Issue 03/2016

Basics
PHA – a polymer family with
challenges and opportunities
At the beginning of the 21 st century the chemical industry
undergoes an accelerated and revolutionary change in
the conversion from hydrocarbons to carbohydrates as
feedstock.
In 2016 it is still small (about 12 % of the chemical industry
is based on carbohydrates feedstock), but growing very fast.
New chemical platforms are being brought to the market, but
still have to prove themselves (like succinic acid, levulinic acid
and CO 2
as examples). Both industrial biotechnology (biocatalytic
conversion, fermentation, downstream processing)
and traditional chemo-catalytic conversion are applied
to convert renewable feedstock to useful chemicals and
polymers.
In this process one sees significant changes in the traditional
value chains for chemicals and polymers. Companies in the
wood, paper, potato, other-agricultural and sugar industries
with strong positions in carbohydrate feedstock and expertise
in industrial biotechnology started to diversify into these
traditional chemical value chains. Also companies active in
waste management (both solid waste, waste water and gas
effluents) work to upgrade the value of their waste streams
(CH 4
biogas, fatty acids, CO 2
and also waste cooking oil),
thus starting to set up after-use value chains for a circular
economy. A challenge at the start of it all is that:
Value chains combine competencies that have
never been associated before
Switching to carbohydrates as feedstock implies a
tremendous innovation promise for the chemical industry.
On the other hand it takes 15 – 20 years for new chemicals
or polymers to become very significant in size, since new
applications come one at the time, while drop-ins penetrate
much faster if they are cost competitive.
An industrial PHA polymer family platform is being
developed since about 25 years now. The platform consists
of a large variety of polymers, each with completely different
properties and based on all raw material sources mentioned
above. Figure 1 shows several PHA polymer examples.
The simplest member, PHB, and its building block 3HB have
apperared in nature for more than 3 billion years already and
are part of the metabolism of many organisms for energy
storage and nutritional value.
PHA products range from amorphous to highly crystalline
and go from high-strength, hard and brittle to low-strength,
soft and elastic, so there is a large property design space
for PHAs. In figure 2 a few differences between some PHA
products are illustrated. However, there are more than
hundred different known building block compositions for
PHAs.
The 3HA building blocks in PHA create sensitivity for
molecular chain scission starting at 160 °C and accelerating at
higher temperatures causing a loss of mechanical properties.
This limits the polymer melt temperatures for processing like
compounding, extrusion and injection moulding. There are
also 4HA building blocks, like 4HB and 4HV, which might have
a positive effect on this temperature sensitivity and so on the
polymer processing window, but that still is hypothetical at
this stage.
During the last decade large scale PHA manufacturing
plants have been built, varying in size between 5,000 and
50,000 tonnes/annum, but it has been troublesome to build
demand for them and to get them base loaded. In 2009
PHA capacity expansion plans for 2015 totaled 920,000
tonnes/annum for all players together, but global sales
volume was still about 1,000 tonnes/annum in 2013.
scl-PHAs P3HB, P4HB, PHBV, P3HB4HB, PHB3HV4HV.
CH 3
O
CH 3
O
CH 3 O C 2 H 5 O
O O
O
x
x
O
O
O y
x
P3HB P3HB4HB PHBV
y
mcl-PHAs PHBH, PHBO, PHBD.
CH 3 O C 3 H 7 O
PHBH:
O
O
x
y
lcl-PHAs Many varieties possible.
scl: short chain length
mcl: medium chain length
lcl: long chain length
In addition PHAs have been
designed with aromatic or C=C
groups in the side chain.
Figure 1:
The PHA products
platform is very diverse.
O
C 7 H 15
O
65
O
C 5 H 11
O
15
O
C 15 H 31
O
O
10
C 9 H 19
O
10
38 bioplastics MAGAZINE [03/16] Vol. 11