Precise control and characterization of surface wettability is critical when applying coatings, inks, adhesives, and other materials to solid surfaces. This is particularly true of plastics which generally exhibit poor wettability due to low surface free energy (SFE). A number of surface modification techniques, such as flame treatment, chemical adhesion promoters, and plasma treatment, can improve wettability, but thorough characterization of the treated surface is needed to demonstrate efficacy as well as optimize the process to reduce costs. In this presentation we demonstrate how contact angle (CA) measurements incorporated into SFE calculations can both optimize the process and provide physical insight into complex surface reactions. We will discuss how this approach is superior to qualitative techniques such as dyne pens while not requiring significant costs associated with rigorous analytical techniques such as FTIR. To illustrate this point we treated polypropylene (PP) samples with air-plasma for between 1-120 s. Subsequently, CA’s of water, diiodomethane and ethylene glycol were measured with an Attention Theta optical tensiometer and SFE calculations, including polar and dispersive components, were made automatically with OneAttension software. While untreated PP exhibited CA &gt;100°, surprisingly only a 3 s treatment reduced the CA to 64°. In that time, SFE increased from 24.2 to 46.8 mJ/m<sup>2</sup> during which a rapid increase in surface polarity indicated efficient surface oxidation. Interestingly, treatment times from 5-15 s reduced polarity and SFE, indicating competition between oxidation and destruction of the surface species. Beyond 15 s, SFE gradually increased again. The effect of storage time on the degradation of SFE was also monitored. Thus, we demonstrate the ability to precisely determine optimum surface treatment protocols using CA and SFE measurements, and gain physical insight into the underlying process.

Quantifying the exposure time-dependent wetting properties of plasma-treated polymer surfaces

2019 Fall National Meeting

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