Nanoparticle dispersions as functional additives can be an interesting alternative to improve the performance of waterborne coatings. This paper illustrates that specifically processed ZnO nanoparticle dispersions called Oxylink™ can increase the chemical resistance of waterborne coatings.
Introduction
The overall market share of environmentally friendly waterborne coatings is still relatively small compared to solventborne alternatives. Yet, waterborne coatings become increasingly important as a technology to reduce VOC emissions and thus cost. Waterborne coatings are pushed worldwide by regulatory bodies for higher market shares. Today, they are used in a wide selection of market segments, such as wood and furniture, but also on non-wood substrates such as metals and plastics for industrial maintenance coatings, coatings for machines and equipment, and metal cans.1 One obstacle that still limits the use of waterborne coatings in various applications is the sometimes poor solvent and humidity resistance of waterborne coatings. In order to overcome these drawbacks, different additives are available, mainly based on silicones or paraffin waxes. However, these additives aren’t universally applicable and can be used only in specific formulations. In addition, they may cause problems later when coated parts have to be over-coated or refurbished.In this paper we present an alternative additive technology, based on ZnO nanoparticle dispersions called Oxylink, which increases the chemical resistance of waterborne coatings, such as durability against solvents, to enable a wider use of waterborne coatings.
It is well known that the dispersion quality critically influences the functionality and performance of inorganic nanoparticles.2 Dispersing nanoparticles is even more intriguing than dispersing other pigments because the resulting composition is a microscopically homogeneous composition with respect to optical and catalytic properties for example. As dispersing nanoparticles is the enabling technology to provide efficient ZnO nanoparticle additives, we start the discussion with a focus on dispersion technology.
Dispersing Nanopowders
Nanopowders have a high specific surface area of up to several hundred m²/ml (Figure 1). Dispersing nanopowders generates new interfaces within the formulation, and the particle-matrix interface needs to be chemically stabilized. Suitable stabilization mechanisms are well known.3Colloids can be electrically stabilized if the pH value can be adjusted accordingly in an aqueous media. The mechanism is fast and efficient. However, many real-world products do not provide this freedom and, consequently, steric and electrosteric stabilization mechanisms are used in addition, or instead, if it comes to stabilizing particles in complex formulations.
Chemical Surface Modification
The chemistry of nanoparticles can be compared to molecular chemistry rather than to the behavior of micron-size particles.4 The surface of inorganic oxides is comprised mostly of OH-functions. Using molecular bifunctional additives, these groups are accessible to a chemical interaction (Figure 2). Examples of suitable reagents include a wide range of chemicals such as: silanes, boranes, carboxylic acids (di-, tri-,), amines, b-diketones, and other chelating agents.The second function of the molecular additive provides stabilization of particles in the product as well as additional chemical reactivity by introducing groups such as amine, C=C double bonds, alcohol or epoxy groups. The correct choice of surface modifier is crucial to realize the respective application. The surface modifier influences the isoelectric point (IEP), polarity and reactivity of the particles as well as solid content and viscosity of the dispersion.
Chemomechanical Process
Dispersing nanopowders requires a deagglomeration step within the process. Chemomechanical processing means carrying out the surface modification reaction under well-defined mechanical conditions. We have found the use of agitator bead mills favorable for the deagglomeration part of the process (Figure 3). Because many parameters influence the selection of the agitator bead mill, there is no standard equipment that can be used in all cases. For instance, product viscosity, cooling options for temperature-sensitive products, availability of materials for the grinding chamber (the stator) and the rotor (e.g., ceramics like ZrO2, Al2O3 or SiC; polymers like PU, PA or PFA; steel), sensitivity of the product with respect to contamination, chemically aggressive products, bead size and thus grinding media separation as well as flow rate need to be considered for the best choice of equipment.When manufacturing dispersions below 100 nm particle size, a recirculation mode is often used because of the demand for rather high energy input into the product. In the case of inorganic oxides, even if only loosely agglomerated, the specific energy requirement to overcome the inter-particle interaction (such as Van-der-Waals) is typically in the range of 1 to 10 kWh/kg product, not to speak of aggregated particles or true grinding of materials.
Oxylink Technology
Oxylink is a chemomechanically synthesized additive based on nanoscaled ZnO dispersed in water. As explained in the previous section, the specific parameters of the process yield a ZnO-based additive formulation. Oxylink has a low viscosity at a solid content of ca. 40 wt.%.We investigated the effect of Oxylink on chemical resistance in a variety of different aqueous dispersion coatings. In general, 1% (solid on solid) of the additive was introduced into the suggested coating formulations with gentle agitation. The resulting formulations were stable and no precipitation was observed.
The coatings were drawn onto glass substrates using a coating knife to obtain a wet film thickness of ca. 100 mm (ca. 4 mils). The slides were dried at 70 ºC. The cloudiness was evaluated visually for each coating system to ensure a good degree of dispersion. Only coatings with no or low haze were included in this study.
The chemical resistance of the films was investigated using MEK double-rub tests. We performed the rub test until the film was continuously destroyed. In an additional evaluation, some coatings were subjected to water vapor for 48 h and visually inspected.
We investigated the solvent resistance as a function of Oxylink concentration in steps of 1, 2 and 3% solids in solids for an acrylic coating (LS 1032, Revertex, see Figure 4). We found that a concentration of only 1% is highly effective in increasing the resistance in MEK double rubs by a factor of ca. 3. Higher additive concentrations result in even higher resistance, however, the correlation is not linear.
Traditionally, additives based on silicones or paraffin/polyolefin waxes have been used to increase the chemical resistance of waterborne coatings. We have therefore formulated waterborne coating systems with wax additives for comparison.
The authors conclude that Oxylink is more widely applicable for increasing the chemical resistance than traditional organic additives like wax dispersions.
Neither the wax nor the inorganic nanoparticle dispersion prevented the increase of haze under the saturated humidity conditions at elevated temperature. However, Oxylink resulted in a formulation that shows a completely reversible hazing effect, and the coating turned completely clear again after drying. This observation stands in contrast to the coating containing a polyolefin wax additive, which was irreversibly affected by the water vapor.
The authors hypothesize that the effect of the inorganic additive is due to a nanoparticle-catalyzed crosslinking effect. Whereas the dispersions usually just dry physically, the additive seems to result in at least a partial chemical crosslinking. Studies have shown5 that mechanical properties such as dry and wet scrape resistance may be improved by a low percentage of nanoscaled ZnO, as well. Presumably, ZnO catalyzes the curing process and leads to a denser network of the polymer structure. ZnO is thus used in a nanoscaled form not only for optical reasons (to reduce haze in clear coatings) but also as it provides a very high surface area.
Outlook
We have found a large influence of specific process parameters on the performance of nanoscaled ZnO as an additive in waterborne coatings. Obviously, a better degree of dispersion yields a better transparency (lower haze). More surprisingly, we found also a strong influence of process parameters on efficiency in chemical resistance. This finding stresses the high importance of dispersion technology. We will use the large parameter space accessible by chemomechanical processing to further optimize the performance of nanoscaled ZnO within the Oxylink platform.Summary
We have presented an additive technology based on ZnO nanoparticle dispersions called Oxylink that increases the chemical resistance of waterborne coatings. We stress the importance of dispersion quality on overall performance and use the chemomechanical process to produce Oxylink from agglomerated nano powders. We investigated the performance of the resulting additive formulation in waterborne acrylic systems and found it highly efficient and broadly applicable, in particular when compared with wax additives.This paper was presented at The Waterborne Symposium sponsored by The University of Southern Mississippi School of Polymers and High Performance Materials and The Southern Society for Coatings Technology, 2009, New Orleans, LA.
