# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 3 Production of Bioplastics from Volatile Fatty Acids
### Physico-Chemical Properties of Bioplastics and Their Applications
#### 3.3.1 Polyhydroxyalkanoates (PHAs)  
Polyhydroxyalkanoates (PHAs) are a family of polymers that predominantly consist of linear polyesters derived from 3-hydroxy fatty acid monomers. In these polymers, the carboxyl group of one monomer forms an ester linkage with the hydroxyl group of another. The diversity of PHA polymers and their wide range of applications stem from variations in chain length and composition, as well as the potential to modify their reactive substituents. Within the PHA family, poly(3-hydroxybutyrate) [P(3 HB)] has been the most extensively studied due to its widespread occurrence. Additionally, P(3 HB) copolymers can be synthesized by co-functionalizing substrates, incorporating monomers such as 3-hydroxyvalerate (3 HV) or 4-hydroxybutyrate (4 HB) into the polymer chain. This enhances the material's properties and broadens its applications [135]. PHAs can be classified based on the number of carbon atoms in the monomer, resulting in the categories of scl-PHA, medium-chain-length PHA (mcl-PHA), and long-chain-length PHA (lcl-PHA). These correspond to 3-5, 6-14, and 15 or more carbon atoms, respectively [166]. To date, more than 90 distinct types of PHAs, comprising a variety of monomer units, have been identified and documented [167]. The following table (Table 4) provides an overview of the main types of PHAs that have been the subject of study, showing their main physio-chemical properties, applications, and other considerations such as their biodegradability. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**PHA Type** & **Description** & **Physio-Chemical Properties** & **Applications** & **Other Considerations** \\ \hline scl-PHA (Short-chain-length) & PHA with monomers of 3 to 5 carbon atoms & The monomer, P(3 HB) is highly crystalline [169] brittle and high melting temperature [135]. & Packaging, fiber, biomedical [168]. & High biodegradability [168]. \\ PHA (168) & [168]. & The monomer, P(3 HB) is highly crystalline [169] brittle and high melting temperature [135]. & Packaging, fiber, biomedical [168]. & High biodegradability [168]. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Overview of some types of PHAs that have been studied, together with a description of their characteristics and main applications. Scl-PHA exhibits a wide range of properties depending on their monomer composition. For example, poly(3-hydroxybutyraterate) [P(3 HB)] is highly crystalline, stiff, and brittle, with a tensile strength like polypropylene [191]. In contrast, poly(4-hydroxybutyrate) [P(4 HB)] is a flexible thermoplastic with tensile strength comparable to polyethylene [191]. Structurally, P(3 HB) forms a compact levorotatory helix and is optically active due to the R-configuration of the chiral center in its monomer unit. Its mechanical properties, such as tensile strength and Young's modulus, are like those of polypropylene [192]. Despite the considerable diversity of PHAs and the resulting heterogeneity of their properties, the following table (Table 5) illustrates a comparison of the physical properties of selected PHAs and non-biobased plastics, such as polypropylene (PP) and low-density polyethylene (LDPE), with data gained from references [161; 193; 194; 195; 196]. \begin{table}
\begin{tabular}{c c c c c} \hline
One of the possible substitutes to avoid dependence on fossil fuels in the manufacture of plastics is PHA, but PHA is currently not cost-competitive with fossil products [197]. PHA polymers have analogous physical and chemical properties to those of common petroleum-based plastics, such as PP or LDPE [193], as shown in Table 5.  
P(3 HB) was the first PHA to be isolated and characterized. Compared to other synthetic polymers, it has a higher barrier permeability than PE and PP. It is also stiffer and less flexible than PP. It has also good permeability barrier properties compared to polyethylene terephthalate (PET) and polyvinyl chloride (PVC). All these properties make it advantageous for use in various packaging applications compared to conventional polymers [198].