Polypropylene car parts could be in rotational molding's future, according to a speaker at a Cleveland rotomolding conference.
``The development of [thermoplastic polyolefin] grades suitable for rotational molding is of great interest, because it would enable rotational molding to enter new, high-volume markets, such as production of automotive parts,'' said Marianna Kontopoulou, assistant professor of engineering at Queen's University in Kingston, Ontario.
Speakers at Advances in Materials and Processes in Rotomolding, held June 9-11 in Independence, reported on new materials, additives and resin grades. The event was sponsored by the Society of Plastics Engineers' Rotational Molding Division and the Cleveland SPE section.
Polyethylene still dominates rotomolding, accounting for 85 percent of the market share, Kontopoulou said. Other resins, including PP, are not used widely. But she pointed out that TPOs are popular in injection molding for automotive parts such as bumper fascias, and in extrusion.
PP is a low-cost plastic, and it offers many good properties.
``But as everybody knows, it suffers from low-impact properties. This is why it has not been widely used in rotational molding,'' Kontopoulou said.
Queen's University researchers tested thermoforming-grade TPOs in rotomolding and investigated how an impact modifier of metallocene-based plastomers affects sintering and rheology of the resin. The metallocene plastomers are better than traditional, rubber-based impact modifiers, she said, since they have comparable molecular weight and are more compatible with PP.
The researchers achieved parts with excellent ductility, she said, adding that powders worked better than resin in micropellet form. With sintering, however, they reported some problems with bubbles and bean holes in parts.
Kontopoulou said Queen's technicians also tried to modify thermoforming-grade TPOs to make them more rotational molding-specific, and got improved sintering, better mechanical properties and better-looking parts.
In other materials papers from the Cleveland conference:
* Randy Fleck, senior technical service engineer for Honeywell Inc.'s Specialty Plastics group in Nazareth, Pa., reported on the company's Capron modified nylon 6 developed for rotomolding.
Some companies already are using nylon, which is one of the few materials that can be rotomolded in pellet form, Fleck said. Good heat resistance, stiffness, toughness and resistance to the permeation of hydrocarbons make nylon an attractive niche player in rotomolding. But normally with nylon, as moisture increases - which can happen with water-cooled stations on rotomolding machines - impact strength increases while stiffness decreases, so the part is ductile. On the other hand, when dry, nylon gets brittle.
The new Capron grade is designed to provide good, dry-as-molded impact strength and stiffness, with ductility, whether or not water is used during the cooling cycle, Fleck said.
In research, Honeywell discovered the new modified grade has good adhesion with PE.
Fleck said rotomolder Centro Inc. of North Liberty, Iowa, developed a process to mold the Honeywell nylon on the exterior, with an inside layer of less-expensive high density polyethylene. Fleck said that could open up new applications for rotomolding, including surge tanks, hydraulic oil reserves, air intakes and chemical tanks.
Honeywell researchers tested parts that were injection molded from standard rotomolding pellets. The researcher also rotomolded test specimens from pellets and a powder.
* Einar Voldner, president of Synergy Polymers Inc. of Hornby, Ontario, introduced Borocene Compact, a ``new powder format'' of metallocene-based PE from Borealis A/S. Synergy represents Borealis in North America.
The resin, which should be available in the United States soon, is available only in black.
Borealis has been making metallocene-based PE in Europe for six years, but the new rotomolding grade does not need secondary pulverizing into fine powder.
Voldner said the resin produces less dust at the molding facility, flows into hard-to-fill mold areas and gives a good surface finish.
An audience member asked whether Borealis makes the powder-grade resin on a single- or twin-screw extruder. Voldner declined to provide details, but he said the company made some modifications to the extruder.
* Urs Stadler, senior staff member of plastics additives at Ciba Specialty Chemicals Corp. in Tarrytown, N.J., discussed several new additives for rotomolding. A phenolic-free antioxidant can prevent thermal degradation, while avoiding the discoloration that can come with phenolic antioxidants.
Stadler said antimicrobial additives, a fairly new development in rotomolding, can retard the growth of bacteria, fungi and algae over the life of a product. But care must be taken, he said, because after weathering, some antimicrobials can cause color changes in the part.