JPS5966424A - Preparation of water absorbing resin - Google Patents

Abstract

PURPOSE:To obtain a completely neutralized resin having improved water absorbing property, by adding an aqueous solution of a hydrogencarbonate of monovalent cation to an aqueous solution of polyacrylic acid having a specific neutralization degree or below (<100%) with the above-mentioned monovalent cation and aluminum ion. CONSTITUTION:Sodium hydroxide is added to neutralize an aqueous solution of polyacrylic acid, e.g. 1% concentration, and an aqueous solution of aluminic acid is added thereto to give an aqueous solution of polyacrylic acid having <100% neutralization degree with the monovalent cation and aluminum ion. A hydrogencarbonate of the above-mentioned cation, preferably sodium hydrogencarbonate, in an amount of a little excess of 100% neutralization degree is added to the aqueous solution to give the aimed water absorbing resin automatically as a polyacrylate neutralized at 100% neutralization degree.

Description

DETAILED DESCRIPTION OF THE INVENTION Water-absorbing resins made by crosslinking polyacrylic acid with aluminum ions have already been proposed, and the present invention relates to industrial production methods and improvements in performance thereof.

When polyacrylic acid salts are produced industrially, it is impossible to obtain a uniform degree of neutralization due to the non-uniformity of the component ratio of the raw materials used and the degree of polymerization. However, non-uniformity in the degree of neutralization affects the performance of the resin. In the water-absorbing resin, it is preferable that 100% of the remaining hydrogen ions cross-linked with an appropriate amount of aluminum oxide ions be replaced with monovalent cations, and it is also preferable that an alkali etc. be present in excess. However, there is a significant difference in the water absorption performance and stability of the ion aqueous solution. Hydrogen carbonates with monovalent cations easily decompose in the presence of acids and generate carbon dioxide gas, but the present invention focused on this phenomenon and investigated the degree of neutralization of polyacrylic acid with monovalent cations. It has been found that by adding less than 100% and then adding a slightly excess amount of bicarbonate to achieve a degree of neutralization of 100%, a 100% neutralized polyacrylate can automatically be obtained. In addition, it has been found that although hydrogen carbonate added in a slight excess remains in the resin as it is, the remaining hydrogen carbonate exhibits a buffering effect in ionic solutions, improving water absorption performance. The hydrogen carbonate to be used is preferably one that is stable in an aqueous solution and in the atmosphere, and an alkali metal salt is preferred, and sodium hydrogen carbonate was used in the preferred practical implementation of the present invention described below.

(Example) A 10% aqueous solution of polyacrylic acid polymerized using persulfate as a catalyst was diluted to make a 1% solution, and after neutralizing it to about pH 6 by adding sodium hydroxide, the solution was 4% based on the solid content of polyacrylic acid. Add part by weight of sodium aluminate dissolved in water, mix thoroughly, and after leaving for 10 hours or more, add a slightly excess amount of sodium bicarbonate until the pH becomes constant, then dry. The PH range is around 9,
This approximately matches the pH of sodium polyacrylate.

The amount of water absorbed by the water-absorbing resin obtained in this way in 1% saline is about 100 to 120 times its own weight, while the performance of the conventional resin is 220 times its own weight. However, it remains approximately constant. Furthermore, in highly concentrated ionized water, higher water absorption performance was obtained with a larger amount of sodium bicarbonate added, which is thought to be due to its buffering effect.

As described above, the method for producing a water-absorbing resin of the present invention is suitable as an industrial production method, and has excellent performance and economical efficiency.

Patent applicant Eslen Kako Co., Ltd.

Claims (1)

[Scope of Claims] A method for producing a water-absorbing resin, which comprises adding a hydrogen carbonate of the cation to an aqueous polyacrylic acid solution in which the degree of neutralization by the 01-valent cation and aluminum ion is less than 100%. (2) The method according to item 0, wherein the monovalent cation is sodium hydroxide and the hydrogen carbonate is sodium hydrogen carbonate.