Surface analysis of polyester materials using time of flight secondary ion mass spectrometry

Abstract

The use of many plastic packaging materials such as PVC, PAN, etc. to protect food and beverages is restricted because of toxicity and recycling problems and there is a need for the development of new packaging materials with good gas barrier properties. Plasma polymerized amorphous carbon hydrogen (a-C:H) films with a thickness of up to 100 nm are promising gas barriers for foodstuff applications when deposited on the outside of poly(ethyleneterephthalate) (PET) films. In order to understand the impact of the surface modification on permeability properties, a characterization method was developed using TOF-SIMS (time of flight secondary ion mass spectrometry) as the principal technique. The chemical and structural nature of plasma modified surfaces are characterized and related to the quality of a-C:H films on PET produced at EMPA (Swiss Federal Laboratories for Materials Testing and Research). In a first step, the mass peaks of TOF-SIMS spectra provided from the substrate materials PET and biopol® a biodegradable packaging material were identified and structural and/or total formulas were assigned. In the case of the biopol® substrate, the citroflex® plasticizer was detected on the polymer surface. Relative signal intensities were utilized to study the distribution of the two monomer units (valerate and butyrate) of biopol®. Principal component analysis (PCA) was used to reveal differences in relative signal intensities among TOF-SIMS spectra obtained from PET substrates provided by different suppliers and using different manufacturing processes. In some cases, the presence of acetaldehyde and surface pre-treatment from the manufacturer were established. In addition, the ion beam modification (dynamic SIMS mode) of the PET substrate material was studied to determine the static limit of TOF-SIMS measurements. In order to get insight into how the sputter process alters organic substrate materials during depth profiling, the decrease of molecular PET fragment intensities was studied. In a second step, existing structural and chemical SIMS parameters were applied and optimized to investigate TOF-SIMS spectra obtained from a-C:H films on PET. Molecular ion fragments (CxHyOz+) revealed a partially oxidized nature of the a-C:H films. In addition, the use of SIMS parameters combined with PCA analysis allowed to correlate poor gas barrier properties with a low degree of saturation (i.e. high aromaticitiy). Low mass and quasi molecular ions were used to identify the a-C:H/PET interface by dynamic TOF-SIMS depth profiling. AFM (Atomic Force Microscopy) and XPS (X-ray Photelectron Spectoscopy) were used as complementary methods to investigate the topography and the chemical composition of the a-C:H films on PET as well as of the substrate material. The partially oxidized nature of the a-C:H films was confirmed, the oxygen contents varying between 12-18%. A correlation between the oxygen content and the plasma treatment conditions of the substrate (application of a rf- or grounded-bias) was established, but no correlation with respect to gas permeability was found. Oxygen is believed to originate from post plasma reactions with air. ERDA (elastic recoil detection analysis) permitted to determine the bulk hydrogen content of the a-C:H films. However, the observed hydrogen content of 40% to 50% did not Vary significantly enough to be correlated to gas barrier properties. RBS (Rutherford Backscattering) allowed us to measure quantitative depth profiles of a-C:H carbon and revealed oxygen also inside of the a-C:H films. In addition, RBS measurements were useful to determine the areal densities of the a-C:H films on PET. The atom number density of the investigated a-C:H films varied between 0.19 and 0.24 g atom/cm3 and are similar to values found in the literature.