Graft Copolymerization of Lignocellulosic Fibers - American Chemical

4 The Ceric Ion Method of Grafting Acrylic Acid to Cellulose

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D. J. McDOWALL, B. S. GUPTA, and V. STANNETT North Carolina State University, Fiber and Polymer Science, School of Textiles, Raleigh, NC 27650

The ceric ion method of grafting acrylic acid to cellulose was extended to rayon filaments. Experimental procedures were designed to optimize grafting yields and minimize homopolymer forma tion. The fiber sample was pretreated with an aqueous solution of ceric ammonium sulfate. This was followed by a washing step to remove excess ceric solution at the fiber surfaces. The graft ing reaction was carried out in toluene to reduce homopolymer formation. Grafting yields of approx imately 40% were obtained with grafting e f f i c i e n cies in excess of 92%. Among the variables examined were the ceric ammonium sulfate concen tration, acrylic acid concentration, reaction time and reaction temperature.

G r a f t i n g r e a c t i o n s provide the p o t e n t i a l for s i g n i f i c a n t l y a l t e r i n g the p h y s i c a l and mechanical p r o p e r t i e s o f a substrate m a t e r i a l . G r a f t i n g possesses great p o t e n t i a l for t a i l o r i n g m a t e r i a l p r o p e r t i e s t o s p e c i f i c end uses. The n a t u r a l abundance and a number o f a t t r a c t i v e p r o p e r t i e s o f c e l l u l o s e appear t o make i t an i d e a l g r a f t i n g s u b s t r a t e . In g r a f t i n g s t u d i e s , the focus g e n e r a l l y i s t o improve s e l e c t e d p r o p e r t i e s without s i g n i f i c a n t l y altering others. Numerous methods have been developed for the g r a f t i n g o f v i n y l monomers to c e l l u l o s e . These have been reviewed i n a number o f papers and a recent monograph ( J , J2, 3 ) . Among the most promising and p r a c t i c a l o f these i s the c e r i c ion technique. T h i s method o f i n i t i a t i o n was f i r s t proposed by Mino and Kaizerman i n 1958 (A) for a l c o h o l s , t h i o l e s g l y c o l s , aldehydes and amines. Schwab e t a l (5) were among the f i r s t t o extend t h i s method to c e l l u l o s i c s u b s t r a t e s . A great d e a l o f i n t e r e s t e x i s t s i n g r a f t i n g a c r y l i c a c i d and methacrylic a c i d onto c e l l u l o s e due t o the p o s s i b i l i t y o f r

©

0097-6156/82/0187-0045$6.00/0 1982 American Chemical Society

In Graft Copolymerization of Lignocellulosic Fibers; Hon, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS


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developing enhanced i o n exchange and water sorbency p r o p e r t i e s . Unfortunately, such a c i d monomers have proven to be very d i f f i c u l t t o g r a f t due to t h e i r s e n s i t i v i t y , which leads to homopolymeriζation and i r r e p r o d u c i b i l i t y . Among o t h e r s , Richards and White (6), Cumberbirch and Holker ( ] ) , and Mansour and Schurz (8, 9, JiO) have g r a f t e d a c r y l i c a c i d onto c e l l u l o s e using a c e r i c ion technique with rather low g r a f t i n g y i e l d s . More r e c e n t l y , Gangneux e t a l (11) developed a method f o r g r a f t i n g a c r y l i c a c i d onto c e l l u l o s e powder, "Solka Floe," f o r use i n t e x t i l e waste treatment. The c e l l u l o s e was treated with c e r i c ion i n aqueous s o l u t i o n p r i o r t o i t s r e a c t i o n with a c r y l i c a c i d . A benzene-acrylic a c i d s o l u t i o n was used f o r g r a f t i n g to reduce homopolymerization. Presumably, the hydrated c e r i c i o n would not d i f f u s e i n t o the monomer s o l u t i o n to i n i t i a t e homopolymerization although the reverse could still take p l a c e . They obtained g r a f t i n g y i e l d s up to approximately 70% accompan ied by 45% homopolymer. In the present i n v e s t i g a t i o n , their method i s extended t o f i b e r s and a d d i t i o n a l emphasis i s placed on the reduction of homopolymerization. Experimental A 50 denier rayon monofilament (~90μ diameter), courtesy o f American Enka, was used as the g r a f t i n g s u b s t r a t e . The samples were Soxhlet extracted i n methanol p r i o r to use. A reagent grade a c r y l i c a c i d , obtained from Fisher S c i e n t i f i c Co. and i n h i b i t e d with p-methoxyphenol, was employed i n t h i s study. The a c r y l i c a c i d was p u r i f i e d p r i o r t o each use with a F a l l i n g Film S t i l l . The c e r i c ammonium s u l f a t e , toluene, methanol, sodium hydroxide, s u l f u r i c a c i d and h y d r o c h l o r i c a c i d used were all o f reagent grade (Fisher S c i e n t i f i c Co.). A low oxygen grade of nitrogen (less than O.5 ppm oxygen), obtained from A i r c o I n d u s t r i a l Gases, was employed. The g r a f t i n g experiments were c a r r i e d out on rayon samples weighing about O.3 grams. The f i b e r sample was placed i n a 50 m i l l i l i t e r f l a s k , stoppered, evacuated with an a s p i r a t o r f o r 1 minute and then flushed with n i t r o g e n . This procedure was repeated three times. Approximately 25 m i l l i l i t e r s of a O.025 M c e r i c ammonium s u l f a t e (CAS) s o l u t i o n i n 1 Ν I^SO^ was i n j e c t e d i n t o the r e a c t i o n v e s s e l . This was followed by three more evacuations and nitrogen f l u s h i n g s . The system was l e f t under a p o s i t i v e nitrogen atmosphere and kept i n the dark at room temperature (21°C) f o r the desired soaking p e r i o d . This allowed the c e r i c s o l u t i o n to d i f f u s e into the rayon f i b e r s p r i o r t o the grafting reaction. A f t e r the incubation p e r i o d , excess CAS s o l u t i o n was rinsed from the f i b e r surface. This step lessened the amount o f surface g r a f t and homopolymer, which may i n h i b i t the g r a f t i n g reaction. The sample was d r i e d on a coarse f r i t t e d g l a s s



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Ceric Ion Method of Acrylic Acid Grafting

McDOWALL E T A L .

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c r u c i b l e by passing a i r over i t for 2 minutes and then r i n s e d i n a solvent for 30 seconds. Excess solvent was removed by again a i r drying the sample for one minute. The sample was then placed i n a clean r e a c t i o n v e s s e l and evacuated with an a s p i r a t o r and flushed with nitrogen three times. Approximately twenty m i l l i l i t e r s o f an a c r y l i c a c i d i n toluene s o l u t i o n was i n j e c t e d i n t o the system. The system was evacuated and purged with n i t r o g e n . The g r a f t i n g r e a c t i o n proceeded under a p o s i t i v e nitrogen atmosphere for the d e s i r e d time at temperature under mild a g i t a t i o n i n a constant temperature water bath. A nonaqueous medium was chosen i n t h i s procedure, as i n the work o f Gangneux e t a l (11), t o i n h i b i t d i f f u s i o n o f CAS out o f the f i b e r , thus lessening homopolymer formation i n the s o l u t i o n , and to increase the a v a i l a b i l i t y o f the a c r y l i c a c i d . Selective experiments were performed t o examine the p o t e n t i a l o f homo polymer formation. A mixture o f CAS s o l u t i o n , water and a c r y l i c a c i d was found t o form homopolymer throughout whereas a mixture of CAS s o l u t i o n , toluene and a c r y l i c a c i d formed homopolymer only i n the aqueous CAS phase. T h i s o f f e r s evidence o f the advantages o f using a two phase system. Toluene was used instead o f benzene, as employed i n the French work (11), f o r safety considerations. A f t e r g r a f t i n g , the sample was washed s u c c e s s i v e l y i n methanol, 1 Ν NaOH, 1 Ν Η 8 0 ^ , 1 Ν NaOH and s e v e r a l times i n water followed by an overnight water soak with some a g i t a t i o n t o remove homopolymer. The samples were converted t o the a c i d form by soaking i n O.1 N HC1 for 2 hours. This was followed by r i n s i n g with water and another overnight soak. The samples were r i n s e d i n methanol, then d r i e d under vacuum at 40°C. The majority o f samples were extracted i n methanol t o remove any remaining homopolymer. Selected samples were a l s o e x t r a c t e d i n water. No s i g n i f i c a n t weight change was observed a f t e r these e x t r a c t i o n s . Percent " g r a f t " was c a l c u l a t e d from the d r i e d weights as the increase i n weight d i v i d e d by the o r i g i n a l weight m u l t i p l i e d by 100. G r a f t i s given i n quotation marks since i n s p i t e o f the rigorous e x t r a c t i o n methods employed i t may still include some occluded homopolymer. The t y p i c a l g r a f t i n g procedure used i n t h i s i n v e s t i g a t i o n i s o u t l i n e d i n Table I . The a c t u a l percent homopolymer was determined i n a number o f cases by preèipitating i t from the washing l i q u o r i n acetone according t o the method o f Chapiro and Sommerlatte (12). With the higher percent " g r a f t s , " 25%, 35% and 40%, the homopolymer represented only 4%, 2% and 8%, i . e . , g r a f t i n g e f f i c i e n c i e s were 96%, 98% and 92% r e s p e c t i v e l y . 2

R e s u l t s and D i s c u s s i o n

for

Several d i f f e r e n t solvents were examined as r i n s i n g agents the f i b e r sample a f t e r soaking i n the CAS s o l u t i o n . Their

American

Chemical

Society Library 1155 16th St. N. W, Washington, D. C. 20036 In Graft Copolymerization of Lignocellulosic Fibers; Hon, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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TABLE I T y p i c a l Experimental 1.

Procedure

Soaked i n CAS s o l u t i o n for 2 h r s . under N at 21 C. Rinsed i n 10/90 MeOH/Toluene. 1 M A c r y l i c Acid i n Toluene added t o substrate under N and reacted for 6 h r s at 35°C with agitation. Product washed with MeOH, 1 Ν NaOH, 1 Ν H^O^, 1 Ν NaOH, and f i n a l l y water. Soaked overnight i n water with a g i t a t i o n . Converted t o a c i d form with O.1 N HC1, washed with water and soaked overnight. Rinsed i n MeOH and d r i e d under vacuum at 40 C. 2

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2. 3. Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on April 3, 2015 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch004

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purpose was t o reduce the c o n c e n t r a t i o n o f CAS and the aqueous medium at the f i b e r s u r f a c e . As seen i n Table I I , methanol, acetone and butanol r e s u l t e d i n s l i g h t l y negative values. T h i s may be due t o very e f f e c t i v e removal o f CAS i n c l u d i n g sorbed s a l t or i n h i b i t i o n o f the g r a f t i n g r e a c t i o n by the s o l v e n t s . Although toluene y i e l d e d the highest g r a f t value, the f i b e r s appeared v i s u a l l y t o have polymer on the s u r f a c e s . Since toluene and the CAS s o l u t i o n are not very s o l u b l e , these r e s u l t s are not s u r p r i s i n g . A mixture o f 10/90 methanol/toluene by volume y i e l d e d high g r a f t i n g r e s u l t s with low surface d e p o s i t i o n . This mixture appeared t o be s u f f i c i e n t l y polar t o remove surface CAS s o l u t i o n yet hydrophobic enough so as not to remove that sorbed i n t o the f i b e r s . A l l remaining samples were r i n s e d i n t h i s solvent mixture.

Percent

TABLE I I "Graft" f o r Solvent Washed Samples

Solvent

Percent " G r a f t "

Methanol Acetone Butanol 10/90 MeOH/Toluene Toluene

-O.2 -2.6 -O.6 39.2 50.2

C o n d i t i o n s : O.025 M CAS, 2 h r s . soaking time, 1 M AA, Reaction time o f 6 h r s . at 35 C. e

The e f f e c t o f oxygen on the g r a f t i n g procedure was a l s o examined. Oxygen i n the CAS soak had no e f f e c t on the percent



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McDOWALL E T A L .


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" g r a f t " whereas the presence o f oxygen during the g r a f t i n g r e a c t i o n reduced g r a f t i n g from 39.2% t o 1.3%. As i s true with all r a d i c a l r e a c t i o n s , the e f f e c t o f oxygen i s important due to i t s pronounced i n h i b i t i n g e f f e c t . In an attempt t o remove remaining t r a c e s o f oxygen, a mechanical vacuum pump was used i n place o f simple a s p i r a t i o n i n the evacuation procedure p r i o r t o the g r a f t i n g r e a c t i o n . No increase i n g r a f t i n g y i e l d s were obtained using the mechanical vacuum pump, t h e r e f o r e , a s p i r a t i o n was used for all ensuing experiments. The e f f e c t o f soaking time i n the CAS s o l u t i o n on percent " g r a f t " i s shown i n Figure 1. I n i t i a l l y , the g r a f t i n g e x h i b i t s an increase and then l e v e l s o f f around 2 hours. Even the extremely long soaking periods o f 1 week (165 hrs.) and 2 weeks (330 hrs.) showed no improvement i n g r a f t i n g . The two hour period was thus adopted as the standard experimental soaking time. Figure 2 shows the e f f e c t o f r e a c t i o n time on percent " g r a f t . " G r a f t i n g increased i n i t i a l l y and then l e v e l e d o f f before s i x hours. Since there i s a l a r g e excess o f a c r y l i c a c i d a v a i l a b l e even a f t e r the longer r e a c t i o n times, t h i s l e v e l i n g o f f i s presumably due t o c a t a l y s t exhaustion. A s i x hour reac t i o n time was used for the remaining experiments. Monomer concentration versus percent " g r a f t " i s examined i n Figure 3. G r a f t i n g increases with the concentration o f a c r y l i c a c i d and reaches a maximum around 1 molar. Further increases i n concentration lead to decreases and f i n a l l y a l e v e l i n g o f f o f g r a f t i n g values. This trend i s i n agreement with the work o f Gangneux et a l (11) and the e a r l i e r work reported by McDowall et a l (13) i n a benzene medium. Although the reasons for t h i s trend are not c l e a r , i t may be due t o the l i m i t e d s o l u b i l i t y o f a c r y l i c a c i d i n toluene. There may be an a s s o c i a t i o n i n t o m i c e l l e s o f the a c r y l i c a c i d i n the toluene at higher concentra t i o n s p o s s i b i l y reducing the e f f e c t i v e monomer concentration a c c e s s i b l e for g r a f t i n g . The a c r y l i c a c i d i n toluene provides a r e s e r v o i r for the g r a f t i n g r e a c t i o n and the rate o f d i f f u s i o n o f the a c r y l i c a c i d w i l l depend upon i t s c o n c e n t r a t i o n . This concentration would i n i t i a l l y increase and then diminish as the a c r y l i c a c i d a s s o c i a t e s . In the French work (11), the maximum monomer concentration was found to be 2 M a c r y l i c a c i d i n benzene. This l a r g e r value may be due t o the enhanced s o l u b i l i ty o f a c r y l i c a c i d i n benzene compared with toluene. C e r i c Ammonium S u l f a t e concentration and percent " g r a f t " are p l o t t e d i n Figure 4. The percent " g r a f t " increased with i n c r e a s i n g c e r i c concentration and reached a maximum value a t about O.040 molar. A f t e r t h i s c o n c e n t r a t i o n , the percent " g r a f t " decreased and began t o l e v e l o f f . T h i s e f f e c t was p r e v i o u s l y observed for other c e l l u l o s i c systems by Schwab e t a l (5) and Cumberbirch and Holker (7). The decrease i n g r a f t i n g with increased c e r i c ion concentration i s b e l i e v e d t o be due t o an increase i n the r a d i c a l termination step i n v o l v i n g the Ce** ion. +




50 h

TIME OF SOAKING IN CAS SOLUTION (hours) Figure 1. The effect of the soaking time in CAS solution on percent "graft." Conditions:O.025M CAS, 1 M AA, reaction time of 10 h at 35°C.

Figure 2. The effect of reaction time on percent "graft." Conditions:O.025M CAS, 2 h soaking time, 1MAA, reaction temperature of 35°C.


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MCDOWALL E T A L .


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50 Y


40\

1 2 3 4 CONCENTRATION OF ACRYLIC ACID (M) Figure 3. The effect of the concentration of AA on percent "graft." Conditions: O.025 M CAS, 2 h soaking time, reaction time of 6 h at 35°C. 50 h

0O25 O.05 O.075 O.1 CONCENTRATION OF CAS (M) Figure 4.

The effect of the concentration of CAS on percent "graft." Conditions: 2 h soaking time, 1MAA, reaction time of 6 h at 35°C.


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Figure 5 shows the e f f e c t o f r e a c t i o n temperature on percent " g r a f t . " G r a f t i n g y i e l d s are very low at 0 C and increase with temperature up to about 35 C. At 45 C, g r a f t i n g decreases and at 55 C i t i s further reduced with obvious degra d a t i o n o c c u r r i n g . The dégradâtive process, although o c c u r r i n g at lower temperatures, becomes more prominent at e l e v a t e d temperatures. R a d i c a l termination may a l s o be i n c r e a s i n g with temperature. T h i s phenomena has a l s o been reported by Hebeish and Mehta (14) with C e r i c Ammonium N i t r a t e (CAN) and a c r y l o n i t r i l e , and by Schwab et a l (5) for both CAS and CAN. Huque e t a l (15) found a decrease i n percent g r a f t with CAS and methyl methacrylate onto jute as the temperature was increased from 30 to 50 C. The n a t u r a l log o f the percent " g r a f t , " a f t e r 6 hours, versus 1/T may be p l o t t e d for the i n i t i a l p o r t i o n o f the curve (0°C to 35 C) i n Figure 5. These values f a l l on a s t r a i g h t l i n e , as seen i n Figure 6. The c a l c u l a t e d a c t i v a t i o n energy was 3.8 Χ 10 J/mol (9.1 kcal/mol). T h i s o v e r a l l a c t i v a t i o n energy r e f l e c t s i n i t i a t i o n , termination and propagation together with degradation. The a c r y l i c a c i d used i n t h i s study was normally d i s t i l l e d once p r i o r t o each use. However, i n an attempt t o increase g r a f t i n g y i e l d s , the e f f e c t o f r e d i s t i l l i n g was examined. Some what higher y i e l d s were obtained with the r e d i s t i l l e d a c r y l i c a c i d . For example, 47.5% g r a f t i n g was obtained by r e d i s t i l l i n g as opposed to 39.2% for s i n g l e d i s t i l l a t i o n . Further d i s t i l l a t i o n s were not performed due to the s u s c e p t i b i l i t y o f e

e

e


e

e

e

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u n i n h i b i t e d a c r y l i c a c i d t o spontaneous p o l y m e r i z a t i o n . The e f f e c t o f monomer p u r i t y w i l l be examined more e x t e n s i v e l y i n f u t u r e work. Conclusion The procedure, as developed from that o f Gangneux et a l (11), gave g r a f t i n g y i e l d s up to 40 percent with very l i t t l e homopolymer. The optimum experimental c o n d i t i o n s found i n the present work are summarized i n Table I I I . Somewhat higher g r a f t i n g y i e l d s , up to 47.5%, were obtained using r e d i s t i l l e d a c r y l i c a c i d . The important features o f the procedure include p r e t r e a t i n g the f i b e r with CAS, removing excess CAS s o l u t i o n from the f i b e r surface with a r i n s i n g procedure and g r a f t i n g i n a hydrocarbon with l i m i t e d water s o l u b i l i t y . The e f f e c t s o f varying monomer c o n c e n t r a t i o n , c e r i c ion concentration and r e a c t i o n temperature on t h i s system are i n agreement with what has been reported i n the l i t e r a t u r e with other v i n y l monomers and c e l l u l o s e (5, T, JM, U , J 5 ) . It should a l s o be noted that the rayon used i n t h i s study was a large denier f i b e r and that higher g r a f t i n g y i e l d s would be expected with smaller diameter f i b e r s , and other c e l l u l o s i c


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50 h

Figure 6. The natural log of percent "graft" in 6 h versus 1/T. Conditions: O.025 M CAS, 2 h soaking time, 1 M AA, reaction time of 6 h.


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GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS Table I I I Summary of Reaction V a r i a b l e s Studied CONDITIONS VARIED CAS Cone (M) Soaking Time i n CAS (hrs) AA Cone (M) Reaction Temp ( C) Reaction Time (hrs)


e

RANGE USED O.013-O.100 O.25-330 O.5-4.0 0- 55 1- 10

OPTIMUM -O.040 2 1.0 35 ~5

m a t e r i a l s with greater surface area. A large denier was s e l e c t e d i n order to f a c i l i t a t e the examination o f the l o c a t i o n of the g r a f t and morphological changes. These together with other p r o p e r t i e s , such as the water sorbency and r e l a t e d p r o p e r t i e s of the sodium s a l t , w i l l be studied and compared with s i m i l a r samples made by a p r e i r r a d i a t i o n g r a f t i n g technique 0 2 ,

J £ ) .

Acknowledgements This work was supported by organized research funds of the School of T e x t i l e s , North C a r o l i n a State U n i v e r s i t y . The rayon f i b e r sample used for g r a f t i n g was s p e c i a l l y manufactured and supplied by Dr. Tom A l l e n of the American Enka Company. The authors g r a t e f u l l y acknowledge t h e i r a s s i s t a n c e s . Literature Cited 1. 2. 3.

4. 5. 6. 7. 8. 9. 10.

Krässig, Η. Α.; Stannett, V. Adv. Polym. S c i . 1965, 4, 111-56. Arthur, J. C., J r . Adv. Macromol. Chem. 1970, 2, 1-87. Hebeish, Α.; Guthrie, J. T. "The Chemistry and Technology of Cellulosic Copolymers"; Springer-Verlag: New York, 1981. Mino, G.; Kaizerman, S. J . Polym. S c i . 1958, 31, 242-43. Schwab, E.; Stannett, V; Rakowitz, D. H.; Magrane, J . K. Tappi 1962, 390-400. Richards, G. N.; White, E. F. T. J. Polym. Sci., Part C 1963, 4, 1251-60. Cumberbirch, R. J. E.; Holker, J . R. J. Soc. Dyers Colour. 1966, 82, 59-63. Mansour, O. Y.; Schurz, J. Sven. Papperstidn. 1973, 76, 258-62. Mansour, Ο. Y.; Schurz, J. Sven. Papperstidn. 1973, 76, 288-91. Mansour, Ο. Y.; Schurz, J. Sven. Papperstidn. 1973, 76, 415-18.


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McDOWALL E T AL.



11.

Gangneux, Α.; Wattiez, D.; Marechal, E. Eur. Polym. J . 1976, 12, 535-41. 12. Chapiro, A.; Sommerlatte, T. Eur. Polym. J . 1969, 5, 707-24. 13. McDowall, D. J.; Gupta, Β. S.; Stannett, V. T.; Williams, J. L.; Zahran, A. H. "Grafting of Acrylic and Methacrylic Acid to Cellulose Materials." National Joint Pacific Conference of ACS, Honolulu, Hawaii, April 1979. 14. Hebeish, Α.; Mehta, P. C. J. Appl. Polym. Sci. 1968, 12, 1625-47. 15. Huque, M. M.; Habibuddowla, M. D.; Mahmood, A. J.; Jabbar, M. A. J . Polym. Sci., Polym. Chem. Ed. 1980, 18, 1447-58. 16. Zahran, A. H.; Williams, J . L.; Stannett, V. T. J. Appl. Polym. Sci. 1980, 25, 535-42. RECEIVED December 17,

1981.


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Graft Copolymerization of Lignocellulosic Fibers - American Chemical

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