Silica Gel Heat Resistants | Professional Guide: The Leap from 200℃ to 300℃ – A Comprehensive Explanation of How to Choose, Use, and Avoid Pitfalls with Silica Gel Heat Resistants

Ordinary silicone rubber has a long-term service temperature of 200℃. At 250℃, it begins to harden, crack, and its mechanical properties drop drastically. In applications such as new energy vehicle battery packs, aerospace seals, and industrial hot stamping rollers, temperatures frequently exceed 250℃, which ordinary silicone rubber simply cannot withstand.

Heat resistant agents are the key to solving this problem. However, choosing the wrong type can lead to issues such as insufficient temperature resistance, unusable products turning red or brown, or even hindering vulcanization despite the addition of the agent. This article clarifies the types, principles, parameters, process matching, and common pitfalls.

01 What are Silicone Heat Resistants? Silicone heat resistant agents are functional additives added to silicone rubber to improve the material's heat resistance and thermo-oxidative stability. Their core objective is to increase the long-term service temperature from 200℃ to 250-300℃ while simultaneously delaying the degradation of mechanical properties at high temperatures.

In a high-temperature, oxygen-rich environment, silicone undergoes two main processes: First, the side groups (methyl groups) are oxidized to form alkyl free radicals. Free radical coupling leads to increased cross-linking density, causing the product to gradually harden and even crack (commonly known as hardening). Second, the main chain undergoes cyclization degradation, generating low-molecular-weight cyclic siloxanes such as D3 and D4, resulting in a decrease in molecular weight and loss of mechanical properties. The role of heat-resistant agents is to slow down these two processes by capturing free radicals, inhibiting oxidation chain reactions, and protecting the silicone main chain.

The technical routes fall into two main directions: Inorganic metal oxides-utilizing redox reactions to absorb free radicals generated by oxidation and regenerating them under the action of oxygen, thus cyclically fulfilling their function; and Organosilicon/organic-forming a protective layer around the silicone main chain through molecular structure design, or directly capturing free radicals. Each route has its applicable scenarios, which will be compared in detail later.

02 Comparison of Four Types

Silicone heat-resistant agents on the market are mainly divided into four categories, with significant differences in principle, effect, and cost:

① Metal oxides-the most classic and widely used type. Iron oxide red (α-Fe₂O₃) is the most commonly used. Adding 2-5 parts per 100 parts of the compound can increase the long-term temperature resistance from 200℃ to 250℃, and reduce the elongation at break at 250℃ by 40%. It is low in cost but will turn the product red/brown. Cerium dioxide (CeO₂) is more effective than iron oxide, with less impact on color (pale yellow), and can be added at 0.5-4 parts. Cobalt oxide, manganese oxide, zinc oxide, and titanium dioxide can also be used. Among them, the TiO₂+Fe₂O₃ binary system (≥0.1% each) can inhibit the formation of formaldehyde and cyclosiloxanes (D4/D5/D6), reducing VOC emissions by 60%-80%.

② Rare earth compounds – high-end heat-resistant solutions. Lanthanide stearates or acetylacetone complexes, and cerium hydroxide (Ce(OH)₃) masterbatch (added 5%–10%), have a TGA initial degradation temperature of approximately 400℃, allowing silicone to remain stable at 300℃. Representative products include lanthanide rare earth heat resistant agents, which offer significant heat resistance but are more expensive, used in aerospace, high-end sealing, and other applications requiring extremely high temperature resistance.

③ Organosilicon heat resistant agents-the first choice for transparent/light-colored products. Hexaphenylcyclotrisilazane (HPCS), added at 0.5%–3.0%, has a TGA initial degradation temperature >450℃, making it one of the highest heat-resistant varieties currently available, without affecting transparency. There are also organic heat-resistant additives (liquid transparent type) containing transition metal ions and thioether bonds, which effectively capture oxidative degradation free radicals, significantly improving thermo-oxidative stability at 280℃ while maintaining mechanical properties and processability. Organosilicon-modified phenolic antioxidants (containing benzotriazole or hindered amine structure siloxane grafts) also belong to this category. Advantages include transparency, no impact on color, and good dispersibility; disadvantages include high cost.

④ Nanocomposite type – a rapidly developing trend in recent years. Nano-iron oxide, nano-cerium oxide, nano-zirconium oxide, etc., have large specific surface areas and high activity. A low addition amount (usually 1-3 parts) can achieve the effect of more than 5 parts of traditional oxides, with less impact on color and transparency. There is also a zirconium hydroxide + yttrium oxide + hexaphenylcyclotrisilazane composite system, which achieves higher temperature resistance through multi-component synergy. Composite oxide heat resistant agents are also a hot patent area.

In short: For low cost, choose iron oxide red (red is acceptable); for temperatures above 280℃, choose rare earth or organosilicon types; for transparency, choose organosilicon types; for low addition amounts, choose nanocomposite types.

03 Matching with the vulcanization system Incompatibility between the heat resistant agent and the vulcanization system can result in reduced temperature resistance or, in severe cases, no vulcanization at all.

Peroxide system (di(2,4-dioxane/di(2,5-dioxane)): Best compatibility, the mainstream choice for heat-resistant silicone rubber. Peroxide-induced C-C crosslinking (Si-CH₂-CH₂-Si) exhibits superior thermal stability compared to addition-cured crosslinking, making it more suitable for heat-resistant applications. Bis-2,5-(2,5-dimethyl-2,5-di-tert-butylperoxyhexane) has a high decomposition temperature and is less prone to scorching, making it suitable for thick products; DCBP (bis-2,5-dimethyl-2,5-di-tert-butylperoxyhexane) is suitable for rapid vulcanization of thin products. When combined with TAIC crosslinking aids (at 1.5–2 times the amount of peroxide), the crosslinking density can be increased, reducing the compression set from 28% to 16% at 200℃ for 70 hours.

Platinum systems (addition-type): Special attention must be paid to the possibility that metal ions and sulfur/phosphorus/nitrogen components in the heat resistant agent may poison the platinum catalyst, leading to incomplete vulcanization. When using acetylene inhibitors, it is essential to ensure that the heat resistant agent does not interfere with the inhibition threshold. Organosilicon heat resistant agents such as HPCS have good compatibility with platinum systems and are the preferred choice for addition-type liquid silicone. Metal oxides require prior small-scale testing to confirm compatibility.

Double vulcanization system (peroxide + KH-CL): Used for phenyl silicone rubber, achieving complete vulcanization with both high strength and good high-temperature resistance, suitable for extreme working conditions above 300℃.

The first step in selection is always to confirm the vulcanization system, then select the type of heat resistant agent; the order cannot be reversed.

04 Three Addition Processes
**Open Mill Mixing:** After dispersing the silicone rubber in an open mill, add the heat resistant agent (which can be added simultaneously with the color masterbatch and vulcanizing agent), and repeatedly roll until uniform. This is the most common method, suitable for solid rubber compounds. Metal oxide heat resistant agents need to be fully dispersed to avoid local agglomeration leading to uneven temperature resistance. Pay attention to mixing temperature control, as some organic heat resistant agents are heat-sensitive.

**Kneader Mixing:** Solid rubber compounds are added to a kneader along with raw rubber, silica, and a structure control agent. After mixing evenly, let stand for 8–36 hours (structure control), then add the heat resistant agent and mix evenly. Suitable for mass production, with better dispersion than an open mill. Many patented formulations of composite oxide heat resistant agents use this process. Liquid silicone rubber (LSR): Add directly to the A/B adhesive and stir until homogeneous. Organic heat resistant agents (liquid type) are best suited for LSR, with an addition amount of 0.5%–3%. This is the simplest process, but care must be taken to degas after stirring to avoid bubbles affecting the density of the product. Metal oxide powder type requires a dispersion masterbatch before addition, otherwise it is prone to sedimentation.

Key Reminder: Heat-resistant silicone rubber must undergo secondary curing (usually 200℃ × 4h) to remove low-molecular-weight substances and fully activate the heat resistant agent to achieve optimal heat resistance. Products without secondary curing will have significantly reduced temperature resistance. Additionally, avoid using metal impurities such as copper and manganese, which promote thermal oxidation and will accelerate the thermal aging of silicone rubber.

05 Key Parameters: When choosing a heat resistant agent, pay close attention to these five key indicators:

① Long-term heat resistance temperature – a core indicator. Ordinary silicone rubber can withstand temperatures up to 200℃. Adding 2-5 parts of iron oxide red can reach 250℃; iron oxide + cerium compound can reach 250-280℃; rare earth/organosilicon heat resistant agents can reach 280-300℃; and phenyl silicone rubber + heat resistant agent can reach 300-350℃. Testing is usually performed according to the hot air aging test (GB/T 3512), measuring the retention rate of mechanical properties after 250℃×72h or 225℃×500h.

② Retention rate of mechanical properties after heat aging-more practically significant than simply measuring temperature resistance. High-quality heat resistant agents should retain ≥60% of tensile strength and ≥50% of elongation at break after 250℃×72h. Ordinary iron oxide red retains approximately 49.4% of tensile strength and less than 80% of elongation at break after 225℃×500h.

③ TGA initial degradation temperature-a thermogravimetric analysis index reflecting the temperature at which the material begins to decompose. HPCS > 450℃, iron oxide-doped TiO₂ approximately 420℃, cerium hydroxide masterbatch approximately 400℃, standard polysiloxane fluid approximately 380℃. The higher the TGA temperature, the greater the heat resistance potential.

④ Dosage – Directly affects cost and physical properties. Iron oxide red 2-5 parts, cerium dioxide 0.5-4 parts, HPCS 0.5%-3%, cerium hydroxide masterbatch 5%-10%, iron oxide-doped TiO₂ 2%-5%, nanocomposite type 1-3 parts. Too low a dosage results in insufficient heat resistance, while too high a dosage affects mechanical properties, increases cost, and may lead to dispersion difficulties.

⑤ Color Influence – Often overlooked but crucial. Iron oxide red turns the product red/brown, cerium dioxide is pale yellow, cobalt oxide is grayish-black, and organosilicon and nanocomposite types have virtually no effect on color. Transparent or light-colored products can only be selected from organosilicon types or surface-treated nanocomposite types.

06 Application Landscape

New Energy Vehicles (41.2%) – Battery pack seals, high-voltage cable insulation, thermal management system seals. In 2025, the consumption of heat-resistant agents used in new energy vehicle battery pack seals reached 1980 tons, a year-on-year increase of 28.9%, making it the largest downstream market. Requirements include long-term temperature resistance of 200-250℃, while also meeting flame retardant and low VOC requirements.

Photovoltaics (28.5%) – Photovoltaic frame adhesives, junction box potting compounds. Outdoor exposure to long-term sunlight and large temperature differences requires both heat resistance and weather resistance.

Consumer Electronics (15.7%) – Smartphone screen bonding and internal component protection (approximately 0.005 kg per phone), home appliance compressor seals and circuit board protection (approximately 2500 tons in 2024). Requirements include light-colored or transparent materials, primarily silicone-based heat-resistant agents.

Industrial Sensors (9.3%) – High-temperature sensor seals, requiring long-term stability.

Medical-grade silicone (5.3%) – Requires FDA/ISO 10993 certification and high purity of heat-resistant agents.

Aerospace – Aircraft and rocket seals, spacecraft door sealing systems; special heat-resistant agents used: 312 tons, up 22.4% year-on-year; requires extreme temperature resistance above 300℃; mainly rare earth and organosilicon compounds.

Other traditional fields – Industrial hot stamping parts (hot stamping plates, hot stamping rollers, hot stamping wheels), low-melting-point alloy molds (lead, tin, zinc alloy jewelry molds), heat-resistant baking pans/silicone kitchenware, wires and cables, hoses, rubber rollers, laminator silicone sheets, silicone insulators, thermally conductive silicone sheets, sealing rings, etc.

07 Common Problems and Troubleshooting Guide
Six Most Common Workshop Problems and Troubleshooting Directions:

Problem

Cause

Solution

Insufficient Temperature Resistance/Hardening and Cracking After High Temperature

Insufficient addition, uneven dispersion, lack of secondary vulcanization, incorrect heat resistant agent type

Increase addition amount; extend mixing time to ensure dispersion; secondary vulcanization at 200℃×4h is mandatory; replace with rare earth or organosilicon agents above 250℃

Product Turns Red/Brown/Color Mismatch

Used coloring heat resistant agents such as iron oxide red or cobalt oxide

Replace with cerium dioxide (pale yellow) or organosilicon agents (transparent/colorless); prioritize organosilicon heat resistant agents for light-colored products

Decreased Transparency/Hazy

Mismatched refractive index between metal oxide filler and silica gel, poor dispersion

Replace with organosilicon Silicon-based heat resistant agents (HPCS or liquid organic type); nano-type requires surface treatment before use.

Incomplete vulcanization/non-vulcanization

Metal ions or sulfur-, phosphorus-, and nitrogen-containing components in the heat resistant agent poison the platinum catalyst.

Replace the platinum system with an organosilicon-based heat resistant agent; for peroxide systems, confirm that the heat resistant agent does not consume peroxide; conduct small-scale testing beforehand.

Decreased mechanical properties/low tensile strength

Excessive heat resistant agent addition (>10 parts), filler agglomeration.

Reduce the addition amount; switch to nano-composite type (low addition amount, high efficiency); optimize the dispersion process (three-roll mill/high shear).

Large compression set

Insufficient crosslinking density, heat resistant agent affects crosslinking.

Add TAIC crosslinking aid to the peroxide system (1.5-2 times the peroxide); optimize the secondary vulcanization process.

08 Selection Points: Six-Question Decision Framework
When selecting a heat resistant agent, first ask six questions:

① What is the target temperature resistance? ① **For temperatures between 200-250℃:** Choose iron oxide red (highest cost-effectiveness); for 250-280℃, choose iron oxide + cerium compound or nano-composite type; for 280-300℃, choose rare earth compounds or organosilicon compounds (HPCS); above 300℃, a combination of phenyl silicone rubber and heat resistant agent is required.

② **Color and transparency requirements:** For red/dark products, iron oxide red is the most economical; for light/semi-transparent products, choose cerium dioxide or nano-type; for completely transparent products, only organosilicon compounds (HPCS or liquid organic heat resistant agents) are suitable.

③ **Long-term or short-term use:** For long-term continuous use (thousands of hours), high retention of mechanical properties after heat aging is required, so rare earth or organosilicon compounds are recommended; for short-term intermittent use, iron oxide red is sufficient.

④ **What vulcanization system:** Peroxide systems have the widest compatibility, and most heat resistant agents can be used; platinum systems must avoid sulfur, phosphorus, nitrogen, and metal ions that interfere with platinum, and organosilicon compounds are preferred; double vulcanization systems are used for the extreme temperature resistance of phenyl silicone rubber.

⑤ **What process:** All types of rubber compounds can be mixed using open mills/kneaders; for LSRs, liquid organic heat-resistant agents or pre-dispersed masterbatches are preferred; note that secondary vulcanization is necessary to activate the heat resistance effect.

⑥ Are there any certification requirements? Medical applications require FDA/ISO 10993 biocompatibility; food contact applications require LFGB/FDA; electronic applications require RoHS/REACH compliance and low VOC (selecting the TiO₂+Fe₂O₃ system can reduce VOC by 60%–80%); automotive applications require compliance with OEM standards.

Once these six questions are clarified, the selection direction will be determined. For the remaining adjustments to the dosage and process optimization, the Yaneng technical team can assist with sample testing.