The residual compression set is one of important indicators of effectiveness of rubber products. The amount of compression set of a vulcanized rubber is related to elasticity and recovery of vulcanized rubber. Elasticity and recovery are two related properties. Some people simply think that better elasticity of rubber, faster recovery and less permanent deformation. This understanding is not enough. When deformation of rubber is caused by stretching of molecular chain, its recovery (or amount of permanent deformation) is mainly determined by elasticity of rubber; if deformation of rubber is accompanied by destruction of network and relative movement of molecular chain, this part, perhaps irreversible, has nothing to do with stability. Therefore, all factors affecting elasticity and recovery of rubber are factors affecting compression set of vulcanized rubber. Factors that affect resilience of rubber include intermolecular strength (viscosity), change or disruption of network structure, and intermolecular displacement.
Elasticity. The elasticity of rubber indicates complexity of internal rotation of rubber molecular chain segments and side groups, or elasticity of rubber molecular chains and magnitude of intermolecular forces. The elasticity of vulcanized rubber is also related to density and regularity of crosslinked mesh. Elasticity and tensile set - We often say that natural rubber has good elasticity, but its tensile set is often very large, which is mainly due to large elongation of natural rubber, which causes damage to network during elongation process and relative displacement of molecular chain is very large, recovery process after a rupture is long and non-recoverable part increases. Compared to permanent elongation set, natural rubber set is not necessarily very large. Impact elasticity or resilience is measured under constant load (or constant energy) conditions, and its elasticity is directly related to degree of crosslinking or modulus of vulcanized rubber, expressing complex elasticity of rubber and viscosity (or absorption).
Compressive shrinkage is measured under constant deformation and is related to elasticity and recovery of rubber. Let me talk about my personal understanding of rubber elasticity and recovery.
First, elasticity of rubber
1. The elasticity of rubber type depends on complexity of internal rotation of rubber molecular chain and magnitude of intermolecular force. Natural rubber, butadiene rubber, butyl rubber, silicone rubber, etc. are considered rubber with good elasticity.
2. The size of molecular weight affects degree of twisting of molecular chain and number of useless ends. Large molecular weight and good elasticity.
3. Chemical composition and structure of copolymerized rubber The elasticity of styrene-butadiene rubber and nitrile rubber decreases with increasing content of styrene and acrylonitrile. In ethylene-propylene rubber, elasticity is best when propylene content is 40%~50%. At this time, resulting copolymer is a random copolymer. If ethylene content exceeds 70%, a longer ethylene block is formed, and a longer ethylene block is formed. Crystals are easily formed and ethylene-propylene rubber loses its elasticity.
Secondly, effect of reinforcing filler on elasticity of vulcanizate
Reinforcing fillers other than carbon black impair rubber elasticity and increase compression set. This is due to sliding of rubber molecules over surface of an inactive filler under stress, which prevents restoration of molecular chain after stress is removed. The use of sizing allows you to significantly improve effect of non-reinforcing filler on elasticity of vulcanizate (improving dispersion and surface activity of filler). Most of literature states that elasticity of vulcanized rubber increases with size of carbon black particles, but effect of amount of filler on elasticity of vulcanized rubber is often ignored. In fact, all types of rubber products have certain requirements for hardness and strength, for example, when using only low-reinforcing carbon black, it is necessary to increase dosage, which will also worsen elasticity and recovery of rubber. In vulcanized rubber with a certain degree of deformation, amount of deformation of molecular chain of filled rubber is larger than amount of macroscopic deformation, and amount of expansion is proportional to degree of filling. An increase in strain will also affect displacement position and recovery of rubber molecular chains, increasing permanent strain. The combination of a suitable reinforcing agent and a suitable mixing process results in an ideal structural shape of rubber compound and results in a highly resilient vulcanized rubber.
3. Softeners and plasticizers
Softeners and plasticizers mcan not only increase elasticity of rubber (reduce force between molecules, increase flexibility of molecular chains), but also improve mobility of molecular chains. However, these two effects can be corrected by a reasonable amount and combination of softeners and plasticizers, as well as appropriate processing methods, to obtain a vulcanized rubber with good elasticity. In some cases, this can have a special effect.
4. Influence of degree of crosslinking of vulcanized rubber and structure of vulcanized rubber on compression set
1. Influence of degree of crosslinking The molecular chain of rubber undergoes relative displacement under long-term stress, resulting in stress relaxation. In some cases, it can even relax to zero. After stress is removed, restorative power of rubber molecule is reduced or even lost, resulting in a permanent loss of shape. A higher degree of crosslinking can reduce displacement and stress relaxation of rubber molecules, maintain a high recovery ability, and reduce residual compression head.
2. vulcanization effect. The compression of vulcanized rubber is usually carried out at a higher temperature. The post-vulcanization effect generated by unspent vulcanizing agent binds deformed rubber molecules with newly formed cross-links, and recovery of rubber molecules after stress relief is difficult, resulting in large permanent deformation. This post-stitching effect is different from degree of stitching mentioned in point 1.
3. Cross-linking Structure and Chemical Stress Relaxation The polysulfide cross-linking bond is oxidized at high temperature for a long time, which leads to breaking of cross-linking bond, which leads to chemical stress relaxation and molecular chain displacement. Broken cross-links form new ones where no force is applied. The increase in compression set caused by relaxation of chemical stress is due to dual effect of displacement of molecular chain and difficulty of recovery of molecular chain. The solution lies in changing structure of cross-links and enhancing antioxidant effect.
5. Influence of residual deformation during low-temperature compression (cold resistance coefficient)
The coefficient of permanent deformation of vulcanized rubber under low temperature compression can still be called elasticity and recovery. The form of expression is crystallization and glass transition of rubber molecular chains. Solution: one is to reduce glass transition temperature of rubber, other is to destroy crystallinity of rubber. The measures taken are different for different grades of rubber. For example, for natural rubber, which crystallizes easily, modifiers or high temperature vulcanization can be used to obtain a certain amount of trans structure and destroy its low temperature crystallinity. For chloroprene rubber and ethylene propilene rubber, it is necessary to choose grades that are difficult to crystallize, and at same time apply frost-resistant plasticizers to reduce their glass transition temperature. For nitrile rubber, cold-resistant plasticizers are mainly used to lower its glass transition temperature, and sometimes some unconventional methods can be used to achieve goal.
6. High hardness vulcanized rubber compression set (Sauer A75° to 90°)
The compressive strength of high hardness rubber is relatively low because a large amount of carbon black is added to rubber to increase hardness, resulting in a decrease in rubber content, a decrease in elasticity, and a decrease in permanent compression deformation. In this case, crude rubber with high Mooney viscosity can be considered, and high structure carbon black can be used to achieve goal of rapidly increasing hardness while maintaining a high rubber content as a bonding method.
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