Hot melts are thermoplastics that are used after melting the polymer beyond its melting point. Hot melts comprise of three components:- a polymer/ blend of polymers
, a tackyfying resin
. Wax and oil are used for the same purpose, the only difference is that oil is liquid and waxes are solid. Apart from these ingredients, hot melts may include an antioxidant
or UV stabilizer
or a pigment and others.
The hot melt should have very low
viscosity in its molten form in order to achieve wetting. Also it must not
cool too rapidly or it will not have time to completely wet the substrate.
Special dispensing equipments are used for the application of hot melt adhesives.
Working of hot melts
Step 1: A hot melt adhesive when heated, melts and becomes a liquid. It is then applied by bringing it between two substrates. Once cooled, it will set and solidify. Cooling of the drop largely depends on the size of the droplet, the nature and temperature of the substrate. In case the substrate is not insulated, it may take a lot of time for the hot melt to cool down.
For e.g. if the hot melt is applied on a cold surface, the hot melt would cool faster, but the adhesion imparted would be weak as the adhesive will not get enough time to fill in the cavities of the substrate. On the contrary if the hot melt was applied on a pre heated surface and then allowed to cool gradually along with the substrate, the adhesion would be stronger.
The second substrate is applied to the adhesive, in order to join the two substrates with the help of hot melt applied between them.
The second substrate is pressed, and one can see the adhesive oozing out, in case the contact surface with the substrate is large. The large surface area
of the substrate allows for quicker cooling of the hot melt
. This hot melt after cooling sets, making it impossible to separate the two substrates.
The horizontal access describes the temperature, which decreases with time. The relationship between temperature and time is never the same.
The vertical axis exhibits the viscosity or the cohesion [modulus] for a period when hot melt is liquid and cohesion for the later stages of the hot melt.