Until recently, those adhesives that use light as a curing mechanism have required exposure to ultraviolet sources for the principle to work. And while the technology has been a resounding success, there are some drawbacks in their use.
One of the most evident is the need for the adhesive to be reached by the ultraviolet light. In many situations this may not be a problem, but where the substrates to be bonded contain a colour that restricts light in the UV part of the spectrum, the curing simply will not take place. In fact, some of the very additives that are used in plastic manufacture to prevent discoloration actually restrict light at the required wavelength.
Some of the difficulties were overcome when UV/visible (UV/V) adhesive technology came on the scene during the 1990s. The adhesives in this range not only worked with UV-blocking and slightly tinted substrates, but also shortened cure times (to around 10 to 20 seconds) and increased the cure-through depths (in excess of 10mm in some instances).
The latest technology to come from adhesives manufacturers involves the use of visible light-curing products. These adhesives incorporate the very latest photo initiators that react wholly with light that is available in visible wavelengths – those in excess of 425nm. Such products not only act faster – less than 10 seconds for cure times is not unusual - but they also have a cure depth of more than 12mm in many instances. Moreover, they offer compatibility with many plastics, glass and metals, and can even be employed for substrates that normally block UV light, such as semi-translucent grades of purple, grey, blue and white.
On the wider health and safety front, it is helpful to know that the use of visible light techniques means protective equipment and other shielding mechanisms are no longer necessary (though safety glasses should be used to protect eyes where high intensity light sources are in use).
Even though the technology is quite new, many designers and production managers are already experiencing the benefits of this new range of adhesives. Alongside the bonding of substrates, which have previously used UV and V/UV products, the new product selection has been found to provide excellent adhesion for materials such as polycarbonate and polyvinylchloride. Beyond that, the visible light adhesives come in low- and high-viscosity formulations to provide flexibility of applications and they generally include fluorescence properties that enable quality control inspection to be easily carried out.
Developed alongside the adhesives has been a new generation of curing units that incorporate both point and flood source systems. These include LED technology that allows the generation of focused wavelengths. Unlike incandescent visible light sources, LEDs do not generate heat or emit a broad spectrum of light that is largely wasted. LED curing systems offer a life of at least 10,000 hours, and their pinpoint accuracy means that they can be used to cure adhesive on small, even miniature, components.
With adhesive qualities of a high standard, added to which are some favourable environmental benefits, it is very likely that more applications will benefit from this new technology. Visible light curing products have a positive future - and that’s good news for design engineers in a wide variety of industries.
- Bob Goss is senior technology specialist at Henkel