Manufacturers of medical devices are tasked with a special challenge: as well as being healthcompatible, the materials they use must allow high-precision production, permanent joining and withstand various sterilisation methods. This especially applies to the adhesives used for assembly.

UV technology has been a reliable method for curing photoreactive adhesives for medical applications. When it comes to short cycle times and fast processes, radiation curing is almost a must. However, a large number of different systems are available and they are perfectly tailored to the respective application. Short innovation cycles induce the need for suitable UV-curing equipment, often reaching the limit of the conventional electrode-emitting technology. UV-LED technology opens up a whole range of opportunities.

The rise of UV-curing epoxies

Until recently, LED-curing adhesives were limited to free radical-curing acrylates. But today, UV-curing-epoxy adhesives can also be advantageous over other chemistries, due to their high bond strength and chemical resistance. Acrylates and UV-curing epoxies can be put together almost arbitrarily to achieve desired properties. Depending on the base resins, additives, modifiers, fillers and adhesion promoters, the resulting adhesive shows large variances; from being flexible and soft with high elongation at break to hard and scratch-resistant with extreme bond strengths, numerous properties and a multitude of application areas are conceivable.

Acrylic adhesives consist of free radical-polymerising monomers or prepolymers. When exposed to UV or visible light, photoinitiators release free radicals, which triggers polymerisation and, thus, curing of the adhesive. As a general rule, the higher the intensity the shorter the curing time. Acrylate polymerisation tends to arise in low-cross-linked thermoplastics or elastomers that demonstrate outstanding softness and elasticity. The reaction with free radicals causes very short curing times of acrylates. In addition to their mechanical bond strength, the rapid curing of the adhesives is important to allow high-volume production, as disposable medical products are often produced in large quantities. The typical applications of acrylic adhesives in medical device assembly are therefore bonding-connecting elements and catheters, needle bonding and bonding plastic enclosures.

UV-curing epoxides are cationic cross-linking resins. Hard and highly cross-linked duromers are always obtained and cause very low migration. Compared with acrylates, epoxies offer some technical advantages, in terms of durability, for example. They are also significantly more stable against environmental and media influences. In addition, epoxies can be used at higher temperatures and have a higher glass-transition temperature.

After curing, epoxy systems usually link with dry and tack-free surfaces, extending their range of application to one-sided protective coatings. Due to the epoxy ring opening during the cross-linking reaction, epoxies have significantly lower shrinkage for better positioning and minimal movement.

Medical applications for UV-curing epoxies and meeting industry standards

UV-curing epoxies that act as medical-grade adhesives can be used to bond medical products, such as endoscopes, catheters, pacemakers, implants, hearing aids, dental technology and much more. Epoxies can also be found in orthopedic devices and in diagnostic equipment, such as MRI machines and ultrasound devices. They have very good temperature, moisture and chemical resistance and can withstand various sterilisation technologies, including steam, ethylene oxide (ETO) and gamma sterilisation. UV-curing epoxies also work well with metals, plastics, glass and other substrates used in medical devices.

The formulation of epoxy-based adhesives allows a broad variety in viscosity control. It is possible to tailor the wetting and flow characteristics from a low viscosity of 100-1,000 millipascal-seconds (mPa-s) to a medium one of 1,000-10,000mPa-s, or paste and gel-like adhesives. In addition, a pronounced thixotropy enables optimal flow, and results in excellent dimension stability and good workability after dispensing/applying the adhesive.

In general, adhesives used for the production of medical devices need to be solvent free and certified to US Pharmacopeia Class VI and/or ISO 10993, in order to ensure the biocompatibility of the finished medical devices. The monomers used for epoxy polymerisation are usually less toxic compared with acrylates. Thus, it is also much easier to implement a biocompatible formulation.

Despite all of these advantages, in the past there was one drawback that reduced the use of epoxies. Due to their chemical modification and the prevailing cross-linking reaction, the cationic polymerisation previously had much longer cycle times. With the recent development of new and innovative photoinitiators, cationic-curing epoxy systems can now also be hardened within seconds.

LED's advantages

In recent years, LED-curing systems have become increasingly important, especially for adhesive applications. This is a trend that will become even more prevalent in the coming years because LED technology has many advantages, including a compact design, lower operating costs, long service life and low heat input. These factors make LED technology more effective and suitable for many applications. In addition, LEDs are mercury and ozone free.

Unlike gas-discharge lamps, LED light sources emit a quasi-monochromatic light spectrum. However, significantly higher intensities can be achieved in this emission range. Curing with LEDs has helped to elevate UV and light-curable adhesive bonding to new levels of advancement. LED devices that are used for curing adhesives are available in different wavelengths, depending on the curing requirements of the adhesive.

Currently, the absorption spectrum of the photoinitiators often partially matches the emission peaks of the LEDcuring systems. A comparison between conventional and new photoinitiators shows that the new initiators have a significantly larger overlap with the emission spectrum of the irradiation sources. Thus, it is now possible to completely cure epoxy resins with LEDs.

The choice of the right wavelength plays a decisive role in curing. High-tech LED-curing devices are available in the wavelengths 365, 385, 395, 405 and 460nm. A 405nm LED, for example, cures adhesives in much deeper layers because it has a longer wavelength.

All Panacol LED-curing epoxy adhesives and sealants can also be cured using conventional UV-curing equipment with a broad spectrum. The new generation of epoxies can therefore be incorporated into existing UV-bonding processes. Equipment manufacturers, such as Hönle, are now offering a new generation of LED-UV equipment with higher output intensities that significantly reduce curing time for adhesives and coatings. Epoxy-based resins can now be cured nearly as fast as acrylates. The combination of modern LED-UV equipment and newly developed epoxies leads to an equally fast – or sometimes even faster – curing as using broad-spectrum UV light.