1177 † To whom correspondence should be addressed. E-mail: sirilux.p@chula.ac.th Korean J. Chem. Eng., 27(4), 1177-1183 (2010) DOI: 10.1007/s11814-010-0168-9 RAPID COMMUNICATION Thiosalicylic acid as a devulcanizing agent for mechano-chemical devulcanization Pipat Thaicharoen*, Patchanita Thamyongkit** , ****, and Sirilux Poompradub*** , **** ,† *Program of Petrochemistry and Polymer Science, Faculty of Science, **Department of Chemistry, Faculty of Science, ***Department of Chemical Technology, Faculty of Science, ****Center of Excellence for Petroleum, Petrochemical, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand (Received 8 March 2009  accepted 27 October 2009) Abstract−A mechano-chemical devulcanization process for vulcanized natural rubber (NV) was investigated. Thiosali- cylic acid was used as a test devulcanizing agent in comparison to diphenyl disulfide as the reference. The optimum condition for devulcanization of NR vulcanizates (NVs) was found to be grinding of the NV and subsequent mixing with a selected devulcanizing agent at 140 o C for 30 min. The degree of devulcanization was indicated by using sol-gel fractions of the devulcanized rubber (DVR). Revulcanized rubber was made by using virgin natural rubber (NR) con- taining DVR at different ratios. The tensile strength of the DVR/NV composite, after revulcanization, decreased by 5-10%, while the elongation at brake was improved by 5-10% at a DVR content of 5-15%. Devulcanization of indus- trial truck tires, as a typical sample of industrial products, was also demonstrated for the practical application of this technique. Key words: Devulcanized Rubber, Devulcanization, Devulcanizing Agent, Vulcanization, Mechano-chemical Devulca- nization INTRODUCTION The process of rubber vulcanization by sulfur is essential for the manufacture of suitable tires for automobiles and aircrafts, which are indispensable to our modern society. Sulfur bridge formation, generated between rubber molecules by this process, affords an elas- tic three-dimensional network. However, the sulfur bridges cannot be naturally removed or decomposed upon disposal of the rubber waste. Attempts to destroy the three-dimensional structure have proved difficult, expensive and energy-consuming. Currently, the amount of global rubber waste has drastically increased, leading to global environmental problems. The devulcanization of rubber waste, es- pecially automobile tires, is the most important issue for the rubber industry, and potential methods of recycling waste tire rubber are of great value and demand. Devulcanization involves the cleavage of the intermolecular carbon-sulfur and/or sulfur-sulfur bonds, leading to shorter polymer chain lengths compared to that of the original, and so resulting in poorer rubber properties [1]. In general, the devul- canization of rubber waste is carried out through physical, chemical or biotechnological processes. In the physical process, rubber wastes are devulcanized with the help of external energy, for example, mech- anical [2], cryomechanical [3,4], microwave [5,6], or ultrasonic [7- 11]. However, these processes lead to a random cleavage of cross- linked and polymeric main chain linkage bonds, leading to a lower quality rubber after revulcanization. In the chemical process, several chemical reclaiming agents have been used for the manufacture of devulcanized rubber, and typical examples include organic disulfides or mercaptans. Apart from these, the use of inorganic compounds has also been reported [12]. In this process, the rubber to be treated is initially swollen in common or- ganic solvents, e.g., toluene, benzene and cyclohexane. However, such solvents are difficult to remove after the reaction. Recently, Kojima et al. reported the effectiveness of supercritical CO 2 as a swelling solvent for the chemical reclaiming process of unfilled and carbon black-filled polyisoprene rubber vulcanizates [13-16]. Resid- ual CO 2 in the rubbery matrix was easily and rapidly removed by re- leasing pressure. Moreover, applications of supercritical n -butanol and toluene have been reported for the decomposition of used rubber into fuel oil [17]. However, a major drawback of the chemical devul- canization process is its high economic and environmental costs. In the biotechnological process, biodegradation of the rubber pow- der has been performed by oxidation-reduction reactions driven by different species of Thiobacillus , Acidithiobacillus , Nacardia, and so on [18-22]. The efficiency and rate of devulcanization were found to be a function of particle sizes. Therefore, this process was exclu- sively and primarily limited to the surface layers of the elastomers [18]. The mechano-chemical process, which applies a mechanical force to the rubber and the addition of the devulcanizing agent, has been shown to be a more efficient approach, in terms of mechanical prop- erties, in several studies [23-25]. In this content, here the mechano- chemical method, starting from grinding of the natural rubber (NR) vulcanizates, is followed by treating the resulting rubber sample by thiosalicylic acid. The potential advantage of thiosalicylic acid as a devulcanization agent is that structurally, thiosalicylic acid combines thiol and car- boxyl functional groups consistent with thiophenol and benzoic acid,