Permanence and AlkalineNeutral Papermaking - ACS Publications

Chapter 1

Permanence and Alkaline—Neutral Papermaking D. J. Priest

Downloaded by UNIV OF NEWCASTLE on February 24, 2017 | http://pubs.acs.org Publication Date: September 28, 1989 | doi: 10.1021/bk-1989-0410.ch001

Department of Paper Science, University of Manchester Institute of Science and Technology, Manchester, P.O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom

The papermaking process i s increasingly being modified so that the sheet i s formed i n a neutral or alkaline aqueous environment, rather than i n an acidic one. Paper made i n this way i s normally longer lasting because acid hydrolysis of the cellulose can no longer occur. However, the reasons for introducing the modified process are largely economic, and the product may not necessarily meet specifications for permanence and durability. This review describes the technicalities of the economic advantages (including easier fibre refining, increased filler content, the use of calcium carbonate fillers, and the availability of cost-efficient neutral sizes), the factors involved i n making a change to neutral/alkaline papermaking, and how all this impinges on producing a satisfactory permanent paper. Paper i s e s s e n t i a l l y a bonded mat or f e l t of r e l a t i v e l y small f i b r e s to which can be added, i f required, f i l l e r s , wet strengtheners, coatings and so on. Although a paper-like material can be produced from many d i f f e r e n t polymeric f i b r e s , paper i t s e l f i s nearly always made using f i b r e s from natural sources, usually, but not exclusively of course, from wood. These natural f i b r e s a l l comprise polysaccharides of one sort or another, predominantly c e l l u l o s e , which are very hydrophilic because they contain many accessible hydroxyl groups. The e s s e n t i a l adhesion between f i b r e s i s a consequence o f hydrogen bonds formed through these hydroxyl groups, as i s the s e n s i t i v i t y o f unmodified paper to d i s i n t e g r a t i o n when wetted by water. In many of i t s uses, paper needs to have resistance to penetration by aqueous f l u i d s such as w r i t i n g inks or the damping solutions used i n lithographic p r i n t i n g . The treatment given to the surfaces of f i b r e s to make them hydrophobic, which i s usually done as the sheet i s being formed, pressed and dried on the papermaking machine, i s known as " i n t e r n a l s i z i n g " , to d i s t i n g u i s h i t from "surface s i z e " applied on a s i z e press part way down the drying section of the machine. 0097-6156/89A)410-0002$06.00A) o 1989 American Chemical Society

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


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Alkaline-Neutral Papermaking

3

Since the early days of machine made paper i n the f i r s t h a l f of the nineteenth century, the most widely applied method of i n t e r n a l s i z i n g has been the use of n a t u r a l l y occurring resinous materials ("rosins") i n conjunction with an aluminium s a l t , usually aluminium sulphate (called "alum" by papermakers). Various forms of r o s i n sizes (rosin soaps, r o s i n emulsions, f o r t i f i e d rosins) have been developed over the years to improve the process, but these variants s t i l l involve the use of alum as a means of ensuring that f i b r e s r e t a i n a layer of size. Aluminium sulphate hydrolyses i n aqueous solution to y i e l d complex hydrated aluminium ions plus hydroxonium ions (Jj 2), and hence a low pH. Papers made using alum/rosin s i z i n g are often said to be " a c i d i c " , although t h i s i s rather imprecise terminology. A complete d e f i n i t i o n , following the related TAPPI standard method (3), i s that paper a c i d i t y i s the extent to which water-soluble materials i n the paper a l t e r the hydrogen-hydroxyl ion equilibrium of pure water causing an excess of hydrogen ions as measured by a commercial pH meter under s p e c i f i e d conditions. The important point i s that the c e l l u l o s e i n these alum/rosin sized papers i s susceptible to acid h y d r o l y s i s , which r e s u l t s i n a lowering of the degree of polymerisation and, eventually, to a serious reduction i n the strength of f i b r e s and to complete embrittlement of the paper. Some recent work i n the w r i t e r ' s laboratory suggests that when alum/rosin papers are made, the hydroxonium ions which lead to the degradation are adsorbed independently of aluminium i o n i c species W. In recent years, increasing attention i s being paid by the paper industry to systems i n which s i z i n g i s accomplished without the need to have the wet end of the machine running at a c i d i c pH values. In these newer systems the pH may be around the neutral point, or be s l i g h t l y a l k a l i n e due usually to the use of calcium carbonate f i l l e r (see below), so they are known as " n e u t r a l / a l k a l i n e " . Papers made i n t h i s way do not y i e l d a c i d i c aqueous extracts and hence degrade more slowly (5, 6). C l e a r l y , t h i s i s of great s i g n i f i c a n c e to those concerned with ensuring that important books and a r c h i v a l documents use paper expected to have a long l i f e , and which w i l l not lead i n 30-150 years time to the enormous problems now being experienced i n l i b r a r i e s and archives with paper made 30-150 years ago (7). However, i t must be recognised that the reasons f o r introducing neutral/alkaline papermaking were not p r i m a r i l y associated with permanence; papers made i n t h i s way do not necessarily meet a l l the requirements f o r permanence and d u r a b i l i t y . Also, the alum/rosin a c i d i c s i z i n g method has been such a dominant force i n papermaking that many other features of the process have been designed around i t and adapted to i t ; the often used term "alum/rosin s i z i n g system" i s e n t i r e l y appropriate. Making the change to n e u t r a l / a l k a l i n e papermaking nearly always involves, as we s h a l l see, much more than throwing a switch or opening a valve. In a previous publication i n t h i s series (8), Hagemeyer set a l k a l i n e papermaking i n the context of future demand f o r paper, and dealt b r i e f l y with some of the technical consequences. Since then, more m i l l s have converted to the new method, and the aim of t h i s chapter i s to inform the reader i n some d e t a i l about the reasons f o r changing to n e u t r a l / a l k a l i n e papermaking, some of the consequences f o r the production and properties of paper, and how the change impinges on


4

HISTORIC TEXTILE AND PAPER MATERIALS U


permanence and d u r a b i l i t y . I t i s important for those concerned with conservation and permanence to be able to communicate with papermakers and others with an awareness of relevant problems. Where possible, l i t e r a t u r e i s c i t e d , but a complete review i s not intended, and some of the comments a r i s e from the w r i t e r ' s past involvement i n some of the i n d u s t r i a l aspects of n e u t r a l / a l k a l i n e papermaking. REASONS FOR CHANGING TO NEUTRAL/ALKALINE PAPERMAKING. As i n most i n d u s t r i a l change, the c h i e f Incentive i s economic, and we need to look at ways i n which the n e u t r a l / a l k a l i n e process gives r i s e to savings i n the cost of production. Four main areas are involved: the f i b r e f u r n i s h , mineral f i l l e r s , the s i z i n g system and the papermaking process i t s e l f . Although f o r convenience these w i l l be discussed i n turn, i t should be noted at the outset that there i s a great deal of i n t e r a c t i o n between the various aspects. FIBRE FURNISH. I t i s w e l l established (9) that when f i b r e s are beaten or r e f i n e d at a neutral or s l i g h t l y a l k a l i n e pH the e f f i c i e n c y of the process i s greater than at the a c i d i c pH of around 4.5 common i n alum/rosin systems. (When running an alum/rosin system i t i s i n e v i t a b l e that much of the stock preparation part of the m i l l operates at low pH because most of the water used i s recycled from the wet-end of the paper machine). The increase i n r e f i n i n g e f f i c i e n c y means, f o r example, that a given l e v e l of strength i n the paper can be obtained f o r a lower expenditure of energy. This i s a major fundamental economic incentive for converting to n e u t r a l / a l k a l i n e papermaking, because large amounts of expensive energy are consued i n r e f i n i n g f i b r e s (_10)* This basic advantage can be exploited i n d i f f e r e n t ways, depending on the p a r t i c u l a r product being made and market requirements (£)• For example: a) The composition of the f i b r e f u r n i s h can be a l t e r e d . The proportion of hardwood pulp might be increased, f o r instance, to give a product with the same strength as before, but with improved formation and opacity. Some cheap, r e l a t i v e l y weak, bleached mechanical pulp might be introduced, or the proportion already used increased, again g i v i n g better uniformity and opacity, and a lower apparent density, but without l o s s of strength. This l a t t e r trend, of course, would not be acceptable i n a permanent grade of paper. b) The p o t e n t i a l l y improved strength can be o f f s e t by increasing the amount of mineral f i l l e r i n the paper, and t h i s i s a common route to f o l l o w , because f i l l e r s are usually much l e s s expensive than the fibrous raw materials they replace, w h i l s t at the same time properties such as brightness and opacity are improved. This important aspect i s discussed more f u l l y i n the next s e c t i o n . c) A product of s i m i l a r composition can be made but simply using l e s s energy i n r e f i n i n g . In f a c t , these three approaches are not mutually exclusive, and a m i l l would need to consider how to combine changes to optimise f i n a n c i a l savings w h i l s t producing a paper acceptable i n q u a l i t y to the p a r t i c u l a r market being served.


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PRIEST

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FILLERS. In addition to being able to use more f i l l e r , a very important feature of running n e u t r a l / a l k a l i n e i s the c a p a b i l i t y of greatly increasing the choice of mineral f i l l e r . This i s because i t becomes e a s i l y possible to use f i l l e r s constituted from calcium carbonate (CaCO-), of which there are many d i f f e r e n t types. In alum/rosin systems, the pH i s low enough f o r chemical reaction with the CaCO~ to occur, producing troublesome evolution of C0 gas, causing f r o t n and foaming and a l t e r i n g the i o n i c c o n s t i t u t i o n and pH of the wet-end c i r c u i t s . 2

CaC0 + 2H0**" — ^ Downloaded by UNIV OF NEWCASTLE on February 24, 2017 | http://pubs.acs.org Publication Date: September 28, 1989 | doi: 10.1021/bk-1989-0410.ch001

3

3

Ca

2 +

+ C0

2

+

3^0

Some attempts have been made i n the past to overcome t h i s d i f f i c u l t y by pre-treating the s l u r r y of carbonate f i l l e r with s p e c i a l starches or water soluble polymers i n order to protect the f i l l e r p a r t i c l e s from a c i d attack f o r long enough to avoid foaming e t c . i f the treated carbonate s l u r r y i s added at a suitable point, the dwell time i n the acid environment i s r e l a t i v e l y short. Although these systems can work w e l l i f properly set up and c o n t r o l l e d , they have not found wide a p p l i c a t i o n , l a r g e l y being superseded by the advent of cost e f f e c t i v e neutral s i z e s , which also avoids the cost of the protecting starch or polymers. However, a p a r a l l e l development i s the a v a i l a b i l i t y of r o s i n s i z e emulsions which are e f f e c t i v e at higher pH's ( i . e . just on the acid s i d e ) , and at least one m i l l i n the UK has been taking t h i s approach to using low additions of alum with carbonate f i l l e r (J2). Once again, the advantage of being able to use carbonate f i l l e r s can be r e a l i s e d i n many d i f f e r e n t ways, depending both on the product and market requirements, and also on the a v a i l a b i l i t y and cost of f i l l e r supplies. Calcium carbonate f i l l e r s are produced either by controlled comminution of n a t u r a l l y occurring materials d i f f e r i n g as widely as chalk, limestone or even marble, or by a chemical process leading to "Precipitated Calcium Carbonates", or PCC»s. Within each type there are a range of products, varying i n p a r t i c l e s i z e and d i s t r i b u t i o n , p a r t i c l e shape, and brightness. D i f f e r e n t materials are produced at d i f f e r e n t locations throughout the world, so a f f e c t i n g detailed l o c a l economics. In Europe, there i s a p l e n t i f u l supply of inexpensive ground chalk f i l l e r , and there i s u s u a l l y an incentive to replace some or a l l of the clay (used i n an acid s i z i n g system) with chalk, and to increase the t o t a l f i l l e r content. However, due regard must be paid to relevant properties of the paper; e.g. large proportions of chalk f i l l e r w i l l increase the o i l a b s o r p t i v i t y of the paper and hence i t s behaviour i n p r i n t i n g processes. Also, although the more e f f i c i e n t a l k a l i n e beating w i l l generally allow r e t e n t i o n of strength at higher f i l l e r l e v e l s , the r e l a t i v e values of d i f f e r e n t types of strength can change, leading to possible d i f f i c u l t i e s i n use. For example, i f burst and t e n s i l e strength remain unaltered, but the paper i s not as s t i f f as before, there i s a danger that sheets w i l l not feed properly i n t o p r i n t i n g machines. In the USA, where there i s not the same supply of cheap ground chalks, i t may be cost e f f e c t i v e to use the more expensive precipitated carbonate, e s p e c i a l l y i f i t can be prepared i n the m i l l , as i s often the case. Through proper c o n t r o l , i t i s possible to make f i n e p a r t i c l e sized uniform products of high brightness, g i v i n g the p o s s i b i l i t y of


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HISTORIC TEXTILE AND PAPER MATERIALS II

replacement, at l e a s t i n part, of very expensive s p e c i a l i t y f i l t e r s such as titanium dioxide (.1^3) • Use of PCC can sometimes also be j u s t i f i e d , even where supplies of cheap chalk are a v a i l a b l e , when making products i n v o l v i n g the use of T10 * In complete contrast, the choice might be a lower brightness coarser f i l l e r where the main aim i s cheapening a product without much a f f e c t i n g i t s o p t i c a l properties, i . e . employing carbonate only as a filler. Using appropriate techniques, and f o r suitable products, i t i s now possible to make s a t i s f a c t o r y papers containing 25-30J w/w of chalk f i l l e r , although 15-20* i n general n e u t r a l / a l k a l i n e p r i n t i n g and w r i t i n g grades i s probably more common. Such high l e v e l s of f i l l e r are not needed f o r supplying an " a l k a l i n e reserve" i n permanent grades of paper; the American National Standard f o r permanence of paper f o r printed l i b r a r y materials proposes a minimum of 2% as calcium carbonate Although the presence of excess f i l l e r i s u n l i k e l y to be detrimental to permanence, i t could mean that the mechanical properties of the paper do not meet the requirements f o r i n i t i a l d u r a b i l i t y - such as those s p e c i f i e d i n the standard.


2

SIZING. C l e a r l y , the key to the increased use of n e u t r a l / a l k a l i n e systems i s the a v a i l a b i l i t y of suitable c o s t - e f f i c i e n t s i z e s . This has come about through the development of synthetic materials which are designed to form chemical covalent bonds with the hydroxyl groups i n the surfaces of f i b r e s (13> 15). In addition to the reactant group, the s i z e molecule also has a hydrophobic portion, usually consisting of short a l k y l chains. The two types of s i z e i n most common use are a l k y l ketene dimers (AKD) or a l k y l succinic anhydrides (ASA); Figure 1 shows the intended s i z i n g reactions. In practice several problems have had to be overcome before t h i s apparently a t t r a c t i v e method of s i z i n g could be implemented efficiently. Since the ketene or the anhydride have to react with hydroxyl groups, they w i l l also react r e a d i l y with water; i . e . the molecules are hydrolysed to give non-reacting carboxylic acids (Figure 2). Some means must therefore be found to permit addition of the s i z e s to the wet-end of a paper machine, and then to ensure that they are retained within the wet paper web i n such a way that an adequate s i z e f i l m i s deposited on f i b r e s i n the dried sheet. This i s made more awkward by the e s s e n t i a l l y hydrophobic nature of the molecules. The means adopted i s to prepare emulsions of the s i z e s , often using c a t i o n i c starch as a s t a b i l i s e r and retention a i d . The storage s t a b i l i t y of these reactive synthetic s i z e emulsions i s also of p r a c t i c a l importance; AKD sizes tend to be delivered by the manufacturer i n emulsion form, w h i l s t ASA i s emulsified on s i t e s h o r t l y before pumping i t i n t o the wet-end. This i s an area where much c o n f i d e n t i a l manufacturer's expertise comes into play. At one time, d i f f i c u l t i e s were encountered with ensuring that the desired degree of s i z i n g developed i n a reasonable time, e s p e c i a l l y with AKD's. With rosin/alum, s i z i n g i s complete i n the r e e l at the end of the paper machine, but with some early AKD s i z e s , water resistance continued to develop f o r some days a f t e r the paper was made, making q u a l i t y control d i f f i c u l t i f not impossible. With newer grades of AKD t h i s problem no longer a r i s e s , provided care i s taken to ensure that temperatures i n the drying section of the paper machine are high enough


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(a) Cril-OH+^-CH

1

R

2

I I


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CeH-0-CO-CHR

Permanence and AlkalineNeutral Papermaking - ACS Publications

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