Hotmelt adhesives are thermoplastic. This means that they become liquid at higher temperatures, returning to a solid state at lower temperatures. When liquid, a measure of the ease with which adhesive flows is its viscosity, and this decreases as the temperature rises. The viscosity of an adhesive is measured using a viscometer and is usually quoted in ‘Poise’ or ‘Centipoise’ (cps).
A lower viscosity adhesive will flow easily, increase glue gun output, reduce stringing, and form stronger bonds. However, they are unsuitable for very porous surfaces, such as foams and fabrics.
A higher viscosity adhesive will be suitable for bonding coarse or uneven surfaces, and for applications where good gap filling and low sag properties are needed. However, higher viscosity adhesives can reduce the output of glue guns, so a more powerful glue gun may need to be specified.
FOSPAT adhesives are designed to run at different temperatures, so we can broadly classify the viscosity as High, Medium or Low, as follows:
| Low Viscosity: 500 – 3000 cps Medium Viscosity: 3000 – 6000 cps High Viscosity: 6000 – 15000 cps |