Journal of the Japan Petroleum Institute, 51, (2), 95-101 (2008) 95

[Regular Paper]

Interaction of Benzoate-type Ultraviolet Absorbers with Hindered Amine Light Stabilizers

Yasukazu Ohkatsu＊, Hiroyuki Takenaka, and Nanae Kamiyama

Dept. of Applied Chemistry, Faculty of Engineering, Kogakuin University, 1-24-2 Nishishinjuku, Shinjuku-ku, Tokyo 163-8677, JAPAN

(Received June 26, 2007)

The interaction of benzoate-type ultraviolet absorbers (UVA) with hindered amine light stabilizers (HALS) was investigated.　Benzoate-type UVA showed antagonism or synergism with HALS, depending on the chemical structures of the UVA.　p-Aminobenzoates exhibited antagonism with HALS to accelerate the photo-oxidation, despite absorbing UV rays with considerably high absorption coefficients.　In contrast, p-hydroxybenzoates showed synergistic photo-antioxidant activity with HALS, despite no or little absorption of ultraviolet rays.　The synergism has been ascribed to conversion into benzophenone-type UVA by a photo-Fries rearrangement.　This mechanism can explain the photo-antioxidant ability of substituted phenyl p-hydroxybenzoates, but not that of alkyl p-hydroxybenzoates not undergoing the photo-Fries rearrangement.　This study proposes a new and comprehensive synergism in which the photo-antioxidant activity of p-hydroxybenzoate is due to the formation of a UV-absorbing intermediate and a photo-antioxidant compound with a catechol structure from the reaction of the benzoate with HALS nitrosonium.

Keywords HALS, HALS nitrosonium, Photo-antioxidant activity, Benzoate-type UVA, Synergism

1.　Introduction The resulting product, with a similar structure to benzo-phenone-type UVA, contributes to photo-stabilization.　

Ultra-violet absorbers (UVA) and hindered amine This mechanism seems plausible, but does not explain light stabilizers (HALS) are often used together for the the synergism of octadecyl 3,5-di-t-butyl-4-hydroxy-photo-stabilization of polymeric materials.　Generally, benzoate, for which no photo-Fries rearrangement is such a combination exhibits synergism, although the feasible, nor the involvement of HALS in the syner-mechanism remains unclear.　One common view of the gism.　The manufacturer of this type of a benzoate has synergistic mechanism is that the HALS show synergis- promoted such synergism, but the mechanism remains tic and/or antagonistic actions with UVA incorporating unknown. intramolecular hydrogen bonding, but the overall inter- This study investigated the interaction of benzoate-action is synergistic1),2).　This synergistic action is based type UVA with HALS and propose a new comprehen-on the reduction of a UVA quinoid compound, inciden- sive synergistic mechanism of both additives, especially, tally produced from the peroxy radical trapping action including the participation of HALS. of UVA, by the HALS, especially HALS NOH, resulting in the regeneration of a new UVA.　In contrast, the antag- 2.　Experimental onistic action is based on accelerating the formation of HALS nitrosonium, which causes useless oxidation of 2. 1.　Reagents UVA. 2. 1. 1.　Benzoate-type UVA

Other types of UVA contain no intramolecular hydro- The benzoate-type UVA, 2,4-di-t-butylphenyl 3,5-gen bonding, such as benzoate-type UVA.　Substituted di-t-butyl-4-hydroxybenzoate (UVA-1), hexadecyl phenyl p-hydroxybenzoates are known to show syner- 3,5-di-t-butyl-4-hydroxybenzoate (UVA-2), ethyl p-gism rather than antagonism with HALS, despite slight aminobenzoate (UVA-5), and 2-ethylhexyl p-N,N-di-absorption of natural UV rays.　For example, 2-t- methylaminobenzoate (UVA-6) were commercial prod-butylphenyl 3,5-di-t-butyl-4-hydroxybenzoate absorbs ucts.　Hexadecyl 3-t-butyl-4-hydroxybenzoate (UVA-3), the UV rays to undergo a photo-Fries rearrangement3).　 hexadecyl 4-hydroxybenzoate (UVA-4), and ethyl

3,5-di-t-butyl-4-hydroxybenzoate (UVA-7) were syn-＊ To whom correspondence should be addressed. thesized as follows. ＊ E-mail: ohkatsu@cc.kogakuin.ac.jp

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2. 1. 1. 1.　Synthesis of UVA-3 Oxalyl chloride (1.27 g, 0.01 mol) was dissolved in

ethyl ether (10 ml), 3-t-butyl-4-hydroxybenzoic acid (1.95 g, 0.01 mol) was added, and the resulting mixture was stirred for 3 h at room temperature.　The reaction solution was cooled at 0℃, n-hexadecanol (2.43 g, 0.01 mol) and pyridine (0.8 g, 0.01 mol) were added, and the mixture was stirred overnight.　Then the sol-vent was removed by evaporation, and the residue was purified by a column chromatography (eluent: methyl-ene chloride).　White fluffy crystals were obtained. Yield: 0.76 g (18.2％) and melting point: 66.4-67.2℃.　FT-IR (KBr): 3263 cm-1 (_OH), 1668 cm-1 (_COO_).　

_1H-NMR (CDCl3, TMS): 0.88 ppm (t, CH3 , 3H), 1.25 ppm (m, _CH2

_, 28H), 1.41 ppm (s, t-butyl, 9H), 4.30 ppm (t, _COO_CH2-, 2H), 6.7-8.0 ppm (benzene, 3H), 7.26 ppm (_OH, 1H). 2. 1. 1. 2.　Synthesis of UVA-4

Synthesis of hexadecyl 3-t-butyl-4-hydroxybenzoate was repeated but using 4-hydroxybenzoic acid in place of 3-t-butyl-4-hydroxybenzoic acid.　The crude prod-uct was purified by a chromatography (column: silica gel, eluent: hexane/ethyl acetate＝6/4), and recrystal-lized from chloroform to obtain white plate crystals. Yield: 2.75 g (38.0％) and melting point: 63.4-64.3℃.　FT-IR (KBr): 3396 cm-1 (_OH), 1687 cm-1 (_COO_).　

_1H-NMR (CDCl3, TMS): 0.88 ppm (t, CH3 , 3H), _1.28 ppm (m, _CH2

_, 28H), 4.30 ppm (t, _COO_CH2_2H), 6.8-8.0 ppm (benzene, 4H), 7.26 ppm (s, OH,

,

1H). 2. 1. 1. 3.　Synthesis of UVA-7

3,5-Di-t-butyl-4-hydroxybenzoic acid (5 g, 0.02 mol) was dissolved in ethanol (100 ml), conc. sulfuric acid (1 ml) was added, and the mixture was refluxed for 5 h.　After the reaction, the mixture was made weakly basic by adding sodium hydrogen carbonate, and extracted with methylene chloride.　The residue was recrystal-lized form hexane to obtain white fluffy crystal. Yield: 2.4 g (43.2％) and melting point: 104.8-105.5℃.　FT-IR (KBr): 3587 cm-1 (_OH), 1693 cm-1 (_COO_).　

_1H-NMR (CDCl3, TMS): 1.01 ppm (t, CH3 , 3H), _1.48 ppm (s, t-butyl, 18H), 4.29 ppm (t, _COO_CH2 ,

2H), 5.90 ppm (s, _OH, 1H), 7.94 ppm (benzene, 2H). 2. 1. 2.　HALS and Derivatives

Bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (ADK stab LA-77, ADEKA Corp.) designated as the HALS in this study was refined by a general method.　In addition, 4-acetoxy-2,2,6,6-tetramethylpiperidine (Ac-HALS) was synthesized according to the procedure described previously4).　HALS nitrosonium was syn-thesized as described previously5). 2. 1. 3.　Other Reagents

Cumene hydroperoxide (CHP) (Nacalai Tesque, Inc.) was used as hydroperoxide.　Chlorobenzene (GODO Solvents. Co.) as solvent was refined by a general method.　Styrene was distilled in vacuo just before

use.　Azo-bis-isobutyronitrile (AIBN) was used as ini-tiator. 2. 2.　Procedures 2. 2. 1.　Decomposition of Cumene Hydroperoxide

(CHP) The reaction solution was prepared by dissolving

HALS, CHP, and UVA in predetermined amounts in chlorobenzene free from oxygen (total amount: 100 ml or 50 ml), and reacted with stirring in an oil bath at 120℃ under diffused light and nitrogen atmosphere.　The reaction was monitored by sampling an aliquot of the reaction solution every several hours, and analyzing it by an iodine titration method: carbon dioxide was bubbled into 10％ acetic acid-isopropyl alcohol solu-tion (5 ml) for 2 min to purge the dissolved oxygen.　Saturated potassium solution (0.5 ml) and the sample (1 ml) were added.　This mixture was stirred at 95℃in an oil bath under carbon dioxide atmosphere to liber-ate iodine, which was titrated with N/1000 sodium thio-sulfate.　The products were analyzed by a gas chroma-tography (GC) [Shimazu GC-17A or GC-14B with FID, and column 400-1HT (Quadrex Corp.)], and a GC-mass spectrometry (GC-MS) [Shimazu QP5050A with the same column]. 2. 2. 2.　Measurement of Photo-antioxidant Ability

The photo-oxidation reaction of styrene in the pres-ence of UVA and/or HALS was performed in chloro-benzene under oxygen atmosphere and UV irradiation from a high pressure mercury lamp (500 W, made by Ushio Inc.).　Ultraviolet irradiation under 290 nm was cut using a UV-cut filter (UV-29 manufactured by AGC Techno Glass Co., Ltd.).　The oxidation reaction was monitored by measuring the amount of oxygen con-sumed with a pressure transducer.　The oxidation of ethyl linoleate in the presence of dihydric phenol was performed in benzonitrile in order to make the reaction system homogeneous: two substrates, styrene and ethyl linoleate, were selected because of their convenient characteristics for kinetically studying antioxidants.　The antioxidant ability was estimated as the ratio (％) of initial oxidation rates obtained in the presence and the absence of additive(s): 0％ represents the complete oxidation inhibition, and 100％ shows no inhibition. 2. 2. 3.　Reaction of HALS Nitrosonium with Ethyl

2,6-Di-t-butyl-4-hydroxybenzoate (UVA-7) HALS nitrosonium (5.0×10-4 mol/l) was reacted

with UVA-7 (5×10-3 mol/l) in the presence of CHP (1 × 10-2 mol/l) in chlorobenzene under nitrogen atmosphere at 120℃.　The CHP was used to make the reaction system resemble the system expected to form under practical conditions.　The products from UVA-7 were analyzed and identified by the same GC-MS method as described in 2. 2. 1.

J. Jpn. Petrol. Inst., Vol. 51, No. 2, 2008

In chlorobenzene under N2 at 120℃, [HALS]＝[UVA]＝10-3 mol/l, [CHP]＝10-2 mol/l.

Fig. 1　Decomposition of CHP by HALS in the Presence of Benzoate-type UVA

3.　Results and Discussion

3. 1.　Effect of Benzoate-type UVA on HALS in CHP Decomposition

The decomposition of hydroperoxide by HALS is very useful for observing the synergistic or antagonistic interaction of UVA with HALS, because the hydro-peroxide decomposition proceeds through a hemolytic process involving an intermediate active species, HALS nitrosonium, and increased decomposition suggests antagonism of UVA with HALS.　A proton donor such as a phenol forms the HALS ammonium salt, which is promptly oxidized to HALS nitrosonium by a hydro-peroxide6).　Therefore, the weak proton donors p-amino-and p-hydroxybenzoate UVA do not show such an antagonistic effect.　2-Ethylhexyl p-dimethylamino-benzoate (UVA-6), for example, can function as light stabilizer as follows:

N UV rays

C_O_C8H17 C_O_C8H17heat

N＋

O O

p-Hydroxybenzoates are also weak proton donors due to the presence of a strong electron-withdrawing sub-stituent (_COO_) on the p-position.

Figure 1 shows the results of CHP decomposition reactions by HALS in the presence of benzoate-type UVA.　Interestingly, p-aminobenzoates, namely UVA-5 and UVA-6, showed remarkable antagonisms with HALS, as recognized by comparing their CHP de-composition curves with the curve of only HALS.　These p-aminobenzoates do not decompose CHP at all in the absence of HALS, but enhanced the CHP decom-position ability of HALS significantly, that is, acceler-ated the formation of HALS nitrosonium.　This phe-nomenon may seem strange, based on the absence of the proton-donation of the UVA.　However, this find-ing may be explained by the following scheme, al-

97

though no precise evidence can be provided:

NHN R COOR’

NR2 COOR’ N＋HR

NR COR’

N＋

O

HR (R＝H or CH3)

The intermediate containing the =N- group is formed by electron transfer between HALS and p-amino-benzoate and is stabilized electronically by the _R effect of the carbonyloxy group.　In addition, the electron transfer reaction may be supported by the high oxida-tion potentials of p-aminobenzoate and p-dimethylamino-benzoate, 1.20 V and 1.09 V (vs. SCE), respectively, which are sufficient to oxidize HALS (oxidation poten-tial: 0.90 V under the same conditions).　The resulting HALS ammonium salt is converted into HALS nitro-sonium with decomposition of hydroperoxide4).　Consequently, such antagonism will be observed.

In contrast, p-hydroxybenzoates (UVA 1-4) did not show significant antagonism with HALS, but rather de-creased the CHP decomposition ability of HALS (Fig. 1).　That is, UVA-4 slightly promoted whereas UVA 1-3 strongly inhibited hydroperoxide decomposi-tion by HALS.　These results may be explained by the idea that p-hydroxybenzoate UVA acts as a very weak proton donor to promote the formation of HALS nitro-sonium, in the presence of phenolic hydrogen, but the resulting nitrosonium will become harmless by a reac-tion with the UVA.　The presence of t-butyl group(s) at the o-position to the hydroxyl group of UVA cumu-latively decreased CHP decomposition. 3. 2.　Photo-antioxidant Effect of Benzoate-type

UVA Measurement of the photo-antioxidant action is one

of the most effective methods to observe the action of UVA.　Figure 2 illustrates the interaction of 2-ethyl-hexyl p-dimethylaminobenzoate (UVA-6) with HALS in photo-oxidation.　In general, the area above the dotted line on the figure suggests antagonism between HALS and UVA, and the area under the dotted line indicates synergism.　Thus, UVA-6 also shows antagonism with HALS in photo-oxidation, although this commercial-ized UVA is often used in cosmetics.　UVA-6 shows a photo-antioxidant action if used alone, probably due to its high absorption coefficient (see, Table 1).　UVA-5 also showed similar antagonism.　That is, p-amino-benzoates cannot control photo-oxidation in the pres-ence of HALS, but rather accelerate the oxidation.　Therefore, during photo-oxidation, HALS nitrosonium may be formed to accelerate the oxidation by homolytic decomposition of hydroperoxides.　We concluded that p-aminobenzoates show antagonism with HALS in both

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CHP decomposition and photo-oxidation. Figures 3 and 4 show the results of photo-oxidations

in the presence of p-hydroxybenzoates and HALS.　UVA-1 is generally accepted to conver into benzo-phenone-type UVA by a photo-Fries rearrangement as shown below:

HOphoto-Friesrearrangement

O＝CHO COO

OH

In contrast, UVA-2 does not undergo such a rearrange-ment.　Figure 5 shows the UV absorption spectra of benzoates used in this study.　Only UVA-1 slightly ab-sorbs ultraviolet rays higher than 290 nm, whereas UVA-2-4 do not.　Therefore, these UVA only slightly inhibit photo-oxidation as illustrated in Figs. 3 and 4, if used alone (see the data at [A]/[A]＋[B]＝100 mol/・％l ).　Nevertheless, UVA-1 and UVA-2 are used as light sta-bilizer together with HALS in practice.　This study also tried to elucidate the reason why a combination of UVA-1 or UVA-2 with HALS shows synergistic light stabilizer action.

The two synergistic curves shown in Fig. 3 present

different features.　First, UVA-1 shows higher syner-gism than UVA-2 possibly due to the additional effect of the photo-Fries rearrangement peculiar to UVA-1.　Second, the molar ratios of benzoate to HALS, showing the maximum synergism are different; at about 65％ for UVA-1 and at about 20％ for UVA-2.　The latter case indica tes tha t UVA-2 can demonst ra te photo-antioxidant ability in the presence of larger amounts of HALS.　In addition, the octadecyl 4-hydroxybenzoates shown in Figs. 3 and 4 suggest that benzoates contain-ing more o-t-butyl group(s) show greater photo-antioxidant action, suggesting a significant role for the t-butyl group in such synergistic actioin.

As shown in Fig. 4, UVA-4 exhibited slight photo-

In chlorobenzene under O2 and UV irradiation at 50℃, [styrene]＝2.0 mol/l, [AIBN]＝10-2 mol/l, [A]＋ [B]＝10-4 mol/l.

Fig. 3　Photo-oxidation of Styrene in the Presence of UVA-1 or UVA-2 (A) and HALS (B)

In chlorobenzene under O2 and UV irradiation at 50℃, [styrene]＝ In chlorobenzene under O2 and UV irradiation at 50℃, [styrene]＝2.0 mol/l, [AIBN]＝10-2 mol/l, [A]＋ [B]＝10-4 mol/l. 2.0 mol/l, [AIBN]＝10-2 mol/l, [A]＋ [B]＝10-4 mol/l.

Fig. 2　Photo-oxidation of Styrene in the Presence of UVA-6 (A) Fig. 4　Photo-oxidation of Styrene in the Presence of UVA-3 or and HALS (B) UVA-4 (A) and HALS (B)

Table 1　Photo-antioxidant Activity of UVA under UV Irradiation

λmax a) Relative initial oxidation rate [％]b)

UVA [nm]

ε[UVA]＝10-4 mol/l [UVA]＝[HALS]＝5×10-5 mol/l

none 100.0 100.0 UVA-5 290 19100 95.0 100.2 UVA-6 310 34400 92.5 100.5

a) [UVA]＝10-4 mol/l in methylene chloride at room temperature. b) In benzonitrile at 50℃; [ethyl linoleate]＝2.0 mol/l, [AIBN]＝10-2 mol/l.

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Fig. 5　UV Spectra of p-Hydroxybenzoate-type UVA (10-5 mol/l in CH2Cl2)

In chlorobenzene under N2 at 120℃, [CHP]＝10-2 mol/l, [HALS nitro-sonium]＝5.0×10-5 mol/l, [UVA]＝5.0×10-3 mol/l.Product A is calculated based on ethyl 2,6-di-t-butyl-4-hydroxy-benzoate.

Fig. 6　Reaction Product of Ethyl 2,6-Di-t-butyl-4-hydroxybenzoate with HALS Nitrosonium in the Presence of CHP

antioxidant action under UV irradiation＞290 nm, but not under UV irradiation＞320 nm.　This phenomenon seems to be completely different from that observed for other octadecyl 4-hydroxybenzoate with t-butyl group(s).　We did not investigate the reason further, but these t-butyl-substituted 4-hydroxybenzoates may also make use of UV rays of such longer wavelengths in the photo-antioxidant action. 3. 3.　Synergistic Mechanism of p-Hydroxybenzoate

with HALS HALS nitrosonium is formed during the CHP de-

composition reaction and also photo-oxidation in the presence of a mixture of HALS and benzoate-type UVA.　Analysis of the products formed by the reac-tion of HALS nitrosonium with benzoate-type UVA is a simple way to investigate this interaction.　In this study, such a method was tried using UVA-7 in place of UVA-2, because the former gives products which can be analyzed by GC-MS.　Figure 6 shows the re-action between HALS nitrosonium and UVA-7 in the presence of CHP.　HALS nitrosonium decomposed

CHP catalytically and also reacted with UVA-7 at the same time to form products derived from UVA-7.　Although all products observed by gas chromatography of the reaction mixture could not be identified, one of the main products derived from UVA-7 was

OH

O COOC2H5 product A

m＋/e＝294 (P, 4), 238 (40), 221 (100), 165 (55), 137 (40), 109 (40):

HOOH OH

O COOC2H5 O HO

294 221 165

OHH

O COOC2H5 O COOC2H5 O COOC2H5

238 221 165

O＋HO COOH O

137 109

This product only accounts for part of the converted UVA-7, but provides important information on the inter-action between p-hydroxybenzoates and HALS.

Interestingly, a new product was formed in a small quantity with decreasing product A, if the reaction mix-ture was irradiated with UV rays for a short time.　This product was identified as

HO

HO COOC2H5 product B

m＋/e＝238 (P, 10), 223 (10), 193 (5), 177 (4), 119 (100), 91 (30)

HO HO HO

COOC＋H2 C＋OHO COOC2H5 HO HO

238 223 193

HO＋ HO＋C O C O C O

177 119

Products A and B were also found in the mixture pro-duced by the photo-oxidation of styrene in the presence of HALS and a p-hydroxybenzoate.

These products and experimental observations sug-gest a synergistic mechanism of benzoates with HALS as shown in Scheme 1.　HALS nitrosonium first removes two electrons from UVA-7 to form 2,6-di-t-butyl-4-ethoxycarbonylphenyl-oxonium ion.　Resonance stabi-

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100

Scheme 1　Interaction of Ethyl 2,6-Di-t-butylbenzoate with HALS Nitrosonium

Table 2　Antioxidant and Photo-antioxidant Activities of Dihydric Phenols

Relative initial oxidation rate [％]

Phenol for antioxidant [％] for photo-antioxidant [％]

10-4 mol/l 10-4 mol/l 5×10-4 mol/l

none 100.0 100.0 100.0 HO

HO10.6 97.3 91.8

OH

HO86.5 100.0 100.0

In benzonitrile at 50℃; [ethyl linoleate]＝2.0 mol/l , [AIBN]＝10-2 mol/l.

lizes the positive charge at the o-position, because the－I effect of the t-butyl group at the o-position stabilizes the cation.　This cation reacts with a hydroxyl anion, probably coming from the decomposition of CHP, to give product A.　The enone structure of product A ab-sorbs UV rays, and the excited carbonyl oxygen ab-stracts hydrogen from the t-butyl at the same time to form butene, resulting in the generation of product B.　This mechanism can explain the maximum synergism action appearing in a HALS-rich mixture with a benzo-ate as shown in Fig. 3.

The formation of the compound with a catechol moiety is very interesting, because catechol has photo-antioxidant activity.　Table 2 illustrates the anti-oxidant abilities of catechol and resorcinol.　Catechol shows much higher antioxidant ability than resorcinol.　In addition, catechol can also inhibit photo-oxidation,

although resorcinol cannot.　This property must be present in a compound like product B.

4.　Conclusion

The interaction of HALS with benzoate-type UVA was studied.　 p-Aminobenzoates showed antagonism with HALS, whereas p-hydroxybenzoates, especially if containing t-butyl group(s), showed synergism with HALS.　This may be explained as follows: (1) p-Hydroxybenzoates react with HALS nitrosonium, and prevent the nitrosonium from accelerating the oxi-dation by hemolytic decomposition of hydroperoxides.　This reaction is not concerned directly with photo-stabilization, but less oxidation results in less useless consumption of UVA. (2) p-Hdroxybenzoates convert to compounds with cat-echol moiety with photo-antioxidant activity in the re-action with HALS nitrosonium.　That is, p-hydroxy-benzoates undergo two electron oxidation by HALS nitrosonium to form a type of quinone species like product A which absorbs UV rays.　This intermediate further undergoes some reactions to finally form a sta-ble and strong antioxidant and photo-antioxidant com-pound with a catechol moiety.

These mechanisms can explain the essential syner-gism of o-hydroxybenzoate-type UVA with HALS.

References

1)

2)

Takenaka, H., Mizokawa, S., Ohkatsu, Y., J. Jpn. Petrol. Inst., 50, (1), 8 (2007). Ohkatsu, Y., Takenaka, H., J. Jpn. Petrol. Inst., in contribution.

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3)

4)

Zweifel, H., “Stabilization of Polymeric Materials,” Springer-Verlag, Berlin (1998), p. 64-65. Yamashita, H., Kawaguchi, A. W., Ohkatsu, Y., J. Jpn. Petrol. Inst., 49, (6), 294 (2006).

5)

6)

Yamashita, H., Banno, K., Ohkatsu, Y., J. Appl. Polym. Sci., 102, 1310 (2006). Yamashita, H., Ohkatsu, Y., Polym. Deg. Stab., 80, 421 (2003).

要　　　旨

ベンゾエート型紫外線吸収剤とヒンダードアミン光安定剤の相互作用

大勝　靖一，竹中　宏行，神山　七恵

工学院大学工学部応用化学科，163-8677 東京都新宿区西新宿1-24-2

ベンゾエート型紫外線吸収剤（UVA）とヒンダードアミン 生成が関与していると考えられてきたが，一方でフリース転位光安定剤（HALS）の相互作用を検討した。ベンゾエート型 を受けないアルキル p-ヒドロキシベンゾエートの光酸化防止UVAは，その化学構造に依存して拮抗作用または相乗作用を 能については十分な説明がなされていない。本研究は，この相示した。p-アミノベンゾエートは，UV光に対して高い吸光係 乗効果が，ベンゾエートの HALSニトロソニウムとの反応に数を有するが，HALSと拮抗作用して光酸化を促進した。これ 起源する p-ヒドロキシベンゾエート UV吸収中間体の生成，おに対し，p-ヒドロキシベンゾエートは，UV吸収能をほとんど よびそのさらなる反応によるカテコール構造を持つ光酸化防止示さないが HALSと相互作用し，顕著な相乗作用を示した。 化合物の生成によって包括的に説明できるという新しい機構を従来，置換アリール p-ヒドロキシベンゾエートの光酸化防止 提案する。能については光フリース転位によるベンゾフェノン型 UVAの

J. Jpn. Petrol. Inst., Vol. 51, No. 2, 2008