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Optical Measurement System for Quality Control of Plastics using Luminescence

Dipl.-Ing. Gregor Gerstorfer

During the polymer extrusion of thermoplastics it can happen that parts of the material partially start to cure. This happens due to increased stress inside the screw extruder and/or locally raised temperatures. This partially curing is unintended because the material is meant to cure only after a number of subsequent processes under certain conditions [1]. The aim is to build a measurement device to estimate the degree of curing not only online but also inline.

The considered polymer is an unsaturated polyester resin (UP) with hydroxylalcylamid as curing agent. As long as the temperature of the polymer is low enough, the crosslinking won't start and the polymer remains as a thermoplastic. When the temperature rises the material starts to cure and the material becomes a thermosetting polymer. The mentioned crosslinking is depicted in Figure 1: the polyester resin has two carboxylic acids at its edges which crosslink with the hydroxylic ends of the curing agent while condensating 2 H2O per reacting carboxylic acid.

Figure 1: Crosslinking of unsattured polyester resin containing carboxylic acid with hydroxylalcylamid as curing agent (see [1], p 12).

This chemical reaction of curing is intended to happen under certain conditions, e.g. in a furnace. But when parts of the material start to cure during the extrusion, the properties of the polymer melt for the subsequent processing steps deteriorate. Hence this quality feature has to be detected and measured.

In this work the luminescence spectrum of a specimen illuminated and excited by a UV source in the UV-spectral part is acquired. It is expected that beside fluorescence also phosphorescence will be observed, both effects are compendiously called lumincescence. After the luminescence spectrum of the illuminated specimen is acquired, lines which are characteristic for the crosslinking process are searched for. In Figure 2 the schematics of the setup for the preliminary investigations is depicted.

Figure 2: Schematic of the setup.

The UV source radiates a broad spectrum of electromagnetic waves, but only the ultraviolet part in the range of λ = 340…380 nm should impinge on the specimen. This is accomplished by a filter that blocks nearly all radiation in the visible and IR range (Baader U-Filter). In Figure 3 the filtered spectrum of the light emitted by the UV source is mapped. To acquire the spectrum a UV/VIS Microspectrometer from Boehringer Ingelheim Microparts in combination with a collimator lens coupled via a fiber is used.

Figure 3: Spectrum of UV source viewed through UV transmitting filter.

For measurements the specimen is placed in front of the collimator lens. For the preliminary investigational measurements, two different samples are used. The first is a solid, cooled down part of polymer melt which is not cured at all. It mainly consists of unsaturated polyester resin, some parts of curing agent and negligible parts of further components. The second sample is from the same polymer melt but cured in a furnace at 180°C for 10 minutes, again cooled down and therefore solid. It is assumed that the second sample is - in contrast to the first sample - completly cured. Both samples show approximately the same thickness of d=1.35mm (not cured) and d=1.45mm (cured).

In Figure 4 the acquired spectra of those samples are plotted in the visible range (λ = 400…700nm). For indication the spectrum of the incident light is plotted, too. One can see that at approximately λ = 490nm the intensities of the sample spectra are higher than the impinging intensity at that wavelength. This is the sought after effect of luminescence. The increase in intensity caused by the crosslinking amounts to 22.9%.

Figure 4: Acquired spectra with cured (red, dashed line) and uncured (green line) samples in comparison to lamp spectrum (blue, dash-dotted line).

The preliminary measurements of luminescence spectra from cured and uncured polymer melt illuminated and excited by a UV source show a sensitivity in the intensity for the degree of crosslinking. Because of the spectrometer exhibits a rather low spectral resolution it is hard to determine wavelengths which are representative for the crosslinking. Therefore in future works a spectrometer with higher spectral resolution will be used. Furthermore the long term aim is to build a setup for conducting an inline measurement.

References:[1] J. Pietschmann, Ïndustrielle Pulverbeschichtung", 3. Auflage, Vieweg+Teubner, 2010.