Properties of PHB

In spite of all these efforts to improve the mechanical properties of PHB and its copolyesters, their commercial application for production of plastic materials are still hindered, due to downstream processing cost and poor yields. 

Due to their fermentative synthesis, natural PHAs are strictly isotactic, featuring exclusively (/ )-configuration at the chiral stereocenter in the main chain. However, PHAs vary in their mechanical properties and can be grouped into two subcategories  

PHAscl (short chain length monomer unit consisting of up to five carbon atoms) 

PHAmcl (medium length side chains monomer unit has more than five carbon atoms) 

Polymer Melting point (°C) Glass transition (°C) Vicat A (°C) Young s modulus (GPa) Tensile strength (MPa) Strain (%) Impact strength (Izod) (J/m) Density (g/cm) 

Polymer Transparency Oxygen barrier (mL/m d bar) Water absorbtion (%) Food contact material B iodegradability  

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The mechanical properties of PHB and its copolymers have been studied extensively [82, 83]. As shown in Table 3, the material properties of PHAs can be readily controlled by adjusting the polymer composition during the fermentation [84]. 

In order to improve the mechanical properties of PHB or poly(3HB-co-3HV), many have reported on blending these biopolymers with other, both degradable as well as non-degradable, materials. However, due to the lack in compatibility between most polymers no substantial improvements in mechanical properties were reported upon, up to now [90]. 

The blending of different polymers is a frequently used technique in industrial polymer production to optimize the material s properties. The biodegradable polymer poly(3-hydroxybutyrate) (PHB) [45, 46], for example, which can be produced by bacteria from renewable resources, has the disadvantage of being stiff and brittle. The mechanical properties of PHB, however, can be readily enhanced by blending with another biopolymer, poly(lactic acid) (PLA) [47]. In order to prepare the optimum blend, it must be noted that the miscibility of different polymers depends on their concentration, the temperature, and their structural characteristics [48]. 

Table 3 repeats the textile physical properties of PHB and PLA fibers conditioned at room temperature for 3 months in relation to the original state. 

In addition, other properties of PHB can be used for example the excellent gas barrier effect, especially against CO2, and its biological efficiency for nitrate elimination during drinking water purification. 

Polymers exposed to the environment are degraded by their hydrolysis, mechanical, thermal, oxidative, and photochemical destruction, and biodegradation [7, 38, 55, 56]. One of the valuable properties of PHB is its biodegradability, which can be evaluated using various field and laboratory tests. Requirements for the biodegradability of PHB may vary in accordance with its applications. The most attractive property of PHB with... 

Other approaches to improve the properties of PHB is the production of derivatives based on PHB via the biosynthesis of copolyesters containing PHB units with other 3-hydroxyalkanoates units [19], such as poly [3-hydroxybutyrate-co-hydroxyvalerate] [PHBV] [20] or poly [3-hydroxybutyrate-co-3-hydroxyhexanoate] [21], with different molar ratios of hydroxycarboxylic acids. This approach has been investigated extensively [22] because it can... 

A good strategy to modify properties of PHB/PHAs in the solid state is blending the polyester with a second polymeric component as summarised in Table 24.3 [30]. This route has been broadly... 

Acetyl tributyl citrate -Weak positive influence on thermal properties of PHB in the melt. [40]... 

Figure 24.4 Mechanical properties of PHB (regarding the grades of PHAs from Metabolix). Adapted from [70].

A lack of mechanical properties of PHB composites is reported but not reflected in the use phase. 

PHB homopolymer, and to some extent even PHBV copolymers, have traditionally suffered from the excessively stiff and brittle nature arising from the relatively high crystallinity of the material. The mechanical properties of PHB and PHBV are said... 

Uefuji M, Kasuya K, Doi Y (1997) Enzymatic degradation of poly[(R)-3-hydroxybutyrate] secretion and properties of PHB depolymerase from Pseudomonas stutzeri. Polym Degrad Stab 58 275-281... 

A comparison of the physical properties of PHB and some of its copolymers with those of polypropylene (PP), and low density polyethylene (LDPE). Most data taken from [3]... 

In conclusion, these types of physical modifications can improve significantly the mechanical properties of PHB, although the extent of these changes is insufficient in terms of its long-term properties. 

Fig. 18.26 Selected mechanical properties of PHB and PHB-rayon composites with designed interphases...

Maiti P, Batt CA, Giannelis EP (2003) Renewable plastics synthesis and properties of PHB nanocomposites. Polym Mater Sci Eng 88 58-59 Maiti P, Batt CA, Giannelis EP (2007) New biodegradable polyhydroxybutyrateAayered silicate nanocomposites. Biomacromolecules 8 3393-3400... 

Some properties of PHB, PHBV copolyesters, and PHO are summarized in Table 12.3. PHB is a highly crystalline material, forming a right-handed helix when crystalline or in chloroform solution. PHBV copolymers with HV content up to 30% have HB units in the crystal lattice, and HV units are excluded. At HV content above 30%, HV occupies the crystal lattice, and HB is excluded. The copolymers have been shown to be random. 

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