Journal of Reinforced Plastics and Composites

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Natural fiber reinforced poly(vinyl chloride) composites: A review H P S Abdul Khalil, M A Tehrani, Y Davoudpour, A H Bhat, M Jawaid and A Hassan Journal of Reinforced Plastics and Composites published online 21 December 2012 DOI: 10.1177/0731684412458553 The online version of this article can be found at: http://jrp.sagepub.com/content/early/2012/12/20/0731684412458553

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Review

Natural fiber reinforced poly(vinyl chloride) composites: A review

Journal of Reinforced Plastics and Composites 0(00) 1–27 ! The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684412458553 jrp.sagepub.com

HPS Abdul Khalil1, MA Tehrani1, Y Davoudpour1, AH Bhat3, M Jawaid4 and A Hassan2

Abstract Materials from renewable resources – also called biomaterials or ‘green’ materials – are presently gaining in importance worldwide. In these times of continuous increases in the price of crude oil and discussion of carbon dioxide (CO2) emissions, conventional plastics have reached a price level and a questionable image which promotes the search of alternatives. Natural fibers are a renewable natural resource and are biodegradable, which is an important characteristic for components that must be disposed of at the end of their useful life. They are recyclable and can be easily converted into thermal energy through combustion without leaving residue. In this study, we will discuss the natural fiber reinforced polyvinyl chloride composites, reinforcing effect, plasticization effect along with modification by coupling agents, properties, and applications based on composite materials. Also, the polyvinyl chloride-based composite materials with specific emphasis on effect of coupling agent, foamed polyvinyl chloride composites, and the effect of natural fiber reinforcement on its material properties will be reviewed. One of the best alternatives is natural fiber reinforced plastics composites. These are composites that are typically filled or reinforced with plant fibers, as well as plastics such as polyvinyl chloride or recently, even bioplastics.

Keywords Polyvinyl chloride, natural fiber, composite, coupling agent

Introduction Poly(vinyl chloride) (PVC) is one of the most commonly used plastics in our society. Its main applications include pipes, electric wires, window profiles, siding, etc. Recently, wood fiber reinforced PVC is becoming more popular because of its acceptable mechanical properties, moisture and fungus resistance, long lifetime, wood-like surface performance, and recyclability.1,2 Some weakness of this material including low impact strength and thermal stability imposes restriction on its application, which signals need for additional research on this important product. Incorporation of natural fiber in a plastic matrix can enhance the modulus of the resulting composites, but decreases impact strength at the same time.3 Considering the relative low impact resistance of the neat PVC matrix, it is of more practical significance to improve the impact strength for PVC/natural fiber composites. Current research in this field has been mainly focused on adding impact modifiers and using coupling agents to improve composite properties.

Coupling agents have been studied for PVC/natural fiber composites to improve their overall properties.4,5 Maleated polypropylene (MAPP) was shown to be able to improve shear strength of PVC/wood composites by 20%.6 Some organic acids were used to increase tensile modulus for PVC/wood fiber composites, but there was no effect on tensile and impact strengths.7

1

School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia 2 Department of Polymer Engineering, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia 3 Department of Fundamental and Applied Sciences, Universiti Teknologi Petronas Malaysia, Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia 4 Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Selangor, Malaysia Corresponding author: HPS Abdul Khalil, School of Industrial Technology, University Sains Malaysia, 11800 Penang, Malaysia. Email: [email protected]



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Journal of Reinforced Plastics and Composites 0(00)

Coupling effectiveness of silane relied on both dispersion solvents and interfacial reaction initiators.8 Few of these coupling agents significantly improved the impact strength of PVC composites. Using a coupling agent to reduce the hydrophilic property of wood fiber is not effective for enhancing the adhesion between PVC and natural fiber.9 Some commonly used impact modifiers for PVC, such as chlorinated polyethylene (CPE), ethylene vinyl acetate (EVA), methacrylate–butadiene– styrene (MBS), and all acrylic elastomer, were proved to be also effective in PVC/wood composites.10 However, some other widespread impact modifiers for polymers and polymer blends, such as styrene– ethylene–butylene–styrene (SEBS), have not been used in PVC/natural fiber composites. SEBS proved to be able to improve the impact strength of the polymer blends,11,12 but no work has been reported on its effect in PVC/natural fiber composites. Beside mechanical properties, water resistance and thermal stability are also important for PVC/natural fiber composites. PVC resin shows relatively high water absorption compared with polyolefin. With the existence of hydrophilic natural fibers, PVC/natural fiber composites tend to have higher water absorption rate than pure PVC, which further affects mechanical properties and structural stability of the composites.13 As a weak point of both of PVC resin and natural fibers, their thermal stability and degradation mechanisms have been investigated with kinetic analysis of thermogravimetric data.6,14 Combining the two components, thermal stability of PVC/natural fiber composites was investigated, but no kinetic analysis has been reported.15 There are a large variety of natural fibers such as rice straw, rice husk (RH), palm, bagasse, hemp, flax, and other agricultural residues.16–18 These cheap natural fibers are normally made from waste part of the eight products. As different natural fibers have different chemical compositions, physical structures, and mechanical properties, variation of properties can be expected from the composites of different natural fibers.19,20 Little effort has been made so far to prepare non-wood fiber (e.g. agricultural fibers) reinforced PVC composites and to improve their properties. With increased wood costs and competition of wood resources from traditional wood sectors, developing alternative, cheap, and environmentally friendly natural fiber sources for plastic composite is highly needed. In this study, the role of various natural fibers as reinforcement in polyvinyl alcohol has been studied. The literature studies have shown no review information on the polyvinyl alcohol with natural fiber as reinforcing material in the polymer composites.

Overview of natural cellulosic fibers The term ‘natural fiber’ used to allocate various types of fibers which naturally come from plants, mineral, and animals.21 In order to clarify our case, the word ‘natural’ might be cited as ‘vegetable,’ ‘cellulosic,’ or ‘lignocellulosic.’ Commercially, important natural fibers are mostly achieved from the stems, leaves, and seeds of the plants. They are mainly consisting of cellulose, hemicelluloses, and lignin along with lower amount of pectin, pigments, and waxes.22 Cellulose, one of the most abundant organic compounds on earth, considered as the main structural component that renders strength and stability to the lignocellulosic fibers. Cellulose molecules are obtained through various stages of extraction of microscale elementary fiber bundles to nanofibrils from various lignocellulosic fibers, as shown clearly by an example of kenaf fiber in Figure 1. Percentage of the cellulose in a fiber affects the ultimate fiber’s application, properties, and also cost of production.22 The properties of cellulosic fibers are strongly influenced by many factors such as area of growth, its climate and the age of the plant, internal fiber structure, chemical composition, cell dimensions, and microfibril angle, which distinct from different parts of a plant as well as from different plants.21,23 Moreover, the mechanical properties and the reinforcing efficiency of natural fibers are related to the nature of cellulose and also their crystallinity. The drawback for using natural fibers as reinforcements is their polar nature which makes them incompatible with non-polar polymer matrices. Also, difficult mixing process caused from poor wettability of the fibers followed by weak interface in the composite.24

Natural fiber reinforced polymer composites Natural fibers have been widely used during recent years as an alternative solution to the ever depleting petroleum sources. However, 100% replacement of natural fiber-based production with petroleum-based material cannot be an economical solution. Finally, combining of petroleum and bio-based resources lead to develop a cost-effective product with diverse applications.24 Natural fiber reinforced composites’ applications have been expanded to many fields. Different types of polymers have been used as matrices for natural fiber composites. Among them, polyester, epoxies, and phenolics as thermosets and PE, polystyrene (PSI), and PP as thermoplastics are the most commonly used matrices. Since the plastics are soft, flexible, and lightweight in comparison to fibers, their combination provides a high strength-to-weight ratio to the resulting



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Figure 1. Microscale elementary fiber bundles to cellulosic nanofibrils from kenaf fiber.

composite.25 From the other point of view, some advantages of using natural reinforcements in composite materials includes biodegradability and renewability, lower specific weight and higher specific strength and stiffness compare with glass fibers (GFs), friendly processing and reduced wearing on tools, better working conditions without skin irritation, good thermal and acoustic insulating properties, etc. On the other hand, some disadvantages might limit their application on an industrial level such as, moisture absorption, limited maximum processing temperature, lower strength properties, in particular, impact strength, poor fire resistance, variable quality depending on several factors such as weather and lower durability which can be considerably improved with fiber treatments.26,27

PVC composites Among the many polymers that have been used as matrices for preparation of composites, PVC is certainly one of the most popular one. Moreover, PVC is one of the most important commodity polymers with many technical applications and vast economic importance.28 The comparison of physical properties of PVC materials with polyolefin materials is shown in Figure 2. PVC production on a technical scale

started at the end of the 1920s from VC and vinyl acetate also form vinyl ethers and acrylic esters.29 Considering to safety and environmental issues, VC can cause serious health problem and the anesthetic property of VC was identified in early 1930s. PVC also introduced as a contaminant material due to releasing harmful substances to the atmosphere such as hydrogen chloride and dioxins during processing or decomposition. Combination of PVC and natural fibers is an interesting alternative due to the ‘ecological friendliness’ of natural fiber. In addition, PVC composites have the wide range of application and their use has grown more rapidly than of other polymer composites. The reason behind this is outstanding chemical resistance of PVC to wide range of corrosive fluids which offer more strength and rigidity than most of the other thermoplastics. Besides, it is easy to fabricate, can last longer and suitable in designed engineering application due to generation of rigid and flexible products.30 Natural fiber-based PVC composites. Hydrophilic character of natural fibers leads to poor resistance to moisture and low interfacial properties between fiber and polymer matrix which reduces their potential as reinforcing agents. To eliminate these problems, fibers are required to be suitably modified with hydrophobic aliphatic and cyclic structures containing reactive functional groups



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Figure 2. Comparison of physical properties of PVC with polyolefin materials. PVC: poly(vinyl chloride).

that are capable of bonding to the reactive hydroxyl groups in the polymer matrix.22,31,32 Therefore, modification of natural fibers attempts to make the fibers hydrophobic and to improve interfacial adhesion between the fiber and the matrix polymer.32,33 Pretreatments of the fiber would clean the fiber surface, change the surface tension and polarity, stop the moisture absorption process, chemically modify the surface, and increase the surface roughness.32,34 Chemical processes, such as alkali treatment or mercerization, silanization, acetylation and acrylonitrile grafting, benzoylation, acrylation, maleated coupling agents, isocyanates, peroxide and permanganate treatment, sodium chlorite, titanate treatment, and finally plasma treatment have achieved various levels of success in improving fiber–matrix adhesion, fiber strength, and fiber fitness in natural fiber reinforced composites.34,35–37 The pretreatment reactions directly influence the cellulosic fine structure of natural fiber and consequently affect the stress–strain behavior, tensile strength, and thermal behavior of natural fibers.32 Physical methods included stretching,38,39 mercerization,22,40 thermotreatment,41 calendering,39,42 and impregnation.22,43,44 The production of hybrid yarns and electric discharge (corona, cold plasma) cannot modify the chemical composition of the fibers; they only change structural and surface properties of the fiber and thus influence the mechanical bondings to polymers.19 In the following subsections, effect of

using different substances and methods in order to improve fiber–matrix adhesion in PVC/natural fiber composites will be summarized.

Chemical modification of natural fiber-based PVC composites Reaction mechanism of coupling agents Most of the physical and mechanical properties of wood polymer composites (WPCs) depend on the interfacial adhesion between the woody materials and the polymer matrix. In order to improve poor interaction between the wood and polymer matrix and overcome the high sensitivities to moisture and heat, various coupling agents have been used to provide a lower surface energy between the natural fibers and the polymer matrix.45 Coupling agents convert the hydrophilic surface of wood fibers to more hydrophobic and improve the strength by creating a chemical bridge between the reinforcement and polymer.9,46 They usually improve the degree of crosslinking in the interface region and offer a perfect bonding.32 The most common coupling agents which have been used in PVC/natural fiber composites are organic acids, anhydrides, isocyanates, and silanes.47 The general reaction mechanism scheme of coupling agent with natural fiber and that of polymer matrix is shown in Figure 3.



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Figure 3. General reaction mechanism of coupling agent.

Silane vs zirconate, titanate, and aluminate Considering to Matuana’s48 study, surface characteristics of the fibers with various coupling agents demonstrated a significant improvement in the strength performance of PVC/wood–fiber composite. Previous studies have also reported that tensile strength of PVC reinforced with cellulosic fibers (wood pulp) decreased using untreated fibers while silane grafted pulp reinforcement enhanced the strength of composite by 51%. Study on PVC/sawdust composite with three different silane coupling agents showed improved mechanical properties compared with untreated composites. Optimized concentrations of coupling agent for tensile and impact properties of the composites was 0.5–1.0 and 1.5 wt%, respectively.49 Abu bakar et al. found that silane and zirconate coupling agents did not increase impact and flexural properties of the acrylic impact modified PVC/oil palm empty fruit bunch (OPEFB) composites due to insufficient interactions. It was exhibited in this study that the water absorption increased proportionally with the fiber content with and without acrylic impact modifier (AIM). However, the water absorption of impact-modified composites was higher than unmodified composites due to the increase in polarity. On the other hand, both of the silane and zirconate coupling agents significantly decreased the water absorption of composites by modification of cell wall of OPEFB fibers and make them less hydrophilic.50 The increase in water resistance of the PVC composite after the benzoylation of the OPEFB fibers in further study of Abu Bakar and Baharulrazi51 also showed an improvement in interfacial adhesion of benzoylated fiber and PVC matrix. In addition, benzolated fibers improved the impact strength, tensile strength, and tensile modulus of the composites compared to the untreated ones. Titanate coupling agent; particularly LICA 12 gave the best enhancement in terms of impact strength for RH ash (RHA) filled modified PVC compared to silane and zirconate coupling agents.52 A processability study

by Hassan and Sivaneswaran53 represented that LICA 12 coupling agent increased the fusion time of the acrylonitrile butadiene styrene (ABS) impact modified RHA/PVC composite but decreased the torque due to agglomeration of the fillers at the presence of a monomolecular layer of a surface coating. Elsewhere, titanate and aluminate coupling agents (ACAs) developed mechanical properties of WPC by improving interfacial compatibility. In that case, the effect of titanate was apparent, while the effect of aluminate was weak. Moreover, it was shown that excessive use of the coupling agents was harmful to properties of WPC.54 In a similar experimental work, wood flour (WF) was surface-treated using titanate coupling agent, oleamide, and polyurethane prepolymer and subsequently blended with PVC. It was found that surface treatments improved not only both mechanical properties and rheological behaviors, but also the compatibility between WF and PVC. However, the WF aggregated at a higher content.55 Ge et al.15 offered a silane coupling agent (-Aminopropyltriethoxysilane (APS)) for PVC/WF composites with pine and bamboo flours content varying from 10 to 50 phr. The incorporation of both bamboo and pine flours significantly improved the stiffness of the composites, while decreasing the tensile strength to some extent. This reduction in tensile strength was attributed to the reduction of interfacial adhesion and homogeneity of composite by loading WF. In addition, pine flour filled composites exhibited better mechanical properties than those filled with bamboo flour with the same loading level and particle size due to well dispersion and alignment tendency of short pine fiber even at a lower loading level inside the composites. Furthermore, thermal analyses indicated that thermal stability of the bagasse fiber (BF)/PVC composite was slightly better than that of pulp fiber (PF)/ PVC composites. -APS coupling agent in an older study was also found to be a suitable adhesion promoter for PVC/wood veneer composites.56 Incorporation of silanes as coupling agents in hardwood aspen fibers



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(chemithermomechanical pulp (CTMP) and sawdust)/ PVC composites proved that apart from the chemical structure of silanes, role of the dispersion aids (e.g. initiators and maleic anhydride (MA) solvent) would be inevitable particularly on mechanical properties of the composites.57 Surface modification of sugarcane bagasse (SCB) by mechanical activation using ACA enhanced the condensation reaction between ACA and hydroxyl groups of the SCB fibers and obviously increased the hydrophobicity of SCB and mechanical properties of PVC composites reinforced by ACA modified SCB. Compared with the unmilled ACA modified SCB/ PVC composite, the flexural strength, tensile strength, and Brinell hardness of the 120 min milled, ACA modified SCB/PVC composite increased significantly.58

Copper amine Upon increasing the concentrations of copper amine coupling agents, flexural modulus of PVC/WF composite decreased slightly but no significant effect on impact strength was found. Incorporation of copper ethanolamine (EA) coupling agent was associated with the lowest flexural modulus, flexural strength, and impact strength of the PVC/WF composite.59 The reasons stated by the researchers were that copper amine coupling agent can generate a complex on the wood surface at high temperature to improve the heat conductivity of WF which cause to reduction in the degradation temperature of WF.60 In addition, EA can degrade the lignin content of WF at high temperatures. Thus, the copper amine coupling agent causes degradation in the structure of WF at high processing temperatures which results in a reduction of mechanical properties of the composite.59 Surprisingly, before this, dissimilar results had been reported for using copper amine as coupling agent in PVC/WF composites. Jiang and Kamdem60 had evaluated the effect of copper amine-treated WF (with optimum Cu concentration range 0.2–0.6 wt% of WF) on mechanical properties of PVC composites. They reported an increase up to around 45% in unnotched impact strength using 0.2 wt% copper amine-treated WF at the 60 wt% PVC loading level in comparison with corresponding samples made with untreated WF. This study also revealed the maximum enhancement of 36% and 40% for flexural strength and flexural toughness, respectively. In a recent study by above authors, the effect of a copper amine treatment of WF on the adhesion and the interfacial shear strength of PVC/WF composite were investigated. The reduction of contact angles between PVC and the wood surface during the treatment with copper amine solution proved that increasing wetting

of PVC on wood surface and improved adhesion was resulted from the mentioned treatment.61 The decomposition temperature of PVC/WF composites was strongly decreased by increasing EA content. This trend could be explained by formation of carboxylic acids during the EA treatment of WF which led to acceleration of hemicelluloses degradation, higher mass loss, and lower thermal stability of components.62

Isocyanate and benzoic acid Kokta et al. studied the effect of poly(methylene (polyphenyl isocyanate)) (PMPPIC) coupling agent on mechanical properties of particle boards made from bagasse and PVC. The compatibility between wood fiber and polymer was enhanced by coating wood fibers with coupling agents. This process produced the polar hydroxyl groups (–OH) of wood fibers to react with coupling agents. Recently, researchers believed that in the above study Kokta et al., isocyanates could not improve the interfacial adhesion of PVC/WF composites, instead, part of the isocyanates which could not react with WF absorbed by PVC resin to produce a semiinterpenetrating polymer network in the PVC matrix. This reaction could improve the mechanical properties of PVC/WF without increasing the interfacial adhesion between PVC and WF. They also hypothesized that a thin layer of coupling agent covered the surface of the WF and caused a reduction in its surface roughness reduced the mechanical interlocking and frictional forces between PVC matrix and WF.59 However, regarding to the recent study of Abdrahman and Zainudin,63 PMPPIC could act as a bonding agent to facilitate the optimum stress transfer at the interface of kenaf fiber and PVC matrix in order to give an optimal mechanical performance to the composites. In another interesting study by Kokta and Maldas64 on the performance of isocyanate as a coupling agent, isocyanate solution was found more efficient than undiluted isocyanate. An approach to the mechanical properties of PVC/wood fiber composites represented the role of isocyanate as a promoter or an inhibitor depending on its concentration. However, at the higher concentration of isocyanate, mechanical properties of the composites exposed to deterioration. In a research study by Wirawan et al.,65 although the unwashed SB/PVC composite gave the highest tensile strength and modulus as compared to treated composites, the incorporation of PMPPIC gave the highest mechanical properties to the composite than benzoic acid and sodium hydroxide treatments. In addition, the unwashed SB/PVC composite attributed the lowest maximum water absorption and low water



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absorption rate to the composite. On the other hand, Saini et al.,66 reported that alkali treatment of the filler, improved SB/PVC composite’s performance strongly including an increase of 48% in tensile modulus, 14% in impact strength, and 10% in thermal stability of the composite compared to neat PVC at a filler loading of 30 phr. Maldas and Kokta in one of their earliest experimental works applied PMPPIC to the PVC composite with particle boards of SB and also hardwood aspen. It was defined that the mechanical properties and dimensional stability of coupling agent-treated particle boards were superior to the non-treated ones. The surface of BF was modified by benzoic acid in a different study and showed an increase in dispersion of BF in PVC matrix after the treatment. Moreover, interface modification dominantly improved the tensile strength but marginally the impact strength of the composite.7

Block copolymers Polymeric coupling agents have been widely examined for their effectiveness in WPCs and they serve more like compatibilizers between the wood phase and the polymer matrix.67 In an interesting study by Rocha et al., hydroxyl groups were introduced in a controlled way into the PVC during the creation of a block copolymer of PVC with poly(hydroxypropyl acrylate) (PHPA). PVC-b–PHPA-b–PVC block copolymer prepared based on living radical polymerization (LRP), was the best coupling agent in this study which at certain loadings enhanced the mechanical properties and thermal stability of the PVC/WF composites. However, for higher loadings, phase separation led to poorer mechanical properties. The reason behind this was increased association between PHPA molecules.68,69 A bit previous, Kim et al.67 produced poly(styreneco-MA)-block-poly(styrene-co-acrylonitrile) {P[(SMA)b-(SAN)]}, with the nitride-masked polishing (NMP) technique at the presence of 2,2,6,6-tetramethylpiperidin-l-oxyl and azobisisobutyronitrile as coupling agent in PVC/bamboo flour composite. It was depicted from the scanning electron microscopy (SEM) images that the mechanical properties of the composite such as tensile strength and tensile modulus were improved with increasing P[(SMA)-b-(SAN)] content. However, for the composite without coupling agent, the WF was easily pulled out from the PVC matrix leading to worse mechanical properties of the composite due to inferior interfacial bonding between the WF and PVC. In another study by the above authors, water uptake property and wrap stability of the same composite were

investigated. They reported improved interfacial bonding between PVC and bamboo flour, also higher water uptake property and wrap stability for the composites in the presence of P[(SMA)-b-(SAN)] coupling agent.70 In another study, POE-g-MAH employed as a compatibilizer into the PVC and wood powder blend and improved the tensile strength, impact strength, processing property, and appearance of the composites compare with those without compatibilizer due to enhancing interfacial strength. In that study, the addition of CPE further improved the mentioned properties.71

Aqueous solutions Wang et al. introduced different types of aqueous solutions as surface treating agents, for moso bamboo particles reinforced PVC composites. The water resistance of the composites followed the order as Alkali treatment > silicate treatment > oxidant treatment on moso bamboo particles and the maximum tensile strength was reached at the concentrations of 5% of solutions. It was also reported that sodium bisulfate (oxidant agent) enhanced the compatibility between cellulose and PVC more than sodium hydroxide (alkali agent) and sodium silicate (silicate agent).72 In Kamel,73 tried to study alkali lignin – a natural wood binder which extracted from bagasse – as coupling agent in PVC/rice straw composite. The addition of lignin up to 7% increased the tensile strength and decreased both percent weight gain and swellability. The main reasons behind these trends were attachment of hydrophobic lignin to the hydroxyl surface of lignocellulose materials and improvement of the binding between the rice straw particles as well as a decrease in the void content.

Chitin and chitosan Acid–base interactions between the chlorine-containing PVC and amino groups on the WF surface were enhanced using chitin and chitosan coupling agents. Structure of chitin and chitosan is similar to the wood structure, which builds up hydrogen bonds between the OH groups of the WF and the Cl groups of PVC leading to the better compatibility between WF and PVC.59 Chitosan-treated composite (up to 0.5 wt%) showed increased flexural strength. Both of chitin and chitosan increased flexural modulus, storage modulus (E0 ), and loss modulus (E00 ) of PVC/WF composites. However, greater improvement in E0 and E00 was observed by incorporation of chitin in the composites.9 The use of various coupling agents and natural fibers associated with PVC has been tabulated in Table 1.



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Table 1. Shows various natural fibers and coupling agents Natural fiber

Coupling agent

References

Pine and bamboo flours Newsprint fiber Wood sawdust

APS APS, DCS, and MAPP N-2 (aminoethyl) 3-aminopropylmethyl (dimethoxysilane, trimethoxysilane, triethoxysilane, trimethoxysilane) APS, MAPE, octyltriethoxysilane Silane and zirconate Titanate (LICA 12 and 38) and silane

Ge et al.15 Matuana74 and Xie et al.46 Sombatsompop and Chaochanchaikul49 and Sombatsompop et al.75

OPEFB RH WF

Mono EA Copper EA solution Pine flour Isocyanate copper EA, thermoplastic starch (TPS) Kenaf fiber PMPPIC Bagasse PMPPIC SCB and hardwood aspen fiber PMPPIC SCB PMPPIC, benzoic acid, and sodium hydroxide Wood pulp Isocyanate, PMPPIC, TDIC, HMDIC, and EIC WF Block copolymers based on LRP (VC–OH, PVC–PHPA, and PVC–WF) Bamboo flour Block copolymers based on NMP P[(SMA)-b-(SAN)] Wood powder POE-g-MAH WF Chitin and Chitosan Rice straw Lignin (alkali) SCB Aluminate WF BF Benzoic acid OPEFB Bamboo Sodium bisulfate (oxidant agent), sodium hydroxide (alkali agent), sodium silicate (silicate agent) SCB NaOH (alkali treatment) WF Titanate, oleamide, polyurethane prepolymer WF Butyl acrylate prepolymer Wood grain Epolene E-43 (a MAPP copolymer with low molecular weight) Epolene G-3015 (a MAPP copolymer with high molecular weight) PEMA

Prachayawarakorn76 Abu bakar et al.50 Sivaneswaran, Abu bakar et al., Hassan and sivaneswaran,53 and Ahmad52 Mu¨ller et al.64 Jiang and Kamdem5,18 Fasihi and Garmabi59 Abdrahman and Zainudin63 Kokta et al.77 Maldas and Kokta78 Wirawan et al.65 Kokta and Maldas64 Rocha et al.68,69 Kim et al.67,70 Chen et al.71 Shah et al.9 Kamel73 Huang et al.58 Huilin et al.79 Zheng et al.7 Abu Bakar and Baharulrazi51 Wang et al.72 Saini et al.66 Liu et al.55 Huilin et al.79 Lu et al.80 Lu et al.80

OPEFB: oil palm empty fruit bunch; WF: wood flour; SCB: sugarcane bagasse; BF: bagasse fiber; RH: rice husk; APS: Aminopropyltriethoxysilane; DCS: dichlorosilanes; MAPP: maleated polypropylene; EA: ethanolamine; MAPE: maleated polyethylene; PMPPIC: poly(methylene (polyphenyl isocyanate)); LRP: living radical polymerization; PVC: poly(vinyl chloride); PHPA: poly(hydroxypropyl acrylate); TDIC: toluene 2,4-diisocyanate; HMDIC: hexamethylene diisocyanate; EIC: ethyl isocyanate; NMP: nitride-masked polishing; P[(SMA)-b-(SAN)]: poly(styrene-co-MA)-block-poly(styrene-co-acrylonitrile); and PEMA: polyethylene maleic anhydride.

Effect of plasticizer on composite material properties For a plasticizer to be effective, it must be thoroughly mixed and incorporated into the PVC polymer matrix. Incorporating different additives can help to modify PVC compositions and obtain desired properties.

For instance, plasticizer plays a crucial role in flexible and semi-rigid PVC formulations. It has influence on processability and physico-mechanical characterization of PVC composite. Different plasticizers will exhibit different characteristics in both the ease with which they form the plasticized material and in the resulting mechanical and physical properties of the flexible



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Table 2. Various plasticizers and their effect on various fibers and additives Fiber and additives

Plasticizers

References

Waste newsprint fibers + calcium stearate lubricant + antimony stabilizer Cellulose whiskers, tin-stabilizer and lubricant (stearic acid-based compound) ESBO + Ba–Zn stabilizer WF + Ba–Cd–Zn, heat stabilizer + lubricant ESBO + BaCl2, ZnSO4 or CaCl2 stabilizers Eucalyptus wood, ground corncob, and spent brewery grain + Cd/Zn heat stabilizer GCFs + stabilizer + UBZ antidegradant OP + benzoyl chloride + Ba–Zn thermal stabilizer + ESBO as co-stabilizer and a lubricant Almond husk, RH, sawdust + Ca–Zn heat stabilizer Wood sawdust, lignin, cellulose (modified with MA) WF + lead stabilizer + processing aid + lubricant WF + GF + Pb–Ba-based organic stabilizer + external lubricant + calcium stearate + calcium carbonate + processing aids

DOP

Matuana81

Di-ethyl-hexyl phthalate fully hydrogenated) or DOP DOP DOP DOP DOP

Chazeaua82 Semsarzadeh et al.83 Djidjelli84 Semsarzadeh85 Georgopoulos86

DOP and TPU DEHP

Leblanc Djidjelli87

Di-isononyl-cyclohexane-1,2 dicarboxylate Plasticizer DOP and EVA DOP

Crespo et al.88–90 Bodirlau91 Marathe Mongkollapkit et al.92

ESBO: epoxidized soy bean oil; WF: wood flour; GCF: green coconut fiber; GF: glass fiber; OP: olive pomace; RH: rice husk; MA: maleic anhydride; DOP: dioctyl phthalate; UBZ: upper basal zone; TPU: thermoplastic urethane; EVA: ethylene vinyl acetate and DEHP: di-2-ethylhexyl phthalate.

Figure 4. End use market for plasticized PVC.93 PVC: poly(vinyl chloride).

product. Several theories by many researchers have been developed to evaluate the observed characteristics of the plasticization process (Table 2). Figure 4 provides an account of end use market scenario for plasticized PVC.

Dioctyl phthalate and epoxidized soy bean oil as plasticizer Semsarzadeh et al.83 studied the use of dioctyl phthalate (DOP) and epoxidized soy bean oil (ESBO) as



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plasticizer in PVC composites. They measured dynamic mechanical properties of the composites, effect of DOP concentration and epoxidization level of SBO in plasticized PVC–ESBO composites. The rheological data of mixing PVC with DOP, SBO, and ESBO exhibited that Tg and plasticization time of PVC may go down and plasticization time of PVC with DOP is at least four times less than PVC with SBO or ESBO. Results showed lower homogeneity for PVC–DOP while a small level of ESBO in DOP/ESBO mixture increases the homogeneity and thus Tg of PVC. A solvable mixture of ESBO with DOP was formed and diminished the degree of blending and torque at balance of PVC. The dynamic mechanical thermal analysis test demonstrated lower tan for PVC–DOP. The dynamic mechanical analysis exhibited that. In a subsequent paper, Semsarzadeh et al.85 investigated about the effect of some salts, different epoxidization level of SBO and mixture of DOP–ESBO as a plasticizer on thermal and mechanical properties of PVC composites. They found that as epoxidization level increases, greater homogeneity and miscibility of PVC–ESBO can be obtain. By addition of DOP to ESBO, hardness and tensile modulus of composite diminish, but impact resistance and elongation go up. Also, they showed that Tg of PVC is linear function of epoxidization of ESBO plasticizer and PVC–ESBO exhibit improved thermal properties.

Effect of different concentrations of plasticizer and filler Crespo et al.88 published a series of papers concerning the use of different fillers in PVC composites and studied how various sizes and percentages of these fillers as well as different levels of plasticizer (40 to 80 phr) affects the mechanical and morphological properties of PVC composites. When filler was almond husk, it was found that as the filler content increases, the tensile strength decreases and the composite becomes stiffer in all filler size. Regardless of particle size and filler level, as plasticizer concentration increases tensile strength, rigidity, and hardness of composites decreases. This trend was seen in plasticized PVC with and without filler. In addition, smaller particles exhibited good mechanical properties because of better dispersion in PVC matrix. At high plasticizer level, greater cavities were formed. They obtained similar results by changing filler to RH89 and sawdust.90

Plasticized PVC filled with newsprint and wood fiber PVC/newsprint fiber composites have been produced and this study examines the effects of the low phr levels of plasticizer (DOP) on the rheological and

mechanical properties of rigid and semi-rigid PVC/ newsprint fiber composites. The influence of low concentration of DOP plasticizer on the rheological and mechanical characterization of composites was evaluated. They found that as the plasticizer concentration increased the melt flow index and impact strength of unfilled and filled PVC composites went up whereas tensile strength and modulus decreased because of plasticization effect. Over all plasticizer levels, by addition fiber to matrix, the melt flow index, and tensile strength at yield diminished while tensile modulus flourished. Elongation at break and toughness of PVC alone increased as the plasticizer amount increased and reduced by incorporation of fibers into matrix as a result of higher brittleness. DOP concentration had not influence on these parameters. Above 11.25 phr of DOP, plasticizer influence became significant.60,94 To evaluate the effect of plasticizer concentration on the void fraction, three types of microcellular foamed composites including PVC, PVC-untreated wood fiber, and PVC silane-treated wood fiber with 13.5, 20 and 30 phr of DOP plasticizer fabricated by Matuana et al. in 1997. Increasing DOP level only influenced on the amount of volumetric expansion of PVC-treated fiber and PVC alone. Low void fraction of PVC-untreated fiber composite ascribed to low level of CO2 regardless of DOP concentration. In addition, they found that by incorporating untreated fibers into matrix, gas can escape easily due to weak adhesion between constituents. Increasing the DOP level, reduced the melt viscosity, flourished gas penetration into polymer, and consequently led to softening of matrix.60,94 Mutuana et al. studied the effect of surface acid–base properties of plasticized PVC and PVC with treated and untreated newsprint fibers on mechanical characterization of the composites. The acid–base factors did not associate with the notched izod impact, tensile modulus, and elongation at break of PVC/newsprint fiber composites. The tensile strength of untreated fibers reduced due to weak dispersion and poor adhesion between matrix and filler. Despite the type of treatment, the tensile modulus of composites with fibers was two times greater than PVC alone. Incorporation of fibers into the composite material reduced elongation at break in comparison to plasticized PVC regardless of surface treatment. The tensile strength of plasticized PVC was more than PVC/newsprint fiber composites. The notched izod impact property of composites with aminosilane-treated fibers was lower than other treated and untreated fibers. Investigation about the effect of different wood fiber surface treatment on adhesion of fibers to PVC composites was done by Matuana et al. in 1998. X-ray photoelectron spectroscopy and contact angel measurement indicated that after treatment, hydrophilic wood surface changed to hydrophobic



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nature and tensile shear strength of wood laminated with PVC went up. Treated fibers exhibited higher contact angle and lower surface tension than untreated fibers. Aminosilane improved wood veneers adhesion to PVC whereas other treatment did not show any improvement in adhesion between matrix and wood.

WF-plasticized PVC Marathe developed composites containing EVA as a polymeric plasticizer, WF and fly ash (FA) as filler and tested their thermal and morphological properties. The results of DSC showed that Tg of composites decreases by addition of EVA due to plasticization effect. On the contrary incorporating WF increases Tg, and therefore Tg of PVC and composite will be close to each other. FA did not exhibit obvious influence on Tg. Infrared (IR) spectroscopy demonstrated the interaction between EVA–PVC and PVC–WF. Good dispersion of these components in the composites revealed by SEM micrographs.95 The attention toward using different WF content in PVC composites in order to explore its effect on the thermal, mechanical, and dielectric properties is the main focus of the paper by Djidjell et al.96 Results showed that as WF content increases, stress and strain at break of composites decreases while permittivity as well as dielectric losses increased due to weak adhesion between PVC and wood. Non-filled plasticized PVC exhibited higher tensile strength and lower dielectric losses as well as permittivity. Decomposition temperature for unfilled plasticized PVC was lower than filled PVC and it attributed to creation of some defects due to using WF in the form of aggregate.

Plasticized PVC filled with other fibers Benzyl-treated and untreated olive pomace (OP) was used by Djidjelli et al. as filler to reinforce PVC composites. Mechanical, thermal, and dielectric behaviors of unfilled PVC and PVC-OP composites were studied. Mechanical analysis revealed that, treated fibers lead to increase elongation and strength at break as compare to untreated ones. It is attributed to good adhesion between fiber and matrix as well as thermoplastic character of filler. Also, from tensile test it was clear that after treatment olive residue withstand plasticization. The data of thermal analysis showed higher degradation temperature for filled PVC than unfilled one. Furthermore, treated fiber–PVC composites exhibited lower weight loss in comparison with PVC alone. Dielectric tests demonstrated that benzylated OP composite can be utilized as insulators. The influence of fiber content and plasticizer type (DOP and thermoplastic polyurethane (TUP)) on

rheological and mechanical properties of PVC–green coconut fiber (GCF) composites was the main parameters investigated by Leblanc et al. PVC–GCF composites were heterogeneous and showed non-linear viscoelastic properties in contrast to pure PVC composite. The complex modulus went up with the GCF concentration. In comparison with PVC alone, impact strength and hardness of composites did not alter much. Comparing SEM micrographs of composites including DOP and TUP exhibited that, TUP led to increase in fiber adhesion to matrix due to higher viscosity and molecular weight. Rheological and mechanical data demonstrated better fiber wetting can achieve by incorporation of TUP with DOP. Georgopoulos et al.97 studied the effect of untreated and pre-treated eucalyptus wood residue, corncob residue, and spent grain low density on composites containing PE (LDPE, low-density polyethyl) and PVC plastisol as matrixes. They evaluated only the effect of untreated eucalyptus wood (shortly EU) and untreated corncob on plastisol PVC. Results showed that as the filler amount increases, hardness goes up. The composite with EU filler displayed better properties. In addition, the plastisol composites indicated improvement in hardness than LDPE composites. The modulus of elasticity of unfilled plastisol PVC was lower in comparison with filled one. It is worth mentioning that at filler level up to 10 phr, LDPE and plastisol PVC composites showed significant reduce in tensile strength and elongation and after that the values of their mechanical characterizations diminished at a noticeably lower degree. Filled plastisol PVC exhibited much higher loading transfer than unfilled one. Bodirlau et al.98 prepared and characterized the composites comprising plasticized PVC and modified wood sawdust, cellulose, and lignin as filler. Fourier transform IR results showed good mixing of carbonyl groups into PVC matrix during blending process. Weight loss of composites containing modified cellulose and lignin was lower than modified sawdust/PVC composite. For the main decomposition temperature, weight loss reduced by increasing the filler level. Thermal stability of composite including modified cellulose was lower than wood sawdust and lignin. They observed more or less uniform distribution of filler in polymer. Also, it seemed that cellulose can better coated by PVC. They found that at high amount of filler, week adhesion at the interface can achieve. Another research by Chazeaua et al. is based on evaluating mechanical properties of plasticized PVC filled with cellulose whisker in rubbery state. Matrix microstructure was studied by small angle neutron scattering (SANS) with and without whiskers and damage phenomena was analyzed. As the filler content increased, the composite elastic modulus boosted.



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Disparity between theory and experimental behavior was ascribed to damage phenomena. SANS test confirmed heterogeneous properties of the matrix. Comparing plasticized and pure PVCs showed that at low-Q, plasticized PVC did not exhibit a scattering curve clearly different from the scattering of PVC/40 DOP. Above Tg, the composite displayed interesting mechanical properties.

Effect of impact modifier The most critical aspects in the development of PVC composites are to achieve a good combination of properties at a moderate cost. Incorporation of some natural fiber in PVC is associated with a decrease in toughness.99,100 Hence, researchers have commonly applied various impact modifiers to overcome this demerit of reinforcing fibers application in PVC composites. The impact modifier in the form of microparticles is dispersed within the molecular structure of PVC. When the PVC products receive impact, these microparticles in the molecular structure absorb the impact energy and prevent damages to the PVC product. CPE, AIM, MBS, and EVA copolymer are several rubbery structures which have been extensively used in wood/PVC (WPVC) composites and their izod impact strength is shown in Figure 5. The effect of various impact modifiers on PVC-based composites is shown in Table 3.

CPE and AIM In case of OPEFB filled PVC, Abu Bakar et al. revealed that physical interactions between OPEFB and polar PVC are relatively weak compared to the strong fiber–fiber interaction caused by hydrogen bonds. This matter followed by weakness of interfacial adhesion resulted in debonding and pulling-out of fiber bundles in the composites. Thus, the impact strength of composite decreased by loading 10 to 40 phr untreated OPEFB fiber content into acrylic impact modified composite.50,102 They also found that impact modifier is effective in enhancing the impact strength of PVC/ OPEFB composites101 but the effectiveness decreases with increasing filler loadings.101,107 The similar trend for impact strength has been reported in Hassan et al.’s107 study upon addition of RH and EFB filled acrylic impact modified PVC-U. It was reported from previous studies of above authors that CPE is the preferred impact modifier at higher filler loading (20 phr and higher) for more effective impact strength enhancement.101 Moreover, AIMbased PVC/OPEFB composites were more stable upon photo-oxidation than CPE impact modified PVC/ OPEFB.102 Also, AIM was more effective than CPE at lower loading (3 and 6 phr).101 Surprisingly, the flexural properties of OPEFB fibers were found differing from one study to another. For instance, the effect of accelerated weathering

Figure 5. Effect of blending impact modifiers.106



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Table 3. Various impact modifiers and its effect on different PVC-based composites Impact modifier

Composite

References

CPE

PVC–OPEFB PVC–WF Wood fibers (hardwood maple) PVC-wood powder Wood fibers (hardwood maple) PVC–OPEFB Wood fibers (hardwood maple) Bagasse, rice straw, and RH PVC–RH

Abu bakar and Hassan,101 Abu bakar et al.,102 and Abu bakar and Hassan103 Guffey and Sabbagh104 Mengeloglu et al.10 Chen et al.71 Mengeloglu et al.10 Abu bakar and Hassan,103 Abu bakar et al.,105 and Abu bakar et al.50 Mengeloglu et al.10 Xu et al.6 Sivaneswaran,76 Abu bakar et al., Ahmad,52 and Hassan and sivaneswaran53

AIM MBS SEBS ABS

CPE: chlorinated polyethylene; AIM: acrylic impact modifier; MBS: methacrylate–butadiene–styrene; SEBS: styrene–ethylene–butylene–styrene; ABS: acrylonitrile butadiene styrene; PVC: poly(vinyl chloride); OPEFB: oil palm empty fruit bunch; RH: rice husk; and WF: wood flour.

environment on PVC/OPEFB composites modified with AIM and CPE (9 phr) showed an increase in flexural strength and a decrease in flexural modulus.102 Meanwhile, in another interesting study by above authors, incorporation of 10 to 40 phr untreated OPEFB fiber content into acrylic impact (9 phr) modified composite was resulted in higher flexural modulus but lower flexural strength of PVC composite.50 In addition, flexural test showed an increase in the flexural modulus by addition of EFB and RH fillers into the unmodified and acrylic impact modified PVC.107 The softening effect of CPE and AIM significantly reduced the modulus and strength of PVC in two more researches.103 Above impact modifiers and OPEFB filler were able to reduce the yield stress of PVC by providing stress concentration sites which resulted in higher energy absorption of PVC composites.103 It was predicted from PVC/OPEFB extrusion that impact strength, yield stress, strain at break, and flexural properties of composite were independent of processing temperature.103,105,108 In another interesting study by the above authors, water absorption of acrylic impact modified OPEFB filled PVC composites was obviously higher than unmodified composites because of enhanced polarity of composite using impact modifier.50 In a different perspective, effect of oil extraction of OPEFB on processability and impact and flexural properties of unplasticized PVC composites was evaluated. It was reported that both of the extracted and unextracted fibers decreased the fusion time and melt viscosity of PVC-U. However, by loading 10–40 phr extracted fiber into the matrix the fusion time was increased. The impact and flexural properties of the composites affected marginally by the fiber extraction.108 It was depicted from the work of Mengeloglu et al.109 that acrylic and CPE impact modifiers had

negative effect on tensile strength and modulus of rigid PVC/wood–fiber composites while elongation at break of the composites was not dependent on the impact modifier. Izod impact strength of this composite increased significantly with the acrylic, CPE, and also MBS impact modifiers. They represented that acrylic and MBS modifiers are more efficient in improving impact resistance of PVC/wood–fiber composites compare to CPE modifier. In the research work done by Guffy and Sabbagh,104 CPE impact modifier significantly improved the processing of PVC/wood composites which resulted in higher output without a loss of surface quality, improvements in melt strength and elongation to break (with low chlorine CPE), and reduction in shear stress and viscosity of composite. Acrylic coating containing cerium dioxide (CeO2) as a ultraviolet (UV) absorber was applied during an aging process in the WF/PVC composites in order to provide the levels of hydrophilicity and mechanical properties of the composites due to the formation of hydrogen bonds between the OH groups on the wood surfaces and water molecules. According to experimental results, tensile and flexural properties of the composites decreased by increasing aging time especially at high condensation temperatures.109

SEBS and ABS SEBS impact modifier has been employed in PVC/natural fiber composites by Xu et al. in a comprehensive study and the influence of fiber type (bagasse, rice straw, RH, and pine fiber) and loading level of SEBS block copolymer on PVC composite properties was investigated. It was found that addition of all four types of natural fibers decreased the impact strength and increased the tensile strength of PVC composites. Meanwhile, SEBS incorporation (up to 5%) made up that reduction of impact strength for PVC/pine



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composite but had negative effect on tensile strength at the same time. In terms of thermal properties, incorporation of natural fibers (treated and untreated) reduced the thermal stability of PVC/natural fiber composites compared with neat PVC. Moreover, SEBS impact modifier loading slightly increased Tg of all the four kinds of PVC/natural fiber composites. Considering the results, addition of SEBS led to the higher water absorption rate for the PVC/natural fiber composites also rice straw offered better mechanical properties than RH, bagasse, and pine.6 Study on processability of the ABS impact modified RHA/PVCU composite demonstrated that fusion time increased on the addition of RHA contents while the torque decreased marginally.53

Effect of concentration, particle size, and various types of natural fibers Study of Joshi and Marathe110 on mechanical properties of highly filled PVC/WF composites indicated a reduction in tensile strength due to poor interfacial adhesion between the hydrophobic PVC and hydrophilic WF; pin-bearing and impact strengths because in absence of a coupling agent, stress transfer between filler and matrix does not occur. On the other hand, thermogravimetric analysis results showed insignificant effect of WF content on the thermal stability of the composites. Thermal investigation of WF (sapwood and heartwood) addition into the PVC exhibited a small but progressive improvement in decomposition temperature of the composites, although the glass transition temperature remained almost constant. It was also found more flexibility for the composite and better interaction and phase dispersion between the components upon the addition of filler content.111 In the recent past, effect of fiber (PF and WF) type and loading level on water resistance and thermal stability of the PVC/wood composites was explored. It exhibited an increase in water absorption by cellulosic filler loadings in the composites, mostly for the PF loading. It was also found a thermal stability improvement for the composite by incorporation of PF and further improvement after addition of WF.112 The effects of WF concentration and particle size was investigated on mechanical properties of PVC/ WF composites in Fasihi and Garmabi’s59 research work. Statistical analysis indicated around 30% decrease for flexural strength and impact strength by loading 38–56 wt% WF content into PVC. The flexural modulus of the composites improved by increasing the WF level up to 50 wt% and above that the flexural modulus decreased due to insufficient dispersion and wetting of the WF surface with the polymer matrix.

In addition, increasing WF particle size or diameter enhanced flexural modulus and strength of the composite because of higher L/D ratio in WF with higher particle size. In another study, the tensile and impact strength of the PVC/wood powder composite decreased and water absorption increased by loading the fiber content.75 PVC/WF composite also shows improved performance over wood in the following properties: termite resistance, weathering aging, less moisture absorption, and ease of installation.113 Incorporation of WFs into the rigid PVC also accelerated the photochemical degradation of the polymeric matrix due to the chromophoric characteristic of the fibers.114 The various types of natural fibers and PVC incorporated with different types of additives have been presented in Table 4.

Moisture content Effect of moisture content on the thermal and mechanical properties of PVC/wood sawdust composite was determined for three different wood sawdust contents. It was revealed that, at low moisture content, the flexural and tensile properties of the composites decreased while the elongation at break increased. Impact strength of the composites at low sawdust contents (16.7 wt%) was considerably improved with moisture content but was independent of that at higher sawdust contents (28.6 and 37.5 wt%). Moreover, a decrease in decomposition temperature of composite was occurred by increasing moisture content while the Tg remained steady.13

Cross section design A study on flexural properties of WPVC composite showed the strong dependency of these properties on the cross section design of composites which resulted from changing the intrinsic properties of the WPVC during the processing. In addition, composites with low density demonstrated lower flexural strength and the rate of wood sawdust loading did not have a significant effect on the flexural properties of the composites.75

Layering or laminating Rigid PVC and WF composites were prepared and the effects of layering and composite composition on their physical properties were studied. It was found that, higher PVC content in the formulation caused a reduction in equilibrium moisture content (EMC), maximum water absorption, water diffusion coefficients, and maximum thickness swelling. Furthermore, laminating



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Table 4. Types of fiber and various additives along with PVC matrix Type of fiber(s)

Polymer matrix and additive(s)

References

Bagasse, rice straw, RH, and pine fiber Bamboo flour WF WF (banana) OPEFB OPEFB

PVC + SEBS

Xu et al.6

PVC + DOP plasticizer + P[(SMA)-b-(SAN)] coupling agent PVC + Zn and Pb stearates + zeolite PVC PVC + benzoyl chloride + NaOH PVC + tin stabilizer + calcium stearate + stearic acid + acrylic polymer + titanium oxide + (acrylic and CPE)

Kim et al.115 Sombatsompop et al.116 Balzer et al.117 Abu bakar et al.118 Abu bakar and Hassan101 Abu bakar and Hassan103 Abu bakar et al.100 Abu bakar et al.50

Wood pulp WF (pine flour)

WF (pine flour and bamboo flour)

WF (maple)

SCB WF and PVC Sugarcane bassage (SB) PF and WF WF WF (hardwood maple) WF (southern pine and maple sapwood lumber) Pine fiber Wood sawdust

WF (hardwood maple)

WFflour

+(Acrylic core-shell) +Silane-based coupling agent (n-octyltriethoxysilane) zirconate-based coupling agent (tris (2-ethylene-diamino) ethanolate) PVC + silane solution PVC + tin stabilizer + PE wax (lubricant) + calcium stearate (lubricant) + processing aid + isocyanate, copper EA and TPS PVC + lead sulfate and lead stearate (heat stabilizer) + acrylic resins (processing aid and impact modifier) + low molecular weight PE wax (lubricant) + APS coupling agent PVC + modifier (compatibilizer/process aid) + PMMA process aid + EBS (lubricant) + calcium stearate (lubricant) + tin stabilizer + CPE impact modifier PVC + ACA PVC + chitin + chitosan-based coupling agents PVC (unplasticized) + PMPPIC coupling agent + 1% solution of sodium hydroxide (NaOH) + benzoic acid PVC PVC + beech veneers MAPE compatibilizer PVC + liquid tin heat stabilizer PVC + tin stabilizer + calcium stearate + paraffin wax + processing aid + impact modifier PVC + impact modifier + Ca/Zn stabilizer PVC + organic complex stabilizer (TSDBL–Pb–Ba) + external lubricant + internal lubricant (calcium stearate) + CaCO3 + processing aids +MMA-co-butyl-acrylate, low Mw MMAco-ethyl-acrylate, high Mw MMA-co-ethyl-acrylate PVC + sodium bicarbonate (BS) and modified azodicarbonamide (AZRV) As CFAs (ICFAs) + tin stabilizer + calcium stearate + paraffin wax + all-acrylic processing aid All-acrylic foam modifier PVC + novel coupling agents Plasticizer + carbon dioxide as a foaming agent

Jiang and Kamdem60 and Beshay et al.119 Fasihi and Garmabi59

Ge et al.15

Guffey and Sabbah104

Huang et al.58 Bhavesh et al.120 Wirawan et al.65 Saini et al.66 Kiani121 Tajvidi and Haghdan122 Matuana and Kamdem114 Andoh et al.123 Augier et al.124 Sombatsompop and Chaochanchaikul125

Sombatsompop and Phromchirasuk126 Mengeloglu and Matuana127 Mengeloglu and Matuana128

Bhavesh

PVC: poly(vinyl chloride); PMMA: poly methyl methacrylate; WF: wood flour; OPEFB: oil palm empty fruit bunch; SCB: sugarcane bagasse; PF: pulp fiber; SEBS: styrene–ethylene–butylene–styrene; DOP: dioctyl phthalate; P[(SMA)-b-(SAN)]: poly(styrene-co-MA)-block-poly(styrene-co-acrylonitrile); APS: Aminopropyltriethoxysilane; CPE: chlorinated polyethylene; EA: ethanolamine; TPS: thermoplastic starch; ACA: aluminate coupling agent; PMPPIC: poly(methylene (polyphenyl isocyanate)); CFAs: chemical foaming agents; RH: rice husk; and MAPE: maleated polyethylene.



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process resulted in an increase in maximum water absorption and maximum thickness swelling and a decrease in the EMC.122

Fungal colonization and discoloration Elsewhere, PVC/WF composite was studied respect to susceptibility to fungal colonization and discoloration. Concerning to the data, WF particulates in rigid composite lumber were not completely encapsulated by the PVC matrix during processing, hence leaving some WF particulates exposed and available for moisture sorption and subsequent fungal colonization.123

Processing aid The mixing torque, shear stress and extrudate swell ratio, and the glass transition and decomposition temperatures increased with increasing processing aid content in PVC/wood sawdust composites. Tensile strength, impact strength, and elongation at break of the composites were mostly dependant on the composite homogeneity and the residue of the unreacted processing aids.129

Concentration particle size and thermal history of SCB An investigation was done on the mechanical properties of SCB/PVC composites respect to the bagasse filler particle size, concentration of filler, and alkali treatment of it. According to the findings, impact strength, tensile strength, and elongation at break decreased while modulus, stiffness, and hardness of the composites increased by addition of filler contents.66 In a recent attempt by Wirawan et al.,130 SB considered as two main components, pith (inner part) and rind (outer part). It was reported that the tensile strength and modulus of rind/PVC composites are higher than the unfilled PVC at composite fiber contents of 30% and 40%. In addition, the SB rind/PVC composites indicated superior stiffness and strength in comparison with the SB pith/PVC composites. Another interesting study of the above authors but only using SB rind/PVC composite was done to evaluate the effects of the thermal history included annealing, quenching, and re-heating to a temperature below Tg on tensile properties of PVC and its composites. They depicted that, in the absence of SB rind fiber, thermal history affected more on the strain at break rather than other tensile properties, e.g. tensile modulus and strength and vice versa at the presence of the fiber. This scenario was associated with the structural order of the polymer after heat treatment.131

Mechanical activation pretreatment In another research work, SB was added to PVC matrix by mechanical activation and a significant improvement in mechanical properties of the composites particularly the tensile strength, flexural strength, and Brinell hardness was observed by increasing the milling time. These properties reached the maximum values when milled for 120 min. However, excessive milling time led to reduction of particle size and poor dispersion of SB in PVC matrix causing poor mechanical interlocking and slight decrease in mechanical properties of the composite.58

Rice straw In a study by Kamel,73 an increase in bending and tensile strength with increasing PVC percent in PVC/rice straw composite was indicated. Briefly, increased internal bond increased density and composite strength. Moreover, NaOH treatment of rice straw lead to improved fiber–matrix interface adhesion also developed mechanical properties.

Fruit seed A renewable material obtained from fruit seed (cumbaru pits) as reinforcing filler in PVC composite showed an increase in the tensile modulus but a decrease in tensile strength and elongation at break. Furthermore, it was not reported any significant changes on thermal properties of the composites by addition of cumbaru filler.28

Bamboo In a comprehensive study by Wang et al.,132 mechanical properties and dimensional stability of bamboo/PVC composites were improved by increasing PVC content due to the lower density and higher porosity and water absorption of bamboo granules compared to PVC as well as some superior properties of PVC like tensile strength.

Foamed PVC composites Some property drawbacks of PVC/wood fiber composites such as, high density, brittleness, and lower impact resistance restrict their full market potential in many industrial applications. Prosperously, foaming process can be fairly effective to overcome mentioned short comings.127

Effect of treated cellulosic fibers More than one decade ago, effect of two cellulosic fibers (wood fiber and newsprint) concentration and



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their silane-based surface modification was investigated on cell morphology and sorption parameters of microcellular foamed PVC/cellulosic fiber composites. In the case of using aminosilanes coupling agent, composites coupled with APS showed better mechanical properties than the uncoupled ones. Besides, treatment with dichlorosilanes (DCS) did not change the tensile strength of resulting PVC composites.46 In a research study by Huilin et al.,79 WF was treated by ACA and butyl acrylate prepolymer to produce PVC/WF foamed composites. It was found that the tensile and impact strengths of the foamed composites are improved by WF surface treatment. Furthermore, although the mechanical properties of composites modified with aluminate were superior to butyl acrylate, the processing properties of the composites with butyl acrylate were better than that of aluminate.

Foaming process and foaming agent Regarding Patterson study,133 even though non-cellular PVC-wood composites have a relatively high specific gravity (about 1.3 g/cc) compared to many other wood products, they can be foamed to prepare composites with much lower density (about 0.6 g/cc) which are suitable for numerous applications. Foaming a PVCwood composite would also improve the economics of some profile parts of the composite efficiently. In a study by Mengeloglu and Matuana,127 two different endothermic and exothermic chemical foaming agents (CFAs) were applied into the rigid PVC/WF composites. Tensile strength and modulus of the composite were deteriorated while elongation at break or ductility was improved by foaming process. A minor decline in impact resistance of the composite also better impact strength for the foams with very fine cells compare to those with larger cells were exhibited. In the previous study by above authors, foaming was successfully performed for the composites of PVC and

WF with high moisture content, using all acrylic foam modifier. The foam modifier provided level of foam density to the composite due to the trapping of evolving gas and interruption of bubble coalescence during the foaming process. Matuana has also demonstrated that cellular morphologies of the foamed PVC/wood fiber composites was strongly dependent on the plasticizer content and wood fibers surface treatment as well as the gas saturation and foaming conditions.

Effect of impact modifier The rate of gas loss during foaming process was increased due to the growth of nucleated cells using impact modifier regardless of modifier type. Thus, impact modification restricted the potential of producing foamed composites with void fractions similar to those obtained in unmodified composites. So, impact modifiers would not be required component in the formulation of foamed neat rigid PVC and rigid PVC/WF composites.134 Several years later, Chinese researchers evaluated the effect of acrylate impact modifier (ACR) on melt rheological properties of foamed WF/PVC composites. They reported that ACR strongly improved the processing rheological properties and impact strength of foamed WF–PVC composites at a relatively low dosage.135 The foamed composites and the type of impact modifier have been tabulated in Table 5.

Hybrid composites The term ‘hybrid composites’ used to allocate polymeric systems in which one kind of reinforcing material is introduced in a mixture of different matrices (blends) or a combination of reinforcing and filling materials are applied in one polymeric matrix or both approaches are associated. The behavior of hybrid composites comes

Table 5. Foamed composites and various additives Foamed composite

Additives

References

WF/PVC WF/PVC Wood fiber/PVC WF/PVC WF/PVC WF/PVC Natural fiber/PVC Wood fiber/PVC

Acrylate impact modifier Impact modifier Plasticizer All-acrylic foam modifier Endothermic and exothermic CFAs ACA and butyl acrylate prepolymer Aminosilanes and DCS coupling agent Silane coupling agent

Bai et al.135 Matuana and Mengeloglu134 Hemmasi et al.136 and Matuana81 Matuana and Mengeloglu Mengeloglu and Matuana127 Huilin et al.79 Xie et al.46 Abu Bakar et al.137 and Matuana et al.138

PVC: poly(vinyl chloride); WF: wood flour; CFAs: chemical foaming agents; ACA: aluminate coupling agent; and DCS: dichlorosilanes.



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Figure 6. Property modification of PVC by polymer alloy. PVC: poly(vinyl chloride).

from the individual components in which more favorable balance between the inherent advantages are attainable.139 Figure 6 shows various modifications in properties of PVC by polymer alloy combination.

PVC/GF/cellulosic fiber GFs are widely used as the reinforcement materials for thermoplastics due to contribution of high strength and stiffness to the composite. However, the addition of the GF has not improved the mechanical properties of PVC/cellulosic material composites unless the cellulosic materials were properly surface modified.140 Effect of GF content and length. In a study by Jiang et al.,140 the addition of type L GFs increased unnotched and notched impact strength of PVC/WF/ GF hybrid composites, significantly. This could be associated with the WF and GFs entanglement to make a three-dimensional GF network. However, type S GFs did not show any positive effects on both impact and flexural properties. In a more recent study, incorporation of E-GF with fiber orientation angle of 0 into the PVC/sawdust composites increased the tensile and flexural modulus, tensile and flexural strength, and impact strength of the GF/WPVC composites for up to16.7 wt% sawdust content while the amount of ultimate elongation did not change for all the E-GF contents.141 In the previous works of the above author, E-chopped strand (ECS) GFs were introduced into WPVC composites and an increase in tensile and flexural modulus and strengths as well as impact strength

was seen by loading levels of GF. On the other hand, the elongation at break was reported to have a slight decrease with increasing GF content.142 In an interesting study on E-GF and three different types of WF fibers has been dry-blended and hot pressed with PVC to form WPVC hybrid composites. It was shown that, wood particles with high aspect ratio, played the role of reinforcing agents by decreasing the specific wear rate of PVC. In addition, the wear properties were found to improve with the addition of 10 phr E-GFs into the WPVC composites.143 Surface-treated GFs. The hybrid behavior of surface-treated GF and hardwood CTMP in PVC composites was also investigated by Maldas and Kokta.144 MA, mixtures of MA and Na-silicate, and isocyanate were employed for pretreatment of the GF. It was obviously indicated that surface-treated wood fibers provided better mechanical properties (except modulus) in PVC composites compare with untreated fibers. However, fiber surface treatment using silicate and MA did not have any significant effect on the mechanical properties of the composites. Sandwich composite. In a novel research study by Mongkollapkit et al.,92 GF/WPVC sandwich composites were produced at the presence of DOP plasticizer. The GF/PVC was used as core layer whereas WPVC was the cover layer. The resulted interface between GF and the PVC core layer led to mechanical properties improvement of the sandwich composites, except for the tensile modulus. Additionally, the incorporation of DOP plasticizer was found to increase the impact



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strength of the composites mostly due to fiber pull-out. However, incorporation of more than 10 pph plasticizer into the composite had the reverse effect on mechanical properties.

PVC/epoxidized natural rubber/cellulosic fibers Epoxidized natural rubber (ENR) in PVC composites acts as a permanent plasticizer for PVC and study on the blends of PVC/ENR has proved that the two polymers are miscible at all composition range.145,146 The potential modification of PVC/ENR blend by electron beam irradiation has brought to the attention by Ratnam et al. in their subsequent publications. The effect of irradiation on the tensile properties of OPEFB fiber reinforced PVC/ENR blends was studied in 2007. It was clearly reported an increase in tensile strength, Young’s modulus, and gel fraction with a concurrent reduction in the elongation at break of the PVC/ENR/OPEFB composites upon electron beam irradiation. The results suggested that grafting of the OPEFB fiber with methyl acrylate did not provide appreciable effect to the tensile properties of the composites in spite of an improvement in adhesion between the fiber and polymer matrix.147 In another interesting study of above authors, enhancement of Young’s modulus, hardness, and flexural modulus of the PVC/ENR blend by the increase in OPEFB loading showed that the OPEFB fiber had the potential to enhance the mechanical properties of PVC/ ENR blends. However, similar to the previous study, the impact strength, ultimate tensile strength (UTS), and elongation at break of the composites were found to decrease with the increase in fiber loading. Furthermore, addition of PMA-g-OPEFB fiber into the blends rendered some flexibility to the composites due to improvement in interfacial wetting between OPEFB and PVC/ENR blend. This graft copolymerization reaction resulted in higher elongation at break and UTS, also a reduction in the flexural and Young’s modulus of the composites.148,149

PVC/nitrile butadiene rubber/cellulosic fibers Nitrile butadiene rubber (NBR) as another common rubbery material was applied as a toughening agent into the PVC to prepare toughened PVC hybrid composite with OP as a filler. Torque rheometer data showed an increase in the storage modulus and the stiffness of the composites by loading filler contents because of the reduction of free volume of the blends, probably due to polar–polar interaction between PVC/ NBR and OP chains. It was also recorded an increase in flexural properties of the composites with the addition of OP filler.150

PVC/LDPE/cellulosic fibers Composites from PVC/LDPE blend reinforced by rubber–wood sawdust had been prepared and modified using three different coupling agents. Tensile modulus of the composite increased with increasing wood contents, while the inverse effect was recorded for tensile strength, elongation at break, and impact strength. After incorporation of silane or maleated polyethylene (MAPE) coupling agents into the compounds, an improvement in mechanical properties of the composites was occurred along with slight decreases in Tg and tan dmax values.151 Lignocellulosic fibrous plant residues introduced as fillers into the thermoplastic polymers. The untreated and pre-treated residues were added to LDPE and PVC plastisol matrices to prepare hybrid composite. Mechanical tests indicated that, loading of LDPE with natural fibers leads to a decrease in tensile strength of the pure polymer while the Young’s modulus increased due to the higher stiffness of the fibers.98 One decade before the above study, cellulose fibers surface-coated with butyl benzyl phthalate (BBP) plasticized PVC had provided excellent processability and dispersion in polystyrene. Resultant single-phase material at the relevant concentrations revealed that BBP is a suitable cosolvent for both PS and PVC. Moreover, it was seen an improved elongation at break and impact strength for the composites mostly due to pulling-out of the fibers.152 In three consequent studies by the same authors, PVC with three different polymers (PP, high-density polyethyl (HDPE), and LDPE) were solution blended using organic solvents in the first research, at the presence of WF, poly(ethyleneco-glycidyl methacrylate) (PE-co-GMA) compatibilizer and nanoclay. It was shown that, tensile and flexural properties, hardness, and thermal stability of the wood/polymer/clay nanocomposites were better than wood/polymer composites while the water absorption was lower.153 The same results were reported using TiO2 and nanoclay together in the same hybride nanocomposite.154 Finally, the third similar study but with using two different components (2,3-epoxy glycidyl methacrylate as compatibilizer and modified MMT by cetyl trimethyl ammonium bromide (CTAB) as filler) also exhibited improved mechanical and thermal stability of WPCs after incorporation of modified filler into the composite.155

PVC/MMT/cellulosic fibers In a different approach to hybrid PVC composite, organomodified montmorillonite (OMMT) and WF, after surface modification with silane coupling agent, were melt blended with PVC to prepare OMMT/surface



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Table 6. Different natural fiber-based hybrid composites involving PVC as a matrix material Natural fiber hybrid composite WF

Additives

PVC + GF

DOP + Pb–Ba stabilizer + external lubricant + calcium stearate + calcium carbonate + processing aids WF (Phragmites karka) PVC + PP + LDPE + HDPE PE-co-GMA compatibilizer + modified nanoclay PE-co-GMA compatibilizer + TiO2 and nanoclay Modified MMT by CTAB + 2,3-epoxy glycidyl methacrylate compatibilizer Wood residues PVC (plastisol) + LDPE DOP + Cd/Zn heat stabilizer Cellulose fibers PVC (plasticized) + PP + PE + PS BBP WF (pine) PVC + MMT Cetylalkyl trimethyl amine bromide (CTABr) + APS + Pb-thermal stabilizer Hardwood (CTMP) PVC + GF DOP + barium acetate (BaAc) stabilizer + PMPPIC + MA + Nasilicate (NaSc) OPEFB PVC + ENR Tribasic lead sulfate stabilizer + Tripropyleneglycol diacrylate crosslinking agent + methyl acrylate (MA) monomer + hydrogen peroxide + ammonium ferrous sulfate Wood sawdust PVC + ECS GF Pb–Ba-based organic stabilizer + lubricant + calcium stearate + calcium carbonate(CaCO3) + amino-silane OP PVC + acrylonitrile nitrile rubber Lead salt stabilizer with 34% acrylic content WF PVC One pack heat stabilizer + external lubricant + calcium stearate (CaSt) + CaCO3 + processing aids PVC + ECS GFs + N– +2(aminoethyl)–3–aminopropyl trimethoxysilane WF PVC + two types of GFs (type L and type S)

References Mongkollapkit et al.92

Deka and Maji153 Deka and Maji155 Deka and Maji154 Georgopoulos et al.97 Gatenholm et al.152 Zhao et al.156 Maldas and Kokta144 Ratnam et al. Raju et al.148 Ratnam et al. Tungjitpornkull et al.157 Tungjitpornkull et al.158 Mousa et al.150 Pattamasattayasonthi et al.110 Jeamtrakull et al.143 Jiang et al.140

PVC: poly(vinyl chloride); WF: wood flour; CTMP: chemithermomechanical pulp; OPEFB: oil palm empty fruit bunch; OP: olive pomace; GF: glass fiber; PP: polypropylene; LDPE: low-density polyethyl; HDPE: high-density polyethyl; PE: polyethylene; ENR: epoxidized natural rubber; ECS: E-chopped strand; DOP: dioctyl phthalate; PE-co-GMA: poly(ethyleneco-glycidyl methacrylate); CTAB: cetyl trimethyl ammonium bromide; BBP: butyl benzyl phthalate; and PMPPIC: poly(methylene (polyphenyl isocyanate)).

treated wood fibre (STWF)/PVC composite. Silanetreated WF enhanced the impact and tensile strengths of WF–PVC composite by 14.8% and 18.5%, respectively. Moreover, although addition of OMMT did not affect the untreated WF/PVC, it could strongly improve the mechanical properties of the treated WF/ PVC composite, for instance, impact strength of the composite was enhanced by 20%. Cone calorimetry also indicated significant improvement in flame retardancy and smoke suppression of composites by the OMMT loading (Table 6).156

Applications PVC is most often used as the thermoplastic matrix in window applications, but other plastics and plastic blends are also used. Although more expensive than unfilled PVC, wood-filled PVC is gaining favor because of its balance of thermal stability, moisture resistance,

and stiffness.159 The detailed applications of rigid PVC and flexible PVC have been presented in Table 7. Schut in 1999 reported application of WPVC composites for window line. He also mentioned to co-extrude of two different profiles. The first one included inside layer of wood-filled PVC and outside layer of unfilled PVC for achieving higher durability and the second one was formed from PVC core and wood-filled PVC at the surface which could be painted.160 Another application of WPVC composite is in decking. Stiffness and balance of thermal resistance are reasons which are favor to use WF–PVC composites for window and door frames.1 Earlier, Bhadauria and Datta161 reported the use of WF–PVC composites for building if they can compete with conventional materials. From 1998 to 2002, the use wood fiber as the filler in plastic-based building products grew from 66% to 80% and PVC resins were expected to average 20% growth per year until 2010.162



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Table 7. Various applications of rigid and flexible PVCs Application

Rigid PVC

Flexible PVC

Construction

Packaging Transport

Window frames, gutters, pipes, cars, house siding, ports, roofing Curtain rails, drawer sides, laminates, audio and video tape cases, records Bottles, blister packs, transparent packs and punnets Car seat backs

Medical

–

Clothing

Safety equipment

Others

Floppy disk covers, credit cards

Water proof membranes, cable insulation, roof lining, green houses Flooring, wall coverings, shower curtains, leather cloth, hosepipes Cling film Under seal, roof linings, leather cloth upholstery, wiring insulation, window seals, decorative trim Oxygen tents, bags and tubing for blood transfusion, drips and dialysis liquids Waterproofs for fishermen and emergency services, life jackets, shoes, Wellington boots, aprons and baby pants Conveyor belts, inflatables, sports

Domestic

PVC: poly(vinyl chloride).

Table 8. Scheme of the three methods of recycling of PVC Recycling type

Process

Product

Mechanical recycling Feedstock recycling Thermal recycling

Material – physical processing material Material – chemical processing material Material – incineration material

Sorting Mixed plastic waste Mixed plastic waste

PVC: poly(vinyl chloride). Seki ‘PVC FACT BOOK,’ Vinyl Environmental Council.

Environmental and recycling issues Combined with natural and biodegradable wood, PVC/ NF composites are definitely more environmentally friendly than pure PVC, which is suspected to create potential hazards in the environment because of its high chlorine component.163 Additionally, both recycled wood and recycled PVC can be used as the raw materials to produce PVC/NF composites. It is obvious that manufacturing and development of composites make a good contribution to solve the environmental problems. At the end of the service life of PVC-containing products, several options are available for disposal and waste management. Of course, the first option is recycling and/or reuse into different useful products (Table 8).164 The problem associated with the above solutions is the number or recycling cycles acceptable and with limited damage on PVC. However, waste generated during manufacture will always have to be disposed of. Landfilling is another easy and simple solution, but the increase cost of landfilling and the potential contamination of ground water from chlorine or chlorine-containing products may limit the use of this option (1 1 6). Incineration is one of the last solutions; it implies the control of the emission of hydrogen

chloride and handling of ashes generated. Some chemical recycling methods are under development in Europe; for example, PVC wastes are treated by a socalled slag bath gasification procedure.163 Through this procedure, wastes can be decomposed to the chemicals for PVC monomer synthesis. Another chemical recycling method is known as Vinyloop process, which involves dissolving PVC wastes in appropriate solvents, and then the PVC is precipitated and recovered from the solvents.163 Furthermore, waste wood, waste paper, and waste plastics are major components of municipal solid waste and offer great opportunities as recycled ingredients in wood fiber–plastic composites. Recycled sources of both wood and plastic are commonly used in WPCs. Both post-consumer and post-industrial materials are used. Finding an inexpensive, yet suitable source, with consistent and sufficient performance can present challenges but the broad use of recycled materials in cellulosic fiber PVC composites demonstrates its feasibility.

Conclusions There is a dramatic increase in the demands for PVC/ NF composites. They are typically used in building



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construction applications, such as window/door profiles, decking, railing, and siding. Various processing techniques especially twin-screw extruders have performed very well in manufacturing these composites. Additives for rigid PVC such as heat stabilizers, processing aids, impact modifiers, lubricants, and pigments are still needed for PVC/NF composites to maintain the PVC matrix properties, facilitate processing, and enhance mechanical properties. The use of antimicrobials is dependent on the WF type and content, microbe types in the application environment, and the environmental humidity. Many coupling agents have been proved to be effective but PMPPIC, APS, MAPP, and copper metallic complex coupling agents for PVC/NF composites have extended better performance. Combining wood with other fillers such as mica or GFs to form hybrid reinforcements can enhance the mechanical properties of PVC/NF composites. UV light resistance and weathering dimensional stabilities of PVC/NF composites are better than those of solid wood. PVC/NF composites are superior to HDPE, PP/NF composites with respect to their higher modulus, good creep resistance, weatherability, and flame retardance. The increased specific density of PVC/NF composites can be reduced through microcellular foaming using chemical blowing agents, such as azodicarbonamide and sodium bicarbonate, or physical blowing agents, like carbon dioxide.

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Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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