Barrier Properties of Biodegradable Bacterial Polyester Films - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 68

an improved processing behavior (Table 1), provides the possibility for technical use
in many applications. Copolymers then exhibited properties much closer to those of
LDPE.
Interesting information can be obtained from the comparison with properties of
various commercial biodegradable polyesters such as PLA (bio-resource based poly-
ester), and PCL, PEA, PBSA, PBAT polymers (petroleum-based polyesters) given in
Table 1. The density of all these polyesters is similar and above unity. Concerning the
glass transition temperature (Tg), most polyesters, including PHB copolymers, exhibit
values below the room temperature explaining the rubbery state of these polymers. An
exception can be made for PLA which presents Tg value higher than the room temper-
ature and thus leading to a glassy state to common temperature. That is why, polyesters
can be considered as replacement of materials made from polyethylene and PLA for
applications involving polystyrene. In addition, the melting temperatures of polyesters
are within a reasonable range allowing a melt transformation of polyesters in order to
tailor film materials. We can also note that the value of elongation at break for polyes-
ter materials is in relation with the glass transition temperature one: the highest value
is obtained for the lowest glass transition temperature as exhibited by PBAT and PBSA
polyesters in rubbery state. The same comment can be made for PCL polymer.
As observed in Table 1 and in the literature, the properties of PHB copolymers
can be adjusted by varying the hydroxyvalerate unit content. An increase of this con-
tent resulted in a decrease of the melt and glass transition temperatures, of the tensile
strength and of the crystallinity. The PHB copolymers are in general much more duc-
tile and elastic than PHB. The crystallinity value for PHB copolymers is thus within a
reasonable range common to thermoplastic polymers. Correlated with the crystallinity,
the decrease of the glass transition temperature involved an increase of the elongation
at break, even if the temperature value did not vary to a large extent. These results are
in relation with the morphology based on a co-crystallization between hydroxyvaler-
ate and hydroxybutyrate units which takes place inside PHB copolymers with a slow
crystallization rate involving thinner crystal lamellae than in PHB.

table 1. Physical and mechanical properties of some conventional polymers and some biodegradable
polyesters.

Glass
transition
temperature
(°C)

Tensile
strength
at break
(MPa)

Melting
temperature
(°C)

Young's
modulus
(MPa)

Crystallinity
(%)

Elongation
at break (%)

Density

PHB 1.25 175 4 60 3.5 0 5

PHBV 7 mol% HV
Monsanto - Biopol
D400G

1.25 153 5 51 900 - 15

PHBV 13 mol% HV - 157.3 0.3 - 1186 25 10

PHBV 20 mol% HV - 145 -1 - 0.8 20 50

LDPE 0.92 110 -30 50 0.2 10 600

PP 0.91 176 -10 50 1.5 38 400