Why PVC micropellets are on a roll in rotomolded auto interiors.

Micropelletized PVC compounds are challenging traditional vinyl plastisols and drysols in roto-molded automotive interior parts. Several applications of this type are being pursued by the Plastics Div. of Teknor Apex Co., Pawtucket, R.I. Technical service engineer John Macaluso says micropellets offer greater formulation flexibility, enabling them to achieve properties not available with plastisols or drysols. Micropellets may even help establish rotomolding as an alternative to injection molding in some cases.

Teknor's micropellets are solid PVC compounds ranging from 0.020 to 0.080 in., a size more convenient for rotomolding than standard 1/8-in. pellets. The tiny pellets enable rotomolders to use materials with a durometer below 55 Shore A while still meeting tough new automotive specs, which reportedly has not been possible with drysols. "Our focus has been on automotive compounds in lower durometers. That's where the benefits lie," says Macaluso. During the past year Teknor has developed six micropellet PVC formulations for rotomolding, one of which is being used commercially while the remainder are being evaluated by customers.

PELLETS ADD FLEXIBILITY

Producing low-durometer, low-fogging vinyl automotive products with good heat-aging properties may require use of high-molecular-weight plasticizers. Drysols containing such plasticizers reportedly do not "dry out" to a free-flowing powder, which leads to poor mold flow and surface imperfections. Drysols also tend to agglomerate in hot, humid atmospheres.

Use of polymeric modifiers to meet automotive property specs is also said to be difficult with drysols or plastisols. First, the additives must be cryogenically ground, a costly step. Second, the additives are hard to disperse evenly in a drysol in order to provide uniform properties, says Macaluso. On the other hand, micropellets compounded by melt extrusion receive thorough homogenization and are flee-flowing no matter what the formulation, he says.

Compared with liquid plastisols, solid micropellets offer easier equipment cleanup and less cross contamination of products. Compounding color into micropellets also results in more uniform color dispersion and less machine contamination than is the case with drysols or plastisols, Macaluso adds.

THREE CASES IN POINT

Macaluso cites a trio of applications that illustrate the advantages of roto-molding with micropellets. Each formulation is aimed at vinyl skins for ann rests, head rests, consoles, and door panels. The first example is a 52 Shore A product with low fogging and high property retention. Macaluso says a drysol could not. be formulated at such a low durometer because it would not dry out and therefore would mold poorly.

The second micropellet formulation, which is in commercial use, is a 60 Shore A compound. It contains an ethylene acrylate modifier resin that improves resistance to heat aging and also provides good paintability, low-temperature flexibility, uv stability, and resistance to polyurethane staining. The modifier, which comes in crumb or pellet form, can be cryogenically ground to permit use in a drysol, but the resulting properties don't match those of micropellets.

The third application started with a 70 Shore A injection molding compound that is widely approved for use in steering wheels, grips, and console skins. Micropellets reportedly make rotomolding a viable alternative for this application. Adding a TP urethane modifier improved the elongation, tear strength, abrasion resistance, and heat-aging property retention. But when the pelletized TPU was cryogenically ground for use in a drysol or plastisol, Macaluso says it compromised the blend's homogeneity and physical properties.