Examination of selected synthesis and room‐temperature storage parameters for wood adhesive‐type urea–formaldehyde resins by ¹³C‐NMR spectroscopy. IV

Abstract

Typical particleboard wood-adhesive urea-formaldehyde (UF) resins, synthesized with formaldehyde/first urea (F/U 1 ) mol ratios of 1.80, 2.10, and 2.40 and the second urea added to an overall F/U ratio of 1.15, in weak alkaline pH, were allowed to stand at room temperature over a period of 50 days.13 C-NMR of time samples taken over the storage period showed gradual migration of hydroxymethyl groups from the polymeric first-urea components to the monomeric second-urea components and also an advancing degree of polymerization of resins by forming methylene and methylene ether groups involving the second urea. These phenomena that varied with the F/U 1 mol ratios used in the resin syntheses due to the varying polymer branching structures resulted in the first step of resin synthesis. Varying viscosity decreases and increases of the resins also occurred. Due to these chemical and physical changes, the particleboards that bonded with the sampled resins showed varying bond strength and formaldehydeemission values, indicating process optimizations possible to improve bonding and formaldehyde-emission performances.

Investigation of Low Mole Ratio UF and UMF Resins Aimed at Lowering the Formaldehyde Emission Potential of Wood Composite Boards

“…Chemical shift values were referenced to the free urea peak at 164.0 ppm. The overall results were very similar to previous reports, and signals were assigned to functional carbon groups, as cited in typical references (Kim 2000(Kim , 2001Kim et al 2001Kim et al , 2003: 47.4, 53.9, and 60.1 ppm to methylenes of types I, II, and III methylenes, respectively; 69.5, 75.7, and 79.1 ppm to methylene-ethers of types I, II, and III, respectively; and 65.2 and 72.0 ppm to types I and II hydroxymethyl groups, respectively. Briefly, I, II, and III types of methylene and methylene-ether groups are defined by whether the two urea nitrogen groups bonded with them are substituted with 0, 1, or 2 formaldehyde-derived groups, respectively.…”

Phenol–formaldehyde‐type resins made from phenol‐liquefied wood for the bonding of particleboard

“…UF resins are low cost and give adequate strength properties, but the boards have a formaldehyde emission problem, which has been proven to be difficult to resolve completely. 4 The use of PF resins in place of UF resins could resolve the formaldehyde emission problem, 5 but there are two major disadvantages. The first disadvantage is the high price of phenol, which is derived from petroleum; a cheaper and readily available replacement is highly desirable.…”

Rheological behaviour of urea-formaldehyde adhesive resins

“…So it is not possible to decide definitely which amounts of these groups are linked to the macromolecular part obtained during polycondensation, and which amounts to the low-molar-mass part obtained in subsequent treatment with secondarily added urea [4]. Because of the equilibrium character of this reaction, the transhydroxymethylation reaction is the key process for understanding the mechanism of heat treating/cooling after polycondensation and the following storing [5][6][7].…”