by Clifford K. Schoff, Schoff Associates
A pre-publication read of Cynthia Challener’s excellent article in this issue on waterborne industrial coatings led me to reflect on my knowledge and experiences in this area. She notes that far fewer commercial waterborne than solventborne industrial products are being used in the United States. People who she interviewed point out many reasons for this, including cost, application difficulties, paint user inertia, and a general attitude that waterborne coatings are not as good as solventborne ones. It is ironic that when I entered the coatings industry almost exactly 43 years ago, I was told that waterborne coatings soon would replace solventbornes in all product areas—Industrial and Automotive as well Architectural. The last-named area certainly is heavily waterborne, and waterborne electrodeposition primers are universal in Automotive along with many waterborne basecoats and a few primer surfacers. However, Industrial has not yet joined the club. There are many paint and resin companies working to change that.
Considering the properties of water, it is not surprising that using it as a solvent or carrier in a coating leads to difficulties. For example, water has a surface tension of 73 mJ/m2 compared to 20–40 mJ/m2 for organic solvents. Low binder surface tension is needed for pigment and substrate wetting and to minimize surface defects. Therefore, additives must be used to lower the surface tension. Surfactant addition is a logical fix, but most surfactants stay behind and make the coating sensitive to water. Use of low surface tension solvents that disappear on drying or baking are a way to get around that, but may raise VOC too much. Most WB coatings need added solvents to compatibilize the aqueous and organic phases and enable coalescence to occur, whereas solventborne coatings are plasticized by the solvents that dissolved the resins and undergo steady solidification. However, newer WB formulations such as those with core-shell latexes with built-in coalescents allow reduced solvent levels.
Considering the properties of water, it is not surprising
that using it as a solvent or carrier in a coating leads to difficulties.
Humidity and temperature on application have considerable effects on film formation and quality of WB coatings. At high humidity, a waterborne film (especially one that is sprayed) is likely to remain wet and initially may be almost soupy, whereas at low humidity, the film may be too dry and have poor appearance. This is why automotive WB basecoats (and WB primer surfacers) are sprayed in booths with temperature and humidity control. This is too expensive for most industrial paint users.
In my experience, WB coatings tend to be more sensitive to oils and other contamination that cause dewetting, craters, and other defects. Therefore, they require better surface preparation and cleanliness than did the older low solids solventborne coatings. However, that is equally true of high solids coatings. Better cleaning and pretreatment is necessary in factory applications for both technologies. WB coatings are more likely to suffer from foam formation, air inclusion, and dissolution of air under pressure. Because of the last-named, airless spray should be avoided. I also have seen popping problems with baked WB coatings. In one case, popping could be prevented by use of a slow tail-end solvent that kept the film open, but, after the bake, enough of that solvent stayed behind that the film was too soft to meet the hardness specification. Regardless of all the difficulties noted here, there are many successful WB industrial coatings, and there will be many more in the future.
Regulations have been the biggest drivers of the adoption of WB industrial coatings, but in the current political climate, it is not clear what the future trend in regulations will be. The administration may move to relax them, but many states will wish to tighten them. Two other factors are the wish for some paint users to be better neighbors and use WB coatings, even if it costs a little more, and increasing customer demand for sustainability. I think that paint users and their customers will ask about sustainability. In addition, green building codes are likely to favor the lower emissions from WB coatings, especially as solvent levels are reduced. Another area that I have not heard mentioned for many years is lower fire insurance premiums for factories using only WB coatings. I recall that, years ago, this was one reason why some customers chose WB, especially when building new plants.
So, the prediction that all coatings would be WB coatings has not held, especially for industrial coatings. However, advances in resin technology and paint formulations plus other factors mentioned above are steadily increasing the market share of WB products in the industrial coating mix.