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The relationship between various physical properties of rubber and crosslinking density

Time:2020/9/28 15:49:02 Reading:12

In the process of rubber products, the physical property experiments that must generally be tested are nothing more than:

1. Tensile strength 2. Tear strength 3. Tensile stress and hardness 4. Wear resistance 5. Fatigue and fatigue failure

6. Elasticity 7. Elongation at break.

Various rubber products have their specific performance and process formula requirements. In order to meet its physical requirements, it is necessary to select the most suitable polymer and compounding agent for reasonable formulation design. First, we must understand the relationship between formulation design and physical properties of vulcanized rubber. The physical properties of vulcanized rubber are closely related to the design of the formula. Different types and dosages of materials used in the formula will produce differences in performance.

1. Tensile strength:

It is the root limit ability of the product to resist tensile failure. It is one of the important indicators of rubber products. The life of many rubber products is directly related to the tensile strength. For example, the durability of the cover rubber of the conveyor belt and the rubber shock absorber is improved with the increase of the tensile strength.

A: The tensile strength is related to the structure of the rubber:

When the divided amount is small, the intermolecular interaction is less valuable. Therefore, when the external force is greater than the intermolecular action, intermolecular sliding will occur and the material will be destroyed. On the contrary, the molecular weight is large, the intermolecular force increases, the cohesive force of the rubber compound increases, and the chain segment is not easy to slide during stretching, so the degree of damage to the material is small. All other factors that affect the intermolecular force have an impact on the tensile strength. Such as NR/CR/CSM, these rubbers have crystalline substituents on the main chain, the valence force between molecules is greatly improved, and the tensile strength also increases. That is one of the main reasons for the good self-reinforcing properties of these rubbers. Generally, as the crystallinity of rubber increases, the tensile strength increases.

B: The tensile strength is also related to temperature: the tensile strength at high temperature is much lower than the tensile strength at room temperature.

C: The tensile strength is related to the crosslink density:

As the cross-linking density increases, the tensile strength increases. After the maximum value is reached, the cross-linking density continues to increase, and the tensile strength will drop significantly. The tensile strength of vulcanized rubber decreases as the cross-linking bond energy increases. Natural rubber that can produce tensile crystallization, the weak bond breaks early, which is conducive to the orientation crystallization of the main bond, so it will have a higher tensile strength. Through the vulcanization system, sulfur vulcanization is used, and the combination of accelerators is selected, DM/M/D can also improve the tensile strength (except for carbon black reinforcement, because of the heat generation effect of carbon black).

D: The relationship between tensile strength and filler:

Reinforcing agent is one of the important factors affecting tensile strength. The smaller the material diameter of the filler, the larger the specific surface area and the greater the surface activity, the better the reinforcement performance. The vulcanizate of crystalline rubber has a monotonous decline because it is self-reinforcing non-crystalline rubber such as styrene butadiene rubber. With the increase of the dosage, the reinforcement performance increases, and overuse will decrease the interest. The low-dispersion rubber reaches its maximum value as the amount increases and can remain unchanged.

E: The relationship between tensile strength and softener:

Adding a softener will reduce the tensile strength, but adding a small amount, generally less than 7 parts in the open kneading machine, and less than 5 parts in the compact kneading machine, will improve the dispersion, which is beneficial to increase the tensile strength. Different softeners have different degrees of reduction in tensile strength. Generally natural rubber is suitable for vegetable oils. Aromatic oil for non-polar rubber such as SBR/IR/BR. For example, paraffin oil and naphthenic oil are used for IIR/EPDM. NBR/CR uses DBP/DOP. etc.

Other methods to improve tensile strength include blending rubber and resin, chemical modification of rubber, and surface modification of fillers (such as cinnamane, etc.)

2. Tear strength:

The tearing of rubber is caused by the rapid expansion of cracks or cracks in the material when subjected to force, leading to destruction.

A: There is no direct relationship between tear strength and stretch: in many cases tear and stretch are not proportional. general

   In this case, crystalline rubber has higher tear strength than amorphous rubber.

B: Tear strength is related to temperature: Except for natural rubber, the tear strength decreases significantly at high temperatures. carbon

   The tear strength of rubber filled with black and white carbon is significantly improved.

C: The tear strength is related to the vulcanization system. Polysulfide bonds have higher tear strength. Sulfur dosage high tear strength

   high. However, excessive sulfur content will significantly reduce the tear strength. It is advantageous to use an accelerator with better flatness

   To improve the tear strength.

D: The tear strength is related to the filling system:

Various reinforcing fillers such as carbon black, white carbon black, Bai Yanhua, zinc oxide, etc., can obtain higher tear strength. Certain coupling agents such as cinnamane can increase the tear strength. Usually adding a softener will reduce the tear strength. Such as paraffin oil will make the tear strength of styrene butadiene rubber extremely disadvantageous. The aromatic oil has not changed much. For example, ester plasticizers used for CM/NBR have much less impact than other softeners.

3. Tension stress and hardness:

Tensile stress and hardness are important indicators of the stiffness of rubber materials. They are the force required for vulcanized rubber to produce a certain deformation. They are related to larger tensile deformations. The correlation between the two is good, and the change law is basically the same.

The greater the molecular weight of the rubber, the greater the effective crosslinking tensile stress. In order to obtain the specified constant elongation stress, the crosslink density can be appropriately increased for rubber with a smaller molecular weight. Any structural factor that can increase the force between molecules. Can improve the ability of the vulcanizate to resist deformation. Such as CR/NBR/PU/NR, etc., have higher constant elongation stress.

Tensile stress and crosslink density have a great influence. Whether it is pure rubber or reinforced vulcanized rubber, as the crosslinking density increases, the elongation stress and hardness also increase linearly. It is usually achieved through the adjustment of vulcanizing agents, accelerators, vulcanizing aids, and active agents. The promotion of sulfur content has a more significant effect on increasing the tensile stress. Polysulfide is beneficial to increase the tensile stress. Fillers can improve the tensile stress and hardness of the product. The higher the reinforcing performance and the higher the hardness, the higher the tensile stress. As the hardness increases, the increase of the filling is higher. On the contrary, as the softener increases, the hardness decreases and the tensile stress decreases. In addition to adding reinforcing agents, there are also alkyl phenolic resins with hardness up to 95 degrees and high styrene resins. Using resin RS, accelerator H and the hardness of the system can reach 85 degrees and so on.

4. Wear resistance:

The characterization of wear resistance is the ability of vulcanized rubber to resist material loss due to surface damage under the action of friction force. It is a mechanical property closely related to the service life of rubber products.

The forms of wear resistance are:

A. Wear and tear:

When rubbing, the uneven, sharp and rough objects on the surface are constantly cutting and rubbing. As a result, the contact points on the rubber surface are cut and broken into tiny particles, which fall off the rubber surface and form abrasion. Abrasion strength is proportional to pressure and inversely proportional to tensile strength. Decrease as resilience increases.

B. Fatigue wear:

The surface of the vulcanized rubber in contact with the friction surface is subjected to periodic compression, shearing, stretching and other deformations in the repeated process, causing fatigue on the rubber surface and gradually producing micro-cracks in it. The development of these cracks causes microscopic peeling of the material surface. The fatigue wear increases with the increase of the elastic modulus and pressure of the rubber, and increases with the decrease of the tensile strength and the deterioration of the fatigue performance.

C: Curly wear:

When the smooth surface under the rubber comes into contact, the uneven surface of the vulcanized rubber will be deformed due to friction, and be torn and damaged, forming a rolled off surface.

The wear resistance is related to the main mechanical properties of vulcanizates. Try to balance the relationship between various properties when designing the formula. The relationship between abrasion resistance and the type of rubber is the greatest, generally speaking NBR>BR>SSBR>SBR (EPDM)>NR>IR (IIR)>CR

The abrasion resistance is related to the vulcanization system, and an appropriate increase in the crosslinking performance can improve the abrasion resistance. The more monosulfur, the better the wear resistance, which is the best reason for the wear resistance of the semi-effective vulcanization system. The wear resistance of using CZ as the first accelerator is better than other accelerators, and the best amount of reinforcing agent will improve certain wear resistance. Reasonable use of softeners will minimize wear resistance. Such as natural rubber and styrene butadiene rubber with aromatic oil.

Effective use of antioxidants can prevent fatigue and aging. Increasing the dispersibility of carbon black can improve wear resistance.

Modification with cinnamane surface treatment agent can greatly improve the wear resistance.

Rubber-plastic blending is used to improve the wear resistance, such as the combined use of nitrile and polyvinyl chloride to produce textile leather knots.

Use nitrile and ternary nylon together, and nitrile and phenolic resin together.

Add solid lubricants and anti-friction materials. For example, adding graphite, molybdenum disulfide, silicon nitride, and carbon fiber to the nitrile rubber compound can reduce the friction coefficient of the vulcanized rubber and improve its wear resistance.

5: Fatigue and fatigue damage:

When the vulcanizate is subjected to alternating stress, the phenomenon that the structure and performance of the material changes is called fatigue. As the fatigue progresses, the phenomenon that leads to material failure is called fatigue failure.

A: Influence of rubber structure, rubber with low glass transition temperature has good fatigue resistance. Rubber with polar groups has poor fatigue resistance. Rubber with large groups or side groups in the molecule, rubber with poor fatigue resistance and regular structural sequence, tends to crystallize, and has poor fatigue resistance.

B: The impact of the rubber vulcanization system. The single-sulfur vulcanization system has minimal fatigue performance and good fatigue resistance. Increasing the amount of crosslinking agent will reduce the fatigue performance of the vulcanized rubber. Therefore, the amount of crosslinking agent should be minimized.

C: The influence of fillers. The smaller the reinforcing performance, the smaller the influence of fillers. The larger the amount of fillers, the greater the influence. Fillers should be used as little as possible.

D: The influence of the softening system is to use non-viscous softeners with low softening points as much as possible; the amount of softeners should be as large as possible, on the contrary, high-viscosity softeners should not be used, such as pine tar has poor fatigue resistance and lipid plasticization The fatigue resistance of the agent is good.

6. Flexibility:

The most valuable property of rubber is elasticity. The high elasticity comes from the movement of rubber molecules, which is completely caused by the conformational changes of the crimped molecules. It can be restored to its original state immediately after removing the external force, which is called an ideal elastomer. The interaction between rubber molecules will hinder the movement of the molecular segments, showing stickiness or viscosity. Therefore, the characteristics of rubber are both elastic and viscous. The factors that affect the elasticity include the size of the deformation, the action time, and the temperature. When the interaction between rubber molecules increases and the regularity of the molecular chain is high, it is easy to produce tensile crystallization, which is beneficial to the improvement of strength and shows high elasticity. Among general-purpose rubbers, natural and butadiene rubber have the best elasticity, followed by nitrile and neoprene. Butylbenzene is inferior to butyl.

A: Elasticity is related to crosslink density:

With the increase of the cross-linking density, the elasticity of the vulcanizate increases and reaches a maximum value, and the cross-linking density continues to increase and the elasticity is showing a downward trend. Properly increasing the degree of fluidization is beneficial to elasticity. In high-elasticity matching, the use of sulfur and CZ together, and the use of promoting D together with vulcanizates have higher resilience and low hysteresis loss.

B: Elasticity is related to filling system:

Increasing the rubber content is the most direct and effective way to improve elasticity. The better the reinforcement, the worse the elasticity is.

C: The relationship between elasticity and softener:

The softener is related to the compatibility of rubber. The smaller the compatibility, the worse the elasticity. Such as natural, butadiene, butyl and paraffin oil, better than naphthenic oil. Butadiene plus DOP is better than naphthenic oil and aromatic oil. Generally speaking, plasticizers will reduce the elasticity of rubber, and plasticizers should be used as little as possible.


7: Elongation at break (elongation):

A: The elongation at break is related to the tensile strength:

Only with high tensile strength and ensuring that it is not damaged during the deformation process can it have a high elongation. Generally, as the tensile stress and hardness increase, the elongation at break decreases, and the resilience and permanent deformation are small, the elongation at break increases. Different rubbers have different elongation at break. Natural rubber has an elongation at break of 1000% when its rubber content is above 80%. Rubber that is prone to plastic flow during deformation will also have a higher elongation. Such as butyl rubber.

B: The elongation at break decreases as the crosslink density increases. When manufacturing high-stability products, the degree of vulcanization should not be too high, and the amount of vulcanizing agent can be reduced slightly. Increasing the amount of filler will reduce the elongation at break. The higher the structure of the reinforcing agent, the lower the elongation at break. Once the amount of softener has been added, a larger elongation at break can be obtained.

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