Laboratory Practice Quick and Easy Analysis of Complex Packaging Films

Due to increased requirements in stability, durability and recyclability, modern packaging films consist of a growing number of thin layers. This is where the previously common infrared spectroscopy reaches its limits. Horiba has unlocked new possibilities using Raman microscopy, which automatically analyses multi-layered films in just a few minutes.

Plastics are indispensable materials in our current economy and daily lives; more than 350 million tons of plastics are produced globally every year. In the EU, almost 40 percent of the output of the entire plastics industry represents packaging.

The EU wants to reduce the amount of (excess) packaging and packaging waste, while increasing the percentage of recycled material used to produce plastics. At the beginning of 2021, it introduced a plastic levy for non-recycled plastic packaging waste, to accelerate the transition to a circular, resource-efficient plastics economy. According to EU Directive 2018/852, from 31st December 2025 onwards at least 65 percent of the weight of all packaging must be recycled, increasing to 70 percent and at least 55 percent for plastic packaging from 31st December 2030.

This new circular strategy poses challenges for the R&D departments and quality control labs of plastic-producing companies. They will need more powerful tools to develop, analyse, characterise and identify the plastics used in packaging films.

Structure of packaging films

Modern packaging films have a complex, multi-layered structure. Food products in particular are subject to strict product safety and quality standards, resulting in intricate packaging requirements. Depending on the application, films must be oxygen-tight, e.g. to prevent meat from oxidising, while a UV-radiation-proof layer ensures that products do not develop discolouration or other undesirable changes. Likewise, a certain degree of stability and tear-resistance is necessary, to ensure that products are not damaged during the production and packaging process.

To incorporate all these functions, modern films consist of 10, 12 or even 15 different layers, each with different properties (Figure 1). These layers can consist of polyethylene, polypropylene and other materials, which are bonded using adhesives. New material formulations, consisting partially of recycled plastics, are also used to enhance the performance of multi-layered structures. Depending on the requirements, the individual layers are between two and 20 micrometres thick. This results in an overall thickness of 25 to 100 micrometres, depending on the respective film. Despite these high functional requirements, ever thinner layers are required, to reduce the quantity of plastic and cut costs – a conflict of objectives that is difficult to reconcile.

Benefits of Raman spectroscopy

Until now, FTIR (Fourier-transform infrared) spectroscopy has been the primary method for analysing films. The drawback of this oft-used infrared spectroscopy is its relatively low spatial resolution, meaning it cannot be used to analyse films less than 10 micrometres thick. To meet the increasing requirements, R&D and QC labs need high-resolution, automated analysis equipment.

The Raman spectroscopy used in the Labram Soleil by Horiba (Figure 4) effortlessly enables measurements within the range of less than 1 micrometre – ideal for ever thinner packaging films as well as all polyester types, such as Pet, Co-Pet and PETGP. Raman can also differentiate between different copolymers, such as PE-Eva, and quantify the Eva content; the spectroscope also analyses acrylate and polyurethane adhesives. In addition, it can differentiate between different thicknesses of polyethylene (high, medium and low thickness).

With Raman spectroscopy, quality control labs can quickly identify errors in film manufacturing and respond immediately. If a defect occurs in one of the many layers of the film, the Labram Soleil can identify the layer using confocal analysis and display it using 3D imaging. This allows inclusions, defects or uneven distribution of adhesives to be easily identified. No preparation of samples is necessary for confocal analysis; a film with seven layers and an overall thickness of 18 micrometres can be fully analysed in only 40 minutes. With an assumed overall thickness of 100 micrometres, a complete scan of the cross-section takes only a few minutes.

If the defective layer is known, it is possible to isolate the problem; for example, it could be related to the temperature range of the corresponding extruder. If the defect is caused by contamination, the problem could be solved by cleaning the extruder. This eliminates the need for time-consuming troubleshooting and hours of lost production time.

Sample preparation in less than five minutes

A decisive advantage of the Horiba application over conventional methods like FTIR spectroscopy is not only its high resolution, but also its high speed. Both sample preparation and the measurement process itself are now much faster: it only takes around five minutes to prepare a sample for Raman spectroscopy. For FTIR spectroscopy, this usually takes multiple hours, as it is first necessary to conduct very expensive microtome sectioning.

Horiba has developed a tool for rapid cross-section analysis, which can be used to prepare even the thinnest films in just a few steps: a piece of film is clamped in the sample holder and a cross-section is cut above the sample holder using a sharp disposable blade; next, the sample holder is fixed onto the microscope stage as usual. The sample holder is extremely precisely designed, to ensure that even the thinnest films are firmly held in place, without being displaced during the cutting process (Figure 2).

Special features of the Horiba Labram Soleil

After the scanning process, the device generates a spectrum that can be used to identify the different components of the film. Horiba has developed a software that fully automatically analyses each spectrum using a database and displays the layer structure, including the layer thickness and material; see Figure 3. A false colour image can be laid over the microscope image, in which each colour represents a polymer. While with other methods, every spectrum has to be individually, manually and laboriously evaluated, now it can be done at the touch of a button. Even dyed polymers can be measured using Raman spectroscopy, although the fluorescence of the dyes disrupts the Raman signal and makes measurement more difficult as well as information about the material, each spectrum provides information about other parameters, such as the degree of branching in the polymer molecules.

Additional areas of application for the Raman spectroscope

The Labram Soleil can be used wherever plastic films of all kinds are produced or processed as well as in the afore mentioned food industry. High-quality packaging is also required in the pharmaceutical industry, as the active ingredients in medical products or medications may not be altered by external influences and must remain sterile.

In product development, new material compositions can be inspected more quickly in practice and adjusted, if necessary. Raman spectroscopy is also commonly used to analyse the material composition of competitor products. The device can analyse robust smartphone films, noise insulation and privacy films, as well as security-related products: for example, the production of identification documents such as identity cards is already assisted by a Raman spectroscope from Horiba. The cards consist of multiple layers, including a hologram, and have a complex structure.

Outlook

With Labram Soleil, R&D departments and quality control labs are optimally equipped for the future, thanks to its extremely low measurement limit of less than 1 micrometre. Films with a thickness of less than 1 micrometre are unlikely to play a major role in the packaging industry, as they can only be produced with considerable effort and lack the necessary strength. Classic FTIR spectroscopy will continue to be used in the future; it is suitable for the measurement of thicker films, and the process is more cost-efficient than Raman spectroscopy.

* *Dr. Ingo Reese studied chemistry at the Christian Albrechts University in Kiel, Germany. In 1999, he received his doctorate in analytical and inorganic chemistry. He has over 20 years of experience in the field of laboratory analysis, especially Raman spectroscopy, and has been Head of Sales and Marketing for the Scientific division at Horiba Jobin Yvon since 2018.

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