Лако-красочные материалы — производство

Hot melts were introduced into the market many years ago and are widely used in the packaging, labelling, woodworking and bookbinding industries but are also used for bonding plastics. Hot melts are 100% solid single-component adhesives that are applied in a molten state and solidify by cooling. They do not contain any carrier material (e. g., water or solvent) and so can offer extremely fast setting times and excellent gap-filling capabilities [6]. Hot melts are available in a variety of forms including tapes, films, pellets, cylinders, cubes and blocks. For small part bonding, they are usually dispensed in ‘stick’ form via a heated hand-gun or in bulk from semi­automatic or automatic application systems.

The two most important characteristics of hot melts are their open time and green strength. The open time is the time that the adhesive remains fluid enough after dispensing the adhesive before it will no longer wet the mating surface and thus create an effective bond. The green strength is a measure of the rate of cure and is often also referred to as the handling strength. These two factors are inexorably related as the longer the adhesive is left ‘open’ on a surface, the thicker it gets and so its ability to wet the surface reduces, thus leading to a drop in bond strength.

Hot melts are easy-to-use, medium-strength adhesives and can be made rigid or flexible. They are thermoplastic materials and so typically have a maximum operating temperature of 90 °C but higher temperature grades are available (see Section 1.6.1).

Hot melts are often used where the bond area is large as they can be difficult to dispense onto small components where the width of the bond is less than 5 mm, as the rheology of these products is such that they have a tendency to ‘string’ during the dispensing cycle. On larger parts, however, where there is a bigger bonded area and more scope for accommodating less precise dispensing criteria, they invariably show faster curing times than cyanoacrylates and certainly considerably improved gap-filling capabilities. One concern for hot melts is the relatively high application temperature (160-180 °C) as this can damage heat sensitive components and incur health and safety issues.