Reactivity, chemical structure, and molecular mobility of urea–formaldehyde adhesives synthesized under different conditions using FTIR and solid‐state ¹³C CP/MAS NMR spectroscopy

Abstract

This study was conducted to investigate the effects of reaction pH condition and hardener type on the reactivity, chemical structure, and molecular mobility of urea-formaldehyde (UF) resins. Three different reaction pH conditions, such as alkaline (7.5), weak acid (4.5), and strong acid (1.0), were used to synthesize UF resins, which were cured by adding four different hardeners (ammonium chloride, ammonium sulfate, ammonium citrate, and zinc nitrate) to measure gel time as the reactivity. FTIR and 13 C-NMR spectroscopies were used to study the chemical structure of the resin prepared under three different reaction pH conditions. The gel time of UF resins decreased with an increase in the amount of ammonium chloride, ammonium sulfate, and ammonium citrate added in the resins, whereas the gel time increased when zinc nitrate was added. Both FTIR and 13 C-NMR spectroscopies showed that the strong reaction pH condition produced uronic structures in UF resin, whereas both alkaline and weak-acid conditions produced quite similar chemical species in the resins. The proton rotating-frame spin-lattice relaxation time (T 1H ) decreased with a decrease in the reaction pH of UF resin. This result indicates that the molecular mobility of UF resin increases with a decrease in the reaction pH used during its synthesis.

Self healing coatings containing dual active agent loaded urea formaldehyde (UF) microcapsules

“…The UF polymer builds up in the acid condensation stage on the oil droplet surface. The methylols reacts with urea and formaldehyde which are still present in the system to give a linear and partly branched molecules with medium and even higher molar masses [41,42]. The type of bond between the urea molecules depends on the reaction condition.…”

Comparison of thermal curing behavior of liquid and solid urea–formaldehyde resins with different formaldehyde/urea mole ratios

“…Our group has been characterizing various features of UF resins, such as hydrolytic stability, morphology, crystallinity, and microstructure [10][11][12][13][14][15][16][17]. Moreover, this work is an extension of our works that have been focused on understanding the impact of F/U mole ratio to properties of liquid [15] or/and cured UF resins [11].…”

Synthesis and properties of multihydroxy soybean oil from soybean oil and polymeric methylene‐diphenyl‐ 4,4′‐diisocyanate/multihydroxy soybean oil polyurethane adhesive to wood

“…This might be attributed to the reaction between the residual ANCO group and water in wet test condition. Park et al 14 reported that urea bond arising from the reaction of ANCO group with water affected the chemical behavior of polyurethane film using FTIR spectroscopy. These results could be attributed to a greater number of ANCO groups in the MHSBO/pMDI resin, which created more bonds between the unreacted ANCO group and the hydroxy group of cellulose in plywood and changed it into urea bonds, biuret bonds, or urethane bonds under the heat of hot-pressing, thereby enhancing its bending properties.…”