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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.
Mass-customization is a megatrend that also affects the wood industry. To obtain individually designed laminates in batch size one efficient printing and processing technologies are required. Digital printing was envisaged as it does not depend on highly costly printing cylinders (as used in rotogravure printing) and allows rapid exchange of the printing designs. In the present work, two wellestablished digital printing approaches, the multi-pass and the single-pass technique, were investigated and evaluated for their applicability in decorating engineered wood and low-pressure melamine films. Three different possibilities of implementing digital printing in the decorative laminates manufacturing process were studied: (1) digital printing on coated chipboard and subsequently applying a lacquered top-coat or melamine overlay (designated as “direct printing”, since the LPM was the printing substrate), (2) digital printing on decorative paper which was subsequently impregnated before hot pressing (designated as “indirect printing, variant A”) and (3) digital printing on decorative paper with subsequent interlamination of the paper between impregnated under- and overlay paper layers during the pressing process (designated as “indirect printing, variant B”). Due to various advantages of the resulting cured melamine resin surfaces including a much better technological performance and flexibility in surface texture design, it was decided to industrially further pursue only the indirect digital printing process comprising interlamination and the direct printing process with a melamine overlay-finishing. Basis for the successful trials on production and laboratory scales were the identification of applicable inks (in terms of compatibility with melamine resin) and of appropriate printing paper quality (in terms of impregnation and imprinting ability). After selection and fine tuning of suitable materials, the next challenge to overcome was the initially insufficient bond strength between impregnated overlay and the ink layers which led to unsatisfactory quality of the print appearance and delamination effects. However, the optimization of the pressing program and the development of a modified impregnation procedure for the underlay and overlay papers allowed the successful implementation of digital printing in the production line of our industrial partner FunderMax.
The isothermal curing of melamine resin is investigated by in-line infrared spectroscopy at different temperatures. The infrared spectra are decomposed into time courses of characteristic spectral patterns using Multivariate Curve Resolution (MCR). It was found that depending on the applied curing temperature, melamine films with different spectral fingerprints and correspondingly different chemical network structures are formed. The network structures of fully cured resin films are specific for the applied curing temperatures used and cannot simply be compensated by changes in the curing time. For industrial curing processes, this means that cure temperature is the main system determining factor at constant M:F ratio. However, different MF resin networks can be specifically obtained from one and the same melamine resin by suitable selection of the curing time and temperatures profiles to design resin functionality. The spectral fingerprints after short curing time as well as after long curing time reflect the fundamental differences in the thermoset networks that can be obtained with industrial short-cycle and multi-daylight presses.
Impregnated paper-based decorative laminates prepared from lignin-substituted phenolic resins
(2020)
High Pressure Laminates (HPL) panels consist of stacks of self-gluing paper sheets soaked with phenol-formaldehyde (PF) resins. An important requirement for such PFs is that they must rapidly penetrate and saturate the paper pores. Partially substituting phenol with bio-based phenolic chemicals like lignin changes the physico-chemical properties of the resin and affects its ability to penetrate the paper. In this study, PF formulations containing different proportions of lignosulfonate and kraft lignin were used to prepare paper-based laminates. The penetration of a Kraft paper sheet was characterized by a recently introduced, new device measuring the conductivity between both sides of the paper sheet after a drop of resin was placed on the surface and allowed to penetrate the sheet. The main target value measured was the time required for a specific resin to completely penetrate the defined paper sample (“penetration time”). This penetration time generally depends on the molecular weight distribution, the flow behavior and the polarity of the resin which in turn are dependent on the manufacturing conditions of the resin. In the present study, the influences of the three process factors: (1) type of lignin material used for substitution, (2) lignin modification by phenolation and (3) degree of phenol substitution on the penetration times of various lignin-phenolic hybrid impregnation resins were studied using a complete twolevel three-factorial experimental design. Thin laminates made with the resins diluted in methanol were mechanically tested in terms of tensile and flexural strains, and their cross-sections were studied by light microscopy.
Here, the effects of substituting portions of fossil-based phenol in phenol formaldehyde resin by renewable lignin from two different sources are investigated using a factorial screening experimental design. Among the resins consumed by the wood-based industry, phenolics are one of the most important types used for impregnation, coating or gluing purposes. They are prepared by condensing phenol with formaldehyde (PF). One major use of PF is as matrix polymer for decorative laminates in exterior cladding and wet-room applications. Important requirements for such PFs are favorable flow properties (low viscosity), rapid curing behavior (high reactivity) and sufficient self-adhesion capacity (high residual curing potential). Partially substituting phenol in PF with bio-based phenolic co-reagents like lignin modifies the physicochemical properties of the resulting resin. In this study, phenol-formaldehyde formulations were synthesized where either 30% or 50% (in weight) of the phenol monomer were substituted by either sodium lignosulfonate or Kraft lignin. The effect of modifying the lignin material by phenolation before incorporation into the resin synthesis was also investigated. The resins so obtained were characterized by Fourier Transform Infra-Red (FTIR) spectroscopy, Size Exclusion Chromatography (SEC), Differential Scanning Calorimetry (DSC), rheology, and measurements of contact angle and surface tension using the Wilhelmy plate method and drop shape analysis.
Here, we study resin cure and network formation of solid melamine formaldehyde pre-polymer over a large temperature range viadynamic temperature curing profiles. Real-time infrared spectroscopy is used to analyze the chemical changes during network formation and network hardening. By applying chemometrics (multivariate curve resolution,MCR), the essential chemical functionalities that constitute the network at a given stage of curing are mathematically extracted and tracked over time. The three spectral components identified by MCR were methylol-rich, ether linkages-rich and methylene linkages-rich resin entities. Based on dynamic changes of their characteristic spectral patterns in dependence of temperature, curing is divided into five phases: (I) stationary phase with free methylols as main chemical feature, (II) formation of flexible network cross-linked by ether linkages, (III) formation of rigid, ether-cross-linked network, (IV) further hardening via transformation of methylols and ethers into methylene-cross-linkages, and (V) network consolidation via transformation of ether into methylene bridges. The presented spectroscopic/chemometric approach can be used as methodological basis for the functionality design of MF-based surface films at the stage of laminate pressing, i.e., for tailoring the technological property profile of cured MF films using a causal understanding of the underlying chemistry based on molecular markers and spectroscopic fingerprints.
Melamine–formaldehyde (MF) resins are widely used as adhesives and finishing materials in the wood industry. During resin cure, either methylene ether or methylene bridges are formed, leading to the formation of a three‐dimensional resin network. Not only the curing degree, but also the chemical species present in the cured resin determine the quality of the final product. Analytical methods allowing a detailed investigation of network formation are of great benefit to manufacturers. In the present work, resin cure of an MF precondensate is studied at different temperatures (100–200 °C) without considering the initial pH as a factor. Isoconversional kinetic analysis based on exothermal curing enthalpies enables calculation of the crosslinking degree at a given time/temperature regime. A semiquantitative determination of the chemical groups present is performed based on solid‐state nuclear magnetic resonance data. Fourier transform infrared spectroscopy has shown to be a fast and reliable analytical tool with high sensitivity toward functional groups and with great potential for at‐line process control.
The data present in this article affords insides in the characterization of a newly described bi-functional furan-melamine monomer, which is used for the production of monodisperse, furan-functionalized melamine formaldehyde particles. In the related research article Urdl et al., 2019 data interpretations can be found. The furan functionalization of particles is necessary to perform reversible Diels-Alder reactions with maleimide (BMI) crosslinker to form thermoreversible network systems. To understand the reaction conditions of Diels Alder (DA) reaction with a Fu-Mel monomer and a maleimide crosslinker, model DA reaction were performed and evaluated using dynamic FT-IR measurements. During retro Diels-Alder (rDA) reactions of the monomer system, it was found out that some side reaction occurred at elevated temperatures. The data of evaluating the side reaction is described in one part of this manuscript. Additional high resolution SEM images of Fu Mel particles are shown and thermoreversible particle networks with BMI2 are shown. The data of different Fu-Mel particle networks with maleimide crosslinker are presented. Therefore, the used maleimide crosslinker with different spacer lengths were synthesized and the resulting networks were analyzed by ATR-FT-IR, SEM and DSC.
Homogeneous and monodispersed furan functionalised melamine-formaldehyde particles were produced. As a precursor, 2-chloro-1,3,5-triazine-2,4-diamine (Mel) was selectively substituted with 2-aminomethyl furan (Fu) units in a convenient one step reaction. The pure reaction product Fu-Mel, which was used without further purification, was reacted with formaldehyde by conventional sol-gel condensation in aqueous medium to yield chemically homogenous, spherically shaped and monodispersed particles. The particles were analysed using ATR-FT-IR, Raman, 1H and 13C NMR spectroscopy, TGA, SEM and DSC measurements. The reactivity of the furan groups located at the particle surface was studied by performing a thermoreversible Diels-Alder cycloaddition reaction with bis-maleimide coupling agents. The formed networks showed thermoreversible behaviour, which was characterised by dynamic IR and DSC measurements.
During curing of thermosetting resins the technologically relevant properties of binders and coatings develop. However, curing is difficult to monitor due to the multitude of chemical and physical processes taking place. Precise prediction of specific technological properties based on molecular properties is very difficult. In this study, the potential of principal component analysis (PCA) and principal component regression (PCR) in the analysis of Fourier transform infrared (FTIR) spectra is demonstrated using the example of melamine-formaldehyde (MF) resin curing in solid state. FTIR/PCA-based reaction trajectories are used to visualize the influence of temperature on isothermal cure. An FTIR/PCR model for predicting the hydrolysis resistance of cured MF resin from their spectral fingerprints is presented which illustrates the advantages of FTIR/PCR compared to the combination differential scanning calorimetry/isoconversional kinetic analysis. The presented methodology is transferable to the curing reactions of any thermosetting resin and can be applied to model other technologically relevant final properties as well.
