Environmental Engineering Reference
In-Depth Information
material recycling facilities (MRFs). The only option is to blend the different
layer into a mixed plastic recyclate of variable quality. Where a single type
of layered film (such as PE/nylon) waste is recovered, the recyclate might be
blended with controlled amounts of virgin PE and filler to obtain a plastic
blend that can replace the original packaging material (Tartakowski, 2010).
Nanoscale fillers such as clays are used to improve the barrier properties
of plastic films. The platelike fillers, in particular, with the relatively higher
tortuosity deliver lower gas permeabilities (Alperstein, 2005). At a volume
fraction Φ of the nanofiller, the permeability
P
of a gas through the polymer
(Picard et al., 2007), depends on the value of tortuosity
τ
of the nanoscale
filler:
The magnitude of tortuosity τ depends on the geometric features of the
nanofiller material.
Transparent high-barrier packaging consisting of a plastic film with a thin
surface layer of vacuum- or plasma-deposited glass has been described
(Landau,2007;LangeandWyser,2003).Whilethesilicalayerimpermeable
in theory, micro-cracking of the glass surface can occur during processing,
and a measurable permeability is often observed with these. There is a need
for ultrahigh barrier plastics that are also transparent in both food and
8.2.2 Additive Migration and Toxicity
Common polymers including plastics are not toxic in the conventional
sense. The long-chain molecules of high polymer are too large in size to
be absorbed via the wall of digestive tract, and enzymes that can digest
polymers into smaller molecules that can be absorbed via the gut are simply
not available in humans. Clean plastics therefore do not pose significant
toxicity to humans; however, this applies only to uncontaminated plastics.
In practice, plastics contain numerous impurities and additives.
Compounded plastics may display toxicity via ingestion because of additives
(low-molecular-weight chemical species) or unintentional contaminants in
the material. Toxicity related to plastics is attributed to the following classes
of contaminants:
