Bimodal pipe

The MWD X CCD correlation exemplified in Figures 2.4 and 2.5 is not desirable for certain applications. A notable example is bimodal pipe resins, where much better mechanical and rheological properties are achieved if the higher molecular weight component has a higher a-olefin fraction than the lower molecular weight component. The reasons for this improved performance are related to the presence of tie molecules, a fascinating subject that is, unfortunately, beyond the scope of this chapter. The reader is directed to the references 2 and 3 at the end of this chapter for more information on this subject. 

The application areas of bimodal polyethylenes are the same as for corresponding uni-modal resins. However, improved product property combinations, such as stiffness-impact balance, result in products with higher performance. For example, without bimodal polyethylene and control over the MWD and comonomer incorporation, development of HDPE pipe materials with higher pressure classification would not have been possible [15, 16]. Another new opportunity is material reduction (source reduction) without compromising the properties of the ready-made plastic products. Depending on the end use, e.g. in film application a thickness reduction of 10-30% is possible. 

The upgraded bimodal high density PE provides certain advantages. Its excellent ductility enables PE pipe to survive an earthquake better than more rigid materials such as PVC or ductile iron. They have a slow... 

Products A wide range of bimodal and unimodal products, with a full control of comonomer distribution, can be produced, with densities ranging from 918 to 970 kg/m3 and melt flowrate from less than 0.1 to over 100. The molecular weight distribution can be controlled from narrow to broad. Advanced properties are tailor-made applications such as pipe strength, film bubble stability as well as high ESCR and stiffness in blow molding. Other special applications include extrusion coating and wire cable. 

Equistar Chemicals L. D. Polyethylene, Bimodal HDPE Ethylene Equistar-Maruzen low-pressure sluury process producees bimodal HDPE resins for film, blow molding, pipe and injection molding 3 1985... 

Reactors used in ethylene polymerizations range from simple autoclaves and steel piping to continuous stirred tank reactors (CSTR) and vertical fluidized beds. Since the 1990s, a trend has emerged wherein combinations of processes are used with transition metal catalysts. These combinations allow manufacturers to produce polyethylene with bimodal or broadened molecular weight distributions (see section 7.6). 

Bimodal PE s (LCB = 0.01- 3 > 7) was lightly crosslinked. The PE-1 was prepared in the hrst reactor, and PE-2 was prepared in the presence of 15 - 65 wt% of PE-1 in the second reactor. The reaction could be carried out in the slurry, solution, or gas phase. blends (p < 885 kg/m ) were used for wrre/cable coating, weather-stripping, seals, foamed articles with closed, open or mixed cells, containers, medical appliances, drapes and coverings, hbers, tapes, tubes, pipes and hoses, bellows, boots, gaiters, footwear, etc. Cree etal, 1998... 

Figure 22 depicts the variation of the pressure drop as a function of mass flow rate for an 80% heavy crude oil OAV bimodal emulsion. It ean be seen that the pressure drop readings are essentially the same for bofli legs up to a critical flow rate at which the pressure drop levels off in Leg 2. In this portion of the pipe, the mass flow rate increases two-fold while the pressure drop remains essentially un-... 

Figure 22 Mass flow rate as a function of pressme drop for a bimodal, 80% heavy cmde oil emulsion, in a once-through open-loop, 21.7 mm diameter pipe. (From Ref. 250.)...

S. Joseph, How Do Bimodal Polyethylene Resins Provide Improved Pipe Properties (Lyondell, 2005)... 

Many different loop reactor configurations are used in industrial processes. The loop can be either in a vertical (Phillips and Spheripol processes [72, 73]) or in a horizontal position (USl process). The pipe can also be bent into multiple legs to increase the reaction volume. Several loops can be arranged in series to produce bimodal polymer, as in the case of Spheripol and Borstar processes [74]. Alternatively, the series of loop reactors can be operated as one single unit to increase average residence time and throughput. 

In 1977 Phillips Petroleum Co. developed blends of Z-N-LLDPE using C2+4 and C2+6 copolymers which were disclosed as suitable for pipe extrusion (Larsen 1982). The same year, Mobil Oil announced reactor blends of PE (copolymer of C2+4 and/or C2+6) for production of blown films that exhibited improved MD/TD tear balance. The latter materials were produced in a multistage, gas-phase, fluidized bed polymerization process with in situ blending. The resulted bimodal MWD resins had 0.35 to about 0.75 wt% of a higher-MW component (Ah et al. 1994). 

Zhou, Z. Hiltner, A. Baer, E., Predicting Long-Term Creep Failure of Bimodal Polyethylene Pipe from Short-Term Fatigue Tests. J. Mater. Sci. 2011,46, 174-182. 

HDPE pipe is used in low pressure applications for transporting potable water, gas, acids, liquid hydrocarbons, oils, salt water, and other chemicals and solvents. Long-term load bearing, or creep, tests are essential to determine the life of PE pipe under pressure. HDPE pipe withstands pressures in short-term burst tests that are several times greater than long-term burst pressures because of the creep phenomenon. Broad molecular weight distribution resin, as obtained from chromium oxide catalysts, are ideal for most pipe applications. The introduction of superior, bimodal type, pipe resins in recent years has extended HDPE pipe applications. 

FIGURE 12.9 TEM image and selected area electron diffraction of CMK-5 material (a) and the proposed structure having a bimodal distribution of mesopores (b). A pore system is formed by the tubular structure of the carbon walls and the second system of pores results from the interconnected carbon pipes. (From Kruk, M. et al., Chemistry of Materials, 15, 2815, 2003. With permission.)... 

The HMW-HDPE with a bimodal MWD is primarily used in film applications to fabricate grocery bags (T-shirt bags) and other merchandise bags. However, polyethylene with a bimodal MWD is also used in premium blow-molding markets and high-pressure pipe applications such as natural gas distribution pipe. 

Nova also reports a single-site catalyst based on an organometallic complex with a metal center (Ti, Hf or Zr), and containing a cyclopentadienyl-type ligand and a phosphinimine ligand with methylalumoxane as activator, that produces bimodal MWD resins in the advanced Sclairtech reactor system [44]. The resins provide premium properties in pipe applications. 

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