The relationship between rubber formula design and performance

The relationship between rubber formula design and performance

1. The relationship between rubber formula design and the 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 do not undergo deformation several times greater than their original length under usage conditions, the actual service life of many rubber products is well correlated with tensile strength.

The results of research on the fracture strength of polymers indicate that the main valence bonds of macromolecules, intermolecular forces (secondary valence bonds), as well as the flexibility and relaxation processes of macromolecular chains, are the intrinsic factors determining the tensile strength of polymers.

Next, we will discuss methods to improve tensile strength from various coordination systems.

1. The relationship between rubber structure and tensile strength

Raw rubber with a relative molecular mass of (3.0~3.5) × 105 is beneficial for ensuring high tensile strength.

When there are polar substituents on the main chain, the intermolecular forces increase, and the tensile strength also increases accordingly. For example, as the acrylonitrile content increases in nitrile rubber, the tensile strength also increases.

As crystallinity increases, the molecular arrangement becomes more compact and ordered, leading to a reduction in pores and microscopic defects. The intermolecular forces increase, making it more difficult for large molecular segments to move, thereby enhancing the tensile strength. After the rubber molecular chains are oriented, the tensile strength in the direction parallel to the molecular chains increases.

2. Relationship between vulcanization system and tensile strength

To achieve high tensile strength, the crosslinking density must be moderate, which means the amount of crosslinking agent used should 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 > monosulfide bond > carbon-carbon bond. The tensile strength decreases as the bond energy of the crosslinking bond increases, because weak bonds with lower bond energy can release stress under stress conditions, reducing the degree of stress concentration and enabling the crosslinked network to uniformly withstand greater stress.

3. Relationship between reinforcement and filling system and tensile strength

The optimal dosage of reinforcing agents is related to the properties of the reinforcing agents, the type of rubber, and other components in the formulation: for example, the smaller the particle size of carbon black and the greater its surface activity, the less dosage tends to achieve maximum tensile strength; when the dosage of carbon black in soft rubber is between 40 and 60 parts, the tensile strength of the vulcanized rubber is better.

4. Relationship between plasticizing system and tensile strength

Generally speaking, when the amount of softener exceeds 5 parts, it will reduce the tensile strength of the vulcanized rubber. For non-polar unsaturated rubbers (such as NR, IR, SBR, BR), aromatic oil has a small impact on the tensile strength of their vulcanized rubber; paraffin oil has an adverse effect on it; and naphthenic oil has an impact that falls between the two. For non-polar rubbers with very low unsaturation, such as EPDM and IIR, it is best to use paraffin oil and naphthenic oil with low unsaturation. For polar unsaturated rubbers (such as NBR, CR), it is best to use ester and aromatic oil softeners.

To enhance the tensile strength of vulcanized rubber, it is more advantageous to choose coumarone resin, styrene-indene resin, high molecular weight oligomers, and high viscosity oils.

5. Other methods to improve the tensile strength of vulcanizate

(1) Blending modification of rubber and certain resins, such as NR/PE blending, NBR/PVC blending, EPDM/PP blending, etc., can improve the tensile strength of the blended rubber.

(2) Chemical modification of rubber: By generating chemical bonds and adsorption bonds between rubber molecules or between rubber and fillers through modifiers, the tensile strength of vulcanized rubber is improved.

(3) Surface modification of fillers: The surface of fillers is treated with surfactants and coupling agents to improve the interfacial affinity between the fillers and rubber macromolecules. This not only aids in the dispersion of fillers but also enhances the mechanical properties of vulcanized rubber.

(II) Modulus at a fixed extension and hardness

Both modulus at a fixed extension and hardness are important indicators of the stiffness of vulcanized rubber, both representing the force required for the vulcanizate to undergo a certain deformation. Modulus at a fixed extension is related to larger tensile deformations, while hardness is related to smaller compressive deformations.

1. The relationship between rubber molecular structure and modulus at a fixed extension

The larger the molecular weight of rubber, the fewer the free ends, the greater the number of effective chains, and the higher the modulus at a given extension.

Any structural factors that can increase the intermolecular forces between rubber macromolecules can enhance the resistance of the vulcanized rubber network to deformation, thereby increasing the modulus at a given extension. For example, structural factors such as the presence of polar atoms or polar groups on the main chain of rubber macromolecules, and crystalline rubbers, increase the intermolecular forces, resulting in a higher modulus at a given extension.

2. Relationship between vulcanization system and modulus at a fixed extension

The influence of crosslinking density on modulus at a fixed extension is quite significant. As the crosslinking density increases, both the modulus at a fixed extension and hardness increase almost linearly.

3. Relationship between filling system and modulus at a fixed extension

The type and amount of filler are the main factors affecting the modulus and hardness of vulcanized rubber.

Both the fixed extension stress and hardness increase with the decrease of filler particle size, with the increase of structure degree and surface activity, and with the increase of filler dosage.

4. Other methods to improve the modulus and hardness of vulcanized rubber

(1) Using phenolic resin/hardener, a three-dimensional network structure can be formed with rubber, making the Shore A hardness of the vulcanized rubber reach 95. For example, using 15 parts of alkyl resorcinol epoxy resin and 1.5 parts of accelerator H can produce a high-hardness bead strip. (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 oligomeric esters to NBR, blending NBR with PVC, or blending NBR with ternary nylons can all achieve a Shore A hardness of 90 for the vulcanized rubber.

(III) Tear strength

Tearing is a destructive phenomenon caused by the rapid expansion and cracking of cracks or fissures in vulcanized rubber when subjected to force. Tearing strength refers to the load per unit thickness that a specimen can withstand before being torn.

There is no direct correlation between tear strength and tensile strength, which means that vulcanized rubber with high tensile strength does not necessarily have high tear strength.

1. The relationship between rubber molecular structure and tear strength

As the molecular weight increases, the intermolecular forces increase, leading to an increase in tear strength; however, when the molecular weight increases to a certain extent, its tear strength gradually tends to stabilize. Crystalline rubber exhibits higher tear strength at room temperature compared to non-crystalline rubber.

At room temperature, NR and CR exhibit high tear strength due to the induced crystallization during tearing of crystalline rubbers, which greatly enhances their strain capacity. However, at high temperatures, the tear strength decreases significantly except for NR. The tear strength of vulcanized rubber filled with carbon black is significantly improved.

2. Relationship between vulcanization system and tear strength

The tear strength increases with the increase of crosslinking density, but after reaching a maximum, further increase in crosslinking density results in a sharp decrease in tear strength.

3. Relationship between filling system and tear strength

As the particle size of carbon black decreases, the tear strength increases. When the particle sizes are the same, carbon black with lower structure is beneficial to tear strength.

Using isotropic fillers, such as carbon black, silica, white carbon black, lithopone, and zinc oxide, can achieve higher tear strength; whereas using anisotropic fillers, such as clay and magnesium carbonate, cannot achieve high tear strength.

Certain modified inorganic fillers, such as calcium carbonate and aluminum hydroxide modified with carboxylated polybutadiene (CPB), can enhance the tear strength of SBR vulcanizate.

4. The influence of plasticizing system on tear strength

5. Generally, the addition of softeners will reduce the tear strength of vulcanized rubber. In particular, paraffin oil is extremely detrimental to the tear strength of SBR vulcanized rubber, while aromatic oil can impart higher tear strength to SBR vulcanized rubber, with an increase in the amount of aromatic oil used.

(IV) Wear resistance

Wear resistance characterizes the ability of vulcanized rubber 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 usage conditions, the surface state of the friction pair, and the structure of the product, but also to other mechanical properties and viscoelastic properties of the vulcanized rubber, as well as other physical-chemical properties. There are many influencing factors.

1. Influence of adhesive type

Among the commonly used diene rubbers, the wear resistance decreases in the following order: BR > solution-polymerized SBR > emulsion-polymerized SBR > NR > IR. The main reason for BR's good wear resistance is its low glass transition temperature (Tg) (-95~105℃), good flexibility of molecular chain, 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 the wear resistance of XNBR is better than that of NBR.

Polyurethane (PU) is one of the most wear-resistant rubbers among all. It exhibits excellent wear resistance at room temperature, but its wear resistance decreases sharply at high temperatures.

2. The influence of the vulcanization system

The wear resistance of vulcanized rubber exhibits an optimal value as the crosslinking density increases. This optimal value is not only determined by the vulcanization system but also related to the amount and structure of carbon black. When increasing the amount and structural degree of carbon black, the rigidity provided by carbon black will increase. To maintain the optimal rigidity of the vulcanized rubber, it is necessary to reduce the rigid part provided by the vulcanization system, that is, to appropriately decrease the crosslinking density. Conversely, the crosslinking density of the vulcanized rubber should be increased.

3. The influence of the filling system

Generally, the wear resistance of vulcanized rubber increases as the particle size of carbon black decreases, and as the surface activity and dispersibility increase.

Both the addition of new process carbon black and the use of silica treated with silane coupling agent can improve the wear resistance of vulcanized rubber.

4. The influence of plasticizing system

Generally speaking, the addition of softeners to rubber compounds will reduce wear resistance. However, when aromatic oil is used in NR and SBR, the loss of wear resistance is less compared to other oils.

5. Impact of protective system

Under conditions of fatigue wear, the addition of appropriate antioxidants can effectively enhance the wear resistance of vulcanized rubber. For instance, 4010NA demonstrates outstanding performance. Besides 4010NA, 6PPD, DTPD, and DPPD/H also exhibit certain effects in preventing fatigue aging.

6. Other methods to improve the wear resistance of vulcanized rubber

(1) Carbon black modifier: Adding a small amount of carbon black modifier containing nitro compounds or other dispersants can improve the dispersibility of carbon black and enhance the wear resistance of vulcanized rubber.

(2) Surface treatment of vulcanized rubber: Using solutions or gases containing halogen compounds, such as liquid antimony pentafluoride and gaseous antimony pentafluoride, to treat the surface of vulcanized rubber such as NBR can reduce the friction coefficient of the vulcanized rubber surface and improve its wear resistance.

(3) Modifying fillers with silane coupling agents. For example, when silica treated with silane coupling agent A-189 is filled into NBR compound, the wear resistance of its vulcanizate is significantly improved. Similarly, when silica treated with silane coupling agent Si-69 is filled into EPDM vulcanizate, its wear resistance can also be significantly improved.

(4) Rubber-plastic blending Rubber-plastic blending is one of the effective ways to improve the wear resistance of vulcanized rubber. For example, blends such as NBR/PVC and NBR/ternary nylon can enhance the wear resistance of vulcanized rubber.

(5) Adding solid lubricants and anti-friction materials. For example, adding graphite, molybdenum disulfide, silicon nitride, carbon fiber, etc. to NBR rubber compounds can reduce the friction coefficient of the vulcanized rubber and improve its wear resistance.

(5) Flexibility

The high elasticity of rubber is caused by the conformational entropy change of coiled 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 "quasi-crosslinking" effect caused by entanglement within the molecular chain increases, so a higher molecular weight is beneficial for improving elasticity.

Polymers composed of flexible molecular chains that are not prone to crystallization at room temperature exhibit better elasticity with greater flexibility in their molecular chains.

2. Relationship between vulcanization system and elasticity

As the crosslinking density increases, the elasticity of the vulcanizate increases and reaches a maximum, and then as the crosslinking density continues to increase, the elasticity tends to decrease. This is because moderate crosslinking can reduce irreversible deformation caused by molecular chain slippage, which is beneficial for improving elasticity. However, excessive crosslinking can hinder the movement of molecular chains, resulting in decreased elasticity.

3. The relationship between filling system and elasticity

The elasticity of vulcanized rubber is entirely caused by conformational changes in rubber macromolecules. Therefore, increasing the rubber content is the most direct and effective way to enhance elasticity. To achieve high elasticity, the amount of filler should be minimized and the raw rubber content should be increased. However, to reduce costs, appropriate fillers should be selected.

4. The relationship between the plasticizing system and the elasticity of vulcanized rubber

The effect of softeners on elasticity is related to their compatibility with rubber. The poorer the compatibility between the softener and rubber, the poorer the elasticity of the vulcanized rubber.

(VI) Fatigue and fatigue failure

And by using appropriate anti-ozone agents, a better protective effect can be achieved.