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Hünnekens, Benedikt

Influence of plasma treatment under atmospheric pressure on surface characteristics and adhesion properties of wood-polymer composites (WPC)

1. Auflage, Juli 2018
Englisch, 166 Seiten, 14,8 x 21,0 cm, 230 g
ISBN: 978-3-86844-978-5
42,00 €
ODER
Influence of plasma treatment under atmospheric pressure on surface characteristics  and adhesion properties of wood-polymer composites (WPC)

Schlagworte

Wood-polymer composites, Atmosphärendruckplasma, Beschichtung von WPC, Oberflächenenergie, Lackhaftung, Adhäsionsverbesserung

Das Buch

This thesis investigates the impact of plasma treatment at atmospheric pressure on the chemical and morphological properties of a series of wood-polymer composites (WPC). Possible differences, depending on polymer/wood-ratio, as well as production-related phenomena, should be additionally identified by means of several surface analysis techniques. The most apparent effect of a surface activation is an improvement in surface wettability which was efficiently analyzed by contact angle measurement. Therefore, in a first step, an adequate approach was identified based on corresponding investigations, including selected WPC formulations and the static sessile drop method. Deriving from these results, the conic section method for drop shape analysis was suggested. Further, the approach of Owens and Wendt for calculation of surface free energy was introduced. Based on this study, these methods were used throughout the thesis. The following chapters focused on the modification of surface chemistry and morphology as well as their impact on adhesion properties. The dominating surface modifications induced by the plasma treatments were shown by an incorporation of new functional oxygen-containing groups (e.g. hydroxyl (OH), carbonyl (C=O) and carboxyl (O=C–OH)), and changes in surface morphology (increase in surface roughness and surface area). Extending the plasma discharge time led to an extension in the resulting effects. Moreover, it could be shown that the surface properties of WPC are very similar to those of the corresponding neat polyolefins, a fact which could be observed for the untreated and treated materials. The extruded samples showed an evenly distributed layer of small nodules throughout the surface after plasma exposure, which sparks the suspicion of the presence of low-molecular-weight oxidized materials (LMWOMs) – a phenomenon which can also be found in the literature for polymers. Despite reports to the contrary, the structures did not have any observable negative impact on the adhesion properties. On the contrary, the reported results supported the assumption that the LMWOMs might enhance the adhesion. The injection-molded samples showed remarkable polymer degradation in the surface and surface-near areas instead of nodules under comparable conditions, but also here, no negative effect on adhesion properties could be proved. Wood particles were exposed, which was suggested by scanning electron microscopy images. The polymer degradation proceeded accompanied by an increase in discharge time. Nevertheless, a negative effect on adhesion properties could be excluded. The adhesion tests showed a significant enhancement in adhesion strength of the coatings on all plasma-modified WPC surfaces considered. As it turned out, the applied plasma treatment of WPC at atmospheric pressure, based on the principle of dielectric barrier discharges (DBD) and ambient air as a process gas, was a very effective method for surface oxidation. Due to similar requirements, an enhanced bondability and printability is also most likely to be achieved by the treatments. A successful treatment can be done within seconds in order to improve the adhesion properties significantly, which was shown by a considerable increase in coating adhesion. The longer the treatment time, the higher the extent of the various stated effects. Further, it was discovered that the plasma effect itself is not stable. This effect is already known from polymer science and called hydrophobic recovery in the corresponding literature. The changes in hydrophilicity of the aged surfaces were verified by water contact angle measurement and found to be a function of time and, in particular, of storage temperature. The recovery rates were higher at higher ambient air temperature during storage (here 60 °C), and no significant differences in recovery rates were found between PP- and PE-based composites. It could be concluded that the aging effect has to be taken into consideration for further processing of pretreated material, and storage at room temperature was recommended.



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