Photooxidation and Photostabilization of Unsaturated Cross-linked

24 Photooxidation and Photostabilization of Unsaturated Cross-linked Polyesters SONG Z H O N G JIAN, JULIA LUCKI, J A N F. R A B E K , and BENGT RÅNBY

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 14, 1985 | doi: 10.1021/bk-1985-0280.ch024

Department of Polymer Technology, The Royal Institute of Technology, S-10044 Stockholm, Sweden

IR-ATR and ESCA spectroscopy have been applied to the study of photo-oxidation and photostabilization of unsaturated polyesters crosslinked with styrene. Results obtained show that both styrene and polyester units of the cured resins are vulnerable to UV oxidative degradation and are easily photo-oxidized. The resin has been stabilized with different types of commercially available photostabilizers, and the best results have been obtained with Tinuvin P as a photostabilizer. Unsaturated polyesters obtained by polycondensation of saturated and unsaturated acids, anhydrides and aliphatic glycols are used as coating resins and as matrices in glass-reiforced products. Styrene is usually added to unsaturated polyesters to act not only as solvent but also as a crosslinking comonomer which reacts with unsaturated groups along the polyester chain. The detailed structure of crosslinked polyesters are, in general, not well known in spite of great efforts made in this field (1-13).This is mainly due to the insolubility of the cured products. In the three dimensional polyester copolymer network, the structural rings are composed of four segments; two segments that are part of the polyester chain and two segments that are composed of polymerized styrene. In the actual polyester network itself, i t is more probable that these polymeric rings of which the network is composed, contain more than four chain segments. The average length of the crosslinks depends upon both styrene concentration and the number and type of reactive double bonds along the polyester chain. The average crosslink consists of two styrene molecules. In general crosslinked polyesters have better light and weathering resistance than uncured polyester resins. Apparently the concentration of unsaturated double bonds is a determining factor. In our previous studies (14) we have shown that under UV irradiation of unsaturated polyesters the primary photoreactions involve excitation of conjugated structures: carbonyl groups ( in ester 0097-6T56/85/0280-0353S06.00/0 © 1985 American Chemical Society Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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bonds)-double bonds-phenylene r i n g s . Secondary reactions occur by complicated mechanisms resulting i n oxidation, chain s c i s s i o n , r a d i c a l termination and crosslinking of structures present i n the photolyzed polyesters. Introduction of styrene segments to the crosslinked polyester structures provides chain structures responsible for the yellowing of UV i r r a d i a t e d r e s i n s . Styrene segments are easily photolyzed with formation of different types of strongly absorbing chromophoric groups (15-20). Because unsaturated polyesters are widely used as material for the mechanical, e l e c t r i c a l and building i n d u s t r i e s , for s k i s , huts, disposable tanks and various containers used outdoors, i t i s important to s t a b i l i z e them against sun l i g h t i r r a d i a t i o n . In t h i s paper we present results of the photostabilization of cured polyesters using commercially produced photostabilizers. Experimental A commercial polyester sample (CDS 2230, No 6462 from Syntes AB, Sweden) containing maleic anhydride (1 mol), isophthalic acid (1 mol), 1,2-propylene glycol (1 mol) and ethylene glycol (1 mol) and hydroquinone (50 ppm) as s t a b i l i z e r was used i n the experiments. Styrene s t a b i l i z e d with 4 - t e r t - b u t y l pyrocatechol (20 ppm) without p u r i f i c a t i o n was added as crosslinking agent, benzoyl peroxide (1 wt-%) as i n i t i a t o r and Co-octate (1 wt-% i n styrene solution) as accelerator. The following composition for thermal curing has been prepared: polyester 70, styrene 15, benzoyl peroxide 1 and Co-octate solution ( i n styrene) 10 ( a l l i n weight p a r t s ) . The following photostabilizers were used: 2-hydroxy-benzophenone (I) (Merck, Germany), 2,2'-dihydroxy-benzophenone (II) ( A l d r i c h , Belgium) and 2(2'-hydroxy-5-methylphenyl)benzotriazole (III) (Tinuvin P, Ciba-Geigy, Switzerland), a l l added at 0.3 wt-%. The samples were cured between glass plates as thin sheets at 60°C for 1 h r , at 80°C for 1 hr or at 130°C for 0.5 hr i n the presence of a i r . Reflection IR spectra were obtained with a Perkin-Elmer computerized spectrometer 580B using a micro MIR accessory at a c r y s t a l angle of 45° incidence. The absorbance values have^been normalized by using the IR band for Ch^ groups at 2930 cm as a standard i n order to overcome variations i n o p t i c a l density resulting from differences i n contact between the polymer films and the ATR c r y s t a l . ESCA c o r e - l e v e l spectra for C ^ and 0^ were recorded with a Leybold-Heraeus spectrometer using A l K ^ ^ excitation r a d i a t i o n . Typical operating conditions for the X-ray gun were 13 kV and 14 mA and a pressure of 3x10 mbar i n the sample chamber. The cured samples were UV i r r a d i a t e d i n an Atlas UVC0N Weatherometer for 10,20,40,60,100 and 200 h r s . g

g

Results and Discussion The mechanism of photo-oxidation at the s o l i d surface of a cured polyester can d i f f e r from that i n the bulk of the s o l i d sample. For instance, the bulk photo-oxidation mechanism i s diffusion c o n t r o l l e d , while the surface i s continually exposed to an abundant

Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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24. JIAN ET AL.


oxygen supply from the a i r . For that reason, the photo-oxidation reactions at the surface occur much more rapidly than i n the bulk. This i s important from a p r a c t i c a l point of view, because many of the properties of cured polyester resins depend s p e c i f i c a l l y upon the nature of the surface of the sample. ATR ( F i g . l ) and ESCA (Fig.2) measurements, which analyze a very thin surface layer of the polymer sample (400-800 nm for ATR and 5-10 nm for ESCA), show that UV exposure of the cured polyester i n a i r gives a gradual increase i n the formation of carbon-oxygen groups such as C - 0 - , HC=0 and -0-C=0.

0

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F i g . l . Kinetics of the formation of C=0 band at 1720 cm from ATR spectra.Key: ( % ) unstabilized sample and ( • ) with I ; ( • ) with II and ( ^ ) with I I I . F i g . 2 shows the high resolution ESCA spectra of the C ^ (Fig.2A) and 0^ ( F i g . 2B) bands of unexposed and photo-oxidized polyester samples. These ESCA spectra have similar character as those of photo-oxidized polystyrene, previously published (21). The Cj, spectrum of polystyrene consists of a single sharp peak at 285 eV°binding energy with an accompanying shake-up structure, centered at around 7 eV of the higher binding energy (22). After UV i r r a d i a t i o n , the 0, signal rapidly increases i n intensity r e l a t i v e to that i n trie C ^ region. Additional peaks at high binding energy, indicating oxidation, appear and increase i n intensity with i r r a d i a t i o n time. Carbon single bonded to one oxygen i s shifted by c a . 1.5 eV to higher binding energy, while the corresponding s h i f t for a carbon single bonded to two oxygens or double bonded to one oxygen i s c a . 3 eV, these s h i f t s being additive (21). Carbons i n hydroxy, ether, peroxide and hydroperoxide groups contribute to the peak at 286.5 eV, while those at 287.9 eV and 289 eV originate from aldehydic, ketonic and ester carbons ( F i g . 2A). The range of binding energies covered by the core levels i s much smaller than for C ^ levels ( F i g . 2 B ) . From the band i t i s evident that more than a single component i s present (two peaks centered a t 533 eV and 534.4-534.6 e V ) . g

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Fig.2.

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5 3 ]

5 2 ge V

(A) and 0^ (B) ESCA spectra of cured polyester at different times of UV i r r a d i a t i o n .

The lower binding energy component i s assigned to double bonded oxygen and to single bonded oxygen i n such groups as alcohols, ethers and peroxides. The higher binding energy i s attributed to single bonded oxygen i n a c i d , ester, hydroperoxide, carbonate, peracid or perester groups (21). Unfortunatelly, the ESCA spectroscopy i s not enough sensitive technique to differenciate C^ and 0^ signals from styrene and other units present i n a complicate! structure of a crosslinked polyester. The k i n e t i c s of photo-oxidation of a crosslinked polyester were studied by ATR ( F i g . l ) and the ESCA (Figs 2-4) measurements. The 0^ / C ^ peak intensity r a t i o as a function of UV i r r a d i a t i o n time (Fig.3) shows that photo-oxidation increases continously with time of i r r a d i a t i o n . The subsequent l e v e l i n g - o f f C ^ / C ^ ESCA peak intensity r a t i o (Fig.4) indicates that a steady s t a t e c o n d i t i o n i n the photo-oxidation i s reached after c a . 100 hrs of UV i r r a d i a t i o n . g

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JIAN ET AL.

Unsaturated Cross-linked Polyesters

F i g . 3 . The r a t i o of the areas of and C, bands from the ESCA spectra. Key: ( 0 ; unstabilized sample and ( A ) with I; ( • ) with II and ( yj ) with I I I .

F i g . 4. The r a t i o of the t o t a l ( £ ) C^ areas for carbon bonded to hydrogen. Key: ( % ) unstabilized sample and ( A ) with I; ( • ) with II and ( ) with I I I . g


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The IR spectra show distinctive change on photo-oxidation. A broad peak at 3200-3500 cm appears as a result_f hydroxy/ hydroperoxide group formation. The peak at 1720 cm emerges and it can be attributed to the formation of carbonyl groups. Formation of these bands is characteristic for the photo-oxidation of both polystyrene (15,23) and polyester (14) samples. Addition of 0.3 wt-% of the photostabilizers (I-III) causes a clear decrease in the rate of photo-oxidation (Figs 1,3,4). The stabilizers show increased effect in the order (I)< (H)< (HI) which is expected from the increased UV absorption of the compounds in this order. Results completed from our experiments show that application of ATR and ESCA techniques do not give direct answer for the question which structures in the photocrosslinked polyester with styrene are more susceptible towards photo-oxidation reactions. Acknowledgment These investigations are part of a research program on the role of commercial additives in the photodegradation of polymers supported by the Swedish National Board for Technical Development, which the authors gratefully acknowledge. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

W.Funke, K.Hamann and H.Gilch, Angew. Chem., 19,596 (1959). W.Funke, S.Knödler and R.Feinauer, Makromol.Chem.,49,52 (1961). W.Funke and H.Janssen, Makromol.Chem., 50, 188 (1961). W.Funke, K.Hamann and H.Roth, Kunstoffe 5,1, 75 (1961). W.Funke, S.Knödler and K.Hamann, Makromol.Chem.,57,192 (1962). W.Funke, S.Knödler and K.Hamann, Makromol.Chem.,53,212 (1962). W.Funke, W.Finus and K.Hamann, Kunstoffe 54,423 (1964). W.Funke, Kolloid-Zeitschr., 197, 71 (1964). W.Funke, Chimia 19, 55 (1965). W.Funke, E.Gulbins and K.Hamann, Kunstoffe 55, 7 (1965). W.Funke,R.Feinauer and K.Hamann, Makromol.Chem.,82,123 (1965). W.Funke,R.Feinauer and K.Hamann, Makromol.Chem.,84,178 (1965). W.Funke, G.Ammon and W.Pechold, Kolloid-Zeitschr.,206,9 (1965). J.Lucki, J.F.Rabek,B.Rånby and C.Ekström,Europ.Polym.J.,17, 919 (1981). 15. J.F.Rabek and B.Rånby, J.Polym.Sci., Al, 12, 273 (1974). 16. J.Lucki, J.F.Rabek and B.Rånby, J.Appl.Polym.Sci.,Polym.Symp., 35, 275 (1979). 17. J.Lucki and B.Rånby, Polym.Degrad.Stabil., 1, 1 (1979). 18. J.Lucki and B.Rånby, Polym.Degrad.Stabil.,1, 165 (1979). 19. J.Lucki and B.Rånby, Polym.Degrad.Stabil.,1, 251 (1979). 20. N.A.Weir,Develop.in Polymer Degradation (Ed.N.Grassie), Appl. Science Publishers, London, 1982, p. 143. 21. J.Peeling and D.T.Clark,Polym.Degrad.Stabil.,3,97 (1980-81). 22. D.T.Clark,D.B.Adams,A.Dilks,J.Peeling and H.R.Thomas, J.Electr.Spectr.Relat.Phenom., 8, 51 (1976). 23. G.Geuskens,D.Bayens-Volant,G.Deleunois, Q.L.Ninh,W.Piret and C.David, Europ.Polym.J., 14, 291, 299, 501 (1978). RECEIVED February 28, 1985


Photooxidation and Photostabilization of Unsaturated Cross-linked

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