WO2020028624A1

WO2020028624A1 - Method for determining the degree of swelling of a polymer using near-ir

Info

Publication number: WO2020028624A1
Application number: PCT/US2019/044600
Authority: WO
Inventors: Suzanne CRAWFORD; John Cook
Original assignee: Cytec Industries, Inc.
Priority date: 2018-08-01
Filing date: 2019-08-01
Publication date: 2020-02-06

Abstract

The present disclosure relates to a method for determining the degree of swelling of polymer fiber, such as polymer fiber used in the manufacturing of composite materials, using infrared spectroscopy, notably near-infrared spectroscopy.

Description

METHOD FOR DETERMINING THE DEGREE OF SWELLING OF A POLYMER

USING NEAR-IR

Cross Reference to Related Applications

This application claims the priority of U.S. Provisional Application No. 62/713108, filed August 1 , 2018. The entire content of this application is explicitly incorporated herein by this reference.

Field of the Invention

The present disclosure relates generally to the use of infrared-based measurements in the production of polymer fiber, and more particularly to a method for determining the degree of swelling of polymer fiber, such as polymer fiber used in manufacturing composite materials.

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 from acrylonitrile is generally produced by a series of manufacturing steps or stages, including polymerization, spinning, drawing and/or washing, oxidation, and carbonization. Polyacrylonitrile (PAN) polymer is currently the most widely used precursor for carbon fibers.

During the precursor spinning process, polymer dope is generally brought into contact with a coagulation bath. Typically, a diffusional interchange occurs between the two phases in which solvent leaves the forming filament as water enters, causing the polymer to phase separate from the solvent and precipitate. A solid fibrillar network is then formed during this period of densification. The inherent nature of the fibrillar structure of the coagulated fiber has a major influence on the tensile properties, abrasion strength, and other mechanical properties of the finished PAN precursor.

These properties are then translated to the resulting carbon fiber. The fiber

denseness/porosity may be measured by evaluating the degree of swelling of the fiber.

In one method for measuring degree of swelling, samples are taken from the

coagulation bath and first centrifuged at 3000 rpm for 15 minutes to remove the adhered liquid from the filament surface. The collected samples are then submerged in a glass beaker/flask containing deionized water, and“washed” for a minimum of 15 minutes. This washing step is then repeated twice more with fresh deionized water to ensure the samples are fully coagulated and solvent has been removed. Once the final wash is completed, the sample is centrifuged again at 3,000 rpm for 15 minutes and weighed to obtain after-wash weight, or W_a. Samples are then placed in an air circulating oven at 110° C. for 3 hours. Following drying, samples are removed from the oven and placed in a desiccator for a minimum of ten minutes. The dried and desiccated samples are re- weighed and the final weight recorded as W_f. The degree of swelling is then calculated using the following relation:

Degree of Swelling (%) = (W_a - W_f)x(100/W_f)

This method correlates the fiber porosity to the liquid uptake. As the coagulated fiber denseness increases, the degree of swelling will decrease proportionally. By measuring the degree of swelling, and noting the change in fiber structure or denseness, it is possible to predict the final carbon fiber performance. It is also possible to modify current spinning parameters, such as coagulation bath concentration, coagulation bath temperature, polymer temperature or polymer concentration to optimize the coagulated fiber denseness. However, each run of this method requires almost 4 hours to complete due to the lengthy drying step. In a continuous process, using such a method to determine degree of swelling would result in the continued formation of a potentially inferior product for hours unnecessarily.

U.S. Patent 6,641 ,915 discloses a method for measuring denseness of an acrylonitrile fiber using iodine absorption. In this method, the amount of iodine absorbed into the fiber is measured. A low amount of iodine adsorption in the fiber is indicative of high denseness and, conversely, a high amount of iodine adsorption in the fiber is indicative of low denseness in the fiber. However, this method requires the use of a lethal solvent (2,4-dichlorophenol) that can cause instantaneous organ failure and, therefore, presents a major health hazard.

Thus, there is an ongoing need for safe and time-efficient methods for measuring the degree of swelling of polymer fiber, particularly during the continuous manufacture of carbon fiber.

Summary of the Invention

It has been discovered that IR, particularly near-IR, may be used to measure the degree of swelling of polymer fiber. The inventive analysis method does not require the lengthy washing and drying steps required in the aforementioned degree of swelling test. The inventive method described herein reduces the testing time from about 4 hours to about 30 minutes, typically about 20 minutes, which means modifications to the coagulation bath to correct the structure can be made much faster and eliminates the formation of inferior product that would occur during the extra 3.5 hours time. Thus, in a first aspect, the present disclosure relates to a method of determining the degree of swelling of a polymer fiber, the method comprising: a) irradiating said polymer fiber with infrared energy over a spectrum of wavelengths in the range of from 400 to 2500 nm, typically from 700 to 2500 nm; b) detecting said infrared energy reflected from said polymer fiber over said spectrum of wavelengths; c) performing multivariate analysis on the spectrum of said reflected infrared energy; d) comparing the results of said multivariate analysis with a predetermined correlation between model infrared energy spectra comprising said spectrum of wavelengths collected from a plurality of model polymer fiber samples, said model polymer fiber samples each comprising a different degree of swelling; and, e) determining the degree of swelling of the polymer fiber based on said

predetermined correlation.

In a second aspect, the present disclosure relates to a process for producing carbon fibers, the process comprising:

1 ) preparing a polymer solution;

2) spinning the polymer solution prepared in step a) in a coagulation bath, thereby forming carbon fiber precursor fibers; 3) drawing the carbon fiber precursor fibers through one or more draw and wash baths, thereby forming drawn carbon fiber precursor fibers that are substantially free of solvent; and

4) oxidizing the carbon fiber precursor fibers of step c) to form stabilized carbon fiber precursor fibers and then carbonizing the stabilized carbon fiber precursor fiber, thereby producing carbon fibers; wherein step 2) further comprises determining the degree of swelling of the carbon fiber precursor fibers formed by performing the method described herein;

or

wherein step 3) further comprises determining the degree of swelling of the drawn carbon fiber precursor fibers by performing the method described herein.

Brief Description of the Figures

FIG. 1 shows the degrees of swelling of coagulated fiber samples as determined by a known method (“swell test”) and by the inventive NIR method described herein.

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.” The first aspect of the present disclosure relates to a method of determining the degree of swelling of a polymer fiber, the method comprising: a) irradiating said polymer fiber with infrared energy over a spectrum of wavelengths in the range of from 400 to 2500 nm, typically from 700 to 2500 nm; b) detecting said infrared energy reflected from said polymer fiber over said spectrum of wavelengths; c) performing multivariate analysis on the spectrum of said reflected infrared energy; d) comparing the results of said multivariate analysis with a predetermined

correlation between model infrared energy spectra comprising said spectrum of wavelengths collected from a plurality of model polymer fiber samples, said model polymer fiber samples each comprising a different degree of swelling; and, e) determining the degree of swelling of the polymer fiber based on said

predetermined correlation.

The method of the present disclosure is performed on polymer fiber. In an embodiment, the polymer fiber comprises a polyacrylonitrile-based polymer. In another embodiment, the polymer fiber is a carbon fiber precursor fiber. In yet another embodiment, the polymer fiber is a carbon fiber precursor fiber that has not been drawn or dried. In still another embodiment, the polymer fiber is a carbon fiber precursor fiber that has not been dried.

The method described herein makes use of IR spectroscopy, typically near-infrared (NIR) spectroscopy. NIR spectroscopy is vibrational spectroscopy which involves overtones and combination bands of the fundamental molecular absorptions found in the near infrared region (700 to 2500 nm or 4000 to 12821 cm ¹). Overtones occur from 700 to 1600 nm at multiples of the frequency. Combinations occur from sharing between two or more fundamental absorptions. Their frequencies are the sums of multiples of each interacting frequency and occur in the 1600 to 2500 nm range. Due to hydrogen being the lightest atom, bonds with hydrogen exhibit the largest vibrations (C- H, N-H, O-H, and S-H). This makes NIR advantageous for analysis of most chemical species.

Thus, any suitable IR spectrometer capable of providing infrared energy to a sample over a spectrum of wavelengths in the range of from 400 to 2500 nm, typically from 700 to 2500 nm, and detecting the reflected infrared energy from the irradiated sample over the specified spectrum of wavelengths may be used to perform the method described herein. The IR spectrometer may be portable or non-portable.

The IR spectrometer suitable for use according to the present disclosure is typically connected to a computer either directly or remotely, for example, via a local area network. Generally, the computer has the ability to store collected IR spectra as well as perform mathematical manipulation of the data comprising the spectra, including multivariate analysis of the spectra.

Multivariate statistical approaches may be used to correlate the statistically determined changes in the plurality of variables (e.g., absorbance and/or reflectance at one or more wavelengths) with one or more second variables, such as a change in a separately measured chemical and/or physical property, typically degree of swelling of polymer fiber.

Suitable multivariate techniques for use in the method described herein include, but are not limited to, quantification methodologies, such as, partial least squares, principal component regression (“PCR”), linear regression, multiple linear regression, stepwise linear regression, ridge regression, radial basis functions, and the like. Multivariate analysis in accordance with the present disclosure may also include suitable multivariate statistical approaches, including classification methodologies, such as, linear discriminant analysis (“LDA”), cluster analysis (e.g., k-means, C-means, etc., both fuzzy and hard), and neural network (“NN”) analysis.

Further, it will be appreciated that there are several data processing methods that may be suitably used to in connection with suitable multivariate statistical approaches including smoothing, taking first and second derivatives of the IR spectra, and peak enhancement methods.

In addition, multivariate analysis of collected IR spectra may include the selection and clustering together of groups of wavelengths on which to perform a regression analysis to determine a corresponding change in the IR spectra (spectrum) (e.g., reflectance) with respect to reference spectra (spectrum). It will be appreciated that an individual IR spectrum may be formed from several IR spectra (e.g., by averaging techniques known in the art). In addition, the raw IR spectra may transformed into second IR spectra by taking first and/or second derivatives and performing smoothing and/or peak

enhancement as well as carrying out regression analysis. For example, manipulation the raw IR spectra by smoothing algorithms prior to or following taking a first derivative and then quantifying a degree of change of the IR spectra from a reference spectrum (similarly processed) according to a regression or partial lest squares analysis may be performed.

In addition, the IR spectroscopy measurement process may include collecting reference IR spectra (including calculated absorbance and/or reflectance) which may serve as a baseline from which to determine relative changes in sample IR spectra by multivariate analysis. In addition, various processing methods as are known in the art may be used to form a single IR spectrum from a collection of a plurality of collected IR spectra, including various averaging techniques, for example to improve a signal to noise ratio, prior to carrying out multivariate analysis to determine a relative change from reference spectrum. It will be appreciated that the relative change may include changes at one or more wavelengths including clusters of wavelengths.

In an embodiment, multivariate analysis is performed on the spectrum of a sample polymer fiber having a degree of swelling to be determined (step c). The results of said multivariate analysis is compared with a predetermined correlation between model infrared energy spectra comprising said spectrum of wavelengths collected from a plurality of model polymer fiber samples, said model polymer fiber samples each comprising a different degree of swelling (step d). The degree of swelling of the “unknown” polymer fiber is then determined on the basis of the predetermined correlation.

The predetermined correlation is obtained by correlating the wavelengths in the spectra of a number of model polymer fiber samples to their respective degrees of swelling, which are determined by any independent method.

Further, the separately measured chemical/physical property, typically, degree of swelling, may be further cross-correlated with another chemical/physical property, for example, concentration.

In an embodiment, the predetermined correlation between model infrared energy spectra comprising said spectrum of wavelengths collected from a plurality of model polymer samples, said model polymer samples each comprising a different

concentration, used in step d) is obtained prior to step a).

The inventive method described herein reduces the time needed to determine the degree of swelling of polymer fiber from almost 4 hours to a time on the order of minutes. In an embodiment, the method is performed within 30 minutes, typically within 20 minutes. The method described herein may further comprise adjusting the degree of swelling of the polymer fiber to a desired level based on the degree of swelling determined in step e). The degree of swelling may be adjusted by modifying, for example, fiber spinning parameters, such as coagulation bath concentration, coagulation bath temperature, polymer temperature or polymer concentration, thereby achieving the desired level of swelling and optimizing the coagulated fiber denseness.

The second aspect of the present disclosure relates to a process for producing carbon fibers, the process comprising:

1 ) preparing a polymer solution;

2) spinning the polymer solution prepared in step a) in a coagulation bath, thereby forming carbon fiber precursor fibers;

3) drawing the carbon fiber precursor fibers through one or more draw and wash baths, thereby forming drawn carbon fiber precursor fibers that are substantially free of solvent; and

or

wherein step 3) further comprises determining the degree of swelling of the drawn carbon fiber precursor fibers by performing the method described herein. Preparing the polymer solution may be achieved according to any method known to those of ordinary skill in the art. One suitable method is a method in which the polymer is formed in a medium, typically one or more solvents, in which the polymer is soluble to form a solution.

Another suitable method is a method in which the polymer is formed in a medium, typically aqueous medium, in which the polymer is sparingly soluble or non-soluble to form a mixture, isolating the resulting polymer, for example, by filtration, and dissolving the resulting polymer in one or more solvents to form a polymer solution.

The polymer is typically a polyacrylonitrile-based (PAN) polymer comprising repeating units derived from acrylonitrile.

The polymer may further comprise repeating units derived from other comonomers. Such repeating units may be derived from suitable comonomers including, but not limited to, vinyl-based acids, such as methacrylic acid (MAA), acrylic acid (AA), and itaconic acid (ITA); vinyl-based esters, such as methacrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), methyl methacrylate (MMA), ethyl methacrylate (EMA), propyl methacrylate, butyl methacrylate, b-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, 2-ethylhexylacrylate, isopropyl acetate, vinyl acetate (VA), and vinyl propionate; vinyl amides, such as vinyl imidazole (VIM), acrylamide (AAm), and diacetone acrylamide (DAAm); vinyl halides, such as allyl chloride, vinyl bromide, vinyl chloride and vinylidene chloride; ammonium salts of vinyl compounds and sodium salts of sulfonic acids, such as sodium vinyl sulfonate, sodium p-styrene sulfonate (SSS), sodium methallyl sulfonate (SMS), and sodium-2-acrylamido-2-methyl propane sulfonate (SAMPS), among others.

The polymer can be made by any polymerization method known to those of ordinary skill in the art. Exemplary methods include, but are not limited to, solution polymerization, dispersion polymerization, precipitation polymerization, suspension polymerization, emulsion polymerization, and variations thereof.

One suitable method comprises mixing a first monomer, typically acrylonitrile (AN) monomer, and a second monomer, typically a co-monomer described herein, in a solvent in which the polymer is soluble, thereby forming a solution. The solution is heated to a temperature above room temperature (i.e., greater than 25 °C), for example, to a temperature of about 40 °C to about 85 °C. After heating, an initiator is added to the solution to initiate the polymerization reaction. Once polymerization is completed, unreacted AN monomers are stripped off (e.g., by de-aeration under high vacuum) and the resulting PAN polymer solution is cooled down. At this stage, the polymer is in a solution, or dope, form.

Examples of suitable solvents include, but are not limited to, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), dimethyl acetamide (DMAc), ethylene carbonate (EC), zinc chloride (ZnChywater and sodium thiocyanate (NaSCN)/water.

In another suitable method, the first monomer, typically acrylonitrile (AN) monomer, and the second monomer, typically a co-monomer described herein, may be polymerized in a medium, typically aqueous medium, in which the resulting polymer is sparingly soluble or non-soluble. In this manner, the resulting polymer would form a heterogenous mixture with the medium. The polymer is then filtered and dried.

The comonomer ratio (amount of one or more comonomers to amount of acrylonitrile) is not particularly limited. However, a suitable comonomer ratio is 0 to 20%, typically 1 to 5%, more typically 1 to 3%.

Suitable initiators (or catalysts) for the polymerization include, but are not limited to, azo-based compounds, such as azo-bisisobutyronitrile (AIBN), 2,2'-azobis[2-(2- imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis(2- methylpropionamidine)dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2- methylpropionamidine]tetrahydrate, 2,2'-azobis[2-(2-imidazolin-2-yl)propane], 2,2'- azobis[2-methyl-N-(2-hydroxyethyl)propionamide], 4,4'-azobis(4-cyanovaleric acid), 2,2’- azobis-(2, 4-dimethyl) valeronitrile (ABVN), among others; and organic peroxides, such as dilauroyl peroxide (LPO), di-tert-butyl peroxide (TBPO), diisopropyl

peroxydicarbonate (IPP), among others.

After the polymer solution is prepared, carbon fiber precursor fibers are formed by spinning the polymer solution prepared in step 1 ) in a coagulation bath.

The term“precursor fiber” refers to a fiber comprising a polymeric material that can, upon the application of sufficient heat, be converted into a carbon fiber having a carbon content that is about 90% or greater, and in particular about 95% or greater, by weight.

To make the carbon fiber precursor fibers, the polymer solution (i.e. , spin“dope”) is subjected to conventional wet spinning and/or air-gap spinning after removing air bubbles by vacuum. The spin dope can have a polymer concentration of at least 10 wt %, typically from about 16 wt % to about 28 wt % by weight, more typically from about 19 wt % to about 24 wt %, based on total weight of the solution. In wet spinning, the dope is filtered and extruded through holes of a spinneret (typically made of metal) into a liquid coagulation bath for the polymer to form filaments. The spinneret holes determine the desired filament count of the fiber (e.g., 3,000 holes for 3K carbon fiber).

In air-gap spinning, a vertical air gap of 1 to 50 mm, typically 2 to 10 mm, is provided between the spinneret and the coagulating bath. In this spinning method, the polymer solution is filtered and extruded in the air from the spinneret and then extruded filaments are coagulated in a coagulating bath.

The coagulation liquid used in the process is a mixture of solvent and non-solvent.

Water or alcohol is typically used as the non-solvent. Suitable solvents include the solvents described herein. In an embodiment, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, or mixtures thereof, is used as solvent. In another embodiment, dimethyl sulfoxide is used as solvent. The ratio of solvent and non-solvent, and bath temperature are not particularly limited and may be adjusted according to known methods to achieve the desired solidification rate of the extruded nascent filaments in coagulation. However, the coagulation bath typically comprises 40 wt% to 85 wt% of one or more solvents, the balance being non-solvent, such as water or alcohol. In an embodiment, the coagulation bath comprises 40 wt% to 70 wt% of one or more solvents, the balance being non-solvent. In another embodiment, the coagulation bath comprises 50 wt% to 85 wt% of one or more solvents, the balance being non-solvent.

Typically, the temperature of the the coagulation bath is from 0 °C to 80 °C. In an embodiment, the temperature of the coagulation bath is from 30 °C to 80 °C. In another embodiment, the temperature of the coagulation bath is from 0 °C to 20 °C.

Step 2) may further comprise determining the degree of swelling of the carbon fiber precursor fibers formed by performing the method of determining the degree of swelling described herein.

In an embodiment, step 2) further comprises determining the degree of swelling of the carbon fiber precursor fibers formed by performing the method of determining the degree of swelling described herein, and adjusting the degree of swelling of the carbon fiber precursor fibers to a desired level based on the degree of swelling determined.

The degree of swelling may be adjusted by modifying, for example, fiber spinning parameters, such as coagulation bath concentration, coagulation bath temperature, polymer temperature or polymer concentration, thereby achieving the desired level of swelling and optimizing the coagulated fiber denseness.

The drawing of the carbon fiber precursor fibers is conducted by conveying the spun precursor fibers through one or more draw and wash baths, for example, by rollers. The carbon fiber precursor fibers are conveyed through one or more wash baths to remove any excess solvent and stretched in hot (e.g., 40° C. to 100° C.) water baths to impart molecular orientation to the filaments as the first step of controlling fiber diameter. The result is drawn carbon fiber precursor fibers that are substantially free of solvent.

In an embodiment, the carbon fiber precursor fibers are stretched from -5% to 30%, typically from 1 % to 10, more typically from 3 to 8%.

Step 3) of the process may further comprise drying the drawn carbon fiber precursor fibers that are substantially free of solvent, for example, on drying rolls. The drying rolls can be composed of a plurality of rotatable rolls arranged in series and in serpentine configuration over which the filaments pass sequentially from roll to roll and under sufficient tension to provide filaments stretch or relaxation on the rolls. At least some of the rolls are heated by pressurized steam, which is circulated internally or through the rolls, or electrical heating elements inside of the rolls. Finishing oil can be applied onto the stretched fibers prior to drying in order to prevent the filaments from sticking to each other in downstream processes.

Step 3) may further comprise determining the degree of swelling of the drawn carbon fiber precursor fibers by performing the method of determining the degree of swelling described herein.

In an embodiment, step 3) further comprises determining the degree of swelling of the drawn carbon fiber precursor fibers by performing the method of determining the degree of swelling described herein, and adjusting the degree of swelling of the drawn carbon fiber precursor fibers to a desired level based on the degree of swelling determined.

The degree of swelling may be adjusted by modifying, for example, fiber spinning parameters in step 2) of the process or other process parameters, such as the amount of total baths, stretches, temperatures, and filament speeds. In step 4) of the process described herein, the drawn carbon fiber precursor fibers of step 3) are oxidized to form stabilized carbon fiber precursor fibers and, subsequently, the stabilized carbon fiber precursor fiber are carbonized to produce carbon fibers.

During the oxidation stage, the drawn carbon fiber precursor fibers, typically PAN fibers, are fed under tension through one or more specialized ovens, each having a

temperature from 150 to 300 °C, typically from 200 to 280 °C, more typically from 220 to 270 °C. Heated air is fed into each of the ovens. Thus, in an embodiment, the oxidizing in step 4) is conducted in an air environment. The drawn carbon fiber precursor fibers are conveyed through the one or more ovens at a speed of from 4 to 100 fpm, typically from 30 to 75 fpm, more typically from 50 to 70 fpm.

The oxidation process combines oxygen molecules from the air with the fiber and causes the polymer chains to start crosslinking, thereby increasing the fiber density to 1.3 g/cm³ to 1.4 g/cm³ In the oxidization process, the tension applied to fiber is generally to control the fiber drawn or shrunk at a stretch ratio of 0.8 to 1.35, typically 1.0 to 1.2. When the stretch ratio is 1 , there is no stretch. And when the stretch ratio is greater than 1 , the applied tension causes the fiber to be stretched. Such oxidized PAN fiber has an infusible ladder aromatic molecular structure and it is ready for

carbonization treatment.

Carbonization results in the crystallization of carbon molecules and consequently produces a finished carbon fiber that has more than 90 percent carbon content.

Carbonization of the oxidized, or stabilized, carbon fiber precursor fibers occurs in an inert (oxygen-free) atmosphere inside one or more specially designed furnaces. In an embodiment, carbonizing in step 4) is conducted in a nitrogen environment. The oxidized carbon fiber precursor fibers are passed through one or more ovens each heated to a temperature of from 300 °C to 1650 °C, typically from 1100 °C to 1450 °C. In an embodiment, the oxidized fiber is passed through a pre-carbonization furnace that subjects the fiber to a heating temperature of from about 300 °C to about 900 °C, typically about 350 °C to about 750 °C, while being exposed to an inert gas (e.g., nitrogen), followed by carbonization by passing the fiber through a furnace heated to a higher temperature of from about 700 °C to about 1650°C, typically about 800 °C to about 1450 °C, while being exposed to an inert gas. Fiber tensioning may be added throughout the precarbonization and carbonization processes. In pre-carbonization, the applied fiber tension is sufficient to control the stretch ratio to be within the range of 0.9 to 1 .2, typically 1 .0 to 1 .15. In carbonization, the tension used is sufficient to provide a stretch ratio of 0.9 to 1.05.

Adhesion between the matrix resin and carbon fiber is an important criterion in a carbon fiber-reinforced polymer composite. As such, during the manufacture of carbon fiber, surface treatment may be performed after oxidation and carbonization to enhance this adhesion.

Surface treatment may include pulling the carbonized fiber through an electrolytic bath containing an electrolyte, such as ammonium bicarbonate or sodium hypochlorite. The chemicals of the electrolytic bath etch or roughen the surface of the fiber, thereby increasing the surface area available for interfacial fiber/matrix bonding and adding reactive chemical groups.

Next, the carbon fiber may be subjected to sizing, where a size coating, e.g. epoxy- based coating, is applied onto the 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 individual tows to reduce fuzz, improve processability and increase interfacial shear strength between the fiber and the matrix resin. Following sizing, the coated carbon fiber is dried and then wound onto a bobbin.

A person of ordinary skill in the art would understand that the processing conditions (including composition of the spin solution and coagulation bath, the amount of total baths, stretches, temperatures, and filament speeds) are correlated to provide filaments of a desired structure and denier. The process of the present disclosure may be conducted continuously.

Carbon fibers produced according to the process described herein may be

characterized by mechanical properties, such as tensile strength and tensile modulus per the ASTM D4018 test method.

The processes of the present disclosure are further illustrated by the following non- limiting examples.

Example 1

Multiple samples of coagulated fiber, spun from a polymer dope, were collected at varied coagulation bath concentrations. The bath temperature was held constant at 4 °C. Due to the varying of the coagulation bath concentration, the coagulated fiber porosity/denseness varied accordingly. The degrees of swelling of the collected samples were then determined using a known method and noted for each condition.

The samples and their corresponding degree of swelling measurements are listed in Table 1.

Table 1.

A Metrohm RapidContent Analyzer using an iris sample centering ring attachment directly on top of the measurement port was used for near-IR (NIR) measurements.

The iris centering ring provided uniformity in sample size and placement. Before the NIR spectrometer could perform quantitative analysis, it was“trained” or calibrated using multivariate methods (chemometrics). This established a relationship between the spectra and the reference data. The process involved selection of a representative calibration sample set, spectra acquisition and determination of reference value, multivariate modeling to relate“spectral variations” to the reference value, and validation of the model.

Each of the representative specimens was measured three times and the measured porosity (swelling % from Table 1 ) was entered for each sample. These measurements provided the reference values used for calibrating the NIR spectrometer. Once the calibration was completed, two samples were measured again as“unknowns.” Based on the calibration, the NIR was able to calculate the degree of swelling value within ± 3 percent, which is exceptional and confirms its capabilities to identify the sample porosity. This variability can be further reduced through additional samples and measurements which will better train the instrument.

Example 2

As in Example 1 , multiple samples of coagulated fiber, spun from a polymer dope, were collected at varied coagulation bath concentrations. The degrees of swelling of the collected samples were determined using both a known method (“swell test”) and the inventive NIR method described herein. The results are depicted in FIG. 1. As shown in FIG. 1 , the degrees of swelling determined by the inventive NIR method were within ± 3 percent of the degrees of swelling determined by another method.

Claims

WHAT IS CLAIMED IS:

1. A method of determining the degree of swelling of a polymer fiber, the method comprising: a) irradiating said polymer fiber with infrared energy over a spectrum of wavelengths in the range of from 400 to 2500 nm, typically from 700 to 2500 nm; b) detecting said infrared energy reflected from said polymer fiber over said spectrum of wavelengths; c) performing multivariate analysis on the spectrum of said reflected infrared energy; d) comparing the results of said multivariate analysis with a predetermined correlation between model infrared energy spectra comprising said spectrum of wavelengths collected from a plurality of model polymer fiber samples, said model polymer fiber samples each comprising a different degree of swelling; and, e) determining the degree of swelling of the polymer fiber based on said

predetermined correlation.

2. The method of claim 1 , wherein the polymer fiber comprises a polyacrylonitrile- based polymer.

3. The method of claim 1 or 2, wherein the predetermined correlation between model infrared energy spectra comprising said spectrum of wavelengths collected from a plurality of model polymer samples, said model polymer samples each comprising a different concentration, used in step d) is obtained prior to step a).

4. The method of any one of claims 1 -3, wherein the polymer fiber is a carbon fiber precursor fiber.

5. The method of any one of claims 1 -4, wherein the polymer fiber is a carbon fiber precursor fiber that has not been drawn or dried.

6. The method of any one of claims 1 -5, wherein the polymer fiber is a carbon fiber precursor fiber that has not been dried.

7. The method of any one of claims 1 -6, wherein the method is performed within 30 minutes, typically within 20 minutes.

8. The method of any one of claims 1 -8, further comprising adjusting the degree of swelling of the polymer fiber to a desired level based on the degree of swelling determined in step e).

9. A process for producing carbon fibers, the process comprising:

1 ) preparing a polymer solution;

4) oxidizing the carbon fiber precursor fibers of step c) to form stabilized carbon fiber precursor fibers and then carbonizing the stabilized carbon fiber precursor fiber, thereby producing carbon fibers; wherein step 2) further comprises determining the degree of swelling of the carbon fiber precursor fibers formed by performing the method according to any one of claims 1 -8;

and/or

wherein step 3) further comprises determining the degree of swelling of the drawn carbon fiber precursor fibers by performing the method according to any one of claims 1 -8.

10. The process according to claim 9, wherein step 2) further comprises determining the degree of swelling of the carbon fiber precursor fibers formed by performing the method according to any one of claims 1 -8, and adjusting the degree of swelling of the carbon fiber precursor fibers to a desired level based on the degree of swelling determined.

11. The process according to claim 9, wherein step 3) further comprises determining the degree of swelling of the drawn carbon fiber precursor fibers by performing the method according to any one of claims 1 -8, and adjusting the degree of swelling of the drawn carbon fiber precursor fibers to a desired level based on the degree of swelling determined.