Degradation of epoxidized natural rubber compatibilized linear low density polyethylene/ soya powder blends: the effect of natural weathering

2.1 Materials and sample preparation

The materials used in this study included LLDPE (ETILINAS LL0209SA) supplied by Polyethylene Malaysia Sdn Bhd, Terengganu, Malaysia. The melt flow index was 0.90 g/10 min and the density was 0.921 g/cm³. Soya powder with a melt flow index of 1.0 g/10 min was supplied by Hasrat Bestari (M) Sdn Bhd, Penang, Malaysia. The average granular size was 12 μm and the protein content was 44.2%.

The mixing process was carried out using the melt blending method in a Haake Reodrive 5000 internal mixer. LLDPE was charged to the chamber for 2 min and followed by the addition of ENR 50. The soya powder was added gradually from 4 to 6 min and mixing was continued until the 10th min. The operating temperature of the internal mixer was maintained at 150°C with a rotor speed of 50 rpm. Table 1 shows the composition of the blends. Then, LLDPE/soya powder blends were molded into 1 mm thin sheets in a hot press at 150°C. The samples were cut into dumbbell shapes according to ISO 527, before being exposed to the natural environment.

2.2 Natural weathering

The natural weathering test was carried out at Universiti Sains Malaysia, Penang for a period of 1 year, from June 2011 to May 2012. The meteorology data, such as average temperature, rainfall, and relative humidity, were obtained from the nearest meteorology station in Butterworth (latitude 5°28’N, longitude 100°23’E). Table 2 shows the data obtained from the meteorology station; an average of the data for each month was taken. The samples were collected after 3 months, 6 months and 1 year. The test was carried out according to ISO 877.2. The samples in a dumbbell shape were placed on an aluminum alloy exposure rack facing south and at an inclination angle of 45°. The weathered samples were washed with distilled water, dried, and weighed until a constant weight in an air-drying oven at 70°C.

2.3 Investigation method of degradation

3.1 Chemical structure changes and CIs

FTIR spectroscopy analysis is a good indicator for the chemical changes in polymer molecules after natural weathering. Figure 1 presents the FTIR spectra of ENR 50 compatibilized blends after 6 months and 1 year natural weathering. The changes of peak can be seen at the broad peak of 1740 cm^-1, indicating the presence of a carbonyl group. Khabbaz et al. [8] studied the environmental degradation of polyethylene film and found that many carbonyl compounds, such as esters and carboxylic acids, existed after exposing to the outdoor environment. The carbonyl groups, such as carbon monoxide and methyl vinyl ketone, were generated through the Norrish type I and II mechanism (Figure 2). Therefore, the broad peak at 1740 cm^-1 indicated the overlapping between the ester bonds during compatibilization [7] and carbonyl products generated after the weathering test. Table 3 summarizes the comparison of CIs between uncompatibilized and ENR 50 compatibilized blends. The chemical changes for uncompatibilized blends after outdoor exposure has been discussed in our previous study [9]. The results (Table 3) show that the CIs of LLDPE/soya powder blends increased after 6 months and 1 year of natural weathering. It was confirmed that the carbonyl product increased depending on the level of degradation. The CI for compatibilized blends was higher than that for uncompatibilized blends. Thus, ENR 50 was not only acting as a compatibilizer, but accelerating the abiotic degradation. This might be due to the elastomer, i.e., ENR 50, which generates free radicals in the presence of sunlight during weathering. It subsequently initiated oxidation of LLDPE according to the typical oxidation scheme for polyolefins [10]. Furthermore, the elastomeric phase is the most oxidizable component, as it contains unsaturated bonds. The higher degradation of ENR 50 compatibilized blends can be further proven in tensile properties.

Figure 1

Fourier transform infrared (FTIR) spectra of unweathered and weathered sample for ENR 50 compatibilized linear low density polyethylene (LLDPE)/soya powder blends.

Figure 2

Schematic diagram for the formation of carbonyl groups through the Norrish Type 1 and Norrish Type II mechanisms.

3.2 Tensile properties

Figure 3 shows the tensile strength of ENR 50 compatibilized LLDPE/soya powder blends over 1 year of natural weathering. It can be observed that the tensile strength reduced with increasing soya powder content, indicating an increase in degradability. After 3 months of weathering, the tensile strength was generally reduced as a function of soya powder content. However, a fragmentation process occurred on the blends with 40% soya powder content. The samples were fragmented and unable to be subjected to tensile test. Based on our previous study [9], soya powder is a highly hydrophilic natural polymer and it can be easily leached out during outdoor exposure. Therefore, the remaining LLDPE can be broken down to smaller segments, by an external force like raining during the weathering test. After 1 year exposure time, the degradation effect was more pronounced as the blends with 20–40 wt% soya powder contents were fragmented. Table 4 illustrates the retention of tensile properties of uncompatibilized and compatibilized blends after weathering. The retention value can further explain the degradation effect as a function of soya powder content and exposure duration. The reduction of tensile strength retention with increasing soya powder content has confirmed that the addition of soya powder can accelerate the degradation process.

Figure 3

Tensile strength and retention of compatibilized blends after different periods of weathering.

The E_b of blends during 1 year of natural weathering is shown in Figure 4. The trend of E_b is similar to that of the tensile strength (Figure 3). The reduction of the E_b is due to the abiotic and biotic effect from the environment. The soya powder leached out and left some pores, consequently created a bigger surface area for further degradation. The mechanism of the degradation of the blends can be explained by a morphological study. Figures 5 A–C and 6 A–C show the morphology of the surface of weathered blends after 6 months and 1 year of natural weathering, respectively. Increasing the soya powder content in the blends created more pores on the weathered surface. The size of the pores is even bigger at high soya powder content. This allows microorganisms, such as fungus, to occupy and consume the polymer. The degradation was more critical after 1 year of natural weathering (see Figure 6). As the exposure period increased, more fungus colonized on the surface of the samples and larger pores were observed. In uncompatibilized blends [9], there were fewer pores and the pore size was smaller compared to ENR 50 compatibilized blends. This can be further confirmed by the E_b retention (Table 4). The retention for compatibilized blends was also lower than for uncompatibilized blends. Apart from the fungus colonization, the pores also underwent photooxidation and thermal oxidation. The creation of carbonyl products, as discussed earlier, has indicated these degradation processes.

Figure 4

Elongation at break and retention of compatibilized blends after different periods of weathering.

Figure 5

Weathered surface (500× magnification) of compatibilized blends with (A) 5 wt%, (B) 20 wt%, and (C) 40 wt% soya powder content after 6 months weathering.

Figure 6

Weathered surface (500× magnification) of compatibilized blends with (A) 5 wt%, (B) 20 wt%, and (C) 40 wt% soya powder content after 1 year weathering.

The Young’s modulus of the blends was generally increased after 1 year of natural weathering (Figure 7). The increment of the Young’s modulus might be due to the formation of radical crosslinking in the presence of sunlight. The formation of crosslinking was confirmed by the carbonyl product generated, as shown in FTIR spectra (Figure 1). The embrittlement of the sample also resulted in the increment of Young’s modulus. The trend is in agreement with the reduction of E_b after weathering. The blends with a high soya powder content (20–40 wt%) were too brittle and fragmented upon outdoor exposure for 1 year. As shown in Table 4, the retention of Young’s modulus of compatibilized blends was lower than that of uncompatibilized blends. This was because during the weathering test, the leach out effect of compatibilized blends was higher and the weathered samples lost rigidity. Thus, the leach out effect in compatibilized blends was more pronounced compared to uncompatibilized blends. This is in agreement with the superior degradability of ENR 50 compatibilized blends.

Figure 7

Young’s modulus and retention of compatibilized blends after different period of weathering.

3.3 Crystallinity study

The crystallinity change upon environmental exposure is one of the indicators for degradation. Figures 8 and 9 show the melting and cooling thermograms of the blends after 1 year of natural weathering. The melting temperature (T_m) did not significantly change after weathering. This confirmed that there was no new crystalline formed during degradation. The DSC data for the uncompatibilized and compatibilized blends after outdoor exposure is summarized in Table 5. The crystalline temperature (T_c) and crystallinity increased as a function of the weathering period, corresponding to a degradation of the amorphous phase in the blends. According to Khabbaz et al. [8], the amorphous phase in the polymer was first attacked during thermal or oxidative degradation, consequently increasing the portion of the crystalline phase in the remaining blends. Therefore, the increase in crystallinity of weathered samples was regardless of new crystalline formation and was solely contributed to by the reduction of the amorphous phase. Soya powder content in the blends played an important role in contributing to the degradation. At the same exposure period, the blends with higher soya powder content exhibited higher crystallinity. This agreed with the study by Lodha and Netravali [11], in which the crystallinity of soya protein isolate resin increased after degradation. In comparison, the crystallinity of compatibilized blends was generally lower (see Table 5). The effect of ENR 50 in enhancing the degradation through the formation of radicals was discussed in a previous section.

Figure 8

Melting thermogram of compatibilized blends before and after weathering.

Figure 9

Cooling thermogram of compatibilized blends before and after weathering.

3.4 Weight loss

Figure 10 shows the comparison of the weight loss of uncompatibilized and compatibilized blends during different periods of natural weathering. The weight loss of compatibilized blends was higher than that of uncompatibilized blends. It can be observed that the weight loss increased dramatically from 30 to 40 wt% soya powder, because the interaction between LLDPE and soya powder became weaker. Consequently, it was much easier for the soya powder to be leached out during the degradation. As mentioned in a previous section, the degradability of compatibilized blends was higher. Therefore, the number of fragmented segments for compatibilized blends was higher and subsequently leached out from the weathered samples. The presence of pores on compatibilized blends indicated the leached out effect. Apart from the abiotic effect, the biotic factor might result in the weight loss of the blends. The existing microorganism such as fungus in the environment can occupy the sample surface and consume the soya powder phase. The colonization of fungus on the sample surface is shown in Figures 5 and 6. The consumption of the soya powder also contributed to the sample weight loss. The results are in agreement with some weight loss studies in LDPE/starch blends [12, 13], in which the starch is a highly hydrophilic material.

Figure 10

Comparison of weight loss for uncompatibilized and compatibilized blends after weathering.

3.5 Molecular weight change

The weight loss study was the gravimetric analysis of the weathered blends. The molecular weight change was used to analyze the degradation of sole LLDPE. It is claimed that in polyolefins/natural polymer blends, only natural polymers can be degraded or consumed by microorganisms [14]. However, the molecular weight of LLDPE/soya powder showed changes after natural weathering in the current study. Table 6 summarizes the Mn and Mw of uncompatibilized and compatibilized blends. Before weathering, the Mn and Mw of LLDPE were 27.7×10³ Da and 97×10³ Da, respectively. The compatibilized LLDPE phase in the blends was the same with the uncompatibilized blends, because there were no co-polymerization and chain breaks during the compatibilization process. In both the uncompatibilized and compatibilized blends, the molecular weight decreased with increasing soya powder content. The results again explained the leach out of soya which created a bigger surface area for the abiotic degradation, such as UV and thermal degradation. In ENR 50 compatibilized blends, the reduction of the molecular weight was greater than in the uncompatibilized blends. This may be due to the fact that the double bond in ENR 50 was more susceptible to photo and thermal degradation. Consequently, the formation of a peroxide radical further attacks the molecular chain of LLDPE.