Characterization of Insoluble Cellulose Acetate Residues

There also was a significant decrease in the diva- lent cation and carboxyl content in the soluble portion. Hence, all of the changes expected with in...
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8 Characterization of Insoluble Cellulose Acetate Residues W. B. RUSSO Fiber Industries, Inc., Charlotte, N.C. 28237 G. A. SERAD Celanese Fibers Co., Charlotte, N.C. 28232

C o m m e r c i a l cellulose acetate is normally manufactured using a very high purity wood pulp because its applications have very c r i t i c a l processing and product requirements. Cellulose acetate films must be c l e a r and free of imperfections that could be caused by undissolved particles during solvent film castings. The preparation of cellulose acetate fibers requires the polymer solution to flow unobstructed through spinneret c a p i l l a r i e s that have extremely s m a l l diameters. A n y undissolved particulate matter that would cause disruptions i n fiber extrusion cannot be tolerated. Since cellulose derived f r o m wood pulp contains im­ purities which do not f o r m acetate esters soluble i n c o m m e r c i a l cellulose acetate solvents ( e . g . , acetone), these impurities must be reduced to an acceptable l e v e l . Consequently, wood pulps used to manufacture cellulose acetate contain 94 to 99 per­ cent alpha-cellulose, the remainder being hemicelluloses. This α - c e l l u l o s e level is higher than that required for the preparation of rayon, cellulose nitrate, or cellulose ethers. Of course, the increased purity results i n an increased cost. Hence, it has long been a goal to develop means to reduce the wood pulp purity requirements for the c o m m e r c i a l preparation of cellulose ace­ tate. This is desirable for the acetate manufacturer, the pulp manufacturer, and the ecology since the higher purity comes at a sacrifice i n wood y i e l d and increased pulp mill effluent treat­ ment requirements. It is well known that insoluble residues obtained f r o m the dissolution of cellulose acetate in acetone are enriched i n hemi­ cellulose acetates (1-4). However, our p r o g r a m to investigate the use of lower purity wood pulps to prepare cellulose acetate by either modifying the pulps or the process required quanti96

8.

RUSSO AND SERAD

tative characterizations. isolate,

quantify,

fractions

Hence,

this study was undertaken

and characterize

to

both soluble and insoluble

of c e l l u l o s e a c e t a t e i n a c e t o n e .

F o u r g e n e r a l c l a s s e s of p u l p ,

representing

nations of w o o d f u r n i s h a n d p u l p i n g p r o c e s s , These included a softwood kraft, kraft,

97

Insoluble Cellulose Acetate Residues

and hardwood sulfite.

various

were

combi-

examined.

softwood sulfite,

hardwood

T h e p u r i t y of the v a r i o u s p u l p s ,

along with typical acetylation grade pulps, are given in Table

T A B L E I:

I.

P u r i t y of A c e t y l a t i o n G r a d e a n d " L o w P u r i t y " W o o d Pulps Alkali S u g a r A n a l y s i s (%)

Solubility

Glucose Mannose Xylose

Purity-RjQ%

" L o w Purity" Grades Softwood K r a f t

85. 6

6.1

8.3

84

Softwood Sulfite

91.2 83. 5

6.9 0.6

1.9

85

91.5

2.4

15.9 6.1

89 87

98. 1

1.4

0.5

96

97.5

0.3

1.2

97

Hardwood Kraft Hardwood Sulfite Typical Acetylation Grade Softwood Sulfite Hardwood Prehydrolyzed Kraft

Fractionation Acetone,

Sequence the c o n v e n t i o n a l s o l v e n t u s e d to

manufacture

c e l l u l o s e a c e t a t e f i b e r s a n d f i l m s , w a s u s e d to p r e p a r e starting solutions.

(wt/wt) a c e t o n e / w a t e r .

the s o l v e n t a n d the c e l l u l o s e a c e t a t e f l a k e . i n the f r a c t i o n a t i o n ( F i g u r e

A 6 g/100

c a l l e d the a c e t o n e / w a t e r s o l u b l e s

saved for analysis.

a

The

(AWS),

combined were

The residue was weighed and likewise c c s o l u t i o n i n 91/9

methylene

000

fresh

and u l t r a -

A p o r t i o n of the r e s i d u e w a s s u b s e q u e n t l y

p a r e d a s a 6 g/100

solution

1) w a s u l t r a c e n t r i f u g a t i o n a t 1 5 ,

centrifugation repeated under s i m i l a r conditions.

for analysis.

cc

T h e f i r s t step

A f t e r d e c a n t i n g o f f the s u p e r n a t e ,

a c e t o n e / w a t e r s o l u t i o n w a s a d d e d to t h e r e s i d u e s supernates,

95/5

T h e w a t e r i n c l u d e s that c o n t a i n e d i n both

o f the c e l l u l o s e a c e t a t e (bone d r y b a s i s ) w a s u s e d . r p m for four hours.

the

The exact solvent composition used was

saved

pre-

chloride/

S O L V E N T SPUN R A Y O N , MODIFIED C E L L U L O S E FIBERS

Acetone Water Soluble Acetone/ Water

Acetone Water Insoluble

Methyle Chloride/ Methanol

Methylene Chloride/ Methanol Soluble

Methylene C h l o r i d e / M e t h a n o l Insoluble

D r y residue P r e p a r e 6% (wt/vol) s o l u t i o n in methylene chloride/methanol U l t r a c e n t r i f u g a t i o n (151.000 r p m f o r 2 h r s ) Decant s u p e r n a t e A d d 150 c c of f r e s h M e C l / M e O r > Repeat steps 3 and 4 C o l l e c t l i q u i d , weigh s o l i d s

6 g m s of bone d r y f l a k e " p e r 100 cc U l t r a c e n t r i f u g a t i o n (1 5,000 r p m f o - 4 h r s ) Decant s u p c r n a t e A d d 150 cc of f r e s h a c e t o n e / w a t e r Repeat steps 2 and 3 C o l l e c t l i q u i d , weigh s o l i d s

2

Figure 1. Fractionation sequence

1.0

1.2

1.4

1.6

Intrinsic V i s c o s i t y

Figure 2. Relationship of cellulose acetate intrinsic viscosity and 6% solution viscosity

8.

RUSSO AND SERAD

methanol.

99

Insoluble Cellulose Acetate Residues

T h e s o l u t i o n w a s u l t r a c e n t r i f u g e d at 15, 000 r p m f o r

two h o u r s .

The supernate was decanted,

fresh methylene

i d e / m e t h a n o l a d d e d , a n d the c e n t r i f u g a t i o n r e p e a t e d . bined supernate, (MCMS),

was saved for analysis.

methylene and

designated methylene

chlor-

The

com-

chloride/methanol

The final residue,

chloride/methanol insolubles (MCMI),

soluble

designated

was weighed

saved.

Analytical

Procedures

A c e t y l V a l u e (defined as percent

combined acetic acid).

A five wt. percent

s o l u t i o n of the c e l l u l o s e a c e t a t e f l a k e

was

p r e p a r e d i n a 91/9

(wt/wt) m e t h y l e n e

solvent.

chloride/methanol

A f i l m a p p r o x i m a t e l y 0. 001 i n c h t h i c k w a s c a s t f r o m the s o l u t i o n a n d a l l o w e d to a i r d r y .

It w a s p l a c e d i n a c u r v e d f i l m h o l d e r ,

h e a t e d to r e m o v e r e s i d u a l w a t e r , spectrophotometer

and scanned i n an infrared

o v e r t h e 2 . 5 to 4 . 5 m i c r o n r a n g e .

The

ratio

of the O H a b s o r b e n c e at 2. 9 m i c r o n s a n d the C - H a b s o r b e n c e 2.4

microns was calculated.

T h i s r a t i o w a s c o r r e l a t e d to

at

acetyl

value by m e a n s of a c a l i b r a t i o n p r o c e d u r e u s i n g a k n o w n standard.

F o r a c e l l u l o s e a c e t a t e of 5 5 . 07 a c e t y l v a l u e ,

the

sample

s t a n d a r d d e v i a t i o n (ten a n a l y s e s ) w a s 0. 0 2 . Filtration Value.

M o i s t u r e c o n t e n t of c e l l u l o s e

samples was determined by oven drying.

A 6 g/100

o f t h e s a m p l e w a s p r e p a r e d u s i n g e i t h e r a 95/5 tone/water vent.

acetate

cc

solution

(wt b a s i s ) a c e -

s o l v e n t o r a 91/9 m e t h y l e n e c h l o r i d e / m e t h a n o l s o l -

T h i s w a s s h a k e n f o r t w o h o u r s to a s s u r e d i s s o l u t i o n .

The

solution was filtered through 30-ply K i m p a k and Canton flannel at 200 p s i g n i t r o g e n p r e s s u r e plugged.

u n t i l the f i l t e r w a s

completely

A f i l t r a t i o n v a l u e w a s d e f i n e d as the g r a m s o f d r y

c e l l u l o s e a c e t a t e p e r c m ^ of f i l t e r a r e a w h i c h c a n be before blockage

filtered

occurs.

Cation Analysis.

Calcium,

magnesium,

and sodium were

determined by atomic absorption using conventional Intrinsic Viscosity.

techniques.

I n t r i n s i c v i s c o s i t y of c e l l u l o s e

acetate

f l a k e s w a s d e t e r m i n e d b y m e a s u r i n g the v i s c o s i t y of a 6 g/100

cc

s o l u t i o n of the f l a k e i n a c e t o n e / w a t e r a n d u s i n g the c o r r e l a t i o n of F i g u r e 2.

Solution viscosity was determined using a

d i r e c t r e a d o u t v i s c o m e t e r ( M o d e l 7-006) at

25°C.

Nametre

S O L V E N T SPUN R A Y O N , MODIFIED C E L L U L O S E FD3ERS

100

C a r b o x y l Content. A n i o n exchange procedure was used f o r the d e t e r m i n a t i o n of the c a r b o x y l c o n t e n t . Flake was treated w i t h H C 1 to c o n v e r t t h e c a r b o x y l g r o u p s to t h e i r a c i d f o r m . C a r b o x y l protons w e r e then l i b e r a t e d by exchange w i t h c a l c i u m acetate and subsequently titrated. S u g a r A n a l y s i s . Q u a n t i f i c a t i o n of the c a r b o h y d r a t e c o n tent w a s a c c o m p l i s h e d b y gas c h r o m a t o g r a p h y of the t r i m e t h y l s i l a t e d s u g a r m o n o m e r s w h i c h w e r e o b t a i n e d b y h y d r o l y s i s of t h e c e l l u l o s e a c e t a t e o r w o o d p u l p (_5, 6 ) . S p e c i a l p r o c e d u r e s w e r e d e v e l o p e d to c h a r a c t e r i z e 7 0 - 1 0 0 m g s a m p l e s . A n accurate d r y sample weight and acetyl value w e r e f i r s t obtained and the f o l l o w i n g h y d r o l y s i s p r o c e d u r e u s e d : 1.

T h e s a m p l e w a s d r i e d at 15-20

f o r about two h o u r s ,

m m pressure and 65°C

cooled i n a dessicator

to r o o m

temperature,

then weighed. 2.

O n e m l o f 77 p e r c e n t s u l f u r i c a c i d w a s a d d e d f o r

v a t i o n a n d s h a k e n f o r o n e to t h r e e h o u r s .

sol-

Extremely discolored

samples were discarded. 3.

T h e s o l v a t e d s a m p l e w a s d i l u t e d w i t h 25 m l o f d i s t i l l e d

w a t e r a n d 5 m l of a 2. 0 m g / m l m y o - i n o s i t o l s o l u t i o n ( r e f e r e n c e sugar). 4.

The solution was then r e f l u x e d f o r four h o u r s .

N e x t , a n e u t r a l i z a t i o n step c o n s i s t e d of p l a c i n g . a 5 m l a l i q u o t of the h y d r o l y z e d s a m p l e i n a c e n t r i f u g e t u b e , a d d i n g 0. 7 g of b a r i u m c a r b o n a t e p o w d e r , a n d p l a c i n g the c o n t e n t s i n t o a v a c u u m o v e n at 6 5 ° C . A f t e r n e u t r a l i z a t i o n , c o i n c i d e n t w i t h the c e s s a t i o n of b u b b l e s , the m i x t u r e w a s c e n t r i f u g e d (2, 000 r p m f o r 10-15 m i n u t e s ) . T h e r e s u l t a n t s u p e r n a t e w a s p l a c e d i n t o a 10 m l p e a r - s h a p e d f l a s k a n d e v a p o r a t e d to d r y n e s s u n d e r r e d u c e d p r e s s u r e w i t h a r o t a r y e v a p o r a t o r (bath t e m p e r a t u r e 2 5 - 3 5 ° C ) . Trimethylsilation was accomplished by adding a 1 m l a m p u l e of T R I - S I L ( P i e r c e C h e m i c a l C o m p a n y ) i n t o the f l a s k a n d p e r m i t t i n g i t to r e a c t f o r o n e h o u r a t 5 0 ° C . The reaction vial w a s c e n t r i f u g e d to s e t t l e t h e p r e c i p i t a t e . A 1 - 2 m i c r o n s a m p l e of supernate w a s u s e d f o r gas c h r o m a t o g r a p h i c a n a l y s i s . Gas chromatograph conditions were: 1. A 4 0 - f o o t , 1/8 i n c h O . D . s t a i n l e s s s t e e l t u b e p a c k e d w i t h D e x s i l 3 0 0 G C (5 p e r c e n t ) o n C h r o m a s o r b W . 2.

Injection p o r t t e m p e r a t u r e of 2 5 0 ° C ,

p e r a t u r e of

detector

tem-

280°C.

3. P r o g r a m s e q u e n c e w a s to i n j e c t a t 1 6 0 ° C w i t h a 12 m i n u t e h o l d t i m e , f o l l o w e d b y a 1 ° C / m i n u t e p r o g r a m r a t e to 250°C.

8. RUSSO AND SERAD

Insoluble Cellulose Acetate Residues

R e s p o n s e f a c t o r s w e r e d e t e r m i n e d f o r a v a r i e t y of s u g a r s , shown i n T a b l e II· T A B L E II: R e s p o n s e F a c t o r s f o r G a s

as

Chromatography RF

Sugar α Glucose 3 Glucose Y Glucose

1.23 1. 22 1.22

ot, β X y l o s e