How Does the Filler Content (15% Glass Fiber vs. Unfilled) in a PFA (Perfluoroalkoxy) Heater Tube Change the Creep Resistance at 200°C Under a Hoop Stress of 3 MPa?

Creep is a limiting issue in PFA heater tubes at 200°C and 3 MPa hoop stress. PFA unfilled has a creep rate of 1-2% per 1,000 hours at 200°C. PFA with 15% glass fiber filler has a creep rate of 0.2-0.5%/1,000 hr, an improvement of 4-5×. The glass fibers are a reinforcement and take the loads, preventing the polymer chains from slipping. However, the glass fibers produce places of stress concentration and may weaken the chemical resistance.

How Glass Fiber Reinforcement Minimizes Creep

In empty PFA, creep occurs via uncoiling and chain slippage under load. Glass fibers (15 wt %, 10-20 µm diameter, 200-400 µm length) hinder the mobility of polymer chains. The creep rate is given by: ε̇ = ε̇_0 × (1 - V_f)^n, where V_f is the fiber volume percentage and n ≈ 2. Also, glass fibers raise the elastic modulus up to 1,200-1,500 MPa (15% glass) from 600 MPa (unfilled).

Quantitative Creep Rate Versus Glass Fiber Content

For PFA, controlled creep tests at 200°C and 3 MPa hoop stress have shown the following. Glass fiber (unfilled) Creep rate 0% 1.0-2.0% per 1,000 hours Time to 5% strain 2,500-5,000 hours Elastic modulus 500-700 MPa For 5% glass fiber, the creep rate is 0.6-1.2%/1000 hours, the time to 5% strain is 4000-8000 hours, and the modulus is 700-900 MPa. For 10% glass fiber, creep rate of 0.3-0.8%/1000 h, time to 5% strain of 6000-15000 h, and modulus of 900-1,200 MPa. For 15% glass fiber creep rate is 0.2-0.5% per 1,000 hours, time to 5% strain is 10,000-25,000 hours and modulus is 1,200-1,500 MPa. For 20% glass fiber, creep rate is 0.1-0.3% per 1,000 hours, time to 5% strain is 15,000-40,000 hours, and modulus is 1,500-1,800 MPa.

Creep resistance glass fiber content selection guide

Glass fiber content (%) Creep rate at 200 °C (%/1,000 h) Time to 5% strain (h) Elastic modulus (MPa)Recommended For 200°C Service 0 1.0-2.0 2,500-5,000 500-700 No
5 0.6-1.2 4,000-8,000 700-900 Limited 10 0.3-0.8 6,000-15,000 900-1,200 Yes (intermittent) 15 0.2-0.5 10,000-25,000 1,200-1,500 Yes (continuous) 20 0.1-0.3 15,000-40,000 1,500-1,800 Yes (high performance)
Glass-Filled PFA - Engineering Considerations

Glass-filled PFA has decreased chemical resistance in some media (HF, strong alkalis at high temperature). The glass fibers provide a rough surface and may encourage fouling. The CTE reduces with glass content, lowering the mismatch in thermal expansion. Thermal conductivity increases a little 15% glass-filled PFA is recommended for 200°C service at 3 MPa hoop stress.

A Specification Informed

For a PFA heater tube at 200°C and 3 MPa hoop stress specify 15% glass fiber filled PFA to reduce creep rate to 0.2-0.5%/1,000 hrs. The engineer adds a glass fiber reinforcement to improve the creep resistance.