Which glue works on which plastic?

Almost every failed plastic repair comes from choosing the adhesive before identifying the plastic. Plastics differ enormously in whether an adhesive can wet the surface at all, and no amount of clamping, roughening or expensive glue overcomes a surface that chemically will not accept one.

Safety

What you'll need

• Whatever the item is, with its recycling mark located
• Abrasive paper, around 120 grit
• Isopropyl alcohol or acetone
• Clamps or tape
• Disposable gloves

Step by step

1. Identify the plastic before anything else.Look for a triangle with a number and letters inside, usually moulded on the underside or inside a lid: PP, PE, HDPE, LDPE, PS, PVC, ABS, PC, PA for nylon, POM for acetal, PMMA for acrylic. Two minutes here decides whether the repair is possible.
2. For ABS, use solvent cement or superglue.ABS is the hard plastic of appliance housings, toys, pipe fittings and many car interior parts. ABS solvent cement dissolves both faces so they fuse into one piece — the strongest possible repair. Superglue works acceptably for small rigid parts, and epoxy gives a reasonable mechanical bond with a keyed surface.
3. For polystyrene, use polystyrene cement.The classic model kit cement. Again it welds rather than glues. Superglue also works but is more brittle and leaves a white bloom on clear parts.
4. For PVC, use PVC solvent cement.Rigid PVC pipe and fittings, and many rigid mouldings. This is a genuine chemical weld and is why plumbing joints made this way do not leak. Flexible PVC also accepts a flexible polymer adhesive.
5. For acrylic, use acrylic solvent cement.Thin, water-like solvent applied along a close-fitting joint by capillary action. It produces an almost invisible weld. Superglue works but fogs clear acrylic, and epoxy leaves a visible line.
6. Accept that polycarbonate, nylon and acetal are difficult.Polycarbonate is attacked by many solvents and can craze and crack later. Nylon and acetal are chemically resistant and very low friction, so adhesives struggle to grip. Expect a mechanical bond from epoxy or a flexible polymer adhesive at best, and design in a screw or a rivet if the joint carries load.
7. Recognise polypropylene and polyethylene and stop.PP and PE — food tubs, buckets, bottle crates, garden furniture, most bumper trims, many toys — have surface energy too low for an adhesive to wet. Nothing ordinary sticks to them. The only real options are a two-part polyolefin adhesive sold with a primer pen, a mechanical fixing, plastic welding with a hot air tool and matching rod, or accepting the item is not repairable.
8. Prepare the surfaces properly whatever you use.Degrease with isopropyl alcohol, key with abrasive paper until the shine is gone, and remove the dust. Mould release agent left on a new moulding will defeat any adhesive.
9. Make the joint fit before you open anything.Dry fit and clamp. Adhesive is not a gap filler; a gap-filling epoxy is far weaker across a gap than a thin bond line in a close-fitting joint.
10. Clamp, and honour the full cure time.Handling strength arrives long before full strength. Epoxy typically reaches full cure in 24 hours and much longer in a cold room. Solvent cement takes a day or more to reach full strength as the solvent leaves the joint.
11. Reinforce anything that carries load.A glued butt joint in thin plastic is weak whatever the adhesive. Add a backing plate of the same plastic bonded behind the break, or embed a piece of fine metal mesh in epoxy across the back.
12. Be honest about what a repair can be.A solvent-welded joint in a matched plastic can be as strong as the original. Every other plastic repair is weaker than the part was, and a repair on a load-bearing or safety-related part — a helmet, a ladder, a child's car seat, a pressurised fitting — should not be attempted at all.

Tips

• The recycling triangle is the single most useful thing on the item. Look for it before opening any tube.
• If there is no marking, a scrap test helps: PP and PE feel waxy and float in water, and a hidden test spot of solvent cement will not soften them at all.
• Solvent cements are plastic-specific. ABS cement will not weld PVC and vice versa.

Common mistakes

• Reaching for superglue for everything — Cyanoacrylate is brittle, has almost no gap-filling ability, and bonds poorly to several common plastics. On anything that flexes or carries load it cracks, usually within days, and it is the wrong product for the plastics where solvent welding would have made a permanent repair.
• Persisting with a polypropylene item — PP and PE cannot be wetted by ordinary adhesives, so no amount of preparation, clamping or expensive glue will hold. Recognising the plastic and either welding it, screwing it or replacing it saves an afternoon.

If it doesn't work

Questions people ask