Cured Urea-formaldehyde Resin Research Articles

An efficient Urea-formaldehyde resin degradation method by hydrogen peroxide and malic acid: At three-mole ratios

Urea-formaldehyde (UF) resin has a high annual output, but the used and waste UF resin poses a challenge for recycling due to the difficulty in effectively decomposition them. In this study, A novel one-step degradation method was proposed, i.e., using eco-friendly malic acid and hydrogen peroxide to degrade cured UF resin residues using carboxylic acid through the hydrothermal oxidation method (HPMA), which was proved to have high cured UF resin degradation efficiency. This method will offer a new path for effectively recycling the UF resin out of service.

Derivative thermogravimetric analysis curve for characterizing the curing process of urea-formaldehyde resins

The curing process of urea-formaldehyde (UF) resin affects the performance of cured UF resin. To characterize the curing process of UF resins, we proposed using the derivative thermogravimetric analysis (DTG) curve to simultaneously disclose the relationship between the mass-loss rate and curing of UF resin during the curing process. In this work, UF resins with F/U mole ratios of 1.60 and 0.95 and MUF resin were synthesized. The TG and differential scanning calorimetry (DSC) curves of all resins at different pH conditions were measured by TG-DSC with an open pan under atmospheric pressure. The DTG curves were obtained by the first derivative of TG curves. The structures of UF resin at different times (or temperatures) of the DTG curve were characterized by FT-IR and X-ray diffraction (XRD). The results show that the temperature of peak 1 of the DTG curve relates to the condensation reactivity of the UF oligomer. The temperature of peak 2 relates to the cross-linking structure of the cured UF resin. The curve between the two peaks reflects the variation in the degree of cross-linking of the UF resin. Analysis by DTG curves is a feasible approach to show the curing process of UF resin. Furthermore, we employed DTG curves to study the effect of NH 4 Cl on the curing process of UF resin. The DTG results show that NH 4 Cl plays different roles for UF resins with different F/U mole ratios.

Synchrotron X-Ray Diffraction Studies on Crystalline Domains in Urea–Formaldehyde Resins at Low Molar Ratio

The crystalline domain of thermosetting urea–formaldehyde (UF) resins at low formaldehyde-to-urea (F/U) molar ratios (≤1.0) is known to be responsible for their poor performance as wood adhesives. Crystallization has been observed in 1.0 F/U UF resins during the addition reaction stage and at the end of the synthesis process (neat UF resins). The crystallinity and X-ray diffraction (XRD) spectra of the uncured neat UF resins, on the other hand, differed significantly from those of the cured neat UF resins, raising the possibility that their crystal structures were also different. This study demonstrates for the first time that the crystalline domains in 1.0 F/U UF resins generated from uncured and cured samples are identical. Despite having a lower crystallinity value, the synchrotron XRD patterns of purified neat UF resins were equivalent to the XRD patterns of cured neat UF resins. Transmission electron microscope images of the cured UF resins showed that the crystals were lamellar structures. This finding suggests that the crystal at low molar ratio UF resins are isotropic polycrystals with random orientation.

Application of Colloid Theory in Urea Formaldehyde Resin

Urea formaldehyde resin is an end-stage thermosetting resin in which urea and formaldehyde are polycondensated into an initial urea formaldehyde resin under the action of alkaline or acidic catalyst, and then an insoluble and non-fusible resin is formed under the action of curing agent or additives. Urea formaldehyde resin has the characteristics of colorless curing adhesive layer, good operation performance, low cost, good bonding performance and water resistance. Its use in the field of wood bonding is increasing. The cured urea formaldehyde resin is lighter in color than phenolic resin, translucent, resistant to weak acid and alkali, good insulation, excellent wear resistance and low price. However, urea formaldehyde resin is easy to decompose in case of strong acid and alkali, and its weather resistance is poor. There is a problem of formaldehyde emission in the manufacturing and use of wood-based panels produced by urea formaldehyde resin, so it must be modified. At present, urea formaldehyde resin is the largest adhesive used in wood industry, especially in the manufacture of various wood-based panels in wood processing industry, urea formaldehyde resin and its modified products account for about 90% of the total amount of adhesive. In this paper, the development and application of colloidal theory in urea formaldehyde resin are introduced, and the significance and Prospect of this theory are prospected.

Condensation reaction and crystallization of urea-formaldehyde resin during the curing process

Urea-formaldehyde (UF) resins were synthesized with different molar ratios and solid contents, and simultaneously they were cured under conditions of different pH values. The curing behaviors of cured UF resins were examined by synchronous thermal analysis (TG-DSC). The crystallinity of cured UF resin was analyzed by X-ray diffractometry. The gel time of cured UF resin was recorded by chemical methods. The results indicate that condensation reactivity leads condensation reaction and crystallization to play various roles during the curing process of UF resin. The condensation reaction and crystallization in the curing process interact to result in different structures of cured resin. A new mechanism for UF resin curing (reaction-crystallization) is thus proposed.

Effects of Hydrolysis on the Removal of Cured Urea-Formaldehyde Adhesive in Waste Medium-Density Fiberboard

The vast production of medium-density fiberboard (MDF) in the world is expected to generate a large quantity of waste MDF after its service life, which requires the recycling of waste MDF (wMDF). This work attempted to investigate the removal of cured urea-formaldehyde (UF) resins adhesive in wMDF using hydrolysis for a possible way of recycling wMDF. The wMDFs were fabricated with two kinds of recycled fibers (RFs): refiner recycled fibers (RRFs) and hammer mill recycled fibers (HRFs) from red and radiata pine. The wMDFs were also produced at different RFs contents, such as 0, 5, 10, 20, 30, 50, and 100%. The panels were then hydrolyzed with water and oxalic acid solution to remove the cured UF resins. The Kjeldahl method was applied to determine the nitrogen (N) content in the panel before and after hydrolysis. Regardless of the wood species and recycling process, the mass loss, pH, and formaldehyde liberation of wMDFs after hydrolysis were greater for oxalic acid than those in water, confirming a greater N content had been extracted by oxalic acid than water. The resin removal became greater as the RFs content increased. In addition, the resin removal was slightly greater in wMDFs made of HRFs than the RRFs. Moreover, red pine RFs gave higher resin removal than radiata pine. These results suggested that a proper combination of the recycling process and additives could make it possible to recycle wMDF panels in the future.Keywords: cured urea-formaldehyde, hydrolysis, medium-density fiberboard, oxalic acid, recycling

Urea Formaldehyde Resin Resultant Plywood with Rapid Formaldehyde Release Modified by Tunnel-Structured Sepiolite.

In order to reduce the cost of plywood and save edible resources (wheat flour), a cheap and resourceful clay, sepiolite, was used to modify urea formaldehyde (UF) resin. The performances of filler-filled UF resins were characterized by measuring the thermal behavior, cross section, and functional groups. Results showed that cured UF resin with SEP (sepiolite) formed a toughened fracture surface, and the wet shear strength of the resultant plywood was maximum improved by 31.4%. The tunnel structure of SEP was beneficial to the releasing of formaldehyde, as a result, the formaldehyde emission of the plywood bonded by UF resin with SEP declined by 43.7% compared to that without SEP. This study provided a new idea to reduce the formaldehyde emission, i.e., accelerating formaldehyde release before the product is put into use.

Hydrolysis and recycling of urea formaldehyde resin residues

Urea formaldehyde (UF) resin residues generate many poisonous chemicals during traditional disposal technologies for organic hazardous wastes. In this study, hydrothermal hydrolysis was developed to decompose, remove, and recycle cured UF resin. Under 140 °C and 2 h of hydrothermal process, 20 mL of either 2 M HCl or 30 w.t. % of formaldehyde solutions can decompose as much as 1.7 g of cured UF resin. In addition, from the attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), gas chromatography–mass spectroscopy (GCMS), X-ray diffraction (XRD), and 1H nuclear magnetic resonance (NMR) analyses, it confirmed that the hydrothermal hydrolysis could hydrolyze cured UF resin. With 2 M HCl, the hydrolysis of cured UF resin can form NH4Cl as the final product. Moreover, formaldehyde solution hydrolysis can fully recycle the decomposed UF resin. The hydrolyzed formaldehyde solution can be directly used as a normal formaldehyde solution for UF resin synthesis. The synthesized resin, which contains about 6 w.t. % of decomposed cured UF resin, has similar chemical structures and bonding performance as normal UF resin. These hydrolysis methods generate no harmful gases and pollutants, nor any pre- or post-treatments, which proves its feasibility in UF resin residues safe removal or disposal and recycle.

Impacts of urea-formaldehyde resin residue on recycling and reconstitution of wood-based panels

Recycling and reuse of wood-based panels has been attracting great attention since it is an environmentally friendly means of managing wood waste. The recycled wood-based panels usually still contain adhesive residue. In the current research, the impact of urea-formaldehyde (UF) resin, one of the most widely used adhesives, on recycling and reconstitution of wood-based products has been evaluated by adding cured UF resin particles into a new adhesive system. The morphology, chemical structure, curing behavior, and bonding quality of an adhesive system containing 0–6% previously cured resin particles of varied sizes (< 600μm) were investigated and compared to those properties of a regular adhesive without any impurity. Three-ply plywood glued with a UF adhesive containing 0–10wt% previously cured resin of varied particle sizes (< 600μm) were prepared and characterized. The results indicated that the resin residue could significantly affect the curing behavior and bonding quality of the new adhesive system. Higher content and greater size of the particles caused more severe deterioration in the bonding strength.

Hydrolytic Removal of Cured Urea–Formaldehyde Resins in Medium-Density Fiberboard for Recycling

This study focused on the removal of cured urea–formaldehyde (UF) resins by hydrolysis of medium-density fiberboard (MDF) at different temperatures and times for MDF recycling. Five aqueous solutions, i.e., two acids, two alkalis, and water, were used for MDF hydrolysis to obtain extract solutions and solid residues for the analysis of mass balance, nitrogen content, and resin removal. As expected, acidic solutions removed the greatest amount of cured resins, followed by water and alkaline. Moreover, temperature had a greater impact on hydrolysis than time. Fourier transform infrared spectra of MDF fibers before and after hydrolysis clearly showed the change in intensity of the amide group in UF resins. Reduction of the amide group was greater with increase in hydrolysis temperature than in time. Statistical analysis results suggested that the hydrolysis of MDF at 80°C for 2 h using oxalic acid solution was optimum condition for the removal of cured UF resins. However, water could be used as hydrolysis agent for the practice of MDF hydrolysis in the future.

Characterization of the crystalline regions of cured urea formaldehyde resin

Urea formaldehyde (UF) resins are widely used thermosetting polymers in adhesives, finishes, molded objects,etc.

Quantification of hydrolytic degradation of cured urea-formaldehyde resin adhesives using confocal laser scanning microscopy

This study reports a method of measuring the extent of hydrolytic degradation of cured urea-formaldehyde (UF) resins trapped in the lumen of softwood tracheids, using confocal laser scanning microscopy (CLSM) which enables quantitative determination of the area of a cured UF resin in the lumen as a function of acid etching time. Two formaldehyde/urea (F/U) mole ratios (1.6 and 1.0) and two hardener levels were employed to compare the extent of hydrolytic degradation of cured UF resins. For comparison, we also measured the mass loss of cured UF resin films after acid etching. Cured UF resins with a 1.6F/U mole ratio resulted in greater hydrolytic degradation than those with a 1.0F/U mole ratio. The hardener level also showed a clear influence on the extent of degradation with a limited consistency. These results indicate that cured UF resins with higher F/U mole ratios are more easily hydrolyzed than those with low F/U mole ratios, which could be ascribed to a more branched network structure of cured UF resins with a high F/U mole ratio.

Micro-Morphological Features of Cured Urea-Formaldehyde Adhesives Detected by Transmission Electron Microscopy

This work examined micro-morphological features responsible for the crystallinity of cured urea-formaldehyde (UF) adhesives, using transmission electron microscopy (TEM) to identify and characterize distinctive crystalline structures in resins obtained with different formaldehyde to urea (F/U) mole ratios and hardener levels. The TEM examination of cured UF resin adhesives impregnated into wood cell lumen revealed the presence of spherical particles with variable diameter and number per unit area. The diameter and number/area of the spherical particles increase for decreasing F/U mole ratio and decrease with an increase in the hardener levels, an effect which is closely related to their crystallinity. Therefore, the present findings suggest that the spherical particles are responsible for the crystallinity of cured UF resin adhesives. The results also indicate that crystalline structures represent an inherent feature of cured UF resin adhesives, particularly for low F/U mole ratios, even though these resins are usually classified as amorphous and cross-linked thermosetting polymers.

Dynamic mechanical analysis of urea formaldehyde resin modified by ammonium pentaborate as wood adhesive

Urea formaldehyde (UF) resins with varied molar ratio of formaldehyde (F) to urea (U), modified by ammonium pentaborate (APB) at different loading level, was analyzed by dynamic mechanical analysis (DMA) and evaluated via bonding properties of its glued plywood. The result indicated that a higher loading of APB made a slower gelling and improved the ΔE′ (the difference of storage modulus) of UF resin with F/U molar ratio of 1.8. Hexamethylenetetramine, generated from the ammonium ion in APB and formaldehyde in UF resin and characterized by the covalent bond connections, was considered as the main reason to improve the rigidity of the cured UF resin system. The bond strength result confirmed the DMA analysis that the addition of APB improved bonding performance of UF resin with higher F/U molar ratio such as 1.8. A specific recommendation loading level of APB was made to modify UF resins, of which 6.0 to 8.0% APB should be used to modify UF resin with F/U molar ratio of 1.8, then 6% and 4% loading level of APB to UF resin with F/U molar ratio of 1.50 and 1.25, respectively. Finally, APB was n'ot suggested to modify UF resin at F/U molar ratio less than 1.20. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

Microstructure of Cured Urea-Formaldehyde Resins Modified by Rubber Latex Emulsion after Hydrolytic Degradation

ABSTRACT This study investigated microstructural changes of cured urea-formaldehyde (UF) resins mixed with aqueous rubber latex emulsion after intentional acid etching. Transmission electron microscopy (TEM) was used in order to better un-derstand a hydrolytic degradation process of cured UF resins responsible for the formaldehyde emission from wood-based composite panels. A liquid UF resin with a formaldehyde to urea (F/U) molar ratio 1.0 was mixed with a rubber latex emulsion at three different mixing mass ratios (UF resin to latex = 30:70, 50:50, and 70:30). The rate of curing of the liquid modified UF resins decreased with an increase of the rubber latex proportion as determined by differential scanning calorimetry (DSC) measurement. Ultrathin sections of modified and cured UF resin films were exposed to hydrochloric acid etching in order to mimic a certain hydrolytic degradation. TEM observation showed spherical particles and various cavities in the cured UF resins after the acid etching, indicating that the acid etching had hydrolytically degraded some part of the cured UF resin by acid hydrolysis, also showing spherical particles of cured UF resin dispersed in the latex matrix. These results suggested that spherical structures of cured UF resin might play an important role in hindering the hydrolysis degradation of cured UF resin.

Swelling Behaviour of Cured Urea–Formaldehyde Resin Adhesives with Different Formaldehyde to Urea Mole Ratios

As a part of understanding of the network structure of urea–formaldehyde (UF) resin adhesives, this study examined the swelling behaviour of cured UF resin adhesives with four different formaldehyde–urea (F/U) mole ratios, using Flory–Rehner thermodynamic theory and field emission-scanning electron microscopy (FE-SEM) to relate the swelling behaviour to consequently induce micromorphological changes. Cured UF resin films before and after acetone extraction were exposed to swelling in dimethyl sulphoxide at three different temperatures. For the first time, this study reported the experimentally determined swelling parameters, such as sol fraction (ωsol), polymer volume fraction (φp), polymer–solvent interaction parameter (χ), and the number average molecular weight between cross-links (Mc), for cured UF resin adhesives. Both ωsol and Mc decreased as the F/U mole ratio increased. But these values increased with an increase in the swelling temperature. The extraction resulted in negative ωsol values, suggesting the removal of a scattered distribution of ωsol in the cured UF resins. The micromorphology helped to explain the differences in the molecular integrity of the resins, indicating a close relationship between the swelling behaviour and the morphological changes after the swelling.

Morphological, chemical and crystalline features of urea–formaldehyde resin cured in contact with wood

The issue whether urea–formaldehyde (UF) resins possess crystallinity when cured in contact with wood has not been clearly resolved, although according to some published work low mole UF resins do not exhibit crystallinity in contact with wood. Here we provide evidence using field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy and wide angle X-ray diffraction (WAXD) that UF resin with a low formaldehyde/urea (F/U) mole ratio does crystallize when cured in contact with wood and show that crystals have various morphologies and sizes and thus can be regarded as polycrystals. Morphological characterization by FE-SEM revealed the presence of globular/nodular particles and crystalline structures. The particles occurred in two distinct populations with respect to their size, and the fractured faces of particles and their associated plate-like structures revealed presence of sub-particles of extremely small size (26.3–56.1nm size range). Some crystalline regions of the resin contained expanded fan-shaped crystals, representing mature crystals. Other crystalline regions displayed features suggestive of emergence of crystals from globular particles. Transmission electron microscopy (TEM) made it possible to obtain electron diffraction (ED) pattern by analyzing directly the resin contained within cell lumens, which also suggested resin crystallization into polycrystals. The micromorphological information presented, particularly on nano-substructures and crystalline features, is novel and provides greater understanding of the architecture and crystalline characteristics of UF resin cured in contact with wood. Furthermore, information included provides evidence of a close relationship of emerging crystals with globular particles. FTIR spectra demonstrated that a quantitative calculation of the methylene linkages formed by cross-linking of UF resins in contact with wood has been advanced in its cure and crystalline structures formed in the cured UF resin. WAXD results also confirmed a polycrystalline nature of UF resin cured in contact with wood. These results corroborate that a low formaldehyde thermosetting UF resin cured in the presence of wood possesses distinct crystalline structures, an aspect of UF resin behavior that remained poorly understood and needed clarification. However, presence of crystallinity, albeit not widespread, may negatively impact cohesive strength of the resin, leading to a poor adhesion performance.

MRT Letter: High resolution SEM imaging of nano‐architecture of cured urea‐formaldehyde resin using plasma coating of osmium

Nanoarchitecture of cured urea-formaldehyde (UF) resins was examined with a field-emission scanning electron microscope (FE-SEM) after coating samples with osmium, which is considered to produce particles of considerably smaller size compared to other metal coatings used in SEM studies. This method enabled comparison of the nanoarchitecture of UF resins of low (1.0) and high (1.6) formaldehyde/urea (F/U) mole ratios to be made, based on imaging of extremely small size particles as part of UF resin architecture, not described before. Imaging revealed presence of relatively large globular particles (148.084-703.983 nm size range) as well as smaller substructures (28.004-39.604 nm size range) as part of the architecture of 1.0-mole UF resin. Globular particles were also present in 1.6 mole UF resin, but of considerably smaller size (14.760-50.269 nm). The work presented demonstrates usefulness of osmium coating in unraveling the intricacies of the nanostructural organization of cured UF resins, prompting wider application of this immensely useful but grossly underutilized metal coating type in high resolution SEM examination of biological and materials samples.

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 (HB) method was also used for comparison. The HB 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 (HM) and reduced modulus (Er) which have been used to calculate the elastic modulus (Es) of cured UF resins. As the F/U mole ratio decreased, the HB 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 HB. The HM value also showed a similar trend as a function of F/U mole ratio. However, the Er and Es did not show a consistent trend as exhibited by HM and HB. Both HM and Er 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 HM and Er 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.

Crystallinity and domain size of cured urea–formaldehyde resin adhesives with different formaldehyde/urea mole ratios

As a part of understanding hydrolytic degradation of cured UF resins responsible for formaldehyde emission, this study investigated the crystallinity and domain size of cured urea–formaldehyde (UF) resins with different formaldehyde/urea (F/U) mole ratios using wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. Both techniques were employed to calculate the crystallinity and domain size of cured UF resins as a function of F/U mole ratio. As the F/U mole ratio decreases, the crystallinity of cured UF resins increases from 26% to 48%. The SAXS provided useful information on the spatial arrangement of the crystalline domains and of their average distance. As the F/U mole ratio decreased from 1.6 to 1.0, the average distance between domains in cured UF resins decreased from 39nm to 34nm while the crystallite size increased from 1.3nm to 5nm. These results suggested that the crystallites were more closely packed with each other in cured UF resins when the F/U mole ratio decreased and the ordered domains were more randomly dispersed in high F/U mole ratio than low F/U mole ratio of UF resins. The greater amount of these ordered crystallites in cured UF resins of low F/U mole ratio was found to be correlated to the improved hydrolytic stability of the cured resins.