On-line monitoring of polymer compounding
Multi-lumen medical tubing made from filled polymers
Filler particles are often added to polymeric materials to improve their properties, e.g. to enhance the mechanical or barrier properties or to improves the aesthetics of products. An example is the use of Barium Sulphate particles added to medical devices to make the devices radio-opaque and clearly visible under x-ray. Unfortunately, the dispersion of Barium Sulphate (BaS) within the polymer matrix is problematic during compounding of the materials, with the BaS particles tending to clump together into micro-sized agglomerates. This may result in unacceptable mechanical and aesthetic defects in products such as multi-lumen medical tubing, where the wall thickness dimensions might be as low as 50mm or below. Clearly, particulate agglomerates at the same scale are unacceptable. We worked with a medical polymer compounder on developing an on-line monitoring solution which could detect agglomerates above a threshold level in real-time during compounding.
Current QA process
At present, when materials are compounded, a sample of the product is taken at regular intervals and compression moulded in a platen press to form a thin film. The film is then manually inspected using optical microscopy to check for agglomerates above the acceptable threshold limit for the client.
There are two major downsides to the current method:
1) The process of pressing the film and optical microscopy check takes a minimum of fifteen minutes to perform – during which time a considerable amount of defective material may have already been produced; and
2) The sampling process is random and does not ensure that the overall quality of the material produced is within specification.
On-line UV-Vis spectroscopy
Our team has considerable experience in the on-line monitoring and control of polymer processes, including monitoring of product viscosity, thermal homogeneity, polymer degradation and additive dispersion. In this case, we used a non-destructive optical technique called UV-Vis spectroscopy, which involves the analysis of the absorption spectrum of the material over the UV to Visible wavelength range. It can be implemented directly into a process, even a high temperature, high pressure, process like polymer compounding with the use of probes with a sapphire window. We had previously proved that UV-Vis spectroscopy was capable of differentiating different particles sizes in the processing of PLA with calcium phosphate fillers (for use in orthopaedic medical devices)
As it was not feasible to fit the probes directly into the process, a bleed-off die was designed to take a small side stream of melt for analysis. This did result in a loss of 0.3% of throughput, however, this was deemed acceptable by the manufacturer.
In order to test the monitoring capabilities of the system, the compounder was run at different torque levels, where a lower torque results in poor mixing and would be expected to result in a high level of agglomerates in the product. In the event, the mixing achieved was actually acceptable at all levels with no differences in agglomeration level detected during optical microscopy. However, later analysis by SEM did show that there were small differences in the average particle size – from 6mm to 8mm. Analysis of the UV-Vis spectrum using multivariable chemometric techniques was able to differentiate between even these modest differences in particle size.
Bleed-off die for application of in-process UV-Vis spectroscopy
We developed and tested a UV-Vis method of agglomeration detection for on-line use in an industrial polymer compounding facility. It was shown that the method could detect even small changes in filler particle agglomeration. The new method has several advantages over the traditional test method including instant feedback and continuous monitoring.
Industrial testing of the system
PCA showing that separation of spectra based on processing conditions is possible (Red points - 70 % torque, Black points - 25 % torque)
If you have any interest in this project or other aspects of monitoring/controlling polymer processes please contact:
Principal Investigator: Dr Marion McAfee, firstname.lastname@example.org
Team Members: Dr Konrad Mulrennan, Albert Weinert, Nimra Munir
Darren A. Whitaker, Fraser Buchanan, Mark Billham, Marion McAfee, A UV-Vis spectroscopic method for monitoring of additive particle properties during polymer compounding, Polymer Testing, Vol. 67, pp392-398, 2018. https://doi.org/10.1016/j.polymertesting.2018.03.030