6.7 Hotmelts

Hotmelt adhesives are 1C solvent-free thermoplastic adhesives that are solid at room temperature and have a low to medium viscosity (750 to 10,000 mPa.s) at dispensing temperatures - typically greater than 195°C (383°F). After dispensing, hotmelt adhesives rapidly cool to form a strong bond. In cooled states, hotmelt adhesives can vary in physical properties from soft, rubbery and tacky to hard and rigid. Hotmelts have excellent long-term durability and resistance to moisture, chemicals, oils and temperature extremes.

The physical forms of hotmelts are blocks, granules, candles and sticks. Hotmelts are applied by a melting machine and they set to a solid state on cooling to achieve the bonding strength. Hotmelts are usually 1C adhesives, which contain 100% solid thermoplastics. When hotmelt adhesives are used, factors such as softening point, melt viscosity, melt index, crystallinity, tack, heat capacity and heat stability must be considered in addition to the usual physical and adhesive strength properties.

Hotmelt types include pressure sensitive adhesive (PSA), non-PSA and reactive polyurethane hotmelt (PUR).

Figure 190 Hotmelt bonding process

General Characteristics

A feature of hotmelts is that on cooling, they very rapidly build up their internal strength. A natural hotmelt familiar to everyone is beeswax, which bees use as a building material. Bond strengths between 1.5 and 3.5 N/mm2 (218-508 psi) can be achieved with industrial hotmelts. These adhesives can be used to create thermally detachable and re-attachable bonded joints due to their adhesive characteristics.

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