How to Identify Whether PTFE High-Temperature Fabric Has Been Over-Sintered or Under-Sintered?
Over-sintering and under-sintering are two opposing quality defects in PTFE high-temperature fabric production — both directly degrading product performance, but manifesting through entirely different failure modes. Identification relies on visual inspection, physical testing, and instrumental analysis working in combination.
Ⅰ. Identification of Over-Sintering (Sintering Temperature Too High or Time Too Long)
Core Cause: Sintering temperature exceeds 390°C, or holding time is too long — causing PTFE to over-melt and flow; crystallinity decreases; molecular chains partially degrade.
Visual Identification Features:
| Inspection Item | Over-Sintered Manifestation | Normal Product Reference |
|---|---|---|
| Surface Color | Deep brown or dark yellow; localized blackening or carbonization spots | Uniform light brown or milky white |
| Surface Texture | Appears glossy/wet; over-flowed PTFE forms irregular surface accumulations | Uniform matte or satin finish |
| Edge Condition | Coating drips or flow marks at edges; non-uniform edge thickness | Clean, straight edges; uniform coating |
| Coating Thickness Uniformity | Non-uniform; thin areas and thick accumulation areas coexist | Uniform cross-width variation ≤ ±8% |
| Fiber Exposure | Fiberglass fibers are pressed deeper into the coating; fabric texture barely visible | Fiber texture still slightly visible through coating |
Physical Testing Identification:
- Tensile Test: Over-sintered products often show reduced tensile strength (PTFE molecular chain degradation) — tensile strength drop > 15% vs. standard; elongation at break significantly increased (coating becomes “soft and ductile”)
- Surface Roughness Test: Ra value increases significantly (> 2.5 μm) — PTFE over-flow creates surface undulation
- Non-Stick Test: EVA peel force may be paradoxically higher than normal product (over-melted PTFE surface microstructure changes; non-stick performance degrades) — EVA peel force > 5 N/25mm suggests over-sintering
- Fold Test: Over-sintered coating is more brittle; fold at 90° and unfold — coating cracking more readily visible than normal product
- Chemical Resistance Test: Immerse in 98% H₂SO₄ for 24h — over-sintered products show greater weight loss than normal (coating porosity increases after over-sintering)
Instrumental Analysis Identification:
- TGA (Thermogravimetric Analysis): Over-sintered products show earlier thermal decomposition onset (Td5% < 310°C vs. normal ≥ 320°C); PTFE molecular chain degradation products detectable
- DSC (Differential Scanning Calorimetry): Melting peak temperature shifts lower (< 325°C vs. normal 327°C); melting peak broadens — indicates crystallinity reduction from over-sintering
- SEM (Scanning Electron Microscopy): Surface shows irregular PTFE accumulation morphology; fiber-coating interface shows PTFE over-flow into fiber gaps causing localized thickness anomalies
Ⅱ. Identification of Under-Sintering (Sintering Temperature Too Low or Time Too Short)
Core Cause: Sintering temperature below 350°C, or holding time insufficient — PTFE emulsion particles do not fully melt and fuse; coating has high porosity; PTFE-fiberglass bonding is weak.
Visual Identification Features:
| Inspection Item | Under-Sintered Manifestation | Normal Product Reference |
|---|---|---|
| Surface Color | Milky white or chalky white; uneven color distribution | Uniform light brown or milky white |
| Surface Texture | Rough “powdery” feel; white powder residue rubs off by hand | Smooth; no powder on hand contact |
| Transparency | Partial translucency; fiberglass fabric clearly visible through coating in transmitted light | Opacity uniform; fabric not clearly visible through coating |
| Coating Continuity | Visible pinholes, micro-voids, or uncoated spots under magnifying glass | Continuous uniform coating; no pinholes |
| Surface Hardness | Noticeably softer than normal; pressed with fingernail leaves permanent indentation | Elastic recovery; no permanent indentation |
Physical Testing Identification:
- White Powder Rub Test (Most Intuitive Field Method): Rub the coating surface vigorously with a clean white cloth 10 times — under-sintered products leave obvious white powder residue on the cloth; normal products leave no white residue
- Water Drop Contact Angle Test: Drop deionized water on the surface — under-sintered products show contact angle < 90° (partially wets surface, indicating poor PTFE sintering); normal products show contact angle > 100°
- Peel Strength Test: Under-sintered products have significantly reduced peel strength — typically < 1.5 N/25mm (vs. normal ≥ 3.0 N/25mm); coating peels easily
- Air Permeability Test: Under-sintered products have higher air permeability than normal (coating porosity increased) — air flow rate > 1.5× normal value at equivalent mesh specification
- Chemical Resistance Test: Immerse in 10% H₂SO₄ for 24h — under-sintered products show visible color change or weight loss; normal products show no change
Instrumental Analysis Identification:
- TGA: Under-sintered products show residual PTFE emulsion dispersant decomposition peaks (typically at 100–200°C) — indicating incomplete sintering
- DSC: Melting peak area smaller than normal (< 80% of normal); crystallinity significantly lower (< 35%); broad, flat melting peak
- FTIR (Fourier Transform Infrared Spectroscopy): Under-sintered products show residual dispersant characteristic absorption peaks — C-H stretching vibration peaks appear around 2,900 cm⁻¹ (absent in fully sintered normal products)
- SEM: Surface shows spherical or semi-spherical PTFE emulsion particle residues (un-fused particles); obvious gaps between particles; fiber surface is not completely covered by PTFE
Ⅲ. Comparative Summary Table — Over-Sintered vs. Under-Sintered vs. Normal Product
| Identification Method | Over-Sintered | Under-Sintered | Normal Product |
|---|---|---|---|
| Surface Color | Deep brown/blackened | Chalky white/uneven | Uniform light brown |
| White Powder Rub Test | No white powder | Obvious white powder | No white powder |
| Water Contact Angle | > 95° (slightly reduced) | < 90° | > 100° |
| Peel Strength | Reduced (1.5–2.5 N/25mm) | Significantly reduced (< 1.5 N/25mm) | ≥ 3.0 N/25mm |
| DSC Melting Peak Temp | < 325°C | Normal range but peak area small | 326–328°C |
| TGA Td5% | < 310°C | Normal but has low-temp decomposition peaks | ≥ 320°C |
| EVA Non-Stick Peel Force | > 5 N/25mm | > 8 N/25mm (extremely poor) | ≤ 3 N/25mm |
| Fold Cracking | Visible cracking | Obvious cracking | No cracking |
| SEM Surface Morphology | Irregular PTFE accumulation | Unfused spherical particles | Dense uniform coating |
Ⅳ. Field Rapid Screening Process (3-Step Quick Assessment)
Step 1: Visual Inspection (30 seconds)
- Color abnormal (too dark or chalky white)? → Suspected over/under-sintering → proceed to Step 2
- Surface has visible flow marks, drips, or pinhole voids? → High probability of sintering defect
Step 2: White Powder Rub + Water Drop Test (2 minutes)
- White powder on cloth → under-sintering confirmed
- Water contact angle < 90° → under-sintering confirmed
- No white powder + contact angle > 100°, but color still abnormal → suspect over-sintering → proceed to Step 3
Step 3: Fold Test + Peel Test (5 minutes)
- 90° fold: coating cracking + peel strength < 2.0 N/25mm → over/under-sintering defective product confirmed → reject
- All three steps normal → preliminary pass; send to laboratory for DSC/TGA verification if high-precision application
