Fillers: making RIM look good.

For that automotive Class A finish, finding the right filler appears to be one key.

As Detroit increases its reliance on plastic body panels, the prospects for RIM appear to hinge partly on selecting or developing fillers that confer the needed stiffness and heat-sag resistance and assist in providing a smooth Class A finish.

"Three years ago, Class A RIM was impossible," says John Phelps, director of the automotive group at Miles, Inc. "The next 5-10 years will hold tremendous opportunity for RIM materials."

Already established in the manufacture of bumper beams, nonappearance parts, and about 60% of the fascia made in North America, RIM is set to compete with SMC in exterior body panels, RIM materials suppliers say, thanks in part to some innovative reinforcing fillers.

General Motors, for instance, reportedly will use mica-reinforced polyurea RIM for the fascia and body panels on the next generation of its Pontiac Firebird and Chevrolet Camaro.

Until now, the most used filler in RIM has been milled glass, such as Owens-Corning's 737 BD. But with increased emphasis being placed on surface quality for new applications, there's disagreement over which reinforcement works best.

Dow Plastics says its research shows that wollastonite--produced primarily by Nyco Minerals and R.T. Vanderbilt Co.--as well as certain micas offer the best combination of properties and price. According to Nyco, its G-RRIM Wollastokup, a specially surface-treated high-aspect-ratio grade is the sole reinforcement specified by GM for RRIM fenders, fascias and side claddings on the APV. And KMG Minerals says RRIM fenders and fascias are the fastest growing market for its HiMod mica. Its new HiMod-270 TABULAR DATA OMITTED grade (34-39 microns average) is specified for such uses.

Yet wollastonite has some limitations. Long-fiber, high-aspect ratio wollastonite tends to produce parts that are more brittle and have lower impact strengths; and it is more abrasive to equipment than some other fillers, says Marty Cornell, Dow senior technology manager for automotive RIM. Cornell says Dow has also found mica to be a viable candidate for automotive RRIM because its platy particles are not subject to linear orientation like wollastonite's acicular (needlelike) particles. "Some body panels are affected by orientation," Cornell says. "The coefficient of thermal expansion is lower in the direction of flow but is higher perpendicular to flow." Mica therefore results in parts with more uniform CLTE and stiffness, as well as very good surface quality. In Dow's tests, KMG's HiMod 270 worked best, Cornell says.

Miles, on the other hand, feels that SRIM reinforced with continuous-strand mat and a fine glass surfacing veil is the way to go. A more expensive alternative to mica or wollastonite, use of glass mat and veil with a polyurethane RIM system can provide a Class A surface comparable to SMC with a 30% weight savings, Miles says (see PT, April '92, p. 19).

NEW HAR WOLLASTONITE

The picture may well change if a new grade of wollastonite from Nyco proves to be as effective as initial tests indicate. The new high-aspect-ratio (HAR), surface-treated wollastonite, called RRIMGlos, is similar to the company's G-RRIM Wollastokup except that it has a uniform particle size of about 4.5 microns versus the 1-10 microns of its G-RRIM. This, RIM processors say, will provide much improved surface qualities. According to Nyco, RRIMGlos provides reinforcing characteristics similar to milled glass together with surface quality in painted parts that is "almost equal" to that of unfilled RIM. Though not expected to become commercially available until next year, RRIMGlos is expected to sell for around $1/lb and RIM formulations containing it could be qualified by major auto makers in time for the 1994 model year.

RIM users say HAR wollastonite approaches the performance level of the filler that so far has proven to be the best reinforcement for providing a Class A surface in every respect except cost. Called "Tismo," the filler is a crystalline potassium titanate microfiber or "whisker" manufactured by Otsuka Chemical Co. of Japan. Those who have tested it say Tismo provides the best distinctness of image (DOI), just below that of painted steel. But it also costs about $5/lb and thus drives up the cost of finished parts to an unacceptable level. And that is probably why there are no known users of Tismo in North America.

But for less than $1/lb, HAR wollastonite gives a DOI just a few points below that of RRIM containing Tismo. Mica-filled RIM DOIs are slightly lower than that; and those provided by wollastonite and milled glass are considerably lower.

More New Products

Wollastonites: Nyco Minerals will introduce commercially next year two new "advanced" high-aspect-ratio, chemically modified wollastonites that reportedly combine good reinforcing properties with excellent part surface smoothness. They are designated RRIMGlos for RIM and Nyglos for general thermoplastics use.

Micas: KMG Minerals, Inc. has six new particle-size distributions of its surface-modified HiMod muscovite micas. These range from HiMod-20 (-20 to +70 mesh) to HiMod-100 (70 microns average).

Hollow microspheres: Microcell Technology, Inc. has introduced the Ceramcel family of hollow ceramic spheres. Eight grades range in diameter from 1.5 to 7.0 mm, with bulk densities of 0.64-1.10 to 0.13-0.18 g/cc, respectively. Spheres can be made from various ceramic compositions.

The PQ Corp. is market testing two new hollow plastic microspheres that confer both resiliency and density reduction. One, PM6545, is an acrylic while the other, PM6445, is PVDC. Effective density of both is 0.02 g/cc, less than one-seventh that of the lightest glass microspheres. They differ in resilience and resistance to temperature and solvents.

Mineral fibers: Industrial Fibers, Inc. has developed Fiberox, a new spun mineral fiber that it says is the cleanest such product produced in North America--referring to freedom from unfiberized shot. Grades are available with under 5% shot. The silicate fibers--in two types derived from blast-furnace slag or from rock--reportedly can improve viscosity, tensile strength and impact resistance while providing reinforcement and fire resistance. Various grades offer fiber diameters of 6-20 microns (12.5 average), average fiber lengths from 0.1 to 4 mm, and aspect ratios up to 700:1. Specific gravity is 2.8 and prices run 14-35|cents~/lb.

Calcium carbonate: H.M. Royal Inc. recently introduced Roy Cal T, a version of micronized oyster shell coated with 1% stearic acid. This product is reportedly unique among calcium carbonates because of its platy structure.

ECC International recently unveiled two ultrafine calcium carbonates, Micro-White 15 and Micro-White 15 SAM. Micro-White 15 has a consistent particle size of 2 microns and reportedly improves gloss and hiding characteristics. Micro-White 15 SAM is organic coated for use in rigid and flexible PVC and filled PP and PE.