Silicone rubber products that harden, soften, or crack after high-temperature use represent a failure mode frequently encountered by technical personnel. Troubleshooting such issues cannot simply be attributed to "choosing the wrong raw gum"—it is first necessary to distinguish the type of environment the product is exposed to.

Core Principle: Identify the Environment First, Then Investigate the Formulation

During thermal aging of silicone rubber, further crosslinking, oxidation, molecular chain scission, rearrangement, depolymerization, and loss of volatile components may occur simultaneously. Which reaction predominates depends on oxygen, moisture, temperature, end groups, fillers, and formulation. The first step is to distinguish whether the product is in an aerobic environment such as hot air, or in a sealed, low-oxygen environment.

Aging Characteristics in Aerobic/Hot Air Environments

In environments with continuous exposure to hot air, methyl side groups may participate in thermo-oxidative reactions, generating free radicals, accompanied by further crosslinking and chain scission. When further crosslinking predominates, common manifestations include increased hardness, decreased elongation at break, reduced resilience, and ultimately embrittlement or cracking. Therefore, "hardening and cracking" in hot air typically requires focused investigation of thermo-oxidative aging, crosslink density, and heat-resistant stabilization systems.

Aging Characteristics in Sealed/Low-Oxygen Environments

In sealed or low-oxygen environments such as O-rings, encapsulated parts, and cable interiors, thermo-oxidative reactions are limited, and siloxane backbone rearrangement, chain scission, or depolymerization involving terminal hydroxyl groups may become more significant. When degradation or depolymerization predominates, products may exhibit decreased hardness, material softening, reduced tensile strength, surface tackiness or loss of load-bearing capacity, and deteriorated sealing performance. Therefore, failure in sealed environments does not necessarily manifest as hardening, nor can improvement measures for hot air aging be directly applied.

Operating Conditions to Confirm Before Material Selection

Before making formulation adjustments or changing raw gums, it is recommended to confirm the following operating condition information: long-term working temperature and short-term peak values; whether the product is in hot air, aerobic, sealed, or low-oxygen environments; whether thermal cycling exists; whether it contacts oil, fuel, steam, or chemical media; the stress state of the product (free, tensile, compressive, or dynamic flexing); the specific manifestation of failure; the current raw gum type, vinyl content, silica, and heat-resistant additives; and structure control agents, vulcanization systems, and post-curing conditions.

Influence of Silica Type

Fumed silica typically has a high specific surface area and strong reinforcing capability, with significant differences among grades in specific surface area, surface hydroxyl groups, moisture content, and surface treatment status. For sealed or moisture-sensitive high-temperature systems, priority can be given to evaluating low-moisture fumed silica with appropriate surface treatment, though specific product technical data sheets should still be verified. Precipitated silica differs from fumed silica products in moisture content, surface hydroxyl groups, and processing performance. If moisture and devolatilization are not properly controlled, the risk of bubbles and performance fluctuations may increase during extrusion or high-temperature processing.

Directions for Heat-Resistant Additives

Iron oxide, titanium dioxide, cerium oxide, zirconium-based or cerium-zirconium composite oxides, and composite heat-resistant stabilizers can be evaluated. Some formulations may use approximately 0.5–4 parts per 100 parts of raw gum as a gradient test range, but this cannot be regarded as a uniform recommendation for all additives—color, vulcanization, mechanical properties, and long-term stability must also be evaluated.

Summary

Troubleshooting high-temperature aging issues in silicone rubber should follow the diagnostic logic of "identify the environment first, then investigate the formulation." The aging mechanisms in aerobic and sealed environments differ, and their solutions cannot be interchanged. When operating conditions are incomplete, it is not appropriate to directly specify a raw gum or compounded rubber grade.

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