Reversion is a specific phenomenon encountered in the vulcanization or use of certain rubber products. The relationship between reversion and sulfur is inseparable, and the use of sulfur vulcanization system is particularly prone to reversion.
After the rubber grommets return to the original, the surface is generally sticky, and the mechanical properties such as strength, elastic modulus, tensile stress and hardness are obviously decreased, and the elongation is increased. Natural rubber, styrene-butadiene rubber, isoprene rubber, butyl rubber, polysulfide rubber and other rubber types will return to the original. However, there is no problem of returning to the vulcanizate using a non-sulfur cross-linking system.
In essence, reversion occurs only in rubber grommets that use a sulfur vulcanization system. In this case, sulfur is bridged by a sulfur bond between rubber macromolecules to complete crosslinking. However, the bond energy of the sulfur bond is low, especially the polysulfide bond, which often cannot withstand the long-term test of high temperature and returns to the original and broken chain. However, the modern rubber vulcanization process tends to be more and more prosperous and productive. Therefore, how to effectively prevent the return to the original has become a common problem faced by the rubber grommets industry.
Return to the original category
Different from the formation mechanism, the rubber return can be divided into two types: reversion and aging.
(1) Vulcanization and reversion
It is well known that unvulcanized rubber is vulcanized to become a vulcanized rubber, and the middle is subjected to heating and heating. In this process, a three-dimensional network structure is gradually formed between rubber macromolecules by the formation of sulfur bonds, but this sulfur bond structure is inferior in ability to withstand high temperatures. With the gradual increase in crosslink density, various physical properties continue to increase. When the positive cure point is reached, the properties of the rubber grommets are optimal and stable, and then enter the vulcanization flat period.
But at the end of the flat period, if the vulcanization continues, it will enter the sulfur phase, which has two very different consequences.
The first one is that the mechanical properties of some rubber types (ie, the above-mentioned natural, isoprene, butylbenzene, butyl, polysulfide) are significantly reduced, and they enter the reversion stage. Therefore, for these rubber species, in essence, the reversion is a macromolecular degradation.
The second is that other rubber types (such as butadiene rubber) have the opposite trend, that is, some mechanical properties (such as tensile stress, tensile strength, hardness, etc.) continue to rise. The phenomenon of returning to the original only occurs in the former type of rubber, so measures to prevent returning to the original are needed. For the latter type of rubber, there is no problem of returning to the original.
It should be noted that even after the returning rubber species represented by natural rubber enters the reversion period, the two processes of cross-linking and degradation coexist, but the tendency of degradation accounts for the upper hand, and the cross-linking is gradually weakened. “There is a change in the sulfurization phase”, so they are collectively referred to as “vulcanization reversion”.