Development of adhesive resin systems with low formaldehyde emission for the wood based panel industry

Abstract

Industrial products that are obtained by mechanically chopped, shredded and ground log wood into fiber, chip or layer and bonding these structures with a resin at high temperature and pressure are named as wood based board or composite wood based panel. Particleboard, medium density fiberboard, plywood, oriented particle board can be listed as the most important wood-based panel types. Formaldehyde is a chemical that has been the subject of a wide variety of industrial processes and is a raw material in the production of urea formaldehyde (UF), phenol formaldehyde (PF), melamine formaldehyde (MF) and melamine urea formaldehyde (MUF) resins. These resins have also been used for years to bind wood fibers and chips in the production of wood-based composite panels. In particular, urea formaldehyde (UF) resin is the most preferred one in the wood-based panel industry due to its cheap, transparent color, fast curing, low viscosity and easy application. However, wood-based composite panels that are produced with formaldehyde containing resins cause free formaldehyde emissions that negatively affect the environment and human health. Emission release can continue for long periods after the production of the panel product, especially under variable temperature and relative humidity conditions. Strict regulations have been imposed on the formaldehyde emissions to be released by wood-based boards produced with binders containing formaldehyde. Emission classes have been defined and the limitations have begun to be secured by legal sanctions. The ultimate aim of the thesis is to press wood-based particleboards with low formaldehyde emission without sacrificing quality. In this thesis, two different studies were carried out with urea glyoxal and urea formaldehyde resins and particleboards pressed with these resins were evaluated. In the first study, glyoxal, an aliphatic dicarbonyl aldehyde was employed instead of formaldehyde for resin synthesis. Urea glyoxal (UG) and urea melamine glyoxal (UMG) resins were synthesized without using formaldehyde and the effects of melamine content were determinated by substituting melamine instead of urea (10% and 20%). The reaction mechanism of urea with glyoxal in acidic environment at elevated temperature is based on two stages. In first step, the nitrogen atom of urea is bonded to the carbon atom of glyoxal, forming the carbon-nitrogen bond. The latter is based on transfer of the hydrogen atom on nitrogen to oxygen atom and forming the hydroxyl bond. The reaction of melamine and glyoxal proceeds as same steps. Usage of melamine within the scope of this thesis study was evaluated in order to increase the cross-linking on the resin due to containing benzene ring and three amine groups on the ring. In order to act as a hardener for urea glyoxal and urea melamine glyoxal resins, an acid ionic liquid that called as N-methyl-2-pyrrolidone hydrogen sulfate was synthesized.Ionic liquids are defined as organic salts that contain a small molecule inorganic/organic anionic structure and a relatively larger cationic structure. These salts have a wide range of uses as polymerization media in various polymerization processes, separation techniques for polymer gel electrodes, and also as catalysts. Ionic liquids are also important for green chemistry as they have low toxicity and volatility properties. FTIR-ATR, DSC, TGA, 13C NMR, SEM and SEM-EDAX studies were carried out for the characterization studies of the synthesized resins. FTIR-ATR spectroscopic experiments, showed characteristic peaks of the resins were determined separately as UG, UMG10% and UMG20%. The temperatures at which resins start to cure, with and without catalyst, were determined with DSC analysis. A small endothermic peak was seen non catalyst resins due to glyoxal, which does not initially react. This peak was not observed in the resins with catalyst due to the ionic liquid reacting with glyoxal. The initial temperature of curing started at 180-200oC and melamine allocation caused decreasing of temperature. Furthermore, decreases were observed in terms of both curing temperatures and enthalpy with the usage of catalyst. Resins that contained melamine, this decrease was more sharply. In this way, it is concluded that both melamine and ionic liquid usage caused the curing reaction to take place more easily. The thermal degradation behavior of the resin samples is determined by TGA and it has been observed that this degradation occurs between 180-300oC.13C NMR, the sequence of carbon atoms of the resin was determined. Resin appearances and elemental compositions were obtained with SEM and SEM-EDAX. The presence of melamine further clarified the crystalline appearance on the surface, while a smooth and homogeneous surface is observed of the system cured with ionic liquid. Particleboards that were pressed with UG, UMG10%, and UMG20% with N-methyl-2-pyrrolidone hydrogen sulphate as a hardener, in 12 mm thickness and density of 700 kg/m3, were evaluated in terms of both mechanical properties and formaldehyde emission. The test specimens, which were subjected to internal bond, bending strength, elastic modulus and surface strength tests, were classified according to the EN 312 standard. All samples fulfilled the P1 classification. According to particleboard results, the best mechanical properties of internal bond and elastic modulus were obtained with 10% melamine substitution, flexural strength and surface toughness values that decreased depending on the amount 20% usage of melamine. Formaldehyde emission was determined according to EN 12460-5 and all particleboard samples were fit with European E1 norms. In addition, using the approaches that given in literature, all of the particleboards fulfill CARB II, E0 and F**** classifications. In this study, UMG10% with acid ionic liquid was recommended as non-formaldehyde resin system. In the second study, urea formaldehyde resin was synthesized and in-situ usage of lignosulfonate and/or 1,4 butanediol diglycidyl ether was investigated. Lignosulfonates are defined as lignin polymers that contain water-soluble sulfonate groups. Glycidyl ethers are chemicals that ended with mono or poly functional oxirane groups with low viscosity. In this study, four different resin syntheses, namely UF, UF-LS, UF-GE and UF-LS-GE, were conducted. Synthesis of UF resin was done by the traditional three stage alkaline-acid-alkaline method. For UF-LS, the same method was followed and calcium lignosulfonate was hydroxymethylated in an alkaline medium to react with excess formaldehyde in the first stage and to participate the polymerization by condensation. For UF-GE, resin synthesis was started in an alkaline environment such as UF, and polymerization was advanced by adding 1.4 butanediol diglycidyl ether in the second step. Polymerization was advanced as a result of ring opening of oxirane rings in an acidic medium and reaction with hydroxymethylated urea. FTIR-ATR, DSC, XRD, 1H NMR, 13C NMR, SEM, and SEM-EDAX analyses were carried out for characterization. FTIR-ATR spectroscopic experiments, the characteristic peaks of the resins were determined separately for UF, UF-LS, UF-GE, and UF-LS-GE. No distinctive peak was observed in the modified resins for this study, apart from the peaks that were seen in UF resin; For this reason, 1H NMR and 13C NMR studies were carried out to determine the structural formulas of the resins, the positions of hydrogen and carbon atoms. The curing temperatures and enthalpies of the resins were determined by DSC analysis. A single exothermic peak seen in a wide range of UF resin showed two different peak structures relatively narrow in the presence of LS or GE. The second peak was attributed to the degradation of the methylene ether bridges. On the other hand, single usage of LS or GE decreased the cure temperatures and significantly lowered the enthalpies. Combinated usage of LS and GE, curing temperature had an increasing trend and a single curing peak was observed. In addition, the combination of LS and GE reduced the enthalpy value by nearly half compared to the standard UF resin. The crystalline structures of the resins were examined and the crystalline regions specific to UF resin were determined by XRD analysis. In this analysis, it can be said that GE had a decreasing effect on crystallinity. Resin appearances and elemental compositions were obtained with SEM and SEM-EDAX. Different agglomeration properties were observed for UF, UF-LS, UF-GE, and UF-LS-GE. Particleboards pressed with NH4Cl as a catalyst on synthesized resins (UF, UF-LS, UF-GE, and UF-LS-GE) with thickness of 16 mm and density of 650 kg/m3 were evaluated in terms of both mechanical properties and formaldehyde emission. The test specimens, which were subjected to internal bond, bending strength, elastic modulus and surface strength tests, were classified according to the EN 312 standard. Particleboards pressed with UF, UF-LS and UF-GE fulfilled the P2 class by meeting the requirements for boards that are used in interior equipment (including furniture) in dry conditions. Especially LS or GE had a positive effect on the internal bond compared to the standard UF resin. Usage of GE for flexural strength, flexural modulus and surface strength tests; for internal bond, the use of LS provides the optimum effect. On the other hand, particleboard samples pressed with the UF-LS-GE met P1 class that is named as the board used in dry conditions for general use. Although the boards show a decreasing trend in flexural strength, flexural modulus and surface durability tests for UF-LS-GE, these properties fulfilled with P2 class however, for internal bond, these particleboards only met the P1 class due to the resin's insufficient bonding with wood. The reason for this situation is considered as relatively high temperature and time required for the bonding of LS and GE during panel formation. In the wood-based panel industry, panels are generally pressed at 180-200oC and 3-5 minutes. During hot press, this temperature reaches only 100-110oC for core layer. These parameters were not fully sufficient for full curing of particle board pressed with UF-LS-GE; however, it is evaluated as P1. Formaldehyde emission was determined according to EN 12460-5 and all particleboard samples comply with E1 norms. Especially, LS modification gave the formaldehyde reduction effect. The effect of GE on formaldehyde emission for particleboard is determined as relatively low. In the combination of LS and GE, it caused an increase in emission as a result of not full curing compared to the standard UF resin. In this study, it can also be interpreted that UF-LS provides F** and CARBI classes by using the approaches that were determined in the literature.