Mechanical and Morphological Properties of Wood Plastic Biocomposites Prepared from Toughened Poly(lactic acid) and Rubber Wood Sawdust (Hevea brasiliensis)

Journal of Bionic Engineering 11 (2014) 630–637

Mechanical and Morphological Properties of Wood Plastic Biocomposites Prepared from Toughened Poly(lactic acid) and Rubber Wood Sawdust (Hevea brasiliensis) Nawadon Petchwattana1, Sirijutaratana Covavisaruch2 1. Division of Polymer Materials Technology, Faculty of Agricultural Product Innovation and Technology, Srinakharinwirot University, Sukhumvit 23, Wattana, Bangkok 10110, Thailand 2. Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand

Abstract The present research aims to utilize the acrylic Core-Shell Rubber (CSR) particles to reduce the brittleness in Wood Plastic Composites (WPC) prepared from poly(lactic acid) (PLA) and rubber wood sawdust (Hevea brasiliensis). Experimental works consisted of two major parts. The first part concentrated on toughening PLA by using CSR particles. Mechanical tests revealed that PLA had become tougher with a more than five times increment in the impact strength when the CSR was added at only 5 wt%. The modified PLA was less stiff with the significant reductions of both elastic and flexural moduli and strengths. The second part focused on producing WPC from the toughened PLA and rubber wood sawdust. The tensile moduli and the strengths of the PLA composites increased with rubber wood content. The composites turned out to be more brittle with reductions of both the impact strength and the tensile elongation at break at all the sawdust contents. Toughening PLA/wood flour with 5 wt% CSR improved both the impact strength and the tensile elongation at break. The toughness enhancement was also depicted by the plastic deformation observed on the surfaces of fractured PLA/CSR/wood sawdust composites. Keywords: wood plastic composites, rubber toughening, poly(lactic acid), rubber wood sawdust Copyright © 2014, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(14)60074-3

1 Introduction Wood Plastic Composites (WPC) is widely known and used as building material such as railings, fences, park benches, window and door frames and indoor furniture over the last two decades[1–3]. WPC offers various benefits such as good mechanical properties, dimensional stability, long service life, low cost as well as less deterioration than that suffered by natural wood[4–7]. Wood sawdust is often utilized as filler in the WPC production because it is cheap, environmental friendly, abundantly available and less abrasive to processing machine[6–9]. Although reinforcing thermoplastics with wood flour has been known to improve stiffness and strength, it also reduces the toughness of the composites especially those with a brittle matrix such as poly(lactic acid) (PLA)[10–12]. At high wood flour content, the PLA Corresponding author: Nawadon Petchwattana E-mail: [email protected]

composites not only tend to become highly susceptible to moisture absorption but also suffer from drastic decrements of the impact resistance[9,13–15]. Numerous reports indicated that the PLA-based WPCs became brittle even when the wood sawdust was added at only small amount. Huda et al.[10] found that the PLA was stiffer and more brittle when wood flour was added at only 20 wt%. The impact strength of the PLA composites dropped by around 20% when kenaf fiber was added by 20 wt%[15]. Moreover, the incorporation of 10 wt% bamboo fiber to PLA was found to decrease the tensile elongation at break by around 50%[13]. Our previous work reported successful toughening of PLA by modifying with rubber particles[16]. Both the tensile elongation at break and the impact strength were raised by 40 times and 5 times respectively when the ultrafine fully vulcanized acrylate rubber was added at 10 wt%[16]. Jaratrotkamjorn et al.[17] found 10% increment in

Petchwattana and Covavisaruch: Mechanical and Morphological Properties of Wood Plastic Biocomposites Prepared from Toughened Poly(lactic acid) and Rubber Wood Sawdust (Hevea brasiliensis) 631

notched Izod impact strength when blending PLA was with 10 wt% natural rubber. Grafting natural rubber with methacrylate was found to improve the impact strength of PLA by 175%[18]. Modifying PLA with acrylic core-shell rubber was reported to increase the impact strength from 17 J·m−1 to 65 J·m−1 when the rubber was added at 10 wt%[19]. Thus, elastic material like rubber, especially acrylic-based rubber, seemed to be an effective toughening agent for PLA-based WPC. The present research aims to utilize Core-Shell Rubber (CSR) particle to reduce the problem of brittleness in the WPC prepared from PLA and rubber wood sawdust (Hevea brasiliensis). The experimental work consisted of two major parts. The first one concentrated on the modification of PLA by CSR to obtain toughened PLA compositions. In this part, PLA was blended with CSR at 0 wt%–10 wt% in order to select the most appropriate PLA/CSR composition for subsequent preparation of the WPC in the second experimental part, which emphasized on the influences of the rubber wood contents from 0 wt% to 30 wt% on the mechanical properties and the morphological characteristics of the toughened PLA.

2 Experimental 2.1 Materials Extrusion grade of PLA (NatureWorks® PLA2003D) with a density of 1.24 g·cm−3 and a melting point of 165 ˚C was used as polymer matrix due to its biodegradability and commercial availability. Fine particles of CSR (ParaloidTM BPM-520, Dow Chemicals Company) were added to PLA and PLA/rubber wood composites for the purpose of toughening. The bulk density and the average particle size of CSR are 0.4 g·cm−3 and 2.5 micrometer respectively. The rubber wood (Hevea brasiliensis) sawdust was obtained from the waste from a sawmill in Ayutthaya, Thailand. The wood flour was further grinded by a ball mill and sieved to the top cut sizes of 500 m. 2.2 Toughening PLA by CSR particles Firstly, the PLA pellets and the CSR impact modifier, ranging from 0 wt% to 10 wt%, were dry-blended using a high speed mixer (Thermo, PRISM Pilot 3) at the speed of 700 rpm for 30 s. Table 1 shows the blend formulations of PLA and CSR particles. To toughen PLA, each mixture was then melt-blended in

Table 1 Formulations of the PLA/rubber wood sawdust and the PLA/CSR/rubber wood sawdust composites

Pure PLA

100

0.0

Rubber wood sawdust content (wt%) 0.0

PLA0.5CSR

99.5

0.5

0.0

PLA1.0CSR

99.0

1.0

0.0

PLA2.5CSR

97.5

2.5

0.0

PLA5.0CSR

95.0

5.0

0.0

PLA7.0CSR

93.0

7.0

0.0

PLA10CSR

90.0

10

0.0

PLA5W

95.0

0.0

5.0

PLA10W

90.0

0.0

10

PLA20W

80.0

0.0

20

PLA30W

70.0

0.0

30

Formulation code

PLA content Core shell rubber (wt%) content (wt%)

PLA5CSR5W

90.0

5.0

5.0

PLA5CSR10W

85.0

5.0

10

PLA5CSR20W

75.0

5.0

20

PLA5CSR30W

65.0

5.0

30

the range of 165 ˚C to 190˚C by a twin screw extruder (Labtech Engineering, LTE20-40). Each of the PLA/CSR compositions was processed using an injection molding machine (Manumold) at 190 ˚C to 200 ˚C under a pressure of 70 bar to prepare standard specimens for subsequent tests and characterizations. 2.3 Preparation of the toughened PLA/rubber wood sawdust composite Based on the impact strength and the tensile elongation at break, the most appropriate PLA/CSR composition was selected among those prepared in section 2.2. Then it was dry-blended and melt-mixed with rubber wood sawdust ranging from 5 wt% to 30 wt% using identical conditions and processing equipment as described in section 2.2. The properties of the unmodified PLA/rubber wood sawdust were also investigated to compare its toughness with those modified with CSR. All the formulations for the PLA/rubber wood and the PLA/CSR/rubber wood composites are listed in Table 1. 2.4 Testing and characterization 2.4.1 Mechanical tests The tensile test of the PLA/CSR and the toughened PLA/rubber wood composites was conducted according to ASTM D 638 by a Universal Testing Machine (INSTRON, 5567). The dimension of each dumbbell test specimen is 165 mm×13 mm×3 mm, the gauge length is 25 mm. The tensile test was performed at a crosshead

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speed of 5 mm·min−1 and a pre-load of 5 N. Rectangular specimens with length of 96.0 mm, width of 12.7 mm and thickness of 3.0 mm were also injected for the flexural test. A three-point bending test with a support span of 48 mm was carried out at room temperature by a Universal Testing Machine (INSTRON, 5567) and following the procedure of ASTM D790. The crosshead speed was set at 1.2 cm·min−1. The impact test was conducted using an Izod impact tester (Yasuda, 190) following the procedure of ASTM D256. All of the impact test specimens were notched at an angle of 45˚. All of the tensile, the flexural and the impact properties were averaged from the results obtained from five tested samples. 2.4.2 Microscopic observation The impact cryo-fracture surfaces of the PLA, PLA/CSR and PLA/CSR/rubber wood composites were observed microscopically by a Field Emission Scanning electron microscope (FE-SEM, Hitachi 4700e) at an accelerating voltage of 2 kV. The fractured surface of each specimen was mounted on an aluminum stub and sputter coated with a thin layer of gold approximately 300 Angstrom in thickness before microscopic observation.

3 Results and discussion 3.1 Toughening of PLA by CSR particles To improve the toughness of PLA, the toughening agent must be dispersed uniformly as small domains with good interfacial adhesion to the matrix. The toughening agent must also be stable under the processing conditions. Only when most of these criteria are fulfilled the toughening agent can effectively improve the overall toughness of the PLA[16]. Table 2 demonstrates the mechanical properties of the CSR-toughened PLA. Generally, PLA became less stiff and tougher

when CSR was added. The addition of only 5wt% CSR increased the impact strength of the PLA by more than fivefold. The increment indicates that the properties of PLA have been raised compared with that of polypropylene (PP)[19,20]. Beyond 5 wt%, the impact strength still increased but it did at lower rate. These increments of the impact resistance were believed to have been induced by the good interfacial adhesion between the CSR and the PLA coupled with the rather well dispersion of the CSR particles throughout the PLA matrix. As a consequence, the rather well-adhered elastic CSR particles helped absorbing the impact energy and consequently retarded the PLA rupture. The impact test results are in good agreement with earlier reports on the rubber-modified PLA[16–18,21,22]. At identical rubber content, CSR was found to exhibit a higher impact strength than those of natural rubber[18], acrylate rubber[16], natural rubber grafted with poly(butyl acrylate)[21] and poly(ether)urethane elastomer[22]. It has been well documented that the addition of an elastic material, such as rubber, to a polymer matrix generally leads to a drop of modulus and strength[17]. The tensile modulus and the strength of the PLA/CSR blends were lower than that of the neat PLA due to the lower rigidity of CSR. However, its tensile strength at 45.31 MPa for the blend containing 10 wt% CSR was still larger than those of commodity polymers such as polyethylene (PE) and PP[23]. The tensile elongation at break of PLA was improved significantly with the increase in CSR content, it surged from 3.08% for neat PLA to 16.1% for the blend containing 5 wt% CSR. Under bending force, the PLA blended with CSR also softened, as was indicated by the decline of the flexural modulus and the flexural strength. The flexural modulus of neat PLA was 3.38 GPa with an associated flexural strength of 89.48 MPa. With

Table 2 Formulations of the PLA/rubber wood sawdust and the PLA/CSR/rubber wood sawdust composites Formulations

Tensile

Impact strength −1

Flexural

(J·m )

Modulus (GPa)

Strength (MPa)

elongation at break (%)

Modulus (GPa)

Strength (MPa)

Pure PLA

20.95±2.94

2.78±0.79

66.43±3.09

3.08±0.77

3.38±0.98

89.48±2.70

PLA0.5CSR

39.21±1.98

2.60±0.37

61.10±2.97

5.83±1.09

2.89±0.34

79.22±1.87

PLA1.0CSR

68.34±3.89

2.55±0.89

58.84±3.21

10.0±5.89

2.68±0.31

73.14±1.59

PLA2.5CSR

93.49±6.98

2.29±0.81

54.02±3.82

13.3±2.80

2.53±0.28

69.41±3.29

PLA5.0CSR

119.3±6.97

2.19±0.48

50.95±1.33

16.1±5.39

2.48±0.73

66.31±3.23

PLA7.0CSR

132.6±7.85

2.09±0.68

48.39±2.89

19.3±6.94

2.40±0.56

65.23±4.73

PLA10CSR

148.1±8.99

2.02±0.05

45.31±1.49

22.9±6.77

2.33±0.70

61.57±3.60

Petchwattana and Covavisaruch: Mechanical and Morphological Properties of Wood Plastic Biocomposites Prepared from Toughened Poly(lactic acid) and Rubber Wood Sawdust (Hevea brasiliensis) 633

the increase in CSR content, both the flexural modulus and the flexural strength dropped, reaching their minimum values at 2.33 GPa and 61.57 MPa respectively when the CSR content reached 10 wt%. Microscopic observation by SEM was employed to observe the fracture morphology of the blends, as illustrated in Fig. 1. The micrograph in Fig. 1a illustrates that PLA fractured in a brittle fashion as demonstrated by its smooth fracture surface. With greater CSR content, the fracture surfaces of the PLA blends showed more localized plastic deformations induced by the CSR. In Figs. 1b–1g, CSR particles were dispersed and embedded quite well within the PLA matrix. Fig. 1h confirmed a good interfacial adhesion between the PLA matrix and the CSR particles, leading to significant improvement of the toughness when CSR was incorporated. Base on the mechanical test results, 5 wt% of CSR seemed to be the most appropriate composition for toughening the PLA matrix for the preparation of the WPC. The 5 wt% CSR was selected as there was no remarkable difference between its toughness (in terms of impact strength and (a) Pure PLA

(e) PLA5.0CSR

10 m (b) PLA0.5CSR

the elongation at break) with the PLA modified by 7 wt% and 10 wt% CSR. 3.2 Effects of rubber wood sawdust on the properties of toughened PLA 3.2.1 Mechanical properties Fig. 2 shows the tensile properties of PLA/rubber wood sawdust and PLA/CSR/rubber wood sawdust composites. The PLA/rubber wood composites failed in brittle mode. Both the tensile modulus and the tensile strength increased as plotted in Figs. 2a and 2b. The PLA/CSR/rubber wood sawdust composites showed a rather similar manner but at lower rate. The tensile moduli of the PLA/rubber wood sawdust and the PLA/CSR/rubber wood sawdust composites increased rapidly with the rubber wood content. Reinforcing PLA and PLA/CSR with the wood sawdust clearly made the composites stiffer and more brittle, this behavior was observed in other WPCs such as PP/Meranti sawdust composites[7], polylactide/pine wood composites[24] and poly(vinyl chloride)/Balau sawdust composites[25]. In comparison, toughening PLA with 5 wt% CSR made the PLA/rubber wood composites less stiff. This was

10 m (f) PLA7.0CSR

10 m (c) PLA1.0CSR

10 m (g) PLA10CSR

10 m

10 m (h) PLA5.0CSR at high magnification

(d) PLA2.5CSR

CSR particles 10 m

1 m

Fig. 1 SEM micrographs of (a) pure PLA and CSR modified-PLA with (b) 0.5 wt%, (c) 1.0 wt%, (d) 2.5 wt%, (e) 5.0 wt%, (f) 7.0 wt% and (g) 10 wt% of CSR.

Fig. 2 Tensile properties of PLA/rubber wood sawdust and PLA/CSR/rubber wood sawdust composites. (a) Tensile modulus, (b) tensile strength and (c) tensile elongation at break.

reflected in the reduction of the tensile modulus by around 25%, 27%, 33% and 64% for the composites reinforced with 5 wt%, 10 wt%, 20 wt% and 30 wt% wood sawdust respectively. With the presence of wood sawdust, the composites became more brittle, as was indicated by the decrements in the tensile elongation at break shown in Fig. 2c. Both of the PLA/rubber wood sawdust and the PLA/CSR/rubber wood sawdust composites suffered a drop in the elongation at break with the increase in rubber wood content. For the PLA/rubber wood composites, the tensile elongation at break decreased from 3% found in the neat PLA to less than 1% when the rubber wood sawdust was added at the maximum content of 30 wt%. Under tension, the rubber wood sawdust not only restricted the PLA chain disentanglement but also obstructed the molecular mobility of the PLA chains[10]. Nevertheless, the tensile elongation at break exhibited relatively small changes when the content of rubber wood sawdust varies between 20 wt% and 30 wt%. Modifying the PLA/rubber wood sawdust composites with CSR tended to improve the elongation at break due to its highly elastic behavior. This behavior had often been found in the elastomer-modified WPC[9,17,21,22,26]. Park and Balatinecz[26] indicated that the addition of wood fiber raised the tensile modulus and strength of PP whereas the addition of Ethylene Propylene Diene Monomer (EPDM) rubber decreased these properties. The tensile elongation at break was found to increase with the EPDM content and was adversely reduced by the addition of wood fibers. The flexural properties of the PLA/rubber wood sawdust and the PLA/CSR/rubber wood sawdust biocomposites are shown in Fig. 3. As with tensile modulus, the flexural modulus was largely dependent on the wood sawdust content in the composites. In general, the flexural modulus was raised with the increase in wood fiber concentration. The flexural modulus of the PLA/rubber wood increased from 3.4 GPa to 7.9 GPa when the wood sawdust was added at 30 wt%. The increment was presumably due to the inclusion of the rigid wood particles. For the PLA/CSR containing 30 wt% rubber wood, the flexural modulus increased from 2.4 GPa to 5.2GPa. The flexural strength in Fig. 3b, exhibited a similar trend to that found in the flexural modulus. Moreover, the flexural strength was raised from 89 MPa to 190 MPa when the rubber wood sawdust was added at 30 wt%. The rubber wood particles acted as rigid filler responsible for the

Flexural modulus (GPa)

Journal of Bionic Engineering (2014) Vol.11 No.4

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634

Fig. 3 Flexural properties of PLA/rubber wood sawdust and PLA/CSR/rubber wood sawdust composites. (a) Flexural modulus and (b) flexural strength.

increase in the stiffness of the PLA. This behavior was expected, based on the rule of mixtures, as the modulus of the wood particles was much greater than that of the PLA matrix. The addition of 5 wt% CSR to the PLA/rubber wood composites reduced the flexural strength from 190 MPa to 130 MPa. Fig. 4 illustrates the effects of rubber wood sawdust content on the energy required to propagate a crack in the PLA/rubber wood sawdust and the PLA/CSR/rubber wood sawdust composites. The notched impact strength of PLA/rubber wood composites decreased slightly from 21 J·m−1 of neat PLA to 17 J·m−1, 13 J·m−1, 10 J·m−1 and 8 J·m−1 when the rubber wood sawdust was added at 5 wt%, 10 wt%, 20 wt% and 30 wt% respectively. This was anticipated as the addition of high wood sawdust content rendered greater possibilities of the wood sawdust to form agglomerates and more pull-outs of wood particles (Figs. 5c and 5d). In comparison, the addition of 5 wt% CSR impact modifier elevated the impact strength of the composites by 299%, 215%, 117% and 134% when the wood sawdust was added at 5 wt%, 10 wt%, 20 wt% and 30 wt% respectively. The apparent rise of the impact strength clearly reflected the toughening ability of the CSR, the rubbery CSR particles had helped to absorb a large amount of the energy upon impact. The CSR was believed to have played an important role in inducing craze formation near the crack tip which retarded the specimen rupture[16]. Similar trends of toughening enhancement were observed

Petchwattana and Covavisaruch: Mechanical and Morphological Properties of Wood Plastic Biocomposites Prepared from Toughened Poly(lactic acid) and Rubber Wood Sawdust (Hevea brasiliensis) 635 (a) PLA5W

120

(c) PLA20W

Remnant of wood particle pulled-out

PLA/rubber wood Agglomerations

PLA/CSR/rubber wood 100

Good interfacial adhesion

Remnant of wood particle pulled-out

80

500 m

60

(b) PLA10W

500 m (d) PLA30W

Wood particle breakage

Remnant of wood particle pulled-out Agglomerations

40 Good interfacial adhesion

20

Remnant of wood particle pulled-out

0

5

10 15 20 25 Rubber wood flour content (wt%)

30

Good interfacial adhesion 500 m

500 m

Fig. 5 Fracture surfaces of the unmodified PLA/rubber wood sawdust composites at various wood sawdust contents.

Fig. 4 Impact strengths of PLA/rubber wood sawdust and PLA/CSR/rubber wood sawdust composites. (a) PLA5CSR5W

in other WPC toughening systems. Huda et al.[27] improved the impact strength of PLA/talc/newspaper fiber composites by treating talc with silane coupling agent. They achieved 17% increment of the impact strength when silane was added at 10 wt%. Wang et al.[28] reported that maleic anhydride grafted ethylene glycedyl methacrylate (MA-g-EGMA) toughened PLA/wood fiber composites by a 20% increase in impact strength. With the presence of 5 wt% Styrene Butadiene Styrene block copolymer (SBS), the impact strength of PLA-based WPC with 30 wt% wood fiber was reported to have increased by 20%[29]. 3.2.2 Microscopic observation SEM micrographs of the impact fractured surfaces of the PLA/rubber wood sawdust and the PLA/CSR/rubber wood sawdust composites are shown in Figs. 5 and 6 respectively. In Fig. 5, some remnants of wood particle pull-out were observed. This indicated some degree of interfacial adhesion between the PLA matrix and the rubber wood particles, which at some places, was stronger than the wood particle. The interfacial action between the PLA matrix and the rubber wood sawdust could be due to the formation of hydrogen bonding between the hydroxyl groups of the rubber wood sawdust and the terminal hydroxyl group[30–32], the carboxylic group[30,33] or the carbonyl group of the ester groups of PLA[30]. Moreover, some agglomerations were observed (Fig. 5d) at high rubber wood sawdust contents, i.e. 20 wt% and 30 wt%.

(c) PLA5CSR20W Good interfacial adhesion Remnant of wood particle pulled-out

Agglomeration Remnant of wood 500 m particle pulled-out (b) PLA5CSR10W Good interfacial adhesion

500 m (d) PLACSR30W

Agglomeration

Remnant of wood particle pulled-out

500 m

Remnant of wood particle pulled-out

500 m

Fig. 6 Rough fracture surfaces of the PLA/CSR /rubber wood sawdust composites at various wood sawdust contents.

As indicated in Fig. 6, the fracture surfaces of the CSR-modified WPC appeared rougher with more localized plastic deformation, which contributed to the improvement of toughness. Some wood particles had been pulled-out, while some remained as agglomerates particles in the composites with high content of wood sawdust. Large agglomerates of wood sawdust led to the reduction of the impact strength and the tensile elongation at break. The morphologies of PLA/CSR/rubber wood sawdust in Figs. 6b and 6c were further magnified to observe the interfacial adhesion between the CSR particles and the PLA matrix, as exhibited in Figs. 7a and 7b respectively. The CSR particles were found to have embedded and adhered well in the PLA matrix, this was the key parameter to effective toughening.

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Xu K, Li K, Tu D, Zhong T, Xie C. Reinforcement on the mechanical-, thermal-, and water-resistance properties of the wood flour/chitosan/poly(vinyl chloride) composites by physical and chemical modification. Journal of Applied Polymer Science, 2014, 131, 9185–9191.

Fig. 7 Highly magnified micrographs showing CSR embedded in the PLA/CSR/rubber wood sawdust composites. (a) PLA/CSR with 10 wt% wood sawdust and (b) PLA/CSR with 20 wt%.

[4]

Tanjung F A, Husseinsyah S, Hussin, K. Chitosan-filled polypropylene composites: The effect of filler loading and organosolv lignin on mechanical, morphological and thermal properties. Fibers and Polymers, 2014, 15, 800–808.

4 Conclusion PLA was toughened by CSR, as was reflected by the more than five times increment in the impact strength. The CSR-modified PLA possessed lower stiffness with the reductions of both modulus and strength. Thus PLA/5 wt% CSR was selected as the matrix for the production of WPC with rubber wood sawdust as reinforcement. With the presence of CSR in the PLA/rubber wood sawdust composites, the impact strength and the elongation at break of the composites were improved significantly. Microscopic observation of the fracture surfaces of PLC to PLA/rubber wood sawdust composites illustrated some remnant sites of wood particle pull-out from the matrix. The interfacial bonding between the wood flour and the PLA matrix was believed to have arisen from the formation of hydrogen bonding between the hydroxyl groups in the rubber wood and the terminal hydroxyl group, carboxylic groups or carbonyl groups of the ester groups in the PLA. The fracture surfaces of the composites with CSR appeared rougher with larger amount of localized plastic deformation, which contributed to the improvement of toughness.

Acknowledgment The authors acknowledge the research grant provided by Srinakharinwirot University (contract no. 005/2558) and supports from Mr. Kittisak Promsuk.

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