The Use of Chemiluminescence in the Study of Paper Permanence

8 The Use of Chemiluminescence in the Study of

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Paper Permanence G. B. KELLY and J. C. WILLIAMS The Library of Congress, Washington, DC 20540 G. D. MENDENHALL and C. A. OGLE Battelle's Columbus Laboratories, 505 King Avenue, Columbus, OH 43201 The chemiluminescence maxima from several samples of paper placed in a preheated oven at constant humidities was shown to obey a linear Arrhenius relationship between about 40 C and 90 C. Cycling between dry and moist air at 70 C increased the chemi luminescence from the paper temporarily after each change. The light emission from the paper was ascribed to radicals produced by bond breakage, thermally in the one case and by humidity-induced swelling and contracting in the other.

The humble papermaker of the Middle Ages made paper which survives today, much of it in good condition. The more scientific papermakers of the last 150 years have made reams of paper which have already crumbled to dust. Many librarians with collections of brittle, unusable books have a natural interest in forecasting the life of paper, which will be less than 50 years for many of the books which they receive today. Forecasting the life of even poor paper is a problem, since paper that lasts half a century is degrading very slowly from the standpoint of an experimental determination at room temperature. The Arrhenius equation, E

log

10

(k / ) = 2

kl

"a 2 - 3 0 3

χ

R

1 1 - ^)

(1)

where k is a specific reaction rate constant at T , E is the activation energy of the reaction, R is the gas constant, 1.9872 cal/deg/mole, and Τ is the absolute temperature, applies in principle to the individual rate processes which collectively govern the loss in paper quality. The most important of these reactions are presumably those involving chain scission in an amorphous region of the cellulose fiber. Broken cellulose chains are free to crystallize; the increase in degree of crystallinity n

n

a

0-8412-0485-3/79/47-095-117$05.00/0

© 1979 American Chemical Society Eby; Durability of Macromolecular Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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results i n a decrease i n p l a s t i c i z i n g water and an increase i n b r i t t l e n e s s . At higher temperatures, cross-linking of cellulose chains with the concommitant elimination of water may give the same result. The l i f e of paper i s commonly predicted by subjecting the paper to accelerated aging at temperatures ranging from 60-100 C, and measuring the decrease of folding endurance at several aging times. Extrapolation of a linear Arrhenius plot of the data to the value of 1/T. corresponding to room temperature then allows one to estimate the rate of loss of folding endurance under ambient conditions. There are d i f f i c u l t i e s i n the use of accelerated aging and the Arrhenius method which have been described i n d e t a i l by Browning and Wink (1) and by Gray φ. The faint chemiluminescence emission from many organic materials has been shown to be a rapid, precise, and empirically useful tool for their characterization, (3) even though the actual nature of the emission i s usually obscure. The l i g h t i s usually ascribed to excited states that are generated i n the termination step of the autoxidation process, although other, physical processes may also give r i s e to such emission i n the s o l i d state. I t was thought that the chemiluminescence emission from paper at series of temperatures could potentially give infomat ion that would bear on the question of continuity of oxidation processes over the entire temperature range of interest. The degradation of paper i s normally caused by both oxidative and acid hydrolytic reactions, so that the results were expected to pertain to the aging alkaline papers i n which the contribution of the hydrolytic mechanism i s minimized. Barrow (4) has already demonstrated that acid papers degrade rapidly and that by treat ment with an alkaline earth metal hydroxide, the rate of degrada tion i s greatly reduced. In the work being reported here, chemiluminescence measure ments were made at Battelle Memorial Institute i n Columbus over the 25-90 C range on papers prepared and supplied by the Library of Congress R&T Laboratory, while oven-aging and c l a s s i c a l endur ance studies were made on the same papers i n the R&T Laboratory over the 70-100 C range. The l a t t e r results w i l l be reported elsewhere. Various treatments were given to the kraft paper selected, i n order to broaden the scope of the investigation. The paper was washed to bring the pH to neutral. Then samples were alkalized with magnesium bicarbonate and with a sequence of calcium bicarbonate and calcium hydroxide. Copper acetate was added to several samples to provide an oxidation catalyst (5).


g

Experimental. Xhe paper used was taken from one r o l l of bleached kraft. The composition and properties of this paper are shown i n Table I .

Eby; Durability of Macromolecular Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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TABLE I.

119 KRAFT PAPER 70 l b s . 0.006 i n . 10.2 kg 5.3 kg 1.73% 3.21% 127 g 120 g 75.2 4.8 24 meq/kg 90% Southern Pine 10% Hardwood 8%, which includes 3% Ti02 0.5% 9 ppm 3 ppm 123 1 ppm

Basic weight (25 χ 38 - 500) Thickness Tensile, 15 mm s t r i p MD CD Elongation at break MD CD Elmendorf tear (single sheet) MD CD Brightness pH (Cold) Titration Fiber Filler Rosin Size Magnesium Copper Iron Cobalt

Sheets of the paper were soaked b r i e f l y i n denatured alcohol i n order to assist wetting. The paper was then washed i n running tap water for three hours, and then air-dried on a nonwoven fabric. Six groups of paper were subjected to the following treat ments: Paper A - this paper was washed as described; Paper Β the washed papers were soaked i n Mg(HCÛ3)2 for one-half hour and dried; Paper C - the washed papers were soaked i n 0.02 g/1 copper acetate solution for 30 minutes and air-dried. They were then soaked for 30 minutes i n the magnesium bicarbonate solution and air-dried. This paper showed 86 ppm copper by atomic adsorption analysis; Paper D - washed papers were soaked i n 0.0159 g/1 copper acetate solution for 30 minutes and a i r - d r i e d . They were soaked i n calcium hydroxide solution for 15 minutes, then calcium bicarbonate solution for 15 minutes and air-dried. This paper showed 64 ppm copper by analysis ; Paper Ε - washed papers were soaked i n 0.0089 g/1 copper acetate solution for 60 minutes and air-dried. Analysis indicated 59 ppm copper; Paper F - washed papers were soaked i n calcium hydroxide solution for 15 minutes, then i n calcium bicarbonate solution for 15 minutes and air-dried. Chemiluminescence. The paper samples were cut into squares 3" on a side (area 9.0 ± 0.6 i n . ) . The samples were placed into a small, circular aluminum oven with an inner chamber 4.5" i n diameter and 1.75" deep. The paper rested at the bottom of an inverted, aluminum weighing cup 2" i n diameter at the base and 0.65" high (to insure equilibrium with a i r temperature). The temperature of the paper was measured with an iron-constantan thermocouple and a Fluke d i g i t a l thermometer. The oven was 2



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maintained at a constant temperature with heating tape and a Foxboro Co. proportional controller. The atmosphere i n the oven was dry a i r (Columbus Oxygen, 2 ppm water), or air-humidified i n accord with the procedure of Gray (2). For the humidification procedure, the a i r was diverted through a glass f r i t immersed i n about 30 ml of water i n a test tube (6" long and 1" i n diameter). The tube was immersed i n a constant temperature bath (Lauda Model K2/RD) that contained water at the appropriate temperature for the experiment. The aluminum tubing that carried the humidified a i r was maintained with heating tape at a temperature between that of the sample oven and the constant-temperature bath. The small oven containing the paper was placed i n a large aluminum box measuring approximately 12" on a side, with the photomultiplier mounted on top. Light from the sample passed through a glass plate on the top of the oven and into the photom u l t i p l i e r system. The unfiltered chemiluminescence emission from the paper was measured with an RCA 4501-V4 12-stage photom u l t i p l i e r with an HP 5300 series measuring system, frequency counter, and digital-to-analog converter. The maximum s e n s i t i v i t y of the photomultiplier tube i s i n the region 400-600 nm. The counter was set to average signals over a 10-second period (to detect large spurious signals), and the counts were monitored continuously as a step function on a strip-chart recorder. A c t i vation energies were derived from the maxima at 40-90 C only. Results and Discussion. temperature above about 40 C the chemiluminescence emission from the paper samples rose to a maximum i n a few minutes and then declined over a period of hours to an emission near the background l e v e l . The time to r i s e to the maximum was longer than the time required to bring the paper to the oven temperature. The maximum counting rates are given i n Table I I , and the associated activation energies are given i n Table I I I . Although additional information about the chemiluminescent process may be obtained from the decaying portion of the curve, and from the spectral d i s t r i b u t i o n of the emitted l i g h t , i n this study only the emission maxima were analyzed. The observed rates of chemiluminescence below 40 C were usually lower than predicted by extrapolation of the Arrhenius plots (e.g., Figure 1). This i s a reasonable result i n view of the curve shape at higher temperatures, which shows a decline i n emission with time. Since the samples were stored at 25 C, they have been under examination conditions since the time of fabrica t i o n , and would i n effect already be i n this post-maximum region at the time of observation. There was observed a s t r i k i n g effect of the chemiluminescence on cycling papers between moist and dry atmospheres (Figure 2). With each change there i s an increase i n chemiluminescence inten s i t y . I t s magnitude decreases s l i g h t l y with successive cycles, indicating the slow exhaustion of some component or configuration of the paper structure. The rate of change i n chemiluminescence A t

o v e n



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Figure 1.

Paper

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Arrhenius plots of chemiluminescence maxima from paper A ( Q ) in humid air and paper C (O)in dry air


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MATERIALS

8.

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Paper

ET AL.

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TABLE I I . MAXIMUM CHEMILUMINESCENCE FROM PAPER SAMPLES IN AIR AT DIFFERENT TEMPERATURES Paper A C dry wèt


a

25 30b 40 50 60 70 80 90 a

B

4.9,2.8 18,24 135,21 230,65 530,160 1260,690 2300,1300 4420,2750

E

F

dry wet

dry wet

C

dry wet

D

dry wet

dry wet

22,4.5 28,52 170,62 370,75 840,300 2000,1000 3800,2110 7720,4600

19,12 38,55 180,29 285,75 650,285 1480,850 3650,2110 6380,3800

12,2.9 8.8,39 150,17 295,50 550,250 1300,700 2900,1350 5080,2520

6.2,2.3 5.0,25 72,17 175,30 385,90 920,300 2050,650 4770,1600

3.7,3.9 11,30 180,24 410,65 800,195 1430,880 3550,1120 2950,3600

Bath temperature 7 ± 1 C. 'Bath temperature 17 ± 1 C. TABLE I I I .

ACTIVATION ENERGIES OF CHEMILUMINESCENCE MAXIMA FOR KRAFT PAPER SAMPLES a

Dry Aging E

Humid Aging E

Washed

16.3 0.99

22.6 0.99

Washed MgC0

17.4 1.0

21.1 0.97

Washed CaC03

15.9 1.0

22.7 0.98

Washed Copper

18.9 1.0

21.4 0.99

Washed Copper-MgC03

17.0 0.99

23.0 0.99

Washed Copper-CaC03

16.4 0.99

23.3 0.98

Paper

3

a

E

a

a

a

(kcal/mole)/r2.



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on cycling i s faster on changing from a dry to a moist atmosphere than from a moist to dry one. This i s possibly associated with lower rates of d i f f u s i o n of water i n the cellulose phase than i n the vapor phase, so that moistening of the paper i s faster than i t s drying. At temperatures of 40 C and above the chemiluminescence from a paper i n a moist atmosphere was always observed to be smaller than i n a dry atmosphere. Possibly the moisture allows the macromolecular segments i n the paper to s l i d e past each other more e a s i l y , so that less bond scission occurs. Other explanations, however, are possible i n terms of an effect of moisture on the e f f i c i e n c i e s of excited state production, or an increased mobility of excited state quenchers. Below 40 C, on the other hand, the maximum emission from samples i n moist a i r was larger i n many cases than i n dry a i r , and we may be dealing with an effect i n volving an increased emission due to the change of atmosphere from the laboratory a i r to that i n the humidified oven (cf. Figure 2). Both the shape of the chemiluminescence versus time curves and the cycling effect can be accommodated by the hypothesis that heat or humidity changes introduce mechanical strains i n the paper which cause homolytic rupture of chemical bonds (Scheme 1). Scheme 1: η η Λ Stress y _RCellulose v

R* + 0

2

R0 ' + Cell-H 2

2R0

2

> R0 2

> C e l l - + R0 H 2

>· termination -*· l i g h t (minor)

The free radicals thus produced combine with oxygen i n the sample and enter into propagation and termination reactions accompanied by the production of l i g h t (3). The effect has been previously noted by Williams (6), who used the free radicals so produced to i n i t i a t e graft polymerization. We note no d i s t i n c t breaks i n the Arrhenius plots of the maxima, which often indicate mechanistic discontinuities i n other systems. (3) Conclusions. A commonly held rule of thumb i s that aging paper for 72 hours at 100 C i s equivalent to 25 years at room temperature. For a f i r s t - o r d e r process, this corresponds to an activation energy of 21.8 kcal/mole (1). We note that the chemi luminescence maxima show activation energies close to this value (Table I I ) . Although this observation i s interesting, the mechanisms associated with the degradation process and with chemiluminescence are not known with s u f f i c i e n t d e t a i l at the molecular l e v e l for us to give this result more than empirical significance.


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The striking increase in chemiluminescence intensity from paper when the humidity is altered supports an earlier suggestion that damage may result from such cyclic treatment (1), and is consistent with the familiar lore that preservation of books and other materials is best achieved under conditions of constant humidity. Acknowledgement. The authors at Battelle-Columbus are grateful for a grant from the Library of Congress in support of this work. Reference List (1) Browning, B. L. and Wink, W. A. (1968), TAPPI, 51, No. 4, 156. (2) Gray, G. C. (1977), Advances in Chemistry Series, 164, 286. (3) Mendenhall, G. D., Ang. Chem. Int. Ed., 16, 225 (1977). (4) Barrow, W. J., "Permanence/Durability of the Book", W. J. Barrow Research Laboratory (1963). (5) Williams, J. C., Fowler, C. S., Lyon, M. S., and Merrill, T. L. (1977) Advances in Chemistry Series, 164, 37 (1977). (6) Williams, J. L . , Verna, G.S.P., and Stannett, V. T., Ind. Eng. Chem. Prod. Res. Develop., 11, 211 (1972). RECEIVED

December 8,

1978.


The Use of Chemiluminescence in the Study of Paper Permanence

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