Variables That Can Influence Adhesive Polymerization
Have you ever been in a situation where you needed confirmation that your adhesive was fully cured before you moved on with your application? Did you know how to tell if it reached full cure? The simple answer is that when a liquid adhesive turns into a solid - as a result of photo initiation, exposure to heat, or when mixed with an activator - it is cured.
There are many variables which directly and indirectly influence the rate and extent of polymerization for a specific adhesive. These may include the following:
* The part substrate(s)
* UV/Visible light transmission through the substrate
* UV/Visible light intensity and exposure time
* Amount of adhesive or activator applied
* Oven temperature and time, etc.
A manufacturer using adhesives must first develop a process that will produce parts that meet all performance expectations. Assemblies must be built at the extremes of the process and be tested at the extremes of their integrity specifications. This is often referred to as process validation. For UV/Visible light-curable adhesives, this work may include destructive and performance testing of parts made with minimum and maximum amounts of adhesive, UV/Visible light intensity, and exposure time allowable by the process.
Once the process is validated and documented, it is critical to maintain it to ensure that all assemblies manufactured to the process, including the extent of cure of the UV adhesive, are exactly alike. To monitor and sustain the process, users may employ radiometers that measure UV and visible light transmission through clear and translucent plastics as well as the intensity of light-curing systems. Meters for dispense equipment, that regulate the amount of adhesive dispensed onto parts, can also be used. It's important to remember that the validation of the polymerization of the light-curable adhesive must be done by the manufacturer since each design and process is unique.
In addition to testing and monitoring, there are a variety of technologies available to help determine if an adhesive is fully cured. Some products on the market, like Dymax adhesives formulated with See-Cure technology, allow the adhesive to appear in a distinctive color in the uncured state to enable easy confirmation of material placement onto substrates. Upon sufficient exposure to UV/Visible light energy under the proper wavelength light source, the adhesive changes to colorless, indicating the material has fully cured. This adhesive color-changing technology was designed to incorporate a safety margin before the color change happens, so is a great way to not only build a process, but have a quality inspection system within the adhesive to tell you if you have reached full cure.
Need help finding the right curing time for your adhesive?
Dymax application engineers can offer manufacturers assistance and analysis of their complex applications and provide guidance on selecting the right adhesive for their process. Contact them now.