Recently, a new class of thermoelastic sulfonated polymers have been described in a number of U.S. Patents, wherein these sulfonated elastomeric polymers have been produced from commercial elastomeric polymers by either sulfonating with a sulfur trioxide complexed with a Lewis base or an acyl sulfate.
The sulfonated elastomeric polymers of these patents were produced from elastomeric polymers having Mooney viscosities of about 50 to about 90 at 212.degree. F. at (1+8) minutes. The high melt viscosities and molecular weights of the initial elastomeric polymers resulted in sulfonated elastomeric polymers having extremely high melt viscosities thereby limiting their use on conventional mixing and fabrication equipment.
Commercial hydrocarbon elastomers such as Vistalon 2504, Vistalon 3708, Vistalon 4608 and Butyl 365 contain some volatile components, most of which is water. The volatile contents depend on polymer type and Mooney viscosity but are generally not below 0.5 weight percent, more normally not below about 0.3 weight percent, and on occasion even lower. Indeed water contents of hydrocarbon polymers vary with the severity of finishing conditions and can vary widely even from bale to bale. When such wet commercial polymers are sulfonated with the reagents described in earlier patents, the incorporation of sulfur is lowered and excesses of reagents are required; however, variable water means variable sulfur levels and off-specification products. In the study of sulfonation reactions, those skilled in the art frequently dry the hydrocarbon polymer further, for example by mixing on a hot rubber mill or by air or vacuum oven drying. Water levels are also significantly reduced by hot extrusion of the commercial polymers, an operation designed to lower the molecular weight and the viscosity of the base polymer.
During the manufacture of elastomeric polymers, the percentage of retained water is increased as the Mooney viscosity decreases. For example, an EPDM terpolymer having a 20 Mooney viscosity at 212.degree. F. generally contains at least 1.0 wt.% of water and could have as high as 3.0 wt.% of retained water.
The two previously taught methods of sulfonation as described in the following patents work readily on elastomeric polymers having less than 0.1 wt.% of water. However, as the percentage of retained water increases, not only does the degree of sulfonation decrease but also the undesirable side reaction of sulfation occurs as will be clearly illustrated in the main embodiment of the present instant invention.
U.S. Pat. No. 3,642,728, herein incorporated by reference, teaches a method for the selective sulfonation of olefinic unsaturation sites of an elastomeric polymer, such as Vistalon 3509 having a Mooney of 90, to form a sulfonated elastomeric polymer. The olefinic sites of the elastomeric polymer are sulfonated by means of a sulfur trioxide complexed with a Lewis base. The sulfonated elastomer can be readily neutralized with a basic material to form an ionically cross-linked elastomer having substantially improved physical properties over an unsulfonated elastomer at room temperature. However, these ionically cross-linked elastomers may be processed like a conventional thermoplastic at elevated temperatures under a shear force in the presence of selected preferential plasticizers which dissipate the ionic associations at the elevated temperatures thereby creating a reprocessable elastomer. If an EPDM contains no moisture and no reagent consuming materials, such as calcium stearate slurry aid used in the plant isolation of the EPDM, the SO.sub.3 complexes will be converted essentially quantitatively to sulfonated polymer.
If an EPDM terpolymer containing 5-ethylidene-2-norbornene as the third monomer having a Mooney viscosity of 20 and more than 0.2 wt.% water is sulfonated with 30 mmoles of reagent per 100 g of EPDM according to this method, the conversion of the SO.sub.3 complex to sulfonate and sulfate is less than 65 percent due to the reaction of the water with the complex to generate sulfuric acid and free Lewis base.
The sulfuric acid may react with the unsaturation sites of the elastomeric polymer to result in the production of hydrolytically and thermally unstable sulfates rather than the desired sulfonates. This is the case when 5-ethylidene-2-norbornene is the third monomer in the EPDM. With less highly substituted olefins the sulfuric acid is relatively unreactive to the available unsaturation and does not take part in further reactions.
U.S. Pat. No. 3,836,511, herein incorporated by reference, teaches an improved process for the sulfonation of the olefinic sites of the elastomeric polymer, wherein the improved sulfonating agent is selected from acetyl sulfate, propionyl sulfate and butyryl sulfate. The neutralizing agents employed to neutralize the sulfonated elastomeric polymers are organic amines. This patent clearly teaches that the acyl sulfate is generated at a molar ratio of acid anhydride to sulfuric acid of 1.4:1 or less. This sulfonating agent works readily on the elastomeric polymers embodied by this patent, namely Vistalon 4608, if these polymers contain less than 0.1 weight percent retained water; and Butyl rubber 268 having a 55 Mooney and less than 0.1 weight percent retained water. When an EPDM terpolymer having a 20 Mooney and more than 0.5 weight percent water (see Table III, Sample F) of the present invention is sulfonated at a molar ratio of acetic anhydride to sulfuric acid of 1.6 or less, a conversion of sulfonating agent to sulfonated elastomeric polymer of less than about 50 percent is realized. The water reacts with the generated sulfonating agent to yield acetic acid and sulfuric acid, neither of which is a sulfonating agent. The reaction rate of water with the sulfonating agent is virtually instantaneous [J. Russell and A. E. Cameron, J. Amer. Chem. Soc., 60, 1345 (1938)]. Acetic anhydride on the other hand reacts slowly with water to form acetic acid ["Techniques of Chemistry, Volume II Organic Solvents", 3rd Edition, J. A. Riddick and W. B. Bunger, Wiley-Interscience, New York, 1970, p. 743]. ##STR9##
U.S. Pat. Nos. 3,779,974; 3,770,682; and 3,912,605 herein incorporated by reference, teach the process method of U.S. Pat. No. 3,836,511, wherein a Butyl rubber having a 55-60 Mooney is sulfonated.
The present invention is concerned with the use of improved sulfonating agents and a process for the sulfonation of an elastomeric polymer having more than 0.2 wt.% water to produce a sulfonated elastomeric polymer, wherein at least 70% of the amount of sulfonating agent present in the reaction zone that does not exceed the stoichiometric amount of sulfonating agent required to react with all available unsaturation in the elastomeric polymer is converted to sulfonate groups in the elastomeric polymer. More preferably the conversion is at least 80% and most preferably 85%, wherein the molar ratio of the sulfonating agent to available unsaturation sites in the polymer is 1.0 or less.