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Carbon fiber strandRelated Patent Categories: Stock Material Or Miscellaneous Articles, Coated Or Structually Defined Flake, Particle, Cell, Strand, Strand Portion, Rod, Filament, Macroscopic Fiber Or Mass Thereof, Rod, Strand, Filament Or FiberCarbon fiber strand description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20050271874, Carbon fiber strand. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a carbon fiber strand suitably used in applications such as resin reinforcing agent and the like. More particularly, the present invention relates to a carbon fiber strand which, when used for production of a composite material (e.g. a carbon fiber-reinforced resin), can give a composite material having sufficient adhesion between the carbon fiber and the matrix resin and being superior in interlaminar shear strength (ILSS) between the matrix resin and the carbon fiber. BACKGROUND ART [0002] Carbon fiber has features of being strong, high in elastic modulus and light as compared with other fibers; therefore, it is in wide use as a reinforcing material for a composite material using a thermoplastic resin or a thermosetting resin as the matrix resin. A composite material reinforced with carbon fiber is light and highly strong and, therefore, is in wide use in various industries including aerospace industry. [0003] The process for producing a composite material using a thermosetting resin includes a process of producing a prepreg of desired shape and molding it. Such a composite material can also be produced by a pultrusion process using a carbon fiber strand and a thermosetting resin, a resin transfer molding (RTM) process, a filament winding (FW) process, a sheet molding compound (SMC) process, a bulk molding compound (BMC) process, a hand lay-up process, etc. [0004] As the thermosetting matrix resin used in production of a thermosetting resin-based composite material, there can be mentioned, for example, epoxy resin, unsaturated polyester resin, vinyl ester resin, and phenolic resin. Of these, epoxy resin, in particular, is preferred because the epoxy resin can produce a composite material well-balanced in heat resistance, physical properties, etc. [0005] As the epoxy resin, there can be mentioned bisphenol A type epoxy resin, polyfunctional epoxy resin produced from a raw material such as tetraglycidylamine, triglycidylamine or the like, novolac type epoxy resin, etc. Bisphenol A type epoxy resin, in particular, is in wide use as a matrix resin because it is superior in adhesivity, physical properties, etc. and is general-purpose. [0006] In producing a prepreg, a carbon fiber strand is processed according to a given procedure. In this processing, the carbon fiber strand is fretted by a guide, etc., which tends to cause fluffing and accordingly makes difficult later handling. In order to avoid this problem, a sizing agent is ordinarily added to the carbon fiber strand. Covering the surface of the carbon fiber strand with the sizing agent can improve the bundle ability and accordingly improve fretting resistance and handleability of the strand. [0007] In general, adhesivity with matrix resin is considered in selection of a sizing agent and, when the matrix resin is an epoxy resin, an epoxy resin is used as the sizing agent (JP-B-62-56266 and JP-A-7-197381). [0008] It was proposed to use a vinyl ester resin as a sizing agent When the matrix resin is an unsaturated matrix resin. In these proposals, however, since no consideration was made as to the adhesivity between sizing agent and carbon fiber, the adhesivity between sizing agent and carbon fiber is insufficient. [0009] Meanwhile, in order to improve the adhesivity between sizing agent and carbon fiber, there were proposed sizing agents having a polar group which may react with a functional group present on the surface of the carbon fiber (JP-A-56-167715, JP-A-63-50573, JP-A-11-93078 and JP-A-2001-20181). A composite material produced using a carbon fiber strand containing such a sizing agent, when measured for its interlaminar shear strength (ILSS), shows a superior value. Incidentally, the interlaminar shear strength (ILSS) of a composite material is a yardstick showing the adhesivity between the carbon fiber and matrix resin constituting the composite matrix. However, in a composite material produced using a carbon fiber containing the above-mentioned sizing agent, when a tension is added thereto in its fiber axis direction, the tension is not dissipated into the whole portion of the carbon fiber and instead is concentrated in a relatively small number of carbon fibers. Consequently, this small number of carbon fibers are broken, it is repeated, and finally the whole composite material comes to be broken. Therefore, the strength of the composite material in the fiber axis direction is not large as expected and its interlaminar shear strength is low currently. DISCLOSURE OF THE INVENTION [0010] In order to solve the above problems, the present inventor made a study on a sizing composition to be added to a carbon fiber. In the course of the study, the present inventor found that when a sizing agent composition is mixed with a curing agent and the mixture is heat-treated to prepare a cured material of the sizing agent composition, the cured material is used as a test sample and measured for dynamic viscoelasticity to obtain a tan .delta. of a relaxation peak (.alpha..sub.tan .delta.) and a tan .delta. of .beta. relaxation peak (.beta..sub.tan .delta.), and there is calculated a product of these two values, i.e. an .alpha..sub.tan .delta. .beta..sub.tan .delta., this product has a large influence on the strength of a composite material produced using a resin and a carbon fiber to which the above-sizing agent has been added, in its normal-to-fiber-axis direction. A further in-depth study by the present inventor confirmed that any of the .alpha..sub.tan .delta. and the .beta..sub.tan .delta. has no correlation with the strength of the composite material in its normal-to-fiber-axis direction. [0011] The present invention has been completed based on the above findings and aims at providing a carbon fiber strand capable of giving a carbon fiber-reinforced resin composite material superior in interlaminar shear strength greatly connected with its breaking strength in its normal-to-fiber-axis direction. [0012] The present invention which achieves the above aim, is as follows. [0013] [1] A carbon fiber strand obtained by impregnating a carbon fiber with a sizing agent composition containing a sizing agent comprising at least two kinds of epoxy resins, wherein the sizing agent composition is such that, when it is mixed with a given curing agent at proportions of 100 parts by mass (the sizing agent composition) and 30 parts by mass (the curing agent) to make a composition for estimation, the composition for estimation is heat-treated at 130.degree. C. for 2 hours, and the resulting cured material for estimation is measured for dynamic viscoelasticity to obtain its tan .delta. of .alpha. relaxation peak and its tan .delta. of .beta. relaxation peak, their product .alpha..sub.tan .delta. .beta..sub.tan .delta. is 0.07 to 0.2. [0014] [2] A carbon fiber strand set forth in [1], wherein the sizing agent composition has a Viscosity of 100 to 10,000 poises at 30.degree. C. [0015] [3] A carbon fiber strand set forth in [1], wherein the sizing agent contained in the sizing agent composition contains a PO/EO block copolymer in an amount of less than 30% by mass relative to the epoxy resins. [0016] [4] A carbon fiber strand set forth in [1], wherein the content of the sizing agent composition is 0.3 to 5.0% by mass. [0017] [5] A carbon fiber strand set forth in [1], which is constituted by 1,000 to 50,000 single fibers. [0018] [6] A carbon fiber strand set forth in [1], wherein the carbon fibers constituting the carbon fiber strand show a surface oxygen concentration ratio O/C of 0.05 to 0.3 when measured by X-ray photoelectron spectroscopy. [0019] The carbon fiber strand of the present invention is constituted by impregnating a carbon fiber strand with a sizing composition which, when cured under particular conditions, shows an .alpha..sub.tan .delta. .beta..sub.tan .delta. of 0.07 to 0.2; therefore, by using this carbon fiber strand as a reinforcing material, there can be produced a carbon fiber-reinforced composite material superior in interlaminar shear strength. BRIEF DESCRIPTION OF THE DRAWING [0020] FIG. 1 is a chart showing an example of the dynamic viscoelasticity curve of a cured material for estimation obtained by curing a sizing composition. Continue reading about Carbon fiber strand... Full patent description for Carbon fiber strand Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Carbon fiber strand patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Carbon fiber strand or other areas of interest. ### Previous Patent Application: Method for manufacturing polarizing film and polarizing film and optical film manufactured by using the method Next Patent Application: Composite particle with a carbodimide resin layer and process for producing the same Industry Class: Stock material or miscellaneous articles ### FreshPatents.com Support Thank you for viewing the Carbon fiber strand patent info. 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