UV epoxy products are adhesive and sealant materials that will polymerize with UV light exposure. Normally UV curing products are limited to applications in which light can reach 100% of the material, however, due to an epoxy's unique cationic polymerization, some "shadow curing" is possible.
This post is elaborates on the concept of cationic shadow curing and aims to answer the following questions:
- What is shadow curing?
- How do UV epoxies achieve a shadow cure?
What Is Shadow Curing?
Shadow curing is simply when a polymer product is able to polymerize in shadowed areas. Of course all heat cure and room-temperature cure products are able to achieve this. However, UV cure products rely on light exposure in order to drive the polymerization.
Typical UV products, such as acrylates, cure via free radical polymerization and require 100% light exposure in order to achieve 100% polymerization. There are a few direct implications of the process:
- UV acrylates can not bond opaque substrates (unless they are formulated with a secondary heat cure - see our UV resin guide to find out more on this).
- If the geometry of a substrate or interface partially shadows a certain section of the material during the light-exposure process, the polymer will not fully complete in these sections.
This limitation is mainly due to the radical species that drives the free radical polymerization reaction. Photoinitiators create free radicals upon exposure to UV light, initiating the polymerization. However, radicals are highly reactive species and typically only last a few moments. As a result, radicals don't exist long enough to migrate into shadowed areas, extending the polymerization to these sections.
How Do UV Epoxies Shadow Cure?
Although the term "UV epoxy" may refer to several different chemistries, a true UV epoxy cures via cationic polymerization.
Photoinitiators upon exposure to the correct UV wavelength will produce a Brönsted or Lewis acid that will initiate the subsequent polymerization reaction. Unlike radicals, acids can exist for up to several days and as a result can migrate within the material, extending the polymerization to shadowed areas.
The following graphic created by Excelitas Technologies outlines the process of UV epoxy shadow curing (the graphic is not to scale).
The degree of polymerization and the depth of cure depends highly on the UV epoxy formulation, as well as the geometry of the application. Some UV epoxy products are specifically designed to maximize the degree of shadow curing.
Most important to realize is that this mechanism exists, and products of this nature are available. Please reach out to use today to learn more about UV epoxy products that might work well for you application specifications.