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Antagonism between transition metal pro-oxidants in polyethylene films

Melissa Nikolic a,*, Emilie Gauthier b, Karina George a,1, Gregory Cash b, Martin D. de Jonge c,Daryl L. Howard c, David Paterson c, Bronwyn Laycock a,b, Peter J. Halley b, Graeme George a

aCooperative Research Centre for Polymers, School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, 1 George Street,Gardens Point Campus, Brisbane Qld 4001, AustraliabCooperative Research Centre for Polymers, School of Chemical Engineering, The University of Queensland, Brisbane Qld 4072, AustraliacAustralian Synchrotron, 800 Blackburn Road, Clayton Vic 3168, Australia

a r t i c l e i n f o

Article history:Received 7 March 2012Received in revised form21 March 2012Accepted 22 March 2012Available online 30 March 2012

Keywords:PolyethylenePro-oxidantTransition metalPhoto-oxidationThermo-oxidationTiO2

a b s t r a c t

The oxidation of linear low density polyethylene (LLDPE) films containing the combination of pro-oxidants titanium (IV) dioxide (TiO2) with either cobalt (II) stearate (CoSt) or iron (II) stearate (FeSt)have been evaluated under accelerated photo- and thermo-oxidative conditions as well as on outdoorweathering. LLDPE containing only surface-compatibilised nano-TiO2 rapidly photo-whitens andembrittles at a low apparent extent of oxidation (as measured by carbonyl index) due to formation ofmicroscopic voids of w150 nm. When CoSt was also included in the film, antagonism occurred shown byembrittlement times longer by w90%, higher carbonyl index and absence of film whitening. In contrast,films containing TiO2/FeSt whitened during photo-oxidation and exhibited lower antagonism with only44% longer times to embrittlement and lower carbonyl index. Antagonism between pro-oxidants was notobserved under dark thermo-oxidative conditions. X-ray Fluorescence Microspectroscopy elementalmaps revealed that the TiO2 nanoparticles were spatially correlated with iron and cobalt metal ionsallowing scavenging of electrons and holes through cycling of the redox states of the metal withoutproducing radical species to initiate polymer oxidation. It is suggested that the antagonism differencesbetween TiO2/CoSt and TiO2/FeSt pro-oxidants is related to the respective reduction potentials of Co3þ/2þ

and Fe3þ/2þ and their effect on the UV conduction and valence band edges of the TiO2 particle. In theseways the photochemistry of TiO2 is suppressed and the photo-oxidative lifetime is governed by thechemistry of the transition metal pro-oxidant.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Linear lowdensitypolyethylene (LLDPE) thinfilm isused formanyapplications and is not easily degraded due to the inertness of thepolyethylene chemical structure. Polyethylene is traditionallydisposedof by collection followedby incinerationor burial in landfill.To augment degradation and to counteract litter, pro-oxidants havelong been researched in which transition metal ions are used topromote auto-oxidation through catalytic hydroperoxide decompo-sition [1]. These films are designed to degrade in the presence ofoxygen, heat and/or UV radiation into small low molecular weightfragments followed by slow evolution into CO2. These pro-oxidantcontaining films are referred to as oxo-degradable althoughrecently it has been demonstrated that the residues after prolonged

oxidation may be assimilated by microbes and the term oxo-biodegradable has often been used [1]. Of particular interest is theapplication of thin oxo-degradable polyolefin films in agriculture toshorten the crop growing period and conservewater [2]. The tailoreddesign of such films alleviates the need for plastic collection anddisposal at the end of their useful lifetime, whilst maintainingoptimal mechanical and barrier properties during their use. In agri-cultural applications, the challenge is to achieve degradation bothabove andbelow the ground ina timeframecompatiblewith the cropgermination, growth and harvest cycle. For the degradationrequirements for LLDPEfilm to be achieved for this application, quitehigh concentrations of typically more than one catalyst type arerequired. In this study we have investigated oxo-degradable LLDPEfilms containing the photo-catalyst titanium (IV) dioxide (TiO2)together with hydroperoxide decomposition catalysts, cobalt (II)stearate (CoSt) and iron (II) stearate (FeSt).

As shown in Equations (1)e(3), transition metals such as cobalt(II) and/or iron (II) are redox catalysts for hydroperoxide decompo-sition (particularly under dark thermo-oxidative aging conditions)

* Corresponding author. Tel.: þ61 7 3138 1108; fax: þ61 7 3138 1804.E-mail address: [email protected] (M. Nikolic).

1 Current address: Queensland Eye Institute, 41 Annerley Road, South Brisbane,Qld 4101, Australia.

Contents lists available at SciVerse ScienceDirect

Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

0141-3910/$ e see front matter Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.polymdegradstab.2012.03.036

Polymer Degradation and Stability 97 (2012) 1178e1188


[3e10] and will be powerful prodegradants since the overall ratelimiting step in oxidation is the decomposition of hydroperoxides(POOH) [11]. It has been shown that several transition metals,incorporated as the stearate into the polymer matrix during pro-cessing, increase the rate of POOH decomposition by orders ofmagnitude, and consequently accelerate the overall rate of poly-ethylene oxidation [3e5,12e16].

M2þ þ POOH/M3þ þ PO� þ HO� (1)

M3þ þ POOH/M2þ þ PO�2 þ Hþ (2)

Overall

2POOH/PO� þ PO�2 þ H2O (3)

A well-known commercially available photoactive form of nano-TiO2, such as Degussa P25, can be used to accelerate the photo-oxidation of polyethylene [17]. Degussa P25 TiO2 is a mixture ofcrystalline structures that isapproximately80%anataseand20%rutilewith a primary particle size of 21 nm [18,19]. The anatase crystallinestructure is themostphotoactive formof TiO2, due to the lower rate ofelectron-hole recombination compared to rutile and brookite crys-talline forms and also due to the higher aptitude for anatase to photo-adsorb and photo-desorb oxygen [20,21]. Fig. 1 illustrates theheterogeneous anodic and cathodic processes that occur once a TiO2particle is illuminatedwith a photon of light with energy higher thanor equal to the band gap energy (3.2 eV) [22,23]. Once illuminated, anelectron is ejected from the valence band (vb) to the conduction band(cb), creating a hole (hvb

þ) at the vb and an electron at the cb (ecb� )which may recombine (Equation (4)), or react with donor (D) (Equa-tions (7 and 8)) or acceptor (A) species (Equation (9)) [22].

Equations (4)e(15) demonstrate the sequence of heterogeneousphotocatalytic reactions that occur at the surface of a UV irradiatedTiO2 nanoparticle [22].

TiO2 /hy

TiO2

�e�cb; h

þvb

�/recombination (4)

TiO2

�hþvb

�þ H2Oads/TiO2 þHO�

ads þ Hþ (5)

TiO2

�hþvb

�þHO�

ads/TiO2 þ HO�ads (6)

TiO2

�hþvb

�þ Dads/TiO2 þ Dþ

ads (7)

HO� þ Dads/Doxid (8)

TiO2

�e�cb

�þ Aads/TiO2 þ A�

ads (9)

Oxidative pathway

TiO2

�e�cb

�þ O2ads þ Hþ/TiO2 þ HO�

24O��2 þ Hþ (10)

HO�2 þ TiO2

�e�cb

�þHþ/H2O (11)

2HO�2/H2O2 þ O2 (12)

H2O2 þ O��2 /HO� þ O2 þHO� (13)

H2O2 þ hy/2HO� (14)

H2O2 þ TiO2

�e�cb

�/HO� þHO� (15)

Ohtani et al. [24] have described the mechanism for the photo-oxidation of polyethylene in the presence of TiO2, whereby a photo-generated hydroxyl radical abstracts a hydrogen from the polymer,as shown in Equation (16). The resulting carbon-centred polymerradical then reacts with O2 and an auto-oxidation mechanismdescribed in Equations (16)e(21) follows:

dCH2CH2CH2dþ O,H/dCH2C, HCH2dþ H2O (16)

dCH2C, HCH2dþ O2/dCH2CHðOO,ÞCH2d (17)

dCH2CHðOO,ÞCH2d

þdCH2CH2CH2d/dCH2CHðOOHÞCH2d

þdCH2_CHCH2d (18)

dCH2CHðOOHÞCH2d/dCH2CHðO, ÞCH2d

þ O,H/dCH2COCH2dþ H2O (19)

dCH2COCH2dþ O2/TiO2;hv intermediates such as HCOOH;

CH3COOH; CH3CH2OH and CH3CHO ð20Þ

/TiO2;hv chain scission with CO2 evolution (21)

Fa et al. [17,19] have reported that unmodified hydrophilic TiO2nanoparticles agglomerate on the micrometer scale within thehydrophobic polyethylene matrix, thereby reducing the photo-activity of the TiO2 due to a reduction in the interfacial areabetween the TiO2 particle and the polyethylene. Accordingly theyhave dispersed the TiO2 in an oxidised polyethylene wax, aidingdispersion and compatibilisation of the TiO2 in the polyethylenematrix. Well dispersed Degussa P25 TiO2 within a polyethylenematrix results in filmwhitening during photo-oxidation, due to theformation of microscopic voids which scatter light [24].

Fig. 1. Heterogeneous processes that occur when a TiO2 particle is illuminated witha photon of energy greater than or equal to the band gap energy [22]. Reproduced withpermission from Elsevier.

M. Nikolic et al. / Polymer Degradation and Stability 97 (2012) 1178e1188 1179


Whilst TiO2 has been shown to be a highly effective photo- pro-oxidant [17,24,25], it has been shown to be very slow in oxidisingLLDPE under dark thermo-oxidative conditions [26e28]. Thecombined impact of photo-oxidation by TiO2 and the thermo-oxidation of transition metal stearate pro-oxidants in poly-ethylene are required for a range of applications, including croppropagation film; therefore the most obvious solution has been toinclude both pro-oxidants (i.e., TiO2 and CoSt or FeSt) together ina polyethylene film. However, the impact of these pro-oxidantcombinations on the photo- and thermo-oxidation of poly-ethylene has not been previously investigated and we report hereconditions under which efficiency is dramatically decreased i.e.combinations are antagonistic compared to the individual pro-oxidants.

2. Materials and methods

2.1. Materials

A resin blend was used to form a base polyethylene matrixwithout pro-oxidant which is suitable for agricultural applicationsas thin film. This mixture comprised two different LLDPE resins(Dow Plastics), a low amount of LDPE (Qenos) and a low molecularweight PIB as a tackifier (Daelim Corporation).

Cobalt (II) stearate and iron (II) stearate were both supplied byAlfa Aesar. The Aeroxide Degussa P25 TiO2 was supplied by EvonikAustralia Pty. Ltd. with a crystalline structure that is approximately80% anatase and 20% rutile, and an average primary particle size of21 nm [18].

2.2. Polymer processing

Prior to blowing, the base resin masterbatch and pro-oxidantwas homogenised by physical mixing, followed by passingthrough a 40 L/D twin, co-rotating Entek extruder with 27 mmdiameter screws. The maximum temperature was 200 �C and thescrew speed was 50 rpm giving a residence time between 3.75 and4.0 min. The extrudate was passed through a hot, die-faced pel-letiser running at 200 rpm producing pellets of approximately5 mm diameter. A total of 300 g of pelletised material was thenadded to a 25 L/D Axon BX25 extruder fitted with a blowing die(215 �C) of 40 mm in diameter and associated tower. The single,25 mm diameter Gateway screw had several cut flights towards theexit end and was run at 28 rpm. The blow-up ratio was a maximumof 3. Table 1 describes the formulation code that corresponds to theconcentration of pro-oxidant in the final film formulation.

Prior to extrusion of films containing Degussa P25 TiO2, the P25was mixed with Sigmacote�, an organosilane obtained from SigmaAldrich, in a weight ratio of 3.0:2.4 and stirred in hexane to renderthe surface hydrophobic and improve its compatibility within thepolyethylene matrix. The solution was left to dry overnight andthen placed under vacuum to remove the residual solvent. MobilDTE heavy oil (0.5%) was added during the extrusion of TiO2formulations to assist the binding of P25 to the resins. The filmthickness of all films was 12 � 2 microns. The films containingorganosilane-treated P25 were transparent, indicating gooddispersion of the nano-TiO2 in the film.

2.3. Accelerated photo-oxidative aging

Polyethylene film samples were mounted onto polystyrene35mm slide holders and exposed to UV light using an Atlas SuntestCPS þ weathering chamber, fitted with a Xenon lamp source andboth coated quartz and solar standard filters. This filter combina-tion was used to simulate the global solar radiation outdoors. The

samples were exposed to a total energy across the spectral range of300e800 nm of 765W/m2, during a continuous 48 h light cycle andblack panel temperature in the range of 49 � 2 �C.

2.4. Accelerated thermo-oxidative aging

After film blowing, duplicate samples of each film formulationwere mounted onto polystyrene 35 mm slide holders and wereaged under ‘dark thermo-oxidative’ conditions in a Conthermdigital series fan-forced oven, maintained at 60 �C, under condi-tions of 100% humidity by being enclosed in desiccators with thebase filled with 20 mL of MilliQ water. Samples were withdrawnevery 48 h and evaluated for film embrittlement and analysedusing FTIR-ATR as described in Section 2.7. It was necessary to blot-dry samples with a lint-free tissue prior to FTIR-ATR analysis, toremove residual water droplets from the surface of the film. Thecarbonyl index (CI) was calculated as described in Section 2.7.

2.5. Outdoor weathering

Two outdoor weathering trials were conducted at Pinjarra Hills,Qld, Australia. The first was a six week study, investigating thechanges in mechanical properties of LLDPE film containing 1 wt%TiO2 pro-oxidant (1Ti/0Co/0Fe) and a control with no pro-oxidantwhich were exposed above-ground on soil. A weather station(Campbell Scientific Inc.) was installed at the trial site with a datalogger, equipped with temperature and humidity probes and a raingauge. A pyranometer (Middleton SK08) was used to measure thetotal solar global radiation across the spectral range 305e2800 nmand recorded with a data logger. The films were exposed above-ground on an organic commercial garden soil (Table 2) and wereremoved weekly to measure the change in the tensile mechanicalproperties in the film (Section 2.8) using an Instron tensile testingmachine.

The second study involved the above-ground exposure of 6 filmformulations: control, 1Ti/0Co/0Fe, 0Ti/1000Co, 1Ti/1000Co, 0Ti/1000Fe and 1Ti/1000Fe. The films were monitored every 2 days, forfilm whitening and embrittlement. The embrittlement pointresulted in fragmentation upon a gentle finger tap, equivalent to an

Table 1Concentrations of pro-oxidants in the final film formulations.

Formulation code TiO2 wt% [Co2þ]a ppm [Fe2þ]b ppm

Control e e e

0Ti/200Co e 200 e

0Ti/1000Co e 1000 e

0Ti/1000Fe e e 10001Ti/0Co/0Fe 1 e e

1Ti/200Co 1 200 e

1Ti/300Co 1 300 e

1Ti/400Co 1 400 e

1Ti/600Co 1 600 e

1Ti/800Co 1 800 e

1Ti/1000Co 1 1000 e

1Ti/1000Fe 1 e 1000

a Incorporated as cobalt (II) stearate.b Incorporated as iron (II) stearate.

Table 2Properties of the commercial garden soil used in outdoor weathering trials.

Texture Coarse sandy clay loam

Colour GreypH 7.2Total organic carbon (%) 2.1Organic matter (%) 4.5

M. Nikolic et al. / Polymer Degradation and Stability 97 (2012) 1178e11881180


elongation at break in the film of 5% or less [29]. The total days,solar global radiation, rainfall and average daily temperature foreach formulation up until the embrittlement point was recordedand the carbonyl index was measured at embrittlement using FTIR-ATR.

2.6. Ultravioletevisible (UVeVis) spectroscopy

Many of the films containing P25 nano-TiO2 whitened duringUV exposure and the method described by Ohtani et al. for char-acterising polyethylene film whitening was used [24]. Thepercentage of light transmittance across the spectral range of200 nme700 nm through each film before aging and at embrit-tlement after photo-oxidation was measured with a Cary 50 ProbeUVevisible Spectrophotometer. The percentage of light trans-mittance at wavelength 585 nm was used as a benchmark tocompare the degree of whitening between formulations.

2.7. Fourier transform infrared e attenuated total reflectancespectroscopy (FTIR-ATR)

IR spectra from 4000 to 525 cm�1, were collected using a Nicolet870 Nexus FTIR spectrometer equipped with a Smart Endurancesingle bounce diamond-window ATR for 32 scans, 4 cm�1 resolu-tion, a gain of 8 and a mirror velocity of 0.6329 cm/s. After initialacquisition using OMNIC software (Thermo-Nicolet, Madison, WI),spectra were manipulated and plotted using a GRAMS/32 softwarepackage (Galactic Corp., Salem, NH). The measurement time foreach spectrum was approximately 60 s. The carbonyl index (CI)peak was measured as the height of the C]O stretching peak at1714 cm�1 divided by the CeH peak height at 1463 cm�1.

2.8. Tensile testing

Film samples were cut into 25� 22mm strips with the long axisin the transverse direction. Analysis was performed on an Instron5543 instrument fitted with a 50 N load cell, equipped withpneumatic grips. The cross head speed of 250 mm/min was chosenbased on the ASTMD882 standard testmethod. Reported values arequoted as the average �1 standard deviation of 6e8 replicatesamples.

2.9. Scanning electron microscopy (SEM)

Samples of film formulations were placed on carbon conductingpads that were then applied to an aluminium stub. The sampleswere sputter-coated with platinum for 100 s using an SPI coater.The Pt-coated samples were examined with a JEOL 6460 SEM(Tungsten filament). An accelerating voltage of 10 kVwas usedwitha working distance of 10e12 mm. Spot sizes are shown on eachmicrograph as the number on the right next to “SEI”. Smaller spotsizes (25e35) were used for magnifications above �5000 andlarger spot sizes (around 45) for lower magnifications. All imageswere captured as TIF files at the highest resolution possible. TIF fileswere post-processed with Paint Shop Pro Version 5 to adjustbrightness and contrast where needed.

2.10. X-ray fluorescence microspectroscopy & Co XANES

X-ray fluorescence microscopy (XFM) and bulk x-ray absorptionnear-edge structure (XANES) were employed in fluorescence modeat the XFM beamline at the Australian Synchrotron. The XFMbeamline is an undulator beamline with a Si(111) monochromatorand a nominal energy resolution (DE/E) of 2 � 10�4 at 10 keV [30].The distribution of Co, Fe and Ti were mapped at 10 keV throughout

the thickness of films 1Ti/1000Co and 1Ti/1000Fe over an area of28 mm� 25 mmwith a step size of 0.2 mm� 0.1 mm (H� V) and witha focussed x-ray spot size of 600 nm � 300 nm (H � V). Aftermapping, Co XANES measurements were performed with a largebeam to avoid any artefacts associated with the relative motion ofthe x-ray beam to the Co nanoparticles. Co XANES were acquiredfrom formulation 1Ti/1000Co after no UV exposure, 6 days accel-erated UV exposure and until embrittlement after an elementalmapping experiment. The XANES spectra were collected between7690 and 7790 eV with a step of 1 eV.

3. Results & discussion

3.1. Defining embrittlement

Fig. 2 illustrates the progression of the decrease in tensileelongation at break for the formulation containing only TiO2 (1Ti/0Co/0Fe), compared to the control film during weathering. Thearrow indicates the embrittlement point reached by the LLDPE film,where the elongation was less than 5% after 17 days. At this pointthe film fractures and fragments under light load. When used asagricultural film the embrittled material cannot be removed andfragments in place. This process is distinguished from film splittingin which there is a loss of properties only in one direction and thesplit agricultural film may remain intact on the soil withoutfragmenting.

3.2. Accelerated laboratory photo-oxidation

3.2.1. Film embrittlementThe time to film embrittlement for photo-oxidised polyethylene

formulations containing combinations of TiO2 and CoSt pro-oxidants are summarised in Table 3 and the most rapid degrada-tion (9 days) occurs with the addition of either 1% TiO2 or 1000 ppmCo2þ. However when the two pro-oxidants were combined at thesame concentration of each, the time to embrittle almost doubles to17 days rather than reducing even further as might be expected.While this is still faster than the 55 days taken for the originalLLDPE to degrade, it indicates that there is antagonism when thetwo pro-oxidants are used together.

The extent of this antagonism has been assessed by reducing theconcentration of CoSt while holding the TiO2 concentration at 1%.

Fig. 2. The change in elongation at break for a TiO2-containing film (1Ti/0Co/0Fe) anda control aged outdoors above-ground on soil. The arrow denotes the embrittlementpoint.



From Table 3 it may be seen that there is an initial reduction in thetime to embrittlement when the concentration is reduced to600e800 ppm Co2þ but it then increases progressively so that evencatalytic concentrations of CoSt (200 ppm Co2þ) increase the timeto embrittle of formulations containing 1% TiO2. Indeed the lifetimeof this combination (21 days) is very close to that with 200 ppmCo2þ alone (23 days) suggesting that the photoactivity of the TiO2 isalmost totally lost.

In order to determine if this effect is restricted to CoSt, the timeto embrittlement for the photo-oxidation of LLDPE formulationscontaining a combination of TiO2 and FeSt pro-oxidants was alsomeasured (Table 3). The presence of 1000 ppm Fe2þ in combinationwith 1% TiO2 only increased the time to embrittle to 13 days from 9days for the TiO2 alone suggesting that not all TiO2 photoactivity islost. FeSt is a much weaker pro-oxidant than CoSt and when1000 ppm Fe2þ alone is added to LLDPE the time to embrittle is 28days.

3.2.2. Film whiteningThe percentage of light transmission at 585 nmwas recorded for

each formulation before and after embrittlement and is shown inTable 4. A whitened film was defined by 5% or less light trans-mittance throughout the film. The percentage of light trans-mittance at embrittlement was used as an indirect measure of TiO2

photoactivity within the film, where lack of film whitening duringphoto-oxidation suggested that the photoactivity of the TiO2 hadbeen reduced.

At embrittlement, the formulation with 1% TiO2 alone wasuniformly white with a light transmission of 0.2% at 585 nm. Forformulations containing CoSt in combination with TiO2, the lighttransmittance was comparable at time zero and embrittlement,

between 59 and 68% (Table 4). LLDPE alone and containing onlyCoSt as pro-oxidant showed a reduction in the light transmittanceat embrittlement, but remained transparent.

When FeSt was used in combination with TiO2, the light trans-mission at 585 nm at embrittlement was reduced to 2.4%,compared to 0.2% for TiO2 alone, however to the eye it was apparentthat both films whitened. The time towhitenwas 38% of the time toembrittle compared to 33% for the sample with TiO2 alone. Theseresults suggest that the extent of whitening of the LLDPE by theTiO2 was not significantly impacted upon by the FeSt consistentwith the observation from the data for time to embrittlement thatnot all TiO2 photoactivity has been lost.

SEM analysis performed on photo-oxidised films 1Ti/0Co/0Feand 1Ti/1000Fe at embrittlement (Fig. 3A and B) revealed a highnumber of microscopic voids with an average diameter ofapproximately 150 nm. It is also apparent from the upper section ofFig. 3A that the microscopic voids have coalesced to form a crackcorresponding to the onset of embrittlement. The scattering of lightby the microscopic voids formed during photo-oxidation isresponsible for the whitening of these films [24]. In contrast to thefilms containing only TiO2, examination of an embrittled, photo-oxidised film with a formulation of 1Ti/1000Co (Fig. 3C), revealedvery few microscopic voids consistent with reduced photoactivityof the TiO2 in the presence of CoSt.

The absence of film whitening for TiO2/CoSt formulations maybe related to the redox potential of Co3þ/2þ metal ions and theireffect on TiO2 band edge potentials. Ohtani et al. [24] found thatheterogeneous photo-induced TiO2 oxidation reactions, shownabove through reactions (10)e(15), are responsible for film whit-ening. When these heterogeneous reactions were inhibited, photo-oxidation of the polyethylene progressed through homogeneousreactions leading to uniform degradation and no whitening.

Antagonism effects have been reported between combinationsof TiO2 and dissolved metal ions during the photo-oxidation ofphenolic compounds in aqueous solutions [31e36]. This antago-nism effect has been described mechanistically by Co3þ and Fe3þ

competing with oxygen for e�cb at the surface of the excited TiO2particle, which consequently reduces the hydroxyl radical forma-tion that would normally occur via Equations (4)e(6). Additionally,the oxidation of the Co2þ or Fe2þ metal ions by a hydroxy radical orhole ðhþ

vbÞ at the surface of the excited TiO2 particle, reduces theoverall concentration of hydroxyl radicals as shown in Equation(22) [22]:

Mðn�1Þþ þ hþvbðHO$Þ/Mnþ (22)

where n ¼ 3; M is Co or FeWhile this mechanism explains the significant decrease in the

rate of embrittlement for TiO2/CoSt formulations, it does notexplain why the antagonism between TiO2 and CoSt was far moresevere than that between TiO2 and FeSt, despite evidence ofoxidation by the formation of carbonyl products.

Brezova et al. [31] found that the influence of metal ions in thepresence of TiO2 on phenol photo-oxidation could be estimated bycomparing the standard reduction potential of the metal ions, suchas Co2þ/3þ and Fe2þ/3þ, to the band edge potentials of TiO2, shownin Fig. 4. The higher redox potential for Co3þ/2þ (1.92 V) comparedwith Fe3þ/2þ (0.77 V) [37], indicates that Co3þ has a higher affinityfor the e�cb ejected from the TiO2 particle and can be more easilyreduced compared to Fe3þ. Equations (23) and (24) show thetransfer of e�cb from the UV irradiated TiO2 particle to the transitionmetal ion and hþ

vb of the TiO2 particle [31]:

Mnþ þ e�cb/Mðn�1Þþ (23)

Table 3Photo-oxidation of polyethylene containing pro-oxidants in an Atlas Suntest.

Formulation code Time to Emb. (Days) �SD (Days) Time to whiten. (Days)

Control 55 1 e

0Ti/200Co 23 1 e

0Ti/1000Co 9 1 e

0Ti/1000Fe 28 1 e

1Ti/0Co/0Fe 9 1 31Ti/200Co 21 1 e

1Ti/300Co 20 2 e

1Ti/400Co 21 1 e

1Ti/600Co 14 2 e

1Ti/800Co 12 2 e

1Ti/1000Co 17 1 e

1Ti/1000Fe 13 1 5

Table 4The change in transmittance at 585 nm for TiO2/CoSt and TiO2/FeSt films at time0 and at embrittlement after aging under accelerated photo-oxidative conditions.

Formulation code t ¼ 0 At embrittlement

%T 585 nm �SD %T 585 nm �SD

Control 86.4 2.4 76.3 0.70Ti/200Co 85.4 0.8 68.2 0.30Ti/1000Co 80.3 0.6 77.1 3.00Ti/1000Fe 82.2 0.3 71.3 3.11Ti/0Co/0Fe 77.6 0.3 0.2 0.11Ti/200Co 73.8 0.7 63.7 1.01Ti/300Co 71.0 0.3 60. 9 3.71Ti/400Co 67.2 0.9 59.0 0.41Ti/600Co 70.7 0.0 63.8 2.41Ti/800Co 66.0 1.0 68.0 0.21Ti/1000Co 70.1 0.4 67.3 0.41Ti/1000Fe 75.7 2.6 2.4 0.8



Mðn�1Þþ þ hþvb/Mnþ (24)

where M ¼ Co or Fe; n ¼ 3The cycling of these redox side reactions shown in Equations

(23) and (24), will decrease the concentration of hydroxyl radicalsformed, so reducing the rate of polyethylene photo-oxidation.

Effectively the initiation of heterogeneous photo-oxidation at thesurface of the TiO2 nanoparticle has been shut down and the onlyoxidation reactions seen are initiated by the metal salt. Thissuggests close association between the TiO2 nanoparticle and themetal stearate in the solid film to enable this scavenging to takeplace.

3.3. X-ray fluorescence microscopy

XFM was used to evaluate the distribution of metal ionsthroughout the films after UV irradiation. Formulation 1Ti/1000Co(Fig. 5) shows the distribution of Co, Fe and Ti ions within the film,as well as an overlay image showing their combined distributions.Scatter plots of these relative concentrations (i.e., normalised to theTi-ion concentration) describe the co-localisation in greater detail,and are included as supplementary information. Examination ofthe overlay map in Fig. 5 and scatter plot SI1 suggest that Ti and Coare strongly co-localised within the film. In addition, the maximumintensity scale for the Fe map shows that there is a low concen-tration of Fe present compared to Co (less than tenfold themaximum of Co) that mostly appears to follow the same spatialdistribution, which are likely to be impurities. There are alsodomains of high density Fe (clearly visible in the overlay as brightblue regions). This low concentration of Fe may be real; alterna-tively, it is possible that the high concentration impurities result inartefactual elemental cross-talk between Co and Fe. The combina-tion of the co-localization of Ti and Co metal ions and the higherredox potential of the Co3þ/2þ would provide an ideal platform forphoto-antagonistic reactions to occur.

The elemental and overlay maps for formulation 1Ti/1000Fe(Fig. 6) again shows the co-localisation of Fe, Co and Ti metal ionswithin the polyethylene film. The maximum intensity scale for theCo map compared to Fe and associated scatter plots (SI 3 & 4 in thesupplementary material), suggest a Co impurity within the film,with some domains composed almost entirely of Co (bright red inoverlay). There is also the possibility of some fluorescence lineoverlap resulting in a low-level Co co-localisation. Photo-oxidation

Fig. 4. Schematic representation of the vb and cb of TiO2 with corresponding photo-chemical generation of reduction (e�) and oxidation sites (hþ) along with the reductionpotentials of Co and Fe [31]. Reproduced with permission from Elsevier.

Fig. 3. SEM micrographs of photo-oxidised LLDPE films at embrittlement. A e 1Ti/0Co/0Fe; B e 1Ti/1000Fe & C e 1Ti/1000Co.



results on formulation 1Ti/1000Fe showed minimal inhibition onthe photochemistry of the TiO2, which may be related to the lowerredox potential of Fe3þ/2þ compared to Co3þ/2þ, even though thereis evidence of Ti and Fe metal ion co-localisation in the film.

3.4. X-ray absorption near-edge structure (XANES)

Fluorescence x-ray absorption near-edge structure (XANES) wasused as a spectroscopic tool to reveal changes in metal ion oxida-tion state and coordination chemistry of Co metal ions in thepresence of TiO2 after UV exposure. Fig. 7 shows the Co XANESspectra obtained for formulation 1Ti/1000Co before UV exposure(control), after accelerated UV exposure for 6 days and at embrit-tlement. The characteristic features of the Co XANES spectra areshown by (A) the small pre-edge peak at 7708 eV, (B) the white lineintensity at 7723 eV, (C) the shoulder region after thewhite line, (D)the post-edge minima between 7749 and 7750 eV and (E) the large

oscillation mostly attributed to the multiple scattering fromneighbouring bonded groups, such as CoeO [38].

Previous studies on tetrahedral Co2þ complexes have attributedthe pre-edge absorptions at z7708 eV to the transition of the 1selectron to the 3d molecular orbital [38e41]. For both UV exposedtraces, the spectra suggest there may be a marginal increase in thefluorescence intensity of this pre-edge feature, compared to thecontrol. The white line peak at 7723 eV for a Co2þ complex oftetrahedral coordination has been reported as the transition of the1s electron to the 4p molecular orbital [38]. Hall et al. [42,43] andRobinet et al. [38] have reported that the absorption intensity of thepre-edge and white line are influenced by the coordination numberof the Co metal ion complex.

The observed decrease in the white line intensity at 7723 eV andthe accompanying increase in the pre-edge intensity following UVexposure suggests a decrease in the cobalt coordination numberwith UV exposure [44], consistent with change in the coordination

Fig. 5. The distribution of Co, Fe and Ti metal ions as well as an overlay distribution map of the 3 metal ions throughout the cross-section of film 1Ti/1000Co after UV irradiation.The images have been scaled to better indicate distribution and are not quantitative. Most particles here indicate a constant admixture of Co, Fe, and Ti although there is alsoa significant population of with almost pure Fe (blue in overlay). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)



from the octahedral CoSt to a tetrahedral complex (possiblyinvolving hydroperoxides) after UV irradiation. The Co XANESspectra also show there has been no change in oxidation state withUV exposure, as previous investigations on the oxidation andreduction of Co metal ions have shown a shift in the energy of thewhite line peak is expected for changes in Co oxidation state[38,39]. There is therefore no evidence for direct chemical reactionbetween CoSt and TiO2 nanoparticles. The antagonism most likelyarises from the competition between the closely associated metalions for the photo-generated electrons and holes at the nano-particle surface so the energy absorbed by the TiO2 is leading tocycling through the oxidation states of the metal without genera-tion of reactive species to degrade the polymer.

3.5. Accelerated laboratory thermo-oxidation

3.5.1. Film embrittlement on heating at 60 �C in the darkAccelerated dark thermo-oxidative experiments were per-

formed on TiO2/CoSt formulations to evaluate the impact of the

combination of TiO2 and CoSt pro-oxidants in the absence of UV onthe oxidation of polyethylene. These simulate the expectedperformance of the pro-oxidants when the agricultural film isburied. The results (Table 5) show that combinations of TiO2 andCoSt pro-oxidants are not antagonistic under dark thermo-oxidative conditions and there was no change in film trans-parency at embrittlement. This result is in agreement with previousstudies, where it was found that TiO2 was a poor pro-oxidant underdark thermo-oxidative conditions and films did not whiten[26e28].

For these formulations, the time to embrittlement wascontrolled by the concentration of CoSt in the film but the depen-dence was not linear with concentration. The rate of polyethylenethermo-oxidation was not significantly different for formulationscontaining 1% TiO2 with up to 600ppm Co2þ. A significant decreasein the time to embrittlement was observed with 800 ppm Co2þ,however at 1000 ppm Co2þ the time to embrittlement increased.According to Black et al. [11], the rate of POOH decomposition isdependent on the pro-oxidant concentration in the LLDPE. The

Fig. 6. The distribution of Co, Fe and Ti metal ions as well as an overlay distribution map of the 3 metal ions throughout the cross-section of film 1Ti/1000Fe after UV irradiation. Theimages have been scaled to better indicate distribution and are not quantitative. Most particles here indicate a constant admixture of Co, Fe, and Ti with a few showinga predominance of Co (red in overlay). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)



POOH decomposition is accelerated up to a critical concentration ofCo2þ, above which the Co2þ may act as a stabiliser [11]. Themechanism for catalytic decomposition requires complexation ofhydroperoxide by the Co2þ and the competing stabilisation reac-tion can occur only when the concentration of CoSt is greater thanthat of hydroperoxide [11]. The data in Table 5 suggest that above800 ppm Co2þ the critical concentration of CoSt is beingapproached so that there is a stabilising effect that reduces theoverall pro-oxidant efficiency of the formulation.

3.6. Carbonyl index at embrittlement

The carbonyl index (CI) at embrittlement for TiO2/CoSt and TiO2/FeSt formulations aged under accelerated photo-oxidative andthermo-oxidative conditions are summarised in Table 6. Fig. 8shows the FTIR-ATR spectra for the photo-oxidised films at thepoint of embrittlement. LLDPE without pro-oxidant showeda similar CI at embrittlement after photo- and thermo-oxidation.When a pro-oxidant is present this is no longer seen. For all TiO2-containing formulations, the CI at embrittlement differed signifi-cantly with type of aging, suggesting mechanistic changes. Forexample, photo-oxidised 1Ti/0Co/0Fe exhibited a significantlylower CI at embrittlement (0.06) compared to both a control film(0.22) and also one that has been thermo-oxidized (0.11).

One explanation for this lower CI at embrittlement is thephotolysis of carbonyl products (such as carboxylic acids) to volatile

products (Equation (25)). As shown earlier, SEM micrographsconfirmed the presence of microscopic voids in photo-oxidised film1Ti/0Co/0Fe, suggesting the formation of volatile species from thepolymer in the vicinity of the nanoparticle surface leading toa lower concentration of oxidation products in the polymer onphoto-degradation.

RCOOH/hy;TiO2 RHþ CO2 (25)

Once cobalt stearate is present in the formulation and photo-antagonism is seen (Table 3), the concentration of oxidation prod-ucts at embrittlement approximately doubles. At the same timethere is no whitening of the film and the production of microscopicvoids is greatly reduced so that at 1000 ppm Co (1Ti/1000Co) novoids are seen (Fig. 3C) indicating that the photochemistry of theTiO2 is totally suppressed. The extent of oxidation at embrittlementis much higher, being greater than when only the CoSt pro-oxidantis present.

Details of the carbonyl region of photo-oxidised formulations atembrittlement are shown in Fig. 8 (with the baselines offset forclarity). The control film (upper curve) shows a pronounced acid(1712 cm�1) peak as indicated by the CI value (0.22) in Table 6, aswell as a neighbouring ester (1735 cm�1) shoulder of significantintensity (CI1735 ¼ 0.20). In contrast, formulations containing pro-oxidant showed an acid and ester shoulder peak of lower inten-sity compared to the control. Further examination of the IR spectraof pro-oxidant-containing films has revealed that both whiteningformulations 1Ti/0Co/0Fe and 1Ti/1000Fe showed a very similar

Fig. 7. Co XANES spectra of formulation 1Ti/1000Co before UV exposure (control), after6 days UV exposure and at embrittlement after UV exposure. (A) Pre-edge peak at7708 eV; (B) White line peak at 7723 eV; (C) Shoulder peak following white linecorresponding to the electronic structure and coordination of the Co; (D) Post-edgeminima between 7749 and 7750 eV; (E) Large oscillation at the start of the EXAFSregion [38]. The inset shows a magnification of the pre-edge region.

Table 5Thermo-oxidation of polyethylene film formulations aged at 60 �C and 100% relativehumidity.

Formulation code Time to Emb. (days) �SD (days)

Control 111 10Ti/200Co 19 10Ti/1000Co 13 11Ti/0Co/0Fe 95 11Ti/200Co 18 21Ti/300Co 18 21Ti/400Co 17 11Ti/600Co 18 21Ti/800Co 11 21Ti/1000Co 15 3

Table 6The CI at embrittlement for TiO2/CoSt and TiO2/FeSt formulations aged underaccelerated laboratory thermo-oxidative and photo-oxidative conditions.

Formulation Photo-Oxidation Thermo-oxidation

CI at Emb. � SD CI at Emb. � SD

Control 0.22 0.03 0.23 0.010Ti/200Co 0.06 0.01 0.40 0.010Ti/1000Co 0.10 0.01 0.21 0.040Ti/1000Fe 0.11 0.01 0.14 0.011Ti/0Co/0Fe 0.06 0.01 0.11 0.011Ti/200Co 0.08 0.01 0.11 0.011Ti/300Co 0.08 0.01 0.22 0.011Ti/400Co 0.09 0.01 0.29 0.011Ti/600Co 0.09 0.02 0.43 0.031Ti/800Co 0.10 0.02 0.40 0.021Ti/1000Co 0.13 0.01 0.36 0.011Ti/1000Fe 0.07 0.01 0.50 0.06

Fig. 8. The carbonyl region of the FTIR spectrum of LLDPE films after photo-oxidationto embrittlement.



carbonyl distribution profile with comparable low intensities ofacid and virtually no esters, due to the volatilization of carbonylproducts to CO2. In contrast, the photo-antagonistic formulation1Ti/1000Co showed a carbonyl profile in close agreement with theCoSt and FeSt alone, with a higher intensity of acid and esterproducts compared to whitening formulations.

For thermo-oxidised formulations containing 1 wt% TiO2 withvarying concentrations of Co2þ, a general trend was observedwhere the CI at embrittlement increased as the concentration ofCo2þ increased within the formulation to values much higher thanthe control film. Allen et al. [28] investigated the thermo-oxidationof a series of LDPE films containing different grades of TiO2 in an airvented oven at 90 �C, measuring the concentration of hydroper-oxides formed during thermo-oxidation. The results from theirstudy on organically coated anatase and rutile TiO2 nanoparticlesdispersed in polyethylene, showed substantially higher concen-trations of POOH in the polyethylene plus TiO2, compared topolyethylene alone. If this is also occurring in our system, thesehydroperoxides are more efficiently decomposed as the concen-tration of Co2þ is increased up to a critical concentration, resultingin an increase in CI at embrittlement. Interestingly, this did notresult in a faster time to embrittlement for films containing 1 wt%TiO2 and CoSt under the conditions used in our study.

3.7. Outdoor weathering

An outdoor weathering trial was conducted on TiO2/CoSt andTiO2/FeSt formulations to determine whether antagonism onpolyethylene photo-oxidation is also observed under the lowerdose rate of natural weathering. The results (Table 7) clearly showantagonism between pro-oxidants, confirming this effect is not anartefact of the accelerated photo-oxidation conditions.

In agreement with accelerated photo-oxidation experiments,formulations 1Ti/1000Co and 1Ti/1000Fe showed longer times toembrittlement compared to TiO2 alone, indicating a reduction inthe photoactivity of TiO2 within the film. The absence of filmwhitening and higher CI at embrittlement for formulation 1Ti/1000Co (0.33) compared to 1% TiO2 alone (0.14) provides furtherevidence of antagonism between TiO2 and CoSt pro-oxidants. Alsoin agreement is the whitening and lower CI at embrittlement forformulation 1Ti/1000Fe (0.08) compared to 1% TiO2 alone. Clearly,the antagonism is so severe that the time taken to embrittle for 1%TiO2 in the presence of 1000 ppm FeSt or CoSt is effectively inde-pendent of the presence of TiO2. However the whitening of thefilms with FeSt indicates that heterogeneous photo-initiation hasoccurred so the lack of an effect on time to embrittle is inconsistentwith observations under accelerated photo-oxidation.

A comparison of the time to embrittlement acceleration factorsfor film aged outdoors to accelerated photo-oxidation trials werevariable (ranging from 1.3 to 2.7) and could not be correlated withpro-oxidant formulation. However, differences in the weatheringfactors outdoors, such as high humidity, rainfall events and fluc-tuations in UV and temperature compared to the controlled and

constant UV, temperature and humidity conditions during accel-erated tests, would contribute to these differences in accelerationfactors between formulations.

4. Conclusions

The effect of a combination of TiO2 with CoSt or FeSt pro-oxidants in polyethylene is to reduce the efficiency of photo-oxidation without affecting the thermo-oxidation efficiency.Differences in the standard reduction potential for Co3þ/2þ (1.92 V)and Fe3þ/2þ (0.77 V) and their effect on the UV band edge of a TiO2nanoparticle, may be responsible for the differences in the antag-onistic impact on the accelerated photo-oxidation of the LLDPE.Antagonistic effects are also seen on outdoor ageing but withdifferences between the magnitude of the effects. The effect of thetransition metal salt is to switch off photo-initiation by hydroxylradicals generated at the surface of the nanoparticle by scavengingof electrons and holes by the redox reactions of the transitionmetals. To achieve optimum control of the lifetime of agriculturalfilm both above and below ground, strategies are required toovercome this antagonism by limiting direct physical interactionbetween the pro-oxidants.

Acknowledgements

The authors would like to thank and acknowledge the Cooper-ative Research Centre for Polymers and Integrated Packaging forfinancial support of this work. Dr Chun-Liang Yeh, MrMarcus Leongand Dr Babak Radi are acknowledged with thanks for performingFTIR-ATR characterisation. The XFM & XANES part of this researchwas undertaken on the XFM beamline at the Australian Synchro-tron, Victoria, Australia.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.polymdegradstab.2012.03.036.

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