From recycling to RTM: SPI Composites meeting preview.
Recent years have seen the emergence of thermoset recycling and liquid composite molding (RTM and SRIM) as major new themes at the annual SPI Composites Institute conference and exhibition. This year's meeting in Cincinnati next month will continue those themes, but the preponderance of new technology discussed will be in a more traditional area - SMC and BMC. Materials and additives for improving toughness and surface quality, flow and cure simulation software, and benefits of continuous-strand mat reinforcement are some other topics. Also among the newsworthy developments are a process combining thermoplastic extrusion in-line with thermoset pultrusion; combined RTM and compression molding; recyclable thermoplastic SRIM; long-fiber epoxy injection molding; microporous epoxy reinforced foams; molded synthetic granite countertops; spray-up composite tools; and thermoplastic honeycomb cores.
SMC/BMC: SMOOTH & TOUGH
Addressing what remains a pre-empinent quality issue in automotive SMC, several technical papers will discuss new resin systems and low-profile additives. Aristech Chemical Corp., Pittsburgh, will present a versatile new resin system for SMC, TMC and BMC, consisting of Aristech's new MR 13006 polyester and Neulon T Plus low-profile additive from Union Carbide Chemicals & Plastics Co., Inc., Danbury, Conn. The system reportedly meets or exceeds General Motors surface requirements in compression and injection molding, in standard and fast-cure formulations, and in a special formulation for grille-opening panels.
Union Carbide will report on three new low-profile additives. One is XLP9004, said to provide injection molded polyester panels with smoothness better than Super Class A SMC compression molded panels, while increasing impact strength over standard injection molded parts.
The second is XLP 57D11, said to give significantly improved deep-color pigmentation with modified, rigid polyester resins in Class A, zero-shrink SMC/BMC. The additive also increases flexibility and toughness of rigid polyesters, Carbide says.
Third, Carbide has a new developmental XLP6070 low-profile agent for highly filled formulations. Thanks to its viscosity-reducing effect, "super-surface" body panels reportedly can be produced with calcium carbonate loadings up to 300 phr. No change in surface quality, thickening behavior, or shrinkage characteristics are observed relative to standard formulations, according to Carbide.
Vetrotex Certain Teed, Valley Forge, Pa., will show that substituting continuous-strand mat for chopped glass in automotive SMC can yield improved surface quality as well as strength. Test plaques and full door panels were molded with standard and mat-reinforced SMC, both at 23% glass content and the same molding conditions (40-50% charge coverage). Lower porosity and waviness resulted with Vetrotex's Unifilo U605 mat, along with higher stiffness. Glass distribution throughout the part was equally uniform, whether chopped or continuous.
Interplastic Corp.'s Commercial Resins Div. in Minneapolis will present a new low-profile SMC/BMC polyester that offers an unusual combination of excellent pigmentability in a one-component system and long-term resistance to low-profile separation before and after compounding. Interplastic plans to extend this technology to thickenable vinyl esters.
Besides achieving Class A finish, making auto body panels tougher and more impact resistant is also a vital goal. Reichhold Chemicals Inc., Durham, N.C., will report on its new toughening agent, a reactive liquid polyester (Dion 31627-00) that is compatible with the matrix polyester and can reportedly yield a Class A surface while improving flexibility and energy absorption and reducing moisture sensitivity in SMC, BMC, TMC and pultrusion. As opposed to "flexibilizing" additives, this one is said to increase toughness without "excessive" loss of stiffness. In tests with several types of polyester resins, replacement of 20-50% of the resin with this additive increased toughness with no loss of dimensional or surface control, Reichhold says. In SMC, Dion 31627-00 reportedly demonstrated low viscosity for easy processing while maintaining constant and reproducible thickening response.
Coating glass strands with a rubber has been tried recently as a means to increase the toughness and resilience of automotive SMC. Joint research at the Dept. of Polymer Science and Engineering at the University of Amherst, Mass., and the Dept. of Materials Science and Engineering at MIT in Cambridge, Mass., involved drawing continuous glass strands through various rubber solutions, curing the coatings, and then chopping the glass and adding it to SMC. This work, sponsored by GenCorp Automotive Div., Akron, Ohio, showed an increase in the strain to first crack from 0.27% to 0.32% and a boost in bending strength from 13,000 psi to 17,000 psi. Force needed to fracture the sheet increased from 4.8 in.-lb to 6.8 in.-lb.
A new family of one-part epoxy adhesives for adhering SMC automotive body panels to metal will be discussed by Lord Corp., Erie, Pa. The heat-activated products require a minimum of surface preparation, Lord says.
For nonautomotive SMC - specifically, sanitary ware - Sherwin-Williams Co.'s Consumer Coating Div., Greensboro, N.C., has developed a one-component, vinyl ester in-mold topcoating system that is injected into the closed mold under pressure after the part is molded. The in-mold coating is virtually free of VOCs and no subsequent painting is required. The resulting 5-7 mil coating reportedly offers high gloss and hardness, and passes ANSI Z124.1 standards for stain, chemical and water resistance. Initial applications have been in sinks, bathtubs and whirlpools, but the company hopes to develop weatherable versions for satellite dishes, tractor hoods, outdoor furniture, and non-Class A auto parts like engine covers. Sherwin-Williams says the coatings can also be extended to BMC injection, RTM, and reinforced thermoplastic sheet stamping.
SMC PROCESS CONTROL &
Production molding of thousands of parts at a commercial installation has
proven that SMC molders can save 13-17% in cycle time through closed-loop press control based on in-mold dielectric monitoring. So says Micromet Instruments, Inc. of Cambridge, Mass., whose ICAM-1000 dielectrometer and SPC software has been used on a 2000-ton press molding a "consumer product" measuring 4 x 5 ft and 1/8 in. thick at an undisclosed customer. The system uses in-mold dielectric sensing to detect when full cure has occurred and then signals the press to open, rather than waiting for a timed interval. The system adjusts automatically for random variations in mold temperature, charge placement, or material uniformity.
In this case a manual press timer set for a 60-sec cure had yielded 100% good parts. But the dielectrometer revealed a standard deviation of 5 sec and an average actual cure time of 50 sec. Thus, by controlling the press according to the actual cure profile on each cycle, an average of 10 sec was saved, while still ensuring consistent part quality. In fact, the system automatically lengthened the mold-closed time in one instance when a platen heater was accidentally turned off, thereby avoiding production of scrap parts.
Micromet's new SPC software can monitor and report full processing data on as many as 1000 parts before it needs resetting by a machine operator. It can be programmed to record partial data on most parts, and full data only at preset intervals such as every tenth part. This increases the time between resetting.
Also designed to shed more light on the SMC molding process is computer modeling software developed at Ohio State University, Columbus, that combines models for mold filling, heat transfer, and curing into one simulation routine. Designed at the University's Dept. of Chemical Engineering and now in use at a few companies, the 3D finite-element modeling program displays such data as the progression of flow fronts and changing profiles of temperature and degree of cure through the thickness of the part. Volumetric shrinkage prediction is in development. While software has been developed in the past to simulate the major aspects of SMC processing, this is believed to be the first time all of the important factors have been combined into a single program.
Besides looking for ways to reuse cured thermosets, industry is also designing composites to be more easily recyclable. Researchers on a federal government-funded project have been working since 1986 on thermoset resin systems containing reversible crosslinks. Initial work has focused on high-value polymers that would economically justify recovery. Latest results of this joint project will be discussed by scientists from Polytechnic University, Brooklyn, N.Y.; the National Renewable Energy Laboratory, Golden, Colo.; and A.J. Power and Associates, Boulder, Colo.
They developed reversible epoxies cured with crosslinking agents that contain disulfide bonds, such as dithiodianiline (DTDA). The bonds can be cleaved by reducing agents (hydrogen and hydrazine), rendering the resin completely soluble. Bonds can be reset by oxidation or by re-crosslinking with a polyfunctional reagent that reacts with the thiol groups formed in reduction. The researchers say the resulting material has thermal and mechanical properties near those of virgin epoxies. A similar approach was applied to reversible polyimides based on the novel disulfide-containing monomer DTDA-BM (a bismaleimide of DTDA). Properties of recycled material could be tailored to be comparable to commercially available systems.
Recycling reinforced thermoplastic composites is much simpler. DSM RIM Nylon, Inc., Augusta, Ga., has been working for over a year on SRIM applications of its Nyrim nylon 6 block copolymer RIM systems. DSM says they offer significantly higher toughness than thermoset SRIM materials, plus the benefit of recyclability. Tests showed that quantities of as little as 2000 lb of nylon SRIM could be recycled into injection molding compound for as little as 30 [cents]/lb. Recycled unreinforced Nyrim, DSM says, has equal or even better properties than virgin material (owing to some increase in molecular weight during reprocessing).
One advantage of Nyrim over thermosets, DSM reports, is greater design flexibility. Unlike thermosets, the inherent toughness of the unreinforced material allows incorporation of resin-rich ribs, bosses, attachment points and even raised decorative patterns.
ADVANCES IN PERFORMING
Preforming for SRIM and RTM continues to be a major subject of R&D. Dow Chemical Co., Midland, Mich., will report on successful laboratory demonstration of a one-minute cycle in producing SRIM preforms with high glass loadings from thermoformable mat. Dow will also discuss its new "cut-and-shoot" process in which preforming, injection, and part trimming take place in the same tool.
New versions of equipment for preforming will be discussed by Cannon USA, Mars, Pa. In order to eliminate production interruptions for frequent changing of thermoformable mat rolls in its Compotec preforming system, Cannon has developed large-volume glass-mat preforming systems with automated roll changes. For structural parts, the mat layers in preforms may all have different orientations and densities or even be of different natures; thus, Cannon has two separate unwinding and cutting stations that slide on rails. As soon as the rolls in one system are empty, it slides out and the second system with fresh rolls slides automatically into place.
However, RTM production of automotive body panels requires only three to four layers of one kind of mat and a surface veil. For this case, only two kinds of mat rolls are used, and three of each kind are mounted at a time. When one roll is empty, the following roll takes its place automatically. Operators are needed only to mount fresh sets of rolls and to remove the mandrels from spent rolls.
And for thick preforms requiring eight or more layers of glass (such as for bumper beams), Cannon now has a heating-station design with three independently controlled substations for the mat to pass through. This eliminates long dwell times of more than 60 sec at a single heating station. Cannon has further developed the forming station by adding an adjustable hold/slip frame separate from the transporting frame, which holds the mat against the lower mold by means of individual spring pushers around its perimeter.
MORE RTM NEWS
Relatively low tooling cost is a major attraction of the RTM process. However, the traditional inexpensive epoxy molds have poor heat-transfer characteristics, resulting in nonuniform mold temperatures during filling and uneven cure exotherm. Researchers from Ohio State University's Depts. of Chemical and Mechanical Engineering in Columbus evaluated thermal characteristics of alternative mold materials and their effect on RTM filling and curing. They found that higher-thermal-conductivity metal molds, such as a chrome/copper alloy and 6061 aluminum, heated and cooled faster than epoxy molds; and the epoxy's low thermal conductivity contributed to larger temperature differences throughout the tool. The Ohio State team also says the long lag time to reheat the epoxy tool following injection adversely influenced the cure reaction. The epoxy mold can store heat better than metal molds, but mold temperatures are more difficult to control.
A problem that has plagued RTM processors is formation of voids in laminates. Work at the Swedish Institute of Composites in Pitea found that the quality of RTM laminates can be significantly improved with vacuum assistance. However, the mold must be vacuum-tight to avoid leakage of air into the laminate. The Swedish team refutes a popular myth that styrene boiling is responsible for poor results when a vacuum is used; the boiling point of styrene, they note, is much higher than normal RTM temperatures.
Another paper from the Swedish Institute suggests a novel technique combining RTM with cold compression molding for cases when viscous resins or high glass contents might make fill times prohibitively long. The idea is to inject resin while the mold is not fully closed; flow resistance is lower since the glass is not highly compressed, reducing its volume fraction. After injection is complete, the mold is fully closed, distributing the resin throughout the remainder of the part.
THERMOPLASTICS IN PULTRUSION
Integrating pultrusion with thermoplastic extrusion is the novel theme of a presentation by Dexter Pultrusions Div. of Dexter Corp., Aurora, Ohio. Thermoset pultruded parts have been unable to attain super-smooth surfaces or high degrees of stain resistance and weatherability without secondary operations to apply paint or surface sealant. Dexter has devised a method to overcome this limitation by combining thermoplastic extrusion in-line with the thermoset pultrusion process. After the pultruded polyester profile emerges from the die, it is surface treated or a solvent-based acrylic adhesive is sprayed on. A small vertical extruder with a crosshead die then applies the thermoplastic coating. After the thermoplastic is applied, the profile enters a cooling bath.
Since Dexter patented this process in July 1990, more than a million ft/yr of thermoset pultruded profiles have been extrusion coated with thermoplastics. Dexter says this technique permits incorporating surface details and complexities that may not be practically achieved via pultrusion.
Dexter notes that in order to minimize problems from offgassing of the hot pultrusion profile after it leaves the die, the pultrusion die entry temperatures are raised to move the exotherm point as far upstream as possible, permitting the lowest possible die exit temperature. Dexter also found that some thermoplastics will not adhere to the polyester pultrusion without pre-application of an acrylic adhesive, but certain styrenics such as ABS and ASA adhere well without it. This was achieved by mixing a thermoplastic into the polyester formulation (several have been found to work) and applying a proprietary surface treatment to the pultrusion after it leaves the die.
NOVEL EPOXY PROCESSING
Two unusual approaches to processing epoxies will be discussed in Cincinnati. One is injection molding long-fiber compounds in order to achieve higher mechanical properties, as has been done with thermoplastics. Case Western Reserve explored this approach, using a RIM-pultrusion approach to impregnate continuous fiber strands, which are subsequently chopped to any desired length (pultrusion impregnation is also used to make thermoplastic long-fiber compounds).
Fiber-reinforced epoxy microporous foams are a new type of composite material to be presented by the Institut fur Werkstofftechnik at the University of Kassel, Germany. The foams are produced by press molding at 175 F, using siloxanes as foaming agents; the latter react with the resin during cure with amino hardeners, releasing hydrogen gas. Such foams have been used to make surfboards and underbody panels for the BMW Z-1 roadster.
MORE NEW RESINS, ADDITIVES
Here's a miscellany of other news to look for at the meeting:
*Reichhold will present an improved low-profile laminating resin for marine applications. Polylite 33-269 can produce consistently higher strength and elongation than orthophthalic and rigid low-profile (DCPD-based) resins, the company says. And, since it reaches full cure faster, it results in a smoother surface finish. Reichhold says higher elongation and toughness should improve crack resistance. Furthermore, Reichhold says the resin's hydrolytic stability exceeds that of orthophthalic and rigid low-profile systems, and secondary bonding is improved, too.
*Dramatic reductions in composite tool fabrication times reportedly can be achieved with a new resin from Reichhold Chemicals. Production-ready tools can be built in under 36 hr, the company says, with a special room-temperature-curing, low-profile polyester sprayed up with glass using a conventional chopper gun. Up to 1/8 in. can be deposited at one time - compared with one or two layers of hand lay-up per day - and subsequent layers can be applied immediately after roll-out without waiting for gel and cure to finish. Lower cost roving is used in place of mat or fabric, and the company says the zero-shrink resin provides less molded-in stress for longer tool life.
*BFGoodrich Specialty Polymers & Chemicals, Cleveland, has a new approach to toughening vinyl esters. Previous efforts have involved pre-reacting BFG's reactive liquid rubbers into the epoxy backbone of the vinyl ester, or mixing a reactive liquid rubber with the vinyl ester prior to curing - the latter being less successful. Now, BFG has found that combining both approaches yields at least fivefold higher toughness than either alone.
*An old material is finding new uses as a core for composite sandwich panels. Norfield Corp., Newtown, Conn., will discuss this new role for its NorCore "stretched polymer" thermoplastic honeycomb. It was developed in the 1970s, stemming from Union Carbide patents that were licensed exclusively to Norfield. A proprietary hot-platen technique transforms a thermoplastic flat sheet (polycarbonate, HIPS or ABS, for example) into a deepdrawn waffle-like structure with 5-10 times lower bulk density and 50-100 times greater stiffness.
*A new kitchen countertop material consisting of a high-performance polyester and crushed granite stone will be presented by Dainippon Ink and Chemicals, Inc. of Japan and its Reichhold subsidiary. DIC Hicerami Stone is produced by formulating a BMC, compression molding it for 8-12 min at 250-300 F and 1300-1700 psi in a 2500-3000 ton press, and polishing and cutting the finished part. Because of the high abrasiveness of coarse crushed granite, a special BMC mixer and "super-hard" steel mold insert are used. The product is said to have better durability, stain resistance, and workability than natural stone.
*And for improved dispersion in cultured marble, the Organic Peroxide Div. of Atochem North America, Inc., Buffalo, N.Y., is developing a finer particle size of its Luperco AFR-400 pumpable BPO paste.
PHOTO : Using a three-step preformer, Dow Chemical Co. has been able to produce thermoformable SRIM preforms with high glass loading in a one-minute cycle time.
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|Title Annotation:||thermoset recycling|
|Date:||Jan 1, 1992|
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