Investigation of UF-resins - the Effect of the Formaldehyde/Urea Molar Ratio during Synthesis
In this project, urea-formaldehyde (UF) resins were produced and investigated. UF resins are commonly used indoors as wood adhesives in, e.g. particle boards, in different furniture applications and flooring. UF resins are produced by alternating methylolation and condensation reactions, thus reacting urea and formaldehyde with each other and creating longer polymeric chains. The number of alternations, i.e. number of condensation reactions can be varied. The focus laid on the effect of the formaldehyde/urea molar ratio during synthesis. This includes the effect of the molar ratio on both the composition and structure and in turn their effect on the properties of the resin. UF resin was synthesized via two different methods. In method one, a reference adhesive was synthesized, this adhesive was produced using three condensation reactions. In method one, another resin was also produced using only two condensation reactions but with the same F/U molar ratios. In method two, only two condensations reactions were performed for each resin. In the start of the reaction, the F/U-molar ratio was varied in three different values. Towards the end, however, different amounts of urea were added to make sure that the final F/U-molar ration was kept constant.
The results showed that the F/U molar ratio during synthesis will have an effect on both the composition of the resin and the structures being formed. With less urea added in the beginning as in the case with a high starting molar ratio, more dimethyl ether bridges and methylol groups, but less methylene bridges were formed in the end-product. This was formed together with a more branched UF structure with a higher polydispersity index. The reason for this probably originates from the more highly substituted amino groups formed due to the lower amount of urea in the beginning in the sample with a high F/U molar ratio. The composition and structure of the resin with a higher F/U molar ratio also seem to favor a stronger tensile strength. The increased amount of methylol groups and the increased PDI are thought to have a larger effect on the increased tensile strength. The shelf life of the finished resin also showed dependency with the F/U molar ratio, since resins produced with a low molar ratio gelled within 24h compared the two other variations of F/U ratios, where it took about 90 days to gel at the same temperature.
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