Liquid Urea-formaldehyde Resin Research Articles

Insights into the development of crystallinity in liquid urea-formaldehyde resins

Differently from most thermoset materials, urea-formaldehyde (UF) resins display the appearance of crystalline domains. In the effort of understanding the mechanism of formation of such crystals, wide angle X-ray diffraction (WAXD), infrared spectroscopy and transmission electron microscopy (TEM) were applied. Liquid UF resins with two different F/U mole ratios (i.e. 1.6 and 1.0) were investigated as a function of hardener level and curing times at room temperature. The WAXD results showed that the liquid UF resin with a low F/U mole ratio had a greater crystallinity than the one with a high F/U mole ratio. An advance in crystal formation in the low F/U mole UF resins was visible, especially in the first phases of curing. However, there were no significant differences in the degree of crystallinity as a function of hardener level. IR spectroscopy highlighted the important role of methylolated species in the formation of crystals. TEM results also confirmed the presence of crystals in all the considered liquid UF resins. The concentration of the hardener and the curing time were critical in shaping morphology and particle dispersion. As a function of the curing conditions, the globular structures present in the samples can aggregate into different morphologies, which can be fibrillar and also lamellar. The obtained results stress the importance of controlling the subtle interplay between crosslinking and formulation for the obtainment and control of the size, quantity and morphology of crystals in UF resins, and therefore for an effective tuning of their properties.

Micro-morphological Features of Liquid Urea-Formaldehyde Resins during Curing Process at Different Levels of Hardener and Curing Time Assessed by Transmission Electron Microscopy

Micro-morphological Features of Liquid Urea-Formaldehyde Resins during Curing Process at Different Levels of Hardener and Curing Time Assessed by Transmission Electron Microscopy

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.

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 (Tp), 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 Tp, 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, Tp 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 (Ea) values calculated by Kissinger method were greater for solid UF resins than for liquid resins. The activation energy (Eα) 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.

Curing characteristics of urea–formaldehyde resin in the presence of various amounts of wood extracts and catalysts

AbstractThe characteristics of urea–formaldehyde (UF) resin curing in the presence of wood extracts and a catalyst [ammonium chloride (NH4Cl)] were investigated by differential scanning calorimetry (DSC). The effects of extracts from 16 wood species on resin curing behaviors were evaluated. A model developed in this study, Tp = 53.296 exp(−9.72C) + 93.104, could be used to predict the resin curing rate in terms of the DSC peak temperature (Tp) as influenced by the NH4Cl content (C). The results indicated that the curing rate of UF resin increased as the catalyst content increased and reached a maximum when the catalyst content ranged from 0.5 to 1.0% (solid basis over liquid UF resin weight). Further increases in the catalyst content had no effect on the resin curing rate. The curing rates of UF resin in the presence of wood extracts increased with decreased pH values or increased base buffer capacities. It was also discovered that the activation energy could not fully explain the resin curing behavior when some species of wood extracts were present, and therefore, the pre‐exponential factor had to be taken into account. The concept of the equivalent catalyst content (ECC) of wood extracts to the NH4Cl content was introduced in this study; ECCs ranged from 0.0022 to 0.0331% among the 16 wood species. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Composite from Wood Fibres Bonded with Renewable Wood Resin

This paper shows the efficiency of a renewable wood resin as a binder for wood fibres in the manufacture of environmentally friendly fibreboard composites. The wood resin was nevertheless partially replaced by urea formaldehyde (UF) resin to make fibreboards more economical and performance-driven. The results demonstrate that nearly all the mechanical properties of the fibreboards were above the minimum requirements specified in the ANSI-AHA standard. The mechanical properties of wood resin bonded fibreboard were inferior to those of the UF resin bonded equivalents. The horizontal density profiles of the composites revealed that the wood resin distribution in the wood fibres could be optimised in presence of liquid UF resin. Wood resin bonded fibreboard showed a lower rate of formaldehyde emission than UF resin bonded fibreboard. The curing conditions were also optimised to achieve improved mechanical performance and dimensional stability for the composite fibreboards.

13C NMR analysis method for urea–formaldehyde resin strength and formaldehyde emission

AbstractA method based on the use of 13C NMR relative peak intensity ratios for different characteristic chemical groups, known or supposed to contribute to urea‐formaldehyde (UF) resin strength and formaldehyde emission is presented. The method relates results obtained by 13C NMR analysis of liquid UF resins with their strength, formaldehyde emission, and percent crystallinity in the resin‐hardened state. Correlation of different peak ratios with experimental results allows the proposal of equations relating the sum of a number of different 13C NMR peak ratios with the three mentioned physical properties of the same resins in their hardened state. Resin strength and percent crystallinity appear to be loosely, inversely related. The equations presented appear to have some applied value in predicting physical properties of hardened industrial‐type UF resins from a single 13C NMR spectrum of the original liquid resins, as well as to render easier comparison between difficult UF resin formulations. This approach has allowed identification of which chemical groups really contribute to the physical properties of the hardened resin, and to what exten. © 1993 John Wiley & Sons, Inc.

Suitability of Muli Bamboo (Melocanna baccifera) for Making Bamboo Mat Plywood

This paper investigated the potentiality of muli (Melocanna baccifera) for making bamboo mat plywood. Bamboo mat plywood with the length, width and thickness of 2.4 m × 1.2 m × 7 mm was made using liquid urea formaldehyde resin. The physical and mechanical properties of such plywood were compared with the existing market plywood. The average density of bamboo mat plywood was much higher compared to the existing market plywood. The modulus of rupture (MOR) of bamboo mat plywood was 3 times and the modulus of elasticity (MOE) was 6 times higher compared to commercial (Bombax ceiba) plywood. Interestingly, the specific MOR and MOE were significantly higher than those of market plywood. The high strength values might be due to the long fiber length of the bamboo. Furthermore, the lamination of decorative thin garjan (Dipterocarpus turbinatus) veneer did not significantly reduce mechanical properties of the products. Bamboo plywood mat as well as garjan laminated bamboo mat plywood showed better performance in respect to thickness swelling, linear expansion and water absorption. Hence, muli bamboo can be a potential alternative source of raw material for the manufacture of plywood materials. Key words: Bamboo mat plywood; Density; Dimensional stability; Modulus of elasticity; Modulus of rupture DOI: http://dx.doi.org/10.3329/bjsir.v46i4.9605 BJSIR 2011; 46(4): 543-548