Influence of the crystalline structure on the fragmentation of weathered polyolefines

doi:10.1016/j.polymdegradstab.2019.109012

Polymer Degradation and Stability

Volume 170, December 2019, 109012

https://doi.org/10.1016/j.polymdegradstab.2019.109012 Get rights and content

Highlights

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Fragment number, size and shape crated during the fragmentation of polyolefin films strongly depends on the crystalline structure of the polymer.
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As demonstrated by AFM force spectroscopy, by limiting the increase of surface rigidity water favors the crack propagation in semi-crystalline polymer.
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For PP and LDPE films, as detected by water contact measurement, oxidation begin in the first days of accelerated weathering but cannot be detected by FTIR spectroscopy because of its limited sensitivity.
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PP oxidizes faster than PE. No difference in term of recrystallization kinetic is observed between the two materials.

Abstract

Blown-extruded low density polyethylene (LDPE) and polypropylene (PP) films were submitted to accelerated weathering in water in order to identify their fragmentation mechanism and to compare the fragments number, size and shapes generated for long irradiation time. The study of the chemical modification during weathering was performed by following the progressive oxidation (FTIR, contact angle), increase of crystallinity (DSC) and surface rigidity (AFM) for the two polymers. It was demonstrated that although the kinetics of degradation is faster for PP than for LDPE films, the same fracture mechanism namely slow crack propagation is observed for both polymers. The analysis of the cracks structures on the surface of the films by AFM indicates that the crack initiation and propagation strongly depends on the crystalline morphology of the polymer. Indeed, for both material cracks propagation mainly occurs in the direction perpendicular to the extrusion direction explaining the elongated shape observed for the LDPE fragments. However, for PP film, the presence of spherulite structures induced the initiation of cracks in other directions (between the lamellae) leading to the formation of an increased number of fragments significantly smaller and with non-elongated shape compared to LDPE. To our knowledge this study is the first to directly link the size, shape distributions of plastics fragments to the crystalline morphology of the weathered polymers. These results point out the importance of taking into account the crystalline morphology to predict plastic fate in the environment.

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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Influence of the crystalline structure on the fragmentation of weathered polyolefines

Highlights

Abstract

Introduction

Section snippets

Materials and methods

PE and PP fragmentation

Conclusion

Declaration of competing interest

Acknowledgements

Mar. Pollut. Bull.

Water Res.

Chemosphere

Polym. Test.

Polym. Test.

Resour. Conserv. Recycl.

Polym. Degrad. Stab.

Polym. Degrad. Stab.

Surf. Coat. Technol.

Polymer

Polymer

Prog. Polym. Sci.