When a PTFE high-temperature adhesive tape is used in a hot environment – say, 200°C for weeks or months – the silicone pressure-sensitive adhesive (PSA) can change. Peel strength may rise, fall, or transfer residue. Cohesion may increase (brittleness) or decrease (cohesive failure).
Not all silicone PSAs behave the same. The differences come down to polymer composition (methyl vs. methylphenyl), crosslinking mechanism (peroxide vs. addition cure), and the ratio of MQ resin to rubber.
Aokai PTFE uses multiple silicone PSA grades in our tapes. This guide explains what happens to peel strength and cohesion after high-temperature aging for each grade, and how to select the right one for your application.
What Happens to Silicone PSA Under Heat – Two Failure Modes
When aged at high temperature (200-250°C), silicone PSAs can undergo two opposing changes:
1. Post-crosslinking (leads to brittleness)
Unreacted functional groups continue to react, increasing crosslink density. Initial peel strength may rise temporarily, then fall sharply as the adhesive becomes too rigid. Static shear strength appears to increase, but holding power and impact resistance deteriorate. The adhesive film becomes hard and cracks.
2. Backbone degradation (leads to peel loss and residue)
The polydimethylsiloxane backbone undergoes cyclization and chain scission. Peel strength drops continuously. The adhesive may transfer to the substrate (residue) or fail cohesively. This is common in low-grade methyl silicone PSAs.
Key modulating parameters:
Phenyl content (higher = better heat resistance)
Crosslink density and curing type (addition cure vs. peroxide cure)
Ratio of MQ resin to silicone rubber
Grade Classifications by Chemical Structure
1. Standard polydimethylsiloxane (methyl-type silicone PSA)
Peel retention after aging (200-250°C): Peel strength rises slightly at first, then declines. After 7 days at 250°C, peel retention is only 50-70%. Performance drops faster at higher temperatures. Degradation comes from cyclization of the dimethyl siloxane backbone.
Cohesion stability: Peroxide-cured versions (e.g., KR-101) have residual initiators that cause uncontrolled post-curing – adhesive hardens and embrittles. Initial shear strength improves, but holding power drops, and cohesive failure occurs under heat. Addition-cured versions are slightly better but still limited by the poor inherent heat resistance of the dimethyl backbone.
Peel retention after aging: Phenyl side chains disrupt the regular packing of dimethyl segments and suppress cyclization degradation. After 7 days at 250°C, peel strength retention exceeds 85%. High-phenyl formulations maintain stable peel properties even at 260-288°C.
Cohesion stability: Excellent. Shear strength changes minimally; holding power retention exceeds 80%. The adhesive layer remains flexible long-term without embrittlement or residue. Addition-cured high-phenyl grades are the premium thermally stable silicone PSA.
3. Special high-peel and low-peel custom grades
High-peel grades (e.g., Dow 7385, typical peel >10N/25mm): High MQ resin loading gives excellent initial tack and peel strength, but at the cost of heat resistance and cohesion stability. They show larger peel drop after aging and are prone to residue due to resin thermal decomposition. Use only for intermittent low-temperature applications.
Low-peel, high-cohesion protective film grades: High crosslink density gives stable peel performance and excellent aging stability, but low initial tack. After aging, they remain cohesive but become hardened, reducing conformability to rough surfaces.
Crosslinking Mechanism – Peroxide vs. Addition Cure (Same Methyl Base)
Even with the same methyl polymer, the curing method dramatically changes aging behavior.
1. Peroxide-cured (conventional early generation)
Residual peroxide triggers uncontrolled post-crosslinking during high-temperature aging.
Peel strength spikes initially, then collapses sharply.
Cohesion rises temporarily, followed by embrittlement and holding power failure.
Batch-to-batch aging consistency is poor.
2. Addition-cured (hydrosilylation curing)
Cure generates no byproducts; crosslink network is precisely controllable.
Post-aging property changes follow a predictable linear trend.
Far superior peel retention and cohesion stability compared to peroxide formulations.
Grade Selection Guidance
Service Condition
Recommended Grade
Why
Continuous high temperature (>200°C), stable peel, zero residue
>85% peel retention, no embrittlement, >80% holding power retention
Intermittent short-term high heat, cost-sensitive
Standard methyl-type, addition-cured, post-bake cured (e.g., 7657)
Acceptable but expect some peel degradation and possible embrittlement over long cycles
Very high initial peel needed, low temperature only
High-peel methyl grade
Not for continuous high heat; expect residue after aging
Protective film, high cohesion, low tack
High-crosslink low-peel grade
Excellent aging stability but hardened and less conformable
Aokai PTFE recommendation: For any PTFE tape that will see sustained temperatures above 200°C, specify an addition-cured methylphenyl silicone PSA. The upfront cost is higher, but it prevents field failures caused by adhesive residue or bond loss.
Summary – Matching Silicone PSA Grade to Thermal Demand
Methyl-type silicone PSA (especially peroxide-cured) is suitable only for intermittent or low-temperature use. After 7 days at 250°C, expect 30-50% peel loss and possible embrittlement.
Methylphenyl (high-phenyl) silicone PSA retains >85% peel strength and flexible cohesion after the same aging. It is the choice for continuous 200-250°C service.
Crosslinking mechanism matters greatly: addition cure (hydrosilylation) is far more stable than peroxide cure.
High-peel grades trade off aging stability for initial tack – avoid them in hot applications.
For demanding high-temperature bonding, Aokai PTFE uses addition-cured methylphenyl silicone PSAs. We can provide peel retention data after 1000h at 250°C upon request.
