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About this tool
Ranks glues and sealants for the two materials you are joining, checking gap fill, water exposure, movement, load and surface energy.
Scores twelve adhesive and sealant families against the specific joint you are making: both substrates, the gap between them, whether it gets wet, whether it moves, whether it carries load and whether it has to be painted. The reasoning follows the three things a bond actually needs — the adhesive must wet the surface (substrates below about 34 mN/m surface energy, like polypropylene at 29 and PTFE at 18, will not accept an ordinary glue), it must suit the gap, and it must tolerate the movement. Strengths quoted are typical lap-shear figures for comparison between families.
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Not just a recommendation — the runners-up appear with the specific reason each falls short.
Flags the plastics no ordinary adhesive can wet before you waste a tube on them.
Warns when a joint is too wide for the glue and when it is too tight for a sealant bead.
Neither takes an ordinary adhesive, because their surface energy is around 29 and 31 mN/m, below the roughly 34 mN/m a glue needs to wet a surface. The practical routes are cyanoacrylate with a polyolefin primer, or a structural acrylic made for low surface energy plastics. Industrially the surface is flame or plasma treated first to raise its energy.
Two-part epoxy, at typical lap-shear values around 20 MPa on prepared steel — far ahead of anything else in a household cupboard. Preparation matters as much as the product: abrade, degrease and bond promptly, especially on aluminium, where the oxide layer reforms within minutes of sanding.
Silicone for anything wet or moving, acrylic only where it will be painted and stays dry. Neutral-cure silicone handles about ±25% joint movement and is waterproof but cannot be painted; acrylic decorator's caulk takes paint but manages under ±10% movement and will shrink and go mouldy in a bathroom within a season.
Usually gap, movement or wetting rather than strength. Super glue and PVA need a fit inside about 0.1 mm; a rigid adhesive across two materials with different expansion rates shears itself apart over a temperature cycle; and any oily, dusty or low-energy surface stops the adhesive making contact at all. Abrading and degreasing fixes more failed joints than upgrading the glue does.