Use of coffee residue as filler in natural rubber

Abstract

The objective of this research is to study the possibility of the use of coffee residue as filler in natural rubber. The various forms of filler were prepared from coffee residue including coffee residue particle (CRP), Si69 treated CRP, char, and activated carbon. The result from a laser particle size analyzer indicated that the particle size of char and activate carbon were comparable and nearly half of that of CRP. However, the BET surface area of char and two activated carbons prepared at 700 and 800°C were only slightly higher than that of CRP. Interestingly, the activated carbon prepared at 600°C had tremendously higher BET surface area than the others. The pH value of char and all activated carbons were also comparable and higher than that of CRP. Among various forms of filler prepared from coffee residue, it was found that CRP showed the lowest reinforcing efficacy. It was not only its largest particle size but also its acidity nature inhibiting curing process, thus giving the vulcanizates with lower crosslink density. The modifying CRP surface by Si69 enhanced the reinforcing ability of CRP. But its reinforcing ability was still low when comparing with the other two forms (char and activated carbon). The presence of char and all three different activate carbons showed nearly similar trend on the cure characteristics, Mooney viscosity, and mechanical properties but different in degree. Importantly, it was observed that amongst fillers prepared from coffee residue, char had highest reinforcing efficacy. This was evident by highest tensile and tear strength, modulus and hardness. The optimum loading for tensile and tear strength is observed at 10 and 20 phr, respectively. Then, the effect of char, highest reinforcing filler from coffee residue, on the cure characteristics, Mooney viscosity, and mechanical properties was compared with the commercial fillers including reinforcing CB (N330), semi-reinforcing CB (N774), and inert calcium carbonate. Char showed similar effect on the cure characteristics and Mooney viscosity as did N774. But the ability to enhance mechanical properties of char was much lower than that of CB both N330 and N774. With the exception of tensile strength, char gave mechanical properties more or less better than CaCO₃. Therefore, coffee residue that was turned into char by carbonization can be used as inert filler for natural rubber.