Cardanol: a green substitute for aromatic oil as a plasticizer in natural rubber

The grafting of cardanol onto natural rubber in the latex stage was carried out successfully at room temperature using cumene hydroperoxide and tetraethylene pentamine. The grafted natural rubber was characterized by FTIR, NMR and GPC. The grafting of cardanol onto natural rubber resulted in a 45.6% increment in the molecular weight without affecting the molecular weight distribution. The Taguchi method was used to optimize the grafting conditions to achieve maximum yield in terms of percent grafting and grafting efficiency. The optimal parameter combination was found to be an initiator concentration of 2 phr, a cardanol concentration of 10 phr, a reaction temperature of 35 degrees C and a reaction time of 10 hours. The percent grafting was found to be 8.25% and the grafting efficiency was 82.5% for the optimum parameter combination. Analysis of variance (ANOVA) method was used to evaluate the percentage contribution of the different control factors on percent grafting and grafting efficiency. Cardanol concentration was found to be the most dominant parameter on grafting efficiency, while initiator concentration was found to play the dominant role on percent grafting. The cardanol grafted natural rubber (CGNR) was found to have a higher molecular weight, a lower Mooney viscosity, a lower Wallace plasticity number and a higher cure rate as compared to the unmodified natural rubber. The physico- mechanical properties of the CGNR vulcanizates were at par with or even better than the gum natural rubber vulcanizates. The rheological characteristics exhibit a better flow behavior as compared to the unmodified natural rubber. Differential scanning calorimetry and dynamic mechanical analysis demonstrate a lowering of the glass transition temperature of the CGNR as compared to the raw natural rubber. This confirms the plasticization effect of the cardanol when grafted onto the natural rubber.