Environmental Engineering Reference
In-Depth Information
3.2.3. Blends
. An established method for changing plastic properties is through blending
with other materials. Several detailed investigations of blends containing poly(3HB) with
other biodegradable polymers have appeared. Such blends are physical mixtures of different
polymers; however, sometimes the two polymers react with one another, compatibilizing the
blend. Blends can be either homogeneous, forming a single thermodynamic phase, or
heterogeneous, comprising two or more phases. Blend properties are dependent, in turn, on
such phase behavior.
Miscible blends containing poly(3HB) have been formed with poly(ethylene oxide) [48,
11, 49], poly(vinyl alcohol), [50] PLA [51, 52], poly(ε-caprolactone-co-lactide) [53],
poly(butylene succinate-co-butylene adipate) and poly(butylene succinate-co-ε-caprolactone)
[54]. Immiscible blends are formed in mixtures of poly(3HB) with poly(β-propiolactone)[55,
13], poly(ethylene adipate) [55], poly(butylene adipate) [56], and poly(ε-caprolactone) [55].
For miscible blends of poly(3HB) with atactic poly(3-hydroxybutyrate), increasing the
weight content of the latter from 0-76 percent increases the elongation to break from 5-500
percent with an accompanying decrease in the Young's modulus and tensile strength [15]. For
the immiscible system of poly(ε-caprolactone) with poly(3HB), in contrast, the decrease in
Young's modulus and tensile strength is not accompanied by an increase in elongation to
break due to macroscopic phase separation [56]. These studies show the continuing
importance of physical property modification by blending as a valuable route towards
improving the properties and therefore increasing the utilization of bioplastics.
4. Research Priorities
PHA development is proceeding in a number of promising directions on both metabolic
engineering and chemical engineering fronts. Fortunately, most of these have the potential for
success both individually and in combination with others, such that no particular obstacle is
currently forming a bottleneck to further progress. The range of physical and thermal
properties achievable with PHAs is still expanding rapidly as new configurations of
copolymers and blends are explored. An on-going challenge will be the ability of the
metabolic engineers to keep pace with the discoveries of the materials scientists, enabling
microbes to synthesize the desired polymers both conveniently and inexpensively. These
efforts can be categorized as follows.
4.1. Investigation of Novel Polymers and Properties
Clearly, a number of modifications of PHA composition have the potential to improve the
plasticity, moldability, heat tolerance, and durability of the resulting plastics to approach
those of conventional thermoplastics. Because of the promising availability and flexibility of
routes to PHA synthesis, and because of increasing oil prices that will enable PHA polymers
to become increasingly cost-competitive, it is a valuable effort to explore the properties of
new PHA-based homopolymers, copolymers, and blends even before microbial pathways to
their syntheses are in place.
