What affects adhesion? As hot melt adhesives set by the loss of heat, the temperature at which they are applied is critical. It must be liquid (molten) enough to wet out the substrates, and be hot enough for assembly before cooling. If the adhesive is not hot enough (in the molten stage), a bond may be made which will fail at a later stage (cold bond). On the other hand, if the adhesive is applied at too high a temperature, it will remain molten for too long, and may come out of the compression stage still soft. Any stress on the bond will then cause failure immediately or produce a weakened bond that will fail at a later stage. Application Tips • Materials being bonded must be clean, dry and free of dust, oil and grease. • Hot melt adhesive must be applied at the correct temperature. • The temperature of the materials and surroundings can affect adhesion: • Too hot and the adhesive takes too long to set • Too cold and the adhesive sets before materials are bonded. • The amount of adhesive applied will greatly affect the bond: • Too little and the adhesive will cool more quickly, perhaps before the bond has been made properly • Too much adhesive prevents the bond cooling down before coming out of compression, allowing it to come apart.
