This article focuses on the chemistry of dirt pickup resistance (DPR) and outlines what steps – including research, product development and testing procedures – need to be taken to provide the proper solutions.
Dow Coating Solutions has ad-dressed the issue of dirt pickup resistance as part of ongoing product development initiatives for more than 40 years. However, due to the trends and market drivers outlined in article one,1 the company recently launched a focused R&D platform to accelerate technology and new product introductions.
As a recap, the need for better dirt pickup resistance is important for several reasons, including:
- industry growth in “softer” elastomeric wall and roof coatings;
- industry demand for low-VOC formulations, which traditionally result in tackier coatings due to reduced glass transition temperatures (Tg); and
- planned construction of high-rise commercial buildings in emerging geographies, most notably in Asia Pacific, which is driving the need for coatings that are easier to clean and maintain.
Measuring Success - Standards for Dirt Pickup ResistanceBefore developing products, scientific methods need to be established that measure dirt pickup resistance and set standards for acceptable and desired levels. This is complicated by several factors, the largest being geography. Dirt and dust differ significantly between continents, based on both environmental and urbanization factors.
So, what is the definition of dirt pickup resistance? We define a coating with good dirt pickup resistance as being able to maintain its original appearance with respect to soiling of the surface due to exterior environmental conditions. At the same time, it is important to measure the change in appearance due to other factors, such as surface aging and weathering.
Currently, the most common method of measuring this characteristic is through gauging the light reflectance of the coating. Since factors other than dirt accumulation, such as paint degradation, can affect reflectance, this test does not provide the ultimate results.
The Chinese government has established a DPR test method requiring multiple steps to simulate real-world scenarios. A painted surface is exposed to 4 hours of UV light; a standard coal ash slurry is applied with a brush and then washed away under a prescribed volume and pressure of water for five cycles. The results can then be correlated, using both change in reflectance tests and optical microscopy methods, to surfaces in actual use. This correlation is important, as the basic characteristics of any paint can change significantly throughout the lifespan due to UV exposure and other environmental factors. These changes are typically oxidation leading to more hydrophilic surfaces. Significant oxidation can lead to extensive chain transfer and crosslinking, which ultimately reduces the integrity of the film.
It is most likely that there are a variety of dirt particles that can adhere to the coatings film. Figures 1-3 reveal the surface characteristics of paint films that have different degrees of DPR and, as a result, different types of soiling levels and amounts. Figure 1 is an optical microscopy image of the surface of a newly cast film of a 42 PC elastomeric wall coating. Figure 2 is the same type of film after having been exposed to 6 months exposure in Thailand at south 45 degrees. Very little dirt has soiled this surface. A similar coating, which has poor DPR, was also exposed under the same conditions (Figure 3). It is quite obvious that the surface has been extensively soiled.
The China test protocol gives product developers a controlled method of measuring dirt pickup resistance for new formulations. While this method is most common in Asia, others around the world are developing their own method. Dow Coating Solutions has its own proprietary method, combining it with forensic data from customers and information gleaned from the company’s test fence program,2 giving the company a strong knowledge platform for current and future R&D efforts into dirt pickup resistance.
What Works? Different Approaches to Dirt Pickup ResistanceImproving dirt pickup resistance is not a new issue. Almost as long as companies have been making coatings, they have been exploring ways to make them less attractive to dirt and dust particles.
The easiest way to achieve good dirt pickup resistance is to raise the glass transition temperature (Tg) of the coating, essentially creating a harder finish faster.3 There are multiple down sides to this approach. First, it is nonviable in elastomeric applications, such as white roof coatings, that require elongation and flexibility. The second downside associated is that increased Tg typically requires higher VOC content in the final coating formulation when compared to formulations utilizing lower Tg coatings. This is in direct opposition to the decrease in VOCs required of paint formulators by government regulations. Figure 4 illustrates the effect of changing the latex Tg in coatings formulations on DPR with lower Tg latexes giving inferior DPR (lower values indicate better DPR). Note here also that UV exposure (measured in hours using a QUV accelerated weathering chamber) can also positively affect DPR by hardening the surface of the film when appropriate UV crosslinkers are present.
Other historic methods include:
- Using highly crosslinked polymers, resulting in a low-tack surface that impedes dirt penetration. While this method is commonly used in automotive coating applications, architectural and elastomeric coatings provide unique challenges, due to the need to retain elongation, making it less viable.4
- Increasing the pigment volume concentration (PVC) is another method of creating a harder surface and increasing dirt pickup resistance by reducing the amount of polymer on the surface of the final coating. However, increasing the PVC significantly past the critical pigment volume concentration (CPVC) produces the opposite effect. More pores result and dirt pickup becomes even more of a problem.5
- Using higher levels of organic opacifiers results in a harder surface with increased hydrophobicity. While dirt pickup resistance is improved, the harder coating makes this method ineffective in elastomeric applications.6
- Photo-catalytic degradation agents are not commonly used because, even though they have been effective in reducing dirt pickup, they can potentially degrade the latex binder as well, reducing the life of the coating.7
- Multi-staged polymers represent a fairly new technology that involves a mixture of polymers with different Tg ranges, resulting in a mix of hard and soft segments. To date, however, this technology has yet to overcome many of the same issues faced by formulators, specifically elongation properties, especially after heat treatment.8,9
- Hydrophobic coating technology is one of the most promising approaches to dirt pickup resistance. In a hydrophobic coating, water beads more readily and maintains a higher surface tension, keeping the water from plasticizing the film. The downside is that coatings utilizing hydrophobic materials tend to streak easily.10
New Research - Developing Advanced Processes, Polymers and AdditivesDow Coating Solutions is exploring new chemistry-based solutions based on multiple approaches to this issue. Addressing this issue from an application and geographic point of view, the company is working with materials that address specific needs without compromising performance.
As an example, in North America, one of the fastest growing needs for dirt pickup resistance is in elastomeric roof coatings. In that market, a solution that inhibits elongation or flexibility will not work. Neither will a solution that significantly increases the VOC level of the formulation since there is a national trend to lower VOC products. A completely new answer is needed to address that market.
Using existing and internal test and measurement protocols, researchers on the dirt pickup resistance technology platform are identifying as many surface characteristics and external factors as possible in these applications. This research is resulting in models upon which the company can launch new product solutions. These models are based on multiple factors, including geographic considerations, dirt type, specific customer requests and others.11,12 Likewise, the company is exploring options across many applications, including low-VOC architectural coatings and Exterior Insulation and Finish Systems (EIFS).
SummaryThere are several hurdles to overcome in developing new dirt pickup resistance solutions. First, new analytical standards and tools have to be developed. Next, each geography and application has specific needs that do not always overlap. Finally, while there are existing solutions on the market, they fall short in areas such as coating elongation and durability.
Improving dirt pickup resistance is critical to furthering advancement in emerging applications worldwide, such as elastomeric roof coatings, low-VOC architectural paints and new construction projects in emerging economies. Continued research and new products will be essential to future initiatives in this area.