Drying is a critical step in PTFE high-temperature fabric production. After fiberglass fabric is impregnated with PTFE emulsion, it passes through a drying oven to remove water and fuse the PTFE particles into a continuous coating.
However, the drying process can introduce a wide range of defects – from surface cracks and pinholes to delamination and warpage. These defects arise from the complex interaction between emulsion properties, substrate structure, and drying parameters.
Aokai PTFE has optimized drying processes across many products. This guide describes the ten most common drying defects, their root causes, and practical solutions.
Coating Cracking and Crazing
Symptoms: Fine or wide cracks spread across the fabric surface; in severe cases, the coating peels off in flakes.
Root causes:
Excessively fast drying speed – rapid water evaporation on the surface forms a dry skin layer. Internal vaporized water breaks through the surface film or generates severe shrinkage stress.
High PTFE solid content – causes massive volume shrinkage during particle accumulation.
Excessively high drying temperature or overstrong hot air – results in surface skinning while the inner layer remains undried, concentrating stress and triggering cracking.
Solutions:
Use a gradient drying profile: lower temperature in the first zone, gradually increasing.
Reduce initial drying intensity (temperature and airflow).
Optimize emulsion solids content – avoid excessively high solids.
Coating Migration (Electrophoretic Migration)
Symptoms: The fabric surface is rich in resin and appears glossy, while insufficient resin fills gaps between internal fiber bundles. Cross-section observation reveals an uneven structure with dense outer coating and sparse inner coating.
Root causes:
Fine PTFE particles migrate to the surface along with water flow as moisture evaporates outward.
Slow drying or single-sided heating aggravates this migration.
Consequences:
Weak coating adhesion.
Degraded final mechanical performance and durability.
Solutions:
Increase initial drying rate to set particles quickly before migration occurs.
Use two-sided heating or better airflow distribution.
Consider higher-viscosity emulsions to reduce particle mobility.
Pinholes and Blisters
Symptoms: Tiny circular holes or raised bubble marks on the coating surface.
Root causes:
Residual microbubbles inside the untreated emulsion.
Air trapped in fiberglass fabric pores expands and escapes upon heating.
Sharp boiling and vaporization of water break through the coating film.
Low-viscosity emulsion, poor substrate wetting, or rapid temperature rise during drying all exacerbate pinhole defects.
Solutions:
Degas the emulsion before impregnation.
Use a defoamer and ensure thorough wetting of fiberglass.
Control initial drying temperature to avoid violent boiling.
Increase emulsion viscosity slightly to resist bubble rupture.
White Spots, Hazing, and Chalking
Symptoms: Local or overall foggy white patches on the coating; powdery residues flake off the surface in serious cases.
Two distinct mechanisms:
White spots / hazing:
Ultra-fast drying generates microcracks that scatter light.
Surfactants migrate and accumulate on the surface, causing moisture-induced whitening.
Incomplete drying leaves residual water and forms watermark-like white blemishes.
Chalking:
Excessively high temperature or direct hot air impact prevents surface PTFE particles from fusing into an integrated film, leaving loose powder.
Unstable emulsion may break and agglomerate into powder clumps during drying.
Solutions:
Avoid excessively high drying temperatures.
Ensure complete sintering – not just drying.
Control surfactant content in the emulsion formulation.
Yellowing and Coking
Symptoms: The coating turns yellow, even with brown scorch marks.
Root causes:
Overhigh drying temperature or prolonged residence time triggers thermal oxidative decomposition of surfactants, thickeners in the emulsion, and residual fiberglass sizing agents.
PTFE itself has excellent heat resistance, but auxiliary additives cannot withstand extreme heat.
Uneven infrared heating creates local overheating zones.
Solutions:
Set drying temperature below the decomposition threshold of all additives (typically <380°C for PTFE sintering, but avoid sustained >350°C).
Ensure uniform heating across the oven width.
Use high-purity, thermally stable additives.
H2: 6. Coating Lifting and Delamination
Symptoms: The coating separates from the fiberglass substrate interface, peelable in large sheets or detachable by rubbing.
Unclean or untreated fiberglass surface leads to poor emulsion wetting.
Uneven shrinkage from single-sided fast drying.
Resin depletion at the substrate interface caused by coating migration forms a weak boundary layer.
Solutions:
Ensure thorough surface cleaning and pretreatment of fiberglass (e.g., heat cleaning to remove sizing).
Use silane coupling agents or adhesion promoters.
Optimize drying profile to reduce shrinkage stress.
Prevent excessive migration.
Orange Peel, Poor Leveling, and Rough Surface
Symptoms: Orange-peel-like uneven textures or overall matte rough surface without gloss.
Root causes:
Poor leveling performance of PTFE emulsion.
Excessively high initial drying temperature rapidly thickens the surface film and eliminates leveling capacity.
Direct hot air flow disturbs the wet coating surface.
Overly prominent fabric weave texture cannot be fully covered by thin coating layers.
Solutions:
Use lower initial drying temperature – allow time for leveling.
Reduce hot air velocity in the first drying zone.
Apply multiple thin coats rather than one thick coat.
Use leveling agents if necessary.
Warpage and Shrinkage Deformation
Symptoms: Uneven fabric surface, curled selvedges, and obvious width shrinkage.
Root causes:
Thermal shrinkage of fiberglass fabric under alternating humid and dry heat conditions, especially with improper warp/weft tension.
Mismatched thermal shrinkage rates between coating and substrate.
Single-sided drying or large temperature gradients create differential shrinkage on two sides of the fabric.
Solutions:
Maintain balanced warp/weft tension during drying.
Use two-sided heating to balance shrinkage.
Pre-shrink the fiberglass fabric before coating.
Surface Dry But Interior Wet – Roll Blocking
Symptoms: The coating feels dry to the touch yet retains internal moisture. Coated layers stick to each other after winding, with loud peeling noise or coating damage during unwinding.
Root causes:
High drying temperature paired with short residence time forms a surface skin that blocks internal water vapor discharge.
Insufficient cooling after exiting the drying oven leaves residual heat and makes surfactants tacky.
Incomplete drying leaves residual water and additives to form sticky films.
Solutions:
Use lower drying temperature with longer residence time – allow moisture to escape gradually.
Ensure adequate ventilation and dehumidification in the oven.
Provide sufficient cooling before winding (target <40°C).
Thick Selvedges and Uneven Coating Thickness
Symptoms: The coating at fabric edges is distinctly thicker than the central area, or striped thickness differences appear across the web.
Root causes:
Fast moisture evaporation at selvedges drives emulsion accumulation and migration toward edges during drying – the “coffee ring effect.”
Uneven hot air distribution creates varied drying rates.
Solutions:
Improve air flow uniformity across the full fabric width.
Use edge-zone temperature control or baffles to reduce edge drying rates.
Adjust coating technique to apply more uniform wet film thickness.
Summary – Key Drying Parameters to Control
Defect
Primary Cause
Prevention
Cracking
Too fast initial drying, skinning
Gradient temperature; slower initial zone
Migration
Slow drying, single-sided heat
Faster initial set; two-sided heating
Pinholes
Trapped air/steam bubbles
Degas emulsion; controlled heating
White spots/hazing
Surfactant migration, microcracks
Moderate drying; good surfactant control
Yellowing/coking
Overheating additives
Lower temperature; uniform heating
Delamination
Shrinkage stress > adhesion
Surface treatment; reduce stress
Orange peel
Poor leveling
Lower initial temp; reduce airflow
Warpage
Uneven tension/shrinkage
Balanced tension; two-sided heating
Roll blocking
Dry surface, wet core
Lower temp + longer time; cool before winding
Thick edges
Coffee-ring effect
Uniform airflow; edge temperature control
Aokai PTFE carefully controls drying parameters – temperature profile, airflow, residence time, and tension – to produce PTFE fabric free of these defects. For custom coating applications, we can optimize drying conditions for your specific product requirements.
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 conveyor belts, seamless bonding machine belts, single-sided PTFE cloth, high-temperature resistant conveyor belts and high-temperature resistant fiberglass fabrics, please contact us:
