Plasma Treatment of Crosslinked Polyethylene Tubes for Improved Adhesion of Water-based Paints

Source

Fernandes, G.F., Amorim, M.K.M., Turri, R.G., Oliveira, P.S., Graff, I.L., Rangel, E.C., Durrant, S.F. “Plasma Treatment of Crosslinked Polyethylene Tubes for Improved Adhesion of Water-based Paints.” Materials Research 22(1), 2019. DOI: 10.1590/1980-5373-MR-2017-0046. Open access.

Key Findings

Substrate and Treatment

• Substrate: high-density crosslinked polyethylene (PE-Xc) tubes (HENCO brand), cut into 12 x 7 x 1 mm plates.
• Note: This is PE-Xc, not standard HDPE. Crosslinked PE has different surface chemistry but similar low surface energy.
• Treatment: RF PECVD plasma (13.56 MHz, 60 W), Ar/CHCl3 gas mixtures at 100 mTorr, 15-minute treatment time.
• CHCl3 partial pressure varied from 0% to 100%.

Paint and Testing

• Paint: white premium acrylic (water-based), interior/exterior rated.
• Applied in coats at 0, 4, and 8 hours; tested next day.
• Adhesion test: ISO 2409 cross-cut and tape test (BS EN ISO 2409:2007).

Results

• Untreated: adhesion rating 4 (very poor---most paint removed by tape).
• Plasma-treated (any CHCl3 concentration > 0): adhesion rating 0 (excellent---no paint removed).
• Water contact angle increased ~20 degrees across the treatment range.
• Surface roughness (Ra) increased with CHCl3 concentration.
• Treatment modifies only the surface skin; bulk mechanical properties preserved.

Mechanism

• Plasma creates polar functional groups (C-Cl bonds from CHCl3) on the PE surface.
• Increased polarity improves wetting by water-based paint.
• Micro-roughening provides additional mechanical interlocking.

Relevance to Solar Fool

This study proves the principle that plasma treatment enables water-based paint adhesion on polyethylene surfaces. Key caveats for selective-coating-on-plastic-tank applications:

1. Substrate difference: PE-Xc is not identical to HDPE or PP used in rotomolded tanks. Crosslinking changes surface chemistry.
2. Lab conditions only: 15-minute RF plasma at 100 mTorr requires vacuum equipment---not practical for large tank surfaces.
3. No durability data: the study tested adhesion one day after painting. No thermal cycling, UV aging, or outdoor weathering data.
4. CHCl3 concern: chloroform in the plasma feed raises environmental and health considerations for production.

The gap remains: no study tests paint adhesion on standard HDPE rotomolded tanks using industrially practical surface treatments (flame or atmospheric plasma) with selective solar absorber coatings under outdoor conditions.