Oxyfluorination of Wood-Fiber Reinforced Thermoplastic Composites to Improve Adhesion

Oxyfluorination was evaluated on a series of wood-fiber reinforced thermoplastic composite (WPCs) formulations as a means to improve the adhesion of a water-based acrylic coating. The oxyfluorination increased the acrylic coating peel load to WPCs by 179%, yielding adhesion levels higher than those obtained on control maple. For oxyfluorinated surfaces, the WPC formulation had a significant impact on the acrylic coating paint. Formulations without maleic anhydride polypropylene coupling agent developed better adhesion than formulations with the coupling agent. Formulations with polypropylene also performed better than formulations with high density polyethylene and so did formulations with pine compared with those with maple. Interestingly, the adhesion improvement imparted by oxyfluorination was higher for those formulations that were least amenable to bonding with the acrylic coating, i.e., those containing the coupling agent and high density polyethylene. Contact angle measurements and ATR-FTIR spectroscopy indicated that oxyfluorination improved wettability with polar liquids by generating new oxygenated groups on the surface. A moderate positive linear relationship (r² = 0.5) was established between the carbonyl/C-H stretch band intensity ratio I_1650cm⁻¹ /I_2915cm⁻¹ in the infrared spectra of oxyfluorinated WPCs and the coating peel strength. It is, therefore, proposed that oxyfluorination improves the adhesion properties of WPCs by oxidizing the surface, and by increasing wettability with polar liquids, thus allowing the development of stronger interfacial forces.

Keywords:

• ATR-FTIR
• Peel adhesion
• Surface activation
• Surface oxidation
• Wettability
• Wood plastic composites

ACKNOWLEDGMENTS

This work was sponsored by the Office of Naval Research, under the direction of Mr. Ignacio Perez, under Grant N00014-03-1-0949. The authors are thankful to Kelly Williams from Inhance/Fluoro-Seal, Ltd., for performing their proprietary oxyfluorination treatment on the WPC samples. Special thanks also go to Kieffer Tarbell from Drew Paints, Portland, OR, USA, for formulating and supplying the acrylic coating.

Notes

^a For comparison, peel load on maple and HDPE averaged 524±64 N/m, respectively [7]. Chromic acid treatment on a similar series of WPCs has increased the average peel load to 637±88 N/m [6].

^b For comparison, advancing contact angle on PP and on HDPE is 95° and 87°, respectively [7].