The formulation of wood coatings based on totally VOC-free materials is difficult. When looking for high blocking resistance, we expect a hard binder. Classical binders have a high lowest film-forming temperature (MFFT), and formulations of these binders need plasticizers or solvents (VOCs) for performance at room temperature. With the so-called core/shell-polymerization, which leads to polymer particles with hard and soft domains, it is possible to produce binders with a high blocking resistance and a low MFFT1. In most cases there are acrylic, styrene and, recently, vinylesters of versatic acid2 that can be used as monomers for such dispersions. Copolymerization of hard and soft monomers enables the combination of flexibility and hardness.

The difficulty during development is not generally getting an appropriate formulation - the big problem is the optimization of the final formula to obtain high film property requirements. This report shows that the copolymerization of small quantities of functional monomers in a normal core/shell formulation can improve certain properties of binders and coatings.

Experimental

Small quantities of functional monomers were used either in the core or in the shell for the polymerization of a formula containing acrylics and the vinylester of versatic acid. Therefore, different emulsions were produced, and one after the other was added into the reactor. We used an anionic phosphate ester as surfactant. The polymerization was done at 80 deg C, and in the post reaction the temperature was increased to 90 deg C. After polymerization we measured the content of coagulum by filtering the dispersion through a 100m filter. A film was produced and dried at 50 deg C. The water uptake was determined by storing the film for 24 hours in water. The particle size was measured in a Coulter N4 plus analyzer. Table 1 contains a typical formula for a wood coating. With these coatings we determined the blocking resistance.

Panels of wood were painted three times with the coating. After drying for three days at room temperature, two panels were placed on top of each other and aged in an oven at 50 deg C. A 1 kg weight was set on this stack, and after two hours we determined the force needed to separate the panels. On a scale from 1 - 5, 1 means very bad (the panels stick together) and 5 is very good (the panels separate without help). For measuring the gloss of the coatings, we made 150m films on Leneta foils. After drying we determined the gloss in a Dr. Lange Reflectometer. For the water resistance, we prepared 1000m films on glass plates and put these plates into a water bath. The storage stability was checked in an oven at 50 deg C for 14 days.

Results and Discussion

The original latex showed a particle size of 108 nm. The water uptake of the dried film was below 10%, and the storage stability of the formulated coating was excellent. It was important to increase the blocking resistance and the gloss of the coating. To optimize these properties, we first replaced the phosphate-containing surfactant with different polymerizable types. This led to an increased water resistance of the coatings, caused by a reduction of the water uptake of the film. Due to a strong increase in the particle size, the gloss of the coatings was unacceptably low. Poor storage stability and high coagulum content also eliminated these raw materials from further studies.

Second, small quantities of crosslinkable monomers with different double bonds were used and copolymerized either in the core or in the shell. Figure 1 shows the blocking resistance and particle size of different monomers in the shell.

We clearly saw an increased blocking resistance at a constant particle size. Only with polyethyleneglycol 400 dimethacrylate (PEG400DMA) the particle size increased to 150 nm and is a little bit higher. The values from water uptake of the dried films and the gloss of the formulated coatings (Figure 2) were not affected. The storage stability of all coatings was acceptable - the viscosity of all coatings remained constant after a storage time of 14 days in an oven at 50 deg C. The coagulum content was approximately 0.2%, which is similar to systems without crosslinkable monomer.

If we look in more detail at the data, we find the best results with the ethoxylated bisphenol dimethacrylate (E(10)BADMA). An extremely high water resistance was found with the diallyl maleate (DAM), a value we would not expect from the water uptake of the dried film. Interestingly, the crosslinkable monomers coploymerized in the core showed no influence. Only the pentaerythritol tetraacrylate (PETA) led to a high-gloss coating. The results indicate that crosslinkable monomers copolymerized in the shell increase the blocking resistance, without changing other parameters.

In the next step we looked at silane monomers. Silane monomers are known to increase the wet adhesion, or the scrub resistance, of paints3. Figure 3 shows the water uptake and the blocking resistance of different silanes copolymerized in the core of the original dispersion.

All silanes that were used, vinyl triethoxy silane (VTES), (3-methacryloyloxy propyl) triethoxy- silane (MPTS) and vinyl triisopropoxy silane (VtiPS), led to improvements. Although the particle sizes remained constant, the gloss of all coatings increased. In contrast to the above-mentioned crosslinkable monomers, the silanes that copolymerized in the shell showed a much lower influence. The storage stability was also acceptable for the silanes. When measuring the content of coagulum, the methacrylic silane had a very high quantity.

The increased blocking resistance, using the crosslinkable monomers, can be explained with the higher crosslinking density. The explanation of the results with the silane monomers in the core is quite difficult. Our results showed that silane monomers copolymerized in the core reduce the water uptake of the films and increase the blocking resistance and gloss of the formulated coatings.

In the last step we used nitrogen-containing monomers. Such monomers are also known to optimize the wet adhesion and the scrub resistance of paints4. We tried 1-(2-methacryloxethyl)imidazolidin-2-one (MAEI), N-(2-allylcarbamato) aminoethyl imidazolidinone (ACAEI) and methacrylamido ethylethylene urea (MAEEU). Figure 5 shows the water uptake of these monomers, copolymerized either in the core or in the shell.

In all cases we found lower values. Lower value particle sizes were also obtained (Figure 6). Surprisingly, this does not lead to an increased gloss of the coatings. In contrast, these values are also decreasing. When using nitrogen-containing monomers such as these, it is necessary to change the surfactant system to compensate the decreased gloss. We concluded that nitrogen-containing monomers in the core or the shell do reduce the water uptake of the dried films, but they also reduce the gloss of formulated coatings.

Conclusion

The use of functional monomers in core/shell latexes increases the properties of the binders and of the formulated coatings. While crosslinkable monomers are efficient in the shell of these latexes, silane monomers need to be copolymerized in the core to increase the parameters. Nitrogen-containing monomers are affecting the blocking resistance when copolymerized in the core or in the shell, but they also reduce the gloss of the coatings, while reducing the particle size. The different results of these monomers are shown in Figure 7.

For more information, contact Dr. Christof Arz, Collano AG, CH-6203 Sempach-Station, Switzerland; e-mail christof.arz@collano.com.

This paper was presented at the 7th Nurnberg Congress, European Coatings Show, April 2003, Nurnberg, Germany.

References

1 BASF, EP 0 184 051; BASF, EP 0332 011; Rohm & Haas, DE 28 11 481
2 Tradename Veova, Resolution
3 Wacker, DE 43 06 831, Development results Resolution
4 Rohm GmbH, DE 39 02 555; Rohm GmbH, EP 0379 892; Rohm & Haas, US 5 212 225; Nacan Products; EP 0 383 592