A guide to modern adhesives
Chris Hall, advanced technologist, industrial adhesives & tapes division, 3M United Kingdom plc
As materials technology continues to evolve, so the requirements on adhesives to effectively bond surfaces of different materials change too. Continued research and development and investment in state-of-the-art adhesive technologies by the leading industry players worldwide mean specifiers can today choose from a vast range of adhesive products each with its own specific features and benefits.
he most commonly used liquid or paste adhesives in industry are formed from either reactive polyurethane (PUR), polyvinyl acetate (PVA), two-part epoxy, hot melt, acrylic or two-part polyurethane. All are suited to
differing applications based on characteristics including their physical properties (and those of substrates), working times, ease and method of application, environmental credentials, heat resistance and the eventual destination and use of the finished bonded product.
Polyurethane – flexible and versatile Reactive polyurethane adhesives are suitable for a wide
variety of applications including wood, plastic and window frame bonding, including bonding muntin bars into glass. They are also widely used in industries including marine and automotive. They tend to be single-part systems – meaning no mixing is required - and are generally very flexible, typically being able to extend up to several times their original length. This, and the high viscosity of many polyurethane products, makes them ideal for gap filling, and use on bumpy and uneven surfaces such as wood. Depending on the product, polyurethane adhesives can
withstand more than 20MPa in shear, with strength build-up usually faster than other adhesive types. The strength and durability of polyurethane adhesives extends to their performance under vibration, thermal expansion and contraction, impact and flexing. When using polyurethane, process steps can often be reduced as there is no need for sanding or damping, reducing time and cost. Hard woods such as maple or Brazilian Ipe can be challenging to work with but moisture-curing polyurethane adhesives are well-suited to these substrates. Polyurethane products are also a good solution in existing applications where blooming or stress cracking is an issue. Polyurethane products do have their limitations, however.
Some of the stronger single part systems may require heat for application - although some water-activated polyurethane products do exist, these are designed for large, uniform areas such as laminates, with close tolerances. Reactive polyurethane adhesives that use moisture to initiate and progress the cure also tend to be unsuitable for bonding in situations where both substrates are metal, for example. Although the initial cure can be a quick process we often find that full cure takes a lot longer. With polyurethane sealants this can mean that the adhesive core can remain weak for an extended period of time, or that the adhesive may be affected by high humidity before full cure. The cure mechanisms used in reactive polyurethanes may also have their own issues regarding health and safety during application and curing.
Where lower shear strength is required, or tighter substrate
tolerances can be guaranteed, a PVA product may be a suitable option. PVA products are cost-effective and highly versatile – when applied, they will either remain as PVA or ‘cross-link’ with the molecules in the substrate to form the bond. Applicable via brush, roller or spray, PVA products do not typically require heat, but usually will not reach handling strength rapidly, requiring anything from 30 minutes to 24 hours, with full bond taking even longer. This means that substrates will often need to be clamped together. PVA products offer limited durability against moisture – though some external durability can be achieved, few reach the universally recognised D4 standard for adhesives. PVA is also widely used as a primer or sealer in conjunction with other adhesive systems.
Two parts providing strength and performance At the other end of the scale, where a guaranteed high-
strength bond is required, two-part epoxy products are the most widely used adhesives. Two-part epoxies have been used since the 1930s in applications such as aerospace, and offer a good bond for most substrate types. Many of them are structural adhesives – designed to form part of a load-bearing structure with high shear strength. A wide range of
performance characteristics are available – some products are more closely related to sealants in their behaviour, while others are designed for optimum performance on modern
124 Fastener + Fixing Magazine • Issue 68 March 2011 Cost-effective sealing with PVA