Abstract:
The present invention relates to a polymeric composition which may be processed with increased scorch safety while retaining good physical properties. The composition of the invention comprises the combination of a specific retarder (vulcanization inhibitor) and at least one of two specific accelerators, and such composition prevents premature vulcanization of the chloroprene while it is being mixed and processed, yet provides excellent cure characteristics at the proper time, producing polymeric products with excellent dynamic properties.

Description:
BACKGROUND OF THE INVENTION 
     This application is a continuation in part of U.S. Ser. No. 693,950 filed Jan. 23, 1985, now abandoned. 
    
    
     1. Field of the Invention 
     The present invention relates to polymeric compositions which may be processed with increased scorch safety while retaining good physical properties. This is of particular benefit when said compositions have a high viscosity. 
     2. Description of the Related Art 
     In the prior art, it is known to increase the load carrying capacity of an endless power transmission belt by incorporating up to 20 phr (parts per hundred rubber) of discrete fibers, by weight, into at least the compound utilized in the compression section of the belt. It has been proposed then, that increasing the amount of fiber in a belt compound (the fiber can be aramid fiber, nylon fiber, cellulose fiber, or similar fiber) that the ability of the belt could be increased even more. It was found, however, that when a chloroprene composition that was used to make a power transmission belt was loaded with about 40 phr fiber, the increased viscosity of the composition and the increased time to mix the fiber into the composition to obtain good fiber dispersion, that the heat generated by mixing and processing caused the composition to cure in the mixer or in subsequent processing. This is commonly referred to as scorching. In order to avoid this problem, it is known to utilize retarders (chemicals that slow down the cure of a polymer) to inhibit scorching. For example, see U.S. Pat. No. 3,780,001 to Son, wherein Son teaches the use of oxamides as retarders and at least morpholines as accelerators in elastomer curing systems. 
     It was found, however, that most common retarders used for chloroprenes, although they increase scorch safety, cause the elastic modulus and other critical properties of the composition for a belt to change such that the overall effect is detrimental to the performance of the belt. For example, Murray, R. M. and Thompson, D. C. in THE NEOPRENES page 28 state that &#34;MBT, MBTS, and RPA No. 3 also retard scorch but have a persistent retarding effect which impairs development of a high state of cure even after prolonged curing&#34;. 
     It is also well known in the art to establish the state of vulcanization by the rheometer data and to establish the scorch safety in a similar manner. For example, U.S. Pat. Nos. 4,209,596 and 4,371,668, incorporated by reference herein, discuss methods of developing compounds and compound blends in the presence of vulcanization agents and retarders showing in particular rheometer curves and related tables of data. The cited references show these methods as well as data developed from stress-strain tensile testing machines. 
     It is also known to provide accelerators or cross-linking agents to enhance the state of cure in chloroprene compositions. For example, Sullivan, in U.S. Pat. No. 3,370,051 teaches the use of 3-substituted, 2-thiazolidinethione accelerators in polychloroprene curing systems but does not discuss the effect of retarders on his novel curing agents. 
     Little, in U.S. Pat. No. 2,989,504, teaches the use of maleimide accelerators in rubber curing systems but also does not discuss the effect of retarders on his novel curing agents nor does Little acknowledge the use of maleimides in chloroprene compositions. 
     These references do not teach or suggest the specific curing system described and claimed herein as they relate to chloroprene compositions and in fact do not teach nor even suggest the use of di-morpholino disulfide as a retarder for chloroprene. 
     It is desirable to provide a retarder for chloroprene compositions which does not inhibit the development of high modulus. It is also desirable to provide an accelerator to produce rapid development of modulus (cure rate) once the composition has had time to flow or mold which does not significantly reduce the scorch time. 
     It is particularly desirable to produce a high viscosity or a high fiber loaded chloroprene composition which is resistant to scorching, yet retains excellent elastic modulus and other critical properties as a composition for a power transmission belt. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a polymeric composition, preferably chloroprene, utilizing di-morpholino disulfide as a retarder, and at least one accelerator selected from N-methyl-2-thiazolidinethione, and N,N&#39;-m-phenylene dimaleimide and mixtures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a reproduction of rheometer curves of compositions comparing various chloroprene retarders known in the art as compared to the use of di-morpholino disulfide as claimed in this invention. 
     FIG. 2 is a reproduction of rheometer curves of the compositions of the preferred embodiments of the invention compared to conventional curing systems for chloroprene. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The polymeric composition of this invention is prepared by mixing together the various materials in a manner which is conventional in the art. These materials include the polymer (preferably chloroprene), at least one metallic oxide, at least one organic acid, antioxidants, antiozonants, and at least one reinforcing filler, a retarder, special processing aids and an accelerator which activates the vulcanization of the composition. Such accelerators are known in the art and may be found in the teachings of Sullivan in his U.S. Pat. No. 3,370,051 or the book by Murray and Thompson. When there is a need for a composition which has increased physical properties, such as durometer, modulus and the like, it is common to incorporate into a conventional chloroprene composition unusually high loadings of fiber and/or carbon black. Such loadings may include up to 100 phr fibers selected from the group consisting of, for example, nylon, aramid, cellulose, polyester, graphite, and glass fibers. The high fiber loading in a polymer composition, or increased concentrations of carbon black, cause the viscosity of the composition to increase in proportion to the amount of the loading. It is obvious to those skilled in the art that increased loadings of either fiber or carbon black cause a reduction in processing safety, otherwise known as scorch. Conventional practice dictates that a retarder be utilized to improve processing safety. The retarder is provided to prevent premature vulcanization of the chloroprene during mixing of the composition because of the heat generated by the mixing of the fibers or carbon black into the composition and because of the extended period of time needed for the mixing due to the high viscosity of the composition. In some subsequent processing steps, including, for instance, calendering and extruding, these high viscosity compositions cause high heat generation and therefore require increased processing safety. 
     Likewise, these high viscosity compositions require longer flow times during molding to completely mold the product. This is accomplished by providing increased scorch safety. 
     Those skilled in the art are familiar with the Monsanto Mooney Viscometer and with the use of this machine and its associated test procedures to determine the scorch of compounds and the parts of the scorch curve. The first part of the curve describes the minimum rotor torque required to shear the rubber compound placed between the platens of the viscometer. The second part of the curve begins after the minimum point and generally relates to the scorching or curing rate of the particular compound at a particular temperature. Values typically used to describe the scorch rate of compounds from the scorch curve are minimum torque, T 1  defined as time to a one point rise after the minimum viscosity point and T 10  similarly defined as time to a ten point rise after the minimum. Increased scorch safety can be readily determined from this scorch curve by observing the increase in time to a one or ten point rise (T 1  or T 10 ). For example, it can be seen from Table 1 that in the 275° scorch test the time to a one point rise T 1  increases from 8.3 minutes for compound B16 to 21.7 minutes for compound B34 where the amounts of the retarder have been increased from 0 phr to 4 phr. Similarly, the Monsanto Oscillating Disc Rheometer (ODR) may be utilized to determine typical values for processing safety, curing rate and maximum ODR torque, a measure of the modulus of the compound. The first upward turn of the curve is the onset of the vulcanization process, and therefore, a delay in this onset is an indication of increased scorch safety. The maximum torque reached in the given time span of the rheometer graph is an indication of the state of cure. Referring now to FIG. 1, which is a graphic representation of the compounds presented in Table 2, note that curve B-26 (MBTS) and curve B-25 (TMTM) show rapid onset of vulcanization whereas curves B-38 (Sulfads) and B-35 (Morfax) show some delayed onset of cure and curve B-32 shows significant delay. The foregoing chemicals are usually used as either curing agents or accelerators in compounds other than chloroprene. Murray and Thompson teach that some of these chemicals, when used in chloroprene, act effectively as retarders. Murray and Thompson also state that at least MBTS also causes a significant reduction in the cured modulus of the chloroprene compound. It can been seen then from FIG. 1 that this decrease in modulus is typical of chemicals used as retarders in chloroprene except in the case of the inventive composition (B-32). 
     In addition to the retarder, an accelerator, which activates the vulcanization of the composition at the proper time, is provided. It has been found that N-methyl 2-thiazoladinethione, and N,N&#39;-m-phenylene dimaleimide and mixtures thereof have the correct accelerating properties such that when they are in a high viscosity chloroprene composition used to make a polymeric product, with the specific retarder mentioned above, that curing is substantially delayed until the vulcanization step, and when vulcanization takes place, the chloroprene polymer achieves optimum properties. 
     The following definitions define ingredients from the tables of compositions on the following pages which illustrate these properties: 
     
         ______________________________________Neoprene   Chloroprene manufactured by DuPontGNA/GRTN762/N650  Carbon blackSilica     Precipitated hydrated amorphous silicaOil        Aromatic processing oil type 102High aromatic oil      High molecular weight oilZnO        French process zinc oxideODP        Octylated diphenylamineMgO        High activity magnesium oxideSulfasan R di-morpholino disulfide sold by MonsantoVulkacit CRV      N--methyl-2-thiazolidinethione sold by MobayMBTS       2,2&#39;Di-Benzothiazyl disulfideETU        Ethylene thioureaTMTM       Tetramethyl thiuram monosulfideMorfax     4-morpholinyl-dithio-benzothiazole disulfide      manufactured by R. T. VanderbiltSulfads    Dipentomethylene thiuram tetrasulfide      manufactured by R. T. VanderbiltHMMM       Powdered 72% active Hexamethoxymethyl-      melamineHVA-2      N, N&#39;--m-phenyl dimaleimide manufactured      by DuPontPolymel 7  Polyethylene manufactured by Polymel Co.Sulfur     Elemental sulfur1/4&#34; Nylon Six denier tire cord chopped to 1/4&#34; lengthSRF 1501   Resorcinol Donor sold by Schenectady ChemicalStearic Acid      Stearic Acid______________________________________ 
    
     The properties desired in delaying the cure of the composition are best graphically illustrated by the position of the cure curve on the abscissa (time axis) of a graph, as is illustrated in FIGS. 1 and 2. It should be noted that the torque units in the ODR test at 320° F., using a 3 degree arc (in the ocillations of the disc) cannot be directly compared with the torque units in the scorch test because different equipment, using different calibrations are used in the two tests. 
     Compositions containing increasing amounts of di-morpholino disulfide were prepared to determine the effect of significantly increased scorch safety on other physical properties. As is illustrated in Table 1, dimorpholine disulfide increases scorch safety without significantly reducing the other physical properties. For example, as the amount of dimorpholine disulfide is increased from 0 phr (B-16) to 4 phr (B-34) a significant increase in processing safety is noted in either the T 1  or T 10  values while the tensile modulus of the compounds is highest at 1 phr, (B-17), but is not significantly lower even at 4 phr, (B-34). This decrease is insignificant compared to the compound containing 0 phr (B-16) while significantly increasing safety from 8.3 minutes T 1  of (B16) to 21.7 minutes T 1  of (B-34). 
     The tests in the tables on the Dynamic Instron were conducted at room temperature, 20 cycles per second compression at a mean stroke of 0.050 for a total of 12000 cycles. The Dynamic Quotient (D.Q.) is expressed as the quotient of compressive modulus and hysteresis. A higher D.Q. is a measure of resistance to fatigue. 
     
                       TABLE 1______________________________________      B16   B17     B-8     B19   B34______________________________________VULKACIT CRV/SULFSAN RDb-6004Neoprene GNA 100     100     100   100   100N650         44      44      44    44    44High Aromatic Oil        7.8     7.8     7.8   7.8   7.8Stearic Acid 1.5     1.5     1.5   1.5   1.5MgO          4       4       4     4     4ODP          3       3       3     3     3ZnO          4       4       4     4     4Sulfur       1       1       1     1     1Vulkacit CRV 0.25    0.25    0.25  0.25  0.25Sulfasan R   --      1       2     3     4Original Stress-StrainCured 30&#39; at 320° F.Tensile, psi 2630    3100    2600  2730  2550Elongation (Percent)         280     295    275   310   27050% Modulus   510     400    350   355   410100% Modulus 1075     900    775   760   825200% Modulus 2475    2250    1925  1875  2000Durometer, D  29      29      34    27    33250° F. &#34;C&#34; Tear         132            127   107   107Mooney Scorch at275°  F.T.sub.1 (time in minutes)        8.3     10.0    11.4  18.0  21.7T.sub.10 (time in minutes)        13.7    17.7    6 pt @                              4 pt @                                    2 pt @                        24&#39;   24&#39;   24&#39;Minimum (torque        12.5    12.5    9.0   9.6   9.5units)ODR TORQUE AT 320° F., 3 DEGREES ARCMinimum (torque        2.75            4.0   3.5   4.0units)Torque At 24&#39;        117 @           102   102.5 98 @        60&#39;                   @ 60&#39; 60&#39;DYNAMIC INSTRONCompressive Modulus        858     865     861   732   711Hysteresis   11.7    12.7    12.3  15.6  3.9D.Q.         73.3    68.1    70    46.9  182______________________________________ 
    
     Compositions containing chloroprene were prepared as known in the art including identical amounts of previously known retarders as well as the chemical di-morpholino disulfide. Table 2 shows representative compositions and FIG. 1 is the graphic comparison of the use of dimorpholine disulfide with identical amounts of several previously used retarders. The retarding effect of di-morpholino disulfide can also be determined from Table 2 by comparing the T 10  values with other compositions. The T 10  value for the composition containing TMTM (B-26) is 13.9 minutes, where as the composition containing MBTS (B-25) is 14.4 minutes. 
     
                       TABLE 2______________________________________      B26   B25     B38     B35   B32______________________________________THE EFFECT OF SULFASAN R ON VULKACIT CRVDb-6004Neoprene GNA 100     100     100   100   100N650         44      44      44    44    44High Aromatic Oil        7.8     7.8     7.8   7.8   7.8Stearic Acid 1.5     1.5     1.5   1.5   1.5MgO          4       4       4     4     4ODP          3       3       3     3     3ZnO          4       4       4     4     4Sulfur       1       1       1     1     1Vulkacit CRV 0.25    0.25    0.25  0.25  0.25TMTM         2       --      --    --    --MBTS         --      2       --    --    --Morfax       --      --      2     --    --Sulfads      --      --      --    2     --Sulfasan R   --      --      --    --    2Original Stress-StrainCured 30&#39; at 320° F.Tensile, psi 3060    2375    2475  2750  2500Elongation (Percent)         280     265    255   305   27550% Modulus   400     450    490   390   350100% Modulus  890    1000    1000  825   775200% Modulus 2250    2375    2225  1950  1925Durometer, D  29      29      34    33    34250° F. &#34;C&#34; Tear      118   127   119Mooney Scorch at275° F.T.sub.1 (time in minutes)        8.8     8.2     12.1  14.3  17.6T.sub.10 (time in minutes)        13.9    14.4    18.9  20.9  4.5 pt                                    @ 24&#39;Minimum (torque        14.0    10.9    10.5  10.0  12.0units)DYNAMIC INSTRONCompressive Modulus        803     771     801   801   706Hysteresis   10.3    9.8     1.5   1.5   3.7D.Q.         78.0    78.7    534   534   191______________________________________ 
    
     Some retarding effect is noted with the use of Morfax (B-38), T 10  equals 18.9, and Sulfads (B-35), where T 10  equals 20.9, but a significant retarding effect is noted using the inventive composition containing di-morpholino disulfide where T 10  is only 4.5 points rise in 24 minutes. It can be seen therefore, that chloroprene compositions containing di-morpholino disulfide can be produced with a significant increase in scorch safety. It will be noted that the invention, curve B-32, shows significant delay in the onset of vulcanization as compared to the previous curves. 
     Table 3 shows that N,N&#39;-m-phenyl dimaleimide (C-4) when used with dimorpholino disulfide provides significantly longer scorch safety than N-methyl-2-thiazolidinethione (C-1) while maintaining the high ODR maximum torque. FIG. 2 shows this increased safety and high maximum torque in graphic representation. Comparing curve (C-4) with (C-1), the table illustrates that the torque at the end of the curve is approximately equal, while the onset of vulcanization is delayed significantly in the composition C-4 using N,N&#39;-m-phenyl dimaleimide. 
     It was found that replacing the ETU and Sulfur with N-methyl-2 thiazolidinethione caused a reduction in physical properties, particularly the 100% modulus (C-6). Replacing the MBTS with di-morpholino disulfide, increased scorch safety and also increased the physical properties near to the properties of the original compound. Comparing the 100% modulus figures shows (C-3) and (C-1) to be nearly equal. The use of N,N&#39;-m-phenyl dimaleimide increased physical properties further raising the 100% modulus from 725 to 850 and a synergistic effect was noted in a further increase in scorch safety from 13.0 to 14.9 minutes T 1 . It is apparent, therefore, that the use of di-morpholino disulfide and either N-methyl 2-thiazolidinethione or N,N&#39;-m-phenyl dimaleimide are useful in retarding the cure rate of chloroprene compositions while not significantly affecting other properties. 
     
                                           TABLE 3__________________________________________________________________________CONVENTIONAL AND INVENTIVE COMPOSITIONS        C3     C6     C1     C4__________________________________________________________________________83CR1013Neoprene GNA 100    100    100    100N762          50     50     50     50MgO           4      4      4      4Stearic Acid 1.5    1.5    1.5    1.5ODP           3      3      3      3Sulfasan R   --     --      2      2MBTS          0.75   2     --     --ZnO           4      5      5      5Vulkacit CRV --     0.5    0.5    --ETU           0.25  --     --     --Sulfur        1     --     --     --HVA-2        --     --     --      2Original Stress-StrainCured 30&#39; at 320° F.Tensile, psi 2350   2270   2290   2450Elongation (Percent)        295    370    290    29050% Modulus  430    300    375    450100% Modulus 800    550    725    850200% Modulus 1800   1250   1650   1860Durometer, D  33     29     31      33Aged 120 Hours at 257° F. (% Change)Tensile      1950           (-17)               2175                  (-4)                      2050                         (-10)                             2250                                (-8)Elongation   75 (-75)               215                  (-42)                      155                         (-47)                             145                                (-50)50% Modulus  1325           (+208)               600                  (+100)                      700                         (+87)                             920                                (+104)100% Modulus 800    1100                  (+100)                      1350                         (+86)                             1700                                (+100)Durometer, D 41 (8 pts)               35 (6 pts)                      35 (4 pts)                             38 (5 pts)250° F. &#34;C&#34; Tear        73            72     98DYNAMIC INSTRONCompressive Modulus        997    735    820    897Hysteresis   7.3    8.8    8.9    7.1D.Q.         13.6   8.4    9.2    12.5Mooney Scorch at 275° F.T.sub.1 (time in min.)        6.8    8.2    13.0   14.9T.sub.10 (time in min.)        10.5   10.6   17.9   22.4Min. (torgue units)        18.75  16.0   16.0   17.75__________________________________________________________________________ 
    
     Table 1, 2 and 3 illustrate the properties of a composition that has not been loaded with fiber, whereas Table 4 illustrates the properties of compositions which have been loaded with fiber. Table 4 again shows the effect of di-morpholino disulfide used with N,N&#39;-m-phenyl dimaleimide or N-methyl-2 thiazolidinethione for increase scorch safety of high viscosity fiber filled compositions. The T 1  values for the compositions show that a significant increase in scorch time of 9.2 minutes for (D-44) to 19.2 minutes for (D-42) can be obtained without sacrificing physical properties. Compare, for instance, the 10% Modulus values of 1800 for (D-44) to 1800 for (D-42) which is the inventive composition. 
     
                       TABLE 4______________________________________        D-44    D-40      D-42______________________________________EFFECT OF SULFASAN R/MVA ON 40 PMR NYLONFILLED NEOPRENE COMPOUNDSDb-9011Neoprene GRT   100       100       100N650           27        27        27Silica         15        15        15Oil            8         8         8MgO            4         4         4Stearic Acid   1.5       1.5       1.5ODP            3         3         3SRF 1501       1.7       1.7       1.71/4&#34; Nylon     40        40        40Polymel 7      6         6         6HMMM           6.7       6.7       6.7ZnO            5         5         5Sulfur         1         1         1ETU            0.25      --        --MBTS           0.75      --        --Vulkacit CRV   --        0.5       --HVA-2          --        --        2.0Sulfasan R     --        4.0       4.0Mooney Scorch at 275° F.T.sub.1 (time in min.)          9.2       16.0      19.2T.sub.10 (time in min.)          16.2      4 @ 24&#39;   2 @ 24&#39;Min. (torque units)          19.25     17.0      17.0Original Stress-StrainCured 30&#39; at 320° F.       MD     CMD     MD   CMD   MD   CMDTensile, psi       1450   5400    1325 5500  1260 4850Elongation (%)       85     52      75   58    75   4710% Modulus 400    1800    390  1600  400  180020% Modulus 550    2750    590  2550  575  282050% Modulus 975    5375    1025 5050  950  5400Durometer, D       43         45         45DYNAMIC INSTRONCompressive Modulus          1751      1660      1542Hysteresis     7.9       9.0       9.3D.Q.           22.3      18.5      16.6______________________________________ CMD as used herein means the orientation of the fiber is in the cross machine direction. MD means that the fiber is oriented in the machine direction. 
    
     In addition, in a composition as shown in Table 4, it is possible to utilize reinforcing fillers up to 250 phr by weight of materials selected from carbon black, amorphous silica, activated clay and chopped fibers and/or mixtures thereof. 
     While present embodiments of this invention and methods of practicing the same have been illustrated and described, it will be recognized that this invention may be otherwise variously embodied and practiced within the scope of the following claims.