OXYMETHYLENE POLYMER MOLDING COMPOSITIONS HAVING ENHANCED IMPACT RESISTANCE AND ARTICLES MOLDED THEREFROM

Abstract: Oxymethylene polymer molding compositions provide articles which exhibit enhanced impact strength properties, especially at low temperatures such as -20°F(-29°C), when such compositions include an elastomeric polyurethane, which is the reaction product of certain proportions of: (i) 4,4′-methylenebis (phenyl isocyanate), (ii) polyoxytetramethylene glycol having a molecular weight of 1500 to 3000, and (iii) at least one diol extender selected from aliphatic straight chain diols having 2 to 6 carbon atoms, bis (2-hydroxy-ethyl) ether of hydroquinone and, bis (2-hydroxy-­ ethyl) ether of resorcinol.

Description: The present invention relates to oxymethylene polymer molding compositions. More particularly, the present invention relates to oxymethylene copolymer molding compositions that contain a certain elastomeric polyurethane,and which may be molded into articles having enhanced impact strength properties. Oxymethylene polymers, having recurring -CH20units have been known for many years. They may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane which is a cyclic trimer of formaldehyde. The usefulness of oxymethylene polymers in molding compositions has also been known for years. For example, U.S. Patent No. 3,275,604 discloses a moldable oxymethylene copolymer comprising oxymethylene groups and oxyethylene groups, with said oxyethylene groups having a -CH2OR group pendant therefrom, which may be prepared by polymerizing a source of oxymethylene units such as trioxane with a glycidyl ether. For certain molding applications, e.g., exterior automobile body parts, molding compositions that impart good impact strength to articles molded therefrom are highly desirable. It is not enough, however, that the molded objects exhibit improved impact strengths at room tem.perature and above, but for many uses they must also retain enhanced impact properties at temperatures as low as -200F (-290C) and lower. For example, low temperature impact retention is especially important when the molded objects are to be used or are frequently present outdoors. Furthermore, it is important that the addition of the impact, modifier to the oxymethylene composition should not reduce the flexural modulus of the resulting molded articles to unacceptable values. In short, what is needed is an impact modified oxymethylene composition that can be molded into articles that have enhanced impact strengths at both room temperatures and low temperatures 'such as -200F (-290c) and lower), and retain commercially useful flexural modulus values. Accordingly, it is an object of the present invention to provide an oxymethylene polymer molding composition that may be molded into articles which exhibit enhanced impact strength. It is another object of the present invention to provide an oxymethylene polymer molding composition that may be molded into articles which exhibit enhanced impact strength and which retain enhanced impact strength values at temperatures as low as -200F (-290C). It is still another object of the present invention that the articles molded from the impact modified oxymethylene compositions have useful flexural modulus values. These and other objects, as well as the scope, nature, and utilization of the present invention, will be apparent to those skilled in the art from the following description and appended claims. SUMMARY OF THE INVENTION In accordance with the present invention, an oxymethylene polymer molding composition which is capable of being molded to form articles exhibiting enhanced impact strength comprises an intimate blend of (a) from 5 to 50 weight percent based upon the total weight of the composition of an elastomeric polyurethane, which is the reaction product of: (i) 4,4'-methylenebis (phenyl isocyanate), (ii) polyoxytetramethylene glycol having a molecular weight within the range of 1500 to 3000, and (iii) at least one diol extender selected from aliphatic straight chain diols having from 2 to 6 carbon atoms, bis(2-hydroxy-ethyl) ether of hydroquinone, and bis(2-hydroxy ethyl) ether of resorcinol, the amounts of phenyl isocyanate and diol extender being as defined below; and (b) from 50 to 95 weight percent based upon the total weight of the composition of an oxymethylene polymer. The amounts of phenyl isocyanate and diol extender are defined as follows: The diol extender is used in the polyurethane reaction in an amount such that there are 0.5 to 2.5 equivalents of extender per equivalent of the polyoxytetramethylene glycol, and the phenyl isocyanate is used in the reaction in an amount such that the ratio of isocyanate groups to total hydroxyl groups (that is, glycol plus diol extender) is in the range from 1.0: 1.0 to 1.08: 1.0. DESCRIPTION OF PREFERRED EMOBODIMENTS OXYMETHYLENE POLYMER The oxymethylene polymer used in the molding composition of the present invention is well known in the art. The polymers are characterized as having recurring oxymethylene groups or units, i.e. -CH2O-. The term oxymethylene polymer as used herein includes any oxymethylene polymer having -CH2O- groups comprising at least 50 percent of the recurring units, for example homopolymers, copolymers, terpolymers and so on. Typically, the homopolymers may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane which is a cyclic trimer of formaldehyde. For example, high molecular weight polyoxymethylenes have been prepared by polymerizing trioxane in the presence of certain fluoride catalysts such as antimony fluoride and may also be prepared in high yields and at rapid reaction rates by the use of catalysts comprising boron fluoride coordinate complexes with organic compounds. Such methods are well known and have been described, for example, in USSN 691,143, filed October 21, 1957, by Hudgin and Berardinelli. The homopolymers are usually stabilized against thermal degradation by end-capping or the incorporation therein of stabilizer compounds. This is described for example in U.S. Patent No. 3,133,896 of Dolce and Berardinelli. Oxymethylene polymers that are particularly adapted for use in the molding compositions of the present invention are oxymethylene copolymers, which may be prepared as described in U.S. Patent No. 3,027,352 of Walling et al by copolymerizing, for example, trioxane with any of various cyclic ethers having at least two adjacent carbon atoms, e.g., ethylene oxide, dioxolane, and the like. Especially suitable oxymethylene copolymers which may be used in the molding compositions of the present invention usually possess a relatively high level of polymer crystallinity,e.g.70 to 80 pen=ent, These preferred oxymethylene copolymers have repeating units which consist essentially of (a) -OCH2- groups interspersed with (b) groups represented bywthesgeneral formula: EMI4.1 wherein each R1 and R2 is selected from hydrogen, lower alkyl and halogen-substituted lower alkyl radicals, each R3 is selected from methylene, oxymethylene, lower alkyl and haloalkyl-substituted methylene, and lower alkyl and haloalkyl-substituted oxymethylene radicals, and n is 0,1, 2 or 3. Each lower alkyl radical preferably has one or two carbon atoms. The -OCH2- units of (a) constitute from 85 to 99.9 percent of the recurring units. The units of (b) may be incorporated into the copolymer during the step of copolymerization to produce the copolymer by the opening of the ring of a cyclic ether having adjacent carbon atoms, i.e., by the breaking of an oxygen-to-carbon linkage. Copolymers of the desired structure may be prepared by polymerizing trioxane together with from 0.1 to 15 mole percent of a cyclic ether having at least two adjacent carbon atoms, preferably in the presence of a catalyst such as a Lewis acid (e.g., BF3,PF5, and the like) or other acids (e.g., HC104,1% H2504,and the like). In general, the cyclic ethers employed in making the preferred oxymethylene copolymers are those represented by the general formula R1CR2 O R1CR2 (RJ )n wherein each R1 and R2 is selected from hydrogen, lower alkyl and halogen-substituted lower alkyl radicals, and each R3 is selected from methylene, oxymethylene,lower alkyl and haloalkyl-substituted methylene, and lower alkyl and haloalkyl-substituted oxymethylene radicals, and n is 0,1,2 or 3. Each lower alkyl radical preferably has one or two carbon atoms. In the copolymerization reaction, the cyclic ether ring is opened up by the breaking of an oxygen-to- carbon linkage. The preferred cyclic ethers used in the preparation of the preferred oxymethylene copolymers are ethylene oxide and 1,3-dioxolane, which may be represented by the formula : EMI5.1 wherein n is 0, 1 or 2. Other cyclic ethers that .- v be employed include 1,3-dioxane, trimethylene oxide, 1,2-propylene oxide,1,2-butylene oxide, 1,3-butylene oxide and 2,2-di-(chloromethyl)-1,3-propylene oxide. The preferred catalyst used in preparing the desired oxymethylene copolymers is the aforementioned boron trifluoride as discussed in the previously mentioned U.S. Patent No. 3,027,352 of Walling et al. Reference may be made to this patent for further information concerning the polymerization conditions,amount of catalyst employed, and the like. The oxymethylene copolymers produced from the preferred cyclic ethers have a structure composed substantially of oxymethylene and oxyethylene groups in a ratio of from 6 to 1 to 1000 to 1. The oxymethylene copolymers that are preferably present in the molding compositions of the present invention are.thermoplastic materials having a melting point of at least 15O0C., and normally are millable or processable at a temperature of from 1800C.to 2000C, They generally have a number average molecular weight of at least 10,000. The preferred oxymethylene copolymers have an inherent viscosity of at least 1.0 (measured at 6O0C. in a 0.1 weight percent solution in p-chlorophenol containing 2 weight percent of alphapinene). Examples of preferred oxymethylene copolymers include a copolymer comprising about 98 percent recurring -OCH2 - groups and about 2 percent recurring -OCH2 CH2 - groups, which may be derived from trioxane and ethylene oxide. Such a copolymer with a melt index (according to ASTM D 1238-82) of about 2.5 g/10 min is particularly suitable. The oxymethylene copolymer component of the molding composition of this invention preferably is an oxymethylene copolymer that has been preliminarily stabilized to a substantial degree. Such stablilizing technique may take the form of stabilization by degradation of the molecular ends of the polymer chain to a point where a relatively stable carbon-to carbon linkage exists at each end. For example, such degradation may be effected by hydrolysis as described in U.S. Patent No. 3,219,623 of Berardinelli. If desired, the oxymethylene copolymer may be end-capped by techniques known to those skilled in the art. A preferred end-capping technique involves acetylation with acetic anhydride in the presence of sodium acetate catalyst. A preferred oxymethylene copolymer is commercially available from Celanese Corporation under the designation CELCON acetal copolymer, and especially preferred is CELCON M25 which has a melt index of about 2.5g/10 min.when tested in accordance with ASTMD1238-82. CELCON is a Trade Mark. With respect to the oxymethylene terpolymer,it may be prepared, for example, by reacting trioxane and a cyclic ether and/or cyclic acetal such as in the preparation of the oxymethylene copolymer, with a third monomer which is a bifunctional compound such as a diglycide of the formula EMI7.1 wherein Z represents a carbon-to-carbon bond, an oxygen atom, an oxy-alkoxy of 1 to 8 carbon atoms, preferably 2 to 4 carbon atoms, and which may be a oxycyclcalkoxy of 4 to 8 carbon atoms, or an oxy-poly(lower alkoxy),preferably of 2 to 4 recurring groups each with 1 or 2 carbon atoms, for example, ethylene diglycide, diclycidyl ether and diethers of 2 mols of glycide and 1 mol of formaldehyde,dioxane or trioxane, or diethers of 2 mols of clycide and 1 mol of an aliphatic diol with 2 to 8 carbon atoms, advantageously 2 to 4 carbon atoms, or a cycloaliphatic diol with 4 to 8 carbon atoms. Examples of suitable bifunctional compounds include the diglycidyl ethers of ethylene glycol, 1,4-butanediol , 1,3-butanediol,cyclobutane-1,3-diol,1,2-propane-diol, cyclohexane-1, 4-diol and 2-dimethyl-4-dimethyl-cyclobutane-1, 3-diol. The butanediol diglycidyl ethers are most preferred. Generally, in preparing the terpolymer of trioxane, cyclic ether and/or cyclic acetal and at least one bifunctional diglycide compound, a ratio of from 99.89 to 89.0 weight percent trioxane, 0.1 to 10 weight percent of the cyclic ether and/or cyclic acetal, and 0.01 to 1 weight per cent of the bifunctional compound is preferred, with the percentage figures being based on the total weight of monomers used in forming the terpolymer. The terpolymers thus obtained are characterized as being essentially white and having a particularly good extrudability. The polymerization of the terpolymer may be carried out according to known methods, that is (in substance) solution or suspension, while using the above-mentioned quantitative proportions of the termonomers. As solvents, there may advantageously be used inert aliphatic or aromatic hydrocarbons,halogenated hydrocarbons or ethers. In some cases, it may be advantageous to use the following quantitative proportions: 99.85 to 89.5 weight percent of trioxane, 0.1 to 10 weight percent of cyclic ether or cyclic acetal, and 0.05 to 0.5 weight percent of diglycidyl ether, the percentage figures being calculated on the total weight of the monomer mixture used for preparing the terpolymer. The trioxane-based terpolymer polymerization is advantageously carried out at temperatures at which trioxane does not crystallize out, that is, at a temperature within the range of -5O0C. to +100 C., depending on the solvent used, and in the absence of a solvent at a temperature within -the range of +200C. to +1000C. As trioxane-based terpolymer polymerization catalysts, all substances may be used which are capable of initiating a cationic polymerization, for example, organic or inorganic acids, acid halides and, preferably, Lewis acids. Of the latter, boron fluoride and its complex compounds, for example, etherates of boron fluoride,are advantageously used. Diazonium fluoroborates are parti cul arly advantageous. The concentration of the catalyst may vary within the limits depending on the nature of the catalyst and on the intended molecular weight of the terpolymer. The concentration of the catalyst may be within the range of 0.0001 to 1 weight percent, calculated on the total monomer mixture, and is advantageously within the range of 0.0001 to 0.1 weight percent. Since the catalyst tends to decompose the terpolymer, the catalyst is advantageously neutralized immediately after the polymerization, for example, with ammonia or methanolic or acetonic amine solutions. The unstable terminal hemiacetal groups may be removed from the terpolymer in the same manner as known for other oxymethylene polymers. Advantageously, the terpolymer is suspended in aqueous ammonia at temperatures within the range of 100C to 2000C., and if desired, in the presence of a swelling agent such as methanol or n-propanol. Alternatively, the terpolymer is dissolved in an alkaline medium at temperatures above 1000C. and subsequently reprecipitated. Examples of suitable solvents are benzyl alcohol, ethylene glycol monoethyl ether or a mixture of 60 weight percent methanol and 40 weight percent water. Examples of suitable compounds having an alkaline reaction are ammonia and aliphatic amines. It is also possible to thermally stabilize the terminal groups of the terpolymers in the absence of a solvent in the melt in the presence of a stabilizer. Alternatively, the terpolymer can be subjected to heterogeneous hydrolysis wherein water, with or without a catalyst, e.g., an aliphatic or aromatic amine, is added to a melt of the terpolymer in an amount ranging from 1 to 50 percent based on the weight of the terpolymer. The terpolymer mixture is maintained at a temperature in the range of from 1700C to 2500C for an appropriate period of time, and then washed with water and dried or centrifuged. A preferred oxymethylene terpolymer is commercially available from Celanese Corporation under the designation U10, and is a butanediol diglycidyl ether/ethylene oxide/ trioxane terpolymer containing about 0.05 weight percent, 2.0 weight percent, and 97.95 weight percent of each component, respectively. The oxymethylene polymers used in the Dresent invention may, if desired, include various additives, for example plasticizers, formaldehyde scavengers, mold lubricants,antioxidants, fillers,colorants.reinforcing agents,light stabilizers, pigments, other stabilizers, and the like, so long as such additives do not materially affect the desired properties (including enhancement of impact strength) of the resulting molding composition and the articles molded therefrom. Examples of suitable formaldehyde scavengers include cyanoguanidine, melamines, polyamides,amine-substituted triazines, amidines,ureas, hydroxyl salts of calcium,magnesium, and the like, salts of carboxylic acids, and metal oxides and hydroxides. Cyanoguanidine is the preferred formaldehyde scavenger. Examples of suitable mold lubricants include alkylene bisstearamide, long-chain amidesh waxes,oils, and polyether glycides. The preferred mold lubricant is commercially available from Glyco Chemical, Inc. under the designation Acrawax C and is alkylene bisstearamide. Acrawax is a Trade Mark. The preferred antioxidants are hindered bisphenols. Especially preferred is 1,6-hexamethylene bis (3, 5-di-t-butyl-4-hydroxyhydrocinnamate), commercially available from Ciba Geigy Corp. under the designation Irganox 259. Irganox is a Trade Mark. A most preferred oxymethylene copolymer for use in the present invention is commercially available from Celanese Corporation under the designation CELCON M25-04. This oxymethylene copolymer has a melt index of about 2.5g/10 min. and contains 0.5 percent by weight Irganox 259, 0.1 percent by weight cyanoguanidine, and 0.2 percent by weight Acrawax C. A most preferred oxymethylene terpolymer is commercially available from Celanese Corporation under the designation U10-11. U10-ll is the previously mentioned U-10 terpolymer stabilized by 0.5 percent by weight Irganox 259 and 0.1 percent by weight calcium ricinoleate. ELASTOMERIC POLYURETHANE The elastomeric polyurethane used in the molding composition of the present invention is the reaction product of (i) 4,4'-methylenebis(phenyl isocyanate), (ii) polycxytetramethylene glycol having a mole cular weight within the range of 1500 to 3000, and (iii) a diol extender selected from aliphatic straight chain diols having from 2 to 6 carbon atoms, bis(2-hydroxy-ethyl) ether of hydroquinone, bis(2-hydroxy-ethyl) ether of resorcinol, and mixtures of any two or more of such diol extenders. Preferably, the polyoxytetramethylene glycol has a molecular weight within the range of 2000 to 2900. The molecular weight of the polyoxytetramethylene glycol as used herein is a number average molecular weight. One may either use individual polyoxytetramethylene glycols which are all within the specified molecular weight ranges,or one may blend lower or higher molecular weight polyoxytetramethylene glycols with other molecular weight polyoxytetramethylene glycols so that the molecular weight of the final blend is within the above specified molecular weight ranges. The term "aliphatic straight chain diols having from 2 to 6 carbon atoms" means diols of the formula HO (CH2 ) nOH wherein n is from 2 to 6 and there is no branching in the aliphatic chain separating the OH groups. The term includes ethylene glycol,1,3-propanediol1 1,4-butanediol,1, 5-pentanediol and 1,6-hexanediol. The preferred diol extenders are 1,4-butanediol, 1,6-hexanediol and the bis (2-hydroxy-ethyl) ether of hydroquinone. The most preferred diol extender is 1, 4butanediol. The polyoxytetramethylene glycol and diol extender are used in the polyurethane reaction medium in a ratio of 0.5 to 2.5 equivalents (e.g., mole equivalents) of the diol extender per equivalent of the glycol. Preferably, the equivalents ratio is from 1 to 2. Most preferably the ratio is from 1.2 to 1.6 equivalents of diol extender per equivalent of the glycol when the polyoxytetramethylene glycol has a molecular weight of about 2000, and especially when the diol extender is an aliphatic straight chain diol. When the hydroquinone or resorcinol extender is used, the equivalents ratio may be lower than the above-mentioned preferred ranges, for example, as low as 0.5 equivalents of the extender per equivalent of the glycol. Regardless of the particular polyurethane reaction method which is chosen, the hydroxyl components (i.e., the polyoxytetramethylene glycol and the diol extender) and the diisocyanate are used such that the overall ratio of isocyanate equivalents or groups to total hydroxyl equivalents or groups (glycol plus diol extender) is within the range from 1:1 to 1.08: 1.0, and preferably is within the range from 1.02:1.0 to 1.07:1.0. The most preferred ratio of isocyanate (NCO) groups to total hydroxyl (OH) groups is within the range from 1.03:1.0 to 1.06:1.0. The term equivalent(s) as used with respect to the polyurethane preparation is based on the hydroxyl and isocyanate groups of the reactants. The thermoplastic polyurethane elastomers of the present invention can be prepared by processes which are conventional in the art for the synthesis of polyurethane elastomers. Illustrative of such processes are those described in U.S. Patents Nos. 3,493,634; 4,169,196; 4,202,957; and 3,642,964. Such processes include the one-shot procedure in which all the reactants are brought together simultaneously, and the prepolymer procedure in which the isocyanate is reacted with the polyoxytetramethylene glycol in a firststep and the isocyanate-terminated prepolymer so produced is subsequently reacted with the diol extender. The oneshot procedure also includes the process in which the diisocyanate has been converted to a quasi-prepolymer by reaction with a very minor amount (i.e.,less than 10 percent on an equivalent basis) of the glycol prior to carrying out the polyurethane forming reaction. The one-shot procedure is the preferred procedure for preparing the elastomeric polyurethanes of the present invention. In a most preferred embodiment, the elastomeric polyurethanes of the present invention are prepared by a continuous one-shot procedure such as that set forth in U.S. Patent No. 3,642,964. In the one-shot procedures (including the continuous one-shot procedure) the reactants are brought together in any order. Advantageously, the polyoxytetramethylene glycol and the diol extender are preblended and fed to the reaction zone as a single component, the other major component being the diisocyanate (or quasi-prepolymer where used). The mixing and reaction of the reactants can be accomplished by any of the procedures and apparatus conventional in the art. Preferably, the individual components or reactants are rendered subsequently free from the presence of extraneous moisture using conventional procedures, for example by azeotropic distillation using benzene,toluene, and the like, or by heating under reduced pressure at a temperature above the boiling point of water at the pressure employed. As is well known by those skilled in the art, water will interfere with the polyurethane reaction by reacting with the diisocyanate and the resulting polyurethane. Thus, moisture levels as low as practical are desired, typically well below 0.1 percent by weight of the total reactants, and more preferably below 0.05 weight percent and even lower. Advantageously, in a one-shot batch procedure, the heated hydroxyl components, either alone or (preferably) together, are subjected to degassing in order to remove residual moisture, entrained bubbles of air or other gases before the polyurethane reaction takes place. This degassing is accomplished conveniently by reducing the pressure under which the components are maintained until no further evolution of bubbles occurs. Typically, such degassing takes from 1 to 3 hours at 850C. to 11O0C. The hydroxyl components are preferably stirred during degassing. The degassed hydroxyl components are then admixed with the diisocyanate (or quasiprepolymer where used), which is typically at about 6O0C. The resulting degassed mixture is then reacted, with stirring, to form the elastomeric polyurethane. The polyurethane reaction is exothermic. Reaction temperatures typically range from 800C. to 2O00C. or higher. The resulting polyurethane is then transferred to suitable molds or extrusion equipment or the like and is cured at a temperature on the order of 200C to 1150C. The time required for curing will vary with the temperature of curing'and also with the nature of the particular composition. The time required in any given case can be determined by a process of trial and error. Typically, the polyurethanes of the present invention may be cured at about 1000C.for about 1 hour and at 200C. to 230C. for 24 hours when a one-shot batch procedure is used. It is frequently desirable but not essential to include a catalyst in the reaction mixture employed to prepare the polyurethane compositions of the present invention. Any of the catalysts conventionally employed in the art to catalyze the reaction of an isocyanate with a reactive hydrogen containing compound can be employed for this purpose; see,for example, Saunders et al, Polyurethane,Chemistry and Technology, Part I, Interscience, New York, 1963, pages 228232; see also Britain et al., J. Applied Polymer Science, 4, 207-211,1960. Such catalysts include organic and inorganic acid salts of, and organometallic derivatives of, bismuth, lead, tin, iron,antim,ony,uranium,cadfum,cobalt, thorium, aluminium, mercury,zinc,nickel,cerium, molybdenum,vanadium, copper, manganese and zirconium, as well as phosphines and tertiary organic amines. Representative organotin catalysts include stannous octoate, stannous oleate,dibutyltin dioctoate and dibutyltin diluarate. Representative tertiary organic amine catalysts include triethylamine, triethylenediamine, N,N',N',N'- tetramethyl- ethylenediam.ine, N,N,N',N',-tetra- ethylethylenediamine, N-methyl- morpholine, N-ethylmorphcllne, N,N,N' ,N'-tetramethylguanidine,N,N,N',N'-tetramethyl-1,3- butanediamine, N,N-dimethylethanolamine and N,N-diethylethan- olamine. The amount of catalyst employed is generally within the range of 0.002 to 2.0 percent by weight based on the total weight of the polyurethane reactants, When the elastomeric polyurethane compositions of the present invention are prepared by the less preferred prepolymer method, the diisocyanate and the polyoxytetramethylene glycol (in whole or in part) are reacted, if desired, in the presence of a catalyst as defined above, in a preliminary stage to form an isocyanate-terminated prepolymer. The proportions of diisocyanate and glycol employed in the preparation of this prepolymer are consistent with the ranges defined above. The diisocyanate and the glycol are preferably rendered substantially free from the presence of extraneous moisture, using the methods described above, before the formation of the prepolymer is carried out. The formation of the prepolymer is advantageously carried out at a temperature of 70GC. to 13O0C, under an inert atmosphere such as nitrogen gas, in accordance with conventional procedures. The prepolymer so formed can then be reacted at any desired time, with the extender diol(and the balance, if any, of polyoxytetramethylene glycol not reacted with the diisocyanate in the first stage) to form the polyurethane elastomers of the present invention. This reaction is carried out advantageously within the range of reaction temperatures specified above for the one-shot procedure. In general, the prepolymer and the diol extender (preferably degassed as described above) are mixed and heated within the required temperature range and then transferred to suitable molds, extrusion apparatus, or the like, and cured as described above for the one-shot procedure. If desired, the polyurethane elastomers of the present invention can have incorporated in them, at any appropriate stage of preparation, additives such as pigments, fillers, lubricants,stabilizers,anticxidants, coloring agents, fire retardants, and so on, which are commonly used in conjunction with polyurethane elastomers. An especially preferred polyurethane elastomer for use in the oxymethylene molding compositions of the present invention is prepared by reacting 1.0 equivalent of polyoxytetramethylene glycol having a molecular weight of about 2000 to 1.5 equivalents of 1,4-butanediol and 2.63 equivalents of 4,4'-methylenebis (phenyl isocyanate). The above polyurethane is admixed with 0.25 percent by weight of Advawax 280, an N,N'-ethylene bisstearmide (lubricant) sold commercially by Cincinnati Milacron Chemicals, Inc., and 0.25 percent by weight of Irganox 1010, a tetrakis (methylene 3-(3',5'-di-tert-butyl- 4'-hydroxyphenyl) propionate) methane (antioxidant) sold commercially by Ciba-Geigy Corp. Advawax and Irganox are Trade Marks. The weight percent of the additives are based on the total weight of the polyurethane reactants. The elastomeric polyurethanes of the present invention are further characterized by a Clash-Berg modulus (Tf) which is generally less than -400C. and preferably less than -600C. The Tg (glass transition temperature) of the polyurethanes is essentially the same value. The preferred polyurethanes for use in the present invention are also generally characterized by Shore A Hardness of 90A or less, and a weight average molecular weight in excess of 100,000. PREPARATION OF MOLDING COMPOSITIONS OF THE PRESENT INVENTION The oxymethylene polymer molding compositions of the present invention comprise from 5 to 50 weight percent, and most preferably from 20 to 40 weight percent of the elastomeric polyurethane. Correspondingly, from 50 to 95 weight percent, preferably from 50 to 85 weight percent,and most preferably from 60 to 80 weight percent of the oxymethylene polymer is present. The above weight percentages are based on the total weight of the molding composition. The molding compositions may suitably be prepared by any conventional procedure that will result in an intimate blend of mixture of the components. Preferably, dry or melt blending procedures and equipment are used. For example, the polyurethane (in the form of pellets,chips or granules) can be dry mixed with the oxymethylene polymer (in the form of pellets,chips,granules or powder) typically at room temperature, and the resulting mixture melt blended in any conventional type extrusion equipment,which is heated to a temperature of from 1800C.to 2300C., and preferably from 185bC. to about 2050C. Preferably, the polyurethane elastomer and oxymethy lene polymer are dried (either separately or together) before being subjected to the intimate blending procedure. The drying can for example be done in desiccated air having a dew point of -300C. to-400C. or lower, at a temperature of from 700C. to 1100C., and preferably above 800C. The drying can also be accomplished in a vacuum oven, for example, at a temperature above 900C. The drying time will depend primarily on the moisture content, drying temperature, and particular equipment employed, but typically is from 2 to 6 hours or more. If the drying is conducted for longer periods of time, such as overnight, the drying temperature should preferably be from 700 C. to 850C. In general, any conventional drying procedure can be used to reduce the moisture content to below 0.1 weight percent, based on the total weight of the pclyurethane and oxymethylene polymer, preferably below 0.05 weight percent, and most preferably below 0.01 weight percent or lower. As is well known to those skilled in the art, water will react with polyurethanes upon processing the polyurethanes at elevated temperatures. The oxymethyiene molding composition resulting from the intimate blending procedure may then be cominuted mechanically, for example by chopping, pelletizing or grinding, into granules, pellets, chips, flakes or powders, and processed in the thermoplastic state, for example by injection molding or extrusion molding, into shaped articles, for example bars, rods, plates, sheets, films, ribbons,tubes and so on. Preferably, the comminuted oxymethylene molding composition is dried (as discussed above) prior to being molded. Objects molded from the oxymethylene molding corpositions of the present invent on exhibit enhanced impact strength or resistance as measured by the Notched Izod impact test, when tested in accordance with ASTM method D256. When oxymethylene copolymers are used in the molding compositions, and especially oxymethylene copolymers having a melt index of about 2.5g./1O min., the resulting molded objects have retained enhanced impact strength values at termperatures as low as -200 F (-290C) and lower and the molded objects even exhibit some ductility in a Gardner type impact test at temperatures as low as -400F (-400C). Because of this enhanced low temperature impact resistance such oxymethylene molding compositions are advantageous for use in forming molded objects that will be subjected to use or storage outdoors such as automobile body parts, tool casings, and so on. The following Examples are given as specific illu-; strations of the claimed invention. It should be understood, however, that the specific details set forth in the Examples are merely illustrative and in no way limitative . All parts and percentages in the Examples and the remainder of the specification are by weight of the total composition unless otherwise specified. EXAMPLE 1 The oxymethylene polymer was a trioxane-ethylene oxide copolymer containing about 98 weight percent of recurring -OCH2- groups and about 2 weight percent of cornonorner units derived from ethylene oxide having the formula -OCH2CH2-. The oxymethylene copolymer was prepared as described above and more specifically in the aforementioned U.S. Patent No. 3,027,352 of Walling et al, and had been hydrolyzed to stabilize the ends of the polymer chains. The oxymethylene copolymer possessed a crystallinity of approximately 75 percent, a number average molecular weight of approximately 35,000, an inherent viscosity (I.V.) of approximately 1.3 (measured at 600C. in a 0.1 weight percent solution in p-chlorophenol containing 2 weight percent of alphapinene), a melting temperature of 1650C., and a melt index of approximately 2.5 g./10 min. when tested in accordance with ASTM nethod D1238-82. The oxyrnethylene copolymer also contained, as a stabilizer package, 0.5 percent by weight of Irganox 259, 0.1 percent by weight of cyanoguanidine and 0.2 percent by weight of Acrawax C. The above weight percentages were based on the total weight of the oxymethylene copolymer plus stabilizer package. Using the reactants shown in Table I below where reactant proportions are shown in equivalents) a series of polyurethane elastomers was prepared using the following one-shot batch procedure. Polyoxytetramethylene glycol (PTMG) was degassed by heating under reduced pressure at 1000C for one hour. To the degassed PTMG was added 1,4-butanediol (1,4-BD) diol extender and the resulting mixture was degassed at 900C under reduced pressure for over one hour. The glycol and 1,4-BD were stirred during degassing. To the degassed hydroxyl group mixture, still at 9O0C, was added 0.005 percent (based on the weight of total reactants, i.e., including the diisocyanÅate) of stannous octoate catalyst followed by the addition of 4,4'-methylenebis (phenyl isocyanate) (MDI). The stannous octoate was added as a 50 percent by weight solution of stannous octoate in dioctylphthalate sold by M & T chemicals under the trade designation T-10. The mixture so obtained was stirred until the viscosity increased sharply, which occurred after about 15 seconds, and was then poured into a shallow aluminium tray. The poured elastomers and tray were allowed to stand at 100 C for one hour and left at room temperature (about 2O0C.) for 24 hours. Thereafter the elastomer was chopped into pieces, granulated and dried for 3 hours at 9O0C. A stabilizer-lubricant package of 0.25 weight percent cf Advawax 28C and 0.25 weight percent of Irganox 1010 was mixed with the PTMG during degassing. The above stabilizerlubricant weight percentages were based on the weight of the total urethane reactants, i.e., the PTMG plus 1,4-BD plus MDI. TABLE 1 Polyurethane Elastomer A B C D E F G H MDI 2.04 2.08 2.34 2.58 2.68 3.0 3.09 3.09 PTWiG 1.0 1.0 1.0 1.0 1.0 1.0 1.0 (M..W.*2000) PTMG - - - - - - - 1.0 (K.W.2900) 1,4-BD 1.0 1.0 1.25 1.5 1.5 2.0 2.0 2.0 NCO/OH 1.02 1.04 1.04 1.03 1.07 1.00 1.03 1.03 + M.W. means number average molecular weight. For purposes of comparison several additional polyurethane elastomers were prepared. The reactants and proportions (in equivalents) are set forth in Table 2 bellow. Elastomer (J) was prepared using the same one-shot batch procedure described above with respect to Table 1, but replacing the polyoxytetramethylene glycol there used by polybutylene adipate diol. Elastomer I was prepared by using a different polybutylene adipate diol. In addition, two elastomers (K and L) were prepared using PTMG having a number average molecular weight of only 1000. Elastomers (I,K,L and M) were prepared by using essentially the same procedure described with respect to Table 1, but carrying out the reaction on a continuous one-shot basis utilizing a Werner-Pfleiderer extruder-mixer, as in Example 2,below and pelletizing as in Example 2. The same stabilizerlubricant package, as described above with respect to Table 1, in approximately the same weight percents was also included in each elastomer. TABLE 2 Polyurethane Elastomer I J K L Mi 3.06 3.09 2.25 2.35 2.05 PTMG (?r 1000) - - 1.0 1.0 PTMG (rw 2000) - - - - 1.0 Polybutylene adipatel.O - (MW 2600) 1,4-BD 2.0 2.0 1.3 1.3 1.0 NWO/OH 1.02 1.03 0.98 1.02 1.0175 * Composition M contained 0.012 equivalent of butyl carbitol, which further reduced the molecular weight of the resulting polyurethane as compared to the same composition without the butyl carbitol. Aliquots of each of the polyurethane elastomers (Tables 1 and 2) were then dry mixed with the above described cxymethylene copolymer and the resulting mixture dried in a dehumidifying dryer (air dew point of -28.90C.) at a temperature ranging from 700C. to 930C. for 4 to 16 hours. Lower temperatures were used for the longer drying times. In each instance, 72 weight percent of the oxymethylene copolymer was used with 28 weight percent of the polyurethane (the weight percentage was based on the total weight of the polyurethane and oxymethylene copolymer). Each resulting polyurethane-oxymethylene mixture was then melt blended in a 1" (2.54cm ) diameter single screw extruder at a temperature of about 1900C. for approximately 20 to 30 seconds, extruded into a strand and pelletized. Polyurethane elastomer K, prior to being mixed and melt blended with the oxymethylene copolymer, was processed in the single screw extruder under the same conditions described above, extruded into a strand and pelletized. The pre-extrusion of elastomer K further reduced its molecular weight. The resulting polyurethane-oxymethylene copolymer pellets were again dried to reduce the moisture level to below about 0.03 weight percent and then molded in a 175 ton Reed Prentice injection molding machine to form bars of 5" x 0.5" x0.25" (12.7 cm x'1.27 cm x 0.635 cm) and ASTM D-638 T-bars for Notched Izod testing. Samples of 0.5" x 0.125" (1.27 cm x 0.3175 cm) were cut frcm the T-bas for the Notched Izod testing. Table 3 below sets forth relevant information concerning the blends and the Notched Izod impact rest results obtained. All Notched Izod impact test results set forth in the Examples 1 through 7 herein were obtained in accordance with ASTM method D 256. In Table 3, the room temperature is identified as 730F (230C), the Notched Izod at that temperature is referred to " (i.e. 0,125" or 0.3175 cm) and to 1/ 4" (i.e. 0.25" or 0.635 cm), and the Notched Izod at -200F (-290C) is referred to " (i.e. 0.125" or 0.635cm). TABLE 3 Corresponding polyurethane from Tables 1 and 2 A B C D E F G H I J K L M*** Oxymethylene Copolymer (wt.%) 72 72 72 72 72 72 72 72 72 72 72 72 72 Polyurethane (wt.%) 28 28 28 28 28 28 28 28 28 28 28 28 28 PIMG (1000) - - - - - - - - - - 1.0 1.0 *BA (2000) - - - - - - - - 1.0 - - - PIMG (2000) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 - - - - - 1.0 *BA (2600) - - - - - - - - - 1.0 - - PIMG (2900) - - - - - - - 1.0 - - - - 1,4-BD 1.0 1.0 1.25 1.5 1.5 2.0 2.0 2.0 2.0 2.0 1.3 1.3 1.0 NCO/OH 1.02 1.04 1.04 1.03 1.07 1.00 1.03 1.03 1.02 1.03 0.98 1.02 1.0175 Room temperature (73 F) Notched Izod 1/8*** 12.4 18.3 17.7 19.0 19.0 12.6 17.4 18.3 19.5 17.0 4.9 18.7 7.5 Room temperature (73 F) Notched Izod 1/4" 6.6 8.5 13.5 13.0 15.0 6.8 6.8 13.0 7.7 15.0 4.9 18.7 7.5 -20 F Notched Izod 1/8" 3.2 3.4 3.6 3.4 4.3 2.9 3.6 3.1 1.2 1.4 - 1.8 3.0 Tf ( C.) # 5 C.**** -70 -70 -67 -65 -65 -62 -62 -70 -38 -42 -50 -48 -70 * polybutylene adipate ** All Notched Izod results are in ft.-lbs/in. notch *** Elastomer M also contained 0.012 equivalent of butyl carbitol **** Estimated Tf For further comparison purposes, the above described oxymethylene copolymer (without any polyurethane modifier) has a room temperature (730F) or 230C Notched Izod (") (i.e. 0.125" or 0.3175 cm) value of from 1.5 to 2.0 and a -200F (-290C) and -400F (-400C) Notched Izod ()"') (i.e. 0.125" or 0.3175 cm) value of about 1.2. EXAMPLE 2 Using the reactants and proportions set forth below a polyurethane elastomer was prepared using essentially the same procedure as in Example 1, but carrying out the reaction on a continuous one-shot basis utilizing a Werner-Pfleiderer extruder-mixer. The stabilizer-lubricant package was added with the reactants to the extruder-mixer. The polyurethane was extruded in the form of strands which were chopped into pellets. An aliquot of the pellets was injection molded in the form of a test sheet (5" x 5" x 060") (i.e. 12.7 cm x 12.7 cm x 0.15 cm) which was subjected to physical testing. The physical properties so determined are set forth below. ELASTOMER COMPOSITION MDI 2.63 (equivalents) PTMG (MW 2000) 1.0 (equivalents) 1,4-BD 1.5 (equivalents) Advawax 280 0.25 wt % (based on total-reactants) Irganox 1010 0.25 wt % (based on total reactants) Stannous octoate 0.025 wt % (based on total reactants) NCO/OH 1.05 PHYSICAL PROPERTIES Hardness, Shore A 75 + 5 Specific gravity 1.09 Tensile Modulus: psi 100% 700 300% 1300 Tensile strength: psi 4500 Tensile set : X 25 Elongation: % 550 at break Melt index, 1900C 1.0 g/10 min. 5 min.preheat,8700 g Tg ( C) about -65 EXAMPLE 3 Using the reactants and proportions (equivalents) set forth below (Table 4) three polyurethane elastomers were prepared using essentially the same one-shot continuous procedure and equipment as in Example 2. The resulting polyurethanes were then extruded in the form of strands which were chopped into pellets. An aliquot of each polyurethane was then melt blended with an aliquot of the oxymethylene copolymer described in Example 1. (A similar 72/28 weight percent blend was used). The mixing, melt blending and drying procedures were as described in Example 1. The resulting dried polyurethane-oxymethylene copolymer blend was then injection molded as described in Example 1 to form the " (0.635 cm) and (ll) (0.3175 cm) thick test specimens in order to perform the Notched Izod impact test. Table 5 sets forth the impact tests results. TABLE 4 Polyurethane Elastomer N O P MDI 2.58 2.63 2.68 PTMG (MW 2000) 1.0 1.0 1.0 1,4-BD 1.5 1.5 1.5 Advawax 280* 0.25 wt.% 0.25 wt.% 0.25 wt.% Irganox 1010* 0.25 wt.% 0.25 wt.% 0.25 wt.% Stannous Octoate* 0.025 wt.% 0.025 wt.% 0.025 wt.% NCO/OH 1.03 1.05 1.07 * wt.% based on total weight of the MDI,PTMG and 1,4-BD. TABLE 5 Polyurethane Elastomer N O P Oxymethylene 72 72 72 copolymer (wt.%) Polyurethane (wt.%) 28 28 28 NCO/OH 1.03 1.05 1.07 Room Temperature(73 F) 17.0 16.0 16.0 Notched Izod 1/8''* Room Temperature (730F) Notched Izod \" 13.5 12.5 13.5 -200F Notched Izod 1/8" 2.9 3.5 2.8 *All Notched Izod results are in ft.-lbs./in.notch. In Table 5, the room temperature is identified as 730F t23 C) and the Notched Izod at that temperature is referred to 1/8" (i.e. 0.125" or 0.3175 cm) and to ·" (i.e. 0.25" or 0.635 cm), and the Notched Izod at -20 F (-290c) is referred to 1/8" (i.e. 0.125" or 0.635 cm). EXAMPLE 4 For purposes of further comparison, a commercially available ester based polyurethane elastomer sold by Mobay Chemical under the designation Texin 591A was blended with the oxymethylene copolymer described in Example 1 in accordance with the same blending procedure set forth in Example 1. A blend of 30 weight percent Texin 591A with 70 weight percent oxymethylene copolymer was used. The resulting pellets were molded as described in Example 1 to form the all (0.635 cm) and 'X" (0.3175 cm) thick bars for testing. The room temperature (730F) Notched Izod results obtained were 4.0 ft.-lbs./in. Notch for both the " (0.635 cm) and &num;" (0.3175 cm) thick samples. EXAMPLE 5 Aliquots of the polyurethane prepared as described in Example 2 were melt blended with aliquots of the oxymethylene copolymer described in Example 1. Table 6 below sets forth the weight percent of the oxymethylene copolymer and polyurethane used to prepare the blends. The mixing, melt blending and drying procedures were as described in Example 1. The resulting dried polyurethane-oxymethylene copolymer blends were then injection molded as described in Example 1 to form the " (0.635 cm) and X" (0.3175 cm) thick test specimens in order to perform the Flexural Modulus and Notched Izod impact tests. Table 6 sets forth the impact tests results and the Flexural Modulus. The Flexural Modulus was measured in accordance with ASTM Test Method D790 using a ' (0.635 cm) thick specimen. TABLE 6 Delrin Q R S T 100-ST* Oxymethylene Copolymer (wt.%) 80 75 70 65 Polyurethane (wt.%) 20 25 30 35 Room Temperature (730F) Notched Izod X'' 7 11 17.5 17 16 Room Temperature (730F) Notched Izod \' 4.5 7.5 13.5 14 7.5 -20 F Notched Izod 1/8" 2.0 2.2 3.5 4.5 2.3 Flexural Modulus (X10ê)·" 200 190 165 150 165 * Delrin 100-ST is a polyurethane modified oxymethylene homopolymer blend sold commercially by Dupont. About 30 to 35 weight percent polyurethane is contained in the Delrin blend. In Table 6, the room temperature is identified as 73eF (230C), the Notched Izod at that temperature is referred to " (i.e. 0.125" or 0.3175 cm) and to " (i.e. 0.25" or 0.635 cm), the Notched Izod at -200F (-290C) is referred to " (0.125" or 0.3175 cm) and the Flexural Modulus is referred to " (i.e. 0.25" or 0.635 cm) EXAMPLE 6 Aliquots of the oxymethylene copolymer as in Example 1 were blended with the polyurethane as in Example 2 in the weight percent ratios shown in Table 7 below. The oxymethylene - polyurethane mixtures were dried in a vacuum oven at 800C. overnight, extruded in a \" (0.75" or 1.905 cm) Brabender single screw extruder and pelletized and the resulting pellet blends were dried again in the vacuum oven under the same conditions as described above, and then molded in a 2.5 oz (71g) Arburg injection molding machine to form 5" x 0.5" x 0.125"' (12.7 cm x 1.27 cm x 0.3175 cm) test bars and ASTM D-638 T-bars for Notched Izod and Tensile Modulus testing. The tensile modulus testing was done on an Instron testing machine. TABLE 7 Blend 1 2 3 4 5 Oxymethylene Copolymer (wt.%) 90 80 70 60 50 Polyurethane (wt.%) 10 20 30 40 50 Room Temperature (730F) Notched Izod all 2.4 4.5 BN* NB NB -20 F Notched Izod X" 1.4 1.6 2.4 7.4 NB -40 F Notched Izod 1/8" 1.6 1.3 1.6 2.4 7.8 Tensile Modulus (X10 psi) 271 205 171 128 102 *NB means no break, i.e. greater than about 16 ft.-lb/in.Notch. In Table 7, the room temperature is indentified as 730F (230C), the Notched Izod at that temperature is referred to " (i.e. 0.125" or 0.3175 cm), the Notched Izod at - 200F (-290C) is referred to X" (i.e. 0.125" or 0.3175 cm), and the Notched Izod at -400F (-400C) is also referred to " (0.125" or 0.3175 cm). EXAMPLE 7 Aliquots of the oxymethylene copolymer as in Example 1 were blended with the polyurethane as in Example 2 (Blend 3) and two comparison polyurethanes having different glass transition temperatures (Tg) (Blends 1 and 2). The copolymer and polyurethanes were dried to reduce the moisture content. The copolymer and polyurethane mixtures were then melt blended, pelletized and dried, and the resulting pellet blends were molded to form all (0.125" or 0.3175 cm) thick test specimens for Notched Izod testing. The Notched Izod test results are set forth in Table 8 below. TABLE 8 Blend 1 2 3 Oxymethylene Copolymer (wt.%) 80 80 80 Polyurethane (wt.%) 20 20 20 Tg* -48.20C -36.80C -640C. Room Temperature (730F) 1.9 3.3 4.5 Notched Izod &num;" -200F Notched Izod 1/8" - - 1.6 * As measured by Dupont DSC 1090 Thermal Analyzer at 200C per minute heating rate. In Table 8, the room temperature is identified as 730F (230C) and the Notched Izod at that temperature is referred to 3'll (i.e. 0.125" or 0.3175 cm), and the Notched Izod at -200F (-290C) is also referred to X'' (i.e. 0.125" or 0.3175 cm). While the invention has been illustrated by various preferred embodiments, it will of course be appreciated that various modifications, substitutions, omissions, and changes may be made.

Claims: CLAIMS 1. An oxymethylene polymer molding composition which is capable of being molded to form articles exhibiting enhanced impact strength, comprising an intimate blend of: (a) from 5 to 50 weight percent (based upon the total weight of the composition) of an elastomeric poly-urethane which is the reaction product of: : (i) 4,4'-methylenebis (phenyl isocyanate), (ii) polyoxytetramethylene glycol having a molecular weight within the range from 1500 to 3000, and (iii) at least one diol extender selected from aliphatic straight chain diols having from 2 to 6 carbon atoms, bis (2-hydroxy ethyl) ether of hydroquinone, and bis(2 hydroxy-ethyl) ether of resorcinol, the diol extender being used in the reaction in an amount of 0.5 to 2.5 equivalents of extender per equivalent of polyoxytetramethylene glycol, and the phenyl isocyanate being used in the reaction in an amount such that the ratio of isocyanate groups to total hydroxyl groups is in the range from 1.0 :1.0 to 1.08 :1.0, and (b) from 50 to 95 weight percent (based upon the total weight of the composition) of an oxymethylene polymer comprising at least 50% of recurring -CH,0-units. 2. An oxymethylene polymer molding composition according to Claim 1 wherein the oxymethylene polymer is selected from: (i) oxymethylene homopolymer, (ii) oxymethylene copolymer comprising 85 to 99.9 percent recurring -OCH2- groups interspersed with groups of the formula: EMI31.1 wherein each R1and Ro is selected from hydrogen, lower alkyl and halogen-substituted lower alkyl radicals, each R3 is selected from methylene, oxymethylene, lower alkyl and halo alkyl-substituted methylene, and lower alkyl and haloalkyl-substituted oxymethylene radicals n is 0,1,2 or 3, each lower alkyl radical having 1 or 2 carbon atoms, the copolymer having a number average molecular weight of at least 10,000 and a melting point of at least 15O0C, and (iii) oxymethylene terpolymer, which is the reaction product of trioxane, a cyclic ether and/or cyclic acetal, and a diglycide of the formula: EMI32.1 wherein Z is selected from a carbon-to-carbon bond, oxygen, an oxyalkoxy of 1 to 8 carbon atoms, and an oxypoly (lower alkoxy). 3. An oxymethylene copolymer molding composition according to Claim 2 wherein the groups represented by the formula: EMI32.2 are incorporated during the step of copolymerization to produce the oxymethylene copolymer by the opening up of a ring of a cyclic ether having adjacent carbon atoms by the breaking of an oxygen-to-carbon linkage the said cyclic ether being represented by the formula EMI33.1 wherein n is 0,1 or 2. 4. An oxymethylene copolymer molding composition according to Claim 2 wherein the oxymethylene copolymer comprises about 98 percent recurring -OCH2- groups and about 2 percent recurring -OCH2CH2- groups and is derived from trioxane and ethylene oxide. 5. An oxymethylene copolymer molding composition according to any of Claims 2-4 wherein the oxymethylene copolymer has a meld index of about 2.5g./10 min. 6. An oxymethylene polymer molding composition according to any of Claims 1-5 wherein the diol extender is used in the-stated reaction in an amount of 1 to 2 equivalents of extender per equivalent of polyoxytetramethylene glycol. 7. An oxymethylene polymer molding composition according to any of Claims 1-6 wherein the polytetramethylene glycol has a molecular weight of 2000 to 2900. 8. An oxymethylene polymer molding composition according to Claim 6 wherein the polyoxytetramethylene glycol has a molecular weight of about 2000 and the diol extender is used in the reaction within the amount of 1.2 to 1.6 equivalents of diol extender per equivalent of polyoxytetramethylene glycol. 9. An oxymethylene polymer molding composition according to any of Claims 1-8 wherein the diol extender is an aliphatic straight chain diol having from 2 to 6 carbon atoms. 10. An oxymethylene polymer molding composition according to Claim 9 wherein the diol extender is 1,4-butanediol or 1,6-hexanediol. 11. An oxymethylene polymer molding composition according to any of Claims 1-10 wherein the phenyl isocyanate is used in the stated reaction in an amount such that the ratio of isocyanate groups to total hydroxyl groups is within the range of 1.02: 1.0 to 1.07 : 1.0. 12. An oxymethylene polymer molding composition according to any of Claims 1-11 wherein the weight percent of elastomeric polyurethane is from 15 to 50, and the weight percent of oxymethylene copolymer is from 50 to 85. 13. An oxymethylene copolymer molding composition according to Claim 12 wherein the weight percent of elastomeric polyurethane is from 20 to 40, and the weight percent of oxymethylene copolymer is from 60 to 80. 14. An oxymethylene copolymer molding composition which is capable of being molded to form articles exhibiting enhanced impact strength, comprising an intimate blend of (a) from 20 to 40 weight percent based upon the total weight of the composition of an elastomeric polyurethane, which is the reaction product of (i) 4,4'-methylenebis (phenyl isocyanate), (ii) polyoxytetramethylene glycol having a molecular weight within the range from 2000 to 2900, and (iii) at least one diol extender selected from aliphatic straight chain diols having from 2 to 6 carbon atoms, the diol extender being used in the reaction in an amount of 1 to 2 equivalents of extender per equivalent of polyoxytetramethylene glycol, and the phenyl isocyanate being used in the reaction in an amount such that the ratio of isocyanate groups to total hydroxyl groups is in the range from 1.03 : 1.0 to 1.06 : 1.0, and (b) from 60 to 80 weight percent based upon the total weight of the composition of an oxymethylene copolymer comprising about 98 percent recurring -OCH2groups interspersed with about 2 percent recurring -OCH2CH2groups, the oxymethylene copolymer having a number average molecular weight of at least 1O,000,a melting point of at least 1500C. and a melt index of about 2.5g./10 min., and being drived from trioxane and ethylene oxide. 15. An oxymethylene copolymer molding composition according to Claim 14 wherein the aliphatic straight chain diol is 1,4-butanediol, the polyoxytetramethylene glycol has a molecular weight of about 2000, and the diol extender is used in the reaction in an amount of from 1.2 to 1.6 equivalents of extender per equivalent of polyoxytetramethylene glycol. 16. An article shaped or molded from an oxymethylene polymer molding composition according to any of Claims 1-15.

Inventors: DRZAL, ROBERT S., EHRLICH, BENJAMIN S., LA NIEVE III, LESLIE H.
Applicants: CELANESE CORPORATION
Application year: 1985
Main classification: C08L59
IPC codes: C08G2, C08L75, C08G18, C08L59