Natural durability of organomodified layered silicate filled wood flour reinforced polypropylene nanocomposites

1 Introduction

In the last decade, natural organic reinforcements, such as cellulose fibers, have slowly penetrated into the market as they offer many advantages over most common inorganic fillers. Cellulose fibers are abundantly available and have lower costs and density. They lead to a reduced wear of processing equipment and are renewable, recyclable, non-hazardous and biodegradable [1]. The replacement of inorganic fillers with comparable cellulose fibers provides weight savings and decreases the cost without reducing the rigidity of the composites. Wood fibers are used most extensively among the cellulose fibers as fillers [2].

Wood fiber/plastic composites (WPCs) can be a cost-effective alternative to many plastic composites or metals in terms of bending stiffness or weight. The wood fibers are non-abrasive so that relatively large concentrations can be incorporated into plastics without causing serious machine wear during blending and processing. The main applications of WPCs are in building products, such as fencing, rails, decking, door and window profiles, decorative trims, etc. These composites are also gaining acceptance in automotive, industrial and marine applications [3, 4].

Cellulosic fibers used in composites are prone to attack by deteriorating factors such as wood decaying fungi, and that fungal decay has a substantial effect on the mechanical and physical properties of composites, studying the effects of fungal decay on the characteristics of such composites calls for immediate attention. A number of researches have focused on the natural durability of such composites as their applications are expanding [5]. Varhey and Lakes conducted research on the variations of mechanical properties of composites made of wood fibers/thermoplastic polymer due to exposure to white and brown-rot fungi. They concluded that because of contact with the studied fungi, the bending strength of the composites decreased. Likewise, with the increase of wood fiber percentage in the studied composites and the increase of exposure time to fungus, their bending strength exhibited a more considerable reduction [6]. Karimi et al. reported that the mechanical properties of composites including bending strength, elastic modulus, and hardness decreased due to exposure to rainbow fungus [7].

Today, polymer/layered silicate (PLS) nanocomposites have attracted great interest, both in industry and in academia, as they often exhibit remarkable improvement in material properties when compared with virgin polymer or conventional micro and macro-composites. This can be achieved when the nanoscale silicate platelets are well dispersed by delamination of the silicate-layered structure throughout the polymer matrices, which results in the so-called exfoliated morphology. Organomodified montmorillonite (OMMT) is the most commonly used layered silicate because of its natural occurrences and beneficial properties (high cationic exchange capacity, high surface area, and large aspect ratio). Essential improvements of physical and mechanical properties including tensile modulus and strength, flexural modulus and strength, thermal stability, fire resistance, and barrier resistance have been observed for various thermoplastic and thermoset nanocomposites at low silicate content [8–10]. Using nanoclay filler in WPC composite has been reported in the literature [11–19]. Effort has been made in the formation of wood polymer nanocomposite to improve such properties so as to meet specific end-use requirements. The aim of this study was to investigate the effect of OMMT loading level on the natural durability properties of polypropylene/wood flour composites exposed to brown-rot fungi (Coniophora puteana), one of the commonest wood deteriorating fungi in Iran. A further aim was to investigate what extent the physical and mechanical properties are affected by the fungus in the presence or absence of an OMMT.

2 Experimental

3 Result and discussion

The average mycelium cover and the results of nail scratching test on the specimens are presented in Table 1. The composites without OMMT exhibited the highest mycelium cover and are in the damage class of 2b–3a. It was also shown that there was a considerable difference with other formulations in the nail scratching test. Composites containing 6 phc OMMT against to brown-rot fungi remained intact in regard to the lowest mycelium cover and damage class.

The effects of OMMT on the weight loss and physical properties of PP/WF composites exposed to brown-rot fungi are listed in Table 2. Furthermore, the statistical comparisons between the values of these samples are summarized in Table 2. According to the variance analysis, the OMMT content had significant effects (p<0.05) on the weight loss and physical properties of samples.

Table 2 shows the weight loss decreases with increase of OMMT content and maximum decrease was observed for 6 phc OMMT filled composites. The conditions essential for fungal growth in wood are food, sufficient oxygen, suitable temperature, and adequate moisture. Decay of wood plastic composites is a function of moisture content, and much of the loss in mechanical properties due to fungal attack can be attributed to moisture absorption. Therefore, the first step in preventing decay is to prevent or limit moisture sorption [4–7]. It seems that the barrier properties of nanoclay with the hydrophobic nature of the clay surface inhibit the oxygen and moisture absorption in the polymer matrix. The mechanism to increase the barrier properties of materials from nanoclay is based on the increase in the tortuous length of the diffusion path through a polymer matrix. And with more tortuous length of the diffusion path through a wood plastic composite subsequently being generated, this decreased the sufficient oxygen and water uptake. Therefore, the insufficient moisture and oxygen caused the condition for fungal growth and its attack would be limited. Consequently, the presence of the nanoclay makes the wood plastic composite less accessible for the fungus through reduction of oxygen content, moisture uptake and nutrient shortage.

Table 2 also shows that the water absorption and thickness swelling decrease with increase of OMMT loading. It seems that the barrier properties of nanoclay fillers inhibit the water permeation in the polymer matrix. Two mechanisms have been reported in an attempt to explain this phenomenon. The first is based on the hydrophobic nature of the clay surface that tends to immobilize some of the moisture; second, surfactant-covered clay platelets form a tortuous path for water transport [21, 22]. The latter barrier property hinders water from going into the inner part of the nanocomposite. It seems that both of the aforesaid mechanisms could be more efficient when the morphology is exfoliated. In other words, in the exfoliated morphology there is more available surface area of nanoclay (with hydrophobic nature) and surfactant-covered clay platelets (tortuous path), so the water transport goes down under the severe conditions. Furthermore, by increasing agglomeration of clay into the samples the accessibility of moisture absorption decreased (this phenomenon is called the zigzag effect). Another reason for less water uptake could be the ability of nanoclay to act as a nucleating agent [16]. Due to such nucleation, the crystallinity of the hybrid composite can be improved by the presence of the nanofiller as a nucleating agent. As the crystalline regions are impermeable, the water absorption is less in the composites.

The effects of OMMT on the mechanical properties of PP/WF composites exposed to brown-rot fungi are listed in Table 3. The statistical comparisons between the values of these samples are also summarized in Table 3. According to the variance analysis, the OMMT content had significant effects (p<0.05) on the mechanical properties of samples.

Table 3 shows the flexural strength and flexural modulus increase with an increase of OMMT up to 3 phc and then decreases. At low weight, for OMMT loadings the enhancement of properties is attributed to the lower percolation points created by the high aspect ratio nanoclays. The increase in properties may also be attributed to the formation of intercalated and exfoliated nanocomposite structures, formed at these loadings of clay [11–15]. At higher weight loading, degradation in mechanical properties may be attributed to the formation of agglomerated clay tactoids [17, 19].

Table 3 also shows the impact strength decreases with an increase of OMMT loading. The decrease in impact strength at higher clay content levels is probably due to the formation of clay agglomerates and the presence of un-exfoliated aggregates and voids [17, 19]. It seems the stronger interfacial interaction between OMMT and the matrix might enable more cracks bridging in the fracture process instead of crack-deflecting, which is the cause of the impact strength decrease for the composites with 3% OMMT.

As can be seen in Table 3, the hardness increases with increase of OMMT content and the highest hardness corresponds to the composite containing 6 phc OMMT. As mentioned, the nanoclay protect the wood plastic composites against fungal attack through reduction of oxygen content, moisture uptake and nutrient shortage which necessities for fungus growth and function. Also, it is known that the fungus only deteriorates the points in the specimens that were more accessible [5–7]. Therefore, we expected that the hardness of the samples increased with an increase of OMMT loading.

The morphology of the nanocomposites samples in this study is characterized by XRD and TEM analysis.

X-ray scattering intensities for composites with different levels of OMMT are demonstrated in Figure 1, where the quantity 2θ=4.76° is related to neat clay with a basal spacing of 18.5 nm. In the sample with 3 phc concentration of OMMT, the peak was shifted to a lower angle (2θ=4.48°, d-spacing=19.70 nm), which implies the formation of the intercalation morphology. The peak related to 6 phc of nanoclay appeared at 2θ=0.54°, with a d-spacing of 19.44 nm. These data show that the order of intercalation is higher at 3 phc OMMT than at 6 phc OMMT concentration.

Figure 1

X-ray diffraction (XRD) patterns of composites with different levels of organomodified montmorillonite (OMMT).

Figure 2 shows the dispersion state of OMMT in the composites, as was made evident by TEM. The dark line represents the intersection of the silicate layers, while the white background corresponds to polypropylene matrix. When the loading level of OMMT into the PP/WF composite was as low as 3 phc (Figure 2A), nanoclay exhibited better dispersion of the clay layers within the polymer matrix than at 6 phc of nanoclay content (Figure 2B). Increasing the level of OMMT to 6 phc, the size of dispersed nanoclay became larger or even aggregated in part (as confirmed by decreased d-spacing from XRD in Figure 1).

Figure 2

Transmission electron microscopy (TEM) micrographs of polypropylene/wood flour/organomodified montmorillonite (OMMT) nanocomposite: (A) 3 phc OMMT; (B) 6 phc OMMT.

4 Conclusions

The following conclusions could be drawn from the results of the present study:

1. The OMMT had significant effects on the natural durability of the studied composite formulations.

2. The flexural strength and modulus increased with an increase of OMMT up to 3 phc and then decreased. However, the impact strength decreased with an increase of OMMT loading.

3. The highest hardness and the lowest weight loss, water absorption and thickness swelling were observed in the composite containing 6 phc OMMT.

4. Morphological findings revealed that intercalation came from the sample with 3 phc concentration of OMMT, which implies the formation of the intercalation morphology and better dispersion than 6 phc.