Decreasing the Content of Free Formaldehyde in Urea-Formaldehyde Resins

Abstract

The possibility of decreasing the content of free formaldehyde in urea3formaldehyde resins by varying the pH and urea3formaldehyde ratio in various synthesis steps was examined. Urea3formaldehyde resins containing no free formaldehyde were prepared.Urea3formaldehyde resins (UFRs) are widely used binders. Their significant drawback is the toxicity caused by the presence of free formaldehyde (FA); its content depends on the preparation conditions [1].The influence of process parameters on the properties and structure of UFRs was examined in numerous studies [23 4], but the problem is far from being solved because of the complexity of the reactions involved in UFR formation. Polycondensation in the urea3FA system is accompanied by many side reactions (hydrolysis, cyclization, addition, etc.), affecting the functional composition of the product. The kinetic features and mechanisms of these reactions are different [5]. An additional factor complicating the mechanism of UFR formation is the polyfunctional nature of urea and FA.In this study, we examined how the process parameters affect the content of free FA in UFRs prepared at varied pH values and urea : FA ratios in particular reaction steps. EXPERIMENTALUFRs were prepared at 82 + 3oC and varied pH in three steps. The urea : FA molar ratio in these steps was 1 : 4, 1 : 1.45, and 1 : 1.33. In the course of the synthesis, we monitored the content of free FA and methylol groups (MGs) [6]. The experimental results are shown in Fig. 1. In run nos. 1 and 2, step I was performed in an acidic solution (pH 4.535.0), and steps II and III, in an alkaline solution (pH 9.5310.0). In run nos. 3 and 4, step I was performed in an alkaline solution (pH 9.5310.0); step II, in an acidic solution (pH 4.535.0); and step III, again in an alkaline solution (pH 9.5310.0). In run nos. 5 and 6, steps I and III were performed similarly to run nos. 3 and 4, and step II, at pH 2.0 32.5. In run nos. 7 and 8, all the three steps were performed in an alkaline solution (pH 9.5310.0). In run nos. 1, 3, 5, and 7, urea was loaded in a single portion in each step; in run nos. 2, 4, 6, and 8, the same amounts of urea were loaded uniformly in equal portions during the course of the process step. A detailed protocol of UFR preparation and the UFR structure were reported in [7,8].Experimental data show that the properties and toxicity of UFRs are appreciably influenced by such process factors as the mode of loading urea, pH of the medium, and reaction time, and also by the concentrations of urea and FA. In the UFR synthesis in an alkaline solution, when urea is loaded in a single portion, the content of free FA is 0.133 0.14, and that of MGs, 18.6 319.8 wt % (Fig. 1d, curves 1 and 1`); when urea is loaded uniformly, the content of free FA is 0.053 0.09, and that of MGs, 18.5319.7 wt % (Fig. 1, curves 2 and 2`). A longer reaction in alkaline solution does not result in an appreciable decrease in the free FA content.In the synthesis of UFR in an alkaline solution, a steady decrease in the free FA content is ...

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