Jiangsu Aokai New Materials, a professional manufacturer of PTFE high-temperature cloth, elaborates on the evolution of PTFE crystalline structure throughout the sintering process of PTFE high-temperature cloth.
I. Destruction of the Original Crystalline Structure
1. PTFE crystalline morphology before sintering
Prior to sintering, PTFE exists as dispersed emulsion particles attached to fiberglass fabric. Each particle contains highly crystalline folded-chain lamellae with tiny, neatly arranged microcrystals.
2. Structural transformation during heating for sintering
Once the temperature rises above the melting point (~327°C), all crystals fully melt. Folded molecular chains unfold and disentangle, transforming into an amorphous melt. Original particle boundaries disappear as particles fuse into a continuous molten phase, completely breaking down the initial granular crystalline structure.
II. Recrystallization During Cooling
1. Formation of spherulitic structures
Upon cooling from the molten state, PTFE rapidly recrystallizes to form spherulites composed of radially oriented folded-chain lamellae extending outward from central nucleation sites. These interpenetrating spherulites build a continuous crystalline matrix, replacing the original packed particle structure.
2. Variation in final crystallinity
PTFE features rigid, highly regular molecular chains, delivering extremely fast crystallization rates even under relatively rapid cooling. The final crystallinity generally ranges from 50% to 70%, marginally lower than that of the original emulsion particles.
III. Evolution of Crystalline Parameters and Crystal Forms
1. Retention of stable crystal phase
After cooling to room temperature, PTFE maintains a stable hexagonal crystal system (room-temperature hexagonal phase), identical to its pre-sintering crystal form. All structural shifts occur in crystal size, perfection and morphology rather than the ultimate crystal phase.
2. Influence of cooling rate on crystal perfection
· Slow cooling yields higher crystallinity, larger spherulites and thicker, more well-defined lamellae, resulting in products with elevated hardness and density.
· Rapid quenching leads to relatively lower crystallinity, finer, less perfect spherulites, and improved flexibility and semi-transparency of the finished material. Slow cooling is standard for PTFE high-temperature cloth to meet mechanical strength requirements.
IV. Interfacial Crystallization Effect on Fiberglass and Overall Significance
1. Transcrystallization phenomenon
When PTFE melt cools on fiberglass surfaces, dense nucleation sites on the fiber surface induce transcrystallization: a columnar crystal layer grows perpendicular to the fiber axis, distinctly different from the spherulite morphology within the bulk coating.
2. Gradient structure from fiber interface to coating bulk
Under the influence of fiberglass, the PTFE crystalline structure forms a gradient transition: a transcrystalline layer adjacent to fibers gradually shifts to spherulitic structures in the inner coating matrix.
3. Decisive impact of structural transformation on PTFE cloth performance
Sintering transforms granular folded-chain PTFE crystals: they undergo melting and structural breakdown, then recrystallize into continuous, dense spherulites (with interfacial transcrystalline layers). This converts loose packed powder particles into an intact continuous film, which is the critical prerequisite for the coating to deliver comprehensive performance including heat resistance, low friction, non-stick property and chemical corrosion resistance.
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