Reactive adhesives

Full information on temperature-stable hot-melt adhesives

These are synthetic adhesives that owe their adhesive power to chemical polymerisation reactions which are activated by heat or by catalysts such as moisture or UV rays. Once polymerised, the adhesive can no longer be melted or dissolved, so the process is irreversible.

They are adhesives based on polymers joined by chemical bonds, which acquire a highly crosslinked final structure. This is directly responsible for the high mechanical and physical strength (force or loads, temperature, etc.) displayed by these materials compared with thermoplastic and elastomer materials. On the other hand, this highly crosslinked structure entails low elasticity, making these materials characteristically brittle. One of the negative aspects of temperature-stable materials is the fact that they cannot be recycled, as once they have solidified or cured it is impossible to return the material to a liquid phase, since temperature-stable materials have the property of not melting or deforming in the presence of temperature or heat.

Polyurethane (PUR) or reactive-urethane adhesives

They are used for high temperatures and high levels of flexibility. Solidification can be quick or prolonged over the space of several minutes; and then with secondary curing under atmospheric moisture or that of the substrate, it continues for several hours, forming crosslinks in the polymer. They display excellent resistance to solvents and chemical products and a low application temperature suitable which makes them suitable for heat-sensitive substrates. After curing they are heat resistant, with working temperatures generally between -30 °C and +150 °C. PUR adhesive glues are suitable for indoor and outdoor use because of their strong thermal resistance and high level of impermeability to moisture.

They are often used in binding, for the automotive and aerospace industries and in plastic bags and filters. They are generally based on prepolymers made from polyols and methylene diphenyl diisocyanate (MDI) or another diisocyanate, with a small number of free isocyanate groups. When these are exposed to moisture they react and crosslink. Resistance to solidification when not yet cured tends to be lower than in non-reactive adhesives; mechanical strength is developed with curing.

Reactive polyolefins (POR)

They are used, like PUR, in applications where the final product requirements include high temperature resistance. Their main feature is good adhesion to non-porous materials such as polypropylene (PP) without the need for pre-treatment.

Although adhesives with a polyamide (PA) base are not strictly reactive adhesives, the difficulty in applying them, their high levels of moisture absorption and their degradation with oxygen from the atmosphere require similar application systems to those used for reactive adhesives. These are considered to be high-performance adhesives for harsh atmospheres and high temperatures, normally applied at more than 200°C, due to their high re-softening point, but they can become degraded and burn during the process. In melted state they can be partly degraded by the oxygen in the atmosphere. They have a wide range of working temperatures, generally displaying adequate adhesion from -40 ºC to 70 °C and, in some compositions, allowing use at 185 °C if they do not have to bear loads.

Resistant to oils and petrol. They display good adhesion to many substrates, such as metal, wood, vinyl, ABS and treated polyethylenes or polypropylenes. Moisture absorption can give rise to the formation of a foaming material during application due to the fact that the water that evaporates during melting creates holes in the adhesive layer, which reduce its mechanical strength.

Products

It is important to note 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.