Accelerated Aging Tests for Determining Permanence of Papers GEORGEA. RICHTER, Brown Company, Berlin, N. H.
T
HE i m p e r m a n e n c e of
p r i m a r y a i m of t h e p a p e r Extended exposures of paper over a 200-day paper and other similar maker to avoid embrittlement period in circulating air maintained at 38" C. cellulosic m a t e r i a l s i s and physical d i s i n t e g r a t i o n results in physical changes that correlaie well largely the result of chemical of the p a p e r i t s e l f , it is still with corresponding changes taking place with oxidation and h y d r o l y s i s that necessary and important that the same types of paper, respectively, when they take place when paper is stored the investigators in this field under ordinary conditions. It i n t e r p r e t changes that occur are subjected to the well-known 100" C., 72is obvious that such degradawhen p a p e r s a r e exposed t o hour, oven test. tion is h a s t e n e d by the usual w e a t h e r i n g a g e n t s in terms A moderate amount of glue applied as a sura c c e l e r a n t s . Increased temof physical sacrifice as well as face coat does not affect the slability of the better p e r a t u r e, cer t a h light rays, in respect to chemical susceptigrade papers when artijcially aged at 100" C. metallic catalysts, and in some bility. cases moisture will speed the The subject of a c c e l e r a t e d There is some evidence that formaldehyde is less o x i d a t i o n . T h e presence of aging tests has provoked condetrimental than alum when used as a glue preacid groups, water, and higher siderable controversy in the last servative. temperatures will hasten the hyd e c a d e , a n d there will probA new series of experiments indicates again drolysis of cellulose. ably continue to be two schools that the folding strength of paper is more susThe chemical decomposition of t h o u g h t o n t h e r e l a t i v e of cellulose in the case of paper merits of such investigations. ceptible to the effect of aging than are the other m a n i f e s t s itself i n several It would seem t h a t t h e v a l u e ordinarily accepted paper tests. well-known ways. Often such of such tests depends largely Severe chemical oxidation of a n unbeaten changes a r e e v i d e n c e d b y a upon t h e consistency of d a t a cellulose fiber is reflected in a marked sacrifice change in color, and, as will be that are obtained when papers in stability of paper produced therefrom. pointed out, the m a t e r i a l in of r e c o g n i z e d s t a b i l i t y and q u e s t i o n m a y either be disacknowledged i n s t a b i l i t y are colored or may a c t u a l l y imsubjected to the tests in auesprove in brightness, depending upon the nature of the im- tion. A large volume of recent"data indicates strongly t h a t purities contained therein. Frequently, profound changes properly devised accelerated aging tests can differentiate the in the chemical properties are accompanied by no appreciable impermanent and the enduring papers, and that such tests change in color. do provide a means of characterizing the cellulose and the When the aging effect is caused by oxidation, there is con- secondary ingredients as well as the severity of the weathering sistently a sacrifice in the alpha-cellulose content, a marked agents. Assuming this to be true, it should be possible to increase in copper number, and often a violent drop in vis- estimate the relative life of the document or manuscript in cosity of the cellulose as measured in a cuprammonium solu- question. The several attempts to evaluate the expected life of papers tion. The same changes occur usually when the degradation is caused by hydrolysis of cellulose, although in such cases include the following typical procedures: the percentage difference is often less marked. (a) An exposure of sheeted samples at somewhat elevated Aside from chemical changes that take place in paper ex- temperatures in the presence of circulating air for suitable periods posed to the ordinary weathering agencies, its physical prop- of time. The so-called 100' C., 72-hour test as practiced in the erties also suffer more or less. Indeed, i t is this secondary laboratories of the Bureau of Standards is typical. Recently test has been supplemented by data that demonstrate the effect of loss of strength that concerns both the maker and this degree of decomposition occurring when paper is exposed to temthe user of paper most seriously. peratures of about:35' C. for 100 to 200 days. I n recent years there has been a great effort made to ex(b) Exposure of samples to artificial light in the presence of tend the present knowledge of those factors which determine air. Various types of ultraviolet sources have been used. There no standardized procedure accepted. the relative permanence of paper, with the hope that such has beenExposure of paper to natural sunlight for extended periods. (c) investigations will lead to the selection and the adoption of Very little quantitative work of this nature has been published. papers that have greatest life expectancy. It is evident that ( d ) Exposure of paper samples to atmospheres that contain acidic constituents. Sulfur dioxide has been the commonly acthe search must comprise: cepted destructive agent. (a) A more complete investigation of the relative severity of (e) Treatments of cellulose fibers in pulp form with concenthe various agencies that are responsible for the injury of cellu- trated alkali and acid solutions. This procedure does not lend lose, in order that papers intended for permanent records will not itself to the procurement of data in which the changes in physical be exposed unnecessarily to the action of such devastating agents. properties of paper can be studied. ( b ) A more ext,ended study of the relative chemical inertness of the in redients that compose the paper in question. The author has already published experimental data that (c) T i e perfection of ways and means for judging the exwere obtained by subjecting a variety of cellulose products pected life of papers in a relatively short time. to accelerated weathering tests. These preliminary data Inasmuch as there are as yet insufficient data to attempt were established both in the case of typical unsized waterleaf or to permit a strict correlation of chemical changes with loss papers and also with a large number of commercial papers in physical strength of papers that are aged by natural or by (2) 3). The series included products representative of rag, accelerated methods, and in view of the fact that it is the sulfite, and purified wood fiber origin. Since that time the 1154
November, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
author has extended this study widely. Part of the later work constitutes the basis of the present publication. Investigations in several laboratories have demonstrated that all types of paper undergo a loss in physical strength when subjected to elevated temperatures in the presence of air. I n recent years there has been some controversy regarding the acceptance of a test procedure which purports to estimate the relative permanence of a paper by measuring the decrease in physical tests when such paper is exposed to a current of circulating air at 100" C. I n general there appears to be ample evidence that the majority of papers composed of high-grade material, such as half-stock properly made from new rags, loses comparatively little in folding strength when exposed to air at 100" C. for 72 hours, whereas the ordinary bleached and unbleached sulfite papers, that hare been proved by experience to possess less durability, are appreciably less resistant to the oxidation by air a t 100" C. Additional data will be of value for a better interpretation of the results obtained by this so-called oven test. The procedures, technic, and definitions of terms are directly comparable to those given in previous publications (1,d, 3). In all cases reported in the present paper, the physical tests were made at 45 per cent relative humidity.
EXPERIMENTAL SERIESI The possibility of abnormal dehydration effects that can occur a t 100" C. has prompted a further search for test conditions under which the paper will be exposed for much longer periods of time a t temperatures that approach those of a normally hot climate. The author has already published preliminary data (2, 3) secured by exposing various papers to air a t 50" and 40" C. Table I lists certain results obtained when various papers were hung freely in circulating air at 38" C. for periods of 100 and 200 days, respectively. The technic and control were much the same as is practiced in the 72-hour exposure a t 100" C. (3, 3 ) . Inasmuch as all previous experience shows convincingly that the folding endurance of paper is more sensitive to change than is the tear strength or the pop test, attention was directed primarily to such fold decreases, although in this particular series the tear values were also determined and are so recorded.
1155
The fold retention data are, on the whole, quite consistent. The papers that have been made from rag or purified wood fiber stocks depreciate less rapidly than- do those of sulfite origin. The results of the groundwood newsprint paper are, however, disturbing, inasmuch as the fold retention is unexpectedly high. It must be noted that, because of the inherent weakness of the sheet in the case of groundwood paper, the fold endurance was determined under 200-gram tension, whereas papers are ordinarily folded under 1000-gram tension. Subsequent work has supported the viewpoint that the groundwood news sheets are peculiarly sensitive to oxidation in the presence of sunlight and somewhat less so when exposed to acid-free air in the absence of light. This apparent discrepancy demands more attention. I n a previous publication the author described still another procedure ( 2 ) whereby papers are exposed to oxidizing atmospheres at temperatures lower than 100" C. and under pressure for periods of 3 to 20 days. For the most part these published tests were made a t 50" C. and under 100 to 200 pounds per square inch (7 to 14 kg. per sq. cm.) pressure. Both dry and moist atmospheres were used, and in some cases oxygen was employed to hasten the deterioration. Since that time many more similar series of experiments have been made with the purpose of again investigating the question of adopting a lower temperature in the aging test. I n some cases the pressure treatments were carried out for 30 days so as to exaggerate the changes that occur. On the whole it was shown that the various papers respond to such a test in much the same order as when subjected to the 72-hour oven test a t 100' C. Inasmuch as the oven test lends itself more readily to laboratory manipulation, it is believed that subsequent experimental data included in this article are justified. The various confirming data obtained when operating under conditions that prescribe pressures above atmospheric have been omitted.
EXPERIMENTAL SERIESI1 Many experimenters believe that when engine-sized papers are given an additional surface size with glue, the stability of the paper is thereby improved. The author shares this belief in those cases where glue has been used moderately and when the base paper is itself relatively unstable. There
TABLEI. ACCELERATED AQINGTESTSIN CIRCELATING AIR AT 38" C. WATERPaper sample
LE.4F WATERPURI- WATEB- LEAF SUL- SWLFIED LEAF NEW FITR FITE RAG WOODSULFITE RAQ BOND BOND BOND F I B E R PAPERPAPER' NO. 16 NO. 2 NO. 16 78 46 35 46 43 44
Basis weight 100-day fold tests: Fold endurance, original paper: With 1252 240 19 273 137 1338 Across 1487 230 27 186 80 1566 Fold endurance, aged paper: 1240 216 17 255 121 1125 With Across 1322 169 25 121 39 1235 Fold retention, 70 94 82 91 79 69 81 ZOO-day fold tests: Fold endurance, original paper: 1495 317 17 223 102 1588 With Across 1716 279 29 199 117 1447 Fold endurance aged paper: With 1284 218 15 210 70 1285 Across 1320 179 25 117 31 1066 Fold retention, % 81 66 87 62 48 77 100-day tear tests: Tear test, original (av. with and across) 204 105 157 162 149 185 Tear test aged paper (av.) 203 99 155 157 134 185 Tear retention, % 100 94 99 96 90 100 ZOO-day tear teats: Tear test, original (av. with and across) 197 100 167 144 138 221 Tear test, aged paper (av.) 200 91 167 132 111 211 Tear retention, Yo 100 91 100 91 81 95 pH, original 5.2 4.6 5.0 4.4 4.6 5.0 The waterleaf new rag paper was one that had been specially prepared for esterification purposes. This will account for low initial folds. b Sulfite bonds were sized with rosin and starch. 0 Rag, bonds were rosin-sized in the beater and then surface-sized with glue. d Purified wood fiber bond No. 1 was beater-sized only. e Purified wood fiber bond No. 2 and No. 3 were beater-sized with rosin and then glue-sized. f Folds on newsprint were made at 200 grams instead of the usual 1000-gram tension.
RAG
BOND No.2 47
PURI-
PKJRI-
FIED
FIED
PURIFIED
WOOD WOOD WOOD RAG FIBER FIBER FIBER BOND BOND BOND BOND NEWSNo. 3 No. l d NO. 2' NO. 3 PRINT 44 43 45 55 31
1922 1671
1287 1156
506 675
834 690
1188 1195
1274f 3637
1965 1508 96
1084 1017 86
512 592 94
727 555 84
1046 957 84
1297 2737 89
2144 1678
1323 1056
755 811
817 887
1146 1422
,.
1734 1255 78
1115 880 84
642 598 79
613 736 79
1257 1150 95
.. .. ..
217 214 99
200 200 100
155 155 100
169 160 95
219 210 96
63 63 100
..
178 182 144 135 212 53 180 175 133 124 190 47 100 96 93 92 90 88 5.0 5.2 5.2 5.0 4.6 .. It comprised relatively long, underbeaten fibero.
1156
Samplea Glue-siaedb Basis weight
PGIffie spln., %
Rosin in base sheet, % ' Original fold endurance: With
I N D U ST R I A L A N D E N G I N E E R I N G C H E M I ST R Y
Vol. 26, No. 11
TABLE11. EFFECTOF GLUESIZINGos STABILITY OF BETTERGRADEPAPERS (Papers were exposed 72 hours at looo C . in circulating air) -1-II--111-1v----. --\'No 52.7 4.8
...
1.2
997 840
Yes 54 5.0 6
1.2 997 1000
No
Yes
KO
52.8 4.8
55.0 5.0
54
1.2
1.2
...
6
661 983
5.0
...
1.2
687 835
Yes 57
5.0
6 1.2 711 1057
No
43 ,..
.., 1.2 213 617
Yes 43.6
NO
44 3.8
...
....
6 1.2
1.2
343 615
1507 663
Yes 46 4.2 6 1.2
VI--
7 -
K O
Yes
48 4.6
51 4.8 6 1.2
....
1.2
1754 1177
771 2880 2832 .4CroSS 737 3229 3513 Fold after 72-hr. oven test: With 817 82 1 590 704 585 679 168 285 1368 1752 2649 2485 Across 572 752 806 608 657 770 481 514 630 996 2804 3035 Fold retention, 7* 75 78 86 87 82 84 79 82 87.6 87.7 89.3 87.0 a All base papers were produced from purified wood fiber except sample V, which was made from equal portions of purified wood fiber and purified leaf fiber and VI which was produced from 100% purified leaf fiber. b Glue-sizing was carried out as a separate operation. The bath was maintained at about 125O F . (51.7' (3,). The surface-sized sheets were air-dried.
is also ample evidence that the oxidation of the cellulose may be retarded by a reasonable protecting surface of glue when the base sheet contains excess rosin, alum, or other deleterious ingredient. An inherently weak paper often gains tremendously in fold value when it is glue-sized; when such paper is then tested by the usual accelerated aging test, there is frequently a great loss in fold value. However, the fold retention alone is in such cases not wholly sufficient in judging the paper. When the increment in fold, as accomplished by glue sizing, is high, the life of the paper may also be improved even though the consequent fold retention as ordinarily defined may be lower than in the case of the unglued paper. In general, a surface sizing of glue does not appreciably affect the life of a basically stable paper unless secondary injurious reagents, such as excess alum, are added simultaneously. The surface film of glue is in itself affected quite materially when exposed to oxidizing influences. It is particularly unstable when oxidation takes place in the presence of sunlight; this effect can be shown by a marked sacrifice in ink resistance, as will be demonstrated in a later article. The 72-hour oven test results in such changes to a lesser degree. Table I1 summarizes a set of data secured to demonstrate again the effect of glue size on the keeping qualities of a paper produced from a high-grade cellulose. The base papers were made from a purified wood fiber according to standard paper mill practice and were beater-sized with about 1.2 per cent rosin. The papers after the surface-sizing treatment contained from 2 to 3 per cent glue. This surface size was applied by the use of a glue bath which tested 6 per cent glue based on solution and 0.5 per cent formaldehyde based on glue. Later experiments indicate a n advantage in the use of formaldehyde over alum as a preservative for glue. This can probably be accounted for by a lower acidity when alum is avoided.
tion will be reported in a later article dealing with sunlight exposures.
EXPERIMENTAL SERIESI11 Although there has been but little dispute regarding the selection of the change in folding endurance (fold retention) as a specific measure of paper stability, it was thought worth while to check again the relative changes that occur in the pop test and in the tensile strength when papers are aged a t 100" C. in circulating air. The data included in Table IV leave little doubt that the fold retention value is most sensitive and that it differentiates the permanent from the more unstable papers. Whereas the sulfite papers suffered major changes in respect to fold value, the sacrifice in tensile strength and pop test is only 10 to 12 per cent below that of the papers of more highly refined stocks. This was also confirmed by sunlight exposure tests to be described in a later publication. TABLEIV. CHANGES IN VARIOUS TESTSWHENPAPERIs AGED AT 100' C. FOR 72 HOURS -ROSIN Rag bond No. 1 Basis weight 44 4.8 $;ding retention, % 8 3 . 6 Pop test retention, % 9 7 . 0 Tensile retention, % 98
+ GPurified L USIZING~ Purified Rag bond No. 2 46 5.2 83.6 99 98
wood fiber paper No. 1 45 6.4 94.0 100 100
wood fiber paper No. 2 46 5.4 99 97 99
+
ROSIN STARCH SIZINQ Sulfite bond No. 1 43 4.4 11.3 84 88
Sulfite bond No. 2 50 4.0 3 84 93
EXPERIMENTAL SERIESIV
There has been a general acceptance of the theory that cellulose chemically degraded by excessive oxidation or hydrolysis will stand up less favorably when aged in paper form. This effect has been frequently observed in the case of rag half-stocks as well as in wood pulp mills. In order to estabAS SECONDARY TABLE111. STUDYOF ALUMus. FORMALDEHYDE REAGENT IN GLUE-SIZE BATH lish typical changes in stability that will result when unbeaten (Papers produced from a purified wood fiber containing 94% alpha-cellulose; cellulose is overbleached, the following experiment was tests made a t 100' C. for 72 hours in circulating air) planned: The pulps included: ( a ) a normal unbleached sulSAMPLE SIZED WITH: fite pulp, ( b ) a commercial grade of purified wood fiber, ( c ) a Glue + alum Glue + formaldehyde 53.5 53.8 Basis weight kraft pulp, and ( d ) a new rag half-stock. 4 . 6 4 . 7 PH I n all cases the fibers were subjected to the action of hypoOriginal fold value: 2285 2529 With chlorite to cause both mild and severe oxidations. Table V 1910 2119 Across shows clearly that overoxidation of pulp will, in turn, render Folding endurance after 72-hr , o v e n test: 2085 2070 With the papers produced therefrom more susceptible to degrada1713 1836 Across 90 84 Folding endurance, % tion as measured by the 72-hour heat test. The value of maintaining an alkaline condition throughout the bleaching A purified wood fiber paper that had been sized in the beater period is apparent. These data were later confirmed by a was selected to demonstrate the difference in stability that similar series in which the chemically treated pulps were exmay be expected when formaldehyde is substituted for alum posed to sunlight. That oxidation takes place when papers are subjected to as the glue preservative. The base sheet was surface-sized on the one hand with a 3 per cent glue solution that contained the 72-hour oven test is demonstrated by submitting a few alum and on the other hand with a similar solution in which typical waterleaf papers to the action of circulating air a t formaldehyde was substituted as the preservative. The 100" C. in the usual manner. Table VI shows the greater alum-containing size yielded a paper of definitely less sta- susceptibility of groundwood to such oxidation than is the bility (Table 111). Further work relating to this same ques- case with the more highly refined fibers.
Covi'er
x".:
Ileioie exposure
I %
After erpoaure Folding endurnnee.
I xi
;
Before erpOsurs
After OXPOIUre Fold retention, %
* Fulds i,, Esse Of
100 g,,'",,dro"d
rem lrindc
0 88
0 00
4 64 6.00
50
I0lY
19 100
45 44
W l L h roduPeii teoairic,.
87JMhlAl*Y
1. Tlie stability of various papers Lias been measured by determining the ret.ention of Eolding endurance when sucli paiiers were aged at 38' C. for 200 days in circulating air. 2 . The results parallel previous tests made by aging curresponding papers at 100" C. for 72 hours in circulating air. llag and Iiiirified ~vwcl-filn~r papers retain a greater percentage US their original folding strength thtm do the solfite slieets. So atteinpt is made at this t h e to explain the pccoliar beliavior oE newsprint rr4:en so exposed. 3. lnasrniicli as a temperatiire of 38" C. is sometimes reached wlreii pnycrs are stored ordinarily and siiice there is good correlntion in t h e deterioration of papers 8.1 38" and
100" C., t.liese findings appear to offer further evidence that the 72-hour oven test at 100" C. is of some value in estimating the probable life of commercial papers. 4. When a high-grado base paper is glue-sized under conditions that will not cause a tremendous drop i n Sold endurance, the permanence as measured by fold retelltion is not materially afiected. The use of Eorrnaldehyde as glue p e servalive and lrardeiiing agent appears to possess soioe advantage over alum. 5 . Additional experiniental data confirm the general belief tlrat the aging of papers manifests itself niost quickly in terms of loss in folding endmance. The pop test and the tear value are a.ffected to a lesser degrce. 6. Tiie life of a paper can be severely and adversely afIeeted if the unbeaten fiber Sroin which the paper is produced is ovcrblenched and contains a relatively liigh percentage of oxyccllulose.
LITERA~W~E CITED Richter. IN". ENG.CHEM.,23, 131, 266 (19dl). ( 2 ) I M . , 23, $71 (19:3l). (8) Kirhtor. J . i k n k l < n I n ~ t .212, , No. 4 (1931). (4) Rielitrr and Sohor, U. S. Patent 1,639,704 (nus.23, 1927) (1)
I I ~ c m v x rAugust ~ 14, 1934
Le GKIMOIRE DE HYPOCRATE BY David Teniers, fhc Younner
No. 47 in the Berolnheimer Series of Alchemical sod Historical Reproductions i s the ninth Teniers naintinp - remoduced . in this collection. The original, 68 by 51 centimeters, i s owned by the Princc ofSalm-Salm, Anhalt. (;ermsnv. and was exhibited at Duessel.
d o d iII i504. It shows the alchemist preparing a potion from directions giuen in the "Conjurin~ Booli of Hippocrates." The anonr;~tus shown differs from that in Te>iers' other paintings. An engraving of this picture by IT. Basan ofParis is also linown.
h drtailod list uf the first t ~ , i ~ treproductiura ~ - ~ i ~ i n heSariea, toeether w i t b lull p~rbicuiarafor obtaining photosraphie copies o i the originals. nppesied in uur isme lor Jsnuhry, 1934, pnge 112. A supplementary list 01 Nos. 37 t o 42. togetlicr w i t h lie. 43 is in our diily. 1984, i ~ ~ up8ge e , SO%. No. 44 l e i n the Aueuat iaaue. page 884, No 45 i n the Septeinhor iseue. p h ~ e967. nnd No. 40 in our October, 1984. issue, page 1110.
