Solvent-free epoxy coatings offer very low environmental impact, but due to the lack of a clear and universal definition of “solvent-free” epoxy coatings, we see a huge range in volatile organic compound content in such coatings. This opinion article discusses this challenge and offers a proposal for a common universal definition for solvent-free-epoxy coating.
There is currently no clear universal definition of what a solvent-free epoxy coating is in the market today. The generic name indicates that the epoxy coating should be “solvent free,” but when looking at solventfree epoxy coatings currently on the market, we see that it’s not always the case. To be able to spray apply such a solvent-free coating through single-feed airless spray equipment, the viscosity needs to be low enough to get a good spray pattern without the need for thinning with solvents. A very common formulation approach for solvent-free epoxies is to use low viscosity liquid bisphenol A or bisphenol A/F epoxies that have been modified with reactive diluents.
The epoxy binder is cured with low-viscosity polyamine or polyamide curing agents that can be delivered with 30% benzyl alcohol as a solvent to further lower viscosity, work as a good epoxy compatibilizer, and accelerate the cure. In many cases, the viscosity needs further reduction to reach optimal application properties, so 10 wt% or more benzyl alcohol solvent can be added as a non-reactive epoxy resin diluent.
Discussion
Without any scientific proof that we are aware of, benzyl alcohol has been claimed to remain as a non volatile in epoxy coatings, but benzyl alcohol is an aromatic alcohol with a boiling point of 205 °C and in our opinion should be classed as a solvent and contribute to the coating’s volatile organic compound (VOC) content.1
Some argue that due to benzyl alcohol’s high boiling point, most of it will not evaporate or diffuse out of the coating film. There is, however, a reason why over the last two decades we have seen solvent-free epoxy coatings for potable water tanks move toward benzyl alcohol-free formulations as it has been observed that over time the solvent diffuses out of the coating film, giving taste and smell to the potable water.2-3 The significantly lower practically determined volume solids compared to the values calculated treating benzyl alcohol as a non-volatile, is a strong indication that it is, in fact, volatile and should be classified as a solvent.
In Table 1, benzyl alcohol contents in the epoxy base and curing agent part of marine solvent-free epoxy tank coatings have been taken from the coating manufacturers' material safety data sheets.
From the table, it is clear that not all paint manufacturers count benzyl alcohol as a solvent that increases VOCs and lowers volume solids of the coating, so we get the unfortunate situation that solvent-free coatings that should have very close to 100% volume solids and VOC 0 g/L can vary anywhere between 95 to 100% solids and 0 to 180 g/L in VOCs. Considering that ultra-high solids epoxy coatings can have up to 97% volume solids and down to 50 g/L in VOCs, a universal common definition of solvent-free epoxy needs to be made so that users can clearly distinguish between solvent-free and ultra-high solids epoxy coatings. In addition, the main advantages of solvent-free epoxy coatings are very low or no VOC emissions, the possibility to apply thick films with little or no film shrinkage, and lower film formation stress.
Based on earlier work we have done more than a decade ago and as a world-leading coating consultancy in marine, protective, and yacht coatings, the authors would propose the following universal definition for a solvent-free epoxy coating:
“A ‘solvent-free epoxy’ coating can be defined as an epoxy paint where all of the non-reactive components of the formulation have an initial boiling point greater than 250 °C at an atmospheric pressure of 101.3 kPa. Benzyl alcohol added to epoxy coatings should be counted as a volatile since it is not reactive and falls within the definition of a solvent according to European Union Paint Directive 2004/42/CE. Benzyl alcohol will also lower the practical volume solids compared to that calculated and often stated on data sheets, when assuming it is non-volatile. At the same time, its inclusion increases the volatile organic compound content of the coating.”
Hopefully, this proposed definition for solvent-free epoxy coatings will start a discussion among relevant stakeholders to agree on a common clear and universal definition.
References
1 “Directive 2004/42/CE of the European Parliament and of the Council of 21 April 2004 on the limitation of emissions of volatile organic compounds due to the use of organic solvents in certain paints and varnishes and vehicle refinishing products and amending Directive 1999/13/EC,” Official J. L 143 (30/04/2004): pp. 0087-0096.
2 J. Romero, et al., “Characterization of Paint Samples Used in Drinking Water Reservoirs: Identification of Endocrine Disruptor Compounds,” J. of Chromatographic Science 40 (April 2002).
3 M. Aamodt, “Eco Friendly Coatings for Potable Water Tanks Offshore,” Offshore Marine Technology (1st Quarter 2010): pp. 20-21.