**3.2. Biodegradability**

polypropylene and polystyrene [3] and products derived from its degradation process are

The monomer of PLA is lactic acid. Although this monomer can synthesize from petroleum, almost all lactic acid available on the market is produced by fermentation. During fermentation a suitable carbohydrate is converted to lactic acid by microorganisms without the presence of oxygen, hence, under anaerobic conditions. Fermentation of sour whey resulted in the

Lactic acid is the simplest α‐hydroxyacid that contains a chiral carbon atom and exists in the following two enantiomeric forms: L‐lactic acid and D‐lactic acid. Monomer forms a stable cycled dimer, that is, lactide. Consequently, dimer presents three different structures, namely L‐lactide, D‐lactide and DL‐lactide. Isotactic, optically active and crystalline homopolymers are obtained if either L‐ or D‐lactide dimers are polymerized. However, DL‐lactide or copoly‐ mers of L‐ and D‐dimers polymerize obtain atactic, nonactive optically and amorphous

Polymerization of this lactic acid is carried out by polycondensation [7], instead of polymeri‐ zation of the dimer that occurs by ring opening polymerization [8]. Polymerization started from lactide dimer allows to obtain high level of molecular mass due to a chain polymerization

Polylactides have a glass transition (*T*g) value around 60°C. This characteristic point refers to a change in the mobility of amorphous chains. Hence, atactic homopolymer shows a value of 60°C, but crystalline homopolymers that have some restriction in the mobility of amorphous phase could present *T*g values up to 70°C depending on the thermal treatment used for

Isotactic polylactides (pure PLLA and PDLA have same properties) crystallize forming a homocrystal, which melts in the range of 160–190°C depending on the molecular mass and shows a crystallinity fraction around 35% [9]. This value is calculated using one of the different values for theoretic melting enthalpy extrapolated from experimental analysis by different

Depending on the crystallization conditions, PLLA can crystallize in α, β or γ polymorphs [13, 14]. The most common form usually is the orthorhombic α crystal [15], while trigonal β form is obtained under high drawing conditions and high temperatures [16, 17]. Besides, γ poly‐

nontoxic for the human body and also do not leave any footprint in the landfills [4].

discovery of lactic acid in 1780, when it was isolated by C. W. Scheele [5].

mechanism and this is the mechanism that is normally used for production.

**2. Synthesis**

134 Composites from Renewable and Sustainable Materials

polymers [6].

**3. Polylactide characterization**

crystallization [9, 10].

**3.1. Physical‐chemical and mechanical characterization**

researches, being the most common values 93.6 [11] and 106 J/g [12].

Ester groups in polylactides allow hydrolytic degradation of polymer chains. The degradation mechanism depends on factors, which can be assigned to two groups: (a) related to material as molecular weight, crystallinity, comonomer structure, porosity, etc; and (b) related to the media: temperature, pH, solute concentration, enzymes, etc. [29].

For bulky materials, there are three kinds of degradation mechanisms: surface erosion, bulk erosion and core‐accelerated bulk erosion [30]. A surface erosion mechanism takes place when the hydrolytic degradation rate of the material surface in contact with water (containing catalytic substances as alkalis and enzymes) is much higher than the diffusion within the material. In contrast, a bulk erosion mechanism occurs when hydrolytic degradation takes place homogeneously, irrespective of the depth from the material surface. As it can be foreseen, the hydrolytic degradation mechanism changes from the bulk to surface erosion when material thickness becomes higher than the critical [31]. On the other hand, some authors report that polylactides degradation mechanism proceeds via core‐accelerated bulk erosion, when the material is thicker than 0.5–2 mm, due to the accelerated degradation sustained by oligomers and monomers trapped and accumulated in the core part of the materials [32]. Hence, depending of the thickness of the PLA piece, the degradation mechanism proceeds via bulk (<0.5–2 mm), core accelerated (between 0.5–2 and 74 mm) and surface erosion (>7.4 cm). In general, chains in the crystalline region are hydrolysis resistant compared to those in the amorphous regions because the access of water molecules to the chains inside the rigid crystalline regions is prohibited. Such crystalline regions are called "crystalline residues."

Concerning to enzymatic degradation, no study of specific enzymes for the biodegradation of polylactides has been reported [33]. Williams reported the enzymatic hydrolysis of polylacti‐ des in the presence of proteases as pronase, bromelain and proteinase K, being the latter a protease with a strong activity in hydrolizing proteins, particularly keratin [34, 35].
