Learning from the PastExamination of paint films has shown that solventborne alkyds achieve their very high gloss by forming a continuous clear top resin layer during drying. This appearance could only be matched with waterborne emulsions by applying a clearcoat. A water-based alkyd-acrylate hybrid has been developed that behaves similarly to solventborne alkyds, giving an alkyd-like gloss and haze with low solvent content.
The substitution of solventborne paints by water-based alternatives has been a major issue in the coatings industry for at least 30 years. Most prognoses about the time scale of the substitution process have turned out to be wrong. The volume of solventborne paints is still huge, even though their market share has decreased over the years.
Now the situation has changed. With the European VOC directive 2004/42/CE having come into force, limits for VOCs have been set; in fact a whole body of legislation has been created, based upon the category of paint and with different values for solvent- and water-based paints.
Whereas the 2007 limits can often be met by reformulation, it will be difficult to meet the 2010 limits. We assume that a major part of solventborne paints will be replaced by water-based paints and we, at BASF, want to participate in that substitution process.
To be successful in this process it is necessary to overcome the technical deficiencies of current emulsion-based paints regarding application properties such as brush drag, body, leveling and open time; or properties of the coating such as gloss level, hardness, stain blocking and resistance to household chemicals.
The main target of these investigations was to match the gloss and haze of solventborne alkyd paints. For emulsion gloss paints, gloss is typically between 40 and 60 units at 20° and 70 to 85 at 60°. Solventborne alkyds, the benchmark, attain 70 to 85 units at 20° and more than 85 at 60°.
Initial Gloss MeasurementsBasically gloss is dependent on two factors: (1) roughness of the surface and (2) refractive index of the paint film.
When comparing an acrylic-based gloss paint and a solventborne alkyd, the following gloss values were obtained:
- acrylate-based paint: 50/78 for 20°/60° angles respectively; and
- solventborne alkyd: 83/92.
What Variables Influence Gloss?The question is, how can surface roughness be reduced and thereby gloss improved? Factors that were investigated during the study included paint flow, film formation and pigment/binder ratio. Variations were implemented on two emulsion gloss paints, one based on an all-acrylic, the other based on a styrene acrylic dispersion, and a solventborne alkyd paint from a paint store for comparison (Table 1).
RheologyRheology is crucial for paint leveling after application and smoothness of the film surface – at least in the wet stage. Rheology can be adjusted by choice of thickeners in the emulsion gloss paint formulation.
High-shear viscosity of the paints was more than doubled and also the
pseudoplasticity was varied to some degree, but gloss remained the same for all
of the all-acrylic based paints. In the case of the styrene acrylic, only the
paint with the lowest viscosity displayed a somewhat lower gloss. All the
others showed equal gloss and haze data.
Film FormationIn general, solventborne alkyds need a much longer time to dry than water-based paints. Additionally, films are formed from solution, whereas in the case of emulsion gloss paints, latex particles have to coalesce to become a more or less uniform film after evaporation of water.
For emulsion gloss paints, longer drying time means that resin and pigment particles have more time to rearrange into a close, regular packing with less surface irregularities. Drying time of the emulsion gloss paints was influenced by varying film thickness – from 100 to 400 µm film applicator gap – and humidity during drying – from 40% to 90% – both at 23 °C. Results for the all-acrylic are shown in Figure 1. There was no change in gloss or haze, even when the extrema – 100 µm, 40% relative humidity and 400 µm, 90% relative humidity – were compared.
Pigment Volume Concentration (PVC)It is well known that PVC definitely has a strong impact on gloss. For the all-acrylic-based paint, as expected, the gloss decreased and haze increased when the PVC was raised (Figure 2). Thus reduction of PVC could be a way to improve gloss and haze. But even at a PVC of 10%, gloss was far below the alkyd level and hiding power became unacceptable.
Morphology of High-Gloss Coatings StudiedTo gain more insight into morphology, the different paint films were investigated using electron microscopy. Topographic images of the paint films were obtained by scanning electron microscopy (SEM). Whereas the solventborne alkyds displayed smooth surfaces free of defects, the all-acrylic and the styrene-acrylic-based gloss paints were full of surface irregularities, corresponding to their reduced gloss.
In Figure 3 (top) the SEM topographic images of the styrene acrylic and the solvent-based alkyd paint are contrasted. In the bottom row the backscattered electron images are shown. Using this technique, titanium dioxide particles are depicted as white spots in the black binder matrix.
In the case of the solventborne alkyd, the white spots were smaller, which means the titanium dioxide was better dispersed. Furthermore, the coarse surface irregularities of the styrene-acrylic-based film could be attributed to large titanium dioxide agglomerates.
In a further step, paint films were embedded in an epoxy resin, cut vertically to the surface and the ultra-thin cuts were subsequently analyzed by transmission electron microscopy (TEM, Figure 4). Again, images of the styrene acrylate are presented on the left; the solventborne alkyd is on the right. Black spots are titanium dioxide particles. White spots are holes resulting from titanium dioxide particles pulled out by the cutting process. The surrounding binder is grey.
When focusing on the surface, i.e., the borderline between paint film and epoxy resin, a remarkable difference becomes obvious: the styrene-acrylic-based paints show many titanium dioxide particles at the surface and partially sticking out of the binder matrix; in the case of the solventborne alkyd the whole surface is covered with a layer of binder.
Solventborne Alkyds - the High-Gloss SecretBasically there are three factors leading to the superior gloss of solventborne alkyds (compared to acrylates).
- (Somewhat) better dispersion of titanium dioxide.
- High refractive index of the alkyd resin.
- Complete coverage of pigment particles at the surface with binder, leading to a smooth surface, which is homogeneous with respect to refractive index, thus minimizing diffuse reflection of light.
Double-Layer Coatings Enhance Waterborne GlossIf the clear layer of binder on top of the film is the key to superior gloss, it should be possible to boost gloss just by applying a layer of binder onto the emulsion gloss paint film. Thus, different binders were cast onto the all-acrylic-based gloss paint as substrate:
- the all-acrylic dispersion;
- the styrene acrylic dispersion;
- a styrene acrylic dispersion II (with a higher content of copolymerized styrene); and
- an acrylate solution polymer.
Surface roughness of all the double layer coatings was below that of the two alkyds. The sample with the acrylate solution polymer had the lowest mean surface roughness, only about 2 µm, but almost the lowest gloss. It seems that below 10 µm surface roughness the refractive index becomes more and more important. With regard to refractive index, styrene acrylates and alkyd resins have a clear advantage.
It is not necessary to have a thick binder layer on top. When the film applicator gap was reduced to 30 µm, corresponding to a dry film thickness of approximately 7 µm, gloss remained unaffected. Unfortunately emulsion gloss paints showed no tendency to separate into a pigmented base and a clear top layer during drying.
Novel, Superior Gloss Alkyd-Acrylate HybridThe main problems encountered with alkyd-acrylate hybrids are:
- Incompatibility leading to grit, coagulum, viscosity increase on storage or just low gloss; and/or
- Retardation of the polymerization or high residual monomer content, when the alkyd is employed during emulsion polymerization.
The main target of the project was to match the gloss of solventborne alkyds. As can be seen from Table 3, the target has been fully achieved: gloss and haze were almost identical to the benchmark, even though data were obtained with a rather simple starting point formulation (Table 4).
Films of the new paint have also been analyzed by means of electron microscopy in the way described above. In the top row of Figure 5 secondary beam (topographic) images are shown. The new gloss paint surface on the right is smoother than the styrene-acrylate-based on the left; pigment particles are well covered with binder.
The ultra-thin cuts on the bottom look somewhat different as a result of RuO4 staining (the styrene acrylate binder in particular displayed an inhomogeneous structure.). In the image for the new paint on the right, the binder layer on the surface is clearly visible. Thus this gloss paint not only matches the gloss of solventborne alkyds, but as with alkyds, the superior gloss level is achieved by a binder layer on top, which develops during drying.
Properties of New Paint Type ExaminedThe hybrid paint was applied on different substrates and tested in comparison to several water-based paints from paint stores based on polymer dispersions or hybrids. Regardless of the substrate, AQAGloss exceeded all competitive paints in gloss or haze performance. But there are other important paint properties besides gloss. Professional painters will appreciate the following: excellent body; superior brush drag; high film build; easy application by PU-foam applicator; low-foaming tendency; safe working hygiene; and easy cleanability.
For DIY painters it will be important that the paint is water-based; contains only a low solvent amount; no heavy metal driers; no oximes; is easy to apply; and that the equipment can be cleaned with water.
On the exposure test fence, the new gloss paint films displayed better gloss retention and reduced yellowing compared to solventborne alkyds. Water uptake was lower than with the styrene acrylic and the paint films remained free of blisters. Furthermore, resistance to household chemicals was greatly improved.
The secret of the high gloss of solventborne alkyds is a clear layer on top, which separates during drying, thus leading to a smooth and homogeneous surface. With a new water-based alkyd-acrylate hybrid, a similar morphology has been realized, resulting in an alkyd-like gloss and haze.
Application properties, stain blocking and water resistance were closer to alkyds than acrylics. The solvent content of the new formulation was far below the limits of the EU-Decopaint directive and, in addition, driers (Co- or Mn-based) and oximes could be omitted.
This paper was presented at the Nürnberg Congress held during the European Coatings Show, Nürnberg, Germany, May, 2007 and organized by the Vincentz Network. See email@example.com.
Results at a Glance
Tests carried out on waterborne gloss emulsions showed that it was not possible to match the high gloss and low haze of traditional alkyds by modifying the formulation or application conditions.
Examination of paint films by electron microscope revealed that the high gloss of solventborne alkyds is produced by a continuous clear resin layer on top, which separates during drying to give a smooth and homogeneous surface.
A new water-based alkyd-acrylate hybrid has been developed that shows a similar morphology to alkyds, resulting in alkyd-like gloss and low haze.
Application properties, stain blocking and water resistance were closer to alkyds than acrylics.
The solvent content of the new paints was well below the limits of the EU-Decopaint directive, and alkyd driers and oximes could be omitted.