Effects 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 affects curing behavior of NR compounds. The effective curing rate and curing efficiency, i.e., torque difference, 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 effects were totally different from those of the CNSL. The D-CNSL behaves like a plasticizer improving chain flexibility and might show dilution effects if used at a high content in the compound formulation. These effects are clearly seen in decreased curing rate and curing efficiency, together with slight losses of tensile properties, tear strength, and thermal stability, with D-CNSL addition.
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The authors would like to acknowledge the financial support from Natural Rubber Innovation Research Institute, Prince of Songkla University (Grant No. SAT601211S). This research was also supported by the Postdoctoral Fellowship from Prince of Songkla University to one of the authors (Miss. Uraiwan Sookyung).