The effect of alkali pretreatment on mechanical and morphological properties of tropical wood polymer composites

Materials and Design 33 (2012) 419–424

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The effect of alkali pretreatment on mechanical and morphological properties of tropical wood polymer composites Md. Saiful Islam a,⇑, Sinin Hamdan a, I. Jusoh b, Md. Rezaur Rahman a, Abu Saleh Ahmed a a b

Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia Faculty of Science, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia

a r t i c l e

i n f o

Article history: Received 13 January 2011 Accepted 21 April 2011 Available online 1 May 2011 Keywords: E. Mechanical G. Scanning electron microscopy G. X-ray analysis

a b s t r a c t In this study, mechanical and morphological properties of wood polymer composites (WPCs) from five kinds of selected tropical light hardwoods namely Jelutong (Dyera costulata), Terbulan (Endospermum diadenum), Batai (Paraserianthes moluccana), Rubberwood (Hevea brasiliensis), and Pulai (Alstonia pneumatophora) were investigated. Methyl methacrylate (MMA) and styrene (ST) vinyl monomer mixture (50:50; volume:volume) was used in preparation of WPCs. Before being impregnated with an MMA/ST monomer mixture, wood species were chemically pretreated with 5% sodium hydroxide (NaOH) solution for the reduction of hydrophilic hydroxyl groups and impurities from the cellulose fibre in wood and to increased adhesion and compatibility of wood fibre to polymer matrix. Monomer mixture (MMA/ST) was impregnated into raw wood and NaOH pretreated wood specimens to manufacture wood polymer composite (WPC) and pretreated wood polymer composite (PWPC). Mechanical tests and microstructural analysis were conducted. Comparison has been made among the properties of raw wood, WPC and PWPC. The result reveals that PWPC yielded better mechanical and morphological properties compared to WPC and raw wood. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Structural wood is the most preferred building and construction material due to its high physical strength, low processing cost and aesthetically pleasing character. But the physical and mechanical properties of wood are readily deteriorated by environmental variation which is the main drawback and limit of its properties [1]. These troublesome inherent properties of wood can be minimized by appropriate chemical treatment such as the formation of wood polymer composite (WPC) [2]. Recently, wood has been treated with a variety of chemicals such as styrene, epoxy resins, urethane, phenol formaldehyde, methyl methacrylate (MMA), vinyl or acrylic monomers to improve its physical, mechanical and biological properties [3–5]. The physical and mechanical properties of wood can also be significantly improved by the impregnation of vinyl monomer mixture [6]. Thermosetting and thermoplastic monomers have been widely used and achieved certain improvements in wood properties, but both showed limitations [7]. Thermosetting-related polymer such as phenolic resins, urea–formaldehyde and melamine–formalde⇑ Corresponding author. Address: Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia. Tel.: +60 14 992251; fax: +60 82 583410. E-mail addresses: [email protected], [email protected] (Md. Saiful Islam). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.04.044

hyde shows improvement in compressive strength properties and moisture-related shrinking and swelling behaviours. However, wood polymer composite treated with these type polymers may be more brittle, and display only marginal improvement in morphological properties. Thermoplastic type monomers such as acrylate or methacrylates, for case in point, do not improve dimensional stability. It has been established that the monomer and its mixture does not form bonds with hydroxyl groups of the cellulose fibres. They simply bulk the void spaces within the wood structure [8]. Since most of the vinyl monomers are non-polar, there is little interaction between these monomers and the hydroxyl groups of the cellulose fibre. Poor chemical and physical interfacial interactions between the wood surface and chemical are two of the most important mechanisms of bond failure [9]. Therefore, the polymer component of the WPC simply bulks the wood structure by filling the capillaries, vessels and other void spaces within the wood. Therefore, one can deduce that if there was the formation of chemical bonds between the impregnated monomers and the hydroxyl groups on the cellulose fibres, the physical and mechanical properties of WPC could be further improved. It has been noted that the better adhesion and interaction between hydrophilic wood fibres and polymer can be improved by pretreatment using varieties of chemicals and reagents such as alkoxysilane coupling agents, diazonium salt, sodium perchlorate, dinitrophenyl hydrazine (DNPH), sodium periodate [8,10–13].

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However, sodium hydroxides (NaOH) are widely used to modify the interface between dissimilar materials such as raw fibres and thermoplastic or thermosetting polymer [14,15]. Alkaline sodium hydroxide removes natural fats and waxes from the cellulose fibre surface thus revealing chemically reactive functional groups like hydroxyl groups. The removal of the surface impurities from the cellulose fibres also improves the surface roughness of the fibres or particles thus opening more hydroxyl groups and other reactive functional groups on the surface [16]. Sodium hydroxide also reacts with accessible –OH groups according to the following proposed chemical reaction [15–17]. 

All oven dried raw wood specimens were placed in a 5% NaOH solution in a reaction vessel at room temperature for 6 h. Specimens were then removed and thoroughly rinsed in distilled water several times after which a few drops of acetic acid were added just to neutralize excess NaOH that could otherwise continue to degrade the wood cell wall components. The extracted wood samples were then oven dried at 100 °C for 24 h. 2.4. Manufacturing of wood polymer composites

þ

Cellulose  OH þ NaOH ! Cellulose  O Na þ H2 O þ impurities

2.3. Pretreatment of wood specimens

ð1Þ

In this study, the effects of alkali pretreatment on some mechanical and morphological properties of selected tropical WPCs in Malaysia have been investigated. Five species of selected Malaysian tropical light hardwood were utilized as starting materials, keeping in mind that they are easily obtainable in the local forests. Major drawback of using these species is their high moisture uptake, and physical and mechanical properties that change with environmental factors. Hydroxyl groups are intrinsic groups of cellulose, that are responsible for the water uptake, a negative characteristic to this purpose, but these groups are responsible for general characteristics of the wood. In order to overcome this problem and to improve the adhesion and compatibility of polymer to the cellulose of wood, the wood samples were chemically pretreated with a 5% alkaline NaOH solution and then impregnated with an MMA/ST monomer mixture to manufacture pretreated wood polymer composite (PWPC). Therefore, the present study dealt with mechanical and morphological properties of WPC pretreated with NaOH. 2. Experimental 2.1. Wood materials Five kinds of tropical wood species, namely Jelutong (Dyera costulata), Terbulan (Endospermum diadenum), Batai (Paraserianthes moluccana), Rubberwood (Hevea brasiliensis), and Pulai (Alstonia pneumatophora) were selected in this study. The densities of these tropical wood species are 380, 450, 455, 480 and 650 kg/m3 for Batai, Jelutong, Pulai, Terbulan, and Rubberwood respectively. Generally these wood species are porous and contain cellulose (40–44%), lignin (18–25%) and hemicelluloses (15–35%). Other polymeric constituents present in lesser and often varying quantities are starch, pectin, and ash for the extractive-free wood. The internal properties of these tropical woods are high vessel diameter (90– 340 lm), medium of the number of vessel present per unit area (1–10%), high fibre length (800–1800 lm), and medium cell wall thickness. These wood species were felled and cut into three bolts of 1.2 m length and subsequently conditioned to air-dry in a room with relative humidity of 60% and ambient temperature of around 25 °C for 3 month prior to testing. The planks were ripped and machined to 300 mm (L)  20 mm (T)  20 mm (R) and 100 mm (L)  25 mm (T)  25 mm (R) specimens for three point bending and compression parallel to the grain test.

For WPC and PWPC manufacturing, raw wood and pretreated wood oven-dried samples were placed in an impregnation vacuum chamber at a vacuum pressure of 10 kPa for 10 min. The MMA/ST mixture was introduced into the chamber as the vacuum pressure was released. The samples were kept immersed in the monomer mixture solution for 6 h at ambient temperature and atmospheric pressure to obtain further impregnation. Samples were then removed from the chamber and wiped of excess impregnate. Samples were wrapped with aluminium foil and placed in an oven for 24 h at 105 °C for polymerization to take place. Weight percentage gain (WPG) of the samples was then measured using Eq. (1):

WPGð%Þ ¼ ½ðW i  W o Þ=W o   100

ð2Þ

where Wo and Wi are oven dried weights of raw wood and monomer mixture impregnated WPC samples respectively. 2.5. Microstructural analysis 2.5.1. Fourier transform infrared spectroscopy (FTIR) The infrared spectra of the raw and modified WPC specimens were recorded on a shimadzu fourier transform infrared spectroscopy (FTIR) 81001 spectrophotometer. The transmittance range of scan was 370–4000 cm1. 2.5.2. Scanning electron microscopy (SEM) The interfacial bonding between the cell wall polymer and monomer mixture were examined using a scanning electron microscope (SEM) (JSM-6701F) supplied by JEOL Company Limited, Japan. The specimens were first fixed with Karnovsky’s fixative and then taken through a graded alcohol dehydration series. Once dehydrated, the specimen was coated with a thin layer of gold before being viewed on the SEM. The micrographs, taken at a magnification of 500 and 1000, are presented in the Section 3. 2.5.3. X-ray diffraction (XRD) In order to assess the effect of alkali pretreatment on surface morphology of the WPC, XRD analysis was applied. 0 A Siemens D500 diffractrometer was used where Cu Ka (k = 1.54 A Å) radiation was employed with 2h varying between 4° and 80° at 5°/min. 2.6. Mechanical test 2.6.1. Bending and compression test In order to characterize mechanical properties of manufactured composites, bending and compression tests were carried out according to ASTM D-143 (1996) [18] using a Shimadzu Universal Testing Machine having a loading capacity of 300 kN. A cross head speed of 2 mm/min was used during the test.

2.2. Alkali and monomer solutions 2.7. Statistical analysis Chemicals used for pretreatment and WPC production were 5% NaOH, and methyl methacrylate/styrene (MMA/ST, 50:50, volume:volume) mixture containing 2% benzyl peroxide catalyst as a polymerization initiator.

The significant difference among raw wood, WPC and PWPC were evaluated by a computerized statistical program (SPSS) composed of analysis of variance (one way anova) and following Tukey

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tests at the 95% confidence level. Statistical evaluations were made on homogeneity groups (HG), of which different letters reflected statistical significance.

(a) 100

NaOH pretreated Wood

3. Results and discussion 3.1. Weight percentage gain (WPG%) The values of WPG for WPC and PWPC samples were measured and are given in Table 1. From Table 1, it can be seen that the WPG of PWPC samples were higher than the WPC samples. These results indicate that after alkali pretreatment, MMA/ST was more easily impregnated in all wood species. This is expected because NaOH reacts with OH groups of cellulose and also removes all impurities from the wood fibre surfaces thus increasing the adhesion and compatibility between wood fibres and polymer, resulting in higher WPG.

Transmittance (%)

80 1642

60

40

1053 1403

1508 1736 1636 1463 1427 1330 1262

3438 2917

1055

3407

Raw Wood

20

0.0 4000

3200

2400

1800

1400

1000

(b) 100

3.2. Microstructural analysis

PWPC

Transmittance (%)

80

3.2.1. Fourier transform infrared spectroscopy (FT-IR) The reactions of NaOH with cellulose in wood fibre yielded cellulose-ONa compound and removed impurities from the fibre surface. This is confirmed by the FTIR spectroscopic analysis as shown in Fig. 1a. The FTIR spectrum of the raw wood clearly shows the absorption band in the region of 3407 cm1, 2917 cm1 and 1736 cm1 due to OAH stretching vibration, CAH stretching vibration and C@O stretching vibration respectively. These absorption bands are due to hydroxyl groups in cellulose, carbonyl groups of acetyl ester in hemicellulose and carbonyl aldehyde in lignin [20]. After pretreatment with 5% NaOH, the characteristic peak at 2917 cm1, 1736 cm1 and 1636 cm1 fully disappeared and there is a slight shift of the AOH peak by about 31 cm1 to the high frequencies obtained. These characteristic peaks may be due to the removal of surface impurities and the formation of cellulose-ONa compound on the wood fibre surface [10,21]. Fig. 1b shows the FTIR spectra of WPC and PWPC. Comparing the FTIR spectra between WPC and PWPC it was found that there is a significant difference in their absorption bands. This is due to the differences of structural and chemical composition within WPC and PWPC. The FTIR spectrum of WPC shows the absorption band in the region of 3435 cm1, 2924 cm1 and 1736 cm1. These absorption bands also existed in raw wood. Therefore, it is clear that MMA/ST do not have the ability to remove these characteristic absorption bands responsible for functional groups from the wood fibre surfaces. On the other hand, no absorption band was found in these above regions for PWPC. This is due to the chemical pretreatment with the NaOH solution.

2271

1421 1265 1160 1629

60

1050

3438 2271 1736

40 2924

1423 1625 1243 1054 1375 1333 1162 1112

WPC

3435

20

0.0 4000

3200

2400

1800

1400

1000

Wood species

Composite type

WPG

St. dev.

Jelutong

WPC PWPC WPC PWPC WPC PWPC WPC PWPC WPC PWPC

29 50 19 38 37 57 13 32 25 44

2.17 2.52 1.7 2.0 2.2 2.53 1.2 1.5 1.3 2.4

Batai Rubberwood Pulai

Mean value is the average of 10 samples, (St. dev. = Stander deviation).

600 400

Wavenumbers (cm-1) Fig. 1. IR spectrum of (a) Raw wood and 5% NaOH pretreated wood and (b) WPC and PWPC.

3.2.2. X-ray diffraction (XRD) The X-ray diffraction patterns of raw wood, WPC and PWPC are given in Figs. 2–4, respectively. As seen in Fig. 2 the patterns of raw wood fibres exhibit three well defined peaks (2h) at 15.1°, 22.8° and 34.7°. The 15.1°, 22.8° and 34.7° reflections correspond to the (1 1 0), (2 0 0) and (0 2 3) or (0 0 4) crystallographic planes, respectively [22]. On the other hand, comparing all the X-ray diffractograms (Figs. 2–4) it is observed that there are some new peaks (2h) of various intensities in the amorphous region 40–75°. These peaks may be

Table 1 Weight percentage gain (WPG) of wood polymer composites (WPC) and NaOH pretreated wood polymer composites (PWPC).

Terbulan

600 400

Wavenumbers (cm-1)

Fig. 2. Typical X-ray diffraction patterns of raw wood.

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due to the incorporation of MMA/ST inside wood and the formation of wood composites. The diffraction patterns of WPC in (Fig. 3) exhibits four new diffraction peaks at 43.6°, 49.1°, 50.9°, and 72.7° whereas, the PWPC (Fig. 4) shows five new peaks of high and sharp intensity at 42.1°, 43.6°, 49.2°, 51.0°, and 72.6°. This result also indicates that the manufactured WPC and PWPC significantly increased the crystallinity of wood as seen by other researchers [23–26]. However, PWPC shows more crystallinity peaks than the raw wood and WPC as shown in (Fig. 4). Such a result is expected because the alkali pretreatment removed all impurities from the wood fibre surface thus increasing the impregnation of MMA/ST and the degree of polymerization inside the wood fibre [21].

Fig. 3. Typical X-ray diffraction patterns of wood polymer composite (WPC).

Fig. 4. Typical X-ray diffraction patterns of pretreated wood polymer composite (PWPC).

3.2.3. Scanning electron microscopy (SEM) Fig. 5i shows a number of void spaces and uneven layers in the raw wood fibre surface which is removable by the suitable chemical treatment [27]. Fig. 5ii depicts the micrograph of WPC while Fig. 5iii is that of PWPC. Fig. 5ii shows clean polymer stands throughout the wood fibres with remarkable gaps between this polymer and the cell walls, while Fig. 5iii shows no noticeable gaps and strong bonds between the polymer and the cell wall. Furthermore, fibrous cellulose material adhered to the surface of the polymer stands. It is thus deduced that the reaction of NaOH with wood cellulose had increased the adhesion and compatibility of the polymer to the cellulose fibres of the wood. 3.2.4. Three point bending test The modulus of elasticity (MOE) and modulus of rupture (MOR) of raw wood, WPC and PWPC were measured and results are given in Table 2. The MOE of WPC and PWPC were higher than those of their raw ones. From Table 2, it is worth noting that the MOE was more significantly affected by the NaOH pretreatment. This result is expected because NaOH reacts with cellulose in wood and enhances the adhesion and compatibility between wood fibres and the polymer resulting in improved MOE. The higher MOE of

Fig. 5. SEM photographs of (i) Raw wood, (ii) WPC and (iii) PWPC.

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Md. Saiful Islam et al. / Materials and Design 33 (2012) 419–424 Table 2 Static Young’s modulus of raw wood, wood polymer composites (WPC) and 5% NaOH pretreated wood polymer composites (PWPC). Wood species

Jelutong

Terbulan

Batai

Rubber

Pulai

Composite type

Untreated WPC PWPC Untreated WPC PWPC Untreated WPC PWPC Untreated WPC PWPC Untreated WPC PWPC

(raw)

(raw)

(raw)

(raw)

(raw)

MOE (GPa)

5.31 7.9 8.52 7.39 10.15 11.14 6.51 9.30 10 11.62 15.20 17.24 4.12 6 6.5

Static bending strength, at 10% MCa St. dev.

HGb

MOR (MPa)

St. dev.

HGb

0.44 0.39 0.49 1.15 0.39 0.40 0.57 0.48 0.40 1.05 0.34 0.32 1.80 0.26 0.53

A B C D E F G H I J K L M N O

46 62 72 60.5 77 88 55.4 73 83 105.8 130 150 37.8 50 58

5.72 3.02 5.60 2.46 5.29 6.86 2.86 4.64 7.31 2.57 1.63 2.03 1.81 7.11 6.61

A B C D E F G H I J K L M N O

Mean value is the average of 10 specimens. a Moisture content. b The same letters are not significantly different at a = 5%. Comparisons were done within the each wood species group. (St. dev. = Stander deviation, HG = Homogeneity Group).

both WPC and PWPC compared to the raw wood were due to the chemical modification and impregnation, which is in accordance with other researchers [28]. In the wood specimens, NaOH reacts with OH groups of cellulose fibre and yielded cellulose-ONa compound and removes the impurities from the wood fibre surfaces thus enhancing polymer loading and degree of polymerization which further increased the MOE of PWPC. It is also apparent from Table 2, the MOE of WPC and PWPC of the Jelutong wood were highest, followed by Pulai, Batai, Terbulan, and Rubberwood respectively. However, for Rubberwood, a small increase was found for its WPC and PWPC due to high density of this species and a little amount of MMA/ST incorporation inside the cell wall, as found by other researchers [29,30]. On the other hand, the MOR of PWPC was higher than those of WPC and raw wood, which is in agreement with previous research [31]. As one can see from Table 2 there was significant improvement of MOR of PWPC for all wood species. These results suggest that the pretreatment enhanced the interfacial bonding strength between wood fibres and polymer. The MOR of WPC and PWPC of Jelutong was highest followed by Pulai, Batai, Terbulan, and Rubberwood respectively. The WPC and PWPC of Rubber wood had lowest MOR observed because of its high density. This indicates that MOR also depends on the wood properties [19]. 3.2.5. Compression test analysis The compressive strength results for the raw wood, WPCs and PWPC were measured and are summarized in Table 3. From Table 3, it is seen that there was a significant increase in compressive strength for both WPC and PWPC of all species. One can also be seen that WPC and PWPC of all species exhibited much higher compressive strength than raw wood samples. These increments were 39.93–54.74% for WPC and 66.04–85.96% for PWPC. It is also apparent that PWPCs had higher (18–21%) compressive strength compared to WPCs. Of the five wood species used, the highest increases of compressive strength were observed in Jelutong followed by Pulai, Batai, Terbulan, and Rubberwood for both WPC and PWPC. Raw wood species failed in compression because of the bulking of relatively thin cell walls due to a long column type of instability. The chemical modification of raw wood puts a coating on the walls which thickens them, thus greatly increasing their lateral stability [32]. This is also expected because MMA/ST mixture has the ability to fill the void spaces and the strong branched

Table 3 Compressive strength of raw wood, wood polymer composites (WPC) and 5% NaOH pretreated wood polymer composites (PWPC). Wood species

Composite type

Jelutong

Untreated WPC PWPC Untreated WPC PWPC Untreated WPC PWPC Untreated WPC PWPC Untreated WPC PWPC

Terbulan

Batai

Rubber

Pulai

(raw)

(raw)

(raw)

(raw)

(raw)

Compressive strength (GPa), at 10% MCa Mean

St. dev.

HGb

2.85 4.41 5.30 3.82 5.50 6.50 3.58 5.20 6.30 2.68 3.75 4.45 2.42 3.70 4.40

0.57 0.33 0.13 0.63 0.29 0.20 0.83 0.30 0.38 0.83 0.26 0.20 1.17 0.21 0.28

A B C D E F G H I J K L M N O

Mean value is the average of 10 specimens. a Moisture content. b The same letters are not significantly different at a = 5%. Comparisons were done within the each wood species group. (St. dev. = Stander deviation, HG = Homogeneity Group).

polymeric situation inside wood, thus forming WPC with improved compressive strength. This enhances the lateral stability of the cell wall. The increase of compressive strength of WPC compared to raw wood was also reported by different researchers [8,29]. All the WPC and PWPC samples had increased in compression resistance. This finding is expected because the incorporation of polymer into the wood reduced the proportion of void spaces in the wood. Thus a greater force was required to deform the WPC specimens. However, the PWPC had the highest compressive strength compared to WPC. This can be attributed to two main reasons, firstly that NaOH pretreatment increased adhesion and compatibility between the wood fibre and the polymer and, secondly that NaOH removes all impurities from the wood fibre surfaces which led to increased degree of polymerization [10].

4. Conclusions The results obtained in this study lead us to the conclusions that:

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1. The mechanical and morphological properties of the WPC were significantly increased by pretreatment with 5% NaOH solution. The mechanical tests also indicated that the PWPC led to significant improvements in MOE, MOR and compressive strength. 2. The MOE, MOR and compressive strength of PWPC were 48– 61%, 41–57% and 66–86% higher than raw wood and 7–10%, 13–17% and 18–20% higher than WPC. 3. NaOH reacts with wood fibres and yielded cellulose-ONa compound and also removed all impurities from the wood fibre surfaces which was confirmed by FTIR spectroscopic analysis. The manufactured WPC and PWPC were also confirmed by FTIR spectroscopic analysis. 4. The significant effects of NaOH pretreatment on the above mentioned properties of PWPC could be explained by the behaviour of the monomer mixture which adhered to the cellulose fibre of the wood as indicated by the scanning electron microscopy and X-ray diffraction patterns. 5. The authors propose that NaOH pretreatment increased the adhesion and compatibility of wood fibre to polymer matrix thus enhancing the degree of polymerization and the degree of crystallinity of wood composite, which significantly increased the mechanical and morphological properties of all selected tropical light hardwoods used in this study.

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