Measuring the Opacity of Plastic Tubing with Vis-NIR Transmission Spectroscopy

Summary

Plastic tubing is available in a variety of opacities ranging from clear to translucent. The transparency of plastic tubing varies for reasons such as providing contrast for visual monitoring of fluid flow, decreasing exposure to ambient light, and making the tubing more distinct for machine vision technology. Visible-NIR transmission spectroscopy is used to assess the amount of frosting applied to plastic tubing to determine if the tubing meets the required opacity level.

Background

Plastic tubing is used everywhere – from the beverage dispenser at your favorite restaurant to the gas and liquid delivery lines in life-saving medical devices.      In many applications that employ plastic tubing, visual contact with the flowing material is required to confirm flow and check for bubbles. Visual monitoring is facilitated by enhancing the contrast between the fluid and plastic tubing. Coatings, frosting and other surface modifications are used to vary the interaction of light with the tubing, making it easier to observe fluid flow. Modular spectroscopy components can be used to assemble a range of setups to measure the interaction of light with plastic tubing. In the case of frosted tubing, where light transmission must be kept within a narrow range to provide the desired tubing characteristics, Vis-NIR transmission measurements provide a straightforward method to assess frosting level.

Measurement Conditions

Samples of five plastic tubes with varying levels of frosting were used for the analysis (Figure 1). Details on the frosting level for each sample are provided in Table 1. Transmission measurements were made using an enhanced sensitivity Vis-NIR spectrometer. The tubes were placed between two collimating lenses in a transmission setup and adjusted until the most reproducible orientation was found.

Table 1: Frost Level Analysis of Plastic Tube Samples.

Figure 1: Although differences in the frosting levels of plastic tubing samples are difficult to distinguish visually, spectroscopic methods are highly reliable in characterizing tubing properties.

Figure 2: Transmission characteristics of plastic tubing samples varied by the amount of frosting applied to the surface of each sample.

Results

The transmission spectra measured for the frosted plastic tubing samples are shown in Figure 2. The transmission intensity measured for these samples correlates with the frost levels reported for the plastic tubing in Table 1. Note that even though the transmission intensity for Samples 1, 2 and 3 is very similar, the transmission spectra are sufficiently different to reject Sample 3 as having insufficient coating. Visual observation of the plastic tubing samples showed that the frosting level for each sample was difficult to distinguish (Figure 1). This underscores the value of spectroscopic transmission measurements to discriminate plastic tubing with very similar frosting levels.

Conclusions

The power of Vis-NIR transmission measurements to discriminate tubing samples is demonstrated by the different transmission intensities measured for plastic tubing samples with similar frost levels. The ability to discriminate samples separated by less than 1 % transmission make this technique a good option for use in QA or QC methods to ensure the plastic tubing has the desired light interaction properties and characteristics.

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