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Melamine formaldehyde (MF) resins are widely used for the gluing and surface coating of wood-based consumer products in the interior design of living environments. MF resins are especially relevant in decorative laminate applications because of their good performance-to-price ratio. In their industrial processing, an important intermediate state is the liquid MF prepolymer that is used for decorative paper impregnation. Here, the drying of impregnated papers is investigated with respect to premature curing. A new method to quantify water release upon drying that allows estimation of the degree of undesired precuring is described. Since curing proceeds via polycondensation, crosslinking brings about the release of water molecules. By thermogravimetric analysis (TGA), drying was studied in terms of water release due to physical drying (elimination of “dilution water”) and chemical crosslinking of the prepolymer to a three-dimensional MF network (elimination of chemically liberated water). The results obtained by TGA/IR spectroscopic analysis of the liberated volatiles show that the emission of water from b-stage MF can be clearly analytically separated into a physical (evaporation of dilution water) and a chemical (liberation via condensation) sequence. TGA experiments were correlated with curing experiments performed with differential scanning calorimetry (DSC) to estimate the residual crosslinking capacities of the impregnated papers. The drying conditions used during the preparation of impregnated decorative papers seemed to significantly affect their remaining reactivity only when harsh drying conditions were used. Upon heat exposure for prolonged time, precuring of the oligomer units results in a shift of the temperature maxima in TGA.
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.
Hardboards (HBs) (wet-process high-density fibreboards) were made in an industrial trial using a binder system consisting of cationic mimosa tannin and laccase or just cationic tannin without any thermosetting adhesive. The boards displayed superior mechanical strength compared to reference boards made with phenol–formaldehyde, easily exceeding the European standards for general-purpose HBs. The thickness swell of most of the boards was slightly greater than the standards would allow, so some optimisation is required in this area. The improved board properties appear to be mainly associated with ionic interactions involving quaternary amino groups in cationic tannin and negatively charged wood fibres rather than to cross-linking of fibres via laccase-assisted formation and coupling of radicals in tannin and fibre lignin.
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.
The powder coating of veneered particle boards by the sequence electrostatic powder application -powder curing via hot pressing is studied in order to create high gloss surfaces. To obtain an appealingaspect, veneer Sheets were glued by heat and pressure on top of particle boards and the resulting surfaceswere used as carrier substrates for powder coat finishing. Prior to the powder coating, the veneeredparticle board surfaces were pre-treated by sanding to obtain good uniformity and the boards werestored in a climate chamber at controlled temperature and humidity conditions to adjust an appropriate electrical surface resistance. Characterization of surface texture was done by 3D microscopy. The surfaceelectrical resistance was measured for the six veneers before and after their application on the particleboard surface. A transparent powder top-coat was applied electrostatically onto the veneered particleboard surface. Curing of the powder was done using a heated press at 130◦C for 8 min and a smooth, glossy coating was obtained on the veneered surfaces. By applying different amounts of powder thecoating thickness could be varied and the optimum amount of powder was determined for each veneer type.
Decorative laminates based on melamine formaldehyde (MF) resin impregnated papers are used at great extent for surface finishing of engineered wood that is used for furniture, kitchen, and working surfaces, flooring and exterior cladding. In all these applications, optically flawless appearance is a major issue. The work described here is focused on enhancing the cleanability and antifingerprint properties of smooth, matt surface-finished melamine-coated particleboards for furniture fronts, without at the same time changing or deteriorating other important surface parameters such as hardness, roughness or gloss. In order to adjust the surface polarity of a low pressure melamine film, novel interface-active macromolecular compounds were prepared and tested for their suitability as an antifingerprint additive. Two hydroxy-functional surfactants (polydimethysiloxane, PDMS-OH and perfluoroether, PF-OH) were oxidized under mild conditions to the corresponding aldehydes (PDMS-CHO and PF-CHO) using a pyridinium chlorochromate catalyst. With the most promising oxidized polymeric additive, PDMS-CHO, the contact angles against water, n-hexadecane, and squalene increased from 79.8°, 26.3° and 31.4° for the pure MF surface to 108.5°, 54.8°, and 59.3°, respectively, for the modified MF surfaces. While for the laminated MF surface based on the oxidized fluoroether the gloss values were much higher than required, for the surfaces based on oxidized polydimethylsiloxane the technological values as well as the lower gloss values were in agreement with the requirements and showed much improved surface cleanability, as was also confirmed by colorimetric measurements.
Clay minerals play an increasingly important role as functional fillers and reinforcing materials for clay polymer nanocomposites (CPN) in advanced applications. Among the prerequisites necessary for polymer improvement by clay minerals are homogeneous and stable Distribution of the clay mineral throughout the CPN, good compatibility of the reinforcement with the Matrix component and suitable processability. Typically, clay minerals are surface-modified with organic interface active compounds like detergents or silanes to obtain favorable properties as filler. They are incorporated into the polymer matrix using manufacturing Equipment like extruders, batch reactors or other mixing machines. In order for the surface modification to survive the stresses and strains during incorporation, the modified clay minerals must display sufficient thermal and mechanical stability to retain the compatibilizing effect. In the present study, thermogravimetry was used in combination with isoconversional kinetic analysis to determine the thermal stability of a silane-modified clay mineral based on bentonite. These findings were compared with the stability of the same clay mineral that was only surfactant-modified. It was found that silane modification leads to significantly improved thermal stability, which depends strongly on the type of silane employed.
The powder coating of wood products as an emerging environmentally sustainable coating technology holds promise in terms of novel product quality features for engineered wood like medium-density fiberboards (MDFs). However, one major limitation currently impeding widespread application of powder coating technology is the availability of MDF panels that are suitable for this process. Typically, special-grade MDF panels are required that are more costly than standard-grade MDF panels to provide reliable coating quality, which makes powder coating economically unattractive for many users. Methods are needed that allow extending the range of available MDF grades. In the present study, three surface pretreatment approaches for MDFs were studied to increase the processability of standard-grade MDF in the powder coating process: atmospheric plasma pretreatment, infrared irradiation, and moisture equilibration in a climate chamber prior to electrostatic powder application. While atmospheric plasma treatment had no beneficial effect on the use of standard-grade MDF panels, both infrared preheating and preconditioning of the panels under controlled temperature–humidity conditions demonstrated that the range of MDF panels suitable for powder coating can be significantly extended by appropriate selection of the pretreatment procedure.
Cyanate esters
(2014)
Cyanate ester resins are an important class of thermosetting compounds that have experienced an ever-increasing interest as matrix systems for advanced polymer composite materials, which among other applications, are especially suitable for highly demanding functions in the aerospace or microelectronics industries. Other names for cyanate ester resins are cyanate resins, cyanic esters, or triazine resins. The various types of cyanate ester monomers share the aOCN functional group that trimerizes in the course of resin formation to yield a highly branched heterocyclic polymeric network based on the substituted triazine core structure. The basic reaction sequence leading to the typical cyanate ester polymer molecule is depicted in Figure 11.1. The curing reaction may take place with or without catalyst.
Decorative laminates are the most important class of surface-finished engineered wood products. However, while there are numerous scientific publications published dealing with the technology of wood, wood-based products and also liquid coating systems, there is practically no scientific research work available in the field of paper-based laminates. In view of an ever increasing global competition it is time to systematically apply and pursue scientific approaches in this field. The present work is based on a knowledge-based manufacturing paradigm. The application of scientific methodology (e.g. instrumental analysis, process analytics, design of experiments, chemometrics, process modeling) to the preparation of decorative laminates covering the whole process chain from resin synthesis to paper impregnation and to final laminate should enable a targeted design of material functionality.