Accelerators, antioxidants, vulcanizers... What are their chemical differences?
In the modern rubber industry, chemical additives are called "seasonings" for rubber. Although they account for a small proportion of the entire formula, they play a decisive role in the performance of the finished product. Among them, accelerators, antioxidants, and vulcanizers are the three most commonly used types of rubber additives. They differ in chemical structure, mechanism of action, reaction path, and purpose of use, and constitute the core system of rubber formulation science.
This article will deeply analyze the differences between the three from multiple angles such as chemical composition, action principle, reaction path, molecular structure characteristics, and usage differences, to help readers establish a clear and systematic understanding.
1. Functional positioning differences: The three have different purposes
Additive type Main function Application stage
Vulcanizing agents provide the "bridge" material required for cross-linking, so that the rubber changes from a linear to a three-dimensional network structure, improving elasticity and mechanical properties Processing stage (vulcanization)
Accelerators accelerate the vulcanization reaction rate, reduce the vulcanization temperature, and improve the efficiency and quality of cross-linking Processing stage (assisting vulcanization)
Antioxidants (Antiozonants) inhibit the aging of rubber during use (oxidation, ozone, heat, light), and extend the service life Use stage (service protection)
Therefore, vulcanizers and accelerators are "chemical reaction promoters", while antioxidants are "reaction inhibitors", and the two have opposite directions of action.
2. Chemical structure differences: Various additives have significant molecular characteristics
1. Vulcanizing agents: sulfur-containing structure is the core
The most commonly used vulcanizer is elemental sulfur (S₈), which also includes insoluble sulfur (IS), organic polysulfides, peroxides, etc.
Molecular structure characteristics: mainly S-S or S-C bonds, with reactive sites;
Main representatives:
Elemental sulfur (S₈): the most traditional vulcanizing agent;
Insoluble sulfur (IS60): higher thermal stability, prevents frosting;
Organic polysulfides (such as TPvS): improve dynamic performance.
2. Accelerators: mainly nitrogen-containing organic compounds
Most accelerators are organic amine derivatives, with nitrogen and sulfur active groups, which can react with sulfur in an intermediate manner.
Common structural types:
Thiazoles (M, DM): thiazole rings in the molecules;
Sulfonamides (CBS, TBBS): improve scorch safety;
Thiurams (TMTD): disulfur structure, which can release sulfur by itself.
Chemically active groups:
Contains N-H, C=S, S-S bonds;
Possess the ability to capture free radicals and promote vulcanization activity.
3. Antioxidants: The structures of antioxidants and anti-ozonants are quite different
Antioxidants are divided into two categories: antioxidants and anti-ozonants, each with a unique structure:
Aromatic amine antioxidants (such as 6PPD, DTPD):
The main body is a diphenylamine structure, and the active hydrogen can react with free radicals;
It can react with ozone to form intermediates to improve ozone resistance;
It has strong dyeing properties and is suitable for black rubber.
Phenolic antioxidants (such as BHT, SP):
It has a phenol structure and can capture free radicals;
It has high stability and good thermal oxidation aging inhibition effect.
Paraffin antioxidant (physical type):
Long-chain alkane molecules can form a protective film to prevent ozone penetration;
It does not participate in chemical reactions and relies on barrier protection.
III. Reaction path differences: The chemical mechanisms are completely different
1. Vulcanizing agent: build cross-linking bonds
Sulfur (S₈) breaks into active sulfur atoms at high temperatures;
Reacts with unsaturated double bonds on the rubber molecular chain to form C–S–S–C type cross-linking bonds;
Achieve the transformation from linear molecules to three-dimensional network structures.
Simplified reaction formula:
Rubber-CH=CH₂+S→Rubber-CH–Sx–CH₂– (cross-linked structure)
2. Accelerator: Participate in intermediate reactions and improve vulcanization efficiency
Accelerators form reaction intermediates (such as active vulcanization intermediates) with sulfur and rubber;
Reduce the starting temperature of vulcanization, shorten the vulcanization time, and improve efficiency;
Not only affects the rate, but also affects the cross-linking density and structure type (single sulfur bond, double sulfur bond, trisulfide bond).
Core reaction path:
Accelerator + sulfur→active intermediate→react with rubber double bond→cross-linking bond generation
3. Anti-aging agent: terminate free radical chain reaction
Rubber aging is mostly caused by free radical reactions, and antioxidants inhibit them through the following mechanisms:
Capture free radicals (such as RO•, ROO•);
Inhibit oxidation chain reactions and stabilize molecular structures;
Absorb ozone and convert it into harmless structures (such as 6PPD);
Form a surface film layer (such as paraffin) to prevent external erosion.
Typical reaction:
R–O•+antioxidant (such as 6PPD) → stable product + prevent chain continuation
IV. Different usage and formulation roles
Additive category Amount added (phr) Addition stage Difficulty of formulation control
Vulcanizer 1–3 Medium to high in basic formula: related to crosslinking density
Accelerator 0.5–1.5 Medium to high in vulcanization system: need to match sulfur dosage
Antioxidant 1–3 Regular addition Medium: match with the use environment
Vulcanizer and accelerator must be used together to form the "core engine" of the vulcanization system;
Antioxidant is closely related to the use scenario, and different types are selected for high temperature, ozone protection, outdoor, static and other conditions;
Formulation engineers need to strike a balance between efficiency, stability and environmental protection.
5. Comparison of environmental and safety performance
Indicators Vulcanizing agent Accelerator Antioxidant
Safety High (such as IS)/Medium (traditional S) Medium/Low (some contain nitrosamines) Medium/Low (aromatic amines may change color)
Environmental protection Some products can meet environmental protection requirements Nitrosamine issues need to be noted Some varieties (such as 6PPD) are subject to strengthened environmental supervision
Substitutes Clean vulcanizing agent substitutes Green accelerators are gradually developed Phenolic antioxidants are more environmentally friendly, and aromatic amines are still irreplaceable
In recent years, international environmental regulations (such as REACH, CA Prop 65) have increasingly stringent supervision of nitrosamine-containing accelerators and aromatic amine antioxidants, and promoting the development of green and non-toxic additives has become the focus of the industry.
6. Technology and formula application prospects
Vulcanizing agent:
Extend to the direction of low sulfur and high efficiency and **non-sulfurized crosslinking (such as irradiation, peroxide);
Insoluble sulfur is still the mainstream, but its stability will be improved under environmental protection requirements
In terms of accelerators:
The research focus has shifted to green accelerator systems, such as thiazole/sulfonamide compound systems that do not contain nitrosamines;
AI-assisted formula selection and scorch-sulfurization balance prediction are taking off.
In terms of antioxidants:
Aromatic amines (such as 6PPD) are being studied in particular because of their oxidation byproducts (such as Quinone) in ozone reactions;
Green, migration-resistant, and low-pollution antioxidants are becoming a hot spot for innovation.
VII. Conclusion: Three types of additives are indispensable, each with a "chemical mission"
Accelerators, antioxidants, and vulcanizers are the "golden triangle" in rubber processing. Although they are all additives, they are completely different in chemical nature, mechanism of action, structural characteristics, and industrial functions.
Vulcanizer: "raw material provider" for cross-linking reactions;
Accelerator: "accelerator and regulator" of reactions;
Antioxidant: "protective shield" during service.
Only with reasonable matching and precise control can these additives play their maximum value in the formula and ensure the performance, life, and environmental friendliness of rubber products.
