Alpha-olefin comonomer

Deliberate addition of alpha-olefin comonomer in an ethylene polymerization reactor leads to the formation... 

In LLDPE, the type of alpha-olefin comonomer determines the length of the SCBs. While being incorporated into the polymer chains, two of the carbon atoms of the comonomer become part of the polymer backbone (Table 1). 

Alpha-Olefin comonomer Carbon content Branch length... 

Kalyan et al. [56] have also studied the effect of alpha-olefin comonomers on the rheological properties and processing of LLDPE. The characteristics of the resins are shown in Table 2. It is found that 1-octene-based LLDPE has the lowest shear viscosity as compared to 1-butene- and 1-hexene-based polymers (Fig. 9). Decrease in power consumption, pressure before the die, temperature in the die, and increase in output has also been found according to shear viscosities of the polymers during tubular film extrusion. 

The level of short-chain (SCB) and long-chain (LCB) branches control the solid resin density of a PE resin. For example, the level of SCB is controlled by the amount of alpha olefin comonomer incorporated into LLDPE resin as a pendant group. The random positioning of the pendant groups disrupts the crystailization process when the polymer is cooled from the molten state, causing the level of crystallinity to decrease with increasing amounts of alpha olefin comonomer. 

Butene. The largest chemical use of n-butenes is 1-butene used in production of LLDPE, which requires alpha-olefin comonomers. Various processes for the production of LLDPE and HDPE incorporate 1 -butene as a comonomer. This accounts for about... 

This increases amorphous content and contributes to LDPE attributes, such as film clarity and ease of processing. As branching increases, density decreases. In LLDPE, incorporation of relatively large quantities of alpha olefin comonomers results in abundant SCB and lowering of density. 

In the 1990s, an improvement for the gas phase process was developed called "condensed mode" operation of Unipol reactors (12). This technique greatly expanded capacity of gas phase reactors and product capability by making it more practical to use higher alpha-olefin comonomers such as octene-1. 

Chemically, LLDPE can be described as linear polyethylene copolymers with alpha-olefin comonomers in the ethylene chain. They are produced primarily at low pressures and temperatures by the copolymerization of ethylene with various alpha-olefins such as butene, hexane, octane, etc., in the presence of suitable catalysts. Either gas-phase fluidized-bed reactors or liquid-phase solution-process reactors are used. (In contrast, LDPE is produced at very high pressures and temperatures either in autoclaves or tubular reactors.)... 

One key feature or limitation of Z-N catalyzed LLDPE and VLDPE resins is the non-uniform or broad composition distribution and broad molecular weight distribution (MWD or polydispersity index of 4 to 5) of the resins. This is due to multi-site nature of Z-N catalyst with differences in each site s ability to incorporate alpha-olefin comonomer. Catalyst sites in Z-N catalyst... 

Another key limitation of Z-N catalysts is the inability to incorporate very high levels of alpha-olefin comonomer such as 1-butene, 1-hexene, or 1-octene to make the density less than about 0.885 g/cm. The lowest density Z-N catalyzed VLDPE resin commercially available today (FLEXOMER from The Dow Chemical Company) has a target density of 0.885 g/cm (approximately 20 wt% crystallinity). Due to this, Z-N catalyzed VLDPE resins cannot be used in applications requiring very low modulus, low shore A hardness (density less than 0.885 g/cm ). Note that logarithm of modulus of polyethylene resins is related to density (degree of crystallinity) ... 

Sun et al. [64] studied the effect of alpha-olefin comonomer content on polyethylene and concluded that models based solely on molecular weight are too simple to describe quantitatively the effect of the short branches. By analogy with Eqs. 2.16 and 2.101, they defined branching factors for short chain branches and found that their data obeyed Eq. 2.111. 

The introduction of short-chain branches into polyethylene by use of an alpha-olefin comonomer yields the commercially important polymer called linear low-density polyethylene (LLDPE). These short branches would also be expected to affect the entanglement molecular weight. Based on previously published data on poly (a-olefins) [10, 53], Fetters etal. [60] developed the following empirical equations for estimating the plateau moduli of polymers of this type, given only the average molecular weight per backbone bond, mj,. 

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