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Hardening, softening or cracking after high-temperature exposure does not necessarily mean that the silicone gum was selected incorrectly. The first step is to distinguish an oxygen-rich hot-air environment from a sealed or oxygen-limited system. Operating temperature, silica type, vinyl content, structure-control agent, heat stabilizer, moisture and post-curing must then be evaluated together.
During prolonged exposure to hot air, methyl side groups may undergo thermo-oxidative reactions accompanied by additional crosslinking and chain scission.
If additional crosslinking dominates, the typical results are increased hardness, reduced elongation, lower resilience and eventual embrittlement or cracking.
Inside O-rings, encapsulated components or cables, oxygen availability may be limited. Backbone rearrangement, chain scission or depolymerization involving terminal hydroxyl groups may then become more important.
If degradation dominates, the rubber may soften, lose strength, become tacky or lose sealing capability. Moisture, hydroxyl groups and catalytic impurities may influence these reactions.
The two environments are not absolute opposites. Small amounts of oxygen, permeating media and temperature gradients may coexist, so the mechanism should be confirmed by testing.
Confirm at least:
Fumed silica generally provides a high specific surface area and strong reinforcement. For sealed systems sensitive to moisture, low-moisture and appropriately surface-treated grades may be evaluated first.
Precipitated silica differs in moisture, surface hydroxyl content, cost and processing behavior. If moisture and venting are not controlled, some extrusion or high-temperature processes may have a higher risk of bubbles.
Neither silica type is automatically superior in every heat-aging condition. The specific grade, gum, curing system and service environment must be tested together.
Vinyl groups affect curing rate and crosslink density. Excessive vinyl content or an unsuitable curing-agent dosage may produce an overly dense network, faster hardness increase and reduced elongation after aging. Insufficient vinyl may lead to incomplete curing or unstable initial properties.
Some methyl-vinyl silicone rubber formulations screen vinyl contents around 0.1–0.2%, but this is not a universal specification. The gum grade, cure curve and aged-property retention must determine the final range.
Possible directions include iron oxide, titanium dioxide, cerium oxide, zirconium-containing oxides, cerium-zirconium mixed oxides and formulated heat-stabilizer packages.
In some formulation studies, approximately 0.5–4 parts per 100 parts of gum may be used as an initial experimental gradient. This must not be treated as a universal recommended dosage. Color, curing behavior, mechanical properties and long-term stability must also be evaluated.
Research references: cerium-ferric mixed oxide and comparison of Fe₂O₃, CeO₂ and CeZrO₂.
Hydroxyl silicone fluids may be used as structure-control agents and must be balanced with the silica system.
Too little may allow the compound to structure and harden during storage. Too much may affect curing, volatile control and heat-aging performance.
Silica moisture, residual catalyst, compound pH, mixer vacuum, devolatilization time and discharge temperature should also be controlled, particularly for sealed high-temperature applications.
Some peroxide-cured silicone rubbers use post-curing around 180–200°C for several hours to remove residual volatiles and stabilize properties.
Higher temperature is not automatically better. Excessive temperature or time may cause over-crosslinking, hardening, loss of elongation or degradation. A condition such as 250°C for three to five hours should not be presented as a general recommendation without product-specific TDS and test support.
Reference: WACKER post-curing guidance.
Compare:
Measure hardness, tensile strength, elongation, mass change, compression set, cracking and color before and after aging.
Applicable references include ISO 188:2023, GB/T 3512—2014, ASTM D573 and GB/T 7759.1—2015.
As a silicone value-chain solutions provider, IOTA Silicone Oil (Anhui) Co., Ltd. can support material screening involving silicone gums, phenyl silicone rubber, silica, structure-control agents and silicone additives. Final formulations and processing conditions must be verified in the actual component.
No. Thermo-oxidation, additional crosslinking, vinyl content, silica, moisture and post-curing can all contribute.
No. Chain scission or depolymerization may instead cause softening and strength loss.
No. The answer depends on the specific grade, moisture, surface chemistry, formulation and service environment.
No. Vinyl content must match the curing system and required crosslink density.
No. Excessive addition may affect color, curing and mechanical properties.
No. Excessive temperature or time can cause hardening, over-crosslinking or degradation.
