The relationship between formula design and performance of high temperature resistant silicone strip
First, the relationship between rubber formula design and physical properties of vulcanized rubber
(1) Tensile strength
Tensile strength characterizes the ultimate ability of vulcanized rubber to resist tensile damage. Although most rubber products do not undergo deformations 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 rupture strength of the polymer indicate that the main valence of the macromolecule, the interaction between the molecules (the sub-valence), and the flexibility and relaxation process of the macromolecular chain are the intrinsic factors that determine the tensile strength of the polymer.
The method of increasing the tensile strength is discussed below from each of the mating systems.
1. Relationship between rubber structure and tensile strength
The raw rubber with a relative molecular mass of (3.0~3.5)×105 is advantageous for ensuring high tensile strength.
When a polar substituent is present in the main chain, the force between the molecules is increased and the tensile strength is also increased. For example, nitrile rubber increases in tensile strength as the acrylonitrile content increases.
As the crystallinity increases, the molecular arrangement will be more closely ordered, the pores and microscopic defects will be reduced, the intermolecular forces will be enhanced, and the movement of macromolecular segments will be more difficult, thereby increasing the tensile strength. After the orientation of the rubber molecular chain, the tensile strength in the direction parallel to the molecular chain increases.
2. Relationship between vulcanization system and tensile strength
In order to obtain a higher tensile strength, it is necessary to make the crosslinking density moderate, that is, the amount of the crosslinking agent is appropriate.
The relationship between the crosslink type and the tensile strength of the vulcanized rubber is decreased in the following order: ionic bond > polysulfide bond > disulfide bond > monosulfide bond > carbon-carbon bond. The tensile strength decreases with the increase of the cross-linking bond energy, 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-linking chain evenly bear more. Big stress.
3. Relationship between reinforcing filling system and tensile strength
The optimum amount of 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 of the maximum tensile strength tends to decrease. When the amount of carbon black of the soft rubber is 40 to 60 parts, the tensile strength of the vulcanized rubber is good.
4. Relationship between plasticizing system and tensile strength
In general, when the amount of the softener exceeds 5 parts, the tensile strength of the vulcanizate is lowered. For non-polar unsaturated rubber (such as NR, IR, SBR, BR), aromatic oil has little effect on the tensile strength of the vulcanizate; paraffin oil has a bad influence on it; the effect of naphthenic oil is between Between the two. For non-polar rubbers such as EPDM and IIR, which have low unsaturation, it is preferred to use paraffin oil and naphthenic oil which have low unsaturation. For polar unsaturated rubbers (such as NBR, CR), ester and aromatic oil softeners are preferred.
In order to improve the tensile strength of the vulcanizate, it is more advantageous to use coumarone resin, styrene-indene resin, polymer oligomer and high viscosity oil.
5. Other methods to increase the tensile strength of vulcanizates
(1) Blending modification of rubber and some resins For example, NR/PE blending, NBR/PVC blending, EPDM/PP blending, etc. can improve the tensile strength of the blended rubber.
(2) Chemical modification of rubber The chemical bond and adsorption bond are formed between the rubber molecules or between the rubber and the filler by the modifier to increase the tensile strength of the vulcanizate.
(3) Surface modification of the filler The surface of the filler is treated with a surface active agent and a coupling agent to improve the interface affinity between the filler and the rubber macromolecule, which not only contributes to the dispersion of the filler, but also improves the mechanical properties of the vulcanizate.
(2) Definite tensile stress and hardness
Both the tensile stress and the hardness are important indicators for characterizing the stiffness of the vulcanized rubber, both of which characterize the force required for the vulcanized rubber to undergo a certain deformation. The tensile stress is related to a large tensile deformation, and the hardness is related to a small compression deformation.
1. Relationship between rubber molecular structure and modulus
The larger the molecular weight of the rubber, the less the free end, the more the effective number of chains, the greater the modulus of elongation.
Any structural factor that increases the force between the rubber macromolecules can improve the ability of the vulcanized rubber network to resist deformation and increase the tensile stress. For example, a structural element such as a polar atom or a polar group or a crystalline rubber on a rubber macromolecular chain increases the intermolecular force, and thus the modulus is high.
2. Relationship between vulcanization system and modulus
The effect of crosslink density on the tensile stress is significant. As the crosslink density increases, the tensile stress and hardness increase almost linearly.
3. Relationship between filling system and modulus
The type and amount of filling are the main factors affecting the tensile stress and hardness of the vulcanizate.
The constant tensile stress and hardness increase with the decrease of the particle size of the filler, and increase with the increase of the structural degree and surface activity, and increase with the increase of the filler content.
4. Other methods to increase the modulus and hardness of vulcanizate
(1) Using a phenolic resin/hardener, a three-dimensional network structure can be formed with the rubber to achieve a Shore A hardness of 95 for the vulcanizate. For example, with 15 parts of alkyl resorcinol epoxy resin / 1.5 parts of accelerator H, can make high hardness bead rubber strip. (2) Adding liquid diene rubber and a large amount of sulfur in EPDM can produce high hardness vulcanizate with excellent vulcanization characteristics and processing properties.
(3) The addition of oligoester in NBR, NBR/PVC blending, NBR/ternary nylon blending, etc. can achieve a Shore A hardness of 90.
(3) Tear strength
The tear is caused by cracks or cracks in the vulcanizate due to rapid expansion and cracking when the force is applied. The tear strength is the load per unit thickness of the specimen when it is torn.
There is no direct relationship between the tear strength and the tensile strength, that is, the vulcanized rubber having a high tensile strength does not necessarily have a high tear strength.
1. Relationship between rubber molecular structure and tear strength
As the molecular weight increases, the interaction between the molecules increases, and the tear strength increases. However, when the molecular weight increases to a certain extent, the tear strength gradually tends to balance. The crystalline rubber has a higher tear strength at normal temperature than the amorphous rubber.
The tear strength of NR and CR at a normal temperature is high because of the induced crystallization which occurs when the crystalline rubber is torn, and the strainability is greatly improved. However, except for NR at high temperatures, the tear strength was significantly reduced. The tear strength of the vulcanizate after filling with carbon black is obviously improved.
2. Relationship between vulcanization system and tear strength
The tear strength increases as the crosslink density increases, but after reaching the maximum value, the crosslink density increases again and the tear strength decreases sharply.
3. Relationship between filling system and tear strength
As the carbon black particle size decreases, the tear strength increases. In the case where the particle diameter is the same, the carbon black having a low degree of structure is advantageous for the tear strength.
The use of isotropic fillers, such as carbon black, silica, Bai Yanhua, lithopone and zinc oxide, can achieve higher tear strength; while the use of anisotropic fillers, such as clay, magnesium carbonate, etc. can not A high tear strength is obtained.
Certain modified inorganic fillers, such as calcium carbonate and aluminum hydroxide modified with carboxylated polybutadiene (CPB), can increase the tear strength of SBR vulcanizates.
4. Effect of plasticizing system on tear strength
5. The addition of a softener generally reduces the tear strength of the vulcanizate. In particular, paraffin oil is extremely detrimental to the tear strength of SBR vulcanizate, while aromatic oil can make SBR vulcanizate have higher tear strength and increase with the amount of aromatic hydrocarbon oil.
(4) Wear resistance
Abrasion resistance characterizes the ability of a vulcanizate to resist material loss due to surface wear under the action of friction. It is a mechanical property closely related to the service life of rubber products. It is not only related to the conditions of use, the surface state of the friction pair and the structure of the product, but also related to other mechanical properties such as vulcanizate and physico-chemical properties such as viscoelastic properties. There are many influencing factors.
1. Glue effect
In the general diene rubber, the abrasion resistance is decreased in the following order: BR>solubilized SBR>milk poly SBR>NR>IR. The main reason for the good wear resistance of BR is its low glass transition temperature (Tg) (-95~105 °C), good molecular chain flexibility and high elasticity. The wear resistance of SBR increases as the molecular weight increases.
The wear resistance of NBR vulcanizates increases with the increase of acrylonitrile content, and the wear resistance of XNBR is better than that of NBR.
Polyurethane (PU) is the most wear-resistant rubber in all rubbers. It has excellent wear resistance at normal temperature, but its wear resistance drops sharply at high temperatures.
2. Effect of vulcanization system
The wear resistance of vulcanizates has an optimum value as the crosslink density increases. This optimum value depends not only on the vulcanization system but also on the amount and structure of the carbon black. When the amount and structure of carbon black are increased, the rigidity provided by the carbon black is increased. To maintain the optimum value of the vulcanizate stiffness, the rigid portion provided by the vulcanization system must be lowered, that is, the crossover is appropriately reduced. The density of the bond, and vice versa, should increase the crosslink density of the vulcanizate.
3. Impact of the filling system
Generally, the wear resistance of the vulcanizate decreases with the particle size of the carbon black, and increases as the surface activity and dispersibility increase.
Filling the new process carbon black and the silica treated with the silane coupling agent can improve the wear resistance of the vulcanizate.
4. Effect of plasticizing system
In general, the addition of a softener to the compound reduces wear resistance. When aromatic hydrocarbon oil is used in NR and SBR, the wear loss is smaller than other oils.
5. Impact of the protection system
Under the condition of fatigue wear, adding appropriate anti-aging agent can effectively improve the wear resistance of vulcanized rubber. If the effect of 4010NA is outstanding, in addition to 4010NA, 6PPD, DTPD, DPPD/H, etc. have certain effects of preventing fatigue aging.
6. Other methods for improving the wear resistance of vulcanizates
(1) Carbon black modifier A small amount of a carbon black modifier or other dispersant containing a nitro compound can improve the dispersion of carbon black and improve the wear resistance of the vulcanizate.
(2) Vulcanized rubber surface treatment using a halogen-containing compound solution or gas, such as liquid antimony pentafluoride, gaseous antimony pentafluoride, the surface of NBR and other vulcanizates can reduce the friction coefficient of the vulcanized rubber surface and improve wear resistance. Consumable.
(3) The silane coupling agent-modified filler, for example, the silica treated with the silane coupling agent A-189, is filled in the NBR compound, and the abrasion resistance of the vulcanized rubber is remarkably improved, and the silane coupling agent Si is used. The -69 treated white carbon black filled EPDM vulcanizate can also significantly improve the wear resistance.
(4) Rubber-plastic blending Rubber-plastic blending is one of the effective ways to improve the wear resistance of vulcanizate. For example, NBR/PVC, NBR/ternary nylon can improve the wear resistance of vulcanizate.
(5) Adding a solid lubricant and a friction reducing material For example, adding graphite, molybdenum disulfide, silicon nitride, carbon fiber, etc. to the NBR compound can reduce the friction coefficient of the vulcanized rubber and improve the wear resistance.
The high elasticity of rubber is caused by the change in the conformational entropy of the crimped macromolecule.
1. Relationship between rubber molecular structure and elasticity
The larger the molecular weight, the smaller the number of free ends that do not contribute to the elasticity; the "quasi-crosslinking" effect caused by the entanglement with each other in the molecular chain increases, and thus the molecular weight is large to contribute to the improvement of elasticity.
A polymer composed of a flexible molecular chain which is not easily crystallized at normal temperature, the greater the flexibility of the molecular chain, the better the elasticity.
2. Relationship between vulcanization system and elasticity
As the crosslink density increases, the elasticity of the vulcanizate increases and a maximum occurs, and then the crosslink density continues to increase, and the elasticity decreases. Because moderate cross-linking can reduce the irreversible deformation caused by the molecular chain slip, it is beneficial to improve the elasticity. Excessive cross-linking can cause the activity of the molecular chain to be blocked, and the elasticity will decrease.
3. Relationship between filling system and elasticity
The elasticity of vulcanizate is completely caused by the conformational change of rubber macromolecules. Therefore, increasing the gelatinization rate is the most direct and effective method to improve elasticity. Therefore, in order to obtain high elasticity, the amount of filler should be minimized and the raw rubber content should be increased. . However, in order to reduce costs, appropriate fillers should be used.
4. Relationship between plasticizing system and vulcanizate elasticity
The effect of the softener on the elasticity is related to its compatibility with the rubber. The poorer the compatibility of the softener with the rubber, the worse the elasticity of the vulcanizate.
(6) Fatigue and fatigue damage
Resistance to damage and the relationship between rubber
From the fatigue failure test of NR and SBR vulcanizates, it is found that the relative advantage of fatigue damage of NR and SBR is transformed when the strain is 120%: the fatigue life of SBR is higher than NR when the strain is less than 120%; At less than 120%, it is lower than NR. The fatigue resistance of NR is just the opposite of SBR.
First, the relationship between rubber formula design and performance
(1) Heat resistance
The so-called heat resistance refers to the ability of vulcanized rubber to maintain its original physical properties under the action of high temperature and long-term heat aging.
1. Rubber choice
A large number of studies have shown that the structural characteristics of heat-resistant polymers are: high molecular chain order; high rigidity; highly rigid structure; large intermolecular force; high melting point or softening point. For example, polytetrafluoroethylene (PTFE), used at a temperature of 315 ° C, fully meets the above structural characteristics.
Currently used as heat resistant rubber are EPDM, IIR, CSM, ACM, HNBR, FKM and Q.
2. Selection of vulcanization system
Different vulcanization systems form different cross-linking bonds. The bond energy and oxygen absorption rate of various cross-linking bonds are different. The higher the bond energy, the better the thermal stability of vulcanizate; the slower the oxygen-absorbing speed, the heat resistance of vulcanizate The better the aging of oxygen.
Among the commonly used vulcanization systems, the peroxide vulcanization system has the best heat resistance.
At present, almost all of the heat-resistant combinations of EPDM use a peroxide curing system. When a peroxide is used alone as a vulcanizing agent, there are problems such as low crosslinking density and low thermal tear strength. It is best to use with some co-crosslinkers.
3. Choice of protection system
Under the conditions of high temperature use of rubber products, the antioxidant may be quickly lost due to volatilization, migration, etc., thereby causing deterioration of product properties. Therefore, in the heat resistant rubber formulation, a small volatile antioxidant or a large molecular weight antioxidant should be used, and it is preferred to use a polymeric or reactive antioxidant.
4. Impact of the filling system
The heat resistance of the inorganic filler is better than that of the carbon black, and the heat resistance of the inorganic filler is preferably white carbon black, zinc oxide, oxidized beauty, aluminum oxide and silicate.
5. Softener effect
Generally, the softener has a low molecular weight and is volatile or migrated at a high temperature, resulting in an increase in the hardness of the vulcanizate and a decrease in elongation. Therefore, the heat-resistant rubber formula should be selected from the varieties with good thermal stability and low volatilization at high temperatures.
(2) Cold resistance
The cold resistance of rubber can be defined as the ability to maintain its elasticity and normal operation at a specified low temperature.
The cold resistance of vulcanizates mainly depends on the two basic physical properties of the polymer, namely the glass transition temperature (Tg) t and crystallization.
For the cold resistance of amorphous rubber, it can be characterized by Tg and Tb (brittle temperature).
For crystalline rubber, Tg and Tb cannot be used to characterize its cold resistance, which can be higher than Tg70~80 °C.
1. Effect of rubber molecular structure on cold resistance
1 Rubber with double bond and ether bond in the main chain, such as BR, NR, CO, Q, has good cold resistance; 2 The main chain does not contain double bonds, and the side chain contains polar groups.