Biermann Ch. Handbook of Pulping and Papermaking

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444 22. PAPERMAKING CHEMISTRY

cal.

As long as excess white water is minimal and

excess white water is treated to remove solid

materials, this is easily achieved.

First—pass retention

First—pass (or one pass) retention is often

separated into various subcategories of total solids

including fiber retention, fines retention, and ash

(i.e.,

filler) retention. The mechanisms of reten-

tion of these fractions are different, so it is useful

to know the retention level of each. The

first—pass fiber retention on paper machines using

unbleached softwood fibers is usually over 98%.

Furnishes using bleached hardwood and softwood

fibers have a first—pass retention of about 95%.

Occasionally the first—pass fiber retention is

below 90 %. The retention of fines is much lower,

however, and should be the focus of retention

strategy. A high first—pass retention means that

there will be less solids in the headbox so that the

drainage rate will increase. (Smaller particles are

the ones less apt to be retained but contribute most

to decreased slurry freeness if they build up in the

system due to low retention.)

Fines also adsorb a higher percentage of

additives such as polymers and sizing agents due

to their high surface area. Additives such as ASA

or AKD will hydrolyze in the white water and

become useless. Hydrolyzed ASA causes pitch

problems and actually decreases the level of sizing

since it acts as a surfactant. Mills with excessive

pitch problems on the paper machine may have to

sewer excess white water rather than use it for

purposes such as wash water in the bleach plant.

An increased

first—pass

retention implies that

particles are held more firmly to the web. This

results in secondary benefits of less sheet two—

sidedness (since particles on the wire side will not

be selectively removed) and decreased felt filling

by fines and fillers. Tanaka et al. (1982) showed

that the clay and Ti02 distribution in the sheet z

direction was more uniform with the use of reten-

tion aids. Systems with high first—pass retention

are often free of slime—producing organisms.

Conversely, high retention values may mean

overflocculafion of filler particles, which leads to

decreased light scattering and lower opacity.

Extra filler may have to be added, the point of

addition of materials may be changed, or the

retention aid changed (by using an alternate prod-

uct).

Ideally, the fillers should flocculate with

fibers and not with each other. Usually there is

some tradeoff between retention of fillers and

opacity. If high levels of fillers are used, if the

filler is inexpensive, and if adequate strength can

be achieved, this will probably not be a problem

(these are the main advantages of alkaline

papermaking for printing papers).

First—pass retention

measurement

First—pass retention (Retpp) can be approxi-

mated by the relative change in solids content

from the headbox (HB) stock to the white water

(WW) tray divided by the initial solids content of

the headbox. This is done by measuring the

consistencies (c) at the headbox and in the white

water. This could be done for total solids or

fractionated samples. This method suffers from

the fact that white water consistency may vary

from place to place in the tray. Also, dilution

from shower water may be appreciable.

Retpp,

% =

(CHB

—

CWW)/CHB

X 100%

A more comprehensive first—pass retention

determination is accomplished by looking at the

material entering the headbox compared to the

sheet coming off the couch roll. Samples are

taken from the headbox slurry, the white water

slurry, and the couch roll. A couch trim sample

can be used if one is confident that its composition

represents that of the entire sheet at that point

(ignoring moisture content variation). TAPPI

Standard 269 pm-85, which uses the Britt jar, and

Standard T 261 cm-90 describe this method in

detail with example calculations. This method

measures the fiber to fines ratio and uses the fibers

as an internal standard with a correction for the

fibers that appear in the white water. Moisture

contents are not critical in the couch sample but

the white water fraction must not be contaminated

with fibers from any other source.

Colloidal retention

Since retention of the fines fraction is of

central importance, the Britt jar (or dynamic

drainage retention jar, page 151 and TAPPI

Standard T 232) is a useftil tool because it mea-

sures retention through a screen with 76 fim holes

without formation of a mat; it measures colloidal

RETENTION, FORMATION, AND DRAINAGE 445

interactions without interference from filtration

mechanisms. (Initially, before filtration of fiber,

the hole size in the forming media of the paper

machine is about 300 fim.) It provides turbulence

and shear forces (via the stirrer) that are always

present during the papermaking process. The Britt

jar consists of a precision controlled stirrer and a

jar with the screen. Three baffles (present in the

one shown Fig. 6-21) may or may not be used.

Even though retention might be high in a

laboratory sheet former, retention may be low

under actual papermaking conditions because of

the turbulence and shear. The reason is that under

certain conditions soft floes may form that, by

definition, break apart under moderate agitation.

Conversely, hard

floes

will withstand a moderately

high level of shear and remain intact during the

sheet forming process (Unbehend, 1976). Obvi-

ously, there is a continuum of floe strengths.

Hard floes may decrease opacity and the

quality of formation in the final sheet. Opacity is

decreased by aggregation of mineral fines in large

clumps, meaning that light travels through fewer

interfaces and is not scattered as much. (Howev-

er, below particle sizes of 0.3 /^m the particles are

so small relative to the wavelength of light that the

light scattering coefficient decreases with decreas-

ing particle size.)

When soft floes are dispersed with high

turbulence, they reform with the same level of

retention in a papermaking system (Fig. 22-7 with

alum).

Hard floes will be retained at a lower level

after they are dispersed with high turbulence (Fig.

22-7,

PAM with and without initial disruptive

turbulence); this is hypothesized as occurring

because polymer loops that extended in solution

are "flattened" onto the surface of the particle and

decrease particle—particle interactions. Also, the

polymer is cleaved into shorter lengths. When

using high molecular weight polymers, one must

be careful to avoid unnecessarily high shear to

achieve maximum retention.

Generally, soft floes form with inorganic salts

and polymers with molecular weights of a few

thousand g/mol or less, whereas hard floes form

with high molecular weight polymers. [A few

metallic elements (such as Al in polyaluminum

chloride, PAC) can form high molecular weight

materials.] Floes that form only by surface patch-

es (using low molecular weight cations without a

high molecular weight anionic bridging polymer)

are apt to be soft floes (Unbehend, 1976). This

means that high speed paper machines using

inorganic fillers will require at least one high

molecular weight polymer in the retention system

or the use of the microparticle retention system.

The behavior of high molecular weight

cationic polyacrylamide has been studied by

Tanaka et al. (1993) in an elegant study. These

workers found that cationic PAM that is first

100

200 400 600

Turbulence, rpm

800 1000 200 400 600 800 1000

Turbulence, rpm

Fig. 22-7. Model floe retention with cations (left) and cationic polymers of decreasing molecular

weight (with and without initial disruptive turbulence in the case of

PAM)

(after Unbehend, 1976).

446 22. PAPERMAKING CHEMISTRY

added to fibers is redistributed to fines in the

presence of hydrodynamic shear (they used poly-

styrene latex particles) within 1—2 min with some

cleavage of the PAM. Much of the PAM was

cleaved from the original molecular weight of

4-10^ to about 4-10^ depending on the amount of

shear and mixing times.

Retention of low molecular weight materials

such as sizing agents and dyes should probably be

considered separately from suspended materials.

Sizing agents can be anchored to the surfaces of

fibers and fines with metallic salts and poly-

elettrolytes. Because of their small dimensions,

they may continue to adhere to surfaces, even

though the particles themselves are separated from

each other (i.e., under turbulent conditions).

The Britt jar can be used with a variety of

stirring speeds to simulate (somewhat) various

degrees of turbulence. Often a corresponding

stirrer velocity (in rpm) can be determined for a

given commercial paper machine.

The above discussion indicates that retention

aids can be classified as inorganic salts or

polyelectrolytes. The pH will have an effect on

the retention system; it should always be moni-

tored and controlled. These materials and the

chemistry of aluminum have been considered in

detail in other sections of this book. Other aspects

of polymers will be covered later in this chapter.

Carbohydrate—based retention

aids

Starches have been described already.

Cationic starches have a much higher level of

retention (and affinity for cellulosic materials with

carboxylate groups) than unmodified neutral

starches, which decreases the biological oxygen

demand in the excess white water. The addition

of tertiary amines often occurs by reaction with

compounds of the form R2NCH2CH2—CI (where

R is an alkyl such as the ethyl group) where the

starch molecule attaches through one of its R'OH

groups by displacing the CI atom. A typical DS is

2 to 5 amines per 100 glucose units (0.02 to 0.05).

Too much cationic starch may decrease retention

of fines by charge reversal. Even so, the relative-

ly low charge density of cationic starch means

they are not as effective retention aids as some

highly cationic synthetic polymers. Cationic

starches are subject to shear damage and are often

added late in the approach system. Commercial

anionic starches include only starch phosphates

(potato starch), which are used from pH 4.5 to 6

with alum. Their absorption may be reversible

unless a cationic promoter (of the quaternary

amine type) is used.

Guar gum (from the seed) is a galactomannan

(similar in structure to the top diagram of Fig. 2-

24).

They exist as rigid rods. Unaltered, they are

retained by hydrogen bonding; when derivatized,

using many of the same approaches used to

derivatize starch, they act as retention aids.

Synthetic

polymer retention aids

Neutral synthetic polymer retention aids

include polyacrylamide (PAM), polyvinyl alcohol,

and polyethylene oxide (PEO). PAM is the most

widely used of these and adds to the dry strength.

PEO forms hard floes with mechanical pulps or in

systems where sulfonated polymers are added.

Polymers are made anionic by carboxylate

groups (RCOO), sulfonate groups (R-SO3) or

phosphate groups (RPO3'). The polymers are

prepared as metallic salts. The most common

anionic polymer is made by introducing acrylic

acid groups to high molecular weight PAM as

mentioned above. Anionic polymers must be used

with cationic intermediates in order to interact

with negatively charged fibers and fines. At pH

4—6,

this may be alum, which can cause charge

reversal on the cellulose fibers. In dual polymer

systems

the

cationic intermediates are small cation-

ic polymers that act by forming patches. In the

microparticle systems the cationic intermediate is

cationic polyacrylamide with anionic bentonite or

cationic starch with anionic colloidal silica. A

recent method is the use of cationic starch with

anionic poly aluminum chloride.

Cationic groups are introduced into polymers

with amines or quaternary ammonium ions (tetra—

substituted nitrogen

atoms).

Rarely sulfonium ions

(tri—substituted sulfonates) or phosphonium ions

(tetra—substituted phosphates) are used. Several

monomers (Fig. 22-8) may be incorporated into

PAM during its polymerization. The copolymers

formed often have a cationic mole percentage

(charge density) as high as 10%. D ADM AC has

the advantage of being a crosslinking agent. This

is necessary since allyl monomers are difficult to

polymerize (Biermann, 1992). In single polymer

systems molecular weights of 1—10 million are

RETENTION, FORMATION, AND DRAINAGE 447

used to allow direct bridging of anionic materials,

but higher molecular weight polymers are very

shear sensitive. If pyridine units are incorporated

(Fig. 22-8), they may catalyze acylation reactions,

such as the binding of AKD to cellulose.

Cationic/anionic polymers usually have more

cationic than anionic functionalities. An example

is potato starch with naturally occurring phosphate

groups corresponding to a DS of 0.002, which is

made cationic with quaternary ammonium salts

corresponding to a DS of 0.03. Cationic/anionic

polymers will work in a wide variety of systems

but are not maximized for any particular system.

Retention systems

Retention by the single additive systems of

flocculation and bridging has been mentioned. In

dual polymer systems, cationic intermediates are

small cationic polymers (such as PEI) that are

added first to form patches on the surfaces of

fibers and fines. Large anionic polymers (PAM)

are added second to bridge these patches to induce

flocculation. This system also adds to dry

strength. This system forms very hard floes that

do not regain their strength fully if they are dis-

rupted by high turbulence.

In the microparticle systems the large anionic

polymer is replaced by bentonite or another colloi-

dal silica with a negative charge. This system is

a recent advancement in papermaking, although

silica was used for AKD retention by Weisgerber

(1958).

It is said to result in relatively small,

reversible floes so that good formation is obtained;

it is discussed in more detail in the section on

practical aspects of alkaline sizing agents.

Formation

Formation is a function of fiber length,

retention aids employed, and turbulence and is

controlled by the size of the floes. Microflocs are

floes with a size on the order of 100 ^m;

macroflocs are floes that are visible to the naked

eye.

Macroflocs are a problem because they can

lead to large local differences in the basis weight

of the final paper. They occur when fibers begin

to clump together. The reader will not be sur-

prised to find that conditions that give good

microflocs (desirable) will also lead to macroflocs.

These conditions include the use of high molecular

weight polymers in bridging systems.

HoCzzzCH—ChU cr

HoC=CH—CHo ^^

'3 H2C:

DADMAC

Diallyldimethyl ammonium chloride 4-Vlnylpyridine

OH,

CH3 0

H2C:

;i=:CH—c-

CH3—\t CH3

;H—

C—

NH— CH^-

CH2

o-CH

[3-(Methacryloylamlno)propyl]trimethylammonlum chloride

Fig. 22-8. Cationic monomers used to form

cationic PAM copolymers.

Control of the floe size can be achieved

through the use of controlled shear forces. This is

accomplished, not by changing the paper machine,

but by controlling where, how, and what polymers

are added to the system. If polymer addition near

the headbox gives large floes, one may consider

polymer addition at the secondary fan pump or

some other place where moderate turbulence

decreases flocculation.

Large polymers probably contribute to forma-

tion by limiting fiber—fiber contacts. A moderate

charge density on the polymer prevents the poly-

mer from coiling in solution. The argument has

been given that large polymers increase the viscos-

ity of the solution so that after turbulence is

induced fibers cannot regroup as quickly.

Drainage, filtration, web consolidation

The mechanics of drainage on the paper

machine have been discussed in detail in Chapter

9. Some additional aspects related to papermaking

chemistry will be mentioned

here.

The freeness of

wood fibers will increase with the addition of

cationic polyacrylamide polymer to the isoelectric

point. Flocculation of fines will also increase the

freeness. Both of these effects will increase the

drainage rate observed on the paper machine.

(Drainage implies the removal of water; another

term, consolidation, views the subject from the

point of view of the fibers.)

High drainage rates mean faster paper ma-

chine operation and/or energy savings at the dryer

section. Since increased solids in the web means

higher web strength, paper machine speed can be

448 22. PAPERMAKING CHEMISTRY

increased without additional web breaks. Faster

drainage also means the sheet will be set more

quickly, before fiber flocculation can take place.

Water removal from a fiber suspension may

occur hy filtration or thickening (Parker, 1972).

In the process of fihration, water is removed from

the bottom of the suspension and the fibers pile on

top of the forming material. A sharp boundary

forms and the undrained suspension on top re-

mains at the same original consistency. In thick-

ening water removal occurs from the entire sus-

pension and the entire suspension

thickens

at about

the same rate with no sharp boundary. Thickening

occurs if the fibers interlock such as by entangle-

ment. The principal drainage mechanism on the

paper machine is initially filtration. (However, the

application of a pressure gradient in the suction

boxes and presses causes drainage by thickening.)

Water removal by filtration results in the

layered nature of paper. (Filtration theory is

discussed in

Perry *s

7th edition starting on pages

19-65 and 20-98), The D'Arcy equation models

flow through a porous media. The equation is:

I dp

rjR dw

where the filtration velocity, U, is a function of

the specific filtration resistance, R, the viscosity of

the liquid, rj, and the pressure gradient, dp/dw. R

is given in the next equation.

Filtration of paper webs has been developed

by Campbell (1947) and Ingmanson (1954, 1959,

1963) based on treatment of the Kozeny—Carman

(1937) equation, which gives R as follows:

R-kS^(±E}I

where

is a constant based on the mat composi-

tion, S is the specific surface area of the solids per

unit volume, E is the mat porosity, and V is the

volume fraction of the web occupied by solids.

Information on drainage is found in Ranee (1980).

Drainage can be increased by other factors as

well. An increased stock temperature gives a

reduced water viscosity and has a similar effect as

in pressing (page 237). The advantages of in-

creased first—pass retention have been discussed.

Retention aids, however, prevent fine material

from clogging the mat of fibers and may help

draw cellulose microfibrils together, decreasing

their hydration. Drainage, then, goes hand in

hand with retention. Surfactants also increase

drainage, but they can cause foaming and interfere

with fiber—fiber bonding and sizing. Entrained

air should be avoided.

An increase in chemically bound water

(hydrogen—bonded, as opposed to free water)

decreases drainage and may result by increased

refining, which also generates fines, and the use of

hygroscopic additives. The low level of chemical-

ly bound water in newsprint may partially explain

why newsprint machines that use a furnish of 100

CSF can operate at speeds in excess of 15 m/s

(3000 ft/min), but machines using mixtures of

bleached softwood and hardwood (which have

higher levels of chemically bound water) with

much higher freeness are limited to much slower

speeds. An extreme example of the problems of

hydration would be drainage from a thick gel.

Sheet density is also a factor in drainage. A

very dense sheet like glassine will presumably

have smaller pores in which the water can flow.

Sheet density is in turn a function of fiber flexibili-

ty, fines content, and other factors that influence

drainage. Too, thick sheets (high basis weight)

will drain more slowly as the water has a thicker

mat through which it must percolate. The high

bulk of mats of mechanical pulp fibers also helps

to explain why newsprint machines can operate at

high speed with a low CSF pulp.

Drainage in vacuum boxes (hivacs) is some-

what different from drainage by gravity. The

reason is that uniform formation increases water

removal since higher vacuums can be achieved.

Holes or areas of low sheet density allow air to

pass easily through the sheet. Therefore, too

much retention aid will give poor drainage on the

hivacs in addition to poor formation. Modern

slotted vacuum boxes work much like foil blades

in causing a pressure gradient and doctoring water

that is pulled away so that rewetting does not

occur. In the early 1960s, Attwood showed this

by comparing dewatering on the paper machine

with his own laboratory dynamic drainage tester.

Sometimes improvements in sheet formation

will result in improved water removal on the

vacuum boxes, shifting the dry line toward the

RETENTION, FORMATION, AND DRAINAGE 449

headbox, while decreasing the drainage before and

after the wet boxes. To a point, refining assists

with formation by making more flexible fibers and

fines,

but increases chemically bound water and

can even make fibers gelatinous. (Good formation

may also hinder initial drainage.) Some additives

also have this effect. A delicate balance of

effective retention additives and induced shear

is required to balance good retention with

appropriate formation and drainage.

Drainage in the press section depends on

capillary action so that water removal is propor-

tional to the square root of the ratio of surface

tension to viscosity of water according to Nissan

(1954).

This value increases about 40% from 38°

to 99°C (100° to 210°F).

Drainage aids

Retention of fines to each other and directly

to larger fibers (flocculation) will result in faster

drainage since pores in the fiber mat will not

become clogged. Most retention aids act as

drainage aids; however, cationic materials (alum

and polymers) seem to be particularly effective at

increasing drainage. Excessive use of cationic

materials will cause charge reversal and may

decrease the drainage rate.

From the above discussion, it is not surpris-

ing that maximum drainage occurs for a particular

type of particle near its isoelectric point. As with

retention, the floes must be strong enough to

withstand the shear forces they will encounter in

the paper machine. For example, while alum may

appreciably increase the observed Canadian Stan-

dard freeness, alum may have a much smaller

effect on drainage and retention on a high speed

paper machine.

22.5 INTERNAL SIZING

Sizing is the ability of paper to resist wetting

by a given fluid. Usually an aqueous fluid is

considered. Internal sizing is important to the

operation of the size press, coating operations,

printing operations, and paper in service.

Davidson (1975) reviewed the topic in detail.

Surface sizing is also used to resist wetting

but has other purposes that internal sizing cannot

achieve. These include controlling the surface

porosity for printing, increasing the surface

strength to avoid linting and to improve the pick-

ing resistance, and improving strength properties.

This explains why many machines use both meth-

ods of sizing.

Inks have wide ranges of compositions and

solids content. Their surface tension can be quite

low relative to water; therefore, paper with

internal sizing that is effective against water may

not have suitable size properties for some inks.

The pulp and paper literature is replete with

unproven dogma (as well as material that contra-

dicts established chemical principles) on internal

sizing. Hypotheses usually come after observa-

tions,

and these are not specifically tested by

additional experiments that may refute them.

Investigators often say that the purpose of

alum in rosin sizing is to orient the rosin with the

hydrophobic tail outward; true, but if one sprays

a sheet of paper with rosin (to insure retention)

and then dries it, there will be no sizing. Rosin

orientation is a consequence of anchoring; no

anchoring, no sizing. (Besides, without anchor-

ing, the carboxylic acid groups would prefer to

hydrogen bond to the surface rather than not.) It

is beneficial to try to think of situations (or experi-

ments) that argue or test the accepted hypotheses.

Unfortunately, interesting stories are accepted too

easily by the pulp and paper industry as fact.

Often what one perceives to be "too complicated"

or "outside of one's understanding" is really

outside of anyone's understanding.

Critical thinking, the difference in knowing

when you are right from when you do not know,

is essential to the scientific process. Whatever the

case,

experimental results from any study (while

ignoring the discussion) may be usefiil.

Attachment of

rosin

size molecules to fiber

The mechanism of attachment of the rosin

size molecule to the paper is an important, poorly

understood, aspect of

sizing.

It was said that rosin

size could not be used above pH 7 until it was

demonstrated that certain polyamines were highly

effective mordants up to pH 10 (Biermann, 1992).

It used to be said that mechanical pulps were more

difficult to size than other pulps, but Zhuang and

Biermann (1993) show that the use of iron—based

mordants is highly effective with mechanical pulps

but ineffective with bleached chemical pulps.

450 22. PAPERMAKING CHEMISTRY

Ligands (of coordinate chemistry) in the liquid

against which sizing is determined (or paper is

subjected to in service) will play an important role

in the stability of the linkage.

The formic acid used in the Hercules size test

(HST) test has been said (in several publications)

to reduce HST values by lowering the ink surface

tension relative to neutral ink. This is an example

of unproven dogma that is easily tested. Work by

Chen and Biermann (1995) refutes this claim.

Formic acid lowers the surface tension of water

from 73 to 71 mJ/m^. The use of a small amount

of hexanol (0.25%) lowers the surface tension

much more than 1% formic acid, yet does not

decrease the HST time to the same extent that

1 %

formic acid does. Furthermore, formic acid and

neutral ink give about the same values for

handsheets sized with AKD (no alum present). In

some cases, formic acid actually takes longer to

penetrate than neutral ink (Chen and Biermann,

1995).

Formic acid was used in the HST test before

ASA and AKD sizing methods were developed.

Early inks used HCl in most formulations. For-

mic acid (a fairly strong acid) was used to simu-

late the HCl in ink formulations while avoiding the

problems of the more volatile HCl that would

make the acidity of ink change with time. The

acid adversely affects the stability of the

rosin—alum linkage in the size complex.

Sizing is not just a property of the paper, but

of the liquid against which sizing is tested. Gess

(1981) has a very useful paper demonstrating this

point. This also presents an opportunity for

studying the size—mordant—fiber interaction by

using a variety of different solutions to test sizing.

For example, fluoride (a strong ligand that reacts

with aluminum) in water destroys alum—rosin

sizing quickly, but has little effect on ASA or

AKD size (Chen and Biermann, 1995).

Practical aspects of

rosin

sizing

Rosin sizing with dispersed rosin requires the

use of cationic starch to help with its retention.

Dispersed size is more expensive than soap size,

but it is said to be more effective and does not

decrease the strength of paper to the same extent

that soap size does. Dispersed rosin size is less

sensitive to the presence of calcium ions than soap

size.

At stock temperatures above 40—45°C, soap

sizes may be more effective than dispersed sizes.

Neutral rosin sizing with dispersed rosin size

and PAC should be used with headbox stock at pH

7 ± 0.2. The overall charge (as measured by

colloidal titration) should be near zero and is

controlled by the addition of cationic polymers.

Retention above 85% is ideal.

Rosin fortified with fumaric acid (trans-

isomer) has better chelating properties than that

fortified with maleic acid (cw-isomer), and most

rosin is fortified with fumaric acid. Traditionally

fumaric acid has been cheaper than maleic acid.

Casein is often used with (anionic or neutral)

dispersed rosin sizes (e.g., Neuphor) to stabilize

the rosin product; the casein itself may improve

the final sheet (as a usefiil binder) but probably

plays a minor role in the size anchoring process in

the final sheet. Cationic dispersed rosin formula-

tions (e.g.. Hi—pHase)are emulsified with various

cationic polymers. In either case the rosin is not

chemically linked to the casein or the cationic

polymer.

Some mills add rosin and alum prior to

refining. The discussion on page 156 indicates

that alum interferes with refining.

Practical

aspects

alkaline

sizing

Some of the advantages of AKD and ASA

are the low melting temperatures of 40—45° that

make them easy to emulsify. The high melting

temperature of stearic anhydride and its relatively

high reactivity make it much harder to use than

ASA or AKD. ASA is more reactive than AKD,

which partially explains why it develops sizing on

machine rather than off machine. There is not a

good theoretical explanation as to how AKD is

able to give improved sizing with time.

One application of ASA size is on newsprint

machines that operate below pH 7. As many

newspaper presses switch to offset printing, some

degree of sizing in the newsprint is required.

Many mills find using alum with mechanical pulps

causes pitch to deposit (by precipitation). Since

ASA is more reactive than AKD, and since lower

pH results in a slower reaction rate for

transesterification reactions, ASA size proves to be

more effective than AKD under these conditions as

AKD takes too long to develop sizing.

INTERNAL SIZING 451

Williams (1991) gives one mill's experience

with conversion to alkaline papermaking with

AKD sizing. AKD slipperiness gave poor

runnability on the winder and converting opera-

tions even though paper gives a higher coefficient

of friction. (Perhaps the coefficient of static

friction is higher, but the coefficient of dynamic

friction is lower; see Tappi Standard T 549 pm-

90).

AKD caused static electricity to build up on

the paper. Size reversion over days and weeks

was also a problem depending on the PCC used,

with larger PCC particles causing fewer problems

but giving lower opacity. A bentonite

microparticle retention system worked better here

than the dual polymer retention aid system of

anionic poly aery lamide with cationic promoter.

Higher retention was achieved with a high level of

retention control without overflocculation; im-

proved drainage and drying are also noted.

The use of alum with AKD provides a reten-

tion aid and detackifying agent for hydrolyzed

AKD to decrease press picking. Improvements

without problems should be possible with addition

up to 2.5 kg/t (5 lb/ton) (on pulp). The alum can

be added to the thin stock when AKD is added to

thick stock. It seems that more mills use alum

with AKD than do not, especially with the dual

polymer retention system.

In alkaline papermaking one will get im-

proved sizing by using higher levels of high DS

cationic starch. Starch at 30 lb/ton will give much

better sizing than 10 lb/ton and 0.35% N will give

much better results than 0.18% N in the starch.

Also,

higher drainage rates are realized. A high

cationic charge also helps with the starch used to

emulsify the ASA or AKD. The molecular weight

of the starch used to emulsify the ASA or AKD is

not critical if adequate emulsification is achieved.

Hexanedioic (adipic) acid (1 to 3% on starch) has

been used to help stabilize ASA emulsions. Chen

and Biermann (1995) show the mechanism of

anchoring for AKD and rosin/alum sizing.

Practical aspects of

retention

systems

The dual retention aid system usually works

with a high molecular weight anionic polyacryl-

amide that is added as a flocculent to the stock

after the pressure screens and a low molecular

weight cationic polymer that is added in the thin

stock as a charge neutralizer/coagulation agent.

A bentonite microparticle retention system

with AKD size (Dixit et al., 1991) works with a

medium to high molecular weight cationic polymer

(starch or polyacrylamide) added early to the

thinstock system. A high surface area anionic

microparticle [2.5 kg/t (5 lb/ton) bentonite] is

injected near the headbox so that microflocs form.

About 0.75 kg/t (1.5 lb/ton) high molecular weight

cationic polymer and 7.5 kg/t (15 lb/ton) cationic

starch with tertiary amines was an effective system

on a machine making 50 g/m^ (34 lb/ream) paper

at 610 m/min (2000 ft/min).

Optimization of this retention system on a

machine making 44 g/m^ (30 lb/ream) paper with

11.25%

UFGC and 2.5% PCC at 600 m/min used

1 kg/t (2.1 lb/ton) HMW cationic polymer added

at the primary fan pump, 9 kg/t (18 lb/ton) cation-

ic starch with tertiary amines, and 2.5 kg/t (5

lb/ton) modified bentonite. The headbox consis-

tency was 0.42%, the FPR was 71% and the

FPAR was 35 %. In mills using clay, one wonders

if clay could be modified to act like bentonite in

the microparticle retention system.

22.6 WET AND DRY STRENGTH

Sections 17.8 and 17.9 describe how fibers

flocculate or bond as a function of consistency in

water. Chapter 6 (aspects of refining) is also rele-

vant since paper strength properties are developed

by refining. Possible mechanisms for fiber to

fiber bonds include mechanical entanglement,

covalent bond formation, and secondary chemical

bonding including ionic, hydrogen, and van der

Waals bonding.

In most paper, hydrogen bonding is the most

important of all of these. Hydrogen bonds are

stronger than the other weak chemical bonds. The

facts that starch and hemicellulose increase the

strength of paper and that paper loses it strength

when wet both indicate hydrogen bonding. Also,

paper made from nonpolar materials has very little

strength. Dry strength additives promote hydro-

gen bonding between fibers (through a polymeric

adhesive such as starch).

Wet strength implies a sheet will maintain a

high level of its strength (usually between 20% to

40%) even when it is saturated with water. Wet

strength additives work by forming covalent bonds

between fibers (through an adhesive) or by form-

452 22. PAPERMAKING CHEMISTRY

ing their own crosslinked network of covalent

bonds. They increase the dry strength as well.

Since these materials are necessarily resistant to

water, they are not used on tissue and other papers

that must be repulped or be water dispersible.

Even small molecules like butane tetracar-

boxylic acid (useful in durable or permanent press

treatments for textiles) can give a wet tensile

strength that is 67% of the dry value, although the

fold strength is lost (Horie and Biermann, 1994).

Dry strength additives

Reynolds (1974) showed that dry strength

additives have a more pronounced effect on inter-

nal bonding in paper at relatively low degrees of

refining. With increased refining, pulp fibers

effectively "make" their own bonding agents by

increased hydration of fibers and increased fiber

flexibility. Dry strength additives can help make

bulky, porous sheets since lower levels of refining

are required. Dry strength aids may allow the use

of higher levels of recycled or hardwood fibers.

Unmodified starch is difficult to retain on the

paper, so cationic starches (with DS of 0.02 to

0.04) have replaced them as the most common dry

strength additive now used. It is typically used at

2 to 4% on pulp.

Polyacrylamide for dry strength development

can be of molecular weight 100,000 to 500,000

(usually with a charge density of 5—10%), while

that used for retention is one to five million.

Consequently, this parameter provides the

papermaker with a degree of control over the

product. Linke (1962) showed 500,000 to be ideal

(for this study's particular conditions), with higher

weights actually decreasing the strength somewhat.

This work also showed that a charge density of

10—12%

(if acrylic acid is used) was ideal for

strength development. PAM had a high effect on

the fold strength.

Anionic PAM is used with

1 %

alum at a pH

of 4 to 4.8. Once PAM is added to the stock

refining must be avoided if alum is present,

otherwise moderate refining is possible. PAM is

not very effective with furnishes containing 50%

or more of mechanical pulps.

Cationic dry strength resins that are

self-

substantive (do not need an alum or polymer pro-

moter for retention) work over a pH of 4 to 8.5

depending on the particular formulation. If rosin

is used it should be set with alum prior to the

addition of the cationic dry strength resin.

Fillers interfere with dry strength resins.

Dry strength resins may not be necessary if wet

strength resins are also used.

Wet strength resins

Anionic PAM dry strength resin can be used

with a cationic polymer (such as a polyamide-

polyamine that is added first) to make wet strength

resins up to a pH near 7. The cationic polymer

decreases the alum requirements.

Wet strength resins have been discussed. In

some systems a partially polymerized material is

added to the wet end during papermaking. (A

second material, such as monomer, may be used

to saturate the paper.) Curing in the dryer section

produces the final polymer. This system,

aminoplasts, is used with urea— or melamine—

formaldehyde (UF or MF), and epoxidized poly-

amine-polyamide resins (Unbehend, 1987).

UF resins have been modified by replacing

some of the urea with amine groups that become

cationic. These are self—substantive. The curing

can continue off—machine for several weeks.

Low pH (« 4.5) helps UF resins cure more

quickly. Like UF, MF must be used under acidic

papermaking conditions. MF tends to impart a

high level of sizing to paper. The dry paper is

also stiffer and stronger in tensile strength. Paper

with MF resin is repulped at pH 4.5 or lower at

elevated temperature.

Epoxidized polyamine—polyamide (EPP or

PPE) resins work from pH 4 to 10 but are best

suited for use at pH 6 to 8. Their cationic nature

means they are well retained without the use of

retention aids. They are more effective than MF

per given level of addition and do not impede

water absorption appreciably. The dry paper is

not much stiffer or stronger. Repulping is done at

pH 10 or above at ambient temperatures. The

epoxy groups of this and other molecules that use

it (such as materials used to prepare cationic

starch) are stored in a stable form and made

reactive before using them (Fig. 22-9). Kymene

is a widely used wet strength resin that is formu-

lated from diethylenetriamine, adipic acid, and

epichlorohydrin to help anchor it to the fibers.

MONITORING WET END CHEMISTRY 453

22.7 MONITORING WET END CHEMISTRY

The properties of all of the materials used in

wet end chemistry should be known as appropri-

ate.

Therefore, most of the quality control tests

and methods discussed in other areas of this book

are applicable. Methods of particular interest to

wet end chemistry will be explored in this section.

They are summarized in Table 22-1.

The large number of tests in this table indi-

cates that only a few of all of

the

possible tests can

be done routinely. If one deals with only one

filler, then an ash content is all that is required to

follow the filler. Some tests might be done to

determine if a certain material is causing a prob-

lem, but is then no longer used. Much ingenuity

must be provided as each mill is different and no

standard outlines exist.

It is well known (Koethe and Scott, 1993)

that cationic polymers cause maximum flocculation

shortly (within a few seconds) after their addition

to fiber slurries. With time (on the order of 10 to

100 or more minutes) zeta potential decreases. It

is thought the cationic polymers diffuse into the

fibers since the decrease in zeta potential is small-

er with higher molecular weight polymers. There-

fore,

cationic polymers should be added relatively

late into the papermaking system.

The composition of the stock includes dis-

solved and suspended solids. Dissolved solids

include polymers, inorganic electrolytes, and

contaminants such as carryover from recycled

fiber and pulping operations. Suspended solids

include fillers and the fibers and fiber fines.

The types of pulps (hardwood or softwood;

mechanical or chemical; CTMP or TMP; size

distribution of materials) are also important. CSF

should be used to monitor refining, but has little

application to papermaking chemistry; CSF is

dominated by fines content, which does not ad-

versely affect paper machine speed to a large

degree. One of the drainage tests should be used

instead. (Industry standardization in this area

would be helpftil.) Other additives are also in-

cluded. First—pass retention of the three main

groups (fibers, fines, and ash) is important.

The properties of the electrolytes include pH,

buffer capacity (often a measure of alum content),

types of ions present (such as aluminum, sulfate,

and water hardness from calcium and magnesium).

HO CI

OK"

H—CHj

-CH—CHj

Stable form Reactive form

Fig. 22-9. Formation of epoxide group.

overall conductivity, and zeta potential of each

type of particle. The properties of each additive

should potentially be monitored separately to

verify that there is no change from batch to batch.

At least some viscosities and colloidal titrations

should be considered.

Zeta potential measurement

The zeta potential can be measured by elec-

trophoresis (Fig. 22-10). The electric field causes

movement of the water, so the measurements must

be made where it is motionless. A microscope

must be used to view the particles and determine

their velocity. The velocity (V) of migration of a

molecule is proportional to the electric field (£,

which is the volts of the field divided by the

distance between the plates, also referred to the

voltage drop per unit length, V/m)) and the charge

of the species (Z) and is indirectly proportional to

the frictional coefficient, /. The velocity can be

related to zeta potential.

y = Ezif

(22-1)

cro"

scope

I ens

c r

fCondenser Lens^

L ight

Source

n /^ !X n\

DC Voltage Supply

-0-lOOOV

Fig. 22-10. Measurement of zeta potential.