Teflon high-temperature cloth manufacturers point out that cooling rate regulation is the most direct and effective method to control crystallinity. After PTFE is completely melted above 380°C, molecular chains arrange into crystal lattices when passing through the maximum crystallization rate temperature range of 310~315°C during cooling. The cooling rate determines the crystal region proportion and microscopic morphology of the coating.
1. Rapid cooling (quenching) for low crystallinity
The sintered cloth surface is rapidly cooled via cold air curtains, cooling rollers or normal-temperature water tanks. The molecular chains are frozen before regular and ordered arrangement, forming a large number of tiny and imperfect crystal grains. The obtained coating features soft texture and excellent toughness with optimized non-stick performance, while its hardness and wear resistance are slightly reduced.
2. Slow cooling (annealing) for high crystallinity
A heat preservation section is arranged at the outlet of the sintering furnace, or the cloth is cooled down with the furnace at an extremely slow rate of 10~50°C/h. This allows sufficient movement of molecular chains to form large and complete spherulites. The coating boasts high hardness, outstanding wear resistance and stable dimensional stability, yet with reduced flexibility and increased brittleness.
II. Auxiliary Process and Material Control
On the basis of cooling rate regulation, the following auxiliary measures can further fine-tune the crystallinity of PTFE coating.
1. Sintering temperature and holding time
The material must be heated to 380~400°C and held for sufficient time to completely melt the original crystals and eliminate thermal history. Inadequate sintering will leave residual crystal nuclei, resulting in excessively high and unevenly distributed final crystallinity.
2. Post-treatment annealing
For coatings with low crystallinity obtained by rapid cooling, secondary crystal growth and internal stress elimination can be realized by constant-temperature treatment at 300~320°C for several hours followed by slow cooling, which moderately increases the coating crystallinity.
3. Raw material molecular weight
High-molecular-weight PTFE dispersion features high melt viscosity and low molecular chain mobility, leading to a slower crystallization rate. Under the same cooling conditions, it is easier to prepare coatings with low crystallinity.
4. Nucleating agent addition
A tiny dosage (0.1%~1%) of inorganic fillers or perfluorinated copolymers can provide a large number of heterogeneous crystal nuclei and significantly refine crystal grains. Though it cannot drastically change the overall crystallinity, it effectively optimizes the crystal microstructure.
III. Comprehensive Management for Multi-Layer Coating Production
Teflon high-temperature cloth is produced by multiple impregnation and sintering processes with gradually accumulated coatings, requiring targeted process control as follows.
1. Interlayer crystallization consistency
Excessively large differences in cooling rate between layers will cause inconsistent interlayer crystallinity and generate internal stress, even leading to coating peeling in severe cases. It is recommended to adopt mild cooling for intermediate layers, and implement targeted rapid or slow cooling only for the outermost layer according to production requirements.
2. Coating thickness effect
Thicker coatings suffer from asynchronous cooling between the surface and inner layer. The inner layer tends to form high crystallinity due to slow heat dissipation. It is necessary to appropriately extend the sintering time to ensure uniform temperature of the entire coating before implementing unified cooling procedures.
3. Tension matching during cooling
The tension applied to the cloth surface during the cooling section affects the orientation of molecular chains, which couples with the crystallization process and jointly determines the final crystal morphology and dimensional stability. Coordinated and precise control is mandatory.
IV. Quantitative Detection of Crystallinity
Differential Scanning Calorimetry (DSC) must be adopted to verify crystallinity after process adjustment. A 5~10mg coating sample is heated at a rate of 10°C/min to test the melting enthalpy. The crystallinity is calculated by dividing the tested melting enthalpy by the theoretical melting enthalpy of 100% crystalline PTFE (approximately 82J/g). Combined with bending resistance, wear resistance and other performance tests, the optimal process window can be determined.
General Operation Principles: To obtain low crystallinity, implement immediate quenching after full sintering; to obtain high crystallinity, keep constant temperature near 310°C followed by extremely slow cooling after full sintering.
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