Selective coatings on plastic

Can Duxberry-style HDPE/PP water tanks be given selective solar absorber surfaces? The research says: selective coatings on plastic are harder than they sound, and the practical path may be two-color molding rather than paint.

The real question is emittance, not absorptance

Flat black paint (carbon-black loaded) already achieves ~0.95 absorptance on any substrate. The value of a “selective” coating is low emittance — reducing thermal re-radiation so the surface retains more absorbed heat. On metal substrates, commercial selective coatings achieve emittance as low as 0.05–0.15. On plastic, the polymer substrate itself has high infrared emittance (~0.9), undermining any coating’s selectivity.

What the literature shows

Cekon et al. (2020) demonstrated a roll-to-roll manufactured polymer-based selective absorber foil achieving 0.95 absorptance / ~0.3 emittance. The polyethylene carrier layer causes the elevated emittance compared to metal-based absorbers (~0.05). The temperature limit is sub-100 °C — well matched to water storage tanks. Environmental impact was 98% lower than metal-based alternatives. This is the most promising published result for polymer-compatible selective absorption.

Thurmalox 250 (Dampney Company) is the most commonly cited selective black paint in the solar DIY community. A BuildItSolar comparative test on aluminum panels found it performed only modestly better than ordinary flat black paint. The manufacturer states thin application and glazed covers are needed for best selectivity. It is formulated for metal substrates; adhesion and performance on HDPE/PP are uncharacterized.

Solkote HI/SORB-II (Solec) is another commercial selective coating. Limited published data shows absorptance 0.8 / emittance 0.45 at 350 °C on metal. No data exists for plastic substrates. The manufacturer should be contacted directly for substrate compatibility guidance.

Two-color rotomolding: the manufacturing path

Rather than painting a finished tank, molding it in two colors may be more practical. The Lysis Technologies in-mould spray system produces sharp color boundaries on polyethylene parts up to 1.7 m, demonstrated in production rotomolding. The process sprays pigmented ink onto the mould interior before charging the resin, producing a part that is (for example) black on one hemisphere and white on the other — sun-facing and room-facing — without post-production painting.

This approach accepts non-selective absorption (carbon-black PE: absorptance ~0.95, emittance ~0.9) on the sun side and uses white on the room side for aesthetics and heat rejection. It sidesteps the adhesion and emittance problems of painting plastic with selective coatings.

Open questions

Adhesion of selective coatings to polyolefins. No source addressed whether any commercial selective paint bonds reliably to HDPE or PP without surface treatment (flame, corona, or primer).
Tank-scale cylindrical rotomolding. The Lysis system was demonstrated on a 1.7 m flat-ish part. Cylindrical tank geometry may behave differently. A rotomolding manufacturer (Duxberry or similar) should be consulted.
Cekon-style foil as a wrap. Could a polymer selective absorber foil be applied as a wrap to an existing white tank? This would combine the foil’s 0.95/0.3 performance with standard tank production.

Surface treatment for paint adhesion

Untreated polyolefins have very low surface energy (~31 dyne/cm for PE), making paint adhesion poor. Three industrial surface treatment methods can enable coating adhesion:

Flame treatment — propane ribbon burners oxidize the surface to ~45 dyne/cm. Longest shelf life on polyolefins (days to weeks). The most practical option for tank-scale production.
Atmospheric plasma — achieves similar surface energy with negligible ozone. More uniform than flame but higher equipment cost.
Pyrosil — deposits a nanolayer of SiO2 on the surface, creating a glass-like bonding surface. Intriguing for solar absorber adhesion but untested in this application.

Vacuum plasma (Fernandes et al. 2019) achieved excellent adhesion (ISO 2409 rating 0) on crosslinked PE, but is impractical for large rotomolded tanks. Chlorinated polyolefin primers work on LDPE but are documented as failing on HDPE.