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178
FOOD PACKAGING TECHNOLOGY
the degree of branching affect properties, such as density, crystallinity, gas and
water vapour barrier, heat sealing, strength, flexibility and processability.
The factors which control polymerisation are temperature, pressure, reaction
time, concentration, chemical nature of the monomer(s) and, of major significance, the catalyst(s). A catalyst controls the rate and type of reaction but is
not, itself, changed permanently. The recent introduction of metallocene (cyclopentadiene) catalysts has resulted in the production of high-performance plastics
and has had a major impact on the properties of PE, PP and other plastics, such
as PS. In some cases, the resulting polymers are virtually new polymers with
new applications, e.g. breathable PE film for fresh produce packing, and sealant
layers in laminates and coextrusions.
It is appropriate to consider PE as a family of related PEs which vary in
structure, density, crystallinity and other properties of packaging importance.
It is possible to include other simple molecules in the structure, and all these
variables can be controlled by the conditions of polymerisation – heat, pressure,
reaction time and the type of catalyst.
All PEs have certain characteristics in common, which polymerisation
can modify, some to a greater and some to a lesser extent, but all PEs will be
different from, for example, all polypropylenes (PP) or the family of polyesters
(PET).
Similar considerations apply to all the plastics listed; they are all families of
related materials, with each family originating from one type or more types of
monomer molecule.
It is also important to appreciate the fact that plastics are continually being
developed, i.e. modified in the polymerisation process, to enhance specific
properties to meet the needs of the:
• manufacture of the film, sheet, moulded rigid plastic container etc.
• end use of the plastic film, container etc.
In the case of food packaging, end use properties relate to performance properties, such as strength, permeability to gases and water vapour, heat sealability
and heat resistance, and optical properties, such as clarity.
Additionally, the way the plastic is subsequently processed and converted in
the manufacture of the packaging film, sheet, container etc., will also have an
effect on the properties of that packaging item.
7.2
7.2.1
Manufacture of plastics packaging
Introduction to the manufacture of plastics packaging
The plastic raw material, also known as resin, is usually supplied by the polymer
manufacturer in the form of pellets. Plastics in powder form are used in some
processes. Whilst some plastics are used to make coatings, adhesives or
additives in other packaging related processes, the first major step in the