Catalyst loading is another important parameter that can significantly affect the yields of degradation products. In this study, three different levels of catalyst load­ing, i. e., 5, 10, and 15 % were investigated for effects on the degradation of HDPE. In this study, the reaction time was fixed at 3 h for HZSM-5 (Si/Al = 80) catalyst. Meanwhile, in case of AlSBA-15 catalyst, initially, the reaction was carried out at a catalyst loading of 5 % for 4 h due to slower reaction to achieve significant level of degradation. As shown in Table 5, a catalyst loading at 10 % yielded the highest amount of light liquid while it produced the least amount of volatile gaseous product as compared to the other two catalysts loadings. The solid coke formation was found to be about 7 % for catalyst loadings of 5 and 10 %. Its yield was slightly higher with 15 % catalyst loading as 7.7 wt. % of the overall weight of residue was obtained.

The trace for waxy compound with 5 % catalyst loading amount was detected, but it was negligible and difficult to be accurately measured. The possible reasons of low conversion of degradation product to liquid were due to the concentration of 5 wt. % catalyst which was considered low to provide sufficient number of active sites for the degradation of large- to moderate-sized polymer chains. However, the optimum catalyst loading was found to be at 10 wt. %. It was further noted that larger amount of catalyst loading might promote faster reaction with higher forma­tion of coke in the residue of degradation products. This might be the reason for the catalyst inactivity throughout the degradation process (Ochoa et al. 1996).

In Fig. 9, the highest amount of liquid degradation product was obtained at 15 wt. % of catalyst loading (23.5 wt. % of the feed polyethylene). It also pro­duced moderate amount of volatile gaseous degradation products (52.2 wt. %) as compared to that obtained using HZSM-5(80). Furthermore, it could prove the theory that zeolitic catalysts such as HZSM-5 promote the production of

gaseous products during degradation of polyethylene. At the same time, it was not considered to be an ideal catalyst for the purpose of maximizing the degradation into liquid yield. However, one of the major disadvantages of using higher cata­lyst loading to increase the liquid product yield was that the formation of higher amount of solid coke in the residue might lead to the poor overall catalyst activity.

It was also observed that at 5 % of catalyst loading, no liquid or gaseous prod­ucts were successfully collected due to the formation of large amount of solid waxy compounds leading to a blockage at the reactor outlet. This was due to the rapid solidification of melted polyethylene degradation products when they were exposed to lower temperature at the reactor outlet. The small amount of catalyst loading was also unable to reduce the activation energy of the degradation process to enable the degradation mechanism to take place rapidly. However, significant formation of solid waxy compound was successfully inhibited at 10 wt. % of cata­lyst loading. Consequently, sufficient amount of liquid and gaseous degradation product was successfully collected. As illustrated in Fig. 9, the maximum liquid yield could be obtained at 15 % of catalyst loading. At the same time, less amount of solid waxy compound might be formed.

Fig. 10 Composition of gas products at 673 K using HZSM-5(80) catalyst over different catalyst loadings

Fig. 11 Composition of gas degradation products using various loadings of AlSBA-15 catalyst at 673 K

Figure 10 shows the data on the gaseous products using HZSM-5(80) catalyst. The highest composition was reported by C4 for 5 and 15 % of catalyst load­ings (27.8 and 33.6 %, respectively). Meanwhile for 10 % of catalyst loading, the highest proportion of C3 carbon chain was 38.4 %. At 5 % of catalyst loading, the amount of catalyst active site was deemed insufficient to cause significant deg­radation of polyethylene. Thus, higher amount of longer carbon chain molecules were produced as they could undergo further cracking reactions into smaller mol­ecules (Pierella et al. 2005). The results also indicated that increasing amount of catalyst would create better uniformity in the products distribution as seen in the case of 15 % of catalyst loading.

Likewise, data regarding catalyst loading and its effects on the degradation of HDPE to gaseous products using AlSBA-15 catalyst at 400 °C are shown in Fig. 11. The highest yield was recorded by the carbon chain of C4 with 15 % of catalyst loading (38.2 %). At 10 % of catalyst loading, its C3 carbon chain composition in the product was 42.1 %. It was also confirmed in earlier findings that the gas prod­uct was more concentrated in the middle of the carbon chain range such as C3 and

Fig. 12 Composition of liquid degradation products by various loadings of HZSM-5 (80) catalyst at 673 K

Fig. 13 Composition of liquid degradation products using various loadings of AlSBA-15 catalyst at 673 K

C4 (Hua et al. 2001). However, no product was detected for Ci products. Similar to earlier case, the use of higher catalyst loading produced more uniform products dis­tribution as in case of HZSM-5 catalyst. By comparing the degradation results using HZSM-5(80) and AlSBA-15 catalysts, it could be concluded that microporous cata­lyst HZSM-5(80) produced more shorter-chain carbon products. At the same time, mesoporous catalyst AlSBA-15 yielded more longer-chain carbon products.

Figure 12 shows the effect of catalyst loading on the distribution of degradation liquid products using HZSM-5(80) as catalyst at 400 °C. The highest composition was recorded with 5 % of catalyst loading to give a C21-C24 carbon chain range of 25.1 %. For the use of 10 and 15 % of catalyst loadings, the compositions of C8—C12 carbon chain range were 34.2 and 30.4 %, respectively. The longest carbon chain range C25+ was the minor product leftover after the degradation process. For this catalyst, it could also be seen that highest product yield was accu­mulated at the lighter end of the C8-C12 and C13-C16 carbon chain ranges.

By comparing to the earlier findings made using HZSM-5(80) catalyst (Koc and Bilgesu 2007), AlSBA-15 mesoporous catalyst showed a more non-uniform distribution (Fig. 13). The highest composition was recorded by a carbon chain

range of C8-C12, i. e., at 10 % catalyst loading (34.2 % yield) while at 15 % catalyst loading, 31.1 % yield of C13-C16 substances was obtained. Similarly, the heaviest carbon chain range, i. e., C25+ again recorded the smallest composition. The difference between the higher end and lower ends was bigger compare to the findings made using HZSM-5(80) microporous catalyst.