Continuous process for manufacturing superabsorbent polymer

#The present invention is directed to a continuous process for making water-insoluble, crosslinked, high molecular weight polymers capable of absorbing and retaining large quantities of aqueous fluids (SAP). These polymers are well known in the art by various names such as superabsorbent polymers, hydrogels, hydrocolloids, and water absorbent hydrophilic polymers, etc. For the purpose of this description the term, “SAP” means superabsorbent polymer, collectively referring to such materials.

#Exemplary superabsorbent polymers include crosslinked, partially neutralized polyacrylic acid (see U.S. Pat. No. 4,654,039); crosslinked, partially neutralized starch-acrylic acid graft polymer (U.S. Pat. No. 4,076,663); crosslinked, partially neutralized copolymer of isobutylene and maleic anhydride (U.S. Pat. No. 4,389,513); saponification product of vinyl acetate-acrylic acid copolymer (U.S. Pat. No. 4,124,748); hydrolyzate of acrylamide polymer or acrylamide copolymer (U.S. Pat. No. 3,959,569); or hydrolyzate of an acrylonitrile copolymer (U.S. Pat. No. 3,935,099). The teachings of the above patents as embodiments of superabsorbent materials are hereby incorporated by reference.

#Superabsorbent polymers find use in many fluid absorption applications with the primary use being in the field of personal care products such as diapers, sanitary napkins, adult incontinent products, absorption pads for medical uses, etc. The largest market for superabsorbent polymers is found in disposable diapers for infants; see e.g. U.S. Pat. Nos. 3,669,103; 3,670,731 or 4,654,039.

#A vertically oriented reactor having a nominal working capacity of 200 liters and an aspect ratio of 3/1 is used in this example. The reactor contains no internal components such as baffling or agitation. The reactor is insulated to maintain the temperature developed during the course of operation. Previous experimentation has left a 90% residual volume of SAP polymer in the reactor, which is the required starting condition for running continuously. The top or head of the reactor is configured with valved opening or ports to 1) enable reactant feeds to be added on a continuous basis, 2) stripped oxygen to be vented continuously, and 3) the mounting of temperature and pressure measuring devices to monitor processing conditions. The bottom of the reactor is configured with a double discharge screw to continuously remove the reacted polymer. In this example there are three reactant components that will be metered into the reactor on a continuous basis. The reactants are combined into a common junction point just prior to entering into the reactor. Additionally, nitrogen is added at this junction point to facilitate removal of dissolved oxygen contained in the reactant feeds. The reactant feeds are 1) monomer feed, consisting of acrylic acid, pentaerythritol triallyl ether crosslinker, and deionized water, 2) hydrogen peroxide and deionized water, and 3) ascorbic acid, V-50 (Wako) and deionized water. The hydrogen peroxide and ascorbic acid function as a redox initiation couple. For this case the monomer concentration is prepared to achieve a 32% acrylic acid concentration in the reactor. In this example, a reactant residence time of 3 hours was selected. The three reactant components are metered into the reactor at a temperature of 10±2° C. and at a feed rate to achieve the required residence time. Simultaneously with the beginning of the addition of reactants to the reactor, polymer is removed by the discharge screw at an equivalent rate. Process control (i.e. residence time) is maintained by controlling the polymer level in the reactor at 90% (180 liters) of the working capacity of the reactor. This is achieved by a load cell upon which the reactor is mounted. Similarly, oxygen removed by the nitrogen stripping is removed from the vapor zone in the top of the reactor. Pressure control is maintained at 3.4 bars. As pressure exceeds 3.4 bars, the pressure is vented to remove the oxygen containing vapors. The 3.4 bar pressure in this example is sufficient to advance the polymer toward the discharge screw.

#Polymer removed from the reactor had an absorbency of 32.2 g/g as measured by the Centrifuge Retention test method. This polymer was subsequently surfaced crosslinked and was retested and found to have a 27.0 gig Centrifuge Retention and a 24.0 gig 0.9 Absorbency Under Load (0.9 AUL).

#A vertically oriented reactor having a nominal working capacity of 200 liters and an aspect ration of 3/1 is used in this example. The reactor contains no internal components such as baffling or agitation. The reactor is insulated to maintain the temperature developed during the course of operation. Previous experimentation has left an 80% residual volume of polymer in the reactor, which is within the required starting range for running continuously. The top of the head of the reactor is configured with valved openings or ports to 1) enable reactant feeds to be added on a continuous basis, 2) stripped oxygen to be vented continuously, and 3) the mounting of temperature and pressure measuring devices to monitor processing conditions. The bottom of the reactor is configured with a double discharge screw to continuously remove reacted polymer. In this example there are three reactant components that will be metered into the reactor on a continuous basis. The reactants are combined into a common junction point just prior to entering the reactor. Additionally, nitrogen is added at the junction point to facilitate removal of dissolved oxygen contained in the reactant feeds. The reactant feeds are 1) monomer feed consisting of acrylic acid and pentaerythritol triallyl ether, a crosslinker, and deionized water, 2) hydrogen peroxide and deionized water, and 3) ascorbic acid, V-50, a thermal initiator and deionized water. The hydrogen peroxide and ascorbic acid function as a redox initiation couple. In this case the monomer concentration is prepared to achieve a 46% acrylic acid concentrate in the reactor. In this example, a reactant residence time of 3 hours was selected. The three reactant components are metered into the reactor at a temperature of 10±2° C. and at a feed rate to achieve the required residence time. Simultaneously with the beginning of the addition of reactants to the reactor, polymer is removed by the discharge screw at a rate equivalent to the rate of monomer addition. Process control (i.e. residence time) is maintained by controlling the polymer in the reactor at 80% (160 liters) of the working capacity of the reactor. This is achieved by a load cell upon which the reactor is mounted. Similarly, oxygen removed by the nitrogen stripping is removed from the vapor zone in the top of the reactor. Pressure control is maintained at 3.4 bars. As pressure exceeds 3.4 bars, the pressure is vented to remove oxygen containing vapor. Polymer removed from the reactor had an absorbency of 28.6 g/g as measured by the Centrifuge Retention test method. This polymer was subsequently surfaced crosslinked and was retested and found to have 24.9 g/g Centrifuge Retention and a 20.0 gig 0.6 Absorbency Under Load (0.6 AUL).