Photochemical Studies of Methacrylate Coatings for the Conservation

13 Photochemical Studies of M e t h a c r y l a t e Coatings for the Conservation of M u s e u m Objects

Downloaded by FUDAN UNIV on March 11, 2017 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch013

ROBERT L. FELLER, MARY CURRAN, and CATHERINE BAILIE Center on the Materials of the Artist and Conservator, Mellon Institute, Carnegie-Mellon University, Pittsburgh, PA 15213 Methyl and ethyl methacrylate polymers, although extensively used in industry, do not possess the solubility characteristics (low polarity) that would make them appropriate for use over tradi tional oil paintings and other organic-based museum objects that might be sensitive to polar solvents such as alcohols, ketones and esters. Poly(n-butyl methacrylate), offered as an artists' varnish in the late 1930's, did not become widely accepted in the war-dis rupted decade that followed. Accordingly, early in 1951, our lab oratory began a detailed study of the higher alkyl methacrylate polymers for potential use as picture varnishes (1). To be suitable for use in museum preservation and restoration practice, picture varnishes must have a number of defined charac teristics: (a) they must not interact with the object in a detri mental way - they must not, for example, soften or dissolve an old painting; (b) they must be easily applied by brushing or spraying; (c) they must have appropriate flexibility and hardness; (d) they must undergo little change in appearance over long periods of time; and (e) they must remain soluble for many generations in solvents of reasonably low polarity so that they can be removed at a later date with little risk to the substrate. We began our search for thermoplastic materials that would fulfill these criteria by inves tigating the variation in the properties of methacrylate polymers as the size of the alkyl group of the alcohol radical was increased from methyl through hexyl (2,3). During our initial accelerated-aging tests in a carbon-arc Fade-ometer®,we found that the higher alkyl methacrylates exhibi ted a tendency to crosslink. Without discoloring - without chang ing in appearance - a picture varnish made with these initially linear polymers could become insoluble and unswellable in toluene and acetone; potentially it would be impossible to remove such a coating in the future without considerable risk to the painting. This was a serious matter that warranted thorough investigation. The purpose of the studies that will be reported here was to determine the influence of (a) temperature, (b) wavelength of ra diation, (c) intensity of radiation, and (d) structure of the alkyl 0097-6156/81/0151-0183$05.00/0 © 1981 American Chemical Society Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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group upon the tendency of a number of the polymers to c r o s s l i n k , as evidenced by the r a t e of formation of i n s o l u b l e matter.


P r e p a r a t i o n of Polymers Both commercial and l a b o r a t o r y - s y n t h e s i z e d polymers were used. Those made i n the l a b o r a t o r y were g e n e r a l l y prepared by s o l u t i o n p o l y m e r i z a t i o n , r e f l u x i n g commercially a v a i l a b l e monomers i n t o l u ene using benzoyl peroxide as the c a t a l y s t . Other preparations were made i n which a z o - b i s - i s o b u t y r o n i t r i l e (AIBN) was used as i n i t i a t o r , ethanol was employed as the r e f l u x i n g medium, and monomers were e s p e c i a l l y synthesized i n the l a b o r a t o r y . These v a r i a t i o n s i n p r e p a r a t i v e procedure d i d not s i g n i f i c a n t l y a f f e c t the ranking of the polymers with respect to t h e i r tendency to c r o s s l i n k , as reported i n Table I. Films of the polymers were g e n e r a l l y cast with a drawdown bar from 20% s o l u t i o n s i n reagent-grade toluene. A l l the data reported i n the accompanying t a b l e s and graphs are based on f i l m s thus coated on aluminum f o i l . The use of the f o i l speeded up the f o r mation of i n s o l u b l e matter 1.9 times over that of coatings on glass and a l s o permitted convenient measurements of changes i n weight upon exposure and e x t r a c t i o n ( 4 ) . Ordinary 0.001"-thick household aluminum f o i l was rubbed f l a t on the surface of g l a s s p l a t e s , using a few drops of acetone to a i d i n the adherence of the f o i l to the g l a s s and to wet the c o t t o n swabs used to c l e a n the f o i l s and rub them f l a t . A f t e r standing f o r 24 hours, the cast f i l m s were baked 48 hours at 70°C. The r e t a i n e d s o l v e n t a f t e r baking was u s u a l l y no more than a few tenths of a percent by weight of r e s i n . The small amount of toluene that remained was considered i n s u f f i c i e n t to a l t e r the c o n c l u s i o n s . Tests have shown, however, that the r e t e n t i o n of a chemically a c t i v e solvent such as t u r p e n t i n e i n the f i l m can shorten the i n d u c t i o n time considerably (5). Major d i f f e r e n c e s i n molecular weight can be expected to i n fluence the r a d i a t i o n dose necessary to give r i s e to i n s o l u b l e matter; because of t h i s , i n t r i n s i c v i s c o s i t y values are given i n Table I to i n d i c a t e that the laboratory-prepared polymers were gene r a l l y s i m i l a r i n this respect. Crosslinking Influence of Wavelength of R a d i a t i o n . Our i n i t i a l i n d i c a t i o n s of c r o s s l i n k i n g were observed during exposures i n an A t l a s E l e c t r i c Devices Co. carbon-arc Fade-ometer® equipped with a Corex D f i l t e r . L a t e r , the same behavior was found when exposures were made i n an A t l a s 600WRC xenon-arc Fade-ometer® having Pyrex-glass f i l t e r s . When c r o s s l i n k i n g was f i r s t encountered, a number of c o l leagues advised us that the phenomenon was not l i k e l y to be of importance i n a museum; there, the n a t u r a l or f l u o r e s c e n t i l l u m i n a t i o n would c o n s i s t only of those wavelengths that would pass through o r d i n a r y g l a s s . We soon found, however, t h a t , although

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

Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981. .59 .21

2t 2s Is Is

i s o b u t y l (Elvacite® 2045)

n-propyl

e t h y l (Elvacite® 2042)

-7:3 ethylmethacry^ate/methy1acrylate (Acryloid B-72)

(b)

(b)

" E l v a c i t e " i s a r e g i s t e r e d trademark o f the duPont Company, " A c r y l o i d " , a r e g i s t e r e d t r a d e mark o f the Rohm and Haas Company and "Fade-ometer", a r e g i s t e r e d trademark of the A t l a s E l e c t r i c Devices Company.

* Polymer 59 i n F i g u r e 3.

no more than 5% i n s o l u b l e matter detected.

(b)

2023

3t = t e r t i a r y hydrogen on t h i r d carbon from a l c o h o l i c oxygen; s = secondary hydrogen.

»

1800

660

126

(a)

.20

1.6

.41

6:4 neopentyl/n-amyl

»

91

.25

3,2s 4,3,2s

n-butyl

110

49

.27

2-methylbutyl

2t

3,4t

1:3 p-methylcyclohexyl/isoamyl*

(a)

35

3t .22

Hours to A t t a i n 50% Insolubility

62°C

4

Intrinsic Viscosity i n MEK, 23°C

(Corex D F i l t e r ) ,

.27

Reactive Hydrogen on A l k y l Carbon Atom No.

3-methylbutyl

A l k y l R a d i c a l , R, i n Poly (R-Methacrylate)

(R)

Tendency o f Methacrylate Polymers to Become I n s o l u b l e

Samples on Aluminum F o i l , Single-Carbon-Arc Fade-ometer

Table I :



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the r e a c t i o n would be very slow under museum c o n d i t i o n s , i t was i n deed a c t i v a t e d by the v i s i b l e as w e l l as by the n e a r - u l t r a v i o l e t r a d i a t i o n . For example, when an u l t r a v i o l e t f i l t e r was introduced that removed most of the r a d i a t i o n below 400 nm, c r o s s l i n k i n g s t i l l took p l a c e , but the r a t e of formation of i n s o l u b l e m a t e r i a l i n the carbon-arc Fade-ometer® was reduced to about one-half. The r a t e of c r o s s l i n k i n g was e v e n t u a l l y shown to vary almost l o g a r i t h m i c a l l y with the s h o r t e s t wavelength i n the i l l u m i n a t i o n , as i l l u s t r a t e d i n Figure 1.(6) I t i s i n t e r e s t i n g to note i n Figure 1 that a number of photochemical degradations f o l l o w much the same r e l a t i o n s h i p : l o s s of weight from alkyd p a i n t f i l m s (_7), degradation of low-grade paper(8), e v o l u t i o n of hydrogen from rubber(9) and development of carbonyl groups i n p o l y ( v i n y l c h l o r i d e ) (10). The reason f o r t h i s p a r t i c u l a r s e n s i t i v i t y to wavelength i s perhaps the a b s o r p t i o n of r a d i a t i o n by f u n c t i o n a l groups present at very low concentrations. L a t e r i n v e s t i g a t i o n s have f u l l y confirmed the c o n c l u s i o n that the c r o s s l i n k i n g o f the higher a l k y l methacrylate polymers w i l l take place on a g a l l e r y w a l l . We have been able to demonstrate that an i n d u c t i o n time of about 11 years occurs before i n s o l u b l e matter begins to form i n commercial normal and i s o b u t y l polymers on a w e l l - i l l u m i n a t e d museum w a l l (1,11). P r o t e c t i o n against such l o s s of s o l u b i l i t y i s one of a number of reasons f o r recommending the use of u l t r a v i o l e t f i l t e r s over windows and over fluorescent-lamp sources i n museums (12). The r e l a t i o n s h i p i l l u s t r a t e d i n Figure 1 should be extended below 313 nm only with c a u t i o n . The s h o r t e r wavelengths may induce p h o t o l y t i c decomposition, r e s u l t i n g i n much more chain s c i s s i o n . Thus, when samples of p o l y ( n - b u t y l methacrylate) were exposed to a high-pressure mercury lamp ( e m i t t i n g intense r a d i a t i o n at 254 nm), we found that, although g e l formation took p l a c e i n the beginning, considerable chain breaking and v o l a t i l i z a t i o n e v e n t u a l l y occurred (13,14). Under t h i s lamp, the f i l m s developed bubbles and b l i s t e r s at an e a r l y stage, perhaps owing to the formation of monomer by unz i p p i n g r e a c t i o n s . We have warned colleagues not to attempt to use lamps that emit 254 nm r a d i a t i o n i n " a c c e l e r a t e d - a g i n g " t e s t s of museum m a t e r i a l s , because the photoactivated mechanism of d e t e r i o r a t i o n may be d i s t i n c t l y d i f f e r e n t from that caused by the near u l t r a v i o l e t and v i s i b l e r a d i a t i o n . For extensive s t u d i e s of the e f f e c t s of 254 nm r a d i a t i o n on a c r y l a t e s and methacrylates, the reader i s r e f e r r e d to the work of Morimoto and Suzuki(14) and of Grassie (15,16), In 1964, Oster r e ported on the c r o s s l i n k i n g of these and other polymers by the near u l t r a v i o l e t , s e n s i t i z e d by the presence of 2-methylanthraquinone (17). Influence of A l k y l Group. Colleagues and manufacturers f u r ther advised us i n 1952 that a c r y l i c s would not undergo c r o s s l i n k i n g because the polymers do not o b v i o u s l y absorb i n the near u l t r a violet. (However, the f i e l d now r e a l i z e s that t r a c e components at very low l e v e l s may absorb r a d i a t i o n i n t h i s range.) Our a d v i s o r s



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may be f o r g i v e n , f o r t h e i r g r e a t e s t f a m i l i a r i t y at the time was with polymers of methyl and e t h y l methacrylate. As the data i n Table I show, we found the t e n d e n c y - t o - c r o s s l i n k to r e s i d e p r i m a r i l y i n the b u t y l and amyl e s t e r s that c o n t a i n t e r t i a r y hydrogens, and s e c o n d a r i l y , i n the n-amyl, n - b u t y l and n-propyl e s t e r s which have secondary hydrogens removed from the backbone of the main chain by the c a r b o x y l i c carbon and oxygen and by two a l k y l carbons (1,18). The normal a l k y l polymers that we prepared may have contained t r a c e s of an i s o p r o p y l , i s o b u t y l or isoamyl impurity; nonetheless, the presence of t e r t i a r y hydrogens i s not necessary to account f o r the tendency to c r o s s l i n k . A six-membered-ring intermediate s t r u c ture, such as proposed by Grassie(15) and others, may a c c e l e r a t e the l o s s of hydrogen atoms from the a l k y l groups. I f so, then the normal b u t y l , amyl and higher e s t e r s should tend to c r o s s l i n k more r e a d i l y than polymers made with the methyl and e t h y l e s t e r s . Thus, Barton(19), who induced c r o s s l i n k i n g i n n - b u t y l and n-nonyl methac r y l a t e polymers through the a d d i t i o n of dicumyl peroxide, c l e a r l y demonstrated that the nonyl e s t e r had a higher c r o s s l i n k i n g e f f i ciency than the polymer with the s h o r t e r s i d e c h a i n . I f the l o s s of s o l u b i l i t y of these i n i t i a l l y l i n e a r polymers takes p l a c e through a f r e e - r a d i c a l - c h a i n process i n which the c r o s s l i n k i n g r e a c t i o n represents the t e r m i n a t i o n step, one may be h e s i tant at f i r s t to e x p l a i n the f a c t that a f r e e r a d i c a l , generated at one p o i n t on a r e l a t i v e l y s l u g g i s h polymer chain, can f i n d a r a d i c a l on a neighboring chain with which to terminate. However, perhaps r e a c t i o n s of the type R* + R H • RH + R • or R00- + R H • ROOH + R can take place r a p i d l y between pendant a l k y l groups along the chain u n t i l a r a d i c a l i s encountered on a neighboring chain; t h i s would e x p l a i n i n p a r t the enhancement of c r o s s l i n k i n g caused by the t e r t i a r y hydrogens being l o c a t e d i n c r e a s i n g l y f u r t h e r from the main c h a i n backbone (Table I; F i g u r e 2 ) . G r a s s i e , Semenov (20), Chien(21), and others, have proposed such i n t r a m o l e c u l a r r e a c t i o n s by pendant groups along the main chain. T

T

f

1-

Influence of I n t e n s i t y of I l l u m i n a t i o n . There i s a strong tendency f o r those who employ i n t e n s e sources of i l l u m i n a t i o n i n a c c e l e r a t e d photochemical aging t e s t s to make quick c a l c u l a t i o n s on the b a s i s of the r e c i p r o c i t y p r i n c i p l e - that i s , to assume t h a t , as the i n t e n s i t y i s lowered, the time f o r e q u i v a l e n t damage w i l l be correspondingly lengthened. For a number of reasons, the r e c i p r o c i t y p r i n c i p l e need not hold t r u e , and i t i s a d v i s a b l e to check t h i s p o i n t . We d i d so by exposing a r e a d i l y c r o s s l i n k a b l e polymer (duPont's Elvacite®2046, a copolymer of normal and i s o b u t y l methac r y l a t e ) under a s e r i e s of i n t e n s i t y - r e d u c i n g screens. The e x p e r i ments were c a r r i e d out i n the xenon-arc Fade-ometer® with Pyrexg l a s s f i l t e r s , i n which the c i r c u l a t i n g a i r was maintained a t 32°C and about 25% r e l a t i v e humidity. Aluminum wire screens were employed to decrease the i n t e n s i t y without s e r i o u s l y a l t e r i n g the s p e c t r a l d i s t r i b u t i o n of the i l l u m i n a t i o n . The r e s u l t s , given i n Table I I , i n d i c a t e that the r e c i p r o c i t y law h e l d true over a 30f o l d range of i n t e n s i t y .



188

PHOTODEGRADATION

Figure 1. Data of various investigators on the influence of the lowest wavelength of irradiance on the rate of a variety of chemical reactions ((%) Feller (6); ((B) Miller (1); O Harrison (S); (O) Bateman (9); (V) Martin/Tilley (10))

J 0

Figure

2.

A N D PHOTOSTABILIZATION

400

O F COATINGS

360 320 280 240 Lowest Wavelength of Irradiance, nm

» I i I I I 1 1 1 1 20 40 60 80 100 Hours in Carbon-Arc F a d e - o m e t e r ® , 6 2 ° C

Comparison of the rate of formation of insoluble matter in polymers based on 3-methyl- and 2-methyl-butyl methacrylate


13.

FELLER

ET AL.

Methacrylate

189

Coatings

Table I I : Check on R e c i p r o c i t y Law: Induction Time f o r Crossl i n k i n g of~Elvacite® 2046 Under Various I n t e n s i t i e s of I l l u mination i n a Xenon-Arc Fade-ometer®With Pyrex-Glass F i l t e r Relative Intensity, I


1.00 .71 .49 .35 .135 .033

Induction Time, T (Hours) ±

15 22 29 40 79 530

IT, 15.0 15.6 14.2 14.0 10.7 17.5 14.5 ± 2.2

Further experiments were conducted under the r e l a t i v e l y m i l d c o n d i t i o n s of exposure to a bank of " d a y l i g h t " f l u o r e s c e n t lamps i n a room maintained a t 70°F (21.1°C) and 50% r e l a t i v e humidity. Under these c o n d i t i o n s , the i n t e n s i t y of the n e a r - u l t r a v i o l e t r a d i a t i o n amounted to 2.5% of the energy i n the v i s i b l e range and the temperature of the samples reached no more than 26°C. Such experiments c l e a r l y demonstrated that commercial normal and i s o b u t y l methacrylate polymers, as w e l l as those that are prepared i n the l a b o r a t o r y by s o l u t i o n p o l y m e r i z a t i o n , develop 50 to 80% i n s o l u b l e matter a f t e r about 9 m i l l i o n f o o t c a n d l e hours of exposure to "dayl i g h t " f l u o r e s c e n t lamps. By e x t r a p o l a t i o n , one may estimate that these coatings would develop the same degree of i n s o l u b i l i t y i n about 50 years of exposure i n a d a y l i g h t e d museum under an average l e v e l of i l l u m i n a t i o n of about 50 footcandles (6). We concluded i n the 1960's that there was no cause f o r immediate alarm concerning the past uses of the normal and i s o b u t y l methacrylate v a r n i s h e s ; the polymers had not been introduced to the museum world u n t i l about 1939. The o l d e s t coatings i n 1960 would not have been more than about 20 years o l d . Nonetheless, i n order to develop a sound understanding o f the p r o p e r t i e s of a c r y l i c r e s i n s that the conserv a t o r of museum o b j e c t s might wish to employ with confidence i n the f u t u r e , a thorough i n v e s t i g a t i o n of the problem was r e q u i r e d . E f f e c t of F i l m Thickness, Oxygen. In the range of 25- to 50microns f i l m t h i c k n e s s , the percentage of i n s o l u b l e matter formed during i n i t i a l stages of c r o s s l i n k i n g decreased w i t h f i l m t h i c k ness. We have f u r t h e r shown that the c r o s s l i n k i n g induced i n these polymers by n e a r - u l t r a v i o l e t r a d i a t i o n i s r e t a r d e d when the f i l m s are exposed i n an i n e r t atmosphere and i s enhanced when the polymer contains oxygen that may have been i n a d v e r t a n t l y introduced i n t o the c h a i n during p o l y m e r i z a t i o n (5).


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Loss i n Weight During the exposure t e s t s , we found, f o l l o w i n g an i n d u c t i o n p e r i o d i n which l i t t l e change was observed, that the f i l m s tended to l o s e weight a t a r a t e that was l i n e a r with time. This behavior was not e x t e n s i v e l y i n v e s t i g a t e d , but we b e l i e v e that the induct i o n time a f t e r which the marked r i s e i n i n s o l u b l e matter occurred corresponded c l o s e l y with the time a f t e r which the d i s t i n c t l o s s i n weight began. The o b s e r v a t i o n of weight l o s s may be of p r a c t i c a l i n t e r e s t i n the s e l e c t i o n of photochemically s t a b l e a c r y l i c c o a t i n g s . A number of years ago, Berg, J a r o s z and Salanthe(22) suggested that coatings that l o s t the minimum of weight during exposure tended to be the most durable. In a q u a l i t a t i v e way, t h i s was borne out i n our r e s u l t s i n the carbon-arc Fade-ometer® (Corex D f i l t e r ) i n which the r a t e of weight l o s s , f o l l o w i n g the i n d u c t i o n p e r i o d , was observed to be as f o l l o w s : commercial normal and i s o b u t y l methacrylate polymers, 1.0 to 2.0% l o s s i n weight per 100 hours; p o l y ( n - p r o p y l methacrylate), 0.6%; Acryloid® B-72 (copolymer of e t h y l methacrylate and methyl a c r y l a t e ) , 0.2%. The ranking i s the same as the t e n d e n c y - t o - c r o s s l i n k reported i n Table I . Ratio of Chain Breaking to C r o s s l i n k i n g Measurement of R a t i o . E a r l y i n our i n v e s t i g a t i o n we encountered a commercial polymer that, although i t e v e n t u a l l y formed about 50% i n s o l u b l e matter i n the Fade-ometer® , d i d not r a p i d l y r i s e to a s t a t e of more than 90% i n s o l u b i l i t y as d i d most of the other polymers then under i n v e s t i g a t i o n . The s i g n i f i c a n c e of t h i s r e s u l t was not f u l l y appreciated at f i r s t ; we r e a l i z e d l a t e r that we were observing a case of simultaneous c h a i n breaking and c r o s s linking. Since then, we(4,11,23), as others before us(24), have found that the equations r e l a t i n g l o s s of s o l u b i l i t y with exposure time that were developed by Charlesby and Pinner(25,26) f o r analyz i n g the e f f e c t s of high-energy r a d i a t i o n a l s o apply i n the case of the photoactivated changes under study here. We have recommended to colleagues that the decrease i n s o l u b l e matter during a c c e l e r a ted-photochemical aging t e s t s be evaluated on a Charlesby l o g - l o g p l o t as seen i n F i g u r e s 3 and 4. For convenience, we f r e q u e n t l y place a s c a l e on these graphs at ten times the g e l dose to i n d i c a t e the approximate r a t i o of c h a i n breaks to c r o s s l i n k s (Figure 3). The l o c a t i o n of the s c a l e d i v i s i o n s at ten times the g e l dose are as f o l l o w s : the r a t i o of breaks to l i n k s , 3/a, i s 1.0 a t a soluble f r a c t i o n of 0.495; i t i s 0.7 a t 0.328, 0.6 at 0.275, 0.5 at 0.225, 0.4 at 0.78, 0.2 at 0.094, and 0.1 at 0.029. Charlesby's method of p l o t t i n g s + /s a g a i n s t 1/6 perhaps provides a more o b j e c t i v e a n a l y s i s of the data: s + /s = 3/ot + 2/6 where s = s o l u b l e f r a c t i o n , 3/ct the r a t i o of degradation to c r o s s -



FELLER

ET AL.

Methacrylate

Coatings

Figure 3. Influence of temperature on the rate of cross-linking of three methacrylate polymers plotted in terms of time relative to the gel dose R * . Insolubility at R / R * = 10 used as a measure of /?/«, the ratio of chain breaks to cross-links formed. For composition of polymer 59, see Table I.

o.i o l — i I i I i i I I i I i I I I i I 40 60 80 100 200 4 0 0 600 1000 Hours Exposure in Xenon-Arc F a d e - o m e t e r ® (Pyrex Filter)

Figure 4. Loss of solubility of Du Pont Elvacite 2046, a 1:1 copolymer of n-butyl and isobutyl methacrylate, with and without UV absorber and antioxidant additives


191

192

PHOTODEGRADATION

AND

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l i n k i n g f o r the case of an i n i t i a l l y random d i s t r i b u t i o n of molecu l a r weights, and 6 i s R/R*, i n which R i s the accumulated exposure time and R* i s the time r e q u i r e d f o r the i n i t i a l formation of i n s o l u b l e g e l , e s s e n t i a l l y the " g e l dose" (25) . We have used graphs of t h i s equation as an a l t e r n a t e method of p l o t t i n g the data and o b t a i n i n g 3/a (1,4). The l o c a t i o n s of the r e f e r e n c e - s c a l e d i v i s i o n s at ten times the g e l dose were c a l c u l a t e d from t h i s equation, using 1/6 =0.1 and the t h e o r e t i c a l slope of 2.0. Influence of Temperature. Panels i n the carbon-arc Fade-ometer® reach 62°C; the b l a c k - p a n e l temperature i n our present 600WCR xenon-arc Fade-ometer® i s 59°C. To l e a r n how s e r i o u s l y temperature a f f e c t s c r o s s l i n k i n g , we b u i l t a water-cooled s u r f a c e upon which to mount our t e s t panels of coated aluminum f o i l d u r i n g exposure i n the carbon-arc equipment. The r e s u l t s of s t u d i e s at v a r i o u s temperatures are shown i n F i g u r e 3. Here we see that, at the usual high temperature of panels i n the carbon-arc Fade-ometer®, 62°C, copolymer No. 59 (based on 1:3 combination of p-methylcyclohexyl and isoamyl methacrylate) and the i s o b u t y l polymer both tend to c r o s s l i n k almost e x c l u s i v e l y ; that i s , the r a t i o o f c h a i n breaking to c r o s s l i n k i n g , $/a, % 0. At a lower temperature, 32°C, considera b l e c h a i n breaking occurs; the r a t i o of l i n k s broken to c r o s s l i n k s formed becomes about 0.3 i n one case, 0.55 i n another. Perhaps much of t h i s behavior can be explained by c o n s i d e r i n g whether the polymers are above or below t h e i r second-order t r a n s i t i o n temperature during i r r a d i a t i o n , but we have not pursued t h i s l i n e of i n v e s t i g a t i o n . As seen i n F i g u r e 3, an isoamyl methacrylate p o l y mer based on a mixture of 2-methyl and 3-methyl e s t e r s , with i t s h i g h l y l a b i l e t e r t i a r y hydrogens, c r o s s l i n k e d almost e x c l u s i v e l y even at 16°C, even though the polymer should have been below i t s Tg (about 26°C) at t h i s temperature. These f i n d i n g s c l e a r l y i l l u s t r a t e how the r e s u l t s of s o - c a l l e d " a c c e l e r a t e d - a g i n g " t e s t s can be a f f e c t e d by the h i g h temperatures i n o r d i n a r y xenon- and carbon-arc equipment and can, t h e r e f o r e , l e a d to erroneous c o n c l u s i o n s r e g a r d i n g the photochemical behavior of m a t e r i a l s at near-normal temperatures (23). Grassie's s t u d i e s on the decomposition of a e r y l a t e and methac r y l a t e polymers at h i g h temperatures may be of i n t e r e s t i n suggest i n g mechanisms by which some of the a l k y l groups may thermally decompose (27), although h i s i n v e s t i g a t i o n s were conducted p r i m a r i l y under vacuum or under i n e r t gases. Petroleum-Soluble Polymers R e s i s t a n t to C r o s s l i n k i n g Under Near Ultraviolet In view of the above f i n d i n g s , what may we choose f o r use as a p i c t u r e v a r n i s h that w i l l have l i t t l e or no tendency to c r o s s l i n k ? Polymers of the p e r f l u o r o a c r y l i c e s t e r s e x h i b i t no such tendency but r e q u i r e s o l v e n t s of questionable appropriateness ( 5 ) Moreover, i f the coatings are used i n d i s c r i m i n a t e l y i n making r e #



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ET AL.

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Coatings

p a i r s to the s t r u c t u r e of a p a i n t i n g , f u t u r e problems regarding adhesion and wetting may be introduced. P o l y ( n - p r o p y l methacryl a t e ) i s a l s o h i g h l y r e s i s t a n t to c r o s s l i n k i n g (Table I) and might be considered, although we do not b e l i e v e that a s u i t a b l e commerc i a l polymer i s a v a i l a b l e . The most s t a b l e r e s i n f o r many of our purposes has proven to be a copolymer of e t h y l methacrylate and methyl a c r y l a t e . This comes as l i t t l e s u r p r i s e ; the Rohm and Haas Company has f o r years s o l d a durable r e s i n based on these two monomers, Acryloid®B-72 (6,28). We have a l s o prepared polymers of s i m i l a r p h y s i c a l propert i e s based on methyl methacrylate and e t h y l a c r y l a t e and have found that t h e i r behavior i s p r a c t i c a l l y the same - the methyl and e t h y l groups apparently do not become s e r i o u s l y i n v o l v e d i n c r o s s l i n k i n g . As reported elsewhere(23), r a t h e r than c r o s s l i n k , Acryloid®B-72 tends to chain break under v i s i b l e and n e a r - u l t r a v i o l e t r a d i a t i o n , although at a very slow r a t e . P o l y v i n y l a c e t a t e i s another polymer used i n the care of museum o b j e c t s that tends more to c h a i n break than c r o s s l i n k under these c o n d i t i o n s ( 2 3 ) , but i t i s not our purpose to d i s c u s s i t s p r o p e r t i e s at t h i s time. One of the recognized challenges i n c r e a t i n g a durable polymer i s to c r e a t e one i n which there i s a proper balance between c r o s s l i n k i n g and chain breaking tendencies. T h i s o b j e c t i v e was d e s c r i bed, pursued and demonstrated i n the work of Maxim and K u i s t (24, 29). These authors showed that a d i s t i n c t maximum i n d u r a b i l i t y was f r e q u e n t l y obtained at an intermediate composition when monomers were copolymerized. Much the same behavior was reported by Graham, Crowne and MacAlpine i n the e v a l u a t i o n of copolymers and t e r p o l y mers of thermosetting a c r y l i c s (30). Maxim and K u i s t s s t u d i e s , which i n v o l v e d a c r y l a t e r a t h e r than methacrylate polymers, confirm our f i n d i n g s that c r o s s l i n k i n g tends to i n c r e a s e with the l e n g t h of the a l k y l s i d e c h a i n . f

I n h i b i t i o n of C r o s s l i n k i n g Our i n i t i a l o b j e c t i v e s i n undertaking these i n v e s t i g a t i o n s were to seek an understanding of the causes of the photochemically induced c r o s s l i n k i n g and to p i n p o i n t a t l e a s t a few thermoplastic polymers - o r i g i n a l l y s o l u b l e i n hydrocarbons no more p o l a r than toluene - that had l i t t l e tendency to undergo l o s s of s o l u b i l i t y under o r d i n a r y museum c o n d i t i o n s . Having achieved these o b j e c t i v e s , we returned our a t t e n t i o n to the matter of i n h i b i t i n g c r o s s l i n k i n g , a p r a c t i c a l p o s s i b i l i t y that had been accomplished through the a d d i t i o n of 2,4-dihydroxybenzophenone, as d e s c r i b e d i n our 1957 p u b l i c a t i o n (18). We have s i n c e found that adding LTDP ( d i l a u r y l t h i o d i p r o p i o n a t e ) and Ciba-Geigy's Tinuvin® 328 u l t r a v i o l e t absorber - each at 1% c o n c e n t r a t i o n r e l a t i v e to the weight of the r e s i n increases the i n d u c t i o n time 10- to 1 5 - f o l d f o r a 1:1 copolymer of normal and i s o b u t y l methacrylate, Elvacite® 2046, during xenon-arc exposure (Pyrex-glass f i l t e r ) (5). C i b a - G e i g y s T i n u v i n ® 7 7 0 alone at 1% c o n c e n t r a t i o n i n t h i s same r e s i n prolongs the i n d u c t i o n time f


194

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i n the xenon-arc Fade-ometer® about 5 - f o l d . When p l o t t e d as we see i n F i g u r e 4, data on l o s s of s o l u b i l i t y w i l l r e a d i l y r e v e a l which combination of u l t r a v i o l e t absorber and i n h i b i t o r a f f e c t s p r i m a r i l y the i n d u c t i o n time, which a f f e c t s the r a t i o of chain breaking to c r o s s l i n k i n g , and which may a f f e c t both (5).


Summary of Major Findings The tendency to c r o s s l i n k under near u l t r a v i o l e t and v i s i b l e r a d i a t i o n has been demonstrated i n polymers prepared from the np r o p y l , n-butyl and i s o b u t y l , n-amyl and isoamyl methacrylate est e r s , p a r t i c u l a r l y i n those w i t h a l k y l groups that c o n t a i n t e r t i ary hydrogens. The r a t i o of chain breaking to c r o s s l i n k i n g has been shown to vary w i t h temperature, the higher temperature f a v o r i n g an i n c r e a s e i n the tendency to c r o s s l i n k . A number of toluenes o l u b l e polymers have been found that e x h i b i t l i t t l e tendency to c r o s s l i n k under the near u l t r a v i o l e t . Moreover, f o r those methac r y l a t e polymers that do tend to c r o s s l i n k , i n h i b i t o r systems can be used to delay the onset of i n s o l u b i l i t y as much as 1 5 - f o l d . We have determined that the r e c i p r o c i t y law ( i n t e n s i t y of i l l u m i n a t i o n times i n d u c t i o n time) e s s e n t i a l l y holds true over a 30-fold range of i n t e n s i t y . Further, the r a t e of formation of i n s o l u b l e matter has been shown to i n c r e a s e l o g a r i t h m i c a l l y w i t h the decrease i n the lowest wavelength of i r r a d i a n c e i n the range of 400 to 300 nm. C r o s s l i n k i n g of these polymers can take p l a c e under the normal c o n d i t i o n s of d a y l i g h t or fluorescent-lamp i l l u m i n a t i o n encountered i n a museum, although at an extremely low r a t e . Importance with Respect to Long-Term Usage These s t u d i e s , sponsored f o r over two decades by the N a t i o n a l G a l l e r y of A r t , have e s t a b l i s h e d some general p r i n c i p l e s that can guide conservators both i n the s e l e c t i o n of durable p r o t e c t i v e coatings and i n the a p p l i c a t i o n , maintenance, and r e p a i r of such c o a t i n g s . Based upon t e s t s made under the r e l a t i v e l y mild c o n d i t i o n s of exposure to " d a y l i g h t " f l u o r e s c e n t lamps and a l s o to natur a l i l l u m i n a t i o n on a g a l l e r y w a l l , we can now reasonably p r e d i c t that - on an a r t g a l l e r y w a l l experiencing about 110,000 footcandle hours of d i f f u s e d a y l i g h t through window glass annually - polymers such as Acryloid® B-72 and p o l y v i n y l a c e t a t e should remain c o l o r l e s s , and s o l u b l e i n the s o l v e n t s i n which they were o r i g i n a l l y s o l u b l e , f o r more than 200 years (31). T r u l y , these are f i r s t - c l a s s m a t e r i a l s to have been placed a t the a r t i s t s ' and conservators' command• Acknowledgements We are g r a t e f u l to the o r g a n i z e r s of t h i s symposium f o r the opportunity to enlarge upon our r e p o r t s o f these i n v e s t i g a t i o n s that have appeared i n a wide v a r i e t y of p u b l i c a t i o n s i n the past,


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publications that have been directed more towards the museum con servator and conservation scientist than to the polymer chemist. Many of the methacrylate polymers used at the beginning of these investigations were prepared by Stuart Raynolds. Richard A. Tauson carried out the initial studies on the effect of tempera ture. The authors wish particularly to thank Dr. John Walker, Di rector Emeritus of the National Gallery of Art, for his great per sonal interest, support, and encouragement in this research. The work of the Center is principally made possible through the gener osity of the Andrew W. Mellon Foundation.


Literature Cited 1. Feller, R. L., Stolow, N. and Jones, E. B., "On Picture Var nishes and Their Solvents", Press of the Times, Oberlin, Ohio, 1959; revised edition of Press of Case-Western Reserve, Cleve land, 1971; both currently out of print. 2. Raynolds, S., "The Dependence of Physical Properties on the Constitution of Alkyl Polymethacrylate-ester Resins", Thesis, Master of Science, University of Pittsburgh, 1954. 3. Feller, R. L., "Identification and Analysis of Resins and Spirit Varnishes", in "Application of Science in Examination of Works of Art", Museum of Fine Arts, Boston, 1959, pp. 51-76. 4. Feller, R. L. and Bailie, C. W., "Studies of the Effect of Light on Protective Coatings Using Aluminum Foil as a Support: Determination of Ratio of Chain Breaking to Cross-linking", Bulletin of the American Group-IIC, 1966, 6, No. 1, 10-12. 5. Feller, R. L., "Problems in the Investigation of Picture Var nishes", in "Conservation and Restoration of Pictorial Art", Eds. Brommelle, N. and Smith, P., Butterworths, 1976, pp. 137144. 6. Feller, R. L., "New Solvent Type Varnishes", in "Recent Advan ces in Conservation", Butterworths, London, 1963, pp. 171-175. 7. Miller, C. D., "Kinetics and Mechanism of Alkyd Photooxida tion", Ind. Eng. Chem., 1958, 50, 125-128. 8. Harrison, L. S., "An Investigation of the Damage Hazard in Spectral Energy", Illum. Eng. (NY), 1954, 49, 253-257. 9. Bateman, L., "Photolysis of Rubber", J. Polym. Sci., 1947, 2, 1-9. 10. Martin, K. G. and Tilley, R. I., "Influence of Radiation Wave length on Photooxidation of Unstabilized PVC", Br. Polym. J., 1971, 3, 36-40. 11. Feller, R. L., "The Deterioration of Organic Substances and the Analysis of Paints and Varnishes", in "Preservation and Conservation, Principles and Practices", Smithsonian Press, Washington, D.C., 1976, pp. 287-299. 12. Feller, R. L., "Control of Deteriorating Effects of Light on Museum Objects", Museum, 1964, 17, 57-98. 13. Feller, R. L., "Speeding Up Photochemical Deterioration", Bulletin royal du Patrimoine artistique, 1975, 15, 135-150.



196 PHOTODEGRADATION AND PHOTOSTABILIZATION OF COATINGS 14. Morimoto, K. and Suzuki, S., "Ultraviolet Irradiation of Poly(alkyl Acrylates) and Poly(alkyl Methacrylates)", J. Appl. Polym. Sci., 1972, 16, 2947-2961. 15. Grassie, N. and MacCallum, J. R., "Thermal and Photochemical Degradation of Poly(n-butyl Methacrylate)", J. Polym. Sci., 1964, 2, Part 2A, 983-1000. 16. Grassie, N., "Photodegradation of Methacrylate/Acrylate Co polymers", Pure and Appl. Chem., 1973, 34, 247-257. 17. Oster, G., "Photochemical Crosslinking of Non-Aqueous Poly mers by Near Ultraviolet Light", J. Polym. Sci., 1964, 2, Part B, Polymer Letters, 1181-1182. 18. Feller, R. L., "Cross-Linking of Methacrylate Polymers by Ul traviolet Radiation", Preprints of papers presented at the New York Meeting, Division of Paint, Plastics and Printing Ink Chemistry, American Chemical Society, Sept., 1957, 17, No. 2, 465-470. 19. Bartoň, J., "Peroxide Crosslinking of Poly(n-alkyl Methacry lates)", J. Polym. Sci., 1968, 6, Part 1A, 1315-1323. 20. Gordon, G. Ya., "Stabilization of Synthetic High Polymers", Israel Program for Scientific Translations, 1964, p. 45. 21. Chien, J. C. W. and Boss, C. R., "Polymer Reactions. V. Ki netics of Autoxidation of Polypropylene", J. Polym. Sci., 1967, 5, Part 1A, 3091-3101. 22. Berg, C. J., Jarosz, and Salanthe, G. F., "Performance of Polymers in Pigmented Coatings, J. Paint Technol., 1967, 39, 436-453. 23. Feller, R. L., "Stages in the Deterioration of Organic Materi als", in Williams, J. C., "Preservation of Paper and Textiles of Historic and Artistic Value", Advances in Chemistry Series No. 164, American Chemical Society, 1977, pp. 314-335. 24. Maxim, L. D. and Kuist, C. H., "The Light Stability of Vinyl Polymers and the Effect of Pigmentation", Off. Dig., Fed. of Soc. for Paint Technology, 1964, 36, 723-744. 25. Charlesby, A. and Pinner, S. H., "Analysis of the Solubility Behavior of Irradiated Polyethylene and Other Polymers", Proc. R. Soc., 1959, A249, 367-386. 26. Charlesby, A., "Atomic Radiation and Polymers", Pergamon Press, London, 1960, pp. 143, 173. 27. Grassie, N., "Recent Work on the Thermal Degradation of Acry late and Methacrylate Homopolymers and Copolymers", Pure and Appl. Chem., 1972, 30, 119-134. 28. DeWitte, E., Goessens-Landrie, M., Goethals, E. J. and Simons, R., "The Structure of 'Old' and N ' ew' Paraloid B-72", Inter national Council of Museums (Paris) Committee for Conserva tion, Zagreb Meeting, 1978, Paper 78/16/3. 29. Kuist, C. H. and Maxim, L. D., "The Ultraviolet Degradation of Scissioning Copolymers", Polymer, 1965, 6, 523-530. 30. Graham, N. B., Crowne, F. R. and MacAlpine, D. E., "A Maximum Durability Prediction Scheme for Thermosetting Acrylics", Off. Dig., Fed. of Soc. for Paint Technology, 1965, 37, 1228-1250. 31. Feller, R. L. and Curran, M., "Changes in Solubility and Re movability of Varnish with Age", Bulletin American Institute for Conservation, 1975, 15, No. 2, 17-48. RECEIVED

October 28, 1980.


Photochemical Studies of Methacrylate Coatings for the Conservation

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