Leaf Coppin
`
`J.B. Yao, Q.L. Wang,
`S.Q. Chen, J.Z. Sun,
`and J.X. Dong Beijing
`Aviation Oil Research
`Institute, Beijing, China
`
`Abstract
`
`Borate Esters Used as Lubricant
`Add it ives
`
`Borate esters possess friction-reducing, antiwear, and anti-
`oxidant characteristics when blended in lubricating oils.
`However, borate esters are susceptible to hydrolysis. The forma-
`tion of a stable five-member ring structure in the ester
`molecules, involving coordination of nitrogen with boron, con-
`tributes substantially to the resistance to hydrolysis of borate
`esters. The susceptibility of borates to hydrolysis can be reduced
`by introducing N,N-dialkylaminoethyl groups with alkyl
`radicals containing more than three carbon atoms. X-ray photo-
`electron spectroscopy and X-ray diffraction reveal that the
`borate ester can be adsorbed on the rubbing surface, and some
`of the adsorbed borate film degrades and forms boron nitride.
`Four-ball wear tests indicate that the combination of oil-soluble
`copper, tin, and cadmium compounds with organoborates gives
`better antiwear properties than the components separately. An
`antiwear synergistic mechanism is postulated in which borates
`with electron-deficient boron p orbitals catalyse the tribo-reduc-
`tion of the metal compounds on the rubbing surfaces, producing
`elemental metals.
`
`Keywords
`
`borate esters, antiwear properties, lubricant additives, synergistic
`effects, hydrolytic stabilisation
`
`INTRODUCTION
`
`In recent years, much effort has been focused on the research
`and development of new types of friction-reducing and anti-
`wear additives for lubricating oils. One such group of additives
`is borate esters.’” It is commonly accepted that these boron-
`containing compounds not only have excellent antiwear and
`antifriction characteristics, but also have good oxidation stabil-
`ity and compatibility with seals. Furthermore, they are non-
`volatile, relatively nontoxic, and have a pleasant odour. How-
`ever, a serious drawback which has restricted the use of borate
`esters in lubricants is their susceptibility to hydrolysis, result-
`ing in the liberation of oil-insoluble and abrasive boric acid.
`
`Lubrication Science 14-4, August 2002. (1 4) 41 5 ISSN 0954-0075 $10.00 + $1 0.00 (1 547/0802)
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`41 6
`
`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`EXPERIMENTAL
`
`.
`
`~
`
`
`
`Kreuz et aL3 studied the extreme-pressure properties of
`tribenzyl borate and found that borate films are heterogeneous
`structures containing both inorganic and organic components.
`Dong and co-workers found that some borates can form BN,
`FeB, and Fe,B on rubbing surfaces.' Dong et al.' and Liu et ~
`1
`showed the existence of borate adsorption films within the fric-
`tional zones. Baldwin' examined the antiwear efficiency of a
`series of sulphur-containing borates and suggested that the
`sulphur, not the boron, plays a major role in the antiwear
`properties of such compounds.
`Oil-soluble organometallic compounds have also been used
`as antiwear lubricant additivesa6 In the present paper, the anti-
`wear synergism of borates and oil-soluble metal compounds is
`addressed, and the mechanism of the synergy is discussed.
`
`Wear tests were performed on a four-ball machine a t 1450 rpm
`at ambient temperature. GCrl5 steel balls (1.00% C, 1.50% Cr,
`0.25% Si, and 0.0% Mn;R, = 62-65) with a diameter of 12.7 mm
`were used. The test times were 10 and 30 min.
`The chemical state of boron on the wear surfaces was
`analysed using X-ray photoelectron spectroscopy (XPS). The
`composition of the wear surfaces was examined by an 18 kW
`rotating anode X-ray diffractometer (XRD); a small quantity of
`substance can be detected directly with XRD due to the high
`energy. The topography and elemental compositions of the
`rubbing surfaces were examined using scanning electron
`microscopy (SEM) and energy dispersive X-ray analysis (EDX).
`Before the XPS and XRD analyses, the wear sample was ultra-
`sonically rinsed with hexane and petroleum ether for 10 min.
`Paraffin oil was used as the base oil. Two borates,
`(C,,H,,O),BOCH,CH,N(~4H~),
`N,N-di-n-butylaminoethyl-didodecyl borate (DBN)
`
`and
`
`CH,SC,,H,,
`
`CHo \
`BOCH,CH,N(C4H,),
`CH,O ' /
`N,N-di-n-butylaminoethyl-3-( 1 '-mercaptododecyl)-l,2-propyl borate (SBN)
`
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`

`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`41 7
`
`and three oil-soluble metal salts, copper oleate (CuOLE), di-
`butyl
`tin
`dilaurate
`(SnDBDL),
`and
`cadmium
`diamyldithiocarbamate (CdDDC), were evaluated in this
`study. The
`two borate additives have been reported
`
`The susceptibility of borates to hydrolysis is due to the exist-
`ence of electron-deficient boron:
`
`--3
`
`HOH
`(RO),BOR + HOH -+ (RO),mR
`(RO),BOH + HOR
`H3B03+3ROH
`
`...
`
`-+
`
`RESULTS AND
`DISCUSSION:
`Hydrolytic stability
`improvement
`
`where R is an alkyl group. Some methods for improving the
`hydrolytic stability of borate esters are the following:
`
`1. A hindered phenol inhibits hydrolytic attack on the
`boron-oxygen bonds. l7
`The hydrolytic stability is improved by the addition of
`2.
`amine compounds which have non-bonding pairs of electrons.
`These amines coordinate with the electron-deficient boron
`atom, preventing hydrolysis."
`The hydrolytic degradation of borates is inhibited by the
`3.
`incorporation of certain diols in the lubricant formulation.
`They react with the boric acid produced by hydrolysis of borate
`esters to form stable five-member ring structures.lg
`However, these methods do not achieve hydrolytic stability in
`use. In addition, a series of other problems, such as poor oil
`solubility and high viscosity, are also encountered.
`A new solution to the problem of the hydrolytic stability
`of borate esters is described here. It can be improved by intro-
`ducing an aminoethyl group into the molecules of such
`compounds to form a stable five-member ring structure
`involving coordination of the nitrogen atom with boron:
`
`Lubrication Science 14-4, August 2002. (14) 417 ISSN 0954-0075 $10.00 + $10.00
`
`~~~~~~~~~~~
`
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`41 8
`
`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`Figure 1 Effect of borate
`(DBN) concentration on
`antiwear properties
`
`0.90 r
`
`0.85
`
`E
`
`0.80
`E $ 0.75
`3
`
`0.70
`
`0.65
`
`2
`
`1
`8
`6
`4
`Concentration, wt.%
`
`0
`
`
`
`where R,, R, are the same or different alkyl groups. The larger
`the attached alkyl groups on the nitrogen, the stronger their
`electron-donating effect; therefore, the electron density of the
`nitrogen atom increases. This may enhance the coordination
`with boron and decrease the possibility of hydrolytic attack on
`the boron-oxygen bonds, potentially also improving the hydro-
`lytic stability of the borate esters. When the alkyl radicals on
`the nitrogen, i.e., Rl', R,', contain more than three carbon at-
`oms, excellent hydrolytic stability of borate esters is possible.
`
`The wear tests were performed on the four-ball tester under a
`load of 392 N. Figure 1 shows the effect of borate (DBN)
`concentration on the antiwear properties. The wear-scar dia-
`meters (WSDs) were obtained after 10 min duration tests. It
`was found that the antiwear capacity increases with increas-
`ing borate concentration, although this tendency is less
`significant for concentrations greater than 5.0 wt.%.
`To investigate the antiwear mechanism of the borate
`ester, the surface film on the four-ball wear scar was analysed
`using XPS. The result is shown in Figure 2. The B 1s binding
`energies on the rubbing surfaces are 192 and 190.3 eV, sug-
`gesting the presence of borate ester and boron nitride.',4,20 The
`
`Antiwear function
`and mechanism of
`berates containing
`nitrogen
`
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`

`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`41 9
`
`Figure 2 Binding energy I
`of B 1s on rubbing
`surface
`
`202
`
`198
`
`194
`
`190
`
`Binding energy, eV
`
`RgureTStandard B 1 s
`XPS of boron nitride
`
`190.3
`
`200
`
`I
`
`190
`Binding energy, eV
`
`180
`
`Figure 4 Powder
`diffraction pattern
`search/match report
`
`40
`
`I
`25-1 033 BN
`
`50
`I
`
`60
`
`70
`
`80
`
`I
`
`28
`
`90
`I
`
`35-1 365 BN
`
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`

`420
`
`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`binding energy of 190.3 eV is consistent with the standard B 1s
`electron binding energy of boron nitride shown in Figure 3.20
`Thus, the form of boron on the rubbing surfaces is borate ester
`and boron nitride.
`To determine the surface composition more precisely, the
`rubbed surface was examined using XRD. The powder diffrac-
`tion pattern searcwmatch report is shown in Figure 4. The
`result shows that the peaks numbered 1,2,3, and 4 match very
`well the standard powder diffraction pattern of boron nitride
`(Powder Diffraction File Nos 25-1033 and 35-13651, strongly
`suggesting the presence of boron nitride on the rubbing
`surface.
`The formation of BN during the rubbing process may in-
`volve the electron-deficient p orbital of boron and the non-
`bonding electron pair of nitrogen of the borate. Under the im-
`pact of exoelectrons produced by rubbing, the borate molecules
`are broken down. The fragments of nitrogen containing non-
`bonding electron pairs can then coordinate with those of boron
`containing a n electron-deficient p orbital and recombine to
`form boron nitride.
`
`Borate esters (DBN or SBN) and oil-soluble metal compounds
`(CuOLE, SnDBDL, or CdDDC) were added to white oil and the
`antiwear properties of the resultant oils were evaluated by the
`four-ball friction tester under a 392 N load. The WSDs of the
`tested balls are reported in Tables 1-3. It can be seen that the
`two borate esters and the three metal salts exhibit good anti-
`wear synergism.
`The chemical states of the surface films on the four-ball
`wear scars were investigated using XPS, SEM, and EDX. The
`XPS results indicated that the copper, tin, and cadmium com-
`pounds were tribo-reduced to elemental copper, tin, and
`
`~ a d m i u m . l l . ~ ~ ~ ' ~ The SEM and EDX results indicated that the
`atomic concentrations of the metals (Cu, Sn, or Cd) produced
`on the boundary layers with borates were greater than those
`
`without b o r a t e ~ . ' ~ ~ ' ~ ~ ~ ~ Based on this, an antiwear synergistic
`mechanism can be postulated in which borates with electron-
`deficient boron p orbitals catalyse the tribo-reduction of the
`metal compounds on the rubbing surfaces. Copper, tin, and
`cadmium are all soft metals, and they could deposit on and/or
`alloy with the surfaces to protect the substrates against wear.
`
`Antiwear synergism
`of borates and oil-
`soluble metal
`compounds
`
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`

`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`42 1
`
`Table 1 Wear-scar diameters with oils containing borates and cop@' oleate (392 N, 30 min)
`
`DBN, wt.%
`
`SSN, wt.%
`
`CuOLE, wt.%
`
`Diameter, mm
`
`0
`4.0
`4.0
`0
`0
`0
`2.0
`2.0
`2.0
`0
`'Calibrated diameter = average diameter - Hertz diameter.
`
`0
`0
`4.0
`0
`2.0
`
`0.726
`0.708
`0.696
`0.508
`0.481
`
`Calibrated
`diameter*
`0.427
`0.409
`0.397
`0.201
`0.182
`
`Table 2 Wear-scar diameters with oils containing borates and dibutyl tin dilaurate (392 N,
`30 min)
`
`DBN, wt.%
`
`SBN, wt.%
`
`SnDBDL, wt.%
`
`Diameter, mm
`
`4.0
`0
`
`0
`4.0
`
`0
`0
`2.0
`2.0
`0
`2.0
`*Calibrated diameter = average diameter - Hertz diameter.
`
`0
`0
`4.0
`0
`2.0
`
`0.726
`0.708
`0.493
`0.354
`0.380
`
`Calibrated
`diameter*
`0.427
`0.409
`0.1 94
`0.055
`0.081
`
`Table 3 Wear-scar diameters with oils containing borates and cadmium diamyl-
`dithiocarbamate (392 N, 10 min)
`
`DSN, wt.%
`
`SBN, wt. %
`
`CdDDC, wt.%
`
`Diameter, mm
`
`2.0
`0
`2.0
`0
`0
`0
`1 .o
`1 .o
`1 .o
`0
`"Calibrated diameter = average diameter - Hertz diameter.
`
`0
`0
`2.0
`0
`1 .o
`
`0.790
`0.774
`0.631
`0.392
`0.395
`
`Calibrated
`diameter*
`0.491
`0.475
`0.332
`0.093
`0.096
`
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`

`422
`
`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`CONCLUSIONS
`
`References
`
`1. Owing to the electron-donating effect of alkyl radicals
`on nitrogen and the existence of a stable five-member ring
`structure, the presence of Nfl-dialkylaminoethyl containing
`alkyl groups with more than three carbon atoms can improve
`the hydrolytic stability of borate esters. It is believed that
`the non-bonding pair of electrons of the nitrogen atom can be
`a donor to the electron-deficient p orbital of the boron to
`inhibit hydrolytic attack on the boron-oxygen bonds of
`borate esters.
`2.
`The borate esters containing nitrogen provide good anti-
`wear properties when used in a lubricant formulation. The
`form of boron existing on the rubbing surfaces is borate ester
`and boron nitride.
`3.
`The combination of oil-soluble copper, tin, or cadmium
`compounds with organoborates, either sulphurised or non-sul-
`phurised, gives better antiwear performance
`than the
`individual components. A synergistic mechanism is postulated
`in which boron atoms with vacant p orbitals catalyse tribo-
`reduction of the metal salts on the rubbing surfaces, producing
`elemental copper, tin, or cadmium.
`
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`
`Lubrication Science 14-4, August 2002. (1 4) 422 ISSN 0954-0075 $1 0.00 + $1 0.00
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`Page 8
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`Anacor Exhibit 2039
`Flatwing Pharmaceuticals, Inc. v. Anacor Pharmaceuticals, Inc
`IPR2018-00171
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`

`

`J.B. Yao, Q.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong: Borate esters used as lubricant
`additives
`
`423
`
`12. Yao, J.B., and Dong, J.X., ‘A tribocatalysis reaction in boundary lubrication -
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`(1995) 231-3.
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`dilaurate’, Lubr. Eng., 52, 7 (1996) 553-6.
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`between borates and Sn(lV) compounds in boundary lubrications’, Rib. fnf., 29, 5
`(1 996) 429-32.
`and
`borates
`synergism of
`‘Antiwear
`15. Yao,
`J.B.,
`diamyldithiocarbamate’, Lubrication Science, 10, 1 (1 995) 59-66.
`16. Yao, J.B., and Xu, Z.D., ‘A proposed mechanism for boundary layer formation
`from organic borates and Cd(ll) compound’, Rib. Lett., 3 (1997) 277-81.
`17. Braid, M., ‘Borate esters and lubricant compositions containing such esters’,
`US patent US 4547302, 1985.
`18. Small, V.R., and Liston, T.V., ‘Diethylamine complexes of borated alkyl
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`19. William, E.W., and Bryan, T.D., ‘Haze-free boronated antioxidant’, US patent
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`
`cadmium
`
`This paper was first presented at the 12th International Tribology Colloquium,
`Technische Akademie Esslingen, Germany.
`
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