the polyether polyols which can be used herein can be selected from amongst any of the polyether polyols heretofore employed in the preparation of polyurethanes . such polyols possess two , and preferably , three or more hydroxyl groups . among the useful polyether polyols are included the poly -( oxypropylene ) glycols , the poly -( oxypropylene ) poly -( oxyethylene ) glycols , the poly -( 1 , 4 - oxypropylene ) glycols and graft copolymers of the poly -( oxypropylene )-( polyoxyethylene ) glycols with acrylonitrile or mixtures of acrylonitrile and styrene . the equivalent weight of these polyether diols can range between 200 to 1000 with a preferred range of 200 to 400 . the polyol may consist of simple polyfunctional alcohols such as glycerine , trimethylolpropane , 1 , 2 , 6 - hexanetriol , or pentaerythritol , or they may consist of polyether triols such as poly ( oxypropylene ) or poly ( oxyethylene ) adducts of the above polyols . the equivalent weight of the polyether polyols may range between 100 to 800 with a preferred range of 100 to 500 . it is also understood that various combinations of diols and polyols may be used . the aromatic polyisocyanates used for the preparation of the hard polyurethane elastomers and optional soft polyurethane prepolymers must contain the isocyanate groups directly bonded to the aromatic nucleus . such aromatic isocyanates include , but are not limited to , 2 , 4 - tolylene diisocyanate ( tdi ), isomeric mixtures of tdi , 3 , 3 &# 39 ;- tolidene 4 , 4 &# 39 ;- diisocyanate ( todi ), 3 , 3 &# 39 ;- dimethyldiphenylmethane 4 , 4 &# 39 ;- diisocyanate , diphenylmethane 4 , 4 &# 39 ;- diisocyanate ( mdi ), mixtures of mdi and adducts of mdi , etc . the elastomers can be prepared by either the prepolymer method in which a prepolymer having a ratio of nco to oh groups greater than 1 : 1 is prepared in a first step followed by chain extension and cross - linking of the prepolymer to form the finished elastomer , or by the one - shot method in which all of the reactants are mixed in a single step to form the final polymer . both methods are well known in the art ( see , for example , polyurethanes , chemistry and technology , part ii . technology by saunders et al ., interscience 1964 ). aromatic isocyanate - terminated prepolymers suitable for preparing the optional soft polyurethane elastomers ( soft prepolymers ) are based on polyether diols alone or combinations of polyether diols or triols , and aromatic polyisocyanates . the same diols and polyols as described above may be used but the average equivalent weight is significantly higher than that used in the preparation of the hard polymer . the preferred range of equivalent weight of the polyethers ( diols or combination of diols and triols ) is 450 to 1500 . they are cured in the same way as the hard prepolymers . the ratio of nco to oh in the preparation of the optional soft isocyanate - terminated prepolymer may range between about 1 . 75 to about 2 . 5 with a preferred range of about 2 . 0 to about 2 . 25 , while the nco / oh of the hard isocyanate - terminated prepolymers should have a free nco content of about 3 . 5 to about 5 . 5 percent , preferably about 3 . 7 to about 4 . 7 percent , and the hard isocyanate - terminated prepolymers , a free nco content of about 9 . 5 to about 14 percent , preferably about 10 to about 13 percent . for the curing ( cross - linking ) of the hard and soft prepolymers , preferred polyols are tertiary amine - containing polyols such as poly ( oxypropylene ) or poly ( oxyethylene ) adducts of diamines or triamines , such as ethylenediamine , diethylene triamine , tolyenediamine , phenylenediamine , or aniline , or any diols , polyols or their blends . advantageously , they are polyols of relatively low molecular weight such as are obtained by condensing propylene oxide with ethylenediamine or pentaerythritol to a molecular weight of about 500 , or of trimethylolpropane or any other base compound to a molecular weight up to 2500 . another preferred curing or cross - linking agent is a hydroxyl - terminated prepolymer . these are prepared essentially the same way as the aromatic isocyanate - terminated prepolymers but the ratio is such that there are free and unreacted hydroxyl groups . the same diols and polyol and aromatic isocyanates can be used , though it is preferred that the prepolymer have a functionality greater than about 2 , which can be obtained by using a polyol having a functionality greater than about 2 and / or an aromaic isocyanate having a functionality greater than 2 . the ratio of oh / nco in the hydroxyl - terminated prepolymers , advantageously , may be in the same range as the nco / oh ratio in the aromatic isocyanate - terminated prepolymers . it will be understood , however , that inasmuch as the cross - linking agent may consist of one or more diols or polyols ( no isocyanate ), the ultimate oh / nco ratio is infinity . another preferred curing or cross - linking agent is a prepolymer - polyol blend . thus , a polyurethane prepolymer , advantageously , one having neither free nco nor free oh groups , can be mixed with a polyol , advantageously a polyol having a functionality of more than 2 , to form a prepolymer - polyol blend . when such a blend is mixed with an aromatic isocyanate - terminated prepolymer in a nco / oh ratio of greater than about 1 , cross - linking is effected both through an nco - oh reaction and through nco - urethane reaction . when a soft elastomer is joined to a hard elastomer to form a unitary whole , the aromatic isocyanate - terminated prepolymers and the cross - linking agent can be mixed in proportions to give an nco / oh ratio of at least about 1 . 05 to about 1 . 0 and preferably not greater than about 1 . 1 to about 1 . 0 . this excess of nco groups ensures a cross - linked polymer which is non - hydrophilic and one which is sufficiently reactive so that the hard non - hydrophilic polyurethane elastomers react chemically with the optional soft non - hydrophilic polyurethane elastomers to form an integral chemical bond between the two . alternatively , the surface of the hard tooth - engaging elastomer element can be coated with a primer formulation prepared by mixing polyisocyanate with polyol and thereafter applying the soft mouth - engaging elastomer formulation to the surface thus coated . upon curing of the soft elastomer formulation , a denture will be provided in which the hard and soft elements are permanently bonded to each other . in order to accelerate the formation of the prepolymers or the cure of both the hard and soft aromatic isocyanate - terminated prepolymers with the cross - linking agents , metal catalysts such as tin catalysts , for example , dibutyltin dilaurate and stannous octanoate , can be used . in the following resin formulations ( all parts by weight ) which are illustrative of the invention herein , the ingredients whose properties are given in the table below were employed . table______________________________________components of hard and soft non - hydrophilicpolyurethane elastomerspolyether average hydro - polyol molecular xylcomponent wt . no . description______________________________________pep 650 600 376 pentaerythritol ( basf - wyandotte ) oxyalkylated with propylene oxide . pep 450 400 560 pentaerythritol ( basf - wyandotte ) oxyalkylated with propylene oxide . 1 , 4 - butane diol 90 1247polyol 1370 5600 30 glycerol oxyalkylated ( wencol , inc .) with a mixture of ethylene oxide and propylene oxide . polyol 1620 100 1160 glycerol oxyalkylated ( wencol , inc .) with a mixture of ethylene oxide and propylene oxide . polyol 41 - 42 700 232 glycerol oxyalkylated with propylene oxide . polyol 11 - 27 6200 27 propylene glycol oxyalkylated with ethylene oxide______________________________________aromatic poly - isocyanatecomponent description______________________________________isonate ( upjohn ) mixture of 60 % diphenyl methane 4 , 4 &# 39 ;- index no . 181 diisocyanate and 40 % higher molecular weight adducts . isonate ( upjohn ) mixture of diphenyl methane 4 , 4 &# 39 ;- diiso - index no . 143 l cyanate and trifunctional adduct of diphenyl methane 4 , 4 &# 39 ;- diisocyanate . ______________________________________othercomponent description______________________________________modaflow ( monsanto ) acrylate polymer used as a surface6 , 000 average tension modifiermolecular weight______________________________________hard polyurethane elastomersformulation ipolyol componentpep 450 67 . 75polyol 41 - 42 32 . 25modaflow q . s . * aromatic isocyanateisocyanate 181 indexed at 1 . 05formulation iipolyol componentpep 650 100modaflow q . s . aromatic isocyanateisocyanate 181 indexed at 1 . 05formulation iiipolyol componentpep 450 100modaflow q . s . aromatic isocyanateisocyanate 181 indexed at 1 . 05formulation ivpolyol componentpep 650 851 , 4 - butanediol 10polyol 41 - 42 5modaflow q . s . aromatic isocyanateisocyanate 181 indexed at 1 . 05soft polyurethane elastomerformulation vpolyol componentpolyol 1370 98 . 5polyol 1620 1 . 5modaflow q . s . aromatic isocyanateisocyanate 181 indexed at 1 . 1polyurethane investment formulationformulation vipolyol componentspolyol 1127 82 . 3pep 650 17 . 7modaflow q . s . aromatic isocyanateisocyanate 143l indexed at 1 . 1______________________________________ * q . s . = quantity sufficient comparison was made between two lower dentures ( horseshoe - shaped ) for resistance to thermal distortion . the dentures were prepared in the same way with the exception that one denture was prepared with a non - aromatic isocyanate ( i . e ., 4 , 4 &# 39 ;- dicyclohexylmethane diisocyanate , as in the process of the prior art ( denture a ) and the other denture was prepared with isocyanate 181 , a mixture of aromatic isocyanates containing 60 % diphenyl methane 4 , 4 &# 39 ;- diisocyanate and 40 % higher molecular weight adducts , in accordance with the invention herein ( denture b ). the cured dentures were each placed in a water bath maintained at a constant 90 ° f . for a period of 7 days . the dentures were then subjected to approximately equal amounts of flexural force . denture a which was prepared with the non - aromatic isocyanate readily flexed to a substantial extent under these conditions while denture b which was prepared with the aromatic isocyanate material remained substantially rigid . both dentures were then placed in an oven maintained at a constant 195 ° f . for two hours and then subjected to flexural force . while denture a exhibited extreme flexibility , denture b retained its original shape with no appreciable distortion . the description whereby dentures are manufactured in accordance with this invention covers three basic situations . in situation a , a hard denture which can be fabricated of hard polyurethane in accordance with this invention or any polymer heretofore used in the making of dentures is supplied by a dental laboratory or dentist to be provided with a soft polyurethane elastomer liner as described herein . in situation b , a wax - up denture is supplied by a dental laboratory or dentist to be made into a denture containing both a hard tooth - engaging polyurethane elastomer element and a soft mouth - engaging polyurethane elastomer element . in situation c , the wax - up denture is made into a denture containing hard elastomer only . upon receipt of the hard denture or wax - up denture as the case may be , the plaster model is sealed ( i . e ., a coating is placed on all exposed plaster surfaces except the bottom ). the denture is then placed in a flask such that the lowest portion of the denture is even with the flask . investment material is then introduced into the flask even with the top of the flask . after the investment has set - up , a mold release agent is applied . the mold release agent is applied to all surfaces , i . e ., investment , denture and teeth . iii . situations b and c ( wax - up to be fabricated into a denture ) the mold release agent is applied to the investment material and a primer , e . g ., a solution of isopropyl alcohol and organosilane coupling agent , is applied to the teeth to provide adhesion to the investment . after the primer or mold release agent has dried ( approximately five minutes ), additional investment material is applied to cover the entire denture . the flask is then completely sealed by fastening a lid thereon . the flask is separated and the denture removed . the denture is then ground out to provide room for the soft polyurethane elastomer . the flask is heated in an oven or a hot water bath to melt the wax . the flask is then separated and hot water is forced into the plaster model and mold cavity to remove all traces of excess wax . all undercuts are blocked out with duct sealant to facilitate the future removal of the denture without destroying the model . mold release agent is applied to the plaster model and the mold cavity . the liquid hard polyurethane formulation ( one - shot ) is introduced into the mold cavity and low spots on the plaster model . the plaster model , acting as a lid , is placed on top of the mold , the entire assembly is placed in a clamp and the clamped mold is placed in an oven heated to 85 ° c . after about five minutes , the assembly is removed from the oven and cooled until comfortable to the touch . the mold is opened and the denture is removed from the investment and plaster model . the denture is trimmed , polished , etc ., to provide the finished product . the denture is ground out to provide room for the application of the soft polyurethane formulation . following the grinding out of the denture , the denture is coated with anhydrous isopropanol or ethanol and air - dried . a primer ( e . g ., 7 . 8 g pep 650 mixed with 10 . 0 g isonate 181 ) is applied to all surfaces of the denture where the soft elastomer is to adhere . the block - out material is removed from the appropos plastic model . mold release is again applied to the mold and plaster model and permitted to air - dry ( approximately five minutes ). the primed denture is then inserted in the mold cavity . liquid soft polyurethane formulation is introduced into the mold cavity and low spots on the plaster model . steps ix and x are carried out to complete the manufacture of the denture . the dentures herein can also be prepared employing any of the other known and conventional techniques known in the art .
a hard , substantially non - hydrophilic polyurethane elastomer denture is disclosed in which the elastomer possesses a hardness of not less than about shore d60 and preferably not greater than about shore d100 . the polyurethane elastomer is prepared from a polyether polyol and an aromatic polyisocyanate in which the isocyanate groups are bonded directly to the aromatic nucleus . the aforesaid hard non - hydrophilic polyurethane elastomer can constitute the entire material of which the denture is made , including the teeth if so desired , or it can be employed as the tooth - holding portion of the denture in combination with a soft , substantially non - hydrophilic elastomer as the mouth - engaging portion of the denture . the hard non - hydrophilic polyurethane elastomer possesses superior resistance to thermal distortion under the conditions prevailing in the mouth .