Cross-linkable filled polyurethanes based on unsaturated cycloaliphatic diisocyanate
This invention relates to cross-linked polyurethanes. According to the present invention there is provided a cured polyurethane elastomer cross-linked by reaction between a cross-linking agent and ethylenically unsaturated groups in the polymer structure wherein the polyurethane comprises urethane-linked repeating units derived from a diisocyanato bicyclo octene and a linear pre-polymer with two isocyanate-reactable end groups. 
     The pre-polymer may be a compound containing a chemically stable linear chain and terminating in groups reactive to isocyanates for example hydroxyl or amino groups. The pre-polymer is preferably a polyether or a polyester and may conveniently be for example any of the aliphatic linear polyethers or polyesters commercially available, some being particularly designed for the production of polyurethanes. Especially preferred polymers include poly(ethylene adipate), poly(propylene adipate), poly(tetramethylene adipate), poly(hexamethylene adipate), poly(ethylene glycol), poly(propylene glycol) and poly(tetrahydrofuran). Mixtures of polymers may be used and also mixtures (or condensation products) of polymers and glycols (or other diols) may also be a convenient form of linear pre-polymer to use in this invention. 
     The diisocyanato bicyclo octene is preferably a bicyclo (3,3,0) octene having isocyanate groups on non-adjacent carbon atoms, especially the 6, 8 carbon atoms of the bicyclo (3,3,0) octene ring system for example the compound having the structure ##STR1## Spatial and configurational isomers are included within the definition of any preferred compound. Diisocyanato bicyclo octene may also be called &#34;the diisocyanate from dicyclopentadiene furoxan&#34; since it may be produced from dicyclopentadiene furoxan as described in U.S. Pat. No. 4,036,869. The 6, 8 diisocyanato bicyclo octene is conveniently prepared from the corresponding furoxan by thermolysis in the presence of sulphur dioxide or via the product of reaction of the furoxan with a vicinal diketone as described in U.S. Pat. No. 4,036,869 and U.S. application of Crosby, Ser. No. 781,869, filed Mar. 28, 1977. 
     It may be appreciated that the diisocyanate will react with the polyester or polyether prepolymer in order to form a polyurethane which will include chemically bound bicyclo octene rings. In the preparation of the polyurethane it is preferable to have a small excess of the linear pre-polymer and thereby minimise the chance of branching or cross-linking from urethane-isocyanate reactions. The residual unsaturation of the bicyclo octene is available for cross-linking and may conveniently be reacted with conventional curing agents for hydrocarbon elastomers, the most widely used examples of which are sulphur and peroxides. These two are preferred, especially sulphur and sulphur-based systems (including sulphur donors) for this invention; but other cross-linking agents reactable with hydrocarbon unsaturation may be used if desired. 
     Before cross-linking, the polyurethane is preferably a thermoplastic material and so may be subjected to fabrication procedures for example extrusion, injection moulding etc. in order to shape an article. The fabricated product may then be cross-linked by reaction with the unsaturated groups and for preference a cross linking agent is incorporated into the composition prior to the final fabrication of the product. The cross-linking may most conveniently be effected by heating the product, the heating preferably being sufficient to cure the polyurethane but not sufficient to deform the shape of the article. However the compositions of this invention may be cross-linked either during, before or after fabrication by choice of suitable curing agents and processing procedures, thus extending their versatility and wide utility as elastomeric products. 
     The cross-linked polyurethanes of this invention are generally elastomers having both a high tensile strength and a high value for elongation to break. The superiority of the new elastomers, as seen by these two properties over comparable polyurethanes cross-linked by means of the similar action of a cross-linking agent on double bonds introduced in the pre-polymer from the acid or polyol portions, is most marked for the unfilled elastomers. There is little need to use a reinforcing filler such as carbon black or fume silica with the polyurethanes of this invention because quite acceptable properties suitable for many applications may be achieved in a non-filled rubber. If it is desired to improve the properties of these new rubbers further, fillers may be incorporated but these, quite suitably, may be cheap fillers, for example calcium carbonate, titanium dioxide or coarse silica. For example a filler such as titanium dioxide may be used to significantly improve the modulus of the elastomer without causing the good elongation to break or tensile strength to suffer unduly. 
    
    
     The invention is illustrated but not limited by the following examples in which all parts and percentages are by weight. Tear strengths quoted in the Examples are &#34;Graves&#34; tear strengths. 
     EXAMPLE 1 
     Preparation of thermoplastic polyurethane 
     Polyethylene tetramethylene adipate (m. wt. 1950) (100 parts) was dried by heating at 80° C. at &gt;5 mm Hg pressure for 30 minutes. 1,4-Butane diol (freshly redistilled and stored under dry nitrogen until used) (13.16 parts), dibutyl tin dilaurate catalyst (1500 ppm with respect to polyethylene tetramethylene diol) and diisocyanate (37.71 parts) were added. The mixture was shaken vigorously for about 1/2 minute than degassed for 5-10 minutes on a rotary evaporator at 70° C., &gt;5 mm Hg. The mixture was cast into a preheated (100° C.) glass mould coated with a silicone release agent. The mould was sealed and heated at 110° C. for 11/2 hours in a hot air oven to cure the urethane. Before removing the resulting thermoplastic urethane from the mould it was allowed to cool to room temperature. 
     The polyurethane prepared in Example 1 was the material used in all the other examples (called &#34;the urethane&#34;). 
     EXAMPLE 2 
     Compression moulding of the urethane 
     The urethane was compression moulded at 125° C. for 15 minutes to give a product with the following properties. (In this and the following examples all the properties quoted were measured at ambient temperature): 
     
         ______________________________________                                    
100% modulus          20 Kg cm.sup.-2                                     
300% modulus          31 Kg cm.sup.-2                                     
Tensile strength     200 Kg cm.sup.-2                                     
Elongation at break  700%                                                 
Hardness             75° (IRHD)*                                   
______________________________________                                    
 *international rubber hardness degrees                                   
 
    
     EXAMPLE 3 
     Curing the urethane 
     The urethane (100 parts) was mixed with zinc oxide (5 parts), sulphur (1.5 parts), zinc diethyldithiocarbamate (1.5 parts), and 2-mercaptobenzthiazole (0.5 parts). The mixing procedure used here and in all the following examples was that the elastomer was first milled on a two roll mill (initially at room temperature) until workable (approximately 40° C.); the other components were then added and milling was continued until they were well dispersed; total milling time being about 10 minutes. 
     The urethane compound was cured under pressure for 30 minutes at 125° C. to give a product with the properties: 
     
         ______________________________________                                    
100% modulus         21.5 Kg cm.sup.-2                                    
300% modulus         38 Kg cm.sup.-2                                      
Tensile strength     &gt;400 Kg cm.sup.-2                                    
Elongation at break  &gt;660%                                                
Hardness             75° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 4 
     Co-curing the urethane with an EPDM rubber 
     The urethane (50 parts) was mixed with an EPDM rubber &#34;Intolan 155&#34; (50 parts), HAF carbon black (25 parts), zinc oxide (5 parts), sulphur (1.5 parts), zinc diethyl dithiocarbamate (1.5 parts), and 2-mercaptobenzthiazole (0.5 parts). The mixture was cured under pressure for 20 minutes at 125° C. to give a product with the properties: 
     
         ______________________________________                                    
100% modulus          21 Kg cm.sup.-2                                     
300% modulus          73 Kg cm.sup.-2                                     
Tensile strength     152 Kg cm.sup.-2                                     
Elongation at break  490%                                                 
Hardness             75° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 5 
     Preparation of a cross-linked urethane filled with carbon black 
     The formulation of Example 3 was prepared except that HAF carbon black (25 parts) was also added. The resulting rubber compound was compression moulded for 45 minutes at 125° C. to give an elastomer with the following properties: 
     
         ______________________________________                                    
100% modulus          39 Kg cm.sup.-2                                     
300% modulus         141 Kg cm.sup.-2                                     
Tear strength         93 Kg cm.sup.-1                                     
Tensile strength     360 Kg cm.sup.-2                                     
Elongation at break  610%                                                 
Hardness             85° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 5A 
     The urethane alone (Example 1) (100 parts) and HAF carbon black (25 parts) were milled together and compression moulded at 125° C. for 10 minutes to give a product with the properties: 
     
         ______________________________________                                    
100% modulus          61 Kg cm.sup.-2                                     
300% modulus         160 Kg cm.sup.-2                                     
Tear strength         88 Kg cm.sup.-1                                     
Tensile strength     160 Kg cm.sup.-2                                     
Elongation at break ca                                                    
                     360% (yielded)                                       
Hardness             90° (IRHD)                                    
______________________________________                                    
 
    
     To this material was then added, by milling, a curing mixture comprising ZnO (5 parts), sulphur (1.5 parts), zinc diethyldithio-carbamate (1.5 parts) and 2-mercaptobenzthiazole (0.5 parts). The material was then compression moulded for a second time at 125° C. (for 20 minutes) to give a cured urethane with the following properties: 
     
         ______________________________________                                    
100% modulus          68 Kg cm.sup.-2                                     
300% modulus         195 Kg cm.sup.-2                                     
Tear strength        119 Kg cm.sup.-1                                     
Tensile strength     220 Kg cm.sup.-2                                     
Elongation at break ca                                                    
                     400% (yielded)                                       
Hardness             93° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 5B 
     The formulation of Example 3 was prepared except that HAF carbon black (50 parts) was also added. The resulting rubber compound was compression moulded for 30 minutes at 125° C. to give a cured urethane with the following properties: 
     
         ______________________________________                                    
100% modulus          77 Kg cm.sup.-2                                     
300% modulus         218 Kg cm.sup.-2                                     
Tear strength        129 Kg cm.sup.-1                                     
Tensile strength     271 Kg cm.sup.-2                                     
Elongation at break  525%                                                 
Hardness             93° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 6 
     Preparation of a cross-linked blend of urethane and natural rubber 
     The formulation of Example 4 was used except that in place of the EPDM was used pale-crepe natural rubber. The resulting rubber compound was compression moulded for 10 minutes at 125° C. to give a product with the properties: 
     
         ______________________________________                                    
100% modulus          24 Kg cm.sup.-2                                     
300% modulus          95 Kg cm.sup.-2                                     
Tensile strength     190 Kg cm.sup.-2                                     
Elongation to break  540%                                                 
Hardness             70° (IRHD)                                    
______________________________________                                    
 
    
     For comparison, a corresponding composition in which 100% of pale crepe were used instead of 50% pale crepe and 50% urethane had an elongation at break of only 350%. 
     EXAMPLE 7 
     Preparation of a cured urethane filled with titanium dioxide 
     The formulation of Example 3 was prepared except that it also contained uncoated filler-grade titanium dioxide (25 parts). The resulting compound was compression moulded at 125° C. for 30 minutes to give a product with the properties: 
     
         ______________________________________                                    
100% modulus          20 Kg cm.sup.-2                                     
300% modulus          48 Kg cm.sup.-2                                     
Tensile strength     400 Kg cm.sup.-2                                     
Elongation at break  690%                                                 
Hardness             75° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 8 
     The urethane (100 parts) and uncoated filler grade titanium dioxide (25 parts) were milled together then compression moulded at 125° C. for 40 minutes to give a product with the properties. 
     
         ______________________________________                                    
100% modulus          50 Kg cm.sup.-2                                     
300% modulus         103 Kg cm.sup.-2                                     
Tensile strength     364 Kg cm.sup.-2                                     
Elongation at break  750%                                                 
______________________________________                                    
 
    
     To this material was then added, by milling, a curing mixture comprising zinc oxide (5 parts), sulphur (1.5 parts) zinc diethyldithiocarbamate (1.5 parts) and 2-mercaptobenzthiazole (0.5 parts). The material was then compression moulded for a second time at 125° C. (for 30 minutes) to give a cured, filled urethane with properties: 
     
         ______________________________________                                    
100% modulus          46 Kg cm.sup.-2                                     
300% modulus         110 Kg cm.sup.-2                                     
Tensile strength     420 Kg cm.sup.-2                                     
Elongation at break  685%                                                 
Hardness             75° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 9 
     The procedure of Example 8 was repeated except that 50 parts of TiO 2  were employed. The properties after pressing at 125° C. for the first time were: 
     
         ______________________________________                                    
100% modulus          21 Kg cm.sup.-2                                     
300% modulus          49 Kg cm.sup.-2                                     
Tear strength         51 Kg cm.sup.-1                                     
Tensile strength     220 Kg cm.sup.-2                                     
Elongation at break  690%                                                 
Hardness             83 (IRHD)                                            
______________________________________                                    
 
    
     and after addition of the curing mixture and pressing again at 125° C. the properties were: 
     
         ______________________________________                                    
100% modulus          28 Kg cm.sup.-2                                     
300% modulus          70 Kg cm.sup.-2                                     
Tear strength         66 Kg cm.sup.-1                                     
Tensile strength     350 Kg cm.sup.-2                                     
Elongation at break  630%                                                 
Hardness             83° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 10 
     The procedure of Example 8 was repeated except that 100 parts of TiO 2  were employed. The properties after adding the curing mixture and pressing for the second time at 125° C. (for 40 minutes) were: 
     
         ______________________________________                                    
100% modulus          45 Kg cm.sup.-2                                     
300% modulus         110 Kg cm.sup.-2                                     
Tear strength         83 Kg cm.sup.-1                                     
Tensile strength     260 Kg cm.sup.-2                                     
Elongation at break  590%                                                 
Hardness             85° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 11 
     The procedure of Example 8 was repeated except that instead of TiO 2 , 25 parts of filler-grade calcium carbonate which was coated with calcium stearate (&#34;Winnofil&#34; S from ICI). The properties after pressing at 125° C. for the first time were: 
     
         ______________________________________                                    
100% modulus          19 Kg cm.sup.-2                                     
300% modulus          30 Kg cm.sup.-2                                     
Tear strength         37 Kg cm.sup.-1                                     
Tensile strength     245 Kg cm.sup.-2                                     
Elongation at break  750%                                                 
Hardness             80° (IRHD)                                    
______________________________________                                    
 
    
     and after adding the curing mixture and pressing again at 125° C. the properties of the cured product were: 
     
         ______________________________________                                    
100% modulus          23 Kg cm.sup.-2                                     
300% modulus          44 Kg cm.sup.-2                                     
Tear strength         47 Kg cm.sup.-1                                     
Tensile strength     350 Kg cm.sup.-2                                     
Elongation at break  660%                                                 
Hardness             77° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 12(a) 
     The formulation of Example 3 was prepared except that Talc (25 parts) was also added. The resulting rubber compound was compression moulded for 30 minutes at 125° C. to give a filled, cross-linked urethane with the following properties: 
     
         ______________________________________                                    
100% modulus         32 Kg cm.sup.-2                                      
300% modulus         54 Kg cm.sup.-2                                      
Tear strength        55 Kg cm.sup.-1                                      
Tensile strength     324 Kg cm.sup.-2                                     
Elongation at break  625%                                                 
Hardness             82° (IRHD)                                    
______________________________________                                    
 
    
     EXAMPLE 12(b, c, d) 
     The formulation of Example 3 was prepared except that fume silica was added in varying amounts. The resulting urethane compounds were compression moulded at 125° C. to give filled, cross-linked products with the following properties: 
     
         ______________________________________                                    
                    12b   12c     12d                                     
______________________________________                                    
pts. silica               25       50   100                               
cure time (min)           45       25    40                               
100% modulus   (Kg cm.sup.-2)                                             
                          28       65   140                               
300% modulus   (Kg cm.sup.-2)                                             
                          70      122   150                               
Tensile strength                                                          
               (Kg cm.sup.-2)                                             
                          451     340   150                               
Tear strength  (Kg cm.sup.-1)                                             
                          77      115   127                               
Elongation at break                                                       
               (%)        640     560   300                               
Hardness       (° IRHD)                                            
                          84       91    98                               
______________________________________                                    
 
    
     EXAMPLE 12(e) 
     The formulation of Example 12b was prepared except that dioctylphthalate plasticiser (25 parts) was also added. The resulting compound was compression moulded at 125° C. for 45 minutes to give a filled, plasticised cross-linked urethane with the following properties: 
     
         ______________________________________                                    
100% modulus          25 Kg cm.sup.-2                                     
300% modulus          65 Kg cm.sup.-2                                     
Tensile strength     318 Kg cm.sup.-2                                     
Tear strength         73 Kg cm.sup.-1                                     
Elongation at break  615%                                                 
Hardness             73° (IRHD)                                    
______________________________________