Polyethylene, Linear Low-density

Abstract

Abstract Introduction LLDPE is the common name for copolymers of ethylene with α-olefin comonomer. The comonomers most frequently used commercially are butene, hexene, and octene. Commercial grade LLDPE resins with 4-methyl-1-pentene (4-MP-1) as comonomer is also available. LLDPE prepared by the conventional Ziegler-Natta (Z-N) catalyst system always exhibit high heterogeneity in the intermolecular distribution of comonomer units along the polymer chains. The branches are preferentially located in the lower molecular weight chains; thus the bulk of LLDPE behaves as if it were a blend of high molecular weight, linear molecules and low molecular weight, branched molecules. LLDPE differs from LDPE principally through a lack of long-chain branching (LCB) and a narrower molecular weight distribution (MWD). New types of LLDPEs based on the metallocene catalyst technology have been introduced recently in the market place. Such LLDPEs are characterized by narrower molecular weight and homogeneous short-chain branching distribution. Some of the metallocene catalyst-based octene-1 LLDPE copolymers made by the Dow Chemical Company are known to have LCB. For the properties of metallocene LLDPE see the entry Polyethylene, metallocene linear low density, in this handbook. LLDPE is commercially available in wide variety of melt indexes (MI) and density ranges. The properties of LLDPE are functions of molecular weight (MW), MWD, density, type, and amount of comonomer. The comonomers are also referred to as short-chain branches (SCB). Consequently, physical and mechanical properties also vary accordingly. Mechanical properties such as tensile, tear, and impact are strongly dependent on the chemical nature of the comonomer type. Therefore, it is difficult to list all properties separately. The values of the properties shown in the following table are given in ranges because of their dependence on molecular structure and type of comonomer and are intended to represent the best published examples of the most commonly used commercial grades of LLDPE resins. The physical properties of extruded materials may vary substantially from those of the compression molded materials. For illustration purposes, a few of the physical properties that depend on the chemical nature of the comonomer are presented in Tables 3, 6, 7, and 8.