BuildItSolar Comparative Test: Thurmalox 250 vs Flat Black Paint

Source

BuildItSolar.com, “Best Type of Black Paint for Solar Collector Absorbers.” Test conducted 27 September 2010. Author: Gary Reysa (site operator). Single hobbyist test, not peer-reviewed.

Test Setup

• Substrate: 6-inch square thin aluminum panels, one per paint
• Paints tested:
  • Rust-Oleum Gloss Protective Enamel 7779 (spray), 0.7 mil thick
  • Rust-Oleum Flat Protective Enamel 7776 (spray), 0.6 mil thick
  • Thurmalox 250 Solar Collector Coating Selective Black, 0.6 mil thick
• Temperature measurement: surface-mount thermocouples on panel backs, read through switch to data logger
• Enclosure: larger than samples, clear acrylic front, intentionally leaky to limit breeze effects
• Conditions: nearly clear sky, ambient 60 F at start, enclosure air about 90-93 F

Results

Temperature Comparisons (author’s averages)

Orientation	Flat	Gloss	Difference
Near direct incidence	155.4 F	148.15 F	~7.3 F
45 degree azimuth	153.58 F	147.13 F	~6.5 F

• Thurmalox fell between flat and gloss, closer to flat
• In a separate small-panel test, Thurmalox was only slightly better than flat black

Reference Values from Literature (cited on page)

Surface	Absorptivity	Emissivity
Black chrome	~0.90	~0.09
Black paint	~0.95	~0.95

Key Observations

• Author suspected Thurmalox was applied too thickly to show its selective benefit
• Dampney (manufacturer) recommends spray-applying a very thin coating for maximum selectivity
• Flat black showed better adhesion when paint was scraped
• The Dampney datasheet says Thurmalox 250 functions with maximum efficiency on glazed collector panels
• For unglazed surfaces, Dampney recommends Endcor 902 latex instead

Thurmalox 250 Product Notes (from manufacturer datasheet)

• Silicone-based, heat-resistant black coating
• Rated to 500 F (260 C)
• Claimed absorptance: greater than 90% of sun’s visible and near-IR wavelengths
• Described as “somewhat selective” — high visible/near-IR absorbency, low far-IR emissivity
• No specific emissivity number published
• Primer (Thurmalox 245) available but inhibits selectivity
• Designed for metal surfaces; no mention of plastic/polymer substrate compatibility

Relevance to Tank Application

Thurmalox 250 is formulated for metal substrates at temperatures up to 500 F. For HDPE/PP tanks:

1. The silicone binder may not adhere well to polyolefin surfaces without primer/surface treatment
2. The 500 F rating far exceeds plastic tank limits (~180 F for HDPE), so thermal rating is not the constraint
3. Adhesion and surface preparation for polyolefins is the main unknown
4. The modest selectivity improvement over flat black paint may not justify the cost and complexity for unglazed plastic applications