Hydrogen-Bond-Induced Crystallization in Low-Molar-Ratio Urea–Formaldehyde Resins during Synthesis

Synthesis of an exfoliated kaolinite-poly(urea-formaldehyde) nanocomposite

In this study, kaolinite-poly(urea-formaldehyde) was successfully prepared through the polymerization of urea intercalated within the kaolinite structure. Polymerization was carried out under ambient conditions by immersing kaolinite-urea in formaldehyde. Evidence of urea intercalation and polymerization was obtained from FTIR, XRD, and thermal analysis (TG-DSC). The XRD pattern of the kaolinite-poly(urea-formaldehyde) composite shows that polymerization induces exfoliation of the kaolinite layers, leading to the formation of a nanocomposite. Textural analysis through nitrogen adsorption on raw kaolinite and kaolinite-urea demonstrates that polymerization mainly occurs within the interlayer. Water resistance tests show that poly(urea-formaldehyde) within the nanocomposite is less sensitive to decomposition when immersed in water compared to pristine poly(urea-formaldehyde). Additionally, a self-healing effect was observed for poly(urea-formaldehyde) in the nanocomposite, indicating that fragments from the decomposition of the intercalated polymer when immersed in water remain trapped within the composite matrix owing to interactions with kaolinite mineral layers. Modeling of kaolinite-poly(urea-formaldehyde) interactions allows for the proposal of a mechanism for the interlayer polymerization of intercalated urea.

Ureido Hyperbranched Polymer Modified Urea-Formaldehyde Resin as High-Performance Particleboard Adhesive

The performance of urea-formaldehyde (UF) resin and its formaldehyde emission is a natural contradiction. High molar ratio UF resin performance is very good, but its formaldehyde release is high; low molar ratio UF resin formaldehyde release is reduced, but the resin itself performance becomes very bad. In order to solve this traditional problem, an excellent strategy of UF resin modified by hyperbranched polyurea is proposed. In this work, hyperbranched polyurea (UPA_6N) is first synthesized by a simple method without any solvent. UPA_6N is then added into industrial UF resin in different proportions as additives to manufacture particleboard and test its related properties. UF resin with a low molar ratio has a crystalline lamellar structure, and UF-UPA_6N resin has an amorphous structure and rough surface. The results show that internal bonding strength increased by 58.5%, modulus of rupture increased by 24.4%, 24 h thickness swelling rate (%) decreased by 54.4%, and formaldehyde emission decreased by 34.6% compared with the unmodified UF particleboard. This may be ascribed to the polycondensation between UF and UPA_6N, while UF-UPA_6N resin forms more dense three-dimensional network structures. Finally, the application of UF-UPA_6N resin adhesives to bond particleboard significantly improves the adhesive strength and water resistance and reduces formaldehyde emission, suggesting that the adhesive can be used as a green and eco-friendly adhesive resource for the wood industry.

Preparation of Novel Biodegradable Polymer Slow-Release Fertilizers to Improve Nutrient Release Performance and Soil Phosphorus Availability

Inspired by the gradual collapse of carbon chain and the gradual release of organic elements into the external environment during the degradation of biodegradable polymers, a novel biodegradable polymer slow-release fertilizer containing nutrient nitrogen and phosphorus (PSNP) was prepared in this study. PSNP contains phosphate fragment and urea formaldehyde (UF) fragment, which are prepared by solution condensation reaction. Under the optimal process, the nitrogen (N) and P₂O₅ contents of PSNP were 22% and 20%, respectively. The expected molecular structure of PSNP was confirmed by SEM, FTIR, XRD, and TG. PSNP can release N and phosphorus (P) nutrients slowly under the action of microorganisms, and the cumulative release rates of N and P in 1 month were only 34.23% and 36.91%, respectively. More importantly, through soil incubation experiment and leaching experiment, it was found that UF fragments released in the degradation process of PSNP can strongly complex soil high-valence metal ions, thus inhibiting the phosphorus nutrient released by degradation to be fixed in the soil and ultimately effectively increasing the soil available P content. Compared with ammonium dihydrogen phosphate (ADP), a small molecule phosphate fertilizer that is easily soluble, the available P content of PSNP in the 20-30 cm soil layer is almost twice that of ADP. Our study provides a simple copolymerization method to prepare PSNP with excellent slow-release N and P nutrients, which can promote the development of sustainable agriculture.

Use of Thymus Plants as an Ecological Filler in Urea-Formaldehyde Adhesives Intended for Bonding Plywood

Innovative adhesive formulations have been developed in the laboratory based on urea-formaldehyde resin by adding medicinal plants to an industrial adhesive formulation containing raw materials: urea-formaldehyde resin, urea, ammonium sulphate and starch. Specifically, Thymus species (Thymus bleicherianus, Thymus capitates, Thymus satureioides, Thymus vulgaris and Thymus zygis) replaced part of the starch and were considered as the second filler in the formulations. The physico-chemical properties of the resulting adhesive formulations, such as: pH, viscosity, gel time, solids content, density, concentration of free formaldehyde and color were measured, and characterized using Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA) and Fourier Transform Infrared spectroscopy (FTIR). In order to evaluate the mechanical performances of adhesive formulations based on plants, plywood panels were produced and their mechanical properties were studied. These mechanical properties included the shear strength, bending strength and the modulus of elasticity. The performance of these panels is comparable to that of plywood panels made using the standard adhesive formulation. From the results obtained, and following the statistical studies, the new adhesive formulations based on plants have the same physico-chemical properties, the same morphologies, and the same mechanical properties. Moreover, the novel adhesives are more viscous, and they have less free formaldehyde content than the commercial formulation.

Degradation of urea-formaldehyde resin residues by a hydrothermal oxidation method into recyclable small molecular organics

Urea-formaldehyde (UF) resin residues and the related product wastes as organic hazardous wastes are difficult to be biodegraded or recycled. In this research, a hydrothermal oxidation method using hydrogen peroxide (H₂O₂) solution has been developed for the degradation and recycling of UF resin residues. The effects of solution concentration, temperature, and time on the degradation efficiency and products of UF resin residues were studied. Under optimal conditions, i.e., 140 °C and 5 wt% H₂O₂ solution, over 75% of UF resin residues was degraded after 3 h. The degradation efficiency is much higher than that of the traditional hydrothermal treatment or acid hydrolysis method. In addition, results from Fourier transform infrared spectroscopy (FTIR), gas chromatography-mass spectroscopy (GC-MS), nuclear magnetic resonance spectroscopy (NMR), and X-ray diffraction (XRD) confirmed that H₂O₂ solution degrades UF resin residues to low molecular compounds, such as alcohols, methylal, and amides. This research provides a novel and high-efficient hydrothermal oxidization process for the degradation of UF resin residues, which might be a promising environmentally friendly and low-cost method for the disposal and recycling of industrial UF resin residues.

Different Zn loading in Urea-Formaldehyde influences the N controlled release by structure modification

Nitrogen fertilization has been a critical factor for high crop productivity, where urea is currently the most used N source due to its high concentration and affordability. Nevertheless, urea fast solubilization leads to frequent losses and lower agronomic efficiency. The modification of urea structure by condensation with formaldehyde has been proposed to improve nutrient uptake by plants and to reduce environmental losses. Herein we show that the co-formulation with Zn strongly modifies the N release (in lab conditions) and, more important, the Zn source—ZnSO₄ or ZnO—has a critical role. Urea–formaldehyde (UF) served as a matrix for the zinc sources, and chemical characterizations revealed that Zn particles influenced the length of the polymeric chain formation. Release tests in an aqueous medium showed that the UF matrix favors ZnO release and, on the other hand, delays ZnSO₄ delivery. Soil incubation with the fertilizer composites proved the slow-release of N from UF, is ideal for optimizing nutritional efficiency. Our results indicated that the ZnO-UF system has beneficial effects for both nutrients, i.e., reduces N volatilization and increases Zn release.

Morphology and Crystallinity of Urea-Formaldehyde Resin Adhesives with Different Molar Ratios

Using formaldehyde and urea as raw materials, a stable urea–formaldehyde resin (UF) is synthesized by the “alkali-acid-alkali” method. Unlike most thermosetting resins, UF often shows the appearance of crystal domains. In order to understand the relationship between the crystal and morphology of UF resin, analysis was carried out with the help of polarizing microscopy (POM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). The changes of two kinds of UF resins with molar ratios (F/U) of 1.4 and 1.0 before and after curing and under the influence of different curing agents and additives were studied. SEM results showed that the UF resins with low F/U (1.0) show spherical or flat structures before and after curing, and the diameter of the spherical structure increases with the increase of the content of curing agent, while in the UF resin with high F/U (1.4) it is difficult to observe the above phenomenon. At the same time, the possible accumulation mode of UF colloidal particles in the process of aggregation is explained, and the curing agent obviously promotes the development of the crystal structure, which may be the reason for the emergence of a large number of spherical particles. XRD results showed that the resin with low F/U has higher crystallinity than the resin with high F/U, indicating that the former shows more crystallization regions, while the latter shows more amorphous structure, and the crystallinity increases with the increase of the curing agent content, but the position of the crystallization peak does not change with the type of curing agent and the amount of curing agent. Observation of the selected area electron diffraction (SAED) pattern obtained by TEM shows that the cured low F/U (1.0) resin has a polycrystalline structure and a body-centered cubic unit cell. FT-IR results showed that the linear segment, branched structure, hydroxymethyl and methylene structure changes in UF affect the formation of crystal structure. This study also shows the possible contribution of hydroxymethylated species to the formation of crystals.