Chemist Clovis Linkous compares different photocatalyst coating formulations.

Research over the years at FSEC has shown that a white roof helps to make a building more energy efficient since lighter colors are more reflective of the sun’s rays.  In particular, white elastomeric roofing material with its 70 percent reflectivity can be an important contributing factor to more energy-efficient buildings and energy and cost savings for their owners. 

However, in Florida as well as in many other hot-climate states, algae growth on the surface of the roofing material causes the color to darken.  Within a short time, the darker color causes the reflectivity to drop to approximately half of what it was, significantly impacting the capability of the roof to keep the building cool and greatly increasing the amount of energy needed to cool the building.

Now, there may be a new strategy using the sun to get rid of that algae problem.  Since 1993, FSEC researcher Clovis Linkous has been studying the effects of using photocatalytic chemistry to destroy the algae that grow on a number of surfaces in Florida’s hot and humid climate. The goal of this work has been to develop photocatalytic paint formulations that can be applied to a surface to protect it from biofouling due to algae growth.  “The idea first hit me one day when I was cleaning my swimming pool,” he explained.  “I thought about this and realized that any solid surface upon which algae collect might benefit from this strategy.”
 
In addition to materials such as roofing tile, shingles and membranes, applications for the photocatalytic paint formulations include stucco walls, swimming pools and outdoor fountains and windows.

Linkous adds that a significant side effect is that disinfection (bacterial inhibition) is also possible with photocatalytic paint, pointing out that counter tops in places like home kitchens or hospital operating rooms could especially benefit from this approach.  

His research has included tests of the photocatalytic power of the sun's rays to effectively inhibit the growth of algae on cement surfaces.  The following photos show the growth of algae after one week, the effect of photocatalytic paint in inhibiting the growth of algae, and the even more successful effect of using a co-catalyst for algae inhibition.

Unprotected cement surface after a one-week exposure to algea attack.	Using phocatalytic paint on cement surfaces provides some protection.
Co-catalyst effect on algae inhibition: virtually no algae have grown on the surface.

He explained that the extent of algae growth on the substrates after one week was determined “via quantification of chlorophyll content of the cellular growth on the surface. The results showed that an unprotected substrate would become completely covered with algae within a week.  By coating the substrate with a photocatalyst such as the P25 brand of TiO2 made by Degussa Corporation, the rate of algae growth was inhibited by about two-thirds.”

As this photocatalytic chemistry continued, he noted that “it became apparent that it could be further improved by adding small amounts of other catalytic substances, or co-catalysts, to the light-absorbing particle surface. The results showed that a Pt-TiO2 formulation could inhibit algae growth by 86 percent, nearly an order of magnitude less  than an unprotected substrate.”

Tungsten tri-oxide (WO3) was also tested as an algae inhibitor. It was found that while WO3 by itself tended to facilitate algae growth, inclusion of almost any  co-catalyst showed an improvement since the baseline for the unmodified photocatalyst is actually a negative effect.

While Pt-TiO2 was found to be effective, researchers wanted to find cheaper co-catalysts that represent a better value in terms of oxidizing power per dollar.  Studies showed that for TiO2, the carbon, cobalt phosphide, and oxidized nickel co-catalysts also improve baseline performance. It has been demonstrated that certain formulations, particularly graphitic carbon-TiO2, can reduce algae growth by nearly 90 percent.

Linkous has recently been conducting research in collaboration with Ross Robertson of Firestone Building Products Company on a method to destroy algae on elastomeric roofing material and retain the high reflectivity to the material. Two types of problems arise from the collection of algae on roofs.  One is aesthetic -- the roof looks as if it needs to be cleaned – and the other is energy related. Traditional cleaning methods such as soap, detergent or chemical solvents have proven to be ineffective in ridding roofs of algae growth. Although they may be temporarily effective in the short term, the algae return.  “Algae may be very simple organisms,” Linkous explained, “but they are extremely difficult to destroy.  The cleaning first appears to get rid of the algae but their strong cellulosic cell walls make them very difficult to kill and they soon begin to grow again.”

Using one-foot squares of new roofing material from Firestone, testing of these paint formulations has been conducted at a test stand in Homestead, Florida.  The Linkous and Robertson study indicated that by modifying elastomeric roofing material such as TPO, Thermoplastic Polyolefin, or PVC, Polyvinyl Chloride, through a bonding process with the photocatalyst TiO2 and its co-catalysts, a new material was created — one that is both algae-resistant and highly reflective.

“We have now demonstrated a genuine solar photocatalytic effect,” Linkous said.  “The next step is to determine whether our formulations can withstand the rigors of the Florida climate, particularly the ultraviolet rays that are absorbed during a hot summer afternoon, as well as the action of wind and water.  The good news is that so far, so good, and I look forward to more research into this area.”
 
For more information on this research, see http://www.fsec.ucf.edu/en/research/environment/microorganism/index.htm.

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