Use of a p,p'-Dinonyldiphenylamine in a composition having a reduced tendency to form sludge in oil

Abstract: A p,p′-dinonyldiphenylamine represented by the following general formula: wherein R₁ and R₂ may be the same or different from each other and each stand for a branched alkyl group having 6 carbon atoms, and a composition containing said amine.

Description: The present invention relates to the use of a branched alkyl-substituted diphenylamine in a composition having a reduced tendency to form sludge in a mineral oil having a low aromatic content or a synthetic oil containing no aromatic ring in its structural unit. Mineral oils having a low aromatic content and synthetic oils, e.g. poly-alpha-olefin oils, which do not contain any aromatic rings in the structural units thereof, are themselves poor in oxidation stability. However, the mineral oils or synthetic oils have a good response to antioxidants and the resulting lubricating oil compositions exhibit high oil stability. However, the problem involved in these compositions is that the solubility of antiozidants in the oils is low. Antioxidants invariably undergo a change in quality in themselves when their function is exhausted during their use, presenting a problem if substances formed by their oxidation are left as a sludge. Japanese Patent Appln. Laid-Open Gazette no. (Sho.) 59-5146 discloses that certain p,p'-dialkyldiphenylamines are useful as additives for lubricants, elastomers and the like, and that such amines exhibit oxidation-inhibiting effects. However, such amines suffer from the disadvantage that the conversion products thereof are poor in oil solubility and thus form sludges. US-A-2493112 discloses a method for alkylation of diphenylamine and the production of diphenylamine alkylates for use as anti-oxidants in rubber compositions. We have made studies in relation to the high oxidation-inhibiting performance of diphenylamines in an attempt to overcome their disadvantage of forming a sludge and have found that when a diphenylamine having a specific structure is added to oil, it exhibits excellent oxidation-inhibiting performance with less sludge formation. According to the present invention there is provided use of a p,p'-dinonyldiphenylamine represented by the following general formula: wherein R 1 and R 2 may be the same or different from each other and each stand for a branched alkyl group having 6 carbon atoms, and nonyl group in the above formula is one derived from a propylene trimer, and an N-p-alkylphenyl-α-naphthylamine represented by the following general formula: wherein R 3 is an alkyl group having 12 or 15 carbon atoms derived from a propylene oligomer as essential components to a mineral oil having an aromatic content of 30% by weight or below and/or a synthetic oil containing no aromatic ring in its structural unit. The present invention will now be described in more detail. The p,p'-dinonyldiphenylamine utilised in the present invention is a compound represented by the following general formula: In the above formula, R 1 and R 2 may be the same or different from each other and each stand for an alkyl group having 6 carbon atoms. According to the present invention, the above groups are each a branched alkyl group derived from a branched nonene, particularly preferably one derived from a propylene trimer. A compound represented by the above general formula wherein R 1 and R 2 are straight-chain alkyl groups even when they have each 6 carbon atoms is unfavorable, because such a compound is liable to settle as a sludge in oil when oxidized. Further, a compound represented by the above general formula wherein R 1 and R 2 have less than 6 carbon atoms even when the groups represented by are those derived from a propylene oligomer is unfavorable, because such a compound is also liable to settle as a sludge in oil when oxidized. Furthermore, a compound as described above wherein R 1 and R 2 have more than 6 carbon atoms is also unfavorable, because the proportion of the functional group in the molecule is too low to exhibit a sufficiently high oxidation-inhibiting power. Particular examples of the groups R 1 and R 2 of the p,p'-dinonyldiphenylamine utilised in the present invention include Although the process for the preparation of the p,p'-dinonyldiphenylamine for the purpose of the present invention may be any arbitrary one, that is, the amine utilised in the present invention can be prepared by any known process, the Friedel-Crafts alkylation process using diphenylamine and propylene trimer is preferable from the standpoint of the ease of synthesis. Examples of the catalyst to be used in such process include metal halides such as aluminum chloride, zinc chloride and iron chloride and acid catalysts such as sulfuric and phosphoric acids, phosphorus pentaoxide, boron fluoride, acid clay and activated clay. Activated clay is particularly advantageous in that the product is not discolored, the conversion is high and the removal of the catalyst is easy. Further, the use of a mixed catalyst system comprising activated clay and aluminum chloride in a weight ratio of the former to the latter of from 3 to 10 brings about a further enhanced conversion. The Friedel-Crafts alkylation is generally carried out according to the procedure which will now be described. 1 mol of diphenylamine, 2 to 20 mol, preferably 3 to 10 mol, of a propylene trimer and 50 to 300g, preferably 100 to 200g, of activated clay are placed in a reaction vessel fitted with a temperature sensor, a nitrogen-gas inlet tube, a reflux condenser and a stirrer and heated to 135 to 142°C with stirring. When aluminum chloride is simultaneously used as a catalyst, one-third to one-tenth as much aluminum chloride as clay used is further added in portions. The reaction is continued at such temperature while sampling the reaction mixture at regular intervals and analyzing the sampled mixture by gas chromatography or infrared spectroscopic analysis. The heating and stirring are continued until the content of unreacted diphenylamine is lowered to 10% or below as determined by sampling. After the completion of the reaction, the reaction mixture is cooled and filtered with suction to remove the catalyst. The filtrate is distilled under a reduced pressure to remove unreacted propylene trimer. The residue is purified by chromatography to obtain the desired p,p'-dinonyldiphenylamine as a viscous transparent liquid. When the p,p'-dinonyldiphenylamine utilised in the present invention is added to a base oil such as mineral or synthetic oil, it serves as an antioxidant for such base oil. The base oil to be used in the present invention must be a mineral oil having an aromatic content of 30% by weight or below and/or a synthetic oil containing no aromatic rings in its structural unit. The mineral oil is generally one having a kinematic viscosity at 40°C of 10 to 10,000 cSt, preferably 20 to 1,000 cSt. It is generally preferable to use a mineral oil prepared by distilling a crude oil to recover a lubricant fraction and purifying the lubricant fraction by any suitable means such as solvent refining, sulfuric acid treatment, hydrogenation or day filtration. It is preferred that the mineral oil has an aromatic content of 30% by weight or below, preferably 20% by weight or below. The term "aromatic content" as used in this specification refers to a value as determined according to ASTM D2549-81. On the other hand, the synthetic oil to be used in the present invention must not contain any aromatic rings in its structural unit and generally has a kinematic viscosity at 40°C of 10 to 10,000 cSt. Particular examples thereof include poly-α-olefin oils prepared by the (co)polymerization of a C 4∼30 α-olefin, such as polybutene and decene-1 oligomer monoesters of an aliphatic monocarboxylic acid with an aliphatic monohydric alcohol, such as butyl stearate and methyl laurate; diesters of an aliphatic dibasic acid with an aliphatic monohydric alcohol, such as di-2-ethylhexyl sebacate, dioctyl adipate and ditridecyl glutarate; esters of an aliphatic polyhydric alcohol with an aliphatic monocarboxylic acid, such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethylhexanoate and pentaerythritol pelargonate; polyalkylene glycols and monoalkyl ethers, dialkyl ethers, monoalkyl esters and dialkyl esters thereof, such as polyethylene glycol and polypropylene glycol; cycloparaffins such as cyclodonecane, hydrindan, dicyclohexyl and tercyclohexyl; alkylcycloparaffins such as dicyclohexylbutane and dicyclohexylpropane and mixtures thereof. The amount of the p,p'-dinonyldiphenylamine utilised in the present invention to be added to the base oil as described above to prepare a suitable composition is preferably 0.01 to 10% by weight, further preferably 0.1 to 5.0% by weight based on the composition. The group R 3 of the N-p-alkylphenyl-α-naphthylamine must be a branched alkyl group derived from a propylene oligomer. If the group R 3 is a branched alkyl group derived from an α-olefin, the resulting compound will have substantially inferior oxidation-inhibiting performance to that of the N-p-alkylphenyl-α-naphthylamine utilised in the present invention. Further, if the group R 3 is a group derived from an oligomer of an olefin other than propylene, such as isobutylene, the resulting compound will be oxidized itself and settle as a sludge in the oil. If the group R 3 has less than 12 carbon atoms, the resulting compound will be liable to settle as a sludge in the oil after it has fulfilled its antioxidant action, even if the group R 3 is a branched alkyl group derived from a propylene oligomer. Furthermore, if the group R 3 has more than 15 carbon atoms, the proportion of the functional group in the molecule will be unfavourably low so as to result in a lower oxidation-inhibiting power. The process for the preparation of such N-p-alkylphenyl-α-naphthylamine is disclosed in detail in Japanese Patent Appln. Laid-Open Gazette No. (Sho.) 62-181396 which has been filed by the applicant of the present invention. The amount of the above N-p-alkylphenyl-α-naphthylamine to be added to a composition comprising the base oil and the p,p'-dinonyldiphenylamine is preferably 0.01 to 10% by weight, further preferably 0.1 to 5.0% by weight, based on the total weight of the composition. If required, the composition utilised in the present invention may contain conventional additives such as antioxidants, detergent-dispersants, viscosity index improvers, pour point depressants, oiliness improvers, wear resistance improvers, extreme pressure agents, corrosion inhibitors, metal deactivators, rust preventatives, defoaming agents, emulsifiers, demulsifiers, bacterioxides and coloring agents. Such additives are described in more detail in, for example, "Junkatsu-yu Gakkai-Shi Vol. 15 No. 6" or Toshio Sakurai "Sekiyu-Seihin Tenkazai (Additives for Petroleum Products)" (Saiwai Shobo). The total amount of the additives to be added is at most 10% by weight, preferably at most 5% by weight, further preferably at most 3% by weight, based on the total weight of the lubricant. The oil composition utilised in the present invention may be used as gasoline engine oil, diesel engine oils such as land diesel engine oil and marine diesel engine oil; turbine oils such as additive-free turbine oil, additive-containing turbine oil, gas turbine oil and marine turbine oil; gear oils such as automotive gear oil, industrial gear oil and automatic transmission oil; metal working oils such as hydraulic fluid, compressor oil, refrigerator oil, cutting oil, grinding oil, plastic working oil, heat treatment oil and electrospark machining oil; sliding guide way oil and bearing oil. Further, the composition can also be used as insulation oil such as transformer oil, breaker oil, cable oil or capacitor oil, anti-corrosive oil, heat transfer oil and grease or vacuum pump oil. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a chart showing the 1 H NMR spectrum of the p,p'-dinonyldiphenylamine utilised in the invention, Fig. 2 is a chart showing the 13 C NMR spectrum of such compound, Fig. 3 is a chart showing the mass spectrum of such compound, and Fig. 4 is a chart showing the IR spectrum of such compound. The invention will be described further with reference to Examples 3 and 5, Example 1 providing a suitable method of preparation of a diphenylamine utilisable in the present invention, Preparations 2 and 4 being given by way of information. Example 1 169g (1 mol) of diphenylamine, 1009g (8 mol) of a propylene trimer and 150g of activated clay having a specific surface area of 235m 2 /g and an acidity of 1.5 mgKOH/g were placed in a four-necked flask, and the resultant mixture was stirred. The mixture was heated in a stream of nitrogen to carry out a reaction at the boiling point of the propylene trimer for 10 hours. After completion of the reaction, the activated clay was filtered out and the filtrate was distilled under a reduced pressure to remove any unreacted propylene trimer. The residue was purified by chromatography to obtain 365g of the desired p,p'-dinonyldiphenylamine having a structure represented by the formula which will be described below, as an isomer mixture in the form of a pale yellow, transparent and viscous liquid. The elemental analysis of the compound so obtained revealed that such compound comprised 85.6% by weight of carbon, 11.4% by weight of hydrogen and 3.2% by weight of nitrogen. The yield based on diphenylamine was 86%. The 1 H NMR spectrum, 13 C NMR spectrum, mass spectrum and IR spectrum of such compound are shown in Figs. 1,2,3 and 4 respectively. Preparation 2 and Comparative Example 1 The conversion product resulting from the oxidation (i.e., after functioning as the antioxidant) of the p,p'-dinonyldiphenylamine prepared in Example 1 was added in an amount specified in Table 1 to a purified mineral oil having an aromatic content of 7% by weight (kinematic viscosity: 34.4 cSt at 40°C) and a poly-α-olefin oil (decene-1 oligomer, kinematic viscosity: 30.7 cSt at 40°C) respectively to determine the solubilities of the conversion product in these oils (Example 2). The results are shown in Table 1. For comparison, as shown in Table 1, a commersially available p,p'-dioctyldiphenylamine antioxidant having alkyl substituents derived from an isobutylene dimer was examined for the solubility of the conversion product thereof in a base oil in a similar manner to that described above (Comparative Example 1). The results are also shown in Table 1. The conversion products resulting from the oxidation of the p,p'-dinonyldiphenylamine and the p,p'-dioctyldiphenylamine were prepared by the method of R.F. Bridger (see J. Org. Chem., Vol. 33, No. 12 (1968)) according to the procedure which will now be described. That is, 0.05 mol of p,p'-dinonyldiphenylamine or p,p'-dioctyldiphenylamine was dissolved in 100 mℓ of acetone and the solution so obtained was cooled to 0°C. After 2.63g of potassium permanganate has been added to the solution portion-wise over a period of 5 hours, the cooling was stopped. The mixture so obtained was allowed to stand at room temperature for 15 hours, while maintaining the system in a nitrogen atmosphere. The reaction mixture was passed through a filter paper to remove manganese dioxide and the filtrate was distilled to remove acetone. The residue was dissolved in 100 mℓ of benzene and the solution so obtained washed with water and distilled to remove the benzene. The product thus prepared was ascertained by IR absorption spectrum, GPC analysis and mass spectrometric analysis to comprise the conversion product resulting from the oxidation of p,p'-dinonyldiphenylamine or p,p'-dioctyldiphenylamine as a main component. Example 3 and Comparative Example 2 In a similar manner to that of Example 2, the conversion product of the p,p-dinonyldiphenylamine prepared in Example 1 and that of N-p-dodecylphenyl-α-naphthylamine (the branched alkyl group being derived from a propylene tetramer) were simultaneously added to the same purified mineral oil and poly-α-olefin oil as those used in Example 2 respectively to determine the solubility of the conversion products (Example 3). For comparison, in a similar manner to that described above, the conversion product of p,p'-dioctyldiphenylamine and that of N-p-octylphenyl-α-naphthylamine were simultaneously added to the same base oil as that used above to determine the solubility of the conversion products in the base oil (Comparative Example 2). The results are shown in Table 1. Preparation Comparative Example 3 and Reference Example 1 In order to determine the oxidation-inhibiting performance of the p,p'-dinonyldiphenylamine prepared in Example 1, the p,p'-dinonldiphenylamine was added, in an amount specified in Table 2, to the same purified mineral oil and poly-α-olefin oil as used in Preparation 2, respectively. The lubricant compositions thus prepared were each subjected to the rotary pump oxidation test according to ASTM D 2272 (test temperature: 150°C, oxygen pressure: 6.3kg/cm 2 , room temperature, copper wire catalyst, 10 mℓ of water). The oxidation-inhibiting performance was evaluated by the time taken until the internal pressure of the vessel was reduced by 1.8kg/cm 2 (Example 4). The results are shown in Table 2. For comparison, the same p,p'-dioctyldiphenylamine as that used in the foregoing Comparative Example 1 and N-p-branched dodecylphenyl-α-naphthylamine were examined for oxidation-inhibiting performance in a similar manner to that described above (Comparative Example 3 and Reference Example 1). The results are also shown in Table 2. As can be understood from the results of Comparative Example 1 shown in Table 1, in Comparative Example 3, the compound listed in Table 2 was added in such a large amount that the conversion product thereof separated out, i.e., the resulting composition was in danger of forming sludge to thereby cause a problem in practical use. Example 5 and Comparative Example 4 In order to determine the oxidation-inhibiting performance exhibited when the p,p'-dinonyldiphenylamine prepared in Example 1 and the N-p-dodecylphenyl-α-naphthylamine according to the present invention are utilised together, the two compounds were added to the same purified mineral oil as that used in Preparation 2 in amounts specified in Table 2 to obtain a composition. This composition was subjected to the same test as that made in Preparation 4 to determine the oxidation-inhibiting performance of this antioxidant system (Example 5). For comparison, the antioxidant system of p,p'-dioctylphenylamine and N-p-octylphenyl-α-naphthylamine as used in Comparative Example 2 was also examined for oxidation-inhibiting performance in a similar manner to that described above (Comparative Example 4). The results of both examinations are shown in Table 2. As can be understood from the results of Comparative Example 2 shown in Table 1, in Comparative Example 4, the compounds listed in Table 2 were added in such large amounts that the conversion products thereof separated out, i.e.,the resulting composition was in danger of forming sludge to thereby cause a problem in practical use. It can be seen from the results shown in Table 1 that when the p,p'-dinonyldiphenylamine is added alone (Example 2),or together with the N-p-alkylphenyl-α-anaphthylamine (Example 3), to the base oil, the conversion product forms virtually no sludge in the base oil, so that such antioxidant systems can be added to the base oil in an enhanced amount. On the other hand, p,p'-dioctyldiphenylamine (Comparative Example 1) and a combination thereof with N-p-octylphenyl-α-naphthylamine (Comparative Example 2) are much Inferior to the antioxidant systems of Examples 2 and 3 in the solubility of the conversion product thereof. Therefore,the use thereof as an antioxidant is likely to form sludge, so that the addition thereof in a large amount is impossible. Further, it can be seen from the results shown in Table 2 that the compositions of Examples 4 and 5 are extremely unlikely to form sludge by virtue of the high solubilities of the p,p'-dinonyldiphenylamine, the N-p-alkylphenyl-α-naphthylamine and the conversion products thereof, so that the amounts of the antioxidant systems added can be arbitrarily controlled. Therefore, when high oxidation resistance is required, the amount of the antioxidant system utilised is increased. On the other hand, the antioxidant system of Comparative Example 3 or 4 cannot produce a lubricant composition having high oxidation resistance equivalent to that of the composition utilised in the present invention, because the amount of the system added is significantly limited owing to its tendency to form sludge, as can be seen from the results of Comparative Example 1 and 2, though the system is nearly equivalent to that of the present invention in respect of its oxidation-inhibiting performance. Further, the compositions of Comparative Example 3 and 4 each contain such a large amount of the antioxidant system that it is in danger of forming a sludge, so that even the oxidation resistance shown in Table 2 cannot be attained in practice. As described above, a specific mineral or synthetic oil is improved in oxidation resistance by the addition of the p,p'-dinonyldiphenylamine alone or together with the N-p-alkylphenyl-α-naphthylamine, with less likelihood of forming a sludge.

Claims: Use of a p,p'-dinonyldiphenylamine represented by the following general formula: wherein R 1 and R 2 may be the same or different from each other and each stand for a branched alkyl group having 6 carbon atoms, and nonyl group in the above formula is one derived from a propylene trimer, and an N-p-alkylphenyl-α-naphthylamine represented by the following general formula: wherein R 3 is an alkyl group having 12 or 15 carbon atoms derived from a propylene oligomer as essential components to a mineral oil having an aromatic content of 30% by weight or below and/or a synthetic oil containing no aromatic ring in its structural unit. Use according to claim 1 characterized in that said p,p'-dinonyldiphenylamine is present in an amount of 0.01 to 10 % by weight and said N-p-alkylphenyl-α-naphthylamine is present in an amount of 0.01 to 10 % by weight of the composition. Use according to claim 1 or 2 characterized in that said mineral oil has a kinematic viscosity of 10 to 10,000 cSt at 40 °C and said synthetic oil has a kinematic viscosity of 10 to 10,000 cSt at 40 °C. Use according to claim 1, 2 or 3 characterized in that said p,p'-dinonyldiphenylamine has the following general formula: wherein R 1 and R 2 may be the same or different from each other and each stand for a branched alkyl group having 6 carbon atoms of the following formula: Use according to any one of the preceding claims characterized in that said N-p-alkylphenyl-α-naphthylamine is N-p-dodecyphenyl-α-naphthylamine.

Inventors: ISHIDA NOBORU, TAKASHIMA HIROYUKI, ISHIDA, NOBORU, TAKASHIMA, HIROYUKI
Applicants: NIPPON OIL CO LTD, NIPPON OIL COMPANY, LIMITED
Application year: 1990
Main classification: C07C211
IPC codes: C10M169, C09K15, C10N30, C10L1, C07C211, C10N20, C10M133