Adhesives from Renewable Resources - ACS Publications - American

Downloaded by NANYANG TECH UNIV LIB on November 5, 2014 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch023

Chapter 23 Starch- and Dextrin-Based Adhesives H a r r y M. K e n n e d y Industrial Section Technical Service Department Grain Processing Corporation 1600 Oregon Street Muscatine, IA 52761

Despite present trends toward use of synthetic polymers developed over the last 10 or 20 years, starches are still being widely used as an adhesive in such applications as the production of paper and paperboard products, warp sizing, and bonding charcoal briquettes. Because of a unique combination of properties and low cost, these adhesives are almost impossible to exclude from many applications, especially those involving the use of hot paste (size) for anchoring fibers. For starch molecules to act as an adhesive, they must be chemically or thermally hydrated. Then, their adhesive character is developed and modified in different ways by chemicals or other additives for different end uses. As renewable resources that are both economical and reliable, starch and dextrin are likely to continue to be significant factors in the adhesive market for many years. I n these m o d e r n , " h i g h t e c h " t i m e s , starch-based adhesives are often relegated to the category o f "other adhesives," w h i l e s y n t h e t i c p o l y m e r s are g a i n i n g h i g h v i s i b i l i t y w i t h their a m a z i n g c a p a c i t y t o b o n d a vast a r r a y o f substrates t h a t enable us t o l a n d m e n o n t h e m o o n a n d even repair l i v i n g tissue a n d bones i n the o p e r a t i n g r o o m . O n e o f t h e faster g r o w i n g p r o d u c t areas is t h a t o f pressure-sensitive adhesives. T h e r e were almost a h a l f - b i l l i o n p o u n d s o f pressure sensitive adhesives sold i n 1985 ( i ) , b u t w i t h a l l t h e p u b l i c i t y o n these newer more versatile p r o d u c t s , most people are left b e l i e v i n g t h a t n o t m u c h s t a r c h is used i n adhesives a n y m o r e . E v e n t h e l i b r a r y paste t h a t was used b y c h i l d r e n for decades i s b e i n g replaced b y p o l y ( v i n y l acetate). B u t t h e fact is t h a t over 3.5 b i l l i o n p o u n d s o f s t a r c h were u t i l i z e d last year t o b o n d p r o d u c t s t h a t are used p r a c t i c a l l y every day. M a j o r markets i n c l u d e t h e paper a n d c o r r u g a t i n g i n d u s t r i e s , s o l i d fiber l a m i n a t e s , w o u n d paper tubes, grocery bags, t e x t i l e s , a n d briquettes. 0097-6156/89/0385-0326$06.00/0 © 1989 American Chemical Society

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

23.

KENNEDY

Starch- and Dextrin-Based Adhesives

327

T h i s chapter's purpose is t o define these " l o w profile" m a r k e t s where starches a n d d e x t r i n s are b e i n g used a n d t o describe h o w the adhesive character of s t a r c h is developed i n different ways for different e n d uses.


Developing Starch's Adhesive Character M o s t of the c o r n s t a r c h sold t o i n d u s t r y comes f r o m wet m i l l i n g p l a n t s t h a t process the f a r m e r ' s shelled c o r n i n such a w a y t h a t pure s t a r c h is recovered. T h e d r i e d s t a r c h is s h i p p e d i n b u l k r a i l cars, t r u c k s , or i n bags t o customers. A f t e r r e c e i v i n g the s t a r c h , the customer m u s t s l u r r y i t i n water at the a p p r o p r i a t e solids content a n d t h e n heat the s l u r r y or a d d caustic t o i t t o g e l a t i n i z e the granules. U n g e l a t i n i z e d s t a r c h is i n c a p a b l e of g l u i n g a n y t h i n g together because the molecules w i t h i n the u n p a s t e d granules are a l r e a d y t i g h t l y b o n d e d t o one a n other. A s the molecules are f o r m e d w i t h i n the g r o w i n g c o r n k e r n e l , t h e y are packed so close t o one another t h a t the ordered arrangements become n e a r l y c r y s t a l l i n e . M o r e precisely, there are c r y s t a l l i n e regions a n d a m o r p h o u s regions w i t h i n the granules. I n order for s t a r c h molecules t o become detached f r o m one a n o t h e r , they m u s t be h y d r a t e d , a process a c c o m p l i s h e d i n water s l u r r y b y u s i n g either t h e r m a l or c h e m i c a l energy. O n l y t h e n w i l l the molecules be separated a n d u n c o i l e d enough to be able to l a t c h o n t o other separate particles a n d act as a n adhesive. T h e m o s t c o m m o n f o r m of c h e m i c a l h y d r a t i o n of s t a r c h is t h r o u g h the use of aqueous a l k a l i . S o d i u m h y d r o x i d e is capable of h y d r a t i n g s t a r c h molecules w i t h o u t heat, b u t a specific m i n i m u m c o n c e n t r a t i o n m u s t be reached for t h a t t o occur (2). A s a s t a r c h s l u r r y is heated to higher a n d higher t e m p e r a t u r e s , a d r a m a t i c increase i n viscosity becomes obvious as the s w e l l i n g granules soak u p m o r e a n d m o r e water u n t i l they b e g i n t o r u p t u r e , so molecules c a n leach out of the granule fragments. O n l y t h e n can the s t a r c h b e g i n to do the j o b of adhesion. I n order for the m a x i m u m a m o u n t of adhesion to take place, the molecules s h o u l d be m a x i m a l l y dispersed i n the p a s t i n g process a n d m a i n t a i n e d i n t h a t state u n t i l j u s t seconds before a p p l i c a t i o n . A d h e r i n g Cellulose Fibers W i t h i n

Paper

T h e largest m a r k e t for s t a r c h is its use as a surface s i z i n g agent for a w i d e v a r i e t y of p a p e r a n d b o a r d p r o d u c t s . T h e purpose of s i z i n g paper is t o anchor the loose or d a n g l i n g fibers t o the b o d y of the sheet i n order to a v o i d t h e i r b e i n g t o r n out d u r i n g the p r i n t i n g process, w h i c h w o u l d have p o o r v i s u a l results. O t h e r objectives of s i z i n g w i t h s t a r c h are t o decrease porosity, increase o i l resistance, a n d increase t o t a l sheet s t r e n g t h . P a p e r is m a d e b y s l u r r y i n g cellulose fibers at a very low c o n c e n t r a t i o n a n d filt e r i n g off most of the water o n the m o v i n g w i r e screen of a F o u r d r i n i e r m a c h i n e .



328

ADHESIVES F R O M R E N E W A B L E RESOURCES

T h e fibers f o r m a wet m a t , w h i c h is t h e n c a r r i e d over hot dryer rolls where the continuous sheet of p a p e r is d r i e d . O f t e n , the p a p e r t h e n goes t h r o u g h a size press where a hot s t a r c h paste is a p p l i e d . M o s t l i k e l y the s t a r c h is u n m o d i f i e d b y the s t a r c h s u p p l i e r a n d converted t o the a p p r o p r i a t e viscosity w i t h a m m o n i u m persulfate i n a t h e r m a l - c h e m i c a l converter. T h i s converted s t a r c h increases the s t r e n g t h of the p a p e r . A p p l y i n g s t a r c h w i t h a size press c a n be expensive, since the sheet has to be d r i e d twice. Nonetheless, a b o u t 1.2 b i l l i o n p o u n d s of s t a r c h were used at the size press last year. If the need for a s t r o n g sheet of paper is the m a i n reason for a d d i n g s t a r c h , there is another place i t m a y be a d d e d w i t h o u t h a v i n g to d r y the p a p e r t w i c e . It also m a y be a d d e d at the wet e n d of the m a c h i n e p r i o r to the cellulose fibers b e i n g p u m p e d t o the continuous w i r e . I n t h i s case, however, the s t a r c h m u s t be m o d i f i e d w i t h a c a t i o n i c reagent t o give i t a p o s i t i v e charge. T h e c a t i o n i c group o n the s t a r c h is r e q u i r e d so each s t a r c h molecule w i l l be a t t r a c t e d to the n e g a t i v e l y charged fibers a n d t h u s be r e t a i n e d o n the w i r e ( w i t h the fibers) rather t h a n b e i n g washed t h r o u g h the w i r e a l o n g w i t h the fines a n d solubles. It is e s t i m a t e d t h a t perhaps 400 m i l l i o n p o u n d s of c a t i o n i c s t a r c h were c o n s u m e d last year i n t h i s use. Making Corrugated Board A p p r o x i m a t e l y 900 m i l l i o n p o u n d s of s t a r c h were used i n the p r o d u c t i o n of c o r r u g a t e d b o x b o a r d last year. M o s t of t h i s was u n m o d i f i e d s t a r c h . T h e paste used o n a c o r r u g a t i n g m a c h i n e is most c o m m o n l y p r o d u c e d b y the " t w o - t a n k " m e t h o d , a process i n v o l v i n g c o o k i n g o u t a s t a r c h paste u s i n g caustic, w h i c h is t h e n used t o " c a r r y " u n p a s t e d s t a r c h t o the b o n d i n g zone. T h e paste is p r e p a r e d by a d d i n g s t a r c h a n d caustic t o water i n the u p p e r m i x e r . T h e water is heated t o 71.1 ° C , w h i c h is sufficient to swell a n d paste the s t a r c h due to the caustic present. S o m e c o o l i n g water is t h e n a d d e d t o the hot paste. T o the lower m i x e r is a d d e d w a t e r , b o r a x , a n d more s t a r c h , a n d the s t a r c h paste f r o m the u p p e r m i x e r is s l o w l y d r o p p e d i n t o the lower one w i t h a g i t a t i o n . T h i s adhesive is t h e n a p p l i e d t o the flute t i p s of the c o r r u g a t e d m e d i u m where the pasted s t a r c h w i l l h o l d the u n g e l l e d s t a r c h granules i n place. W h e n the l i n e r sheet is b r o u g h t i n t o contact w i t h the flute t i p s a n d the resultant " s a n d w i c h " is heated by a r o l l or s t e a m t a b l e , the granules are s w o l l e n . T h i s produces a n e x t r e m e l y h i g h solids paste w i t h a n e x t r e m e l y h i g h tack t h a t b o n d s the sheets together, a l l o w i n g the c o r r u g a t o r t o r u n at very h i g h speeds. B o n d i n g Pigments to P a p e r A b o u t 600 m i l l i o n p o u n d s of s t a r c h were used i n 1986 as p a p e r - c o a t i n g b i n d e r s , a n d a g a i n , most were u n m o d i f i e d b y the s t a r c h s u p p l i e r (3). T h e p a p e r m i l l s convert the s t a r c h t o the necessary viscosity b y either e n z y m e or the t h e r m a l c h e m i c a l c o n v e r t i n g process. H y d r o x y e t h y l s t a r c h is also used q u i t e successfully as is some o x i d i z e d s t a r c h .


23.

KENNEDY


329


T h e m a j o r f u n c t i o n of the s t a r c h is to b o n d the p i g m e n t (clay, c a l c i u m c a r b o n a t e , t i t a n i u m d i o x i d e , etc.) t o the p a p e r surface, m a k i n g the sheet w h i t e r , b r i g h t e r , a n d s m o o t h e r for p r i n t i n g . I n a d d i t i o n t o b o n d i n g s t r e n g t h , s t a r c h also provides the p r o p e r r h e o l o g i c a l characteristics t o the c o a t i n g color. T h i s enables the color t o t h i n d o w n under the shear of the c o a t i n g b l a d e so the c o a t i n g c a n be a p p l i e d s m o o t h l y to the f a s t - m o v i n g web of p a p e r . A n o t h e r m a j o r f u n c t i o n of the s t a r c h is t o increase the w a t e r h o l d i n g capacity of the c o a t i n g . T h i s reduces the tendency for the b i n d e r s y s t e m (there is u s u a l l y a cobinder present) t o m i g r a t e excessively d u r i n g the h i g h - t e m p e r a t u r e d r y i n g process. A n y a l t e r a t i o n i n the b i n d e r d i s t r i b u t i o n w i t h i n the c o a t i n g can d r a s t i c a l l y affect the appearance a n d performance of the coated sheet. Sizing Textile Y a r n I n the past several years, between 100 a n d 150 m i l l i o n p o u n d s of starches a n d d e x t r i n s have been used by the t e x t i l e i n d u s t r y for w e a v i n g a n d f i n i s h i n g c l o t h , a l m o s t 8 5 % of w h i c h is a c i d - m o d i f i e d a n d d e r i v a t i z e d starches. V e r y l i t t l e dext r i n or u n m o d i f i e d s t a r c h is used i n w a r p s i z i n g . S i z i n g y a r n involves the a p p l i c a t i o n o f a hot s t a r c h paste (or "size") t o the y a r n p r i o r t o i t s b e i n g woven i n t o f a b r i c . O t h e r m o d i f y i n g agents m a y be present, b u t s t a r c h has been f o u n d to be the most cost-effective b i n d e r t o use for c o t t o n y a r n . Besides i t s low cost, the value of s t a r c h lies i n i t s a b i l i t y t o p r o v i d e a p l i a b l e film o n the y a r n surface w i t h o u t excessive p e n e t r a t i o n . T h e need for t h i s filming a c t i o n is t h a t i t effectively ties d o w n the loose c o t t o n fibers or ( " h a i r s " ) o n the y a r n surface. If t h i s is not done, the hairiness of the w a r p y a r n s w i l l cause a c l i n g i n g of adjacent fibers d u r i n g the s e p a r a t i o n of the i n d i v i d u a l y a r n s r a t h e r t h a n a l l o w i n g t h e m t o be c l e a n l y separated t o f o r m a n open " s h e d " t h r o u g h w h i c h the weft y a r n c a n pass u n o b s t r u c t e d . I n the n o r m a l l o o m o p e r a t i o n , the m e c h a n i s m c a r r y i n g the weft y a r n t h r o u g h the shed w i l l encounter c l i n g i n g w a r p y a r n s a n d sever t h e m , causing a n i m m e d i a t e s h u t d o w n o f the l o o m . T o o m a n y l o o m stops m a y cause a t e x t i l e m i l l t o suffer e c o n o m i c losses (4)Bonding Charcoal Briquettes A l m o s t 100 m i l l i o n p o u n d s of s t a r c h are n o w b e i n g used to b o n d c h a r c o a l d u s t i n t o e a s y - t o - h a n d l e , p i l l o w - s h a p e d b r i q u e t t e s . I n order t o get t h e p r o p e r s t r e n g t h characteristics, f r o m 5 % to 8% of the c h a r c o a l weight m u s t b e s t a r c h ; a n d t o be a n effective b i n d e r , the s t a r c h m u s t be cooked-out before a d d i t i o n t o the c h a r c o a l . T h u s , i t m a y be either a pregelled s t a r c h t h a t has been cooked a n d d r i e d by the s t a r c h m a n u f a c t u r e r , or i t m a y be a g r a n u l a r s t a r c h t h a t the b r i q u e t t e r m u s t paste. U n m o d i f i e d c o r n s t a r c h processed t h r o u g h a j e t cooker has proven to be the most effective b i n d e r for t h i s m a r k e t .


330

ADHESIVES F R O M RENEWABLE R E S O U R C E S


Other Adhesive Applications T h e a m o u n t of s t a r c h a n d d e x t r i n s o l d to the " o t h e r " adhesive m a r k e t s c a n be r o u g h l y e s t i m a t e d at f r o m 200 t o 300 m i l l i o n p o u n d s . It m a y even be i n excess o f 300 m i l l i o n p o u n d s due t o the i n e x a c t r e p o r t i n g procedures for the m a n y specific e n d uses t h a t comprise t h i s m a r k e t area. A l t h o u g h , as p r e v i o u s l y p o i n t e d o u t , the m a n y s y n t h e t i c adhesives developed d u r i n g the past t w o decades are r e p l a c i n g the n a t u r a l adhesives t o a large extent i n a n u m b e r of areas, starch-based adhesives are a l m o s t i m p o s s i b l e to remove f r o m some m a r k e t s because of specific properties a n d cost savings. S o m e of those m a r k e t s are discussed below. A d h e s i v e s f o r P a p e r B a g s . P a p e r bags g l u e d w i t h s t a r c h or d e x t r i n pastes range f r o m s m a l l sacks for a few pieces of c a n d y or h a r d w a r e i t e m s t h r o u g h g r o cery bags t o m u l t i w a l l paper sacks used for s h i p p i n g 50 t o 100 p o u n d q u a n t i t i e s of c h e m i c a l s , b u i l d i n g p r o d u c t s , s t a r c h , etc. T w o adhesives are used t o p r o d u c e a sack like a grocery b a g : a side s e a m adhesive a l l o w i n g the paper to be f o r m e d i n t o a t u b e , a n d a b a g b o t t o m adhesive sealing the b o t t o m . T h e large m u l t i w a l l bags require a t h i r d t y p e o f paste i n a d d i t i o n . T h i s is a cross-paste used to glue the several plies o f paper together to f o r m a l a m i n a t e d sheet of t w o - t o six-sheet thicknesses before the t u b e is f o r m e d . T h e s e a m a n d cross-pastes are b o t h n o r m a l l y based o n the r a t h e r t a c k y a n d f a s t - s e t t i n g b o r a t e d d e x t r i n s , are of m o d e r a t e viscosity (2,000 t o 4,000 c P ) at r o o m t e m p e r a t u r e , a n d 2 0 % t o 3 0 % solids ( 5 ) . T h e cross-paste u s u a l l y has some m i n e r a l filler or even some l a t e x t o prevent b l e e d i n g t h r o u g h t o the bags above or below. A d h e s i v e b l e e d i n g w i l l cause the stacked bags t o s t i c k together d u r i n g d r y i n g . H i g h - f l u i d i t y starches m a y also be used to m a k e s e a m a n d cross-pastes, a n d a b o u t 1% s o d i u m c h l o r i d e m a y be a d d e d (based o n the t o t a l adhesive) to give greater adhesive s t a b i l i t y a n d i m p r o v e the d e w a t e r i n g o f the b o n d d u r i n g d r y i n g . A u r e a - f o r m a l d e h y d e resin m a y be a d d e d for water resistance as w e l l as p o l y ( v i n y l alcohol) or p o l y ( v i n y l acetate). (6). B a g b o t t o m paste must be viscous enough to i n h i b i t flow u n t i l shear is a p p l i e d b y the a p p l i c a t o r rolls. It m u s t have a " s h o r t " character so i t w i l l not be t h r o w n d u r i n g high-speed a p p l i c a t i o n a n d m u s t have a h i g h degree of tack so the green b o n d w i l l not allow the f o l d e d b o t t o m t o o p e n u p before s t a c k i n g ( 7 ) . N a t u r a l l y , the f i n a l d r y b o n d must be very s t r o n g t o s u p p o r t b a g contents. P r e g e l l e d starches, d e x t r i n s , a n d a c i d - m o d i f i e d starches have a l l been used to prepare g o o d b o t t o m pastes, b u t u n m o d i f i e d c o r n s t a r c h has p r o v e n t o be v e r y effective as w e l l . T h e f o r m u l a t i o n o f such a n adhesive w i t h water resistance is given i n T a b l e I. T h i s s l u r r y is heated t o 90 ° C a n d h e l d for 15 m i n u t e s before c o o l i n g . A t r o o m t e m p e r a t u r e , the paste s h o u l d be 60,000 to 70,000 c P . A d h e s i v e s f o r L a m i n a t i n g . S o l i d f i b e r b o a r d for boxes a n d s h i p p i n g c o n t a i n ers is t o u g h a n d d u r a b l e due t o a c o m b i n a t i o n o f t w o or m o r e plies of p a p e r b o a r d b o n d e d w i t h a s o l i d fiber l a m i n a t i n g adhesive (8). W a r p i n g can be a p r o b l e m ,


23.

KENNEDY

331

Starch-and Dextrin-Based Adhesives T a b l e I. B a g B o t t o m P a s t e F o r m u l a Ingredients

P a r t s by W e i g h t

Unmodified starch P o l y ( v i n y l alcohol) P o l y ( v i n y l acetate) Soap


Water

12.6 4.5 0.9 0.1 81.9

since the m u l t i p l e plies c a n cause significant s t r a i n w h i l e the adhesive is d r y i n g . Therefore, t h i s adhesive is u s u a l l y h i g h solids (50-55%) a n d contains a r a t h e r h i g h level (30-50%) of p l a s t i c i z e r (sugar, s o d i u m n i t r a t e , urea) t o reduce s t r a i n s . N a t u r a l l y , a g o o d d r y b o n d is r e q u i r e d , a n d water resistance is so i m p o r t a n t t h a t u p t o 2 5 % of a u r e a - f o r m a l d e h y d e resin m a y be used. T h e h i g h solids are achieved b y u s i n g h i g h - s o l u b l e w h i t e d e x t r i n s or h i g h - f l u i d i t y starches (6). M o r e recently, blends of p o l y ( v i n y l a l c o h o l ) , s t a r c h a n d clay are used t o p r o v i d e b o t h g o o d water resistance a n d i m p r o v e d tack for faster l a m i n a t i n g speeds (6). A m o r e m a r k e t a b l e s h i p p i n g c a r t o n m a d e of lightweight coated p a p e r or a l u m i n u m f o i l l a m i n a t e d t o the top side a n d p r i n t e d before c o n s t r u c t i o n of the b o x requires a more general purpose adhesive f o r m u l a t e d at h i g h enough solids to have g o o d tack a n d n o w a r p . T h e adhesive s h o u l d also be i n c a p a b l e of b l e e d i n g t h r o u g h the t o p sheet. T h e s e adhesives are m a d e a c c o r d i n g to the f o r m u l a i n Table II.

Table II. General Purpose L a m i n a t i n g Adhesive Ingredients W h i t e d e x t r i n ( h i g h soluble) Clay Urea B o r a x (10 m o l ) Water

P a r t s by W e i g h t 20.2 13.5 6.7 5.0 54.6

T h i s f o r m u l a m u s t also be cooked a n d c o o l e d p r i o r to a p p l i c a t i o n

(8).

A d h e s i v e s f o r C a s e a n d C a r t o n S e a l i n g . O n c e the c o r r u g a t e d b o a r d or the solid

fiberboard

boxes are c o n s t r u c t e d , they m u s t be g l u e d , filled w i t h p r o d u c t ,

a n d the top flaps folded d o w n a n d glued closed before s h i p m e n t . T h e adhesive


332

ADHESIVES F R O M RENEWABLE RESOURCES

for t h i s a p p l i c a t i o n m u s t be of m o d e r a t e b u t stable v i s c o s i t y (1,000 t o 3,000 c P ) w i t h h i g h tack a n d g o o d b o n d (5). A l t h o u g h s y n t h e t i c s have p r a c t i c a l l y t a k e n over t h i s m a r k e t , b o r a t e d d e x t r i n adhesives are s t i l l b e i n g used. A t y p i c a l finished, p a s t e d out f o r m u l a t i o n is s h o w n i n T a b l e I I I .

Table III. Case and C a r t o n Adhesive


Ingredients W h i t e d e x t r i n ( h i g h soluble) S o d i u m metaborate B o r a x (10 m o l ) Water

P a r t s by W e i g h t 37.5 5.0 1.3 56.2

P o l y ( v i n y l alcohol) m a y also be a d d e d i f water resistance is r e q u i r e d . A n other f o r m u l a t i o n t h a t does not require c o o k i n g t o develop i t s adhesive c h a r a c t e r i s t i c is based o n the c o l d , caustic conversion o f a n a c i d - m o d i f i e d s t a r c h (75 fluidity) (6). C l a y filler m a y be a d d e d t o any case or c a r t o n s e a l i n g adhesive i f the b o a r d b e i n g g l u e d is t o o p o r o u s . A d h e s i v e s f o r T u b e W i n d i n g . M a n y p r o d u c t s , such as cores, cans, cones, a n d containers, are p r o d u c e d f r o m p a p e r tubes. C o n v o l u t e w o u n d tubes or the c o n t i n u o u s l y p r o d u c e d s p i r a l w o u n d tubes require different glues. C o n v o l u t e t u b e w i n d i n g is a c c o m p l i s h e d b y feeding p a p e r b o a r d t h r o u g h a t w o - s i d e d adhesive a p p l i c a t i o n s y s t e m t o a r o t a t i n g m a n d r e l whose shape determines w h e t h e r the t u b e is h e x a g o n a l , o c t a g o n a l , or r o u n d . T h e w i d t h of the p a p e r b o a r d w i l l d e t e r m i n e t u b e l e n g t h , or different finish lengths m a y be c u t . W i n d i n g the b o a r d a r o u n d the m a n d r e l the proper n u m b e r of t i m e s gives the t u b e the desired w a l l thickness to p r o v i d e a p p r o p r i a t e s t r e n g t h . P r e g e l l e d starches m a y be m i x e d o n site, b u t p r e p a r e d pastes based o n regular or b o r a t e d d e x t r i n s are often p u r chased f r o m the adhesive suppliers (5). S p i r a l w o u n d tubes are p r o d u c e d b y means o f m u l t i p l e plies fed t h r o u g h adhesive s t a t i o n s a n d w o u n d a r o u n d the m a n d r e l at a n angle so the n e w l y f o r m e d t u b e c o n t i n u o u s l y slides d o w n the m a n d r e l u n t i l cut t o the desired l e n g t h . F a s t e r b o n d i n g pastes are r e q u i r e d for acceptable p r o d u c t i o n . T h e t y p i c a l adhesive is based o n a h i g h solids, b o r a t e d d e x t r i n , b u t o x i d i z e d a n d h y d r o x y e t h y l starches are also used (9). A d h e s i v e s f o r B o t t l e L a b e l s . T h e adhesive used t o glue a l a b e l t o a glass, p o l y e t h y l e n e , or t r e a t e d p o l y p r o p y l e n e b o t t l e has l o n g been c a l l e d a " j e l l y g u m , " w h i c h describes the adhesive's consistency w h e n i t is ready t o be a p p l i e d - a heavy, g u m m y , r u b b e r y , t a c k y character. T h e h i g h tack is necessary t o keep u p w i t h the high-speed b o t t l i n g m a c h i n e s a n d to stop the labels f r o m s l i d i n g once they are p l a c e d o n the b o t t l e (10). T h e solids of these pastes are a b o u t 4 0 % t o 5 0 % a n d the v i s c o s i t y 80,000 t o 150,000 c P at r o o m t e m p e r a t u r e (6). L a b e l s m u s t also be able t o be washed off i f b o t t l e s are r e t u r n a b l e a n d reused (11). B o t h d e x t r i n s a n d a c i d - m o d i f i e d starches have been used for t h i s a p p l i c a t i o n ,


23.

KENNEDY

333


b u t starches are necessary for any degree of water resistance (8). A c i d - m o d i f i e d , w a x y starches are very effective b o t t l e l a b e l adhesives. O n e f o r m u l a t i o n is l i s t e d in Table I V .


Table I V . Bottle Label Adhesive Ingredients W a x y s t a r c h (40 f l u i d i t y ) Urea Sodium nitrate Water

Parts by Weight 38.0 3.0 3.0 56.0

S o m e beer b o t t l e m a n u f a c t u r e r s have a d d e d p o l y ( v i n y l alcohol) t o the adhesive t o ensure resistance t o iced water yet r e t a i n wash a b i l i t y i n the b o t t l i n g opera t i o n . O t h e r s have used the s t a r c h paste t o e x t e n d the m o r e w a t e r - r e s i s t a n t casein adhesives (9). N e w e r adhesives are b e i n g developed based o n double t r e a t m e n t s of starches such as h y d r o x y a l k y l ethers of o x i d i z e d s t a r c h , b l e n d i n g these p r o d u c t s w i t h other specified p o l y m e r s (12,13) or a d d i n g s y n t h e t i c resins (U)A d h e s i v e s f o r E n v e l o p e s . A d h e s i v e s o n envelope flaps m u s t o b v i o u s l y be remoistenable a n d thus are often based o n h i g h - s o l u b l e w h i t e d e x t r i n s or c a n a r y d e x t r i n s f r o m regular or w a x y c o r n s t a r c h . N o r m a l l y a r o u n d 5 5 % t o 6 5 % solids, a b o u t 2,000 t o 10,000 c P , these adhesives m a c h i n e w e l l d u r i n g a p p l i c a t i o n a n d d r y f a i r l y q u i c k l y . T h e d r i e d f i l m o n the flap m u s t be glossy, low i n color, n o n c u r l i n g , a n d n o n b l o c k i n g i n h u m i d weather (5). A general f o r m u l a is given i n T a b l e V . T h e s o d i u m bisulfite acts as a color reducer, a n d the p o l y (ethylene g l y c o l ) is a n o n h u m e c t a n t p l a s t i c i z e r (6).

T a b l e V . E n v e l o p e F l a p Seal A d h e s i v e Ingredients W a x y d e x t r i n ( h i g h soluble) S o d i u m bisulfite P o l y ( e t h y l e n e glycol) Water

Parts by Weight 63.0 1.0 0.5 35.5

T h e adhesive for the seams o n envelope backs m u s t also be at a h i g h solids to prevent s t r i k e t h r o u g h , allow fast p r o d u c t i o n speeds, a n d p r o v i d e l a y - f l a t . D e x t r i n s or pregelled starches m a y be used, b u t are u s u a l l y h e a v i l y p l a s t i c i z e d w i t h the lower cost h u m e c t a n t - t y p e plasticizers since b l o c k i n g is not a factor o n


334

ADHESIVES F R O M R E N E W A B L E RESOURCES

the back side. P o l y ( v i n y l acetate) m a y be a d d e d to increase the solids (10). flexible f o r m u l a is l i s t e d i n T a b l e V I (6).

A

Table V I . Envelope Back Seam Adhesive


Ingredients P r e g e l l e d s t a r c h (low viscosity) Dextrose Sodium nitrate Urea P o l y ( v i n y l acetate) Water

P a r t s by W e i g h t 30-35 10-35 8-15 0-6 5-15 27-30

A d h e s i v e s f o r G u m m e d P a p e r a n d T a p e . R e m o i s t e n a b l e adhesives for p a p e r a n d t a p e are not u n l i k e those used for envelope front seals. P r o d u c i n g g u m m e d labels or t r a d i n g s t a m p s has c o n v e n t i o n a l l y been a m a t t e r of a p p l y i n g a h i g h - s o l i d s (50%) cooked d e x t r i n paste t o the p a p e r . T h e p r o b l e m of p a per shrinkage c a u s i n g c u r l i n g is u s u a l l y dealt w i t h b y r u n n i n g the sheet over a breaker b a r t h a t fractures the b r i t t l e film, m a k i n g i t d i s c o n t i n u o u s a n d n o n c u r l i n g . M o r e frequently, producers are a p p l y i n g a b i n d e r dissolved i n a solvent " c a r r y i n g " the c o l d water-soluble s t a r c h or d e x t r i n t h a t produces a d i s c o n t i n u ous film not susceptible t o c u r l i n g . S e a l i n g tape has 35 t o 90 p o u n d s per r e a m of k r a f t p a p e r as a s u b s t r a t e , whereas, reinforced sealing tape is based o n a b o n d e d l a m i n a t e of k r a f t p a p e r , r e i n f o r c i n g fibers, a n d k r a f t p a p e r . T h e adhesives a p p l i e d to these substrates m a y be t h i n - b o i l i n g , w a x y starches alone, or b l e n d e d w i t h a soluble d e x t r i n (5). M o r e recently, blends of a soluble d e x t r i n w i t h o x i d i z e d p o t a t o or a h y d r o x y p r o p y l ether of a n o x i d i z e d p o t a t o s t a r c h are b e i n g used (9). A l s o , the acetate or succinate of a n o x i d i z e d w a x y s t a r c h m a y be used (15) as w e l l as s p e c i a l l y p r o d u c e d w a x y s t a r c h a c r y l a m i d e graft c o p o l y m e r p r o d u c t s (16). A d h e s i v e s f o r W a l l p a p e r . Homeowners can s t i l l b u y the s m a l l packages of wheat s t a r c h to m i x paste for h a n g i n g w a l l p a p e r . P a s t e p r e p a r e d by adhesive m a n u f a c t u r e r s is m o r e easily a p p l i e d d i r e c t l y f r o m the container a n d is w i d e l y used b y professionals. P r e p a r e d w a l l p a p e r pastes m u s t o b v i o u s l y have g o o d viscosity s t a b i l i t y , have g o o d wet t a c k , a n d g o o d s l i p properties t o allow caref u l s e a m a l i g n m e n t w h i l e h a n g i n g . P r e g e l l a t i n i z e d starches a n d a c i d - m o d i f i e d starches are b e i n g used t o prepare these pastes, a n d b o r a x adds g o o d t a c k w h i l e plasticizers p r o v i d e the s l i p r e q u i r e d . C l a y m a y be a d d e d for ease i n s t r i p p i n g p a p e r l a t e r . A t y p i c a l paste f o r m u l a is s h o w n i n T a b l e V I I (6). A d h e s i v e s f o r M i s c e l l a n e o u s U s e s . S t a r c h is also used t o prepare c h i l d r e n ' s l i b r a r y paste. A b l e n d of low-soluble w h i t e d e x t r i n a n d u n m o d i f i e d c o r n s t a r c h is cooked u p w i t h glycerine a n d water to a b o u t 5 5 % solids a n d set back to o b t a i n a firm b u t s m o o t h t e x t u r e (6). C o r n s t a r c h is also used i n the p r o d u c t i o n


23.

KENNEDY

335


of cigarette s e a m paste at t h e c i g a r e t t e - m a n u f a c t u r i n g p l a n t . T h i s adhesive is a p r o p r i e t a r y b l e n d o f ingredients, b u t i t s base is a very h i g h - s o l i d s paste o f u n m o d i f i e d s t a r c h t h a t is allowed t o set back overnight so i t is capable o f b e i n g e x t r u d e d onto a n a p p l i c a t o r wheel.

Table V I I . Prepared Wallpaper Paste


Ingredients

Parts by Weight

P r e g e l l e d s t a r c h (acid modified) Clay

25.0 20.0

Urea S o d i u m metaborate Water

3.75 1.25 50.0

Conclusions S t a r c h is s t i l l q u i t e w i d e l y used as a n adhesive i n o u r m o d e r n , " h i g h t e c h " w o r l d . Its adhesive properties are developed differently for different p r o d u c t s , a n d starch-based adhesive is used i n a large variety o f a p p l i c a t i o n s . F r o m t h e s t a n d p o i n t o f its b e i n g a renewable resource, a reliable performer, a n d a low-cost raw m a t e r i a l , s t a r c h w o u l d seem t o be a n adhesive ingredient o n t h e m a r k e t for a long time into the future. Literature Cited 1. Fries, J. A. Paper, Film and Foil Converter 1984, 58(1), 48. 2. Leach, H. W.; Schoch, T. J.; Chessman, E . F. Starke 1961, 13, 200. 3. Ducey, M. J. Pulp and Paper 1985, p. 83. 4. Strauss, M . D.; Pettey, D. A. In Sizing: Keystone to Quality Fabrics, Warp Sizing Symposium, No. 40, AATCC, Charlotte, NC, 1987, p. 75. 5. Jarowenko, W. In Handbook of Adhesives, 2nd Edition, Skeist, I., Ed.; Van Nostrand Reinhold Co., New York, 1977, Chap. 12. 6. Kennedy, H. M.; Fischer, A. C. In Starch: Chemistry and Technology, 2nd Edition, Whistler, R. L.; BeMiller, J. N.; Paschall, E. F., Eds.; Academic Press, Inc., New York, 1983, Chap. XX. 7. Caesar, G. V. In Handbook of Adhesives, Skeist, I., Ed.; Reinhold, New York, 1962, p. 176. 8. Dux, E.F.W. In Starch: Chemistry and Technology, Whistler, R. L.; Pacshall, E . F., Eds.; Academic Press, Inc., New York, 1967, Vol. II, Chap. XXIII. 9. Converter, 1983, 20(1), 14, 16, January. 10. Lazarus, D. M . In Adhesion 7, Allen, K. W., Ed., Applied Science Publishers Ltd., London, 1983, Chap. 10. 11. Converter, 1979, 16(10), 16, October.


336

ADHESIVES F R O M RENEWABLE RESOURCES

12. Andres, J.; Hasenkamp, R.; Merkel, H. U.S. Patent 4 462 838, 1984. 13. Andres, J.; Hasenkamp, R.; Merkel, H. U.S. Patent 4 464 202, 1984. 14. Jannusch, L. C. U.S. Patent 4 440 884, 1984. 15. Bovier, Ε. M.; Carter, J. A. U.S. Patent 4 231 803, 1980; Adhesives Technology Devel opments Since 1979, Gutche, M., Ed.; Noyes Data Corp., New Jersey, 1983, p. 178. 16. Kaspar, M . L.; Lowey, J. F. U.S. Patent 4 322 472, 1982; Adhesives Technology Devel opments Since 1979, Gutcho, M., Ed.; Noyes Data Corp., New Jersey, 1983; p. 179.


RECEIVED May 24, 1988


Adhesives from Renewable Resources - ACS Publications - American

Recommend Documents