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Plastics Profile

Polystyrene Shows Its Versatility

Polystyrene is a clear amorphous polymer that exhibits high stiffness, good dimensional stability, electrical insulation properties and low specific gravity. General purpose polystyrene (GPPS), also referred to as crystal polystyrene, offers exceptional clarity and rigidity. GPPS is available in a wide range of melt flow rates in high-heat, high-molecular weight and general-purpose grades.
High impact polystyrene (HIPS) is made by adding butadiene rubber to styrene during the polymerization process. HIPS is opaque and provides improved impact properties while maintaining good stiffness. The versatility of HIPS and the sparkling clarity of GPPS make both materials ideal for an range of end-use applications, including: food packaging, food service-ware, office supplies, point-of-purchase signs and displays, TV cabinets/housings, housewares and consumer goods, refrigerator trim/liners, small appliance parts, toys, medical products and packaging, building insulation, cosmetics packaging, and CD jewel boxes.
Advantages over competitive resins
Polystyrene offers several advantages over competitive resins, including higher stiffness, ease of processing and exceptional clarity. Due to the low density (1.04) of polystyrene, yields 20-30 percent greater than alternative resins, such as polyester, can be realized. There is less mold shrinkage in injection molding and thermoforming processes with polystyrene than other crystalline thermoplastics, such as polypropylene and polyester. Performance characteristics allow the material to be used in some engineering resin applications. It is available in flame-retardant and UV-resistant compounded formulations for use in television cabinets, computer housings, printers and other electronic applications.
Extrusion techniques
Excellent thermal stability and high melt strength make polystyrene ideal for extrusion. It offers a wide processing window, with a melt temperature range of 390F to 460F. In addition, polystyrene is heat stable and does not degrade easily.
Polystyrene resins are ideal for sheet extrusion at a variety of gauge thicknesses, ranging from 0.005 inch to 0.125 inch, but can also be extruded in thicker sections up to 0.250 inch. Polystyrene extrusion resins have melt flow rates in the range of 1.0 to 4.0. HIPS is primarily extruded into sheet for applications such as cups, lids, point of purchase displays, signs, packaging and other applications.
High heat crystal polystyrene is also used in extrusion of oriented polystyrene (OPS). OPS sheet is produced through a mechanical process of biaxial orientation of the extruded sheet, which results in a very clear, tough and durable sheet. The OPS sheet is thermoformed into products such as clear bakery packaging (cake domes, cookie trays and lids, etc.) and take-out deli packages.
Coextrusion: Polystyrene materials can be coextruded with different polystyrene materials or other resins. Examples include HIPS with a crystal polystyrene cap layer for high gloss and aesthetics and HIPS with a cap layer of color-concentrated HIPS for a bright surface color. Coextrusion combinations with other resins can utilize ABS, SAN, NAS, ZYLAR, ZYNTAR&Mac226; SB block copolymers and other materials. Polystyrene can be easily colored at the extrusion machine to obtain the desired color. Natural high-impact polystyrene is an opaque, white material.
Foam Extrusion: Polystyrene also can be foamed as it is extruded into sheet or rollstock. A variety of blowing agents are used including hydrocarbons (pentane, butane and isomers of these hydrocarbons), HCFCs and carbon dioxide. Foam materials are widely used in food service and building insulation applications.
Forming techniques
Polystyrene can be thermoformed or vacuum formed using standard aluminum tooling and equipment. The thermoforming and trim tooling need to be designed with dimensions that will incorporate the typical polystyrene mold shrinkage rate of 0.004-0.007 inch/inch. Polystyrene’s low shrinkage provides good dimensional stability and part dimension repeatability.
It does not absorb moisture, so drying is not required prior to extrusion or thermoforming. During the forming process, material surface temperatures range between 250F to 300F, depending on the depth of draw and the forming rate. Because of its amorphous nature, polystyrene has good sheet integrity for forming operations.
Secondary operations
HIPS materials are commonly die cut, sheared or stamped to cut the larger sheets down to size for thermoforming blanks or to cut out specific shapes for an end use application, such as point of purchase signs and displays. HIPS extrusion sheet grades generally have low angel hairs and fines generated from stamping or die cutting processes. HIPS can be machined with programmable CNC routers or by hand. It also can be drilled using a standard drill press or hand-held drill using drill bits designed for use with plastic.
Polystyrene has good surface tension for adhesion with other materials. It is easily printed on using both conventional and UV ink processes. The surface tension of HIPS is 35 to 37 dynes/cm2. Polystyrene can be Coronna treated for obtaining even higher surface tension of 47 to 55 dynes/cm2. Coronna treating is commonly used as a pretreatment for lithography printing. Flame treating, another process used to increase surface tension prior to printing on the part, is also a common pretreatment.
Other secondary operations include using adhesives, thermal and ultrasonic welding, hot stamping and painting. Both GPPS and HIPS accept a variety of adhesives for bonding polystyrene to itself or to other resins. Recommended adhesives include cyanoacrylates, two-part acrylics, two-part polyurethanes, and two-part epoxies. w
Holly Wilson is the Marketing Manager, Polystyrene Durables at Nova Chemicals. For more information, contact Nova Chemicals at (866) 275-6682 or www.novachem.com

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