INFRARED ANALYSIS OF SINGLE AND MULTILAYER FILMS IN THE PRODUCTION AREA

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1 INFRARED ANALYSIS OF SINGLE AND MULTILAYER FILMS IN THE PRODUCTION AREA Sandy Rintoul Wilks Enterprise, Inc. South Norwalk, CT Scott Cobranchi Sealed Air Corporation Duncan, SC Nina Tani Sealed Air Corporation Duncan, SC ABSTRACT There are many areas in production that can benefit from a rapid onsite measurement: determining which side is which on multilayer films to be laminated, checking rolls of film in the warehouse, measuring an individual layer within a film in the production area, raw material verification, and formulation discrimination are few examples. A new concept portable infrared spectrometer enables the user to obtain infrared spectra on single layer or multiple layer films outside of the laboratory. Polymers such as nylon, EVOH, EVA, and polyethylene have characteristic absorbances in the mid infrared range that can be used for material identification or for quantitative measurements. This new spectrometer utilizes a detector array and a linear variable filter in order to produce spectral information much like an FTIR (fourier transform infrared spectrometer). Unlike the FTIR, the spectrometer is rugged, has no moving parts, fits in the palm of a hand, is a fraction of the cost and is easy to operate. INTRODUCTION Analysis is crucial to producing quality products. Any manufacturer knows that their reputation and ultimate success depends on giving the customer the product they expect. Each step in the production process requires analysis. FTIR has proven itself to be a reliable measurement technology for film analysis. Many polymers have characteristic and well-delineated absorption bands in the mid infrared region. FTIRs are often used in the quality control laboratory as a means to identify incoming raw materials or QC outgoing products. However, due to the complexity, cost and environmental requirements, it is impractical to move them into the field or along side a production line. They also typically require a trained technician or chemist. Having a portable measurement system gives the option of getting immediate results (where the measurement is needed) on-site; at the loading dock, production line or in the warehouse. Problems can be caught before they become expensive errors. ANALYSIS METHOD Variable Filter Array IR Spectrometer FTIRs contain delicate interferometer mechanisms requiring precise optical alignment. In addition, the long optical path typically requires a controlled ambient environment in order to preserve stability. The net result is that FTIR spectrometers are complex devices that have been restricted to installation in the benign

2 environments such as a quality control laboratory. With the introduction of infrared filter based instruments in the 1970 s, infrared measurements could then leave the laboratory and be done in the field. Filter based analyzers have fewer or no moving parts and have much shorter exposed optical paths, making them more robust and portable. The principal limitation is most of them are fixed wavelength instruments and are typically dedicated to a specific application. For film identification and analysis, a spectral range is much more useful than a single wavelength analysis. Two recent developments have made possible the design of a filter based infrared spectrometer capable of measuring a spectral range with the advantage of no moving parts and no exposed optical path. These are pyroelectric detector arrays and linear variable filters (LVFs). Pyroelectric detectors can be used throughout the entire IR range from visible to the far infrared and do not need to be cooled. LVF s are wedge shaped interference filters that gradually change in wavelength transmitted from one end to the other. A LVF typically covers an octave in wavelength (ie: microns ( cm -1 ) or microns ( cm -1) ). For a majority of the film analysis or identification applications the microns ( cm -1) ) spectral range is optimal. While a variable filter array infrared spectrometer does not have the high resolution and detection of an FTIR, it does offer a light weight, rugged and portable solution to on-site applications at a much lower cost. Film Analysis with a Variable Filter Array IR Spectrometer The mid-ir region of the infrared spectrum, especially the fingerprint region ( cm -1 ) is very useful in solving real world analysis problems in the film industry. Organic functional groups have characteristic absorption bands in this region that allow them to be quantified by the strength of their absorption. In addition, calibration data in the mid-ir region is more generic and less matrix sensitive than that in the near infrared region of the spectrum. For example, nylon has a strong infrared absorption bands at approximately 1670 and 1335 cm -1 (6 and 7.5 microns, see Figure 1) that are unique from the polyethylene or tie layers typically found in protective packaging films. Since absorbance is directly proportional to thickness or concentration, it is possible to measure internal nylon layers in a film using a VFA-IR spectrometer. Another example is the analysis of ethylene vinyl acetate (EVA). EVA is a copolymer of ethylene and vinyl acetate (VA). Commercial grades often run from 3% to 40% vinyl acetate. As shown in Figure 2, the amount of EVA present in a copolymer or blend can be determined from the infrared absorbance for EVA at 1742 cm -1.

3 Figure 1 Infrared Spectrum of Nylon using a VFA-IR Spectrometer Figure 2 Infrared Spectrum of EVA using a VFA-IR Spectrometer % Transmission 'EVA 3%VA' 'EVA 12% VA' Wavelength (µm) Sample Setup for Film Analysis with a VFA-IR Spectrometer Two different sampling platforms can be used for single layer and multilayer film analysis. For measurement of film thickness, identification of unknown films, and quality control of the finished product, a transmission system with a card holder is optimal, see figure 3. With this setup a film is placed on a card and the card is inserted into the VFA-IR spectrometer. A beam of infrared light is passed through the sample and measured by the detector array. Infrared absorbances at different wavelengths can then be used to measure the thickness of a particular film layer or to identify a specific film.

4 Figure 3 VFA-IR Spectrometer schematic for Card Holder For applications such as multilayer film orientation and incoming materials verification, a sample plate with an ATR (attenuated total reflection) crystal can be used. Samples are place directly on the ATR crystal. An elongate infrared source illuminates the ATR sample plate, penetrates approximately 1 micron into the film sample and is then focused onto the detector array with the variable filter (figure 4). Because of the small depth of penetration, only the exterior film layer in contact with the sample plate is measured. The resulting characteristic spectrum can then be used to verify the material or film orientation. Figure 4 VFA-IR Spectrometer schematic for ATR Sample Plate Sample Surface Internal Reflections Hemicylinder Hemicylinder Source ATR Crystal Detector Array Linear Variable Filter VFA SPECTROMETER ANALYSIS APPLICATIONS There are a number of applications for a small, hand-held infrared spectrometer that would be useful for production facilities. Although similar applications can be applied to any number of industries, the following discussion will focus on those for the films and polymer industry, particularly as they apply to flexible packaging. Examples will include verification of incoming raw materials, quality control of finished goods, manufacturing and customer support, as well as identification of unknowns. Incoming raw materials verification As with most industries, raw material verification is critical to avoid costly manufacturing problems. For example, it is not uncommon to receive a variety of polymers in railcar shipments at a production facility. From these railcars, the resin is typically pumped into silos for distribution to numerous production lines throughout the facility. Since silos can contain well over 100,000 pounds of material, it is critical that they are not contaminated with an incorrect resin. As a first line of defense, samples can be analyzed at remote locations prior to off-loading. In this way, additional verification can be done without having to bring samples to a centralized laboratory and delaying the off-loading of the raw material.

5 Admittedly, no form of spectroscopy will be able to detect differences between samples that do not result in spectroscopic differences. However, remote spectroscopic analysis can prevent the gross mistakes that can cripple production. For instance, a resin vendor may supply various grades of EVA to the same facility. Spectroscopic analysis can be used to verify the grade prior to transfer to the storage location prior to use. See Figure 2. Quality control of the finished product Similar to raw material verification, it is prudent to monitor production of finished goods in order to avoid long production runs that may be making inconsistent or unacceptable product. As an example, films used to package perishable products typically contain polymer layers that act as a functional barrier to oxygen. If that layer was inconsistently manufactured or left-out during the campaign, the end result would be unacceptable product. At the very least, the material would have to be scrapped. The worst case scenario is that the problem goes undetected and the material is used by the customer. Obviously, this could result in a very costly error. Spectroscopic analysis can be used in support of other quality control methods in order to avoid this situation. In addition, having a portable, simple-to-use infrared spectrometer at the production line assures that any problems can be detected much faster. Also, material can be analyzed more frequently than sending samples to a central laboratory. Manufacturing and Customer Support One major advantage of the VFA-IR spectrometer over standard infrared spectrometers is the portability of the instrument. The small size and robust design make it ideal to bring to remote locations in order to perform an analysis. For example, a packaging company may offer several hundred different formulations in their portfolio. Unfortunately, no matter how good the logistical controls may be, production runs can accidentally be mislabeled, resulting in the wrong film being sent to the customer. If this customer mixed these misidentified rolls with their normal inventory, it might become necessary to replace the entire inventory at great expense in order to assure that the customer does not use the incorrect material. However, a portable VFA-IR spectrometer that has the capability to discriminate between the different formulations, would allow the packaging company to analyze each roll prior to putting it in service at the customer s location. Not only would this avoid having to replace the entire inventory but this on-the-fly verification of the correct formulation would allow the customer to maintain production without a major interruption. Figure 5 is an example typical spectral result of two films that may appear similar in appearance but have vastly different functional properties. Spectroscopic analysis would prove relatively simple were it to be necessary to determine which of these two films should be used.

6 Figure 5 Comparison of two different film formulations % Transmission 'Formulation A' 'Formulation B' Wavelength (µm) Another example for manufacturing support is using the VFA-IR spectrometer equipped with an ATR in order to analyze the outside layers of a film. Laminated films are very typical in the packaging industry. In most cases, the two films that are to be laminated together are produced at different time or even at different facilities. When two different films to be laminated are not symmetrical, it is critical that both have the correct orientation before the adhesive and laminate is applied. If the top and bottom consist of two different layers such as ethylene vinyl acetate (EVA) and polyethylene it should be relatively easy to determine the film orientation. The analysis can be done using a variable filter array infrared analyzer with an ATR crystal (Figure 4). With a stored library of known samples, a film can be quickly identified by its characteristic spectrum. For the operator, a piece of film is pressed onto the crystal, the button is pressed and 15 seconds later he knows which side is which.

7 Figure 6 Overlaid Spectra of Polyethylene and Ethylene Vinyl Acetate % Transmission 'Polyethylene' 'Ethylene Vinyl Acetate' Wavelength (µm) As a last example for manufacturing support, the VFA-IR spectrometer can be used for quantitative analysis in order to analyze either raw materials or finished products. Figure 7 shows the relative absorbance for a series of barrier structures containing a core layer of barrier material with increasing layer thickness. As can be seen in the plot, it is very easy to determine the thickness of the internal barrier layer and determine whether the film would perform adequately. Figure 7 Relationship of Film Thickness to Peak Area Peak Area y = x R 2 = Relative Barrier Thickness

8 Identification of unknowns Lastly, the VFA-IR spectrometer data can be treated like any raw, spectroscopic data file. This includes the exportation of data into standard spectroscopic software packages. With this capability, it makes it possible to build and store libraries of standard materials as well as perform subsequent searches for identification of unknowns. As an example, Figure 8 shows an unknown spectrum that has been imported into a standard spectroscopic software package and searched against a series of library spectra. Although not exhaustively tested, results so far appear to be consistent enough as to allow for excellent results and accurate identifications. Figure 8 Library Search for Unknown Material In addition, the ability to export the data to other software packages allows the spectroscopist to not only work with the data in a format that they feel most comfortable with but also opens up the possibility for more advanced data treatments. CONCLUSION The development of linear variable filters and detector arrays has opened up numerous possibilities for moving analytical measurements out of the laboratory into the production area. With on-site measurements for raw materials, quality control, identifying unkowns, and customer support, manufacturers can avoid costly waste and errors that could cut into their profit margin.

9 2005 PLACE Conference September Las Vegas, Nevada Infrared Analysis of Single and Multilayer Films in the Production Area Sandy Rintoul Wilks Enterprise, Inc. Scott Cobranchi Sealed Air Corporation Nina Tani Sealed Air Corporation Infrared Analysis of Single and Multilayer Films in the Production Area Moving analysis out of the laboratory 1

10 Into the production area Benefits of onsite measurements At the loading dock Production line In the warehouse In the customer s warehouse Infrared Measurement of films FTIR a proven measurement technology for film analysis. Polymers have characteristic and well-delineated absorption bands in the mid infrared region Complexity, cost and environmental requirements make it impractical to move FTIR s into the field or along side a production line. 2

11 Infrared Measurement of Films Calibration data in the mid-ir region is more generic and less matrix sensitive than near infrared Films such as Nylon, EVOH, EVA, and polyethylene have characteristic absorbances in the mid infrared VFA Spectrum of Various Polymers Two new developments Detector arrays Linear variable filters (LVFs( LVFs) Allow for a filter based infrared spectrometer capable of measuring a spectral range with the advantage of no moving parts and no exposed optical path 3

12 Linear Variable Filter (LVF) Typically covers an octave in wavelength microns ( cm -1 ) microns ( cm -1 ) Detector Array 64 Pixel pyroelectric detector array with the linear variable filter mounted on top Array Assembly Advantages Rugged and portable No moving parts No exposed optical path-not affected by environmental conditions Low cost 4

13 Two Sample Platforms for Different Applications ATR (attenuated total reflection) system Transmission system Card Holder ATR Optical System Sample Surface Internal Reflections Hemicylinder Hemicylinder Source ATR Crystal Detector Array Linear Variable Filter Transmission Optical Setup Card Holder with Film Linear Variable Filter Elongated Source Detector Array 5

14 VFA Applications Multilayer film to be laminated Raw material verification Quantify film thickness Measuring individual layer within a film Quality control of finished product Identification of Unknowns Multilayer film to be laminated % Transmission 'polyethylene' 'EVA 9% VA' Wavelength (µm) Multilayer film to be laminated Which side is which % Reflectance 'unknown film side A' 'unknown film side B' Wavelength (µm) 6

15 Raw Material Verification EVA is -copolymer of ethylene and vinyl acetate (VA) Commercial grades often run from 3% to 40% vinyl acetate. Often necessary to verify grade for a particular function or application Raw Material Verification % Transmission EVA 3%VA EVA 12% VA Wavelength (µm) Measuring an Individual Layer within a Film Since absorbance is directly proportional to thickness or concentration, it is possible to measure internal layers in a film using a VFA-IR spectrometer. 7

16 Nylon has a strong infrared absorption bands 1670 and 1335 cm-1 1 (6 and 7.5 microns) unique from polyethylene or tie layers typically found in protective packaging films % Reflectance Nylon Wavelength Quantifying film thickness Peak Area y = x R 2 = Relative Barrier Thickness Quality control of finished product Qualify finished product or formulation verification % Transmission Formulation A Formulation B Wavelength (µm)

17 Quality control of finished product Check for mislabeling for rolls of film in the warehouse, avoiding the wrong film being sent to the customer. Trouble shoot at customers facility On-the the-fly verification of the correct formulation allows the customer to maintain production without interruption Identification of Unknowns Infrared Analysis of Single and Multilayer Films in the Production Area 9

18 Thank You Sandy Rintoul Wilks Enterprise, Inc. Scott Cobranchi Sealed Air Corporation Nina Tani Sealed Air Corporation Please remember to turn in your evaluation sheet... 10