Elsevier

Applied Surface Science

Volume 315, 1 October 2014, Pages 426-431
Applied Surface Science

Enhancement of surface properties on commercial polymer packaging films using various surface treatment processes (fluorination and plasma)

https://doi.org/10.1016/j.apsusc.2014.05.163Get rights and content

Highlights

  • Two different surface treatment processes were investigated in this work.

  • Both processes drastically change the composition induced on the surfaces.

  • Direct fluorination is identified as an efficient way to adjust surface properties.

  • Plasma processes result in a specific enhancement of the surface properties.

  • The pristine polymer surface has been successfully improved.

Abstract

Before considering their combination on commercial packaging films, two surface treatments processes were investigated. Indeed, direct fluorination and plasma processes are currently recognized as effective processes to improve polymer surface properties. The aim of this first work is to elucidate mechanisms that occur on the treated surface. The modifications of the surface layer were characterized using various complementary spectroscopy techniques such as Fourier Transform Infrared (FTIR) spectroscopy, high resolution solid state Nuclear Magnetic Resonance (NMR) with 19F nucleus which are suitable to determine the nature of bonding and specific groups formed during the process. X-ray Photoelectron Spectroscopy (XPS) was also achieved to extract the surface chemical compositions. In addition, surface properties of the treated films were studied by specific measurements of surface energy in order to reveal surface parameters such as rugosity and chemical composition which could be adjusted. All these results underline that the layer induced regardless of the two processes plays a key role in the enhancement of the surface properties.

Introduction

Nowadays, polymer films are commonly used for packaging due to many advantages, such as their low cost and low processability but exhibit also few drawbacks such as poor adhesion, low printability, low barrier properties and low chemical resistance that require a continuous improvement firstly at the lab scale. All these features explain why these materials are currently used for this kind of application. To remedy all these defects, it is now possible to obtain the whole articles from special polymers or using multilayers packaging films, each layer adding one property. However, these kinds of process remain restricted as a result of the high cost and a high complexity of synthesis of these materials.

This present paper will focus on the changes applied on the surface by both direct fluorination and plasma treatments on a specific polymer which is devoted to packaging applications. These two processes have been identified as effective surface treatments to improve polymer surface properties. More broadly, surface treatments are recognized as one of the most efficient solution in order to modify commercial polymer film surfaces. Control of both the topography and the chemical composition are the key parameters to define the efficiency of each method. For a large-scale industrial use, these processes must be the cheapest and safest as possible. The damages on the treated surface should be limited and properties of the bulk must be unchanged.

The first solution is developed from a particular reactant, fluorine, due to its high reactivity. Direct fluorination is known to improve polymers surface properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. It consists of a diffusion-controlled process which may be described as a heterogeneous reaction between a gas (pure F2 or mixed with other inert gases) with the polymer surface. This method modifies only the upper surface layer to reach a hydrophilic/phobic surface depending on the used gaseous mixture. The bulk properties remain unchanged. This process, a safe technology without solvent, displays a lot of advantages which allow it to be used for industrials applications. Different fluorination conditions (temperature, duration, dilution, composition of the gaseous mixture, gas flux) were investigated on a specific commercial packaging polymer film.

The second strategy consists in the use of plasma treatments. As for direct fluorination, these treatments are recognized as efficient to enhance polymer surface properties [12], [13]. By this way, several experiments were carried out in order to identify the best conditions to modify the pristine surface for the targeted properties (printability; hydrophilicity, gas barrier; hydrophobicity). This technique does not attempt the bulk properties. First, etching treatment using Argon plasma has been developed. Reactive treatments with specific gaseous mixture plasma (O2, N2) have been then tested to modify not only the topography on the surface but at the same time the chemical composition of the treated surface.

The final aim of the study is the combination of fluorination and plasma processes. The present study appears as a preliminary step.

Section snippets

Materials

High Density Polyethylene films [Pristine_HDPE] were commercialized for packaging applications dedicated to food or agricultural domains. The industrial films were used as received and were corona treated on one side. All developed surface treatments have been performed on the other face as initially identified by the manufacturer. Thickness of the pristine polymer was equal to 17 μm. Its density was estimated by ISO 1183 methods at 0.948 g cm−3.

HDPE-direct fluorination

Direct fluorination using gaseous molecular fluorine

Results and discussion

The main objective of this work is to compare the two surface treatments, i.e. direct fluorination and plasma. The different mechanisms occurred on the treated surfaces were then investigated.

Fig. 1 displays FTIR spectra recorded using ATR mode. Only the upper surface is analyzed with such ways. This method appears then as a good way in order to underline the surface modifications and identify the chemical changes on the surface. Vibration mode of –CF– groups (–CHF–, –CF2–, –CF3–) between 1300

Conclusion

This present work demonstrates that direct fluorination is an effective process to modify both surface chemical composition and roughness. These two parameters could be adjusted. This process consists to maintain a gas flux during a duration which must be adjusted. As an excess of molecular fluorine gas acts on the surface, phenomena of hyperfluorination were identified. Direct fluorination, a diffusion controlled process, changes the upper fluorine layer with the identification of fluorinated

Acknowledgements

The authors wish to gratefully acknowledge the “Région Auvergne” and the “FEDER” for their financial support within the context of a specific scholarship: “Bourse Recherche Filière” (FLUOPLAST project).

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