ABSTRACT

This disclosure provides three-dimensional and thin film morphologies of fluoro-polymer nanocomposites with adjusted friction properties, which contain inorganic nanomaterials as low-friction additives. In particular, this disclosure provides Polyvinylidene fluoride (PVDF) based polymers, which contain MoS ₂ -based nanomaterials and the method of adjusting friction properties of PVDF based polymers.

FIELD OF THE INVENTION

The present invention relates generally to the field of polymer nanocomposites and more particularly to methods of adjusting the physical properties of thermoplastic high- performance fluoro-polymers, and especially friction properties of Polyvinylidene fluoride based polymers with use of MoS ₂ -based nanotubes

BACKGROUND OF THE INVENTION

Polyvinylidene fluoride (PVDF) is a highly non-reactive thermoplastic fluoro-polymer with a high thermal stability up to 175 °C,. It is usable in a wide range of applications that depend on its particular phase of crystallization, such as piping products, insulators for premium wires, binder material for composite electrodes for lithium ion batteries, membranes in biomedicine, components for the pharmaceutical and food processing industry, as piezoelectric and pyroelectric materials, etc. PVDF can be found in five crystal forms (α,β,γ,δ,ε) and in three of these (β, γ, δ) the highly polar structure is oriented in such a way to make PVDF piezoelectric (A. Lovinger, Macromoleculesl982, 15, 40- 44.). It has a relatively high PVDF-PVDF coefficient of friction in the range 0.25-0.45 that limits its application as friction-intensive or self-lubricative coatings or as protective barrier coatings. Inorganic solid lubricant molybdenum disulfide (MoS ₂ ) is a known lubricant, which has been applied extensively for decades. The easy mutual gliding of M0S2 layers along (001) basal planes and surface inertness of the MoS ₂ (001) basal planes give it its low friction properties. The MoS ₂ in usual plate-like form which can be synthesized or exploited as a natural mineral, is widely used as a dry lubricant or an oil or grease additive. Unfortunately, the high-hardness edges of crystal layers are prone to oxidation, which reduces the efficiency of lubrication, especially in humid environment. Thin flakes with a high active surface and with a relatively low number of unsaturated bonds at edges are therefore preferable. Standard use of MoS ₂ platelets as additive for friction reduction and recent discoveries of new morphologies of MoS ₂ have opened the route to prepare new PVDF-based nanocomposite films containing MoS ₂ nanotubes or exfoliated MoS ₂ nanotubes for self-lubricative and protective barrier coatings.

Curved, self-terminated shapes of MoS ₂ as nanotubes and fullerene-like particles with a nano-onion morphology (Margulis L, Salitra G, Tenne R, Talianker M: Nested fullerene- like structures .Nature 1993, 365: 113-114) allow the"elimination" of edges. They are intensively investigated with regards to their particular appropriateness for a new generation of lubricants. Under mechanical stress the nanoparticles slowly deform and exfoliate transferring MoS ₂ nano-sheets onto the underlying surfaces (third body effect), and continue to provide effective lubrication until they are totally exfoliated (Chhowalla M, Amaratunga GAJ: Ultra low friction and wear MoS ₂ nanoparticle thin films. Nature 2000, 407: 164-167).

Different morphologies and combinations of MoS ₂ nanotubes and nano-onions synthesized from M ₆ C _y H _z , 8.2<y+z<10, where M is a transition metal (Mo, W, Ta, Nb), C is a chalcogen (S, Se, Te); H is a halogen (I), were disclosed by US 8,007,756 B2.

Spontaneous partial exfoliation of MoS ₂ nanotubes' walls in pristine material (Remskar M, Virsek M, Mrzel A: The MoS ₂ nanotube hybrids. Appl Phys Lett 2009, 95: 133122-1- 133122-3.) and thin walls of MoS ₂ nanotubes (M. Remskar, A. Mrzel, M. Virsek, M. Godec, M. Krause, A. Kolitsch, A. Singh, and A. Seabaugh, Nanoscale Res. Lett. 6, 26 (2011)) enable a low energy-cost running-in process in friction process. Exfoliation of MoS ₂ nanotubes can be achieved by chemical means through by intercalation of highly reactive molecules, such as butyllithium (Visic et al. Nanoscale Research Letters 2011, 6:593) or by mechanical means during the gliding process.

The PVDF/MoS ₂ composites disclosed by: a)US 5,024,882 where MoS ₂ was added to PVDF as powder particles < 40 μπι for materials for use in composite sliding surface bearing and process of manufacturing the material; b) US 201 1/0045309 AI, where MoS ₂ of non-disclosed shape was used as lubricant alone or in combination with PVDF for adjusting the friction coefficient of a metallic workpiece; c) US 5,976,190, where MoS ₂ of non-disclosed shape was imbedded into an intermediate layer between a tube socket and tube made of light metal both consisting orthopaedic clamped connection; d)US 6,106,936, where MoS ₂ was described as metal compound having a laminar structure only and used as a component of sliding material for sliding bearing material; e) US 6,528,143 Bl, where MoS ₂ of non-disclosed shape composed a plastics overlay on a metallic backing layer; f) US20130071623 Al, where MoS ₂ of non-disclosed shape composed a solid lubricant dispersed within the polymer.

In all above listed PVDF/MoS ₂ composites standard macrosized or micro sized MoS ₂ platelets were used.

The PVDF/M0S2 composites, where MoS ₂ are discribed as nanotubes are dislosed by: g) US 20080248201 and US20080249221 Al, where PVDF is listed indirectly as a member of polyvinylidene halides and MoS ₂ as a posible nanofiller among a wide range of materials for low friction coatings, but with no data presented on this particular composition; h) US20060233692 Al, where a method is dislosed whereas a metal alloy substrate can be directly coated with nanotubes, among them also MoS2 nanotubes are listed, and applying a polymeric coating thereover. PVDF is dislosed as polymer binder of carbon nanotubes.

PVDF/MoS ₂ nanocomposites, where MoS ₂ is in cylindrical geometry of nanotubes or as exfoliated MoS ₂ nanotubes and mixed into disolved PVDF before a coating preparation and tested for their friction properties have not been disclosed yet.

Tribology testing using MoS ₂ nanotubes as additive in Polyalphaolefin (PAO) oil have shown significantly reduced friction and improved wear behavior in the boundary- lubrication conditions between steel counterparts. The coefficient of friction was decreased more than 2-fold, while the wear was reduced by as much as 5 to 9-fold (M. Kalin, J. Kogovsek, M. Remskar, Wear 280/281, 36-45 (2012), doi: 10.1016/j.wear.2012.01.01 1). The improvement with respect to the base oil was found to be greater for rough surfaces, where the coefficient of friction was reduced by up to 65%, compared to about 40% reduction for smooth steel surfaces, under boundary- lubrication conditions. The friction was found mainly independent on the surface roughness (J. Kogovsek, M. Remskar, A. Mrzel, M.Kalin,Tribology International 61(2013)40-47).

The MoS ₂ nanotubes introduced into an isotactic polypropylene (iPP)decreased coefficient of friction for 15 % and wear for more than 50 % (M. Naffakh, M. Remskar, et al., J. Mater.Chem. 22, 17002-17010 (2012).). This is the only polymer-MoS ₂ nanotube composite reported to date.

SUMMARY OF THE INVENTION

This disclosure provides three-dimensional and thin film morphologies of fluoro-polymer nanocomposites with adjusted friction properties, which contain inorganic nanotube-based nanomaterials as low-friction additives. The term nanotube-based nanomaterials means nanomaterials which occur in cylindrical geometry, or are derived from cylindrical geometry by using mechanical or chemical methods. In particular, this disclosure provides a method of adjusting friction properties of PVDF based polymers with MoS ₂ -nanotube- based as inorganic low-friction additives. Friction of the PVDF/MoS ₂ nanotube-based nanomaterials is substantionaly reduced with respect to PVDF coatings without the said additives.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is to provide three-dimensional and thin film morphologies of fluoro-polymer nanocomposites with adjusted friction properties, which contain inorganic nanotubes-based nanomaterials as low-friction additives. According to the invention the object is solved by a method for adjusting the friction coefficient of PVDF and PVDF-based polymers by incorporation of MoS ₂ .

With the method according to the invention, the MoS ₂ nanotube-based nanomaterials are added to PVDF in form of a solution in an appropriate solvent or PVDF in melt form, and further homogenized by means of mechanical stirring.

Nanocomposite films may be prepared by various techniques applied to a polymer- nanoparticle mixture in liquid or plasticized state. Two of them are: a) solution casting on a suitable substrate by means of doctor blade applicator; b) spin-coating on a suitable substrate. The films are cured at different heating regimes with or without application of an atmosphere with a controlled composition.

Using the described application methods, the PVDF/MoS ₂ nanotube-based nanomaterials in shape of films and coatings with thicknesses in the range between 10 μπι and 500 μιη were prepared. Controlled crystal structure and morhology of the nanocomposites may be prepared using different co-additives in a role of inhibitors of agglomeration and sedimentation of MoS ₂ nanotube-based nanomaterials and by varying of application and curring conditions.

The so-prepared nanocomposites were tested for their physical and chemical properties, particularly for their crystal structure, surface morphology, distribution of MoS ₂ -nanotube- based nanomaterials inside the PVDF-based polymers, and thickness of the nanocomposite films.

The so-prepared nanocomposite coatings were tested for the friction properties in flat-on- flat and ball-on-disc geometries. Friction of the the PVDF/MoS ₂ nanotube-based nanocomposites was substantionaly reduced with respect to PVDF-based coatings without the MoS ₂ nanotube-based additives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 shows X-ray diffraction patterns of PVDF/MoS ₂ films with 0 wt. % (label-0), 1 wt. % (label- 1), and 2 wt.% (label-2) of M0S ₂ nanotubes. The diffraction peaks corresponding to MoS ₂ are labelled with *. Other peaks correspond to PVDF. FIG2 is an optical micrograph taken in transmision mode showing homogeneous distribution of MoS ₂ nanotubes inside the PVDF film containing 0.5 wt. % of MoS ₂ nanotubes.

FIG3 is scanning electron microscopy image of the upper surface of a PVDF/MoS ₂ nanotube film containing 2 wt.% of MoS ₂ nanotubes showing porous structure.

FIG4 is a scanning electron microscopy image of the lower surface (at the interface with glass substrate) of a PVDF/MoS ₂ nanotube film containing 1 wt.% of MoS ₂ nanotubes.

FIG5 shows results of friction tests in a flat-on-flat geometry for PVDF/MoS ₂ nanocomposite films with (a) 0 wt. %; (b) 1 wt.%, and (c) 2 wt.% of MoS ₂ nanotubes, with polished stainlessteel AISI 316. as counterpart.

FIG6 shows results of friction tests in ball-on-disk geometry for PVDF/MoS ₂ nanocomposite films with (a) 0 wt. %, (b) 2 wt. %, and (c) 16.7 wt.% of MoS ₂ nanotubes, and stainless steel AISI 316 ball.

The following examples serve to illustrate the invention but do not restrict it. EXAMPLE 1 :

PVDF was added to dimethylformamide (DMF) in 20 wt.% and dissolved for 24 hours with low speed stirring and heating up to 50 degree C. The homogeneous PVDF/DMF solution was mixed using a magnetic stirrer for 15 minutes prior to addition of nanotubes. MoS ₂ nanotubes in 0.5 wt. %, 1 wt. %, and 2 wt. % with respect to wt. of PVDF were added into the PVDF/DMF solution and mixed using a magnetic stirrer for additional 30 minutes. Then the so-produced dispersion was sonicated for 30 minutes in an ultrasonic bath at 40 kHz and 200 W. A homogeneous dispersion of nanotubes in the polymer solution was obtained. The dispersion was cast on a glass plate and drawn by a doctor blade with solution film thickness of 300 μιτι, moved by means of a film applicator (Erichsen). The films were dried at 22 °C and at 50 % relative humidity for 24 hours. After the drying the films were removed from the glass plate. The film surface, which was exposed to the air during the drying is designated as the upper surface. The film surface, which formed at the film/glass interface is designated as the lower surface.

X-ray diffraction (XRD) was performed at room temperature with a D4 Endeavor diffractometer (Bruker AXS) using a quartz monochromator Cu K l radiation source (λ = 0.1541 nm) and a Sol-X energy dispersive detector. The angular range 2 Θ was chosen from 6° to 73° with a step size of 0.04° and a collection time of 4 s. Crystal structure characterization of the PVDF/MoS ₂ nanotube films by x-ray diffraction as represented on FIG1 confirmed the presence of MoS ₂ in the film. The MoS ₂ peaks are marked with (*). Other peaks correspond to γ phase of PVDF. The peak at 16.8° corresponds to a doubled unit cell of γ-phase of PVDF.

Homogeneous distribution of the MoS ₂ nanotubes in PVDF containing 0.5 wt. % MoS ₂ nanotubes is represented by the optical micrograph taken in reflection mode shown in FIG2.

Morphologies of the PVDF/MoS ₂ nanocomopsites were studied by scanning electron microscope FE-SEM, Supra 35 VP, Carl Zeiss. The upper surface of PVDF/MoS ₂ nanotube films are porous and composed of sphelurites as it is represented by FIG3. The MoS ₂ nanotubes are not visible because they are covered by PVDF. The lower surface of the PVDF/MoS ₂ nanotube film contains MoS ₂ nanotubes, which are covered with a thinner layer of PVDF than the upper surface. MoS ₂ nanotubes in the lower surface become visible when higher accelerating voltages in scanning electron investigation are applied, as it is shown in FIG4.

The thickness of the PVDF and PVDF/MoS ₂ films and surface roughness were measured by a profilometer (Form Talysurf Series,Taylor-Hobson Ltd.), with resolution: in x- direction 0.25 μιη, in y-direction 1 μιη, in z-direction 3 nm. Pure PVDF films were 18.7 μηι ± 1 μιη in thickness. PVDF/MoS ₂ films which contained 1 wt.% of MoS ₂ nanotubes were 22.3 μιη ± 1 μη thick. PVDF/MoS ₂ films which contained 2 wt.% of MoS ₂ nanotubes were 30 μηι ± 1 μιη thick. Upper surface of pure PVDF film and PVDF/MoS ₂ nanotube films were attached to the holder of the tribometer with carbon adhesive tape. The lower surfaces of the PVDF and PVDF/M0S2 nanotube films were put into the contact with polished stainless steel AISI 316. The friction experiments were performed using a CF-800XS friction tester (RDM Test Equipment). The load applied to the friction holder was 223g. The calculated contact presure was 83 kPa. The gliding of the PVDF and PVDF/MoS ₂ nanotube films on the stainless steel AISI 316 was in the reciprocal mode. Only in forward direction the friction was mesured. Relative velocity was 300 mm/min. Each path was 300 mm in length. Friction was tested at normal room conditions: relative humidity in the range: 43-52% and in the temperature range: 20-25 °C. Friction tests were performed in a flat-on-flat geometry for PVDF/MoS ₂ nanocomposite films with 0 %, 1 wt.% and 2 wt.% of MoS ₂ nanotubes. The results are represented in FIG5. The presence of MoS ₂ nanotubes in the PVDF films eliminated running-in peaks which are typical for pure PVDF films. The presence of MoS ₂ nanotubes decreased coefficient of friction. After 40 m sliding distance the coefficient of friction for pure PVDF/AISI 316 contact was 0.42, for PVDF/1 wt.% MoS ₂ nanotubes/ AISI 316 was 0.31 and at the PVDF/2 wt.% MoS ₂ nanotubes / AISI 316 contact was 0.11. The 1 wt.% of MoS ₂ nanotubes in PVDF reduced friction by more than 20 % with regard to pure PVDF. The 2 wt.% of MoS ₂ nanotubes in PVDF reduced friction by more than 70 %.

The friction test results obtained in flat-on-flat geometry, graphically represented in FIG5, indicate that the MoS ₂ nanotubes added to PVDF as described in the invention strongly reduce friction at the PVDF/MoS ₂ nanotube-AISI 316 contact.

EXAMPLE 2

PVDF was added to dimethylformamide (DMF) in 20 wt.% and dissolved for 24 hours under gentle mechanical stirring. Then the homogeneous PVDF/DMF solution was mixed using a magnetic stirrer for 15 minutes. Then the MoS ₂ nanotubes in 2 wt. % and 16.7 wt.% with respect to wt. of PVDF were added into the PVDF/MoS ₂ solution and mixed using a magnetic stirrer for additional 15 minutes. Then the so-produced dispersion was sonicated for lh 45' in ultrasonic bath at 40 kHz and 200 W. The PVDF and PVDF/MoS ₂ nanotube-based coatings were prepared by solution drop casting directly on AISI 316 disks polished to arithmetical mean surface rougness Ra of 2 micrometers. The coatings were dried at 22 °C and at 50 % relative humidity until constant mass were reached.

The so-produced pure PVDF coatings on AISI 316 disks were 50μπι in thickness. PVDF/MoS ₂ coatings on the AISI 316 disks which contained 2 wt.% of MoS ₂ nanotubes were 50μηι thick. PVDF MoS ₂ coatings which contained 16.7 wt.% of MoS ₂ nanotubes were 60μπι thick.

Friction testswere performed with a standard ball bearing, 6 mm in diameter, made of stainless steel AISI 316 as counterpart to PVDF and PVDF/MoS ₂ nanotube coatings. The arithmetic mean surface rougness Ra of the ball was 200 nm, load applied to the ball was 1 N, radius of the circular path was 5.2 mm, contact pressure was 0.2 MPa, and velocity of the ball with respect to the disk was 1 cra/s.

Results of friction tests in ball-on-disk geometry, graphically represented in FIG6, indicate that the MoS ₂ nanotubes added to PVDF as decribed in the invention, reduce friction at the contact between PVDF/MoS ₂ nanotube composite and AISI 316. The PVDF/MoS ₂ nanotube coating with 2 wt.% of MoS ₂ nanotubes revealed reduced friction by 7 % with respect to pure PVDF coating. The PVDF/MoS ₂ nanotubes coating with 16.7 wt.% of MoS ₂ nanotubes revealed reduced friction in the first 12 m of sliding by 73% with respect to pure PVDF coating. After the sliding length of 12 m the coefficient of friction gradually increased to 0.43 and slowly approached 0.47 at 77 m of sliding.

Friction test results obtained in ball-on-disk geometry, indicate that the MoS ₂ nanotubes added to PVDF as decribed in the invention strongly reduce friction at the contact between PVDF/M0S2 nanotube composite and AISI 316.