Holik H. (ed.) Handbook of Paper and Board

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stages move the sample under the sensor and the sensor passes the captured

height data to the computer for evaluation.

Another surface property of paper is the abrasion resistance. The mechanical

abrasion resistance of surfaces is determined in the friction wheel process (DIN 53

109–93). In this process, the amount of abrasion which is obtained by abrading the

conditioned or wet sample with an abrasion wheel of defined quality under defined

conditions is measured.

12.7

Optical Properties

An object, e.g., a paper surface, is termed white when the illumination intensity

and the absorption capacity of the surface are independent of the wavelength.

Deviations confer a more or less pronounced color shade on the surface.

In the paper industry, a special process is used to characterize the brightness

because this is one of the most important optical properties of paper. The determi-

nation of the reflectance factor (ISO brightness) is based on ISO 2470 (1999). For

this test, a filter is used which has an intensity maximum at a wavelength of

457 nm. The reflectance (blue component) measured in a reflectometer under

specified conditions is known as brightness. It is expressed as a percentage of the

brightness of a white standard.

Another optical property of paper is its transparency, i. e., a measure of its light

transmittance. It is calculated from the reflectance factors R

, R

, and R

(w)

, which

are determined in accordance with DIN 53 147 (1993). The reflectance factor of the

individual sheet on a completely black background is R

, R

is the reflectance

factor of the individual sheet on a white background, and R

(w)

is the reflectance

factor of the white base.

Most white papers and paperboards, and therefore also secondary fiber materi-

als, currently contain an optical brightener. Brightness measurements for such

materials depend on the relative proportion of UV radiation in the illuminant used

for the determination. The standard test methods for ISO brightness have not

defined the standard illuminant for use in the determination. The relative amount

of UV has also not been defined. As a result, widely different R457 reflectance

factors exist for the same kind of fluorescent material. The problem has recently

been solved. A revised ISO method, ISO 2470, states that the UV radiation of the

illumination must correspond to the relative amount of UV in the standard illumi-

nant C when measuring fluorescent objects.

The opacity is a measure of light-tightness. It is defined in ISO 2471 (1998) as the

ratio of the reflectance factor R

to the reflectance factor R

. Both reflectance

factors are determined in accordance to DIN 53 145 (2000). R

is measured as the

reflectance factor of an individual sheet on a completely black background and R

as the reflectance factor of an “infinitely” thick stack of the same paper.

12 Testing of Paper and Board472

12.8

Printing Properties

The printing properties of paper result from complex interactions between print-

ing ink, printing process, and paper. Practice-oriented printability tests must be

performed to evaluate these properties. Test printers are also suitable for this pur-

pose. Instruments for offset printing, gravure printing, and flexographic printing

can be used with standard printing inks under laboratory conditions to test the dry

pick resistance or the wet pick resistance of papers. Missing dots, mottling and ink

penetration properties can also be tested. Conversely, the behavior of various print-

ing inks towards standard papers can also be evaluated.

Using an image analyzer for testing printed image and printing process has

increased during the last decade. This is due to the improved capability of the

analyzers and because the importance of the quality of the printed image has

increased. The image analyzer and its use in paper testing has already been men-

tioned earlier in connection with paper tests. The image analyzer is used to meas-

ure the uniformity of paper (print mottle, number of missing dots, and properties

of dot), surface properties of paper (fiber rising, picking, and contact angle meas-

urement), and many other properties of paper, i. e., width of cracking at the fold.

Furthermore, defined proof copies produced with the test printers can be used to

test full ink coverage, color density, color gloss, shade, abrasion resistance, stack-

ing ability, and contact yellowing.

More reliable results obviously come from tests that closely simulate the actual

printing process. That is why tests developed for full-scale printing machines give

the best predictions of the actual printability properties of a paper [5].

12.9

Behavior towards Liquids

The behavior of liquids towards paper is characterized by the processes of wetting

and penetration. In both cases, the characteristic physical property is the surface

tension. This value can be measured directly and tensiometrically in the case of

liquids and indirectly, via the contact angle of test liquid droplets, in the case of

solids such as paper. A liquid wets the surface of paper only if its surface tension is

lower than that of the paper. The same holds for the wetting of the capillary walls

upon penetration of liquids into the capillaries of the paper.

If the wetting and the penetrating capacity of liquids are to be changed, the surface

tension of the paper must also be changed. This is achieved, for instance, by sizing

the paper, a process which must fulfill the requirements regarding printability with

aqueous inks. According to DIN 53 126 (2001), paper is considered to be printable

if a standard ink line drawn with an adjusted drawing pen has neither run nor

penetrated into the paper after 24 h.

The water absorption W

(Cobb) (ISO 535, 1991) refers to the amount of water

that is absorbed by a certain area of paper on one-sided contact for a specified

12.9 Behavior towards Liquids 473

exposure time. The time of exposure to water must be chosen such that a sufficient

amount of water enters into the fiber matrix, but does not penetrate to the opposite

side of the sample.

In the determination of grease permeability (DIN 53 116), red-colored palm kernel

oil is used as the testing agent. The passage of fat through the sample under

specified conditions is then evaluated.

12.10

Exclusion of Gases and Vapors

As a rule, papers have only a limited ability to exclude gases and vapors. Of partic-

ular importance are the air permeability (e.g., for filtration properties) and the

water-vapor permeability.

There are two standardized test methods available for the determination of the

mean air permeability: the Bendtsen method (ISO 5636-3, 1992) and the Schopper

method (ISO 5636-2, 1984).

A gravimetric method for the determination of water-vapor permeability is de-

scribed in ISO 12572 (2001). This method is suitable for building materials. For

foils, laminated paper and board also DIN 53 122-1 (2001) is recommended.

References

1 R. Korn, F. Burgstaller, Papier- und Zell-

stoffprüfung, 2nd edn., Springer-Verlag,

Berlin 1953.

2 K. Frank, Taschenbuch der Papierprüfung,

Eduard Roether Verlag, Darmstadt 1958.

3 R. E. Marks (ed.), Handbook of Physical and

Mechanical Testing of Paper and Paperboard,

Marcel Dekker, New York 1983.

4 J.-E. Levlin, L. Söderhjelm, Pulp and Paper

Testing, Vol. 17, ISBN 952-5216-17-9 Fapet

Oy, Helsinki 1999.

5 A. Mackay and A. Wright, Correlation be-

tween laboratory printability testing and com-

mercial printing, 50

Appita Annual General

Conference, APPITA, Rotura, 1996, p. 405.

References

474

Book and Paper Preservation

Manfred Anders

13.1

Introduction

Most modern paper lacks the durability and permanence that has characterized its

use since its earliest application in recording the history of man and his civiliza-

tion. Although fundamental papermaking techniques have changed little in nearly

2000 years, two innovations in the 19th century, namely (i) the use of alum as a

component of size, and (ii) the replacement of cotton fiber by wood pulp, are

responsible for this predicament.

There has been an unbelievable development in the production of documents

since the invention of printing with movable letters by Gutenberg. Within a short

period of time, the necessary fibers for paper production could no longer be

supplied by rags. After an intense search wood was discovered as an alterna-

tive. Ground wood pulp was used and later also chemical pulp, whereas in the

latter the lignin is removed in a chemical process and so a “woodfree” pulp is

produced.

The beginning of industrial paper manufacture dates from the middle of the

19th century, and with the development of paper machines, a new sizing method

which could be integrated into the mechanical process became necessary. The

sizing of paper is necessary for its writing property, otherwise it would act like

blotting paper. The “sizing in the mass”, wherein rosin is precipitated onto the

fibers by means of acid-forming alum or aluminum sulfate in the pulp, was part of

the mechanical paper manufacture until the 1980s. Today, a pH neutral and syn-

thetic sizing agent is used. Papermaker’s alum easily forms sulfuric acid on expo-

sure to water in air; in turn this acid easily catalyzes the depolymerization of

cellulose. Further, cellulose from wood pulp is more susceptible to this attack than

linen- or cotton-derived fiber. Today it is accepted that the inherent acidity of such

papers accounts for 85–90% of the destruction in book papers, resulting in a half-

life for fold endurance of an estimated 7.5 years [1].

William Barrow was one of the first to describe the problem and correctly iden-

tify the cause at the Virginia State Library [2] beginning in the 1930s. Prior to his

work, book and paper conservators relied as much on art as on science. While

Handbook of Paper and Board. H. Holik (Ed.)

ISBN: 3-527-30997-7

475

highly skilled, these professionals often selected materials and treatment without

adequate consideration of the long-term consequences.

The discovery and the use of wood pulp (1840 by Keller) and acid sizing, mostly

with alum or aluminum sulfate (1807 by Illig), are the main reasons for the cur-

rent problems of deterioration of books in libraries und archives. Even at that time

it was known that acid-sized wood pulp paper was not as stable against aging as

traditionally made rag paper. Today the books most threatened by deterioration are

those that were made in the last 150 years and not, as many people may assume,

the older works from centuries ago.

There has been a fundamental change in papermaking in recent years. The

development of synthetic sizing enables the industry to produce paper with neutral

to slightly alkaline pH. With calcium carbonate as a filler, this paper has a suffi-

cient alkaline reserve against acid contaminants from the environment. The paper

has a better aging permanence than previous modern papers.

The main cause for paper deterioration in libraries and archives is the acid

catalyzed hydrolysis of cellulose, the ingredient that gives permanence to the pa-

per. The aging permanence of paper is closely connected with the acid concentra-

tion in the paper. The priority in preservation measures is in most cases the deaci-

dification of acid paper. Today, more than 70 years after Barrow’s pioneering work,

mass deacidification of books and archive materials has become a commercial

reality.

13.2

Mechanisms of Paper Deterioration (Fig. 13.1)

13.2.1

Paper Deterioration by Aging

The aging mechanisms can be divided into biological, physical and chemical proc-

esses. While using and storing paper appropriately, only the chemical processes

are decisive. Paper nearly universally involves cellulose, although its source and

methods of processing have evolved steadily throughout the centuries. As produc-

tion costs dropped significantly, the application of paper dramatically expanded so

that its traditional applications, particularly its use in books and official records,

were dwarfed by other functions. Such widespread use of paper reduced the em-

phasis on permanence.

The sizing of paper continues to be the predominant cause of paper deteriora-

tion. In the early 1800s, about the time of Robert’s invention of the fourdrinier

machine for continuous paper manufacturing, gelatin sizing was abandoned in

favor of alum or alum-rosin blends. Papermaker’s alum, Al

(SO

)

· 18 H

O, is

easily hydrolyzed by water to form sulfuric acid. Acid catalyzed hydrolytic attacks

cause degradation of cellulose by random scission of the hemiacetal links, leading

to breakage of chain linkages (Fig. 13.2). Even a few scissions per molecule cause a

substantial loss in physical properties [3]. In 1851, the discovery of the soda process

13 Book and Paper Preservation476

for separating cellulose from wood added a further dimension to the problem

since paper derived from wood pulp is more susceptible to acid attacks than the

cotton cellulose obtained from linen and rags.

Fig. 13.1 The main decomposition reactions of paper fibers.

Fig. 13.2 Acid catalyzed hydrolysis of cellulose.

13.2 Mechanisms of Paper Deterioration

477

The lack of paper permanence is attributable to both internal and external influ-

ences [4]. In addition to sizing materials, internal factors include the type of fiber,

coatings, and the presence of acidic and metallic compounds. External variables

are the conditions during storage and use. Heat and humidity accelerate the deteri-

oration of paper, and atmospheric pollution is frequently the source of external

acid attacks. A comparison of identical volumes stored in the New York Public

Library and the Royal Library in The Hague proved the importance of proper

storage, as the New York books were found to be in a far more advanced stage of

deterioration [5].

It was not until 1926, when Gösta Hall from Sweden developed a satisfactory

method for accelerated aging [6], that researchers had the means to simulate the

effects of the natural aging process. Although Edwin Sutermeister of the S. D.

Warren Company recognized as early as 1901 that stable paper should contain an

alkaline filler, it was left to William Barrow of the Virginia State Library to pull the

disparate elements of early researchers together in a scientific analysis of the prob-

lem [7].

As acid catalyzed hydrolysis of cellulose is responsible for up to 90% of paper’s

loss of permanence, researchers have concentrated on deacidification as the pri-

mary solution to the impermanence of modern papers. Paper deacidification in-

volves introduction of a strong base to convert acid species to their corresponding

neutral salts, and the establishment of a neutral buffer to protect the paper against

future acidcatalyzed damage. The first recorded attempt to neutralize paper was

by Arthur Church at the Victoria and Albert Museum in London who, in 1891,

advocated the immersion of paper in a methanolic solution of barium hydroxide

[8].

Subsequent attempts to develop effective deacidification focused on aqueous

techniques which offered the additional benefit of modest strengthening through

re-establishment of hydrogen bonding. Otto Schierholz, working for the Ontario

Research Foundation in Toronto, secured a patent in 1936 for his work which

involved dipping paper in a solution of alkaline earth bicarbonates [9]. In 1943

Barrow described a two-step process, involving immersion in a calcium hydroxide

bath and subsequent treatment with calcium bicarbonate. Commentators differ as

to whether Barrow was aware of the earlier Canadian work. Further refinement of

the technique included the 1957 development by James Gear et al. at the United

States National Archives of a one-step process involving magnesium bicarbonate,

and the use of the more soluble magnesium hydroxide by scientists at the Library

of Congress in 1978. While effective and proven, these techniques involve pains-

taking care as they are applied to single sheets.

13.2.2

Oxidative Deterioration Processes

Although the above mentioned acid catalyzed hydrolysis is the main cause for the

deterioration of cellulose and the connected loss of permanence, the influence of

oxidation cannot be neglected. The oxidative damage is much more complex in its

13 Book and Paper Preservation478

various chemical reactions, synergistic effects and secondary reactions and not at

all completely researched.

Cellulose can be oxidized not only by various oxidants like pollution gases from

the air (ozone, NO

, etc.) and bleaching agents in the wood pulp production, but

also by atmospheric oxygen. In the oxidation by atmospheric oxygen an additional

activation or catalysis is needed.

Acids as well as bases have a catalyzing effect. The catalysis by heavy metal ions

is really effective. A well-known example of this mechanism is the ink corrosion

which leads to a total destruction of paper in the area of ink due to the excess of

iron ions in the ferro-gallic ink and the acid. Some of the most valuable manu-

scripts are affected (Fig.13.3 and 13.4). Temperature and electromagnetic radia-

tion, especially UV rays, have an activating effect.

Fig. 13.3 Approximately 200 years old manuscript attacked by

ink corrosion.

Fig. 13.4 Approximately 200 years old manuscript attacked by

ink corrosion.

13.2 Mechanisms of Paper Deterioration

479

The observed reactions are often summarized under the term “autoxidation” as

reaction products of such oxidation processes: organic acids like ethanoic acid,

formic acid, oxalic acid etc., cause a further acidification of the paper. Due to these

oxidation processes the acid catalyzed hydrolytic deterioration of cellulose is ac-

celerated.

13.2.3

Alterations due to Paper Aging

13.2.3.1 Yellowing

Paper ages by becoming yellow and brittle. An intensification of the yellowing is

observed with continuous aging. There are various reasons for the yellowing of

paper. Many of the reaction products of the aging processes, such as oxidation

products, condensed furan derivates etc. are dark colored. More and more of these

colored reaction products are produced with continuous aging. In wood-contain-

ing, i.e. lignin-containing papers, the yellowing is extremely intense.

13.2.3.2 Embrittlement of Paper

The alterations in paper that result from aging can be seen most obviously on the

basis of the angles of cracks after a strain-to-crack measurement. The ratio be-

tween individual fiber strength and the bonding strength between the fibers

changes during the aging of paper. The strength of individual fibers decreases due

Fig. 13.5 Scanning electron microscope pictures of the angles

of cracks of non-aged (a) and aged (b) paper after a strain-to-

stress measurement.

13 Book and Paper Preservation

480

to the shortening of cellulose molecules as the result of deterioration mechanisms

in the course of aging. On the other hand the inter-fiber bonding is strengthened

by linking reactions and salt bonds: the fibers are linked one to the other inflexibly

and strongly. The fiber-fiber bonds are developed on the fiber surface so that the

structure of their surface, where an increasing number of reactive groups are

developed by oxidation processes, is decisive. As a consequence of aging inflexible

bonds between fibers are created. In the case of a mechanical strain the attacking

forces cannot be spread to nearby fibers because the fibers are linked inflexibly.

During a mechanical strain, like in tearing, in an aged paper the deformation

energy is spread less in the paper but concentrates on individual fibers that are

already weakened and then break.

If new paper is torn the fibers slide apart without breaking. This can be shown

with pictures from a scanning electron microscope of the angles of cracks of aged

and non-aged paper after a strain-to-stress measurement (Fig. 13.5). The fibers of

the new paper only slid apart whereas in the aged paper they broke. This means

that the fiber-fiber bonds in new paper and the individual fibers in aged paper are

the mechanically weaker points.

13.3

Development of Mass Deacidification Processes

13.3.1

Overview

There are many statements in literature about different deacidification systems

and agents, its advantages, use and also comparative examinations [10–16]. In the

treatment a long-term acid protection (exogenous influences) is to be achieved and

therefore an additional alkaline buffer is deposited in the paper [17–23]. In the

following the most important methods are introduced.

On the one hand the deacidification systems can be divided into single sheet

methods and mass deacidification methods. On the other hand the methods can differ

in the polarity of the solvents used. Polar solvents have a swelling effect on cellu-

lose, or rather paper, and can dissolve some of the colors and inks and therefore

mostly nonpolar solvents are used in mass deacidification systems. Single sheet

methods with water as a treatment medium are the classical methods.

Mass preservation is designed to meet the needs of national and major research

libraries and archive groups. With a mean collection deterioration rate of 4.7% [24]

and limited resources, a treatment based on painstaking manual work is incon-

sistent with the need. Further, greater emphasis has been placed on nonaqueous

and gas-phase deacidification techniques to compensate for some of the intrinsic

limitations of aqueous methods.

Nonaqueous deacidification systems contain a deacidification agent and an or-

ganic solvent that serves as the carrier. The solvents offer the advantage of rapid

penetration and drying. The latter is particularly important in minimizing the

13.3 Development of Mass Deacidification Processes 481