Demand in the architectural coatings market requires new polymers to meet strict hardness properties while maintaining low coalescent demand. In the past, a polymer’s hardness and blocking properties could be improved simply by using higher-Tg monomers and increasing the volatile coalescent amount used in formulating the polymer. Due to changes in environmental awareness, the VOC limitations of architectural paints require less coalescent, less solvent or use of a low-VOC coalescent.1 Meeting the desired hardness at low coalescent demand becomes even more difficult in deep-base paint systems with high loadings of low-VOC colorants that exacerbate the problem. The most common issue with performance in deep-base paint systems with low-VOC colorants is the tackiness and “green feel”. The term green feel typically refers to the paint remaining sticky to the touch even after long cure times. The low-VOC colorants introduce many surfactants, dispersants, humectants and coalescing agents that contribute to this issue. Tackiness is similar to green feel, however this term is used for short-term stickiness that fades over time, and is defined as the ability to form a connection of measurable strength to a substrate under pressure after a short contact time.2 As a result of these issues, there is a market requirement for a low-VOC-capable polymer that functions in low-VOC, colored, deep-base paint systems with low to almost no tack or green-feel. This unmet market need must also be balanced with other common paint properties.
Another important feature of the described low-VOC paint systems is the hardness profile. There are many different methods for evaluating the hardness of a coating, and the variety of tests may lead to different conclusions about the hardness performance.3 Some commonly used tests are Koenig hardness, pencil hardness, block resistance, print resistance, scrub resistance and various tack methods. Each of these methods does not always correlate with the others, which can lead to different conclusions for the hardness of the coating. As a result, the ultimate hardness profile of the coating must be balanced depending upon the application or function required by the end-use consumer. Some other important properties of architectural paints include oil and lanolin resistance, chemical resistance, abrasion resistance, scrape resistance, flexibility and cleanability. If the coatings are to be used in exterior applications they must also withstand heat, temperature changes, moisture, oxygen, sunlight and freeze-thaw cycles.3 A fundamental understanding of the polymer design as it affects the polymer functionality is critical to meet the current demand for architectural coatings. This study develops a test method to measure surface tack to accelerate polymer development for an architectural, high-gloss, interior and exterior paint application with an emphasis on performance in deep-base systems with high loadings of low-VOC colorants. This application space is very difficult for current polymer technology to meet the demand of hardness and low tack while maintaining the other previously described properties. Much polymer development focuses on increasing the hardness of the polymer with the assumption that the tack will improve as well; however, this is not always the case. This study compares the different measurements of hardness and tack to evaluate if a direct correlation can be made. There are many references in literature for different methods to measure tack, including Zapon, peel tack, rolling ball tack and probe tack, as well as subjective hand feel of the coating. Many of these test methods have limitations and are not consistent. This study utilizes the use of a modified probe-tack test on a tensile tester to replace the other subjective tack tests with a quantitative and reproducible measurement for tack. The tensile tester was fitted with a probe fixture to quantify the tack performance of deep-base paint systems with high loadings of low-VOC colorants. Correlation analysis between results of the probe tack test and other hardness tests was used to make conclusions on the overall hardness performance of industry-leading paints and polymers.