PP, PE or PVDF: Picking a Thermoplastic for Acid Service
Thermoplastic reactors and tanks are cheap, corrosion-resistant and easy to fabricate — until the temperature or the oxidizer exceeds what the polymer can take. The failure is usually creep or stress cracking, not bulk chemical attack, and it happens at flanges and welds.
Where each polymer sits
| Polymer | Continuous temp | Strong acids | Oxidizers | Weak point |
|---|---|---|---|---|
| PE (HDPE) | ~60 °C | Good (HCl, dilute H₂SO₄) | Poor | Low temperature ceiling; creep |
| PP | ~90–100 °C | Good (HCl, H₂SO₄, HF) | Poor | Stress cracking; embrittles when cold |
| PVDF | ~130–140 °C | Good | Moderate | Cost; permeation of some solvents |
Temperature is the real limit
All three soften progressively. A PP vessel rated for 100 °C will creep under a flange load long before it shows visible chemical attack. If your process runs at 85–95 °C, specify PP with a prudent wall thickness and support the vessel properly; if it runs above 100 °C, move to PVDF or a lined steel vessel.
Stress cracking is the silent failure
Moulded-in stress, poor weld beads and residual solvents combine to crack PP under load — often months after commissioning, with no warning. Design with generous radii, anneal welded fabrications where possible, and avoid hanging heavy agitators from a plastic head.
When to leave thermoplastics
Hot nitric acid, hypochlorite, chromic acid and other strong oxidizers attack PP and PE. If your liquor contains them, use PVDF, PTFE-lined steel, or glass-lined steel. Do not accept a supplier’s assurance that "PP handles acids" without the specific oxidizer and temperature.
Send the liquor composition, temperature and pressure, and we will confirm the polymer and the flange/gasket package — the joint usually fails first.
Joining and repair usually decide the choice
The polymer data sheet compares chemical resistance, but what determines whether a vessel survives five years is how it is joined and whether that joint can be repaired on site. PP and PE are hot-gas welded and the weld is 70 to 80 per cent as strong as the parent material; PVDF welds to near-parent strength but needs controlled temperature and a clean environment, which is difficult in a plant workshop. A material that cannot be repaired locally becomes a lead-time item the first time it is damaged.
Stress relief matters more than most specifications admit. Welded thermoplastic vessels carry residual stress at the seam, and in chloride or oxidising service that is exactly where environmental stress cracking starts, often within months. Annealing after fabrication, generous fillets instead of sharp corners, and supports that do not point-load the shell are worth more than a step up in resin grade. The best-value decision in acid service is frequently a lower-cost polymer with a better joint detail, not a higher-cost polymer with the same weld.
Equipment for this step
Tell us your feed assay, flow rate and target recovery and we will size the unit: request a quote.