Blocked melamine–urea–formaldehyde resins and their usage in agglomerated cork panels

Caprolactam and o‐p‐toluenesulfonamide are tested as chain‐growth blockers for melamine–urea–formaldehyde (MUF) resins, in an attempt to reduce the crosslinking density of the cured resin and hence improve its flexibility. Agglomerated cork panels, for which flexibility is a technical demand, were produced with the modified resins and tested. The blockers were added at three different steps in the synthesis process: methylolation, condensation, and at the end of the synthesis. Besides evaluation of standard properties, resins were characterized using gel permeation chromatography and Fourier transform infrared. Blocked resins showed better storage stability and improved water tolerance, especially when caprolactam was employed. When used as binders in agglomerated cork panels, the blocked resins allowed for significantly better flexibility, evaluated in terms of mandrel bending test. The tensile resistance of the panels remained well within the desired limits for this type of material. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46663.     

Influence of the synthesis conditions on the curing behavior of phenol–urea–formaldehyde resol resins

The curing behavior of synthesized phenol–urea–formaldehyde (PUF) resol resins with various formaldehyde/urea/phenol ratios was studied with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The results indicated that the synthesis parameters, including the urea content, formaldehyde/phenol ratio, and pH value, had a combined effect on the curing behavior. The pH value played an important role in affecting the shape of the DSC curing curves, the activation energy, and the reaction rate constant. Depending on the pH value, one or two peaks could appear in the DSC curve. The activation energy was lower when pH was below 11. The reaction rate constant increased with an increase in the pH value at both low and high temperatures. The urea content and formaldehyde/phenol ratio had no significant influence on the activation energy and rate constant. DMA showed that both the gel point and tan δ peak temperature (T_tanδ) had the lowest values in the mid‐pH range for the PUF resins. A different trend was observed for the phenol–formaldehyde resin without the urea component. Instead, the gel point and T_tanδ decreased monotonically with an increase in the pH value. For the PUF resins, a high urea content or a low formaldehyde/phenol ratio resulted in a high gel point. The effect of the urea content on T_tanδ was bigger than that on the gel point because of the reversible reaction associated with the urea component. Too much formaldehyde could lead to more reversible reactions and a higher T_tanδ value. The effects of the synthesis conditions on the rigidity of the cured network were complex for the PUF resins. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1368–1375, 2005     

Deeper insights into the thermal properties of epoxy thermoset resins using torsion measurements

The glass transition temperature (T_g) of epoxy thermosets is a critical material property that depends on the component chemistry, the final cross-link density, and processing conditions. This study incorporates dynamic mechanical analysis (DMA) testing with a torsion clamp geometry on a TA Instruments DHR-2 and differential scanning calorimetry (DSC) to characterize five different two-component epoxy-amine systems. Investigation of the T_g dependence on DMA frequency and heating shows that lowering the frequency from 1 to 0.01 Hz results in a T_g very similar to that measured using DSC, while a heating rate of 0.3°C/min using DMA gives a T_g comparable to the DSC measured value at 30°C/min. The DMA technique reveals secondary relaxation transitions and peak broadening in the tan(δ) plots of poorly mixed epoxy blends, quantified using full width at half maximum (FWHM) of tan(δ) peaks, and are indicative of a non-homogeneous cross-linked network and off-ratio blending, respectively. The increase in the FWHM due to poor mixing ranges from 8% to 96%. These parameters are easily measurable and quantifiable in DMA, but are not observed in DSC. The additional DMA insights are valuable for process development and failure analysis, and can improve the understanding of epoxies.     

Differential scanning calorimetry characterization of urea–formaldehyde resin curing behavior as affected by less desirable wood material and catalyst content

Differential scanning calorimetry was applied to investigate the curing behavior of urea–formaldehyde (UF) resin as affected by the catalyst content and several less desirable wood materials (e.g., wood barks, tops, and commercial thinnings). The results indicate that the reaction enthalpy of UF resin increased with increasing catalyst content. The activation energy and peak temperature of the curing UF resin generally decreased with increasing catalyst content at lower levels of catalyst content. However, with further increases in catalyst content, the changes in the activation energy and peak temperature were very limited to nonexistent. The hydrolysis reaction of the cured UF resin occurred during the latter stages of the curing process at both lower level (<0.2%) and higher level (>0.7%) catalyst contents. This indicates that there existed an optimal range of catalyst content for the UF resin. The curing enthalpy of the UF resin decreased with increasing wood raw materials present due to the effect of diffusion induced by the wood materials and the changes in the phase of the curing systems. This suggests that the curing reactions reached a lower final degree of conversion for the wood–resin mixtures than for the UF resin alone. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2027–2032, 2005     

Hydrolytic stability of cured urea‐formaldehyde resins modified by additives

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 1.4, except the modified UF resins with pMDI. The hydrolytic stability of modified UF resins by sulfur compounds (sodium bisulfate and sodium hydrosulfite) decreased with an increase in their level. However, both acrylamide and pMDI were much more effective than two sulfur compounds in terms of hydrolytic stability of modified UF resins. These results indicated that modified UF resin of the F/U mole ratio of 1.2 by adding acrylamide was the most effective in improving the hydrolytic stability of UF resin. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Low‐volatility acetals to upgrade the performance of melamine–urea–formaldehyde wood adhesive resins

1,1,2,2‐Tetramethoxyethane (TME), a high boiling point acetal derived from glyoxol, lhas been shown to upgrade the performance of melamine‐urea‐formaldehyde (MUF) and some UF resins used for wood adhesives. This affords the possibility of decreasing the percentage of resin used in the preparation of wood panels without volatilizing the TME acetal used.     

Evaluation of curing and thermal behaviors of konjac glucomannan–chitosan–polypeptide adhesive blends

An environmentally friendly wood adhesive has been achieved through novel blending of konjac glucomannan and chitosan with polypeptide. Tensile tests reveal an optimal curing temperature of 130°C. Both viscoelastic properties of the adhesive blends during curing process, and the structural variation of chemical components induced by curing temperature, were defined by using dynamic rheometry, X‐ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. Results show both storage and loss modulus increase markedly from 90 to 105°C because of the reactions occurred between konjac glucomannan, chitosan, and polypeptide with formation of amide. Increased polypeptide content not only strengthens intermolecular hydrogen bonds but also enhances covalent reactions occurred between the components. Thermogravimetric and differential scanning calorimetry analyses indicate good miscibility of components in the adhesive blends and improved thermal stability with added polypeptide. Hydrogen bonds between polysaccharides and polypeptide break at 125 ± 10°C. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42202.     

Optical characterization and differential scanning calorimetry studies of carboxymethyl cellulose–nickel chloride composite system

Carboxymethyl cellulose (CMC) films doped with nickel chloride hexahydrate have been prepared by casting technique. The phase transitions and thermal stability of the prepared samples were investigated by differential scanning calorimetry and thermogravimetry. The optical absorption was recorded at room temperature in the wavelength range of 190–2500 nm. From the absorption edge studies, the values of the Urbach energy (E_e) were found to be 0.58 eV in case of the pure polymer; however, the Urbach energy values were found to be in the range of 0.64–1.0 eV under additional different percentages of nickel chloride. These energy values indicate that the model based on random fluctuations of the internal fields associated with structure disorder is preferable and transitions are made between band tails. Refractive index, complex dielectric constants have also been determined. Color properties of the prepared samples are discussed in the frame work of CIE L*u*v* color space. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Differential scanning calorimetric examination of melamine–formaldehyde microcapsules containing decane

The main objective of this study was to investigate the composition of microcapsules and the degree of curing of melamine–formaldehyde (MF) resin, which formed a shell of microcapsules, by the use of differential dynamic calorimetry (DSC). For this purpose, decane was chosen as core material. The microencapsulation of decane with MF resin was carried out at different temperatures and pH values. The temperature and pH value were kept constant during the process. The composition of the microcapsules and the degree of curing of the shell material were studied during and after the microencapsulation process. DSC analysis, in combination with scanning electron microscopy analysis, was revealed as an effective tool for the investigation of the microencapsulation process with MF resin. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

In this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (_gelT_g)], the glass‐transition temperature of the uncured material (without crosslinking; T_g0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower T_g0 and _gelT_g values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Model‐free kinetics: Curing behavior of phenol formaldehyde resins by differential scanning calorimetry

Isoconversional analysis was used to treat nonisothermal DSC data and yield the dependence of activation energy on conversion during the curing process of PF resins. The shape of the dependence revealed that the curing process of PF resins displayed a change in the reaction mechanism from a kinetic to a diffusion regime. In the kinetic regime a comparative DSC experimental analysis between monomer mixtures and PF resins showed that the addition reactions between phenol and formaldehyde had been mostly completed during the synthesis of PF resins and that the main kinetic reactions contained parallel condensations in the curing process. For the diffusion regime a modified equation for the diffusion rate constant, k_D = D₀ exp(?E_D /RT + K₁α + K₂α²), is proposed. This equation is in good agreement with the experimental dependence of E_α on α in the diffusion regime, which shows the effect of both temperature and conversion on diffusion. A prediction of the conversion advancement with the reaction time under isothermal condition for PF resin has been made. This prediction can be useful in practical applications for evaluating isothermal behavior of thermosetting systems from nonisothermal experimental data. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 433–440, 2003     

Cure kinetics of epoxy resins by differential scanning calorimetry technique

The curing reactions of epoxy resins with aliphatic amine are investigated using the differential scanning calorimetry technique with a single dynamic scan. The rate of the reaction was followed over the temperature range 30–250°C, and the activation energy and the order of the reaction are determined using four different computational methods. The activation energy for the various epoxy systems is observed in the range 40–76 kJ mol^?1 and the order of the reaction is observed to be ? 1·0.     

Fibers from urea–formaldehyde resins

The preparation of fibers from aqueous urea–formaldehyde resins has been investigated; a dry spinning process has been developed based on the extrusion of catalyzed resin into a drying chamber at 180–220°C, producing a multifilament yarn at spinning speeds of up to 600 m/min. A range of UF filaments was produced with diameters between 10–70 μm; the tenacities of spun filaments were 6–10 cN/tex, initial moduli were 220–340 cN/tex, and elongation at break was 4–10%. The best tensile properties resulted from conditions that produced the smallest diameter fibers. Postspin heat treatment improved the tenacity to 14 cN/tex and the elongation to 20%. Spinnability improved with increased viscosity of the spinning solution and increased cell temperature, while tenacity and elongation increased with increasing cell temperature and spinning stretch. A correlation was found between TGA weight loss (between 105 and 200°C) and fiber tenacity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 64–74, 2000     

Differential scanning calorimetry and dynamic mechanical analysis of amine-modified urea–formaldehyde adhesives

Urea–formaldehyde (UF) resins are susceptible to stress rupture and hydrolytic degradation, particularly under cyclic moisture or warm, humid conditions. Modification of UF resins with flexible di- and trifunctional amines reduces this problem. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the thermal behavior of modified and unmodified adhesives to identify the physical and morphological factors responsible for the improved performance. A UF resin modified by incorporating urea–capped poly(propyleneoxidetriamine) during resin synthesis exhibited a higher cure rate and greater cure exotherm than the unmodified resin. Resins cured with a hexamethylenediamine hydrochloride curing agent had slower cure rates than those cured with NH₄Cl. DMA behavior indicated that modified adhesives were more fully cured and had a more homogeneous crosslink density than unmodified adhesives. DMA behavior changed with storage of specimens at 23°C and 50% relative humidity, after previous heating for approximately 20 min at 105°C to 110°C. The initial changes were postulated to occur because of physical aging (increase in density) and continued cure. These were followed by physical breakdown (microcracking) and possibly cure reversion. © 1995 John Wiley & Sons, Inc.     

Cure kinetics of the cardanyl acrylate–styrene system using isothermal differential scanning calorimetry

The curing kinetics of styrene (30 wt %) and cardanyl acrylate (70 wt %), which was synthesized from cardanol and acryloyl chloride, was investigated by differential scanning calorimetry under isothermal condition. The method allows determination of the most suitable kinetic model and corresponding parameters. All kinetic parameters including the reaction order, activation energy E_a and kinetic rate constant were evaluated. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2034–2039, 2002     

Uron and uron–urea‐formaldehyde resins

The favored pH ranges for the formation of urons in urea‐formaldehyde (UF) resins preparation were determined, these being at pH's higher than 6 and lower than 4 at which the equilibrium urons ↔ N,N′‐dimethylol ureas are shifted in favor of the cyclic uron species. Shifting the pH slowly during the preparation from one favorable range to the other causes shift in the equilibrium and formation of a majority of methylol ureas species, whereas a rapid change in pH does not cause this to any great extent. UF resins in which uron constituted as much as 60% of the resin were prepared and the procedure to maximize the proportion of uron present at the end of the reaction is described. Uron was found to be present in these resins also as linked by methylene bridges to urea and other urons and also as methylol urons, the reactivity of the methylol group of this latter having been shown to be much lower than that of the same group in methylol ureas. Thermomechanical analysis (TMA) tests and tests on wood particleboard prepared with uron resins to which relatively small proportions of urea were added at the end of the reaction were capable of gelling and yielding bonds of considerable strength. Equally, mixing a uron‐rich resin with a low F/U molar ratio UF resin yielded resins of greater strength than a simple UF of corresponding molar ratio indicating that UF resins of lower formaldehyde emission with still acceptable strength could be prepared with these resins. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 277–289, 1999     

Morphology and chemical elements detection of cured urea–formaldehyde resins

As a part of understanding the hydrolysis of cured urea–formaldehyde (UF) resins that has been known as responsible for the formaldehyde emission, leading to sick building syndrome, this study attempted to investigate the morphology and to detect chemical elements of the cured UF resins of different formaldehyde/urea (F/U) mole ratios and hardener (NH₄Cl) levels, using field emission‐scanning electron microscopy and energy‐dispersive spectroscopy. Cured UF resins of low F/U mole ratio showed spherical structure whose diameter increased with an increase in the hardener level, whereas this was not observed for high F/U mole ratio UF resins regardless of the hardener levels. The energy‐dispersive spectroscopy results showed five different chemical elements such as carbon, nitrogen, oxygen, chloride, and sodium in cured UF resins. The chloride distribution assumed as the presence of residual acid in the cured UF resins suggested that the hydrolysis of cured UF resins could initiate at the sites of chlorides on the surface of the spherical structures. As the hardener level increased, the quantities of both carbon and oxygen decreased, whereas those of nitrogen and chloride increased as expected. But the quantity of sodium was within measurement error. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effects of acid hydrolysis on microstructure of cured urea‐formaldehyde resins using atomic force microscopy

This study investigated the effect of acid hydrolysis on the microstructure of cured urea‐formaldehyde (UF) resins using atomic force microscopy (AFM) to better understand its hydrolytic degradation process which has been known to be responsible for the formaldehyde emission of wood‐based composite panels. The AFM was scanned on both outer surface and facture surfaces of the thin films of cured UF resins that had been exposed to the etching of dilute hydrochloric acid to simulate their hydrolysis process. The AFM images showed two distinctive parts, which were classified as the hard and soft phases in cured UF resins. For the first time, this study reports the presence of thin filament‐like crystalline structures on the fracture surface of cured UF resin. The soft phase of cured UF resins by ammonium chloride was much more easily hydrolyzed than those cured by ammonium sulfate, indicating that hardener types had a great impact on the hydrolytic degradation behavior of cured UF resins. The surface roughness measurement results also supported this result. The results of this study suggested that the soft phase was much more susceptible to the hydrolysis of cured UF resin than the hard phase. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Optimal designs for constant‐heating‐rate differential scanning calorimetry experiments for polymerization kinetics: nth‐order kinetics

Optimal designs have been constructed for differential scanning calorimetry (DSC) experiments conducted under constant‐heating‐rate conditions for materials that are a priori assumed to follow nth‐order kinetics. Two different operating scenarios are considered, including single‐scan and multiscan DSC experiments for eight different kinetic parameter combinations representing a range of typical polymeric curing reactions. The resulting designs are studied to determine which kinetic model parameters are influential in determining the optimal design. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010