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Unimodal HMW-HDPE hits high film output rates.

Tests conducted by Novacor Chemicals Ltd. of Calgary, Alberta, may dispel the notion that HMW-HDPE with a unimodal molecular-weight distribution can't run at competitive output rates on high-speed blown film lines.

During a 24-hr trial on Reifenhauser Film Systems's lab line in Peabody, Mass., a Novacor film-grade unimodal HDPE ran as a 0.6-mil film at rates of 652 lb/hr on a 80-mm, 30:1 extruder fitted with an 225-mm. die. That rate translates to just over 23 lb/hr/in.

The unimodal resin, Novapol HF-W946-HP, was developed two years ago specifically for blown film and has a 10.5 M.I. and 0.945 density. Novacor technical-service specialist Mike Jones notes that "conventional wisdom" had pegged output rates for unimodal resins at only 10-12 lb/hr/in. In the past, he explains, that rate wasn't a problem, since many older lines don't exceed that output anyway.
FILM PROPERTY & PERFORMANCE DATA ON UNIMODAL & BIMODAL HDPE(a)
 Unimodal(b) Bimodal
Avg. Thickness, mil 0.6 0.6
45|degrees~ Gloss 12 9
Dart Impact, g/mil 184 318
Puncture, in.-lb/mil 11 11
Elmendorf Tear, psi
MD 11 10
TD 92 117
Yield Strength, psi
MD 5200 5700
TD 4200 4100
Tensile Strength, psi
MD 10,900 13,700
TD 7800 7700
Elongation, %
MD 370 390
TD 650 630
Secant Modulus, psi
MD 99,500 121,100
TD 102,800 128,900
EXTRUSION PERFORMANCE
 Unimodal Bimodal
Screw rpm 91 88
Ouput, lb/hr 652 617
Specific output,
lb/hr/rpm 7.2 6.3
Current, amp 240 213
Melt Temp, F 425/428 430/446
Melt Pressure, psi 6850 8290
a Novacor data. b Unimodal is Novacor's HF-W946-HP.


LONGER SCREW HELPS

Reifenhauser v.p. Matthew Bangert attributes the competitive rates primarily to the long L/D of the RT 80-30 extruder, which can provide a relatively low instantaneous shear rate. It apparently handled the higher torque requirements of the shear-sensitive unimodal resin without sacrificing production rates or stability, he explains. In fact in similar trials, Novacor could not get the unimodal resin to run on two brands of shorter barreled extruders without gauge or port-flow problems. Moreover, the Reifenhauser test run demonstrated the ability to switch the extruder feed between unimodal and bimodal resins without disrupting the production of usable film, according to Bangert.

Unlike more common bimodal HDPE film resins, Novacor's unimodal resin is produced in a single-stage process at a generally lower manufacturing cost than bimodal resins, which commonly require two-stage reactors. However, Novacor market-segment manager Richard Levesque says the unimodal resin is priced competitively with bimodals and sells on its desirable physical properties (see table). These properties include higher flexibility, as revealed by lower secant modulus, and better sealability than comparable bimodal products--both of which add up to the ability to do away with LLDPE blending, the company says. The unimodal also showed higher gloss levels, an advantage in highly printed bag applications.
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Title Annotation:high molecular weight-polyethylene
Author:Ogando, Joseph
Publication:Plastics Technology
Date:Feb 1, 1993
Words:487
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