Comprehensive Analysis of Silicone Rubber: Types, Formulations, Manufacturing Processes and Full-Scale Applications

I. Classification of Silicone Rubber

• Heat-vulcanized silicone rubber (HTV): With a molecular weight of approximately 400,000 to 800,000, it requires cross-linking and curing with peroxides at high temperatures of 110~180°C. It is mainly used to manufacture molded and extruded products. China's annual output of HTV is about 500,000 tons.
• Room-temperature-vulcanized silicone rubber (RTV): With a lower molecular weight (10,000 to 100,000), it can be cross-linked and cured at room temperature, making it convenient to use and suitable for on-site construction. It is further divided into one-component RTV (cured by air moisture) and two-component RTV (cured by mixing base rubber and curing agent). Currently, China's annual output of RTV is about 1.2 million tons, accounting for more than 70% of the total silicone rubber output.

II. Core Properties of Silicone Rubber

1. Ultra-wide Temperature Range Adaptability: It has the widest operating temperature range among all rubbers, reaching -100~350°C. Vinyl silicone rubber or low-phenyl silicone rubber can still maintain elasticity after thousands of hours of hot air aging at 250°C or hundreds of hours at 300°C; low-phenyl silicone rubber vulcanizate can still be elastic after dozens of hours of aging at 350°C, with a glass transition temperature as low as -140°C. It can be used for instantaneous high-temperature thermal protection coatings on the inner walls of rocket nozzles.
2. Excellent Aging Resistance: After several years of outdoor exposure in a free state, its properties show no significant changes. Its ozone resistance, oxygen resistance, light resistance and weather aging resistance are far superior to ordinary organic rubbers.
3. Outstanding Electrical Insulation Properties: Its electrical insulation properties change very little when exposed to moisture, frequency changes or temperature increases. The silicon dioxide generated after combustion is still an insulator. Its corona resistance life is 1000 times that of polytetrafluoroethylene, and its arc resistance life is 20 times that of fluororubber, making it extremely reliable for high-voltage applications.
4. Special Surface Properties and Physiological Inertness: It has low surface energy, with a water absorption rate of only about 1% after long-term immersion in water, and its physical properties do not decrease. It has good mildew resistance and does not adhere to most materials. It is tasteless, non-toxic, has no adverse effects on the human body, and causes minimal reaction with body tissues.
5. High Gas Permeability: At room temperature, its permeability to nitrogen, oxygen and air is 30~40 times higher than that of natural rubber. It also has selective gas permeability, with a carbon dioxide permeability about 5 times that of oxygen.
6. Excellent Biomedical Properties: Its molecular structure characteristics give it good biocompatibility and controllable degradability, which have been verified by a large number of animal and human clinical trials.

III. Formulation System of Heat-Vulcanized Silicone Rubber (HTV)

1. Three Principles of Formulation Design

• Silicone rubber is a highly saturated raw rubber, usually cannot be vulcanized with sulfur, and must use organic peroxides as vulcanizing agents. Therefore, the rubber compound must not contain active substances that can react with peroxide decomposition products (such as channel black, certain organic accelerators, etc.).
• Silicone rubber products are mostly used at high temperatures, so the compounding agents must have high-temperature stability. Inorganic oxides are usually selected as reinforcing agents.
• Silicone rubber is prone to cleavage and rearrangement of siloxane bonds under the action of trace amounts of acid or alkali, leading to a decrease in heat resistance. Therefore, the acidity and alkalinity of the compounding agents must be strictly controlled.

2. Vulcanization System

• General-purpose peroxides: With high activity, they can vulcanize various silicone rubbers, such as benzoyl peroxide (BPO) and dicumyl peroxide (DCP).
• Vinyl-specific peroxides: With lower activity, they can only vulcanize vinyl-containing silicone rubbers, such as 2,5-dimethyl-2,5-di-tert-butylperoxyhexane (DBPH), which gives vulcanizates better heat resistance.

3. Reinforcement System

• Strong reinforcing fillers: With a particle size of 10~50 nm and a specific surface area of 70~400 m²/g, they are mainly synthetic silicon dioxide (silica). Among them, fumed silica has the best reinforcing effect, producing rubber compounds with high mechanical strength and good electrical properties; precipitated silica has slightly lower reinforcing effect but better heat aging resistance and lower cost.
• Weak reinforcing fillers: With a particle size of 300~1000 nm and a specific surface area below 30 m²/g, such as diatomaceous earth, quartz powder, titanium oxide, etc. They are mainly used to reduce costs and adjust the hardness of the rubber compound.

4. Functional Additives

• Structure control agents: Prevent the "structuring" (hardening, decrease in plasticity) of rubber compounds reinforced with fumed silica during storage. Commonly used ones include diphenylsilanediol, low-molecular-weight hydroxyl silicone oil, silazanes, etc.
• Heat-resistant additives: Significantly improve the hot air aging performance of silicone rubber. The most commonly used is ferric oxide (dosage 3~5 parts), and metal oxides such as manganese, zinc and nickel also have similar effects.
• Colorants: Inorganic colorants are commonly used for silicone rubber, such as iron oxide (red), titanium dioxide (white), chromium trioxide (green), carbon black (black), ultramarine (blue), etc.

IV. Formulation System of Room-Temperature-Vulcanized Silicone Rubber (RTV)

1. One-Component RTV Silicone Rubber

• Curing mechanism: Relies on moisture in the air to initiate cross-linking reactions, curing gradually from the surface to the interior. At room temperature, it can cure to a depth of 2~4 mm in 24 hours, and complete curing takes more than 7 days.
• Typical formulation: 107 room-temperature-vulcanized silicone rubber raw rubber (base material) + cross-linking agent (such as methyltriethoxysilane) + catalyst (such as organotin compounds) + reinforcing fillers (fumed silica, calcium carbonate) + coupling agent (to improve adhesion).
• Characteristics: Easy to use, no need for metering and mixing, suitable for on-site sealing and bonding, but has a slow curing speed and difficulty in deep curing.

2. Two-Component RTV Silicone Rubber

• Curing mechanism: Divided into condensation type and addition type. Condensation type cross-links through dealcoholization and dehydration reactions between the base rubber and cross-linking agent under the action of a catalyst; addition type cross-links through an addition reaction between vinyl silicone rubber and hydrogen-containing silicone oil under the action of a platinum catalyst.
• Typical formulation: Component A (base rubber + reinforcing fillers + additives), Component B (curing agent + catalyst). Mix evenly in proportion before use.
• Characteristics: Fast curing speed, capable of deep curing, low shrinkage, good dimensional stability, suitable for potting, mold making and other scenarios. Addition-type two-component RTV has no by-products and better environmental protection.

V. Manufacturing Process Flow of Silicone Rubber

1. Manufacturing Process of Heat-Vulcanized Silicone Rubber (HTV)

1. Raw rubber synthesis: Using dimethyldichlorosilane as the main raw material, cyclic monomers are prepared through hydrolysis, cracking and purification, and then polymerized under the action of a catalyst to obtain high-molecular-weight polydimethylsiloxane raw rubber.
2. Mixing: Add the raw rubber to an internal mixer, then add structure control agents, reinforcing agents and functional additives in sequence. Mix evenly and filter to remove impurities.
3. Preforming: According to the shape of the product, extrude or mold the rubber compound into blanks.
4. Vulcanization: Divided into two stages. The first stage vulcanization is carried out at 110~180°C to initially shape the rubber compound; the second stage vulcanization is kept at 200~250°C for 2~4 hours to remove residual peroxide decomposition products and improve product performance.
5. Post-treatment: Trim, inspect and package the vulcanized products.

2. Manufacturing Process of Room-Temperature-Vulcanized Silicone Rubber (RTV)

1. Base material preparation: Add 107 raw rubber, reinforcing fillers and dispersants to a planetary mixer, disperse at high speed under vacuum, grind and filter to obtain the base material.
2. Packaging:
   • One-component RTV: Mix the base material with cross-linking agent, catalyst and coupling agent evenly under vacuum, and directly package into sealed rubber tubes.
   • Two-component RTV: Package the base material as Component A; mix the curing agent, catalyst and appropriate fillers evenly, and package as Component B.
3. Curing: When in use, one-component RTV is directly extruded for construction and cures automatically after contacting air; two-component RTV is mixed evenly in proportion, degassed, then poured or coated, and can be cured at room temperature in tens of minutes to several hours.

VI. Full-Scale Applications of Silicone Rubber

1. Automotive Industry

2. Electronics and Electrical Industry

3. Construction Engineering

4. Medical and Health Care

5. Aerospace and National Defense

6. Daily Necessities and Home Appliances

Summary