Bio-based Polymers and Materials

NATHALIE BEREZINA AND SILVIA MARIA MARTELLI

1.1 Introduction

Biomaterials have gained attractiveness in the last decades due to both ecological and economic concerns. Increased pollution, and especially visible pollution, has first driven the scientific and industrial communities to look at biofragmentable and biodegradable substitutes for traditional petroleum-based non-biodegradable materials. Then the dramatic increase of oil prices before the economic crisis of 2007 influenced the move from the biodegradable to the bio-based. Finally, the compliance of the obtained materials with thermo-mechanical constraints has turned interest to the partially bio-based materials.

Bio-based materials can be obtained mainly by two different ways: the direct production of polymers or the production of bio-based monomers and their further (bio)chemical polymerization. The direct production of biopolymers can be achieved by microorganisms (polyhydroxyalkanoates, PHA), by algae (alginate etc.), by superior plants (pectin etc.) or by several types of producers, e.g. cellulose is produced by superior plants but also by bacteria, chitosan is produced by crustacean but also by fungi.

Whatever the producer of biopolymers, the main difficulty is to trigger its composition. Indeed, the obtained material has to comply with the thermomechanical constraints of its anticipated usages, and these characteristics are strongly related to the monomeric composition of the polymer and its size. This common problem of biopolymers does not have a unique solution. In the case of PHA, several microorganisms can produce the same polymer and the modification of the feeding substrates influences the monomeric composition of the polymers produced by the same microorganism, whereas important differences are found in the monomeric composition of the poly/ oligo-saccharides produced by different (micro)organisms. The regulation of the size of the biopolymers appears even more complicated; although PHA can be obtained with important masses, polysaccharides are actually mainly oligosaccharides. In any case, the main difficulty consists in making the (micro)organism perform the biosynthesis we want consistently and repeatedly.

To circumvent this problem, one can be tempted to make more controlled chemical polymerization with the bio-based monomers. Thus the production of bio-based monomers was also developed. However, the polymerization of bio-based monomers often asks for more development, as in the case of polylactic acid (PLA) and of polybutylene succinate (PBS); moreover, the thermo-mechanical needs for the expected applications are hardly reached with these polymers. Therefore, two more options can be foreseen: the production of partially bio-based materials (Sorona®) or the production of bio-monomers identical to the already existing and improved petroleum-based (ethylene, isobutylene, caprolactam etc.).

In this chapter we discuss the main biomaterials produced by these different methods as well as the achieved improvements and remaining bottlenecks in their production, modifications and applications.

1.2 Direct Production of Biopolymers

1.2.1 PHA

Polyhydroxyalkanoates (PHA) were discovered in 1926 by Maurice Lemoigne as energy storage materials in Bacillus megatherium and Bacillus mesentericus vulgatis. Still, they had to wait until the 1960s and for Cupriavidus genera (previously referred as Hydrogenomonas, Alcaligenes, Ralstonia and Wautersia) to be extensively studied. Indeed, the accumulation of the PHA by this genera has appeared to be more effective. Moreover, the first petrol crisis and further ecological issues increased the awareness and the interest in the bio-based materials.

Several PHA producing microorganisms as well as several types of PHA were discovered. The whole PHA family can be sub-divided into three main categories: the short-chain length PHA (PHASCL), the medium-chain length PHA (PHAMCL) and rarer PHA (Figure 1.1). The structural differences inside the PHA family imply deep differences in their thermo-mechanical properties. Thus, PHASCL mainly composed by polyhydroxybutanoates (PHB) and poly(hydroxybutanoate-co-valerate) (PHBV), are crystalline polymers, which are rather brittle and stiff, with high melting points (near 160–180 °C) and low glass transition temperature (between -5 and 0 °C), whereas the PHAMCL are thermoplastic elastomers with low crystallinity and tensile strength with high elongation to break (400–700%).

1.2.1.1 PHASCL

PHASCL are the most studied biopolymers among the PHA family. Numerous improvements of their production have been achieved during these last decades. These improvements mainly concerned the selection of wild-type strains (121 g L-1 of PHB was thus achieved using Cupriavidus genera), the engineering of strains (161 g L-1 of PHB was reported with E. coli (XL1-Blue) strain), the feeding strategy 'nutrient limited' versus 'nutrient sufficient' conditions, with the latter having been recently proved to be most efficient for the main producing strains (33 times productivity enhancement).

Also, the growth and the PHA accumulation on wastes and by-products have been paid important attention in recent years, in order to enhance the economic and sustainable efficiencies. Thus, different alternative substrates were tested – such as vinasse, oil palm frond juice, soybean oil, waste glycerol and other by-products from the biodiesel industry. Unfortunately up to now these strategies have not shown comparable productivities as an artificial carbon source (only 67.2 g L-1 of PHB were produced when soybean oil was used as a substrate).

Even if the Cupriavidus genera remains predominant in the production of PHASCL, other genera were also discovered and studied in recent years: Bacillus cereus, Brevundimonas vesicularis, Sphingopyxis macrogoltabia, Nostoc muscorum, Synechocystis sp., Herbaspirillum seropedicae, Haloferax mediterranei etc.

The important issues of the control of the biopolymer composition, the relative abundance of the 3-hydroxybutanoate (3-HB) and 3-hydroxyvalerate (3-HV), was also addressed by different feeding strategies, namely the choice of the...