Budding technologies for composites detailed in SAMPE papers.

Budding Technologies for Composites Detailed in SAMPE Papers

As a follow up to our SAMPE show news report (PT, June '90), here's a summary of some interesting developments for processors found in the many technical papers presented at the conference. Companies discussed novel methods for molding and curing advanced materials, R&D of fiber-placement machinery, and activities on the frontiers of materials research.

PROCESSING INNOVATIONS

Using an autoclave to perform isostatic, "isoclave" molding for in-situ fabrication of thermoplastic composite sandwich structures in one heating cycle was the subject of a paper from Sundstrand Corp., Rockford, Ill. In isoclave molding, all of the skin and core materials are placed in the mold in an unconsolidated state. Then, pressure required to consolidate the sandwich skin is developed internally by the thermal decomposition of chemical blowing agents mixed with the core.

A high-pressure autoclave can be made into an isoclave by adding a hopper and a transfer tube in the autoclave chamber. The transfer tube connects the hopper containing a portion of the core materials to the mold that contains the remaining core and unconsolidated skin materials. For those who already have an autoclave, this method reportedly yields cost savings over other in-situ molding methods requiring additional machinery, such as powder compaction, vacuum bagging and foam expansion fabrication.

Lockheed Aeronautical Systems Co., Burbank, Calif., is working with integrally heated tooling for non-autoclave production of thermoplastic parts. The company says that by using an unheated pressure vessel in lieu of an autoclave, the requirements for capital, skilled labor, vacuum bagging materials, and energy consumption are reduced dramatically. Laminate cycle times can be only 30-40% of those experienced with autoclaves, according to Lockheed.

The use of an in-situ sensor and a Fourier transform infrared (FTIR) spectrometer to monitor autoclave curing of epoxy composites was discussed by Foster-Miller, Inc., Waltham, Mass. A thin sapphire fiber embedded in the center of the prepreg lay-up serves as a sensor and is connected via fiber-optic cable to a FTIR spectrometer outside the autoclave.

An electron-beam composite curing process has been developed by Aerospatiale, France, a supplier of rocket motor casings for the French military. The process uses a combination of electron beams and x-rays--the latter permitting curing of thicker laminates, although at slower speeds. The most commonly used resins are radiation-sensitive acrylics, although the company also uses BMIs modified with reactive diluents. Aerospatiale is currently setting up production facilities for this process and plans to be able to cure large structures up to 33 ft long and 13 ft diam. by mid-1991.

A numerical model for optimizing pultrusion operating parameters was presented by Shell Development Co., Houston. The model incorporates a one-dimensional finite-difference simulation of the pultrusion process. Based on target parameters such as degree of cure at outlet and/or pultrusion speed, the model presents optimal temperature set points along the die and/or other parameters. Shell says that the program is small enough to be run on a desktop computer.

Heat-transfer analysis of pressure-formed thermoplastic composites at the Virginia Polytechnic Institute and State University, Blacksburg, Va., has resulted in a computer code that predicts the time required for autohesive bonding of the ply interfaces of a polysulfone composite under nonisothermal processing conditions.

A new process for post-forming thermoplastic composites is being studied by Lord Corp., Erie, Pa. The company uses internally heated, matched molds in a press to post-form PPS/carbon-fiber I-beams to an angle of 30 [degrees], with a 6-in. inside radius. Lord says that further research is needed to evaluate the effects of this process on material strength.

The use of exterior molds for filament-wound composite structures, in order to avoid post-processing steps such as machining, grinding, and sanding was discussed by Advanced Composites, Salt Lake City. After a wound part is placed in a rigid exterior mold, the mandrel is inflated to apply sufficient pressure to remove trapped air bubbles and voids and duplicate the surface of the mold. This procedure is desirable for non-uniform shapes that would otherwise require complex post-finishing operations to obtain the desired outside finish. In parts where the inside surfaces require tight dimensional tolerances, a precision mandrel is required.

FIBER-PLACEMENT MACHINERY

A new, computer-controlled fiber-placement machine that uses up to seven axes of notion has been produced by Hercules, Inc., Magna, Utah. The machine transports prepreg tow material from a refrigerated creel through a "ribbonizing" delivery head. The delivery-head roller is said to be close enough to the part to place the material with almost no tension. This feature reportedly allows the placement of material into complex, concave surfaces and in axial orientation, which is not possible with conventional filament winding. Hercules has also developed technology for adding single prepreg tows and varying the band width while ply thickness remains constant, thus allowing the fabrication of parts with varying cross-sections and constant thickness.

In conjunction with Hercules and McDonnell-Douglas Aircraft Co. (St. Louis), Northrop Corp. of Hawthorne, Calif., has developed another automated method of placing prepreg tows on complex-shaped tools. The fiber-placement process includes independent tow delivery, compaction at the laydown point, tow cut/start, and robotic control of the applicator position. Tension is kept low to allow the fabrication of concave shapes.

Material is placed by a Cybotech G-100 six-axis gantry robot with a working range of 42.6 x 16.4 x 6.6 ft. The system also includes a Silicon Graphics 3130 workstation, Adept IC robot controller, Sun Sparc 3390 process controller, and sensors to monitor and control the fiber placement process.

Researchers at the University of Delaware in Newark have developed a robotic filament-winding system for complex geometries. The system uses a Hewlett-Packard SRX workstation to control a Steuble-Unimation Puma 762 robotic manipulator. The most recent development is a head for thermoplastic winding. The head uses two nitrogen gas torches and air cylinders to supply heat and consolidation pressure.

In addition to the robot arm's six axes for fiber delivery, a seventh axis for mandrel manipulation is provided by a belt-driven lathe retrofitted with a servomotor. An in-house computer simulation program is used to develop mandrel shapes and place fibers on the computer screen. Another in-house program simulates the motions of the robot manipulator and its attached payout tool, to expose possible problems such as collisions with the mandrel or other objects in the robot's work space.

Automated Dynamics Corp., Troy, N.Y., has made several improvements on ROWS, its robotic winding system. The design of the gas torch has been modified to reclaim some previously wasted energy. Additionally, incoming gas is now preheated, allowing higher temperatures at lower power settings, and a cooler outside surface, which reportedly allows the torch to be handled more easily. The company also says it has developed an improved heat-transfer model for more accurate temperature control, a feedback element for consolidation pressure control, a refined cut-and-splice mechanism with an air-cylinder-actuated cutting blade for stopping material flow without halting machine movement, and a larger robotic end-effector to allow production of larger parts.

ADVANCES IN EPOXIES

By dispersing insoluble acrylic elastomers in epoxies, Dow Chemical Co., Midland, Mich., says it can enhance toughness without the loss of heat resistance typically experienced with reactive liquid polymer tougheners. Polymerizing the acrylic particles prior to curing the epoxy solves this problem, according to Dow, with minimal increase in viscosity. Other reported advantages include better control of the modifier particles' size, molecular weight, morphology, and surface functionality.

ICI Fiberite Composite Materials, Tempe, Ariz., provided further information on its thermoplastic-toughened epoxy resins, offered under the 977 trade name. The company says the 977-1 system has been formulated for maximum toughness and that it can be cured for 2 hr at 350 F to provide 180 F hot/wet service. Two other products balance toughness with higher temperature performance. The curing of the 977-2 system can now reportedly be optimized to provide up to 220 F hot/wet service, and the 977-3 grade up to 270 F.

ICI Fiberite also presented data on two novel, thermoplastic-toughened cyanate ester resins, X54 and X54-2. The X54 system has been evaluated on both glass fabric and unidirectional graphite tape, and reportedly offers a balance of toughness and hot/wet properties up to 300 F. Demands for greater toughness are said to be met by the X54-2 formulation.

Shell Development Co., Houston, is also developing thermoplastic-toughened epoxies, using primarily polyetherimide. Shell is also studying the toughening of BMIs with polyetherimide, polyhydantoin, and polyarylene ether.

A new epoxy for RTM has been introduced by 3M Co.'s Aerospace Materials Dept., St. Paul, Minn. The PR 500 resin system is a one-part, room-temperature-stable epoxy having a glass-transition temperature of 400 F when cured for 2 hr at 350 F. PR 500 also reportedly has low moisture absorption, and provides toughness not typically found in high-modulus, high-[T.sub.g] resins. 3M says the new material's viscosity drops to under 100 cp during processing, making it advantageous for RTM.

POLYIMIDE DIRECTIONS

Researchers from the University of Tennessee at Knoxville, and NASA Langley Research Center, Hampton, Va., are studying the use of freeze-drying for solvent removal to improve the morphological characteristics of interpenetrating polymer networks (IPNs). Typically, evaporation is used to remove the solvent from an IPN solution. In experiments with a BMI linked to a reactive-endcapped polyimidesulfone, the freeze-dried IPNs reportedly yielded higher [T.sub.g]'s than those produced via evaporation.

A new, toughened BMI resin system from American Cyanamid, Way N.J., is available in carbon prepreg for Cycom X3135 is said to provide lanates free of microcracking and 350 hot/wet performance.

Another new tough BMI prepreg system, Rigidite X5260, from BASF Structural Materials, Inc., Charlotte, N.C., is said to combine the high-temperature performance of BMIs with the damage tolerance of thermoplastics. The new material is also said to have improved thermo-oxidative stability compared to other BMIs.

There were quite a few developments related to NASA Langley's thermoplastic polyimide (LARC-TPI) at SAMPE. NASA researchers described a system for making LARC-TPI prepregs in which carbon-fiber tows are spread open in a fluidized bed and impregnated with thermoplastic powder. The powder is melted by radiant heating and adheres to the fibers. The process is said to produce uniform prepreg tows without imposing severe stress on the fibers or requiring lengthy high-temperature residence times for the polymer.

Foster Miller Inc., Waltham, Mass., says it has improved processing of LARC-TPI by using a thermoplastic liquid-crystal polymer (LCP) as a processing aid at levels of 10-30% by weight. LCP lowered the polyimide melt viscosity enough to allow extrusion at lower temperatures, and it reduced the coefficient of thermal expansion of extruded film to near zero in the direction of primary orientation.

An improved polymerization process for LARC-TPI has led to a new powder version from Mitsui Toatsu Chemicals, Inc., Tokyo. The original powder grades, tradenamed 1000 and 2000 series, were melt-fusible, but not melt-processable. The new polymerization process reportedly allows greater control of molecular weight and MW distribution in LARC-TPI powders, making possible a new 1500 series that is said to be thermally stable and melt-processable. Melt flow is said to be comparable to that of PEEK, polysulfone, and polyethersulfone. Lockheed Engineering & Sciences Co., Hampton, Va., and Old Dominion University, Norfolk, Va., in conjunction with NASA, have evaluated the new material.

A new PMR-type addition polyimide with reportedly improved toxicity, outlife, and tack retention compared with PMR-15 has been introduced by Ferro Composites, Los Angeles. The new non-MDA material, CPI-2310, has a reported temperature tolerance of 600 F and is available in glass, quartz, or carbon-fiber prepregs.

THERMOPLASTIC PREPREGS

To verify the reprocessability of ICI Fiberite's APC-2 PEEK/carbon-fiber thermoplastic prepregs, researchers at the University of Dayton, Ohio, performed a "worst case" processing test. After slowly heating and cooling the material in an autoclave, they reported that multiple processing cycles in extreme conditions did not degrade the material's thermal or mechanical properties significantly.

ICI announced a new offering of APC-2 in pre-consolidated sheet form. The sheets are either vacuum-consolidated to be used in processes such as matched-metal stamping or hydro-rubber forming, or autoclave-consolidated to provide a near-net-shape part. The material was previously offered only in prepreg tow and tape forms.

FIBER DEVELOPMENTS

Researchers at the University of Akron, Ohio, are developing a process for electrostatic dry-powder prepregging of carbon fibers. Ultrafine polymer powders of 5-10 microns are aspirated and electrostatically charged. The charged powder adheres to grounded carbon fibers, which are spread by a venturi laminar-flow air tunnel and a convex roller. The powder is then sinterfused to the fiber in an oven. Thus far, the Akron researchers have worked with polysulfone, PMR-15, polyamide-imide (Amoco's Torlon), and LARC-TPI powders. They are currently seeking to increase the line throughput from 1.6 ft/min to over 6.5 ft/min.

BASF announced that it has been able to produce carbon fibers with a variety of cross-sectional shapes, such as rectangular, tetralobal, and hexalobal. (This follows recent developments in glass-fiber geometries--see PT, April 90.) The latest efforts have been directed toward the development of a trilobal fiber.

Researchers at Concordia University in Montreal have developed a method to achieve a stronger interface between cellulosic fibers and thermoplastics. Use of cellulosic fibers in thermoplastics composites has been limited by the poor compatibility between the fibers and polymers. Silanes have been used to coat the fibers, but this reportedly has not led to significantly improved composite properties.

In this new method, the surface of the cellulosic fibers is modified by silane grafting (covalent bonding) using benzoyl peroxide as a catalyst. This reportedly leads to improved bonding between the coupling agent and fibers. Tensile strength of both PVC and PS reinforced with these fibers is said to be significantly improved over versions filled with conventionally silane-coated fibers.

PHOTO : In-situ "isoclave" fabrication of thermoplastic composite sandwich structures is performed

PHOTO : in one heating cycle via a process from Sundstrand. An autoclave is equipped with a hopper

PHOTO : connected to the mold via a transfer tube.

PHOTO : At the University of Delaware, the motions of a robotic filament winder are simulated to

PHOTO : expose possible problems such as collisions with the mandrel. Other research includes the

PHOTO : development of a head for thermoplastic winding.

PHOTO : McClean-Anderson's filament winding compaction roller is said to provide the constant

PHOTO : consolidation pressure needed to work with thermoplastic resins. Our June 1990 SAMPE

PHOTO : report described other filament winding innovations as well.