What are the effects of web tension control during the coating process of Teflon high-temperature tape on the flatness of the PTFE substrate and the uniformity of the adhesive coating?

I. Effects on the Flatness of the PTFE Substrate

1. Plastic Stretching and the "Necking" Effect

PTFE material has a low elastic modulus. Once the tension exceeds its elastic limit, irreversible plastic stretching occurs, resulting in longitudinal elongation and transverse narrowing. After the tension is released, the substrate cannot recover to its original shape, leading to slackness, longitudinal wrinkles, and uneven thickness. In severe cases, transverse wavy patterns may form.

2. Creep (Cold Flow) Accumulation Over Time

PTFE undergoes slow plastic flow under sustained tension. During the coating process, the substrate softens when heated inside the drying oven, and creep intensifies. This uneven elongation becomes "frozen" after cooling, resulting in surface unevenness or longitudinal ridges, which cannot be eliminated by subsequent processes.

3. Uneven Transverse Tension

Poor parallelism of guide rollers or inaccurate edge guiding can cause tension differences between the edges and the center, leading to "curled edges," central blistering, or single-side warping. Periodic tension fluctuations can also produce alternating transverse "tight-loose marks" on the substrate surface, directly compromising flatness.

II. Effects on the Coating Uniformity of the Adhesive Layer

1. Tension Fluctuations Alter the Coating Gap

In knife-over-roll coating, tension directly determines the degree of contact between the substrate and the doctor blade. When tension suddenly increases, the substrate is pressed tighter, reducing the coating weight; when tension decreases, the substrate loosens, increasing the coating weight. Such fluctuations create periodic variations in adhesive layer thickness along the web direction, known as "crosswise streaks" or "zebra marks."

2. Substrate Stretching Affects Adhesive Spreading

Excessive tension thins the PTFE film and generates micro-wrinkles, which are then replicated by the adhesive coating on the uneven substrate surface. After tension is released and the film shrinks back, the adhesive layer may develop micro-cracks due to uneven stress. For fiberglass fabric substrates, excessive tension enlarges yarn interstices, causing excessive adhesive penetration, insufficient surface retention, and a rough coating surface.

3. Coupled Effects During Heating and Drying

PTFE has a high coefficient of thermal expansion. Inside the drying oven, if multi-zone tension settings are mismatched, the heat-softened substrate undergoes secondary stretching, distorting the wet adhesive layer along with it, resulting in a thinner and uneven coating after drying. Improper tension in the cooling zone can also cause shrinkage wrinkles in the semi-dry adhesive layer.

III. Key Control Strategies

1. Unwinding and Coating Sections

Extremely low and constant tension should be applied, just sufficient to prevent the substrate from wrinkling, allowing it to enter the coating zone in a natural state and avoid pre-stretching.

2. Drying and Cooling Sections

Precise fine-tuning via floating rollers should be used to actively compensate for thermal expansion and contraction of the substrate, and high tension in the high-temperature zones must be strictly avoided.

3. Rewinding Section

Taper tension must be adopted, gradually reducing the nip pressure as the roll diameter increases, to prevent damaging the coated surface or "transferring" unevenness from the underlying layers to adjacent layers.

IV. Summary

Precise segmented tension control is the core factor in balancing the inherent characteristics of PTFE substrates—namely low friction and susceptibility to creep—with the requirement for adhesive layer uniformity. It directly determines the ultimate performance of Teflon tape in applications such as high-temperature resistance and release properties.

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

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