Demonstration of the Power Law Model Through Extrusion

Extrusion is an industrial process that transforms polymers and other materials into defined shapes, such as tubing and pipes for applications as diverse as car parts and toys. It is studied at the small scale prior to the design of industrial machines. Common materials for extrusion are polyolefins, polyethylene, and copolymers. During extrusion, the thermal plastic material, known as solid feed, is transported, mixed, and melted. The substance is passed through a mold known as the die, after which it cools and resumes to the non-pliable properties. Simple lab extruders can be used to investigate various parameters affecting the polymer output using a power law model. Furthermore, relationships between operating conditions and deviations from theoretical behavior, as well as extrudate shape, can be established. This video will illustrate how an extruder works, how to operate it, and how to use the power law model to evaluate the process.

The extruder consists of a hopper, which feeds in the polymer granules, a barrel, composed of a cylindrical chamber with resistive heating elements to control the different temperature zones, and a helical screw that rotates around the center line. The channels of the screw are widest at the feeder to promote mixing and melting. However, the channels become increasingly narrow and shallow along the length of the screw. The screw is designed to ensure steady transport from the feeder, while accounting for the reduction in volume and build-up and pressure as the feed melts. The behavior of a molten polymer depends on the temperature, pressure, and the viscosity, which is the ratio of shear stress to shear rate. For most polymers, viscosity decreases with both temperature and shear rate, making them non-Newtonian fluids. Specifically, polymer melts are usually viscoelastic and their flow is described by a power law model. The power law contains two empirical constants. M is the modulus of viscosity and strongly temperature-dependent. And n may also vary with temperature. The power law constants can be calculated from the volumetric flow rate, pressure, and geometry. The flow rate is established by weighing the die output over two time intervals. Now that you know how an extruder works, let's apply the power law model in a real experiment.

The thermoplastic material used in this experiment is a high density polyethylene copolymer, which contains links of both ethylene and a long chain olefin. To start, turn the exhaust to on. Take the polymer pellets and fill the hopper of the extruder. Ensure that the motor switch is off and then turn the main switch to on. The temperature settings should be adjusted to the material in use. Set the temperature of zone one to around five to 20 degrees Celsius above the melting point of the polymer, which is approximately 200 degrees Celsius. Set the temperature of zone three, which is the temperature of the cylindrical die, between 220 and 250 degrees Celsius. Finally, set the temperature of zone two to be between zones one and three. Check the temperature of all heated zones to see if they reached the desired set-point. Once set-points are reached, wait for a minimum of one hour, a phase called heat-soak. Heat-soak ensures melting of any residual solid polymer, which otherwise can exert excessively high pressure on the die, resulting in unsteady flows.

Turn the motor to on. Set the desired speed using the switch starting with low RPM. And gradually increase the speed as the polymer is seen exiting the die until the lowest desired speed is reached. Do not exceed 3,000 psi die pressure. Run the extruder for 10 minutes after the desired speed has been reached. Periodically check the hopper to ensure it has enough resin pellets. Pre-weigh the pans to be used for sample collection. Put on safety gloves. Using scissors, carefully cut the very hot extrudate into a pre-weighted pan and weigh the mass of polymer that was extruded between measured time intervals to calculate the flow rate. Measure the diameter of the extrudate ribbon with a micrometer. Using the speed controller, adjust the set-point to a new setting and wait for 10 minutes. Collect samples and data as performed previously. To obtain the data set at different temperatures, lower the speed and use the temperature controllers to adjust the set-point of the zones. Wait for 15 minutes before collecting the samples.

Turn off both the extruder motor switch and the main switch. Using the mass rate and the melt density of the polymer, calculate the volumetric flow rate, Q. Use the power law to determine the modulus of viscosity, m, and the power law index, n, that best characterize the material at a given die temperature. The linchpin between these two equations is the momentum balance, which relates shear stress to the pressure drop across the barrel. Combine these three equations into a differential equation that can be solved to yield volumetric flow rate. Linearize this equation and use both linear and nonlinear regression to find m and n and compare the results. Now, let's analyze the data and examine how well it is fitted by the power law model and whether it is consistent with the model at all.

The linear regression to the power law model is seen in this graph, which depicts the relationship between the pressure, P, and the flow rate, Q. The coefficient of determination shows a good fit. The power law index, n, and modulus of viscosity, m, indicate that this is a pseudoplastic, that is, as shear rate increase, viscosity decreases. It is over 10 million times more viscous than water at room temperature, and 10,000 times more viscous than glycerin. The flow rate appeared to have some slight affect on the die swell ratio, but not on polymer slippage. In summary, it shows that the power law model, in conjunction with the momentum equation, suitably describe the flow of this non-Newtonian fluid, indicating the flow and viscosity changes in response to screw speed and temperature.

A variety of extrusion techniques exist that are used in both industrial skill processes and benchtop research to create various types of products, ranging from pipes and plastics to biomaterials. Extruders convert polymers into simple shapes. They can also mix non-polymeric additives to the polymer blend. Additives are added in order to modify the mechanical properties of the final product, often imparting more toughness. Examples include plasticizers, antioxidants, and flame retardants. Inorganic additives, such as talc or carbon, are of limited use because they do not melt. Extrusion is also the basis for 3D printing, a process in which a thermoplastic ink exits from a nozzle and is deposited on a surface in many layers to create a three-dimensional material. This versatile technique is being explored in bioengineering applications to bio-print tissue-specific cell constructs. Another key use for extruders is to feed products to an injection mold, which forces the material into a mold cavity using pressure. It is similar to die-casting. This process creates more specialized products and is therefore limited in its range of application. Besides piping, tubing, and packaging materials, extrusion is also commonly used for food processing. Products, such as bread, pasta, confectioneries, cereals, or pet foods, are extruded in mass quantities. Products high in starch content are commonly processed in food extrusion because of their moisture and viscosity profiles.

You've just watched JoVE's introduction to polymer extrusion. You should now understand the process of extrusion, how the flow, speed, and temperature can affect the process, and how to apply the power law model to evaluate it. Thanks for watching.