The relationship between rubber formula design and physical properties of vulcanized rubber
（1） Tensile strength
The tensile strength characterizes the ultimate ability of vulcanized rubber to resist tensile failure. Although the vast majority of rubber products do not undergo deformation several times longer than their original length under service conditions, the actual service life of many rubber products is closely related to their tensile strength.
The results of studying the fracture strength of polymers indicate that the main valence bond of macromolecules, intermolecular forces (secondary valence bonds), as well as the flexibility and relaxation process of macromolecular chains, are internal factors that determine the tensile strength of polymers.
Below, we will discuss methods for improving tensile strength from various coordination systems.
1. The relationship between rubber structure and tensile strength
The relative molecular weight is (3.0-3.5) × 105% raw rubber is beneficial for ensuring high tensile strength.
When there are polar substituents on the main chain, the intermolecular force increases and the tensile strength also increases. For example, with the increase of acrylonitrile content, the tensile strength of nitrile rubber increases.
As the crystallinity increases, the molecular arrangement becomes more tightly and orderly, reducing pores and micro defects, enhancing intermolecular forces, and making the movement of macromolecular chains more difficult, thereby improving tensile strength. When the rubber molecular chain is taken backwards, the tensile strength in the direction parallel to the molecular chain increases.
2. Relationship between vulcanization system and tensile strength
To achieve high tensile strength, the crosslinking density must be moderate, that is, the amount of crosslinking agent should be appropriate.
The relationship between the type of cross-linking 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 cross-linking bond energy, as weak bonds with smaller bond energy can release stress under stress, reduce the degree of stress concentration, and enable cross-linked network chains to uniformly withstand larger stresses.
3. Relationship between reinforcement filling system and tensile strength
The optimal dosage of reinforcing agent is related to its properties, adhesive type, and other components in the formula: for example, the smaller the particle size of carbon black, the greater the surface activity, and the dosage tends to decrease when reaching the maximum tensile strength; When the amount of carbon black used in soft rubber is 40-60 parts, the tensile strength of vulcanized rubber is better.
4. Relationship between plasticizer system and tensile strength
Overall, when the amount of softener exceeds 5 parts, it will reduce the tensile strength of the vulcanizate. For non polar unsaturated rubbers (such as NR, IR, SBR, BR), the influence of aromatic oil on the tensile strength of their vulcanizates is relatively small; Paraffin oil has adverse effects on it; The impact of naphthenic oil is somewhere between the two. For non polar rubbers with low unsaturation, such as EPDM and IIR, it is best to use low unsaturation paraffin oil and naphthenic oil. For polar unsaturated rubbers (such as NBR, CR), it is best to use esters and aromatic oil softeners.
To improve the tensile strength of vulcanized rubber, it is more advantageous to choose coumarone resin, styrene indene resin, polymer oligomers, and high viscosity oil.
5. Other methods to improve the tensile strength of vulcanized rubber
(1) Blending modification of rubber and certain resins, such as NR/PE blending, NBR/PVC blending, EPDM/PP blending, can all improve the tensile strength of the blended rubber.
(2) The chemical modification of rubber generates chemical and adsorption bonds between rubber molecules or between rubber and fillers through modifiers to improve the tensile strength of vulcanized rubber.
(3) The surface modification of fillers involves the use of surfactants and coupling agents to improve the interfacial affinity between fillers and rubber macromolecules. This not only helps with the dispersion of fillers, but also improves the mechanical properties of vulcanizates.
（2） Constant tensile stress and hardness
The constant elongation stress and hardness are important indicators to characterize the stiffness of vulcanized rubber, and both represent the force required for vulcanized rubber to undergo certain deformation. The tensile stress is related to larger tensile deformation, while the hardness is related to smaller compressive deformation.
1. The relationship between rubber molecular structure and elongation stress
The larger the molecular weight of rubber, the fewer the free ends, the more effective chain numbers, and the greater the tensile stress.
Any structural factor that can increase the intermolecular force of rubber can improve the resistance of the vulcanized rubber network to deformation and increase the tensile stress. For example, structural factors such as polar atoms or groups on the main chain of rubber macromolecules, crystalline rubber, etc. increase intermolecular forces, resulting in higher tensile stress.
2. Relationship between vulcanization system and constant elongation stress
The influence of crosslinking density on the tensile stress is relatively significant. As the crosslinking density increases, the tensile stress and hardness almost linearly increase.
3. Relationship between filling system and constant elongation stress
The variety and amount of filling are the main factors affecting the elongation stress and hardness of vulcanized rubber.
The tensile stress and hardness increase with the decrease of filler particle size, the increase of structural degree and surface activity, and the increase of filler dosage.
4. Other methods to improve the elongation stress and hardness of vulcanized rubber
(1) By using phenolic resin/hardener, a three-dimensional spatial network structure can be formed with rubber, resulting in a Shore A hardness of 95 for vulcanized rubber. For example, using 15 parts of alkyl resorcinol epoxy resin/1.5 parts of accelerator H1.5 can produce high hardness bead rubber strips. (2) Adding liquid diene rubber and a large amount of sulfur to EPDM can produce high hardness vulcanized rubber with excellent vulcanization characteristics and processing performance.
(3) Adding polyester, NBR/PVC blending, NBR/ternary nylon blending, and other methods to NBR can all achieve a Shore A hardness of 90 for vulcanized rubber.
（3） Tear strength
Tearing is a destructive phenomenon caused by the rapid expansion and cracking of cracks or openings in vulcanized rubber under stress. Tear strength is the load per unit thickness that a specimen bears when 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 tearing strength increases; However, as the molecular weight increases to a certain extent, its tearing strength gradually tends towards equilibrium. The tear strength of crystalline rubber at room temperature is higher than that of amorphous rubber.
At room temperature, the tear strength of NR and CR is relatively high, which is due to the induced crystallization generated during the tearing of crystalline rubber, greatly improving the strain capacity. However, at high temperatures, except for NR, the tear strength significantly decreases. The tear strength of the vulcanizate filled with carbon black was significantly improved.
2. Relationship between vulcanization system and tear strength
The tear strength increases with the increase of crosslinking density, but after reaching the maximum value, the crosslinking density increases again and the tear strength sharply decreases.
3. Relationship between filling system and tear strength
As the particle size of carbon black decreases, the tear strength increases. When the particle size is the same, carbon black with low structural degree is beneficial for tear strength.
Using isotropic fillers such as carbon black, white carbon black, white Yanhua, lithopone, and zinc oxide can achieve high tear strength; However, using anisotropic fillers such as clay and magnesium carbonate cannot achieve high tearing strength.
Some modified inorganic fillers, such as calcium carbonate and aluminum hydroxide modified with carboxylated polybutadiene (CPB), can improve the tear strength of SBR vulcanizates.
4. The effect of plasticizer system on tear strength
5. Generally, adding softeners will reduce the tear strength of vulcanized rubber. Especially, paraffin oil is extremely detrimental to the tear strength of SBR vulcanizates, while aromatic oil can make SBR vulcanizates have higher tear strength, as the amount of aromatic oil increases.
（4） Wear resistance
Wear resistance characterizes the ability of vulcanized rubber to resist material loss caused by surface wear under frictional force. It is a mechanical property closely related to the service life of rubber products. It is not only related to the service conditions, surface state of friction pairs, and structure of the product, but also to other physical and chemical properties such as mechanical properties and viscoelastic properties of vulcanized rubber, which are influenced by many factors.
1. The influence of adhesive type
In general diene rubber, the wear resistance decreases in the following order: BR>solution polymerized SBR>emulsion polymerized SBR>NR>IR. The main reason for the good wear resistance of BR is its low glass transition temperature (Tg) (-95-105 ℃), good molecular chain flexibility, and high elasticity. The wear resistance of SBR increases with the increase of molecular weight.
The wear resistance of NBR vulcanizate increases with the increase of acrylonitrile content, and XNBR has better wear resistance than NBR.
Polyurethane (PU) is the rubber with the best wear resistance among all rubbers. It has excellent wear resistance at room temperature, but its wear resistance will sharply decrease at high temperatures.
2. Effect of vulcanization system
The wear resistance of vulcanized rubber has an optimal value as the crosslinking density increases, which not only depends on the vulcanization system but also on the amount and structure of carbon black. When increasing the dosage and structural degree of carbon black, the stiffness provided by carbon black will increase. To maintain the optimal value of the stiffness of the vulcanizate, it is necessary to reduce the rigid part provided by the vulcanization system, that is, appropriately reduce the crosslinking density. Conversely, the crosslinking density of the vulcanizate should be increased.
3. Impact of filling system
Usually, the wear resistance of vulcanized rubber increases with the decrease of carbon black particle size and the increase of surface activity and dispersibility.
Both filling with new process carbon black and treating with silane coupling agent can improve the wear resistance of vulcanized rubber.
4. Effect of plasticizer system
Generally speaking, adding softeners to the rubber compound will reduce its wear resistance. When aromatic oils are used in NR and SBR, the wear resistance loss is smaller than other oils.
5. Impact of protective systems
Under the condition of fatigue wear, adding appropriate anti-aging agents can effectively improve the wear resistance of vulcanized rubber. For example, 4010NA has outstanding effects. In addition to 4010NA, 6PPD, DTPD, DPPD/H, etc. have certain effects in preventing fatigue aging.
6. Other methods to improve the wear resistance of vulcanized rubber
(1) Adding a small amount of carbon black modifier containing nitro compounds or other dispersants to the carbon black modifier can improve the dispersion of carbon black and improve the wear resistance of vulcanizates.
(2) The surface treatment of vulcanizates using solutions or gases containing halogen compounds, such as liquid antimony pentafluoride and gaseous antimony pentafluoride, can reduce the friction coefficient of the vulcanizate surface and improve wear resistance.
(3) The use of silane coupling agent to modify fillers, such as silica treated with silane coupling agent A-189, can significantly improve the wear resistance of the vulcanizate when filled in NBR compound. EPDM vulcanizate filled with silica treated with silane coupling agent Si-69 can also significantly improve its wear resistance.
(4) Rubber plastic blending is one of the effective ways to improve the wear resistance of vulcanized rubber. For example, NBR/PVC, NBR/ternary nylon, etc. can improve the wear resistance of vulcanizates.
(5) Adding solid lubricants and wear reducing materials such as graphite, molybdenum disulfide, silicon nitride, carbon fiber, etc. to NBR rubber can reduce the friction coefficient of vulcanized rubber and improve its wear resistance.
（5） Elasticity
The high elasticity of rubber is caused by the conformational entropy change of curled macromolecules.
1. The relationship between rubber molecular structure and elasticity
The larger the molecular weight, the fewer the number of free ends that do not contribute to elasticity; The entanglement within the molecular chains leads to an increase in the "quasi crosslinking" effect, so a large molecular weight is beneficial for improving elasticity.
A polymer composed of flexible molecular chains that are not easily crystallized at room temperature. The greater the flexibility of the molecular chains, the better their elasticity.
2. Relationship between vulcanization system and elasticity
As the crosslinking density increases, the elasticity of the vulcanizate increases and reaches its maximum value. Subsequently, the crosslinking density continues to increase, while the elasticity shows a downward trend. Moderate crosslinking can reduce irreversible deformation caused by molecular chain slip, which is beneficial for improving elasticity. Excessive cross-linking can hinder the activity of molecular chains, resulting in a decrease in elasticity.
3. Relationship between filling system and elasticity
The elasticity of vulcanized rubber is completely caused by the conformational changes of rubber macromolecules, so increasing the rubber content is the most direct and effective method to improve elasticity. Therefore, in order to achieve high elasticity, the amount of filler should be minimized as much as possible and the raw rubber content should be increased. But in order to reduce costs, appropriate fillers should be selected.
4. Relationship between plasticizer system and elasticity of vulcanized rubber
The effect of softeners on elasticity is related to their compatibility with rubber. The poorer the compatibility between softeners and rubber, the poorer the elasticity of vulcanized rubber.
（6） Fatigue and fatigue failure
The relationship between resistance to damage by labor and rubber seed
From the fatigue failure tests of NR and SBR vulcanizates, it was found that at a strain of 120%, the relative advantage of NR and SBR in fatigue failure resistance transformed: SBR had a higher fatigue life frequency than NR when the strain was less than 120%; And below 120%, it is lower than NR. The fatigue damage resistance of NR is exactly opposite to that of SBR.
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