Measuring Film Thickness and Coatings

Composition Measurements vs Thickness Measurements

NIR sensors are most widely used to measure the chemical composition of materials based on specific light wavelength absorption. These measurements are mostly surface measurements, and accuracy depends on the material surface being representative of the total material. Sensor incident angle, distance to product, and beam area are not usually significant to this measurement.

Thickness measurement is very different from composition analysis and depends on the total transmission of the light through the entire product thickness. Reflection sensors such as the Sensortech NIR-7000 rely on the backscatter of near-infrared light from molecules within the product or require a reflective background to return the light effectively in a double pass through the product. The paper substrate often provides a good reflective backing when measuring a coating. Either method is generally effective and requires the use of a wavelength that will be absorbed by the product, compared to a less absorbed wavelength. Polymer film products are hydrocarbon-based and require a C-H absorbing wavelength. Different wavelengths are used depending on the film or coating thickness.

While physical variations such as incident angle and product distance have little impact on surface constituent measurements, they seriously impact thickness measurements.

Figure 1 shows a light path normal to the product (90°) and at an incident angle Φ. The path length through the product is: t / cosф (Path length will vary as incident angle).

Sensor Placement

Most films or coatings tend to be glossy and highly reflective. This first surface reflection is useless in measurement, but the highly reflected energy will saturate the detector. For this reason, the instrument is almost always mounted at an angle off perpendicular so the first surface reflection is not returned to the instrument.

If the instrument is not firmly mounted to a rigid structure, movement may cause instability due to a varying incident angle. When measuring a web product between rollers, web flutter will cause a change in incident angle and subsequent measurement fluctuation. Figure 2 illustrates this phenomenon with exaggerated flutter.

Flutter is minimized by positioning the sensor directly over a roller, as in Figure 3. Another benefit is that the roller acts as a good reflector. Natural process variation will give rise to continual thickness variation about a setpoint. These variations may be averaged out with filtering (damping). Instrument response is inherently linear.

Summary

NIR reflectance gauges are a valid alternative to nuclear gauging in film thickness and coating applications, provided care is taken in sensor mounting. Optimal accuracy is achieved with the following:

1. The sensor must be mounted on a rigid stand to minimize movement.
2. The sensor should be at an incident angle of 15° - 20°, eliminating the first surface reflectance. Once calibrated, this angle must remain constant.
3. The sensor beam should be directed onto a roller to minimize web flutter.
4. Sufficient filtering (damping) should be applied to average out short-term fluctuations.