c. CAPACIDAD DE USO MAYOR

PLA is a biopolymer produced from renewable sources with excellent mechanical properties and easy processing. PLA has the disadvantage that it is currently not recycled for reuse, but instead sent to composting. Being a polymer obtained from food sources, it is questionable to use it as a material for a single use. The ability to reuse and recycle PLA should have clear environmental, social and economical benefits.

The main application of PLA is in food packaging, making it necessary to have excellent resistance to hydrolytic degradation because it is in continuous contact with moist food. If resistance to hydrolytic degradation of the recycled material were much lower than that of the virgin material, the possible application of mechanical recycling would be questionable. Therefore, the overall object of this project was to study and compare the behaviour of the polymer in virgin and recycled state on the absorption of water and, therefore, the hydrolytic degradation.

The resistance to hydrolytic degradation in PLA was studied by FTIR analysis, as well as UV- Vis spectroscopy, thermogravimetric analysis, gravimetric analysis and viscosity measurements. When analysed by FTIR spectroscopy, interactions between the water and the polymer was established, it follows that there are four types of associations: very strongly bound water, strongly bound, weakly bound and very weakly bound (or free). The area of the band corresponding to hydration exhibited that, while the recycling process did not have a significant effect on the water absorption, the montmorillonite clay did.

UV-Vis spectroscopy of the liquid where the PLA samples were immersed showed the appearance of bands with maxima at 290, 248 and 228 nm. These bands revealed the existence of products such as oligomers of low molecular weight, lactide, and lactic acid, which are formed during the hydrolytic degradation. The UV-Vis measurements indicate that the absorbance increase with immersion time. Further, UV-Vis spectroscopy shows that the recycling process does not significantly affect the absorbance, nor does the presence of clay in the material. But due to the magnitude of deviations in the measurements, it's hard to conclude anything.

The gravimetric analysis of virgin and recycled materials showed that at short absorption times, water occupies the free volume and joins the polymer by hydrogen bonds and Van der Waals forces. However, the observed increase in absorption at longer times can be explained by the phenomenon of hydrolysis, resulting in the generation of hydrophilic species in the material, which make the absorption gradually increase.

The diffusion coefficients obtained by the Fickian fit of the water absorption were lower in the virgin and recycled materials than in the recycled material including the cleaning step. The diffusion coefficients were calculated to be 8.4·10-13, 1.3·10-12 and 7.1·10-12 m2/s, for the virgin, recycled and recycled, including the cleaning step, materials without clay respectively. For the materials with clay, they were calculated to be 3.4·10-13, 7.3·10-13, and 3.5·10-12 m2/s for PLAV- 302, PLAR-302 and PLARC-302 respectively. The water absorption at equilibrium does not show a significant difference between the three materials.

The measured intrinsic viscosity indicates that the immersion in water causes hydrolytic degradation in the material that results in a decrease of the average molecular weight. The degradation is much more pronounced in the material with montmorillonite clay than in the material without clay, which may be because the silicates in the clay act as catalysts for the hydrolytic degradation. The recycled material including the cleaning step has a slightly lower viscosity and therefore a lower average molecular weight than the recycled material and the virgin material, because of degradation during ageing, reprocessing and also the exposure to the alkaline medium in which it was cleaned. Furthermore, degradation is faster in the recycled material than in the virgin material, due to the previous degradation, favouring their hydrolytic degradation during immersion.

Thermogravimetric analysis indicates that the thermal stability decreases with immersion time due to degradation. Small differences in stability can be seen in the recycled material and the virgin material. However, recycling, including the cleaning step also show slight inferior thermal stability to the recycled material without the cleaning step. Clay present in the polymer will cause faster degradation and therefore reduced thermal stability compared to the material without clay.

acting as a barrier. Finally, recycling causes a slight degradation of the virgin material, and when the cleaning step is included it also causes a small degradation concerning the recycled material without the cleaning step, but the influence is not of high magnitude.

These results appear to show that the properties of the mechanically recycled samples of PLA depend on the steps included in the recycling process. When no demanding cleaning steps are included, the recycled material shows a behaviour very similar to that observed in the virgin material, thus indicating that this recycled material can be used in packaging. When a demanding cleaning step is included in the recycling process, the recycled material shows degradation and a decrease in the resistance to hydrolysis, although the worsening is not severe. These results support the feasibility of the mechanical recycling of PLA.

As a consequence of the results obtained during this thesis, ideas for future research has emerged related to the study of mechanically recycling PLA. First, it would be interesting to expand the information regarding hydrolytic degradation of PLA by employing additional techniques such as X-Ray Diffraction or Differential Scanning Calorimetry. Secondly, the effect of hydrolytic degradation on other material properties such as optical properties and mechanical stability could be characterised. Thirdly, the effect of temperature on the kinetics of hydrolysis could be studied by measuring at different temperatures below the glass transition temperature. Finally, it would be interesting to examine the effect that recycling has on the composting of PLA.