Patent Publication Number: US-2006014862-A1

Title: Vulcanizing latex compounds without the use of metal oxide activators or a zinc based accelerator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application 60/588,846 filed Jul. 16, 2004. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to latex compounds that may be cured without the use of a metal oxide and/or without the use of zinc based accelerators. The latex compounds may comprise zinc based antioxidant materials to provide zinc to the compounds. The invention further relates to methods for vulcanizing latex films without the use of metal oxide and/or without the use of zinc based accelerators.  
      2. The Related Art  
      Metal oxides, such as zinc oxide, can be used for vulcanizing latex compounds. Specifically, metal oxide functions as an activator for sulfur cure systems and a donor to the metal chloride crosslinks in a polychloroprene. For example, use of zinc oxide donates to zinc chloride crosslinks in a polychloroprene. Other metal oxides which may also be used for vulcanizing latex compounds include magnesium oxide and lead oxide.  
      Zinc oxide has a specific gravity of about 5.5 and settles quickly in water dispersions and latex compounds. Other metal oxides generally experience the same phenomena in water dispersions and latex compounds. As such, containers of latex dispersions comprising metal oxides, such as zinc oxide, must be vigorously remixed before each use because the metal oxide generally settles to the bottom of the container. Also, when latex dispersion master batches of accelerator, antioxidant and metal oxide, like zinc oxide, are prepared, frequent remixing is required which is difficult and time consuming. Furthermore, when hold and dipping tanks of the type used in the latex dipped goods industry are cleaned, the evidence of metal oxide accumulation and settling is pronounced, as in the case of zinc oxide. The same evidence is generally found in baths of latex compound in the manufacture of latex thread.  
      Metal oxides, particularly zinc oxide, have an important role in the activation of a latex compound. Accordingly, homogeneous consistency within the dispersion is needed for proper vulcanization of the latex material, however, accurate levels of the metal oxide are difficult to maintain due in part to the specific gravity of the metal oxide, which is particularly relevant with zinc oxide.  
      Another consideration with zinc oxide is that the level of zinc oxide in natural rubber and synthetic latex compounds is primarily responsible for dynamic and continual viscosity increases which affect the shelf life of such compounds. This consideration is pertinent to other metal oxides as well. Without the zinc oxide (or other metal oxides), as well as zinc based thiazole or dithiocarbamate accelerators, viscosity fluctuations would not be a consideration and natural rubber and synthetic latex compounds could be held for a very long time in storage.  
      Prior to the invention, the function of zinc based material in, for example, an antioxidant system having zinc based synergist, was thought only to enhance the performance of the antioxidant. Indeed, in the prior art, zinc 2-mercaptotoluimidazole compounds are said to improve high temperature performance of compounds comprising thermoplastic polymers.  
      In this specification, all parts and percentages are on a weight by weight basis unless otherwise specified.  
     SUMMARY OF THE INVENTION  
      The invention relates to the surprising discovery that latex compounds can be effectively vulcanized without the use of metal oxide and/or without the use of zinc based accelerators. In an aspect of the invention, the latex compounds comprise a zinc based antioxidant material, for example an antioxidant system having a zinc based synergist which provides zinc to the compound. The zinc based synergist provides multiple functions in a latex compound; such as 1) antixoidant synergist, 2) an activator for sulfur cured compounds and 3) as a zinc source for zinc chloride crosslinks in polychloroprene. Acceptable physical properties can be obtained from compounds comprising no metal oxide and no zinc based accelerator.  
      Methods for curing latex films without metal oxide and/or without zinc based accelerators are also within the scope of the invention. In one aspect of the invention, the method generally concerns heating latex films comprising zinc based antioxidant material (e.g. synergist). The method generally comprises the steps of forming a film of a latex compound having no metal oxide or zinc based accelerator, such as films of compounds comprising zinc based antioxidant material, and heating the film.  
     DETAILED DESCRIPTION OF THE INVENTION  
      The latex compounds of the invention do not comprise either metal oxide, such as zinc oxide, magnesium oxide, lead oxide or combinations thereof. The latex compounds may also not comprise a zinc based accelerator. In an embodiment of the invention, the latex compounds comprise a zinc based antioxidant material (e.g. synergist). The amount of zinc based antioxidant synergist material may be about 0.1 parts per hundred rubber (hereinafter referred to as “phr”) or less, to about 5.0 phr, or more. In an aspect of the invention, the zinc based antioxidant synergist material functions as an activator for a curing system for latex compounds.  
      In an embodiment of the invention, the zinc based antioxidant materials comprise a synergist selected from the group consisting of zinc 2-mercaptotoluimidazole (“ZMTI”) and zinc 2-mercaptobenzimidazole (“ZMBI”), or combinations thereof. A zinc based antioxidant system useful in the invention may comprise a combination of phenolic antioxidant (phenol) and ZMTI. VANOX® SPL, which is a mixture of phenolic antioxidant and ZMTI available from R. T. Vanderbilt Co. Inc., Norwalk, Conn., USA (“Vanderbilt”) may be used, alone or in addition to separate phenolic antioxidant (such as VANOX® L from Vanderbilt) and zinc 2-mercaptotoluimidazole (such as VANOX® ZMTI from Vanderbilt) in the compound.  
      The latex compounds generally comprise heat curable natural or synthetic rubber type materials. The latex compounds may comprise one or more of the materials selected from the group consisting of natural rubber, polyisoprene, polychloroprene, acrylonitrile, styrene-butadiene, sulfur cured butyl, and the like, and combinations thereof. Useful polyisoprene materials include KRATON® polymers, such as KRATON® IR-401, from the KRATON® Polymers Group of Companies having offices in Houston, Tex., USA. Neoprene polychloroprene from DuPont Dow Elastomers, Wilmington, Del., USA may be used. The zinc based antioxidant synergist material forms part of the elastomer curing system, as well as providing other beneficial properties, such as, for example, enhanced physical properties, high temperature and aging resistance, whiter and brighter colors and film odor reduction to objects cured from the latex compounds.  
      In an embodiment of the invention, the latex compound comprises a liquid dithiocarbamate having a water soluble base, such as sodium, ammonium, triethylamine, piperdine and the like. In a preferred embodiment, the liquid dithiocarbamate having a water soluble base may be a blend of sodium dibutyldithiocarbamate (“NaDTC”) and sodium mercaptobenzothiazole (“NaMBT”).  
      The latex compounds may further comprise additional accelerator materials such as thiourea, including but not limited to trimethyl, diethtyl, ethylene, diphenyl and the like. The thiourea may be dibutylthiourea, in particular 1,3-dibutylthiourea, available under the trade name THIATE(® U from Vanderbilt.  
      The latex compounds may further comprise any number of additives. Non limiting examples of such additives are latex modifiers, thickeners, fire retardants, plasticizers and the like. Useful latex modifiers include sodium lauryl sulfate (DARVAN® WAQ from Vanderbilt) and sulfated methyl oleate (DARVAN® SMO from Vanderbilt). Useful fire retardants include antimony oxide and useful thickeners include sodium polyacrylate thickeners, such as Paragum-231 from Para-Chem, Simpsonville, S.C., USA.  
      In addition to the benefits of overcoming the disadvantages of using zinc oxide, as well as other metal oxides, in latex compounds, another benefit is recognized. A common practice in the glove and balloon dipping industry is to dilute latex compounds to about the 25% to 35% range. This is done to limit the amount of pick-up on the former and to control viscosity increases during maturation, especially in natural rubber and polyisoprene latex applications. By using a viscosity stable latex compound, such as the compounds described herein, higher concentrations of latex compound in the dip tanks are possible. Lower concentrations of coagulant might be possible in the coagulation tanks. For some applications, it may be possible to eliminate the coagulant.  
      Viscosity stable latex compounds afford the manufacturer the luxury of choosing among a wide range of thicknesses and the opportunity to adjust the concentrations of coagulant and latex for optimum pick-up. An application for which this would be an advantage is in the manufacture of teats or baby bottle nipples. Straight dipping applications such as in the manufacture of condoms could benefit. Lineman, household and supported gloves like industrial gloves could be manufactured without multiple dips. Latex thread could be manufactured to greater cross sectional dimensions. Also, in certain applications, such as in the manufacture of teats or baby nipples, the elimination of metal oxides, such as zinc oxide, as well as zinc based accelerators from the formulation is desirable.  
      Elimination of metal oxides, such as zinc oxide and zinc based accelerators, from the latex compounds will provide environmental benefits. The elimination of the metal oxides and zinc based accelerators will reduce the heavy metal content of waste water effluent from facilities where the latex compounds are manufactured into useful goods.  
      The invention encompasses latex compounds which can be cured without metal oxide, such as zinc oxide, and/or zinc based accelerators, comprising an antioxidant system having zinc based materials. The zinc based materials may be selected from the group consisting of ZMTI and ZMBI, and the antioxidant may further comprise phenolic antioxidant. The invention also encompasses a method of curing a latex film without metal oxide, such as zinc oxide, and/or zinc based accelerators comprising the steps of forming a film from a latex compound comprising an antioxidant having zinc based materials and heating the film for a period of time. In an embodiment of the invention, the film can be heated up to about 1 hour at a temperature up to about 150° C., for example for about 5 minutes to about 45 minutes at about 90° C. to about 150°. The invention further encompasses a curing system for latex compounds wherein zinc based antioxidant synergist provides zinc to the system and acts as an activator, including systems wherein the antioxidant synergist is ZMTI and/or ZMBI. 
    
    
     EXAMPLES 1-3  
      Synthetic polyisoprene latex compounds having the compositions set forth in Table 1 were formulated by methods known in the art. All components set forth in Table 1 are reported as phr. Example 2 is a comparative example which includes zinc oxide. The liquid dithiocarbamate (WB-7) used in the compounds listed in Table 1 is a blend of NaDTC and NaMBT. A water dispersion of 2-mercaptotoluimidazole was used in Example 3. Note that in Examples 1-3, as well as other examples in this specification, zinc oxide and sulfur are identified as being from various suppliers. Zinc oxide and sulfur are ubiquitous in the market and the origin of these materials are not pertinent to the function of the examples, however, zinc oxide is generally available from Zinc Corporation of America, Monaca, Pa., USA and sulfur is generally available from R. E. Carroll Inc., Trenton, N.J., USA.  
      Films made from compounds of Examples 1-3 were cured at different conditions as set forth in Table 2. The curing conditions were 30 minutes at 100° C. and 20 minutes at 120° C. The tensile strength, 300% modulus and elongation of each cured film were tested in accordance with ASTM Test Method D412-98a (2001) e1 “Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomer-Tension” which is incorporated herein by reference in its entirety. The results are set forth in Table 2.  
                           TABLE 1                                          Ingredient           Example   Supplier or                                 Components   1   2   3   Tradename                                         Polyisoprene   100   100   100   KRATON ®, IR-401       latex       Sodium lauryl   0.25   0.25   0.25   DARVAN ® WAQ       sulfate       Zinc Oxide   —   0.5   —   Various       Sulfur   1.5   1.5   1.5   Various       Blend of   2   2   —   VANOX ® SPL       phenolic AO       and ZMTI       2-mercaptoto-   —   —   1   VANOX ® MTI       luimidazole       Phenolic   —   —   1   VANOX ® L       antioxidant       Dibutyl Thiourea   1   1   1   THIATE ® U       Liquid   1   1   1   Vanderbilt       dithiocarbamate       (WB-7)       Sulfated   0.5   0.5   0.5   DARVAN ® SMO       methyl oleate                  
 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Results 
               
               
                   
                 Example 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Test Applied 
                 1 
                 2 
                 3 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Films cured for 30 min. at 100° C. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Tensile Strength, MPa 
                 28.2 
                 28.2 
                 15 
               
               
                   
                 300% Mod., MPa 
                 1.3 
                 1.3 
                 1.2 
               
               
                   
                 Elong., % 
                 1080 
                 970 
                 960 
               
            
           
           
               
               
            
               
                   
                 Films cured for 20 min. at 120° C. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Tensile Strength, MPa 
                 24.5 
                 26.8 
                 18.5 
               
               
                   
                 300% Mod., MPa 
                 1.2 
                 1.4 
                 1.2 
               
               
                   
                 Elong., % 
                 990 
                 950 
                 930 
               
               
                   
                   
               
            
           
         
       
     
      Table 2 shows that for polyisoprene latex compounds, satisfactory physical properties can be obtained with or without the use of zinc oxide or a zinc based accelerator as long as the zinc source, ZMTI, is present. This is verified by Example 3 that contains no ZMTI and had poorer physical properties than Example 1. This is surprising in that zinc comprises only about 13%, by weight, of the ZMTI molecule.  
     EXAMPLES 4-8  
      Natural latex rubber compounds having the compositions set forth in Table 3 were formulated by methods known in the art. All components set forth in Table 3 are reported as phr. Example 4 is a comparative example which includes zinc oxide. The liquid dithiocarbamate used in the compounds listed in Table 3 is WB-7, a blend of NaDTC and NaMBT.  
      Films having thicknesses of about 25 mils to about 40 mils were made from compounds of Examples 4-8. The films were cured for 20 minutes at 100° C., after which the tensile strength, 300% modulus and elongation of each cured film were tested applying ASTM Test Method D412-98a (2001) e1. The results are set forth in Table 4.  
      Based on the testing of the cured films, the samples containing no source of zinc, Example 6, and no dithiocarbamate from the liquid dithiocarbamate, Example 7, were slow to improve their physical properties. Therefore, Examples 6 and 7 were not further tested or considered.  
      The remaining cured compounds, Examples 4, 5 and 8, were subjected to mild heat aging at 70° C. for 7 days. The heat aged samples were tested for tensile strength, 300% modulus and elongation applying ASTM Test Method D412-98a (2001) e1.  
      The testing of the heat aged samples indicates that the physical properties of the compounds improved compared to the testing done prior to aging. Thus, the samples were initially undercured because at normal aging temperatures, fully cured natural rubber latex films would have deteriorated to some degree rather than continue to crosslink.  
      An appropriate state of cure for the film thicknesses of about 25 mils to about 40 mils would require a slightly longer dwell time, a slightly higher temperature or both. The wall thickness of latex examination gloves is generally about 10 mils, wherein the applied cure conditions would have been adequate. Nevertheless, none of Examples 4, 5 or 6 displayed deterioration following the normal heat aging procedure employed in the latex glove industry. The testing demonstrates that natural rubber compounds can be cured to form products with acceptable physical properties without using zinc oxide or a zinc based accelerator (Example 5) and that, with natural rubber, the liquid dithiocarbamate has a more pronounced effect on crosslinking than thiourea (compare Examples 7 and 8).  
                           TABLE 3                                      Example   Ingredient                                         Components   4   5   6   7   8   Supplier                                                 NR LATZ   100   100   100   100   100   Getahindus (M)                               Sdn Bhd       Sodium lauryl sulfate   0.25   0.25   0.25   0.25   0.25   DARVAN ® WAQ       Zinc oxide   0.5   —   —   —   —   Various       Sulfur   1.5   1.5   1.5   1.5   1.5   Various       Blend of phenolic AO   2   2   —   2   2   VANOX ® SPL       and ZMTI       Dibutyl Thiourea   1   1   1   1   —   THIATE ® U       Liquid dithiocarbamate   1   1   1   —   1   Vanderbilt       (WB-7)       Sulfated methyl oleate   0.5   0.5   0.5   0.5   0.5   DARVAX ® SMO                  
 
     
       
         
           
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Results 
               
               
                   
                 Example 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Test Applied 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Curing for 20 mins @ 100° C. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Tensile, MPa 
                 18.1 
                 16.9 
                 13.4 
                 7 
                 16.8 
               
               
                   
                 300% Mod., MPa 
                 2 
                 1.5 
                 1.1 
                 0.8 
                 1.3 
               
               
                   
                 Elong., % 
                 690 
                 730 
                 740 
                 780 
                 770 
               
            
           
           
               
               
            
               
                   
                 Aged for 7 days @ 70° C. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Tensile, MPa 
                 23.1 
                 21.1 
                   
                   
                 22.9 
               
               
                   
                 300% Mod., MPa 
                 2.6 
                 2 
                   
                   
                 1.7 
               
               
                   
                 Elong., % 
                 630 
                 720 
                   
                   
                 790 
               
               
                   
                   
               
            
           
         
       
     
     EXAMPLES 9-13  
      Polychloroprene latex compounds having the compositions set forth in Table 5 were formulated by methods known in the art. All components set forth in Table 5 are reported as phr. Example 9 is a comparative example comprising zinc oxide. Polychloroprene Neoprene from DuPont Dow Elastomers, was used in the compounds formulated for Examples 9-13. The liquid dithiocarbamate used in the compounds listed in Table 5 is WB-7, a blend of NaDTC and NaMBT. The thickener was Paragum 231 from Para-Chem and is added in amounts as necessary to reach optimum viscosity for each application (for example exam gloves, household gloves, thread, balloons, condoms and the like), as would be understood by one skilled in the art.  
      Films made from compounds of Examples 9-13 were cured at different conditions as set forth in Table 6. The curing conditions were 30 minutes at 120° C. and 45 minutes at 120° C. The tensile strength, 300% modulus and elongation of each cured film were tested applying ASTM Test Method D412-98a (2001) e1. After the testing, the films cured for 30 minutes at 120° C. were heat aged for 22 hours at 100° C. and then again tested for tensile strength, 300% modulus and elongation by applying ASTM Test Method D412-98a (2001) e1. The results are set forth in Table 6.  
                           TABLE 5                                      Example   Ingredient                                         Components   9   10   11   12   13   Supplier                                                 Polychloroprene latex   100   100   100   100   100   DuPont Dow       Sodium lauryl sulfate   0.25   0.25   0.25   0.25   0.25   DARVAN ® WAQ       Zinc oxide   5.0   —   —   —   —   Various       Sulfur   1.5   1.5   1.5   1.5   1.5   Various       Blend of a phenolic   2   2   —   2   2   VANOX ® SPL       AO and ZMTI       Dibutyl Thiourea   1   1   1   1   —   THIATE ® U       Liquid dithiocarbamate WB-7   1   1   1       1   Vanderbilt       Sulfated Methyl Oleate   0.5   0.5   0.5   0.5   0.5   DARVAN ® SMO                         Thickener   as required ——————————     Para-chem                  
 
     
       
         
           
               
               
             
               
                   
                 TABLE 6 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Results 
               
               
                   
                 Example 
               
            
           
           
               
               
               
               
               
               
            
               
                 Test Applied 
                 9 
                 10 
                 11 
                 12 
                 13 
               
               
                   
               
            
           
           
               
            
               
                 Curing for 30 min @ 120° C. 
               
            
           
           
               
               
               
               
               
               
            
               
                 Tensile, MPa 
                 26 
                 25.4 
                 20.9 
                 17.5 
                 15.3 
               
               
                 300% Mod., MPa 
                 2 
                 2 
                 1.8 
                 1.8 
                 1.5 
               
               
                 Elong., % 
                 810 
                 810 
                 830 
                 810 
                 850 
               
            
           
           
               
            
               
                 Curing for 45 min @ 120° C. 
               
            
           
           
               
               
               
               
               
               
            
               
                 Tensile, MPa 
                 29.3 
                 29.3 
                 22.3 
                 25.5 
                 21.9 
               
               
                 300% Mod., MPa 
                 2 
                 2 
                 1.6 
                 1.8 
                 1.6 
               
               
                 Elong., % 
                 790 
                 790 
                 840 
                 800 
                 860 
               
            
           
           
               
            
               
                 Curing for 30 min @ 120° C. and then aging for 22 hrs at 100° C. 
               
            
           
           
               
               
               
               
               
               
            
               
                 Tensile, MPa 
                 34 
                 29.9 
                 34.6 
                 26.6 
                 29.6 
               
               
                 300% Mod., MPa 
                 2.4 
                 2 
                 2.2 
                 2.1 
                 1.7 
               
               
                 Elong., % 
                 700 
                 710 
                 700 
                 670 
                 760 
               
               
                   
               
            
           
         
       
     
      The results of the polychloroprene film testing were surprising considering that polychloroprene is known in the art for its dependence on substantial quantities of metal oxide to achieve a satisfactory cure. For example, excellent physical properties were obtained in Example 10 without the use of zinc oxide or a zinc based accelerator. The results imply that sufficient zinc is supplied by the ZMTI in the antioxidant system to establish zinc chloride crosslinks in addition to the sulfur crosslinks.  
      The results of testing the compound of Example 11 show that the zinc from the ZMTI may have more of an affect on the cure rate and cure quality for polychloroprene films than 5.0 phr of zinc oxide (Example 9). The physical property results for Example 10, which did not contain zinc oxide, are virtually equivalent to the results from testing of Example 9. However, the physical property results from the testing of Example 11, without ZMTI, are significantly lower.  
      The results further indicate that the acceleration system of thiourea and liquid dithiocarbamate, WB-7, is synergistic. Using either the thiourea alone in the compound of Example 12 or using the dithiocarbamate alone in the compound of Example 13 produced average results for polychloroprene latex, however, the compound of Example 10, which is comprised of both the thiourea and liquid dithiocarbamate produced excellent physical properties in the polychloroprene latex film, which is an indicator of the synergy achieved in the invention in that neither accelerator alone is as effective as the two in combination.  
      As discussed above, the samples of Examples 9-13 cured for 30 minutes at 120° C. were subjected to heat aging for about 22 hours at about 100° C. Heat aging of cured polychloroprene latex films generally tends to cause degradation evidenced by continual crosslinking that increases tensile strength and modulus ultimately resulting in polychloroprene latex films becoming hard and brittle. None of the heat aged samples of the invention (e.g., Examples 10-13), however, showed significant decrease in performance properties. The only tested property which decreased in value was elongation. The polychloroprene latex film of Example 10 had the smallest increase in tensile and modulus than the other compounds and retained the most of its original physical properties after heat aging. These aspects of the results from these examples are particularly surprising and dispel the common notion that zinc oxide in abundance is needed to protect the chloroprene polymer from heat degradation. The limited amount of zinc, then, supplied by the synergist, must limit the amount of crosslinking in the polymer, thereby maintaining the original properties and retarding the onset of hard and brittle films.