Pergamon Radiat. Phys. Chem. Vol. 46, No. 4-6 , pp. 871-874, 1995

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A COMPARATIVE STUDY OF THERMAL AND MECHANICAL STABILITIES OF GAMMA IRRADIATED ETHYLENE - ETHYL ACRYLATE AND ETHYLENE -

VINYL ACETATE COPOLYMERS

Murat ~;en, Olgun Gtiven

Hacettepe University, Department of Chemistry 06532 Beytepe, Ankara TURKEY

ABSTRACT

Ethylene-ethyl acrylate and Ethylene-vinyl acetate copolymers were irradiated in ambient conditions with ), -rays. The influence of the chain scission, crosslinking and relative changes in crystallinity on the thermal and mechanical properties were investigated and a correlation has been tried to find between the thermal and mechanical stabilities of copolymers. For the two copolymers, among various mechanical properties evaluated, the best correlation was found between the toughness (energy to break point) and the time required for 10% weight loss.

INTRODUCTION

The radiation stability of polymeric materials is generally determined by referring to the changes in their mechanical properties upon irradiation. The determination of radiation-induced deterioration in various mechanical properties requires howewer, rather expensive mechanical testing apparatus and considerable amount of materials. On the other hand, thermal stability or resistance of plastics are relatively easy to determine and require extremely small amount of materials (Giiven and Basan, 1985). An article on the comparative study of thermal and mechanical stabilities of gamma irradiated poly(methyl methacrylate), polypropylene and poly (vinyl chloride) was published by Giiven and Uzun (Giiven and Uzun, 1993). The authors indicated that there is a correlation between the thermal and mechanical properties of "t- irradiated polymers and mechanical properties can be referred from thermal properties. In this study a similar approach was made to find a relation between the thermal and mechanical stabilities of EEA and EVA copolymers irradiated up to 130 kGy dose.

EXPERIMENTAL

Ethylene-ethyl acrylate (EEA; 15% ethyl acrylate) and Ethylene-vinyl acetate (EVA; 13% vinyl acetate) copolymers used in this investigation were commercial products obtained from Atochem Comp., Germany. They were all additive free and used as received. EEA and EVA films were prepared by hot pressing. Oxidation of polymers during the preparation of films was continuously checked by FI'IR spectroscopy. The polymeric films with an average thickness of 500 I~m were irradiated in air at a dose rate of 0.58 kGy/h in a Gammacell 220 type 60Co y-irradiator at ambient temperature. Isothermal thermogravimetric measurements were made with a Dupont 951 Thermogravimetric Analyzer. Weight loss of the samples were determined in a dynamic oxygen atmosphere. Heat of fusion of the films were obtained by using the Differential Scanning Calorimeter model DSC 910, of Dupont Instruments. Each sample, approximately 3 mg in weight, was heated at a rate of 5.0 C/min. up to 150 °C. The relative crystaUinitiy ;Cc based on the heat of fusion was calculated from the formula;

Xc=AI-I/AI-Ic x 100 871

872 Murat ~;en and Olgun Giiven

Where AH and AI-Ic are the heat of fusion of the sample and 100% crystalline PE per weight (taken as 277.7 J/g) respectively (Matsui et al. 1992). Stress-Strain tests of polymeric films were performed on a series IX model Instron Mechanical Testing apparatus with a crosshead speed of 100 mm/min, and gauge length of 25 mm, grip distance of 50 mm. At least five samples were tested for every mechanical property measurement.

RESULTS AND DISCUSSION

The typical weight loss curves of original and irradiated EEA and EVA films at 185°C are given in Figs 1 and 2 respectively. As shown in these figures the thermal stability of the films decreased with increasing irradiation dose. This has been attributed to the formation of radiation induced oxidation products within the chains. The magnitude of decrease in the thermal stability of EEA was higher than EVA. In this study various criteria were applied to the thermal stability of films such as the time required for 10% weight loss to be reached at 185°C(t10%), degradation rate(Rd)( defined as the slope of the linear declining portion of the isothermal degradation curve, up to about 10% weight loss) and induction time(ti). Generally in all thermograms a period of appearent inactivity was observed followed by a rapid uptake of oxygen and subsequent decomposition. The induction time is the time interval till the end of appearent inactivity (Gal et al., 1983).

105 ........................

1 O0

95 ̧

90

B5 160.0 k G y ~

2~ 4~ 6b 8~ 1o~ 1~o 14o TimQ (mSm)

Fig. 1 lsothermal(T=185*C) weight loss curves of EEA irradiated to various doses

105-

100

~O~" 95

90

85 0 160

~ NNNN~ kGyl 28.0 kGy\

130.0 kGy

2b 4b 6b " Bb " 16o " 12o 14o TimQ (min)

Fig. 2 Isothermal(T=185*C) weight loss curves of EVA irradiated to various doses

9th International Meeting on Radiation Processing 873

The changes in the relative crystallinity of irradiated EEA and EVA films are given in Fig. 3. Apparently ~ of irradiated f'llms decreased with increasing irradiation dose. The magnitude of the decrease in crystallinity of EEA was higher than EVA. The maximum percent reduction in the crystallinity was 21% for 130 kGy irradiated EEA film and 10% for EVA. Relative changes at crystallinity and main chain structure as resulted from irradiation affected the thermal and mechanical properties of these copolymers.

26

.~ 24

~ 22

2o

o 18

14

12

• EEA

I I I I I I

0 20 40 60 80 100 120 140

Dose(kGy) Fig. 3 Changes of crystallinity in irradiated EEA and EVA films as a function of dose

The changes of mechanical properties with irradiation were followed by taking typical stress- strain curves. The mechanical properties evaluated in this study were tensile strength at break, elongation at break and toughness. The elongation at break and toughness of EEA and EVA films decreased with irradiation dose Figs 4 and 5. A slight increase with dose has been observed for tensile strength at break values. This can be related to the decrease observed in crystallinity of the samples.

e%120 1"[ ~ A ,Ik _A

^ ^

80

60

40

20

0 0 20 40 60 80 100 120 140 160

Dose(kGy)

Fig. 4 Relative changes in some mechanical properties and thermal stability of EEA with dose: (/k) Elongation at break, (O) 10% weight loss at 185°C, (m) Induction time, (A) Tensile strength at break, (s) Degradation rate, (O) Toughness

874 Murat ~en and Olgun Giiven

120

100

80

60

40

20

0

~ " ,lk

[] m

0 20 40 60 80 100 120 140 160

Dose(kGy)

Fig. 5 Relative changes in some mechanical properties and thermal stability of EVA with dose: (A) Elongation at break, (O) 10% weight loss at 185°C, (a) Induction time, (A) Tensile strength at break, (m) Degradation rate, (O) Toughness

The mechanical property that suffered most from irradiation is found to be the toughness for both EEA and EVA. The toughness values showed a sudden decrease upon irradiation. Although both polymers are of crosslinking type, very low dose rates applied in this study had probably caused the formation of a considerable amount oxidative products on the polymer chains and subsequently extensive chain scissions as well. All these are assumed to cause the decrease observed in toughness values.

In conclusion among the various thermal and mechanical properties tested, a close correlation was found for toughness and time required for 10% weight loss values of irradiated copolymers.

REFERENCES

Gtiven, O. and Basan, S. (1985). Thermal Degradation of T-Irradiated Poly(vinyl chloride). Polvm. Des. and St~b,, 11, 45-53.

Giiven, O. ancl Uzun, C. (1993). A Comparative Study of Thermal and Mechanical Stabilities of Gamma Irradiated PMMA, PP and PVC. Radiat. Phys. Chem., 42, 1047-1050.

Matsui, T., Shimoda, M. and Osajima, Y. (1992). Mechanical Changes of Electron Beam Irradiated Ethylene-Vinyl Acetate CopolymerfEVA) film(I). Polymer International. 29, 85- 90.

Gal, O., Novakovic, Lj., Markovic, V. and Stannett, V.T. (1983).Thermogravimetric Studies of the Thermooxidative Stability of Irradiated and Unirradiated Polyethylene Part I. Effect of Antioxidant. Radiat. Phys. Chem.. 22, 627-634.