As materials technology evolves, so too do the requirements placed on adhesives to effectively bond the surfaces of these different materials. Chris Hall, advanced technologist, Industrial Adhesives & Tapes Division 3M, looks into the options available
Continued research and development, and investment in state-of-the-art adhesive technologies, means specifiers can today choose from a vast range of products, each of which offers its own features and benefits. So what is currently available?
Reactive polyurethane adhesives, for example, are suitable for a wide variety of applications – such as wood, plastic and window frame bonding, including bonding muntin bars into glass. They are also widely used in industries like marine and automotive.
Tending to be single-part systems – meaning no mixing is required – these 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.
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.
Polyurethane products are also a good solution in existing applications where blooming or stress cracking is an issue.
Where lower shear strength is required, or tighter substrate tolerances can be guaranteed, PVA products may be a suitable option. These 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.
When a guaranteed high-strength bond is required, two-part epoxy products are the most widely used adhesives, offering a good bond for most substrate types.
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 aircraft and Formula 1 cars. Flexibility, toughness, fluidity and thixotropic qualities can all be varied according to customer requirements.
With these, open time – the window of time available to bring the surfaces together – can vary between 30 seconds and several hours. Full cure times can be anything between one and three days, but some rapid products have a ‘time to handling’ as short as three minutes, although a few hours is more common. This can be a disadvantage in high throughput applications, particularly given the typically higher costs of two-part epoxy adhesives compared with PVA.
A strong bond
For highly demanding applications requiring a structural strength bond, acrylic adhesives deliver a rapid result at reasonable cost. These are structural in performance and reach handling strength very quickly, typically in less than an hour. Full cure usually occurs within 24 hours. They also require less surface preparation than epoxy products, typically cope well with oily surfaces, and need minimal mixing.
While most are two-part products that require mixing, acrylic ‘bead on bead’ products also exist with each part applied to the substrates, which are then pushed together.
Acrylics are ideal for metals, plastic and composite substrates, while highly modified products are available for low surface energy plastics.
Hot melt adhesives, meanwhile, offer a versatile solution for where the requirement is more for fixing than bonding. Generally designed for non-critical and temporary fixing applications, hot melt products are non-curing systems which generally reach handling strength quickly after being dispensed. Suitable for almost all substrates, minimal surface preparation is usually required.
Whatever the application, a few simple steps should be followed when specifying adhesives, and the characteristics and requirements of the substrates to be bonded will always play a key role in the decision.
When calculating cost, the specifier should not only look well beyond the purchase price of the adhesive, but the costs of additional surface preparation, cleaning, operator training, adhesive heating and energy requirements, and any necessary PPE, must also be factored in.
Consideration must also be given to the required production throughput – and so the time to handling strength and to full cure of potential adhesive solutions – and how soon each component will need to undergo further processing after application of the adhesive.
Consulting an adhesive specialist, who can advise on the correct product and how to get the best out of it, from the early design stages, will help ensure optimum product selection and ultimately the effectiveness of the process.
T: 08705 360036