Лако-красочные материалы — производство

The mechanism of the initial stages of the reaction of melamine with formaldehyde leading to the formation of methylol melamines is very similar to that of urea. The reaction mechanism of the acid-catalyzed condensation reactions of methylol melamines to form polymers and resins has been elucidated by Sato and Naito [4]. Melamine and formalde­hyde react similarly to urea and formaldehyde, although basic differences are evident in the reaction rates and mechanism. The primary products of reaction are methylolmela — mines, and evidence indicates that such compounds are formed only at ambient or higher temperature except in acid pH ranges. The reaction is reversible throughout the pH range. Its forward rate is proportional to either [melamine][HCHO] or [melamine][H+CHOH] or [melamine+][HCHO], according to the pH used.

Methylolmelamine forms ‘‘dimers’’ by condensation with melamine under neutral and acid conditions (70°C); this process is irreversible. The initial hydroxymethylation is very rapid. Its rate is determined by the condensation of conjugated acids of methylolme — lamines with melamine. The reaction rate is proportional to [melamine]2[HCHO] [5]. When the [mineral acid]/[melamine] ratio is 0.0 to 1.0, the early stage hydroxymethylation of melamine is dependent on the concentration of the melamine molecule (base species) MH and its conjugated acid MH3 in the following manner [6]:

rate = kH2O[MH][HCHO] + kH[MH^][HCHO] + kMH+[MH^][MH][HCHO]+

+kMH[MH]2[HCHO]

in the absence of added acid, when the ratio [mineral acid]/[melamine] is = 0, the rate of the reaction can thus be represented as

rate = kH2O[MH][HCHO] + WMH]2[HCHO]

The condensation reaction has been studied by investigating the kinetics of the initial stage of the condensation of di — and trimethylolmelamine (MF2 and MF3) in the pH range 1 to 9. Regardless of pH, the initial rate is equal to [4]:

rate = k[MFn]2 (with n = 2 or 3)

In the presence of mineral acid, the main reaction at the early stage of the condensation is the reaction between the methylolmelamine molecule and its conjugated acid (MFnH+) [7]. This was found at an [acid]/[MFn] (n = 2 or 3) ratio lower than 1.0 (pH 2.7). With an [acid]/[MFn] ratio higher than 1.0 to 1.2 (pH<2), the main condensation takes place between the conjugated acids themselves.

At equal pH values the condensation rate of trimethylolmelamine is considerably faster than that of dimethylolmelamine. This is the opposite of the rates of mono — and dimethylolurea. This means that while the nitrogen of the amido group in the case of urea is more reactive and therefore more nucleophilic than the nitrogen of the amidomethylol group, the opposite is true in the case of melamine. The reaction for MF2 is primarily between the carbon of the methylol group next to the nitrogen in HM+CH2OH, and the nitrogen of the amino group in MCH2OH. For MF3, the condensation is mainly between the carbon of the methylol group next to the charged nitrogen in H+MCH2OH, and the nitrogen of the aminomethylol group in MCH2OH [4]. The condensation rate therefore increases with the increasing electrophilicity of the carbon of the methylol group and the increasing nucleophilicity of the nitrogen of the amino group or aminomethylol group.

Therefore in MF3 the carbon in HM+CH2OH is more electrophilic than the same carbon in MF2. On the other hand, the nitrogen of the aminomethylol group in

HM+CH2OH of MF3 is less nucleophilic, and therefore less reactive, than the nitrogen of the amino group of MF2. The effects of the carbon and nitrogen atoms are consequently opposite to each other in the MFn condensation. Since the effect of the carbon is greater than the effect of the nitrogen on the reaction rate, MF3 condenses faster than MF2. At lower pH values the effect of the nitrogen becomes negligible and MF3 is even faster than MF2 in condensing to polymers.

The difference between the kinetic behavior of urea and melamine can be ascribed to the different effect of the nitrogen atom in the two compounds. With regard to the for­mation of methylol compounds as a result of hydroxymethylation, the functionality of melamine has been observed to be 6 against formaldehyde [1,8]. Similarly, melamine reacts easily with formaldehyde to form MF3; it also forms MF6 in concentrated formaldehyde [1,4,8]. For example, urea readily forms dimethylolurea, but forms trimethylolurea with marked difficulty [1,8] and never forms tetramethylolurea. These results suggest that the nitrogen of the amidomethylol group in methylolurea is considerably less nucleophilic than the nitrogen of the amido group in urea. However, the nitrogen of the aminomethylol group in methylolmelamine is not markedly less nucleophilic than the nitrogen of the amino group in melamine. Presumably, this is due to the difference in basicity between urea and melamine. The same is also true of their condensation reactions.