Floor Coatings with UV LED Curing: A Focus on Performance and Properties

Floor coating compositions were prepared from monofunctional, difunctional and trifunctional acrylates, as well as urethane acrylates (UA), and used to determine the effect of double-bond equivalent weight (DBEW) mercapto-based resin, high-viscosity UA and LED photoinitiators on final cured film properties when cured under an atmosphere of air vs. nitrogen. The least amount of yellowing was observed using a combination of TPO/ITX as the UV LED photoinitiator package. Scuff, stain and abrasion properties sought in floor coatings were met using LED/UV 365 nm and 385 nm lamps. The use of nitrogen inerting resulted in an improvement in surface properties, such as stain resistance and scuff resistance. Thermal and mechanical properties, such as tensile elongation and storage modulus, are described.

Introduction
Armstrong’s first UV-cured floor coating was introduced in 1976, and coating systems that provide improvements in resisting stains and abrasions have continued to evolve for the past 40 years. In 1976, the company introduced its first UV-curable coating, purchased from W.R. Grace, based on “thiol-ene” chemistry for Solarian tile. The coating brought to market a no-wax “do it yourself” installation. This encompassed the reaction between difunctional norebornenes with R = 6 with tetrathiol. A schematic of the “thiol-ene” cure process is shown in Figure 1. These systems cured quite well due to the nature of the thiol-ene reaction that mitigated oxygen inhibition1,2. As published by Hoyle and Johnson, oxygen has little effect on the polymerization rate of thiol-ene free radical polymerization process. Upon addition of oxygen to the carbon-centered radical formed from the radical propagation step, the peroxy radical readily abstracts a hydrogen from the thiol, producing a thiyl radical that adds to the carbon-carbon double bond, thereby reinitiating the propagation step (Scheme 1).3 These successes with the photoinitiated polymerization of thiol-enes led to the first truly large-scale uses of radiation curing in the US, ensuring the expanding future of this green technology. For a wide variety of reasons, both economic and technical, thiol-ene photocuring gave way to acrylate-based photocurable systems. Objections at the time to thiol-ene-based ultraviolet-curable resins included odor, as well as the incorrect perception that all thiol-ene coatings were subject to rapid yellowing and discoloration upon weathering.

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

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. 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 percent elongation at break, DMA properties to determine level of cross-linking and thermal analysis to determine glass transition temperatures.

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