a comparative study of uv aging characteristics of poly(ethylene-co-vinyl acetate) and...

POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2005; 16: 61–66

Published online 16 November 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.538

A comparative study of UV aging characteristics of

poly(ethylene-co-vinyl acetate) and poly(ethylene-co-vinyl

acetate)/carbon black mixture

Mehmet Copuroglu and Murat Sen*Polymer Chemistry Division, Department of Chemistry, Hacettepe University, 06532, Beytepe, Ankara, Turkey

Received 17 May 2004; Revised 9 July 2004; Accepted 5 September 2004

In this comparative study, the effect of carbon black (CB) on the UV aging characteristics of

poly(ethylene-co-vinyl acetate) (EVA) was investigated. EVA, containing 13% vinyl acetate (VA),

and poly(ethylene-co-vinyl acetate)/carbon black mixture (EVA/CB), containing 13% VA and 1%

CB, were aged by means of UV light with a wavelength in the vicinity of 259nm, in air, up to

400hr. Sol-gel analyses were made to determine the percentage gelation of both virgin and aged

samples. FT-IR measurements were performed to follow the chemical changes which took place

in the samples during aging. Dynamic and isothermal thermogravimetry studies were performed

for determination of the thermal stabilities of virgin and aged samples.

Sol-gel analysis results showed that EVA itself has a tendency to form a gel under UV irradia-

tion. EVA/CB, however, becomes a gel to a smaller extent, comparatively, under the same condi-

tions. As a result of FT-IR measurements, some oxidation products such as ketone, lactone and

vinyl species were observed through UV ageing of EVA and EVA/CB. Thermal analysis experi-

ments exhibited that the thermal stabilities of EVA and EVA/CB decreased, to a similar extent

through UV aging. Copyright # 2004 John Wiley & Sons, Ltd.

KEYWORDS: poly(ethylene-co-vinyl acetate); carbon black; aging; degradation; FT-IR

INTRODUCTION

Restricted properties and limited use of homopolymers

alone, has given rise to exploration of composites, copoly-

mers, blends, etc. Copolymers such as poly(ethylene-co-vinyl

acetate) (EVA), ethylene-butyl acrylate, ethylene-ethyl acry-

late have wide range of usages in different industries. Among

the numerous ethylene copolymers, due to its wide range of

properties depending on its vinyl acetate (VA) content, EVA

has become one of the most useful copolymers in the trans-

portation industry as an insulator, in the electric industry as

a cable insulator, in the shoe industry as soles, and in many

other industries as an hot melt adhesive, a coating, etc. Sev-

eral pieces of work have looked at EVA itself and also its

degradation characteristics.

One of the earliest studies on UV irradiation of polymers,

was performed by Geuskens et al.1 They investigated and

compared the influences of UV light and gamma irradiation

on poly(vinyl acetate) (PVA). They estimated the products of

photolysis and radiolysis of PVA. They also proposed some

mechanisms for chemical changes undergoing during photo-

lysis and radiolysis of PVA.

Lacoste and Carlsson2 undertook a study of gamma-,

photo-, and thermally-initiated oxidation of linear low

density polyethylene (LDPE), and compared the detailed

oxidation products for different initialization types. They

observed common oxidation products, such as hydroper-

oxide, ketone, vinyl and ester groups, for all types of

initialization but with quantitative differences.

Artificial ageing studies on high density polyethylene by

UV irradiation in the presence of air was performed by

Carrasco et al.3 They observed structural modifications and

chemical changes during UV irradiation; and these were

explained via chain breaking, branching, crosslinking and

oxidation phenomena.

Recently, in order to investigate the synergism between

carbon black (CB) and different stabilisers, Pena et al.4 studied

the photodegradation of LDPE. They concluded some results

concerning synergetic and antagonistic effects, in which CB

played an important role for all combinations.

Ageing characteristics of EVA and poly(ethylene-co-vinyl

acetate)/carbon black mixture (EVA/CB) are very important

because of their usages in the fields where the products are

exposed to sunlight and/or heat. In some countries, like in

Turkey, climate has a great effect on the aging of EVA. EVA/

CB with 13% VA and 1% CB is a widely used polymer in

particular by Turkish State Railways (TCDD) due to its elastic

structure and insulation property. In our previous study,5 we

investigated the accelerated thermal aging characteristics of

Copyright # 2004 John Wiley & Sons, Ltd.

*Correspondence to: M. Sen, Polymer Chemistry Division,Department of Chemistry, Hacettepe University, 06532,Beytepe, Ankara, Turkey.E-mail: [email protected]

EVA and EVA/CB; and we concluded that CB is a very

effective stabilizer against thermal degradation of EVA at

moderate temperatures. In this comparative study; acceler-

ated UV aging characteristics of EVA (13% VA) and EVA/CB

(13% VA and 1% CB) were investigated.

EXPERIMENTAL

MaterialsEVA, containing 13% VA and of density 0.9288 kg l�1, was

supplied by Elf-Atochem Co. in the form of granules. EVA/

CB plates with contents of 13% VA and 1% CB were obtained

from Panel Co., Inc., Turkey. EVA, used in the preparation of

this EVA/CB mixture, obtained from Elf-Atochem Co.;

whereas masterbatch (PE Black 99209) from Viba Co., Italy,

in the form of 50% dispersion of CB, type SRF, in LDPE.

Xylene, used as a solvent for EVA and EVA/CB and for the

determination of gelation, was obtained from Merck.

Aging of materialsEVA granules, and EVA/CB specimens of dimensions

3.4 mm� 3.9 mm with 1.9 mm thickness were irradiated

with UV light with an intensity of 12,000 lm at a distance of

15 cm, in air, at ambient temperature; and were aged at dif-

ferent intervals of time up to 400 hr.

Analyses

Determination of percentage gelationsInorder to investigate the influence of UV light on the gela-

tions of EVA and EVA/CB, sol-gel analyses were performed.

Xylene was used as a solvent in the soxhlet extractor and

was fluxed through each sample for 14 hr. Gel percentages

were calculated gravimetrically according to the following

equation:

% Gel ¼ m=m0 � 100

where m0 and m are the masses of a sample before and after

extraction, respectively.

FT-IR studiesFT-IR studies were carried out by means of Nicolet 520 model

spectrometer. Samples were pressed in a hotplate at about

1008C for 10 sec in order to obtain film forms. Samples, cross-

linked to a high extent, were scraped to obtain tiny particles

which were then mixed with KBr.

Thermogravimetric analysis (TGA)Thermogravimetric analyses were performed by utilizing Du

Pont Instruments-Thermal Analyzer, Model 951. Dynamic

thermogravimetric studies were carried out under nitrogen

atmosphere; and 108C/min heating rate was used. Dynamic

thermogravimetric study results indicated the thermal

stabilities of virgin and aged EVA and EVA/CB. Two types

of isothermal thermogravimetric studies were performed.

One was in nitrogen atmosphere at 3508C, and was used to

determine the percentages of volatile degradation products

of virgin and aged samples. The other was under oxygen

atmosphere at 1958C; and was used to determine the

thermo-oxidative stabilities of virgin and aged EVA and

EVA/CB.

RESULTS AND DISCUSSION

Sol-gel analysesBefore investigation of chemical changes in the structure of

EVA and EVA/CB, the effect of UV aging on the gelation of

EVA and EVA/CB was investigated. The effect of UV aging

on the percentage gelations of EVA and EVA/CB are given in

Fig. 1. As shown in Fig. 1 percentage gelation increased up to

88% for EVA with UV aging time, whereas up to merely 11%

for EVA/CB.

Gelation in polymers is generally referred to as cross-

linking of macromolecules by means of covalent bonds.

Sharp increase in the percentage gelation indicates that EVA

can crosslink easily by UV irradiation. Crosslinking most

likely occurs due to combinations of the macromolecular

radicals formed during UV irradiation, as explained in the

literature for polyolefins.6–8 Since the main chains of EVA

(with a content of 13% VA) macromolecules resemble those of

polyethylene (PE), it shows similar behavior to that of PE. An

explanation proposed by Geuskens et al.1 for PVA, might also

be valid for EVA. This mechanism is shown in Scheme 1.

The extent of this mechanism is small when compared to

that of PVA, of course, due to the rare occurrence of acetate

groups on the main chains of EVA macromolecules.

In order to determine the crosslink and the chain scission

reaction yields occurring during UV aging of both EVA

and EVA/CB, the usual Charlesby–Pinner equation was

modified. Charlesby–Pinner equation,9 (sþHs¼ po/qoþ 2/

(qou2,0 D)), has been used by many researchers so far for

simultaneous determination of the crosslinking and the chain

scission reaction yields of polymers being irradiated by

ionizing radiation. In this equation, s is the sol fraction, po is

the chain scission yield (average number of main chain

scissions per monomer unit and per unit dose), qo is

crosslinking yield (proportion of monomer units crosslinked

per unit dose), u2,0 is initial weight (average degree of

polymerization), and D is irradiation dose. Although this

equation was derived from by Charlesby–Pinner for deter-

mination of crosslinking and chain scission yields for

irradiated polymers such as gamma rays or electron beam,

it is assumed that this general equation can also be used for

the simultaneously and UV crosslinked and degraded

polymers by simple replacement of dose with aging time.

Figure 1. Variation of percentage gelation with UV irradia-

tion time.

Copyright # 2004 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2005; 16: 61–66

62 M. Copuroglu and M. Sen

Replacing the UV aging time instead of irradiation dose of

ionizing radiation in the Charlesby–Pinner equation, it

becomes as follows:

sþps ¼ po=qo þ 2ðqou2;0tÞ

where t is aging time.

In this equation, u2,0 was calculated as the ratio of weight

average molecular weight (obtained from Elf-Atochem as

95,000 g mol�1) to average weight of a monomer unit

(calculated as 30.8), and was found to be 3084. Plots of the

reciprocal of aging time, 1/t, versus sþHs; yield straight

lines (with a regression coefficient, r¼ 0.972 for EVA and

0.999 for EVA/CB). and po/qo and qo values were calculated

from the intercept and the slope of the lines, respectively.

As a result of the calculations, po and qo (per hour) were

found to be 2.84� 10�3 and 8.20� 10�3, respectively, for

EVA; and 5.73� 10�2 and 3.21� 10�2, respectively, for EVA/

CB. Although the values for EVA/CB differ from those of

EVA, when they are evaluated individually, one can

conclude that EVA has a tendency to form crosslinks rather

than chain scission, since po/qo¼ 0.346. However, with a

value po/qo¼ 1.79, EVA/CB seems to be a mixture which

cannot crosslink readily upon UV irradiation. These results

are parallel with those obtained for thermally aged EVA and

EVA/CB,5 since the materials showed similar tendencies.

FT-IR studies and UV degradation mechanismsof EVA and EVA/CBEVA and EVA/CB have almost the same spectra, because CB

itself has no remarkable characteristic absorption bands and

thus, does not influence the spectrum of EVA. The FT-IR spec-

tra of virgin and UV aged EVA and those of EVA/CB are

given in Figs. 2 and 3 respectively, in the range 1950–

1550 cm�1. Main functional groups changes were observed

in this region. In order to calculate the absorbance value of

a band which overlapped with another band or bands in

the FT-IR spectra, a band separation software program was

used. Absorption at 723 cm�1 (A723) is assigned to –CH2–

CH2–CH2– sequences. It was supposed that, during UV

degradation, the amount of these methylene sequences

remains almost constant with respect to the other bands at

which significant changes occurred. Therefore, regarding

the absorption at this wavenumber as constant, changes at

any other band can be monitored.

The main UV degradation product of EVA is acetic acid

which forms due to ester elimination. Mechanism of ester

elimination is likely to be that given in our previous paper;5 so

it is anticipated that the absorption at 1740 cm�1 (A1740)

should decrease with UV ageing. The formation and the

Scheme 1.

Figure 2. FT-IR spectra in the range 1950–1550 cm�1 of

virgin and UV aged EVA. Numbers on the curves represent

UV aging time (in hours).

Figure 3. FT-IR spectra in the range 1950–1550 cm�1 of

virgin and UV aged EVA/CB. Numbers on the curves

represent UV aging time (in hours).

UV aging characteristics 63


destruction of aldehydes (Schemes 2 and 3 in ref. 5) should be

taken into account. In the case of EVA, the decrease in the

absorption at 1740 cm�1 was observed merely up to around

50 hr in EVA (Fig. 4a). From this point on, a continuous

development of aldehydes is predominant up to around

150 hr. The ratio is almost constant between 150–400 hr. In the

case of EVA/CB (Fig. 4a), the formation of aldehydes is

predominant in the early stages of UV aging. The formation

rate of aldehydes after around 100 hr is very slow when

compared to that of EVA, which results in a decrease in the

absorbance at 1740 cm�1. It can be concluded that the

presence of CB can prevent the formation of aldehydes to

some extent. The amount of aldehydes in the virgin EVA/CB

is twice that of EVA. This might be due to some oxygen-

containing groups which could be formed in EVA/CB during

its manufacturing stages.

Ketone formation was observed in UV aged EVA and

EVA/CB. Mechanisms of ketone formation via degradation

(either non-oxidative or oxidative) were proposed in the

literature,6,10,11 Variation of ketone groups was followed by

the ratio of absorbance at 1715 cm�1 (A1715) to absorbance at

723 cm�1. As can be seen from Fig. 4(b), ketone formation in

EVA increased continuously until the end of aging. In EVA/

CB, the profile of the change is very similar to that of EVA.

Another species formed as a consequence of UV aging is

lactone. The mechanism given in Scheme 6 in ref. 5 was

proposed for lactone formation associated with a back-biting

process in the VA moieties by the acetate groups forming

methane. Lactone formation in UV aged EVA and EVA/CB

was followed by the ratio of absorbance at 1780 cm�1 (A1780)

to absorbance at 723 cm�1. The change of A1780/A723 with UV

ageing time is given in Fig. 4(c). Lactone formation increased

rapidly in the initial stage of the aging (up to around 100 hr).

This formation is almost constant between 100–250 hr. After

that step, an increase again is observed up to 400 hr.

However, the amount of formation of lactone is less than

that of ketone in every step of aging, since ketone formation is

easier than lactone formation due to the conformational

arrangements. The presence of CB prevents this formation to

a small extent; most probably by hindering the intermole-

cular back-biting processes of macromolecular segments

required for the formation of lactones.

Samples, which were subjected to UV aging (except CB

containing ones), exhibited yellowing behavior during this

procedure. It is well known that yellowing (or even darker

colors from brown to black) is a consequence of formation of

unsaturated groups.6,8 These unsaturated groups are gen-

erally vinyl, vinylidene, and trans-vinylene groups which are

responsible for coloring. Mechanisms of formations of trans-

vinylene, vinyl, and vinylidene are given in Schemes 7 and 8

in ref. 5 and also in ref. 12. The results of normalization for the

band at 910 cm�1 (A910) for which the vinyl groups formed in

UV aged EVA and EVA/CB are shown in Fig. 4(d). Vinyl

content in EVA increased continuously up to around 100 hr.

After that time, a slight decrease was observed until 400 hr

which might be due to the increase in the crosslinking ratio. In

the case of EVA/CB, however, this formation is hindered by

CB due to its radical delocalizing property. The formation of

Figure 4. Some chemical changes observed by means of FT-IR spectroscopy during UV

aging. (a) Changes occurred at 1740 cm�1 during UV aging of EVA and EVA/CB. (b) Evolution

of keto groups at 1715 cm�1 during UV aging in EVA and EVA/CB. (c) Evolution of lactone at

1780 cm�1 during UV aging in EVA and EVA/CB. (d) Vinyl formation at 910 cm�1 during UV

aging in EVA and EVA/CB.



vinylidene and trans-vinylene were not clearly observed due

to high noise to signal ratio. This resulted from the difficulties

in the preparation of thin films from crosslinked granules.

Influence of UV aging on the thermal stabilitiesof EVA and EVA/CBFigures 5 and 6 show the dynamic TGA thermograms of vir-

gin and UV aged EVA and EVA/CB, in nitrogen atmosphere,

respectively. As shown in Figs. 5 and 6 virgin EVA and EVA/

CB show the typical step degradation profile with the initial

stage involving acetic acid evolution and the second invol-

ving main chain degradation.10,13 Figures 5 and 6 indicate

that, when aging time increases, degradation profile changes.

Degradation at both stages becomes easier. Both first and sec-

ond degradation steps are shifted to lower temperatures in

the UV aged EVA and EVA/CB. As a consequence, CB failed

to prevent EVA from UV degradation contrary to that in the

case of thermal degradation.5

Degradation activation energies (E) of EVA and EVA/CB

were calculated by using Freeman–Carroll equation and

the data obtained from derivatives of dynamic thermograms

of UV aged EVA and EVA/CB were used in the calcula-

tions.14

� logðd%=dTÞ� log ð1 � cÞ ¼ n� E

2:3R� �ð1=TÞ� log ð1 � cÞ

whereT is the temperature in Kelvin,n is the order of reaction,

R is the gas constant which has a value of 8.314 J mol�1 K�1, c

is the conversion ratio and is equal to (m0�m)/m0, where m0

is the initial mass and m is the mass at any time.

When D(1/T)/Dlog(1� c) versus Dlog (d%/dT)/Dlog

(1� c) is plotted, the slope of this plot gives the value for E

and the intersection of ordinate indicates the order of

reaction. The c and T values were obtained from derivatives

of thermograms. The effect of UV aging time on the E values

of EVA and EVA/CB are shown in Fig. 7. TheEvalues of EVA

and EVA/CB decreased continuously with increasing UV

aging time. Oxidation and chain scission are the main reasons

for this decrease. The chain scission may occur by a seven-

membered ring transition state without acetal group

elimination. Main chain scission may also occur by the

fragmentation of the polymer alkoxy radical or as the

disproportionation of the polymer alkyl radical.8 Decreases

in the thermal stability and the E value of an oxidized

polymer and the effect of molecular weight on these

parameters have been well described in the literature by

many researchers.15–17

As can be seen from Fig. 5 due to overlapping of first and

second degradation steps, it is not easy to interpret and

determine the amounts of readily decomposing groups from

dynamic TGA thermograms. For a detailed analysis of the

first stage of degradation, isothermal TGA analyses were

performed in nitrogen atmosphere. Isothermal-nitrogen

thermograms of UV aged EVA and EVA/CB were obtained

at 3508C. This temperature is sufficient to break down all

kinds of oxygen-containing groups. Figure 8 shows the

percentages of volatile products obtained from these thermo-

grams at the end of 70 min. Fig. 8 also indicates that the

amount of volatile products increases with UV aging time.

EVA/CB has low resistance to thermo-oxidation; and the

change in EVA/CB during UV irradiation is similar to that in

EVA, contrary to thermally aged EVA/CB.5 These results are

consistent with the results of FT-IR and dynamic thermo-

gravimetry studies.

Isothermal thermogravimetry studies which were carried

out in oxygen atmosphere give important information

concerning the degradation characteristics of materials.

Isothermal thermogravimetry studies in oxygen atmosphere,

were performed at 1958C. Weight increase in the initial stage

Figure 5. Dynamic thermograms of UV aged EVA in

nitrogen atmosphere. Numbers on the curves indicate the

UV aging time (in hours).

Figure 6. Dynamic thermograms of UV aged EVA/CB in

nitrogen atmosphere. Numbers on the curves indicate the UV

aging time (in hours).

Figure 7. Variation of degradation activation energies with

UV aging time for EVA and EVA/CB.

UV aging characteristics 65


of degradation indicates the oxygen penetration into the

polymer structure.18,19 EVA has an oxygen uptake capacity

which diminishes through UV aging time and then falls

completely to zero. This can be explained by the rapid

formation of decomposable oxygen-containing species such

as aldehydes or ketones through UV ageing. When this

formation reaches an equilibrium, uptake of oxygen stops.

Oxygen uptake capacity falls to zero in EVA/CB quicker than

that of EVA. Oxidation, undergoing during UV aging, also

makes EVA/CB relatively more easily degradable with

increasing UV irradiation time. Lower degree of crosslinking

in EVA/CB during UV aging is another reason for much

more facile decomposition. Moreover, time to reach the 10%

mass loss increased sharply after around 116 hr in EVA

(Fig. 9). This increase might be due to the higher crosslinking

of EVA macromolecules and the formation of relatively more

sustainable oxygen-containing groups such as carboxylic

acids. In EVA/CB, time period for 5% mass loss decreases

suddenly up to 24 hr, then remains unchanged (Fig. 9).

CONCLUSIONS

In order to investigate the UV aging characteristics of EVA

and EVA/CB, samples were subjected to UV irradiation at

ambient temperature and in air up to 400 hr. Sol-gel analyses

showed that EVA has a tendency to crosslink under UV irra-

diation.

FT-IR spectra of virgin and UV aged EVA and EVA/CB

showed that, some chemical changes took place in EVA and

EVA/CB during UV ageing (such as the formation of ketone,

lactone and vinyl species).

Dynamic thermograms, which were carried out in nitrogen

atmosphere, indicated that the thermal stabilities of EVA and

EVA/CB decreased during UV aging. Isothermal TGA

analyses, performed at 3508C and in nitrogen atmosphere,

indicated that the concentration of volatile products

increased during UV aging both in EVA and EVA/CB. As a

result of isothermal thermogravimetry studies, which were

carried out at 1958C and in oxygen atmosphere, the oxygen

uptake capacities of EVA and EVA/CB decreased through

UV aging time and fell to zero eventually. Time to reach the

10% mass loss in EVA increased sharply after around 116 hr,

whereas time passed for 5% mass loss in EVA/CB decreased

suddenly up to around 24 hr with UV irradiation time; then it

remained almost constant.

As a conclusion, all these studies show that, 1% CB is not a

very effective stabilizer against UV degradation of EVA at

moderate temperatures.

AcknowledgmentsThe authors would like to thank Prof. Dr Olgun Guven for the

laboratory facilities he supplied and for his encouraging and

constructive advice during this study.

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Figure 8. Percentage of volatile products against UV

irradiation time.

Figure 9. Time required to reach the 10 and 5% mass

losses for UV aged EVA and EVA/CB, respectively.



a comparative study of uv aging characteristics of poly(ethylene-co-vinyl acetate) and...

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