Vol.:(0123456789) 1 3 Journal of Polymers and the Environment https://doi.org/10.1007/s10924-018-1227-2 ORIGINAL PAPER Efects of Cashew Nut Shell Liquid and Its Decarboxylated Form on the Properties of Natural Rubber Uraiwan Sookyung 1  · Anoma Thitithammawong 1  · Charoen Nakason 2  · Charoen Pakhathirathien 3  · Woothichai Thaijaroen 4 © Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract Efects of addition of raw cashew nut shell liquid (CNSL) or its decarboxylated form (D-CNSL or cardanol) on curing characteristics, physical properties and temperature scanned stress relaxation were investigated. The study revealed that the CNSL containing high anacardic acid concentration acts as an acid activator and strongly afects curing behavior of NR compounds. The efective curing rate and curing efciency, i.e., torque diference, crosslink density, and activation energy for curing, clearly increased with 2–5 phr of CNSL. Furthermore, physical properties and thermal stability also improved. However, when the acidity was removed by forming D-CNSL, the efects were totally diferent from those of the CNSL. The D-CNSL behaves like a plasticizer improving chain fexibility and might show dilution efects if used at a high content in the compound formulation. These efects are clearly seen in decreased curing rate and curing efciency, together with slight losses of tensile properties, tear strength, and thermal stability, with D-CNSL addition. Keywords Natural rubber · Cashew nut shell liquid · Decarboxylated cashew nut shell liquid · Renewable material · Curing activator Introduction Cashew nut shell liquid (CNSL) is a by-product from the cashew industry, and is a renewable natural material. The natural raw CNSL mainly composes of phenolic compounds with diferent alkenyl side chains. The phenolic compounds include mostly anacardic acid, along with some cardol, cardanol, and 2-methyl cardol [1] (the chemical structures are shown in Fig. 1). The alkenyl side chains have 15 carbon atoms with unsaturated sites of monoene, diene, or triene (non-conjugated) types. The CNSL has as its advantages cheap price, abundant availability, and biodegradability. Also, there are unique structural features, such as active positions of double bonds and phenolic hydroxyl group. Therefore, CNSL is considered as a primary material in the synthesis of many polymers, for example polyurethanes [2], polyether [3] and phenolic resins [4, 5]. Moreover, some researchers have applied CNSL as an additive in rubber and polymer formulations to promote thermal stability, to plas- ticize, and as antioxidant [6–8]. A large number of modifed CNSL products have been developed, mostly by frst decarboxylating the CNSL. The decarboxylation of CNSL produces structural change from anacardic acid to cardanol, before further reactions such as sulfonation, epoxidation, esterifcation, etc. [1]. Examples of modifed CNSL products in rubber applications are phos- phorylated cardanol prepolymer (PCP) used as a plasticizer [9, 10], thermal resistance promoter [11], and coupling agent enhancing fller–rubber interactions [12, 13]. It has also been reported that NR-grafted-cardanol, cardanol-formaldehyde * Uraiwan Sookyung uraiwan.sookyung@gmail.com * Anoma Thitithammawong anoma.t@psu.ac.th 1 Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani 94000, Thailand 2 Faculty of Science and Industrial Technology, Prince of Songkla University, Suratthani Campus, Suratthani 84000, Thailand 3 Department of Science, Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani 94000, Thailand 4 National Metal and Materials Technology Center (MTEC), Pathum Thani 12120, Thailand