Bimodal MWDs weight distributions

Description The CX process uses two polymerization reactors in series. The products have bimodal molecular-weight distribution (MWD), where MWD and composition distribution is freely and easily controlled by adjusting the operating conditions of two reactors without changing the catalyst. 

Unique PVDF properties have been observed after polymerization in supercritical carbon dioxide [38,51-53,55] at certain conditions. The polymer synthesized in supercritical carbon dioxide exhibits a bimodal molecular weight distribution (MWD), as illustrated in Fig. 4 [52]. At molar VF2 feed concentrations less than about 1.9 M, the polymer has a unimodal distribution, at the conditions of Fig. 4. As the monomer concentration is increased, the distribution becomes broader and bimodal. Changes in temperature, pressure, and residence time also have an effect on the MWD [51,52]. In Fig. 4, ris the average residence time (the reactor volume divided by the inlet volumetric flow rate). 

Application The UNIPOL Polyethylene (PE) Process produces the widest array of linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HOPE) having unimodal or bimodal molecular weight distribution (MWD) using a single, low-pressure, gas-phase reactor. 

Both processes can produce unimodal and bimodal molecular weight distributions. Currently, bimodal MWDs may have to be produced in dual reactor systems. They are energy intensive and require more capital and increase the control complexity. Some licensors claim now to achieve similar product quality with a single reactor by using a dual site catalyst with bimodal capability. 

Another important characteristic of oxo acids as initiators is that they yield polymers with a bimodal molecular weight distribution (MWD) under suitable conditions. We observed this particular MWD for the first time in the polymerization of styrene by acetyl perchlorate (AcClO ) the MWD curve c in Fig. 1 shows a typical example. Subsequently, Pepper reported a similar MWD for polystyrene obtained with perchloric acid The bimodal MWD is another manifestation of the strong interaction of oxo acid-derived counteranions with propagating carbocations as discussed below. 

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

In order to compare the results of an analysis technique with the true solution it is essential that the MWD be known before the analysis is started. Consequently, computer simulated data generated by assuming a molecular weight distribution and subsequently using Equation 3 to generate g(x) for a series of t values, was used for analysis. Five different MWDs were used, four of which were generated using GEX functions as models (two GEX functions for bimodal peaks), and Table I shows the GEX parameters used for each of these MWDs. 

Description The Hostalen process is a slurry polymerization method with two reactors parallel or in series. Switching from a single reaction to a reaction in cascade enables producing top quality unimodal and bimodal polyethylene (PE) from narrow to broad molecular weight distribution (MWD) with the same catalyst. 

According to the MC simulation, the high molecular weight, narrow distribution component consists of the largest polymer molecule in each polymer particle, and the bimodal MWD is formed because of the limited space effects. 

Polymer Characterization. Molecular weight distributions (MWD s) were obtained from a Waters model 201 ALC/GPC using micro-styragel columns with pore sizes of 500, 10, 10, 10 and 10 A. Calibration was with polystyrene standards in THF solution. Fractionation of bimodal MWD polymers was achieved by use of a Knauer CPC using styragel columns of 10, 10, 10 and 10 A. Tacticity information was obtained from NMR spectra using a 270 MHz Bruker instrument. 

Metallocene LLDPEs are relatively difficult to process because of narrow molecular weight distribution (MWD) when compared to conventional Ziegler LLDPEs. Metallocene catalyst based octene-1 LLDPE copolymers made by the Dow Chemical Company are known to process better as a result of their long-chain branched (LCB) structure, referred to as Dow Rheology Index (DRI) numbers.The LCB is also responsible for improved melt strength in mLLDPEs.Exxon has also addressed the processibility issue with advanced performance terpolymers.LCB bimodal mLLDPE resins are commercially available from BP Chemicals.Such mLLDPEs are produced by BP s proprietary gas phase fluid bed technology called Innovene technology. 

Molecular weight distribution (MWD) bimodal, each peak with a narrow MWD... 

Polyethylene for pipe applications has a relatively high molecular weight and a broad molecular weight distribution. Some premium grades have a bimodal MWD. Table 6.10 summarizes the product specifications for several suppliers of pipe-grade polyethylene for various applications. 

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