Linear low density polyethylenes

The general structure of linear low density polyethylene is shown in Fig. 18.2 c). Linear low density resins are copolymers of ethylene and 1-alkenes principally 1-butene, 1-hexene, and 1-octene. Comonomer levels range from approximately 2 to 8 mole %. This family of polyethylene is widely known as LLDPE. Linear low density polyethylenes are polydisperse with regard to molecular weight and branch distribution. 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

Packaging films (extruded, blown, and cast) are used in pressure-sensitive tapes and electrical applications and as replacements for cellophane and glassine films in liners for cereal boxes, wraps for snack foods, cigarettes, bread, and cheese. Blow-molded containers are used in applications where the higher use temperature of polypropene is needed (compared to HDPE), such as in packaging of syrups that are hot-filled. 

Fiber products account for about 15% of polypropene consumption. The products range from continuous filaments for carpeting and rope to melt-blown fibers for nonwoven goods. Specific applications include outdoor carpets, yarns for upholstery and automobile seats, and replacements for canvas in luggage and shoes, disposable goods (diapers, surgical gowns), ropes, and cords. 

The chemical iadustry manufactures a large variety of semicrystalline ethylene copolymers containing small amounts of a-olefins. These copolymers are produced ia catalytic polymerisation reactions and have densities lower than those of ethylene homopolymers known as high density polyethylene (HDPE). Ethylene copolymers produced ia catalytic polymerisation reactions are usually described as linear ethylene polymers, to distiaguish them from ethylene polymers containing long branches which are produced ia radical polymerisation reactions at high pressures (see Olefin POLYMERS, LOWDENSITY polyethylene). 

Densities and crystallinities of ethylene—a-olefin copolymers mosdy depend on their composition. The classification ia Table 1 is commonly used (ASTM D1248-48). VLDPE resias are usually further subdivided iato PE plastomers of low crystallinity, 10—20%, with densities ia the range of 0.915—0.900 g/cm, and completely amorphous PE elastomers with densities as low as 0.860 g/cm.  

Commercial production of PE resias with densities of 0.925 and 0.935 g/cm was started ia 1968 ia the United States by Phillips Petroleum Co. Over time, these resias, particularly LLDPE, became large volume commodity products. Their combiaed worldwide productioa ia 1994 reached 13 X 10 metric t/yr, accouatiag for some 30% market share of all PE resias ia the year 2000, LLDPE productioa is expected to iacrease by 50%. A aew type of LLDPE, compositioaaHy uniform ethylene—a-olefin copolymers produced with metallocene catalysts, was first introduced by Exxon Chemical Company in 1990. The initial production volume was 13,500 t/yr but its growth has been rapid indeed, in 1995 its combiaed production by several companies exceeded 800,000 tons. 

The large number of commodity and specialty resias collectively known as LLPDE are in fact made up of various resias, each different from the other in the type and content of a-olefin in the copolymer, compositional and branching uniformity, crystallinity and density, and molecular weight and molecular weight distribution (MWD). 

Crystallinity and Density. These two parameters, which are closely related, depend mosdy on the amount of a-olefin in the copolymer. Both density and crystallinity of ethylene copolymers are also influenced by their compositional uniformity. Eor example, for LLDPE resias with different a-olefin (1-hexene) content, the density (g/cm ) is as follows  

In the solution reactor, the polymer is dissolved in a solvent/comonomer system. Typically, the polymer content in a solution reactor is controlled at between 10 and 30 wt-%. A hydrocarbon solvent in the range of C6 to C9 is typically used as the diluent in the solution process. 

In the recycling stage, the following purge flows are taken away from the process  

The fluidised bed reactor used for the production of LLDPE is the same as for the production of HD PE described above in Section 3.2.3.2 and is not described again. A number of fluidised bed reactor plants are designed to produce both HDPE and LLDPE according to market requirements but, in general, plants tend to operate and produce one t)q)e only. 

Catalyst Ziegler or metallocene Ziegler-Natta or metallocene 

Most of the processes applied for the production of polypropylene are very similar to the ones used to produce high density polyethylene. Nevertheless, this section describes the most important and most widely used processes for the production of polypropylene. Generally, two different types of processes are apphed in the production of polypropylene  

From the point of view of establishing realistic comonomer distributions for various LLDPE resins it is clear that data obtained through analytical TREF is 

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. 

It is important from the point of view of clarifying the relationship between structure and properties for LLDPE resins to be able to define in which molecular weight species the comonomer molecule are concentrated. This is because of the impact the comonomer distribution can have on polymer morphology. In the dynamic process of crystallization, large molecule with little comonomer will exert quite a different morphological influence than a large molecule with many branches. This in turn raises the practical question of whether there is variation in the nature of comonomer incorporation depending on the catalyst system and process used and to what extent are observed resin properties differences a consequence of this. 

The molecular weight dependent of comonomer content in LLDPE has been evaluated to some degree in a number of studies. Wild, Ryle and Knobeloch [25] 

The studies of Mirabella and Ford [26], Hazlitt and Moldovan [12] and Wilfong and Knight [29] have also noted the decresing comonomer content with increase in molecular weight for LLDPE resins. However, detailed inspection of these as well as the other studies cited leads one to conclude that it is the extraordinarily broad SCB distribution that constitutes the most remarkable characteristic of commercial LLDPE resins. 

The melt rheology of LLDPE differs from that of LDPE, so that processing equipment for the latter cannot be used without modification. Consequently, much interest in blends has been generated these are discussed later in this chapter. 

Film appears to be the main application area at present. 

These copolymers are thought to be random copolymers, which means that the comonomer is randomly distributed along the chains [76]. In other words, the probability of finding a 

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One of the mam uses of the linear a olefins prepared by oligomerization of ethylene is in the preparation of linear low density polyethylene Linear low density polyethylene is a copoly mer produced when ethylene is polymerized in the presence of a linear a olefin such as 1 decene [H2C=CH(CH2)7CH3] 1 Decene replaces ethylene at random points in the growing polymer chain Can you deduce how the structure of linear low density polyethylene differs from a linear chain of CH2 units ... 

OLEFIN POLYTBRS - POLYETHYLENE - LINEAR LOW DENSITY POLYETHYLENE] (Vol 17)... 

Linear Low Density Polyethylene. Films from linear low density polyethylene (LLDPE) resias have 75% higher tensile strength, 50% higher elongation-to-break strength, and a slightly higher but broader heat-seal initiation temperature than do films from LDPE. Impact and puncture resistance are also improved over LDPE. Water-vapor and gas-permeation properties are similar to those of LDPE films. 

The majority of spunbonded fabrics are based on isotactic polypropylene and polyester (Table 1). Small quantities are made from nylon-6,6 and a growing percentage from high density polyethylene. Table 3 illustrates the basic characteristics of fibers made from different base polymers. Although some interest has been seen in the use of linear low density polyethylene (LLDPE) as a base polymer, largely because of potential increases in the softness of the final fabric (9), economic factors continue to favor polypropylene (see OlefinPOLYMERS, POLYPROPYLENE). 

High density polyethylene. Linear low density polyethylene. 

Low Pressure Linear (Low Density) Polyethylene" in ECT3rd ed., under "Olefin Polymers," Vol. 16, pp. 385—401, byj. N. Short, Phillips Petroleum Co. 

AH higher a-olefins, in the presence of Ziegler-Natta catalysts, can easily copolymerise both with other a-olefins and with ethylene (51,59). In these reactions, higher a-olefins are all less reactive than ethylene and propylene (41). Their reactivities in the copolymerisation reactions depend on the sise and the branching degree of their alkyl groups (51) (see Olefin polya rs, linear low density polyethylene). 

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