TWI Knowledge Summary

Adhesive Bonding - Light-curable Adhesives

by Colin McLean

Introduction

Photocurable (or light-curable) adhesives, which cure when exposed to light, are composed of a number of chemical species. In common with other adhesive formulations, they typically contain low-medium molecular weight polymers, - oligomers -, that impart a major influence on the physical and mechanical properties of the cured system, e.g. whether it is hard or soft, rigid or flexible. In addition to the oligomers, monomers are added to modify the viscosity of the uncured adhesive and control cross-link density during subsequent polymerisation. Optional additives in the adhesive composition can include adhesion promoters, wetting agents, pigments and fillers for example. Finally, there is one component that is unique to this class of adhesives, a photoinitiator.

How do they work?

The photoinitiator, typically representing less than 5% of the adhesive formulation, is best regarded as a dormant hardener/curing agent. In the uncured, unexposed adhesive it is unreactive towards the rest of the components in the adhesive. However, when the adhesive is exposed to light of the correct wavelength, and sufficient intensity, the photoinitiator absorbs this radiation and is converted into a highly reactive species that rapidly initiates copolymerisation of the oligomers and monomers. Progression of this copolymerisation reaction is what we observe as hardening, or curing, of the adhesive, i.e. conversion from a liquid to a solid state.

This feature, where the adhesive only cures upon exposure to light, is sometimes referred to as 'command cure' or 'cure on demand' and means that, as long as the adhesive is not inadvertently exposed to light wavelengths that will initiate curing, pot life of the adhesive is essentially unlimited, thereby giving ample time for positioning of the substrates to be bonded.

The wavelength of light required to initiate curing typically falls in the long-wavelength UV region of the electromagnetic spectrum, between 325 and 380nm. In industrial applications of these adhesives, such UV light is provided by medium-pressure mercury lamps incorporated into sophisticated curing systems designed to deliver the light either in a focused spot, strip or defocused area ('flood') configuration. With appropriately-formulated adhesives, visible light can also be used to effect cure. Blue light, at a wavelength of 470nm, is usually used and offers advantages in terms of safety and the ability to effect cure of relatively deep sections or filled/pigmented adhesives.

Although perhaps obvious, it is important to note that the spectral emission of the curing system must match the spectral absorption of the photoinitiator(s) present in the adhesive. Suppliers of photocurable adhesives either provide information on recommended wavelength and intensity of light on the technical data sheet of the adhesive, or on request.

Adhesive chemistries

The vast majority of photocurable adhesives on the market today are based on acrylate/methacrylate chemistry. The 'toolbox' of such raw materials available to adhesive formulators is very large and this translates into quite a number of suppliers offering adhesives with a very diverse range of properties. In general, this chemistry allows for very rapid cure times, (less than one second in the case of adhesives for DVD production, for example), but any adhesive exposed to air, at the edges of bonded parts for example, can suffer from an effect known as 'oxygen inhibition', where diffusion of atmospheric oxygen inhibits the curing reaction resulting in a tacky and/or greasy surface (a few micrometers thick) on the otherwise cured adhesive. A number of approaches are available to circumvent this effect (e.g. inert gas blanketing during cure).

Less common, but gaining increasing acceptance in a number of applications, are photocurable adhesives based on epoxy chemistry. These adhesives do not fully cure as rapidly as their acrylate counterparts, - often a thermal post-cure is required to achieve optimum properties -, but can provide better adhesion strengths, particularly on certain metals, and do not suffer from the oxygen inhibition effect.

Seeing the light!

As the curing process is initiated by light, this means that at least one of the substrates to be bonded must be transparent to the initiating wavelength(s). This is often not as apparent as it may seem! For example, many transparent plastics inherently absorb UV light, and this absorbance will be increased if the plastic contains anti-yellowing additives (typically used in outdoor- and medical-grade plastics). Conversely, relatively thin, white alumina, opaque to the human eye, will transmit sufficient blue light to allow the use of visible light-curable adhesives in the assembly of fibre optic connectors, while the ability of blue light to penetrate highly-filled, opaque adhesives allows the use of the latter as dental filling composites in preference to mercury amalgams.

In view of the above, photocurable adhesives will not cure if they are shadowed from the initiating light. The terms 'dark cure' or 'shadow cure', used by some adhesive suppliers, suggest otherwise, but these actually refer to adhesives that continue to cure once the initiating light has been removed, i.e. epoxy-based adhesives. However, the curing reaction does not progress into bond areas initially shadowed from the incident light. For such joint configurations, photocurable adhesives are available with secondary cure capability, typically effected by heat although other options, e.g. moisture- or anaerobic-cure, can also be provided.

For bonding two substrates that are both opaque to the initiating radiation, a handful of epoxy-based photocurable adhesives are available that have a 'delayed cure' feature. With these the adhesive is applied to the bonding surface of one substrate then given a relatively short exposure to the initiating light. This short exposure 'activates' the adhesive which remains in a liquid state for a short period of time, - the so-called 'open time' -, during which the second substrate is attached. Once the open time has expired the adhesive begins to cure, bonding the two substrates.

Market drivers

Photocurable adhesives are increasingly used in a number of industrial applications, but perhaps the most important sectors which benefit from this technology are microelectronics, photonics/optoelectronics and medical device assembly. In these areas the principal advantages of photocurable adhesive systems are: active alignment assembly permitted by the 'command cure' feature, very rapid non-thermal cure, ease of automation, and reduced energy requirements provided by compact curing systems. The benefits such features provide, in terms of enhanced productivity, mitigate the relatively higher price of photocurable adhesives compared to many other adhesives types.

In addition, environmental legislation on elimination of volatile organic compounds (solvents) and reduction in the carbon footprint of industrial processes, is increasingly forcing the evaluation and subsequent adoption of photocurable adhesive technology in many product assembly applications.