Effects of wood flour addition and coupling agent content on mechanical properties of recycled polystyrene/wood flour composites

Abstract

The effect of filler addition and coupling agent content on the mechanical properties and morphology of wood plastic composites was examined. Using wood flour as the reinforcement filler and recycled expanded polystyrene as the thermoplastic matrix polymer, a particle-reinforced composite was prepared using a co-rotating twin-screw extruder. In the sample preparation, four levels of filler loading (10, 20, 30, and 40 wt%) and three levels of coupling agent content (1, 2, and 4 wt%) were used. The influence of particle size and particle morphology was also evaluated. The results showed that short fibers provide a higher specific surface area, enlarging the contact surface area with the polymer matrix. A wood flour loading of more than 20 wt% caused a decrease in the mechanical properties studied. The addition of 2 wt% of coupling agent improved the interfacial adhesion between the polymer matrix and wood flour and increased the flexural and impact strengths of the lignocellulosic composites. Addition of a coupling agent content greater than 2 wt% caused a reduction in the mechanical properties evaluated. A morphological study revealed that the positive effect of coupling agent on the interfacial adhesion of wood flour reinforced the expanded polystyrene composites.

Introduction

The need for materials possessing particular characteristics for specific purposes, while at the same time being renewable and environment-friendly, is increasing, dueto a lack of resources and increasing levels of environmental pollution.^1,2 Lignocellulosic plastic composites developed with recycled polymers and wood waste provide an alternative, leading to a reduction in the use of materials from non-renewable resources.^3–7 These composites have several advantages over traditional mineral-filled plastic composites: low density, low cost, lower energy consumption during manufacturing, non-abrasive behavior during processing, renewability, and biodegradability.^8–10

The use of lignocellulosic fibers shows some drawbacks such as degradation at relatively low temperatures (around 200°C)^2,11 and low compatibility between the hydrophilic character of the polar lignocellulosic filler and hydrophobic character of the non-polar polymer matrix. Due to strong intermolecular hydrogen bonding between the lignocellulosic fibers, they tend to agglomerate during mixing with the polymer matrix in the compounding process. ¹² The low compatibility and interfacial adhesion of composites lead to low mechanical and thermal properties of the final products.

Enhancement of the compatibility between the hydrophobic thermoplastic polymer and the hydrophilic lignocellulosic filler has attracted much attention from researchers as the interfacial adhesion plays an important role in determining the mechanical properties of the composites. ¹³ The addition of a coupling agent can represent a useful tool for achieving such adhesion. ¹⁴ Maleic anhydride-grafted thermoplastic polymers are the most common coupling agent used to improve the interfacial adhesion between lignocellulosic fillers and a thermoplastic polymer matrix.^13–15

The objective of this study was to evaluate the effect of the wood flour loading and coupling agent content on the mechanical properties and morphology of composites developed with recycled expanded polystyrene (r-EPS) and wood flour. In addition, an investigation of the wood flour particle size, shape, and surface morphology was carried out.

Experimental

Materials

The expanded polystyrene waste was obtained from a sorting unit called Associação de Recicladores Serrano, Caxias do Sul, Brazil, and it had a melt flow index (MFI) of 20 g/10 min (200°C/5 kg). Commercial-grade virgin polystyrene N1921 supplied by Innova S/A with an MFI of 20 g/10 min (200°C/5 kg) was also used for comparison studies. Wood flour of Pinus elliottii was obtained from Madarco Co., Caxias do Sul, Brazil. The poly(styrene-co-maleic anhydride) oligomer, SMA, used as a coupling agent was supplied by Sartomer Company, Exton/USA. The coupling agent, SMA2000, contains 30 wt% of maleic anhydride groups and a weight average molecular weight of 7500 g/mol.

Composite preparation

The methodology for recycling expanded polystyrene waste has been described in a previous paper. ¹⁶ The wood flour was dried in an oven at 105°C for 24 h, before being used in the composite formulations. Composite formulations were processed in a co-rotating twin-screw extruder at 200 rpm. The nine barrel temperature zones were controlled at between 160°C and 190°C. Specimens for dynamic mechanical tests were injection molded at a barrel temperature of 180°C and mold temperature of 40 ± 2°C. The formulations of the composite samples are given in Table 1.

Table 1.

Wood plastic composite formulations (percent by weight).

Sample code	r-EPS content	Wood flour content	Coupling agent content
r-EPS	100	0	0
M100	90	10	0
M200	80	20	0
M300	70	30	0
M400	60	40	0
S201	79	20	1
S202	78	20	2
S204	76	20	4

Wood flour particle size distribution

The wood flour particle size distribution was determined based on ASTM D1921 – Method A. A sample of 50 g of previously dried wood flour was placed on a mechanical sieving device with selected sieves. The measurements were carried out in triplicate.

Mechanical testing

The flexural tests were performed according to ASTM D790 at a flexural speed of 1.5 mm/min using an EMIC DL 3000 testing machine. Izod impact strength was measured with a CEAST Resil 25 pendulum using unnotched specimens according to ASTM D256. Each test value was calculated as the average of at least five independent measurements.

Morphological study

Studies on the morphology of wood flour and composites were carried out using a SHIMADZU Superscan SS-550, scanning electron microscope (SEM). The cryofracture surface specimens were sputter-coated with gold before the analysis in order to eliminate electron charging.

Results and discussion

Wood flour characterization

Fiber size and shape are among the most important factors related to composite materials. ¹⁷ The effective surface area, which influences the mechanical properties inversely depends on particle size and shape. Its means that the same amount of smaller particles expose more effective surface area than the bigger particles if the particle has the same shape. Figure 1 shows the particle size and shape of the wood flour used in the composite formulations where different shapes can be observed. However, most of the particles are of longitudinal shape, characteristic of fibers. The particle size distribution is shown in Figure 2. It was observed that approximately 80% of all fibers were distributed in the range 40–150 µm, although the distribution varied. Also, the distribution was found to be almost bimodal. Short and tiny fibers are preferable for the development of composite formulations, ⁵ since these provide a higher specific surface area and a greater surface area of contact with the polymer matrix which usually produces better fiber–matrix adhesion when the coupling agent is used. Also, an increase in the mechanical properties is observed.^18,19 Thus, the swelling decreases and breaks during processing are reduced. ⁵

Figure 1.

SEM micrograph of fiber geometry.

Figure 2.

Particle size distribution.

The fiber surface morphology plays a vital role in the case of composite materials. ¹⁷ The external surface features of the wood flour can be observed in Figure 3. The wood fiber surface is rough, which can provide mechanical anchoring to the polymer composite matrix and thus lead to improved mechanical properties compared to particles with smooth surfaces.

Figure 3.

Surface morphology of wood flour.

Effect of wood fiber loading

The typical flexural stress versus strain curves of wood flour/r-EPS composites are shown in Figure 4. As can be seen in the figure, the addition of wood flour increased the composite stiffness, but decreased the flexural strength.

Figure 4.

Flexural stress–strain curves for composites with different wood flour contents.

Table 2 gives the flexural strength, flexural strain, flexural modulus, and impact strength values of virgin polystyrene (PS control), r-EPS/wood composites containing different wood flour loadings, respectively. The PS control exhibits higher mechanical properties than r-EPS, probably because the r-EPS is mechanically recycled ¹⁶ before it is used in the development of the composites, which can cause chain scission and reduction of mechanical properties. ²⁰ It can be clearly observed in Table 2 that increasing the wood fiber content increased the composite flexural modulus, but decreased the values for the flexural strength and flexural strain. The changes in the flexural properties of recycled polystyrene/wood composites, due to the addition of wood fibers, can be considered as follows. The flexural modulus progressively increases with filler content, probably due to the fact that the wood fiber is more rigid than the polymer matrix.^19,21 The decrease in flexural strength and flexural strain with increasing filler loading can be attributed to poor dispersion of the fibers in the matrix. The wood fibers tended to form agglomerates,^1,19 due to strong inter-fiber hydrogen bonding, and thus dispersion of the individual fibers was inhibited as the filler content increased. Furthermore, the poor interfacial bonding between the hydrophilic filler and hydrophobic matrix also led to decreased flexural strength.^19,21,22

Table 2.

Flexural and impact properties of recycled polystyrene/wood flour composites without coupling agent.

Sample	Flexural strength (MPa)	Flexural strain (%)	Flexural modulus (MPa)	Impact strength (J/m)
PS control	53.66 ± 2.78	1.59 ± 0.19	3734 ± 207	129.70 ± 7.24
r-EPS	46.10 ± 1.53	1.56 ± 0.06	3315 ± 189	123.77 ± 6.96
M100	52.35 ± 0.93	1.51 ± 0.02	3621 ± 62	96.87 ± 4.82
M200	54.96 ± 2.71	1.22 ± 0.04	4072 ± 22	95.98 ± 1.62
M300	47.43 ± 2.72	0.97 ± 0.02	4844 ± 99	78.62 ± 4.00
M400	46.51 ± 1.75	0.89 ± 0.04	5725 ± 86	71.47 ± 5.85

The Izod impact strength of the composites decreased as the filler content increased, as can be seen in Table 2. The introduction of reinforcements will introduce weak interfacial regions and the concentration of stress at the fiber ends, thereby decreasing the impact strength. ²³ The wood filler reduced the polymer chain mobility and therefore its ability to absorb energy during fracture propagation. The poor interfacial bonding between the filler and matrix causes micro-cracks to occur at the point of impact, which cause cracks to easily propagate in the composite,^24,25 reducing the impact strength of the composites with no improvement in the interfacial adhesion between the wood fiber and polystyrene matrix.

Effect of coupling agent content

Considering practical applications, the mechanical properties of composites are of major importance. For short fiber-reinforced thermoplastic composites, the adhesion between the fiber and the matrix plays a fundamental role in achieving good mechanical properties. ²⁶

The effect of the use of a coupling agent on the composites was studied using 1, 2, and 4 wt% of SMA with a wood flour content of 20 wt%. Typical flexural stress–strain curves for composites with 20 wt% of wood flour and different coupling agent contents are shown in Figure 5. Some qualitative information on the nature of the fiber–matrix interface could be obtained by comparing the results from the stress–strain curves. ²⁷ The curves showed that the addition of 1 and 2 wt% of coupling agent caused an increase in the flexural strength and flexural modulus compared to the composite without coupling agent. On the other hand, the addition of 4 wt% of coupling agent caused a more pronounced reduction in the flexural strength and flexural modulus.

Figure 5.

Flexural stress–strain curves for composites with 20 wt% of wood flour with and without coupling agent.

The flexural and impact properties increased with addition of 1 and 2 wt% of coupling agent compared to the composite without coupling agent, as can be seen in Table 3. This behavior can be attributed to the reaction between the hydrophilic –OH groups of the filler and the acid anhydride groups of the SMA,^12,27–29 as can be seen in Figure 6. The greatest effect of SMA was observed for the composite containing 2 wt% of coupling agent, showing an improvement of 23% in the flexural strength. The flexural strain remained practically unchanged on addition of different amounts of SMA. The flexural modulus was improved in composites with the addition of coupling agent. Composites with 2 wt% of SMA presented flexural modulus values 18% higher than composites without coupling agent. The improvement in the interfacial adhesion and enhanced stress transfer from the polymer matrix to the stiffer wood flour caused an increase in the flexural modulus.

Table 3.

Flexural and impact properties of recycled polystyrene/wood flour composites with coupling agent.

Sample	Flexural strength (MPa)	Flexural strain (%)	Flexural modulus (MPa)	Impact strength (J/m)
M200	54.96 ± 2.71	1.22 ± 0.04	4072 ± 22	95.98 ± 1.62
S201	64.52 ± 1.99	1.33 ± 0.25	4525 ± 23	99.45 ± 4.59
S202	67.43 ± 2.62	1.54 ± 0.08	4806 ± 94	96.49 ± 5.65
S204	51.28 ± 3.83	0.95 ± 0.06	4455 ± 72	95.23 ± 2.60

Figure 6.

Hypothetical model of interactions at the interface of composites with SMA.

The impact strength was, in general, higher for composites with SMA than for the composite without coupling agent, as presented in Table 3. The presence of SMA improved the wood flour dispersion and led to a more uniform distribution of the applied stress. Therefore, more energy for debonding and fiber pullout is required^22,30,31 and thus the impact strength increases. The composite with 2 wt% of SMA had higher impact strength than the composites with 1 or 4 wt% of SMA. This composite thus absorbs more energy before break than the other two compatibilized composites.

For composites with 4 wt% of SMA, the mechanical properties were inferior to those of the composites without coupling agent. This behavior may be attributed tothe migration of excess SMA around the fibers, causing self-entanglement amongthe coupling agent rather than the polymer matrix resulting in slippage.^27,32 The same trend was observed by Rana et al. ³³ in jute fiber composites reinforced by a polypropylene matrix where composites with 4 wt% of maleated polypropylene showed a slight decrease in the mechanical properties. Probably, it is due to the migration of a higher content of coupling agent around the fiber surfaces, that ocassionates slippage of the polymer matrix.^32,33 On the other hand, it is possible that the SMA excess may be dispersed in the polymer matrix; thus, acting as a plasticizer because its molecular weight is lower in comparison with the matrix, and consequently there is a reduction in the mechanical properties.^26,29,34 Nevertheless, satisfactory results were obtained with 2 wt% of SMA.

The morphology of the fracture surfaces of the r-EPS/wood composites without SMA showed that the surface of the wood flour is very smooth and gaps between the filler and matrix can be seen, Figure 7(a), which indicates poor bonding^1,12 between the polystyrene and wood flour. On the other hand, the morphology of the fracture surfaces of the composites with 2 wt% of SMA showed that the surface of the wood flour is very rough with traces of polymer matrix attached to it, Figure 7(b). This indicates strong interfacial adhesion and good wetting^12,23 between the wood flour and polystyrene.

Figure 7.

SEM micrographs of composites with 20 wt% of wood flour (a) without and (b) with coupling agent.

Conclusions

Based on the results of this investigation, wood flour with small particle size, between 40 and 150 µm, can provide a higher specific surface area and a larger effective surface area for wetting with the polymer matrix. The addition of more than 20 wt% of wood flour causes a decrease in the mechanical properties studied. This behavior may be associated with the formation of some particle agglomerates due the intermolecular hydrogen bonds, which reduces the mechanical properties. The addition of a coupling agent was shown to improve the interfacial adhesion and increase the flexural and impact strengths. Satisfactory results were obtained with 2 wt% of SMA. The SEM micrographs confirm better adhesion between the wood flour and recycled polystyrene. The addition of more than 2 wt% coupling agent content caused a reduction in the mechanical properties studied.