STUDY OF OXO-BIODEGRADABLE POLYETHYLENE DEGRADATION IN SIMULATED SOIL
L. B. Gomes1, J. M. Klein 1,R. N. Brandalise2,M. Zeni2,
B. C.Zoppas2,A. M. C. Grisa2
Rua Francisco Getúlio Vargas, 1130. CEP 95070-560 – Caxias do Sul/
1UFRGS - UniversidadeFederal do Rio Grande do Sul;
2UCS - Universidade de Caxias do Sul
ABSTRACT
This study aims to evaluate the influence of pro-oxidant additive and accelerated aging on the degradation of polyethylene (PE)films in simulated soil, in accordance with ASTM G160-03. Films with pro-oxidant additive were studied, before and after 72 hours of accelerated aging. The films were initially characterized by analysesof Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR) (to evaluate the Carbonyl Index (CI)). The films were exposed for 30, 60 and 90 days in simulated soil, with controlled moisture and soil pH. The results showed the degradation of polyethylene films through an increase of CI in samples with additive and accelerated aging after 30 days of exposure, and a decrease, after 60 and 90 days, indicating the uptake of material oxidation by-products by microorganisms. The polyethylene films without pro-oxidant additive after accelerated aging showed greater structural and surface modifications, as compared to films with the additive.
Keywords:accelerated aging, biodegradation, microorganisms, polyethylene,
pro-oxidant additives.
INTRODUCTION
Lately, there is a growing search for solutions to minimize environmental problems caused by polymeric materials, when disposed improperly. Researches have been conducted in order to control the lifetime of polymeric materials, or even accelerate its degradation after disposal. Light and temperature are the primary initiators for oxidation of PE’s containing photo and pro-oxidant. In the presence of oxygen these additives initiate the polymer degradation by producing free radicals that react with molecular oxygen and possibly the carbonyl and ketone groups in the oxidized polyethylene (1-4).
Every year, about 140 million tons of synthetic polymers are produced and 30% of this amount is used as packing (5,6). In Brazil, PE contributes with 48.9% of all the thermoplastic polymers discarded.It is used, in its various forms, in the manufacture of plastic bags and of packaging of various products. These products require a long time to decompose after use (7-9). In order to minimize the generation of waste that is inert to degradation, it stands the development of polymers of short duration, those with the possibility of having their degradation process accelerated with the incorporation of pro-oxidant additives (10). The pro-oxidant additives are based on transition metals such as nickel (Ni), cobalt (Co), iron (Fe), manganese (Mn) and cerium (Ce) and are responsible for accelerating the photo and thermal oxidation’s processes of polymers (abiotic degradation), preceding biotic degradation processes (11-14). There are many standards for assessing the biological degradation of polymeric materials in different ways. The standard ASTM G160-03, used in this study, defines the method to evaluate microbiological susceptibility of nonmetallic materials exposed to natural soil conditions (15). This study aims to evaluate the influence of the pro-oxidant additive and of the accelerated aging on the degradation of PE films in soil, in accordance with ASTM G160-03.
MATERIALS AND METHODS
The blue polymeric films,with and without pro-oxidant additive, were produced with high density polyethylene (HDPE), melt flow index (MFI) 0.35 g.10min-1, and linear low density polyethylene (LLDPE), MFI 0.71 g.10min-1, in accordance with ASTM D1238-04c and ASTM D1505-03 (16, 17).
The blue pigment was developed by Multicolor Indústria e Comércio de Pigmentos LTDA., MFI 25 ± 3 g.10min-1, concentration of 18.6% and thermal resistance 260°C.
The simulated soil, which was used to assess the degradation of the polymer samples by the action of microorganisms, has been developed in accordance with ASTM G160-03 standard(15).
The blue films were produced on asingle screw extruder, double fillet, with the ratio diameter/length (D/L) of 1.3, screw diameter of 45 mm, temperature profile of 142, 195, 195, 183 and 160 ºC, thickness of 5 µm, with and without pro-oxidant additive, PEOXand PE respectively (Tab. 1).
Tab. 1 – Formulation of polymeric films.
Code / HDPE(% wt) / LLDPE
(% wt) / Pigment
(% wt) / Additive
(% wt)
PEOX / 59.11 / 39.41 / 0.49 / 0.99
PE / 59.70 / 39.80 / 0.50 / -
The accelerated aging of the blue films was performed in accordance with
ASTM G154-00 (16) adapted, alternating for every 4 hours, condensing atmosphere at 40°C and ultraviolet (UV) radiation at 60°C, for 72 hours. It was used a UV chamber, produced by Comexim Matérias Primas Indústria e Comércio LTDA., with eight fluorescent lamps,mercury vapor (TL40W/12RS - PHILIPS), peak wavelength of 313 nm.
Each part of the simulated soil, black earth, grit sand and horse manure was mixed for 20 minutes on a Horbach concrete mixer (Figure 1).
(a) /
(b) /
(c) /
(d)
Fig. 1 – Simulated soil preparation: grit sand (a), grit sand and black earth (b), grit sand, black earth and horse manure (c) and final aspect of simulated soil (d).
The polymer samples were buried in polypropylene (PP) cups and placed in low density polyethylene (LDPE) greenhouse (Fig. 2).
(a) /
(b) /
(c)
Fig. 2 – PP cups with the simulated soiland the polymeric samples (a), greenhouse dispose (b) and greenhouse (c).
After 30, 60 and 90 days of simulated soil, the polymer samples were removed and washed in distilled water. The liquid from the washing process was centrifuged for 10 minutes in a centrifuge Fanem - Baby I Model 206 BL. The supernatant was discarded and 0.1 mL of the sediment was seeded with a platinum spatula onSabourauddextrose agar and incubated at 25°C for seven days in a bacteriological culture incubator, model 502, Orion, Fanem.
The identification of fungi present on the polymer films in the sediment seeded, was performed by macro and microscopic features with an Axiostar/Zeiss light microscope, magnification 100 and 400X. It was used as reference specific literature and taxons copies of the mycology collection of the Medical Mycology Laboratory of the Caxias do SulUniversity(19, 20).
The morphology of the samples before and after the simulated soil was analyzed by Scanning Electron Microscopy (SEM) SSX-550,Superscan, Shimadzu, with 15 kV accelerating voltage (after a deposition of carbon on the surface).
The films, before and after exposure to the process of simulated soil, were analyzed by Fourier Transform Infrared Spectroscopy (FTIR). The equipment used was a spectrophotometer Nicolet, Impact IS10 Transmission. Carbonyl Index (CI) of the samples was calculated in accordance with Eq. A, with A1 corresponding to the reading area of carbonyl ketone, ester and/or aldehyde bands and A2 corresponding to the reading area of PE characteristic bands (21).
CI = A1(1715 to 1740 cm-1) / A2(1475 cm-1) (A)
RESULTS AND DISCUSSION
The infra-red spectra of PE and PEOX films with and without degradation by accelerated aging and simulated soil are showed in Fig. 3. The structural analysis by FTIR of the samples of PEOX after exposure to accelerated aging made possible to identify the formation of a band between 1714 and 1740 cm-1, characteristic of C=O bond (carbonyl), especially for the PEOX samples after simulated soil and 72 h of exposure, indicating more than one oxidation product. These absorption bands were related to the vibrational stretching of ketone (1715 cm-1) and aldehydes and/or esters groups (1733 cm-1) (8, 21).
Fig. 3 - FTIR spectra of PE and PEOX films before and after 72 hours of accelerated aging and 30, 60 and 90 days in simulated soil.
For PEOX samples, it was observed an increase in CI 30 days after exposure in simulated soil, and a decreased after 60 and 90 days, attributed to the consumption of carbonyl groups by microorganisms, indicating polymer chain scission by Norrish type I degradation mechanism or by ester formation (14).
The microbiological tests performed before and after the 30, 60 and 90 days of simulated soil indicated the presence of microorganisms, characterized as colonies of: (a) Geothrichum spp, (b) Mucor spp, (c) Rhizopus spp (d) Trichoderma spp, (e) Aspergillus spp, (f) Aspergillus niger (Fig. 4).
(a) /
(b) /
(c)
(d) /
(e) /
(f)
Fig. 4–Fungi identified as (a) Geothrichum spp, (b) Mucor spp, (c) Rhizopus spp (d) Trichoderma spp, (e) Aspergillus spp, (f) Aspergillus niger(400X).
Morphological analysis of polymer films shows some important aspects used to describe the phenomenon of biodegradation of polymers, such as biofilm(22).In the samples of PE and PEOX exposed to simulated soil it was observed adherence and colonization of a complex mixture of microorganisms, surface erosion and presence of fruiting bodies and hyphae, characteristics that indicate the biodegradation of the polymer films (Fig. 5).
(a) /
(b) /
(c) /
(d)
Fig. 5 - SEM micrographs of the surface of the PE and PEOX samples after simulated soil, (a) and (b), hyphae formation, (c) complex mixture of microorganism and (c) surface erosion.
CONCLUSIONS
The degradation of PEOX films was observed after exposure in simulated soil, although the degradation was more evident by the action of accelerated aging promoted in the samples than by the action of the pro-oxidant additive, confirmed by the PEOX results without accelerated aging.
The variation of CI in the samples of PEOX during exposure to simulated soil is due to the action of microorganisms on the polymer structure that indicate degradation/biodegradation.
In the samples of PE and PEOX exposed to simulated soil it was observed adherence and colonization of a complex mixture of microorganisms, surface erosion and presence of fruiting bodies and hyphae, characteristics that indicate the biodegradation of the polymer films.
The addition of pro-oxidant additives, when combined with accelerated aging, may be an alternative to the spread of the oxidation of polyethylene, but extremely specific conditions were required for the uptake of oxidation by-products by microorganisms in simulated soil.
ACKNOWLEDGEMENTS
The authors acknowledge, Multicolor Indústria e Comércio de Pigmentos LTDA. and financial support from the Caxias do Sul University (UCS).
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