What are hot melt adhesives made of?
- Hot melt adhesives contain no solvents and no water.
- They set by loss of heat. This gives them a very fast setting speed.
- They can bond two non-porous materials.
- Polymers give the adhesive strength and a degree of flexibility.
- Resins give the immediate hot-tack of the adhesives.
- Wax thins the adhesive so that it can be applied.
Antioxidants help preserve the adhesive at high temperatures.
How do hot-melts work?
- Hot-melts are applied to a material in the liquid (molten) state.
- The time between applying the adhesive and then bringing the second material in contact is called open time.
- When the second material is brought into contact, the adhesive cools down and solidifies very quickly.
- The compression of materials is very important at this stage.
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.
- 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.