Metallocene Linear Low-Density Polyethylene

Transcription

1 Metallocene Linear Low-Density Polyethylene By: Paul M. German, ExxonMobil Chemical Company Introduction Metallocene linear low-density polyethylene (mlldpe) resins are manufactured by copolymerizing ethylene with selected alpha-olefin comonomers using a metallocene based catalyst system, instead of the traditional Ziegler Natta type catalysts that are normally used for conventional LLDPE. Several low-pressure processes are used to produce mlldpe resins, including gas-phase fluidized bed, gas-phase stirred bed, liquid- phase slurry, and liquid-phase solution. The high pressure tubular or autoclave type process can also be used to make mlldpe, but this is less common. Most mlldpe polymers are characterized by short-chain branching induced by the comonomer type, and little or no long-chain branching as seen Figure The distinguishing features of most mlldpe are narrower molecular weight distribution (MWD) and more uniform comonomer distribution (CD) than conventional LLDPE. Comonomers used in mlldpe resins can include butene, hexene, 4-methyl pentene, and octene. Within each comonomer type film physical properties correlate to melt index and density. The comonomers that are most commonly used commercially today are hexene and octene. Some producers are developing mlldpe polymers with long chain branching using metallocene mixed or composite catalysts, but a detailed discussion of all resins is beyond the scope of this manual. Polymer Characterization The distinguishing features of most mlldpe are narrower molecular weight distribution (MWD) and more uniform comonomer distribution (CD) than conventional LLDPE. These differences in molecular architecture of the mlldpe provide the polymer with significant improvement in film physical properties such as seal initiation temperature, puncture resistance, tensile strength, and dart impact. These property improvements are typically made with some sacrifice in resin processability. Typical mlldpe polymers exhibit higher viscosity in shear flow than same MI conventional LLDPE. Figure 31-2 shows the comparison of a conventional LLDPE viscosity curve compared to a mlldpe at similar polymer molecular weights. The LLDPE shear thins at the higher shear rates more than mlldpe. The low shear viscosity of mlldpe is lower than LLDPE because of its narrower MWD.

2 Figure Polyethylene Polymers.- Molecular Comparison HP-LDPE LLDPE mlldpe Figure Structural differences between polyethylene families. Figure 31-2 LLDPE vs mlldpe Shear Rheology at 230 o C Viscosity (PA*s) 1000 Film Melt Index m LLDPE 1MI /.918d Z-N LLDPE 1MI/.918 Extrusion Shear rate (s-1)

3 Most film extruders operate from about 300 to 600 1/sec shear rates, thus the mlldpe will process slightly more stiffly than LLDPE and require slightly more torque (power), higher amps, higher melt temperatures, and higher pressures. For many mlldpe resins, processing is such that a 1 MI mlldpe processes like a 0.7MI conventional LLDPE. Another key difference between mlldpe polymers and LLDPE is the comonomer distribution or CD, which is also shown in Figure mlldpe resins have a more ideal or isolated comonomer distribution along the polymer chain. This results in a more uniform polymer with less high comonomer molecules and less slightly branched 'higher density' molecules. This improved, more uniform CD is a primary source of mlldpe property improvements including lower melting point, improved optics and higher toughness performance. Typical mlldpe Applications and End Uses The mlldpe resins are used in a wide variety of packaging film and other applications. Many LLDPE and VLDPE applications have been replaced with mlldpe polymers, which allow reduction in film thickness or the use of lower cost materials due to their enhanced toughness. Applications include the following: Trash bags Stretch film Cling film Frozen food packaging Sandwich Bags Heavy-duty sacks Ice bags Milk bags Bag-in-box Produce bags Generic packaging Liners Shrink wrap Lamination films The improved toughness properties, which include, puncture, tensile, and dart impact strength for mlldpe polymers provide benefits in a wide spectrum of applications. When the application requires specific performance properties, the selection of the proper grade of mlldpe to match the fabrication requirements with the film property needs is important. Table Blown film property comparison.

4 Test Unit LDPE LLDPE mlldpe Property Basis C6 C6 Melt Index ASTM g/10 min D1238 Resin Density 3 g/cm DSC Melting Peak ( C) Gauge micron Tensile Strength MD TD ASTM D882 MPa % Secant Modulus MD TD ASTM D882 MPa Elmendorf Tear MD TD ASTM D1922 g Dart Impact F50 g > 1000 Haze ASTM D1003 % Gloss, 45 ASTM D2457 Films made at 10 lb/hr/in output rate, 2.5:1 BUR. 60 mil die gap used for LLDPE and mlldpe, 30 mil die gap used for LDPE. 70 Figure 3 Blown Film Tensile Comparison Tensile atbreak MD D882 (MPa) Tensile atbreak TD D882 (MPa) Ultimate Tensile LDPE C4-LLDPE C6-LLDPE mlldpe

5 Typical Polymer Properties The physical properties of mlldpe vary with melt index, density, and comonomer type. The mlldpe polymers discussed in this chapter are narrower MWD polymers without long chain branching. Table 31-1 gives typical blown film mlldpe properties compared with conventional LLDPE and LDPE controls. The film properties of mlldpe are influenced by the intrinsic polymer properties, the comonomer type, and the conditions of the fabrication process. Commercial mlldpe resins available today generally use octene or hexene comonomer, providing similar properties and end use performance at a given MI and density. Tensile Strength The tensile strength of mlldpe films increases as resin molecular weight increases (indicated by lower resin MI). Higher alpha-olefin comonomers have about 20% percent higher tensile strength compared to butene copolymers. The increase in tensile strength of percent for mlldpe polymers over LLDPE polymers is quite significant as shown in Figure This is especially true for packaging films and bags that are used with stresses in the TD direction, the direction of greatest tensile improvement. Yield Strength The yield strength of mlldpe films depends primarily upon the base resin density. Base resin density is a function of comonomer level. Yield strength is an important criterion for many film applications such as heavy-duty bags, grocery sacks, trash bags, produce bags, and ice bags. mlldpe and LLDPE films can experience high creep behavior when stressed. Increasing the film yield strength by increasing resin density will increase the level of stress the film will support. Dart Impact The dart impact strength of mlldpe films is dependent upon a number of factors including resin density, comonomer type, molecular weight, and film fabrication conditions. For mlldpe, dart impact typically correlates with resin crystallinity such that dart impact decreases as resin density is increased. The highest dart impact values are normally seen for resins at 0.918g/cc density. At a 0.918g/cc density, the butene comonomer resin will have half the dart impact strength of hexene and other higher alpha-olefins copolymers. The dart impact strength of mlldpe is typically 2 to 10 times higher than similar LLDPE film. Elmendorf Tear mlldpe films typically have slightly lower, but more balanced MD and TD Tear than similar LLDPE films. This is due to the faster relaxation times of these resins as discussed below. mlldpe film tensile, dart impact and puncture toughness are significantly higher such that lower measured tear values typically do not compromise end use performance. Relaxation Time and Film Orientation

6 The mlldpe polymers have faster relaxation times, due to their narrower MWD, which produces a difference in the effects of fabrication variables on molecular orientation when compared to conventional LLDPE films. Since mlldpe have much faster molecular relaxation times they are less sensitive to orientation than conventional LLDPE and more balanced tear and tensile properties are obtained at equivalent processing conditions. The mlldpe films can be readily fabricated on typical LLDPE equipment to provide good film properties. General Processing Guidelines There are several basic considerations, for running mlldpe resins on extrusion equipment designed for LLDPE. The torque and power requirements are slightly higher for mlldpe. Some common machine limitations might be motor current limitations, high pressures, high melt temperatures, melt fracture, and higher line speeds. mlldpe is more viscous than the same MI LLDPE as seen in the rheology curves in Figure 31-2, and thus mlldpe can require about 10 percent more extruder drive torque to tum the screw at equivalent output and temperature. The higher viscosity of mlldpe will increase the pumping efficiency of the screw (measured as lb/hr/rpm) by up to 10 percent because of reduced pressure flow back down the screw. The pressure drop through the die will increase for the mlldpe polymer. Die designs for mlldpe have less restrictive flow channels for a reduced die pressure to allow higher polymer flow rates. The die gap and land lengths used for LLDPE will usually work acceptably in mlldpe films. The mil die gap typically used for LLDPE is also acceptable for mlldpe and will minimize melt fracture. Melt temperatures tend to run slightly higher for mlldpe resins. This can be a limitation because mlldpe have slightly lower melt strengths, thus too high a melt temperature will restrict the ability to cool the bubble (excessive cooling air impingement) and maintain bubble stability. Enhanced cooling can be obtained with dual-lip air rings, internal bubble cooling (IBC), or stacked air rings. Melt temperatures for a l.0 Melt Index mlldpe resin will normally run in the 420 to 480 F range. Line speed limitations can become a concern, if the mlldpe polymer allows sufficient downgauging capability such that the haul-off drives are unable to reach the increased speeds. mlldpe resins have in some applications allowed downgauging from 25 to 50 percent compared to LLDPE films. The selection of extrusion equipment will depend to a large extent on the type of application. mlldpe can be fabricated on a variety of extruder screws, including both singleflighted and barrier screws. Some general suggestions for mlldpe processability include the following: l. The extruder L/D can range from 24:1 to 32:1. L/D higher than 32/1 is generally not used to avoid excessive work input and possible temperature buildup. Barrier screw design is highly preferred, for its efficient melting and mixing.

7 2. The screw compression ratio can vary from 2.5 to 3.0. The mlldpe polymer does not typically require higher shearing because its high viscosity will generate appropriate melt temperatures in the extruder. 3. High specific outputs at lower melt temperatures typically using barrier (solids channel-melt channel) screw designs have been successful in processing mlldpe resins. 4. Mixing sections have been successful in providing barriers to unmelt polymer passing through the extruder and for dispersing additives, pigment and blend. 5. Die flow channels are designed more open for the higher viscosity of the mlldpe polymer at the expected flow rates. Die gaps generally range from 50 to 120 mils to reduce melt fracture. 6. The extensional viscosity of mlldpe allows easy drawdown of the melt. The air ring should be a dual-lip design or internal bubble cooling (IBC) to maximize cooling without destabilizing or wrinkling the bubble. 7. Slitting knives should be coated such as ceramic or carbide, due to the higher toughness of the mlldpe. Blends Blends of LDPE in mlldpe decrease extrusion pressure, extruder drive amps, and melt temperature during the fabrication process. The LDPE will also reduce pumping efficiency, but the improved melt strength of the blown film bubble may allow higher specific output (lb/hr/in of die circumference). LDPE blends will also reduce the tendency for melt fracture to occur. Table 31-2 lists the resins used for a blend study of mlldpe and LDPE. The extrusion and film data is given in Figures 31-4 through This data is generally typical of mlldpe blends such that tensile strengths are reduced when compared to 100 percent mlldpe, MD tear strengths will decrease. However, optical properties and processability can be improved with blends. mlldpe is frequently blended with appropriate MI LLDPE. The maximum toughness properties of mlldpe are available when it is used as 100% layer. Blends with LDPE typically result in an averaging of resins properties as shown in the enclosed Figures.

8 Table 31-2 Blend Study Resins Resin Designation Nominal MI Nominal Density Description m-lldpe Gas phase metallocene catalyzed linear hexene copolymer mlldpe LLDPE Conventional Ziegler-Natta linear hexene copolymer LD High pressure LDPE; lower branching content LD High pressure LDPE; high branching content MAXIMUM EXTRUSION Nominal 0.75 mil Film Nominal 3.0 mil Film Max Output (lb/hr) Max. Output (lb/hr)

9 Figure 5 Dart Impact Nominal 0.75 mil Film Nominal 3.0 mil Film 26" Dart Impact (gm/mil) " Dart Impact (gm/mil) Impact for neat mlldpe Figure 6 MD Elmendorf Tear Nominal 0.75 mil Film Nominal 3.0 mil Film MD Tear (gm/mil) MD Tear (gm/mil)

10 MD Ultimate Tensile Nominal 0.75 mil Film Nominal 3.0 mil film MD Ultimate Tensile (psi) MD Ultimate Tensile (psi) PACKEX.PRE Haze Nominal 0.75 mil film Neat LLDPE 3% LD-1 Haze (%) % LD-1 10% LD-1 20% LD mlldpe LLDPE 3% LD-2 5% LD-2 10% LD-2 20% LD-2 Nominal 3.0 mil Film 25 Neat LLDPE 20 3% LD-1 Haze (%) m-lldpe LLDPE 5% LD-1 10% LD-1 20% LD-1 3% LD-2 5% LD-2 10% LD-2 20% LD-2

11 Gloss (45 degree) Nominal 0.75 mil Film m-lldpe LLDPE Neat LLDPE 3% LD-1 5% LD-1 10% LD-1 20% LD-1 3% LD-2 5% LD-2 10% LD-2 20% LD-2 Nominal 3.0 mil film m-lldpe LLDPE Neat LLDPE 3% LD-1 5% LD-1 10% LD-1 20% LD-1 3% LD-2 5% LD-2 10% LD-2 20% LD-2

12 Melt Fracture The mlldpe polymers have a narrow MWD which generate high shear stress. Melt fracture is a classical surface roughness, which degrades the appearance of the film and usually occurs at some critical shear stress. Although melt fracture is a concern in extrusion of mlldpe resins, techniques developed for LLDPE have proven successful with mlldpe. Methods to reduce melt fracture include the following: 1. Increasing the melt temperature to reduce the high shear stress encountered in the die land regions. The limitation on how much melt temperature can be increased will depend upon bubble stability concerns(blown films). Normal melt temperatures would range from o F ( o C). 2. Reducing the shear stress in the die gap and land region by increasing the die gap. The die exit land is an area of high shear stress in the film forming process. (Die gap is typically to 60 to 120 mils.) 3. Increasing die temperatures at the top zone to reduce the severity of melt fracture. It is recommended to run die temperatures at the melt temperature of the LLDPE. Good results have been obtained by increasing the top die zone temperature to 20 to 40 o F (-11 to 22 o C) above the melt temperature. 4. Processing aids can allow higher specific output rates without melt fracture. The processing aids have affinity for the die metal, coating the flow surface and reducing the shear stress level at the polymer-die interface. Some common additives such are abrasive pigments, antiblock additives, and UVI can interfere with the processing aids. Excessive processing additives can result in printing and treating problems and should be avoided. 5. Reducing output rates will also reduce shear stress and can eliminate melt fracture. However, it is interesting to note that determining the onset of melt fracture is sometimes difficult because it is path dependent. Higher rates can be obtained if the critical rate is approached from the low side. After melt fracture is encountered in the die, output rates must be greatly reduced before nonfractured film will return. Blown Film vs. Cast Film The properties of a 1.0 MI, density mlldpe resin in blown and cast films is surprisingly similar. The fast relaxation time of mlldpe resins prevents much orientation from occurring at typical extrusion conditions. The key improvements in cast film are the optical properties of gloss and haze. Table 3 compares a 1.0 MI, density mlldpe in blown and cast film.

13 Table 31-3 mlldpe 1MI/ 0.918g/cc Cast Film Blown Film Tensile Yield (psi) MD 1, at 200% MD 3, Ultimate Tensile (psi) MD 12, Break Elongation (%) MD Elmendorf Tear MD (g/mil) TD (g/mil) Dart Drop Method A (g/mil) Gauge (mil) Average Puncture Break Energy (in-lbs/mil)

14 Bibliography Resources 1. Bibee, D.V., "Prediction of LLDPE Blown Film Performance," 1987 Polymers, Laminations and Coatings Conference Proceedings, TAPPI PRESS. Atlanta, p Schurzky, K.G., "Design Parameters for LLDPE Film Resin Selection," 1984 Polymers, Laminations and Coatings Conference Proceedings, TAPPI PRESS, Atlanta, p Lee, C.D., Plastics Eng. (12) :43 (1988). 4. Veazey, E.W., "LLDPE Blown Film Equipment Design," p Ramamurthy, A.V., J. of Rheology 30(2): 337 (1986). 6. Krumm, K., "LLDPE Extrusion Features in the Blown Film and Cast Film Process," p Kwack, T.H., "A Study on Blown Film Extrusion of Linear Low-density Polyethylene," p Constantin, D., Poly. Eng. Sci. 24(4): 268 (1984). 9. Helmy, H., "Optimizing Equipment Design for Linear Low Density Polyethylene Blown Film," p Fraser, W. A., "Film Extrusion of Low Pressure LDPE," SPE Antec Proceedings, p Bonotto, S., "LLDPE Blends for Film Extrusion," 1982 Paper Synthetics Conference Proceedings, TAPPI PRESS, Atlanta, p Veazey, E.W., "Linear Low Density Polyethylene Processability-Where Are We Today?" 1981 Paper Synthetics Conference Proceedings, TAPPI PRESS, Atlanta, p Helmy, H., "Blown Film Technology for Linear Low Density Polyethylene and Its Blends," 1982 Paper Synthetics Conference Proceedings, TAPPI PRESS. Atlanta, p Bode, W., "Equipment and Processing Update for the Blown Film Extrusion of Linear Low Density Polyethylene," 1988 SPE Antec Proceedings, p Haber, A., "Morphology and Orientation in Polyethylene Tubular Blown Films," 1987 SPE Antec Proceedings, p Huang, T.A., "Deformational and Temperature History Comparison for LLDPE and LDPE Elements in the Bubble Expansion Region of Blown Films," 1985 SPE Antec Proceedings, p Constantin, D., "LLDPE Melt Rheology: Stretchability, Extrusion Defects," p Kalyon, D. M., "Microstructure Development in the Blown Film Extrusion of Linear Low Density Polyethylenes as Affected by Alpha-olefin Comonomers." 1986 SPE Antec Proceedings, p. 741.

15 19. Cady, L. D., "The Role of Comonomer Type and Distribution in LLDPE Product Performance," p Johnson, J., "An Examination of LLDPE as It Relates to the Blown Film Process," 1982 Paper Synthetics Conference Proceedings, TAPPI PRESS, Atlanta, p Arvedson, M, "Combining Processability and Toughness with Minority Blends of LDPE in Metallocene LLDPE", 1997 Pack Expo, Chicago, IL 22. Trudell, B. "Exceed mlldpe Hexene Copolymers Structure/ Property Relationships", September 1995, Schotland Conference 23. Bailey, P. "Stretch Film from mpe-lld", October 1997, Kunstsoffe 24. Simpson, D. "Product Performance of mlldpe Resins with Improved Processability" September 1997, Schotland Conference 25. German, P. "Stretch Film Background and Basics" August 1998, TAPPI PLC Conference 26. Mavridis, H. "New High Performance LLDPE for Blown Films", TAPPI PLC Conference, August 1998