Synthesis–structure–performance relationship of cocondensed phenol–urea–formaldehyde resins by MALDI‐ToF and 13C NMR (original) (raw)
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Typical particleboard wood-adhesive urea–formaldehyde (UF) resins, synthesized with formaldehyde/first urea (F/U 1) mol ratios of 1.80, 2.10, and 2.40 and the second urea added to an overall F/U ratio of 1.15, in weak alkaline pH, were allowed to stand at room temperature over a period of 50 days. 13 C-NMR of time samples taken over the storage period showed gradual migration of hydroxymethyl groups from the polymeric first-urea components to the monomeric second-urea components and also an advancing degree of polymerization of resins by forming methylene and methylene ether groups involving the second urea. These phenomena that varied with the F/U 1 mol ratios used in the resin syntheses due to the varying polymer branching structures resulted in the first step of resin synthesis. Varying viscosity decreases and increases of the resins also occurred. Due to these chemical and physical changes, the particleboards that bonded with the sampled resins showed varying bond strength and formaldehyde-emission values, indicating process optimizations possible to improve bonding and formaldehyde-emission performances.
Journal of Applied Polymer Science, 2003
Typical particleboard wood-adhesive urea-formaldehyde (UF) resins, synthesized with formaldehyde/first urea (F/U 1 ) mol ratios of 1.80, 2.10, and 2.40 and the second urea added to an overall F/U ratio of 1.15, in weak alkaline pH, were allowed to stand at room temperature over a period of 50 days. 13 C-NMR of time samples taken over the storage period showed gradual migration of hydroxymethyl groups from the polymeric first-urea components to the monomeric second-urea components and also an advancing degree of polymerization of resins by forming methylene and methylene ether groups involving the second urea. These phenomena that varied with the F/U 1 mol ratios used in the resin syntheses due to the varying polymer branching structures resulted in the first step of resin synthesis. Varying viscosity decreases and increases of the resins also occurred. Due to these chemical and physical changes, the particleboards that bonded with the sampled resins showed varying bond strength and formaldehydeemission values, indicating process optimizations possible to improve bonding and formaldehyde-emission performances.
Urea-formaldehyde resins characterized by thermal analysis and FTIR method
Journal of Thermal Analysis and Calorimetry, 2008
Urea-formaldehyde (UF) resins are the most used polycondensation resins today, in manufacturing particleboards. UF resins possess some advantages such as fast curing, good performance in the panel, water solubility and low price. However, the main chemical bonds of the UF resins macromolecules are hydrolysis sensitive. This causes low water and moisture resistance performance and subsequent formaldehyde release from the UF-bonded panels. A multitude of pathways have been explored for the improvement of UF resins' behavior relating either to their synthesis procedure or application parameters during panel manufacture. In this study, two UF resins (a conventional and an innovative one produced at very low pH and temperature conditions) were analyzed for their specifications and characterized with TG-DTA technique in dynamic heating conditions and FTIR measurements both in their pre-polymer and cured state.
European Polymer Journal, 2014
The issue whether urea-formaldehyde (UF) resins possess crystallinity when cured in contact with wood has not been clearly resolved, although according to some published work low mole UF resins do not exhibit crystallinity in contact with wood. Here we provide evidence using field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy and wide angle X-ray diffraction (WAXD) that UF resin with a low formaldehyde/urea (F/U) mole ratio does crystallize when cured in contact with wood and show that crystals have various morphologies and sizes and thus can be regarded as polycrystals. Morphological characterization by FE-SEM revealed the presence of globular/nodular particles and crystalline structures. The particles occurred in two distinct populations with respect to their size, and the fractured faces of particles and their associated plate-like structures revealed presence of sub-particles of extremely small size (26.3-56.1 nm size range). Some crystalline regions of the resin contained expanded fanshaped crystals, representing mature crystals. Other crystalline regions displayed features suggestive of emergence of crystals from globular particles. Transmission electron microscopy (TEM) made it possible to obtain electron diffraction (ED) pattern by analyzing directly the resin contained within cell lumens, which also suggested resin crystallization into polycrystals. The micromorphological information presented, particularly on nanosubstructures and crystalline features, is novel and provides greater understanding of the architecture and crystalline characteristics of UF resin cured in contact with wood. Furthermore, information included provides evidence of a close relationship of emerging crystals with globular particles. FTIR spectra demonstrated that a quantitative calculation of the methylene linkages formed by cross-linking of UF resins in contact with wood has been advanced in its cure and crystalline structures formed in the cured UF resin. WAXD results also confirmed a polycrystalline nature of UF resin cured in contact with wood. These results corroborate that a low formaldehyde thermosetting UF resin cured in the presence of wood possesses distinct crystalline structures, an aspect of UF resin behavior that remained poorly understood and needed clarification. However, presence of crystallinity, albeit not widespread, may negatively impact cohesive strength of the resin, leading to a poor adhesion performance.
Urea formaldehyde UF adhesive resins for wood
Urea-formaldehyde (UF) resins are the most important type of adhesive resins for the production of wood based panels. They convince by their high reactivity and good performance in the production and by their low price, however they lack in water resistance of the hardened resin owing to the reversibility of the aminomethylene link and hence the susceptibility to hydrolysis. This need can be overcome by introducing other components like melamine to the UF resin molecules. The former problem of subsequent formaldehyde emission can be considered as solved owing to the decrease of the content of formaldehyde in the resins during the last two decades. Modern laboratory test methods enable a deep insight into the chemical structure and the gelling and hardening behaviour of the resins.
Eur. Chem. Bull, 2023
Urea formaldehyde adhesives are widely used in wood composite industries to make wood based panel products, However to achieve better bonding strength and low VOC emission panel products the modification of resin further required. The objective of the study was to manufacturing of modified urea formaldehyde resin with polymeric isocyanate (pMDI) for manufacturing of better water resistant wood composite and resin properties study for manufacturing of wood composite products. In order to improve the water resistance properties, urea-formaldehyde resin are commonly modified with melamine or phenol. In this study water resistance properties of urea formaldehyde resin was enhanced by using Yamandur R-11(polymeric isocyanate (PMDI) having terminal isocyanate-CNO group appx. 12,5 %) with ureaformaldehyde resin. A typical composite adhesive was manufactured by mixing emulsifiable polymeric isocyanate with urea formaldehyde resin at a different concentration starting from 1%, 2.5%, 5.0%, 7.5% and 10% on the basis of liquid Urea Formaldehyde resin. Adhesive properties of UF /pMDI hybrid resin like viscosity, gel time ,FTIR spectroscopy, free formaldehyde of the resin and other resin property like formaldehyde emission of the board were analyzed. The results showed that the hybrid adhesive consisting of UF resin and emulsifiable polymeric isocyanate has shown better adhesive properties than the conventional Urea formaldehyde resin for manufacturing of wood composite products and reduces formaldehyde emission to maintain better indoor airquality for safer environment.
Structure formation in urea-formaldehyde resin synthesis
The structure formation in acid promoted polycondensation and particularly in alkaline post-treatment with urea was studied by ¹³C NMR spectroscopy in the three-step synthesis of urea-formaldehyde resins. Trishydroxymethyl urea was identified as the constituent of the mixture of hydroxylmethylated urea derivatives obtained in alkaline conditions. The formation of methylenes linked to secondary or tertiary amino groups occurs only in acidic conditions by branching chains in the reaction with mono-and 1,3-bishydroxymethyl urea. Bishydroxymethyl groups do not take part in acid promoted polycondensation. The final structure of resins depends mostly on the migration of formaldehyde from bishydroxymethyl groups to urea with formation of mono-hydroxymethyl urea as the first preferred compound. A greater amount of oligomers with methylenes adjacent to secondary amino groups and containing singly bonded urea is a sign of a thorough heat treatment. A steady content of methylenes linked to tertiary amino groups is observed in heat treatment and ageing of resins.
Hydrolytic stability of cured urea‐formaldehyde resins modified by additives
Journal of Applied Polymer Science, 2009
Urea‐formaldehyde (UF) resins are prone to hydrolysis that results in low‐moisture resistance and subsequent formaldehyde emission from UF resin‐bonded wood panels. This study was conducted to investigate hydrolytic stability of modified UF resins as a way of lowering the formaldehyde emission of cured UF resin. Neat UF resins with three different formaldehyde/urea (F/U) mole ratios (1.4, 1.2, and 1.0) were modified, after resin synthesis, by adding four additives such as sodium hydrosulfite, sodium bisulfite, acrylamide, and polymeric 4,4′‐diphenylmethane diisocyanate (pMDI). All additives were added to UF resins with three different F/U mole ratios before curing the resin. The hydrolytic stability of UF resins was determined by measuring the mass loss and liberated formaldehyde concentration of cured and modified UF resins after acid hydrolysis. Modified UF resins of lower F/U mole ratios of 1.0 and 1.2 showed better hydrolytic stability than the one of higher F/U mole ratio of 1....
Journal of Adhesion Science and Technology, 2018
In industry, formalin and urea formaldehyde condensates (UFC) are used for the preparation of urea-formaldehyde adhesives. The aim of this study was to evaluate the characteristics of a UF resin prepared with formalin (42% concentration) and two kinds of UFC with various urea content (F/U $ 5 and 4.3 malar ratio). The resins were prepared with final molar ratio F/U ¼ 1.1. The UF resin with formalin showed better mechanical properties than those with UFC. The 13 C NMR and FT-IR results indicated that methylene, methylene ether and urons groups in a UF resin prepared with formalin were present in higher proportion than observed in industrial UF resins. The DSC (differential scanning calorimetry) results showed that the curing temperature of a UF resin prepared with formalin is lower than that of a UF resin prepared with UFC. The mechanical properties of MDFs prepared with the three types of adhesives showed that MDFs bonded with a UF prepared with formalin yields a better mechanical properties than MDFs prepared with a UF resin based on UFC. Moreover, resin synthesized with UFC (F/U ¼ 5) result in higher IBS (internal bond strength) MOR (modulus of rupture), MOE (modulus of elasticity), TS (thickness swelling) and formaldehyde emission (FE) than resin synthesized with UFC (F/U ¼ 4.3). The MDF bonded with the UF2 resin presented poor results, with the highest TS and the lowest IBS, MOE, MOR and FE.
Urea-formaldehyde Resins Synthesis, Modification and Characterization
IOSR Journals , 2019
Urea-formaldehyde resins (UF resins) were prepared by a two-stage reaction. The comparative analysis were made of the resulting laboratory samples with some commercial samples. Some of the drawbacks of UF resins, such as poor adhesive strength, especially in moisture or humid environments and high free formaldehyde emission (FE).In order to reduce the FE, lowering urea/formaldehyde (U/F) mole ratio in the synthesis of the UF resin was done. In this study, synthesis of UF resins was carried out following the conventional alkaline-acid two-step reaction with a second addition of urea, resulting in the following U/F mole ratio: 1:2.60, 1:2.70, 1:2.30, 1:2.04, 1:1.97, 1:2.13 and 1:2.90. Remarkably, the free formaldehyde was found to decrease with increased mole U/F mole ratio, solid content and ash content.The key parameter in the decrease of formaldehyde emission is lowering the urea/formaldehyde (U/F) ratioin the synthesis of UF resins. This leads to a reduction in the content of crosslinking groups in cured UF resins, lowering the strength and water resistance of adhesive joints in boards. The laboratory synthesized UF resins samples were modified using different types of alcohols. N-butyl alcohol modified UF resins samples were found to possess better properties generally.