Test method for measuring non-visible set-off from inks and lacquers on the food-contact surface of printed packaging materials

Abstract

The main objective was to develop a technique to expose spots of invisible set-off of inks and lacquers on the food-contact surface of food-packaging materials. Set-off is the unintentional transfer of components of printing inks from the outer printed surface onto the food-contact surfaces. The target sensitivity was 20 µg cm⁻² and the technique should be capable of examining large areas of printed substrate for no more than 4% coverage by set-off. These requirements equate to an ability to detect a worst-case migration potential of less than 50 µg kg⁻¹. Other objectives were the industrial requirements that the equipment should be inexpensive, should be easy to use by existing personnel and should preferably be non-destructive with a clear criterion for pass or fail. The approaches investigated included chemical analysis of solvent extracts, Fourier-transform infrared spectroscopy and microbeam analytical techniques, but these were found to be cumbersome and had only limited success. The objectives were achieved using an optical approach to excite and observe luminescence from invisible set-off. In model experiments, resins were applied to different substrates (plastic, paper and cartonboard). For a given resin on a given material, the key to success was to maximize the discrimination between the luminescence from the resin and that from the substrate by selecting the optimal combination of exciting wavelength and viewing goggles with selective wavelength filters. The required level of detection (20 µg cm⁻²) was achieved or exceeded for all ten resins tested on three different plastics. It was also achieved for two different papers and in all but four cases of the resins on three different cartonboards. Quantitation was achieved by the use of a calibration palette prepared using different quantities of resin spotted onto the relevant blank packaging material.

Keywords:

• Flexible food-packaging materials
• plastics
• paper and board
• migration

Acknowledgements

The authors gratefully acknowledge funding from the former Ministry of Agriculture, Fisheries and Food (now DEFRA) and latterly from the Food Standards Agency. A final research report on this work (Project FS2340), including a protocol and guidelines, has been placed in the FSA's Library and Information Service. The authors also thank the numerous representatives from industry who provided samples and useful information. They acknowledge in particular Roger Saunders of Sun Chemical Ltd, David Easterby of Coates Lorilleaux, Mike Zobel of the Flexible Packaging Association, and Bob Dartnell of Foster and Freeman Ltd. Finally, they thank Detective Constable MacKenzie of the Tayside Police Department for helping us with our enquiries. The statements and conclusions in this paper are the responsibility of the authors alone.