Hotmelt adhesives (HMA), also known as hot glues, are 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 typically takes only a few seconds after dispensing and once cooled to room temperature, these thermoplastic adhesives reach full strength. 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 a variety of methods including extruding, rolling, or spraying the adhesive. Manual application is usually done with hand held electrical hot glue guns. The adhesive is supplied in solid form as cylindrical glue sticks that are melted in the gun. Joining of the parts is carried out either immediately after application or after reheating the solidified adhesive beads or layers. 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.
Traditional hot melts have several limitations; they are temperature sensitive, that is, they soften at elevated temperatures and consequently loose strength and at low temperatures, they become brittle. They are also susceptible to creep which can cause joint failure when exposed to stress. Most hot melt adhesives are applied quite hot at temperatures between 100 to 230°C (212 to 450°F), and therefore, cannot be used on temperature sensitive substrates. Another draw back is the short open time, which is the time before the liquid adhesive solidifies.
Hot melts are formulated with numerous thermoplastics, including ethyl vinyl acetate (EVA) for general purpose bonding, polyolefins for difficult-to-bond plastics, styrene block copolymers (SBC) for good low temperature flexibility, high elongation and improved heat resistance, polyamides for severe environments, and reactive urethanes and silicones for elevated temperature and/or high flexibility requirements. These thermoplastics are typically blended with a variety of other raw materials including tackifying resins, waxes, plasticizers, oils, 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 during service and during 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. The newest classes of hot melts are reactive urethanes and silicones. These types of hot melts combine the advantages of traditional hot melts with those of reactive structural adhesives. They undergo a secondary moisture cure upon cooling or are cured by ultraviolet radiation that causes the adhesive to crosslink which provides higher strength and flexibility and results in more durable bonds.
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 commodity uses include shoe-making (bonding toecaps and shoe soles), disposable diapers and sanitary napkins bonding, and bookbinding. Hot melts are also extensively used in the textile industry for the manufacture of non-woven materials, and in the furniture industry for various laminating applications such as veneer surrounds and edging. The automotive industry employs hot melts to assemble various products including bonding of carpeting and seat covers and the electronics industry uses hot melts to bond coil windings and coil ends among many other applications.