Rotomolding officials and product designers heard updates on barrier layers, polymer microspheres, additives that speed up the molding cycle and cryogenic grinding, during the Rotational Molding by Design Conference, which drew 168 people to Cleveland June 6-8.
They also heard a resin expert advise that, when changing materials, to test, test, test!
The Society of Plastics Engineers sponsored the meeting. These papers were from a session on materials and process technology:
* Research from Houston-based Atofina Petrochemicals Inc. shows potential for barrier-layer tanks rotomolded from metallocene polyethylene with a layer of rotomolding-grade nylon or rotomolding grade of polyvinylidene fluoride.
The barrier layer cuts permeation of gases out of the tank, improves resistance to chemicals and high temperatures, and boosts toughness, said Gregory O'Brien, research and development manager at Atofina's Technical Polymers Division.
Potential applications include fuel tanks for small off-road vehicles, marine fuel tanks, chemical tanks and tanks that hold hydraulic fluid.
The result is a superior tank that ``molds like polyethylene,'' O'Brien said.
For the nylon-layer study, researchers did trials on a Ferry shuttle machine. Nylon 11 resin was placed in a drop-box inside the mold, and released after the PE outer layer was fairly well-formed during molding. O'Brien said the two layers showed excellent adhesion.
Researchers found it is important to use water cooling to eliminate shrink voids on the corners, he said.
Researchers conducted the PVDF work on an STP shuttle machine. It was found to have even higher barrier performance than nylon 11. PVDF also works for applications requiring flame-retardant materials, since it will not support combustion, he said.
PVDF also has good impact properties, similar to high density PE.
* Polymer microspheres expand when heated, so they make a good alternative to blowing agents for rotomolding, according to a graduate student at McMaster University.
David D'Agostino is completing his master's degree at McMaster in Hamilton, Ontario. His specialty: microspheres.
Benefits of using the polymer microspheres include excellent surface properties. Molders also can use glass microspheres - tiny glass beads that remain the same size and don't expand. D'Agostino said the glass microspheres give better stiffness than polymers. But the polymer version makes parts with greater elongation at break, while achieving greater density reduction.
D'Agostino investigated microspheres in rotomolding of polyethylene and polypropylene. Good candidates for this foaming technique are resins that have low melting temperatures, low viscosity and low melt elasticity, he said.
With typical foaming, the rotomolder adds a blowing agent, a gas that creates bubbles to form a cellular part. A polymer microsphere contains a hydrocarbon glass, so each one expands during the mold cycle, but it remains intact as a sphere. That means microspheres are more precise, D'Agostino said.
``Microspheres always remain as close cells. With conventional foaming - picture a bubble being blown, and there is the potential for bubbles to be formed together'' and form big voids, he said. Also, the surface finish is better with microspheres.
Applications include toys, recreational products, insulation and automotive. ``There's a potential to reduce the weight in cars today,'' he said.
* Research from Ciba Specialty Chemicals Corp. shows rotomolding cycle time can be cut about 10 percent by using an additive package that includes a process stabilizer now under development, by speeding up the sintering/densification stage during molding.
Faster densification comes because the additives allow the parts to cure at a lower temperature - therefore it doesn't take as long to mold, the research said.
``Cycle time reduction has been a hot-button issue in the rotational molding industry for some time now,'' said James Botkin, industry manager for Ciba's automotive plastics additives segment, in Tarrytown, N.Y.
A special committee created by the Association of Rotational Molders International found that cycle times can be reduced significantly by using internal air pressure or internal cooling, and by monitoring the air temperature and pressure to improve process control.
Also, new grades of linear low density polyethylene made using single-site catalyst technology show faster densification than conventional grades.
But Botkin said those solutions to accelerate densification ``are not completely satisfactory,'' since molders may be reluctant to mold under pressure because of flash and safety concerns. The new resin grades are not yet available globally.
Ciba's study examined how densification speed is impacted by processing stabilizers - typically combinations of antioxidants used to prevent degradation during exposure to heat and oxygen during rotomolding. Researchers tested traditional additive packages and ``new'' formulations now under development. The package for all test samples also included a hindered amine light stabilizer and zinc stearate.
On a clamshell laboratory machine, they molded sample parts from a high-viscosity grade of LLDPE recommended for tanks. Total part cure time was 35 seconds with the new additive package, down from 38 or 39 seconds for the additive package that included the conventional process stabilizers. Botkin said Ciba is planning more research on how stabilizers impact cycle time.
* Jon Ratzlaff of Chevron Phillips Chemical Co. LP, delivered a simple message. No. 1: Not all polyethylene is the same. No. 2: Rotomolders should test incoming material and run impact tests on finished parts.
``An additive package, or a difference in resins, can change your cycle time,'' he said. Off-spec resin can make parts with a big variation. And when replacing resins, don't assume the new one will have the same impact strength and molding parameters, he said.
``You need to go out and test it. Obviously, you would like to get a polyethylene with a wide processing window. But if you don't - you've got to be aware of that,'' said Ratzlaff, senior plastics engineering specialist at Chevron Phillips, in Bartlesville, Okla.
It's also important for molders to maintain the exact cycle time, he said. That's not easy to do in rotomolding, when a machine operator may leave for a bathroom break while the mold spins in the oven.
Tests by Chevron Phillips showed longer heating time can dramatically reduce the part's impact strength with certain resins. ``The impact can vary a lot. It depends what happens in the oven,'' Ratzlaff said. ``If you go from 10 minutes to 12 minutes, your impact goes to zero,'' he said.
* Cryogenic grinding of materials now used in slush-molded automotive instrument panels and front grills could be used for rotomolding, according to Midwest Elastomers Inc. of Wapakoneta, Ohio.
Like rotomolding, slush molding results in bubbles forming as the resin melts. Reduction of the ``bubble issue'' can improve particle distribution. That's where cryogenic grinding comes in.
Cryogenic grinding creates very fine particles. When slush molded, the plastic fills in cracks and crevices.
The authors - Louis Heckman, sales manager for Midwest Elastomer's Plastics Grinding Division, and William G. Jacobs Jr., quality assurance manager - said cryogenic grinding may open rotomolding up to new materials, including thermoplastic polyurethane, thermoplastic rubber, and alloys of TPU and TPR with PVC.