JPH0260681B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a water-swellable substance that maintains high water absorption capacity and has reversible water absorption and water release properties. The polymerization methods and physical properties of acrylic acid, alkali metal acrylates, and ammonium acrylates are already well known, and depending on the polymerization conditions, water-insoluble or water-swellable polymers can be produced. However, these are recognized as phenomena associated with abnormal reactions during polymerization, and no effective proposals have been made for the purpose of producing swellable polymeric substances with high water absorption capacity. . Generally known methods for obtaining high-performance swelling polymers include a method using a crosslinking agent such as divinylbenzene or methylenebisacrylamide, and a method derived from an ester copolymer. Completely neutralized polyacrylic acid with an appropriate degree of crosslinking exhibits a maximum swelling rate of over 1000 times in deionized water, but conventional methods have low productivity and are not economical, as well as the use of crosslinking agents. The use of such materials may pose health and safety problems. The present invention provides a method for producing a self-crosslinking swellable material by neutralizing and polymerizing acrylic acid monomers, which is free from safety and health problems and is highly productive and economical. During polymerization of acrylic acid, if the polymerization temperature rises abnormally, crosslinking occurs, resulting in a water-insoluble polymer. The present inventor has demonstrated that a polymer obtained by polymerization in the presence of an appropriate amount of monovalent cations exhibits water-swellability, and a polymer obtained by polymerization in the presence of a large amount of monovalent cations exhibits water-soluble properties. This fact indicates that monovalent cations control the crosslinking of polyacrylic acid. The present inventor discovered that when ammonium ions are present in an acrylic acid aqueous solution, the pH is adjusted to be maintained in the range of 6 to 9, and polymerization is carried out so that the peak temperature during the polymerization reaction is 106°C or higher, the water-swelling property It was discovered that a polymer can be obtained, and the method of the present invention was developed. In the method of the invention, ammonium, i.e. ammonium hydroxide, ammonium carbonate, ammonium bicarbonate or other salts are used to neutralize acrylic acid;
Adjust the pH of the acrylic acid aqueous solution to be in the range of 6 to 9. Furthermore, mainly ammonium compounds,
The pH of the aqueous solution can also be controlled by adding an alkali metal hydroxide, carbonate or bicarbonate as a pH adjuster. In the method of the present invention, the polymerization temperature is set at 106°C or higher,
As the polymerization temperature increases, the molecular weight of the polymer decreases, so it is controlled to 200°C or less, and for practical purposes, the polymerization temperature is kept within 160°C. The heat of polymerization of acrylic acid is 18.5Kcal/mol
(at 25°C), and the heat of polymerization of sodium acrylate also shows a slightly lower level of heat generation, so in order to maintain the polymerization temperature within a constant temperature range, it is necessary to perform controls such as removing the heat of polymerization. When polymerizing acrylic acid in an aqueous solution system, the polymerization temperature is controlled by the balance between the latent heat of vaporization of water in the polymerization system and the amount of heat generated by polymerization of acrylic acid. In other words, the larger the amount of acrylic acid monomer in the polymerization system, the larger the amount of polymerization heat generated, and the temperature tends to increase.
The amount of latent heat of vaporization of water increases, which tends to prevent temperature rise. However, changes in the pressure within the polymerization system affect the evaporation temperature of water and, by extension, the temperature of the polymer, and the degree of influence of external temperature also varies depending on the shape of the polymerization reactor, so that water evaporates in the reaction system. The amount of water condensed and refluxed, the amount of heat dissipated to the outside, etc. are affected, which in turn significantly affects the polymerization temperature. The appropriate polymerization temperature varies depending on the water retention performance required of the polymerization product and the PH range of the polymerization aqueous solution.
By selecting the temperature within the range of 106°C to 160°C, a high-performance polymer can be stably produced. However, in the method of the present invention, the amount of acrylic acid, the amount of ammonium compound, and the amount of water in the polymerization solution are extremely closely related to the peak polymerization temperature reached during polymerization, and a simple method of selectively adjusting them can be used. By controlling the peak polymerization temperature, a polymer having desired swelling performance can be obtained. In the method of the present invention, initiators such as benzoyl peroxide, acetone peroxide, and azobisisobutyronitrile, redox catalysts such as peroxides, persulfates, and sulfites, iron, tin nickel, magnesium, etc. Using selected catalysts. Furthermore, in the method of the present invention, a monomer such as acrylamide is added to an aqueous monomer solution to be polymerized, and this is polymerized.
The method described in the patent application specification No. 108362, in which salts of alkaline substances are added and polymerized, is also effective. In addition, when carrying out the method of the present invention, by impregnating a base material such as paper or fiber with a monomer aqueous solution to be polymerized and then polymerizing it, a composite of paper, fiber, etc. and a swellable substance can be easily produced. Can be done. In carrying out the method of the present invention, there is a discontinuous method in which polymerization is completed in a single reaction vessel, and a discontinuous method in which polymerization is completed while a plurality of reaction vessels are used and the polymerization liquid is continuously moved through these reaction vessels. In addition to the method of pressing, a method of using a heated roll, a method of using a belt conveyor, etc. can be adopted. In a continuous polymerization method using a belt conveyor, monomer and added salt solutions are supplied from each tank to a solution tank while adjusting the mixing ratio to a predetermined ratio using a flow rate adjustment valve, and a soluble catalyst is supplied in advance as a solution from the solution tank. Add to the solution while adjusting the flow rate. The solution to which the catalyst has been added is supplied from the final stage solution tank onto the belt conveyor at a predetermined flow rate by a flow rate adjustment valve. The polymerization process area on the belt conveyor can be surrounded by an outer cover, and in this case, unreacted monomers and water (both in the vapor phase) in the outer cover are collected by suction and transferred to the final stage solution tank or on the belt conveyor. can be returned to the solution supply. It is sufficient to apply the heat necessary for starting the polymerization by an appropriate means only at the start of the heat operation, and once the polymerization starts, heat of polymerization is generated, so there is no need to supply heat from the outside. Once the polymerization is completed on the belt conveyor, the polymer may be transferred to the next drying or cooling step, if necessary. Since conditions can be set so that drying by heating is not necessary, dehydration can be achieved simultaneously with cooling by using a cooling process, surrounding this process area with an outer cover, and creating a negative pressure state by drawing air from inside. The solution bath can be cooled if necessary. Further, the water content can be adjusted in advance by adjusting the concentration of the added salt solution, but only water may be supplied separately. In the polymerization method using a belt conveyor as described above, the amount of polymerization solution in the reaction system is controlled by adjusting the supply flow rate of the solution onto the belt conveyor (controlling the flow rate adjustment valve) and adjusting the moving speed of the conveyor belt. can be determined.
Furthermore, if a product in which paper or fiber is impregnated with a polymer is required, the impregnated material may be placed on a belt. Of course, a predetermined amount of solution may be placed in a container and polymerized while being transferred by a belt conveyor, or a pan conveyor in which containers are connected in an endless manner may be used instead of the belt conveyor. The method of the invention can also be carried out by spray polymerization methods. In this case, an endless belt or roller can be rotated in a closed tank, the sprayed solution can be attached to the belt surface or roller surface, and after polymerization, it can be carried out of the tank and taken out with a scraper. Further, the inside of the tank is heated to bring the temperature of the polymer to a required temperature, and the temperature can be easily controlled by heating the belt or roller. The polymerization conditions and water absorption performance of the products in Examples and Comparative Examples for clarifying the embodiments of the patented invention are shown in Tables 1 and 2, respectively. The present invention is not limited to these examples.

【table】

【table】

[Table] In the results of the Examples, there were generally many products with good water absorption performance when the polymerization temperature was relatively low. Regarding the water absorption performance of electrolyte-containing aqueous solutions, polymers with low pH during polymerization were generally better than polymers with high pH. Polymers obtained by polymerizing a polymer solution with a pH around neutrality at high temperatures generally have a high water absorption rate, and this seems to be related to the degree of polymerization. Another method for improving the water absorption swelling rate of a polymer is to mix a fatty acid ester into the polymer as appropriate. As described above, the method of the present invention adds an appropriate amount of ammonium compound and water to acrylic acid monomer to obtain a high-performance swellable polymer material in a simple manner, and can be done safely and with high productivity using simple manufacturing equipment. It can be operated. In addition, ammonium polyacrylate is highly reliable in terms of safety and toxicity, and the polymer material produced by the method of the present invention, which does not require the addition of a crosslinking agent and has a similar chemical structure, can be used in the medical industry, food industry, agriculture, etc. horticulture field,
It can be used in a wide range of other fields such as industrial fields and consumer goods. The method of the present invention is thus expected to be widely used in various industrial fields as water absorbing agents, water retention agents, humidity regulators, fireproofing/extinguishing agents, flocculants, wastewater treatment agents, ion exchange agents, etc. It provides extremely useful substances industrially.

Claims (1)

[Claims] 1. A solution of ammonium and water added to acrylic acid is polymerized at a pH of 6 to 9 at a polymerization temperature of 106°C to
A method for producing a swellable substance, which comprises adjusting the amount of each component and the amount of a polymerization solution so that the temperature is within the range of 160°C, and polymerizing in the absence of a crosslinking agent. 2. The method according to claim 1, wherein the ammonium is ammonium hydroxide, ammonium carbonate or ammonium bicarbonate. 3. The method according to claim 1 or 2, wherein an alkali metal hydroxide, carbonate or bicarbonate is added to the solution as a pH regulator.