Although raw rubber also has some useful application characteristics, it also has many shortcomings, such as low strength and small elasticity; cold is hard, hot is sticky; easy to age and so on. As early as the 1840s, it was discovered that the rubber can be cross-linked by heating it together with sulfur. Therefore, until now, although rubber can be cross-linked not only with sulfur, but also with many other chemical cross-linking agents and physical-chemical methods, in the rubber industry, it has been accustomed to call the cross-linking of rubber “vulcanization”, while The plastics processing industry sometimes refers to the crosslinking reaction as curing. Vulcanization greatly improves the properties of raw rubber, expands the application scope of rubber, and lays a foundation for the large-scale industrial production and application of rubber.
Rubber vulcanization is one of the main technological processes in the processing of rubber products, and it is also the last processing procedure in the production of rubber products. In this process, the rubber has to undergo a series of complex chemical changes, from a plastic compound rubber to a highly elastic or hard cross-linked rubber, so as to obtain more perfect physical and mechanical properties and chemical properties, improve and widen The use value and application range of rubber materials. Therefore, vulcanization is of great significance to the manufacture and application of rubber and its products.
First, the concept of vulcanization
Vulcanization refers to a semi-finished product made of a certain plastic and viscous rubber (raw rubber, masticated rubber, compounded rubber) through appropriate processing (such as calendering, extrusion, molding, etc.) through chemical factors under certain external conditions. (such as vulcanization system) or physical factors (such as gamma rays), the process is re-transformed into soft elastic rubber products or hard rubber products, so as to obtain performance. During the vulcanization process, external conditions (such as heating or radiation) cause a chemical reaction between the raw rubber and the vulcanizing agent or between the raw rubber and the raw rubber in the compound component, and the linear rubber macromolecules are cross-linked to form a three-dimensional network. macromolecules with a similar structure, as shown in the figure below.
Through this reaction, the properties of the rubber are greatly improved, and the rubber products have obtained physical and mechanical properties and other properties that can meet the needs of product use. The essence of vulcanization is cross-linking, that is, the process of transforming the linear rubber molecular structure into a spatial network structure.
The substance that vulcanizes the rubber is called the vulcanizing agent of the rubber (also known as the cross-linking agent). In addition, cross-linking agents are peroxides (such as DCP, bis-2,5-2,5-dimethylbishexane, BP), metal oxides (such as zinc oxide, magnesium oxide, lead oxide, calcium oxide) , resin, amine substances. In most cases, the vulcanization speed is very slow and has no use value. It is also necessary to add vulcanization accelerators and accelerators that can increase the vulcanization speed of rubber, reduce the vulcanization temperature, reduce the amount of crosslinking agent, and improve the degree of vulcanization. Active vulcanization activator. In most cases, two or more accelerators are used together to obtain better results. Commonly used active agents include inorganic active agents and organic active agents.
Rubber vulcanizing agent, rubber vulcanizing powder, rubber vulcanization process
The external conditions that promote the transformation of rubber vulcanization are the process conditions necessary for vulcanization, that is, vulcanization temperature, time and pressure. Therefore, the reasonable determination and strict control of vulcanization process conditions are the key links in determining the quality of rubber products.
To be precise, vulcanization is a process in which macromolecules are cross-linked by a cross-linking agent or a cross-linking initiator under the conditions of rubber temperature, time and pressure, which changes the rubber from a plastic state to an elastic state. .
Second, the performance change of rubber before and after vulcanization
After vulcanization of rubber, most of its physical, mechanical and chemical properties have changed, and the changes are shown in the table below.
However, the performance of rubber does not always increase or decrease with the increase of vulcanization degree. The main physical and mechanical properties of the rubber compound change as shown in the figure below.
The change in the properties of the rubber during the vulcanization process is the result of the change in the molecular structure. The unvulcanized raw rubber is a macromolecule with a linear structure, and the molecular chain can move freely. It can produce large deformation and plastic flow, and can be dissolved in some solvents, so it is soluble. In the vulcanized rubber macromolecules, various parts of the molecular structure have formed a network structure to varying degrees, which restricts the relative movement of the macromolecules, resulting in higher tensile strength, lower elongation, and elasticity of vulcanized rubber than raw rubber. big wait.
(1) Constant elongation strength
Through vulcanization, the single molecules of the rubber are cross-linked, and with the increase of the cross-linking density, the external force required to produce a certain deformation (such as stretching to 200% or 300% of the original length) increases, and the vulcanized rubber increases. hard. For a certain rubber, when the test temperature, the shape of the test piece and the elongation are constant, the constant elongation strength is inversely proportional to MC (average molecular weight of rubber molecules between two cross-linking bonds), that is, proportional to the degree of cross-linking. This shows that the degree of crosslinking is large, that is, the smaller the average molecular weight of the segments between the crosslinks, the higher the tensile strength.
Like the tensile strength, the hardness of the rubber gradually increases with the increase of the degree of crosslinking. The hardness is measured under a certain deformation, so the above-mentioned conditions about the tensile strength are basically applicable to the hardness.
(3) Tensile strength
The tensile strength is different from the tensile strength and hardness. It does not continuously increase with the increase of the number of cross-linking bonds. For example, when the degree of cross-linking reaches an appropriate value, if the cross-linking degree reaches an appropriate value, the tensile strength will be reversed. will decline. In the hard rubber with a high amount of sulfur, the tensile strength decreases and then rises again until it reaches the level of the hard rubber.
(4) Elongation and permanent deformation
The elongation of rubber decreases with the increase of cross-linking degree, and the permanent deformation also has the same law. Rubber with reversion, such as natural rubber and butyl rubber, will gradually increase in elongation and permanent deformation due to the continuous decrease of crosslinking degree after overvulcanization.
When the unvulcanized rubber is subjected to external force for a long time, plastic flow mainly occurs, and the rubber molecules basically have no tendency to return to the original position. After the rubber is vulcanized, the crosslinking makes the molecules or segments fixed, and the deformation is constrained by the network. After the external force is eliminated, the molecules or segments try to restore the original conformation and position, so the vulcanized rubber shows great elasticity. With an appropriate increase in the degree of crosslinking, this reversible elastic recovery is more pronounced.
In the industrial production process, rubber vulcanization is usually done using a vulcanization tank (also called an autoclave). The vulcanization tank can precisely control the vulcanization temperature, vulcanization time and vulcanization pressure in the vulcanization process, and realize batch processing of products through multi-stage process control.
According to the heating method, the vulcanization tank can be divided into steam vulcanization tank, electric heating vulcanization tank, heat transfer oil vulcanization tank, etc. The suitable heating method is selected according to the actual situation of the manufacturer’s workshop. The vulcanization tank is a medium and low pressure pressure vessel, and has certain technical requirements for product quality and production operations. When choosing a vulcanization tank equipment, manufacturers need to choose a professional pressure vessel manufacturer.