POLYISOCYANATE COMPOSITIONS AND RIGID POLYURETHANE FOAM THEREFROM

Abstract: Polyisocyanate composition which has an average isocyanate equivalent weight of from 140 to 280 and comprises an isocyanate-terminated prepolymer is disclosed. The prepolymer is obtained by reacting an excess of a polyisocyanate containing from at least 35 weight percent methylene diphenylisocyanate isomers, wherein the weight ratio of the 4,4'- to 2,4'- isomer is from 98:2 to 50:50, with a polyoxyalkylene polyol or mixture thereof which has an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 500 and an average molecular weight of at least 1100. The compositions show improved miscibility with rigid polyurethane foam polyols resulting in processing benefits and foams having a fine cell structure and enhanced thermal insulation characteristics.

Description: POLYISOCYANATE COMPOSITIONS AND RIGID POLYURETHANE FOAM THEREFROM This invention relates to a polyisocyanate composition comprising an isocyanate- terminated prepolymer and the preparation of closed-celled rigid polyurethane foam therefrom. Foamed plastic materials such as rigid polyurethanes have found wide applicability in fields wherein their light weight, relatively high strength to weight ratio and lowthermal conductivity provide distinct advantages over other materials. Generally such polyurethane foam can be prepared by reacting a polyisocyanate with a polyether or polyester polyol in the presence of a blowing agent. Polyols most frequently used for preparation of rigid polyurethane foam have hydroxyl equivalent weights of from 75 to 150. Use of polyols having hydroxyl equivalent weights significantly greater than this being considered to provide foams having inferior dimensional stability, unattractive strength to weight ratios, and frequently a relatively high degree of open cells. If a rigid polyurethane foam is to display attractive thermal insulation properties it is desirable to have a foam which consists essentially of a closed-celled structure. Such closed-cells retain within the foam the blowing agent used in its preparation and confer its thermal conductivity characteristics to the foam. Until recently, desirable blowing agents for preparing the foam have been halogenated and particularly fully halogenated chlorofluorocarbons (CFCs) having a boiling unit of up to 100°C such as, for example, trichlorofluoromethane (Refrigerant-1 1). The use of many such fully halogenated CFCs is now strongly counseled against in view of their current association with the depletion of ozone from the earths' upper atmosphere. Environmentally friendlier blowing agents now considered for use are those substances which are only partially halogenated hydrogen-containing or "soft" chlorofluorocarbons such as for example dichlorotrifluoroethane (Refrigerant-123), dichlorofluoroethane (Refrigerant 141 b) and tetrafluoroethane (Refrigerant 134a). These alternative types of blowing agents frequently do not have such low thermal conductivities as the fully halogenated CFCs and therefore it is necessary to ensure that polyurethane foam prepared in the presence of such blowing agents has a high percentage of closed-cells and preferably a fine cell structure. From the general mechanics of thermal conductivity, itis known that thermal insulation performance of a foam may be improved by having a fine cell structure. Preparation of rigid polyurethane foam requires that the polyisocyanate and polyols reacting together be miscible with each other or alternatively be mixed very efficiently. Otherwise a foam having a course, large, cell structure and high proportion of open cells may result. The polyisocyanate and polyol components frequently used to prepare rigid polyurethane foam are not generally noted for good mixing characteristics. Fortheir efficient mixing it generally requires the viscosity reducing effect and/or compatibilizing influence provided by the presence of the physical blowing agent. If water is used in moderate amounts as blowing means then there Is frequently insufficient physical blowing agent present to provide an acceptable viscosity reduction thereby allowing for the efficient mixing of reacting components. It would therefore be desirable to develop a process for the manufacture of rigid closed-celled polyurethane foam in which the reacting components have an enhanced miscibility and/or ease of mixing. Itwould be also desirable that foam resulting from such a process has a fine, closed-celled r structure. To this purpose the use of polyisocyanate compositions comprising a prepolymer is contemplated. Polyisocyanate compositions comprising a prepolymer component are widely disclosed in the literature and include patent publications such as, for example, GB 2,021 ,605; GB 1,444,192; DE 2,913,126; DE 2,513,793; EP-22,617; EP-1 1 1,1-21 and U.S. Patent 4,945, 117. However, these disclosures are principally concerned with polyisocyanate compositions comprising a prepolymer of 4,4'-, and 2,4'-methylene diphenylisocyanate (hereinafter also referred to as 4,4'- and 2,4'-MDl) with polyols of bearing 2 to 3 isocyanate reactive groups and having hydroxyl equivalent weights of above 500. Such polyisocyanate compositions are eminently suitable for preparing open-celled flexible polyurethane foams. U.S. Patents 3,153,002; 3,524,825; 3,846,347 and 4,742,089 disclose polyisocyanate compositions suitable for preparing rigid polyurethane foam which comprises a toluene diisocyanate prepolymer of sucrose- or sorbitol-initiated rigid polyether polyols. U.S. Patent 4,248,975 discloses rigid polyurethane foams prepared using a polyisocyanate composition comprising prepolymers of toluene diisocyanate or 4,4'-MDI with sucrose and/or sorbitol-initiated polyols calculated as having a molecular weight below 1000. U.S. Patent 4,087,389 discloses polyisocyanate compositions comprising prepolymers obtained by reaction of a dexixose/ethylene glycol initiated polyether poiyol having a molecular weight of less than 1000 with polymethylene polyphenylisocyanate. U.S. Patent 3,728,288 discloses preparation of rigid polyurethane using polymethylene polyphenylisocyanate compositions comprising a prepolymer obtained by reaction of the polyisocyanate with polyether polyols of four to eight functionality and calculated as having molecular weights of less than 1000. U.S. Patent 4,972,004 discloses polyisocyanate compositions for preparation of rigid polyurethane foam comprising limited amounts of MDI (30 to 45 weight percent), polymethylene polyphenylisocyanate (28 to 67 weight percent) and a MDI based prepolymer (3 to 27 weight percent) an adduct of a polyol having at least two isocyanate reactive groups and a molecular weight of below 1000. It has now been found that urethane modified polyisocyanate compositions obtained by reacting a high molecular weight polyol with the isocyanate may be used to prepare rigid polyurethane foam having attractive physical properties. In a first aspect, this invention is a polyisocyanate composition that has an average isocyanate equivalent weight of from 140 to 280 and which comprises: (a) from 10 to 85 weight percent of an isocyanate- -terminated prepolymer; (b) up to 50 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (c) up to 40 weight percent of a methylene diphenylisocyanate, characterized in that the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 35 weight percent methylene diphenylisocyanate isomers, wherein the weight ratio of the 4,4'- to 2,4'-isomer is from 98:2 to 50:50, with a polyoxyalkylene polyol or mixture thereof having an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 500 and an average molecular weight of at least 1 100. In a second aspect, this invention is polyurethane foam-forming system comprising: (a) a polyisocyanate composition as described above; (b) a polyether or polyester polyol which has an hydroxyl number of from 200 to 700; and (c) a blowing agent comprising water. In a third aspect, this invention is a process for preparing a rigid closed-celled polyurethane foam by intimately mixing under reaction conditions a polyether or polyester polyol which has an hydroxyl number of from 200 to 700 with a polyisocyanate composition in the presence of urethane catalyst and a blowing agent containing water characterized in that: (a) the polyisocyanate composition, present in an amount to provide an isocyanate index of from 80 to 250, is as described above; and (b) the blowing agent is present in an amount to provide for a foam having a density of from 15 to 80 kg/m2. In a fourth aspect, this invention is a rigid closed-celled polyurethane foam obtainable from the above process. Surprisingly, the resulting foams have attractive pnysical properties including a fine, closed-celled structure, good dimensional stability and attractive strength to weight ratio as well as enhanced thermal insulation. Preferabl the polyisocyanate composition has an average isocyanate equivalent weight offrom 150, more preferably from 160, and up to 240, more preferably up to 230 and comprises the prepolymer in from 12, more preferably from 30, and preferably up to 70, more preferably up to 50 percent by total weight of the composition. As mentioned, the remaining part of the composition when not prepolymer is constituted by a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule and/or methylene diphenylisocyanate. The methylene diphenylisocyanate preferably is present in from 5 to 40, and more preferably from 15to 40, percent by total weight of the composition. The polymethylene polyphenylisocyanate preferably is present in an amount from 10 to 48, and more preferably from 15 to 45 percent, by total weight of the composition. The presence of a polymethylene polyphenylisocyanate provides the polyisocyanate composition with an enhanced functionality leading to greater Crosslin king within a polymer network obtained therefrom. Preferably the polymethylene polyphenylisocyanate has an average of from 2.3, more preferably from 2.5, and preferably up to 3.5, more preferably up to 3.1 isocyanate groups per molecule. The isocyanate-terminated prepolymer present in the polyisocyanate composition is obtained by reacting a certain polyoxyalkylene polyol or mixture thereof with a molar excess of a polyisocyanate containing atleast35, preferably from 50, and more preferably from 70, and upto 100 weight percent methylene diphenylisocyanate (MDI) isomers. The MDI isomers include 4,4'-MDI, 2,4'-MDI and 2,2'-MDl. Advantageously the MDI consists essentially of 4,4'- MDl and 2,4'-MDI in a weight ratio of from 98:2 to 50:50, preferably from 95:5 to 60:40 and more preferably from 90:10 to 70:30. When not constituted in its entirety by MDI the remaining part of the polyisocyanate present when preparing the prepolymer can be a carbodiimϊde-modifϊed MD[ or preferably polymethylene polyphenylisocyanate. The polyoxyalkylene polyol or mixtures thereof used to prepare the prepolymer is characterized in that it has an average of from 4 to 8 hydroxyl groups per molecule, a hydroxyl equivalent weight of less than 500 and an average molecular weight of at least 1 100. Preferably the polyoxyalkylene polyol has a molecular weight of at least 1200; and a hydroxyl equivalent weight which is preferably less than 400, and more preferably less than 300. Preferably the average hydroxyl content of the polyol, or mixtures thereof, is from 4.3 to 8, more preferably from 5 to 8 and most preferably from 6 to 8 hydroxyl groups per molecule. Polyoxyalkylene polyols suitable for use in preparing the prepolymer may be obtained by any art recognized processes such as, for example, disclosed byWurtz, Encyclopedia of Chemical Technology, Vol. 1 , pp. 257-262, published by Interscience Publishers, Inc. (1951) or U.S. Patent Nos. 1,922,459 and 3,040,076. Generally, an alkylene oxide(s) is polymerized at superatmospheric pressure in the presence of an initiator bearing active hydrogen atoms and a strongly basic material such as an alkali metal hydroxide or tertiary amine. Exemplary of suitable initiators bearing such active hydrogen atoms are polyols, polyether adducts of polyols, polyamines and other compounds having a plurality of active hydrogen atoms per molecule, such as are described in column 2 of U.S. Patent 4,500,422. Preferred initiators for use in preparing the polyoxyalkylene polyols include α-methylglucoside, α-hydroxyethylglucoside, sorbitol and sucrose or mixtures thereof. The preferred initiators may also be used in admixture with coinitiators containing less than four active hydrogen atoms per molecule providing that such a mixture has an average of from 4 to 8 active hydrogen atoms per molecule. Suitable coinitiators for use in admixture with the above include water, ethylene glycol. propylene glycol, butylene glycol, glycerine, 1 , 1 ,1-trimethylolpropane and 1 ,1 , 1- trimethylolethane. Illustrative of suitable polyoxyalkylene polyols for preparation of the prepolymer are those obtained by reacting sucrose and/or sorbitol with propylene oxide and/or ethylene oxide in amounts sufficient to obtain a product having an average molecular weight of at least 1 100 and equivalent weight of less than 500. Prior to use of the polyol in preparing the prepolymer any basic material or salts thereof should be removed from the polyol by a suitable process such as, for example, filtration over an appropriate adsorbing substance or ion exchange substance including magnesium silicate. Removal of such material is desirable to provide isocyanate compositions containing such prepolymers that exhibit favorable storage stability characteristics. The polyisocyanate composition in its entirety may be prepared by separately obtaining and blending the prepolymer with polymethylene polyphenylisocyanate and/or methylene diphenylisocyanate. Or alternatively, preparing the prepolymer from an isocyanate already comprising polymethylene polyphenylisocyanate and/or methylene diphenylisocyanate. In a preferred embodiment of the invention the prepolymer is prepared by reacting a polyoxyalkylene polyol with a polyisocyanate consisting essentially of methylene diphenylisocyanate and the resulting prepolymer subsequently blended, as desired, with polymethylene polyphenylisocyanate and/or methylene diphenylisocyanate. Rigid, closed-celled, polyurethane foam can be prepared from a foaming system which contains the above described polyisocyanate composition, a polyether or polyester polyol which has a hydroxyl number of from 200 to 700 and a blowing agent comprising water. The blowing agent is present in an amount to produce a foam having a density of from 15 to 80, preferably from 15 to 40, and more preferably from 18to 35 kg/m2. The polyisocyanate composition is present in an amount to provide for an isocyanate index of from 80 to 250, preferably from 90 to 150 and more preferably from 95 to 130. An isocyanate index of 100 corresponds to one isocyanate group per isocyanate-reactive hydrogen atom present including those of polyol and water in the foaming system. As mentioned, the polyisocyanate composition is reacted with a polyether polyol or polyester polyol which has a hydroxyl number of from 200 to 700, preferably from 250 to 600, and more preferably from 250 to 500. Examples of these and other suitable isocyanate- reactive materials are described more fully in U.S. Patent 4,394,491 , particularly in columns 3-5 thereof. Most preferred for preparing rigid foams, on the basis of performance, availability and cost, is a polyether polyol prepared by adding an alkylene oxide to an initiator having from 5 2 to 8, preferably 3 to 8 active hydrogen atoms. Exemplary of such polyols include those desigijated by the trademark VORANOL and include VORANOL 202, VORANOL 360, VORANOL 370, VORANOL 446, VORANOL490, VORANOL 575, VORANOL 800, ail sold by The Dow Chemical Company. Other most preferred polyols include alkylene oxide derivatives of Mannich condensates, as taught, for example, in U.S. Patents 3,297,597; 4,137,265 and 10 4,383,102, and aminoalkylpiperazine-initiated polyethers as described in U.S. Patents 4,704,410 and 4,704,411. Other isocyanate-reactive substances which may be present in addition to the polyester or polyether polyol include polyhydroxy-terminated acetal resins, hydroxyl- -terminated amines and polyamines. The polyether polyol reacted with the polyisocyanate composition to give polyurethane can be the same as the polyoxyalkylene polyol used in the 15 preparation of the prepolymer. As mentioned, the blowing agent comprises water. Although the entire blowing agent requirement ma be provided for by water this is not optimal when seeking to prepare foam having both attractive thermal insulation and dimensional stability properties. Accordingly, it is preferred to employ a blowing agent which comprises both water and a 20 physical blowing agent wherein the amount of water is such to provide for at least 10, and preferably at least 40, mole percent of the blowing requirement and the physical blowing agent provides for at least 10, preferably for at least 30, and more preferably for at least 40 percent of the total blowing requirementto give foam of the desired density. Most preferably water is presentin an amount to provide for 40 to 60 mole percent of the blowing 25 requirement. To this purpose, advantageously the water is present in an amount of from 0.5 to 10, preferably from 2, more preferably from 3 and preferably up to 6 parts by weight per 100 parts by weight of the polyol. Suitable physical blowing agents include inert volatile organic substances having a boiling point of generally from -50°Cto 100"Cand include "hard" chlorofluorocarbon (CFC), 30 "soft" CFCand polyfluorocarbon (FC) substances. Hard CFCs are fully halogenated substances in contrastto "soft" CFCs which contain at least one hydrogen atom. Exemplary of "hard" CFCs include Refrigerant 11, Refrigerant 12, Refrigerant 13, Refrigerant 14, Refrigerant 1 13, Refrigerant 114, Refrigerant 115, and Refrigerant 116. Exemplary of "soft" CFCs include Refrigerant 21, Refrigerant 22, Refrigerant 123, Refrigerant 123a, Refrigerant 124, Refrigerant 35 124a, Refrigerant 133 (all isomers), Refrigerant 141b, Refrigerant 142 (all isomers), and Refrigerant 151. Exemplary FCs include Refrigerant 134 and 134a, Refrigerant 125, Refrigerant 143, Refrigerant 152, Refrigerant 356 and perffuorinated alkanes such as perfluoropentane and perfluorohexane and their mono- ordi- hydrogen-containing equivalents and polyfluoroethers. Further details are provide in for example, AmericanSociety of Heating, Refrigerating, and Air Conditioning Engineers (Atlanta, Georgia) ASHRAE Standard 34-78 where the method of coding such substances is explained and exemplified. Preferred for use in this present invention include the soft CFCs particularly Refrigerant 22, Refrigerant 123 (all isomers), Refrigerant 141 b, Refrigerant 142 (all isomers) and the polyfluorocarbon substances especially Refrigerant 134a, perfluoropentane and perfluorohexane and mixtures thereof. One or more catalysts for promoting the reaction of the polyol and/or water with the polyisocyanate is present. Any suitable catalyst may be used, including tertiary amine compounds and organometallic compounds. Exemplary tertiary amine compounds include triethylenediamine, N-methylmorpholine, pentamethyldiethylenetriamine, tetramethylethyl- enediamine, 1-methyl-4-dimethylaminoethylpiperazine, 3-methoxy-N-dimethylpropylamine, N-ethylmorpholine, diethylethanolamine, N-cocomorpholine, N,N-dimethyl-N',N'-dimethyl isopropylpropylenediamine, N,N-diethyl-3-diethylaminopropylamine, and dimethyl benzylamine. Exemplary organometallic catalysts include organomercury, organolead, organoferric and organotin catalysts, with organotin catalysts being preferred among these. Suitable tin catalysts include stannous chloride, tin salts of carboxylic acids such as dibutyltin di-2-ethyl hexanoate, as well as other organometallic compounds such as are disclosed in U.S. Patent 2,846,408. A catalyst for the trimerization of polyisocyanates such as an alkali metal alkoxide, alkali metal carboxylate, or quaternary amine compound, may also optionally be employed herein. Such urethane and optional trimerization catalysts are used in an amount which measurably increases the rate of reaction of the polyisocyanate. Typical amounts are from 0.001 to 3 parts of catalyst per 100 parts by weight of polyol. Other auxiliaries useful in producing polyurethanes include surfactants, pigments, colorants, fillers, fibers, antioxidants, flame retardants, and stabilizers. In making polyurethane foam, it is generally highly preferred to employ a minor amount of a surfactant to stabilize the foaming reaction mixture until it cures. Such surfactants advantageously comprise a liquid or solid organosilicone surfactant such as disclosed in U.S. Patents 2,834,748; 2,917,480; 3,505,377; 3,507,815; 3,563,924 and 4,483,894. Typically, from 0.2 to 5 parts of the surfactant per 100 parts by weight polyol are sufficient for this purpose. Exemplary of suitable commercially available surfactants include products designated by the trademark TEGOSTAB such as TEGOSTAB B-8427, B-1049 and B-1048 supplied by Th. Goldschmidt AG. In making a polyurethane foam, the polyol(s), polyisocyanate and other components are contacted, thoroughly mixed and permitted to expand and cure into a cellular polymer. The particular mixing apparatus is not critical, and various types of mixing head and spray apparatus are conveniently used. Suitable equipment is discussed by J. H. Saunders and K.C. Frisch in "Polyurethanes Chemistry and Technology" Volumes I and II, R.E. Kπeger Publishing Company, Inc., ISBN 0-89874-561 -6. Suitable procedures for the preparation of polyurethane foams are discussed in U.S. Patents RE 24514 and 3,821,130, and GB Patent 1,523,528. The polyurethane foam of this invention is useful in a wide range of applications, such as in spray insulation, appliance foam, rigid insulating boardstock, laminates, and many 5 other types of rigid foam. The following examples are provided to illustrate the invention but are not intended to limit the scope thereof. All parts and percentages are given by weight unless otherwise indicated. Where reported, thermal conductivity properties, K-factor, of the foams so 10 obtained are measured 1 day after preparation in the parallel-to- -rise direction using samples having dimensions of approximately 20 x 20 x 3 cm. The conductivity is measured using an Anacon Model 88 Thermal Conductivity Analyzer having mean plate temperatures of 10.2°C and 37.8°C. Lower values (mW/MK) indicate better thermal insulative properties. The averagefoam cell diameter is determined from a thin section of 15 foam using a polarized-light optical microscope together with a Quantimet 520 Image Analysis system. The accuracy of the measurement is considered to be ± 20 microns. Example 1 MDI-based isocyanate-terminated prepolymers, Prepolymers I and II, having an _ n isocyanate content of 18.1 and 19.8 weight percent, are independently obtained by reacting a hexafunctional, sorbϊtol-oxypropylene polyol (hydroxyl equivalent weight 192, calculated molecular weight 1152), withaπ excess of methylene diphenylisocyanate (MDI) containing 85 weight percent 4,4'-MDI and 15 weight percent of 2,4'-MDI. Resulting Prepolymer I is estimated to contain 10.6 weight percent non reacted MDI and Prepolymer ll 16.4weight ._ percent non reacted MDI. The so obtained prepolymer is blended with VORANATE '" M220, a polymethylene polyisocyanate composition available from The Dow Chemical Company and containing 41 weight percent methylene diphenylisocyanate and 59 weight percent polymethylene polyphenylisocyanate, and the resulting polyisocyanate composition used to prepare _- polyurethane foam. Table I presents the proportions of prepolymer and VORANATE " " M220, and the calculated weight percent of (A) prepolymer, (B) polymethylene polyphenylisocyanate, and (C) methylene diphenylisocyanate in the resulting polyisocyanate composition and its final average isocyanate equivalent weight. The resulting polyisocyanate compositions are used to prepare foam. Formulation and some foam properties are indicated in Table 1. Comparative foams are prepared from the same polyether polyol and polyisocyanate components but not by a prepolymer route. The polyisocyanate composition used to prepare the comparative foam is characterized by absence of the prepolymer. Foams obtained according to the present invention display a reduced cell size particularly where water provides a significant percentage of the total blowing agent requirement. The thermal insulation properties of the foams obtained according to this invention are better than the comparative foams as evidenced by the lower K-factor. Table 1 O H I * Not an example of this invention ® Not observed as foam collapsed during preparation © Same polyol as used in the preparation of Prepolymers I and II Example 2 An MDI-based isocyanate-terminated prepolymer, Prepolymer III having an isocyanate content of 20.4 weight, percent is obtained by reacting a hexafunctional, sorbitol- oxypropylene as described in Example 1 with an excess of a methylene diphenylisocyanate (MDI), containing 50 weight percent 4,4'-MDI and 50 weight percent of 2,4'-MDI. Resulting Prepolymer III is calculated to contain 17.8 weight percent non reacted MDI. The so obtained prepolymer is blended with VORANATE " M220 and the resulting polyisocyanate composition used to prepare polyurethane foam. Table 2 presents the _ proportions of prepolymer and VORANATE " M220, and the estimated weight percent of (A) prepolymer, (B) polymethylene polyphenylisocyanate, and (C) methylene diphenylisocyanate in the resulting composition and its final average isocyanate equivalent weight. The resulting polyisocyanate compositions are used to prepare foam; foaming formulation and some foam properties are indicated in Table 2. Comparison of Example 1 and Example 2 indicates that 1 - increasing the quantity of 2,4'-MDI results in an enhanced cell size reduction. 0 5 0 5 Table 2 I * Not an example of this invention ® Not observed as foam collapsed during preparation ©Same polyol as used in the preparation of Prepolymer III Example 3 An MDi-based isocyanate-terminated prepolymer, Prepolymer IV having an isocyanate content of 18.6 weight percent is obtained by reacting a polyoxyalkylene polyol composition with an excess of a methylene diphenylisocyanate (MDI), containing 85 weight percent 4,4'-MDI and 15 weight percent of 2, '-MDI. The polyoxyalkylene polyol composition has an average functionality of 4, a hydroxyl equivalent weight of 286, a calculated molecular weight of 1 150 and contains 67 mole percent of a glycerine-initiated oxypropylene polyol having a hydroxyl equivalent weight of 333, and 33 mole percent of a sorbitol initiated oxypropylene polyol equivalent weight 192. Resulting Prepolymer IV is calculated to contain 14.4 weight percent non reacted MDI. The so obtained prepolymer is blended with VORANATE'" M220 and the resulting polyisocyanate composition used to prepare polyurethane foam. Table 3 presents the proportions of prepolymer and VORANATE ' " M220, and the estimated weight percent of (A) prepolymer, (B) polymethylene polyphenylisocyanate, and (C) methylene diphenylisocyanate in the resulting polyisocyanate composition and its final average isocyanate equivalent weight. The resulting polyisocyanate compositions are used to prepare foam; foaming formulation and some foam properties are indicated in Table 3. Table 3 * Not an example of this invention ®Not observed as foam collapsed during preparation © Same polyol blend as used in the preparation of Prepolymer IV Example 4 A polyisocyanate composition with an isocyanate equivalent weight of 162, comprising 33 weight percent prepolymer, is obtained by reacting sorbitol-oxypropylene polyol as described in Example 1 with an excess of VORANATE '" M220 which contains 59 weight percent polymethylene polyphenylisocyanate, and 41 weight percent methylene diphenylisocyanate wherein the weight ratio of 4,4'-MDI to 2,4'-MDI is 98:2. Properties of foam obtained with this polyisocyanate composition are given in Table 4. Table 4 No an examp e o s nven on ® Same polyol as used to prepare the Prepolymer ©Not determined Example 5 In this example a comparative prepolymer is prepared and polyurethane foam prepared therefrom. The comparative prepolymer is an MDI-based isocyanate-terminated prepolymer, Prepolymer V, having an isocyanate content of 18.4 weight percent is obtained by reacting a trifunctional glycerine-oxypropylene polyol having a molecular weight of 1000 with an excess of a methylene diphenylisocyanate (MDI), containing 85 weight percent 4,4'-MDI and 15 weight percent of 2,4'-MDI. Properties of foam obtained with this polyisocyanate composition are given in Table 5. All foams obtained by reaction of a polyisocyanate composition containing Prepolymer V are observed to collapse or shrink emphasizing the importance of polyol functionality when preparing the prepolymer. * Not an example of this invention ©Not observed as foam collapsed during preparation ©Not observed as oam collapsed on storage ® Same polyol as used to prepare Prepolymer V 5 0 5

Claims: CLAIMS : 1. A polyisocyanate composition that has an average isocyanate equivalent weight of from 140 to 280 and which comprises: (a) from 10 to 85 weight percent of an isocyanate- -terminated prepolymer; (b) up to 50 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (c) up to 40 weight percent of a methylene diphenylisocyanate, characterized in that the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 35 weight percent methylene diphenylisocyanate isomers, wherein the weight ratio of the 4,4'- to 2,4'-isomer is from 98:2 to 50:50, with a polyoxyalkylene polyol or mixture thereof having an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 500 and an average molecularweight of at least 1 100. 2. A polyurethane foam-forming system comprising: (a) a polyisocyanate composition that has an average isocyanate equivalent weight of from 140 to 280 and contains: (i) from 10 to 85 weight percent of an isocyanate- -terminated prepolymer; (ii) up to 50 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (iii) up to 40 weight percent of a methylene diphenylisocyanate, wherein the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 35 weight percent methylene diphenylisocyanate isomers with the weight ratio of the 4,4'- to 2,4'-isomer being from 98:2 to 50:50, with a polyoxyalkylene polyol or mixture thereof having an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 500 and an average molecular weight of at least 1 100; (b) a polyether or polyester polyol which has an hydroxyl number of from 200 to 700; and (c) a blowing agent comprising water. 3. A process for preparing a rigid closed-celled polyurethane foam by intimately mixing under reaction conditions a polyether or polyester polyol which has an hydroxyl number offrom 200 to 700 with a polyisocyanate composition in the presence of urethane catalyst and a blowing agent containing water characterized in that: (a) the polyisocyanate composition, present in an amount to provide an isocyanate index offrom 80 to 250, has an average isocyanate equivalent weight of from 140 to 280 and comprises: (i) from 10 to 85 weight percent of an isocyanate- -terminated prepolymer; (ii) up to 50 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (iii) up to 40 weight percent of a methylene diphenylisocyanate, wherein the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 35 weight percent methylene diphenylisocyanate isomers with the weight ratio of the 4,4'- to 2,4'-isomer being from 98:2to 50:50, with a polyoxyalkylene polyol or mixture thereof having an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 500 and an average molecular weight of at least 1 100; and (b) the blowing agent is present in an amount to provide for a foam having a density of from 15 to 80 kg/rπ2. 4. A rigid closed-eel led polyurethane foam obtainable by the process of Claim 3. 5. Theinventϊon as claimed in Claims 1 to 3 wherein the polyisocyanate composition comprises the polymethylene polyphenylisocyanate in an amount offrom 10 to 48 weight percent and the methylene diphenylisocyanate in an amount of from 5 to 40 weight percent. 6. The invention as claimed in Claims 1 to 3 wherein the polyoxyalkylene polyol used to prepare the prepolymer has a hydroxyl equivalent weight of less than 400. 7. The invention as claimed in Claims 1 to 3 wherein the prepolymer is obtained from a polyisocyanate comprising at least 50 weight percent methylene diphenylisocyanate isomers. 8. The invention as claimed in Claims 1 to 3 wherein the prepolymer is obtained from a polyisocyanate containing the 4,4'- and 2,4 r -MDl isomers in a weight ratio offrom 95: 5 to 60r40. 9. The invention as claimed in Claims 1 to 3 wherein the polyisocyanate composition has an average isocyanate equivalent weight of from 150 to 240 and comprises: (a) from 12 to 50 weight percent of an isocyanate-terminated prepolymer; (b) up to 10 to 48 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (c) up to 5 to 40 weight percent of a methylene diphenylisocyanate, characterized in that the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 50 weight percent methylene diphenylisocyanate isomers, wherein the weight ratio of the 4,4'- to 2,4'-isomer is from 95:5 to 60:40, with a polyoxyalkylene polyol or mixture thereof having an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 400 and an average molecular weight of at least 1 100. 10. A process as claimed in Claim 3 wherein the polyisocyanate composition is present in an amount to provide an isocyanate index of from 90 to 150 and wherein at least 40 mole percent of the total blowing agent requirement is provided by water. AMENDED CLAIMS [received by the International Bureau on 22 February 1993 (22.02.93) original claim 6 cancelled ; original claims 1-3 and 9 amended ; other claims unchanged (2 pages)] 1. A polyisocyanate composition that has an average isocyanate equivalent weight of from 140 to 280 and which comprises: (a) from 30 to 85 weight percent of an isocyanate- -terminated prepolymer; (b) up to 50 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (c) up to 40 weight percent of a methylene diphenylisocyanate, characterized in that the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 35 weight percent methylene diphenylisocyanate isomers, wherein the weight ratio of the 4,4'- to 2,4'-ϊsomer is from 98:2 to 50:50, with a polyoxyalkylene polyol or mixture thereof having an average offrom 4to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 400 and an average molecularweightof at least 1 100. 2. A polyurethane foam-forming system comprising: (a) a polyisocyanate composition that has an average isocyanate equivalent weight offrom 140 to 280 and contains: (i) from 30 to 85 weight percent of an isocyanate- -terminated prepolymer; (ϊi) up to 50 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (iii) up to 40 weight percent of a methylene diphenylisocyanate, wherein the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 35 weight percent methylene diphenylisocyanate isomers with the weight ratio of the 4,4'- to 2,4'-isomer being from 98:2 to 50:50, with a polyoxyalkylene polyol or mixture thereof having an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 400 and an average molecular weight of at least 1100; (b) a polyether or polyester polyol which has an hydroxyl number of from 200 to 700; and (c) a blowing agent comprising water. 3. A process for preparing a rigid closed-celied polyurethane foam by intimateiy mixing under reaction conditions a polyether or polyester poiyol which has an hydroxyl number of from 200 to 700 with a polyisocyanate composition in the presence of urethane catalyst and a blowing agent containing water characterized in that: (a) the polyisocyanate composition, present in an amount to provide an isocyanate index of from 80 to 250, has an average isocyanate equivalent weight of from 140 to 280 and comprises: (i) from 30 to 85 weight percent of an isocyanate- -terminated prepolymer; (ii) up to 50 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (iii) up to 40 weight percent of a methylene diphenylisocyanate, wherein the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 35 weight percent methylene diphenylisocyanate isomers with the weight ratio of the 4,4'- to 2,4'-isomer being from 98:2 to 50:50, with a polyoxyalkylene polyol or mixture thereof having an average of from 4 to 8 hydroxyl groups per molecule and an hydroxyl equivalent weight of less than 400 and an average molecular weight of at least 1 100; and (b) the blowing agent is present in an amount to provide for a foam having a density of from 15 to 80 kg/m 2 . 4. A rigid closed-celled polyurethane foam obtainable by the process of Claim 3. 5. The invention as claimed in Claims 1 to 3 wherein the polyisocyanate composition comprises the polymethylene polyphenylisocyanate in an amount of from 10 to 48 weight percent and the methylene di phenyl isocyanate in an amount of from 5 to 40 weight percent. 6. Cancelled. 7. The invention as claimed in Claims 1 to 3 wherein the prepolymer is obtained from a polyisocyanate comprising at least 50 weight percent methylene diphenylisocyanate isomers. 8. The invention as claimed in Claims 1 to 3 wherein the prepolymer is obtained from a polyisocyanate containing the 4,4'- and 2,4'-MDi isomers in a weight ratio of from 95:5 to 60:40. 9. The invention as claimed in Claims 1 to 3 wherein the polyisocyanate composition has an average isocyanate equivalent weight of from 150 to 240 and comprises: (a) from 30 to 50 weight percent of an isocyanate-te ' rminated prepolymer; (b) up to 10 to 48 weight percent of a polymethylene polyphenylisocyanate having more than two isocyanate groups per molecule; and (c) up to 5 to 40 weight percent of a methylene diphenylisocyanate, characterized in that the prepolymer is obtained by reacting an excess of a polyisocyanate containing at least 50 weight percent methylene diphenylisocyanate isomers, wherein the

Inventors: GRUENBAUER HENRI J M, GRUENBAUER, HENRI, J., M.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1992
Main classification: C08G18
IPC codes: C08G18