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.     

Effect of alkylresorcinols on curing behaviour of phenol-formaldehyde resol resin

Thermal behaviour of cure-accelerated phenol-formaldehyde (PF) resins was studied using the addition of commercial mixture of water soluble oil shale alkylresorcinols (AR) to PF resin, 5-MR being as model compound. The acceleration effect of AR is based on the promotion of condensation of resin methylol groups and subsequent reaction of released formaldehyde with AR. Commercial PF resins SFŽ-3013VL and SFŽ-3014 from the Estonian factory VKG Resins have been used. The chemical structure of resins was characterised by ¹³C NMR spectroscopy. TG-DTA analysis was carried out using labsys^TM instrument Setaram. By TG-DTA measurements, the shift of exothermic and endothermic peaks and the changes of mass loss rate in the ranges of 1.5–10 g AR/100 g PF resin were studied. The effect of AR on the curing behaviour of PF resins was also followed by gel time. Testing of the plywood when using PF resin with 5 mass% of AR shows that the press time could be reduced by about 15%.     

Changes in curing behaviour of aminoresins during storage

Summary The curing behaviour of commercial UF and MUF resins, stored at room temperature nearly up to gelation, is studied by simultaneous TG-DTA technique and structural changes of resins are also followed during aging. On the basis of ¹³C NMR spectra, the main chemical reaction during UF resin storage is the formation of methylenes and dimethylene ethers linked to secondary amino groups. Aging of resins results in a decrease of cure rate which is related to lower concentration of active functional groups and decrease in molecular mobility. On DTA curve, the resin with higher content of methylol groups reveals the curing exotherm earlier. With decreasing methylol content during storage, the peak maximum of exotherm is shifted to higher temperature value. Advanced polycondensation and sedimentation processes during storage produce partly locked in macromolecule structure water, and the water evaporation endotherm on DTA curve shifts to considerably higher temperature. The aged MUF resins are chemically less changed than UF resins and the aging process mainly involves noncovalent network formation due to complex molecular structure.     

Site-specific C content by quantitative isotopic C Nuclear Magnetic Resonance spectrometry: A pilot inter-laboratory study

Isotopic ¹³C NMR spectrometry, which is able to measure intra-molecular ¹³C composition, is of emerging demand because of the new information provided by the ¹³C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic ¹³C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular ¹³C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic ¹³C NMR was then assessed on vanillin from three different origins associated with specific δ¹³C_i profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ¹³C_i in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results.     

Solid state 13C NMR study of hexa(methoxy methyl)melamine and melamine-formaldehyde selfcondensates

The structure and molecular dynamics of self-condensation products of three different melamine resins based on hexa(methoxy methyl)melamine are studied by ¹³C NMR in the solid state. The application of direct (DP) and cross-polarization (CP) pulse sequences shows that uncured and cured melamine resins are motionally heterogeneous systems, with the mobile and rigid parts consisting of the same basic structural units. Viscous lightly cured samples based on commercial melamine-formaldehyde resins contain low-molecular weight species which act as plasticizers. In these resins, three motionally different methyl groups are observed. Their existence is confirmed either by spectral deconvolution or by fitting the experimental signal intensities to the cross-polarization dynamics with the assumption of two cross-polarization-transfer rates. Fast cross-polarizing rigid methyl groups are accompanied with the spinning sidebands. On the basis of DP and CP relaxation measurements, quantitative results for three major structural units are calculated. Problems concerning the ¹⁴N-¹³C quadrupolar interactions and quantitative analysis are discussed. ©1995 John Wiley & Sons, Inc.     

Melamine–formaldehyde resins: Constitutional characterization by fourier transform 13C-NMR spectroscopy

The random chemical structures of melamine–formaldehyde resins, including methylated melamine–formaldehyde resins and urea–melamine formaldehyde resins, were investigated by ¹³C-NMR spectroscopy (Fourier transform). All the combined formaldehydes, methylol and methyl ether groups, methylene structures, and dimethylene ether structures were assigned. A ¹³C chemical shift of methylene carbon occurred by substitution of other constituents of the methylene group for a proton of the adjacent monosubstituted nitrogen atom, as shown in a ¹³C-NMR spectrum of urea–formaldehyde resins. It was found that the chemical shift of each corresponding carbon of both melamine resins and urea–melamine resins was almost superimposed with that of urea resins.     

Oxidation of C-labeled ethyl linoleate monitored and quantitatively analyzed by C NMR

The oxidation of ¹³C-labeled ethyl linoleate (¹³C-EL), a model compound for alkyd resins, was investigated by ¹³C NMR in the presence of Co(II)-2-ethylhexanoate (Co-EH), Mn(acac)₃ (acac = acetylacetonate), and Mn(acac)₃ in combination with 2,2′-bipyridine (bpy), respectively. The use of ¹³C-EL allows us, in an unprecedented way, to reveal the individual evolution of hydroperoxides (ROOH) and peroxy (ROOR) links by ¹³C NMR and to quantify the oxidation intermediates during the oxidation. Mn(acac)₃ appeared to be less effective in decomposing ROOH than Co-EH and the Mn(acac)₃/bpy combination. Quantitative analyses were attempted for a few major ¹³C peaks.     

13C and19F NMR study of α,β-difluorostyryl derivatives of transition metal carbonylates. A method of signal assignment based on the carbon—fluorine spin-spin coupling constants

The¹³C and¹⁹F NMR spectra ofZ- andE-isomers of β-X-substituted α,β-difluorostyrenes (X=F, Cl, CpFe(CO)₂, Re(CO)₅, Re₂(CO)₉Na) were studied. Direct and long-range (across 1–5 bonds) spin-spin coupling constants and the (¹³C−¹²C) isotope shifts in the¹⁹F NMR spectra were determined. The study of the¹³C satellites in the¹⁹F NMR spectra of substituted difluorostyrenes permitted assignment of the¹³C NMR signals of the vinylic carbon atoms. Similarly, the signals in¹⁹F NMR spectra were assigned based on coupling constants of fluorine withipso-carbon. These assignments were found to be in good agreement with the data available from the literature (X=F, Cl). The developed approach was applied to the elucidation of the structure ofZ−PhCF=CClFe(CO)₂Cp. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya. No. 8, pp. 1575–1579, August, 1998.     

Mercury(II) cyanide complexes with alkyldiamines: solid-state/solution NMR,computational, and antimicrobial studies

Mercury cyanide complexes of alkyldiamines (1–6), [Hg(L)(CN)₂] (where L?=?en (1,2-diaminoethane), pn (1,3-diaminopropane), N-Me-en, N, N′-Me₂-en, N, N′-Et₂-en, and N, N′-ipr₂-en), have been synthesized and characterized by elemental analysis, IR, ¹³C, and ¹⁵N solution NMR in DMSO-d₆, as well as ¹³C, ¹⁵N, and ¹⁹⁹Hg solid-state NMR spectroscopy. Complexes 1 and 2 have been studied computationally, built and optimized by GAUSSIAN03 using DFT at B3LYP level with LanL2DZ basis set. Binding modes of en and bn (where bn?=?1,4-diaminobutane) toward Hg(CN)₂ are completely different. Complexes with en and pn show chelating binding to Hg(II), while bn behaves as a bridging ligand to form a polymeric structure, [Hg(CN)₂-bn]_∞ [B.A. Al-Maythalony, M. Fettouhi, M.I.M. Wazeer, A.A. Isab. Inorg. Chem. Commun., 12, 540 (2009).]. The solution ¹³C NMR of the complexes demonstrates a slight shift of the ?C≡N (0.9 to 2?ppm) and ?C–NH₂ (0.25 to 6?ppm) carbon resonances, while the other resonances are relatively unaffected. ¹⁵N labeling studies have shown involvement of alkyldiamine ligands in coordination to the metal. The principal components of the ¹³C, ¹⁵N, and ¹⁹⁹Hg shielding tensors have been determined from solid-state NMR data. Antimicrobial activity studies show that the complexes exhibit higher antibacterial activities toward various microorganisms than Hg(CN)₂.     

On the structure and cure acceleration of phenol-urea-formaldehyde resins with different catalysts

Phenol-urea-formaldehyde (PUF) resins with different catalysts [calcium oxide (CaO), sodium carbonate (Na₂CO₃), zinc oxide (ZnO), and magnesium oxide (MgO)] were prepared to accelerate the cure of the resin at low temperature. The cure-acceleration effects of catalysts on chemical structure and cure characteristics of PUF resins were investigated by using both liquid ¹³C nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The liquid ¹³C NMR analysis indicated that the catalyst such as CaO seemed to present a retarded effect on the polycondensation reaction of phenolic components with urea units, while the Na₂CO₃ appeared to promote the self-condensation reaction of phenolic methylol groups at para position toward the formation of para-para methylene linkage. Both ZnO and MgO in PUF resins promoted self-condensation reaction of para methylol groups and condensation reaction of ortho methylol groups with para methylol groups. The catalysts such as Na₂CO₃, ZnO, and MgO can make PUF resins cure at a low temperature. Among these catalysts, the MgO had the most significant accelerating effect on polycondensation and cure reaction of PUF resin.     

The DEPTQ+ Experiment: Leveling the DEPT Signal Intensities and Clean Spectral Editing for Determining CHn Multiplicities

We propose a new ¹³C DEPTQ⁺ NMR experiment, based on the improved DEPTQ experiment, which is designed to unequivocally identify all carbon multiplicities (Cq, CH, CH₂, and CH₃) in two experiments. Compared to this improved DEPTQ experiment, the DEPTQ⁺ is shorter and the different evolution delays are designed as spin echoes, which can be tuned to different ¹J_CH values; this is especially valuable when a large range of ¹J_CH coupling constants is to be expected. These modifications allow (i) a mutual leveling of the DEPT signal intensities, (ii) a reduction in J cross-talk in the Cq/CH spectrum, and (iii) more consistent and cleaner CH₂/CH₃ edited spectra. The new DEPTQ⁺ is expected to be attractive for fast ¹³C analysis of small-to medium sized molecules, especially in high-throughput laboratories. With concentrated samples and/or by exploiting the high sensitivity of cryogenically cooled ¹³C NMR probeheads, the efficacy of such investigations may be improved, as it is possible to unequivocally identify all carbon multiplicities, with only one scan, for each of the two independent DEPTQ⁺ experiments and without loss of quality.     

Quantitative analysis of deuterium using the isotopic effect on quaternary C NMR chemical shifts

Quantitative analysis of specifically deuterated compounds can be achieved by a number of conventional methods, such as mass spectroscopy, or by quantifying the residual ¹H NMR signals compared to signals from internal standards. However, site specific quantification using these methods becomes challenging when dealing with non-specifically or randomly deuterated compounds that are produced by metal catalyzed hydrothermal reactions in D₂O, one of the most convenient deuteration methods. In this study, deuterium-induced NMR isotope shifts of quaternary ¹³C resonances neighboring deuterated sites have been utilized to quantify the degree of isotope labeling of molecular sites in non-specifically deuterated molecules. By probing ¹³C NMR signals while decoupling both proton and deuterium nuclei, it is possible to resolve ¹³C resonances of the different isotopologues based on the isotopic shifts and the degree of deuteration of the carbon atoms. We demonstrate that in different isotopologues, the same quaternary carbon, neighboring partially deuterated carbon atoms, are affected to an equal extent by relaxation. Decoupling both nuclei (¹H, ²H) resolves closely separated quaternary ¹³C signals of the different isotopologues, and allows their accurate integration and quantification under short relaxation delays (D1 = 1 s) and hence fast accumulative spectral acquisition. We have performed a number of approaches to quantify the deuterium content at different specific sites to demonstrate a convenient and generic analysis method for use in randomly deuterated molecules, or in cases of specifically deuterated molecules where back-exchange processes may take place during work up.     

Quick analysis of composition of semi-aromatic copolyamide via 13C NMR study

Standard high resolution ¹³C NMR spectra of PA10T, PA6T, PA106, and PA66 were obtained by a nonacidic solvent mixture of HFIP and CDCl₃. Several chemical shifts were found extremely sensitive to the polyamide type. According to the standard spectra, semi-aromatic copolyamides comprising PA10T, PA6T, PA106, and PA66 units could be distinguished. The ratio of each polyamide component in the copolyamide was determined through the integration of the methylene carbon peak associated with the amine group. ¹³C NMR analysis results were consistent with the theoretical values and copolyamide hydrolysis test results, making ¹³C NMR analysis quite reliable on the quick composition analysis of semi-aromatic copolyamides. Based on this technique, several commercial semi-aromatic copolyamides were further examined and their compositions were easily determined.     

Modification Study of Aluminum sec-Butoxide by Acrylic Acid

A new way of modifying aluminum sec-butoxide (Al(OBu^s)₃) is proposed. This synthesis is carried out by reacting Al(OBu^s)₃ dissolved in tetrahydrofuran with an unsaturated acid, viz. acrylic acid.The structure evolution of Al(OBu^s)₃ with increasing acrylic acid amounts is investigated by infrared, ¹H NMR, ¹³ C NMR, and ²⁷Al NMR spectroscopies and viscosity measurements. Information obtained suggests that the exchange reaction occurring between butoxy groups and acrylate ligands is stopped for an acid/alkoxide molar ratio within the range 1.6–1.7. This value leads us to assume that the dominant trimeric species of precursor is preserved after modification. Moreover, ²⁷Al NMR analysis only reveals the presence of hexacoordinated Al sites in the structure of the modified Al(OBu^s)₃.Evidence of the acrylic acid reaction with sec-butanol released during the alkoxide modification is also proved by the infrared and ¹³ C NMR data. However the produced ester amount can be considered as negligible.     

Biosynthesis of aspergiolide A, a novel antitumor compound by a marine-derived fungus Aspergillus glaucus via the polyketide pathway

Aspergiolide A, a novel antitumor compound, was produced by a marine-derived filamentous fungus Aspergillus glaucus. The biosynthesis of it was unambiguously determined by feeding experiments using [l-¹³C]sodium acetate, [2-¹³C]sodium acetate, and [1,2-¹³C₂]sodium acetate precursors followed by ¹³C NMR spectroscopic investigation of the isolated products. Analysis of the patterns of ¹³C-enrichment revealed that all 25 carbon atoms in skeleton of aspergiolide A were derived from labeled acetate. And among them, 12 carbon atoms were labeled from the carboxylic group of acetate, while the other 13 carbon atoms were labeled from the methylic group of acetate. Besides, the labeling pattern of [1,2-¹³C₂]sodium acetate feeding experiment demonstrated that 12 intact acetate units were incorporated in aspergiolide A by polyketide pathway.     

Analysis of silicone resins by methods of 29Si NMR spectroscopy

Summary Technical silicone resins were characterized by ²⁹Si NMR in liquid and solid state and their relaxation behaviour were investigated. Relationships between resin structure and relaxation are studied.     

Crystalline O,O′-di-sec-butyl and O,O′-diethyl dithiophosphate platinum(II) complexes: Synthesis, C and P CP/MAS NMR, single crystal X-ray diffraction studies and thermal behaviour

Crystalline bis(O,O′-di-sec-butyldithiophosphato)platinum(II) was prepared and studied by means of ¹³C, ³¹P CP/MAS NMR spectroscopy and single-crystal X-ray diffraction. The unit cell of the platinum(II) compound is comprised of one centrosymmetric mononuclear molecule [Pt{S₂P(O-sec-C₄H₉)₂}₂], in which the dithiophosphate groups display structural equivalence in both ³¹P NMR and XRD data. A pair of the dithiophosphate ligands exhibit the same S,S′-bidentate chelating structural function and form two planar four-membered chelate rings, [PtS₂P], in this molecule. The planar configuration of the [PtS₄] chromophore in structure 1 is governed by the dsp²-hybrid state of platinum(II). The structural states of the dithiophosphate groups in two different samples of complex 1 (one crystallised from ethanol and the other one precipitated from an aqueous solution) are all characterised by almost rhombic ³¹P chemical shift tensors. The observed essential dispersion of the ³¹P NMR chemical shift is caused by a coexistence of six optical isomers of molecule 1. The thermal behaviour of this compound was studied by means of simultaneous thermal analysis (a combination of TG and DSC) under an argon atmosphere. The thermal behaviour shows that the mass of 1 is lost in three steps, involving successive thermal decompositions of the organic and inorganic parts of this compound with platinum(II) dithio-meta-phosphate and reduced metallic platinum as the intermediate and the final products, respectively.     

Thermogravimetric study of 8-hydroxyquinoline 5-sulfonic acid–melamine–formaldehyde terpolymer resins-II

The title terpolymer (8-HQ5-SAMF-II) is synthesized by the condensation of 8-hydroxyquinoline 5-sulfonic acid (8-HQ5-SA) and melamine (M) with formaldehyde (F) in the presence of acid catalyst and using 2:1:3 M proportions of the reacting monomers. The synthesized terpolymer resin is then characterized by different physicochemical techniques viz. number average molecular mass determination, intrinsic viscosity determination, and spectral studies like UV–Visible, IR, ¹H NMR, and ¹³C NMR spectra. The morphology of synthesized terpolymer was studied by scanning electron microscopy (SEM). The thermogravimetry of the terpolymer resin prepared in this study has been carried out by non-isothermal thermogravimetry technique in which sample is subjected to condition of continuous increase in temperature at linear rate. Thermal study of the resin was carried out to determine their mode of decomposition and relative thermal stabilities. Thermal decomposition curves were studied carefully with minute details. The Freeman-Carroll and Sharp-Wentworth methods have been used in the present investigation to calculate thermal activation energy and different kinetic parameter of the terpolymer resins. Thermal activation energy E _a calculated with the two above-mentioned methods are in close agreement. The advantage of Freeman-Carroll method is to calculate both the order of reaction n and energy of activation in one single stage by keeping heating rate constant. By using data of thermogravimetry, various thermodynamic parameters like frequency factor Z, entropy change ΔS, free energy change ΔF, and apparent entropy S* have been determined using Freeman-Carroll method.     

Long-range13C—19F NMR spin-spin coupling in some α-trifluoromethyl-substituted polychloropyridines

High resolution¹³C NMR spectra of a series of -trifluoromethyl-substituted polychloropyridines were studied. Long-range¹³C—¹⁹F NMR spin-spin coupling through four and five bonds involving carbon atoms of the pyridine ring and the fluorine atom of the CF₃ group was found.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1973–1974, October, 1995.