Abstract
This paper presents the impact of different processes for producing urea–formaldehyde resins and their roles on the physico-mechanical properties and the formaldehyde emission of the resulting particleboards. Five resins were produced: four via the alkaline–acid process (Resin A to D) and one using the strongly acid process (Resin E). The differences between the syntheses are mainly related to different formaldehyde/urea molar ratios during the synthesis, temperatures, and the number of urea load addition. The molecular weight distribution of the resins was monitored by gel permeation chromatography/size exclusion chromatography and the unreacted oligomers were followed by high-performance liquid chromatography. The shear strength of adhesive joints was evaluated using automated bonding evaluation system. Particleboards produced with these resins were analyzed according to the usual standards for mechanical tests and formaldehyde emission. The resins differ in some characteristics, namely percentage of unreacted oligomers, chemical composition, viscosity, and reactivity. At a pressing time of 120 s, the internal bond of the particleboards was similar for all the resins produced using the alkaline–acid process (≈ 0.60 N mm−2) but differed from that obtained using the strongly acid process (≈ 0.40 N mm−2). However, formaldehyde emissions were apparently independent of the synthesis process.
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