SD Warren Bulletin Coated Papers for Web Offset Printing

7/29/2019 SD Warren Bulletin Coated Papers for Web Offset Printing

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Bulletin No.

CoatedPapers for

Web OffsetPrinting

A



WARREN

How w ill it print?

Coated Papers for Web Offset Printing is beingissued by S.D. Warren Com pany to aid the graphicarts community in dealing with the com plexit ies of theprinting process. Information contained in thisbooklet combines the findings of scientists and theobservations of experienced craftspeople.No scientist will claim that existing knowledge iscomplete, and no sincere craftsperson will pose as afinal authority. The text of this bulletin, therefore ,represents merely the considered opinions ofexperienced and thoughtful analysts.



CoatedPapers for

eb OffsetPrinting

ow to Select the

e, High Qualityrinting

If you were printing a catalogtomorrow and its quantity exceeded25,000, chances are you'd chooseweb offset as your method .

One of the fastest grow ing of m odernprinting tech nolog ies, if not thefastest, web offset is a regular cho ice

for medium to long runs of catalogs,direct mail and promotionalbrochures, magazines, textbooks andhardcover trade boo ks.

Web offset offers the printing buyerquality, flexibility, and econo my.It is generally tho ught to havesurpass ed sh eetfed offset in ink gloss.. .and to provide greater flexibilitythan gravure in a ccom mod atingchanges in copy and variations inlength of run. Equally important are itsgreat speed and complete in-line

finishing capa bilities. Together theymake web offset a "bes t-bu y" in themedium to long-run category.

To fully exploit these advan tages,however, you must use a paperdesigned to meet the demands ofthe web offset pro cess... demandswhich are considerable.

This booklet will...

• Help you to understand thesedemands

• Show what properties a pa per musthave in order to fulfill them

• Describe how different

manufacturing processes andmaterials can affect the suitability ofa pape r for high quality, high v olumeweb offset printing

Normal paper costs can range from30% to 50% of a production budget.That percentage may be increasedby waste if paper isn't up to thedema nds of the job. It's clear,therefore, that you must e xercise thesame care in selecting paper that youwould in planning copy, typograp hyphotos, artwork, and design.

For paper is the sho wcase thatenables these other elemen ts not onlyto get your message across , but to doit with style and impact. In manycases, this may be as impo rtant as thespec ifics of what you have to say.



mm

i



HowThis Book isOrganized

So you can fully app reciate thechallenges your paper faces whenbeing printed by web offset, we beginthis book by describing the mostcomm on web offset jobs, the spec ificoperations involved, and thecharacteristics that a paper must

have for top performa nce duringthese operations.

The Prominence ofWeb Offset Printing

The Web Offset

>ss

Then you are introdu ced to differentpaper types and have an opportunityto observe in a variety of printingsimulations how each performs undethe stresses of web offset.

Page 24 Types of Paper

for Web OffsetPrinting

luirements

Finally you learn the five majorcharacteristics to consider inchoosing a paper for web offsetprinting.

Page 30 Summary

buil



TheProminenceof Web OffsetPrinting

The use of web offset is growing inconjunc tion w ith two trend s. First,advance s in its own techno logy haveresulted in better print quality and ingreater economy on shorter runs.Secon d, the m arkets that web offset isbest suited to are expand ing atsteady rates, particularly in the fieldsof specialty magaz ines, direct m ailmaterials, and catalog s.

When mu lticolor web offset wasintroduced thirty years ago, paperwaste was high be cause of thecomplicated makeready processneeded to compensate for less thanideal materials. Inks were frequentlyinconsistent. Many papers were oflow b rightness. Plates were short livedand plate making cum bersome. Theresult was often a finished produ ctthat looked weak and gray, with co lors

that might vary througho ut a run.

•mwv

- Speed, economy, and a superior finished prohave come with automation and better platespapers, and inks. Yet it is the skilled pressmancontrols the system and determines ultimate



Running at speeds of up to 25,000 impressions perhour, a web press with in-line finishing can print,fold, and bind within just that single hour w hat itmight take all day to accomplish by a sheetfed pressand separate finishing.

0

Web offset and gravure a re today's two fastestgrowing printing methods, with web offsetaccounting for the largest sha re of overall printingsales. Steady growth in such markets as books,specialty magazines and demographic editions ofweekly magazines, plus catalogs and direct mailmaterials in quantities of under one million shouldcontinue this trend. Source: GATF.

B Magazines/Periodicals Today the situation has changedcompletely. Improved inks, blankets,papers, and registration monitoringsystems have vastly improved pressrunnability. Automation and bettermaterials have boosted plateperformance and durability...therebycutting makeready t ime and paperwaste, whic h, in turn, have reducedthe minimum qu antity for aneconomical run to 25,000 copies.

As a consequence of thesedevelopments, the demand for weboffset printing has grown to the po intwhere it is dominant in severalmarkets. It is now the preferredmethod for printing specialtymagazines and most others withcirculations or demographic editionsof between 25,000 and 1,000,000;

and (in the same quantity range) forcatalogs , direct mail materials, andvirtually every type of caseb oun d a ndtrade-quality book.

Because these markets themselvesare growing with various economicand social trends, web sales shouldcontinue to rise. Only gravure iskeeping pace by expanding withmarkets where extremely long runsare involved. In terms of the totalprinting market, however, gravure'spercentag es are still quite small whencompared to those of web offset.



TheWeb OffsetProcess

Each of the four to six units on a typicalblanket-to-blanket web offset press prints bothsides of the web at the same time... squeezing thepaper between two blankets with considerablepressure.

B

In the drying oven, ink solvents are flashed off at atemperatures that exceed 450°F, which raise thepaper's own temperature to almost 300°F. Thenin the chill rolls that follow, its temperature isimmediately dropped by about 200°F and the inkis set.

Imagine a sheet of pap er more than ahundred feet long. Now picture itbeing stretched and contracted...hitwith water and ink...driven from roomtemperature to nearly 300°F in lessthan three-quarters of a second.. .dro pp ed to under 100°F in lessthan one.

That is a fair image of what pape rendures as it passes through themulticolor web offset press...acolossal m achine able to print apape r roll at over 25 mph. In athree-shift day this can mean morethan enough paper for a round tripbetween New York and Washington!

The ordeal begins as paper is pulledfrom the infeed to the first printingunit. (Figure E) There water along w ithink is transferred to the paper. Du ringthe transfer, the paper adheres toblanket to such an extent that theforces requ ired to pull the paperfrom blanket produce a hard snap.(Figure A)

Under the pressure exerted by eachsuccessive unit, paper stretches.Then, after the last of the four, five, orsix units that a press may have, thepaper enters an ink drying oven andshrinks by as much as an eighth of aninch across a 35" web.

Air temperatures in this oven canexceed 450°F, the level needed toflash off ink solvent and papermoisture. But in this pro cess, thepaper's own surface temperatureof 250°-300°F is raised h igh enoughto burn your h and. (Figure B)

Cooling the paper and setting theinks is the job of the chill rolls, wh ichcome next. They drop the paper'stempe rature by almost 200°F in lessthan a seco nd.

Splicing and feeding in a new paper web isaccomplished without stopping the press by theaction of the dancer rollers (or automatic splicers)located just before the infeed, which adjust paperflow until the new web has reached full runningspeed.

Notice that on the press you see here, two webs arebeing run. The one shown in black is printed by thefirst unit (black or special matched color) only. Th eweb shown in white follows the usual path througha five-unit press.

Accumulators

AutomaticSplicers

Black Unit orA , ,. Special PMSAutomatic c £ | o r U n i tInfeed

CyanUnit

Pre



With an in-line finishing s etup, a job can go frompaper roll to completed brochure with cover letter,

reply card, and envelope addressed for mailing inone operation. Here the paper web is being s lit intotwo or more ribbons, which will be turned, stacked,and realigned one over the other in preparation foradditional operations.

The plow folder (foreground) folds the ribbons oneor more times along their line of travel.Otherfinishing operations include gluing, perforating,die-cutting, imprinting, trimming and cutting-to

meet whatever requirements the buyer stipulates.

Oven

i3lackJnit

r"--~j

:- -. %

•

/

i

HighTemperatureDryers

: i 3

ChillRollers

On-PressChopper Folder

As if surviving all this weren't testenough of a paper's strength andresiliency many web presses nowinclude in-line finishing equipm ent.Thus a job can go from paper roll tocomplete mailer...folded, gathered,perforated, stitched, stapled orglued...with cover letter, reply card,

address label and envelope...all in asingle operation. (Figures C and D)

This is convenient, of course, andecon om ical. But in-line finishingplaces such additional burdens onpaper that, when m aking your papercho ice, you must think not only of whawill happen on press, but also of thefollow-up.

To be specific, supp ose that yo u'reabout to release a new produ ct withgreat potential for being s old by directmail. You've dec ided on a mailing

package that wil l include-along withenvelope, letter, business reply cardand a variety of other ins ert s-afull-color circular cut to a sheet size of17" x 22'.' Itwill be folded twice,perforated for a coup on , and have aplastic medallion tippe d onto its cover

When selecting a paper for thiscircular, what should you look for?

Strength

Smoothness and Printability

Brightness

Opacity

And, of course, Affordability.

In the next section of this booklet,you'll learn how to identify each.



PaperRequirementsfor Web OffsetPrinting

Types of paper strength and printing problemsavoided.

Strength

If paper cann ot be run through apress without breaks or ruptures, thequestion of how well it handles an inkfilm becom es a cadem ic. The mostbasic printing requirement, therefore,is strength.

Paper strength is multidimensio nal,meaning that it is a com bination ofproperties, some internal and somerelating to its surface. (Figure A)Internally, for exam ple, there isMullen or b ursting s trength, which isthe ability to withstand punc ture.Although not a major printingconcern, it can be cruc ial in foldin g,perforating, and other in-line finishingoperations.

The importance of tear strength,

another internal property, was implicitin the prece ding discussio n of strainsimposed by the web offset process.For these same reasons, paper needstensile strength.. .to withstand thestresses that cause breaks in the web.

Problem Avoided

Puncture

Bond,internal

Blister anddelamination

Fiber puff ormicro-blistering

Holes and hickeys

Bonding strength is necessary first toprevent delaminating and blistering,particularly in areas of heavy inkcoverag e. Second , it helps prevent

fiber puff, a condition that occurswhen m oisture laden fibers or fiberfragments expand under the surfaceof halftones and heavy solids while inthe ink drying oven. Where bonds areweak, these fibers break apart fromeach other and, although minute, theyare visible to the naked eye whenviewed by strong side light.

On its surface, paper needs print (orpick) strength. Lack of print strengthis evidenced by "holes" where thereshould be ink. These occu r whe reverthe surface is weak, allowing coa tingand/or individual fibers to be pickedoff by the blanket. They rema in on theblanket and produce white spotswhen caused by loose coating orfiber. Hickeys or spec ks w ill app earon every impression until either theywork their way up throug h the inktrain or the press is stopp ed and theyare washed off. (Figure B)

Smoothnessand Printability

After determining that a paper canmake its way through the press intacthe next considerations are uniformitof process co lor and quality ofhalftones, line art, and type. Heresmoothness and a suitably tightsurface...a paper's printability...areprime requirements, for they largelydetermine what happen s to an inkwhen applied.

If images are to appe ar sharp andwell detailed, the ink film must be laidown so that dots are of uniformshape and thickness; edges of typeand line art must be clean andregular. If colors are to be vivid,pigments and resins in the ink mustremain on the paper's surface, not

drain into the body of the sheet.

Light is also scattered and co lorsweakened by a rough paper. This isbecause of its many non-uniformsurfaces. Black ink film can beespecially disappointing. Formaximum intensity, black must abs olight; without a smooth surface tosupport a uniform and non-scatterinfilm, it turns gray. (Figure C)



B

A paper m ust have good print strength to performsuccessfully on a web offset press. When printstrength is lacking, fibers or coating can be pulledfrom the paper surface by the ink on to the b lanket,producing a white void in print. If the fibers orcoating are transferred to the plate it will print asa pick out as shown here.

Unless a paper's surface is smooth and tight,halftone dots spread, reducing contrast andhalftone detail, as shown in the print on the right.



In this simulation, it's easy to see that brightness, orthe amount of light a paper reflects, has a majorinfluence on color intensity. Compare the print onthe left, made to appear as it would on a highbrightness paper, with the one on the right, made toappear a s it would on a paper of low brightness.

Brightness

Another property that good printquality depends on is brightness,which is generally though t of as ameasure of how much light a paperreflects. Brightness determinescontrast, of course, by making blacks

and type stand out. But it is even moreimportant for intensifying color.

This is because multicolor printinginvolves transparent inks, which aredesigned to produce virtually anycolor of the spectrum with only cyan,magen ta, and yellow. Beingtransparent, such inks act like filters,transmitting certain lightwaves andabsorbing others. So to produceoptimum color, transparent inks mustbe illuminated not only by lightstriking their surface, but also by light

reflecting back through them fromthe paper itself.

To understand the proc ess, think of apane of green glass (a filter) beingheld up against different backgrounds:a white, a gray, and a black . Wh ichbackground makes the glass appearbrighter? The white o ne, of course.Why? Because it reflects m ore lightback throu gh the glass than either ofthe other two.

Paper provides illumination in exactlythe same way The b righter the paper,

the more light it reflects, and the moreintense the inks appear.

10



Opacity

Opacity prevents type and imagesprinted on one side of the paperfrom showing through to the otherside or to the adjacent page s. Thisis an important property becauseshow-through is distracting to the

reader and can also interfere with thetonal qualities of delicate halftones.

In order to have adeq uate opacity, apaper must block light from passingthrough it and being abso rbed bytype or ima ges on the reverse side. Itis this "reverse-side absorption" thatproduces show-through. To preventit, a paper must reflect or scatterlight from its surface or from withinits interior.

Opacity can also be influenced by apaper's weight, brightness, andinternal forma tion.

Affordability

For superior results in high volumemulticolor w eb offset printing, a papemust have all the properties that havebeen des cribed: strength, a smoothand printable surface, brightness anopacity.

And because paper represents sucha large share of the overall prod uctiobudg et, it must also p rovide theseproperties at a price/quality level thawill be considered a good value.

This is the challenge of papermaking.. .the incentive for m anufacturers tocontinue sp ecializing their linesand impro ving the value of eachprodu ct. Today a paper buyer hasmore choices than ever before and,consequently, needs a more thorougunderstanding of paper types andhow each one attempts to ach ieve agood balance between printingrequirements and market value.



Wood,Fiber and

The distances between these growth ringsrepresent the amoun t of new material that a treeadds each year. The dark outline of the ringsthemselves are composed of thick walled summerfibers.

Wood is 50% cellulose fiber, the basic componentof paper. The remaining 50% is a combination ofmaterials, which, when used in printing paper, maydegrade its quality.

Paper consists mainly of cellulosefibers, wh ich, in their natural state, aretubular structures much finer than ahuman hair and shorter than a grainof rice.

Although any plant is a potentialsource of cellulose for papermaking,

woo d has proved to be the mostpractical. Cellulose fibers acco unt for50 % of the mass of dry wood, givinga tree rigidity and often serving asits fluid transport system. Another30% or more of the wood is lignin,the "glu e" that binds fibers together.What remains is a com bination ofcarbohydrates, proteins, gums,resins, and fats used in facilitatingvarious growth and maintenanceprocesses. (Figure B)

In the spring and sum mer of ea chyear, a tree ad ds a new layer of thesematerials. Spring fibers, form edduring the time of the fastest gro wth,have large ce nters and thin walls.Summer fibers, formed when thegrowth rate has slow ed, have sm allercenters, thicker walls, and are visibleas the outline of the growth rings in atree's cross section. (Figure A)

All fibers run parallel with the trunk.In softwood trees (such as spru ce,fir, and pine), they average 3 to 5millimeters in length and are theprimary wood ce lls, functioning bothto conduct fluids and to providesuppo rt. (Figure C) In hardwood tree(such as oak, poplar, and ash), fibersare shorter, ranging from V2 to 3millimeters, and provide suppo rt onlySap and water conduction is

performed by much larger vesselcells, which are compose d of thenon-cellulose resins and gums.(Figure E)

12



Cellulose fibers, which run parallel w ith the trunk,are shaped like miniature drinking straws, 100 timesgreater in length than in width. In softwood, thesefibers are the primary wood cells. Spring fibers havelarge centers and thin walls. Summer fibers havesmall centers and thick walls. The l ine of summerfibers is visible as the annual growth ring.

In hardwood, large vessel cells appear am ong thecellulose fibers. Unlike softwood, in which fibersprovide both support and fluid conduction, herethey are for support only.Vessels carry the waterand sap. (Figures C an d E courtesy SyracuseUniversity Press).

Fibers are bonded together by lignin, which runsbetween their outer walls in a layer called themiddle lamella.

Individual whole wood fibers. Top: softwood,averaging be tween 3 and 5 millimeters in length.Bottom: hardwood, averaging between V2 and 3millimeters.

Most of the lignin in woo d is foun d inthin layer, the middle lamella, that runbetween the fiber walls. (Figure D) Amajor challenge in papermaking is toseparate the fibers at this po int andpreserve as many of them aspos sible, intact and free of lignin and

the other wood "impurities." Successhere is important because fiber lengtand purity are two key variables indetermining how strong and howbright a pa per w ill be. (Figure F)

1



A

Chemical pulp is produced by cooking eithersoftwood or hardwood chips with a dissolvingagent in a pressurized digester. Lignin is softenedthen extracted with other impurities and theprocessing chemica ls. Lignin and the impurities aburned in the chemical recovery process and thesteam produced is used for cooking more chips.The chemicals are recycled.

Stone groundwood pulp is produced by forcingwhole softwood logs against a grindstone. Althou

the pulp is then washed, many impurities remain.

Pulping

onverting whole wood into separatecalled pulp ing. As previously

it is a critical man ufacturingfor it determines several of a

paper's most important qualities.

chemical an d mechanical.

hemical pulping begins with chips,

e "co oke d" under pressure in aagent to soften their lignin.

in length. (Figureremoves

most of the lignin (and the othernon-cellulose materials as well), fiberis virtually all that the re sulting pulp

Even though it eliminates impurities,

% of the original dry w ood for useing m aterial. And that

50 % of non-cellulose impuritiess not wasted .

l beco mes the fuel for

The chemicals themselves ared after one use. Each is

"reactivated," then routedinto the process.

In mechan ical pu lping, fibers areseparated by physical abrasion.(Figure B) The most traditional of themechanical methods, stonegroundwo od, employs a circulargrindstone against which whole logsare forc ed. Because fibers are tightly

bound by lignin, this action g enerallycauses them to fragmen t an d tear,sometimes in chunks, rather than toseparate cleanly. The result is a pulpwith a high pe rcentage of short,broken fibers, fiber d ebris, and fiberchunks or bun dles. (Figure D)

Although this pulp is then washed an dscreened, very little of the lignin orother impurities are rem oved, whichmeans a high yield (usually 90% ormore of each log) but also a highpercentage of materials that may

seriously d etract from paper quality.The stone groundwo od processsuffers from two additional p roblem s.First, it cannot use chips (which aremore convenient to work w ith thanlogs). Seco nd, it is principallydependent on long fibered andexpensive softwood (becausehardwoo d fibers, which are naturallyshorter, would be groun d too small formost applications in papermaking).

A me chanical m ethod that is able touse both chips and hardwood is

thermo-mechanical pulping (IMP).The essential difference between itand stone ground woo d is that in TMRwood is preconditioned by heat andpressure before it is grou nd, whichsoftens its lignin. The grinding itselftakes place under pressure, whichfurther softens the lignin. Bothprocesses (heat and pressure) makefiber separation less damaging.(Figure E)

A Chemical Pulp

B Stone Groundwoo d Pulp



Most fibers in chemical pulp are whole

ashed out.

ibers in stone groundwood pulp are fragmented

es m ake up a considerable percentage of

While each of the mechanicalprocesses mentioned herehas unique fea tures , all involvegrinding. In gen eral usa ge,therefore, even TMP is oftenreferred to as "groundwood."Throughout this bulletin,unless groundwo od is

referred to specifically as"stone groundwo od," it shouldbe understood to include TMP.Also note that wh en the term"free shee t" is use d, it refers toa paper that is mad e exclusivelychemical pulp, that is , onethat is free from groundwood.

TMP, l ike chemical pulp, uses wood chips. They ared to soften their lignin, then ground under

Thermo -Mechanical Pull

TMP, therefore, contains a higherpercentage of long, unbroken fibersthan stone groundwood, althoughthey are shorter on the average andmore are fractured than those inchemical pulp. Because impurities in

TMP remain, the yield is high; but asstone groundw ood, they deg radepulp quality and undermine to somedegree the potential advantages oflonger fibers. An additional dra wba cof TMP is its very high ene rgyrequirements. Heat recovery systemmay someday help to defray this cosbut at pres ent, there are only a fewmills in North Ame rica where suchsystems have been installed. Toreduce the cost of energ y yet stillexploit the advantages of "thermalsoftening," another mechanicalprocess has recently been introducepressurized groundwood. Likestone ground woo d, it uses whole logbut they are grou nd in a highpressure atmosphere, creating hightemperatures that soften the ligninand liberate more whole, undamagefibers.

Other mechanical processes (semi-

mechanical, chemi-mechanical,

and chemical thermo-mechanical)are used in the p roduction ofcorrugated board, newsprint, andpacka ging materials, but none ofthese is yet involved, to a significantextent, in the production of betterquality coated printing papers.

Pressurized Refining



A

(The sheets shown here were made from chemicalpulp at four stages of bleaching.) The end-productof pulping is brown sto ck (top), which is thoroughlystained by dissolved lignin. Chlorination (first)dissolves external impurities. Extraction removesinternal impurities (second) but recolors the surface.At the end of the hypo stage (third) the pulp is asemi-bleached yellow. The final stage (fourth),brightens the pulp from yellow to blue white.

B

Bleaching groundwood pulp is generally a simpl

single-stage process. Unbleached groundwood(top) is somewhat lighter than chemical brownstock because lignin is not dissolved in themechanical process. Yet because it is softened bheat in grinding, there is still some residue of lignalong with a variety of other impurities clinging tothe fibers. Bleached groundwood (bottom) isbrighter than unbleache d but usually less brightthan bleached chemical. That shown here has abrightness level of about 70-75.

Bleaching

After p ulping , fibers are washed toremove dissolved lignin and screenedto remove fiber chunks or b undles.But washing does not affect the ligninthat has stained the fibers a dark

brown. Because of this staining, theend-product of pulping is usuallyreferred to as "bro wn stock." B efore itcan be used in the man ufacture ofbetter printing papers, it must bebleached.

In the making of a free sheet,bleaching can be a four- to six-stagproces s. It begins w ith chlorinationwhich dissolves surface impurities.Then there is extraction, which opethe fiber and removes impurities frowithin. In that p roc ess , however, thefiber's surface is "rec olo red " so it mlook almost as dark, som etimes evedarker than it did at the previous staIt takes another step, whitening, toreduce this stain to a se mi-bleac hepale yellow. The final step in makinga free sheet, brightening, turns fiberbright-white. (Figure A)

Bleaching groundwood is generallysimpler, single-stage proce ss. Whe

complete, most bleachedgroundwood pulp is only as bright asemi-bleached chemical pulp.(Figure B)

16



.the celluloseinto a thin,

ular mesh. D epending

ed or still tubular, whole orflexible or stiff, t ightly packed

wood impurities, or spe cial

he mechanical processes of

between the fibers directlyther

of cellulose fiberbond s pos sible. First, it is

ter, be com ingven gelatinous, but without

nd, the fiber's structurethat of a rope, having walls

ore, they m ust conform to one

t. The strength of their bo nd is

ore than just the fibers'or surfac e.

ed with pulping and blea ching.in large

er, where fibers beginThe softer and more flexible

come , of course, the m orearea they develop an d the

Refining

Refining co ntinues the pro cess. In aslurry con taining abo ut 95% water,pulp is pum ped between serratedmetal disks, which collapse the fibersand rupture their outer walls. Thus

flexibility is increase d, and m any ofthe fibrils unravel, swelling with waterto create additional bonding sites.

At this point, there are importantdifferences between chemical andgroundwood pulp and their relativesuitability for papermaking.

In refined che mical pulp, the m ajorityof fibers are whole, collapse d, highlyfibrillated, and very flexible. These arecharacteristics that produce a strongpaper. And because there is a goodmix of shorter hardwood fibers amongthe longer softwood, such a paper willhave a smooth surface.

Also in chemical pulp, lignin and otherwood impurities have been dissolvedand washed away and the purecellulose fibers have been bleachedto a bright white shade. It will takeonly a few add itives.. .internal sizingfor water re sistance, dye for sha de,and/or some mineral pigment foropacity and greater brightness...tocomplete the com bination ofmaterials, known as furnish.

With stone groundwood, however,additives serve not only these butseveral additional function s. Since fewof its fibers have remained w hole,groundwo od furnish lacks longexpanses of bonding area that givepaper strength. And not having been

softened as much as those in achem ical p ulp, these fibers are stiffand do not as easily conform to oneanother. Moreover, fiber bundles,which are difficult to screen outentirely provide relatively no bondingpower at all.

17



Additives such as those shown here (top, calciumcarbonate, a pigment which is also used forbrightening and protection against acid reactions;middle, titanium dioxide, a pigment and brighteningagent; bottom, clay, a pigment) are used to facilitatemanufacturing processes and achieve a variety ofend-use properties.

in ground woo d, therefore,s to a large degree on "fines,"

are small pieces of fiber that

also improve d by the

this additional material gives

itself. Usually it is blended withoportion of 50% of a

the darkening effects

el needed by a printing paper,

calcium carbona te and clay)t be added .

Finally, as with c hem ical pulp , internsizing is used to pro vide resistancewater penetration.

Refined TMP contains a higherpercentage of whole fibers than stogroundwo od because of the

preliminary thermal-softening of itslignin. But this prevents neither fibetearing nor the presence of manyfines. Softened lignin actually is aproblem in itself since it adheres inthick coat to the separated fibers,reducing their ability to abso rb watecollapse, and become well fibrillateFor this reason, TMP depen ds evenmore than stone groundw ood on finand additives for bonding ability anstrength.

Brightness in TMP may also be mor

of a problem , for the heat ap plied inpulping actually darkens the lignin.Again, a pigment such as titaniumdioxide is called for, plus sizing asrequired by all pulps.

Like stone groundw ood, TMP iscom bined with chemical pulp inbetter printing p apers, but fornewsprint it is sometimes used alon

1



Pre

CleanerHeadbox

Refiner • • •



Dryer

20



B

paper machine seen from the "wet en d." At theleft is the headbox, from which furnish is

The web then moves into the

reduced to about 65%. The largest component of

right. Here the paper is dr ied and sometimes

end." This web is

d either prior to additional finishing(calendering, coating, etc.) or for cuttingIt is 290" wide. The

webs at3,000 feet-per-minute.

the P aper Web

n be as long as

y tuned instrumentsin any industry. Yet they are

pos ed essentially of just four

adbox , the wire, the

cribe d, it is called ais 99% water. The type of

ndw ood , or TMP) has virtually nothe mec hanics or the

involved in making a

Paper is formed during the first fewfeet of travel on the machine. (FigureA) Furnish is fed from the headboxonto the wire, a continuous fine-meshscreen where about 20 % of the wateris drained off.

From the wire, the new p aper webpasses into the press se ction, whereit is squeezed between several pairsof rollers that reduce water content toabout 65%. Then the web begins itstrip through the dryer section, whichis the largest com ponen t of themachine. Here the web has its watercontent red uced to about 5% as ittravels through scores of steamheated drums.

On some m achines, coaters are alsolocated in this dryer section as shownin the diagram below.

B l a d e C o a t e r( f i rst s ide ) Dr ye r

B l a d e C o a t e r( seco nd s ide ) D r ye r



A film coater applies m aterials with low solidscontent to both sides of the paper simultaneous

BA blade coater applies a thick coat of material onside of the sheet at a time. Excess is removed byflexible trailing blade, which also smooths thecoating material.

Coating

Coating is the next step in themanufacture of papers intended forbrochures, annual repo rts, and betterquality m agazines. This is becau sethe surface of even the finest pa per

remains somewhat rough and porousas it forms on the ma chine and beginsto dry.

Also present may be some poorlybonded fibers, which can be p ickedoff by a tacky ink and de pos ited on anoffset blanket.

Neither the addition of sizing nor aperfect blend of fibers (short with longin chem ical pu lp, fines with fragm entsin mechanical) is sufficient by itself tofill all the crevices and to create asurface that is tight, flat, and smoothenough to produce superior halftones.

For that, coating is req uired.Coating m aterial contains a mixture ofpigments (like clay or calciumcarbonate) and adhesives or binders(like starch, casein and latex).App lying this m aterial to paper is likepainting wo od; it fills the pores andforms a new surface smoother thanthe original.

Coating is applied either as part of thebasic paperm aking process (machinecoating) or as a later and separatestep (off-machine c oating). Withinthese two general technologies thereare many variations, the princ ipalones being film coating, blade coating,and a combination of both.

Film coating is actually a pre-co at,like wood primer, althou gh it is theonly coating that some papersreceive. Coating material is a ppliedsimultaneously to both sides of thepaper web by a small press locatedwithin the drying section of themachine. (Figure A) Because thematerial is applied and smoothed by

rollers only, it must be low solids incontent (aga in, like wood primer).Much of a film co ating, therefore, isabso rbed into the paper, leaving o nlya thin layer on the surfa ce. (Figure C)

22



Because of the low solid content of the film coating(magenta), most of it flows into the paper (yellow),leaving a thin, uneven layer on the surface. Anink film (blue) printed on this coating will also berelatively uneven. (Highly magnified cross section)

With its high solids content, most of the bladecoating (magenta) remains on the paper's surface in alayer that can be made up to one-tenth of its thickness.An ink film (blue) printed h ere will be highly uniform.Papers coated by both blade and film have ofcourse, an even thicker, smoother surface. (Highly

magnified cross section)

1 r

Paper

Blade coating is more like spre adingbutter or applying the thick finalcoat of paint that leaves a board'ssurface smooth and even. With thismethod, the paper receives a heavyapplication of material one side at atime. Excess is removed and theremaining material is smoothed by a

flexible trailing blade . (Figure B)Some of this co ating, too, is abso rbedinto the paper. But because of its highsolids content, most remains on thesurface to form a layer that can be upto one-tenth the thickness of thepaper itself. (Figure D)

What's the best surface of all?When a paper is both film an d bladecoated, it's sealed by the first andsmoothed by the second . And w hensuch co atings are given the benefitof being app lied to a smoo th, well

formed paper base, you can lookforward to halftone effects that aretruly spectacular.

Paper

2



Typesof Paperfor Web OffsetPrinting

Tests indicate that a free sheet is generally higher inboth internal strength and surface strength thanpaper made from either stone groundwood pulp orTMP. Here is a chart summarizing the results oftests conducted in Warren's Research Laboratorythat compared the print strength of 11 differentpapers, 5 coated free sheets and 6 coatedgroundwood-content sheets.

Strength

Both internal strength (Mullen, tensile,and bond) and surface strength (printand resistance to fiber puff) arerelated directly to fiber bo nding. Thepaper that is the best bo nded shouldalso be the strongest.

Whenever the principal paper typesare tested (free, stone ground wo od,TMP), the free sheet, which has beenshown to have the greatest b ondingpotential, exhibits superior strength inthose strength categories that affectweb press performance. (Figure A)

It breaks and tears less often thangroundwood or TMP (indicating hightensile and tear), is less subject toblistering (high internal bond), andgenerally performs better during mostin-line finishing operations (high

tensile and Mullen).

For evidence of a free sheet's greatersurface streng th, one can look tocom parisons of the same print havingbeen run on a free sheet and then ona groundwood content sheet.Although the results demonstrated onthe opposite page will not beduplicate d exactly every time one of

these papers is used, they are typicalenough to indicate a strong tendency.

Lack of print strength shows up aseither white specs or "hicke ys" in thesame locations, impression afterimpression, throughout a run. Thiscondition is often referred to as"picking." It is caused by loose fibersor bits of coa ting which are notwell bonded and, therefore, canbe picked off by the tack of theink and left on the blanket. Correctingthis problem means delays and paper

waste.The press must be stopped andwashed, unacceptable impressionsdiscarded, and ink tack reduced,possibly affecting the quality of theprint. (Figure B)

Fiber puff, commonly associated withgroundwood-content paper andsometimes called "heat setroughening," is noticeable only afterthe printed paper passes through thedrying ovens. It appears as a surfaceroughness, like the raised grain of

wood, particularly in areas of heavyink coverag e. It is cause d by theescap e of paper moisture in the formof steam, which exerts an outwardpressure. Where bo nds are weak andfibers stiff, they will pop up, makingtheir shapes visible on the paper'ssurface. (Figure C)

Coated Free Coated Groundw oo

2 4



B

To resist the pull of offset inks, which are high intack, a paper must have good print strength, theproduct of strong bonds among the fibers on itssurface. When surface fibers are not well bonded,the paper suffers from picking, e videnced by thepick ou t (caused b y a tiny bit of loose coating) on theinset at the upper right.

Stiff fibers that are poorly bon ded become"unseated" by steam released during oven dryingand pop up so their shapes are visible on thepaper's surface. W hen viewed by strong side light

these fibers can actually be seen to cast shadowsamong the halftone dots, as in the print o n the right.Compare this with the print on a free sheet, left.

Both prints are greatly enlarged.



On the smooth, tight surface created by bladecoating (left), a halftone has good color and detail

The film coated surface (right) allows ink topenetrate and diffuse, light to scatter and thereforto lessen color intensity.

Smoothnessand Printability

If you want type to be crisp and clean ,halftones to be faithful to original art,and solids to be uniform in color, it'sessential to choose a paper whosesurface is tight and smooth enough toprovide good ink holdout and goo dink lay

As explained earlier, these prope rtiesare primarily the function of co ating,and a good coating can be ap plied tovirtually an y paper...although thebetter the paper surface you beginwith, the better your final results.

From the comp arison in Figure A ofblade an d film co ating, it is obv iousthat the thick, high solids co ntent ofthe blade coating creates a m uch

smoother, tighter surface than the thin,low solids co ntent of the film co ating.Note the greater density, contrast, anddetail in the halftone. (Figure A)

For the purpose of demonstration,these prints were air dried afterprinting and not passed through theoven as would be the ca se in anactual run. It isthen-in theoven-thatthe strength of the pa per surface itselfwould have an effect, as shown in thenext pair of prints.

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26



B

A bright sheet gives halftones a vividness (left) thatis not matched by a less bright one (right).

The print on a simulated free sheet (right) remainsbright even after prolonged exposure to light. Theprint on a simulated groundwood-content sheet(left) has yellowed because of deterioration overtime of the non-cellulose materials w ithin that sheet.

Brightness

There are actually two types ordegrees of brightness. Initialbrightness ensures v ivid halftonesand legible type at the moment ofimpression. Long-term brightnessmaintains the usefulness and goo d

appearance of a printed piece overan extended pe riod of time and undera variety of co nditions.

A free sheet, because it containsneither lignin nor other discoloringimpurities, can be b leached to ahigh degre e of initial b rightness. Asis evident in the demonstrations(Figure B), it provides both contrastand color enhancement.

A sheet containing groundwood pulpand, consequently, an abundance ofnon-cellulose materials, may notachieve this level of brightness. Unlessadditives are included to ensureequivalent brightness, halftones a ndtype printed on it suffer bycomparison.

Impurities not only impose a ceilingon how bright a paper can be madebut are also unstable, oxidizing in thepresence of light, heat, and suchcommon modern-day air contaminantsas ozone and the oxides of nitrogenand sulfur. Less affected by impurities,the free sheet holds its value for a

much longer time. (Figure C)

2



A

These tests on 50 lb. paper show no c learcorrelation between the percentage of mecha nicalpulp and opacity... contrary to the widespreadbelief that groundwood content is the primarydeterminant of opacity. Actually, opacity is theproduct of many ingredients and factors.

In chemical pulping the wood yield is not as high asin groundwood. But because it is less energyintensive (actually producing energy as aby-product), chemical fiber does not have to costappreciably more than groundwood fiber.

Opacity

More a produ ct of fine tuning than ofpulping method or basic paper type,opa city is a highly con trollable feature.The papermaker, not the pro portionof groundw ood to chemical pulp,determines how opaque a paperwill be.

It is true that ground wo od pulp hasrelatively high opa city b ecause ofits lignin content and broken fibers.Lignin absorbs more light thancellulose, and fragme nts a nd finesoffer more surface for light to reflectfrom, diffusing its intensity.

However, tests show that the re is noclear correlation between thepercentage of mechanical pulp inpaper and opacity (Figure A)

Other properties commonlyassociated with opacity are weightand color. But as with perce ntage ofground woo d c ontent, they are not thedeciding factors. A paper can be lightin weigh t, very b right, and still be asopaq ue as another pape r that isheavier and less bright. Aga in, it mustbe remembered that there are manydifferent ways to control paper opa city

Affordability

The cost of paper is largely determ inedby how much a manufacturer pays forthree comm odities: 1) wo od, 2) energyused to convert it, and 3) materialsadded to obtain certain printing andend-use properties. To be competitive,a manufacturer must balance thesecommodities skillfully. Higher thanaverage costs for one of them willhave to be offset by savings derivedfrom the others.

It has already been shown thatgroundwood pulps, because theyretain non-cellulose materials,

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% Groundwood

represent a woo d yield of more than

90%. Chem ical pu lp, which is free ofimpurities, utilizes only a bout 50% .Yet this initial "penalty" is modera tedsomewhat by the fact that thechemical process can use chips andhardwood...which are relativelycheap...while mechanical pulpingrelies (except for TMP) on whole logsand softwood...which are the mostexpensive so urces of fiber.

The yield disad vantage is furthercompensated for by energy savings.Today 40% of the energy ne eded to

run a large and mo dern pulp and

Yield Energy Energy FibeIntensity By- Qualit

product

paper mill (either chemical ormechanical) can be obtained throughthe use of biomass: burning the bark,l imbs, and other refuse not utilized fotheir fiber. Add itionally, chem ical millscan generate still another 5 0% of theenergy by ca pturing waste impuritiesfrom raw woo d plus steam andrecycled chemicals from the pulpingproce ss. (Figure B)

This means that the cost of energyin chemical pulping may amount toa very small pe rcentage of thecost of wood. By contrast, in stonegroundwood pulping, energy cancost just as much as wo od; and in

TMP, it usually costs more. (Figure C)Having less but still some influenceon the cost of paper are theadditives needed to improve ink andwater resistance, internal strength,brightness, opacity, and otherproperties.

Here again, the manufacturers ofchem ical pa per are able to offset thehigher wood costs.

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Because of energy efficiency, better utilization ofmaterials, and econom ies o f scale, it is nowpossible for a modern pulp and paper mill toproduce a free sheet at a cost usually comparableto the higher grades of coated groundwoods.

Free sheets, becau se of their"built-in" strength and brightness,need far fewer, if any, additives toreach the desired levels of theseproperties. Only in the area of opac ityare additives of much significance .

When all present-day papermakingcosts are considered the differencesbetween free sheet and groundwoodpapers are m uch less than aregenerally su ppose d.

2



Summary To resist the pull of offset inks, which a re high intack, a paper must have good print strength (leftwhich comes from strong bonds among the fiberon its surface. Without well bonded surface fiberpaper may exhibit picking (right). This is anexample of ust one of the kinds of strength a freesheet provides.

B

On the smooth, tight surface created by bladecoating (left), a halftone has good color and detaThe film coated surface (right) allows ink topenetrate and diffuse, light to scatter and therefoto lessen color intensity.

To give superior results in highvolume m ulticolor web offsetprinting, a paper mu st exhibit fourproperties:

Strength

Paper needs strength b oth forrunnability and printability. It must beable to endure the rigors not only ofprinting but also of in-line finishing.

Internal strength will reduce thechanc e of breaks, tears, and blistering.Surface strength will prevent fiber puff(or heat-set roughening) and picking.

Underlying the many types of strengththat have been discussed are fibersthat are whole and we ll bon ded . Theyhave not been fragmented andshortened in pulping...which would

Smoothness andPrintability

Printability also depends on howsmooth a pa per's surface is, which a function both of paper stock and oits coa ting.

A smooth coated sheet will providegood dot formation, high ink holdouand co ntrolled reflection of light.But if the fibers and fiber bon ds o nthe surface of the pa per are notsufficiently stron g, even the best ofcoatings will often begin to deteriorand show either picking or fiber pufbefore a job is off press.

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have reduced the expanse of their

bon ding area. They are neithercoated with a film of lignin nor are theysurrounded by lignin particles or otherimpurities...conditions which wouldimpede bonding.

Such fibers are typically produ ced bychemical pulping and the paper m adefrom them is a free sheet.

As you have seen, assurance of a

base strong en ough to protect andsustain a paper's coating is generalbetter provided by a free sheet thanby a paper containing groundwood

3 0



A free sheet can be bleached to a brightness thatgives halftones a vividness (left) that is not matchedby many groundwood content sheets (right). Andthe free sheet remains bright even after prolongedexposure to light and air pollutants. Impurities ma kethe difference.

Brightness

Brightness creates contrast andintensifies color. This seco nd functionis particularly im portant be cause ofthe transparent inks used in offsetprinting. They are illuminated by lightreflecting back through them by the

paper itself. Thus the brighter thepaper (i.e., the more light that itreflects), the brighter the colorswill be.

If a paper is to be made bright initiallyand then to remain b right over a longperiod of time and under a variety ofenvironmental co nditions, it must be

Opacity

Opacity prevents type and imagesprinted on one side of the paperfrom show ing through to the otherside or to the following sheet. This isan important property becauseshow-through is distracting to the

reader and can also interfere with thetonal qualities of delicate halftones.

In order to have adequate opacity, apaper must block light from passingthrough it and being absorbed bytype or images on the reverse side. Itis this "reverse-side a bso rption" thatproduces show -through. To prevent it,a paper must either: 1) reflect orscatter light from its surface or withinits interior, or 2) abs orb light with thematerials of which it is com pos ed.

Such properties c an be built into many

different types of pape rs. Contrary tothe belief that high groundwoodconten t is the only way to achieve highopacity, manufacturers of free sheetscan now offer papers that are lightin weigh t, very bright, and still highlyopaque. This is because groundwo odcontent is only one of the factors thatgo into determining opacity.

free of lignin and all other discolo ringimpurities. Because it is only in the

chemical process that theseimpurities are rem oved, the free sheetis naturally brighter than groundwood(requiring few a dditives, such astitanium)...and it remains brighterduring the life of a printed piece.

3



In making a free sheet, a wide variety of economicalwood types can be used, chemicals can berecycled, and 90% of the energy neede d to run amodern mill can be obtained by using impuritiescontained in wood, steam (generated by thepulping process itself), plus bark and slash fromwoodlots and the millyard.

A Chem ical Pulp Conclusion

In addition to how well a pa permeasures up in these four p roperties,a buyer must also co nsideraffordability.

Suppo se that you are considering two

papers. The first is known to p rodu cemuch better results than the sec on d,but it is pric ed subs tantially higher. Ifcost is your only conce rn, then b uyingthe less expensive one m ight bejustified.

But when it's not just co st, but also thequality of your finished produ ct that'simportant, then the better performer isusually the wiser cho ice.

And w hen the prices of these twopapers are not significantly different,the choice is clear.

Advances in papermakingtechnology, particularly in the areas ofenergy and raw materials utilization,have appreciably reduced the costdifferential between some free sheetsand the better groundwoo d-contentpapers.

Which the n is the better buy?As the demonstrations andinformation given in this bulletinhave shown, the multicolor weboffset performance of the freesheet is superior in virtually everway...making it generally t he

superior value.

32



Bibliography

J.H. Ainsworth,Paper: The FifthWonder, Seco nd, Kankauna,Wisconsin: Thomas Printing &Publishing Co. Ltd., 1959

Douglas Atack, "FundamentalDifferences in Energy R equirements

Between the Mechanical PulpingProcesses," Svensk Papperstidning,November 1981

C. Bauer, "Thermo-Mechanical Pulpis a Good C hoice for P apermakers,"Southern Pulp & Paper, March 1983

Wilfred A. Cote, Papermaking Fibers,Syracuse, New York: SyracuseUniversity Press, 1980

Ingrid Fineman, "The Influence ofBleached Mechanical Pulp on SomePrintability Properties of High QualityPrinting Paper,"SvenskPapperstidning, June 1975

GunnarGavelin,"Thermo-Mechanical Pulp isDifferent," Pulp & Paper International,

March 1976

Gustaff Jo hnsson, "HowGroundwood Can Reduce Costs,"Paper, March 1983

Joseph Kurdin, "In Search of TMPStrength at Groundwood EnergyCosts," Pulp and Paper Journal,

November 1982

Ullta-Britt Mohlin, "DistinguishingCharacter of TMP," Pulp & PaperCanada, Vol. 78, No. 12, Decemb er1977

Jack Perry, "Web Offset P rintingPapers," TAPPI, Vol. 55, No. 5, May1972

Robert P lankinton, "Heatset

Roughening of Coated Paper," TAPPICoating C onference, May 1972

Printing Industries of America (WebOffset Section), 1980 Annual MeetingProcee dings, Arlington, Virginia,September 1980

Robert Reed, What the Printer ShouldKnow About Paper, Pittsburgh,Pennsylvania: Graphic Arts TechnicalFoundation, 1976

Charles Shapiro, Editor, TheLithographer's Manual, Fifth Ed ition,

Pittsburgh, Pennsylvania: GraphicArts Technical F oundation, 1974

PA . Tarn Doo and R.J. Kerekes,"The Flexibility of Web Pulp Fibers,"Pulp & Paper Canada, Vol. 83, No. 2,March 1982

Graphic Arts Technical Foundation,Techno-Economic Forecast No. 3:

Growth of Web Offset andRotogravure - The Impact ofTechnological D evelopment1978-1982, Pittsburgh, P ennsylvania,1977

Printed in U.S.A. on Lustro Web Gloss 80 lb. and Lustro Gloss Cover 100 lb.© 1987 S.D. Warren, a Subsidiary of Scott Paper Com pany 3



WarrenPaperMerchants

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CaliforniaFresno

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Unijax, Inc.

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Western Paper Co.


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Dixon Paper Co.

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Seneca Paper Co.

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Baldwin Paper Co.

Bulkley Dunton


Marquardt & Co., Inc.Ail ing and Cory

Seneca Paper Co.

Ail ing and Cory

Seneca Paper Co.

Ail ing and Cory

34



Caskie Paper C o., Inc.

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Virginia Paper Co.

Caskie Paper C o., Inc.

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Virginia Paper Co.

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Nationwide Papers

Ris Paper Co ., Inc.

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Cordage Papers/Columbus Division

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Ris Paper Co., Inc.

Commerce Paper Co.

City Western Paper Co.

Mead Merchants

Western Paper Co.


Ail ing and Cory

Ail ing and Cory

Ail ing and Cory


Ail ing and Cory


Ail ing and Cory

Ail ing and Cory

Carter Rice

The Rourke-Eno Paper Co., Inc.

Dil lard Paper Co.

Caskie Paper Co ., Inc.

Dil lard Paper Co.

Virginia Paper Co.


Dil lard Paper Co.

TennesseeChattanooga

Knoxvil le

Memphis

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TexasAmaril lo

Austin

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Houston

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Export and ForeignNewYork,N.Y.

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WashingtonSeattle Zellerbach Paper Co.

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Documents

SD Warren Bulletin Coated Papers for Web Offset Printing