Biomedical Engineering Reference
In-Depth Information
utilization of the starch component, but the polyolefi n fraction will nevertheless
persist in the environment. Can these materials be called “biodegradable”?
11.3
Defi ning Biodegradability
In 1992, an international workshop on biodegradability was organized to bring
together experts from around the world to achieve areas of agreement on
defi nitions, standards, and testing methodologies. Participants came from manu-
facturers, legislative authorities, testing laboratories, environmentalists, and
standardization organizations in Europe, United States, and Japan. Since this
fruitful meeting, there is a general agreement concerning the following key
points [12] .
1)
For all practical purposes of applying a defi nition, material manufactured to
be biodegradable must relate to a specifi c disposal pathway such as compost-
ing, sewage treatment, denitrifi cation, and anaerobic sludge treatment.
2)
The rate of degradation of a material manufactured to be biodegradable has
to be consistent with the disposal method and other components of the
pathway into which it is introduced, such that accumulation is controlled.
3)
The ultimate end products of aerobic biodegradation of a material manufac-
tured to be biodegradable are CO
2
, water, and minerals and that the intermedi-
ate products include biomass and humic materials. (Anaerobic biodegradation
was discussed in less detail by the participants.)
4) Materials must biodegrade safely and not negatively impact the disposal
process or the use of the end product of the disposal.
As a result, specifi ed periods of time, specifi c disposal pathways, and standard test
methodologies were incorporated into defi nitions. Standardization organizations
such as CEN, ISO, and ASTM were consequently encouraged to rapidly develop
standard biodegradation tests so these could be determined. Society further
demanded nondebatable criteria for the evaluation of the suitability of polymeric
materials for disposal in specifi c waste streams such as composting or anaerobic
digestion. Biodegradability is usually just one of the essential criteria, besides
ecotoxicity, effects on waste treatment processes, etc.
In the following sections, biodegradation of polymeric materials is looked upon
form the chemical perspective. The chemistry of the key degradation process is
represented by Eq. (11.1) and (11.2), where C
polymer
represents either a polymer or
a fragment from any of the degradation processes defi ned earlier. For simplicity
here, the polymer or fragment is considered to be composed only of carbon,
hydrogen, and oxygen; other elements may, of course, be incorporated in the
polymer, and these would appear in an oxidized or reduced form after biodegrada-
tion depending on whether the conditions are aerobic or anaerobic, respectively.
