111
and van Loosdrecht 2004; Verlinden et al. 2007; Muralidharan et al. 2013), but has
inferior mechanical properties than petroleum-based plastics as it is brittle and has
a high crystallinity degree (Castilho et al. 2009; Visakh 2014). Nevertheless, polyhydroxyalkanoates have many advantages such as their tunable mechanical and
physical properties as well as a low environmental impact (Dietrich et al. 2017).
Moreover, copolymers of polyhydroxybutyrate and polyhydroxyvalerate are known
to be less permeable to oxygen than polyethylene and polypropylene (Salehizadeh
and van Loosdrecht 2004). In the food packaging field, this property is important as
it reduces the need for antioxidants.
The mechanical properties of polyhydroxyalkanoates vary significantly and can
be tuned based on the type of substrate, bacteria, and fermentation conditions used
to derive varying homo- and co-monomer units (Dietrich et al. 2017). Moreover, the
mechanical properties of a bioplastic are strongly connected to the average molecular weight of the polymer (Salehizadeh and van Loosdrecht 2004). The homopolymer polyhydroxybutyrate is highly crystalline (70–80% crystallinity), meaning it
has a low impact strength and low resistance to brittle failure (Salehizadeh and van
Loosdrecht 2004; Bugnicourt et al. 2014). It was also reported that polyhydroxybutyrate displays a high tensile strength with a value between 30 and 43 MPa (Verlinden
et al. 2007; Bugnicourt et al. 2014). On the other hand, polyhydroxybutyrate-covalerate has better mechanical properties than polyhydroxybutyrate due to an
increase in flexibility, toughness, and strength influenced by the addition of
hydroxyvalerate units in the polyhydroxyalkanoates polymer (Salehizadeh and van
Loosdrecht 2004). It was also discovered that the molar fraction of hydroxybutyrate
units increased the crystallinity and made the material less ductile because of the
short chain length of the polyhydroxybutyrate monomers (Li et al. 2016). Glass
transition temperature is found to be ranging from −52 °C to 4 °C. The melting
temperature was recorded around 175–177 °C, while the thermodegradation temperature is in the range of 227 °C and 256 °C (Bugnicourt et al. 2014; Tan et al.
2014). Pure polyhydroxybutyrate was found to have a very low resistance to temperature as it can decompose at room temperature (Bugnicourt et al. 2014). It was
also discovered that the existence of valerate in the polyhydroxyalkanoates chain
increases their thermal stability (Verlinden et al. 2007). Table 5.1 summarizes some
mechanical properties, temperature transition points, and crystallinity values of
various types of polyhydroxyalkanoates.
Polyhydroxyalkanoates have a molar mass that varies according to the producing
organism, growth environment, and extraction method employed (Bugnicourt et al.
2014). Usually, it is high in the range of 100–1000 kDa. However, the highest molar
mass achieved by polyhydroxyalkanoates was about 20,000 kDa and reported to be
produced by recombinant Escherichia coli (Castilho et al. 2009). However, the
chain length has a more significant role and affects many properties of the polyhydroxyalkanoates including melting temperature, glass transition temperature,
hydrophobicity, crystallinity, and mechanical strength (Castilho et al. 2009; Li et al.
2016). While the scl-polyhydroxyalkanoates are highly crystalline and display high
melting temperature, the mcl-polyhydroxyalkanoates have low melting temperatures and low crystallinity. In general, polyhydroxyalkanoates display low
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…
and van Loosdrecht 2004; Verlinden et al. 2007; Muralidharan et al. 2013), but has
inferior mechanical properties than petroleum-based plastics as it is brittle and has
a high crystallinity degree (Castilho et al. 2009; Visakh 2014). Nevertheless, polyhydroxyalkanoates have many advantages such as their tunable mechanical and
physical properties as well as a low environmental impact (Dietrich et al. 2017).
Moreover, copolymers of polyhydroxybutyrate and polyhydroxyvalerate are known
to be less permeable to oxygen than polyethylene and polypropylene (Salehizadeh
and van Loosdrecht 2004). In the food packaging field, this property is important as
it reduces the need for antioxidants.
The mechanical properties of polyhydroxyalkanoates vary significantly and can
be tuned based on the type of substrate, bacteria, and fermentation conditions used
to derive varying homo- and co-monomer units (Dietrich et al. 2017). Moreover, the
mechanical properties of a bioplastic are strongly connected to the average molecular weight of the polymer (Salehizadeh and van Loosdrecht 2004). The homopolymer polyhydroxybutyrate is highly crystalline (70–80% crystallinity), meaning it
has a low impact strength and low resistance to brittle failure (Salehizadeh and van
Loosdrecht 2004; Bugnicourt et al. 2014). It was also reported that polyhydroxybutyrate displays a high tensile strength with a value between 30 and 43 MPa (Verlinden
et al. 2007; Bugnicourt et al. 2014). On the other hand, polyhydroxybutyrate-covalerate has better mechanical properties than polyhydroxybutyrate due to an
increase in flexibility, toughness, and strength influenced by the addition of
hydroxyvalerate units in the polyhydroxyalkanoates polymer (Salehizadeh and van
Loosdrecht 2004). It was also discovered that the molar fraction of hydroxybutyrate
units increased the crystallinity and made the material less ductile because of the
short chain length of the polyhydroxybutyrate monomers (Li et al. 2016). Glass
transition temperature is found to be ranging from −52 °C to 4 °C. The melting
temperature was recorded around 175–177 °C, while the thermodegradation temperature is in the range of 227 °C and 256 °C (Bugnicourt et al. 2014; Tan et al.
2014). Pure polyhydroxybutyrate was found to have a very low resistance to temperature as it can decompose at room temperature (Bugnicourt et al. 2014). It was
also discovered that the existence of valerate in the polyhydroxyalkanoates chain
increases their thermal stability (Verlinden et al. 2007). Table 5.1 summarizes some
mechanical properties, temperature transition points, and crystallinity values of
various types of polyhydroxyalkanoates.
Polyhydroxyalkanoates have a molar mass that varies according to the producing
organism, growth environment, and extraction method employed (Bugnicourt et al.
2014). Usually, it is high in the range of 100–1000 kDa. However, the highest molar
mass achieved by polyhydroxyalkanoates was about 20,000 kDa and reported to be
produced by recombinant Escherichia coli (Castilho et al. 2009). However, the
chain length has a more significant role and affects many properties of the polyhydroxyalkanoates including melting temperature, glass transition temperature,
hydrophobicity, crystallinity, and mechanical strength (Castilho et al. 2009; Li et al.
2016). While the scl-polyhydroxyalkanoates are highly crystalline and display high
melting temperature, the mcl-polyhydroxyalkanoates have low melting temperatures and low crystallinity. In general, polyhydroxyalkanoates display low
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…