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REPRINTED FROM MODERN PLASTICS - October 1999
Blowing agents for engineering resins are cost effectiveDeveloping blowing agents for engineering thermoplastics is more tricky than commodity resins, but with resin prices up to $50/kg, any way to cut weight in half or more has to be considered seriously. Developing blowing agents for engineering thermoplastics is more tricky than commodity resins, but with resin prices up to $50/kg, any way to cut weight in half or more has to be considered seriously. The trickiness arises because the blowing has to decompose at a higher temperature - much higher in the case of resins like polyphenylene sulfide - and formation of water during decomposition may be a critical issue, since it can cause polymers like polycarbonate to depolymerize. Some suppliers of chemical blowing agents adjust formulations to suit the need. For example, Reedy International, Keyport, NJ, offers a grade of Safoam endothermic blowing agents containing a dryer. Ajax, ON, is also readying several new products. Others, like Ticona, Frankfurt, have devised totally new compounds. And work with physical blowing agents, notably at Trexel, Woburn, MA, is producing startling results. As Robert Heinz, research associate for foam extrusion at the IKV, the plastics processing institute in Aachen, Germany, notes, foamed engineering thermoplastics are creating new applications. "With high heat resistance and excellent insulating properties, the foams can be used for insulating purposes in areas with high thermal loads." Reedy cites increasing activity in PET foam food packaging. One customer is already making and forming PET foam sheet, and Reedy will license the technology. Interest from food companies is "substantial." The IKV is doing tests for Ticona on a blowing agent called PPSO, polyphenylene sulfoxide, that can be used for plastics with high melting points like PPS. It is actually derived from PPS. PPSO can be supplied as a fine yellowish powder with an average particle size ranging from 5 to 50 microns. It decomposes at between 320 and 380 degrees C, depending on ho wit is synthesized, creating mostly sulfur dioxide, but also carbon dioxide and carbon monoxide. A total of 1 g of PPSO generates 80 to 100 ml of gas. "This might be below other chemical blowing agents, but together with the high decomposition temperature, PPSO should open new fields of application," says Heinz. In extrusion tests at the IKV, profiles were made with a closed-cell structure with an even cell distribution and an average cell size of 0.5 mm. The IKV is also doing foaming trials with nylon 46, PEI, and PES. Trexel's MuCell technology is based on the use of gases (nitrogen or carbon dioxide) in a supercritical state in the melt. When they come out of solution, they produce bubbles under 10-micron diameter. First applied to extrusion, the technology is increasingly being used in injection molding, and with that, its use with engineered resins is growing. Last year, Trexel licensed Engel, Schwertberg, Austria, to offer the process on its equipment. In recent weeks, Arburg's North American subsidiary in Newington, CT, and Milacron Inc., Batavia, OH, have followed suit. According to Dan Sczcurko, VP for business development at Trexel, "the real dollar advantage comes from the process savings." He says in a recent molding trial, cycle time was cut by 66% compared to processing solid material. Machines with lower clamp forces can also be used. Costs can be further cut by reducing sprue and runner diameters. Szczurko says that with blowing agents that rely on the heat of the process to generate the gas, there is "absolutely no control" over how the decomposition goes off in the mold or, in extrusion, when the melt exits the die. But with MuCell, in which the supercritical fluid diffuses into the plastics melt to form a single-phase solution, release of pressure produces instantaneous nucleation and a completely homogeneous structure. Sczcurko also says that the cost of MuCell is favorable. A licensee processing engineering resins on a 2000-kN (225-ton) unit pays $15,000/yr for the first seven years, after which they have free access. Payback in a recent case study - a 40%-filled nylon automotive bracket - was two months. Material cost per part was $0.34 versus $0.47 for the solid molding, cycle time was 26s versus 51s, and machine size was 2000 kN compared to 3000 kN. Return to Top | Return to Magazine Articles 98-99 |
