Polyhydroxyalkanoate-based Multiphase Materials - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 121

carbon economy and carbon cycle. The human activities have resulted in a
huge and rapid release of CO 2 (and other greenhouse effect gases) in the
atmosphere which cannot be fully compensated by photosynthesis activity
and dissolution in the oceans. It results in a large accumulation of carbon
dioxide in the atmosphere; this greenhouse gas, along with CH 4 (also re-
ferred to as GHG), is contributing to global warming. People are now aware
that efforts have to be made to re-balance the carbon cycle by reducing the
amount of CO 2 in the atmosphere. Part of the carbon cycle re-balancing
concept is based on the development and manufacture of products based on
renewable and biodegradable resources. By collecting and composting bio-
degradable plastic waste, we can generate much-needed carbon-rich com-
post: humus materials. These valuable soil inputs can go back to the
farmland and ''reinitiate'' the carbon cycle. Then, the plants' growth con-
tributes to reducing CO 2 atmospheric accumulation through photo-
synthesis. Besides, composting is an increasing key point to maintain the
sustainability of the agricultural system by reducing the consumption of
chemical fertilizers.

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6.1.2 Biodegradability and ''Compostability''
According to ASTM standard D-5488-94d, a biodegradable article is capable
of undergoing decomposition into carbon dioxide, methane, water, in-
organic compounds, or biomass by enzymatic degradation in microbes.
''Compostability'' is similar to biodegradation and, as per ASTM, requires
the plastic to break down into biomass, CO 2 , and water at the same rate as
water. Hence, biodegradation is the degradation of an organic material
caused by biological activity, mainly micro-organisms' enzymatic action.
This leads to a significant change in the material chemical structure. The
end-products are carbon dioxide, new biomass and water (in the presence of
oxygen: aerobic conditions) or methane (when oxygen is absent: anaerobic),
as defined in the European Standard EN 13432:2000. There are different
media (liquid, inert, or compost medium) to carry out and analyse bio-
degradability. Depending on the type of standard to be followed (ASTM, EN),
different composting conditions (humidity, temperature cycle) must be
realized to determine the compostability level. 3 Therefore, a comparison of
the results obtained from different standards seems to be dicult or im-
possible. It is important henceforth, that the conditions of humidity and
temperature be specified before mentioning the duration required for the
biodegradation of the product.
Another aspect that we must also take into account is the amount of
mineralization as well as the nature of the residues (commonly called ''by-
products'') left after biodegradation. 4 The accumulation of contaminants
with toxic residues in the compost can cause plant growth inhibition in
these products, which must serve as fertilizers. The key issue is to determine
the environmental toxicity level for these by-products, which is known as
eco-toxicity. 5 Even if the biodegradation behaviour of a polymer is dicult to

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