Dicumyl peroxide cross-linking of nitrile rubbers with different content in acrylonitrile

ABSTRACT: The scope of this article is the study of peroxide curing of two nitrile rubbers with low and high nitrile content. The peroxide efficiency can be much higher than one, and the polymer structure determines the mechanism of cross-linking. In the rubber with low nitrile content, the peroxide radical may give rise to a polymerization reaction between adjacent double bonds generating a heterogeneous network with a negative effect on the vulcanizate properties. On the contrary, in the nitrile rubber with high nitrile content, this negative effect it is not present or is present to a lesser extent, and their vulcanizates show good physical properties. Key words: rubber; cross-linking density; mechanical properties
The cross-linking of rubber with organic peroxide is of considerable and practical interest. The peroxides produce vulcanizates with physical properties such as high modulus, high hardness, and low compression set and, of course, their heat aging properties are far superior to sulfur cure systems. On the other hand, the peroxide systems have disadvantages, the vulcanizates present low tensile and tear strengths, a slower rate of cure, and lack of delayed action during cure. These factors have drastically restricted their use in diene rubbers. Peroxides interact with polymers in a variety of ways. The effect that a peroxide has on the crosslinking reaction depends on the polymer nature, type, and concentration of peroxide, reaction temperature, and reactivity of other components that might be present (i.e., antioxidants). The peroxide reaction consists of several competing mechanisms, and the properties of the final cure state will depend on the balance between these often opposite reactions. The mechanism of peroxide vulcanization has been the subject of important reviews.1–5 The cross-linking reaction involves the homolytic decomposition of the peroxide molecule to produce two radical fragments. 6 Next, these radicals remove hydrogen atoms from the polymer forming a polymer radical in what is called the hydrogen abstraction reaction. Also, these peroxide radicals could add to a double bond present in unsaturated rubbers and generate a polymer radical. Finally, two polymer radicals combine to form a covalent cross-link. Although nominally this reaction is relatively simple, the reaction usually is quite complex due to numerous side reactions that can occur. Some of these reactions may increase the cross-link, making the peroxide radical more efficient. Thus, the generated macroradical can react with an unsaturated rubber chain by addition to a double bond. This would lead to a new macroradical with the ability to react again with other double bonds or with another radical in a termination reaction. Basically, the peroxide radicals could react in both ways by addition and abstraction, with a diene rubber. Both ways involve the formation of bonds between polymer chains, but there are important differences in the effect that each one could have on the properties of the final vulcanizates.
Many unsaturated rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), polybutadiene rubber (BR), and butadiene-acrilo-nitrile rubber (NBR) contain a varying degree of unsaturation in the polymer backbone or in pendant positions.1 A peroxide radical potentially could react by addition to a double bond or by abstraction of an allylic hydrogen. The number of moles of cross-links that can be formed from a mole of peroxide is called peroxide efficiency.