The packaging materials used for food are glass, metal, plastic, fibre-based and bio-based materials. Nofima Mat has most competence in and focus on primary packaging, mainly with plastic materials.
Glass is used to some extent as packaging for certain food products. Advantages of glass include its good protection of the product, its integrity (good barrier), its lack of smell and taste, its safety from a health point of view and its recyclability. Disadvantages are that glass is heavy, fragile and requires long production runs.
Metal packaging consists mainly of steel, aluminium and tin and is mainly used for preserved and semi preserved food, beer, soft drinks and certain dry products. Advantages of metal include that it is environmentally friendly and recyclable, is impermeable to gas and light, can be subjected to a wide range of temperatures, is ductile and gives good mechanical protection. Disadvantages are that it is dependent on impermeable soldering and/or folding and is liable to corrosion and also that one cannot see the contents.
Fibre packaging is a generic term for materials made from cellulose fibre, including paper, cardboard, corrugated board and solid board. Fibre packaging is sometimes used in combination with plastic materials. In addition to the strength of the fibre packaging, this will also be more resistant to humidity. An important advantage of fibre based packaging is that it is made from renewable raw materials and can be recycled.
Plastic materials are the fastest growing group and already represent the majority of packaging (plastic packages 50% of all goods).
Advantages of plastic include the wide variety of possibilities and properties it offers, that it is possible to tailor its properties to suit different purposes, it has low weight and is relatively cheap and easy to produce and shape. Technical disadvantages of plastic are that it can react with the packaged product and does not provide an absolute barrier against gas and vapour (as glass does for example).
What are plastics / polymers?
Plastic packaging consists of polymers, which are long chains made up of small, individual links: monomers. The type of monomer and the molecular structure (whether the polymer chain is linear, branched, cross-linked, whether it is made up of one type of monomer [homopolymer] or several types of monomer [copolymer] and how the lateral groups or branches are arranged) decide many of the polymer's properties.
Pure polymers are not suitable as packaging materials. Additives (such as antioxidants, UV stabilisers, pigments, anti-fog agents etc.) are used to make the production of polymers easier or to change or improve properties.
Choice of plastic material
It is the product's requirements for protection and the desired quality and/or shelf life that decide the choice of material. Parameters that must be considered include appropriate safety, migration, neutrality of taste and smell, barrier qualities against oxygen, CO2, water vapour, aroma and light, temperature during filling, storage, distribution, machine suitability (serviceability and sealing properties), reuse/recycling and price.
In some cases where there is no very stringent requirement for barrier properties, such as an O2 barrier, a single polymer (single layer material) may be sufficient. These may be materials like polyethylene (PE), polypropylene (PP) or polystyrene (PS). (In general we can say that materials with an oxygen transmission rate [OTR] of 10-100 [ml O2/m2 24 hr, 1 atm] are considered to be good barriers, OTR 1-10 as very good barriers and <1 as extremely good barriers). Which you choose will be decided by the requirements for barrier properties, ductility, processing method and price.
Polymers with different properties are often combined so as to obtain a material that is a barrier against humidity and O2 and may be easily sealed together and shaped.
The different polymers may be laminated together using glue or they may be coextruded and joined with a so-called tie layer (which is a type of polymer and not defined as glue).
Laminates - compound materials
Some materials, such as PE and PP, are very good barriers against humidity, but not against O2. Other materials like PA, EVOH, PVdC, PET and PA are good O2 barriers, but many of these are sensitive to humidity.
A common solution is to have a centre layer that is a good O2 barrier, protected on one or both sides by materials that are good water vapour barriers.
Bio based materials, biomaterials, bioplastic, biopolymers: These are all names used for this type of material.
These materials originate from nature, are renewable and are not created from fossil raw materials. The term biodegradable is also used, but this describes only the functionality of the waste handling of the material and says nothing about the source of the raw materials. Many biomaterials are biodegradable, but others are not. (Some materials based on fossil raw materials may also be biodegradable.)
Biomaterials are based on agricultural and forestry products and on marine sources. Biopolymers for packaging purposes may be created in several ways:
- By direct extraction from biomass. That is from lipids, proteins and polysaccharides. Polysaccharides are the largest group and consist of starches, for example from potatoes, maize, wheat, rice, cellulose, guar gum, pectin and chitin/chitosan.
- Chemical processes, such as hydrolysis of bio based raw materials, in which bio monomers are created that become the building blocks of biopolymers such as polyesters and polylactate.
- Polymers produced from organisms, such as bacterial cellulose or PHA (polyhydroxyalcanoates).
Some types of biomaterials
Three types of biopolymers dominate the market (representing about 90% of the biopolymer market).
These are starch based materials, cellulose and PLA (poly lactic acid).
- Starch based materials represent the major part of the bioplastic market. These are competitive when compared with petrochemical materials, in both mechanical properties and economically. Their disadvantage is that they are sensitive to humidity, brittle and liable to degradation. Their properties may be improved with additives and by mixing with biodegradable co polyester.
- Cellulose is the biopolymer there is most of in nature. These products, such as cellophane for example, have good mechanical properties and are good oxygen barriers - in their dry state, but they are sensitive to humidity and cannot be thermoplastically welded.
- PLA is transparent and similar to conventional plastics like PE, PP and PET in characteristics (physical and chemical properties). This means it can also be easily processed using standard equipment that already exists for production of conventional plastics (film blowing, thermoforming, injection moulding).
Another interesting material is PHA (polyhydroxyalcanoates), a group of aliphatic polyesters. PHA is made by microbial fermentation of sugar or lipids and the most common is Poly-3-hydroxybutyrate (PHB). This polymer is a polyester whose characteristics are similar to the petrochemically produced PP, but is somewhat stiffer and more brittle.