Food Packaging Materials

Basically, Packaging Science is Materials Science. As a consequence, the right choice of a package largely depends on the performance of the packaging materials used. These performances may be expressed in a chemical and physical way. On the other hand, a technical description in terms of material chemistry can be useful. This book introduces the chemical peculiarities of four different material categories: plastics, cellulosic, ceramic and metals. Nevertheless, a large variability of different items are obtained in these four categories by means of even small chemical changes. Moreover, important changes in the final properties and performances of food packaging materials can be obtained by means of chemical modifications. Finally, the current situation of food packaging materials is briefly described in terms of global amount of unit sales worldwide.

The term ‘ceramic’ refers to a category of materials that can be all defined as inorganic and non-metallic, and whose chemistry is mainly based on silicon structures. Substantially, these products are subdivided in two categories, depending on the main component: glass on the one hand, and minor ceramic materials like china, porcelain and earthenware on the other side. Common glasses can have different chemical composition, leading to their own colour, thermal or mechanical resistance. Silicon, commonly represented as silica, is found as silica or silicates. With relation to the crystalline structure, silicon’s four valences lead to a structural unit in which each silicon atom is located at the centre of a tetrahedron. For this reason, crystalline structures are typical and commonly found in all silicon-based ores. Glass packages are well known because of their optical, thermal and mechanical properties. On the other side, non-glass packages are commonly subdivided in two subgroups: porous earthenware and non-porous porcelain. Each different sub-type shows peculiar properties with notable importance when speaking of food packaging applications.

The majority of known elements are metals. With relation to food packaging, these elements are used in a very pure form (aluminum) and as metal alloys (steel, tin plate). Five fundamental characteristics of metals make them particularly feasible for packaging: compactness, high density, unrivalled toughness, malleability and high thermal conductivity. In addition, the easiness of a selective collection of the metallic waste (due to magnetic behaviour and high density values) and the opportunity of thermal recycling without any loss of the original performances should be noted. As a result, three main classes of metallic materials for food packaging applications are currently available on the market: aluminum, coated plates (tinplate, tin-free steel, polymer-coated and steels) and stainless steel plates. Each material has interesting features on the one hand and peculiar disadvantages on the other side, when speaking of food packaging applications and other topics such as simple logistic considerations (oxidation, etc.).

The most abundant biopolymers in the biosphere are carbohydrates polymers which account for three-fourth of the global available biomass. Cellulose is, by far, the most abundant and widely spread carbohydrates polymer and the first renewable organic material. The packaging industry uses cellulose-based materials on a very large scale: these matters represent the biggest part of the whole packaging materials in all countries. Actually, cellulosic packaging is quite a broad category of different types including both primary and secondary packages, as well as wrapping materials and containers. In fact, even if the basic chemical structure of cellulose is the same in all the different cellulosic materials, their final structure can be significantly dissimilar. This variability, due to a combination of cellulose biosynthesis conditions and technological features, allows to obtain different packaging products: papers, boards, regenerated cellulose (cellophane), moulded cellulose, etc. General features of main cellulosic packaging materials are strictly linked with cellulose chemistry.

Words ‘plastics’ and ‘polymers’ are used quite often, particularly in the packaging sector, as synonymous even if they do not have the same meaning. These macromolecules are composed of many repeated subunits, i.e. definitely polymers. Actually, plastic packaging materials are predominately constituted of polymers (70–99 %) containing always various amounts of additives, such as plasticisers, antioxidants, pigments, antistatic, fillers and many other compounds. These chemicals are essential to provide the expected functionality; for this reason, final products are not definitely polymers. When speaking of food packaging applications, all starting substances, as well as finished plastics materials must have regulatory approvals, based on their specific chemical and toxicological features. Hereafter, the chemistry and general information about food packaging polymers are discussed here regarding two arbitrary categories: synthetic-oil derived polymers—polypropylene, polystyrene, polyvinyl chloride, etc.—on the one hand, and oil-derived and biodegradable bioplastics on the other side.

A single material is used rarely alone in the manufacturing of final packages, in particular when speaking of flexible packaging. Various materials can be used in order to assemble a structure with interesting properties (logistic advantages, ameliorated shelf lives of packaged products, opportunities for recycling and environmental impacts). This reflection should address more attention to the possible development of high-performing packages which can extend shelf lives and better protect foods. The most important technologies used to arrange together different materials in order to achieve more performing packages—multilayer structures, composites, polymer blends and alloys—are shortly described in this chapter with emphasis on chemical aspects.

Chemistry is not only important in the production of packaging materials. Important reactions may take place or must occur during practical uses, when packages are filled with food and beverages and, after their use, addressed to recycling processes. For various reasons, these chemical changes can be very important; as a result, the most relevant ones of these modifications—corrosion, cracking, fractures, weathering, etc.—should be shortly discussed. Corrosion is usually referred to metals and, more rarely, to concrete, polymers and glasses. This complex phenomenon depends on different variables. Also, biodegradation and compostability have to be discussed when speaking of food packaging materials. Chemical resistance can be indirectly described in terms of stability to oxidation, resistance to corrosion and other performances. In addition, peculiar abuse tests are available when speaking of the resistance of materials under the combined effects of a stress and aggressive environmental. Consequently, modifications of weight, dimensions, mechanical properties and visual appearance are evaluated in order to express a rate of chemical resistance.