The process by which Ultraviolet (UV) light is used to initiate a photochemical reaction that generates a crosslinked network of polymers is known as UV curing.
UV light curable resins, being eco-friendly and versatile are being developed to reduce environmental harm. UV curable coatings have many advantages over conventional curing systems as they consume less energy and equipment space, produce less waste, emit fewer gases, have higher productivity (faster curing) and lower temperature treatment. UV curable resins generally do not contain organic solvents that have adverse effects on the environment. Their usage results in reduction of energy consumption and air pollution.
The main disadvantage when it comes to UV light induced curing systems is the limited depth of light penetration which depends on the spectral distribution and wavelength and generally does not exceed a few millimeters. The light induced cure is therefore limited to adhesives, inks and coatings. At least one substrate must be able to transmit light, in the case of adhesive applications. Systems with shaded areas usually require a secondary cure mechanism.
UV curable resins typically consist of:
Oligomers
Monomers (acting as diluents)
Photo-polymerization initiators
Coinitiators (reducing agents, spectral sensitizer)
Additives (plasticizers, stabilizers, pigments, antioxidants)
Most light cure resins present in the market are based on acrylates or free radical curing acrylic compounds. Acrylates are the most versatile curing systems when product properties are taken into consideration. They can, however, leave a tacky surface layer as they are subject to oxygen inhibition. This means that the oxygen in the air can stop the molecules at the surface from polymerizing.
Apart from free-radical curing systems, vinyl ethers, cationic systems based on cyclic ethers, and other heterocyclic compounds are used. For light-induced cationic cure, a limited number of oligomers and monomers are commercially available. The photoinitiators used in this cure may be corrosive and toxic. Cationic systems are easily poisoned by moisture present in the air, but they are not subjected to air inhibition, unlike free-radical curing acrylates. A wide range of acrylic-functionalized oligomeric resins is available today, which all have widely varying properties, including polyethers, silicones, aromatic and aliphatic urethanes, epoxy, and polyester resins.
Acrylated Urethanes: UV curable resins based on acrylate modified urethanes are an important category of oligomeric resins. These pre-polymers are prepared by reacting low molecular weight polyether or polyester diols with an excess of aromatic or aliphatic diisocyanates. The acrylic functionalized urethane is then formed when the remaining diisocyanate functionalities are capped with an acrylic monomer such as hydroxyalkyl acrylate. Reactive diluents with one or more vinyl groups are generally added to decrease the viscosity of the pre-polymer. The pre-polymers are then crosslinked in a free radical type polymerization by exposure to UV radiation. UV-curable acrylated urethane systems have excellent all-round performance properties on many different substrates. They usually have high elasticity, very good film hardness, abrasion resistance and high cure rates depending on the pre-polymer used. Because of their superior flexibility and durability, they are often used as coatings for flexible packaging and resilient floor coatings. Other application segments include liquid optical clear adhesives, curable gaskets, cardboard and paper coatings and printing inks.
Acrylated Epoxies: Epoxy acrylates are extremely popular UV curing systems. When exposed to UV radiation, they undergo quick radical-induced crosslinking. They are used extensively in lithographic inks and varnishes, printed circuit boards, concrete, plastic and wood coatings. They have excellent overall properties, fast cure rates and good adhesion performance due to the presence of polar hydroxyl and ether groups in the epoxy backbone structure. They also provide good resistance to corrosion. Aliphatic and aromatic epoxies are used as UV curings. 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 epoxidized fatty acid oils such as linseed or soybean oil and acrylates of epoxy novolacs.
Acrylated Silicones: Silicones are generally known for their excellent release properties and good resistance to weathering, heat and chemicals. They have good adhesion to different substrates such as plastics and metals and have a broad service temperature range (-60°F to 500°F). Additional favourable properties include good optical clarity, abrasion and scratch resistance, excellent non-yellowing and electrical properties. Silicone acrylate UV curable resins are used as protective coatings for optical fibers and as potting and encapsulation of sensitive electronic elements such as capacitors, sensors, inductors, connectors, relays and switches. They are also used as UV-curable adhesives for applications where outstanding toughness, abrasion resistance and flexibility is required. Unfortunately, this class of adhesives usually has low bond strength which limits their usefulness.
Acrylated Polyesters: Acrylated polyesters are used mainly in paper and wood coatings and printing inks, and are produced in a wide range of viscosities and reactivities. They are favorable resin systems as they are relatively inexpensive and have wide variation of properties due to the large number of esters available. Both saturated and unsaturated esters are present in the market. Acrylated polyester UV curing resins have all-round performance but only average properties when compared to other resin systems.They are usually lower in viscosity than epoxy resins and more versatile, can be used in a larger variety of formulations and are more compatible with other pre-polymers.
Applications of UV Resins
UV curing resins are used in the ink, coating, and adhesive industry in a variety of applications. They have become popular resin systems due to their ease of application, high productivity, and low impact on the environment. They are utilized when quick drying and curing is required. They are used extensively in inks for screens, lithographic printing, print varnishes as well as in products for potting and encapsulation of sensitive electronic elements. Other important applications include UV curable acrylic adhesives, scratch-resistant wood, plastic and concrete coatings.
C&E offers a wide variety of eco-friendly UV-curable technologies. We strive to create excellence by offering top-quality solutions to meet our customers' requirements. Contact us to understand our UV-Curable Oligomers and Monomers and how our products can be utilized for specific applications.
uv@cel.co.in
By Ashni Arun on March 23, 2022