| 摘要 |
<p>Infusible and insoluble melamine-formaldehyde condensation products having a mean particle size <1m are prepared by reacting an aqueous solution of melamine and formaldehyde or an aqueous solution of a melamine-formaldehyde precondensate, in the presence of a protective colloid in the pH range 6 to 8 until a solid phase is formed. Reaction is preferably carried out at elevated temperature, e.g. at the boiling point of the solution. 1.5 to 6 mols. of HCHO may be used per mol. of melamine. Other aminoplast forming compounds (e.g. urea and its derivatives and other amino-triazines) may also be present. Examples of protective colloids are starch, tragacanth, gelatine, agar,-agar, polyvinyl alcohol, the sodium salt of carboxmethylcellulose and a hydrolysed polymer of maleic anhydride and styrene. The amount of protective colloid used may be 0.1 to 10% by wt. based on the total amount of aqueous reaction mixture. The starting solution may contain from 5 to 25% by wt. of resin-forming constituents. The precipitation reaction may be carried out at the boiling point of the reaction mixture.</p> |
