiber reinforced plastic composites. However, poor compatibility between natural fiber and matrix has limited its development. Therefore, it is necessary to improve their interfacial adhesion to improve the comprehensive properties of composites. In this work, sisal fibers were subjected to an alkali/polyvinyl alcohol coating treatment by an ultrasonic impregnation method, and the sisal/high-density polyethylene composite was prepared by a twin-screw extruder. The Fourier transform infrared spectroscopy was used to characterize the modification effect of sisal fiber. The surface morphology of sisal fiber and the interfacial morphology of sisal/high-density polyethylene composites were observed. The mechanical properties and water absorption of sisal/ high-density polyethylene composites were also studied. The results show that alkali/polyvinyl alcohol coating compound treatment can effectively improve the interfacial adhesion between sisal fiber and high-density polyethylene, improve the mechanical properties of composite, and reduce water absorption. Alkali/polyvinyl alcohol coating compound treatment is a very environment-friendly, cost-effective fiber modification method when compared with traditional modification methods. It is helpful for the development and application of natural fibers reinforced composites.

Keywords

Sisal fiber composites interfacial adhesion mechanical properties water absorption

Introduction

As a result of their excellent properties, plastics are widely used in various industries. However, just as they have brought great convenience to our lives, they have also caused many problems. Most plastics are non-degradable, and burning them will produce a lot of toxic substances, causing the world to suffer from “white pollution”. According to the reports, scientists have even found a large amount of microplastics in the Mariana Trench, the deepest trench in the world. Due to the pressure of associated environmental concerns and energy issues, people have been paying more and more attention to their impact on the environment while continuously improving the performance of materials. Since Yamamoto Ryoichi of the University of Tokyo in Japan proposed the concept of environmental materials (Environment Conscious Materials, Econ-materials) in 1992,¹ environmental materials have become one of the most popular and important research interests in the field of new materials in recent decades. Natural fibers, such as hemp fiber, wood fiber, bamboo fiber, etc., have the advantages of low price, recyclability, and natural degradation when compared to synthetic fibers.^2–5 Therefore, natural fibers can be utilized as potential substitutes for synthetic fibers to reinforce polymer matrix. Among them, sisal fiber possesses higher cellulose content which is the main factor to improve the mechanical properties and it does not absorb moisture easily.⁶ It can be widely used in packaging, household, automotive, construction, agriculture and other industries.^7–10 Some studies have shown that sisal fiber can be added into polymer matrix as reinforcement to obtain better performance.¹¹ Vimalanathan et al.¹² had studied the reinforcing effect of sisal fiber in the polyester matrix, and the results showed that the composite reinforced by fibers with the length of 20 mm and volume fraction of 20% produced the best mechanical properties.

However, sisal fibers are hydrophilic due to the large number of hydroxyl groups, while most polymers are hydrophobic. Therefore, the compatibility between the two phases is poor, which will cause poor interfacial adhesion between sisal fibers and polymers.^13,14 Due to weak interfacial adhesion, stress cannot be transferred from the matrix to the fiber, where the stress concentration occurs, affecting the comprehensive performance of final products.^15–17 At the same time, during the development of this kind of composite, it was easy for the hydrophilic sisal fiber to absorb surrounding water, which will gradually destroy the interfaces of the various components in the composite and reduce the mechanical performance.^18–20 Thence, the most problem to be solved when preparing sisal fiber/polymer composite is how to improve the interfacial adhesion between sisal fiber and polymer matrix.

Surface modification of nature fibers can improve the interfacial adhesion.^21–25 Sepe et al.²⁶ has studied the influence of chemical treatments on mechanical properties of hemp fiber reinforced polymer composites. It was reported that the fibers treated with alkali and trimethoxy silane had improved the interfacial adhesion between the fibers and polymer matrix, while the tensile and flexure strength of the composites had been improved dramatically. Zhao et al.²⁷ have studied the effects of sisal fiber content and compatibilizers on the properties of sisal reinforced high-density polyethylene (HDPE) composites. Many researchers have proved that the increase of fiber content could improve the mechanical properties of composites. Pre-impregnated compatibilizer (maleic anhydride grafted HDPE, MAPE) can improve the interfacial adhesion between fibers and HDPE, and increase the mechanical strength of the composites. Li et al.²⁸ studied the effect of surface treatments on adhesion between the sisal fiber and the HDPE matrix. Microscopic morphology showed that KMnO₄ and dicumyl peroxide (DCP) could produce rough surface of sisal fiber, and mechanical interlocking could be formed between the fibers and matrix, further increasing the mechanical properties of the composite. Sreekumar et al.²⁹ treated sisal fibers with permanganate, bensoilation and silainization separately and investigated the effects of those surface treatments on the interface compatibility between fibers and matrix. As a result, after permanganate treatment, the tensile and flexure strength of sisal fiber polyester composites increased by 36% and 25%, respectively.

However, traditional coupling, esterification, grafting, and other treating methods have some disadvantages such as high cost and environmental pollution. The development of environment-friendly, simple, and low-cost modification method will be beneficial to the promotion and development of natural fiber composites. Polyvinyl alcohol (PVA) is a kind of safe, environment-friendly, and water-soluble polymer that can be used as fiber treatment agents. Partially alcoholized PVA molecules contain hydrophobic molecular chains and hydrophilic hydroxyl groups, which have a good compatibility between hydrophilic cellulose. The hydroxyl groups of PVA can be cross-linked with formaldehyde, borax, and other substances. Therefore, PVA can be used to modify natural fiber to improve the interfacial adhesion between the fiber and polymer matrix. Wang et al.^30,31 used PVA to modify kenaf and hemp fiber in order to strengthen polypropylene. The results showed that the PVA modification effectively improved the interfacial adhesion between the fiber and PP. The mechanical strength and thermal stability of the composites could be significantly improved, but the water absorption rate decreased.

In this paper, sisal fibers were subjected to alkali/PVA coating treatment with an ultrasonic impregnation method and along with traditional alkali treatment. The sisal/HDPE composites were prepared, and the effects of PVA coating modification on the interfacial adhesion, mechanical properties, and water absorption of sisal/HDPE composites were investigated. We propose an environment-friendly and cost-effective method to modify sisal fiber to improve the interfacial adhesion between the fiber and the polymer matrix, and it would be helpful for the development of natural fiber reinforced composites.

Experimental

Materials

Sisal fiber was bought from Guangxi sisal group Co., Ltd. HDPE (grade JHMGC100S) was manufactured by Sinopec with the melt flow index of 6.8 g/10 min. PVA (industry grade, 1788) with the molecular weight of 1700 produced by Sinopec Shanghai Petrochemical Co., Ltd. was used to treat sisal fiber. NaOH (analytically pure) supplied by Sinopharm was used to treat sisal fiber. The mixture of polyethylene wax and stearic acid (the ratio of 1:1 by weight) from Sinopharm was used as lubricant.

Experimental procedure

Surface treatment for sisal fibers

Alkali treatment: The sisal fibers were soaked in NaOH solution concentration of 10% for 1 h at room temperature (25 °C). The concentration of sisal fiber is 15% by weight. Then it was washed with deionized water until pH equals to 7 and dried in an oven at 80 °C for 12 h.

PVA coating treatment: PVA and deionized water were placed in a beaker at a concentration of 5% at room temperature, and stirred for 30 min. The beaker was then placed in water bath at 75 °C, and stirred for 1 h until the PVA was completely dissolved. Then, the alkali-treated sisal fiber was immersed in the PVA aqueous solution, and the beaker was placed in an ultrasonic container for 30 min. The sisal fiber was then taken out, and dried in an oven at 80°C for 12 h.

Figure 1 illustrates the surface treatments of sisal fiber under ultrasonic wave effects. The alkali treatment by NaOH in solution removed the impurities around the sisal fiber and made the surface of sisal fiber rough. Further immersing in PVA solution under the action of ultrasonic wave, the PVA molecular could easily permit the cavities on or in the fiber. After solidification the PVA could strongly adhere to the celluloses. The tight contact between sisal fiber and PVA could make an excellent mechanical performance of the composite.

Preparation of sisal/HDPE composites

The HDPE resin, pretreatment sisal fiber with the length of 6 mm and lubricant (polyethylene wax and stearic acid with the mass ratio of 1) were mixed in a high-speed mixer for 5 min. Then the mixture was added to the hoper of a twin-screw extruder to make pellets by melt blending. The diameter for the screw was 65 mm and the length diameter ratio were 44. The temperature of the extruder was set to 180 °C, 180 °C, 185 °C, 185°C, 185°C, 190°C, 190°C, 190°C, 185°C for each zone. The sisal fiber/HDPE composites samples were obtained by injection. The injection machine for the sample was 160 Ton manufactured by Hitian Machinery Company from China. The temperature used for the sample injection were 185°C, 190°C, 190°C, 200°C for the barrel and 210 °C for the nozzle. The injection pressure was 5.8 MPa and the hold temperature time was 10 second.

Performance testing

Micro-morphological characterization

The surface morphology of the fibers and the cross-sectional morphology of the composites were observed with a scanning electron microscope (SEM, JSM-67000F, Japan Electronics, Japan). In order to obtain good conductivity, the samples were gold sprayed. The composites samples were brittlely fractured after being placed in liquid nitrogen for 30 min.

2. Chemical structure characterization

With reference to the national standard GB/T 32198–2015, the chemical structures of sisal fiber before and after modification were analyzed by Fourier transform infrared spectrometer (FTIR, Nicolet 380, Thermo Scientific, USA).

3. Tensile strength of monofilament fiber

With reference to the national standard GB/T 31290–2014, the tensile strength of the monofilament fiber was tested with a microcomputer controlled universal testing machine (LWK-250, Tianyuan testing equipment, China).

The cross-sectional area of the monofilament was calculated from the fiber diameter measured by SEM. The sample structure is shown in Figure 2. A total of 20 repeated samples were made for the average calculation. The tensile speed is 10 mm/min.

4. Mechanical performance of composites

The microcomputer controlled universal testing machine was used to test the tensile strength and flexural strength of the composites, with reference to the national standards GB/T1040-1992 and GB/T 9341–2008, respectively. Moreover, the impact strength was tested by an electronic cantilever impact testing machine, with reference to GB/T 1843–2008. The samples for tensile, flexural, and impact strength test are shown in Figure 2, with dimensions of 150 mm ×10 mm × 4 mm, 80 mm × 10 mm × 4 mm, and 80 mm ×10 mm × 4 mm, respectively. And the testing speed for tensile and flexural strength test were 20 mm/min and 10 mm/min.

5. Water absorption test

The water absorption of the composites was tested with reference to national standard GB/T1462-2005. The dimensions of samples were 50 mm × 50 mm × 4 mm. The samples were completely immersed in deionized water at 23°C. The water absorption W (%) of the sample was calculated from equation (1)

W = \frac{M_{1} - M_{0}}{M_{0}} \times 100%

(1)

where M₀ is the initial mass of the sample after drying, the unit is g. M₁ is the mass of the sample after immersion in water, the unit is g.

Results and discussion

Surface morphology of sisal fiber

Figure 3 shows digital photos and SEM images of sisal fibers. The surface morphology of sisal fiber was changed significantly after modification. It can be seen from Figure 3(a) that the untreated sisal fiber is white, and the surface is rough with many impurities wrapped as can be seen in the SEM. Figure 3(d) shows that the fiber surface is covered with a lot of impurities, while the fiber structure is almost invisible. As indicated in Figure 3(b), the alkali-treated sisal fibers turned yellow and pliable, with the diameter decrease from 0.24 to 0.19 mm. From Figure 3(e) it can be drawn that the impurities on the fiber surface were removed thoroughly, and the fiber structure is clear. The grooves are obvious. This rough structure and clean surface of sisal fibers contribute to the wetting of the matrix and further enhances the mechanical adhesion to the matrix.³² Figure 3(c) shows that the surface of the sisal fiber treated with alkali/PVA coating is covered with a layer of translucent PVA. The cured PVA increased the fiber hardness. SEM in Figure 3(f) clearly shows that the fiber surface is tightly covered with a layer of PVA, and the outline structure of the fiber becomes blurred.

FTIR

Figure 4 shows the infrared spectrum of sisal fibers. The chemical composition of sisal fiber has changed dramatically after modification. The spectrum of the untreated fiber reflects its main components.^33,34 The peak at 3423 cm⁻¹ is ascribable to the stretching of the O-H group in cellulose of sisal fibers. The characteristic peak at 2921 cm⁻¹ represents the C-H stretching of methyl and methylene in cellulose and hemicellulose. Furthermore, the characteristic peak at 1064 cm⁻¹ represents the C-O stretching in the glycosidic bond of cellulose.¹⁵ The peak at 1393 cm⁻¹ represents the C-H bending shock in cellulose. However, in the spectrum of alkali-treated fibers, the peak at 1732 cm⁻¹ disappeared. This represents the C = O stretching in the ester of the fiber.²⁶ It shows that the alkali treatment removed the ester impurities such as pectin on the surface of sisal fiber, which can be reflected in SEM of Figure 3(e). It helps to improve the interfacial adhesion between sisal fiber and polymer matrix.^32,35 It also can be seen, the intensity of the band around 1250 cm⁻¹ was reduced significantly after the alkali treatment, where the reduction can be attributed to the stretching vibration of -O-C-C and bending vibration of O-H, indicating the removal of lignin and intensity reduction of polarity. In the spectrum of alkali-treated fibers, there are no changes in other characteristic peaks, indicating alkali treatment did not destroy the cellulose structure of sisal fibers. While in the spectrum of alkali/PVA coating treated fibers, the peak at 3423 cm⁻¹ is weakened, representing the hydroxyl group –OH on the fiber surface. It shows that the number of hydroxyl groups on the surface and the polarity of sisal fibers decreases. In addition, the peak at 806 cm⁻¹ represents the stretching peak of olefin -CH bond. The peak at 1433 cm⁻¹ is a characteristic peak caused by the bending vibration of CH-OH. This indicates that PVA has successfully coated on the surface of sisal fibers, as is shown in Figure 3(f).

Tensile strength of monofilament fiber

Surface modification has an effect on the tensile properties of sisal fibers. As is shown in Figure 5, the tensile strength of the alkali-treated sisal fiber was 563.68 MPa, increasing by 5.0% compared with untreated sisal fiber of 537.04 MPa. Because the alkali treatment removed impurities on the surface of the sisal fibers. The treated sisal fibers became flexible, and the number of fibers per unit area increased. Although the tensile stress decreased after the alkali treatment, the tensile strength of the fiber increased.³⁶ The alkali treatment caused fibrillation of the sisal fiber, and further the cellulose rotation angle reduced. Thus, the fiber orientation was enhanced, improving the tensile strength.^37,38

The tensile strength of the alkali/PVA coating treated fiber was 594.93 MPa, with an increase of 10.8% over the untreated fibers. When sisal fibers were coated with PVA, the fiber bundles were firmly bonded together and further improved the mechanical performance. As indicated in Figure 5, the tensile moduli of sisal fibers slightly decreased when they were treated by NaOH, but it was greatly improved after being coated with PVA plastic.

Micromorphology

The micromorphology of the cross sections of the composites can directly reflect the interfacial adhesion. The SEM images of the cross section show that the interfacial adhesion between sisal fibers and HDPE matrix has been greatly improved after surface modification. As shown in Figure 6(a), the gap between the untreated sisal fiber and the HDPE matrix is large, and the fiber of the cross section of the composites is pulled out. The poor adhesion between sisal fibers and HDPE matrix always leads to poor mechanical performance. It can be seen in Figure 6(b) that the gap between the alkali-treated fibers and HDPE matrix becomes smaller, and the interfacial adhesion is improved. As can be seen in Figure 6(c), there is little gap between the alkali/PVA coating sisal fibers and HDPE matrix. The fiber turns out to be broken at the cross section without being pulled out. It shows that the interfacial adhesion between sisal fibers and HDPE matrix has been greatly improved.

Tensile strength of composites

The surface treatment for sisal fibers enhanced the interfacial adhesion between sisal fibers and matrix, further increasing the tensile strength of the composite significantly. The tensile properties of sisal fiber/HDPE composites are shown in Figure 7. With the increase of sisal fiber content, the tensile strength of the composites increased continuously. When the content of sisal fiber was 12%, the tensile strength reached the maximum value. At this time, the tensile strength of the untreated sisal fiber composite was 38.57 MPa, while the alkali-treated sisal fiber composite was 40.49 MPa, increasing by 4.98% over that of the untreated sisal composite. The tensile strength of the alkali/PVA coating treated sisal fiber composite was 45.84 MPa, increasing 18.85% over that of the untreated sisal fiber composite. As the content of sisal fiber continued to increase, the tensile strength of the composites decreased; because the sisal fiber has a high modulus, and it played a reinforcing role. However, as the sisal fiber continued to increase, when exceeding 12%, the fiber agglomeration occurs and the dispersion turns poor. During processing, the fluidity of the composite becomes poor and turns difficult to process. The fiber aggregation destroyed the continuity of the resin matrix, causing serious stress concentration, which will lead to the decline of the tensile strength of the composites.

Figure 7 shows that the tensile strength of the alkali/PVA coating-treated sisal fiber composites was better than the alkali-treated sisal fiber composites when the content of sisal fiber remains the same. The surface of the untreated sisal fiber was covered with impurities such as pectin. As can be seen in Figure 3(d) and Figure 6(a), the interfacial adhesion between the fibers and HDPE matrix was poor, as a result, the tensile strength of the composite was relatively low.

After the alkali treatment, the impurities such as pectin on the surface were dissolved by NaOH. The surface of sisal fibers becomes rough, which lead to the increase of contact area between sisal fibers and HDPE matrix. As illustrated in Figure 3(e) and Figure 6(b), the molten HDPE entered the pores of the sisal fibers, which improved the interfacial adhesion between sisal fibers and HDPE matrix.

The surface of alkali/PVA coating-treated sisal fiber was covered with a layer of partially alcoholized PVA. The PVA molecule contained hydrophobic molecular chains and hydrophilic hydroxyl groups. During processing, sisal fibers would volatilize formaldehyde for higher temperature, while PVA and sisal fibers would undergo hemiacetal reaction with formaldehyde. An intermolecular acetal reaction would occur (as shown in Figures 8 and 9). Thus, the PVA can be fixed on the surface of sisal fibers.^30,31,39 In addition, the PVA molecule contained hydrophobic molecular chain, bringing excellent compatibility between hydrophobic HDPE. As a result, the interfacial adhesion between sisal fibers and HDPE matrix was greatly improved. The tensile strength of the composites improved. It can also be seen from Figure 6(c), the interfacial adhesion between sisal fibers and HDPE matrix has been greatly improved.

Flexural strength

The flexural strength of sisal fiber/HDPE composites is shown in Figure 10. With the increase of sisal fiber content, the flexural strength increased firstly and then decreased. When the sisal fiber content reached 12%, the flexural strength obtained the maximum value. The flexural strength of the untreated sisal fiber composite was 22.59 MPa, while the alkali-treated sisal fiber composite was 23.66 MPa, increasing by 4.74% over that of untreated sisal fiber composites. The flexural strength of the alkali/PVA coating sisal fiber composite was 26.86 MPa, increasing by 18.90% over that of untreated sisal fiber composite. This can be attributed to the good dispersion and well orientation of sisal fiber in the composites. The high-strength sisal fiber behaved as rigid bone in the composite, which can withstand the transferred extender force. When the fiber content continued to increase, the fiber agglomerated and the matrix continuity was destroyed. Stress concentration is prone to occur under higher pressure, bringing about a decline of the flexural strength.

The flexural strength of the alkali/PVA coating-treated sisal fiber HDPE composites reached a peak value at the same sisal fiber content, higher than that of alkali-treated and the untreated sisal fiber composite. Because the alkali/PVA coating-treated sisal fibers had the best interfacial adhesion with the HDPE matrix. The sisal fibers could be uniformly dispersed in the HDPE matrix.

Impact toughness

The toughness of the composite is related to that of the matrix and the reinforcements, as well as the interfacial adhesion between the two phases.⁴⁰ It can be seen from Figure 11 that with the addition of sisal fiber, the impact toughness of the composites decreased. Because sisal fiber was a rigid material, its toughness was lower than that of HDPE. The addition of sisal fibers destroyed the continuity of HDPE matrix. Therefore, the toughness of the composite was reduced. Figure 11 also shows that the toughness of the alkali/PVA coating-treated sisal fiber composite had decreased less than that of the untreated sisal composite. Because the surface of the alkali/PVA coating-treated sisal fiber was covered with PVA. PVA had a better toughness and impact strength than sisal fibers. On the other hand, PVA greatly improved the interfacial adhesion between sisal fibers and HDPE matrix, further improving the continuity of the composites. Alkali-treated sisal fibers improved the interfacial adhesion between fibers and HDPE matrix, and improved the overall continuity of the composite material. However, it did not increase the toughness of the fibers. The interfacial adhesion between untreated sisal fibers and HDPE matrix was relatively poor, and the gap at the interface was large. Therefore, the composite was more likely to fail when subjected to a certain level of impact.

Water absorption

The composition and pore structure of sisal fibers results in higher water absorption for the composites. After absorbing water, the sisal fibers are easy to separate from the HDPE matrix, leading to the degradation of the performance of the composites. Therefore, it is necessary to reduce the water absorption of the composites by different modification treatments. Figure 12 shows the water absorption of sisal fiber/HDPE composites with the filling content of 12%. The water absorption of the composites has decreased significantly after the surface modification of sisal fiber. With the increase of soaking time, the water absorption of the composite increased firstly and then tended to be stable. The water absorption of untreated sisal composite was 2.03%, while that of the alkali-treated sisal fiber composite was 1.57%, 22.67% lower than the untreated sisal fiber composite. The water absorption of the alkali/PVA coating treated sisal fiber composite was 1.33%, 34.48% lower than the untreated sisal fiber composite. As is shown in Figure 6, there is a large gap between the untreated sisal fiber and the HDPE matrix. The structure of the fiber itself has many pores, leading to the higher water absorption of the composite. The gap between the alkali-treated sisal fiber and the HDPE matrix became smaller with a reduction of water absorption. The interfacial adhesion between the alkali/PVA coating-treated sisal fiber and HDPE matrix was better, as a result the water absorption was lower than that of others.

Conclusion

In this study the sisal fibers were subjected to alkali/PVA coating treatment with ultrasonic impregnation method, and the sisal/HDPE composite was prepared by using a twin-screw extruder. From the experimental results, it can be concluded: alkali/PVA coating treatment was a very effective fiber modification method. Alkali treatment removed impurities on the surface of sisal fiber, and PVA coating treatment greatly improved the interfacial adhesion between sisal fibers and HDPE matrix. Thereby, the mechanical properties of the composites were improved but the water absorption was decreased. The experimental results verify that alkali/PVA coating treatment is an environment-friendly, cost-effective method to modify sisal fibers. It would promote the development and application of natural fiber reinforced composites.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by National Natural Science Foundation of China under grant 51973056, the key research and development plan of Hunan technology department under grant 2017WK2042, scientific research projects of Hunan provincial department of education under grant 18A258 and 18C0524, and Hunan provincial scientific research and innovation project under grant CX20190843 and CX20190844.

ORCID iD

Can Hu

References

Zhang

Meng

Treatment study of noil ramie fiber with PVC. Fiber Reinf Plast/Compos 2002; 4: 13–15.

Saba

Tahir

Jawaid

A review on potentiality of nano filler/natural fiber filled polymer hybrid composites. Polymers 2014; 6: 2247–2273.

Xie

Hill

Xiao

, et al. Silane coupling agents used for natural fiber/polymer composites: A review. Compos Part A 2010; 41: 806–819.

Goh

Yap

Agarwala

, et al. Recent progress in additive manufacturing of fiber reinforced polymer composite. Adv Mater Technol 2019; 4: 1–22.

Elanchezhian

Vijaya Ramnath

Ramakrishnan

, et al. Review on mechanical properties of natural fiber composites. Mater Today 2018; 5: 1785–1790.

Senthilkumar

Saba

Rajini

, et al. Mechanical properties evaluation of sisal fibre reinforced polymer composites: A review. Constr Build Mater 2018; 174: 713–729.

Xie

, et al. A new biodegradable sisal fiber–starch packing composite with nest structure. Carbohydr Polym 2018; 189: 56–64.

Jiang

Zeng

Experimental estimation of relaxation behavior for hemp fiber reinforced polypropylene composite with torque rheometer. Macromol Res Epub ahead of print 10 December 2019. DOI: 10.1007/s13233-020-8059-3.

Sanjay

Madhu

Jawaid

, et al. Characterization and properties of natural fiber polymer composites: A comprehensive review. J Clean Prod 2018; 172: 566–581.

10.

Karus

Kaup

Natural fibres in the European automotive industry. J Ind Hemp 2002; 7: 119–131.

11.

Ibrahim

Jamiru

Sadiku

, et al. Mechanical properties of sisal fibre-reinforced polymer composites: a review. Compos Interface 2016; 23: 15–36.

12.

Vimalanathan

Venkateshwaran

Srinivasan

, et al. Impact of surface adaptation and Acacia nilotica bio filler on static and dynamic properties of sisal fiber composite. Int J Polym Anal Charact 2018; 23: 99–112.

13.

Zhou

Fan

Chen

Interface and bonding mechanisms of plant fibre composites: An overview. Compos Part B 2016; 101: 31–45.

14.

Venkata Krishna

Kanny

The effect of treatment on kenaf fiber using green approach and their reinforced epoxy composites. Compos Part B 2016; 104: 111–117.

15.

Wang

Memon

Elwathig

, et al. Effect of jute fiber modification on mechanical properties of jute fiber composite. Materials 2019; 12: 1226–1236.

16.

Sinha

Narang

Bhattacharya

Mechanical properties of natural fibre polymer composites. J Polym Eng 2017; 37: 879–895.

17.

Devireddy

SBR

Biswas

Physical and mechanical behavior of unidirectional banana/jute fiber reinforced epoxy based hybrid composites. Polym Compos 2017; 38: 1396–1403.

18.

Zabihzadeh

SM.

Water uptake and flexural properties of natural filler/HDPE composites. BioResources 2009; 5: 316–323.

19.

Demir

Atikler

Balköse

, et al. The effect of fiber surface treatments on the tensile and water sorption properties of polypropylene–luffa fiber composites. Compos Part A 2006; 37: 447–456.

20.

Chandrasekar

Ishak

Sapuan

, et al. A review on the characterisation of natural fibres and their composites after alkali treatment and water absorption. Plast Rubber Compos 2017; 46: 119–136.

21.

Tabil

Panigrahi

Chemical treatments of natural fiber for use in natural fiber-reinforced composites. J Polym Environ 2007; 15: 25–33.

22.

Fiore

Scalici

Nicoletti

, et al. A new eco-friendly chemical treatment of natural fibres: Effect of sodium bicarbonate on properties of sisal fibre and its epoxy composites. Compos Part B 2016; 85: 150–160.

23.

Suardana

NPG

Piao

Lim

, et al. Mechanical properties of hemp fibers and hemp/pp composites: effects of chemical surface treatment. Mater Phys Mech 2011; 11: 1–8.

24.

John

Anandjiwala

RD.

Recent developments in chemical modification and characterization of natural fiber-reinforced composites. Polym Compos 2008; 29: 187–207.

25.

Saba

Paridah

Jawaid

Mechanical properties of kenaf fibre reinforced polymer composite: A review. Constr Build Mater 2015; 76: 87–96.

26.

Sepe

Bollino

Boccarusso

, et al. Influence of chemical treatments on mechanical properties of hemp fiber reinforced composites. Compos Part B 2017; 133: 210–217.

27.

Zhao

Li Robert

Bai

SL.

Mechanical properties of sisal fiber reinforced high density polyethylene composites: Effect of fiber content, interfacial compatibilization, and manufacturing process. Compos Part A 2014; 65: 169–174.

28.

Interfacial studies of sisal fiber reinforced high density polyethylene (HDPE) composites. Compos Part A 2008; 39: 570–578.

29.

Sreekumar

Thomas

Saiter

, et al. Effect of fiber surface modification on the mechanical and water absorption characteristics of sisal/polyester composites fabricated by resin transfer molding. Compos Part A 2009; 40: 1777–1784.

30.

Wang

Ren

Liu

Effect of hemp fiber modification on the properties and volatile organic compounds emission of its reinforced polypropylene composites. Acta Mater Compos Sin 2018; 35: 804–814.

31.

Wang

Lin

Long

, et al. PVA crosslinking modification effect on the properties of natural fiber reinforced polypropylene composites. Acta Mater Compos Sin 2017; 34: 1902–1910.

32.

Mwaikambo

Ansell

MP.

Chemical modification of hemp, sisal, jute, and kapok fibers by alkalization. J Appl Polym Sci 2002; 84: 2222–2234.

33.

Dai

Fan

Investigation of the dislocation of natural fibers by Fourier-transform infrared spectroscopy. Vib Spectrosc 2011; 55: 300–306.

34.

Yang

Yan

Chen

, et al. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007; 86: 1781–1788.

35.

Herrera-Franco

Valadez-González

A study of the mechanical properties of short natural-fiber reinforced composites. Compos Part B 2005; 36: 597–608.

36.

Sun

Jia

Properties of natural cotton stalk bark fiber under alkali treating. J Appl Polym Sci 2012; 125: E534–E539.

37.

Bledzki

Gassan

Composites reinforced with cellulose based fibers. Prog Polym Sci 1999; 24: 221–274.

38.

Siregar

Salit

Rahman

MZA

, et al. The effect of alkali treatment on the mechanical properties of short pineapple leaf fibre (PALF) reinforced high impact polystyrene (HIPS) composites. J Food Agric Environ 2010; 8: 419–429.

39.

Teramoto

Saitoh

Kuroiwa

, et al. Morphology and mechanical properties of pullulan/poly(vinyl alcohol) blends crosslinked with glyoxal. J Appl Polym Sci 2001; 82: 2273–2280.

40.

Huda

Drzal

Mohanty

, et al. Effect of fiber surface-treatments on the properties of laminated bio-composites from poly(lactic acid) (PLA) and kenaf fibers. Compos Sci Technol 2008; 68: 424–432.