Background The present invention provides improved prepreg tiles useful in making tools

(also called molds) for forming structural parts for vehicles and other applications.

Tiles are composites of reinforcing fiber and resin. The reinforcing fibers are insoluble within the resin matrix, generally acting to stiffen and strengthen and thus reinforce the resin matrix after curing. The combined physical and chemical properties of the matrix resin and the reinforcing fibers in composite materials are generally such that when the combination is cured, the resultant composite articles have considerable strength and relatively light weight characteristics which enable such components to find many applications in many industries including aerospace, vehicles, and marine applications. The resin(s) and fiber(s) are carefully chosen to produce a composite material and article with desired characteristics.

Prior to the present invention, tiles were manufactured using a square edge. Referring to FIGS.s 1 a and 1 b and Comparative A below, tiles 10 comprise eight layers 12 with a square edge 14. Two tiles 10 were contacted to form a butt joint 16. However, this approach resulted in no connectivity between the tiles when making a tool.

Tiles 20 comprising eight layers 22 were then manufactured with a 18.4 degree cut in order to increase overlap and connection between tiles 20. Referring to FIGS.s 2a and 2b and Comparative B below, tiles 20 were cut at 18.4 degrees and contacted to form joint 24. However, this approach wasted valuable tile material and provided only limited connectivity between tiles.

Tiles 30 comprising eight layers 32 were then manufactured with an overlap to increase connection between tiles. Referring to FIGS.s 3a and 3b and Comparative C below, one tile 30 overlapped another tile 30 to form overlap joint 34. However, this approach also wasted valuable tile material and produced undesirable tool thickness variation which adds significant cost in custom alterations for any backing structure attachment. Thus, the tool manufacturing industry needs a tile with edges providing increased connectivity between adjacent tiles.

Summary

The present invention provides prepreg tile comprising at least two adjacent fiber-reinforced resin layers, arranged one on top of another, wherein each of the layers has an edge that forms an offset stagger relative to the adjacent layer. The present tiles are useful in making tools (also called moulds) for forming structural parts for vehicles and other applications.

Brief Description of the Drawings

FIG. 1 illustrates a known prepreg tile butt joint. FIG. 2 illustrates a known prepreg tile 18.4 degree joint.

FIG. 3 illustrates a known prepreg tile overlap joint. FIG. 4 illustrates the present prepreg tile.

FIG. 5 illustrates the present preprep tile.

Detailed Description

The phrase "offset stagger" as used herein means that the edges of adjacent layers of a tile are offset from each other by a distance. The preferred offset distance is at least about 5 mm, and in some embodiments, from about 5 mm to about 10 mm.

Fibers: The prepregs in the tile are composite materials comprising reinforcing fibers and resin. The reinforcing fibers are made of carbon, glass, aramid, ceramic, or other suitable materials. The reinforcing fiber may be made of a blend of at least two of the preceding materials. The reinforcing fiber may also be a combination of two fibers made of different materials. The reinforcing fiber may be continuous, discontinuous, unidirectional, twisted, and/or intertwined.

The fibers in a reinforcing fiber layer are preferably in the form of continuous fibres, filaments, tows, bundles, sheets, plies, or combinations thereof. The precise specification of the fibers, for instance their orientation and/or density, can be specified to achieve the optimum performance for the intended use of the prepregs. Continuous fibers may adopt any of unidirectional (aligned in one direction), multidirectional (aligned in different directions), non-woven, woven, knitted, stitched, wound, and braided configurations. Woven fibre structures may comprise a plurality of woven tows, each tow composed of a plurality of filaments, e.g. thousands of filaments. In further embodiments, the tows may be held in position by cross-tow stitches, weft-insertion knitting stitches, or a small amount of resin binder, such as a thermoplastic resin. In one preferred embodiment, the layer(s) of reinforcing fibres used in the present invention comprise woven fibre structures comprising a plurality of woven tows arranged substantially orthogonally. In a further preferred embodiment, the layer(s) of reinforcing fibres used in the present invention comprise fibre structures wherein the fibres are arranged unidirectionally. In a further preferred embodiment, the layer(s) of reinforcing fibres used in the present invention comprise fibre structures wherein the fibres are arranged in other orientations, such as tri-axial wherein fibres are arranged in three directions, such as 0°, +60°, -60.°

The reinforcing fibers are preferably selected from, but not limited to, fibers of glass (including Electrical or E-glass), carbon (particularly graphite), aramid, polyamide, high-modulus polyethylene (PE), polyester, poly-p-phenylene- benzoxazole (PBO), boron, quartz, basalt, ceramic, and combinations thereof. Carbon fiber is particularly suitable. For the fabrication of high-strength composite materials, e.g. for aerospace and automotive applications, it is preferred that the reinforcing fibres have a tensile strength of greater than 3500 MPa.

Resin: The curable resin may be selected from curable thermosetting resins conventionally known in the art. The formulation of the curable resin can be specified to achieve the optimum performance for the intended use of the tiles. The resinous material used in the present invention is preferably a curable resinous material. The resinous material may comprise thermoset resin such as at least one of bismaleamide ("BMI"), cyanate ester, epoxy, polybenzoxazine, phenolic resin, or vinyl ester. Suitable curable resins may be selected from the group consisting of an epoxy resin, an addition-polymerisation resin (for instance a bis-maleimide resin), a benzoxazine resin, a formaldehyde condensate resin (especially a formaldehyde- phenol or urea-formaldehyde resin), a vinyl ester resin, resins of, 1 ,3,5-triazine-2,4,6- triamine (melamine), a cyanate resin, an isocyanate resin, a phenolic resin and mixtures of two or more thereof. Preferably the curable resins are selected from epoxy, phenolic or cyanate ester resins, particularly epoxy and phenolic resins, and particularly epoxy resins. An epoxy resin is preferably an epoxy resin derived from the mono or poly-glycidyl derivative of one or more of the group of compounds consisting of aromatic diamines, aromatic monoprimary amines, aminophenols, polyhydric phenols, polyhydric alcohols, polycarboxylic acids and the like, or a mixture thereof. Examples of addition- polymerisation resins are acrylics, vinyls, bis- maleimides, and unsaturated polyesters. Examples of formaldehyde condensate resins are urea, melamine and phenols.

The particularly preferred epoxy resins may be monofunctional, difunctional, or multifunctional epoxy resins. As used herein, the term "multifunctional" epoxy resin is a resin which has a functionality of greater than two. Preferred multifunctional resins are at least trifunctional, typically trifunctional or tetrafunctional, although epoxy resins having greater functionality may also be used, for instance those having 5 or 6 epoxy groups. The term "multi-functional" encompasses resins which have non-integer functionality, for instance epoxy phenol novolac (EPN) resins, as known in the art. The epoxy resin may comprise monofunctional, difunctional and/or multifunctional (typically trifunctional or tetrafunctional) epoxy resins. Preferably the curable resin comprises one or more difunctional epoxy resin(s) (and preferably at least two difunctional epoxy resin(s)) optionally in combination with one or more multifunctional (typically trifunctional or tetrafunctional) epoxy resin(s). In a preferred embodiment, the curable resin comprises one or more difunctional epoxy resin(s) (and preferably at least two difunctional epoxy resin(s)) optionally in combination with one or more trifunctional epoxy resin(s) and/or one or more tetrafunctional epoxy resin(s). In a further preferred embodiment, the curable resin comprises one or more multifunctional epoxy resin(s) (typically trifunctional and/or tetrafunctional).

Suitable difunctional epoxy resins include those based on: diglycidyl ether of Bisphenol F, Bisphenol A (optionally brominated), phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, glycidyl ethers of aliphatic diols, diglycidyl ether, diethylene glycol diglycidyl ether, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidised olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof. Difunctional epoxy resins are preferably selected from diglycidyl ether of Bisphenol F (DGEBF), diglycidyl ether of Bisphenol A (DGEBA), diglycidyl dihydroxy naphthalene, or any combination thereof.

Suitable trifunctional epoxy resins, by way of example, may include those based upon phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, aromatic epoxy resins, aliphatic triglycidyl ethers, dialiphatic triglycidyl ethers, aliphatic polyglycidyl ethers, epoxidised olefins, brominated resins, triglycidyl aminophenols, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof.

Suitable tetrafunctional epoxy resins include Ν,Ν,Ν',Ν'-tetraglycidyl diamino diphenylmethane (TGDDM) and N,N,N',N'-tetraglycidyl-m-xylenediamine.

Thus, an epoxy resin may be selected from Ν,Ν,Ν',Ν'-tetraglycidyl diamino diphenylmethane (e.g. grades MY 9663, MY 720 or MY 721 ; Huntsman); Ν,Ν,Ν',Ν'- tetraglycidyl-bis(4-aminophenyl)-1 ,4-diiso-propylbenzene (e.g. EPON 1071 ; Shell Chemical Co.); N,N,N',N'-tetraglycidyl-bis(4-amino-3,5-dimethylphenyl)-1 ,4- diisopropylbenzene, (e.g. EPON 1072; Shell Chemical Co.); triglycidyl ethers of p- aminophenol (e.g. MY 0510; Ciba-Geigy); diglycidyl ethers of bisphenol A based materials such as 2,2-bis(4,4'- dihydroxy phenyl) propane (e.g. DE R 661 (Dow), or Epikote 828 (Shell)) and higher molecular weight diglycidyl ethers of bisphenol A based materials such as those with an epoxy equivalent weight of 400-3500g/mol (e.g. Epikote 1001 and Epikote 1009), and Novolak resins preferably of viscosity 8- 20 Pa s at 25°C; glycidyl ethers of phenol Novolak resins (e.g. DEN 431 or DEN 438; Dow); diglycidyl 1 ,2-phthalate (e.g. GLY CEL A-100); or diglycidyl derivative of dihydroxy diphenyl methane (Bisphenol F) (e.g. PY 306; Ciba Geigy). Other epoxy resin precursors include cycloaliphatics such as 3',4'-epoxycyclohexyl-3,4- epoxycyclohexane carboxylate (e.g. CY 179; Ciba Geigy) and those in the "Bakelite" range of Union Carbide Corporation.

The resinous material may also comprise thermoplastic resin including polymers and co-polymers such as at least one of polyolefins including polymethylene, polyoxymethylene, polyethylene, polypropylene, high density polyethylene, and low density polyethylene; polystyrene; acrylonitrylstyrene; acrylonitrile-butylacrylate-styrene polymers; butadiene; polyvinyl chloride; acrylics; polyarylene ethers; polysulfones including polyethersulphone, polyphenyl sulfone, and polysulfone; polyphenylene sulfide; polyetherimide, polyetheretherketone, polyaryletherketone, polyphenylene sulphide, polysiloxane; polyimides; liquid crystal polymer; polycarbonate, polyphenylene oxide; styrene maleic anhydride; polyamides including polyamide-imide, polycaprolactam, polyphthalamide, nylon 12, nylon 66, nylon 6/6, nylon 6/6/t, nylon 4/6, and nylon 10/10; polyesters including polyethylene terephthalate, polybutylene terephthalate, and polycyclohexylene- dimethylene terephthalate; styrene maleic anhydride; polyacrylonitrile; polyoxymethylene (polyacetal); or various types of rubbers or elastomers including thermoplastic elastomers, polyisoprene, polybutadiene, polyisobutylene, polychloroprene, butadiene-styrene, butadiene-acrylonitirile or silicones, polymethylmethacrylate; and blends of the preceding with each other. Prepreg Tile Formation: A prepreg may be manufactured by any suitable technique known in the art, such that the curable resin is contacted with the fibrous reinforcing agent under conditions of temperature and pressure sufficient to cause the curable resin to flow and infuse or impregnate the fibers. The term "impregnate" refers to the introduction of a curable resin composition to reinforcement fibres so as to introduce the curable resin between the interstices of the fibres and/or fully or partially encapsulate the fibres. Thus, the pre-preg is prepared by the general method of: (a) providing a dry fibre layer of reinforcing fibers, and (b) impregnating the dry fiber layer with a curable resin. The individual prepreg layers are cut to size and laid up such that the edge of each layer is offset from the edge of the adjacent layer by a distance, resulting in a tile with an offset stagger. At least two resin- impregnated layers are used in the present prepreg tile. Preferably, the number of resin-impregnated layers in the present prepreg tile ranges from 2 to 16. More preferably, the number of layers in the present prepreg tile ranges from 5 to 8.

The present prepreg tile having an offset stagger provides a shortened process time and less scrapped material because connections between tiles have been incorporated into the tile and no additional machining or trimming of the tile is required. With the speed and low cost required by the automotive industry, this invention is particularly attractive for automotive applications. The present prepreg tile may be conveniently supplied in flat packages rather than the current products supplied in roll form.

Prepreg Use: To form a molded article, a plurality of the present prepreg tiles is laid up into or onto a mold (often referred to as molding tool) in a stacking arrangement to form a prepreg lay-up. The prepreg tiles within the lay-up may be positioned in a selected orientation with respect to one another, e.g. 0°, 45 ^° , or 90°. The prepreg tiles comprise multiple layers of woven or unidirectional fibers that are impregnated with resin. Each of these individual layers used in the tile may contain fibers oriented at a selected angles, e.g. 0°, 45 ^° , or 90°, with respect to the largest dimension (typically defined as the length) of the layup. Once in place, the prepregs in the lay-up are cured as described hereinbelow.

The present invention provides increased connectivity between prepreg tiles during lay-up without restricting the tiles' ability to conform to the tool geometry and to allow load transfer between the tiles in the cured structure. As a result, the present invention reduces the risk of poor prepreg tile layup, reduces the prepreg tile layup time, and reduces prepreg tile scrap and prepreg tile cutting time.

After the desired or pre-determined number of prepregs has been laid in or on the mold, the plurality of prepregs are cured, preferably thermally cured. Curing is preferably effected while the prepregs are located in or on the mold, and preferably while the prepregs are compressed in a mould cavity, preferably a heated mould- cavity, preferably an isothermally heated mould cavity. Thus, as described above, curing is preferably effected in a press-molding process where the mold tool is at a fixed temperature (isothermal tooling). In the present invention, thermal curing is conducted at a cure temperature of at least 100°C, preferably at least 120°C, preferably greater than 120°C, preferably at least 125°C, preferably at least 130°C, and preferably in the range of from about 130°C to about 150°C. Preferably, thermal curing is conducted using a cure cycle having a duration of no more than 30 minutes, preferably no more than 15 minutes, preferably no more than 10 minutes, preferably no more than 5 minutes. The cure cycle duration as defined herein is the period for which the plurality of prepregs is subjected to the pre-determined cure temperature. The cure cycle duration does not include the ramp phase or the cool-down phase. As noted hereinabove, the present invention is primarily directed to press-moulding processes where the mould tool is at a fixed temperature (isothermal tooling), and heats the prepregs as rapidly as possible.

In an alternative embodiment, thermal curing may be conducted in an oven or autoclave, and may be conducted under vacuum (for instance in a vacuum bag as known in the art), suitably conducted at elevated pressure, for instance at a pressure of from about 2 to about 10 bar. In this embodiment, the cure temperatures and cure cycle durations described hereinabove are also applicable, but typically the heating and cooling rates are controlled. Preferably, the heating rate during the ramp phase is from about 1 to about 5°C/min, preferably from about 1 to about 3°C/min. Preferably, the cooling rate in the cool-down phase is from about 1 to about 5°C/min, preferably from about 1 to about 3°C/min to 60°C.

Molded Tool Use: The molded tools are suitable as forms for component fabrication for aerospace, automotive, and other industrial applications. The tools prepared by the present invention are particularly suitable as low to mid- volume parts, in which cost, speed, and reproducible quality of production are paramount. The present invention provides a process in which the cutting and handling of prepreg is greatly simplified, providing advantages of efficiency and economy. The layup time according to the present invention is significantly reduced, allowing a reduction in the tool cost and allowing for faster transition from tool fabrication to component fabrication, or shorter lead time for tooling availability so production can be initiated more quickly.

The following non-limiting examples serve to illustrate the present invention.

Comparatives and Inventive Examples For the Comparatives and Inventive Examples, a 380 gsm carbon fabric was used with bismaleamide. Eight layers were used in each tile and the thickness of each tile was 50 mm. Each tile was cured at 6 hours at 180°C at 6 bars.

Each prepreg tile was tested for tensile and flexural performance. For the tensile testing, the width of the tested fabric was 25 mm, the length of the tested fabric was 238 mm, the gauge length was 138 mm, the modulus was 0.1 to 0.3 % strain, and the displacement rate was 2mm/min. For the flexural testing, the width of the tested fabric was 10mm ± 0.2, the length of the tested fabric was 100mm ± 1 , the span was 80mm ± 0.5, the upper radius was 12.5mm ± 0.1 , the lower radius was 5mm ± 0.1 , the modulus was 10-50% Fmax, and the displacement rate was 5mm/min. For flexural testing, the center of the joint was aligned with the center of the test span and the central loading point was located on the flat (or tool side) face of the tile. The results are in the Table below. Comparative A prepreg tiles are shown in FIGS.s 1 a and 1 b. FIG. 1 a is a one dimensional view of tiles 10 comprising eight layers 12 with a square edge 14. Two tiles 10 form a butt joint 16. FIG. 1 b is a two dimensional view of FIG. 1 a.

Comparative B prepreg tiles are shown in FIGS.s 2a and 2b where a knife was used to cut the tiles. FIG. 2a is a one dimensional view of tiles 20 comprising eight layers 22 cut at 18.4 degrees. Two adjacent tiles 20 form a 18.4 degree joint 24. FIG. 2 b is a two dimensional view of FIG. 2a.

Comparative C prepreg tiles are shown in FIGS.s 3a and 3b where overlap occurs. FIG. 3a is a one dimensional view of tiles 30 comprising eight layers 32 and one tile 30 overlaps another tile 30 to form overlap 34. The overlap distance is 50 mm. FIG. 3b is a two dimensional view of FIG. 3a. As reported in the Table, the mechanical properties are not as favorable as those achieved with Inventive Example 1 .

Inventive Example 1 prepreg tiles are shown in FIGS. 4a and 4b. FIG. 4a has eight layers 42 in tile 40 where the stagger offset of adjacent layers is shown to be 6 mm. The total stagger offset for tile 40 is 42 mm. FIG. 4b is a two dimensional view of FIG. 4a. As shown in Table 1 below, Inventive Example 1 has superior mechanical properties compared with Comparatives A, B, and C. The present invention advantageously provides both edge and in-plane layer by layer contact to yield a substantial mechanical improvement.

Inventive Example 2 prepreg tiles are shown in FIG. 5 similar to FIG. 4 with a gap between tiles as such may happen in a manufacturing facility rather than the preferred Inventive Example 1 . TABLE 1

It is to be understood that various modifications and variations of the present invention will become apparent to those skilled in the art upon reading the specifications and the disclosed invention is intended to cover such modifications and variations as fall within the scope of the claims and equivalents.