Metallocene-based LLDPE

LLDPE with narrow molecular weight distribution exhibits a lower, a sharper melting point [3], better hot tack and heat seal properties as well as higher clarity and better impact resistance (Fig. 3), tensile strength (Fig. 4) [11], and lower levels of alkane-soluble components. The most distinguishing characteristic of metallocene-based LLDPEs is that they are not restricted by the current immutable property relationships that are... 

B.P. Chemicals [81] have also reported the new metallocene-based LLDPE (with 1-hexene comonomer) having much superior properties than those produced by using the Ziegler-Natta Catalyst. 

With the discovery and development of metallocene-based LLDPEs with narrow MWD and high molecular weight, there has been a flurry of investigations with these polymers, because they exhibit sharkskin melt fracture at quite low and industrially limiting production rates. The objective of such studies is to increase the rate of production with... 

EFFECT OF DIE DESIGN AND WALL SLIP ON DIE DROOL PHENOMENON FOR METALLOCENE BASED LLDPE THEORETICAL AND EXPERIMENTAL... 

The effect of die design and wall slip on the die drool phenomenon was investigated for metallocene based LLDPE. It has been found that die exit opening and wall slip can significantly reduce the die drool phenomenon. Moreover, theoretical research has revealed that die drool onset can be explained by the negative/non-monotonic pressure profile generated inside the die and/or at the die exit region due to melt elasticity. 

INVESTIGATION OF FILM BLOWING PROCESS STABILITY FOR METALLOCENE BASED LLDPE WITH DIFFERENT LEVEL OF LONG CHAIN... 

LLDPE resias are produced ia iadustry with three classes of catalysts (11—14) titanium-based catalysts (Ziegler), metallocene-based catalysts (Kaminsky and Dow), and chromium oxide-based catalysts (Phillips). 

Metallocene-based polyethylene does not offer the lower production costs associated with LLDPE. Hence there will be a price premium for the new materials but this is felt to be justified in view of their improved property profile. 

Table 1 Characteristics of a metallocene (m) and a Ziegler (Z) octane-based LLDPE polymers... 

INTRODUCTION New types of linear low-density polyethylenes (LLDPE) based on the metallocene catalyst technology have been introduced recently in the market place. Metallocene-based Ziegler-Natta catalysts utilize a new synthetic approach for the polymerization of poly(a-olefins).( 5) Metallocene precatalysts are based primarily on group IV transition metals (primarily titcuiium and zirconium straddled by a pair of cyclic alkyl molecules) and require a coactivator, which is typically methylalumoxane but certain acids containing noncoordinating anions as bases also work well. 

As a result of the controlled long-chain branching in otherwise linear polymers, the processability of these resins is claimed to be significantly enhanced compared to the standard LLDPE and EP rubber materials. Envircmmental stress crack resistance of metallocene polyolefin is also claimed to be significantly better. A wide range of densities (0.86-0.93), flexural modulus (10-100 MPa), melting points (60-120 °C), and melt flow index (0.5-125) is available in the commercial metallocene-based ethylene copolymers. 

Fig. 7.29. dE/d 2(2) for (a) a 20/80 blend of HDPE-D and heterogeneous (Ziegler-Natta-catalyzed) ethylene-hexene LLDPE copo ymer en an ( ) a 25/75 blend of HDPE-D with a metallocene-based copolynier. (Reproduced with permission from [191]. Copyright 2002 American Chemical Society.)... 

Recent progress has also been made in the area of homo- and copolymerization of polar monomers. For instance, metallocene catalysts copolymerize ethene with 11-undecene-l-ol to yield hydroxy-functional LLDPE exhibiting improved adhesion [29]. Metallocenes based upon lanthanoids, as developed by Yasuda et al, copolymerize non-polar and polar monomers to produce new families of block copolymers, e.g., polypropylene-block-polymethylmethacrylate [30]. 

Exxon Mobil Chemical Escorene LDPE, Escorene LLDPE, Escorene EVA, Escorene ULTRA EVA (EVA copolymers, 12-33% EVA) Exceed 1012 CA (new metallocene-based VLDPE grade (912 kg/m ) with exceptional toughness and strength and dart impact). Exact 4041 (ULDPE). 

Metallocene based polyolefins and polyolefin elastomeric compositions (POE) are often noted in supplier product literature to exhibit useful (and commercially viable) properties in blends with other polyolefins. Metallocene based polyethylene has also been noted to have the potential to replace the LLDPE/LDPE blends that are employed in a myriad of film applications [32 ]. PP/metaUocene ethylene-a-olefin copolymer blends for car interior applications have been noted [33 ]. MetaUocene-LLDPE blends with HDPE have been proposed for heavy-duty industrial grade bags as well as food packaging films [34]. Exxon-Mobil product literature notes the utility and ease of blending the metallocene LLDPE Exceed with other polyolefins. PP/POE blends showed similar mechanical properties and lower viscosity than the more conventional PP/EPDM blends [35]. Polyolefin plastomers (POP) imder the tradename Affinity GA were introduced by Dow as a polymer blend additive to improve the flow without loss of... 

Mention has already been made in this chapter of metallocene-catalysed polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal compounds, usually zirconium or titanium. Incorporated into a cyclopentadiene-based structure. During the late 1990s several systems were developed where the new catalysts could be employed in existing polymerisation processes for producing LLDPE-type polymers. These include high pressure autoclave and... 

Ziegler-Natta catalysts are widely used in the production of high-density and linear low-density polyethylene (HDPE and LLDPE). More than half the world production of HDPE and over 90%o of LLDPE is based on Ziegler-Natta catalysts, although increased use of metallocene and other single-site catalysts is expected throughout the next decade. 

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