The Application of Modified Starches at the Size Press

8/12/2019 The Application of Modified Starches at the Size Press

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NORTHERN DIVISIONMEETING HELD AT THE ENGINEERS CLUB, MANCHESTER

6th MARCH 1959Mr. P.A. DUXBURY in the Chair

THE PPLIC TIONOF MODIFIED ST RCHES

T THE SIZ PRESSA. HARSVELDT

SynopsisThe paper gives a survey of a number of important factors thatmust be borne in mind in the application of modified starches t thesize press The causes of the coating pattern or tracking re discussedand methods are given to eliminate this pattern The conversion

methods of starches are discussed and advantages and disadvantagesin connection with special applications re indicatedPurpose of surface sizing

Surface s1zmg imparts to writing papers increased strength,improved rattle and better surface smoothness. For printings, thesize press is principally used for coating to obtain higher gloss andbetter smoothness after supercalendering.A correctly located size press may also contribute essentially to aproduction increase of the machine-for example, writings can bemade with free-beaten stuff and consequently at higher speed;strength and rattle in this case are obtained by surface sizing in thesize press.Printings will benefit also by the higher speed made possible byuse of free-beaten stuff. Part of the fillers can now be added in thesize press and the addition of filler to the beater can be reduced inquantity or dropped altogether.

f one wanted the fibres to absorb the fillers added in the beater,none at the size press, the degree of beating would have to beincreased, which would mean lower operational speeds and troubleslike cockling and wrinkling.

Mr Harsveldt is Research and Laboratory Director, a \ i m 1 4 - J r - , z e t r y 1 i : p < \ t i s W ~ /N.V., Holland t I :S l U I H __ \399 Nuaml Vennootscnap

W A S C H O L J ' , ' ~ ;CHEM\SCHE FABfilt i\FNF 0 X H


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400 MODIFIED STARCHES AT THE SIZE PRESShe modem size press

Fig. 1 shows the size press mounted between the first and seconddrying sections and indicates a number of particulars that areimportant for size press operation as follows.I Feeding the paper into the size pressThe feeding-in angle shall not be greater than 5 to the horizontal.f the angle is greater the surface area available for impregnationwith the same quantity of size delivered will be reduced the sizeflowing quickly to both sides in the direction of the upper cylinder

of the size press.f the angle is too acute the size will flow on the sheet in theopposite direction of travel and the area of paper coated with starchsolution will be greater.

FIG.

SIZEPRESSIN T IE

PAPERMACll tYE

2 Feeding of the paper into the drying sectionThree important things are to be observed hereJ The paper when emerging from the size press should not adheretoo long to the upper roller.2 The paper shall be given sufficient time for expansion-in otherwords the run after the size press should not be too short. n thisrespect fine papers will present greater difficulties than mediumgrade and mechanical papers.3 The paper should not be dried too quickly; in the first twocylinders therefore the steam pressure should be kept low andthese cylinders are run without dryer felts in a number ofpapermills.After the size press but not long before the paper enters thesecond drying section a Mount Hope cylinder should be provided

to smooth out any wrinkles. This cylinder should be located so thatthe paper contacts the cylinder at an angle less than 30.


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MODIFIED STARCHES AT THE SIZE PRESS 401The rolls of the size press are generally located one verticallyabove the other, but the top roll may also be placed a little out ofvertical with the bottom roll, as a consequence of which the surfacearea covered with starch solution is somewhat larger on the upperside of the paper. The pressure per linear inch will be lower inthis case.A size press with horizontally mounted rolls has been designed.This requires little space on the papermachine, which is certainly

an advantage.Hardness of rubber rolls

Little can be found in the literature.on the degree of roll hardnessin size presses. t will be quite clear that two non-resilient rollslike cast iron) are unsuitable, as the line of contact will be very thin.

rom a very good article on size presses,


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MODIFIED STARCHES AT THE SIZE PRESS 4 3used a rheological approach for the property of length. A shortcoating colour-possessing a low plastic viscosity and a high yieldvalue will show shorter necks. A long coating colour with toohigh a plastic viscosity-will tend more to form necks and producea heavier pattern on the coated paper.

n the case of a coating with very low water retention applied tothe same paper the penetration of adhesive and water into thepaper will cause a substantial increase in solids and flow resistanceof the coating next to the paper with the result that the film willsplit rather close to the roll surface. Hence the web will carry awaya film much heavier than that left on the roll. When water retentionis extremely low coating may become inoperable over the wholearea the solids content of the ilm become so high that dilatantbehaviour occurs resulting in erratic film splitting in the dilatantlayer and a sharply ribbed pattern. The applicator roll then emergesfrom the nip carrying deposits of unsized pigment. A series ofexperiments were undertaken on our laboratory size press to establish the dependency of water retention and patterning.

For these trials starch/clay colours at 4 per cent. solids withvarying percentages of a thin oxidised starch were applied on a fullrosin-sized mechanical printing.

Water retention and viscosities o 40 per cent. solidsstarch/clay coloursTABLE 1

Per cent. o starch Water retention Viscosityon the china clay sec. centipoises

15 36 1518 48 721 57 2424 67 2827 82 31

The data given in Table 1 indicate an increase in the plasticviscosities with increasing percentages of starch. All the coatingcolours gave Newtonian rheograms so that patterning could only becorrelated to viscosity and wet film thickness. t appears that increasing the proportion of starch which means increasing the waterretention reduced the sharply ribbed coating pattern to a minimum.


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404 MODIFIED STARCHES AT THE SIZE PRESSAs soon as the wet film thickness and the viscosities increasedtoo much, the normal coating pattern appeared again. The ex

planation of this phenomenon is that the higher percentages of starchinhibit excessive drainage of adhesive and water in the paper, soleaving enough fluid colour to give reasonably satisfactory filmsplitting. Too high a percentage of starch, however, increases theviscosity too much, giving longer necks and more coating pattern.Application to the paper surface

In operating a size press, it is imperative for its economic performance to have an adequate after-section to remove the water taken upup to 30 g./sq. m.).1 Effect o the weight per unit area

The pick-up per unit area is fairly uniform over the paper sheetand is independent of the weight applied.

When increasing the weight, the drying section before the sizepress will as a rule be heavily loaded, whereas the after-section isloaded to a smaller extent. With low weights per unit area, a higherspeed will usually be chosen; the drying section before the size presswill be sufficient, but the after-section will now have to evaporatemore water, because a larger surface of paper passes through thesize press in a given time.2 Effect o rosin siZing

An unsized paper will absorb much more water than will a sizedpaper. There is decreasing absorption with increasing degree ofsizing.3 Viscosity and solids content o the starch solution

These two characteristics are closely related. The viscosity of thestarch solution will increase hyperbolically in higher concentrations,when the temperature remains unaltered.Fig. 3 shows the increase in viscosity for three different temperatures, if the concentration is also increased. When theconcentration is graphed against the logarithm of the viscosity, a

straight line Fig. 4 is obtained.t higher starch concentrations, the viscosity and also the starchabsorption by the paper will increase. Furthermore, the starchabsorption by the paper increases more than one would expect fromthe increase in the solids content of its solutions.


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MODIFIED STARCHES AT THE SIZE PRESS

2

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Increase in l/i .s 'cosityo / modif ' led .starchFI G 3

by increasing' concentro;c1on

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Increase in viscosit Iof mo di l i ed sfCJre>h

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406 MODIFIED STARCHES AT THE SIZE PRESSThis explains why, when working with higher solids, more waterhas to be evaporated than when working with lower solids. With

water only, the amount absorbed is t a minimum: as soon asstarch has been dissolved in it, the amount of water absorbed ythe paper will increase with advancing viscosity./.8.

I

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.Orf, p /ck ,u ,o ci 9 cSl/nS t vlS co.r/t51s i z e d , b se pcS1pers)

' ~ - - - - - ~ - - - - - V ~ i ~ s _ c _ o _ s 1 _ c - ' y _ i n _ c . : . . . , o _ .ISO

Dry pi k vp ag-CJ/nsc /ofT ol viscositys./zed 601 se ,bc>1j:1er.s

FI G 6

loq oF v/sco.s-i ty n c J;>.4567891o 5 6 7 8 9 10

To confirm this conclusion further on an experimental size press,trials were run with starches of different viscosities in a singleconcentration.

I


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MODIFIED STARCHES AT THE SIZE PRESS 407With a 11 in. paper width, the machine was run at speeds of

9 ft./min.; on two different offset papers of approximately 90 g./m.,quantities of 1 2 g./sq. m. were applied (Fig. 5 .The curves show the characteristic hyperbolic relationship betweenabsorption and viscosity. The steeper line in Fig. 6 shows the smallincrease of the absorption with the viscosity, if the offset paper isalso engine-sized with 2 per cent. starch.

4 ffect o the degree o beatingWhen beaten to a low degree, the porosity ofa paper will be higher

and the absorption of a starch solution in the size press will naturallybe higher.5 Temperature o the starch solution

As the viscosity of starch solutions rises with falling temperature,the paper will absorb more of the solution upon cooling.Generally speaking, at least 30 35 per cent. of the total dryingcapacity should be found in the after-section.

The requiremen.ts for a starchAny native starch is composed of two components linearmacromolecules called amylose and branched macromolecules calledamylopectin. Chemically, both polymers consist of units of glucosethat are linked to one another. n such glucose chains, the hydroxylgroups of one molecule have a strong affinity for the hydroxyl groupsin a second molecule, especially if the two molecules are closelyadjacent. This is possible with amylose, because straight chains canlie parallel to one another. This occurs in a starch solution uponcooling and ageing the linear chains gravitate towards each otherand the hydroxyl groups keep the chains close together like bridges.f the starch solution is diluted, the chain bundles will settle as aninsoluble precipitate; if the solution is a concentrated one, however,the whole substance will gel. This phenomenon is called retrogradation, going back to the solid form. With the branchedcomponent, the amylopectin, only a few hydroxyl groups can comeinto proximity, as parallel orientation is not easy: therefore, solutionsthat contain amylopectin exclusively cannot gel.

Native starches cannot be used in the size press, because of theirrather high viscosity, either for starch coating or for surface sizing.t is necessary to have starches that are carefully converted to therequired degree with low viscosity, but still having good adhesivepower.


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408 MODIFIED STARCHES AT THE SIZE PRESSA low viscous solution penetrates more deeply into the paper thandoes a highly viscous one and will consequently better meet therequirements. One can work with a higher solids content and obtaingreater tensile strength improvement, though decomposition should

not proceed too far, otherwise the film-forming properties of thestarch and its adhesive power will be lost.There are various processes to decompose native starches. One ofthem s the hydrolytic decomposition under the influence of acids.

t has been found, however, that these thin-boiling starches retaina great deal of amylose and consist of very little amylopectin. Thisaccounts for the high tendency to retrogradation.This is a disadvantage, if a deeper penetration into the paper iswanted or if a coating colour with a long period of stability isrequired. Hydrolytically decomposed starches, however, may alsohave their advantages for example, optimum closing of the papersurface with low penetration depth is required.In another process of conversion, oxidation is the principle involved. Thin-boiling starch types, possessing very high stability,result. The oxidising agent, sodium hypochlorite, does not destroy

the chained amylopectin molecule, but introduces highly solublecarboxyl groups and carbonyl groups into the chains, which repressthe tendency to retrogradation. This method enables the preparationof the ideal types for normal surface sizing and for starch pigmentcoating.n the third place, roasted dextrins are known. Brimhall 1943)proved that dextrinising s accompanied by a diminishing of thelinear chain molecule and that the number of chained moleculesincreases. During the dextrinising process, however, chain fragments

will also come into existence and, being so small, they dissolve evenin cold water.As a result, it is not advisable to use roasted dextrins for thesurface sizing of offset papers and writing papers, because the starchfilm is in contact with dampening water on the offset press andbecause too high a sensitiveness to water in writings may cause thedetrimental feathering. Finally, the papermaker can decomposethe native starch himself by means of enzymes. This methodrequires careful control, but will give good filming properties and

adhesive power if well conducted. Unfortunately, the low molecularsugars also formed cause non-drying films.In recent years, other starch derivatives have found some interestfor surface sizing-etherified and esterified starches, in which thehydroxyl groups in various places have been substituted by other


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MODIFIED STARCHES AT THE SIZE PRESS 409chemical groups. The advantages brought about by such substitutes re

J By substitution of the hydroxyl groups, the orientation of parallelamylose chains is hampered. This means improved stability.2. The kind of group introduced may have an essential effe t on thefilming properties.Apart from the fact that films of etherified and esterified starchesare generally less brittle, a polar substituent (like carboxyl, sulphonicacid, glycollic acid) will make starch films more resistant to oil and -grease, which makes these products interesting for greaseproofpapers and boards.On the other hand, it will be understood that the substitution ofhydroxyl groups by hydrophobic ether groups or ester groups will

impart to the films improved oil absorptivity, as it is known thathigher rosin sizing of a paper will also give this.I have attempted to summarise the principles of size press work.Although much depends on the mechanics of the size press, theparticular type of starch used plays an important role, too.

REFERENCE1 Currie, D. C., Wyatt, M. C. and Taylor, J. E., A Short Discussion of SizePressing and Tub-sizing : Pulp and Paper Mag. Can. 1956, 57 (3), 274


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410 MODIFIED STARCHES AT THE SIZE PRESSDIS USSION

MR. J WINTERBURN: Do the horizontally mounted rolls not give abetter nip condition, owing to less camber being needed and thesame natural deflection being present in both rolls?I object to the expander roll being called a Mount Hope cylinder,as this type of expander was invented by William Mycock ofLittleborough and has been known for a great number of years in thetextile trade as the Mycock expander.MR. A. HARSVELDT: f we consider the case of a pair of rubbercovered rolls that have been ground straight face and that are loadedby a force being applied to their journals, the result will be a bendingof the rolls under the application of the load, resulting in increasedpressure near the ends of the roll. This gives rise to a nip with smallwidth at the centre of the roll and wider at the ends of the roll.The simplest method of correcting this situation is to increase thethickness of the rubber in the centre of the roll, tapering it outtowards the ends. When this is dom\ we have equal pressure acrossthe nip and an even nip is obtained.Mounting the rolls horizontally or vertically doe " not change thefact that we have to compensate for the deflection of the rolls underload by increasing the thickness of the cover in the centre of the roll.Thank you for the comment on the expander roll.MR. F. BRIDGE: Has Mr. Harsveldt investigated the effect offurnish

on the quality of the starch to be used in the size press operation?Hardwood pulps are now being used more extensively along withsoftwood pulps aDd it would be interesting to know whether this facthas to be taken into account when deciding the most suitable typeof starch.What is Mr. Harsveldt s opinion on the best type of surface forthe drying cylinder immediately following the size press? In thisconnection, am particularly interested in the reduction of stickingtroubles and fouling of the cylinder surface by starch and claydeposits.MR. HARSVELDT: The composition of the furnish of the base papercan alter the starch pick-up. In papers containing esparto, the pickup is normally higher than in those grades when hardwood pulpsare used. Softwood pulps will pick up more than will hardwoodpulps. Another factor plays a role, too. t will depend on themake-up of the furnish to what degree the base sheet has to be


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DISCUSSION 4

rosin-sized. We have seen that the degree of rosin sizing in the basepaper can alter the starch pick-up. Generally the composition of thefurnish together with the degree of rosin sizing decide the strengthof the starch solution and the molecular magnitude of the starchto be used.I think the best surface for the drying cylinder immediatelyfollowing the size press would be a chrome-plated finish. This isseldom done; usually the first two dryers following the size pressrequire doctors to remove the build-up of coating material from theirsurface. Sometimes a baby dryer operating at reduced temperatureto avoid sticking and provided with an oscillating doctor is installedimmediately after the size press.

MR. J. A. HAAS: Could Mr. Harsveldt indicate how pick-up of thestarch solution is influenced by its contact time with the paper weband also by the moisture content of the web entering the size press?MR. HARSVELDT: n a fully rosin-sized base paper the pick-up ofthe starch solution is not affected by the moisture content of the sheetas it enters the size press. f he rosin sizing is dec eased the moisture

content of the sheet plays an important role. The most favourableweb moisture content is 6 20 per cent. Below and above thismoisture content absorption of size decreases. n U.S.A. practicethe sheet is fed in at 3 17 per cent. moisture content.With the same quantity of starch solution present for impregnationthe pick-up will increase with its contact time with the paper web.MR. F. GLEDHILL: What angle does Mr. Harsveldt recommend forthe sheet entering the starch press on a papermachine making fine

papers at say 400 ft./min.? MR. HARSVELDT: The angle at which the paper web enters the pressnip affects the height of the size barrier on the top side of the sheet;Each mill must decide by trial and error the angle most suited toits own conditions. Making the angle more acute gives greaterpick-up making it steeper gives less pick-up. Nobody can recommend a particular angle without having tried it out.MR. R. TOMLINSON: What diameter would you make the starchpress rolls?MR. HARSVELDT: The average diameter of the size press rolls is24 in. Small diameter rolls give splashing troubles but give lesscoating pattern when starch/clay coatings are applied.


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412 MODIFIED STARCHES AT THE SIZE PRESSMR. B. H. TURNER: J What is the most effective method of controlling the starch pick-up on the papermachine?2 ls it usual to drive both the top and bottom rolls in a size press?3 Should the expander roll be driven and, if so, at what speed inrelation to the paper?MR. HARSVELDT: J The most effective method of controlling thestarch pick-up in the size press on a given base paper is controllingthe solids and viscosity of the starch solution.A direct measurement on the paper could be done by incorporatingradioactive tracers in the size and measuring them in the sheet withradioactive counting techniques.2 Usually, the bottom roll of the size press is driven and the toproll is driven by contact with the bottom roll and the paper. l t is agood practice to drive both the top and bottom roll. When the pressis loaded and speeds are high, the top roll has a tendency to slip

on tbe starch size or coating on the paper. This can be avoided bydriving the top roll individually: if so, the roll speed is normallymaintained slightly below paper speed.3 The task of the expander roll is to eliminate wrinkles andcreases in the sheet and to ensure that the sheet is correctly presented

to the first after-drying cylinder.There should be as little tension on the paper as possible, so itwould appear a good practice to drive the expander roll independently at a slightly higher speed than the paper.MR. A. L OKELL: Did Mr. Harsveldt find that, despite the lowerspeed of the paper 9 ft./min.) through his experimental size press

installation, he was able to correlate the results obtained on theexperimental press with those obtained at normal speeds on a fullsize papermachine?MR. HARSVELDT: The logarithmic proportion between viscosity andpick-up has been established in papermachine practice. This isreferred to in a paper given by Dr. Arneberg in summer 1957 atBaden-Baden, Germany not yet published).MR. A. BIRTWISTLE: What type of size press would Mr. Harsveldtprefer to use for applying a clay coating vertical or horizontal?

f a horizontal press is preferred, what type of rolls would he suggestbe used?


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DISCUSSION 413MR. HARSVELDT: For applying starch/clay coating, I prefer a sizepress with hard rolls (one metal roll and one hard rubber roll or

two hard rubber rolls) that can be loaded up to a pressure of350 lb./linear in. In the classical type of size press with verticallymounted rolls, it is quite usual to apply high pressure. The horizontalsize press does not allow pressure at the moment; but, if thehorizontally mounted rolls are both hard and it is designed to applyhigh pressures, a horizontal size press would do very well forstarch/clay coatings.MR. J H POTTER: When starch is oxidised by sodium hypochlorite,

carboxyl groups are introduced into the molecule. The introductionof these groups into the starch makes films of such starches morehydrophilic than a modified starch with no carboxyl groups introduced. Furthermore, there is a chemical reaction between electrolytesand oxidised starches resulting in a marked fall in pH value. Sucha reaction will occur when oxidised starches are applied to paperscontaining even minute traces of electrolytes. In advocating the useof oxidised starches for general size press application, are you notgoing against two of the primary requirements of the lithographer-namely, a paper surface not unduly weakened by water and a stablepH value greater than 4.8? .The speaker s statements on roasted dextrins cannot be allowedto pass without comment. Present day materials, produced byroasting, do in fact possess film solubilities much lower than starchesconverted by hydrochlorite oxidation. I must disagree therefore withstatements that roasted starches are unsuitable for sizing offsetpapers and writing papers.I should also like to point out to the papermakers present that thelow powder solubility of oxidised starches beam no relationship tothe solubility of its film when applied to paper.

MR. HARSVELDT: t is perfectly right that introducing carboxylgroups into the starch molecule makes the starch more hydrophilic;however, on the average, only one carboxyl g1oup is introduced intoten glucose units, just sufficient to eliminate the set-back propertiesof the starch. Work can then be done with a starch having as higha molecule magnitude as possible and still possessing a goodstability. Because of the size of the molecule, the water resistance issufficient for offset printing.Oxidised starches are supplied at a pH value between 6 5 and 7This means that the carboxyl groups are neutralised, giving partlythe sodium salt of a carboxylic acid; when electrolytes are present


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4 4 MODIFIED STARCHES AT THE SIZE PRESS

in fair quantities the sodium salt cannot behave like a cationexchanger as the carboxylic acid would certainly behave. Theconclusion s th t starches having a small part of their hydroxylgroups substituted by carboxylic groups do not give any noticeableexchange of sodium ions with hydrogen ions when neutralised.On the use of roasted dextrins that contain a certain percentage ofcoldwater soluble fragments of starch 1 stressed that these smallfragments effect the film solubility. ln n oxidised starch themolecular size s homogeneous because of the wet manufacturingprocess.

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The Application of Modified Starches at the Size Press

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