A study of the effects of kaolin, solid filler on the processing, mechanical, and dynamic properties of some industrial rubbers cured with novel sulphur cure system

Two novel methods for the sulphur vulcanisation of NR, BR and EPDM rubbers using N-tert-butyl-2-benzothiazole sulphenamide (TBBS) accelerator and zinc oxide (ZnO) activator have been developed. In one method, the optimum loading of TBBS and ZnO were measured for some sulphur-filled NR, BR and EPDM rubbers. The cure systems for the NR were (S/TBBS/ZnO), (1/1.5/0.2), (2/1.5/0.3), (3/1.5/0.25), and (4/3.5/0.2), for the BR, (0.5/1.75/0.2) and (1/3/0.2), and for the EPDM, (1/1/0.075). The cure was very efficient in spite of reducing the amount of TBBS and ZnO chemicals. In another method which used a single additive component in the form of a powder (TBBS/ZnO: 350mg/1g), the loading of the powder in NR was raised increasingly from 0.63 to 5.63 phr, the scorch time was unchanged and the optimum cure time reduced at 1.25 phr powder. The rate of cure accelerated at 1.25 phr powder. The crosslink density reached its maximum value at 5.63 phr powder. This method reduced the TBBS and ZnO requirement in the cure system by 85wt%. Solid kaolin filler pre-treated with a sulphur-bearing mercaptosilane was used to reinforce NR, BR and EPDM rubbers. For NR, to react the sulphur in the silane on the kaolin surface with the rubber chains and optimise the reaction between the two, 16 phr TBBS and 0.2 phr ZnO were added to the kaolin-filled rubber. The hardness and Young s modulus increased and compression set decreased when up to 3 phr elemental sulphur was included in the kaolin-filled rubber with 16 phr TBBS and 0.2 phr ZnO. The tensile strength, elongation at break, stored energy density at break, and tear energy of the rubber vulcanisate reduced when elemental sulphur was added. Notably, the inclusion of elemental sulphur was the key factor in controlling the rubber properties. In an extended work, 60 phr silane pre-treated kaolin was mixed with NR, BR and EPDM and the rubbers were cured using the novel cure systems developed earlier. The effect of 140 phr kaolin on the properties of NR was also investigated. For NR, the hardness increased by 64% when 60 phr kaolin was added and the trend continued rising by another 28% when the loading of kaolin reached 140 phr. Similarly, the Young s modulus rose by 170% with 60 phr kaolin and then by an extra 148% when the full amount of kaolin, i.e. 140 phr, was reached. The tensile strength and tear energy were unchanged and the elongation at break and stored energy density at break deteriorated by a total of 65% and 34%, respectively with 140 phr kaolin. The compression set of the unfilled rubber was 41%, and it then rose to 64% and 71%, when 60 and 140 phr kaolin was added, respectively. For BR, the hardness increased by 23% and for EPDM, by 34%, respectively when 60 phr kaolin was incorporated in the rubbers. For BR, the tensile strength, elongation at break and Young s modulus rose by 759%, 256% and 114%, respectively. The compression set of the unfilled BR was 9.4%, and subsequently rose to 26% when 60 phr kaolin was mixed with the rubber. For EPDM, the tensile strength, elongation at break and Young s modulus improved by 964%, 332% and 71%, respectively. For BR, the stored energy density at break and tear energy were increased by 2442% and 536%, respectively and for EPDM, by 3133% and 1479%, respectively. The compression set of the unfilled EPDM was 39%, and afterward increased to 48% with 60 phr kaolin. Kaolin was found to be extending or non-reinforcing filler for the strain-induced crystallising NR and highly reinforcing for the non-crystallising BR and EPDM.