Influence of fungal infection on the long-term water absorption and morphological behavior of bagasse fiber/polypropylene composites at different exposure times

2.1 Materials

The polymer matrix used in this study was PP with a melt flow index of 7–10 g/10 min and a density of 0.90 g/cm³ (supplied by Bandar Imam Petrochemical Co., Bandar-Abbas, Hormozgan, Iran). The lignocellulosic material used as the reinforcing filler in the composite was fresh bagasse (Saccharum officinarium L.) from sugarcane fields in the south of Iran (Ahvaz, Khuzestan). The bagasse was chopped with a laboratory chopper and then soaked in tap water for 1 h. Then the soaked bagasse was steamed under 8 psi pressure and 170°C temperature. The steamed bagasse was then pulped by using a laboratory atmospheric refiner. The BFs average 100 mm in length. Maleic anhydride polypropylene (MAPP) was obtained from Eastman Chemical Products, Inc. (Kingsport, TN, USA), as Epolene G-3003^TM polymer with 8% acid anhydride and a molecular weight of 103,500. It was used as coupling agent.

2.2 Composite preparation

Before preparation of samples, BF was dried in an oven at 65±2°C or until it reached a constant weight. The mass ratio of BF to PP was controlled at 40:60 percent by weight for all blends. The amount of coupling agent was fixed at 2% for all formulations. The mixing was carried out in a rectangular mold box with a size of 36 cm×31 cm and manually formed. The mats were then compressed on aluminum cauls in a hot press (Oil Hydraulic Press, Burkle L100, Stuttgarter, Germany) at a temperature of 190°C using a pressure of 3 MPa for 6 min. This was the amount of time necessary to reach the melting temperature of the PP in the core of the panels. Teflon films were used to avoid the adhesion of MAPP to the stainless surface of the mold. Stops were used to produce panels at a given thickness. All panels were pressed to an average target thickness of 10 mm. The target density was 1.02 g/cm³. The edge of the panel had a lower density than the rest of the panel. Thus, the edges were trimmed off, and the final panel size was 32 cm×27 cm. This allowed for less variability in density (hence properties) within each panel. Finally, specimens were conditioned at 23°C and 50% relative humidity over 40 h according to ASTM D 618.

2.3 Fungus culture

Malt extract agar was used as the culture medium. The medium was supplied by Merck (NJ, USA) and was used at a concentration of 48 g/L in all laboratory tests. Purified white-rot (Trametes versicolor) and brown-rot (Coniophora puteana) fungi were used in this study as the biological degradation agents. The white-rot and brown-rot fungi were transferred to Petri dishes containing malt extract agar under sterile hood using sterile pincers near an alcohol burner. The dishes were kept at 25°C for 1 week until the culture medium was fully covered by the fungus. The cultured fungus was transferred into Kolle dishes containing the culture medium that were incubated for 14 days at 25°C. Then, to prevent direct contact of the specimens with the culture medium, the specimens were mounted over two 3-mm platforms and were placed in the Kolle dishes. The dishes containing the fungus and the specimens were then stored in an incubator for 8, 12, and 16 weeks at 25°C and 75% relative humidity.

2.4 Measurements

Water uptake tests were carried out according to ASTM D 7031 specification. Specimens with a dimension of 20 mm×20 mm×20 mm were cut for the water uptake measurement. Five replicates were used for each sample code. To ensure the same moisture content for the specimens before each test, all the specimens were oven-dried at 102±3°C. The weight and thickness of dried specimens were measured to a precision of 0.001 mm. The specimens were then placed in distilled water and kept at room temperature. For each measurement, specimens were removed from the water, and the surface water was wiped off using blotting paper. Weight and thicknesses of the specimens were measured at different time intervals during the long-time immersion. The measurements were terminated after the equilibrium weights and thicknesses of the specimens were reached. The values of the water absorption in percentage were calculated using Eq. (1).

where WA(t) is the water absorption at time t, W₀ is the oven-dried weight and W(t) is the weight of specimen at a given immersion time t.

For the weight (mass) loss measurement, dry weights of the specimens were measured after 24 h at 103±2°C and weight losses were calculated using the following equation:

where M_b and M_a denote the oven-dry weights prior to and after incubation with fungi, respectively.

The morphology of composites was characterized using scanning electron microscopy (SEM, Model LEO 440i, Oxford, UK) at 15 kV accelerating voltage. Samples were first frozen in liquid nitrogen and fractured to ensure that the microstructure remained clean and intact, and then coated with a gold layer to provide electrical conductivity.

3.1 Long-term water absorption behavior

The water absorption curve is illustrated in Figure 1, where the percentage of water absorption is plotted against time for all samples. It is clearly seen that generally water absorption increases with immersion time; this trend continued up to where no more swelling was attained. Time to reach the saturation point was not the same for all formulations.

Figure 1

Effect of fungal decay on the long-term water absorption of BF/PP composites at different exposure times.

As can be seen in Figure 1, the water absorption of BF/PP composites was actually much higher after both types of rooting and was significantly higher than that of control samples. Various possible reasons for greater water absorption and moisture penetration into the samples exposed to white- and brown-rot fungi in an incubator could be proposed, where the main mechanism is the diffusion of water molecules inside the microgaps between polymer chains [19, 20]. It might alternatively be proposed that rooting created channels for the water into the composite structure. Two mechanisms by which this could occur are capillary transport into the gaps and flaws at the interface between fibers and polymer, and transport by microcracks in the matrix, which formed during the compounding process. Other possible mechanism may involve diffusion behavior (Fickian diffusion process) in the polymer matrix of composites.

Figure 1 also shows that the water absorption of white-rotted samples was higher than that of brown-rotted and control samples for all weeks of fungal exposure times. It is well known that white-rot fungi deplete all components of the wood cell wall during decay, but some species cause selective removal of the lignin in wood. In both the decay types, the lignin in the wood cell wall being decayed is completely depleted, but some white-rot fungi can degrade the lignin in the wood preferentially to cellulose [19, 20]. It can be said that the removal of lignin as a hydrophobic component tends to favor increased water absorption of white-rotted samples.

Another interesting result in Figure 1 is that the water absorption of BF/PP composites increases with increase of exposure time to fungal decay. It is well known that the void content was increased by increase of exposure time to fungal decay; consequently, more voids will speed the penetration of water into the depth of the composites. Also, the presence of more voids helps to make the WPC much more accessible for the moisture uptake due to increasing of hydrogen bonds between hydroxyl groups of wood flour and water molecules. Furthermore, the fungal decay changes the cell morphology, and chemical structure increases the water absorption in BF/PP composites with the porous structure formation. Also, it seems that the void volume in the composite material system, which was affected by exposure time to fungal decay, could have increased the capacity for accommodating more water molecules and thereby increased the hygroscopic behavior.

In general, there are three known mechanisms for water transport in polymer composites: Fickian diffusion, relaxation controlled and non-Fickian or anomalous. The dominant mechanism depends on factors such as chemical structure of the polymer, dimensions and morphology of the wood flour and polymer-filler interfacial adhesion. These cases can be distinguished theoretically by the shape of the sorption curve represented by the following equation [13, 14]:

where M_t is the water absorption at time t, M∞ is the water absorption at the saturation point and k and n are constants. The amount of the n is different for the following cases: in Fickian diffusion, n=0.5; relaxation n>0.5; and anomalous transport 0.5<n<1.

The coefficients (n and k) are calculated from the slope and intercept of the log plot of M_t/M∞ versus time, which can be drawn from experimental data.

An example of the fitting of the experimental data for white-rot and brown-rot fungi at different exposure times is given in Figures 2 and 3, respectively.

Figure 2

Diffusion case fitting for BF/PP composites against white-rot fungi at different exposure times.

Figure 3

Diffusion case fitting for BF/PP composites against brown-rot fungi at different exposure times.

The values of k and n resulting from the fitting of all formulations are shown in Table 1. The n values are similar for all formulations and close to n=0.5. Therefore, it can be concluded that the water and moisture absorption of all formulations approach the Fickian diffusion case. Table 1 also shows the water diffusion coefficients for all formulations. The results show that the fugal decay incorporated with exposure time caused an increase in the water diffusion coefficients into BF/PP composites. The control sample and composite after 16 weeks of exposure to white-rot fungi exhibited the lowest and highest water diffusion coefficients, respectively.

The water diffusion coefficient is the most important parameter of the Fick’s model and shows the ability of water molecules to penetrate inside the composite structures. At early stages and short times (typically M_t/M∞≤0.5), the diffusion process is presented as follows [16]:

where L is the thickness of the specimen and D is the diffusion coefficient.

The influence of fungal decay on the weight loss of BF/PP composites at different exposure times is presented in Table 2. As can be seen, the weight loss of samples increases with exposure to fungal decay. The biodegradability of PP is very limited [16–20]. Therefore, calculation of weight loss was based on bagasse filler because this represents the predominant fungal food source in the composite. It can also be concluded that only BF in surface regions would be accessible to fungal attack [19, 20]. Furthermore, white- and brown-rot fungi caused a higher weight loss of 7.15% and 6.3% for 16 weeks, respectively. The possible reason for greater weight loss of samples exposed to white-rot fungus in an incubator could indicate a difference in decay mechanism. Even small mass losses in the early stages of decay are associated with significant strength losses, especially when the wood is colonized by brown-rot fungi [16–20].

3.2 Electron microscopy

SEM micrographs showing the fracture surfaces of the composites against fungal decay at different exposure times are given in Figure 4. As can be seen, the extent of degradation increased with increasing exposure time to fungus. These results were consistent with the observed losses of mass. Structure decomposition of samples was considerable after 16 weeks of exposure.

Figure 4

SEM micrograph of the fracture surfaces in the composites against fungal decay at different exposure times. (A) Control sample, (B) 8 weeks, (C) 12 weeks, (D) 16 weeks.