What is UV LED Curing?
UV LED stands for UltraViolet light being emitted by Light-Emitting Diodes. UV curing has traditionally been accomplished using mercury-vapor lamps but with the advent of LEDs capable of producing UV radiation, curing devices using LEDs are rapidly finding their way onto the market.
UV curing is a photopolymerization process that uses UV energy to change a liquid to a solid. Upon absorption of the UV energy, the photoinitiator produces free radicals which initiate cross-linking between resins and cure or solidify the ink or coating.
An increasing number of players in packaging coatings and inks (e.g. overprint varnishes, flexographic printing, digital inks), wood coatings and adhesives (e.g. labels, displays, medical, electronics, industrial) are replacing their traditional mercury lamps by UV LED. Industries beyond those already mentioned are also eager to bring LED curing benefits to their applications. This trend is driven by improvements in process efficiency, economics as well as safety and environmental aspects:
- Compact and scalable design of product line
- Instantaneous turning on and off of the curing unit - no warm-up times
- Constant output
- Ease of handling heat-sensitive substrates due to low operating temperatures
- Reduced electricity consumption
- Increased lifetime – LED chips last ~30,000 operating hours
- Reduced maintenance
- No UVC and UVB wavelengths
- Low operating temperatures
- No ozone emission, no mercury
UV Cure with medium pressure Hg lamps vs UV LEDs The same, but not quite…
Compared to the output of standard medium pressure mercury UV lamps, which ranges from UVC to infrared wavelengths, UV LEDs emit very intense light in a very narrow range of UVA and visible light wavelengths. As a consequence, the number of useable photoinitiators is limited, and formulations based on them perform well in through-curing but have underperforming surface cure.
relative intensity of UV light sources (illustrative)
Combining high quality LED lamps with the proper photoinitiator is the first ingredient behind the setting up of a LED curing line. Given the options available on the market at this moment, this needs to be supplemented by the use of specific resins that improve surface curing. In partnership with clients and UV lamp suppliers, Allnex has developed a range of resins that covers this need. In combination with the selection of the proper LED machine and photoinitiator, these resins are helping companies to successfully set up LED UV curing systems. In low energy UV curing systems, where the intensity of traditional Mercury lamps is reduced in order to save energy, the same challenges are encountered. As such, below products will also bring solutions for low energy curing systems.
Below resins are exceptionally reactive under LED lamps. Next to conventional performance properties, the selection of resins in a formulation will be based on the balance between high reactivity with low viscosity.
|Functionality||Viscosity (cP @ 25 °C)||Tg (°C)||Tensile Strength (psi)||Elongation at break (%)||Notes|
|EBECRYL® 5781 (bio-based, not available in EU)||2||450||162||1,740||<1||Hard, scratch-resistant, low viscosity and shrinkage|
|EBECRYL® 895||5.5||7,500||Hard, scratch-resistant and high cross-link density|
|EBECRYL® 3703||2||̴ 30,000||57||5,900||47||Amine modified, flexible, good pigment wetting, good chemical resistance.|
|EBECRYL® 3600||2||̴ 20,000||59||12,300||8||Amine modified, high gloss and hardness, low color, excellent chemical resistance|
|EBECRYL® 3418||2||̴ 20,000||4,067||15||High flexibility, gloss, toughness, adhesion and superior chemical and scratch resistance|
|EBECRYL® 8811||2||̴ 450,000||44||1,986||66||Flexible and tough, good abrasion resistance, non-yellowing|
|EBECRYL® 4858||2||7,000||54||5,700||3.5||Low viscosity, good hardness, low color, good impact resistance|
|EBECRYL® 8465||3||2,100 at 60°C||36||3,726||50||High viscosity, tough, excellent exterior durability, non-yellowing|
|EBECRYL® 8210||3.5||4,500||68||6,400||2.1||Low viscosity, low color, high gloss, scratch and abrasion resistance|
|EBECRYL® 4201||4||7,000||12||870||15||Aliphatic, low viscosity, high flexibility and outstanding abrasion resistance|
|EBECRYL® 4501||4||̴ 6,000||9||870||15||Aromatic, low viscosity, high flexibility, high coin and scratch resistance|
|EBECRYL® 1291||6||̴ 80,000||80||1,070||1||High gloss, scratch and abrasion resistance|
|EBECRYL® 2221||6||28,500||49||8,000||3||PETIA-free aromatic, low color, high gloss, hardness, solvent resistance|
|EBECRYL® 5129||6||̴ 20,000||30||9,100||4||Flexible, high gloss, good scratch and abrasion resistance|
Regulation friendly for tin, heavy-metals* and quinones *As defined by C.O.N.E.G’s Toxic in Packaging Legislation, the ASTM Standard Consumer Safety Specification on Toy Safety F 963 (ASTM F 963-08), or the EU Directive 94/62/EC (and amendments) on packaging and packaging waste
The use of following diluents to reduce viscosity will contribute to increased reactivity under LED curing.
|Functionality||Viscosity (cP @ 25 °C)||Notes|
|EBECRYL® 110||1||̴ 20||Excellent viscosity reduction and flexibility|
|EBECRYL® 130||2||160||aliphatic di-acrylate, increases Tg of formulation|
|EBECRYL® 160||3||80||Good flexibility while retaining hardness|
|EBECRYL® 85||3.6||̴ 160||Amino modified polyether acrylate, giving low odor and high gloss. Recommended for low migration applications.|
|EBECRYL® 45||4||160||Polyether acrylate, improves hardness and solvent resistance of formulation. Recommended for low migration applications.|
|EBECRYL® 892||4||̴ 140||Polyester acrylate, giving low odor and high surface hardness|
Alternatively, Allnex UCECOAT® range of waterborne UV resins is very well suited for LED curing. Waterbased systems will inherently reduce oxygen inhibition, and because of that display improved surface cure. Additional information on such systems can be found here. Within Allnex UCECOAT® range of waterborne UV resins, UCECOAT® 7200 stands out for its high reactivity.
|Solid Content||Viscosity (cP @ 25 °C)||pH||Notes|
|UCECOAT® 7200||65||<700||3-5||High solid content, high scratch, abrasion and chemical resistance. Can be combined with EBECRYL® P115 as neutralizing amine and as LED booster.|
|UCECOAT® 7210||65||<1,000||2-5||High solid content, recommended as blending partner for UCECOAT® 7200 to adjust flexibility|
Finally, the following LED boosters can be added to improve surface cure. These additives will mitigate oxygen inhibition to improve surface cure.
|Viscosity (cP @ 25 °C)|
|EBECRYL® LED 02||110|
|Amino modified acrylates|
|EBECRYL® P115||̴ 20|
|EBECRYL® P116||̴ 20|
|EBECRYL® 7100||̴ 1,200|