Bioplastic blends: challenges and opportunities

Morphology and properties of polylactide and polyhydroxyalkanaotes blends

Objective of the thesis of Thibaut Gérard

The objective of this PhD work is to provide a systematic and detailed study of the morphology and properties of polylactide and polyhydroxyalkanaotes blends prepared in the molten state. Polylactide is derived from renewable resources such as corn or sugar and polyhydroxyalkanoates are produced by bacterial fermentation of sugar or lipids.

This work was performed in the frame of the Industrial Chair on Bioplastics, organised by CEMEF/MINES ParisTech and supported by Arkema, L’Oreal, Nestlé, PSA and Schneider Electric through the “Fondation des Industries Minérales, Minières et Métallurgiques Françaises”.


Melt mixing for PLA and PHA blends

Polylactide (PLA) and polyhydroxyalkanoates (PHA) are polymers made from renewable resources. Despite their growing competitiveness with conventional petrol-based polymers, they still have some disadvantages such as poor (fragile) mechanical properties. Blending is a potentially prospective way of obtaining new materials with improved properties, which can overcome the drawbacks of the pure components. 

In this work, PLA and PHA blends were prepared via melt mixing over the entire composition range. In particular, their rheological behavior, miscibility, morphology, mechanical properties and the influence of physical aging on their tensile properties were studied.

Nodular morphology of a PLA/PHA blend (weight ratio 70-30) observed by scanning electron microscropy


3 Research's Results

The results can be divided in several parts: 

1)     Properties of the initial components. The sensibility of PHA to thermal degradation was highlighted; this strongly influences the choice of processing parameters.

2)     Blends: miscibility and morphology. The importance of the interfaces between PLA and PHA was revealed with the rheological behavior of mixtures at low frequencies. The differential scanning calorimetry showed that the two polymers are immiscible. Two different types of morphologies (nodular and co-continuous) depending on blend composition have been observed by scanning electron microscopy and optical microscopy in reflection. An example of the morphology of PLA/PHA 70:30 blend is shown in the figure.

3)     Mechanical properties of blends and aging. The impact resistance and the tensile properties of the PLA/PHA blends with or without impact modifier was studied. The addition of impact modifier allows increasing the impact strength from 2 kJ/m² to 10 kJ/m². The addition of a small amount of dispersed phase in the initially brittle PLA matrix increases considerably its elongation at break. However, the obtained ductile properties decrease in time. The influence of the type and the amount of dispersed phase on aging of blends was studied; some dispersed phases allow keeping PLA ductile for several months. A qualitative model was proposed.

Enthusiastic for these promising polymers:

Thibaut Gérard is a doctor of MINES ParisTech since Aug. 29th, 2013. His thesis was focused on the preparation and the characterization of polymer blends made from renewable resources.

Thibaut obtained his chemical engineer diploma at the Ecole Européenne de Chimie, Polymères et Matériaux (ECPM) of Strasbourg in 2009. During his studies, he specialized in Polymer Materials and carried out several training periods in the research center of different big companies (Lafarge, BASF, Arkema). He found there his taste for research and decided to prepare a PhD thesis at Mines ParisTech work at the Centre de Mise en Forme des Matériaux [Center for Material Forming] (CEMEF) in Sophia-Antipolis.


Thibaut chose his PhD subject because of the numerous challenges in the development of new bio-based materials. He was very enthusiastic to work within the Industrial Chair in Bioplastics in order to progress in the knowledge of these promising polymers. During three years, he used many different experimental techniques to deeper understand the interaction mechanisms between polylactide and polyhydroxyalkanoates and to characterize their properties. He acquired new scientific, technical and managerial skills useful for his future job in the industry.