Increasing filament count of carbon fiber tows

Abstract

The filament count of carbon fiber tows is increased by spreading and merging the carbon filaments of a first tow with the carbon filaments of a second tow. The carbon filaments of the first and second tows are spread apart using a gas stream. The spread tows are aligned with each other and contact each other as they pass over the grid in the presence of the same or different gas stream, thereby forming entangled tows. The entangled tows are then formed into a combined tow. The modulus and denier of the carbon filaments in the first and second tows can be the same, or they can be altered to provide a combined tow having a wide range of filament counts and filament combinations.

Description

TECHNICAL FIELD

[0001] The present invention relates generally to carbon fiber tows comprised of carbon filaments. More specifically, the present invention relates to a method of increasing the filament count of any given carbon fiber tow by physically introducing one or more other carbon fiber tows directly into the tow. BACKGROUND

[0002] Composite materials include a fiber reinforcement structure and a resin matrix as two primary components. Composite materials generally have a relatively high strength-to-weight ratio. As a result, composite materials are used in the aerospace industry where the high strength and relatively light weight of composite structures are of particular importance.

[0003] Carbon fibers are a commonly used fiber reinforcement material for composite materials. Carbon fibers are typically provided as a multifilament yarn, which is commonly referred to as a "tow." Carbon fiber tows typically contain between 1,000 and 50,000 individual filaments. Commercially available carbon fiber tows contain, for example, about 3,000 filaments (3K), 6,000 filaments (6K), 12,000 (12K) filaments, or 24,000 (24K) filaments. The linear weight of an individual carbon filament is typically in the range of 0.02 to 0.5 milligrams per meter.

[0004] The linear weight of a carbon fiber tow depends on the number of filaments, their linear weight, and the weight of any coatings or other tow treatments. Hex AS4 is a type of carbon fiber tow available from Hexcel Corporation (Dublin, CA). The 3K version of Hex The linear weight of an AS4 carbon fiber tow is approximately 0.21 grams per meter, while the linear weight of the 6K and 12K versions is approximately 0.43 grams per meter and 0.86 grams per meter, respectively. Hex IM7 is another type of carbon fiber tow, which is also available from Hexcel Corporation (Dublin, CA). Hex IM7 carbon fiber tows have a higher linear weight than Hex AS4 carbon fiber tows have a higher tensile strength and tensile modulus. The 6K Hex The linear weight of an IM7 carbon fiber tow is approximately 0.22 grams per meter, while the linear weight of the 12K version is approximately 0.45 grams per meter.

[0005] The filaments in a carbon fiber tow are not twisted in the same manner as traditional yarns. Rather, the filaments in a carbon fiber tow are substantially parallel to one another. As a result, carbon fiber tows tend to have a flat, ribbon-like cross-sectional shape. The carbon filaments are entangled with one another to provide the tow with some cohesiveness, so that it can be handled and processed to some extent without falling apart. The cohesiveness of a carbon fiber tow is typically measured by determining the "spreading" of the tow. Tow evaluation testers are commercially available that are used to determine the properties of many carbon fiber tows, including spreading. For example, a tow evaluation tester is commercially available from Textechno H. Stein GmbH & Co. KG (Moenchengladbach, Germany) under the trade designation ROVING TEST. Tow evaluation testers or devices are common in the textile industry.

[0006] In a typical tow evaluation test, spreading is determined by placing a tow under a predetermined tension and passing it over a series of spreading bars. The width of the tow is measured before the spreading bars (W1), and immediately after the spreading bars (W2). The spreading of the tow is equal to W2 / W1. Typical spreading values (W2 / W1) for carbon fiber tows are in the range of 1.2 to 2.2, depending on the filament count, the linear weight of the filaments, the type of filaments, the degree of entanglement of the filaments, and post-formation handling, such as the application of a size or other coating.

[0007] Carbon filaments are relatively fragile. As a result, carbon filaments are susceptible to damage during the production of carbon fiber tows and subsequent handling. Tow fuzz or fuzziness refers to broken filaments and fluff that can occur on the surface of a carbon fiber tow. Test equipment is commercially available for measuring the amount of fuzz present on the surface of a carbon fiber tow. The amount of fuzz is expressed as a fuzz count or fuzz value. An exemplary test equipment for measuring the fuzz count of a carbon fiber tow is the FRAY VIEW yarn defect visualization equipment available from Lenzing Instruments GmbH & Co. KG (Gumpen, Austria). The FRAY VIEW equipment measures the fuzz count using an optical sensor and a high resolution digital camera.

[0008] The fuzz count of a carbon fiber tow will vary depending on a number of factors, including the manner in which the tow is processed and handled, the filament count, the linear weight of the filaments, the type of filaments, and post-formation handling, such as the application of a size or other coating. In general, it is desirable for a carbon fiber tow to have as low a fuzz count as possible. However, in certain instances, a certain level of filament breakage is desired. For example, broken filaments in a carbon fiber tow have been used to increase the Z-directional electrical conductivity of carbon fiber composite laminates. In these instances, the fuzz count is measured and monitored to ensure that the carbon fiber tow has the desired level of filament breakage. SUMMARY

[0009] According to the present invention, the filament count of a first carbon fiber tow is increased by physically combining a second carbon fiber tow with the first carbon fiber tow to form a combined tow. It has been discovered that two carbon fiber tows can be physically combined to provide a cohesive combined tow that exhibits a spreadability that is close to that of the first and second tows. It has further been discovered that such a combined tow can be formed without unduly increasing the number of damaged filaments present in either the first or second tow.

[0010] The present invention is based on a method for increasing the filament count of a carbon fiber tow to form a combined tow, wherein a first tow is comprised of first carbon filaments and has a first tow width, the first tow is passed into a spreading / laying zone, at the spreading / laying zone, the first tow is subjected to a first gas stream to form a spread first tow. A second tow is comprised of second carbon filaments and has a second tow width, the second tow is also passed into the spreading / laying zone, at the spreading / laying zone, the second tow is subjected to the first gas stream to form a spread second tow.

[0011] As a feature of the present invention, the spread first tow and the spread second tow are brought into contact with one another to form a contacted spread tow, the contacted spread tow is passed over a grid such that the first and second tows contact one another and the grid. A second gas stream, which can be part of the first gas stream, is passed through the grid to provide a layup of the first and second carbon filaments to form a laid-up tow having a laid-up tow width. The laid-up tow is then formed into a combined tow.

[0012] As another feature of the present invention, the first and second tows include a spreading / laying up enhancer that is applied to the tows prior to spreading in the spreading / laying zone. It has been discovered that the use of a spreading / laying up enhancer enables the level of filament spreading, layup and entanglement required to form a suitable combined tow to be achieved without generating an undue number of broken filaments or fuzz.

[0013] The present invention can be used to form a combined tow wherein the first and second tows have an equal number of the same type of carbon filaments. As an additional feature, combined tows having a variety of properties, such as tensile modulus, tensile strength and linear weight, can be prepared by using first and second tows having different filament count numbers and / or types of carbon filaments.

[0014] The present invention is directed not only to a method for increasing the filament count of a carbon fiber tow, but also to combined tows prepared according to the present invention, including unidirectional carbon fiber tapes and woven carbon fiber products. The present invention is also directed to uncured composite materials, such as prepregs, prepared using the combined tows. The present invention is also directed to composite parts prepared from the uncured composite materials.

[0015] The above described and many other features and attendant advantages of the present application will become better understood from the following detailed description when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a partial schematic side view of an exemplary method for increasing the filament count of carbon fiber tows according to the present application.

[0017] Figure 2 is Figure 1 is a partial schematic top view of the exemplary method shown.

[0018] Figure 3 is a side view of a portion of an exemplary grid showing downward displacement of carbon filaments caused by gas flow through the grid during entangled tow formation.

[0019] Figures 4A-4E are cross-sectional views showing the starting tow and different tow configurations at various stages during the method.

[0020] Figure 5 is a detailed partial cross-sectional view of a preferred exemplary consolidation reel used in forming entangled tows into combined tows.

[0021] Figure 6 is a representative cross-sectional view of a portion of an exemplary hybrid combined tow comprising a first tow having carbon fiber filaments that are different than the carbon fiber filaments in a second tow. DETAILED DESCRIPTION

[0022] In Figure 1 and 2 A preferred exemplary system for performing the method of increasing the filament count of carbon fiber tows according to the present application is shown at 10 in FIG. 1. First tow 12 is fed into spreading / laying-up zone 14 (drawn in dashed box) by feed roll 16. Spreading / laying-up zone 14 can be an enclosed space, such as a chamber, or can be an open space in which spreading and laying-up of the tows is performed. Any type of carbon fiber tow composed of 2,000 to 50,000 carbon filaments can be used as first tow 12. Preferred carbon fiber tows are commercially available and have a filament count of 3K (about 3,000 filaments), 6K (about 6,000 filaments), or 12K (12,000 carbon filaments).

[0023] First tow 12 can have a variety of tensile strengths and tensile moduli. For example, carbon fiber tows used as first tow 12 preferably have a tensile strength of 750 to 900 ksi, a tensile modulus of 35 to 45 Msi, a strain at failure of 1.5 to 2.5%, a density of 1.6 to 2.0 g / cm3, and a diameter of 0.5 to 1.5 mm.3 density of 1.78 g / cm3and a linear weight of 0.446 g / m (4,014 denier). Hex 3 IM7 carbon fiber tow is generally considered to be a medium modulus carbon fiber tow. Any medium modulus carbon fiber tow having similar properties to the Hex IM7 carbon fiber tow is preferred for use as the first tow 12.

[0024] A preferred carbon fiber tow for use as the first tow 12 is Hex 12K IM7 carbon fiber tow having a tensile strength of 810 ksi, a tensile modulus of 40 Msi, a failure strain of 1.9%, a density of 1.78 g / cm 3 , and a linear weight of 0.446 g / m (4,014 denier). Hex IM7 carbon fiber tow and any other carbon fiber tow having similar properties. The Hex IM7 carbon fiber tow is generally considered to be a medium modulus carbon fiber tow. Any medium modulus carbon fiber tow having similar properties to the Hex

[0025] IM7 carbon fiber tow is preferred for use as the first tow 12. A preferred carbon fiber tow for use as the first tow 12 is Hex 12K AS4 carbon fiber tow having a tensile strength of 640 ksi, a tensile modulus of 33.5 Msi, a failure strain of 1.8%, a density of 1.79 g / cm 3 , and a linear weight of 0.858 g / m (7,722 denier). Any lower modulus carbon fiber tow having similar properties to the Hex AS4 carbon fiber tow is also preferred for use as the first tow 12.

[0026] Figure 4 shows simplified cross-sectional views of the tows at locations (A)-(E) within the system 10, as indicated by Figure 1 and Figure 2 The simplified cross-sectional view of the first tow 12 at location (A) is shown in Figure 4. At location (A) prior to any spreading or commingling, the first tow 12 has a width T1 of 1-4 mm, preferably 1.5-2.5 mm. The feed roll 16 includes a groove 18 that is configured to maintain the desired T1 and provide alignment of the first tow 12 with the second tow 20.

[0027] The second tow 20 can be the same carbon fiber tow as the first tow 12 or can be a different tow having a different filament count and having filaments of different properties, so long as the carbon fiber tow used for the second tow 20 has properties falling within the properties outlined above as applicable to the first tow 12. First and second fiber tows are considered the same for the purposes of this specification if the filament count, tensile strength, tensile modulus, and linear weight of the first tow are within 10% (preferably within 5%) of the filament count, tensile strength, tensile modulus, and linear weight, respectively, of the second tow. First and second fiber tows are considered different for the purposes of this specification if the filament count, tensile strength, tensile modulus, or linear weight of the first tow differs by more than 10% (preferably more than 5%) from the filament count, tensile strength, tensile modulus, or linear weight, respectively, of the second tow.

[0028] Exemplary combinations of different carbon fiber tows are: 1) the tensile modulus of the first tow is the same as the tensile modulus of the second tow and the filament count of the first tow is different from the filament count of the second tow; 2) the tensile modulus of the first tow is different from the tensile modulus of the second tow and the filament count of the first tow is the same as the filament count of the second tow, and 3) the tensile modulus of the first tow is different from the tensile modulus of the second tow and the filament count of the first tow is different from the filament count of the second tow.

[0029] Preferred exemplary combinations of first and second tows (first tow / / second tow) include the following: 1) Hex 12K IM7 carbon fiber tow / / Hex 12K IM7 carbon fiber tow; 2) Hex 6K IM7 carbon fiber tow / / Hex 12K IM7 carbon fiber tow; 3) Hex 12K AS4 carbon fiber tow / / Hex 12K AS4 carbon fiber tow; 4) Hex 6K AS4 carbon fiber tow / / Hex 12K AS4 carbon fiber tow; 5) Hex 12K IM7 carbon fiber tow / / Hex 12K AS4 carbon fiber tow; 6) Hex 12K AS4 carbon fiber tow / / Hex 12K AS4 carbon fiber tow; 7) Hex 6K AS4 carbon fiber tow / / Hex 12K IM7 carbon fiber tow; and 8) Hex 6K IM7 carbon fiber tow / / Hex 12K AS4 carbon fiber tows.

[0030] For exemplary purposes, the description herein is directed to Hex 12K IM7 carbon fiber tows and Hex 12K AS4 carbon fiber tows, and sets forth the preferred embodiments of the present application. The present application can also be used to increase the filament count of many other types of commercially available carbon fiber tows, such as increasing the filament count of Hex AS2C, Hex AS4C, Hex AS7, Hex IM8, Hex IM9, Hex IM5, and carbon fiber tows having similar properties available from other commercial sources.

[0031] A simplified cross-sectional view of the second tow 20 at position (A) is also shown in FIG. 4. At position (A), prior to any spreading or intermingling, the width T2 of the second tow 20 is 1-4 mm, preferably 1.5-2.5 mm. The feed roll 22 includes a groove 24 that is configured to maintain the desired T2 and provide alignment of the second tow 20 over the first tow 12.

[0032] The first and second tows 12 and 20 are preferably provided to their respective alignment spools 16 and 22 from standard spools (not shown) of carbon fiber tow. If desired, the system 10 can be incorporated into an existing carbon fiber production line, where the carbon fiber tows can be fed directly to the alignment spools 16 and 22 without first being wound onto storage spools.

[0033] The spreading / intermingling zone 14 includes an inlet 26 and an outlet 28, a left side 30, a right side 32, a top 34, and a bottom 36. The first tow 12 includes a plurality of first carbon filaments extending in a first tow longitudinal direction, as indicated by arrow 38. The first tow 12 has a first tow filament count that is preferably 3K to 24K. The second tow 20 includes a plurality of first carbon filaments extending in a second tow longitudinal direction, as indicated by arrow 40. The second tow 20 has a second tow filament count that is also preferably 3K to 24K.

[0034] The speed at which the first and second tows are fed through the inlet 26 into the spreading / intermingling zone 14 can be as low as 2 meters / minute and as high as 10 meters / minute. Below 2 meters / minute, the tow feed speed results in filament breakage and damage, and the cohesiveness of the combined tow does not increase. Above 10 meters / minute, the tow feed speed does not allow sufficient intermingling and entanglement to provide a combined tow having the desired low spreadability. For 6K-12K carbon fiber tows, the preferred tow feed speed is 5-8 meters / minute.

[0035] At point (B) within the spreading / laying-up zone 14, the first gas stream 42 traverses the first and second tows 12, 20 laterally as indicated by arrow 42 to provide lateral spreading of the tows. The first gas stream is preferably a stream of air. Other gases can be used provided that they do not adversely affect the tows. For example, in cases where an inert environment is required, a gas stream consisting essentially of nitrogen can be used.

[0036] The rate of the first gas stream 42 and the longitudinal directional tension applied to the first and second tows 12, 22 at point (B) are selected to provide gas-induced spreading or aerodynamic spreading of the tows. The downward rate of the first gas stream 42 should be 10 to 25 meters per second, preferably 15 to 20 meters per second. The longitudinal tension on the first tow 12 should be 50 to 150 grams. The longitudinal tension on the second tow 20 should also be 50 to 150 grams. For 6K and 12K carbon fiber tows, the preferred tension at point (B) is 90 to 110 grams.

[0037] At point (B), as the two tows aerodynamically spread to form a spread first tow 44 and a spread second tow 46, the second tow 20 remains aligned directly above the first tow 12. The spread first tow 44 has a spread first tow width (ST1) and the spread second tow has a spread second tow width (ST2). During the spreading operation, the tows do not contact each other. It has been found that contact between the tows prior to completion of the spreading step has a negative effect on the subsequent laying-up of the tows.

[0038] The tow feed speed, the first gas stream rate, and the tow tension are selected to provide the level of spreading necessary to form a coherent combined tow of the first and second tows. The first tow 12 should be spread such that the ratio of ST1 to T1 is 5:1 to 25:1. For 6K and 12K carbon fiber tows, the ratio of ST1 to T1 is preferably 8:1 to 12:1. The second tow 20 should be spread such that the ratio of ST2 to T2 is 15:1 to 40:1. The ratio of ST2 to T2 is preferably 20:1 to 30:1.

[0039] When the first and second tows are identical carbon fiber tows, the second tow 20 tends to spread more than the first tow 12 because the second tow is located directly above the first tow 12 in the first gas stream 42. It has been found that initially wider tows tend to spread more in the first gas stream than narrower tows. It has also been found that tows having larger diameter filaments tend to spread more in the first gas stream.

[0040] The orientation, width, and filament dimensions of the filaments in the first gas flow are selected such that the ratio between ST2 / T2 and ST1 / T1 is 1.5:1 to 5:1. The ratio between ST2 / T2 and ST1 / T1 is preferably 2:1 to 3:1. When both the first and second filaments are 6K or 12K carbon fiber filaments, the preferred ratio between ST2 / T2 and ST1 / T1 is 2.5:1. The tension applied to the filaments affects the gas-induced spreading. Preferably, the tension applied to the first filament 12 is equal to the tension applied to the second filament 20 (±10%). However, the tensions applied to the first and second filaments can be varied together or independently to achieve the desired levels of ST1 / T1 and ST2 / T2, which are necessary to provide the desired ST1 / T1 to ST2 / T2 ratio.

[0041] The first and second bundles of filaments 44 and 46 are laid out into contact with each other and at point (C) into grid 50 to form a contacted bundle of filaments 48. The contacted bundle of filaments 48 has a width (CST). As the contacted bundle of filaments 48 moves through grid 50, it is exposed to a second gas flow, indicated by arrow 52, ​​which passes through grid 50. The second gas flow 52 preferably has the same source as the first gas flow 42, such that the second gas flow is a portion or part of the first gas flow. Therefore, the parameters described above for the first gas flow 42 also apply to the second gas flow 52. Preferably, the first and second gas flows are identical to provide a single gas flow for laying out and merging the first and second bundles at the same gas flow rate. However, there may be cases where it is desirable to use the first gas flow to lay out the first and second bundles and then use a different second gas flow to merge the bundles.

[0042] At point (C), the first and second spread filament bundles come into contact with each other and with the grid 50. A second gas flow 52, ​​combined with the movement of the filament bundles on the grid 50, results in the formation of an entangled filament bundle 56 having a width (ET). The width of the entangled filament bundle 56 as it crosses the grid 50 preferably remains relatively close to the width of the contacted spread filament bundle 48, such that the ratio between CST and ET is maintained between 0.9:1 and 1.1:1. The tension applied to the entangled filament bundle 56 on the grid 50 and the rate of the second gas flow 52 are selected to keep the CST / ET within a desired range.

[0043] A partial cross-sectional view of a preferred exemplary grid 50 is shown in Figure 3The grid 50 includes bars shaped as rods 54 spaced a distance (GS). The number of bars, cross-sectional shape, size, and spacing of the bars in the grid can be varied to achieve the desired level of entangling. The downward force of the gas stream applied to the entangled tow 56 results in a downward displacement "D" of the entangled tow between the bars. The tension on the entangled tow 56, the rate of the second gas stream 52, and the GS are selected to achieve a "D" that provides sufficient entangling without causing excessive breakage of the filaments. If the tension is too low, the tow will be pulled too far into the grid and be damaged. If the tension is too high, the desired level of spreading and entangling will not occur.

[0044] The grid 50 should have 10 to 20 grid rods 54 extending parallel to each other between the left edge 30 and the right edge 32 of the spreading / entangling zone 14. The diameter of the rods 54 should be 2 to 5 mm and the GS should be 3 mm to 9 mm. D should be 0.5 mm to 5 mm to achieve sufficient entangling of the filaments without increasing the pile count. A preferred grid for entangling 6K and 12K carbon fiber first and second tows will have 12 to 16 rods each having a diameter of 2 to 3 mm and a GS of 4 to 6 mm. When using this preferred grid to entangle 6K and / or 12K carbon fiber tows, the tow tension and the rate of the second gas stream 52 are selected so that D is 1 mm to 3 mm.

[0045] The grid bars 54 are shown as rods having a circular cross-section. Other types of grid bars are possible, such as those having a non-cylindrical arcuate cross-sectional shape or a square / rectangular cross-section. When using grid bars having a non-cylindrical cross-sectional shape, care must be taken to control the other process parameters to ensure that sufficient entangling is achieved without causing excessive filament breakage.

[0046] The entangled tow 56 is removed from the entangling / spreading zone 14 through the outlet 28. The entangled tow 56 is formed into the combined tow 58 by passing the entangled tow 56 over a consolidation spool 60 having a grooved circumference 62 designed to reduce the width of the entangled tow 56 to the width (CT) of the combined tow 58. The grooved circumference 62 has a concave shape designed to provide the desired reduction from ET to CT without adversely affecting the entangled filaments.

[0047] A detailed partial cross-sectional view of an exemplary consolidation spool 60 is shown in Figure 5 The consolidation spool 60 is designed for forming 6K to 24K entangled tows 56 into combined tows 58. The consolidation spool 60 has a radius (RR) of 30 to 60 mm, a width (RGW) of the groove 62 of 10 to 25 mm, and a depth (GD) of 4 to 10 mm.

[0048] The tension applied to the entangled tow 56 remains at a level that, in combination with the grooves 62, provides the desired reduction in tow width as the entangled tow 56 passes to the consolidation spool 60. For a combined tow comprising 6K to 24K carbon filaments, the width of the combined tow (CT) should be 2 to 5 mm. The tension on the entangled tow 56 between point (D) and the consolidation spool 60 should be 300 to 500 grams.

[0049] It has been discovered that the organic oil-based surfactant applied to the first and / or second tow prior to point (B) acts as a spreading / coalescence enhancer (SCE) to provide the desired level of spreading and coalescence of the tow without damaging the tow. When subjected to the same degree of spreading and coalescence, the pile count of the combined tow tends to be significantly higher when the SCE is not used.

[0050] The SCE can be any organic oil-based surfactant commonly used as a softener in the textile industry. Exemplary SCEs include alkoxylated castor oil triglycerides, such as ethoxylated castor oil triglycerides. A preferred exemplary SCE is a polyoxyethylene castor oil solution available from Henkel Corporation (Dusseldorf, Germany) under the trade name TROLOX. For example, the spreading and coalescence of 12K IM7 first tow and 12K IM7 second tow was improved in quality when the tow was treated by first passing through a bath of TRILOX 5918 solution (1% solution) and then dried at 135°C.

[0051] The SCE can be applied to the first tow and / or second tow at any time prior to point (C). For example, the tow can be passed through a bath of SCE and dried on-line between point (A) and point (B). Alternatively, the SCE can be applied to the tow at any time prior to the tow passing over the alignment spools 16 and 22.

[0052] Any sizing agent conventionally applied to carbon fiber tow can also be applied to the combined tow 58. Such conventional sizing agents include vinyl ester sizing agents, epoxy-based sizing agents, phenolic sizing agents, polyurethane sizing agents, and the like. When sizing is desired, it should be applied only after the first and second tow have been spread and coalesced. It has been discovered that applying a conventional sizing agent of the type described above to the tow at any point in the system prior to point (D) has an adverse effect on the spreading and coalescence of the tow. Preferably, the sizing, if any, is applied at a point after the entangled tow 56 exits the spreading / coalescing zone 14.

[0053] The combined tow 58, whether sized or not, is wound onto a tape reel 64 for storage and further use. The combined tow 58 can be used in the same manner as any other carbon fiber tow to make a variety of composites. For example, the combined tow 58 can be formed into unidirectional tapes or woven fabrics and combined with uncured thermoplastic or thermoset resin matrices to form a variety of prepregs and molding compounds that can be cured / molded into composite parts.

[0054] A preferred practice implementation of increasing the filament count of a carbon fiber tow using the same type of carbon filaments is as follows: The combined tow 58 is prepared using the system 10, where the first tow 12 is a Hex 12K IM7 carbon fiber tow and the second tow 20 is a Hex 12K IM7 carbon fiber tow. Both T1 and T2 are 2 mm and the linear velocity through the inlet 26 is 3 meters / minute, with a tension of 100 grams on each of the first and second tows. Both the first and second tows are passed through a 1% solution of TRYLOX 5918 at a linear velocity and dried at 135°C, then spread and laid down in the spreading / laying down zone 14. The first and second air streams 42 and 52 are at the same rate of 20 meters / second. The ratio of ST1 to T1 is 10:1 and the ratio of ST2 to T2 is 25:1. The grid 50 contains 14 grid bars 54. Each grid bar 54 is 2.3 mm in diameter and the GS is 4.7 mm. The tension on the entangled tow 56 is 400 grams and the width of the combined tow 58 (CT) is 3 mm. It should be noted that the parameters of the foregoing method are approximations and each parameter can vary by ± 15% without adversely affecting the formation of the combined tow 58 with the desired spreading and loft count.

[0055] The spreading of the combined tow 58 (24K filaments) prepared as described above is measured using a tow evaluation apparatus similar to the ROVING TEST tow evaluation apparatus. The spreading of the combined tow 58 is 1.7 (± 10%). The spreading of a standard Hex 24K IM7 carbon fiber tow is also measured on the same tow evaluation apparatus. The spreading of a standard Hex 12K IM7 carbon fiber tow is 2.0 (± 10%).

[0056] The above examples show that the combined tows produced according to the present application have sufficient cohesion such that their spreadability is equal to or less than the spreadability of a standard commercially available tow having the same filament count. Preferably, the spreadability of the combined tow is equal to or less than the spreadability of a standard commercially available tow having the same filament count when measured using the same test equipment. However, the methods and systems of the present application can also generally be used to produce combined tows wherein the spreadability of the combined tow is within 25% of the spreadability of a similar commercially available tow having the same filament count when measured on the same test equipment.

[0057] Using a test equipment similar to the FRAY VIEW yarn defect visualization equipment, the spreadability and pile count of the combined tows (24K filaments) and standard Hex The pile count of the 12K IM7 carbon fiber tow was 24. The pile count of the combined tows was 24.5, which is less than 25% higher than the pile count of the standard Hex The pile count of the 12K IM7 carbon fiber tow was 24. The pile count of the combined tows was 24.5, which is less than 25% higher than the pile count of the standard Hex

[0058] Preferred practice embodiments for producing hybrid combined tows, wherein different carbon filaments are used to increase the filament count of the carbon fiber tow, are as follows: producing a combined tow 58 using system 10, wherein the first tow 12 is a Hex 12K IM7 carbon fiber tow, the second tow 20 is a Hex 12K AS4 carbon fiber tow. T1 is 2 mm, T2 is 2.5-3 mm. Otherwise, the system and method parameters are the same as described in the preferred embodiments above. The ratio of ST1 to T1 is 10:1, and the ratio of ST2 to T2 is 30:1. CT is 4 mm. These are also approximate values, as are the parameters mentioned above, and can vary by ±15%.

[0059] The hybrid combined tows and commercially available Hex 12K IM7 carbon fiber tow can be measured in the same manner as the 12K IM7 / / 12K IM7 combined tows and commercially available Hex The spreadability and pile count of the 12K AS4 carbon fiber tow. The spreadability of the hybrid combined tows will be 2.0 (±10%), and the pile count will be 24.5 (±10%). The spreadability of the 12K AS4 carbon fiber tow will be 2.0 (±10%). This example shows that a 12K IM7 / / 12K AS4 hybrid combined tow can be produced according to the present application that has a spreadability (cohesion) that is superior to the spreadability of a Hex 12K IM7 and Hex 12K AS4.

[0060] The hybrid combined tows and commercially available Hex 12K AS4 can also be measured as in the above-described examples. Compared to the commercially available Hex 12K IM7 or Hex The pile count of 12K AS4 is less than 25% higher.

[0061] Hex The filament diameter of 12K IM7 carbon fiber tows is less than that of Hex The filament diameter of 12K AS4 carbon fiber tows. Microscopic observation of the cross-section of the hybrid combination tow was made to determine the uniformity of the filament intermingling based on different filament diameters. It has been found that the hybrid combination fiber has a substantially uniform cross-sectional mixture of IM7 and AS4 filaments. This uniform intermingling of filaments is a particular advantage according to the present application when combining carbon fiber tows having different properties. A representative cross-sectional view of the hybrid combination tow is shown in Figure 6 wherein the IM7 filaments (5.2 microns in diameter) are shown as 12 and the AS4 filaments (7.1 microns in diameter) are shown as 20.

[0062] It should be noted that the present application is not limited to combining a first tow with a second tow, but can also be used to increase the filament count by aligning, spreading and intermingling 3 or more tows. When combining more than two tows, the various conditions and parameters described above with respect to combining a first tow with a second tow can be varied to achieve the desired level of intermingling of the multiple tows and the spreadability of the combined tow.

[0063] Having thus described exemplary embodiments of the application, those skilled in the art will note that the disclosure given herein is illustrative only and that various other alternatives, adaptations and modifications can be made within the scope of the present application. Accordingly, the present application is not limited to the embodiments described above, but only by the claims below.

Claims

1. A method for increasing the filament count of a carbon fiber tow by forming a combined tow, the method comprising the steps of: providing a first tow comprising a plurality of first carbon filaments extending in a first tow longitudinal direction, the first tow having a first tow filament count and a first tow width; providing a second tow comprising a plurality of second carbon filaments extending in a second tow longitudinal direction, the second tow having a second tow filament count and a second tow width; passing the first tow into a tow spreading / laying zone in the first tow longitudinal direction, the tow spreading / laying zone having an inlet, an outlet, a left side, a right side, a top, and a bottom, wherein the first tow passes through the inlet at a feed speed; passing the second tow into the tow spreading / laying zone in the second tow longitudinal direction, wherein the second tow passes through the inlet at the feed speed, the second tow aligned above the first tow, wherein the second tow is not in contact with the first tow; providing a grid positioned at the bottom of the spreading / laying zone, the grid comprising a plurality of grid bars extending parallel to each other between the left and right sides of the spreading / laying zone; providing a first gas stream flowing in a direction from the top to the bottom of the spreading / laying zone; contacting the first tow with the first gas stream, thereby spreading the first carbon filaments apart a sufficient distance to provide a spread first tow having a spread first tow width, wherein the ratio of the spread first tow width to the first tow width is from 5: 1 to 25: 1; contacting the second tow with the first gas stream, thereby spreading the second carbon filaments apart a sufficient distance to provide a spread second tow having a spread second tow width, wherein the ratio of the spread second tow width to the second tow width is from 15: 1 to 40: 1, and wherein the spread second tow width is greater than the spread first tow width; contacting the spread first tow with the spread second tow to form a contacted spread tow; providing a second gas stream flowing in a direction from the top to the bottom of the spreading / laying zone, wherein the second gas stream flows through the grid; passing the contacted spread tow over the grid in the presence of the second gas stream to provide a lay of the first carbon filaments with the second carbon filaments to form a tangled tow having a tangled tow width; and forming the tangled tow into a combined tow having a combined tow width and a combined filament count, wherein the ratio of the tangled tow width to the combined tow width is from 5: 1 to 40: 1, and wherein the combined filament count is greater than the first tow filament count or the second tow filament count.

2. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the first tow and / or the second tow comprises a spreading / laying synergist.

3. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the first tow and the second tow have the same tensile modulus and the same filament count.

4. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the tensile modulus of the first tow is the same as the tensile modulus of the second tow, and wherein the filament count of the first tow is different than the filament count of the second tow.

5. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the tensile modulus of the first tow is different than the tensile modulus of the second tow, and wherein the filament count of the first tow is the same as the filament count of the second tow.

6. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the tensile modulus of the first tow is different than the tensile modulus of the second tow, and wherein the filament count of the first tow is different than the filament count of the second tow.

7. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the linear weight of the second carbon filaments is greater than the linear weight of the first carbon filaments.

8. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the width of the spread second tow is 4 to 6 centimeters.

9. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the spread first tow is under a longitudinal tension of 50 to 150 grams, and the spread second tow is under a longitudinal tension of 50 to 150 grams.

10. The method for increasing the filament count of a carbon fiber tow according to claim 1, wherein the rate of the first gas stream is substantially the same as the rate of the second gas stream.

11. The method for increasing the filament count of a carbon fiber tow according to claim 1, comprising the additional step of applying a sizing agent to the entangled tow or the combined tow to form a sized combined tow.

12. The method for increasing the filament count of a carbon fiber tow according to claim 2, comprising the additional step of applying a sizing agent to the entangled tow or the combined tow to form a sized combined tow.

13. The method for increasing the filament count of a carbon fiber tow according to claim 1, comprising the additional step of combining the combined tow with an uncured resin matrix.

14. The method for increasing the filament count of a carbon fiber tow according to claim 13, comprising the additional step of curing the uncured resin matrix.

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