Preparation and characterisation of wood flour/polystyrene composites by subcritical fluid-assisted reactive extrusion

Abstract

In this work, subcritical fluids are applied to twin-screw extruders as a novel design for the preparation of wood flour/polystyrene composites. Subcritical fluids have suitable process conditions, excellent swellability and diffusibility. Therefore, the subcritical fluids in the extruder system can alleviate the thermal degradation of wood floor, reduce the viscosity of the resin and strengthen the mass transfer rate. Wood flour evenly distributed in the polystyrene matrix in the presence of subcritical n-propanol. The best adhesion between wood flour and PS is shown when 10 wt-% MAPS is added. MAPS was confirmed to be grafted onto wood flour. In addition, the thermal stability and crystallinity of wood flour and the composites are studied. Mechanical tests proved the effectiveness of subcritical fluids, particularly subcritical n-propanol, in improving the mechanical properties of the composites. The extrusion temperature and content of MAPS show optimum values at 190°C and 10%, respectively.

Keywords

Polystyrene wood flour compatibiliser subcritical fluid-assisted extrusion mechanical properties morphology

Introduction

Currently, environmental and resource issues are attracting increasing attention. Recent studies show that single-use plastic and wood products result in environmental pollution and waste of resources. The emergence and development of wood-plastic composites (WPC) alleviate these problems because plastic can come from waste products and wood flour can come from processing residues [1, 2]. Wood fibres are biodegradable and easily available [3]. Polymer matrixes have characteristics of easy processing and environmental resistance [4]. As an environmentally friendly material, it is now widely used in automotive interiors, outdoor buildings and park plank ﬁelds [5].

Polystyrene (PS) is widely used as a resin matrix for composites due to its higher rigidity and heat resistance relative to other common thermoplastics, such as polyethylene and polypropylene. The environmental pollution caused by PS-based composites is also much less than that caused by polyvinyl chloride-based products [6]. However, PS has poor impact strength. Extensive research has been conducted to improve the toughness of plastic through the use of block copolymers or the addition of elastomers. Rubber systems, such as ethylene-propylene-diene [7], styrene–ethylene/butylene–styrene [8] and natural rubber [9], have been proven to have good toughening effects on polymers.

WPCs show many problems in the preparation process. Hydrophobic polymers are poorly compatible with hydrophilic wood flour, which greatly affects the mechanical properties of their composites [10]. To overcome this problem, many researches have been performed to improve the interfacial adhesion of WPCs. Surface modification is a method to reduce the hydrophilicity of fibres via heat treatment [11], alkali treatment [12] or esterification treatment [13]. Another method is adding additives that are amphiphilic substances, such as titanate [14], silane coupling agents [15] and maleic anhydride grafted polymers [16]. The addition of maleic anhydride grafted polymers has been studied by many researchers as a simple and effective method.

Supercritical fluid treatment technology is an environmentally friendly method that has rapidly developed. Supercritical fluids have been used for extraction [17], recycling of plastics [18] and desulphurisation of waste rubber [19] due to their high solubility, low viscosity and high diffusion rate. Kojima et al. [20] used supercritical carbon dioxide (scCO₂) combined with desulphurisation aids to study the desulphurisation of waste rubber. They found that supercritical carbon dioxide contributes to the diffusion of desulphurisation aids into the waste rubber. The desulphurisation effect is more significant under the cooperation of supercritical carbon dioxide. Supercritical fluids mainly include carbon dioxide, water, and alcohols for they are nontoxic, chemically inert and readily available [21-23], these products have excellent properties of both liquids and gases under supercritical conditions. The special combination of gas-like viscosity and liquid-like density makes it an excellent reaction assistance medium [24]. However, high temperature and pressure are required to reach supercritical fluid conditions. Subcritical fluids, as an alternative, only need mild temperature and pressure but have characteristics analogous to those of supercritical fluids [25]. Subcritical fluids have temperatures above the boiling point but below the critical temperature and pressure. So the subcritical state can be easily achieved and maintained in the experiment.

In this work, a novel design that incorporates subcritical fluids during reactive extrusion is used to prepare wood plastic composites. The effects of subcritical fluid (water, ethanol and n-propanol)-assisted reactive extrusion on the properties of the WPCs are investigated and compared to the conventional extrusion process. The effects of different content of maleic anhydride grafted polystyrene (MAPS) in wood flour/PS composites were studied. Styrene butadiene rubber (SBR) was added to improve the toughness of the composites.

Experimental

Materials

Eucalyptus wood flour was purchased from Shanghai Yuncan Wood Products (Shanghai, China). The PS used in this study was supplied by BASF-YPC Company with a melt flow index and density of 3 cm³/10 min (200°C/5 kg) and 1.05 g cm⁻³, respectively. MAPS was obtained from Dongguan Shenghao Plastic Materials Co. Ltd. Ethanol and n-propanol were supplied by Shanghai Lingfeng Chemical Co. Ltd, China. SBR-1502 was purchased from Petrochemical Company of Jilin, China and used in this study.

Processing conditions

The wood flour was dried in an oven at 105 ± 2°C for approximately 12 h. Then, the wood flour, PS, SBR, and MAPS were weighed and stirred in a high-speed mixer for 10 min to obtain a homogenous mixture. The ratio of wood flour to the PS matrix was six to four. The content of SBR was 10 wt-%. A co-rotating twin screw extruder (TE-35, Coperion Keya Machinery, and China) was used for compounding. The structure of the twin screw extruder with the fluid device is shown in Figure 1. The screw diameter was 35 mm, and the aspect ratio was 44. There were seven zones in the extruder barrel, in which the temperature was set as 170, 175, 180, 190, 190, 190 and 180°C in order. The subcritical fluids were injected into the die through the vacuum pumps (GF1.5-50-S4, Qingdao Lubike Pump Industry, and China). The extracted product was pelletised by a cutter. The granulation product was plasticised at 150°C in the two roll mills. Then, the flat vulcanising machine was employed to press sheets at 150°C under 10 MPa for 5 min and then cooled by a cool press machine for 10 min to obtain plates (150 × 150 × 4 mm³). Then, the plates were cut into standard samples.

Characterisations

Mechanical properties

Tensile and flexural properties were tested according to the ISO 527-2:1993, ISO 178:2001 standard respectively. Loading of the tensile specimens in the tests was performed at rate of 50 mm min⁻¹. Flexural properties were measured with a crosshead speed of 2 mm/min. Unnotched impact properties were determined following ISO 180:2000. Each value obtained represented the average of five samples.

Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) was used to analyse the effect of fluid condition and compatibilisers on the chemical structure of extrudate. The extruded pellet of WPCs was extracted by xylene in a Soxhlet Extractor for 12 h then dried. The spectra were recorded by FTIR device (Nexus 670, American Nicolet Company) with a resolution of 2 cm⁻¹, scan number of 32, and scan range of 4000–400 cm⁻¹.

Scanning electron microscopy

The composites were fractured after infiltration into liquid nitrogen. The fractured surfaces were coated with gold to eliminate electron charging, and then investigated by Scanning electron microscope (Model JSM-5900LV, Japan) operated in secondary electron mode at a beam current of 100 μA with an accelerating voltage of 15 kV.

Thermogravimetric analysis

Thermogravimetric analysis was performed on TGA equipment (Netzsch STA449C/6/F) to investigate the thermal properties of wood flour, PS and their composites. About 4 mg of dried samples were scanned from room temperature to 600°C at a heating rate of 10°C min⁻¹ under nitrogen atmosphere.

X-ray diffraction

The crystallinity of specimens was estimated using X-ray analysis (X'TRA, Switzerland) with monochromatic CuKa1 radiation (λ = 0.15406 Å) at 40 kV, 40 mA. Samples were scanned at 2θ range of 10–50° at room temperature. The scan rate was 5° min⁻¹. The crystallinity index (I_Cr) was calculated based on the Segal's method. (1) Where I₀₀₂ is the maximum intensity of diffraction at a 2θ of 22.5° and I_am is the intensity of diffraction at a 2θ of 15.6°. I₀₀₂ represents both crystalline and amorphous regions of the material, whereas I_am is the peak for the amorphous regions.

Results and discussion

FTIR analysis

The FTIR spectra of different samples are shown in Figure 2. The area ratio of characteristic peak/reference peak of the FTIR spectra is shown in Table 1. The broad peak at 3437 cm⁻¹ is associated with the –OH stretching mainly present in cellulose. Sample d has the lowest absorption peak intensity here, indicating that the hydroxyl groups in the wood flour are more involved in the interfacial reaction. The absorption bands at 2940 and 2852 cm⁻¹ are assigned to the –CH stretching vibrations of the methyl and methylene groups, respectively. The peaks that appear at 1715 cm⁻¹ are attributed to the C=O stretching vibration of the carbonyl group [26]. The intensity of this peak increases from Figure 2(a–d), which demonstrates that subcritical fluids can promote the interfacial reaction. The effect of subcritical n-propanol is the most obvious. Figure 2(e) exhibits the lowest intensity for this peak. This result indicates that MAPS participates in the esterification reaction. All infrared spectra show an intense absorption peak at 1633 cm⁻¹ due to the hydrophilic character of wood flour [27]. The intensity at this peak decreases in the presence of subcritical fluids. This reduction indicates that subcritical fluids are effective in liquefying hemicellulose to reduce the water absorption of the composites. The polarity of the wood flour decreases, so the interfacial adhesion of wood flour and PS matrix tends to be improved. The absorption peaks at 1460 cm⁻¹ are ascribed to the symmetrical bending vibration peaks of the methylene group. The peaks at 1383 cm⁻¹ are attributed to the bending vibration of the methyl group. The peak value at 1161 cm⁻¹ is attributed to C–O–C symmetric stretching vibrations [28]. The intensities of the peaks for samples prepared in the presence of subcritical fluids (Figure 2(b–e)) increase, which demonstrate that subcritical fluids can protect the structure of wood flour from destruction. The relative intensities of the bands at 899 cm⁻¹ indicate the C–H bending of cellulose does not change, revealing that cellulose is only slightly affected by the subcritical fluid [29].

Table 1

The area ratio of characteristic and reference peak of wood flour/PS composites in infrared spectra.

Sample	A _{Characteristic peak} /A _{Reference peak}
Sample	A ₃₄₃₇ /A ₁₄₆₀	A ₁₇₁₅ /A ₁₄₆₀	A ₁₆₃₀ /A ₁₄₆₀	A ₁₁₆₁ /A ₁₄₆₀
a	187.3	29.4	9.86	1.08
b	180.9	45.21	8.21	1.21
c	177.1	49.98	7.76	1.33
d	168.8	67.43	6.56	1.62
e	247.9	38.88	11.24	2.01

SEM analysis

Figure 3 shows SEM micrographs of different samples. As is shown in Figure 3(a), wood flour is coated by the PS matrix. Some cracks exist between the wood fibre and the PS matrix. The broken fibres remain in the PS matrix. This suggests good adhesion between wood flour and PS. However, many wood fibres are agglomerated into bundles. In Figure 3(b–d), the fibres are completely covered and infiltrated by the PS matrix. The wood flour is evenly distributed in the PS matrix, which indicates that the liquefaction and swelling effect of subcritical fluids can promote the distribution of wood flour in the resin. A few holes are found in Figure 3(b), while there are almost no holes in Figure 3(c,d). The fractured surface of the wood flour is found to be rougher in Figure 3(d) than in Figure 3(b) or (c). This difference demonstrates that subcritical n-propanol is more efficient in improving the interfacial adhesion and distribution of wood flour and the PS matrix. Figure 3(e) shows the SEM image of the sample without MAPS under subcritical n-propanol. The wood fibres are evenly dispersed, but there are many holes and protruding wood fibres on the fractured surface. This result indicates that the wood flour is weakly bonded to the PS matrix, so the wood fibres are easily pulled out. In addition, the SBR rubber particles are clearly visible on the fractured surface in Figure 3(e) but are blurred on the fractured surface of the samples containing MAPS, proving that MAPS plays an important role in enhancing the interface between SBR and the PS matrix.

XRD analysis

Figure 4 shows the XRD patterns of different samples: 1-wood flour, 2-WPCs without subcritical fluid but with 10% MAPS, 3-WPCs with subcritical n-propanol and 10% MAPS, and 4-WPCs with subcritical n-propanol but without MAPS. The crystallinity index of the different samples is shown in Table 2. The wood flour and composites show similar XRD patterns with two diffraction peaks at 2θ values of approximately 15.8 and 22.4°. The peak at approximately 2θ = 22.4° corresponds to both the crystalline and amorphous regions of the composites, while the peak at approximately 2θ = 15.8° only represents the amorphous region [30]. The crystallinity index (I_Cr) of the samples (patterns 1–4) is calculated as 34.46, 38.17, 41.93, and 38.96%, respectively. The crystallinity in cellulose appears due to the formation of inter- and intramolecular H-bonding by the hydroxyl groups. The amorphous region is mainly composed of hemicellulose and lignin [31]. Untreated wood flour contains many noncrystalline components. There is no shear stress to break the amorphous region, which results in a low crystallinity of wood flour. For the sample without subcritical fluids but containing 10 wt-% MAPS (pattern 2), esterification happens between MAPS and wood flour, which leads to the random cellulose orientation alignment. The result is an increase in the crystallinity value. The increased crystallinity index of the sample with 10 wt-% MAPS in the presence of subcritical n-propanol (pattern 3) can be attributed to the degradation of amorphous portions such as hemicelluloses and lignin [32]. Subcritical n-propanol promotes the liquefaction of hemicelluloses and lignin into small molecules that can be dissolved into subcritical n-propanol. The removal of some hemicelluloses and lignin from diffuse portions leads to a decrease in the degree of polymerisation but an increase in crystallinity. The crystallinity of pattern 4 is lower than that of pattern 3, which indicates that the esterification reaction is beneficial for increasing the crystallinity of the composites.

Table 2

Crystallinity index of wood flour and wood flour/PS composites.

Sample	Intensity at 15.8°	Intensity at 22.4°	Crystallinity index (%)
1	27350	41850	34.46
2	31800	51433	38.17
3	20142	34683	41.93
4	16658	27292	38.96

TG analysis

The thermal properties of wood flour, PS and the composites are assessed via thermogravimetric analysis. Figure 5 depicts the TG (a and b) and DTG (c) curves of different specimens. The relevant dates of the weight losses and the temperatures are shown in Table 3. Wood flour is vulnerable to thermal decomposition. Cellulose and hemicellulose in wood flour have different thermal stabilities due to their different structures [33]. The degradation of hemicellulose occurs at 200–300°C, while the temperature range of 300–400°C is related to the depolymerisation of cellulose [34]. PS exhibits only one decomposition step in the temperature range from 400 to 500°C.

Table 3

Thermal properties of different samples in a nitrogen atmosphere.

Sample	C_150°C^a (%)	T_max1^b (°C)	R_Tmax1^c (°C)	T_max2^b (°C)	R_Tmax2^c (°C)	C_500°C^a (%)
1	92.55	362.56	0.757	–	–	23.75
2	99.99	–	–	433.28	3.05	0.11
3	97.03	361.73	0.43	451.09	0.87	15.03
4	98.75	362.55	0.44	454.49	0.88	15.13
5	97.11	360.71	0.43	451.05	1.06	15.62

^aThe char residue at 150°C or 500°C; ^bThe maximum weight loss temperature; ^cThe weight loss rate at T_max.

All samples except PS show some weight loss within the temperature range of 35.0–150.0°C due to evaporation of residual moisture in the wood flour. The addition of MAPS or subcritical fluid is helpful in reducing the hydrophilicity of the wood flour. The weight loss of the composite specimens (samples 3–5) mainly occurred in two sequential steps. The first decomposition step corresponds to the degradation of wood flour in the temperature range from 200 to 400°C. And the degradation of the polymer matrix of the composites at 400–500°C is categorised as the second step. The degradation process of the composites (i.e. the first step) is delayed relative to wood flour, indicating the thermal stability increases. This increase is attributed to the liquefaction of hemicellulose and lignin by subcritical n-propanol and the chemical linkage between wood flour and MAPS. Good interfacial adhesion can promote heat transfer. The degradation of the composites is significantly different from that of PS in the temperature range of 400–500°C. The thermal stability of the composites increases with the addition of MAPS or subcritical n-propanol. Sample 4 exhibits better thermal stability than sample 3 or sample 5. The comparison can be seen clearly in Table 3. This information indicates that subcritical fluids are useful in improving the thermal stability of composites due to their liquefaction of thermally unstable hemicellulose. Meanwhile, esterification improves the interfacial interaction, which is efficient for heat transfer. The residue content of the materials at 500°C (C_500°C) is shown in Table 3. It is noted that PS has almost no residue at 500°C, which suggests that the residue originates from a component of wood flour. The char residue of sample 3 is the lowest among the three composites, which can be explained by the fact that the part component of wood flour is liquefied under subcritical n-propanol. Overall, the addition of MAPS in the presence of subcritical n-propanol improves the thermal stability of the composite more than using MAPS or subcritical n-propanol separately.

Mechanical properties

Figure 6 shows the mechanical performances of samples with or without subcritical fluids and with 10 wt-% MAPS added. As shown in Figure 6, there is an optimum temperature for WPCs. When the temperature increases, the movement of the PS molecular chain accelerates, and the fluidity increases, which enhances PS contact with wood flour. The interface between the two phases is enhanced, and the mechanical properties of the WPCs are improved. However, under the action of screw shear and high reaction temperature, wood flour is susceptible to coking, and the WPCs show less reinforcement. The TG analysis shows the thermal weight loss of wood flour. The wood flour begins to degrade at around 200°C. The co-rotating twin screw produces shear heat during extrusion. So the enhancement of wood flour is reduced due to degradation after 190°C. Therefore, the mechanical properties of the composites decreased. The composites with and without subcritical fluids exhibit the best mechanical properties at 190°C.

Compared with the blank sample, the samples with subcritical fluids (water, ethanol and n-propanol) show significantly improved mechanical properties. This is because the subcritical fluids as reaction media have excellent permeation and dissolution properties for wood flour. Lignocellulose is complex and mainly composed of cellulose, hemicellulose and lignin. Hemicellulose exhibits high activity for liquefaction and degradation due to its high degree of branching and amorphous structure. Thus, the wood flour shows low polarity and high surface roughness. In addition, because of the swelling effect of subcritical fluids, MAPS penetrates the interface between the wood flour and PS, which leads to improved interface adhesion. PS exhibits higher fluidity in the presence of subcritical fluids, which facilitates processing and a more uniform mixing of PS and wood flour [35, 36]. The chances of MAPS participating in the interfacial reaction increase, and the interface adhesion is improved. The most effective subcritical fluid is n-propanol. This may be because n-propanol has a lower dielectric constant than water and ethanol. The lower the dielectric constant, the easier it is to dissolve non-polar substances. The enhancement of wood flour is mainly provided by cellulose. The subcritical state of n-propanol dissolves poorly soluble substances such as alkali lignin and extracts in wood flour, which result in a higher proportion of cellulose. Long-chain alkyl alcohols exhibit higher efficiency in wood fibre liquefaction in subcritical conditions with shorter reaction times. Efficient liquefaction of subcritical n-propanol removes more unstable structural parts of wood flour during shorter extrusion processes [37]. At the same time, the surface roughness of the wood flour increases, so the interface reaction is more complete. When the temperature is 190°C and extruded under subcritical n-propanol, the composite has tensile strength, flexural strength, flexural modulus and unnotched impact strength values of 34.4, 61.41, 5327 MPa and 10.49 kJ m⁻², respectively.

The mechanical properties of wood flour/PS composites with different contents of MAPS are shown in Figure 7. After the addition of MAPS, the mechanical performances of the composites show great improvement. The tensile strength, flexural strength, flexural modulus and unnotched impact strength show the same growth trend, reaching a maximum when the MAPS content is 10%. This phenomenon can be attributed to esterification enhancing the combination of wood flour and the PS matrix. The PS grafting segments of MAPS are tightly bound to the PS matrix by chain winding. The compatibility between SBR and PS improves with the addition of MAPS. In addition, due to the swelling effect of subcritical n-propanol, MAPS penetrates the interior of the wood flour. Esterification takes place in the interior of the wood flour, which causes a deeper reaction. The esterification reaction is shown in Figure 8. Appropriate amount of MAPS reacts at the interface to form a monolayer. As the contents of MAPS increase, the excess MAPS accumulate at the interface to form a multi-molecular layer. These extra MAPS do not participate in the interface reaction. On the contrary, it will hinder the stress transfer of the monolayer and cause stress concentration. So the mechanical properties of composites decreased. These results suggest that MAPS can improve the mechanical performance of WPCs, but excess MAPS may reduce the stiffness and strength of the composites. 10 wt-% of MAPS is the best dosage.

Conclusions

Wood flour/PS composites were successfully prepared via subcritical fluid-assisted extrusion. The subcritical fluids used as the reaction media are helpful for wood flour and SBR distribution in the PS matrix due to the liquefaction and swelling effect. Subcritical n-propanol results in the most obvious improvement in the performance of composites. The utilisation of an optimum temperature (190°C) is effective for both composites with or without subcritical fluids. Wood flour is not easy to coke, and PS shows high liquidity. The addition of MAPS enhances the mechanical properties due to the improved interfacial adhesion between wood flour and PS matrix. The interface between SBR and the polymer matrix shows significant improvement with the addition MAPS. This result is supported by SEM analysis. When 10 wt-% MAPS is used in the presence of subcritical n-propanol at 190°C, the tensile strength, flexural strength, flexural modulus and unnotched impact strength of the composites can reach 34.4, 61.41, 5327 MPa and 10.49 kJ m⁻², respectively. FTIR analysis indicates that an esterification reaction takes place between wood flour and MAPS. The SEM results confirm the ameliorative interfacial adhesion between wood flour and PS after MAPS is added, especially with 10% MAPS in the presence of subcritical n-propanol. Enhanced thermal stability and crystallinity of the wood flour/PS composites are observed to be better with the addition of both 10 wt-% MAPS and subcritical n-propanol than using MAPS or subcritical n-propanol separately, as demonstrated by TGA and XRD analyses.

Footnotes

Disclosure statement

All the authors do not have any possible conficts of interest.

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