DEGRADATION OF PLASTICS IN THE ENVIRONMENT - Plastics and Environmental Sustainability - page 244

Adhering to a standardized test protocol such as the relevant ASTM
standards in assessing degradability helps avoid some of this variability.
Naming the test method used to assess biodegradability increases the
credibility of the claim (e.g., chitin was found to be 80% biodegradable
in seawater in 60 days when tested according to ASTM D6691-09.
Though there is no guarantee that inter-laboratory data will be strictly
comparable, specifying the standard at least informs how exactly the
test was carried out.
The ASTM has developed 26 standards related to test methods on
degradation of plastics materials. These are shown in Table 6.4 .
3. How much faster should the rate of biodegradation of a plastic material
be for it to be reasonably called “biodegradable?” The biodegradation
must be rapid enough to be readily observable experimentally; the
mineralization experiment must be completed in a reasonable timescale
of weeks or months rather than years. A specific rate or the extent of
biodegradation might be agreed upon for this purpose; the ASTM
D5338-93 as well as European standard (EN 13432) requires a material
to be at least 90% biodegraded in less than 6 months. The ASTM D5988
requires mineralization ≥70% at 25 ± 2°C within 6 months.
Alternatively, the rate might be benchmarked to the biodegradation of
selected natural materials such as dead dry biomass or a biopolymer
such as cotton. (ASTM D5338-93 allows a comparison with the rate for
cellulose as the criteria for biodegradability.) However, the suitability of
cellulose as a positive control is not clear (Mezzanotte et al., 2005).
4. The terms biodegradable plastics and bio-based or bio-derived plastics
were already defined in Chapter 4 . Plastics can be classified in terms of
the source of raw materials into four classes: plastics based on
fossil-fuel feed stocks, biopolymers made by living organisms, modified
biopolymers, and bio-based plastics derived from renewable biomass
feedstock. Members of each of these classes can be either inherently
“biodegradable” or “recalcitrant” when placed in an appropriate biotic
environment. Biodegradability is a property or characteristic of plastics
and is independent of the feedstock it is based on. This distinction was
discussed in detail in Chapter 4 and is further illustrated in Figure 6.11 .