US20140205831A1

US20140205831A1 - Flexible reinforcing fiber yarn pre-impregnated with resin

Info

Publication number: US20140205831A1
Application number: US14/235,564
Authority: US
Inventors: Markus Schneider; Silke Stüsgen; Silke Witzel; Bernd Wohlmann
Original assignee: Toho Tenax Europe GmbH
Current assignee: Teijin Carbon Europe GmbH
Priority date: 2011-07-29
Filing date: 2012-07-20
Publication date: 2014-07-24

Abstract

A pre-impregnated yarn having a bundle made of reinforcing fiber filaments impregnated with a first resin composition infiltrated into the pre-impregnated yarn and at least partially connected via the first resin composition. The first resin composition contains at least two bisphenol A epichlorohydrin resins H1 and H2 in a weight ratio H1:H2 of 1.1 to 1.4, and an aromatic polyhydroxy ether P1. The pre-impregnated yarn has a second resin composition on the bundle outer side in the form of adhesive particles or drops. The second resin composition is solid at ambient temperatures and has a melting temperature in the range from 80 to 150° C. The bundle interior and at least 50% of the surface of the bundle outer side are free of the second resin composition.

Description

The invention relates to a pre-impregnated yarn consisting of a bundle of reinforcing fiber filaments having a bundle interior and a bundle outer side, wherein the reinforcing fiber filaments are impregnated with a first resin composition infiltrated into the pre-impregnated yarn, which composition can be multiply melted and converted to a solid state by cooling to ambient temperature. The invention further relates to a textile structure which comprises a yarn of this type.
Components made from fiber composite materials are increasingly used, especially in the aircraft and space industrial sectors, yet also e.g. in machine building, wind power, or the automotive industry. Fiber composite materials often offer the advantage of lower weight and/or higher strength over metals. An essential aspect thereby is the inexpensive production of this type of resilient and yet lightweight composite material components at the same time. In view of the resistance, i.e. the rigidity and strength, the volume percent of the reinforcing fibers and especially also the direction of the reinforcing fibers have a decisive effect on composite material components.
A commonly used manufacturing method is currently based on the so-called prepreg technology. In this case, the reinforcing fibers, such as glass fibers or carbon fibers, are arranged for example parallel to one another, embedded in a matrix resin, and processed into sheet-like semi-finished products. For component manufacture, these sheets are cut according to the component contour and laminated layer-by-layer into a tool by machine or by hand while taking into account the orientation of the reinforcing fibers as required by the component load. Subsequently, the matrix is cured under pressure and at temperature in an autoclave. Prepregs (abbreviation for pre-impregnated fibers) already have as a rule the two components (fiber and matrix resin) in the final mixture ratio and are therefore already bending resistant as a semi-finished product. In order to prevent premature, undesired reactions, this material must additionally be stored under cool conditions and at the same time has only a limited storage period. Due to the bending stiffness and the production as wide rolled goods, the applications for prepregs are limited to large-surface and virtually flat components. The matrix resin already present does not allow for textile processing or laying of the prepregs without folds, for example along narrow radii or on strongly contoured geometries.
Examples for achieving improved textile processing with impregnated yarn products are described e.g. in U.S. Pat. No. 5,275,883 and U.S. Pat. No. 4,614,678, which disclose reinforcing fiber yarns provided with a coating. According to these documents, the reinforcing fiber yarns are initially loaded with a mixture of a polymer powder and subsequently coated with a sheath preferably made from a thermoplastic polymer in order to stabilize the polymer powder in the interior. These yarn materials do indeed have a certain flexibility; however, as a result of the continuous thermoplastic sheath they are still relatively rigid, and therefore are only of limited suitability for e.g. further textile processing methods.
Similar products are disclosed in EP-A 0 554 950 A1, which relates to a method in which initially an open yarn bundle of reinforcing fibers is impregnated with a thermoplastic polymer powder and subsequently the impregnated fiber bundle is provided with a continuous sheath made of a thermoplastic polymer. The resulting sheathed bundle is calendered at a temperature above the softening temperature of the thermoplastic, after which the bundle is finally cut into granular form. The granules serve for producing composite components via methods like extrusion or injection molding.
In EP-A-0 814 916 B1, so-called yarn prepregs (“towpregs”) are described, which are suitable for use in textile preform processes, wherein, among others, weaving, braiding, or knitting processes or winding methods (“filament winding”) belong to textile preform processes of this type, or the yarn prepregs can be processed into short-cut material. The yarn prepregs from EP-A-0 814 916 B1 comprise a plurality of fibers as well as a coating made of matrix resin, wherein the fibers are structured in an arrangement of inner fibers, which are substantially free of matrix resin, and outer fibers, wherein the outer fibers are at least partially embedded in a non-continuous sheath made of the matrix resin. The production of the yarn prepreg takes place by applying powdery particles of the matrix resin to the outer fibers and subsequently partially melting the matrix resin particles. The matrix resin used can be a thermoset or a thermoplastic material.
U.S. Pat. No. 6,228,474 also deals with the production of yarn prepregs made of reinforcing fiber bundles impregnated with an epoxy resin composition, wherein the resin proportion in the yarn prepregs lies in the range from 20 to 50 wt. %. The epoxy resin composition in one embodiment can comprise two or more epoxy resins and be a mixture of monofunctional or bifunctional epoxy resins with trifunctional or polyfunctional epoxy resins. This epoxy resin composition further comprises fine particles of a rubber material that is insoluble in the epoxy resins as well as curing agent components for the epoxy resins.
Yarn prepregs must have a sufficiently high proportion of matrix resin, typically more than 15 vol. %, in order to allow for consolidation to a component structure that is substantially free of cavities or pores without requiring the addition of further matrix resin. Yarn prepregs of this type do indeed have a higher flexibility over sheet-like prepregs. However, they can only be further processed in a limited way in textile processes due primarily to the high matrix resin proportion. In addition, the presence of the matrix resin often leads to an increased tackiness of the yarn prepregs, which results in an increased complexity during handling of these yarn prepregs. In addition, as a rule, permanent cooling of the yarn prepregs is required up until the time of processing in order to prevent an uncontrolled curing of the matrix resin. Finally, yarn prepregs have disadvantages in the production of three-dimensional structures and, for example, cannot be repeatedly reformed.
Increasingly, fiber composite components made from reinforcing fibers are produced via so-called near-net-shape fiber preforms. Essentially, these fiber preforms are textile semi-finished products in the form of two- or three-dimensional configurations made from reinforcing fibers, in which, in further steps for producing the fiber composite component, a suitable matrix material is introduced via infusion or injection, also by application of vacuum. Subsequently, the matrix material is cured at, as a rule, increased temperatures and pressures into the finished component. Known methods for infusion or injection of the matrix material are the so-called liquid molding (LM) method, or methods related thereto such as resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin film infusion (RFI), liquid resin infusion (LRI), or resin infusion flexible tooling (RIFT). For these applications, reinforcing fibers are used that are not yet provided with matrix resin in the amount required for the later composite component because the matrix material is, as previously stated, introduced into the finished fiber preform in a subsequent process step. On the other hand, it is advantageous if the fiber material used for the production of the fiber preforms already is impregnated with e.g. small amounts of a plastic material, i.e. a binder material, that is e.g. curable or becomes rigid at reduced temperature, in order to improve the fixing of the reinforcing fibers in the fiber preform and to impart a sufficient stability to the fiber preform.
WO 98/50211 relates to reinforcing fibers coated with a binder material and suitable for use in the production of fiber preforms, to which fibers the binder material is applied in the form of particles or discrete regions on the surface of the reinforcing fibers. The binder material consists of 40 to 90 wt. % of a thermoset resin and 10 to 60 wt. % of a thermoplastic, which is tailored to the matrix material used in the later fiber composite component produced from the fiber preform. The binder material applied to the reinforcing fibers is rigid and non-tacky at ambient temperatures. According to WO 98/50211, the reinforcing fibers thus coated or, e.g. the woven fabrics produced therefrom, have a good drapability. According to WO 98/50211, the individual yarn strands can initially be provided with the binder material and subsequently processed into woven fabrics. The yarns from WO 98/50211 are not suitable for the production of flat yarn strands with a fixed yarn width, which flat yarn strands would be amenable to an automated, direct processing into fiber preforms. In addition, the reinforcing yarns coated with binder material from WO 98/50211 can have in part relatively high proportions of binder material of up to 20 wt. %, which can result in significantly impaired impregnation behavior in the fiber preforms produced therefrom.
Pre-impregnated yarns for the production of fiber preforms are also described in WO 2005/095080. Regarding the yarns of WO 2005/095080, the filaments of the pre-impregnated yarns are at least partially connected via a resin composition, wherein the yarns have 2.5 to 25 wt. % of the resin composition in relation to the total weight of the yarns, wherein the resin composition is composed of a mixture of at least two epoxy resins, and wherein the epoxy resins are different with respect to their epoxy value and molecular weight. The weight ratio of the epoxy resins in the mixture is selected such that the resin composition has an epoxy value between 550 and 2100 mmol/kg of resin. Alternatively, a mixture of three bisphenol A epichlorohydrin resins is proposed with defined characteristics of the resins with respect to epoxy value, molecular weight, and melting point. The resin compositions are selected such that they can be multiply melted and can be converted again to a solid state by cooling to ambient temperature, and that the yarns impregnated therewith are non-tacky at ambient temperature, yet are tacky at increased temperatures. However, it has been shown that yarns impregnated with the resin compositions from WO 2005/095080 do not have a sufficient tackiness for all applications, e.g. for applications in which yarns are laid over each other at an angle of for example 90°.
There is therefore a need for improved pre-impregnated yarns for the production of fiber preforms. It is therefore the object of the present invention to provide improved pre-impregnated reinforcing fiber yarns of this type, in particular for use in the production of fiber preforms.
The object is achieved by a pre-impregnated yarn consisting of a bundle of reinforcing fiber filaments with a bundle interior and a bundle outer side,

- wherein the reinforcing fiber filaments are impregnated with a first resin composition infiltrated into the pre-impregnated yarn and the filaments of the pre-impregnated yarn are at least partially connected via the first resin composition, and
- wherein the first resin composition contains at least two bisphenol A epichlorohydrin resins H1 and H2 in a weight ratio H1:H2 of 1.1 to 1.4,
  - wherein H1 has an epoxy value of 1850 to 2400 mmol/kg and an average molecular weight M_Nof 800 to 1000 g/mol and is solid at ambient temperatures, and
  - H2 has an epoxy value of 5000 to 5600 mmol/kg and an average molecular weight M_Nof <700 g/mol and is liquid at ambient temperatures, wherein the pre-impregnated yarn is characterized in that
- it has 0.1 to 2% wt. % of the first resin composition in relation to the total yarn weight, and the first resin composition further contains an aromatic polyhydroxy ether P1, which has an acid value of 40 to 55 mg KOH/g and an average molecular weight M_Nof 4000 to 5000 g/mol, and that
- the pre-impregnated yarn has a second resin composition on the bundle outer side in the form of particles or drops adhering to the reinforcing fiber filaments,
  - wherein the second resin composition is solid at ambient temperatures, has a melting temperature in the range from 80 to 150° C. and is present on the bundle outer side in a concentration of 0.5 to 10 wt. % in relation to the total weight of the pre-impregnated yarn,
  - wherein at least 50% of the surface of the bundle outer side is free of the second resin composition, and
  - wherein the bundle interior is free of the second resin composition.

It has been shown that the yarn pre-impregnated in such a way possesses excellent dimensional stability and can be multiply melted and converted to a solid state or quasi-solid state by cooling to ambient temperature. In addition, the resin for the yarns according to the invention can be selected such that the yarn to be coated therewith is non-tacky at ambient temperature. In this case, a non-tacky state is understood as a state, such as is e.g. also present for commercially available standard carbon fibers and which enables a problem-free unwinding up e.g. from bobbins. Therefore, a yarn of this type then can be not only wound up, but also stored in the wound up state while retaining its textile characteristics and can even be unwound again after a long storage period at ambient temperature. For example, the yarn according to the invention can be unwound without problems after 12 months storage time and shows at most negligible changes for the characteristics of strength, elastic modulus, and elongation at break measured according to DIN 65 382. In a preferred embodiment, the first and/or the second resin composition is free of curing agents.
The yarn according to the invention can be a yarn made of short fiber filaments or a yarn made of endless filaments. In the case that the yarn consists of endless filaments, the number of filaments can lie preferably in the range from 6000 to 48,000 filaments, and particularly preferably in the range from 12,000 to 24,000 filaments. Likewise, yarns having a linear density in the range from 400 to 32,000 tex are preferred, and particularly preferred yarns are those having a linear density in the range from 800 to 16,000 tex.
In a further preferred embodiment, the yarn according to the invention is obtained from pitch, polyacrylonitrile, lignin, or viscose pre-products, or the yarn is an aramid, glass, ceramic, or boron fiber yarn, a synthetic fiber yarn or a natural fiber yarn, or a combination of one or more of these fibers. The yarn according to the invention is particularly preferably a carbon fiber yarn.
As previously stated, the yarn according to the invention has a high dimensional stability, wherein this is understood to mean that the yarn has a stable, fixed yarn width or a stable ratio of yarn width to yarn thickness that remains unchanged even if the yarn according to the invention is held unsupported over large distances under tension or is further processed in textile processes. Due to this excellent dimensional stability, automated processing, for example automated laying to form fiber preforms, is enabled. In addition, the fixed and consistent yarn width of the yarns according to the invention leads to more stable adhesion of superimposed yarns during the production of fiber preforms. It was found that the dimensional stability of the yarn according to the invention is essentially a result of the first resin composition, with which the pre-impregnated yarn is infiltrated, wherein the proportion of the aromatic polyhydroxy ether P1 plays a major role. In a preferred embodiment, the first resin composition thereby contains the bisphenol A epichlorohydrin resins H1 and H2 in a weight ratio to the aromatic polyhydroxy ether P1, (H1+H2):P1, of 0.05 to 0.8. It was observed in tests that weight ratios lower than 0.05 can lead to increased yarn abrasion. Weight ratios greater than 0.8 in contrast lead to yarns with an excessively low dimensional stability. In view of the dimensional stability on the one hand and the drapability on the other, it is also advantageous if the first resin composition is present in a concentration of 0.4 to 1.2 wt. % in relation to the total weight of the pre-impregnated yarn.
The yarn according to the invention can be brought in a simple way into the shape of a flat band already in the production process of the yarn, in that the initially impregnation-free, easily spreadable yarn is fed into and through a bath via suitable spreading devices and impregnated with the first resin composition. The first resin composition thereby connects the filaments of the yarn at least partially and ensures a very good consolidation. In addition, due to its composition, the first resin composition imparts a high dimensional stability to the spread and now impregnated yarn, by which means the ribbon shape remains unchanged and the yarn can be wound up, e.g. on spools, in this shape after the application of the second resin composition. Later, then, the inventive pre-impregnated yarn can be processed, without additional measures such as the transition via suitable spreading devices, by means of routine laying methods to produce textile structures, such as two- or three-dimensional fiber preforms or two-dimensional structures, for example in the form of unidirectional wovens or multiaxial composites. The high dimensional stability enables an advantageous embodiment of the pre-impregnated yarn, such that said yarn is available as a flat band, which has a ratio of yarn width to yarn thickness of at least 20. in a particularly preferred embodiment, the flat band has a ratio of yarn width to yarn thickness in the range of 25 to 60.
Due to the second resin composition applied to the bundle outer side, it is achieved in the pre-impregnated yarns according to the invention, that these are non-tacky at ambient temperatures and can be e.g. wound up as described. At increased temperature, however, a high tackiness is achieved due to the second resin composition, which tackiness also leads to a high stability of the structure of the fiber preform after cooling, even in structures in which the yarns according to the invention are laid superimposed over one another at an angle. When using the yarn according to the invention, preforms can therefore be produced without requiring the costly addition of binder material for fixing the yarns, wherein a better binding still results between the yarns than in a preform of the prior art.
At the same time, it was found that in the indicated concentration of the second resin composition, in particular the type of application of the second resin composition in the form of particles or drops adhering to the reinforcing fiber filaments, wherein at least 50% of the surface of the bundle outer side is free of the second resin composition and wherein the bundle interior is free of the second resin composition, leads to pre-impregnated yarns with high flexibility and good drapability. It thereby is shown to be advantageous when the particles or drops adhering to the reinforcing fiber filaments have a size less than 300 μm, and particularly advantageous if they have an average size in the range from 20 to 150 μm.
To achieve in particular the characteristics of the present yarn with respect to its tackiness or its adhesive strength, the second resin composition contains in a preferred embodiment at least 50 wt. % of a bisphenol A epichlorohydrin resin H3 with an epoxy value of 480 to 645 mmol/kg and an average molecular weight M_Nof 2700 to 4000 g/mol, an aromatic polyhydroxy ether P2, a polyimide, a polyethylene, an ethylene copolymer such as an ethylene vinyl acetate (EVA) copolymer or a thermoplastic polyurethane resin or mixtures of these compounds, wherein these compounds have a melting temperature in the range from 80 to 150° C. Thereby, embodiments are also comprised, in which e.g. the bisphenol A epichlorohydrin resin H3 is a mixture of two or more bisphenol A epichlorohydrin resins, so long as the mixture has an epoxy value of 480 to 645 mmol/kg and an average molecular weight M_Nof 2700 to 4000 g/mol, and a melting temperature in the range from 80 to 150° C.
Particularly preferably, the second resin composition contains the previously mentioned compounds in a ratio of at least 80 wt. % and more particularly preferably at least 90 wt. %. In a particularly suitable embodiment, the second resin composition consists of the indicated compounds or mixtures of the indicated compounds.
The aromatic polyhydroxy ether P2 used in the second resin composition and the aromatic polyhydroxy ether P1 contained in the first resin composition can be the same or different. However, the condition must be fulfilled for the aromatic polyhydroxy ether P2 that it has a melting temperature in the range from 80 to 150° C.
To achieve a sufficiently high adhesive strength of the pre-impregnated yarns for the production of the fiber preforms, the second resin composition has a good adhesive force or adhesive strength above the melting temperature thereof. In a preferred embodiment, the second resin composition has an adhesive strength or adhesive force of at least 5 N at a temperature 20° C. above the melting temperature, in relation to an adhesive surface with a diameter of 25 mm. The determination of adhesive force or adhesive strength is carried out basing on ASTM D 2979. In this case, the adhesive force is considered to be the force that is required to separate a sample of the second resin composition from an adhesive surface shortly after bringing the second resin composition and the adhesive surface into contact under a defined load and temperature and during a defined time. The details of the determination will be given later.
In view of the total characteristics of the inventive pre-impregnated yarns and especially in view of achieving good impregnation characteristics for the fiber preforms produced from the yarns during later infusion or injection with matrix resin, it is advantageous when the concentration of the second resin composition is greater than that of the first resin composition. It is likewise advantageous when the total concentration of the first resin composition and the second resin composition lies in the range from 2 to 7 wt. % in relation to the total weight of the pre-impregnated yarn.
In principle, any technology is suitable for the infiltration of the first resin composition into the yarn or the impregnation of the yarn with the first resin composition, which technology supports a fast and complete wetting of the reinforcing fiber filaments of the yarn with the first resin composition. Methods of this type are described for example in EP 1 281 498 A. For example, the yarn can be sprayed with a dispersion or an emulsion of the first resin composition. A film of the resin dispersion or resin emulsion can also be applied to a smooth roller or into the grooves of a roller and the yarn can be pulled over the smooth roller or through the grooves of the roller. Preferably, the yarn is fed through a bath which contains a dispersion or emulsion of the first resin composition. It is likewise possible that the yarn is successively impregnated with the individual components of the first resin composition, for example, in that the yarn is successively fed through different dispersion baths which contain the individual components of the first resin composition. In this case, the yarn provided for the impregnation step can be initially spread by means of a suitable spreading device to the desired width so that the individual fibers or individual filaments are easily accessible for impregnation. Preferably, the bundle of the yarn to be impregnated is brought into the shape of a flat band having the ratio of yarn width to yarn thickness desired for the final inventive pre-impregnated yarn.
In principle, any liquid mixture is suitable as the liquid phase for the previously indicated resin dispersion or resin emulsion, which liquid mixture forms a stable dispersion or emulsion with the inventive resins. Among these liquid mixtures, in particular those which are aqueous and have a low VOC (volatile organic content) are suitable for emission protection reasons. The components of the first resin composition are thereby present advantageously as particles in the micrometer range, particularly preferably with a size less than 0.1 μm.
Naturally, the application amount of the first resin composition, in relation to the total weight of the yarn, can be adjusted via the speed with which the yarn is e.g. fed through a bath that contains the dispersion of the first resin composition, via the immersion length, and via the resin concentration in the bath. In this case, the speed with which the yarn is fed through the bath lies preferably in the range of 120 to 550 m/h, particularly preferably in the range of 150 to 250 m/h. The immersion length lies preferably in the range from 0.2 to 1 m. The resin concentration in the dispersion in relation to the weight thereof lies preferably in the range of 2 to 35 wt. % and particularly preferably in the range from 2 to 7 wt. %.
Subsequent to the impregnation of the yarn with the first resin composition, the yarn or the yarn bundle is loaded on the outer side thereof with the second resin composition. In this case, after the impregnation, the second resin composition is applied in the form of a powder to the outer side of the preferably still moist yarn bundle. The application of the second resin composition can take place e.g. via powder scattering methods, via aqueous dispersion or via fluidized bed processes, as is e.g. described in U.S. Pat. No. 5,275,883 or U.S. Pat. No. 5,094,883, wherein the particles can be preferably electrostatically charged, as is the case in electrostatic powder scattering.
The second resin composition present in particle form has a particle size distribution, wherein in a preferred embodiment the particle size distribution, as determined by laser diffractometry, has characterizing values for the particle size D50 for the average particle size in the range of approximately 20 to 120 μm and D90 in the range from 70 to 160 μm. Particularly preferable is a particle size distribution with a D50 value in the range of 30 to 100 μm and a D90 value in the range from 85 to 155 μm.
A drying temperature in the range of 100 to 160° C. has been shown to be particularly suitable for drying the yarn provided with the first and the second resin compositions. By this means, the second resin composition is simultaneously melted and forms island-shaped particles or drops adhering to the bundle outer side.
The production of the inventive pre-impregnated yarn can be integrated into the production process of the initial yarn, i.e. the impregnation of the yarn with the first resin composition and the application of the second resin composition on the yarn can directly follow the production process for the yarn provided. However, an initial yarn which is e.g. wound up on a spool can also be provided in a separate process with the first resin composition and subsequently with the second. It is likewise possible that an initial yarn which is impregnated with the first resin composition is provided wound up on a spool and is then furnished with the second resin composition in a separate process step.
The pre-impregnated yarn according to the invention can be used advantageously for the production of textile structures like fiber preforms.
A further, underlying object of the present invention is therefore achieved by a textile structure which comprises the previously-described yarns according to the invention, wherein the yarns preferably are connected to each other via the second resin composition at points of mutual contact. In a preferred embodiment, the textile structure is a fiber preform.
Although wovens can also be produced from the yarns according to the invention, said wovens, following melting and re-solidification of the resin compositions, resulting in e.g. an exceedingly non-slip fiber preform, it is advantageous to construct fiber preforms of this type directly from the yarns according to the invention because the yarns can thereby be positioned in the direction in which, during the use of a composite component produced from the fiber preform according to the invention, the highest mechanical loads are expected.
Thus, in a preferred embodiment of the fiber preform according to the invention, the yarns are arranged unidirectionally, by which means the preform can be further processed into a composite component, during the use of which the maximum mechanical load is expected to be in this one direction of the yarns.
In a further preferred embodiment of the fiber preform according to the invention, the yarns are arranged bidirectionally, tridirectionally, or multidirectionally, by which means the preform can be further processed into a composite component, during the use of which the maximum mechanical load is expected to be in these two or more directions of the yarns.
In addition to the previously mentioned flat embodiments of the fiber preform according to the invention, the uni-, bi-, tri-, or multidirectionally arranged yarns can be wound around a body having, e.g. a cylindrical shape, such that a three-dimensional fiber preform results.
Further, an embodiment of the fiber preform according to the invention is preferred in which the yarns according to the invention were chopped (short-cut) into short pieces, and the pieces can be oriented in all spatial directions. By this means, this fiber preform is particularly suited for the production of a composite component, during the use of which mechanical loads can arise in all spatial directions.
Preforms according to the invention preferably can be produced by a method comprising the steps

- a) Provision of at least one of the yarns according to the invention,
- b) Arrangement of the at least one yarn in a configuration which corresponds to the configuration of the desired fiber preform,
- c) Heating of the configuration resulting from step b) to a temperature above the melting point of the second resin composition, and
- d) Cooling the configuration resulting from step c) to at least below the melting point of the second resin composition.

In a preferred embodiment of this method, the configuration resulting from step b) is simultaneously compacted during the heating in step c).
The fiber preform according to the invention or the fiber preform produced according to the inventive method detailed above shows a pronounced anti-slip property because the yarns of the fiber preform according to the invention are connected to one another at least via the second resin composition. Therefore, the fiber preform according to the invention is easily handled, which is advantageous in particular during the further processing thereof into a composite component.
When the fiber preform according to the invention or the fiber preform produced according to the inventive method should have openings, these openings can be realized by appropriate arrangement of the yarns and thus without any cutting losses. Thus, an expensive and labor-intensive cutting is avoided and no waste is generated. By this means, the production of composite components having openings is simplified and reduced in price.
Further, by using the yarn according to the invention instead of a textile fabric during the production of the fiber preform according to the invention or the fiber preform produced according to the method detailed above, the yarn can be positioned in the directions in which, during use of the subsequently produced composite component, the highest mechanical loads are expected.
For example, in a preferred embodiment of the method for producing a fiber preform, yarns according to the invention are arranged unidirectionally in step b) such that following step d) a fiber preform according to the invention results in which the yarns are unidirectionally arranged.
In a further preferred embodiment of the method for producing the inventive preform, the yarns according to the invention can be laid either in bi-, tri-, or multidirectional layers in step b) in a configuration that corresponds to the desired fiber preform. Yarns according to the invention can be used exclusively therein. Likewise, within a layer of yarns, only a part can consist of yarns according to the invention and the rest can be yarns, the filaments of which have no resin coating or have common yarn preparations used to improve the processability of carbon fibers. The yarns configured in the indicated way are heated in step c) of the inventive method at a temperature that is above the melting point of the second resin composition, whereby the yarns are compacted if necessary. By this means, the yarns become tacky. After cooling to at least below the melting point of the second resin composition in step d), an inventive preform is generated in which the yarns are arranged bi-, tri-, or multidirectionally.
In a further preferred embodiment of the method for producing the inventive preform, the yarns according to the invention are cut into short pieces, which have e.g. a length of 1 to 1000 mm, preferably I to 40 mm, and the short yarn pieces are placed into a mold in step a). Afterwards, in step b) of the inventive method, the short yarn pieces are heated to a temperature above the melting point of the second resin composition, by which means the short yarn pieces become tacky, and are thereby compacted if necessary. After cooling to at least below the melting point of the second resin composition in step d), a fiber preform according to the invention is generated in which the yarns according to the invention are present as short yarns having isotropic directionality.
The fiber preform according to the invention, or the fiber preform produced according to the inventive method can be used advantageously, due to the previously specified reasons, to produce a composite component that comprises a matrix, which is selected from one of the groups of polymers, metals, ceramics, hydraulically setting materials, and carbon, wherein thermoplastics like polyamides, copolyamides, polyurethanes and the like, or duromers such as epoxides are suitable as the polymer matrix, steel (alloys) or titanium are suitable for the metal matrix, silicon carbide and boron nitride are suitable as the ceramic matrix, cement or concrete is suitable as the hydraulically setting material, and graphite is suitable as the carbon matrix.
The yarns according to the invention are arranged in the resulting composite components in the direction in which, during use of the composite component, the greatest mechanical loads are expected. Thus, use of the yarns according to the invention and of the fiber preform produced therefrom leads to composite components in which the directionality of the yarns is custom adapted to the expected mechanical loads.
The invention will be described in more detail using the following examples and comparative examples. In so doing, the following methods of analysis will be used:
The epoxy value of the epoxy resins used is determined according to DIN EN ISO 3001:1999.
The molecular weight is determined by means of GPC analysis according to DIN 55672 after calibration with polystyrene (with tetrahydrofuran as the eluent).
The acid value in mg KOH/g is determined by titration with potassium hydroxide according to DIN 53240-2.
The particle size distribution is determined by means of laser diffractometry according to ISO 13320. The D50 and D90 parameters for the particle size are subsequently determined from the particle size distribution.
The melting temperature is determined by means of DSC according to DIN 65467.
The adhesive strength or adhesive force of the second resin composition is determined at a temperature of 20° C. above the melting temperature, based on ASTM D2979. The adhesive strength or adhesive force is measured as the force that is required to separate a sample of the second resin composition from an adhesive surface shortly after bringing the second resin composition and the adhesive surface into contact under a defined load and temperature and during a defined time. For this purpose, a measuring apparatus is used, such as the MCR 301 rheometer (Anton Paar GmbH), which is equipped with corresponding force sensors and suitable for tensile tests. The determination of adhesive strength or adhesive force thereby takes place with a plate/plate measuring geometry using plates made of aluminum (AlCuMgPb, Wst.-Nr. 3.1645, EN AW 2007) and with a plate diameter of 25 mm.
Approximately 5 g of the resin composition to be tested (preferably in powder form) is applied at ambient temperature to the lower plate of the plate/plate measuring system. Shortly before the contact of the sample material by the upper plate, the plates of the measuring system are brought together to a distance of approximately 2.025 mm. The sample is subsequently heated by means of a suitable temperature control device (e.g. Peltier temperature control system) to the required measuring temperature of 20° C. above the melting temperature of the second resin composition to be tested. After reaching the measuring temperature, the plates of the measuring system are brought together until contact with the sample material at 2 mm and the sample material is pressed together at a constant force of 10 N for 5 s.
Subsequently, the upper plate is moved upward at a constant withdrawal speed of 2 mm/s and a constant temperature, and the force required thereby is constantly measured. The maximum value of the force required to pull the plates apart is used as the measurement for the adhesive strength or adhesive force of the sample tested.
The determination of the adhesive strength of the pre-impregnated yarns is done based on DIN EN 1465:2009. For this purpose, five pieces of yarn are laid superimposed above one another and placed alternating against one another at 0° orientation in a receiving mold such that they lie on top of one another with one end thereof in the middle of the mold with an overlap length of 2 cm. The adhesive surface A results from the overlap length and the width of the yarns used. The stack of yarn pieces is treated for 5 minutes in an oven at an oven temperature lying 20° C. above the melting temperature of the second resin composition, whereby the stack is loaded in the middle region thereof with a weight having a mass of 2 kg. By this means, the second resin composition is activated, i.e. it starts to melt. After cooling, the test body thus produced is subjected to a shear-tensile test, in which the ends of the test body are pulled apart at a test speed of 10 mm/min. The shear-tensile strength characterizing the adhesive strength of the yarns is determined from the resulting maximum force F_max[N] and the adhesive surface area A [mm²] according to the formula
$Shear - tensile strength = \frac{F_{\max} [N]}{A [{mm}^{2}]}$
The concentration of the resin composition, in relation to the total weight of the yarn and resin composition, is determined via extraction by means of sulfuric acid/hydrogen peroxide according to EN ISO 10548, Method B.

EXAMPLE 1

A carbon fiber filament yarn with a linear density of 800 tex and 12,000 filaments was fed dry at a speed of approximately 100 m/h at a thread tension of 1800 cN through a bath containing an aqueous dispersion of a first resin composition. The bath was conditioned to a temperature of 20° C. The aqueous dispersion contained a first epoxy resin H1 and a second epoxy resin H2 as the first resin composition in a concentration of 1.6 wt. %, wherein the weight ratio of resins H1 and H2 was 1.2. The first epoxy resin H1 had an epoxy value of approximately 2000 mmol/kg and an average molecular weight M_Nof 900 g/mol, and was solid at ambient temperature; the second epoxy resin H2 had an epoxy value of approximately 5400 mmol/kg and an average molecular weight M_Nof <700 g/mol, and was liquid at ambient temperature. The aqueous dispersion further contained a linear aromatic polyhydroxy ether P1 in a concentration of 14.4 wt. % with an acid value of 50 mg KOH/g, and an average molecular weight M_Nof 4600 g/mol, which polyhydroxy ether was solid at ambient temperature.
After traversing the bath containing the aqueous dispersion of the first resin composition, the yarn infiltrated with the first resin composition was dried at a temperature of 150° C. After drying the carbon fiber filament yarn had the first resin composition, comprising the components H1, H2, and P1, in a concentration in the range of 0.6 to 0.8 wt. % in relation to the yarn impregnated with the first resin composition, and showed a good compactness of the yarn, i.e. the filaments of the carbon fiber filament yarn were at least partially connected to each other via the first resin composition.
Directly subsequent to the drying, the yarn impregnated with the first resin composition was fed through a second bath containing a second aqueous dispersion. The second aqueous dispersion likewise had the first resin composition, but in a concentration of 0.5 wt. %. Further, the dispersion contained a second resin composition in a concentration of 6.75 wt. %, which comprised an epoxy resin H3 according to this example. The epoxy resin H3 had an epoxy value of 500 to 645 mmol/kg and an average molecular weight M_Nof 2900 g/mol, and was solid at ambient temperature. The adhesive strength or adhesive force of the second resin composition was determined to be 10 N. The epoxy resin H3 was present in the dispersion in the form of a powder having an average particle size D50 of 70 μm and a D90 of 125 μm.
After leaving the second bath, the yarn now loaded with the first and the second resin compositions was dried in that it was fed through two horizontal driers arranged in series and was dried there at a temperature of 200° C. and 220° C., respectively. The resulting pre-impregnated yarn had the first and the second resin compositions in a total concentration of 4.8 wt. % in relation to the total weight of the pre-impregnated yarn. The finished pre-impregnated yarn showed island- or drop-shaped adhesions of the second resin composition on the outer side while the yarn interior was free of the second resin composition. The pre-impregnated yarn had a stable form with a ratio of yarn width to yarn thickness of 38. The adhesive strength of the impregnated yarn was good. A force of 553 N was required to separate the unidirectionally adhered yarns, which resulted in a shear-tensile strength of 4.03 N/mm2.

EXAMPLE 2

This proceeded as in Example 1. Unlike Example 1, the concentration of the first and second epoxy resins H1 and H2 was 1.65 wt. % and the concentration of the linear aromatic polyhydroxy ether P1 was 14.85 wt. %.
The epoxy resin H3 from Example 1 was likewise used as the second resin composition. Unlike Example 1, the carbon fiber filament yarn loaded with the first resin composition was fed, still wet, after exiting the bath containing the dispersion of the first resin composition, without drying, through a conventional powder coating chamber, in which the second resin composition was applied to the yarn infiltrated with the first resin composition via powder coating. By this means, the concentration of the second resin composition on the outer surface of the yarn was controlled via conventional measures such as volume flow of the particles of the second resin composition and exhaust airflow.
After exiting the powder coating chamber, the carbon fiber filament yarn provided with the first and second resin compositions was dried at a temperature of 120° C. After drying, the pre-impregnated yarn obtained had a concentration of H1, H2, and P1 (first resin composition) of 0.7 to 0.9 wt. % and of H3 (second resin composition) of 2.4 to 2.6 wt. %, in each case in relation to the total weight of the pre-impregnated yarn. The finished pre-impregnated yarn showed island- or drop-shaped adhesions of the second resin composition on the outer side while the yarn interior was free of the second resin composition. The pre-impregnated yarn had a stable form with a ratio of yarn width to yarn thickness of 48. A force of 429 N was required to separate yarns adhering to each other, resulting in a shear-tensile strength of 3.68 N/mm². The yarns of this example thus had a good adhesive strength.

EXAMPLE 3

This proceeded as in Example 2. Unlike Example 2, a mixture of the epoxy resin H3 and a copolyamide was applied as the second resin composition via powder coating to the yarn infiltrated with the first resin composition. The epoxy resin H3 had an epoxy value of 500 to 645 mmol/kg and an average molecular weight M_Nof 2900 g/mol, and was solid at ambient temperature. The copolyamide was an aliphatic copolymer based on caprolactam and laurolactam. The copolyamide had a particle size distribution with an average particle size D50 of 50 μm, a D90 of 100 μm, and a molecular weight of 10,000 g/mol. It was solid at ambient temperature and had a melting point of approximately 135° C. Both components of the second resin composition together were present at a mixture ratio of 1:1 in an average particle size D50 of 45 μm and a D90 of 125 μm and were applied in this composition in the powder coating chamber to the yarn infiltrated with the first resin composition. The adhesive strength or adhesive force of the second resin composition was determined to be 16 N.
After exiting the powder coating chamber, the carbon fiber filament yarn loaded with the first and second resin compositions was dried at a temperature of 140° C. After drying, the pre-impregnated yarn obtained had a concentration of H1, H2, and P1 (first resin composition) of 0.7 to 0.9 wt. % and of H3 and the copolyamide (second resin composition) of 4.3 to 4.5 wt. %, in each case in relation to the total weight of the pre-impregnated yarn. The finished pre-impregnated yarn showed island- or drop-shaped adhesions of the second resin composition on the outer side while the yarn interior was free of the second resin composition. The pre-impregnated yarn had a stable form with a ratio of yarn width to yarn thickness of 27. The adhesive strength was determined to be 678 N and the shear-tensile strength was 4.44 N/mm2.

COMPARATIVE EXAMPLE 1

A carbon fiber filament yarn with a yarn linear density of 400 tex and 6000 filaments was fed dry at a speed of approximately 240 m/h at a thread tension of 340 cN through a bath with an aqueous dispersion, conditioned to a temperature of 20° C., of a resin composition comprising two bisphenol A epichlorohydrin resins H1* and H2*. The aqueous dispersion contained the first epoxy resin H1* in a concentration of 8.4 wt. % and the second epoxy resin H2* in a concentration of 6.9 wt. %; the weight ratio of resins H1* and H2* was 1.2. The first epoxy resin H1* had an epoxy value of approximately 2000 mmol/kg and an average molecular weight M_Nof 900 g/mol, and was solid at ambient temperature; the second epoxy resin H2* had an epoxy value of approximately 5400 mmol/kg and an average molecular weight M_Nof <700 g/mol, and is liquid at ambient temperature.
After traversing the bath containing the aqueous dispersion of the resin composition made of H1* and H2* (residence time=12 s), the yarn infiltrated with the epoxy resins H1* and H2* was dried at a temperature decreasing from 250° C. to 140° C. After drying, the carbon fiber filament yarn had the resin composition, comprising the components H1* and H2*, in a concentration in the range of 1.2 to 1.4 wt. % in relation to the yarn impregnated with the first resin composition.
Directly subsequent to the drying, the yarn impregnated with the resin composition comprising H1* and H2* was fed through a second bath containing an aqueous dispersion of a third bisphenol A epichlorohydrin resin H3*. The epoxy resin H3* had an epoxy value of 515 mmol/kg and an average molecular weight M_Nof 2870 g/mol, and a melting point of 120-130° C. The second aqueous dispersion has the epoxy resin H3* in a concentration of 3.8 wt. %, wherein the epoxy resin H3* in the dispersion had a particle size in the range from 0.35 to 0.8 μm. The dispersion medium consisted of a mixture of 76 wt. % water and 24 wt. % 2-propoxy ethanol.
The residence time of the yarn in the second bath was 15 seconds. After leaving the second bath, the yarn now loaded with the resins H1*, H2*, and H3* was dried, in that it was initially dried in a vertically arranged drier at 300° C. and subsequently in a horizontally arranged drier at 330° C. This resulted in yarn impregnated with the resins H1*, H2*, and H3* with a total concentration of the resins of 3.6 wt. % in relation to the total weight of the pre-impregnated yarn. This showed thereby that the resin H3* was distributed equally across the entire yarn cross section. At the same time, the yarn of the comparative example had no particles or drops of the epoxy resin H3* adhering to the outer side thereof; instead, the epoxy resin H3* was also distributed uniformly across the surface in the form of a film.
The pre-impregnated yarn had a high rigidity and a comparatively round shape with a ratio of yarn width to yarn thickness of 3.75. The adhesive strength of the yarns of the comparison example was insufficient. A force of 99 N was required to separate yarns adhering to each other, which results in a shear-tensile strength of 2.32 N/mm².

EXAMPLE 4

The pre-impregnated yarn obtained according to Example 1 was wound on a metal plate, the two faces of which were each covered with a separating film, by means of a laboratory winding system at a thread speed of 23.1 mm/s and a thread tension of 400 cN in each case up to the edge of the metal plate. The faces of the metal plate had dimensions of 280×300 mm², and each had first and second winding axes, respectively, in the middle between the opposing edges.
Initially, a first winding layer with a fiber mass per unit area of 267 g/m²with a 90° orientation to the first winding axis was generated on the two sides of the metal plate. Afterwards, the metal plate was rotated by 90° such that the already present winding layer was oriented parallel to the second winding axis. In the next step, under identical winding conditions, a further winding layer with a 90° orientation to the first winding layer was applied to the already present winding layer. In this way, a layered structure with a 0° thread layer and a 90° thread layer resulted on each of the two sides of the metal plate. The previously described winding process was repeated until in each case four winding layers lay on top of each other on the two faces of the metal plate, which layers had alternating 0° and 90° thread orientations.
Subsequently, the winding layers on the two faces of the metal plate were each covered with a separating film. The metal plate was thereafter conditioned, complete with both respective four-layer winding structures and the separating films, in a press for 5 min at a surface pressure of 2 bar and a temperature of 125° C. The resulting pressing was cooled to below the melting point of the second resin composition (epoxy resin H3). Afterwards, the two winding packets were cut apart at the end faces of the metal plate and the four separating films were removed. In this way, two preforms resulted with a respective four-layer structure alternating between 0° and 90°, i.e. with a bidirectional arrangement of the yarns. The nominal thickness per thread layer was 0.25 mm.
The preforms were very dimensionally stable due to the high adhesive strength of the pre-impregnated yarns used and could be handled problem-free for further processing. In addition, after inserting the preforms in a mold, a good impregnation capability of the preforms was determined during the injection of the matrix resin.

COMPARATIVE EXAMPLE 2

This proceeded as in Example 4. However, the yarns obtained according to Comparative example 1 were used as the pre-impregnated yarns.
The preforms of the comparative example had low dimensional stability due to the low adhesive strength of the pre-impregnated yarns used. The handling during further processing was revealed as problematic due to the instability. In addition, during injection of matrix resin, a worsened impregnation capability of these preforms was determined.

EXAMPLE 5

A square piece having an edge length of 200 mm was cut from a preform analogous to that produced in Example 4 and inserted into a mold having equal edge lengths and a height of 2 mm, which square piece however had an 8-layer structure with fiber layers only in the 0° orientation. An epoxy resin (type RTM6, Hexcel), previously heated to 80° C., was injected into the mold so that a composite with a fiber volume proportion of 60 vol. % results. The preform now impregnated with resin was cured at 180° C. A composite laminate resulted having an eight-layer structure with an orientation of the fibers in the 0° direction.
Test bodies were taken from the composite laminate to determine the inter-laminar shear strength (ILSS) according to DIN EN 2563 and the compressive strength and the compressive modulus according to DIN EN 2850. It was shown that the mechanical characteristics of the composite laminate produced with the pre-impregnated yarns of the present invention were at the same level as corresponding characteristics of a laminate based on standard carbon fiber yarns (Tenax HTS40 F 13 12 K 800 tex, Toho Tenax Europe GmbH), even though the concentration and the composition of the resin application of the yarns according to the invention differed significantly from the concentration and composition of the standard carbon fiber yarns.

Claims

1. A pre-impregnated yarn comprising:

a bundle of reinforcing fiber filaments with a bundle interior and a bundle outer side,

a first resin composition comprising at least two bisphenol A epichlorohydrin resins H1 and H2 in a weight ratio H1:H2 of 1.1 to 1.4, and

a second resin composition on the bundle outer side in the form of particles or drops adhering to the reinforcing fiber filaments,

wherein:

the reinforcing fiber filaments are impregnated with the first resin composition infiltrated into the pre-impregnated yarn and the filaments of the pre-impregnated yarn are at least partially connected via the first resin composition,

H1 has an epoxy value of 1850 to 2400 mmol/kg and an average molecular weight M_Nof 800 to 1000 g/mol and is solid at ambient temperatures,

H2 has an epoxy value of 5000 to 5600 mmol/kg and an average molecular weight M_Nof <700 g/mol and is liquid at ambient temperatures,

the pre-impregnated yarn has 0.1 to 2 wt. % of the first resin composition in relation to the total weight of the yarn,

the first resin composition further contains-comprises an aromatic polyhydroxy ether P1, which has an acid value of 40 to 55 mg KOH/g and an average molecular weight M_Nof 4000 to 5000 g/mol,

the second resin composition is solid at ambient temperatures, has a melting temperature in the range from 80 to 150° C. and is present on the bundle outer side in a concentration of 0.5 to 10 wt. % in relation to the total weight of the pre-impregnated yarn,

at least 50% of the surface of the bundle outer side is free of the second resin composition, and

the bundle interior is free of the second resin composition.

2. The pre-impregnated yarn according to claim 1, wherein the first resin composition comprises the bisphenol A epichlorohydrin resins H1 and H2 and the aromatic polyhydroxy ether P1 in a weight ratio (H1+H2):P1 of 0.05 to 0.8.

3. The pre-impregnated yarn according to claim 1, wherein the first resin composition is present in a concentration of 0.4 to 1.2 wt. % in relation to the total weight of the pre-impregnated yarn.

4. The pre-impregnated yarn according to claim 1, wherein the second resin composition comprises at least 50 wt. % of a compound selected from the group consisting of a bisphenol A epichlorohydrin resin H3 with an epoxy value of 480 to 645 mmol/kg and an average molecular weight M_Nof 2700 to 4000 g/mol, an aromatic polyhydroxy ether P2, a polyamide, a polyethylene, an ethylene copolymer, a thermoplastic polyurethane resin, and mixtures thereof.

5. The pre-impregnated yarn according to claim 1, wherein the second resin composition has an adhesive strength of at least 5 N at a temperature of 20° C. above the melting temperature of the second resin composition, in relation to an adhesive surface with a diameter of 25 mm.

6. The pre-impregnated yarn according to claim 1, wherein the particles or drops of the second resin composition adhering to the reinforcing fiber filaments have a size less than 300 μm.

7. The pre-impregnated yarn according to claim 6, wherein the particles or drops of the second resin composition adhering to the reinforcing fiber filaments have an average size in the range from 20 to 150 μm.

8. The pre-impregnated yarn according to claim 1, wherein the concentration of the second resin composition is greater than that of the first resin composition.

9. The pre-impregnated yarn according to claim 1, wherein the total concentration of the first resin composition and second resin composition is from 2 to 7 wt. % in relation to the total weight of the pre-impregnated yarn.

10. The pre-impregnated yarn according to claim 1, wherein the first resin composition and/or the second resin composition is free of curing agents.

11. The pre-impregnated yarn according to claim 1, wherein the yarn comprises synthetic or natural fibers selected from the group consisting of carbon fibers that are obtained from pitch, polyacrylonitrile, lignin, or viscose pre-products, aramid fibers, glass fibers, ceramic fibers, boron fibers, and a combination thereof.

12. The pre-impregnated yarn according to one or more of claim 1, wherein the pre-impregnated yarn is present as a flat band that has a ratio of yarn width to yarn thickness of at least 20.

13. The pre-impregnated yarn according to claim 12, wherein the flat band has a ratio of yarn width to yarn thickness in the range from 25 to 60.

14. A textile structure comprising the pre-impregnated yarns according to claim 1.

15. The textile structure according to claim 14, wherein the pre-impregnated yarns are connected to each other at points of mutual contact at least via the second resin composition.