It is well known that the compression set of vulcanizates is an important physical and mechanical performance index of rubber products, which determines the performance and service life of rubber products. As an important part of the air spring, the performance and service life of the rubber have the most direct impact on the air spring!
The determinants of compression set are many and complex, not only depending on the raw rubber, but also on the formulation and process. Therefore, we must carefully analyze and study and find out the internal factors to find out the solution.
First of all, it depends on the variety of air spring rubber raw rubber raw materials, because raw rubber is the most important raw material for rubber products. If there is no raw rubber, it will become “no rice”. The different types of raw rubber have different structures, and their properties are different when they are different in structure.
Classification of raw rubber, crystalline and non-crystalline; polar and non-polar; saturated and unsaturated; self-reinforcing and non-self-reinforcing; thermoplastic Thermoplastic and so on, in short, different structures, different compositions, different genes, and different properties.
The order of compression permanent deformation of various raw rubber:
In general, the elastic rubber with high elasticity and high strength is self-reinforcing, and its deformation is easy to recover, and the compression set is small; while the structure has many side chains, many branches, and many genes, The resistance is large, it is not easy to recover after deformation, and the residual deformation is large. For example, BR, NR, CR, FKM have smaller compression set, while TR, IIR, and SBR have larger compression set. Because SBR has the largest hysteresis loss, its compression set is large.
Followed by the rate of gelatin. If the rubber content is high (60% or more) and the filler is small, the voids in the spatial network structure of the crosslinked bonds of the vulcanized rubber after vulcanization are large, and are easily collapsed after being stressed, and the compression permanent deformation is large. The rubber content is low (30% or less), the filler is many, and the void of the space network structure of the vulcanized rubber after vulcanization is small. After being stressed, the stiffness is large and it is not easy to be deformed, so the compression deformation is small. If the rubber content is medium, the compression set is centered, between the two.
The following are the hardness, degree of vulcanization, type of crosslinks, carbon black, shape of filler particles, and the like.
The hardness of the compound depends on the amount of vulcanizing agent, reinforcing agent and filler. Generally speaking, if the amount is high, the hardness of the vulcanized rubber is high, the compression set is small, and the product is suitable for the high hardness product; while the amount of the vulcanized rubber is low, the compression set is large, and the compression set is large, suitable for For low hardness products. It should be noted that a rubber product having a high hardness has a small shrinkage ratio and a rubber product having a low hardness, and has a large shrinkage ratio. The high-hardness rubber products above 85° have an increase in shrinkage. Therefore, it should be considered when designing the formula.
When the amount of reinforcing agent or filler (hardening) is medium (moderate), the compression set is centered.
The degree of vulcanization of the compound:
The degree of vulcanization of the compound determines the physical and mechanical properties of the vulcanizate, as well as the size of the compression set. Generally speaking, if the vulcanization degree of the rubber compound is high, the cross-linking density of the vulcanized rubber is large, the void structure of the vulcanized rubber has small voids, is not easily deformed, and is easily recovered when deformed, so that the compression permanent deformation is small; When the pressure is low, the cross-linking density of the vulcanized rubber is small, and the space network structure of the vulcanized rubber has a large void, is easily deformed after being subjected to force, and has a large elongation, is easily deformed, and is difficult to recover, so that the compression permanent deformation is large; Moderate to medium, with a low crosslink density and a central compression set.
Type of cross-linking bond of vulcanized rubber:
We know that the vulcanization system of the rubber compound is different. The structure of the cross-linking bond of the vulcanized rubber is different. The structure of the cross-linking bond is different, and the bond energy and bond length of the cross-linking bond are different. It determines the physical and mechanical properties of the vulcanized rubber. It also determines the compression set after the force is applied. Generally speaking, the bond of the cross-linking bond can be low, and the longer the bond length, the deformation is easy after the force is applied, the recovery after deformation is also slow, and the compression permanent deformation is large; the bond of the cross-linking bond can be high and the bond length is long. The shorter the force is, the less deformed after the force is applied, the recovery after deformation is also faster, and the compression permanent deformation is smaller. For example, the bond energy of the polysulfide bond (-C-Sx-) is low (54 kcal/mol), the bond length (2.04×n, A) is large, and the compression set is large; and the carbon-carbon bond (-CC-) bond The energy is high (84 kcal / mol), the length of the key is short (1.54A), and the compression set is small.
The following are the compression permanent deformations of vulcanizates of different vulcanization systems and cross-linking types:
Sulfur vulcanization system>Semi-effective vulcanization system>Effective vulcanization system>Resin vulcanization system>Organic peroxide vulcanization system.
Type of crosslink key:
Polysulfide bond (-C-Sx-C-)> disulfide bond (-C-S-S-C-)>monosulfide bond (-C-S-C-)>ether bond (-C-O-C-)>carbon-carbon bond (-C-C-).
Carbon black, filler shape:
We know that in order to obtain excellent physical and mechanical properties of rubber products, a certain amount of reinforcing agent is generally added to improve the tensile strength, tensile stress and wear resistance of the vulcanized rubber. In addition, in order to reduce the production cost, it is also necessary to add some fillers which can increase the volume to increase the volume and reduce the gel content. However, the particle structure and morphology of the reinforcing agent and filler play an important role in the physical and mechanical properties of the vulcanizate, especially the compression set. For example, carbon black particles have large surface roughness and many small porosity on the surface, which is easy to form a carbon black gel (absorbed or contained rubber) with the rubber compound, which can improve the tensile strength, tensile stress, hardness and resistance of the vulcanized rubber. The grinding performance, etc., its ability to resist compression set is large, and the compression set is small. In addition, the particle shape of the filler is compatible with the rubber compound, and has some reinforcing effect on the vulcanized rubber, which can increase the modulus of the vulcanizate and also reduce the compression set of the vulcanizate. In general, the particle size of the carbon black and filler to the compression set of the vulcanizate is as follows:
Needle-like, spindle-shaped (magnesium carbonate, clay, hydrous calcium silicate, etc.) > flaky particles (calcium carbonate, talc, graphite, etc.) > round spherical (all kinds of carbon black, new process carbon black, etc.).
Therefore, when selecting the reinforcing agent carbon black and the filler, it should be selected according to the different performance requirements of the rubber product. For example, if you press out the product, you can choose fast-pressing carbon black (FEF), general furnace black (CPF) and semi-reinforcing furnace black (SRF), hard clay (soft clay has large particle size, can only increase capacity, can not Reinforcement); wear-resistant products, can choose high wear-resistant furnace black (HAF), white carbon black, hard clay and so on. In this way, the vulcanized rubber has a certain physical and mechanical properties, and has a low compression set, and can obtain a better use effect, so that the rubber product has better performance and service life.
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