METHOD FOR CHEMICALLY ADHERING A DIENE RUBBER TO A PIEZOELECTRIC POLYMER

Abstract

The present invention relates to a method for chemically adhering a diene rubber to a piezoelectric polymer, the method comprising the steps of: a) providing a piezoelectric polymer having at least one surface, b) providing a rubber component having at least one surface and comprising a sulfur cross-linkable rubber composition, c) introducing oxygen-containing functional groups on the at least one surface of the piezoelectric polymer, d) reacting the oxygen-containing functional groups with a compound comprising thiocyanate groups, and e) contacting the surface of the piezoelectric polymer obtained of step d) with the surface of the rubber component, and cross-linking.

Claims

1. A method for chemically adhering a diene rubber to a piezoelectric polymer, the method comprising the steps of: a) providing a piezoelectric polymer having at least one surface, b) providing a rubber component having at least one surface and comprising a sulfur cross-linkable rubber composition, c) introducing oxygen-containing functional groups on the at least one surface of the piezoelectric polymer, d) reacting the oxygen-containing functional groups with a compound comprising thiocyanate groups, and e) contacting the surface of the piezoelectric polymer obtained of step d) with the surface of the rubber component, and cross-linking.

2. The method according to claim 1, wherein oxygen-containing functional groups are provided by treatment with oxygen plasma.

3. The method according to claim 1, wherein the reacting the oxygen-containing functional groups in step d) is a silanisation of an oxygen-treated surface with a silane comprising thiocyanate groups and ethoxy and/or methoxy groups.

4. The method according to claim 1, wherein the silane comprising thiocyanate groups and ethoxy and/or methoxy groups is selected from the group of trimethoxy(3-thiocyanatopropyl)silane or triethoxy(3-thiocyanatopropyl)silane and mixtures thereof.

5. The method according to claim 3, wherein the the silanisation is carried out for a time period in a range of ≥1 minute to ≤2 days, preferably a range of ≥23 hours to ≤25 hours.

6. The method according to claim 1, wherein the piezoelectric polymer is selected from the group of polyvinyldiene fluoride (PVDF) polymer or co-polymers selected from polyvinyldiene fluoride (PVDF), polyvinylidene fluoride-trifluoroethylene (P(VDF-co-TrFE)) copolymer, poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)), poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)), or a polyamide, liquid crystal polymer or poly(p-xylylene), and mixtures of these polymers.

7. The method according to claim 1, wherein the rubber is a diene rubber selected from the group of natural rubber (NR), isoprene rubber (IR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR) or a mixture thereof.

8. The method according to claim 1, wherein the rubber composition is an electrically conductive rubber composition comprising a conductive component selected from the group of carbon black, single-wall carbon nanotubes or multi-wall carbon nanotubes), graphene or a mixture thereof.

9. The method according to claim 8, wherein the electrically conductive rubber composition comprises single-wall carbon nanotubes in a range of ≥0.01 wt% to ≤2 wt%, preferably a range of ≥0.2 wt% to ≤0.6 wt%, based on a total weight of 100 wt% of the sulfur cross-linkable rubber composition.

10. The method according to claim 1, wherein the method is for producing a piezoelectric device, comprising the steps of: a) providing a piezoelectric polymer having first and second opposing surfaces, b) providing a first and a second rubber component each having at least one surface and each comprising a sulfur cross-linkable electrically conductive rubber composition, c) introducing oxygen-containing functional groups on the first and second surface of the piezoelectric polymer, d) reacting the oxygen-containing functional groups with a compound comprising thiocyanate groups, and e) contacting the first and second opposing surfaces of the piezoelectric polymer obtained of step d) with the surfaces of the first and second rubber component each comprising a sulfur cross-linkable electrically conductive rubber composition, whereby the piezoelectric polymer is sandwiched between the sulfur cross-linkable rubber surfaces, and cross-linking.

11. The method according to claim 10, wherein the piezoelectric polymer and the first and second rubber component comprising a sulfur cross-linkable electrically conductive rubber composition each have the form of a layer, and wherein in step e) the piezoelectric polymer layer is sandwiched between the conductive rubber layers, and cross-linking results in a three-layered piezoelectric device, wherein the three-layered piezoelectric device is attached using an adhesive to an inner liner of a tyre.

12. The method according to claim 10, wherein the piezoelectric polymer and the first rubber component comprising a sulfur cross-linkable electrically conductive rubber composition have the form of a layer and the second rubber component is a rubber component comprising a sulfur cross-linkable electrically conductive rubber composition of a tyre.

13. The method according to claim 10, wherein the piezoelectric polymer and first and second rubber component each have the form of a layer, and wherein the second rubber component has a second surface contacting an electrically conductive or non-conductive rubber component of a tyre.

14. The method according to claim 12, wherein the rubber component of the tyre is an inner liner.

15. A tyre comprising a piezoelectric device, obtained by the method according to claim 1.

Description

EXAMPLES

[0044] The invention will be further described with reference to the following examples and figures without wishing to be limited by them.

[0045] FIG. 1 shows the XPS spectra of pure PVDF film, and PVDF films after oxygen plasma treatment and silanization by thiocyanate-silane.

[0046] FIG. 2 shows a peel test of cured samples prepared from silanized PVDF/conductive rubber and non-silanized PVDF/conductive rubber.

[0047] FIG. 3 shows the result of a fatigue test of the piezoelectric patch in a sandwich configuration of PVDF/C.sub.ref using a time sweep analysis with a double shear mode at 2% dynamic strain.

[0048] Example 1: Preparation of piezoelectric devices

[0049] Step 1.1 Providing a piezoelectric polymer

[0050] The piezoelectric polymer PVDF was supplied from PolyK Technologies LLC, USA, in a form of thin film with A4 in size and a thickness of 100 μm. The electrical and chemical properties of this polymer film are shown in the following Table 1.

TABLE-US-00001 TABLE 1 Properties of the PVDF film as provided by the supplier PolyK Technologies LLC: PVDF film with 100 μm thickness Properties Value Charge coefficient, d.sub.31 30 pC/N Charge coefficient, d.sub.33 −30 pC/N Melting Temperature 170-175° C. Tensile strength*  400-600 MPa Young Modulus* 2300 MPa Elongation at break* 20-30%

[0051] Step 1.2 Providing rubber components having at least one surface and comprising a sulfur cross-linkable rubber composition

[0052] Non vulcanized rubber sheets of four different conductive rubber compositions C.sub.ref, C.sub.1, C.sub.2 and C.sub.3, were prepared according to the table 2 below:

TABLE-US-00002 TABLE 2 components of conductive rubber compositions C.sub.ref, C.sub.1, C.sub.2 and C.sub.3 C.sub.ref C.sub.1 C.sub.2 C.sub.3 amount amount amount amount Component: (phr) (phr) (phr) (phr) NR 25 25 25 25 SBR 25 25 25 25 BR 50 50 50 50 Filler 60 60 60 60 Processing oil 14 14 14 14 Curing agent 10 10 10 10 Anti Degradation agent 7 7 7 7 SWCNT/TDAE (10 wt % 0 4 8 12 SWCNT in TDAE)

[0053] The NR rubber used was TSR 20 grade.

[0054] The SBR rubber used was SBR 1502.

[0055] The BR rubber used was a Ni catalyzed Butadiene Rubber.

[0056] The filler used was carbon black N 330.

[0057] Abbreviations used are: TDAE (treated distillate aromatic extract; processing oil); SWCNT (Single Wall Carbon Nanotubes.

[0058] The mixture denoted C.sub.ref comprised carbon black as conductive component. In the three samples denoted C.sub.1, C.sub.2 and C.sub.3, respectively, 2, 4 and 6 wt% of a paste of highly conductive nano-fillers comprising 10 wt% of Single Wall Carbon Nanotubes (SWCNT) in a low aromatic plasticizer (TDAE) was added to the mixture C.sub.ref via a two roll-mill to improve conductivity. For good dispersion of the nano-fillers, the compound was passed and rolled in the mills for ten times.

[0059] Step 1.3: introducing oxygen-containing functional groups on the surfaces of the piezoelectric polymer

[0060] The PVDF surface was cleaned by chloroform (99.5%, Sigma-Aldrich, St. Louis, Mo.) before treatment with oxygen plasma. The cleaned PVDF film was treated with an oxygen plasma treatment using a Plasma-Prep II (SPI Supplies, West Chester, USA) that contained a plasma vacuum chamber, in which the PVDF film was placed. A mechanical vacuum pump (Oerlikon, Lafert S.p.A., Italy) reduced the pressure inside the chamber to around 100-200 mTorr. At the optimal pressure, oxygen gas was pumped into the chamber. RF power at 13.56 MHz was applied to the chamber. This excited and charged the oxygen molecules and therefore created the oxygen plasma radicals ready to react on the PVDF surface. The PVDF film was continually treated for 15 minutes at room temperature (20±5° C.).

##STR00001##

[0061] Step 1.4: Silanisation of the oxygen-treated surface with a silane comprising thiocyanate groups and ethoxy and/or methoxy groups

[0062] After oxygen treatment the PVDF was silanised with a thiocyanate based silane, namely 3-thiocyanatopropyltriethoxysilane (Si-264, Evonik Industries AG, Germany).

##STR00002##

[0063] The physical and chemical properties of Si-264 as provided by the supplier Evonik Industries AG are summarised in Table 3 below.

TABLE-US-00003 TABLE 3 Physical and chemical properties of Si-264. Si 264 Sulphur Content 12.5% Average molecular weight 263 g/mol Density 1.00 g/cm.sup.3

[0064] For the silanization procedure, the PVDF film and silane S-264 were introduced into a desiccator under a vacuum atmosphere at room temperature for 24 hours. Inside the desiccator, the film was fixated with a holder to fully expose the surface of the film to silane reactive vapour. 3 ml of Si-264 silane was kept into a small Petri dish and placed close to the film.

[0065] The ethoxy groups of the silane reacted with the active functionalities on PVDF surface with released ethanol as a by-product. Another end group was thiocyanate group introducing sulphur atom to the surface of the film which can react with rubber molecules during vulcanization, expected to give chemical bonds or strong adhesion to the interfaces as shown in scheme 3 below.

##STR00003##

[0066] Step 1.5: Contacting the oxygen-treated and silanised surface of the piezoelectric polymer with the surface of the rubber components, and cross-linking vice

[0067] To prepare piezoelectric devices, the silanized PVDF film of step 1.4 was cured together with the layers of conductive rubber compound of step 1.2. The vulcanization behaviour of the compounds was analysed using a Rubber Process Analysis (RPA 2000, Alpha Technologies, USA). The optimal cure time, i.e. the time, when the cure torque reaches 90% of the maximum cure state, known as t.sub.c90, was taken as an input for the vulcanization step. The samples were cured at 100 bar in a Wickert press (Landau in der Pfalz, Germany) to the measured optimal cure time and temperature.

[0068] Four piezoelectric patches were fabricated in a sandwich configuration with the 0.1 mm thick PVDF film inserted in between two sheets of conductive rubber compounds C.sub.ref, and C.sub.1, C.sub.2 and C.sub.3, respectively, and cured. The resulting cylindrical specimen had a sandwich-like configuration with a diameter of 10 mm and a thickness of 4,1 mm. In the following, the resulting piezoelectric devices are denoted C.sub.ref, C.sub.1, C.sub.2 and C.sub.3 according to the respective conductive compounds. The chemical bonds, adhesive strength and bond stability between the two components were investigated using an X-ray photoelectron spectroscopy (XPS), a T-Peel test using a universal tensile testing machine, respectively.

[0069] Example 2: Determination of the surface functionalisation of the PVDF surface

[0070] In order to confirm the chemical modification on the PVDF surface, XPS spectroscopy was used to analyze the surface chemistry. Chemical components on the surface of PVDF film before treatment, after oxygen plasma treatment and after silane-treatment were analysed using an X-ray photoelectron spectroscopy or XPS (Quantera SXM, USA). The XPS measurements were performed before and after surface treatment with oxygen plasma and after salinization with thiocyanate silane with two different reaction times, i.e. 45 minutes and 24 hours. The PVDF films were irradiated with a monochromatic x-ray beam, i.e. A1 with Kα=1486.6 eV, and a spot size of 100 μm. Each spectrum was compared with the spectra of the pure PVDF without treatment that was washed and cleaned with chloroform before analysis. All samples were analysed on four different points to compare the peak area and quantify the standard deviation of the values.

[0071] FIG. 1 shows the spectra of pure PVDF, O2-treated PVDF and silanized PVDF using two different silanization times, i.e. 45 min and 24 hours. Table 4 summarises the peak area of the specific elements with standard deviations from each spectrum.

TABLE-US-00004 TABLE 4 Integral peak area of elements on the surface of PVDF films derived from XPS analysis. C 1s N 1s O 1s F 1s Si 2p Peak Peak Peak Peak Peak Sample Area Area Area Area Area PVDF pure 54.20 1.35 8.66 33.24 2.05 PVDF/O.sub.2- plasma 48.54 5.01 11.32 30.37 4.42 PVDF/Si 264 49.69 2.01 13.19 29.09 4.56 (45 min) PVDF/Si 264 77.75 2.85 10.97 1.11 4.31 (24 h)

[0072] As can be taken from the FIG. 1, the PVDF film purified with chloroform showed an outstanding peak of fluorine (F) at 688 eV with an integral area of 33.2, i.e. 33.2 atomic percent, and two peaks of carbon (C) at 286 to 290 eV with a peak area of 54.2, i.e. an amount of 54.2 atomic percent. The spectrum of pure PVDF showed a small peak of oxygen (O) at 533 eV, but the spectrum of O.sub.2-treated PVDF showed an increased concentration of oxygen on the surface, the integral area and thus the elemental composition was increased from 8.7 to 11.3. Due to increasing the silanization time from 45 minutes to 24 hours, the amount of F remarkably decreased from 29.09 to 1.11 atomic percent. This confirms that the fluorine atoms were replaced and covered by silane molecules, giving the silane-modified surface of PVDF film.

[0073] It is assumed that the ethoxy groups of the silane reacted with the active functionalities on the PVDF surface, while the thiocyanate group which introduced sulphur atom to the surface of the film, was able to react with rubber molecules during vulcanization.

[0074] Example 3: Determination of adhesive strength between PVDF and conductive rubber

[0075] Adhesive strength between the PVDF film and the elastomeric compounds was investigated using a T-peel test. The adhesion test performed was according to ASTM D413 test standard, which is, in particular, for the application of rubber property-adhesion to flexible substrate. The measurement was carried out using a tensile testing machine (Zwick Z1.0, Zwick/Roell Nederland, the Netherlands), following the test standard described in ISO 5893.

[0076] The composition C.sub.refwas used for the T-peel test study and a comparison between test specimen of cured two-layered samples of non-silanized PVDF/conductive rubber C.sub.refand silanized PVDF/conductive rubber C.sub.refwas made. Five samples were tested and the data were averaged. FIG. 2 shows the forces used to peel the two components of the respective specimen apart. As can be taken from the FIG. 2, the results show that surface modification of the PVDF film by silane significantly improves the adhesion between PVDF and the conductive rubber compound.

[0077] Example 4: Determination of the durability of the adhesion between PVDF and conductive rubber

[0078] Durability of the piezoelectric device is very important since a tyre will be used for several years. Thus, the piezoelectric device needs to have good durability under the dynamic mechanical conditions in a pneumatic tyre. Since the piezo-patch PVDF/C.sub.refhas to withstand the rolling conditions of tyres, a time sweep analysis DMA (Dynamic Mechanical Analysis) technique with double shear deformation mode was carried out. The dynamic mechanical conditions were measured according to ASTM D5418-99. A sandwich patch was prepared into the double shear sample and tested with a DMA Eplexor9(Netzsch Gabo Instruments GmbH, Ahlden, Germany). With this mode, two piezoelectric sandwich patches (10 mm width and 4 mm length) were glued using a cyanoacrylate adhesive (Sicomet 7000, Henkel AG, Germany) to three metal cylinders. After that, the sample was clamped horizontally in the DMA and the double shear mode test was performed to check the durability.

[0079] FIG. 3 shows the results from durability test of the piezoelectric sample under a dynamic mechanical condition with a double shear mode technique.

[0080] As can be taken from the FIG. 3, owing to a constant dynamic strain applied during the measurement, a constant value of the dynamic force was observed over the testing time. This shows that the adhesion between the PVDF film and the conductive rubber compound C.sub.refwas rather stable. This confirms that the adhesion between the components of the piezoelectric patch is reliable.