PEER-REVIEWED ARTICLE bioresources.com Ooi et al. (2013). “HCl-treated oil palm ash composites,” BioResources 8(4), 5133-5144. 5133 The Effect of Hydrochloric Acid Treatment on Properties of Oil Palm Ash-filled Natural Rubber Composites Zhong Xian Ooi, Hanafi Ismail,* and Azhar Abu Bakar The properties of non-treated and hydrochloric acid (HCl)-treated oil palm ash (OPA)-filled natural rubber (NR) composites were investigated in terms of swelling behavior, rubber-filler interaction, ageing resistance, dynamic mechanical analysis, and thermal stability. The incorporation of OPA resulted in a lower degree of swelling in the NR composites, which was even lower after HCl treatment. Concerning the rubber-filler interaction, the HCl-treated OPA had better interfacial interaction with the NR matrix than that of non-treated OPA, resulting in higher crosslink density and improved ageing resistance. The dynamic mechanical properties of NR composites were better with the incorporation of HCl- treated OPA compared to non-treated OPA, in that they showed a higher storage modulus and lower mechanical loss factor. Thermogravimetry analysis revealed that the HCl treatment process did not affect the thermal stability of OPA-filled NR composites. Keywords: Oil palm ash; Natural rubber; Treatment; Rubber-filler interaction; Dynamic mechanical properties Contact information: Division of Polymer Engineering, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia; * Corresponding author: hanafi@eng.usm.my INTRODUCTION Ash, usually regarded as waste, can be obtained from the residue of combustion sources that generate steam for electrical power plants. Various kinds of ash have been obtained, e.g., coal fly ash, rice husk ash, and oil palm ash. In this study, oil palm ash (OPA) was used because Malaysia is the second largest producer of palm oil in the world, after Indonesia (Santosa 2008; Mekhilef et al. 2011). According to Teoh (2010), there has been significant growth in the production, consumption, and market share of palm oil due to its cost competitiveness. Therefore, it is expected that more oil palm trees will be planted and mature in the very near future. Along with the increasing demand for palm oil, OPA has also become highly abundant and is usually dumped in open fields, leading to severe environmental problems. The utilisation of OPA in natural rubber composites has been studied, and the material has showed a reinforcing effect when very low OPA loading was used, as reported in a previous study (Ooi et al. 2013a). In a particulate-filled natural rubber system, the interfacial adhesion between natural rubber matrix and OPA plays an important role in enhancing the required mechanical properties. Therefore, modification of the surface properties of OPA particles is required to enhance the rubber-filler interaction as well as the dynamic mechanical properties. The ability to further improve the mechanical properties and ageing resistance of natural rubber vulcanisates is of great interest for this research.