Forming a bond? Selection matters

Ensuring products are correctly bonded is a key aspect of the manufacturing process.Poor adhesive selection or application is likely to impact heavily on overall quality, durability and performance, as Chris Hall, advanced technologist, Industrial Adhesives & Tapes Division,3M United Kingdom, explains

There is a vast array of adhesives on the market and, once specifiers have chosen between a water-based or solvent-based product, they must decide on a contact, single-part or two-part adhesive, and whether it should be applied by brush, spray or roller. It is imperative to get these decisions right to avoid costly mistakes.

Commercially, there is a requirement to strike a balance between adhesive cost, the requirements of production deadlines, and the time needed for products to reach the desired bond strength.

While it is generally true that in terms of bonding strength and quality, you get what you pay for, overspecification can impact on performance, with too strong a bond not allowing the required flexibility during use of the finished product.

Water-based or solvent-based? The benefits of solvent-based adhesives have always been the speed of solvent evaporation and the potential for high strength, but increased environmental and health and safety concerns are beginning to have a greater impact on adhesive selection.While solvent-based adhesives are still the preferred option in applications involving rubber, for almost every other application, regardless of the combination of substrates, there is usually a water-based adhesive available which, if used correctly, will deliver the required bond.

Water-based adhesives are non-flammable when wet and do not emit toxic fumes - meaning they are less demanding in terms of the PPE required - and usually require less capital outlay for their application within the factory. Conversely, solvent-based or polyurethane reactive (PUR) adhesives which contain isocyanates require greater outlay to ensure safety in the application area.

The speed with which the products must be bonded will govern the type of adhesive chosen. All adhesives will have a differing 'open time' - the time available to bring the surfaces together before the adhesive sets - as well as a 'time-to-handling-strength' and a 'time-to-full-cure'.

Where immediate bonding is needed - either because of high throughput or because further processing is required - a contact adhesive is the best option.Waterbased contact adhesives often outperform solvent-based products when it comes to elevated temperature applications.

Otherwise, specifiers have a choice between single-part PUR adhesives, which are usually pre-heated and then applied to one surface, and a two-part epoxy adhesive, comprising two separate components which react chemically and start to cure when mixed.

The properties of the substrates are key to the specification process.Using a water-based adhesive to join wood to metal will require thorough drying of the metal before application to negate risk of corrosion.When bonding foam, heat-applied products can melt foams, while a solvent-based contact adhesive may contain chemicals which can dissolve or damage the foam.

Application matters When selecting between application methods, the total surface area to be covered must be considered. Spraying requires the preparation (and cleaning afterwards) of a spray gun but is generally more economical for larger areas and offers better control over adhesive thickness. For smaller areas, brush or roller application is preferable as there is less waste and no requirement for spray gun preparation and cleaning. Apart from a few PUR adhesives, most modern adhesives are suitable for spraying, although with solventbased contact adhesives, potential health and safety issues can arise as a proportion of the solvent will diffuse into the air before the adhesive reaches the product surface.

Where a contact adhesive is used, pressure will be required after application to allow the surfaces to bond. Flexible materials can be treated with a hand roller, while solid sections will require either a hydraulic press (where the surfaces being joined are relatively small) or, for larger surfaces, a 'nip' roller, which consists of two continuously rotating wheels with a space between them through which the workpiece is passed. This lends itself well to continuous operation and, thus, to high throughput applications. The subsequent processes required after application can impact significantly on cost - of both equipment and labour - and so should be considered alongside the adhesive costs when specifying.

Getting the advice of a specialist adhesives supplier from an early stage will go a long way towards guaranteeing correct product selection. Once the decision on adhesive type and application method has been made, training operatives in effective preparation, handling and use will ensure the product delivers to expectations.