Photostabilization of Bisphenol A-Epichlorohydrin Condensation

Downloaded by PURDUE UNIV on August 25, 2016 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch008

8 Photostabilization of Bisphenol A - E p i c h l o r o h y d r i n Condensation P o l y m e r s Fluorescence and Model Compound Studies S. PETER PAPPAS, LESLIE R. GATECHAIR, ELLIS L. BRESKMAN, and RICHARD M. FISCHER Polymers and Coatings Department, North Dakota State University, Fargo, ND 58105 U. K. A. KLEIN Institute for Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 7000 Stuttgart 80, West Germany Previous studies have provided evidence for resonance energy transfer (RET) from excited singlet states of aromatic polymers 1 and 2 to 2-hydroxybenzophenone light stabilizers. The evidence was derived from fluorescence intensity measurements on polymer films as a function of stabilizer concentration. After correcting for absorption of exciting and emitted light by the stabilizer (screening and radiative energy transfer, respective ly), the resulting experimental quenching efficiencies exhibited good agreement with predicted RET quenching efficiencies, based on Forster kinetics. 1

2

3

During the course of these studies, it was found that fluorescence intensity from the polymeric films rapidly decreased on continued excitation in a fluorescence spectrophotometer (ca. 30% loss in 1 min for 1). Herein, we (1) elaborate further upon the fluorescence loss studies, (2) provide direct evidence for RET from fluorescence lifetime measurements, and (3) present preliminary findings on the photochemistry of model compounds for polymer

1.

The results support the conclusion, from previous

studies, that the effectiveness of added stabilizer decreases with time due to formation of a photoproduct(s) from the polymer which competes in RET, and is less able to dissipate the resulting excitation energy. 1

Results and Discussion Fluorescence Intensities. The fluorescence from polymeric films of bisphenol A-epichlorohydrin condensate 1^ (Eponol-55-B-40, 0097-6156/81/0151-0109$05.00/0 © 1981 American Chemical Society Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


110

PHOTODEGRADATION

A N D PHOTOSTABILIZATION

O F COATINGS

S h e l l Chemical Company), e x h i b i t s a maximum at 300 nm, c o r r e s ponding t o that o f the model chromophore a n i s o l e . The f l u o r e s cence i n t e n s i t y decreases monotonically with i n c r e a s i n g concent r a t i o n o f 2,4-dihydroxybenzophenone (DHB) and, furthermore, decreases with time on continued e x c i t a t i o n (274 nm) i n the spectrophotometer. The fluorescence l o s s with time may be r e solved i n t o two exponential decays. I n i t i a l l y , a r e l a t i v e l y r a p i d fluorescence l o s s i s observed w i t h i n 20 sec, followed by a slower l o s s . Loss constants f o r the i n i t i a l (k^) and secondary (k^) exponential decays f o r 1.5 urn f i l m s (on glass s l i d e s ) c o n t a i n i n g v a r y i n g concentrations o f DHB are provided i n Table I ( e n t r i e s 1-3). The i n i t i a l l o s s constants are seen t o decrease more markedly with i n c r e a s i n g DHB concentration than the secondary constants. In order t o determine the e f f e c t o f a i r on fluorescence l o s s , f r e e f i l m s o f polymer 1_ (15 urn t h i c k ) were placed i n a quartz cuvette, which was evacuated p r i o r t o e x c i t a t i o n i n the fluorescence spectrophotometer. Although the i n i t i a l l o s s constant was not determined a c c u r a t e l y , both constants (entry 4) were s u b s t a n t i a l l y s m a l l e r i n vacuo r e l a t i v e t o a i r . F l u o r e s cence l o s s from correspondingly t h i c k f i l m s i n a i r i s provided i n entry 5. TABLE I. Fluorescence DHB M x 10 1. 2. 3. 4. 5. a

0 2.3 9.2 0 0

b

3

Constants - from Polymer 1_ Films 3

I n t e n s i t y Loss

Film Thickness (urn) 1.5 1.5 1.5 15 15

3

kj

x 10 (sec~ )

9.9 4.6 2.8 chemical—>therma1 1 r

0 H^o 0|

'C>

HO

energy):

0-H o C•

0

HO

10

RET to photoproduct:

1

P* +

PP l p* P

i p*

+

P

P

> Photodegradation

SCHEME II OH OH P h - 0 - C H - i H - C H - 0 - P h - ^ Ph-0- + .CH -CH-CH -0-Ph254 nm OH ->CH -C-CH -0-Ph CH =fc-CH -0-Ph Ph-OH + 2

2

2

2

2

3

2

2

4

Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

COATINGS

8.

PAPPAS

ET

AL.

Bisphenol

A-Epichlorohydrin

Polymers

113

formation o f these products i s provided i n Scheme I I . I n i t i a l C-0 bond homolysis has been p o s t u l a t e d as the primary step i n the photochemistry o f a r y l e s t e r s and e t h e r s , based on f l a s h photolysis s t u d i e s . Subsequent H - t r a n s f e r (which may occur w i t h i n a solvent cage) followed by k e t o n i z a t i o n o f the r e s u l t i n g enol y i e l d s the products. The importance o f phenol formation by the proposed pathway was probed by i r r a d i a t i n g l,3-diphenoxy-2-methyl-2-propanol (5) under the same c o n d i t i o n s . Compared to 3^, the r a t e o f phenol formation was approximately 2 times slower. Since the Ht r a n s f e r step i n Scheme II i s not a v a i l a b l e to _5, the r e s u l t s provide support f o r the scheme as an important, but not s o l e , pathway f o r phenol formation. I r r a d i a t i o n o f 3^ and _5 with an a i r purge r e s u l t e d i n f a s t e r r a t e s o f phenol formation (ca. 5-fold) r e l a t i v e t o N . These f i n d i n g s p a r a l l e l the a c c e l e r a t e d fluorescence i n t e n s i t y l o s s from polymer 1^ f i l m s i n a i r as compared to the r e s u l t s i n vacuo (see Table I ) .


1 0

2

OH Ph— 0— C H —

C — C H o — 0 - Ph

I

CH

3

5 I r r a d i a t i o n o f 3^ at longer wavelengths (>280 nm) p r o v i d e d phenyl formate (6) as a major v o l a t i l e product, together with minor amounts o f phenol and phenoxyacetone (4), as w e l l as other products. A p o s s i b l e pathway f o r formation o f phenyl formate by o x i d a t i o n and subsequent cleavage i s provided i n Scheme I I I . Phenoxyacetic a c i d {T) was a l s o i d e n t i f i e d as a minor product by mass-gc a n a l y s i s . P h o t o l y s i s o f phenoxyacetone (4) and phenoxyacetic a c i d ( 7 ) y i e l d s phenol together with photo-Fries products (also shown i n Scheme I I I ) . At present, the relevance o f these r e s u l t s to photodegradation o f condensates H s a matter o f s p e c u l a t i o n . Of p a r t i c u l a r i n t e r e s t i s i d e n t i f i c a t i o n o f the photoproduct quencher(s) (PP, Scheme I ) . P o s s i b l e candidates are s a l i c y l i c a c i d d e r i v a t i v e s , which e x h i b i t the r e q u i s i t e a b s o r p t i v i t y at about 300 nm, and which may be formed by o x i d a t i o n o f orthophoto-Fries products (Scheme I I I ) , as i l l u s t r a t e d i n eq. 1. 1 2

1 3

(1)


114

PHOTODEGRADATION

AND

PHOTOSTABILIZATION

OF

COATINGS

S C H E M E III OH I

Ph-0-CH -CH-CH -0-Ph


2

Ph-O-C-H

2

(Major)

3

hv/O

z

>280 n m OH Ph-0-CH -C-H

Ph-0-CH-CH-CH -0-Ph i :

2

z

7" O H Ph-0-CH -COOH

(Minor)

2

7

OH

O II

Ph-0-CH -C-CH 2

O

hi/

II

>

3

PhOH

OH

+

o

CH CCH 2

3

+

OH Ph-0-CH -COOH 2

hv

PhOH

+

|Q

J—CH COOH 2

7 £

+

£-


8.

PAPPAS

ET

Bisphenol

AL.

A-Epichlorohydrin

Polymers

115


Experimental Materials. Bisphenol A - e p i c h l o r o h y d r i n condensate _1 (Eponol-55-B-40, S h e l l Chemical Co.) was p r e c i p i t a t e d from chloroform s o l u t i o n by a d d i t i o n of methanol t h r e e successive times p r i o r to u t i l i z a t i o n . Polymer f i l m s o f 1.5 and 15 ym were cast onto g l a s s p l a t e s from chloroform s o l u t i o n . 2,4-Dihydroxybenzophenone was obtained from A l d r i c h Chemical Co. and used as r e c e i v e d . 1,3-Diphenoxy-2-propanol (3) was prepared from phenol and e p i c h l o r o h y d r i n (l-chloro-2,3-epoxypropane), as p r e v i o u s l y described,8 and r e c r y s t a l l i z e d three times from 2-propanol t o y i e l d white c r y s t a l s , m.p. 82-82.5°C. The nmr spectrum i n CDCl- (Varian EM-390 spectrometer) e x h i b i t e d resonances ( i n ppm (6) r e l a t i v e to t e t r a m e t h y l s i l a n e ) at 3.0 (1H, doublet, J=5 Hz), 4.1 (4H, doublet, J=5 Hz), 4.3 (1H, m u l t i p l e t , J=5 Hz), and 6.8-7.4 (10H, m u l t i p l e t ) , which are assigned to the h y d r o x y l , methylene, methyne, and a r y l hydrogens, r e s p e c t i v e l y . l,3-Diphenoxy-2-methyl-2-propanol (5) was prepared from phenol and 2-methylepichlorohydrin (l-chloro-2-methyl-2,3epoxypropane) by the above method and obtained as an o i l , which e x h i b i t e d a s i n g l e peak on gas chromatographic (gc) analysis. In conformance with the proposed s t r u c t u r e , the nmr spectrum i n CDC1 e x h i b i t e d a m u l t i p l e t at 6.8-7.4 ppm (aromatic hydrogens), and s i n g l e t s at 4.0, 3.1 and 1.4 ppm, corresponding to the methylene, hydroxyl and methyl hydrogens, r e s p e c t i v e l y . 2-Methylepichlorohydrin 4 a s obtained from epoxidation o f m e t h a l l y l c h l o r i d e with meta-perbenzoic a c i d , by a standard procedure. 5 8

3

1

W

1

Fluorescence Studies. Fluorescence s p e c t r a o f f i l m s on glass p l a t e s were obtained with a Perkin-Elmer MPF-3 spectrof l u o r i m e t e r . A p r e v i o u s l y - d e s c r i b e d phase f l u o r i m e t e r ^ was u t i l i z e d f o r f l u o r e s c e n c e l i f e t i m e determinations. I r r a d i a t i o n Studies. I r r a d i a t i o n o f 1,3-diphenoxy-2propanol (3), 0.01 M i n a c e t o n i t r i l e , was conducted at 254 nm, u t i l i z i n g a 2.5-W low pressure Hg immersion lamp (PCQ9G-1, U l t r a v i o l e t P r o d u c t s ) , and at wavelengths longer than 280vnm, u t i l i z i n g a Hanovia 450-W high-pressure Hg immersion lamp (Type L) and 9700 Corex f i l t e r s l e e v e . I r r a d i a t i o n o f 1,3diphenoxy-2-methyl-2-propanol (S), 0.01 M i n a c e t o n i t r i l e , was a l s o conducted at 254 nm. The p h o t o l y s i s v e s s e l s were equipped with a gas i n l e t , serum-capped opening f o r a l i q u o t removal, and a water-cooled condenser. The i r r a d i a t e d s o l u t i o n s a l s o contained d e c a l i n (1.5 x 10-3 M ) , which was u t i l i z e d as an i n t e r n a l standard f o r gc a n a l y s i s (Varian Aerograph 2400, flame i o n i z a t i o n d e t e c t o r , 6 x 1/8" columns o f OV-101 (1.5%) on Chromosorb G). Product a n a l y s i s was a l s o conducted with a V a r i a n mass spectrometer f


116

PHOTODEGRADATION AND PHOTOSTABILIZATION OF COATINGS

(112-8) interfaced with a gc (3700) and data collection system (SS 200). Acknowledgement We are grateful to Professors M. Hauser and H. E. A. Kramer for stimulating discussions, the Deutscher Akademischer Austauschdienst (DAAD) for a research grant (to SPP), DeSoto, Inc. for financial assistance, and Shell Chemical Company for generously supplying resins.


Literature Cited 1. Breskman, E.L.; Pappas, S.P. J. Coatings Technol., 1976, 48 (622), 34. 2. Breskman, E.L., Ph.D. Thesis, North Dakota State University, 1976. 3. Förster, Th., Discuss. Faraday Soc., 1959, 27, 7; North, A.M.; Treadaway, M.F., Eur. Polymer J., 1973, 9, 609. 4. Ware, W.R., "Creation and Detection of the Excited State," Lamola, A.A., Ed., Vol. I, Part A, 1971, pp. 269-283. 5. Haar, H-P.; Hauser, M. Rev. Sci. Instrum., 1978, 49, 632. 6. Pivovarov, A.P.; Pivovarova, T.S.; Lukovnikov, A.F. Polymer Sci. (USSR), 1973, 15, 747. 7. Klöpffer, W. Adv. Photochem., 1977, 10, 311 8. Minor, W.F.; Smith, R.R.; Cheney, L.C. J.Amer.Chem.Soc., 1954, 76, 2993. 9. Paterson-Jones, J.C.; Percy, V.A.; Giles, R.G.F.; Stephen, A.M. J. Appl. Polym. Sci., 1973, 17, 1877. 10. Kalmus, C.E.; Hercules, D.M. J.Amer.Chem.Soc., 1974, 96, 449. 11.

Dirania, M.K.M.; Hill, J. J.Chem.Soc.(C), 1968, 1311

12. Kelly, D.P.; Pinhey, J.T. Tetrahedron Lett., 1964, 3427. 13.

Kelleher, P.G.; Gesner, B.D. J.Appl. Polym.Sci., 1969, 13, 9; Gesner, B.D.; Kelleher, P.G., ibid., 1969, 13, 2183.

14. DePuy, CH.; Dappen, G.M.; Eilers, K.L.; Klein, R.A. J.Org. Chem., 1964, 29, 2813. 15. Fieser, L.F.; Fieser, M. "Reagents for Organic Synthesis," Vol. I, John Wiley and Sons, Inc., 1968, pp. 135-6. RECEIVED

September 16, 1980.


Photostabilization of Bisphenol A-Epichlorohydrin Condensation

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