Abstract:
Polyurethane (PUR) cast elastomers are produced from NCO-functional prepolymers based on 2,4′-MDI satisfying specific compositional requirements and amine-based chain extenders and/or crosslinking agents.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to polyurethane (PUR) cast elastomers made from NCO-functional prepolymers based on 2,4′-MDI and amine-based chain extenders and/or crosslinking agents and to a process for their preparation.  
         [0002]     MDI (diphenylmethane diisocyanate) is a technically important group of poly-isocyanates; it has a very heterogeneous composition in terms of its structure and comprises (a) monomer grades characterized in that they have two aromatic structural elements bonded via a single methylene bridge, and (b) higher oligomers having more than two aromatic structural elements and possessing more than one methylene bridge, which are referred to as polymeric MDI.  
         [0003]     Monomeric MDI contains predominantly the 4,4′ and 2,4′ isomers as a consequence of its synthesis. The 2,2′ isomer also occurs to a lesser extent, but is largely of no technical value.  
         [0004]     The ratio of monomeric MDI to polymeric MDI, and the proportions of the 2,4′ and 4,4′ isomers in monomeric MDI, can be varied within wide limits by varying the conditions of synthesis of the precursor.  
         [0005]     The crude MDI obtained in the MDI synthesis is separated substantially by distillation, it being possible, depending on technical expenditure, to separate off either almost isomerically pure fractions with proportions of 4,4′-MDI, for example, of more than 97.5 wt. %, or isomer mixtures with, for example, proportions of 4,4′-MDI and 2,4′-MDI of about 50 wt. % in each case.  
         [0006]     In the past, because of technical conditions, pure 2,4′ isomer was commercially available only in very limited quantities, if at all. Recently, however, more effort has been devoted to making this isomer available in high purity as well. A basic reason for this effort is the difference in reactivity of the 2- and 4′-NCO groups of 2,4′-MDI, in a way similar to the differences in reactivity of the 2- and 4-NCO groups of 2,4-toluene diisocyanate (TDI).  
         [0007]     These differences in reactivity allow or facilitate the synthesis of monomer-poor NCO prepolymers. NCO prepolymers are materials with terminal NCO groups which are obtained by reacting a polyol with a polyisocyanate using a molar excess of NCO, based on the NCO-reactive groups, at a temperature of from room temperature to about 100° C. Depending on the initial molar proportions, NCO prepolymers prepared in this way always contain free monomeric diisocyanate.  
         [0008]     In the case of 2,4-TDI, the driving force behind the preparation of monomer-poor to practically monomer-free NCO prepolymers is justified by its high vapor pressure and the resulting health hazards. NCO prepolymers based on aliphatic diisocyanates, e.g. hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI), are to be regarded as even more critical in this context. This aspect is also relevant to MDI, although to a markedly reduced extent because its vapor pressure is lower than that of TDI. Moreover, reducing the monomer content of the prepolymer results in polyurethanes that are softer than those prepared from monomer-containing NCO prepolymers.  
         [0009]     Monomer-poor NCO prepolymers can be prepared in several different ways: 
        a.) Removal of the free monomeric diisocyanate by technically expensive film evaporation or short-path evaporation. This is independent of whether the diisocyanates used have NCO groups of the same or different reactivity. Entraining agents, for example, can also be used for this purpose.     b.) Use of diisocyanates with NCO groups of different reactivity or NCO groups of the same reactivity, and specially chosen stoichiometric proportions, e.g. molar proportions of NCO to NCO-reactive groups of less than 2:1, and/or optionally under special catalysis conditions.     c.) Combinations of both of the above-described a.) and b.) processes, e.g., in such a way that the proportion of free monomeric diisocyanate is initially limited to a certain extent by process b.) and then minimized further by process a.).        
 
         [0013]     Such combination processes can be useful when the viscosity of the prepolymers is to be minimized. The disadvantage of process b.) is basically that reactions with stoichiometric proportions (particularly proportions of less than 2:1) lead to increased pre-extension, inherently resulting in a marked increase in the viscosity of the reaction product.  
         [0014]     WO 01/40340 A2 (Crompton Corp.) gives examples of such combinations wherein, in a first step, the diisocyanate is converted to an NCO prepolymer with the concomitant use of a selectivity-increasing catalyst, and the prepolymer is then freed of excess monomer by film evaporation.  
         [0015]     Particularly critical applications, for instance in the food sector, are affected by the matter of industrial hygiene, which applies to a high degree to TDI and also to MDI. This is indicated by numerous patents dealing with monomer-poor MDI prepolymers, e.g., WO 03/006521 (Henkel KGaA), WO 03/033562 (Henkel KGaA), WO 03/055929 (Henkel KGaA), WO 03/051951 (Henkel KGaA), WO 93/09158 (Bayer AG) and EP 0 693 511 A1 (Bayer AG).  
       SUMMARY OF THE INVENTION  
       [0016]     The object of the present invention was therefore to provide polyurethanes based on 2,4′-MDI which have processing advantages compared with the state of the art without sacrificing the mechanical properties of the product. Such processing advantages include longer casting times and lower prepolymer viscosities.  
         [0017]     Surprisingly, it has now been found that, in terms of mechanical properties (e.g., abrasion, ultimate strength, tear propagation resistance, elongation at break), valuable PUR products are obtained from NCO prepolymers based on 2,4′-MDI with a 2,4′ isomer content of at least 85 wt. % and a proportion of free monomeric MDI in the prepolymer of at least from about 1 wt. % to 20 wt. %, preferably of from 2 to 18 wt. %, most preferably from at least 3 wt. % to 15 wt. %, based on the prepolymer. The low viscosity of the NCO prepolymers is a further advantage. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     NCO prepolymers are understood hereafter as meaning NCO prepolymers which have been prepared from pure 2,4′-MDI, containing at least 1 wt. % and a maximum amount of 20 wt. % of free monomeric diisocyanate, based on the prepolymer, which MDI has not been extracted or distilled.  
         [0019]     Pure 2,4′-MDI is understood hereafter as meaning MDI grades which have a 2,4′ isomer content of at least 85 wt. %, preferably of at least 90 wt. %, more preferably of at least 95 wt. %, and most preferably of at least 97.5 wt. %.  
         [0020]     The present invention provides polyurethane elastomers obtainable (by the casting process) from 
        a) one or more NCO prepolymers based on diphenylmethane diisocyanate with a 2,4′ isomer content of at least 85 wt. %, preferably of at least 90 wt. % and more preferably of at least 95 wt. %, the proportion of free monomeric 2,4′-MDI being from at least 1 wt. % to 20 wt. %, preferably from 2 to 18 wt. % and most preferably from 3 to 15 wt. %, based on the NCO prepolymer, and on one or more polyols having OH numbers of from about 20 to about 200 mg KOH/g and functionalities of from about 1.95 to about 2.40, preferably from about 1.96 to 2.20,     b) one or more amine-based chain extenders and/or crosslinking agents, preferably aromatic amine-based chain extenders and/or crosslinking agents, and     c) optionally, auxiliary substances and additives.        
 
         [0024]     The polyurethanes of the present invention are superior to those within the current state of the art because they have particularly favorable combinations of advantageous properties with respect to prepolymer viscosity, casting time and mechanical and mechanico-dynamic properties.  
         [0025]     The present invention also provides a casting process for the preparation of the polyurethane elastomers according to the invention. In the process of the present invention, 
        A) a prepolymer is formed by reacting diphenylmethane diisocyanate (MDI) with a 2,4′ isomer content of at least 85 wt. %, preferably of at least 90 wt. % and more preferably of at least 95 wt. % with one or more polyols having OH numbers of from about 20 to about 200 mg KOH/g and functionalities of from about 1.95 to about 2.40 to give NCO prepolymers with a proportion of free monomeric 2,4′-MDI of from 1 wt. % to 20 wt. %, preferably of from 2 to 18 wt. % and most preferably of from about 3 to 15 wt. %, based on the NCO prepolymer, and     B) one or more amine-based chain extenders and/or crosslinking agents and optionally auxiliary substances and additives 
 
 are added to the prepolymer from A) to produce the elastomer product. 
       
 
         [0028]     Preparation of elastomers by the casting process is an important use for NCO-terminated prepolymers. The NCO prepolymers are either reacted with a chain extender directly after their preparation or after being cooled to a lower temperature (storage temperature) and stored for the purpose of chain extension at a later stage.  
         [0029]     The synthesis of PUR elastomers from prepolymers is favorable because part of the heat of reaction is already produced during the synthesis of the prepolymer, thereby reducing the exothermic heat of the actual polymer synthesis. This has a favorable effect on the rate of molecular weight build-up and allows longer casting times, a processing advantage.  
         [0030]     In one particularly preferred embodiment of the process for preparation of the PUR elastomers by the prepolymer process, the prepolymers are first degassed by application of a reduced pressure at room temperature or elevated temperature, and then stirred with a chain extender, usually at elevated temperature.  
         [0031]     In the process of the present invention, the prepolymer is preferably heated to a temperature of from 60° C. to 110° C. and degassed under vacuum, with stirring. The chain extender and/or crosslinking agent is then added in liquid form, optionally after having been heated to a temperature typically of at least 5° C. above its melting point. The reaction mixture is cast into preheated molds (preferably heated to 90° C. to 120° C.) and cured at 90° C. to 140° C. for about 24 hours.  
         [0032]     Polyols suitable for use in the practice of the present invention include polyether polyols, polyester polyols, polycarbonate polyols and polyetherester polyols having hydroxyl numbers of from 20 to 200 mg KOH/g, preferably of from 27 to 150 mg KOH/g and most preferably of from 27 to 120 mg KOH/g.  
         [0033]     Polyether polyols are prepared from an initiator molecule and epoxide, preferably ethylene oxide and/or propylene oxide, by either alkaline catalysis or double metal cyanide catalysis, or optionally by alkaline catalysis and double metal cyanide catalysis in a stepwise reaction, and have terminal hydroxyl groups. Initiators which can be used to prepare suitable polyether polyols for use in the practice of the present invention include the compounds with hydroxyl and/or amino groups known to those skilled in the art, and water. The functionality of the initiator(s) is at least 2 and at most 4. Of course, it is also possible to use mixtures of several initiators. Mixtures of several polyether polyols can also be used in the practice of the present invention.  
         [0034]     Polyether polyols can be hydroxyl terminated oligomers of tetrahydrofurane.  
         [0035]     Polyester polyols may be prepared in known manner by the polycondensation of aliphatic and/or aromatic polycarboxylic acids having from 4 to 16 carbon atoms, optionally their anhydrides and optionally their low-molecular esters, including cyclic esters, with the reaction component used being predominantly low-molecular weight polyol(s) having from 2 to 12 carbon atoms. The functionality of the structural components for polyester polyols is preferably 2, but can also be greater than 2 in individual cases, the components having functionalities greater than 2 only being used in small amounts so that the arithmetic number-average functionality of the polyester polyols ranges from 2 to 2.5, preferably from 2 to 2.1.  
         [0036]     Polyetherester polyols may be prepared by the concomitant use of polyether polyols in the synthesis of polyester polyols.  
         [0037]     Polycarbonate polyols may be produced by known processes, e.g., by polycondensation of carbonic acid derivatives (e.g. dimethyl or diphenyl carbonate or phosgene) and polyols.  
         [0038]     Preferred chain extenders are aromatic amine-based chain extenders such as diethyltoluenediamine (DETDA); 3,3′-dichloro-4,4′-diaminodiphenylmethane (MBOCA); isobutyl 3,5-diamino-4-chlorobenzoate; 4-methyl-2,6-bis(methylthio)-1,3-diaminobenzene (Ethacure 300); trimethylene glycol di-p-aminobenzoate (Polacure 740M); and 4,4′-diamino-2,2′-dichloro-5,5′-diethyldiphenylmethane (MCDEA). MBOCA and isobutyl 3,5-diamino-4-chlorobenzoate are particularly preferred. Aliphatic amine-based chain extenders can likewise be used (concomitantly).  
         [0039]     It is also possible to use auxiliary substances and additives such as catalysts, stabilizers, UV stabilizers, hydrolysis stabilizers, emulsifiers, and dyestuffs and color pigments that are preferably capable of incorporation.  
         [0040]     Examples of suitable catalysts are: trialkylamines, diazabicyclooctane, tin dioctanoate, dibutyltin dilaurate, N-alkylmorpholine, lead, zinc, calcium or magnesium octanoate and the corresponding naphthenates and p-nitrophenate.  
         [0041]     Examples of suitable stabilizers are Broensted and Lewis acids, such as hydrochloric acid, benzoyl chloride, organomineral acids (e.g., dibutyl phosphate), and also adipic acid, malic acid, succinic acid, tartaric acid and citric acid.  
         [0042]     Examples of UV stabilizers and hydrolysis stabilizers are 2,6-dibutyl-4-methylphenol and sterically hindered carbodiimides.  
         [0043]     Dyestuffs capable of incorporation are those which possess Zerewitinoff-active hydrogen atoms, i.e. atoms which can react with NCO groups.  
         [0044]     Other auxiliary substances and additives include emulsifiers, foam stabilizers, cell regulators and fillers. A survey of such auxiliary substances and additives can be found in G. Oertel, Polyurethane Handbook, 2nd edition, Carl Hanser Verlag, Munich, 1994, chap. 3.4.  
         [0045]     The polyurethane elastomers of the present invention can be used in a very wide variety of applications, e.g., as elastic moldings produced by the casting process, as well as in coatings and adhesive bonding agents produced by a spraying process, e.g., in parking deck coating systems, concrete repairs and corrosion protection.  
         [0046]     The invention will be illustrated in greater detail with the aid of the Examples which follow.  
       EXAMPLES  
       [0047]     Methods of Measurement Used:  
                                                   DIN   ISO/ASTM       Property   Dimensions   standard   standard                   Hardness   [Shore]   DIN 53505   ISO 868       Stress   [MPa]   DIN 53504   ISO 527       Ultimate strength   [MPa]   DIN 53504   ISO 527       Elongation at break   [%]   DIN 53504   ISO 527       Tear propagation resistance   [kN/m]   DIN 53515   ISO 527       Abrasion   [mm 3 ]   DIN 53516   ASTM D 1242       Density   [g/mm 3 ]   DIN 53420   ISO 1183       Permanent set, PS   [%]   DIN 53517   DIN ISO 815                  
 
 Chemicals Used: 
        Polyester Polyol 1: poly(ethylene-co-butylene) adipate having an OH number of 56 mg KOH/g which is commercially available from Bayer MaterialScience AG; nominal functionality 2.0     4,4′-MDI: 4,4′-diphenylmethane diisocyanate which is commercially available under the name Desmodur® 44M from Bayer MaterialScience AG; 98.5 wt. % 4,4′-isomer     2,4′-MDI: 2,4′-diphenylmethane diisocyanate (laboratory product) from Bayer MaterialScience AG; 98.5 wt. % 2,4′-isomer     Isobutyl 3,5-diamino-4-chlorobenzoate: RC-Crosslinker 1604 commercially available from Rheinchemie, Rheinau.        
 
       Example 1  
     Preparation of MDI-based Ester Prepolymers  
       [0000]     Instructions for the Preparation of Prepolymers Using Prepolymer 2 as an Example (Table 1):  
         [0052]     25 parts by weight of 2,4′-MDI were heated to 70° C. in a stirred flask under nitrogen and stirred rapidly with 100 parts by weight of dehydrated Polyester Polyol 1 heated to 70° C. The reaction was allowed to proceed for 2 hours and the physical properties of Prepolymer 2 were determined (See Table 1.).  
                                                                               TABLE 1                           Formulations of MDI-based ester prepolymers (according       to the invention and Comparative Examples)                1 C   2   3 C   4 C   5   6 C                        Polyester Polyol 1   [parts by weight]   100   100                       Prepolymer 1 1     [parts by weight]               100       100       Prepolymer 2 2     [parts by weight]           100       100       4,4′-MDI   [parts by weight]   25       10   10       2,4′-MDI   [parts by weight]       25           10   10       NCO (theoretical)   [wt. % of NCO]   3.36   3.36   6.1   6.1   6.1   6.1       NCO (experimental)   [wt. % of NCO]   3.4   3.44   6.2   6.1   6.17   6.15       Free MDI   [wt. %]   4.8   3.1   11.9   13.4   11.9   13.4       Viscosity at 70° C.   [mPas]   10,600   4800   2900   6200   2900   6400                 C: Comparison              1 Prepolymer 1: Reaction product of 100 parts by weight of Polyester Polyol 1 and 25 parts by weight of 4,4′-MDI              2 Prepolymer 2: Reaction product of 100 parts by weight of Polyester Polyol 1 and 25 parts by weight of 2,4′-MDI             
 
         [0053]     Comparison of the viscosity values for MDI prepolymers with a theoretical NCO content of 3.36 wt. % shows the advantages of the prepolymer based on 2,4′-MDI (Prepolymer 2, according to the invention) over the 4,4′ analogue (Prepolymer 1 C, not according to the invention).  
         [0054]     Mixing of either of these two prepolymers with additional MDI to attain NCO contents of 6.1 wt. % of NCO (theoretical) obviously gives in all cases prepolymers with lower viscosities than the starting prepolymers (Prepolymers 3 C, 4 C, 5 and 6 C in Table 1). It is further seen that the equally low viscosity of Prepolymers 3 C and 5 (in each case, 2900 mPas at 70° C.) is not sufficient for advantageous processing (e.g., casting time) to casting elastomers. Only Prepolymer 5 could advantageously be processed further to an elastomer (See Tables 2 and 3.).  
       Example 2  
     Preparation of Casting Elastomers According to the Invention from Prepolymers 2 and 5 of Example 1  
       [0000]     Instructions for the Preparation of Casting Elastomers Using Casting Elastomer A as an Example:  
         [0055]     100 parts of Prepolymer 2 were degassed at 90° C. under vacuum, with slow stirring, until free of bubbles. This degassed prepolymer was then stirred with 9.05 parts of isobutyl 3,5-diamino-4-chlorobenzoate preheated to 100° C., and the reacting homogeneous melt was cast into molds preheated to 110° C., having dimensions corresponding to the testing standards. The melt was then heated for 24 hours at 110° C. and the mechanical properties listed in Table 2 were determined.  
                                                                                                                                                                   TABLE 2                           Formulations, preparation and properties of the casting elastomers according to the invention                Casting elastomer                No.   A   B   C   D   E                        Formulation and preparation:            Prepolymer   No.   2   2   2   2   5           [parts by weight]   100   80   60   40   100       Prepolymer   No.       5   5   5           [parts by weight]       20   40   60       NCO (theoretical)   [wt. % of NCO]   3.36   3.9   4.46   5.04   6.1       Prepolymer temperature   [° C.]   90   90   90   90   85       Viscosity of prepolymer mixture,   [mPas]   2030   1940   1750   1600   1200       90° C.       Isobutyl 3,5-diamino-4-   [parts by weight]   9.05   10.5   12.0   13.6   16.4       chlorobenzoate       Temperature of crosslinking agent   [° C.]   100   100   100   100   100       Index (theoretical)       107   107   107   107   107       Casting time   [s]   225   165   105   105   60       Peeling time   [min]   8   8   7   7   5       Mold temperature   [° C.]   110   110   110   110   110       Post-heating temperature   [° C.]   110   110   110   110   110       Post-heating time   [h]   24   24   24   24   24            Mechanical properties:            Hardness   [Shore A]   91   92   93   97   99           [Shore D]   37               49       Stress 10%   [MPa]   3.61   4.22   5.26   6.45   9.23       Stress 100%   [MPa]   6.5   6.9   7.5   8.3   10.0       Stress 300%   [MPa]   9.9   10.0   11.4   12.0   14.3       Ultimate strength   [MPa]   43.31   36.3   44.4   42.6   46.0       Elongation at break   [%]   683   607   591   616   609       Tear propagation resistance,   [kN/m]   62.8   67.3   71.6   83   99.2       Graves       Impact resilience   [%]   47               47            Formulation and preparation:            Abrasion (DIN)   [mm 3 ]   59   57   62       52       Density   [g/mm 3 ]   1.214   1.218   1.224       1.214       PS 22° C.   [%]   25.4           64   36.7       PS 70° C.   [%]   47.4           61   56.4       Storage modulus G′ [MPa]   at 0° C.   36.0   48.4   70.6   86.5   139           at 20° C.   28.2   36.9   53.3   65.9   108           at 50° C.   24.6   31.2   43.9   53.1   84.8           at 80° C.   24.3   29.4   41.0   47.4   74.9           at 110° C.   25.5   29.6   40.7   45.5   70.2       Loss factor, tan δ   at 0° C.   0.1302   0.1246   0.1170   0.1045   0.0903           at 20° C.   0.0768   0.0789   0.0756   0.0734   0.0690           at 50° C.   0.0484   0.0494   0.0497   0.0542   0.0543           at 80° C.   0.0302   0.0318   0.0318   0.0392   0.0389           at 110° C.   0.0177   0.0193   0.0193   0.0259   0.0270       Tan δ   max.   −36   −36   −36   −36   −36       Tan δ   min.   130   130   130   130   130       Loss modulus G″ [MPa]   at 0° C.   4.69   6.0   8.26   9.04   12.5           at 20° C.   2.16   2.9   4.03   4.84   7.46           at 50° C.   1.19   1.5   2.18   2.88   4.61           at 80° C.   0.74   0.9   1.30   1.86   2.91           at 110° C.   0.45   0.6   0.79   1.18   1.89       Softening point   [° C.]   190   195   195   210   195                  
 
       Example 3  
     Preparation of Casting Elastomers not According to the Invention from Prepolymers 1 C, 3 C, 4 C and 6 C of Example 1  
       [0056]     The preparation was carried out as described under Example 2. The formulations and properties of these cast elastomers are reported in Table 3.  
                                                                                                                                                   TABLE 3                           Formulations, preparation and properties of the cast elastomers not according to the invention                Cast elastomer                No.   F   G   H   I                        Formulation and preparation:            Prepolymer   No.   1 C   3 C   4 C   6 C           [parts by weight]   100   100   100   100       NCO (theoretical)   [wt. % of NCO]   3.36   6.1   6.1   6.1       Prepolymer temperature   [° C.]   100   90   90   90       Viscosity of prepolymer, 90° C.   [mPas]   4530   1200   2710   2720       Isobutyl 3,5-diamino-4-chlorobenzoate   [parts by weight]   9.05   16.4   16.4   16.4       Temperature of crosslinking agent   [° C.]   100   100   100   100       Index (theoretical)       107   107   107   107       Casting time   [s]   75   30   60   60       Peeling time   [min]   9   4   3   4       Mold temperature   [° C.]   110   110   110   110       Post-heating temperature   [° C.]   110   110   110   110       Post-heating time   [h]   24   24   24   24            Mechanical properties:            Hardness   [Shore A]   83   99   99   99           [Shore D]   31   49   48   48       Stress 10%   [MPa]   1.92   9.91   9.09   8.45       Stress 100%   [MPa]   4.0   10.2   9.06   8.6            Formulation and preparation:            Stress 300%   [MPa]   8.8   16.0   13.8   12.3       Ultimate strength   [MPa]   10.3   51.5   35.1   33       Elongation at break   [%]   325   538   543   589       Tear propagation resistance, Graves   [kN/m]   14.9   89.6   89.5   87.8       Impact resilience   [%]   50   48   47   46       Abrasion (DIN)   [mm 3 ]   101   69   46   55       Density   [g/mm 3 ]   1.205   1.228   1.228   1.228       PS 22° C.   [%]   8.5   45.9   45.1   47       PS 70° C.   [%]   16.2   66.8   57.8   67.4       Storage modulus G&#39; [MPa]   at 0° C.   16.6   177   140.4   138.8           at 20° C.   14.6   137   110.5   101.8           at 50° C.   15.0   107   87.6   78.0           at 80° C.   15.8   94.0   78.7   66.2           at 110° C.   16.2   87.2   74.8   60.8       Loss factor, tan δ   at 0° C.   0.1295   0.0870   0.0807   0.0976           at 20° C.   0.0428   0.0665   0.0605   0.0735           at 50° C.   0.0169   0.0544   0.0468   0.0616           at 80° C.   0.0097   0.0417   0.0358   0.0488           at 110° C.   0.0075   0.0309   0.0231   0.0352       Tan δ   max.   −33   −36   −36   −33       Tan δ   min.   110   130   190   140       Loss modulus G″ [MPa]   at 0° C.   2.15   15   11.32   13.55           at 20° C.   0.63   9.08   6.68   7.48           at 50° C.   0.25   5.81   4.10   4.81           at 80° C.   0.15   3.92   2.82   3.23           at 110° C.   0.12   2.69   1.73   2.14       Softening point   [° C.]   165   195   230   200                  
 
         [0057]     The advantages of the present invention are clear upon comparison of Tables 2 and 3.  
         [0058]     At comparable prepolymer temperatures (starting temperature) and comparable NCO contents, i.e. comparable formulations, the casting times of the prepolymers according to the invention (Table 2) are up to 3 times longer than those of the systems not according to the invention (Table 3), which represents a clear processing advantage. The particularly favorable combinations of the properties of “long casting time” and “low prepolymer viscosity” are only achieved by the systems according to the present invention.  
         [0059]     The cast elastomers of the present invention also exhibit advantages with respect to their mechanical properties:  
         [0060]     If, for example, the PUR prepared from Prepolymer 2 (Cast Elastomer A, Table 2) is compared with a PUR prepared from Prepolymer 1 C (cast Elastomer F, Table 3)—both prepolymers having the same NCO value of 3.36 wt. % of NCO—Cast Elastomer A (an example of the present invention ) has better ultimate strength, elongation at break, tear propagation resistance and abrasion.  
         [0061]     If the PUR prepared from Prepolymer 5 (Cast Elastomer E, Table 2) is compared with a PUR prepared from Prepolymers 3 C, 4 C and 6 C (Cast Elastomers G, H and I, Table 3)—all the prepolymers having the same NCO value of 6.1 wt. % of NCO—Cast Elastomer E (an example of the present invention) has comparable good ultimate strength, elongation at break, tear propagation resistance, abrasion and permanent set, within the limits of experimental error.  
         [0062]     The same also applies in terms of the mechanico-dynamic properties (storage and loss moduli and loss factor).  
         [0063]     The PUR-forming systems of the present invention exhibit a unique combination of advantageous properties with respect to prepolymer viscosity, casting time and mechanical and mechanico-dynamic properties.  
         [0064]     Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.