How to solve the rubber vulcanization technology?

1. Important technical conditions in rubber vulcanization

In the vulcanization process of rubber products, the time, temperature and pressure of vulcanization are the main factors that constitute the vulcanization process conditions. These factors have a decisive influence on the quality of product vulcanization and are usually called the "three elements of vulcanization". This is also an important technical condition in rubber vulcanization work. 1. Vulcanization temperature Vulcanization temperature is one of the basic conditions for the vulcanization reaction of rubber. It directly affects the speed of the vulcanization reaction and the physical and mechanical properties of the vulcanized rubber, thereby affecting the quality of the product. Like general chemical reactions, the rubber vulcanization reaction depends on temperature. As the temperature rises, the vulcanization reaction speed increases, the production efficiency is high, and it is easy to generate more low-sulfur cross-links; on the contrary, the vulcanization temperature is low, the vulcanization speed is slow, the production efficiency is low, and it is easy to generate more polysulfide cross-links. Obviously, to obtain high production efficiency, a higher vulcanization temperature should be used as much as possible. The level of the vulcanization temperature depends on the rubber variety and vulcanization system in the rubber formula, and is also related to factors such as the shape, size, thickness and other factors of the product and the process conditions of the factory. In fact, the vulcanization temperature cannot be raised unlimitedly. Generally, the higher the vulcanization temperature, the lower the physical and mechanical properties of the rubber. Too high a temperature will cause the cracking of the rubber molecular chain, which will reduce the performance and make process control difficult. Therefore, the choice of vulcanization temperature should be comprehensively considered according to the type of product, the type of rubber and the vulcanization system. Generally, the vulcanization temperature of natural rubber and general synthetic rubber is in the range of 120-190°C. Rubber is a poor conductor of heat. Especially for thick rubber products, it is difficult to make the inner and outer layers of rubber reach the flat range of the vulcanization curve at the same time by high-temperature vulcanization. Sometimes, the surface of the product is over-sulfurized when the inside is under-sulfurized or just vulcanized. Moreover, the higher the vulcanization temperature, the easier it is for this phenomenon to occur and the greater the degree of over-sulfurization on the surface. Therefore, in order to ensure the uniform vulcanization of multi-component products and thick-walled products, in addition to fully considering the vulcanization flatness of the rubber in the formulation design, the choice of vulcanization temperature should also consider a lower degree of vulcanization or a gradual heating method. For thin-walled products with simple structures, the vulcanization temperature can be higher. Different vulcanization systems have different vulcanization characteristics, some require high activation temperatures, and some require low activation temperatures. Therefore, the vulcanization temperature suitable for it should also be selected according to the vulcanization system in the formula. When the active temperature of the accelerator is low or the product requires high tensile strength, low tensile stress and hardness, the vulcanization temperature can be lower; when the active temperature of the accelerator is high or the product requires high tensile stress and hardness, low elongation, the vulcanization temperature can be higher. In addition, the high temperature resistance of various rubbers is also different. Therefore, choosing an appropriate vulcanization temperature is of great significance for ensuring product quality and improving vulcanization efficiency. 2. Vulcanization pressure Vulcanization pressure refers to the pressure per unit area of rubber products during the vulcanization process. Pressure is one of the conditions for rubber vulcanization. Generally, rubber products need to be subjected to a certain pressure during vulcanization. Vulcanization pressure can be divided into normal pressure and high pressure. Usually, normal pressure vulcanization is suitable for thin-walled products such as tape, while molded products require higher pressure. This is because during the vulcanization process, some volatiles in the rubber (such as moisture in the raw materials, processing aids mixed in during the processing of semi-finished products, etc.) will be released at high temperatures, and some air trapped between the layers of multi-layer products will also escape, which will have an adverse effect on the products. Therefore, high pressure must be applied during vulcanization to allow volatiles and air to be discharged from the mold vents. In addition, under pressure conditions, the solubility of the rubber in gas increases, which also reduces the generation of microscopic bubbles. The methods of vulcanization pressurization include: using a hydraulic pump to transfer pressure to the mold through a flat vulcanizer, and then from the mold to the rubber plate for pressurization; direct pressurization by the vulcanization medium, such as steam pressurization; pressurization by compressed air, that is, hot air pressurization; injection molding and individual vulcanizer pressurization, etc.   The main effects of vulcanization pressure are as follows:

(1) Preventing bubbles from forming in the product during the vulcanization process and improving the density of the rubber. During vulcanization, the water and other volatile substances contained in the rubber, as well as the hydrogen sulfide gas that may be formed during the vulcanization reaction, will all volatilize and escape at high temperatures, causing bubbles in the rubber. If a vulcanization pressure greater than the internal pressure that may escape from the rubber is applied during vulcanization, the generation of bubbles can be prevented and the compactness of the rubber can be improved.

(2) Make the rubber easy to flow and fill the mold cavity. In order to produce products with clear and full patterns, the rubber must be able to flow and fill the mold cavity. In particular, during the vulcanization induction period when the rubber is in an uncrosslinked state, the effect of vulcanization pressure is more obvious.

(3) Improve the compactness of the rubber and improve the adhesion between the rubber and the skeleton material. During vulcanization, as the vulcanization pressure increases, the depth of rubber penetration into the fabric layer increases, thereby improving the adhesion between the rubber and the fabric layer and the flexural resistance of the product.

(4) Helps improve the physical and mechanical properties of the vulcanized rubber. Tests show that with the increase of vulcanization pressure, some physical and mechanical properties of vulcanized rubber, such as strength, fatigue resistance, and wear resistance, are correspondingly improved. However, too high vulcanization pressure will also be detrimental to the performance of rubber, because high pressure, like high temperature, will accelerate the thermal degradation of rubber molecules, which will reduce the performance of the rubber. Excessive pressure also places high demands on equipment, causes greater damage to equipment, and has high power costs. The selection of vulcanization pressure should be based on the rubber formula, rubber plasticity, product structure and process. The general principle is that the plasticity of the rubber is large, the vulcanization pressure should be small; the product is thick, has many layers, and has a complex structure, and the vulcanization pressure is large; the product is thin, the vulcanization pressure should be low, and even room temperature vulcanization can be used.

3. Vulcanization time Like many chemical reactions, the progress of the vulcanization reaction also depends on time. Under the action of a certain vulcanization temperature and pressure, only after a certain vulcanization time can the vulcanization degree that meets the design requirements be achieved. Usually, the vulcanization time of rubber products should be within the range of the rubber compound reaching positive vulcanization, and should be selected according to the performance requirements of the product, and should also be adjusted according to the thickness of the product and the existence of the cloth layer skeleton. When the formula and vulcanization temperature of the rubber compound are determined, the positive vulcanization time of the semi-finished product (test piece) rubber compound can be first determined by the physical and chemical method, the conventional physical and mechanical property method or the vulcanizer method. Then the vulcanization time of the finished product is determined according to the positive vulcanization time of the test piece. At present, shortening the vulcanization time of the finished product and improving the vulcanization efficiency are the needs of rubber companies. There are two main ways: ①Increase the vulcanization temperature; ②Adjust the formula. When the rubber compound formula is the same, the vulcanization time is restricted by the vulcanization temperature. Since the vulcanization time and the vulcanization temperature are mutually restricted, when the vulcanization conditions change, the equivalent vulcanization time and the equivalent vulcanization effect method that can reflect the internal connection between the two are usually used to solve the problem. Here is only a brief introduction to the equivalent vulcanization time. During the vulcanization process, the time it takes for a product to achieve the same vulcanization effect at different vulcanization temperatures is called the equivalent vulcanization time, also known as the equivalent vulcanization time or equivalent vulcanization time. Vulcanization time, how long does rubber vulcanization take? Vulcanization can be designed as fully automatic or semi-automatic control vulcanization according to demand. Vulcanization is automatically controlled by computer electrical appliances, which can accurately control the vulcanization pressure, vulcanization time and vulcanization temperature of rubber products. The control system has a pre-stored process function. After setting the process parameters, the system automatically controls the air intake, heating, insulation, pressure replenishment and pressure relief of the equipment. According to the different vulcanized products, it can be divided into hose vulcanization, rubber roller vulcanization, rubber shoe vulcanization, tape vulcanization, rubber plate vulcanization and other vulcanization categories. The heating method of vulcanization generally uses steam heating, electric heating and thermal oil heating to increase the vulcanization temperature, and uses air intake to replenish pressure. 2. Changes in performance during rubber vulcanization Vulcanization is the last process in the rubber industry production and processing. In this process, a series of chemical reactions occurred in the rubber, which turned the linear polymer into a three-dimensional network structure, thus obtaining valuable physical and mechanical properties and becoming a valuable engineering material.

Vulcanization refers to the chemical reaction between the raw rubber and the vulcanizing agent in the rubber compound under heating or irradiation conditions, and the linear polymer cross-linked to become a macromolecule with a three-dimensional network structure, and the physical and mechanical properties and other properties of the rubber compound undergo fundamental changes.

In industrial production, the vulcanization cross-linking reaction is generally completed under certain temperature, time and pressure conditions. These vulcanization conditions are usually called the three elements of vulcanization. Implementing vulcanization conditions in production, correctly selecting vulcanization equipment and selecting heating media are all important technical contents in the vulcanization process.

In the process of vulcanization, a series of properties of the rubber compound have undergone significant changes. Taking test pieces with different vulcanization times for various physical and mechanical properties, it can be seen that after the tensile strength, tensile stress, elasticity and other properties reach their peak values, the vulcanization time is extended, and their values decrease, while the hardness remains unchanged. The properties such as elongation and permanent deformation gradually decrease with the extension of vulcanization time. After reaching the minimum value, they slowly rise again with continued vulcanization. Other properties such as heat resistance, wear resistance, and anti-swelling resistance all improve with the increase of vulcanization time.

The change of rubber properties during vulcanization is the result of changes in molecular structure during vulcanization. Unvulcanized raw rubber is a linear macromolecule, whose molecular chain has the independence of movement, showing great plasticity, high elongation, and solubility; after vulcanization, the rubber macromolecule produces transverse bonds between the molecular chains to form a spatial network structure. Therefore, in addition to the secondary bond force between molecules, there is also a primary bond force at the junction of the molecular chains, resulting in the vulcanized rubber having greater tensile strength, higher tensile stress, smaller elongation and greater elasticity than the raw rubber, and losing solubility and only limited swelling. Therefore, the changes in the properties of the rubber during the vulcanization process should be examined in conjunction with its structural changes. The general rules of changes in several main physical and mechanical properties of rubber (taking natural rubber as an example) are as follows:

1. Elongation stress When the rubber is not vulcanized, the linear molecules can flow more freely, and within the plastic range, they show non-Newtonian flow characteristics. However, as the degree of vulcanization deepens, this flow freedom becomes smaller and smaller, and the deformation force required for fixed length stretching becomes larger and larger. This is the "elongation stress".

2. Tensile strength The tensile strength of soft rubber gradually increases with the increase of crosslinking degree until the highest value appears. When further vulcanized, the tensile strength drops sharply after a flat area. In hard rubber with a large amount of sulfur, the tensile strength drops and then rises until it reaches the level of hard rubber.

3. Elongation The elongation of rubber gradually decreases with the increase of crosslinking degree.

4. Compression set The compression set of rubber also gradually decreases with the increase of crosslinking degree. For rubber with vulcanization reduction, its compression set gradually increases after positive vulcanization.

5. Elasticity The elasticity of rubber comes from the reversible change of the micro-Brownian motion position of the macromolecular flexible chain segment. Due to this characteristic, a small external force will cause it to deform greatly.

When in a plastic state, the displacement of rubber molecules is irreversible, but after the rubber molecules are cross-linked, they are relatively positioned with each other, resulting in a strong tendency to reset. However, when the degree of cross-linking continues to increase, the tendency to reset after deformation is reduced due to the excessive relative fixity between macromolecules. Therefore, when the vulcanized rubber is severely over-sulfurized, the elasticity is weakened, and the elasticity of the elastomer changes to the elasticity of the rigid body.

6. Hardness The hardness of the vulcanized rubber increases rapidly after the start of vulcanization, reaches the maximum value at the positive vulcanization point, and then remains basically constant.

7. Anti-swelling Unvulcanized rubber, like other polymers, swells in certain solvents and absorbs solvents until it loses its cohesion. When rubber molecules enter the solution, swelling will only occur when the osmotic pressure of the solvent on the rubber is greater than the cohesion of the rubber molecules.

8. Air permeability: As the degree of rubber cross-linking increases, the gaps in the network structure gradually decrease, and the ability of gas to pass and diffuse in the rubber decreases due to the increase in resistance. Therefore, the air permeability of fully vulcanized elastomers is better than that of under-vulcanized ones.

9. Heat resistance: The heat resistance is best during positive vulcanization.

10. Wear resistance: After the start of vulcanization, the wear resistance gradually increases and reaches the highest level during positive vulcanization. Under-vulcanization or over-vulcanization is not good for wear resistance, but over-vulcanization is less affected.

There are three differences between natural rubber and synthetic rubber in vulcanization:

1) Natural rubber contains more non-rubber hydrocarbon substances, such as albumin, albumin decomposition products, amines, etc., which play a role in accelerating vulcanization, while there are no such substances in synthetic rubber; on the contrary, synthetic rubber contains a certain amount of resin or fatty acids left over from the polymerization process, which play a role in delaying vulcanization.

2) The main chain structure of natural rubber has more double bonds, and the vulcanization speed is faster than that of synthetic rubber, so synthetic rubber must use more accelerators and less sulfur when compounding.

3) Natural rubber is mainly a linear macromolecular structure, while synthetic rubbers such as styrene-butadiene rubber and nitrile rubber are prone to intramolecular crosslinking due to their high degree of branching, which makes the vulcanized rubber hard.

In addition, the side groups of the rubber chain also affect the vulcanization speed. For example, the polarity of the acrylonitrile group is greater than that of the styrene group, so the vulcanization speed of nitrile rubber is greater than that of styrene-butadiene rubber.