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News > Dynamic industry > Selecting thermally conductive silicone rubber and thermal grease

Selecting thermally conductive silicone rubber and thermal grease

High thermal conductivity does not necessarily mean low thermal resistance.

Recent industry reports continue to identify traction batteries, energy storage systems and high-density data centers as important demand areas for thermal interface materials. For businesses, customers usually care about more than the single thermal conductivity value in promotional literature: can the material maintain stable thermal management despite actual assembly gaps, compression, pump-out, volatilization, aging and batch variation? This article explains selection through operating conditions, material systems and validation methods.

1 Determine whether the product needs heat transfer alone or heat transfer and sealing

Thermal grease, thermal gel, thermal pads, thermally conductive silicone rubber and thermally conductive potting materials can all transfer heat, but have different structural functions. First establish whether the material must also accommodate assembly tolerances, maintain compression recovery, block moisture, secure components or provide electrical insulation.

Material form	Main role	Priority criteria	Common risks
Thermal grease	Fill microscopic gaps and reduce interfacial contact thermal resistance	Initial thermal resistance, pump-out, volatilization, application properties	Long-term drying, migration, pump-out
Thermal gel	Combine compliance, gap filling and reworkability	Curing/non-curing state, compression, recovery, rheology	Excessive flow, edge squeeze-out
Thermal pad	Heat transfer at a defined thickness and convenient assembly	Thickness tolerance, compressive stress, thermal conductivity, insulation	Poor contact, insufficient compression
Thermally conductive silicone rubber/potting material	Heat transfer, fixation, sealing and environmental protection	Thermal conductivity, hardness, shrinkage, adhesion, aging resistance	Stress, cracking, difficult rework

2 Why high thermal conductivity may not mean low system thermal resistance

Thermal conductivity generally describes the bulk material under specified test conditions. System cooling also depends on complete interface filling, void formation, contact retention under changing pressure and temperature, and the overall thermal path between the heat source, heat sink and enclosure.

1. An overly hard material may fail to fill microscopic interface gaps, increasing actual contact thermal resistance.

2. An overly soft or mobile material may develop pump-out, edge squeeze-out or local drying over time.

3. Excessive filler loading may raise thermal conductivity while worsening viscosity, application properties and reliability.

4. Laboratory results cannot replace system validation when specimen thickness, pressure or temperature differs from actual assembly conditions.

5. Electrical insulation, flame retardancy, low volatility and long-term heat resistance often need to be achieved alongside the thermal target.

3 Operating parameters to confirm before selection

Operating factor	Information to confirm	Effect on selection
Assembly gap	Minimum, maximum and tolerance distribution	Determines thickness, flow and accommodation of compression
Operating temperature	Ambient conditions, peak temperature, thermal cycling	Affects viscosity, pump-out, volatilization and aging
Mechanical conditions	Compression force, vibration, impact, displacement	Affects recovery, pump-out, cracking and interface retention
Electrical requirements	Volume resistivity, breakdown and creepage distance	Determines whether an electrically insulating thermal system is needed
Application method	Dispensing, spreading, screen printing, automated coating or compression molding	Determines viscosity window, thixotropy and cure rate
Rework requirements	Whether disassembly, rework and recoating are permitted	Affects selection of curing or non-curing materials
Environmental and compliance requirements	VOC, volatile substances, flame retardancy and customer restricted substances	Affects fillers, additives, catalysts and formulation selection

4 How silica and silicone components affect processing

Thermally conductive systems usually contain a silicone matrix, thermally conductive fillers, reinforcing or rheology-modifying components, a crosslinking/catalyst system and any necessary surface-treatment materials. Silica may not be the main heat-conducting component, but can significantly affect viscosity, thixotropy, storage stability, sedimentation resistance and the application window.

For formulation screening, IOTA9001 fumed silica from IOTA may be considered for rheology control and reinforcement. See:

IOTA9001 fumed silica product page

This link is a formulation raw-material page; it does not mean that the raw material alone is a thermal interface material or a finished thermally conductive product.

When adjusting filler structure to improve dispersion, viscosity or interface stability, also evaluate surface treatment, matrix viscosity, filler loading and application method. Do not change just one filler while leaving the rest of the formulation unchanged.

5 Designing a validation program

1. Use the current qualified material as the reference and fix assembly thickness, pressure and curing or standing time.

2. Record thermal conductivity, interfacial thermal resistance, viscosity/rheology, volatiles, pump-out or sagging together.

3. For thermally conductive silicone rubber, add hardness, tensile strength, elongation, compression set and adhesion/sealing tests.

4. For thermal grease, add extrusion, dispensing, spreading and rework assessments, plus condition checks after thermal cycling.

5. Validate thermal cycling with the actual heat source, heat sink, enclosure and assembly pressure.

6. Track changes in batch, storage time, packaging and production equipment.

7. Any change in thermal filler, silica, silicone oil viscosity, catalyst or crosslinker should trigger a formulation-change assessment.

Validation level	Main purpose	Suggested output	Ready for scale-up
Material	Confirm thermal conductivity, rheology and basic reliability	TDS, COA, basic test results	No
Formulation	Confirm compatibility of filler, matrix and additives	Formulation window, application parameters, performance comparisons	No
Component	Confirm actual interfaces and thermal paths	Thermal resistance, temperature rise, cycling and appearance	Close, but further validation is still needed
Mass production	Confirm batch consistency and process stability	First-batch, consecutive-batch and change records	After approval conditions are met

6 Common mistakes

1. Ranking only by thermal conductivity without considering actual thermal resistance and assembly interfaces.

2. Comparing only a single laboratory test, without observing thermal cycling or long-term pump-out.

3. Changing only the thermal filler without readjusting the matrix, rheology and curing system.

4. Treating a raw-material page as a compliance or performance commitment for a finished thermal product.

5. Testing only material specimens, without validating production components and mass-production equipment.

6. Overlooking electrical insulation, flame retardancy, low volatility and rework requirements.

7 Procurement and supplier-audit recommendations

Audit item	Documents to obtain	Key assessment
Product identity	Complete grade, production site, TDS, SDS, COA	Do samples and production supplies come from the same source?
Formulation boundaries	Main components, restricted substances, change management	Do formulation changes require revalidation?
Test methods	Methods for conductivity, thermal resistance, volatilization, rheology and reliability	Are different suppliers' data comparable?
Delivery capability	Lead time, packaging, storage/transport and safety stock	Can continuous production be supported?
Technical support	Laboratory trials, pilot trials, failure analysis and on-site cooperation	Can issues be resolved promptly?

8 How IOTA can support thermal-material projects

IOTA Silicone Oil (Anhui) Co., Ltd. positions itself as a solutions provider across the silicone industry chain. Around silicone oils, silica, silicone rubber, silanes, catalysts and other silicone additives, it can help screen candidate directions based on matrix viscosity, rheology control, filler dispersion, interface stability and final application conditions.

Suitability of a specific grade for a thermal grease, thermally conductive silicone rubber or electronic-component project still requires confirmation against the customer's assembly gap, temperature, pressure, electrical requirements, application process and final validation results. Without complete operating-condition information, no single grade is directly promised to cover every application.

FAQ

Does higher thermal conductivity always mean a better material?

No. System thermal resistance also depends on interface filling, thickness, pressure, voids, pump-out and long-term reliability.

Can thermal grease and thermally conductive silicone rubber replace each other?

Generally, no direct substitution is possible. Their application methods, fixation, reworkability and long-term mechanical retention differ.

Is silica a thermally conductive filler?

Silica usually contributes more to reinforcement, rheology and structural control. Whether it serves as the main thermal filler depends on the formulation and cannot be generalized.

Why does changing silica affect dispensing?

Silica affects viscosity, thixotropy, structure development, dispersion and storage stability. The processing window usually needs reassessment after replacement.

Should thermal materials also be tested for electrical insulation?

In batteries, power supplies, power devices and other electrical components, insulation and breakdown-related properties generally need confirmation according to design requirements.

Can a supplier's TDS replace component testing?

No. A TDS mainly describes material-level properties. Suitability must still be validated with production components under actual assembly conditions.

Products related to this article

1. IOTA9001 fumed silica: a candidate raw material for reinforcement, rheology and structural control in silicone rubber or silicone formulations. Suitability for a thermally conductive system requires validation against the matrix, fillers, viscosity and application requirements.

IOTA9001 fumed silica product page

2. Silicone oils, silicone rubber and silicone additives: combinations can be screened according to viscosity, compliance, rheology, curing and interface requirements of thermal-management materials. Refer to IOTA's public product pages and valid technical documentation for specific grades and parameters.

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