Polymer Rheology: Quick Overview Including what, why and how for polymer developments

What is it?

Rheology is very sensitive to small changes of the material’s polymer structure. Polymers constitute the basic materials of the rubber and plastic industries and are of vital importance to the textile, petroleum, automobile, paper, and pharmaceutical industries. Their viscoelastic properties determine the mechanical performance of the final products of these industries, and also the success of processing methods at intermediate stages of production. In order to give the plastic object its desired shape, polymers are typically melted, processed in the liquid state, and then solidified. One of the crucial aspects in the engineering of polymer processing operations is therefore the study of the flow behavior of the polymeric liquids. In viscoelastic materials the presence of liquid-like behaviour depends on the properties of and so varies with rate of applied load, i.e., how quickly a force is applied.

Why?

Rheology is capable of evaluating even small changes in polymer structures and hence it is suitable for polymer characterization. The rheology-structure relationship is critical for the development of new materials. Polymer rheology in the liquid state aims at understanding the complex flow behavior of these materials so as to model and optimize processing operations. The complexity of the materials also couples with that of the processing flow conditions. Rheological studies reduce the flow complexity to a set of basic simple flows. The rheology of the material needs to be controlled within the specified range to ensure a smooth operation process. The structure– rheology relationship is critical for the polymer development during processing. The main objective of the design engineer is to tweak the material structure to ensure better processing performance without affecting the final product’s performance. Owing to its sensitivity to changes in material’s structure, rheology can also be used to control the process.

How?

The experimental characterisation of a material's rheological behaviour is known as rheometry, although the term rheology is frequently used synonymously with rheometry, particularly by experimentalists. Measurements of the rheological properties offer a fast and reliable way to determine molecular weight distribution and long-chain branching, which, in combination with the processing conditions, have a decisive influence on the end-use product properties. Theoretical aspects of rheology are the relation of the flow/deformation behaviour of material and its internal structure, and the flow/deformation behaviour of materials that cannot be described by classical fluid mechanics or elasticity. The set of rheological measurements in simple, well-defined flow conditions constitutes the rheometry, and includes both shear and extensional flows, the latter being encountered in many shape-forming operations like fiber spinning, blow molding, etc. The viscoelastic behavior of the material is typically characterized by the storage (elastic) modulus, G′(ω), and the loss (viscous) modulus, G″(ω), which respectively characterize the solid-like and fluid-like contributions. The ratio between loss and storage modulus, named the “phase angle” or “tang δ”, is also a useful parameter for viscoelastic characterization.

NOTE: Rheological data can help to optimise products and process conditions, thereby saving costs and minimising potential waste.

Some common and useful process and parameters for Polymer rheology

Melt Flow Index/Rate: Useful for checking whether the polymer grade is as specified, especially if processing problems are occurring.

Rotational rheometry: It Is for information on molecular weight distribution that can be used for processing, thermal stability or degradation study. Also useful in assessment of changes in viscosity as a function of temperature or shear rate.

Capillary rheometry: Measures viscosity at a range of temperatures to help optimise processing conditions. It can differentiate between polymers with similar Melt Flow Index but different molecular weight distributions.

Microfluidic rheometry: Microfluidic rheometry is a relatively new rheological technique closely aligned to capillary rheometry. It is especially relevant for characterizing low viscosity polymeric solutions at ultra-high shear rates, in excess of 1,000,000 s-1, and finds practical application in the analysis of drinks, inkjet inks, personal care products and other types of material, when rotational rheometry may be less suitable.

Microrheology: It can be used to investigate the viscoelastic profile of a material over a very wide frequency range and over much shorter timescales than can be accessed using rotational rheometry, and with smaller sample volumes.

Rheological tests on polyethylene: Rheological measurements provide information about the chemical properties of PE. For example, the molar mass of PE can be determined by measuring its zero shear viscosity with the help of frequency sweeps.

Rheological tests on polypropylene: To see how PP (or glass-fiber-reinforced PP) reacts to mechanical stress at various temperatures, DMA is used. The main aims of such a test are to see at what point the polymer starts to soften (glass transition temperature, Tg), and up to which temperature the polymer can still resist a certain mechanical load.

Rheological tests on polystyrene: To investigate the short-term and long-term behavior of a polystyrene melt, a frequency sweep can be carried out using an oscillatory rheometer. Frequency sweeps are oscillatory tests performed at a constant amplitude and variable frequencies.

Rheological tests on polyurethane: PUs are manufactured by mixing the two liquid components, the isocyanate and the polyol, and dispensing the mixture into a mold.The sample’s viscosity and other properties can be analyzed over the entire curing process by using a rheometer with a parallel-plate measuring system.

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