Protecting from Oxygen (and from other gases)

As mentioned in previous chapters, the benefits provided by innovation in the paper and board business are connected with the possible chemical modification of fibres, with coupling with other materials (composites), as well as with special surface treatments. Today paper and board can substitute polymers by virtue of their enhanced ability to act as barriers to gases, while displaying an active, antimicrobial, photocatalytic, etc. behaviour. The desired barrier effect is primarily against oxygen. Considerable interest was aroused in the past by substances like artificial or natural antioxidant additives, capable to act as a barrier to oxygen by preventing its action, as well as by solutions that prevent oxygen contact with food, such as vacuum and modified atmosphere. One interesting option today is nanocellulose, a strong barrier to oxygen and other gases that can truly compete with resins currently in use to produce composite materials. A water barrier is also required for paper and board. Surface treatments are therefore available for paper, enhancing its resistance to water contact without affecting its recyclability and biodegradability properties. Also important is the grease barrier. Greaseproof paper can be obtained through the enhanced disaggregation of the fibre mesh, which can be achieved either chemically or mechanically. Alongside mass treatment, surface coatings are also available. These act as a physical barrier to grease by means of paraffin, fluorinated emulsions, or silicone-based composites. Coatings based on polymer solutions have a severe drawback – they prevent paper recyclability. Therefore latest research in the paper and board packaging pipeline is aimed at producing new grease-barrier coating options, to be used without jeopardizing the medium’s recyclability. In these natural coatings, the “active” part, i.e. the fraction responsible for the desired barrier properties, is made up of biodegradable substances, including polysaccharides, proteins, and fatty acids. Microbial proliferation is another important cause for food decay. Bacterial contamination occurs, in particular, on the product’s surface, and incorporating antimicrobial agents in packaging allows to reduce the amount of preservatives added to food and to modulate their release. At present, research is carried out based on a variety of approaches: these range from a release mechanism, where the antimicrobial agent is incorporated in the material and migration to food occurs through diffusion and distribution with a decreasing effect in time, to a contact-based mechanism, where the antimicrobial agent is immobilized on the material and the antimicrobial action is displayed at the interface, thus ensuring a long-lasting effect. Abundant research is carried out on chitosan, another natural substance and a structural component of the shell of certain crustaceans. It is a biodegradable material with excellent antimicrobial properties. Lastly, several studies focus on titanium dioxide, which displays both an antibacterial and a photocatalytic action and is therefore applied in the paper and board sector on food packaging, as well as on other products, such as air purification devices.