US2882229A

US2882229A - Lubricant compositions

Info

Publication number: US2882229A
Application number: US531823A
Authority: US
Inventors: Eugene E Richardson
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1955-08-31
Filing date: 1955-08-31
Publication date: 1959-04-14
Anticipated expiration: 1976-04-14

Description

United States Patent LUBRICANT COMPOSITIONS Eugene E. Richardson, Hammond, Ind., assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana No Drawing. Application August 31, 1955 Serial No. 531,823

7 Claims. (Cl. 25232.7)

This invention pertains to improved lubricant compositions and more particularly pertains to internal combustion engine lubricant compositions which inhibit cam follower pitting'and/or wear and which inhibit the sticking of hydraulic valve-lifters.

Many modern automobile engines are designed with hydraulic valve-lifters for smoother and more eflicient engine operation. In the operation of hydraulic valvelifters, the lifter rise on the cam shaft converting the rotary motion of the cam into a reciprocating motion, which in turn opens and closes the valves. The bottom of the valve-lifter is known as the cam follower. The hydraulic part of the valve-lifter functions by means of a plunger on the inside of the valve lifter barrel, together with a ball-check and a spring. To obtain higher efficiency and greater horsepower, larger valves, higher valve spring pressures and camshafts which produce more rapid valve opening and closing are employed. In the operation of such engines, greatly increased pressures are encountered where the camshaft lobes come into contact with the faces of the valve-lifters resulting in excessive and severe cam and lifter wear as well as pitting of the cam followers. This wear and/or pitting cannot be adequately inhibited by most present-day internal combustion engine crank-case lubricants. Surprisingly, the type of anti-wear properties which will inhibit piston ring and/or cylinder wear are not necessarily the same as the anti-wear properties necessary to prevent cam and lifter wear and/orpitting; hence many lubricants which are effective in inhibiting ring and cylinder wear are ineffective in preventing cam and lifter wear. A further complicating factor in this problem is caused by the materials used in making the camshaft and valve-lifters since different metals and metal combinations are used for this purpose in the various engines. Camshaft and valve-lifter combinations of difierent metals create different requirements on the'lubricant in that certain types of anti-wear additives may be effective with some cam-lifter metal combinations but ineffective with other metal combinations. Furthermore, because of the small clearances between the plunger and the barrel of hydraulic valvelifters, extremely small amounts of deposits such as varnish and/or rusting may cause sticking of the valvelifter. Hence, it is essential for good engine operation to provide a lubricant which will, in addition to inhibitwear and/ or pitting, also inhibit the formation of varnish and/or rusting in the valve-lifter.

It is an object of this invention to provide a lubricant composition which will not cause sticking of hydraulic valve-lifters and which will inhibit cam follower wear and/or pitting. Another object of the invention is to provide a crank-ease lubricant which will be substantially free 'of any tendency to cause valve-lifter sticking and which will have cam follower anti-pitting and anti-wear properties. Another object of the invention is to proi ce vide an improved lubricant for internal combustion engines. Still another object of the invention is to provide a method of preventing the sticking of hydraulic valvelifters and the pitting and/or wear of cam followers in and 5.0% of an oil soluble mercaptau e.g. dodecyl mercaptan or disulfide e.g. dodecyl disulfide. The polymerization products have a molecular weight between about 400 and 2000.

The unsaturated fatty acids which may be polymerized are those natural or synthetic mono-carboxylic acids which generally will have two or more unsaturated linkages. If natural fatty acids are employed they will usually have sixteen to twenty-six carbon atoms, most z frequently eighteen carbon atoms, but if synthetic unsaturated fatty acids are used they may have a lesser or greater number Of carbon atoms. Examples of the natu ral fatty acids are those such as linoleic, linolenic,

ricinoleic (which upon heating forms linoleic acid), linoleaidic, elaidolinolenic, eleostearic, arachidonic, eicosatrienaic, cetoleic, docosatrienoic and the like. The free fatty acids can be polymerized either thermallyv or with the assistance of catalysts. In US. 2,482,761 Goebel disclosed the polymerization of free fatty acids by hydrolyzing a fat or an oil, adding a small portion of water, and heating in a pressure vessel until substantially all of the diand tri-unsaturated fatty acids present polymerize. The resultant product is then heated at a reduced pressure to distill off vaporizable constituents, leaving behind the polymerized unsaturated fatty acids. The.

polymerization reaction is carried out at a temperatureof about 330 to 360 C. for about three to eight hours at a pressure varying between .75 and 500 p.s.i.g. The

polymerization product may consist of monomers, dimers,-

trimers, and higher polymers of the unsaturated fatty acids.

merization products thereof, followed by hydrolysis to.

yield the corresponding polymers of the acids. A large source of the polymerized unsaturated fatty acids are those residual acids obtained by methanolysis of the semi-drying or drying type oils such as castor oil, soybean oil and others listed supra, polymerizing the methyl esters, removing unpolymerized compounds, saponifying; the residual esters and freeing polymerized acids therefrom. Such a procedure is described in Us. 2,450,940 to Cowan et al. The products of catalytic polymerization of semi-drying oils such as the BF polymerization products of soybean oil, cottonseed oil, or the hke also produce polymers suitable for use in the invention.

It should be understood that while various polymerized unsaturated fatty acids may be used, they do not alllprovide the same effect, and indeed there may be pronounced Various fats or oils such as sardine oil, linseed oil, soydifferences when used in the composition of this invention. A highlypreferred source of the polymerized unsaturated fatty acids is obtained as a by-product still residue in the manufacture of sebacic acid by the dry distillation of castor oil in the presence of sodium hydroxide. A method of obtaining such by-product still residues in the manufacture. of sebacic acid is described in US. 2,470,849 issued to WL'E. Hanson on May 24, 1949. The mixture of high molecular weight unsaturated fatty acids comprises monomers, dimers, trimers and higher polymers in the ratio, of from about 45% to about 55% of a monomers and dimers fraction having a molecular weight in the range of from about 300 to 600, and from about 45% to about 55% of a trimers. and higher polymer fraction having a molecular weight in excess of 6,00. The fatty acid polymers result in part from a thermal polymerization of fatty acid type constituents of the castor oil, and in part from other reactions, such as the inter-molecular esterification, of such acid to form high molecular weight products. The acid mixture, which is. mainly a mixture of polymeric long chain polybasic carboxylic acids, is further characterized by the following specifications:

Acid No.150 to 164 Saponitication No.l75 to 186 Free fatty acids,.75 to 82% Iodine, .valuc-44 to 55 Non-.saponifiables-Jfi to 5 A fatty acid mixture such as above described is marketed by the W. C. Hardesty Company under the trade name "13-50 Acids, and as VR-l Acids" by Rohm and HaasCompany.

The polymerization, products of the unsaturated fatty acids may have a molecular weightbetween about 400 and: 2000. Those polymers having a molecular weight higher thanabout500, and especially those having molecular weights averaging about 800 or higher are particularly preferred for use in this invention. Especially preferred polymerized unsaturated acids are the polymerization products of linoleic acid having a molecular weight between about 400 and 2000, wherein the polymerization products consist of more than about 40% of the trimer and higher molecular weight polymers of linoleic acid with the remainder consisting primarily of monomers and dimers of linoleic acid. Thepolymerization products of the unsaturated; fatty acids should be used inan amount between about 0001 and 5.0%. in the oil, preferably between about 0.05 to 2.0%, for example about 0.2%. The amount ofpolymerized fatty acid, as well as the amounts of the mercaptan or disulfide used, may be-varied dependingupon the oiljn which it is employed, the materials used in makingthe cam shaft and valve-lifters, the valve spring pressures and other factors which affect the degree of pitting and wear.

The oil soluble organic sulfur compounds which are used in combination with the polymerized unsaturated fatty acids in the oil, are those aliphatic mercaptans, and disulfides which are oil soluble. Thus, aliphatic marcaptans having from two to twenty-four Carbon atoms, especially those having between eight and. sixteen carbon atoms, may be used. The mercaptansmay be primary-, secondary-, or. tertiary-aliphatic mercaptans. having straight or branched alkyl chains. Mercaptans such as, butyl mercaptan, hexyl mercaptan, octylj mercaptan, nonyl mercaptan, dodecyl mercaptan, tridecyl mercaptan, hexadecyl mercaptan and the like may be used; While normal dodecyl mercaptan is very satisfactory, tertiary dodecylimercaptan is preferred since it is more cheaply manufactured;

Similarly, the oil soluble disulfides which are employed. are. aliphatic disulfides having from about two or four carbon atoms up to about forty carbon atoms,,preferably between about 15 and. 30' carbon atoms. The aliphatic side chains may be straight or branched, and'the disulfides may e seco dary or t t a y. i fid S mi ar o diis-v 4 similar aliphatic side chains may be contained in the disulfide. Disulfide such as hexyl disulfide, butylhexyl disulfide, nonyl disulfide, dodecyl disulfide, hexadecyl disulfide, nonyldodecyl disulfide, and others may be used. A preferred disulfide is tertiary dodecyl disulfide.

The mercaptan or disulfide may be employed in amounts between about 0.001 and 5.0%, preferably between about 0.05 and 2.0%. As in the case of the polymerized fatty acid, the amount of the mercaptan or disulfide may be varied depending upon the intensity of pitting and wear which normally occurs and which is due to the factors such as valve-spring pressures, the materials used in making the, cam-shaft and valve-lifters, and the oil employed. A suitable amount of the mercaptan or disulfide is usually about 0.2%

Although the polymerization products of unsaturated fatty acids together with the oil soluble mercaptans or disulfides may suitably be used by themselves in a base oil to overcome pitting, wear, and/ or rusting, they are usually used in combination with detergent type additives, espe cially those which contain sulfur and/ or phosphorus and sulfur, which function to inhibit. the formation of sludge and/or varnish like products deposits in the engine and/ or about the valves and rings of the engine. Additives of, this type are usually used in amounts of' from about 0.002% to about 10%, and preferably from about 0.01% to about 5% Among the phosphorus and sulfur-containing addition agents are the neutralized reaction products of a phosphorus sulfide and a hydrocarbon, an alcohol, a ketone, an amine or an ester. Of the. phosphorus sulfide reaction product additives, the neutralized reaction prodnets. of a phosphorus sulfide, such as a phosphorus pentasulfide, and a hydrocarbon of the type described in U. S. 2,316,082, issued to C. M. Loane et al. April 6, 1943, are preferred.

As taught in the US. 2,316,082 patent, the preferred hydrocarbon constituent of the reaction is a mono-olefin hydrocarbon polymer resulting from the polymerization of low molecular weight mono-olefin hydrocarbons, such as propylene, butenes, amylenes or copolymers thereof; Such polymers may be obtained by the polymerization of mono-olefms of less than 6 carbon atoms in the presence of a catalyst, such as sulfuric acid, phosphoric acid, boron fluoride, aluminum chloride or other similar halide catalysts of the Friedel-Crafts type.

The polymers employed are preferably mono-olefin polymers or mixtures of mono-olefin polymers and isomono-olefin polymers having molecular weights ranging from about 150 to about 50,000 or more, and preferably from about 500 to about 10,000. Such polymers can be obtained, for example, by the polymerization in the liquid isomono-olefins, such as butylene and isobutylene at a temperature of from about F. to about 100. F. in

the presence of a metal halide catalyst of the Friedel- Crafts type, such as for example, boron fluoride, aluminum chloride, and the like. In the preparation of these polymers, a hydrocarbon mixture containing isobutylene, butylenes and butanes recovered from petroleum gases, especially. those gases. produced in the cracking of petroleum oils in the maufacture of gasoline canbe used.

Another suitable polymer is that obtained by polymerizing in. the liquid phase, a hydrocarbon mixture comprising substantially C hydrocarbons in the presence of an aluminum chloride-complex catalyst. The catalyst. is preferably prepared by heating aluminum. chloride with isooctane. into the bottom of the reactor and passed upward through the catalyst layer, while a temperature of from about.

50' F; to about F. is maintained in the. reacton to any desired molecular weight, preferably. from. about.

500 to aboutl,000,or-. higher.

The hydrocarbon mixture is introduced stock residuums, lubricating oil'distillates, petrolatums,

or parafiin waxes, can be reacted with the'ph'o'sphor'us sulfide, as well as high molecular weight olefinic hydrof carbons such as cetene (C and cerotene (C 5), melene (C andmixed high molecular weight alkenes obtained by crackingpetroleum oils..

Other preferred olefins suitable for the preparation of the phosphorus sulfide reaction products are olefins having at least carbon atoms in the molecule of which from about 13 carbon atoms to about l8'carbon atoms, and preferably at least 15 carbon atoms, are in a long chain. Such olefins can be obtained by the dehydro genation of alkyl halides, preferably long chain alkyl halides, particularly halogenated paraflin waxes.

As a starting material there can be used the polymer or synthetic lubricating oil obtained by polymerizing unsaturated hydrocarbons resulting from the vapor phase cracking of paralfin waxes in the presence of aluminum chloride which is fully described in US. Patents 1,955,260, 1,970,402 and 2,091,398.

, Other hydrocarbons that can be reacted with a phosphorus sulfide are aromatic hydrocarbons, such as for example, benzene, naphthalene, toluene, xylene, diphenyl and the like, or an alkylated aromatic hydrocarbon, such as for example, benzene having an alkyl substituent having at least 4 carbon atoms, and preferably'at least 8 carbon atoms, such as a long chain paraifn wax.

The phosphorus sulfide-hydrocarbon reaction product can be readily obtained by reacting a phosphorus sulfide, for example P 8 with the hydrocarbon at a temperature of from about 200 F. to about 500 F., and preferably from about 200 F. to about 400 F., usingfrom about 1% to about 50%, and preferably from about 5% to about of the phosphorus sulfide in the reaction.' It is advantageous to maintain a non-oxidizing atmosphere, such as for example, an atmosphere of nitrogen above the reaction mixture. Usually, it is preferable to use an amount of the phosphorus sulfide that will completely react with the hydrocarbon so that no further purification becomes necessary; however, an excess amount of phosphorus sulfide can be used and separated from the product by filtration or by dilution with a hydrocarbon solvent, such as hexane, filtering and subsequently removing the solvent by suitable means, such as by distillation. If desired, the reaction product can be further treated with steam at an elevated temperature of from about 100 F. to about 600 F.

The phosphorus sulfide-hydrocarbonreaction product normally shows a titratable acidity which is neutralized by treatment with a basic reagent. The phosphorus sulfide-hydrocarbon reaction product when neutralized with a basic reagent containing a metal constituent is characterized by the presence or retention of the metal constituent of the basic reagent. Prior to neutralization the reaction product can be hydrolyzed and clayed to remove inorganic acids of phosphorus as described in US. 2,688,612 issued to R. W. Watson September 7, 1954.

The neutralized phosphorus sulfide-hydrocarbon reaction product can be obtained by treating the acidic reaction product With a suitable basic compound, such as hydroxide, carbonate oxide or sulfide of an alkaline earth metal or an alkali metal, such as for example, potassium hydroxide, sodium hydroxide, sodium sulfide, calcium oxide, lime, barium hydroxide, barium oxide, etc. Other basic reagents can be used, such as for example, ammonia or an alkyl or aryl-substituted ammonia, such as amines. The neutralization of the phosphorus sulfidehydrocarbon reaction product is carried out preferably in a non-oxidizing atmosphere by contacting the acidic reaction product either as such or dissolved in a suitable solvent, such as naphtha with a solution of the basic agent. As an alternative method the reaction product can be treated with solid alkaline compounds, such as KOH, NaOH, Na CO ,'Ca'0,"BaO, BflOl-ljg; Ned-and the like, at an elevated temperature of from about F. to about 600 F. Neutra-lized reaction products containing a heavy metal constituent, such as for example, tin, titanium, aluminum, chromium, cobalt, zinc, iron, and the like, can be obtained by reacting a salt of the desired heavy metal with the phosphorus sulfide-hydrocarbon reaction product which has been treated with a basic reagent such as above-described. v I

Other phosphorus sulfide reaction products which can be used are the reaction products of a phosphorus sulfide and a fatty acid ester of the type described in US. 2,399,243; the phosphorus sulfide-degras reaction products of US. 2,413,332; the reaction product of an alkylated phenol with the condensation product of P 5 and turpentineof US. 2,409,877 and US. 2,409,878; the re: action product of a phosphorus sulfide and stearonitrile of US. 2,416,807, etc. j Thepolymerization products of the unsaturated fatty acids, the oil soluble mercaptans or disulfides, and the sulfur and/ or phosphorus and sulfur-containing detergenttype additive can be added individually to the base motor oil or they can'be pr-mixed in the desired proportions and the resultant mixed product added. to the base oil. For example,'from about 1 to 25 of the polymerized acids, from about 1 to 25% of the oil soluble mercaptan or disulfide, and from about 98 to, about 50% of the detergent-type additive can be pre-mixed at'a temperature of about 70 to F. and the mixture then added to the base oil. If desired, concentrates of a suitable lubricating oil base containing" 10 to about 50% or more of the herein-described mixture of polymerized acids and mercaptans or disulfides, with or without the detergent-type additive, may be used forblending with other hydrocarbonlubricating oils in the proportions de sired for the particular conditions of use to provide a finished product containing the amounts of the individual constituents previously indicated to be useful. I The lubricating oil may also contain otheradditives such'as VI improvers, bearing corrosion inhibitors, rustihhibitors andthelike. 1

While detergent-type additives effectively inhibit'the formation of sludge and/ or varnish-like productsin the engine, they do not possess anti-wear properties and corrosion and/ or rust inhibitingproperties. However,,when used in combination with the herein-described mixture of polymerized unsaturated fatty acids and mercaptan's or disulfides, complete engine'protection is obtained. is demonstrated by the data in the table which follows wherein performance testsfor evaluating the ability ,of the lubricating oil to inhibit pitting. and wear are'shown. The test used is known as theTestestablished by the General Motors Research Division. .The testisinade on a 1953 Chevrolet Power Glide engine equipped with chilled iron lifters, forged steel cams, steel rocker arm shafts, malleable iron rocker arms and operating at 3150 r.p.m. with a load of 30 brake horsepower, an oil sump temperature of 255 F., and a water outlet temperature of 200 F. Dual valve springs are installed to give an assembled static valve spring load of 240 pounds at 0.330 inch valve opening. The test is continued for 24 hours at the end of which the cam followers are inspected for pitting and over-all wear. The effectiveness of the additives for preventing pitting of the cam followers is shown in the table which follows. In this table each sample consisted of an SAE 10W-30 oil containing 3.3% of a barium-containing neutralized reaction product of P 8 and a polybutene of about 1000 molecular weight, 0.75% of a sulfurized dipentene (35% sulfur), and 10 p.p.m. silicone polymer anti-foam agent. Various additives having anti-pitting and anti-wear properties were added to difierent samples of the oil containing the barium salt,

7 glfurized dipcntene and-silicone polymer. The results ow: v

--L-.S5 .engine tests Connen-;

SarnploNo. AntkPittingrAnti-Wear tration, Lifters Additive Wt. Fitted ;Percent- 1 None. p Y J 10 2-, 13-50 cm. "0.25 '3 3.. -tert-Dodeeyl ll/Iercaptan 0.40 4 c r None tort-DodocylMercaptan 0.20 5 F. staid -0. 16 No n-Dodecyl-Mercaptan- 9-0. 20 n8 6 D-so Acid. 0. is N tert-Dodecyldlsulfide.- 0.25

Sample .1 .shows that when no anti-.pitting-anti-wear additive is added to the oil, 10 of the l2eam .follower surfaces .of the hydraulic valve lifters become pitted. Samples2 and '3-show that although the D-SO acid -(inspect'ionsand methodof manufacture .of which have been detailed ,pneviously) and the oil soluble mercaptan are somewhat .ei'r'ect'ive individually for reducing the .number of pitted cam followers, the additives individuallyare not-able to eliminate pitting. Samples 4, '5, and .6 show that .whenthe polymerized unsaturated fatty ,acid and the oilsoluble-mercaptan ordisulfide are used in combination in ,theflubricating oil, there is no pitting .at all .ofthe cam fo'llowersurfaces.

While the present inventionjhas been described by the use o'flthe mixture .of fatty acids in petroleum lubricating oils, other lubricating oil bases can be employed such as hydrocarbon oils, .natural Qrsynthetic such as .those obtained bythe polymerization of olefines, as 'well as synthetic lubricating oils of the 'alkylene' oxide type, for example Ucon Oils marketed by Carbide *and Carbon Corporation, and the polycafboxylic 'acid ester type oils such'as 'the oil-soluble esters of adipic acid, sebacic acid, azelaic acid, etc. a Unless .otherwise stated, ithe percentages stated herein and in the claims .are .weight percentages.

Although the present invention has been described with reference to .specific ,preferred embodiments thereof, .the invention is not to the considered .as limited -.thereto :but incl de t s ene .such m i at o a d sariations as .come within .the spirit :of .;the appended claimsdai 1. A lubricating .oil composition comprising .a major proportion of a mineral lubricating oil between about 0.002 and1i0% of an alkalinefearth containing neutralized reaction product of a phosphorus sulfide and an olefin 'polymer,-"between about 05001 and 510% of the polymerization products of unsaturated fatty acids having from 16 "to 26 l carbon atoms, said polymerization produets'having a molecularweight between400 and 2,000,

and between about 0.001 and 5.0% .of an oil soluble organic sulfur compound selected .from the class con.- sisting of .aliphaticimercaptans having from .8 to .16 carhonatomsaud aliphatic disulfides having from to carbon atoms. f.

. .2. The composition of. cla m l ..in which the unsaturated fatty acid is linoleic acid.

3. The composition .of .claim 1 wherein the organic sulfur compound .dodecyl mercaptan.

I 4. TIhe composition of claim 1 wherein the organic sulfur compound is .terhdodecyl disulfide.

5. The composition of claim 2 wherein the polymerization products of .linoleic acid consist of more than about of the .trimer and higher molecular weight polymers, the remainder being essentially monomers and dimers of linoleic acid.

.6. A lubricating .oil composition comprising a major proportion .of a mineral lubricating oil base, between about 0.05 and 2.0% of the polymerization products of linoleic acid having a molecular weight between about 400 and 2000, said polymerization products consisting of more than about 40% .of the trimer and higher molecular weight polymers .of linoleic acid, between .about 0.05 and 2.0% of .tert-dodecyl mercaptan, and between about 0;.002 .and 10% of .a barium-containing neutralized reaction product of .a phosphorus sulfide and an olefin hydrocarbon polymer.

7. An addition agent-for a mineral lubricating oil composition comprising a concentrated solution of a mineral lubricating oil containing from about 10 to of a mixture consisting essentially of an alkaline earth-convtain'ing neutralized reaction product of a phosphorus .sulfide and ,an olefin polymer, the polymerization products of unsaturatedfatty acids having from 16 to 26 carbon atoms, said polymerization products having a molecular weight'between 400 and 2000, and an oil soluble organic sulfur compound selected from the class consisting of aliphatic mercaptans having from 8 to 16 carbon atoms and .aliphatic ,disulfides having from 15 to .30 carbon atoms, said concentrated solution being capable of dilution with a mineral lubricating oil to form a compounded lubricatingoil containing between about 0.002 and 10% of said alkaline earth-containing neutralized reaction product and phosphorus sulfide and an olefin polymer, between about 0.001 and 5.0% of thepolyrneriza'tion products of the fatty acids, and between 0.001 and 5.0% of said oilsoluble organic sulfur compound.

References Cited in the file of this patent UNITED STATES .PATENTS ;2,-546,552 Loane et-;al .Mar. 27, ,1951 12,631,979 :McDermott Mar. .17, 19.5.3 -2-, .69.l;002 Bartlesonet al. Oct. 5, 1954 2,123,236 Asseflzet a1. Nov. 8,, 195.5

Claims

1. A LUBRICATING OIL COMPOSITION COMPRISING A MAJOR PROPORTION OF A MINERAL LUBRICATING OIL, BETWEEN ABOUT 0.002 AND 10% OF AN ALKALINE EARTH CONTAINING NEUTRALIZED REACTION PRODUCT OF A PHOSPHORUS SULFIDE AND AN OLEFIN POLYMER, BETWEEN ABOUT 0.001 AND 5.0% OF THE POLYMERIZATION PRODUCTS OF UNSATURATED FATTY ACIDS HAVING FROM 16 TO 26 CARBON ATOMS, SAID POLYMERIZATION PRODUCTS HAVING A MOLECULAR WEIGHT BETWEEN 400 AND 2,000, AND BETWEEN ABOUT 0.001 AND 5.0% OF AN OIL SOLUBLE ORGANIC SULFUR COMPOUND SELECTED FROM THE CLASS CONSISTING OF ALIPHATIC MERCAPTANS HAVING FROM 8 TO 16 CARBON ATOMS AND ALIPHATIC DISULFIDES HAVING FROM 15 TO 30 CARBON ATOMS.