Hot melt is an adhesive, with 100% solids. It’s applied in molten state and becomes stronger with cooling down to room temperature. Because of the high difference between application temperature, common around 160°C, and the room temperature, 20°C, hot melt adhesives are quick setting.
Using this feature, it’s important to know about critical properties and parameters of hot melt adhesives.
STANDARD HOT MELT ADHESIVES ARE THERMOPLASTIC
Most hot melt adhesives are EVA or PO (Polyolefine, Metallocene) based. In most formulations, these adhesives contain a considerable amount of tackifier resins and waxes. Tackifier resins will improve adhesion and the green strenght of an adhesive. Waxes are mostly added to decrease viscosity for application purposes, and make the final hot melt quicker setting.
Some other thermpoplastic grades may be PolyAmide or PolyEsther hot melt adhesives. This typical grades are usually not a blend of polymers.
REACTIVE HOT MELT ADHESIVES (PUR)
PUR hot melt adhesive combine the quick setting o hot melt adhesive with the durable bond of a curing adhesive. Most reactive hot melt adhesives are based on PUR (Polyurethane). Polyurethane adhesives are not a typical blend, but more a polymer with some minor additives as adhesion promotors, anti oxidants, UV Stabilizers and flame retardants. These additives may add value to the bond where already a PUR hot melt apparently was needed to create a more durable bond. A special group of reactive hot melt adhesives are the POR hot melt adhesives.
TEMPERATURE INFLUENCES AND SETTINGS
Temperature is a very important parameter for hot melt adhesives. The temperature of the hot melt adhesive itself is important to make a good application and machine runnability. The temperature of the substrate may be to high, resulting in slow setting speed, or to low, which may result in a poor bond. In addition, bonds realized by hot melt adhesives may be influenced by high or low temperatures. At high temperatures bonds may become more flexible and finally even fail. For thermoplastic hot melt adhesive critical temperatures may vary typically from +40°C to +90°C, of course depending on the hot melt grade used. Critical temperatures may also be faced from -40°C up tot +5°C, because hot melt adhesives may lose flexibility and therefore easy crack and loose adhesion to substrates.
TYPICAL PARAMATERS OF HOT MELT ADHESIVES
Most typical parameters of hot melt adhesives are it’s melting point and viscosity. These are standard parameters mentioned on datasheets. Other specific properties are open time, setting speed and adhesion strength, but these values depend on the substrates and application environment.