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.     

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.     

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.     

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.     

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.     

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     

脲醛树脂合成反应过程的FTIR研究

利用傅里叶变换红外(FTIR)光谱研究了脲醛树脂合成中不同结构形成和变化情况. 对FTIR谱图解析和分析的结果表明, 在脲醛树脂预聚物合成过程中, 随着甲醛与尿素反应的进行, 红外谱图特征峰发生有规律的变化, 酰胺II带特征峰波数逐渐低移, 有更多羟甲基、醚键和—NHCH2—结构基元形成, 固化反应使上述结构基元相应减少. 着重分析了不同甲醛与尿素摩尔比(nF/nU)下尿素与甲醛反应产物结构的变化, 随nF/nU的增加, 酰胺II带特征峰波数逐渐低移, 碱性条件下醚键和—NHCH2—结构基元明显增加, 在较强酸性条件下Uron环、亚甲基桥和羟甲基显著增加, 而—NHCH2—结构基元含量下降.     

Hardness evaluation of cured urea–formaldehyde resins with different formaldehyde/urea mole ratios using nanoindentation method

To understand the influence of formaldehyde/urea (F/U) mole ratio on the properties of urea–formaldehyde (UF) resins, this study investigated hardness of cured UF resins with different F/U mole ratios using a nanoindentation method. The traditional Brinell hardness (H_B) method was also used for comparison. The H_B of cured UF resin films with different F/U mole ratios was determined after exposing the films to different post-curing temperatures. The nanoindentation method was employed for these films to measure Meyer hardness (H_M) and reduced modulus (E_r) which have been used to calculate the elastic modulus (E_s) of cured UF resins. As the F/U mole ratio decreased, the H_B decreased continuously, indicating a less rigid network structure in low F/U mole ratio UF resins. The higher the post-curing temperature, the greater the value of H_B. The H_M value also showed a similar trend as a function of F/U mole ratio. However, the E_r and E_s did not show a consistent trend as exhibited by H_M and H_B. Both H_M and E_r showed much greater variation in the coefficient of variation (COV) at lower F/U mole ratios 1.0 and 1.2, indicating a more heterogeneous composition of these resins. Linear relationships between H_M and E_r indicate that heterogeneity of the surface composition of samples contributes greatly to variations in the measured values. This variability is discussed in terms of crystal structures present in the cured UF resins of low F/U mole ratios.     

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.     

酸性条件下脲醛树脂中不稳定结构的研究

通过较强酸性介质中尿素和甲醛的反应, 在不同条件下合成了透明的脲醛树脂溶液, 利用液体13C NMR研究了甲醛与尿素摩尔比对最终树脂中不稳定结构的影响.     

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.     

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.     

网状多级孔结构氧化铁的制备、合成机理及其光催化性质

以九水合硝酸铁为原料, 利用改进的聚合诱导胶体聚集(PICA)的方法制备出三维网络状多级孔结构氧化铁(HPH). 此结构的制备关键是在合成过程中尿素和甲醛聚合生成脲醛树脂(UF). 脲醛树脂一方面在铁的羟基氧化物生长过程中与之杂化形成杂化产物Fe-UF, 另一方面则进一步聚合, 形成脲醛树脂微球(UFM). 脲醛树脂微球作为模板诱导杂化产物Fe-UF在其表面的聚集. 微球与微球之间则由于表面存在的脲醛树脂间的聚合会相互交联形成网络状结构. 经过煅烧处理后, 脲醛树脂及脲醛树脂微球的分解导致不同尺寸孔结构的生成. 光催化研究结果表明, 产物对罗丹明B的降解效率是商用纳米氧化铁的4倍.     

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...     

Cutoff performance of Escherichia coli by charged and noncharged polyacrylonitrile ultrafiltration membranes

Cutoff performance of Escherichia coli (E. coli) was studied by three types of polyacrylonitrile (PAN) ultrafiltration (UF) membranes without and with charge groups of sulfonate sodium salt (SSS) and trimethylammonium chloride (TMA). These UF membranes were prepared by phase inversion method in water coagulation bath with various concentrations of dimethylsulfoxide (DMSO) and used for the E. coli cutoff experiments under 2.5 kPa applied pressure. With the increase of the polymer concentration in the DMSO cast solution, the pore size of the molecular size exclusion effect of the resultant UF membrane decreased. For UF experiments of E. coli suspension solution with 10⁷ colony forming unit/unit volume (cfu/ml), the permeability of the bacteria through the membrane was in the range of about 10⁻³% in PAN homopolymer membranes. It was found that E. coli permeation through copolymer UF membranes with SSS and TMA groups was completely restricted. Difference of the E. coli cutoff performance in these UF membranes was discussed in comparison with membrane filtration properties such as molecular sieve effect, permeation rate of solute and membrane morphology.     

Evaluation of anion exchange resins Tulsion A-30 and Indion-930A by application of radioanalytical technique

Radioanalytical technique using ¹³¹I and ⁸²Br was employed to evaluate organic based anion exchange resins Tulsion A-30 and Indion-930A. The evaluation was based on performance of these resins during iodide and bromide ion-isotopic exchange reactions. It was observed that for iodide ion-isotopic exchange reaction by using Tulsion A-30 resin, the values of specific reaction rate (min^?1), amount of iodide ion exchanged (mmol), initial rate of iodide ion exchange (mmol/min) and logK _d were 0.238, 0.477, 0.114, and 11.0, respectively, which was higher than 0.155, 0.360, 0.056, and 7.3, respectively as that obtained by using Indion-930A resins under identical experimental conditions of 40.0°C, 1.000 g of ion exchange resins and 0.003 M labeled iodide ion solution. Also at a constant temperature of 40.0°C, as the concentration of labeled iodide ion solution increases 0.001 to 0.004 M, for Tulsion A-30 resins the percentage of iodide ions exchanged increases from 59.0 to 65.1%, and from 46.4 to 48.8% for Indion-930A resins under identical experimental conditions. The identical trend was observed for both the resins during bromide ion-isotopic exchange reactions. The overall results indicate that under identical experimental conditions, Tulsion A-30 show superior performance over Indion-930A resins. The results of present experimental work have demonstrated that the radioanalytical technique used here can be successfully applied for characterization of different ion exchange resins so as to evaluate their performance under various process parameters.     

Studies on cellulose acetate-carboxylated polysulfone blend ultrafiltration membranes--Part I

Hydrophilic polysulfone ultrafiltration (UF) membranes were prepared from blends of cellulose acetate with carboxylated polysulfone of 0.14 degree of carboxylation. The effects of blend polymer composition on compaction, pure water flux, water content and membrane hydraulic resistance (R_m), have been investigated to evaluate the performance of the membranes. The performance of the blend membranes of various blend polymer compositions were compared with that of membranes prepared from pure cellulose acetate and blends of cellulose acetate and pure polysulfone. The hydrophilic cellulose acetate-carboxylated polysulfone blend UF membranes showed better performance compared to membranes prepared from pure cellulose acetate and blends of cellulose acetate and pure polysulfone.     

Etherification of glycerol with tert-butyl alcohol catalysed by ion-exchange resins

The reaction of glycerol with tert-butyl alcohol in the liquid phase on acid Amberlyst-type ion-exchange resins was studied. The influence of temperature, mole ratio n(TBA)/n(G), water and swelling of gel, and macroreticular type of polymer catalysts on etherification reaction was investigated. The most favourable reaction temperature is 75°C. The conversion of glycerol and yield of glycerol tert-butyl ethers has increased with the mole ratio n(TBA)/n(G). Dry form of macroreticular catalysts provided the best results. Etherification reaction of glycerol with isobutylene in non-aqueous conditions gives the highest yield of desired ethers. The influence of water was studied. The gel forms of ion-exchange resins have very low catalytic activity. It can be concluded that water has an inhibition effect on ion-exchange resins. By comparing the gel and macroreticular forms of Amberlyst ion-exchange resins it can be concluded that very acid forms of macroreticular ion-exchange resins with a high degree of crosslinking are more active catalysts for the studied reaction due to their pores which are sufficiently large so that the voluminous tert-butyl ethers of glycerol can be formed. It was estimated that tert-butyl alcohol as tert-butylation agent is not suitable for etherification of glycerol with the formation of di-and triethers.     

Thermal study of phenol–formaldehyde resin modified with cashew nut shell liquid

In this study, an experimental phenol–formaldehyde resin with 20% phenol replacement by cashew nut shell liquid (CNSL) was studied and compared with a conventional phenol–formaldehyde resin synthesized totally from petrochemical raw materials. The resins were characterized with standard lab analysis for their physicochemical specifications, while their thermal properties were studied with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). For comparison reasons pure CNSL and wood were also included in the TGA study. A DSC study conducted both for the neat resins and the system wood–resin as to examine the effect of wood on the curing performance of the resins in the real time conditions of their usage at the wood-based panels industry.The adhesion strength of these resins was investigated by their application in plywood production. The plywood panels were tested for their shear strength and wood failure performance while their free formaldehyde emissions were determined with the desiccator method. It was proved that although the neat CNSL modified PF resin (PCF) cures at longer time and higher temperature than a conventional PF resin, wood affects it more significantly, resulting in the evening of their curing performance. This is a novel finding that manifests the possibility of replacing a convention PF resin by a CNSL modified one in the plywood production, without changing any of their production conditions and with improvement to their overall properties.     

Characterization in the Toughening Process of CTBN Modified Epoxy Resins Induced by Electron Beam Radiation

Epoxy resins are widely utilized as high performance thermosetting resins for many industrial applications but characterized by a relatively low toughness. Electron beam (EB) curing of polymer resins has a number of advantages over conventional thermal curing, such as shorter curing time, low energy consumption, low cure temperature, dimensional stability, reduced manufacturing cost. In the present work liquid carboxyl-terminated butadiene acrylonitrile (CTBN) copolymers containing 8% acrylonitrile is added at different contents to improve the toughness of diglycidyl ether of bisphenol A (DGEBA) epoxy resins using triarylsulfonium hexafluoroanimonate as a photointiator. The EB irradiation was conducted 5 kGy to 250 kGy in nitrogen. The physics properties of CTBN modified epoxy resins were examined by determine gel content, DMA (dynamic mechanical analysis), UTM (Instron model 4443), SEM (scanning electron microscopy).