Properties of UreaFormaldehyde Resins Synthesized at Different Molar Ratios

The formaldehyde/urea (F/U) molar ratio is one of the main parameters determining the formulation of urea-formaldehyde (UF) resins. The molar ratio largely determines the structure of the main components of the resin (UF oligomers, methylol derivatives of urea), as well as the properties of the resins and materials manufactured using them. The aim of this work is to research the structure of the UF resins synthesized at different F/U molar ratios; to assess the effect of the molar ratio on the properties of the resins and particleboards manufactured using them. It was found that the low molar ratio UF resin (F/U = <1.1) consist mainly of linear oligomers containing a insignificant amount of type II methylol groups. During curing of low molar ratio resins, a network structure with an insignificant amount of cross-bridges is formed, which negatively affects the water resistance of particleboards. With an increase in the F/U molar ratio, the number of branches of UF oligomers increases, also the content of methylol groups increases and the reactivity of the resins increases. Cured medium and high molar ratio UF resins (F/U = 1.1–1.2 and >1.2) are distinguished by a dense cross-linked structure and ensure comparatively high physical and mechanical properties of particle-boards. At the same time, an increase in the F/U molar ratio leads to a significant increase in the formaldehyde content in particleboards.

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