Holik H. (ed.) Handbook of Paper and Board

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Substances of different chemical composition and different biological origin are

used for fluorescent whitening of paper. Derivatives of diaminostilbene disulfonic

acid have proved the most popular in industry because of their fastness properties.

The active ingredient content is generally between 20 and 27%. Their central part

is disulfonated diaminostilbene. The types differ in the number of sulfonic acid

groups in the side groups. The high-substantivity types with only 2 sulfo groups

make up about 11% of the market while tetrasulfonated derivatives with medium

substantivity are about 80%. The rest is low-substantive, hexasulfonated types.

The paper industry today needs fluorescent whitening agents to obtain high

degrees of whiteness at reasonable cost, especially if the FWA (OBA) are combined

with shading dyestuffs. This in turn leads to sharper contrasts in the printed image

and thus helps to reduce the toner consumption of copiers, for example. Similarly,

the color brilliance of color prints is improved. For application in the stock, high or

medium-substantivity types are generally used, being 1% of the commercial-grade

product. For surface application in the size or film press, medium or low-sub-

stantivity types are used, the normal addition being 1.5% and in exceptional cases

up to 3.5%. Low or medium-substantivity FWA are also the preferred products for

coating application, the amount required being up to 1.5% up to 3.5 % in excep-

tional cases. A maximum amount of FWA can be used in each application as the

shade becomes greener. This will lead to a graying effect on the brightness.

3.6.3

Chelating Agents – Complexing Agents [3–5]

The presence of heavy metals negatively influences many production processes

and paper properties. In particular, they interfere with the bleaching agents in

chemical and mechanical pulp production, reducing their effect, so the task of

chelating agents is to counteract this detrimental impact. Chelating agents can also

reduce or stop uncontrolled decomposition of a hydrogen peroxide solution, which

would result in loss of bleaching effectiveness.

Chelating agents that contain amino and carboxyl groups mask metal ions effec-

tively. Chemical compounds of this type are nitrilotriacetic acid (NTA), ethylene-

diaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and

hydroxyethylethylenediaminetriacetic acid (HEEDTA). Other complexing agents

include the soluble salts of oxalic acid, citric acid, tartaric acid, gluconic acid,

amines, and ammonia.

Both the demand for increased brightness and the technology trend to elemental

chlorine-free and total chlorine-free bleached pulps has led to an increased use of

chelating agents today, dominated by DTPA and EDTA.

In view of environmental protection, there exists a controversial discussion

about the biodegradability of chelating agents. The results of newer investigations

show that, under certain conditions, EDTA and, with some restrictions, DTPA, are

fairly biodegradable. Their degree of biodegradability is still under discussion. It is

strongly influenced by pH (optimum at pH 8.5–9), by ultraviolet light and by the

effectiveness of adapted microorganisms. Up to 90% biodegradability of EDTA is

reached in some pulp and paper mills.

3 Chemical Additives82

Of less importance are chelating agents based on polyphosphates, phospho-

nates, and hydroxycarboxylates as well as nitrogene free products like citrate, ta-

trate and gluconate.

3.6.4

Sizing Agents [2, 3, 9–11]

3.6.4.1 Fundamental Aspects of Paper Sizing

Sizing makes the native fiber network hydrophobic and thus prevents or reduces

the penetration of water or other aqueous liquids into the paper. Sizing prevents

the spreading and strike through of ink or printing colors. Papermaking fibers

have a strong tendency to interact with water. This property is important for the

development of strong interfiber hydrogen bonds, especially during drying, and is

also the reason why paper loses its strength when rewetted. A high absorbency is

important for a few paper grades such as toweling and tissue. Also corrugated

medium paper must be “absorbent” to a certain degree to convert properly in the

corrugating process. On the other hand such properties are disadvantageous for

many paper grades e.g. liquid packaging, top layer of corrugated board, writing

and printing papers and most of the specialty papers. The water and liquid absorb-

ency can be reduced by the addition of sizing agents to the paper stock and/or by

their application to the paper surface.

Since the invention of paper about 2000 years ago, it has been treated or sat-

urated with mucilage and rice starch for surface glazing properties, so that people

could write on it. After 1280 A.D. animal glue was the principal sizing agent.

Alum (potassium aluminum sulfate) was used to harden the applied glue. Alum

rosin sizing entered the scene in 1806 (Moritz Friedrich Illig). In 1876, paper-

maker’s alum, Al

(SO

)

, was introduced. Since the 1950s the various forms of

rosin size (paste, dispersed, fortified), alkyl ketene dimmer size (AKD), alkenyl

succinic anhydride size (ASA) and polymers mainly based on styrene acrylate and

styrene maleinate (PSA) have come to the market. Today, beside starch for strength

improvement (Fig. 3.2) and polymer binders for paper coating, sizing agents are

the most important quality improving additives in paper manufacturing. Amongst

the specialty chemicals, sizing agents represent a share of 10% in dry mass

(Fig. 3.3.) and a total worldwide turnover of approximately 950 V 10

€.

3.6.4.2 Product Classes

The main classes of chemicals and their consumption as a proportion of the total

worldwide usage of 350000 tons dry p. a. is shown in Fig. 3.10. Because of the

trend to neutral or slight alkaline papermaking and the increasing use of calcium

carbonate as a filler and coating pigment, the proportion and also the absolute

volume of rosin will decline further and the synthetic products will steadily

grow.

3.6 Functional Chemicals 83

3.6.4.2.1 Rosin-based Sizing Agents

These are still the most widely used of all wet-end sizes, because they are cheap to

produce and are produced from regenerative raw materials. They are mainly used

in acid and much less in pseudo-neutral pH conditions. They have the advantage

that the degree of sizing can be controlled very easily, and the sizing effect is fully

cured when the paper leaves the machine. In practical use there are different

preparations of rosin, for which the following terms are used:

• Free rosin: Resin acids whose carboxyl groups are in the completely protonated

form.

• Dispersed size: High (95–100%) free rosin dispersion of resin acids. The dis-

persed size particles are stabilized by surfactants, starch, polymers, or pro-

teins.

• Rosin soap size: Rosin size composed of the sodium salts of resin acids. Formed

by neutralizing (saponifying) resin acids with NaOH or some other sodium

base, such as Na

. Different forms of soap sizes are available, including

paste, extended, and dry sizes.

• Extended size: A 50% solids product consisting of a 50:50 mixture of completely

neutralized resin acids and urea.

• Fortified size: Rosin size that has been reacted with fumaric acid or maleic anhy-

dride to form the Diels-Alder adduct. The reaction product contains extra car-

boxyl groups and produces more efficient sizing response than the unreacted

resin acids. It is used as a starting material for other size products.

• Dry size: A 100% neutralized, dry rosin size product that dissolves easily in

water.

• Paste size: An 80 % neutralized rosin size product marketed as a 70–80% solids

paste.

3.6.4.2.2 Synthetic Sizing Materials

These were only developed in the second half of the last century. Driving forces

have been the increasing use of the cost and performance effective calcium carbon-

Fig. 3.10 Product groups of sizing agents worldwide in 2004

(% shares of solid or active substances).

3 Chemical Additives

ate as filler and coating pigment and also the need to produce paper with high and

permanent paper strength. To enable its use the paper has to be produced under

alkaline conditions. Other paper grades must have very hard sizing in order to

resist edge wicking in liquid packaging applications or photographic base paper, or

to exhibit hot water resistance for gypsum board. Many mills also seek an alter-

native sizing procedure to the wet-end application, e.g. in the form of a surface

treatment in the drying section, to obtain higher efficiency.

• Alkyl Ketene Dimer (AKD) was the first synthetic size and appeared in the patent

literature in 1953. Structurally, alkyl ketene dimers are unsaturated lactones. In

the manufacturing process a frequently used synthesis route is: the acid chloride

of a carboxylic acid is prepared (fatty acids, C

16–22

homologs treated with, e.g.

phosphorus trichloride, phosphorus pentachloride, thionyl chloride or phos-

gene), followed by intermolecular lactone ring condensation via a labile inter-

mediate carboxylic acid obtained by dehydrohalogenation of the acid chloride

with triethanolamine in an organic solvent. Newly developed processes work

without any solvent. To handle and to use AKD in a paper mill, the wax must be

converted into tiny particles (0.5–2 mm) dispersed in water. The emulsification is

usually effected in a hot (75–90 °C) solution containing the cationic starch stabi-

lizer (alternatively a cationic polymer e.g. polydadmac, polyvinylamine) and a

small amount of surfactant, e. g. sodium lignin sulfonate, to which are fed AKD

flakes. After melting the AKD, the mixture is forced through a microfluidizer

and cooled. Small amounts of a promoter (cationic polymer with low molecular

weight and high charge density) and biocide can also be included. The solid

content of the commercial products varies between 6 and 30%. Some scientists

believe that AKD reacts with the hydroxy groups of cellulose to form b-keto

esters.

• Alkenyl Succinic Anhydride (ASA), as a synthetic sizing agent for paper, was first

synthesized in 1974. It is composed of an unsaturated alkenic hydrocarbon back-

bone coupled to succinic anhydride. It is usually manufactured in two stages:

First an unsaturated linear or branched olefin, e.g. a 1-alkene, is isomerized by

moving its double bond randomly from its a-position. This will yield an ASA

product that is liquid at room temperature, a condition important for easy emul-

sification of the size on application in the paper mill. In the second step, the

mixture of isomerized alkenes is reacted with maleic anhydride to produce the

ASA raw material. Normally ASA is a yellowish oily product. This 100% active

substance can be stored as such for a long time but must be well protected from

water or humidity.

• Polymeric Sizing Agents (PSA) are mainly based on styrene-acrylates, polyure-

thanes or styrene-maleic acid anhydride. They are either water-based dispersions

or aqueous solutions. These products were developed around 1960. Since that

time an increasing number of size-presses for strength improvement with

starch have been installed.

The styrene-maleinate polymers are anionic, aqueous solutions with a solid con-

tent of 20–30% and an alkaline pH (mainly containing the ammonium salt).

They are specially designed for surface application. Their advantages are very

3.6 Functional Chemicals 85

good compatibility with all other anionic chemicals (e.g. native and anionic

starches, CMC, optical brighteners, anionic polyacrylamides) and high stability

in the size-press or in water doctors. The disadvantages of styrene-maleinate

polymers are that the sizing effect depends on a certain amount of alum in the

base paper, the improvement of printability on wood-free white papers is lim-

ited, and these products are not suitable for wet-end addition (internal sizing).

In respect of this, the cationic styrene-acrylate dispersions are much more com-

monly used. They possess high paper stock affinity and the sizing effect is

largely independent of the alum content and pH in the stock suspension. There-

fore they can be used for nearly all paper grades, depending on the cationic

density of the polymer. With increasing cationic density a reduction in the opti-

cal brightening and the paper brightness will occur. Most products of this class

are suitable for wet-end and surface application. The sizing effect is developed

within the paper machine. The sizing is improved by intense contact drying on

hot metal surfaces (e. g. drying cylinders). The sizing obtained is resistant to

acids and alkalis. The surface properties of the paper are improved, especially

pick strength and printability and linting and dusting are also reduced.

3.6.4.3 Application Advice

• Rosin: The average addition rate is 1% solid calculated on paper, the individual

rate depending on the required degree of sizing. With these products this degree

can be adjusted very precisely and is fully obtained directly after the paper has

been dried. The product is usually added to the stock suspension upstream of

the headbox and then fixed chemically, in the simplest case with aluminum

sulfate (alum). The retention of rosin size is in the region of 70 % if the usual

fixing agents are employed. Rosin sizes are not usually suitable for surface ap-

plications.

• AKD: As little as 0.005–0.008% reacted AKD is believed to yield sufficient

monomolecular coverage of the paper stock, while dosage rates are considerably

higher and typically vary from 0.05 to 0.2%, calculated as solid AKD wax based

on paper stock. The stability to hydrolysis of most commercial products has been

improved significantly in recent years. The ketone resulting from the hydrolytic

decomposition of AKD is a solid (melting point 80–85 °C) which can be retained

in the sheet but does not contribute to sizing. The risk of the formation of

deposits is relatively low. For cost efficient sizing, the curing (orientation of AKD

particles) must be initiated appropriately because chemical reaction alone is not

sufficient. The curing can be improved by drying the paper web at high contact

temperatures and to a very low moisture content e.g. before the size press,

typically to 1–3%, especially on fast-running machines. Another way to speed

up the curing before converting is to carry out AKD sizing in the presence of a

so-called promoter resin, e.g. polyamide epichlorhydrin, modified polyethyleni-

mine, polyvinylamine.

• ASA: This will not dissolve in water and, prior to application in papermaking, it

must be emulsified on site at the paper mill. There, a small amount (3–6%) of

3 Chemical Additives86

an activator is added, plus cationic starch and a synthetic cationic polymer which

serve as stabilizers. Activators are surface-active agents that promote effective

emulsification at low mechanical energy. Emulsion stability is best with syn-

thetic polymer, and sizing efficiency is best with cationic starch, improving with

the amount of starch used. The ratio of starch to ASA is usually in the range 2:1

to 4:1. Emulsification is primarily carried out in a continuously operated special

automated equipment, and can be affected by both low and high shear proce-

dures. Particle sizes in the range 0.5 to 2 mm can be obtained. Emulsification

imparts a cationic charge on the oil droplets which helps to increase the stability

as well as improving ASA retention on anionic fibers and sizing efficiency. Lin-

ear hydrocarbons are more hydrophobic than branched ones. However, the

greater the hydrophobicity of the ASA molecule, the more difficult is the prepa-

ration of a stable quality ASA emulsion.

ASA undergoes the normal reactions of acid anhydrides. Of particular interest

in conjunction with sizing is the reaction with alcoholic hydroxy groups to yield

an ester, and the hydrolysis with water. Both reactions occur in the papermaking

system. ASA is highly reactive, and the reactions proceed rapidly and irreversi-

bly. Although this would provide satisfactory development of sizing on a paper

machine, the hydrolysis of ASA is undesirable. The hydrolysis accelerates with

pH, time, and temperature and leads to deposits and runnability problems on

the paper machine. In order to limit the hydrolysis of ASA emulsions, the pH

can be lowered immediately after the emulsification by addition of aluminum

sulfate.

High solids retention is extremely important in ASA sizing. If this is guaranteed

and ASA is applied correctly, a relatively small dosage of 0.1–0.2 % based on

solid furnish is required to produce the desired degree of sizing. Careful selec-

tion of retention aid is therefore an important part of the ASA sizing process.

The great advantage in favor of ASA is the high rate of cure in comparison with

AKD size. The reaction with cellulose hydroxy groups takes place rapidly in the

dryer section of the paper machine at less than 5% web humidity. More than

90% of the attainable sizing potential is, in most cases, achieved before the size

press unit. The reaction rate is influenced by the same parameters as the hydrol-

ysis, that is primarily by pH, previous residence time, and stock temperature.

The best conditions obviously depend upon the balance between the reaction

rates with cellulose hydroxy groups and hydrolysis of ASA, and optimum sizing

can be achieved at neutral rather than at higher pH, depending on the full set of

system conditions.

• PSA: For internal sizing (wet-end addition) quantities of 0.5 to 1% (solid product

on dry paper stock) are required to obtain a hard sized sheet, depending on the

nature of the stock. The product is best metered continuously to the stock sus-

pension before the pressure screens. The polymer must be prediluted with an

in-line mixer to a concentration of 1–5%. A cationic retention aid, e. g. modified

polyethylenimine, cationic polyacrylamide or polyvinylamine improves the fixa-

tion/retention and therefore gives a better sizing effect.

3.6 Functional Chemicals 87

For surface application, first the compatibility with the other chemicals used in

the preparation and also the shear stability have to be checked (e.g. with starch,

optical brightener, dyes). It is advisable to meter the undiluted polymer by

means of a piston pump to the circuit of the size-press, e.g. before the feed

pump. To achieve a good effect, the PSA must penetrate sufficiently into the

paper, e. g. an adequate quantity of sizing solution must be taken up. Quantities

of 2–4 g l

–1

solid polymer in combination with 40–80 g l

–1

starch are normally

sufficient to achieve a hard sized paper sheet; 1–2 g l

–1

solid polymer is suffi-

cient for final sizing of a paper that has been presized internally.

3.6.4.4 Requirements and Measurements of Sized Papers

The purpose of sizing is to modify the surface of the fibers to control penetration

of aqueous liquids into the paper. The penetration usually correlates with the ab-

sorbency, repellency (hydrophobicity), and spreading of the aqueous liquid con-

cerned. Control of these important properties may be required for three pur-

poses:

1. Control of the penetration rate of the aqueous phase in a converting operation

such as size press treatment, coating, glueing (influences also the machine

runnability).

2. Control of liquid absorption or wetting in a printing process.

3. Control of the serviceability of many grades of paper and board, e.g. milk/juice

carton, packaging papers, wallpaper, printing writing papers, etc.

Besides a certain degree of water and ink resistance the papermakers and their

customers often look for improvements in dimensional stability, surface strength

(pick and rub), internal bond, linting, dusting, stiffness, smoothness, porosity, or

friction coefficient, depending on the converting process and use of the paper.

Several test methods are used to measure the degree of water and ink resis-

tance:

• Water drop absorption: the time required for defined droplets of water to be ab-

sorbed by paper.

• Ink flotation test: the time required for writing ink to penetrate through a floating

piece of paper and to change the color of the upper surface.

• Hercules sizing test: the decrease in reflectance of the opposite side of a paper

sheet which has been covered with a given amount of ink.

• Cobb test: the amount of water absorbed by a given area of paper in a specified

period of time.

• Pen and ink feathering test: the extent of spreading of lines drawn on a paper

sample with a steel pen and ink.

• Contact angle: the tangential angle from the horizontal which the base of a drop

of liquid develops when carefully placed on a paper sheet.

• Edge penetration/edge wicking test: the lineal penetration of liquid in the in-plane

dimension of the paper.

3 Chemical Additives88

3.6.5

Dry-Strength Resins (DSR) [3, 9–11, 22–24]

Some of the specific mechanical properties important for paper are tensile

strength, tearing resistance, folding endurance, bending resistance, burst and sur-

face strength, internal bond and compression resistance. It is generally accepted

that the major factors contributing to dry strength development in paper manu-

facture include Van der Waals forces, hydrogen bonding and ionic bonding. Be-

cause of the special effect of water on paper strength, it is common to distinguish

between “dry” and “wet” strength properties of paper. The two subjects are ob-

viously related, but wet strength will be treated as a separate topic. Paper strength

is affected by many furnish and process variables. On the furnish side, longer

softwood fibers produce stronger papers than shorter hardwood fibers. Fillers re-

duce strength. Alkaline pH conditions in the wet-end produce stronger papers,

especially after aging, than do acid pH conditions. On the process side, both in-

creased refining and wet pressing increase paper strength. The basic factors that

influence paper strength are: individual fiber strength, interfiber bond strength,

the number of interfiber bonds (bonded area) and the distribution of fibers (sheet

formation). While the first factor cannot be influenced by strength additives, the

remaining three factors can be strongly influenced by such products.

Many water soluble, hydrogen bonding polymers will act as dry strength ad-

ditives. In fact, wood fibers contain their own natural dry strength additive in the

form of hemicelluloses. It is well-known that the removal of hemicelluloses from

wood fibers makes it more difficult to develop their bonding characteristics.

• Starch: In general starch derivatives represent the most common and by far the

largest amount of dry strength additives (Fig. 3.1).

• Vegetable Gums: Much less important but also used are water soluble vegetable

gums, such as locust bean gum and guar gum. These highly hydrophilic poly-

mers have chemical structures which are similar to cellulose, enabling them to

participate in extensive hydrogen bonding with fiber surfaces. The natural mate-

rials are nonionic and are not retained by fibers to any extent. Consequently,

successful commercial products all have cationic groups attached to the main

chain, which increases the attraction between gum molecules and fibers and

results in improved polymer retention. Due to the combination of high retention

and effective dry strength enhancement only 0.1–0.35% of the material has to be

added in most instances.

• Polyacrylamide Resins: The fully synthetic DSR are of growing importance. In

Japan they already form the largest proportion of the total consumption of dry

strength resins. Their worldwide consumption is an average of 300 g dry poly-

mer per ton of paper or 3% of the total amount of specialty chemicals (see

Fig. 3.3). In the group of synthetic dry strength resins the polyacrylamide-based

products are still dominant today. Since anionic polyacrylamides are negatively

charged, they are not directly attracted to paper fibers. A cationic substance, such

as alum or polyamide epichlorohydrin resin, must be used to promote their

retention. To avoid the need for a cationic promoter, it is possible to incorporate

3.6 Functional Chemicals 89

cationic groups (e.g. methacryuloyloxethyl trimethyl ammonium methosulfate,

dimethyldiallyl ammonium chloride, vinyl benzyl trimethyl ammonium chlo-

ride, 3-acrylamido-3-methyl butyl trimethyl ammonium chloride) directly into

the polyacrylamide backbone by copolymerization. When used as dry strength

additives, typically 10% of the monomers will be charged and their molecular

weight will be between 100000 and 500 000. This range is low enough that the

polymers will not bridge between particles and cause flocculation, and high

enough to retard migration of the polymer into the fiber pores with concomitant

loss of activity.

• Polyvinylformamide/Polyvinylamine Resins (PVF/PVAm): Recently developed new

types of polymers in the form of polyvinylformamide and polyvinylamine are

coming into use. All these water soluble polymers contain primary amino

groups that can form hydrogen bonds with surface cellulose molecules in fibers

and improve interfiber bonding. These polymers enable the papermaker to

achieve combinations of paper properties that cannot be obtained through refin-

ing alone. For example, strength properties can be improved without affecting

sheet bulk and/or appearance properties.

The new DSRs are produced by polymerizing vinylformamide and then hydro-

lyzing it. This results in a chain type macromolecule with primary amino groups

without using an additional monomer. These polymers can be varied within a

very wide range of molar mass and charge density to optimize the performance.

Medium or low charged polyvinylamines of medium molecular length give high

performing dry strength resins. Further improvements in cost-performance can

be achieved by product combinations, e.g. low/middle charged cationic poly-

vinylamine plus low molecular, middle/high charged anionic polyacrylamide or

cationic polyvinylamide plus anionic polyvinylformamide.

These products have no remaining monomers, and do not contain formalde-

hyde or organically bound chlorine, therefore they do not release chlorine to the

effluent. The environmental advantages of this new group of DSRs is underlined

by the fact that they have been approved by the German health authorities (BfR-

Bundesinstitut für Risikobewertung) for food packaging paper and board. PVF

and PVAm products also conform to the requirements of the United States Food

and Drug Administration (FDA) regulation 21 CFR 176.170 (Components of

paper and board in contact with aqueous and fatty foods), according to its cur-

rent status. It may be added at levels of up to 1.5% solid polymer, expressed as a

proportion of the dry, finished paper.

Polyacrylamide- and polyvinylamine-based DSRs are mainly supplied as ready to

use aqueous solutions or emulsions (10–40%) or as water soluble powders that

must be dissolved prior to use. No other preparatory steps are necessary. The

most beneficial way of application is to meter them continuously to the thick

paper stock at a point of thorough mixing, e.g. at stock dilution. Addition rates

of 0.1 to 0.5% of solid material are adequate for most uses. Excessive use can

overcationize the stock suspension and reduce the effectiveness of the DSR and

of other cationic additives.

3 Chemical Additives90

3.6.6

Wet Strength Resins (WSR) [2, 9–11, 21–24]

Certain types of paper can only fulfil their purpose if they have adequate wet

strength. Such papers include, for example, filter papers, hygienic papers, papers

for bags, label papers, wallpapers, laminate base papers, packaging papers for

moist goods and all papers which, in the course of further processing and use, risk

breaking when rewetted. The required wet strength (up to 50% of the dry paper

strength can be retained) is obtained with the aid of wet-strength resins. For ex-

tremely high wet-strength properties the most common WSR are urea formal-

dehyde resins (UF-resins) and melamine formaldehyde resins (MF-resins), These

chemicals need acid pH conditions and the presence of alum in the papermaking

process. For neutral pH conditions polyamide-epichlorohydrin resins (PAE-resins)

are mainly used (e.g. for hygiene and laminate papers); polyethylenimine products

are used for specialty papers such as industrial filter papers and shoe board.

The total consumption of wet strength resins, together with insolubilizers for

coating (see 3.6.9.3.5.2), accounts for about 0.07% of the worldwide paper produc-

tion or 7% of all specialty chemicals, calculated on the active ingredient (Fig. 3.3).

Over the past years, consumption of PAE-resins has increased overproportionately.

This is partly due to the trend from acid manufacturing conditions to the neutral

pH range, where polyamide-epichlorohydrin resins are more effective than urea

formaldehyde and melamine formaldehyde resins. However, the increasing im-

portance of PAE-resins is no doubt also largely a result of the formaldehyde con-

troversy of the early eighties. PAE-resins account today for about 45%, urea for-

maldehyde resins 15%, melamine formaldehyde resins 10 %, glyoxal resins 15 %

and the remaining 15% are others, e. g. ammonium zirconium carbonate (as in-

solubilizer) and newly developed products e.g. polyvinylamines.

There are two theories regarding the mechanism of wet strength. The first states

that the wet strength effect is due, at least in part, to a reaction between the resin

and the cellulose, which leads to the formation of ether bonds. The second theory

assumes that the wet-strength resins crosslink on exposure to heat in the dryer

section to form a three-dimensional network, wrap themselves around the points

where the fibers intersect and thus protect the points of intersection from water

penetration and swelling. Given the short contact times with the steam-heated

cylinder surfaces (less than a second in the case of the yankee cylinder used in

hygienic paper production), wet-strength agents require a high level of reactivity to

allow crosslinking to take place and bonds to form. At the same time, the wet-

strength resins have to have a selective effect if they are not to react with the

surplus hydroxy groups in the paper stock suspension. Therefore a healthy balance

between reactivity and selectivity has to be found, so that the chemical reactions

(crosslinking, formation of covalent bonds) are not completed at the end of the

paper manufacturing process but continue during storage until maximum wet

strength is reached one to three weeks later. This gradual curing should not be

looked upon as a disadvantage as it is essential for good recycling of the paper

machine broke.

3.6 Functional Chemicals 91