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High-melt-strength PP permits foam sheet extrusion.

High-Melt-Strength PP Permits Foam Sheet Extrusion

A new polypropylene grade is designed to overcome the resin's traditional melt-strength weakness, which has largely precluded its application in extruded low-density foams. The material, which does not yet have a trade designation, is slated for commercialization later this year by Himont U.S.A. Inc., Wilmington, Del.

At this year's SPE ANTEC conference in Dallas, Himont research engineer Mark B. Bradley described the new resin as a PP homopolymer developed to produce low-density foam, using a variety of hydrocarbon blowing agents, on conventional tandem extrusion equipment such as is used to produce polystyrene foam sheet. Bradley said the resin is nucleated and has a crystallinity comparable to standard PP grades.

The new PP grade is not an alloy, but rather a "100% homopolymer with a modified rheology that results in high melt strength," according to Bradley. Though details of the material technology are proprietary, he explained that the new PP grade has an "elastic characteristic in the melt phase," offering a greater processing window that enables it to achieve the proper foam cellular structure, while virtually eliminating voids. The processing window of the material (melt temperature of the resin as it exits the die for foaming) is between 330 F and 340 F, compared with an virtually nil range for standard PP.

Priced at about 80^/b, the material is expected to be used mainly for packaging such as microwaveable food containers. Though not designed specifically to compete with PS foam, PP does offer inherently superior heat and chemical resistance and toughness, Bradley pointed out.

WHY IT WORKS FOR FOAMS

The new polymer has an elastic type of behavior in the melt phase with higher than normal extensional viscosity, permitting the material to be foamed. "Normally when you pull PP in the melt phase it tends to thin, and viscosity decreases," he noted. "The melt phase for this grade has considerable elasticity, which represents a break-through for PP." Unstable cell growth or non-uniform cell size occurs if a polymer's viscosity decreases rapidly with increasing stress, he explained.

Foaming experiments were conducted on a single-screw, vented, 24:1 L/D extruder with 38-mm diam., which was modified to simulate the functions of a tandem system. Himont engineers observed that when a blowing agent was injected into a conventional PP melt stream, an unstable condition resulted, and the blowing agent exited through the die without mixing with the polymer. Although the melt initially expanded as it left the die exit, the cell walls soon ruptured, causing total collapse of the foamed structure. This was attributed to the inherent low extensional viscosity of the conventional PP.

By comparison, the new high-melt-strength PP grade resisted cell-wall rupture and was foamed successfully. The researchers observed retention of the blowing agent and significant expansion and density reduction, while little or no collapse or shrinkage was recorded. This permitted formation of a uniform, closed-cell structure.

The resin is in the final stages of testing with potential customers, and represents "about 10 years" of technical devleopment for Himont, according to Bradley. The modified-rheology material technology behind the new foamable PP grade is also being applied to new grades that are said to provide new and unique capabilities for PP in extrusion coating and large-part thermoforming and blow molding (more details next month). (CIRCLE 1)
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Title Annotation:polypropylene
Author:Gabriele, Michael C.
Publication:Plastics Technology
Date:Jul 1, 1990
Words:553
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