Abstract
The objective of this research was to provide a useful approach of polymer synthesis for accelerating the fast cure of phenol-urea-formaldehyde (PUF) resin as wood adhesive by optimizing its structure and composition. The PUF resins containing high contents of very reactive groups such as para-methylol groups were synthesized by reacting methylolurea, phenol, and formaldehyde in the presence of magnesium oxide (MgO) as catalyst. The effects of synthesis parameters including F/(P + U), OH/P, and MgO/P mole ratios on the structure, composition, curing characteristics, and their relationships of PUF resins were investigated. The results indicated that MgO seemed to be an efficacious catalyst for PUF resin synthesis and promote its faster cure. The increase in the F/(P + U) mole ratio or/and OH/P mole ratio appeared to be beneficial for the formation of para-methylol groups and cocondensed methylene linkages between phenolic methylol groups and urea units, and for the removal of unreacted urea. In case of Catalyst/P mole ratio, an appropriate dosage of added metal-ion was very important for synthesizing the high-content reactive groups of PUF resins, otherwise leading to the reverse effects.
Highlights
Phenol-formaldehyde (PF) resins are obtained in the reaction of phenol and formaldehyde in aqueous alkaline solutions
The addition reactions occur between phenol and formaldehyde, and urea and formaldehyde, respectively
The main reactions are condensation including self-condensation and co-condensation of methylolphenol, methylolurea, and urea, which contribute to the main structure and the increased polymer chain of PUF resin
Summary
Phenol-formaldehyde (PF) resins are obtained in the reaction of phenol and formaldehyde in aqueous alkaline solutions. This type of resin has a wide range of commercial applications in industrial products such as molding compounds, coatings, flame retardants, and wood adhesives [1]. Urea component addition in PUF resin has lead to additional reactions, additional structures and more complicated compositions. These variations in structures and compositions had profound influences on the cure characteristics of the resins
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