Terpolymer of ethylene, vinyl acetate and isobutylene useful as pour point depressants in distillate oils
This application is a continuation-in-part of application Ser. No. 678,945, filed Dec. 6, 1984 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to terpolymers and more particularly to terpolymers of ethylene, vinyl acetate and isobutylene useful as pour point depressants and/or flow improvers in distillate fuel oils. 
     The serious concern over supplies of petroleum has led to increased interest in diesel engines for passenger automobiles because of their proven economy in operation. Distillate oil fuels for automotive use must be made suitable, in cold weather, for uninterrupted service. Pour point depressants to promote cold temperature pour are added by the producer of the distillate or may be sold in automobile supply outlets, for direct addition by the consumer. In either case, a clear pour point additive is desirable from a marketing standpoint. Often a multiadditive system is prepared which combines pour point effectiveness with other functions such as corrosion inhibition and detergency to reduce nozzle deposits. 
     The use of ethylene-vinyl acetate copolymers to improve the low temperature pour characteristics of distillate oils has long been known. U.S. Pat. No. 3,048,479 describes the use of ethylene-vinyl acetate copolymers containing about 20 percent by weight vinyl acetate to improve the pour of distillate fuels. Such polymers are prepared in a solvent such as benzene at low pressures of around 800 psi using ditertiary butyl peroxide. U.S. Pat. No. 3,627,838 describes a process for manufacturing an ethylene-vinyl acetate copolymer containing 28 to 60 weight percent vinyl acetate where the monomers are reacted at a temperature of 280° F. to 340° F. under a pressure of 700 to 2000 psi in a solvent with azo compounds or peroxides such as ditertiary butyl peroxide. Similar processes are described in U.S. Pat. No. 3,126,364 and U.S. Pat. No. 3,254,063. A further modification in the use of ethylene-vinyl acetate copolymers as pour depressants is shown in U.S. Pat. No. 3,961,916 wherein two synthetic ethylene-vinyl acetate copolymers differing by at least 5 weight percent vinyl ester content are separately prepared and blended to produce a pour depressent additive. 
     A slightly different approach is disclosed in U.S. Pat. No. 4,087,255 which describes the preparation of ethylene-vinyl acetate copolymers for use as pour point depressants by reacting the monomers in an inert solvent at relatively low temperatures (70° to 120° C.) and pressures of 700 to 3000 psig. Polymers produced in this manner, having a molecular weight around 1000-2900, show reduced branching characterized by 2-methyl side branches per 100 methylene groups. 
     Other patents disclose the use of terpolymers of ethylene, vinyl acetate and monolefinically unsaturated polymerizable monomers for use as pour point depressants. For example, U.S. Pat. No. 3,467,597 discloses terpolymers of ethylene, vinyl acetate and butylenes. U.S. Pat. No. 3,638,349 discloses copolymers of ethylene and vinyl acetate wherein up to 20% of the copolymer can be other polymerizable unsaturated monomers. U.S. Pat. No. 4,178,950 discloses terpolyumers of ethylenevinyl acetate-butylene prepared by solution polymerization and of a number average molecular weight of about 5,000 to about 80,000, preferably 12,000 to about 60,000. 
     Improved pour point depressants having better clarity compared to presently used EVA copolymers, which are hazy at room temperature, and which suffer no adverse effects insofar as compatibility and response in distillate fuels are very much desired. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, polymers which are of improved clarity compared to presently used EVA copolymers (which are very hazy at room temperature) are provided. The clearer polymers of the present invention suffer no adverse effects insofar as compatibility and response in distillate fuels. They offer improved performance because the degree of branching can be controlled by blending or by direct synthesis. The polymers can be produced without the need for inert solvents, which must be removed before the product can be used. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been found that clear ethylene-vinyl acetate polymers can be produced herein by utilizing isobutylene as a monomer along with vinyl acetate. Since isobutylene-is incorporated in the polymer chain, a terpolymer containing ethylene, vinyl acetate, and-isobutylene is produced. The product, produced as described below, is a sparkling clear liquid at room temperature. When compared with an ethylene-vinyl acetate copolymer of similar viscosity-and-vinyl acetate content, the latter product is very hazy. Moreover; surprisingly, polymers with controlled amounts of the isobutylene terpolymer are found to be effective in lowering the pour point of distillate fuels while also possessing other requisite properties, such as good cold filter plugging point (CFPP) performance and good filterability. Specifically, ethylenevinyl acetate-isobutylene terpolymers with methyl per 100 methylene group ratios of about 2 to about 15, preferably about 4 to about 12, are found to perform better and have better clarity than copolymers of similar vinyl acetate content. 
     The ethylene-vinyl acetate-isobutylene terpolymers of the present invention are prepared by nonsolution, high pressure polymerization. In general these procedures involve free-radical polymerization in a stirred autoclave reactor designed to operate at high pressures of ethylene in a continuous manner. The ethylene pressure may vary from about 10,000 psig to about 35,000 psig, pressures of about 19,000 psig to about 30,000 psig are preferred. Vinyl acetate monomer is introduced into the aforesaid stirred autoclave reactor at a flow rate sufficient to produce a product containing about 30 to about 45 weight percent of combined vinyl acetate. The rate of isobutylene introduction depends on the rate of vinyl acetate introduction, and may range from about 0.1 to about 10 times the rate of vinyl acetate monomer flow rate to the reactor; flow rates of isobutylene to the reactor that are preferred will be about 0.1 to about 5 times the rate of vinyl acetate monomer flow rate. In addition, since the ethylene-vinyl acetate-isobutylene terpolymers of this invention are desirably of relatively low molecular weight, having a viscosity of about 100 to about 300 centipoise (cP) as measured at 140° C. in a Brookfield Thermosel viscometer, a suitable chain transfer agent may also be introduced into the reactor, despite the fact the isobutylene, in addition to acting as a monomer, also shows chain-transfer activity. Lower molecular weight methyl ketones and aldehydes are employed as chain transfer agents. Examples of the useful ketones are acetone, methyl ethyl ketone, methyl isobutyl ketone, and the like; examples of the useful aliphatic aldehydes are formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, and the like. Acetone and acetaldehyde are preferred, acetaldehyde being especially preferred. 
     When acetone is employed as chain transfer agent, its flow rate to the reactor will range from about 0.3 to about 1 times the flow rate of vinyl acetate monomer; a flow rate of about 0.6 to about 0.85 times the VA flow rate being preferred. When acetaldehyde is used as the chain transfer agent, its flow rate can be lower, from about 0.01 to about 0.3, preferably about 0.04 to about 0.1, times the flow rate of vinyl acetate monomer to the autoclave reactor. 
     The polymerization process of this invention is carried out at temperatures of about 225° to about 475° F.; a temperature of about 250° to about 450° F. is preferred. The temperature profile over the reactor may be held relatively constant or it may be relatively broad, as much as 150° F. in certain instances. 
     Free-radical initiators are employed in the process of the invention. In general, these are peroxygen compounds, for example, hydroperoxides, dialkyl peroxides, peroxyacids and estefs of peroxy acids and typically, include tert-butyl hydroperoxide, di-tert-butyl peroxide, peracetic acid, tert-butyl peracetate, tert-butyl perisobutyrate, tert-butyl perpivalate (also known-as pertrimethylacetate), tert-butyl peroctoate, di-sec-butyl peroxydicarbonate, and the like. 
     Preferred initiators are tert-butyl perpivalate, tert-butyl peroctoate and di-sec-butyl peroxydicarbonate. Two or more initiators may be used in a given polymerization. 
     When the polymerization process of this invention is performed as disclosed hereinabove, a terpolymer product is obtained that contains from about 30 to about 45 weight percent combined vinyl acetate, preferably from about 30 to about 40 weight percent vinyl acetate. The remainder of the terpolymer product will consist of ethylene and isobutylene of which ethylene makes up the major proportion. The isobutylene content is manifested largely in terms of a methyl to 100 methylenes ratio; as determined by proton nuclear magnetic resonance (proton NMR). A typical ethylene-vinyl acetate copolymer prepared by the process of this invention will exhibit a methyl to 100 methylenes ratio of about 2, but terpolymers containing increasing combined isobutylene content will have methyl to 100 methylene ratios in the range of about 2 to about 15, preferably from about 4 to about 12. At higher methyl to 100 methylene ratios, the terpolymers of this invention are no longer effective as pour point depressants. Thus, the methyl to 100 methylene ratio is very critical. 
     The molecular weight of the terpolymers of the invention is also an important property in relation to their performance as pour point additives to distillate fuel oils. Molecular weights may be determined by vapor pressure osmometry or by size exclusion chromotography (SEC). Both number average molecular weights (Mn) and weight average molecular weights (Mw) may be determined for the products of this invention by the SEC method. The Mn of the useful terpolymers of this invention ranges from about 400 to about 1200, preferably from about 500 to about 900; while the Mw of these terpolymers ranges from about 1500 to about 3000, preferably from about 2000 to about 2500. 
     As pointed out hereinabove, the ethylene-vinyl acetate-isobutylene terpolymers of this invention are surprisingly found to exhibit improved clarity relative to ethylene-vinyl acetate copolymers of similar vinyl acetate content. In fact, the terpolymers of this invention, prepared under conditions that favor maximum incorporation of isobutylene, are transparent, viscous liquids but, unfortunately, do not sufficiently depress the pour point of distillate fuels. Only those with a methyl to 100 methylene ratio of about 2 to about 15, preferably about 4 to about 12, provide the desired pour point depressing action. Nevertheless, these latter ethylenevinyl acetate-isobutylene terpolymers with controlled isobutylene content still possess greatly improved clarity relative to ethylene-vinyl acetate copolymers of similar vinyl acetate content, which are very hazy materials. Since pour point additives are generally handled and dispensed as concentrated solutions in a hydrocarbon solvent, said solutions of ethylene-vinyl acetate copolymers are also very hazy, whereas the corresponding solutions of the ethylene-vinyl acetate-isobutylene terpolymers of this invention, having controlled isobutylene contents to give methyl to 100 methylene ratios of about 2 to about 15, preferably about 4 to about 12, will exhibit greatly improved clarity. 
     The terpolymers of the present invention act as pour point depressants when effective amounts of the terpolymer are added to distillate fuel oils. Useful amounts of the terpolymer range from about 100 to about 1000, preferably about 200 to about 800, ppm by weight of the fuel oil being treated. 
     The terpolymers of this invention may be used alone as the sole additive or in combination with other oil additives, such as corrosion inhibitors, antioxidants, sludge inhibitors, etc. 
     The invention will be further understood by reference to the following examples 
     COMPARATIVE EXAMPLE 1 
     Ehtylene and vinyl acetate were pumped to a stirred high pressure autoclave and reacted at a pressure of 30,000 psig. The catalyst used was t-butyl perpivalate. Temperature in the reactor was 331° F. at the bottom and 333° F. at the top. The vinyl acetate was pumped at rate of 63 gal/hr. Acetone (a chain transfer agent) was pumped, to give a low molecular weight polymer, at a rate of 47 gal/hr. The final product had a vinyl acetate content of 36.3% and a viscosity of 163 cP at 140° C. It was a very hazy liquid at room temperature. 
     This EVA copolymer was tested in a No. 2 fuel oil with a distillation range of 193° F. to 320° F. The ASTM D-97 pour point of the base fuel was -10° C. At a level of 200 ppm, this EVA copolymer reduced the pour point to -16° C. However, while the product showed good pour response in the fuel, it failed to satisfy the fluidity test at -20° F. which is required for commercial products. The fluidity test measures the quantity of a premeasured amount (40 ml) of oil which flows through a small orifice in three minutes after being kept at the test temperature for two hours. A flow of 32 ml (80%) of the 40 ml aliquot of treated oil is required for a satisfactory rating. This product, when tested at a level of 900 ppm in oil gave only a 6 percent recovery. Hence, while it would act to depress the pour point, it was not satisfactory for commercial use. 
     EXAMPLE 2 
     Ethylene and vinyl acetate were pumped to a stirred autoclave reactor designed to operate at high pressure. The reaction was carried out at 20,000 psi using t-butyl perpivalate as the catalyst. The feed gas temperature was 129° F. and the temperature in the reactor ranged from 327° F. at the bottom to 383° F. at the top. The vinyl acetate was pumped at 30 gallons per hour and isobutylene was added at about 95 gallons per hour to produce a low molecular weight terpolymer. The terpolymer product had a vinyl acetate content of 39.2%, a viscosity at 140° C. of 402 cP and a CH 3  /100 CH 2  ratio of 19.9, as determined by proton NMR. This product was water white and clear with no haze at room temperature. 
     When tested in number 2 fuel oil, the terpolymer gave no response. It was completely ineffective in lowering the pour point. 
     The remarkable clarity of the isobutylene terpolymer can be shown by comparing the haze readings with a typical EVA copolymer. Measurements were made using a Coleman Model 9 Nepho-Colorimeter which is designed to measure haze. Since these pour depressants are sold in solvent to make handling and dilution easier, the haze measurements were made in 50% solutions of the polymer in HiSol 10 solvent. The results were: 
     
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                       EVE Terpolymer
            EVA Copolymer
                       (Example 2)
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Haze-Nephelos Units
              408          5.5
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     EXAMPLE 3 
     A terpolymer, in accordance with the present invention, was prepared in the high pressure stirred autoclave by reacting ethylene and vinyl acetate with controlled amounts of isobutylene. The reactor pressure was 19,000 psi and temperature ranged from 273° F. to 436° F., bottom to top. The catalyst used was di-secbutyl peroxydicarbonate. The vinyl acetate was pumped at 102 gph (gallons per hour) and the isobutylene at 103 gph. The product had 35.6 percent vinyl acetate and a viscosity of 167 cP at 140° C. The haze reading was 212 compared to 408 for the similar EVA copolymer. To further characterize the terpolymer, the CH 3  /100 CH 2  ratio was determined by proton NMR and found to be 4.9. The typical value for an EVA copolymer is 2.0. Molecular weights were determined by Size Exclusion Chromatography (SEC) and determined to be as follows: Mn=770, Mw=2100. 
     This product was tested in two different fuels with the following results: 
     
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              POUR POINT °C.
                100 ppm    300 ppm
Base Fuel       Terpolymer Terpolymer
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Fuel 1  -13         -28        -32
Fuel 2  -10         -22        -32
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     These-data show a good response when the additive was used in the fuel. 
     The terpolymer was tested for other important properties with the following results: 
     
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Test
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                   Fuel 1 + 100 ppm
                                Fuel 1 + 300 ppm
         Base Fuel 1
                   Terpolymer   Terpolymer
Cold Filter
         -7° C.
                   -14° C.
                                -22° C.
Plugging           Fuel 2 + 100 ppm
                                Fuel 2 + 300 ppm
Point    Base Fuel 2
                   Terpolymer   Terpolymer
         -9° C.
                   -16° C.
                                -17° C.
       % Recovery with Terpolymer
         100 ppm   200 ppm      Base Fuel
Fluidity Test
         81        92           0
Fuel 1
         Flow - °C. at 600 ppm
         terpolymer         Base Fuel
Low Temper-
         -15                -10
ature Flow
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     These results show that the terpolymer is an effective pour depressant under vehicle use conditions. 
     EXAMPLE 4 
     A terpolymer was synthesized in the same equipment as Example 3 except that acetaldehyde was used as a modifier in addition to the isobutylene. The reactor temperature ranged from 275° F. at the bottom to 420° F. at the top. Pressure was 22,400 psig. The catalysts used were di-sec-butyl peroxydicarbonate and t-butyl peroctoate. The VA feed rate was 105 gph; the isobutylene feed rate was 28 gph and the acetaldehyde feed rate was 7 gph. The product had a vinyl acetate content of 36.4% and a viscosity of 158 cP at 140° C. The CH 3  /100 CH 2  ratio was 7.9. The sample was tested in fuel oil with the following results: 
     
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Test                Result
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CFPP                -24° C.
Fluidity Test       82%
Low Temperature Flow Test
                    -22° C.
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     EXAMPLE 5 AND COMPARATIVE EXAMPLE 6 
     A terpolymer of ethylene, vinyl acetate and isobutylene was prepared by feeding ethylene gas, at a temperature of 120° F. and a pressure of 22,600 psi, vinyl acetate at a rate of 230 gallons per hour and isobutylene at a rate of 65 gallons per hour into a high pressure autoclave reaction. The reactor was maintained at an average temperature of 340° F. and at a pressure of 22,600 psi. 
     The product meets the requirements for a commercial pour depressant additive. 
     Obviously, other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention which are within the full intended scope of the invention as defined by the appended claims.