Polymer Blog Series: Why perform rheology testing for polymers?

Optimising polymer product performance through in-depth understanding of the rheological properties

17 July 2018

Polymer rheology testing is the study of how the stress in a material or force applied is related to deformation and flow of the material. The physical properties of polymer formulations can be described using a variety of rheological terms and as these properties can directly impact the performance of the product, it is important to gain a full comprehensive understanding through laboratory testing.

Rheological data can help to optimise products and process conditions, thereby saving costs and minimising potential waste. It can provide useful information right across the polymer product lifecycle from raw materials suppliers through to end-of-life recycling and so is relevant to a range of stages of a stakeholders' supply chain including:

Raw materials:

It can be useful for materials quality testing and selection, and new materials development.

Plastic resin manufacturing and compounding / processing:

Rheology testing can give insight required for process development, optimisation, problem solving and performance testing.

Converting into finished products:

Rheology testing helps to monitor quality control and optimizes the process for new materials or new applications.

Market launch and ongoing use:

Supporting post marketing launch rheology is useful for continuing performance monitoring, quality control and problem solving. It can also provide invaluable information in litigation or polymer failure issues.

End-of-life recycling:

Rheologyaids development of the recycling process to help ensure that the polymer properties are retained during the reprocessing cycle.

Rheology tests are typically performed while the polymer is in the melt phase or while the polymer has been dissolved in a solvent for intrinsic viscosity and relative viscosity. Typical instrumentation includes capillary, rotational and controlled stress rheometers which can be used to measure various parameters of interest thus providing data which can help to solve many polymer-related problems, examples of which are given below:

Poor Surface Finish (films/mouldings)

The possible rheological cause could be high elasticity or excessive shear and/or tension thinning behaviour; alternatively, an inappropriate MWD. To confirm the cause the following rheological measurements can be made: Capillary rheometry, Die Swell analysis, Rotational rheometry, Stress relaxation tests.

Tape/Fibre Breakages or Bubble bursts

Indication: A potential rheological cause would be an inappropriate extensional viscosity behaviour.

Analytical approach: Capillary rheometry experiments at 2 or more temperatures can be conducted to confirm the cause.

Low Strength/Fibre Coalescence

Indication: Viscosity too low.

Analytical approach: Capillary or Rotational rheometry can be carried out in to confirm the cause.

Edge Curl/Excessive shrinkage

Indication: Melt memory. Excessive elasticity. MWD too broad.

Analytical approach: Rotational rheometry. Die Swell. Stress relaxation.

Film Splits/fibrillation

Indication: Viscosity too low. Elasticity too high.

Analytical approach: Rotational rheometry. Die Swell. Stress relaxation

Delamination on coextrusion. 

Indication: Mismatch in flow properties of components.

Analytical approach: Monitor temperature dependence of rheological properties

Uneven thickness (films/fibres)

Indication: Excessive tension thinning behaviour.

Analytical approach: Capillary rheometry experiments at 2 or more temperatures can be conducted in order to confirm the cause.

Die Drool/deposits      

Indication: MWD too narrow. Viscosity too low.

Analytical approach: Melt stability analysis can help to determine the cause of this problem

To resolve processability problems it is critical to be able to 'home in' on the likely causes and therefore ensure that the correct approach is selected to make meaningful measurements – saving the customer both time and money. Extensive expertise and understanding of the chemistry of common synthetic polymers enables our experts to drive Insight into the meaning behind the data which is invaluable to help solve the many varied polymer-processing related problems. 

Learn more by accessing our On-Demand Webinar.

This On-Demand Webinar summarises the various rheological tools available to characterise materials ranging from high performance plastic composites to dermal fillers. Examples of specific case studies will be presented which demonstrate how rheology has contributed to an improved understanding of the effect of polymer structure (either by accident or design) and inherent properties on processing behaviours.

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Dave is Senior Research Scientist in the Rheology Laboratory at Intertek Wilton.Dave studied applied science to first degree level at Sheffield Hallam University before being employed by the University of Nottingham and studying the effect of polymer coatings on metals as part of a M Phil degree. He joined ICI in 1988 as a polymer melt rheologist and is now a technique leader. Dave's main interests are the processing behaviours of polymeric materials from melts to pastes and adhesives. He is also an active member of the IUPAC working party on the structure and properties of commercial polymers. He has published in the scientific literature as well as presenting papers at International Rheology conferences.