332
Chapter 19 · Plastics from Nature - Biopolymers
19
Polyhydroxyalkanoates are still comparatively
expensive bioplastics, with a price of several
euros per kilogram. Substrate costs account for a
significant part of these costs, as e.g. glucose or
sucrose are high-value carbohydrates. In addition, only a fraction (approx. 25%) of the carbon
is converted into polyester at all: The majority is
used for the reproduction of the bacterium and
its metabolism. The use of carbon-containing
waste such as whey or wood hydrolyzates, for
example, could reduce costs.
Cork/Suberine
Cork is produced in industrially significant
amounts exclusively from the bark of the cork
oak (lat. quercus suber). It is briefly mentioned
here because its main component, so-called
suberine, is a natural polyester (. Fig. 19.9).
Suberine comprises about 50% of the composite material cork, among other typical
wood components, which we have already discussed in this book: Lignin, cellulose, hemicelluloses and other organic compounds.
Suberine is not structurally uniform and its
structure is not yet fully known. It has typical structural patterns which have an aromatic
character and, analogous to lignin, are attributable to phenylpropane subunits. In addition,
the structure of the suberine contains longchain fatty acids, dicarboxylic acids and polyhydroxycarboxylic acids, which are linked
by ether and lactone bonds and can also be
unsaturated.
As a natural polyester, suberine is not isolated
and is used exclusively in the form of cork. About
300,000 tons of cork are produced annually,
mainly in Portugal, which has the largest cork
oak forests in the world. The largest area of application for cork is still bottle closures, of which
about 13 billion are produced annually. Cork,
which has been known for about 3000 years, is
still the preferred material because of its special
properties: It is extremely elastic, light, thermally
stable and almost impermeable to gases and liquids. Due to an elevated proportion of tanning
agents, it also rots very slowly. Cork is also used
as an insulating material, for damping and floors.
In the latter applications, waste from cork processing can also be reused, or partially recycled
material can be incorporated, which is ecologically very advantageous.
and the incorporation of 4-hydroxyalkanoates)
depend on the type of bacterium and the carbon
source available.
The most important method to obtain PHA
is bacterial fermentation. In addition to this, it is
also possible to use genetically modified plants or
enzymatic catalysis. However, both variants are
not yet sufficiently developed to be competitive.
For example, fermentation is carried out with
the gram-negative rod bacterium alcaligenes eutrophus for the production of PHB. The yield is up to
90% PHB based on the dry mass of the cells. The
process takes place in two stages, either locally or
temporally separated: First, the bacterial population is increased in a first growth fermenter under
optimized conditions (oxygen and nutrient presence). Subsequently, the production of PHA under
limited nutrient conditions is initiated in a second
fermenter. After fermentation periods of one and
a half to two days, production rates of approx.
1–3 g per liter and hour are achieved.
The most common process for isolating PHA
from the concentrated fermentation broth is
solvent extraction. PHA is well soluble in warm
chloroform and insoluble in methanol so that
PHA can be obtained in high purities. However,
the use of these low-boiling and partially chlorinated solvents is toxicologically and ecologically questionable.
Pure PHB has similar properties to petrochemically produced polypropylene (PP), so
that the use of PHB in simple consumer articles such as shampoo bottles or carrier bags was
investigated. However, the high melting point
of this polyester is unfavourable for secure processing, for example, in injection molding. The
incorporation of longer side chains into the
polymer structure, for example, by 3-hydroxyvaleric acid, provides a copolymer, the so-called
polyhydroxybutanoate/polyhydroxy-valerate
(PHBV), with a lower melting point and lower
crystallinity. The proportion of comonomer can
be varied by selecting the bacterium used for
fermentation and the carbon source (glucose,
sucrose, starch, glycerol, etc.).
All PHAs show a common feature, they are biodegradable, formed by bacteria for storage, their
structures are best suited for bacterial degradation!
This can be done in the presence of oxygen, e.g. on
the compost (aerobic), or in the absence of oxygen
(anaerobic), e.g. underwater or on a landfill.
Chapter 19 · Plastics from Nature - Biopolymers
19
Polyhydroxyalkanoates are still comparatively
expensive bioplastics, with a price of several
euros per kilogram. Substrate costs account for a
significant part of these costs, as e.g. glucose or
sucrose are high-value carbohydrates. In addition, only a fraction (approx. 25%) of the carbon
is converted into polyester at all: The majority is
used for the reproduction of the bacterium and
its metabolism. The use of carbon-containing
waste such as whey or wood hydrolyzates, for
example, could reduce costs.
Cork/Suberine
Cork is produced in industrially significant
amounts exclusively from the bark of the cork
oak (lat. quercus suber). It is briefly mentioned
here because its main component, so-called
suberine, is a natural polyester (. Fig. 19.9).
Suberine comprises about 50% of the composite material cork, among other typical
wood components, which we have already discussed in this book: Lignin, cellulose, hemicelluloses and other organic compounds.
Suberine is not structurally uniform and its
structure is not yet fully known. It has typical structural patterns which have an aromatic
character and, analogous to lignin, are attributable to phenylpropane subunits. In addition,
the structure of the suberine contains longchain fatty acids, dicarboxylic acids and polyhydroxycarboxylic acids, which are linked
by ether and lactone bonds and can also be
unsaturated.
As a natural polyester, suberine is not isolated
and is used exclusively in the form of cork. About
300,000 tons of cork are produced annually,
mainly in Portugal, which has the largest cork
oak forests in the world. The largest area of application for cork is still bottle closures, of which
about 13 billion are produced annually. Cork,
which has been known for about 3000 years, is
still the preferred material because of its special
properties: It is extremely elastic, light, thermally
stable and almost impermeable to gases and liquids. Due to an elevated proportion of tanning
agents, it also rots very slowly. Cork is also used
as an insulating material, for damping and floors.
In the latter applications, waste from cork processing can also be reused, or partially recycled
material can be incorporated, which is ecologically very advantageous.
and the incorporation of 4-hydroxyalkanoates)
depend on the type of bacterium and the carbon
source available.
The most important method to obtain PHA
is bacterial fermentation. In addition to this, it is
also possible to use genetically modified plants or
enzymatic catalysis. However, both variants are
not yet sufficiently developed to be competitive.
For example, fermentation is carried out with
the gram-negative rod bacterium alcaligenes eutrophus for the production of PHB. The yield is up to
90% PHB based on the dry mass of the cells. The
process takes place in two stages, either locally or
temporally separated: First, the bacterial population is increased in a first growth fermenter under
optimized conditions (oxygen and nutrient presence). Subsequently, the production of PHA under
limited nutrient conditions is initiated in a second
fermenter. After fermentation periods of one and
a half to two days, production rates of approx.
1–3 g per liter and hour are achieved.
The most common process for isolating PHA
from the concentrated fermentation broth is
solvent extraction. PHA is well soluble in warm
chloroform and insoluble in methanol so that
PHA can be obtained in high purities. However,
the use of these low-boiling and partially chlorinated solvents is toxicologically and ecologically questionable.
Pure PHB has similar properties to petrochemically produced polypropylene (PP), so
that the use of PHB in simple consumer articles such as shampoo bottles or carrier bags was
investigated. However, the high melting point
of this polyester is unfavourable for secure processing, for example, in injection molding. The
incorporation of longer side chains into the
polymer structure, for example, by 3-hydroxyvaleric acid, provides a copolymer, the so-called
polyhydroxybutanoate/polyhydroxy-valerate
(PHBV), with a lower melting point and lower
crystallinity. The proportion of comonomer can
be varied by selecting the bacterium used for
fermentation and the carbon source (glucose,
sucrose, starch, glycerol, etc.).
All PHAs show a common feature, they are biodegradable, formed by bacteria for storage, their
structures are best suited for bacterial degradation!
This can be done in the presence of oxygen, e.g. on
the compost (aerobic), or in the absence of oxygen
(anaerobic), e.g. underwater or on a landfill.