Open Access

Powder coatings provide several advantages over traditional coatings: environmental friendliness, freedom of design, robustness and resistance of surfaces, possibility to seamlessly all-around coating, fast production process, cost-effectiveness. In the last years these benefits of the powder coating technology have been adopted from metal to heat-sensitive natural fibre/ wood based substrates (especially medium density fibre boards- MDF) used for interior furniture applications. Powder coated MDF furniture parts are gaining market share already in the classic furniture applications kitchen, bathroom, living and offices. The acceptance of this product is increasing as reflected by excellent growth rates and an increasing customer base. Current efforts of the powder coating industry to develop new powders with higher reactivity (i.e. lower curing temperatures and shorter curing times; e.g. 120°C/5min) will enable the powder coating of other heat-sensitive substrates like natural fibre composites, wood plastic composites, light weight panels and different plastics in the future. The coating could be applied and cured by the conventional powder coating process (electrostatic application, and melting and curing in an IR-oven) or by a new powder coating procedure based on the in-mould-coating (IMC) technique which is already established in the plastic industry. Extra value could be added in the future by the functional powder toner printing of powder coated substrates using the electrophotographic printing technology, meeting the future demand of both individualization of the furniture part surface by applying functional 3D textures and patterns and individually created coloured images and enabling shorter delivery times for these individualized parts. The paper describes the distinctiveness of powder coating on natural fibre/ wood based substrates, the requirements of the substrate and the coating powder.

The interfacial compatibility between polymers and nanoclay fillers as well as the thermostability of both components are important characteristics for processing them into polymer composites. While the polymer component is often grafted using common polymerization reactions, the nanoclay component is usually surface modified by surfactant treatment to improve compatibility. In the present study, the polymer ethylene vinyl alcohol and a nanoclay filler based on natural bentonite are both surface modified by different silanes, 3-glycidoxypropyltrimethoxysilane and methacryloxymethyltrimethoxysilane and their interfacial properties are investigated by inverse gas chromatography. The silane-modified samples had improved interfacial properties as reflected by a significant increase in dispersive and specific surface energies. Lewis acidities were determined using chloroform and 1,4-dioxane as polar probes and showed a good match between polymer and nanofiller interfaces. Lewis acidity was generally lower after silane-modification. Silanization yielded increased thermal stability of the treated samples. Thus, silanization led to improved compatibility and enhanced thermal stability which facilitates further processing.

Powder coating of engineered wood panels such as medium density fibreboards (MDF) is gaining industrial interest due to ecological and economic advantages of powder coating technology. For transferring powder coating technology to temperature-sensitive substrates like MDF, a thorough understanding of the melting, flowing and curing behaviour of the used low-bake resins is required. In the present study, thermo-analysis in combination with iso-conversional kinetic data analysis as well as rheometry is applied to characterise the properties of an epoxy-based powder coating. Neat resin and cured powder coating films are examined in order to define an ideal production window within which the resin is preferably applied and processed to yield satisfactory surface performance on the one hand and without exposing the carrier MDF too high a temperature load on the other hand to prevent the panel from deteriorating in mechanical strength. In order to produce powder coated films of high surface gloss – a feature that has not yet successfully been realized on MDF with powder coatings – a new curing technology, in-mould surface finishing, has been applied.

Ethylene terephthalate and ethylene naphthalate oligomers of defined degree of polymerization were synthesized via chemical recycling of the parent polymers. The oligomers were used as defined building blocks for the preparation of novel block-co-polyesters having tailored sequence compositions. The sequence lengths were systematically varied using Design of Experiments. The dispersive surface energy and the specific desorption energy of the co-polymers were determined by inverse gas chromatography. The study shows that polyethylene terephthalate-polyethylene naphthalate (PET-PEN) block-co-polyesters of defined sequence lengths can be prepared. Furthermore, the specific and dispersive surface energies of the obtained block-co-polyesters showed a linear dependence on the oligomer molecular weight and it was possible to regulate and control their interfacial properties. In contrast, with the corresponding random-block-co-polyesters no such dependence was found. The synthesized block-co-polyesters could be used as polymeric modifying agents for stabilizing PET-PEN polymer blends.

The fiber deformations of once-dried, bleached and never-dried unbleached kraft pulps were studied with respect to their behavior in high- and low-consistency refining. The pulps were stained with congo red to experimentally highlight areas where the arrangement of the fibrils was altered by refining such as dislocated zones or slip planes. The stained fibers were analyzed with conventional Metso Fiberlab but also with a novel prototype measurement device utilizing a color imaging setup. The local intensity of the stain in the fiber was expressed as degree of overall damage (Overall fiber damage index, OFDI). The rewetted zero span tensile index (RWZSTI) was used to verify the OFDI with respect to the pulp strength. High consistency refining resulted in a clear increase in the number of kinks which negatively influenced the pulp strength. The OFDI which was used to detect the intensity of local fiber defects also responded accordingly. A higher OFDI resulted in a lower pulp strength. Low consistency refining removed a significant amount of kinks and resulted in an increase in fiber swelling. A slight increase in fibrillation and a significant increase in flake-like fines were also observed. The OFDI, however, was not reduced in low consistency refining as it would be expected by the removal of less severe dislocations. One reason proposed here is that low consistency refining created new fiber pores that allowed the dye to penetrate into the fiber wall similarly as it does in the zones of the dislocations.