Patent Application
20060121805
1. Field of the Invention
The present invention relates to textiles. More particularly, the present invention relates to fabrics useful as reinforcement in composite structures and the resulting structures.
2. Description of the Related Art
A known method of forming reinforced plastics articles and composites is to lay a mat of a non-woven or woven glass fiber or other reinforcement and wet the mat of material with a synthetic resin material, to cure the resin, and to remove the molded article from the mould. When a greater thickness of reinforced composites material is required in the molded article, then further mats of reinforcing material are laid upon the first, wetted, and cured. Also, plates of reinforced composites materials may be formed suitable for cutting to form a desired article. Textile fabric materials useful as reinforcement in such composite structures typically are either woven or stitch-bonded fabrics, using yarns of fibers such as fiberglass, carbon, or aramid (Kevlar) fibers. To form a useful article, each layer of the textile material must be fully wetted by the synthetic resin material before curing to avoid the creation of voids in the article, reducing its strength and integrity.
The advantage of a woven fabric in composite structure is that the fabric is very pliable. This characteristic is advantageous when laying the fabric inside of an open or closed mold, where the resin is either applied or injected. The disadvantage of woven fabric in a composite structure is that a weave creates weak places in the yarn. This is due to how the yarns must go up-and-down in a weave forming crimps. These crimps create weak locations, in the finished composite structure.
For a non-woven fabric, such a stitch-bonded fabric, the yarns are non-crimped, i.e., there is no repeated up-and-down orientation of the yarn as in a woven fabric. The yarns in a non-woven, stitch-bonded fabric can be laid-out in a fabric machine direction (warp of 0 degrees), perpendicular to the machine direction (weft or filling of 90 degrees), or at a +45/−45 degree angle to the machine direction. The yarns in this type of fabric are non-crimped. The disadvantage, however, of this type of fabric is that the warp-knitted yarns may be pulled out during wetting with resin, thereby causing the 0 degree/90 degree/+45 degree/−45 degree yarns to fall apart. To overcome this problem the manufacturers of this type of fabric put addition warp-knitting yarn in place, however this inhibits the flow of liquid resin throughout the fabric to fully wet the fabric as required for a desirable composite structure.
It would be desirable to provide a non-woven fabric for reinforcement of composite structures wherein the non-woven yarns can be laid-out relative to the machine direction +90 degree/−90 degree wherein the yarns are held in place without stitch bonding with the physical integrity to avoid pulling out or attendant resin wetting problems.
Thus a non-woven, unidirectional, multi-axial fabric for reinforcement of composite structures solving the aforementioned problems is desired.
The non-woven, unidirectional, multi-axial layered fabric for reinforcement of composite structures of the present invention provides for holding the non-woven yarns as laid-out relative to the machine direction of +90 degree/−90 degree by adhesion of polymeric adhesive applied to the non-woven yarns. The adhesive layer on the yarns dissolves as liquid resin is applied to form a composite structure, the polymeric coating dissolving in the liquid resin. The polymeric adhesive dissolves to allow the resin to wet the yarns, forming new bonds with the yarns and curing to create the desired composite structure. The polymeric adhesive coating is directly applied to first web of yarns by applying the adhesive to the yarn in a finish bath, applying heat to the filament yarn to cure the polymeric coating and cooling to form a cured coating directly on the filament yarn. Then a second layer of polymeric adhesive coated filament yarn is laid on top of the first layer of filament yarn and heat is applied to form a +90 degrees/−90 degrees layer of adhesive film-covered filaments forming layered yarn fabric.
During the resin application process, the resin penetrates the filament fabric layers and uniformly dissolves in the resin, as the filament fabric layers become part of the hardened cured resin matrix. Useful polymeric adhesive materials include but are not limited to low molecular weight acrylic, polyester or polyurethane for the finish bath.
Filament yarns useful in the present invention include but are not limited to those made of aramid, boron, carbon, fiberglass, nylon, PBO, PEN, polyester, and polyethylene.
A web of netting material is applied in a similar manner for added physical stability of the inventive reinforcement fabric. The web of netting material may be applied to the upper surface of the second web and the lower surface of the first web.
In some applications, a single web of coated filament yarn cured with the netting material web is adequate as a composite reinforcement fabric and is within the scope of the present invention.
In some applications, the first and second coated filament yarn webs may be cured and employed as a reinforcement fabric without the netting web and is within the scope of the present invention.
When manufacturing a composite structure, a bottom layer of the inventive reinforcement fabric is laid in a mold and liquid resin is applied. As the resin penetrates the reinforcement fabric (wet out), another layer of the reinforcement fabric is applied over the bottom layer and more liquid resin is added to wet this reinforcement fabric layer. This process is repeated, until the desired thickness is achieved. The netting holds the top layer of filament yarn in place as the liquid resin is applied and penetrates the fabric. When the next layer of reinforcement fabric is added, the bottom layer of filament yarns is held in place between netting material and the newly applied layer of filament yarns.
The netting material is preferably made of fiberglass filament yarns and the netting material held together by a polymeric adhesive coating that also dissolves in the liquid resin. The netting material has negligible effect on the hardened resin matrix composite product.
It is an aspect of the invention to provide improved reinforcement fabric materials and composite products thereof for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.
These and other aspects of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The present invention is non-woven, unidirectional, multi-axial layered fabric for the reinforcement of composite structures. The term non-woven, unidirectional, multi-axial layered fabric is defined as adhering layers of non-woven fabric having filament yarns laid at relative angles, for instance, 0 degrees/90 degrees relative to a fabric-making machine.
The inventive reinforcement fabric employs a first web of coated, of non-woven, unidirectional fabric webs of filaments, the coating being of a low molecular weight polymer adhesive material. The fabric has a second layer of non-woven, unidirectional fabric of filaments having a coating of the same adhesive material upon the filaments thereof and overlaid at an axial angle relative to the first layer of non-woven, unidirectional fabric, the layers of fabric being held by the adhesive between the first layer of fabric and the second layer of fabric. The axial angle is preferably about 90 degrees. Filaments useful in the non-woven, unidirectional fabric include filaments of boron, carbon, and fiberglass, and aramid, nylon, PBO, PEN, polyester and polyethylene polymers. The adhesive film on each layer of non-woven, unidirectional fabric of filaments is formed by treating in a finish bath of a low molecular weight polymer of acrylic, polyester, or polyurethane polymers.
The fabric has a first layer of filament netting overlaid on the second layer of non-woven, unidirectional fabric. The netting layer adheres to the second layer by the adhesive coating applied to the second layer. Upon application of a liquid resin to the non-woven, unidirectional, multi-axial fabric, the liquid resin dissolves the polymer adhesive coating and wets the filaments of the first and second layers of fabric. The resulting fabric is heated to drive off water or other solvent from the finish bath by heating lamps or rollers and then cooled and rolled to form the product inventive reinforcement fabric. The resulting reinforcement fabric exhibits a high degree of wetting by liquid resin when is applied thereto, thus resulting in a superior composite product. Due to the layer or layers of netting, the resulting reinforcement fabric also exhibits physical strength and stability(pulling of fibers) during its manufacture, handling, and during lay-up with resin to form a composite material.
The fabric thus formed may lack physical integrity sufficient to avoid pulling, torqueing, or unraveling of the filaments during application of liquid resin by ordinary means, such as by rollers, to manufacture products. Known methods of retaining the integrity of the non-woven fabric include cross-stitching or knitting which inhibits the wetting of the filaments of the fabric with resin and thus weakens the resulting composite.
The physical integrity of the inventive reinforcement fabric is preferably increased by applying a netting layer over at least one of the upper and lower surfaces of the fabric before the drying step. The netting material acts as a carrier and to assist in the holding-down of the side-by-side yarns during the resin application process. The netting is held in place by the tackiness of the wet polymer film before drying and is bonded to the fabric surfaces during the heating step for drying, and the cooling step, resulting in the finished composite reinforcement fabric product.
The netting material is a woven or non-woven filament fabric formed by knitting of or adhesion between filaments of yarns employing an adhesive film of a low molecular weight polymer of acrylic, polyester, or polyurethane polymers, which will dissolve in a corresponding resin. The spacing of the yarns in the netting material may be within the range of about 5 mm to about 75 mm. The netting material fabric is laid at angles ranging from 0 degrees to 90 degrees with respect to the second layer of non-woven, unidirectional fabric, and preferably at an average angle of about 45 degrees with respect to said second layer of non-woven, unidirectional fabric.
The reinforcement fabric of the present invention may include a second netting layer of filaments laid on the underside of the first layer of non-woven, unidirectional fabric, the netting layer adhering to the second layer by the thin adhesive film thereon. The netting layers of filaments preferably include filaments of fiberglass.
A preferred method of forming the non-woven, unidirectional, multi-axial fabric for the reinforcement of composite structures of the present invention include:
(1) Placing the high-tenacity filament yarns in a creel stand or warp beam;
(2) Drawing the filament yarns through a reed;
(3) Through tension, laying the individual filaments from the reed side-by-side to form a first web of reinforcement material;
(4) treating the first web of reinforcement material in a finish bath forming a thin, tacky adhesive polymeric film thereon;
(5) drawing netting fabric against the web of reinforcement material on either the upper or lower side of the first web forming a unidirectional fiber/netting web;
(6) drawing the resulting unidirectional fiber/netting web over a heating element to flash-off solvent and set the polymeric film to form a product unidirectional filament fabric;
(7) drawing the product fabric through a cooling zone; and
(8) winding the product fabric onto a primary roll.
If desired, the following steps may be taken to form a multi-axial reinforcement fabric material:
(9) placing the primary roll across a second web of side-by-side reinforcement yarns such as the product of step 4, above and repeating the heating and cooling steps of steps 6 and 7, above.
(10) winding the product fabric onto a product roll.
If desired, an additional web of netting fabric may be added by employing a unidirectional fiber/netting web obtained as a product of step (5) above as the second web in step (9) and the resulting product fabric onto a product roll.
Preferably, the netting fabric is added on the upper side of the upper unidirectional fiber/netting web and on the lower side of the second web of step (9) to form a multi-axial, unidirectional product reinforcement fabric having netting webs on both the upper and lower surfaces thereof. The second netting layer of fiberglass filaments provides additional physical stability, the netting layer adhering to the second underside of the first layer by tackiness of the thin film of polymer thereon.
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In the case of composite reinforcement fabrics employing fiberglass or carbon filament yarns or the like, a heated calendar roll may be employed for the handling of the fabrics since a severe angle around a small roller useful with a heating zone would damage those types of yarn. For these types of yarn, a flat heated surface is useful as a heating zone.
A finished composite article is constructed by (1) laying a first web portion of the non-woven, unidirectional, multi-axial fabric of the present invention on a supporting mold (2) applying a liquid resin of a polyester, epoxy, styrene, or vinyl ester to the first web portion so as to uniformly wet the fabric, (3) overlaying a second web portion of the non-woven, unidirectional, multi-axial fabric of the present invention upon the first web portion, (4) applying liquid resin to the second web portion so as to uniformly wet said fabric, (5) repeating the overlaying and liquid resin application steps to form a laid-up article of the thickness desired, and curing the laid-up article to form a finished composite article.
Alternatively, layers of pre-impregnated, dried and cooled intermediate webs may be cut as appropriate and laid at 0 degrees/90 degrees on a heated surface to form a bi-axial finished reinforcing fabric. This method is particularly useful for relatively brittle yarn filaments such as fiberglass and carbon filaments.
An example of a polymer and liquid bath is Eastmans's WD-30 polyester. Polyester solids at a wt. percent level of 30% are suspended in water to form the polymer/liquid bath. These polyester solids are compatible with polyester resins and dissolve therein in the practice of the present invention. Other polymers which may be useful include epoxy polymer/liquid baths and epoxy resins, and phenolic polymer/liquid baths and phenolic resins.
Two types of fabric were manufactured on a pilot machine of van Wees, Tilburg, The Netherlands. The main characteristic of the pilot machine is that it has a 1,700 mm wide×541 mm diameter heated drum. The yarns were drawn from a creel, through a reed, through an impregnation roller(the polymer bath) for a adding a binder, and onto a heated Teflon-coated drum that was wrapped with the carrier. The yarns advanced by pitch (depending on the width of the yarn) as the yarns complete one revolution around the drum, and the individual filaments of the yarns laid side-by side until the yarns wrapped completely around the drum. The yarn/fabric was then cut perpendicular to the wrap around the drum, turned ninety, degrees, and re-attached to the drum. The process was than repeated to give a 0 degree/90 degree fabric. A table of process parameters follows:
As can be seen from the table above, S/295 (which is the equivalent in weight and construction to the common fiberglass woven fabric, S/7782) did not have binder added to the yarns: only the pre-impregnated web (carrier) held the yarns together. This was done not only to demonstrate that binders do not have to necessarily used, but it was discovered that the binder was incompatible to the yarn finish and would not wet-out properly when the resin was applied. This demonstrated that a binder resulting in compatible yarn finishes must be chosen for specific resins.
In the case of actual manufacturing, the heated drum from the pilot process may be replaced by a heated calender or flat, heated Teflon-coated surface to flash-off the water from the binder.
In some applications, a single web of coated filament yarn cured with the netting material web is adequate as a composite reinforcement fabric and is within the scope of the present invention.
In some applications, the first and second coated filament yarn webs may be cured and employed as a reinforcement fabric without the netting web and is within the scope of the present invention.
In some applications, more than two coated filament yarn webs within a range of 0 degrees/90 degrees relative to each other may be cured employed as a reinforcement fabric and is within the scope of the present invention.
The filament yarns employed in the present invention preferably posses a tenacity exceeding 7.0 grams per denier, and are uncrimped.
Examples of articles which may be produced according to the present invention include sports equipment such as rackets and boat hulls, automobile panels, and aircraft wings and fuselage.
The laid-up article may be cured by any conventional means such as vacuum bag pressure and heat, or pressure autoclave to form the finished composite article.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
