WO2021194585A9

WO2021194585A9 - Process for pre-treating carbon fiber with plasma containing carbon dioxide, and composite materials made therefrom

Info

Publication number: WO2021194585A9
Application number: PCT/US2020/065838
Authority: WO
Inventors: Quan Shi; Alan Thomas; Suzanne CRAWFORD
Original assignee: Cytec Industries Inc.
Priority date: 2019-12-20
Filing date: 2020-12-18
Publication date: 2021-10-28
Also published as: WO2021194585A2; WO2021194585A3

Abstract

The present disclosure relates to a process for treating carbon fiber, the process comprising: a) exposing one or more carbon fibers to plasma comprising carbon dioxide, and/or a reactive species thereof, thereby forming one or more pre-treated carbon fibers; and b) exposing the one or more pre-treated carbon fibers formed in step a) to a further surface treatment, thereby forming the treated carbon fiber. The present disclosure also relates to a composite material comprising one or more treated carbon fibers obtained by the process described herein and a matrix resin.

Description

PROCESS FOR PRE-TREATING CARBON FIBER WITH PLASMA CONTAINING CARBON DIOXIDE, AND COMPOSITE MATERIALS MADE THEREFROM

Cross Reference to Related Applications

This application claims the priority of U.S. Provisional Application No. 62/951430, filed December 20, 2019, the entire content of which is explicitly incorporated herein by this reference.

Field of the Invention

The present invention relates to the field of carbon fiber surface treatment in which the carbon fiber is pre-treated with plasma comprising carbon dioxide prior to a further surface treatment, and reinforced composite materials made from such modified carbon fibers.

Background

Carbon fibers have been used in a wide variety of applications because of their desirable properties, such as high strength and stiffness, high chemical resistance and low thermal expansion. For example, carbon fibers can be formed into a structural part that combines high strength and high stiffness, while having a weight that is significantly lighter than a metal component of equivalent properties. Increasingly, carbon fibers are being used as structural components in composite materials for aerospace industrial and automotive applications, among others. In particular, composite materials have been developed wherein carbon fibers serve as a reinforcing material in a resin or ceramic matrix.

Carbon fiber-reinforced resins or composites are generally formed by incorporating carbon fibers into a matrix resin. Methods for enhancing the interaction between carbon fiber and matrix resin, such as through electrochemical treatment or plasma treatment of carbon fibers, are known. However, in industrial settings where carbon fiber is manufactured and/or processed in large amounts in a continuous manner, such methods are often inefficient, difficult to implement and control, or result in damage of the carbon fiber being treated.

Thus, there is an ongoing need for new or improved processes chemistries for enhancing the interaction between carbon fiber and matrix resin, thereby enhancing the properties, typically mechanical properties, of the reinforced resin or composite material containing carbon fiber.

Summary of the Invention

This objective, and others which will become apparent from the following detailed description, are met, in whole or in part, by the methods and/or processes of the present disclosure.

In a first aspect, the present disclosure relates to a process for treating carbon fiber, the process comprising: a) exposing one or more carbon fibers to plasma comprising carbon dioxide, and/or a reactive species thereof, thereby forming one or more pre-treated carbon fibers; and b) exposing the one or more pre-treated carbon fibers formed in step a) to a further surface treatment, thereby forming the treated carbon fiber.

In a second aspect, the present disclosure relates to a composite material comprising: one or more treated carbon fibers obtained by the process described herein, and a matrix resin.

Brief Description of the Figures FIG. 1 shows the interfacial shear strength (IFSS) of carbon fiber subjected to the inventive process described herein in comparison to carbon fiber subjected to only plasma comprising carbon dioxide and/or a reactive species thereof, and carbon fiber exposed to acetic acid, acrylic acid, allylamine, C0₂/acrylic acid, or methane/ammonia each without CO2 plasma pre-treatment.

Detailed Description

As used herein, the terms “a”, “an”, or “the” means “one or more” or “at least one” and may be used interchangeably, unless otherwise stated.

As used herein, the term “comprises” includes “consists essentially of” and “consists of.” The term “comprising” includes “consisting essentially of and “consisting of.”

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this specification pertains.

As used herein, and unless otherwise indicated, the term “about” or “approximately” means an acceptable error for a particular value as determined by one of ordinary skill in the art, which depends in part on how the value is measured or determined.

In certain embodiments, the term “about” or “approximately” means within 1 , 2, 3, or 4 standard deviations. In certain embodiments, the term “about” or “approximately” means within 50%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1 %, 0.5%, or 0.05% of a given value or range.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10; that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Because the disclosed numerical ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations.

Throughout the present disclosure, various publications may be incorporated by reference. Should the meaning of any language in such publications incorporated by reference conflict with the meaning of the language of the present disclosure, the meaning of the language of the present disclosure shall take precedence, unless otherwise indicated.

In the first aspect, the present disclosure relates to a process for treating carbon fiber, the process comprising: a) exposing one or more carbon fibers to plasma comprising carbon dioxide, and/or a reactive species thereof, thereby forming one or more pre-treated carbon fibers; and b) exposing the one or more pre-treated carbon fibers formed in step a) to a further surface treatment, thereby forming the treated carbon fiber.

In step a), one or more carbon fibers is/are exposed to plasma comprising carbon dioxide, and/or a reactive species thereof to form one or more pre-treated carbon fibers. The form of the carbon fiber suitable for use in the presently-described process is not particularly limited. Suitable carbon fiber may be made from rayon, pitch or polyacrylonitrile (PAN). In some embodiments, the PAN-based carbon fiber may be ultra-high strength. The carbon fiber may be in the form of a single filament or in a multifilament form, for example, in the form of a tow. The tow size, which is the number of filaments that make up the tow, may be in the range of 0.5 to 450K filaments. The type of carbon fiber, often based on the carbonization temperature used to form the carbon fiber, is also not particularly limited. Suitable carbon fibers, prior to treatment by the process described herein, are the result of carbonization at a temperature of at least 1300 °C, at least 1350 °C, or at least 1380 °C For example, suitable carbon fibers, prior to treatment by the process described herein, are the result of carbonization at a temperature from 1300 to 2800 °C, typically 1350 to 1700 °C, more typically 1380 to 1500 °C, still more typically 1400 to 1500 °C. The one or more carbon fibers may be standard modulus carbon fibers, intermediate modulus carbon fibers, or high modulus carbon fibers.

As used herein, exposure time refers to the time during which the one or more carbon fibers are exposed to the plasma. In an embodiment, the exposure time in step a) is from 0.5 second to 2 minutes, typically 1 to 10 seconds.

The plasma suitable for use in step a) comprises carbon dioxide and/or a reactive species thereof. Such plasmas may be generated according to methods known to those of ordinary skill in the art from a precursor gas, which may comprise one or more gases. For example, suitable plasmas may be generated using direct current, alternating current, dielectric barrier discharge (DBD) typically with radio frequency, and microwave, and may be thermal or more preferably non-thermal (i.e. , “cold” plasma). The power used for plasma generation is not particularly limited. However, excessive power may fragment and destroy the reactive species used in the process. Generally, suitable plasma power is from 100 W to 800 W, typically from 100 W to 200 W. The precursor gas used in step a), which may be one or more gases, comprises carbon dioxide. The ordinarily-skilled artisan would understand that when the plasma is formed from the precursor gas, at least a portion of the carbon dioxide gas contained therein gives rise to reactive species, such as radicals, that are capable of modifying the surface of the carbon fiber being pre-treated.

The precursor gas used in step a) may further comprise one or more inert gases, typically non-oxidative gases. Examples of suitable inert gases that are non- oxidative include, but are not limited to, helium (He), argon (Ar), and nitrogen (N2). In an embodiment, the precursor gas comprises Ar, N2, or a mixture thereof, typically argon, and carbon dioxide.

In step a), the precursor gas and the plasma made therefrom are free of oxygen (O2) gas. As used herein, the phrase “free of oxygen gas” means that there is no external addition of oxygen gas and that there is no detectable amount of oxygen gas that may be observed by analytical techniques known to the ordinarily-skilled artisan, such as, for example, gas or liquid chromatography, spectrophotometry, optical microscopy, and the like. High exhaust velocity to the fiber surface is required to transport short-lived reactive oxidative species, such as those derived from oxygen gas. The gas velocity of an oxidative environment can damage the carbon fiber, significantly reducing tensile strength and other mechanical or physical properties. Therefore, the presence of oxygen gas, either alone or in a mixture, such as air, is undesirable.

Step a) may be conducted in a batch process or a continuous process. As used herein, a continuous process refers to a process in which the carbon fiber is conveyed through one or more processing steps a single work unit at a time without any breaks in time, substance, or sequence and can be incorporated into the continuous production of carbon fibers. This is in contrast to a batch process, which would be understood as being a process that comprises a sequence of one or more steps that performed in a defined order and in which a finite quantity of material is treated at the end of the sequence, which must be repeated in order to produce another batch of treated material. In an embodiment, step a) of the process is continuous.

The plasma pressure used in step a) is not particularly limited. As used herein, plasma pressure refers to the pressure at which the plasma is maintained. In an embodiment, the plasma pressure is 10 to 200 mTorr, typically 25 to 150 mTorr, more typically 30 to 100 mTorr. In another embodiment, the plasma pressure is at atmospheric pressure. Atmospheric pressure refers to ambient pressure at sea level, which is considered to be 760 Torr, or 1.013 bar. However, the ordinarily- skilled artisan would recognize that atmospheric pressure may vary locally as a result of, for example, climate and altitude. Thus, suitable pressures may vary between about 0.5 bar below and about 0.5 bar above atmospheric pressure.

The apparatus used to expose the one or more carbon fibers to plasma comprising carbon dioxide and/or a reactive species thereof in step a) may be any apparatus known to those of ordinary skill in the art that is capable of generating the said plasma from a precursor gas and maintaining the plasma at the desired plasma pressure, such as those described herein, while allowing the plasma to contact the carbon fiber to be treated.

Step b) of the process involves exposing the one or more pre-treated carbon fibers formed in step a) to a further surface treatment, thereby forming the treated carbon fiber. The further surface treatment may be any surface treatment known to those of ordinary skill in the art known to improve one or more properties, such as mechanical properties, of carbon fibers and composite materials made therefrom. For example, the further surface treatment may be an electrochemical treatment or even another plasma treatment in which the plasma comprises gases and/or reactive species different from those used in step a).

In an embodiment, the further surface treatment is a plasma treatment.

In another embodiment, the further surface treatment is a plasma treatment comprising exposing the one or more pre-treated carbon fibers formed in step a) to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group, carbon dioxide, ammonia, amines, carboxylic acids, esters, alkanes, or a mixture thereof, with the proviso that carbon dioxide may be present only in combination with an organic compound comprising at least one vinyl group, ammonia, amines, carboxylic acids, esters, alkanes, or a mixture thereof.

In this embodiment, the plasma suitable for use in step b) is generated from a precursor gas, which may comprise one or more gases, different from the one used in step a). This second precursor gas comprises an organic compound comprising at least one vinyl group, carbon dioxide, ammonia, amines, carboxylic acids, esters, alkanes, or a mixture thereof, with the proviso that carbon dioxide may be present only in combination with an organic compound comprising at least one vinyl group, ammonia, amines, carboxylic acids, esters, alkanes, or a mixture thereof.

A suitable organic compound comprising at least one vinyl group is a compound having the formula: wherein

Ri, R₂, and R₃ are each, independently, H or halogen;

L is a bond, -(CH₂)_n-, -X-, or -(C=X)-; wherein n is an integer from 1 to 10 and X is selected from the group consisting of O, N, and S;

R₄ is H, alkyl, -NR₅R₆, or -ORs, wherein R₅ and R₆ are each, independently, H or alkyl.

In another embodiment, Ri, R₂, and R₃ are each H.

In yet another embodiment, Ri, R₂, and R₃ are each independently selected from the group consisting of F, Cl, Br, and I; typically Ri, R₂, and R₃ are each F.

In an embodiment, L is a bond, -(CFhV, -X-, or -(C=X)-; wherein n is an integer from 1 to 5, typically 1 , and X is O.

In another embodiment, R₄ is alkyl, typically fluoroalkyl, more typically perfluoroalkyl.

In one embodiment, R₄ is -NH₂.

In another embodiment, R₄ is -OH.

In yet another embodiment, Ri, R₂, and R₃ are each H;

L is -CH₂- or -(C=0)-;

R₄ is -NH₂ or -OH.

In another embodiment, Ri, R₂, and R₃ are each F;

L is a bond or -O-;

R₄ is perfluoroalkyl, typically perfluoromethyl.

Suitable amines include, but are not limited to, primary amines, secondary amines, and tertiary amines having at least one (C₁-C₆) alkyl group. Suitable carboxylic acids include, but are not limited to, carboxylic acids having at least one (C₁-C₆) alkyl group, such as acetic acid, propionic acid, butyric acid, and the like. Suitable esters include, but are not limited to, compounds formed by reacting carboxylic acids having at least one (C₁-C₆) alkyl group, such as acetic acid, propionic acid, butyric acid, and the like, with (C₁-C₆) alkyl alcohols, such as methanol, ethanol, propanol, and the like. Suitable alkanes include, but are not limited to, (C₁-C₆) alkanes, such as methane, ethane, propane, butane, pentane, hexane, and the like.

In an embodiment, the further surface treatment in step b) is a plasma treatment comprising exposing the one or more pre-treated carbon fibers formed in step a) to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group, typically acrylic acid, allylamine, and/or perfluorinated methyl vinyl ketone, optionally with carbon dioxide, a carboxylic acid, typically acetic acid, or ammonia with an alkane, typically methane.

In another embodiment, the further surface treatment in step b) is a plasma treatment comprising exposing the one or more pre-treated carbon fibers formed in step a) to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group, typically acrylic acid, allylamine, and/or perfluorinated methyl vinyl ketone.

The precursor gas that may be used in step b) may further comprise one or more inert gases, typically non-oxidative gases, as described herein. In an embodiment, the precursor gas that may be used in step b) comprises Ar, N₂, or a mixture thereof, and an organic compound comprising at least one vinyl group, typically acrylic acid, allylamine, and/or perfluorinated methyl vinyl ketone. As with step a), the precursor gas and the plasma made therefrom suitable for use in step b) are free of oxygen (O2) gas.

Step b) may be conducted in a batch process or a continuous process. In an embodiment, step b) is conducted in a continuous manner.

The plasma pressure that may be used in step b) is not particularly limited. In an embodiment, as with step a), the plasma pressure that may be used in step b) is 10 to 200 mTorr, typically 25 to 150 mTorr, more typically 30 to 100 mTorr. In another embodiment, the plasma pressure that may be used in step b) is at atmospheric pressure.

The apparatus used to expose the one or more pre-treated carbon fibers to the plasma when the further surface treatment in step b) is a plasma treatment may be any apparatus known to those of ordinary skill in the art that is capable of generating the said plasma from a precursor gas and maintaining the plasma at the desired plasma pressure, such as those described herein, while allowing the plasma to contact the carbon fiber to be treated.

Generally, other than pitch-based fibers and rayon-based carbon fibers, carbon fiber is produced from acrylonitrile in a series of manufacturing steps or stages, including polymerization, spinning, drawing and/or washing, oxidation, and carbonization. The apparatus suitable for conducting the inventive treatment may be configured to be in line within such a manufacturing process, for example, following the carbonization step and before the sizing, drying and winding steps. It can also be appreciated by those of ordinary skill in the art that carbon fiber that has been manufactured and packaged can be unpackaged and subjected to the process described herein. For example, spooled carbon fiber may be unwound, subjected to the process described herein, optionally conveyed through further processing steps, such as a sizing step, and then re-wound on a spool for storage. In an embodiment, steps a) and b) of the process described herein are both performed following the carbonization step and prior to any sizing, drying and/or winding steps. In another embodiment, step b) is performed immediately following step a) without any intervening steps. The treated carbon fiber may optionally be subjected to sizing, where a size coating, is applied onto the treated fiber. Sizing may be carried out by passing the fiber through a size bath containing a liquid coating material. Sizing protects the carbon fiber during handling and processing into intermediate forms, such as dry fabric and prepreg. Sizing also holds filaments together in the case of individual tows to reduce fuzz, improve process ability, among other advantages. Thus, in an embodiment, the process further comprises applying a sizing agent, which is typically comprised of at least one components or the combination of epoxy, polyurethane, polyamide and polyimide resins, to the one or more treated carbon fibers.

In the second aspect, the present disclosure relates to a composite material comprising: one or more treated carbon fibers obtained by the process described herein, and a matrix resin.

Composite materials may be made by molding a preform and infusing the preform with a thermosetting resin in a number of liquid-molding processes. Liquid-molding processes that may be used include, without limitation, vacuum-assisted resin transfer molding (VARTM), in which resin is infused into the preform using a vacuum-generated pressure differential. Another method is resin transfer molding (RTM), wherein resin is infused under pressure into the preform in a closed mold. A third method is resin film infusion (RFI), wherein a semi-solid resin is placed underneath or on top of the preform, appropriate tooling is located on the part, the part is bagged and then placed in an autoclave to melt and infuse the resin into the preform.

The matrix resin for impregnating or infusing the preforms described herein is a curable resin. “Curing” or “cure” in the present disclosure refers to the hardening of a polymeric material by the chemical cross-linking of the polymer chains. The term “curable” in reference to a composition means that the composition is capable of being subjected to conditions which will render the composition to a hardened or thermoset state. The matrix resin typically is a hardenable or thermoset resin containing one or more uncured thermoset resins or thermoplastic resin. Suitable thermoset resins include, but are not limited to, epoxy resins, oxetanes, imides (such as polyimide or bismaleimide), vinyl ester resins, cyanate ester resins, isocyanate-modified epoxy resins, phenolic resins, furanic resins, benzoxazines, formaldehyde condensate resins (such as with urea, melamine or phenol), polyesters, acrylics, hybrids, blends and combinations thereof. Suitable thermoplastic resins include, but are not limited to polyolefins, fluoropolymers, such as polyvinylidene fluoride (PVDF), perfluorosulfonic acids, poly amid-imides, polyamides, such as PA-66, polyphthalamide (PPA), polyesters, polyketones, such as polyaryletherketone (PAEK), polyetherketoneketone (PEKK), and polyether ether ketone (PEEK); polyphenylene sulfides (PPS), polyvinylidene chlorides, sulfone polymers, hybrids, blends and combinations thereof.

Suitable epoxy resins include glycidyl derivatives of aromatic diamine, aromatic mono primary amines, aminophenols, polyhydric phenols, polyhydric alcohols, polycarboxylic acids and non-glycidyl resins produced by peroxidation of olefinic double bonds. Examples of suitable epoxy resins include polyglycidyl ethers of the bisphenols, such as bisphenol A, bisphenol F, bisphenol S, bisphenol K and bisphenol Z; polyglycidyl ethers of cresol and phenol-based novolacs, glycidyl ethers of phenol-aldelyde adducts, glycidyl ethers of aliphatic dials, diglycidyl ether, diethylene glycol diglycidyl ether, aromatic epoxy resins, aliphatic polyglycidylethers, epoxidised olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or combinations thereof.

Specific examples are tetraglycidyl derivatives of 4,4'-diaminodiphenylmethane (TGDDM), resorcinol diglycidyl ether, triglycidyl-p-aminophenol, triglycidyl-m- aminophenol, bromobisphenol F diglycidyl ether, tetraglycidyl derivatives of diaminodiphenylmethane, trihydroxyphenyl methane triglycidyl ether, polyglycidylether of phenol-formaldehyde novolac, polyglycidylether of o-cresol novolac or tetraglycidyl ether of tetraphenylethane. Suitable oxetane compounds, which are compounds that comprise at least one oxetano group per molecule, include compounds such as, for example, 3-ethyl-3[[(3- ethyloxetane-3-yl)methoxy]methyl]oxetane, oxetane-3-methanol, 3,3-bis- (hydroxymethyl) oxetane, 3-butyl-3-methyl oxetane, 3-methyl-3-oxetanemethanol, 3,3-dipropyl oxetane, and 3-ethyl-3-(hydroxymethyl) oxetane.

The curable matrix resin may optionally comprise one or more additives such as curing agents, curing catalysts, co-monomers, rheology control agents, tackifiers, inorganic or organic fillers, thermoplastic and/or elastomeric polymers as toughening agents, stabilizers, inhibitors, pigments, dyes, flame retardants, reactive diluents, UV absorbers and other additives well known to those of ordinary skill in the art for modifying the properties of the matrix resin before and/or after curing.

Examples of suitable curing agents include, but are not limited to, aromatic, aliphatic and alicyclic amines, or guanidine derivatives. Suitable aromatic amines include 4,4'-diaminodiphenyl sulphone ( 4,4'-DDS), and 3,3'diaminodiphenyl sulphone (3,3'- DDS), 1 ,3-diaminobenzene, 1 ,4-diaminobenzene, 4,4'-diammodiphenylmethane, benzenediamine(BDA); Suitable aliphatic amines include ethylenediamine (EDA), 4,4'-methylenebis(2,6-diethylaniline) (M-DEA), m-xylenediamine (mXDA), diethylenetriamine (DETA), triethylenetetramine (TETA), trioxatridecanediamine (TTDA), polyoxypropylene diamine, and further homologues, alicyclic amines such as diaminocyclohexane (DACH), isophoronediamine (IPDA), 4,4' diamino dicyclohexyl methane (PACM), bisaminopropylpiperazine (BAPP), N- aminoethylpiperazine (N-AEP); Other suitable curing agents also include anhydrides, typically polycarboxylic anhydrides, such as nadic anhydride, methylnadic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, endomethylene-tetrahydrophtalic anhydride, pyromellitic dianhydride, chloroendic anhydride and trimellitic anhydride.

Still other curing agents are Lewis acid: Lewis base complexes. Suitable Lewis acid: Lewis base complexes include, for example, complexes of: BCl3:amine complexes, BF3:amine complexes, such as BF3:monoethylamine, BF3:propylamine, BF3:isopropyl amine, BF3:benzyl amine, BF3:chlorobenzyl amine,

BF3:trimethylamine, BF3:pyridine, BF3:THF, AICb THF, AlCb acetonitrile, and ZnCI₂:THF.

Additional curing agents are polyamides, polyamines, amidoamines, polyamidoamines, polycycloaliphatic, polyetheramide, imidazoles, dicyandiamide, substituted ureas and urones, hydrazines and silicones.

Urea based curing agents are the range of materials available under the commercial name DYHARD (marketed by Alzchem), and urea derivatives, such as the ones commercially available as UR200, UR300, UR400, UR600 and UR700. Urone accelerators include, for example, 4,4-methylene diphenylene bis(N,N-dimethyl urea) (available from Onmicure as U52 M).

When present, the total amount of curing agent is in the range of 1 wt % to 60 wt % of the resin composition. Typically, the curing agent is present in the range of 15 wt % to 50 wt %, more typically in the range of 20 wt % to 30 wt %.

Suitable toughening agents may include, but are not limited to, homopolymers or copolymers either alone or in combination of polyamides, copolyamides, polyimides, aramids, polyketones, polyetherimides (PEI), polyetherketones (PEK), polyetherketoneketone (PEKK), polyetheretherketones (PEEK), polyethersulfones (PES), polyetherethersulfones (PEES), polyesters, polyurethanes, polysulphones, polysulphides, polyphenylene oxide (PPO) and modified PPO, polyethylene oxide) (PEO) and polypropylene oxide, polystyrenes, polybutadienes, polyacrylates, polystyrene, polymethacrylates, polyacrylics, polyphenylsulfone, high performance hydrocarbon polymers, liquid crystal polymers, elastomers, segmented elastomers and core-shell particles.

Toughening particles or agents, when present, may be present in the range 0.1 wt % to 30 wt % of the resin composition. In an embodiment, the toughening particles or agents may be present in the range 10 wt % to 25 wt %. In another embodiment, the toughening particles or agents may be present in the range from 0.1 to 10 wt%. Suitable toughening particles or agents include, for example, Virantage VW10200 FRP, VW10300 FP and VW10700 FRP from Solvay, BASF Ultrason E2020 and Sumikaexcel 5003P from Sumitomo Chemicals.

The toughening particles or agents may be in the form of particles having a diameter larger than 20 microns, to prevent them from being incorporated into the fiber layers. The size of the toughening particles or agents may be selected such that they are not filtered by the fiber reinforcement. Optionally, the composition may also comprise inorganic ceramic particles, microspheres, micro-balloons and clays.

The resin composition may also contain conductive particles such as the ones described in PCT International Publications WO 2013/141916, WO 2015/130368 and WO 2016/048885.

The mold for resin infusion may be a two-component, closed mold or a vacuum bag sealed, single-sided mold. Following infusion of the matrix resin in the mold, the mold is heated to cure the resin.

During heating, the resin reacts with itself to form crosslinks in the matrix of the composite material. After an initial period of heating, the resin gels. Upon gelling, the resin no longer flows, but rather behaves as a solid. After gel, the temperature or cure may be ramped up to a final temperature to complete the cure. The final cure temperature depends on the nature and properties of the thermosetting resin chosen. Thus, in a suitable method, the composite material is heated to a first temperature suitable to gel the matrix resin, after which the temperature is ramped up to a second temperature and held for a time at the second temperature to complete the cure.

An advantage of using carbon fibers treated by the process described herein in a composite material is the enhancement of the interfacial shear strength (IFSS). As used herein, interfacial shear strength refers to the strength of the interfacial adhesion between the matrix resin and carbon fibers. Improved IFSS is indicative of improved the translational tensile strength and compression strength of the bulk composite material. IFSS may be determined by any method known to those of ordinary skill in the art. For example, a suitable method is the so-called “single filament fragmentation” (SFF) test. Generally, a single fiber is embedded in matrix resin to form a dogbone coupon. Then the testing coupon is strained on a device that is capable of pulling the coupon to cause the embedded fiber to fragment. At the conclusion of the test, the number of fiber fragmentations is counted and a critical fiber length (l_c) is calculated by dividing the gauge length (i.e. , length of fiber embedded in the dogbone coupon) by the number of fragments. Without wishing to be bound by theory, it is believed that the smaller the l_c, the stronger the fiber/matrix interface is. SFF test is generally performed with 9 replicates for each sample.

IFSS, T, of the carbon fibers is then calculated according to the following relation:

wherein d is the fiber diameter, and O_f is the fiber tensile strength.

In an embodiment, the interfacial shear strength is in the range of from 1.6 to 9 ksi, typically from 3 to 7 ksi.

The methods and processes, including materials useful therefor, according to the present disclosure are further illustrated by the following non-limiting examples.

Example 1.

Standard modulus (SM) carbon fiber was subjected to the inventive process in which the carbon fiber was first exposed to plasma comprising carbon dioxide and/or a reactive species thereof to form pre-treated carbon fiber, which was subsequently subjected to a different plasma treatment in which the plasma comprised acetic acid, acrylic acid, allylamine, or methane/ammonia. Carbon fiber subjected to only plasma comprising carbon dioxide and/or a reactive species thereof, and carbon fiber exposed to acetic acid, acrylic acid, allylamine, C0₂/acrylic acid, or methane/ammonia each without CO2 plasma pre-treatment were used for comparison. The plasma pressure was maintained between 10 to 200 mTorrfor both plasma treatments. The results are summarized in FIG. 1.

As shown in FIG. 1 , the carbon fiber exposed to acetic acid, acrylic acid, allylamine, C0₂/acrylic acid, or methane/ammonia each without CO2 plasma pre-treatment exhibited interfacial shear strength similar to untreated carbon fiber. However, when the carbon fiber were pre-treated with C0₂-containing plasma and subsequently subjected to a different plasma treatment in which the plasma comprised acetic acid, acrylic acid, allylamine, or methane/ammonia, a dramatic increase in interfacial shear strength was observed.

Example 2.

Standard modulus (SM) carbon fibers were subjected to the inventive process in which the carbon fibers were first subjected to an atmospheric plasma treatment with plasma containing an inert gas and CO2 under various conditions (T able 1 ). The pre-treated carbon fiber was then subjected to a further atmospheric plasma treatment with plasma containing an inert gas and an organic compound comprising at least one vinyl group, i.e. , allylamine (AIA), under various conditions (Table 2).

The apparatus used for the CO2 pre-treatment was a single spot configuration with one plasma chamber discharging into a single afterglow chamber under inert non oxidizing gas followed by a second apparatus of the same configuration in series in a continuous process for the subsequent plasma treatment. Untreated fiber and fiber treated only with AIA without CO2 pre-treatment are included for comparison.

Table 1.

Table 2.

The surface elemental changes of the treated carbon fibers were measured with X- ray photoelectron spectroscopy (XPS). Carbon fiber tow tensile strength was measured in accordance with ASTM D4018 standard, as a method to insure lack of fiber damage and integrity of the inherent properties (strength, modulus, density and liner density) of the tow. The IFSS of the carbon fibers were determined using the single filament fragmentation test with ERONQ828 as the matrix resin and m- phenylenediamine (m-PDA) as hardener. The results are summarized in Table 3 below.

Table 3.

Little change was observed in the tow tensile properties of the fiber indicating that all surface treatment conditions did not damage the material. On plasma treating the fiber with AIA, the IFSS improved relative to untreated fiber. Further improvements in the IFSS were observed when pretreating the fiber with CO2 followed by plasma treatment with AIA. Example 3. Standard modulus (SM) carbon fibers were subjected to the treatment procedure described in Example 2, except that perfluorinated methyl vinyl ether (CF₃-O- CF=CF₂; MVE) was used instead of AIA. Untreated fiber and fiber treated only with MVE without CO₂ pre-treatment are included for comparison. The process conditions used are summarized in Table 4 and 5 below.

Table 4.

Table 5.

XPS was used to measure relative changes in the elemental composition of the fiber surfaces. Carbon fiber tow tensile strength was measured in accordance with ASTM D4018 standard, as a method to insure lack of fiber damage and integrity of the inherent properties (strength, modulus, density and liner density) of the tow. The fiber performance of plasma treatment for SM fiber after plasma surface treatment with MVE are summarized in Table 6 below.

Table 6.

XPS results indicated significant increases in surface fluorine content with little change in surface nitrogen or oxygen, for both fiber types. A statistically insignificant difference in tensile strength between untreated and plasma-treated fiber indicating no fiber damage due surface treatment.

Claims

WHAT IS CLAIMED IS:

1. A process for treating carbon fiber, the process comprising: a) exposing one or more carbon fibers to plasma comprising carbon dioxide, and/or a reactive species thereof, thereby forming one or more pre-treated carbon fibers; and b) exposing the one or more pre-treated carbon fibers formed in step a) to a further surface treatment, thereby forming the treated carbon fiber.

2. The process according to claim 1 , wherein the one or more carbon fibers, prior to treatment, are the result of carbonization at a temperature of at least 1300 °C, at least 1350 °C, or at least 1380 °C.

3. The process according to claims 1 or 2, wherein the one or more carbon fibers, prior to treatment, are the result of carbonization at a temperature from 1300 to 2800 °C, typically 1350 to 1700 °C, more typically 1380 to 1500 °C, still more typically 1400 to 1500 °C.

4. The process according to any one of claims 1-3, wherein the one or more carbon fibers are standard modulus carbon fibers, intermediate modulus carbon fibers, or high modulus carbon fibers.

5. The process according to anyone of claims 1-4, wherein the exposure time in step a) is from 0.5 second to 2 minutes, typically 1 to 10 seconds.

6. The process according to any one of claims 1 to 5, wherein step a) of the process is continuous.

7. The process according to any one of claims 1 to 6, wherein the plasma pressure in step a) is 10 to 200 mTorr, typically 25 to 150 mTorr, more typically 30 to 100 mTorr.

8. The process according to any one of claims 1 to 6, wherein the plasma pressure in step a) is at atmospheric pressure.

9. The process according to any one of claims 1 to 8, wherein the further surface treatment is an electrochemical treatment or a plasma treatment, typically plasma treatment.

10. The process according to any one of claims 1 to 9, wherein the further surface treatment is a plasma treatment comprising exposing the one or more pre-treated carbon fibers formed in step a) to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group, carbon dioxide, ammonia, amines, carboxylic acids, esters, alkanes, or a mixture thereof, with the proviso that carbon dioxide may be present only in combination with an organic compound comprising at least one vinyl group, ammonia, amines, carboxylic acids, esters, alkanes, or a mixture thereof.

11 . The process according to any one of claims 1 to 10, wherein the further surface treatment is a plasma treatment comprising exposing the one or more pre treated carbon fibers formed in step a) to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group, typically acrylic acid, allylamine, and/or perfluorinated methyl vinyl ketone, optionally with carbon dioxide; a carboxylic acid, typically acetic acid; or ammonia with an alkane, typically methane.

12. The process according to any one of claims 1 to 11 , wherein the further surface treatment in step b) is a plasma treatment comprising exposing the one or more pre-treated carbon fibers formed in step a) to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group, typically acrylic acid, allylamine, and/or perfluorinated methyl vinyl ketone.

13. The process according to any one of claims 1 to 12, wherein the further surface treatment is a plasma treatment conducted at 10 to 200 mTorr, typically 25 to 150 mTorr, more typically 30 to 100 mTorr.

14. The process according to any one of claims 1 to 12, wherein the further surface treatment is a plasma treatment conducted at a plasma pressure at atmospheric pressure.

15. The process according to any one of claims 1 to 14, further comprising applying a sizing agent to the one or more treated carbon fibers formed.

16. One or more treated carbon fibers obtained by the process according to any one of claims 1-15.

17. A composite material comprising: one or more treated carbon fibers obtained by the process according to any one of claims 1-15 or one or more treated carbon fibers according to claim 16, and a matrix resin.

18. The composite material according to claim 17, wherein the interfacial shear strength is in the range of from 1 .6 to 9 ksi, typically from 3 to 7 ksi.