surface sizing basics

8/12/2019 Surface Sizing Basics

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urfacesizing bSurface sizes he lp provide arange of characteristics tothe paper sheet Peter deClerck looks at some of thefundamentals of surfacesizing and the ways thatcareful application canimprove paper qualities

Peter de Clerck ab ove ) is Technical Ser-vice Manager with Avebe Far East) PteLtd in Singap ore

THE IN LINED SIZE PRESSubber

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e function of surface sizing isquite different from that of in-ternal sizing. Whereas the lat-ter provides water repellancy, surfacesizing is used primarily to keep col-loidal solutions such as writing inksfrom penetrating the paper surface.Water repellancy is undesirable forwriting papers or commercial rulingswhere migration of the water into thesheet is the main method of inkdrying, leaving the colloidal pigmentparticles on the sized surface. Hencegood internal sizing is no guarantee ofgood writing qualities.Surface sizing is also necessary forpaper to be used with oil-vehicleprinting inks. The desired effect is to%lter &t the ink pigments while al-lowing the oil vehicle to migrate intothe body of the paper. Slow inkdrying, resulting in set-off ' andsmearing, may occur if the migrationis too slow.Why surface sizePhysical properties: The appliedstarch penetrates into the sheet andincreases inter-fibre bonding, result-ing in increased burst an2 tensilestrengths together with a very largeimprovement in fold strength. The in-creased strength may allowthe mill to use a cheaper fur-

nish, or to reduce its refiningand so achieve easierdrainage on the wire and bet-ter hydrostability in the sheet,giving fewer cockles and lesscurl.Another effect is the im-provement of surfacestrength or scuff resistance.This reduces fluffing onthe printing press, a particularproblem with offset lithoprinting. Fibre picking on theafter-dryers is reduced. Stiff-ness is improved and ir per-meability reduced. Alow-viscous starch solutionmay also be applied to

b sicssaturate the paper and give loweropacity, as for example in glassinepapers.Erasability: This requires surfaceresistance to abrasion and inkpenetration with the ability to retainboth a good surface and sizing aftererasure. Good internal and surfacesizing are both needed to providegood erasability. Surface sizing im-proves erasability by reducing inkpenetration and increasing abrasionresistance by promoting inter-fibrebonding. The ideal condition is astrong, well-bonded body stock.

il resistance: Surface sizing in-creases the contact angle against oil,providing a relatively continuous,non-porous film impervious to oilpenetration.The oil resistance properties areimportant for gloss ink printing, as thesurface absorbency must be carefullyregulated. If too little surface size isused the paper absorbs too much inkduring printing. This gives a high inkdemand and a flat, unattractive print.Conversely, too much surface sizeresults in excessive ink holdout andslow ink drying which causes set-off' and smudging. A maximum of

1.5 2 g/m2 starch per side is recom-mended.Writing qua lities: The surface filmfilters out the pigments in ordinarywriting inks, so preventing feather-ing . The size reduces the porosity ofthe sheet surface, improves thesmoothness and surface strength, andcements down any protruding fibres.The surface properties are also im-portant for printing, as modernrapid-drying inks dry either throughpenetration of the vehicle into thepaper or by solvent evaporation. Mostof these inks contain a water-misciblenon-volatile surfactant to promote ink

penetration. Many flexographic inksare of this type.Pre-coating: Surface sizing maybe applied to give the controlled sur-face characteristics required for a


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subsequent operation. This is espe-cially useful where the very demand-ing blade coater is used.The more uniform surface absorp-tion of surface sized papers helps toreduce print mottling due to unevenbinder migration in all types of coatedpaper. Since binder migration isreduced to a controlled level, highercoating solids contents are then pos-sible. Sometimes pigments are addedto the surface size to give a pre-coator a lightly pigmented surface.Starch selectionThe properties of the finishedpaper product must be consideredwhen selecting starch for the sizepress.Linerboard requires a high ring-crush value and hence good stiffness.This is best achieved by main-taining the starch on the surfaceof the sheet. Hence a relativelyhigh viscous starch is applied toensure only limited penetration.For printing paper good sur-face strength and generalstrength are required. A mediumviscous starch ensures greaterstarch penetration while main-taining surface strength.The main requirement of en-velope paper is good drystrength. Maximum starch vpenetration is required for rein-forcement of inter-fibre bonding,so a low viscosity starch is recom-mended.

Sheet factorsThe factors affecting pick-up arethe moisture content, internal sizing,smoothness, permeability and com-pressability of the sheet. Formation isalso important as any surface applica-tion emphasises rather than concealsany unevenness or other problem.Entering moisture content Maxi-mum pick-up occurs at about 10sheet moisture. At very low moisturecontents the surface energy of cel-lulose is reduced and the paper is dif-ficult to wet. Many mills do run sizepresses with sheet moistures down to2 , but this is invariably a sign ofproblems further up the papermachine that result in a poor moistureprofile. Overdrying is very expensivein energy as the extra heat has to beforced into the sheet through a dry

Size press designSize press design has passedthrough various stages. Today the in-clined size press is most common. Itgives a more direct sheet run than thehorizontal size press and is easier tofeed, but maintains most of thebenefits. For example, it eliminatesthe problem of dirt and other materialfalling from the lead roll into the sizepress and reduces the tendency forwrinkling to occur at the turning roll.More recently the diameter of thesize press rolls has been increased toimprove the equality of pond depth onthe inclined size press and to improvepond stabililty. This was becoming aproblem for small diameter size pressrolls turning at high speed.A recent innovation is the use of agate-roll or fountain coater to meterthe size solutions onto the size pressrolls. This eliminates the size pondand allows very accurate size applica-tion. However, problems have oc-curred with insufficient penetrationof the starch solutions so applied.

Absorption ( )

1 15 1 2 3Moisture of sheet entering size press Rosin kgn

ariations in w t pick-up and rosin absor

amount of rosin to such furnishes toeliminate or greatly reduce variationsin size press pick-up. Particular careis required when neutral sizing isemployed as the development of thesize is slower on machine. Often thepre-dryer temperature profile must beadjusted for these grades. For Cobb60values before the size press wegenerally recommend as follows:well-sized: c25 low viscosityoxidised starchmedium-sized: 25 medium vis-cosity oxidised starchpoorly-sized: >25 high vis-cosity oxidised starchPermeability compressability:Anelastic, porous sheet may act like asponge in the size press nip. The morepermeable the sheet, the greater willbe the starch penetration and hencethe pick-up. A free-beaten stock

results in the greatest pick-up, asdoes a low size solution vis-cosity.Smoothness The smoothnessof the sheet affects the pick-up atthe size press by trapping andimmobilising a layer of size solu-tion on the paper surface. Thesmoother the paper surface thethinner this boundary layer. Theroughness of the surface also hasa profound effect on the rate ofn wetting of the paper and on the

depth of penetration. Wettingand penetration are increased byusing low solution viscosities,while the boundary layer is increasedwith high solution viscosities.cellulose surface, which is n ex-tremely good insulator.It is usually inadvisable to run theingoing sheet at the moisture levelcorresponding to maximum sizepick-up because this increases theload on the after dryers and results ina loss of production. Size pressesusually run at 5-8 sheet moisture,with the size solution being adjustedto give the desired pick-up. However,increased concentration increases theviscosity, so a lower viscosityproduct may be necessary. Sincelower-viscosity products have lowerbinding power, a compromise is re-quired.Internal sizing is particularly im-portant for producers of waste-basedpaper. Initially, internal sizing causesa rapid drop in starch pick-up at thesize press, but a minimum pick-up isquickly reached and maintained asthe level of internal sizing is increased(see graph).

This shows the rapid decrease inpick-up achieved on adding the first 1kglton of rosin. Since wastepaper isan extremely variable commodity, itis often valuable to add a small

Solution factorsThe viscosity, temperature andconcentration of a solution are all in-terdependent; t is not possible to alteronly one factor and not affect theothers.For a given starch solution the vis-cosity may be reduced by loweringthe concentration or raising thetemperature. This will invariablyalter the surface tension of the solu-tion and hence its ability to wet andpenetrate the sheet.By changing the starch it is pos-sible to attain the same viscosity at thedesired temperature but at a differentconcentration. The pick-up of starchsolution (wet-pickup) at the size pressmay then be the same, but the drypick-up will be different due to thedifferent solution concentrations. Forsize press application, such a narrowband of viscosities is used that therelationship of viscosity to degree ofmodification may be consideredlinear for a given temperature and


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concentration.Three points are significant whenconsidering a change of concentra-tion to increase pick-up:Increased concentration will in-crease the solution viscosity and alterthe level of penetration. A lower-vis-cous starch may be required toachieve the penetration necessary toimpart the desired properties to thesheet.The increased viscosity mayproduce runnability problems soagain a lower-viscous starch may beneeded.lower-viscous starch will havelower binding power.

Another area of concern is therope-feed system. This must beproperly designed not only to ensureeasy feeding of the size press, but alsoto avoid the problems of size makingup on the ropes.Speed: Speed directly affects thetime available for absorption in thenip and also affects the behaviour ofthe size press. Increased speedpumps more starch solution nto thenip of the size press, increasing thelocal pressure and so forcing morethrough the nip. This effect is knownas hydrodynamic metering. Hencespeed increases may result in more orless pick-up, depending on which ef-fect is dominant.As machine speed is increasedfrom a very low level, the wet pick-updecreases with the absorption time.At a certain point the hydrodynamic

STARCH PICK UP: PILOT PLANT TRIAL RESULTSHydro TotalSheet Absorption dynamic wet

Viscosity Speed term term term pick upCP mlmin glm2 gm 2 g m2 g/m214 210 17.5 3.5 0 9 21 918 183 17.5 2 2 2 O 21.733p . 21 0 17.5 0 3 3.6 21 476 90 17.5 0 0 9 0 26.5

Source: Howarth Hoyland UMIST UK.

quipmentfactorsNip loadings vary from 5-22kglcm in the horizontal size press to4- 3 kglcm in the vertical size press.The rolls must be accuratelycrowned for the operatingpressure used, especially in avertical size press, becausewith too little crown the sheetwill be baggy in the centre.Increased nip loading shouldtheoretically reduce pick-upas more starch solution is ex-pressed from the paper inmid-nip. Iodine staining ofpaper cross sections haveconfirmed that at high niploadings greater penetrationof the starch occurs eventhough the overall pick-up isreduced. This is somewhatacademic as it is not usual to vary thenip loading during normal operation.Roll hardness and diameter are notreadily changed and should be con-sidered in the initial installation of thesize press.Pond depth is important as itgoverns the contact time and hencethe time available for absorption tooccur. Pick-up increases with ponddepth and the effect is more notice-able on slower machines. Pond depthmay only be attended by altering theend dams and is limited by the sizepress configuration and roll diameter.Roll sugaces should be smoothand in good condition, beingreground as necessary. Roll smooth-ness will affect the position of filmsplitting on the emerging paper web.Sheet tension is important, as un-even tension on the sheet leaving thesize press will rekult in uneven migra-tion of the starch into the paper.Starch feed is extremely impor-tant. Failure to ensure sufficient feedpipes across the width of the sizepress will result in uneven pick-up.

metering effect becomes thedominant factor and then pick-up in-creases with speed. Hence the wetpick-up curve passes through a mini-mum. The machine speed at whichthis minimum occurs depends onmany factors, including the starchviscosity and type of paper.At low speed, where absorption isthe main method of wet pick-up, alower viscosity solution gives ahigher wet pick-up. At higher speedsa higher solution viscosity results in athicker film being metered throughthe nip, giving a greater wet pick-up.Mechanism of size pick-up: Onentering the size pond there is a slightdelay before the sheet surface iswetted. Absorption then commences.The amount absorbed depends on thetime available ie pond depth andmachine speed), the solution vis-cosity, the paper type, porosity anddegree of internal sizing. The wettingtime of the paper surface acts todecrease the time available for ab-sorption and is a function of surfacesmoothness and dryness of the sheet.Within the size pond, as the web

approaches the press nip thehydrodynamic pressure increases andsome liquid is forced into the pores ofthe paper. The sheet is compressedand air is forced from the sheet. Onexiting the nip the sheet will expand,taking in liquid like a moist sponge.Obviously the porosity, permeabilityand bulk of the paper and its compres-sibility are factors governing theamount of liquid absorbed, as is thesize solution viscosity.As the web leaves the size pressrolls the liquid film carried throughthe nip between the roll and the papersurface splits, probably by a processof filamentation and cavitation. Ex-actly where the split occurs dependson the relative surface roughness ofboth the paper and the size press rolls,as a rougher surface traps more solu-tion. The surface roughness is also

important in determining thethickness of the starch filmpassing through the sizepress nip.The whole process of ap-plication and metering a sizesolution takes place in onlya fraction of a second.Solution viscosity is im-portant in thehydrodynamics of the sizepress nip. The greater theviscosity the greater is thepressure generated in thesize pond by the rotatingrolls. At high speeds thepressure generated may beenough to eject the entire contents ofthe nip. This is usually preceded byjumping of the size pond. Largerrolls are used on high speed size pres-ses mainly because their largerdiameters do not close the nip sorapidly, resulting in a more gradualpressure build-up and hence a morestable size pond.In conclusionIt should be noted that in all caseswhere the absorbency factor is notphysically limited by low sheetweight, the sheet substance has noeffect on size press wet pick-up for agiven set of sheet characteristics anda given size press operating regime.There are many factors influenc-ing the performance of the size press,a significant number of which arefixed by equipment factors or opera-tional necessity. For the mill, the big-gest benefits may be obtained bypaying attention to the characteristicsof the ingoing web and to the qualityand properties of a properly selectedsize press solution.

surface sizing basics

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