Hot Melt Adhesives

Hot melt adhesive (HMA), also known as hot glues, are generally 100% solids formulations based on thermoplastic resins.

They are solid at room temperature and can be softened, reshaped, and dispensed upon heating above their softening point. Solidification takes only seconds after dispensing, or in other words, these thermoplastic adhesives reach full strength once they cool to room temperature.

This minimizes clamp time and allows for rapid assembly. Hot melt adhesives are also easy to clean with low to no toxicity and the application and bonding can easily be automatized. Furthermore, they have excellent gap filling capabilities and bond a wide variety of porous and non-porous substrates including pre-painted steel and polyolefin plastics.

Hot melts can be applied by extruding, rolling, or spraying the adhesive and manual application is usually done with hand held electrical hot glue guns.

Because traditional hot melts are thermoplastics they can be repeatedly melt and cooled to solidify. Since these adhesives loose their ability to bond in the liquid state, joint parts can be thermally detached and re-bonded. Hot melt adhesives usually have long shelf-life and most can be disposed of without special precautions.

However, traditional hot melts have several limitations; they are usually temperature sensitive, that is, they soften at elevated temperatures and consequently loose strength and become brittle at low temperatures.

They are also susceptible to creep which can cause joint failure when exposed to (noticeable) stress. Most hot melts are applied quite hot at temperatures between 100 to 230°C (212 to 450°F), and therefore, most cannot be used on temperature sensitive substrates. Another draw back is the short open time, which is the time before the liquid adhesive solidifies.

Traditional and high performance structural hot melts are formulated with many (advanced) thermoplastics, including ethyl vinyl acetate (EVA) for general purpose bonding, polyolefin for difficult-to-bond plastics, styrene block copolymers (SBC) for good low temperature flexibility, high elongation and higher heat resistance, polyamides for severe environments, and reactive urethanes and silicones for elevated temperature and/or high flexibility requirements.

These thermoplastics are blended with a variety of other raw materials including tackifying resins, waxes, plasticizers, fillers, and antioxidants, which are incorporated into the base hot melt resins to enhance adhesive performance.

Natural or synthetic tackifiers are usually added to some formulations to alter or improve adhesion (tack), surface wetting, open time, and polymer flexibility of the adhesive. Waxes help to reduce pellet blocking, and reduce melt viscosity, and/or enhance the tack of the adhesive. Antioxidants are used to prevent/minimize oxidation and decomposition of the hot melt and to aid in processing and to improve storage stability.

In the last two decades, new structural hot melts have been developed that overcome some of the drawbacks and limitations of traditional hot melts. These new adhesives have been designed to reduce manufacturing cost and to improve manufacturing efficiency.

Hot melts are used in many different industries for a wide variety of applications. They are capable of bonding many different substrates including rubbers, plastics, metals, ceramics, glass and wood. The packaging industry is one of the major users. Hot melts are used for carton sealing and assembly of corrugated boxes and paperboard cartons, and for labeling applications.

Other uses include shoe-making (bonding toecaps and shoe soles), disposable diapers and sanitary napkins bonding, bookbinding, manufacture of non-woven materials, and various laminating applications in the furniture industry (veneer surrounds and edging). The automotive industry employs hot melts to assemble various products including bonding of carpeting and seat covers. The electronics industry makes also use of hot melts, for example to bond coil windings and coil ends.

All information from

http://polymerdatabase.com/Adhesives/Hot%20Melt%20Adhesives.html