Lots of new composite materials and fibers at SAMPE meeting.

Anticipated sharp spending cuts at the Pentagon, the best customer for advanced composites during the 1980s, might well cause trepidation among composite materials producers and fabricators. However, the mood at the recent 36th annual SAMPE conference and exhibition in San Diego suggested that the pain is not yet being felt. Although unlikely to offset retrenchments in defense, exhibitors saw some cause for optimism in the expanding number of applications in other markets. Emphasis on applications from microwave antennas on spaceships to medical prosthetics, golf clubs, fishing poles, and tennis rackets suggests composites manufacturers are looking towards other markets, feeling they have some time yet before the momentum of current military programs winds down.

According to the Suppliers of Advanced Composite Materials Association (SACMA) in Washington, D.C., poundage consumption of composites is, at least for a while, expected to maintain the 10%-20% annual growth rate it enjoyed for most of the 1980s, with growth peaking in 1995 or 1996. After that, SACMA says, there will probably be 5%-7% growth in each of the following 10 years.

In the meantime, the SAMPE show indicated that suppliers are still prolifically generating new thermoset and thermoplastic materials, reinforcement fibers, and processing methods.

BCB FOR ADVANCED

COMPOSITES

Dow Chemical Co., Midland, Mich., introduced an experimental benzocyclobutene (BCB) resin for advanced composites. Dow first introduced these resins only about a year ago, and then only for high-temperature electronic components as an alternative to polyimides (see PT, Jan. '90, p. 13; April '90, p. 45; June '90, p. 107). But Dow's new XU 35040.00L is aimed at composites, where it reportedly can provide short-term hot/wet service at 525 F, long-term oxidative stability at 500 F, and fairly good impact damage resistance. [T.sub.g] is 644 F. Dow thinks BCB should be attractive to airframe manufacturers looking for performance midway between bismaleimide (BMI) and PMR-15 polyimide.

In the five papers Dow presented on the new resin at SAMPE, BCB's excellent processability was emphasized. It is a one-component system that cures without evolving volatiles. The monomer is a solid that melts at 306 F and "flows like water." It also shows little or no change in viscosity for 100-150 min at 320 F. This is said to provide easy fiber wetting and impregnation in RTM-type processes. In one variation, the solid monomer was placed in a hot mold together with a fiber preform. Upon heating, the resin flows and impregnates the fiber.

Because of BCB's highly exothermic nature, Dow recommends using a prepolymer for RTM rather than monomeric form. Prepolymer has a lower melting point (266 F) and higher viscosity. It reaches an RTM-processable viscosity of 200 cps at 320 F and shows no viscosity increase at that temperature for 3 hr. Test panels were cured in the mold at 437 F for 2 hr and then post-cured free-standing for 2 hr at 482 F. Carbon-fiber composites maintained 80% of their interlaminar shear strength at 450 F and 90% of compressive strength at 325 F. (CIRCLE 9)

SPOTLIGHT ON CYANATE ESTERS

In terms of new resins, cyanate esters seemed to be the stars of this show. In separate efforts, researchers at Dow Chemical Co., Midland, Mich., and the Specialty Chemicals Div. of Rhone-Poulenc Inc., Cranbury, N.J., have been working on rubber toughening of cyanate esters. Dow's experimental XU 71787.07 is a blend of its earlier XU 71787.02 and 10% of an experimental core-shell rubber modifier to enhance damage tolerance. Use of the elastomer raised compressive strength after impact by 24% without appreciable increase in resin viscosity or lower glass-transition temperature. (Dow says the rubber is "also surprisingly compatible with most epoxy formulations.") The blend has a hot/wet service temperature of 350 F. Dow is aiming the resin at space structures and cryogenic applications. However, the company has found that composite toughness is quite sensitive to the precise grade of carbon fiber used. Even between two intermediate-modulus fibers, some measures of damage tolerance can differ by a factor of two. (CIRCLE 10)

Rhone-Poulenc uses two hydroxyl-terminated polybutadiene/acrylonitrile oligomers to toughen its AroCy cyanate ester monomers and prepolymers. At the show, the company introduced developmental RTX-366 dicyanate monomer and REX-378 prepolymer, both rubber-modified. At 5% to 20% concentrations, the reactive rubbers increase fracture toughness two to seven times. The company chose reactive rubbers because when compared with particulate tougheners such as core/shell latex particles and powdered thermoplastics, in-situ reactives phase-separate homogeneously during cure and are not subject to "fiber sieving" or gravitational segregation. (CIRCLE 11)

Like the rubber-modified resins, a new group of modified aromatic cyanate ester resins from ICI Fiberite, Tempe, Ariz., offers low moisture absorption and strong electrical properties. Available in three grades, the new Fiberite 954 resins have [T.sub.g.'s] of 330-660 F and are targeted at aerospace applications. All three grades cure at 350 F, and all three can be prepregged via hot-melt or solution techniques.

Fiberite 954-1 is a controlled-flow cyanate resin with 300 F wet service capability, which can be formulated for either autoclave or press molding. Typical applications, ICI says, include radomes, antennas, and other components requiring very low dielectric and loss-tangent properties.

Fiberite 954-2 also has 300 F wet service temperature. Toughened with ICI's proprietary thermoplastic technology, the resin is aimed at applications where inpact resistance, light weight, and good dielectric properties are required, such as secondary aircraft and space structures, and cryogenic tanks.

Fiberite 954-3 has a -200 F to +250 F service temperature. It reportedly reduces microcracking and lowers moisture absorption, ICI says, making it applicable to primary and secondary space structures. (CIRCLE 12)

A similar group of cyanate esters was introduced by Bryte Technologies, Milpitas, Calif. The three grades in the BTCy series cure at 250 F or 350 F. Like most of the other new cyanate esters, BTCy-1, BTCy-2, and BTCy-3 reportedly offer good electrical properties, low moisture absorption, and improved resistance to microcracking. BTCy-1 has a [T.sub.g] of 518 F and continuous hot/wet service temperature of 350 F. BTCy-2 has a continuous-service temperature of 300 F, [T.sub.g] of 375 F, and 0.6% moisture absorption at 212 F saturation. BTCy-3 offers continuous-service temperatures of 200 F cured and 250 F post-cured. All the resins are aimed at a broad range of applications from aircraft and spacecraft to high-performance electrical substrates. (CIRCLE 13)

In a paper at the conference, BASF Structural Materials, Charlotte, N.C., reported on its newest "Metlbond" cyanate esters, also targeted at the next generation of radomes and microwave applications. Compared with epoxies and bismaleimides, these new matrix materials--available in glass and quartz prepregs, adhesive film, and syntactic foam--reportedly offer a three-fold reduction in loss tangent; 10% reduction in loss dielectric constant; electrical properties unaffected by frequency and temperature changes; low moisture absorption; and good wet electrical properties. Their high mechanical properties allow minimum skin or laminate thickness, BASF says (see Fig. 1). (CIRCLE 14)

Now there are even cyanate ester foams available as moldable core materials. Hexcel Corp., Dublin, Calif., discussed preparation of a foamable cyanate ester in the form of a thin film cast from a mixture of resin, blowing agent, surfactant, and other additives. This film is cut to a desired shape and then placed in a hot mold or free foamed. At 338-410 F, it melts and then foams and cures simultaneously. Foam density can be controlled by the amount of blowing agent and the foaming temperature.

Cyanate ester foams of 0.10 g/cc density were compared to PVC and polymethacrylimide (PMI) foams of the same density, which are commonly used as sandwich cores in the aerospace industry. Cyanate foam had the lowest compressive strength at room temperature but the highest at 300 F, and retained almost 50% of its initial compression strength at 351 F, when even the PMI foam had yielded. Cyanate foam had a [T.sub.g] of 388 F. From a thermal degradation standpoint, cyanate ester foam was also much more stable. In addition, the cyanate foam had the lowest moisture absorption and by far the lowest smoke generation in the NBS smoke chamber. Dmc was only 61 for cyanate foam vs. 483 for PVC and 102 for PMI. (CIRCLE 15)

EPOXIES FIGHT BACK

While some see cyanate esters as a replacement for epoxies, SAMPE showed there's still a lot of room for improvements in the latter. For example, a pair of two-part epoxy/anhydride systems targeted at the same radome and microwave market that attracts the cyanates was introduced at the show by Advanced Resins Inc., Grand Junction, Colo. Built chemically upon a flexible polybutadiene chain, Ricotuff and Ricotuff LV (low viscosity) reportedly broaden the application potential for epoxies. They have oxygen contents of 10%-12%, compared with the 20% in cycloaliphatic epoxies and 23%-30% in bisphenol-A resins and epoxy novalacs. This reportedly gives them very good electrical properties. They also adhere to virtually all laminate fibers and metals and offer good chemical and moisture resistance, Advanced Resins says. (CIRCLE 16)

Dow Chemical Co.'s new Tactix 177 bisphenol-A-based resin and Tactix 178 hardener are aimed at RTM and open-mold processes where a high [T.sub.g] is not required. The system reportedly offers processors low viscosity and good fiber wetout, making it applicable to composites with high fiber content. Reported cure time is 30 min at 266 F plus 3 hr at 122 F. (CIRCLE 17)

Researchers from Shell Development Co. in Houston and Belgium claim to have achieved a new level of epoxy performance with the development of a resin system with Tg of 392 F, high fracture toughness, and good processability. To achieve such performance, Shell blended its Epikote Resin 1079 and Epikote 1071 with its SLRES 3051, an epoxy toughened by being reacted with a high-molecular-weight, immiscible polymer. Epikote 827, a low-viscosity diglycidyl ether of bisphenol-A, was added to the systems in order to reduce the viscosity. Shell's SLRES aromatic amine curing agent was added to provide a stable, tacky hot-melt system that can be prepregged easily at 167 F. These system also boast storage stability of at least two months at 0 F, working life of 18-30 days at room temperature, and pot life of at least 1 hr at 257 F. [T.sub.g.'s] are 392-406 F dry and 360-383 F wet. (CIRCLE 18)

In addition to new resins, novel curing techniques offer promise of yielding dramatic property improvements in epoxies. Recent research at the U.S. Air Force Astronautics Lab at Edwards Air Force Base, Lancaster, Calif., and the University of Dayton, Ohio, has confirmed earlier work done by Soviet scientists suggesting that the ultimate tensile strength, strain to failure, and toughness of a neat diglycidylether of bisphenol-A (DGEBA) epoxy system could be significantly increased when the system was simultaneously cured and exposed to a 3700 Oersted or greater magnetic field oriented perpendicular to the material's test axis. The process did not work, the researchers stressed, when the magnetic field was below 3700 Oersted or oriented parallel to the resin specimen. (CIRCLE 19)

Also, a method for eliminating curing shrinkage in carbon-reinforced epoxy and thereby potentially increasing impact-damage tolerance, was described in a paper given by Dr. Barbara F. Howell of the David Taylor Research Center, Annapolis, Md. According to Howell, copolymerization of 11.7% dinorbornene spiroorthocarbonate (DNSOC) with epoxy can eliminate shrinkage and, in some cases, produce expansion. The addition of DNSOC to DGEBA epoxies causes a tenfold reduction of interfacial tensile stress between carbon fibers and the epoxy matrix and also reduces longitudinal stresses on the fibers as indicated by reduced waviness in the cured composite. Also, Howell reported, matrices containing DNSOC don't show the colored strain pattern seen under polarized light with identically cured epoxy, indicating the DNSOC reduces strain in the matrix as well as near the fibers. (CIRCLE 20)

In the same vein, an epoxy system that reportedly results in warp-free carbon-fiber composites with higher mechanical performance due to lower residual thermal stress between fiber and matrix or between cross plies was reported by Mitsubishi Rayon Co., Ltd., of Nagoya, Japan. By reducing residual thermal stress, Mitsubishi says, flexural strength can be increased by as much as 30% and warpage in fishing rods made with the new system was reduced by about 25%. (CIRCLE 21)

NEWS IN CARBON FIBERS

Unsatisfactory shear strength of carbon-fiber composites is commonly attributed to a lack of bonding between the matrix and filaments. Researchers at the Georgia School of Technology in Atlanta found that interlaminar shear strength could be increased two or threefold by electromechanically surface treating the fibers with ammonium nitrite solution for about 10 min. This was sufficient to obtain maximum uniform oxidation across the fiber bundles without changing the surface morphology or mechanical properties of the fibers. Reportedly, the resulting carboxylic and phenolic functional groups on the fibers remain stable at temperatures up to 842 F, indicating the treatment is suitable for carbon fibers incorporated in composites with high-temperature curing resins and thermoplastic matrices. (CIRCLE 22)

In the show exhibits, several firms introduced new carbon fibers. Courtaulds Advanced Materials, Sacramento, Calif., unveiled Grafil 58-650, an aerospace-grade fiber with 58 million psi tensile modulus and 650,000 psi tensile strength; and Grafil 33-600 WD, a wide, flat tow for prepregging. Designed for use in nonaerospace applications such as sporting goods, the high strength and modulus of the tow allow use of prepregs as light as 70 g/sq m, Courtaulds says. (CIRCLE 23)

Hercules Inc., Wilmington, Del., introduced its latest carbon fiber, AS4C, a commercial grade of its AS4 aerospace fiber. AS4C is a high-strength, high-strain, PAN-based fiber that's been surface-treated and sized to improve its interlaminar shear properties, handling characteristics, and structural properties. Applicable to weaving, prepregging, filament winding, pultrusion, and molding compounds, the fiber is aimed at applications such as sporting goods, a Hercules spokesman says. Tensile strength is 550,000 psi, tensile modulus is 38 million psi and the fiber has 1.6% strain to failure. It costs between $10 and $15/lb.

PROGRESS IN THERMOPLASTICS

Although not yet at the point where it can be called thriving, the thermoplastic composites business keeps growing slowly. For example, Phillips 66 Co.'s Advanced Composites Div., Bartlesville, Okla., reported that Boeing Co. in Seattle recently selected Phillips' Avtel L PPS/glass prepreg sheet as the material for the skin of its Advanced Interdiction Weapon System (AIWS) missile. Boeing is said to be forming a 14-ft skin every 12 minutes. Phillips introduced the Avtel L series at last year's SAMPE show; it's based on a special PPS resin with enhanced fracture toughness (see PT, May '90, p. 12). Avtel L is aimed at subsonic aerospace applications. A Phillips spokesman says the company also expects to release a carbon-fiber sheet grade later this year. (CIRCLE 24)

High-temperature thermoformable composites of its Declar polyetherketoneketone (PEKK) resin were discussed in a paper by Du Pont Co., Wilmington, Del. The so-called "LDF" (Long Discontinuous Fiber) sheet of Declar with about 58% by volume of Hercules' AS-4 carbon fiber in unidirectional alignment reportedly can be thermoformed into aerospace parts that retain sufficient crystallinity to resist aerospace fluids. Tg is 313 F by DSC (compared with 291 F for ICI's PEEK) and 374 F by DMA. Formed parts reportedly retain 95% of original flexural modulus at 302 F, 60% at 374 F. (CIRCLE 25)

Declar is reportedly one of the few thermoplastics that meet aircraft flammability and smoke toxicity requirements for cabin interiors. (Du Pont says Declar and PEEK have identical rates of heat release by the OSU test.) Unreinforced Declar sheet for thermoforming into aircraft and other parts is being extruded by Sheffield Plastics Inc., Sheffield, Mass. Deep draws of 3-8 in. have been accomplished without cracking, Sheffield says. (For other properties, see Fig. 2). (CIRCLE 26)

Amoco Performance Products Inc., Atlanta, introduced a commingled-fabric thermoplastic prepreg that reportedly exhibits exceptional drapability in shaping complex forms and deep-draw parts. Preliminary tests of new Radel 8320, a combination of fiberglass and carbon reinforcements together with filaments of Radel polyarylsulfone, show it produces thermoplastic composites with low smoke and heat release, good toughness, and improved dimensional stability. Processing temperatures are below 700 F. (CIRCLE 27)

Another new commingled thermoplastic fabric was introduced by Asahi Kasei Carbon Fiber Co. Ltd., Tokyo. The two versions of WIP (Web Interlaced Prepreg) use interlaced thermoplastic and carbon fiber or TP/fiberglass filaments to give it enough drapability to be compression molded on curved molds at lower pressures and shorter cycles than most other co-woven prepregs, Japanese researchers reported. They went on to explain that the number of binding points between the TP and fibers is so great that WIP can be cut, sheared, or punched without damage to its edges. They said the fabrics can be molded with a variety of matrices, including PEEK, PEI, PPS, PET, and nylon 6. (CIRCLE 28)

Idemitsu Kosan Co., Ltd., Chiba, Japan, and the Center for Composite Materials at the University of Delaware in Newark jointly presented experimental work with pitch carbon-fiber prepregs that were powder impregnated with Idemitsu's relatively new engineering thermoplastic, a polyethernitrile designated ID300. Powder-impregnated tows were wound on an aluminum plate, consolidated by hot pressing into a sheet, and then diaphragm-formed into hemispherical shapes using polyimide film as the diaphragm. ID300 is an injection moldable, semicrystalline polymer with high heat and chemical resistance. Its Tg is 293 F and melting point is 644 F. The base resin has a specific gravity of 1.32, 24-hr water absorption of 0.07%, tensile strength of 19,000 psi, flexural strength of 27,550 psi, flex modulus of 537,000 psi, notched Izod impact strength of 0.7 ft-lb/in., and unnotched Izod of 5.1 ft-lb/in. (CIRCLE 29)

The latest in thermoplastic honeycomb core materials is a fusion-bonded product made of GE's Ultem polyetherimide by Supracor, Sunnyvale, Calif. It's available neat or reinforced with glass, aramid, or carbon fibers. The material is said to meet 1990 FAA regulations for flammability and smoke emission. (CIRCLE 30)