A comparative study of multimodal vs. bimodal polyethylene pipe resins for PE-100 applications

Abstract

In this paper several high density polyethylene pipe resins are compared to polyethylene resins made in the Phillips Loop-Slurry Process (single-reactor), using a catalyst of chromium on modified aluminophosphate (Cr/AlPO 4 ). A brief description of the Cr/AlPO 4 system is presented along with polymer chain architecture (i.e., topology) and other structural aspects of polymers made from these catalysts. The physical properties of these resins and their fabricated pipe processing/performance properties were compared to both PE80 and conventional bimodal type PE-100 resins. Results from this study show that high-density polyethylene resins made from Cr/AlPO 4 catalysts possess primary structural attributes that are truly unique for chromium-catalyzed resins. These multimodal resins have very high molecular weights (MW; M w > 400 kg/mol) and exceptionally broad MW distributions (MWD; M w /M n > 50). Topologically, these resins were found to have uniform short chain branching distributions across the MWD; as in polymers made from single site metallocene catalysts) and significantly reduced levels (10 times less) of long chain branching when compared to resins made using chromium catalysts on conventional supports. Furthermore, results from rheological, fabricated pipe processing, and performance studies suggest that these resins should be especially well suited for high performance pipe applications. Pipe samples made from these resins were found to meet PE-100 requirements including low temperature impact properties. POLYM. ENG. SCI., 45: 1203-1213, 2005.

Effect of small amount of ultra high molecular weight component on the crystallization behaviors of bimodal high density polyethylene

“…The result shows that the high molecular component (UHMWPE) can effectively elevate the viscosity close to zero shear. Previous studies [4] have shown that Newtonian (zero shear) viscosity can successfully be used to model the effects of gravity-induced sag in commercial large-diameter pipe processing and an inverse relationship between zero shear viscosity and gravity-induced sag was found. Therefore, we concluded that with a small amount of UHMWPE, the zero shear viscosities of HDPE/UHMWPE composites could be elevated and the happening of gravity-induced sag during pipe manufacturing may effectively be avoided.…”

“…However, compared with unimodal resins, bimodal PE-100 resins lack the melt elasticity due to the absence of both long chain branching (LCB) and the very high molecular weight tail [4]. The result is that PE-100 resins are limited in producing large diameter, thick wall pipes because the molten polymers tend to sag before hardening.…”

“…The molecular feature that allows these bimodal resins to exhibit improved properties is not the shape of the molecular weight distribution (MWD) but the preferred incorporation of comonomers in the long polymer chains within the second reactor during a cascade polymerization process. This way a polymer blend is formed with low-molecular weight ethylene homopolymer components and high-molecular weight ethylene-1-alkene copolymer components [2][3][4]. Bohm [3] has recently concluded that in a bimodal PE-100 resin, the high-molecular weight copolymers form amorphous regions and act as tie molecules that connect the crystal lamella mainly formed by the low-molecular weight homopolyethylene.…”

Polyethylene loss of ductility during oxidation: Effect of initial molar mass distribution

“…In order to reach higher mechanical properties for specific applications, PE with high molecular weight had been commercialized [7,8]. The main drawback of high molecular weight PE's is their high melt viscosity which limits their processability.…”

Automatic Synthesis of Multimodal Polymers

“…[1][2][3] There is often an advantage to having two or more distinct polymer subpopulations. The production of multimodal polymers is frequently achieved by blending polymers made in separate reactions, [4][5][6] often in separate reactors. [7][8][9] When working with polymers for in vivo applications such as those used for drug delivery, the use of An automatic molecular weight controller is used in semi-batch reactions to produce final polymers that contain distinct subpopulations with different molecular weight distributions (MWD).…”