News in composite materials, processing and recycling.

The recent annual composites conference of ASM International, Materials Park, Ohio, and the Engineering Society of Detroit covered a range of timely topics, including advanced thermoplastic composites, thermoset recycling, so-called "3-D" fabric reinforcements, and real-time process monitoring of liquid composite molding (LCM) processes such as SRIM and RTM. Here is a synopsis of some of the more newsworthy presentations.

HIGH-PERFORMANCE RTP SHEET

Du Pont Co., Wilmington, Del., described an aerospace-grade formable thermoplastic sheet based on its "XTC" technology. Du Pont XTC sheet of long-glass-reinforced PET was introduced for cost-sensitive automotive uses last year (see PT, April '92, p. 15). This sheet is produced in a papermaking type of process that results in a porous scrim of fibers coated with resin. The porous quality facilitates rapid hot-air heating of the sheet to forming temperature.

For high-performance applications, Du Pont has made XTC-type sheet of 1/4-in. AS4-type carbon fibers and its polyetherketoneketone (PEKK) thermoplastic resin. Offering fuel and flame resistance, as well as static dissipation, Du Pont considers this compression moldable composite to be a lower cost alternative to aluminum in semi-structural aircraft parts. Physical properties of an isotropic panel containing 30% by volume of carbon fiber include a density of 1.44 g/cc, tensile strength of 39,440 psi, tensile modulus of 3.52 million psi, compressive strength of 47,415 psi, shear strength of 17,110 psi, and surface resistivity of 519 ohm/sq.

ADVANCES IN RECYCLING

Researchers at Union Carbide Chemical and Plastics Co. Inc., Danbury, Conn., reported that incorporation of small amounts of organosilicon chemicals increased mechanical properties in polypropylene filled with reground SMC scrap, in recycled Azdel PP/glass-fiber sheet (from GE Plastics, Pittsfield, Mass.), as well as in virgin PP/glass-fiber mixtures. The additives were Carbide's Ucarsil PC-1A or a blend of Ucarsil PC-2A and PC-1B at a 3:1 ratio. Adding 2% of the organosilicon mixture to reground Azdel containing 38% glass boosted tensile strength in injection molded parts from 6850 psi to 9650 psi. Flexural strength rose from 11,240 psi to 15,350 psi; and notched Izod impact strength increased slightly from 1.0 ft-lb/in. to 1.3 ft-lb/in.

Rogers Corp., Manchester, Conn., discussed its work in recycling phenolics. According to Rogers, cured phenolics can be ground and used as filler in fresh phenolic compounds, resulting in mechanical properties that range from unaffected to significantly increased. Rogers incorporated 4-15% phenolic regrind containing approx. 50% glass content. Rogers found that the smaller the particle size of the regrind, the better the results. Production-scale trials used a virgin phenolic with 40% glass and 10% regrind. Tensile strength at 302 F increased from 8279 psi to 9265 psi, while flexural strength at the same temperature jumped from 17,660 psi to 18,995 psi.

In an effort to make lighter weight yet stronger building materials, the Department of Civil and Environmental Engineering at Michigan State University in East Lansing has experimented with replacing as much as 50% of the aggregate in concrete with recycled HDPE. Using a sulfonation process that helps adhere the recycled plastic to the cement matrix, the MSU team was able to stop microcracks in the concrete and improve its impact resistance. The quality of the sulfonation-induced bonding with the cement matrix, the study showed, was more important to the ultimate results than the recycled HDPE characteristics. Therefore, the researchers concluded, it is probable that different types of plastics would produce comparable results.

3-D REINFORCEMENTS

Two innovations from overseas were the top items in reinforcement news at this year's conference. First, Parabeam b.v. of the Netherlands (represented in North America by the FRP Supply Div. of Ashland Chemical Inc., Columbus, Ohio) revealed more details on its novel Parabeam 3-D woven fiberglass fabrics (see PT, Jan. '92, p. 69). Three grades, 3, 4.5 and 6 mm thick, respectively, consist of two woven E-glass fabrics connected by vertical E-glass fibers. They are designed to be used in conjunction with other reinforcements to produce lightweight laminates with polyester, vinyl ester or epoxy matrices by spray-up, hand lay-up or pre-impregnation processes. Because of the fabric's hollow nature, less resin is required to wet it, and the exotherm during production will be low, preventing warping after demolding.

A laminate produced from a layer of 4.5-mm thick Parabeam between two layers of continuous-strand mat (300 g/|m.sup.2~) weighed half as much as, and was slightly stiffer than, a laminate reinforced with six layers of heavier continuous-strand mat (450 g/|m.sup.2~). Bending strength of the Parabeam laminate was 87.75 lb, compared with 112.5 lb for the six-layer mat laminate.

MONITORING SRIM/RTM PROCESSES

Molders and researchers generally agree on the importance of monitoring the liquid composite molding process (SRIM or RTM) to detect mold fill and resin cure, so as to reduce material waste and minimize cycle times. The most common and reliable way to do this has been through mounting reusable thermocouples at the center of the laminate to evaluate peak exotherm and to ensure that resin exotherm isn't suppressed by the mold. This method, though, leaves a small hole in the laminate that may be a source of weakness.

In an attempt to overcome this problem, K.N. Kendall at the Ford Research Laboratories, Dearborn, Mich., has tried a few other ways of monitoring the LCM process. Ford found that non-intrusive, real-time process monitoring was possible through permanent mounting of thermocouples, pressure transducers, or dielectric sensors in a mold. Temperature and pressure sensors (especially the latter) can provide useful information on the arrival of resin at a point in the mold, mold filling, and cure, and can do so at modest cost, but require user definition and development of both hardware and software. For more money, flush-mounted dielectric sensors offer the only system available commercially as a complete hardware and software package, Kendall said, providing SPC records and a reliable indication of when a part is ready to be demolded. Ford used the ICAM 1000 dielectric sensing system from Micromet Instruments, Inc., Cambridge, Mass.