Collection: Sealant Adhesive

A polymer adhesive sealant performs two jobs at once: it bonds the parts together and it seals the joint. Unlike a rigid adhesive it stays flexible once cured and absorbs movement, which makes it essential whenever the two materials being joined do not expand at the same rate.

The 15 references in this range

• Polyurethane: Sikaflex 221, 223 and 252, 3M 540 sealant. See our polyurethane sealants.
• MS polymer hybrid: Sikaflex 268 and 521, STP Sikaflex 521, Stik MS60, 3M 760 and 3M 780. See our hybrid sealants.
• Silicone: Sikasil SG-500. See our silicone sealants.
• Butyl: SikaLastomer 710 TX and preformed 831E. See our butyl sealants.
• Flooring: Sikaflex PRO-3 SL, self-levelling.

Polyurethane, hybrid or silicone

Polyurethane is the strongest mechanically and the most abrasion resistant. It can be painted. It does yellow under UV and contains isocyanates, which calls for ventilation at the workstation.

MS polymer hybrid contains neither isocyanate nor solvent, does not yellow, can be painted and adheres to a very wide range of substrates without primer. Its final mechanical strength stays below that of a structural polyurethane and its price per litre is higher.

Silicone holds over a very wide temperature range and resists UV perfectly. But it cannot be painted, and above all it leaves a silicone trace on the substrate that nothing will adhere to afterwards, paint included. That point alone rules it out on a part meant to be reworked.

Butyl never fully hardens. It remains a permanent sealing product for joints that move, not a structural adhesive.

Sizing the joint

A sealant bead should be neither too thin nor pressed hard against the back of the joint. Two rules cover most cases.

Minimum thickness governs the ability to absorb movement: a flexible joint squeezed below two millimetres has no material left to deform and tears at the first expansion.

The width to depth ratio should stay around two to one. A joint that is too deep works badly, because the material at the back resists deformation instead of taking part in it. That is what a backer rod is for.

When to use a primer

On a porous substrate, on glass, on baked paint or on most plastics, an adhesion promoter changes the order of magnitude of the bond. Respect the stated flash-off time: too short and solvent stays trapped under the joint; too long and the active layer stops being reactive.