Laser Speckle Spectroscopy Image Analysis for High Pressure and High Temperature Treatment Discrimination on LDPE, HDPE BOPP, BOPA and PET Polymer Layers Used for Food Packaging

The reflection and scattering of a coherent laser beam at the optically rough packaging surface, shows a speckle pattern which can be collected by a digital CCD camera and subsequently analyzed by image processing algorithms. This paper presents an optical laser speckle algorithm for the analysis of the surface roughness of different types of flexible packaging films of low-density polyethylene (LDPE), high-density polyethylene (HDPE), biaxial oriented polyamide (BoPA), biaxial oriented polypropylene (BoPP) and polyethylene terephthalate (PET) as a marker to the application of high pressure and high temperature treatments employed in food packaging industry. For every type of polymer, three samples with high pressure treatment of 680 MPa applied for a period of 10 minutes, three samples with high temperature treatment of 90 C applied for a period of 10 minutes and three samples with no treatment application were prepared to ensure structural differences on the polymer surface and the repeatability of the experiments. For the surface analysis, the laser beam was pointed into five different regions (R1 to R5) of every polymer sample analyzed. To ensure a bigger amount of information from the polymer structure, the frame grabber was programmed so that the total acquisition time per region and sample was 5 seconds, at a frame rate of 8.2 pictures per second, generating 41 images per region. A total of 205 speckle images per sample were grabbed and subsequently pre-processed, cross-correlated and analyzed. For this purpose, a similarity pattern analysis algorithm was developed, to distinguish among the different treatment classes analyzed (no treatment, high temperature treatment and high pressure treatment). This development takes advantage of the cross-correlation of intensity curves arising from treatment types. We hypothesize that cross-correlation coefficients can be exploited to enhance the identification of different treatments types for the different packaging polymers. As shown in the final results, the cross-correlation coefficient arising from different treatment types were lower than those of intensity curves arising from the same treatment. Remarkable differences were shown in BoPA, where the cross-correlation coefficient shows big differences between different treatments. These differences between treatments were in the order of 0.980 for the same treatment, 0.742 between nontreated and high-temperature treatment and 0.388 between nontreated and high pressure treatment. It shows the possibility of using the algorithm investigated as a marker for the discrimination between different treatments on plastics. Practical Application: The laser/optical analysis method developed in this investigation, was found to be appropriate for the analysis of the differences in the surface roughness of different flexible packaging films, treated with industrial food conservation methods such as high temperature and high pressure. This investigation gives the possibility of applying an online treatment judgment method without any losses. Simple upgrading of existing production lines can help to prevent erroneous processing and increase the quality and monitoring of the treatment application.