KnowHOW team explains: Glue is something that joins two different materials by surface attachments. Adhesion is most effective when the adhesive is in intimate contact with the adherents.

This occurs when the adhesive makes the adherents completely wet. For this, it must be a liquid at some point of formation of the bond between the two.

Once the adhesive is in contact with the substrate and has at some point been a liquid in its sandwich position, it must pass through a transition to become a tough layer.

Glues are suitable for porous substrates. Commercially they are available in the liquid form and are packed in airtight containers to prevent premature hardening. After the adherents are made wet, the adhesives are transformed from the liquid to the solid state by simple evaporation of the solvent through the porous substrate. This causes the glue to harden through exposure to air.

There are other methods for preparing glues as well. Adhesive compounds like epoxy-amines and isocyanate-polyols are used after heating a surface and they harden when the surface gets cold again. But it is pressure that does the trick for the glue in gum tapes and it hardens when the pressure is gone.

The question was sent by Anindita Sengupta from Gariahat