Seite/Page: 1 Perfect layers Coalescents are critical in every waterborne paint formulation. Both the concentration and the type of cosolvent can have a profound effect on film properties. Specially designed, solvent-free, secondary polyol emulsions, so called NOVOS emulsions, have been used to determine the effect of various co-solvents on the film performance of waterborne, two-component, polyisocyanate crosslinking coatings. An advantage of the emulsions is their significantly improved blisterfree film thickness (BFFT) compared to traditional secondary emulsions. A novel class of waterborne two-component isocyanate curing resins Tijs Nabuurs Willem-Jan Soer Wendy van Bavel Jelle van der Werf Waterborne two-component, polyisocyanate curing coatings suffer from two distinct disadvantages compared to their solventborne counterparts. Firstly, in solventborne coatings, the polymer and crosslinker are both soluble in the solvent of choice. In waterborne coatings, however, they both have to be emulsified in water. This can lead to serious issues [1] particularly with the crosslinker. When hydrophobic crosslinkers are used, high shear needs to be applied for emulsification to prevent inferior film properties [2]. Very often, the crosslinker is dissolved in co-solvents prior to addition to the binder to improve miscibility. An alternative is to use hydrophilically modified polyisocyanates. This does not always give the required chemical resistance. Secondly, whilst the chosen solvent in solventborne coatings the solvent is inert to the crosslinker, in waterborne coatings, the isocyanate group can readily hydrolyse (see Figure 1). Even though in general the rate constants of the reaction between aliphatic polyisocyanates and water are lower than with primary alcohols [3], the excess of water compared to polymer bound hydroxyl groups can cause significant hydrolysis. Hydrolysis is a problem The disadvantage of polyisocyanates hydrolyzing is twofold. When an isocyanate group reacts with water, an unstable carbamic acid group is formed. This readily dissociates to form an amine group and carbon dioxide. The amine group will subsequently react with a second isocyanate group to form a urea bond (see Figure 1). Hence, hydrolysis leads to crosslinking, but at the expense of a second crosslinking group. For this reason waterborne, two-component, polyisocyanate crosslinking coatings are always formulated with more isocyanate than hydroxyl groups. The carbon dioxide formed is a problem especially in thick films. At a dry film thickness of more than 50 µm, blisters may form that affect both the aesthetics and coherence of the film. Acrylic polyol emulsions for two-component, polyisocyanate crosslinking can be prepared using two processes. So-called primary emulsions are made via emulsion polymerisation. An advantage of such emulsions is that particles are formed in situ and no emulsification step is required. Because they can have high molecular weights, physical drying times are generally short. A disadvantage is that, due to inferior flow and leveling, film appearance is not always perfect. Secondary emulsions are made using a process in which firstly the polymer is first made by solution polymerisation in an organic solvent. Then, the polymer solution is emulsified in water after neutralisation of the polymeric acid groups with a base. In this step, good processing is dependent on the polymer solution viscosity. Hence, the polymer is limited in molecular weight or much organic solvent need to be used. Since this is highly undesirable, the molecular weight is generally limited to approximately 50 kg/mole. An advantage of the polymerisation in organic solvents is a more homogeneous incorporation of functional groups, such as hydroxyl or acid groups. As the molecular weights for secondary emulsions are generally lower than in primary emulsions and relatively high levels of organic solvents are used, the film appearance of secondary acrylic polyol emulsions crosslinked with polyisocyanates is very good. Significantly higher gloss levels and better DOI are achieved than with primary emulsions. The low molecular weights also show disadvantages. Firstly, tack-free times are generally much longer than for primary emulsions. Secondly, higher hydroxyl concentrations are needed to give the same crosslinking density as higher molecular weight resins. Finally, because of the way that secondary emulsions are made, to date, they have always contained organic solvents. This greatly influenced the freedom of the paint formulator in selecting both the optimal solvents and total VOC levels. In this article, the effect of co-solvents in waterborne twocomponent, polyisocyanate curing binders on various film properties are looked at. In order to achieve this, secondary emulsions were prepared that were essentially free of any solvent. Waterborne two-component, polyisocyanate curing binders based on secondary emulsions often contain coalescents. They are chosen as their high boiling points enable effective solution polymerisation. The disadvantage is, however, that because of their high boiling point, the evaporation rate is very slow. In Figure 2 the concentration of the combination of butyl glycol and "Solvesso 100" in two-component, polyisocyanate curing films is shown as a function of time. After 24 hours less than 80% of the coalescents had evaporated. Solvent-free emulsion The use of co-solvents in waterborne paints is under debate; the focus is primarily on environmental issues. However the coalescent to be used in for the chosen
Seite/Page: 2 application is very relevant. For waterborne, twocomponent, polyisocyanate crosslinking coatings, this topic has only been discussed to a limited extent [3,4]. The main point has been the use of protic coalescents. Isocyanate reactive solvents do not appear to affect film performance negatively. The effect of a using co-solvents (see Table 1), with various evaporation rates and different degrees of solubility in water, in two-component, polyisocyanate curable binders was tested. Here new, solvent-free, waterborne, polyol emulsions were used. These so-called NOVOS emulsions allow selection of the optimal coalescent for film performance and give the formulator increased flexibility of achieving the desired VOC level. An overall coalescent concentration of 8 wt-% on total emulsion was used in all samples. For the purpose of comparison, secondary emulsions were used based on three different polymer compositions with OH levels of 3.3 %, 4.2 % and 5.0 % (corresponding to OH numbers of 108.9, 138.6, and 165.0 mg KOH/g of solid resin, respectively).table 2 is an overview of application properties for the most relevant coalescents for the three different emulsions. The results compare as expected. With increasing hydroxyl number, chemical resistances and König hardness increased. Due to the higher concentration of polyisocyanate in these films tack-free times also increased. Logically, for all secondary emulsions, drying times increased as the evaporation rate of the coalescent decreased. Films containing solvents with slow evaporation rates also gave significantly softer films. Surprisingly, using solvents with a high evaporation rate gave shorter tackfree times than those of solvent-free systems. The cause is probably solvent retention in the drying film. However, a comparison of the masses of drying films with slow or fast solvents, or even no solvent at all, shows that the water evaporation rate was affected by the choice of solvent. This can be seen in Figure 3 using the polyol with an OH number of 3.3 %. In the first 30 minutes of film formation, when using "Dowanol MPA", the rate of evaporation of water was significantly increased compared to the film comprising the slower evaporating "Solvesso 100". The weight loss of the film cast without any co-solvents lies between those of the slow and fast solvent. Hence, besides the possible effect of solvent retention on tack-free time, films containing fast evaporating solvents also give faster water evaporation. Only butyl digycol, which evaporates extremely slowly, or to a lesser extent "Dowanol DPM", showed a significant decrease in chemical resistance and surface hardness compared to the faster evaporating coalescents. The main reason for this is probable co-solvent retention in the film. The use of protic solvents led to sediment formation, grains, hazy films or even gellation. Hence, formulating these types of binders with, for instance, butyl glycol or butyl diglycol is not advisable. This seems to be in contrast to results reported in literature [4]. When the hydroxyl number was increased to 4.2 % or even 5 %, no stability problems are observed. Apparently, the higher hydroxyl numbers provide enough extra stabilisation to prevent settlement. However, they still give films with grains or a haze. Coalescents with different solubility in water, showed no clear differences. For the two emulsions with the lower hydroxyl number, the reference coalescent combination gave significantly longer tack-free times than only butyl glycol. This could mean that decreased water solubility of the coalescent does indeed lead to longer drying times. The gloss readings in Table 2 show that film formation of the solvent-free polyol emulsions was good. The extent of film formation was investigated further using AFM. In the topology pictures in Figure 4, polyol emulsions formulated with the standard solvent combination (4 wt-% of "Solvesso 100" and 4 wt-% of butyl glycol) were compared to those of the solvent-free film. As can be seen film formation was perfect in both cases. This can also be judged from the height differences in the z-range, which was 11 nm for both films. Hence, under the microscope, film formation in the absence of co-solvents is as good as when using powerful coalescents. Careful choice of co-solvent Thus in general, co-solvents need to be carefully selected for use in waterborne two-component, polyisocyanate crosslinking coatings. For some applications, properties such as drying times, gloss values or chemical resistances may need to be adapted. This is achieved by selecting the proper solvent or mixture of solvents. Solvent-free, zero- VOC secondary emulsions alone can already provide an interesting set of film properties, for example, high levels of gloss, König hardness and chemical resistances. Surprisingly high BFFT (Blister Free Film Thickness) values were observed for all solvent types. Due to the specific process by which the new emulsions are prepared, there are reduced concentrations of low-molecular weight, acid rich chains (see Figure 5). Using gradient HPLC, the chemical composition distribution of polyols was compared before and after methylation of the acid groups. In order to draw conclusions from Figure 5, the upper two plots need to be compared with the lower curve. It is clear that the chemical composition distribution of the new emulsions hardly changed upon methylation. The plot of the polymer prepared using to the typical solution polymerisation process in butyl glycol/"solvesso 100" changed markedly in the middle section. This results from the methylation of acid rich material which was originally not visible in the HPLC plot. The principle difference between the former and the new process in terms of chemical composition distribution is that the new process for making emulsions yields far less acid rich material. In earlier articles the role of acid rich material in the formation of carbon dioxide blisters has been discussed [5]. High concentrations of polymer chains with a high acid load result in an increased concentration of water in the drying film. This water causes more extensive hydrolysis of polyisocyanates during cure, and thus blister formation. Hence, in the new process, less acid rich chains are formed. This will result in reduced sensitivity to hydrolysis and a higher film thickness before the onset of blisters. References [1] Bui H., Dvorchak M., Hudson K., Hunter J., Eur. Coat. Jnl., 1997 J. 97, p. 476. [2] Melchiors M., Sonntag M., Kobusch C., Jürgens E., Prog. Org. Coat., 2000, 40, p. 99. [3] Wicks Z., Wicks D., Rosthauser J., Prog. Org. Coat., 2002, 44, p. 161. [4] Trapani A., Wood K., Wood T., Pitture e Vernici Eur., 1995, 71 (9), p. 14. [5] Nabuurs T., Pears D., Overbeek A., Prog. Org. Coat., 1999, 35, p. 129.
Seite/Page: 3 Figure 1: The desired reaction in two-component, polyisocyanate curing coatings (I) and reactions occurring upon hydrolysis of the polyisocyanate (II)
Seite/Page: 4 Figure 2: Solvent concentration as a function of time in a two-component, polyisocyanate curing film. Starting concentration was 9 wt-% of a 50/50 mixture of butyl glycol and "Solvesso 100"
Seite/Page: 5 Figure 3: Weight loss of drying films of polyols with a hydroxyl number of 3.3 %, formulated at an NCO:OH ratio of 1.2.*
Seite/Page: 6 Figure 4: Profile plots of solvent containing (A) and solvent-free (B) two-component, polyisocyanate cured films
Seite/Page: 7 Figure 5: LC plots of methylated polyols formed by solution polymerisation in butyl glycol/"solvesso 100" (upper) and by the new process yielding NOVOS emulsions (middle), compared to plot of polymers before methylation (lower)