A large number of UV light curable resins have been developed to meet the stringent regulations in Europe and North America and to reduce air pollution as well as energy consumption. Their use as well as their popularity has grown significantly over the last forty years.1 UV induced cure has many advantages over conventional cure in terms of less energy consumption and equipment space, reduced waste, less emission, higher productivity (fast cure) and lower temperature treatment. In addition, UV curable resins usually do not contain any organic solvents that have an adverse effect on the environment. The primary limitation of light-induced curing system is the limited light penetration depth which will depend on the wavelength and spectral distribution and usually does not exceed a few millimeters. This means that light induced cure is limited to inks, coatings and adhesives and in the case of adhesive applications, at least one substrate must be able to transmit light. Thus, assemblies with shaded areas will require a secondary cure mechanism.
Typical UV curable resins consist of oligomers, monomers (which act as diluents), photo-polymerization initiator, coinitiators (spectral sensitizer, reducing agents etc.) and various additives such as stabilizers, antioxidants, plasticizers, and pigments. The majority of commercial light cure resins are based on free radical curing acrylic compounds (acrylates). Free radical curing compositions are the most versatile curing systems in regard to product properties and monomers / oligomers available on the market. However, free radical acrylic systems are subject to oxygen inhibition, meaning oxygen in the air can stop the molecules at the surface from polymerizing, leaving a tacky surface layer.
A large number of acrylic-functionalized oligomers are commercially available including polyester and epoxy resins, aliphatic and aromatic urethanes, silicones and polyethers. Besides free-radical curing systems, cationic systems based on epoxy and/or vinyl ether compounds are employed. However, only a limited number of monomers and oligomers are available for light induced cationic cure. Thus, the versatility in tailoring properties is rather limited. Furthermore, the photoinitiators used in cationic cure can be somewhat toxic and corrosive. Cationic systems are not subjected to air inhibition but are easily poisoned by moisture in the air.
Epoxy acrylates are very popular UV curing systems. They undergo fast radical induced crosslinking when exposed to UV radiation.
They are extensively used in lithographic inks and varnishes, as well as in printed circuit board, wood, concrete and plastic coatings. They have good all round properties combined with fast cure rates and exceptional adhesion performance due to the presence of polar hydroxyl and ether groups in the epoxy backbone structure. They also have outstanding corrosion resistance.
Both aromatic and aliphatic epoxies are employed. The most common acrylated and methacrylated epoxy oligomers are based on diglycidyl ether of bisphenol A (DGEBA) and derivatives of it. Other important epoxy acrylates include epoxidised fatty acid oils such as soya or linseed oil and acrylates of epoxy novolacs.
Acrylated polyesters are produced in a wide range of
reactivities and viscosities and are mainly used in printing inks,
wood and paper coatings. They are very attractive resin systems due
to their relative low cost and due to the large number of esters
available which allows for a wide variation of properties. Both saturated and
unsaturated esters have been reported in the literature.
This class of UV curing resins has often reasonable all-around performance but only average properties when compared to the other resin systems.They are typically lower in viscosity than epoxy resins and more versatile because they are more compatible with other prepolymers and therefore can be used in a larger variety of formulations.
One of the most important UV curable resins are based on acrylate modified urethanes. Typical prepolymers of this class are made by reacting low
molecular weight polyester or polyether diols with an excess of a aliphatic or aromatic diisocyanates. The remaining diisocyanate functionalities are then capped with an
acrylic monomer such as hydroxyalkyl acrylate which results in acrylic functionalized
urethane. Reactive diluents with one or more vinyl groups are often added to reduce the viscosity of the prepolymer. The prepolymers
are then cross-linked in a free radical type polymerization by exposure to UV radiation.
UV-curable acrylated urethane systems have good all-around performance properties on many substrates. They usually have excllent film hardness, high elasticity and abrasion resistance and high cure rates depending on the prepolymer used. Because of their superior durability and flexibiltiy, they are often used as resilient floor coatings and as coatings for flexible packaging. Other applications inclde printing inks, liquid optical clear adhesives, curable gaskets as well as paper and cardboard coatings.
Silicones are well known for their excellent release properties
as well as for their good heat, weathering, and chemical
resistance. They also have good adhesion to many substrates
including metals and plastics and have a very broad service temperature
range of about -60°F to 500°F (-50°F - 250°F).
Other important properties include good scratch and abrasion resistance, non-yellowing,
excellent optical clarity, and good/excellent electrical
This class of UV curable resins is often used as protective coatings for optical fibers and as potting and encapsulation of sensitive electronic elements such as sensors, connectors, capacitors, inductors, switches and relays. They also find (limited) use as UV-curable adhesives for demanding applications where outstanding flexibility, toughness and abrasion resistance is required. However, this class of adhesives has usually low bond strength which limits their usefulness.
COMMERCIAL UV Curing Resin Systems
Major manufacturers of UV curable monomers and resins are BASF, DSM, Covestro, Allnex, Nippon-Gohsei, Hitachi, Sartomer (now Arkema) and Soltech.
UV curing resins are used in a variety of applications in the coating, ink and adhesive industry. They have become very popular resin systems due to their high productivity, ease of application, and low(er) impact on the environment. They are often the best choice when fast drying and curing is needed. They are extensively used in inks for UV flexo, screen, and lithographic printing, in over print varnishes as well as in products for potting and encapsulation of sensitive electronic elements. Other important applications include scratch resistant wood, concrete and plastic coatings and UV curable acrylic adhesives.
1According to Credence Research and P&S Market Research, the UV curable resins and formulated products market has reached more than 3 billion US$ in 2014 and is projected to grow to over 5 billion US$ by 2021.