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Measuring layer thickness in coextrusions by interferometry.

Measurement of layer thicknesses in multilayered plastic films, sheets, and coatings is a difficult problem and has been a challenge to instrument designers for quite some time. Until recently, the only universal method for measuring individual layer thickness separately has been the microscopic method. This method is slow, requiring tedious sample preparation, and cannot be automated or used on-line. Infrared absorption and FTIR methods can be used on-line, but they lack the capability of individually measuring two layers of the same material in A-B-C-B-A constructions.

This article describes an interferometric gage, the Topwave LayerGauge, that does not have these limitations with transparent films and coatings. It has been commercially available for a couple of years. Although the principle is not applicable to on-line use, off-line measurement has been automated. The instrument rapidly and accurately analyzes a cut strip of film or coating-a full thickness profile in the cross-machine direction can be measured and printed in minutes. The instrument's programmability enables dozens of different material combinations to be measured without any hardware modifications. The examples given indicate the variety of applications in which LayerGauges have been successfully used.

Principles of Operation

A narrow beam of ordinary white light is directed into the material to be measured. As shown in Fig. 1, interfaces cause light reflections, provided that the layers to be measured are transparent or semitransparent to the white light and that there are slight differences between the refractive indices of the adjacent layers.

The detector senses light reflections and, by a special optical and electronic arrangement, converts these reflections into a sensor signal. This signal, as shown in Fig. 2, consists of peaks separated by electronic noise. Each peak of the detector signal represents the location of the corresponding layer interface; thus, the separation of the two adjacent peaks is proportional to the layer's optical thickness. The instrument calculates thickness by dividing optical thickness by the material's refractive index.

The optical arrangment of the LayerGauge resembles that of the traditional Michelson interferometer, as illustrated in Fig. 2. The beam splitter divides the light beam into two identical beams: reference beam and measuring beam. The measuring beam travels to the sample and is reflected from its upper surface and the layer interfaces. The reference beam is bent at the beam splitter to a movable mirror. On the way back, the two beams are combined in the detector.

In the balance situation, both beams travel the same distance and arrive simultaneously the detector. Both beams are in phase and reinforce each other, producing a peak in the sensor signal. As soon as the mirror is deflected from balance, the signal level drops because the light beams are now out of phase and interfere with each other. When the mirror is moving outward, peaks are obtained from the upper layer, from the internal layer interfaces, and from the bottom layer, respectively. The optical thickness of a layer is the difference of the mirror positions corresponding to the two adjacent peaks. Mirror positions must be accurate in order to calculate layer thickness. In actual operation, the mirror is moved back and forth continuously and the instrument's built-in computer automatically analyzes the sensor signal and calculates layer thickness using the refractive indices of each layer's materials. Physical Requirements There are some special features and requirements for instrumentation and materials:

* The technique itself is linear and does not require calibration for different materials.

* Refractive indices, required for this procedure, can be found in the literature.

* Individual layers of the same material can be measured individually provided that the layers are separated by a different material: for example, A-B-C-B-A.

* The layers to be measured must be more or less transparent to light. Suitable applications are transparent or semitransparent multilayer films, and transparent or semitransparent coating on nontransparent material, such as paper, metal, foil, or pigmented plastics.

* The sample must be stationary during measurement of one point for 1 to 10 seconds, depending on its thickness. This makes it difficult to apply the technique for on-line analysis.

* Off-line analysis, however, can be automated by moving the film sample between measurements and stopping it for each measurement.

Instrument Description

The Topwave LayerGauge consists of the following components:

* measuring unit with built-in microcomputer;

* film bench for automatic scanning through a cut strip of film or sheet;

* display terminal for the operator;

* floppy disk drives for storage of measuring software, measuring recipes, and results;

* printer for numerical and statistical reports; and

* digital plotter for graphic presentations.

Nearly the entire operation of LayerGauge is software controlled. The user builds measuring recipes for all different material combinations. Each recipe consists of all the information the computer needs to perform the analysis: location of measuring points, refractive indices, method of reporting, etc. Recipes are stored automatically on a floppy disk for recall on demand.

Results can be reported numerically, as shown in Fig. 3, or graphically, as shown in Fig. 4. Floppy-disk storage allows later analysis and reporting of results. Data can also be sent to an external computer for further statistical analysis.

Special care had to be taken in designing the sample holder in order to maintain perpendicularity between layer interfaces and light beam. Fig. 2). The film sample is wound around an unwinding roll. A rubber-coated pulling roll compresses the film sample against a polytetrafluoroethylene plate and moves the sample a preset distance after each measurement. A metal push-up ensures that the film is always the same distance from the measuring unit.


In practice, Topwave LayerGauges have been best used in combination with online measuring systems. Each production line has its on-line system for monitoring total-thickness variation and possibly the amount of the barrier material in the film. One LayerGauge serves several lines and is used to check the calibration of the on-line systems and to measure the actual layer thickness profiles at certain intervals, typically once per roll.

Refractive indices of materials that have been successfully used with LayerGauges are listed in the Table. These data are the only "calibrations" necessary when a new measuring recipe is being prepared.

LayerGauges have been used in the following applications:

Packaging Coextrusion

* filled layers

* barrier layers

* tie layers

* regrind layers

* heat seal layers

* layers with slip, antiblock, and antistat

* additives


* film only

* film and foil

* film and paper

Coated products

* coatings on paper

* coextruded coatings on paper, film,

* and foil

User Examples

In the following examples, the old testing methods usually included 1 to 4 measured points per sample while in LayerGauge testing, 25 to 500 points per sample are measured.

Case A

Product: Paper coating-single layer and multilayer.

Old testing time: 24-hr dissolving test.

With LayerGauge: 3 min. Now able to measure each roll produced.

User: Production.

Case B

Product: Polyolefin A-B-A coextrusion, no tie layers.

Old testing time: 4 hr.

With LayerGauge: 15 min.

User: R&D, soon to be transferred to production.

Case C

Product: Barrier films with PA and EVOH layers.

Old testing time: Unknown.

With LayerGauge: 15 min.

User: Production supervisor of each shift.

Case D

Product: Coextruded barrier coating on paper board, 4- and 5-layer polyolefins, PET, ties, and EVOH.

Old testing time: Unknown.

With LayerGauge: 20 min.

User: R&D.

Case E

Product: Wide variety of structures (>50). From 3 to 8 layers, many polymer types, cast and blown film.

Old testing time: Average 2 hr.

With LayerGauge: 15 min.
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Article Details
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Author:Tormala, Sauli
Publication:Plastics Engineering
Date:Mar 1, 1990
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