Friction- and Wear-Reducing Properties of Multifunctional Small

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Friction and wear reducing properties of multifunctional small molecules Lelia Cosimbescu, Nick Demas, Joshua W Robinson, and Robert A. Erck ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13271 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Applied Materials & Interfaces

Friction and wear reducing properties of multifunctional small molecules L. Cosimbescu,†*N. G. Demas,‡ J. W. Robinson,† R. A. Erck‡ †

Pacific Northwest National Laboratory, Richland, WA

‡

Argonne National Laboratory, Lemont, IL

Abstract Nitrogen- and oxygen-containing compounds were designed empirically and subsequently synthesized, and their rheology, friction and wear performance as multifunctional base oils (MFBOs) was evaluated. Two of the compounds displayed good viscosity/rheology profile without the addition of polymeric viscosity modifiers, displaying high viscosity indexes (VI) above 200. Furthermore, all three MFBOs had lower coefficients of friction compared to well established and accepted benchmarks. The most significant advancement is their impressive wear improvement, by a factor of 5.5 to 70 compared to either of the benchmarks, which is attributed to the polar nature of the base oil which promotes boundary lubrication with metal surfaces. Moreover, the compounds were easily synthesized in one step from commercial starting materials. This work demonstrates that careful design could provide the features/performance/functions of base oil, rheology modifiers (or viscosity index improvers), anti-wear and friction reducing additives, all-in one molecule. Keywords: hexahydrotriazine, base oils, friction modifiers, Nitrogen containing, polyesters, antiwear; Introduction Fuel efficiency continues to be an important topic in today's world because of dwindling petroleum resources, costs associated with non-renewable energy such as rising gas prices and ‘carbon footprints’, and national security derived from dependence on oil-rich nations to meet energy needs. In engines, one approach that is being pursued by many lubricant researchers and manufacturers is to use lower viscosity oils. Further reduction of oil viscosity will present many technical hurdles, including the vulnerability of engine components to premature failure, and adequate long-term tribological performance of the lubricant, due to a shift in the lubrication 1 ACS Paragon Plus Environment

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regime from hydrodynamic and mixed to boundary, which will negatively impact engine components (i.e. wear). In an effort to address these issues, researchers have explored and evaluated unconventional base oils such as polyesters, poly alpha olefins, polyalkylene glycols. In fact, many exceed the performance requirements of conventional lubricants in terms of volatility, reduced ash-formation and deposits, viscosity, stability, and lifetime. Despite these advances, mineral oil-based lubricants have the strongest presence in the lubricant market due to an existing infrastructure, their low cost, and adequate performance.1 Other types of base oils have been investigated as potential feed stocks. Increased awareness regarding environmental protection is driving the development of sustainable green materials. Among environmentally friendly feed stocks, vegetable oils are cheap and abundant. Their use as starting materials offers low toxicity and inherent biodegradability.2 The innate economic value of the vegetable oil may be increased by the conversion of the readily available olefins to epoxides or other electron rich functional groups. One example is soybean oil, which is epoxidized and used as is, or processed further, for enhanced performance.3-5 Despite these advances, functionalized or modified vegetable oils do not find widespread use in the automotive sector. Synthetic lubricants have unique performance attributes which vary depending on the chemical composition, structure, and molecular weight of pure or formulated compounds. One of the main advantages is their stability. This is due in large part to the consistency and uniformity of their molecules as well as the lack of aromatic structures. Another benefit of synthetic lubricants is the increased viscosity index and low pour-point properties. Of the many available synthetics in the market, esters have been used successfully in the lubrication of moving surfaces under heavy loads or harsh conditions and are the preferred base stock in many applications.6,7 Esters are generally compatible with mineral oils, which gives them the advantage that they can be blended with mineral oils to boost performance for specific applications. Whether the synthetic base fluid is a polyalphaolefin, polyalkylene glycol, or polyester/polyol ester, they all still require additive technology and are additized accordingly to improve the many requirements of lubrication. Ideally, a formulated oil should consist of a minimum number of additives necessary to perform its intended function.

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This work, to our knowledge first of its kind, explores the potential of designing multifunctional lubricants as base oils which themselves provide acceptable rheology, friction and wear performance without the use of additives. Novel, low-molecular-weight materials suitable for base stock applications were prepared in one step from available starting materials. The neat compounds were characterized for viscosity (rheology), friction, and wear against four commercial benchmarks. The exact structure of the base oils in the benchmark materials is unknown, at best, we have a general idea of the class of base oils they may fall under. Hence the benchmarks are only used as a performance gauge for our experimental compounds. Experimental Section General considerations, characterization and synthetic experimental details, NMR and MS spectra, optical and profilometry wear images are available in the SI. Results and Discussion Molecular Design Strategy Introduction of oxygen in the molecular structure is a known methodology to build base oils such as polyesters, polyol esters and polyalkylene glycols (PAGs), while the introduction of nitrogen to obtain amides or hindered amines is less common. MFBO1 is a triester which was selected and prepared in-house to provide a baseline comparison to one of the benchmarks thought to have similar molecular structure. MFBO2 is an amphiphilic compound which has both polar functionality (hexahydrotriazine core) and lipophilic tails in which the latter afforded compatibility with conventional oils. MFBO3 is the hexahydrotriazine of Jeffamine XTJ435, which is a relatively hydrophobic experimental primary amine from Huntsman. Hexahydrotriazines have not been explored for base oil applications. Their design is based on the fundamental principle of friction modifiers structures, comprising of a polar head and a lipophilic tail. In this case, the polar head is a hexahydrotriazine, while the three tails are identical long alkyl chains, as illustrated in Figure 1. These compounds have been utilized as H2S scavengers8 or as fuel additives9, and in the biocide industry as formaldehyde releasers. 10 The synthetic pathways and structures of the three analogs are illustrated in Figure 1.



a.

b.

c.

Figure 1. Synthetic schemes and structures of base oil candidates under study. a. MFBO1 from trimethylolpropane: oleoyl chloride in THF and trimethylamine, RT. b. MFBO2 from dodecylamine and formaldehyde, in refluxing tolene; c. MFBO3 from XTJ-435 under similar conditions as b., illustrates an approximate structure as the lipophilic alcohol group is polydispersed and is a mixture of C12- C14. Material Synthesis The commercial viability of any new synthetic oil against petroleum-based oils depends greatly on the commercial availability of inexpensive starting materials. As such, the esterification of trimethylolpropane with a number of unique acid chlorides varying in both alkyl chain length (C12 and C16) and alkyl chain saturation (oleic acid chloride) was explored. The saturated long chain compounds were waxy at room temperature, therefore were not ideal candidates for this study and were not investigated further. Esterification of trimethylolpropane is a common strategy among synthetic polyesters producers, such as Nyco, which have a full line of saturated, unsaturated polyesters in their portfolio; however these commercial compounds are proprietary 4 ACS Paragon Plus Environment

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and therefore their chemical composition and structures are not public knowledge. Alternatively, technical information on trimethylolpropaneoleate (TMPO) is available, though finding all the rheological and tribological data measured with a configuration similar to our ‘in-house’ set-up was not possible. Therefore, this compound was prepared from trimethylolpropane with oleoyl chloride and its performance was assessed using in-house protocols. The hexahydrotriazines were prepared from equimolar amounts of a primary amine, dodecylamine or Jeffamine respectively, and formaldehyde, in refluxing toluene. The reaction was driven to completion by the removal of water via Dean-Stark distillation. Several hexahydrotriazines, particularly those derived from octadecyl amine, proved to be waxes at room temperature and were not suitable as base oil candidates The study compounds were easily prepared in one step from commercial starting materials (or intermediates), and purification consisted of only one or two aqueous washes, a step which is compatible with a scalable process. All three compounds were prepared in small scale (3-5g) to probe the efficiency of the approach and then scaled up to 40-50g to produce enough material to enable rheology, pour point, and tribological evaluation. No efforts were made to ultra-purify the materials. Detailed procedures are included in the SI. Structure-Properties Considerations Many researchers demonstrated that organic friction modifier molecules adsorbed on rubbing metal surfaces form monolayers which mitigate direct metal-to-metal contact, thereby reducing friction.11 There is an extensive list of esters reported to be beneficial as friction modifiers.12 The review by Li et al. presents extensive examples of amines, amides, imides, imidazoles, ionic liquids, used as friction modifier additives. There are no reports of triazine or hexahydrotriazine additives. Typically, friction modifiers are not tested as neat materials but rather they are components in an oil or oil-like matrix (hexadecane, dodecane), to reduce friction of the overall composition. In brief, these components only perform one function, friction reduction. In contrast, our candidates possess molecular moieties which can contribute to lubricity, viscosity, surface interaction and thus friction and wear reducing properties. MFBO1 and 2 were selected for study because they have high VIs, while MFBO3 was selected due to its structural similarity to MFBO2, although it only has a moderate VI. These compounds will likely adopt a 5 ACS Paragon Plus Environment


conformation with either the polar ester group or hexahydrotriazine ring pointed towards the surface (red spheres) and the oleophilic groups somewhat perpendicular to the surface, as illustrated in Figure 2.

Figure 2. Potential conformation and surface interaction of the base oil candidates with metal surfaces; A: only one ester group forms close contact with the surface (MFBO1); B: hexahydrotriazine forms close contact with the surface via one N atom (MFBO2 and 3); C: Hexahydrotriazine ring forms a close contact via all three N (MFBO2 and 3), with the ring sitting on the surface It is possible that all three esters in MFBO1will have an affinity for and physisorb on the surface, or only one. These various scenarios are depicted in Figure 2, as a, b, and c. Similarly, the pointof-contact with the surface for the hexahydrotriazine would be expected to occur via at least one nitrogen atom (Figure 2b). Due to the geometry of the ring (cyclohexane), it is reasonable to assume that the molecule will occasionally interact with the surface via a chair conformation with all three nitrogen atoms close to the surface (Figure 2c). MFBO2 is only slightly crowded around the nitrogen, having a methylene attached directly to it, however MFBO3 is the more sterically hindered of the two, having the isopropyl group attached to the nitrogen. Based on the steric hindrance of the nitrogens in the hexahydrotriazine core alone, MFBO2 is likely to have a lower coefficient of friction and potentially produce less wear. The extent of this interaction was qualitatively determined indirectly from the amount of friction and wear protection provided by each analog and those results are covered in the Friction and Wear Performance section. Characterization


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The three compounds were characterized by NMR (both 1H and 13C NMRs are available in the Supporting Information - SI) and their purity qualitatively assessed by 1H NMR. MFBO1 and 2 have a clean spectrum with a minimum purity of 95%, due to the uniformity of the starting materials, each being synthesized from a single clean compound. However MFBO3 was prepared from polydispersed intermediate, which in turn was prepared from a mixture of alcohol starting materials which varied in alkyl chain length from 12 to 14. The intrinsic polydispersed nature was observed in the NMR spectra as a large number of oligomers. The molecular weights of MFBO1 and 2 were determined by electrospray ionization and the fragmentation pattern as well as the parent ion is consistent with a single main component. MFBO3 had a mass-to-charge (m/z) pattern consistent with polydispersion of a low molecular weight polymer which made it impractical to confirm molecular weight. The HPLC chromatograms can be found in the SI. Viscosity/Rheology The goal of this work was two-fold: to achieve a multifunctional base oil that provided an acceptable VI, a reasonable coefficient of friction, and reduced wear, without the aid of additives; and to generate viable candidates with competitive performance compared to wellaccepted benchmarks spanning a variety of structures and formulations. Viscosity, friction, and wear were measured and compared with those four industrial benchmarks. The authors were not intending to optimize performance and thus did not follow industry protocols towards evaluating lubricants. The benchmarks are only used as performance metrics and not to make structural comparisons with our compounds; these structure-property relationships are impossible to define in benchmarks, as the components and formulations are proprietary, hence unknown. For the MFBOs under study, it was important that at least two if not three of the performance metrics (VI, COF, wear) were better than those of the benchmarks. The results of viscosity measurements at 40 and 100°C in centistokes as well as the derived VIs from those values, are reported in Table 1. The dynamic viscosities (centipoise; cP) of the neat and blended oils were measured by a spindle Brookfield viscometer at controlled temperatures of 40 and 100 °C. These values were converted into kinematic viscosity (KV: centistokes, cSt) by dividing them by the density of the oil solution, and are also reported in Table 1. The benchmarks are all commercial products and the data reported below is openly available from specification sheets or safety data 7 ACS Paragon Plus Environment


sheets (SDS). The benchmarks span a variety of oils. Benchmark 1 is a conventional, petroleumbased fully formulated oil (5W30 GF5), which contains viscosity modifiers, friction modifiers, anti-wear additives, detergents, etc. Benchmark 2 is an additive-free polyalphaolefin base oil (PAO4), expected to have a high COF and increased wear, based on the general performance of PAOs. Benchmark 3 is a saturated trimethylolpropane ester of linear acid (Nycobase 7300), bearing a close structural similarity to MFBO, and it is the basis for which it was chosen as a comparative example. Benchmark 4 is a top-tier synthetic 2-stroke chainsaw engine oil (Nycolube 248) which contains a special complex ester and a high performance package of additives improving its lubricity and oxidation resistance. The latter is also a fully formulated oil. Table 1: Base oil properties: rheology data, density, and pour point Sample ID

KV40 (cSt)

KV100 (cSt)

Density VI (g/mL)

Pour point (°C)

MFBO1

43.2

9.56

0.909

215

-48

MFBO2

25.7

6.35

0.852

214

-12

MFBO3

46.31

7.01

0.903

133

*

Benchmark 1 (5W30 GF5)

63.7

10.8

0.862

161

-42

Benchmark 2 (PAO4)

19

4.1

0.820

126

-66

Benchmark 3 (Nycobase 7300)

14

3.4

0.959

120

-66

Benchmark 4 (Nycolube 248)

39

7.8

0.895

175

-42

*not measured; KV40: kinematic viscosity at 40 °C; KV100: kinematic viscosity at100 °C; VI: viscosity index In analyzing the viscosity data it was evident that two of the synthesized base oils displayed an outstanding viscosity index without the aid of additives, much higher than any of the benchmarks. Even the fully formulated oils which have added polymers to meet a viscosity grade and improve the overall VI were well below of those which we prepared (Benchmark 1 with a VI of 175 and Benchmark 4 with a VI of 160 versus a VI of roughly 215 for both MFBO1 and 8 ACS Paragon Plus Environment

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MFBO2). Surprisingly, MFBO2 and 3 demonstrated very different viscosity index values although they had very similar chemical compositions, structures, and molecular weights. One of the factors affecting viscosity is the extent to which the molecules are linked or bound: the greater the inner interaction, the greater the resistance to deformation, i.e. flow. MFBO2 and MFBO3 differ in their polydispersity, which should not affect the resistance to thermal deformation, in other words their VI. However, if one thinks of a liquid as a loosely bound stack of molecules, MFBO2 has a less sterically hindered triazine and the molecules can allow tighter inner interactions. In the case of MFBO3 however, the triazine has an isopropyl group which might be just enough to disrupt such close ring to ring interactions, or stacking and therefore affect the resistance to deformation. Furthermore, MFBO3 contains oxygens which too can disrupt packing of long chains alkyl. Polarity and molecular weight within this range do not appear to play a significant role in viscosity behavior, which was observed by the two very different MFBO1 and 2 which had very close VI values. Two of the compounds appear to meet the high VI criteria and could be viable self-standing base oils. Moreover, MFBO1 and benchmark 3 have the same core structure (trimethylol propane) but differ in the long alkyl chain, resulting in very different KV40, KV100 and VIs. It is evident that the structure of the ester has a great impact on the viscosity of the base oils. Friction and Wear Performance Organic friction modifiers are typically long chain surfactants with polar end groups, such as carboxylate, alcohol, amine, amide, imide, ionic liquid, boron or phosphorus-containing, which either physically adsorb onto or chemically react with the metal surfaces to form monolayers. They work by forming a protective film, which improves the tribological performance of the lubricant. Active elements such as sulfur, phosphorus, molybdenum play an important role in improving the tribological properties of friction modifiers.13-17 In recent years, there is an increased demand in the development of sulfur- and phosphorus-free environment-friendly friction modifiers which do not adversely affect their wear and friction performance. Thus, boron-based and nitrogen-containing compounds such as amines, amides, and imides, are thought to be the most promising friction modifiers .12,18 In light of these general guidelines, it seems sensible to build a base fluid which contains only nitrogen and oxygen as the polar surface anchors. 9 ACS Paragon Plus Environment


In order to demonstrate the validity of the approach, the friction and wear were evaluated. The setup used to measure friction was a ball on flat configuration, both hardened type 52100 steel Rc62, using a constant load of 15.6 N, at a frequency speed of 2Hz, in reciprocating motion at 220 mm stroke, for 1h. The tests were run at room temperature (RT, ~20-23 °C) and at 100 °C. The speed and load of the tests produced boundary lubrication, therefore, there are no concerns of different viscosities among the MFBOs and benchmarks. The coefficient of friction (COF) results are illustrated in Figure 3(a, b).

a.

b.

Figure 3. COF of MFBO1-3 at room temperature and 100 °C (a) and COF of Benchmarks 1-4 at 100 °C (b), both run under the same conditions for 1h. The friction plot for each sample is an average of two runs. The general trend for the synthesized oils was their COF at RT was low and steady, while at 100 °C, COF was higher and less uniform. In contrast, Benchmark 1 and 2 have a higher but steady COF at 100 °C while Benchmarks 3 and 4 have an erratic behavior. MFBO1 had the lowest (0.07-0.08) and most steady COF at both temperatures, for the duration of the test, making it an ideal base oil candidate. MFBO2 and 3 had a similar COF at room temperature, steady and relatively low at around 0.085. At 100 °C, the COF was higher (as high as 0.12) and less uniform, but not as high as the benchmarks. Notably, all experimental base oils demonstrated a superior friction behavior versus all benchmarks, including those which are fully formulated and presumably contain friction modifiers.


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After the tests were run, both balls and flats were analyzed to quantify the magnitude of the wear at RT and 100 °C. The optical micrographs of the balls and flats at room temperature and 100 °C are shown in the SI, with an illustrative example in Figure 4. From the optical micrographs it can be qualitatively observed that MFBO1 had the lowest wear and no tribofilms formation, while MFBO2 and MFBO3 produced tribofilms. MFBO1-100 °C

a.

MFBO2-100 °C

b.

MFBO3-100 °C

c.

Figure 4. Optical micrographs of the wear scars of the balls for MFBO1 (a), MFBO2 (b) and MFBO3 (c), at 100 °C. Similar tests were carried out at 100 °C only, on the four benchmarks for comparative purposes. The ball wear scars for all four benchmarks are shown in Figure 5. All micrographs were examined for evidence and magnitude of wear as well as tribofilm formation. Tribofilm formation has a critical role in the longevity of moving parts.19 The tribofilms formed by our compounds are ash-free, and do not contain any metals or particles.

a.

b.



c.

d.

Figure 5. Optical micrographs of the wear scars on the balls, for Benchmark 1 (a), Benchmark 2 (b), Benchmark 3 (c) and Benchmark 4 (d) at 100 °C. From the optical micrographs alone, the only material among the benchmarks that shows evidence of tribofilms formation is Benchmark 1, as indicated by the yellow-blue streaks. It is not possible to quantitatively assess the amount of wear on the ball from optical micrographs, because there can be loss of material, or material gain as a tribofilms, or both. For example, MFBO2, the ball displays a wear circular scar where material was removed, but also blue/brown marks due to material deposits as tribofilms (Figure 4b). The correct evaluation of the volume lost or gained on the ball was performed via profilometry, with the application of a gold reflective coating on the wear surface. A representative profilometry image of wear on a ball is shown as an example, for MFBO2, at room temperature and 100 °C, in Figure 6. All profilometry images, including those of the four benchmarks and three experimental compounds at 100 °C are available in the SI.

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b. Figure 6. Profilometry images of MFBO2 worn balls with reflective coating applied, after friction tests at room temperature (a) and 100 °C (b) The data resulting from this analysis for two tests of study compounds and benchmarks is summarized in Table 2. Table 2. Summary of ball wear results for the experimental candidates MFBO1, 2 and 3 Base Oil

Volume

MFBO1

MFBO1

MFBO2

MFBO2

MFBO3

MFBO3

RT

100 °C

RT

100 °C

RT

100 °C

3100

2700

+500 gain

2600

1060

+3220 gain

3300

1300

700

+1200 gain

500

+23400gain

(µm3) Test 1 Volume (µm3) Test 2 The standard deviation of the measurements is in the order of 1300 µm3 Table 3. Summary of ball wear results of the four benchmarks at 100 °C Base Oil

Benchmark 1

Benchmark 2

Benchmark 3

Benchmark 4

Volume

16,200

193,000

20,600

42,00

15,300

*

*

*

3

(µm ) Test 1 Volume (µm3) Test 2 *Not measured



Only Benchmark 1 was tested twice, and the results demonstrate consistency and repeatability. Benchmark 2 displays, as expected from a typical PAO, very high wear, so there was no reason to repeat. Benchmark 3 and 4 are a polyester base oil and polyester finished oil respectively, and they both produce medium wear. The main findings were that room temperature sliding did not produce any detectable material loss or grooving in MFBO1, however these effects are observed at 100 °C in the case of MFBO2 and MFBO3. The data in Tables 2 and 3 shows that the synthesized MFBOs have lower friction as base oils, compared to commercial standards. MFBO1 produces no deposits and little wear and low COF, both at RT and 100 °C. This compound, although has some structural similarities to Benchmark 3, has far superior friction and wear behavior, outperforming two fully formulated oils, Benchmark 1 and Benchmark 4. Benchmark 2 was expected to have poor wear performance, as most PAOs have. Overall, MFBO1 has wear, a factor of 5.5-70 lower than any of the benchmarks, depending which benchmark it is compared to. This is a serendipitous result, as this compound was prepared with the goal of having a direct comparison with a commercial product (Benchmark 3) and was expected to have similar performance. Moreover TMPO is a known compound, but it is sold under various trade names depending on the supplier and its exact friction and wear behavior is not publicly disclosed. The synthetic pathway is likely different, and that too can contribute to performance differences. MFBO2, one of the hexahydrotriazine candidates has medium friction, produces negligible wear at room temperature, and little wear or tribofilms at 100 °C. The 100 °C test data was variable, but still much lower wear compared to benchmarks is observed, similar to MFBO1, by a factor of 5.5-70 lower wear. MFBO3, the polydispersed hexahydrotriazine produces very little wear at room temperature and significant deposits at 100 °C, under the form of tribofilms. This compound has the highest wear of all target compounds, but no higher than the benchmarks. Overall, all three prepared MFBOs have substantially lower friction and wear than the common benchmarks which span various types of base oils such as mineral-based, polyster and polyalphaolefin, fully additized or neat. Conclusion


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Several compounds with dual chemical functionality were synthesized and investigated as potential multifunctional base oils, and their viscosity/rheology, friction, and wear properties were evaluated. One of the compounds is an unsaturated polyester (tri-ester) while the other two compounds are hexahydrotriazenes that have not been previously exploited as lubricant components. Viscosity, friction, and wear results of these compounds were compared to four commercial benchmarks to gauge their performance against various commercial products as well as understand their potential value in the market place. Not only did two of the analogs have an exceptional viscosity index (VI of 214 for MFBO1 and a VI of 215 for MFBO2) higher than any of the commercial base oils or fully formulated oils, , but they also demonstrated lower friction as well as a remarkable wear suppressant type behavior. Surprisingly, MFBO1 outperformed all the others across the board: displayed lowest friction both at room temperature and 100 °C (COF between 0.07 and 0.075); showed very low wear at both low and high temperatures. .One of the nitrogen bearing compounds also demonstrated a very high VI, lower COF and reduced wear than all the benchmarks. Given that these are main attributes required from a fully-formulated oil, these two compounds are good candidates as multifunctional base oils and would require minimal amount of additives if any. Acknowledgement: This project was funded by the Office of Vehicle Technology (VTO) of the U.S. Department of Energy (US DOE), (under contract No. VT0604000-31909 and VTO604000-27029). A portion of this research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. PNNL is operated by Battelle for the U.S. DOE (under Contract DE_AC06–76RLO 1830). The authors cordially acknowledge contributions from Dr. Anil Shukla (PNNL) for performing electrospray ionization mass spectrometry, and Dr. Samantha Burgess for providing 1H and 13CNMR measurements. We thank Nyco for generously donating base oils samples for screening purposes and Huntsman for Jeffamine XTJ435 which was sent as a free sample to be used for research purposes only. AUTHOR CONTRIBUTIONS



L.C. synthesized the materials and J.R. measured viscosity and characterized the materials. N.D. and R.E. conducted friction, wear, and profilometry measurements and analyzed results. L.C. proposed the original concept and analyzed results. All authors reviewed and edited this manuscript. Supporting Information Includes detailed syntheses of all materials and characterization considerations (general experimental considerations, rheology, and tribology conditions), 1H and 13C NMRs and mass spectra of each compounds; optical images of balls and flats from wear experiments of all compounds tested, candidates and benchmarks. This material is free of charge via the internet at http://pubs.acs.org. COMPETING FINANCIAL INTEREST STATEMENT The authors declare no competing financial interest. REFERENCES 1.

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Wen, H.; Shi, Y G.; Fan, G Z.; Xu, P F.; Jiang, Y X.; Su, B.; Gong, H.F. The progress of vegetable oil-based lubricants. Material Science and Engineering: Proceedings of the 3rd Annual 2015 International Conference on Material Science and Engineering (ICMSE2015, Guangzhou, Guangdong, China, 15-17 May 2015). CRC Press, 2016.

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Panchal, T M.; Patel, A,; Chauhan, D. D.; Merlin, T.; Patel, J V. A methodological review on bio-lubricants from vegetable oil based resources. Renew. Sust. Energ. Rev. 2017, 70, 6570.

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McNutt, J. Development of biolubricants from vegetable oils via chemical modification. J. Ind. Eng. Chem. 2016, 36, 1-2.


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TOC – Representative example of a hexahydrotriazine and its wear profilometry image


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Friction- and Wear-Reducing Properties of Multifunctional Small

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