Urea/formaldehyde Resins
2005; Elsevier BV; Linguagem: Inglês
10.1016/b978-081551515-9.50007-x
Autores Tópico(s)Carbon dioxide utilization in catalysis
ResumoPublisher Summary Urea/formaldehyde glue resins are the most important type of urea/formaldehyde-resins. Formaldehyde is available in many forms; paraform and formalin solutions are the commonly used forms. Formaldehyde is also available as gas. Typically, formalin solutions are the preferred source of formaldehyde. Urea is the standard nitrogen-containing component in urea/formaldehyde resins. Resins with improved properties can be obtained by substitution of urea with melamine. Sulfitation of the methylol groups can improve the resin properties. Another approach is the co-condensation with amines and the introduction of urea-terminated amines. Melamine improves the resistance against attack by humidity and water, especially at elevated temperatures. The synthesis of a urea/formaldehyde (UF) resin proceeds via the methylolation of urea and condensation of the methylol groups. The reaction can be conducted in an aqueous medium because of the good solubility of both urea and formaldehyde. The production of UF resins is usually achieved in three stages. First, methylolation—urea reacts with aqueous formaldehyde under alkaline conditions at temperatures up to 100°C. Second, condensation—the condensation of methylols in slightly acidic medium yields oligomers with different molar mass and various functionalities. The condensation is then stopped by adding alkaline substances. Third, post treatment—evaporation of excess water and formaldehyde or addition of secondary urea to decrease the ratio of formaldehyde to urea. The multistage process is useful to fulfill the requirements of retaining the reactivity and the strength of the cured resin under the condition of minimal emission of formaldehyde during service.
Referência(s)