Fiber, particle and/or aggregate reinforced composites have found a broad utility in structures requiring high strength-to-weight ratios. Resin reinforced composites used in such products have usually been produced by forming a lay-up, e.g., layers or piles of particles, aggregates and/or unidirectional or multi-directional fabrics made of glass or graphite fibers, impregnated with resin and cured. This resin impregnation is generally done by either a wet lay-up or dry lay-up technique. In the wet layup process the reinforcement materials are pre-impregnated with a resin and curing catalyst and partially cured.
Such "prior composites" are positioned upon a forming tool or a mold surface which, in its simplest form may comprise a flat plate. In the dry lay-up method, the fiber reinforcement is laid up dry on a forming tool or a mold surface and the resin may be applied by conventional methods know to the art, e.g., brushing, spraying or coating, etc.
After lay-up is prepared, heat and pressure are applied. The heat cures the resin and the pressure compresses the layers, thereby preventing air and other gases, and volatiles, from forming voids within the composite as the resin cures. The curing process is generally carried out in an autoclave.
While monolithic structures formed of fiber reinforced-resin composites processed in the manner described above are satisfactory for some environments, for other environments, they have certain disadvantages. For example, in using the fibers pre-impregnated with resin, gases trapped between the layers when the resin is being cured form weakening voids in the resultant monolithic structure.
Another disadvantage with prior composites is the need to store such materials at low temperatures to prevent curing prior to use. Conventionally, pre-impregnated fiber reinforced layers. which are usually in the form of relatively wide tape or fabric on rolls prior to being laid up, are stored in a refrigerated environment. A further disadvantage is that, even at low temperatures, the resin will cure and-may become unstable and must be discarded.
The disadvantages in the dry lay-up process include the requirement of more resin and greater assembly times. The manner of resin application by brushing or spraying wastes resin and time by requiring the removal of excess resin before the curing process. Additionally, some manual smoothing may be necessary and the sticky resin makes it difficult not to avoid displacement of the reinforcement fibers, thereby adding to production delays.
Vacuum bags techniques are well known in the prior art and encompass both the dry lay-up and dry lay-up processes. After the reinforced fiber is laid up on a forming tool or mold surface, a flexible gas impervious sheet, liner or bag is used to form a sealed vacuum envelope over the lay-up, a heat curable or peroxide catalyzed resin is introduced into the envelope and a vacuum is drawn on the interior envelope space. The vacuum induces an internal collapse or pressure of the film envelope against the mold surface and forces the fiber mat or fabric to follow the mold pattern and pushes or pulls out voids. Thereafter, heat is applied to cure the resin.
The internal collapse of the vacuum envelope restricts the resin from freely flowing through the fiber mat or fabric which has a tendency to trap air or vapors between the vacuum film envelope and the composite structure so as to result in low reinforcing fiber-to-resin ratio and non-uniformity. This reduces production rates and increases production failures and costs.
Some of the presently known vacuum bag techniques avoid several of the above discussed problems by employing a breather fabric with a plastic film which is positioned between the dry lay-up and the inside of the vacuum bag and barrier materials to prevent resin from reaching and plugging the vacuum lines in the bag. The breather bag functions to prevent the outer bag from collapsing completely on the lay-up.
Another approach to preventing bag closure is disclosed in U.S. Pat. No. 4,902,215 to Seeman, which is incorporated herein by reference. Seeman relates to a resin distribution medium comprising spaced-apart plastic monofilaments which are non-resin absorptive and run in a criss-cross fashion and an open array of separated raised segments providing vertically oriented spaced-apart apart props or pillars to prevent closure between the inner face of the flexible sheet and the upper surface of the dry lay-up. The open pillar-like structure and lateral openings between these pillars provide channels for resin to flow over the entire distribution medium without an entrainment of air or volatiles. Both the breather fabric and the reusable resin distribution system of Seeman require additional expense in equipment and production time.
Another approach to improving vacuum bag resin transfer molding techniques involves an improved vacuum bag. U.S. Pat. No. 5,129,813 to Shepherd, also incorporated herein by reference, discloses a non-porous material having a three dimensional pattern defining a plurality of interconnected channels which partially collapses upon the evacuation of the vacuum bag and causes the three-dimensional pattern to relax into the locally flat two dimensional configuration with interconnecting channels. The completely evacuated vacuum bag is in direct contact with the entire surface of the lay-up and the interconnected channels provide free flow of resin and avoids entrapment of gas and volatiles pockets. The flexible film used in making the vacuum bag also can be reused.
In yet another alternative approach, U.S. Pat. No. 4,942,013, to Palmer et al, incorporated herein by reference, teaches a process and system for vacuum impregnation of a fiber reinforcement, such as carbon cloth, with a resin to produce a resin-fiber composite. Liquid resin enters into an arrangement or system comprising a fiber reinforcement layer uniformly across the width thereof and along the length of the fiber reinforcement layer. A fiber reinforcement layer is placed on a tool. A porous parting film is applied over the fiber reinforcement layer and a bleeder layer, such as polyester or nylon, is applied over the parting film. A non-porous film is placed over the bleeder layer, a breather cloth, such as fiberglass, is then applied over the non-porous film, and a vacuum bag is place over the entire assembly and sealed to the mold surface. Liquid resin is fed to the assembly within the vacuum bag. An assembly for spreading the resin receives the liquid resin and distributes it initially across the panel adjacent one end of the reinforcement layer after the vacuum has been applied to the assembly. The liquid resin is drawn through the bleeder cloth and through the fiber reinforcement layer from one end to the opposite end thereof to completely impregnate it. The resin catalyst system is designed so that it will commence to gel the resin when the liquid resin has completely impregnated the fiber reinforcement layer.
U.S. Pat. No. 4,132,755, to Johnson, also incorporated herein by reference, teaches another alternative technique for the manufacture of a composite article. A permeable reinforcing material, such as fiberglass, is deposited on a mold or on a structure to be reinforced. A preferably flexible sheet of perforated material is then placed over the reinforcing material, such that the marginal areas of this sheet are preferably sealed to the mold so as to define an outer chamber. The inner chamber is effectively connected to a vacuum source so as to draw the impervious sheet and along with it the perforated sheet, against the reinforcing material. The outer chamber is connected to a source of catalyzed resin so that the-resin is caused to flow from the outer chamber into the reinforcing material through the perforations of the sheet of perforated material. In this manner the material is substantially evenly impregnated with resin without escape of resin fumes into the surrounding atmosphere.
Perhaps most pertinent to the instant improvement, is U.S. Pat. No. 5,166,007, to Smith et al, also incorporated herein by reference. It teaches a patch or repair assembly. The assembly comprises at least one photo-curable pre-impregnated fabric, a UV (ultra-violet) transparent release film on top of the fabric and a UV blocking film over the release film. When applied to a work surface, the assembly forms a secondary wet-to-dry resin bond to the work surface.
With the advancement of the plastics industry in general, and PLP in particular, the fabrication of relatively large FRP structures has evolved into significant acceptance. With the emergence of very large, highly loaded plastic composites, FRP fabricators have been faced with the realities of placing ever increasing amounts of materials and labor at risk during the curing process. To minimize such risk they have shifted to structures which comprise a plurality of panels or subassemblies which can be field assembled. However, prior to the instant invention, fabricators had to rely only upon the use of mechanical fasteners or secondary bonding techniques (e.g., wet-to-dry surface resin bonding) to assemble such sections or subassemblies. Experience has proven that both of these means provide a less than acceptable bond. It has long been known that a primary bond (e.g., wet-to-wet surface resin bonding) provides a union which, unlike mechanical or secondary bonds, provides a strength which rivals that of any other section of the structure. Other advantages of primary bonds include, but are not necessarily limited to: (a) they are less susceptible to contamination; (b) they enable selective fiber orientation across the bond line, thus enabling cross hatching of fibers across the bond line which provide for higher structural efficiencies; (c) they preclude the need for adhesives thereby eliminating the cost and separate curing cycle required by such an adhesive; (d) they eliminate the need for elaborate jigs and fixtures often required for the often precise fitting of a secondary bond; (e) they normally provide for a much simpler inspection than that required by a secondary bond; (f) they substantially preclude the need for surface preparation which would otherwise be required by a secondary bond; and, (g) they are much stronger than secondary bonds.
Thus there exists a long felt need in the art of FRP fabrication by which relatively large structures, such as plastic tanks, can be fabricated by assembling discrete panels or sections which are joined together with primary, wet-to-wet resin bonds.