Optical adhesives offer a clear answer

21 Sep 2004

Want to bond a lens or a fibre-optic component? Detlef Heindl describes the differences between various optical adhesives so that you don't end up in a sticky mess.

From Opto & Laser Europe October 2004

Optical adhesives are a vital, but often hidden, ingredient of photonic systems. Today a wide range of commercial products are available to fix optical fibres, bond and coat lenses, and encapsulate LEDs, among other applications. However, it is important to choose carefully to optimize the adhesion as well as the optical and physical properties for your specific use.

The big difference between the two main categories of optical adhesives, epoxy resins and ultraviolet (UV) curable compositions, is how they are prepared and cured. Epoxies are often supplied as two separate components which harden when they are mixed together, thanks to a chemical reaction. There are also single-component epoxies which are cured by exposure to heat, such as 121-148ºC for 1h or even longer. In contrast, light-curable adhesives harden when illuminated by short-wavelength (usually blue or UVA) light.

The choice of which to use depends on the volume and time requirements of the application and the desired properties. For small, infrequent jobs in a laboratory environment where the curing time is not an important consideration, an epoxy might be the adhesive of choice.

However, in circumstances where time is a critical factor, a large number of parts need to be bonded on a regular basis, or processes are automated, an acrylic UV-curable adhesive is the better choice. Unlike epoxies, which can take 24h or longer to cure at room temperature, acrylic UV curables harden in just a few seconds when exposed to light of an appropriate wavelength.

The other advantage of UV adhesives is that there is no need to mix multiple components and no limitation on pot life (the length of time before the adhesive goes off after it has been mixed). For example, a fast-acting two-part epoxy may have a pot life as short as 10 min. The remainder of this article will focus on the technology, application and properties of UV-curable adhesives.

Two main types of light-curable adhesive are available: acrylate-based materials and epoxy (cationic) systems. Acrylates have a wider range of physical properties (viscosity, appearance and adhesion) than epoxies.

UV-curable acrylates are comprised of oligomers, monomers, additives and photoinitiators. These formulations can be tailored to provide optically clear resins with specific refractive indexes, adhesion and resistance to UV. Oligomers are medium-length polymer chains that contribute to the adhesive's tensile strength, modulus and elongation. Monomers provide adhesion to different substrates. Additives can provide a wide range of viscosities to allow for easier dispensing, or can be incorporated as fillers for extremely low shrinkage.

An acrylate does sometimes suffer from surface tack (stickiness) due to a reaction with atmospheric oxygen. In most cases, however, surface tack can be eliminated by adjusting the light intensity, wavelength or duration of the curing process.

Where tackiness is an issue, cationic epoxies offer an advantage (no oxygen inhibition) but often have a longer curing process that can be impeded by moisture and humidity.

Adhesives designed for use in the optical path typically offer a very high clarity - in excess of 90% across the visible and near-infrared wavebands - and come with a wide refractive index range (1.40-1.60). They also have a low linear shrinkage of 0.1% or less and are often compatible with glass, plastic, metals and ceramics. Outgassing, the release of gas from the adhesive during the curing process, can now be as low as 10^-6g per gram of adhesive.

Aid from additives High cross-linked densities, normally required for low-outgassing epoxies, typically produce overly rigid polymers that are not suitable for durable bonding. Now, however, additives in acrylic UV optical adhesives can increase cross-linking substantially, which effectively "traps" volatile components within the adhesive matrix and lowers thermal outgassing. This is important where the engineers are looking for low outgassing, such as in the assembly of sealed lasers.

All light-curable adhesives rely on using the energy provided by light to trigger a curing reaction. The formulations include a range of photoinitiators that are sensitive to light of different wavelength ranges. While many are designed for UVA light (340-380nm), some also respond to visible light. This can help accelerate the curing time as more light energy is absorbed, and it can help increase the cure depth. Typical cure speed for acrylic adhesives is 1-15s. Heat is sometimes used to speed up the curing or assist it in areas that are hard to illuminate.

Specialized light-curing equipment is available for the curing process and ultimately the selection depends on the area and speed of the cure. Users have a choice between spot lamps, flood lamps and focused beam systems to create an illumination area ranging from a 5mm-diameter spot to a large 20x20cm footprint. In order to achieve complete and high-quality curing it is important that the system's spectral output and intensity are carefully matched to the adhesive's absorption maxima.

Although a lamp may boast a large light intensity it is necessary to check how much of that lies in the required wavelength band.

Today, spot intensities ranging from 4000 to 20,000mW/cm² across the UVA waveband are available. By contrast, a focused beam system may typically deliver a 2.5x15cm beam with an intensity of between 250 and 2500mW/cm². For the illumination of larger areas, flood systems that offer a 20x20cm footprint with an intensity of 30-70mW/cm² are available.

As for the application of the adhesives, the options include dispensing, spraying, brushing and roll coating. The best method is often determined by the viscosity of the adhesive and the size and type of the part to be treated. Adhesives are commonly available in a wide range of package sizes - everything from a syringe or cartridge containing 10g to a 1kg bottle or a 200kg drum.

To ensure consistent, high-performance bonding the temperature and humidity of the curing environment and stored materials must be kept from reaching unacceptable levels. Increases in temperature can cause a significant drop in the viscosity of the adhesive, changing its dose and flow.

As is the case with all coatings and adhesives, it is crucial to ensure that the parts to be bonded are not contaminated by dirt, dust, oil or chemicals since this can cause poor adhesion. Ideally variations in the size, chemistry or surface quality of the part to be bonded should be kept to a minimum.

To ensure continuous and repeatable process quality, users should be aware that the bulbs (lamps) in curing systems gradually degrade. As a result, the light intensity of a system decreases as the bulb gets older. The amount of degradation depends on the age of the bulb, the number of times that the bulb is switched on and off, and the operation temperature. Even though the bulbs have a high lifetime expectancy it is essential that the intensity of a curing system is checked at regular intervals by a radiometer to ensure that it is sufficient. To help prolong the life of the bulb do not switch the system on and off repeatedly - if the equipment is to be used within 2h of finishing a process it is better to leave the lamp on.

Our advice is to choose a curing system that delivers a maximum intensity level that is three to five times higher than the lower limit needed for your application. We also suggest that you check the curing bulbs regularly and change them when the measured intensity is 50% above the lower limit. For example, if you require 20mW/cm² for 10s to give a satisfactory cure then choose a system providing 100mW/cm² and change it when it reaches 30mW/cm².