Thermoplastic adhesives

Full information on thermoplastic hot-melt adhesives

A thermoplastic adhesive becomes deformable or flexible at relatively high temperatures and melts when it is heated (application). It hardens into a vitreous transition state when it cools sufficiently (gluing). These are generally polymers with a high molecular weight, which have strong molecular bonds. After initial cooling and solidification, it can be heated and melted again, although its physical properties gradually change if it is melted several times (thermal history). These properties are generally reduced as the bonds weaken.

The most commonly used type, given its cost and broad range of applications, is EVA-based adhesive (ethyl-vinyl-acetate). It provides enough strength at 30-50 °C, but its use is limited below 60-80 ºC and it has low creep resistance under load and low humidity resistance (not suitable for products for outdoor use). It is widely used in packaging applications.

A particular case of a thermoplastic adhesive is the self-adhesive group (PSA), whose main feature is its high tack, even when cold. These adhesives are also called styrene copolymer or rubber-based adhesives. They display good flexibility at low temperatures and high levels of stretching and heat resistance. They are often used in pressure-sensitive adhesive applications in which the composition remains tacky even when solidified. Normally used in labelling, non-woven, automotive and assembly applications.

Polyolefins

Polyolefins, specifically amorphous polyolefins (APO or APAO) are compatible with many solvents, adhesives, waxes and polymers and are widely used in many adhesive applications.

Hot-melt APO adhesives display high resistance to combustible materials and acids and moderate heat resistance. They are tacky, soft and flexible, with good adherence and longer open time than crystalline polyolefins. They tend to be less viscous when melted, with better adhesion, and sometimes slower open and fixing times than EVA adhesives.

APOs have relatively low cohesion; under mechanical load, most of the pressure is dissipated by the stretching and loosening of the polymer chains, and only a small fraction reaches the adhesive-substrate interface. Lack of cohesion is therefore one of the most common defects in APO-based adhesives. The main advantage in comparison with EVA is greater thermal resistance (reactivation temperature of 115 ºC to 130 ºC). They are not suitable for outdoor use. Their greater thermal resistance means a greater melting energy demand.

Products

It should be noted that many of the products used with temperature-stable adhesives can be used with thermoplastic adhesives. In these cases, the division is by frequency of use with this type of adhesive.