Research on the low pressure glow discharge plasma -acting mechanisms on polymer films. Optimization of film-polyethylene foam laminates for uses in automotive industry.

摘要

In recent years a remarkable growth in the polymeric films industry for engineering applications has been observed due to the excellent combination of properties that can be achieved such as easy processability, surface finishing versatility, excellent barrier behaviour, balanced properties (mechanical, thermal, electrical...). All these have enabled the extensive use of, polymers in applications of high technological content on a film form (packaging, automotive and aircraft industry, electric and electronic equipment, health care...). Some films exhibit good adhesion properties but the most generalized situation is that polymer films show poor adhesive properties since they are characterized by low surface energy values, chemical inertness and smooth surface. For this reason polymeric films need, in many cases, some additional treatment to raise surface activity, thus enhancing wettability and, consequently, the adhesive properties.;In this research work, low pressure glow discharge plasma with different gases (O2 and N2) has been used to increase wettability in different polymer films (low density polyethylene and polyurethane) in order to obtain laminates with polyethylene foams for uses in automotive industry. The improvement on adhesion properties has been estimated by surface energy values which have been determined using contact angle measurements for different exposure times and different test liquids. In addition, plasma-treated-samples have been subjected to an aging process to determine the durability of the plasma treatment since most of the polar species generated by the plasma treatment are highly unstable. Characterization of the surface changes due to the plasma treatment has been carried out by means of Fourier transformed infrared spectroscopy (FTIR) to determine the presence of polar species and X-ray photoelectron spectroscopy (XPS). In addition to this, scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis have been used to evaluate changes in surface morphology and roughness.;The results show that low pressure plasma improves wettability in' the different polymer films for low exposure times and for both gases (O 2 and N2), and no significant changes can be observed at longer exposure times. The main plasma-acting mechanism is surface activation by insertion of polar species which are highly unstable thus promoting an aging process with a considerable decrease in the wettability achieved with the plasma treatment; this process is known as "hydrophobic recovery" and it is very important in order to obtain good and durable laminates. Also some changes in surface topography (roughness changes) have been detected which have a positive effect on surface wettability together with surface activation.;Laminates with polyethylene foam exhibit good mechanical properties and also high durability of the adhesive joints in aggressive conditions, (humidity and temperature) has been observed. As a general conclusion, it is important to highlight that low pressure plasma treatment is an interesting and environmentally efficient method to improve adhesion properties of polymer films for technical applications.