Some studies on starch-urea-acid reaction mechanism

Starch reacted with urea in the absence or presence of mineral acids or mineral acid salts at 150degreesC for various reaction durations. The acids used were phosphoric, nitric, and sulphuric and the salts used were ammonium sulphate, ammonium nitrate, ammonium chloride and magnesium sulphate. The reaction extent was followed by monitoring nitrogen content. Carrying out the reaction of starch and urea in the presence of mineral acid or mineral acid salt gives rise to higher extents of reaction and in turn, products with higher nitrogen content and higher solubility %. The extent of reaction depends upon the nature and concentration of added acids or salts as follows: (a) sulphuric acid > nitric acid > phosphoric acid; (b) ammonium sulphate > ammonium nitrate > ammonium chloride > magnesium sulphate. The solubility of reaction products attains a value of 100% on using acids while it never exceeds 68% on using the mineral acid salts within the studied range. The nitrogen content of reaction product prepared in the presence of mineral acids is higher than the theoretical value in its absence, if all urea reacts with starch specially at longer durations indicates that there are side reactions which lower ammonia liberation. Based on detailed investigation of the starch-urea-phosphoric acid system through monitoring the nitrogen and phosphorus contents as well as the liberated ammonia, it is envisaged that the reaction products comprises of starch carbamate, starch carbarnate phosphate, starch phosphate and ammonium phosphate. It is further envisaged that the acid performs two main functions. Firstly, it prevents the amide groups of starch carbamate from reacting with adjacent amide or hydroxyl groups of the modified starch molecule through blocking these amide by the acid thereby ceasing crosslinking. Secondly, it degrades starch molecule via hydrolysis thereby lowering the molecular size of starch that enhance its solubility. (C) 2002 Elsevier Science Ltd. All rights reserved.