How to extrude HMW-LDPE film.

Extrude HMW-LDPE Film

HMW-LDPE is not brand new, though it is still unfamiliar to most film producers. It has been offered commercially on a small scale for several years, but may have been "ahead of its time" when first unveiled in the early 1980s (see PT, Nov.'84, pp. 47, 55). Last year, the USI Div. of Quantum Chemical Corp., cincinnati, felt the time was right to "reintroduce" this material, since the expanded use of long-stalk HDPE blown film machinery has created a new window of opportunity for HMW-HDPE. Quantum, believed to be the only producer of this material, currently offers three grades of Petrothene HMW-LDPE resin, ranging from 0.25 to 0.5 melt index.. Two grades are homopolymers (0.925 density), while the third is an EVA copolymer. Petrothene NA 351 (0.3 MI) is suitable for a broad range of applications. NA 355 (0.5 MI) is recommended where clarity is most important or where better drawdown is required. NA 357 (4.5% VA content) is for uses demanding exceptional toughness, low-temperature properties, or low sealing temperatures. These high-strength resins are also suitable for combination with other plastics through coextrusion or blending.

With long-stalk equipment now more prevalent in the market, the resin may be re-examined by processors as a means of boosting their equipment's utilization as well as tapping into niche markets such as merchandise bags, clarity shrink film, high-gloss coextruded film, heavy textiles and heavy-duty produce bags. Other potential applications include ice bags, quickfreeze food bags, drum liners and clarity film where downgauging is desired.

COMPARISON WITH

OTHER FILMS

HWM-LDPE's higher viscosity than conventional LDPE and LLDPE provides excellent bubble stability. The combination of high viscosity and molecular chain branching gives rheological properties that permit extrusion under a broad range of operating conditions.

Because of its unique properties, HMW-LDPE can compete in applications with a variety of resins. Compared with conventional PE clarity packaging films, HMW-LDPE is stiffer, has better tensile, puncture and impact strengths, and equivalent optical properties. Extrusion can be with either long-stalk or "pocket" bubble, better optical properties usually resulting with the latter.

HMW-LDPE also can compete in heavy-duty film applications. When compared with a standard 7.5% EVA copolymer of 0.5 MI, used for heavy-duty liner film, HMW-LDPE has lower impact strength but better tear resistance (both MD and TD), puncture resistance and stiffness. Compared with an LDPE homopolymer of 0.3 MI and 0.918 density, HMW-LDPE film impact properties are similar, tear and stiffness again are better, and optical properties are much better.

Table : HMW-LDPE VS. OTHER PE FILMS

Conditions            HMW-LDPE       LLDPE            HMW-HDPE
Melt Temperateture    Lowest         Medium           Highest
Long Stalk            Desirable      Not Normally     Necessary
Blow-Up Ratio         3:1            2:1              3.2:1
Die Gap               40 mil         >89 mil          40 mil
Melt Strength         Excellent      Poor             Excellent
Optical Properties    Excellent      Poor             Very Poor
Impact Strength       Very Good      Good             Excellent
Elmendorf Tear        Low, but       High and         Low MD,
                      Balanced       Balanced         High TD
Stiffness             Good           Good             Excellent
Softness              Good           Good             Poor
MVTR                  Good           Good             Excellent

Dart-drop impact strength of HMW-LDPE per mil of thickness is higher than that of conventional LDPE, and even higher than LLDPE. This emphasizes the orientation effect of long-stalk extrusion. Standard LDPE and LLDPE impact/mil values remain relatively constant with increasing gauge. HMW-LDPE has remarkably high impact/mil for films under 1 mil, decreasing with gauge, and eventually leveling off at about 2 mils. HMW-LDPE also has better optical properties than the other two high-strength resins, HMW-HDPE and LLDPE.

Table : HMW-LDPE & HMW-HDPE IN LONG-STALK EXTRUSION

                          Petrothene   Petrothene
                          NA 351       LY600
Properties                HMW-LDPE     HMW-HDPE

Melt Index,

  g/10 min.               0.3          8.0 HLMI
Density, g/cc             0.925        0.951
Film Gauge, mil           0.75         0.75
Haze, %                   6.0          High
Gloss, 45 [degrees]       67           Low
Drop dart, g              238          414

Total Energy Dart

Drop, ft-lb

  Body                    1.9          2.2
  Creased                 1.7          2.1

Tens. Str., psi

  MD                      6090         11,600
  TD                      5900         9010

Elongation, %

  MD                      130          260
  TD                      320          380

1% Secant Mod., psi

  MD                      32           128
  TD                      33           144

Processing Conditions

Output, lb/hr             116          120
Screw Speed, rpm          175          125
Pressure, psi             2420         5100
Drive, amps               23           31
Melt Temp., [degrees] F   396          397
Blow-Up Ratio             3.2:1        3.2:1
Stalk Height, in.         28           30

LONG-STALK EXTRUSION

HMW-LDPE can be blown into film by either of two standard bubble shapes: a conventional "pocket" bubble, used with LDPE gives high clarity and high output, but film cannot readily be drawn down to 1.5 mils, unless blended with LLDPE. Long-stalk extrusion, characteristic of HDPE, provides twice as much impact/mil and permits drawdowns to 0.5 mil, though clarity is somewhat lower. These two methods may be obtained by changing the type of air ring used in the process.

A dual-lip air ring pulls the bubble down into the air-ring pocket in the conventional bubble shape. A single-lip air ring allows the bubble to rise in a stalk and expand when the viscoelastic forces of the melt have been overcome by the drawing forces. During this rapid expansion just below the frost line, significant molecular orientation is created that optimizes the film's impact strength. This effect is enhanced by the resin's high melt viscosity and molecular weight.

HMW-LDPE can be extruded through grooved-feed extruders developed for HWM-HDPE long-stalk film. Gauges usually range between 0.5 and 1.5 mil. HMW-LDPE long-stalk extrusion uses the same die gaps - about 40 mils - as for HMW-HDPE. Extruded under these conditions, melt temperatures can be as low as 370-380 F, which is about 50 [degrees] lower than for HMW-HDPE. In addition, there is a less tendency for melt-temperature override with HMW-LDPE because less shear heat is generated in the extruder than with HWM-HDPE.

Output per screw rpm of HMW-LDPE is lower than with HMW-HDPE, meaning that extruder screw speed would have to be increased in order to achieve equivalent outputs through grooved-feed extruders. Normally this isn't a problem since HME-LDPE generally runs cooler and at a lower backpressure than HMW-HDPE.

For example, HMW-HDPE was run on quantum's 100-mm laboratory grooved-feed extruder with a 4-in. diam. die at 120 lb/hr at 125 rpm; to match this output with HMW-LDPE, screw speed was increased to 175 rpm.

HMW-LDPE has the high melt strength required for a long stalk. Good gauge control is possible, with no special die adjustments necessary, other than standard gap adjustments in order to get a symmetrical bubble.

Long-stalk film optical properties and puncture resistance are unaffected by blow-up ration (BUR). However, dartdrop impact increases dramatically with BUR. MD tear strenght and stiffness decrease with BUR. Since these resins are stiffer than normal, a film blown with a large 4:1 BUR has a modulus similar to a normal 0.922-density homopolymer film. Optimum properties are achieved at about 3:1 BUR.

TANDEM AIR-RING BENEFITS

Several years ago, the former Chemplex Co., now merged into Quantum, found that a tandem air ring was one way to improve output rate and clarity in HMW-LDPE long-stalk extrusion (see PT Nov. '84, pp.47, 55). In the tandem setup, the primary air ring is a conventional single-lip type mounted directly onto the die. The secondary ring is a dual-lip model attached 30 to 51 in. above the first ring. A moderately shallow lip was found to give the best results. The secondary air ring "locks" the bubble in place, increasing stability and providing up to 14% higher throughput rates, while reducing gauge variation by about 60%. And while film toughness and strength were somewhat reduced by the tandem air-ring method, these properties remained well above levels of conventional clarity LDPE, suggesting potential downgauging. Clarity of HMW-LDPE is also better with dual-air-ring long-stalk extrusion than by any other method. On the other hand, it can be somewhat tricky to set up a dual air ring to achieve these benefits, and some trial and error may be necessary.

USING LDPE FILM LINES

Standard LDPE film extrusion lines can also be used to run HMW-LDPE. Although it is difficult to extrude below 1.5 mil in the pocket, favorable properties can be attained at all gauges. Pocket extrusion results in significantly less TD orientation than long-stalk extrusion, so film impact strength may be lower. Optical properties of HMW-LDPE generally improve when going from long-stalk to pocket extrusion. Compared with typical LDPE clarity films, HMW-LDPE extruded in the pocket has better impact, stiffness and strength properties, with equivalent opticals.

To improve drawdown of HMW-LDPE when extruding a conventional pocket bubble, 10-30% LLDPE can be dry-blended into the resin. Film gauges as low as 0.5 mil have been reached in Quantum's applications research laboratory, where HMW-LDPE is blended with 30% of a hexene LLDPE with a 2.5 MI and 0.918 density at a 3:1 BUR. The addition of the 30% LLDPE had little effect on film haze or gloss, although "see-through clarity" (NAS) was decreased relative to 100% HMW-LDPE. Tensile and tear strengths were improved; and though total dart-drop impact energy decreased, puncture resistance improved with addition of LLDPE.

COEXTRUDED HMW-LDPE

HMW-LDPE also is a good candidate for coextrusion. When used as the outside layer in HMW-HDPE structures, the low-density resin provides high gloss and a good heat seal. And since it's a natural in long-stalk extrusion, the coextruded material runs with a more stable bubble, which gives better gauge control. It also allows the coextrusion to be run with a high frost line, providing excellent orientation.

PHOTO : High strength of HMW-LDPE film suggests uses in heavy produce bags, ice bags, quick-freeze bags, drun liners, shrink films, heavy textile, and even downgauged clarity films.