Polyurethane foams, formed by the reaction of a polyisocyanate with a polyhydroxyl-containing compound in the presence of a suitable catalyst, are widely accepted as padding materials for cushions in furniture, automobiles and the like. Polyurethane foams are also used in sponges and for other uses that require liquid absorption properties, such as personal care and hygiene items, and also for specialty packaging.
The art and science of producing polyurethane foams involves controlling polymerization while liberating a blowing agent to produce a cellular mass. The gas that blows the foam also fills the polyurethane foam cells. Commonly used blowing agents are carbon dioxide, fluorocarbons and methylene chloride. The resultant density of a polyurethane foam is controlled by the quantity and efficiency of the blowing agents. While carbon dioxide may be generated as a blowing agent through the reaction of water with the isocyanate, the use of low-boiling inert liquids, in particular fluorocarbons, to augment or replace the chemical blowing action has lead to certain property advantages in the final foams, such as low thermal conductivity characteristic of the trapped fluorocarbon gas.
However, the chlorofluorocarbons (CFCs) used as blowing agents, and for other purposes, are now suspected to be linked to the depletion of ozone in the upper atmosphere where the generally inert CFCs are ultimately decomposed by ultraviolet light. To avoid this problem, polyurethane resins blown only with carbon dioxide from the reaction with water have acquired renewed interest.
Rates of reactions of the foaming ingredients are controlled by the nature of the catalysts employed. It is recognized that tertiary amines are added to control the reaction of water and isocyanate to liberate carbon dioxide. Polymerization of polyol and isocyanate is catalyzed by metal salts, in particular, tin salts as an example. In order to produce a good foam, the rates of polymerization and blowing must be carefully balanced by adjusting the amounts of tertiary amines and tin catalysts.
In addition, the major parameters regulating flexible polyurethane foam hardness are both the relative amount of each segment in the hard segment/soft segment ratio and their distribution in the polymer chain. A balance of hard and soft segments is necessary to produce the optimum physical properties.
A typical formulation for a flexible foam would consist of 100 parts of a polyol for the soft segment and 3 parts of water for the hard segment and CO.sub.2 generation. From the stoichiometry, it is apparent that at water levels of 3 parts and above that the hard segment becomes a significant percentage that overpowers the softening characteristics of the polyols. Because of the high ratios of water in the formulation, the water-isocyanate reaction takes place very rapidly.
Many attempts have been made to control the water-polyolisocyanate reactions in order to improve the quality of the finished product. For example, to slow down the rate of the water-isocyanate reaction, amine salts are often used as catalysts. A specific example is the formate salt of triethylenediamine. The cream time (water reaction) is delayed slightly to permit the polymerization reaction (polyol) to proceed.
It has been suggested to use hydrated salts in polyurethane formulations. For example, German Patent No. 947,833 describes apparently rigid porous foamed materials obtained from the reaction products of diisocyanates with high molecular weight polyesters containing at least two groups capable of being substituted by isocyanate radicals. The addition of salts containing water of crystallization or surfactants having adsorbed water to the above mixture generates carbon dioxide which acts as the blowing agent. These salts and surface-active substances are used as propellants instead of water or low-molecular organic hydroxyl compounds. Examples for such propellants are borax containing water of crystallization and humid activated carbon.
U.S. Pat. No. 3,169,826 relates to a method of preparing a hydrated magnesium carbonate, and incidentally mentions the particular salt produced liberates water at a relatively low rate and at a temperature below about 100.degree. C. to thereby control the rate of foaming during the reaction between polyesters and polyisocyanates to produce a polyurethane foam. Finely divided man-made calcium sulfate dihydrate was found to be particularly useful in the production of cellular polyurethanes because it liberates water over a narrow temperature range, according to British Patent No. 1,214,478.
Also of interest is the abstract to Japanese Kokai Patent Document No. 58-67,713 (Chemical Abstracts 99:213682e, 1983) which describes rigid polyurethane foams made from isocyanate-terminated prepolymers and compounds containing water of crystallization. Specifically mentioned is a mixture of Na.sub.2 SO.sub.4.10H.sub.2 O and Na.sub.2 SO.sub.4.
Additionally of note is U.S. Pat. No. 4,882,363 which discusses a process for the production of fluorocarbon-free, rigid polyurethane foams from a reaction mixture based on (a) a polyisocyanate component containing at least one aromatic polyisocyanate, (b) a polyhydroxyl component, (c) blowing agents, which may be water and/or carbon dioxide, (d) catalysts and (e) zeolite adsorbents. The zeolitic absorbents may be synthetic faujasite zeolites having the general formula EQU (M.sup.I, M.sup.II.sub.0.5).sub.2 O.Al.sub.2 O.sub.3.y SiO.sub.2.z H.sub.2 O
in which M.sup.I represents sodium or potassium cations; M.sup.II represents calcium or magnesium cations; y has a value of 2 to 6, those of the x type having a y-value of 2 to 3 and those of the y type having a y-value of 3 to 6; and z represents 0 or a number of up to 5.5 (x type) or 0 or a number of up to 8 (y type).
Of lesser importance is Japanese Patent Application No. 28427/75 which describes a process for preparing polyurethane foams by adding a foaming agent such as a fluorocarbon, and an amine and/or organotin compound as a catalyst to a polyol and an isocyanate, characterized by adding at least one of hydrous alkali, metal compounds thereto. These hydrous alkali metal compounds include sodium borates, sodium phosphates, sodium carbonates, alum, etc. These materials are used as co-catalysts with no expectation or suggestion that the foaming reaction is affected. Fluorocarbons are still used as a blowing agent.
Additionally, there is German Patent No. 1,173,240 which relates to the use of known zeolite molecular sieves charged with amine catalysts in a process for the preparation of polyurethane foams. An inert, hydrated salt is used together with the tertiary amine forming catalyst and finely divided crystalline molecular sieves having a diameter of not more than 20 .ANG.-units. The inert hydrated salts for the German Patent release water at a temperature of approximately 50.degree. to 150.degree. C., such as crystallized barium chloride (BaCl.sub.2.2H.sub.2 O), crystallized manganous chloride (MnCl.sub.2.4H.sub.2 O), crystallized calcium sulfite (CaSO.sub.3.2H.sub.2 O) or crystallized calcium lactate ((CaC.sub.3 H.sub.5 O.sub.3).sub.2.5H.sub.2 O). It is noted that sodium salts or potassium salts are less suited in this process.
However, some of the formulations of these documents do not avoid the use of fluorocarbons and many do not broadly address polyurethanes in that they are limited to rigid foams. Additionally, the hydrated salts are either acidic or basic. The problem with the acidic salts if used in proportions to be effective water sources is that they tend to tie up the amine groups of the catalyst prematurely. The problem with the basic salts is that too many side reactions tend to occur and more trimerization happens than is desirable. Additionally there is the problem of deterioration in resultant polyurethane foam properties when any additive is employed and the same is true with the hydrated salts. It would be useful to discover an improved polyurethane composition employing only water as a blowing agent which would not have its properties degraded at all, and particularly not in the manner of the prior approaches, but which would have water generated in a controlled manner.