NBSIR 74-499

An Analysis of the Aging of Paper:

Possible Reactions and Their Effects

on Measurable Properties

W. K. Wilson and E. J. Parks

Paper Evaluation Section

Institute for Materials Research

National Bureau of Standards

April 26, 1974

Interim Report

Prepared for National Archives and Records Service

Washington, D. C. 20408

NBSIR 74-499

AN ANALYSIS OF THE AGING OF PAPER:

POSSIBLE REACTIONS AND THEIR EFFECTS

ON MEASURABLE PROPERTIES

W. K. Wilson and E. J. Parks

Paper Evaluation Section

Institute for Materials Research

National Bureau of Standards

April 26, 1974

Interim Report

Prepared for National Archives and Records Service

Washington, D. 0. 20408

v.. >

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

CONTENTS

1. INTRODUCTION 1

2. POTENTIAL CHEMICAL AND PHYSICAL CHANGES

IN CELLULOSE AND PAPER 3

2 . 1 Overview

.

3

2.2 Hydrolysis 4

2.3 Oxidation 6

2.4 Crosslinking 7

2.5 Changes in Lateral Order 8

2.6 Thermal Decomposition 10

3. PROBABLE EFFECTS OF CHEMICAL AND PHYSICAL

CHANGES ON MEASURABLE PROPERTIES OF PAPER .... 11

3.1 Overview 11

3.2 Hydrolysis 11

3.3 Oxidation 12

3.4 Crosslinking 13

3.5 Changes in Bonding 14

3.6 Thermal Decomposition 15

4. SOME SPECIAL SITUATIONS 16

4.1 Overview 16

4.2 Metals as Catalysts 16

4.3 Effect of Chlorinated Extractives 17

4.4 Migration of Impurities 17

4.5 Air Pollutants 17

5. RESULTS OF SOME RECENT STUDIES ON

ACCELERATED AGING OF PAPER 19

6. SUMI4ARY 21

7. RECOMMENDATIONS 23

8. REFERENCES 25

1 . INTRODUCTION

At the request of the National Archives and Records

Service, this laboratory is examining the variables associated with the accelerated aging of paper in an attempt to

arrive at reasonable conditions for an aging method. Emphasis

has been on (1) the preparation of laboratory handsheets under

carefully controlled conditions, (2) precisely defined and

controlled aging conditions, and (3) measures of change that

reflect what actually occurs during the aging of paper, including the effects of oxygen and moisture.

The permanence of paper and laboratory aging have been

discussed extensively in the literature. Pertinent articles,

including reviews, have been authored by Spinner [1], StubchenKirchner [2] , Neimo [3] , Browning and Wink [4] , Luner [5]

,

Millett [6], Kleinert [7], Smith [8], Barrow [9], Graminski

[10], Byrne and Weiner [11], Kantrowitz [12], Venter [13],

Cardwell [14], and Gray [15]. Much of this work was directed

toward color reversion.

The permanence of paper can be studied from the standpoint of (1) composition, (2) environment, and (3) significance of test data with respect to changes that occur during

aging. These three approaches are interdependent. Under

composition one might list:

(a) aldehyde

(b) ketone

(c) carboxyl

(d) salt of carboxyl

(e) hemicelluloses

(f) lignin

(g) fiber, fiber length and degree of polymerization

(h) crystallinity , or order

(i) acid incorporated during manufacture

(j) other additives.

Under environment one might list:

(a) oxygen and ozone

(b) moisture

(c) smog and other impurities in the atmosphere

(d) light

(e) heat.
2

Reactions that might occur during the natural aging of

paper include:

(a) hydrolysis

(b) oxidation

(c) crosslinking

(d) changes in order, or crystallinity , due to reported

changes in moisture content

(e) photolysis

(f) photosensitization

(g) photo-oxidation

Thermal decomposition must also be considered during accelerated aging, but it probably does not occur during natural

aging unless the storage temperature is excessively high.

This report reviews the structure and basic reactions

of cellulose in relation to the aging of paper and the effects

these reactions might have on the measurable properties of

paper. Also included is a discussion of some special reactions

that normally occur due to environmental conditions, but are

greatly influenced by composition of the paper.
3

2. POTENTIAL CHEMICAL AND PHYSICAL CHANGES

IN CELLULOSE AND PAPER

2 . 1 Overview

Hundreds of articles have been written describing

reactions of cellulose dispersed in aqueous solutions of

various reagents. The actual reactions that occur during

the aging of paper have received little attention in the

literature. This is no surprise for "solid state" reactions

of this type are not easy to follow. The effects of these

reactions are evident from changes in physical and chemical

tests, but these tests seldom describe what happens. Functional group analysis (aldehyde, carboxyl, ketone) is elusive,

and Browning and Wink [4] found little change in functional

group content during aging. However, the metal on the

carboxyls greatly influences the course of decomposition

during differential thermal analysis [16]

Moisture relationships in the degradation of cellulose

have not been properly identified. Supposedly, at low moisture

contents, water exists on cellulose as a monomolecular layer

[17] . The differential heat of swelling (AH) and the free

energy changes (AF) vary greatly with moisture content and

with temperature. The relative humidity necessary to maintain

a given moisture content increases somewhat with temperature.

At constant relative humidity, the absolute vapor pressure of

water increases dramatically with increase in temperature.

This, in turn, decreases the partial pressure of oxygen. At

accelerated aging temperatures, near 90°C, the rate of exchanrre

of moisture with cellulose in equilibrium with a given vapor

pressure of water would be orders of magnitude higher than

at room temperature. Whether this higher rate of exchange

of moisture and the differences in AH and AF would simply

speed up reactions that occur at room temperature or cause

other reactions to occur, is a matter for speculation.

Hydrolysis, oxidation, crosslinking , and changes in

lateral order, or crystallinity , are possible reactions

during the natural aging of paper. In addition, thermal

decomposition must be considered during accelerated aging.

The basic reactions that can occur during natural aging are

fairly limited, but the conditions that make these reactions

possible and their effects on paper properties are more

extensive.
4

Degradation reactions may occur in the fiber and in the

bonding areas between the fibers. As some physical tests

are affected by changes in bonding (e.g., crosslinking as

measured by wet strength) more than by changes in the fiber

(depolymerization and its effect on tensile strength) or vice

versa, this must be taken into consideration in the evaluation of data. Hydrolysis, oxidation, and crosslinking would

occur in both the fiber and in the fiber bonding areas. In

order to separate the effects of crosslinking in the fiber

and in the bonding areas, it would be an interesting experiment to achieve crosslinking in fibers and then make paper

from them. As there would be little crosslinking in the

bonding area, one could separate the effects of crosslinking

in the fiber and in the bonded area.

The effect of additives introduced during the papermaking process may have a profound effect on the behavior of

paper during aging. For example, an additive may perform

well as a bonding agent but degrade with time. Some wet

strength resins are in this category. Another additive might

perform well as a bonding agent, and be more stable than the

cellulose

2 . 2 Hydrolysis

The acetal linkages in cellulose are fairly stable in

neutral and alkaline media, but are easily hydrolyzed in

acid (rate increasing with hydrogen ion activity) [18, p. 991-

1006]. Although the homogeneous (cellulose in solution)

hydrolysis of cellulose is a random process [19, p. 122],

heterogeneous (cellulose fiber dispersed in acid) hydrolysis

is more complex. The bonds are not equally accessible in the

crystalline and less well ordered areas. Peeling reactions

occur in hot alkali [20, p. 596].

The hemicelluloses are much more readily accessible and

occur in the amorphous state in wood [20, p. 164], but may

precipitate out in the crystalline state on cellulose fibers

during lignin removal [20, p. 510, 608]. Model compounds

related to the hemicelluloses are hydrolyzed at a much higher

rate than cellulose [20, p. 119; 21]. As long as the hemicelluloses are in the amorphous state, their rates of hydrolysis will be much greater than that of cellulose. When crystallized onto cellulose, some hemicelluloses probably do not
5

hydrolyze appreciably faster than cellulose [20, p. 510].

As a large fraction of the bonding material in wood pulp

papers may consist of hemicelluloses , their chemistry is

germane to the stability of paper, especially with respect

to the stability of interfiber bonding.

The hydrolysis of carbohydrate structures is enhanced

by the presence of oxidized groups [20, p. 123, 124]. This

probably explains the synergistic effect of oxygen and moisture

in the degradation of cellulose [22] and cellulose acetate [23].

The most significant work on the effect of chemical

structure on hydrolysis that is germane to the aging of cellulose has been compiled by Sharpies [18, p. 993-996]. Oxidation and cleavage at the No. 2 and No. 3 carbon atoms in the

anhydroglucose ring (Figure 1) leads to the formation of

aldehydes or carboxyls. These structures cause an increase

in the rate of hydrolysis, even when reduced to the dialcohol.

Most dramatic is the discovery of evidence that an aldehyde

group on the No. 6 carbon atom may increase the rate of acid

hydrolysis as much as 70 times [24] . This effect can be

eliminated by reduction of the aldehyde back to hydroxyl with

sodium borohydride. Wood cellulose is hydrolyzed at a higher

rate than cotton cellulose, and it has been suggested that

the reason for this is oxidation during pulping to give an

aldehyde on the No. 6 carbon. When this is coupled with the

higher rate of hydrolysis of the hemicelluloses, it is obvious

that cotton fiber is likely to be more resistant to hydrolysis

than wood pulp fiber.

It has been demonstrated that glycolic acid, a product

of cellulose oxidation and hydrolysis, can react with glucose

under certain conditions to give reversion products [25].

Thus, the aging of paper may involve a multitude of reactions

l)etween decomposition products and between decomposition

products and cellulose. Rapson has shown that color reversion

is caused by degradation products of oxidized cellulose [26].
6

2 . 3 Oxidation

Little definitive work has been done on oxidation of

carbohydrate structures during the aging of paper. Using

the generally accepted formula for cellulose in Figure 1,

one can review the possibilities [19, p. 140-183].

(1) The primary alcohol group on No. 6 carbon can be

oxidized to aldehyde or carboxyl.

(2) The secondary alcohol groups on carbons in the

No. 2 and 3 position can be oxidized to ketone, either

separately or simultaneously.

(3) These same groups can be oxidized to aldehyde with

cleavage of the ring, and then to carboxyl.

(4) Although not an oxidative reaction, the sensitivity

of the 2,3 dialdehyde to alkali is worth noting. The aldehyde

in the No. 3 position is two carbon atoms removed from the

ring oxygen. This is an example of the general case where

alkaline cleavage can occur if an ether linkage is separated

by two carbon atoms from an electronegative group, i.e.,

aldehyde. A similar instability would occur with an aldehyde

group in the No. 6 position, or a keto group in the No. 2 or

No. 3 position. Further oxidation of the 2,3 aldehydes to

carboxyls stabilizes the structure.

The above oxidation reactions normally take place with

cellulose dispersed in an aqueous solution of oxidant. Oxidation reactions that occur during the natural aging of paper

would be expected to proceed with oxygen or ozone as oxidant.

A subtle example of oxidation that is devastating to paper

(but does not directly involve paper in the oxidation process)

is the oxidation, catalyzed by manganese and iron impurities

in the paper [27] , of sorbed sulfur dioxide to sulfur trioxide.

Kleinert [28] has shown that peroxide formation occurs

during the accelerated aging of wood pulps at just under 100°C.

Aging at high humidity produced much more peroxide than aging

at low humidity. Strangely enough, aging at high humidity in

nitrogen produced more peroxide than aging in air. Although

much of the peroxide could be removed by washing with water,

most remained with the pulp. While it is possible to follow

aging by peroxide formation, peroxide also is destroyed during

the process [29]. Kleinert [29] also showed that peroxide

formation was several times greater in the fines than in the

longer fibers in the pulp, and that peroxide was formed during

air drying of pulp at SS^C.
7

Peroxide as an intermediate in the aging of paper should

receive more attention, for cellulose basically is an ether.

Ethers undergo oxidative degradation through peroxide formation [30, p. 77]. The transition metals, including iron

which occurs as a common impurity in cellulose, catalyze the

decomposition of hydroperoxides [30, p. 42] and could be a factor in the degradation of paper.

The above oxidation reactions of cellulose also occur in

the hemicelluloses . Some monomer units that occur in the

hemicelluloses are shown in Figure 2, and it is obvious that

the units are very similar to anhydroglucose , the monomer

unit of cellulose. The hemicelluloses normally are more

accessible than cellulose and the building units may react

much more rapidly with some reagents [20, p. 119; 31]. Therefore, this fraction of wood pulp paper should be more important

to oxidative stability than the cellulosic fraction, especially

as it is responsible for much of the interfiber bonding.

The fines probably represent the most vulnerable fraction

of paper with respect to degradation. This is indicated by

the much greater generation of peroxide in fines, as mentioned

above. Fiber fraction studies show no real differences in

cnemical composition among the various fractions of chemical

pulps except for the fines [32, p. 642]. The fines in a

bleached sulfite pulp contained most of the ash and hemicellulosic material, which was reflected in greater alkali solubility, hiaher copper number, and lower viscosity.

2.4 Crosslinlcing

Although crosslinking of cellulose has been known for

years, it has been recognized only recently as a reaction

that occurs during the aging of paper [10]

Crosslinking in paper occurs at elevated temperature

and is a consequence of conditions that could be described

as accelerated aging, both in moist and in dry gases [33, 34,

35]. Back [33] has postulated that wet strength can develop

as a result of a reaction between an aldehyde group on one

chain with a hydroxyl group on a neighboring chain to form

a hemiacetal. Presumably, a ketone on carbons 2 or 3 could

form a hemiketal. Unless steric considerations prevent it,

an aldehyde or ketone could form an acetal or ketal by

reacting with adjacent hydroxy Is with the elimination of water.
8

Back [33] has mentioned the possibility of formation of

ether linkages between hydroxyls of adjacent chains by elimination of water. This certainly is a possibility, for this

reaction takes place in an acidic solution of alcohol at

around 110-130°C [36]. At higher temperatures an alkene is

formed.

Ruffini [34] has presented evidence showing that crosslinking occurs by the formation of an ester linkage between

a carboxyl in one chain and a hydroxyl in a neighboring chain.

He postulates that uronic acid groups in the hemicellulose

fraction and, in unbleached sulfite pulps, sulfonic acid

groups in the lignin, are responsible for the high strength

level of paper made from certain pulps. The sheets Ruffini

made from the experimental pulps were dried for three minutes

at 130°C. As it should be possible for esters to be formed

at this temperature, it would be interesting to prepare sheets

by air drying without heat to determine the effect of the

drying process on ester formation.

Regardless of the mechanism, wet strength development

is catalyzed by aluminum sulfate, zinc chloride, ferric

chloride, acids, and cupric nitrate [33, 35]. Back [33] has

reported that the generation of wet strength is catalyzed by

low pH and by metal ions with a high redox potential. He

also found that salts that produce wet strength at a high

rate also cause a rapid decrease in wet strength. This is

especially true of those salts that release acids at elevated

temperatures.

Improved bonding of cellulose fibers occurs after exposure

to oxygen and ozone in a corona discharge. The bonding is

further improved by soaking in 0.1 M sodium hydroxide

after exposure to the corona discharge. These effects have

been attributed to surface oxidation of the cellulose [37]

2 . 5 Changes in Lateral Order

Changes in lateral order of cellulose in paper would

be expected to affect most physical properties of paper.

The discussion here will be confined to change in order that

may occur in the bonding areas of paper. Recrystallization

has been reported to occur during heterogeneous hydrolysis

[18, p. 996; 38]. It has been reported that partially decrystallized cellulose recovered most of its crystallinity

when exposed to moist air [39]. Extensive changes in
9

physical properties of paper over a period of 36 years,

apparently without changes in fiber strength [40], are difficult to explain without assuming some change in the order

of the system. Cellulose fibers change dimensions with changes

in moisture content, and this is reflected in hygroscopic

dimensional instability in paper. When pulp is dried and

rewetted, the physical properties (wetting, strength, moisture

regain) are never quite the same [18, p. 724; 41], The properties of paper made from "broke," the waste paper from the

paper machine, are significantly different from those of the

paper from which it was made [42]. Luner [5] found that

cycling of temperature and, therefore, relative humidity and

moisture content, had an effect on physical properties of

paper during accelerated aging.

From the above, it appears that, if conditions are

favorable, crystallinity in the fiber may increase in water

suspension or in a moist atmosphere. At the same time, the

"order" that exists in the bonding regions of paper may decrease with time. This could seriously affect some physical

properties without damaging the fiber. A possible explanation

for this is that crystallization occurs with the exclusion

of water, but requires the presence of water for the necessary

niobility, whereas fiber to fiber bonding occurs through water

bridges. With cycling of relative humidity, these water

bridges probably can relax with time and affect the properties of paper that depend most on interfiber bonding. If

some agent, such as rosin size, is present to enhance these

bridaes, they are less likely to relax with time. Chemical

degradation with time in a sizing-bonded paper would affect

those properties dependent on fiber integrity.

It has been reported [43] that dialdehyde cellulose

(carbons 2 and 3) and monocarboxyl cellulose (carbon 6)

exhibit higher moisture regains than the unoxidized control.

On the other hand, the moisture regain of dialcohol cellulose

(reduced dialdehyde cellulose) was less than the unoxidized

control. The heat of interaction of the samples with water

showed the same trend. However, the levels of oxidation

necessary to achieve these changes in moisture regain and

heat of wetting were far higher than those which occur in

the aging of paper. Therefore, oxidation is not likely to

affect "order" or moisture regain in the bonding areas of

paper as a primary effect.

The effects of destruction of bonding depend, in part,

on the fiber length. In a long-fibered paper, fold, for

example, might be affected hardly at all; but in a shortfibered paper, fold would be drastically reduced.
10

2 . 6 Thermal Decomposition

A property that often is overlooked in the accelerated

aging of paper is the purely thermal instability of cellulosi

and noncellulosic carbohydrate structures. This thermal instability should be more pronounced with increasing amounts

of oxidized cellulose and with increasing amounts of hemicelluloses. Thermal decomposition at various temperatures

has been discussed by Schwenker [44] , who showed that thermal

degradation occurs even below lOO^C.
11

3. PROBABLE EFFECTS OF CHEMICAL AND PHYSICAL

CHANGES ON MEASURABLE PROPERTIFS OF PAPER

3 . 1 Overview

The changes that might be expected to occur during the

accelerated aging of paper are given in Table 1 along with

anticipated effects on various laboratory tests. Many of

the reactions must occur concurrently or consecutively.

Some may be isolated by excluding oxygen or moisture (or

oxygen and moisture) from the aging atmosphere. This also

may confuse the situation by preventing synergistic reactions

from occurring.

The effects of various changes are listed as primary

or secondary. A decrease in degree of polymerization (DP)

is a primary effect of hydrolysis. Decrease in wet strength

probably would be a secondary effect resulting from a general

weakening of the structure.

3 . 2 Hydrolysis

During accelerated aging, hydrolysis could be expected

to affect test properties as indicated in Table 1.

(a) Degree of polymerization (DP) would be reduced by

random attack in accessible areas.

(b) Organic acids could be generated from carbohydrate

structures, and protonic acid from aluminum complexes (if

present)

(c) Aldehyde content would be increased, as one aldehyde

group is formed with each hydrolytic chain break.

(d) Carboxyl/ ketone, and peroxide should not be affected.

(e) Moisture regain should be relatively unaffected by

chain scission, as the latter occurs in the accessible areas

and none of the material is lost. This is in contrast to

heterogeneous hydrolysis in acid solution where the hydrolyzed

material becomes soluble and is lost. The remaining cellulose

is highly crystalline and has a low moisture regain [18, p. 736].

(f) Alkali solubility should increase depending on the

extent of hydrolysis. If hydrolysis is known to be the only

reaction taking place, alkali solubility is an excellent

diagnostic test.
12

(g) Fold, tear, burst, tensile, elongation, and tensile

energy absorption (TEA) would decrease. Modulus should

decrease somewhat, but not extensively. Tensile, elongation,

and TEA are increasingly sensitive, in that order, to a decrease

in DP. This is illustrated in Figure 3 [10] , which shows

typical load-elongation curves for an unaged specimen and

an aged specimen of the same paper, whose retention of tensile

strength, elongation, and tensile energy absorption are approximately 65 percent, 35 percent, and 25 percent, respectively.

In this particular sample, modulus has increased due to crosslinking.

(h) Zero span would be decreased by fiber degradation,

due to decrease in DP.

(i) Wet strength would decrease as a secondary effect

due to general weakening of the structure, or to hydrolysis

of crosslinks as a primary effect.

(j) Blue reflectance would not be expected to change

if the cellulose is unaltered. However, this is never the

case, and humic substances may be formed during hydrolysis

[19, p. 23]. These may be dark in color and, therefore,

would reduce the blue reflectance.

3 . 3 Oxidation

Oxidation of carbohydrate structures during aging of

paper could be expected to affect test properties as indicated in Table 1.

(a) A direct decrease in DP could be expected, partly

as a result of increased sensitivity of oxidized structures

to hydrolysis. Although viscosity measurements are a function

of DP, viscosity solvents for cellulose usually are alkaline

and are likely to cause chain scission [19, p. 147]. An

alternate approach is to nitrate the cellulose and dissolve

in acetone for measurement of viscosity [45], but this procedure has its own set of problems, especially with wood

pulps [ 46 ]

.

(b) Carboxyl, aldehyde, ketone, and peroxide would be

expected to increase, although aldehyde might also decrease,

depending on the specific reaction. Low molecular weight

oxidation products with an appreciable vapor pressure would

be lost. Peroxides would be expected to form and decompose,

so for a given set of experimental conditions, peroxide

concentration should stabilize at a steady state value.
13

(c) Moisture regain should decrease as a secondary

effect due to crosslinking reactions [18, p. 857].

(d) Alkali solubility should increase with oxidation

and should be a sensitive test, but not a diagnostic one.

(e) All of the physical properties, except wet strength,

should decrease with oxidation, especially with the synergistic

action of oxidation and hydrolysis.

(f) Blue reflectance should decrease with oxidation.

Saturated organic compounds (no double bonds) absorb in the

ultraviolet, and the absorption moves up to the blue end of

the visible spectrum only after the formation of a conjugated

system of double bonds [47] . As it is not possible to form

a conjugated system between anhydroglucose units in cellulose,

because of the presence of an oxygen bridge, a conjugated

system can exist only on carbons 1, 2, and 3 in the anhydroglucose ring, and on 1 only when it is terminal. Therefore,

absorption in the visible due to oxidation of cellulose and

hemi cellulose is likely to occur only in the blue. Oxidation

of hemicelluloses in wood pulp is far more probable than

oxidation of cellulose.

3.4 Crosslinking

Crosslinking could be expected to affect the tests

listed in Table 1 as follows:

(a) Some crosslinking should increase the molecular

mass, and eventually crosslinking may render the material

insoluble [48] . This may or may not show up in a viscosity

determination, as some viscosity solvents m.ay destroy the

crosslinks.

(b) Carboxyl [34] could be consumed in ester formation,

and aldehyde and ketone [33] could be consumed in acetal and

ketal formation, respectively.

(c) Moisture regain should be decreased by crosslinking

[18, p. 857]. The structure would become more rigid and less

accessible to moisture.

(d) Alkali solubility should decrease unless the crosslinks are decomposed in alkali [48]
14

(e) Fold, tear, and burst deteriorate with crosslinking

[49]. Changes in tensile strength will depend on the extent"

of crosslinking. Elongation decreases with crosslinking [50],

modulus increases [10], and TEA should decrease as a function

of decrease in elongation. Zero span should not be affected

except as a secondary effect of fiber degradation if crosslinking occurs due to the presence of an acid catalyst.

(f) Wet tensile increases significantly with crosslinking [ 33 ] .

(g) Blue reflectance should not be affected.

3 . 5 Changes in Bonding

A discussion of decrease in order in the bonding area

will be confined to (1) decrease in bonding due to decrease

in "order" of the cellulose in the bonding area (conjecture

at the moment) , and (2) degradation of bonds formed by sizing

agents. Concepts of the nature of fiber bonding and of sheet

formation are changing rapidly but a good introduction to the

subject is given by Casey [32]. Graminski [51] has shown

that the film-like material created by the "fines" in paper

affects properties, especially durability, in a way not previously anticipated. Apparently, failure occurs first in

the "fines" area and not in the fiber to fiber bonding.

The bonding in paper depends on a host of variables

such as type of fiber, extent of beating, wet pressing,

sizing agents, carboxyl groups, zeta potential, and many

others. McKee [42] has shown that repulping degrades those

properties that are a direct function of fiber strength and

fiber bonding, but those properties such as tearing strength,

that are inversely related to bond strength, increase with

the number of repulpings. This is not the same situation as

degradation of bonding in a sheet of paper, but it may indicate that tearing strength would increase, or decrease more

slowly, if degradation of bonding were the dominant reaction

during aging.

As so little is known about selective degradation

reactions in the bonding area of paper, the indications in

Table 1 are conjecture only. Bonding is discussed because

degradation of bonding and its effect on the properties of

aged paper have been largely overlooked.
15

3 . 6 Thermal Decomposition

As a result of thermal decomposition, one would expect

DP, all physical properties, and blue reflectance to decrease.

Carboxylic acid content probably would decrease due to decarboxylation.
16

4. SOME SPECIAL SITUATIONS

4 . 1 Overview

The above discussion is basic to an understanding of

the natural aging of paper and the effects of aging on paper

properties. Several specific situations related to the

aging of pulp and paper deserve special attention, such as

catalytic action of metals, generation of acid from unsuspected sources, and the potential of air pollutants for

causing damage to paper. Although these examples of degradation are generated mostly by environmental factors, papers

that are damaged most extensively have the seeds of their

own destruction built into them.

4 . 2 Metals as Catalysts

Although the deleterious effect of sulfur dioxide on

paper has been long recognized [52] , Langwell apparently

was the first to show that the oxidation of sulfur dioxide

was catalyzed by metallic impurities such as manganese and

iron in paper, to sulfur trioxide [27] . Hudson and Milner

[53] were able to detect the pickup of sulfur dioxide within

two hours using radioactive sulfur as a tracer. Hudson [54]

,

Smith [55], and Scribner [56] have shown that books stored

in libraries in urban areas are more acid than books stored

in rural areas. Pickup of sulfur is especially noticeable

at the edges of pages in books, and Langwell [27] showed that

the sulfate content was higher at the edges. This was noted

earlier by Jarrell [57] . The effect of certain metals that promote crosslinking

already has been mentioned.

Several investigators [58, 59, 60] have shown that metals

act as catalysts in brightness reversion.
17

4.3 Effect of Chlorinated Extractives

Bergman et al [61] have shown that removal of extractives

car reduce color reversion. It also has been shown that

chlorinated lignin [62] and chlorinated extractives [63]

cause serious brightness reversion during accelerated aging.

Chlorinated extractives release hydrochloric acid at comparatively low temperatures and cause brightness reversion of

pulp during storage.

4 . 4 Migration of Impurities

Barrow [64] has reported on the migration of various

impurities in paper. He showed that acid could migrate from

one sheet to another and appreciably decrease the folding

endurance. Examples of migration of impurities from sheets

containing groundwood to fully bleached papers are quite

common. It is not known whether the migration of colored

impurities affects permanence.

4 . 5 Air Pollutants

The effect of air pollutants on the stability of paper

has received little attention in the laboratory, with the

exception of sulfur dioxide. The problem could be more

serious than is suspected, for urban atmospheres contain

several compounds that could be injurious to paper [65]

The possibility that certain metals may catalyze reactions

of air pollutants with paper, or that some functional groups

may cause paper to be more susceptible, cannot be overlooked.

For various large cities in the U.S., mean values for 1964

for various gaseous pollutants in parts per million are as

follows [66 ]

Range

of Means Maximum

Carbon monoxide 5 -12 6 0

Hydrocarbons 2-3 20

Sulfur dioxide 0.01- 0.18 1.62

Nitric oxide ' 0.03- 0.09 1.35

Nitrogen dioxide 0.03- 0.06 0.79

Total oxidants 0.02- 0.03 0.85
18

Aldehydes may run as high as 1.87, with a normal range

of 0.05-0.60. Ozone may run as high as 0.90 with a normal

range of 0.05-0.65.

Nitrogen dioxide has been found to degrade cotton,

partially acetylated cotton [67] , and cellulose acetate film

[23]. From these examples, it is obvious that air pollutants

other than sulfur dioxide can give trouble.
19

5. RESULTS OF SOME RECENT STUDIES ON

ACCELERATED AGING OF PAPER

Venter [13] conducted an extensive investigation of the

aging of paper. All of his conclusions are germane only to

the samples he used and to his aging conditions, but the

following of his conclusions appear generally pertinent:

(1) Alkali solubility is a very useful test, for changes

in alkali solubility may show up, due to formation of alkalisensitive linkages, before changes in physical properties.

(2) Degradation reactions that occur during the aging

of paper are random and are not "peeling" or "unzipping."

(3) Chain length distribution provides very useful

information, but the test is most time-consuming.

(4) Sharp changes in slopes of plots of degradation

rates against temperature occur at about 70 °C with tear,

fold, and alkali solubility.

(5) Copper number (and, therefore, reducing power)

increases with time of aging.

(6) The presence of rosin does not cause paper to be

less Ftable during aging when change in alkali solubility

is used as a criterion.

Cardwell [14] studied several variables associated with

the aging of handsheets and commercial papers, and some of

his conclusions are as follows:

(1) If a crosslinking reaction occurs during the aging

of paper, paper with a high bonding volume should develop

crosslinks more rapidly during aging. (This probably is the

reason that papers made from highly beaten stock are considered

to be less stable than papers made from mildly beaten stock.)

(2) ". . . more information is needed on the chemical

and physical reactions contributing to loss in paper permanence

over a temperature range before the Arrhenius relationship

can be used with confidence to predict paper permanence."

(3) An increase in interfiber bonding—presumably crosslinking--occurs during aging. (Cardwell suggests that water

bridges may be replaced by direct hydrogen bonding between

cellulose chains. It is more likely that disappearance of

water bridges would "relax" bonding, so crosslinks would be

formed by some other mechanism.

)
20

(4) The three tests for best predicting the stability

of paper are tensile energy absorption, tensile breaking

length, and force to give 300 folds. (On the basis of changes

that occur in the force-elongation curve during aging, the

sensitivity and usefulness of tensile breaking length might

be questioned.

)

(5) Cycling of temperature and relative humidity

increased the rate of degradation over that which occurred

under constant conditions of temperature and relative ) humidity

Neither Browning and Wink [4] nor Venter [13] found an

increase in carboxyl content during accelerated aging. As

any increase in carboxyl probably would be small, and as

oxidized structures may become alkali-sensitive or soluble

due to chain scission, the analytical tools presently available

are not likely to be capable of measuring, or detecting,

changes in carboxyl due to natural or accelerated aging.

Several investigators [4, 13, 23, 15, 68] have used

Arrhenius plots as a tool in the evaluation of accelerated

aging data. This approach has merit only if properlv used within

the limitations of (1) knowledge of the reactions involved

and (2) mathematical treatment of data 169, 70, 71]. A far

more useful approach should be to probe the reactions that

occur during natural aging and devise accelerated aging

conditions that cause these same reactions to occur.
21

6 . SUMMARY

(1) The predominant chemical reactions in accelerated

aaing are hydrolysis, oxidation, and crosslinking . In some

cases, especially waterleaf papers, a decrease in bonding

order may be predominant.

(2) Hydrolysis and oxidation are synergistic.

(3) Hydrolysis in a heterogeneous system is especially

enhanced by an aldehyde on the No. 6 carbon of anhydroglucose

(4) Hydrolysis is catalyzed by acid.

(5) Aluminum catalyzes degradation.

(6) Aluminum, other metals, and acids enhance and also

destroy crosslinking.

(7) Under proper experimental conditions, aldehydes

and carboxyls enhance crosslinking.

(8) Cycling of moisture content degrades physical

properties of paper.

(9) Some degradation reactions decrease moisture regain.

(10) Moisture greatly enhances some types of degradation,

(11) The "fines" in paper contribute significantly to

the degradation of paper.

(12) It is obvious from a review of the literature on

accelerated aging of paper that:

(a) Accelerated aging conditions that would

satisfy all possibilities are impossible to define.

(b) An accelerated aging test does not tell

what a paper will be like after 25 or 50 years of

storage. It only provides information concerning the

ranking of different samples with respect to storage

properties.

(c) Some conclusions can be drawn concerning

aging conditions.
22

(d) Some conclusions can be drawn concerning

useful tests for evaluation of aging.

(e) Published data do not give a coherent picture

of the aging of paper.

(f) Most of the data are of limited utility

because the samples were not adequately defined.

(g) Arrhenius plots must be used with caution in

the evaluation of stability.

(h) The correlation of natural aging with accelerated aging is never likely to be perfect, or even high,

as different natural aging conditions produce different

results [55 , 72]

(i) Extensive mathematical treatment of data,

and predictions of stability based on these treatments,

are not likely to be very fruitful.
23

7. RECOMMENDATIONS

(1) The following tests should be useful in determining

what happens during the aging of paper:

(a) Zero-span tensile (fiber strength)

(b) Wet tensile (crosslinking)

(c) pH (generation of acid)

(d) Alkali solubility (some chain scission and

alkali sensitivity)

(e) Functional group content (oxidation)

(f) Molecular chain length distribution (chain

scission and randomness of chain scission)

(g) Peroxide formation (probably oxidation, but

peroxides apparently can be formed in cellulose in the

absence of air)

(h) Oxygen sorption (oxidation)

(2) The following tests should be useful for detecting

changes during the aging of paper, but are not particularly

useful for identifying the nature of the change:

(a) Folding endurance

(b) Tearing strength

(c) Elongation

(d) Tensile energy absorption

(e) Alkali solubility

(f) Copper number

(g) Viscosity

Alkali solubility appears under both (1) and (2) . Without additional information, it belongs under (2); with information on functional group content, change in zero-span

tensile, viscosity, and peroxide content, it can be evaluated

properly and belongs under (1)
24

(3) Other tests that may be somewhat less useful are:

(a) Tensile strength

(b) Burst

(4) The following general conditions for accelerated

aging should be evaluated:

(a) Low temperature (50°-70°C)

(b) Cycling of relative humidity, probably between

0 percent and 70 percent on hourly cycles.

(c) Oxygen atmosphere

(5) General guidelines for the composition of permanent

record paper (not for specification purposes) are as follows:

(a) Low functional group content (aldehyde,

carboxyl, ketone)

(b) Alkaline filler, or pH in the neutral range

(c) Stable sizing material

(d) Large percentage of long fibers (2-3 mm)

(e) Low percentage of fines

:' (f) No lignin

(g) No aluminum salts during manufacture

(6) Recommendations for future work:

(a) Examine papers that have been in storage for

some time and attempt to identify the changes that have

taken place during storage.

(b) Devise one or more accelerated aging methods

that cause these same changes to occur.

Work is in progress on both of these tasks.
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,

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26

13. Venter, J.S.M., The Chemistry and Mechanism of Paper

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York State College of Forestry (1973)

15. Gray, G. G. , An Accelerated Aging Study Comparing Kinetic

Rates vs. TAPPI Standard 453, Tappi 52 (2): 325 (1969).

16. Parks, E. J. and Hebert, R. L. , Thermal Analysis of Ion

Exchange Reaction Products of Wood Pulps with Calcium

and Aluminum Cations, Tappi 55 (10): 1510 (1972).

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18. High Polymers, Vol. V, Cellulose and Cellulose Derivatives,

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19. Ott, E., Spurlin, H. M. , and Grafflin, M. W. , Cellulose

and Cellulose Derivatives, second edition. Part 1, Vol. V

of High Polymers, Interscience (1954).

20. Rydholm, S. A., Pulping Processes , Interscience, New

York (1965).

21. Wolfrom, M. L. and Thompson, A. in w. w. Pigman, editor.

The Carbohydrates, Academic Press, New York, 1957, p. 208.

22. Waller, R. C, Bass, K. C. , and Roseveare, W. E., Degradation of Rayon Tire Yarn at Elevated Temperatures, Ind . Eng. Chem. 40 : 138 (1948) .

23. Wilson, W. K. and Forshee , B. W. , Degradation of Cellulose

Acetate Films, SPE Journal 15 , 146 (Feb. 1959).

24. Lindberg, B. , Synthesis and Acid Hydrolysis of PseudoCellobiouronic Acid, Chimie et Biochimie de la Lignine,

de la Cellulose et des He'micelluloses , Actes du Symposium

International de Grenoble (July 1964), p. 303.

25. Petersson, G. , Pettersson, S., and Samuelson, 0., Formation of Glucopyranosylglycolic Acids During the Hydrolysis

of Cellulose, Svensk Papperstidn. 72 (7): 222 (Apr. 15,

1969) .
27

26. Rapson, W. H. and Corbi , J. C, Colour Reversion Caused

by Degradation Products of Oxidized Cellulose, Pulp and

Paper Magazine of Canada 65 ; T-459 (Nov. 1964)

27. Langwell, W. H. , The Permanence of Papers, Tech. Section

of the British Paper and Board Makers' Assoc., Inc. 29 : 21 (1952).

28. Kleinert, T. N. and Marraccini, L. M. , Aging and Color

Reversion of Bleached Pulps, Part 2. Influence of Air

and Moisture, Svensk Papperstidn. 66 ; 189 (1963),

29. Marraccini, L. M. and Kleinert, T. N., Aging and Color

Reversion of Bleached Pulps, Part 1. Peroxide Formation

During Aging, Svensk Papperstidn. 65 : 126 (1962)

30. Scott, G., Atmospheric Oxidation and Antioxidants,

Elsevier (1965) .

31. Wilson, W. K. and Padgett, A. A., Reaction of Sodium

Chlorite with Some Aldoses and Modified Celluloses, Tappi

38_ (5) : 292 (1955) .

32. Casey, J. P., Pulp and Paper, second edition. Vol. II,

Interscience (1960)

33. Back, E. L. , Thermal Auto-Crosslinking in Cellulose

Material, Pulp and Paper Magazine of Canada 68 ; T165

(April 196Trr

34. '^uffini, G., Improvement in Bonding of Wood Pulps by the

Presence of Acidic Groups, Svensk Papperstidn. 69 (3):

72 (Feb. 15, 1966).

35. Cohen, W. E., Stamm, A. J., and Fahey, D. J., Dimensional

Stabilization of Paper by Catalyzed Heat Treatment, Tappi

£2 (11) : 904 (1959) .

36. Szmant, H. H. , Organic Chemistry , Prentice-Hall (1957),

p. 241.

37. Goring, D.A.I, and Suranyi, G. , Improved Bonding of

Cellulose and Other Polymers by Surface Treatment with

a Gas, Pulp and Paper Magazine of Canada , Convention

Issue, 149 (1969)

38. Howsmon, J. A., Water Sorption and the Poly-phase Structure

of Cellulose Fibers, Textile Research J. 19: 152 (1949).
28

39. Caulfield, D. F. and Steffes, R, A., Water-Induced

Recrystallization of Cellulose, Tappi 52 (7): 1361 (1969).

40. Parks, E. J., Graminski, E. L. , and Wilson, W. K.

,

unpublished data.

41. Britt, K. W. , Handbook of Pulp and Paper Technology , Van Nostrand Reinhold, second edition (1970), p. 30.

42. McKee, R. C, Effect of Repulping on Sheet Properties

and Fiber Characteristics, Paper Trade J. 155: 34

(May 24, 1971).

43. Kuniak, L. , Alince, B. , Masura, V, and Alfoldi, J.,

Changes in the Fine Structure of Cellulose Upon Oxidation, Svensk Papperstidn. 72 (7): 205 (April 15, 1969).

44. Schwenker, R. F., Jr. and Beck, L. R. , Jr., Study of

the Pyrolytic Decomposition of Cellulose by Gas Chromatography, J. Poly. Science (2): 331-34 (1963), Part C,

Polymer Symposia, Proceedings of the Fourth Cellulose

Conference

45. D 1716, ASTM Standard Method of Test for Cellulose Chain

Length Uniformity by Fractional Precipitation of Cellulose

Nitrate, 1973 Annual Book of Standards, p. 521.

46. Timell, T. , Fractional Precipitation as a Method for

Estinating the Chain-Length Distribution of High-Molecular

Weight Celluloses, Svensk Papperstidn. 58 , 1 (1955).

47. Glasstone, S., Textbook of Physical Chemistry, second

edition. Van Nostrand (1946), p. 584.

48. Wilson, W. K. , Harvey, J. L. , and Padgett, A. A., Effect

of Melamine Resin on Chemical Tests of Paper, Tappi 34

(9) : 410 (1951)

49. Stamm, A. J., Dimensional Stabilization of Paper by

Catalyzed Heat Treatment and Cross-Linking with Formaldehyde, Tappi 42 (1): 44 (1959).

50. Back, E. L. and Klinga, L. O., Reactions in Dimensional

Stabilization of Paper and Fibre Building Board by Heat

Treatment, Svensk Papperstidn. 66 ; 745 (1963)
29

51. Graminski, E. L. , The Effect of Flexing on the Mechanical

Properties of Paper, paper presented at the Fifth Fundamental Research Symposium, Cambridge, England, Sept. 1973.

52. Kimberly, A. E. and Scribner, B. W. , Summary Report of

National Bureau of Standards Research on Preservation of

Records, NBS Miscellaneous Publication M154 (March 16,

1937)

53. Hudson, F. L. and Milner, W. D., The Permanence of Paper;

The Use of Radioactive Sulfur to Study the Pick-U|5 of

Sulfur Dioxide by Paper, Paper Tech . 2\ 155 (1961)

54. Hudson, F. L. , Acidity of 17th and 18th Century Books

in Two Libraries, Paper Tech. 8^: 189 (1967).

55. Smith, R. D. , A Comparison of Paper in Identical Copies

of Books, Restaurator , Supplement 2 (1972).

56. Scribner, B. W. , The Preservation of Records in Libraries,

The Library Quarterly 4; 371 (1934).

57. Jarrell, T. D. , Hankins, J. M. , and Veitch, F. P.,

Deterioration of Book and Record Papers, Technical Bulletin No. 541, USDA (Nov. 1936).

58. Spinner, I. H. , MacKinnon, M. H. , and Lilley, J. W. , The Effects of Iron and Copper on the Brightness and

Brightness Reversion of Oxidized Cellulose, Pulp and

Paper Magazine of Canada , Convention Issue, T114 (1966).

59. Wultsch, Von F. , Limontschew, W. , and Schindler, G.

,

Stabilitat der Weisse gebleichter Zellstoffe, Das

Papier 17; 326 (1963).

60. Spinner, I. H. , Brightness Reversion; A Critical Review

With Sugaestions for Further Research, Tappi 45 (6) : 495

(1962).

61. Bergman, A. J., Hartler, N. , and Stockman, L. , Effect of

Extractives on the Yellowing of Pulps, Svensk Papperstidn . 6j5 (15) : 547 (1963)

62. Howard, E. J. and Histed, J. A., Pulp Brightness Reversion;

Influence of Residual Lignin on the Brightness Reversion

of Bleached Sulfite and Kraft Pulps, Tappi 47 (11) : 653

(1964).
30

Croon, I. and Olsson, J., Some Findings on the Influence

of the Extract on Pulp Ageing, Svensk Papperstidn. 67

(21): 850 (1964).

Barrow, W. J. , Migration of Impurities in Paper,

Archivum _3' 105 (1953) . Wessel, C. J., Environmental Factors Affecting the Permanence of Library Materials, The Library Quarterly 40 ; 39 (1970).

Stern, A. C, Air Pollution , Vol. 1, second edition,

pp. 32, 33, Academic Press, New York (1968).

Schreiber, W. T. , Bullock, A. L., and Ward, W. L.

,

Resistance of Partially Acetylated Cotton Fabric to

Nitrogen Dioxide and Hydrogen Chloride, Textile Res. J . 24: 819 (1954).

Stamm, A. J., Thermal Degradation of Wood and Cellulose,

Ind. Eng. Chem . 48^: 413 (1956).

Arni, H. T. , The Significance of the Correlation Coefficient for Analyzing Engineering Data, Materials Res . and Stds . 11 (5): 16 (1971).

Zelen, M. , Factorial Experiments in Life Testing,

Technometrics 1 (3) : 269 (Aug. 1959)

Hahn, G. J., Don't Let Statistical Significance Fool You,

Chemtech, p. 16 (Jan. 1974)

Hudson, F. L. and Edwards, C. J., Some Direct Observations

on the Aging of Paper, Paper Tech. 1_: 27 (1966) .
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.
Figure 3. Typical force-elongation curves for aqed and

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FORCE-ELONGATION

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ELONGATION -%

NBS-1 14A iREv. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-4992. Gov't Accession

No.

3. Recipient's Acces.sion No.

4. TITLE AND SUBTITLE

An Analysis of the Aging of Paper: Possible

Reactions and Their Effects on Measurable

Properties

5. Publication Date Anril L974

6. Performing Organization Code

7. AUTHOR'S)

W. K. Wilson and E. J. Parks

8. Performing Organ. Report No. NBSIR. 74-499

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3000442

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Archives and Records Service

Washington, D.C. 20408

13. Type of Report & Period Covered

Interim

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. It document includes a significant

bibliography or literature survey, mention it here.)

Various reactions that cellulose can undergo are reviewed

in relation to their bearing on the aging of paper. The principal

reactions of cellulose are hydrolysis, oxidation, crosslinkina,

change in lateral order in the bonding area, and, during accelerated aging, thermal decomposition. The effects these various

reactions might have on tests that are available for evaluating

changes that occur during the aging of paper are reviewed. Some

special examples of these reactions that occur during natural

aging, because of special composition characteristics of the

paper, are discussed. Suggestions are made concerning most

sensitive tests for use in detecting changes in paper and tests,

regardless of sensitivity, available for determining what happens

during aging. Some general guidelines regarding specifications

for permanent record papers are discussed.

17. KEY ^'ORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name: separated by semicolons)

Accelerated aging; aging; aging of cellulose; aging of paper; cellulose;

cellulose aging; natural aging; paper; paper aging; tests for' paper.

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