WO2021137859A1

WO2021137859A1 - Low friction graphene rubber composition

Info

Publication number: WO2021137859A1
Application number: PCT/US2019/069076
Authority: WO
Inventors: Frederic Vautard
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2021-07-08

Abstract

The subject matter of the present invention relates to a rubber mix having decreased frictional coefficient comprised of a graphene filler having a platelet lateral size of 0.2 microns to 120 microns and a volume graphene filler concentration measured as a percent volume concentration of the total rubber composition equal to or greater than 13 percent for platelet lateral sizes of 0.2 microns up to but not including 0.4 microns, 10 percent for platelet lateral sizes of 0.4 microns up to but not including 3 microns, and 1 percent for platelet lateral sizes of 3 microns or greater.

Description

LOW FRICTION GRAPHENE RUBBER COMPOSITION

FIELD OF THE INVENTION

[0001] The subject matter of the present invention relates to a rubber mix having a low coefficient of friction. In particular, the subject matter of the present invention relates to a rubber composition having a decreased coefficient of friction under wet conditions when compared to silica or carbon black-based rubber compositions having the same rigidity.

BACKGROUND OF THE INVENTION

[0002] There is a need for a rubber composition having low friction and low adherence. A rubber composition having low friction and low adherence could be used to construct conveyor belts, high performance sealants, O-rings, gaskets, bushings, packings, washers or seals.

[0003] Rubber compositions contain elastomer chains interspersed with non-elastomer fillers. The non-elastomer fillers are added to alter the physical and chemical properties of the rubber composition, such as, for example, altering the electrical or thermal conductivity, friction, tear resistance or increase resistance to degradation by chemicals and/or light. A common rubber filler is carbon black, used to enhance the rubber’s friction, wear resistance, and resistance to degradation by UV radiation.

[0004] Graphene is a relatively newly explored form of carbon, most notably characterized by having a single layer of atoms in a two-dimensional hexagonal lattice.

This form can typically be produced by Chemical Vapor Deposition. The term “graphene” has been also been used both in the academia and the industry to describe a group of materials derived from graphite that are technically not graphene particles, as described above, as they usually are particles made of several layers with defects in their carbon structure. As used herein, the term “graphene” reflects the broader definition of materials and includes graphene oxide, reduced graphene oxide, and particles of graphite or expanded and exfoliated graphite down-sized to nano scale by milling (and particularly ball-milling).

[0005] Reduced Graphene Oxides (“RGO”) are made of graphite heavily damaged by an oxidation/reduction process. The final carbon structure is made of areas of aromatic/crystalline structure separated by amorphous areas. This results in very undulated/wrinkled platelets because of the defects in the carbon structure (amorphous areas, and oxygen-containing functional groups as well as 5-7 carbon rings in the aromatic structure). These structures are described in an academic paper “Atomic Structure of Reduced Graphene Oxide” by C. Gomez-Navarro (Nanoletters, 2010). A picture obtained by Scanning Electron Microscopy(“SEM”)of a RGO platelet can be seen in FIG 1.

[0006] Exfoliated Graphite NanoPlatelets (“GNP”) possess an aromatic/crystalline structure which is more prominent than in RGO, as GNP particles are exfoliated without the damaging effects of oxidation. In contrast with RGO, the platelets of GNP particles are very straight, with sharp angles on their edge as shown in the SEM picture of FIG. 2. [0007] The density of defects in the carbon structure leading to the different platelet shapes can be measured by Raman spectroscopy with the ID/IG ratio, ID and IG being the intensity of the peaks located at 1360 cm^-1 and 1580 cm -¹, respectively. The two peaks basically correspond to the defects in the carbon structure (ID) and to the crystalline part (IG). RGO typically led to values of ID/IG superior to 0.8 and GNP showed values below 0.5. This was described in detail in PCT patent application publication WO 2019133442 Al.

[0008] When added as a filler to a rubber composition, graphene platelets have been found to increase the electrical conductivity of the rubber as compared to carbon black. Graphene platelets have also been found to increase the rubber compositions thermal conductivity, making graphene an interesting filler for use in rubber compositions in products where electrical conductivity and thermal conductivity are desired.

[0009] When added as a filler to a rubber composition, graphene platelets have been found to increase adherence and friction. For example, Gratomic Inc. of Toronto, Ontario, Canada state that graphene improves traction in bike tires. (Website: “https://gratomic.ca/graphene-tires” accessed December 30, 2019). When used in automotive tires, Gratomic Inc. claims improved braking distances by 40% compared to “ premium tires.” (Website: “https://www.european-rubber-joumal.com/news/technology- gratomic-graphene-tires-outperform-household-brand-tires" accessed December 30 , 2019). [0010] Directa Plus (Como, Italy), a manufacturer of GNP, disclosed in a patent application on silica/GNP hybrid rubber composites for tire treads, Patent Publication US 2018/0215904 Al, a finding of improved friction with the addition of graphene particles. The platelets reportedly had a lateral size between 0.05 and 50 μm. The wet grip was measured with bicycle tires and shows an improvement when GNP is added to silica, claiming the improvement in wet grip is due to the GNP. The GNP used in that specific example has a lateral size comprised between 10 and 15 μm.

[0011] In an academic article “Characterization and Application of Graphene Nanoplatelets in Elastomers” by Kliippel et al., Advanced Polymer Science 275 (2017) 319-360, reports similar frictional performance properties of rubber mixes containing GNP versus carbon black. The authors of this study compare wet friction behavior for both compounds and conclude on pages 354 and 355 of the study that "wet friction behavior for both compounds exhibits no major differences” between carbon black and GNP filled rubber.

[0012] While increased friction and adhesion has particular advantages for certain rubber products, rubber compositions having reduced friction and reduced adhesion would be particularly useful in other rubber products. Indeed, there is a need for a rubber composition having low friction and low adherence. A rubber composition having low friction and low adherence could be used to construct conveyor belts, high performance sealants, O-rings, gaskets, bushings, packings, washers or seals where friction and adhesion would be detrimental to product performance.

SUMMARY OF THE INVENTION

[0013] Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

[0014] In one exemplary embodiment, a rubber composition having low friction properties is provided having a graphene filler. The rubber composition exhibited lower friction properties as the graphene particle size increased and/or graphene particle volume increased. Particularly the rubber composition is comprised of graphene filler having a platelet lateral size of 0.2 microns to 120 microns and a volume graphene filler concentration measured as a percent volume concentration of the total rubber composition equal to or greater than 13 percent for platelet lateral sizes of 0.2 microns up to but not including 0.4 microns, 10 percent for platelet lateral sizes of 0.4 microns up to but not including 3 microns, and 1 percent for platelet lateral sizes of 3 microns or greater.

[0015] The wet adherence of all such graphene-based rubber products was significantly below the one corresponding to carbon black-based rubber products, and therefore also significantly below the one corresponding to silica-based rubber products, at iso-rigidity. [0016] Among the different graphene products, large GNP, when incorporated at relatively high concentrations, starting at 13 vol. % for the larger ones, led to very low wet adherence.

[0017] It was also found that there was a concentration effect of the filler. GNP with a smaller lateral size can lead to a similar decrease of wet mu obtained with larger lateral size GNP if their concentration is higher.

[0018] Such a rubber composition having a low friction property could be advantageous in the case of applications for which anti-adherence/low friction is targeted including tire components, conveyor belts, high performance sealants such as O-rings, and structural parts containing rubber in motion and rubbing against harder parts, like arm- bushings in a suspension system, etc.

[0019] These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS [0020] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

[0021] FIG. 1 provides a SEM image of an RGO N002-PDR material from Angstron Materials (now Global Graphene Group).

[0022] FIG. 2 provides a SEM image of a GNP XGnP-M-15 material from XG Sciences.

[0023] FIG. 3 shows the wet traction performance of the rubber composite comprising the corresponding filler assessed by a proprietary wet traction test method. G*(60°C) = 1 MPa.

[0024] FIG. 4 shows the wet traction performance of the rubber composite comprising the corresponding filler assessed by a proprietary wet traction test method. G*(60°C) = 2 MPa.

[0025] The use of identical or similar reference numerals in different figures denotes identical or similar features. DETAILED DESCRIPTION OF THE INVENTION [0026] The present invention provides a rubber composition having a graphene filler exhibiting reduced friction and adhesion properties. For purposes of describing the invention, reference now will be made in detail to embodiments and/or methods of the invention, one or more examples of which are illustrated in or with the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features or steps illustrated or described as part of one embodiment, can be used with another embodiment or steps to yield a still further embodiments or methods. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0027] As used herein, "phr" is “parts per hundred parts of rubber by weight” and is a common measurement in the art wherein components of a rubber composition are measured relative to the total weight of rubber in the composition, i.e. parts by weight of the component per 100 parts by weight of the total rubber(s) in the composition.

[0028] As used herein, elastomer and rubber are synonymous terms.

[0029] As used herein, “based upon” is a term recognizing that embodiments of the present invention are made of vulcanized or cured rubber compositions that were, at the time of their assembly, uncured. The cured rubber composition is therefore “based upon” the uncured rubber composition. In other words, the cross-linked rubber composition is based upon or comprises the constituents of the cross-linkable rubber composition.

[0030] Reference will now be made in detail to embodiments of the invention. Each example is provided by way of explanation of the invention. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield still a third embodiment. It is intended that the present invention includes these and other modifications and variations.

[0031] In order to assess wet adherence performance, a proprietary method is used. It is a confidential laboratory wet traction evaluation method which has been found to correlate with the wet traction performance of rubber tires on a road surface.

[0032] It is usual to relate the wet traction performance to the energy dissipation measured during a temperature sweep of a Dynamic Mechanical Analysis (DMA) test. For the purposes of the disclosure of the invention set forth herein, we used the area under the tan delta peak comprised between -40°C and +50°C.

[0033] For comparison of the rubber compositions, the concentration in filler were adjusted so that the same level of rigidity is achieved for all rubber composites. That is, typically, the same values of G*(60°C) are obtained for the rubber compositions compared. This is done because rigidity itself will influence the level of adherence. Because the different graphene fillers reported here are more or less rigidifying, two levels of G*(60°C) were targeted: 1 MPa and 2 MPa. The rigidity of the composition is a major factor having an influence on the level of friction/adherence between the rubber mix and the surface and it is important to account for this affect when assessing frictional and adherence properties of rubber compositions.

[0034] Example 1 :

[0035] Several rubber compositions were constructed with a target modulus of G*(60°C) = 1 MPa. Each of the rubber compositions contained solution SBR at 100 phr, Zinc Oxide at 2 phr, steric acid (“SAD”) at 1.2 phr, N'-phenyl-p-phenylenediamine (“6PPD”) at 2 phr, sulfur at 1.5 phr and N-cyclohexyl-2- benzothiazole sulfenamide (“CBS”) at 1.5 phr and either a carbon black filler or a graphene filler in an amount sufficient to obtain the target modulus. Three types of RGO were used: RGO N002-PDR and RGO N002-PDE, both manufactured by Angstron Materials (now Global Graphene Group, Dayton, OH), and RGO Vor-X manufactured by Vorbeck Materials, Jessup, MD. RGO being very rigidifying, only a low concentration was necessary (1 vol. %). Four types of GNP were used: GNP 4119 manufactured by Asbury Carbons, Asbury, NJ, XGnP- M-5, XGnP-M-25, and XGnP-R-100, each manufactured by XG Sciences, Lansing, MI. Typically the small GNPs (4119 reference, 1 pm in diameter) are less rigidifying than the larger ones (5 and 25 pm in diameter for XGnP-M-5 and XGnP-M-25, respectively), so larger concentrations were necessary (10 vol. % versus 5 vol. %). One exception is related to the very large platelets (100 pm in diameter) which result in low rigidity (13 vol. % was necessary to obtain 1 MPa). The carbon black (CB) reference was N234. Graphene fillers were added at the concentrations given in Table 1.

[0036] The rubber formulations were prepared by mixing the components, except for the sulphur and the accelerator, in a Banbury mixer operating between 30 and 90 RPM until a temperature between 130 degrees Celsius and 165 degrees Celsius was reached. The accelerator and sulfur were added in the second phase on a mill. [0037] The characteristics of the filler, the wet traction performance of the corresponding rubber composite, as well as the associated tan delta indicator are displayed in Figure 3 and Table 1.

Table 1: Filler properties, wet traction and energy dissipation associated to the corresponding rubber composites.

[0038] The wet traction performance of all “graphene rubber composites was systematically below the wet traction of a carbon black N234-based rubber mix of equivalent rigidity (value of m coefficient at 25°C-Table 1). This is a surprising result given literatures support of graphene generally having an increasing effect on wet traction. The very large platelets (XGnP-R-100) led to even much lower wet traction. This is an unexpected result considering that the DMA indicator (tan delta-Table 1) does not match the evolution of m.

[0039] Example 2.

[0040] In this second example, several rubber compositions were constructed with a target modulus of G*(60°C) = 2 MPa. Each of the rubber compositions contained solution SBR at 100 phr, Zinc Oxide at 2 phr, SAD at 1.2 phr, 6PPD at 2 phr, sulfur at 1.5 phr and CBS at 1.5 phr and either a carbon black filler or a graphene filler in an amount sufficient to obtain the target modulus. Three types of RGO were used: RGO N002-PDR and RGO N002-PDE, both manufactured by Angstron Materials (now Global Graphene Group, Dayton, OH), and RGO Vor-X manufactured by Vorbeck Materials, Jessup, MD. RGO being very rigidifying, only a low concentration was necessary (2 to 4 vol. %). Three types of GNP were used: GNP 4124 manufactured by Asbury Carbons, Asbury, NJ, XGnP-C- 300 and XGnP-M-15, each manufactured by XG Sciences, Lansing, MI. Their concentration was between 13 and 15 vol.%. The carbon black (CB) reference was N234. Graphene fillers were added at the concentrations given in Table 2.

[0041] The rubber formulations were prepared by mixing the components, except for the sulphur and the accelerator, in a Banbury mixer operating between 30 and 90 RPM until a temperature between 130 degrees Celsius and 165 degrees Celsius was reached. The accelerator and sulfur were added in the second phase on a mill.

Table 2: Filler properties, wet traction and energy dissipation associated to corresponding rubber composites.

[0042] XGnP-M-25 and XGnP-R-100, used in the first group of samples, cannot give a rigidity of 2 MPa with a concentration of 15 vol. %, which was the limit considered here (to keep the cohesion of the green rubber to a practical level). XGnP-M-15 platelets were used, and they were the largest platelets leading to the right rigidity value. Based on the m coefficient value obtained with XGnP-M-5 and XGnP-M-25 at 5 vol. %, we can reasonably consider that the m coefficient would be 0.52 with XGnP-M-15 at 5 vol. % (Table 1). Regarding the small exfoliated graphite nanoplatelets (diameter inferior to 1 pm), a higher surface area was needed to achieve 2 MPa. As mentioned before, RGO being very rigidifying, small concentration were necessary (between 2 and 4 vol. %). But the target in rigidity was high enough to show that the level of rigidification was different depending on the grade of RGO.

[0043] Based on the m coefficient values, an increase in RGO (PDR, PDE, Vor-X) concentration leads to higher wet traction (compare Tables 1 and 2). The same trend was observed with small exfoliated graphite nanoplatelets (4124, XGnP-C-300).

[0044] On the contrary, large GNPs (XGnP-M-15) led to lower values of m coefficient at higher concentration (therefore lower wet traction performance). The evolution of wet adherence in function of filler concentration is different based on the nature of the filler (RGO, small GNP, large GNP). This is important information to claim novelty.

[0045] All “graphene-like” fillers led to lower wet traction performance compared to carbon black N234 when formulated at iso-rigidity as shown in FIG.4.

[0046] The general conclusion is that large GnP platelets, that is, with a diameter larger than 15 pm, lead to low wet adherence, and that phenomenon is amplified with high concentrations.

[0047] Selected combinations of aspects of the disclosed technology correspond to a plurality of different embodiments of the present invention. It should be noted that each of the exemplary embodiments presented and discussed herein should not insinuate limitations of the present subject matter. Features or steps illustrated or described as part of one embodiment may be used in combination with aspects of another embodiment to yield yet further embodiments. Additionally, certain features may be interchanged with similar devices or features not expressly mentioned which perform the same or similar function. [0048] The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.” Also, the dimensions and values disclosed herein are not limited to a specified unit of measurement. For example, dimensions expressed in English units are understood to include equivalent dimensions in metric and other units (e.g., a dimension disclosed as “1 inch” is intended to mean an equivalent dimension of “2.5 cm”).

[0049] As used herein, the term “method” or “process” refers to one or more steps that may be performed in other ordering than shown without departing from the scope of the presently disclosed invention. As used herein, the term "method" or "process" may include one or more steps performed at least by one electronic or computer-based apparatus. Any sequence of steps is exemplary and is not intended to limit methods described herein to any particular sequence, nor is it intended to preclude adding steps, omitting steps, repeating steps, or performing steps simultaneously. As used herein, the term "method" or "process" may include one or more steps performed at least by one electronic or computer-based apparatus having a processor for executing instructions that carry out the steps.

[0050] The terms "a," "an," and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. The terms "at least one" and "one or more" are used interchangeably. Ranges that are described as being "between a and b" are inclusive of the values for "a" and "b." [0051] The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.

Claims

WHAT IS CLAIMED IS:

1. A rubber composition based on at least one diene elastomer, comprising: a diene elastomer; a crosslinking system; a filler comprising of graphene platelets having: a platelet lateral size of 0.2 microns to 120 microns; and a volume concentration measured as a percent volume concentration of the total rubber composition equal to or greater than:

13 percent for platelet lateral sizes of 0.2 microns up to but not including 0.4 microns;

10 percent for platelet lateral sizes of 0.4 microns up to but not including 3 microns;

1 percent for platelet lateral sizes of 3 microns or greater.

2. The rubber composition of claim 1 wherein the filler of graphene platelets have a lateral size of 0.2 microns to 80 microns.

3. The rubber composition of any of the above claims wherein the diene elastomer is selected from the group consisting of polybutadienes, synthetic polyisoprenes, natural rubber, isoprene copolymers, butyl rubber, butadiene copolymers including styrene- butadiene and mixtures of these elastomers.

4. The rubber composition of any of the above claims wherein the crosslinking system is a sulfur.

5. The rubber composition of any of the above claims further comprising of an anti- degradant.

6. The rubber composition of claim 5 wherein the anti-degradant is comprised of 6PPD and zinc oxide

7. The rubber composition of any of the above claims, wherein the diene elastomer is a styrene butadiene rubber.

8. The rubber composition of claim 7 wherein the styrene butadiene rubber is obtained by solvent polymerization.

9. The rubber composition of any of the above claims wherein the graphene platelets are Reduced Graphene Oxide platelets.

10. The rubber composition of any of claims 1-8 wherein the graphene platelets are exfoliated graphite nanoplatelets.