PVC composites

Composites: Part B 43 (2012) 2721–2729

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Effects of wood constituents and content, and glass fiber reinforcement on wear behavior of wood/PVC composites q Supreeda Jeamtrakull a, Apisit Kositchaiyong a, Teerasak Markpin a, Vichai Rosarpitak b, Narongrit Sombatsompop a,⇑ a Polymer Processing and Flow (P-PROF) Group, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi (KMUTT), Thongkru, Bangmod, Bangkok 10140, Thailand b V.P. Wood Co., Ltd., 25/5 Moo 4, Soi Suksawad 66, Thongkru, Bangmod, Bangkok 10140, Thailand

a r t i c l e

i n f o

Article history: Received 26 April 2011 Received in revised form 18 August 2011 Accepted 23 August 2011 Available online 3 May 2012 Keywords: A. Glass fibers B. Mechanical properties A. Natural fiber composites B. Wear

a b s t r a c t In flooring applications, experimental data and insight from scientific investigations on wear properties of wood/polymer composites (WPCs) are important for engineers to understand how to design and formulate WPC materials with high resistance to wear. In this work, three different types of wood flour – namely Xylia kerrii Craib & Hutch., Hevea brasiliensis Linn., and Mangifera indica Linn. – were utilized and incorporated into poly(vinyl chloride) (PVC) with a fixed content (10 phr) of E-chopped strand glass fiber. The physical, mechanical and wear properties, in terms of specific wear rate, were then assessed as a function of wood content and sliding distance. The experimental results suggested that the addition of wood flour increased the flexural modulus and strength up to 40 phr; beyond this concentration, the flexural properties decreased. Hardness was not affected by the addition of wood flour. The mechanical and wear properties of WPVC composites were found to improve with the addition of the E-glass fiber. Xylia kerrii Craib & Hutch. wood exhibited the lowest specific wear rate for non-reinforced WPVC composites, whereas Hevea brasiliensis Linn. wood showed the lowest specific wear rate for the glass fiber reinforced WPVC composites. The longer the sliding distance, the greater the specific wear rate in all cases. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Wood flour was initially used for purposes of cost reduction, and for reducing large amounts of natural fiber wastes in both structural and non-structural applications. Recently, wood flours have been used for reinforcement in polymeric materials, offering wood/polymer composite (WPC) products with low density and better dimension stability. Wood/polymer composites (WPCs) have been developed so that they can now be utilized for building and construction, and in automotive, gardening and outdoor products as well as in marine applications [1], although their mechanical strengths are still lower than those of natural woods. A large number of research works on WPC have been published; most of these have investigated the mechanical, thermal and morphological properties of WPC, and how they are affected by the addition of synthetic fibers and nanoparticles [2–6], processing techniques and conditions [7,8], the cross-sectional design of shaping dies [9–11], chemical and physical treatments of wood surfaces [12,13], and the type of thermoplastics used [11]. These published q Part of this manuscript was presented at the 7th Asian-Australasian Conference on Composite Materials (ACCM#7) 15–18 November 2010, Taipei, Taiwan. ⇑ Corresponding author. Tel.: +66 2 470 8645; fax: +66 2 470 8647. E-mail address: [email protected] (N. Sombatsompop).

1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.04.031

works [2–13] have aimed to improve the mechanical properties of WPC in structural applications, in order to compete with the natural woods. The two-body abrasion process that is traditionally applied in woods includes a sanding process for surface finishing; and the use of loose particles in three-body abrasion becomes a major problem in the wear resistance of flooring and construction materials [14,15]. The wear behavior of WPC materials is a relatively new area of study, and could be considered to incorporate three separate components: wood, polymer, and WPC. The wear behaviors of woods [16,17] and polymers [18–20] have been extensively studied, whereas few reports have been published on the wear behavior of WPC materials [1,21]. Works by Ohtani and Kamasaki [16] and Ohtani et al. [17] suggested that different types of timber woods resulted in different wear rates obtained, and that this was dependent on the direction of the applied pressure on the wood samples. None of the existing literature has investigated the wear behavior of composite wood flour and thermoplastic materials in light of the effect of wood species. A few recent works [1,21] have attempted to study the wear properties of WPC. Kim et al. [1] revealed that the wear resistance of wood impregnated with polyethylene glycol (PEG) was higher than that impregnated with lubricant oil, release agent, and epoxy. This was attributed to the low friction coefficient of PEG. Aurrekoetxea et al. [21] studied

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wear resistances of pine wood, polypropylene, and wood-reinforced polypropylene (PP) using a ‘‘block-on-ring’’ tribometer. The results suggested that the wear rate of pine wood/PP composite was the lowest, whereas that of neat PP was highest. The wear mechanism for PP was a combination of a plastic flow and melting phenomenon. For pine wood, the microfissuring and delamination induced by deformation were the main parameters for the micromechanism of wear. In wood/PP, the wear mechanism of the wood reinforcement was different from that of neat wood, since the collapsed PP-filled wood fibers in the composites reduced the

Fig. 1. FTIR spectrum for wood particles.

buckling, microfissuring and delamination inherent in the cellular structure of the wood fibers. As stated earlier, published works on the wear behavior of woods and polymers alone have been extensively available [16– 20], whereas reports on WPC materials have been relatively rare [1,21], suggesting that the subject of the wear behavior of WPC is still open for wide discussion – one of the main reasons being that the wood flour could be incorporated in a wide range of thermoplastics, depending on their applications and weathering resistances. For example, wood/olefin composites are widely used in Western countries, whereas wood/PVC composites are preferable in Asian countries. Knowledge of the wear properties of WPC materials is very important, especially for flooring and construction applications. More experimental data from scientific investigations are required to cover all possible applications of WPC composites produced by various polymers, and also for the engineer to understand how to design WPC material formulations with high resistances to wear. Hence the objective of the present work was to investigate the mechanical properties and wear behavior of wood/PVC composites, using three different types of wood flour – Xylia kerrii Craib & Hutch., Hevea brasiliensis Linn., and Mangifera indica Linn. – with and without the addition of E-chopped strand (ECS) glass fiber. These wood flour particles and the glass fiber were dryblended and compression-molded with poly(vinyl chloride) (PVC) to form wood/PVC (WPVC) composite sheets. The mechanical properties and wear behavior of WPVC composite sheets were then evaluated for the effects of wood type and concentration, with and without the addition of ECS glass fiber. A wear mechanism for WPVC composites during wear tests was proposed, based on analysis of the morphological characteristics of the worn surfaces.

Fig. 2. Physical morphologies of wood particles: (a) XK, (b) HB, (c) MI.

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2. Experimental 2.1. Raw materials

49

3 28 69 40.5 28.5

71 SEM of wood timbers

Shore D

60

6 27 67 42 25 Hardness

Composition content (%)

XK wood

Ethanol:toluene (2:1), 100 °C, 5 h 72% Sulfuric acid, water bath 100 °C, 2 h 96% Acetic acid and sodium chlorite, water bath 70–80 °C, 3 h 17.5% Sodium hydroxide, room temperature, 15 min Whemicellulose(wt.%) = Wholocellulose(wt.%)  Wcellulose(wt.%)

Silane-treated wood flour was dried at 80 °C for 24 h in a hot oven for elimination of residual humidity. WPVC composite specimens were directly prepared by blending PVC and treated wood flour at wood concentrations of 0, 20, 40 and 60 phr, using a high speed mixer. The dry-blended wood/PVC compound was then filled in a steel mold having a cavity size of 180  180  3 mm3, and was plate-shaped by a compression molding machine (LP-S20 laboratory press, Labtech Engineering, Bangkok, Thailand) at a preheating temperature of 180 °C and 18 MPa compression pressure for 7 min, before being cooled down to room temperature for 5 min. For all glass fiber reinforced WPVC composites, 10 phr glass fibers were used, and their blending procedures were the same as used for preparation of WPVC composite specimens.

Extractives Lignin Holocellulose Cellulose Hemicellulose

2.3. Preparation of wood/PVC composite specimens

Extraction conditions

All woods were characterized in terms of constituents and initial hardness properties. Analysis of the constituents of the woods was performed using solvent extraction method, whose experimental procedures can be found elsewhere [23]. Hardness properties of all woods were measured using wood strip samples, the sample preparation and test procedures following ASTM 2240-05 (Shore D hardness).

Constituent

2.2. Constituents, initial hardness and morphology of wood particles

Table 1 Constituents of wood particles, initial hardness and physical morphologies for different timber woods.

2.1.3. Glass fiber reinforcement E-chopped strand (ECS) glass fibers (3 mm long and 13 lm in diameter) were used to improve the mechanical and wear properties of wood/PVC composites. The E-glass fibers used were supplied by Pongpana Co., Ltd. (Bangkok Thailand). A silane coupling agent, 3-methacryloxypropyl-trimethoxysilane (KBM503) having an average molecular weight of 248.4, was coated on the glass fiber surface. A fixed content of ECS glass fibers (10 phr of PVC compound) was introduced in the wood/PVC (WPVC) composites. Our previous findings [2,8] suggested that 10 phr E-glass fibers could give a relatively good and consistent mixture with WPVC composite for mechanical property improvement in the compression molding process.

HB wood

2.1.2. Wood flours Three different types of wood flour particles – namely Xylia kerrii Craib & Hutch. (XK), Hevea brasiliensis Linn. (HB), and Mangifera indica Linn. (MI) – were used. Their chemical structures were characterized by Fourier transform infrared spectroscopy (ATRFTIR), using a Nexus 470 (Thermo-Nicolet Ltd., USA). All woods were supplied by Pongsiri Co., Ltd. (Ratchaburi, Thailand). The wood content in all PVC compounds varied from 0 to 60 parts per hundred (phr) by weight of PVC powder. Wood particles were chemically surface-treated with 1.0 wt.% N-2(aminoethyl)-(3-aminopropyl)trimethoxysilane (Mw = 222.4); details regarding the experimental procedure can be found in our previous work [22].

5 29 66 45 21

MI wood

2.1.1. Poly(vinyl chloride) or PVC Suspension PVC was supplied by V.P. Wood Co., Ltd. (Bangkok Thailand) in the form of powder (trade name, SIAMVIC 258RB) having a K value of 58. The PVC powder was dry-blended with various necessary additives whose compound formulations were the same as used in our previous work [22].

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Table 2 Effects of type and content of wood flour on mechanical and physical properties of WPVC composites. Wood

Mechanical properties

Physical property

Type

Content

Hardness (Shore D)

SD

Flexural modulus (MPa)

Flexural strength (MPa)

SD

Roughness (lm)

SD

XK

0 20 40 60

78 78 81 81

1.7 1.1 1.8 1.0

1219 1765 2081 2103

17 33 34 32

4.1 2.8 3.1 5.8

3.2 3.8 4.1 4.3

0.4 0.2 0.4 0.5

HB

0 20 40 60

78 78 79 80

1.7 1.6 1.6 1.3

1219 1755 1936 1576

17 33 32 25

4.1 1.4 2.4 5.2

3.2 4.0 4.4 4.5

0.4 0.8 0.3 0.9

MI

0 20 40 60

78 81 82 82

1.7 1.0 1.5 1.1

1219 1778 2251 2130

17 30 38 33

4.1 2.9 3.3 5.6

3.2 4.1 4.2 4.2

0.4 0.2 0.3 0.3

(a) Sliding distance of 0.5 km

(b) Sliding distance of 1.0 km

(c) Sliding distance of 1.5 km

(d) Sliding distance of 2.0 km

Fig. 3. Effects of type and content of wood flour on specific wear rate of WPVC composites at various sliding distances: (a) 0.5 km, (b) 1.0 km, (c) 1.5 km, (d) 2.0 km.

2.4. Characterizations 2.4.1. Mechanical properties The mechanical properties of WPVC composites and glass fiber reinforced WPVC composites were investigated through flexural properties and Shore D hardness (ASTM D2240, 2005). The flexural properties were determined according to ASTM D790 (2003): specimen dimensions of 15  120 mm2, support span of 86 mm, and a

crosshead speed of 1.7 mm/min. It should be noted that the mechanical property results reported in this work were averaged from 10 independent determinations. 2.4.2. Composite roughness Surface roughness (Ra) of WPVC composites and glass fiber reinforced WPVC composites were measured using a SV-3000 profilometer (Mitutoyo, Kawasaki, Japan). A conical stylus with a

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Fig. 4. SEM micrographs of worn surfaces of WPVC composites for different wood types at 60 phr wood content at a sliding distance of 2.0 km: (a) XK, (b) HB, (c) MI.

spherical tip of 2 lm radius and 60° cone angle was used; the stylus speed was 0.5 mm/s. The roughness results reported in this work were averaged from five independent determinations. 2.4.3. Specific wear rates Wear tests were performed using a Taber 5130 Abraser (Taber Industries, North Tonawanda, NY) with abrasive wheels (CS-17) which were attached to a load of 250 g, following ASTM D4060 (2007) for determinations of specific wear rates of neat PVC, WPVC, and glass fiber reinforced WPVC composites. The specimen dimensions used were 120  120  3 mm3. The sliding distance varied from 0.5 to 2.0 km. The specific wear rate (K) was measured by determination of weight loss of the test specimen, which can be calculated using the following equation [24]:

K¼

DM LF q

ð1Þ

where Dm is the weight loss (mg), L is the sliding distance (m), F is the applied load (N) and q is specimen density (g/cm3). 2.4.4. Scanning electron microscope (SEM) investigation Morphologies of wood particles and worn WPVC composite surfaces, with and without E-glass fiber in WPVC composites, were investigated using a JSM-6301F scanning electron microscope (JEOL, Tokyo, Japan) at 15 kV accelerating voltage. Details of the experimental procedure and sample preparation can be found in a previous work [25].

mechanical properties of wood/PVC (WPVC) composites obtained were not caused by the differences in the chemical functional groups among these three wood flours. Fig. 2 shows the initial morphologies for XK, HB and MI wood particles by SEM technique. All types of woods had an average particle size of 100–250 lm. The aspect ratios for XK, HB and MI wood flour were measured to be 4.78, 1.63 and 4.96, respectively. The constituents and initial hardness results of wood flours used are shown in Table 1. It can be observed that all woods had different proportions of cellulose and hemicellulose, and their hardness results obtained from the original wood timbers. It has been established that the cellulose portion enhances the rigidity of the wood: the greater the cellulose content, the higher the hardness [14,17]. The opposite effect is found for hemicellulose content. XK wood appeared to have the lowest hemicellulose content (66%), the highest cellulose content (45%), and the highest hardness value (71 Shore D); thus XK is regarded as a hard wood. MI had the highest content of hemicellulose (69%) and the lowest initial hardness (49 Shore D), and was regarded as a soft wood in this work. The SEM texture surfaces for all woods were also different: i.e. HB and MI woods were relatively less compact (high porosity) compared with XK wood. It should be noted that the differences in aspect ratios, constituents and initial hardness values, as well as the textures of these three woods, will be taken into account when analyzing the differences in the mechanical and wear properties of WPVC. 3.2. Physical and mechanical properties of WPVC composites

3. Results and discussion 3.1. Chemical, morphological and physical characterizations of wood flours used The chemical structures of XK, HB and MI by FTIR technique are shown in Fig. 1. It can be seen that all wood flours used had similar chemical functional groups, suggesting that the differences in the

Table 2 shows the physical roughness and mechanical properties of WPVC composites containing three different types of wood particles and various wood contents. The results showed that the hardness values for WPVC composites with three different wood types were very similar, the differences being within an acceptable experimental error range (+/2.0%), although the initial hardness values for these three woods (Table 1) were different. The

+9 +1 6 2 1.0 0.2 0.3 0.5 3.5 4.2 4.0 4.2 0.4 0.2 0.3 0.3 3.2 4.1 4.2 4.2 +122 +36 +21 +34 39 41 46 45 78 81 82 82

1.7 1.0 1.5 1.1

80 80 83 83

0.9 1.6 1.6 1.1

+3 1 +1 +1

1219 1778 2251 2130

4.1 2.9 3.3 5.6 0 20 40 60 MI

2407 2798 3416 3545

3.3 3.0 5.5 4.2

+97 +57 +52 +66

17 30 38 33

4.1 2.9 3.3 5.6

3.3 3.0 5.5 4.2

+9 14 16 14 1.0 0.4 0.2 0.6 3.5 3.5 3.7 3.9 0.4 0.8 0.3 0.9 3.2 4.0 4.4 4.5 +122 +57 +61 +93 39 51 51 48 78 78 79 80

1.7 1.6 1.6 1.3

80 80 82 82

0.9 0.8 1.0 1.1

+3 +3 +4 +3

1219 1755 1936 1576

4.1 1.4 2.4 5.2 0 20 40 60 HB

2407 2896 3175 3604

3.3 2.6 3.5 3.1

+97 +65 +64 +129

17 33 32 25

4.1 1.4 2.4 5.2

3.3 2.6 3.5 3.1

1.0 0.3 0.2 0.5 3.5 3.8 3.7 4.0 0.4 0.2 0.4 0.5 3.2 3.8 4.1 4.3 +122 +50 +47 +61 39 49 49 51 1219 1765 2081 2103 +3 +4 +1 +2 0.9 1.0 1.1 1.4 80 81 82 83 1.7 1.1 1.8 1.0 78 78 81 81

% Change SD With 10% GF SD

4.1 2.8 3.1 5.8 0 20 40 60 XK

2407 3253 3344 3425

3.3 3.5 1.3 4.1

+97 +84 +61 +63

17 33 34 32

4.1 2.8 3.1 5.8

3.3 3.5 1.3 4.1

SD With 10% GF With 10% GF Without GF

SD Without GF

Hardness (Shore D) Content Type

With 10% GF

SD

% Change

SD

SD

% Change

SD

Roughness (lm)

Without GF Without GF

Physical property

Flexural strength (MPa) Flexural modulus (MPa) Mechanical properties Wood type

Table 3 Effects of type and content of wood flour on mechanical and physical properties of WPVC composites without and with glass fiber.

+9 +1 9 9

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% Change

2726

differences in the hardness results were because the hardness of the WPVC composites was measured on the surface of composites, whereas the wood particles were embedded underneath and within the PVC matrix. However, it was also found that increasing wood content tended to result in higher hardness. Similar behavior was also observed for roughness result and flexural modulus. The flexural strength increased with increasing wood content up to 40 phr, and then slightly decreased at a wood content of 60 phr (except for HB wood). The increases in the flexural properties indicated PVC reinforcement by the wood particles, whereas the decreases in the flexural properties probably resulted from difficulty in dispersing the wood particles around the PVC matrix due to high wood content [2,26–28]. Regarding the effect of wood type, it was found (Fig. 2) that for any given wood contents, the flexural properties decreased in the order of: MI, XK and HB wood. Since the chemical functional groups on the surfaces of these three wood types were the same, the differences in flexural properties found in this work were not caused by the differences in chemical functional groups on the wood surfaces. Therefore, the results should be explained in terms of differences in physical forms and constituents of these three woods. The higher flexural properties of WPVC with MI and XK woods were caused by the higher aspect ratios of the MI and XK woods (see Fig. 2). The higher flexural properties for MI as compared with XK wood was associated with the high porosity of MI wood, which allowed the PVC to penetrate into the fiber body during processing; this would probably form a physical interlocking, and result in greater flexural properties of the WPVC composite. 3.3. Specific wear rate of WPVC composites Fig. 3a–d shows the specific wear rates for WPVC composites with XK, HB and MI woods at different wood contents (0–60 phr), and for sliding distances of 0.5, 1.0, 1.5 and 2.0 km. In general, as the sliding distance increased the specific wear rate appeared to increase (as one would expect). For 0.5–1.0 km sliding distances, the specific wear rates of WPVC composites for all wood contents were not different, taking the experimental error bars into account. However, the effects of wood type and content became more pronounced as the sliding distances were increased to 1.5 and 2.0 km. The addition of wood particles into the PVC matrix enhanced the wear resistance of the WPVC composite, indicating the wood reinforcement in the PVC matrix. This view was in line with the results of hardness, roughness and flexural properties as given in Table 2. The increased wear resistance of the WPVC composites by the addition of wood was because the wood particles were more rigid than PVC and had the ability to resist the force exerted on the WPVC sample. At high sliding distances (1.5–2.0 km), it was found that the specific wear rates for WPVC composites with XK and MI woods appeared to decrease up to 40 and 20 phr, respectively, and then started to increase at higher wood contents. The reductions of the specific wear rates were caused by the wood reinforcement, as mentioned earlier. The optimal wood content of XK wood to obtain the highest wear resistance (lowest specific wear rate) of WPVC composites was higher than that of MI, due to the greater stiffness of XK wood compared to MI wood (see Table 1). Above the optimal wood contents (40–60 phr for MI and 60 phr for XK), the specific wear rates increased. This was probably caused by the three-body abrasion effect [29], where wood particles escape from the WPVC surface during abrasion testing. The abrasive action of hard wood debris is exhibited on the surface of the WPVC sample, as seen in SEM micrographs in Fig. 4 showing the scratch lines on the worn surface of an XK-filled PVC composite sample. The differences in the magnitude of increase in the specific wear rates between XK and MI were related to the differences in the initial hardness of these two woods, as indicated in Table 1.

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(a) without glass fiber at 0.5 km

(b) with glass fiber at 0.5 km

(c) without glass fiber at 2.0 km

(d) with glass fiber at 2.0 km

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Fig. 5. Effect of type and content of wood flour on specific wear rate of WPVC composites at two different sliding distances: (a) without glass fiber at 0.5 km, (b) with glass fiber at 0.5 km, (c) without glass fiber at 2.0 km, (d) with glass fiber at 2.0 km.

In the case of the specific wear rate result for WPVC composite with HB wood flour in Fig. 3, it was noticeable that the specific wear rate gradually decreased with increasing wood content, and showed no optimal wood content for specific wear rate under the experimental conditions used in this work. Again, the effect was more evident for higher sliding distances (1.5–2.0 km). The wear behavior for WPVC composites with HB were different from those with XK and MI woods, because the HB wood had a much lower aspect ratio and tended to give relatively low reinforcement [30–33] as opposed to XK and MI woods. It should be stressed again that the differences in the properties of WPVC composites found in this work were not caused by the differences in chemical functional groups on the wood surfaces.

3.4. Effect of glass fiber reinforcement Table 3 shows the physical and mechanical properties of WPVC composites without and with 10 phr glass fiber reinforcement for three different types of wood particles and various wood contents. The results clearly indicate that the addition of 10 phr glass fiber into WPVC composites greatly improved the flexural modulus and strength – by 52–129% and 21–93%, respectively, depending on the type and content of wood flour added. However, the hardness and roughness of the WPVC composites hardly changed with the addition of 10 phr glass fiber, due to the embedding action of wood particles within the PVC matrix.

Fig. 5a and b shows a comparison of specific wear rates for WPVC composites at a sliding distance of 0.5 km, without and with 10 phr glass fiber reinforcement; and Fig. 5c and d illustrates the specific wear rate results for a sliding distance of 2.0 km for WPVC composites without and with 10 phr glass fiber reinforcement. The results clearly indicate that the addition of glass fiber resulted in an improvement of the wear resistance (reduced specific wear rate) of WPVC composites, the effect being more pronounced for a sliding distance of 2.0 km. Among the three wood types used, HB appeared to give the lowest specific wear rate for the glass fiber reinforced WPVC composites. This result was different from that for the WPVC samples without glass fiber given in Fig. 3, which indicated that XK gave the lowest specific wear rate. The decreased specific wear rate of the glass fiber reinforced WPVC specimen at 2.0 km in Fig. 5d is supported by SEM micrographs showing the appearances of the glass fiber on the worn WPVC surface with the addition of 40 phr XK wood to resist the abrasive forces exerted (see Fig. 6). This seems to confirm that the addition of glass fiber can enhance the mechanical properties and wear resistance of wood/PVC composites. 4. Conclusions The experimental results suggested that the differences in the mechanical properties and wear behavior of WPVC composites found in this work were associated with the aspect ratio and constituents of the wood particles, and the initial wood hardness

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(a) without glass fiber at 0.5 km

(b) with glass fiber at 0.5 km

(c) without glass fiber at 2.0 km

(d) with glass fiber at 2.0 km

Fig. 6. SEM micrographs for worn surfaces of WPVC composites with 40 phr XK wood flour at two different sliding distances: (a) without glass fiber at 0.5 km, (b) with glass fiber at 0.5 km, (c) without glass fiber at 2.0 km, (d) with glass fiber at 2.0 km.

values of the three woods used. The hardness of WPVC composites did not change with increasing wood content, whereas the flexural properties of WPVC composites tended to increase with wood content up to 40 phr. Mangifera indica Linn. gave the WPVC with highest flexural properties. The wood particles with high aspect ratio (Xylia kerrii Craib & Hutch., and Mangifera indica Linn.) acted as reinforcement by reducing the specific wear rate of PVC. The optimum content of wood particles for highest wear resistance was dependent on the constituents of wood, aspect ratio and sliding distance. Xylia kerrii Craib & Hutch. at 40 phr gave the lowest specific wear rate for WPVC composites at a sliding distance of 2.0 km, whereas Hevea brasiliensis Linn. gave the lowest specific wear rate for the glass fiber reinforced WPVC composites. The addition of glass fiber increased the flexural modulus and strength by 52– 129% and 21–93%, respectively, as well as the wear resistance. The effect of glass fiber was more pronounced at a sliding distance of 2.0 km. The longer the sliding distance, the greater the specific wear rate of the WPVC composites, for WPVC samples both with and without E-glass fiber. Acknowledgments The authors thank the Thailand Research Fund (TRF Senior Research Scholar; RTA5280008) and V.P. Wood Co., Ltd., for financial support throughout this work. Sincere thanks are expressed to the Office of the Higher Education Commission (OHEC) under the National Research University (NRU) Program. References [1] Kim SS, Yu HN, Hwang IU, Lee DG. Characteristics of wood–polymer composite for journal bearing materials. Compos Struct 2008;86:279–84. [2] Tungjitpornkull S, Chaochanchaikul K, Sombatsompop N. Mechanical characterization of E-chopped strand glass fiber reinforced wood/PVC composites. J Thermo Compos Mater 2008;50:535–50.

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