Adhesives that react when exposed to ultra-violet light are becoming increasingly popular in high volume manufacturing because of the rapid and selective curing they offer. Goods being produced with them can avoid the rate-limiting step of being placed in an oven that heat-cured adhesives demand. UV-cured adhesives are also normally solvent-free, which is important for manufacturers looking to limit their emissions of volatile organic compounds. One trade off against these advantages is the complexity added from the need to ensure no undesired exposure to the light wavelengths used for curing. Also, the fact that the adhesive needs to be irradiated limits how thick a layer can be dispensed. The interior of thicker layers would not receive any UV-light to initiate its curing, leading to sub-optimal bonding. Another issue has been encountered in the famous Blackberry smartphones produced by Research in Motion, where UV adhesives rapid curing still has not been rapid enough.1 The phone maker adhesively bonds each key to its stem. Because these did not cure sufficiently quickly, they instead flowed and created an unwanted bonds to its surroundings. This excess adhesive caused some Blackberries to have stiff keypads, however the Canadian firm values the benefits of these adhesives enough to redesign its products to prevent these problems. An area that RIM and its adhesive suppliers might look at to solve its problems is the photoinitiator, the light-sensitive catalyst that all UV-cured material systems rely on. The radical-cured acrylate and cationically-cured epoxy systems that remain among the dominant UV-cured adhesive products demand different photoinitiators. For epoxies the photoinitiator is normally a carbonyl-containing compound such as benzophenone. For acrylates it is commonly an aryldiazonium salt like phenyldiazonium tetrafluoroborate.2 Ironically, epoxy/acrylate hybrids also use aryldiazonium photoinitiators as they are actually a subtype of radical-cured acrylates. These hybrids are especially popular, because they have excellent adhesive and non-yellowing properties, flexibility, hardness and chemical resistance.
By taking a fresh look at how these adhesives can be catalysed researchers are now tackling some of the problems seen in UV-cured acrylates, via "dual cure" mechanisms. This approach sacrifices some curing speed advantage by adding another catalyst to the formulation that will react upon heating. The second catalyst converts the linear acrylate chains into a stronger three-dimensional network.3 One team using this approach recently included Korean scientists at the LCD module division of Samsung and chemical producer Kolon. They were especially interested in the problem of "creep" - the movement of adhesive after it is cured. They included a dicyandiamide latent curing agent in a UV-cured adhesive that produced a polymer network with greater adhesion strength after heating.4 Another possible way to create a three-dimensional acrylate network is through what is known as "hyperbranching". This generally involves attaching the adhesive polymer to an underlying dendritic carbohydrate core, where each monomer bonds to two further monomers. As well as producing a stronger adhesive, this can accelerate UV-initiated curing. A team of Indian researchers has also recently shown that in a hyperbranched polyurethane/acrylate hybrid adding ZnO nanoparticles increases crosslinking density still further.5 |
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