High-solids (HS) industrial coatings are those which have a solvent content less than 30%. More specifically, if the solids content of a coating is greater than 80 volume % (vol%) or 60 weight % (wt%) it is categorized as an HS one. This means all other coatings with a solids content less than 80 vol% are conventional low-solids (LS) coatings. Figure 1 shows an over simplified illustration of LS and HS coatings.
HS coatings are being used by the automotive, aerospace and marine industries.
Note that the powder coatings have 100 vol% solids because they typically do not have solvent(s). In this post, the focus is on solventborne HS coatings with the aim to discuss briefly the parameters influencing the film formation process in HS coatings.
Why we need HS Coatings?
The short, sweet and ‘green’ answer to this question is that we reduce emissions of volatile organic compounds (VoCs) by employing HS coatings. Solids or Volume-solids (VS) of a coating indicates how much of the applied wet paint film will end up as a dried/cured (solid) film. Therefore, the higher the VS, the lower the VoC of a given coating i.e., the coating is ‘relatively green’ for the environment.
This is especially true for solventborne coatings. For a waterborne coating, this can be true equally. Waterborne coatings have water as the main solvent. Consequently, they already have low VoC (but are not necessarily VoC-free).
In addition to VoC reduction, if the user/customer has to apply one coat instead of two (or more) to achieve the desired DFT (and the properties) by using HS coating, the overall cost of paint application is reduced (see Figure-2). Furthermore, some (environmental) savings might also be associated with the production process of HS coatings.
Is the Chemistry of LS and HS Coating Really Different?
With respect to the underlying chemistry, the main binders and curing agents used in LS and HS coatings are similar. The major difference is in the size of the molecules used i.e., molecular weight of the resins used in HS coating is lower than that of the resins used in LS coatings. Relatively small sized resin molecules (or oligomers) used in HS coatings cause reduction in the overall viscosity of the formulation which helps in reducing solvent(s) used and, consequently, increase the solids content of the coating (while reducing the VoC of coating).
In addition to the size, the presence and the number of reactive functional groups on (some or all of) the LMW molecules is a critical parameter to adjust in HS coatings. According to Hill et al., the presence of reactive functional groups is about three times more important in HS coatings than in the conventional LS coatings. The reason being the fact that one wants to get the right chain extension with the lowest possible amount of solvent added to the coating.
Using different non-functional hydrocarbon resins, extender grade and thixotropic agents also helps in increasing the solids content of the coatings while influencing the viscosity depending upon their properties. Therefore, in most cases, LMW and/or functional molecules are preferred over other formulation handles for increasing the solids content.
The presence of reactive functional groups is about three times more important in HS coatings than in the conventional LS coatings.
As an example, European Patent EP2931822A2 reports achieving HS epoxy-amine coating by blending a conventional bisphenol A based epoxy resins with low molecular weight 1,4-cyclohexanedimethanol epoxy resin. Several other examples exist in the patented and open literature covering epoxy, polyurethane (PU), acrylics etc coatings.
Parameters Influencing Film Formation in HS Coatings
As mentioned above, HS coatings have high content of low molecular weight chemical compounds. This translates into the fact that film formation and final mechanical properties of HS coatings can be very different from their conventional LS coatings if not formulated carefully. Therefore, it is a challenging task to formulate HS coatings whose performance is like their LS analogs. After spraying, the wet film properties play a crucial role in the performance of the coating during its entire service life. A brief summary of the important parameters influencing wet film properties of the HS coatings is presented below.
Molecular Weight Development & Viscosity
Functionality of the low molecular weight (LMW) chemicals or oligomers used in HS should be selected carefully. If the functionality of the LMW chemical is one, dangling ends can be produced in the cured network. These dangling ends act as plasticizers impacting the mobility of the chains and, hence, properties (i.e., the service life) of the coating. Typically, the used LMW chemicals should be at least difunctional. Correct functionality selection is also important for the molecular weight build-up and crosslinking during cure. This is linked to the glass transition (Tg) development during (and after) cure. Molecular weight build-up during cure impacts the viscosity of the coating. Very high molecular weight build-up will increase the viscosity of the coating while very low molecular weight build-up can keep the film tacky for a long time. In most cases, both are unwanted (no one wants either a high viscous coating or a tacky film for a long time after application). As a result, an optimum functionality of the LMW chemical provides a good balance between the viscosity and molecular weight development of the HS coating.
More functional groups and reduced level of solvent increase the surface tension of the HS coatings. Consequently, film defects can occur if the surface of the substrate is not prepared in a way that it has a surface free energy similar to (or comparable with) the surface tension of the HS coating. Cratering or crawling has been reported to occur in such cases. Therefore, formulation of HS coatings need optimization with respect of surface tension.
Due to increased number of reactive functional groups and reduced solvent amount, the pot of life of HS coatings is typically lower than their conventional LS equivalents. For the same reason, storage stability of HS coatings is also influenced.
The thickness of the adsorbed layer (of resin and other additives) around the surface of the pigment particles is among the important parameters controlling good dispersion of pigment particles in the continuous phase of a coating. It is believed that when the LMW resin molecules are used, the thickness of the adsorbed layer around the pigment particles is not adequate. As a result, chances of pigment flocculation in HS coatings are significantly higher than in their LS conventional analogs.
Different Solvent Evaporation Rate
In sprayed coatings, sagging is more difficult to control for HS than for the conventional LS coatings. Existing evidence suggests that this is due to the difference in the solvent evaporation rates of HS and LS coatings during spray application. During the travel of atomized droplets between the spray gun and the substrate being coated, HS coating loose less solvent than the conventional LS coatings. This results in a film with a low viscosity from an HS sprayed coating and a more viscous from an LS sprayed. Therefore, the film from an HS sprayed coating has a high tendency to sag. So far, despite the presence of different explanations (colligative effect of low mole fractions, Tg related etc.), it is not well-understood why there is a different solvent evaporation rates are different between the HS and LS coatings. In a nutshell, the HS coatings need to be formulated carefully to avoid sagging issues.
In conclusion, the control over film formation process in HS coatings to obtain a defect free dried/cured film can be challenging. Defect free dried/cured film is extremely important in achieving the maximum desired performance of a given HS coating. Moreover, correctly formulating an HS coating can be challenging. The choice of LMW chemicals/resins has to be made in such a way that the delicate balance between the reaction/drying kinetics, surface properties, processability and final coating properties is maintained.