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:

Allnex
  • 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
AllnexSavings
  • Reduced electricity consumption
  • Increased lifetime – LED chips last ~30,000 operating hours
  • Reduced maintenance
Allnex
  • 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
Polyester acrylates
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
Epoxy acrylates
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
Urethane Acrylates
EBECRYL® 8811 Allnex 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 Allnex 4 7,000 12 870 15 Aliphatic, low viscosity, high flexibility and outstanding abrasion resistance
EBECRYL® 4501 Allnex 4 ̴ 6,000 9 870 15 Aromatic, low viscosity, high flexibility, high coin and scratch resistance
EBECRYL® 1291 Allnex 6 ̴ 80,000 80 1,070 1 High gloss, scratch and abrasion resistance
EBECRYL® 2221 Allnex 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

AllnexRegulation 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

 For further questions about UV LED curing or Allnex LED-curable products, please contact us here