bioplasticsMAGAZINE_0903

Basic
The Development
Poly Hydroxy
Fig.1 Microbial Cells containing PHA
Fig.2 PHA powder
Article contributed by
Dr. Jim Lunt,
V.P. Sales and Marketing,
Tianan Biologic,
Wayzata, Minnesota, USA
As discussed in preceding issues of bioplastics MAGAZINE -Poly Hydroxy
Alkanoates or PHA’s represent an emerging class of biopolymers
which are presently produced through the fermentation of natural
sugars, vegetable oils or fatty acids. These materials are unique in the
field of renewable resource based biopolymers in that they represent the
only class of polymers which are converted directly by microorganisms from
feedstock to the polymeric form - no additional polymerization steps being
required. The product in the form of microscopic granulates is extracted from
the microbial cells (Fig. 1) and used either as the powder (Fig. 2) directly or
converted to pellets for ease of shipping and handling. Also during the melt
conversion to the pellets, additives such as an antioxidant and nucleating
agent to accelerate crystallization, can be incorporated Although attracting
recent pilot/commercial scale attention by companies such as Tianan
Biologic, Telles, Meredian and others, interest in PHA’s has spanned many
decades. Today there are actually over 300 known microorganisms capable
of producing PHA’s [1] and over 10 monomer combinations that can produce
PHA’s with widely different properties. In terms of commercial interest- poly
3 hydroxy butyrate-co-valerate (PHBV)-Tianan Biologic, poly 3 hydroxy -co-4
hydroxy butyrate, (PHB)-Telles, and poly 3 hydroxy butyrate-co-hydroxy hexanoate
(PHBH) – Meredian, are probably the most well known polymers.
The History of PHA
The ability of micro-organisms to produce and store a PHA within their
cells was first observed by Beijerinck in 1888. He observed inclusions within
the bacterial cells but could not identify their structure. In 192 Lemoigne,
using Bacillus megaterium, identified the polymer to be poly 3 hydroxy
butyrate (PHB). It would appear little more was done in this area for another
30 years until in 198 McCrae and Wilkinson observed that bacteria stored
PHB in their cells. When the carbon to nitrogen ratio in the fermentation
medium was high and when the external carbon source was depleted [2],
they consumed the PHB as a food and energy source. From this point, the
fact that a biopolymer could be produced within a microbial cell, and become
a source of intracellular reserve material, created significant interest among
microbiologists and biochemists. The interest was still, however, essentially
academic. The primary focus was directed on understanding the polymers
significance on the functioning of the microorganisms and how external factors
affected the rate of production and re-utilization by the microorganisms.
Around 193, as oil prices climbed, this academic interest took on a more
practical focus. In 19, ICI in the UK began to investigate if PHB could be
commercially produced using glucose as the feedstock. They developed a
practically viable process but the economics were so completely unattractive
that this initiative was terminated and the technology was divested.
PHA Technology
The manufacture of PHA’S involves providing a microorganism a carbon
feed source such as dextrose or glucose along with suitable nutrients, such as
nitrogen, phosphorus or oxygen which encourage growth and multiplication of
3 bioplastics MAGAZINE [03/09] Vol. 4