What Causes Cracking in the PTFE Impregnation Process?

As a professional source manufacturer of PTFE high-temperature fabrics, Jiangsu Aokai New Materials will explain the causes of cracking in glass fiber cloth after impregnation with PTFE (polytetrafluoroethylene) emulsion. This is a common process issue in composite material manufacturing, which usually occurs during drying (removal of moisture and surfactants) or high-temperature sintering (melting PTFE particles into a continuous film).

1. Severe Mismatch of Thermal Expansion Coefficients

This is the fundamental physical cause.

There is a huge difference in thermal expansion coefficients between glass fiber and PTFE:the linear expansion coefficient of glass fiber is approximately 5×10⁻⁶/℃,while that of PTFE is about 100×10⁻⁶/℃ to 200×10⁻⁶/℃ (roughly 20 times that of glass fiber).

During cooling (from a sintering temperature of approximately 380℃–400℃ down to room temperature), PTFE shrinks far more significantly than the glass fiber framework. Since glass fiber cloth acts as a rigid skeleton, PTFE is constrained during shrinkage, generating substantial internal tensile thermal stress. When this stress exceeds the tensile strength of the PTFE film or the interfacial bonding strength between PTFE and glass fiber, cracking occurs to release the stress.

2. Improper Control of Drying and Sintering Processes

Heating parameters of the impregnated material directly affect cracking:

· Excessively fast heating rate: Especially at around 100℃ (rapid vaporization of moisture) and 200℃–250℃ (decomposition of surfactants in the emulsion and the crystalline transition zone before PTFE sintering). Rapid heating causes internal stress and gas impact from instantaneous volatilization of solvents and additives, resulting in coating cracking.

· Excessively fast cooling rate: Direct exposure to ambient air for rapid cooling after sintering creates intense instantaneous shrinkage stress due to large temperature differences, which readily induces fine network cracks.

· Excessively high sintering temperature or overly long sintering time: This causes degradation of PTFE molecular weight and excessive fluidity, making the material more susceptible to cracking under stress.

3. Issues in Impregnation Process and Emulsion Concentration

The thickness, uniformity and penetration depth of the impregnated layer directly affect cracking sensitivity:

· Excessively thick single impregnation layer: In pursuit of thickness or production efficiency, an overly thick PTFE coating is applied in one pass. During drying and sintering, a thick coating has a longer path for solvent removal and undergoes greater volumetric shrinkage (PTFE emulsion typically has a solid content of about 60%, meaning roughly 40% moisture must be removed), which frequently causes surface cracking.

· Uneven impregnation: Localized excessive PTFE buildup or uneven emulsion distribution on the fabric surface leads to concentrated shrinkage stress.

· Insufficient emulsion penetration: If glass fiber cloth is not dewaxed or poorly surface-treated, PTFE emulsion fails to fully penetrate into fiber bundles and only forms a superficial “skin film”. This film has weak anchoring with the substrate and tends to detach or crack when heated.

4. Substrate-Related Factors

· Unde-waxed glass fiber cloth: Glass fiber cloth contains sizing agents (paraffin, epoxy, etc.) from the weaving process. Without high-temperature dewaxing or chemical cleaning before impregnation, these organic substances carbonize and volatilize during subsequent sintering, generating gas that ruptures the coating or weakens the bonding between PTFE and glass fiber, resulting in cracks.

· Structure and thickness of glass fiber cloth: Thicker and denser glass fiber cloth (such as thick satin fabric) exhibits higher rigidity and stronger restraint on PTFE shrinkage. Meanwhile, deeper grooves between fiber bundles tend to cause stress concentration and cracking at groove edges during drying.

The above information is provided by Jiangsu Aokai New Materials Technology Co., Ltd.

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