CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority benefit of U.S. Provisional Application Serial Number 62/913,897 filed 11 October 2019, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

[0002] The present invention generally relates to the field of materials, and more specifically to graphene -reinforced polyamide composites.

BACKGROUND

[0003] Polyamide or “Nylon” is an important class of plastic materials with a wide spectrum of applications. It is a family of synthetic polymers based on polyamides which are a group of polymers that contain a recurring amide group (-CONH-) in their molecular structure. The materials include a variety of types and hence exhibit a broad range of properties. Nylon-based composites with different functionalities have significantly expanded the market and are replacing traditional materials for advanced applications such as in automotive underhood and for 3D printing.

[0004] Polyamides may contain one monomer that carries both the acid and amine functionalities, which is called AB type. They can also contain two monomers with one containing the amine functionality while the other containing the acid functionality. These are called A ABB type since two monomers are required for polyamide formation. A and B stand for the functional groups -N¾ and -COOH, respectively. Polyamides are conventionally identified by the number of carbon atoms present in the monomers. Consequently, AABB type of polyamides are differentiated by two numbers: the first one corresponds to the number of carbon in the diamine and the second one to that in the diacid, as shown in formula [1] and [2]. Examples are Nylon 6,6, Nylon 6,10, Nylon 6,12, etc. For the AB type, however, there is only a single number representing the number of carbons such as Nylon 6, Nylon 11, and Nylon 12. [0005] Nylon-n: -[-(NH-CO)-(-CH ₂ ) _n-i -]- [1]

[0006] Nylon-m,n: -[-(NH-CO)-(CH ₂ )n-2-(CO-NH)-(CH ₂ )m-]- [2]

[0007] Engineering Nylons are categorized in several groups: (i) Highly reinforced Nylons, (ii) Halogen-free flame retardant Nylons, (iii) Abrasion resistant Nylons, (iv) Heat resistant Nylons, (v) Nylon alloys and blends, and (vi) Conductive Nylons.

[0008] This underscores the fact that by manipulating the composition and microstructure of Nylon materials, one can significantly modify their properties for specific applications. Many traditional reinforcements or fillers such as carbon fibers, glass fibers, mineral fibers, wood fibers, metallic fibers, aramid fibers, talc, silica, and calcium carbonate particulates have been used to improve the performance of Nylons. In recent years, graphene-based materials, as a new class of nanofillers have received a lot of attentions for composite applications.

[0009] One aspect of the instant invention is to reinforce polyamides with graphene-based materials. Graphene is an allotrope of carbon in the form of a two-dimensional, atomic-scale, honey-comb lattice in which one atom forms each vertex. Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner. Graphene is the basic structural element of other allotropes, including graphite, charcoal, carbon nanotubes and fullerenes. Graphene is a highly versatile material with unique properties in terms of mechanical strength, electrical conductivity and thermal conductivity. The flexibility of graphene makes it perfectly suited for coatings and application in membranes and thermoplastics. The desired properties of graphene are at least partly dependent on the two dimensional structure of the carbon allotrope.

[0010] As used herein, graphene is defined as a two dimensional material constructed by close-packed carbon atoms including a single-layer graphene, double-layer graphene, multi layer graphene, graphene nanoplatelets, functionalized graphene, doped graphene, graphene oxide, reduced graphene, and a combination thereof.

[0011] As used herein, single-layer graphene is defined as a single layer of close-packed carbon ato s.

[0012] As used herein, double-layer graphene is defined as a stack graphene of two layers. [0013] As used herein, multi-layer graphene, is defined as a stack graphene of 3-10 layers

[0014] As used herein, graphene nanoplatelet is defined as a stack of graphene of more than

10 layers.

[0015] As used herein, graphene oxide is defined as one or more graphene layers with various oxygen-containing functionalities such as epoxide, carbonyl, carboxyl, and hydroxyl groups and a typical C:0 ratio around 2.

[0016] As used herein, reduced graphene oxide is defined as graphene oxide that has been chemically or thermally reduced with a total oxygen content of typically in the range of 10%- 30% depending on the extent of reduction.

[0017] As used herein, functionalized graphene is defined as graphene, few layer graphene, graphene nanoplatelets, graphene oxide, and reduced graphene oxide that are attached certain functional groups at their surfaces or edges. The functional groups include, but are not limited to, epoxide, carbonyl, carboxyl, hydroxyl, and amine, etc.

[0018] As used herein doped graphene is defined as graphene and graphene oxide that are doped in their crystal structures of certain metallic or non-metallic elements such as nitrogen, fluorine, oxygen, etc. The graphene materials can be made by chemical or mechanical exfoliation of graphite. The graphene materials can also be made by oxidizing graphite with or without a reduction step.

[0019] Among them, graphene nanoplatelet is a preferred reinforcement filler wherein the graphene nanoplatelet is a new type of nanoparticles made from graphite. These nanoparticles consist of small stacks of graphene that are 1 to 15 nanometers thick, with diameters ranging from sub-micrometer to 100 micrometers. U.S. Patent Publication 2010/0092809 describes an exemplary process for forming exfoliated graphite nanoparticles.

[0020] Graphene enhanced polymer composites can be made by a compounding process. Compounding is a process of blending plastics with other additives. Compounding changes the physical, thermal, electrical, or aesthetic characteristics of the plastic. The final product is called a compound or composite. Compounding starts with a base resin or polymer. The resins have unique characteristics that make each suitable for use in certain applications. By incorporating an extensive range of additives, fillers, and reinforcements, a wide range of properties can be achieved in conductivity, flame retardancy, wear resistance, structural properties, and pre-coloration. The additives are based on performance criteria. For example, glass fibers can be added at various levels to increase stiffness in a resin that is more flexible than desired.

[0021] One of the compounding methods is melt-compounding which is done in several steps. Resin and additive(s) are fed through an extruder where they are combined. The melted compound exits the extruder in strands about the diameter of yarn. These strands are cooled and cut into pellets. The pellets are thoroughly inspected and quality checked before being delivered to customers for use in injection molding or extrusion to produce composite structures, fibers, fabrics, films, pipes, rods, and sheets.

[0022] As used herein, a pellet is defined as small particles typically created by compressing an original material with a particle size in the range from several microns to several centimeters. [0023] As used herein, a fiber is defined as a one-dimensional substance that is significantly longer in their length than in their width with a diameter typically from several microns to several millimeters.

[0024] As used herein, a yam is defined as a long continuous length of interlocked fibers, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery, or rope making.

[0025] As used herein, a fabric is defined as textile materials made through weaving, knitting, spreading, crocheting, or bonding of fibers or yams that may be used in production of further goods such as clothes.

[0026] As used herein, a film is defined as a thin continuous polymeric material with a thickness typically in the range of several microns to hundreds of microns.

[0027] As used herein, a sheet is defined as a thick continuous polymeric material with a thickness typically in the range of several microns to several millimeters.

[0028] While there has been substantial progress made in graphene-reinforced polyamide composite materials, there continues to be a need for improved graphene-reinforced polyamide composite materials and the processes to make them.

SUMMARY OF THE INVENTION

[0029] A graphene -reinforced polyamide composite is provided. The graphene is one or more of: graphene, multi-layer graphene, graphene nanoplatelet, graphene oxide, reduced graphene oxide, doped graphene, functionalized graphene, and a combination thereof. The graphene is incorporated in a polyamide resin by melt compounding graphene having a C:0 ratio in between 85:15 to 99, wherein graphene is incorporated as a masterbatch with a compatibilizer as the matrix of the masterbatch. [0030] A method is provided for introducing and evenly dispersing graphene nanoplatelets for use in polyamide 6 (PA6). The method includes loading a PA6 powder and one or more fillers into an extruder, and loading exfoliated graphene platelets (xGnP) into a feeder. Adjusting a feeder rate for a desired dispersion weight percentage of the xGnP in an end product masterbatch. Adding a compatibilizing additive to the PA6 and one or more fillers in the extruder, and heating the PA6 and the one or more fillers to form a molten thermoplastic. Dispensing the xGnP from the feeder into the molten thermoplastic to form a melted compound, where the melted componnd exits the extruder in strands. Cooling the strands and cutting the strands into pellets that form the masterbatch.

BRIEF DESCRIPTION OF THE DRAWINGS [0031] The present invention is further detailed with respect to the following drawing that is intended to show certain aspects of the present of invention, but should not be construed as limit on the practice of the invention, wherein:

[0032] FIG. 1 is a flowchart of a method for introducing and evenly dispersing graphene for use in PA6, thermoplastics, and masterbatch composite materials in accordance with embodiments of the invention;

[0033] FIGS. 2A-2C show the improvement in tensile modulus (FIG. 2A) and tensile strength (FIG. 2B) and modulus of graphene-enhanced PA 6 composite with the addition of 0.25 and 2.4 wt. % addition of a graphene, the results summarized in tabular form in FIG. 2C; and

[0034] FIGS. 3A-3C show the improvement in flexural modulus (FIG. 3A) and flexural strength (FIG. 3B) and modulus of graphene-enhanced PA 6 composite with the addition of 0.25 and 2.4 wt. % addition of a graphene, the results summarized in tabular form in FIG. 3C. DETAILED DESCRIPTION

[0035] The present invention has utility as a graphene-reinforced polyamide composite material and the process to make it. Embodiments of the invention overcome the aforementioned limitations by effectively dispersing graphene-based nanofillers in polyamide resins in the melt compounding manufacturing process. The graphene-based nanofillers are selected from a group of single-layer graphene, double-layer graphene, multi-layer graphene, graphene nanoplatelet, doped graphene, graphene oxide, reduced graphene oxide and a combination thereof.

[0036] In some inventive embodiment, the filler is first mixed and compounded with the polymer to fabricate a masterbatch via a melt compounding process. Masterbatch is a concentrated mixture of pigments and/or additives encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape. The melt compounding process uses shear forces to ensure a homogenous distribution of graphene in the polymer systems. [0037] Melt compounding is the most industrially accessible process for manufacturing of polymers including polyamide masterbatches. When utilizing melt compounding for incorporating graphene nanoplatelets, the most significant challenge is obtaining a uniform dispersion of graphene nanoplatelets in the polymer. In this process, graphene can be pre-mixed with the polymer resin before the melt-compounding. However, this method does not provide enough control to achieve a uniform dispersion of graphene in the resulting masterbatch. At high loadings of graphene, the viscosity of the molten material causes difficulty in mixing graphene powder uniformly into the molten material. Graphene-based fillers can also be added by gravimetric or volumetric feeding.

[0038] Often, other additives are also incorporated in the composite during melt compounding. One of the additives is for compatibilization. Compatibilization in polymer chemistry is the addition of a substance to an immiscible blend of polymers that will increase their stability. Polymer blends are typically described by coarse, unstable, phase-separated morphologies. This results in poor mechanical properties. Compatibilizing the system will make a more stable and better blended phase morphology by creating interactions between the two previously immiscible polymers. Not only does this enhance the mechanical properties of the blend, but it often yields properties that are generally not attainable in either single pure component (Chen, C., & White, J. (1993). Compatibilizing Agents in Polymer Blends: Interfacial Tension, Phase Morphology, and Mechanical Properties. Polymer Science and Engineering, 33(14), 923-930).

[0039] According to some aspects of the present invention, the minimum amount of compatibilizer in a composite, X _m in is defined in the following equation:

Xmin = 0.01Y X Z where Xmin is minimum amount of compatibilizer in composite (wt%), Y is an amount of graphene in composite (wt%), and Z is the surface area of graphene (m ² /g). In other aspects of the present invention, the maximum amount of compatibilizer in the composite, X _max is defined in the following equation:

Xmax = Y X Z where X _max is the maximal amount of compatibilizer in composite (wt%), Y is an amount of graphene in composite (wt%), and Z is the surface area of graphene (m ² /g).

[0040] One aspect of present invention is to enhance mechanical properties of polyamide polymers using graphene-based nanofillers. In one exemplary embodiment, graphene nanoplatelet is added to polyamide 6 (PA6) to obtain an enhanced PA6/graphene composite. Polyamide 6 also known as Nylon 6 is a semicrystalline polyamide. Unlike most other Nylons, PA6 is not a condensation polymer, but instead is formed by ring-opening polymerization; this makes it a special case in the comparison between condensation and addition polymers. PA6 has good thermal stability, mechanical strength, and high yield stress due to the hydrogen bonds that can be formed between chains. PA6 is wrinkle-proof and highly resistant to abrasion and chemical exposure to both alkalis and diluted acids. PA6 is a significant engineering thermoplastic material used in many applications in automotive, aircraft, electronic, clothing, machinery, and other industries. The material has improved creep resistance over PA6,6, another important material of the Nylon family, but has a lower modulus and working temperature. It also absorbs moisture more rapidly but has improved processability. Therefore, there exists a need for improving PA6 performance. Introducing and evenly dispersing graphene in PA6, either directly, or through a masterbatch, is one of the approaches.

[0041] Another aspect of the present invention is to enhance thermal properties of polyamide polymers using graphene-based nanofillers. For example, PA6 has a lower operation temperature range as compared to PA6,6. It is often desirable to further improve the heat- distortion temperature (HDT) of PA6 and use the enhanced material to replace the more expensive PA6,6 for automotive under-hood applications. Adding graphene-based nanofillers can help improve both mechanical and thermal properties of PA6 due to the superior properties of graphene.

[0042] In another exemplary embodiment of improving both mechanical and thermal properties, polyamides such as PA6, PA12, and PA11 are composited with graphene nanoplatelet filler for 3D-printing applications. The improved thermal and mechanical properties of graphene-enhanced polyamides allow quick heating and cooling during the 3D- printing while maintaining the structural stability of the printed part, thereby improving the quality and throughput of the process.

[0043] Another aspect of the present invention is to enhance electrical properties of polyamide polymers using graphene-based nanofillers.

[0044] Another aspect of the present invention is to enhance flame-retardant properties of polyamide polymers using graphene-based nanofillers. [0045] Another aspect of the present invention is to enhance barrier properties of polyamide polymers using graphene-based nanofillers.

[0046] Another aspect of the present invention is to enhance lubrication properties of polyamide polymers using graphene-based nanofillers.

[0047] In some inventive embodiments, graphene-based filler is used in a PA6 composite or masterbatch by a melt compounding process wherein the graphene filler is added to an extruder by pre-mixing, gravimetrical feeding, or volumetric feeding. The inclusion of well dispersed graphene fillers improves the mechanical, thermal, electrical, flame retardant, and barrier properties of PA6.

[0048] It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.

[0049] In some inventive embodiments, a masterbatch is produced in the first step by melt compounding with pre-mixing, gravimetric feeding, or volumetric feeding, followed by a second step of compounding to the final composition at a specified let-down ratio. A masterbatch with a uniform dispersion of graphene fillers can more efficiently reinforces the polymer resin, providing superior properties over a masterbatch without proper dispersion. [0050] Absent a chemical modification of the graphene nanoplatelets, the polyamide, or both; attempts to compound the graphene nanoplatelets uniformly into a PA6 matrix simply through mechanical mixing of the graphene nanoplatelets into a polymer melt are unsuccessful. Further, chemical modification of components adds complexity to the process. Some additives such as an appropriate compatibilizer can help the dispersion of graphene nanoplatelets and their interaction with polyamide resins. [0051] Another approach is to modify graphene with certain functional groups to improve the dispersion and compatibility of graphene in polyamide resins. Yet another approach is to decorate graphene with certain metallic, ceramic, or polymeric nanoparticles wherein the nanoparticles enhance the interaction between the nanofillers and polyamide resin.

[0052] Embodiments of the invention provide an accurate and consistent mixing method to disperse graphene fillers in a polyamide resin to create a masterbatch via melt compounding at a level of from 1 to 20 total weight percent. This is accomplished with resort to a compatibilizing additive, feeding of graphene fillers into polyamide pellets or powders, or a combination thereof.

[0053] Compatibilizing additives operative herein includes polymers modified to include a maleic anhydride content. A maleated polymer is present in an inventive composition and is characterized by a graft polymer in which maleic anhydride is graft copolymerized with a polymer. Maleated polymers operative in the present invention illustratively include a maleic anhydride grafted copolymer (also referred to herein as maleated) of polypropylene; maleated polyethylene, both low density (LDPE) and high density (HDPE) forms thereof; maleated etheylene octene; maleated copolymers of styrene/ethylene/butylene/styrene (SEBS) copolymer; maleated functionalized HDPE; maleic anhydride grafted polystyrene; and a combination of any of the aforementioned with one another or disparate materials. Methods of maleating polymers are conventional to the art, as detailed illustratively in the following references. Premphet, K; Chalearmthitipa, “Melt Grafting of Maleic Anhydride onto Elastomeric Ethylene-Octene Copolymer by Reactive Extrusion”, S. Polymer Engineering and Science; Newtown Vol. 41, Iss. 11, (Nov 2001): pp: 1978+. Zhang, L; Zhang, Y. “Functionalization of HDPE/EPDM with Maleic Anhydride through High Shear Stress Initiation”, Hecheng Shuzhi Ji Suliao/China Synthetic Resin and Plastics. 28. (2011), pp: 9-12. [0054] Typically, the degree of maleation is between 0.1 and 5 maleic anhydride content as weight percent of the maleated polymer. In still other inventive embodiments, the degree of maleation is between 1 and 4 weight percent of the maleated polymer. Typically, a maleated polymer is present in an inventive formulation in an amount of between 0.1 and 10 total weight percent and in some embodiments, between 1 and 5 total weight percent.

[0055] Additional additives common to the industry are readily accommodated by an inventive formulation with these additives typically including pigments, colors, chemical or gas blowing agents, lubricants, thermal stabilizers, oxidation stabilizers, and plasticizers, with each of the additives being present in amounts typically ranging from 0.1 to 5 total weight percent for each.

[0056] In some inventive embodiments, additional fillers can also be used in combination with non-graphene-based fillers. Those additional fillers and reinforcements illustratively include rubber modified polypropylene; mineral fillers such as talc, calcium carbonate, mica, wollastonite, magnesium oxide, kaolin, nanoclay; noncellulosic fibers such as fibers of glass, carbon, aramid, PET polyester, viscose and ceramic. Rubber modified polypropylenes illustratively including rubbers of ethylene butene, ethylene octene, styrene butadiene, EPDM, latex, and SEBS copolymer. Typically, an inclusion filler or reinforcement is present in an amount of between 0 and 20 parts by weight per part by weight of polyamides.

[0057] In some inventive embodiments, the PA6 is selected to have a melting temperature greater than the melting temperature of the compatibilizing additives. In still other inventive embodiments, the PA6 has a melt flow index of between 1 and 100 dg per minute, while in other inventive embodiments, the melt flow index is between 10 and 70 dg per minute. As the subsequently detailed examples provide, enhanced thermal, mechanical and structural properties obtained in the resultant article include increased tensile strength at break as measured at 23° Celsius and a cross-head speed of 50 millimeters per minute relative to a comparable composition lacking the compatibilizing additives. Surprisingly, the processability of the PA6 resin to include the homogenous distribution of graphene nanoplatelets is increased through incorporation the compatibilizing additives results in processing at temperatures below the conventional processing window for a particular polymer are noted absent the compatibilizing additives.

[0058] In a specific embodiment a mix of PA6 pellets (if required a mixture of a compatabilizer and a coupling agent with PA6 to change the surface energy of polymer/graphene) and graphene nanoplatelets are used to fabricate the masterbatch. With the objective of improving the level of dispersion of graphene nanoplatelets into matrix, PA6 powders are utilized. Compared to pellets, PA6 powders have a higher surface area per volume ratio. Mixing powders with graphene nanoplatelets improve the distribution of graphene nanoplatelets into the PA6 matrix, resulting in a more uniform dispersion in the masterbatch. Achieving a more uniform dispersion in the masterbatch results in superior mechanical properties of the masterbatch.

[0059] An embodiment of an inventive method 10 is generally shown in FIG. 1 for introducing and evenly dispersing graphene nanoplatelets for use in polyamide 6 (PA6). Thermoplastics and masterbatch composite materials includes loading a PA6 powder and one or more fillers into an extruder (Block 12). Exfoliated graphene platelets (xGnP) are loaded into a feeder (Block 14). The feeder rate is adjusted for a desired dispersion weight percentage of the xGnP in the end product masterbatch (Block 16). A compatibilizing additive is added in some inventive embodiments to the PA6 and one or more fillers in the extruder (Block 18). The PA6 and one or more fillers are heated to a molten state and the xGnP is dispensed from the feeder into the molten thermoplastic (Block 20). The melted compound exits the extruder in strands that are cooled and cut into pellets that form the masterbatch (Block 22). Other Embodiments

[0060] While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the described embodiments in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes may be made in the function and arrangement of elements without departing from the scope as set forth in the appended claims and the legal equivalents thereof.

[0061] Exemplary embodiment 1

[0062] Polyamide PA6/graphene composites were made using a melt compounding process. The PA6 with a melt flow index of 47 was used. Graphene nanoplatelets (xGnP™ C-300, XG Sciences, Inc) at a loading of 10 wt%, together with a maleic anhydride compatabilizer at a loading level of up to 10% were incorporated into the extrusion process through a gravimetric feeder at a certain rate to make a masterbatch. A twin-screw extruder was used with an operation temperature of ~250°C and a co-rotation speed of 200 rpm. The masterbatch pellets were further let-down to form PA6/graphene composites at different graphene loadings and injection molded into testing samples for mechanical testing. The results were compared with PA6 obtained under the same condition without graphene. As shown in FIGs. 2A-2C, at the graphene loading of 0.25 wt%, addition of graphene nanoplatelets improved the tensile strength by 11.7% and tensile modulus by 16.5%, respectively. At the graphene loading of 2.4 wt%, the improvement of tensile strength and modulus were 11.8% and 20.0%, respectively.

[0063] The addition of graphene nanoplatelets also improved flexural properties of PA6 materials. As shown in FIGs. 3A-3C, at the graphene loading of 0.25 wt%, addition of graphene nanoplatelets improved the flexural strength by 6.9 % and tensile modulus by 5.6%, respectively. At the graphene loading of 2.4 wt%, the improvement of tensile strength and modulus were 9.2% and 11.1%, respectively.