Suspension Polymerization Process for Producing Polytetrafluoroethylene (PTFE)

Suspension Polymerization Process for Producing Polytetrafluoroethylene (PTFE)

I. Specific Process for Suspension Polymerization of PTFE

1. Process Overview

Suspension polymerization involves polymerizing tetrafluoroethylene (TFE) in an aqueous medium using a free radical initiator to form PTFE particles. The final product is a white granular resin (medium or fine particles), requiring drying and post-processing for applications such as compression molding.

2. Process Steps

(1) Raw Material Preparation

• Monomer:
  • Tetrafluoroethylene (TFE, C₂F₄), high purity (>99.9%), with inhibitors (e.g., terpenes) removed.
• Medium:
  • Deionized water, free of impurities (e.g., metal ions).
• Initiator:
  • Persulfates (e.g., ammonium persulfate, potassium persulfate) or organic peroxides.
• Additives (Optional):
  • Dispersion stabilizers (e.g., fluoride salts such as NH₄PF₆).
  • Buffers (e.g., sodium bicarbonate to maintain pH 6–8).

(2) Reactor Preparation

• Use a stainless steel high-pressure reactor with stirring, temperature control, and pressure monitoring systems.
• Purge the reactor with nitrogen to remove oxygen and minimize explosion risk.

(3) Polymerization Reaction

• Charging: Add deionized water, initiator, and additives into the reactor.
• Monomer Introduction: Introduce TFE gas slowly under 1.5–3.0 MPa pressure.
• Reaction Conditions:
  • Temperature: 50–90°C (typically 70–80°C).
  • Stirring Speed: 200–500 rpm.
  • Duration: 4–12 hours (depends on target molecular weight).

(4) Reaction Termination

• Stop monomer feed when polymerization is complete (judged by pressure drop).
• Vent excess gas and cool the reactor.

(5) Post-Processing

• Separation: Filter or centrifuge to isolate PTFE particles.
• Washing: Rinse with deionized water to remove residual initiators.
• Drying: Dry at 100–150°C (avoid temperatures >260°C to prevent decomposition).
• Grinding/Sieving: Process into medium (20–300 μm) or fine particles.

3. Process Parameter Reference

II. Key Chemicals

1. Tetrafluoroethylene (TFE, C₂F₄)

• Role: Sole monomer for PTFE synthesis.
• Properties:
  • Colorless gas, boiling point -76.3°C.
  • Highly reactive and flammable.
• Requirements:
  • Purity >99.9%, stored in inhibitor-containing cylinders to prevent polymerization.

2. Initiators

• Common Types:
  • Ammonium persulfate (APS): Water-soluble, generates radicals via thermal decomposition.
  • Potassium persulfate: Stable for high-temperature reactions.
  • Organic peroxides (e.g., benzoyl peroxide).
• Dosage: 0.01–0.1% of monomer mass.

3. Water Medium

• Requirements: Deionized water with conductivity <1 μS/cm to avoid contamination.

4. Additives (Optional)

• Dispersion Stabilizers:
  • Fluoride salts (e.g., NH₄PF₆) for particle uniformity.
• Buffers:
  • Sodium bicarbonate to maintain pH 6–8.

III. Precautions

1. Safety Measures

• TFE Explosion Risk:
  • Purge oxygen with nitrogen.
  • Avoid sparks or high temperatures (>25°C).
• Toxic Gas Release:
  • Decomposition above 260°C may release perfluoroisobutylene (PFIB), a highly toxic gas.
  • Operate in ventilated areas with exhaust treatment.
• Personal Protective Equipment (PPE):
  • Protective clothing, masks, and gloves required for operators.

2. Process Control

• Temperature Control:
  • <50°C: Slow reaction.
  • >90°C: Reduced molecular weight.
• Pressure Control:
  • >3.5 MPa: Equipment risk.
  • <1.5 MPa: Causes uneven dispersion.
• Stirring Speed:
  • Inadequate stirring: Leads to particle agglomeration.
  • Excessive stirring: Air ingress may inhibit polymerization.

3. Product Quality Control

• Particle Size: Adjust stirring speed and initiator concentration.
• Residual Initiator Removal: Ensure thorough washing.
• Moisture Content: Ensure complete drying to prevent molding defects.

4. Waste Management

• Wastewater: Neutralize before discharge.
• Exhaust Gas: Recover unreacted TFE via condensation or incineration.

IV. Chemical Reaction Principle

1. Initiation

(NH4)2S2O8→2SO4−⋅+2NH4+(NH₄)₂S₂O₈ → 2SO₄⁻cdot + 2NH₄⁺

(Persulfate decomposes into sulfate radicals)

2. Chain Propagation

R⋅+nCF2=CF2→R−(CF2−CF2)n⋅Rcdot + nCF₂=CF₂ → R-(CF₂-CF₂)ₙcdot

(Radicals react with TFE monomers, forming a growing polymer chain)

3. Chain Termination

R−(CF2−CF2)n⋅+R−(CF2−CF2)m⋅→R−(CF2−CF2)n+m−RR-(CF₂-CF₂)ₙcdot + R-(CF₂-CF₂)ₘcdot → R-(CF₂-CF₂)ₙ₊ₘ-R

(Polymer chains combine, forming high-molecular-weight PTFE).

4. Final Product

• High-molecular-weight PTFE (10⁶–10⁷ g/mol).