Key takeaways from this newsletter:

• Dirt pickup resistance is the ability of a coating to resist dirt accumulation.
• A self-cleaning coating goes a step further than dirt resistant coatings by incorporating mechanisms to released adhered dirt.  Self-cleaning coatings belong to the class of coatings referred to as smart coatings.
•  A hard surface will lead to less dirt pickup.
• High PVC (pigment volume concentration) and hydrophilicity lead to more shedding of dirt and self-cleaning
• Self-cleaning and dirt-resistant paints are superior to typical exterior paints because they maintain the appearance of exteriors, reduce the need for frequent repainting, and support sustainability.

Smart coatings were addressed in the July 2024 Polaris Chemical Consulting Newsletter.(1)  A smart coating is a coating which has the ability to respond to environmental changes, and in so doing offers additional functional value over the traditional attributes of protection and decoration.  One such class of coatings is self-cleaning coatings which are designed to stay clean with minimal human intervention.  They are similar to, but distinct from, coatings that resist dirt pickup, which have been under development since at least the 1960s.

Exterior coatings need to be soft and flexible to accommodate the dimensional changes in wood that occur when exposed to varying weather conditions.  However, soft exterior coatings can become dirty with time as airborne dirt sticks to the soft surface.  A patent issued to du Pont in 1967 addresses this exterior paint dirt pickup issue.  The patent covers paints consisting of relatively soft latex copolymers in combination with photoreactive benzophenone.  The reaction initiated by UV light at the surface leads to a crosslinking of the coating which is both crack and dirt pickup resistant.  Presumably the exposure of the coating to UV light leads to a “layering” of the paint, where the surface is hard and resistant to dirt pickup and the remainder of the film remains flexible and tacky which is needed for good adhesion and crack resistance.(2)

Numerous offerings are on the market claiming dirt pick up resistance.(3,4,5)

The concept of dirt pickup resistance implies that the coating will not easily accumulate dirt or is at least resistant to it; however, it does not address the removal of dirt that does adhere. Self-cleaning coatings go a step further by incorporating mechanisms to release adhered dirt. One such mechanism involves using rainwater to clean the surface, and there are products available that utilize this approach.(6,7)  Another approach to self-cleaning is the use of photocatalytic titanium dioxide in inorganic coatings.(8)  When exposed to sunlight the titanium dioxide breaks down water into hydroxyl radicals which then degrade organic species at the surface. 

So how do you design a self-cleaning coating?  Work by researchers Wagner and Baumstock had shown that PVC (pigment volume concentration ) and binder selection were the most important determinants of DPUR.(9)  And work by Smith and Wagner demonstrated that as PVC (under the critical pigment volume concentration or CPVC) increases dirt pick up decreases; less tacky polymer at the surface leads to less dirt pickup.(10)

There is a very good article on the prediction of dirt pickup resistance which correlates real world dirt pickup performance with various laboratory methods.(11) The dirt pickup process can be broken down into three events: dirt deposition onto the surface, adhesion and entrenchment of the dirt, a shedding and release of the dirt. Experiments were designed to replicate each stage, and their significance was assessed by comparing the results with real-world data.  Real-world dirt pickup data was collected by exposing paint panels in Guangzhou, China, and Chennai, India. Significant differentiation between these paints, all claiming dirt pickup resistance, was observed in the exposure testing. The main conclusions of the paper are as follows:   

• Importance of dirt deposition. One laboratory test for predicting dirt pickup involves dusting surfaces with powders. In this study, paints were exposed to carbon black in an aerosol chamber. The test showed minimal performance differences, suggesting it is not a reliable predictor of DPUR, as the same paints exhibited significant differentiation during exposure testing.
• Importance of precipitation or water induced surface creep.  This experiment is designed to study what happens to a dirty surface if exposed to precipitation.   Panels freshly dusted with carbon black were immersed in two types of water: pH 3 and DI water.  Samples were allowed to dry and removal of dirt with tape peels was done.  The results of this test did not correlate with outdoor exposure tests.
•  Another test examining water induced surface creep. A common DPUR test is the application of a slurry of dirt (carbon black, iron oxide, etc.) to a test panel, allowing it to dry and then washing it with light rubbing.  These slurry tests did not correlate with the real world results in Guangzhou and Chennai.
• Thermal Creep.  Paints were dusted with carbon black and then placed in an oven at 45 oC for three days.   The samples were allowed to cool to room temperature and then loose carbon black was removed with tape.  The results here correlated well with the real world exposure data.
• Dirt release.  Higher PVC favors carbon black shedding as does a more hydrophilic surface. 

The conclusions of the article and implications for design are:

• Thermal surface creep is a major contributor to the DPUR performance of an exterior coating, and the dusting and slurry based tests are not as significant.  A harder surface will lead to less dirt pickup.
• High PVC and hydrophilicity lead to more shedding of dirt and self-cleaning.

Self-cleaning and dirt-resistant paints are superior to typical exterior paints because they maintain the appearance of exteriors, reduce the need for frequent repainting, and support sustainability.

(1) July 2024 Newsletter (polarischemical.com) Polaris Chemical Consulting website. “July 2024 Newsletter-Smar Coatings and Beyond. (https://www.polarischemical.com/july-2024-newsletter)

(2) Trevor B. Hill, “Latex Paint Containing Benzophenone,” U.S. Patent 3,320,198, May 16, 1967.

(3) SuperPaint® Exterior Acrylic Latex Paint - Sherwin-Williams  SuperPaint-Sherwin Williams website.  (https://www.sherwin-williams.com/homeowners/products/superpaint-exterior-acrylic-latex)

(4) Exterior Paint and Primer All-in-One | BEHR ULTRA® Exterior Paint | Behr  Behr Ultra Exterior Paint-Behr Website. (https://www.behr.com/consumer/products/exterior-paint-and-primer/behr-ultra-exterior)

(5) Residential Roof Coatings | Rust-Oleum (rustoleum.com) Rustoleum website. (https://www.rustoleum.com/product-catalog/consumer-brands/roofing/coatings/750-elastomeric-roof-coating)

(6)  Emerald Rain Refresh Exterior Acrylic Latex Paint | SherwinWilliams  Emerald Rain Refresh-Sherwin Williams Website.  (https://www.sherwin-williams.com/homeowners/products/emerald-rain-refresh-exterior-acrylic-latex-paint)

(7) LOXON Self-Cleaning Acrylic Coating | SherwinWilliams Loxon Self-Cleaning Coating-Sherwin Williams Website.  (https://www.sherwin-williams.com/painting-contractors/products/loxon-selfcleaning-acrylic-coating)

(8) EXOCOAT 151B | Self-Cleaning Nano Titanium Oxide Coating  Exocoat 151B-Exocoat Website.  (https://exocoat.eu/products/exocoat-151b)

(9) Wagner O. and Baumstark R., “How to Control Dirt Pick-Up of Exterior Coatings,” Macromol. Symp., (187), 447-458 (2002).

(10) jctNOV96-Smith.pdf Alan Smith and Oliver Wagner, “Factors Affecting Dirt Pickup in Latex Coatings,” Journal of Coatings Technology, Vol. 68, No. 862, November, 1996.  (https://www.paint.org/wp-content/uploads/2021/09/jctNOV96-Smith.pdf )

(11) Scott Brown, Michael Diebold, Daniel Kraiter, Carlos Velez, and Peter Jernakoff, “Towards a Comprehensive Understanding of Dirt Pickup Resistance,”  Coatings Tech, p. 14, June 2020.

(photo credit:  Jorge Urosa, pexels.com)