Accelerating the degradation of polyethylene composite mulches

19 May 2017
Ying Luo, Xianming Dong, and Chaoqun Zhang
Films containing poly(methyl methacrylate)-modified titanium dioxide nanoparticles exhibit enhanced photocatalytic oxidation and biodegradation.

Polyolefins are widely used in agricultural applications (e.g., for greenhouse, mulching, or tunnel films). Indeed, 80% of plastic mulches are reportedly made from polyolefins (e.g., polyethylene).1, 2 Although this material can promote good harvests—by providing protection from insect pests and by creating suitable growth environments (e.g., in terms of soil temperature and moisture conservation)—the disposal of most polyolefin plastic mulches is problematic. The low cost and difficulty of recycling polyolefins means that the mulches are usually released directly to the surrounding environment at the end of their service life, but this can lead to serious waste and pollution problems (because polyolefins are non-degradable).3, 4 It is therefore highly desirable to develop an efficient, environmentally friendly, and degradable polyolefin material for sustainable use in soil environments.

Oxo-biodegradation (e.g., abiotic and biotic degradation) of polyethylene has previously been demonstrated as a promising route to solve this plastic pollution problem.3, 5–8 In this method, pro-oxidant additives (e.g., titanium dioxide, TiO2, nanoparticles) are introduced into the polyethylene matrix to accelerate the rate of abiotic degradation.9, 10 The pro-oxidants react—under UV irradiation—with water and oxygen to form hydroxyl radicals that can then initiate the degradation of polyethylene. As a result, the smaller hydrophilic molecular fragments can be further biodegraded in the presence of micro-organisms (e.g., bacteria, fungi, and algae). This whole process, however, is usually very slow and is limited by the formation of nanoparticle agglomerates.

As part of our ongoing efforts to design and produce novel degradable polyethylenes, we have investigated the use of poly(methyl methacrylate), PMMA, as a graft on the surface of TiO2 nanoparticles to hasten the polethylene photo-oxidation process.11 This hydrophilic coating promotes water adsorption and therefore more water is available for photo-oxidation. In addition, we have unexpectedly found that this hydrophilic modification of TiO2 nanoparticles improves their dispersibility and compatibility within the polyethylene matrix (thus resulting in further enhanced photocatalytic oxidation).

For our experiments, we used samples of pure low-density polyethylene (LDPE), as well as LDPE composites containing either TiO2 nanoparticles (TiO2/LDPE) or PMMA-modified TiO2 nanoparticles (TiO2-g-PMMA/LDPE). We performed photocatalytic oxidation of the films under ambient air in a UV lamp box. For each sample, we created three replicated discs and weighed them every 24 hours during a 415-hour irradiation cycle. We measured a 39.6% weight loss for the TiO2-g-PMMA/LDPE film, and a 96.5% reduction in average molecular weight, after 415 hours. These results are much better than for the pure LDPE sample (weight and average molecular weight reductions of 0.5 and 46.3%, respectively).

From scanning electron microscope (SEM) images of our films (see Figure 1), we observe several morphological features that suggest the extent of oxidation of the TiO2-g-PMMA/LDPE sample was much higher than that of the pure LDPE and TiO2/LDPE films (after 415 hours of UV irradiation). In particular, we find that the surface of the pure LDPE film is very smooth and unchanged by the irradiation. In contrast, the structure of the composite films was substantially destroyed by the irradiation process. We also see a number of cavities and cracks on the surface of the composite films, which are created by the escape of volatile products from the LDPE matrix. The cavities are larger on the TiO2-g-PMMA/LDPE surface than on the TiO2/LDPE film.


Scanning electron microscope (SEM) images of the low-density polyethylene (LDPE) film (a) before and (b) after UV irradiation for 415 hours. Images of irradiated LDPE composite films that contain (c) titanium dioxide (TiO2) nanoparticles and (d) poly(methylmethacrylate)-modified TiO2 nanoparticles are also shown (TiO2/LDPE and TiO2-g-PMMA/LDPE, respectively).11

In a subsequent part of our study, we aseptically transferred our irradiated films onto agar plates and inoculated them with selected fungal strains (previously isolated from soil and purified). We then incubated the samples at 28°C, and used SEM images (see Figure 2) to analyze colony growth after 30 days. The SEM images reveal profuse fungal growth on both of the composite films, but no colonization of the mycelia-forming fungus on the pure LDPE film. Furthermore, the more vibrant growth of mycelia on the surface of the TiO2-g-PMMA/LDPE composite film indicates more robust fungal metabolic activity than on the TiO2/LDPE sample.


SEM images of the pure LDPE, TiO2/LDPE, and TiO2-g-PMMA/LDPE composite films after fungal metabolic action.11

In summary, we have studied the use of PMMA-surface-modified TiO2 nanoparticles as additives to promote the degradation of polyethylene. We find that composite films made from LDPE and these modified nanoparticles exhibit much greater weight loss after UV irradiation than pure LDPE samples. In addition, we observe vibrant fungal growth on our irradiated composite samples, which can lead to further biodegradation of the films. In our future work, we plan to evaluate the impact of photo-oxidized and biodegraded LDPE fragments on the environment and on human health. We would also like to test the use of the resultant plastic mulches in the field, as well as investigate the scale-up of our approach.


Authors

Ying Luo
South China Agricultural University

Ying Luo, Xianming Dong, and Chaoqun Zhang are faculty members whose work is focused on biodegradable polyethylene films.

Xianming Dong
College of Materials and Energy, South China Agricultural University

Chaoqun Zhang
College of Materials and Energy, South China Agricultural University


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DOI:  10.2417/spepro.006909