Michael E. Mackay

A novel processing aid for polymer extrusion: Rheology and processing of polyethylene and hyperbranched polymer blends

The use of hyperbranched polymers (HBPs) as a processing aid for linear low density polyethylene (LLDPE) was investigated. Various generation (or pseudo-generation) HBPs were used which had either 16 carbon atom alkanes or a mixture of 20/22 carbon atom alkanes on the end groups. In addition, the degree of end group substitution was studied. Blends of up to 10% HBP content were mixed via extrusion at 170 °C to produce 1 mm diameter fibers. Processability, surface appearance and the rheological properties of the blends were evaluated. It was found the power requirement for extrusion was significantly decreased as a result of reduced blend viscosity, and also, the mass flow rate for a given extruder speed was greater than virgin LLDPE for all HBP blends. Melt fracture and sharkskin of the blends was successfully eliminated, and minimal preprocessing time was required for the effect to take place. Surface analysis using x-ray photoelectron spectroscopy and transmission electron microscope techniques were per...

Rheological properties of poly(lactides). Effect of molecular weight and temperature on the viscoelasticity of poly(l‐lactic acid)

The dynamic viscoelastic behavior of Poly(l-lactic acid) (PLLA), with molecular weights ranging from 2,000 to 360,000, have been studied over a broad range of reduced frequencies (approximately 1 × 10-3 s-1 to 1 × 103 s-1), using time-temperature superposition principle. Melts are shown to have a critical molecular weight, Mc, of approximately 16,000 g/mol, and an entanglement density of 0.16 mmol/cm3 (at 25°C). PLLA polymers are noted to require substantially larger molecular weights in order to display similar melt viscoelastic behavior, at a given temperature, as that for conventional non-biodegradable polymers such as polystyrene. The reason for this deviation is suspected to be due to steric hindrance, resulting from excessive coil expansion or other tertiary chain interactions. PLLA melts show a dependence of 0 on chain length to the 4.0 power (M[stack 4.0W ]), whilst J[stack 0e ] is independent of MW in the terminal region. Low molecular weight PLLA ( 40,000) shows Newtonian-like behavior at shear rates typical of those achieved during film extrusion.

Film blowing of linear low-density polyethylene blended with a novel hyperbranched polymer processing aid

The use of hyperbranched polymer (HBP) as a processing aid for linear low-density polyethylene (LLDPE) in the tubular film blowing process was investigated. Through the addition of HBP, sharkskin was successfully eliminated without significantly changing the overall physical properties of LLDPE films. Also, there was a minimum of 40% enhancement in processing rate with addition of 0.5 wt% HBP. The study showed that HBP and LLDPE are immiscible, and HBP has a tendency to migrate to the surface, subsequently, it seems to form a lubricating layer between the metal surfaces and the bulk material. This phase separation between HBP and LLDPE results in an HBP-rich surface, which has a high potential to create unique surface properties tailored to various applications. Rheological analysis indicated that excessive slip was present in HBP/LLDPE suggesting that the onset of slip is not the cause of sharkskin. On the contrary, it may be partially responsible for the elimination of sharkskin.

An explanation of the rheological properties of Boger fluids

Fluide obtenu en dissolvant du polyisobutene dans du kerosene puis addition de polybutene-1 (Hyvis 3)

Wall slip and shear stresses of polymer melts at high shear rates without pressure and viscous heating effects

We present experimental data demonstrating wall slip of high temperature polymer melts at high shear rates (up to ~70 s) without pressure and viscous heating effects. Results for polydisperse (linear low density polyethylene) and monodisperse (polystyrene) melts are presented. For the monodisperse case two distinct regimes of slip are seen: (1) a low shear rate regime in which the slip velocity increases slowly with shear rate (weak slip) and (2) a high shear rate regime in which the increase is dramatic (strong slip). Through scaling of our shear stress/shear rate data we demonstrate that the crossover between the two regimes occurs when the bulk polymer chains are effectively disentangled. These findings are in qualitative agreement with molecular models of slip which invoke an entanglement-disentanglement scenario for the transition from weak to strong slip. The polydisperse system shows less critical slip behaviour and the slip velocity increases; yet, the increase is less dramatic than the monodisperse system

Analysis of entry flow to determine elongation flow properties revisited

The minimisation technique proposed by Binding (J. Non-Newtonian Fluid Mech., 27 (1988) 173) was used in our Generalised Engineering Bernoulli Equation framework to relate the entry pressure and stress power. We arrived at a final result similar to Bindings using assumed kinematics. Through subsequent assumptions to the kinematics we finally arrive at a result exactly equivalent to Cogswells technique (Trans. Soc. Rheol., 16 (1972) 383). Thus, these two techniques are related in this general framework. The techniques were used to predict elongation flow properties of a polymer melt and polymer solution. The results for the polymer melt clearly show Cogswells technique is adequate at high elongation rates. All these techniques require minimisation of the stress power with respect to the flow volume and discussion is given as to the validity of this minimisation technique. In addition, the approximate variational technique we propose gives clears limits as to when a technique, such as Cogswells, can be applied.

The effect of molecular mass and temperature on the slip of polystyrene melts at low stress levels

The slip of monodisperse polystyrene melts next to a solid, inhomogeneous, metal substrate (stainless steel) is measured at small stress levels for a variety of temperatures. A critical stress, below which no slip occurs, is not seen and the polystyrene melts used here slip at all stress levels. The slip velocity is quantified by the slip length (b, equal to the slip velocity divided by the shear rate) and friction coefficient (k, equal to the slip velocity divided by the shear stress). The slip length shows complicated dependence with both the molecular mass and temperature, however, when converted to the friction coefficient a master curve with molecular mass results for temperatures above 170 °C. The data are compared to contemporary theories for slip. It is concluded that none of the present theories accurately represent the data and that the number of adsorbed molecules are in a dynamic equilibrium which affects the slip behavior. Analysis of the force on the adsorbed molecules during shear demonstrates this may cause adhesive failure which contributes to the slip.

Comparison of entry flow techniques for measuring elongation flow properties

Abstract The elongation viscosity of linear low density polyethylene and low density polyethylene were estimated using converging flow and stagnation flow devices. Results were obtained from the converging flow rheometer using the analyses proposed by Binding [1] and Cogswell [2] . These results were compared with an estimate of uniaxial elongation viscosity obtained with the stagnation flow rheometer similar to that proposed by Mackay et al. [3] . The results show that the elongation viscosities determined by both flow geometries are comparable. Also, the measured elongation viscosity is affected to a small degree by geometric properties for converging flow.

A model of plastics recycling: Does recycling reduce the amount of waste?

Abstract A model for recycling plastics is made to ‘phase in’ recycling over a given period. The recycled plastic can either replace virgin plastic or enter a new market where plastics are not normally used. The former is denoted as ‘true recycling’ and the latter, ‘new market recycling’. It is shown that ‘true recycling’ will eventually reduce the amount of waste by the same amount of plastic that is recycled. On the other hand, ‘new market recycling’ will not reduce the amount of plastic waste and after a given period the overall result is that there is no effect on the amount of plastic waste generated. This is due to the fact that plastics in general have a finite lifetime.

Comparison of shear stress and wall slip measurement techniques on a linear low density polyethylene

Abstract Small gap, torsional, parallel plate experiments allow the measurement of shear stress and slip velocity at shear rates comparable to those measured in slit and capillary rheometers. Using this technique we compare data from parallel plate measurements, where effects of pressure and temperature are negligible, to those from pressure driven flows where they are not. The shear stress for all techniques agrees within experimental error at low shear rates. At higher shear rates the slit and capillary measurements diverge from the parallel plate measurements highlighting the effect of pressure and temperature on viscosity. Also, breakdown of the Cox–Merz rule at slightly higher shear rates is apparent when comparing dynamic and steady shear data from the parallel plate experiments. Since the complex viscosity diverges in the same manner as the viscosity derived from pressure driven flow it seems as if agreement of the complex viscosity with the viscosity derived from pressure driven flow is fortuitous. With the capillary geometry, there is no sign of slip effects at low stress levels probably due to experimental limitations while the torsional rheometer shows slip at all shear stresses. There is no critical stress for the onset of slip but there is a critical stress where slip increases dramatically.