Preparation and characterization of water soluble and crosslinkable cellulose acrylate

Preparation and characterization of water soluble and crosslinkable cellulose acrylate Mu-Shih Lin* and An-Jong Chen Department of Applied Chemistry, National Chiao-Tung University, Hsinchu, Taiwan 300, Republic of China (Received 8 August 1991; revised 5 March 1992) The reaction of cellulose with acryloyl chloride in LiCl/N,N-dimethylacetamide resulted in a water soluble cellulose acrylate. The degree of substitution per anhydroglucose unit was determined from the titration of the double bond with bromine. Crosslinking of the acryloyl moieties was carried out in aqueous solution using K2S208/FeSO4 as redox initiator. The original cellulose, cellulose acrylate and the crosslinked cellulose acrylate were characterized by i.r., wide-angle X-ray diffraction, d.s.c., t.g.a, and solid state 13C n.m.r. Crosslinked cellulose is insoluble in water and the gel fraction was measured with a Soxhlet extractor. The fundamental properties of the crosslinked cellulose acrylate films were evaluated. Experimental results indicated that this water soluble and crosslinkable cellulose acrylate is a promising candidate as a water-borne coating material. (Keywords: cellulose; crosslinking; water soluble)

INTRODUCTION

Preparation of cellulose acrylate

Cellulose is the most abundant naturally occurring polymer. Cellulose and its derivatives are of great industrial importance. However, the regularity of the cellulose chain and the extensive hydrogen bonding between hydroxy groups in adjacent chains cause cellulose to be a tightly packed crystalline material 1. As a result, it is insoluble, even in hydrogen-bonding solvents. Furthermore, although cellulose is a linear polymer, it is infusible, with decomposition occurring before a sufficient number of hydrogen bonds are broken 2. Thus, cellulose cannot be processed in the melt or in solution. In spite of this, several cellulose derivatives with less hydrogen bonding can be processed. Most of these commercially interesting derivatives are chemically modified from the native cellulose in heterogeneous reaction mixtures. The reaction conditions are normally difficult to control. In recent years, several solvent mixtures for cellulose have been reported 3-6. In this paper, we report a water soluble cellulose derivative, cellulose acrylate, prepared from the esterification of cellulose with acryloyl chloride in homogeneous LiCI/ N,N-dimethyl acetamide (DMAc) solution. This product is crosslinkable and is a promising candidate as a water-borne coating material.

Cellulose (4 g, 0.025 mol, based on the anhydroglucose (AHG) unit) was added to a solvent mixture which was prepared by dissolving LiC1 (6 g) in DMAc (100 ml). The mixture was stirred and heated to 160°C under nitrogen atmosphere for 1.5 h and the resultant mixture became a homogeneous gold coloured solution. The solution was then cooled down to room temperature and charged into a three-necked flask equipped with a stirrer. Pyridine (6 ml) in DMAc (20 ml) was added to the flask as an acid acceptor. DMAc solution (60ml) containing acryloyl chloride (12.08 ml, 0.149 mol) was then added dropwise with constant stirring. Different batches were prepared for various reaction times (12, 36 and 48 h, respectively). The reaction mixture was then diluted with water and poured into a vessel containing methanol (500ml). White cellulose acrylate was coagulated in quantitative yield. The product was filtered, washed with methanol and dried at 60°C under vacuum for 2 days.

EXPERIMENTAL

Materials Cellulose, acryloyl chloride, LiC1, DMAc and pyridine were purchased from Merck Co. Cellulose, LiCI and pyridine were used without further purification. Acryloyl chloride and DMAc were purified by distillation under reduced pressure. K2S20 8 and FeSO4 were used as received from Janssen Co. * To whom correspondenceshould be addressed 0032-3861/93/020389~05 © 1993 Butterworth-Heinemann Ltd.

Degree of substitution The degree of substitution (DS) per A H G unit was calculated from titration data for the double bond. Titration was carried out by dissolving the cellulose acrylate (0.5000g) in deionized water (120ml), and titrating with bromine. The end point was taken as when the bromine colour persisted. The DS per A H G unit was calculated according to the relationship:

DSx[

W(1-a)

~_

159+(3-0S)+550SI

W' 159.8

where W is the precise weight of cellulose acrylate, a is the fraction of water in the cellulose acrylate, which can be read directly from the t.g.a, thermogram, and W' is the exact weight of bromine consumed in the titration.

POLYMER, 1993, Volume 34, Number 2

389

Preparation and characterization of cellulose acrylate." M.-S. Lin and A.-J. Chen

Crosslinking of cellulose acrylate The dried cellulose acrylate (lg, 4.57x 10-3mol, based on the AHG unit) was dissolved in deionized water (30 ml) containing FeSO4 (0.0161 g, 5.77 × 10- 5 mol). To this clear solution, KzS20 8 (0.0156g, 5.77× 10-5mol) in water (10ml) was added to induce the crosslinking reaction of the acryloyl moiety. A portion of this reaction mixture was put into an i.r. liquid cell (CaFz window) and the decreasing absorption of the double bond at 1610cm -1 was monitored. The change in solution viscosity during crosslinking was measured with a Brookfield viscometer. For a coating solution, the pot-life is defined as the time from the initial cure to the point where the solution viscosity increases abruptly so that the flow becomes difficult, and poor brushing of the coating is observed. The gel fraction was measured with a Soxhlet extractor, and the uncrosslinked portion of cellulose acrylate was continuously extracted with water for 60 h.

a

c-2,5,5

C--I

I I t I I I 180 160 140 120 I 0 0 8 0 ppm

b

Instruments Solid state 13C n . m . r , spectra were recorded with a Bruker MSC 200 using a 54.7° magic angle at 3500revmin -1. Tetramethylsilane was used as the internal standard. Wide-angle X-ray diffraction (WAXD) patterns were obtained using a Shimadzu X-ray diffractometer (model XD-5). D.s.c. scans and t.g.a, thermograms were recorded with DuPont 1090 and DuPont 951 instruments, in nitrogen atmosphere at a heating rate of 10°C min- 1.

I 40

I 20

c-z,3,5

Film properties Clear film was obtained by casting the cellulose acrylate onto a glass plate from its aqueous solution containing the K2S2Os/FeSO4 redox initiator. The film properties of the crosslinked cellulose acrylate were evaluated according to ASTM standard test methods 7.

1 60

C-8

c-7

C - 6'61(Subst')

(s.b,,.)

A c-9

, A

A

180 160 140 120 lO0 80 ppm

60

40

20

C

Sat. C - 9

RESULTS AND DISCUSSION The solid state 13C n.m.r, spectra of the original cellulose, cellulose acrylate and crosslinked cellulose acrylate are shown in Figures la, b and c, respectively. For the original cellulose, C-I, C-4 and C-6 have chemical shifts at 105.9, 89.3 and 65.1 ppm, respectively, while C-2, C-3 and C-5 have chemical shifts at ~72-75ppm 8. The cellulose acrylate shifts the substituted C-l, C-4 and C-6 peaks to 104.5, 83.0 and 57.9 ppm, respectively, while the substituted C-2, C-3 and C-5 are overlapped at 35.9 ppm. The unsubstituted C-2, C-3 and C-5 are overlapped at 73.1 ppm. New peaks at 170.6, 145.9 and 129.5 ppm are assigned as C-7, C-8 and C-9 on the acryloyl moiety as shown below: 6 CHzOH

6

CH20R

°\/o\ OH

OR

0 [I

where R=H or - C C H = C H 2 78 9 Cellulose

390

Cellulose acrylate

POLYMER, 1993, Volume 34, Number 2

I

I

I

I

I

I

I

I

I

180 160 140 120 I00 80 60 4 0 20 ppm Figure 1 Solid state 13C n.m.r, spectra of (a) cellulose, (b) cellulose acrylate and (c) crosslinked cellulose acrylate

For the crosslinked cellulose acrylate (Figure lc), the newly developed peak at 22.4 ppm is believed to be due to saturated C-9. The peak around 35.9 ppm now splits into a doublet, presumably because of incomplete cure. The DS values per AHG unit for various batches of cellulose acrylate obtained at different reaction times are given in Table 1. The i.r. spectra of cellulose and cellulose acrylate are shown in Figures 2A and B, respectively. Absorption of the carbonyl group and the double bond in the cellulose acrylate can be observed at 1730 and 1610cm -1. The WAXD patterns of cellulose and cellulose acrylate are given in Figures 3A and B, respectively. The diffractogram of the original cellulose shows crystalline peaks at 20=14.8, 16.4 and 22.5°, which are characteristic of cellulose 1 9 ' 1 ° . The cellulose acrylate shows no characteristic peak in the diffractogram, indicating loss of crystallinity and, therefore, is soluble in water. This amorphous polymer was also evidenced by the transparency of the cast film.

Preparation and characterization of cellulose acrylate. M.-S. Lin and A.-J. Chen Table 1 Measured DS per AHG unit for cellulose acrylate by titration

r

Reaction 3612 time (h)

DS 11.75.05

~"-

48

1.80

.¢1 c

=o o ,¢1 o

/ / I

I

4000

5500

3000

I

I

I

I

2500

2000

1500

I000

I

1900

500

Figure 2 I.r. spectra of (A) cellulose and (B) cellulose acrylate

I

40

i

I

i

[

30

20

20

I

I

1700

1600

Wavenumber

(cm -I }

Wovenumber

I

1800

i

I

I0

(deg)

Figure 3 WAXD patterns of (A) cellulose and (B) cellulose acrylate

1500

(cm - I )

Figure 4 I.r. absorption change of the C ~ C bond at 1610cm-1 in cellulose acrylate: (A) before crosslinking; (B) during crosslinking; (C) after crosslinking

The crosslinking reaction of the cellulose acrylate in aqueous solution was induced by the redox initiator FeSOg/K2S20 8. Figure 4 shows the successive changes in i.r. absorption for the C ~ C bond at 1610cm -1 during crosslinking. A residual absorption peak at 1610cm -1 after gelation is observed (Figure 4C). The thermal characterization of cellulose, cellulose acrylate and crosslinked cellulose acrylate is shown in Figures 5a-f. Eventually all of the samples contained ~ 6 - 8 % water which was very difficult to remove. The property of moisture absorption for cellulose acrylate is believed to be due to incomplete esterification of the original cellulose, leading to some residual hydroxy groups as indicated by the i.r. spectrum in Figure 2B. Figures 5a and b show the d.s.c, scan and t.g.a. thermogram for cellulose, where the onset of melting temperature occurs at 305°C, followed immediately by decomposition. Figures 5c and d show the d.s.c, scan and t.g.a, thermogram of amorphous cellulose acrylate, where no apparent melting point can be identified. The first

POLYMER, 1993, Volume 34, Number 2

391

Preparation and characterization of cellulose acrylate. M.-S. Lin and A.-J. Chen

b

a

2

I00

o

E

8O --2

~ ~o

o

--4

~-6

40

I

20 -8

I

I

I

40

I

I

t 20

I

I

200

I

280

200

Temperature (°C)

400

600

Temperature (°C)

d

I

I

P

0

iI i I

E --I I00

o~_2

m

I i,

80

"6 - 3

60

I

-4 -5

I

I

I

I

60

20

I

I

I00

I

l

140

40 20

I

180

0

200 400 600 Temperature (oC)

Temperature (°C)

e i Ii h

2

o g

3

II , ~

Ii

I00

It

80

--2

j

t

"r --4

60

;

--6

40

'

"

I

I

I

--8

2O

1

60

I

I

I00

I

I

I

140

I

20

1

180

I 0

200

,

I

I

I 6OO

400

Temperature (°C)

Temperature (°C)

Figure 5 D.s.c. scans and t.g.a, thermograms, respectively, of (a, b) cellulose, (c, d) cellulose acrylate and (e, f) crosslinked cellulose acrylate

onset of decomposition temperature occurs at 186°C, followed by a second stage of decomposition. The d.s.c. scan and t.g.a, thermogram of crosslinked cellulose acrylate are shown in Figures 5e and f, where the onset of decomposition temperature occurs at 194°C, slightly higher than the 186°C for the uncrosslinked precursor. The broad exothermic peaks around 100°C in the d.s.c. scans are believed to be due to the evaporation of the absorbed water, which can be confirmed from the initial weight loss in the corresponding t.g.a, curves. Figure 6 shows the plot of viscosity versus curing time for cellulose acrylate (1 g) for various amounts of KzS208 in water (30ml). The molar ratio of the redox pair, FeSO4/K:S208, was kept at a 1:1. This clearly demonstrated that the pot-life is dependent on the level of curing agent. A pot-life of 45min was found when 0.0156g of K2S208 (5.77 × 10- s mol) was used in conjunction with 0.0161 g of FeSO4 (5.77 x 10 -s mol) as initiator (Figure 6B). Figure 7 shows the plot of gel fraction versus the amount of K2S2Oa for cellulose acrylate with a DS of 1.05. The gel fraction increases with increasing amount of curing agent. A gel fraction of 80% or higher can be reached. The cellulose acrylate thus obtained is soluble in water,

392

POLYMER, 1993, Volume 34, Number 2

4000

~,k,,~k~ ' A

3600

/

3200

/

2800 2400

k

2000

.~ >

c

16oo 1200

/

800 400 ~ 0

_l|_|s--i

/ j C a " )x ~It~L-_t~ - - - - - - - * -x] I0

20

/

! A

- I -I--I-I-l--I

30

I

40

I

I

50

60

65

Time (rain)

6 Plot of viscosity v e r s u s curing time for cellulose acrylate (1 g) in water (30 ml) containing various amounts of K2S208: (A) 0.0088 g; (B) 0.0156 g; (C) 0.0278 g

Figure

Preparation and characterization of cellulose acrylate. M.-S. Lin and A.-J. Chen IOO

Table 2 Film properties of crosslinked cellulose acrylate

90

i l l t ~ t / t /

80 t

g "~ 50

/

3o 20 10

f

/

7O

I

©

I I0

5

I 15

I 20

I 25

I 50

I 55

I 40

I 45

50

KzS20e (xlO3) (wt %)

Figure 7 Plot of gel fraction versus the amount ofK2S20 8 for cellulose acrylate (1 g) in water (30ml) 200

160

o 120

D-

80

>

4O

i f.// t

0

Result

Resistant to: Alcohol Water (DS = 1.05) Water (DS= 1.75) Water (DS = 1.80) Benzene Aliphatic hydrocarbons Acids (HC1, H2SO~) Alkalis U.v. Heat Adhesion to glass Hardness Gloss (°)

Good Fair Good Good Good Good Slight yellowing Yellowing Good Good Good 84.2 Shore A 30

I

/ I S

~

Test

I

t

2

I

4

i

I

6

i

cellulose acrylate c o n c e n t r a t i o n in a q u e o u s solution. Its a q u e o u s s o l u t i o n shows g o o d flow p r o p e r t y . S o m e w a t e r - b o r n e c o a t i n g recipes were f o r m u l a t e d a n d tested. F o r e x a m p l e , the p l o t of a n a q u e o u s s o l u t i o n of cellulose acrylate ( l g ) in w a t e r (30ml) c o n t a i n i n g different a m o u n t s of K 2 S 2 0 8 at v a r i o u s t e m p e r a t u r e s v e r s u s the d r y i n g time is shown in F i g u r e 9. The crosslinked cellulose acrylate film was e v a l u a t e d a c c o r d i n g to A S T M test m e t h o d s 7 a n d its film p r o p e r t i e s are listed in T a b l e 2. The w a t e r resistance of the c r o s s l i n k e d cellulose a c r y l a t e film with a D S of 1.05 per A H G unit was found to be fair with wrinkles being o b s e r v e d after the test. W h e n the D S was raised to 1.75 or higher, the w a t e r resistance was g o o d a n d no a p p a r e n t defect was f o u n d after the test. CONCLUSIONS

I

t

8

I

I0

i

I

12

i

I

14

Cellulose acrylate (wt%)

Figure 8 Plot of viscosity versus cellulose acrylate concentration in aqueous solution

T h e h o m o g e n e o u s r e a c t i o n of cellulose with acryloyl chloride in L i C 1 / D M A c resulted in an a m o r p h o u s cellulose acrylate which was b o t h w a t e r soluble a n d crosslinkable. C l e a r film can be cast from its a q u e o u s solution. W h e n the D S per A H G unit is 1.75 or higher, the crosslinked film d e m o n s t r a t e d g o o d w a t e r resistance a n d o t h e r film p r o p e r t i e s a n d , therefore, is a p r o m i s i n g c a n d i d a t e for a w a t e r - b o r n e c o a t i n g material.

2O ACKNOWLEDGEMENT T h e a u t h o r s w o u l d like to express their g r a t i t u d e to the N a t i o n a l Science C o u n c i l of the R e p u b l i c of C h i n a for financial s u p p o r t u n d e r c o n t r a c t n u m b e r N S C 80-0405E009-06.

i 0

REFERENCES 1 2 3 50

I O0 150 Drying time (min)

200

250

Figure 9 Dependence of drying time on the amount of K2S208 at various temperatures: (A) 80°C; (B) 60°C; (C) 40°C; (D) 18°C

4 5 6 7

w a t e r / m e t h a n o l (50/50 v/v) a n d w a t e r / e t h a n o l (50/50 v/v), but i n s o l u b l e in m e t h a n o l , e t h a n o l , c h l o r o f o r m , ethyl acetate, ethyl ether, acetone, D M A c a n d N , N - d i m e t h y l f o r m a m i d e . F i g u r e 8 shows the p l o t of viscosity versus

8 9 10

Simon, I., Glasser, L. and Scheraga, H. A. Macromolecules 1988, 21, 990 Dawsey, T. R. and McCormick, C. L. J. Macromol. Sci., Rev. Macromol. Chem. 1990, 30, 4005 McCormick, C. L. and Licvhatowich, D. K. J. Polym. Sci., Polym. Lett. Edn 1979, 17, 479 McCormick, C. L. US Pat. 4 278 790, 1981 Hudson, S. M. and Cuculo, J. A. J. Macromol. Sci., Rev. Macromol. Chem. 1980, 18, 1 Isogai, A., Ishizu, A. and Nakano, J. J. AppL Polym. Sci. 1984, 29, 2097 Sward, G. G. (Ed.) 'Paint Testing Manual', 13th Edn, ASTM, Philadelphia, 1972 Kondo, T., Isogai, A., Ishizu, A. and Nakano, J. J. Appl. Polym. Sci. 1987, 34, 65 Lewin, M. and Roldox, L. G. Text. Res. J. 1975, 45, 308 Dobb, M. G. and Sefain, M. Z. Text, Inst. 1976, 67, 229

POLYMER,

1993, Volume

34, Number

2

393