ORI GIN AL

Effect of nano-SiO2 on properties of wood/polymer/claynanocomposites

Rashmi Rekha Devi • Tarun K. Maji

Received: 7 February 2011 / Published online: 3 March 2012

� Springer-Verlag 2012

Abstract Wood polymer nanocomposites (WPNC) based on nano-SiO2 were

prepared by impregnation of styrene acrylonitrile copolymer (SAN), SiO2 nano-

particles modified with c-trimethoxy silyl propyl methacrylate (MSMA), and

nanoclay into wood. The structure of modified SiO2 nanoparticles and WPNC was

characterized by Fourier transform infrared spectroscopy (FTIR). XRD analysis

showed the delaminated structure of SAN/SiO2/clay-treated wood composites. The

synergistic effect of nano-SiO2 and nanoclay was investigated. Thermal stability of

SiO2 nanoparticles decreased after modification, while that of wood treated with

SAN, SiO2, and nanoclay improved. Morphological characteristics were examined

by scanning electron microscopy (SEM). Mechanical properties, water uptake (%),

dimensional stability, hardness, and flammability were found to improve due to

incorporation of SiO2 and nanoclay into wood polymer composites. Maximum

improvement in properties was observed in the wood polymer composites con-

taining SiO2 and nanoclay at the ratio of 1:1.

Introduction

Wood is an important ecomaterial, playing an irreplaceable role in resources,

energy, and environmental aspects addressing the survival and development of

human society. In the past decades, the research and development of environment

materials of the wood composites with different thermoplastic polymers in the

presence of cross-linking agent, which emphasized the enhancement in dimensional

stability, superior mechanical strength, and elevated thermo stability, were done

(Banks and Lawther 1994; Kumar 1994; Rowell 1983). Vinyl monomers and

R. R. Devi � T. K. Maji (&)

Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, India

e-mail: tkm@tezu.ernet.in

123

Wood Sci Technol (2012) 46:1151–1168

DOI 10.1007/s00226-012-0471-1

various impregnation compounds such as phenol, urea, melamine formaldehyde

resin, polyethylene glycol, and isocyanate resins were used for wood treatment.

Nano-based treatments present new opportunities to enhance wood attributes

more effectively for different applications in the horizon of wood modification

processes. Nano-modified wood polymer composites could be a promising new

approach to obtain effective products with better physical, thermal, and mechanical

properties. The preparation of wood plastic nanocomposites based on water-soluble

melamine urea formaldehyde (MUF) resin, phenol formaldehyde resin, and

aluminosilicate nanofillers has been reported in the literature (Cai et al. 2008;

Zhao and Lu 2008).

In polymer composite, SiO2 nanopowder is one of the widely used fillers. To

increase the hydrophobicity of the inorganic silica particle, the surface is modified

by different silane compounds (Hu et al. 2004). It was reported that nano-SiO2 was

beneficial in improving the bonding strength of UF adhesive and subside its free

formaldehyde emission (Lin et al. 2005). Therefore, it could be expected that

nanomaterials could be applied to modify the properties of inferior wood as well

(Shi et al. 2004).

It is well documented that the loading of clay into polymer enhances mechanical,

barrier, and flame-retardant properties. Most authors conclude that these lamellar

nanoparticles have to be used in combination with other additives in order to meet

fire resistance requirements (Le Bras et al. 2004). The surface characteristics of

nanopowders play a key role in their fundamental properties from phase

transformation to reactivity. Due to a large fraction of surface atoms and a higher

surface area, a nanopowder would be expected to be more active. A dramatic

increase in the interfacial area between fillers and polymer can significantly improve

the properties of the polymer (Song 1996). Apart from improvement in thermo-

stability and flame retardancy caused by the addition of metal oxides nanoparticles,

the enhancement in other properties like mechanical and fire resistance due to the

synergistic effect with nanoclays has also been achieved due to its nanometric sizes

(Laachachia et al. 2005).

With this in view, wood polymer nanocomposites have been prepared by

impregnation of styrene acrylonitrile copolymer in the presence of c-trimethoxy

silyl propyl methacrylate (MSMA)-modified nano-SiO2 and nanoclay into Simul

wood (Salmalia malabarica). The synergistic effect of nano-SiO2 and nanoclay on

dimensional stability, thermal, mechanical, and flame resistance has been

investigated.

Materials and methods

Simul wood (Salmalia malabarica) was collected locally. Styrene obtained from

Merck (Mumbai, India) was purified by following standard procedure (Ashraf et al.

2009). Acrylonitrile was purchased from Merck (Germany), and 2, 20–azo bis

isobutyronitrile (AIBN) obtained from Merck (Germany) was used as received.

Nanoclay (Nanomer, surface modified by 15–35 wt% octadecylamine and 0.5–5

wt% amino propyl triethoxy silane), SiO2 nanopowder, \5 to 15 nm, and

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c-trimethoxy silyl propyl methacrylate (MSMA) were purchased from Sigma-

Aldrich (USA). All other chemicals used were of analytical grade.

Sample preparation

The Simul wood samples used for the study were prepared from clear defect-free

wood and cut into blocks of 2.5 cm 9 1 cm 9 2.5 cm (radial 9 tangential 9 lon-

gitudinal) for dimensional stability, water uptake, and hardness tests.

For tensile strength measurement, the samples were prepared according to ASTM

D638. The samples were cut into 10 cm 9 0.5 cm 9 2 cm using standard method.

For bending strength measurement, the blocks were cut into dimensions of

1 cm 9 1 cm 9 10 cm (radial 9 tangential 9 longitudinal) according to ASTM

D790.

Surface modification of nano-SiO2

The surface of SiO2 nanoparticles was activated by a silane coupling agent

c-trimethoxy silyl propyl methacrylate (MSMA). Typical method was followed for

modification of nano-SiO2 as reported in the literature (Hong et al. 2009).

Preparation of styrene acrylonitrile copolymer

To prepare styrene co-acrylonitrile pre-polymer, a mixture of monomers styrene and

acrylonitrile in the molar ratio of 2:3 was taken and prepared according to the

procedure used in the laboratory (Devi and Maji 2012).

Dispersion of SAN/SiO2/nanoclay

Both the nanofillers in appropriate amounts were allowed to swell in THF for 4 h in

a round-bottom flask by constantly stirring and then sonicated for 15 min. The

solution was further dispersed in SAN pre-polymer followed by sonication for

another 15 min. This dispersion was taken for subsequent study.

Preparation of wood/SAN/MMT nanocomposite

Wood samples were preliminarily dried in an oven at 105�C until constant weight

before treatment, and dimensions and weight were measured. The samples were

then placed in an impregnation chamber followed by application of load over each

sample to prevent them from floatation during addition of pre-polymer. Vacuum

was applied for a specific time period to remove the air from the pores of the wood

samples prior to addition of pre-polymeric mixture. Now the dispersion of SAN pre-

polymer, MSMA-modified SiO2 nanopowder with nanoclay and initiator, or

dispersion of SAN pre-polymer with MSMA-modified SiO2 nanopowder and

initiator, or that of SAN pre-polymer and initiator was added from a dropping funnel

to completely immerse the wood samples. The samples were then kept in the

chamber at room temperature for another 4 h after attaining atmospheric pressure.

Wood Sci Technol (2012) 46:1151–1168 1153

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This is the minimum time to get maximum polymer loading, which showed

maximum improvement in properties. After impregnation, samples were taken out

of the chamber, and excess chemicals were wiped from the wood surfaces. The

samples were then wrapped in aluminum foil and cured at 90�C for 24 h in an oven.

This was followed by drying at 105�C for another 24 h. The cured samples were

then Soxhlet extracted using chloroform to remove homopolymers, if any, formed

during polymerization. Finally, the samples were dried, and the dimensions were

measured by using slide caliper, and weights were taken.

FTIR study

The treated and untreated samples were ground, and FTIR spectra were recorded by

using a KBr pellet in a Nicolet (model Impact 410) FTIR spectrometer.

Thermogravimetric analysis

Thermal properties of the untreated and treated wood samples were measured by

using a thermogravimetric analyzer, Metler (model TA 4000), at a heating rate of

10�C/min up to 600�C under nitrogen atmosphere.

X-ray diffraction (XRD) studies

The XRD studies were done by Rigaku X-ray diffractometer (Miniflex, UK) using

Cuka radiation (k = 0.154 nm) at a scanning rate of 2–30� of 2h.

Weight percent gain

Weight percent gain (WPG) after polymer loading was calculated according to

formula as reported in a previous study (Devi and Maji 2012).

Volume increase (%) after impregnation

Percentage volume increase after curing of wood samples was calculated as follows:

%Volume increase ¼ Vt � Voð Þ=Vo � 100

where Vo is the oven dry volume of the untreated wood and Vt is the oven dry

volume of the treated wood.

Hardness

The hardness of the samples was measured according to ASTM D2240 method

using a durometer (model RR12) and expressed as shore D hardness.

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Morphological study

Scanning electron microscope (SEM) study

The morphological features of untreated and treated wood samples were observed

using scanning electron microscope (JEOL JSM-6390 LV) at an accelerated voltage

of 5–10 kV. Fractured surface of some of the selected wood samples coated with

platinum was used for the study.

Mechanical properties

The flexural strength and flexural modulus of the samples were measured by UTM-

HOUNSEFIELD, England (model H100 K-S) with a cross-head speed of 2 mm/min

and by calculating the modulus of elasticity (MOE) and modulus of rupture (MOR)

according to ASTM D790 method.

The tensile strength and tensile modulus were examined by using UTM-

HOUNSEFIELD, England (model H100 K-S), with a 5-KN load cell and cross-head

speed of 10 mm/min according to ASTM D638.

Limiting oxygen index test

Limiting oxygen index (LOI) is defined as the minimum concentration of oxygen,

expressed as percent volume, in a flowing mixture of oxygen and nitrogen that will

support flaming combustion of a material initially at room temperature. The method

used was according to ASTM D2863-77 and calculated as per formula given in a

previous communication (Devi et al. 2011).

Water uptake test

Water uptake test was done by following the method as reported in the literature

(Devi et al. 2004).

Dimensional stability test

Swelling in water

Dimensions of the oven-dried samples were measured and conditioned at room

temperature (30�C) and 30% relative humidity. Final placement of the samples was

done in distilled water, and then dimensions were remeasured after different time

intervals. Swelling was considered as a change in volume and calculated according

to the procedure (Devi et al. 2004).

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Anti-swelling efficiency

The anti-swell efficiency (ASE) index was determined to evaluate dimensional

stability of treated wood specimens and calculated according to the procedure

described in the literature (Devi and Maji 2008).

Results and discussion

The optimum conditions for impregnation, at which maximum improvement in

properties was observed, were as follows: vacuum: 508 mm Hg, time of

impregnation: 4 h, AIBN: 0.75% (w/v), SAN (mL):100, THF (mL): 20, and

modified SiO2: 0.5–2.0% (w/v). Nanoclay: 0.125–2.0% (w/v).

Effect of variation of SiO2/nanoclay on polymer loading (WPG%), volume

increase, and hardness

Related results are shown in Table 1. Polymer loading (WPG%), volume increase

(%), and hardness were found to enhance with the incorporation of either SiO2,

nanoclay, or a combination of SiO2/nanoclay into wood. The addition of SiO2 alone

improved all the properties. The higher the amount of SiO2, the higher was the

improvement. The improvement was further enhanced by the addition of nanoclay

along with SiO2. The improvement took place up to the addition of a certain amount

of nanoclay. Beyond that it deteriorated. The increase in both weight gain (%) and

volume was due to the filling of the void spaces in wood by polymer and

nanoparticles. The higher the amount of nanoparticles, the more was the space

occupied. The overall improvement in hardness was due to the restriction in the

mobility of the polymer chain inside the clay interlayers. Besides this, the well-

dispersed SiO2 nanoparticles could act as filler, which also reinforced the polymer

matrix and increased hardness values. Hardness values were found to decrease

beyond the addition of SiO2/nanoclay (0.5% each). The improvement of properties

in wood polymer composites loaded with SiO2 was more compared to that of

composite loaded with similar level of nanoclay. This might be due to the higher

interaction between wood and polymer by modified SiO2 compared to nanoclay.

FTIR study

FTIR spectra of (a) unmodified SiO2 nanoparticles, (b) MSMA-, and (c) MSMA-

modified SiO2 nanoparticles are presented in Fig. 1. The absorption peaks (curve-

1a) at 3,464, 1,089, and 474 cm-1 were ascribed to the vibrations of –OH and

Si–O–Si groups in the SiO2. The FTIR spectrum of MSMA (curve-1b) showed that

the absorption peaks at 2,953, 1,720, 1,454, and 1,326 cm-1, which were attributed

to the vibration of –CH2, C=O, C=C, and C–H groups of MSMA. In the FTIR

spectrum of MSMA-treated nanoparticles (curve-1c), all the characteristic absorp-

tion peaks of SiO2 and MSMA appeared (Kang et al. 2009).

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Figure 2 shows the FTIR spectra of (a) untreated wood, wood treated with

(b) SAN, (d) SAN/SiO2 (0.5%), (e) SAN/SiO2 (2.0%), and (f) SAN/SiO2 (0.5%)/

nanoclay(0.5%). The curve-2c and curve-2f represent the spectra of SAN-SiO2

polymer composite and nanoclay, respectively. The pure wood was characterized by

the absorption bands (shown in curve-2a) appearing at 3,425 cm-1 (OH stretching),

1,740 cm-1 (C=O stretching of acetylated xylem), and 1,257 cm-1 (C–O stretching

of acetyl groups), respectively. The FTIR spectrum of organically modified

Table 1 Effect of variation of SiO2 on polymer loading (WPG %), volume increase and hardness

Sample particulars Weight gain (%) Volume increase (%) Hardness (Shore D)

Untreated – – 40 (±3.6)

Treated with SAN/THF/SiO2/nanoclay

100/20/0/0 34.1 (±3.6) 3.0 (±1.6) 42 (±0.8)

100/20/0.5/0 98.5 (±0.6) 5.4 (±0.5) 54 (±1.0)

100/20/2.0/0 109.3 (±0.4) 8.3 (±0.7) 70 (±0.1)

100/20/0.5/0.125 110.5 (±0.9) 7.2 (±1.6) 63 (±1.4)

100/20/0.5/0.5 124.9 (±0.2) 10.5 (±0.6) 69 (±2.0)

100/20/0.5/2.0 91.0 (±1.0) 7.5 (±1.0) 66 (±1.0)

100/20/0/0.5 52.9 (±0.8) 5.3 (±0.4) 49 (±0.5)

* Each value represents the average of three samples

Fig. 1 FTIR spectra of (a) unmodified SiO2 nanoparticles, (b) MSMA, and (c) MSMA-modified SiO2

nanoparticles

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nanoclay (curve- 2f) showed the presence of bands at 3,467 cm-1 for –OH

stretching, 2,927 and 2,852 cm-1 for –C–H stretching of modifying hydrocarbon,

1,619 cm-1 for –OH bending, 1,031, 530, and 460 cm-1 for oxide bands of metals

(Al, Mg, Si, etc.). SAN-treated wood (curve-2b) showed the absorption bands at

3,026 cm-1 (aromatic –CH stretching), 2,927 cm-1 (–CH2 stretching), 2,242 cm-1

(C:N stretching), and 764 cm-1 (aromatic C–H wagging) (Yap et al. 1991). In the

spectra of SAN-SiO2 polymer composite (curve-2c) and SAN-SiO2-treated wood

composites (curve-2e and curve-2f), the characteristic peaks for wood, SAN, and

SiO2 were observed. In the spectrum of SAN/SiO2 (0.5%)/nanoclay (0.5%)-treated

wood sample, all the characteristic absorption bands for MSMA-modified SiO2

and nanoclay became prominent. The intensity of the band in the region

3,500–3,200 cm-1 was slightly increased. The increase in intensity of the band

reflects the increased hydrogen bonding between the lattice hydroxyls and polymer

(Gunister et al. 2007).

Thermogravimetric analysis (TGA)

Figure 3a shows TGA curves of unmodified and modified SiO2 nanoparticles. A

remarkable difference in thermal behavior was noticed. It was observed that a

weight loss (%) of 8.18 and 4.67 occurred (between 30 and 260�C) for unmodified

and modified SiO2 nanoparticles, respectively. Furthermore, in the thermal

degradation process, 32.89 and 19.86% weight loss was observed for modified

and unmodified nanoparticles, respectively. In general, the weight loss was more in

Fig. 2 FTIR of (a) untreated wood and treated with (b) SAN/wood (c) SAN/SiO2 polymer composites,(d) SAN/SiO2 (0.5%)-, (e) SAN SiO2 (2.0%)-, (f) nanoclay-, and (g) SAN/Si02 (0.5%)/nanoclay (0.5%)-treated wood samples

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modified SiO2 compared to unmodified SiO2. This was due to the decomposition of

grafted MSMA from the surface of SiO2. These results indicated that MSMA

interacted with SiO2 nanoparticles and improved the hydrophobicity of the surface

of SiO2 nanoparticles (Shu-rui et al. 2008).

Table 2 shows the initial decomposition temperature (Ti), maximum pyrolysis

temperature (Tm), temperature of decomposition (TD) values at different weight

losses for different samples, and residual weight (%) (RW) for untreated and

polymer-treated wood samples with or without SiO2 and nanoclay as summarized in

Fig. 3b. Ti value was highest for SAN/SiO2 (0.5%)/nanoclay (0.5%)-treated wood

polymer composite (WPC). The Tm values of SiO2-treated samples were more

compared to those of other samples treated with either nanoclay or SiO2/nanoclay

Fig. 3 a TGA of (a) unmodified SiO2 nanoparticles and (b) MSMA-modified SiO2 nanoparticles andb TGA of (a) wood, (b) SAN, (c) SAN/SiO2 (0.5%)-, (d) SAN/nanoclay (0.5%)-, (e) SAN/SiO2 (0.5%)/nanoclay (0.125%)-, (f) SAN/SiO2 (0.5%)/nanoclay (2.0%)-, and (g) SAN/SiO2 (0.5%)/nanoclay (0.5%)-treated wood samples

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combination. RW (%) value of wood polymer composite treated with SiO2/nanoclay

(0.5:0.5) was found less compared to untreated wood, but highest among the treated

samples.

TD values of polymer-treated wood samples were observed higher than those of

virgin wood samples. TD values increased further on inclusion of silane-modified

SiO2 and nanoclay. The higher TD values might be due to the combining effect of

cross-linked structure formed by MSMA present in SiO2 and the presence of silicate

layers. The cross-linked structure reduced the rate of decomposition of degradable

components in the WPNC. The presence of silicate layer acted as a barrier and

hindered the diffusion of volatile decomposition products (Das and Karak 2009;

Deka and Maji 2010; Boonkrai and Aht-Ong 2010). More detailed information

regarding the influence of the incorporation of the nanomaterials on the thermal

stability could be drawn from Fig. 4. Curves 4(c) and 4(d) represent wood samples

treated with SAN/SiO2 (0.5%) and SAN/nanoclay (0.5%), respectively, while

curves 4(e–g) were for SAN/SiO2 (0.5%)/nanoclay (0.125%)-, SAN/SiO2 (0.5%)/

nanoclay (2.0%)-, and SAN/SiO2 (0.5%)/nanoclay (0.5%)-treated wood samples,

respectively. For all these systems examined, the decomposition temperature was

maximum for the SAN/SiO2 (0.5%)/nanoclay (0.5%)-treated wood samples. But

beyond that it decreased. The clay might exist in the agglomerated form at higher

percentage of loading. This could reduce the tortuous path compared to that of lower

nanoclay-loaded WPC for the diffusion of volatile products.

X-ray diffraction studies

A broad diffraction peak at 23.5� for the amorphous SiO2 particle was reported (Wa

et al. 2009). There was no noticeable difference in diffraction peak after

modification. This confirmed their non-crystalline nature. This was in accordance

with the findings by Ahmad et al. (2007).

Figure 4 shows the plots of intensity versus scattering angle, 2h, of untreated

wood, SAN polymer, and wood treated with SAN/SiO2 (0.5%), (d) SAN/nanoclay

Table 2 Thermal analysis of untreated and treated wood

Sample particulars TiaTm

bTm Temperature of decomposition

(TD) in �C at different weight

loss (%)

RW (%)

20 40 60 80

Untreated 225 235 337 284 318 342 – 22.0

Treated with SAN 212 324 356 283 314 365 447 4.76

SAN/SiO2 (0.5%) 215 347 391 284 328 381 407 7.98

SAN/nanoclay (0.5%) 228 321 391 290 316 371 446 8.09

SAN/SiO2 (0.5%)/nanoclay (0.125%) 230 337 369 294 335 381 416 11.1

SAN/SiO2 (0.5%)/nanoclay (0.5%) 254 343 393 313 372 406 433 16.5

SAN/SiO2 (0.5%)/nanoclay (2.0%) 235 335 362 289 329 380 413 10.5

Ti value for initial degradation; aTm value for 1st step; bTm value for 2nd step

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(0.5%), and (e) SAN/SiO2 (0.5%)/nanoclay (0.5%) in downward direction. The

crystalline peak of cellulose occurred at 2h of 23.05� (curve-4a). Curve-4(b) rep-

resents the diffractogram of SAN polymer, which was amorphous in nature. The

addition of SiO2 into wood did not induce any crystallinity in these polymers within

the wood matrix (curve-4c) (Ahmed et al. 2007). But the incorporation of nanoclay

into wood resulted in shifting of crystalline peak of cellulose from 23.05� to 22.65

(curve-4d) of wood. It could be said that either the full expansion of the MMT

gallery occurred, which was not possible to detect by XRD, or the MMT layers

became delaminated and no crystal diffraction peak appeared (Lu et al. 2006)

(curve-4d). Curve-4e represented the XRD patterns of wood composites containing

SAN/SiO2 (0.5%)/nanoclay (0.5%). The nanoclay layers were also exfoliated by the

disappearance of the nanoclay diffraction peak (Yu et al. 2007). It is noteworthy that

curve-4f showed diffraction peak of nanoclay at 2h = 4.1�.

Morphological studies

Scanning electron microscope (SEM) study

Figure 5 shows the SEM micrographs of untreated (Fig. 5a) and treated wood

samples (Fig. 5b–f). For untreated wood, the empty cell wall, the pit, and

parenchyma are seen. In treated wood, these empty spaces were occupied by the

SAN polymer (Fig. 5b), SAN/SiO2 (0.5%) (Fig. 5c), and SAN/nanoclay (0.5%)

(Fig. 5d) materials. The impregnated nanoparticles as white spots were either

located in the cell walls or filled in the cell lumen. The presence of nano-SiO2 and

nanoclay in the wood pit and wood cell wall was seen (Fig. 5e, f).

Mechanical property study

Table 3 represents the tensile and flexural strength of treated and untreated wood

samples. The values reported were the average values of three readings. It was

observed that wood treated with SAN caused an improvement in tensile as well as

flexural properties. Both the tensile strength and flexural strength increased

Fig. 4 X-ray diffractograms(a.u. denotes arbitrary unit) of(a) untreated wood, (b) SANpolymer, (c) SAN/SiO2 (0.5%),(d) SAN/nanoclay (0.5%),(e) SAN/SiO2 (0.5%)/nanoclay(0.5%), and (f) nanoclay

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drastically when silane-modified SiO2 was added. The flexural and tensile properties

were improved throughout the studied percentage (%) of SiO2 loading. SiO2

nanoparticles were distributed uniformly in the composites. The MSMA-modified

SiO2 increased the interaction between wood and SAN polymer resulting

in stiffening of the composites. As a result, improvement in properties was

observed. The effect of silane coupling agent on the mechanical properties of

bamboo fiber–filled natural rubber composites was reported by Ismail et al. (2002).

The properties were further improved after incorporation of clay and SiO2

nanopowder. The silicate layers of nanoclay acted as a reinforcing agent that binds

the polymer chain inside the gallery space and hence restricted the mobility of the

polymer chain. SiO2 nanoparticles also increased the adhesion between wood and

SAN polymer. The influence of restriction in the mobility of polymer chain and

increase in adhesion caused significant enhancement in mechanical properties. The

Fig. 5 Scanning electron micrographs of a untreated and b SAN, c SAN/SiO2 (0.5%)-, d SAN/nanoclay(0.5%)-, e SAN/SiO2 (0.5%)/nanoclay (0.5%)-, and f SAN/SiO2 (0.5%) nanoclay (2.0%)-treated woodsamples

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deterioration of mechanical properties at higher clay loading might be due to the

agglomeration of clay nanoparticles.

Limiting oxygen index test

Limiting oxygen indices (LOI) of treated and untreated wood samples are also

shown in Table 3. Untreated wood samples produced a candle-like flame and

generated little char, whereas SAN- and SAN/clay/SiO2-treated samples produced a

small localized flame and generated more char. Moreover, it was observed that

SAN-treated samples showed higher LOI (%) value than untreated samples. The

addition of MSMA-modified nano-SiO2 and SAN polymer into wood resulted in

more improvement in LOI (%) value. Besides this, the inorganic silica nanoparticles

acted as a thermal barrier. The formation of network structure as well as the

presence of inorganic SiO2 in WPNC might have caused the reduction in

flammability characteristics. Giudice and Pereyra (2007) have found the improve-

ment in fire properties by impregnation of silicate into wood. The LOI value

improved further on addition of nanoclay with nano SiO2. The value increased up to

the addition of SiO2/nanoclay (0.5:0.5), after which it decreased again. It was

reported that organically modified clay and silica modified with 3-amino propyl

trimethoxy silane improved the flame-retardant properties of ABS nanocomposites

(Boonkrai and Aht-Ong 2010). The nanoclay produced silicate char on the surface

of WPC, which increased the flame resistance property of the composite (Camino

et al. 2005). The tortuous path provided by the silicate layers had better barrier

property to the oxygen and heat which delayed the burning capacity of the

composite. Moreover, modified SiO2 was also assisted in improving the LOI value

as explained earlier. At higher clay loading, the tortuous pathway provided by the

clay decreased due to agglomeration which resulted in decrease of barrier property

and LOI value.

Table 3 Flexural, tensile properties and LOI (%) of untreated and treated wood

Sample particulars Flexural Properties Tensile Properties LOI (%)

MOE

(MPa)

MOR

(MPa)

Tensile

modulus

(MPa)

Tensile

strength

(MPa)

Untreated 2,779 (±12.7) 31.7 (±0.7) 846 (±12.7) 27.6 (±5.7) 19.4 (±1.0)

Treated with SAN 4,387 (±10.8) 35.6 (±0.2) 913 (±4.7) 34.0 (±1.9) 22.2 (±0.5)

SAN/SiO2 (0.5%) 5,355 (±9.7) 64.2 (±0.5) 1,098 (±2.7) 47.3 (±0.7) 26.6 (±1.0)

SAN/nanoclay (0.5%) 5,308 (±7.7) 62.9 (±0.8) 987 (±1.7) 40.9 (±1.2) 25.0 (±1.0)

SAN/SiO2 (0.5%)/

nanoclay (0.125%)

5,687 (±20.7) 64.9 (±0.6) 1,183 (±2.3) 49.0 (±0.9) 27.7 (±3.0)

SAN/SiO2 (0.5%)/

nanoclay (0.5%)

6,290 (±7.7) 67.9 (±1.1) 1,574 (±1.7) 50.6 (±1.4) 30.5 (±1.0)

SAN/SiO2 (0.5%)/

nanoclay (2.0%)

5,290 (±11.7) 63.1 (±0.4) 1,101 (±3.4) 48.7 (±3.0) 28.8 (±0.5)

* Each value represents the average of three samples

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Water uptake study

Figure 6 represents the water uptake of untreated wood and treated WPNC samples.

In both treated and untreated samples, water absorption increased with increasing

time of immersion. Untreated samples absorbed more water than the treated

samples. Wood samples treated with SAN copolymer decreased the water uptake

throughout the studied time period. MSMA-modified SiO2-treated wood showed

lower water absorption because of the cross-links formed by the interaction of

double bond of MSMA and hydroxyl group present in modified SiO2 with SAN

copolymer and hydroxyl group of wood, respectively.

With the addition of nanoclay, the water uptake percentage further decreases.

The silicate layers of the clay provide tortuous path for the water transport, which

increased the barrier property that leads to a decrease in water uptake capacity.

The surface of the modified nanoclay had a tendency to immobilize some of the

moisture. The layer of modified nanoclay enhanced tortuous path for water

transport and thereby resulted in a decrease in water diffusivity (Cai et al. 2008).

Improvement in water resistance due to introduction of melamine urea formal-

dehyde resin and nanofiller in wood was reported in the literature (Zhang et al.

2007). The addition of nanoclay and SiO2 decreased further the water uptake

capacity. The least water uptake was observed for WPNC having SiO2/nanoclay at

0.5:0.5 ratio. WPNC treated with SiO2/nanoclay (0.5:2.0) showed less water

absorption capacity compared to WPNC treated with SiO2/nanoclay (0.5:0.5). The

agglomeration of clay decreased the tortuous pathway for water causing an

increase in water absorption.

Fig. 6 Water uptake (%) of (a) untreated wood and treated with (b) SAN, (c) SAN/SiO2 (0.5%),(d) SAN/SiO2 (0.5%)/nanoclay (0.125%), (e) SAN/SiO2 (0.5%)/nanoclay (2.0%), (f) SAN/SiO2 (0.5%)/nanoclay (0.5%), and (g) SAN/nanoclay (0.5%)

1164 Wood Sci Technol (2012) 46:1151–1168

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Dimensional stability test

The results showing the effect of swelling in water vapor for untreated and treated

wood samples at room temperature (30�CC) for different time periods are shown in

Fig. 7. As expected, wood treated with SiO2 and nanoclay particles showed least

swelling. The explanation was similar to the one discussed earlier.

Anti-swelling efficiency

The results of anti-swelling efficiency of wood samples are shown in Table 4. Anti-

swelling efficiency was found to decrease with the increase in time. The anti-

swelling efficiency was more in the case of WPC.

Improvement in dimensional stability of wood composites over virgin wood sample

might be due to the deposition of polymer. The polymer was less hygroscopic than

wood, and less water would be absorbed during humid conditions. The dimensional

stability was further improved by addition of MSMA-modified SiO2 nanoparticles.

The addition of nanoclay with SiO2 further enhanced the dimensional stability. The

tortuous pathway and cross-links caused by the presence of nanoclay and SiO2 might

be helping WPNC to absorb less water. As a result, dimensional stability would be

improved. Treatment of Scots pine with different types of organo alkoxy silane

increasing the ASE was reported in the literature (Panov and Terziev 2009).

Conclusion

Wood polymer nanocomposites (WPNC) were prepared by the treatment of styrene

acrylonitrile copolymer/SiO2/nanoclay intercalating mixture. FTIR study showed

Fig. 7 Volumetric swelling (%) of (a) untreated wood and treated with (b) SAN, (c) SAN/SiO2 (0.5%),(d) SAN/SiO2 (0.5%)/nanoclay (0.125%), (e) SAN/SiO2 (0.5%)/nanoclay (2.0%), (f) SAN/SiO2 (0.5%)/nanoclay (0.5%), and (g) SAN/nanoclay (0.5%)

Wood Sci Technol (2012) 46:1151–1168 1165

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the presence of characteristic bands for c-trimethoxy silyl propyl methacrylate

(MSMA) in modified SiO2 nanoparticles, which confirmed that surface modification

of nano-SiO2 by c-trimethoxy silyl propyl methacrylate (MSMA) was successful.

FTIR study also showed the incorporation of modified SiO2 and nanoclay into

wood/SAN composite. TGA study showed that modified SiO2 nanoparticles were

less thermally stable. The incorporation of 0.5phr each of SiO2 and nanoclay into

wood exhibited maximum thermal stability. XRD analysis showed that incorpora-

tion of SiO2 and nanoclay resulted in an exfoliated structure. The interaction

between wood, SAN, SiO2, and nanoclay was studied by SEM, which showed the

presence of nano SiO2/nanoclay in the wood cell wall and cell lumens. Maximum

improvement in flexural, tensile properties of water uptake and dimensional stability

were observed for SiO2 (0.5%)/nanoclay (0.5%)-treated wood samples. The LOI

value improved due to incorporation of SiO2 and nanoclay into wood/SAN

composite.

Conflict of interest The authors have declared that they have no conflict of interest.

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Table 4 Anti-swell efficiencies of treated wood

Time

(h)

SAN SAN/

SiO2

(0.5%)

SAN/

nanoclay

(0.5%)

SAN/SiO2 (0.5%)/

nanoclay

(0.125%)

SAN/SiO2 (0.5%)/

nanoclay (0.5%)

SAN/SiO2 (0.5%)/

nanoclay (2.0%)

0.5 56.8

(±0.4)

77.8

(±0.1)

83.7

(±0.2)

75.6 (±0.2) 90.1 (±0.1) 79.4 (±0.2)

2 62.7

(±0.1)

71.8

(±0.2)

79.2

(±0.1)

70.3 (±0.1) 80.9 (±0.4) 76.5 (±0.3)

4 32.8

(±0.3)

55.3

(±0.1)

64.8

(±0.4)

53.8 (±1.0) 65.3 (±0.1) 56.8 (±0.2)

24 33.4

(±0.4)

35.7

(±0.3)

42.0

(±0.1)

34.7 (±0.3) 52.1 (±0.4) 39.1 (±0.1)

48 32.0

(±0.5)

31.7

(±0.4)

41.2

(±0.2)

29.1 (±0.8) 51.2 (±0.5) 39.1 (±0.1)

72 20.7

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33.8

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44.4

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39.4

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46.1

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35.7 (±0.4) 57.0 (±0.5) 40.9 (±0.4)

144 25.1

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39.7

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46.0

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34.7 (±0.4) 58.9 (±0.6) 40.9 (±0.4)

168 46.6

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53.9

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61.2

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