Elucidation of the accelerated sulfur vulcanization of bio oil‐extended natural rubber compounds

Abstract

The elucidation of the role of bio-oils on the accelerated sulfur vulcanization of natural rubber (NR) compounds is discussed in this study. Two types of bio-oil, palm oil and soybean oil, were studied in direct comparison with a distillate aromatic extract oil (DAE) as a reference. The scorch and cure times of the bio-oil-extended NR compounds were shorter than those containing DAE. The use of bio-oils gave a higher cure reaction rate constant along with a lower activation energy than the use of DAE. The attenuated total reflectance-Fourier transform infrared spectroscopy analysis revealed that the fatty acid segment of the bio-oils can react with zinc oxide to give zinc carboxylate, which is then involved in and promotes the vulcanization reaction. The use of bio-oils to increase the rate of vulcanization strongly influenced the crosslink density of the obtained NR vulcanizates, yielding NR vulcanizates with a lower crosslinking density. It is proposed here that the bio-oils might consume the curing agent via the reaction between their own unsaturated fatty acid and sulfur. This was supported by the increased viscosity of the oils after exposure to sulfur at a high temperature. The tensile strength and elongation at break of the bio-oilextended NR compounds were lower and higher, respectively, than the NR extended with DAE oil due to the lower crosslink density of the bio oil-extended NR vulcanizates.

Unfilled Natural Rubber Compounds Containing Bio-Oil Cured with Different Curing Systems: A Comparative Study

“…Figure 4 shows a typical torque-time curve of NR compounds containing processing oils with peroxide curing systems and compared with those using sulfur curing system from [16]. The curing system showed a strong influence on the cure curve behavior of the NR compounds throughout the curing period in this study.…”

“…Two types of bio-oils, i.e., PO and SBO, locally produced in Thailand were used. The viscosities of PO and SBO were 233 and 213 cP, respectively, as measured by a Brookfield KU-2 Viscometer [16]. Tudalen 65 (H&R ChemPharm, Bangkok, Thailand), with aromatic (45%), naphthenic (26%), and paraffinic (29%) components, was used as DAE oil.…”

“…Sulfur-cured rubber compounds were prepared as previously described [16]. For peroxide-cured rubber compounds, unfilled NR compounds containing different types of processing oils, i.e., DAE, PO, and SBO, were prepared as shown in Table 1.…”

“…Polymers 2022, 14, 2479 2 of 16 In addition to the different crosslink types, the levels of crosslink density within the rubber matrix also affect the rubber properties, including hardness, tensile strength, elongation at break, tear strength, compression set, tension set, creep, fatigue resistance, and swelling resistance. The crosslink density slightly changes several properties, including abrasion resistance, gas permeability, glass transition temperature (low temperature flexibility), electrical conductivity, heat conductivity, and chemical stability [1].…”

“…The properties of unfilled natural rubber compounds containing bio-oils, i.e., palm oil (PO) and soybean oil (SBO), using sulfur curing system have been reported by authors who demonstrated that the bio-oils had a huge influence on the properties of natural rubber compounds, such as cure characteristics, crosslink density, and mechanical properties [16]. So, this research work aimed to study the properties of unfilled NR compounds containing bio-oils cured with peroxide systems and to comparatively discuss with those cured with sulfur system [16]. Two types of bio-oils, PO and SBO, were used, and DAE-based petroleum oil was selected as a reference.…”