Influência do inibidor de pré-vulcanização nas propriedades de composições de borracha natural não vulcanizadas

Abstract

Vulcanization is a crosslinking process in which the chemical structure of the elastomer is changed by the interlinking of the long chains of the polymer, resulting in a three-dimensional network. The tire repairing industry sells unvulcanized rubber products with relatively short shelf life. These products are vulcanized later, by the retreaders. For this reason, there is a big interest in increasing this shelf life without affecting the quality of the material. In other words, ensuring the stability of its rheometric properties. The objective of this work is to analyze the use of different sulfur-curing systems in the stability over time of the properties of natural rubber unvulcanized compositions. In this work, twelve compositions were prepared: six formulations using the 2,2-dithiomercaptobenzothiazole accelerator (MBTS) and another six using the accelerator N-cyclohexyl-2-benzothiazole sulphenamide (CBS). For each accelerator, three vulcanization systems were formulated: conventional, efficient and semi-efficient. The same formulations were also prepared including the prevulcanization inhibitor (PVI) N-cyclohexylthiophthalimide (CTP). The cure properties of the compositions were periodically analyzed over six months. Regarding the maximum torque, parameter related to the crosslink density, the CBS presented higher values for all the acceleration systems compared to the MBTS. Regarding the type of accelerator used, the MBTS had greater acceleration stability over the six months. CBS showed a longer safety time in the first months, losing this characteristic over time. The use of PVI reduced the scorch time. However, the optimum vulcanization time, t90, was increased. After evaluating the type of accelerator and vulcanization system under standard conditions (23 °C) and selecting only one system (semi-efficient, CBS with and without PVI), this system was studied under more severe climatic variations, simulating the conditions which these formulations are submitted during transportation and storage. In this second part of the study, the need to use PVI was confirmed, mainly in situations where unvulcanized materials are stored under 45 °C. In this case, the use of PVI allowed an increase of about 38% in the shelf life when compared with material under the same conditions without PVI. Other properties were not affected by the use of this pre-vulcanization inhibitor additive. In general, it can concluded that the rheometric behavior over the shelf life of unvulcanized composition depends directly on the type of accelerator used, the use of the pre-vulcanization inhibitor, and the type of vulcanization system used.