• Kuusipalo, Jurkka
• Räsänen, Jari P.
• Penttinen, Tapani
• Tuominen, Mikko
• Lahti, Johanna

Abstract

<p>The most important function of a packaging material is to shield the product inside the package. Extrusion coated papers and paperboards are generally used in various consumer packages like food, medical and cosmetic packages. Extrusion coatings give a barrier against water, water vapour, aroma, grease, oxygen, etc. In addition to barrier properties, heat sealability and printability are important properties in packaging applications. From the point of view of printing, the dense and impervious structure of extrusion coatings is challenging: printing inks and toners do not penetrate into the coatings. The durability of the printed image is significant, because the image must withstand various converting operations when the package is constructed. The most common method for obtaining good ink or toner adhesion is to oxidise the surface. Surface treatments are used to change the chemical composition, increase surface energy, modify surface morphology and topography, or remove contaminants and weak boundary layers. Two widely used methods are corona discharge treatment and flame treatment. These processes generally cause physical and chemical changes in a thin surface layer without affecting the bulk properties. Treatments will increase surface energy and also provide polar molecular groups necessary for good bonds between ink/toner and polymer molecules. In addition to printability, surface treatments also affect the sealing properties, i.e. initial heat sealing temperature, initial hot tack temperature, sealing window and seal strength of extrusion coatings. Both the sealability of packaging material and the tightness of the seal are critical points in the manufacturing process of packages and of the final package. The printability must be obtained without losing the sealability properties. In the first part of this research (TAPPI European PLACE 2007), surface energy, printability and sealability of low density polyethylene (PE-LD) coated paperboard after flame and corona treatments were studied. In this second part of the study, the research is extended to other polyolefins, i.e. high-density polyethylene (PE-HD) and polypropylene (PP). The surface chemistry is evaluated with contact angle measurements and X-ray photoelectron spectroscopy (XPS) measurements. Scanning electron microscopy (SEM) and optical profilometry are used to study the topographical and morphological changes on the surfaces. Furthermore, the heat sealing and hot tack properties, and water vapour barrier properties of the extrusion coatings are evaluated. The aim of this study is also to evaluate the printability of the extrusion coatings and to map out the role of surface modification in print quality formation. This study has concentrated on digital printing, particularly on the dry toner-based electrophotographic printing process. Flame treatment decreases the contact angle of water on PE-LD, PE-HD and PP coated papers more than corona treatment, but the lowest contact angle is obtained when the treatments are used simultaneously (i.e. co-effect of the treatments). Flame treatment deteriorates the sealability properties of PE-LD coated paper, whereas corona treatment improves sealability for example by decreasing the minimum heat sealing temperature. The sealability properties of PE-HD and PP coated papers are improved not only by corona treatment, but also by flame treatment. Flame treatment significantly improves the water vapour barrier of PEs. Where printability is concerned, it can be noticed that all the treatments improve rub-off resistance with PEs. With PE-LD flame is the most effective, and with PE-HD corona. With PP, the co-treatment gives the best result. Morphological changes in micro- and nano- scale were most observed on the flame treated PE-LD surface, whereas the electret phenomenon was observed on PE-LD, PE-HD and PP surfaces only after corona treatment.</p>

Topics

• density
• impedance spectroscopy
• morphology
• surface
• polymer
• scanning electron microscopy
• x-ray photoelectron spectroscopy
• Oxygen
• extrusion
• strength
• chemical composition
• durability
• surface energy
• profilometry