Mechanical properties of PLA

The mechanical properties of PLA, which have been extensively studied, are dependent on the production process and the amorphous or semi-crystalline state of the sample. Amorphous PLA presents a tensile modulus between 2.05 

The comparison of two grades of PDLLA, one for general purpose and the other one for injection, showed a possible difference in the modulus which can be due to a molecular mass discrepancy. Moreover, the comparison of the mechanical properties of oriented and unoriented films depicted an increase in the elongation at break with orientation. 

At room temperature, the mechanical properties of PLA are close to the one of PS but smaller than the one of PET (Table 8.5). Polyolefins present reduced stress at yield compared to PLA but the strain at break of LDPE and HOPE are much higher than the one of PLA. Compared to another biobased polymer, poly(hydroxybutyrate) (PHB), PLA shows better mechanical properties with higher modulus of elasticity and stress at yield. 

The influence of plasticizers on the mechanical properties of PLA has been extensively studied. Various plasticizers have been tested with PLA such as, glycerol, PLA oligomers, poly(ethylene glycol) monolaurate [98], 

Polymer Modulus of Elasticity (GPa) Stress at Yield (MPa) Strain at Break (%) 

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Epoxidized oils were also used to modify PLA Ali et ah (2009) reported that its use as a plasticizer to improve flexibility. Thermal and scanning electron microscope analysis revealed that epoxidized soybean oil is partially miscible with PLA. Rheological and mechanical properties of PLA/epoxidized soybean oil blends were studied by Xu and Qu (2009) Epoxidized soybean oil exhibited a positive effect on both the elongation at break and melt rheology. Al-Mulla et al. (2010b) also reported that plasticization of PLA (epoxidized palm oil) was carried out via solution casting process using chloroform as a solvent. The results indicated that improved flexibility could be achieved by incorporation of epoxidized palm oil. 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

Table 8.5 Comparison of mechanical properties of PLA to synthetic polymers.

The mechanical properties of PLA can be enhanced by orientation, generally in machine direction orientation with a draw ratio from 2 to 3, and in transverse direction with a draw ratio from 2 to 4 [178]. Especially toughness... 

Table 11.9 Thermal and mechanical properties of PLA plasticized with different citrate esters...

In general, the incorporation of the G5-glass particles in the polymer improves the flexural mechanical properties of PLA, modifies its degradation behavior, and induces interesting changes in the material surface morphology. 

Table 11.1 Mechanical properties of PLA (material datasheet by Biomer for L9000)...

The products can also be recycled after use for second time and they can even be hydrolyzed back into lactic acid which is the basic monomer. Nevertheless, the recycled lactic acid can be re-introduced into the polymerization process of PLA. The last possibility is to introduce PLA into the natural life cycle of aU biomass where it degrades into carbon dioxide and water. Thus, the diversity of PLA becomes obvious as it can be recycled and also decomposed like all other organic matter. Besides that, it will do no harm if burned in an incineration plant or introduced into a classical waste management system (Jacobsen et al. [1]). The typical values of mechanical properties of PLA are displayed in Table 11.1. 

Despite its origin form the nature, PLA s good stiffness and strength has enabled it to compete with other existing chemically based commodity plastics. Previous study on the mechanical properties of neat PLA by Jacobsen et al. [1] showed that PLA has great potential to be a substitute polymer for petroleum based plastics. The respective values of mechanical properties of PLA [2] with comparison of other petroleum based plastics e.g. polypropylene (PP) [3], polystyrene (PS) [4], high density polyethylene (HOPE) [5], polyamide (PA6) [6] shown in Fig. 11.2. 

Fig. 11.2 Mechanical properties of PLA and other commodity plastics, a Young s modulus, b Tensile strength, c Elongation at break...

In similar research, Huda et al. [34] evaluated the effect of the addition of silane treated and untreated talc as the filler on the mechanical properties of PLA/... 

Electrospinning of PLA and copolymers [12] and the PLA stereocomplex [145,146] has been used to prepare nanofibres or nanomats. Similar to the effect of pores on the mechanical properties of PLA-based materials, the formation of nanomats effectively lowers the E and elevates the 8b value. The morphology of the fibres and nanomats and the mechanical properties are affected by the applied voltage, the effluent rate and concentration of the solution, the type of solvent and so on (Figure 8.12 [147]). 

Takayama, T., Todo, M. and Tsuji, H. (2011) Effect of anneahng on the mechanical properties of PLA/PCL and PLA/PCL/LTI polymer blends. Journal of the Mechanical Behavior of Biomedical Materials, 4,255-260. 

Wan Y.Z., Wang Y.L., Li Q.Y., Dong X.H., Influence of surface treatment of carbon fibers on interfacial adhesion strength and mechanical properties of PLA-based composites, J. Appl. Polym. Sci., 80, 2001, 367-376. 

Mechanical Properties of PLA/PGA Nano-/Microfibrillar Polymer-Polymer Nanofibrillar Composites... 

Bledzki, A.K., Jaszkiewicz, A., and Scherzer, D. (2009) Mechanical properties of PLA composites with man-made cellulose and abaca fibres. Composites Part A, 40 (4), 404-412. 

Fig. 18.23 Selected mechanical properties of PLA-rayon composites with designed interphases...

Fig. 18.25 Selected mechanical properties of PLA (black) and its rayon composites with standard (blue) and weak green) interphase in comparison to glass fiber reinforced PP (red)...

A.L.M. Sis, N.A. Ibrahim, and W.M.Z.W. Yunus, Effect of (3-aminopropyl)trimethoxysi-lane on mechanical properties of PLA/PBAT blend reinforced kenaf fiber. Iranian Polym. J. 22(2), 101-108 (2013). 

Lactic acid bears an asymmetric carbon and is mainly produced by bacteria at nearly enantiopure (5) (l) configuration. The improvement of thermo-mechanical properties of PLA and more specifically of its heat resistance require the combination of pure P(l)LA and P(d)LA in so-called stereocomplexes.The production of the (R)-lactic acid was therefore studied, even if until now the strains producing this enantiomer are rarely reported and the production remains lower than for the (5) counterpart. ... 

Finally, the mechanical properties of PLA-electrospun mats obtained by using different solvents were studied by Liao et al In particular, fibres obtained from a solution of PLA and chloroform (CHCI3) were prepared by using a collector disc with a centrifugal field of 1800 rpm. The experimental conditions used in this case, indicated with (a) in Table 6.4, are voltage 15 kV, flow-rate 0.25 mL h and distance between the needle and the cylinder 200 mm. Other samples, indicated with [h] in Table 6.4, were obtained from a PLA solution with a solvents mixture and a collector disc with centrifugal field 1800 rpm, flow-rate 0.25 mL h , 30 kV and distance between the needle and the cylinder 200 mm. From these results, it is worthwhile to note that the solvent and its evaporation rate strongly influence the PLA-electrospun fibres behaviour. The best results were obtained when a mix of tetrahydrofuran and chloroform was used. 

Several elastomer matrices have been used to improve the mechanical properties of PLA, but special attention has been paid to polyurethane elastomer (PU) ° and natural rubber (NR). Lt is well known that thermoplastic polyurethane elastomers are widely used in a diverse range of implantable medical devices due to a unique combination of toughness, durability, flexibility, biocompatibility and biostability. Ln addition, it... 

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