335
19
Today, polylactic acid is one of the most
promizing biogenic plastics and is given a high
growth potential. It is already commercially
available in large quantities and relatively cheap
(approx. 2€ kg −1 ). Besides its biodegradability,
PLA is also biocompatible, which makes it interesting for medical applications.
PLA is able to replace established polyolefins
such as polyethylene, polypropylene or polystyrene in certain applications. This is because
established manufacturing processes can be used.
The most important fields of application are
short-life consumer goods such as food packaging, films and bags. Polylactic acid is also suitable for the manufacture of disposable tableware;
however, it is not very heat resistant. Hot coffee
in a PLA cup is not possible. An already established approach to compensate for the unfavourable properties of polylactic acid is the use
of mixtures with other biogenic polymers or
the production of composites. There are many
other applications for PLA, and many more will
undoubtedly be added in the future.
Other Biogenic Monomers
In addition to lactic acid, other biogenic polyester and polyamide monomers can be produced
from renewable raw materials, e.g. by fermenting glucose (7 Chap. 6) or from oleochemicals.
Dicarboxylic acids will first be presented because
they can be used for both polyesters and polyamides (. Fig. 19.12).
Succinic acid (1,4-butanedicarboxylic acid)
is the first to be mentioned. It is produced from
glucose or glycerol fermentation and CO 2 using
certain microorganisms isolated from rumen.
A first large-scale plant with an annual capacity
of 10,000 t was started up in 2014. Succinic acid
is converted into polyesters with diols and into
polyamides with diamines. The most important
product is the polyester polybutylene succinate
(PBS), a polymer with 1,4-butanediol as alcohol component. The latter can also be biogenic
so that PBS can be made entirely from renewable raw materials. Today, both succinic acid and
1,4-butanediol are still preferably produced petrochemically, but more and more capacities are
being established based on renewable raw materials. PBS is biodegradable and biocompatible.
Structurally similar dicarboxylic acids are the
unsaturated compounds, itaconic acid, which
milk products such as yoghurt and buttermilk, in
many fermented foods and in pickled cabbage. In
the human body, it is responsible for hyperacidity
of the muscles during extreme exertion. Lactic
acid has an asymmetric carbon atom and therefore occurs in two different enantiomers, l- and
d-lactic acid.
Today, approximately 0.5 million tons of lactic acid are produced per year, with almost the
entire production capacity based on fermentation. A number of substrates containing carbohydrates such as sugar, maize starch or molasses
serve as raw material sources for fermentation. In
addition to PLA synthesis, lactic acid and its salts
(so-called lactates) are mainly used in food technology as preservatives and acidity regulators.
Other important applications include pharmaceuticals, cosmetics and leather production.
Only biogenic lactic acid is used for the production of polylactic acid; PLA based on petrochemicals is not produced. In principle, there
are several synthesis options, although industrially, it is mainly one route that has prevailed
(. Fig. 19.11). Lactic acid is obtained as calcium,
sodium or ammonium lactate from the previously purified fermentation broth. The addition
of sulfuric acid subsequently releases lactic acid
resulting in considerable amounts of coproducts
(gypsum and calcium lactate).
Lactic acid is then prepolymerized to form
oligomers with molecular weights of about 1000–
5000 Da. These, in turn, are catalytically split
into lactide (also called dilactide because this is
the cyclic dimer of lactic acid), which is obtained
by distillation in high purity (bp. 285.5 °C).
Subsequently, it is converted into polylactic acid,
preferably with homogeneous transition metal
catalysts.
The “detour” via lactide is particularly useful because melt polymerization can be carried
out without a solvent. Also no water splits off
as a by-product which would otherwise have to
be separated during the polymerization process
in order to achieve high molecular weights. The
molecular weights of PLA thus reach values of
approx. 100,000 Da.
One of the most important properties of PLA
is its good biodegradability. However, PLA also
meets the relevant standards only under industrial composting conditions. PLA is therefore not
suitable for domestic compost.
19.2 · Biopolymer Representatives
19
Today, polylactic acid is one of the most
promizing biogenic plastics and is given a high
growth potential. It is already commercially
available in large quantities and relatively cheap
(approx. 2€ kg −1 ). Besides its biodegradability,
PLA is also biocompatible, which makes it interesting for medical applications.
PLA is able to replace established polyolefins
such as polyethylene, polypropylene or polystyrene in certain applications. This is because
established manufacturing processes can be used.
The most important fields of application are
short-life consumer goods such as food packaging, films and bags. Polylactic acid is also suitable for the manufacture of disposable tableware;
however, it is not very heat resistant. Hot coffee
in a PLA cup is not possible. An already established approach to compensate for the unfavourable properties of polylactic acid is the use
of mixtures with other biogenic polymers or
the production of composites. There are many
other applications for PLA, and many more will
undoubtedly be added in the future.
Other Biogenic Monomers
In addition to lactic acid, other biogenic polyester and polyamide monomers can be produced
from renewable raw materials, e.g. by fermenting glucose (7 Chap. 6) or from oleochemicals.
Dicarboxylic acids will first be presented because
they can be used for both polyesters and polyamides (. Fig. 19.12).
Succinic acid (1,4-butanedicarboxylic acid)
is the first to be mentioned. It is produced from
glucose or glycerol fermentation and CO 2 using
certain microorganisms isolated from rumen.
A first large-scale plant with an annual capacity
of 10,000 t was started up in 2014. Succinic acid
is converted into polyesters with diols and into
polyamides with diamines. The most important
product is the polyester polybutylene succinate
(PBS), a polymer with 1,4-butanediol as alcohol component. The latter can also be biogenic
so that PBS can be made entirely from renewable raw materials. Today, both succinic acid and
1,4-butanediol are still preferably produced petrochemically, but more and more capacities are
being established based on renewable raw materials. PBS is biodegradable and biocompatible.
Structurally similar dicarboxylic acids are the
unsaturated compounds, itaconic acid, which
milk products such as yoghurt and buttermilk, in
many fermented foods and in pickled cabbage. In
the human body, it is responsible for hyperacidity
of the muscles during extreme exertion. Lactic
acid has an asymmetric carbon atom and therefore occurs in two different enantiomers, l- and
d-lactic acid.
Today, approximately 0.5 million tons of lactic acid are produced per year, with almost the
entire production capacity based on fermentation. A number of substrates containing carbohydrates such as sugar, maize starch or molasses
serve as raw material sources for fermentation. In
addition to PLA synthesis, lactic acid and its salts
(so-called lactates) are mainly used in food technology as preservatives and acidity regulators.
Other important applications include pharmaceuticals, cosmetics and leather production.
Only biogenic lactic acid is used for the production of polylactic acid; PLA based on petrochemicals is not produced. In principle, there
are several synthesis options, although industrially, it is mainly one route that has prevailed
(. Fig. 19.11). Lactic acid is obtained as calcium,
sodium or ammonium lactate from the previously purified fermentation broth. The addition
of sulfuric acid subsequently releases lactic acid
resulting in considerable amounts of coproducts
(gypsum and calcium lactate).
Lactic acid is then prepolymerized to form
oligomers with molecular weights of about 1000–
5000 Da. These, in turn, are catalytically split
into lactide (also called dilactide because this is
the cyclic dimer of lactic acid), which is obtained
by distillation in high purity (bp. 285.5 °C).
Subsequently, it is converted into polylactic acid,
preferably with homogeneous transition metal
catalysts.
The “detour” via lactide is particularly useful because melt polymerization can be carried
out without a solvent. Also no water splits off
as a by-product which would otherwise have to
be separated during the polymerization process
in order to achieve high molecular weights. The
molecular weights of PLA thus reach values of
approx. 100,000 Da.
One of the most important properties of PLA
is its good biodegradability. However, PLA also
meets the relevant standards only under industrial composting conditions. PLA is therefore not
suitable for domestic compost.
19.2 · Biopolymer Representatives