Novel POSS based nanohybrids for improving tribological properties of liquid paraffin

Abstract

Polyhedral oligomeric silsesquioxane (POSS) based nanohybrids were prepared via “thiol–ene click” reaction between vinylPOSS and alkyl mercaptan. The structure and composition of these hybrids were characterized by ¹H nuclear magnetic resonance (¹H NMR), thermo-gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The effects of carbon chains of mercaptan and concentration variations of POSS nanohybrids on the tribological performance improvements of liquid paraffin (LP) were investigated. According to our experimental results, the optimal concentration of POSS nanohybrids in LP was 0.40 wt%, corresponding to the lowest friction coefficients of 0.099 (for OS-POSS additive) and 0.101 (for OD-POSS additive) respectively. POSS nanohybrids can achieve homogeneous dispersion in LP, while a hard, stable Si–O cage in their molecular structure provided effective wear resistance. These nanohybrids were expected to be useful for more lubricating oil systems, which possess great scientific values and promising application prospects compared with pure solid or liquid lubricating additives.

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1. Introduction

Friction is a general phenomenon in nature and industry, while lubrication is a principal ways to improve energy efficiency and mechanical durability. On the surface of friction pairs, there are “ridges”, “valleys”, “asperities” and “depressions” at micrometer or nanometer scale. Accordingly, more effective controls or reductions of the friction in mechanical systems are important for a sustainable consideration. Lubricating oils are widely used in most kinds of machines, and how to enhance the lubricating properties has been an important challenge in the field of industrial friction, in which many researchers have done a lot of research work in the past decades.^1–6

With the rapid developments of nanoscience and nanotechnology in recent years, nanomaterials gradually become to be a new and important choice for lubricant additives and they are expected to play more important roles in the area of friction and wear.^7–11 For example, Kinoshita's group showed that well dispersed graphene oxide (GO) in pure water provided rather a low friction coefficient of 0.05 with no obvious surface wear.¹² Zhang et al. modified ZrO₂ nanoparticles by using KH-570, and the results indicated that the modified nanoparticles could enhance the anti-wear and friction-reduction properties effectively.¹³ Zin et al. demonstrated that single-walled carbon nano-horns used as lubricating additives and reduced friction coefficient of the base oil.¹⁴ On the other hand, lots of nanoparticles were easy to agglomerate in organic system,^15–17 which restricted their further application and better effectiveness as lubricant additives. Novel nanomaterials possess special physical and chemical properties and outstanding surface,^18–23 which provided more possibilities for novel effective design of lubricant additives with high performance.

As one kind of silicon-based nanomaterials, polyhedral oligomeric silsesquioxanes (POSS) were consisted of oxygen and silicon atoms which were linked to well-defined nanostructures.^24–26 Generally, POSS was regarded as the smallest silica,²⁷ possessed a chemical formula as (RSiO_1.5)_n with a diameters ranged from 0.5 nm to 1.5 nm,^28,29 where “R” stands for hydrogen or any alkyl, alkylene, aryl, arylene groups, or organo-functional derivatives of them. Among kinds of POSS structures, octameric silsesquioxanes [(RSiO_1.5)₈] are the most widely investigated and numerous derivatives with tailored functional groups generated from its cubic structure.^30,31 Hence, an inorganic core (Si₈O₁₂) in POSS cage can be functionalized by seven inert groups and a unique functional group, or eight functional groups, which are capable of polymerization or cross-linking. The presence of organic functionalities makes it soluble in varieties of organic solvents or integrated organic systems.^32,33 Numerous POSS based nanomaterials or nanocomposites have been designed and prepared, and expected properties had been realized.^34–36 More importantly, the existing research proved that POSS itself had a great potential in the tribological design of materials: Luca grafted POSS–NH₂ onto GO sheets, which exhibited a reduced friction coefficient for applications in lubricant coatings;³⁷ Bhushan found that the addition of small amounts of POSS could improve the friction and wear resistance of polymer and polymer composites.³⁸ In addition, Misra found that POSS nanocomposites exhibit reduced friction compared to neat polymer.³⁹ Undoubtedly, POSS based nanomaterials or nanocomposites have great potentials to be used as lubricant additives to achieve better tribological performance for their nanosize effects, steady Si–O structures, excellent organic compatibility and easily being functional designed. To the best of our knowledge, there is no report of POSS using as additives for lubricant oils till now. In this work, novel POSS nanohybrids were prepared via “thiol–ene click” reaction between vinylPOSS and alkyl mercaptan, and the products were used for decreasing friction properties of liquid paraffin (LP) mixtures.

2. Experimental

2.1 Materials

Octavinyl polyhedral oligomeric silsesquioxane (vinylPOSS) were provided by the Nanohybrid Plastics. Azobisisobutyronitrile (AIBN) was purchased from Shanghai National Medicine Group Chemical Reagent Co., Ltd. 1-Dodecanethiol and 1-octadecanethiol was acquired from Aladdin Industrial Corporation (Shanghai, China). All chemicals were used without further purification.

2.2 Techniques

XRD patterns were obtained by using a Rigaku K/max-γA X-ray diffractometer with a Cu Kα (λ = 1.5415 Å) at the scanning rate of 0.02° s⁻¹; ultraviolet-visible spectroscopy was recorded by using a UV-3600 spectrometer. FTIR spectra were obtained by using a MAGNA-IR 750 spectrometer. Thermal gravimetric analysis (TGA) was performed on a Netzsch STA-409c Thermal Analyzer under a 50 × 10³ mm³ min⁻¹ nitrogen or air flow with the heating rate of 10 °C min⁻¹; ¹H nuclear magnetic resonance (¹H NMR) spectra were recorded on a Bruker DRX 400 MHz spectrometer in CDCl₃ at room temperature. Scanning electron microscopy (SEM) and Energy Dispersive Spectra (EDS) of worn surface were carried out on an FEI Helios Nanolab 600i DualBeam system and an FEI Inspect F50 instrument respectively.

The tribological properties of the liquid paraffin mixtures were carried out on a MW-W1A vertical universal friction and wear tester (Jinan, China) with a pin-on-disk sliding pair. It worked at a rotating speed of 200 rpm under a constant load of 250 N for the testing duration of 60 min; friction pins used in this study were made of 45 steel (diameter: 4.8 mm; hardness: 44–46 HRC; surface roughness: 0.0125 μm). Al₂O₃ disk (diameter: 42 mm; thickness: 5 mm; surface roughness: 0.0125 μm) was made by Suzhou Crystal Element Company in China. The friction and wear tests were performed at least three times under the same condition so as to minimize data scattering, the final friction data was obtained by calculating average value of FC of different times.

2.3 Synthesis of POSS nanohybrids

The synthesis is illustrated in Scheme 1, detailed preparations are as followings:

Scheme 1 Synthesis of vinylPOSS derivatives via a thiol–ene click reaction.

Preparation of OS-POSS. 1.16 g (1.83 mmol) vinylPOSS, 2.97 g (14.67 mmol) 1-dodecanethiol and 60 mL toluene were mixed and transferred into a three-necked round-bottom flask. The reaction mixture was stirred and bubbled in nitrogen for 30 minutes. After that, 30.0 mg AIBN was dissolved in toluene and the solution was purged in nitrogen for another 15 minutes. Then the reaction system was heated to 80 °C and then maintained for 5 hours. After the reaction was completed, unreacted 1-dodecylthiol was removed by vacuum distillation and the resulting nanohybrid was rinsed by acetone for several times.

Preparation of OD-POSS. 0.58 g (0.92 mmol) vinylPOSS and 60 mL toluene together with 2.11 g (7.34 mmol) 1-octadecanethiol were added into a three-necked round-bottom flask. The used amount of initiator AIBN was 21.10 mg, other experimental methods and procedures were similar to the preparation of OS-POSS.

3. Results and discussion

3.1 Characterizations of OS-POSS and OD-POSS

FT-IR spectra of POSS before and after modification are shown in Fig. 1. In the curve for vinylPOSS, a wide peak at 3400 cm⁻¹ belongs to O–H stretch attributed to hydroxyl group from water, while broad, multiple peaks between 3000 cm⁻¹ and 3300 cm⁻¹ correspond to the stretching vibration of vinylPOSS; absorbing peaks at 1004 cm⁻¹ and 1120 cm⁻¹ attribute to Si–O–Si structure in POSS cage. In the curves for the products after being modified by 1-dodecanethiol and 1-octadecanethiol (OS-POSS and OD-POSS), adsorbing peaks at 2850 cm⁻¹ and 2919 cm⁻¹ related to the stretching modes of –CH₃ and –CH₂– from alkyl mercaptan can be observed; at the same time, the peaks over 3000 cm⁻¹ related to the unsaturated alkene in vinylPOSS and a peak at 1670 cm⁻¹ related to C=C in vinyl disappeared, while a peak at 760 cm⁻¹ due to the bending vibration of Si–CH₂– can be found.

Fig. 1 FT-IR spectra of (a) vinylPOSS, (b) OS-POSS, (c) OD-POSS, (d) 1-dodecanethiol, (e) 1-octadecanethiol.

Fig. 2 shows ¹H NMR spectra of vinylPOSS and vinylPOSS nanohybrids. The assignments of these chemical shifts are listed as followings: vinylPOSS: ¹H NMR (CDCl₃): δ = 5.88–6.16 ppm (–CH=CH₂); OS-POSS: ¹H NMR (CDCl₃): δ = 0.84–0.92 ppm (–CH₃), 0.97–1.07 ppm (–SiCH₂–), 1.19–1.33 ppm (–CH₂(CH₂)₈CH₃), 1.33–1.44 ppm (–SCH₂CH₂CH₂–), 1.52–1.63 ppm (–SCH₂CH₂CH₂–), 2.47–2.53 ppm (–SiCH₂CH₂–), 2.54–2.62 ppm (–SiCH₂CH₂–); OD-POSS: ¹H NMR (CDCl₃): δ = 0.89–0.92 ppm (–CH₃), 1.01–1.06 ppm (–SiCH₂–), 1.20–1.34 ppm (–CH₂(CH₂)₁₄CH₃), 1.34–1.42 ppm (–SCH₂CH₂CH₂–), 1.54–1.62 ppm (–SCH₂CH₂CH₂–), 2.50–2.54 ppm (–SiCH₂CH₂–), 2.55–2.63 (–SiCH₂CH₂–).

Fig. 2 ¹H NMR (in CHCl₃) spectra for (a) vinylPOSS, (b) OS-POSS, (c) OD-POSS, (d) 1-dodecanethiol, (e) 1-octadecanethiol.

The disappearances of the chemical shift at 5.88–6.16 ppm (–Si–CH=CH₂) and the appearances of the chemical shifts at 0.90–1.28 ppm (–SiCH₂–) and 2.52–2.60 ppm (–CH₂SCH₂–) in OS-POSS support the success of thiol–ene click reaction. Moreover, the integral ratio of the chemical shift at 0.90–1.28 ppm (–SiCH₂–) to that of the chemical shift at 0.84–0.92 ppm (–CH₃) is about 2/3. The same consequences can also be seen in OD-POSS ¹H NMR spectra. These results indicate that most of vinyls in vinylPOSS have participated in the thiol–ene click reaction and novel mercaptan modified POSS nanohybrids were successfully synthesized. In addition, the chemical shift at 7.28 ppm corresponds to the residual solvent peak, and the chemical shift at 1.60 ppm related to water in CDCl₃.

TGA curves for POSS nanohybrids are shown in Fig. 3. The initial mass loss in POSS nanohybrids attribute to the vaporization of adsorbed water. T_d (temperature corresponding to 10% mass loss) of POSS nanohybrids is higher than vinylPOSS, which clearly indicates that long carbon chains of mercaptan have been grafted onto POSS cages. When all samples are heated up to 700 °C, the weight loss for vinylPOSS, OS-POSS, and OD-POSS are 65.1%, 74.3%, and 79.2% respectively, caused in large part by that the mass ratio of silicon and oxygen components in vinylPOSS is bigger than that in OS-POSS or OD-POSS. In the TGA curve for POSS, there is a sharp mass drop between 250 °C and 310 °C. However, TGA curves for POSS nanohybrids show a slow decline at first; when the temperature is higher than 340 °C, the decomposition rate begins to increase rapidly. The terminated decomposition temperature for vinylPOSS and POSS nanohybrids can achieve 500 °C, while OS-POSS and OD-POSS are more stable due to the long chain alkyl bonded on POSS cages.

Fig. 3 TGA curves for (a) vinylPOSS, (b) OS-POSS, (c) OD-POSS.

3.2 Dispersion stability of POSS nanohybrids

OS-POSS and OD-POSS nanohybrids are solids in room temperature, but they will translate into liquid state when heated to 61.5 °C or 67.5 °C respectively. These characteristics make OS-POSS and OD-POSS nanohybrids be easily dispersed in LP by gentle heating and vigorous stirring. As shown in Fig. 4A and B, when the concentration of POSS nanohybrids in LP were increased by a step of 0.20 wt%, the strength of the characteristic peak at 300 cm⁻¹ increases almost linearly. According to Lambert–Beer law, UV-vis absorbance is proportional to the concentration, thus the results indicate that POSS nanohybrids possess a very good dispersion performance in LP.

Fig. 4 (A) UV spectra for LP mixture containing (a) 0.20 wt% OS-POSS, (b) 0.40 wt% OS-POSS, (c) 0.60 wt% OS-POSS, (d) 0.80 wt% OS-POSS, (e) 1.0 wt% OS-POSS. (B) UV spectra for LP mixture containing (a) 0.20 wt% OD-POSS, (b) 0.40 wt% OD-POSS, (c) 0.60 wt% OD-POSS, (d) 0.80 wt% OD-POSS, (e) 1.0 wt% OD-POSS.

In addition, the characteristic peak of Si–O–Si at 1120 cm⁻¹ in FT-IR spectra for LP mixtures containing POSS nanohybrids (Fig. 5A and B) continue to increase, which are in good agreement with the UV-Vis spectra. In our experiments, solid POSS nanohybrids can be converted into colorless transparent liquids under slight heating. As a kind of organic–inorganic material, POSS nanohybrids are different from common solid or liquid lubricating additives, which can be dissolved into LP completely. Thus OS-POSS and OD-POSS are organic–inorganic nanohybrid materials, which can achieve phase transformation by heating. On the other hand, OS-POSS and OD-POSS contain a hard and stable Si–O cage in each molecule, which can provide effective wear resistance and possess thermal stability compared with pure liquid lubricant additives. This special property is beneficial to its complete dissolution in LP and to the improvement of tribological performance.

Fig. 5 (A) FTIR spectra for (a) pure LP and LP mixture containing (b) 0.20 wt% OS-POSS, (c) 0.40 wt% OS-POSS, (d) 0.60 wt% OS-POSS, (e) 0.80 wt% OS-POSS, (f) 1.0 wt% OS-POS. (B) FTIR spectra for (a) pure LP and LP mixture containing (b) 0.20 wt% OD-POSS, (c) 0.40 wt% OD-POSS, (d) 0.60 wt% OD-POSS, (e) 0.80 wt% OD-POSS, (f) 1.0 wt% OD-POSS.

3.3 Tribological properties and related mechanism

Fig. 6 shows friction coefficient changing tendency via POSS nanohybrid concentrations in LP. Similar changing tendency can be found for sample containing both OS-POSS and OD-POSS. Taking OS-POSS as an example, the friction coefficient curve displays a ‘U’ shape and the lowest friction coefficient is 0.099, corresponding to the concentration of 0.40 wt%. The reduction comparing with pure LP (friction coefficient: 0.130) is about 23%. However, the friction coefficient gradually increases with additive concentrations increasing after the most optimal point at 0.40 wt%. Similarly, the optimal concentration for OD-POSS is 0.40 wt% and the corresponding value of friction coefficient is about 0.102 with a reduction of 21% when compared with pure LP. Being a kind of nanomaterials, POSS nanohybrids possess big surface area, which can form lubrication film on the frictional surface. At lower concentrations, a homogenous and continuous film cannot form on the frictional surface. With the concentration increasing to a certain value (0.40 wt% in our experiment), a fine and continuous protective film can form on the frictional surface, which could prevent the friction pairs from direct contact and improve the tribological properties. When the additive concentration continues to increase, excessive nanohybrids in LP will result in random aggregation on the surface of the friction pairs, which is not advantageous to the improvement of friction performance furthermore.

Fig. 6 Friction coefficient for LP mixtures containing POSS nanohybrids at different concentration (a) OS-POSS and (b) OD-POSS.

Fig. 7 gives the real-time changing tendency of the friction coefficient of LP, LP mixtures containing vinylPOSS, mercaptan and POSS nanohybrids. At the very beginning, the friction coefficient of each system starts from a similar value. With time increasing, the friction coefficients of LP and LP mixture containing 0.40 wt% of vinylPOSS, 0.40 wt% of 1-dodecanethiol and 0.40 wt% of 1-octadecanethiol increase remarkably. Meanwhile, these friction coefficient curves give quite larger fluctuations during the friction time. The friction coefficient for pure LP gradually increases to 0.146. Similarly, the friction coefficients for LP mixtures containing vinylPOSS, 1-dodecanethiol and 1-octadecanethiol increase to 0.141, 0.143 and 0.141 respectively; while those for LP mixtures containing OS-POSS and OD-POSS slowly decrease to 0.097 and 0.093. Similar phenomena can be found in the changing tendency of the average friction coefficient for LP and LP materials (in the inserted picture of Fig. 7).

Fig. 7 Friction coefficient as a function of friction time (a) pure LP and LP mixture containing (b) 0.4 wt% vinylPOSS, (c) 0.4 wt% 1-docosanethiol, (d) 0.4 wt% 1-octadecanethiol, (e) 0.4 wt% OS-POSS, (f) 0.4 wt% OD-POSS. Inserted image the average friction coefficients of POSS nanohybrids, vinylPOSS, LP and mercaptan.

At the same time, SEM images for the frictional surface after friction experiments using pure LP (Fig. 8a) or LP mixtures as a medium give supplementary evidences of friction coefficient changing tendency. In the SEM images for LP and LP mixtures containing vinylPOSS (Fig. 8b) and 1-dodecanethiol (Fig. 8c) cases, severely worn surface can be observed and materials on the surface are stripped by a strong shear force. Being different with the above results, the SEM image for LP mixtures containing OS-POSS (Fig. 8d) only shows a shallow and tidy friction trace, which indicates that it is advantageous to improve the frictional performance of LP.

Fig. 8 SEM images for the frictional surface after friction experiment using pure LP or LP mixtures as a medium. (a) Pure LP and LP mixture containing (b) 0.4 wt% vinylPOSS, (c) 0.4 wt% 1-docosanethiol, (d) 0.4 wt% OS-POSS.

As we known, the molecular weights of 1-dodecanethiol and 1-octadecanethiol are smaller than LP. When they are added into LP, it may result in a slight decrease of the mixture viscosity, which is not good for reducing the friction coefficient. On the other hand, as a solid additive, vinylPOSS does not dissolve in paraffin, and it possesses poor dispersion stability in organic phase, thus it even plays a negative role during the friction experiments. According to our results, both OS-POSS and OD-POSS can decrease the friction coefficient of LP mixtures, while OS-POSS works a little better than OD-POSS. From a perspective of molecular structure, vinylPOSS has an inorganic Si–O core surrounded by 8 vinyl groups, which are saturated by mercaptan with carbon chains after “thiol–ene click” reaction. Eight flexible long carbon chains form three-dimensional dendrimer-like structures, which affords the nanohybrids with good organic compatibility and better dispersion when compared with common solid additives (such as GO and MoS₂). Meanwhile, the inorganic Si–O core can confer ideal hardness and possess excellent thermal stability. These properties make POSS nanohybrids very suitable to be used as a lubricant additive for improving the friction performance.

On the other hand, POSS nanohybrids can also be regarded as the smallest silicon oxide pellet coated by organic carbon chain. When the pin rolls on the ceramic disk after one circle, “peaks” and “grooves” at micro- or nano-meter scale on the frictional surfaces may contact each other directly, as shown in Fig. 9a. POSS nanohybrids can work as nano ball bearings which translate the sliding friction of pin with ceramic disk into rolling friction, as shown in Fig. 9b. In addition, POSS nanohybrids possess small size and large surface area, which can easily fill the micro-scratches and form lubrication film on the on the frictional surface in the friction process. In this way, the self-repairing of the abrasive surface is partially realized, as shown in Fig. 9c.

Fig. 9 The illustrated process for POSS nanohybrids improving friction properties.

Micro-area element analysis of friction surface is an effective tool to understand the friction-reducing mechanism.^40,41 In our work, the suggested mechanism can be partly supported by surface element analysis by EDS. Fig. 10a–c shows the EDS spectra of wear scars for the steel pin lubricated by pure LP, LP containing 0.40 wt% of OS-POSS and LP containing 0.40 wt% of OD-POSS respectively. Element Al is observed on the surface lubricated by pure LP (Fig. 10a), which can be attributed to the direct contact and friction between steel pin and Al₂O₃ disk. Element Si is observed on the surfaces lubricated by LP containing 0.40 wt% of OS-POSS and OD-POSS (Fig. 10b and c), which indicates that stable protective films made up of POSS nanohybrids have formed on the frictional surface. This film could separate the direct contact between asperities, thereby improving the tribological properties of LP.

Fig. 10 The EDS spectra of wear scars for the steel pin lubricated by pure LP (a), LP containing 0.40 wt% of OS-POSS (b) and LP containing 0.40 wt% of OD-POSS (c) respectively.

4. Conclusions

Novel POSS based nanohybrids have been prepared via “thiol–ene click” reactions between vinylPOSS and alkyl mercaptan. As one kind of novel additive, they are effective for improving the tribological properties at rather a low concentration when compared with pure solid or liquid lubricating additives. These POSS nanohybrids are expected to be beneficial for more lubricating oil systems, which possess great scientific values and promising application prospects. This work tries to walk a step towards fundamental understanding of the friction reducing mechanism generated by these POSS based additives, which may be due to the synergistic effects of many factors.

Acknowledgements

This work is financially supported by the Natural Science Foundation of China (U1332134, 51635004), the Natural Science Foundation of Suzhou (SYG201329), the Fundamental Research Funds for the Central Universities (3202006301, 3202006403), the Qing Lan Project and the International Foundation for Science, Stockholm, Sweden, the Organization for the Prohibition of Chemical Weapons, The Hague, Netherlands, through a grant to Lei Liu (F/4736-2), the Natural Science Foundation of Jiangsu Province (BK20150505) and the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF15A11).

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