Environmental Engineering Reference
In-Depth Information
obvious is that comparisons are made on an equal weight basis. For
example, material substitution from rigid PVC to a polyolefin, for pressure
pipe application, might be considered desirable, as a Rank V plastic is being
replacedbyaRankIplastic.But,thewallthicknessandthereforetheweight
of resin used in the two pipes to achieve the same performance (pressure
rating) will be very different. Also, the health impacts (especially those
from migrants into food in contact with plastic) cannot be assessed on the
basis of broad classes of polymers. Substituting a plastic lower in rank with
one that is higher does not always result in a choice that is better for the
environment. When all factors are taken into account PVC, the worst rated
resin, may in fact be desirable in specific medical applications.
A second limitation is that additives used in its formulation often determine
the environmental desirability of a plastic material used in a given
application. While the pure polymers are nontoxic, the toxicity of leachates
from compounded plastics must also be considered in developing rankings.
Additives and residual monomers are responsible for toxicity outcomes that
vary widely within a single class of plastic. Their health impacts often
overshadow those due to residual monomers. Phthalates are used in some
PVC products such as medical tubing but not in others (such as potable
water pipes), just as brominated flame retardants are used only in some
formulations of PS foam (EPS) but not in injection-molded PS cutlery. A
practical assessment of the acute toxicity of water extracts of various plastic
products (1-3 days at 50°C) to
Daphnia magna
sp. shows that for 42%
of the 48-hour extract tested, the EC50 values (the concentrations causing
toxicity effects in 50% of the test organisms) were below 250 g plastic/l
(Lithner et al., 2012). All extracts from plasticized PVC (but not rigid PVC)
were toxic at concentration of only 17-24 g plastic/l illustrating the toxicity
of plasticizers. The exposure of plastics at high temperatures for extended
periodsoftimeisnotcommonineverydayuse,andthetestisanaccelerated
leaching test. However, the mortality of
Daphnia
stored at ambient
temperatures in water contained in food-grade LDPE and HDPE was found
to be significantly higher than in glass container controls (Rkman-Filipovic
et al., 2012).
Overall, the use of ranking systems regardless of the life cycle analysis it is
based on, can be misleading. Sustainability assessments cannot be made on
broadclassesofplasticsorevencategoriesoftheircompounds.Ineachcase,
the environmental cost of using plastics must be balanced with the societal
benefits afforded by the products. What is particularly useful is a ranking
