Rubber process performance and formula design

Rubber is an important material used in the daily production and life of human society. Rubber can be divided into natural rubber and synthetic rubber according to the source and preparation method. The types commonly used in industrial production and daily life include natural rubber, butadiene rubber, styrene butadiene rubber, EPDM rubber, butyl rubber, etc. The advantages and disadvantages of material performance are different.
The process performance of the rubber compound has a great relationship with the formula design of rubber products, and even the formula design of rubber products can awaken the quality of the rubber to a large extent. Therefore, to study the relationship between rubber product formulation design and rubber process performance, and then to improve the formula and process, is an effective measure to improve the performance and quality of rubber materials.

The relationship between rubber formulation design and performance

1. The relationship between rubber formulation design and physical properties of vulcanized rubber
(1) Tensile strength
Tensile strength characterizes the ultimate ability of vulcanized rubber to resist tensile failure. Although most rubber products will not undergo deformation several times larger than the original length under the conditions of use, the actual service life of many rubber products has a good correlation with the tensile strength.
The results of studying the breaking strength of polymers show that the main valence bond of macromolecules, the intermolecular force (secondary bond), the flexibility of the macromolecular chain, and the relaxation process are the internal factors that determine the tensile strength of polymers.
The methods to improve the tensile strength are discussed below from the various coordination systems.

1. The relationship between rubber structure and tensile strength
The crude rubber with a relative molecular mass of (3.0~3.5)×105 is beneficial to ensure high tensile strength.
When there are polar substituents on the main chain, the intermolecular force will increase, and the tensile strength will also increase. For example, as the content of acrylonitrile increases, the tensile strength of nitrile rubber increases.
With the increase of crystallinity, the molecular arrangement will be more compact and orderly, so that the pores and micro-defects will be reduced, the intermolecular force will increase, and the movement of macromolecular segments will be more difficult, which will increase the tensile strength. After the rubber molecular chain is oriented, the tensile strength in the direction parallel to the molecular chain increases.
2. The relationship between vulcanization system and tensile strength
To obtain higher tensile strength, the cross-linking density must be moderate, that is, the amount of cross-linking agent must be appropriate.
The relationship between the type of crosslinking bond and the tensile strength of vulcanized rubber decreases in the following order: ionic bond>polysulfide bond>disulfide bond>single sulfur bond>carbon-carbon bond. The tensile strength decreases with the increase of the bond energy of the cross-linked bond, because the weak bond with smaller bond energy can release the stress under the stress state, reduce the degree of stress concentration, and make the cross-linked chain can evenly bear the high Great stress.
3. The relationship between reinforcing filling system and tensile strength
The optimal dosage of the reinforcing agent is related to the nature of the reinforcing agent, the type of rubber and other components in the formulation: for example, the smaller the particle size of carbon black, the greater the surface activity, and the amount used when the maximum tensile strength is reached tends to decrease. ; When the amount of carbon black for soft rubber is 40-60 parts, the tensile strength of vulcanized rubber is better.
4. The relationship between plasticizing system and tensile strength
In general, when the amount of softener exceeds 5 parts, the tensile strength of the vulcanized rubber will be reduced. For non-polar unsaturated rubber (such as NR, IR, SBR, BR), aromatic oil has little effect on the tensile strength of its vulcanized rubber; paraffin oil has a bad effect on it; naphthenic oil has an influence between Between the two. For non-polar rubbers with very low unsaturation, such as EPDM and IIR, it is best to use low-unsaturation paraffin oil and naphthenic oil. For polar unsaturated rubber (such as NBR, CR), it is best to use ester and aromatic oil softeners.
In order to improve the tensile strength of the vulcanizate, it is more advantageous to use coumarone resin, styrene-indene resin, high molecular oligomer and high viscosity oil.
5. Other ways to improve the tensile strength of vulcanizates
(1) Rubber and some resin blending modification, such as NR/PE blending, NBR/PVC blending, EPDM/PP blending, etc. can improve the tensile strength of the blend.
(2) Chemical modification of rubber. Modifiers are used to generate chemical bonds and adsorption bonds between rubber molecules or between rubber and fillers to improve the tensile strength of vulcanizates.
(3) Filler surface modification Use surface active and coupling agents to treat the filler surface to improve the interface affinity between the filler and the rubber macromolecule, which not only helps the filler dispersion, but also improves the mechanical properties of the vulcanizate.
(4) Constant elongation stress and hardness
Tensile stress and hardness are both important indicators to characterize the stiffness of vulcanized rubber, both of which characterize the force required for a certain deformation of vulcanized rubber. The tensile stress is related to the larger tensile deformation, and the hardness is related to the smaller compression deformation.
1. The relationship between rubber molecular structure and constant elongation stress
The greater the molecular weight of the rubber, the fewer free ends, the greater the number of effective chains, and the greater the tensile stress.
All structural factors that can increase the force between rubber macromolecules can improve the ability of the vulcanized rubber network to resist deformation and increase the constant elongation stress. For example, structural factors such as polar atoms or polar groups on the main chain of rubber macromolecules, crystalline rubber and other structural factors increase the intermolecular force, so the tensile stress is higher.
2. The relationship between vulcanization system and constant elongation stress
The crosslink density has a significant effect on the tensile stress. As the crosslinking density increases, the tensile stress and hardness increase almost linearly.
3. The relationship between filling system and constant elongation stress
The type and amount of filling are the main factors affecting the tensile stress and hardness of the vulcanizate.
Tensile stress and hardness both increase with the decrease of filler particle size, increase with the increase of structure degree and surface activity, and increase with the increase of filler content.
4. Other ways to improve the tensile stress and hardness of vulcanizates
(1) The use of phenolic resin/hardener can form a three-dimensional network structure with rubber to make the Shore A hardness of the vulcanized rubber reach 95. For example, 15 parts of alkyl resorcinol epoxy resin/1.5 parts of accelerator H can be used to make high-hardness bead strips.
(2) Adding liquid diene rubber and a large amount of sulfur to EPDM can produce high-hardness vulcanizates with excellent vulcanization characteristics and processing properties.
(3) Adding all-polyester to NBR, NBR/PVC blending, NBR/ternary nylon blending and other methods can make the Shore A hardness of the vulcanized rubber reach 90.
3. tear strength
Tearing is a damage phenomenon caused by cracks or cracks in the vulcanized rubber that rapidly expand and crack when subjected to force. Tear strength is the load per unit thickness of the sample when it is torn.
There is no direct relationship between tear strength and tensile strength, which means that vulcanizates with high tensile strength may not necessarily have high tear strength.
1. The relationship between rubber molecular structure and tear strength
As the molecular weight increases, the intermolecular force increases and the tear strength increases; but when the molecular weight increases to a certain extent, the tear strength gradually tends to balance. The tear strength of crystalline rubber at room temperature is higher than that of amorphous rubber.
The tear strength of NR and CR is higher at room temperature. This is because the induced crystallization produced when the crystalline rubber tears greatly improves the strain capacity. However, with the exception of NR, the tear strength is significantly reduced at high temperatures. The tear strength of the vulcanizate after filling with carbon black is obviously improved.
2. The relationship between vulcanization system and tear strength
The tear strength increases with the increase of the cross-linking density, but after reaching the maximum value, the cross-linking density increases again, and the tearing strength drops sharply.
3. The relationship between filling system and tear strength
As the carbon black particle size decreases, the tear strength increases. In the case of the same particle size, carbon black with low structure is beneficial to tear strength.
The use of isotropic fillers, such as carbon black, white carbon black, Bai Yanhua, lithopone and zinc oxide, etc., can obtain higher tear strength; while using anisotropic fillers, such as clay, magnesium carbonate, etc., it cannot Obtain high tear strength.
Certain modified inorganic fillers, such as calcium carbonate and aluminum hydroxide modified with carboxylated polybutadiene (CPB), can improve the tear strength of SBR vulcanizates.
4. Effect of plasticizing system on tear strength
5. Generally, the addition of softeners will reduce the tear strength of the vulcanized rubber. In particular, paraffin oil is extremely unfavorable to the tear strength of SBR vulcanizates, while aromatic oil can make SBR vulcanizates have higher tear strength, which increases with the amount of aromatic oil.