Floor Coatings with UV-LED Curing – Additional information

Floor Coatings with UV-LED Curing – Additional information

By Gary Sigel, Ph.D., senior principal scientist

Armstrong Flooring

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UV-LED Characteristics

Since 1977, Armstrong Flooring has used conventional medium pressure Hg vapor arc lamps to cure acrylate-based 100% solids UV coatings. The operating technology of conventional UV lamps is based on Arc Lamp UV. The classic type of medium pressure vapor lamp source is the electrode type, or arc lamp. The bulbs are fabricated from quartz tubing with a small amount of mercury and sometimes other inert gases, vacuum sealed between two metal electrodes. An arc generated between the electrodes produces a plasma that gives emission of light from 200nm to 450nm. The emitted light in the UVA, UVB and UVC spectral regime is absorbed by specific photoinitiators that leads to polymerization by initiation, propagation, and termination steps. The heat generated by medium pressure lamps in the form of UV and IR energy can be a problem for flooring substrates if there is not a proper cooling design in place for the arc lamp unit.4

Several papers have addressed the drawbacks of conventional medium pressure Hg vapor lamps compared to LED semiconductor light sources. In LEDs, as excited electrons relax, energy is emitted in the form of photons where the emitted photon is based on the material used for the construction of the LED.5 Narrow wavelengths are directly emitted by the current input where the spectrum is a monochromatic radiation in defined wavelengths, e.g. 365nm, 385nm, 395nm,and 405nm (Scheme 2).

There are several reasons why Armstrong Flooring would be interested in utilizing UV LED over UV technology, including lower processing temperatures for wood and dimensionally sensitive substrates such as rigid or plasticized films used in the manufacture of tiles and 12-foot-wide sheet goods used for both commercial and residential applications.6 In the wood industry, excess heat mainly comes from 5 or more UV banks of lamps after each coating station as well as carrier belts (Scheme 3). The excess heat in the board can lead to blisters in the soft grain or open grain areas as shown in Figure 2. This can occur due to residual uncured stain and/or coating that originates from a layer in the coating structure, as depicted in Figure 3, that upon exposure to the IR component of the UV lamps comes to the surface prior to cure. The result is a finish exhibiting cured stain blisters or coating blow out through the soft grain of the wood due to the porous nature of the wood.

Coating requirements for flooring are very different from other coating systems used in the UV/EB industry, such as inks, varnishes, coatings on furniture and plastics, and wall coverings. Several surface properties of the coating must be retained after UV initiated polymerization including stain resistance to common household stains, ease of cleaning the surface after spills on the floor or extended wear, scuff resistance to rubber heels or other types of footwear and ability to retain original aesthetics after every day wear from foot traffic.

Several papers have been written on the topic of mitigation of oxygen inhibition in UV LED curing processes. These papers have focused on similar themes to improve the effects of oxygen inhibition including processing changes or chemical reactivity. Process changes include nitrogen inerting, higher peak irradiance or barrier films to mitigate oxygen diffusion. Changes in formulation to improve surface cure include chemistry changes to increase viscosity, higher functionality acrylate materials to increase double bond equivalent weight of formulation, use of photoinitiators specific to desired wavelength or increase concentration, utilization of wax additives, use of liquid-liquid barriers and addition of chemicals that mitigate oxygen inhibition such as thiols, amines or ethers.7-10

UV LED surface cure has been documented by several authors using FTIR spectroscopy. Bowman conducted studies on the effects of oxygen inhibition by FTIR based on a number of techniques to mitigate the effects of oxygen inhibition. These techniques include:

1. the use of high intensity irradiation sources to increase the initiation rate by increasing the production of primary radicals such that it becomes much greater than their consumption by oxygen,
2. formulation changes to have higher cross linking as determined by double bond equivalent weights,
3. polymerize the samples in an inert environment whereby the oxygen is eliminated from the polymerization,
4. chemical additives in the form of thiols, amines or ethers from backbone structures (e.g. ethoxylation of acrylates), and
5. co-initiators such as ITX or photoinitiators designed to absorb light energy in the 395nm to 408nm regime such as phosphine oxides MAPO, BAPO, and more recently 3-ketocumarins.11

This paper will explore methods to mitigate oxygen inhibition by utilizing reactive chemicals and nitrogen atmosphere to improve surface cure for UV/LED cured floor coatings. The types of materials studied can be broken down into high viscosity urethane acrylates, mercapto modified polyester acrylate resin, and LED photoinitiators within a base formulation for each series of formulations (Tables 1, 2). Within these comparative studies using a base formulation with the same materials, the effect of atmosphere processing conditions (i.e., air vs. nitrogen) on degree of cure, glass transition (Tg), mechanical properties, and scratch, scuff, and stain performance will be described. Properties of cured UV/LED coatings include double bond conversion determined by FTIR, mechanical tests to determine % elongation at break, DMA properties to determine level of cross linking, and thermal analysis to determine glass transition temperatures.

Three studies are presented:

1. Effect of double bond equivalent weight (DBEW) within the formulation on degree of cure, mechanical testing, and performance data (Table 3, purple).
2. Effect of photoinitiator type, phosphine oxide based TPO, TPO-L vs. 3-ketocumarin on cure of LED formulations (Table 4, yellow).
3. Effect of high viscosity urethane acrylate and thiol based acrylate on LED cure (Table 5, green).

References

1. J.S. Ross, L.W. Leininger, G.A. Sigel, and D. Tian; “A Brief Review of Radiation Cure Systems Used in Flooring;” RadTech Conference Proceedings; 241 – 250; Baltimore, MD; April 9-12, 2000.
2. “Armstrong Wood Coatings Quality Journey,” J.S. Ross and G.A. Sigel; RadTech Report, May/June2006, pgs. 39-47.
3. Charles Nason, M. Cole, CE Hoyle, Sonny Johnson, Fusion Systems, Inc. “Photocuring of Hard Thiol-Enes,” RadTech 2002 (Scheme used to show oxygen reaction)
4. Kijoi, Ed, “Wood Coating with UV-LED Curing; A focus on Heat,” Issue 2 2014 RadTech Report.
5. G.A. Sigel, L.W. Leininger, E.A. Malkowski and J.S. Ross “Issues Associated With Wide Web Curing” RadTech Conference Proceeding; 494-503, Baltimore, MD; April 9-12, 2000.
6. G.A. Sigel, L.W. Leininger, E.A. Malkowski and J.S. Ross, “Gloss Banding Phenomena In Wide Web Applications,” RadTech Conference Proceedings: 373-385; Indianapolis, IN: April 29–May 1, 2002
7. V. Landry, P. Blanchet, G. Boivin, Jean Bouffard, M. Vlad, “UV-LED Curing Efficiency of Wood Coatings,” Coatings 2015, 5, 1019-1033.
8. Jennings, Sara, “UV-LED Curing Systems: Not Created Equal,” Presented at 2016 RadTech International).
9. Jo Ann Arceneaux, “Mitigation of Oxygen Inhibition in UV-LED, UVA and Low Intensity UV Cure,” UV+EB Technology, Vol. 1 No. 3, pp48-56.
10. Jo Ann Arceneaux, “Oligomer Solutions for UV Curable Inkjet and 3D Printing Applications,” RadTech Conference Proceedings; Chicago, IL, May 16-18, 2016.
11. R.P. Karsten, “Implementing UV LED Curing for Wood Coatings,” Phoseon Technology p.7. Available online: http://www.phoseon.com/uploads/pdfs/documentation/uv-led-curing-for-wood-coatings.pdf (accessed on 31 July 2015). Coatings 2015, 5 1033
12. E.V. Sitzmann, “Critical Photoinitiators for UV-LED Curing: Enabling 3D Printing, Inks and Coatings.” In Proceedings of the Radtech UV.EB West 2015, Redondo Beach, CA, USA, 10 March 2015.
13. A. Freddi, M. Morone, G. Norcini, “Design of New 3-ketocoumarins for UV/LED Curing,” UV+EB Technology, Vol 2 No 3, pp46-51.