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.