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Rubber Thermooxidation Induction Period Antioxidants Protection Factor Antioxidant Effectiveness Synergy

The aim of this paper is to present a method for the evaluation of the stabi-lizing effect of various antioxidants and their mixtures in rubber matrices. The method is based on three criteria, viz. the protection factor, antioxidant effec-tiveness, and S-factor enabling to as-sess the synergy/antagonism in the sta-bilizer mixture. For the treatment of the experimental results, a procedure of obtaining the kinetic parameters from non-isothermal differential scanning calorimetry measurements, based on the dependence of onset temperature of oxidation peak on heating rate, was applied. Some of our previous results on the evaluation of the antioxidants performance in rubber matrices are briefly reviewed as case studies de-monstrating usefulness of the method.

Untersuchung der Leistungsfä-higkeit von Antioxidantien für Kautschukmatrizen mit der Dif-ferentialleistungskalorimetrieKautschuk Thermooxidation Indukti-onszeit Antioxidantien Schutzfaktor Anitoxidantienwirksamkeit Synergie

Ziel ist es, eine Methode für die Bewer-tung des Stabilisierungseffekts von ver-schiedenen Antioxidantien und ihren Mischungen in Kautschukmatrizen zu präsentieren. Die Methode basiert auf drei Kriterien, nämlich derm Schutzfak-tor, der Antioxidantienwirksamkeit und dem S-Faktor, um die Einschätzung ei-nes Synergismus/Antagonismus in einer Stablisatormischung zu ermöglichen. Für die Behandlung der experimentellen Ergebnisse wurde eine Vorgehensweise für das Erhalten der kinetischen Para-meter aus nicht-isothermen differential-kalorimetrischen Messungen, die auf der Abhängigkeit der Onset-Temperatur des Oxidationssignals von der Heizrate basiert, angewendet. Einige unserer frü-heren Ergebnisse hinsichtlich der Leis-tungsfähigkeit von Antioxidantien in Kautschukmatrizes werden kurz als Fall-studie nachgeprüft, um die Brauchbar-keit der Methode zu zeigen.

Figures and Tables:By a kind approval of the authors.

IntroductionAgeing of materials represents one of the most serious problems of everyday life leading to the degradation of their properties. The most common ageing causes are heat, oxygen, radiation and ozone. The combination of oxygen as the reactant and heat as the energy source is a major factor in material degradation. Elastomeric materials are especially sen-sitive to thermooxidative degradation. The thermal oxidation of elastomers is an autocatalytic radical chain process where the oxidation products are car-boxylic acids, ketones, aldehydes, epox-ides, etc. Hence, the oxidation leads to changes in molecular structure that sub-sequently bring about changes in physi-cal and physico-chemical properties of elastomers.

In most cases, the oxidation processes occurring in the condensed phase exhibit an induction period (IP) – a stage where seemingly no chemical reactions take place [1]. The induction period of oxida-tion is determined as the time of a sud-den increase in the rate of oxidation. At the end of induction period, also a sud-den change in material characteristics mostly takes place so that the length of induction period is often considered as a relative measure of material thermooxi-dative stability [1, 2]. Since the oxidation is an exothermic process and is accompa-nied with the mass loss and evolution of light, thermoanalytical methods such as the differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermogravimetry (TG) or chemiluminis-cence can be employed for its study. In the case of rubber matrices, the heat evolved during thermooxidation is relia-bly measured by DSC or DTA.

The rate of degradation processes un-der application conditions (temperature) is usually very slow. Therefore, to esti-mate the stability of materials, a sample is mostly subjected to an accelerated test under standardized conditions. The most common means of accelerating the oxi-dation is the heating of material. Two types of oxidative induction tests are used in practice: (i) the oxidation induc-

tion time (OIT) and (ii) the oxidation on-set temperature (OOT) [3]. In the case of OIT, the sample is held at a preset con-stant temperature and the time of the sudden change of measured signal (exo-thermal effect) is detected. The OOT is determined as the onset temperature of oxidation while the temperature is raised at a constant heating rate. The standard tests for induction period determination are predominantly carried out under iso-thermal conditions. However, under iso-thermal conditions, the peak measured using DSC or DTA is often flat and its on-set, corresponding to the end of induc-tion period, cannot be determined un-ambiguously. Additionally, a significant period of time is required to achieve the constant elevated temperature which brings about systematic errors of meas-urements. In our previous studies [1, 4, 5] it has been shown that, contrary to the problems associated with the OIT deter-mination, the transformation peak of the processes at various heating rates is dis-tinct so that OOT can be measured accu-rately and unambiguously. Hence, a method has been proposed for the deter-mination of the kinetic parameters of induction periods from the onset tem-peratures of nonisothermal DSC runs using linear heating rates [1].

Performance of Antioxidants for Rubber Matrices evaluated by differential Scanning Calorimetry

AuthorsZuzana Cibulková, Peter ŠimonBratislava, Slovak Republic Corresponding author:Peter ŠimonInstitute of Physical Chemistry and Chemical PhysicsFaculty of Chemical and Food TechnologySlovak University of TechnologyRadlinského 9812 37 Bratislava, Slovak RepublicE-Mail: peter.simon@stuba.sk peter.simon@stuba.sk

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The aim of this work is to review and summarize some of our recent applica-tions of the above mentioned method employing both Arrhenius and non-Ar-rhenian temperature functions in the evaluation of the thermooxidative stabil-ity of rubber matrices.

Theoretical partProcesses in condensed phase, such as thermooxidation, are very complex in general. Their mechanisms are very often unknown or too complicated to be char-acterised by a simple kinetic model since they tend to occur in multiple steps with different rates. For the accurate descrip-tion of the kinetics of a complex process, each elementary reaction step should be described by its own kinetic equation so that a set of (many) differential kinetic equations should be solved. For obtain-ing the kinetic parameters needed in the set of kinetic equations, concentrations of all intermediates and products should be measured. This procedure would be very laborious and hardly feasible. There-fore, much simpler methods, based on the single-step approximation, are ap-plied for the description of the kinetics of the condensed state processes.

Rate of the processes in condensed state is generally a function of tempera-ture and conversion:

( )d ,d

Ttα

α= Φ (1)

Within the single-step approximation it is assumed that the function Φ in Eq. (1) can be expressed as a product of two separable functions independent of each other, the first one, k(T), depending only on the temperature, T and the other one, f(α), depending only on the conversion of the process, α. The rate of the process can be then formally described as [6]:

( ) ( )dd

k T ftα

α= (2)

Equation (2) is called the general rate equation. Indeed, it resembles a single-step kinetic equation, even though it is a representation of the kinetics of a com-plex condensed-phase process. The sin-gle-step kinetics approximation thus re-sides in substituting the set of kinetic equations by the sole single-step kinetics equation. Equation (2) represents a mathematical formulation of the single-step kinetics approximation [6, 7].

The temperature function k(T) is most frequently expressed by the Arrhenius relationship:

kexp - Ek A

RT =

(3)

where Ak is the preexponential factor, E is the activation energy, T is absolute tem-perature and R is the gas constant.

Combining Eqs. (2) and (3), after sepa-ration of variables and integration under assumption that the conversion is the same for any temperature regime, one can obtain:

i

0

d1exp

t tBA T

=

∫ (4)

where ti is the length of induction period, i.e. the oxidation induction time, and the constants A and B are given:

( ) ( )i

k

F F 0 EA B RA

α −= = (5)

F(α) in Eq.(5) is the primitive function of 1/f and αi is the conversion of the reac-tions occurring during IP and correspond-ing to the end of IP. The conversion αi is assumed to be the same for any temper-ature regime which is the main idea of the isoconversional methods [8].

Eq. (4) enables to assess the induction period of thermooxidation for any time/temperature regime. The physical mean-ing of the denominator in Eq. (4) can be simply demonstrated for a case of iso-thermal processes where the denomina-tor is a constant equal to the length of the induction period at a given tempera-ture, i.e., OIT. Thus the temperature de-pendence of OIT can be expressed as:

i exp Bt A T = (6)

For the linear increase of temperature in DSC or DTA measurements, the furnace temperature can be expressed as fol-lows:

f 0T T tβ= + (7)

where Tf is the furnace temperature, T0 is the starting temperature of the meas-urement and β stands for the heating rate. Combination of Eqs. (4) and (7) gives the result [1, 9]:

i

0

dexp

T TBA T

β =

∫ (8)

where Ti is the onset temperature of the oxidation peak, i.e. the oxidation onset temperature.

The principal goal of stability studies is to extrapolate the kinetic data, ob-

tained from accelerated stability tests, to the application conditions. The extrapo-lation is almost exclusively carried out using the Arrhenius temperature func-tion. In recent years, it has been shown in several papers that the polymer degrada-tion does not obey the Arrhenius kinetics [10–13]. It has been found out that the Arrhenius extrapolation mostly gives un-realistically long estimates of thermooxi-dative stability [14]. In [6, 14, 15] it has been shown that, due to the complexity of the thermooxidative degradation of polymers, the temperature function can be hardly considered the rate constant. Therefore, there is no reason to be con-fined to the Arrhenius temperature func-tion. In [14] two other non-Arrhenian temperature functions were suggested. It has been documented in [14] that ap-plication of these functions instead of the Arrhenius equation gives more real-istic estimates of the material durability corresponding most with experience. The best estimates have been obtained using the Berthelot-Hood equation [16]:

( )kexpk A DT′= (9)

where and D are kinetic parameters. Combination of Eq. (9) and Eq. (2) after some manipulations leads to:

0

d1exp( )

it tA DT

= ∫ ′ − (10)

where parameter A′ is defined as:

i

k

( ) (0)F FAA

α −′ =′

(11)

For isothermal conditions it can be ob-tained:

i exp( )t A DT′= − (12)

For the measurements with constant heating rate, from the equations (7) and (10) it can be obtained:

( )i1 ln 1T A DD

β′= + (13)

In the degradation tests it has been often observed that the dependence of the onset oxidation temperature on the heating rate obeys the following equa-tion [17]:

( )i 1 exp aT T β∞ = − − (14)

where T∞ and a are parameters. The ex-trapolation of accelerated thermooxida-

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tive tests, based on Eq. (14), has been tested for 26 data sets [17]. Description of the data applying Eqs. (13) and (14) is of similar quality.

ExperimentalThe calorimeter Perkin Elmer DSC-7 was employed to study the thermooxidative stability of the samples under non-iso-thermal conditions in oxygen atmos-phere with the flow rate of 50 ml/min. Heating rates were 1, 3, 5, 7, 10 and 15 K/min. The starting temperature of oxida-tion was determined as the onset tem-perature of the oxidation peak.

The samples were prepared in the Re-search Institute of Chemical Technology in Bratislava within the framework of close mutual cooperation.

Case studies and discussion

A. Thermooxidation of polyisoprene rubber stabilized with p-phenylenedi-amines The aim of the study was to explore the antioxidant activity of several N,N‘-substi-tuted in polyisoprene rubber (PIR) ther-mooxidation and the influence of antioxi-dant concentration on its activity [18, 19].

All compounds studied were N,N‘-substituted p-phenylenediamines with various structure of the substituents R1 and R2 as seen in Scheme 1. The charac-teristics of antioxidants studied are sum-marized in Table 1. More details can be found in [18, 19].

For the treatment of the experimental results Eq. (8) was used. The kinetic pa-rameters A and B have been obtained by minimizing the sum of squares between experimental and theoretical values of oxidation onset temperature by the sim-plex method [15] and the integration in-dicated in Eq. (8) was carried out by the Simpson method. These dependencies together with the values of kinetic pa-rameters can be found in [18, 19].

Protection factorsAs it has been already mentioned in the introductory part, the extrapolation of absolute values of the lengths of induc-tion periods can lead to non-realistic es- Fig. 1: Temperature dependences of PFs for PIR with various content of 6PPD.

1

Scheme 1: Structure of N,N‘-substituted p-phenylenediamines

1 Characteristics of the antioxidants under studyAntioxidant Substituent R1 Substituent R2 Summary fromula M/g mol-1

DPPD CHN .

6PPD CHN .

IPPD CHN .

SPPD CHN .

MBPPD CHNO .

CPPD CHN .

o-cumyl derivative of 6PPD

CHN .

p-cumyl derivative of6PPD

CHN .

Dusantox L 6PPD 60% + p-cumyl6PPD 40% – –

CH3 C H CH2 CHCH3 CH3

CH3 C H CH2 CHCH3 CH3

CH3 C H CH2 CHCH3 CH3

CH3 C H CH3

CH3C CH3

CH2

CH3O

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timations. A better estimation can be obtained using the ratio of the lengths of induction periods of stabilized and un-stabilized PIR, since it is expectable that the same structural units are responsible for the degradation both in stabilized and unstabilized PIR. This ratio is called the protection factor (PF) [18]:

( )( )matrix

AOxmatrix

i

i

ttPF +

= (15)

For antioxidants, PF should be greater than one. The greater is the value of PF, the better is the antioxidant perfor-mance. It follows from Eq. (6) that the length of induction period depends on temperature; hence, the protection fac-tor depends on temperature as well. Fig. 1 shows the dependences of PF on temperature for the samples stabilized with various concentrations of 6PPD. It can be seen that the values of PFs are greater than one in the temperature range of PIR practical use. Similar de-

pendences could be obtained for SPPD. Dependences of PFs on temperature for antioxidants DPPD, IPPD, MBPPD and CPPD are depicted in Fig. 2. The highest values of PFs were obtained for PIR with the addition of DPPD and SPPD.

Antioxidant effectiveness In order to compare the effect of individ-ual stabilizers, a new criterion, antioxi-dant effectiveness, AEX, has been intro-duced in [18]:

AEX PFX

=−1 (16)

where X is the concentration of antioxi-dant in PIR expressed as the relative mass fraction of the antioxidant in rub-ber, i.e. in phr. In a molar scale, the effec-tiveness AEM can be defined as follows:

AEM PFm

=−1 (17)

where m is the concentration of antioxi-dant in polymer matrix expressed in mol

kg-1. Both criteria bring about a normali-zation of the protection factor so that the values of PF for various stabilizer content can be mutually compared. The values of AEX can be more illustrative for practitioners whereas the values of AEM may be advantageously applied by re-searchers dealing with the mechanism of antioxidant effect. The relationship between AEX and AEM is following:

AEM M AEX= 10 . (18)

where M is the molar mass of the anti-oxidant expressed in mol kg-1.

Since PF depends on temperature, al-so both AEX and AEM are functions of temperature. Values of antioxidant ef-fectiveness in the order of decreasing values of AEX for the temperature 180°C are summarized in Table 2.

From Table 2 it is seen that the order of AEM values slightly differs from the order of AEX. Eq. (18) indicates that the differences in both orders is brought about by the differences in molar mass of antioxidants. The highest values ob-tained for DPPD are not surprising since its high antioxidant efficiency is well-known. However, a weak spot of DPPD is its low solubility in rubber [20]. Relatively high antioxidant effectivness of Dusan-tox L, which is a mixture of 6PPD and its p-cumyl derivative indicates a synergistic effect of its components. The lowest, al-most zero antioxidant activity, has been obtained for CPPD.

B. Antioxidant effect of p-phenylenedi-amines in styrene-butadiene rubber (SBR)In this study the antioxidant activity of selected seven p-phenylenediamines (PP-Ds) in SBR were assessed and the results were compared with the results obtained for the same antioxidants in PIR matrix [21]. In the case of 6PPD, the effect of its concentration was also investigated and the samples with the following relative mass ratios of 6PPD have been prepared: 0.022; 0.055; 0.064 and 0.232.

The kinetic parameters A and B were obtained from the dependence of OOT on heating rate using Eq. (8) by the same procedure as described above. The values of kinetic parameters A and B for all samples are listed in [21]. The protection factors (PF) of the compounds were calculated according to Eq. (15). Then, the values of PFs were used to calculate the antioxidant effectiveness, AEX, which enables to compare the stabilizing effect

Fig. 2: Temperature dependences of PFs for PIR stabilized with DPPD, MBPPD, IPPD and CPPD.

2

2 Values of antioxidant effectiveness AEX and AEM calculated for 180°CAntioxidant AEX AEM / kg mol-1

DPPD 283 738Dusantox L 129 395SPPD 122 351

6PPD 103 277p-cumyl 6PPD 97 375IPPD 78 177o-cumyl 6PPD 68 263MBPPD 50 152CPPD 0 0

3 Relative mass ratio of the antioxidant in SBR matrix (X) and antioxidants effec-tiveness AEX at 180°C Antioxidant X AEX (180°C)

DPPD 0.014 4579

IPPD 0.003 1467

Dusantox L 0.100 726

6PPD 0.064 626

MBPPD 0.147 524

p-cumyl 6PPD 0.146 351

o-cumyl 6PPD 0.140 62

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of individual antioxidants. The antioxi-dant effectiveness AEX of individual sta-bilizers at 180°C in the order of decreas-ing values are listed in Table 3.

Comparison of Tables 2 and 3 shows a dramatic effect of the rubber matrix on the antioxidant effectiveness. Except for o-cumyl 6PPD, the absolute values of AEX are several times higher for SBR matrix than for PIR matrix. The orders of the AEX values also differ. The highest stabilizing effect of DPPD can be observed both for PIR and SBR matrix [18]. Similar values of AEX were also obtained for o-cumyl 6PPD for both PIR and SBR matrices. The high value of AEX for Dusantox L in SBR matrix indicates that also in this matrix a syner-gism exists between 6PPD and its p-cu-myl derivative. Surprisingly, totally differ-ent values have been calculated for MBPPD and 6PPD. While MBPPD mixed with SBR exhibited good stabilizing ef-fect, this is not true for MBPPD in the PIR matrix. In this case, MBPPD exhibited the lowest stabilizing effect.

C. Stabilization effect of different antioxidants and their binary mixtures in SBRIt is a common practice to use combina-tions of stabilizers in order to provide ef-fective stabilization of the material both during the processing and for long-term stability when the product is exposed to various environmental effects. Mixtures of stabilizers are used with the aim of obtaining an overall synergistic effect where the observed effect of the combi-nation is greater than the sum of the ef-fects of individual stabilizers. However, both additive or antagonistic effects may also result as a consequence of using mixtures of stabilizers. Therefore, it is important to investigate possible inter-actions between various antioxidants and stabilizers which have the potential for being used in the stabilization of ma-terials [22].

The presented research was concerned with a study of the type of cooperation between antioxidants in the binary mix-tures in the stabilization of styrene-buta-diene rubber. In [23] the stabilizing effect of antioxidant binary mixtures of several disubstituted p-phenylenediamines with phenothiazine were evaluated.

The antioxidants consisted of four substituted diphenylamines, phenotia-zine and their binary mixtures. Struc-tures of the antioxidants tested and composition of the samples are given in Table 4 and in [23].

The values of the kinetic parameters A´ and D have been obtained from de-pendences Ti versus β by the non-linear least squares method using Eq. (13); their values are summarized in [23]. Pro-tection factors and antioxidant effective-ness have been calculated using Eqs. (15) and (16), respectively. The values of AEX are summarized in Table �.

The obtained results indicate that at 130°C, among the individual anti-oxidants in the first group, phenothiazi-ne (A1) exhibits the highest protective effect, higher by an order of magnitude than derivatives of diphenylamine. There are only small differences in the protecti-ve effects of diphenylamine structures in A2, A3, A4 and A5 comparing to that of phenothiazine. Among them, the high-est value can be assigned to A3. In the case of mixtures, presence of phenothia-zine (A1) in the mixtures (samples 7 and 10) led to higher values of their PFs. The

temperature of 25°C represents an extra-polation very far from the range of the measured values of OOTs. Taking into account values of the coefficients of vari-ability of the kinetic parameters, one should be very careful to draw trustwor-thy conclusion from the values of protec-tion factors for this temperature. Howe-ver, it is possible to conclude, that the effect of phenothiazine is comparable to the stabilizing effect of the diphenylami-ne derivatives. The weakest protective effect can be observed in the case of the diphenylamine structure in A4.

Antioxidant synergismThe stabilizer effectiveness offers the pos-sibility of quantitative characterization of the stabilizer synergism. If there is no in-teraction between the stabilizers in the mixture, the resulting stabilizer effective-ness should be a weighed average mean of the effectiveness of single stabilizers:

Values of of AEX, AEXcalc and S-factors calculated for phenothiazine mixtures at 2� and 130°CSample AEX(2�°C) AEX(130°C) AEXcalc(2�°C) AEXcalc (130°C) S(2�°C) S(130°C)

A1 420 2840 – – – –A2 97.6 202 – – – –A3 498 318 – – – –

A4 562 167 – – – –A5 308 262 – – – –A1 + A2 574 1960 259 1520 122 synergism 29 synergismA3 + A5 286 272 403 290 -29 antagonism -6 additivityA4 + A5 406 394 435 214 -7 additivity 84 synergismA1 + A5 534 1370 364 1550 47 synergism -11 additivity

Characteristics of the antioxidants and their mixturesSample Antioxidant Ratio in mixture w/w

Code Structure M /g mol-1

1 SBR unstabilized

2 A1 199.28 pure

3 A2 287.41 pure

4 A3 281.44 pure

5 A4 393.66 pure

6 A5 281.44 pure

7 A1 + A2 – – 1:18 A3 + A5 – – 1:19 A4 + A5 – – 1:110 A1 + A5 – – 1:1

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calc i ii

AEX w AEX= ∑ (19)

where wi is the mass fraction of a given antioxidant in the mixture. A type of co-operation between the antioxidants (synergism, antagonism or additive ef-fect) was estimated using S-factor [25]. The S-factors were evaluated using the equation [25]:

calc

calc

100%AEX AEX

SAEX

−= g (20)

In Eq. (20) the fact is taken into account that both synergy or antagonism are de-viations from additivity, either positive or negative. If the value of S is positive, an-tioxidants in the mixture exhibit syner-gistic effect, for the value of S < 0, the effect is antagonistic and in the case of S = 0 the cooperation is additive. The val-ues of S are listed in Table 6. The values of S-factors are obtained from the treat-ment of experimental data so that they convey some error. Taking into account these uncertainties, we estimate that the values of S in the range ± 20% corre-spond to additive cooperation.

From the values of S in Table 6 it can be seen that the best results have been obtained for the mixture of A1 + A2, where the synergism can be observed at both temperatures. The worst combina-tion appeared to be A3 + A5 where the antagonistic effect has been observed for the lower temperature and at higher temperatures the type of cooperation was only additive.

A similar study was carried out in [24] where combinations of the commercial antioxidant 6PPD with diphenylamine derivatives and heterocyclic compounds on the thermooxidative stability of SBR were tested.

ConclusionsThe present work summarizes the results of the study of thermooxidative stability of rubber matrices and the influence of various antioxidants and their mixtures on the stability of rubber. For the treat-ment of the experimental data a non-isothermal isoconversional method based on the dependence of onset oxida-tion temperature on the heating rate has been applied. The method gives kinetic parameters enabling to calculate the lengths of induction periods of samples at a chosen constant temperature.

Three new criteria for the assessment and comparison of the stabilizing effect of various additives and their mixtures in

rubber materials are presented, i.e. the protection factor, antioxidant effective-ness and S-factor. Based on these criteria, it is possible to select the most suitable antioxidant in the rubber matrix. In addi-tion, these criteria make it possible to design a mixture of stabilizers with a synergistic stabilizing effect.

The method presented here can be applied also for other polymers and sta-bilizers. For example, evaluation of the residual stability of polyurethane auto-motive coatings, equivalence of Xenotest and desert weathering tests and the synergism of stabilizers were studied in [25]. Residual stability of pol-yurethane automotive coatings meas-ured by chemiluminescence and equiva-lence of Xenotest and Solisi ageing tests were the topics of interest in [26]. Paper [27] deals with the thermooxidative sta-bility of polymethyl methacrylate con-taining nanoparticles of silica/titania and silica/zirconia. In [28], effects of supported metallocene catalyst active center multiplicity on antioxidant stabi-lized ethylene homo- and copolymers were studied, etc.

AcknowledgementFinancial support from the Slovak Research and Development Agency (APVV-15-0124) is gratefully acknowledged.

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[15] Šimon P. J. Therm. Anal. Cal.2006. 84. 263.[16] Laidler K. J. J Chem Educ. 1984; 61: 494.[17] Šimon P. Material stability predictions apply-

ing a new non-arrhenian temperature func-tion. J. Therm. Anal. Cal. 2009, 97, 2. 391.

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[20] Lehocký P., Syrový L., Kavun C. M. Rubber Chem’01. Brussels, 2001, Paper 18.

[21] Cibulková Z., Šimon P., Lehocký P., Kosár K., Chochulová A. DSC study of antioxidant ac-tivity of selected p-phenylenediamines in styrene-butadiene rubber. J. Therm. Anal. Cal. 2009, 97, 2, 535.

[22] Zweifel H., Maier R.D., Schiller M. Plastics additives handbook(6th ed.).Munich: Carl-Hanser Verlag, 2009. 28

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[25] Šimon P., Fratričová M., Schwarzer P., Wilde H.W. Evaluation of the residual stability of polyurethane automotive coatings by DSC Equivalence of Xenotest and desert weath-ering tests and the synergism of stabilizers. J. Therm. Anal. Cal. 2006, 84, 3. 679.

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[27] Šimon P., Zhong W., Bakoš D., Hynek D., Chemical Papers 62 (2008) 176: Thermooxi-dative stability of polymethyl methacrylate containing nanoparticles of silica/titania and silica/zirconia.

[28] Atiqullah M., Cibulková Z., Černá A., Šimon P., Hussain I., Al-Harthi M.A. & Anantawar-askul S., Journal of Thermal Analysis and Calorimetry 119 (2015) 581: Effects of sup-ported metallocene catalyst active center multiplicity on antioxidant stabilized ethyl-ene homo- and copolymers.