Journal of Materials Exploration and Findings (JMEF) Journal of Materials Exploration and Findings (JMEF)
Volume 2 Issue 2 Article 1
7-10-2023
The Effect of Heat Treatment and Chemical Treatment on Natural The Effect of Heat Treatment and Chemical Treatment on Natural Fibre to The Durability of Wood Plastic Composites – A Review Fibre to The Durability of Wood Plastic Composites – A Review
Nuratiqah Asyiqin Mohd Nasharudin
Universiti Teknologi MARA Cawangan Pahang, 2019403406@uitm.edu.my Nur Fatihah Sulaiman
Universiti Teknologi MARA Cawangan Pahang, 2019402844@uitm.edu.my Nurul Aziemah Mohammad
Universiti Teknologi MARA Cawangan Pahang, 2019402434@uitm.edu.my WAN NOR RAIHAN WAN JAAFAR
Universiti Teknologi MARA, raihanjaafar@uitm.edu.my Falah Abu Dr.
Universiti Teknologi MARA, falah@uitm.edu.my
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Nasharudin, Nuratiqah Asyiqin Mohd; Sulaiman, Nur Fatihah; Mohammad, Nurul Aziemah; WAN JAAFAR, WAN NOR RAIHAN; Abu, Falah Dr.; and Surip, Siti Norasmah (2023) "The Effect of Heat Treatment and Chemical Treatment on Natural Fibre to The Durability of Wood Plastic Composites – A Review," Journal of Materials Exploration and Findings (JMEF): Vol. 2: Iss. 2, Article 1.
DOI: 10.7454/jmef.v2i2.1023
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Durability of Wood Plastic Composites – A Review Durability of Wood Plastic Composites – A Review
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The authors would like to thank Universiti Teknologi MARA for assistance and matching grant (100-RMC 5/3/SRP 036/2021) in completing this project.
Authors Authors
Nuratiqah Asyiqin Mohd Nasharudin, Nur Fatihah Sulaiman, Nurul Aziemah Mohammad, WAN NOR RAIHAN WAN JAAFAR, Falah Abu Dr., and Siti Norasmah Surip
This review is available in Journal of Materials Exploration and Findings (JMEF): https://scholarhub.ui.ac.id/jmef/
vol2/iss2/1
Journal of Materials Exploration and Findings 2(2) 35-47 (2023)
Received 3rd February 2023/Revised Date 10th April 2023/ Accepted 17th April 2023
JOURNAL OF MATERIALS EXPLORATION AND FINDINGS
https://scholarhub.ui.ac.id/jmef/
doi: https://doi.org/10.7454/jmef.v2i2.1023
The Effect of Heat Treatment and Chemical Treatment on Natural Fibre to The Durability of Wood Plastic Composites – A Review
Nuratiqah Asyiqin Mohd Nasharudin1, Nur Fatihah Sulaiman1, Nurul Aziemah Mohammad1, Wan Nor Raihan Wan Jaafar1, a, Falah Abu2 and Siti Norasmah Surip2
1Faculty of Applied Science, Universiti Teknologi MARA Cawangan Pahang, Kampus Jengka, 26400 Bandar Tun Abdul Razak Jengka, Pahang, Malaysia
2Faculty of Applied Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia Corresponding author’s email:araihanjaafar@uitm.edu.my
Abstract. The application of WPC is not only limited to indoor applications but has been extended to exterior applications where properties of WPC could compromise during service life. WPC was derived from wood fiber of various sizes to reinforce polymers. Wood fiber has the advantage of a cheaper price, being abundantly available, and ease of processing, however, the critical factor is its hydrophilic nature, where moisture absorption is likely to occur. Thus, treatments for reducing the hydrophilicity of wood fibers are applied. By treatment of wood fiber, the surface impurities were removed, leaving the roughened surface of fibers, thus providing a good surface for fiber-matrix interaction. The purpose of this study is to systematically review the effect of heat treatment and chemical treatment on wood fiber on the durability of WPC. The literature cited was searched in Scopus with related keywords – leading to 4292 articles. Article search was then limited from 2012 to 2021 and led to 443 articles to identify and analyze focusing on the durability of WPC. After fine-tuning, only 30 articles were considered to answer the research question in this study. The finding reveals that the treatment of wood fiber has a significant effect on the durability of WPC. Heat and chemical treatment were observed to improve the interfacial adhesion and resistance to fungal attack and the mechanical and physical properties of WPCs.
Keywords: Chemical treatment; Durability; Heat treatment; Natural fiber; WPC 1. Introduction
Wood-plastic composites (WPC) are combinations of wood-based components such as fibers or lumber particles with polymers to manufacture composite matter. WPC can be formed using either thermoplastic or thermosetting polymers that are usually called separate materials (Gardner, Han & Wang 2015). In general, wood fibers are used as reinforcement materials while plastic is used as fine materials that bind together with composite materials where it used mostly 56% natural fibers and 44% of thermoplastic (Matuana & Stark 2015). WPCs have the capability to be molded into any desired shape. The most common polymer matrices used in processing wood-plastic composites (WPC) are polypropylene (PP), polyethylene (PE) and polyvinyl chloride (PVC) (Soccalingame et al. 2016). Thermoplastic is a synthetic plastic that can be softened by heating it and hardened by the cooling process while thermosetting are denote substances that permanently shape when heated. Polymer is characterized as plastic when it is mixed with other substances such as stabilizers, plasticizers, or other additives in it (Jais, Omar & Rus 2016). It is
also an organic material that consists of a very long chain of multiple simple units which makes the molecular weight of polymers materials high. The production of the wood-plastic composite (WPC) includes extrusion, injection molding, and thermoforming.
Natural fiber as the main component of wood plastic composite works to reinforce the polymer materials. Natural fiber has many advantages when bonded with plastic which is low cost, biodegradable, low abrasive toward machinery process, and high filling capability (Mazzanti & Mollica 2015). However, natural fibers are hydrophilic in nature and thus have a higher tendency for moisture absorption. This leads to incompatibility between plastic and the reinforcement agent which consequently reduces the performance of WPC. Thus, modification of natural fibers through heat and chemical treatment on natural fiber is necessary to be done to produce high-quality WPC.
WPC is widely us as a construction building material such as flooring, decking, and siding.
Besides that, WPC is also used in the production of furniture, housewares, and office device.
Other than that, it is also used in the automotive industry. For instance, it is used in the production of the gearshift knobs of automobiles and sometimes it is used in the production of engine components. According to (Ebe & Sekino 2015), the United States WPC production has reached 980 000 yearly, and it was expected to increase to 1540000 tons per year. Other resources from (Friedrich & Lubile 2016) stated that Europe’s WPC production has increased from 16700 to 260000 per year, and it continuously grew up to 450000 tons in 2020. Both authors have claimed that WPC has gained popularity in the industrial sector.
The uses of WPC are mostly for exterior applications, leading to the discussion of its durability in terms of resistance to biological attack, weathering performance, and moisture absorption. To enhance the mechanical and physical properties of WPC, the durability of WPC on exterior application desires to be established to guarantee the WPC's service life.
WPC is widely used as an outdoor application. Due to this factor, wood-plastic composites’
durability became a center of attention to ensure WPC service life and consumers’ safety. Wood- Plastic Composites have durability in structural aspects, especially in ensuring the safety of the building’s standpoint and aesthetic aspects of WPC’s surface appearance (Gardner, Han & Wang 2015). The material properties that impacted the structural durability of WPC are based on its mechanical properties, physical properties, and biological degradation while the aesthetic durability of WPC’s material properties is impacted by weathering, food stains, and biological attacks. Thus, the durability of wood-plastic composites has been confirmed before their application in the industrial field. Even though WPC can be recycled, those material’s properties impacted WPC will eventually decrease the life service of the WPC (Kuka et al. 2020). Hence, this study enhances the knowledge and understanding of the effectiveness of heat treatment and chemical treatment on natural fiber to the durability of wood-plastic composites. Although different treatments will be discussed, the concept of treatment is similar despite different methods used, to remove the impurities and prepare the fiber for more bonding sites. Figure 1 below shows the principle of treatment on fiber.
Figure 1 Illustration of the principle of treatment on fiber to remove impurities and prepare the fiber for more bonding site. Source: (Ren et al. 2019)
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Figure 1 The discovery of this study would be beneficial to researchers and industrial manufacturers in widening the knowledge of the durability of wood-plastic composites during service life. Moreover, the study about the effect of heat and chemical treatment on natural fiber on the durability of the wood plastic composite can ease the raised issue about the dropped performance of WPC after several years of service.
2. Effect of Heat Treatment on Natural Fibre to The Durability of WPC
Heat treatments are mostly used to increase the mechanical properties, physical properties, and thermal stability attributes of the final product (Nasser et al. 2017). Moreover, to improve the performance and compatibility of wood fiber and matrix, heat treatment was applied to the wood particles (Mansour et al. 2017). Various types of heat treatment involved treating natural fiber such as drying, steaming, and vapors dry. Heat treatment on natural fiber is purposely to discard the moisture where it can be done by several methods such as heating in a hot air oven at 105oC for 24 hours (Gunjal, Aggarwal & Chauhan 2020), using an electro-thermostatic drum- wind drying oven or air-circulation oven for at 80oC - 90oC for 12 hours to 24 hours to eliminate moisture (Zhong et al. 2020; Mazzanti & Mollica 2017), and dried in a vacuum oven at 105oC with gauge pressure of 80kPa for 24 hours (Chan et al. 2019). Koffi et al. 2021 also reported heat treatment where fibers were placed in steam with a temperature above 100°C. As a result of heat treatment, the surface of the fiber becomes smoother with less stiffness and a more concentrated distribution. This is due to the mechanical disruption by the explosion, resulting in fibrillation of the cellulosic component of the fiber. In addition, heat treatment cause cell wall breakdown which had significant potential for bonding interaction with hydrophobic thermoplastic due to the removal of the lignin that covered fiber surfaces (Olakanmi & Strydom 2016). Table 1 listed the references with different heat treatment methods. Although different methods were used, the results were similar where moisture has been removed from the natural fiber used in the studies.
Table 1 Heat treatment at different methods with their references
References Method of heat treatment Result of treatment (Gunjal, Aggarwal & Chauhan
2020)
Heating natural fiber in hot air oven at 105 °C for 24 hours
Moisture has been removed from natural fiber.
(Zhong et al. 2020; Mazzanti &
Mollica 2017)
Electro-thermostatic drum-wind drying oven
(Chan et al. 2019) Vacuum oven at 105 °C with gauge pressure of 80 kPa for 24 hours
Besides that, heat treatment helps in the prevention of fungal decay. The weight loss for WPCs reinforced heat-treated wood is much lower than untreated wood composites because heat-treated wood improves the decay resistance of the composites (Aydemir et al. 2019). It was proved in the studies conducted by (Mansour et al. 2017) where Juniperus procera wood fiber was heat-treated at 70°C and exposed on the invasion of Trichoderma Harzianum. The studies were to determine the distinguishing characteristics such as wood hardness, termite, and fungal disease resistance hence the result shows absence of T. Harzianum leading to the decay fungi was observed in WPC-based heat-treated J. procera. Thus, there is no record of reduction in its
weight. Other studies conducted by Nasser et al. 2017, show that heat treatment extracted the composition in fiber such as starch, sugars, and lipids which resulted in the reduction of T.Harzianum’s growth. The antifungal efficacy of the composites was increased by heat treatment of the wood because the mass loss decreased as the heat treatment temperature was raised, according to the decay test (Aydemir et al. 2019).
However, heat treatment with higher temperatures can cause an alteration to the wood properties. By using high temperatures of 140 °C, 155 °C, and 170 °C, heat treatment can cause the composition of chemical wood’s components to degrade, affecting the qualities of the WPC manufactured such as mechanical, physical, thermal, and mold resistance (Nasser et al. 2017).
The components of wood fiber decompose in a wide temperature range, 200 to 650 °C (Barton- Pudlik et al. 2017; Motoc, Bou & Pop 2018). This is because the main components of the wood fiber, such as cellulose, hemicellulose, and lignin degrade in the range of 275 °C to 350 °C, 150 °C to 350 °C and 250 °C to 500 °C, respectively (Barton-Pudlik et al. 2017). According to Yang et al.
2007, hemicellulose will degrade at the temperature of 220-315 oC, cellulose will degrade at 300- 400 oC, and lignin has a large range of temperature degradation between 150-900 oC. The application of heat treatment should be selective to avoid the degradation of chemical components that contribute to the bonding and strength properties of the composites. This is proved when maximum weight loss due to thermal degradation has been detected in nano- fibrillated cellulose (NFC) when the heat reaches 320 °C (Platnieks et al. 2020). Thus, WPC reinforced with heat-treated wood fiber has a higher potential to be applied for outdoor applications. However, temperature ranges during treatment are necessary to control.
3. Results and DiscussionEffect of Chemical Treatment on Natural Fibre to The Durability of WPC
3.1. Acetylation Treatment
Acetylation treatment is a famous research area in enhancing the performance of moisture on natural fiber and WPC. It was applied to treat natural fibers mostly using acetic or propionic acid with or without the addition of an acidic catalyst at elevated temperature (Olakanmi &
Strydom 2016). The moisture sorption of wood fibers was decreased through acetylation treatment (Matuana & Stark 2015). Approximately 60%-75% of natural fiber contains cellulose.
Hence, through acetylation treatments, acetic anhydride was reacted to the hydroxyl groups on cellulose in the fibers cell wall to produce acetylated fibers (Matuana & Stark 2015; Segerholm, Ibach & Westin 2012). The replacement of acetyl groups with the hydroxyl group in the fibers cell wall enhances the strength of interfacial adhesion of the natural fibers with the matrix to help in the moisture absorption reduction (Olakanmi & Strydom 2016). This indicates good dimensional stability to the wood material due to swollen state (Segerholm, Ibach & Westin 2012). Thus, acetylation treatment can improve the condition between wood fiber and matrix and improve the strength of interfacial adhesion to increase the strain at failure (Gardner, Han & Wang 2015).
This can be proved where acetylating pinewood showed a reduction from 22% to 8% of its moisture content at 90% of the relative humidity which is the temperature at 27°C (Matuana &
Stark 2015). The same article also proved that a wood fiber that filled with PP composite absorbed more moisture than acetylated wood fiber that filled with PP composite after soaking for about 34 days where wood fiber filled PP composited and acetylated wood fiber filled PP composite absorbed 5% and 2.5% moisture, respectively (Matuana & Stark 2015).
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Furthermore, Segerholm, Ibach & Westin 2012 examined the moisture sorption properties by monitoring the weight gain in periods of 20 months for acetylated fiber. This study exposed the samples in the climate room with a temperature setup of 27°C and 90% of relative humidity (RH). Both acetylated medium density fiberboard (MDF) fibers and acetylated flour were prepared using 20% of wood acetyl content. The result is shown in Figure 2 that the lowest rate of moisture sorption is PLA acetylated MDF fiber followed by PLA acetylated flour which also achieves a minimum level of moisture weight gain among other PLA samples. In addition, the PLA acetylated MDF fiber and PLA acetylated flour undergoes a preconditioning test to measure the moisture content of the composites after water soaking in 2 weeks. The result showed that PLA acetylated MDF fiber has the lowest moisture content followed by the PLA acetylated flour with 2.2 % and 2.3%, respectively (Segerholm, Ibach & Westin 2012). The difference in the percentage of moisture content after soaking between the PLA acetylated MDF fiber and PLA acetylated flour with another sample was recorded in Table 2. Based on these results, it is important to treat natural fibers through acetylation treatment to prevent damage to WPC materials. Untreated natural fibers can cause damage such as cracking of wood particles. This is due to interface failure when wood weight increases as a result of exposure to moisture that causes excessive swelling which leads to stress in the matrix and also creates the formation of microcracks (Matuana & Stark 2015). Swelling produced by moisture absorption resulted in changes in the compositions' thickness and volume of WPC (Koivuranta et al. 2017).
Figure 2 Water vapor sorption for Pure PLA, PLA unmodified MDF fiber, PLA Acetylated MDF fiber and PLA Acetylated. Source: (Segerholm, Ibach & Westin 2012)
Table 2 Heat treatment at different method with their references
Material Moisture Content After Soaking (%)
Pure PLA -0.1
PLA – Unmodified MDF Fibre 3.7
PLA – Acetylated MDF Fibre 2.2
PLA – Acetylated Floor 2.3
Source: (Segerholm, Ibach & Westin 2012)
Originally, the polymer in ideal WPCs products will help in resisting absorption of moisture by the incorporated wood particles. Thus, in principle, this would improve WPC’s resistance to fungal degradation to the point where no fungal attack would occur. However, numerous elements can degrade the durability of newly manufactured WPCs, particularly surface qualities, throughout the expected life duration of several more years in use. WPC materials must be tested for their product quality in terms of the resistance to fungal deterioration promptly (Naumann, Stephan & Noll 2012). Hence, through the prevention of water absorption by acetylation treatment, it gives the advantage to stop the occurrence of decay by fungi and microorganisms (Matuana & Stark 2015). This is because most fungi prefer to rot wood at moisture levels exceeding fiber saturation, which is typically about 25-30%, the minor rise in wood moisture content to 7.7% in the overall dimension of the test specimen was likely insufficient to speed up fungal decay (Naumann, Stephan & Noll 2012). Gardner, Han & Wang 2015, stated that incorporated WPCs with fungicide-treated woods have a greater performance to resist decay than the untreated ones. Weight reduction is an important measurement to evaluate the decay of solid wood (Ashori, Behzad & Tarmian 2013; Friedrich & Lubile 2016). Thus, the mass loss of WPC after 32 weeks of fungal incubation in the soil to wood decay was used to determine wood degradation (Segerholm, Ibach & Westin 2012). It was found that the highest mass loss was caused by Trametes Versicolor (white rot), followed by Coniophora puteana (brown rot) which caused slightly less mass loss and WPC’s mass loss was negligible due to the presence of Alternaria alternata (surface mold) (Naumann, Stephan & Noll 2012).
Fungal degradation reduces the strength of WPC, and the weight reduction of material corresponds to the MOR losses (Friedrich & Lubile 2016). The more weight loss of WPC, the more reduction of stiffness which can drop the performance of WPC and its durability. The Terrestrial Microcosms Test (ENV 807) was conducted by Segerholm, Ibach & Westin 2012 to determine the durability of WPC using acetylated fiber toward the resistance of decay. The samples were placed in two places which are in compost soil (TMC1) and forest soil (TM2) for 32 weeks (Segerholm, Ibach & Westin 2012). The result stated in Table 3 showed that PLA acetylated MDF fiber and PLA acetylated floor undergo small mass losses lower than 1% and composites with acetylated fiber have less mass loss than composites with unmodified fiber (Segerholm, Ibach & Westin 2012).
The only cause for weight reduction would be microorganism attack on wood components that could be attributable to the effect of moisture, which has the potential to advance the biodegradation structural changes in the polymer matrix because the fibers are quick to detect the absorption of water thus the more water absorption in the composite material results in more water reaching the wood parts by imperfections (Tazi, Erchiqui & Kaddami 2015).
Table 3 Heat treatment at different method with their references
Material
Mass Loss (%)
TMC 1 TMC 2
PLA – Unmodified MDF Fibre 0.8 0.7
PLA – Acetylated MDF Fibre 0.0 0.0
PLA – Acetylated Floor -0.1 -0.2
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3.2. Alkaline Treatment
Alkaline treatment is also one of the chemical treatments to enhance the quality of natural fiber to the durability of WPC. The disruption of hydrogen bonding in the wood flour network structure and the removal of lignin, wax, and oils covering the external surface of the fiber cell wall were attributed to the fiber or matrix adhesion process (Olakanmi & Strydom 2016). Based on Gardner, Han & Wang 2015 the dispersion of wood flour in PP has been improved through alkaline treatment and its effect in increasing the performance of mechanical properties of WPCs. The effect of the alkaline treatment using sodium hydroxide (NaOH) on the natural fibers has been explored. Figure 3 shows the leaching of the layer of waxy cuticle when soaking in 4 wt% NaOH concentration with a time increment to 150 minutes. As a result, the microstructural pores of the wood flour became clearer thus the surface of the wood fiber became soft and curly (Olakanmi & Strydom 2016). This can be beneficial to the improvement of the aesthetic durability of WPC.
Figure 3 SEM images showing the surface morphologies of (a) untreated wood fibres; and wood fibres treated for (b) 30 min; (c) 90 min; (d) 150 min in 4 wt% NaOH solution. Source:
(Olakanmi & Strydom 2016).
In the article, Jiang et al. 2018a stated that the incorporation of the alkali-treated Sorghum Straw (SS) fibers into the PVC matrix increases the durability of the SS/PVC composites rather than using untreated SS Fibers. In the point of mechanical evaluation, the good impact of the relationship between the fiber and the matrix by better flowability of the matrix has led to the larger MOE which indicates the larger tensile strength of WPC (Sarabi et al. 2012). This statement was proved by experimenting using air-dried SS fibers that were soaked in 0.5, 2.5, 4.5, and 6.5 % concentrations of NaOH solution at a temperature of 100°C for about 1 hour. After that, the treated fibers were segregated from liquids through the filtration process, and using deionized water, the treated fibers were washed until the rinse solution turned neutral. The fibers then were dry in the oven at 90°C for 24 hours (Sarabi et al. 2012).
In terms of chemical structure, the alkali treatment could improve mechanical interlocking interfaces which lead to the enhancement of desirable properties (Olakanmi & Strydom 2016;
Zhong et al. 2020). 4.5% NaOH concentration (medium level of concentration) produces high
crystallinity and strength of the fiber. This is lead to the strong interfacial bonding between fiber and matrix/polymer which served to improve the wear resistance of the composites resulting in stronger load-bearing fiber. Therefore, more energy for debonding of the fiber-matrix interface is required (Jiang et al. 2018a). WPC modulus is influenced by the interactions and adhesion of the matrix and filler (Sarabi et al. 2012). The increment of fiber crystallinity was due to the reduced amount of active hydroxyl groups, resulting in a stronger fiber-matrix interaction. In addition, higher crystallinity reduces the imperfections of the bonding interface where the transmission of stress between the fiber and matrix occurred by strong mechanical locked adhesion (Ebe & Sekino 2015).
As shown in Figure 4, the broad intensity at the range of 3500-3300 cm-1 was due to the O-H stretching vibration of hydroxyl groups. The reduction of the peak can be seen with the increasing NaOH concentration where some peaks almost completely disappeared at higher NaOH concentrations. Hence, it reduced the hydrophilicity and the polarity of SS fiber. Based on FTIR spectra, it is proved that some functional groups related to the hemicellulose, cellulose, pectin, or lignin in the SS fiber are discarded through alkali treatment when the reduction of the peak can be seen at 1740 and 1248 cm-1 (Jiang et al. 2018a). One of the famous characteristics of wood fiber is hydrophilic, meaning it attracts water. Thus, the alkaline treatment decreased the characteristic of hydrophilic, hence making it more permeable to water and preventing the degradation of wood fiber to maintain the durability of WPC products.
Figure 4 The FTIR spectra of treated and untreated-alkali SS Fibre. Source: (Jiang et al. 2018a)
Furthermore, the high crystallinity of wood fibers is important in producing composite materials because fiber strength and stiffness are related to its crystallinity resulting in high resistance of the relative motion of the WPC. The XRD spectra in Figure 5 showed that the crystallinity of 4.5% alkali-treated SS fiber is 59.3% compared to the untreated SS fiber which is 53.5%. This maximum crystallinity value was due to the discarded of the hemicellulose, cellulose, pectin, or lignin in the SS fiber which allows for easier fibrils rearrangement in the direction of extension deformation, leading to increased tensile strength and modulus (Jiang et al. 2018a).
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Figure 5 The XRD spectra of treated and untreated-alkali SS Fiber. Source: (Jiang et al. 2018a) In contrast, Jiang et al. 2018a stated that strong alkali strikes the dominant component of fibers which leads to the loss of its crystallinity and conformation which affects its modulus and strength of tensile. Alkali-treated fiber with a concentration of 6.5 % NaOH can result in fracture surfaces with clear boundaries forming between fibers and the matrix, as well as some pull-out fiber and voids. This effect could be seen by the presence of micro-cracks and poor adhesion interaction, which lead to ineffective strain stress transferred from fiber to matrix, thus resulting in poor mechanical properties. On the other hand, for the durability of water resistance, strong alkali shows great impact whereas wood fiber treated with 6.5% NaOH concentration shows poor polarity and lacks -OH components. This result was supported by an SEM micrograph as shown in Figure 6 where excess NaOH concentration caused surface fractures, pull-out fibers, and the formation of voids on fiber surfaces.
Figure 6 SEM micrograph of the tensile fracture surface of untreated SS fibre-reinforced PVC and 6.5% alkali-treated SS fibre-reinforced PVC. Source: (Jiang et al. 2018a)
3.3. Other Chemical Treatment
Other chemical treatments such as esterification and benzylation have been reported previously. However, not many researchers use the esterification method. Gardner, Han & Wang 2015 mention that esterification using acetate, propionate, and benzoate can result in better mechanical properties, improved resistance to fungi, and enhanced weathering performance.
Furthermore, through benzylation treatment, WPC’s water absorption was reduced, but it did not affect the tensile strength of WPC. In addition, stearic acid and cellulose palmitate has improved the homogeneity and processability of the composites Gardner, Han & Wang 2015 According to Jiang et al. 2018b, 0.8 wt. % stearic acid acts as a degradation inhibitor because it can close the interfacial gaps and restrict moisture absorption by combining with 0.5 wt. % palmitic acid. Samples with long periods of exposure show a minor increase in moisture absorption. This showed when the optimized WPC with 0.8 wt. % stearic acids (SA) and 0.5 wt. % palmitic acid (PA) has lower moisture absorption than the unoptimized WPC. The mechanical and physical qualities of the optimized (0.8 SA - 0.5 PA) WPC also are superior to those of the unoptimized WPC. Nevertheless, due to alternate simulated seawater and acid rain degradation conditions, mechanical and physical qualities such as tensile, flexural, hardness loss, and compressive strength decreased. Also, degradative water (seawater and acid rain), attacks fiber that causes damage to the structure and properties of the fiber. Besides that, wear resistance of benzylation-treated WPC also decreases as time exposure decreases but results in better interfacial bonding that prevents de-bonding of the fiber from the matrix and improves matrix strength.
Moreover, natural fiber can be treated using acidified sodium chlorite (ASC) through the delignification of fiber. Chen et al. 2016, mentioned that when HDPE is blended with wood fiber that has been delignified, the crystallinity of the HDPE would increase. The presence of lignin reduces the crystallinity of HDPE when exposed to the elements because lignin absorbs UV radiation, causing free radicals to form, thus inducing further reactions. Comparing non- extracted control and extracted wood fiber-based composites, the wood loss from delignified WPC composites was slightly lower. In addition, the WPCs formed from fiber treated with toluene/ethanol, acetone/water, or hot water extracted, the wood fiber shows values in the change of lightness (ΔL*) and change of color (ΔE*) when compared to the composite that made from untreated control. The rise in ΔL* showed that chemicals that reflected yellowish and reddish light were degraded, resulting in a bleaching impact in the composite samples. In addition, the increased value of ΔE* is caused by the photodegradation of structure in wood components, particularly conjugated structures, which break down under UV light and water spray. The absence of lignin in natural fiber has a positive effect on WPC’s color change. Hence, there was no significant difference in ΔL* and ΔE* values between those formed from delignified wood fiber. It is possible that extractive degradation influenced the color changes of WPCs during xenon-arc accelerated weathering to some extent. This is understandable because the structures in wood components are likely to photodegrade without discrimination. WPC samples created from delignified wood fiber had a higher flexural modulus of rupture (MOR) and modulus of elasticity (MOE) in general than those prepared from non-extracted WF. Fabiyi &
McDonald 2012 studied delignified pine fibers as the best WPC because it was not badly damaged when compared to untreated pine fiber and acetone-extracted pine fiber. The results show that the lowest L* was found in delignified pine fibers WPC, while the highest L* was found in acetone-extracted fiber WPC. Thus, it is proved that the highest value of L* found in acetone- extracted fiber is because of the lignin degradation.
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4. Conclusions and Recommendations
Heat treatments usually can be found during the sample's preparation such as drying, steaming, and vapor drying the wood composites. Thus, the effect of heat treatment on natural fiber on the durability of wood-plastic composites has been reviewed which are helped in enhancing the strength of interfacial adhesion of the natural fibers with the matrix, reducing the moisture absorption that leads to resistance to fungal degradation, improving mechanical and physical properties of WPC. This study has also shown that the chemical treatments found are acetylation, alkaline, and other chemical treatments such as esterification and benzylation.
Hence, the effect of chemical treatment on natural fiber on the durability of wood-plastic composites has been analyzed where it was found that chemical treatment helped in enhancing the strength of interfacial adhesion of the natural fibers with the matrix, reducing the moisture absorption that leads to resistance to fungal degradation, improved mechanical and physical properties and better surface appearance of WPC. In conclusion, the objective of this paper has been achieved where treatment on fibers is necessary to improve the properties of WPC.
Hence, after completing this paper, a few recommendations are as followed:
1. Studies regarding the testing should be carried out on wood fiber to produce better WPC material with long-term durability features, especially for exterior applications.
2. The bibliometric analysis on the distribution of articles and authors can be specified to view the distribution and trend of this topic.
Acknowledgments
The authors would like to thank Universiti Teknologi MARA for assistance and matching grant (100-RMC 5/3/SRP 036/2021) in completing this project.
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