Curing of urea-formaldehyde resins on awood substrate

TG-DTA analysis method was used to study the curing behaviour of urea-formaldehyde (UF) adhesive resins in the presence of a wood substrate. The cure process was followed using a Setaram labsys^TM instrument in flowing nitrogen atmosphere by varying the ratio of resin and wood. Resin cure was catalysed with 2% of NH₄Cl. Curing tests were performed in the open standard platinum crucibles and in the sealed glass capsules. To characterise the reactivity of curing system, the peak temperatures in DTA curve and the mass loss values in TG curve were taken as the apparent indices. The main attention was paid to phenomena which actually take place in curing of UF resins during manufacturing of particleboards. Reactivity of the curing system depends mostly on methylol content of resin and can be adequetly evaluated by the maximum temperature of exothermic peak. The wood substrate has a substantial influence on the resin and water diffusion in system causing the changes in water/resin separation and water evaporation conditions. The water movement in curing adhesive joint was a confusing parameter in determining the peak positions. The rate of mass loss on a wood substrate is higher as compared to curing UF resin alone.     

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.     

TG-DTA study of melamine-urea-formaldehyde resins

The thermal behaviour of MUF resins from different suppliers with different content of melamine was studied, along with the ¹³C NMR spectroscopic analysis of resin structure and the testing of particleboards in current production at Estonian PB factory Pärnu Plaaditehas AS. The chemical structure of resins from DMSO-d₆ solutions was analysed by ¹³C NMR spectroscopy on a Bruker AMX500 NMR spectrometer. The melamine level in different MUF resins is compared by the ratios of carbonyl carbon of urea and triazine carbon of melamine in ¹³C NMR spectra. Curing behaviour of MUF resins was studied by stimultaneous TG-DTA techniques on the Labsys? instrument Setaram. The shape of DTA curves characterisises the resin synthesis procedure by the extent of polymerisation of UF and MF components and is in accordance with structural data.     

Preparation and properties of epoxy resins cured with silyl esters of phenol novolak and cresol novolak

The curing agents of epoxy resin, trimethylsilyl ethers of phenol novolak (TMSPN) and cresol novolak (TMSCN) were prepared by refluxing phenol novolak and cresol novolak respectively, with the mixture of hexamethyldisilazane and chlorotrimethylsilane in THF. The curing reaction of epoxy resin with these curing agents and the thermal properties of cured resins were examined. The Tg values of epoxy resins cured with TMSPN were a little higher than those cured with TMSCN. The maximum of Tg is 118°C for TMSPN-cured epoxy resin against 112°C for TMSPN-cured epoxy resin. The water absorption of hydrophobic epoxy resins cured with TMSPN was a little lower than those cured with TMSCN. The clear decrease of water absorption is attributed to the difficulty of the micro-void formation caused by the more tight primary structures of TMSPN. The water absorption at 25°C containing trimethylsilyl groups is about one-tenth of that of epoxy resins cured with conventional curing agents and even one-half of that of the epoxy resins cured with active esters. The low water absorption is attributed to the presence of trimethylsilyl groups, which are more hydrophobic than ester groups, and to the absence of hydroxyl groups of the cured resins. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal behaviour of hydroxymethyl compounds as models for adhesive resins

Urea–formaldehyde (UF) and phenol–formaldehyde (PF) resins are the most widely used wood adhesives. The first stage in resin manufacturing is the formation of methylol derivatives which polycondensation leads to building the tridimensional network. Understanding the behaviour of methylol compounds in curing provides useful information for developing appropriate resin structures. Thermal behaviour of N,N′-dihydroxymethylurea, 2- and 4-hydroxymethylphenols, urea and phenol as model compounds for UF, PF and phenol–urea–formaldehyde (PUF) resins was followed by TG-DTA method. The measurements were carried out by the labsys instrument Setaram at 30–450 °C in nitrogen flow. The characteristic signals for model compounds and for some reaction mixtures were measured by high resolution ¹³C NMR spectroscopy.     

Cure study of addition-cure-type and condensation-addition-type phenolic resins

By the incorporation of propargyl and methylol groups on to novolac backbone, a series of addition-curable phenolic resins and condensation-addition dual-cure type phenolic resins (novolac modified by propargyl groups referred as PN, and novolac modified by propargyl and methylol groups simultaneously referred as MPN) were synthesized. The processing characteristics, thermal cure and catalytic cure behavior for both resins were investigated mainly by means of viscosity measurement and non-isothermal differential scanning calorimetry (DSC) techniques. The effect of propargyl and methylol content of PN and MPN, the molecular weight and the configuration of the parent novolac, on the processing and cure behavior was studied in details. Processing parameters and curing kinetic parameters were obtained. Both resins exhibit excellent processing properties. Thermal cure of PN resins possessed one cure mechanism and that of MPN resins possessed two cure mechanisms according to DSC analysis. The dual-cure-type mechanism made MPN resins superior to PN resins in terms of a mild and controllable cure process. Compared with thermal cure, catalytic cure of PN resins showed lower initiation temperature and cure temperature by about 60 °C. These novel resins have a bright prospect of application as matrix for thermal-structural composite materials.     

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%.     

Phenolic resins bearing maleimide groups: Synthesis and characterization

Novel phenolic novolac resins, bearing maleimide groups and capable of undergoing curing principally through the addition polymerization of these groups, were synthesized by the polymerization of a mixture of phenol and N‐(4‐hydroxy phenyl)maleimide (HPM) with formaldehyde in the presence of an acid catalyst. The polymerization conditions were optimized to get gel‐free resins. The resins were characterized by chemical, spectral, and thermal analyses. Differential scanning calorimetry and dynamic mechanical analysis revealed an unexpected two‐stage curing for these systems. Although the cure at around 275°C was attributable to the addition polymerization reaction of the maleimide groups, the exotherm at around 150 to 170°C was ascribed to the condensation reaction of the methylol groups formed in minor quantities on the phenyl ring of HPM. Polymerization studies of non‐hydroxy‐functional N‐phenyl maleimides revealed that the phenyl groups of these molecules were activated toward an electrophilic substitution reaction by the protonated methylol intermediates formed by the acid‐catalyzed reaction of phenol and formaldehyde. On a comparative scale, HPM was less reactive than phenol toward formaldehyde. The presence of the phenolic group on N‐phenyl maleimide was not needed for its copolymerization with phenol and formaldehyde. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 641–652, 2000     

Examination of selected synthesis parameters for typical wood adhesive‐type urea–formaldehyde resins by 13C NMR spectroscopy. I

Selected synthesis parameters of typical wood adhesive‐type urea–formaldehyde (UF) resins were examined using the ¹³C NMR spectroscopy. The monomeric hydroxymethylureas and methylene–ether derivatives formed in the initial alkaline reaction polymerize in the subsequent acidic reaction by forming methylene bonds and cleaving some hydroxymethyl groups as formaldehyde. For typical resin syntheses at F/U ratio of 2.10, the resulting UF polymer is found to be a number‐averaged pentamer having 3.25 polymer chain branches with about 80% of chain ends bonded to hydroxymethyl groups and the rest being free amide groups. When the second urea is added during the cooling period, about 67% of hydroxymethyl groups cleave from the UF polymeric components and the freed formaldehyde reacts with second urea to form monomeric hydroxymethylureas. This hydroxymethyl group move is suppressed when the second urea is added at low temperatures, suggesting that wood adhesive‐type UF resins are composed of monomeric and polymeric UF components having hydroxymethyl functional groups in varying proportions. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 995–1007, 1999     

Structural characteristics of p‐t‐Octylphenol formaldehyde resole resins using molecular simulation

p‐t‐Octylphenol formaldehyde resole resins have two linkage types of methylene‐ and dimethylene ether‐linkages and have three terminal types of hydrogen, methylol, and o‐methylene quinone. Variation of structural characteristics of the resins due to different types of linkages and terminals were studied using molecular dynamics and molecular mechanics. The structural characteristics of the methylene‐bridged resins were intramolecular hydrogen bonds between hydroxyl groups of the adjacent p‐t‐octylphenols. In the dimethylene ether‐bridged resin, the intramolecular hydrogen bonds between oxygen atoms of the dimethylene ether‐linkages and hydroxyl groups of the neighboring phenolic units were found. For the resins with both methylol terminals, one of both terminals of the resins was hidden at the center of the molecule when the resin size is large. The number of hydrogen bonds in the resins with the methylol terminal was larger than for the resins with the o‐methylene quinone terminal. Variation of the structural characteristics of the resins by dehydration of the terminal methylol was discussed. Using the calculated results, dissociation of the dimethylene ether linkage and crosslinking reaction of rubber chains by the resin were explained. Copyright © 2002 John Wiley & Sons, Ltd.     

Effect of three boron flame retardants on thermal curing behavior of urea formaldehyde resin

The purpose of the study was to investigate the effects of three kinds of flame retardant (FR), boric acid, zinc borate, and borax on the thermal curing behavior of urea–formaldehyde (UF) resin. Both pH value and gel time were measured to study the curing characters of the UF resin with different loading levels of FR. In addition, differential thermal analysis was also used to obtain kinetic analyses parameter. The results showed that boric acid decreased pH value of UF resin, and reduced gel time of the UF resin. There are no significant changes of the UF resin curing characters with different loading levels of FR. The activation energies for curing reaction of UF resins in the presence of boric acid, zinc borate, and borax, were 84.37, 84.41, and 118.4 kJ/mol, respectively, higher than that of the control one (75.38 kJ/mol). All FRs showed adverse effect on the curing behavior of the UF resin.     

Acetylene-grafted resins derived from phenolics via azo coupling reaction

Novel phenolic resins with high ethynyl contents were realized via azo coupling reaction between phenol units and diazonium of 3-ethynylaniline. If Novolac and high-ortho Novolac resins were used as the starting materials directly, the ratio of ethynyl groups to phenolic rings was ca 70%; while the ratio was 100% for the resin from Friedel-Craft polycondensation of 4-(3-ethynylphenyl)salicyl alcohol. All the resins were readily soluble in acetone and ethanol, and meltable at temperatures below 100 °C. The resins underwent thermal addition-type cure with a broad exotherm of around 140-280 °C, and the starting curing temperature showed a downward drift with increase in the ethynyl content. The thermal properties of the cured resins, determined from thermogravimetric analysis (TGA), were considerably superior to those of Novolac- and Resole-type phenolic resins. The initial decomposition temperatures were at ca 400 °C, and the anaerobic carbon yields were ca 80% for all the resins.     

Adhesives and coatings based on phenolic/epoxy resins

Low molecular weight epoxy resin based on bis (4‐hydroxy phenyl) 1,1 cyclohexane was prepared and modified with various types of the prepared phenolic resins. Phenol–, cresol–, resorcinol–and salicylic acid–formaldehyde resins were used. The optimum conditions of formulation and curing process were studied to obtain modified wood adhesives characterized by high tensile shear strength values. This study indicated that the more suitable conditions are 1:2 weight ratio of phenol–or cresol–formaldehyde to epoxy resin in the presence of phthalic anhydride (20 wt%) of the resin content as a curing agent at 150°C for 80 min. Resorcinol–or salicylic acid–formaldehyde/epoxy resins formulated at 1:2 weight ratio were cured in the presence of paraformaldehyde (20 wt%) at 150°C for 60 min. The effect of the structure of phenolic resins on the tensile shear strength values of formulated resin samples, when mixed with the epoxy resins and cured under the previously mentioned optimum conditions for different times, was investigated. Metallic and glass coatings from the previous resins were also prepared and evaluated as varnishes or paints. Copyright © 1999 John Wiley & Sons, Ltd.     

Thermal behaviour of urea-formaldehyde resins during curing

Urea-formaldehyde (UF) resins are the most widely used polycondensation resins today in manufacturing particleboards. The performance of UF resins in their processing is greatly influenced by curing characteristics. The cure process has been monitored by TG-DTA technique on a Setaram labsysTM instrument in dynamic heating conditions at different heating rates. Commercial UF resins from different suppliers used in Estonian particleboard factories were selected for TG-DTA measurements. Experiments were carried out without and with catalysts. Ammonium chloride and ammonium sulphate were used. Curing characteristics were evaluated both for fresh and aged resins. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photonic Patterns Printed in Chiral Nematic Mesoporous Resins

Chiral nematic mesoporous phenol‐formaldehyde resins, which were prepared using cellulose nanocrystals as a template, can be used as a substrate to produce latent photonic images. These resins undergo swelling, which changes their reflected color. By writing on the films with chemical inks, the density of methylol groups in the resin changes, subsequently affecting their degree of swelling and, consequently, their color. Writing on the films gives latent images that are revealed only upon swelling of the films. Using inkjet printing, it is possible to make higher resolution photonic patterns both as text and images that can be visualized by swelling and erased by drying. This novel approach to printing photonic patterns in resin films may be applied to anti‐counterfeit tags, signage, and decorative applications.     

用红外光谱法对热固性酚醛树脂固化过程的研究

用红外光谱法研究了四种热固性酚醛树脂在固化过程中的化学变化。结果表明热固性酚醛树脂的固化过程,首先是羟甲基发生缩合反应,缩合反应形成醚键是主要的。热固性酚醛树脂在进一步固化时(150—200°)出现了羰基,它的出现不是亚甲基氧化所致,而是部分醚键断裂形成了醛羰基。对热固性酚醛树脂的固化过程的机理进行了讨论。     

Thermal characterization of carbonate rocks,Kozani area,North-western Macedonia,Greece

The curing of a thermoreactive alkyd-melamine-formaldehyde resin system was investigated by rheologycal, TG and TMA-analysis, in order to construct the time-temperature-transformation diagram. The points of the gelation curve were determined by measuring the increase in viscosity during isothermal curing at different temperatures. A power-function could be fitted to the gelation curve, which is suitable to estimate gelation at any curing conditions, as well as to establish storage conditions. The reaction in the resin matrix was followed by monitoring the loss of mass during isothermal curing at different temperatures. The final section of the resulted iso-curing temperature (iso-T _cure) diagrams could be fitted with logarithmic functions, which may be used for estimating the conditions needed to a given, desirable mass loss, i.e. conversion. The steepness of the curves increases with temperature suggesting the forthcoming of degradation during cure with increasing temperature. From these data the iso-mass loss curves of the TTT-diagram were constructed. For determining the iso-Tg curves of the TTT-diagram isothermal curing was carried out in a drying oven at different temperatures, followed by TMA measurements. The iso-Tcure diagrams served to determine T _{g ￥}, and to construct the iso-T _g curves of the TTT diagram. Vitrification curve is far beyond conditions of storage, curing and degradation, meaning that the resin matrix is in rubbery physical state before, during and after the cure. Curing conditions resulting degradation can also be estimated from the TTT-diagram. This revised version was published online in August 2006 with corrections to the Cover Date.     

Curing kinetics and thermal stability of diglycidyl ether of bisphenol

Curing kinetics of diglycidyl ether of bisphenol-A (DGEBA) in the presence of varying molar ratios of aromatic imide-amines and 4,4′-diaminodiphenylsulfone (DDS) were investigated by the dynamic differential scanning calorimetry. The imide-amines were prepared by reacting 1 mole of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (B) with 2.5 moles of 4,4′-diaminodiphenyl ether (E)/ or 4,4′-diaminodiphenyl methane (M)/ or 4,4′-diaminodiphenylsulfone (S) and designated as BE/ or BM/ or BS. The mixture of imide-amines and DDS at ratio of 0:1, 0.25:0.75, 0.5:0.5, 0.75:0.25 and 1:0 were used to investigate the curing behaviour of DGEBA. The multiple heating rate method (5, 10, 15 and 20°C min⁻¹) was used to study the curing kinetics of epoxy resins. The peak exotherm temperature was found to be dependent on the heating rate, structure of imide-amines as well as on the ratio of imide-amine: DDS used. A broad exotherm was observed in the temperature range of 180–230°C on curing with mixture of imide-amines and DDS. Curing of DGEBA with mixture of imide-amines and/or DDS resulted in a decrease in characteristic curing temperatures. Activation energy of curing reaction as determined in accordance to the Ozawa’s method was found to be dependent on the structure of amine. The thermal stability of the isothermally cured resins was also evaluated using dynamic thermogravimetry in a nitrogen atmosphere. The char yield was highest in case of resins cured using mixture of DDS: BS (0.25:0.75; EBS-3), DDS: BM (0.5: 0.5; EBM-2) and DDS: BE (0.5: 0.5; EBE-2).     

Urea-formaldehyde resins characterized by thermal analysis and FTIR method

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 mositure 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.