Damaging Effects of Visible and Near-Ultraviolet Radiation on Paper

Chapter 4

Damaging Effects of Visible and NearUltraviolet Radiation on Paper 1

S. B. Lee , J. Bogaard, and R. L. Feller

Downloaded by UNIV LAVAL on April 27, 2016 | http://pubs.acs.org Publication Date: September 28, 1989 | doi: 10.1021/bk-1989-0410.ch004

Research Center on the Materials of the Artist and Conservator, Mellon Institute, Carnegie—Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213 Little data have been available concerning chain breaking or an increase in the degree of oxidation of cellulose during exposure to the visible and near -ultraviolet radiation emitted by ordinary "daylight" fluorescent lamps under moderate conditions of temperature and humidity, both during exposure and during subsequent thermal degradation. The present investigation, involving papers of little or no lignin content -- an unbleached and bleached kraft pulp as well as filter paper -- revealed moderate immediate effects of exposure as well as sensitiza tion towards subsequent thermal degradation. Inter vention of an ultraviolet filter noticeably reduced, but did not prevent, deterioration both during exposure and during subsequent aging of 50% RH and 90°C. Continuous exposures of 800,000 to 1,300,000 footcandle hours were involved. Although i t i s commonly stated that exposure of paper t o l i g h t i s p o t e n t i a l l y harmful, l i t t l e information i s a v a i l a b l e concerning how much and what kind of "harm" i s done under conditions of moderate temperature, humidity and i l l u m i n a t i o n . I t was decided, therefore, to study the e f f e c t s of exposure to "daylight" fluorescent lamp l i g h t , our p a r t i c u l a r source emitting only 3.8% near u l t r a v i o l e t , and to express the results i n terms of footcandle hours of exposure. Measurement of h o t - a l k a l i - s o l u b l e (HAS) matter would r e f l e c t the degree of oxidation of paper s i m i l a r to the betterknown measurement of copper number. (1.) In addition, changes i n degree of polymerization (DP) were determined. e r r e n t address: Preservation Research and Testing Laboratory, Library of Congress, Washington, DC 20540

0097^6156789Λ)410-0054$06.00Λ) e 1989 American Chemical Society

Zeronian and Needles; Historic Textile and Paper Materials II ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

4. LEE ET A L

Damaging Effects of Viable and Near-Ultraviolet Radiation

Exposure can result thereafter i n increased thermally-induced changes. In order to evaluate t h i s aspect of the p o t e n t i a l harm that exposure to l i g h t represents, the exposed sheets were subjected to thermal aging at 90°C and 50% RH and the changes i n HAS matter and DP measured. Thus, not only were the immediate changes that took place during exposure monitored but the e f f e c t of exposure upon subsequent thermally-induced d e t e r i o r a t i o n were determined as well. This i s b a s i c a l l y the same technique of i n v e s t i g a t i o n used by Launer and Wilson i n 1943. (2)


EXPERIMENTAL CHARACTERISTICS OF STOCK PULPS. The c h a r a c t e r i s t i c s of the bleached (BP) and unbleached (UBP) kraft pulps used i n these studies have been previously described.Q) P r i n c i p a l l y the BP pulp contained about 0.24% l i g n i n and an i n i t i a l s o l u b i l i t y i n hot 1%sodium hydroxide of about 4.7% (HAS matter). The UBP pulp contained 4.4% l i g n i n and 2.7% HAS matter. Whatman No. 42 f i l t e r paper, having only about 1% HAS matter, was chosen to represent a c e l l u l o s e of high p u r i t y . MEASUREMENTS OF PROPERTIES. Water-leaf handsheets at a basis weight of about 75 g/m^ were prepared from the BP and UBP pulps using a Williams Standard Sheet Mould. S o l u b i l i t y i n hot-l%-sodium hydroxide was determined by TAPPI method T212 om 83. Intrinsic v i s c o s i t y i n cuene (cupraethylenediamine solution) and, therefrom, the DP were determined according to ASTM Standard D1795. V

EXPOSURE TO LIGHT SOURCES. As previously describedQ) exposures took place under a bank of s i x General E l e c t r i c 48-inch high-output "daylight" fluorescent lamps. The spectral power curve f o r such a lamp and the tolerance i n i t s correlated color temperature has been published.(4). Spectral d i s t r i b u t i o n data have also been provided by Harrison.(£) Measurements with a c a l i b r a t e d International Light IL700 research radiometer indicated that, of the t o t a l m i l l i w a t t s of v i s i b l e and n e a r - u l t r a v i o l e t irradiance, the lamps emitted about 3.8% i n the near u l t r a v i o l e t . A separate sensor head provided readings i n terms of footcandles (1 footcandle - 10.67 l u x ) . The lamps were mounted 3 1/2 inches above samples placed on a wire shelf i n a room maintained at 50% RH and about 23°C. Owing to an unavoidable heating e f f e c t of the lamps, the t e s t papers reached a temperature of about 29°C. Exposures were also c a r r i e d out under a P l e x i g l a s UF-3 sharp-cut-off f i l t e r (Rohm and Haas Company) which transmits less than 5% energy below 400 nm. Under the f i l t e r the luminous i n t e n s i t y was reduced about 10%; i n the figures t h i s correction has been made f o r the reduced e f f e c t i v e time of exposure. Exposures were only to one side of the test sheets; previous studies indicated that the same net exposure on two sides would have much the same r e s u l t . (£) THERMAL AGING. Aging at 90°C and 50% RH took place i n a humidityc o n t r o l l e d Blue M oven.


55

56

HISTORIC TEXTILE AND PAPER MATERIALS Π


RESULTS UNBLEACHED PULP. I n i t i a l l y , the development of h o t - a l k a l i - s o l u b l e (HAS) matter i n water-leaf handsheets prepared from unbleached pulp (UBP) was followed both during exposure and during subsequent thermal aging of the exposed sheets at 90°C and 50% RH. The thermally-induced generation of HAS matter i n the exposed t e s t sheets was compared to that which occurred i n an unexposed c o n t r o l . An increase i n the degree or rate of generation of HAS matter during thermal aging subsequent to exposure was taken as an i n d i c a t i o n that p o t e n t i a l damage had been done to the paper as the result of the exposure. As seen i n Figure 1, the sheets, which had been exposed to a t o t a l of about 800,000 footcandle hours, developed a greater amount of HAS matter upon subsequent thermal aging than d i d the unexposed c o n t r o l . Exposure to v i s i b l e radiation alone (exposure under a Plexiglas UF-3 u l t r a v i o l e t f i l t e r ) resulted i n less generation of HAS matter than when the sheets were exposed to u n f i l t e r e d r a d i a t i o n . I f one may envision thermal deterioration of the papers to occur i n at least two p r i n c i p a l stages(£), then the results shown i n Figure l b suggest that exposure to l i g h t tends to increase the extent of thermal degradation that occurs subsequently i n an i n i t i a l stage or stages. Thereafter, as suggested by the dashed l i n e s , thermally-induced oxidation proceeds at a rate s i m i l a r to that experienced by the unexposed c o n t r o l . If t h i s interpretation of the results i s v a l i d , one may state that exposure of the UBP sheets to "daylight" fluorescent lamplight tends to cause some immediate photochemical damage, more oxidative i n character than d i r e c t l y causing chain scission.(2) This i s r e f l e c t e d i n a r i s e i n HAS matter during exposure and also during an i n i t i a l rapid stage of thermal d e t e r i o r a t i o n . Thereafter, the rate of generation of HAS matter at 90°C and 50% RH resembles somewhat the rate exhibited by unexposed controls. With removal of the u l t r a v i o l e t portion of the irradiance from the "daylight" fluorescent lamps by use of a Plexiglas UF-3 f i l t e r , the extent of subsequent generation of HAS matter i s reduced to about one-half the rate without the f i l t e r . The immediate damage i n t h i s paper, which contained about 0.24% l i g n i n , was reduced, but not eliminated, by removal of u l t r a v i o l e t r a d i a t i o n . BLEACHED PULP. The i n i t i a l experiment was followed by s i m i l a r exposures of handsheets prepared from a bleached pulp (BP). Again the U F - 3 - f i l t e r e d s i t u a t i o n resulted i n a lesser development of HAS matter (Figure 2a). The i n i t i a l photochemical and subsequent thermally-induced changes i n HAS matter were less extensive than i n the case of the unbleached pulp (2a and b). Owing to the very low l i g n i n content i n t h i s p a r t i c u l a r pulp, the v i s c o s i t y i n cuene solutions could be determined and from t h i s the f a l l i n DP of the c e l l u l o s e estimated (Figure 2c and d). In Figure 3, a plot of the change i n inverse of the degree of polymerization (1/DP) with time indicates that measurable chain breaking may have occurred even when the UF-3 f i l t e r was i n t e r posed. The lower degree of photochemical damage under the UF-3 f i l t e r i s also r e f l e c t e d i n the subsequent rate of chain breaking


LEE ET A L

Damaging Effects of Visible and Near-Ultraviolet Radiation

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Damaging Effects of Visible and Near-Ultraviolet Radiation on Paper

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