Chapter 18 Surface Treatment

7/27/2019 Chapter 18 Surface Treatment

1/70

Handbook for Pulp &

Technologists

Chapter 18: Surface Treatment


2/70

I.Siing

Sizing operations are carried out primarily to provide paper with

resistance to penetration by aqueous solutions. The treatment also

provides better surface characteristics and improves certain physicalproperties of the paper sheet, such as surface strength and internal bond.

Two basic methods of sizing are available to the papermaker: internal

sizing and surface sizing. These are used either as sole treatments or in

combination. Internal sizing utilizes rosin or other chemicals to reducethe race of water penetration by affecting the contact angle. Surface

sizing typically utilizes starch particles to tilt in the surface voids in the

sheet, reducing pore radius and therefore the rate of liquid penetration.

The relative advantages and disadvantages of surface sizing in relationto internal sizing are summarized in Table !". Internal sizing was

considered in Section #.$ and will not form part of the present

discussion.


3/70

I.Siing


4/70

I.Siing

Surface sizing is most commonly applied on"machine at a station

between dryer sections, which is referred to as the %size press%. &or

board grades, sizing solutions may be applied at the machine calenderstack. &or the highest"quality grade papers, an off"machine operation

known as tub sizing may be utilized. The most common material used in

surface sizing solutions is starch, either cooked or in a modified form

'e.g., o(idized or enzyme"converted). *ften wa( emulsions or specialresins are added to the starch solution. *ther agents may be used as well

to provide specific strength and optical improvements.


5/70

I.1 Con!entional Sie Press

Sizing solution is commonly applied within a two"roll nip+ hence the

term, size press. The traditional size press configurations are categorized

as vertical, horizontal, or inclined as illustrated in &igure !". In eachcase, the obective is to flood the entering nip with sizing solution+ the

paper absorbs some of the solution and the balance is removed in tire

nip. The overflow solution is collected in a pan below the press and

recirculated back to the nip. The vertical configuration provides theeasiest sheet run, but the pond depth of solution in each nip is unequal.

The horizontal size press arrangement solves the problem of unequal

lop" and bottom"side absorption by providing identical pond forms on

either side of thesheet. The inclined configuration is a compromise, and

was developed to avoid the rather awkward vertical run of the sheet inthe horizontal size press.


6/70



7/70


The retention time of the sheet in the pond and nip of the size press is

very brief, and consequently, the operation stud be carefully controlled to

ensure that the requisite amount of solids is absorbed uniformly acrossthe sheet. -t the same time, the amount of water absorption should be

minimized so that the steam requirement for subsequent drying is

maintained at the lowest level. The main variables affecting size press

performance are summarized in Table !". /ach size press application is

unique and must be evaluated and optimized with respect to its own

peculiar demands and conditions.


8/70



9/70


There are two basic mechanisms for incorporating sarch solutions into the sheet at the

size press. The first is the ability of the sheet to absorb the size

solution+ the second is the amount of solution film passing through the nip and the manner

in which the paper and roll surfaces separate. &actors that favor

greater absorption are low solution viscosity 'higher solution temperature), low machine

speed, high sheet moisture, high sheet porosity, and low level of internal sizing. The factors

favoring greater film thickness include high sheet roughness and low nip pressure.

The sheet moisture content has a significant effect on solution pickup. -lthough higher

sheet moisture promotes absorption, the level is typically controlled at 0 to #1 or less to

ensure an even moisture profile and to keep the sizing agent nearer to the surface.

*f the factors affecting pickup, solution viscosity and solids content are the most easily

manipulated. The solids concentration, however, is usually

maintained at the highest manageable level consistent with the desired viscosity in order

to minimize the amount of water that must be subsequently evaporated.


10/70

I." Con!entional Sie Press #imitations

-t higher paper machine speed, the pond in a conventional size press begins

to absorb kinetic energy from the converging web and roll surfaces. -s the

sheet moves more rapidly toward the nip, the nip pressure causes e(cesssolution to flow backward and2or upward with greater force. /ventually the

hydrodynamic forces become sufficient to cause the solution to break the

surface of the pond and splash out of the nip. This turbulence results in uneven

pickup of solids across the machine.

-lso at higher speed, a greater quantity of solution remains on the surfaces

between the paper and each roll at the e(it of the size press nip. -s the sheet

leaves the nip, each film of solution is split unevenly into two layers, part

remaining with the paper and part with the roll. -gain, the result is uneven

pickup of solids.-nother difficulty with the conventional size press, which becomes more

severe at higher machine speed, is matching the speeds of the two rolls.

3inute speed differences can lead to paper surface marking and increased web

breaks.


11/70

I.$ Sie Press %odifications

Slight modifications in design have allowed conventional size presses

to operate successfully at somewhat higher speeds. &or e(ample, larger"

diameter rolls keep the pond turbulence more manageable. 4lastic bafflesare sometimes used to isolate the sizing pond from the high"speed

surfaces of the paper and rolls.

To avoid pond problems altogether, many modern paper machines are

now utilizing some version of the gate roll size press. This design,illustrated in figure !", has an offset pond on each side which is not in

contact with the sheet. The offset pond feeds a metering nip which

controls the amount of solution going to the second nip. The second nip

controls the uniformity of the film which is finally transferred to the sizepress roll. -ll the rolls in the gate roll train are run at different speeds to

minimize film split patterning.


12/70



13/70


5ith the gate roll design, it is possible to increase the starch solution

concentration which reduces the evaporative load on the %after dryers%

and keeps the starch on the sheet surface. 6evertheless, problemsassociated with film splitting are still present, and may be amplified by

the higher starch solution concentration ' ). -lso, supplanting a

conventional size press with a gate roll design involves replacing two

rolls with si(, thus contributing toward higher initial investment and

increased maintenance costs.

The roll maintenance and film"split pattern problems of the gate roll

press led to the development of blade or rod metering size presses.

In most designs, short"dwell coater heads are used to supply the sizingmaterial and either a bent blade or red is incorporated into the head to

control the wet film thickness '&igure !"$). '7efer to ne(t Section

for discussion of short"dwell applicators and blade2rod metering

equipment.)


14/70



15/70

I. Coating 'pplications

-lthough its original purpose and primary application is for applying

sizing solutions to paper, the functionality of the size press has been

greatly e(panded. Today, this equipment is also used for pigmentedcoatings and other specialized surface applications+ however, the

descriptive term, size press, has been retained.

Some producers of printing papers are now including pigment in the

starch application as a means of adding value to their products. 5ith thismethod, some of the pigment penetrates into the sheet, and overall

pigment pickup is limited to about $ g2m per side. 6onetheless, the

treatment is sufficient to significantly improve printing properties, and

these so"called pigmented papers are finding a market with customerswho are reluctant to pay a high premium for coated or supercalendered

grades.


16/70

I.( )ature of Starch

Starch is a carbohydrate synthesized in corn, tapioca, potato and other

plants by polymerization of de(trose units. The polymer e(ists in two

forms: a linear structure of about #88 units and a branched structure ofseveral thousand units. The linear polymer, called %amylose%, constitutes

91 of normal corn starch, while the branched polymer, called

%amylopectin% makes up the remaining 9$1. &ractionated starches are

available for special uses.

Starch is supplied as a white, granular powder which is insoluble in

cold water because of the polymeric structure and hydrogen bonding

between adacent chains. owever, when an aqueous suspension is

heated, the water is able to penetrate the granules and causes them toswell, producing a ;gelatinized% solution or paste, depending on the

concentration.


17/70

I.( )ature of Starch

=nfractionated and unmodified starch, called %pearl starch%, is %thick"boiling% 'i.e., viscous) and

has a tendency toward gelling or setback, even without cooling. Setback is avoided by using 881

amylopectin '%wa(y starch%) which forms a clearer paste and is non"gelling+ however, a loss of

sizing efficiency results because the linear fraction contributes more toward film formation.

>ower viscosity and setback resistance are achieved by using chemically" or thermally"modified

starches. &or e(ample, a %thin"boiling; low"viscosity starch can be prepared at the mill by enzyme

conversion, with film formation properties and setback resistance unaffected. ?epending on the

properties desired, a number of chemical methods may be utilized by the supplier to modify the dry

starch product. *r alternativety, the user can employ a suitable conversion process 'e.g., enzyme,

thermal, thermal2chemical) when preparing the sizing solution.

In practice, a low"viscosity starch solution of 0 to 81 solids is used at the traditional size press

in order to achieve starch pickup of 98 "8 Ibs per ton

of product. Somewhat higher concentrations are used with gate roll presses. - higher viscosity

starch can be used at the calender stack, where a more limited penetration is desired and a pickup

rate of 8 lbs per ton is more typical.


18/70

I.* Starch Preparation

Starch cooking may be either batch or continuous. The batch cooking

operation '&igure !"0) is always carried out in all open agitated vessel where

the pre"measured mi(ture of water and dry stall is heaved by direct steaminection or by circulation through a heat e(changer. The solution is heated to !!

" @$8 < and held at this temperature for 8 to $8 minutes. 5here live steam is

used, it is essential that turbulent mi(ing conditions are maintained at the point

of inection to provide even heating. The starch slurry should be kept below the

boiling point to avoid foaming and spates. Since cooking temperatures arelimited, premodified starches are normally used in batch systems.

-t the heart of most continuous systems is an eductor which utilizes high"

velocity steam energy for both mi(ing and heating. In the so"called thermal et

system, the steam is totally dispersed into the slurry and then retained in a

pipeline long enough for the steam to fully condense. The temperature rise is

e(tremely rapid, and steam pressure is controlled to provide the required final

temperature. Since the et cooker system is pressurized and can operate at

elevated temperatures, it can be used for thermal conversion of unmodified

starch.


19/70

I.* Starch Preparation


20/70

I.+ Calender Sie 'pplication

eavier paper and boards are commonly surface"sized at the calender

stack in order to get improved calendering action and obtain a smooth.

Scuff"resistant surface for printing. -pplication of the size solution isusually by means of a water bo( with a reinforced rubber lip contacting

the calender roll 'refer to &igures !"# and !"A). The solution is carried

on the roll surface into the calender nip where it is applied to the board.

*ne or more bo(es may be fitted to one side of the stack, and the same

number to the other side if equal absorption is desired. The pickup of

solution is controlled in part by the roughness and absorbency of the

sheet: therefore, more material will he picked up from the bo(es

installed at the top of the stack where the sheet is less compacted. -fter"

calender drying is required to evaporate the water that is added with thesize solution. *ften, an additional calendering treatment is given to the

sized sheet after it is dried.


21/70



22/70


I 8 T b Si i


23/70

I.8 Tub Siing

The ultimate in sizing treatments is achieved by tub sizing. ere the

sheet is run through a shallow bath containing a solution of starch and

other additives, and the e(cess solution is removed by passing the sheetthrough a light nip. Initial drying of the sized sheet is usually

accomplished by hot air impingement to avoid disturbing the size film.

In tub sizing, the obective is not only to improve surface properties,

but also to impregnate the sheet sufficiently to improve such propertiesas ply bond, burst, stiffness and tensile strength. Tub sizing is sometimes

carried out on"machine, but better results are obtained with an off"achine

operation.

II Pi C i


24/70

II. Pigment Coating

The advancing technologies of printing and packaging have placed

greater demands on the surface of the paper sheet. To meet the more

stringent requirements, many paper surfaces are coated with suitablepigment"rich formulations to provide improved gloss, slickness, color,

printing detail, and brilliance. The coating can be applies either on"

machine or off"machine, depending on product requirements and

operating philosophy.


25/70

II. Pigment Coating

The mineral pigment used in coatings is similar to a filler 'see Section

#.$), but is usually somewhat finer+ it is mi(ed with adhesives and

other components to hold it onto the paper surface and provide suitablefinish and rub"resistance.

The applied coating tends to fill in the void areas 'hollows) on the

surface of the paper sheet '&igure !"9). -fter drying and calendering,

the coating provides a smooth, even surface for printing. owever, asatisfactory coated sheet can be produced only if the base sheet '%coating

raw stock;) is well"formed and free from defects. 6o surface treatment

can compensate for poor raw stock, the quality requirements of which

are usually more stringent than for uncoated papers. The base sheet isusually sized prior to the coating operation to control the receptivity of

the surface to a particular coating mi(.

II Pi t C ti


26/70

II. Pigment Coating

II 1 C ti , l ti


27/70

II.1 Coating ,ormulations

The variety of coating formulations is awesome. It is not unusual for a

coating mi(ture to contain more than ten ingredients, and some blends

could contain more than fifteen different constituents. In many cases, theformulations have evolved and developed in response to more stringent

requirements with respect to both the paper coating itself and the

handling properties of the coating dispersion 'referred to simply as

%color% in the mills). 5hile the list of possible ingredients is large, the

components can be conveniently grouped into three general categories:

pigments, binders, and additives, us summarized in Table !"$.

II 1 C ti , l ti


28/70


II 1 C ti , l ti


29/70



30/70


The purpose of the binder 'or adhesive) is to cement the pigment particles firmly to the

paper surface and to each other. Binders are used at the lowest level consistent with the

end"use requirements of the product, It must be kept in mind that the final dried coating is

not a continuous film, but rather a porous structure of pigment particles cemented togetherat their points of contact. If too

much binder is used, the voids begin to fill in and some light scattering capability is lost.


31/70

II." Coating -itchen

Since the different components of a coating mi(ture vary greatly in physical and chemical

characteristics, it is common practice to disperse and store each component separately and

then mi( together in the desired proportions. These operations are usually carried out in a

centralized facility known as the %coating kitchen%.

Some of the components must be prepared for use. &or e(ample, if pigments and binders

are purchased and stored in dry font, dispersions or solutions must be made up. -ssuming

that all components are ready for use, the sequence of operations is to meter each

component from storage into a high"viscosity mi(er according to a preset order for each

formulation, mi( thoroughly, and e(tract into agitated holding tanks. In a modern system,the components are added by pushing a series of buttons or inserting punch cards into a

computer console. The ingredients are then automatically metered either by weight or

volume. &lowsheets for batch and continuous systems are shown in &igures !"! and !"@.

Strainers are used at nearly every stage in the preparation and use of coating color to

ensure that foreign material and over"size solids cannot be applied to the product or buildup on equipment surfaces. =ndispersed or oversize pigment, flocculated pigment, or

undissolved adhesives can arise at various steps in the process and must be removed before

they cause problems.

II " Coating -itchen


32/70


II " Coating -itchen


33/70


II $ heolog/ of Coating Suspensions


34/70

II.$ heolog/ of Coating Suspensions

7heology is the science of the deformation and flow properties of

matter. &luid mechanics usually considers only the behavior of

6ewtonian fluids, which are characterized by constant viscosity.owever, starch solutions and pigment suspensions are among a host of

rheologically interesting materials that e(hibit strain"rate dependency,

i.e., in which the viscosity is nonlinear and varies with the flow. The

rheology of coating suspensions must be controlled so that the coating

color can be easily pumped and also perform adequately under the high"

shear conditions of the coating application system.

Because of their potential impact on coating system design and

development, it has been necessary to quantify the flow behavior and

properties of a wide class of dispersions. This has led to the

accumulation of a bewildering amount of e(perimental data, and to a

framework of practical fundamentals for the coating specialist '$)+ but a

full understanding of the behavior of interacting dispersions is still

lacking.

II 0n %achine !s 0ff %achine


35/70

II. 0n%achine !s. 0ff%achine


36/70

II. 0n%achine !s. 0ff%achine

II ( Coaters


37/70

II.( Coaters

- wide variety of single"sided coaters, as well as a good selection of equipment

for simultaneous two"sided application, is offered by equipment

manufacturers. Some paper products require coating only on one side of the

sheet+ a one"sided application is generally easier to control and simplifies the

subsequent drying operation. 3any double"sided coated grades are produce

stations with dryers in between. 3ost corner designs incorporate the following

features:

) uniform application of color to the entire paper surface.

) metering or attenuating the coating layer to control its weight or thickness.

$) smoothing and evening the surface.

The first on"machine coating was carried out using equipment adapted fromthe size press. In the 3assey print roll coater '&igure !"8), the color is supplied

by a series of metering or transfer rolls that smooth out the coating color and

spread a evenly by the time it reaches the two large applicator rolls.

Typically, one or more of the metering rolls oscillate. The pressure is varied

between rolls to control the amount of coating transferred.

II ( Coaters


38/70

II.( Coaters

II ( Coaters


39/70

II.( Coaters

In the air"knife coater '&igure !" ), lift sheet picks tip the coating

mi(ture from an applicator roll running in a trough of color. The sheet

then passes over a backing roll, where a sharp et of air impinges on thesheet, evening out the coating layer and blowing off the e(cess. In this

design, it is vital that the air et be oriented at the correct angle and be of

uniform intensity across the entire width of the machine.

The principle of the rod coater is illustrated in &igure !". -gain,the sheet picks up the coating mi(ture from an applicator roll. ere, the

doctoring and smoothing function is performed by a small"dianieter roll

or wire"wound rod which rotates in the opposite direction to the travel of

the web. Since web tension is used to maintain pressure against the rod,

this coating method is generally limited to heavier weight products that

can withstand the tensile loading.

II ( Coaters


40/70

II.( Coaters

II ( Coaters


41/70

II.( Coaters

II ( Coaters


42/70

II.( Coaters

Since their introduction in the @#8Ds, blade waters have undergone e(tensive

development, and a large number of designs are currently being employed. In all

cases, the web is given a generous application of color, and the e(cess is

removed by using a metal blade. In sonic designs, the blade tip is bevelled at the

same angle as the blade orientation: and the lip rides on a thin film of coating

and performs the metering and smoothing function. In other designs, the blade is

very thin and is fle(ed against the web. Cenerally, the so"called bent"blade

designs allow higher coat weights and are less prone toward scratching. In allcases, the angle and pressure of the blade against the metal or rubber"covered

backing roll determine the weight of coating retained by the sheet.

*ne of the earliest blade coaters was the pond or puddle"coater as illustraed in

&igure !"$. In more recent designs, the blade is separate from the applicator,

leading to the designation of trailing blade coater. 7ecent variants are the

inverted blade coater '&igure !"0) and the vacuum blade roster '&igure !"

#). ?esigns which coat both sides at the same time are the Billblade coater

'&igure !"A and !"9) and the opposed"blade coater '&igure !"!).

II ( Coaters


43/70

II.( Coaters

II.( Coaters


44/70

II.( Coaters

II.( Coaters


45/70

II.( Coaters

II.( Coaters


46/70

II.( Coaters

II.( Coaters


47/70

II.( Coaters

II.( Coaters


48/70

II.( Coaters

II.( Coaters


49/70

II.( Coaters

- specialized off"machine technique known as cast coating is used to

produce paper of e(ceptional gloss and smoothness. ere, the wet coated

paper is pressed into contact with a large"diameter, highly"glazedcylinder 'called a Eankee cylinder or machine"glazed cylinder) during

the drying phase '&igure !"@). Creat care must be taken at all steps

of the process. The base sheet must be fairly porous so that water vapor

given off during the drying of the coating can pass through it. The binder

must have the ability to adhere to the hot chrome surface when wet, and

then when dry, to separate without picking or plucking.

II.( Coaters


50/70

.( Coate s

II.* Short23ell Concept


51/70

p

In the coater designs reviewed thus far, a measurable time lag occurs

between the application of color and the metering2smoothing operation.

whether carried out by an air knife, rotating rod, or blade. -lthough theelapsed time is short, it is sufficiently long for some of the water and

binder to migrate into the sheet+ and thus the composition of the color

that is doctored from the surface is slightly different from that retained

on the sheet. This situation leads to inconsistent coating composition,

which has been recognized for some time as a significant problem.

-lthough the impetus for development of a short"dwell coater design

e(isted for some time, it was not until the early @!Ds that a

breakthrough design was introduced by


52/70

p

II.* 2r/ing of Coatings


53/70

/ g g

Sometimes, conventional steam cylinder methods are used for the drying of coatings

'refer back to Section 9.). 3ore often, other methods trust be used to avoid

disturbing the coating film. The two methods employed most often are hot air

impingement and infra"red drying '0).

igh"velocity convective hoods placed over conventional steam cylinders are a

popular method of drying single"sided coatings. Tunnel"drying is another approach,

suitable for both single" and double"coated sheets. In this method, the air temperature is

controlled to suit the drying requirements and speed of the machine, while the paper is

carried through the tunnel on rollers, supported on foils, or held up by air"impingement.

- complete on"machine coating system consisting of two single"sided coating stations

followed by their respective dryers is depicted in &igure !". - similar system for

off"machine coating is shown in &igure !".

-n infra"red emitter 'usually gas"fired) provides a compact, high"intensity heatsource which transfers its energy without any physical contact ideal for the drying of

coatings. owever, since the infra"red radiation unit supplies only a source of heat, air

must also be provided to carry away the moisture evaporated from the coating. Some

drying units, therefore, combine infra"red and air"impingement principles for more

efficient operation.



54/70

/ g g



55/70

/ g g

II.+ ,actors 'ffecting Coated Sheet Properties


56/70

g p

&ive general factors are of importance in determining the nature and unifonnity of the coating

layer'#):

) surface properties of the raw stock.

) composition of the coating.

$) method of coating.

0) method of drying.

#) e(tent of supercalendering.

The surface properties of the raw stock influence the formation of the coated layer in two

ways. Surface roughness has a significant impact on the coating thickness uniformity+ while

surface absorptivity determines the composition of the actual coating layer. 5hen the coating

first contacts the paper surface, capillary forces within the sheet structure cause a movement of

water"soluble components into the smaller pores of the sheet, leaving behind at the surface 'byfiltration action) a formulation richer in pigment particles.

The type and amount of hinder in the coating formulation has a pronounced effect on coating

structure because it influences the rate of fluid penetration into the raw stock, the degree of

filling between pigment particles, and the rate of drying. The basic structure of the coating layer

is more fundamentally related to the size and shape of the pigment particles and the degree of

packing.



57/70

g p

-dditives in the coating mi( will normally determine the fle(ibility of the dried

adhesive and the subsequent reorientation of pigment particles during

supercalendering.

&igure !"$ depicts magnified cross"sections of raw stock coated by three common

techniques. The air"knife coater tends to deposit a uniform layer that follows the

contours of the base sheet 'i.e., good coverage). The roll coater provides good

coverage, but patterning defects are introduced from the film"splitting. Blade coating

results in good filling in of the surface valleys, but the uniformity of coating layer

thickness is sacrificed to obtain increased smoothness. /ach coating process forms a

somewhat different layer+ hence, some coating operations employ multiple coating

steps to combine the advantages of two or more methods.

The effects of drying conditions on coating structure can be significant. If the coating

dries too quickly, those areas with a thicker deposit or a slower rate of absorption willretain a higher proportion of adhesive, and thereby yield a different coating structure.

-s the speed of the coating operation increases, and the time interval between

application and drying becomes shorter, this problem can be more severe.



58/70

g p



59/70

?uring drying, a considerable amount of shrinkage occurs in the

thickness of the coating layer. The e(tent of shrinkage is mainly a

function of the solids content of the initial dispersion, but is also affectedby the shape of the pigment, the degree of dispersion, and the physical

properties of the binder. The shrinkage is undesirable because a portion

of the original sheet roughness returns, as shown in &igure !"0+ this

problem is most severe with low coat weights.

Supercalendering is often carried out on coated sheets to compact the

coating structure and develop a greater level of smoothness. If the

coating structure is not uniform, it is likely that the supercalendering will

further emphasize the non"uniformity. -reas of the coating structure

relatively rich in adhesive will not develop as high a gloss as adacent

areas, and the sheet will e(hibit a finely"mottled appearance.



60/70



61/70


62/70



63/70

III. SuperCalendering


64/70

The typical stand"alone supercalender consists of a series of rolls arranged

vertically, with alternating hard metal rolls and soft rolls made from

compressed fibrous material. The web of paper is fed from an unwind stand

into the top of the stack, through each nip, and out the bottom into a rewind

unit. Fuite often, the web is fed around lead rolls into each nip to prevent air

entrapment that could cause creasing. - typical arrangement is shown in &igure

!"9. This equipment is used to develop smoothness and gloss in such

products as coated and uncoated high"quality printing papers.3achine calendering has been discussed previously 'see Section 9.). &or

many grades of paper, the required degree of smoothness can be achieved with

hard steel nips. owever, if a smooth, highly"glazed surface is required without

over"compaction, then a different type of calendering action is required. *n"

machine soft"nip calenders are now being successfully utilized for intermediate"

quality grades. owever, for production of the highest"quality printing papers,

there is no satisfactory substitute for supercalendering.



65/70



66/70

The unique results achieved with supercalendering are due to the intermediate

fibrous rolls 'sometimes referred to as howls or filled rolls) which possess elastic or

plastic properties. 5ith the application of load on the nips, the metal rolls cause a

depression or deformation on the fibrous rolls at the point of contact, and thedeformation spreads out on either side of the nip '&igure !"!). 5hen rotated, this

spread area will creep because of the constant effort of the material to return to its

normal shape. This flow causes a relative motion of the filled roll surface against the

metal roil surface, thus producing rolling friction, which helps to give the polishing and

smoothing effects. The intensity of action is governed by the amount of plastic flowand by the nip contact pressure.

In contrast to the hard"nip machine calender, the supercalender characteristically

converts part of the applied energy into heat through deformation of the soft rolls.

Typically, about #1, of the drive power goes into roll heating. =ntil recently, this

heating was viewed as undesirable because it reduced the life of conventional filledrolls. owever, the development of modern %controlled temperature rolls% 'refer back

to Section 9.) has made possible the utilization of heat for better control, particularly

when optimizing production 'A).



67/70



68/70

5hen the rolls alternate all the way down the stack, a high finish is

imparted only to the surface in contact with the steel rolls. Such an

arrangement is known as a single"finishing supercalender. The placementof two consecutive steel or fiber rolls in the middle of the stack has the

effect of reversing the nips and giving a similar finish to both sides of the

sheet. - stack with a reversing nip '&igure !"@) is known as a double"

finishing supercalender.

-n important aspect of supercalender operation is to select the proper

material for the filled rolls, which are made to a specified hardness. The

composition is a mi(ture of cotton and other cellulosic fibers, sometimes

containing up to #1 wool fibers. The traditional method of bowl

preparation is to cut composition material into donut"shaped discs which

are assembled and compressed onto a shaft, and then fitted onto a lathe

and ground to a fine polish.


69/70



70/70

The 6orth -merican paper industry has been actively looking for

alternate soft roll materials for supercalenders. In recent years, some

polymer cover rolls, which do not generate heat and which are markresistant, have replaced cotton"filled rolls in less critical applications.

6ew soft"roll technology is e(pected to proceed along with the

development of on"line soft calendering.

Supercalendering is almost always carried out as a separate 'i.e., off"machine) operation because of the delicate nature of the filled rolls.

These rolls are easily damaged or dented by torn paper or lumps from

any source getting into the nips. To prevent marking of the paper surface,

any damaged rolls must be immediately replaced. The frequent

shutdown, and delays would be intolerable on a papermachine or coater.

Documents

Chapter 18 Surface Treatment