CARBON FIBER

Cross Reference to Related Applications

This application claims the priority of U.S. Provisional Application No. 62/951453, 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 treatment in which tailored grafting structure is built on the carbon fiber surface and composite materials, typically reinforced composite materials, having tailored interphase structure made from such treated 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 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. An interphase layer may be formed between each carbon fiber and the matrix resin, and the strength of the bond formed between the carbon fiber and the interphase layer and/or the strength of the bond formed between the interphase layer and the matrix resin may affect the properties, such as mechanical properties, of the carbon fiber-reinforced resin or composite material.

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 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 continuous process for treating carbon fiber, the process comprising:

Exposing one or more carbon fibers to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group and/or other gas that can modify the surface of the one or more carbon fibers to promote interphase formation at a plasma pressure of 10 to 200 mTorr, thereby forming one or more treated carbon fibers.

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, wherein an interphase layer is disposed between each carbon fiber and the matrix resin.

Brief Description of the Figures

FIG. 1 shows the schematic front view (a) and side view (b) of a vacuum plasma chamber suitable for use in the process described herein.

FIG. 2 shows the interfacial shear strength (IFSS) of standard modulus (SM) carbon fibers subjected to the inventive treatment process (30 seconds exposure time) described herein using various plasmas, each containing an organic compound comprising at least one vinyl group and/or other gas that can modify the carbon fiber surface to promote interphase formation, and the comparison with untreated carbon fibers as a control.

FIG. 3 shows the tensile strength of the standard modulus carbon fibers subjected to the treatment process described herein using plasma having CO2 and acetic acid (30 seconds exposure time) with untreated carbon fibers as a control.

FIG. 4 shows the interfacial shear strength (IFSS) of intermediate modulus (IM) carbon fibers subjected to the vacuum plasma treatment process (2 min exposure time) described herein using various plasmas, each containing an organic compound comprising at least one vinyl group and/or other gas that can modify the carbon fiber surface to promote interphase formation within a composite matrix, and the comparison with untreated carbon fibers as a control.

FIG. 5 shows the tensile strength of the intermediate modulus carbon fibers subjected to the vacuum plasma treatment process (2 min exposure time) described herein using various plasmas, each containing an organic compound comprising at least one vinyl group and/or other gas that can modify the carbon fiber surface indicating the lack of fiber damage from such process, and the comparison with untreated carbon fibers as a control. 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 “and/or” used in a phrase in the form of “A and/or B” means A alone, B alone, or A and B together.

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. As used herein, the terminology "(Cx-Cy)" in reference to an organic group, wherein x and y are each integers, means that the group may contain from x carbon atoms to y carbon atoms per group.

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 continuous process for treating carbon fiber, the process comprising:

Exposing one or more carbon fibers to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group and/or other gas that can modify the surface of the one or more carbon fibers to promote interphase formation at a plasma pressure of 10 to 200 mTorr, thereby forming one or more treated carbon fibers.

As used herein, a continuous process for treating carbon fiber 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 accordance with the inventive process disclosed herein, the one or more carbon fibers are continuously exposed to plasma comprising reactive species derived from an organic compound comprising at least one vinyl group and/or other gas combinations that can modify the carbon fiber surface to promote interphase formation. The exposure of the one or more carbon fibers to the plasma may be achieved by continuously conveying the carbon fiber through any apparatus known to those of ordinary skill in the art capable of generating the desired plasma and allowing the plasma to contact the carbon fiber while maintaining the desired pressure. For example, the treatment may be conducted in a vacuum plasma chamber. Generally, other than those that are pitch-based and rayon-based, carbon fiber is produced from acrylonitrile in a series of manufacturing steps or stages, including polymerization, spinning, drawing and/or washing, oxidation, and carbonization. Therefore, the process and/or the apparatus used to carry out the process may be configured to be in-line within such a manufacturing process, for example, following the carbonization step and before downstream steps, such as sizing, drying and winding steps.

In an embodiment, the process described herein is conducted in a vacuum plasma chamber, such as the one shown in FIG. 1 . The vacuum plasma chamber (3) shown in FIG. 1 may be placed in the carbon fiber manufacture process following carbonization and prior to sizing. Carbon fiber, typically in the form of tow (4), enters and exits the vacuum plasma chamber through vacuum sealing valves (1). The carbon fiber tow undergoes one or more wraps on guiding wheels (2) to achieve a certain exposure time in the vacuum plasma chamber.

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 continuous process described herein. For example, spooled carbon fiber may be continuously unwound, conveyed through the vacuum plasma chamber, optionally conveyed through further processing steps, such as a sizing step, and then re wound on a spool for storage.

As would be apparent to the ordinarily-skilled artisan, the one or more carbon fibers are directly exposed to the uniformly distributed plasma intensity in the vacuum plasma chamber, which allows for uniform treatment of the carbon fiber. This is in contrast to indirect exposure methods in which the plasma is generated in a separate location and blown onto the carbon fiber from a fixed direction. As indicated herein, the plasma pressure is 10 to 200 mTorr within the chamber. However, plasma pressures of 25 to 150 mTorr, typically 30 to 100 mTorr, are also suitable.

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 is from 1 second to 5 minutes, typically 1 to 60 seconds. As would be appreciated by persons of ordinary skill in the art, operating parameters such as the line speed, or the speed by which the carbon fiber goes through the plasma, and the area of plasma exposure, may be adjusted such that the exposure time falls within the ranges disclosed herein.

The plasma suitable for use in the process described herein comprises reactive species derived from an organic compound comprising at least one vinyl group and/or other gas that can modify the carbon fiber surface to promote interphase formation. 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, typically with radio frequency, and microwave, and may be thermal or non-thermal (i.e., “cold” plasma). It would be understood by those of ordinary skill in the art that the reactive species derived from the organic compound comprising at least one vinyl group and/or other gas that can modify the carbon fiber surface to promote interphase formation in the plasma would be generated from a precursor gas comprising the said organic compound comprising at least one vinyl group and/or the said other gas.

In an embodiment, the plasma comprises reactive species is derived from a compound having the formula: wherein

Ri, R ₂ , and R ₃ are each, independently, H or halogen;

L is a bond, -(CH ₂ ) _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;

R ₄ is H, alkyl, -NR ₅ R ₆ , or -ORs, wherein R ₅ and R ₆ are each, independently, H or alkyl.

In another embodiment, Ri, R ₂ , and R ₃ are each H.

In yet another embodiment, Ri, R ₂ , and R ₃ are each independently selected from the group consisting of F, Cl, Br, and I; typically Ri, R ₂ , and R ₃ 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, R ₄ is alkyl, typically fluoroalkyl, more typically perfluoroalkyl.

In one embodiment, R ₄ is -NH ₂ .

In another embodiment, R ₄ is -OH.

In yet another embodiment, Ri, R ₂ , and R ₃ are each H;

L is -CH ₂ - or -(C=0)-;

R ₄ is -NH ₂ or -OH.

In another embodiment, Ri, R ₂ , and R ₃ are each F;

L is a bond or -O-;

R ₄ is perfluoroalkyl, typically perfluoromethyl.

In some embodiments, the plasma comprises reactive species derived from acrylic acid and/or allylamine.

The precursor gas may comprise a gas that can modify the carbon fiber surface to promote interphase formation. Interphase formation may be observed when the treated carbon fiber is used to manufacture a composite material and the composite material is then tested using known methods, such as scanning electron microscopy (SEM). A gas that can modify the carbon fiber surface to promote interphase formation may be selected from the group consisting of carbon dioxide, ammonia, amines, carboxylic acids, esters, and alkanes. Suitable amines include, but are not limited to, primary amines, secondary amines, and tertiary amines having at least one (C ₁ -C ₆ ) alkyl group. Suitable carboxylic acids include, but are not limited to, carboxylic acids having at least one (C ₁ -C ₆ ) 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 (C ₁ -C ₆ ) alkyl group, such as acetic acid, propionic acid, butyric acid, and the like, with (C ₁ -C ₆ ) alkyl alcohols, such as methanol, ethanol, propanol, and the like. Suitable alkanes include, but are not limited to, (C ₁ -C ₆ ) alkanes, such as methane, ethane, propane, butane, pentane, hexane, and the like. The gas that can modify the carbon fiber surface to promote interphase formation may also be a combination of two or more of the gases described herein. In an embodiment, a gas combination that can modify the carbon fiber surface to promote interphase formation is carbon dioxide and carboxylic acid, typically acetic acid. In another embodiment, a gas combination that can modify the carbon fiber surface to promote interphase formation is ammonia and alkane, typically methane.

The precursor gas may further comprise other gases, typically non-oxidative gases, including, but not limited to, helium, argon, other inert gases, and nitrogen (N ₂ ). 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. However, suitable carbon fibers, prior to treatment by the process described herein, are the result of carbonization at a temperature from 1200 to 2800 °C, typically 1200 to 1700 °C, more typically 1300 to 1500 °C. The one or more carbon fibers may be standard modulus carbon fibers or intermediate modulus carbon fibers.

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 processability, among other advantages. Thus, in an embodiment, the process further comprises applying a sizing agent to the one or more treated carbon fibers. Suitable sizing agents include, but are not limited to, epoxy resin, polyurethane resin, polyamide resin, polyimide resin, and combinations thereof.

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, wherein an interphase layer is disposed between each carbon fiber and the 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, perfluorosulfonic acids, poly amid-imides, polyamides, polyesters, polyketones, polyphenylene sulfides, 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 HF, AICl3:acetonitrile, and ZnCI ₂ :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.

Without wishing to be bound by theory, it is believed that a long-chain structure is grafted or deposited onto the one or more carbon fibers during the treatment process described herein. The grafting or deposition of such a long-chain structure on the surface of the one or more carbon fibers results in an interphase layer that bonds to both the carbon fiber as well as the matrix resin when a composite material is produced. Therefore, an interphase layer is formed between each carbon fiber and the matrix resin in the composite material. The properties of the interphase layer can be controlled, or tailored, to the type of carbon fiber and matrix resin to be used in the composite material, for example, by modifying the parameters described herein. For example, the thickness of the interphase layer can be modulated by modulating the exposure time. The interphase layer is believed to have intermediate mechanical properties that allow it to crack, rather than the carbon fiber and/or the matrix resin, during the fracture of the composite material.

An advantage of the tailored interphase resulting from 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 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 lc, 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 fibers were subjected to the plasma treatment process described herein using the vacuum plasma chamber illustrated in FIG. 1 for 2 minutes using various plasmas, each containing an organic compound comprising at least one vinyl group and/or other gas that can modify the carbon fiber surface to promote interphase formation. The power for plasma generation was 250 watt. The IFSS for the treated standard modulus carbon fibers were determined using the single filament fragmentation test with an epoxy resin (available as EPON®828 from Hexion) as the matrix resin and m-phenylenediamine (m-PDA) as hardener. For comparison, untreated standard modulus carbon fibers were used. The IFSS results are shown in FIG. 2.

As shown in FIG. 2, the standard modulus carbon fibers treated with plasma containing an organic compound comprising at least one vinyl group and/or other gas that can modify the carbon fiber surface to promote interphase formation showed significantly higher IFSS than the untreated control fibers. The tensile strength of one plasma-treated fiber was tested and compared to that of the untreated control fibers. The results are shown in FIG. 3. It could be seen that the plasma treatment showed no harm or damage to the tensile strength of the fibers.

Example 2.

Intermediate modulus (IM) carbon fibers were subjected to the plasma treatment described herein using various plasmas, each containing acrylic acid, allylamine, ammonia, or carbon dioxide. The IFSS for the treated intermediate modulus carbon fibers were determined using the single filament fragmentation test with epoxy resin (ERONQ828) as the matrix resin and m-phenylenediamine (m-PDA) as hardener.

For comparison, untreated intermediate modulus carbon fibers were used. The IFSS results are shown in FIG. 4.

As shown in FIG. 4, the IM carbon fibers treated with plasma containing acrylic acid, allylamine, ammonia, or carbon dioxide showed significantly higher IFSS than control, with the IM carbon fibers treated with plasma containing acrylic acid showing the highest level of IFSS. The tensile strength of the plasma treated fibers was tested and compared to the untreated control fibers. The results are shown in FIG.

5. It can be seen that the plasma treatment showed no harm or damage to the tensile strength of the fibers.

Example 3.

IM carbon fibers that had been treated with plasma comprising acrylic acid were embedded in epoxy resin (available as EPON 836® resin from Hexion) with polypropylene glycol-based monoamine (available as Jeffamine® from Huntsman) as hardener. Then the fiber along with the epoxy resin matrix was etched from the top by Focused Ion Beam (FIB) to observe the interphase structure under Scanning Electron Microscopy (SEM). Another sample with untreated control carbon fibers was prepared in exactly the same way. It was observed that the adhesion between untreated fibers and epoxy resin matrix was very poor while the acrylic acid plasma treated carbon fiber showed excellent bonding with the resin matrix and the presence of an interphase layer was clearly apparent.

Example 4. A tow of IM carbon fibers (12K filament count) treated with plasma having acrylic acid was embedded in epoxy (EPON 836® resin) with polypropylene glycol-based monoamine (Jeffamine®) as hardener. The tow was then pulled out from the resin matrix with a universal testing machine (Instron®). The morphology of the pulled-out tow was observed under SEM. It was observed that there was more resin remaining on the surface of the treated fibers after being pulled out from epoxy resin than on the surface of untreated fibers, which were relatively clean after being pulled out from the epoxy resin. Such a result is indicative of better adhesion between the fiber and resin in the case when the fibers were treated according to the inventive process. The surface morphology of the fibers on the pulled-out tow was also examined under higher magnification. It could be seen that the interphase layer formed in Example 3 was broken and showed irregular cracking structure when the tow was pulled out from the resin matrix. Such interphase cracking was believed to be responsible for the increase in IFSS test. EDX mapping, which tracks oxygen that only exists in the resin, was done to the same region to confirm the structure of the interphase. The observation of oxygen on the surface of the of the pulled-out treated tow is indicative of partially coverage of the carbon fiber by the resin bonded to it and confirms that the interphase layer cracked along the interphase layer on the carbon fibers.