Effect of different curing systems on the mechanical and physico-chemical properties of acrylonitrile butadiene rubber vulcanizates

Abstract

In the present study, the effect of different curing systems including sulfur, dicumyl peroxide, dicumyl peroxide/coagent and radiation/coagent on the mechanical and physico-chemical properties of acrylonitrile butadiene rubber (NBR) was studied. In order to correlate, the effect of curing systems on rubber, the comparison was carried out at comparable value of volume fraction of rubber in swollen gel (V_r) for NBR vulcanizates. Mechanical properties like tensile strength, elongation at break, modulus, Young’s modulus, tearing strength and abrasion loss of vulcanizates have been followed up for comparison. In addition, physico-chemical properties like swelling ratio, soluble fraction, and cross-link density were investigated. On the other hand, the effects of fuel, thermogravimetric analysis, and thermal ageing have been studied.

Introduction

Acrylonitrile butadiene rubber (NBR) has excellent oil resistance. However, shows no self-reinforcing effect, as there is no crystallinity, but when used in combination with reinforcing fillers, vulcanizates with excellent mechanical properties can be obtained from NBR [1]. Vulcanization occurs by a chemical agent, such as sulfur or peroxide. Alternatively, high-energy radiation, such as electron beam or gamma radiation can be used to vulcanize rubbers [2].

The use of organic peroxide as a cross-linking agent through a free radical process is also largely developed. The vulcanization rate is controlled essentially by the decomposition of the peroxide at a given temperature [3]. Compared with sulfur vulcanization, crosslinking by peroxides is a relatively simple process, with physical properties such as high modulus, low compression set and heat ageing properties superior to sulfur cure systems. On the other hand, the peroxide crosslinking has many disadvantages, such as low tensile and tear strength, and flex resistance, which have restricted their use in diene rubbers. Many unsaturated rubbers, such as natural rubber (NR), styrene–butadiene rubber (SBR), butadiene rubber (BR), and acrylonitrile butadiene rubber (NBR), contain a varying degree of unsaturation in the polymer backbone or in pendant positions. Peroxide radical could potentially react by addition to a double bond or by abstraction of an allylic hydrogen, and both mechanisms occur concurrently in the vulcanization of unsaturated elastomers [4].

The use of coagents in conjunction with peroxides to cure elastomers has been common practice in the rubber industry for many years. Coagents are typically multifunctional vinyl monomers that are highly reactive toward free radicals and readily graft to elastomer chains to form a complex crosslinked network. These coagents with peroxide are used to improve the physical properties and processability of peroxide-cured elastomers. Also, they increase not only the crosslinking efficiency of the vulcanization process but the cross-link density as well [5].

During this last decade, the crosslinking of rubbers by means of electron beams has strongly developed in place of the use of cross-linking agents, such as sulfur or peroxides. NBR belongs to the crosslinking type rubbers when exposed to high-energy radiation [6]. Compared with the conventional chemical processes such as peroxide [7] or sulfur [8] induced vulcanization used for crosslinking rubber, radiation crosslinking has the advantages of being faster and being more versatile, leads to more uniform crosslinking, consumes less energy, and occupies less floor space for processing.

Inherently waste free nature of the technology makes it less polluting than the conventional technologies. The disadvantage is that the physical properties of radiation vulcanized rubber were adversely affected by the high crosslinking dose required. To overcome this problem, several authors [9] have reported that polyfunctional monomers PFMs (coagents) such as multifunctional acrylates and methacrylates are useful to obtain optimum mechanical properties at lower dose levels. These PFMs form a network structure with polymeric materials at a lower dose because of its higher reactivity [10] and the resulting structure is useful for the improvement of mechanical properties as well as thermal stability [11] .

This article is a comparative study between the different techniques to vulcanize NBR rubber.