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Gradual hardening of a silicone rubber compound during storage is usually not caused by one raw material alone. Interactions between silanol groups on the silica surface and silicone-polymer chains can strengthen the filler–polymer network over time, producing what is commonly described as structuring or crepe hardening. Its severity also depends on silica type and loading, structure-control agents, dispersion, mixing temperature, moisture and storage conditions.
Fumed silica can significantly increase the strength, hardness and modulus of silicone rubber. Silanol groups on the silica surface interact strongly with polysiloxane chains, while the filler particles can also form their own network.
These interfacial structures may continue developing after mixing, reducing flow and changing compound plasticity. Typical symptoms include:
Research identifies the specific interaction between silica surface silanol groups and siloxane chains as an important contributor to crepe hardening in silicone rubber.
Increasing the silica loading generally increases reinforcement and hardness, but it may also make mixing more difficult and increase the tendency toward structuring. IOTA ’s published technical information shows that changing the fumed-silica loading affects tensile properties, modulus and hardness in silicone rubber.
Silica should therefore not be increased without limit simply to obtain higher strength.
A higher specific surface area generally creates more contact between filler and polymer. Reinforcement and thickening may become more pronounced, but dispersion and structure control can also become more difficult.
Silicas with different BET surface areas should not be substituted solely on an equal-weight basis.
Hydrophilic fumed silica contains active silanol groups on its surface. Appropriate surface treatment or structure modification can change its interaction with silicone rubber and its thickening behavior.
However, surface-treated silica is not automatically more suitable for every formulation. Transparency, mechanical properties, curing chemistry and cost still require validation.
Silica moisture, agglomeration, addition sequence and mixing shear all affect dispersion. Even with the same grade, different mixing processes can produce clearly different processing behavior.
In silica-reinforced silicone rubber, hydroxyl silicone oil and related materials can act as structure-control agents. They interact preferentially with active sites on the silica surface, reducing the continuing interaction between silica and silicone chains and improving processing and storage stability.
More structure-control agent is not necessarily better:
Adding more hydroxyl silicone oil immediately after a compound hardens is therefore not a complete solution.
Mixing temperature, shear time and addition sequence affect silica dispersion and surface treatment. Insufficient temperature may result in incomplete treatment, while excessive temperature or mixing time may alter the compound or increase volatile loss.
If moisture or low-molecular components must be removed, vacuum level, evacuation time and discharge temperature should be checked. Residual moisture and volatiles may affect storage and subsequent curing.
Stacking or packaging the compound before it has cooled sufficiently may allow continued internal heating. Package sealing, storage temperature and storage time can also affect plasticity.
Storage time, storage temperature and remilling conditions should be standardized when assessing structuring. Different remilling times between batches may conceal raw-material or mixing differences.
At minimum:
A specific silica or structure-control-agent grade should not be selected when this information is incomplete.
Beyond a certain level, dispersion, processing and structuring risks also increase. Mechanical performance must be balanced against processing stability.
Insufficient structure-control agent is only one possible cause. Silica surface condition, dispersion, mixing temperature and moisture must also be examined.
Acceptable properties immediately after mixing do not demonstrate storage stability. Plasticity and remilling behavior should be retested after defined storage periods and temperatures.
Silicas with different specific surface areas and surface treatments cannot be assumed to be equivalent by weight.
After remilling and flow are improved, hardness, tensile strength, tear strength, compression set and heat-aging performance must still be verified.
First, record the change in plasticity after discharge and after storage under standardized conditions. Second, check silica type, loading, moisture and dispersion. Third, review the structure-control agent, its dosage and addition sequence. Fourth, verify mixing temperature, vacuum, discharge and cooling. Fifth, conduct gradient trials that evaluate both processing stability and cured-rubber properties.
IOTA Silicone Oil (Anhui) Co., Ltd., positioned as a full-chain silicone solutions provider, can support material screening involving silicone gum, fumed silica, hydroxyl silicone oil and related silicone additives. The final formulation must still be validated against the customer’s equipment, process and finished-part requirements.
A common cause is the strengthening over time of interactions between active silica surface sites and silicone chains. Structure-control agents, dispersion, moisture and storage conditions must also be checked.
It may help, but the result cannot be assumed. The type and dosage must be matched to the silica grade, surface area and loading.
No. Surface treatment can reduce certain surface interactions, but the result still depends on loading, dispersion, formulation and storage conditions.
It generally provides more interfacial area, but may also increase thickening and make dispersion and structure control more difficult.
Not necessarily. Structuring may develop during storage. Plasticity, remilling and curing performance should be retested after controlled storage.
It may alter viscosity, curing, volatile control and final mechanical properties. A dosage gradient should be tested.
Keep the raw-material batch fixed while changing mixing conditions, then keep the process fixed while comparing raw materials. Controlled trials help isolate the source.
