INSTITUTE OF HEAVY ORGANIC SYNTHESIS UL. ENERGETYKÓW 9, 47-225 KĘDZIERZYN-KOŹLE, POLAND STANISŁAW KUDŁA e-mail: stankud@icso.com.pl INFLUENCE OF INTERACTIONS BETWEEN DICUMYL PEROXIDE AND PHENOLIC ANTIOXIDANTS ON THE PROPERTIES OF CROSSLINKED POLYOLEFINES INTRODUCTION Reactions of polyolefines crosslinking, especially of polyethylene, are widely applied in commercial scale. Crosslinking under the influence of organic peroxides is of special significance. In order to ensure greater stability of products made of such plastics, an addition of different stabilizers is applied, among which very important are phenol-like compounds with steric hindrance, including also compounds containing sulphur in their molecule. However, simultaneous use of peroxide and phenolic compound may involve certain number of chemical reactions, which may change their effectiveness. Reactions between oxidizing compounds and phenols have been investigated many years ago and are widely described [1]. It is well known that these reactions lead to the oxidative coupling of phenols, and result thereof is the formation of compounds which have the nature of oligomers, and creation of coloured products which have an unfavourable impact on certain applications of plastics. It is admitted that the mechanism of phenols coupling can be of radical as well as non-radical nature. In case of polymeric systems, the radical mechanism is predominant. It seems however, that some detailed problems have not been definitively solved yet. This can be even testified by the fact that in the newest literature still appear publications concerning these problems [2-3]. We are of opinion that it is just the case of reactions which occur between phenolic antioxidants and organic peroxides at the temperatures lower than the temperature of rapid radical decomposition of peroxide [4-5]. It is obvious that greater significance have reactions occurring in conditions which are favourable to the rapid decomposition of peroxide, consequently to the crosslinking of the material. Nevertheless, knowledge of the behaviour of substances being discussed at lower temperatures, is important as well. Since many years, research work within the scope of degradation and stabilization of polyolefine plastics, including crosslinking grades, is carried out by our team [6-9]. As concerns this work it relates to the influence of interactions between organic peroxides and phenolic antioxidants on some important properties of crosslinked ethylene polymers. The main target of this work was to answer the question: does efficiency of both substances, that is peroxide as a polymer’s crosslinking agent and antioxidant, change in the consequence of the reactions taking place between these two compounds at temperatures lower than the temperature of rapid, radical decomposition of peroxide? EXPERIMENTAL PART We examined substances applied at commercial scale: two antioxidants - Lowinox 22M46 (AO1) and Irganox 1081 (AO2) and one peroxide, namely dicumyl peroxide (DCP). Antioxidants used have very similar chemical constitution, they differ from each other by one atom only. They have been already examined by us in our previous works. Lowinox 22M46 (AO1) Irganox 1081 (AO2) Reactions between dicumyl peroxide and one of the two selected phenolic antioxidants were carried out at the temperatures from 90 to 110C, so substantially lower than the temperature of rapid radical decomposition of dicumyl peroxide which takes place at the temperature higher than 125C. Molar ratio peroxide : antioxidant was from 3:1 to 8:1, reaction time from 10 to 60 minutes; air or nitrogen atmosphere was applied. No solvent was used, as the dicumyl peroxide melts at lower temperature (about 38C) and the antioxidants used dissolved well in the molten peroxide. After having performed the reaction, products obtained were introduced into polymer. We determined the influence of interactions between peroxide and antioxidant, taking place in conditions as mentioned above, on the properties of peroxide-crosslinked polymers: LDPE and EVA UL220. For that purpose the neat polymer was homogenized with additives, using rolling mill and then crosslinked in the press at the temperature of 180C. Crosslinked polymers were evaluated from following points of view: tensile properties, degree of crosslinking and thermooxidative resistance (OIT method). As reference samples, systems containing peroxide and antioxidant were prepared, which were not subjected to mutual reactions. RESULTS All numerical results of the work are considered as relative values, calculated in respect of results obtained for the reference samples. Therefore, „100%” result means that there was not any difference between the values obtained for the system containing peroxide and antioxidant after mutual reaction, and the results obtained for the reference system. In the case of the gel point measurements results obtained at different reactions conditions did not differ considerably, and moreover, there were not found any regularity, therefore, all results have been averaged for each of investigated polymers. The same is valid for tensile properties values. Results are shown in table 1. Table 1. Influence of DCP-AO interactions at 110C on the gel index and tensile properties of crosslinked LDPE and crosslinked EVA UL220 copolymer One can see that reactions between peroxide and antioxidant practically do not cause decrease of activity of peroxide as crosslinking agent for both polymers investigated. On the other hand, tensile properties slightly drop. The biggest decrease, by 10% at the average, occurred in the case of maximum tensile stress, measured for the crosslinked EVA copolymer. Reactions examined in this work exert greater influence on the thermooxidative stability of crosslinked polymers. It can be stated at the same time that in all examined tests this influence was advantageous, that is thermooxidative resistance of crosslinked systems containing product of reaction between peroxide and antioxidant is greater than the resistance of systems containing the same quantities of additives, which however have not been subjected to earlier mutual reaction. The results are presented on Fig. 1 for LDPE and Fig. 2 for EVA. Figure 1. Influence of DCP-AO interactions at 110C, at different molar ratio (DCP:AO) and different atmosphere on the thermooxidative stability of crosslinked PELD. The values presented are relative change of oxidation induction time (215C) in comparison with reference samples (100%). We can see that in the case of crosslinked LDPE, particularly advantageous results have been obtained for antioxidant AO 2, especially at higher values of the molar ratio peroxide : antioxidant. Practically in all cases products of reactions occurring in nitrogen atmosphere ensured better thermooxidative protection than analogous compounds obtained in the air atmosphere. In the case of the system: dicumyl peroxide – AO 2 antioxidant, used at molar ratio 8 : 1, being subjected to mutual reaction during 60 minutes at 110C in nitrogen atmosphere, thermooxidative resistance of crosslinked LDPE was achieved, estimated as the value of OIT at 215C, almost 2.5-times higher than the resistance guaranteed by the system having identical composition, but not subjected to the reaction. The next figure shows above mentioned dependences for crosslinked EVA UL220 copolymer. General regularities are similar, although in this case, values’ increases of the oxidation induction time are little lower. Figure 2. Influence of DCP-AO interactions at 110C, at different molar ratio (DCP:AO) and different atmosphere on the thermooxidative stability of crosslinked EVA UL220 copolymer. The values presented are relative change of oxidation induction time (215C) in comparison with reference samples (100%). CONCLUSIONS We carried out research on reactions occurring between dicumyl peroxide and two phenolic antioxidants at the temperatures lower than the temperature of rapid decomposition of this peroxide. Influence of temperature, molar ratio of the reagents, reaction time and the presence of atmospheric oxygen on this reactions have been determined. We found that between dicumyl peroxide and investigated phenolic antioxidants reactions occur at the temperature from 90 to 110C, causing formation of coloured products. The probable initiator of these reactions are radicals present even at low temperature in used compounds, although we can't exclude completely the non-radical reactions in this systems. The paper concerning this problem is in preparation. Colour intensity of these products is lesser if oxygen is removed from the reaction atmosphere. Reactions taking place between dicumyl peroxide and either of the two antioxidants used, in the conditions examined by us, do not cause decreasing of peroxide ability to crosslinking of LDPE or EVA copolymer. Above-mentioned reactions increase the ability of investigated antioxidants to thermooxidative protection of both crosslinked ethylene polymers. LITERATURE 1. McDonald P., Hamilton G., Mechanisms of Phenolic Oxidative Coupling Reactions, in: Oxidation inOrganic Chemistry, part B, Trahanovsky W. ed. Academic Press, New York 1973 2. Pospišil J., Habicher W., Zweifel H., Pilař J., Nešpurek S., Piringer O.; Polym. Polym. Compos. 10(1) (2002) 37-47 3. Vulic I., Vitarelli G., Zenner J.; Polym. Degrad. Stab. 78 (2002) 27-34 4. Kudła S., Poskrobko J.; Polymer Processing Society 18-th Annual Meeting, Guimaraes, Portugal, June 2002, Conference Proceedings 5. Kudła S., Herzog W.; Polymer Processing Society Europe/Africa Reginal Meeting, Athens, Greece, September 2003, Abstracts Book, p.212 6. Uhniat M., Kudła S., Polym. Degrad. Stab. 71 (2001) 69-74 7. Uhniat M., Sudoł M., Kudła S.; Polym. Degrad. Stab. 71 (2001) 75-82 8. Uhniat M., Kudła S., Dziwiński E., Sudoł M.; Polym. Degrad. Stab. 71 (2001) 83-91 9. Uhniat M., Kudła S., Zemlak M., Balcerowiak W.; Polimery 49 (2004) 180-186 Relative values, % Gel index Tensile properties Min. Mean Max. Tensile strength (max) Elongation at break LDPE 94 99 106 84 96 111 82 98 EVA UL220 100 101 55 90 117