Читать книгу Packaging Technology and Engineering - Dipak Kumar Sarker - Страница 27

The plastic packaging used across the globe is made from processing various products from crude oil and gas. However, only about 5% of global oil resources are used in the production of plastic and an astonishing mere 3.5% of this small amount is lavished on the production of plastic packaging. Plastic can be used for both packaging body and closures and, with the aid of designer input with ever less material, to produce even more packaging. As a consequence of their ductility, malleability (plasticity), and ease of shaping, ‘plastics’ remain one of the most popular classes of materials for universal packaging needs. Plastic use as a ubiquitous packaging material did not start until the 1950s; it gained momentum up to the 1970s and is now globally a matter of prime concern with respect to its ineffective disposal and frequent single use.

Acrylonitrile (AN) and its related family of packaging materials are often used when pliability is required. There are many different types of AN‐based plastics and synthetic rubbers. Materials in this grouping include acrylonitrile–butadiene–styrene, a terpolymer (three different monomers) with extremely good chemical resistance and flexibility, or the copolymers styrene–acrylonitrile, which is more thermally resistant than polystyrene (PS) alone, and polyacrylonitrile or Creslan 61, which is thermally resistant but also possesses some unique metal‐binding properties. On combustion, as part of the energy‐recovery processes of waste plastics or thermal recycling because of the acrylonitrile (AN) group (CH₂=CH–C≡N), the compounds are known to liberate cyanide gas and carbon monoxide. In opposition to AN, polycarbonate (PC) packaging is easy to process, cover, and shape with heat‐forming capability. These types of plastic have a wide area of usage in the modern production sector, where toughness and a durable character with respect to impact are required. Consequently, PC plastic is now used for ampoules and ‘plastic glass’ mimics. This polymer is very transparent and light transmitting – actually being better than most types of glass. Water bottles used in homes and babies' bottles populating nurseries around the globe are made from PC material. The best property of PC lies in its durability to impact, which is why it is also used for prefilled syringes and industrial safety glasses.

The PE group of packaging materials represents the single biggest category of plastic used in packaging but also universally across all sectors. Recycled PE is used for milk bottles, medicine bottles, and many general containers and can account for up to 61% of all plastics in the recycling stream. HDPE is a tough, malleable, abundant, and cheap material but its natural opacity due to light scattering means it cannot be used in products where transparency is needed. Nevertheless, it is one of the most widely used plastics of all those that are currently available to manufacturers. HDPE, which is a particularly tough version of PE, is also utilised for tubs used for cheeses and butter and boil‐in‐the‐bag food products and may account for as much as 29% of all plastics. Low‐density polyethylene (LDPE) is a semi‐opaque, tough, durable plastic but with an elastic, easy‐to‐cut character. LDPE plastics are used mostly in pack‐film materials by virtue of being smooth, elastic, and relatively transparent. LDPE plastics are also routinely used in the manufacture of bags and in the elastic lids of many types of jars. This type of plastic may account for an incredible 32% of all plastics and, along with HDPE, accounts for a significant portion of environmental plastics and micro‐plastics.

PET packaging, depending on the thermal treatment, is an amorphous (transparent), semi‐crystalline (opaque), flexible, and valuable packaging material (representing 9% of all plastics used). Depending on PET film thickness it may be rigid or semi‐rigid and this can dictate its possibilities for end use. At a density of around 1.39 g/cm³ (cf. 2.7 g/cm³ for aluminium and about 2.8 g/cm³ for glass) it is a lightweight material that has excellent gas and humidity barrier properties. Simultaneously, it is mechanically tough and highly resistant to impact, making it ideal for bottles such as liquid pharmaceutical containers and carbonated drinks bottles as well as jars and trays. The semi‐crystalline form of PET, known as CPET, is used almost exclusively for oven‐ready meal trays because of its high thermal resistance. The now common PET bottle was first invented in 1973 but has since spread to use in some ‘plastic cans’ that consist of a transparent or printed PET body and aluminium lid, often with a pull‐ring (Minuman, Invento, Lino, and Sino Packaging). The most important advantage of PET usage is that it possesses sound multiple recycling characteristics; consequently, greater use of this plastic presents greater possibilities for more routine plastic recycling. Some recently discovered species of bacteria are thought to be able to digest PET as a food source; this opens up more avenues for improved recycling or disposal by species found in the natural environment that degrade the waxy and wax‐like materials.

The next cluster of packaging materials includes PP, PS, and polyvinyl chloride (PVC). PP as a packaging material is resistant to chemicals, heat, and moisture. It is a plastic that has moderate rigidity, being used for ketchup bottles, medicine bottles, yogurt pots, and lids. It has the lowest density among plastics used in packaging and accounts for up to 11% of all plastics. PS packaging can be seen in rigid containers and in expanded insulating foam. In a non‐expanded form it is a very tough, highly transparent, and bright plastic used for protective packaging (egg cartons and meat trays) and may account for up to 10% of all plastics. PVC packaging exists in two forms – hard and elastic varieties (constituting 5% of all plastics) – and is often used to make bottles for vegetable oil and shampoo; it is also used in pharmaceutical push‐out‐packs. PVC was initially discovered by Henri Victor Regnault and later refined for potential use by Eugen Baumann. Approximately 40 years later in 1926 Waldo Semon mixed different additives into PVC to make it more pliable; this resulted in an easier‐to‐process material and allowed its widespread use (5% of all plastics). Concerns in the 1970s over the vinyl chloride (C₂H₃Cl) monomer (frequently referred to as VCM), bisphenol A ((CH₃)₂C(C₆H₄OH)₂), and dioxin (dioxin‐like compounds, e.g. 1,4‐dioxin) present in the material have somewhat spoilt the reputation of PVC as the superpackaging material that it is. From the 1970s, VCM exposure was linked to a rare form of liver cancer, known as angiosarcoma. The US Environmental Protection Agency classified VCM as a known human carcinogen from this time onwards, with factory workers being the most common victims of VCM over‐exposure.