Influence of the crystalline structure on the fragmentation of weathered polyolefines
Introduction
Due to their many interesting properties and low production costs, plastics are extremely popular materials. Unfortunately, their end of life is poorly managed at the moment and this material constitutes today one of the main environmental pollutants. As shown in a recent study [1], since the fifties almost 8300 million metric tons of virgin plastics have been produced. As of 2015 only, approximately 6300 Mt of plastic waste had been generated and 79% was accumulated in landfills or the natural environment. Apart from the aesthetic nature of marine litter pollution, it has also been reported to be the cause of injuries and other health concerns besides adverse economic and social impacts. It is known that plastic items gradually degrade into smaller pieces (meso-, micro- or even nanoplastics) through a combination of photo-degradation, thermal-degradation, mechanical erosion and the possible action of fungi or bacteria [2]. Microplastics with sizes < 1 mm are the most dominant type of plastics found in marine environments and are usually made of polyethylene or polypropylene [1].
Because fragmentation of polymeric material is a ubiquitous process present in many natural and technological processes, there is a body of literature dealing with fracture and fragmentation processes [3]. However, plastic fragmentation in natural ecosystems is not a single event but rather a continuously evolving process taking place in extremely dynamic media. As a consequence, dedicated fundamental research on plastic fragmentation under weathering leading to quantitative predictive tools should be a priority [4].
Until now most studies have focused on the abundance of microplastics to assess the risks associated with their presence in the environment [5]. However, these studies do not take into account the highly heterogeneous nature of semi-crystalline polymers at a microscopic scale. Indeed, polymers such polyethylene and polypropylene present a high heterogeneity due to the presence of aggregates of crystal lamellae connected by amorphous regions, these latter consisting of a network of non-extended entangled chains, loops, and tie-molecules [6]. Very recently, Gaillard et al. [7] have demonstrated the influence of this heterogeneity on the enzymatic erosion kinetic of biodegradable semi-crystalline polymers.
In a previous work we have shown that the fracture of weathered polyethylene films in air or in water media follow a crack propagation mechanism [8]. Since crack propagation is known to occur preferentially in the amorphous region [3], it seems clear that the fracture of polymer and the shape and number of fragments will strongly depend on the crystalline morphology of the polymer. In this paper we study the fragmentation mechanism of “additives free” LDPE and PP films weathered in water and try to link the shape, number and size of the fragments to the crystalline morphology of the initial polymer.
Section snippets
Materials and methods
Low density polyethylene (LDPE) and polypropylene (PP) 20–26 μm thick films were obtained by a blow-extrusion process using manufactured pellets (Alcudia PE-003 and HC101BF) that were, respectively, provided with the mention: does not contain any additives and contains no slip, antiblock, antistatic additives or nucleating additives. No additives could be detected either by Thermogravimetric Analysis (TGA). Differential Scanning Calorimetry showed that the LDPE and PP films were
PE and PP fragmentation
Whereas the LDPE and PP films have similar properties in term of thickness, semi-crystalline nature and fabrication process, the two polymers did not fragment with the same kinetics under accelerated weathering. Indeed, after several tests under the same weathering conditions, LDPE fragmentation onset is always greater than 30 days (≈155500 kJ/m2) while those of the PP was systematically less than 13 days (≈67392 kJ/m2). In addition, Fig. 2 shows that for the same weathering conditions (i.e.
Conclusion
In this work the fragmentation mechanism of LDPE and PP films, prepared by blown extrusion, and put under accelerated weathering in water was studied. Under the same weathering conditions, the chemical modification of the material (i.e. oxidation and crystallization) was followed. By combining water contact measurements and FTIR analysis it was demonstrated that the oxidation starts from the first days of weathering and that the oxidation of PP films was 1,49 to 1,75 times faster than for PE.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by the French National Research Agency ANR through NANOPLASTICS (ANR-15-CE34-0006-02) and BASEMAN (ANR-15-JOCE-0001-01) projects. We also would like to thank Nadine Auriault, Roxane Noblat and Anne Leclerc from CTTM (Centre de Tranfert et Technologie du Mans) for their help with extrusion process and Cécile Brault for her help in figure drawing. We would like to acknowledge the spectroscopy plateform of IMMM.
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