Lubricating Properties of Pentaerythritol Partial-Esters*

1 SCIENTIFIC PAPER

by Toshio Sakurai**, Seiichiro Hironaka**, Mineo Furuta***, and Yasuo Watanabe**

Summary: Pentaerythritol partial-esters were synthesized and their physical properties, thermal stabilities, heat of adsorption, and lubricity were investigated. It was found that the hydroxylgroups involvedin the partial-esters exerted some important effectson their properties. The partial-esters showed higher viscositiesthan their respectivefull-esters due to the association of their molecules. They adsorbedstrongly onto metal surfaces to control corrosionof the metals, and to give better boundarylubricity.

tetra-ester which contained some uncomplete 1 Introduction esters was reacted with caprylate acid chloride. The use of Type II esters such as neopentyl The remaining polar compounds as contaminants polyol esters as aircraft turbine engine lubricants were removed from these esters by percolation has rapidly increased, and these esters have through alumina and Florisil columns. These replaced Type I esters, the conventional dibasic esters were classified into mono-, di-, tri-, and acid esters, because of their better thermal stability tetra-esters by the experimental saponification properties. Requirements for lubrication under values which agreed with those calculated. severe condition of the present aircraft have led Pentaerythritol 2-ethylhexanoates were also to the development of polyphenyl ethers or C- prepared and purified by the same procedures. ethers, Type III lubricants. There are many Trimethylolethane and neopentylglycol capry- studies made on such properties as thermal and lates were prepared by the same procedures, and oxidative stabilities of neopentyl polyol esters1)～4). purified by distillation under reduced nitrogen However, most of them have been made for atmosphere at 231℃/0.2mmHg and 17-173℃/ full-esters. It is assumed that partial-esters are 0.2mmHg, respectively, and then percolated good boundary lubricants because of their higher through an alumina-silica gel column. adsorption characteristies due to the hydroxyl Di-2-ethylhexyl sebacate (DOS), a commercial groups involved in the molecules. product, was percolated through an alumina- In the present study, the physical properties, silica gel column for purification. thermal stabilities, heat of adsorption, and lubri- 2.2 Infrared Spectroscopy city of pentaerythritol partial-esters were investi- The association of the ester molecules which is gated in comparison with their full-esters and due probably to hydrogen bonding was investiga- di-2-ethylhexyl sebacate. ted by infrared spectroscopy of ester-carbon tetra-

2 Experimental chloride solutions. The effect of temperature on association was also studied with a high tem- 2.1 Materials perature cell. Pentaerythritol caprylates were prepared by 2.3 Heat of Adsorption esterification procedures. Amberlyst-15 was used The heat of adsorption of ester molecules from as the catalyst and toluene as azeotropic solvent. their solutions onto solid surfaces were measured The reaction products were fractionated into with a flow-microcalorimeter, Microscal Ltd., mono-, di-, tri-, and tetra-esters with a falling London. The apparatus and method used for film molecular still. However, it was difficult determinations of heat of adsorption were describ- to obtain high purity tetra-ester by molecular ed previously5). distillation, and to accomplish it the fraction of 2.4 Thermal Stability * Received December 26, 1975. The thermal stability of esters was pursued with ** Tokyo Institute of Technology (O-okayama, Meguro-ku, Tokyo 152) the apparatus as shown in Fig. 1. The test *** Nippon Oil Co., Ltd. fluid, under an atmosphere of inert nitrogen, was

Volume 18, No. 1, May 1976 2 Sakurai, Hironaka, Furuta and Watanabe:

Table 1 Properties of the Pentaerythritol Caprylates

Table 2 Properties of Esters for Thermal Stability Tests

diameter SUJ-2). The ball and ring were thoroughly cleaned with benzene reflux in a soxlet apparatus and later dried in vacuum for

Fig. 1 Apparatus for Thermal Stability Tests 2hrs. Experiments were carried out at 30℃ under the load conditions of 200, 1,000 and 2,000g at confined by liquid seal in the glass apparatus and sliding speeds of 15 and 60cm/sec. The wear tested at given conditions of heating time and scar diameters of the balls were measured after temperature. Volatile decomposition products a sliding distance of 450m. The seizure load were leaked from this test system kept at a con- was measured at 30℃, using 10 second-runs stant pressure through the liquid seal. at a sliding speed of 60cm/sec. at constant in- Details of esters used for thermal stability tests cremental loadings of 0.5kg. are given in Table 1 and 2. The tests were carried out under the following conditions: test 3 Results and Discussion times=6 and 8hrs, test temperatures=320℃ 3.1 Properties of Pentaerythritol Caprylates and 280℃, respectively. Iron (JIS H 3141) and The physical properties of pentaerythritol capry- copper (JIS G 3141) specimens were as catalysts. lates prepared are shown in Table 1. The The changes in physical properties of esters after saponification values of these esters corresponded test were examined under Japanese Industrial to those calculated except for monoester. Visco- Standards. The changes in the compositions of sity, density, and refractive index decreased with esters tested were determined by the use of Model increase in the degree of esterification, whereas JGC-20K temperature-programmed gas chro- viscosity index increased. matography, JEOL Co., Ltd. Pour point was independent of the degree of 2.5 Lubricity esterification, i. e. the molecular weight of the Tribological behaviors of lubricants were ester. The pour point of monoester was meas- examined by using a ball-on-ring friction machine ured as viscous flow point, and the pour point of which consisted of a ball (0.45mm diameter , the other esters as solidifying point. (These esters SUJ-2) sliding against a rotating ring (52mm contain some of the others, but they have rich

Bulletin of The Japan Petroleum Institute Lubricating Properties of Pentaerythritol Partial-Esters 3

Fig. 2 Viscosity and Structure of Neopentyl Polyol Ester components respectively.) Their pour points might be discussed as follows. Pour points of mono- and triesters having unsymmetrical molecular structures might be lower than those of di- and tetra-esters having symmetric structures, and tetra- ester have a higher pour point than the diester. Therefore, it may be that pour points of these esters are more strongly affected by the symmetric nature of the molecules rather than the molecular weights. Viscosity, in general, increases with increase in the molecular weight. However, viscosities of pentaerythritol caprylates decreased with increas- ing molecular weight. Viscosities of these esters may be strongly affected by the hydroxyl groups involvedjin the molecules, that is, the degree of association of their molecules. The relation between the molecular structure and viscosity of neopentyl polyol esters is shown in Fig. 2. The viscosity of neopentylglycol caprylate was 8.57cSt at 37.8℃, while trimethylolethane caprylate, introducing another ester group into neopentylglycol caprylate resulted in 18.43cSt due to the increase in the molecular weight. Fig. 3 IR Spectra of Pentaerythritol Caprylates in Furthermore, pentaerythritol tricaprylate with a Various Concentrations hydroxyl group instead of methyl group of trimethylolethane caprylate gave 42.19cSt. On centrations were prepared for IR spectra meas- the other hand, the order of viscosity of penta- urements. IR spectra of the hydroxyl groups of erythritol caprylates with the number of hydroxyl these esters are shown in Fig. 3. Absorption of groups is: mono->di->tri->tetraester. This fact hydroxyl groups of mono- and di-esters was at suggests that viscosity is independent of molecular comparatively low wave numbers, 3,423 and weight. The effect of the hydroxyl group on the 3,440cm-1, respectively, that is, polymeric ab- viscosity of these esters was discussed from the sorptions, but the absorption of tri-ester was at results of infrared spectroscopy and molecular a higher wave number, 3,509cm-1. Each ester weight measurements. diluted with carbon tetrachloride gave a sharp 3.2 Infrared Spectroscopy absorption of free hydroxyl groups at 3,630cm-1. Carbon tetrachloride solutions of pentaery- Absorptions of hydroxyl groups of mono- and thritol mono-, di-, and tri-esters in various con- di-esters shifted gradually to a higher wave number,

Volume 18, No. 1, May 1976 4 Sakurai, Hironaka, Furuta and Watanabe:

Fig. 5 IR Spectra of Carbonyl Group of Pentaery- thritol Caprylates

Table 3 Molecular Weights of Pentaerythritol Caprylates by Cryoscopic Method

Fig. 4 IR Spectra of Pentaerythritol Caprylates at tetra-ester resulted in a sharp absorption at 1,740 Various Temperatures cm-1, the others showed another absorption at 1,720cm-1, which became stronger with increas- 3,510cm-1, with decreasing concentration of the ing number of hydroxyl groups. It is assumed solution, while the absorption of tri-ester 3,509cm-1 that absorption at 1,720cm-1 may be that of hardly changed. Therefore, it is considered that the polymeric hydrogen bond. It may be con- 3,510cm-1 by dilution is the absorption of the cluded that the unreacted hydroxyl groups of intramolecular hydrogen bond, and that at these partial-esters greatly affect their viscosity 3,509cm-1 of neat tri-ester is the doublet of the characteristics due to intermolecular association. absorptions of dimeric association and intra- 3.3 Molecular Weight molecular hydrogen bond. Thus mono- and Molecular weights of pentaerythritol carylates di-esters associated strongly with the hydrogen measured by cryoscopic method using benzene as bond of hydroxyl groups to give high viscosities, the solvent are shown in Table 3. The molecu- while the association of tri-ester was weak and lar weight of tetra-ester was obtained by extra- its viscosity was very low. poration, but the molecular weights of the other The effect of temperature on the association esters were determined as their values at 8.0% of the ester molecules is shown in Fig. 4. The by weight. Experimental values for mono- and association of tri-ester was not substantially in- di-esters were 2.4 and 1.6 times the calculated fluenced by temperature, but that of di-ester was ones, respectively. It is suggested, therefore, greatly affected. This suggests that, at low that mono- and di-esters form bigger molecules by temperatures the viscosity of di-ester is very high their strong association. This fact confirms the with strong association and it decreases at higher results of the IR spectra study. The value temperatures because of breaking-up of the obtained for tri-ester corresponded to the calculated association, resulting in poor viscosity index. one because of its weak association. On the other hand, the tri-ester yielded a better 3.4 Heat of Adsorption viscosity index with less effect of temperature. Heat of adsorption of pentaerythritol caprylates Absorptions of pc=0 for the four pentaery- in n-heptane onto iron oxide are shown in Table thritol caprylates are shown in Fig. 5. While the 4. As the number of hydroxyl groups involved

Bulletin of The Japan Petroleum Institute Lubricating Properties of Pentaerythritol Partial-Esters 5

in a molecule increased, the heat of adsorption characteristics of pentaerythritol partial-esters increased. The adsorption activity of tetra- were investigated for comparison with those of ester on iron oxide surface was lower because full-esters and di-2-ethylhexyl sebacate (DOS). only ester bondings were involved, while the other Experimental results at 280 and 320℃ in the esters gave larger heat of adsorption because of absence of metallic catalysts are shown in Table the effects of their active hydroxyl groups. From 5. In the case of DOS, at 320℃, its change in this, it may be considered that the hydroxyl viscosity and increase in acid value were remark- groups involved in the partial-esters influence able because of the thermal decomposition shown strongly their adsorptive activity and boundary by Kraft, and a large quantity of sediments lubiricity. yielded, but pentaerythritol n-alkyltri- and n- 3.5 Thermal Stability alkyltetra-esters were more stable than DOS. At Neopentyl polyol esters as synthetic lubricating 280℃, DOS was as stable as n-tri-ester. However, oils have been favored compared with others n-alkyldi-ester was not so stable as n-alkyltri-ester because of their superior thermal stability. There and yielded more pentaerythritol as sediment. are many studies on the relation between thermal It is considered that the remarkable decrease in stability and molecular structure of full-esters, viscosity of n-alkyldi-ester was due to its con- but only few studies on partial-esters. Thermal version to tri- and/or tetra-esters as will be de- scribed later. Pentaerythritol partial-esters were

Table 4 Heat of Adsorption of Pentaerythritol unstable as compared with pentaerythritol full- Caprylates from n-Heptane onto α- esters, trimethylolethane caprylate and neopentyl- Fe2O3 at 0.03mol/l glycol caprylate. This may suggest that partial- esters condensed easily because of the intramolecular hydrogen bonding between the hydroxyl and carbonyl or hydroxyl groups. Table 6 shows the changes in physical pro-

Table 5 Thermal Stability Characteristics of Esters

Table 6 Therrnal Stability Characteristics of Esters in the Presence of Catalysts

Volume 18, No. 1, May 1976 6 Sakurai, Hironaka, Furuta and Watanabe:

Table 7 Analysis of Pentaerythritol n-C8 or i-C8 Esters by GLC

Fig. 6 Effect of Addition of Pentaerythritol Capry- lates on Iron Corrosivity of DOS

perties of various esters at 280℃ in the presence of metallic catalysts. The remarkable effects of Table 8 Analysis of Pentaerythritol Capryrates after the catalysts on thermal stability of esters were Thermal Stability Tests by GLC not existent, but the weight loss of iron was marked and its quantity with DOS was largest, 0.56mg/cm2. The order of weight loss of iron by pentaerythritol esters is: di-

Table 9 Effect of Water on Thermal Stability of Pentaerythritol tri-Caprylate

Table 10 Seizure Load with Various Esters

Fig. 7 Wear Scar Diameter vs. Load for Various Esters

carboxylic acid groups have been studied2),8),9). However, there are only few studies on lubricity of the effect of degree of esterification. Lubricity of pentaerythritol partial-esters was investigated and compared with that of its full-esters, DOS, and hvdrotreated oils. Figures 7 and 8 show the effects of load and hydroxyl group on wear at sliding speeds of 15 and 60cm/sec, respectively. Seizure loads for the esters are shown in Table 10. Synthetic oils such as pentaerythritol caprylates and DOS give better load-carrying properties in comparison Fig. 8 Wear Scar Diameter vs. Load for Various with hydrofinished oils. The order of lubricity of Esters these esters is: di->tri->tetra-ester>DOS. Thus, the excellent lubricity of partial-esters may be due to their higher heat of adsorption, that is, partial- stability, heat of adsorption, and lubricity. esters adsorb more strongly on metal surfaces to Partial-esters gave higher viscosities because of form an excellent lubricating film. the association of the molecules due to their hydroxyl groups. Partial-esters adsorbed intense- 4 Conclusion ly on metal oxide surfaces to suppress corrosion of Pentaerythritol partial-esters were investigated the metals at high temperatures and give better in comparison with typical synthetic lubricating lubricity. oils such as its tetra-esters and di-2-ethylhexyl Partial-esters, especially mono- or di-ester, in- sebacate. The hydroxyl groups involved in the troduced some problems as a lubricating base partial-esters were found to have profound oil because of the change in its composition ratio influence on their physical properties, thermal by thermal reaction. However, it may be ex-

Volume 18, No . 1, May 1976 8 Sakurai, Hironaka, Furuta and Watanabe: Lubricating Properties of Pentaerythritol Partial-Esters

3) Klaus, E. E., Tewksbury, E. J., Fietelson, S. S., pected that tri-ester or mixtures of tri- and tetra- ASLE Trans., 13, 11 (1970). esters will be good lubricating oils because the 4) Klaus, E. E., Fenske, M. R., Lubrication Eng., 14, change in viscosity is controlled and the relevant 266 (1958). thermal characteristics have been developed. 5) Hironaka, S., Yahagi, Y., Sakurai, T., Bull. Japan Petrol. Inst., 17, (2), 201 (1975). References 6) Muphy, C. M., Ravner, H., Timmons, C. O., J. Chem. Eng. Data, 6, 135 (1961). 1) Gunderson, H., Harf, A. W., "Synthetic Lubricants," 7) Barnum, E. R., Larson, R. G., Wachter, A., Cor- (1962) Reinhold Publishing Co., New York. rosion, 4, 423 (1948). 2) Barns, R. S., Fainman, M. A., Lubrication Eng., 8) Hall, J. M., .ASLE Trans., 14, 292 (1971). 13, 454 (1957). 9) Hall, J. M., Ravner, H., ibid., 16, 291 (1973).

Bulletin of The Japan Petroleum Institute