JP2018507963A - Carbon fiber obtained from silicon-treated polyolefin precursor fiber - Google Patents

Abstract

Carbonaceous fibers are prepared from polyolefin fibers. Stabilize polyolefin fiber and treat with silicon source. In one example, the silicon source is siloxane. In one example, the polyolefin fibers are stabilized by sulfonation. In one example, the carbonaceous fiber has 90-100 weight percent carbon, 0.1-1 weight percent silicon, 0.1-1 weight percent nitrogen. [Selection figure] None

Description

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

  Carbon fiber meets the ever-increasing needs. The global production of carbon fiber in 2010 is 40 kilometric tons (KMT) and is expected to grow to 150 KMT in 2020. Industrial growth is expected to contribute significantly to this growth, and low cost is essential for application. Conventional methods for producing carbon fibers rely on polyacrylonitrile (PAN) that is solution spun into fibers, oxidized and carbonized. About 50 percent of the cost of carbon fiber is related to the polymer itself and the cost of solution spinning.

  In an effort to produce low cost industrial grade carbon fibers, various groups have studied alternative precursor polymers and methods for making carbon fibers. Precursor alternatives to PAN fibers included cellulose yarn, nitrogen-containing polycyclic polymer, and pitch. Since each precursor must be designed for the particular precursor chemistry, preparing carbon fibers from each different precursor involves challenges specific to that precursor and carbonization process.

  More recent efforts include research using stabilized polyolefin (S-PO) fibers such as sulfonated polyethylene fibers. For example, US 4,070,446 and WO 92/03601 both teach methods for sulfonation of polyethylene fibers and subsequent conversion to carbon fibers, or even conversion to graphitized carbon fibers. Generating carbon fibers using S-PO fibers is a relatively new technology and historically has lower tensile strength and Young's modulus compared to carbon fibers from other known precursors. Carbon fiber has been produced. High temperature graphitization of S-PO fibers (typically above 2000 ° C.) can help increase the Young's modulus of the resulting carbon fibers, but at the same time increases processing costs and complexity.

  Lubricating oils such as silicon-based lubricating oils are used in the production of carbon fibers. Such lubricating oils are known to be beneficial when added to a carbon fiber process, such as before an air oxidation process or before a carbonization process. It is generally accepted that such coatings do not substantially penetrate the carbon fiber internal structure.

  Carbons from S-PO fibers such as sulfonated polyolefin fibers that increase the Young's modulus and preferably the tensile strength of the resulting carbon fibers without the need to heat to temperatures exceeding 2000 ° C., preferably above 1800 ° C. It would be desirable to provide a process for making fibers.

  A carbonaceous article is prepared from a polyolefin processed article. Stabilize the polyolefin processed article and treat with silicon source. In one example, the silicon source is siloxane. In one example, the processed article is stabilized by sulfonation. In one example, the carbonaceous article includes fibers having 90-100 weight percent carbon, 0.1-1 weight percent silicon, 0.1-1 weight percent nitrogen.

  Test method refers to the latest test method as of the priority date of this document when the date is not indicated with the test method number. Reference to a test method includes reference to both a test association and a test method number.

  “And / or” means “and or alternatively”. Unless otherwise specified, all ranges include endpoints.

  “Elastic modulus” and “Young's modulus” are synonymous.

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

  “Carbon fiber” is a fiber containing more than 70 wt% carbon, preferably 80 wt% or more carbon, and even more preferably 90 wt% or more carbon, and the carbon weight is 20 times or more hydrogen weight, preferably Is over 50 times.

  “Graphite fiber” is a form of carbon fiber, which is characterized by a regular arrangement of hexagonal carbocycles, ie a crystal-like structure and order. Carbon fibers become more graphitic in nature as the amount of hexagonal rings and organized organization in carbon fibers increases.

  A “graphitized carbon fiber” is a carbon fiber that exhibits a certain degree of crystal-like structure and order.

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

  Polyolefin fibers that are chemically modified to be S-PO may be polyolefin homopolymers or multipolymers, including multipolymers containing both olefins and non-olefins. As used herein, “multipolymer” refers to a polymer of two or more types of monomers, such as copolymers, terpolymers, and higher order polymers. Desirably, the polyolefin fiber is a homopolymer or copolymer comprising one or any combination or two or more of ethylene, propylene, butadiene, and / or styrene units.

  Polyethylene homopolymers and multipolymers, especially copolymers, are particularly desirable polyolefin fibers. Preferred polyethylene copolymers include ethylene / octene copolymer, ethylene / hexene copolymer, ethylene / butene copolymer, ethylene / propylene copolymer, ethylene / styrene copolymer, ethylene / butadiene copolymer, propylene / octene copolymer, propylene / hexene copolymer, propylene / Butene copolymer, propylene / styrene copolymer, propylene / butadiene copolymer, styrene / octene copolymer, styrene / hexene copolymer, styrene / butene copolymer, styrene / propylene copolymer, styrene / butadiene copolymer, butadiene / octene copolymer, butadiene / hexene copolymer, Butadiene / butene copolymer, butadiene / propylene copolymer, pig Ene / styrene copolymers, ethylene / ethylidene norbornene copolymer, ethylene / propylene / ethylidene norbornene copolymer, or combinations of two or more thereof may be mentioned. As used herein, octene may refer to any octene isomer. In one example, octene refers to 1-octene.

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

  The polyolefin multipolymer can have any monomer unit configuration. For example, a polyolefin multipolymer may be a linear or branched chain, an alternating polymer within a monomer unit or block of monomer units (such as a diblock or triblock polymer), a graft multipolymer, a branched copolymer, a comb copolymer, a star copolymer, or Any combination of two or more thereof may be used.

  The polyolefin fiber and the S-PO fiber may have any cross-sectional shape such as a circular, oval, star, triangular, rectangular, and square hollow fiber.

  The S-PO fiber is desirably a sulfonated polyolefin fiber. Sulfonated polyolefin fibers are polyolefin fibers containing sulfate functional groups that are stabilized by being sulfonated. Any means for sulfonating polyolefin fibers is suitable for preparing sulfonated polyolefin fibers for use in the process of the present invention. For example, suitable means for sulfonating polyolefin fibers expose the polyolefin fibers 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. It depends. Preferably, the sulfonated polyolefin fiber is prepared 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 is a stepwise process that exposes polyolefin fibers to a first sulfonating agent and then to a second sulfonating agent, optionally, a third and optionally more sulfonating agents. It may be. The sulfonating agent in each step may be the same or different from any other step. Typically, sulfonation occurs by passing polyolefin fibers through one or more vessels containing a sulfonating agent.

  One desirable method for sulfonating polyolefin fibers is to treat the polyolefin fibers with fuming sulfuric acid (first step), then with concentrated sulfuric acid (second step), and then with a second concentrated sulfuric acid treatment (third step). ) To process. The temperature during each of the three steps may be the same or different from each other. Preferably, the temperature during 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. As examples of suitable temperatures, the first step is zero ° C. or higher, preferably 30 ° C. or higher, more preferably 40 ° C. or higher and desirably 130 ° C. or lower, preferably 100 ° C. or lower. The second step is desirably 105 to 130 ° C, and the third step is desirably 130 to 150 ° C. The residence time of each step can be from 5 minutes to 24 hours.

  It is desirable to treat S-PO fibers with a siloxane source. Suitable siloxane sources include siloxanes, cyclic siloxanes, or polydiorganosiloxane oligomers (cyclic or linear hydroxyl terminated). In one example, an aqueous siloxane solution is selected as the siloxane source to treat the S-PO fiber by exposing the S-PO fiber to the aqueous siloxane solution. The concentration of siloxane in the aqueous siloxane solution is typically from 0.1 weight percent to 1.0 weight percent. Most preferably, the siloxane solution is a saturated siloxane solution at the temperature of exposure to S-PO fibers. In another example, a siloxane solution stock solution is used to treat S-PO fibers. Commercially available siloxanes such as Advalon CF 3295 (octamethylcyclotetrasiloxane) are preferred.

  It is desirable to expose the S-PO fibers to the silicon source for a sufficient period of time in order to incorporate sufficient silicon into the S-PO fibers so as to obtain a silicon concentration in the S-PO fibers of at least 1 mole percent. In addition, it is desirable to incorporate sufficient silicon within the S-PO fiber to obtain the final carbon fiber as described below with a description of the carbon fiber. The treatment time of the S-PO fiber with the siloxane source can range from 0 to 180 minutes.

  In order to convert S-PO fibers to carbon fibers, S-PO fibers treated with a siloxane source are heated in an inert atmosphere. Heating in an inert atmosphere prevents oxidative degradation during carbonization of the S-PO fibers. The inert atmosphere contains less than 100 parts per million oxygen based on total atmospheric weight. The inert atmosphere may contain an inert gas (a gas that does not oxidize PO fibers during the heating process). Examples of suitable inert gases include nitrogen, argon, and helium. The inert atmosphere may be a vacuum at a pressure lower than 101 kilopascals. The oxygen level can be determined by purely purging with one or more inert gases, by purging with inert gas and pulling a vacuum, or during the heating to undesirably oxidize the S-PO fibers. It can be reduced by pulling a sufficiently low vacuum to reduce the oxygen level to a concentration low enough to prevent.

  In order to carbonize the S-PO fiber, the S-PO fiber is heated to a temperature of 1000 ° C. or higher in an inert atmosphere. Preferably, the S-PO fiber is heated to a temperature of 1150 ° C. or higher, more preferably 1600 ° C. or higher, and even more preferably 1800 ° C. or higher in an inert atmosphere. The heating may be up to a temperature of 2000 ° C. or higher, 2200 ° C. or higher, 2400 ° C. or higher, and even 3000 ° C. or higher. In general, however, the heating is to a temperature of 3000 ° C. or less. Because higher temperatures can convert the fibers to graphite fibers having higher strength, higher Young's modulus, or both than non-graphitic carbon fibers, in some cases to carbonize S-PO fibers. Higher heating temperatures are desirable.

  Heat the fiber as long as necessary to obtain the desired properties. In general, the longer the fibers are heated, the more complete the carbonization and the more aligned the carbon. In general, the duration of heating is a balance between heating the fiber long enough to obtain the desired fiber properties while processing the fiber sufficiently quickly to be commercially feasible.

  Treatment of S-PO fibers with siloxane prior to carbonization results in higher strength, higher Young's modulus, or higher strength and higher than carbon fibers from S-PO fibers carbonized at the same carbonization temperature without siloxane. It is observed that it is possible to produce carbon fibers having both a Young's modulus. For the purposes of the present invention, strength refers to tensile strength. It is observed that the mass yield of carbon fibers is improved overall when treated with siloxane, as compared to fibers prepared similarly but not treated with siloxane.

  In one example, a carbonaceous article having fibers having 90-100 weight percent carbon is provided. In one example, a carbonaceous article having fibers having 94.4 to 100 weight percent carbon is provided. In one example, a carbonaceous article having fibers having 0 to 1 weight percent silicon is provided. In one example, a carbonaceous article having fibers having 0.004 to 0.410 weight percent silicon is provided. In one example, a carbonaceous article having fibers having 0 to 1 weight percent nitrogen is provided. In one example, a carbonaceous article having fibers having 0 to 0.9 weight percent nitrogen is provided.

In the embodiment, calculated according to the following equation mass yield Y _CF carbon fiber.

In this equation, ρ _CF is the calculated linear density of the carbonaceous article, ρ _FA is the calculated linear density of the processed article before processing, and S _S (stabilized shrinkage) is Calculated according to the equation in the middle, and S _C (carbonization shrinkage) is calculated according to the equation herein.

  In the example, the shrinkage of the stabilized article is calculated according to the following formula:

  In this formula, the length of the S-PO article refers to the length of the processed article after the stabilization step, eg after sulfonation. In this equation, the length of the processed article refers to the length of the processed article prior to any processing step.

  In the examples, the shrinkage of the carbonaceous article is calculated according to the following formula:

In this formula, the length of the carbonaceous article refers to the length of the processed article after carbonization. In addition to the mass yield of carbon fiber, it is also of interest to quantify the relative change in mass yield% Y _CF and the relative change in linear density% ρ _CF. For this calculation, we determine a linear density [rho _CF and carbonization shrinkage S _C for untreated polyethylene (superscript C) samples and treated polyethylene (superscript T) specimen. Thus, the relative improvement in mass yield carbon fiber linear density is defined as:

  Since the same starting SPE material is used in comparative tests, the terms of polyethylene linear density and sulfonated shrinkage are offset.

  The percentages used in these examples are on a weight basis unless otherwise specified.

  Example 1

Polyethylene / 1-octene copolymer (density = 0.955 g / cm ³ , MI = 30 g / 10 min, 190 ° C./2.16 kg) was melt-spun into 1758 filaments (diameter = 8.8 microns, toughness = 4.54 g) / Denier, breaking strength = 7.61%) is formed. The fiber tow is then sulfonated in a 4 tank continuous process under constant tension. The first tank contains 6% fuming sulfuric acid (maintained at 120 ° C.). The second tank contains 1% fuming sulfuric acid (maintained at 120 ° C.). The third tank contains 95-98% sulfuric acid (95-98% sulfuric acid, obtained from EMD, UN1830, SX1244-6, lot 51320, MW98.08, CAS: 766493-9, maintained at 140 ° C.) . Each bath is maintained at a constant temperature. The fourth tank contains deionized water. In the first tank, an average fiber tension of 151.5 gf (weight gram) is maintained. In the second tank, the third tank, and the fourth tank, an average fiber tension of 125 gf is maintained. The fiber tows pass sequentially through the first tank, the second tank, the third tank, and the fourth tank so that the fiber tows stay in each tank for 15 minutes, thereby providing a stabilized processed article. Go ahead. The seven segments of the stabilized workpiece were then treated with Advalon CF 3295 (from Wacker) at room temperature without tension for various lengths of time, thereby providing seven siloxane-treated stabilized workpieces. Provide the goods. Each segment is then carbonized in a continuous furnace at a maximum of 1150 ° C. with a tension of 0.5 N to obtain a carbonaceous article having the properties specified in Table I.

  Example 2

Polyethylene / 1-octene copolymer (density = 0.955 g / cm ³ , MI = 30 g / 10 min, 190 ° C./2.16 kg) was melt-spun into 1758 filaments (diameter = 9.1 microns, toughness = 3.989 g) / Denier, breaking strength = 8.51%) is formed. The fiber tow is then sulfonated in a 4 tank continuous process under constant tension. The first tank contains 20-30% free fuming sulfuric acid (maintained at 50 ° C.). Both the second tank and the third tank contain 95 to 98% sulfuric acid (95 to 98% sulfuric acid, manufactured by EMD, UN1830, SX1244-6, lot 51320, MW 98.08, CAS: 766493-9, The second tank is maintained at 120 ° C. and the third tank is maintained at 140 ° C.). Each bath is maintained at a constant temperature. In the first tank, a constant fiber tension of 250 gf (weight gram) is maintained. In both the second tank and the third tank, a constant fiber tension of 150 gf is maintained. The fiber tows sequentially pass through the first tank, the second tank, the third tank, and the fourth tank so that the fiber tows stay in each tank for 60 minutes, thereby providing a stabilized processed article. Go ahead. The stabilized segment of the processed article is then processed according to three processing regimes.

  In Regime 1, the four segments (A, B, C, D) of the stabilized workpiece are not treated with the siloxane-containing solution. Here, the fourth tank is a deionized water tank having a residence time of 60 minutes.

  In regime 2, the stabilized and processed four segments (E, F, G, H) are treated with Advalon CF 3295 (from Wacker) at room temperature in a fourth tank with a residence time of 60 minutes. The tension is maintained at 150 gf and after processing, the segments are dried in-line with a nitrogen drying jet (˜0.25-1 minutes), thereby providing four siloxane treated stabilized workpieces.

  In regime 3, the stabilized four segments (I, J, K, L) are treated with Advalon CF 3295 (from Wacker) at room temperature in a fourth tank with a residence time of 60 minutes. The tension was maintained at 150 gf, thereby providing four siloxane treated stabilized workpieces.

  Each segment prepared according to regimes 1, 2, and 3 is then carbonized at a tension of 0.5 N in a continuous furnace at the temperature listed in Table II to produce a carbonaceous article having the properties described in Table III. obtain. It should be noted that the segment of regime 1 is a control that measures the data of other regimes, so these segments have no strength or elasticity improvement data.

  Example 3

Polyethylene / 1-octene copolymer (density = 0.955 g / cm ³ , MI = 30 g / 10 min, 190 ° C./2.16 kg) was melt-spun into 1758 filaments (diameter = 8.5 microns, toughness = 4.434 g). / Denier, breaking strength = 8.483%) is formed. The fiber tow is then sulfonated in a 4 tank continuous process under constant tension. The first tank contains 20-30% free fuming sulfuric acid (Acros, maintained at 50 ° C.). Both the second tank and the third tank contain 95 to 98% sulfuric acid (95 to 98% sulfuric acid, manufactured by EMD, UN1830, SX1244-6, lot 51320, MW 98.08, CAS: 766493-9, The second tank is maintained at 120 ° C. and the third tank is maintained at 140 ° C.). Each bath is maintained at a constant temperature. The fourth tank contains deionized water. In the first tank, a constant fiber tension of 292.5 gf (weight gram) is maintained. In both the second tank, the third tank, and the fourth tank, a constant fiber tension of 194 gf is maintained. The fiber tows sequentially pass through the first tank, the second tank, the third tank, and the fourth tank so that the fiber tows stay in each tank for 60 minutes, thereby providing a stabilized processed article. Go ahead. The eight segments (M, N, O, P, Q, R, S, T) of the stabilized processed article were then subjected to various lengths listed in Table IV by Advalon CF 3295 (from Wacker) at room temperature. Treated without tension for a period of time, thereby providing eight siloxane treated stabilized workpieces. Each segment is then carbonized in a continuous furnace at a temperature listed in Table IV at a tension of 0.5 N to obtain a carbonaceous article having the properties described in Table V and Table VI. It is observed that tensile strength and Young's modulus are improved by the introduction of silicon.

  As summarized in Table VII, the carbon, carbon, hydrogen, nitrogen, and sulfur content of the carbonaceous article is determined by elemental analysis (where ND is the undetectable amount of a given element or. Refers to less than 5 weight percent). Of particular interest here is the silicon concentration in the fiber. The silicon composition of the carbonaceous article was determined by X-ray fluorescence (XRF) -direct fiber analysis, XRF-melt bead analysis, and inductively coupled plasma-emission spectroscopy (ICP-AES) and is summarized in Table VIII. As used herein with respect to XRF-melt bead analysis, ND refers to an undetectable amount of a given element or less than 0.014 weight percent. As used herein for ICP-melt bead analysis, ND refers to an undetectable amount of a given element or less than 0.05 weight percent. The silicon content measured with each technique is consistent. Further silicon analysis is reported by XRF-direct fiber analysis. The silicon composition determined by XRF-direct fiber analysis of the carbonaceous articles prepared according to the given examples is summarized in Table IX. Untreated fibers, ie fibers not treated with a silicon source such as Advalon, were measured by XRF as having an undetectable amount of silicon (about 0.01 wt% silicon). This value is used as a reference for comparison. It is observed that the Si concentration decreases (constant processing time) as the carbonization temperature increases.

  Table X summarizes the surface chemical composition of carbonaceous articles prepared according to a given example, as determined by X-ray photoelectron spectroscopy (XPS). As used herein with respect to XPS analysis of surface chemical composition, ND refers to an undetectable amount of a given element or less than 0.1 atomic percent.

  Using a scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-XEDS), the section of the prepared carbonaceous article is scanned to characterize the radial distribution of silicon. The distribution of silicon is surprising and unexpected. At 1150 ° C., there is a silicon coating on the carbon fibers. At 1800 ° C., SEM-XEDS reports that the silicon is uniformly distributed throughout the fiber, thereby indicating that the silicon is mobile within the fiber at higher carbonization temperatures.

  As reported in Table XI, EDS is used to determine the silicon content at various depths within the fiber at different carbonization temperatures. As used in Table XI, the location of measurement refers to the distance between the outer edge of the fiber and the center of the fiber measured along the radius of the fiber, for example 25% is the distance between the outer edge and the center of the fiber. Refers to 25% of the distance between. Table XI shows that the migration of silicon within the fiber is temperature dependent. At lower carbonization temperatures, the silicon is primarily at the outer edge of the fiber, and at higher carbonization temperatures, the silicon is generally uniformly distributed.

  Example 4

Polyethylene / 1-octene copolymer (density = 0.955 g / cm ³ , MI = 30 g / 10 min, 190 ° C./2.16 kg) was melt-spun into 1758 filaments (diameter = 8.0 microns, toughness = 4.957 g). / Denier, breaking strength = 7.273%). The fiber tow is then sulfonated in a 4 tank continuous process under constant tension. The first tank contains 20-30% free fuming sulfuric acid (Acros, maintained at 50 ° C.). Both the second tank and the third tank contain 95 to 98% sulfuric acid (95 to 98% sulfuric acid, manufactured by EMD, UN1830, SX1244-6, lot 51320, MW 98.08, CAS: 766493-9, The second tank is maintained at 120 ° C. and the third tank is maintained at 140 ° C.). Each bath is maintained at a constant temperature. The fourth tank contains deionized water. In the first tank, a constant fiber tension of 292.5 gf (weight gram) is maintained. In both the second tank and the third tank, a constant fiber tension of 194 gf is maintained. The fiber tows sequentially pass through the first tank, the second tank, the third tank, and the fourth tank so that the fiber tows stay in each tank for 60 minutes, thereby providing a stabilized processed article. Go ahead.

  The stabilized segment of the processed article is then processed according to two processing regimes.

  In Regime 1, the four segments (A, B, C, D) of the stabilized workpiece are not treated with the siloxane-containing solution.

  In regime 2, four stabilized and processed segments (E, F, G, H) are treated with Advalon CF 3295 at room temperature for 100 minutes.

  All segments are then carbonized to 1150, 1400, 1600, or 1800 ° C. according to the conditions listed in Table XII. Segment characteristics are reported in Table XIII.

  Example 5

  In this example, samples A to H of Example 4 are used. The obtained carbon fiber is subjected to a thermal oxidation resistance experiment using TGA. Control carbon fibers and silicon-containing carbon fibers are heated to 25-800 ° C. at 10 ° C./min in an air environment. The measured properties of the segments are reported in Table XIV.

Claims (10)

  A process that involves treating polyolefin fibers with a silicon source.   The process of claim 1, wherein the polyolefin fiber is a stabilized polyolefin fiber.   The process of claim 2, further characterized in that the stabilized polyolefin fibers are stabilized by sulfonation.   4. The polyolefin fiber according to claim 1, further characterized in that the polyolefin fiber is one or any combination of ethylene, propylene, butadiene and / or styrene units or a copolymer comprising two or more. The process described in the section.   The process according to any one of claims 1 to 4, further characterized in that the silicon source comprises a siloxane, a cyclic siloxane, or a polydiorganosiloxane oligomer (cyclic or linear hydroxyl terminated).   The process of claim 5, wherein the silicon source is an aqueous siloxane source.   The process of any one of the preceding claims, wherein the aqueous siloxane source comprises an aqueous siloxane bath having a siloxane concentration of at least 0.1 weight percent siloxane.   8. The method of any one of claims 1-7, further comprising heating the fiber to a temperature of 1000C or higher in an inert atmosphere so as to convert the stabilized polyolefin fiber to carbon fiber. process.   9. The process of claim 8, further characterized by disposing sufficient silicon on the stabilized polyolefin fiber such that the resulting carbon fiber has a silicon concentration of 1 mole percent or greater.   A carbonaceous article comprising fibers having 90-100 weight percent carbon, 0.1-1 weight percent silicon, 0.1-1 weight percent nitrogen.