12 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 Introduction Silicone, or as per IUPAC nomenclature polysiloxane, represents a wide variety of polymeric chains and networks constructed around a backbone of Si-O-Si atoms. Their synthesis and manufacturing process is complex, starting from the reduction of quartz with carbon to yield elemental silicon. Then using a fluid bed reaction process silicon and methylchloride form a mixture of chlorosilanes with the majority being dichlorodimethylsilane. Hydrolysis of distilled chlorosilanes and subsequent polymerization result in polysiloxanes. The Si-O bonds of siloxanes are > 30 % stronger than C-C bonds of hydrocarbons. The Si-O-Si angle is wider compared to C-C-C; this gives to the molecule great flexibility (Figure 1). The strength, length, and flexibility of silicon bonds impart many unique properties, including low melting Aus Wissenschaft und Forschung * Dr. rer. nat. Manfred Jungk Dr. rer. nat. Christian Kranenberg Aleksandra Nevskaya, Dow Corning GmbH, Rheingaustr. 34, 65201 Wiesbaden Figure 1: Structure of polydimetylsiloxane (PDMS) Latest Development of Polysiloxane Structures Utilizing New Model Design M. Jungk, C. Kranenberg, A. Nevskaya* Eingereicht: 12. 11. 2015 Nach Begutachtung angenommen: 10. 2. 2016 Siloxane finden Einsatz als Spezialschmierstoffe und haben aufgrund der limitierten Schmierwirksamkeit keine breite Anwendung im Vergleich zu den kohlenwasserstoffbasierten Grundölen. Es wurden Copolymere auf Basis von Polyaklylphenylsiloxanen und Polyalkylfluoroalkylsiloxanen synthetisiert, welche diese Limitierung überbrücken. Diese neuen Colpolymere sind synthetisierbar in verschiedenen Viskositäten um als Industrieschmierstoff Anwendung zu finden. Die Eigenschaften dieser neuen Copolymere kann durch das Verhältnis von Polyalkylphenylsiloxan zu Polyalkylfluoroalkylsiloxan variiert werden. Ferner weisen sie Löslichkeit von Additiven auf, wie sie bei Kohlenwasserstoff basierten Grundölen zum Einsatz kommen. Nach einer Einführung beschreibt dieser Artikel ein Modell zur Vorhersage von tribologischen Daten auf Basis der Siloxanmolekuelstruktur, sowie Formulierungsansätze der neuen Copolymere bis hin zu Schmierfetten. Schlüsselwörter Silikone, Siloxane, synthetische Grundoele, Phenylfluorosilikon Copolymer, Hochtemperaturschmierstoff, Modell Molekülstruktur, Rheology, Tribology Siloxanes are used as lubricants in special applications but have not found broad use due to limitation of some of their varieties with respect to lubricating properties. Newly developed copolymers of polyalkylphenyl siloxanes and polyalkylfluoroalkyl siloxanes show properties that overcome these limitations. Copolymers of polyalkylphenyl siloxanes and polyalkylfluoroalkyl siloxanes are available in viscosities that make them suitable as lubricants for industrial applications. The properties of these new copolymers can be adjusted by varying the ratio of polyalkylphenyl siloxanes and polyalkylfluoroalkyl siloxanes. Another limitation of siloxanes was their solubility of additives used in lubricant formulations based on hydrocarbon fluids. This paper describes that the newly developed copolymers are receptive to conventional lubricant additives and can also be used for preparing lubricating greases. Keywords Silicone, siloxanes, synthetic base oils, phenylfluorosilicone copolymer, high temperature lubricant, model molecular structure, rheology, tribology Kurzfassung Abstract T+S_5_16 29.07.16 11: 27 Seite 12 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 temperature, fluidity, low glass transition temperature, and increased compactness. With these properties siloxanes can find many diverse applications in different industries. For the use as lubricating base fluid their exceptional oxidative stability and temperature-viscosity indices stand out versus other synthetic lubricant base fluids such as polyalphaolefines, polyalkylenglycols, polyolor dibasic-esters. This paper describes a model that has been developed to tailor siloxane structures to tribological needs and shows results of newly developed siloxane based fluids. Model Based Siloxane Structure Design: From Molecule to Tribology Recently Dow Corning worked in cooperation with Northwestern University in USA and constructed a modeling system for designing and developing future siloxane structures with excellent lubricity characteristics such as high load carrying capacity and strong film formation capability. The objective was to identify the relationship between molecular structure and rheological/ tribological properties to find structures with superior lubricity characteristic. Figure 2 shows schematically the concept and main parameters used during the creation of the model. The first step of the model was to calculate the rheological properties using molecular parameters of the structure. For the research different siloxane structures were investigated. For their nomenclature the letters are used throughout this paper where Q is the percentage of functional branch content, L is the length of alkyl branch (carbon atoms), J is the type of branch (Alkyl, Phenyl, Cyclic, Fluoro) and Z indicates atomic Length (silicon & oxygen atoms) of the polymer. Figure 3 shows the flow of properties and equations used to calculate rheological properties: specific volume, viscosity and pressure-viscosity coefficient. Knowing molecular structure and molecular weight the Van der Waals volume ν w can be easily calculated. The molecular packing factor is the quotient of specific Van der Waals volume over measured specific volume v 0 . The specific volume at room temperature and atmospheric pressure ν 0 can be calculated using molecular packing factor. The viscosity (η 0 ) can be calculated using the structure-viscosity equation of Berry and Fox which includes the parameters of radius of gyration and monomeric friction which is described on the right hand side of Figure 3. As an outcome we have got the Tait Equation of state (v(T, P)) which best describes the pressure and temperature dependence of the specific volume and the Tait Doolittle equation (η (T, P)) which is used to calculate the temperature and pressure dependence of the viscosity. During the experimental part film thickness and film friction were measured with the PCS EHL instrument. Lubricants were tested using a ball-on-disk machine to determine the effects of molecular structure on boundary friction and wear. Total friction is the summary of two components: asperity and film friction. In hydrodynamic lubrication, viscosity alone determines the coefficient of friction and film thickness. In elastohydrodynamic lubrication, friction and film formation are decoupled and three properties become influential in optimization of lubricants, i. e. the viscosity at atmospheric pressure and the pressure-viscosity index influence overall film formation while limiting shear stress is the 13 Aus Wissenschaft und Forschung Figure 2: Modeling system: from molecule to tribology Figure 3: Modeling system: from molecular to rheological ( ) Z J L Q v , , , 0 ( Z J L Q , , , 0 η MPa P K T 1 . 0 298 = = MPa P K T 1 . 0 298 = = Molecular Structure - Specific Volume Packing Factor W v ( ) P T v , Equation of State Molecular Structure - Viscosity ( ) P T , η Viscosity (T,P) Shear Viscosity ( ) γ η ! s 0 η 0 v ( ) c X F Viscosity Variation ! " v η T+S_5_16 29.07.16 11: 27 Seite 13 Molecular properties Tribological Properties •Molecular structure •Molecular mass distribution •Asperity friction Fil f i ti •Molecular mass distribution •Film friction Rheological properties Tribological parameters •Volume structure and •Geometry Volume Pressure Temperature •Viscosity structure and Viscosity Pressure Temperature P i it i d y •Poisson ratio •Young Modulus •Load S d •Pressure viscosity index •Speed 14 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 primary contributor to the elastohydrodynamic friction coefficient. Figure 4 shoes the GUI (graphical user interface) of the algorithm that was programmed on experimental data and equations from the literature. Polydimethylsiloxane (PDMS) is the basis and as branching structure one can choose from Alk y l ( PA M S ) , F l u o r o (PFMS), Cyclohexyl (PCMS) or Phenyl (PPMS). Input variables are temperature, branch length (for Alkyl), polydisperity, load, branch content (% ), polymer length, speed, slide to roll ratio, ball and disk (disk is set at 100 to simulate infinite flat surface) diameter, Young’s Modulus of ball and disk, Poisson Ratios of ball and disk, as well as surface finish of ball and disk. The output data are the last 2 rows with 8 boxes, where molecular mass, volume, viscosity and pressure-viscosity are the molecular to rheological data and film thickness, Hertzian pressure, stress and total friction are the rheological to tribological data. Results Figure 5 shows two graphs with the film thickness values versus entrainment speed for phenylfunctional siloxane (PPMS 90) and alkylfunctional siloxane (A100-12) at different temperatures. Ring branches (aryl PPMS) show nearly Newtonian behavior. High monomeric friction allows a relatively low molecular mass (Mw = 1990 g/ mol for PPMS 90) to build viscosity, so shear thinning is low. On the right hand side of the Figure 5 we can see a different behavior for alkylfunctional fluids. Low monomeric friction requires a relatively high molecular mass (Mw = 29900 g/ mol for A100-12) to build viscosity and shear thinning is high. So, linear branches may exhibit temporary shear-thinning. Figure 6 shows the coefficient of friction versus film thickness for three different structures. Data for polycyclohexylmethylsiloxane PCMS 50, polyphenylmethylsiloxane PPMS 50 and standard polydimethylsiloxane PDMS are shown at two different temperatures. For PCMS the CoF does not change much by temperature. In case of PPMS and PDMS CoF drops down at higher temperatures. This means that cyclohexyl siloxanes have greater thermal stability in the fluid film region. As we could see from these examples siloxanes are adaptable to diverse application, e. g. cycloalkysiloxanes as traction fluid or alkylsiloxanes as energy efficiency lubricants. Phenyl- / Fluoro- Siloxane Copolymer Lubricants As shown above the chemistry of siloxanes is very diverse. The most common siloxane structure is polydi- Aus Wissenschaft und Forschung Figure 4: Graphical User Interface Figure 5: Newtonian and Non-Newtonian siloxanes PPMS: Phenyl-methyl PPMS 90 (90% Phenyl) 303 K 348 K 398 K " PAMS: Alkyl-methyl A100-12 (100% Dodecyl) " 303 K 348 K 398 K T+S_5_16 29.07.16 11: 27 Seite 14 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 methylsiloxane which finds application in lubricants as well but it has limited wear resistance. Phenyl functional group inside the siloxane molecule provides additional thermal and oxidation resistance but does not improve the lubricity. Trifluoropropyl substituted siloxanes exhibit reasonable wear protection and load carrying capacity, though their oxidation stability is not as good as that of the above mentioned phenyl substituted materials. Copolymerization technology allows to combine different siloxane monomers with specific properties to copolymer fluids. These new fluids contain properties of both siloxane monomers included in the copolymer fluid. The balance of monomer ratio allows the development of specific properties and the degree of polymerization is used to make copolymers in a broad range of viscosity. We used this technology to develop copolymers containing phenylmethyl siloxanes and trifluoropropylmethylsiloxane. These phenyl- / fluorocopolymers (Ph/ F copolymers Figure 7) combine the excellent temperature stability of polyphenylmethylsiloxane with the enhanced wear resistance properties of polytrifluoropropylmethylsiloxanes. Compared to the letter nomenclature from above, Z is the sum of x and y, Q is for the copolymer 100 % where the remainder of the balance would be methyl before and J presents 2 types of branching. Properties of Ph/ F copolymer neat fluids The thermal stability of three different ratios has been determined by thermal treatment in closed and open cups at 250 °C. We measured the viscosity on a weekly base. 15 Aus Wissenschaft und Forschung Figure 8: Thermal stability of copolymer fluids Figure 6: EHD friction for cyclic branched silicones Figure 7: Structure of Ph/ F copolymer fluid " Friction at 303 K and Σ=0.5 Friction at 398 K and Σ=0.5 $" T+S_5_16 29.07.16 11: 27 Seite 15 16 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 Figure 8 shows the results which indicate, that the viscosity of Ph/ F copolymers is almost stable over a period of more than 20 days when stored in open cups. Copolymers with higher phenyl content (75: 25) demonstrate a better stability than such with higher fluoro content (25: 75). In closed cups, where no evaporation of the copolymer occurs, the viscosity is nearly stable over a period of more than 100 days. Table 1 shows the thermal stability of copolymer fluids compared to other lubricating fluids: Ph/ F copolymer fluids show viscosity indices (VI) of about 230-240, which are similar to high phenylated siloxane fluids and fluorosiloxane fluids. Compared to other lubricating fluids these values are higher than most of the hydrocarbon fluids and branched PFPE fluids. Some additional comparison tests have been made by using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both methods indicate superior thermal stability of Ph/ F copolymers compared to PAO, polyolester and fluorosiloxane fluids. Table 2 shows our investigations on wear resistance according to DIN 51350-3. We report the wear scars as average of the three steel balls after applying a load of 400 N and 800 N for 1 hour test duration. Whereas no data could be generated for phenylmethylsiloxane fluid (based on the high load), we could measure wear scars for all three investigated copolymer ratios. The results reflect the assumption that copolymer fluids with higher fluoro contents have smaller wear scars. In addition to our investigations in wear resistance we also measured the coefficient of friction (CoF) on a SRV test machine according DIN 51834-4 and compared these Aus Wissenschaft und Forschung Table 1: Comparison of Ph/ F copolymer fluids with typical lubricating fluids Fluid Ph/ F Ph/ F Ph/ F PAO Polyol- Phenylmethyl- Fluoro- PFPE PFPE 75: 25 50: 50 25: 75 ester polysiloxane silicone (branched) (linear) Viscosity Index 229 239 242 155 145 220 241 108 338 TGA* (250 °C) 98.6 % 99.0 % 99.0 % 97.4 % 96.5 % 99.1 % 97.2 % 93.4 % 99.3 % DSC** (onset temperature) 282 °C 283 °C 277 °C 205 °C 203 °C 366 °C 246 °C 348 °C 330 °C Evaporation*** Not Not (200 °C, 7d) 0.13 % 0.47 % 0.65 % 7.85 % 13.38 % 1.46 % 18.50 % tested tested * TGA: 30-500 °C, 10°/ min, Air 60 ml/ min ** DSC: 30-500 °C, 10 °C/ min, Air, 60 ml/ min *** internal test method Table 2: Wear resistance of neat copolymer fluids Fluid Ph/ F ratio 400 N load 800 N load average wear scar diameter Ph/ F copolymer fluid 75: 25 1.53 mm Not measurable 50: 50 1.48 mm 2.82 mm 25: 75 0.55 mm 1.81 mm Polyphenylmethylsiloxane Not measurable Not measurable Polytrifluoropropylmethylsiloxane 1.18 mm 1.17 mm Table 3: SRV measurement results (DIN 51834-4, T = 50 °C, Freq. = 50 Hz, Load = 300 N, Stroke = 2 mm) Fluid Ph/ F ratio µ (f15) µ (f30) µ (f90) µ (f120) Ph/ F copolymer fluid 75: 25 0.129 0.128 0.126 0.125 50: 50 0.128 0.128 0.119 0.118 25: 75 0.121 0.118 0.112 0.111 Polytrifluoropropylmethylsiloxane reference 0.109 0.104 0.098 0.096 Polyphenylmethylsiloxane reference not measurable T+S_5_16 29.07.16 11: 27 Seite 16 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 results with polyphenylmethylsiloxane and polytrifluoropropylmethylsiloxane fluids. Table 3 lists the CoF after 15, 30, 90 and 120 minutes operating time. These results demonstrate, that the CoF of copolymer fluids are slightly higher than those of polytrifluoropropylmethylsiloxane fluids. Compared to conventional polysiloxane fluids, they show much better results as we couldn’t measure polyphenylmethylsiloxane at applied loads of 300N. Ph/ F copolymer fluids with additives Further investigations with Ph/ F copolymers indicate, that the additive acceptance of this new class of lubricants is much better than with polyphenylmethylsiloxane and polytrifluoropropylmethylsiloxane fluids. Table 4 for example shows the improvements in wear resistance (DIN 51350-3) by using commercial available additives compared to the neat copolymer fluid (Ph/ F ratio: 50: 50). Greases with Ph/ F copolymers The next step was to focus further investigations on the development of greases using Ph/ F copolymer fluids. These greases can be prepared by using single and complex soap thickener systems like Li-complex-soaps, but also by using polyurea, PTFE and other kind of thickeners. As an example we show results of greases thickened by Li-complex-soaps and PTFE in Table 5. 17 Aus Wissenschaft und Forschung Table 4: Wear resistance improvements by additives (DIN 51350-3) Additivation % 400 N load 800 N load Chemistry average wear scar diameter (change compared to particular neat fluids) antimony o,o-dialkylphoshorodithionate 2.5 % 0.55 mm (- 69 %) 1.03 mm (- 59 %) zinc diamyldithiocrbamate 2.5 % 0.51 mm (- 71 %) 1.07 mm (- 57 %) 1.0 % 0.66 mm (- 55 %) 1.29 mm (- 54 %) dithiocarbamate, ashless 2.5 % 0.62 mm (- 58 %) 1.16 mm (- 59 %) 1.0 % 0.65 mm (- 56 %) 1.98 mm (- 30 %) dialkylpentasulfide 2.5 % 0.82 mm (- 45 %) 2.07 mm (- 27 %) 1.0 % 0.76 mm (- 49 %) 1.21 mm (- 57 %) zinc dialkyldithiophosphate 2.5 % 0.66 mm (- 55 %) 1.16 mm (- 59 %) 1.0 % 1.48 mm (± 0 %) 1.89 mm (- 33 %) amin alkylisooctylphosphate 2.5 % 1.22 mm (- 18 %) 1.98 mm (- 30 %) Table 5: Copolymer grease properties Characteristic Test method Li-complex grease PTFE grease Ph/ F ratio 50 / 50 50 / 50 Base oil viscosity (40°C) 246 cSt 790 cSt Additives No additives No additives Penetration DIN ISO 2137 258 / 261 / 282 / 303 324 / not tested (unworked / 60 / 10k / 100k strokes) Bleed / Evaporation (24 h / 200 °C) ASTM D 6184 0.46 % / 1.17 % 4.58 % / 0.44 % Dropping point IP 396-2 340 °C 332 °C Flow pressure DIN 51805 925 mbar (-35 °C) 1125 mbar (- 40 °C) Emcor corrosion (7d) DIN 51802 0 4 Water resistance (3 h, 90 °C) DIN 51807 1 0 DIN 4 ball test (ok load) DIN 51350 1700 N 1900 N DIN 4 ball test (wear scar 400N, 1 h) DIN 51350 1.05 mm 1.02 mm FE 9 (F10 / F50) DIN 51821-2 Not tested 52 h / 69 h (B, 6000 rpm, 1500 N, 220 °C) T+S_5_16 29.07.16 11: 27 Seite 17 18 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 Ph/ F-copoylmer greases show excellent high temperature resistance and are suitable in applications where a broad service temperature range is needed. Their preparation is similar to process of greases using polydimethylsiloxanes, polyphenylmethylsiloxanes or polytrifluoropropylmethylsiloxanes. Non additivated Li-complex greases based on Ph/ F copolymer show low bleeding behavior and already good corrosion protection at high temperatures. The wear resistance of copolymer greases is much better compared to other siloxane based grease. Similar to the neat Ph/ F copolymer fluids their performance can be increased by adding appropriate additives. Applications Polydimethylsiloxane based greases and compounds find use as O-ring and valve lubricant, damping grease, plastic gear lubricant or brake caliper grease. Phenyl groups on the siloxane molecule provide additional thermal and oxidation resistance but do not improve lubricity. Polyphenylmethylsiloxane based greases are used in metalto-metal applications requiring high temperatures such as clutch release bearings or requiring slip prevention such as overrunning clutches. Trifluoropropyl substituted siloxanes exhibit reasonable wear protection and load carrying capacity, though their oxidation stability is not as good as that of the above mentioned phenyl substituted materials. Typical applications of polytrifluoropropylmethylsiloxanes are for pumps, mixers or valves in the chemical industry and circuit breakers. Ph/ F copolymer based greases thickened with PTFE show good results (as shown in table 5) at high temperatures (220 °C) in FE 9 bearing application tests. Based on this characteristic, bearings running at high and/ or broad temperature ranges might be an interesting application for this kind of greases. Outlook Newly synthesized temporary shear thinning polyalkylmethylsiloxanes, high traction polycyclohexylmethylsiloxanes and the high temperature Ph/ F copolymer siloxanes are examples for the trobological potential of siloxanes beyond the specialty use of polydimethylsiloxanes, polyphenylmethylsiloxanes, polytrifluoropropylmethylsiloxanes. The model in form of the graphical user interface and its algorithm can be used to optimize the chemical structure for a specific tribological need. Besides tailoring siloxane fluids for specific tribological challenges as described above, the researcher could extend the model to other types of chemical structures besides silicones, explore the impact of mixtures and additives to the model or introduce functional groups (S, N, P) to the side groups. Ph/ F siloxane copolymer greases are defining a new class of lubricating siloxane fluids which solve some limitations of the current used polysiloxanes. Their flexible structure defined by the ratio of phenyl and fluoro functional groups allows the design of specific fluids at high thermal stability and improved wear resistance. The good solubility of commercial available additives allows to create lubricants for a broad range of applications. References [1] Zolper, T.J., Li, Z., Chen, C., Jungk, M., Marks, T.J., Chung, Y.-W., Wang, Q.: Lubrication properties of polyalpha-olefin and polysiloxane lubricants: molecular structure-tribology relationships. Tribol. Lett. 48, 355- 365 (2012) [2] Zolper, T.J., Li, Z., Jungk, M., Stammer, A., Stoegbauer, H.,Marks, T.J., Chung, Y.-W., Wang, Q.: Traction characteristics of siloxanes with aryl and cyclohexyl branches. Tribol. Lett 49, 301-311 (2013) [3] Zolper, T.J., Seyam, A.M., Chen, C., Jungk, M., Stammer, A., Stoegbauer, H., Marks, T.J., Chung, Y.-W., Wang, Q.: Energy efficient siloxane lubricants utilizing temporary shear-thinning. Tribol. Lett. 49, 525-538 (2013) [4] Zolper, T.J., Seyam, A.M., Chen C., Jungk, M., Stammer, A., Chung, Y.-W., Wang, Q.: Friction and Wear Protection Performance of Synthetic Siloxane Lubricants. Tribol. Lett. 51, 365-376 (2013) [5] Zolper, T.J., Jungk, M., Marks, T.J., Chung, Y.-W., Wang, Q.: Modeling polysiloxane volume and viscosity variations with molecular structure and thermodynamic state. Journal of Tribology 136(1), 011801/ 1-011801/ 12 (2014). [6] Nevskaya, A., Jungk, M., Kranenberg, C., Weber, V.: Silicone base fluids for high temperature lubricants. EU- ROGREASE 3, 15-23 (2014). Aus Wissenschaft und Forschung Anzeige Nutzen Sie auch unseren Internet-Novitäten-Service: www.expertverlag.de mit unserem kompletten Verlagsprogramm, über 800 lieferbare Titel aus Wirtschaft und Technik T+S_5_16 29.07.16 11: 27 Seite 18