Surface modification of low density polyethylene (LDPE) film by low pressure O₂ plasma treatment

Abstract

In this work, low pressure glow discharge O₂ plasma has been used to increase wettability in a LDPE film in order to improve adhesion properties and make it useful for technical applications. Surface energy values have been estimated 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. 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 such as carbonyl, carboxyl and hydroxyl groups. In addition to this, atomic force microscopy (AFM) analysis has been used to evaluate changes in surface morphology and roughness. Furthermore, and considering the semicrystalline nature of the LDPE film, a calorimetric study using differential scanning calorimetry (DSC) has been carried out to determine changes in crystallinity and degradation temperatures induced by the plasma treatment. The results show that low pressure O₂ plasma improves wettability in LDPE films and no significant changes can be observed at longer exposure times. Nevertheless, we can observe that short exposure times to low pressure O₂ plasma promote the formation of some polar species on the exposed surface and longer exposure times cause slight abrasion on LDPE films as observed by the little increase in surface roughness.

Introduction

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 behavior, balanced properties (mechanical, thermal, electrical, etc.). All these have enabled the extensive use of the so-called “technical” and “high performance” polymers in applications of high technological content on a film form (packaging, automotive and aircraft industry, electric and electronic equipment, health care, etc.) [1]. Many of these applications require good adhesion properties [2], [3], [4] to enhance good mechanical performance and allow replacing aluminium and steel parts on secondary structures; in this sense, surface phenomena regarding adhesion behavior acquire special relevance on materials study and characterization. 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 [5], [6], [7], [8]. This situation is particularly remarkable in non-polar polymers such as polyethylene (PE); non-polar polymers need surface treatment to enhance good adhesion properties. If we consider that the different adhesion mechanisms are based mainly on chemical interaction and mechanical interlock, surface treatments must focus on the addition of active species and on the generation of certain surface roughness to improve adhesive performance. In the last years a great interest about material preparation and modification has occurred. The basic principle is that it is possible to change the surface properties without changing the bulk properties of the material. There are many different methods to modify the surface properties of polymeric films such as chemical, thermal, mechanical and electrical (plasma) treatments [2], [9], [10], [11], [12]. Recently, research on the use of plasma treatments has grown in interest [13], [14], [15] since they are environmentally efficient. Low pressure plasma (glow discharge) is a popular technique since it is a dry process and allows better uniformity in the modified surface, so it is widely used for industrial applications [3], [16], [17]. Depending on the gas used for plasma generation and on the general conditions it is possible to activate a polymeric surface by inserting active species (mainly polar groups), surface abrasion or etching, cross-linking processes [6] or, in many cases, combined effects can be obtained. The use of low pressure conditions allows plasma treatments at low or moderate temperatures; in this way, the aggressiveness of the plasma treatment is considerably reduced and consequently degradation occurs in a less extent. It is possible to generate glow discharge by either radio frequency (RF) or microwave (MW) but at industrial level RF excitation is preferred for surface modification [5], [8], [11], [17]. The different species present in the plasma induce the formation of free radicals in the polymeric chains and in this way it is possible to insert or interlock certain functional groups on the polymeric surface; this will have a positive effect on the functionalization/activation of the polymer surface [16] and will improve surface adhesion properties.

The present study uses RF oxygen plasma to improve the intrinsic low wettability of a low density polyethylene (LDPE) used for technical applications on a film form. The use of O₂ plasma promotes surface modification by both surface activation and slight etching as the main plasma mechanisms. The functionalization of the LDPE surface is characterized by FTIR–ATR analysis and the evolution of the surface contact angles is observed as a function of the exposure time. The evolution of the polar and dispersive contributions to LDPE solid surface energy is determined in terms of the treatment time. Moreover, the durability of the plasma treatment is evaluated by subjecting the sample-treated to an aging process. The etching effects of plasma are analyzed using SEM and AFM analysis to characterize morphology changes caused by the plasma treatment.