Empirical study and prediction of contact angle and surface free energy of commonly used plastics with pillar-like structure

An effective approach is proposed to estimate liquids' contact angles on five commonly used plastics, polyethylene terephthalate, polypropylene, high-density polyethylene, low-density polyethylene, and polyvinyl chloride, with pillar-like structures. A change in liquid droplets' three-phase contact line due to surface roughness has been proposed in literatures. In this article, contact length ratio, σ, was used as a parameter corresponding to a specific dimension of the pillar-like structure. Wettability of these rough plastics and their surface free energy were investigated by liquids with various polarities—de-ionized water (polar), ethylene glycol (monopolar), and α-bromonaphthalene (apolar). The effects of pillar-like structures on liquids' contact angles and plastics' surface free energy were studied, and the results reveal that both effects are linear in the range of σ = 1.0 to 1.96. Linear regression models are hence proposed to predict liquids' contact angles, and accuracies are confirmed by less than 6% error for most plastic–liquid combinations. Plastics' surface free energy is also predicted by linear regression models, and the results agree with existing experimental data. Plastic–liquid interactions were also studied, and the results further validate predictions of plastics' surface free energy. In addition, plastics' polarity alteration due to effects of pillar-like structure were analyzed and reported in this article. Copyright © 2014 John Wiley & Sons, Ltd.