USE OF MICROWAVE ENERGY TO AID IN ALTERING THE SHAPE AND IN POST-PRODUCTION PROCESSING OF FIBER-REINFORCED COMPOSITES Background of the Invention Field of the Invention The invention relates generally to the use of microwave energy to aid in altering the shape of fiber-reinforced composites and, more specifically, to a method for melting the thermoplastic polymer of a fiber-reinforced composite using surprisingly low amounts of microwave energy to permit the composite to be altered in shape by, for example, bending, torsional movement or twisting, molding, clamping, shearing, self- expansion, and the like. The invention also relates to the use of microwave radiation to improve the bonding of over-molded components to composites.

Background of the Prior Art Fiber-reinforced composites are in wide use in a large variety of applications.

Such composites have the advantages of being lightweight, strong, resistant to corrosion, and thermally and electrically insulative. A disadvantage of fiber-reinforced composites is that while they may be shaped during the initial process when the composite is being formed, once formed they are difficult to alter in shape or further process.

It is well known to produce fiber-reinforced composites by drawing fibers into a pultrusion device, impregnating the fibers with a matrix material, most commonly a thermoset or thermoplastic resin, and forming the structure in a heated die. The fibers used in the composites, typically glass and other ceramic fibers, are commonly manufactured by supplying the ceramic in molten form to a bushing, drawing the fibers from the bushing, applying a chemical treatment, such as a size, to the drawn fibers, and then gathering the drawn fibers into a strand. Composites formed by pultrusion have the fibers oriented in the longitudinal direction and consequently have a high strength in tension.

The chemical treatment applied to the fibers generally is comprised of both a film former and a coupling agent. The film former creates a film or layer of polymeric material around each fiber and acts as a handling and processing aid. When incorporated into a pultrusion, the coupling agent helps to create a bond betwecn the fiber and the

matrix of the composite. The coupling agent at times may also help the film former react or interact with the resin system being used. Film formers used in thermosetting-type chemical treatments are commonly low molecular weight monomers, and are preferably heat-curable without generating a substantial amount of water or other solvent vapor.

Typical monomers used include polyester alkyd, epoxy resin, and a combination of glycidyl ether functional sufficient to form a film on the fibers yet not constitute an epoxy resin. Other suitable monomers disclosed include urethane, vinyl ester, amic acid, Diels Alder reactive species, and molecules that can undergo Cope rearrangement. With thermoset-type film formers, a heating step is preferably used to cure or at least partially cure the chemical treatment. Film formers used in thermoplastic-type chemical treatments commonly comprise at least one low molecular weight thermoplastic polymeric material that has a relatively low viscosity at elevated temperatures. The viscosity of the film former is low enough to adequately wet the fibers.

Organosilanes are one of the most commonly used coupling agents. Examples of specific silanes used in sizes are gamma-aminopropyltriethoxysilane, gamma- methacryloxypropyltrimethoxysilane, and gamma-glycidoxypropyltrimethoxysilane.

It is desired that the chemical treatment be compatible with the resin system used as the matrix material in the composite, particularly in creating the desired mechanical properties between the fibers and the matrix material. A chemical treatment is considered compatible with the resin if it is capable of interacting with and/or reacting with the resin.

Chemicals known as compatibilizers may be added to the chemical treatment to improve the interaction and/or reaction between the chemical treatment and the resin. Examples of compatibilizers are set out in U. S. Patent No. 5,972, 503 (incorporated herein by this reference). The chemical treatment may also include one or more processing aids to facilitate the use of the chemical treatment at some point during the manufacturing process or to optimize one or more properties of the composite article. Processing aids include viscosity reducers to thin the film former.

The resin can be a thermoset, either one-or two-part set, or a thermoplastic, i. e., one that is depolymerizable and repolymerizable. There are advantages and disadvantages to each resin. One disadvantage of thermoplastic resin is that, as discussed in U. S. Patent Nos. 5,891, 560 (incorporated herein by this reference), a relatively high

temperature is required in order for the resin to be applied to the glass fibers and such temperatures can cause damage to the composite if it is applied for too long of a time.

The'560 patent addresses this specific issue by use of a thermoplastic that has a low melt viscosity and which is applied to the fiber bundle as a melt from a heated conduit.

Fiber-reinforced composites have been made with a wide range of matrix materials. Thermoplastic polymers in common use include thermoplastic polyurethanes, an example of which is commercially available under the trade name ISOPLASTTM from The Dow Chemical Company.

The possibility of melting or depolymerizing thermoplastic composites allows, under certain circumstances, for the shape of composite articles to be altered after manufacture. Heating in ovens or autoclaves, flame treatment, and the application of light energy are all used to at least partially depolymerize the thermoplastic matrix material. Each method of depolymerization has its disadvantages. Ovens require large residence times in order to heat a composite of any substantial thickness and result in undesirably large areas of the composite article being depolymerized. Flame treatments work well on the surface of article and to a shallow depth, but will typically result in degradation of the composite material in outer layers before the temperature in internal layers has been brought high enough for sufficient depolymerization; flame treatment also will result in a large area of the composite being heated. Radiant, or light, energy can be focused into a small area, but does not typically penetrate the composite article to a sufficient extent to allow for sufficiently uniform heating between inner and outer layers of the article.

It is well known that certain composites will at least partially absorb energy in the microwave range. Microwaves have the advantage of being subject to focusing onto small areas of the article being treated and have the potential to penetrate a composite article more uniformly than energy in the visual or infrared range. They also have a high energy density and so can rapidly heat those materials that are absorbing. Additionally, if the material being heated is comprised of materials with differing absorptions, the possibility exists of selective heating through differential absorption.

An early use of microwave energy for the processing of polymeric materials was in the vulcanization of rubber, a process developed during the 1960's. The rubber was

heated by the microwave radiation through coupling with the carbon black fillers that were typically already present in the rubber formulations. By adjusting the carbon grade and concentration in rubber formulations and constructing multilayered products, rubber compounders were able to control the heating patterns through the products.

The absorption of microwave radiation by a polymer is affected principally by the reorientation of dipoles in the imposed electric field. Materials with the greatest dipole mobilities will exhibit the most efficient coupling, a property that is dependent on the dipole strength, its mobility and mass, and the matrix state of the dipole. Microwave coupling to a given dipole, accordingly, will be greater in a liquid, decreased in a rubber, and even further decreased in a glassy or crystalline polymer. The absorption of microwave radiation of a polymer can change, for example, during a processing cycle, with temperature changes, if solvent is removed, or if a reaction occurs which changes the type and concentration of dipoles. Many polymers contain functional groups, such as epoxy, hydroxyl, amino, cyanate, and so forth, which can form strong dipoles.

An approach that has been taken to enhance the response of polymeric materials to microwave energy has been to include particles and fibers that are conductive or which have dielectric properties significantly different from the matrix polymer. Examples of these inclusions are glass fibers, metal oxides, carbon black, carbon or metal fibers, and metal flakes, spheres or needles with sizes ranging from 0.1 to 100 um. The surfaces of conducting inclusions interact strongly with the microwave radiation. This effect has been used to improve the adhesion at the interface between the fibers and the matrix resin and result in improvement in the fracture properties of the composite following microwave processing. Aluminum powder added to an epoxy resin has been used in a method for joining polymers and polymeric composites by increasing the cross-linking in the composite, and similar results have been found for carbon-black filled epoxy resin systems.

Early studies on the microwave processing of polymer-matrix composites using wave-guide applicators indicated that materials with conducting fibers (specifically, carbon fibers) was limited to unidirectional composites with less than about 32 plies (approximately 7-8 mm thick). These results were attributed to the high reflectivity of the fibers and the resulting poor penetration depth of the microwave radiation into the

composite. Others have been able to cure up to 72-ply composites using a single-mode resonant cavity which applied the microwave energy in pulses and was controlled by feedback from a temperature probe. Further, carbon-fiber reinforced PEEK thermoplastic was found to absorb enough power from a tunable single-mode applicator to melt the matrix so that it could be bonded to a consolidated laminate.

However, no process is known for using microwave energy to efficiently depolymerize the matrix material of a fiber-reinforced composite so as to permit the shape of the composite to be altered after its manufacture without adversely affecting the properties of the composite. The ability to heat a fiber-reinforced composite locally to melt the thermoplastic resin will also give the ability to weld two thermoplastic pieces together. The development of a process for efficiently depolymerizing a composite article using microwave energy would overcome many of the disadvantages of prior art heating or depolymerization methods.

Summary of the Invention The invention consists of a method for preparing a fiber-reinforced composite that has a surprisingly high absorption of microwave energy and subjecting the composite to microwave energy to sufficiently depolymerize the matrix material of the composite to selectively alter the shape of the article. The composite has an absorption of microwaves that is between about 2 and about 20 times greater than that of conventional composites.

Articles made from the composite may be efficiently and uniformly heated in desired areas to permit the operation of many diverse manufacturing steps heretofore not practicable or feasible.

A ceramic fiber, such as a glass roving, is used that has been chemically treated with a size that is compatible with polyester, vinyl ester, and epoxy resin systems. The treated fiber is heated to a temperature during pultrusion that results in a change in the dielectric properties of the composite. One method of manufacturing the composite is following the general method as disclosed in the'560 patent, but using a glass roving having a different chemical treatment, specifically a silane-based size intended for thermoset pultrusion but used with a thermoplastic polyurethane matrix. The resulting composite absorbs microwave energy between about 100 and about 700% more efficiently than a composite material manufactured according to the specific example

recited in the'560 patent wherein a roving developed to be compatible with thermoplastic resins, specifically polypropylene and polyamide systems is described.

Accordingly, there is disclosed a process by which a fiber-reinforced composite material is manufactured which has a surprisingly high absorption of microwave radiation. The high absorption of microwaves permits articles manufactured from the novel composite material to be conveniently and efficiently altered in shape after initial formation. Such articles may also be selectively secured to other articles, particularly other thermoplastic articles, by the convenient and efficient use of microwave energy to melt the thermoplastic matrix and thereby permit welding of the composite article to the other articles.

Long and short fiber reinforced thermoplastic components are produced by means other than pultrusion. Long fiber reinforced thermoplastic pellets used in injection molding can be produced using a pultrusion compounding operation very similar to the Edwards'560 process, with the primary differences being that the pultruded profiles are relatively small profiles that are chopped into pellets (0.5 inches to 1.5 inches long) shortly after they cool. These pellets are subsequently used in injection molding operations.

Short fiber reinforced thermoplastic parts can be produced by loading chopped fibers with neat resin in an extruder used as part of the injection molding process.

Either of the above resin types can be used in overmolding as well as in regular injection molding operations. There may be some advantage if the overmold resin is improved using the modified fibers. There also may be some advantage if pultruded or moded parts are enhanced using modified fibers. Other processes are available that allow, for example, the use of continuous strand mats in thermoplastic resin parts. In all of the above cases, the fibers could be pre-treated to enhance the composite's response to microwave energy.

Brief Description of the Drawings Fig. 1 is cross-sectional view of a piece of pultruded fiber-reinforced composite rod which has been treated with microwave radiation to expand an end portion of the rod and a polymer over-mold surrounding the end portion which is prevented from being pulled off of the rod during use by the expanded end portion.

Fig. 2 is a side view of a piece of pultruded fiber-reinforced composite rod which has been exposed to microwave radiation until the resin matrix depolymerized whereupon the rod was bent at the site of the depolymerization.

Fig. 3 is a side view of two pieces of pultruded fiber-reinforced composite rod which have been welded together by the use of microwave radiation.

Fig. 4 is a cross-sectional view of a composite rod to which a component has been over-molded and wherein microwave heating was used to improve the cross-linking of the over-molded part and the composite.

Fig. 5 is a cross-sectional view of a composite rod to which a component has been over-molded and wherein microwave heating was used to expose fibers above the surface of the composite to improve the consolidation of the over-molded part and the composite.

Figs. 6a and 6b are top and side views, respectively, of a composite rod which has been heated by microwave energy at a central portion and then mechanically deformed.

Fig. 7 is a diagrammatical view of a fiber-reinforced composite illustrating a melt zone caused by microwave radiation at the interface between the fibers and the matrix of the composite.

Detailed Description of Preferred Embodiments A composite material within the scope of the present invention may be produced using the apparatus as described in the'560 patent, although it may also be produced using other known pultrusion apparatus. Rovings of glass or other ceramic fiber are passed through a preheat station to bring them to a temperature near that of the thermoplastic resin to be applied to improve setting of the fibers by the resin and to prevent cool down of the resin as it comes into contact with the fibers. A thermoplastic resin is applied to the fibers, typically by an extruder which has heated the resin to approximately its melt temperature. The coated fibers are pulled through a die which shapes and cools the composite.

In a preferred embodiment, the glass roving is 366 Type 30'8'AG-207 (2,400 tex) roving sold by Owens Coming. The roving is coated with a proprietary sizing system that is identified by the manufacturer as silane-based sizing that is designed to have excellent adhesion with polyester, vinyl ester and epoxy resin systems. The sizing

system is formulated for use with thermoset resins but is used in the present invention with thermoplastic resins. The sizing is believed to be comprised of approximately 66% epoxies, 15% surfactants, and 10% silanes, with the balance being polyethylene glycol monooleate and butoxyethylsterate (probably processing aids). The rovings are pulled through the 2-meter long preheat station that has been set at a minimum 230° C, although temperatures exceeding 300° C may be used. A breaker bar is used to loosen the strands of the roving but does not break the sizing off of the rovings.

The resin used is ISOPLASTTM polyurethane engineering thermoplastic resin sold by The Dow Chemical Company. The resin is extruded using a 35 mm extruder with a standard screw and barrel, although other extruders may be used. The heat profile in the extruder runs from approximately 285° C to approximately 300° C. The speed of the extruder is adjusted to stay in synchrony with the speed at which the rovings are being pulled through the die. The resin is fed by the extruder into the input end of the die which has been heated to a temperature of approximately 288° C. A cooling die is in contact engagement with the impregnation unit to prevent swelling of the coated fiber as it exits the impregnation unit. The cooling die is cooled to approximately 2° C.

The resulting composite is surprisingly absorptive of microwave radiation. When compared to a composite formed under identical conditions but using R43S (Owens Coming) glass rovings, the composite of the present invention is between about 2 and about 20 times more efficient at absorbing microwave energy. In one experiment, a 350 watt microwave oven was used to heat two rods of identical profile made by the two described methods. A portion of the rod made with the Type 366 rovings reached temperature at or near the melt temperature after approximately 40 seconds, while it took between about 6 and 7 minutes to similarly heat the other rod.

The R43S roving was developed to be compatible with thermoplastic resins, specifically polypropylene and polyamide. While Owens Coming does not reveal the specifics of the formulation of its sizing systems, it is believed that low molecular weight thermoplastic polymers are used as film formers in the sizing system of the R43S roving whereas epoxy monomers are believed to be used as film formers in the sizing system of the Type 366 roving.

Without wishing to be bound by any theory, it is believed that the temperatures of the roving reached in the preheat station are sufficient to alter one or more physical or chemical properties of the sizing system used on the Type 366 roving. Since the organosilanes have degradation temperatures that are at least several times any maximum temperature that might be experienced by the roving in the preheat station, no material change is believed to take place to the silanes. On the other hand, it is believed that the film former, thought to be monomer epoxies, are degraded, perhaps through oxidation, in the preheat station. In any event, one or more of the dielectric properties of the composite are changed so that it more strongly absorbs microwave radiation. One explanation may be that the epoxies are oxidized, leaving carbon particles (it was noted that initial samples of the composite of the present invention were noticeably darker that the composite formed under identical conditions but using the thermoplastic compliant sizing system). These carbon particles are likely distributed in the formed composite about the interface between the fibers and the matrix resin. Since carbon particles are known to increase the coupling of the microwave radiation to the composite material, it is expected that the microwave radiation would cause heating about the interface between the fibers and the matrix resin. Accordingly, temperature profiles in the composite would initially be high at the interface surfaces and so the matrix resin would tend to melt or depolymerize first at the surfaces of the fibers. A fiber-reinforced composite article is illustrated in Fig. 7, generally at 10. The article 10 includes fibers 12 within a matrix resin 14 and illustrates melting or depolymerization of the matrix resin 14 along the fibers 12 in the areas indicated at 16. This would be particularly advantageous to forming of the composite 10 through bending, twisting, clamping, shearing, molding or other mechanical forming processes, in that the fibers 12 throughout the volume of the composite 10 in the heating zone would be relatively free to move with respect to the matrix in the areas 16 even though the matrix material distant from the fibers may not have reached the melt temperature.

With the surprisingly high absorption of microwave energy, it becomes practical to use microwaves to melt desired regions of a fiber-reinforced composite allowing the composite to be formed in the heated or melt zone and for the composite to be welded to other components that have also been heated to a melt temperature. Accordingly, even

though heretofore pultrusion has been limited to the formation of longitudinal composite articles having a fixed cross-section, these articles can now be formed into more complex shapes by bending, forming, and other mechanical processes. For example, microwave heating of a free end portion of a composite article 10 will act to permit the composite to release the energy of consolidation during forming, resulting in the self-expansion of the free end portion 18 (Fig. 1). This expanded diameter end portion 18 may then be used to assist in the retention of an over-molded component 20. The present method for retaining of an over-molded part requires that notches be formed in the composite rod to provide a surface for consolidation of the over-molded part; unfortunately, the creation of the notches requires an extra machining step and reduces the strength of the composite rod through the removal of material. Alternatively, a composite rod 10 may be heated in a limited zone along its length and then bent at an angle, such as is illustrated in Fig. 2.

Further, two of the composite rods 10a and 1 Ob may be simultaneously heated and the melt zones brought together, resulting in the formation of a weldment 22 at the conjoined melt zones (Fig. 3).

Another use of the novel material of the present invention is in the use of microwave heating to assist in the securement of over-molded components through pre- melting of the composite prior to the over-molding step. In over-molding of components on composites, the composite is inserted into a mold cavity, the mold cavity is closed, and the polymeric material for the over-molded component is injected into the mold cavity around the composite. Consolidation of the over-molded polymeric material to the composite material would be assisted if at least a portion of the surface of the composite was heated to or near to the melt temperature of the matrix immediately prior to insertion into the mold cavity. This will result in improved cross-linking between the composite matrix and the polymer used in the over-molded component. Substantially the same result can be obtained if the over-molded part is molded on the composite followed by heating and melting of the composite at the interface of the over-molded component until an adjacent portion of the over-molded component also melted so that cross-linking of the two elements could occur. The finished products of both of these processes are illustrated in Fig. 4, showing a fiber-reinforced composite rod 10, a thermoplastic overmold 24, and zones of increased cross-linking 26. Alternatively, the microwave

radiation may be used to heat the matrix material 14 in a region on the surface of the composite 10 causing fibers 12 to be exposed above the surface of the composite 10 (Fig.

5). If the microwave-treated composite 10 is now used in an over-molding process, the exposed fibers 12 will be consolidated in the over-molded polymer 26, thus greatly improving the bond between the over-molded component 26 and the composite 10.

Microwave heating can also be used to assist in mechanical deformation of the composite. For example, the composite could be heated to at or near the melt temperature in a region and then deformed by conventional mechanical working such as bending, twisting, clamping, shearing, forming or the like. A specific example is illustrated in Figs. 6a and 6b, which illustrates flattening of a composite rod 10 in a central region 28. Such a step may be used to prepare rods for joining together, as in Fig.

3, whereby the flattening of a region 28 increases the surface area of the rods 1 Oa and 1 Ob in contact engagement and thereby the strength of the joint.

Another aspect of the invention is the ability to selectively adjust the microwave absorptivity of the composite by manufacturing the composite out of rovings some of which provide enhanced microwave absorption and some of which do not. The two kinds of rovings could be uniformly distributed throughout the profile of the composite, thereby adjusting the absorptivity of the composite selectively between a low absorptivity (all of the rovings are of a type that do not lend high absorbance) and a high absorptivity (all of the rovings are of a type that do lend high absorbance). Alternatively, the rovings of the two types could be selectively distributed in regions of the profile so as to make some regions more highly absorptive than others. For example, highly absorptive rovings could be selectively distributed in an annular region (considered in cross-section) of the profile near the surface, thereby selectively heating the composite near its surface. This may be preferable when bonding an over-molded component, such as is described with respect to Figs. 4 and 5.

Although the invention has been described with respect to a preferred embodiment thereof, it is to be also understood that it is not to be so limited since changes and modifications can be made therein which are within the full intended scope of this invention as defined by the appended claims.