DESC:FIELD OF INVENTION
The current invention generally relates to methods for coating a substrate with graphene-based nanocomposites, and in particular to methods for coating ultrahigh molecular weight polyethylene-graphene nanocomposites on a substrate to enhance ballistic performance, abrasion resistance, cut resistance and/or stab resistance of the substrate.

BACKGROUND
Polymer compositions are being increasingly used in a wide range of areas that have traditionally employed the use of other materials. The development of lightweight, high strength, and high modulus polymer fibres has introduced a new era of soft body, stab and puncture resistant materials that has obsoleted conventional forms of protection offered by metal-based materials.

Typically, high strength fibres like Aramid (Kevlar®) and Ultrahigh molecular weight polyethylene (Spectra® and Dyneema®) are used in the manufacture of fabric-based body armor to provide ballistic resistance, as these fibres possess high strength, high energy absorption, and lightweight properties. However, they do not meet the standards against stab attacks. The performance of these fabric-based body armors can be further enhanced by providing a composite coating on the surface.

Graphene is a substance composed of pure carbon in which atoms are positioned in a hexagonal pattern in a densely packed one-atom thick sheet. In relation to its thickness, it is about 100 times stronger than the strongest steel. Yet its density is dramatically lower than any steel, with a surfacic mass of 0.763 mg per square meter.

UHMWPE has good mechanical, physical and tribological properties, such as: extreme hardness and durability, good chemical resistance, abrasion resistance, impact resistance, being easy to fabricate and a very low coefficient of friction. UHMWPE-graphene composites are being developed for many applications, such as aerospace, industrial, and biomedical applications.

Existing methods for manufacturing UHMWPE-graphene nanocomposites for coating applications suffer from various drawbacks, namely melting of the polymer material at high temperature; damage of the substrate to be coated and uneven deposition. Non-uniform coating of the substrate result in poor performance of the coated substrate. Further, when composites based on ceramic materials, such as SiC, is desired, high temperatures are required that can only be provided by propylene or acetylene flames, which requires special equipment and complicated procedures. Therefore, there is a need in the market for a method for coating UHMWPE-graphene nanocomposites that overcome some of these shortcomings.

SUMMARY
Embodiments of the present invention provide methods for coating a substrate with a UHMW polyethylene-graphene nanocomposite comprising blending in solid form UHMW polyethylene and a graphene component to form a UHMWPE-graphene composite blend. The composite blend is sieved to a particle size in the range of about 100 microns to about 200 microns to form a coating mixture. The coating mixture is then supplied through a thermal spray apparatus so as to deposit UHMW polyethylene-graphene nanocomposite on the substrate to form the coating. In one embodiment, UHMWPE has a molecular weight in the range of 3,00,000 to 8,00,000 AU. In another embodiment, the graphene component is present in a range of about 1% to about 20% by weight in the UHMWPE-graphene composite blend.

The method of the present invention, in one embodiment, comprises blending UHMWPE and the graphene component at 40 to 100 rpm for a time in the range of 1 to 4 hours and at temperature less than or equal to about 80 ?. In one embodiment, the graphene component is graphene selected from few layered graphene (FLG), graphene nano sheets, ammonia-functionalized graphene, carboxylic acid-functionalized graphene, argon-functionalized graphene and any combinations thereof. In another embodiment, the graphene component further comprises silicon carbide (SiC) in the form of SiC fibres or SiC microfibres.

In one embodiment, supplying the coating mixture comprises supplying the coating mixture through a thermal spray apparatus at a flow rate of 0.5 to 3 kilograms per hour and operating the thermal spray apparatus at a temperature in the range of about 100 ? to 300 ?.

According to embodiments of the present invention, the substrate to be coated with UHMWPE-graphene nanocomposites is selected from a group consisting of aramid fabrics, nylon fabrics, carbon fibre fabrics, fibre glass fabrics and UHMWPE fabrics can be coated with UHMWPE-graphene nanocomposites.

DETAILED DESCRIPTION
The following description and examples illustrate some exemplary embodiments of the disclosed invention in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present invention.

The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.

As used herein, the term “ballistic” refers to ability of the material to stop or mitigate the impact of projectile and/or bullet.

The term, “cut resistance”, as used herein refers to material property defined by the amount of energy or force required to cut through a material using a moving blade.

As used herein, the term “abrasion resistance” refers to the ability of a material to withstand abrasion.

The term “stab resistance” as used herein refers to resistance of the material to stabbing using sharp objects such as knives.

The term “nanocomposites” as used herein, refers to nanosized particles of graphene incorporated into a matrix of UHMWPE. The terms “nanocomposites” and “composites” have been used interchangeably and it is to be understood that the usage of composites with reference to graphene implies graphene nanocomposites. It is to be appreciated that the addition of nanosized graphene particles results in drastic improvement in properties of the polymer matrix that can include mechanical strength, toughness, electrical and/or thermal conductivity.

The present invention provides methods for coating a substrate with ultrahigh molecular weight polyethylene (UHMWPE)-graphene nanocomposite. The UHMWPE used in this method can be a homopolymer of ethylene or a copolymer of ethylene with a co-monomer which is another alpha-olefin or a cyclic olefin both with generally between 3 and 20 carbon atoms. Examples include propene, 1 -butene, 1 -pentene, 1 -hexene, 1 -heptene, 1 -octene, cyclohexene, etc. The use of dienes with up to 20 carbon atoms is also possible, e.g., butadiene or 1 -4 hexadiene.
The UHMWPE of the present invention has molecular weight in the range of 3,00,000 to 8,00,000 atomic units (AU). In one embodiment, the molecular weight is in the range of 5,00,000 to 7,50,000 atomic units.

When a composite component is incorporated in a polymer matrix, such as in UHMWPE, the resultant composite may have better mechanical property depending on factors like processing technique, choice of composite component and concentration of composite component. In the present invention, nanoparticles of graphene are incorporated into UHMWPE matrix to form nanocomposites.

Graphene is known for its strength and lightness which makes it ideal for ballistic application. When used to strengthen polymers, graphene in any form increases polymer toughness by inhibiting crack propagation.

The graphene-component of the present invention is selected from few layered graphene, graphene nanosheets, ammonia-functionalized (NH3-functionalized) graphene, carboxylic acid functionalized (-COOH-functionalized) graphene, argon functionalized (Ar-functionalized) graphene and any combinations thereof. The term "graphene" refers to single layer of carbon atoms densely packed into a fused benzene-ring structure. As used herein, the term “few layered graphene” (FLG) refers to graphene with 2 to 10 layers. The term graphene nanosheets refers to two-dimensional nanostructures of graphene with thickness in a range of 1 to 100 nanometers (nm).

Graphene is typically produced in bulk by exfoliation and reduction of graphite oxide, where the graphite oxide is prepared by the oxidation of graphite. The graphene can be further functionalized using chemical agents to derive functionalized graphene such as ammonia-functionalized graphene (NH3-functionalized), carboxylic acid functionalized graphene (COOH-functionalized) and argon functionalized graphene (Ar-functionalized). It is to be appreciated that functionalization of graphene improves dispersion and or adhesion to UHMWPE matrix as compared to neat graphene.

Silicon carbide (SiC) is a heat-resistant and high-strength material. It is believed that incorporating SiC in graphene-UHMWPE nanocomposite may enhance the stab resistance of the resultant coating. In one embodiment, the graphene component in addition to graphene includes SiC. The SiC is included in the form of SiC fibres and/or SiC microfibres.

As used herein, the term “SiC fibre” refers to polycrystalline SiC having a particle length greater than 5 µm and a particle width of less than 3 µm and an aspect ratio of more than 3. The term “SiC microfibre” refers to sub-micron sized SiC fibres.

The method of the present invention comprises blending in solid form the graphene component and UHMW polyethylene to form a UHMWPE-graphene composite blend. The blending can be carried out using mixers known in the art, such as a turbo mixer. The blending can be carried out as a batch or by a continuous process. In a batch mixing process, UHMWPE and graphene component are stored in different hoppers and a pre-set amount of each component is filled in a mixing vessel to be blended. In a continuous process, all the components are fed simultaneously in to the mixing vessel.

In one embodiment, desired amount of UHMWPE in solid form, preferably in powder form, is taken in a mixer along with desired quantity of graphene component. The blending step is very critical to ensure uniform dispersion of graphene component in UHMWPE. Inventors, through exhaustive experimentation varied mixing parameters such as temperature, speed and time of mixing to achieve uniform dispersion thereby enhancing the property of final coating. A particular advantage of the present method is dry blending of the UHMWPE and graphene component to form the composite. In the prior art this has been achieved by solvent-based blending that uses additional chemicals, or by melt blending by supplying energy. The method of the present invention is thus a much greener method.

In one embodiment, blending is carried out at a temperature equal to or less than about 80 ?. In another embodiment, the blending is carried out at room temperature, i.e., 25?. In yet another embodiment, mixing is performed at a temperature in a range of about 25? to about 80?. The blending is continued for a time period of about 1 hour to about 4 hours at speeds of 40 to 100 revolutions per minute (rpm) to form the UHMW PE-graphene composite blend.

The UHMWPE-graphene composite blend formed is sieved to a particle size in the range of about 100 microns to about 200 microns to form a coating mixture. The sieving was performed using ASTM standard sieve series to get the desired particle size. In one embodiment, the particle size achieved is 150 microns using ASTM No.100. Sieving the blend to a uniform particle size helps in the subsequent coating step to form uniform thickness coating.

In one embodiment, the graphene component is present in a range of about 1% to about 20% by weight in the coating mixture. In another embodiment, the graphene component is present in a range of about 1% to about 10% by weight in the coating mixture. In one embodiment, the graphene component further includes SiC in a range of about 0.1% to about 10% by weight in the coating mixture.

The coating mixture is supplied through a thermal spray apparatus so as to deposit UHMW polyethylene-graphene nanocomposite on a substrate to form the coating. Typically, thermal spray method involves generation of a gas flow which is used to accelerate particles of the coating material to be deposited and to direct them towards the surface of the substrate to be coated, where they impact and on which they remain adhered. The interaction of the gas flow with the particles to be deposited (heat exchange, chemical reactions and transfer of mechanical moment), defines the features of the process and, ultimately, the nature and quality of the coatings generated.

In the thermal spray apparatus of the present invention, the coating mixture is supplied in the form of powder where it is contacted with a fuel gas and on their exit from a nozzle of the thermal spray apparatus form melted or molten spray. The melted or molten spray is propelled at high speed on to a substrate placed in front of the nozzle to form the coating. The nozzle traverses repeatedly over the surface of the substrate to form the coating. The coating can be applied in single layers or in multiple layers, with a typical layer thickness of 10 to 20 microns. Thermal spray parameters such as gas flow, powder injection rate and distance from nozzle to substrate are adjusted to optimize coating to get uniform thickness.

In one embodiment, fuel gas is commercially available LPG. Other fuel gases may be employed. The temperature that is achieved in the thermal spray apparatus, in one embodiment, is between 100 and 300 ?. In one embodiment, gas flow or pressure is in a range of about 2 to 6 bar. In one embodiment, the coating mixture is injected at a rate of 0.5 to 3 kilograms per hour. A coating thickness of 70 to 200 microns can be achieved using the methods of the present invention. The coating may be carried out in inert atmosphere or in open air atmosphere.

Suitable substrates that can be coated using this technique include aramid fabrics, nylon fabrics, carbon fibre fabrics, fibre glass fabrics and UHMWPE-based fabrics. Suitable aramid fabrics include fabrics made of p-aramid, m-aramid or any combinations thereof, such as commercially available Kevlar® and Twaron® series. In one embodiment, the substrate is aramid fabric. Example UHMWPE-based fabrics include Dyneema® from DSM and Spectra® from Honeywell.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims.

EXAMPLE
Example 1
Preparation of NH3-functionalized graphene-UHMWPE coating mixture: About 750 gm of UHMWPE was taken in a jar which has the capacity of 1 litre. Added 7.5 gm (1%) of NH3-functionalized graphene into this. The mixing was performed at 25 rpm for a time of about 2 hours. The NH3-functionalised graphene/UHMWPE composite formed was sieved using ASTM No. 100 (150 µm mesh size) to get fine NH3-functionalized graphene-UHMWPE coating mixture.
Example 2
Preparation of COOH-functionalized graphene/UHMWPE coating mixture: About 750 gm of UHMWPE was taken in 1 liter jar. Into this was added 7.5 gm (1%) of COOH-functionalized graphene. The mixing was performed at 25 rpm for a time of about 2 hours. The COOH-functionalised Graphene/UHMWPE composite formed was sieved using ASTM No. 100 (150 µm – mesh size) to get fine COOH functionalized graphene-UHMWPE coating mixture.
Example 3
Preparation of Ar functionalized graphene/UHMWPE coating mixture: About 750 gm of UHMWPE was taken in a 1 liter jar. Into this was added 7.5 gm (1%) of Ar-functionalized graphene. The mixing was performed at 25 rpm for a time of about 2 hours. The Ar-functionalised graphene/UHMWPE composite formed was sieved using ASTM No. 100 (150 µm – mesh size) to get fine Ar-functionalized graphene-UHMWPE coating mixture.
Example 4
Preparation of SiC fibre/graphene-UHMWPE coating mixture: About 750 gm of UHMWPE and 7.5 g (1%) of graphene was taken in a 1 liter jar. Into this was added 7.5 gm (1%) of SiC in fibre form. The mixing was performed at 25 rpm for a time of about 2 hours. The SiC fibre/graphene-UHMWPE composite formed was sieved using ASTM No. 100 (150 µm – mesh size) to get fine SiC fibre/graphene-UHMWPE coating mixture.
Example 5
Preparation of SiC microfibre/graphene-UHMWPE coating mixture: About 750 gm of UHMWPE and 7.5 g (1%) of graphene was taken in a 1 liter jar. Into this was added 7.5 gm (1%) of SiC in microfibre form. The mixing was performed at 25 rpm for a time of about 2 hours. The SiC microfibre/graphene-UHMWPE composite formed was sieved using ASTM No. 100 (150 µm – mesh size) to get fine SiC microfibre/graphene-UHMWPE coating mixture.
Examples 1-5 were repeated in Commercial scale using Industrial 3-Dimensional shakers. The material loading were adjusted to get uniform mixing. It was observed that 20-25% hollow space was necessary above the shaker for better mixing.
Example 6
Coating of Graphene/UHMWPE composite on Aramid fabric:
Polymer Thermal Spray (PTS) technique was used to coat Graphene/UHMWPE coating mixtures on Aramid fabics of 125 GSM, 150 GSM and 200 GSM respectively. The coating mixture was taken into the spray gun powder feeder. Commercial LPG flame torch was used as coating mixture carrier. The air pressure to the powder spray gun was created by the compressor and monitored with the help of flow meter to maintain at 4 bar. The powder was sprayed at a flow rate of 30 liters per minute. A 180 degree angle was maintained between powder spray gun and flame torch tip. The distance between flame torch tip and Aramid fabrics were kept at 30 centimeters. The experiments were conducted in open atmosphere. The composite was coated in a systematic approach from left to right, or right to left, or from top to bottom, or from bottom to top to obtain uniformly coated Aramid fabrics.
The Graphene/UHMWPE coated Aramid fabric obtained were tested for ballistic applications acording to NIJ IIIA Standard test method. The coated fabrics qualifed NIJ IIIA test with 20-25 % lighter weight. The strength of coated Aramid fabric was found to be 20% more than that of uncoated Aramid fabrics.
The cut resistance test was conducted and the graphene/UHMWPE coated Aramid fabric was found to have 30% better performance than uncoated Aramid fabric. Abrasion resitance test performed also showed 40% improvement in performance compared to uncoated Aramid fabrics.
Example 7
Coating SiC fibre/graphene UHMWPEnanocomposite on Aramid fabric:
Polymer Thermal Spray (PTS) technique was used to coat SiC fibre/graphene UHMWPE coating mixture on Aramid fabics of 125 GSM, 150 GSM and 200 GSM respectively. The coating mixture was taken into the spray gun powder feeder. Commercial LPG flame torch was used as coating mixture carrier. The air pressure to the powder spray gun was created by the compressor and monitored with the help of flow meter to maintain at 4 bar. The powder was sprayed at a flow rate of 30 liters per minute. A 180 degree angle was maintained between powder spray gun and flame torch tip. The distance between flame torch tip and Aramid fabrics were kept at 30 centimeters. The experiments were conducted in open atmosphere. The composite was coated in a systematic approach from left to right, or right to left, or from top to bottom, or from bottom to top to obtain uniformly coated Aramid fabrics.
The SiC/graphene UHMWPE coated Aramid fabric thus obtained were tested for stab proof applications using NIJ Standard 0115.00 level 2 test method. The stab resistance of SiC/graphene UHMWPE nanocomposite coated Aramid fabric was found to be 15-20% better than uncoated Aramid fabric. Ballistic test was conducted according to the NIJ A Standard test method and qualified with 15-20% lighter weight.
,CLAIMS:1. A method for coating a substrate with ultrahigh molecular weight polyethylene (UHMWPE)-graphene nanocomposite comprising:
blending in solid form UHMWPE and a graphene component to form a UHMWPE-graphene composite blend;
sieving the UHMWPE-graphene composite blend to a particle size in the range of about 100 microns to about 200 microns to form a coating mixture; and
supplying the coating mixture through a thermal spray apparatus so as to deposit UHMWPE graphene nanocomposite on the substrate to form the coating.

2. The method as claimed in claim 1, wherein the UHMWPE has a molecular weight in the range of 3,00,000 to 8,00,000 AU.

3. The method as claimed in claim 1, wherein the graphene component comprises graphene selected from few layered graphene (FLG), graphene nano sheets, ammonia-functionalized graphene, carboxylic acid-functionalized graphene, argon-functionalized graphene and any combinations thereof.

4. The method as claimed in claim 3, wherein the graphene component further comprises SiC microfibres, SiC fibres or any combinations thereof.

5. The method as claimed in claim 1, wherein the graphene component is present in a range of about 1% to about 20% by weight in the UHMWPE-graphene composite blend.

6. The method as claimed in claim 5, wherein the graphene component is present in a range of about 1% to about 10% by weight in the UHMWPE-graphene composite blend.

7. The method as claimed in claim 1, wherein blending the UHMWPE and graphene component comprises mixing at 40 to 100 rpm for a time in the range of 1 to 4 hours and at temperature less than or equal to about 80 ?.

8. The method as claimed in claim 1, wherein supplying the coating mixture through a thermal spray apparatus comprises supplying the coating mixture at a flow rate of 0.5 to 3 kilograms per hour and operating the thermal spray apparatus at a temperature in the range of about 100 ? to 300 ?.

9. The method as claimed in claim 1, wherein the substrate is selected from a group consisting of aramid fabrics, nylon fabrics, carbon fibre fabrics, fibre glass fabrics and UHMWPE fabrics.

10. The method as claimed in claim 9, wherein the substrate is aramid fabric.