A contribution of understanding the stability of commercial PLA films for food packaging and its surface modifications

Poly(lactic acid) (PLA) is a biodegradable and renewable polyester, which is considered as the most promising eco-friendly substitute of conventional plastics. It is mainly used for food packaging applications, but some drawbacks still reduce its applications. On the one hand, its low barrier performance to gases (e.g. O2 and CO2) limits its use for applications requiring low gas transfer, such as modified atmosphere packaging (MAP) or for carbonate beverage packaging. On the other hand, its natural water sensitivity, which contributes to its biodegradation, limits its use for high moisture foods with long shelf life. Other biopolymers such as wheat gluten (WG) can be considered as interesting materials able to increase the PLA performances. WG is much more water sensitive, but it displays better gas barrier properties in dry surroundings. This complementarity in barrier performances drove us to study the development of multilayer complexes PLA-WG-PLA and to open unexplored application scenarios for these biopolymers. This project was thus intended to better understand how food components and use conditions could affect the performances of PLA films, and how these performances could be optimized by additional processing such as surface modifications (e.g. corona treatment and coatings). To that aim, three objectives were targeted: - To study the stability of industrially scale produced PLA films in contact with different molecules (CO2 and water) and in contact with vapour or liquid phases, with different pH, in order to mimic a wide range of food packaging applications. - To better understand the impact of some industrial processes such as corona or hot press treatments on PLA. - To combine PLA with WG layer to produce high barrier and biodegradable complexes. Different approaches coming from food engineering and material engineering were adopted. PLA films were produced at industrial scale by Taghleef Industries with specific surface treatments like corona. Wheat gluten films, coatings and layers were developed and optimized at lab scale as well as the 3-layers PLA-WG-PLA complexes. Different technologies able to mimic industrial processes were considered such as hot press, high pressure homogenization, ultrasounds, wet casting and spin coating. The physical and chemical properties of PLA films were then studied at the bulk and surface levels, from macroscopic to nanometer scale. The functional properties like permeability to gases (e.g. O2 and CO2) and water, gas and vapour sorption, mechanical and surface properties were also investigated. Exposed to CO2, PLA films exhibited a linear sorption behaviour with pressure, but the physical modifications induced by high pressure did not affect its use for food packaging. However, when exposed to moisture in both liquid and vapour state (i.e. environments from 50 to 100 % relative humidity (RH)), PLA was significantly degraded after two months at 50 °C (accelerated test) due to hydrolysis. This chemical deterioration was evidenced by a significant decrease of the molecular weight, which consequently induced a loss of transparency and an increase of the crystallinity. The hydrolysis was accelerated when the chemical potential of water was increased, and it was surprisingly higher for vapour compared to liquid state. In addition, pH did not affect the rate of hydrolysis. Knowing much better the limitation of PLA films, the challenge was to improve its functional properties by combining them with WG, as a high gas barrier bio-sourced and biodegradable polymer. The use of high pressure homogenization produced homogeneous WG coatings, with improved performances. This process was thus selected for making 3 layer complexes by assembly of a wheat gluten layer between two layers of PLA, together with corona treatment and hot press technologies. Corona treatment applied to PLA physically and chemically modified its surface at the nanometer scale. It induced an enhancement of the PLA surface tension and of its polar contribution, as well as its barrier properties of around 20 %. A strong influence of the hot press technology on PLA was also observed. Hot press induced further crystallization in PLA, increasing its overall barrier properties to water and oxygen of approximately 60 %. Hot press was also found to be a suitable technology for producing the PLA-WG-PLA complexes. It induced a particular restructuration in WG coatings, which generated a reduced adhesion between layers, but unexpectedly improved the barrier properties to gases of the complexes. The barrier properties of PLA-WG-PLA complexes to water were improved from 10 to 20 times compared to WG, depending on the RH differential, while those to O2 and CO2 were also improved of same magnitude compared to PLA. This improvement can be considered satisfactory, considering that the transfer rates were determined in realistic conditions for food packaging application (50 % RH), and were comparable to high barrier conventional plastics such as poly(ethylenterephtalate) (PET), polyamide 6 (PA 6) or rigid poly(vinyl chloride) (PVC). This project thus showed the real potential of biopolymer complexes such as PLA-WG-PLA as an eco-friendly and sustainable strategy for substituting conventional plastics for high barrier requirements for food products. These findings gave strong basis for going further on the processing aspects for industrial applications. Additionally the results of this project could motivate the investigations on multilayer complexes with PLA using other biopolymers/hydrocolloids with similar high barrier performances, coming from agro industrial waste or by products.