High-Performance, Zero-VOC Waterborne Acrylic Topcoat

Introduction

In order to address the VOC elimination issue, paint manufacturers have formulated coating systems that contain zero VOCs and hazardous air pollutants (HAPs) for the zero-VOC market. Among these coatings are acrylic latex water-based paints designed for architectural interior applications. Although formulators are working hard to meet the performance requirements of the protective coatings market, usually the zero-VOC latex acrylics do not perform as expected as an anticorrosive coating for exterior applications.

There are two main approaches to this challenge. First, is to choose the acrylic latex system that is engineered to form a film without coalescent solvents; second, is to find all additives that are zero VOC. These materials should also improve blocking resistance and hardness to impart better weather durability and dirt pick-up resistance for an optimized formulation.

A zero-VOC acrylic topcoat designed for protective coating application has been formulated with no VOCs or HAPs. It is based on a new, innovative latex that has very small particle size and can form a film at low temperature without coalescent solvents. Utilizing all unique, zero-VOC additives, the new development offers high gloss, fast cure, excellent corrosion resistance, weathering durability, early moisture resistance, freeze-thaw resistance and dirt pick-up resistance that is comparable to a high-VOC commercial acrylic.

Experiment and Discussion

Binder System
The key parameters in selecting a water-based acrylic latex that can be formulated into a zero-VOC paint were the choice of particle size and glass transition temperature (Tg) of the polymer.(1) The major properties of a zero-VOC acrylic paint, including the film formation, hardness and blocking resistance, are influenced by the particle size and Tg of the acrylic latex. Good film formation at difficult conditions, such as below 40 °F/50% RH, will require a very soft polymer to ensure optimal polymer flow. This implies that the polymer will need a glass transition temperature that is well below 40 °F, which will result in poor block resistance and insufficient hardness.

To achieve the required film properties, a high Tg is required. Care should be taken in the selection of a high Tg polymer. If it is too high this will result in poor film formation, cracking, delamination and high MFFT. Normally the term “soft” refers to a polymer with a Tg<32 °F; the term “hard” refers to a polymer with a Tg>122 °F.(2) Another area to consider would be the performance properties of zero-VOC acrylic paints, which are dependent on the particle size of the acrylic latex. Generally, the smaller the particle size of the acrylic latex, the better the film formation. Smaller particle size helps the acrylic latex in forming a continuous solid film where latex particles are packed together without the need of co-solvent.

Figure 1 Click to enlarge

Plasticizers in a Zero-VOC Acrylic Coating
Plasticizers are commonly used in low-VOC acrylic formulations, especially the zero-VOC versions. Plasticizers work as a non-volatile coalescing agent in the zero-VOC formula. Plasticizer has been shown to effectively increase polymer flow and reduce the minimum film formation temperature (MFFT) of the zero-VOC paint. The film formation was considerably improved when a plasticizer was added to the formula (Figure 1).

Table 1 Click to enlarge

The use of a plasticizer lessens the possibility of film cracking and delamination; in many instances, the performance of the finished film was improved. However, great care must be taken in the choice and level of plasticizer. An improper plasticizer can cause softness, haziness and low gloss of the finished film (Table 1). The plasticizer selected in this zero-VOC acrylic topcoat gives the finish high gloss, toughness, low-temperature (40 °F/50% RH) cure properties and a comparable dirt pick-up resistance to the commercial high-VOC acrylic.

Pigmenting in a Zero-VOC Acrylic System

Table 2 Click to enlarge

Dispersing pigment into a solvent-free system such as a zero-VOC waterborne acrylic presents unique challenges. Zero-VOC latex systems are not inherently good media for achieving the degree of dispersion required for high-gloss finishes, so identification and optimization of the proper dispersing agent is extremely important. Initially, three different dispersing agents were evaluated at various usage levels (Table 2). A good dispersing agent must have good compatibility with the binder and other additives in the formula. For high-gloss finishes, the dispersing agent must provide not only good pigment wetting but also pigment de-flocculation and electrostatic stabilization to achieve high gloss at both 20º and 60º illumination angles. The dispersing agent Sample 3 provided a uniform electrical charge to pigments thus avoiding pigment flocculation. Once the proper dispersing agent is identified and optimized, the new zero-VOC acrylic topcoat achieves a gloss >85 @ 60º and >70 @ 20º, as well as good color acceptance and pigment stability.

Freeze-Thaw Stability of a Zero-VOC Acrylic Coating

Table 3 Click to enlarge

Zero-VOC formulations often have inherently poor freeze-thaw stability, due to the lack of co-solvent. In latex paint, water is a continuous phase in which small complex micelles of polymer are dispersed. As this continuous phase solidifies under the influence of low temperature, the dispersed particles can be fused together and coagulated. Thawing can also cause problems in the latex system because each component of the formulation tends to become soluble at a different temperature; this results in a non-homogeneous system. The ideal method used in the formulation of zero-VOC latex paints to reduce the temperature at which the latex paint will freeze is to add a water-miscible additive such as a non-ionic surfactant. These non-ionizing, larger organic molecules will reduce the density of the hydrogen-bonding network to prevent development of large ice crystals by lowering the freezing point of the zero-VOC acrylic paint (Table 3).

Dirt Pick-Up Resistance of a Zero-VOC Acrylic Coating

Table 4 Click to enlarge

In general, zero-VOC acrylic coatings do not have excellent dirt pick-up resistance due to the soft resin and additives such as plasticizers used in the formulation. Plasticizers are known to have a strong effect on film formation of zero-VOC coatings and always have to be optimized according to the final paint properties, especially the dirt pick-up resistance. As the results show, the higher the loading level the worse the dirt pick-up resistance. However, certain types of flow and leveling agents are able to improve dirt pick-up resistance on zero-VOC acrylic coatings. Since the flow and leveling agent helps the acrylic latex to lower the surface tension by forming a tough, smooth film, the need for higher loading levels of plasticizer can be reduced. By using the proper loading level of plasticizer along with carefully selected flow and leveling agent, the newly developed zero-VOC acrylic topcoat achieves good dirt pick-up resistance that is comparable to the high-VOC acrylic latex coating (Table 4).

Coating Performance

Table 5 Click to enlarge

Mechanical Properties
The zero-VOC acrylic topcoat described in this paper has excellent mechanical properties as shown in Tables 5-7.

Early Moisture Resistance

Table 6 Click to enlarge

The new acrylic topcoat has very good early moisture resistance as tested in a mist box. This test determines the effect of water misting on an applied coating film during various stages of the coating cure cycle. The mist box cabinet is equipped with six fog nozzles utilizing tap water and has a panel rack for supporting test specimens at an angle of 30º from vertical.

After the coating is applied at recommended film thickness, samples cured for 2, 4 and 6 h and longer intervals if necessary, are exposed to the mist cabinet for 24 h. The panels are removed from the mist box and compared to control panels cured at 77 ºF/50% RH. Degree of blistering is checked immediately and after recovery using ASTM D 714.

Table 7 Click to enlarge

Gloss readings are taken according to ASTM D 523 and compared to the control panels. Adhesion checks per ASTM D 3359 are made at 1 and 24 h recovery periods. The zero-VOC acrylic topcoat showed excellent early moisture resistance after only 2 h cure (Table 8).

QUV Resistance
The QUV (ASTM D 4587) chamber utilizes a 4 h light cycle of intense light, using UVA 340 lamps at 60 ºC followed by 4 h condensation on the coating surface at 50 ºC.

Under these conditions, the new acrylic topcoat showed excellent gloss retention of greater than 70% after 1000 h exposure (Figure 2) and color change of less than 3.5 delta-E (Figure 3). This is better than the commercial high-VOC acrylic topcoat.

Figure 2 Click to enlarge

Corrosion Weathering Resistance
ASTM D 5894 describes a cyclic weathering test that includes wet/dry and light/dark cycles to mimic natural exposures. Test panels are exposed to alternating periods of one week in a QUV chamber and one week in a cyclic fog/dry chamber. The QUV/condensation cycle is 4 h UV at 60 ºC and 4 h condensation at 50 ºC, using a UVA-340 lamp. The fog/dry chamber runs a cycle of 1-hour fog at 35 ºC and 1 h dry-off at 35 ºC. The fogging electrolyte is a relatively dilute solution consisting of 0.05% sodium chloride and 0.35% ammonium sulfate.

Figure 3 Click to enlarge

The new acrylic topcoat applied direct to a sand-blasted panel cured for 7 days at room temperature (77 ºF/50% RH) was rated as 10 with no blistering, and rusting was rated as 9 after 5 cycles. This performance is comparable to the commercial high-VOC, high-performance acrylic and better than the other commercial high-VOC acrylic topcoat that was also tested (Table 9).

Table 8 Click to enlarge

The new acrylic topcoat was also applied direct to sand-blasted panel, cured for 14 days at low temperature (40 ºF/50% RH), and was rated as 9 with blistering; rusting was rated as 9 after 5 cycles. This performance is better than both of the commercial high-VOC acrylic topcoats that were also tested (Table 10). The results show that this new zero-VOC acrylic topcoat has excellent low temperature cure properties that allow this topcoat to be applied at low temperature and still provide good performance.

Table 9 Click to enlarge

Salt Fog Resistance
In the salt fog test (ASTM B 117) the test panels are continuously exposed in the cabinet to a wet and dark salt fog environment. The fogging electrolyte solution consists of 5% sodium chloride sprayed at 95 °F (35 °C). The new acrylic applied over a commercial acrylic latex primer was rated as 10 with no rusting, blistering was rated as 9 and scribe performance was rated as 8 with 1/32th inch scribe creep after 1500 h (Figure 4).

Figure 4 Click to enlarge

This performance is comparable to the commercial high-VOC, high-performance acrylic topcoat over the commercial acrylic latex primer system (Figure 5).

Condensing Humidity Resistance
Testing was conducted in accordance with ASTM D 4585, where condensation is produced by exposing one surface of a coated panel to a heated, saturated mixture of air and water vapor, while the reverse side of the panel is exposed to the cooling effect of room temperature air. The chamber utilizes the water temperature of 100 °F and cooling temperature of 77 °F. The zero-VOC acrylic applied direct to sand-blasted panels cured for 7 days at room temperature was rated as 10 with no blistering and no rusting (Figure 6).

Figure 5 Click to enlarge

This performance is better than both of the commercial high-VOC acrylic topcoats that were also included in this evaluation (Figure 7).

Recommended Systems

Figure 6 Click to enlarge

The new zero-VOC acrylic topcoat can be applied directly to concrete, metal and primed substrates. The coating is compatible with a wide variety of primers including water-based acrylics, solvent-based alkyds and zinc-rich primers. These systems offer comparable exterior durability and corrosion resistance to high-VOC, high-performance acrylic topcoat and primer systems. Table 11 shows examples of recommended systems.

Figure 7 Click to enlarge

Conclusion

Table 10 Click to enlarge

A high-performance, zero-VOC acrylic topcoat has been formulated based on a new innovative acrylic resin that has a fine particle size and a proper Tg without adding solvent or additives that contain VOCs or HAPs materials. The new development offers excellent corrosion resistance, weathering durability, early moisture resistance, freeze-thaw resistance and dirt pick-up resistance that is suitable for protective coating applications. The performance properties of the new zero-VOC acrylic topcoat are comparable to the commercial high-VOC, high-performance acrylic.