0.4 MBAA (0/o)

3000

)

1000

500

0.6 0

Fig. 1.3 Effects of the amount of the crosslinker added on the degree of grafting and water retention value of P/PULP-AM-Hyd.

Plots:

(

⁰

)

, degree of grafting;

(

^e

)

, water retention value.

-C)

...

C)

� Q)

>

(TJ

c 0

�

c

+J

Q) 0::: d)

�

d)

+J

�

crosslinker that had been added.

Fig. 1.4 shows the absorbencies of the graft copolymers

on FILM. The WRV increased with an increase in the DG and reached about 150 g/g. In regard to the effect of the species of the branch polymers, the WRVs of the graft copolymer of PAA on FILM(FILM-AA) and the graft copolymer of PAA·PAM on FILM(FILM-AA·AM) were about the same at a similar DG, but that of the partially hydrolyzed graft copolymer of

PAM on FILM(FILM-AM-Hyd) was lower than those of the others.

It is considered that these results are ascribed to the amount of carboxyl groups of the graft copolymer; that is, as shown in Tables 1.3 and 1.4, the branch polymer in FILM­

AA·AM is mainly composed of PAA, and the amounts of carboxyl groups of FILM-AA and FILM-AA· AM are larger than that of FILM-AM-Hyd. Incidentally, the graft copolymers on FILM were broken into gel particles after swelling.

The absorbencies of the graft copolymers on NWF are shown in Fig. 1. 5. The results were similar to those for the graft copolymers on FILM. The WRV increased with an increase in the DG and reached about 250 g/g. The WRVs of

� ²⁵⁰

...

C)

Q) :J

� c:

� 0 Q) c:

�

Q) 0::

a-� cu

�

150

50

Chapter 1

0 200 400

Degree of Grafting ( % )

Fig. 1.4 Effect of the degree of grafting on the water retention value of the graft copolymers on FILM.

Plots: ( 0), FI LM-AM-Hyd;

(L.

) , FILM-AA;

( 0

^{) , FILM-AA · AM .}

-C)

250

...

C)

4)

�

�

.Q

c:

+J c:

d)

&!

+J

'-+J d)

�

150

50

0 200 400

Degree of Grafting ( 0/o )

Fig. 1.5 Effect of the degree of grafting on the water retention value of the graft copolymers on NWF.

Plots: (0), NWF-AM-Hyd;

(D.),

_NWF-AA;

(D),

NWF-AA· AM.

Chapter 1 thickness and 1-6 times larger in area.

The graft copolymers on FILM and NWF had lower absorbencies than those on the pulps did. In spite of no addition of the cross linker, the graft copolymers on the FILM and NWF were prevented from being water-solubilized because the FILM and NWF trunk polymers had been shaped into sheet forms. This shaping caused the depression of the extension of the molecular chain of the graft copolymer, and the WRVs of the graft copolymers on the sheets were smaller than those of the graft copolymers on the pulps. The graft copolymers on NWF had higher absorbencies than those on FILM, and the formers retained the sheet forms after swelling while the latters were broken into gel particles.

It is considered that the reason of these results is that NWF has higher hydrophilicity and flexibility than FILM did.

1.3.3 Hygroscopicity of Graft Copolymer

The time courses of moisture sorption at 20 oc and 66%

RH for the graft copolymers on NWF and the original NWF are shown in Fig. 1.6. The graft copolymers could absorb moisture more rapidly than the original NWF did, and they absorbed 80-95% of the moisture for each equilibrium moisture gain within 10 hours. The equilibrium moisture gains of the graft copolymers were about 3-5 times as great as that of the original NWF. In regard to the effect of the species of the branch polymers in the graft copolymer with a similar DG in the vicinity of 300%, the moisture gain decreased in the order of NWF-AA, NWF-AA· AM, and NWF-AM-Hyd.

The order of the moisture gain as well as that of the absorbency coincided with the order of the amount of carboxyl groups in the graft copolymers.

Fig. 1.7 shows the moisture sorption isotherms at 20 oc for the graft copolymers on NWF and the original NWF. The graft copolymers could absorb much moisture than the original NWF did at every RH. The order of the moisture gains was the same as shown in Fig. 1.6. The effect of the

r"\

�

v r:::

(TJ

C!J

Q)

� ::J

+""

(/) 0

�

60

40

20

0

�----�--�----�--�---�

1 10 100

Time (h)

Fig. 1.6 Time courses of moisture sorption at 20 oc and 66% RH for the graft copolymers on NWF and the original NWF.

Plots: (0), NWF-AM-Hyd (DG, 330.1%);

(�

), NWF-AA (DG, 315.2%);

(0

) , NWF-AA· AM (DG, 307. 2%);

( e

) , original NWF.

Chapter 1

100

,...

�

\o,J

r::

60

·-ro

<-'

a>

L. :J

+-' C/)

·a � 20

0 50 100

Relative Humidity ( Ofo )

Fig. 1.7 Moisture sorption isotherms at

20

_{oc for}

the graft copolymers on NWF and the original NWF.

Plots: (0), NWF-AM-Hyd

(330.1%

grafting);

(6),

NWF-AA

(315.2%

grafting);

(0), NWF-AA· AM

(307. 2%

grafting);

( e

) , original NWF.

250

""

�

'V

c:

co

150

(.!J

(1) J...

o+-ol ::J CJ) 0

�

50

0 50 100

Relative Humidity ( Ofo)

Fig. 1.8

Moisture sorption-desorption isotherms at 20

ac

for

NWF-AA·AM

(307.2% grafting).

Plots: (

0

) , sorption process;

(

e

) , desorption process.

Chapter 1

higher humidity was exposed to lower humidity, the moisture sorption was indicative of the reversible behavior with hysteresis. The moisture gain at 98% RH reached about 240 wt%, and the difference in the moisture gains between sorptive and desorptive processes at 72% RH was about 25%.

Chapter 1 1.4 Conclusion

The cellulosic super absorbent polymers were synthesized by the graft copolymerization of AM and AA onto several types of cellulosics such as NBKP, P/PULP, CE/PULP, FILM, and NWF by using of the eerie ion initiation method, with or without post-hydrolysis. These graft copolymers were prepared as the super absorbent polymers in fibrous and sheet forms instead of the conventional super absorbent polymers in powdery and granular forms.

The partially hydrolyzed graft copolymers of PAM on pulps had higher absorbencies than those on starches and the commercial super absorbent polymers from starch and synthetic polymers did. The cellulose molecule seems to act effectively in the swelling of the cellulae graft copolymer. Their WRVs were affected by the degree of grafting and the amount of the crosslinker added, i.e., the amount and the crosslinking density of the grafted branch polymer. Under a condition of fixed amount of the cross linker added, the maximum WRV appeared at about 200%

grafting. Under a condition of fixed degree of grafting, the maximum WRV appeared at 0.30 wt% addition of MBAA. The maximum WRVs of NBKP-AM-Hyd, P/PULP-AM-Hyd, and CE/PULP-AM-Hyd were 2692.8 g/g, 3010.2 g/g, and 3034.6 g/g, respectively. The absorbency of the graft copolymer on water-soluble pulp was higher than those of the graft

copolymers on water-insoluble pulps. The absorbency of the graft copolymer with shorter fiber length was higher than

that with longer fiber length.

The graft copolymers on FILM and on NWF were prevented from being water-solubilized in spite of no addition of the crosslinker. The WRVs of the graft copolymers on FILM and NWF increased with an increase in the DG and reached about 150 g

/

g and 250 g

j

g, respectively. Their WRVs were smaller than those of the graft copolymers on the pulps because of the shaping in sheets. The graft copolymers on NWF with higher hydrophilici ty and flexibility than FILM had higher absorbencies than those on FILM, and the formers retained the sheet forms during swelling while the latters were broken into gel particles. In regard to the effect of the species of the branch polymers, the WRVs of the partially hydrolyzed graft copolymers of PAM were smaller than those of the graft copolymers of PAA and PAA·PAM.

The graft copolymers on NWF could absorb much moisture more rapidly than the original NWF. The effect of the species of the branch polymers on the hygroscopicity was

humidity.

Chapter 1 When the graft copolymer that had absorbed moisture at higher humidity was exposed to lower humidity, its moisture sorption was indicative of the reversible behavior with hysteresis. The moisture gain at 98% RH reached about 240 wt%, and the difference in the moisture gains between sorptive and desorptive processes at 72% RH was about 25%.

The cellulose graft copolymers synthesized are to be expected for the development of new materials and for the utilization in various fields.

Chapter 2

Effects of the Crosslinking Density and the Amount of Branch Polymer on the Swelling Behavior of Cellulose Graft Copolymer

2.1 Introduction

The capability of the super absorbent polymer is usually evaluated by

solutions containing

the absorbencies for water electrolytes, because the

and super absorbent polymer is generally utilized in the sanitary and medical fields or the agricultural and horticultural fields [1-9, 11-25]. The absorbencies for solutions containing electrolytes may be important rather than the absorbency for pure water.

The swelling behavior in solution or solvents of various kinds of the ionic polymer networks, which absorb less water than the super absorbent polymers, have been reported in many articles [26-43, 44-53, 59-72]. Though the

Chapter 2 the types of the cellulose trunk polymers and by the species of the grafted branch polymers. As to the branch polymer,

the amount of the cross linker added at the graft copolymerization and the degree of grafting, i.e. ^, the crosslinking density and amount of the branch polymer affected the absorbencies of the graft copolymers. Their effects were very pecurior; that is, the graft copolymers had maximum absorbencies with an increase in the degree of

grafting, and an optimum amount of crosslinker for the absorbency existed.

the addition of It would be important to clarify the effects of the crosslinking density and amount of the branch polymer on the swelling behavior of the cellulose graft copolymer.

Therefore, the effects of the crosslinking density and amount of the branch polymer on the swelling behavior of the cellulose graft copolymer in aqueous NaCl solutions were discussed.

2.2 Experimental

2.2.1 Synthesis of Sample

A bleached kraft pulp from softwood with shortened fiber length(P/PULP) was used as trunk polymer. Other chemicals were the same ones as in Chapter 1. The graft copolymerization of acrylamide(AM) onto P/PULP and the alkaline post-hydrolysis of the graft copolymer of crosslinked polyacrylamide(PAM) on P/PULP(P/PULP-AM) were carried out in similar manners as described in Chapter 1.

2.2.2 Measurement of Absorbency

The water retention value(WRV) and saline retention values(SRVs) for aqueous NaCl solutions with various concentrations of the partially hydrolyzed P/PULP-AM(P/PULP­

AM-Hyd) were measured by using of a similar method as described in Chapter 1. The characteristics such as degree of grafting (DG), degree of hydrolysis (DH), and WRV of the samples used in this chapter are shown in Table 2.1.

The swelling behavior of P/PULP-AM-Hyd was compared with that of a partially hydrolyzed crosslinked

Chapter 2

Table 2.1 Characteristics of P/PULP-AM-Hyds Used in This Chapter

Grafting Conditions Characteristics

Sample Weight Amount of Grafting

of AM MBAA time D G D H WRY

(g) (wt% of AM) (h) (%) (%) (g/g)

A 10.0 0.10 5.0 338.7 63.2 2635.2

B 10.0 0.30 3.0 314.8 65.4 3010.2

c 10.0 0.60 3.0 294.2 70.7 1772.8

D 6.0 3.00 1.0 52.2 58.4 500.2

E 10.0 3.00 1.0 116.0 62.3 802.3

F 10.0 3.00 3.0 148.1 58.6 463.3

Grafting conditions: weight of P/PULP, 2.0 g; cone. of eerie ammonium nitrate, 3.32x10-3 mol/L; cone. of nitric acid, 0.24 mol/L; total volume, 80 mL; temp., 40 ac.

ferrous sulfate were dissolved in 50 mg of water. The two solutions were mixed and allowed to polymerize for 1 hour at

20 a C • After polymerization, the hydrogel of the

polyacrylamide(PAM) was washed with water to remove any residual impurities. One gram of the hydrogel of PAM was added to 50 g of 0.5 N aqueous solution of sodium hydroxide and allowed to stand for 48 hours at 20 a c 0 Then, the hydrogel of PAM-Hyd washed with water to remove the alkali.

The DH and WRY of PAM-Hyd were 56.5% and 634.7 g/g, respectively.

Chapter

2

2.3. Results and Discussion

2.3.1 Swelling Behavior in aqueous NaCl solutions

Fig.

2.1

shows the effect of the concentration of NaCl in external solution on the SRV for PIPULP-AM-Hyd(Sample C).

The PIPULP-AM-Hyd had a high absorbency for water, and its WRV reached

1772.8

gig. By the addition of a small amount of NaCl to water, the absorbency steeply decreased, and the SRV for

0.01

wt% aqueous NaCl solution was

493.5

gig. With further increase in the concentration of NaCl, the SRV gradually decreased. The SRVs for

0. 10

wt% and 0. 90 wt%

aqueous NaCl solution were

338.0

gig and

249.6

gig, respectively.

It is well-known that the swelling phenomenon of the ionic polymer network can be explained by Flory's theory of swelling representing by the following equation

[73]:

(f'/3

=

(

1

I

2

X

i

I

v u

X

1

I S* ¹

/

2 ) 2 + (

1

I

2

- ^X 1 )

I v

1

Vel Vo

⁽

2

.

1

)

where Q is the swelling ratio at equilibrium,

iiVu

is the concentration of fixed charge referred to the unswollen

network, S* is the ionic strength in the external solution,

(1/2 - X1) I V1

represents the network-solvent affinity, and

Ve/Vo

is the crosslinking density of the network.

I n E q.

2 1

. , t h e f . ¹ rs t t erm,

( 1

I

2

^{x �} . I

v

^{u x}

1

I S* ¹

/

2 ) ' represents the ionic osmotic pressure of the polymer network. The ionic strength in solution, S*, is

2 000

�

0>

...

0>

.._....

1500

Q.) :J

-ro >

c

1 000

0

...

500

c Q.)

...

Q.) c::

a>

c

250

(/)

ro

0 0.25 0.50 0.75 1. 00 Concentration of NaCI ( ^{0/o )}

Fig. 2.1 Effect of the concentration of NaCl on the saline retention value of P/PULP-AM-Hyd(Sample C).

proportional to solution(Eq. 2.2):

the

S* =

concentration

1 2

Chapter 2 of electrolytes in

( 2 . 2 )

where S* i s the ionic strength in so l u t ion ( mol

I

kg ) ^{, z .1} and

Ci are the charge number and concentration of ion spesies i

in solution, respectively.

The highly water-absorptive cellulose graft copolymer is one of the polymer network containing ionic groups, and its swelling phenomenon as shown in Fig. 2. 1 also can be explained by Flory's theory. With an increase in the concentration of NaCl, i.e. , the ionic strength, the difference in the osmotic pressure between the internal and external solution of the hydrogel, which was represented by the first term in Eq. 2. 1. decreased. As a result, the expansion of the polymer network depressed, and the SRV decreased. Such a decrease in the absorbency with an increase in the concentration of NaCl is an unavoidable

property of the super absorbent polymer.

In this instance, provided that Flory's equation(Eq.

2.1) is quantitatively applicable to the swelling behavior of P/PULP-AM-Hyd, the value of QP/3 should linearly increase with an increase in the value of 1/S*. To examine whether Flory's equation can be quantitatively applicable to the swelling behavior of P/PULP-AM-Hyd or not, the value of

(f'/3

was plotted against the value of 1/ S*, assuming that the value of Q is equal to the SRV. The result is shown in Fig.

2. 2.

^{The plot for PAM-Hyd gave a good linear}

relationship, but that for the P/PULP-AM-Hyd did not give a linear relationship. That is, the swelling behavior of the P/PULP-AM-Hyd did not quantitatively follow Flory's theory.

The relationship between the SRVs and the concentrations of NaCl is presented in Fig.

2.3

in the form of semilogarithmic plot for the P/PULP-AM-Hyd(Sample C)

[ 6 6] . The relationship is linear in the NaCl concentration

range from 0.01 wt% to 10.00 wt%. In Chapter 1, it becomes apparent that the absorbencies and hygroscopicities of the cellulose graft copolymers are affected by the types of the cellulose trunk polymers and the species of the grafted branch polymers, the degree of grafting, and the amount of the crosslinker added. The NaCl concentration dependence of the absorbency would be also affected by both conditions of the branch polymer and the trunk polymer. In what follows, the effects of the crosslinking density and amount of the branch polymer on the swelling behavior of the graft

M I

0

,...

20

><

10

M ' l{)

0

0 0

I I I I I

20 40

1 1 s·

Chapter 2

60

Fig. 2.2 Plots of QP/3 against 1/� for P/PULP-AM-Hyd (Sample C) and PAM-Hyd.

Plots: (0), P/PULP-AM-Hyd(Sample C);

(e),

PAM-Hyd.

r"\

500

C)

"""

v400

0')

Q) ::J

>300

ro c:

0

� c:

�200

Q) 0:::

Q)

.E 100 (/)

ro

10-1 10° 101

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