Environmental Engineering Reference
In-Depth Information
Thermoplastics, in contrast, get soft when exposed to heat and can be shaped into objects that
retain their shape once cooled to room temperature. These are the type of plastics used to make
packaging by using extrusion, injection molding, blow molding, thermoforming, injection
blow molding, extrusion blow molding, and stretch blow molding.
With exception of cellophane that is obtained from cellulose, most plastic precursors are
derivatives of petroleum or natural gas that are converted by chemical processes into precursor
monomers. More recently, plastics started being produced from renewable sources including
polylactate, polyhydroxybutyrate, starch derived plastics, polyamide 11 & 12, and bio-derived
polyethylene.
The most common petroleum-based resins used for food packaging are polyethylene
terephthalate (PET, PETE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), low-
density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS). Other resins are used
for reusable containers and oven-baking containers. Many packages are made from an only one
resin; however, especially with films, to achieve desired properties (e.g., enough mechanical
strength, moisture barrier, light barrier, and oxygen barrier), two or more films, which can be
plastic or other films such as aluminum, are laminated or coextruded into one single film.
Despite public concerns about migration of unreacted monomers, plasticizers, and addi-
tives from packaging to the food products as well as environmental concerns about disposal,
petroleum-based plastics are the most popular packaging material. Plastic-made packaging
has several advantages such as low weight, sturdiness, chemical inertness, good optical prop-
erties, thermosealability, microwaveability, and the possibility of molding the package in
many shapes and sizes (Marsh and Bugusu, 2007). Paradoxically, one virtue of nonbiode-
gradable plastics is their high chemical stability that makes their interaction with the food
product negligible. This chemical stability becomes a problem at the moment of disposal.
In the group of resins obtained from renewable resources, cellophane and polylactate are
the most commonly used. Cellophane is an old material that has been produced since the
1930s. It is obtained by treating sulfite paper pulp with sulfur dioxide and carbon disulfide that
transform the cellulose into a viscous material called
viscosa
. The viscosa is extruded through-
out a slit to form a sheet into a bath containing sulfuric acid and sodium sulfate where it
becomes cellulose hydrate. The sheet is then washed, bleached, treated with a plasticizer
(glycerol), and dried on heated rollers (Fellows, 2000). Cellophane is used for baked goods,
nuts, candies, dried fruits, spices, and cake mixes.
In the past decade, polylactate, or polylactic acid (PLA), became a commercial alterna-
tive to petroleum-based plastics for the production of specific types of food packaging and
eating utensils. PLA is produced by polymerization of the L-lactide monomer, which is
a lactone obtained by condensation of two L-lactic acid molecules (Drumright et al.,
2000). Lactic acid is produced via fermentation using any sugary substrate, but most com-
monly it is made from corn starch in the United States and from cane sugar in other parts
of the world.
PLA can be used for the production of:
Plastic bags.
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Clamshell-type packaging for baked goods, produce, and salads.
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Disposable cutlery products including forks, knifes, and spoons.
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Disposable drinking cups and straws.
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More recently some companies started offering bottled water in PLA-made bottles and
dried products such as coffee in bags made of a laminated PLA film and other films.
