Interaction of the thermal decomposition products of azodicarbonamide with some antioxidants in polyethylene

Polymer Degradation and Stability 23 (1989) 193-199

Interaction of the Thermal Decomposition Products of Azodicarbonamide with some Antioxidants in Polyethylene Czestaw Latocha & Maria Uhniat Institute of Heavy Organic Synthesis 'Blachownia', 47-225 K~dzierzyn-Ko21e,Poland (Received 10 May 1988; accepted 26 May 1988)

ABSTRACT The interactions of four commercial antioxidants--lrganox 1010, Vanox SKT, Cyanox 1790 and Goddrite 3114--with the solid products of thermal decomposition of azodicarbonamide are described. The synergistic and antagonistic effects are found to be dependent on the structure of the an tio xidants.

Azodicarbonamide (ADCA) is one of the widely used agents for chemical foaming processes in thermoplastics. Thermal decomposition of A D C A at approx. 200°C leads to the formation, of about 40 wt% of a solid product as well as gaseous products. The main components (65 wt%) of the former are urasol and isocyanuric acid. OH

I

HN-

NH

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I

O=C,.N...C=O

N"~S"~N II

I

HO--C..N~C--OH

H

urasol

isocyanuric acid

In the structures of urasol and isocyanuric acid, there are reactive hydrogen atoms in the form of imide or hydroxyl groups which are specific groups to amine and phenol type antioxidants. 193 Polymer Degradation and Stability 0141-3910/89/$03-50© 1989 ElsevierSciencePublishers Ltd, England. Printed in Great Britain

194

Czestaw Latocha, Maria Uhniat

There are no data in the literature on the influence of solid decomposition products of ADCA on the inhibiting power of antioxidants in polyolefines. This paper presents the results of the examination of the thermooxidative resistance of foam made from low density polyethylene which also contains, besides ADCA, one of the following four commercial antioxidants: Irganox 1010, Vanox SKT, Cyanox 1790, Goodrite 3114. The results are compared with those obtained for polyethylene which was stabilized by these substances but without any blowing agent.

EXPERIMENTAL Materials L D P E - - P o l i t e n II 003/GO (Chemical Works 'Blachownia', Poland)

Blowing agent--Genitron AC/3 (azodicarbonamide), (Fisons Industrial Chemicals, UK). Antioxidants

--Irganox 1010 (tetra-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] of pentaerythrite), (Ciba-Geigy, Switzerland) --Vanox SKT (isocyanurate of tri-N-I-3,5-di-t-butyl-p-hydroxyhydrocinnamic acid ester with ethyl alcohol]), (R. T. Vanderbild Comp. Inc., USA) --Cyanox 1790 (tri-N-[2,6-dimethyl-3-hydroxy-4-t-butylbenzyl isocyanurate]), (Cyanamid B.V., Netherlands) --Goodrite 3114 (tri-N-[3,5-di-t-butyl-4-hydroxybenzyl isocyanurate]), (B. F. Goodrich Chem. Comp., USA). Procedure

Mixtures of polyethylene with a blowing agent (5.0wt%) and/or an antioxidant (0.1 wt%) were made by the rolling method at 130°C. The mixtures which contained ADCA and antioxidant were subjected to a foaming process using a DTA isothermal method at 210°C in an argon atmosphere. 1 The oxidative induction time (ti) was determined for all the materials by isothermal DTA in the temperature range 190-220°C. A Mettler TA 2000A instrument was used.

Interaction of the thermal decomposition products of azodicarbonamide

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200

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2.12 2"16 l I T × 103(K -1) Fig. 1. Dependence of the oxidative induction time on temperature for: O, PE + Irganox 1010 (0-1wt%); x, PE + Irganox 1010 (0-1 wt%) + solid products of A D C A (5-0wt%) decomposition.

RESULTS A N D DISCUSSION Figures 1-4 show the dependences of the oxidative induction time on the temperature (as In t i vs lIT plots); for polyethylene containing an antioxidant and with and without the solid products of decomposition of ADCA. It is obvious that in all four instances the effectiveness of

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2 12 2.16 x 103(K-1) Fig. 2. Dependence of the oxidative induction time on temperature for: ©, PE + Vanox SKT (0.1 wt%); x , PE + Vanox SKT (0"l wt%) + solid products of A D C A (5"0wt%) decomposition. lIT

Czeslaw Latocha, Maria Uhniat

196

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Fig. 3. Dependence of the oxidative induction time on temperature for ©, PE + Cyanox 1790 (0'l wt%); x , PE + Cyanox 1790 (0"1 wt%) + solid products of A D C A (5"0wt%) decomposition.

stabilization of the foamed polyethylene is different from that of the material which had not been subjected to the foaming process. It should be emphasized that, within the temperature range examined, the solid products of ADCA decomposition do not exhibit any stabilizing properties. Consequently, an interaction between the solid ADCA decomposition products and the antioxidants takes place. The activation energies of the inhibition of the oxidation of the polymer were calculated from the straight l~nes in Figs 1-4 and are presented in Table 1. 220

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Fig. 4. Dependence o£ the oxidative induction time on temperature for: O, PE + Goodrite 3114 (0'1wt%); x , PE + Goodrite 3114 ( 0 . 1 w t % ) + solid products of A D C A (5-0wt%) decomposition.

hlteraction of the thermal decomposition products of azodicarbonamide

197

TABLE 1

Activation Energies of the Inhibition of Polymer Oxidation

Antioxidant

E (kJ/mol) Polyethylene

Foamed polyethylene

250-0 246.2 165-5 163.4

160"1 110"4 178'5 151.7

lrganox 1010 Vanox SKT Cyanox 1790 Goodrite 3114

As a quantitative measure of the interaction between the antioxidant and the solid A D C A decomposition products, the cooperative action factor (0) was obtained as for stabilizing systems. 2 Figure 5 shows the temperature dependence of the cooperative action factors in the form In 0 vs lIT. Interactions of the solid ADCA decomposition products with Irganox 1010 and Vanox SKT in polyethylene are similar in nature but differ considerably from the nature of interaction with Cyanox 1790 and Goodrite 3114. Below 216°C for Irganox 1010 and below 214°C for Vanox SKT, an antagonistic effect was found, the magnitude of which rises with decrease of temperature. Cyanox 1790 and Goodrite 3114, on the other hand, show 220

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--

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Fig. 5. Dependence of the cooperative factor on temperature for the systems PE + solid products of ADCA decomposition + antioxidant (a-d); a--Irganox 1010, b~Vanox SKT, c--Cyanox 1790, d--Goodrite 3114.

198

Czeslaw Latocha, Maria Uhniat

synergistic effects with solid ADCA decomposition products, the magnitudes of which do not vary much with temperature. In searching for the reasons for such behaviour of antioxidants in the presence of solid decomposition products of ADCA, two aspects of the structures of the stabilizers should be considered. (1) Irganox 1010 may be regarded as being a methane derivative with four substituent groups and Vanox SKT, Cyanox 1790 and Goodrite 3114 as derivatives of cyanuric acid. (2) As far as the structural groups present between the phenol ring and the structural nucleus of antioxidants are concerned, two pairs may be considered: Irganox 1010 and Vanox SKT are compounds in which there is a propionic acid ester group between the phenol ring and the structural nuclei of these antioxidants, and Cyanox 1790 and Goodrite 3114 are compounds in which the phenol ring is bonded to the structural nucleus by means of a methylene group only. From a comparison of the magnitudes of the activation energies for polyethylene which contains only the antioxidants (Table 1), it is evident that they may be divided into the same two pairs as in item (2) above: Irganox 1010 and Vanox SKT (E = 250.0 and 246.2 kJ/mol, respectively) and Cyanox 1790 and Goodrite 3114 (E = 165.5 and 163.4 kJ/mol, respectively). The observed similarity of interaction behaviour of these pairs of antioxidants with the solid ADCA decomposition products lead us to the conclusion that the differentiation of the antioxidants as in item (2) above is valid. Thus, the groups which exist between the phenol ring and structural nucleus of the antioxidants have a vital influence on the delocalization of the non-paired electron of the phenoxy radical formed and, consequently, on its stability and reactivity. Phenomena of antagonism and synergism, similar to those which occur in the thermooxidation stabilization of foamed polyethylene as described above, were also observed in the photostabilization of polypropylene containing the UV stabilizer Tinuvin 770 and an antioxidant. An antagonistic effect was observed in the case ofa Tinuvin 770 + Irganox 1010 stabilizing system, while in case of a Tinuvin 770 + Goodrite 3114 system there was a synergistic effect. 3- 5 In the structure of Tinuvin 770, as in some solid ADCA decomposition products, reactive hydrogen atoms bonded to nitrogen atoms are present. The cause of the observed antagonism and synergism phenomena which appears due to the interaction of the antioxidants with compounds that contain imide groups can be attributed to the diversity of the reactivity of phenoxyl radicals with the N-oxy radicals. Thus, there exists the possibility of forming different intermediate products.

In teraction of the thermal decomposition products of azodicarbonamide

199

REFERENCES 1. Latocha, Cz., Uhniat, M. & Schneider, Z. Conference: 'Stabilization of Plastics and Chemical Fibres', Rabka, 1984. 2. Latocha, Cz., Uhniat, M. & Balcerowiak, W., Poly. Deg. andStab., 7 (1984) 189. 3. Allen, N. S., Poly. Deg. and Stab., 2 (1980), 129. 4. Allen, N. S., Poly. Deg. and Stab., 3 (1980/81), 73. 5. Allen, N. S., Gardette, J. L. & Lemaire, J., Poly. Photochem., 1 (1981) 111.