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Synergism Between Phenolic Antioxidants in Autoxidation

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Synergism Between Phenolic Antioxidants in Autoxidation 22 Journal of the Japan Petroleum Institute, 54, (1), 22-29 (2011) [Regular Paper] Synergism between Phenolic Antioxidants in Autoxidation Yasukazu OHKATSU＊ and Fumie SUZUKI Dept. of Applied Chemistry, Faculty of Engineering, Kogakuin University, 1-24-2 Nishi-shinjuku, Shinjuku-ku, Tokyo 163-6877, JAPAN (Received March 2, 2010) Combinations of two types of phenolic antioxidants were examined to assess synergistic or antagonistic inter- action from both radical scavenging rate constant (kinh) and radical scavenging number (n). The synergism of two types of phenols was observed especially with comparatively lower and similar oxidation potentials. In syn- ergism of kinh, a phenol with lower oxidation potential simply acts as a peroxy radical scavenger, whereas the other phenol with higher oxidation potential acts as a hydrogen donor in the regeneration of the former phenol from its phenoxy radical. On the other hand, synergism of n is seen for less or non-hindered phenols, especially with 4-methoxy substituent, and is controlled by the reaction conditions, such as solvent polarity. This is interpreted as a molecular association of phenols formed through hydrogen bonds and hydrophilic interactions, resulting in obstruction of coupling of the phenoxy radicals. As a result, 3,5-di-t-butyl-4-hydroxy-toluene (BHT) and 2-t-butyl-4-methoxyphenol (3 : 2 by mole ratio), and BHT and 4-methoxyphenol (3 : 2 by mole ratio) were esti- mated to show total synergism of 1.41 and 1.35 times, respectively, compared with the simple sum of the anti- oxidant activities in a hydrophobic system, such as chlorobenzene. These results are important for reduction of costs as well as environmental pollution by allowing decreased use of phenols. Keywords Phenolic antioxidant, Synergism, Antagonism, Kinetic treatment, Cost reduction 1. Introduction the number of radicals scavenged by a phenolic moirty, and kinh is the radical scavenging rate constant. Larger Polymeric materials such as plastic and rubber are n and/or kinh values will indicate better antioxidant extremely important materials in the contemporary properties. world. However, these materials gradually deteriorate d[O]2 kp[RH] R i under the influence of external factors such as light, − = = Ri ()1 (1) dt nkinh[AH] heat, and oxygen, which mainly cause autoxidation as a inh radical chain reaction. Control of this autoxidation Most antioxidants generally show some interactions mainly depends on methods to stop the chain reaction in combined use. Phenolic antioxidants are reported or to scavenge chain carriers. to interact with many other types of antioxidants, but In general, phenols are used as antioxidants for stabi- the interactions between different types of phenols re- lizing many polymeric products and petroleum1), main unclear. The synergism of two phenols has been because of their effective scavenging of peroxy radicals reported2),3), but the details are unknown. However, if responsible for polymer deterioration. Scheme 1 shows a general autoxidation mechanism in the pres- ence of phenolic antioxidants. The radical (R・) gener- ated in the initiation step of the deterioration promptly reacts with oxygen to form a peroxy radical, which ab- stracts a hydrogen atom from the substrates to form a hydroperoxide and R・. This sequence forms the chain reaction. Antioxidant AH can react with peroxy radi- cals faster than substrate RH, resulting in interruption of the chain reaction. In this mechanism, the oxygen uptake rate can be represented by Eq. (1), in which n is Ri＝the rate of initiation step n＝the number of peroxy radicals scavenged by a phenol kinh＝the rate constant of peroxy radical scavenging step by a phenol ＊ To whom correspondence should be addressed. ＊ E-mail: [email protected] Scheme 1 Autoxidation in the Presence of Phenolic Antioxidant J. Jpn. Petrol. Inst., Vol. 54, No. 1, 2011 23 the required amounts of phenols can be reduced by such benzene as solvent, to estimate the association of phe- synergism, both costs and environmental pollution can nols in the solvent. be reduced. 2. 4. GC Measurement The present study investigated the interaction of two The consumed amount of phenols was measured by a types of phenolic antioxidants to try to identify cheap conventional gas chromatograph (GC) method using a and environmentally friendly combinations of phenolic 400-1HT column (Qudrex Corp.) to evaluate the inter- antioxidants. action of two types of phenols. 2. Experimental 3. Results and Discussion 2. 1. Phenolic Antioxidants 3. 1. Effect of Substituents on Characteristics of Seven types of phenolic antioxidants were purified Phenolic Antioxidants by conventional methods, so that the melting point was Table 1 shows the oxidation potentials and anti- within 0.5℃ of that reported for authentic samples: oxidant characteristics of the seven phenols. The oxi- 3,5-di-t-butyl-4-hydroxyanisole (BHA), 2-t-butyl-4- dation potentials are based on oxidation of the phenolic methoxyphenol, 4-methoxyphenol, BHT, 2-t-butyl-4- group, and apparently depend on the types and numbers methylphenol, 4-methylphenol, and 2-t-butylphenol. of substituents. It increased with lower steric hin- 2. 2. Redox Potential drance of the phenol group. The redox potential of each phenol was measured Electron donating substituents with higher electro- under nitrogen atmosphere using cyclic voltammetry philic substituent constant (σ＋) at the o- and p-positions (CV) using a saturated calomel reference electrode enhance the activity of phenols, and a proposed mecha- (SCE), a Pt working electrode, and a Pt counter elec- nism for the hydrogen abstraction of a peroxy radical trode. The measured solution contained 0.1 mol/dm3 from a phenol is proposed as shown in Scheme 25). of tetra-butylammonium perchlorate as electrolyte and The hydrogen abstraction starts with electron transfer 10-3 mol/dm3 of phenol in acetonitrile. from the oxygen atom of the hydroxyl group to a per- 2. 3. Antioxidant Activity oxy radical, and ends by elimination of the phenolic The antioxidant activity of phenols was measured in hydrogen as a proton. This mechanism suggests that a chlorobenzene as follows: autoxidation of styrene phenol with low oxidation potential, which is oxidized 3 (2 mol/dm ) was carried out in the presence of phenolic easily, can be expected to have high kinh as an excellent antioxidant(s) using α,α’-azo-bis-isobutyronitrile (AIBN) antioxidant. The mechanism might be correct under (10-2 mol/dm3) as an initiator at 50℃ under oxygen. certain conditions, but Table 1 shows that phenols with The reaction was followed by measuring the uptake of low oxidation potential do not necessarily have high kinh oxygen, and analyzed in terms of n and kinh as defined on the basis of the nature and number of substituents. by Eq. (1). BHT was mainly used as the reference an- Peroxy radical can more easily approach the hydroxyl tioxidant. In addition, autoxidation was carried out group of a phenol with fewer o-substituent(s), resulting using benzonitrile or acetophenone in place of chloro- in easier formation of the intermediate shown in the Table 1 Oxidation Potential and Antioxidant Activities of Phenolic Antioxidants Phenolic Oxidation k Phenolic Oxidation k inh n inh n antioxidant potential [V] [×104 dm3・mol-1・s-1] antioxidant potential [V] [×104 dm3・mol-1・s-1] OH OH 0.90 1.8 1.8 1.27 1.7 1.7 OCH3 CH3 OH OH 0.99 3.6 2.2 1.29 3.3 1.6 OCH3 CH3 OH OH 1.04 3.2 1.6 1.42 5.3 1.3 OCH3 CH3 OH 1.45 3.4 1.4 J. Jpn. Petrol. Inst., Vol. 54, No. 1, 2011 24 Scheme 2 Hydrogen Abstraction from Phenol by Peroxy Radical appears as a solid line: the interaction is called “syner- gism” for a solid line above a broken line and “antago- nism” for a solid line below the broken line. The de- gree of “synergism” is assessed semi-quantitatively by the area between the broken and solid lines: the larger area is represented by a double circle. In Fig. 2, the vertical axis shows the kinh or n value obtained experi- mentally, and the horizontal axis shows the mixed mole ratios of the two phenols. The observed intera ctions are arranged to form inter- action maps (Figs. 3(a) and 3(b)), in which the interac- tion modes of kinh and n values are plotted against the redox potentials of phenols. Comparison of the inter- action modes of kinh with n shows that kinh has prefera- ble synergism in a wider combination of phenols. In Fig. 1 Relationship between kinh and n of Phenols particular, strong synergism of kinh was observed for combinations of phenols with lower oxidation poten- tials, such as phenols with a methoxy group on the parenthesis of Scheme 2. Therefore, such a chemical 4-position. In contrast, the combination of phenols structure should govern the reaction of Scheme 2 and with 4-methyl group and/or with a high difference of result in high kinh for the phenol. In fact, phenols show their redox potentials showed no interaction or antago- a higher kinh in the order of no o-＞one o-＞two o- nism. substituent(s). Therefore, the activity of a phenol is Figure 4 shows the representative synergism and likely controlled by the o-substituent rather than the antagonism profiles for kinh and n, respectively, to rec- oxidation potential. ognize how such phenomena are observed in the pres- Figure 1 shows the relationship between kinh and n ence of two types of phenolic antioxidants. The left of of the seven phenols. 4-Methyl-phenols showed some the Fig. 4 shows strong synergism for kinh of a combi- relationship between higher kinh and lower n. A simi- nation of 4-methoxyphenol and BHT, and the right of lar relationship is also observed for commercialized Fig.
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