The Influence of Surface Treatment on the Polyolefin Coating

Low-density polyethylene (LDPE) and polypropylene (PP) are widely used polymers in packaging industry. These polymers differ to one and other by their polymer structures: LPDE has long branched body, when PP is a linear polymer. Branched structure gives LDPE flexibility that PP lacks due to its high level of crystallization. The aim of this research was to compare these polymers in extrusion coating and find out if there are certain procedures in surface treatments that could improve PP’s properties in post-coating stage. For instance PP has high melting temperature and low surface energy that prevents its use in extrusion coating. In extrusion coating a thin film of the molten polymer is extruded though a flat tie and then pressed towards the substrate. The substrate in this study was paper and the studied treatments were flame, corona, and plasma focusing on heat sealing, hot tack, surface energy and water vapour transmission (WVTR). The study was divided into four different case studies: 1. The effect of flame treatment, 2. The barrier effect of plasma and corona, 3. The effect of the treatment combination, 4. The influence of time and conditions. In the results, it was shown that PP has better water vapour transfer resistance throughout the study points and in comparison to LDPE it also reacted more on the surface treatments in case of WVTR. LDPE had higher surface energy in nearly every study point despite the surface treatment and its reaction on treatments was greater. LDPE’s and PP’s heat sealability temperatures were relatively close to each other in treated study points, though LDPE’s reference value was remarkably lower. If flame treatment is combined with plasma or corona, the best results for both used polymers took place in the treatment order where the flame treatment was performed first: sealing temperature, surface energy and WVTR enhanced. In the study of time condition contact angles grow in logarithmic scale in both studied polymers and the highest growth happens in hotter and humid conditions. LDPE’s and PP’s sealing temperatures follow relatively similar curve in function of time. /Kir1