LLDPE with a Broad MWD

Most LLDPE resins manufactured have a relatively narrow molecular weight distribution as indicated by Melt Flow Ratio values (MFR = FI/MI) of 16-30 or polydispersity values of 2-4. Films produced from this type of polyethylene have higher dart impact and tear properties as shown in the Table 6.5. However, LLDPE resin with a MWD broader than LDPE is used in geomembrane applications such as landfill covers and caps and other heavy-gauge film applications. 

The LLDPE with a broad MWD can be produced in a single reactor with a Cr-based catalyst or in tandem reactors such as two gas-phase reactors 

Chevron Phillips manufactures several broad MWD LLDPE resins in a slurry loop reactor using a Cr-based catalyst that generates 1 -hexene, in situ, used as comonomer. Table 6.6 summarizes some property data for this type of polyethene film. 

In landfill applications, this type of film needs to have a very high environmental stress crack resistance to withstand degradation by various waste components found in a landfill. 

Using UNIPOL II process technology. Union Carbide introduced easy flow LLDPE products that had better processability than LLDPE with a narrow MWD and LDPE with an intermediate MWD. Union Carbide reported processability and product physical property data for these new resins [25]. Some of the data is summarized in Table 6.7 which compares a UNIPOL II easyflow resin with a high-pressure LDPE. 

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PE mixtures constitute an important part of commercial blends. The information in Appendix, Table 18.12, suggests five major steps in the production of commercial PE, which result in different types of PE, commercially available at present, viz., 1935 - LDPE with LCB 1950 - LLDPE or HOPE catalyzed by, e.g., CraOg, NiO, M02O3, or CoO 1953 - the Ziegler-Natta (Z-N) catalysis for HOPE, UHMWPE, and LLDPE with a broad MWD and heterogeneous comonomer placement 1975 - the metallocene catalysis producing narrow MWD and homogeneously distributed SCB and 1997 - post-metallocene catalysis that leads to PE copolymers with adjustable MW and MWD as well as copolymerization of olefins with polar monomers and macromers. 

Solution polymerization in pentane (C5) or hexane (Cg). The mixed catalyst had high activity. The mixed catalysts, phosphinimine + Z-N, provided LLDPE with a broad MWD... 

Table 6.6 Typical specifications and properties for LLDPE with a broad MWD.

The width of molecular weight distribution (MWD) is usually represented by the ratio of the weight—average and the number—average molecular weights, MJM. In iadustry, MWD is often represented by the value of the melt flow ratio (MER), which is calculated as a ratio of two melt indexes measured at two melt pressures that differ by a factor of 10. Most commodity-grade LLDPE resias have a narrow MWD, with the MJM ratios of 2.5—4.5 and MER values in the 20—35 range. However, LLDPE resias produced with chromium oxide-based catalysts have a broad MWD, with M.Jof 10—35 and MER of 80-200. 

Disclosure of the Ziegler catalyst, used for industrial production of HDPE since 1954—1955. The Ziegler-Natta (Z-N) catalyst is mainly a binary mixture of a titanium halide and an organoaluminum compound producing mainly LLDPE copolymer with a broad MW distribution (MWD). Academic and industrial modifications of the Z-N catalyst started right away, e.g., by Elston in DuPont... 

Figure 6.13 Rheological comparisons (190°C) for LDPE with a relatively broad MWD and LLDPE with a relatively narrow MWD.

Countries produciug commodity LLDPE and their capacities, as well as production volumes of some U.S. companies, are Hsted iu Table 5. Iu most cases, an accurate estimate of the total LLDPE production capacity is compHcated by the fact that a large number of plants are used, iu turn, for the manufacture of either HDPE or LLDPE iu the same reactors. VLDPE and LLDPE resius with a uniform branching distribution were initially produced in the United States by Exxon Chemical Company and Dow Chemical Company. However, since several other companies around the world have also aimounced their entry into this market, the worldwide capacity of uniformly branched LLDPE resins in 1995 is expected to reach a million tons. Special grades of LLDPE resins with broad MWD are produced by Phillips Petroleum Co. under the trade name Low Density Linear Polyethylenes or LDLPE. 

To improve the end use performance and make the processability easy, control of MW and MWD as well as the use of.more than one comonomer has been reported for LLDPE [28]. Union Carbide s high MW-LLDPE with broad MWD is a 1-hexene-based resin, and its film provides superior (about 30-50% higher) tensile strength, puncture resistance, and dart impact strength than conventional 1-hexene-based resin, but with lower tear resistance in the transverse direction. The broad MWD makes the resin processability easy on the conventional extruder. 

Phillips catalysts also have lower reactivity ratios toward a-olefin incorporation, therefore are not used to produce LLDPE. However, they are excellent catalysts for HOPE and dominate the market for this resin. HDPEs made with Phillips catalysts have a very broad MWD, often with PDIs of 10 or higher. 

Once the very broad comonomer distribution and relatively narrow MWD was recognized, detailed structure studies of LLDPE focused on determining the location of the comonomer among the various molecules that make up a LLDPE product. To answer this question fractionation is necessary and as Nakano and Goto [15] pointed out a dual fraction by both molecular weight and comonomer content will ultimately be necessary for complete information. The reason for this is illustrated in their imaginary example shown in Fig. 25. This shows how two resins with the same MWD and SCB distributions can be made up of quite different molecular species. 

Hosoda [19] also compared the detailed branching distribution of a LLDPE with that obtained for a LDPE resin. In addition to the 3-D representation, Hosoda expressed the distribution data in the alternate form, that of a contour map. The contour maps for the two resin types are shown in Fig. 43. Once again one observes the extremely broad distribution of comonomer in the LLDPE. This particular sample appears to be tri-modal in character with surprisingly little molecular weight dependence on branching. The LDPE on the other hand is very broad in MWD but extremely narrow in SCB distribution. Here also, no attempt was made to relate the structure to any resin properties, although in another... 

Pont de Nemours, Phillips, Celanese, and DuPont of Canada Ltd. The first were LDPE/HDPE blends, but later LLDPE/LDPE blending dominated the film-blowing application. Even a small amount of LDPE significantly improved LLDPE processability as well as the heat sealing, puncture resistance, tensile, and optical properties. However, for >35 % LLDPE in LDPE modification of film-blowing line was needed. Also LLDPE/LLDPE blends with broad MWD showed improved processability... 

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