Abstract:
An apparatus and method for stretch breaking fibers wherein the formation of relatively small (lengths less than 30 microns) fractured fibers is substantially reduced. This reduction is achieved by applying a dampening fluid to the bundle of fibers during stretch breaking to dampen shockwaves generated during the fracturing process. The fractured fiber bundle may be subsequently treated with a sizing material to improve bundle cohesiveness and handling characteristics.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods and apparatus for stretch breaking fibers. More particularly, the present invention involves stretching and breaking bundles of relatively brittle fibers using methods and apparatus that reduce the amount of small fiber filaments which are produced during the stretch breaking process. 
     2. Description of Related Art 
     Composite materials are well known and widely used in applications where a high strength and light weight material is required. Composite materials are typically composed of fibers that are embedded in a polymer resin matrix. Glass and carbon are two of the most popular fiber materials. Epoxy and phenolic resins are two of the most popular resin matrix materials. The fibers used in composite materials can be arranged in a wide variety of configurations depending upon the desired final properties of the composite. For example, fibers can be randomly oriented in the resin matrix or they can be woven into a wide variety of fabric patterns. 
     In many applications, multiple fibers are combined to form yarn that is woven to form fabric which is impregnated with resin and cured to form the final composite. In many situations, it is desirable to use yarn which contains continuous unbroken fibers. However, there are a number of situations where yarn containing broken fibers or filaments are desirable. For instance, yarn containing discontinuous fibers is useful in situations where the composite material is formed into complex shapes. Fabric made from yarn containing discontinuous fibers is easier to form around tight bends than fabric made using continuous fibers. 
     A common method for forming yarn with discontinuous fibers is referred to as “stretch breaking”. Typical stretch breaking methods involve coating a fiber bundle with various viscous lubricants and stretching the bundle until the individual fibers break or fracture into multiple fragments. In some methods, the fiber bundle is subjected to breaker bars during stretching to facilitate fracturing of the fibers. The resulting fractured yarn is used in the same manner as unbroken yarn except that it must be handled more carefully to prevent the yarn from falling apart. The viscous lubricant is designed, among other things, to help keep the bundle of fractured fiber together. Exemplary stretch breaking methods are described in U.S. Pat. Nos. 4,759,985 and 4,825,635. 
     Although the existing methods for stretch breaking fibers are well suited for their intended purpose, there is a continuing need to improve upon such methods. For example, many of the stretch break methods produce a large number of relatively short (ie. less than 30 microns long) fiber fragments. The generation of a relatively large number of short fiber fragments reduces the strength of the final composite material. In addition, the short fibers tend to separate out from the fractured bundle during stretching and during subsequent handling. This not only causes potential pollution problems, but also results in loss of mass from the yarn. Further, the amount of viscous lubricants used to coat the fibers must be carefully controlled. In most applications, the amount of lubricant is kept below one percent by weight of the total yarn bundle weight. If too much lubricant is used, the adhesion of the fractured fibers to the resin matrix can be adversely affected. If too little lubricant is used, the broken bundle will lack the desired cohesiveness and may not be further processed (i.e., the bundle falls apart). 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and apparatus are provided for stretch breaking fibers wherein the formation of relatively short filaments is reduced and the need for viscous lubricants is eliminated. The present invention is based upon the discovery that the shock waves generated during fracturing of stretched fibers cause the formation of a high proportion of relatively short (i.e., less than 30 microns long) filaments. It was discovered that the application of a sufficient amount of dampening fluid, such as water, to the stretched fiber bundle provides sufficient dampening of the shock waves to reduce the amount of relatively short filaments which are formed. 
     The present invention also includes the addition of compatible coatings to the stretch broken bundles to increase bundle cohesiveness and ensure bundle integrity as they are handled subsequent to stretch breaking. 
     One aspect of the present invention involves apparatus for stretch breaking fibers to provide bundles of fractured filaments having different lengths. The apparatus includes a bundle anchoring device which anchors the bundle or “tow” of fibers at a first end of the bundle. The apparatus further includes a bundle pulling device which pulls on the bundle of fibers at a second end to produce a stretched bundle extending between the first and second ends of the bundle. The amount of stretching provided by the bundle pulling device is sufficient to break the fibers to form fractured filaments having different lengths. The apparatus further includes a fluid applicator which applies a dampening fluid to the bundle of fibers. A sufficient amount of dampening fluid is applied to the bundle in order to provide dampening of the shock waves generated along substantially the entire length of the stretched bundle. It was discovered that saturating substantially the entire length of the stretched bundle of fibers provides sufficient dampening of the shock waves to reduce the amount of relatively short fiber filaments formed during fracturing of the fibers. 
     As a feature of the present invention, water is a preferred dampening fluid, because it is extremely inexpensive and can be removed easily by evaporation. Further, it was discovered that the tendency of the stretch-broken tows or yarn to fall apart could be reduced by leaving a small amount of water in the fiber bundle. The damp fractured bundle is then treated with compatible coatings or sizing materials to increase the cohesiveness of the bundle sufficiently so that the integrity of the bundle is not compromised during subsequent handling. 
     Another aspect of the present invention involves methods for stretch breaking fibers to provide bundles of fractured filaments having different lengths. The method includes the step of anchoring a bundle of fibers at a first end with an anchoring device, such as a pair of rollers. In a second step, a pulling device, such as a pair of rollers, is provided for pulling on the bundle of fibers to provide sufficient stretching to fracture the fibers to form filaments having different lengths. As a feature of the invention, the method includes applying a sufficient amount of a dampening fluid to the stretched bundle of fibers to provide dampening of the shock waves along substantially the entire length of the stretched bundle. In a final step of the method, the dampening fluid is removed from the bundle after formation of the fractured filaments and/or a compatible coating is applied to increase the cohesiveness of the bundle to provide a stable product which can be processed further without falling apart. 
     The present invention is also directed to the bundles of stretch-broken fibers formed using the above-summarized apparatus and method. In addition, the invention covers the textiles and composite materials that include bundles of stretch-broken fibers as described above. 
     The present invention is an improvement over existing methods and apparatus in that stretch breaking of fibers is provided wherein the number of relatively short fractured filaments is reduced and wherein the resulting fractured fiber bundles are free of viscous lubricants. 
     The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of an apparatus for stretch breaking fibers in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides methods and apparatus for stretch breaking a wide variety of fibers. The types of fibers which may be stretch broken include glass fibers, carbon fibers, silicon carbide and other ceramic fibers. Preferred fibers are those which are relatively brittle. Such fibers typically will fracture when they are stretched to 3 percent elongation or less. Exemplary fibers are high modulus fibers which have a modulus of about 30 Msi or more. Carbon fibers having a modulus of between about 30 and 50 Msi are particularly preferred. The individual fibers may range in diameter from 3 to 10 microns. Bundle sizes which are amenable to treatment include those bundles, tows or yarn bundles which contain from 3,000 to 50,000 individual filaments or fibers in each bundle. In general, the present invention may be used in stretch breaking of any fiber bundles which are presently being stretch broken utilizing the known procedures as described in previously-mentioned U.S. Pat. Nos. 4,759,985 and 4,825,635. 
     An apparatus in accordance with the present invention is shown generally at  10  in FIG.  1 . The apparatus is designed for stretch breaking of fibers to provide bundles of fractured filaments having different lengths. The apparatus  10  includes a bundle anchoring device shown generally at  12 . The bundle anchoring device includes a pair of rollers  14  and  16  which are pressed against the bundle of fibers  18  to provide anchoring thereof. The use of rollers  14  and  16  to anchor the bundle at the first end of the bundle  20  is well known in the art. Other types of anchoring devices may be used if desired. 
     The stretch breaking apparatus  10  further includes a bundle pulling device shown generally at  22 . The bundle pulling device  22  preferably includes a pair of rollers  24  and  26  which are pressed against bundle  18  and rotated as shown by arrows  28  to provide pulling of the bundle  18 . As is known in the art, the pair of pulling rollers  24  and  26  are rotated slightly faster than the anchoring rollers  14  and  16  to provide desired stretching of the bundle  18  between the bundle first end  20  and the bundle second end  30 . The amount of stretching is carefully controlled depending on the particular type of fiber being stretch broken. The amount of stretching is selected to provide a bundle of fractured filaments  32  having different lengths. The stretch broken bundle exits the apparatus at  10  in the direction as shown by arrow  34 . Typically, a control device  36  is provided for controlling the relative rotational speeds of rollers  14 / 16  and  24 / 26  to ensure accurate and reproducible stretching of the fibers to the point where fracturing occurs. 
     The temperature at which fracturing is carried out is not critical. For most operations, room temperature is suitable. Temperatures may be varied, if desired, depending upon the particular dampening fluid being used. 
     In accordance with the present invention, it was discovered that shockwaves are generated during breakage of fibers. These shockwaves were found to be responsible for the generation of a relatively large number of small (less than 30 microns in length) fiber fragments. These fiber fragments are undesirable in that they tend to fall from the stretched fiber bundle resulting in reduction in bundle mass and strength. Further, the small fibers tend to become airborne and must be captured and removed in order to prevent possible pollution problems. 
     The shockwaves generated during breakage are dampened utilizing a dampening fluid. The dampening fluid is contained in a reservoir  38 . The dampening fluid is preferably applied to the bundle  18  utilizing one or more nozzles  40 . The nozzle(s) is preferably located so that dampening fluid is applied at the first end  20  of bundle  18 . As the bundle moves between the rollers in the direction of arrow  34 , the bundle  18  becomes completely saturated with dampening fluid as shown at  42 . The amount of dampening fluid  42  which is applied through nozzle(s)  40  is sufficient to completely saturate the bundle  18  over substantially the entire distance between the anchoring rollers  14 / 16  and pulling rollers  24 / 26 . To ensure that sufficient dampening fluid is present to dampen out the shockwaves, it is preferred that the amount of dampening fluid be sufficient so that dampening fluid drops from bundle  18  as represented by droplets  44 . The droplets of dampening fluid  44  are recovered in a container  46 . The recovered dampening fluid  48  may be recycled to reservoir  38  or discarded. The term “substantially the entire distance” means at least 90% of the distance between the location where the bundle  18  is anchored by the anchoring rollers  14 / 16  and the location where the bundle  18  is stretched by pulling rollers  24 / 26  (i.e., 90 percent of the length of the stretched bundle). More preferably, “substantially the entire distance” means at least 95% of the length of the bundle being stretched. 
     The pulling rollers  24  and  26  are thrust against bundle  18  with sufficient pressure to not only provide sufficient traction to pull the fiber bundle  18 , but also to squeeze or wring excess dampening fluid from the bundle as shown at  50 . It is possible to squeeze substantially all of the dampening fluid from bundle  18 , to produce a stretched bundle  32  containing little if any dampening fluid. However, it is preferable to squeeze only a portion of the dampening fluid from the bundle to produce a stretched bundle  32  having a residual amount of dampening fluid left therein. 
     Water is a preferred dampening fluid. Water was found to provide adequate dampening of the stretched fiber to substantially reduce the shockwaves generated during fiber fracture. In addition, water is easily removed from the stretched fiber bundle by evaporation. Preferably, deionized or reverse osmosis water is utilized wherein impurities in the water are reduced. Use of such purified water ensures that no contaminants are introduced into the fiber bundle  18  which might adversely affect surface properties of the final bundle. However, in many situations untreated tap water may be used with acceptable results. 
     In accordance with the present invention, it was discovered that structural integrity of the stretch bundle  32  is optimized by leaving between 10 to 20 weight percent of water in the bundle. Preferably, the amount of water remaining in the bundle after it is passed through rollers  24  and  26  is about 15 percent. This amount of water was found to provide some cohesiveness for the fractured fiber bundle  32  so that it is more easily handled. 
     Suitable dampening fluids in addition to water are organic and inorganic fluids which are capable of dampening shock waves. In general, a suitable dampening fluid is one which is capable of dampening shockwaves generated during fiber pulling and which can be easily removed from the fiber bundle without adversely affecting the physical or chemical properties of the final fiber bundle. If desired, the dampening fluid may contain certain additives which are intended as a coating or surface treatment for a particular application or use of the final stretched fiber bundle. 
     The apparatus and methods of the present invention have been shown with the dampening fluid being applied only at the first end of the fiber bundle. It will be understood by those skilled in the art that any method for applying the dampening fluid to the fiber bundle is suitable provided that the fiber bundle is completely saturated along substantially its entire length between the anchoring rollers  14 / 16  and pulling rollers  24 / 26 . For example, multiple application nozzles located along the entire length of stretched fiber bundle may be used. Alternatively, the entire fiber bundle may be immersed in a reservoir of dampening fluid. For simplicity, however, it was found that sufficient dampening fluid could be applied using a single nozzle applying a sufficient amount of dampening fluid so that the fiber bundle becomes completely saturated along its entire length between the two pairs of rollers as the bundle travels through the apparatus. 
     As known in the art, the stretched fiber bundle may be manipulated with breaker bars (not shown) which are designed to facilitate fracturing of the fibers. A wide variety of breaker bar configurations are possible ranging from simple bar structures that hit the fibers to more complex structures which manipulate the fibers so as to provide desired levels of fiber fracturing. In accordance with the present invention, it was discovered that the use of dampening fluid produces fractured fiber bundles which are substantially free of fractured filaments that are less than 30 microns long. “Substantially free” means about 1 percent or less. 
     The damp stretch broken fiber bundle  32  can be handled to some degree without affecting integrity. However, upon drying, the fractured bundle is more difficult to handle. Accordingly, it is preferred to treat the damp fractured bundle with a sizing material that is compatible with the dampening fluid and which increases the cohesiveness of the fiber bundle. For example, water-based epoxies are a preferred coating or sizing material when water is used as the dampening fluid. Other coatings that are commonly used as sizing materials may be used. The sizing material is preferably applied as a mist or fine spray. The amount of sizing added is between about 0.3 weight percent to about 3.0 weight percent. Amounts in the range of about 0.5 to 1.5 weight percent are preferred. In general, the amount of sizing applied to the bundle will be sufficient to increase the cohesiveness of the bundle so that it can be handled without falling apart while at the same time not saturating the bundle. 
     The coated or “sized” fiber bundle may be dried and then wound onto a spool or otherwise further processed. For example, as is well known, the sized fiber bundles can be woven to form any number of desired fabric structures. The fabric structures are impregnated with a suitable polymer resin such as epoxy resin, phenolic resin, biomaleimides (BMI), vinyl esters and polyesters, and other thermosetting and thermoplastic resins. The sized fiber bundle may be used in the same manner as previous stretch broken fiber bundles to form a wide variety of composite materials where the stretch broken fibers are embedded in a resin matrix. The stretch broken fibers are especially useful in forming complex composite material structures which include relatively sharp bends. 
     Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the above preferred embodiments and examples, but is only limited by the following claims.