Cellulosic Adhesives - ACS Symposium Series (ACS Publications)

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Chapter 21 Cellulosic Adhesives David N.-S. H o n Wood Chemistry Laboratory Department of Forestry Clemson University Clemson, SC 29634

Cellulose is an old polymer with new industrial applications. The derivatization of cellulose has opened up tremendous production and marketing possibilities for the adhesives industry. Various important adhesives have been derived from cellulose ethers. The structure and molecular size of cellulose and their influence on swelling and solubility are important considerations in the preparation of cellulose derivatives for adhesive applications. Modern cellulosic adhesives derived from grafted copolymers and polyblends are also proving very useful. Since t h e energy a n d c h e m i c a l r a w m a t e r i a l c r u n c h o f 1973, m a n y a l t e r n a t i v e s to p e t r o l e u m a n d other i m p o r t e d c h e m i c a l stocks have been e x p l o r e d .

What

are t h e needs o f t h e adhesive i n d u s t r y ? W h a t other sources c a n b e t a p p e d ? C e l l u l o s e , a p o l y s a c c h a r i d e p r o d u c e d i n great a b u n d a n c e i n n a t u r e , i s a p r i m e c a n d i d a t e as a r a w m a t e r i a l for use i n adhesives because o f i t s a v a i l a b i l i t y a n d r e l a t i v e l y l o w cost a n d because o f i t s readiness t o b e converted i n t o a v a r i e t y of useful adhesive p r o d u c t s .

I n essence, cellulose i s a wonder m a t e r i a l w i t h a

p r o m i s i n g future ( i ) . Structure and Molecular Weight C e l l u l o s e i s a polydisperse, l i n e a r s y n d i o t a c t i c p o l y m e r . Its basic m o n o m e l i c u n i t is D-glucose, w h i c h l i n k s successively t h r o u g h a g l u c o s i d i c b o n d i n the β c o n f i g u r a t i o n between c a r b o n 1 a n d c a r b o n 4 o f adjacent u n i t s t o f o r m l o n g c h a i n l,4-/?-glucans. F i g u r e 1 shows a s t r u c t u r a l d i a g r a m o f a p o r t i o n o f a cellulose c h a i n . Because o f t h e /^-configuration o f t h e i n t e r m o n o m e r l i n k s , the glucose u n i t s effectively alternate u p a n d d o w n i n t h e c h a i n . Hence, scientists consider cellobiose as the r e p e a t i n g u n i t o f cellulose, o n w h i c h a s y n d i o t a c t i c c o n f i g u r a t i o n o f the m a c r o m o l e c u l e is f o r m e d . T h e size o f the cellulose molecule 0097-6156/89/0385-0289$06.00/0 « 1989 American Chemical Society

In Adhesives from Renewable Resources; Hemingway, Richard W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


290

ADHESIVES F R O M RENEWABLE RESOURCES

.

HoCOH

,

QH

ι

.

H

2

C

0

H

«

F i g u r e 1. T h e p a r t i a l m o l e c u l a r structure of cellulose.


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Cellulosic Adhesives

o c c u r r i n g i n n a t u r e , i n d i c a t e d b y i t s degree o f p o l y m e r i z a t i o n ( D P ) or c h a i n l e n g t h , is dependent h e a v i l y o n i t s source ( T a b l e I). I n some cases, the D P m a y exceed

10,000.

T a b l e I. Degree o f P o l y m e r i z a t i o n o f C e l l u l o s e f r o m


V a r i o u s Sources Source A. xylinum Bagasse B a s t fibers C o t t o n fibers C o t t o n linter F l a x fibers P u l p cellulose (bleached) R a m i e fibers Rice straw Valonia W o o d fibers

Degree o f P o l y m e r i z a t i o n 2,000 - 3,700 700 1,0008,000 1,0007,000 500 9,000-

900 5,000 14,000 5,000 8,000 2,100 11,000

700 - 800 25,000 - 27,000 8,000 - 9,000

C e l l u l o s e never occurs i n pure f o r m ; i n softwood a n d h a r d w o o d , i t c o n s t i tutes a b o u t 40 to 5 0 % of the weight, i n flax 70 to 8 5 % , whereas, cottonseed h a i r s , w h i c h are the purest source, c o n t a i n more t h a n 9 0 % ( T a b l e I I ) . I n these m a t e r i a l s , cellulose macromolecules serve as a s t r u c t u r a l m a t e r i a l w i t h i n the c o m p l e x architecture of the p l a n t cell w a l l s . C o m m e r c i a l p r o d u c t i o n o f cellulose is concentrated o n the h i g h l y pure sources like c o t t o n or easily harvested sources like w o o d . Adhesives and Adhesion A d h e s i v e s h o l d two surfaces together b y developing i n t e r n a l or cohesive s t r e n g t h . In order to h o l d surfaces together, adhesives must be a p p l i e d to the substrate i n a fluid f o r m to wet, spread, a n d penetrate the surface c o m p l e t e l y a n d leave no voids. Hence, adhesives m u s t be low i n viscosity at the t i m e o f a p p l i c a t i o n . In order t o p r o v i d e s t r o n g cohesive s t r e n g t h , the adhesive m u s t be set o r solidified by either c o o l i n g , c r o s s l i n k i n g r e a c t i o n , or e v a p o r a t i o n o f solvents, d e p e n d i n g o n whether the adhesive is h o t m e l t ( t h e r m o p l a s t i c ) , thermoset, or solvent-based. T h i s also i m p l i e s t h a t i n order to p r o v i d e enough b o n d i n g s t r e n g t h , a n adhesive m u s t be a p o l y m e r w i t h h i g h m o l e c u l a r weight. A s a r u l e of t h u m b , the higher the m o l e c u l a r weight, the higher the b o n d i n g power. H o w e v e r , for t h e r m o p l a s t i c s , as for cellulosic adhesives, the higher the m o l e c u l a r weight, the more difficult i t becomes t o process the adhesives. Hence, the m o l e c u l a r weight range of t h i s t y p e of p o l y m e r is carefully chosen to represent the best c o m p r o m i s e


292


between p r o c e s s a b i l i t y a n d final properties. F o r adhesive a p p l i c a t i o n , m a n y a d hesives a n d h o t m e l t s have a lower m o l e c u l a r weight. T h e adhesives w i t h lower m o l e c u l a r weight have better s o l u b i l i t y i n solvents, a w i d e r range of c o m p a t a b i l i t i e s w i t h other resins a n d p l a s t i c i z e r s , a n d lower m e l t i n g or softening p o i n t s .


T a b l e I I . N a t u r a l Sources o f C e l l u l o s e Source

Cellulose Content (%)

Bagasse Bamboo Cotton Flax Hemp Jute Kapok Ramie Straw Wood

35 409070-

45 55 99

75 7 5 - 80 6 0 - 65 7 0 - 75 7 0 - 75 4 0 - 50 4 0 - 50

C e l l u l o s e is a p o l y m e r t h a t meets these requirements as a n adhesive. H o w ever, due t o i t s s e m i c r y s t a l l i n e s t r u c t u r e , h i g h l y h y d r o g e n - b o n d e d cellulose c a n not be dissolved easily i n c o n v e n t i o n a l solvents, a n d i t c a n n o t be m e l t e d before i t b u r n s . T h i s is because the a t t r a c t i v e forces a n d s t a b i l i t y o f c r y s t a l structures are greater t h a n those t h a t result f r o m i n t e r a c t i o n between p o l y m e r a n d solvent. Hence, cellulose i t s e l f is not s u i t a b l e for use as a n adhesive. T h e same c a n be s a i d of regenerated cellulose. I n order to m a k e cellulose soluble or m e l t a b l e , the h y d r o g e n b o n d s m u s t be broken (i.e., cellulose molecules m u s t be m o r e flexible a n d possess h i g h entropy, so t h a t they can be separated e a s i l y ) . C e l l u l o s e is a s e m i c r y s t a l l i n e p o l y m e r . I n the c r y s t a l l i n e r e g i o n , the l i n e a r p o l y m e r i c chains are h e l d together b y r e l a t i v e l y s t r o n g i n t e r m o l e c u l a r h y d r o gen b o n d s . D e r i v a t i v e s , g r a f t i n g , a n d p o l y b l e n d i n g of cellulose can reduce the s t r e n g t h o f i n t e r m o l e c u l a r b o n d s , m a k i n g cellulose soluble i n water a n d i n organic solvents. S o m e cellulose derivatives are also m e l t a b l e under heat. A s a m a t t e r of fact, cellulose derivatives are a very g o o d class o f t h e r m o p l a s t i c a d hesives. T h e y c a n be used i n the f o r m of s o l u t i o n s , dispersions i n w a t e r , or solids. T h e i r properties are i n m a n y cases influenced b y factors such as m o l e c u l a r weight a n d degree o f c h e m i c a l s u b s t i t u t i o n a n d , i n c o p o l y m e r s , the m o n o m e r r a t i o ; i n graft c o p o l y m e r s , the degree of g r a f t i n g .


21.

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293

Swelling and Dissolution of Cellulose


A p o l y m e r dissolves i n two stages. F i r s t , solvent molecules diffuse i n t o the p o l y m e r , s w e l l i n g i t t o a gel state. T h e n , the gel g r a d u a l l y disintegrates, a n d the molecules diffuse i n t o the s o l v e n t - r i c h regions. I n essence, s o l u b i l i t y a n d s w e l l i n g of a p o l y m e r i n a solvent d e p e n d u p o n c o m p e t i t i v e i n t e r m o l e c u l a r a t t r a c t i o n between solvent a n d p o l y m e r molecules versus adjacent p o l y m e r molecules. If the interactive force between the p o l y m e r molecule a n d solvent molecule is stronger t h a n the p o l y m e r - p o l y m e r secondary force, the p o l y m e r w i l l i n i t i a l l y swell a n d dissolve. A l t h o u g h cellulose molecules h o l d together s t r o n g l y v i a i n t e r m o l e c u l a r a n d i n t r a m o l e c u l a r h y d r o g e n b o n d i n g s , some l i q u i d s c a n penetrate c e l l u lose c o m p l e t e l y a n d thus w i l l cause i n t r a c r y s t a l l i n e as well as i n t e r c r y s t a l l i n e s w e l l i n g . W a t e r is a g o o d s w e l l i n g agent for cellulose; however, s w e l l i n g o n l y occurs either i n i n t e r c r y s t a l l i n e regions or o n the surfaces o f the c r y s t a l l i t e s a n d the gross s t r u c t u r e . W a t e r does not penetrate the c r y s t a l l i n e r e g i o n . T h e r e are m a n y reagents, such as a l k a l i m e t a l h y d r o x i d e s , salts i n s t r o n g l y a l k a l i n e s o l u t i o n , some i n o r g a n i c acids a n d salts, a n d c e r t a i n amines a n d related c o m p o u n d s t h a t c a n cause i n t r a c r y s t a l l i n e s w e l l i n g of cellulose. I n order to m a k e cellulose soluble or m e l t a b l e , c h e m i c a l reagents m u s t be i n t r o d u c e d i n t o the cellulose t o destroy the i n t e r m o l e c u l a r h y d r o g e n b o n d i n g . O n c e the o r i g i n a l h y d r o g e n b o n d s have been b r o k e n a n d i n t r a m i c e l l a r s w e l l i n g is achieved, the cellulose h y d r o x y Is are capable o f r e a c t i n g like o r d i n a r y a l i p h a t i c h y d r o x y l groups. Hence, s o d i u m h y d r o x i d e is a p o p u l a r l y used agent t o swell cellulose p r i o r t o s u b s t i t u t i o n reactions. G e n e r a l l y , the s u b s t i t u t i o n of h y d r o x y l groups i n cellulose b y a b u l k y reagent allows s e p a r a t i o n of cellulose chains so t h a t a solvent m a y penetrate a n d solvate the molecules. T h i s a c t i o n has been f o u n d useful i n b r i n g i n g cellulose i n t o s o l u t i o n or l o w e r i n g i t s m e l t i n g p o i n t . C o n s e q u e n t l y , based o n t h i s p r i n c i p l e , m a n y adhesive a n d c o a t i n g m a t e r i a l s have been prepared f r o m cellulose by different s u b s t i t u t i o n reactions. T w o m a j o r groups are cellulose esters (2-4) a n d cellulose ethers (2-4). T h e s u b s t i t u t i o n of h y d r o g e n a n d h y d r o x y l groups w i t h a p o l y m e r b y a graft c o p o l y m e r i z a t i o n r e a c t i o n has also been f o u n d useful i n b r i n g i n g cellulose i n t o s o l u t i o n a n d l o w e r i n g its m e l t i n g p o i n t . T h e g r a f t i n g of a c r y l o n i t r i l e w i t h subsequent h y d r o l y i s produces a water-soluble cellulose-acrylic a c i d g r a f t - c o p o l y m e r . O c c a s i o n a l l y , the b l e n d i n g of cellulose w i t h a c o m p a t i b l e p o l y m e r has also been f o u n d useful i n lowering the softening p o i n t of cellulose or i n c r e a s i n g i t s a p p l i c a b i l i t y i n use as a n adhesive. F o r instance, the b l e n d i n g of cellulose w i t h n a t u r a l r u b b e r to m a k e s u r g i c a l adhesives has been c o m m e r c i a l l y successful. Hence, the adhesives i n d u s t r y has a w i d e v a r i e t y of cellulosic p r o d u c t s f r o m w h i c h t o select a n d create a n end p r o d u c t w i t h v a r y i n g adhesive properties. Cellulose Derivatives: Esters and Ethers Since cellulose is a p o l y h y d r o x y l a l c o h o l , i t can undergo esterification a n d etheri f i c a t i o n m o d i f i c a t i o n s . T h e properties o f the derivatives depend h e a v i l y o n the


294



t y p e , d i s t r i b u t i o n , a n d u n i f o r m i t y o f the s u b s t i t u e n t groups. F o r each β-0-Dg l u c o p y r a n o s y l r i n g , there are three h y d r o x y l groups available for the n u c l e o p h i l i c s u b s t i t u t i o n r e a c t i o n . R e a c t i o n s at these sites c a n o c c u r either o n a one-to-one basis or w i t h f o r m a t i o n of side chains d e p e n d i n g o n choice of reagent employed t o m o d i f y the cellulose. T h e t e r m "degree of s u b s t i t u t i o n " ( D S ) is used t o i d e n t i f y the average n u m b e r of sites reacted per r i n g . T h e m a x i m u m value is 3, c o r r e s p o n d i n g to the n u m b e r of h y d r o x y Is available for r e a c t i o n . W h e n s i d e - c h a i n f o r m a t i o n is possible, the t e r m " m o l a r s u b s t i t u t i o n " ( M S ) is used t o denote the l e n g t h of side c h a i n , a n d the value c a n exceed 3. C e l l u l o s e E s t e r s . C e l l u l o s e contains p r i m a r y a n d secondary h y d r o x y l groups. Hence, cellulose esters c a n be m a d e w i t h a l l i n o r g a n i c a n d o r g a n i c acids. T r a d i t i o n a l l y , cellulose esters are m a d e b y a c o n t r o l l e d a c i d - c a t a l y z e d r e a c t i o n be tween a n a c i d or a c i d a n h y d r i d e a n d the h y d r o x y l groups of cellulose. T h e r e a c t i o n requires the absence of water for c o m p l e t i o n because i t is a reversible r e a c t i o n . T h e general r e a c t i o n scheme c a n be i l l u s t r a t e d as s h o w n i n Scheme 1.

Cellulose + A c i d

c a t

-^

Cellulose Ester +

s t

H 0 2

Scheme 1

H i s t o r i c a l l y , the first t h e r m o p l a s t i c s y n t h e t i c adhesive was the cellulose i n organic ester, cellulose n i t r a t e . Schonbeir ( 5 ) is generally regarded as h a v i n g discovered cellulose n i t r a t e i n 1845 b y n i t r a t i n g cellulose w i t h a m i x t u r e of n i t r i c a n d s u l f u r i c acids. M o s t early w o r k was a i m e d at u t i l i z i n g cellulose n i t r a t e i n explosives, b u t l a t e r , i t f o u n d use i n p l a s t i c , adhesives, a n d c o a t i n g a p p l i c a t i o n s . T o d a y , i t is s t i l l one of the most i m p o r t a n t adhesives. C e l l u l o s e n i t r a t e can be prepared b y t r e a t i n g h i g h l y p u r i f i e d cellulose w i t h a m i x t u r e of n i t r i c a n d s u l f u r i c a c i d (6). T h e r e a c t i o n scheme m a y be represented as s h o w n i n Scheme 2.

R-OH + H N 0

3

H

^

4

R-O-NO2 +

H 0 2

Scheme 2

F o r adhesive a p p l i c a t i o n , the a c i d m i x t u r e is m a d e u p of n i t r i c a c i d ( 2 5 % ) , sulfuric a c i d ( 5 5 % ) , a n d water ( 2 0 % ) . T h e f u n c t i o n of s u l f u r i c a c i d is to remove the water of r e a c t i o n so t h a t n i t r a t i o n m a y be c a r r i e d to the desired degree m o r e readily. T h e various p r o d u c t s m a y be characterized by n i t r o g e n content, w h i c h corresponds t o the degree of s u b s t i t u t i o n . T h e n i t r o g e n content also de termines the s o l u b i l i t y of cellulose n i t r a t e . W i t h 11.8 to 1 2 . 2 % n i t r o g e n content,


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cellulose n i t r a t e w i l l be soluble i n esters, ketones, e t h e r - a l c o h o l m i x t u r e s , a n d g l y c o l ethers. It w i l l also have excellent a r o m a t i c h y d r o c a r b o n tolerance b u t less tolerance for a l i p h a t i c h y d r o c a r b o n s . W i t h 11.3 t o 1 1 . 7 % n i t r o g e n content, it w i l l have a p p r o x i m a t e l y the same solvency; however, i t w i l l also tolerate h i g h percentages of l o w - m o l e c u l a r weight a n h y d r o u s alcohols. W i t h 10.9 t o 1 1 . 2 % n i t r o g e n content, i t c a n dissolve i n a l c o h o l . I n p r a c t i c e , the n i t r a t i o n o f c e l l u lose is allowed to proceed o n l y far enough t o give a n average d i n i t r a t e (nitrogen content r a n g i n g f r o m 10.7 to 1 2 . 2 % ) , since the t r i n i t r a t e is explosive. Because of i t s w i d e range o f s o l u b i l i t y , cellulose n i t r a t e has become a p o p u l a r " h o u s e h o l d " cement. It is a waterproof, clear, flexible adhesive for use w i t h p l a s t i c s , c l o t h , w o o d , p a p e r , c h i n a , glass, m e t a l , a n d leather. A m e d i u m or h i g h viscosity t y p e cellulose n i t r a t e is generally used w i t h solvents t h a t are f a i r l y r a p i d i n e v a p o r a t i o n r a t e . A p l a s t i c i z e r is used t o give flexibility. S e v e r a l c o m m e r c i a l grades of cellulose n i t r a t e w i t h characteristic properties are l i s t e d i n T a b l e I I I . C e l l u l o s e acetate is u n i v e r s a l l y recognized as the m o s t i m p o r t a n t o r g a n i c ester of cellulose. It is w i d e l y used i n plastics a n d textiles b u t finds o n l y l i m i t e d a p p l i c a t i o n i n adhesives a n d coatings. It c a n be prepared by r e a c t i n g h i g h p u r i t y cellulose w i t h acetic a n h y d r i d e , u t i l i z i n g acetic a c i d as the solvent a n d sulfuric a c i d as a c a t a l y s t (3) as s h o w n i n Scheme 3.

R-OH + ( C H C O ) 0 3

2

H

^

A

R-O-COCH3 + CH3COOH

Scheme 3

T h e degree o f n i t r a t i o n o f cellulose n i t r a t e c a n be r e g u l a t e d b y choice of r e a c t i o n c o n d i t i o n s ; however, the degree o f a c e t y l a t i o n is not r e g u l a t e d u n t i l t r i acetate is o b t a i n e d . T h i s is because, i f a c e t y l a t i o n is i n t e r r u p t e d before complete esterification, a heterogeneous m i x t u r e of the triacetate a n d unreacted cellulose w i l l result. T h e diacetate is n o r m a l l y o b t a i n e d by p a r t i a l h y d r o l y s i s of the t r i acetate so t h a t p r o d u c t s w i t h various degrees o f esterification are o b t a i n a b l e . A s a n adhesive, i t is used i n s o l u t i o n w i t h o u t a d d i t i v e s or fillers. It finds use i n b u i l d i n g models a n d j o i n i n g p l a s t i c s , leather, w o o d , a n d c h i n a . P l a s t i c i z e d cellulose acetate films have f o u n d use for p r o t e c t i n g a r c h i v a l d o c u m e n t s . A l a m i n a t e of the d o c u m e n t a n d a sheet of cellulose acetate film are m a d e u n d e r heat a n d pressure. T h e cellulose acids act not o n l y as a n adhesive b u t also i n g i v i n g p r o t e c t i o n against s o i l i n g , a g i n g , a n d m e c h a n i c a l abuse. C e l l u l o s e acetate films can a l l be a c t i v a t e d by b r u s h i n g t h e m w i t h acetone a n d t h e n l a m i n a t e d w i t h d o c u m e n t s w i t h o u t u s i n g heat a n d pressure. C e l l u l o s e esters of b u t y r i c a n d p r o p i o n i c acids have l i m i t e d adhesive use. However, cellulose caprate, h a v i n g a refractive i n d e x near t h a t of glass a n d g o o d resistance t o p h o t o c h e m i c a l change, is a useful h o t m e l t o p t i c a l cement for the m a n u f a c t u r e o f c o m p o u n d lenses.


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T a b l e I I I . C h a r a c t e r i s t i c s of C o m m e r c i a l C e l l u l o s e N i t r a t e A d h e s i v e s Bond Adhesives C o .

Manufacturer/Supplier Commercial name

Bond Adhesives

Co.

G. C. Electronics

Bond

Bond

Electronic

9164

5275

C e m e n t 34-2S

S p e c i a l t u b e for V e r s a t i l e , t o u g h W a t e r p r o o f , vibration f i l m s , resists controlled resistant b o n d water dispensing

Features

Substrates

1. M i n e r a l 2. G l a s s

Wood Leather

3. M i n e r a l 4. P l a s t i c

Plastic Glass Metal

5. Characteristics 1. C o l o r

Clear

Clear

Metal Paper Textile

Clear-light straw

2. W e i g h t per g a l l o n 7.30

7.20

7.90

3. S o l i d s content, % 4. Solvent

17.0 Tin

23.0 BAc, EAc, Tin

5. 6. 7. 8.

Flammable

25.0 Ac Flammable

>6 m o / R T T h i n syrup

>2 y r / R T M e d i u m syrup

24 h r / R T Brush, dip,

30 m i n 24 h r / R T B r u s h , knife,

tube

tube

(lb/gal)

F l a s h point (°C) Storage c o n d i t i o n s Form C u r e conditions

9. A p p l i c a t i o n procedure

-9 >2 y r / R T Liquid 10-15 m i n / R T Brush


21.

297


HON

T a b l e I I I . C h a r a c t e r i s t i c s of C o m m e r c i a l C e l l u l o s e N i t r a t e A d h e s i v e s


(continued)

Manufacturer/Supplier

G. C. Electronics

G. C. Electronics

Service Plastic C e m e n t 10-324 C e m e n t 10-302

Commercial name

General P u r p o s e 45-2

Features

Universal type, Waterproof, quick d r y i n g waterproof, w i l l not become quick d r y i n g brittle

Substrates

G. C. Electronics

Waterproof, quick d r y i n g , strong, hard, resists v i b r a t i o n

1. P a p e r 2. L e a t h e r

Plastic Textile

Metal Plastic

3. M e t a l 4. G l a s s 5. C e r a m i c

Wood

Wood Paper Ceramic

Characteristics 1. C o l o r

Clear-light straw

2. W e i g h t per g a l l o n (lb/gal) 3. S o l i d s content, 4. Solvent 5. F l a s h p o i n t 6. Storage 7. F o r m

%

(°C)

conditions

Clear-light

Clear-light

straw

straw

7.90 7.90 30.0 23.0 23.0 BAc, EAc, T i n BAc, EAc, T i n BAc, EAc, Tin

7.90

-9 >2 y r / R T

-9 >2 y r / R T

-9 >2 y r / R T

Liquid

Liquid

Liquid

8. C u r e c o n d i t i o n s

10-15 m i n / R T

10-15 m i n / R T

10-15 m i n / R T

9. A p p l i c a t i o n procedure

Brush

Brush

Brush



298


Interestingly, m i x t u r e s of esters of cellulose such as the a c e t a t e / b u t y r a t e a n d a c e t a t e / p r o p i o n a t e are i n m a n y ways s u p e r i o r t o the s t r a i g h t acetate i n h a v i n g , for instance, lower water a b s o r p t i o n a n d greater flexibility. T h e y have f o u n d use i n adhesive a p p l i c a t i o n . C e l l u l o s e a c e t a t e - b u t y r a t e is p r e p a r e d by u s i n g a m i x t u r e of acetic a n h y d r i d e a n d b u t y r i c a n h y d r i c w i t h s u l f u r i c a c i d as c a t a l y s t , a n d t h e n the p r o d u c t is s l i g h t l y h y d r o l y z e d . D e p e n d i n g o n the r e a c t i o n c o n d i t i o n s , various p r o d u c t s m a y be o b t a i n e d . C e l l u l o s e a c e t a t e - b u t y r a t e c a n be used i n h o t m e l t adhesives or dissolved i n ketone-ester solvent m i x t u r e s . It has been used i n the m a n u f a c t u r e of safety glass. T h e c o m p o s i t i o n of a t y p i c a l c o m m e r c i a l grade of cellulose a c e t a t e - b u t y r a t e for adhesive a p p l i c a t i o n is s h o w n in Table I V .

T a b l e I V . P r o p e r t i e s of T y p i c a l C o m m e r c i a l G r a d e s of C e l l u l o s e A c e t a t e - B u t y r a t e for A d h e s i v e A p p l i c a t i o n Acetyl Content

Butyryl Content

Hydroxy Content

(%) 17 26 37

(%)

(%) 29.5 20.5 13.0

1.0 2.5 2.0

Degree of E s t e r i f i c a t i o n ( D S ) Acetyl Butyrate Hydroxy 2.1 1.4 0.95

0.7 1.1 1.65

0.2 0.5 0.4

A d d i t i o n a l i n f o r m a t i o n o n p h y s i c a l a n d c h e m i c a l properties of cellulose esters is s u m m a r i z e d i n T a b l e V . Cellulose Ethers.

C e l l u l o s e ethers are f o r m e d w h e n cellulose, i n the pres

ence of a l k a l i or as a l k a l i cellulose, is treated w i t h a l k y l or a r y l a l k y l h a l i d e s . T w o types of r e a c t i o n are employed i n the p r e p a r a t i o n of cellulose ethers. T h e most c o m m o n is n u c l e o p h i l i c s u b s t i t u t i o n . M e t h y l a t i o n of a l k a l i cellulose w i t h a m e t h y l h a l i d e is a n e x a m p l e of t h i s t y p e . T h e other t y p e of etherification re a c t i o n is M i c h a e l a d d i t i o n . T h i s r e a c t i o n proceeds b y way of a n a l k a l i - c a t a l y z e d a d d i t i o n of a n a c t i v a t e d v i n y l group t o the cellulose. T h e r e a c t i o n of aery Ion i t r i l e w i t h a l k a l i cellulose is a t y p i c a l e x a m p l e . T h e general r e a c t i o n is o u t l i n e d i n Scheme 4. R - O + C H = C H - C N —•> R - 0 - C H - C H - C N R - 0 - C H - C ^ H - C N + H 0 — • R - 0 - C H - C H - C N + ΟΗ$ e

2

2

e

2

2

2

2

Scheme 4

C e l l u l o s e ethers have f o u n d p o p u l a r a p p l i c a t i o n s due to t h e i r s o l u b i l i t y c h a r acteristics. T h e i n t r o d u c t i o n of a s m a l l n u m b e r of a l k y l (e.g., m e t h y l or e t h y l )


21.

299


HON

T a b l e V . P h y s i c a l a n d C h e m i c a l P r o p e r t i e s of C e l l u l o s e E s t e r s Cellulose Nitrocellulose

Acetate

Cellulose Acetate B u t y r a t e High Acetyl High Butyl

B u l k i n g value,


(gal/lb)

0.0704

0.0925

0.104

0.0965

Specific g r a v i t y

1.70

1.30

1.25

1.17

Tensile s t r e n g t h , [psi ( 1 - m i l film)]

9,000-16,000

8,500-11,000 2,500-6,500

10-50

4-55

40-60

155-220

235-255

240

165

Ketones, esters

Alcohols, aromatic hydrocarbons

Elongation, [psi ( 1 - m i l film)] Softening point, Solubility

0

F

E s t e r s , ketones, K e t o n e s ether alcohols, g l y c o l ethers

Resin

Good

Limited

Limited

Wide

Excellent

Limited

Limited

Wide

Resistance to: W e a k acids

Fair

Good

Good

Poor

S t r o n g acids

Poor

Poor

Poor

Poor

W e a k alkalies

Poor

Good

Poor

Poor

S t r o n g alkalies Water Sunlight Heat

Poor

Poor

Poor

Poor

Good Fair Fair

Good Good Very good

Good Excellent Very good

Good Excellent Fair

V i s c o s i t y range

Wide

Limited

Fair

Fair

Flammability

High

V e r y low

Low

Low

C o l o r of

Water white

Water white Water white Water white

Wide

Limited

compatibility Plasticizer compatibility

film

Use of adhesives

Fair

Wide



300


groups i n t o the cellulose molecule sufficiently opens u p the s t r u c t u r e to p e r m i t s o l u b i l i t y i n aqueous s o d i u m h y d r o x i d e . A s s u b s t i t u t i o n increases, the p r o d u c t s become soluble i n decreasing concentrations o f a l k a l i , a n d at l e n g t h , they are soluble i n water. A s the n u m b e r o f a l k o x y groups increases, the p r o d u c t s be come less soluble i n water a n d m o r e soluble i n p o l a r o r g a n i c solvents. A t higher degrees of s u b s t i t u t i o n , s o l u b i l i t y i n p o l a r o r g a n i c solvents declines, whereas, s o l u b i l i t y i n n o n p o l a r solvents increases. D u r i n g the past 20 years, cellulose ethers have progressed f r o m m a t e r i a l s o f l a r g e l y e x p e r i m e n t a l a n d d e v e l o p m e n t a l i m p o r t a n c e t o p r o d u c t s of considerable i n d u s t r i a l i m p o r t a n c e . T h e m o s t i m p o r t a n t p r o d u c t s t h a t have been used i n adhesive a p p l i c a t i o n are l i s t e d i n Table V I .

Table V I . Important Cellulose Ethers Used in Adhesive Applications Substance

Symbol

Carboxymethylcellulose Ethylcellulose Methylcellulose Hydroxyethylcellulose Hydroxypropylcellulose Ethylhydroxyethylcellulose Hydroxy butylmethylmethylcellulose Hydroxyethylmethylmethylcellulose

CMC EC MC H EC HPC EHEC HBMC HEMC

M e t h y l - a n d ethylcelluloses are prepared by r e a c t i n g p u r i f i e d w o o d p u l p or c o t t o n linters h a v i n g a h i g h α-cellulose content w i t h aqueous s o d i u m h y d r o x ide a n d t h e n w i t h m e t h y l chloride or e t h y l chloride a c c o r d i n g to the f o l l o w i n g scheme:

ROH + NaOH

—•

ROH-NaOH

R O H - N a O H (complex)

—•

R O N a + CH3CI —> RONa + CH CH C1 3

2

RONa +

H 0 2

ROCH3 +

—• ROCH CH 2

NaCl 3

+

NaCl

Scheme 5

H y d r o x y e t h y l c e l l u l o s e ( H E C ) a n d h y d r o x y p r o p y l c e l l u l o s e ( H P C ) are pre p a r e d by r e a c t i n g c o t t o n linter or w o o d p u l p w i t h aqueous s o d i u m h y d r o x i d e , a n d the r e s u l t i n g a l k a l i cellulose is reacted w i t h ethylene oxide a n d propylene oxide, respectively.


21.

HON


301

ROH + NaOH

—• RONa + H 0 2

A

RONa + C H - C H 2

2

—

R-(OCH -CH )-OH

—•

R-0-CH -CH(OH)-CH

2

2

A RONa + CH -CH - C H Downloaded by CHINESE UNIV OF HONG KONG on February 17, 2016 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch021

3

2

2

3

Scheme 6 T h e h y d r o x y l groups can also undergo r e a c t i o n w i t h ethylene o x i d e ; hence, two or m o r e p o l y ( o x y m e t h y l e n e ) u n i t s can f o r m p o l y m e r s as s h o w n i n Scheme 7.

R - 0 - C H - C H - O H 4- η CE ^CE 2

2

2

R-0-(CH -CH -0)„+i-H

—•

2

2

2

Scheme 7 L i k e w i s e , the secondary h y d r o x y l g r o u p i n the h y d r o x y p r o p y l g r o u p c a n u n d e r g o h y d r o x y p r o p y l a t i o n t o give a side c h a i n :

Λ R-0-CH -CH(OH)CH 2

+

3

CH -CH-CH 3

2

1 R-0-CH -CH-CH I 2

3

0-CH -CH(OH)-CH 2

3

Scheme 8 T h e r e a c t i o n o f a l k a l i cellulose w i t h a m i x t u r e o f e t h y l c h l o r i d e a n d e t h y lene oxide can p r o d u c e e t h y l h y d r o x y e t h y l c e l l u l o s e ( E H E C ) , as i l l u s t r a t e d i n Scheme 9.

/

0

R-C-θ + C H - C H C 1 + C H - C H 2

2

OCH CH OH 2

2

—-»

2

R OCH CH 2

3

Scheme 9


302


T h e r e a c t i o n of a l k a l i cellulose w i t h a m i x t u r e of m e t h y l chloride a n d p r o p y lene oxide c a n p r o d u c e h y d r o x y p r o p y l m e t h y l c e l l u l o s e ( H P M C ) . OCH

Ο R-O

e

3

+ CH3CI + C H 3 - C H - C H 2 — • R Ô-CH -CH(OH)-CH


2

3

Scheme 10

I n general, these groups of cellulose ethers have been used for t h e i r i n n a t e adhesive properties a n d t o p r o v i d e t h i c k e n i n g to adhesive f o r m u l a t i o n s . T h e y are used for p l y w o o d adhesives, i n d u s t r i a l adhesives, w a l l p a p e r paste, l i b r a r y paste, a n d l a t e x adhesives. F o r e x a m p l e , m e t h y l c e l l u l o s e is used i n some a d hesives as a n a d d i t i v e t o c o n t r o l viscosity, especially i n the heat-cure p h e n o l f o r m a l d e h y d e glues a n d other hot-pressing adhesives. H y d r o x y e t h y l c e l l u l o s e is used as a n ingredient i n p o l y v i n y l acetate emulsions, where i t acts as a thickener and protective colloid. C e l l u l o s e ethers have also been used i n the ceramic i n d u s t r y ( 7 ) . Since t h e i r appearance i n 1959, water-based cellulose ethers have replaced solvent-based adhesives. T h e adhesives used for ceramic tile are r e a d y - m i x e d p r o d u c t s based o n n a t u r a l or s y n t h e t i c r u b b e r , p o l y v i n y l acetate, a n d other resins, a n d they a l l c o n t a i n cellulose ethers of one k i n d or another (e.g. M C , E C , H P M C , H E M C , H E C ) . T h e s e cellulose ethers reduce water loss, m o d i f y the v i s c o s i t y of the m i x , a n d c a n p r o v i d e excellent adhesion for dry, very porous tiles. S o d i u m c a r b o x y l m e t h y l c e l l u l o s e ( N a - C M C ) is also a water-soluble a n i o n i c l i n e a r cellulose ether. It is prepared b y t r e a t i n g cellulose w i t h aqueous s o d i u m h y d r o x i d e followed b y reaction w i t h s o d i u m chloracetate as s h o w n i n Scheme 11.

ROH + NaOH + ClCH COONa 2

—•

R O C H C O O N a + NaCl + 2

H 0 2

Scheme 11

C M C has been w i d e l y used as a n o n s t a i n i n g w a l l p a p e r adhesive. It has also been used as a n adhesive i n the paper a n d t e x t i l e i n d u s t r i e s . C h a r a c t e r i s t i c s of C M C t h a t are i m p o r t a n t for this a p p l i c a t i o n are i t s ease of " s l i p / ' n o n s p o i l i n g p r o p e r t y , h i g h adhesive efficiency, a n d ease of m a k e u p . C M C has f o u n d use i n the ceramics i n d u s t r y where its a b i l i t y t o b i n d a n d s u s p e n d m a t e r i a l s d u r i n g various stages of m a n u f a c t u r e is i m p o r t a n t . It is used i n glazes for s a n i t a r y w a r e , s t r u c t u r a l t i l e , a n d dinner ware.


21.

HON


303

Graft C o p o l y m e r s of Cellulose C e l l u l o s e esters a n d cellulose ethers are p r e p a r e d based o n the s u b s t i t u t i o n of cellulose h y d r o x y l groups w i t h short c h a i n regents. C e l l u l o s e c a n also be modified by introduction of long chain polymer(s) onto its m a i n chain. T h e


p r o d u c t s are m o s t l y grafted c o p o l y m e r s , a n d i n some cases, block c a n also be m a d e .

copolymers

M a n y different m e t h o d s o f g r a f t i n g have been developed. B y far the greatest effort has been v i a free-radical v i n y l - p o l y m e r i z a t i o n routes. T h e general r e a c t i o n scheme is s h o w n below:

Initiation

In — •

In

0

In© + R H — • R

+ M

0

Propagation R M

0

—•

+ M

R R M

—•

R M

0

R

0

+ R

R

0

+ R M

R

0

+ RM+1

—+ 0

0

RM+1

4- n M — • 0

0

R(M)„+i

R-R

—+ —

R-MR R-RM+1

In = i n i t i a t o r , R H = cellulose, M =

monomer

Scheme 12

V i n y l m o n o m e r s t h a t c a n be grafted to cellulose to achieve adhesive p r o p e r ties are a c r y l i c a c i d , a e r y l o n i t r i l e , m e t h y l m e t h a c r y l a t e , a n d m a n y others. G r a f t c o p o l y m e r s of cellulose derivatives have also f o u n d use as adhesives. F o r e x a m ple, v i n y l a c e t a t e - g r a f t e d h y d r o x y e t h y l c e l l u l o s e c a n be used as a n adhesive for p a c k a g i n g a n d tile (

Cellulosic Adhesives - ACS Symposium Series (ACS Publications)

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