Advanced composites.

Advanced Composites

Faster and more economical processing through automation and computerization are key themes. So are proliferation of high-tech materials, composite toughening methods, and bonding techniques.

Threatened cutbacks in military spending have not yet been reflected in any slackening of the energetic pace of R&D on advanced composite materials and processing methods. Automation, computer process analysis and control, and simplified processing techniques continue the push toward lower manufacturing costs and higher quality. Thermoplastic composites are still where much of the actions is, with new developments in pultrusion, filament winding, and prepreg lay-up. In-situ formation of thermoplastic foam-core sandwich structures is a new area attracting attention. And there are still plenty of new high-performance resin chemistries appearing in the labs, along with novel approaches to toughening brittle thermosets. Composite welding, adhesive bonding, and fiber/resin coupling are still other areas of recent innovation.

Much of the following news roundup is based on several technical conferences over the past few months, which provide a good overview of what's new in this field. More information is available from the published proceedings of the following: Structural Composites Design and Processing Technologies, sponsored by ASM International and the Engineering Society of Detroit (ESD), in Detroit last October; Fabricating Composites '90, sponsored by the Society of Manufacturing Engineers (SME), also in Detroit in October, Advancements in Materials for Polymer Composites, sponsored by the Society of Plastics Engineers (SPE) Southern California Section and Advanced Polymer Composites Div., in Los Angeles in October; 22nd International SAMPE Technical Conference in Boston in November; Composites in Manufacturing 10, by SME in Anaheim, Calif. in January; and the Society of the Plastics Industry (SPI) Composites Institute 46th Annual Conference in Washington, D.C., last month.

PROGRESS IN AUTOMATION

Cincinnati Milacron's newly developed Fiber Placement Processing System, the FPX, can double the productivity rate of hand lay-up and automatically steer fibers along non-geodesic paths. The seven-axis machine, Milacron says, can even place fibers along paths not achievable by manual lay-up methods or automated tape-laying systems. Milacron recently completed development of a production version of this machine, which was first announced two years ago (see PT, July '89, p. 27). Two FPX machines have now been sold to General Electric Co.'s Aircraft Engine Group, Hooksett, N.H., to be used to lay up fan blades.

Using a fiber-placement head on a robotic arm (p. 50), the FPX heats the tows until they are tacky to ensure adhesion. It can lay up to 24 individual 1/8-in.-wide graphite/epoxy tows. This, along with its ability to cut and restart tows individually, lets the machine lay up contoured ply shapes, construct tapered shapes without material buildup, and create localized buildups if necessary. Its ability to cut and start tows reduces the amount of material wasted when the band has to be turned around. And the FPX's in-process compaction capability, Milacron says, allows the machine to work on concave surfaces with a minimum of debulking.

Milacron is not alone in developing automated fiber-placement technology. Automated Dynamics Corp., Troy, N.Y., has developed what it calls "a system for fiber placement of thermoplastic composites on complex shapes," which was discussed both at the SAMPE and ASM/ESD meetings. The system (cover photo and p. 51) works on the same principle as Milacron's FPX, using a patent-pending hot-gas torch to heat the junction of the incoming tow and the previous ply to a temperature higher than the melt temperature of the material used in the two. A compaction roller then applies pressure to the tows for a predetermined period of time to ensure they remain in place.

The automated fiber-placement concept has also been applied to preform fabrication for RTM. An innovative preform manufacturing system developed by Cooper Composites was presented at the ASM/ESD conference. Called the Near Net Fiber Placer, the new system can reportedly place fiber into any preprogrammed shape, fiber angle and areal weight. The computer-controlled machine is available with four to 12 heads with each head capable of laying fiber at a rate of 9 ft/min. It lays down the fiber on a flat surface in a series of tight zigzags that can duplicate even complex shapes with holes and cutouts. The present system has a manufacturing field of 25 x 30 in., but Cooper says a larger version is under development.

The Near Net Fiber Placer ([N.sup.2]FP) converts CAD drawings of parts into digital machine language. First, the CAD representation of a 3-D part is "unfolded" into a "layflat" of 2-D shapes. These data, Cooper says, are stored on a disc that controls such variables as tow angle, the weight of each ply laid up and the tows' fiber volume. The system can replicate the structure of any fabric for a given ply, and multiple plies are stitched together. When completed, the layflat preform can be folded up into the original 3-D shape of the part.

STREAMLINING THE PROCESS

In an effort to streamline the structural RIM (SRIM) process by reducing the number of steps involved, Dow Chemical researchers from Freeport, Texas, reported at the ASM/ESD meeting on a patent-pending system that allows preforming of mat reinforcement, glass trimming, and resin impregnation to be accomplished in a single mold. Dubbed the "Cut-N-Shoot" process by Dow, the new technique holds the mat in position around the mold-cavity parting line by a frame built into the upper mold half. Air cylinders around the perimeter of the frame provide the proper degree of tension and slip, in order to prevent wrinkling as the mat is shaped by bringing the two mold halves together. In the final stage of mold closing, a shear edge on the core half both trims the mat to the part shape and serves as a resin seal while permitting air venting during injection.

Another effort at streamlining composites fabrication is the experimentation with "dieless forming" being done at Stanford University in Stanford, Calif., and presented at SAMPE. The idea behind dieless forming, Stanford researchers say, is to have a small, economical machine be able to form large components from sheets of continuous-fiber materials. The process is roughly analogous to roll forming of metals. It's designed to form long components having no curvature in the lengthwise direction, such as those used in aircraft wings. The key to the machine is the use of two fixed and two moving "cluster rollers," each consisting of six small rollers that can spin independently and rotate together in a wheel arrangement. The workpiece is formed progressively, a little at a time, as it passes between the rollers in an induction-heated forming zone. This allows longitudinal bends to be introduced at the edge of a flat workpiece and spread in the transverse direction.

An improved method of trapped-rubber molding (TRM) that eliminates the trouble and expense of autoclaving was presented at the Texas SME meeting by Dow Corning Corp. TRM has advantages over vacuum bagging when molding complex shapes, because the elastomer (typically silicone rubber) serves as a pressure-transfer medium, conforming itself intimately to the part and distributing pressure evenly in all directions. The critical limitation of most prior TRM work has been the necessity of precisely calculating ullage, or the thermal expansion of the silicone rubber in order to provide the desired pressure. Not only was this difficult, but the rubber never expanded to the same degree from one cycle to the next, owing to progressive shrinkage and changes in properties.

Three years ago, United Technologies Corp. patented a Therm-X process, using an autoclave filled with powdered silicone rubber as a pseudo-fluid medium. By heating or cooling the wall of the autoclave separately from the composite part, the pressure imparted by the rubber could be precisely controlled independent of part temperature. Ullage calculations were no longer necessary (see PT, Dec. '88, p. 31).

Because this technique is not only the property of UTC, but also still requires an autoclave, Dow Corning patented a non-autoclave technique that it is making freely available. This method involves casting a silicone rubber (such as Dow Corning's Silastic E) against the tool, which has been laid up with wax to the thickness of the composite laminate. During subsequent molding of the composite, the rubber is completely enclosed on all sides (see diagram, p. 52). A cooling platen controls the rubber's expansion, and thus the pressure against the laminate, independent of the curing temperature, as is true of the Therm-X process. Company spokesmen say it would be impossible to vacuum bag such a complex part as the sine-wave panel shown in the schematic. The company's highly instrumented lab tool registered equal pressure from the rubber against both horizontal and vertical surfaces of the laminate.

UPDATING THE AUTOCLAVE

Meanwhile, new vacuum-bag materials can avoid the tendency to "bridging" over complex shapes that limits applicability of traditional nylon films. CR&M Inc. has applied for a patent on its TFN high-stretch bagging films. At SAMPE, the firm described these blown films of urethane or copolyester thermoplastic elastomers. Their stretchability allows them to conform to virtually any shape.

Another improvement over traditional autoclave procedures would be closed-loop control of resin content in the laminate. Researchers at Michigan Technological University showed at the ASM/ESD meeting that this could be accomplished by continuously monitoring laminate thickness and feeding back the result to control pressure. Experiments showed that conventional open-loop processing was most sensitive to changes in temperature, while closed-loop control successfully counteracted those variations.

Dow Corning has good news for users of elastomer tooling, which is often torn or damaged. Brand-new X3-6339 silicone rubber tooling material reportedly has almost twice the tear strength of conventional elastomers (over 140 lb/in.). It's also translucent after cure, revealing laminate voids that might otherwise require x-ray analysis for detection.

In addition, Dow Corning's new X1-4052 X14052 Barrier Coating protects silicone tools from attack by amines in epoxy resins. It's a reactive liquid material that's wiped on the tool like a release coating. It reportedly increases tool life by 50% on its own, and doubles tool life when used together with a conventional release coating.

THERMOPLASTIC PROCESSING

Thermoplastic Pultrusions, Inc. reported at the SAMPE meeting on an ultrasonic process for rapid, low-stress pultrusion of thermoplastic composites. Using a variety of resin/fiber combinations, ranging from a powder preform of PEEK/graphite to PP/glass, the company has been able to process 10-20 times faster than other reported rates for thermoplastic pultrusion (up to 20 ft/min for 60-mil rod) while frequently maintaining void contents of less than 2%. In the process, material is passed first through a heating die, then an optional preforming die, followed by the consolidation die and an optional cooling die. The material is heated to or just below its melting point when it enters the consolidation die, which is held at the same temperature. A relatively low die temperature is said to prevent resin sloughing problems during consolidation. The key to this process is that ultrasonic energy is applied in the consolidation die to induce resin flow. Because consolidation of the pultruded material is greatly enhanced by the ultrasonic activation, consolidation dies are very short--only 1-2 in. long. Besides being less expensive to manufacture, shorter dies reduce pull-force requirements and permit even brittle, high-modulus fibers to be pultruded without breaking.

Thermoplastic pressure vessels have been successfully fabricated with a polar winding system developed at Thiokol Corp. The diagram on p. 53 shows the company's third-generation experimental system using infrared and quartz heating lamps with a standard unheated mandrel. Separate delivery systems provide hoop and polar winding of PEEK and PPS graphite-prepreg tows. Air cylinders provide up to 700 psi compaction force. Thiokol hopes one day to use the system to wind rocket-motor casings, though processing speeds must be improved substantially to make it cost-competitive with epoxy thermoset filament winding.

Computer modeling of diaphragm forming of thermoplastic continuous-fiber prepregs is a joint research effort between ICI Composite Structures and the University of Delaware (Newark). As reported at the SPI conference, the software predicts the fiber geometry and thickness profile in the formed part. This can be used for formability analysis of a proposed tool--i.e., determining whether the prepreg can be formed to that shape without buckling. As shown in the accompanying schematics, the system can also be used to design a sheet blank that will produce minimum waste by "unfolding" the trimmed shape of the formed part.

In other thermoplastics news, interim results from an ongoing collaborative study by ICI Advanced Materials and the Southwest Research Institute in San Antonio, Texas, show that thermoplastic composites can be more effective fire barriers than some thermosets. Cone calorimeter tests defined by the U.S. Navy's MIL STD 2202 showed that ICI's APC-2 prepreg had more than triple the time to ignition and less than one-tenth the peak heat-release rate of an epoxy composite. A PES composite had about the same time to ignition as the epoxy, but addition of a proprietary ICI mineral coating almost tripled that time.

HOW TO MAKE A SANDWICH

At least two firms are working on insitu, one-step methods of forming thermoplastic foam-core sandwich panels via "foam forming." We reported two years ago on work by Mobik GmbH of Germany, in which a solvent-impregnated core sheet of polyetherimide (PEI), PES or polycarbonate can be laid up in a mold with face sheets of similar thermoplastic prepregs. When sufficiently heated, the core will soften and expand, generating internal pressure to form a 3-D shape that can have varying thickness, and simultaneously bonding the face sheets (see PT, April '89, p. 45). Mobik presented an update paper at the ASM/ESD meeting.

Taking the idea one step further, Sundstrand Advanced Technology Group, a research company, revealed at the same meeting results of experiments in using the pressure of the expanding thermoplastic core to impregnate dry-fabric face sheets, in addition to bonding the sheets to the core and forming a 3-D shape. Sundstrand researchers said they took this approach because of the high cost of thermoplastic prepregs and commingled fabrics, the difficulty of placing such materials into a mold for a complex structure, and the high pressure (300 psi) required to consolidate commingled fabrics, which frequently results in microcracks.

Sundstrand experimented with PEEK and nylon 12 powders, as well as glass, aramid and carbon fibers. One fabrication method was simply to lay up the resin-powder core material together with chemical foaming agents at the center of a sandwich of dry reinforcing fabric plies, with or without interleaved resin films. This lay-up was vacuum bagged inside a closed mold that served as an autoclave. The second approach was to put the lay-up and mold inside an autoclave with a hopper of resin connected by a feeder tube to the core of the lay-up. Thus, resin could be added to or subtracted from the core during molding, so as to control the internal pressure and final density of the core.

The expanding-film core principle is also being applied to thermoset composites. Y.L.A., Inc. produces new MicroPly syntactic films of epoxy, polyester, polycyanate or phenolic resin containing both a foaming agent and 3M Scotchlite Glass Bubbles. These films expand when heated and co-cure against prepreg face sheets. The glass microspheres act as nucleants, providing a fine, uniform cell structure for low final density. One application of MicroPly films is in sporting goods. They are used by Ektelon in San Diego to make racquetball racquets, and are qualified by a division of Schwinn Bicycle Co., Chicago, for making frames of rugged mountain bikes.

A somewhat different approach to in-situ sandwich-panel formation was discussed by Bayer AG (the German parent of Mobay Corp.) at SAMPE. The company combines thermoplastic prepreg face sheets with a core of compressible, loosely knitted fabric impregnated with the same thermoplastic, and consolidates the whole into a single, solid, flat sheet. When heated, the compressed fibers of the core fabric will spring back, causing the laminate to expand and fill a mold. Bayer is developing glass and carbon-fiber composites with nylon 6 and PPS resin.

BETTER ADHESION, BONDING

A new method of electromagnetic induction bonding of thermoplastic advanced composites without the need for metallic "susceptor" implants was introduced at the Anaheim SME meeting by Heltra, Inc. Heltra found that a fabric of commingled PEEK/carbon fiber yarn serves as an effective susceptor for bonding ICI's APC-2 PEEK/carbon-fiber prepregs. Heltra co-molded a layer of its Filmix commingled fabric onto the surfaces of APC-2 panels; then the panels were pressed together over an induction coil to form a strong bond.

Incidentally, Ameritherm Inc. introduced at SAMPE an 8-lb RF induction heating coil designed for handheld or robotic operation. It operates at 50-200 kHz and generates 2.5-25 kw.

For adhesive bonding of thermoplastic composites, some form of surface preparation is needed. Lockheed Aeronautical Systems Co. evaluated several methods. At the ASM/ESD meeting a researcher reported that gas plasma treatment, among others, provided relatively good results; but the speaker said that corona discharge also looks like "a very promising method." Unlike plasma treatment, corona discharge does not produce microcracking of the composite surface, which may indicate the possibility of stronger bonds.

To help in adhesive selection and use, the U.S. Army Materiel Command sponsored creation of a computer database on adhesives' properties, surface preparations, and test methods, which has been fully operational since 1989. Described at SAMPE, the AMC Adhesives Database at the Army R&D center in Picatinny Arsenal, N.J., includes data on design and manufacturing of parts to be adhesive bonded.

Two papers at the ASM/ESD meeting introduced new methods of enhancing adhesion of ultra-high-molecular-weight polyethylene fibers to thermoset resin matrixes. Allied-Signal Corp.'s Fibers Div., which produces Spectra-1000 UHMW-PE fibers, found that passing the fiber through a mixture of fluorine and nitrogen gases in a stainless-steel tube reactor creates a partially fluorinated surface layer that enhances adhesion. In an epoxy composite, the treated fiber gave a five-fold improvement in flexural modulus over untreated fiber, fourfold improvement in single-filament pull-out strength, and 170% increase in short-beam shear strength.

Researchers at Michigan State University reported that while both plasma and corona treatment of UHMW-PE fibers improve adhesion to the matrix, they also frequently result in degradation of fiber mechanical properties. New research with ion-beam irradiation of Spectra-1000 fibers showed improved adhesion to epoxy, as well, but with a difference. Electron microscope examination of plasma-treated fibers shows considerable surface roughening with many small pits or cavities. While these would produce good mechanical interlocking with the resin, they could also serve as stress-concentrating flaws, thus accounting for the reduced fiber tensile strength reported for such treatments.

As a new approach, MSU researchers found that ion-beam irradiation of Spectra-1000 improved adhesion to epoxy--apparently by removing low-molecular-weight species from the surface--without any fiber surface roughening. This treatment reportedly gave excellent retention of fiber tensile properties.

NEW MEANS OF TOUGHENING

Increasing the toughness or damage resistance/tolerance of composites has been a consistent theme in recent materials development, and one of the major motivating forces behind development of thermoplastic composites and thermoplastic/thermoset blends. Some recent experiments have explored methods of "inhomogeneous toughening" of discrete layers or regions in the laminate. One reason for exploring this approach, according to Dr. Paul A. Steiner of Lockheed's Palo Alto Research Laboratories, is that prepregging and other processing requirements limit the amount of thermoplastic that can be incorporated into a thermoset matrix, and thus the amount of toughening that can be imparted. Steiner noted at the SPE meeting that American Cyanamid has developed a method of interleaving discrete thermoset prepreg and thermoplastic resin layers to enhance delamination resistance, but at a sacrifice of stiffness, which requires a thicker, heavier overall laminate.

Steiner obtained encouraging results with an intermediate approach, whereby thermoplastic PEEK and polyethersulfone (PES) particles were dispersed in thermoset cyanate ester resin mainly in the interlaminar region between prepreg plies. This was accomplished by using large enough thermoplastic resin particles (11 microns) to be trapped in the outer regions of the prepreg by the carbon-fiber fabric.

A similar approach was discussed at SAMPE by researchers from the University of Washington and Boeing Commercial Airplanes, Seattle. They investigated dual-pass impregnation of prepregs, in which the first pass wets the fibers with thermoset matrix, and the second pass adds a thin layer of thermoset/thermoplastic blend to the prepreg surface.

A different approach, involving discrete layers of a ductile metal between epoxy/graphite prepreg plies, was investigated at the Composite Materials Research Lab of the State University of New York (Buffalo). It was found that addition of low-melt-temperature tin-lead alloy particles between prepreg layers during composite lay-up improved fatigue life 100-fold but had little effect on tensile, flexural, compressive or impact properties. These results indicate very good bonding between the metal and composite layers, the researchers concluded. The alloy accounted for 27-37% of the composite weight and 5-7% of its volume.

Still another approach reportedly can reduce or eliminate problems of microcracking after thermal cycling in epoxy/carbon-fiber laminates. Amoco Performance Products found a solution in ultra-thin (1.5-mil) prepregs, of which 32 plies equal the thickness of a laminate containing only eight conventional 5.6-mil plies. Ultra-thin prepregs also permit fabrication of lighter spacecraft components, whose minimum weight is often limited by available prepreg thickness, not by concerns of strength of stiffness. Processors should be aware, however, that thin prepregs have a wider range of resin-content variability, must be handled more carefully to prevent breaking the carbon fibers, and tend to have higher tack, making it difficult to reposition a mislaid ply. In this work--reported at the SPE conference--Amoco also demonstrated lower water absorption (and consequent outgassing) with a new modified epoxy system, ERL-1939-3.

Efforts continue to toughen normally brittle bismaleimide (BMI) resin systems. An approach based on addition of low-molecular-weight additives with similar, compatible chemistries was reported at SAMPE by researchers at Winona State University. They utilized an aniline-endcapped BMI in 50/50 blends with a standard MDA-BMI, the goal being to improve toughness without significant losses in mechanical and thermal properties.

NEW SPECIALTY RESINS

Improvements in process-ability, heat resistance, moisture absorption, and low toxicity are the drivers for several recent materials developments.

A new class of low-viscosity thermosets for RTM and filament winding was recently developed by Dow Chemical Co., Midland, Mich. Called acetylene-chromene terminated (ACT) resins, the new materials reportedly can be prepared very simply from low-cost precursors, have high [T.sub.g]'s (above 644 F), excellent electricals, high flexural modulus (up to 600,000 psi) and low moisture uptake, giving them greater than 450 F wet performance. The resins' thermal stability in air at 400 F is said to be much better than BMIs, and they cure by addition polymerization without liberating volatiles. A typical cure schedule might be 2 hr at 400 F, followed by 2 hr at 482 F and a 1-hr post-cure at 527-572 F.

Ciba-Geigy introduced a new product at SAMPE that's said to offer a major advance in processability of high-performance tetrafunctional epoxies. Araldite XU MY 722 is a liquid with room-temperature viscosity of 8000-12,000 cp, compared with around 50,000 cp for Ciba-Geigy's MY 720 and 721 tetrafunctionals. It's thus suited to a variety of applications, including RTM, filament winding, and tooling. In short, it's said to combine the processability of a basic liquid epoxy with the performance properties of MY 720 and 721, including a similar [T.sub.g] of 468 F, ability to withstand continuous exposure to 350 F dry and 250 F wet, and elongation of 1.6%. Its stiffness is somewhat greater than other tetrafunctionals, but it can also be flexibilized with additives. Also, the new resin's lower reactivity gives increased working time. Typical gel time is 55 min at 356 F, vs. 20 min for other tetra-functionals, and pot life at 212 F is 5 hr instead of 2 hr. This can be accelerated with catalysts or high-reactivity resins. Because of its low viscosity, MY 722 is easily modified with more viscous or solid materials in order to achieve particular properties.

A new PMR-type polyimide similar to PMR-15 is based on a proprietary fluorinated monomer of low toxicity in place of methylene dianiline (MDA), a suspected carcinogen. BP Research Centre in England and BP Chemicals (Hitco) developed this polymer, code named B1, which can withstand long-term use at 482 F, and has mechanical properties and elevated-temperature property retention similar to PMR-15, though B1's thermal stability is greater at 572 F.

American Cyanamid Co. recently obtained an exclusive license to use Rohr Industries' patented method for synthesizing an improved PMR-15 polyimide with reduced levels of MDA. Rohr, based in Chula Vista, Calif., says its new process does not adversely affect the product's composition, shelf life, nor processing characteristics.

Also, NASA Lewis Research Center reported at SAMPE the long-term thermal-oxidative stability of PMR-15 can be increased from its current 600 F up to 700 F by modifying its chemical structure with para-aminostyrene substituted for the usual nadic ester endcap.

PHOTO : Manufacturing economies obtained through process automation, computerization, and simplification will be essential to broadening advanced composites' appeal in general industrial applications.

PHOTO : Cincinnati Milacron says its FPX system can double the productivity of hand lay-up by automatically placing fibers along paths not achievable by manual lay-up or automated tape-laying systems.

PHOTO : Robotic fiber placement system developed by Automated Dynamics Corp. uses a hot-gas torch to fuse the incoming tow and the previous ply and a compaction roller to apply pressure to the tows to ensure they remain in place.

PHOTO : Research into dieless forming has shown that a small machine can form large components from continuous-fiber materials. The process is similar to roll forming of metals. (Stanford University)

PHOTO : Trapped-rubber molding system from Dow Corning uses a cast silicone rubber to create pressure, without need for an autoclave. Water-cooled upper platen controls rubber expansion--and thus pressure--independent of the cure temperature of the laminate.

PHOTO : Still experimental, Thiokol Corp.'s thermoplastic polar winding system uses infrared heating and high-pressure air compaction cylinders, allowing the use of an unheated mandrel. Thiokol hopes to someday use this system to manufacture rocket-motor casings.

PHOTO : Computer analysis of thermoplastic diaphragm forming models fiber geometry in the formed part (left) and also shows how to create minimum trim waste by "unfolding" the final trimmed shape (center) into the optimum blank shape (right). Software was developed by ICI Composite Structures and the University of Delaware.