Low-density polyethylene films treated by an atmospheric Ar–O₂post-discharge: functionalization, etching, degradation and partial recovery of the native wettability state

Abstract

To optimize the adhesion of layers presenting strong barrier properties on low-density polyethylene (LDPE) surfaces, we investigated the influence of argon and argon-oxygen atmospheric pressure post-discharges. This study was performed using x-ray photoelectron spectroscopy, atomic force microscopy, optical emission spectroscopy (OES) and dynamic water contact angle (WCA) measurements. After the plasma treatment, a slight increase in the roughness was emphasized, more particularly for the samples treated in a post-discharge supplied in oxygen. Measurements of the surface roughness and of the oxygen surface concentration suggested the competition of two processes playing a role on the surface hydrophilicity and occurring during the post-discharge treatment: the etching and the activation of the surface. The etching rate was estimated to about 2.7 nm s −1 and 5.8 nm s −1 for Ar and Ar-O2 post-discharges, respectively. The mechanisms underlying this etching were investigated through experiments, in which we discuss the influence of the O2 flow rate and the distance (gap) separating the plasma torch from the LDPE surface located downstream. O atoms and NO molecules (emitting in the UV range) detected by OES seem to be good candidates to explain the etching process. An ageing study is also presented to evidence the stability of the treated surfaces over 60 days. After 60 days of storage, we showed that whatever the O2 flow rate, the treated films registered a loss of their hydrophilic state since their WCA increased towards a common threshold of 80°. This 'hydrophobic recovery' effect was mostly attributed to the reorientation of induced polar chemical groups into the bulk of the material. Indeed, the relative concentrations of the carbonyl and carboxyl groups at the surface decreased with the storage time and seemed to reach a plateau after 30 days. IntroductionPolymers such as polyethylene present excellent physical and chemical bulk properties but exhibit poor surface adhesion features. The deposition of a subsequent layer onto a low-density polyethylene (LDPE) surface can be achieved for specific applications, for instance a layer acting as an oxygen barrier for food packaging. In order to improve the adhesion of a subsequent layer onto this polymer, a pretreatment of its surface is therefore usually performed. One of the most promising techniques is the plasma activation which-for instance-consists of exposing the sample surface to the post-discharge of a radio frequency (RF) plasma torch. These plasma treatments can be carried out with different gases and vapours like O2, N2, NH3, H2O, CO2, air and noble gases [1][2][3][4]. Depending on the experimental conditions, it is considered that a plasma treatment leads to one of the four following effects: cleaning (removing of organic contaminants), etching (removing of polymer material and degradation of polymer), cross-linking (formation of free radicals and branching of macromolecules) and functionalization (formation of new chemical functions) [5][6][7][8][9][10]. ...

Molecular Surface Analysis and Depth-Profiling of Polyethylene Modified by an Atmospheric Ar-D2O Post-Discharge

“…The LDPE film was treated by an atmospheric radiofrequency (RF) plasma torch (AtomfloTM 250D) from SurfX Technologies LLC . The schematic representation of the experimental device, called “showerhead plasma torch” because of its configuration, is shown elsewhere .…”

“…Indeed, according to recent studies on the air aging of LDPE treated by atmospheric post‐discharges, the most significant hydrophobic recovery happens in the first 2–3 days after the plasma treatment. However, the LDPE samples conserved for a few months never go back to their native hydrophobic state …”

“…A known route to improve the adhesion of the polyolefins consists in the so‐called oxidative functionalization, usually obtained by the use of O 2 as reactive gas, that induces the grafting of O‐containing functional groups. The literature reports a series of recent works carried out with atmospheric Ar/O 2 or He/O 2 post‐discharges for improving the hydrophilicity of LDPE or HDPE (high density polyethylene) . This result is obtained by the grafting of three polar functionalities –C–O, CO and O–CO– whose densities on the polymer surface decrease with increasing the treatment time.…”

“…This result is obtained by the grafting of three polar functionalities –C–O, CO and O–CO– whose densities on the polymer surface decrease with increasing the treatment time. This suggests that an erosion process, competitive to the functionalization, occurs in flowing post‐discharges . Furthermore, higher hydrophilicity values are obtained for sample‐torch gaps <15 mm, while at distances >20 mm from the plasma source the effectiveness of the polymer treatment drops dramatically.…”

Surface Analysis and Ultra‐Shallow Molecular Depth‐Profiling of Polyethylene Treated by an Atmospheric Ar‐D2O Post‐Discharge

“…The LDPE film was treated by an atmospheric RF plasma torch (Atomflo 250D) from SurfX Technologies LLC . The torch consists in two parallel 126‐holes circular electrodes.…”