the paradox of papermaking - nc state university : webauth : login

Paradox of Papermaking Page 0

THE PARADOX OF PAPERMAKING MARTIN A. HUBBE, ORLANDO J. ROJAS North Carolina State University · Raleigh, NC 27695-8005 _________ Department of Wood and Paper Science Box 8005 Raleigh, NC 27695-8005

Marty Hubbe is an Associate Professor in the Department of Wood and Paper Science at North Carolina State University. He received his Bachelors degree in Chemistry from Colby College in 1976, an M.S. degree in paper technology from the Institute of Paper Chemistry in 1979, and a PhD. in Chemistry from Clarkson University in 1984. Prior to joining the faculty of NC State University he worked at the central research centers of American Cyanamid Corp. (now Cytec Industries) and International Paper. His interests include the colloidal chemistry of papermaking, surface charges, and polyelectrolytes. Orlando Rojas is an Assistant Professor in the Department of Wood and Paper Science at North Carolina State University. He received his B.Sc. degree in Chemical Engineering from Universidad de Los Andes (ULA, Venezuela) in 1985, a M.S. degree in Chemical Engineering from ULA in 1993 and a PhD. in Chemical Engineering from Auburn University in 1998. His postdoctoral research was conducted in the Dept. of Chemistry, Physical Chemistry of the Royal Institute of Technology (Stockholm) and he was a faculty member of the Department of Chemical Engineering of ULA. His interests include interfacial phenomena and surface and colloid science and the study of adsorption behaviors of surfactants and polymers at interfaces.

ABSTRACT

Students and educators in chemical engineering, are you aware of the paper industry and

its impact in our society? With retirements and with changing technology there is a

continual need for new technical and scientific skills to face the challenging goals of our

times. The purpose of this article is to introduce some intriguing aspects of papermaking

technology. The paradoxical nature of the papermaking process is sure to capture your

interest and imagination.

KEY WORDS Papermaking Paper Dispersion Flocculation


Students in chemical engineering who enroll in courses such as papermaking often find themselves

startled with the richness and breadth of phenomena that concurrently take place in related processes.

Gas, solid, and liquid phases are put into contact in different states of dispersion, where surface and

colloidal forces, together with hydrodynamic effects shape the final outcome, i.e. the familiar sheet of

paper. Even more perplexed is the instructor who, while teaching, finds him/herself explaining a series of

events that are full of paradoxes.

To begin with, while librarians expect paper to last for hundreds of years,[1-2] most paper either gets

thrown away or recycled within a matter of days or weeks. Whereas paper is one of the least expensive

manufactured items, its production involves use of some of the most expensive systems of equipment.[3-4]

Paper is among the most recyclable and environmentally compatible products – made mainly from

naturally renewable materials,[5] but at the same time the industry has faced great pressure related to its

environmental impact.[6-10]

Though each of the items just mentioned raises some interesting questions, the focus of the present

article is on some especially paradoxical issues related to the process itself. There are some apparent

contradictions inherent in the papermaking process, which continue to make this a fascinating field of

science and art. Even as we begin to understand the principles behind what at first appears to be magic,

we owe profound respect to the craftspeople in China and elsewhere[11-12] who discovered and developed

this subtle and economically important process.

PARADOX ONE: DIVIDE TO COMBINE

Wood and paper are both solid materials, composed mainly of polysaccharides – cellulose and

hemicellulose.[13] Both wood and paper contain at least 5% of water, though wood can contain

considerably more in a living tree and before it is dried. In addition, both wood and paper are composed

of fibers that are firmly adhering to adjacent fibers.

As shown in Fig. 1, the first step in papermaking is to destroy every one of those inter-fiber bonds in

the original wood. This is done at considerable expense and effort. The most widely used process for

converting wood into papermaking pulp, the so-called kraft process, commonly involves dissolution 50 to

60% of the solid material [14]. What makes the kraft process particularly impressive is the toughness,


insolubility, and high resistance to chemical attack on the part of lignin, which is the phenolic substance

that holds the fibers together in the wood. All of this is accomplished by a process that recovers most of

the chemicals used in cooking, and also generates an excess of high-pressure steam and electricity from

the heat evolved from the unused components of the fibers.[14-15]

Though less impressive from a chemical standpoint, the other way of liberating wood fibers from each

other also involves drastic action. Mechanical approaches to turning wood chips or logs into

papermaking fibers require a huge amount of energy, usually between 5 and 10 megajoules per kilogram

of fibers, on a dry basis.[14,16]

The next step, after converting the wood material to pulp, involves adhering the fibers back together.

A typical papermaking fiber is about 1-3 mm long and has a length-to-thickness ratio of about 50-100.[17]

During the process of forming a sheet of paper these fibers have a tendency to lie in layers, each fiber

being approximately parallel to the plane of the sheet. Hydrodynamic effects, as well as tension on the

wet sheet as it is being pressed and as it starts to be dried, can further impose a preferential orientation in

the direction of manufacture.[18-19] It has been estimated that a typical fiber in paper crosses about 20-40

similar fibers.[20] Adhesion at each of these crossing points has a dominant effect on the strength of the

resulting paper.

PARADOX TWO: ADD WATER, THEN TAKE IT AWAY

Immense amounts of water are added to the papermaking process, even if one just considers the

initial separation of wood into a suspension of fibers. Then, as illustrated in Fig. 2, the water is taken away

again. Both the kraft process and mechanical pulping processes involve dilution of the fibers to a solids

content of about 3 to 10%. It is usual to add much more water just before the paper is formed into a

sheet and dried. Papermakers refer to this highly diluted condition as the “headbox solids” or “headbox

consistency,” since the headbox is the last part of the paper machine that is visited by the fiber

suspension before water gets taken out during the paper forming process. The most common values of

headbox consistency lie within a range of about 0.3 to 1.2%.[21-22] Taking 0.5% as an example, this

implies that the papermaker needs to pump roughly 200 mass units of water for every unit of fiber, on a

dry basis.


Why do papermakers use such high dilution? The answer can be traced again to the high length-to-

thickness ratio of fibers, giving them a tendency to become entangled as “flocs” in a flowing

suspension.[23-26] It has been proposed that flocculation can be mostly avoided by diluting the suspension

enough so that most fibers are able to rotate about their center-points without touching another fiber.

Based on the lengths and masses of typical papermaking fibers, the solids level required to approach this

theoretical condition, even if the fibers are lined up on an artificially regular array, would be less than

about 0.02% solids.[27]

In actuality, the levels of headbox consistencies used in most papermaking operations seldom are as

low as these theoretical numbers. Rather, papermakers need to strike a compromise between a desire to

minimize flocculation and the expense and difficulty of recirculating so much water. Though headbox

consistencies in the range 0.3 to 1.2%, as mentioned earlier, imply very frequent collisions among fibers,

tending to produce some fiber flocs, the uniformity of the resulting paper tends to be satisfactory for most

end-uses.

The most massive and outwardly impressive part of a paper machine is devoted to removal of almost

all of that water that was used to dilute the fibrous suspension. Though details of paper machine systems

are discussed elsewhere,[4,14] several features of this equipment are especially notable. These include

the forming fabric, which is essentially a continuous screen or pair of screens upon which the paper is

initially dewatered. Adjustments in the angle impingement of the jet of fiber suspension onto the fabric,

and also the relative speeds of the jet and the fabric can be used to partly break up the flocs of fiber,

yielding more uniform paper.[28] As the wet paper proceeds down the moving fabric surface, it

experiences a series of pulses of vacuum and pressure. These pulses not only help in the process of

water removal, but they also tend to improve paper’s uniformity of formation.[28-29] Stationary devices

known as hydrofoils and forming blades are often used to pull water from the wet mat of fibers. Gradually

increasing vacuum pulls yet more water from the paper. Most of the water is removed in this stage, and

the solids content of the paper web may reach 15-20%. To remove more water, the damp paper is

pressed against felt surfaces as it passes through the nips between large solid rolls. Finally, in the dryer

section of the machine, the paper typically passes around multiple, steam-heated cylinders to evaporate

most of the remaining water. Because evaporation requires much more energy, compared to the


previous dewatering steps, it is important that the paper enter the dryer part of the paper machine with as

high solids level as practical. Usually about 4 to 8% of moisture content is left in the final paper so that it

will be as close as practical to equilibrium with the expected relative humidity when it is used, a practice

that tends to minimize curl problems in the paper.[30]

PARADOX THREE: SWELL WITH WATER TO DEHYDRATE AND SHRINK

Recently we were asked a question from someone in charge of making a relatively heavy grade of

paper on a modern machine. He wanted to know, “What can I do to reduce the amount of water

contained in the fibers?” What was left unsaid was the fact that this papermaker still wanted to achieve a

high level of inter-fiber bonding within the product. Past studies have shown a high correlation between

fiber’s state of swelling, as represented by its water-holding ability, and the tensile strength properties of

paper made from those fibers.[31-33] According to theory, a more swollen fiber has a more flexible surface,

and it is able to develop a higher proportion of bonded area under a given set of conditions for forming,

pressing, and drying the paper sheet.[20,34] This situation is illustrated in Fig. 3, showing how papermaking

fibers become swollen during the papermaking process, but they can end up “shrunk” relative to their

original perimeter.

But the more swollen fiber can be more difficult and more expensive to dry. That is because it is very

difficult to remove the last bit of water that is held within the cell walls of papermaking fibers, except by

evaporation. Though papermakers apply intense pressure, as the wet paper sheet passes between steel

rolls or “extended nips”,[35-36] some water remains in the cell walls of the compressed fibers.[37-38] The

average dimension of micropores in which such water is held in a kraft pulp fiber has been estimated to

be in the range of about 1 to 50 nm. [39-41] If one models such capillaries as cylinders, the pressure

exerted by meniscus (capillary/surface tension) forces is predicted to be in the range 4 to 200 MPa, a

range which partly overlaps the range of pressures that papermakers use to squeeze water out of a paper

sheet in press nips[14] before it is dried by evaporation.

Papermakers’ holy grail, a visionary, but seemingly impossible goal, would be to find a type of fiber

that has a highly conformable surface, in the wet state, but a low amount of water held within the fiber

walls. Though the search for such fibers generally has led to frustration, two contrasting solutions to this


dilemma are worth considering. One notable approach involves the use of relatively high-yield fibers,

such as the mechanical pulp fibers mentioned in Paradox One. The lignin and hemicellulose

components, which account for over half of the dry mass of such fibers, have softening points within a

temperature range of about 50 to 200 oC,[42-43] depending on moisture content, which is close to the

temperature that paper reaches during a typical drying operation.[44] Thermal deformation, allowing fibers

to develop a higher proportion of bonded area, has been especially noted in the case where high-yield

fibers are subjected to certain modern drying practices that achieve higher-than-typical combinations of

moisture and temperature.[45-46]

Research has shown a fascinating inter-relationship between the strength of paper and its ability to

scatter light and resist that show-through of print images.[20] The reason that these two variables are

connected is that the relative amount of light scattering is roughly proportional to the air-solid interfacial

area within paper. In areas where fibers are bonded tightly together, light can pass between the two of

them without scattering. So one of the penalties of relying on either refining or plasticization as the chief

means of increasing paper’s strength is that the paper tends to become more transparent, and it might

not meet the customer’s specifications.

To minimize the loss of opacity just described, papermakers can use a completely different

approach to increasing the bondable nature of fiber surfaces. Rather than to make the whole fiber

more flexible, the common approach is to add water-loving polymers as dry-strength agents to the

fiber slurry. The function of these dry-strength agents is to increase the tenacity of bonding within

areas where the fibers contact each other[47-49] and possibly to increase the area over which bonding

takes place. There has been a debate as to whether additives such as cationic starch or acrylamide

polymers can increase the relative area of bonding between fibers.[50-52] However, if that were true,

then one would expect the resulting paper to be more translucent, as discussed. Rather, an analysis

based on light scattering tests revealed very little increase in optically bonded area.[53] Thus the main

contribution to paper strength, due to the polymeric additives, is related to the strength per unit of

bonded area. Apparently, any effect of dry-strength polymers to fill in spaces between rough surfaces

of fibers must happen at a molecular scale, smaller than the limits of detection of optical methods.

We have to keep in mind that the light scattering method relies on the fact that a fiber surface element


appears bonded if there is another fiber surface at a distance smaller than half the wavelength of

light. This doesn't guarantee that the two fibers are bonded chemically, since the bonding distance is

shorter. Irrespective of the case under consideration it is concluded that the interaction between light

and the paper network is closely related to the bonding degree. Both light absorption and scattering

are the same properties that define the brightness and opacity of paper. Therefore the relationship

between optical and mechanical strength in paper is not surprising.

PARADOX FOUR: MAKE THE FIBERS FLEXIBLE TO MAKE THE PAPER STIFF

Although some producers of paper will argue that the primary benefit that they provide their

customers is a surface on which to print images or messages, there are many grades of paper where

“support” is a function having at least equal importance. Paper bags provide a good example. Though

many grocers prefer plastic bags, due to their handles, their resistance to water, and their low cost,

customers can clearly tell the difference; only the paper version will be stiff enough to stand up by itself,

once it is opened. As another example, xerographic copy paper has to meet a certain minimum stiffness

level, or it will tend to jam in the machine. Boxes made of paperboard also need to have sufficient

resistance to bending and crushing in order to fulfill their role.

So what is the first thing that papermakers do to the fibers? As illustrated in Fig. 4, they convert them

to flexible ribbons. In the tree, fibers can be envisioned as little tubes, with closed, pointy ends. Based

on the principles of mechanics, the tubular shape offers a high resistance to bending, relative to the mass

of solid material.[54] Instead of taking advantage of this inherent resistance to bending, papermakers

subject the fibers to a number of processes that make them more conformable. The combined effects of

kraft pulping and refining makes the fibers flexible enough to collapse. Refining involves passage of fiber

slurries between rotating steel plates with raised bars. The fibers are repeatedly compressed and

sheared as they pass between these bars, causing internal delamination, as well as fibrillation of the

outer layers of the fibers.[14,55-56] A recent study in our laboratory showed that refining increased the

flexibility of wet, unbleached softwood kraft fibers by a factor of between 6 and 19.[57]

Research has shed insight on how the orientations of fibers, the bonds between them, and the

degree to which the paper is held in tension during drying relate to the final properties of the paper.[58-60]


To simplify the analysis it was shown that the in-plane mechanical properties of a thick sheet of paper can

be reproduced by laminating many thin sheets.[61-62] An idealized model, involving 2-dimensional random

networks of fibers is then able to explain many of paper’s characteristics. In the simplest network

approach (fibers are assumed to be randomly distributed and correlations between fibers are neglected) it

is found that the local value of the number of fiber crossings can be described by simple probability

distributions. From these distributions one can easily calculate the average number of fibers crossing at

any given point in the network. This is the so-called coverage, c. The coverage can be measured from

sheet cross-sections by determining the number of bonds that intersect a reference line and this gives a

precise measure of the effective number of fiber layers in a sheet. Typical values of coverage for printing

papers are 5-20 (layers of fibers).

A more challenging issue to deal with quantitatively is paper’s directional nature. For instance,

paper’s strength in the direction of manufacture tends to be considerably higher, compared to the cross-

directional strength.[63] Briefly stated, the factors that mainly account for this directionality are (a) a

tendency of fibers to become aligned in the direction of manufacture due to hydrodynamic shear as the

paper is being formed,[63-65] and (b) forces exerted on the paper during the process of drying.[63] Tensile

forces exerted by the rotating dryer can keep the paper from shrinking, especially in the direction of

manufacture, adding to the elastic modulus of paper in that direction.

There probably will never be a completely satisfactory explanation as to why papermakers so often

fail to take advantage of the inherent stiffness of native, uncollapsed fibers. Ribbon-like fibers, as used by

papermakers, can be advantageous in terms of achieving a high proportion of bonded area.[20] It appears

that the increased inter-fiber bonding is so important that it offsets the possible advantage of keeping the

fibers in their native shape. Perhaps the next generation of papermakers will figure out a way to achieve

high levels of inter-fiber bonding without collapsing most of the fibers into ribbons.

PARADOX FIVE: DISPERSE EVERYTHING WELL, BUT RETAIN THE FINE PARTICLES

The fifth paradox to consider is deeply engrained in the art of papermaking. The function of a

dispersant chemical is to help achieve and maintain a uniform suspension of fine particles – avoiding the

formation of agglomerated material. The latter could either hurt the uniformity of the paper product, cause


abrasion, or form deposits on some of the papermaking equipment. Some of the kinds of materials that

need to be dispersed before they are added to the papermaking process include mineral fillers, sizing

additives (see later), and certain biocides and colorants. Chemical products used to avoid undesired

agglomeration of the fine particles include phosphates, low-mass acrylate copolymers, and a wide range

of nonionic surface-active agents (surfactants).[66-67] Such chemicals adsorb onto the solid surfaces and

increase the electrostatic and/or steric repulsion forces,[66] keeping the particle from colliding and sticking.

It is worth noting that some dissolved and colloidal materials originating from the wood also can play the

role of dispersants due to their negative charge.[68]

As shown in Fig. 5, the papermaker’s perspective changes abruptly when the time comes to form the

dispersed fibers and finely divided substances into a wet sheet of paper. Typical mesh fabrics, upon

which paper is formed, are composed of polyester monofilaments.[69] Though there is a wide variety of

forming fabric designs, including double- and triple-layer fabrics, the openings between adjacent filaments

is approximately 0.1 to 0.3 mm, big enough to allow passage of non-fibrous materials. This “fines”

fraction may contain, in addition to some of the wood-derived solids, also mineral fillers and sizing

emulsion particles. Even through, in principle, some of the fine particles may be retained by mechanical

filtration in the mat of wet fibers, experience has shown that the efficiency of such mechanical retention

tends to be low in the absence of flocculating chemicals.[70] Poor retention of these materials produces

not only a lower productivity in terms of mass balance, but also filtered water that is more difficult to treat

for recirculation.

Perhaps surprisingly, the kinds of chemical treatments that have been found to be most effective for

increasing the retention efficiency during paper formation work according to a different principle,

compared to dispersants. Mere neutralization or removal of repulsive forces between surfaces[71] does

not provide nearly the strength of attachments needed to resist the strong hydrodynamic forces inherent

in the formation of paper on a modern machine.[72-75] Strong forces tending to detach fine particles from

fibers develop as water is removed from the sheet by gravity, and by the repeated vacuum and pressure

forces.[76-83] It is because of this that fine materials tend to be washed out of those layers of a paper

product which were closest to the forming fabric during the production process.[84-85]


The chemicals found to be most effective for retention of fine particles are the very-high-mass

acrylamide copolymers, having molecular masses in the range of about 5 to 20 million grams per

mole.[70,86-87] This is roughly 1000 times larger in molecular mass compared to common polymeric

dispersant molecules. The monstrous size of retention aid polymers allows them to bridge between

surfaces of adjacent solids, extending beyond the range of repulsive forces, including those components

of the repulsive forces induced by dispersant treatments. Various different bridging mechanisms have

been studied.[88-95] The effectiveness of very large molecules has also been attributed to the fact that

multiple points of attachment occur simultaneously, so that adsorption of the polymer onto a surface is

very difficult to reverse.

It is reasonable to ask, “Do dispersants interfere with the performance of retention aids?” In general,

the answer is “yes.” Many studies have shown diminished effectiveness of cationic acrylamide copolymer

retention aids in systems that contain substances that can act as dispersants.[96-98] This is particularly

observed in the case of wood-derived anionic colloidal materials.[68,96-98] To overcome this kind of effect,

papermakers often use highly-charged cationic materials such as aluminum sulfate, polyaluminum

chloride (PAC), polyamines, polyethyleneimine (PEI), and similar chemicals. In addition to their role as

charge-neutralizers, such additives also can serve as anchoring sites for anionic retention aid

molecules,[99-102] or as site-blockers to enhance the effectiveness of cationic retention aid molecules.[103]

Cationic retention aids exhibit a surprising degree of compatibility with nonionic dispersants. The

latter often consist of long hydrophilic ethylene-oxide chains, having the repeating unit (-CH2-CH2-O-).

These are attached either to an alkyl or aromatic hydrocarbon group, or to a water-hating propylene oxide

chain. An example of the compatibility between such nonionic dispersants and cationic retention aids can

be seen in a patented system for control of wood pitch deposition in paper machine systems.[104] This

system consists of adding a nonionic surfactant to the furnish to disperse the pitch particles (to keep them

from colliding and building up to objectionable size), and then treating the slurry with a cationic acrylamide

copolymer retention aid.

At the opposite extreme, one can consider the use of a nonionic retention aid system based on

polyethylene oxide (PEO) and a cofactor.[92-95] Such systems can be almost unaffected by changes in the

amounts of anionic colloidal materials and other anionic dispersants in the system.


Though the strategies mentioned in the previous two paragraphs are useful for illustrating some

principles, it is far more common for papermakers to follow a strategy of minimizing the amounts of

dispersants that are ever added to the papermaking system – knowing that their effects will need to be

reversed later on, when the retention aid polymers are added. The goal is to keep the amounts of

dispersant no higher than the minimum needed to maintain uniform suspensions of such materials as

calcium carbonate filler. This strategy can explain at least part of the success of on-site production of

precipitated calcium carbonate (PCC) filler.[105-106] Relative to ground calcium carbonate (GCC), PCC

requires relatively little dispersant, as long as it is made on site and kept agitated for the relatively short

time between its production and use.[107-108]

PARADOX SIX: CHEMICALLY FLOCCULATE THE FIBERS AND THEN DISPERSE THEM

As odd it may seem, one of the first things that often happens after the papermaker has flocculated

the suspended material with very-high-mass acrylamide copolymers, as just described, is that the furnish

passes through a screening device that rips apart 100% of the polymer-induced attachments between

fibers. This circumstance is illustrated in Fig. 6. Studies have shown that breakage of contacts between

the fibers can irreversibly degrade the high-mass polymeric flocculants.[109-112] The main function of a

pressure screen is to prevent large objects, such as incompletely cooked bits of wood, from getting into

the product.[113] The slots in the type of screen typically used in these applications have widths of about

0.15 to 0.45 mm,[113] which is large enough to allow passage of a single fiber, but too small to allow

passage of fibers that are bound together by polymers.

Though “pre-screen” addition of retention aid polymers, as just mentioned, is very common, many

papermakers choose to maximize the efficiency of these flocculants by adding them just after the

screening operation.[114] Depending on the type of headbox, and other details of the machinery, the

papermaker then selects a suitably low dosage of retention aid to achieve almost the same result –

redispersal of most of the fibers from each other before the paper sheet is formed.

To add yet another layer to the riddle, many paper machine systems, especially in the manufacture of

printing papers, use something called microparticles, which partly reflocculate the papermaking furnish

again.[115-117] These additives include colloidal silica dispersions and montmorillonite clay, a suspension


of extremely thin plate-like particles. The common feature is that microparticles all have a very high ratio

of surface area to mass, usually in excess of 100 m2/g. One very common strategy for microparticle use

involves pre-screen addition of a cationic acrylamide retention aid, as just mentioned, followed by post-

screen addition of the microparticle additive.[115-116,118] When microparticles are added in this way, the

little particles are able to bridge between the fragments of retention aid polymers remaining on the

adjacent fiber surfaces and connect them together again. That fact that papermakers seem to vacillate

between inducing flocculation and then deflocculation of the papermaking stock can start to make one

wonder about what they are really trying to achieve.

One explanation for the papermaker’s odd practice of flocculating fibers, and then immediately

deflocculating them again is the fact that hydrodynamic forces are much better able to detach larger

objects from each other, compared to their ability to detach a small object, either from a larger object or

from other like-sized objects.[74,119-121] It’s a matter of leverage. Though only something like a screen

device can ensure complete deflocculation of the papermaking furnish, if only for a moment,

hydrodynamic forces in the headbox of a paper machine have the potential to achieve selective breakage

of polymer bridges. Modern paper machines often employ hydraulic headboxes, in which hydrodynamic

shear and extensional flow fields have been designed in such a way as to maximize uniformity of the

resulting paper.[122-123] Recent work suggests that it is possible, in principle, to select conditions of

retention aid treatment that are more than sufficient to retain small particles, such as mineral fillers, on

cellulosic surfaces, but most contacts between fibers will be separated from each other as the furnish

passes through the high-shear zones of the headbox.[74,121,124-125]

PARADOX SEVEN: WATER-BORNE TREATMENTS TO MAKE PAPER WATER-RESISTANT

Many different kinds of paper have to be able to resist water to perform their intended function, but

the fibers themselves are generally water-loving. In a process called “internal sizing,” papermakers add

materials they call “sizing agents” to the aqueous mixture of fibers and other materials so that the

resulting paper becomes hydrophobic.[17,126-127] These sizing agents have to be either water-dispersible or

water-soluble in order to become well mixed with the papermaking stock. Science fiction? No. This is

commonly accepted practice within the paper industry.


While there have been many detailed studies of the molecular mechanisms of different internal sizing

systems,[126-132] little of which will be repeated here, it is important to emphasize two key molecular events

which seem to underlie the seemingly impossible transformation of water-loving fibers in an aqueous

environment to water-repellent paper once the same materials are dried. One of these events is the

orientation and anchoring of sizing molecules, due to the ways in which these molecules interact with

fiber surfaces. The other event involves the way some sizing molecules become distributed over the

surface of fibers when the paper heated to evaporate the water that remains after the paper has been

formed and pressed.

The concept of “anchoring and orienting” can perhaps best be illustrated by the case of rosin soap

products. As the word soap implies, rosin soap is a water-dispersible, sudsy material. Though rosin

products contain a mixture of different compounds, most of them have between one and three water-

loving carboxylate groups per molecule. In addition, the remainder of a typical rosin molecule consists of

water-hating hydrocarbon material. When dispersed in water, the rosin soap exists not as a true solution,

but as micelles. In other words, groups of rosin soap molecules associate with each other so that the

water-hating parts are generally facing each other to avoid contact with the water. In order to achieve a

sizing effect, an aluminum compound, such as aluminum sulfate, is added to the papermaking furnish.

As shown in Fig. 7, the aluminum ions interact with the carboxylate groups, causing the rosin to

precipitate onto the fiber surfaces. It has been proposed that the alum keeps the sizing molecules

oriented on the fiber surfaces such that the water-hating hydrocarbon portions face outwards from the

fiber surface.[130,133-134]

The other common types of internal sizing agents work differently, and the key molecular events

occur during drying at high temperature. If you were to add either rosin emulsion size or alkylketene

dimmer (AKD) sizing agents to a papermaking furnish, and then gradually dry the paper at room

temperature over night, very little hydrophobicity would develop.

Although many authors have used words like “spreading” to describe how emulsified sizing agents

become distributed over the exposed surfaces of paper during the drying process, recent evidence favors

a different mechanism. It is true that rosin acids, AKD, and alkenylsuccinic anhydride (ASA) sizing agents

all exist as liquids at the temperatures found in the dryer sections of paper machines, these droplets of


liquid tend to remain localized at the fiber surfaces, rather than spreading out as a mono-molecular

layer.[135-139] It has been proposed that the lack of spreading is due to formation of so-called “precursor

films” adjacent to the bulk of sizing material.[138-139] The very low surface energy achieved in areas

covered by such films impedes spreading of the droplets of hydrophobic material. In addition, studies

have shown that only a fraction of the surface area needs to be covered with sizing molecules to achieve

a high level of water resistance.[140-142]

Despite the relatively low vapor pressures of AKD and other emulsified sizing agents, even at the

temperatures adjacent to drying cylinders on a paper machine, there is circumstantial evidence of vapor-

phase migration. For example, if one forms a sheet of wet paper in the presence of sizing agents, and

then dries that sheet of paper in a stack with unsized paper sheets in an oven, a significant sizing effect

can become distributed throughout the stack, with results depending on the location of a sheet relative to

the treated sheet.[140,143-144] Perhaps the answer to this puzzle involves the relatively short distances over

which the vapors of sizing agents need to migrate. The distances that sizing agent vapors need to

migrate are even less if one considers the fact that the process of forming the paper results in micron-

sizing droplets or particles of sizing material distributed in a semi-random manner over the surface of

each fiber.

A further perplexing phenomenon is observed when papermakers add polymeric sizing compounds to

the starch solutions that are applied to the surface of dry paper at so-called size press operations. These

polymers, which include styrene maleic anhydride (SMA) copolymers, are dispersible in the aqueous

starch solution. Apparently the ratio of water-loving maleic acid salts, versus hydrophobic styrene groups,

is enough to achieve either solubility, or a micellization effect that closely resembles solubility. When the

starch film is dried, however, droplets of water will not spread over the paper surface. To explain this

effect, it has been proposed that the sizing copolymers migrate to the surface of the starch film, as it

dries, and that hydrophobic styrene groups face outwards from the paper surface.[145-146]

CONCLUDING REMARKS

After going through all these seven paradoxes, it becomes evident that making paper is not as simple

as it may seem, and there is plenty of room to further our understanding. The science of papermaking


offers an abundance of opportunities for fundamental inquiry on the biological, material, and chemical

fronts. At the educational level it is a subject where one can put into practice all it is learned in allied

subjects of chemical engineering, including mass, energy and momentum transfer, colloid and surface

science, materials science, and chemistry. Many potential career opportunities are available to new

chemical engineers who enjoy paradoxes. Possible career roles for engineers can be as diverse as

process engineering and optimization, product development, research, technical sales, and mill

management. Although it is foreseen that the nano-bio-techno waves will make an impact in

papermaking and paper composites, the main process by which paper is made probably will remain the

same, since all paradoxes coexist in perfect harmony. No wonder it took many centuries to our

papermaker predecessors to get to where we are now.

REFERENCES 1. Wilson, W. K., and Parks, E. J., “Comparison of Accelerated Aging of Book Papers in 1937 with 36 years of Natural Aging,” Restaurator 4, 1-55 (1980) 2. Waterhouse, J. F., “Monitoring the Aging of Paper,” in Paper Preservation, TAPPI Press, Atlanta, 53-57 (1990) 3. Yin, R., “Industry Characteristics and Investment Decisions: Alternative Approach to Pulp and Paper Production,” Tappi J. 81 (1), 69-74 (1998) 4. Atkins, J., “The Modern Paper Machine. Part 2: Coated and Fine Paper,” Solutions! 86 (12), 31-34 (2003) 5. Jorling, T., “The Forest Products Industry: A Sustainable Enterprise,” Tappi J. 83 (12), 32-35 (2000) 6. Axegård, P., Dahlman, O., Hanlind, I., Jacobson, B., and Morck, R., “Pulp Bleaching and the Environment – The Situation in 1993,” Nordic Pulp Paper Res. J. 8 (4), 365-378 (1993) 7. Munkittrick, K. R., Servos, M. R., Carey, J. H., and van der Kraak, G. J., “Environmental Impacts of Pulp and Paper Wastewater: Evidence for a Reduction in Environmental Effects at North American Pulp Mills Since 1992,” Water Sci. Technol. 35 (2/3), 329-338 (1997) 8. Möbius, C. H., and Cordes-Tolle, “Paper Industry on the Way to Integrated Environmental Protection: Wastewater Treatment,” Papier 53 (10A), V60-V65 (1999) 9. Wahaab, R. A., “Evaluation of Aerobic Biodegradability of Some Chemical Compounds Commonly Applied in Paper Industry,” Bull. Environ. Contam. Toxicol. 64, 558-564 (2000) 10. Anon., “Nutrient Forms in Pulp and Paper Mill Effluents and their Potential Significance in Receiving Waters,” NCASI Tech. Bull. 832, 25 pp. (2001) 11. Hunter, D., Papermaking, The History and Technique of an Ancient Craft, Dover Publ., New York (1974) 12. Collings, T., and Milner, D., “New Chronology of Papermaking Technology,” Paper Conservator 14, 58-62 (1990) 13. Lewin, M., and Goldstein, I.S., Wood Structure and Composition, Marcel Dekker, New York (1991) 14. Smook, G.A., Handbook for Pulp and Paper Technologists, 2 Ed., Angus Wilde Publ., Vancouver (1992) 15. Aziz, S., and Arafat, N., “Pulp Manufacturing Energy Survey – TAPPI Alkaline Committee,” Proc. TAPPI 1997 Pulping Conf., 455-462, TAPPI Press, Atlanta (1997) 16. Law, K.-N. Insights on the refining mechanism,” Tappi J. 1(1), 4-7 (2002) 17. Scott, W. E., Principles of Wet End Chemistry, TAPPI Press, Atlanta (1996)


18. Ross, R.F., and Klingenberg, D.J., “Simulation of Flowing Wood Fiber Suspensions,” J. Pulp Paper Sci. 24 (12), 388-392 (1998) 19. Niskanen, K., Kajanto, I., and Pakarinen, P., “Paper Structure,” in Niskanen, K., Paper Physics, Ch. 1, 14-53, Fapet Oy, Helsinki (1998) 20. Page, D., “A Theory for the Tensile Strength of Paper,” Tappi 52 (4), 674-681 (1969) 21. Beirmann, C. J., Handbook of Pulping and Papermaking, Academic Press, San Diego (1996) 22. Weise, U., Terho, J., and Paulapuro, H., “Stock and Water Systems of the Paper Machine,” in Paulapuro, H., Ed., Papermaking. Part 1, Stock Preparation and Wet End, Ch. 5, 125-190 Fapet Oy, Helsinki (2000) 23. Waterhouse, J. F., “Effect of Papermaking Variables on Formation,” Tappi J. 76 (9), 129-134 (1993) 24. Kerekes, R. J., “Perspectives on Fiber Flocculation in Papermaking,” 1995 Intl. Paper Phys. Conf., 23-31, TAPPI Press, Atlanta (1995) 25. Dodson, C. T. J., “Fiber Crowding, Fiber Contacts, and Fiber Flocculation,” Tappi J. 79 (9), 211-216 (1996) 26. Beghello, L., and Eklund, D., “Some Mechanisms that Govern Fiber Flocculation,” Nordic Pulp Paper Res. J. 12 (2), 119-123 (1997) 27. Kerekes, R. J., and Schell, C. J., “Characterization of Fiber Flocculation Regimes by a Crowding Factor,” J. Pulp Paper Sci. 18 (1), J32-J38 (1992) 28. Manson, D. W., “The Practical Aspects of Formation,” Wet End Operations Short Course Notes, TAPPI Press, Atlanta (1996) 29. Hubbe, M.A., Tripattharanan, T., Heitmann, J.A., and Venditti, R.A., “The 'Positive Pulse Jar' (PPJ): A Flexible Device for Retention Studies,” Paperi Puu 86 (2004) accepted 30. Green, C., “Curl in Paper,” Appita J. 53 (4), 272-273, 275 (2000) 31. Thode, E. F., Bergomi, J. G., and Unson, R. E., “The Application of a Centrifugal Water-Retention Test to Pulp Evaluation,” Tappi 43 (5), 505-512 (1960) 32. Jayme, G., and Büttel, “The Determination and Meaning of Water Retention Value (WRV) of Various Bleached and Unbleached Pulps,” Wochenbl. Papierfabr. 96 (6), 180-187 (1968) 33. Anon., “Water Retention Value (WRV),” TAPPI Useful Method UM 256 (1981) 34. Robinson, J. V., “Fiber Bonding,” in Casey, J. P., Ed., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. 2, 915-961, Wiley-Interscience, New York (1980) 35. Worsick, A., “Developments in Press Technology,” Paper Technol. 35 (5), 30-35 (1994) 36. Wahlström, B., “Wet Pressing in the 20th Century: Evolution, Understanding, and Future,” Pulp Paper Can. 102 (12), 81-88 (2001) 37. Maloney, T. C., and Paulapuro, H., “The Centrifugal Compression Value,” Tappi J. 82 (6), 150-154 (1999) 38. Ahrens, F., Alaimo, N., Nanko, H., and Patterson, T., “Initial Development of an Improved Water Retention Value Test and its Application to the Investigation of Water Removal Potential,” TAPPI 99 Proc., 37-47, TAPPI Press, Atlanta (1999) 39. Stone, J. E., and Scallan, A. M., “A Structural Model for the Cell Wall of Water-Swollen Wood Pulp Fibers based on their Accessibility to Macromolecules,” Cellulose Chem. Technol. 2 (3), 343-358 (1968) 40. Berthold, J., and Salmén, L., “Effects of Mechanical and Chemical Treatments on the Pore-Size Distribution in Wood Pulps Examined by Inverse-Size-Exclusion Chromatography,” J. Pulp Paper Sci. 23 (6), J245-253 (1997) 41. Alince, T., and van de Ven, T. G. M., “Porosity of Swollen Pulp Fibers Evaluated by Polymer Adsorption,” in Baker, C. F., Ed., The Fundamentals of Papermaking Materials, Vol. 2, 771-788, Pira Int’l., Leatherhead, Surrey, UK (1997) 42. Back, E.L., and Salmén, N.L., “Glass Transitions of Wood Components Hold Implications for Molding and Pulping Processes,” Tappi J. 65 (7), 107-110 (1982) 43. Salmén, N.L., Kolseth, P., and de Ruvo, A., “Modeling the Softening Behavior of Wood Fibers,” J. Pulp Paper Sci. 11 (4), J102-J107 (1985) 44. Garvin, S. P., and Pantalea, P. F., “Measurement and Evaluation of Dryer Section Performance,” Proc. TAPPI Engineering Conf., Book 2, 125-133 (1976) 45. Kunnas, L., Lehtinen, J., Paulapuro, H., and Kiviranta, A., “The Effect of Condebelt Drying on the Structure of Fiber Bonds,” Tappi J. 76 (4), 95-104 (1993) 46. Retulainen, E., “Condebelt Press Drying and Sustainable Paper Cycle,” Paperi Puu 85 (6), 329-333 (2003)


47. Marton, J., and Marton, T., “Wet End Starch: Adsorption of Starch on Cellulosic,” Tappi J. 59 (12), 121-124 (1976) 48. Zhang, J., Pelton, R., Wågberg, L., and Rundlöf, M., “The Effect of Charge Density and Hydrophobic Modification on Dextran-Based Paper Strength Enhancing Polymers,” Nordic Pulp Paper Res. J. 15 (5), 440-445 (2000) 49. Pelton, R., Zhang, J., Wågberg, L., and Rundlöf, M., “The Role of Surface Polymer Compatibility in the Formation of Fiber/fiber Bonds in Paper,” Nordic Pulp Paper Res. J. 15 (5), 400-406 (2000) 50. Hofreiter, B. T, “Natural Products for Wet-End Addition,” in Casey, J. P., Ed., Pulp and Paper Chemistry and Chemical Technology, Vol. III, Wiley-Interscience, 3rd Ed., New York (1980) 51. Spence, G. G., “Application of Wet- and Dry-Strength Additives,” in Spence, G. G., Ed., Wet- and Dry-Strength Additives – Application, Retention, and Performance, TAPPI Press, 19-47, Atlanta (1999) 52. Tiberg, F., Daicic, J., and Fröberg, J., “Surface Chemistry of Paper,” in Holmberg, K., Ed., Handbook of Applied Surface and Colloid Chemistry, Ch. 7., 123-173, Wiley, New York (2001) 53. Howard, R. C., and Jowsey, C. J., “The Effect of Cationic Starch on the Tensile Strength of Paper,” J. Pulp Paper Sci. 15 (6), J225-J229 (1989) 54. Gere, J. M., and Timoschenko, S. P, Mechanics of Materials, 3rd Ed., PWS Publ. Co., Boston (1984) 55. Baker, C. F., “Good Practice for Refining the Types of Fiber Found in Modern Paper Furnishes,” Tappi J. 78 (2), 147-153 (1995) 56. Batchelor, W. J., Martinez, D. M., Kerekes, R. J., and Ouellet, D., “Forces on Fibers in Low-Consistency Refining: Shear Force,” J. Pulp Paper Sci. 23 (1), J40-J45 (1997) 57. Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., “Effects of Sugar Addition Before Drying on the Wet-Flexibility of Redispersed Kraft Fibers,” J. Pulp Paper Sci. 30 (1), 29-34 (2004) 58. Kim, C. Y., Page, D. H., El-Hosseiny, F., and Lancaster, A.P.S., “Mechanical-Properties of Single Wood Pulp Fibers. 3. Effect of Drying Stress on Strength,” J. Applied Polymer Sci. 19 (6), 1549-1562 (1975) 59. Zhang, G., Laine, J. E., and Paulapuro, H., “Characteristics of the Strength Properties of a Mixture Sheet under Wet Straining Drying,” Paperi Puu 84 (3), 169-173 (2002) 60. McDonald, J. D., Pikulik, L. I., and Daunais, R., “On-Machine Stress-Strain Behavior of Newsprint,” J. Pulp Paper Sci. 14 (3), J53-J58 (1988) 61. Kallmes, O.; Corte, H., and Bernier, G., “The Structure of Paper. V. The Bonding States of Fibers in Randomly Formed Papers,” Tappi J. 46 (8), 493-502 (1963) 62. Deng, M., and Dodson, C. T. J., Paper: An Engineered Stochastic Structure, Tappi Press, Atlanta 1994 63. Niskanen, K., Kajanto, I., and Pakarinen, P., “Paper Structure,” Ch. 1 in Niskanen, K., Ed., Paper Physics, Fapet/Tappi, Atlanta, 1998 64. Schulgasser, K., “Fiber Orientation in Machine-Made Paper,” J. Materials Sci. 20 (3), 859-866 (1985) 65. Kallmes, O., Bernier, G., and Perez, M., “A Mechanistic Theory for the Load Elongation Properties of Paper,” Pap. Technol. Ind. 18 (7), 222, 225-228, (8), 243-245, (9), 283-285, (10), 328-331 (1977) 66. Hanu, W. M., “Dispersants,” in Kirk-Othmer Encyclopedia of Chemical Technol., 4th Ed., Vol. 8, 293-311, Wiley-Interscience, NY (1993) 67. Lynn, J. L., Jr., and Bory, B. H., “Surfactants,” in Kirk-Othmer Encyclopedia of Chemical Technol., 4th Ed., Vol. 23, 478-541, Wiley-Interscience, NY (1993) 68. Sundberg, A., Ekman, R., Holmbom, B., and Grönfors, H., “Interactions of Cationic Polymers with Components in Thermomechanical Pulp Suspensions,” Paperi Puu 76 (9), 593-598 (1994) 69. Kilpenläinen, R., Taipale, S., Marin, A., Kortelainen, P., and Metsäranta, S., “Forming Fabrics,” in Paulapuro, H., Ed., Papermaking. Part 1, Stock Preparation and Wet End, Ch. 7, 253-282, Fapet Oy, Helsinki (2000) 70. Horn, D., and Linhart, F., “Retention Aids,” in Roberts, J. C., Ed., Paper Chemistry, Ch. 4, 44-62, Blackie, Glasgow, UK (1991) 71. Walkush, J.C., and Williams, D.G., “The Coagulation of Cellulose Pulp Fibers and Fines as a Mechanism of Retention,” Tappi 57 (1), 112-116 (1974) 72. Britt, K. W., “Mechanisms of Retention during Paper Formation,” Tappi 56 (10), 46-50 (1973) 73. Britt, K. W., and Unbehend, J. E., “New Methods for Monitoring Retention,” Tappi 59 (2), 67-70 (1976) 74. Hubbe, M. A., “Retention and Hydrodynamic Shear,” Tappi J. 69 (8), 116-117 (1986)


75. Tripattharanan, T., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., “Effect of Idealized Flow Conditions on Retention Aid Performance. 2. Polymer Bridging, Charged Patches, and Charge Neutralization,” Appita J. 57 (2004) accepted 76. Lindberg, L., “Pulsed Drainage of Paper Stock,” Svensk Papperstidn. 73 (15), 451-454 (1970) 77. Walser, R., Eames, J. D., and Clark, W. M., “Performance Analysis of Hydrofoils with Blades of Various Widths,” Pulp Paper Mag. Can. 71 (8), T183-T187 (1970) 78. Tam Doo, P. A., Kerekes, R. J., and Pelton, R. H., “Estimates of Maximum Hydrodynamic Shear Stresses on Fiber Surfaces in Papermaking,” J. Pulp Paper Sci. 10 (4) J80-J88 (1984) 79. Britt, K. W., Unbehend, J. E., and Shidharan, R., “Observations on Water Removal in Papermaking,” Tappi J. 69 (7), 76-79 (1986) 80. Kiviranta, A., and Paulapuro, H., “The Role of Fourdrinier Table Activity in the Manufacture of Various Paper and Board Grades,” Tappi J. 75 (4), 172-185 (1992) 81. Swerin, A., and Ödberg, L., “Flocculation and Floc Strength – From the Laboratory to the FEX Paper Machine,” Papier 50 (10A), V45-V47 (1996) 82. Räisänen, K., Karrila, S., and Maijala, A., “Vacuum Dewatering Optimization with Different Furnishes,” Paperi Puu 78 (8), 461-467 (1996) 83. Baldwin. L., “High Vacuum Dewatering,” Paper Technol. 38 (4), 23-28 (1997) 84. Räisänen, K. O., Paulapuro, H., and Karrila, S. J., “The Effects of Retention Aids, Drainage Conditions, and Pretreatment of Slurry on High-Vacuum Dewatering: A Laboratory Study,” Tappi J. 78 (4), 140-147 (1995) 85. Zeilinger, H., and Klein, M., “Modern Measuring Methods for the Cross-Sectional Filler Distribution,” Wochenbl. Papierfabr. 123 (20), 903-910 (1995) 86. Schiller, A. M., and Suen, T., “Ionic Derivatives of Polyacrylamide,” Ind. Eng. Chem. 48 (12), 2132-2137 (1956) 87. Norell, M., Johansson, K., and Persson, M., “Retention and Drainage,” in Neimo, L., E., Papermaking Chemistry, Ch. 3, 42-81, Fapet Oy, Helsinki (1999) 88. La Mer, V. K., and Healy, T., “Adsorption-Flocculation Reactions of Macromolecules at the Solid-Liquid Interface,” Rev. Pure Applied Chem. 13 (Sept.), 112-133 (1963) 89. Swerin, A., and Ödberg, L., “Some Aspects of Retention Aids,” in Baker, C.F., Ed., The Fundamentals of Papermaking Materials, Pira International, Leatherhead, UK, Vol. 1, 265-350 (1997) 90. Petäjä, T., “Fundamental Mechanisms of Retention with Retention Agents. Part 1. Electrolyte and Single Polymer Systems,” Kemia-Kemi 7 (3), 110-130 (1980) 91. Petzold, G., Buchhammer, H.-M., and Lunkwitz, K., “The Use of Oppositely Charged Polyelectrolytes as Flocculants and Retention Aids,” Colloids Surf. A. 119 (1), 87-92 (1996) 92. Lindström, T., and Glad-Nordmark, G., “Network Flocculation and Fractionation of Latex Particles by Means of Polyethyleneoxide-Phenolformaldehyde Resin Complex,” J. Colloid Interface Sci. 97 (1), 62-67 (1984) 93. Xiao, H., Pelton, R., and Hamielec, A., “Retention Mechanisms for Two-Component Systems Based on Phenolic Resins and PEO or New PEO-Copolymer Retention Aids,” J. Pulp Paper Sci. 22 (12), J475-J485 (1996) 94. Van de Ven, T. G. M., and Alince, B., “Association-Induced Polymer Bridging: New Insights into the Retention of Fillers with PEO,” J. Pulp Paper Sci. 22 (7), J257-J263 (1996) 95. Kratochvil, D., Alince, B., and Van de Ven, T. G. M., “Flocculation of Clay Particles with Poorly and Well-Dissolved Polyethylene Oxide,” J. Pulp Paper Sci. 25 (9), 331-335 (1999) 96. Lindström, T., Söremark, C., Heinegård, C., and Martin-Löf, S., “The Importance of Electrokinetic Properties of Wood Fiber for Papermaking,” Tappi 57 (12), 94-96 (1974) 97. Wågberg, L, and Ödberg, L., “The Action of Cationic Polyelectrolytes Used for the Fixation of Dissolved and Colloidal Substances,” Nordic Pulp Paper Res. J. 6 (3), 127-135 (1991) 98. Nurmi, M., Byskata, J., and Eklund, D., “On the Interaction between Cationic Polyacrylamide and Dissolved and Colloidal Substances in Thermomechanical Pulp,” Paperi Puu 86 (2), 109-112 (2004) 99. Moore, E. E., “Charge Relationships of Dual Polymer Retention Aids,” Tappi 59 (6), 120-122 (1976) 100. Petäjä, T., “Fundamental Mechanisms of Retention with Retention Agents. Part 2. Dual Polymer Systems,” Kemia-Kemi 7 (5), 261-263 (1980) 101. Wågberg, L., and Lindström, T., “Some Fundamental Aspects of Dual-Component Retention Aid Systems,” Nordic Pulp Paper Res. J. 2 (2), 49-55 (1987)


102. Petzold, G., “Dual-Addition Schemes,” in Faranato, R. S., and Dubin, P. L., Colloid-Polymer Interactions: From Fundamentals to Practice, Ch. 3, 83-100, Wiley Interscience, New York (1999) 103. Swerin, A., Glad-Nordmark, G., and Ödberg, L., “Adsorption and Flocculation in Suspensions by Two Cationic Polymers – Simultaneous and Sequential Addition,” J. Pulp Paper Sci. 23 (8), J389-J393 (1997) 104. Capozzi, A. M., and Rendé, D. S., “Particle Management: Effective Stickies Control Approach,” Proc TAPPI 1994 Pulping Conference, 643-654, TAPPI Press, Atlanta (1994) 105. Gill, R. A., “The Behavior of On-Site Synthesized Precipitated Calcium Carbonates and Other Calcium Carbonate Fillers on Paper Properties,” Nordic Pulp Paper Res. J. 2 (4), 120-127 (1989) 106. Fairchild, G. H., “Increasing the Filler Content of PCC-Filled Alkaline Papers,” Tappi J. 75 (8), 85-90 (1992) 107. Sanders, N. D., and Schaefer, J. H., “Comparing Papermaking Wet-End Charge-Measuring Techniques in Kraft and Groundwood Systems,” Tappi J. 78 (11), 142-150 (1995) 108. Suty, S., Alince, B., and van de Ven, T. G. M., “Stability of Ground and Precipitated CaCO3 Suspensions in the Presence of Polyethylenimine and Salt,” J. Pulp Paper Sci. 22 (9), J321-J326 (1996) 109. Sikora, M. D., and Stratton, R. A., “The Shear Stability of Flocculated Colloids,” Tappi 64 (11), 97-101 (1981) 110. Tanaka, H., Swerin, A., and Ödberg, L., “Transfer of Cationic Retention Aid from Fibers to Fine Particles and Cleavage of Polymer Chains under Wet-End Papermaking Conditions,” Tappi J. 76 (5), 157-163 (1993) 111. Hubbe, M. A., “Reversibility of Polymer-Induced Fiber Flocculation by Shear. 1. Experimental Methods,” Nordic Pulp Paper Res. J. 15 (5), 545-553 (2000) 112. Tripattharanan, T., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., “Effect of Idealized Flow Conditions on Retention Aid Performance. 1. Cationic Acrylamide Copolymer,” Appita J. 57 (2004) accepted 113. Bliss, T., “Screening in the Stock Preparation System,” in TAPPI Stock Preparation Short Course Notes, TAPPI Press, Atlanta (1996) 114. Hubbe, M. A., and Wang, F., "Where to Add Retention Aid: Issues of Time and Shear," TAPPI J. 1 (1), 28-33 (2002) 115. Langley, J.G., and Litchfield, E., “Dewatering Aids for Paper Applications,” In Proc. TAPPI Papermakers Conf., TAPPI Press, Atlanta (1986) 116. Andersson, K., and Lindgren, E., “Important Properties of Colloidal Silica in Microparticulate Systems,” Nordic Pulp Paper Res. J. 11 (1), 15-21 (1996) 117. Hubbe, M. A., “Microparticle Programs for Drainage and Retention,” In Microparticles and Nanoparticles in Papermaking, TAPPI Press, Atlanta (2004) 118. Main, S., and Simonson, P., “Retention Aids for High-Speed Paper Machines,” Tappi J. 82 (4), 78-84 (1999) 119. McKenzie, A. W., “Structure and Properties of Paper. XVIII. The Retention of Wet-End Additives,” Appita 21 (4), 104-116 (1968) 120. Hubbe, M. A., “Detachment of Colloidal Hydrous Oxide Spheres from Flat Solids Exposed to Flow. 2. Mechanism of Release,” Colloids Surf. 16 (3-4), 249-270 (1985) 121. Hubbe, M. A., “Reversibility of Polymer-Induced Fiber Flocculation by Shear. 1. Experimental Methods,” Nordic Pulp Paper Res. J., 15 (5), 545-553 (2000) 122. Kiviranta, A., and Paulapuro, H., “Hydraulic and Rectifier Roll Headboxes in Boardmaking,” Paper Technol. 31 (11), 34-40 (1990) 123. Bonfanti, J. -D., Roux, J. -C., and Rueff, M., “Hydraulic Headbox S – Technology and Industrial Results,” Wochenbl. Papierfabr. 128 (20), 1372-1375 (2000) 124. Hubbe, M. A., "Detachment of Colloidal Hydrous Oxide Spheres from Flat Solids Exposed to Flow. 4. Effects of Polyelectrolytes," Colloids Surf. 25 (2-4), 325-339 (1987) 125. Rojas, O. J., and Hubbe, M. A., “The Dispersion Science of Papermaking,” J. Dispersion Sci. Technol. 25(6), 713-732 (2004) 126. Hodgson, K. T., “A Review of Paper Sizing Using Alkyl Ketene Dimer versus Alkenyl Succinic Anhydride,” Appita J. 47 (5), 402-406 (1994) 127. Neimo, L., “Internal Sizing of Paper,” in Neimo, L., Ed., Papermaking Chemistry, Ch. 7, 151-203, Fapet Oy, Helsinki (1999)


128. Wasser, R. B., “The Reactivity of Alkenyl Succinic Anhydride: Its Pertinence with Respect to Alkaline Sizing,” J. Pulp Paper Sci. 13 (1), J29-J32 (1987) 129. Ödberg, L., Lindström, T., Liedberg, B., and Gustavsson, J., “Evidence for β-Ketoester Formation during the Sizing of Paper with Alkylketene Dimers,” Tappi J. 70 (4), 135-139 (1987) 130. Marton, J., “Mechanistic Differences between Acid and Soap Sizing,” Nordic Pulp Paper Res. J. 4 (2), 77-80 (1989) 131. Bottorff, K. J., “AKD Sizing Mechanism: A More Definitive Description,” Tappi J. 77 (4), 105-116 (1994) 132. Isogai, A., “Mechanism of Paper Sizing by Alkylketene Dimers,” J. Pulp Paper Sci. 25 (7), 251-255 (1999) 133. Strazdins, E., “Interaction of Rosin with some Metal Ions,” Tappi 46 (7), 432-437 (1963) 134. Ehrhardt, S. M., and Gast, J. C., “Cationic Dispersed Rosin Sizes,” Proc. TAPPI 1998 Papermakers Conf., 181-186, TAPPI Press, Atlanta (1988) 135. Lee, H. N., “The Microscopical Mechanism of Rosin Sizing,” Paper Trade J. 103, T386-T390 (1936) 136. Garnier, G., Wright, J., Godbout, L., and Yu, L., “Wetting Mechanism of Alkyl Ketene Dimers on Cellulose Films,” Colloids Surf. A 145 (1-3), 153-165 (1998) 137. Wang, F., Tanaka, H., Kitaoka, T., and Hubbe, M. A., “Distribution Characteristics of Rosin Size and their Effect on the Internal Sizing of Paper,” Nordic Pulp Paper Res. J. 15 (5), 80-85 (2000) 138. Seppänen, R., and Tiberg, F., “Mechanism of Internal Sizing by Alkyl Ketene Dimers (AKD): The Role of the Spreading Monolayer Precursor and Autophobicity,” Nordic Pulp Paper Res. J. 15 (5), 452-458 (2000) 139. Shen, W., and Parker, I. H., “A Study of the Non-Solid behavior of AKD Wax,” Appita J. 56 (6), 442-444 (2003) 140. Swanson, J. W., and Cordingly, W., “Surface Chemical Studies on Pitch. 2. The Mechanism of the Loss of Absorbency of Self-Sizing in Papers Made from Wood Pulps,” Tappi J. 42 (10), 812-819 (1959) 141. Davison, R. W., “The Chemical Nature of Rosin Sizing,” Tappi 47 (10), 609-616 (1964) 142. Garnier, G., and Yu, L., “Wetting Mechanism of a Starch-Stabilized Alkyl Ketene Dimer Emulsion: A Study by Atomic Force Microscopy,” J. Pulp Paper Sci. 25 (7), 235-242 (1999) 143. Back, E. L., and Danielsson, S., “Hot Extended Press Nips as Gas-Phase Reactors: Hydrophobization with ASA,” Tappi J. 74 (9), 167-174 (1991) 144. Yu, L., and Garnier, G., “Mechanisms of Internal Sizing with Alkyl Ketene Dimers: The Role of Vapor Deposition,” in Fundamentals of Papermaking Materials, Vol. 2, 1021-1046 (1997) 145. Batten, G. L., Jr., “A Papermaker’s Guide to Synthetic Surface Sizing Agents,” in Proc. TAPPI 1992 Papermakers Conf., 12-20, TAPPI Press, Atlanta (1992) 146. Garnier, G., Duskova-Smrckova, M., Vyhnalkova, R., van de Ven, T. G. M., and Revol, J.-F., “Association in Solution and Adsorption at an Air-Water Interface of Alternating Copolymers of Maleic Anhydride and Styrene,” Langmuir 16 (8), 3757-3763 (2000)


FIGURES AND CAPTIONS

Fibers bound together in wood

Fibers bound together in paper

Dispersed fibers

Fig. 1. A view of the papermaking process as breaking the inter-fiber attachments in wood, just to re-establish them again later as paper

Papermaking process

Paper machine

White water

Fillers, etc.

Wood

Recycled paper

Suspension of wood pulp fibers

Polymer

Process water

Colloidal silica

New paper

← Dry → Wet ← Dry →

Fig. 2. A view of the papermaking process as a matter of making the fibers wet, and then having to dry them again


Fiber not swollen

Increased swelling of fiber

Fiber less swollen than at first

Refining Drying

Fig. 3. A paradoxical aspect of papermaking: Fibers are made to swell in water, but they shrink again even more after the paper has dried.

Stiff, hollow fiber

Refining

Cut-away views

Flexible, ribbon-like fiber

Paper-making

Fig. 4. Papermakers do not take full advantage of the inherent stiffness and strength of hollow-shaped fibers, but rather convert them into ribbons.


Undispersed particulate matter

Apply shear & dispersants.

Dispersed particulate matter

Add a retention aid polymer.

Agglomerated particulate matter

Fig. 5. A schizophrenic aspect of papermaking, wanting everything well dispersed, but also wanting the fine particles to adhere together when the sheet is being formed

Dispersed fibers and fine particles

Add very-high mass flocculant.

Flocculated fibers and fine particles

Apply hydrodynamic shear.

Dispersed fibers with particles attached

Fig. 6. Papermakers often add the retention aid polymers just before the furnish is subjected to high hydrodynamic shear, partly reversing the flocculating effect.


Water-loving surface of fiber

Fiber

Add micelles of rosin soap.

Add alum.

Al

Fiber

Water-hating surface of fiber

AlAl Al Al Al Al Al

Fig. 7. One way that papermakers achieve the impossible – using a water-borne additive to convert water-loving surfaces to water-resistant surfaces

the paradox of papermaking - nc state university : webauth : login

Documents