Study of a component of the urea-formaldehyde resin, is insoluble in water
A method of chemical analysis of a component of urea-formaldehyde resin (UFR) which is insoluble in water has been developed. The resin was synthesized at the molar ratio of the starting components urea : formaldehyde = 1 : 2, temperature 90 °С, initial pH = 7,0...8,0 pH in acidic stage 4,0...4,3. For isolation of the water-insoluble component, UFR was mixed with a large amount of water. The precipitate was washed with water and dissolved in a 40% potassium iodide solution. In the resulting solution, hydroxymethyl groups and free formaldehyde were oxidized with iodine in an alkaline medium. Excess of iodine was titrated with a solution of sodium thiosulfate. As a result of the reaction, a white flake-like precipitate was formed, which was filtered out and analyzed. The total content of formaldehyde and urea was determined in the sample. To do this, the analyzed composition was placed in a round-bottomed flask equipped with a direct condenser and a dropping funnel. A measured amount of 45% phosphoric acid was poured into the dropping funnel and added drop by drop to the flask. The flask was heated on a metal tile, and the released formaldehyde and accompanying water were collected in a measuring flask. After the end of the process, the released formaldehyde was determined. Urea was determined using the urease- hypochlorite method, in which urease hydrolyzes the remaining urea to ammonia and carbon dioxide. Then all the formed ammonia was determined by its color reaction with sodium hypochlorite and converted to urea. Separating the obtained masses of urea and formaldehyde by their molecular masses, we obtained a molar ratio of urea : formaldehyde in an insoluble precipitate equal to 1: 1.5. The smallest molecule that meets this condition is an oligomer in which four carbamide molecules are connected with three methylenester bonds, i.e. they contain six formaldehyde molecules. However, oligomers with a small molecular weight were highly soluble in water. Water-insoluble oligomers are those with a large mass that is significantly higher than the average level. Calculations showed that the average number of urea and methylene-ether links in UFR oligomers was 10, while the maximum number can reach 122. The study of the curing process of the UFR component, insoluble in water, by differential thermal analysis showed that it, like UFR, took place in three stages. However, the temperatures of endothermic peaks differed. Thus, the peak of the second stage of UFR curing was found at the level of 241.0 °C, and for an oligomer that was insoluble in water, it corresponded to 244.2 °C. There were even greater differences in the peak temperatures of the third stage of curing: 274.4 °C for UFR an,288.2 °C for the water-insoluble oligomer. It has been obvious that the water-insoluble oligomer significantly has slowed down the UFR curing process.