In two of our previous articles, “Understanding UV Curing Adhesives” and “Cationic Epoxies- Advantages” we discussed the benefits of single component light curing adhesives. UV/light curing adhesives use energy from visible light or UV radiation to initiate polymerization. However, areas that are not exposed to radiation will not cure. This weakness has led to hybrid systems that allow for dark sectional curing through a secondary mechanism. Hybrid systems come in two forms, those that continue the use of the acrylate chemistry and those that incorporate new chemistries.
Light radiation is not the only avenue for creating the free radicals that causes acrylate polymerization. Free radicals can be created using e-beam curing technology or heat.
Electron beam technology, otherwise known as E-beam, uses an electron accelerator to project an electron at its target. Unlike UV or light radiation, no photoiniator is required. This is because a free radical, used in acrylate curing mechanisms, provides an unpaired electron to initiate polymerization while the e-beam directly provides that unpaired electron. E-beam can penetrate certain substrates that would absorb light. The depth of penetration is directly linked to the energy used by the electron accelerator.
Heat can also generate free radicals. Heat generating initiators (e.g. peroxides) decompose at high temperatures to produce free radicals. Due to the elevated temperatures, the acrylate polymerization occurs very rapidly. Heat induced reactions prevent dark section cure issues. Heat polymerization can also be used alongside light induced polymerization to guarantee complete and homogeneous curing.
Isocyanates, which is one of the two functional parts of a urethane adhesive, can react with moisture to become polyureas as discussed in the article “Urethanes”. This technique can be used, and combined with, acrylate chemistry to allow dark sectional cure providing that there is moisture present. After the moisture reaction finishes the adhesive will have strong moisture resistance. As such the adhesive will become a barrier where the initial moisture penetration existed. Aside from moisture resistance and dark section curing, another major benefit to using a secondary moisture reaction is improved bond with difficult substrates.
Vividcure 76011 is one part dual cure UV Urethane. This product bonds well to difficult plastics such as santoprene, delrin, polypropylene, nylon, and PBT. It has had success many markets, including automotive.