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Comparison of biodegradability of various polypropylene filmscontaining pro-oxidant additives based on Mn, Mn/Fe or Co
Stphane Fontanella a, Sylvie Bonhomme a, Jean-Michel Brusson e, Silvio Pitteri d, Guy Samuel f,Grard Pichon g, Jacques Lacoste a,h, Dominique Fromageot a, Jacques Lemaire a, Anne-Marie Delort b,c,*aCentre National dEvaluation de Photoprotection (CNEP), Universit Blaise Pascal, 63 174 Aubire cedex, FrancebClermont Universit, UBP, ICCF, F-63000 Clermont-Ferrand, FrancecCNRS, UMR 6504, F-63177 Aubire, Francee Total Petrochemicals Research Feluy, Zone industrielle C, B-7181 Feluy, BelgiumdBasell Poliolefine Italia, 44100 Ferrara, ItalyfAssociation NEOSAC, F-43600 Ste Sigolne, FrancegGroupe Extrusion Soufflage de Ste Sigolne (E3S) F-43600 Ste Sigolne, FrancehClermont Universit, ENSCCF, ICCF, F-63000 Clermont-Ferrand, France
a r t i c l e i n f o
Article history:Received 26 October 2012Received in revised form28 December 2012Accepted 3 January 2013Available online 11 January 2013
a b s t r a c t
The biodegradability of two polypropylene films with low content of ethylene (a statistical copolymer(PPs) and a block copolymer (PPb) with balanced additions of phenolic antioxidant and pro-oxidantsbased on Mn, Mn/Fe or Co was studied. Abiotic pre-treatments by accelerated artificial photooxidationand thermooxidation representing about 3e4 years of outdoor weathering, including 3e4 months ofexposure to daylight and 3 years in soil were followed by FTIR and SEC measurements. When a controlledoxidation was reached in the films, they were inoculated, in a second step, with the strain Rhodococcusrhodochrous in mineral medium and incubated up to 180 days. The metabolic activity of bacteria wasevaluated by measuring ATP content, ADP/ATP ratio and cell viability. Complementary 1H NMR experi-ments were conducted on the incubation media, with and without cells, in order to monitor the con-sumption of soluble compounds excreted from the oxidized polymers by R. rhodochrous cells. The mainconclusions are that the Co derivatives (with Co content 150 ppm) must be considered toxic forR. rhodochrous. PP films containing pro-oxidants based on Mn and Mn Fe give positive results for thebiotest (low ADP/ATP ratio, post-development in Petri dishes). However the biodegradability of oxidizedPP films is less efficient in comparison to oxidized PE films (see paper published in this journal). Thisobservation may be correlated with the accumulation in the incubation media of oxidized oligomers thatcannot be metabolized rapidly by the bacterial cells and/or by the residual crystallinity of PP derivatives.
2013 Published by Elsevier Ltd.
The replacement of inert and non biodegradable materials bybiodegradable alternative is significantly increasing in hydro-biodegradable materials and oxobiodegradable polyolefin mate-rials. However the domain of oxobiodegradable materials ispresently essentially limited to HDPE, MDPE, LDPE formulations.Addition to PE films of pro-oxidant agents, basically transitionmetal salts or complexes inducing photo and thermal oxidation,area widespread technique largely used in industry. The fine
balance of antioxidant and pro-oxidant contents guarantees thatafter the preset period of service life, relatively fast abiotic oxida-tion begins. As a consequence, the material loses its mechanicalproperties and disintegrates into small fragments, providing ananswer to the problem of visual pollution by plastic litter that isconstantly in the center of public attention.
More importantly, the oxidized fragments of PE films wereproven to be slowly biodegradable as reported in extensive reviewson the topic [1e3]. The biodegradability of oxidized PE films hasbeen assessed by two different approaches: complex media (soil,compost, sludge) with microbial consortia, or pure microbialstrains in controlled mineral medium.
In complex media the biodegradability of PE containing pro-oxidant can be evaluated by a conventional approach, which eval-uates the mineralization percent based on CO2 formation. These
* Corresponding author. CNRS, UMR 6296 F-63171 Aubire, France. Tel.: 33 47340 77 14; fax: 33 473 40 77 17.
E-mail address: [email protected] (A.-M. Delort).
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier .com/locate /polydegstab
0141-3910/$ e see front matter 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2013.01.002
Polymer Degradation and Stability 98 (2013) 875e884
Author's personal copy
studies weremainly performed by the groups of Chiellini [4e6] andJakubowicz [7, 8]. Usually the microbial population was not iden-tified, except in the recent study of Jakubowicz in 2011 . Exten-sive information can be found in the review of Ammala .This conventional approach is the basis of some internationalstandards .
An alternative approach is to use pure strains: the most fre-quently studied microbial strains belong to the bacterial generaRhodococcus, Pseudomonas, Arthrobacter, Bacillus, Streptomyces or tothe fungi genera Phanerochaete, Penicillium. Aspergillus [1,3]. Forpure cultures some authors assessed the biodegradability of theoxidized PE films by visualizing the microorganisms by microscopy[9e12] or by measuring the change of crystallinity of thematerial .
In the approach developed by our group, the biodegradability ofoxidized particles is evaluated in controlled experimental condi-tions i.e. experiments with identified microbial strains in an aque-ous medium formulation with defined chemical compounds. Tomonitor both growth and development of microbial culturesadenosine triphosphate (ATP) and adenosine diphosphate (ADP)concentrations were assayed. ATP is the key molecule of all livingcell energetic metabolism. Its level reflects metabolic activity ofa culture. This test method is extremely sensitive and quantitative,and it allowed us to show that the cell populations in the presenceof the oxidized PE samples were in a better energetic state than thecontrol cultures that did not contain any polymer. It suggested thatthe cells were able to grow and to maintain their energetic statusover months [14,15]. This approach, using Rhodococcus rhodochrousATCC 29672 has been validated recently as the French AgreementAFNOR AC T 51-808 for the evaluation of oxidized PE materials .
Although the biodegradability of PE films containing pro-oxidants is now well documented, to our knowledge no paperhas been published on the biodegradability of polypropylene filmscontaining pro-oxidants.
The main goal of this work was to evaluate the biodegradabilityof polypropylene films containing different pro-oxidant additivesusing the same methodology based on ATP, ADP/ATP test (alreadysuccessfully applied to PE films); the idea is to check additive effi-ciency on both oxidation/fragmentation and biodegradation. Twotypes of PP matrices were examined: a statistically random PPcopolymer at low content of ethylene (PPs) and a block PPcopolymer at a low content of ethylene (PPb). Three different pro-oxidant additives PP1, PP2 and PP3 containing various metal salts(based on Mn/Fe, Co and Mn respectively) were tested and com-pared with additive free PPs and PPb used as reference samples.
The objective of the study was to compare these various filmsto determine which factors are more favorable for theirbiodegradability.
In addition to ATP content and ADP/ATP ratio measurements,cell viability was assayed.
Complementary 1H NMR experiments were conducted on theincubation media, with and without cells, in order to monitor themetabolism of soluble compounds excreted from the oxidizedpolymers by R. rhodochrous cells.
Moreover size exclusion chromatography (SEC) measurementshave been done on all the studied polymer samples initially, afterabiotic oxidation and after biodegradation.
2. Materials and methods
2.1. Tested materials
Statistical PP copolymers at low content of ethylene (PPs) weresupplied by Total Petrochemicals, block PP copolymers at lowcontent of ethylene (PPb) were supplied by Lyondell Basell. Bothsamples were modified with an iron based photo-inducer supply-ing radicals through a photooxidoreduction process, and/or witha cobalt or manganese based thermo-inducer, organometallic type,catalyzing the primary hydroperoxide decomposition, and werelater extruded into transparent films with thickness ranging from51 to 63 mm (Table 1).
To antagonize the prodegradant activity of both photo- andthermo-inductors during the first year of storage and use underindoor conditions, phenolic antioxidant was also added to theblends, extending the thermal-induction period at 60 C in the darkto over 400 h.
Metal contents in each film were determined by inductivelycoupled plasma/atomic emission spectroscopy (ICP/AES) with anOriba/Jobin Yv