Thermal behaviour of melamine-modified urea–formaldehyde resins

Thermal behaviour of industrial UF resins modified by low level of melamine was followed by TG-DTA technique on the labsys ^TM instrument Setaram together with the ¹³C NMR analysis of resin structure and testing boards in current production at Estonian particleboard factory Pärnu Plaaditehas AS. DTA curve of UF resin which has been cocondensed during synthesis with even low level of melamine shows the shift of condensation exotherm and water evaporation endotherm to considerable higher temperatures. The effect of melamine monomer introduced to UF resin just before curing was compared. The effect of addition of urea as formaldehyde scavenger was studied.     

Thermal behavior of modified urea–formaldehyde resins

The thermal stability of pure urea–formaldehyde resin (PR) and modified urea–formaldehyde (UF) resins with hexamethylenetetramine-HMTA (Resin 1), melamine-M (Resin 2), and ethylene urea (EU, Resin 3) including nano-SiO₂ was investigated by non-isothermal thermo-gravimetric analysis (TG), differential thermal gravimetry (DTG), and differential thermal analysis (DTA) supported by data from IR spectroscopy. Possibility of combining inorganic filler in a form of silicon dioxide with UF resins was found investigated and percentage of free formaldehyde was determined. The shift of DTG peaks to a high temperature indicates the increase of thermal stability of modified UF resin with EU (Resin 3) which is confirmed by data obtained from the FTIR study. The minimum percentage (6%) of free formaldehyde was obtained in Resin 3.     

Comparison of thermal curing behavior of liquid and solid urea–formaldehyde resins with different formaldehyde/urea mole ratios

This study was undertaken to compare thermal cure kinetics of urea–formaldehyde (UF) resins, in both liquid and solid forms as a function of formaldehyde/urea (F/U) mole ratio, using multi-heating rate methods of differential scanning calorimetry. The requirement of peak temperature (T _p), heat of reaction (ΔH) and activation energy (E) for the cure of four F/U mole ratio UF resins (1.6, 1.4, 1.2 and 1.0) was investigated. Both types of UF resins showed a single T _p, which ranged from 75 to 118 °C for liquid resins, and from 240 to 275 °C for solid resins. As the F/U mole ratio decreased, T _p values increased for both liquid and solid resins. ΔH values of solid resins were much greater than those of liquid resins, indicating a greater energy requirement for the cure of solid resins. The ΔH value of liquid UF resins increased with decreasing in F/U mole ratio whereas it was opposite for solid resins, with much variation. The activation energy (E _a) values calculated by Kissinger method were greater for solid UF resins than for liquid resins. The activation energy (E _α ) values calculated by isoconversional method which showed that UF resins in liquid or solid state at F/U mole ratio of 1.6 followed a multi-step reaction in their cure kinetics. These results demonstrated that thermal curing behavior of solid UF resin differed greatly from that of liquid resins, because of a greater branched network structure in the former.     

Sulfonated Kraft lignin addition in urea–formaldehyde resin

Journal of Thermal Analysis and Calorimetry - Urea–formaldehyde (UF) resins are widely used over the world as an adhesive, principally in the civil construction, but its use leads to...     

Direct measurement of surface adhesion between thin films of nanocellulose and urea–formaldehyde resin adhesives

Cellulose - When applying an adhesive to wood, the chemical heterogeneity of the wood cell walls makes it difficult to understand the contribution they make to the interfacial adhesion between the...     

Thermal degradation of lignin–phenol–formaldehyde and phenol–formaldehyde resol resins

Resol resins are used in many industrial applications as adhesives and coatings, but few studies have examined their thermal degradation. In this work, the thermal stability and thermal degradation kinetics of phenol–formaldehyde (PF) and lignin–phenol–formaldehyde (LPF) resol resins were studied using thermogravimetric analysis (TG) in air and nitrogen atmospheres in order to understand the steps of degradation and to improve their stabilities in industrial applications. The thermal stability of samples was estimated by measuring the degradation temperature (T _d), which was calculated according to the maximum reaction rate criterion. In addition, the ash content was determined at 800 °C in order to compare the thermal stability of the resol resin samples. The results indicate that 30 wt% ammonium lignin sulfonate (lignin derivative) as filler in the formulation of LPF resin improves the thermal stability in comparison with PF commercial resin. The activation energies of degradation of two resol resins show a difference in dependence on mass loss, which allows these resins to be distinguished. In addition, the structural changes of both resins during thermal degradation were studied by Fourier transform infrared spectroscopy (FTIR), with the results indicating that PF resin collapses at 300 °C whereas the LPF resin collapses at 500 °C.     

α-Thioureidoalkylation of urea heteroanalogs

α-Thioureidoalkylation of urea heteroanalogs such as thiosemicarbazide, amino-guanidine, sulfamide, and sulfonamides with 4,5-dihydroxyimidazolidine-2-thiones has been studied. Previously unknown 4,5-bis[thiosemicarbazido(guanidinoamino)]imidazolidine-2-thiones, 5,7-dialkylperhydroimidazo[4,5- e][1,2,4]triazine-3,6-dithiones, 4,6-diethyl-5(3H)-thioxotetrahydro-1 H-imidazo[4,5- c][1,2,5]thiadiazole 2,2-dioxide, and 1,3-dialkyl-4-[guanidinoimino(arylsulfonylimino)]imidazolidine-2-thiones have been synthesized.     

Does urea denature hydrophobic interactions?

It is generally accepted that clusters of hydrophobic moieties in water fall apart when urea is added in substantial amounts. We performed atomistic molecular dynamics simulations of hydrophobic solute pairs and found evidence that urea molecules act as "glue" bridging these pairs thereby holding them together. The picture is quite general as it applies to aliphatic-aliphatic as well as aromatic-aromatic interactions. The implications of this finding on the role of urea as a protein denaturant are discussed.     

N,N''''-bis(3-Chlorophenyl)urea

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Synthesis and thermal decomposition of Cr–urea complex

In this work, Cr–urea complex ([Cr(NH₂CONH₂)₆](NO₃)₃) was synthesized by direct solid-state reaction of chromium nitrate and urea, and its thermal decomposition reaction was studied for the first time to explore the possibilities of using the complex as precursor to nanosized chromium oxide. The formation of [Cr(NH₂CONH₂)₆](NO₃)₃ is confirmed from infrared spectroscopy and elemental analysis. Thermogravimetric and differential thermal analysis of the compound show a three-stage thermal decomposition in the temperature range from 190 to 430 °C. The result of X-ray diffraction (XRD) shows that the [Cr(NH₂CONH₂)₆](NO₃)₃ decompose at ~300 °C into α-Cr₂O₃ nanopowder with an average crystallite size of 33 nm.     

Infrared lines in β-quinol clathrates of methyl-isocyanide and formaldehyde

We have used diffuse reflectance FTIR measurements to identify the vibrational band positions of formaldehyde and methyl-isocyanide enclathrated in β-quinol. The CN stretch in the methyl-isocyanide exhibits two distinct frequencies whose relative line strengths are a function of temperature. This observation indicates that there are two energetically close states for the CH₃NC orientation in the β-quinol cage.     

(AEDPH3)·(BtaH): a novel supramolecular plaster with formaldehyde adsorption and formaldehyde/ultraviolet ray-induced luminescence switching performance

A novel supramolecular plaster, (AEDPH(3))·(BtaH) (1), is synthesised and characterized. The supramolecular plaster is easy to synthesise and process, and displays good mechanical properties. It can adsorb and eliminate formaldehyde (HCHO) with high efficiency and exhibits very interesting HCHO/ultraviolet ray-induced luminescence switching.     

Bulk properties and hydration numbers of components of water–salt,water–urea,and water–urea–salt systems

With an increase in the concentration of additives, the hydration numbers of compounds decrease. Thus, in a saturated 54.6% solution, urea loses approximately 3/4 of the initial amount of water, forming an aquacomplex of the composition (NH₂)₂CO?H₂O. In a supersaturated 44% solution, the sodium chloride aquacomplex is dehydrated by 2/3, and in a supersaturated 67% solution, sodium sulfate is dehydrated by 5/6. The density of these solutions is 1.354÷1.360 g/cm³ (44% NaCl) and 1.800÷1.849 g/cm³ (67% Na₂SO₄). In a saturated urea solution, NaNO₃, NaCl, and Na₂SO₄ complexes lose 53÷55% of hydration water. It is shown that the interactions in the binary water–urea system somewhat increase the hydration number of the salts (structural hydration). The hydration water density, a structurally important characteristic, increases in the series of solutions of urea, NaNO₃, NaCl, and Na₂SO₄. In the same series of additives, the excess volume of binary water–urea and water–salt systems becomes more negative.     

Interlayer-type Crystal Structure of N-（1-Adamantyl） urea

1 INTRODUCTION In the context of supramolecular chemistry, mo- lecules are joined together by intermolecular interac- tions to form a supramolecule whose physical pro- perties largely depend on the orientation and packing of molecules in the crystal structures[1]. The adaman- tane is a kind of cage alkane with high symmetry and stable framework. Its derivatives are extensively applied in the fields of medicine, macromolecular materials, aviation and so on due to the unique structures and …     

o-Cresol,thiourea and formaldehyde terpolymer – A cation exchange resin

o-Cresol–thiourea–formaldehyde terpolymer resin was synthesized through the condensation of o-cresol and thiourea with formaldehyde in the mole ratio 1:3:5 in the presence of 2 M hydrochloric acid as a catalyst. The resulting copolymer was characterized with IR and ¹H NMR spectral data. The average molecular weight of the resin was determined by Gel permeation chromatography. Thermal study of the resin was carried out to calculate the activation energy (Ea), enthalpy of activation (H³), entropy of activation (S³), free energy of activation (G³), and pre-exponential factor (A) of various steps of thermal decomposition of the terpolymer. The Dharwadkar and Kharkhanavala method has been used to calculate thermal activation energy and thermal stability. The chelation ion-exchange properties were also studied with the batch equilibrium method. The chelation ion-exchange properties of the copolymer was studied for Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺ and Cd²⁺ ions. The resin was proved to be selective chelating ion-exchange copolymer for certain metals. The study was carried out over a wide pH range and in media of various ionic strengths.     

Enzymatic catalysis of urea decomposition: elimination or hydrolysis?

We present a high-level quantum chemical study of possible reaction mechanisms associated with the catalytic decomposition of urea by a bioinorganic mimetic of the dinickel active site of urease. We chose the phthalazine-dinickel complexes of Lippard and co-workers, because these mimetics have been shown to hydrolytically degrade urea. High-level quantum chemical methodologies were utilized to identify stable intermediates and transition-state structures along several possible reaction pathways. The computed results were then used to further analyze what may occur in the active site of urease. Valuable information on the latter has been extracted from experimental data, computational approaches, and unpublished molecular dynamics simulations. On the basis of these comparative studies, we propose that both the elimination and hydrolytic pathways may compete for urea decomposition in the active site of urease.     

Hydration numbers of α-alanine in an aqueous urea solution

Literature data on the apparent molar volumes ϕ of alanine in water and aqueous urea solutions at 298 K are analyzed. It is shown that the slope of the ϕ dependence on the alanine concentration is not dependent on the urea concentration. The standard partial volume of alanine increases linearly with the increase in the urea concentration (wt.%). The structural characteristics of hydrated complexes of alanine (hydration number, molar volume of water inside and outside the hydration sphere, and proper volume of alanine in solution) are given. The hydration number of alanine decreases by a factor of two in passing from water to a saturated (20m) urea solution. The effects of urea additions on the hydration numbers of alanine and glycine are compared.     

The preparation and characterization graphene-cross-linked phenol–formaldehyde hybrid carbon xerogels

A new synthesis route based on polycondensation of phenol and formaldehyde cross-linked by graphene oxide (GO) was developed. Wet gel after gelation was converted into an organic xerogel by ambient pressure drying to obtain GO-cross-linked phenol–formaldehyde (PF) organic xerogels (GOCPFOX). Graphene-cross-linked PF carbon xerogels (GCPFCX) were produced by carbonization. The morphology and chemical structure of GOCPFOX and GCPFCX were analyzed. The electrochemical behavior of GCPFCX as an electrode material in electric double-layer capacitors (EDLCs) was investigated. Results show that the high mechanical strength of GO increased the gel skeleton strength; thus, organic xerogels exhibit very low drying shrinkage. Scanning electron micrographs indicated that addition of GO altered the gel structure. Thus, when GO was added into the PF solution, the PF molecular chains were anchored on the surface of GO by chemical and physical interaction. The GCPFCX-10 sample achieved the highest specific surface area, mesoporous volume, and specific capacity with 378 m²/g, 0.56 cm³/g, and 116 F/g, respectively. Hence, GCPFCX is a potential material for EDLCs owing to its low production cost and ability to avoid supercritical drying.     

Graphene enhanced α-MnO2 for photothermal catalytic decomposition of carcinogen formaldehyde

Formaldehyde (HCHO) causes increasing concerns due to its ubiquitously found in indoor air and being irritative and carcinogenic to humans. Photothermal-catalysis developed in recent years has been considered as a significant strategy for enhancing catalytic activity. Manganese oxides, compared with its strong thermocatalytic activity, generally suffer from much lower photocatalytic activity make its photochemical properties less concerned. Herein, α-MnO₂ nanowires were composited with the graphene oxide (GO) via mechanical grinding and co-precipitating method, respectively. α-MnO₂/GO nanohybrids prepared by co-precipitating method exhibits excellent activity, achieving 100% decomposition of HCHO with the solar-light irradiation at ambient temperature. It is found that, besides the photo-driven thermocatalysis, the photocatalysis mechanism made a major contribution to the decomposition of HCHO. The incorporation of GO, on the one hand, is beneficial to improve the optical absorption capacity and photothermal conversion efficiency; on the other hand, is conductive to electron transfer and effective separation of electrons and holes. These synergistic effects significantly improve the catalytic activity of α-MnO₂/GO nanohybrids. This work proposes a new approach for the utilization of solar energy by combining manganese oxides, and also develops an efficient photothermal-catalyst to control HCHO pollution in indoor air.     

Reaction of γ-dicarboxylic acids amides and imides with trifluoromethanesulfonamide and formaldehyde

Three-component condensation of trifluoromethanesulfonamide with paraformaldehyde and succinamide depending on the reaction conditions led alongside bis(trifluoromethanesulfonamido)methane to the formation of a substitution product, bis[(trifluoromethylsulfonyl)aminomethyl]succinamide, or to a cyclization product, N-[trifluoromethylsulfonyl)aminomethyl]succinimide. The attempt to obtain the latter by the reaction of the trifluoromethanesulfonamide sodium salt CF₃SO₂NHNa with N-chloromethylsuccinimide unexpectedly resulted in N,N-bis(succinimidomethyl)-trifluoromethanesulfonamide. Analogously the reaction of CF₃SO₂NHNa with N-chloromethyl-phthalimide gave N,N-bis(phthalimidomethyl)trifluoromethanesulfonamide. The reaction of CF₃SO₂NHNa with succinimide and phthalimide in water and alcohol solution resulted in the ring opening and further transformation of the formed monosubstituted N-(trifluoromethylsulfonyl)amides of succinic and phthalic acids.