PFA (perfluoroalkoxy) heater tubes are manufactured by extrusion. The cooling rate after extrusion determines the degree of crystallinity. Water quenching produces amorphous PFA with low crystallinity (10-20%), while air cooling produces semi-crystalline PFA with high crystallinity (40-50%). In 98% sulfuric acid at 120°C, amorphous PFA has higher resistance to environmental stress cracking because the amorphous structure is more flexible and can absorb stress without cracking. Water-quenched PFA has 3-5 times longer stress crack initiation time than air-cooled PFA.

The Mechanism of Crystallinity Influence on Stress Cracking

Semi-crystalline PFA has lamellar crystals that act as stress concentration points. Under applied or residual stress in aggressive environments, cracks initiate at crystal boundaries. Amorphous PFA has no such crystalline regions, so stress is distributed more evenly. The crack propagation rate follows: da/dt ∝ (crystallinity)^1.5. Higher crystallinity leads to faster cracking.

Quantitative Stress Crack Resistance as a Function of Cooling Rate

Controlled testing in 98% H₂SO₄ at 120°C at 50% of yield stress has established the following for PFA. For water-quenched PFA (crystallinity 10-20%, amorphous), the stress crack initiation time is 1,000-2,000 hours, and the cracking is minimal. For air-cooled PFA (crystallinity 40-50%, semi-crystalline), the initiation time is 200-400 hours, and cracking is severe. For slow-cooled PFA (crystallinity 50-60%), the initiation time is 50-150 hours, and cracking is rapid. For annealed PFA (crystallinity 60-70%), the initiation time is <50 hours, and cracking is immediate.

Cooling Rate and Crystallinity Selection Guide

Making an Informed Specification

For a PFA heater tube in 98% sulfuric acid at 120°C, specify water quenching after extrusion to achieve low crystallinity (10-20%). By controlling the cooling rate, the engineer maximizes resistance to environmental stress cracking.