Influence of additives on some physical properties of high density polyethylene-I. commercial antioxidants

Fur. Polym. J. Vol. 23, No. 9, pp. 723-727, 1987 Printed in Great Britain. All rights reserved

0014-3057/87 $3.00+0.00 Copyright © 1987 PergamonJournals Ltd

INFLUENCE OF ADDITIVES ON SOME PHYSICAL PROPERTIES OF HIGH DENSITY POLYETHYLENE--I. COMMERCIAL ANTIOXIDANTS ALFONSO J. CHIRINOS PADRON,* MARIA A. COLMENARES,ZIMBUL RUBINZTAIN and Luls A. ALBORNOZ Laboratorio de Polimeros, Centro de Quimica, IVIC, Apartado 21827, Caracas 1020-A, Venezuela (Received 19 August 1986)

Abstract--The variations in mechanical properties of processed high density polyethylene, when antioxidants are added, is examined using tensile and melt flow measurements. The results show that there is an optimum concentration for each antioxidant at which changes in mechanical properties during processing are inhibited. At relativelyhigh concentrations, there is a loss in tensile properties due primarily to decreased solubility of the antioxidant. The latter can cause changes in morphology in some cases. All these conclusions are supported by electron microscopy and DSC studies which show that the changes can be satisfactorily explained using degradation inhibition and solubility arguments. INTRODUCTION

Tensile measurements

Variations in mechanical properties has been used to monitor polyolefin degradation both for accelerated [1-3] and natural wheathering conditions [4, 5]. The reports have shown that, although reliable, the changes observed in mechanical properties are quite complex in comparison to those exhibited by other methods such as oxygen uptake [6, 7], and carbonyl development [7, 8]. All previous work has concentrated on changes in mechanical properties during ageing [1-5]. However, there have been no reports on the effect of additives in these properties, particularly as the additive concentration is increased. The one exception being carbon black which has been extensively studied [9]. The present work describes how small variations in antioxidant concentration can have a dramatic effect on the material properties and, in particular, on the modulus of elasticity of high density polethylene (HDPE). This parameter, which measures the "stiffness" of the material, undergoes changes during processing that can be inhibited by commercial antioxidants only if the appropriate concentration is chosen.

Dumbshell-shaped specimenswere prepared according to ASTM D 638 and D 1708. The samples were cut from sheets prepared by moulding the HDPE pellets for 5 min, at 150°C. Tensile tests were performed in an Instron (Model 1113) Tensile Tester. Melt flow measurements

Melt flow data was obtained in a Monsanto 3501 Capillary Rheometer according to ASTM D 1238 (Condition E). Thermal oxidation

Thermal oxidation was carried out in a Du-Pont 990 Thermal Analyzer using a Du-Pont DSC cell. The measurements were performed in sealed capsules which were heated in air until thermal decomposition was observed in the thermogram. Electron microscopy

Scanning electron microscopy was carried out in a Philips SEM 500. The samples were metallized using a goldplatinum alloy. RESULTS AND DISCUSSION

The effect of thermal processing on the tensile modulus of unstabilized HDPE is shown in Fig. 1. EXPERIMENTAL There is a decrease in modulus during the first 15 min followed by a subsequent increase in this parameter Materials as the processing continues. These changes can be Unstabilized HDPE (Altaven 1300-J) from commercial explained by a decrease in molecular weight during sources was used as supplied. The antioxidants, the initial stages of the processing operation due to tri(2,4-ditert.-butylphenyl) phosphite (Hostanox VP PAR 24); butyric acid [3,3'-bis(3-tert-butyl-4-hydroxyphenyl) oxidative degradation [10-13]. As the processing conethylene ester], (Hostanox 03); and di-octa-decyl disulphide tinues (for 20 min and beyond), the oxygen concen(Hostanox SE 10), were all kindly supplied by Hoechst AG, tration in the chamber drops and crosslinking reacWest Germany. Carbon black (Grafto1300) was supplied by tions predominate causing an increase in molecular Sandoz de Venezuela. The additives were incorporated into weight and a concurrent increase in the modulus the polymer by mixing in a Haake EU-5 Rheometer (at [10-13]. 150°C) for 10rain. The results shown in Fig. 1 are interesting since extrapolation to zero processing time gives a value of ~ 13 GPa for the modulus of the virgin polymer. As *To whom all correspondence should be addressed. the material is provided in the form of pellets (or 723

724

ALFONSOJ. CItiRINOSPADR6Net al.

12

Concentration/wt 2 I

%

3 I

4 ]

5 I

I0 ,'~ 8

~6 2

~4 2-

I

I

I0

I

20

1'f

A

30

15

"

Processing time/min

Fig. l. Effect of thermal processing on the tensile modulus of unstabilized HDPE. otherwise in the form of powder), a 5 min moulding time is necessary to prepare the sheets from which the samples are cut (see experimental section). Whether or not the observed value for the modulus is close to that obtained by extrapolation ( ~ 13 GPa) depends on the use of an adequate antioxidant and its chosen concentration. This is exemplified by Figs 2-5, which show the effect of antioxidant concentration on the tensile and flow properties of HDPE containing Hostanox VP PAR 24, Hostanox 03, Hostanox SE 10 and carbon black respectively. In general, there is an initial increase in the modulus and a subsequent

0

Concentration/wt % 2 3

1 I

I

I

4

5

I

I

12

11 ! 0

I

I

]

I

I

1

2

3

4

5

Concentration/wt %

Fig. 3. Variations in the tensile modulus and MFI of HDPE (processed for 10min at 150°C), with concentration for Hostanox 03. decrease at high concentrations for all antioxidants. A corresponding initial decrease in MFI is also observed. This behaviour is consistent with the inhibition of the degradation occurring during the processing and moulding operations described above.

( 14 o

n

1 I

Concentration/wt % 2 3 I

I

4

5

I

I

12

)t0 0

~

'~,

~ e

6

4

15I

1°I

14

~

13

5 ,

•

~

14

....--.J

9

e

0

lZ =S 11 4 ~ / I "~0

~

1

, I 2

J 3

I 4

,1 5

Concentration/wt %

Fig. 2. Variations in the tensile modulus and MFI of HDPE (processed for 10min at 150°C), with concentration for Hostanox VP PAR 24.

,O'o

1 2

I 3

I 4

I 5

Concentration/wt %

Fig. 4. Variations in the tensile modulus and MFI of HDPE (processed for 10rain at 150°C), with concentration for Hostanox SE 10.

Commercial antioxidants

725

~

15

r ~6 ._o

,t2

~.

B ~J

5

4

10 I 1

0

I 2

I $

I 4

5

Concentration/wt %

Fig. 5. Variations in the tensile modulus and M F I of HDPE (processed for 10min at 150°C), with concentration for carbon black.

The increase can thus be assigned to a antioxidant process at low additive concentrations which will maintain the material properties at levels comparable to the virgin polymer samples. Large variations in the crystallinity of HDPE which could lead to an increase in modulus were ruled out, since DSC studies did not show any significant changes in this parameter. As the concentration is increased beyond 0.2%, the values for the modulus show a steady decrease. There are three possible reasons for this effect. Firstly, at high concentrations, direct reaction of the antioxidant with oxygen lowers the efficiency of the inhibitor, thus: [14-16] HIn + 02 ~ HO2 + In HO 2 or

In + RH ~ R + reaction products

where HIn = antioxidant.

?

of

240

This behaviour has previously been observed for antioxidants in photostabilization studies [17]. Secondly, at high concentration, there is an increased importance of pro-oxidant transformation products which may be formed during the processing operations and can participate in oxidative degradation [17-19]. These observations are supported by the results in Fig. 6, which shows the variation in degradation temperature with stabilizer concentration in HDPE. It can be seen that none of the antioxidants has a great influence in Tn and that, as the concentration increases, the curves reach a "plateau". This behaviour may be due to the reasons outlined above and is supported by previous results in photostabilization studies [17-19]. Thirdly, limitations in the solubility may cause a decrease in efficiency at high levels. This last point is most clearly exemplified by Hostanox 03 at concentrations />2.0%. This

J

f

~e

220

~X

q

2OO 180 1600

i 1

[ 2

[ 3

I 4

I 5

Concentration / wt %

Fig. 6. Variations in the degradation temperature (TD) with antioxidant concentration in HDPE (processed for 10rain at 150°C), containing: (--O--): Hostanox 03; (--O--): Hostanox VP PAR 24; (--f-I--): Hostanox SE 10 and ( - - I - - ) : carbon black.

726

ALFONSOJ. CHIRINOSPADR6N et al.

Fig. 7. Fracture zone of an HDPE sample containing 2% w/w Hostanox 03 at (a): 380 × and (b) 1500 ×. The antioxidant agglomerates can clearly be seen. is shown by the micrographs in Fig. 7, which demonstrate that the antioxidant is exuded from the polymer and exists as a completely separate phase which does not mix with the matrix. This behaviour was reflected in the material properties of these samples which showed fragile fracture with no elongation (no "necking"). The fragile behaviour is due

to the change in morphology of H D P E when high antioxidant loadings are used. Figure 8 shows the fibrillar nature of the fracture zone in the unstabilized H D P E sample. When high concentrations of Hostanox 03 are used, not only is the antioxidant exuded (Fig. 7) but the fibrillar nature in some regions of the fracture zone disappears (Fig. 9). Since the fibrils are

Fig. 8. Fracture zone of an unstabilized HDPE sample at (a): 100 ×, and (b): 380 x, showing the fibrillar nature of the material when stressed.

Fig. 9. Different fracture zones obtained in an HDPE sample containing 2% w/w Hostanox 03 at (a): i00 x and (b): 380 ×.

Commercial antioxidants responsible for the elasticity of the PE samples, their disappearance in some areas results in fragile behaviour. In the case of carbon black, a reinforcement effect is observed at concentrations >t 1% (Fig. 5). This is a well-known phenomenon, which has been reported by other workers, and for which various explanations and theories have been put forward [9].

CONCLUSIONS The results show that commercial antioxidants can significantly change the material properties of HDPE. Thus, at optimum concentrations, they can inhibit the degradative reactions which take place in the polymer and cannot be detected by more conventional methods such as i.r. analysis (there was no significant changes in the i.r. spectra of the various H D P E samples). At high antioxidant concentrations, the ability of the latter to inhibit these changes decreases for the reasons already mentioned. In the case of Hostanox 03, its high incompatibility with the matrix at these levels can cause it to aggregate and exist as a separate phase. This can produce dramatic changes in the morphology of the samples and thus in the tensile strength. Part II of this study shows that with "u.v. stabilizers" this behaviour is not observed and that the reason for this seems to be the greater compatibility of these compounds with the polymer. Acknowledgements--The authors thank Hoescht AG for kindly supplying the samples of Hostanox VP PAR 24, Hostanox 03 and Hostanox SE 10. The authors are also grateful to the Venezuelan Council for Scientific and Technological Research (CONICIT), for financial support of this project. Finally, the authors thank Dr Norman S. Allen of

727

Manchester Polytechnic, U.K. for reading the manuscript, and making useful corrections. REFERENCES

1. S. Yano and M. Murayama. J. appl. Polym. Sci. 25, 433 (1980). 2. J. Pabiot and J. Verdu. Polym. Engng Sci. 21, 32 (1981). 3. M. Raab, L. Kotulak, J. Kolarik and J. Pospisil. J. appL Polym. Sci. 27, 2457 (1982). 4. G. Akay, T. Tincer and H. E. Ergoz. Eur. Polym. J. 16, 601 (1980). 5. F. P. La Mantia. Eur. Polym. J. 20, 993 (1984). 6. P. Vink. In Degradation and Stabilization of Polyole_fins (Edited by N. S. Allen), p. 213. Applied Science, London (1983). 7. P. K. Bandyopadhyay, M. T. Shaw and R. A. Weiss. Polym. Plast. Technol. Engng 24, 187 (1985). 8. N. S. Allen, A. Chirinos Padron and J. H. Appleyard. Polym. Deg. Stab. 4, 223 (1982). 9. B. B. Boonstra. In Reinforcement of Elastomers (Edited by G. Kraus), p. 529. Wiley-Interscience, New York (1965). 10. C. Decker, F. Mayo and H. Richardson. J. Polym. Sci., Polym. Chem. Edn 11, 2879 (1973). 11. M. U. Amin, G. Scott and L. M. K. Tillekeratne. Eur. Polym. J. I1, 85 (1975). 12. K. B. Chakraborty and G. Scott. Polymer 18, 98 (1977). 13. K. B. Chakraborty and G. Scott. Eur. Polym. J. 13, 731 (1977). 14. I. A. Shlyapnikova. Vysokomolek. Soedin. 8, 846 (1966); ibid. pp, 1401, 1405. 15. B. A. Gromov. Vysokomolek. Soedin. 6, 1865 (1965). 16. J. C. Johnson and R. B. Walter. J. Polym. Sci., Macromolec. Rev. 15, 323 (1983). 17. N. S. Allen, A. Chirinos Padr6n and J. H. Appleyard. Polym. Deg. Stab. 5, 323 (1983). 18. N. S. Allen, A. Chirinos Padr6n and J. H. Appleyard. Eur. Polym. J. 20, 433 (1984). 19. A. J. Chirinos Padr6n. Ph.D. Thesis, Manchester Polytechnic, U.K. (1984).