Tersiyer butil katekol, alkillenmiş tri fenil fosfat ve karışımlarının CBR kauçuğundaki oksitlenmeyi yavaşlatıcı etkilerinin dsc yöntemi ile incelenmesi

Abstract

CBR, sentetik olarak elde edilen elastoînerlerin en önemlilerinden biri olup, başta otomobil lastiği üretimi ol mak üzere çeşitli,.alanlarda yaygın bir biçimde kullanılmakta dır. Diğer polimerik maddelerde olduğu gibi, CBR'de oksi jenle reaksiyona girerek oksit lenebilmekte ve özelliklerinde istenmeyen değişiklikler meydana gedmektedir. Bu nedenle, CBR ile çeşitli oksitlenmeyi önleyici maddeler kullanılmaktadır. Bugün Dünya pazarlarında çok sayıda ve çeşitli yavaşlatıcılar pazarlandığindan, en uygun yavaşlatıcı ve/'veyV karışımlarının belirlenmesinin hem' teknik hem de ekonomik' açldan önemi bu lunmaktadır. Bu çakışmada, Petkim, Petrokimya A.Ş.'nde üretilen GBR ile yavaşlatıcı olarak 2,6-ditersiyer butil katekol (Deenax) ve tri (mono-ve- di-nonil-f enil karışımı) fosfit (polygard) ve karışımları kullanılması halinde, CBR'nin oksitlenmeye daya nıklılığının etkilenişi "differential scanning calorimetry", DSC yöntemiyle incelendi ve ilgili aktivasyon enerjileri he saplanarak elde edilen sonuçlardan CBR'nin depolama süresi belirlendi. Bu nedenle, zaman zaman İR spektroskopisi yönte minden de yararlanıldı. Ayrıca, seçilen % 1 yavaşlatıcı kon santrasyonunun uygunluğu belirlendi ve elde edilen sonuçlar, yaptırılan spesif ikasyon testleriyle kontrol edildi. Deneyler IV için hazırlanan yavaşlatıcı karışımı katılmış örneklerdeki yavaşlatıcı miktarının kantitatif tayini için ise UV spektros- kopisi yönteminden yararlanıldı. üç asama halinde yürütülen çalışmalar sonunda; - DSC termogramlarında endotermik veya egzotermik her hangi bir reaksiyonun gözlenemediği sürenin gerçek yavaşlatılmış oksitlenme süresi olduğu belirlendi, - Deenax (Dnx), Polygard (Fig) karışımlarındaki etken yavaşlatıcının Dnx olduğu saptandı, - Dnx/Plg, 55/45 karışımında sinerjistik bir karışımın meydana geldiği belirlendi, - Dnx/Plg 40/60 karışımının etkenliğinin, % 100 Dnx' in etkenliğiyle yaklaşık aynı olduğu belirlendi, - CBR'nin sıcaklıkla değişen üc aşamalı bir oksitlenme kinetiği gösterdiği saptanarak, ilgili aktivasyon enerjileri düşük sıcaklıktan yüksek sıcaklığa doğru, sırasıyla, E.fl - 17.5, 27.0, 42.0 kcal/mol olarak he saplandı, - Dnx/Plg, 40/60 karışımından % 1 içeren CBR'nin 25 ± 5°C'de altı ay depolanabileceği belirlendi, - Dnx/Plg, 40/60 örneklerine oranla yavaşlatıcı etken liğinin değişmediği sıcaklıklarda, yapılacak DSC ça lışmalarıyla, oda sicaklarındaki yavaşlatılmış ok sitlenme surelerinin, DSC yöntemiyle kolaylıkla be- lirlenebileceği saptandı.

Most of the commercial elastomers are produced from 1,3-diens synthetically as Cis-1,4 polybut-adiene rubber (CBR) which is a rather important one. Since CBR replaces styrene - butadiene rubber (SBR) and/or natural rubber in the formula tions to improve properties, it has a special importance for the rubber tire industry and thus, is utilized in this area extensively. Production of footwear, transport belts, "V" belts, hoases, floor tiles, ebonite, etc. are other important application areas of CBR. CBR is alsa used in the production of antishock polystyrene and acrylonitrile-butadiene-styrene (ABS) plastics to improve their impact properties. As in the case of other polymeric materials, with physical and chemical factors, CBR oxidises by reacting with oxygen. Since oxidation effects the properties of CBR in an undesirable way, various antioxidants are used to retard or preferably inhibit its oxidation during its usage under the application conditions. Today, many antioxidants are marketed which are mainly phonolic, alkyl aryl phosphite, dithiocar- bonate and thiazyl derivatives. Furthermore, their number on the market keeps increasing. Therefore, selection of the most suitable antioxidants and/or their mixtures is very important from both technical and echonomical points of views. In our work, we determined the effectiveness of 2,6 - - VI ditertiarybutyl cathecol (Deenax) and tri (mono-and-di-nonyl- phenyl mixture) phosphite (Polygard) and their mixtures as antioxidants when they are used with CBR. The relevant experiments are done mainly by differential scanning calori- metry (DSC). From time to time however, IR spectroscopic techniques are also utilized. An UV spectroscopic method is used to determine the amount of the antioxidants in the samples prepared, quantitatively. The samples for our experiments are prepared from the CBR solution containing 13 % CBR which is produced by the CBR Plant of Petkim Petrokimya A.§. Deenax (Dnx) and Polygard (Pig) are desolved in benzene to obtain the desired Dnx/Plg value and then, this solution is added to the CBR solution to obtain total amount of 1 % antioxidant (Dnx + Pig) in CBR. The CBR solution containing the antioxidants is then poured slowly ower the rolls of a tworoll mixer whose rolls are kept at 85 C to evaporate the solvent. To secure total evaporation of the solvent and homogenenous distribution of the anti oxidants in CBR, the sample is milled a few times and then it is obtained as a sheet. One gramme samples are cut from the CBR sample prepared thus, 0.3 gramme of which is used to determine, the amount of the total antioxidant in the sample quantitatively by UV spectroscopy. If the value obtained is 1 ± 0.1,% 0.025 gr samples of equal sfzes are cut which are then placed in the DSC 'capsules. These capsules are kept open for air-rubber.interaction during the isothermal DSC studies to obtain the relevant thermograms. From the rest of the sample a 2.5 % CBR solution in benzene is prepared from which CBR films of equal thicknesses on KBr ift cells are prepared by dropping equal amounts of this solution on the KBr cells each time and then evaporating the solvent in vacuum. The film thicknesses are checked by IR absorbance measurements at 1458 cm. The films vıı obtained thus are used for IR studies. At the begining of our work, by IR and DSC experiments carried out in parallel, it is found that the real retarded oxidation period is the period during which no chemical reac tion, either endothermic or exothermic, is observed on the DSC thermograms. It is also found that, for comparative studies only, the time necessary for the formation of the peak at the DSC thermograms can also be used as the retarded oxidation period. At the second stage of our work, various CBR samples containing 1 % total antioxidant of different Dnx/Plg values are prepared and their DSC thermograms at various temperatures are obtained. When the retarded oxidation periods obtained from the DSC thermograms are compared with the Dnx/Plg values of the related samples, the following conclusions are reached. - The more effective antioxidant in the mixture of 2,6-ditertiary butyl cathecol (Dnx) and tri (mono - and-di-nonyl-phenyl mixture) phosphite (Pig) is 2,6- ditertiary butyl cathecol. - For 2,6-ditertiaryl butyl cathecol (Dnx) and tri (mono-and-di-nonyl-phenyl mixture) phosphide (Pig) mixtures, at Dnx/Plg, 55/45, a synergistic mixture is obtained. In this mixture, tri (mono-and-di-nonyl - phenyl mixture) phosphite is synergist for 2,6 - ditertiarybutyl cathecol. - When Pig is added to Dnx which is the more effective antioxidant, up to Dnx/Plg, 70/30 no considerable change in the effectiveness of Dnx is observed. When more Pig is added, the overall effectiveness in creases which reaches a maximum value at Dnx/Plg, 55/45. Further addition of Pig to Dnx, lowers the effectiveness from the maximum value obtained at Dnx/Plg, 55/45 and when Dnx/Plg is 40/60, the anti oxidant effectiveness obtained is almost equal to the effectiveness of Dnx. Since Dnx is about 2.5 times more expensive than Pig, this conclusion has v a rather important echonomical value. The retarded oxidation period is a function of reaction rate constants of the reactions taking place in this period, initial concentrations and percent conversions. Therefore, for an oxidation temperature, t = f(C2, C2, X)/Kc for CBR samples containing equal amount of antioxidant of particular Dnx/Plg value, f(C-, C», X) = B, Constant can be written. Therefore, _B If we write the Arrhenius equation for Kc> the overall rate constant and plug it In the above equation for t lnt. ln B + Ea 1 is obtained. Taking above equation into consideration, in our work, by plotting "ln" of retarded oxidation periods obtained from DSC thermograms against =» Arrhenius plots for various Dnx/Plg values are obtained and activation energies are calculated from them. The values obtained indicates that, around our experiment temperatures (120-170 C), CBR has two - IX different kinetic steps whose activation energies are, E = 27.5 kcal/mol for lower and E = 42 kcal/mol for higher temperatures. However, taking into account the facts that calculations of retarted oxidation periods for ambient temperatures using 27.5 kcal/mol gives rather unrealistic va lues such as^ 120 years for 25°C and CBR, like many other polymeric materials, might have a third kinetic step with a lower activation energy, further experiments are carried out at temperatures lower than 120°C. In this case, IR spectros copic techniques are utilized and an activation energy value of E = 17.5 kcal/mol is calculated from the Arrhenius plot obtained for the third kinetic step which seems to take place near ambient temperatures (T < 80°C). From E » 17.5 kcal/mol, the retarded oxidation period at 25 ± 5°C for CBR containing 1 % antioxidant (Dnx/Plg, 40/60) is calculated as six months. Therefore, the following conclusions are reached. - The activation energies calculated from the Arrhenius plots are in good agreement with the values given in the literature for polymeric materials. -i Results of DSC studies to evaluate antioxidant effectiveness which are carried out at relatively high temperatures, can not be directly corrolated to room temperatures. However, the results obtained can be used for comperative evaluations. - CBR containing 1 % Dnx/Plg, 40/60 can be stored for six mounths at 25 ± 5 C in contact with air. To compare antioxidant effectivenesses, the values obtained from DSC studies for the effectiveness (retarded oxidation periods) of 1 % Dnx/Plg of Dnx/Plg, 40/60 is assumed 100 % for each temperature. The retarded oxidation period of this sample corresponding to each temperature of experiment is then devided by the retarded oxidation periods of other samples (Dnx/Plg, 30/70, 20/80, 10/90) at the same tempera ture, and thus, an antioxidant effectiveness value for each temperature and for each sample is obtained. When antioxidant effectiveness values are platted against temperature, the graph obtained indicates that at high temperatures, antioxidant effectiveness values change with temperature. At relatively lower temperatures (120-140°C) however, it is seen that the antioxidant effectiveness is almost constant and does not change with temperature. Assuming that the constant effective ness values obtained for 120-140°C can be correlated to room temperatures, antioxidant effectiveness values of 68 %, 58 % and 45 % at 25°C for the samples of Dnx/Plg 30/70, 20/80 and 10/90 are obtained respectively. In the third stage of our work, experimental studies are carried out to check the results obtained in the second stage. Before proceeding for the third stage experiments however, suitability of the 1 Z total antioxidant usage is verified by controlling if the "maximum critical limit", which is an important parameter in antioxidant usage, is exceeded. For this purpose, CBR samples containing 0.5-1 % antioxidant (Dnx/Plg, 30/70) are prepared and their DSC thermograms at different temperatures are obtained. From the graphs obtained by plotting the retarded oxidation periods, obtained from these thermograms, against temperature, it is seen that linearity of affectiveness change with the change of concentration is still retained at 1 % usage of Dnx/Plg and therfore, the "maximum critical limit" is not exceeded. After this result, CBR samples containing 1 % Dnx/Plg, 40/60 are prepared and they are stored- at 25 + 5 C in contact with air. Periodical specification tests of these samples indicated that even after 9.5 mounths they retained their quality test result values. However, between 6 and 7.5 months, - xı - the samples developed a pale yellow to yellow color which was due to a rather thin oxidised layer on their surfaces. This result is found to be in good agreement with the approximately 6 month storage time at 25 + 5°C obtained from the Arrrhenius plot for the same CBR sample. After confirming the storage time for CBR samples containing 1 % Dnx/Plg, 40/60, as six months, experiments are carried out to confirm the storage time at 25 + 5 C for the samples containing 1 % Dnx/Plg, 20/80 which was obtained as 3.5 months from the antioxidant effectiveness calculations. For this purpose IR spectroscopic techniques are utilized and the retarded oxidation periods for the samples prepared are determined at 70-100 C. From the Arrhenius plot obtained, the retarded oxidation period for the CBR containing 1 % Dnx/Plg, 20/80 at 25 + 5 C is calculated as 3.5 months which is in very good agreement with the results obtained from the anti oxidant effectiveness calculations. The conclusion reached from the third stage studies is that, by parrying out DSC experiments at temperatures where comparative effectivenesses of the samples to that of Dnx/Plg, 40/60 samples is constant, retarded oxidation periods at room temperature of other CBR samples containing various amounts of Dnx/Plg or some other antioxidants can easily be determined.