Performance and structures of urea-formaldehyde resins prepared with different formaldehyde solutions

Xu, Gaoxiang; Liang, Jiankun; Zhang, Bengang; Wu, Zhigang; Lei, Hong; Du, Guanben

doi:10.1007/s00226-021-01280-y

Cited by 8 publications

(4 citation statements)

References 25 publications

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“…If we consider only a pure urea as a source of amino groups, then after adding the modifier-curing agent to the resin, the molar ratio F/U shifts downward. In [32][33][34][35], the resin synthesis showed that a decrease in the molar ratio leads to an increase in the number of methylene bridges in UF oligomers, and this pattern is true for reactions in both alkaline and acidic environments. It is noteworthy that mostly increase amount just methylene bridges type I, that is in reactions involved mainly the terminal methylol groups.…”

Section: Results and Analysesmentioning

confidence: 91%

Usage of the New Modifier-curing Agent in Plywood Technology: The Influence to Urea-formaldehyde Resin Curing and Formaldehyde Emission

Ivanov

Ekaterincheva

Kalashnikov³

et al. 2023

Period. Polytech. Chem. Eng.

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One of the urgent scientific and technical objectives in the technologies of plywood and wood boards is the search for ways to reduce of hot pressing time without increasing the formaldehyde emission from finished products. To solve this problem was developed the new modifier-curing agent MC-4SF, is mainly a product of interaction of citric acid with urea and ammonia. Compared to traditional ammonium salts, the modifier-curing agent combines the properties of both direct and latent catalysts. Determination of the composition of residual methylol groups in the aqueous extracts obtained by treating the resin cured at 100 °C showed that the modifier-curing agent provides relatively high hydrolytic stability of the UF-polymer during extraction. Spectra of solid-state 13C NMR showed that in resins cured with MC-4SF increased the compound of methylene bridges compared to resins cured with standard catalysts. It is possible that the amino groups of urea (or its derivatives) included in the modifier-curing agent, react with the methylol groups of UF oligomers, fitting urea into the structure of the resulting polymer. Thus explains the increased hydrolytic stability and reduced toxicity of the cured resin. Manufacturing tests of nine-layer plywood made with a modifier-curing agent showed that replacing ammonium sulfate with MC-4SF allows a significant reduction in pressing time at 110 °C without loss of quality of the finished product. With the same pressing time, it was possible to increase the line shear strength by 14% and to reduce formaldehyde emission by 45%.

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Section: Results and Analysesmentioning

confidence: 91%

Usage of the New Modifier-curing Agent in Plywood Technology: The Influence to Urea-formaldehyde Resin Curing and Formaldehyde Emission

Ivanov

Ekaterincheva

Kalashnikov³

et al. 2023

Period. Polytech. Chem. Eng.

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“…Typically, the average fracture energy for mode I fracture in commercial syntheticurea formaldehyde adhesives falls within the range of 0.1 to 0.2 N/mm. For mode II fracture, this value extends between 0.2 to 0.4 N/mm [34,35]. In contrast, the bio-based adhesive showcased substantially elevated fracture energy values when compared to these conventional synthetic-urea-formaldehyde adhesives indicating its potential to enhance the durability and dependability of wooden products.…”

Section: Fracture Testsmentioning

confidence: 96%

Characterization of Densified Pine Wood and a Zero-Thickness Bio-Based Adhesive for Eco-Friendly Structural Applications

Jalali,

Borges,

Carbas

et al. 2023

Materials

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This study investigates a sustainable alternative for composites and adhesives in high-performance industries like civil and automotive. This study pioneers the development and application of a new methodology to characterize a bio-based, zero-thickness adhesive. This method facilitates precise measurements of the adhesive’s strength and fracture properties under zero-thickness conditions. The research also encompasses the characterization of densified pine wood, an innovative wood product distinguished by enhanced mechanical properties, which is subsequently compared to natural pine wood. We conducted a comprehensive characterization of wood’s strength properties, utilizing dogbone-shaped samples in the fiber direction, and block specimens in the transverse direction. Butt joints were employed for adhesive testing. Mode I fracture properties were determined via compact tension (CT) and double cantilever beam (DCB) tests for wood and adhesive, respectively, while mode II response was assessed through end-loaded split (ELS) tests. The densification procedure, encompassing chemical and mechanical processes, was a focal point of the study. Initially, wood was subjected to acid boiling to remove the wood matrix, followed by the application of pressure to enhance density. As a result, wood density increased by approximately 100 percent, accompanied by substantial improvements in strength and fracture energy along the fiber direction by about 120 percent. However, it is worth noting that due to the delignification nature of the densification method, properties in the transverse direction, mainly reliant on the lignin matrix, exhibited compromises. Also introduced was an innovative technique to evaluate the bio-based adhesive, applied as a zero-thickness layer. The results from this method reveal promising mechanical properties, highlighting the bio-based adhesive’s potential as an eco-friendly substitute for synthetic adhesives in the wood industry.

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“…In this study, the crosslinking modification mechanism of UF for tannin adhesives was explored by taking resorcinol as a tannin A ring model compound [34][35][36][37] and dimethylol urea as a UF model compound from the perspective of the reaction of such model compounds. At present, most scholars believe that the copolycondensation reaction exists in the alkaline stage [38][39][40][41][42], and the research direction mostly focuses on the influence on the final resin properties, while the co-polycondensation reaction conditions have been investigated less. Co-polycondensation structures remain ambiguous, and the co-polycondensation conditions and mechanism have not been clearly described [43][44][45].…”

Section: Introductionmentioning

confidence: 99%

Crosslinking Mechanism of Tannin-Based Adhesives Based on Model Compounds: Copolycondensation of Resorcinol with Dimethylol Urea

Liang,

Li,

Zhong

et al. 2024

Forests

Self Cite

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This study focuses on the competition reaction rules of a system containing resorcinol (as a tannin model compound) and dimethylol urea (as a urea–formaldehyde resin model compound) under various alkaline and acidic environments. The aim is to investigate the crosslinked modification mechanism of urea–formaldehyde resin with tannin adhesive. The study delves into the competitive relationship between self-condensation polymerization reactions and co-condensation polymerization reactions. It specifically highlights the conditions for the copolycondensation reaction of dimethylolurea and resorcinol and validates its rationality through an examination of the resorcinol–urea–formaldehyde system’s reaction rules. The results show that (1) under strongly acidic conditions, the activity of carbocation intermediates produced by hydroxymethyl resorcinol for the resorcinol phenol ring is higher than the electrophilic reactivity of nitrogen atoms on hydroxymethyl urea, which is more beneficial for the resorcinol–formaldehyde self-polycondensation reaction, and the co-polycondensation structures do not play a dominant role. (2) Under weakly acidic conditions, the co-polycondensation structures are evidently advantageous over self-polycondensation structures, and the degree of the co-polycondensation reaction is positively correlated with pH below the neutral point of resorcinol. (3) Under alkaline conditions, the self-polycondensation between resorcinol and formaldehyde is dominant in the system. (4) The concentration of hydroxymethyl urea carbocation is the key factor to determine the degree of the co-polycondensation reaction.

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Performance and structures of urea-formaldehyde resins prepared with different formaldehyde solutions

Cited by 8 publications

References 25 publications

Usage of the New Modifier-curing Agent in Plywood Technology: The Influence to Urea-formaldehyde Resin Curing and Formaldehyde Emission

Usage of the New Modifier-curing Agent in Plywood Technology: The Influence to Urea-formaldehyde Resin Curing and Formaldehyde Emission

Characterization of Densified Pine Wood and a Zero-Thickness Bio-Based Adhesive for Eco-Friendly Structural Applications

Crosslinking Mechanism of Tannin-Based Adhesives Based on Model Compounds: Copolycondensation of Resorcinol with Dimethylol Urea

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