Process Analysis of Polymer Formulations

Up to the early 1990s, in most plastics production plants, a small sample of plastic was taken from the compounding area and hand-carried to a QC laboratory for evaluation. The polymeric material in a particulate form was then usually hot pressed into a melt film or dissolved in a solvent for mid-IR spectrophotometry. Off-line mid-IR evaluation of the plastic s composition and concentration of constituents would take from one to two hours. Process monitoring using IR requires short measurement times. Analysis of IR output data is relatively rapid, lending itself to on-line processes. 

In this context, the use of a dispersive IR-spectro-photometer is only recommended when one wavelength is measured. Advantages of dispersive sys- 

Alternatively, it is possible to install fibre optic probes directly in the main stream in-line while the IR spectrophotometer remains remotely in a low vibration laboratory environment. In-line analysers, which do not remove any sample from the line, have the minimum possible lag time and do not change the sample physically or chemically from its nature in the process. Recently, bundles of 500 /xm optic fibres have been developed for the 5000-900 cm (2000-11,000 nm region), which permit transmission of IR energy over distances of several metres. Lowry et al. [76] have evaluated fibre-optic cables that might prove useful in FTIR remote sampling applications. The various optical fibres (chalcogenide, silver halide, heavy metal fluoride or sapphire) differ in their spectral window [77]. Due to the thermal stability and the spectral window, sapphire fibres are considered suitable for in-line characterisation of polymer melts in a production line (e.g. in an extruder head) as an alternative to discontinu-ously operating conventional off-line transmission IR spectroscopy of polymer films [78]. 

Quantitative information may be obtained by calculating the ratio of the peak height of CH2 and CH3 bands, thus compensating for different sample thickness. Quantitative determinations are always possible via FTIR but do require the building of a robust calibration matrix. In fact, via on-line EUR, additives can be quantitated effectively from low ppm ranges as found in the case of most resin producers, to relatively high percentage levels as prepared by compounders and masterbatch suppliers. 

The required short transmission path length due to strong IR absorption bands causes problems with 

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