New-generation PP's excel in processing & performance.

New-Generation PP's Excel In Processing & Performance

Thermoforming and blow molding of large and small parts, extrusion coating, and thin-wall injection molding are some of the processes that will benefit from new polypropylenes that utilize new polymerization technology to produce hitherto unprecedented processing and performance characteristics. So say officials of Himont U.S.A., Inc., Wilmington, Del., about a number of developmental and newly commercial grades.

HIGH-MELT-STRENGTH FAMILY

In early October, Himont plans to start up a plant in Varennos, Quebee, with more than 150-million-lb/yr capacity for producing a new family of resins with novel rheological characteristics. These materials, which have been available from a multimillion-lb/yr pilot plant since 1988, all exhibit higher melt strength, or extensional viscosity, than normal PP's, according to market development manager William R. Curtis. He explains that these materials show strain hardening, or increasing resistance to extension, as the melt is drawn, unlike conventional resins, which become easier to draw further the more they are drawn.

Included in this new family are two new melt-thermoformable PP's, Pro-fax PF-613, a 0.3 MFR homopolymer, and Pro-fax SD-613, a 0.3 MFR impact copolymer. A paper given at the recent SPE ANTEC in Dallas by researchers Kathleen E. McHugh and K. Ogale, showed that these high-molecular-weight grades, require 20-30 sec longer to sag the same amount as standard PP's at forming temperature; that they can be formed within a much wider operating window permitting up to 30 [degrees] F higher forming temperatures (to 375 F); and that at those higher melt-forming temperatures the new resins produce containers with a more uniform wall thickness and greater average thickness.

Curtis thinks this could permit forming stress-free retortable food containers from wider webs perhaps as much as 60 in., up from 50 in. today. Wider webs also suggests the potential for PP to break through into large-part industrial forming applications. A commercial example is a wheel-well component for a new 1991 car line, in which two parts are formed together from 60-mil copolymer sheet of 3 x 5 1/2 ft. This intricate part has areas of draw up to 4 in. deep. "Nobody has been forming PP in such large areas," says Curtis. Forming trials are being conducted on refrigerator liners in China, he notes, adding that PP's, not attacked by the new HCFC blowing agents used in refrigerators', urethane insulation, whereas HCFC's appear more aggressive to ABS and HIPS than the ozone-damaging conventional CFC's. (CIRCLE 5)

Himont is also investigating large-part blow molding applications (another rarity in PP) for these resins, whose melt strength prevents parison sag. Already commercial or nearly so are automotive seat backs and windshield-washer reservoirs, and a 1-gal handled jug--where lower taste and odor transfer are said to be advantages over HDPE. (CIRCLE 6)

NEW EXTRUSION POTENTIAL

Another member of this high-melt-strength family is an as-yet undesignated homopolymer that's said to be the first PP that can be extruded into low-density foam on conventional equipment. Also included are a pair of extrusion coating grades. Pro-fax PF-611 homopolymer (40 MFR) was unveiled at last fall's TAPPI conference. It now has a sister product, Pro-fax SD-621 copolymer (12 MFR), which offers advantages of even easier processing, greater flexibility, and lower seal initiation temperature. Unlike conventional PP's, which must be blended with LDPE for processability, and are run at only 500-600 ft. min, these new materials have been run without LDPE blending at up to 1300 ft. min on Himont's lab line and one customer reports 2300 ft/min on a production line. Himont's Curtis thinks these materials will run four times faster than some LDPE's, without pinholes and with good coating-weight uniformity. (CIRCLE 7)

FOR TOUGH, CLEAR BOTTLES

For extrusion blow molding bottles where a combination of impact, clarity and cost is important. Himont has a new developmental random copolymer that's said to be especially good for larger containers, especially handleware up to 1 gal. Pro-fax SD-354 has clarity equal to or slightly better than standard 2.0-MFR random copolymers, but also has 5-10 times the impact resistance at refrigerator temperatures (11 F), according to Donald A. Goodridge, Himont's industry manager for packaging. Table 1 compares SD-354 to a typical random copolymer of the same flow rate; although these results are based on injection molded test specimens, the improved impact resistance of this new grade has been confirmed in a number of applications over the last two years, says Goodridge. (CIRCLE 8)

THIN-WALL INJECTION MOLDING

For thin-wall injection molded containers, Himont has a pair of "next-generation" products designed to compete with HDPE and existing PP's. Goodridge says that in Europe, PP is the material of choice for thin-wall packaging, whereas HDPE is king in North America, except in yogurt containers.

Goodridge hopes to change that with Pro-fax SD-242, a 35-MFR impact copolymer that he says flows better than 60-90 MI HDPE's. His claim is supported by Himont serpentine flow tests that show 14 in. of flow for SD-242, vs. 10.5-11.5 in. for three HDPE's from 60 to 93 MI. This enables molding thin-wall containers with higher impact strength, owing to less molded-in stress, and with faster cycles, says Goodridge. In addition, the higher stiffness of PP (215,000 psi flex modulus for SD-242, vs. 105,000-160,000 psi for HDPE) reportedly enables further downgauging, with all its attendant economic and environmental benefits (source reduction).

Although tests on injection molded bars show that SD-242 beats HDPE in tensile strength (4000 psi), HDT (a 66 psi (225 F), and drop-weight impact both at room and freezer temperatures, the real proof of the pudding, Goodridge says, is container impact testing. In Table 2, drop-weight impact testing on containers from identical molds shows that SD-242 beats HDPE and standard PP's at room and cold temperatures. (CIRCLE 9)

Himont also has a "next generation" resin to compete with existing PP homopolymers and homopolymer/copolymer blends already used in yogurt cups. It's a 110-MFR copolymer that's already commercial in Europe and is expected to become commercially available here by the end of this year. It's also a member of Himont's new Valtec family of faster-melting PP's made with a proprietary coating of additives on the outside of the spherical granules.

Valtec HE289 is said to be the first PP of such high flow to be available here for injection molding, because previous technology yielded excessively brittle resins at such high flow rates. On the other hand, actual container impact tests (Table 3) show HE289 to produce higher toughness at ambient and refrigerator temperatures than a PP homopolymer and a PP homo/copolymer blend at 24 and 28 MFR, respectively. (Serpentine flow for HE289 is 19 in., vs. 13 in. for the competing PP's).

Valtec HE289 maintains stiffness (260,000 psi flex modulus) in the same range as lower-flow PP's. Its tensile strength (4700 psi) and HDT (113 F @ 66 psi) are also in the same range as competing PP's. HE289 also boasts low odor and taste, which can be a problem with competing "controlled-rheology" PP's.

Although Valtec HE289 may sell for more than standard PP's (Valtec grades generally cost 5% more than pellets), Goodridge expects this to be off-set by at least 25% faster cycles with the high-flow material. This was substantiated in trials of a new four-cavity mold for a 1-lb dairy container, conducted by the moldmaker, Husky Injection Molding Systems, Ltd., Bolton, Ontario. On a 225-ton Husky toggle press, cycle times were reduced to 3.5 see with Valtec HE289, from 5 sec with a 28-MFR standard PP--a 30% improvement.

Himont has begun sampling a 35-MFR version of this material to see whether it can provide many of the same advantages, and to help ascertain what level of flow the U.S. market desires. (CIRCLE 10) [Tabular Data 1 to 3 Omitted]