Indian Journal of Fibre & Tcxtile Rescarch Vol . 30, June 2005, pp. 207-2 1 0

Study on superabsorbent polyacry lonitri le­

based fibre

X i aoyu Hu & Changfa X i ao"

Department of Material Science, Tianj in Polytechnic University, 300 I 60 Tianj in , People's Republ ic of China

Received I I March 2004; revised received and accepted 25 A ugllst 2004

Superabsorbent polymer fibres with maximum water absorb­en'cy were produced using acrylonitri le and methyl methacrylate as I"ilOnOmers, N-hydroxymethyl acrylamide as potential crossl inking agent, dimethyl sulfoxide as solution and a7.0-bis­isobutyronitrile as init iator. The copolymer solution was thcn spun using dry-wet spinning method with water as coagUlation bath. The fibres thus produced were heated to get crossl inking structure and thei r surfaces were hydrolyzed hy .:l kal ine solution. The i nfluence of hydrolyzing conditions, such as temperature and concentrations of alkali nc solution and N-hydroxymcthyl ac­rylamide, on the fibre structure and properties was also studied using FrIR, DSC, DMA and SEM techniques. The changes in storage modulus, Tg and surface structures of fibres were also studied. The superabsorbent polymer of about 40g/g water ab­sorbency was obtained using N-hydroxymethyl acrylamide con­centration of equal to about 10wt%monomer, alkal i concentration of about 1 5wt% and hydrolyzing t ime of about 5 min .

Keywords: Polyacrylonitrile fibre, Superabsorbent polymer, Water absorbcncy

IPC Code: lnt. CI.7 A6 I L l5/00, D06M 1 3/00

Superabsorbent polymers (SAPs) are water-insoluble hydrophilic polymers having the abi l i ty of absorbing large amount of water (> 10 times of their own weight). Nowadays, though SAP particles have been widely used in various fields l •2, their shape restricts their applications. To overcome this disadvantage, the production of fibrous SAPs draws great attention of the researchers over the world. SAP fibres not only have the characteristic of high absorptive speed, but also can be converted into a wide range of textile structures, which make them far more suitable for specialist applications3 . Recently, there are some reports about the preparation of SAP fibres4.5, but work on polyacrylonitrile-based SAP fibre is relatively less.

"To whom ali the correspondence should be addressed. Phone: 24528 1 38; Fax: +86-22-24528000; E-mai l : cfx iao@ljpu.edu.cn

In the present work, a kind of SAP fibre based on polyacrylonitrile (PAN) whose surface can absorb water evenly (water absorbency, 40g/g) and core can keep the fibre ' s form and strength has been produced.

Acrylonitri le (AN) was purified by decompressed disti l lation . Azo-bis-isobutyronitrile (AIBN) was puri­fied by recrystal l ization from aqueous alcohol (95wt%). N-hydroxymethyl acrylamide (NHMA) (analytical grade) as potential crosslinking agent and dimethyl sulfoxide (DMSO) as solution were com­mercially procured and used as such. Methyl­methacrylate (MMA) (analytical grade) was used as monomer and NaOH (analytical grade) was used to make the solution alkaline to hydrolyze the fibres. All solutions were prepared with distil led water.

AN, NHMA, MMA, DMSO and AlBN were put into a three-open flask (500m1) in a certain ratio under N2 atmosphere. The mixture was heated to 65°C and the temperature was maintained as such for 4h to ob­tain a yellow and viscous solution. After spinning by the dry-wet spinning method with water as coagula­tion bath, the fibres were put into an oven at about 1 70° - 1 80°C for 10 min to obtain proper crosslinking structure. They were then hydrolyzed by the alkaline solution of different concentrations for different time at 1 00°C to obtain SAP fibres.

A sample from the SAP fibres was immersed in water at room temperature unti l equil ibrium had reached. Absorbability was determined by weighing the swollen fibres that were allowed to drain for 1 0 min. The water absorbency (Q) was calculated using the fol lowing equation:

Q(g/g)=( W2-WI )/Wt where WI and W2 are the weights of fibres before and after water absorption respectively . Absorbency is expressed as the ratio of retained water i n the fibres to the weight of the dried fibres.

The infrared transmission spectra of the samples were recorded on a Bruker Vector-22 spectroscope using a method of ATR.

The DMA (Dynamic Mechanical Analysis) curves were obtained by Netzsch DMA242. The temperature range covered in this analysis was 40°-340°C at a heating rate of 5°C/min and the stress frequency was 1 0Hz.

208 INDIAN 1 . FIBRE TEXT. RES., JUNE 2005

The DSC (Differential Scanning Calorimetric) curves were obtained using Perkin Elmer DSC-7. The temperature range covered in this analysis was 40°-340°C at a heating rate of 20°C/min.

The morphology of the sample was observed by KYKY-2800 (China) scanning electron microscope.

Whether PAN i s crosslinked or not, it has no abil­i ty to absorb water. Only after getting hydrolyzed and after the -CN group on the chain changed partly (or all) into hydrophilic -CONH2 and -COOH, it be­comes SAPs. The major reaction is given below 6:

Fig. I confirms the structural change in hydrolyz­ing process. Peak at 2240cm" is due to -C=N­stretching and wide peak at 3274cm" is due to stretching of O-H from -COOH7. The alkaline hy­drolysis of the copolymer could be verified by the disappearance of -C=N- stretching band and the ap­pearance of carboxamide and carboxylate bands at 1660 cm' l and 1 549 cm, I respectively 8 (Fig. l b) .

Fig. 2 confirms the change i n fibre' s surface mor­phology before and after hydrolyzing. Hydrolyzation influences the fibre ' s surface morphology greatly. The surface of fibre is found to be smoother before hy­drolyzing than after hydrolyzing, which has porous structure (Fig. 2b). This is because the fibres are partly crosslinked. The parts crosslinked perfectly would not be destroyed easily but the uncrosslinked parts would be destroyed or even dissolved by alka­line solution. The change in Tg is shown in Eg. 3. I n

DMA curves, Tg can be denoted by the x-coordinate of the tano peak. Fig. 3 shows that the hydrolyz�r�s decreases the Tg of polymer. This may be due to thl fact that: ( i ) the hydrolyzing decreases the number of

1 0 2 r------------------,

� 1 0 0 u C e<: .E 9 8 E if) c '" � 9 6 � o

9 4 3 5 0 0

( 0 )

3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 - I

Wavenumber, em

1 0 0 0

Fig . J-ATR-FTIR spectra of fibres [Ca) unhydrolyzed and (b) hydrolyzed)

-CN group and hence the forces between macromole­cules decrease; ( i i ) hydrolyzing destroys parts of the fibre ' s crosslinking structure; and ( i i i ) hydrolyzing temperature ( 1 00°C) makes some parts of the oriented chai I 1 S 1I l1orien lccl .

Fig. 2-Surface morphologies of crossl i nked fibres rCa) unhy­

. drolyzed and (b) hydrolyzed]

c.o \:l ro f--

50 1 00 1 50 Temperature, oc

200

Fig. 3-Curves of tan b from DMA of crossli nked fibres rCa) un­hydrolyzed and (b) hydrolyzed)

SHORT COMMUNICATION 209

The circulation of PAN's macromolecules causes the exothermic peaks (Fig. 4). For the longer hydro­lyzing time, the peak moves towards lower tempera­ture. This phenomenon might also be attributed to the destruction of crossl inking structure.

Fig. 5 shows that the water absorbency i ncreases as the NaOH concentration (CNaOH) i ncreases and finally reaches a certain value. This could be ascribed to an increase in hydrophilic functional groups caused by the increase in CNaOH in the same hydrolyzing time.Fig. 6 shows that the effect of hydrolyzing time on water absorbency is similar to the effect of CNaOH on water absorbency. The decrease in water absorb­ency after the maximum could be attributed to the over-hydrolyzing of the polymer that partly destroys the fibre's polymer gel network.

Potential crosslinking agent is such an agent that could merely be (partly or not) crosslinked during polymerization. After the copolymer solution is spun into fibres, the crosslin king structure could be ob­tained by heating. Fig. 7 shows the I R spectra of polyNHMA and the fibres before and after crosslink­ing. Peak at 1 023cm- 1 is due to the c-o stretching of -CH3-OH group9. After crosslinking, the peak is weak­ened (Fig. 7c). The crosslinking mechanism might be the cause of disappearance of -CH3-OH and the ap­pearance of -CHrO-CHr. Liu and Rempel lO studied the polymer similar to polyNHMA and found that there is no peak at 1 040cm-1 before crosslinking, while after crosslinking the peak appears. The reasons for the difference between the two results are still not clear.

Fig. 8 shows the effect of NHMA concentration on water absorbency. With the increase in NHMA con­centration, the water absorbency i ncreases because of the more perfect polymer gel network. The decrease

(a)

o (b)

] u.l (c)

150 1 75 200 Temperature, °C

Fig. 4---DSC curves of crosslinked fibres [(a) unhydroJyzed (b) hydrolyzed for 4min and (c) hydrolyzed for I l min]

J:!l 50 OJ) � 40 c: II.) 12 30 o CI) � 20 ...

.--- 1

/ •

/ • . / ---­

°0�--�5--�1 0��1 5��2�0--�25

� 1 0 :3

CN•OH' wt%

Fig. 5-Effect of CNaOH on water absorbency [NHMA 15wt% monomer, hydrolyzed for 8min]

./ . - 11 .............. . G' 20 ,/ _ c: • i 1 5 / i ':

/.

°0L-�2��4��6--�B--�1 0� Hydrolyzing time ,min

Fig. 6-Effect of hydrolyzing time on water absorbency [NHMA 7wt% monomer, CNaOH 15wt%]

1 0 0r-------------------------� 9 0 8 h.r---.... c:: 8 0

� 'E 7 0 r-.---__ � 6 0

� 5 0

'#. 4 a 3 �2�S�0�2�0�0�O -1�7�5�0-1�S�0�0--1 2�S�0�1�0�0 0��7 5�O

-1 Wavenumber (em ) Fig. 7-ATR-FTIR spectra of polyNHMA (a), and the fibres uncross linked (b) and crosslinked (c) [NHMA 1 5wt% monomer]

60 .---------------------, 01) On 50 /� ;;:.. <..> c II.) .0 .... 0 CI) .0 '" t-Il.) � :3

40

30 20

1 0

o ..... o 2

."..... 4

/.

6 B 1 0 1 2 1 4 NHMA conc. ,wt%monomer

Fig. 8- Effect of NHMA concentration on water absorbency [CNaOH 15%. hydrolyzed for 4min]

2 1 0 I N DIAN J . FI B R E TEXT. R ES . , JUNE 2005

25 50 75 1 00 125 150 1 75 200 225 Temperature, "C

Fig. 9-Curves of tan () from D M A of tibres I (a) without N H M A

and ( b) containing 1 5wt% monomer NHMA ( u ncrossl inked l l

I L , :il

Telllpcr<l!ure, "C 100

Fig. 1 0-E' and tan 0 curves from DMA of fibres [(a) un­crossl inked and (b) crossl inked] (NHMA 1 5wt% monomer)

in water absorbency after maximum could be attrib­uted to the over-crosslinking of fibre polymer gel network, which hinders the stretching of polymer chains in the network. This conclusion is consistent to the Flory ' s theory " .

Figs 9 and 1 0 show the influence of NHMA on the thermal behaviour of fibre polymer. It can be ob­served from Fig. 9 that the copolymer' s Tg is higher by using NHMA as crosslinker. This shows that some parts of the copolymer are crosslinked during polym­erization , thus the movement of chains is hindered.

Fig. 1 0 also shows that the Tg of copolymer increases again after the copolymer is heated. This proves that the crosslinking process takes place during heating. At the same time, the E' (storage modulus) of the fi­bre i ncreases.

Enhanci l lg alkaline concentration has the same ef­fect as it was with increasing hydrolyzing time on improving watcr absorbency; the excessi ve hydrolysis would however decrcase water absorbency. Hydro­lyzing makes the Tg of the copolymer tB move to­wards lower temperature and finally a kind of fibre with porous surface is obtained. I n addition, the mechanism of the crosslinking process involves the appearance of -CH2-0-CHT and crosslinking makes the 7� of copolymer and E' of the fibre higher. The study reveals that to obtain the SAP fibres with max water absorbency of about 40g/g, the NHMA con­centration is found to be equal to about 1 0wt%monomer and hydrolyzing time and CNaOH are controlled to be about Smin and 1 5wt% respectively .

References Sannino A, Esposito A, De Rosa A, Cozzol i no A, Ambrosio L & Nicolais L, J Biol1led Mater Res Part A, 67(2003) 1 0 1 6.

2 Masuda F, PolvlII Maler Sci Eng Pmc, A CS Oil' PO/VIII Ma­ler Sci Ellg, 69 ( \ 993) 484.

3 Akers P J, Tech Text lnt, 6 ( 1 997) 20.

4 Dohrn W, Bu ttner P & Knobelsdorf C, Chell1 Fibres IlIf, 52

( 2002) 405.

5 Kim Y J. Yoon K J & Ko S W, J App/ PO/Yin Sci, 78 (2000)

1 797.

6 Yang M C & Tong J H, J !v1e1l1br Sci, 132 ( 1 997) 63.

7 Lim K Y, Yoon K J & Kim B C, ELlr PO/Yin J, 39 (2003)

2 t 15 .

8 Lim 0 W, Whang H S, Yoon K J & Ko S W, J App/ Po/ym

Sci, 79 (200 1 ) 1 423.

9 Aggarwal P, Thenlloci1illl Acta, 340 ( 1 999) 195.

1 0 Liu Z S & Rempel G L , J App/ PolYIIl Sci, 64 ( 1 997) 1 345.

I I Flory P J, Prillciples of Polymer ChemisflY (Cornell Univer­sity Press, New York), 1980, 589.