Statement of Government Interest

This invention was made under a NFE- 10-02991 between The Dow Chemical Company and UT-Batelle, LLC, operating and management Contractor for the Oak Ridge National Laboratory operated for the United States Department of Energy. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a process for producing carbon fibers.

Introduction

Carbon fibers serve an ever increasing need. The world production of carbon fiber in 2010 was 40 kilometric tons (KMT) and is expected to grow to 150 KMT in 2020.

Industrial-grade carbon fiber is forecasted to contribute greatly to this growth, wherein low cost is critical to applications. The traditional method for producing carbon fibers relies on polyacrylonitrile (PAN), which is solution-spun into fiber form, oxidized and carbonized. Approximately 50 percent of the cost is associated with the cost of the polymer itself and solution- spinning.

In an effort to produce low cost industrial grade carbon fibers, various groups studied alternative precursor polymers and methods of making the carbon fibers. Precursor alternative to PAN fibers have included cellulosic yarns, nitrogen-containing polycyclic polymers and even pitch. Preparing carbon fibers from each different precursor entails challenges unique to the precursor and the carbonization process for each precursor has to be designed for the chemistry of the particular precursor

More recent efforts have included work with stabilized polyolefin (S-PO) fibers such as sulfonated polyethylene fibers. US4070446 and WO 92/03601, for example, both teach methods for sulfonation of polyethylene fibers and subsequent conversion to carbon fibers and even further conversion to graphitized carbon fibers. The use of S-PO fibers to produce carbon fibers is relatively new technology and historically has produced carbon fibers with lower tensile strength and Young's modulus compared to carbon fibers from other known precursors. High temperature graphitization (typically in excess of 2000 degrees Celsius (°C)) of S-PO fibers can help increase the resulting carbon fiber Young's modulus, but also increases the processing cost and complexity.

Work with PAN fibers has revealed that boron can be an effective catalyst for graphitizing carbon fibers to increase the fiber modulus. However, as the following references reveal, the required graphitization temperature is still quite high even when the fiber includes boron catalyst. Moreover, the references reveal that boron can actually cause a reduction in tensile strength unless heating above 2300°C.

Ya Wen et al., Materials and Design 36, 728-734 (2012) presents data that demonstrates treating PAN fibers with boric acid treated results in an increase in Young's modulus after heating to temperature greater than 1250°C, but the tensile strength of the fibers is reduced unless heated to temperatures in excess of 2300°C.

GB 1295289 reports that boron can serve as a catalyst for facilitating rapid graphitization of certain polymer fibers at temperature ranges of 1800-3200°C. GB 1295289 identifies as suitable precursor fibers PAN, cellulosic and nitrogen-containing polycyclic polymer fibers. The boron catalyst is shown by examples to produces carbon fibers having an increased Young's modulus relative to similar fibers prepared without boron when PAN fibers are heated in excess of 2200°C.

Other references also identify boron as a suitable graphitization catalyst for producing graphite fibers with improved properties provided the graphitization temperature exceeds 2000°C. See, for example, DE1949830A1, JP3457774B2, JP3303424B2, and Cooper, GA, Mayer RM, Journal of Materials Science 6 (1971) 60-67.

PAN fibers have a different chemical structure from S-PO fiber precursor. It remains unclear in what way boron would affect conversion of S-PO fibers to carbon fibers, or if it would affect such a conversion at all.

It is desirable to provide a process for creating carbon fibers from S-PO fibers, such as sulfonated polyolefin fibers, that increases Young's modulus and preferably also tensile strength of the resulting carbon fiber without requiring heating to temperatures in excess of 2000°C, or even 1800°C. BRIEF SUMMARY OF THE INVENTION

The present invention provides a solution to the need for a process for creating carbon fibers from stabilized polyolefin fibers, such as sulfonated polyolefin fibers, that increases Young's modulus and preferably also tensile strength of the resulting carbon fiber without requiring heating to temperatures in excess of 2000°C, or even 1800°C.

Surprisingly, the present invention is a result of discovering that boron serves as a catalyst during carbonization of stabilized polyolefin fibers. Without being bound by theory, the boron likely acts as a graphitization catalyst in the S-PO fiber and facilitates graphitization of the S-PO fiber during carbonizing at temperatures even below 2000°C and even below 1800°C. What is even more surprising is that boron uniquely affects carbonization of S-PO fibers in a way that results in an increase in both Young's modulus and tensile strength at temperatures below 1800°C. This result is in contrast to how boron catalyzes PAN during carbonization as noted above in the Background. Catalytic effects of boron in carbonizing S-PO fibers are previously unknown. Similarly, catalytic effects of boron at temperatures below 1800°C sufficient to increase both Young's modulus and tensile strength are previously unknown.

In a first aspect, the present invention is a process comprising treating a stabilized polyolefin fiber with a boron source followed by heating the fiber to a temperature of

1000°C or higher in an inert atmosphere so as to convert the stabilized polyolefin fiber into a carbon fiber.

The process of the present invention is useful for preparing carbon fibers from polyolefin fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 -4 illustrate wide angle X-ray scattering analysis data for sulfonated polyolefin fibers of Comparative Example C and Example 6 during carbonization. DETAILED DESCRIPTION OF THE INVENTION

Test methods refer to the most recent test method as of the priority date of this document when a date is not indicated with the test method number. References to test methods contain both a reference to the testing society and the test method number. The following test method abbreviations and identifiers apply herein: ASTM refers to ASTM International (formerly American Society for Testing and Materials); EN refers to European Norm; DIN refers to Deutsches Institut fiir Normung; and ISO refers to International Organization for Standards. "And/or" means "and, or as an alternative". All ranges include endpoints unless otherwise indicated.

"Elastic modulus" and "Young's modulus" are interchangeable.

The process of the present invention is useful for preparing carbon fibers, preferably graphitized fibers from stabilized polyolefin fiber.

"Carbon fiber" is a fiber comprising an excess of 70 wt , preferably 80 wt or more, still more preferably 90 wt or more by weight of the fiber and wherein the carbon weight exceed hydrogen weight by a factor of twenty or more, preferably fifty or more.

"Graphite fiber" is a form of carbon fiber this is characterized by ordered alignment of hexagonal carbon rings - a crystal-like structure and order. A carbon fiber becomes more graphite in nature as the amount and organized arrangement of hexagonal rings increases in the carbon fiber.

"Graphitized carbon fibers" are carbon fibers demonstrating some degree of crystallike structure and order.

Stabilized polyolefin (S-PO) fibers are polyolefin fibers that have been chemically modified so as to experience less than 10 weight-percent (wt%), more preferably less than 5 wt , or even more preferably less than 1 wt and preferably no detectable hydrocarbon loss at temperatures up to 600°C by thermogravimetric analysis based on fiber weight. Polyolefin (PO) fibers can be converted into S-PO fibers by crosslinking, oxidizing (for example air oxidation) or sulfonating the polyolefin fibers.

The polyolefin fiber that is chemically modified so as to become an S-PO can be polyolefin homopolymer or multipolymer, including multipolymers comprising both olefins and non-olefins. Herein, "multipolymer" refers to polymers of more than one type of monomer such as copolymers, terpolymers and higher order polymers. Desirably, the polyolefin fiber is a homopolymer or copolymer comprising one or any combination or more than one of ethylene, propylene, butadiene and/or styrene units.

Polyethylene homopolymer and multipolymers, particularly copolymers, are especially desirable polyolefin fibers. Preferable polyethylene copolymers include ethylene/octene copolymers, ethylene/hexene copolymers, ethylene/butene copolymers, ethylene/propylene copolymers, ethylene/styrene copolymers, ethylene/butadiene copolymers, propylene/octene copolymers, propylene/hexene copolymers, propylene/butene copolymers, propylene/styrene copolymers, propylene butadiene copolymers, styrene/octene copolymers, styrene/hexene copolymers, styrene/butene copolymers, styrene/propylene copolymers, styrene/butadiene copolymers, butadiene/octene copolymers, butadiene/hexene copolymers, butadiene/butene copolymers, butadiene/propylene copolymers,

butadiene/styrene copolymers, or a combination of two or more thereof.

The polyolefin is desirably a multipolymer, preferably copolymer of ethylene and octene.

Polyolefin multipolymers can have any arrangement of monomer units. For example, the polyolefin multipolymer can be linear or branched, alternating in monomer units or blocks of monomer units (such as diblock or triblock polymers), graft multipolymer, branch copolymers, comb copolymers, star copolymers or any combination of two or more thereof.

The polyolefin fiber and S-PO fiber can be of any cross- sectional shape such as circular, oval, star-shaped, that of a hollow fiber, triangular, rectangular and square.

The S-PO fibers are desirably sulfonated polyolefin fibers. Sulfonated polyolefin fibers are polyolefin fibers that are stabilized by being sulfonated and comprising sulfate functionalities. Any means of sulfonating a polyolefin fiber is suitable for preparing the sulfonated polyolefin fiber for use in the process of the present invention. For example, a suitable means of sulfonating a polyolefin fiber is by exposing the polyolefin fiber to a sulfonating agent such as concentrated and/or fuming sulfuric acid, chlorosulfonic acid, and/or sulfur trioxide in a solvent and/or as a gas. Preferably, prepare the sulfonated polyolefin fiber by treating the polyolefin fiber with a sulfonating agent selected from fuming sulfuric acid, sulfuric acid, sulfur trioxide, chlorosulfonic acid or any combination thereof. Sulfonation can be a step-wise process during which a polyolefin fiber is exposed to a first sulfonating agent and then a second sulfonating agent and the, optionally, a third and optionally more sulfonating agents. The sulfonating agent in each step can be the same or different from any other step. Typically, sulfonating occurs by running a polyolefin fiber through one or more than one bath containing a sulfonating agent.

One desirable method for sulfonating a polyolefin fiber is to treat the polyolefin fiber with fuming sulfuric acid (first step), then with a concentrated sulfuric acid (second step) and then by a second concentrated sulfuric acid treatment (third step). The temperature during each of the three steps can be the same or different from one another. Preferably, the temperature in the first step is lower than the temperature during the second step. Preferably the temperature during the second step is lower than the temperature during the third step. Examples of suitable temperatures include: for the first step: zero degrees Celsius (°C) or higher, preferably 30°C or higher and more preferably 40°C or higher and at the same time desirably 130°C or lower, preferably 100°C or lower; desirably 105-130°C for the second step and desirably 130-150°C for the third step. Residence times in each step can range from 5 minutes or more to 24 hours or less.

Treat the S-PO fiber with a boron source. Suitable boron sources include boric acid, phenyl boronic acid. It is desirable to use an aqueous boric acid solution as the boron source and to treat a S-PO fiber by exposing the S-PO fiber to the aqueous boric acid solution. The concentration of boric acid in the aqueous boric acid solution is typically 0.09 moles per liter (M) or higher, preferably 0.1 M or higher, more preferably 0.2 M or higher, 0.3 M or higher, 0.4 M or higher, even 0.5 M or higher. Most preferably the boric acid solution is a saturated boric acid solution at the temperature of exposure to the S-PO fiber.

It is desirable to expose the S-PO fiber to a sufficient concentration of boron source for a sufficient period of time so as to incorporate sufficient boron with the S-PO fiber to obtain a boron concentration in the final carbon fiber as described below with the description of the carbon fiber.

Heat the S-PO fiber that has been treated with a boron source in an inert atmosphere in order to convert the S-PO fiber into a carbon fiber. Heating in an inert atmosphere prevents oxidative degradation of the S-PO fiber during carbonization. An inert atmosphere contains less than 100 parts per million by weight oxygen based on total atmosphere weight. The inert atmosphere can contain inert gasses (gasses that will not oxidize the PO fiber during the heating process). Examples of suitable inert gasses include nitrogen, argon, and helium. The inert atmosphere can be a vacuum, that is a pressure lower than 101 kiloPascals. The oxygen level can be reduced purely by purging with one or more than one inert gas, by purging with inert gas and drawing a vacuum, or by drawing a low enough vacuum to reduce the oxygen level to a low enough concentration to preclude an undesirable amount of oxidation of the S-PO fiber during heating.

Heat the S-PO fiber in an inert atmosphere to a temperature of 1000°C or higher in order to carbonize the S-PO fiber. Preferably, heat the S-PO fiber in an inert atmosphere to a temperature of 1150°C or higher, more preferably 1600°C or higher, still more preferably 1800°C or higher. Heating can be to a temperature of 2000°C or higher, 2200°C or higher, 2400°C or higher and even 3000°C or higher. However, generally heating is to a temperature of 3000°C or lower. Higher heating temperatures are desirable for carbonizing S-PO fibers because higher temperature can convert the fiber to a graphite fiber having higher strength, higher Young's modulus or both than non-graphite carbon fiber. One of the surprising results of the present invention is that graphitization (that is, formation of crystal structure in a carbon fiber) can be achieved from a S-PO fiber by heating to only 1800°C or lower. That is, the present invention provides for graphitization of a S-PO fiber without heating the S-PO fiber to temperatures above 1800°C.

Heat the fiber as long as necessary to achieve desired properties. Generally, the longer a fiber is heated the more complete the carbonization and more aligned the carbon becomes. Generally, the duration of heating is a balance of processing the fibers fast enough to be commercially viable while still heating long enough to achieve desired fiber properties.

After heating the S-PO fiber and converting it to a carbon fiber it is desirable for the carbon fiber to have a boron concentration of at least 0.3 mole-percent (mol ), preferably 0.35 mol or more, more preferably 0.5 mol or more, yet more preferably one mol or more, even more preferably 2.5 mol or higher, still more preferably 2.8 mol or higher, yet more preferably 3 mol or higher, still even more preferably 3.3 mol or higher and even yet more preferably 3.6 mol or higher. Typically, the concentration of boron is 10 mol or less, more typically 5 mol or less in the final carbon fiber. Boron concentration is relative to the total moles of elements in the carbon fiber. Determine boron concentration in the carbon fiber by inductively coupled plasma (ICP) analysis according to the method set forth below in the Examples section.

The present invention is a result of discovering that treating a S-PO fiber with boron prior to carbonization allows production of carbon fibers having higher strength, higher Young's modulus or both higher strength and higher Young's modulus than carbon fibers produced from S-PO fibers that do not contain boron and that are carbonized at the same carbonization temperature. For the sake of this invention strength refers to tensile strength. Characterize tensile strength and Young's modulus according to ASTM method CI 557. Examples

Melt-spin a polyethylene /1-octene copolymer (melt index of 30; density of 0.9550 grams per milliliter; polydispersity of 3.0) into a continuous fiber tow containing 1700 filaments (tenacity of 4.4 grams per denier; elongation to break of 8.4%; diameter of 8.2 microns). Sulfonate the fiber tow in a 4-bath continuous process under 25 megaPascals (MPa) tension in the first bath and 15 MPa tension in the subsequent baths. Feed the fiber tow at a rate that corresponds to a residence time of approximately 60 minutes in each bath. The first bath is 20 mole-percent (mol%) fuming sulfuric acid at 50°C. The second bath is 96 mol% sulfuric acid at 120°C. The third bath is 96 mol% sulfuric acid at 140°C. The fourth bath is deionized water for Comparative Examples A and B, aqueous boric acid (BA) solution of various concentrations for Examples 1-5 and 0.082 molar aqueous phenyl boronic acid (PBA) for Example 5. From the fourth bath, spool the fiber.

Note, Comparative Example B and Example 5 are made from a different sample of PO fiber on a different day than Comparative Example A and Examples 1-4. Therefore, comparisons of results are most accurate when comparing Comparative Example A and Examples 1-4 and Comparative Example B with Example 5.

Carbonize the sulfonated fiber by passing a 10 centimeter (four inch) sample through a continuously nitrogen-purged three-zone carbonization furnace with heated zone temperatures of 650°C, 950°C 1150°C. Run the sample fibers through the furnace with 5.5 MPa tension and for a total resonance time of 14 minutes. For carbonizations of 1200°C and higher (see Table 1), further pass the sample through a continuously nitrogen purged single zone KYK furnace with 5.5 MPa tension and a total resonance time in the hottest zone of 2.5 minutes.

Resulting strength and Young's modulus values for the resulting carbon fibers for the Examples are in Table 1 given in units of gigaPascals (GPa). Concentration of boron in the resulting carbon fiber is given for select examples in mol% based on fiber mole composition as determined by ICP analysis.

Conduct ICP analysis using the following procedure. Prepare samples by acid digestion using single reaction chamber microwave digestion technology with a Milestone UltraWave digestion system. Transfer approximately 10 milligrams of carbon fiber into a quartz digestion tube and add 0.5 milliliters of high purity deionized water rand two milliliters of concentrated nitric acid. Pre-pressurize the reaction chamber with nitrogen at 4 mega Pascals (40 bar) and heat samples to 200-250°C with microwave energy to perform the digestion. After digestion, dilute the sample to 15 milliliters with high purity deionized water. Analyze the sample using an inductively coupled plasma emission spectrometer (ICP-OES). Calibrate the ICP-OES suing dilutions of a certified aqueous boron standard over a range of approximately 1-10 micrograms/gram and a 5% nitric acid blank. Prepare the calibration standards and samples based on a weight basis. Run the samples against this calibration. Numerous rinses and calibrations are done along with quality control checks to ensure an absence of carry over throughout the measurement run due to the tendency of boron to cause memory effects. Accuracy was confirmed by spiking a fiber sample containing no added boron with a certified aqueous boron standard and doing the analysis.

Table 1

NM= not measured. The data of Table 1 reveals measurable increases in Young's modulus and/or strength of a carbonized sulfonated PO fiber that has been carbonized at temperatures ranging from 1150°C to 2400°C. Increases in Young's modulus and strength are evident a broad range of boron treatment concentration and with a broad range of boron

concentrations in the resulting carbon fiber.

Prepare pre-carbonized fibers for Comparative Examples B and 6 in like manner as Comparative Example A and Example 3, respectively except use 45.7 centimeter (18 inch) fiber samples instead of 10 centimeter (four inch) fiber samples. Analyze Comparative Example B and Example 6 by wide-angle X-ray diffraction (WAXD) while carbonizing under a helium atmosphere. Maintain the fibers under 163 MPa tension during the WAXD analysis and conduct the analysis while heating the fibers to determine change in crystal structure with heating.

WAXD characterization for cos φ, La, Lc and d ₀ 02 is in Figures 1-4 respectively. Figure 1 illustrates that the degree of orientation in the fiber undergoes a greater degree of orientation at lower temperatures when boron is present. Figure 2 further illustrates that the length of crystallites is greater at lower temperatures when boron is present. Figure 3 illustrates that the spacing between crystal layers is smaller at lower temperatures (therefore, the crystal structure is more pure) at lower temperatures when boron is present. Figure 4 illustrates that the number of crystal sheets is higher at lower temperatures when boron is present. The WAXD data confirms that graphitization is more extensive at lower temperatures in boron treated sulfonated PO fibers than in boron-free sulfonated PO fibers.