Effects of Environmental Humidity on PTFE Emulsion Impregnation

In the actual production process of PTFE high-temperature cloth by Jiangsu Aokai New Materials, environmental humidity is usually controlled within a specific range of 40%–60% RH, combined with temperature regulation. The evaporation rate is adjusted to be slightly lower than the capillary transport rate of water in the emulsion within the capillary network, ensuring full penetration and leveling of the emulsion while achieving defect-free, highly dense PTFE sintered films.

The core influence of environmental humidity on PTFE emulsion impregnation lies in the dynamic balance between water evaporation rate and emulsion stability. PTFE emulsion is typically a non-Newtonian fluid composed of PTFE resin particles (dispersed phase), surfactants, stabilizers, and water (continuous phase). As an environmental parameter, humidity regulates the volatilization kinetics of the solvent (water), exerting fundamental effects as follows:

1. Fundamental Effects on the Impregnation Process

The impregnation stage requires the emulsion to effectively penetrate into the pores or fiber gaps of the substrate (e.g., glass fiber cloth, metal mesh).

Low humidity environment: Water evaporates too quickly, causing the emulsion to “surface-dry” rapidly on the substrate. This significantly increases the apparent viscosity of the emulsion at the interface, hindering capillary penetration of resin particles into the deep substrate and resulting in “insufficient impregnation” — that is, residual air pockets inside the substrate. Meanwhile, rapid evaporation removes large amounts of heat, which may cause a sudden local temperature drop, destabilizing the emulsion thermodynamically and inducing surfactant precipitation, leading to “orange peel” or uneven thickness in the impregnated layer.

High humidity environment: Water evaporation is inhibited, and the emulsion remains in a low-viscosity wet state on the substrate for an excessively long time. For vertical or inclined substrates, sagging easily occurs under gravity; for highly hygroscopic substrates, high humidity may hinder gas discharge inside the substrate, forming microbubbles that develop into pinhole defects during subsequent sintering.

2. Fundamental Effects on Final Film-Forming Quality

PTFE film formation generally involves two stages: drying (water removal) and high-temperature sintering (particle melting). Humidity mainly affects the structure of the green body during drying, which is “inherited” into the final sintered film.

Cracking and internal stress: Film formation of PTFE resin particles relies on capillary pressure during water evaporation to compress particles into a dense packed layer. Excessively low humidity causes a sharp increase in capillary pressure gradient and excessively fast drying shrinkage, generating large tensile internal stress within the green body. Since PTFE lacks self-healing ability before sintering, such stress directly leads to microcracks or macroscopic cracking in the green body. These cracks cannot be eliminated after sintering, resulting in film leaks or reduced mechanical strength.

Surfactant migration and residue: Surfactants (such as alternatives to perfluorooctanoic acid) contained in commercial PTFE emulsions migrate with water evaporation during drying. A high humidity environment extends the drying window, allowing sufficient time for surfactants to accumulate on the surface, forming a “surface oil slick” or hard skin. If this layer is not fully decomposed or volatilized in the early sintering stage, it will be carbonized and encapsulated inside the film, causing yellowing, reduced density, and significantly weakened adhesion between PTFE and the substrate (poor interfacial bonding due to contaminant isolation).

Film density and porosity: An ideal PTFE film should be pinhole-free and dense. Improper humidity control leading to residual moisture is a major hazard. Under high humidity, if water is not completely removed during the heating and drying stage, residual moisture vaporizes instantaneously during subsequent sintering (approx. 327–400°C) with drastic volume expansion, creating blowholes or interlayer blisters in the partially melted film. Conversely, under extremely low humidity, rapid surface drying forms a sealed skin that blocks internal water evaporation, also causing blisters or delamination during sintering due to accumulated vapor pressure.

In principle, the influence of environmental humidity on PTFE emulsion processing is essentially an intervention in water evaporation kinetics:

· Low humidity (excessive dryness) tends to cause: poor penetration, blocked leveling (shrinkage cavities), drying cracks, and uneven distribution of surfactant residues.

· High humidity (excessive moisture) tends to cause: sagging, increased pinholes, excessive surfactant floating, blistering after sintering, and reduced adhesion.

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

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