Performance Of Urea-formaldehyde Resins Research Articles

Sustainable bio-based dialdehyde cellulose for transforming crystalline urea–formaldehyde resins into amorphous ones to improve their performance

Sustainable bio-based dialdehyde cellulose (DAC) was employed to transform crystalline urea–formaldehyde (UF) resins into amorphous ones for simultaneouly improving their adhesion strength and formaldehyde emission. Serial samples of the UF resins modified with DAC during the resin synthesis were extracted to understand the chemical reactions between the DAC and UF species. Fourier transform infrared, 1H-nuclear magnetic resonance (NMR), and 13C NMR spectroscopies confirmed the occurrence of reactions between the DAC and UF species. As the synthesis proceeded, the crystallinity of the modified UF resins decreased from 51.7% to 17.4%, transforming the crystalline domains into amorphous ones. Thermograms showed that the DAC in the modified UF resins was decomposed at temperatures over 200 °C as degraded form, resulting in a lower cross-linking density than that of the neat UF resins. The adhesion strength of the modified UF resins was statistically similar to that of the neat UF resins, and the formaldehyde emission of the modified UF resins dramatically decreased to ∼ 64.6%. These results evidence the significant application potential of bio-based DAC in improving the sustainability and performance of UF resins.

Effects of Compounded Curing Agents on Properties and Performance of Urea Formaldehyde Resin

The effects of three compounded curing agents on the properties and performance of the urea-formaldehyde (UF) resin were investigated in this study. The compounded curing agents were prepared by mixing ammonium chloride with hexamethylenetetramine, citric acid, and oxalic acid respectively at a ratio of 1:1, named N-H, N–CA, and N–OA, respectively. The curing process, crystallinity, and physical properties were measured, and the three-ply plywood was fabricated to measure its prepress strength, wet shear strength, and formaldehyde emission. Results showed that the compounded curing agents N–CA and N–OA enhanced the initial viscosity, crosslinking density and thermal stability of UF resin. Additionally, the prepress strength of the plywood bonded by UF resin with N–CA and N–OA increased by 82 and 111% respectively compared to the UF resin with NH4Cl, and the wet shear strength increased by 14 and 16%, the formaldehyde emission decreased by 19 and 42% respectively. However, owing to the short pot-life of these curing agent limited their storage time, the curing agents N–CA and N–OA should be applied to fabricate plywood in winter for obtaining a better bond strength and a lower formaldehyde emission. While the UF resin with N–HT showed a suitable pot-life, so it could be applied to fabricate plywood in summer for long time storage and avoiding procuring problem.

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

Influence of uron resins on the performance of UF resins as adhesives for plywood

Uron resin, a kind of urea-formaldehyde (UF) resin containing much more uron structure, were prepared. Several modied UF resins were obtained from mixing uron resins with normal UF resins as well as synthesizing UF resin with uron resin as raw material. � is work demonstrated that the introducing of the uron structure reduced free formaldehyde content of UF resin and formaldehyde emission levels in bonded plywood panels signicantly, the former were reduced by 76% and 84% in the latter. � e synthesis time of uron resin can be shortened to 8 hours. � e test result of free formaldehyde content, formaldehyde emission levels and bond strength indicated that specimen of 20 parts uron resin synthesized in 8 hours mixed with 100 parts UF resins (F/U molar ratio of 1.3) had a potential advantage to industrial application. � e benecial eect of uron resin on the performance of UF resin can be attributed to the opening of uron cycle structure and the following reaction with free formaldehyde and its oligomeric glycol forms.

Preparation and Fertilizer-Permeation Performance of Urea-Formaldehyde Resins Modified by Montmorillonite

Urea-formaldehyde (UF) resins modified by montmorillonite(MMT) were prepared for coating materials of controlled-release fertilizers. The influences of MMT on the mechanical properties and fertilizer-permeation performance of UF resin were studied. The mechanical properties of modified UF resins are enhanced by the addition of MMT. The shear strength of modified UF resin with 3% MMT increases by nearly 20% and reaches the maximum. The urea permeation rate of modified UF resin membranes decreases with the increasing of membrane thickness and increases with the increasing of MMT content.

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.

Urea-Formaldehyde Resins and Free Formaldehyde Content

Specifications of wood adhesives must be in correlation with the requirements for the corresponding wood products, for which they will be used. Formaldehyde emission of wood products bonded with urea-formaldehyde resin based adhesives is strictly regulated by standards and there is a compromise is between formaldehyde emission and performance, such as strength, or water resistance. Since values of formaldehyde emission depend on the test method used, in Europe urea-formaldehyde resins for adhesives may generally be classified according to HCHO emission in the particleboard rating of Emission 0 to Emission I class (E-0 to E-1). According to DIN EN 120, particleboard quality E1 emits <6.5 mg/100 g dry article determined with the perforator method. Although a great number of factors effect the formaldehyde emission of the cured products, such as the hardener system, the type of wood etc., the emission of formaldehyde is in strict correlation with the free formaldehyde content of the resin before the curing process. E1 emission class can be achieved, if the free formaldehyde content of the resin is lower than 0.2% by mass. Urea-formaldehyde resins containing. higher than 0.5% free formaldehyde by mass exceed emission class E2, and are not accepted. Since the free formaldehyde content of the urea-formaldehyde resin effects the emission of formaldehyde in the cured product, low formaldehyde content must be ensured during resin synthesis. This can be achieved by properly selecting synthesis conditions as well as raw materials. The quality of raw materials is an essential and determining factor for the synthesis of urea formaldehyde resins. The principal changes which may take place in the formaldehyde solution on storage are the polymerization and precipitation of the polymer, Cannizzaro reaction, methylal formation, oxydation to formic acid, condensation to hydroxyaldehydes and sugars. Any of these reactions are detrimental to product quality. The state of formaldehyde is also an essential factor, since the reaction of poly(methylene glycol)s with urea leads to methylene ether linkages resulting in emission of formaldehyde during storage and later on during the process of curing. Hydrolysis, isomerisation and decomposition of urea may take place simultaneously during improper storage conditions, such as high humidity, high temperatures, industrial atmosphere. The side products formed affect the reaction with formaldehyde during the synthesis resulting in high free formaldehyde content of urea-formaldehyde resins. The relation between synthesis conditions, free formaldehyde content and performance of urea-formaldehyde resins are discussed in detail.

Effects of Reaction pH on Properties and Performance of Urea-Formaldehyde Resins

Effects of Reaction pH on Properties and Performance of Urea-Formaldehyde Resins