Abstract:
An improved, abrasion-resistant rice hulling roll comprises a core with coating of a nitrile rubber compound, particularly a carboxylated nitrile rubber compound. The hulling rolls of this invention are characterized by substantially improved wear life as compared to conventional rolls made of styrene-butadiene rubber (SBR). The nitrile rubbers are also useful in other wear-resistant applications.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to improved rubber products with better abrasion resistance. More particularly, this invention concerns the use of nitrile rubbers, in particularly, carboxylated nitrile rubbers with proper additives to make long life rubber products, e.g., rice hulling rolls. 
     2. Brief Description of the Prior Art 
     It is known in the art that current hulling rolls are made of styrene-butadiene rubber (SBR) with proper additives. Typically SBR needs sulfur, accelerators, antioxidants, antiozonants, activators, fillers, and softeners or extenders. The ingredients are mixed in internal mixers or on mills, and calendered, molded and cured in conventional equipment. Mixing procedures vary with the compounds. In general, the rubber, zinc oxides, antioxidant, and stearic acid are mixed; then the filler is added in portions with the softeners or oil. For rice hulling rolls, in order to maintain the quality of the rice, Hi-Sil is usually used as the filler instead of carbon black. It is general practice to dump and cool the batch. The second phase includes mixing in all the other ingredients, with the accelerator and sulfur being added last. Mixing is continued until the sulfur is well dispersed. The hulling rolls are usually produced from stock. First the core must be carefully applied to the core and rolled down by machine or by hand. Special care must be taken to prevent the incorporation of air into the roll either between the plies or as blisters in the calendered stock. The roll is built to sufficient size to allow it to be ground to a smooth surface at the desired diameter. The built roll is then wrapped tightly with nylon tape for vulcanization in an open steam. The vulcanized roll, after cooled, is machined to the proper size. The SBR rubber roll made as described above is currently used in all the rice mills. It is obvious that the rice hulling depends on abrasion to strip the rice shell. Further, in the hulling process, heat is generated. It is well known that the SBR roll does not have good heat resistance. When the shelling temperature is increased, the roll life will be decreased. 
     SUMMARY OF THE INVENTION 
     It is accordingly one object of the invention to provide a rubber roll with improved abrasion resistance and thermal resistance. Other objects and advantages of the present invention will become apparent as the description proceeds. 
     In satisfaction of the foregoing objects and advantages, this invention provides a class of rubber compounds, noticeably, nitrile rubber compounds, particularly carboxylated nitrile rubber compounds, to fulfill the necessary enhancements. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention involves the use of nitrile rubbers, more importantly carboxylated nitrile rubbers in the manufacture of rice hulling rolls. 
     Nitrile rubbers have excellent abrasion resistance. And the additional carboxyl groups in the carboxylated nitrile rubbers enhance the abrasion resistance even at elevated temperature. The vulcanized nitrile rubber compounds also have superior heat resistance. Therefore, the nitrile rubber compounds, particularly carboxylated nitrile rubber compounds, should be excellent products to be used as hulling rolls, e.g., rice hulling rolls. 
     Nitrile rubbers are amorphous like SBR, need reinforcement by fillers in order to obtain optimum properties. These fillers may be carbon black or mineral types. Carbon blacks give the best balanced reinforcement; but mineral fillers are used where colors other than black are required, for example, for the purpose of rice hulling. In addition, plasticizers, tackifiers, and processing aids may be added to improve the flow or knitting characteristics of the compounds. A plasticizer may further secure some end product properties. Antioxidants and activators may also be added to improve scorching phenomena. Also, pigmentary filler may be added to provide desirable color. Further other elastomers may be added to reduce cost and/or to improve the processing characteristics. 
     Nitrile rubber compounds are vulcanized in essentially the same manner as SBR and natural rubbers. Sulfur is usually used as vulcanizer. However, sulfur is less soluble in nitrile rubber than in SBR or natural rubber, an increase in accelerator is required. Carboxylated nitrile rubbers are vulcanized in a manner similar to standard nitrile rubbers plus the additional reaction of the acid groups. Zinc oxides or magnesium oxides can be used to react with the acid group. If the reaction is too fast and scorch problems are encountered, special types of zinc oxides or magnesium oxides can be used to slow down the reaction. 
     In accordance with this invention, there are provided rice hulling rolls of abrasion resistant nitrile rubber compounds, particularly carboxylated nitrile rubber compounds, which comprise: carboxylated nitrile rubber from 50 parts to 100 parts made up with other elastomers, which can be other nitrile rubbers, styrene-butadiene rubbers, or natural rubbers, to have 100 parts total rubber by weight. The filler is 0 to 150 parts, preferably 20 to 120 parts. The plasticizer is 0 to 35 parts. The tackifier is 0 to 35 parts. The processing aid is 0 to 5 parts. The antioxidant is 0 to 7 parts. The activator is 0 to 7 parts. The vulcanizer is 0.5 to 7 parts. The accelerator is 0 to 10 parts. A combination of zinc oxides and magnesium oxides is used in the amount of 1 to 25 parts, preferable 2 to 15 parts. If necessary, 0 to  20 parts of pigmentary filler may be added. Note that the formulation is described based on 100 parts of total elastomers, and given in parts by weight. 
    
    
     The following examples are presented to illustrate the invention but are not to be considered as limited thereto. Again in the examples and throughout this specification, parts are by weight unless otherwise indicated. 
     EXAMPLE I 
     The following samples illustrate the excellence of the nitrile rubber, particularly the carboxylated nitrile rubber, in the abrasion resistance. 
     
                       TABLE I______________________________________        1A       1B     1CSample No.     Parts by Weight______________________________________Chemigum NX775 100        75     75Chemigum N683B --         25     --SBR 1502       --         --     25Hi-Sil 215     30         30     30Hi-Sil EP      20         20     20Diisodecyl phthalate          5          5      5Durez 31802    12         12     12Oleic Acid     3          3      3Wingstay L     2          2      2PASCO 558T     5          5      5Unads          1.5        1.5    1.5Sulfur, spider brand          1.5        1.5    1.5Wingstay 100   0.5        0.5    0.5HEXA powder    0.75       0.75   0.75______________________________________ 
    
     Chemigum is the trade name of nitrile rubber produced by Goodyear Rubber &amp; Tire Company. Chemigum NX775 is a carboxylated nitrile rubber while Chemigum N683B is one kind of the nitrile rubbers. Durez is a phenol formaldehyde polymer which is a good tackifier for nitrile rubbers and HEXA powder is hexamethylenetetramine which is a phenolic resin accelerator. HEXA powder is added to provided additional cross-link reaction with the tackifier, Durez 31802. Both HEXA powder and Durez 31802 (Phenol Formaldehyde polymer) can be obtained from Occidental Chemical Company. Wingstay L is an antioxidant (Reaction product of para-cresol &amp; dicyclo-pentadiene). It is a product of Goodyear Rubber &amp; Tire Company. PASCO 558T is a zinc oxide, and can be obtained from Harwick Chemical Corporation. Unads is the trade name of tetramethylthiuram monosulfide. It is a good accelerator for nitrile rubber vulcanization, and can be obtained from R. T. Vanderbilt Company. Hi-Sil is precipitated hydrated amorphous silica. Wingstay 100 is mixed diaryl p-phenylenediamine. The source of the other chemicals can be found in the &#34;Blue Book&#34; of Rubber World Magazine. 
     The above formulas were mixed and vulcanized. The following properties were detected: 
     
         ______________________________________Sample          1A         1B     1C______________________________________Hardness, Shore A            91         90     91PICO Abrasion Index           665        438    175______________________________________ 
    
     EXAMPLE II 
     The following samples illustrate that various additives can be used. 
     
                       TABLE II______________________________________             2A   2BSample              Parts by Weight______________________________________Chemigum NX775      100    85Budene              --     15Cab-O-Sil M5, densed               10     10Hi-Sil EP           40     40Titanium Dioxide    10     10DSC-18              0.5    0.5Stearic Acid        2      2Sunolite 240        1.5    1.5Dioctyl Phthalate   25     25Santicizer 8        5      5Wingstay L          2      2PASCO 558T          6      6Unads               2.5    2.5Sulfur, spider brand               1.5    1.5Methyl Tuads        0.25   0.25______________________________________ 
    
     Budene is the trade name of polybutadiene produced by Goodyear Rubber &amp; Tire Company. DSC-18 is mercapto-silane coupling agent, and can be obtained from Harwich Company. Stearic acid is an activator; also has the function of plasticizer and softener. Sunolite 240, a blend of petroleum waxes, is an antioxidant agent, and can be obtained from Witco Chemical Company. Santicizer 8 is a sulfonamide type plasticizer and softener produced by Monsanto Company. Methyl tuads is the trade name of tetramethylthiuram disulfide. Similar to unads, it is a good accelerator for nitrile rubber vulcanization, and can be obtained from R. T. Vanderbilt Company. Wingstay L is an antioxidant. It is a product of Goodyear Rubber &amp; Tire Company. Hi-Sil is precipitated hydrated amorphous silica. Cab-o-sl is fumed colloidal silica. The source of the other chemicals either has been discussed or can be found in the &#34;Blue Book&#34; of Rubber World Magazine. 
     The above formulas were mixed and vulcanized. The following properties were detected: 
     
         ______________________________________Sample              2A     2B______________________________________Hardness, Shore A    77     78PICO Abrasion Index 213    149______________________________________ 
    
     Note that the plasticizer content in the samples in Example II is much higher than that in the samples in Example I, and titanium dioxide, a pigmentary filler, is also included. 
     EXAMPLE III 
     Based on the results shown in Examples I and II, the following formula is chosen for the field test. 
     
                       TABLE III______________________________________           Parts by WeightSample          3______________________________________Chemigum NX775  100Hi-Sil 233      50Diisodecyl phthalate           5Durez 31802     12Oleic Acid      3Wingstay L      2PASCO 558T      6Unads           1.5Sulfur, spider brand           1.5Wingstay 100    0.5HEXA poWder     1.0______________________________________ 
    
     Hi-Sil 233 is used only because of the availability at the time of test. The formula is mixed and used to make rice hulling rolls. The rolls are vulcanized and consumed in rice milling machine. These rolls last must longer than the rolls, which are made of SBR compounds, currently used by the rice milling plants. 
     The composition of this invention may be further formulated with a wide variety of elastomers, fillers, plasticizers, tackifiers, processing aids, antioxidants, pigmentary fillers, accelerators, vulcanizers and other ingredients. 
     In view of the foregoing description and examples, it will become apparent to those of ordinary skill in the art that equivalent modifications thereof may be used without departing from the spirit and scope of this invention.