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Food Package Engineering 327

Low-density polyethylene (LDPE) Polyvinyl chloride (PVC)
High-density polyethylene (HDPE) Polyvinylidene chloride (PVDC)
Polypropylene (PP) Nylon
Polyethylene terephthalate (PET) Ethylene vinyl alcohol (EVOH)
Polystyrene (PS) Polysulfone
Polyvinyl alcohol(PVOH) Polyimide
Polyphenylene sulfide Polyvinyl fluoride
Ethylene-vinyl acetate copolymer (EVA)

TABLE 11.1 Most Commonly Used Plastics for Packaging

homopolymer and copolymer. The former is synthesized using a sin-
gle type of monomer, for example, polyethylene from an ethylene
monomer; the latter is synthesized using more than one type of
monomer, for example, ethylene vinyl acetate produced from a mix-
ture of ethylene and vinyl acetate monomers. In addition, two or
more polymers (homopolymers or copolymers) are blended to obtain
a polymer with desirable properties. The most widely used thermo-
2,6
plastic used for packaging are listed in Table 11.1.

Polymer Processing
Extrusion Low-cost synthetic thermoplastics can be converted into
various shapes such as films, bags, bottles, trays, and other forms
using different polymer-processing techniques. Extrusion is one of the
most common methods of plastic processing. This process is used to
produce cast films, blown films, multilayer films, coatings, and lami-
nated structures. Extrusion involves melting a thermoplastic resin
with an extruder, which consists of screw fitted in an electrically
heated metal barrel, a hopper for feeding the resin, a motor for rotat-
ing a screw, and a die where the polymer melts exits in certain forms
(Fig. 11.1). The heat from the barrel softens the polymeric material and
rotating screw melts the resin pellets and forces the melted polymer
through the die. The melting of the resin is mainly due to the mechan-
ical energy input to the screw, which compresses and shears the resin
pellets and dissipates frictional heat to melt the pellets. The screw is
the most important element of the extruder, and different designs are
used for extruding different polymers. Extruder screws are character-
ized by their length-to-diameter (L/D) ratios and their compression
ratios, this being the ratio of the volume of one flight of the screw at
the feed end to the volume of one flight at the die end. L/D ratios
commonly used for single-screw extruders are between about 15:1
and 30:1, whereas the compression ratio varies from 2:1 to 4:1. 1,2
Extruded thermoplastic is converted into films by two processes:
cast film extrusion and blown film extrusion. In cast film extrusion,