Essay good chemistry, m hubbe

Opportunities in Wet-End Chemistry: Feature Essay, Posted Oct. 2001

Good Chemistry - Looking towards the Future of

Papermaking Additives

Martin A. Hubbe

Dept. Wood & Paper Sci., N.C. State Univ., Box 8005, Raleigh,

NC 27695-8005

Citation (public domain):

http://www4.ncsu.edu/~hubbe/new/goodchem/

There's an old story about a public hearing in which paper

company executives were describing their plans for a green-field

mill. A spokesperson ended her presentation with a listing of the

maximum levels of various substances in the liquid effluent

from the proposed plant. "Our effluent water will have a

biological oxygen demand of less than 10 parts per million, and

it will have a pH of 7." At this point someone near the back of

the room stood up and said, "I am a citizen of this town, and I

will insist that the pH value be reduced to zero before the water

is discharged!"

Part of our challenge as papermakers is to maximize the

efficiency of our operations and make them increasingly eco-

friendly. But, as illustrated by the story above, we also need to

be proactive in explaining the steps we are taking as an industry.

Our challenge is to educate our fellow citizens that chemicals,

used appropriately, are absolutely essential in this effort and that

they also can be safe to use.

What about the chemicals that one adds at the wet end of a paper

machine? The public sometimes associates the word "chemical"

with words like "pollutants," "emissions," "toxicity," or

"hazard." As noted in an article by Reinbold (1994), "the public

no longer views technology as something beneficial." Some

advocates for the environment have described paper as a

"chemical cocktail." The goal of this essay is to consider how

we, as papermakers, can do more in the years ahead to minimize

environmental impacts and also to achieve a more favorable

impression in the eyes of the public.

Fig. 1. Full description given by

Gottsching (1993)

Fig. 2. Full description given by

Gottsching (1993)

Where do we look for answers? In my opinion there are

basically three answers to our situation as suppliers and users of

papermaking additives. I will spend the rest of this essay

expanding on each one of them in turn. The first answer is for us

in the industry to show that each additive to a paper machine has

a clear and beneficial role. The second answer is to demonstrate

progress in understanding and minimizing environmental

impacts of specific papermaking additives. The third answer is

to envision the types of chemical additives and their uses in a

hypothetical future paper mill. Our ideal paper mill of the future

should be both profitable and as nearly "invisible" as possible in

terms of its impact on the environment.

Part 1 - A Purpose for Each Wet-End Chemical

Think about your reaction when you see a really long list of

ingredients on the container of a processed food item. Do you

ever read down through the list and wonder whether all of those

odd-sounding chemical items are really needed? It's far worse

for those who happen to be allergic to one or more of those

additives. Unlike packaged food, paper products come with no

list of ingredients. Except for some factors that I will discuss

below, we are in a situation somewhat resembling the years

before food labels. It is possible to list about 3000 different

kinds of chemicals that have been proposed for use in

papermaking (Reinbold 1994). Ingredient labels for paper

products may or may not be a good idea; but it is also clear that

there is an opportunity for the paper industry to tell the public

what is used in paper and why.

Progress in explaining the environmental consequences of

papermaking additives already has been achieved in a series of

publications that appeared in the early 1990's. First, an article by

Reinbold (1994) clarifies just how few chemicals papermakers

actually use. If one ignores brand designations, differences in

concentration, and minor variations in molecular mass or

composition, then only about 200 individual chemicals are

commonly added to paper machines, not 3000. The relatively

low number of chemical additives used in papermaking is

consistent with the fact that this industry mainly makes low-cost,

high-volume products; we simply can't afford to use superfluous

chemicals.

An article by Göttsching (1993) makes the further point that

papermaking practices are generally compatible with the

environment. If one were to omit all chemical additives from a

papermaking process, then the consequences would include

larger increases in emissions of solids, biological oxygen

demand, and even of noxious gases - resulting from uncontrolled

growth of slime in paper machine systems. This article, together

with a publication by Webb (1993) give an excellent run-down

of the main types of chemical additives and the status of each of

these additive relative to various environmental impacts.

Saving Energy: Let's take a closer look at how wet-end

additives can reduce the energy required used in papermaking.

Removal of water uses by far the largest component of that

energy. Most water is removed by gravity drainage, application

of vacuum, inertial effects, and pressing. However, most of the

energy is expended during a subsequent process, drying by

evaporation (Hersh 1981; Specht 1992). Approximately 2 to 9

million BTU are required per ton of product, to evaporate water.

Substantial savings in energy can be achieved by shifting a

greater proportion of the water removal to the preceding unit

operations of forming and pressing (Manson 1980; Nelson 1981;

Manfield 1986; Marley 1990). One way to accomplish this goal

is to accelerate dewatering with chemical additives. There has

been much work in this area (Auhorn 1982; Allen, Yaraskavitch

1991; Litchfield 1994; Raisanen et al. 1995; McGregor, Knight

1996). I it generally agreed that each 1% increase in solids

content of a paper web should yield about a reduction of 4 to 5%

in the net drying load (Shirley 1980; Nelson 1981; Auhorn 1982;

Strawinksi 1985; Marley 1990). Pulp mills are often net

producers of energy in the form of steam or electricity, but

savings in the energy of drying has the potential to either

decrease the consumption of fossil fuels or decrease the

production of greenhouse gases.

The goal sounds great, but what chemicals are we talking about

in terms of additives? Three classes of chemicals stand out as

the major drainage chemicals in current use (Allen,

Yaraskavitch 1991; McGregor, Knight 1996; Scott 1996). These

three classes are often called "coagulants," "flocculants," and

"microparticles." Coagulants used in papermaking are generally

multivalent or polymeric compounds of high positive charge

density. Commonly used coagulants include aluminum sulfate

("papermakers' alum"), polyamines, and polyethyleneimine

(PEI). The word "coagulate" implies that the negative surface

charges of suspended solids, fibers, and colloidal material are

neutralized, removing the electrical repulsion between these

surfaces. Flocculants complete the process of bringing fine

particles together; the most widely used type of flocculants in

the paper industry are very high mass copolymers of acrylamide

(Horn, Linhart 1991). Amounts typically less than 0.05% based

on product mass are sufficient to increase the retention of fine

particle in paper as it is being formed. Microparticles are tiny

negatively charged particles such as colloidal silica, bentonite,

or highly branched carboxyl compounds; they interact with

cationic polyacrylamides or cationic starch to further promote

dewatering (Langley, Litchfield 1986; Knudson 1993; Honig et

al. 1993; Andersson, Lindgren 1996; Swerin et al. 1996). A

common characteristic of all of these drainage-promoting

chemicals is that, to perform their function, they adsorb onto the

surface of solids in the papermaking furnish. That means that

these chemicals tend to be retained well in the paper; relatively

little of it remains in liquid effluent from paper machines, even

before wastewater treatment.

Defoamer chemicals affect many aspects of papermaking, in

addition to drainage, but it is the drainage benefits that have the

clearest connection with environmental impact. A study by

Brecht and Kirchner (1959) was among the first to clearly show

that air bubbles in a stock suspension can have an effect very

similar to that of fiber fines in slowing the rate of drainage from

a paper web. Especially in the case of heavier weights of paper

or paperboard, higher levels of fines or bubbles can be expected

to clog the drainage channels in a wet sheet of paper (Gess 1989,

1991). Defoamers are added to the wet end in the form of

emulsions; little droplets of oily material spread rapidly on

bubble surfaces and cause the bubbles to coalesce. The result is

less entrained air coming out of the headbox. In principle,

improvements in drainage can be converted into dryer paper

going into the wet-press section. In turn, a dryer sheet coming

into the press section makes it possible to load the presses more

without squashing the sheet. The happiest situation is when

increased pressing results in a stronger, better-consolidated

sheet, with less water remaining to be evaporated. The wild card

in this situation is whether the resulting sheet still has enough

caliper so that it can be calendared to meet a specified

smoothness.

Decreasing Effluent Loads: A remarkable aspect of the "art" of

papermaking is that paper is formed on a relatively coarse,

continuous screen fabric; typically the openings in the fabric are

large enough so that between about 5 and 50% of the solids

delivered to the forming section are capable of passing through

those openings. The small particulate material in paper, the

"fines," may consist mostly of wood byproducts (Brecht, Klemm

1953; Scott 1986; Gess 1991; Luukko, Paulapuro 1999; Rundlöf

et al. 2000). Even before it is refined, a typical kraft pulp

contains about 5 to 10% by weight of such things as tiny

parenchyma cells, used for food storage or conduction. The

process of refining pulp - passing the pulp slurry between

counter-rotating metal plates or cones having raised bars - is

necessary to develop the bonding ability of fibers for most

grades of paper, but refining also increases the level of fines in

the slurry. But all of these wood-derived fines can be

overwhelmed by fine material of a different type, the mineral

fillers (Bown 1998). Calcium carbonate and clay are the major

types of fillers used, and they make it possible to achieve

opacity targets with less total materials.

To understand how retention aid chemicals can impact the

environment it is worthwhile to view papermaking operations as

the first step in a multi-step water clarification process (Leitz

1993). Though there is a great deal of overlap between

"retention chemicals" and the fore-mentioned "drainage

chemicals," the emphasis of a retention program is to increase

the relative proportion of fine materials that stay with the wet

paper web as it is being formed (Jaycock, Swales 1994; Gess

1998). The very-high-mass acrylamide copolymers,

polyethylene oxide in combination with phenolic cofactors, and

also high-mass acrylamides, in combination with microparticles,

can be very effective retention systems, even in some cases

where the surfaces of the suspended matter are far from being

neutral in charge. Higher retention efficiency implies that less

solid material is present in the water that drains from the paper.

The traditional name that papermakers used to describe the

filtrate water from papermaking is "white water." A generation

ago it used to be more common for white water to contain so

much clay, titanium dioxide, and air bubbles that it looked like

milk. Now, largely thanks to chemical additives, together with

screen devices called save-alls, solids levels of white water are

kept under control and nearly all of the fine material eventually

ends up as paper.

Avoiding Waste of Fibers: You may not think of strength aids

as fiber-saving chemicals, but you should. Consider the case of

recycled office waste fibers. Such fibers tend to loose a

significant fraction of their bonding ability each time they are

dried and reslurried (Lindström, Carlsson 1982; Klungness,

Caulfield 1982; Howard, Bichard 1992; Nazhad, Paszner 1994;

Zhang et al. 2001). The loss in bonding ability has been

attributed to essentially irreversible closure of pores in the cell

wall (Stone, Scallan 1966), resulting in a loss of flexibility of the

fiber surfaces (Paavlilainen, Luner 1986). Strength

specifications become more difficult to achieve. One approach is

to try to make up for the strength loss by increased refining.

However, the furnish is likely to already have a relatively low

freeness, so there comes a point where more refining is not the

answer. Rather, papermakers tend to use increased levels of

strength additives, such as cationic starch or acrylamides

(Marton 1980; Strazdins 1984; Howard, Jowsey 1989; Smith

1992; Iwasa 1993; Glittenberg et al. 1994).

Another situation in which strength additives can "save fiber"

arises in the case of paper grades that are specified by strength

rather than basis weight. Such is the case for containerboard

grades made in accordance with the Rule 41 criteria (Gutmann

et al. 1993). Briefly stated, the rule allows a producer to

decrease the basis weight of a product as long as the combined

board still meets various strength goals, such as crush resistance.

In practice, papermakers use a combination of refining practices,

dry-strength additives, and sometimes size-press addition to

make the premium-strength board and take advantage of Rule 41

(Smith 1992).

Can Chemicals Added Initially Benefit Recycling? It has been

shown that strength-enhancing chemicals added to never-dried

kraft fiber can also have a beneficial effect after the same fibers

are recycled (Higgins, McKenzie 1963; Grau et al. 1996;

Laivins, Scallan 1996; Zhang, Hubbe 2000). Treatments found

to be effective included cationic starch and combinations of

cationic and anionic polymers. Results were consistent with the

ability of such chemicals to act as inter-fiber bonding agents -

both in the initial paper and also in the recycled paper, even

when no additional polymeric material was added during the

second generation of papermaking.

Losses in fiber bonding ability due to drying, aging, and

recycling of paper made from kraft pulp may be minimized by

alkaline papermaking conditions. Some benefit of alkaline

conditions may result from reduced hydrolysis of cellulose

macromolecules (Wilson, Parks 1983; Nazhad, Paszner 1994).

Further benefits may be associated with reduced closure of pores

in the cell walls (Lindström, Carlsson 1982), and reduced

stiffening of fibers. Though papermakers adjust pH values in

various different ways, one type of additive stands out in terms

of adjusting the pH to minimize damage to fibers. Give up? That

additive is calcium carbonate filler. Recent recommendations for

archival papers require at least two percent calcium carbonate to

make sure that the paper remains buffered in a weakly alkaline

pH range to make it resistant to gradual embrittlement

(McComb, Williams 1981; Kelly, Weberg 1981; Anon. 1993).

Work by Pycraft and Howarth (1980) shows further that over-

drying of virgin paper is likely to harm the properties of the

fibers, if they are to be used later for recycled paper.

The Sludge Dewatering Press is Like a Little Paper

Machine: Recycling of paper requires more fossil fuels or

electrical energy, compared to new pulp and paper from wood or

sawmill waste. The recycling of paper also can produce a lot of

waste sludge. Nevertheless, recycling usually is regarded as

having a favorable net impact on the environment (Pajula, Kärnä

1995; Jorling 2000). A key goal of increased recycling helps

keep the rate of tree harvesting below the growth rate of new

trees.

Saving land-fill space is another motivation to recycle paper: it

turns out that chemicals can play a beneficial role in helping to

achieve this goal. The reason is that sludge from wastepaper

recycling can contain a lot of water (Dorica, Allen 1997;

Kantardjieff 2000). The water content adds to the weight of

sludge to be discarded, and it also makes it more fluid-like, not

the ideal characteristic for building a stable landfill. Chemicals

coagulants such as poly-aluminum chloride (PAC), essentially

the same coagulants used in paper formation, can be used to

assist pressing more water from sludge (Ghosh et al. 1985; Leitz

1993; Pawlowska, Proverb 1996). Side benefits of sludge

dewatering may include a) more stable, solid-like sludge, b) the

colloidal materials in the sludge will tend to be insolubilized in

polyelectrolyte complexes and precipitates, and c) the sludge

will be more valuable as a fuel source, if that option is

considered (Harila, Kivilinna 1999). In principle well coagulated

waste sludge is expected to have reduced rates of leaching.

Part II - Minimizing the Environmental Impacts of Each

Type of Additive

"You work for the paper industry? Then maybe you can explain

that smell when I drive into [you fill in the place name]." To put

the present discussion into context it is worth noting that most

recent public concern has been directed at issues other than

papermaking additives. Rather, greater attention has been

directed towards issues of pulping, tree harvesting practices,

paper recycling, and, yes, air emissions (Vasara 2001). Another,

possibly more authoritative measure of environmental concern

comes in the form of legislation. Pulping and bleaching have

been center-stage in the so-called "cluster rule" regulations

(Vice, Carroll 2001). While keeping this context in mind, we

still have to seriously consider the potential impacts of

papermaking additives, if and when they enter the environment.

The good news is that substantial progress has been

accomplished in the area of papermaking additives with respect

to their toxicity, their biodegradability, and their ability to be

removed from the water phase during wastewater treatment

(Jorling 2000; Hamm, Göttsching 1994; Swann 2000). Later in

Part II we will consider various papermaking additives, focusing

on their potential hazards.

Fig. 3. See article by Goettsching

(1993)

Fig. 4. Full description given by

Vasara (2001)

A subtle, and often overlooked influence on chemical additives

for papermaking comes in the form of Material Safety Data

Sheets. "MSDS" information often is kept in orange or yellow

loose-leaf notebooks, adjacent to places where industrial

chemicals are being used. As noted by Allen (1991), these

documents have encouraged a trend towards greater awareness

of what it being added to paper machines. Toxicity and safety

information in MSDS has provided a starting point for making

improvements, and making substitutions toward less toxic

materials.

After toxicity, perhaps the second most serious issue is

biodegradability of chemical additives for papermaking.

Essentially all excess water from US paper mills undergoes

wastewater treatment before it is discharged. Bacterial action

during the secondary wastewater treatment converts many

organic chemicals into benign forms, and most of the biological

oxygen demand (BOD) is consumed. Some approximate rules to

predict biodegradability have been proposed. For instance,

compounds that contain chlorine, nitrogen, sulfonic acid, or azo-

groups are more likely to resist breakdown during water

treatment (Hamm, Göttsching 1994). Other factors that appear to

hurt biodegradability include toxicity, long chain length of

polymers, branching, and chemical substituents along polymer

chains. Unsubstituted alkyl chains also resist biological

degradation (Swann 2000). The problem with persistent

chemicals is that they might have the potential to accumulate in

the environment or in particular organisms.

Wet-strength agents: Environmental concerns about wet-

strength chemicals are often associated with their monomer

composition, possible residual monomers, and the possibility of

regenerating these monomers and releasing them into the

environment. The traditional wet-strength resins most often used

for acidic papermaking conditions are based on formaldehyde

(Dulany 1989; Espy 1995; Spence 1999). Possibly in response to

these concerns, the usage of phenol-formaldehyde and

melamine-formaldehyde resins has decreased dramatically in the

US paper industry. Poly-amidoamine-epichlorohydrin (PAAE)

resins have been replacing the formaldehyde resins in most

paper applications requiring durable wet-strength (Espy 1995;

Fischer 1996; Spence 1999).

Besides the issues with biodegradability, users of wet-strength

agents face two additional concerns. First, difficulties in

repulping wet-strength paper increase the likelihood that the

fiber will be sent to landfills after its first use. Second, if

papermakers decide to repulp the wet-strength paper, one needs

to be concerned about the chemicals used as repulping aids.

Hypochlorite bleach is sometimes used to repulp wet-strength

broke (Espy 1992; Fischer 1997). Elevated pH or temperature

also may be required to redisperse the fibers. At a minimum,

recycling of wet-strength paper is likely to require higher energy

input in the repulping operation. That means that there is an

environmental price to wet-strength treatment; sometimes the

price is paid in terms of increased landfilling, sometimes in

increased water treatment requirements, and sometimes in

increased energy expenditures. The ideal, in terms of wet-

strength treatments, would be to find a non-toxic, biodegradable

material that provided efficient, durable wet-strength under

conditions of use, but which also repulped easily under slightly

higher temperatures and hydrodynamic shear conditions in a

repulping operation.

Dyes: Papermaking colorants tend to have relatively poor

biodegradability (Webb 1993; Wahaab 2000). Fortunately there

has been a trend towards dyes with relatively high affinity for

solid surfaces. That means that the dyes tend to leave the paper

machine as part of the product, not in the water to be treated. In

addition, dyes entering the wastewater plant tend to be removed

with biological sludge (Webb 1993). High affinity onto solid

surfaces is generally achieved by development and use of

relatively large, planar molecules - the so-called "direct" dyes.

Affinity for fibers is further promoted by the trend for more use

of cationic direct dyes, in cases where these are appropriate.

Jackson (1993) noted that dye suppliers can minimize adverse

environmental impacts by careful selection of adjunct materials

used to stabilize liquid dyes.

Acrylamide copolymers: Considering their benefits in reducing

the waste of unretained fines, it is easy to love retention aids.

Copolymers of acrylamide are the most widely used very-high-

mass flocculants to promote fine-particle retention. On the one

hand, acrylamide products are expected to contribute much less

to biological demand (BOD), compared to the amounts of starch

products needed to render equivalent benefits in terms of either

retention or dry strength (Iwasa 1993). On the other hand, they

are not easily biodegradable (Webb 1993), as is to be expected,

based on their molecular mass (Hamm, Göttsching 1994). The

maximum permissible level of monomers present in acrylamide

copolymers is 750 ppm, compared to 100 ppm in the case of

other polymers (Swann 2000). Acrylamide products have

received the more lenient limits due to their history of 40 years

of use in the paper industry without evidence of harm.

Another issue to consider is the use of mineral oil as the

continuous phase of common retention aid emulsion products

(Swann 2000). Oil introduced with retention aids probably is

mostly adsorbed by fibers, with no adverse effects. However, it

is possible to imagine a bad effect resulting from the following

sequence: a) a low-grade mineral oil, having a significant

aromatic content, is used in the formulation; b) some of the same

paper is recycled in a batch that includes colored papers; and c)

the paper is bleached with elemental chlorine (Fleming 1995;

Lancaster et al. 1992). Fortunately, this combination of

circumstances is probably rare these days due to the use of

purified, alkyl mineral oils and the elimination of elemental

chlorine from most pulp bleaching operations in the US

(Deardorff 1997).

Another way to address concerns about oils in retention aid

products is to eliminate them from the formulation. One of the

side-benefits of oil-free formulation can be a substantial

reduction in shipping weight and transportation costs for a given

amount of active materials. Many water-in-oil retention aid

emulsions have active solids contents in the range of 25 to 50%

(Horn, Linhart 1991). Dry granular or "bead" acrylamide-type

flocculants, which have been available for many years, have

nearly 100% active content. If it weren't for the perceived

convenience of pumpable liquid formulations it is likely that dry

products would enjoy more widespread use. An especially

elegant solution to this dilemma involves a dispersion of

acrylamide-copolymer particles in aqueous solutions (Feng et al.

2001). Normally such copolymers would dissolve in water, but

the ion concentrations can be adjusted to prevent this from

happening.

Highly cationic copolymers: Efforts by papermakers to

conserve fiber resources and water have led to increased usage

of highly charged cationic polymers. One of the ways to

conserve fiber resources is to use high-yield pulps, such as

thermo-mechanical pulp (TMP). Wood pitch from TMP can be a

source of tacky deposits on papermaking equipment, forcing the

mill to shut down often for cleanups (Back, Allen 2000).

Another way to conserve fiber is through recycling. Wood pitch

is less of a problem with recycled fibers, but the problem is

replaced by stickies from pressure-sensitive adhesives and

coating latex (Hsu 1997; Douek et. al. 1997; Venditti et al. 1999;

Wilhelm et al. 1999). Some of the tackiness problems can be

minimized by use of talc (Braitberg 1966; Allen et al. 1993).

Also the furnish usually can be treated with highly charged

cationic materials such as polyethylene-imine (PEI),

polyamines, or poly-diallyldimethyl-ammonium chloride (poly-

DADMAC). Such cationic treatments can help to bind the tacky

materials to fibers so that they can be purged from the system

(Gill, 1993; Fogarty 1993; Shetty et al. 1994; Magee, Taylor

1994; Moormann-Schmitz et al. 1994). Highly cationic

polymers or soluble aluminum compounds are used for the

neutralization of excess anionic colloidal charge in papermaking

furnish - often the first key step in optimization of drainage and

retention systems (see references cited in Part I). Yet another use

of highly cationic polymers is in the spraying or forming fabrics

or press felts to inhibit deposition of tacky substances from the

paper (Allen 1991; Sawada 1997); here again, the use of these

agents is helping in the effort to use wastepaper and high-yield

pulps, both of which are worthy environmental goals.

Highly substituted, synthetic polymers of the type used for

precipitation of tacky materials and the neutralization of excess

colloidal charge are not expected to be highly biodegradable

(Hamm, Göttsching 1994). For example Wahaab (2000)

observed very poor biodegradability in the case of a commercial,

highly cationic polymer used for treatment of forming fabrics.

One step towards addressing concerns about possible

environmental impacts of highly cationic polymers is to avoid

using more than is needed. For instance in the spraying of

forming fabrics it is possible to minimize the chemical use by

proper dilution and by use of a well-designed spray boom

(Sawada 1997). When used to neutralize excess colloidal charge,

it is possible to avoid overdose of highly cationic polymers by

carrying out online or laboratory charge titrations with streaming

current instruments (Bley 1992; Stitt 1998; Phipps 1999; Gill

2000; Rantala, Koskela 2000; Chen et al. 2001). Charge control

to the neutral range has the advantage of tending to maximize

precipitation of most polymers and fines onto fiber surfaces,

reducing the amounts of polymeric and colloidal substances that

are sent to the wastewater treatment system.

Recently there is yet another option to consider, the use of

highly cationic polymers based on starch or other natural

products. Already a highly cationic polymer based on starch has

been used for charge neutralization and optimization of wet-end

operations (Vihervaara, Paakkanen 1992). Presumably such

materials might be more easily biodegraded, compared to their

synthetic counterparts. "Not necessarily so," says Reinbold

(1994). Rather, there is a wide range of variability in the

biodegradation of both natural and synthetic polymers.

Biocides: Conventional slimacides are highly toxic. They have

to be to perform their function. Many do not break down readily

during treatment of wastewater (Webb 1993). Concerns over

these types of biocides have resulted in pressure against biocide

use for papermaking in Sweden (Swann 2000). One of the goals,

then, is to develop biocides that do their job and then self-

destruct (Allen 1991).

Enzymes are very good at self-destruction. The fragile nature of

enzymes is due to the fact that they consist of complex proteins

with many loops and coils that have to fit together in an exact

way to perform some kind of function. Even moderate changes

in pH or temperature can temporarily or permanently destroy the

enzyme's activity. Enzymes such as amylases are already used

for cleaning up deposits on starch-preparation equipment and

paper machine wet-ends (Swann 2000).

Another way to minimize the need for toxic agents to control

slime involves biodispersants (Crill 1993). Biodispersants make

sense because bacteria attached to surfaces, the so-called

"sessile" bacteria, tend to cause more problems than freely

floating bacteria in paper mill systems. Although it is premature

to expect that biodispersants can eliminate the need for toxic

biocides, or of oxidizers such as chlorine dioxide, it is

reasonable to expect the dependency on such materials to be

reduced.

Starch: Starch products probably wouldn't even be included in

the present discussion, but for the fact that the paper industry

uses so much of them. The largest proportion of starch is added

to the surface of paper at the size press or in coating

formulations. Additional starch is commonly added at the wet

end in levels up to about 1% on paper mass. Native,

underivatized starch is close to ideal in terms of its

biodegradability (Hijiya 1999). In addition to providing strength

and helping certain retention aid programs, starch products also

are based on a renewable resource. The most common grade of

starch used in the US is a byproduct of processing corn

sweetener for soft drinks and other processed foods. The trouble

is, size-press starch often makes up 1 to 5% of the mass of

various paper products. This is certainly true of printing papers.

Since the kinds of starch most often used at the size press are

poorly attached to fibers, large amounts of starch can become

solubilized through the repulping of dry-end broke. Such starch

is likely to be a major contributor to BOD of liquid effluent from

the mill. In other words, the problem is in the large amount of

starch products in the effluent water, not in their rate of

degradation in a well-run biological wastewater treatment

system.

Work carried out by Roberts et al. (1987) showed a very

effective way to minimize BOD contribution of starch in

effluent from paper machine systems. The answer is to use

cationic starch (Webb 1994). Roberts showed a case in which

about 85% of cationic starch was retained at neutral pH, whereas

only about 10% was retained when the experiments were

repeated with unmodified starch. It should come as little surprise

that most starch now added at the wet-end of paper machines is

either cationic or amphoteric (i.e. having both positive and

negative charged groups attached to the chain). The down side is

that cationization of starch appears to make it less biodegradable

(Hamm, Göttsching 1994). In summary, the higher retention of

cationic starch and its good, though not perfect biodegradability

make it highly beneficial in terms of overall environmental

impact of paper mills.

Sizing Agents: Internal sizing agents are truly remarkable in

their ability to transform the nature of paper, even when the

added dosages are typically well below 1% of the dry mass of

product. The chemical composition of wood-derived fibers

makes them highly water-loving. Paper uses for cups, bags,

cartons, and various printing applications can require that it

resist water absorption and penetration.

Rosin size has been criticized for its toxicity and for the fact that

rosin sizing usually requires the use of aluminum compounds

(Webb 1993). But rosin products can claim a positive attribute

not shared by the common alternative sizing agents; rosin is a

byproduct of wood pulping. Rosin is a renewable, biodegradable

material (Webb 1993). There is an interesting balance between

rosin's efficiency and its biodegradability; most rosin is reacted

with maleic or fumaric anhydride to produce "fortified" rosin

size. The fortified size is more storage-stable and more efficient

in use. However, it also is less biodegradable than natural rosin

(Webb 1993).

Though it still is worth considering environmental implications

of rosin size products, there has been a strong trend over the past

20 years towards alkaline papermaking conditions and the use of

calcium carbonate filler (Gill, Scott 1987; Laufmann et al.

2000). Values of pH higher than about 7 make it increasingly

harder to size paper with conventional rosin products (Liu 1993;

Schultz, Franke 1996; Wang et al. 2000). Fortunately, two

widely used "alkaline sizing agents" are available.

Alkenylsuccinic anhydride (ASA), which is very popular for

production of printing papers and gypsum board liner, is a

byproduct of petroleum (Webb 1993). By contrast, alkylketene

dimer (AKD) is made from fatty acids, a renewable resource. In

either case, alkaline sizing agents tend to be much more efficient

than rosin in terms of the amounts needed to reach equivalent

levels of resistance to fluids.

Surfactants: Some surface-active materials are added to paper

intentionally, whereas others come along for the ride as

stabilizers for other chemicals or as residuals from de-inking. If

we use a broad definition, then the list of intentionally added

surfactants would include sizing agents (e.g. rosin soap size),

components of certain defoamers (i.e. water-insoluble

surfactants), certain deposit-control additives, and debonding

agents used in certain tissue products. Various nonionic and

fatty-acid-based surfactants are used in flotation de-inking

(Johansson, Ström 1998; Rao, Stenius 1998) and for the

agglomeration of xerographic toners (Darlington 1989;

Heitmann 1994; Bast-Kammerer, Salzburger 1995). Nonionic

surfactants also are used to stabilize such additives as retention

aid emulsions, dyes, and certain sizing agents.

Probably the most obvious adverse environmental impact of

surfactants would be cases of visible foam. But the more serious

impacts should be evident to anyone who has opened their eyes

in soapy water, or when shampooing. Has anyone interviewed a

fish on this subject?

Issues of toxicity and persistence have been raised in the case of

non-ionic surfactants (Hamm, Göttsching 1994). Nonylphenol-

ethyoxylate products have been replaced, especially in Europe,

due to concerns about their toxicity (Swann 2000). Linear alkyl

(or alcohol) ethoxylates have taken their place in many

applications. Though the latter are not regarded as toxic, the

saturated alkyl chains tends to make them poorly biodegradable

(Swann 2000). Perhaps the next logical extension is to use

unsaturated aliphatic (alkenyl) poly-ethers. Alternatively,

perhaps the most economical solution is to do a better job at

removing surfactants before effluent water is discharged.

Chelating Agents: The most common function of chelating

agents such as diethylenetriaminepentaacetate (DTPA) in

papermaking is to keep certain metal ions from interfering with

peroxide bleaching of mechanically defibered pulps. Strictly

speaking this is not an issue of wet-end chemistry; usually the

pulping and bleaching operations are regarded as separate from

papermaking. That matter aside, the problem with chelating

agents is that they resist biodegradation (Göttsching 1993;

Hamm, Göttsching 1994; Reinbold 1994). The potential adverse

effect of persistent chelating agents follows from their likely

tendency to interfere with natural uses of calcium and other

metals in aquatic organisms. Since peroxide bleaching is often

used for recycled pulp, especially when it contains mechanical

fibers, there is active interest in finding biodegradable

alternatives to chelating agents. One approach is to use

sequesterants such as silicates. In layman's terms, a sequesterant

is something that binds objectionable metal ions less efficiently

than a chelating agent, but enough to permit peroxide bleaching.

Since the byproducts of peroxide bleaching tend to be non-toxic,

it would be highly beneficial to find other ways of increasing its

efficient use in pulp mixtures that are likely to contain

manganese, iron, and other divalent transition metal ions.

While on the subject of metals, it is worth considering the

environmental consequences of heavy metals in effluent from

paper mills. In the past there were concerns about heavy metals

in various printing inks. As noted by Göttsching (1993),

papermakers have to work with their associates in publishing

and converting companies to avoid contaminating the waste

fiber supply with persistent hazardous materials. D'Souza et al.

(1998) observed that between 75% and 100% of various metals

entering a paper mill system by way of waste paper were

removed as a component of sludge. However, the levels of metal

in the sludge, and also in the product, were both below the level

of concern.

Part 3 - A Vision for the Future

"I don't know what they do in those buildings next door. They

seem to do a lot of business and process a lot of waste materials.

They seem to ship a lot of product. They always keep their lawn

mowed and the people are always polite." My vision for the

future paper mill is that it should be "invisible" in terms of its

effects on the environment. Neighbors, from urban people to

rural fish, ought to hardly notice its presence. The goal of Part 3

is to consider what kinds of wet-end additives and related

processes are likely to take place in that paper mill.

Fig. 5. Paper technologist thinking of

word "chemistry."

Fig. 6. Using the other dictionary

definition of "chemistry"

"Fiber-Friendly"

A lot of effort and capital goes into the production of fibers from

wood, as well as from alternative fiber sources such as sugar

cane residues (bagasse), straw, and cotton. These are renewable

resources. When managed properly, every tree that gets

converted into pulp for paper products gets replaced by new

planting and new growth of trees or other fibrous materials.

Actually, the situation is even a bit more complicated than that.

Rather than using all trees cut from the forest, the paper industry

gets much of its wood fiber in the form of used paper an waste

from lumber mills and related operations (Smith 1984; Kramer,

Jurgen 1998).

Even before one considers the effect of papermaking additives,

plant fibers already have the following highly desirable

attributes: a) they easily bond to each other without needing any

glue; b) they can easily be redispersed in water and formed into

recycled paper; c) they do not originally contain toxic materials;

and d) after they have become too degraded or contaminated to

be worth recycling, they still can be used for energy generation

(Göttsching 1993; Delefosse 1993; Norris 1998; Weigard 2001).

By using fibrous waste products to fuel power boilers at the

paper mill it is possible to displace some of the need for fossil

fuels and also reduce landfill requirements. Landfilling of paper

products can result in production of greenhouse gases such as

methane (Wiegard 2001), so it makes more sense to use waste

wood products for fuel and leave more petroleum, natural gas,

and coal reserves in the ground.

Having said all these nice things about plant fibers, especially

those from wood, one of our high priorities as an industry ought

to be aimed at preserving their quality and in continuing to use

renewable plant fibers as the main component in our products.

Calcium carbonate is known to inhibit aging of paper by

buffering the pH in the alkaline range. By contrast, acidic paper

tends to become embrittled during drying and storage, and the

cellulose molecules gradually suffer hydrolysis (McComb,

Williams 1981). In addition to its beneficial buffering ability,

calcium carbonate may be preferred over clay products due to a

relatively high purity of its deposits, so that mining of CaCO3

generates less volume of "pits" in the ground and "piles" of

tailings (Webb 1993). In cases where sludge from treatment of

paper mill wastes is used for compost, the calcium carbonate

provides useful pH buffering.

Mineral fillers, though abundant, are non-renewable, so there

seems little point in trying to load up paper with high

percentages of calcium carbonate, beyond what is needed to

achieve opacity and smoothness specifications; rather it has been

suggested that papermakers ought to concentrate on achieving

high smoothness and covering the paper with relatively thin

layers of mineral-based coatings (Lindström 1994; Swann

2000). In that way any printing inks are likely to adhere to the

coating materials and the fibers can be more readily recovered

"clean" when the resulting wastepaper is de-inked and recycled.

A recent project at North Carolina State University has involved

efforts to minimize or compensate for loss of bonding ability of

kraft fibers when they are dried (Zhang et al. 2000). The vision

that comes out of this type of work is that fibers ought to be

treated gently during each cycle of papermaking. One of the key

strategies in this regard is to avoid excessive drying

temperatures or very low moisture contents, i.e. "over-drying"

(Pycraft 1980). It also is recommended to avoid excessive

energy or intensity of pulp refining (Baker 1995). In this regard,

dry-strength chemicals such as cationic starch can help to

achieve strength objectives with moderate savings in refining

energy. Our recent work indicates that the proportional effect of

dry-strength additives added to never-dried pulp may be greater

when the fibers are recycled, compared to their effect on the

initial paper.

"Water-Friendly"

In a paper mill of the future I envision that not only fiber, but

also water, is handled as a precious resource. Future mills are

likely to be choosing between the following two alternatives: a)

continue the gradual trend of many years towards operation with

less and less fresh water per unit of product (Springer 1978;

Swann 1999); and b) operate with zero or very little discharge of

liquid effluent - in a so-called "closed water cycle" mode

(Pietschker 1996; Wiseman, Ogden 1996). In either case I

envision that paper mill operations will increasingly turn to their

own wastewater treatment systems as a source of "fresh" water.

The logic is as follows: Some level of treatment is required even

of "clean" water from rivers or springs to remove sand, humic

acids, and to control microbes. On the other hand, a paper mill

will already have expended considerable effort in purifying the

wastewater; it may be cleaner in some respects than untreated

"fresh" water. In fact, some system already in place to condition

white-water for internal re-use are very similar to conventional

primary clarification of wastewater (Sugi 1997).

Not only are future paper mills likely to reuse some of their

wastewater, but it appears likely that some of them will

essentially "bring the wastewater treatment plant into the paper

mill." The motivation for this trend is a need to control the

build-up of biological oxygen demand and colloidal materials - a

probable consequence of increased recycling of both fibers and

water (Pietschker 1996; Zhang 1999). Successful applications of

this type of technology have been reported (Delefosse 1993;

Norris 1998). In some cases it is possible to justify the cost of

such processes as ultrafiltration (Norris 1998) and ozonization to

purify water to be reused in papermaking, whereas the same

treatments would be considered too expensive if the wastewater

were to be discharged (Demel, Kappen 1999). Combinations of

aerobic and anaerobic treatment have been recommended to

minimize the volume of sludge (Göttsching 1993; Demel,

Kappen 1999). Compact reactors for biological treatment may

make sense in terms of minimizing the volume of water as paper

mills begin to incorporate these operations as part of their

system (Tenno, Paulapuro 1999; Gubelt et al. 2000). The most

important attribute of paper chemicals, in order to be compatible

with the biological treatment systems just described, is

biodegradability. Some progress has been made in this area

(Hamm, Göttsching 1994; Wahaab 2000), but much more work

is needed.

Another essential part of efforts to reduce water usage is to

select combinations of additives that tend to "self-purge"

themselves from the system by becoming retained on fibers. In

principle that implies avoiding substances like simple salts,

sugars, and oils that have little affinity for fibers, even in the

presence of coagulants or retention aids. In isolated cases it may

make sense to remove excess salts by evaporation or reverse

osmosis (Wigsten 1995; Norris 1998; Tenno, Paulapuro 1999).

In principle it is possible to maximize the retention of both

colloidal and fibrous materials in paper by control of highly

cationic additives to achieve near neutral zeta potential (Bley

1992; Moormann-Schmitz 1994), followed by very-high-mass

flocculants to collect primary particles into a particles large

enough to be mechanically retained (Horn, Linhart 1991). It is

remarkable the extent to which these principles parallel those

used in treatment of fresh water and wastewater (Leitz 1993).

"Frugal of Energy and Raw Materials"

I envision the ideal paper mill of the future as being frugal in

terms of energy and raw materials. Losses of fine materials can

be reduced to very low percentages in the paper forming process

by use of an effective retention aid program on a paper machine,

plus the use of a saveall to recover fine material from white

water. Closing up the water system it is possible to conserve

heat (Springer 1978; Wigsten 1995). Hot water promotes more

rapid drainage, and extra heat energy has to be supplied to the

extent that fresh water is used. In these respects the paper

industry already seems to be doing a very good job.

Though the paper machine tends to be frugal, relatively large

amounts of fiber fines, fillers, and fibers can be lost when paper

is recycled (Paula, Kärnä 1995; Dorica, Allen 1997; Kantardjieff

2000). It is likely that much of such waste consists of ink,

colloidal materials, and fiber fines too small to be of much value

in papermaking. However there may be opportunities to recycle

the mineral content of waste paper or of sediment in the

clarifiers at paper mills. Studies have shown that it is possible to

"burn off" various organic materials and recover gray filler

particles that are useful for paper products with intermediate

brightness targets (Sohara, Westwood 1997; Johnston et al.

2000; Moilanen et al. 2000; Wiseman et al. 2000).

Further savings in energy, per ton of paper, are likely to come in

two areas. The use of chemicals to promote dewatering and

reduce the need to evaporate water was discussed already in an

earlier section. It is possible that further savings will be achieved

by reducing the amount of water that needs to be pumped. Said

another way, it will be possible to save electrical energy

expended at the fan pump by increasing the typical consistency

of headbox furnish. Higher-consistency forming has been

considered in various publications (Case 1990; Waris 1990).

Already, modern headbox designs have been helpful in being

able to still form uniform paper with slightly less water

(Kiviranta, Paulapuro 1990). But it seems that chemical

additives will be needed to that minimize fiber flocculation at

the higher solids levels. Conventional "formation aid" strategies

can have a devastating effect on drainage (Wasser 1978; Lee,

Lindström 1989). This is an area of wet-end chemistry that may

become important in the future.

"High-Tech"

It seems that no vision of the future ought to be complete

without the words "high tech." In terms of papermaking

chemicals, the key "high tech" trends to look out for will include

automation, new sensors, bio-engineered processes or additives,

and nano-technology. Recently it seems that nano-technology is

a growth area for research. In fact, papermakers have been

involved in nano-technology for many years. How big are the

colloidal silica "microparticles" used in drainage-enhancing

programs? The answer is "usually about 1 to 5 nm" (Moffett

1994; Andersson, Lindgren 1996; Swerin et al. 1996). So, in

fact, we already use nano-technology.

Bio-tech solutions are recently becoming important in the use of

enzymes for deposit control and slime control (Webb 1994).

Enzymes also can be used to reduce the cationic demand of

process water, especially in cases involving thermomechanical

fiber (Buchert et al. 1996). In the future we can expect to see

more progress in the use of enzymes to assist with strength

development and to promote more rapid drainage (Eriksson et

al. 1997).

The large-scale, continuous, capital-intensive nature of

papermaking operations make them attractive subjects for

improved process control strategies. The last couple of decades

have brought substantial progress in the development and

implementation of tray-water solids sensors (Bernier, Begin

1994; Artama, Nokelainen 1997). These have made it possible

to even out swings in first-pass retention by varying the addition

rates of retention aids. However, in at least one case it was

shown that the demand for retention aid was strongly correlated

with variations in cationic demand of the furnish (Tomney et al.

1997). Therefore it makes sense to control cationic demand as

well, with a goal of getting closer to the root cause of the

variations. Significant progress has been achieved in online

charge control, especially with automated streaming current

titrating devices (Tomney et al. 1997; Gill 2000; Rantala,

Koskela 2000). In principle the same type of data can be

obtained more accurately and reliably by a new streaming

potential titration method (Hubbe 1999). Other devices that are

likely to become more common, especially in large papermaking

facilities, include automated freeness testers (Lehtikoski 1991),

online evaluation of fiber flocculation (Wågberg 1985; Alfano et

al. 1998; Hubbe 2000), and automated monitoring of bio-films

related to the growth of slime deposits (Robertson, Rice 1998;

Dickinson 1999; Flemming et al. 2000).

Earning the "Good Chemistry" Label

The goal of "good chemistry" with the public, with our clients,

and with our investors will require a long-term approach. We

have to face the fact that papermaking is a highly capital

intensive enterprise with long lead times for new construction

and replacement of existing facilities. We cannot expect to keep

up with every new change in focus of environmental issues

(Vasara 2001). The challenge will be continue to make

meaningful, practical improvements in our practices affecting

the environmental even through changes of issues and economic

cycles.

So what about the "chemistry" between the paper industry and

the public? We need to encourage an atmosphere of "working

together" on environmental issues for the sake of long term

progress is exemplified by a Wisconsin initiative to create a

private-public partnership (Schmidt 1998). A focus on headlines

sometimes can lead to a view that our society is highly polarized

around issues of environmentalism versus profitability

(Reinbold 1994). However, a more cautious analysis of public

opinions reveals that the bulk of the American public tends to

see issues in a much more balanced light, compared to their

politicians (Wolfe 1998). Technical people in the paper industry

have a responsibility to be environmental advocates. Some

worthy environmental goals for papermakers include a)

continuing to rely mainly on a renewable, recyclable resource -

wood fibers, b) taking steps to avoid deforestation - by

replanting, recycling, and avoiding waste, c) minimizing water

pollution by careful development, selection, and use of wet-end

chemicals, and d) minimizing energy use. As noted by Siekman

(1998), some environmentally sound practices can be profitable,

in addition to their intrinsic benefits.

No, it probably wouldn't do much good to place an ingredients

label on each sheet of paper, or even on each ream wrap, carton,

or jumbo roll. But already there have been proposals to label

certain products as "eco-friendly" (Rogers 1993). For instance,

such labels could be awarded by an independent agency based

on a point system, with part of the score coming from such

issues as wet-end chemical practices, bleaching practices, or the

amount of energy used in the life-cycle of a product. Ideally this

ought to be a voluntary system, something like the ISO

certifications of paper mill practices. In that way, paper

companies will have the incentive to get their products certified

so that they have the right to label their products as "eco-

friendly."

Achieving "good chemistry" on the paper machine and with the

public will require more than good intentions. It will require

significant technological input and a long-term commitment on

the part of us in the industry to continue to make the needed

progress.

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