Elizabeth Takacs

The effect of multiple extrusion passes during recycling of high density polyethylene

High density polyethylene blow modling resins have been identified as a primary material for solid waste minimization and recycling. An experimental study into the effect of multiple extrusion cycles on the properties of a virgin homopolymer, virgin copolymer, natural post consumer, and mixed color post consumer blow molding resin was conducted. Rheological properties such as shear and elongational viscosity and elastic modulus were studied in the context of changes experienced during recycling. The G9–G0 (elastic storage and loss modulus) crossover point was used to measure relative changes in the polydispersity index and molecular weight distribution (MWD). It is also shown that extrudate swell and sag change after multiple extrusion passes. Environmental stress crack resistance was also measured. A rationale for the significant decrease in the environmental stress crack resistance of the virgin copolymer resin is presented. The results are analyzed in terms of known degradation mechanisms such as chain scission and crosslinking, and their relationship to the molecular structure.  1997 John Wiley & Sons, Inc. Adv in Polym Techn 16: 11–24, 1997 tend to be application-specific measures. RheologiBackground Information and cal characterization of high density polyethylene Literature Review (HDPE) involves measuring the viscosity and viscoelasticity of the molten polymer. Unlike simple fluP ids, these properties for HDPE have a strong depenolyethylene, like all polyolefins, is described dence on temperature, shear rate, molecular weight, by its density, strength, molecular weight and MWD. There are two primary flow mecha(MW), crystallinity, and melt flow or rheological nisms, shear and elongational flow. Shear flow decharacteristics. Other properties such as environscribes the response of the polymer to an imposed mental stress crack resistance (ESCR) and optics shearing force. Elongational flow is the response of the polymer to an imposed stretching or pulling force. Although not independent of each other, they Correspondence to: A. T. P. Zahavich  1997 by John Wiley & Sons, Inc. CCC 0730-6679/97/010011-14 MULTIPLE EXTRUSION PASSES both have their own viscosity measures. At very he 5 9(n 1 1)(DPe) 32hċ2 (4) low shear rates (ċ , 1022 s21), a molten polymer behaves roughly as a Newtonian fluid. Beyond this point the shear stress of molten HDPE has a nonlinand: ear relationship with shear rate. As the shear rate increases the rate of increase in shear stress de« 5 4hċ2 3(n 1 1)DPe (5) creases and this is described as shear thinning. Within the range of typical extrusion shear rates (5 , ċ , 1000 s21) this relationship can be described Similar to the importance of elongational viscosby a power law expression: ity, viscoelasticity manifests itself in the behavior of the molten polymer in the parison. Viscoelasticity h 5 mċn21 (1) determines the elastic recovery of a polymer after a load is applied to cause viscous creep. This pheThe coefficient, m, is a measure of the viscous nature nomenon is time and temperature dependent. As a of the polymer. The power law index, n, is a measure load is applied the molecules tend to disentangle of the effect of shear thinning; as n approaches 1, and align themselves in the direction of the load. the fluid tends to be Newtonian. When the load is removed the molecules relax. The shape of the MWD and the average molecular The time-dependent stress relaxation modulus, weight are directly related to the melt flow properG(t), is defined as the ratio of an applied shear stress, ties and processing performance, in terms of extrut, to the shear strain, c: sion output rate and pressure.1 As the length of the molecule increases the resistance to flow increases. G(t) 5 t c (6) As the breadth of the distribution increases there is a greater tendency to shear thinning.2 In a blow molding process HDPE is extruded into When a sinusoidal rotational shear strain is applied, a molten tube called a parison. After the bottle molds as done in dynamic measurements, G is a function are clamped around the parison air is blown into of the rotational speed, g, and as a complex function the parison which expands it to conform to the mold. can be decomposed: Minimal stretching of the parison prior to clamping and the ability of the parison to expand during blow G(g) 5 G9(g) 1 iG0(g) (7) molding is critical. The elongational flow characteristics, described by the elongational viscosity, he , which results in the following complex function for is a measure of how well the parison will resist the shear stress: stretching and conform to the mold during blowing. By definition: t 5 c [G9sin(gt) 1 G0cos(gt)] (8) The elastic or storage modulus, G9(g), is in phase he 5 Fe/A « (2) with the applied strain and measures the mechanical energy stored during each rotation. The loss moduwhere Fe is the elongational force applied over the lus, G0(g), is out of phase with the applied strain cross-sectional area, A, normal to the flow, and « is and measures the amount of mechanical energy disthe rate of elongation or stretch rate.3 The measuresipated per cycle. ment of he is difficult, but it has been shown by The Relationship between viscoelastic properties Laun and Munstedt4 that, at very low shear rates, and MW characterization has been reported widely.6 where the shear flow is near Newtonian, the Trouton However, it has usually been in the direction of MW relationship holds for LDPE where: predicting visco-elasticity. Within the last 15 years, Tuminello,7 Tuminello and Cudre-Mauroux,8 Yu (3) he 5 3h(ĉ) and Ma,9 and others have successfully used dynamic measurements to describe MW and MWD properties. More specifically, Shang10 used the terminal At higher shear rates, Cogswell5 proposed that he is related to the shear rate, shear viscosity, and enzone crossover point, Gc(g), of G9(g) and G0(g) to predict the polydispersity, the ratio of the weight trance pressure loss, DPe:

Rotational Foam Molding of Metallocene Catalyzed Polyethylene: CBA Screening and Process Characteristics

The objective of this work is to investigate the effects of different chemical blowing agents (CBAs) and processing conditions on the cellular structure of foamed metallocene polyethylene and characterize an appropriate blowing agent. An experimental study was conducted to produce metallocene polyethylene foams in dry-blending-based rotational foam molding. The critical processing parameters that optimize the foam structure have been identified through modifications of the molding conditions. The physical properties and cellular structure of the final foamed parts were also examined. The foaming performance of exothermic and endothermic CBAs was studied. It was revealed that selecting a suitable CBA is crucial as the foam structure depends significantly on the properties of the blowing agent. Exothermic blowing agents resulted in greater foam density reduction and exhibited a wider processing window compared to endothermic blowing agents. It was found that a balance between different properties of the blowing agent is required to achieve control over the foam structure.