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As a part of understanding the hydrolysis of cured urea-formaldehyde (UF) resins that has been known as responsible for the formaldehyde emission, leading to sick building syndrome, this study attempted to investigate the morphology and to detect chemical elements of the cured UF resins of different formaldehyde/urea (F/ U) mole ratios and hardener (NH 4 Cl) levels, using field emission-scanning electron microscopy and energy-dispersive spectroscopy. Cured UF resins of low F/U mole ratio showed spherical structure whose diameter increased with an increase in the hardener level, whereas this was not observed for high F/U mole ratio UF resins regardless of the hardener levels. The energy-dispersive spectroscopy results showed five different chemical elements such as carbon, nitrogen, oxygen, chloride, and sodium in cured UF resins. The chloride distribution assumed as the presence of residual acid in the cured UF resins suggested that the hydrolysis of cured UF resins could initiate at the sites of chlorides on the surface of the spherical structures. As the hardener level increased, the quantities of both carbon and oxygen decreased, whereas those of nitrogen and chloride increased as expected. But the quantity of sodium was within measurement error.

Effects of adding urea to the strand board core-layer phenol-formaldehyde (PF) resin were investigated in conjunction with cure-accelerating catalysts. Ten percent urea based on the liquid resin weight was added at the beginning, at three different middle stages of polymerization, and at the end of PF resin synthesis. No significant cocondensation between the urea and PF resin components occurred as identified by 13 C NMR analyses, which corroborated well with the curing and strand board bonding performance test results. The various urea addition methods resulted in resins that slightly differ in the various tests due to the urea's temporary holding capacity of formaldehyde. The preferred method of urea addition was found to do it in the later part of PF resin synthesis for convenience, consistency, and slightly better overall performance. Some cure-accelerating catalysts were shown to reduce the thickness swelling of strand boards. This study showed the usefulness of adding some urea to strand board core-layer binder PF resins of replacing higher cost phenolic components with lower cost urea.

ABSTRACT:The effects of posttreatments of particleboard adhesive-type urea-formaldehyde resins were studied. The resins were synthesized with formaldehyde/first urea (F/U 1 ) mol ratios of 1.40, 1.60, 1.80, 2.10, and 2.40 and then the second urea was added to give a final formaldehyde/urea ratio of 1.15 in alkaline pH. The resins were posttreated at 60°C for up to 13.5 h and the 2.5-h heat-treated resin samples were stored at room temperature for up to 27 days. Resins sampled during the posttreatments were examined by 13 C-NMR and evaluated by bonding particleboards. In the posttreatments, hydroxymethyl groups on the polymeric resin components dissociated to formaldehyde and reacted with the second urea, and methylene and methylene-ether groups were formed from reactions involving the second urea. Methylene-diurea and urea groups bonded to UF polymers were identified. As a result, the viscosity of the resins initially decreased but later increased along with the cloudiness of the resins. Bond-strength and formaldehydeemission values of particleboard varied with posttreatment variables as well as with the F/U 1 mol ratios used in the resin syntheses. The results would be useful in optimizing resin synthesis and handling parameters. Various reaction mechanisms were considered.