UV curable adhesives are generally 100 percent solid formulations. They are widely used in hotmelt and semi-structural adhesives and offer many advantages over traditional hotmelt and solvent-based adhesives including no solvent emissions, smaller manufacturing footprint, lower energy cost and and improved production efficiency due to very fast cure.
The two most common types of resins used in UV curable adhesives are free-radical curable vinyl esters and acrylates and cationically curable cycloaliphatic compounds such as epoxides and oxitanes. The cation type resins are typically more expensive but offer better overall performance including better heat resistance, less shrinkage and superior strength, whereas free radical type adhesives cure (much) faster.
An important application of UV curable resins are pressure sensitive hotmelt adhesives for tapes and clear labels. Both free-radical curable acrylates and cationically curable
epoxy resins are
extensively used in these applications. The cation type adhesives have excellent clarity and moisture resistance comparable to solvent based acrylic adhesives whereas the performance of free-radical type hot melt adhesives is comparable to emulsion based pressure sensitive acrylic adhesives, which are generally inferior to solventborne acrylics.
A major drawback of UV curable adhesives is that at least one
mating part has to be transparent and not block UV light.
Free radical photo-polymerizable systems are often formulated with acrylate functionalized oligomers. Typical backbone resins include
aliphatic and aromatic urethanes
epoxides and its copolymers
aliphatic polyesters
acrylics
Acrylic adhesives formulated with acrylated acrylics provide good resistance to oxidation, yellowing, solvents and UV. They also bond to a large number of substrates including many difficult to bond materials. Aliphatic ether- and ester-based urethanes have similar good resistance to yellowing. They also possess high toughness, flexibility and good overall durability including excellent chemical and UV resistance. The ether-based urethanes have better hydrolytic stability whereas the ester-based urethanes have higher heat resistance. Another important class of UV curable acrylate functionalized resins are epoxy acrylates. These resins proivde superior heat and chemical resistance, higher strength and excellent weatherability. They also offer fast cure.
Depending on the application, UV curable resin compositions might also contain mono- and multifunctional monomers such as isobornyl acrylate and dimethyl acrylamide. They are added as diluents to lower the viscosity and to improve cure. Multifunctional monomers improve chemical resistance, cure speed, and shear strength but lower the peel and impact strength. They also increase shrinkange which can cause adhesion issues.
Free radical type UV-curing compositions usually cure in a matter of seconds but only in the presence of radiation due to the very short life time of free radicals. Typically, only thin coatings (tapes) and bondlines (adhesives) in the range of 5 - 30 mils can be cured because not enough photons penetrate beyond this depth to achieve full cure. Another major drawback of free-radical curing systems is oxygen inhibition. Atmospheric and dissolved oxygen in the resin reacts with free radicals forming less reactive peroxy radicals. This not only reduces the rate of polymerization but also can result in incomplete cure which produces a tacky surface. Common strategies to avoid or reduce air inhibition include high cure dosage and radiation intensity, shorter wave length, high concentration of photo initiators, cure in inert atmosphere, addition of waxes that bloom to the surface, and use of dye sensitizers.
Cationically polymerizable resins are fomulated with cylcoaliphatic monomer and oligomers. The initiating moities of cationic photopolymerization are Bronsted or Lewis acids which are generated by UV radiation of suitable photoinitiators (onium salts). Unlike free radicals, the initiating moities are chemically stable for a very long time. As a consequence, polymerization, once initiated, continues for a very long time and even in the absence of light (dark cure). Thus, these resins can be cured with a much lower UV dose and are suitable for even thick bondlines and coatings due to dark cure. Full strength, however, is only reached after several hours.
Cationically photo-polmerizable resin systems have generally a better overall performance than free radical curable resins and thus, are suitable for a wider range of applications.
Important cationically photopolmerizable resins include aliphatic epoxides, cyclic ethers (oxitanes), and cyclic sulfides. The epoxide terminated oligomers are more common than the others. They offer good adhesion to many substrates, reduced shrinkage, and good heat resistance.
Unlike free-radical polymerization, cationic polymerization is not inhibited by oxygen. However, cationic photo-polymerization is inhibited by bases and water which act as chain transfer agents.