Wettability of polyethylene terephthalate melt on steel substrates and the effect of cooling rate on polymer amorphicity

Abstract

The effect of surface roughness and temperature on wettability of polyethylene terephthalate (PET) melt on steel substrates is assessed. A plot of contact angle, Φ, Vs. time, τ, revealed the significant effect of temperature on wetting compared to surface roughness. The surface free energy of the steel substrate decreased by about 22% with increase in surface roughness from 0.21 to 3.8 μm. The measured surface tension of the polymer melt was 31.36 mN m−1 at 260°C. The variation in the cooling rate of the solidifying molten PET resulted in varying degree of polymer opacity. The opacity variations were quantified as a function of pixel intensity. The opacity of the solidified PET melt drops sharply declines at lower cooling rates before stabilizing at higher rates. No significant difference in the mean pixel intensity is observed between PET melts solidified at different substrate temperatures. Three distinct regimes in PET melt solidification are identified: (i) increased crystallization propensity at lower cooling rates (<8°C min−1), (ii) dominance of amorphous characteristics at higher rates (>199°C min−1), and (iii) a transitional regime at intermediate cooling rates.