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Polymer Degradation and Stability 102 (2014) 33e40

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Polymer Degradation and Stability

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

Study of the effect of natural antioxidants in polyethylene:Performance of b-carotene

Dóra Tátraaljai a,b, Luca Major a,b, Enik}o Földes a,b,*, Béla Pukánszky a,b

a Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, H-1519 Budapest, P.O. Box 286,Hungaryb Laboratory of Plastics and Rubber Technology, Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics,H-1521 Budapest, P.O. Box 91, Hungary

a r t i c l e i n f o

Article history:Received 19 September 2013Received in revised form15 January 2014Accepted 10 February 2014Available online 19 February 2014

Keywords:PolyethyleneNatural antioxidanta-Tocopherolb-CarotenePhosphoniteActive packaging

* Corresponding author. Institute of Materials anResearch Centre for Natural Sciences, Hungarian ABudapest, P.O. Box 286, Hungary.

E-mail address: foldes.eniko@ttk.mta.hu (E. Földes

http://dx.doi.org/10.1016/j.polymdegradstab.2014.02.00141-3910/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The effect of b-carotene on the behaviour of polyethylene stabilised with a-tocopherol and a phosphoniteantioxidant was studied under processing and storage conditions. The amount of b-carotene rangedbetween 0 and 2000 ppm. The polymer was characterised by different methods after processing thenduring and after storage at ambient temperature in light and dark. b-Carotene hinders the oxidation ofpolyethylene and does not increase the chain extension reactions during processing, though more vinylgroups and phosphonite molecules react. b-Carotene colours polyethylene strongly already at low con-centrations. The reactions of the polymer and b-carotene are affected strongly by the storage conditions.The presence of b-carotene does not influence the stabilising efficiency of the primary and secondaryantioxidants. In dark the molecular structure of the polymer does not change appreciably, while thereactions of b-carotene lead to an increase in the yellowness index. In light the molecular characteristicsof polyethylene undergo significant changes indicating long chain branching. The polymer fades rapidlyafter an induction period. The length of the induction period is not influenced by light. The rate of thedegradation reactions of b-carotene during storage is controlled by its concentration and film thickness.Visible autoaccelerated decomposition in light renders b-carotene candidate as an indicator in activepackaging materials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polyethylene is one of the main components of modern foodpackaging materials used for various purposes. This polymer is oflow density, translucent, water-resistant, harmless for humanhealth, recyclable, and cheap. However, polyethylene has to beprotected against degradation which occurs as an effect of heat,shear, oxygen and/or UV light during processing and application.The degradation of polyethylene starts at the weak sites of thepolymer chains (double bonds, branching points) with the forma-tion of alkyl radicals which react rapidly with oxygen yieldingperoxy radicals. These abstract hydrogen atoms from the polymerbackbone and the hydroperoxide groups formed decompose fast toreactive oxy and hydroxyl radicals resulting in an auto-accelerated

d Environmental Chemistry,cademy of Sciences, H-1519

).

12

degradation process [1e5]. Comparing the effect of unsaturationson the thermal- and photo-oxidative stability of polyethylenerevealed that the number of unsaturated groups affect significantlythermal oxidation, while their concentration does not influence therate of photo-oxidation [6]. The degradation of polyethylene can beinhibited by antioxidants. Combinations of hindered phenols andphosphorous antioxidants are usually used as processing stabil-isers. The phosphorous antioxidant inhibits degradation reactionsby decomposing hydroperoxide groups to alcohols and it reacts alsowith carbon-centred, peroxy and oxy radicals [1,7,8]. Its efficiency isaffected strongly by its chemical composition and thermal stability,as well as the amount of oxygen present [7e10]. Hindered phenolsterminate autoxidation by trapping alkylperoxy, alkoxy and alkylradicals [11e14]. The efficiency of the phenolic antioxidant dependson the number of hydroxyl groups and the nature of substituents atthe orto and para positions to the phenolic OH group [15,16]. Thesynergistic effect of phenol/phosphorous antioxidant combinationscompared to single antioxidants can be partly attributed to thereduced rate of consumption of each type of stabiliser during theprocessing of polyethylene [10].

D. Tátraaljai et al. / Polymer Degradation and Stability 102 (2014) 33e4034

Different synthetic antioxidants are used for stabilisation inpractice. The stabilisation of polyethylene with natural antioxi-dants for packaging materials is a new challenge both from thescientific and the industrial point of view. Replacing syntheticphenolic antioxidants by them could stop worries caused by thepossible harmful effects of their reaction products on humanhealth [17]. Active packaging films can be produced by incorpo-rating natural antioxidants into the polymer matrix resulting inquality improvement in different ways. Some examples: 1) Theshelf-life of oxygen-sensitive products can be prolonged by usingradical scavenging natural antioxidants [18e21]. 2) The migrationof antioxidants (like vitamins, flavonoids) from the packagingmaterial to the packaged goods may be also beneficial. 3) Most ofthe natural antioxidants are colourful substances. Intelligentpackaging materials may be produced with their application, asthe loss in colour can indicate the expiration of guarantee for theproduct.

b-Carotene is one of the colourful carotenoids with antioxidantactivity. It is a strongly-coloured red-orange pigment present inmany plants and fruits, like carrots, pumpkins, or sweet potatoes. b-Carotene is the pro-vitamin of vitamin A and is widely used as acolourant in foods and beverages. It is a non-polar highly conju-gated lipophilic hydrocarbon compound (Table 1). The elevenconjugated C]C double bonds serve as a chromophore moiety inthe molecule, which is responsible for the light absorption and thecolour of b-carotene [22]. Accordingly, b-carotene has a character-istic UVeVIS spectrum, in which the position of the principal ab-sorption maximum (lmax) depends on the configuration [23,24]

Table 1Chemical composition of the antioxidants studied.

Commercial name Chemical composition

b-carotene

a-tocopherolOH

O

Sandostab P-EPQ

Diphosphonite >70% P

O

O

Monophosphonite w20% PO

O

Phosphite <10% P

O

OO

and oxidation of the molecule [25,26], as well as on the type ofthe solvent used [25,27] according to the changes in transitionenergies between the ground and excited states.

The melting point of b-carotene is higher than 180 �C, butmelting is not reversible and the true melting point cannot bedetermined [25]. Pure crystalline b-carotene (purified by fractionaldissolution) is black but undergoes fast direct oxidation by air in thecrystal phase even at low temperatures (�80 �C) in dark. Oxidationresults in a change of colour to orange or red, the powder becomessimilar in appearance and composition to laboratory samplesavailable commercially [25]. The oxidation of b-carotene withmolecular oxygen is an autocatalytic free radical process whichleads to the formation of different epoxycarotenoids, apocarotenals,small molecular mass products, and some oligomeric/polymericmaterials, some of them representing the key aroma compounds incertain flowers, vegetables, and fruits [28e33]. The initial site of theoxygen attack on the b-carotene molecule occurs at the terminaldouble bonds resulting in predominant formation of epoxides[30,34]. These and long chain b-apocarotenals formed in the firststage of oxidation decompose to lower molecular mass carbonyl-containing compounds in further oxidation reactions [30]. Thepathway of cleavage of the molecule (central or random) is affectedby the surrounding medium (type of solvent), as well as theabsence or presence of a phenolic antioxidant [35]. Phenolic anti-oxidants, like BHT and a-tocopherol, inhibit the oxidation of b-carotene [30,35]. b-Carotene is more resistant to heat than to ox-ygen [36,37]. Besides isomerisation of cis- to trans-isomer, a fewhigh molecular mass components are formed at high temperatures

Molecularmass (g/mol)

537

431

P

O

O

1035

595

647

Fig. 1. UVeVIS absorbance spectra of polyethylene processed with a-tocopherol/phosphonite antioxidant package and b-carotene of 0 (d), 250 (e e), 500 ($$$$), 1000($$e$$), 1500 (——), and 2000 (e$e) ppm. Absorption intensities are related to 100 mmfilm thickness.

D. Tátraaljai et al. / Polymer Degradation and Stability 102 (2014) 33e40 35

in nitrogen atmosphere, while polymerisation and decompositionreactions take place simultaneously in air [28,36]. As an effect of UVirradiation the C]C and CeC bonds of the hydrocarbon chain breakleading to a loss of colour [22]. The UV stability of b-carotene can beimproved by the addition of a photostabiliser which absorbs lightbelow 465 nm [22,38].

b-Carotene is considered as a chain-breaking antioxidant, as itexhibits high reactivity towards carbon-centred, peroxy, alkoxy,and NO2 radicals [39]. It can even prevent the photosensitisation ofhuman skin [40]. According to Ozhogina and Kasaikina [31] b-carotene reacts primarily by the addition of free radicals to itsconjugated system. The radical-trapping behaviour of b-carotenedepends on the partial pressure of oxygen [41,42]. It proved to be agood radical-trapping antioxidant at partial pressures lower thanthe pressure of oxygen in normal air. At higher oxygen pressures, b-carotene loses its antioxidant activity and shows an autocatalytic,prooxidant effect, particularly at relatively high concentrations. Themixtures of b-carotene, a-tocopherol, and ascorbic acid providebetter protection against protein oxidation than any of the com-ponents alone [42,43].

Although the antioxidant activity of b-carotene has beeninvestigated mainly in food and medical sciences, some experi-ments have already been carried out also in polymeric materials.Abdel-Razik [44] found that b-carotene increases the thermo-oxidative stability of ABS copolymer and explained the effect bythe formation of biradicals which react rapidly with oxygen mol-ecules inhibiting the formation of alkoxy and peroxy radicals fromthe polymer. López-Rubio and Lagaron [45] observed the UV sta-bilising effect of b-carotene in biopolyesters [poly(lactic acid),polycaprolactone, and polyhydroxybutyrate-co-valerate]. b-caro-tene plasticise these biopolymers already at small concentrations(0.4 w/w %) and a part of the effect is retained even after UVeVISirradiation.

Preliminary experiments run in our laboratory revealed that b-carotene improves the processing stability of polyethylene. Thepositive effect is further enhanced by the addition of a phosphonitesecondary antioxidant; the efficiency of this additive combinationproved to be similar to that of the synthetic phenolic antioxidantpackage frequently used in industrial practice [46]. Gradual loss ofthe original orange colour of films stored in an artificially lightedroomwas observed in the experiment, which was attributed to thedecomposition of b-carotene. As a-tocopherol inhibits oxidation ofb-carotene [30,35], and a phosphonite antioxidant enhances itsstabilising efficiency [46], the effect of b-carotene on the charac-teristics of polyethylene was investigated in the presence of theseadditives in this work. Two main questions were studied: 1) theeffect of the concentration of b-carotene on the processing stabilityof polyethylene; 2) changes in polymer characteristics duringstorage at ambient temperature.

2. Experimental

The experiments were carried out with the Tipelin FS 471 gradeethylene/1-hexene additive-free copolymer powder of TiszaChemical Group Ltd. (TVK) (PE; melt flow index of the powder:0.32 g/10 min, nominal density: 0.947 g/cm3) polymerised with aPhillips catalyst. The polymer was stabilised with 500 ppm a-tocopherol (Ronotec 201; Hoffmann-La Roche), 2000 ppm phos-phonite antioxidant (Sandostab P-EPQ; Clariant), and differentamounts of b-carotene (purum, �97%, UV, Sigma) ranging from 0 to2000 ppm.

The polymer powder was homogenised with the additives intwo steps. a-tocopherol dissolved in acetone and the phosphoniteantioxidant was added gradually to the polymer powder undermixing at a rate of 500 rpm for 10 min in a high-speed mixer

(Henschel FM/A10). The blend was dried at ambient temperatureand after the evaporation of acetone b-carotene was mixed to itunder the same conditions as previously. Subsequently the powderwas extruded and pelletised using a Rheomex S 3/400 25 L/D singlescrew extruder attached to a HAAKE Rheocord EU 10 V driving unitat a rate of 50 rpm and barrel temperatures of 180, 190, 190, and190 �C. Plates of 1 mm and films of 100 mm thickness werecompression moulded from the pellets using a Fontijne SRA 100equipment.

The chemical structure of the polymer and the concentration ofthe phosphonite stabiliser were determined by Fourier trans-formed infrared (FT-IR) spectroscopy (Tensor 27, Bruker). Five par-allel measurements were carried out in each case. The spectrum ofthe nascent polymer powder was recorded by Diffuse ReflectanceFourier Transform Infrared (DRIFT) spectroscopy [47]. The con-centration of the residual phosphonite and that of the vinyl andcarbonyl groups in the processed polyethylene were determinedfrom the absorbance spectra of compression-moulded films [9].The UVeVIS spectra of 100 mm thick films were recorded in thewavelength range of 200e600 nm using a Unicam UV 500 typespectrophotometer. The absorption spectra are complex, as showedby Fig. 1. The absorption band between 200 and 300 nm can beassigned mainly to a-tocopherol, although b-carotene dissolved incyclohexane has also a small band in this region. The main ab-sorption band of b-carotene is located between 400 and 600 nm(blue-green region), and the band between 300 and 400 nm orig-inates also from b-carotene; it is the cis-band region [23,24]. Theconcentration of residual b-carotene in polyethylene films wascalculated from the absorption intensity at 456 nm after calibrationin cyclohexane solutions. This method yields approximate valuesfor the concentration of b-carotene in polyethylene due to the effectof different factors (e.g., isomerisation, oxidation) on the UVeVISspectra. The colour of the samples was measured with a HunterlabColourquest 45/0 apparatus on 1 mm thick plates and the yellow-ness index (YI) was used for characterisation. The UVeVIS spectraand YI were measured after processing, and after ageing the sam-ples at ambient temperature in a container placed in a chest ofdrawers (in dark) and in the window of a room illuminated bynatural light (in light) for different lengths of times. The melt flow

D. Tátraaljai et al. / Polymer Degradation and Stability 102 (2014) 33e4036

index (MFI) of the samples was determined according to the ASTMD 1238-79 standard at 190 �C with 2.16 kg load using a GöttfertMPS-D tester running five parallel measurements. The residualthermo-oxidative stability of the polymer was characterised by theoxidation induction time (OIT) measured in a Perkin Elmer DSC-2apparatus at 200 �C in oxygen flow.

Fig. 3. Correlation between the vinyl group concentration and the melt flow index ofpolyethylene before and after processing with additive packages containing a-tocopherol/phosphonite antioxidant package and different amounts of b-carotene.

3. Results and discussion

3.1. Processing stability

The polymer and the additives participate in different chemicalreactions during the processing of polyethylene. These reactionsare influenced significantly by the antioxidant combination and theinitial concentration of b-carotene. Without any antioxidant thevinyl content of the nascent polymer powder drops from 0.91 to0.66 vinyl/1000 C after processing. The protecting effect of the a-tocopherol/phosphonite antioxidant pair is well reflected in thereduced change to 0.88 vinyl/1000 C at 0 ppm b-carotene. With theaddition of b-carotene to the antioxidant package the concentrationof vinyl groups decreases slightly reaching a value of 0.84 vinyl/1000 C at 2000 ppm b-carotene concentration. The residual con-centration of phosphonite antioxidant decreases, as well. Similarlyto our earlier studies on polyethylene stabilisation [10], the numberof vinyl groups changes linearly with the residual amount ofphosphonite. b-Carotene also participates in the chemical reactionstaking place during the processing of polyethylene. The loss in b-carotene derived from the absorption intensities at 456 nm in theUVeVIS spectra (Fig. 1) increases linearly with its initial concen-tration and it is around 50e60 wt % at each additive package. Fig. 2shows that the reactions of vinyl groups and phosphonite aredirectly related to those of b-carotene.

The melt flow index of the nascent polymer powder (0.32 g/10 min) decreases to 0.13e0.14 g/10 min independently of the b-carotene content after processing (Fig. 3). This is a smaller changethan without any stabiliser where MFI drops to 0.06 g/10 min. Theindependence of MFI from b-carotene concentration indicates thatthe reactions of vinyl groups do not always lead to the chainextension of the polymer, though the latter is characteristic for thedegradation of Phillips type polyethylenes [10]. This can be attrib-uted to the radical trapping ability of b-carotene at low oxygenconcentrations [31,41,42]. b-carotene protects the polymer fromoxidation during processing. In the absence of b-carotene the

Fig. 2. Effect of the reactions of b-carotene on the consumption of phosphonite anti-oxidant (C) and the decrease in the vinyl group concentration of polyethylene (-)during processing at 190 �C.

polymer oxidises slightly (0.042 mmol C]O/mol PE), but carbonylgroups are not formed when the concentration of reacted b-caro-tene is �200 ppm. These results can be explained by the reactivityof b-carotene towards oxygen and different radicals (carbon-cen-tred, oxy) [30,31,39]. b-Carotene reacts fast with molecular oxygenpresent in small concentrations in the extruder and hinders theformation of peroxy macroradicals which are the precursors ofoxidised derivatives. The reactivity of b-carotene towards oxygenleads to an opposite effect at high oxygen concentrations. The OITmeasured at 200 �C in pure oxygen atmosphere decreases withincreasing concentration of unreacted b-carotene (Fig. 4), whichconfirms that b-carotene looses its antioxidant activity and has aprooxidant effect at large oxygen concentrations [41,42].

b-carotene colours polyethylene already at small concentrationsdue to its eleven conjugated C]C double bonds absorbing light inthe blue-green region. Although the absorption intensity at 456 nmincreases linearly with the amount of b-carotene added to thepolymer (Fig. 1), the yellowness index determined from the colourcoordinates changes non-linearly (Fig. 5a). 250 ppm b-caroteneincreases the yellowness index from 2.5 to 135 and turns the visiblecolour from colourless to orange-yellow.With further addition of b-

Fig. 4. Correlation between the residual concentration of b-carotene after processingand the thermo-oxidative stability of polyethylene measured at 200 �C in oxygen.

Fig. 5. Effect of the concentration of b-carotene on the colour of polyethylene pelletsstabilised with a-tocopherol/phosphonite antioxidant combination; a) yellowness in-dex, b) visual colour.

Fig. 7. Discolouration of 1 mm thick polyethylene sheets processed with a-tocopherol/phosphonite antioxidant package and b-carotene of 0 (*), 250 (>), 500 (6), 1000 (B),1500 (7), and 2000 (,) ppm during storage at ambient temperature in dark.

D. Tátraaljai et al. / Polymer Degradation and Stability 102 (2014) 33e40 37

carotene the colour deepens gradually and turns from orange-yellow to red-orange (Fig. 5b). At the same time yellowness indexincreases more moderately and reaches a value of YI ¼ 175 at a b-carotene concentration of 2000 ppm. This character of the yel-lowness index indicates limited solubility of b-carotene in poly-ethylene. From the concentration dependence of YI we can deducethat the solubility is below 200 ppm.

Summarising the results we can conclude that b-carotene incombination with a-tocopherol and phosphonite antioxidants in-hibits the oxidation of polyethylene, does not affect its melt flowindex, increases the number of phosphorous stabiliser molecules

Fig. 6. UVeVIS spectra of polyethylene films of 100 mm thickness stabilised with500 ppm a-tocopherol þ 2000 ppm phosphonite þ 1500 ppm b-carotene stored atambient temperature in dark for 0 (d), 8 (e e), 20 (e$e), and 64 (e$$) days.

participating in chemical reactions and colours the polymer to or-ange during processing.

3.2. Storage at ambient temperature

The film and sheet samples were stored at ambient temperaturein dark and in light for various lengths of times. UVeVIS spectra andyellowness index were measured regularly as a function of storagetime.

Some of the UVeVIS spectra taken from the film (100 mm thick)containing 1500 ppm b-carotene stored in dark for different timesare shown in Fig. 6. In the first period (8 days) isomerisation of b-carotene is revealed by the decrease in the intensity of the ab-sorption band between 300 and 400 nm (characteristic of the cis-isomer) and the increase of the one appearing between 400 and600 nm. The maximum of the main band shifts from 456 nm to460 nm. This introduces some uncertainties in the determination ofthe concentrations of b-carotene from the UVeVIS spectra but

Fig. 8. Effect of the concentration of b-carotene on the time elapsing before acceler-ated change in the yellowness index (induction time) of 1 mm thick polyethyleneplates stored at ambient temperature in dark (:) and light (-).

D. Tátraaljai et al. / Polymer Degradation and Stability 102 (2014) 33e4038

deviations do not exceed 10%. In the further period of storage indark slow loss in the b-carotene concentration occurs which is lessthan 150 ppm after 133 days. The occurrence of some reactions ofa-tocopherol is indicated by the increase in the intensity of theabsorption between 250 and 300 nm and the shift of the peakmaximum from 272 nm to 270 nm.

The yellowness index values of 1 mm thick plates stored in darkare shown as a function of time in Fig. 7. An increase of YI can beobserved even for the sample stabilised with a-tocopherol/phos-phonite antioxidant combination without b-carotene. In this lattercase yellowing of polyethylene can be explained by the reactions ofa-tocopherol, as its transformation products are more colouredthan the parent antioxidant [48]. YI values of the polymer pro-cessed with b-carotene increase sigmoidally with storage timeindicating autocatalytic reactions. The time elapsed before the startof accelerated change in YI (induction time) increases withincreasing residual concentration of b-carotene measured afterprocessing (Fig. 8). The differences in YI measured before and afterstorage in dark for 182 days are DYI z 25e30 at b-carotene con-centrations below 600 ppm and decrease at higher b-carotenecontents. The shift of the main UVeVIS absorption peak of b-carotene to higher wavelengths and the increase in YI can beattributed to the reaction of b-carotene with oxygen. As the ter-minal double bounds of b-carotene are less stable than the centralone (because of resonance) [49] and the CeH bond is theweakest atthe a-position next to the double bond [50], we may assume thatthe oxidation starts there. One or two peroxy radicals then hy-droperoxide groups are formed which participate in further re-actions. The reaction products formed in dark do not containcarbonyl groups. The changes in colour characteristics confirm thatthe reactions take place in the terminal rings of the molecule andmay originate from an increase in the number of conjugated doublebounds in the reaction products or from a decrease in solubility andseparation of reacted molecules from the solution state. Thedecrease in DYI with increasing concentration of b-carotene above600 ppm suggests higher probability of the latter case. Furtherexperiments are in progress to explore the mechanism of reactionsin dark.

The chemical structure and the melt flow properties of thepolymer do not change significantly during storage in dark for 8

Fig. 9. UVeVIS spectra of 100 mm thick polyethylene films processed with a-tocoph-erol/phosphonite antioxidant package and 1500 ppm b-carotene stored at ambienttemperature in light for 0 (d), 8 (e e), 15 ($$e$$), 20 ($$$$), 22 (——), and 25 (e$e)days.

months. Although vinyl groups participate in some reactionsresulting in a slight decrease in their concentration(�Dcvinyl ¼ 0.03e0.07 vinyl/1000 C) but the polymer does not ox-idise and its melt flow index remains practically constant(DMFI ¼ 0.01 g/10 min). From these results we can conclude thatthe reactions of additives leading to some deepening of colourduring storage in dark do not affect the stabilising efficiency ofantioxidants which controls the characteristics of polyethylene.

The degradation of b-carotene in light is fast, occurs heteroge-neously, and results in fading of polyethylene. The time of thecomplete loss of colour depends on the concentration of b-caroteneand the thickness of the sample. Fig. 9 shows some UVeVIS spectraof 100 mm thick films processed with 1500 ppm b-carotene andstored in light at ambient temperature for different times. First (8days of storage) isomerisation of b-carotene occurs revealed by thedecrease in the intensity of the absorption band in the region of300e400 nm (cis-isomer) and the increase of the band appearingbetween 400 and 600 nm. The maximum of the main absorptionband shifts from 456 nm to 460 nm and then to 462 nm duringfurther storage. Fast decomposition of b-carotene is revealed by thedecrease and finally the complete disappearance of the absorptionband between 300 and 600 nm in 25 days. The maximum of theabsorption band of a-tocopherol shifts gradually from 272 nm to267 nm indicating its participation in chemical reactions. Theconcentration of b-carotene determined from the intensity of themain absorption peak is plotted as a function of storage time inFig. 10. We can see that the reactions of b-carotene are autoaccel-erated and the increase in the initial concentration results in anincrease of decomposition rate. Changes in the yellowness index of1 mm thick plates confirm the autoaccelerated nature of b-carotenedecomposition in light (Fig. 11). YI of the polymer stabilised with a-tocopherol/phosphonite antioxidants increases gradually as afunction of storage time due to the reactions of a-tocopherol. In thepresence of b-carotene, following an induction time the yellownessindex drops unexpectedly fast and almost independently of theconcentration of b-carotene to a value corresponding to the poly-mer processed without this additive. Two points must beemphasised here. The length of the induction period, as well as theoverall decomposition time of b-carotene do not depend only on itsconcentration measured before storage but also on the thickness of

Fig. 10. Residual concentration of b-carotene in 100 mm thick polyethylene filmsprocessed with a-tocopherol/phosphonite antioxidant package and b-carotene of 250(A), 500 (:), 1000 (C), 1500 (;), and 2000 (-) ppm, stored at ambient temperaturein light.

Fig. 11. Yellowness index of 1 mm thick polyethylene plates processed with a-tocopherol/phosphonite antioxidant package and b-carotene of 0 (*), 250 (>), 500(6), 1000 (B), 1500 (7), and 2000 (,) ppm, stored at ambient temperature in light.

D. Tátraaljai et al. / Polymer Degradation and Stability 102 (2014) 33e40 39

the polymer sample. The time of complete decomposition of b-carotene in light is around 30 days in 100 mm thick films inde-pendently of the initial concentration (Fig. 10), while it changesfrom 80 to 170 days with increasing initial concentration in 1 mmthick plates (Fig. 11). It is also remarkable, that the length of theinduction time of YI is similar in dark and light as shown in Fig. 8.This reveals that the reactions of b-carotene are governed primarilyby the composition of the additive package and the thickness of thesample, and depend less on the presence of light in the inductionperiod. As we deduced above, peroxy radicals and hydroperoxidegroups in the allylic position to the terminal double bonds can beexpected to form in the induction period of oxidation. In this stagethe rate of oxidation must be determined primarily by the con-centration of oxygen which decreases with increasing film thick-ness, i.e., with the decrease in the surface/volume ratio. The elevenconjugated double bonds of b-carotene absorb a part of visible lightand get into excited state. In this state the molecule becomes more

Fig. 12. Correlation between the concentration of reacted vinyl groups and carbonylgroups formed in polyethylene processed with a-tocopherol/phosphonite/b-caroteneadditive packages and stored in light for 8 months.

sensitive to oxygen attack. After the induction period the rate ofoxygen uptake accelerates and decomposition becomes the maindirection of degradation in light.

The radiation of light does not influence only the reactions of theadditives but also the characteristics of polyethylene measuredafter storage for 8 month. The concentration of vinyl groups de-creases more (�Dcvinyl ¼ 0.12e0.14 vinyl/1000 C) than in dark andmelt flow index decreases significantly (DMFI ¼ �0.11 g/10 min)independently of the concentration of b-carotene. This change canbe attributed to the reactions of vinyl groups leading to extension ofpolymer chains after total consumption of the phosphorous anti-oxidant. Carbonyl groups form, their concentration increases withb-carotene content. Fig. 12 reveals that the formation of carbonylgroups is accompanied by a decrease in the number of vinyl groupsparticipating in reactions. This and the independence of MFI fromthe concentration of b-carotene indicate that b-carotene oxidisesprimarily and protects the polymer from degradation in someextent.

We can conclude that storage in light results in reactions of theunsaturated groups of polyethylene leading to long chain branch-ing. After an induction time b-carotene decomposes in oxidationprocesses which protect the polymer from degradation in someextent and result in fast fading of polyethylene.

4. Conclusions

Studies of the effect of b-carotene in polyethylene stabilisedwith a combination of a-tocopherol and phosphonite reveals thatthe additive with its eleven conjugated double bonds colours thepolymer significantly but does not reduce the stabilising efficiencyof antioxidants either during processing or in the course of storageat ambient temperature. b-Carotene reacts with molecular oxygenboth at high and ambient temperatures. It hinders the oxidation ofpolyethylene completely under the processing conditions whichcannot be avoided with the usually applied phenolic/phosphorousantioxidant packages. The direction of reactions of b-carotene atambient temperature is controlled by the absence or presence oflight, as well as the concentration of oxygen. The molecular struc-ture of polyethylene is primarily determined by the stabilising ef-ficiency of antioxidants and is not affected significantly by thereactions of b-carotene during storage at ambient temperature. Thecolour of the polymer depends on the nature of the reactionproducts of b-carotene. Deepening of orange colour occurs in dark,while the polymer fades rapidly in light after an induction period.This special character of colour changes could be utilised indeveloping functional packaging materials with indicator function.

Acknowledgements

The authors express their thanks toTVK and personally to ÁgnesOrbán-Mester for providing nascent polyethylene powder for theexperiments. The National Research Fund of Hungary (OTKA K77860 and K 101124) is greatly acknowledged for the financialsupport of the research.

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