The relationship between rubber formula design and usage performance
The relationship between rubber formula design and usage performance
（1） Heat resistance
The so-called heat resistance refers to the ability of vulcanized rubber to maintain its original physical properties under high temperature and long-term thermal aging.
1. Selection of rubber
Numerous studies have shown that the structural characteristics of heat-resistant polymers are: highly ordered molecular chains; High rigidity; Having a highly rigid structure; High intermolecular forces; Having a high melting point or softening point. For example, polytetrafluoroethylene (PTFE), with a usage temperature of 315 ℃, fully conforms to the above structural characteristics.
Currently, EPDM, IIR, CSM, ACM, HNBR, FKM, and Q are commonly used as heat-resistant rubbers.
2. Selection of vulcanization system
Different curing systems form different cross-linking bonds, and the bond energy and oxygen absorption rate of each cross-linking bond are different. The higher the bond energy, the better the thermal stability of the vulcanizate; The slower the oxygen absorption rate, the better the heat and oxygen aging resistance of the vulcanizate.
Among the commonly used vulcanization systems, the peroxide vulcanization system has the best heat resistance.
At present, almost all the heat resistant blends of EPDM use a peroxide vulcanization system. When using peroxide alone as a vulcanizing agent, there are problems such as low crosslinking density and low thermal tearing strength. It is best to mix with certain co crosslinking agents.
3. Selection of protective system
Under high temperature usage conditions, rubber products may experience rapid loss of anti-aging agents due to volatilization, migration, and other reasons, resulting in deterioration of product performance. Therefore, in the formulation of heat-resistant rubber, low volatile antioxidants or high molecular weight antioxidants should be used, preferably polymerized or reactive antioxidants.
4. Impact of filling system
Inorganic fillers have better heat resistance than carbon black. Among inorganic fillers, white carbon black, zinc oxide, aluminum oxide, and silicate have better heat resistance.
5. Effects of Softeners
Generally, softeners have low molecular weight and are prone to volatilization or migration at high temperatures, resulting in an increase in hardness and a decrease in elongation of vulcanized rubber. Therefore, in the formulation of heat-resistant rubber, varieties with good thermal stability and non volatile properties at high temperatures should be selected.
（2） Cold resistance
The cold resistance of rubber can be defined as the ability to maintain its elasticity and function normally at a specified low temperature.
The cold resistance of vulcanized rubber mainly depends on the two basic properties of the polymer, namely the glass transition temperature (Tg) t and crystallization
The cold resistance of amorphous rubber 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 Tg at 70-80 ℃.
1. The effect of rubber molecular structure on cold resistance
① Rubber containing double and ether bonds in the main chain, such as BR, NR, CO, Q, has good cold resistance; ② Rubber with no double bonds in the main chain and polar groups in the side chain, such as ACM, CSM, FKM, has the worst cold resistance; ③ Rubber with a main chain containing double bonds and a side chain containing polar groups, such as NBR and CR, has a moderate cold resistance; ④ Non polar rubber EPDM and IIR with low unsaturation have better cold resistance than SBR, NBR, and CR.
2. Effect of plasticizers
Plasticizers are the most influential additives on cold resistance besides raw rubber. Adding plasticizers can reduce the Tg of rubber, improve its cold resistance, lower the relaxation temperature of polymers, reduce the stress generated during deformation, and thus achieve the goal of preventing brittle failure.
3. Effect of vulcanization system
The chemical bonds generated by cross-linking cause an increase in Tg, which is detrimental to cold resistance, as the activity of molecular segments after cross-linking is limited, reducing the flexibility of the molecular chain. Another explanation is that as the crosslinking density increases, the volume of free segments in the network structure decreases, thereby reducing the mobility of molecular segments.
4. Impact of filling system
The effect of fillers on cold resistance depends on the structure formed by the interaction between the filler and rubber. Different physical adsorption bonds and strong chemical adsorption bonds will be formed between activated carbon black particles and rubber molecules, forming a raw rubber adsorption layer (interface layer) on the surface of carbon black particles. The inner layer of this interface layer is in a glass state, and the outer layer is in a glass state. Therefore, the adsorbed rubber Tg increases, and it cannot be expected to improve the cold resistance of vulcanized rubber by adding fillers.
（3） Oil resistance
Oil resistance refers to the ability of vulcanized rubber to resist oil. When rubber products come into contact with oil for a long time, the following two phenomena occur: ① oil penetrates into the rubber, causing it to swell or increase in volume; ② Some soluble additives in the rubber compound are extracted by oil, resulting in shrinkage or volume reduction of the vulcanizate.
1. Selection of rubber
(1) The volume change and tensile strength retention rate of various rubbers with fuel resistance are shown in Table 9-37 after being immersed in a mixture of isooctane and aromatic compounds (gasoline and benzene) (volume ratio 60:40) at 23 ℃ for 46 hours.
The order of fuel resistance in polar rubber is FKM>CO>NBR>ACN>CPE>CR.
FMVQ and FKM have the best resistance to hybrid fuels; NBR takes second place, and with the increase of acrylonitrile content, the resistance to mixed fuel improves; ACM has the worst resistance to mixed fuel.
(2) Mineral oil resistance belongs to the category of non polar oils, with only polar rubber resistant to mineral oil, while non polar rubber is not resistant to mineral oil.
NBR is a commonly used mineral oil resistant rubber, and its oil resistance increases with the increase of acrylonitrile content.
(3) Synthetic lubricating oil is composed of two parts: basic liquid and additive. The basic liquids mainly include synthetic hydrocarbons, esters of dicarboxylic acids, phosphate esters, compounds of silicon and fluorine, etc.
Common additives include antioxidant, corrosion inhibitor, detergent, dispersant, foam inhibitor, anti extrusion agent, viscosity index improver, etc. Usually, the chemical properties of most additives are relatively active and have a greater chemical corrosiveness to rubber. Sulfur and phosphorus compounds in antioxidants and anti extrusion agents can cause severe hardening of NBR, and amines can severely corrode FKM.
2. Effect of vulcanization system
As the crosslinking density increases, the intermolecular force increases, the network structure of the vulcanizate is dense, the free space decreases, and the oil is difficult to diffuse. Therefore, the amount of crosslinking agent should be appropriately increased and the joint density should be submitted.
3. Effects of fillers and plasticizers
When the dosage of fillers and plasticizers increases, the swelling rate of the vulcanizate decreases. Because swelling is mainly caused by the infiltration of oil into the vulcanized rubber network, increasing the amount of fillers and plasticizers reduces the volume fraction of rubber in the rubber material, which helps to improve the swelling resistance. Usually, the higher the activity of the filler, the stronger the binding force with the rubber, and the smaller the volume swelling of the vulcanized rubber.
4. Selection of protective system
Oil resistant rubber products are often used in hot oil at higher temperatures, so the stability of antioxidants in oil is very important. If the antioxidant in vulcanized rubber is extracted in oil, the heat aging performance of the product will be greatly reduced.
（4） Chemical corrosion resistance
When rubber products come into contact with chemicals, oxidation often causes the decomposition of rubber and additives, resulting in corrosion or swelling of vulcanized rubber. These chemicals mainly consist of various acid, alkali, and salt solutions, which mainly appear in an aqueous solution state.
Coordination system with chemical corrosion resistance
(1) The selection of corrosion-resistant rubber should have a high saturation and try to eliminate or reduce active substituents, or introduce some substituents to stabilize the active parts of the rubber molecular structure.
(2) One of the important measures to improve chemical corrosion resistance is to increase the crosslinking density and increase the elastic modulus of vulcanized rubber in the vulcanization system.
(3) The filling agent selected for the chemical corrosion resistant adhesive formula of the filling system should be chemically inert, resistant to reaction with chemical corrosive media, non corrosive, and free from water-soluble electrolyte impurities.
(4) The plasticizer system should use plasticizers that are not easily extracted by chemicals and do not react chemically with them. For example, esters and vegetable oil plasticizers are prone to saponification in alkaline solution, and are often extracted in hot alkaline solution, resulting in volume shrinkage of the product and even loss of working ability.
（5） Vibration damping
The main performance indicators of vibration damping rubber are: ① the static stiffness of vulcanized rubber, that is, the elastic modulus of vulcanized rubber; ② Damping performance of vulcanized rubber, i.e. damping coefficient tan δ；③ The dynamic modulus of vulcanized rubber. In addition to the key performance indicators mentioned above, fatigue, creep, heat resistance, and metal bonding strength should also be considered.
1. Selection of rubber
The damping performance of damping rubber mainly depends on the molecular structure of the rubber. For example, when the volume of side groups introduced on the molecular chain is large, it hinders the movement of the chain segment, increases the internal friction between molecules, and increases the damping coefficient tan δ Increase. The presence of crystals can also reduce the damping characteristics of the system, such as mixing crystalline IR into CIIR with good damping effect and using the damping coefficient tan of the adhesive δ It will decrease as the IR content increases.
Tan δ The order from large to small is: IIR>NBR>CR, SBR>Q, EPDM, PU>NR>BR. Tan of NR δ Although relatively small, NR has the best comprehensive properties such as fatigue resistance, heat generation, creep, and metal adhesion, so it is also widely used in vibration damping rubber.
2. Effect of vulcanization system
Vulcanization system and stiffness, tan of vulcanized rubber δ、 Both heat resistance and fatigue resistance are related. The fewer sulfur atoms in a general vulcanized rubber network, the stronger the cross-linking bond, and the higher the elastic modulus of the vulcanized rubber δ The smaller.
In SBR, as the amount of sulfur increases, the static stiffness increases, the damping coefficient decreases, and the dynamic stiffness remains basically unchanged.
3. Impact of filling system
Filling system and dynamic modulus, static modulus, tan of vulcanized rubber δ There is a close relationship. When the vulcanizate undergoes deformation under force, internal friction occurs between the rubber molecular chain segment and the filler or between the filler and the filler, which increases the damping of the vulcanizate. The smaller the particle size of the filler, the larger the specific surface area, the larger the contact surface with rubber molecules, the more physical nodes, the greater the thixotropy, and the greater the hysteresis loss generated in dynamic strain. Therefore, the smaller the particle size of the filler, the greater the activity, and the greater the damping, dynamic modulus, and static modulus of the vulcanizate.
In order to improve the damping characteristics of vibration damping rubber as much as possible and reduce the dependence of creep and performance on temperature, special fillers such as vermiculite and graphite are often used in high damping vibration isolation rubber.
4. Effect of plasticizer system
As a plasticizer for damping rubber, if the damping peak is required to be widened, plasticizers that are incompatible with the rubber or have only a certain degree of solubility should be used.
（6） Electrical insulation
Electrical insulation is generally characterized by insulation resistance (volume resistivity and surface resistivity), dielectric constant, dielectric loss, and breakdown voltage.
1. Selection of rubber
The electrical insulation of rubber mainly depends on the polarity of rubber molecules. Usually, non polar rubbers such as NR, BR, SBR, IIR, EPDM, Q have good electrical insulation. It is a commonly used type of electrical insulation adhesive.
2. Effect of vulcanization system
Different types of cross-linking bonds can produce different dipole moments in vulcanizates, resulting in different electrical insulation properties. Taking NR as the basis for soft insulation rubber, it is more suitable to use a low sulfur or sulfur free vulcanization system. It is best to use a quinone oxime vulcanization system for IIR based electrical insulation rubber.
3. Impact of filling system
In general, the amount of fillers used in the formula of electrical insulation rubber is relatively large, so it has a significant impact on the electrical insulation of vulcanized rubber. Carbon black, especially those with high structure and large specific surface area, can easily form conductive channels when used in large amounts, significantly reducing electrical insulation. Therefore, in electrical insulation rubber, carbon black is generally not used except as a colorant.
4. Selection of Softeners
Low voltage electrical insulation rubber based on NR, SBR, and BR usually uses paraffin oil to meet the requirements, with a dosage of 5-10 parts.
5. Selection of protective system
Electrical insulation rubber products, especially those that are resistant to high voltage, must withstand the effects of high temperature and ozone during use. Therefore, when designing the formula of electrical insulation rubber, attention should be paid to selecting a protective system to extend the service life of the products. Generally, amine and para phenylenediamine antioxidants are used, and appropriate anti ozone agents are used to achieve good protective effects.
Relevant equipment used in rubber product factories:
75L dual hydraulic mixer
