Factors Affecting Compression Permanent Deformation of Vulcanized Rubber and Its Control

Nov 09, 2021

Compression set is one of the important performance indicators of rubber products. The size of the compression set of vulcanized rubber is related to the elasticity and recovery of vulcanized rubber. Elasticity and recovery are two interrelated properties. Some people often simply think that the elasticity of rubber is good, its recovery is fast, and its permanent deformation is small. This understanding is not enough. When the deformation of the rubber is caused by the extension of the molecular chain, its recovery (or the size of the permanent deformation) is mainly determined by the elasticity of the rubber; if the deformation of the rubber is accompanied by the destruction of the network and the relative movement of the molecular chain, this part It can be said to be unrecoverable, it has nothing to do with elasticity. Therefore, all factors that affect the elasticity and recovery of rubber are factors that affect the compression and permanent deformation of vulcanized rubber. The factors that affect the rubber recovery ability include intermolecular force (viscosity), network structure change or destruction, and intermolecular displacement.
Elasticity-The elasticity of rubber indicates the ease of rotation within the rubber molecular chain and side groups, or the flexibility of the rubber molecular chain and the size of the intermolecular force. For vulcanized rubber, its elasticity is also related to the density and regularity of the cross-linking network. Elasticity and tensile permanent deformation-we often say that natural rubber has good elasticity, but its tensile permanent deformation is often very large. This is mainly due to the large elongation of natural rubber, which causes network damage during the elongation process And the relative displacement of the molecular chain is very large, the recovery process after fracture is long and the unrecoverable part increases. If the permanent deformation of constant elongation is compared, the permanent deformation of natural rubber is not necessarily very large. The impact elasticity or resilience is measured under constant load (or constant energy) conditions. Its elasticity is directly related to the degree of crosslinking or modulus of the vulcanizate. It expresses the rubber elasticity and viscosity (or absorption). comprehensive.
Compression set is measured under constant deformation conditions, and its value is related to the elasticity and recovery ability of rubber.

1. The elasticity of rubber
1) The type of rubber elasticity depends on the difficulty of internal rotation of the rubber molecular chain and the size of the intermolecular force. Such as natural rubber, butadiene rubber, butyl rubber, silicone rubber, etc. are considered rubbers with good elasticity.
2) The size of the molecular weight affects the degree of curling of the molecular chain and the number of useless ends. The molecular weight is large and the elasticity is good.
3) The chemical composition and structure of copolymer rubber, styrene butadiene rubber, nitrile butadiene rubber, with the increase of styrene and acrylonitrile content, the elasticity becomes worse. In ethylene-propylene rubber, the elasticity is best when the propylene content is 40% to 50%. The copolymer formed at this time is a random copolymer. If the ethylene content exceeds 70%, a longer ethylene block will be formed. It is easy to form crystals and make the ethylene propylene rubber lose its elasticity.

2. The effect of reinforcing fillers on the elasticity of vulcanizates
Non-carbon black reinforcing fillers will damage the elasticity of the rubber and increase the compression set. This is related to the fact that rubber molecules slide on the surface of the inactive filler under stress, and after the stress is removed, the recovery of the molecular chain is hindered. The application of coupling agents can greatly improve the impact of non-reinforcing fillers on the elasticity of vulcanizates (improve the dispersibility and surface activity of fillers). Most of the literature says that with the increase of the carbon black particle size, the elasticity of the vulcanized rubber is enhanced, but the influence of the filling amount on the elasticity of the vulcanized rubber is often ignored. In fact, various rubber products have certain hardness and strength requirements. For example, when low-reinforcing carbon black is used alone, the amount needs to be increased, which will also damage the elasticity and recovery of the rubber. In a vulcanized rubber with a certain amount of deformation, the amount of deformation of the filled rubber molecular chain is greater than the amount of macroscopic deformation, and the expanded value is proportional to the amount of filling. The increase in the amount of deformation will also affect the displacement and recovery of the rubber molecular chain, and increase the permanent deformation. The use of appropriate reinforcing agents and appropriate mixing processes can obtain the ideal structure of the rubber compound and obtain highly elastic vulcanized rubber.

3. Softeners and plasticizers
Softeners and plasticizers can not only increase the elasticity of rubber (reduce the force between molecules and increase the flexibility of molecular chains), but also increase the mobility of molecular chains. However, these two effects can be adjusted by the reasonable dosage and combined use of softeners and plasticizers, as well as appropriate processing techniques, to obtain a vulcanized rubber with good elasticity. On some occasions, it can have special effects.

Fourth, the effect of the degree of crosslinking of vulcanized rubber and the structure of vulcanized rubber on compression set
1) The influence of the degree of crosslinking. Under the long-term stress, the relative displacement of the molecular chain of the rubber molecular chain will occur, resulting in stress relaxation. In some cases, it can even relax to zero. After the stress is removed, the recovery ability of the rubber molecules will be reduced or even lost. Produce permanent deformation. A higher degree of cross-linking can reduce the displacement and stress relaxation of rubber molecules, maintain a higher recovery capacity, and reduce compression set.
2) The effect of vulcanization The compression set of vulcanized rubber is usually carried out at a higher temperature. The post-vulcanization effect of the unconsumed vulcanizing agent causes the deformed rubber molecules to be bound by the newly formed cross-linking bonds, and the recovery of the rubber molecules after the stress is removed is hindered, resulting in a larger permanent deformation. This post-crosslinking effect is different from the degree of crosslinking mentioned in point 1.
3) Cross-linked structure and chemical stress relaxation The polysulfide cross-linked bond is oxidized at high temperature for a long time, and the cross-linked bond is broken, resulting in chemical stress relaxation and displacement of the molecular chain. The broken cross-links form new cross-links where there is no force. The increase in compression set caused by the chemical stress relaxation is caused by the dual effects of molecular chain displacement and hindered molecular chain recovery. The solution is to change the cross-linked structure and strengthen the antioxidant effect.

5. Influence of low-temperature compression permanent deformation (cold resistance coefficient)
The factors of low-temperature compression permanent deformation of vulcanized rubber can still be said to be elasticity and recovery. The manifestation is the crystallization and vitrification of the rubber molecular chain. The solution is: one is to reduce the glass transition temperature of the rubber; the other is to destroy the crystallinity of the rubber. For different rubber varieties, the measures taken are different. For example, for natural rubber that is easy to crystallize, modifiers or high-temperature vulcanization can be used to produce a certain amount of trans structure and destroy its low-temperature crystallinity. For chloroprene rubber and ethylene-propylene rubber, it is necessary to select the varieties that are difficult to crystallize, and apply cold-resistant plasticizers to reduce their glass transition temperature. For nitrile rubber, cold-resistant plasticizers are mainly used to reduce its glass transition temperature. Sometimes, some unconventional methods may be used to achieve the goal.

6. Compression permanent deformation of high hardness (Sauer A75° to 90°) vulcanized rubber
The compression permanent of high-hardness rubber is relatively poor, because in order to increase the hardness, a large amount of carbon black is added to the rubber, which causes the rubber content to decrease, the elasticity decreases, and the compression set is also reduced. In this case, raw rubber with a higher Mooney viscosity can be considered, and high-structure carbon black can be used to achieve the purpose of rapid increase in hardness while maintaining a high rubber content. On the other hand, it can also be considered to increase the crossover of the vulcanization system. The way of linking density.

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