Abstract:
A method is disclosed for the vulcanization of acrylic gums having active halogen groups, comprising adding thereto a mixture of trithiocyanic acid, a metal salt of an aliphatic organic acid and a metal dialkyl dithiocarbamate and applying heat to the resulting mixture. The resultant vulcanizate does not require conventional oven post cure processing.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a non-postcure vulcanizing composition and the elastomers made therefrom More particularly, the present invention relates to a non-postcure polyacrylate vulcanizing system. The invention further relates to a method of producing elastomers from the claimed non-postcure vulcanizing composition. 
     BACKGROUND OF THE INVENTION 
     Polyacrylic elastomers have many properties including good heat and oil stability which make them desirable for the production of various rubberlike products such as, for example, gaskets, hoses, conveyor belts, valve seals, packings, oil seals and the like. 
     However, polyacrylic elastomers are difficult to vulcanize even when they contain vulcanization sites such as active halogen or epoxy groups. Generally, to achieve optimum properties the molded parts require a postcure process in an oven to complete their cure. This is due to the cure rate being too slow due to the use of conventional curing agents. If the processing temperature is increased to accelerate the cure time during molding, the elastomers cure prematurely (scorch) which is not desired for proper molding. The postcure process which is therefore necessary involves capital expense, and additional labor and energy costs. 
     U.S. Pat. No. 3,622,547 disclosed a vulcanizable acrylic composition having an improved cure rate. It was said to provide a vulcanizable composition containing an acrylic elastomer having active halogen groups an improved state of cure and improved aging properties. This was accomplished by providing a vulcanizable composition containing an acrylic gum and a combination of a dithiocarbamic acid derivative and trithiocyanuric acid in an amount sufficient to vulcanize the composition. 
     The compositions disclosed in U.S. Pat. No. 3,622,547 and other prior art publications, however, cannot very well meet the two critical needs that a non-postcure system requires, namely a fast cure rate and good scorch protection. It is well known in the art that scorch protection is probably the most important factor in elastomer production. Scorchiness leads to premature curing of the elastomer, causing poor flow of the gums from which the elastomers are produced and consequent clogging of the molding apparatus. Reducing the processing temperature and/or lowering the curative content in the gums do improve scorch protection but instead lower the cure rate to unacceptably low levels. 
     In recent years, the trend has focused upon the development of non-postcure curative systems. For example, these systems are disclosed in the following publications. 
     R.M. Montagne, &#34;Advances in acrylic elastomer cure technology&#34; Rubber World, Volume 199, No. 3, 20 (1988). 
     E. Lauretti et al., &#34;Enichem Acrylic Elastomers&#34;, PRI Rubber Conference (1984). 
     L.M. Centric et al., Detroit Rubber Group Technical Meeting, Detroit, MI (Oct. 13, 1988). 
     It is therefore an object of the present invention to provide a curative mixture that, after compounding with an acrylic gum in the usual manner, provides good scorch protection while still providing relatively high cure rates such that postcuring is not required. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to novel vulcanizable compositions which do not require postcuring, said compositions comprising 
     (a) at least one acrylic gum having active halogen groups; 
     (b) trithiocyanuric acid; 
     (c) at least one metal salt of an aliphatic organic acid; 
     (d) a metal dialkyl dithiocarbamate, and 
     (e) optional fillers and other inert additives. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As understood in the art, components (b), (c) and (d) are often referred to as curatives, curative modifiers and accelerators, or more generically, vulcanizing agents. 
     The present invention contemplates broadly all vulcanizable compositions containing an acrylic gum (a) having active halogen groups, including those elastomers disclosed, for example, in U.S. Pat. Nos. 3,201,373 and 3,312,677. More specifically, the invention contemplates the use of acrylic gums prepared by polymerizing alkyl acrylates and alkoxyalkyl acrylates, for example ethyl acrylate, butyl acrylate, methoxyethyl acrylate and the like with a chlorine or bromine containing comonomer such as, for example, vinyl chloroacetate, or vinyl benzyl chloride. 
     Preferably used in the practice of the present invention are polyacrylate gums containing from about 0.1 wt% to about 2.0 wt% of active halogen atoms, which are most preferably chlorine atoms. 
     Component (b) useful in the practice of the present invention is trithiocyanuric acid, the structure of which is well known. 
     The metal dialkyl dithiocarbamates (b) useful in the practice of the present invention are those of the formula ##STR1## wherein M is a divalent metal ion; and R 1  and R 2 , which may or may not be different, each is an aliphatic radical, an alicyclic radical, or an aryl-substituted alkyl radical. 
     Preferably, M is a divalent metal ion selected from the group consisting of zinc, copper, cadmium, lead, bismuth, iron and selenium ions. Most preferably, M is a zinc ion. 
     Preferably, R, 1  and R 2 , which may or may not be different, are selected from the group consisting of aliphatic or acyclic radicals having from 1 to about 12 carbon atoms, alicyclic radicals having from 5 to about 8 carbon atoms, a benzyl radical and a pentamethylene radical. Most preferably, R 1  and R 2  are selected from the group consisting of alkyl radicals having from 2 to about 4 carbon atoms and a pentamethylene radical. 
     The metal dithiocarbamates useful in the practice of the present invention therefore include zinc dimethyldithiocarbamate; zinc diethyldithiocarbamate, zinc dibenzyldithiocarbamates; zinc pentamethylene dithiocarbamate and the like. 
     The metal salts of aliphatic organic acids (c) useful in the practice of the present invention are mono-, di- and tri- aluminum salts of organic acids. There include salts of both long chain acids, such as fatty acids (stearic acid, oleic acid, etc.) and short chain acids (acetic acid, butyric acid, hexanoic acid, etc). 
     Preferably, an aluminum salt of a long chain acid is employed in the practice of the present invention. Most preferably, aluminum distearate is utilized. 
     The components (b), (c) and (d) are typically used in the practice of the present invention, on a weight percentage basis, as set forth in the ratios below. 
     Component (b): about 10 about 35 wt. % 
     Component (c): about 5 about 80 wt. % 
     Component (d): about 10 about 70 wt. % 
     Preferably, components (b), (c) and (d) are present in the following ratios set forth below: 
     Component (b): 15-25 wt. % 
     Component (c): 20-60 wt. % 
     Component (d): 25-55 wt. % 
     Components (b)-(d) are used in the practice of the present invention such that component (b) is present in amounts ranging from about 0.3% about 3.0% (on a weight basis) of component (a). Preferably, components (b)-(d) are present such that component (b) is present in amounts ranging about 0.6% to about 1.5% on the same basis. 
     The vulcanizable compositions provided by the present invention may also include carbon blacks, stearic acid, fillers, antioxidants, polymeric binders and similar well known additives. 
     The compositions of the present invention may be prepared by conventional compounding techniques. For instance, the various components may be mixed together on a two-roll mill or in a Banbury mixer. Through use of the present invention a compounded mixture of components (a)-(d) including optional fillers and other additives (e), exhibit very stable properties when stored at room temperature. Further, upon vulcanization, good cure rates coupled with excellent scorch protection are exhibited. Finally, following vulcanization, the elastomers produced exhibit excellent scorch long term stability in a variety of environments. 
     For convenient application and better effectiveness of the curative (components (b-d)), it is preferred that components b-d be premixed before compounding with the gum (a) and any fillers or other additives (e). Premixing may be accomplished with the components in a powdered form, or more conveniently, they may be premixed with an inert type of &#34;binder&#34; into a free flowing, dust-free bead form or in a. rubbery masterbatch form. Binding dusty powders into beads or producing other masterbatch forms for industrial applications for convenience as well as health concerns is well known in the industry and does not require elaboration herein. 
     The present invention is further illustrated through reference to the following Examples which should not be construed as limiting the scope of the present invention. All percentages are weight percentages unless otherwise stated. 
     EXAMPLE 1 
     A mixture consisting of 16.5 wt. % of trithiocyanuric acid, 32.8 wt % of zinc dimethyldithiocarbamate, 25.7 wt. % of aluminum distearate and 25 wt. % of a polymeric binder was produced into free flowing dust-free beads through use of a mixer. 
     A vulcanizable composition was then produced by compounding the following components: 100 parts of a polyacrylate gum made from ethvl acrylate, butyl acrylate and vinyl chloroacetate having a chlorine content of 0.3%, 60 parts of a N-550carbon black, 2 parts of a substituted diphenylamine antioxidant, 1 part of a stearic acid processing aid, 2 parts of a high molecular weight fatty acid ester processing aid (Strucktol Co.-WB222) and 4.5 parts of the previously described masterbatch. 
     EXAMPLE 2-6 
     The compounded vulcanizable composition of Example 1 was then transferred to a mold and subjected to a pressure at 200°0 C. for the period of time noted in Table 1. No postcure was employed. Properties exhibited by the vulcanizates so produced are noted therein. 
     
                       TABLE I______________________________________Example No.  2       3       4     5     6______________________________________Presscure = min.        0.5     0.75    1.0   2.0   3.0@ 200° C.Original Vulcan-izate PropertiesShore A Hardness        68      69      72    71    71100% Modulus, MPa        5.4     5.3     6.0   6.6   6.3Tensile Strength        9.7     9.7     9.6   10.2  9.9MPaElongation, %        250     250     222   212   210Compression Set*,%, Plied22 Hrs. @ 150° C.        24      27      25    27    2170 Hrs. @ 150° C.        37      33      27    27    23Air-oven Aging,70 Hrs. @ 150° C.Change in Hard-        +3      +2      +2    +2    +2ness, ptsChange in Ten-        +1      +3      +3    -5    -3sile, %Change in    +1      +3      +1    +14   +15Elongation, %AFT** 70 Hrs.@ 150° C.Change in Hard-        -8      -8      -9    -8    -8ness, ptsChange in Ten-        -1      -2      +1    -5    -2sile, %Change in    +1      - 1     +1    -5    -2Elongation, %Volume Change, %        +6      +4      +4    +4    +4______________________________________ *ASTM 0D395, Method B **AFT denotes Dextron ™ automatic transmission fluid 
    
     It can be seen that even after as little as 0.5 minutes of cure, the physical properties of the resulting elastomer, except for compression set, are as good as those of elastomers cured for longer cure times. After 1 minute of cure, all properties are seen to be excellent.