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Figure 2.4. Relationship between the crystallinity, density, and extent of chain branching.

Already, a second generation of LLDPE resins based on metallocene catalysts (sometimes referred to as mLLDPE resins) is becoming available. This new catalyst system allows the production of LLDPE as well as very low-density polyethylene (VLDPE) resins of exceptionally narrow molecular weight distribution, with controlled amounts of short-chain branching. The microstructure and therefore the mechanical properties of mLLDPE resins differ substantially from those of LLDPE made using conventional Zieglar-Natta type of catalysts. The polydispersity of the mLLDPE resins approach the most probable value of 2.0 (compared to the value of 20-50 typically obtained for LDPE and 4-15 for HDPE). The narrower distribution of molecular weights in a resin generally leads to better mechanical properties such as increased toughness or hardness and better low-temperature impact strength. However, the same features also makes it difficult to melt and process the resin. Introducing low levels of certain co-monomers as well as controlled amounts of chain branching can often alleviate some of these difficulties.

The amount of chain branching in polyethylene is an important determinant of the properties of the resin. For instance, important material properties such as the hardness, or the initial tensile modulus, decrease almost linearly as the extent of chain branching increases. The presence of branch points (as well as co-monomers) in the chain reduces the likelihood of crystallite formation at those points, reducing the overall crystallinity as well as density of the resin. The different grades of polyethylenes and its common copolymers used in popular applications and their general characteristics are listed in Table 2.4.

Recent market studies indicate very good prospects for polyolefins in general (and polypropylene in particular) in the near term. Of the polyethylenes LLDPE is presently growing at the fastest rate while HDPE is continuing to compete

Figure 2.5. Schematic representation of levels of chain branching encountered in different types of polyethylenes.

effectively with vinyl in the pipe market [4]. It is also used extensively in bottles, containers, drums, and houseware applications where its high modulus and very low permeability is particularly desirable. The high-density variety has the highest stiffness among polyethylenes but molded parts and thick films are opaque compared to the translucent LDPE products. The latter grade therefore finds a considerable market in films for packaging (including plastic-coated paperboard packages) and agricultural use as mulch films, greenhouse films and irrigation pipes. These uses represent about two thirds of the U.S. market for

Table 2.4 Different Types of Polyethylenes Used in Packaging Applications and Their Properties

Type Comonomer

Densitya

Melt Index

Pb

Applications

LDPE None

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