Best of Both Worlds
Waterborne acrylic polyurethane has excellent application, performance characteristics
In the finishing industry, “green” is a term used (or sometimes over-used) to describe the move toward more environmentally friendly and safe coating materials and processes. Increasingly strict federal, state and local environmental and worker safety regulations have required coating formulators to look at modifications to solvent-based coating formulations or find solutions using water-based technology.
According to Randy Funston, president of Excalibur Paint & Coatings Ltd. (Wichita Falls, TX), development of green coating materials has been the main project of the company’s R & D laboratory since its inception nearly a decade ago.
“We set out to develop a protective coating that would meet demands on two global fronts: the needs of the commercial industry and the green coating movement to address air quality and health and safety factors,” Funston says.
To do it, Excalibur assembled a team of three developmental chemists with a combined 120 years’ experience in the paint and coating industry. The result of their efforts is Aqua-Thane, a patent pending, 2K water-borne acrylic polyurethane technology with performance properties said to meet or exceed those of high-performance solvent-borne systems without the associated health and safety hazards.
“Prior to the introduction of Aqua-Thane, the coatings industry had been unsuccessful in developing a two-part, water-borne polyurethane system that has a VOC of less than 50 g/L and the performance characteristics of a solvent-based coating,” says technical director Craig Murphy. “Initial problems included foaming and gassing when wet film thickness exceeded 2–3 mils, primarily due to the reaction of isocyanate with water. Furthermore, the systems were unable to achieve the desired flow that is required in larger-scale commercial painting. The lack of flow would result in an eggshell-type appearance.”
According to Murphy, the sensitivity of water-based systems necessitated years of testing to find raw materials that were compatible and worked synergistically with the binder system. Surface treatment of pigments in regard to the type of dispersant and additives used also affected a number of physical properties including film coalescence, stability, viscosity and gloss, he says.
“Most manufacturers sought easier solutions by modifying solvent-based technology, which resulted in high-solids, polyaspartic and polysiloxaine technology,” Murphy says. “Formulators produced these materials primarily by reducing solvent, introducing low-viscosity reactive diluents, adding exempt solvents and pigment modification, including moving to lower oil-absorption materials.”
Funston says these reformulated solvent-based materials offered many benefits towards the environment, but also required modified application techniques and equipment requirements due to viscosity and pot-life limitations. “Larger pumps and sometimes plural-component equipment were required to facilitate these modified products,” he explains. “In many instances, performance properties were also sacrificed.”
According to Funston, Excalibur’s development team faced these and other challenges in formulating the material. Other key performance issues that had to be addressed included:
- Chemical resistancen Corrosion Resistance
- Storage Stability
- Crosslink density
- Air entrapment during manufacturing
- Gloss and Color Retention
- VOC content.
Excalibur says Aqua-Thane addresses these concerns, easily achieving wet film build of 4–8 mils without foaming, gassing or flow problems. The material is also said to exhibit exceptional depth of image and have good resistance to abrasion, solvents and chemicals while maintaining hardness and flexibility. Application is similar to that of conventional, higher-VOC solvent-based systems of the past, with no need for specialty airless equipment is eliminated.
According to the company, Aqua-Thane also exceeds solvent-based polyurethanes in adhesion. It can be applied directly to metal (DTM) or over primer on a wide variety of substrates, including hot- and cold-rolled steel, hot-dip or electrogalvanized steel, aluminum, PVC and polypropylene, stainless steels and composite materials.
Funston says the material’s adhesion and performance characteristics lend it to a wide variety of potential applications, including aircraft, computer cabinets, land and offshore drilling platforms, rail, transportation and wind generation.
The table shows performance properties for hot-rolled steel test panels prepared by applying Aqua-Thane material at dry film thickness of 3 mils. Panels were allowed to cure for a week inambient laboratory conditions before testing.
|Wet tape adhesion||ASTM D 3359||Pass|
|Hardness—pencil test||ASTM D 3363||(2H) Pass|
|Impact||ASTM D 2794|
|Direct intrusion >30 inch-lb||>60 Pass|
|Flexibility—GE impact||>40% Elongation||>60% Pass|
|Low-temperature flexibility||No cracking—1-inch bend||Pass|
|Humidity resistance||No blistering, adhesion loss, defects etc.||Pass|
|Weather resistance||ΔE <1.0 after 500 hr xenon arc exposure||0.25 Pass|
|Gloss 80 minimum||Pass|
|Weather Resistance||ASTM D 523, 2,000 hr|
|Color difference <3 ΔE||Pass|
|60º specular gloss 85 minimum >88||Pass|
|Solvent resistance||50 MEK rubs||Pass|
|Fluid Resistance||Lubricating oil, 24 hr at 250º F Pass|
|Jet fuel (JP-5), 7 days at room temperature||Pass|
|Hydraulic oil, 24 hr at 150º F||Pass|
|Surface finish||No irregularities or orange peel||Pass|
|Salt fog||ASTM B 117, 1500 hr|
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