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Polymer Degradation and Stability

Characterization of oxidation progress by chemiluminescence:A study of polyethylene with pro-oxidant additives

Marek Koutny

′a ,*,Tereza Va ′clavkova ′a ,Lyda Matisova ′-Rychla ′b ,Jozef Rychly ′b a Department of Environmental Protection Engineering,Faculty of Technology,Tomas Bata University in Zlin,76272Zlin,Czech Republic

Polymer Institute,Slovak Academy of Sciences,84236Bratislava,Slovak Republic

a r t i c l e i n f o

Article history:

Received 22January 2008

Received in revised form 23April 2008Accepted 14May 2008

Available online 22May 2008

Keywords:

Non-isothermal and isothermal chemiluminescence Rate of oxidation Carbonyl index

Hydroperoxides concentration Polyethylene Pro-oxidants

a b s t r a c t

Non-isothermal chemiluminescence measurements in nitrogen and isothermal measurements in oxygen were used for the evaluation of degradation in pre-oxidized polyethylene either pure or containing Mn-based pro-oxidant additives.The results were compared with infrared spectroscopy data.Chem-iluminescence measurements of pure polyethylene and polyethylene with additive made it possible to calculate the set of rate constants,based on the Bolland–Gee oxidation scheme.The oxidation rate constants of polyethylene with additive were signi?cantly higher,while the activation energy of the process appeared lower (65kJ mol à1),than those of pure polyethylene.The method provides an access to study oxidation processes during the induction period of oxidation when infrared spectroscopy cannot provide suf?cient information.

1.Introduction

Polyethylene (PE)is not as frequently studied by chem-iluminescence (CL)as is polypropylene [1–5].This is obviously due to the less intense light emission related to the higher thermo-oxidation stability of the former.Two basic approaches can be used to characterize polymers and their oxidation status by CL.Oxidation and measurement of CL under oxygen or air can be done directly inside a chemiluminescence instrument.In the case of PE,CL then manifests itself as a double sigmoidal increase of the light intensity,or the oxidation is done outside the instrument and CL is sub-sequently recorded during a non-isothermal run under nitrogen atmosphere.The latter approach is usually used for the monitoring of transient structures in polymers.

The idea has been proposed that the strongest light emission is related to polymer chain hydroperoxidation on b -positions next to electron-withdrawing groups such as carbonyls,carboxyls,amides,hydroxyls,etc.[6].The CL signal measured in nitrogen re?ects the concentration of peroxidic groups in the polymer,regardless of the fact that such hydroperoxides are usually considered to be only

a fraction of total titratable peroxides [7].This may be due to the

link between the rate of secondary peroxyl radical disproportion-ation,the ultimate process probably responsible for the light emission,and the rate of hydroperoxide decomposition,which is the initiation step of the proposed mechanism (Scheme 1).Though a complete understanding of the CL phenomenon in polymers is not yet available and an effort to describe the process represents a challenge for those who want to predict the remaining service life of polymer materials,such a technique may provide useful in-formation on the extent of polymer stabilization,on initial molar mass effects and even on rate constants of individual reactions in the course of polymer degradation [8–11].

Polyethylene (PE)?lms with pro-oxidant additives are newly emerging materials bringing together the solution of plastic litter problems with the continual use of existing production and pro-cessing technologies.Their primary use can be found in agricultural mulching ?lms and packaging ?lms as well as in other products with a limited lifetime,e.g.bags [12–15].Since pro-oxidants ac-celerate initiation of oxidation we might believe that CL measured will be much more intense than from reference sample and the method proves itself as a valuable tool to study such materials.Until now,the progress of oxidation in polyethylene containing pro-oxidants was investigated only by FTIR spectroscopy,eventually complemented with data from gel permeation chromatography.For the induction phase particularly,the above methods do not

*Corresponding author.

E-mail addresses:mkoutny@ft.utb.cz (M.Koutny

′),upolrych@savba.sk

(L.Matisova ′-Rychla ′),upoljori@savba.sk (J.Rychly ′

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Polymer Degradation and Stability 93(2008)1515–1519

provide enough information which could be important,e.g.for the

precise formulation of the material.The chemiluminescence tech-nique which was introduced only recently for the estimation of PE degradation due to pro-oxidants [2]appears,therefore,as a one of alternative approaches how to monitor the degree of oxidation in the above type of materials.2.Experimental 2.1.Material

The investigated sample was low density polyethylene Bralene RB 0323transparent ?lm 50–55m m thick containing micro-milled lime Omyalen 2021P (15%w/w)as ?ller and a commercial pro-oxidant additives Addi?ex HE (5%w/w)based on manganese ions.The isothermal oxidation experiments were performed with polyethylene powder Marlex TR-161,free of any additive.2.2.Chemiluminescence and FTIR measurements

The sample material was cut into strips 10cm long and 2cm wide which were then incubated at 60,70,or 80 C in air.At times predetermined for each temperature,the strips were taken out and their transmission FTIR spectra (Mattson 3000,UNICAM,UK)and non-isothermal CL runs were recorded.Carbonyl index was calcu-lated as the ratio of absorbance values at 1713and 1465cm à1.CL measurements were performed on the photon counting instrument Lumipol 3manufactured at the Polymer Institute of Slovak Acad-emy of Sciences,Bratislava,Slovakia.The instrument dark count rate was 2–4counts/s at 40 C.Three milligrams of the sample oxidized at a given temperature was placed onto an aluminium pan,and analyzed under oxygen or nitrogen at gas ?ow of 3l/h.Iso-thermal experiments were carried out at temperatures 110–150 C,non-isothermal runs from 40to 180 C were done at the heating rate of 2.5 C min à1.The intensity of CL was expressed as counts s à1per mass of the sample.3.Results and discussion

As already indicated,the CL–time curves for thermal oxidation of polyethylene have a double sigmoidal shape with an auto-ac-celerating increase (Fig.1).One can see that the shape of the double waves depends on temperature;below 130 C,both waves in a double wave apparently coalesce.Whilst at 150 C,the transition between the ?rst and the second wave occurs at 55%of the total CL intensity;at 130 C,it is at 28%only.CL attains a maximum after a certain time period and then decays.Fig.1also provides the possibility to compare susceptibilities of the pure and the pro-oxidant containing PE to oxidation.The effect of pro-oxidants is evidently signi?cant.The oxidation of PE with additive at 80 C in air (changes in carbonyl index)appears to be faster than that of pure PE at 110 C in oxygen.The oxidation curves of PE with additive (Fig.2)have a typical auto-accelerating shape.Initial induction periods are 25,9and 5days at 60,70and 80 C,respectively.

To describe theoretically the kinetics of changes in CL intensity and carbonyl index evolution,we assumed the validity of the Bol-land–Gee mechanism of hydrocarbon oxidation.The sequence of reactions occurring in oxidized PE can be then depicted as in Scheme 1.

In the proposed mechanism,the initiation step is the reaction of hydroperoxide decomposition providing alkoxyl,alkyl and in the presence of oxygen ultimately peroxyl radicals which are converted back to hydroperoxides,which thereby accumulate in the system and the rate of oxidation increases autocatalytically.The prevailing termination pathway of free radicals is that of peroxyl radical disproportionation.The majority of authors are convinced that,even at relatively low concentration levels of hydroperoxides,the bimolecular decomposition of hydroperoxides is the rate limiting step [16].

Provided that transition metal ions like Mn 2tplay a role in the decomposition of hydroperoxides,the so-called Haber Weiss cycle of hydroperoxide decomposition may be adopted (Scheme 2).The stoichiometry of the Haber Weiss cycle of hydroperoxide decomposition obtained as a simple sum of both reactions

https://www.wendangku.net/doc/a711488171.html,parison of chemiluminescence runs at temperatures from 110to 150 C in oxygen atmosphere for pure polyethylene (solid line)and carbonyl index evolution at 80 C in air for pro-oxidant containing polyethylene (-).Points ($)correspond to the theoretical ?t of the oxidation runs with Eq.(2).Carbonyl index increases for poly-ethylene with additive at 80 C in air was ?tted with Eq.(1),solid line.Arrows indicate the corresponding vertical

axis.

Fig.2.Carbonyl index evolution of the pro-oxidant containing polyethylene during ageing in air atmosphere at three different

temperatures.

Scheme 1.

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′et al./Polymer Degradation and Stability 93(2008)1515–15191516

Scheme 2is a bimolecular reaction corresponding to the ?rst line of Scheme 1.

According to Ref.[16],the concentration of hydroperoxides [POOH]can be described by the following equation:

?POOH ?

1tY exp eàk 1t T

(1)

where

X ??POOH N

(1a)

Y ?

?POOH N à?POOH 0

?POOH 0

(1b)

?POOH N ?k 4?PH ???????????k 6k bi 2

p (1c)

k 1?k 4?PH

??????k bi

k 6

s (1d)

Except for the paper by George and Celina [9],there were no attempts to analyze the isothermal kinetics of CL signal under ox-ygen atmosphere.In our last papers [10,11],we have shown that isothermal CL runs up to the maximum value of light emission may be well described and ?tted by the following function:

I ?

A exp eàk 1t T?1tY exp eàk 1t T (2)

which corresponds to the rate of hydroperoxide decomposition (Eq.(3))

d ?POOH d t ?A 1exp eàk 1t T

?1tY exp eàk 1t T 2

(3)

Here A and A 1are proportionality constants and k 1and Y follow from Eq.(1).

One can easily see that the differentiation of Eq.(1)with respect to time yields a term corresponding to Eq.(3),i.e.to the rate of hydroperoxide decomposition.

Eq.(1)was used for ?tting the carbonyl index increase,with Eq.(2)for CL–time runs.This re?ects the fact that carbonyl index is related to hydroperoxide concentration while CL intensity is related to the rate of hydroperoxide decomposition.When applying Eq.(2)to experimental CL–time runs we have neglected the ?rst sigmoidal wave in the double wave.As it is evident from Figs.1and 2,the ?t of CL–time runs with Eq.(2)and carbonyl index evolution with Eq.(1)was satisfactory.

On the basis of functions (1)and (2),the complex rate constant k 1was calculated from obtained experimental curves (Table 1)and plotted as the Arrhenius plot (Fig.3).As it was supposed,PE con-taining pro-oxidants has signi?cantly higher values of k 1and a lower value of activation energy (65kJ mol à1)when compared with pure PE which has an activation energy of oxidation of 104kJ mol à1.The latter value is in accordance with the activation energy of hydroperoxide decomposition during oxidation of

polyole?ns and with Ref.[8]where temperature interval 150–190 C was examined for polyethylene (?lled circles in Fig.3).

Fig.4shows the effect of pro-oxidants on CL intensity–time runs in oxygen.Provided that PE was oxidized 1day at 80 C in air when the carbonyl index is still very low (Fig.2)the CL runs at 150 C have qualitatively similar shape as reference sample of pure PE.A shift of both sigmoidal waves to lower times was observed.However,after 11days of oxidation when carbonyl index is high,the sample is largely degraded and behind the induction period,the CL intensity at 150 C in oxygen becomes at least by one order of magnitude higher.The signal attains its maximum and decays.Double sig-moidal shape was not apparent (Fig.4).

The possible reasons for the appearance of relatively complex shape of oxidized polyethylene CL signals were discussed in detail by Broska and Rychly [4].They found that the ?rst increase of the light emission corresponds to the decay of vinyl group concentra-tion.One should,however,also take into account the fact that the ?rst increase is reduced with decreasing temperature (Fig.1).When the deconvolution of the original CL intensity–time course at 150 C is done as the superposition of two Gaussian curves,the ?rst one accounts for only 10%of the total area (Fig.5).One may also notice that the ?rst wave of the double sigmoidal wave appears after some delay when the evolution of the oxidation process represented by the second wave is already in progress.

Samples aged for a certain time in air within temperature interval from 60to 80 C were subsequently examined by non-isothermal CL measurements in nitrogen atmosphere (Fig.6).Thus the immediate comparison of CL runs for the respective points

Scheme

Table 1

Rate constant k 1of hydroperoxide decomposition determined from oxidation of pure (chemiluminescence)and with additive (carbonyl index)polyethylene ac-

Fig.3.Arrhenius plot of the oxidation rate constants determined for pure (1)poly-ethylene and (2)polyethylene with additive from chemiluminescence (1)and carbonyl index measurements (2).Data from paper [8]corresponding to the rate constant k 1(Eq.(1d))in the temperature interval from 150to 190 C are indicated by ?lled circles and dotted line.

M.Koutny ′et al./Polymer Degradation and Stability 93(2008)1515–15191517

carbonyl index vs.time curves may be provided.The applied temperature ramp spanned the interval from 40to 180 C at the rate of 2.5 C min à1.Within the induction period of oxidation,as controlled by carbonyl index measurement,the course of CL intensity during temperature run may be characterized by an in-crease until maximum temperature 180 C is attained.With the time of ageing the increase is shifted towards lower temperatures and when the ageing time gets over the induction period the in-crease disappears and is replaced by a distinct and broad peroxidic peak extending from 60to 130 C (Fig.6).This peroxidic peak is disrupted slightly by the signal of melting polyethylene crystallites during which the geometry of the sample changes.The data from steady increase of CL intensity for PE in inert atmosphere appearing above 150 C and modi?cations of the curve shape during the progress of a sample ageing were not used for PE characterization until now except of paper by Broska and Rychly [4].There the au-thors assume that the steady increase of CL at higher temperatures in nitrogen might be due to scissions of C–C bonds on defect macromolecular chains.The alkyl radicals recombine and the released heat transfers suitable acceptors (carbonyl groups)to an excited state.An alternative explanation may be that temporary

crosslinking events,via C ]C double bonds,occur.Such process is exothermic probably capable of exciting chromophores present in PE from previous treatments.The disappearance of the described increasing signal with the progress of oxidation and peroxidic peak formation indicates that the polymer chains become too short and bonds in the main chain cannot be further split or the crosslinking cannot take place.Courses of CL intensity during non-isothermal runs under nitrogen for samples aged at 70 C for 3,6and 12days and carbonyl index evolution curve at 70 C are compared in Fig.7.The area below the peroxidic peak corresponds to the relative concentration of hydroperoxides in aged samples.Their relative content increases with days of PE ageing to a maximum and then decays.However,the maximum value increases with decreasing temperature (Fig.8),which is in agreement with the classical Bol-land–Gee scheme of hydrocarbon oxidation (see Eq.(1b)–(1d)).Provided that stationary concentration of hydroperoxides [POOH]N corresponds to the maximum value of the area below the peroxidic peak obtained during non-isothermal chem-iluminescence measurements,we can see why this maximum

value

https://www.wendangku.net/doc/a711488171.html,parison of oxidation runs for pro-oxidant containing polyethylene and pure

polyethylene at 150 C in

oxygen.

Fig.5.Deconvolution of double chemiluminescence intensity–time run in oxygen at 150 C in

oxygen.

Fig.6.Non-isothermal chemiluminescence runs in nitrogen atmosphere with pro-oxidant containing polyethylene after different periods of ageing at 60 C in air.Numbers denote days of

ageing.

Fig.7.Non-isothermal chemiluminescence runs under nitrogen atmosphere (lines)for pro-oxidant containing polyethylene after different periods of ageing at 70 C in air.Numbers denote days of ageing.The evolution of carbonyl index is displayed in an opposite time scale of the upper axis,points (-).Arrows indicate corresponding axes for a particular curve.

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′et al./Polymer Degradation and Stability 93(2008)1515–15191518

should increase with decreasing temperature.The resulting tem-perature coef?cient for the term (Eq.1c )is obviously negative:

E [POOH]N ?E 4à1/2E bi à1/2E 6;here E 4z 30–40kJ mol à1,E bi z 90–120kJ mol à1,and E 6z 10–15kJ mol à1.4.Conclusions

The examination of pre-oxidized PE by isothermal and non-isothermal chemiluminescence methods appears to be useful for the description of PE degradation progress.During the induction period,as estimated by FTIR,the subsequent CL measurements under nitrogen atmosphere display a steady increase of light emission with increasing temperature.However,with the progress of oxidation this increase is shifted to lower temperatures and,after the induction period it almost disappears and quite well developed hydroperoxide peaks may be observed under such conditions.After the induction period,the hydroperoxide concentration ?rst in-creases with the time of sample ageing,reaches a maximum,and then decreases.The value of the maximum is inversely proportional to the temperature of sample ageing.These observations were found to be in good agreement with the Bolland–Gee oxidation mechanism.Oxidation rate constants of PE with Mn ion-based

pro-oxidant additives are signi?cantly higher than those of pure PE

while the activation energy is lower (65kJ mol à1).Acknowledgments

This study was supported by the Czech Ministry of Education,Project No.MSM 7088352101and by grant Agency VEGA SAV,Projects No.2/5109/27and 2/6115/27.References

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Fig.8.Evolution of hydroperoxide peak areas corresponding to hydroperoxide con-tents in polyethylene during sample ageing at respective temperatures.

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