Cellulose Derivatives - American Chemical Society

Chapter 7

Cationization of Cellulose Fibers in View of Applications in the Paper Industry

Downloaded by UNIV OF OTTAWA on March 18, 2013 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/bk-1998-0688.ch007

E. Gruber, C. Granzow, and Th. Ott Institute of Macromolecular Chemistry, University of Technology Darmstadt, Alexanderstrasse 10, D-64283 Darmstadt, Germany

Three methods were presented to render pulps cationic: • direct reaction of epichlorohydrin and a tertiary amine • coupling of oligo-ionomers • grafting of cationic monomers Advantages of such cationic pulps are: • • • •

more effective than soluble cationic celluloses not sensitive towards pH-changes very versatile more economic

The direct reaction of epichlorohydrin and a tertiary amine is catalyzed by hindered tertiary amines (possible auto catalysis). A wide range of different products carrying various chemical groups (different polarity, accessibility, charge density) can be achieved by this method. Disadvantages of this reaction are, that the reaction proceeds also within the fiber and that it causes cross linking. The method of coupling ionomers to the fiber yields higher charge densities, but surface selectivity is still poor. Surprisingly a higher surface charge has an adverse effect on retention of anionic filler particles. For radical grafting besides charged monomers neutral comonomers have to be used (e.g. acrylamide). This method exhibits the best surface selectivity. As paper aids cationic pulps excel at • high total retention effectiveness • good strength properties • good drainability 94

©1998 American Chemical Society In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


95 Cellulose fibers normally are negatively charged. This has a strong bearing on properties of a paper stock, which predominately consists of anionic species like pulp fibers, fillers, and additives. Repelling interactions between such negatively charged particles hamper the process of flock formation, which may cause problems for water drainage and particle retention. It is common practice in paper making to add cationic polymers to improve flock formation and particle retention. However such soluble polyelectrolytes may also exhibit some disadvantages. One draw back is the viscosity contribution of soluble polymers, which in most cases also depends on pH and salt concentration in the aqueous medium. Polymeric aids are also very sensitive to dosage. They should be applied in stoichiometric concentrations, over dosage leads to a change from a negative to a positive net charge for all species present, resulting in another repelling interaction. This is demonstrated on a soluble cationic cellulose. Figure 1 shows, that the effectiveness of soluble ammonium alkyl cellulose as a retention aid passes a maximum as a function of dosage . The maximum shifts to higher dosages, when the DS increases. A similar effect is found with most other cationic polymers used as paper additives . The reason is, that a flexible polyelectrolyte may easily approach anionic charges on the surface of either a fiber or filler particle, thus effectively neutralizing their electrical charge. A stiff polyelectrolyte on the other hand will only form a minor fraction of ion pairs, such aggregates will still carry naked anions as well as „unused" cations. Figure 2 demonstrates schematically this different situation. If there are only stiff charge carriers flock formation is more efficient, as there are always free ions available. Bridging capacity of cationic compounds will also depend on the accessibility of the cationic groups. Evidently they should sit on the surfece and have some mobility to be able to form contact ion pairs. The chemical nature of the cationic groups will also be of importance as it will control the degree of hydration. Based on such considerations there should be a potential for cationic fibers for paper making. Such fibers could offer some benefits over soluble polyelectrolytes: 1

23

• • • • •

improving filler retention support drainage adsorb anionic trash do not disperse into white and waste water are more easily biologically degradable 4

Stone and Rutherford have already described cationization of cellulose fibers by using glycidyl ammonium salts in 1969. Krause and Kâufer suggested to use such cationic pulps as paper making additives and investigated their basic properties . This paper describes some investigations based on such previous work extending it by using different chemical routes to obtain cationic pulp fibers. 5

In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


ammoniumalkyl cellulose [ mmol/kg]

Figure 1 : Retention effect of a soluble ammonium - alkyl - cellulose (trimethyl glycidyl ammonium cellulose)

soluble poly cations retention

flock formation retention

flocculation drainage

cationic fibers Figure 2: Interactions among electrically charged fibers, pigments, and polymers


97

Different types of cationic pulp fibers In order to study morphological effects we prepared three different kinds of grafted fibers illustrated schematically in Figure 3. Direct addition reaction yields glycidyl ammonium derivatives of cellulose, where charged groups are tightly bond to the cellulose material. By coupling oligomers short side chains carrying cations can be introduced. Finally by grafting cationic polymer chains somewhat longer and more accessible side chains of ion carriers could be attached to the cellulose fibers.


Chemical routes to cationic pulp fibers Simultaneous reaction of epichlorohydrin and a tertiary amine Normal cationization uses glycidyl trimethyl ammoniumchloride as a reactant. To obtain a wider range of different cationic functionalities a one step method can be applied, which starts from epichlorohydrin and any tertiary amine (Equation 1). Equation 1: One step cationization of a polysaccharide Cf Rl Rl Posac—OH

X

+P>^ C1 (

+

Yl—R

Posac—Ο

^—N—

i

R

2

R/ Polysaccharide

^v.

Cationic Polysaccharide

Epichlorohydrin

As there are also some carboxylic functions present in natural polysaccharides, besides ethers, esters and salts may be formed (Equation 2). Equation 2: Possible side reactions with carboxylic groups (Ester linkage) Ο

Ο Posac—C* X)H

Posac—C* x

cr

o-

Polysaccharide Tertiary Amine

Ammonium salt

To get some insight in the mechanism of these reactions, methyl-ct-Dglucopyranoside and glucuronic acid were used as model substances. These were reacted with epichlorohydrin and triethyl amine and the resulting products were analyzed by N M R and M A L D I mass spectroscopy. The conclusions, drawn from these investigations are listed in Table 1.


2

98


Table J: Features of one step cationization Feature

Method

Result

yield

MALDI

formation of ethers

NMR

formation of uremic esters formation of uremic ammonium salts formation of advancement products crosslinking

NMR NMR

if stoichiometrically applied, epichlorohydrin is consumed quantitatively C2, C3, C6 rather similar in reactivity insignificant positive

MALDI

few oligomers

reaction in solution of dimethyl acetamide / LiCl

some crosslinking occurs

We should expect, that the reaction proceeds predominately on the surface of the fiber. However the kinetics of the reaction (see Figure 4) show that both, a surface reaction and penetration into the depth of the fiber occurs. A slow diffusion controlled reaction of accessible areas within the fibers follows a swift reaction on the surface. The ratio between surface and bulk reaction depends on catalysts applied and the size of the ligands of the amine. The degree of surface cationization can be determined by polyelectrolyte titration (see Figure 5). Cationization bv 0U20 - ionomers By coupling a oligo-ionomer to the fiber higher charge densities can be achieved. Such oligomers are synthesized according to Equation 3: Equation 3: Synthetic route for preparing oligo - ionomers



Figure 3: Types of cationic pulp fibers, prepared by different methods

0

2

4

6

8

10

12

Figure 4: Kinetics of amine/epichlorohydrin cationization (ECH = epichlorhydrin; T E A = triethyl amine; DABCO = 1,4diazabicyclo[2.2.2]octane; G M A C = glycidyl trimethyl ammonnium chloride; IMIZ = 4-methylimidazole) In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


100

Figure 5: Comparison of some different degrees of cationization obtained (colorimetric titration according to reference ) 6

Figure 5 shows, that cationization by oligo-ionomers is most effective, the surface selectivity however is still poor (app. 10%). As an example of technical features of such pulps the retention of filler (CaC0 ) was studied. The fiber stock used as model of paper stock contained unmodified pulp, modified pulp, and filler. Figure 6 shows the results obtained with pulp slurries, which contained the same amount of cationic charge equivalent but different charge densities of the cationic pulps added. 3

It can be seen, that filler retention capacity decreases with the charge density applied. Oligomer grafted pulps had the highest surface charges but were specifically least effective. To achieve optimal filler retention a rather high feed of charged fibers is needed, each of which should carry only a moderate surface charge. Among differently substituted ammonium ions trimethyl compounds are most effective. Grafted cationic pulp

Heterogeneous grafting Another way to render cellulose cationic was pursued by grafting cationic monomers to fibers. The reaction consists of a radical polymerization of an unsaturated monomer (Equation 4) starting at a radical, generated on the fiber surface. Equation 4: Grafting on cellulose (Cell- = cellulose radical)

η


101 Table 2: Substances usedfor grafting

Name

Short

Diallyl dimethyl ammoniumchloride

DADMAC

Formula Μ

\·Λ·

Me'


[3-(Methacryloylamino)propyl]-trimethylammonium chloride

" ^ ^ ^

Ο MAPTAC

TA 0

Acrylamide

AAM

The cellulose radical may be generated by chemical activated initiation of grafting: • redox systems (e.g. Ce IV or Fe II) • chemical oxidation (e.g. ozone, peroxide) In these investigations Ce (IV) was used as a matter of convenience. The substances used for grafting are listed in Table 2. As fiber material bleached beech sulfite pulp was applied. Trials to graft charged monomers alone to cellulose however were not successful. With D A D M A C practically no grafting reaction was observed. But even with the neutral monomer acrylamide only very low grafting yields could be achieved. Surprisingly both monomers together reacted vigorously to a graft copolymer with copolymeric side chains. The reaction is summarized in Equation 5. Equation 5: Cationic grafting onto cellulose

Pure (meth)acryl ammonium compounds like M A P T A C also do not graft to cellulose but contrary to allyl ammonium salts, they do not copolymerize easily with acrylamide (Figure 7). In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


102

8

10

12

14

beating time [min] Figure 6: Filler retention as a function of beating time at various degrees of cationization (ECH = epichlorhydrin; G M A C = glycidyl trimethyl ammonnium chloride)

1 1 1 N-content [mmol/kg]

• AAM+DADMAC 5+1 mol/kg

I1

11

1

Ο A A M 6 mol/kg

—

s .--^

•T 0

2

Δ AAM+MAPTAC 5+1 mol/kg

-

ο DADMAC 6 mol/kg

1

• MAPTAC 6 mol/kg 4

6

8

10

12

14

ι

r 16

» 18

reaction time [h]

Figure 7: Influence of monomer type and reaction time on grafting yield ( A A M = acrylamide; D A D M A C = diallyldimetyl ammoniumchloride; M A P T A C = [3-(methacryloylamino) propyl] trimethylarnmoniumchloride)


103 Technical features of grafted cationic pulp fibers


Retention effects

As expected cationic pulps operate as retention aids to negatively charged filler particles. The grafted cationic pulp was tested in a fiber stock containing calcium carbonate. The effect on retention of such filler particles was compared with a soluble cationic polymer (PDAMAC), which contained the same charged groups like the grafted pulp. As seen in Figure 8 at lower dosage of the polyelectrolyte the soluble polymer (PDADMAC) is more effective than the grafted pulp (BuSiKat). At higher dosages however grafted fibers are more active and do not show any sign of saturation and over dosage. From this experiment we may conclude, that cations fixed to fibers are specifically less effective than chain bond ones, but there is no self inhibition and a higher total amount of filler can be fixed. Mechanical properties

Each chemical modification may damage the pulp fibers leading to losses of mechanical strength. In addition to that, the modified surface charge will have an influence on the fiber to fiber interactions and on the formation of the sheet. To test the influence of modified pulps, sheets were made from a mixture of modified and unmodified pulp fibers. As shown in Figure 9 the strength of the paper is augmented by the content of grafted pulp. This may be caused by stronger fiber flocks formed and may be also an indication, that the strength of the fibers as such is not hampered. In practice however this positive effect may be camouflaged by the retention effect, which will lead to a higher filler content in papers containing cationic pulp. To evaluate this issue, filled papers were made using either grafted pulp or a synthetic polyelectrolyte (PDADMC) as a retention aid. Figure 10 shows, that the mechanical strength decreases as normal, when the filler content is increased. However this decline is comparable with fibers and soluble polyelectrolytes. Drainage effects

As drainage of a paper slurry depends among others on flock formation, it has to be expected, that grafted pulp fibers will influence drainability. As a general information on drainage properties Schopper-Rieglerfreenesswas measured as a function of feed of grafted pulp and beating time. The results are summarized in Figure 11. As can be seen, cationic pulps drain more readily and exhibit slightly higher beating resistance. This may suggest, that flocks of higher densities are formed by fibers of opposite charge. This leads to slightly more substantial inhomogeneities in the paper. In fact this can be confirmed by image analysis of microphotographs. Opacity and, as shown, mechanical strength however does not suffer by these stronger variations of fiber density.


104


80

0

5

10

15

20

Figure 8: Retention of C a C 0 by cationic graft pulps compared to a soluble cationic polymer (PDADMAC = poly diallyldimetyl ammoniumchloride; BuSiKat = beech sulphite pulp grafted by PDADMAC) 3

Figure 9: Mechanical strength of lab sheets without filler (made from grafted + ungrafted beech sulphite pulp)


105

breaking length [m 5000


4500 4000 3500 3000

• Bu Si Kat ° PDADMAC

2500

retention [%]

2000 ί

ο

10

J|0_

_30_ 10

.m.

15 %CaCOj

Figure 10: Breaking length as a function of filler concentration (PDADMAC = poly diallyldimetyl ammoniumchloride; BuSiKat = beech sulphite pulp grafted by PDADMAC)

5 10 15 20 cone, of cations / dry fiber [mmol/kg]

25

Figure 11 : Influence of cationic pulps on the drainability andfreenessoffiberstock suspensions


106 Acknowledgment This work was sponsored by a grant AIF 10210 Ν by the German „Arbeitsgemeinschaft Industrieller Forschungseinrichtungen" and by the German Papermakers Association („Verband der Deutschen Papierfabriken"). Literature Cited 1

2


3

4

5

6

Ott, G., Thesis Darmstadt 1992 Swerin, A.; Sjödin, U.; Ödberg, L.; Nordic Pulp Paper Res. J. 8 (1993) No. 4, 389 - 396 Klix,J.,Thesis Darmstadt 1991 Stone, F.W., Rutherford, J.M.; US-Patent 3 472 840 (1969) Käufer, M.; Thesis, Darmstadt 1982; Käufer, M., Krause, Th.; Schempp, W.; Das Papier 34 (1980), Nr. 12, 575579 Käufer, M., Krause, Th.; Schempp, W.; Das Papier 35 (1981) Nr. 10A, V33V38 Käufer, M., Krause, Th.; Das Papier 37 (1983), Nr. 5, 181-185 Käufer, M., Das Papier 35 (1981) Nr.12, 555-562 Gruber, E.; Ott, Th.: Das Papier 49 (1995), Nr. 6, 289 - 296


Cellulose Derivatives - American Chemical Society

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