when it comes to bonding plastic, there is a similarly broad selection of adhesives, writes Colin Chapman. It would be tempting to think that any adhesives would join any plastic - but, in fact, it takes a detailed look at each technology to determine the best for the job in hand. In general terms, five adhesive types are suitable for plastics bonding.
Cyanoacrylates, available in viscosities ranging from liquids to thixotropic gels, feature high peel, shear and tensile strength and impact resistance. Thermally resistant cyanoacrylates provide excellent bond strength retention at temperatures up to 100°C. UV curing cyanoacrylates are a recent development. Acrylics are two-part adhesives with a resin and activator/hardener or a single part UV cured product. The latter is a solvent-free liquid that turns to thermoset plastic in about 2 to 60 seconds when exposed to an appropriate source, with cure depths in excess of 13mm possible. Both cyanoacrylates and acrylics are suitable for small part bonding, but in broad terms, the following technologies are more suited to larger applications.
Epoxy adhesives are typically two-part systems that cure at room temperature. There are also one-part pre-mixes that use a heat cure. Once cured, epoxies form tough, rigid thermoset polymers that achieve adhesion with a good depth of cure to a wide variety of substrates. Polyurethanes are supplied as one and two-part systems that cure to form thermoset polymers with good solvent and chemical resistance. They are extremely versatile and can range from extremely soft elastomers to rigid, extremely hard plastics with good impact and abrasion resistance. These adhesives bond well to most unconditioned substrates, but may require the use of solvent-based primers to achieve high bond strengths. Shown here, polyurethanes are being used in the assembly of plastic aseptic tanks
Finally, Silane-modified adhesives are solvent-free, one-part products that produce good weathering and ageing resistance. The properties of these adhesives are similar to those of polyurethane - but without need of primers.
Surveys have revealed that adhesives are the most versatile permanent joining method for plastics. That's not to say other joining procedures do not have a place in modern production processes, but when it comes to a solution to suit a wide variety of substrates and assembly methods, adhesives offer a solution that is readily accepted.
Colin Chapman is with Henkel Loctite