In the production of PTFE high-temperature tape, the silicone pressure-sensitive adhesive is applied as a solvent-based solution. As the coated tape moves through the drying oven, solvent evaporates, leaving behind a solid adhesive layer. The rate at which this happens – solvent volatilization rate – is a core process parameter that determines whether the final adhesive surface is smooth, defect-free, and consistently thick, or covered in pinholes, craters, and streaks.
Aokai PTFE has optimized coating processes across thousands of production runs. This guide explains how volatilization rate affects leveling, pinhole formation, and crater defects – and how to control it for high-quality PTFE tape.
How Volatilization Rate Affects Leveling – The Viscosity Trade-Off
Leveling is the ability of a wet adhesive film to eliminate coating streaks, transverse stripes, and orange-peel texture, forming a smooth surface. The driving force is surface tension; the resistance is viscosity.
1. Too fast – surface skinning stops leveling
When solvent evaporates too quickly, the solvent concentration at the surface drops rapidly. Silicone PSA (high-molecular-weight polysiloxane solution) is extremely concentration-sensitive. The surface layer viscosity rises exponentially – even forming a high-viscosity “skin” almost instantly.
This skin locks the underlying uncured adhesive in place. The adhesive can no longer flow to level out streaks or blade marks. Irregular textures generated during coating are permanently fixed.
2. Too slow – sagging and dewetting
If solvent volatilizes too slowly, the adhesive layer remains a low-viscosity liquid for too long. While this allows time for leveling, gravity causes:
Sagging (adhesive runs downward on vertical surfaces)
Thick edges (adhesive pools at the edges of the coated area)
Dewetting – on low-surface-energy PTFE substrates (critical surface tension only 18-20 mN/m), prolonged wetting can actually cause the adhesive to shrink and pull away from the substrate, ruining surface smoothness.
How Volatilization Rate Causes Pinholes
Pinholes are tiny holes through the adhesive layer. They form when solvent or air bubbles become trapped and burst through a rigidified surface before the adhesive can reflow.
1. Mechanism 1 – boiling solvent trapped under the skin
When the coated tape enters the drying oven, a sharp temperature rise combined with ultra-fast solvent volatilization cures the film surface first. Solvent trapped inside the layer vaporizes violently, forming bubbles. Once internal bubble pressure breaks the rigidified surface skin, the ruptured liquid film cannot reflow and heal – leaving permanent pinholes or volcano-like craters.
2. Mechanism 2 – bubbles cannot escape
Tiny air bubbles are always entrained during coating and adhesive feeding. They require sufficient time to float up, rupture, and heal at low viscosity. If the surface skins over quickly due to fast drying, bubbles cannot escape and are fixed as pinholes.
3. Mechanism 3 – poor wetting of micro-pits
Early rapid volatilization spikes adhesive viscosity sharply, depriving the solution of the ability to slowly fill micro-pits and expel interfacial air. This results in dense pinhole-like wetting defects after drying.
How Volatilization Rate Causes Craters (Dimples)
Craters are small, round depressions on the adhesive surface, distinct from pinholes. They are caused by local surface tension imbalances.
1. Mechanism 1 – condensed water droplets
Rapid solvent volatilization absorbs massive vaporization heat, sharply cooling the wet film and substrate surface. If the temperature drops below the ambient dew point, water vapor in the air condenses into micro-droplets on the wet film.
Surface tension of water: ~72 mN/m
Surface tension of silicone adhesive: 20–25 mN/m
This huge difference creates a local surface tension gradient that drives surrounding adhesive to shrink away from the droplets, leaving craters after drying.
2. Mechanism 2 – contaminant-induced craters
Low-surface-tension contaminants (silicone micro-droplets, grease) inevitably exist in the adhesive or the working environment. These impurities form low-surface-tension centers on the wet film. Under the Marangoni effect, adhesive flows outward from these centers, creating depressions.
Moderate volatilization allows depressions to self-heal via adhesive backflow.
Overly fast volatilization deprives the liquid of fluidity before healing – craters are permanently locked.
3. Mechanism 3 – dewetting craters from insufficient wetting
PTFE substrates have critical surface tension of only 18–20 mN/m – right at the threshold of feasible wetting. The silicone adhesive needs a sufficient low-viscosity window to spread and wet the surface thermodynamically. If solvent volatilizes too fast, the adhesive loses fluidity before complete wetting. During drying, volume shrinkage and surface tension force the adhesive to contract, leaving micro-craters at poorly wetted zones – visible as a pockmarked surface.
Solutions – How to Control Volatilization for Defect-Free Coating
The ideal solvent volatilization strategy uses gradient balance:
Use a blend of solvents with different boiling points:
High-boiling, slow-evaporating good solvents (toluene, xylene, or higher-boiling aromatics/esters) – main component – ensures solubility and adequate leveling time.
Small amount of medium/low-boiling fast-drying solvents – blended to adjust overall drying speed.
3. Gradient temperature control in the drying oven
Low temperature at inlet – slow initial volatilization for leveling.
Medium temperature in middle zone – controlled drying.
High temperature at exit – final rapid shaping.
Avoid sharp heating at the inlet section – this causes instant skinning.
4. Ancillary measures
Control ambient humidity – prevent water condensation on the wet film.
Online corona treatment of the PTFE substrate – raises surface energy, improves wetting, reduces dewetting craters.
Summary – Key Control Points for Coating Quality
Defect
Primary Cause
Prevention
Orange peel / streaks
Too fast initial volatilization (skinning)
Slow first-stage drying; use slower solvents
Sagging / thick edges
Too slow volatilization
Accelerate drying; reduce coating thickness
Pinholes
Solvent trapped under skin; bubbles cannot escape
Gradient heating; reduce initial temperature
Condensation craters
Surface cooling below dew point
Control humidity; avoid excessive cooling
Contaminant craters
Impurities + fast volatilization
Clean environment; slower initial drying
Wetting craters
Poor PTFE wetting + fast dry
Corona treatment; slower initial drying
Aokai PTFE optimizes coating parameters – solvent blend, drying profile, and substrate pretreatment – to produce PTFE tape with smooth, defect-free adhesive layers. Contact us for process support or custom coating specifications.
If you wish to learn more about detailed specifications, application scenarios and customized solutions for our full product range, including PTFE high-temperature fabrics, PTFE high-temperature tapes, PTFE mesh belts, seamless bonding machine belts, single-sided PTFE cloth, high-temperature resistant conveyor belts and high-temperature resistant fiberglass fabrics, please contact us:
