Water-absorbent, high capacity polyurethane foams

Abstract: A water absorbent, relatively dimensionally stable foam is formed from a prepolymer prepared from a polyoxyethylene containing diol, a crosslinking agent and a polyisocyanate. The preferred polyisocyanate is methylene bis(cyclohexyl isocyanate). The foam also demonstrates the ability to hold substantial amounts of water, even under pressure, and can be used in various personal and medical care products such as diapers and surgical sponges.

Description: The present invention relates to water absorbent polyurethane foams having a high water capacity, processes for their preparation, and articles of manufacture comprising them. The prior art discloses various compositions and methods used to make polyurethane prepolymers and water absorbent polyurethane foams resulting from such prepolymers. Recently, there have been attempts to achieve highly water absorbent foams which also exhibit dimensional stability, e.g. ability to retain shape and rigidity, and hold the absorbed water even under pressure. See Non Wovens Market and Fiber Structures Report , p. 3, July 14, 1989. This ability to retain water under pressure also reflects the foam's load bearing characteristics. Further, materials possessing these characteristics have also been recognized as useful in products such as surgical dressings, diapers or bedpads. See U.S. Patent 3,939,123 to Matthews et al, Col. 1, lines 13-23. One known method for making prepolymers for water absorbent foams is to make the prepolymer with hydrophilic components such as polyoxyethylene oxide containing polyols. See U. S. Patent 3,903,232 to Wood and Frisch (equivalent to FR-A-2,261,293). However, when wet, such foams undergo a large amount of swelling and water plasticization of the polymer matrix. Further, Wood and Frisch disclose that they can compress their foams to form soft dense materials and upon exposure to water, regenerate them to the original volume, e.g. fifteen to twenty times expansion. See Column 2, lines 38-42 and lines 52-57. Because of this expansion and the accompanying swelling and plasticization, the Wood et al foams lose their structural rigidity and release substantial amounts of water when handled. Thus their ability to hold water is diminished. Also see U.S. Patent 4,160,076. (hydrophilic polyurethane sponge which absorbs six times its weight in water) More recent methods achieve a water absorbent foam by incorporating hydrophilic components into the prepolymer. For instance, U.S. Patent 4,738,992 to Larson and Hedrick discloses the incorporation of ionic hydrophilic functional groups, such as sulfonate groups, into a polyurea/urethane, isocyanate terminated prepolymer used to make water absorbent sponges. Of course, the hydrophilicity of the functional groups enhances the overall water absorption properties of the foam. See also U.S. Patent 4,638,017 to Larson and Hedrick. The addition of hydrophilic polycarbonyl moieties, e.g. those found on acrylates, to the polyurethane structure of a foam represents a similar approach to Larson and Hendrick's method. See U.S. Patent 4,725,628 to Garvey et al. See also U.S. Patent 4,725,629 to Garvey and Pazos. Water absorbent foams also have been made using hydrophobic materials. For instance, U.S. Patent 4,377,645 to Guthrie and Arquette discloses foaming prepolymers wherein isocyanate capped prepolymers are prepared from hydrophobic methylene diphenyl isocyanate, (MDI), trimethyolpropane and a polyoxyalkylene diol. While the foam resulting from the MDI terminated prepolymer only swells 19% by volume, it is only able to hold a little over 20 times its weight in water. These foams are suitable for household sponges and substrates for flocked wall coverings, as well as suitable for flexible foams used in medical or personal care items. See also U.S. Patents 4,384,050 and 4,384,051 to Guthrie. To enhance a foam's dimensional stability, additives have been used in making water absorbent foams. Typically, additives are also used to improve the durability and the load bearing characteristics of the foam. A popular method for achieving these characteristics is the addition of reinforcing agents. For instance, U.S. Patent 4,314,034 to Fulmer and Vollmerhausen discloses the addition of polyester fibers to provide structural rigidity. See also U.S. Patent 4,201,846 to Kehr et al. (polyvinyl alcohol fibers used for reinforcement in polyurethane foams) and U.S. Patent 3,959,191 to Kehr and Marans (addition of solid materials such as fibrous materials to improve load bearing characteristics.) However, the prior art approaches for achieving water absorbency and dimensional stability have disadvantages. For example, the super absorbent foams disclosed by Larson and Hedrick are made by the attachment of ionic groups to the urethane prepolymer and thus involves additional chemical processing to achieve the desired water absorbent properties. Garvey et al's use of the methacrylates results in foams which undergo undue swelling when wet. The foams made from MDI and reinforcing agents, while they have relatively good structural stability, do not have sufficient water capacity characteristics because the reinforcing agents increase foam density and deplete void space. As mentioned above, MDI-based foams without reinforcing agents were only able to hold water in an amount up to about twenty times their weight. US-A-4,137,200 discloses hydrophilic crosslinked polyurethane foam prepared from a blend of a monomeric polyol and polyoxyalkylene glycol (the -OH groups of which are capped by a polyisocyanate, such as toluene diisocyanate) and an aqueous reactant. GB-A-2,108,136 likewise discloses hydrophilic polyurethane foam, but which contain methylenediphenyl isocyanate or a derivative thereof, as the sole source of isocyanate. Accordingly, it is an object of the invention to obtain foams having improved water capacity properties. It is also an object of this invention to obtain a foam which holds water under pressure. It is an object of the invention to obtain foams with improved water absorbent properties, yet which undergo relatively little swelling. It is an object of the invention to provide a novel method to produce water absorbent foams. It is also an object to produce a novel water absorbent foam composition as well as an object to use the novel foams in articles of manufacture which require good water absorbent and wicking properties, as well as structural stability. These and other objects will be apparent from the following description. The present invention provides a water absorbent polyurethane foam having a water capacity of 30 to 50 grams of water per gram of foam and which retains substantial amounts of water under pressure which foam is derivable from an isocyanate group-containing prepolymer obtainable from (a) a polyoxyethylene-containing diol having a number average molecular weight of 2,500 or less, and comprising at least 50% by weight oxyethylene groups; (b) a crosslinking agent which is a polyol with molecular weight less than 1,000 and having 3 or 4 hydroxyl equivalents per mole, and (c) methylene bis(cyclohexyl isocyanate). The invention further provides a process for making a water absorbent foam having a water capacity of from 30 to 50g of water per gram of foam, which process comprises: (a) reacting (i) a polyol component comprising a polyoxyethylene-containing diol having a number average molecular weight of 2,500 or less and comprising at least 50% by weight oxyethylene groups, and a crosslinking agent which is a polyol having 3 or 4 hydroxyl equivalents per mole and a molecular weight of less than 1,000 with (ii) methylene bis(cyclohexyl isocyanate) to form a prepolymer and (b) then reacting the prepolymer prepared in (a) with water to form said foam. The invention further relates to an article of manufacture comprising a water absorbent polyurethane foam of the invention. Prepolymers used to make the foams of the invention are prepared using the well known prepolymer approach and can be made with polyoxyethylene containing diols and particular polyisocyanates. Crosslinking agents are also used to make the prepolymer. The resulting prepolymer is then foamed in water and surfactants. The polyoxyethylene containing diols used are those having an average molecular weight, i.e. number average, of 2,500 or less. The diol should also have at least 50%, preferably 80% and most preferably substantially 100% by weight, oxyethylene groups. By using a diol with higher oxyethylene content, hydrophilicity and, ultimately, good water absorption is maintained. A preferred diol is a polyoxyethylene diol from Union Carbide which has a molecular weight of between about 950 and 1,050 and is commercially available as Carbowax 1,000. However, other polyethylene glycol formulations having higher or lower molecular weights can also be used. As mentioned earlier, when a diol such as Carbowax 1,000 is used, a crosslinking agent is required to provide sufficient crosslinking and strength to the final foam. Typical crosslinking agents include low molecular weight polyols having 3 or 4 hydroxyl equivalents per mole. The crosslinking agents include aliphatic polyhydroxy alcohols and generally have a molecular weight of less than 1,000, preferably less than 500. Examples are trimethylolethane, trimethylolpropane, glycerol, triethanolamine, pentaerythritol or mixtures thereof. The preferred crosslinking agent is trimethylol propane (TMP). In general the molar ratio of diol to TMP should be in the range of about 1:1 to 4:1, with the preferred ratio being about 2:1 to 3:1. Of course, the higher the molecular weight diol used, the more crosslinker needed to get the proper crosslinking. By using the crosslinking agent in these proportions, sufficient hydroxyl equivalents are present for reaction with the isocyanate function groups of the polyisocyanates. Thus, the addition of the crosslinking agent to the diol and the polyisocyanate provides a prepolymer which has a number of branches and terminal isocyanates. The resulting terminal isocyanate groups are thus present in sufficient numbers to provide a relatively strong crosslinked foam. The isocyanate methylene bis(cyclohexyl isocyanate) is available as Desmodur W from Mobay, Corp. The polyol component, i.e. the diol and hydroxyl containing crosslinking agent, and the polyisocyanate component are preferably mixed at a ratio of about 1:1 by weight. Catalysts can also be used to make the prepolymer. Examples include organo tin compounds such as stannous octoate, stannous oleate, dibutyltin dilaurate and dibutyltin di-2-ethyl hexoate. Tertiary amine catalysts such as triethyl amines may also be used. Such catalysts are disclosed in Flexible Polyurethane Foams: Chemistry and Technology ; Woods, pp. 50-55, 1982, the contents of which are incorporated herein by reference. The amount of catalyst employed varies between 0.0005% and to about 0.1% by weight of catalyst to the weight of all components of the prepolymer, i.e. polyol, crosslinking agent and polyisocyanate. The prepolymer is formed, using the above mentioned proportions, at a temperature in the range of 50°C to about 90°C and a residence time in the range of 0.5h to 48h, depending on the amount and type of catalyst used. Preferably the reaction is maintained at about 70°C throughout the reaction. Further, during the prepolymer forming reaction, it has been helpful to measure the isocyanate level by titration periodically during the reaction. From these periodic measurements, one can determine the theoretical point at which all the hydroxyl groups of the polyol (and crosslinking agent) will have reacted with an isocyanate. If the theoretical point has not been reached, additional reaction time will be required. However, if the reaction is carried out too far, the viscosity of the resulting prepolymer increases to the point where it becomes difficult to mix the prepolymer with water. After the prepolymer is made, it is added to water to produce the foam. Surfactants can also be added to adjust the texture and appearance of the foam, i.e. such as the cell size, shape, etc., as well as preventing the formation of splits in the resulting foam buns. Examples of surfactants include silicone-polyether copolymer surfactants such as L-520 from Union Carbide and DC-198 from Dow Corning. Fluorocarbon copolymers such as the Zonyl (from Dupont) or Fluorad (from Minnesota Mining and Manufacturing) surfactant families can also be used. The polyether silicones are preferred when water absorbent foams containing small, fine cell-sized foams are desired. Polyoxyethylene/polyoxypropylene block copolymer surfactants such as Pluronic surfactants can also be used. The preferred Pluronic surfactants, e.g. L-62, L-92 and L-122, contain 20 percent by weight or less of polyoxyethylene and are preferably employed in 1-2% solution, the use of which is well known to one skilled in the art of foam formation. It is also often desirable to add about 0.5 to 2.0% by weight of an inorganic base to catalyze the foaming reaction for aliphatic isocyanate-based prepolymers. Sodium bicarbonate or sodium carbonate are two examples of such catalysts. The prepolymer and surfactant containing aqueous solution are then mixed in a high shear mixer in a volume amount ratio of prepolymer:water of about 2:1 to 6:1, with a ratio of about 3:1 preferred. The resulting foams typically have a low density and have been shown to be water absorbent. While the density of the foams obtained have been in the range of 0.020 to 0.060 preferably 0.020 to 0.040 g/cm³, higher density foams should also show good water absorbency. The foams demonstrate high water absorbency as measured by vertical wicking rates of up to about 3cms in one minute. The capillary wicking tests used are similar to those used to measure vertical wicking in paper products, e.g. the well known Klemm test. The wicking results for the foams tested are found in Table I. The water capacities of the foams are found in Table II and are expressed as the amount(s) of water absorbed per gram of foam. As indicated by the results in Table II, the invention shows improved water capacity than that of prior art foams, e.g. improvements of up to two times as much capacity. Examples F and G also indicate improvements in water absorption properties, e.g. wicking. In one embodiment, the foams of the invention retain from 30 to 50 grams of water per gram of foam, under a pressure of 1.4 kPa (0.2 psi). In a further embodiment, the foams of the invention retain from 20 to 30 grams of water per gram of foam. The foam's dimensional stability is evidenced by its load bearing characteristics and rigidity when wet; i.e. it does not collapse under its own weight as well as the weight of the water. Further when wet, the foam transforms from a flexible structure to one that is quite firm and undergoes only a relatively small amount of swelling after complete saturation. In addition, after it has been wetted and water has been absorbed, the foams show the ability to retain a substantial amount of water, even under pressure. See Table III. In view of the above properties, it is envisioned that the invention can be used in various personal and medical care areas. Personal products include incontinence pads, tampons, sweat bands and household sponges. Medical care products include surgical sponges, wound dressings, surgical drapes and mattress covers. The following examples illustrate the basic concepts of the invention, but are in no way to be used to limit the scope of the claims below. Example A - Prepolymer 1 (P1) A mixture of 500g (0.5 moles) of Carbowax 1000 and 33.5g (0.25 moles) of trimethylolpropane was degassed by heating for two hours @ 70°C and about 266 Pa (2 Torr). To this was added 542g (2.07 moles, 18% excess) of Desmodur W, i.e. methylene bis(cyclohexyl isocyanate). The temperature was raised to 70°C and 0.5g of stannous octoate was added as catalyst. After 35 minutes, the isocyanate content of the reaction product was 2.19 meq/g and the product was poured into a bottle for storage. After a few days at ambient temperature, the isocyanate content and viscosity had stabilized at 2.00 meq/g and 730 Pas (73,000 cp) at 25°C. Example B - Foam 1 (F1) A foam was made from P1 by mixing 100g of P1 with 33g of water containing 1.6g of sodium bicarbonate and 0.7g of Dow Corning's polyether-polysiloxane surfactant DC-198. The dried foam had a fine-celled structure and a density of 0.037 g/cc. After soaking in water, the foam contained approximately 35 times its weight of water and its volume had swelled by 50%. Upon application of 1.4 kPa (0.2 psi), the foam retained 34g water per gram of foam. See Tables II and III. Example C - Prepolymer 2 (P2) In another illustration, three batches of a prepolymer were made by mixing 1000g (1.0 mole) of Carbowax 1000 and 67.0g (0.5 mole) of trimethylolpropane and degassed as in the previous example. To this mixture was added 1084 g (4.14 moles) of Desmodur W, the temperature was raised to 70°C and 1.0g of stannous octoate was added. Of the several preparations made, each had an isocyanate content of 2.15-2.25 meq/g at the completion of the reaction. The products were poured into bottles for storage as described in Example A. Example D - Foam 2 (F2) Approximately two quarts of P2 were loaded into the "prepolymer (isocyanate) side" of a UNIPRE Type M-10/2 two-component meter, mix, and disperse system equipped with two five-liter stainless steel tanks, each tank connected to a metering pump drive unit controlled by two separately adjustable electric motors driving independent metering pumps, connected by means of teflon-lined hoses to a high capacity mixing head containing a high-shear dynamic mixer containing a rotor and stator. (Ashby-Cross Co., Inc., Topsfield, MA). Approximately two quarts of an aqueous solution containing two wt% Pluronic L-62 surfactant and 2% sodium bicarbonate was used. By a computer controlled metering system a 3:1 (volumetric) ratio of prepolymer-to-aqueous was metered through the mix head and into an open container, where the foaming reaction proceeded. The dry foam had a density of 0.030 g/cc and had a very fine celled structure composed primarily of open cells. Immersion of the foam into an aqueous saline solution led to sinking of the foam into the water and an immediate retention of 38-40 g saline solution/g polymer-foam in three different samples. A volume expansion of about 40% was observed after immersion. After gentle application and releasing of pressure on the foam while submerged in water, the samples of foam were able to hold 41-45g of water per gram of foam. Upon application of 1.4 kPa (0.2 psi), 40-43g of water per gram of foam remained and upon application of 6.9 kPa (1 psi) 22g-29g of water per gram of foam remained. See Tables II and III. Example E - Foam 3 (F3) P2 was again used to make a foam except an aqueous phase of 2% Pluronic L62 and 2% sodium bicarbonate and a 2:1 volumetric ratio of prepolymer:aqueous as controlled by the computer was used. When dry, the foam had a density Of 0.050 g/cc and a higher portion of open cells than the foam of Example D, e.g. F2. After submersion the foam contained 31g of water per gram of foam. At 1.4 kPa (0.2 psi) the foam retained 30-31g of water per gram of foam. This foam exhibited vertical wicking of saline solution of 2.0-2.7 cm in 1 minute, 4.7-5.2 cm in 20 minutes, and 8 cm in three hours. Example F A foam (C1), was produced according to Example 7 of U.S. Patent 4,377,645. The procedure of that Example is as follows: Prepolymer was produced using a mixture of Carbowax 1000 (412g, 0.4 mole) and trimethylolpropane (9.05g, 0.0675 mole) dried by heating for two hours at 70°C under a reduced pressure of 266 Pa (2 Torr). To the dried and degassed polyol mixture was added 507.5g of Isonate 143L (3.55 equivalents of isocyanate): Isonate 143L is a methylene bis(phenyl isocyanate) based isocyanate product of Upjohn Polymer Chemicals having an isocyanate functionality of approximately 2.1. The temperature was maintained at 70°C for 2 1/2 hours, after which time the isocyanate content of the product was found to be 2.67 milliequivalents per gram. The prepolymer weighed 928g. It had an initial viscosity at 25°C of 19 Pas (19,000 cp). After a storage test for two weeks at 80°C, the viscosity had increased only to 30 Pas (30,000 cp). The production of a foam using this prepolymer was as follows: To a beaker were added 700g of prepolymer heated to 35°C and 686g of water heated to 35°C. They were stirred together for fifteen seconds with a drill motor and then poured into a mold. A dried foam product was obtained. A piece of foam weighing 2.6 g was allowed to float on water for one day. It was observed that only the bottom surface was wet and that no wicking had occurred in the foam's interior. The foam was then compressed under water to expel as much air as possible. The foam was then removed from the water and allowed to drain for five minutes. It was reweighed and was found to hold 23.4 g of water per gram of foam (See Table III) and observed that the foam had expanded 19% in volume. When squeezed, it released the water easily due to the softness of the foam, but required several minutes to return to its original wet dimensions because of its lack of resilience in the wet state. Example G A prepolymer produced according to Example F (i.e. using the procedure of U.S. Patent 4,377,645) was converted into a foam (C2) by mixing the prepolymer on an Ashby-Cross Foam Machine with an equal volume of water containing 2% by weight of Pluronic 25-R2 as a surfactant. After drying, the foam had a density of 0.047g/cc and has cells somewhat smaller than C1 of Example F. A piece of the resulting foam weighing 2.6g was floated on water and required three hours to become saturated. It held 22.8g of water per gram of foam (See Table III) and its volume expanded by 19%. When wet, C2 was much like C1, but C2 was somewhat harder to squeeze because of its finer cell structure. Recovery of original dimensions required even more time to return to its original dimensions than C1. Water Absorbency Foam Wicking Height (cm) Elapsed Time (min) F2 1.2 1 2.4 20 F3 2.0-2.7 1 4.7-5.2 20 8.0 180 Foam Water Capacity (O kPa,) (g water/g foam) F1 35 F2 41-45 F3 31 C1 23 C2 23 Foam Pressure [kPa (psi)] Water Retention g water/g foam F1 1.4 (0.2) 34 F2 1.4 (0.2) 40-43 F2 6.9 (1.0) 22-29 F3 1.4 (0.2) 30-31

Claims: A water absorbent polyurethane foam having a water capacity of 30 to 50 grams of water per gram of foam and which retains substantial amounts of water under pressure, which foam is derivable from an isocyanate group containing prepolymer obtainable from (a) a polyoxyethylene-containing diol having a number average molecular weight of 2,500 or less, and comprising at least 50% by weight oxyethylene groups; (b) a crosslinking agent which is a polyol with molecular weight less than 1,000 and having 3 or 4 hydroxyl equivalents per mole, and (c) methylene bis(cyclohexyl isocyanate). A foam according to claim 1 wherein the diol comprises at least 80% by weight oxyethylene groups. A foam according to claim 2 wherein the polyoxyethylene-containing diol is polyethylene glycol. A foam according to any one of the preceding claims wherein the crosslinking agent is trimethylolpropane. A foam according to any one of the preceding claims which has a density of from 0.020 to 0.060g/cm³. A foam according to claim 5 which has a density of from 0.020 to 0.040g/cm³. A foam according to any one of the preceding claims which is derivable from a prepolymer obtainable by reacting the polyol component (i) and the isocyanate component (ii) in a ratio of about 1:1 by weight. A foam according to any one of the preceding claims which is derivable from a prepolymer wherein the molar ratio of diol:crosslinking agent is from 1:1 to 4:1. A foam according to claim 8 wherein the molar ratio is from 2:1 to 3:1. A foam according to any one of the preceding claims which absorbs water to a vertical wicking height of about 1.2cm in one minute and about 2.4cm in twenty minutes. A foam according to any one of claims 1 to 10 which absorbs water to a vertical wicking height of 2.0-2.7cm in one minute, 4.7-5.2cm in twenty minutes and about 8cm in three hours. A foam according to any one of the preceding claims which retains 30-50 grams of water per gram of foam under 1.4 kPa (0.2 psi). A foam according to any one of the preceding claims which retains 20-30 grams of water per gram of foam under 6.9 kPa (1.0 psi). A process for making a water absorbent polyurethane foam having a water capacity of from 30 to 50 grams of water per gram of foam, which process comprises: (a) reacting (i) a polyol component comprising a polyoxyethylene-containing diol having a number average molecular weight of 2,500 or less and comprising at least 50% by weight oxyethylene groups, and a crosslinking agent which is a polyol having 3 or 4 hydroxyl equivalents per mole and a molecular weight of less than 1,000 with (ii) methylene bis(cyclohexyl isocyanate) to form a prepolymer and (b) then reacting the prepolymer prepared in (a) with water to form said foam. A process according to claim 14 wherein sodium bicarbonate or sodium carbonate is used in step (b) as a foaming catalyst. A process according to claim 14 or 15 wherein a surfactant is added in the foam forming step (b). A process according to any one of claims 14 to 16 wherein stannous octoate is used in step (a) as a catalyst for the formation of prepolymer. A process according to any one of claims 14 to 17 for the preparation of a foam as claimed in any one of claims 2 to 13. An article of manufacture comprising a water absorbent polyurethane foam as claimed in any one of claims 1 to 13.

Inventors: GRASEL TIMOTHY GERARD, GUTHRIE JAMES LEVERETTE, GRASEL, TIMOTHY GERARD, GUTHRIE, JAMES LEVERETTE
Applicants: HAMPSHIRE CHEMICAL CORP, HAMPSHIRE CHEMICAL CORPORATION
Application year: 1990
Main classification: C08G18
IPC codes: C08J9, C08G18, C08G101