BR112021017379B1 - A SEW-IN MULTIAXIAL REINFORCEMENT - Google Patents

Abstract

A SEW-IN MULTIAXIAL REINFORCEMENT. The present invention relates to a stitched multiaxial reinforcement and a method of producing a stitched multiaxial reinforcement. The stitched multiaxial reinforcement of the present invention can be used in all applications where reinforcements are generally required and especially in such applications where Vacuum Infusion Technology or Resin Transfer Molding (RTM) Technology to distribute the resin in the mold is used. The stitched multiaxial reinforcement of the present invention is especially applicable in the manufacture of wind turbine blades, boats, sports equipment, storage tanks, bus, trailer, train and truck panels, etc., and generally in all structures that are subject to tensions in more than one direction.

Description

Field of Technique

[0001] The present invention relates to a sewn multiaxial reinforcement. The stitched multiaxial reinforcement of the present invention can be used in all applications where reinforcements are generally required and especially in such applications where Vacuum Infusion Technology or Resin Transfer Molding (RTM) Technology to distribute the resin in the mold is used. The stitched multiaxial reinforcement of the present invention is especially applicable in the manufacture of wind turbine blades, boats, sports equipment, storage tanks, bus, trailer, train and truck panels, etc., and generally in all structures that are subject to tensions in more than one direction.

Fundamentals of the Technique

[0002] When manufacturing composite and laminated products using various fibers such as glass, carbon and aramid fibers, as well as flax, hemp, jute, kenaf, basalt and other natural fibers, etc. for the manufacture of, for example, boat, automotive panels, buses, trains, tow and trucks or wind turbine parts, for example, manufacturing begins with the production of an appropriate fiber reinforcement as a woven or knitted structure, which may have a unidirectional or multiaxial orientation. The structures are then placed in a mold used to manufacture the intermediate or final product. The mold is, of course, shaped like the final product, which means the shape can sometimes be very complicated and requires substantial shaping of the reinforcement when placed in the mold. Typically, several layers, up to dozens of layers, of reinforcements are placed on top of each other in the mold and a thermosetting resin such as epoxy mixed with hardener or unsaturated polyester resin or vinylester resin is introduced into the mold to form a composite article reinforced with fiber. The resin can also be thermoplastic such as PA (polyamide) or CBT (Cyclic Polybutylene Terephthalate) or similar. Practice has shown that when the final product has to withstand high mechanical loads, unidirectional layers of reinforcing roving, which can be held together by stitching, are a preferred choice in its manufacture. In this case, where there are mechanical loads in more than one direction, multiaxial reinforcements are a preferred choice. These unidirectional layers of a reinforcement are made from roving or tow, often called reinforcement fibers.

[0003] Multiaxial reinforcement is formed by two or more layers of reinforcing strands, in which the strands in one layer are unidirectional, but the strands in adjacent layers form a certain angle, generally 45, 60 or 90 degrees, although other orientations wick can be used well. The construction of the reinforcement depends on the weight of the target area and the tex number of the strands. For example, if a high area weight or gram weight is desired, a thick roving (e.g. with E-glass 2400 tex) is used, and where a low area weight reinforcement is desired, a thin roving (e.g. example, with E-glass 600 tex) is used in its manufacture.

[0004] The final product, i.e., cured laminated structure can be made from a series of such multiaxial reinforcements, arranging the layers of reinforcements so that, in the final product, the strands of the layers are oriented in at least two different directions of According to the loads, the laminated construction is subjected to or by first manufacturing fabrics of several layers of unidirectional reinforcements so that the strands of adjacent layers form a certain angle and subsequently using the fabrics thus formed in the production of the final product. These fabrics are called biaxial, triaxial, quadraxial, etc. fabrics depending on the number of different fiber orientations in them.

[0005] A multiaxial reinforcement formed by at least two reinforcing layers of unidirectional reinforcing strands itself is inherently unstable in nature, as the strands of each reinforcing layer run in only one direction. To be able to handle such reinforcement, your strands must be anchored or glued together properly. The prior art knows, in principle, two different mechanical methods for this purpose.

[0006] A method of securing strands by means of sewing (e.g., warp knitting). The sewing threads form knitting loops, that is, stitches, that hold the actual reinforcing strands in place in the reinforcement. The stitches are formed by various knitting elements, for example by needles, which penetrate the layer or layers of reinforcing rovings in accordance with the known warp knitting technique. The stitches can form several well-known patterns, such as chain or knitting, etc. Sewing thread is typically, but not necessarily, textured or untextured polyester (polyethylene terephthalate) filament thread having a thickness of about 34 dtex to about 167 dtex and comprising dozens of filaments, typically, for example 24 or 48 filaments.

[0007] Another mechanical method is to use the weaving technique to anchor the longitudinal warp threads, i.e. reinforcing strands, with light weft threads in their respective places. As weft yarns, both uncoated and coated hot melt yarns have been used. After heating and cooling, the hot melt binder gave the reinforcement considerable stability. However, the weaving alternative is no longer considered favorable as the reinforcing rovings form folds when crossing the weft threads, leading to lower stress concentrations and mechanical properties than the sewn versions. Hot melt binder yarns have been found to create local disturbances in die curing and are no longer favored in commerce. Typically, weft yarns are multifilament yarns that lay flat under compression, regardless of whether they are hotmelt yarns or not.

[0008] Sewn reinforcements are well known and have some good properties. Firstly, its transverse stability is good because the sewing threads, although they run mainly longitudinally, form such patterns, like knitting, that give the unidirectional strands the structural integrity necessary for reinforcement. Secondly, the gusset is easy to position in the pattern (i.e., it allows the gusset to follow the contours of the pattern), as the sewn gusset is often very flexible if the sewing parameters are chosen correctly, such as stitch length, needle gauge and thread tension, just to name a few as examples.

[0009] The use of points, however, can also cause problems. The problem can be seen when infusing a stack of sewn reinforcements, i.e. the so-called preform, with resin. The distribution of resin in fiber bundles is surprisingly slow and irregular in both directions, i.e., in a direction parallel to the reinforcing strands and in a direction transverse to the reinforcing strands. The above discovery is surprising, as at first glance a stitched reinforcement appears to include flow passages in three dimensions. The stitches when tightened around a bundle of rovings open flow passages through the reinforcement. Also in the direction of the sewing threads parallel to the reinforcement surface, the strands are pressed together so that flow passages on the reinforcement surface are created. And also in the direction of the strands, the tightening of the points would appear to form longitudinal flow passages on the surface of the reinforcement. One would expect that when one reinforcement is placed on top of another in the mold, the reinforcement stack would include a three-dimensional network of flow passages, which would ensure rapid resin flow and penetration, as well as rapid wetting of the reinforcement stack. reinforcements. However, as already mentioned above, this is not the case. The main reason is that before the resin feed into the mold begins, the reinforcement stack in the mold is subjected to compression. Compression causes the ribs to be pressed against each other by a force such that, as the points of the ribs are not vertically directly above each other, but their positioning is random, the “free” strands (i.e. strands, which are not under compression by a point) between the points of one reinforcement are pressed into the point of a neighboring reinforcement. As a result, the flow passage towards the surface of a reinforcement is more or less completely filled with “free” strands, preventing efficient resin flow towards the surface of a reinforcement. As for the part of a stitch where the sewing thread is in the Z direction, the flow path remains in the pile, perhaps a little smaller, but still. However, now that the flow passages in the surface direction of a reinforcement are substantially closed, the flow passage in the Z direction remains filled with air, which is very difficult to remove. This easily results in the presence of gas bubbles in the final product, which naturally reduces the quality and strength properties of the final product.

[0010] As good permeability of the resin is vital for the practical execution of the molding process, it is normally accelerated using the pressure difference when feeding the resin into the mold. It is common practice to apply Vacuum Infusion technology or Resin Transfer Molding (RTM) technology to distribute resin throughout all reinforcing layers of the mold. However, sometimes, despite various measures such as vacuum and/or high supply pressure, small air cavities tend to remain in the reinforcement, significantly reducing the strength properties of the laminate. The main reason for the air cavities is the tight positioning of the strands against each other in the reinforcement, so that their permeability to the resin is in the transverse and longitudinal directions of the reinforcement strands, as well as in the limited Z-direction. In view of the above, new ways to improve both gas removal from the reinforcement stack and the permeability of the reinforcement to the resin should be investigated.

[0011] One way to improve the permeability of the reinforcement is to provide the reinforcement with flow passages for resin, the flow passages allowing the resin to flow quickly into the reinforcement. Numerous ways of arranging resin flow passages in the reinforcements or between reinforcements in a stack of reinforcements, for example, so-called infusion fabrics, can be found in the prior art. However, it has been learned that the use of such flow passages is not very efficient as the vacuum applied in the infusion phase tends to displace or pull wicks from neighboring areas or ribs and even change their positions to fill the flow passages/cavities. flow.

[0012] A traditional way of organizing resin flow channels in a reinforcement is discussed in US-A-2005/0037678 (Mack et al.). The document discloses an open grid fabric, which is formed of thick unidirectional rovings, which are sewn together so that such a mat of unidirectional rovings is formed such that an open space is left between adjacent parallel rovings. Another optional structure is to form the grid fabric of two layers of roving, where the rovings of one layer are arranged perpendicular to those of the other layer. The strands are again sewn together so that an open grid fabric is formed. The US order open grid fabric is used as an infusion fabric, placing it between the reinforcing layers of a laminate to ensure the unobstructed flow of resin between the reinforcing layers to moisten neighboring reinforcing layers in the Z direction. The problems in constructing the US application relate to the open grid fabric being a separately manufactured product and the rovings used in the fabric. The separate manufacturing step increases the costs of manufacturing a reinforcement, and the use of rovings, which are not bonded or twisted, means that in order to provide some open space in the open grid fabric, the rovings must be very thick ( see the discussion in connection with Figure 1 b concerning a flattening of the roving under compression). Thick wicks increase both in weight and expense in those parts of the product that are needed only for a secondary purpose, i.e., resin flow. The relatively high proportion of strands in one direction which is not necessary in view of the strength and fatigue aspects of the product makes the product neither commercially nor in market terms attractive. Another disadvantage of this type of infusion fabric is that it causes areas of higher resin content in the final product than in parts containing only reinforcement layers, that is, the product is not cohesive.

[0013] The prior art also knows biaxial infusion fabrics which are manufactured from two layers of fabric rovings in themselves, which are first stretched in the diagonal direction so that one layer is transformed into a -45 degree layer and a another in a layer of +45 degrees, and after that sewn together. Both fabric layers comprise thick glass fiber rovings in the weft direction and thick glass fiber yarns in the warp direction. The warp direction threads bind the rovings into relatively loose round bundles. When the layers of fabric are distorted, the warp direction threads tighten and bind the strands together more tightly (equal to an almost round cross-section of the bundles). As a good example of yarn tightness, it may be mentioned that the longitudinal weft yarns can be rotated, distorting, 30 and 45 degrees from the weft direction, but no further 60 degrees. The two layers of stretched fabric are then placed one on top of the other so that the strands of the layers run in different orientations, resulting in a biaxial product after sewing. The product is marketed for use as an infusion fabric, which has resin flow channels in the direction of the wicks.

[0014] However, the above infusion fabric has some problems in its structure, use and operation. In practice, in this type of products, there is a clear correlation between the flow capacity of the resin and the gram weight, so that the greater the gram weight, the worse the flow capacity (unless the tex number of the wicks is changed). The reason is that flow passages in the product are formed between the wicks and by increasing the gram weight the number of wicks is increased, so the open area between the wicks is naturally reduced. Another option would be to increase the size or tex number of the strands, but as strands are only available in 300 tex, 600 tex, 1200 tex, 2400 tex, 4800 tex etc., it is not always possible to find a good match. Often a step of one wick size resulting in less adequate flow to the next wick size possible without increasing gram weight results in an infusion fabric, which is difficult to handle due to having such clear gaps between the wicks. that the fabric has, in practice, no stiffness. Correspondingly, increasing the gram weight decreases the resin flow capacity (if only the wick number is increased) or increases the open flow area (if the wick size is increased) in the infusion fabric so that he loses his integrity. In other words, in many applications an infusion fabric must be used, which is not exactly what is desired, but a compromise between resin flow capacity and gram weight. Another characteristic that must be taken into consideration is the non-uniformity of the final product, that is, reinforcement. If the infusion fabric contains more open cavities than the reinforcing layers themselves, which often happens, in the final product the open cavities are filled with resin. Therefore, the proportion of resin is clearly higher at the locations of the infusion fabric(s), causing areas of weaker strength in the reinforcement.

[0015] The prior art also knows of other structures in which the wicks are used in basically the same way as in the US document, the wicks being, however, surrounded by a polyester thread wound around the wicks to make them non-compressible. . By making the wicks rigid in the above-mentioned manner, adequate resin flow properties are obtained, but the manner also has disadvantages. Firstly, winding the polyester (PE) thread around the strands is not done for free. Secondly, the availability of applicable roving is very limited, so the diameter of the PE roving yarn package cannot be chosen freely. In practice, the last problem with PE-wound roving prevents the user from choosing the best possible diameter for their application. Thus, it can be seen that the use of wicks in infusion products or infusion medium results in at least one of the several steps of the method, manual labor, use of material being wasted, etc., all of which means increase and, of in a way, unnecessary expenses.

[0016] Another problem related to the use of fiberglass roving in open grid fabrics or any other infusion products is the fact that fiberglass or any other reinforcing fiber such as aramid fiber or carbon fiber , has a much higher modulus (generally above 50 GPa) than (generally below 10 GPa, typically on the order of 3 to 4 GPa) of the resin used when curing the reinforcement. The difference in moduli means that as soon as the reinforcement begins to support a load, stress peaks are created in such reinforcing fibers of the infusion product that are transverse to the actual reinforcing fibers or rovings of the reinforcing layers. Stress peaks are subject to fatigue stress and act as starting points for fatigue cracks.

[0017] The various problems related to the use of roving as a means of creating a flow channel are addressed in EP-B1-2874803 (Bergstrom), which discloses a stitched unidirectional or multiaxial reinforcement for the manufacture of fiber-reinforced composites by a process resin transfer molding and vacuum infusion molding process, the sewn multiaxial or unidirectional reinforcement comprising at least one layer of continuous unidirectional strands disposed in the reinforcement and mono- or multifilaments, the mono- or multifilaments being arranged transverse to the unidirectional strands and forming to the sides thereof flow passages extending from one edge of the sewn multiaxial or unidirectional reinforcement to its opposite edge to facilitate, when wetting a stack of reinforcements with resin, the flow of resin in a direction transverse to the direction of the reinforcement. unidirectional strands, at least one layer and the mono- or multifilaments being joined together by means of sewing, the mono- or multifilaments having, under compression, a diameter of 70 to 300 μm. The mono- or multifilaments of the EP patent being made of such a polymer that has a low modulus, that is, close to that of resin, where mono- or multifilaments are considered as non-reinforcements.

[0018] The reinforcement discussed in the above document is intended for use in the production of wind turbine blade spar tables, which have a length of tens of meters and a width of a few tens of centimeters. Thus, by organizing the resin feed into the mold along the entire length of the stringer table, i.e. on one side of the stack of reinforcement layers, the resin only needs to flow a few tens of centimeters to impregnate or moisten the entire stack. of reinforcement layers. Thus, the wetting distance is so short that even a relatively slow soaking speed is considered acceptable. One reason for the slow speed of impregnation is the small diameter of the bonded mono- or multifilaments, the small diameter being necessary in view of the strength properties required by stringer tables, i.e. the use of the smallest possible diameter minimizes the risk of microcracks. on the laminate of the stringer table.

[0019] The use of monofilaments as a means of creating flow passages for the resin also presents some drawbacks. First, the thicker monofilaments are used, the less flexible they are, which makes it difficult to place the infusion or booster product in positions that require sharp bends in the product. Secondly, the monofilaments may not be reliably fixed in the resin, so there is a risk that they will break away from the resin and move in their longitudinal direction, leaving an open cavity in the final product.

[0020] Thus, the aforementioned prior art EP document uses flow passage forming means arranged in a transverse direction to the reinforcing fibers or strands. The flow passage forming means may be formed by a single monofilament or a bonded multifilament, i.e., a bundle of filaments. Such construction is limited, in practice, to allowing resin flow in only one direction, which cannot be considered sufficient in the manufacture of articles with a complex shape or large area.

[0021] EP-B1-2918398 (Grove-Nielsen) discusses a fiber-reinforced composite for the manufacture of a component for a wind turbine, comprising a plurality of first fibers, the fibers being arranged in a unidirectional or biax configuration, a plurality of second fibers, the second fibers being disposed perpendicular to a longitudinal direction of the first fibers, and a resin impregnating the first and second fibers, wherein a modulus E of the resin is equal to a modulus E of the second fibers so that When the fiber is reinforced the composite is stretched lengthwise, allowing the second fibers to contract at the same rate as the resin.

[0022] To be more specific, EP-B1-2918398 discusses, on the one hand, a unidirectional reinforced composite having the second fibers transverse to the first reinforcing fibers or the second fibers zigzagging through the layer(s) of the first reinforcing fibers, or, on the other hand, a biaxial reinforced composite where the reinforcing fibers are arranged at angles of +/- 45 degrees to the direction of the second fibers and the second fibers being fixed to the two layers of the first reinforcing fibers through sewing.

[0023] However, EP-B1-2918398 does not teach the purpose of its second transverse fibers. But as the document clearly teaches that a separate vacuum distribution layer, i.e., an infusion layer, is arranged on top of the fiber material, it is quite clear that the second fibers of the fiber material are only used to give stability. transverse to the reinforcement. But even if the second fibers create resin flow passages, the zigzag of the monofilaments in the form of the EP document does not seem reasonable, as the resin infusion normally occurs from one side of the reinforcement to the other, and arranging the monofilaments in orientation other than parallel to each other forms both widening channels into which the resin can hardly enter (since there is almost no opening between the monofilaments) and converging channels in which the converging monofilaments and the converging space itself add resistance to flow and reduce the speed at which the resin is capable of advancing.

[0024] In other words, the prior art suggests, on the one hand, the use of multifilament yarns or rovings arranged in two directions transverse to each other and stitched together to form an open grid fabric to provide an open grid between the reinforcing layers of a reinforcement, and on the other hand, the use of linked mono- or multifilaments to organize flow passages for resin in a single direction transverse to the direction of the reinforcement strands.

[0025] However, practice has shown that today's sewn reinforcements present several problem areas, such as: - the flow channels provided in or in connection with the reinforcements are designed for long and narrow objects where the resin only needs to flow in one direction, i.e. the shortest route through the object, or for more or less round or square objects where it is sufficient for the resin to flow at a similar rate in all directions, hence the prior art does not consider objects with such a complex shape that requires different infusion rates in different directions, - the use of prior art fabrics as infusion media is limited to a few applicable grams - combinations of resin flow capacity, - in infusion fabrics of the prior art, the resin flow channels were in the direction of the wicks, which does not ensure adequate and reliable infusion of resin throughout the product, the applicability of wicks as a means of organizing flow channels in reinforcement is very limited, - wicks have problems in their compressibility, in their resin infusion and in their diameter, as discussed above, - prior art infusion fabrics often contain large open areas, which are, also in the final product, filled with resin, making causing the final product to have a non-uniform structure with changes in strength values, - when monofilaments are used as a medium to create resin flow passages, products requiring relatively long resin flow passages and/or high infusion speed require the use of relatively thick monofilament. The thick monofilaments make the product relatively rigid, so the product cannot be easily bent into a mold that requires sharp bends, and - the use of roving as a means of arranging the flow channels requires several production steps, which means increased production time and costs.

[0026] The use of bonded mono- or multifilaments proved to be an ideal way of providing resin flow channels in connection with unidirectional reinforcements in the production of wind turbine blade spar tables that it was decided to test their use in connection with other types of products, i.e. multi-axial products requiring load-bearing capacity in more than one direction and having a wider area compared to stringer tables. It was soon discovered that the resin flow capacity of the type of bonded mono- or multifilaments used in EP-B1-2874803 was not sufficient in the manufacture of truck or bus panels or boat hulls, just to name a few options. One reason was that in such panels or hulls the resin impregnation or wetting distance from one edge of the product to its opposite edge thereof is many times greater than in corresponding spar table applications. Another reason was that such panels or shells have shapes that require bending or molding of the reinforcement, so reinforcement layers or infusion layers with thick monofilaments which make the layer relatively rigid cannot be used. To overcome the problems, it was decided to improve the flexibility of the product, i.e. a reinforcing layer or an infusion fabric, by applying twisted multifilament yarns as a means of creating flow passage, which improves the flow capacity of the resin by increasing the diameter of the twisted multifilament yarns without sacrificing the flexibility of the yarns.

[0027] Then, it was surprisingly discovered that the risk of microcrack formation in multiaxial reinforcements was considerably reduced, although the diameter of the twisted multifilament strands increased. The conclusion was that because the various panels, boat hulls or sandwich laminates were subjected to loads in different directions and therefore required the use of multiaxial reinforcements, that at least one reason for the discovery is that in a biaxial product or multiaxial, the strands of neighboring layers, in any case, are slightly bent as they pass one another, so that the possible additional bending or twisting caused by the twisted multifilament yarns no longer has, in practice, much influence. Another factor that reduces the tendency for microcracks to form refers to weight optimization, since the various panels, for example, do not have weight requirements as stringent as stringer tables. It was thus learned that twisted multifilament yarns having a larger diameter can be used in multiaxial reinforcements, in which resin flow channels can be widened and thus the resin flow velocity increased. Definitions

[0028] The following illustrative explanations are provided to facilitate the understanding of certain terms frequently used in the specification and claims discussing the present invention. The explanations are provided as a convenience and are not intended to limit the invention.

[0029] Area weight - Weight (mass) per unit area of a fabric.

[0030] Binder — A polymeric material in various forms, such as powder, film or liquid. Binders can be made from one or several individual binders with different characteristics in chemical or physical properties such as rigidity, melting point, polymeric structure, Tg, etc. The binder is used to join the fiber structure together to form a web and ultimately reinforcement. Suitable binders are thermoplastic epoxies, co-polyesters, unsaturated bisphenolic polyesters or mixtures thereof, just to name a few examples.

[0031] Fabric - a flexible woven material consisting of a network of natural or man-made fibers, often referred to as thread or yarn. Fabrics are formed, for example, by weaving, knitting, crocheting, knotting, needling or pressing fibers (felt).

[0032] Filament - A yarn composed of a single continuous structure typically made of synthetics (including cellulose-based materials) and natural materials such as viscose, silk, polyamide (e.g. nylon), polyethylene terephthalate (PET), polypropylene (PP), polybutylene terephthalate (PBT) etc. When used, it appears in multifilaments formed by a bundle of filaments.

[0033] Infusion product/layer - a fabric or non-woven fabric, which has channels to facilitate the flow of resin in the plane of the product. It is used by placing between reinforcement layers to introduce resin into the entire area of reinforcement so that the resin can be easily absorbed by neighboring reinforcement layers.

[0034] Laminate - A material that can be constructed by impregnating one or more reinforcing layers using appropriate resin and hardener mixture and allowing it to harden by chemical reaction or temperature cooling. Laminate is a fiber-reinforced structure made from a matrix reinforced with fine fibers of, for example, glass, carbon, aramid, etc. The matrix can be epoxy, a thermosetting plastic (most often epoxy, polyester or vinylester) or a thermoplastic. Common end uses of fiberglass reinforcements include boats, automobile parts, wind turbine blades, etc.

[0035] Matrix - material that joins the reinforcements to form a composite. Composites use specially formulated polymers such as thermosetting epoxy, unsaturated vinylester or polyester resin, and phenol formaldehyde resins or a thermoplastic resin (see “Polymer”) just to name a few examples.

[0036] Monofilament - A thread composed of a single continuous filament normally made of synthetic material, such as polyamide (e.g. nylon), polyethylene terephthalate (PET), polypropylene, polybutylene terephthalate (PBT) etc. Specifically meant in this description is a yarn functioning alone, that is, as a unitary structure, as opposed to a single filament, which, when brought together into a bundle of filaments, forms a multifilament yarn.

[0037] Multifilament - A line or thread composed of a multitude of continuous filaments typically made of synthetic material, such as polyamide (nylon), polyethylene terephthalate, polypropylene, polybutylene terephthalate, etc. Especially, in connection with the present invention, a distinction has to be made between loose compressible multifilaments formed from separate filaments and bonded multifilaments where the filaments are bonded together to make the bonded multifilament behave like a monofilament.

[0038] Polymer - Generally includes, for example, homopolymers, copolymers, such as, for example, block, graft, random and alternating copolymers, terpolymers, etc., and mixtures and modifications thereof. Furthermore, unless specifically limited otherwise, the term “polymer” includes all possible geometric configurations of the material. These configurations include, for example, isotactic, syndiotactic and random symmetries.

[0039] Reinforcement - a web composed of reinforcing fibers, the fibers being anchored to each other by appropriate means. Often manufactured as continuous webs. There are several ways to manufacture reinforcement in unidirectional or multiaxial or random orientations, for example, through the textile processing techniques of weaving, knitting, braiding and sewing or by bonding with an appropriate binder.

[0040] Reinforcement fibers - fibers used in conjunction with a matrix in the manufacture of composite materials. Fibers are generally man-made such as glass (including all its variants), carbon (with all its variants) or aramid, which can be used as both continuous filaments and non-continuous fibers. A wide variety of natural fibers have also been used, such as sisal, linen, jute, coconuts, kenaf, hemp or basalt, just to name a few.

[0041] Resin Transfer Molding (RTM) - A process with two mold surfaces whereby a resin is pumped typically at low viscosities and low or high pressures into a closed mold assembly often containing a dry reinforcement preform , that is, to infuse resin into the preform and to make a fiber-reinforced composite part.

[0042] Roving - a long, narrow untwisted bundle of continuous fibers or filaments, particularly glass fibers. In this application, it is synonymous with tow, where the fiber selection contains not only glass fibers, but also carbon, basalt and aramid fibers, most generally artificial continuous fibers.

[0043] Roving group or tow group - one or more tows or wicks that are spaced closely together.

[0044] Sewing Thread — Thread formed, for example, by 24 or 48 individual filaments of textured polyester. The sewing thread typically used in the manufacture of unidirectional reinforcements typically has a linear mass density of 76 or 110 dtex. The diameter of an individual filament is typically 5 to 10 μm.

[0045] Tex Number - An SI unit of measurement for the linear mass density of yarns and is defined as the mass in grams per 1,000 meters. Tex is more likely to be used in Canada and Continental Europe, while denier remains more common in the United States and the United Kingdom. The unit code is “tex”. The most commonly used unit in connection with man-made synthetic fibers is actually the decitex, abbreviated dtex, which is the mass in grams per 10,000 meters.

[0046] Textile - general definition for various types of articles, including sheets, webs, fabrics and mats having one or more layers, the layers being formed by uni- or multi-directional lines.

[0047] Thermoplastic - A polymer that is fusible, softening when exposed to heat and generally returning to its unsoftened state when cooled to room temperature. Thermoplastic materials include, for example, polyvinyl chlorides, some polyesters, polyamides, polyfluorocarbons, polyolefins, some polyurethanes, polystyrenes, polyvinyl alcohol, caprolactams, ethylene copolymers, and at least one vinyl monomer (e.g., polyvinyl acetates ethylene), cellulose esters and acrylic resins.

[0048] Thermosetting — A polymeric material that cures irreversibly. Curing can be done through heat (generally above 200 Celsius), through a chemical reaction (two-part epoxy, for example), or through irradiation such as electron beam processing.

[0049] Line — twisted bundle of filaments or unitary fibers, threads.

[0050] Tow - In the composites industry, a tow is an untwisted bundle of continuous filaments and refers to artificial fibers, mainly carbon fibers (also called graphite). Tows are designated by the number of fibers they contain, for example, a 12K tow contains about 12,000 fibers. Here synonym for lock.

[0051] Transverse handling stability - Force that prevents a unidirectional reinforcement from deforming or tearing into pieces. Required when positioning a gusset in a mold on top of another gusset and moving the gusset in a direction transverse to its longitudinal direction.

[0052] Transverse direction - Any direction that is not parallel to the reference direction, preferably deviating at least 5 degrees, more preferably at least 10 degrees, more preferably at least 15 degrees from the referenced direction.

[0053] Unidirectional reinforcement (UD) - Reinforcement in which all strands or tow run in the same direction, in this particular case in the longitudinal direction, but a UD reinforcement can also be transverse, that is, oriented in the 90° direction. These strands are often in prior art UD reinforcements bonded by sewing and typically using some additional light layer of chopped yarns or continuous multifilament yarns to hold the strands together and to prevent tearing, or by weaving where the strands of weft provide structural stability. Weft yarn can also be hot melt coated. Another way to join the wicks or tugs together is to use a binder, for example a thermoplastic or thermosetting binder. Also in this case, additional stabilization layers mentioned above can be used.

[0054] Vacuum infusion - A process that uses a one-sided mold that shapes the final product. At the bottom is a rigid mold and at the top is a flexible membrane or vacuum bag. When vacuum/suction is applied to the mold cavity, air escapes from the cavity, after which the resin can be infused by suction (or additionally aided by a small overpressure on the feed side - a characteristic feature of light RTM) to completely moisten the reinforcements and eliminate all air voids in the laminated structure.

[0055] Wetting distance — The position of the flow front or, in fact, the distance measured from the location where the resin entered the reinforcement stack to the current position.

[0056] Yarn - A long continuous length, often twisted, multifilament, suitable for use in textile production, sewing, crochet, knitting, weaving, sewing, embroidery and rope making. Yarns can be made from continuous or non-continuous natural or synthetic fibers.

[0057] Z direction - The direction perpendicular to the plane of the layer or stack of layers, that is, thickness direction.

Brief Summary of the Invention

[0058] An object of the present invention is to provide a solution to at least one of the problems discussed above.

[0059] Another objective of the present invention is to develop a new multiaxial sewn reinforcement having excellent resin permeability in more than one direction transverse to the orientations of the reinforcement filament.

[0060] A further objective of the present invention is to develop a new sewn multiaxial reinforcement in which the flow of resin in different directions can be controlled.

[0061] Yet another objective of the present invention is to accelerate the production of multiaxial reinforcements by being able to produce a multiaxial reinforcement in a single production step.

[0062] Yet another objective of the present invention is to develop a new infusion product whose resin flow capacity and gram weight can be chosen freely, that is, independently of each other.

[0063] The necessary resin permeability of the reinforcement and the necessary gas removal from the reinforcement of the present invention are ensured in accordance with a preferred embodiment of the present invention using at least two sets of arranged twisted multifilament yarns, in a preferred alternative of the present invention, between the unidirectional and transversely oriented reinforcing layers themselves to form flow passages for resin to organize the free flowing area in more than one direction transverse to the direction of the reinforcing strands for both air to escape from the reinforcement and for the resin to efficiently impregnate or moisten the product.

[0064] At least one of the problems of the prior art is solved and at least one of the objectives obtained by means of a stitched multiaxial reinforcement for the manufacture of fiber reinforced composite by a resin transfer molding process and a resin transfer molding process. vacuum infusion, the stitched multiaxial reinforcement comprising at least a first layer of continuous unidirectional roving reinforcement having a first axial direction, a second layer of continuous unidirectional roving reinforcement having a second axial direction, and a first set of highly twisted multifilaments formed from a bundle of filaments joined together by twisting the filaments at least 100 turns per meter or coated with an adhesive; the first axial direction and the second axial direction leaving an angle between them; wherein the first set of bonded multifilament yarns is formed from highly twisted multifilament yarns positioned in the first reinforcing layer at an angle different from the first axial direction, a second set of highly twisted multifilament yarns being disposed between the first and second reinforcing layers. reinforcement (20, 32) in a second direction transverse to a first direction of the first set (26) of highly twisted multifilament yarns, the highly twisted multifilament yarns are unreinforced yarns having a modulus of less than 10 GPa, and the first and The second reinforcing layer and the highly twisted multifilament yarns of the first set and the second set are bonded together by means of stitching to form a stitched reinforcement, wherein the highly twisted multifilament yarns form resin flow passages in their sides.

[0065] Other characteristic features of the sewn multiaxial reinforcement of the present invention are disclosed in the attached patent claims.

[0066] With the present invention at least some of the following advantages can be achieved - the permeability of sewn multiaxial reinforcements is improved to such a level that it facilitates good resin flow, - the use of twisted multifilament threads in infusion products ensures that resin flow capacity and gram weight can be chosen freely, independently of each other, for all applications, - the distance that the resin advances in the transverse direction in a certain period of time is increased considerably, in experiments carried out for at least least 2 times, - the time required for impregnation is reduced considerably, in experiments carried out to at least a quarter of the time required with prior art reinforcements, - the resin flow passages in two different directions ensure that the resin reaches all parts of the reinforcement even if a channel in the direction of primary resin flow may for some reason be blocked, - more uniform reinforcement due to a more uniform distribution of resin throughout the reinforcement, - the use of highly twisted multifilament yarns ensures fixation of the multifilaments to the resin, as the specific surface area of the twisted threads is, having a type of a threaded surface, much larger than that of a monofilament for the resin to attach to, and - the properties of resin flow in different directions can be adjusted by varying at least one of the direction and diameter of the twisted multifilament yarns.

Brief Description of the Drawing

[0067] In the following, the sewn multiaxial reinforcement of the present invention and the method of its production are discussed in more detail with reference to the accompanying figures, in which Figures 1a and 1b schematically illustrate a comparison between the behaviors of a highly of the present invention and a prior art multifilament under compression between two layers of strand reinforcement,

[0068] Figure 2 schematically illustrates the production process of the sewn multiaxial reinforcement according to a preferred embodiment of the present invention,

[0069] Figures 3a to 3c schematically illustrate cross sections of the biaxial reinforcement manufactured in the manner discussed in Figure 2, and

[0070] Figure 4 compares a prior art stitched reinforcement with two stitched reinforcements of the present invention in view of the resin flow distance in the transverse direction.

Detailed Description of the Drawings

[0071] Figures 1a and 1b schematically illustrate the cross-sectional comparison between the behaviors of a twisted multifilament yarn of the present invention, and a prior art multifilament yarn (such as that used in US-A-2005/0037678) under Vacuum infusion process compression between two layers of wick reinforcement. Figure 1a illustrates a cross-section of two overlapping reinforcing layers 2 and 4 made of bundles of unidirectional rovings sewn by means of cross stitching (not shown) together as a reinforcement having a twisted multifilament yarn 6 disposed perpendicular to the UD rovings between the layers 2 and 4 of the same. Figure 1b shows the same reinforcing layers 2 and 4 made of bundles of unidirectional rovings sewn by means of transverse stitching together as a reinforcement having a multifilament thread 8 disposed perpendicular to the rovings between layers 2 and 4 thereof. Figure 1a shows how the twisted multifilament yarn further pushes or holds the strands of the stiffeners 2 and 4 apart so that open flow passages 10 are formed between the stiffeners 2 and 4 to the sides of the twisted multifilament yarn 6. Figure 1 b shows the strands of the reinforcements 2 and 4 pushed apart in a manner equal to Figure 1a, that is, the thickness of the two reinforcements with the transversely twisted multifilament yarn 6 and the multifilament 8 is the same. However, it can be seen that the multifilament yarn 8 required to push or hold the strands apart is of an entirely different size and cross-sectional area. It has transformed into an oval or flat shape under compression so that in practice there are no true flow passages 12 on the sides of the multifilament yarn 8.

[0072] The reason is that multifilament yarns are made of tens, hundreds or even thousands of individual filaments. When the multifilament yarn in Figure 1 b is subjected to compression pressure, that is, in the vacuum infusion stage, in the mold, the filaments of the multifilament yarns are forced to move laterally so that the dimension in the Z direction of the Multifilament yarn is a fraction of a twisted multifilament yarn.

[0073] A common understanding was that wires with high twist, on the order of 100 TPM or 150 TPM (TPM = turns per meter) or more, could be effective in resisting the compressive effect of the vacuum. An equally accepted understanding was that these highly twisted yarns cannot be used in the composites industry due to their negative effect on the mechanical properties of the composite. Furthermore, the highly twisted multifilament yarns were believed to be rigid in character, leading to kinks in the reinforcement of the UD strands. Therefore, when stranded yarns were even considered, their twist was normally kept relatively low, i.e., on the order of 20 to 40 TPM.

[0074] Consequently, the use of twisted multifilaments was in no way considered advisable, since the low-twist multifilament is capable of flattening and thus losing its ability to form flow passages for resin. The only way to provide the reinforcement with the necessary resin passages would be to increase the size of the multifilament, which inevitably leads to increased weight of the multifilament. The increase in weight, on the one hand, increases the costs of the multifilament and, on the other hand, increases the material that does not participate in the basic task of reinforcement, that is, the load-bearing capacity of the composite.

[0075] To solve the problems mentioned above, the use of a highly twisted multifilament is taken into consideration. Therefore, the term already used above “bonded multifilaments” refers in this invention to a multifilament, which is formed by a bundle of filaments connected together by twisting the filaments at least 100 turns per meter, preferably more than 150 TPM and more preferably more than 200 TPM. “Glued multifilament” is then called highly twisted multifilament yarn. The use of such a highly twisted multifilament yarn reduces the weight of the added material, as the cross section of a highly twisted yarn is substantially round.

[0076] When using such highly twisted multifilament yarns, the diameter, or actually the Z-direction dimension, of the highly twisted multifilament yarns is in the order of 50 to 2000 μm, preferably 100 to 1000 μm, more preferably between 150 to 900 μm, 200 to 800 μm. Other preferred ranges are 500 to 1000 μm, 500 to 900 μm and 500 to 800 μm.

[0077] Thus, to ensure that the flow passages formed by the highly twisted multifilament yarns are as efficient as possible in relation to the amount of foreign matter brought into the reinforcement by the highly twisted multifilament yarns, the highly twisted multifilament yarns must be as compact as possible, which means that their aspect ratio (width/height ratio) must be equal to or less than 2.0, preferably less than 1.5, most preferably as close to 1.0 as possible, when the Highly twisted multifilament yarns are subjected to vacuum, that is, to compression during the moistening or impregnation phase. Aspect ratio 2 means, for example, two highly twisted multifilament yarns arranged side by side.

[0078] Highly twisted multifilament yarns are made from materials such as polyester (e.g. PET or PBT), polyamide (PA), co-polyamide or copolyester (co-PET) filaments, generally synthetic or natural polymers with a modulus less than 10 GPa, so that the modulus is close enough to that of the resin, so that the risks of stress concentrations and fatigue stress cracks can be avoided. The modulus of the resin is normally on the order of 3 to 3.5 GPa. The filaments of the highly twisted multifilament yarns are, preferably but not necessarily, coated with a sizing agent, which improves the bonds of both filaments to each other and the highly twisted multifilament yarns to the matrix resin. In the following table, the moduli of various materials appearing in fiber-reinforced composites are given.

[0079] The table shows the enormous differences, for example, in the moduli, density and elongation of, on the one hand, non-reinforcing fibers (polyester) and resin, and on the other hand, reinforcing fibers.

[0080] Regarding the size of the highly twisted multifilament yarns, as well as their positioning in the reinforcement layers, i.e., their lateral distance from each other, all these characteristics (among others) must be carefully considered with a view to adequate impregnation and wet - outside the resin reinforcement pile. The resin flow passages formed on the sides of the highly twisted multifilament yarns must not be too open to give the resin enough time to impregnate the strands and not flow directly from the side of the reinforcement pile where the resin is introduced on the opposite side of the pile reinforcement. Naturally, the smaller the distance between adjacent highly twisted multifilament yarns, the more open, i.e., larger cross section, the transverse flow passages on the sides of the highly twisted multifilament yarns can be, and vice versa. Another thing that must be taken into consideration is the thickness or gram weight of the reinforcing layer. The thicker the reinforcing layer, the longer it takes to properly moisten the reinforcing layer with resin. With the present invention it is possible to adjust the permeability of the reinforcement to ensure that the individual reinforcement fibers are well impregnated and no dry areas or voids are left between the fibers.

[0081] An ideal property of the polymeric material for highly twisted multifilament yarns is that the material does not delay curing or have a negative effect on the chemical, thermal or mechanical properties of the resin, which forms the matrix. In the experiments carried out, highly twisted multifilament yarns of polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyamide (PA), co-polyamide or co-PET, or other non-reinforcement materials were used. Therefore, other materials that function in the desired way and have a low enough modulus can also be used.

[0082] Another preferred optional property of polymeric material for highly twisted multifilament yarns is that the material is at least partially soluble in the resin. However, the solubility must be so weak or slow that the reinforcement has time to be impregnated by the resin before the highly twisted multifilament strands “disappear” or “collapse”. However, the advantage of at least partially soluble highly twisted multifilament yarns is that the channels formed by the highly twisted multifilament yarns disappear, and the product as well as the matrix becomes even more homogeneous than when using high-twisted multifilament yarns. highly twisted non-soluble multifilament.

[0083] Figure 2 schematically illustrates the production process of the sewn multiaxial reinforcement according to a preferred embodiment of the present invention. Fabrication of a stitched multiaxial reinforcement proceeds as follows. To begin, a first reinforcing layer of 20' unidirectional rovings (preferably but not necessarily glass fiber or carbon fiber rovings or tow or aramid fibers as well as linen, hemp, jute, sisal, coir, kenaf, basalt or other natural fibers) is formed by pulling the strands 20' from the packages 22 and arranging them side by side or at a controlled distance from each other, depending on the target area weight in a first axial direction, here +45 (optionally also, for example, 0 or +60) degrees. From now on, the word “wicks” is used to refer to all tow, roving, fiber, etc. which are used in the manufacture of unidirectional reinforcements. Thus, the strands are arranged side by side in one or more layers of strand reinforcement.

[0084] Next, in the first layer of reinforcement 20, a first set 26 of highly twisted multifilament yarns running in the machine direction is placed from the feeding means 24, that is, in a direction of displacement of the reinforcement to be produced, or at least in a direction transverse to 20' of the first reinforcing layer 20. Next, a second set 28 of mono- or multifilaments is placed from the feed means 30 into the first reinforcing layer 20 and the first set 26 of highly twisted multifilament yarns, preferably, but not necessarily, transverse to the first axial direction of the first reinforcing layer 20. Naturally, the order in which the first and second sets of highly twisted multifilament yarns are placed in the first layer of reinforcement 20 can be chosen freely. Thereafter, a second reinforcing layer 32 of unidirectional strands 32' is formed by pulling them from the packages 34 and arranging them side by side or at a controlled distance from each other depending on the target area weight in a second axial direction. here -45 (optionally also, for example 90 or -60) degrees. Thus, the highly twisted multifilament yarns of both sets 24 and 26 are left between the two reinforcing layers 20 and 28, whereby the highly twisted multifilament yarns of the first and second sets are in direct contact with each other. The highly twisted multifilament yarns of the present invention have a diameter of 50 to 2000 μm, preferably 100 to 1000 μm, and more preferably 150 to 900 μm, 200 to 800 μm. Other preferred ranges are 500 to 1000 μm, 500 to 900 μm and 500 to 800 μm. The diameter, or thickness in the Z direction, is ideal in case the viscosity of the infused resin curing agent mixture is at the level of 200 to 350 mPas at room temperature. If the viscosity clearly differs from this, it may be necessary to adjust the distance between the highly twisted multifilament strands or the diameter/thickness in the Z direction thereof. Here, the word “transverse” mainly means any non-parallel direction. However, if and when some margin of safety is required, the word “transverse”, for example, with regard to the direction of highly twisted multifilament yarns, means any direction that deviates at least 5, 10 or 15 degrees from the direction of the first and second axial directions of the reinforcing strands of the first and second reinforcing layers. The purpose of the diversion is to prevent the highly twisted multifilament strands from orienting locally between the strands, whereby the objective of facilitating resin flow would be lost or at least seriously compromised. The same angular displacement also applies to the directions of the first and second sets of highly twisted multifilament yarns, that is, they should preferably, but not necessarily, form an angle of at least 5, 10 or 15 degrees, as well.

[0085] However, as a second supplementary embodiment of the present invention, it should be understood that the sets of highly twisted multifilament yarns can be arranged so that the first set of highly twisted multifilament yarns has a filament orientation transverse to the first direction axial direction of the first reinforcing layer but parallel to the second axial direction of the second reinforcing layer. In a corresponding manner, the second set of highly twisted multifilament yarns may have a filament orientation parallel to the first axial direction of the first reinforcing layer but transverse to the second axial direction of the second reinforcing layer. Here the idea is, of course, that the highly twisted multifilament yarns of the first set of highly twisted multifilament yarns, due to their stiffness, are not able to bend in the open area between two adjacent highly twisted multifilament yarns of the second set of highly twisted multifilament yarns to reach the surface of the second reinforcing layer and thus block the flow of resin between the mono- or multifilament and the reinforcing layer and vice versa. Naturally, the smaller the distance between neighboring highly twisted multifilament yarns in one set, the less stiffness is required from the highly twisted multifilament yarns in the other set. Therefore, to optimize the resin flow properties of highly twisted multifilament yarns, the diameter and stiffness thereof, as well as the distance between neighboring highly twisted multifilament yarns, must be taken into consideration and the best combination of these must be selected. best way to meet the requirements of each particular application.

[0086] The highly twisted multifilament yarns of the second set 28, and also those of the first set 26, if the direction of the first set 26 is not parallel to the direction of displacement of the reinforcement to be produced, can be arranged in the first reinforcement layer 20 using a well-known yarn transport system of multiaxial production machines, i.e. by means of a tool that moves transversely back and forth above the first layer of reinforcement, placing a certain number of highly twisted multifilament yarns in the first layer of reinforcement at a time. Laying can, for example, be facilitated with a servo linear motion manipulator with highly twisted multifilament yarn feeding organization.

[0087] An advantageous feature of the present invention is that both sets of highly twisted multifilament yarns are placed in the first reinforcing layer 20 in a straight, parallel formation each, that is, the highly twisted multifilament yarns of the first set 26 run linearly and uniformly in the desired direction, preferably mainly in the direction of displacement of the reinforcement 38 to be produced, and the second set 28 of highly twisted multifilament yarns runs linearly and uniformly from one edge of the reinforcement layer 20 to its opposite edge, in In other words, highly twisted multifilament yarns, for example, do not form loops normally found in knitting patterns. Essentially straight, that is, the linear and flat formation of highly twisted multifilament yarns through the unidirectional reinforcing strands ensures the shortest resin flow time between the reinforcing edges, as the straight line is the shortest path between two points. Regardless of the actual location and direction of the highly twisted multifilament yarns of the two sets, they are arranged at regular intervals, i.e., at about 2 to 50 mm, preferably 5 to 25 mm, more preferably at about 5 to 15 mm. lateral distance or spacing from each other. The exact distance must be optimized according to the viscosity of the resin and the gram weight of the reinforcement layer, just to name a few variables.

[0088] However, it should be understood that the above discussion refers to the simplest method of manufacturing a multiaxial reinforcement, in this case a biaxial reinforcement. Basically, the same method can be applied in the manufacture of reinforcements having several unidirectional reinforcement layers in themselves. If a multiaxial reinforcement with more than two reinforcement layers is to be manufactured, so many additional means for forming new reinforcement layers are required. Furthermore, as it is preferable, but not always necessary, that both transverse sets of mono- or multifilament yarns should be placed between each pair of reinforcing layers, each additional reinforcing layer requires feeding means for both sets of multifilament yarns. highly twisted. Furthermore, when laying highly twisted multifilament yarns, it must be remembered that this should be neither parallel nor nearly parallel with the reinforcing strands of the nearest reinforcing layer, i.e. the reinforcing layer on which they are resting. but preferably, but not necessarily, inclination of at least 5, 10 or 15 degrees should be arranged between them. However, the highly twisted multifilament yarns can be paralleled with the reinforcing strands of the most remote reinforcing layer.

[0089] As a third supplementary embodiment of the present invention, which brings at least one major advantage, a reinforcing structure containing the two sets of highly twisted multifilament yarns and possibly some other features discussed in connection with the present invention, further comprises Furthermore, sets of highly twisted multifilament yarns having different diameters. The different diameters help control the flow of resin between the reinforcement layers. If, for example, we assume that the diameter of the first set of highly twisted multifilament yarns is 300 μm and that of the second set of highly twisted multifilament yarns is 600 μm, the experiments carried out show that the resin advances in the direction of the multifilament yarns highly twisted thicker faster or moves a certain distance faster. When using such highly twisted multifilament yarns, the diameter, or actually the dimension in the Z direction, of the highly twisted multifilament yarns of the set of highly twisted multifilament yarns having a smaller diameter can vary between 50 and 1000 μm, preferably between 150 and 900 μm, more preferably between 200 and 700 μm. This characteristic can be used when the reinforcement to be produced has a complex and broad shape. The detailed reinforcement structure of the third embodiment is discussed in more detail in connection with Figures 3b and 3c.

[0090] It should also be understood as a fourth supplementary embodiment of the present invention that highly twisted multifilament yarns can be placed on the upper and/or lower surface(s) of the reinforcement, as well, i.e. , highly twisted multifilament yarns can not only be found between the reinforcing layers. The same rules for placing the highly twisted multifilament yarns on the top and bottom surfaces of the reinforcements as when placing them between the reinforcement layers apply here as well, i.e., the orientation of the highly twisted multifilament yarns may not be the same. of the reinforcing layer strands left between the sets of highly twisted multifilament yarns.

[0091] According to a fifth supplementary embodiment of the present invention, a biaxial reinforcement may have highly twisted multifilament yarns in the 0 direction at the bottom, a reinforcement layer in the +45 degree direction on the highly twisted multifilament yarns, then a reinforcement layer in -45 degree direction and finally another set of highly twisted multifilament yarn in a 90 degree direction.

[0092] According to a sixth supplementary embodiment of the present invention, another biaxial reinforcement may have highly twisted multifilament yarns in the 0 direction at the bottom, a reinforcement layer in the +45 degree direction on the highly twisted multifilament yarns, then another set of highly twisted yarns multifilament yarns twisted in the 90 degree direction and finally a reinforcing layer in the -45 degree direction.

[0093] According to a seventh supplementary embodiment of the present invention triaxial and quadraxial reinforcements can be discussed. A triaxial reinforcement is produced by adding a third layer of reinforcement having strands in a third axial direction either below the first layer of reinforcement or on top of the second layer of reinforcement, when compared to the production discussed in connection with Figure 2. A reinforcement Quadraxial is produced by adding a third layer of reinforcement having strands in a third axial direction below the first layer of reinforcement and a fourth layer of reinforcement having strands in a fourth axial direction on top of the second layer of reinforcement when compared to the production discussed in connection with Figure 2. Additionally, if desired or deemed necessary, at least one set of highly twisted multifilament yarns may be provided between the third reinforcing layer and the nearest adjacent reinforcing layer, as well as between the fourth reinforcing layer and the nearest adjacent reinforcing layer. The same rules as in the above embodiments, that is, the highly twisted multifilament yarns of each set of highly twisted multifilament yarns disposed between two reinforcing layers may not be parallel with the strands of the nearest reinforcing layer, but preferably , but not necessarily, inclination of at least 5, 10 or 15 degrees must be arranged between them, apply here too.

[0094] After a desired number of reinforcing layers and a desired number of sets of highly twisted multifilament yarns are placed on top of each other, the stack of reinforcing layers is taken to a bonding step 36, where the reinforcing layers are reinforcement and the sets of twisted multifilament yarns placed therebetween are stitched, points 38 shown by dashed lines, together to form a unitary reinforcement 40 having strands in multiaxial configuration. Thereafter, the multiaxial reinforcement 40 is rolled onto 42 for delivery to a customer.

[0095] As shown schematically in Figure 1a of the prior art the highly twisted multifilament yarns 6 used between reinforcement layers 2 and 4 to improve the permeability of the reinforcement stack to the resin in the transverse direction and air removal between the reinforcement stack The layers create small flow passages 10 on both sides and between the unidirectional reinforcing strands.

[0096] The reinforcement stack of the present invention as shown in Figures 3a to 3c operates in the infusion phase so that the infusion resin flows through the flow passages 10' transversely to the reinforcement wicks 32' and then penetrate between individual reinforcing strands or filaments and fast resin flow, safe and good impregnation. During infusion, the advancing resin pushes remaining air bubbles along the chambers or cavities within the reinforcing structure into the flow passages and ultimately out of the product. Both resin advancement and air removal can be further facilitated by pressurizing the resin feed in case rigid top molds are in use as in RTM or RTM Light (although rarely used) at the first ends of the flow passages and/or organizing vacuum at opposite ends of flow passages. Now that the highly twisted multifilament yarns of the two sets are arranged transversely to each other, the mono- or multifilaments can be oriented in the desired direction, whereby the direction of resin flow can be controlled better than in prior art products. This is especially true if the diameter of the first set of highly twisted multifilament yarns is different from that of the second set of highly twisted multifilament yarns. In other words, the resin flow can, for example, be oriented in the direction of the shortest or longest dimension of the product to be manufactured, depending on the application.

[0097] Figures 3a to 3c schematically illustrate, on the one hand, the effect that the highly twisted multifilament yarns arranged transversely to each other bring to the reinforcement and, on the other hand, the effect that the variable diameter of the highly twisted multifilament yarns brings. twisted causes. Figure 3a shows a cross-section of the reinforcement of the present invention taken in the axial direction of the strands 32' of the second reinforcement layer and that of the first set 26 of highly twisted multifilament yarns, the strands 20' of the first reinforcement layer and the second set 28 of highly twisted multifilament yarns being oriented perpendicular to both strands 32' of the second reinforcing layer and the highly twisted multifilament yarns of the first set 26 of highly twisted multifilament yarns. Both strands 32' of the second reinforcing layer and the first set 26 of highly twisted multifilament yarns are shown to bend under compression in the infusion step. In the embodiment of Figure 3a the highly twisted multifilament yarns of both sets 26 and 28 have the same diameter. Figure 3b is similar in all other respects, but the first set 26' of highly twisted multifilament yarns now has a smaller diameter than the second set 28 of highly twisted multifilament yarns. By comparing the exemplary figures, it is easy to see that the thinner highly twisted multifilament yarn (from the first set 26') bends more and thereby slightly reduces the cross-sectional flow area of resin flow channels 10' in the direction of the thicker highly twisted multifilament yarns (from second set 28). However, what is more important is that the cross-flow area in the direction of the finer highly twisted multifilament yarns (of the first set 26') is reduced further, since the plied strands 32' of the second reinforcing layer are , at least, almost able to contact those 20' of the first reinforcing layer at point restricted than in the direction of the thicker highly twisted multifilament yarns (from the second set 28). Basically the same is shown in Figure 3c where the cross section is taken such that the first thinner set 26' of highly twisted multifilament yarns comes from the left towards the observer and the second thicker set 28 of highly twisted multifilament yarns from the right to the viewer.

[0098] Figure 4 is a graph comparing the resin flow or moisture properties of six biaxial reinforcements manufactured in accordance with the method discussed in Figure 2 and using the highly twisted multifilament yarns as the means of creating flow passage. of resin for a standard biaxial reinforcement. In other words, the reinforcement layers in all examples were formed by two unidirectional reinforcement layers of +1 -45 degrees, resulting in reinforcement with an area weight of 600 g/m2. The standard biaxial reinforcement (Example A) had no means of forming flow passages in the product. In all examples (B to G) according to the present invention, the reinforcing layers had two sets of highly twisted multifilament yarns placed at orientations of 0 and 90 degrees to each other. The experiment was carried out in such a way that seven different biaxial reinforcements were prepared. The same unidirectional strands, the same sewing thread and the same type and type of stitch were used to manufacture the reinforcements. The only difference was in the diameters of the sets of highly twisted multifilament wires that were placed (in examples B to G) with a spacing of 10 mm and arranged at an angle of 0 and 90 degrees in relation to the direction of displacement of the reinforcement to be produced. between the two layers of UD strand reinforcement. The properties of the highly twisted multifilament yarns used in the experiment can be seen in the following table.

[0099] For the experiment, an 80 cm by 80 cm sheet of biaxial reinforcement of the present invention was cut from each biaxial reinforcement so that the strands formed angles of +/- 45 degrees to the sides of the sheet and the highly twisted multifilament the wires were parallel to the sides of the sheet, that is, at angles of 0/90 degrees in examples B to G. In each experiment, the sheet was placed in a test mold comprising a sheet of glass at the bottom so that the film plastic covering the reinforcement. The packaging was made hermetically with the usual sealing compound. Then the mold was subjected to a vacuum of -0.95 bar to remove air for 10 minutes, after which epoxy resin with a viscosity of 300 mPas was introduced transversely into the reinforcing strands in the mold at an ambient temperature of 23° W. A graph was drawn recording the wetting distance that the resin advanced as a function of time.

[00100] Figure 4 illustrates the wetting distance that the resin traveled as a function of time. In the graph, the X axis shows the time used for impregnation and the Y axis the distance that the resin was able to advance. The position of the flow front normally follows the well-known Darcy's law, where the position is inversely proportional to the square root of the time. Thus, there is a certain maximum value, which can be infinitely approached, but never reached. The difference in permeability determines the actual distance from the flow front, i.e. the wetting distance, if other parameters such as viscosity and temperature are kept constant. Bottom graph A represents a standard biaxial reinforcement where there was no means of forming resin flow passage. It took about 31 cm for the resin to advance in 35 minutes. Graph B represents a biaxial reinforcement where the diameter of the highly twisted multifilament strands was 0.25 mm, and during the same time of 35 minutes the resin advanced approximately 56 cm. Graph C represents a biaxial reinforcement where the diameter of the highly twisted multifilament threads was 0.35 mm, with the resin advancing approximately 76 cm in 35 minutes. Graph D represents a biaxial reinforcement where the diameter of the highly twisted multifilament threads was 0.45 mm, with the resin advancing approximately 81 cm in the same 35 minutes. Graph E represents a biaxial reinforcement where the diameter of the highly twisted multifilament strands was 0.55 mm, and for the resin to advance 80 cm, it only took about 20 minutes. Graph F represents a biaxial reinforcement where the diameter of the highly twisted multifilament strands was 0.65 mm, and for the resin to advance 80 cm, it only took about 14 minutes. And graph E represents a biaxial reinforcement where the diameter of the highly twisted multifilament strands was 0.75 mm, and for the resin to advance 80 cm, it only took about 7 minutes. As can be seen, for example, in Figure 4, by doubling the diameter of the highly twisted multifilament yarn from 0.35 mm of Example C to the 0.65 diameters of Example F or to 0.75 of Example G reduces the Wetting time from about 38 minutes to 14 or 7 minutes. In other words, it appears that the wetting speed, in practice, on average, quadruples when the diameter of highly twisted multifilament yarns is halved. The experiments carried out also show that, if a long soaking distance is required, the use of relatively thick highly twisted multifilament yarns in accordance with the present invention reduces the soaking time to about a quarter compared to prior art stiffeners. . When compared with Example A, the wetting distance of 33 cm can be achieved in about 4 minutes with a diameter of 0.55, in about 3 minutes with a diameter of 0.65, and in about 2 minutes with a diameter of 0.75 mm. Another way to increase the wetting distance in a given period of time is to reduce the distance between adjacent highly twisted multifilament yarns to 5 mm, for example.

[00101] The above experiments clearly show the great advantage that the new design of having flow passages in two transverse or non-parallel directions causes. And as already discussed above, it is not only a matter of “high speed” infusion which increases production speed significantly, but also a matter of very efficient gas removal from the reinforcement stack ensuring void free laminate with no dry or semi dry areas. -impregnated, and a matter of a laminate that has better strength and fatigue properties than prior art laminates used for the same purposes.

[00102] The multiaxial reinforcement of the present invention can be used with all types of infusion methods, including, but not limited to, vacuum, RTM Light or RTM infusion methods. Other cases of lamination where resin impregnation is critical or otherwise retarded by well-arranged fibers or other materials are present in the laminate structure, such as sandwich materials, fire retardant materials, fillers, pigments, etc., where the viscosity of the resin can be extremely high, can be improved through the reinforcement of the present invention.

[00103] The multiaxial reinforcements of the present invention can be used in the manufacture of preforms or final products, that is, laminates such as, for example, wind turbine blades, boats, sports equipment, storage tanks, buses, trailers, trains and truck panels, etc. Preforms can be manufactured from at least two unidirectional reinforcement layers in themselves, placing the reinforcement layers one on top of the other so that their axial directions form an angle (for biaxial reinforcements preferably, but not necessarily +/- 45 degrees, +/- 60 degrees or 0/90 degrees), positioning the highly twisted multifilament yarns in at least two transverse directions between the reinforcing layers so that the direction of the highly twisted multifilament yarns does not be parallel to the axial dimension of the strands of the nearest reinforcement layer, sewing the multiaxial reinforcement, and finally using appropriate binder to join the reinforcement to form the preform.

[00104] In a similar way, a laminate can be manufactured from the multiaxial reinforcement of the invention or from the preform discussed above. In the laminate manufacturing method, at least two multiaxial reinforcements, or preforms, are placed on top of each other in the mold, a cover is positioned on the multiaxial reinforcements, the mold is closed, and a pressure difference is provided to evacuate the mold air and to impregnate the multiaxial reinforcements with resin.

[00105] The multiaxial reinforcement of the present invention can also be used in connection with the manufacture of sandwich laminates. Sandwich laminates are formed by at least one outer layer, which is arranged on a single-thickness face or central layer. Typically, these laminates, however, have two outer layers arranged on both opposite faces of a thick core or ply. The outer layer(s) is (are) formed by one or more multiaxial reinforcements of the present invention optionally arranged in connection with one or more other reinforcement layers. Preferably, but not necessarily, the multiaxial reinforcement of the present invention acts as an infusion medium, introducing resin into the entire area of the reinforcement to be absorbed by the other optional reinforcement layers. Such a sandwich laminate can be used on panels of buses, trucks, trailers or boats. In such laminates, the thickness or core layer disposed in connection with an outer layer or between the outer layers may be formed from at least one of PVC, PE and balsa foams. The multiaxial reinforcement of the present invention can also be used in structures where more than two layers of reinforcement are required, such as in bus or trailer floors or boat bottoms.

[00106] Another optional use can be found in laminated structures that replace the use of prior art screens. Screens are open mesh structures, which are positioned on one or both sides (top or bottom) of a stack of reinforcements in a mold. The purpose of the screens is to allow resin to be introduced quickly across the entire surface of the reinforcement, from where resin infusion into the entire reinforcement stack should occur. However, using screens has several disadvantages. Firstly, the screen must be removed from the mold before the resin can cure, which means manual labor, for example. Secondly, the screen once used cannot be used again as the resin cures on the screen. And thirdly, a considerable amount of resin sticks to the screen and is also wasted. Now, when placing the biaxial reinforcement of the present invention between layers of other reinforcements, it works like a screen, that is, it spreads the resin throughout the reinforcement like the screen does, but it does not have any of the weak points of screens, since which forms one of the reinforcing layers that can remain in the armor. The only compensation might be slightly heavier weight.

[00107] In addition to the attached claims, the present invention also covers the following aspects.

[00108] According to one aspect of the present invention, the highly twisted multifilament yarns of each set are arranged at a spacing of 2 to 50 mm from each other.

[00109] According to another aspect of the present invention, the highly twisted multifilament yarns 6 have an aspect ratio of less than 2, preferably less than 1.5.

[00110] According to another aspect of the present invention, the strands 20', 32' of the reinforcing layers 20, 32 are man-made or natural fibers, that is, fibers such as glass, carbon, aramid, basalt, linen, hemp, jute, linen.

[00111] According to another aspect of the present invention, highly twisted multifilament yarns are made from polyester (e.g., PET and PBT), polyamide (PA), co-polyamide or copolyester filaments (co-PET), generally man-made or natural polymers with a modulus of less than 10 GPa.

[00112] According to another aspect of the present invention, the first and second set 26, 26', 28 of highly twisted multifilament yarns form flow passages 10' for resin to the sides of the highly twisted multifilament yarns thereof. , and thereby facilitating, when wetting a pile of reinforcements 38 with resin, the flow of resin in directions transverse to the direction of the unidirectional strands 20', 32'

[00113] According to another aspect of the present invention, unreinforced wires having a modulus of less than 20 GPa, preferably less than 10 GPa, more preferably substantially the same as that of the resin used.

[00114] According to another aspect of the present invention, the multifilaments of the highly twisted multifilament yarns are twisted at least 100 turns per meter, preferably more than 150 TPM, more preferably more than 200 TPM.

[00115] Of course, the invention is not limited to the examples mentioned above, but can be implemented in many other different embodiments within the scope of the inventive idea. It is also clear that the features in each embodiment described above may be used in connection with the other embodiments whenever possible.

Claims (14)

1. Stitched multiaxial reinforcement for the manufacture of fiber reinforced composite by a resin transfer molding process and a vacuum infusion molding process, the stitched multiaxial reinforcement (38) characterized by the fact that it comprises at least: • a first reinforcing layer (20) of continuous unidirectional rovings (20') having a first axial direction, • a second reinforcing layer (32) of continuous unidirectional rovings (32') having a second axial direction, and • a first set ( 26) of highly twisted multifilaments formed from a bundle of filaments joined together by twisting the filaments at least 100 turns per meter or coated with an adhesive; • the first axial direction and the second axial direction leaving an angle between them; wherein • the first set (26) of bonded multifilaments is formed of highly twisted multifilament yarns positioned in the first reinforcing layer (20) at an angle different from the first axial direction, • a second set (28) of highly twisted multifilament yarns twisted being disposed between the first and second reinforcing layers (20, 32) in a second direction transverse to a first direction of the first set (26) of highly twisted multifilament yarns, • the highly twisted multifilament yarns are unreinforced yarns having a modulus of less than 10 GPa, and • the first and second reinforcing layers (20, 32) and the highly twisted multifilament yarns of the first set (26) and the second set (28) are connected to each other by means of seam (36) to form a sewn reinforcement, • in which the highly twisted multifilament threads form resin flow passages on their sides. 2. Sewn multiaxial reinforcement, according to claim 1, CHARACTERIZED by the fact that the second set (28) of highly twisted multifilament threads is arranged between the first and second reinforcement layers (20, 32) in the same direction than the first set (26) of highly twisted multifilament yarns. 3. Sewn multiaxial reinforcement, according to claim 1 or 2, CHARACTERIZED by the fact that the first and second sets (26, 28) of highly twisted multifilament threads are disposed between the first and second reinforcement layers (20, 32). 4. Sewn multiaxial reinforcement, according to claim 1 or 2, CHARACTERIZED by the fact that the sewn reinforcement comprises the first and second layers of reinforcement arranged on top of each other and sets of highly twisted multifilament threads on the outer surfaces of the reinforcing layers. 5. Sewn multiaxial reinforcement, according to claim 1 or 2, CHARACTERIZED by the fact that the sewn reinforcement comprises, in a direction perpendicular to the plane of the reinforcement, a first set of highly twisted multifilament threads, a first layer of reinforcement, a second set of highly twisted multifilament yarns and a second layer of reinforcement. 6. Sewn multiaxial reinforcement, according to claim 1, CHARACTERIZED by the fact that the highly twisted multifilament threads of the first set (26) and those of the second set (28) are transverse to the axial direction of the reinforcing strands forming the layer nearest booster (20, 32). 7. Sewn multiaxial reinforcement, according to claim 1, CHARACTERIZED by the fact that the highly twisted multifilament threads of the first set (26) and those of the second set (28) are parallel with the axial direction of the reinforcing strands forming the most remote reinforcing layer (20, 32). 8. Sewn multiaxial reinforcement according to any one of claims 1 to 7, CHARACTERIZED by the fact that the highly twisted multifilament threads of the second set (28) extend from an edge of the sewn multiaxial reinforcement (38) to the its opposite edge. 9. Sewn multiaxial or unidirectional reinforcement, according to any one of 1 to 8, CHARACTERIZED by the fact that the highly twisted multifilament threads of the first and second set (26, 28) are arranged at an angle of 5 degrees or more for the first and second axial directions. 10. Sewn multiaxial or unidirectional reinforcement, according to any one of claims 1, 2, 4 to 10, CHARACTERIZED by the fact that in the first direction and in the second direction they are arranged at an angle of 5 degrees or more from each other. 11. Sewn multiaxial reinforcement according to any one of claims 1 to 10, CHARACTERIZED by the fact that the highly twisted multifilament threads (6) have a diameter of 50 to 2000 μm, preferably between 100 and 1000 μm, and more preferably between 150 and 900 μm. 12. Sewn multiaxial reinforcement according to any one of claims 1 to 11, CHARACTERIZED by the fact that one (26') of the sets of highly twisted multifilament yarns has a different diameter from the other set (28) of highly twisted multifilament yarns. twisted. 13. Sewn multiaxial reinforcement according to any one of claims 1 to 12, CHARACTERIZED by the fact that one (26') of the sets of highly twisted multifilament threads has a diameter ranging between 50 and 1000 μm, preferably between 150 and 900 μm, more preferably between 200 and 800 μm. 14. Sewn multiaxial reinforcement according to any one of claims 1 to 13, CHARACTERIZED by the fact that the multiaxial reinforcement (38) having an upper surface and a lower surface, and a third set of highly twisted multifilament threads being arranged in at least one of the upper surface and the lower surface of the multiaxial reinforcement (38).