Thermoformers may find new opportunities in higher-performance applications with the arrival of thin-gauge PPS sheet from Penn Fibre Plastics (PFP). Highly crystalline PPS has until now resisted extrusion at less than 0.25-in. thickness due to its poor melt strength and the resulting sheet’s tendency toward brittleness. Assisted by a pair of development partners, PFP now claims to have mastered PPS thin-sheet extrusion. The company sees this as paving the way for thermoformers to begin penetrating into chemical-tank liners, mass-transit panels, and medical-handling devices, as well some automotive under-hood parts.
PPS has good potential for replacing aluminum, steel, and currently used high-temperature thermoplastics in forming large shaped parts, says Michael Gehrig, general manager at PFP, a subsidiary of Germany’s Ensinger, a long-time supplier of PPS rod and shapes. PPS products offer high heat and chemical resistance, outstanding mechanical strength, and inherent flame retardancy.
“PPS is a semi-crystalline material with a sharp melting point, so extruding it into thin sheet is a challenge,” Gehrig says. Yet he reports success in recent thermoforming trials of thin PPS sheet by Magee Plastics, a maker of panels used in aircraft, railcars, and buses. Another partner is Ticona LLC, the developer of a line of formable PPS resins and compounds used by PFP to make the sheet used Magee’s trials.
Novel PPS materials
PPS reportedly fills a wide cost-performance gap between nylon and more high-tech materials in the existing range of thin-gauge thermoformable engineering thermoplastics, says Ed Hallahan, Ticona’s marketing manager for Fortron PPS. PVDF and PFA fluoropolymer sheets generally cost more per pound than PPS sheet and are around 30% denser. Similarly, sheets of polyetherimide (PEI, supplied by GE Advanced Materials as Ultem) and polyethersulfone (PES, Solvay Advanced Polymers’ Radel) are considerably more costly than PPS.
On the other hand, formable nylon sheet, while less costly than the other options, is relatively unproven and to date has gained only modest traction in a handful of under-hood automotive parts.
In response, Ticona has launched a broad and growing line of PPS resins and compounds tailored for use in formable sheet. These materials are based on linear PPS resins that are said to have higher inherent melt strength than conventional branched PPS. Ticona also adopted a special impact modifier to enhance sheet toughness. Some grades have 15% and 30% glass reinforcement.
PFP uses Ticona’s new materials for its line of PPS flat sheet and rollstock in thicknesses of 0.01 to 0.25 in. PPS processes at 575 to 600 F, so the new sheets are readily handled by equipment designed to thermoform other high-temperature thermoplastics.
A promising opening for PPS forming is in chemical tank liners for trucks, railcars, and giant storage tanks. Today, these are typically formed or fabricated out of a fluoropolymer, then given a fabric backing that adheres to the metal tank. “Cracking this market would put formed PPS on the map,” predicts Gehrig, who notes that PPS provides 30% weight savings versus PVDF, while also boosting throughput and providing corrosion resistance equal to or better than that of PVDF and PFA.
Another market amenable to PPS forming is automotive under-hood parts like engine covers and air-intake manifolds that are currently injection molded of nylon or other high-temperature plastics. Best prospects are in the after-market sector, where volumes are relatively low. In that niche, the economics of low tooling costs and short lead times favor thermoforming over molding.
PPS substitutions take off
Magee Plastics, a manufacturer of aircraft interior systems, has long designed and made sun visors, window shades, stowage bins, stowage doors, ceiling and sidewall panels, and other parts in metal or plastic for the airlines and aircraft manufacturers. The company uses many processes, including sheet-fed vacuum forming of PEI, PES, and fluoropolymers. Its equipment incorporates special high-heat ovens for processing materials at up to 600 F.
“We formed 0.093 and 0.125-in. PPS sheets without problems,” says Sheridan Kelly, Magee’s sales manager. He says conversion of two existing parts to formed PPS is now under review. These are an emergency evacuation chute and a lower sidewall (kick) panel that are currently compression molded out of PPS and phenolic, respectively.
The incentive in both cases is to reduce tooling delivery time and cost, Kelly says. Other benefits of PPS are outstanding flame resistance, toughness, and dimensional stability. In the future, the part design will probably be revised to take advantage of PPS’s downgauging potential.
Magee says additional conversions of aircraft parts to PPS from thermoformed PEI and PES are likely to occur as a result of potential cost and density savings. Magee also uses ABS and PVC for bus and railcar interior panels due to the less stringent flammability requirements in those vehicles. Formable PPS would be a strong candidate if regulatory requirements in those sectors became more rigorous.
Meanwhile, Chevron Phillips Chemical (CPChem) is equally committed to developing formable PPS materials, although its approach differs from that of Ticona. Jay Blackburn, CPChem’s manager of new products and technology, says the firm is adapting its branched PPS resins for thermoforming (and also for blow and rotational molding) partly through use of higher molecular weights to enhance melt strength. Another approach is to alloy PPS with an ethylene copolymer elastomer that results in a tougher and more ductile material without sacrificing other PPS properties, Blackburn says.
CPChem is working with sheet extruders to develop PPS sheet as thin as 0.02 in. for use in reusable and sterilizable medical containers (surgical trays) in place of existing glass and steel versions. The company also is developing PPS sheet materials suitable for mass-transit panels. CPChem sees further potential in panels for clean rooms, where PPS’s inertness and purity would be beneficial.