Commonly used accelerators in the rubber production process.

There are thousands of compounding agents used in rubber production, and their roles in rubber are quite complex. They not only determine the physical and mechanical properties and service life of vulcanized rubber products but also affect the processing performance of the rubber compound and the quality of semi-finished products. The same compounding agent can have different effects in different raw rubbers, and different compounding agents can have different effects in the same raw rubber. Even the same compounding agent can have multiple roles in the same type of rubber. Therefore, we can only classify them based on their main roles in rubber into vulcanizing agents, vulcanization accelerators, activators, anti-aging agents, anti-scorching agents, reinforcing fillers, softening plasticizers, and other specialized compounding agents.

Vulcanizing agents: Substances that can react with rubber to cause vulcanization or cross-linking are collectively referred to as vulcanizing agents, also known as cross-linking agents.

There are many types of vulcanizing agents, and they are continuously increasing. The vulcanizing agents that have been used include sulfur, selenium, tellurium, sulfur-containing compounds, metal oxides, peroxides, resins, quinones, and amines.

Since the invention of rubber vulcanization, sulfur has been the main vulcanizing agent for natural rubber and most diene-based synthetic rubbers. Although many new types of vulcanizing agents have emerged that significantly improve the performance of rubber products, they are generally more expensive, so sulfur remains the primary choice. Metal oxides are specialized vulcanizing agents for chloroprene rubber. Other non-sulfur compound vulcanizing agents, except for selenium and tellurium, are mainly used for the vulcanization of saturated rubbers and special synthetic rubbers, with peroxides being the most important.

Vulcanization accelerators

Substances that can accelerate the vulcanization reaction speed, shorten vulcanization time, lower vulcanization temperature, reduce the amount of vulcanizing agents used, and improve or enhance the physical and mechanical properties of vulcanized rubber are collectively referred to as vulcanization accelerators, abbreviated as accelerators.

Accelerators can also improve the efficiency of the vulcanization process and the quality of vulcanized rubber, ensure uniform vulcanization of thick products, and reduce product costs. There are many types of accelerators used in the rubber industry, which can be divided into two main categories based on their chemical composition and properties: inorganic accelerators and organic accelerators. Currently, organic accelerators are generally used, except in a few cases. Due to their strong promoting effect, good vulcanization characteristics, and excellent physical and mechanical properties of vulcanized rubber, they are rapidly developed and have a wide variety.

According to the chemical structure of accelerators, they are usually divided into eight categories: thiazoles, thiuram, sulfenamides, guanidines, dithiocarbamates, aldehyde amines, xanthates, and thioureas.

Using accelerator M as a standard, they can be further divided into ultra-fast, fast, quasi-fast, medium, and slow types. Internationally, it is customary to use accelerator M (mercaptobenzothiazole) as the standard. Those with a vulcanization speed faster than M are classified as fast or ultra-fast, those with a vulcanization speed lower than M are classified as medium or slow, and those with a vulcanization speed equal to or close to M are classified as quasi-fast. The accelerators used in natural rubber and most synthetic rubbers follow this classification method.

Accelerators can also be classified based on acidity and alkalinity. Those that are acidic themselves or produce acidic products after reacting with hydrogen sulfide are classified as acidic accelerators, such as thiazoles, thiuram, dithiocarbamates, and xanthates; those that are basic themselves or produce basic products after reacting with hydrogen sulfide are classified as basic accelerators, such as guanidines, aldehyde amines, and amines; those that are neutral under both conditions are classified as neutral accelerators, such as sulfenamides and thioureas. This classification method is often used in production or when accelerators are used together.

In an accelerator combination system, there should be one main accelerator, referred to as the primary accelerator or first accelerator, while another or two others are auxiliary, referred to as secondary accelerators or second and third accelerators.

Activators

Vulcanization activators, abbreviated as activators, are substances that participate in the vulcanization reaction of rubber, enhance the activity of accelerators, fully exert their effectiveness, reduce the amount of accelerators used, increase vulcanization speed, and improve the degree of cross-linking, thereby affecting the cross-linking structure and improving the physical and mechanical properties of vulcanized rubber. There are many types of vulcanization activators used in rubber, which can be divided into inorganic and organic categories. The most commonly used are zinc oxide and stearic acid, used in amounts of 35 parts and 1.5-2.0 parts, respectively. Zinc oxide also has a certain reinforcing effect on natural rubber. Stearic acid has a softening and plasticizing effect on the rubber compound, helping to disperse carbon black.

Protective system compounding agents

During storage and use, rubber and its products gradually lose elasticity and service value due to various external factors such as heat, oxygen, ozone, divalent metal ions, mechanical forces, light, high-energy radiation, chemical substances, and mold. This phenomenon is called aging. To extend the service life of products, certain substances must be added to rubber to inhibit or delay the aging process, collectively referred to as anti-aging agents for rubber.

There are many types of anti-aging agents, which can be divided into physical anti-aging agents, chemical anti-aging agents, and reactive anti-aging agents based on their protective principles; and into antioxidants, anti-ozonants, flexural crack inhibitors, harmful metal inhibitors, ultraviolet absorbers, etc., based on their protective functions.