The Chemistry of Solid Wood - American Chemical Society

9 Chemistry of Adhesion R. V. S U B R A M A N I A N

Downloaded by UNIV OF LEEDS on June 19, 2016 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch009

Department of Materials Science and Engineering, Polymeric Materials Section, Washington State University, Pullman, WA 99164

The basic features of the structure, properties, and modification of polymeric adhesives are discussed as they relate to adhesive behavior with special reference to wood substrates. Adhesive properties and bond performance, including durability, are derived from chemical composition (functional groups), molecular organization (branching, molecular weight distribution, cross- l i n k i n g ) and physical state (elastomer, thermoplastic, thermoset, crystalline). The various types of adhesives include those derived from naturally occurring polymers—carbohydrates, proteins, and natural rubber— and those derived from the multitude of synthetic polymers—phenolics, epoxies, acrylics, elastomers, urethanes, etc. The transformation of liquid adhesive to a solid bond calls for curing reactions of reactive components or film formation from emulsion, solution, or melt. The properties of adherend surfaces play a critical role in the wetting and bonding of substrates as do modifications of interfacial interactions by chemical surface treatments and coupling agents. Resistance to environmental effects of humidity, temperature, microbial attack, etc. determines the durability of bonds.

. A D H E S I V E B O N D I N G PLAYS A VITAL R O L E i n m a t e r i a l s e n g i n e e r i n g — t h e design, fabrication, a n d application of n e w and i m p r o v e d products f r o m a v a r i e t y o f m a t e r i a l s . T h e s c i e n c e of a d h e s i o n has d e v e l o p e d into a multidisciplinary study i n v o l v i n g the chemistry and physics of a d h e r e n d surfaces a n d adhesives a n d the fracture m e c h a n i c s of a d hesive joints. B e c a u s e the adhesives i n use are v i r t u a l l y all p o l y m e r s , a n u n d e r s t a n d i n g o f p o l y m e r s c i e n c e is r e q u i r e d , i n c l u d i n g t h e c h e m istry of p o l y m e r i z a t i o n reactions, a n d the rheology, deformation, a n d f r a c t u r e b e h a v i o r o f p o l y m e r s (1-3). T h i s c h a p t e r d i s c u s s e s t h e c h e m i s t r y o f a d h e s i o n as i t r e l a t e s to t h e m a n y a s p e c t s o f a d h e s i v e b o n d formation and performance. 0065-2393/84/0207-0323/$07.50/0 © 1984 American Chemical Society Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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In focusing attention o n w o o d b o n d i n g , w e are d e a l i n g w i t h nature's o w n u n i q u e material whose sophisticated structure and c o m p l e x i t i e s a r e at t h e s a m e t i m e b a f f l i n g a n d c h a l l e n g i n g . T h u s , t h e r e a r e s e v e r a l c o m p l i c a t i n g f a c t o r s i n t h e s t u d y o f w o o d as a n a d h e r e n d : the species; h e a r t w o o d ; sapwood; earlywood; latewood; surface planes i n radial, tangential, longitudinal, or intermediate directions; p H ; porosity; m o i s t u r e content; a n d extractives are all capable of m o d i f y i n g t h e b o n d i n g p r o p e r t i e s o f w o o d (4, 5). O f t h e s e , s o m e , like p H , moisture content, or extractives, have a m o r e direct i n f l u e n c e o n a d h e s i v e c h e m i s t r y a n d n e e d to b e c o n s i d e r e d c a r e f u l l y .


Adhesive Bond Formation A d h e s i v e s w e t , f l o w , a n d set to a s o l i d d u r i n g b o n d f o r m a t i o n . T h e t r a n s f o r m a t i o n f r o m l i q u i d a d h e s i v e to s o l i d b o n d can be a c h i e v e d i n a n u m b e r o f w a y s . W h e r e t h e a d h e s i v e is a p o l y m e r , t h e i n i t i a l s t a r t i n g m a t e r i a l is a l i q u i d m o n o m e r o r p r e p o l y m e r t h a t , u n d e r the conditions of b o n d i n g w i t h heat, pressure, and/or catalyst, p o l y m e r i z e s t o t h e s o l i d p o l y m e r i n t h e g l u e l i n e . I t is a l s o u s u a l t o a p p l y s o l u t i o n s o f p r e f o r m e d p o l y m e r s i n s u i t a b l e s o l v e n t s to t h e faces o f a d h e r e n d s , a n d a l l o w b o n d f o r m a t i o n to t a k e p l a c e w i t h e v a p o r a t i o n o f s o l v e n t . A l t e r n a t i v e l y , p o l y m e r s t h a t c a n b e m e l t e d o r soft e n e d t o f l o w at e l e v a t e d t e m p e r a t u r e s c a n b e a p p l i e d as hot-melt adhesives to f o r m t h e b o n d o n c o o l i n g . W i t h p o r o u s a d h e r e n d s l i k e w o o d , p e n e t r a t i o n o f t h e p o r e s b y l i q u i d o r m o l t e n a d h e s i v e s is a n i m p o r t a n t factor i n b o n d formation. T h e t r a n s f o r m a t i o n of l i q u i d m o n o m e r s to s o l i d a d h e s i v e i n volves an increase i n m o l e c u l a r size a n d m o l e c u l a r w e i g h t t h r o u g h polymerization. T h e increase i n molecular weight of the p o l y m e r i c a d h e s i v e is r e s p o n s i b l e f o r t h e a t t a i n m e n t o f a d e q u a t e m e c h a n i c a l properties, cohesive strength, impact strength, etc., and should, t h e r e f o r e , b e a l l o w e d t o p r o c e e d to t h e r e q u i r e d l e v e l s d u r i n g b o n d formation. I n p o l y m e r adhesives s y n t h e s i z e d separately for adhesive a p p l i c a t i o n f r o m s o l u t i o n o r as h o t m e l t , t h e r e q u i r e d s o l u b i l i t y , s o f t e n i n g t e m p e r a t u r e , viscosity of m e l t s , a n d f l o w properties are d e p e n d e n t u p o n the molecular weight of the p o l y m e r and further, on the m o lecular weight distribution. T h e presence of low molecular weight p o l y m e r a m o n g the p o l y m e r molecules of higher molecular weight facilitates easier m e l t i n g a n d f l o w i n g of the adhesive. F u r t h e r m o r e , b e c a u s e w o o d is a p o r o u s a d h e r e n d , l o w m o l e c u l a r w e i g h t p o l y m e r s a c h i e v e b e t t e r p e n e t r a t i o n of the pores to e n h a n c e the r e s u l t i n g a d h e sion. Thermoplastic and Thermosetting Polymers. Polymer adhe sives that dissolve i n solvents, or that soften a n d f l o w o n h e a t i n g a n d

Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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s o l i d i f y o n c o o l i n g , a r e a b l e t o d o so b e c a u s e t h e l o n g - c h a i n p o l y m e r molecules are essentially linear, perhaps w i t h occasional branches, b u t are not c h e m i c a l l y b o n d e d to each other. B e c a u s e of t h e i r l o n g chains and high molecular weights, the p o l y m e r molecules have physical entanglements w i t h each other. A s the temperature of the p o l y m e r is r a i s e d , v a r i o u s s e g m e n t s o f t h e l o n g p o l y m e r m o l e c u l e a c q u i r e m o b i l i t y at a t e m p e r a t u r e c a l l e d t h e glass t r a n s i t i o n t e m p e r a t u r e , T . B e l o w t h i s t e m p e r a t u r e , l a r g e - s c a l e s e g m e n t a l m o b i l i t y is f r o z e n , a n d t h e a m o r p h o u s p o l y m e r is i n t h e g l a s s y s t a t e ; a b o v e T , t h e o n s e t o f s e g m e n t a l m o b i l i t y e n a b l e s t h e p o l y m e r to r e s p o n d q u i c k l y to a n y a p p l i e d stress, m o v e i n t o t h e r u b b e r y state, a n d e x h i b i t t h e t y p i c a l e l a s t i c i t y o f e l a s t o m e r s . P o l y m e r f l o w at s t i l l h i g h e r t e m p e r a t u r e s o c c u r s as a c o n s e q u e n c e o f t h e v i g o r o u s m o v e m e n t s o f various s e g m e n t s that shift t h e c e n t e r of g r a v i t y of the p o l y m e r m o l e c u l e p r o g r e s s i v e l y . S u c h a p o l y m e r is t h e r m o p l a s t i c , a n d f o r m s t h e basic constituent of soluble or hot-melt adhesives. g


g

A p o l y m e r is i n t h e g l a s s y s t a t e at r o o m t e m p e r a t u r e i f t h e T is a b o v e r o o m t e m p e r a t u r e , a n d i t b e h a v e s as a n e l a s t o m e r i f t h e T is considerably b e l o w r o o m temperature. T h e Τ of a p o l y m e r depends on the freedom of rotation available about bonds i n the p o l y m e r chain. F l e x i b l e p o l y m e r s are m a d e b y the i n t r o d u c t i o n of groups a b o u t w h i c h r o t a t i o n is f a c i l e , l i k e - O - , - S i - O - S i - , C H - S - , = C H - C H - . Discussions of different adhesive systems provide a n u m b e r of examples of such groups b e i n g i n t r o d u c e d i n otherwise intractable p o l y m e r s to m a k e t h e m f l e x i b l e a n d t o u g h . Plasticizers are also e m p l o y e d to m a k e r i g i d p o l y m e r s p l i a b l e . P l a s t i c i z e r m o l e cules, by their interaction w i t h p o l y m e r chains, reduce interchain interactions and consequently, their rigidity. g

g

2

2

In c o n t r a s t to t h e l i n e a r t h e r m o p l a s t i c p o l y m e r s , w h i c h are s o l u b l e a n d fusible, the c r o s s - l i n k e d n e t w o r k p o l y m e r s are i n s o l u b l e and infusible. T h e y are f o r m e d from p o l y m e r i z i n g systems c o n t a i n i n g m o n o m e r s or p r e p o l y m e r s w i t h a functionality of three or more. A g o o d e x a m p l e is t h e p h e n o l - f o r m a l d e h y d e r e s i n s y s t e m s . T h e c r o s s l i n k i n g reaction takes place i n the b o n d u n d e r a p p l i e d pressure a n d heat, a n d the w h o l e adhesive b o n d m i g h t consist of only one super giant molecule. S u c h resins are, therefore, called thermosetting resins. T h e cross-link density a n d the spacing of cross-links along the p o l y m e r chain d e t e r m i n e the relative flexibility of the rigid network s t r u c t u r e s , w h i c h a r e n a t u r a l l y m o r e r e s i s t a n t to h e a t , s o l v e n t s , a n d c h e m i c a l attack t h a n u n c r o s s - l i n k e d p o l y m e r s . F l e x i b i l i z i n g p o l y m e r s o r fillers are also a d d e d to t h e r m o s e t t i n g resins to i m p r o v e t o u g h ness a n d r e d u c e brittleness of the adhesive. An

i m p o r t a n t a s p e c t o f f o r m i n g a c r o s s - l i n k e d s t r u c t u r e is t h e


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extent of reaction of the available functional groups. As the p o l y m e r m o l e c u l e g r o w s i n size its T m a y rise a b o v e t h e t e m p e r a t u r e o f c u r e . S e g m e n t a l m o b i l i t y is t h e r e f o r e l o s t , a n d t h e r e r e m a i n a f e w f u n c tional groups i n the p o l y m e r chain i m m o b i l i z e d and incapable of r e a c t i o n at t h a t t e m p e r a t u r e . H o w e v e r , o n r a i s i n g t h e t e m p e r a t u r e a g a i n , m o l e c u l a r m o b i l i t y is r e s t o r e d a n d m o r e f u n c t i o n a l g r o u p s c a n react to f o r m m o r e cross-links a n d a stronger n e t w o r k s t r u c t u r e . T h e i n c r e a s e i n b o n d s t r e n g t h o b s e r v e d o n p o s t - c u r i n g is a t t r i b u t e d to fuller utilization of the reactive groups present.


g

Molecular Forces Between Adherend and Adhesive. The var ious theories of adhesion i n v o k e the occurrence a n d interplay of p h y s i c a l a n d c h e m i c a l i n t e r a c t i o n s across t h e a d h e r e n d - a d h e s i v e i n t e r f a c e , as w e l l as t h e d e f o r m a t i o n b e h a v i o r o f t h e a d h e s i v e (6, 7). T h e r e f o r e , b o n d f o r m a t i o n d e p e n d s u p o n the d e v e l o p m e n t of intermolecular attraction, b o t h w i t h i n the b u l k of the p o l y m e r and between adhesive and adherend. O f t h e d i f f e r e n t t y p e s o f forces r e s p o n s i b l e for i n t e r m o l e c u l a r a t t r a c t i o n , t h e f o r e m o s t a r e t h e L o n d o n o r d i s p e r s i o n f o r c e s t h a t act b e t w e e n a l l atoms a n d account for v i r t u a l l y a l l of the m o l e c u l a r at traction or cohesion i n all molecules except the very polar molecules ( d e s c r i b e d l a t e r ) . D i s p e r s i o n f o r c e s a r e s h o r t - r a n g e i n t e r a c t i o n s , ef f e c t i v e at a b o u t 4 Â , a n d r a p i d l y d e c r e a s e w i t h t h e s i x t h p o w e r o f the distance b e t w e e n molecules. Therefore, the adhesive p o l y m e r m o l e c u l e m u s t b e f l e x i b l e e n o u g h to c o m e w i t h i n t h i s r a n g e o f i n teraction w i t h the r i g i d a d h e r e n t surface u n d e r the conditions of b o n d formation. A n o t h e r interaction occurs between dipoles i n molecules. D i poles arise w h e n the electrons of a c h e m i c a l b o n d b e t w e e n atoms are not shared equally, thus creating positive and negative charge centers i n the m o l e c u l e . T h e i n t e r a c t i o n forces b e t w e e n p e r m a n e n t dipoles of polar molecules d e p e n d on the strength of the two dipoles, and decrease w i t h the sixth p o w e r of the distance between their centers. Clearly, the dipolar interaction of p o l y m e r i c adhesives w i l l be strong w h e n they carry polar chemical groups. A particularly strong type of dipolar attraction results w h e n the p o s i t i v e c e n t e r is a s s o c i a t e d w i t h t h e h y d r o g e n a t o m a t t a c h e d to a n e l e c t r o n e g a t i v e a t o m , m o s t c o m m o n l y n i t r o g e n o r o x y g e n , as i n t h e following examples: δ + δ- c = o

δ + δH - N -

δ- δ + - Ο - Η

δ- δ + Ο-Η

T h i s p r o t o n s h a r i n g b e t w e e n e l e c t r o n e g a t i v e a t o m s is c a l l e d t h e h y d r o g e n b o n d . It o c c u r s i n p o l y m e r s c a r r y i n g a m i d e ( - C O N H - ) , c a r -


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b o x y l ( - C O O H ) , or h y d r o x y l ( - O H ) groups, a n d contributes signif i c a n t l y t o t h e a d h e s i o n t o p o l a r s u b s t r a t e s o f a d h e s i v e s s u c h as p r o t e i n s , s t a r c h , p o l y v i n y l a l c o h o l , e p o x y r e s i n s , p h e n o l i c s (8), a n d e s p e c i a l l y to w o o d w h i c h has a n a b u n d a n c e o f h y d r o x y l g r o u p s . T h e m o l e c u l a r f o r c e s a r e s e c o n d a r y o r v a n d e r W a a l s f o r c e s . It is a l s o c o n c e i v a b l e t h a t p r i m a r y v a l e n c e f o r c e s f o r m c h e m i c a l b o n d s , either covalent or ionic, between adhesive and adherend. T h e con t r i b u t i o n o f c o v a l e n t b o n d s t o b o n d s t r e n g t h is a s u b j e c t o f g r e a t , i f s o m e t i m e s c o n t r o v e r s i a l , i n t e r e s t (6).


Adhesives for Wood T h e v a r i e t y o f w o o d p r o d u c t s has i n c r e a s e d e n o r m o u s l y o v e r t h e p a s t d e c a d e s as m o r e a d h e s i v e s h a v e b e e n f o u n d f o r b o n d i n g . T h e most i m p o r t a n t p r o d u c t s i n t e r m s of v o l u m e are p l y w o o d , particle b o a r d , a n d fiber b o a r d . B u t a d h e s i v e l y b o n d e d p r o d u c t s r a n g e f r o m t i n y articles of j e w e l r y to giant l a m i n a t e d t i m b e r s s p a n n i n g h u n d r e d s o f f e e t (5). T h e m o d i f i c a t i o n o f a d h e s i v e p r o p e r t i e s t o s u i t t h e d i f ferent application requirements requires a sound understanding of the basic c h e m i s t r y of adhesives. In the f o l l o w i n g discussion, o n l y the most w i d e l y u s e d adhesive types are d e s c r i b e d . T h e s e are the u r e a - f o r m a l d e h y d e ( U F ) resins, m e l a m i n e - f o r m a l d e h y d e ( M F ) resins, p h e n o l - f o r m a l d e h y d e (PF) resins, diisocyanates, polyisocyanates, polymers a n d copolymers of v i n y l acetate, a n d p o l y a m i d e s . T h e s e are all p r e d o m i n a n t l y t h e r m o setting resin systems. S o m e general observations can be made regarding the above t h e r m o s e t t i n g r e s i n s y s t e m s b e f o r e d i s c u s s i n g t h e i r c h e m i s t r y (9). T h e cohesive strength of these t h e r m o s e t t i n g resins exceeds the t e n sile s t r e n g t h of w o o d . T h e adhesive p r e p o l y m e r resins have h i g h e n o u g h p o l a r i t y a n d l o w e n o u g h v i s c o s i t y to p e n e t r a t e i n t o t h e m i cropores of w o o d a n d b r i n g about mechanical anchorage of the a d h e s i v e at t h e o u t s e t o f t h e b o n d i n g p r o c e s s . T h e p o l a r g r o u p s a r e capable of f o r m i n g strong h y d r o g e n bonds w i t h the h y d r o x y l groups in w o o d . T h u s s t r o n g d i p o l e i n t e r a c t i o n s are f o r m e d i n a d d i t i o n to secondary van d e r Waals forces. P r i m a r y c h e m i c a l b o n d s can be formed b y c h e m i c a l reactions between functional groups i n w o o d and those i n P F , M F , or U F resins or diisocyanates. R e s i n types vary m a r k e d l y i n t h e i r w a t e r r e s i s t a n c e w h i c h c o u l d affect t h e i r a p p l i c a t i o n in exterior or i n t e r i o r e n v i r o n m e n t s . T h e P F resins are the most d u r a b l e , a n d t h e U F r e s i n s , t h e least. T h e c o r r e l a t i o n of these p r o p erties w i t h their c h e m i c a l structure w i l l be made clear i n the study of their chemistry. Apart from the synthetic thermosetting polymer adhesives, o t h e r p o l y m e r s s u c h as p o l y v i n y l a c e t a t e a n d p o l y a m i d e s a r e u s e d i n


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m u c h s m a l l e r q u a n t i t i e s . A d h e s i v e s o f n a t u r a l o r i g i n , s u c h as a n i m a l , casein, soybean, starch, a n d b l o o d glues, are n o w largely s u p p l a n t e d b y t h e m o r e v e r s a t i l e s y n t h e t i c p o l y m e r s . H o w e v e r , c a s e i n is u s e d f o r s t r u c t u r a l l a m i n a t i n g (5). P h e n o l i c c o m p o u n d s d e r i v e d f r o m t r e e s , s u c h as t a n n i n s , a r e i n c r e a s i n g l y u s e d f o r w o o d b o n d i n g , m o s t l y i n c o m b i n a t i o n w i t h s y n t h e t i c a d h e s i v e c o m p o n e n t s (JO, 11). Phenolic Resin Adhesives. P F r e s i n adhesives are l o w m o l e c ular weight prepolymers formed from phenol and formaldehyde, and a r e o f t h e t h e r m o s e t t i n g t y p e . T h e p o l y m e r i z a t i o n is c o n t r o l l e d b y t h e a c i d o r a l k a l i n e c o n d i t i o n s , i . e . , p H , at w h i c h i t is c o n d u c t e d . A n o t h e r i m p o r t a n t r e a c t i o n v a r i a b l e is t h e m o l a r r a t i o o f p h e n o l to formaldehyde. B y varying the time and temperature of reaction, and b y c h o o s i n g catalysts a n d p h e n o l s of v a r y i n g reactivity, a w i d e variety of a d h e s i v e s can b e p r e p a r e d for m a n y a p p l i c a t i o n s a n d c o n d i t i o n s . RESOLES. Resoles are p h e n o l i c resins p r o d u c e d u n d e r alkaline conditions w i t h a m o l a r excess of f o r m a l d e h y d e , H C H O , over p h e n o l i n t h e r e a c t i o n m i x t u r e . T h e i n i t i a l r e a c t i o n is t h e s u b s t i t u t i o n o f p h e n o l w i t h m e t h y l o l ( - C H O H ) g r o u p s , as s h o w n i n F i g u r e 1, b o t h at t h e ortho (I) a n d para (II) p o s i t i o n s . F u r t h e r m o r e , b e c a u s e m o r e t h a n 1 m o l o f f o r m a l d e h y d e is u s e d f o r e a c h m o l e o f p h e n o l , p r o d u c t s c a r r y i n g t w o o r t h r e e m e t h y l o l g r o u p s (ΠΙ, IV) a r e a l s o f o r m e d . T h e ortho.para substitution ratio depends on the type of c a t a l y s t a n d p H , a n d d e c r e a s e s f r o m 1.1 at a p H o f 8 . 7 t o 0 . 3 8 at a 2

Figure 1. Mixture of reaction products in resole formed from phenol and formaldehyde.


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p H o f 1 3 . 0 (9). A l k a l i a n d a l k a l i n e - e a r t h m e t a l h y d r o x i d e s c o n s i d e r a b l y e n h a n c e ortho s u b s t i t u t i o n i n t h e o r d e r M g > C a > S r > B a > L i > N a > Κ (9). T h e c h e l a t i n g s t r e n g t h o f t h e m e t a l c a t i o n h a s a p r o n o u n c e d c a t a l y t i c e f f e c t o n d i r e c t i n g ortho s u b s t i t u t i o n , a n d c a t ions o f t r a n s i t i o n metals F e , C o , M n , etc. are e v e n m o r e effective. T h e i n i t i a l f o r m a t i o n o f t h e s e m e t h y l o l - s u b s t i t u t e d p h e n o l s is followed b y their reaction w i t h each other and w i t h unreacted p h e n o l to g i v e a c o m p l e x m i x t u r e o f m o l e c u l e s o f d i f f e r e n t s i z e s a n d d e g r e e s o f b r a n c h i n g i n w h i c h t h e p h e n o l i c n u c l e i a r e c o n n e c t e d to e a c h o t h e r through m e t h y l e n e links, - C H - , or m e t h y l e n e ether bridges, - C H - 0 - C H - , as e x e m p l i f i e d i n F i g u r e 1 ( V ) . 2


2

2

T h e e x t e n t to w h i c h t h e m e t h y l o l p h e n o l s l i n k a n d g r o w d e p e n d s upon the conditions of reaction, p H , time, and temperature. In ad d i t i o n , t h e m o l e r a t i o o f f o r m a l d e h y d e to p h e n o l i n t h e r e a c t i o n m i x t u r e , is a n i m p o r t a n t c o n t r o l l i n g f a c t o r . T h e p r o p e r t i e s o f r e s o l e a d hesive resins can be v a r i e d or r e p r o d u c e d only by careful control of t h e s e e x p e r i m e n t a l c o n d i t i o n s . M e t h y l o l f o r m a t i o n is a c c e l e r a t e d b y increasing the p H . M e t h y l o l groups, w h i c h subsequently form the m e t h y l e n e l i n k s l e a d i n g to t h e g r o w t h o f the m a c r o m o l e c u l e , i n c r e a s e w i t h i n c r e a s i n g m o l a r r a t i o o f f o r m a l d e h y d e to p h e n o l . U n d e r a n y o n e set o f c o n d i t i o n s , c o n t i n u e d h e a t i n g o r h e a t i n g at h i g h e r t e m perature results in products of higher viscosity and lower water sol u b i l i t y . R e s o l e s c a n t h u s b e f o r m e d as w a t e r - s o l u b l e l i q u i d r e s i n s o f l o w m o l e c u l a r w e i g h t , a b o u t 1 5 0 , o r as g r i n d a b l e s o l i d s o f m o l e c u l a r w e i g h t a r o u n d 1000. A n a l k a l i n e c a t a l y s t l i k e N a O H is u s e d , w i t h f o r m a l d e h y d e t o p h e n o l r a t i o v a r i e d f r o m 1:1 t o 3:1 ( u s u a l l y 1 . 8 - 2 . 4 : 1 ) f o r p a r t i c l e boards. A p a r t from catalyzing the hydroxymethylation of phenol, N a O H also serves to p r o v i d e t h e r e q u i r e d w a t e r s o l u b i l i t y o f t h e r e s i n , e v e n at a h i g h m o l e c u l a r w e i g h t , t h r o u g h t h e f o r m a t i o n o f s o d i u m p h e n o x i d e s . F u r t h e r m o r e , it accelerates the c u r i n g of the resin. Resins are p r e p a r e d c o m m e r c i a l l y that have extraordinarily l o w m o n o m e r c o n t e n t (less t h a n 0 . 1 % o f e i t h e r p h e n o l o r f o r m a l d e h y d e ) (9). T h e m o l e c u l a r w e i g h t d i s t r i b u t i o n o f t h e t y p i c a l r e s i n s h o w s v e r y small amounts of bis(hydroxymethyl) and tris(hydroxymethyl) phenols, w i t h the b u l k of the product b e i n g larger condensation p r o d u c t s (9). R e s o l e r e p r e s e n t s an i n t e r m e d i a t e stage i n t h e progress of t h e reaction of p h e n o l a n d formaldehyde. T h e final product, i d e a l i z e d i n t h e c r o s s - l i n k e d n e t w o r k s h o w n i n F i g u r e 2 , is f o r m e d b y h e a t i n g the resole to b r i n g about c r o s s - l i n k i n g t h r o u g h the m e t h y l o l groups a l r e a d y p r e s e n t i n t h e r e s i n . F u r t h e r a d d i t i o n of reactants to resole is t h e r e f o r e u n n e c e s s a r y . T h e c u r i n g r e a c t i o n c a n a l s o b e b r o u g h t



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Figure 2. Illustration of the cross-linked structure of phenol-formaldehyde resin. about b y the a d d i t i o n o f s t r o n g acids to the resole i n s t e a d o f b y h e a t i n g it. M o s t l y h y d r o c h l o r i c , p h o s p h o r i c , p-toluenesulfonic, or phenolsulfonic acids are u s e d . C o r r o s i o n of m e t a l substrates a n d l o n g t e r m attack on w o o d a d h e r e n d s are p r o b l e m s associated w i t h the use of a c i d h a r d e n e r s for p h e n o l i c adhesives. A l t h o u g h the a c i d - c u r i n g of r e s o l e a d h e s i v e s is t e c h n i c a l l y n o t i m p o r t a n t , t h e r e a c t i o n o f p h e n o l a n d f o r m a l d e h y d e , u n d e r i n i t i a l l y a c i d i c c o n d i t i o n s , is u s e d to p r o d u c e a n o t h e r class o f p h e n o l i c r e s i n i n t e r m e d i a t e s , novolaks. NOVOLAK. In the acidic p H range, the reaction of p h e n o l and f o r m a l d e h y d e o c c u r s as a n e l e c t r 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 . I n i t i a l l y t h e r e a c t i o n p r o d u c e s m e t h y l o l p h e n o l ( I - I V , F i g u r e 1) as u n d e r a l k a l i n e c o n d i t i o n s . H o w e v e r , t h e m e t h y l o l g r o u p is u n s t a b l e u n d e r acid conditions a n d q u i c k l y forms methylene bridges by further reac t i o n w i t h p h e n o l . B y u s i n g p h e n o l i n e x c e s s , i . e . , at a f o r m a l d e h y d e : p h e n o l ratio less t h a n one, l i n e a r m o l e c u l e s w i t h t e r m i n a l p h e n o l groups are f o r m e d (VI). T h e m o l e c u l a r w e i g h t of these poly m e r i c c h a i n s is u s u a l l y l e s s t h a n 2 0 0 0 . B e c a u s e t h e y a r e u n c r o s s l i n k e d , novolaks are s o l u b l e a n d t h e r m o p l a s t i c . S p e c i a l catalysts, s u c h

VI


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as b i v a l e n t m e t a l a c e t a t e s , n e e d to b e u s e d to o b t a i n high-orthosubstituted novolaks, i n w h i c h the p h e n o l i c n u c l e i are l i n k e d p r e d o m i n a n t l y t h r o u g h t h e o r f / i o - p o s i t i o n (see V I ) (12). I n t h e a b s e n c e o f t h e s e s p e c i a l c a t a l y s t s , para l i n k s a r e a l s o f o r m e d . T h e m e c h a n i s m o f t h e s e l e c t i v e o r f / i o - h y d r o x y m e t h y l a t i o n is f o r m a t i o n o f m e t a l i o n chelates. T h e novolak resin, u n l i k e resoles, does not contain any free m e t h y l o l g r o u p s . T h e r e f o r e , i t is i n c a p a b l e o f f u r t h e r r e a c t i o n w i t h o u t the addition of m o r e formaldehyde. Consequently, h a r d e n i n g of no volaks to t h e i n f u s i b l e , c r o s s - l i n k e d p r o d u c t s h o w n i n F i g u r e 2 c a n be achieved only by further addition of formaldehyde, or formalde h y d e donors. T h e usual f o r m a l d e h y d e donors are paraformaldehyde, or almost invariably, hexamethylenetetramine (methenamine) V I I , b o t h o f w h i c h d e c o m p o s e to f o r m a l d e h y d e u n d e r t h e r e a c t i o n c o n d i t i o n s . E i t h e r o f t h e s e c a n c r o s s - l i n k t h e l i n e a r n o v o l a k s to f o r m m e t h y l e n e bridges that p r o d u c e the n e t w o r k structure s h o w n i n F i g u r e 2. T h e h i g h - o r t T i o - s u b s t i t u t e d n o v o l a k s a r e m o r e r e a c t i v e i n t h i s c r o s s - l i n k i n g s t e p b e c a u s e t h e r e l a t i v e l y m o r e r e a c t i v e para p o s i t i o n h a s b e e n l e f t f r e e d u r i n g t h e i r p r e p a r a t i o n , a n d is n o w a v a i l a b l e for r e a c t i o n w i t h f o r m a l d e h y d e o r h e x a m e t h y l e n e t e t r a m i n e . RESORCINOL RESINS. T h e reactivity of p h e n o l w i t h formaldehyde is g r e a t l y i n c r e a s e d w i t h t w o h y d r o x y l g r o u p s o n its n u c l e u s ( r e s o r c i n o l V I I I ) . R o o m t e m p e r a t u r e p o l y m e r i z a t i o n is o b s e r v e d w i t h o u t the n e e d for any catalyst. T h e rate of reaction goes t h r o u g h a m i n i m u m at a p H o f 3 . 5 a n d i n c r e a s e s at l o w e r o r h i g h e r p H v a l u e s . To m a k e a u s e f u l a d h e s i v e , p r e p o l y m e r s , s i m i l a r to n o v o l a k s , a r e p r e -

VII

OH



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p a r e d f r o m r e s o r c i n o l a n d f o r m a l d e h y d e at a l o w m o l e r a t i o , — 0 . 5 0 . 7 , of the aldehyde. T h e p r e p o l y m e r w i t h the reactive resorcinol n u c l e i c a n t h e n b e c u r e d , e v e n at a m b i e n t c o n d i t i o n s , w i t h f u r t h e r a d d i t i o n o f f o r m a l d e h y d e , u s u a l l y as p a r a f o r m a l d e h y d e , to f o r m t h e adhesive bond. T h e s e n s i t i v i t y o f r e s o r c i n o l r e s i n c u r i n g to c h a n g e s i n p H has interesting consequences i n the variability of b o n d strength w i t h spe cies o f w o o d b o n d e d . S p e c i m e n s of E n g l i s h oak, for e x a m p l e , w h i c h gave l o w strength w h e n b o n d e d b y resorcinolic resins, w e r e found to g i v e a n a q u e o u s extract o f p H 3 . 7 , close to the p o i n t w h e r e t h e r e a c t i v i t y o f t h e r e s i n is a m i n i m u m ( 3 ) . T h i s r e t a r d a t i o n o f c u r i n g b y the a c i d i t y of oak c o u l d b e o v e r c o m e b y w a s h i n g the s p e c i m e n w i t h s o d i u m acetate after w h i c h the s t r e n g t h p r o d u c e d b y r e s o r c i n o l i c r e s i n a d h e s i v e w a s e x c e l l e n t . T h e effect o f e x t r a c t i v e s o n g e l a tion of c u r i n g resins w i l l be discussed again later. Modifications can be made i n phenolic resins by using alkyl sub s t i t u t e d p h e n o l s t o p r o d u c e l e s s r e a c t i v e r e s i n s , to i m p r o v e c o m p a t i b i l i t y w i t h r u b b e r , a n d to i m p r o v e m o i s t u r e resistance, e l e c t r i c a l p r o p e r t i e s , etc. APPLICATION

OF

PHENOL— FORMALDEHYDE

RESIN

ADHESIVES.

The

w o o d i n d u s t r y is a m a j o r o u t l e t f o r p h e n o l - f o r m a l d e h y d e ( P F ) a d hesives a n d uses about 2 5 % of t h e P F r e s i n p r o d u c e d i n m a k i n g particle boards a n d p l y w o o d s , a n d i n structural b o n d i n g . I n these a p p l i c a t i o n s , r e s o l e s a r e u s e d a l m o s t e x c l u s i v e l y , as d r y films, l i q u i d resins, or p o w d e r e d resins. W o o d contains a variety of reactive func t i o n a l g r o u p s , i n a d d i t i o n to h y d r o x y l s , capable of b o t h h y d r o g e n b o n d i n g a n d c h e m i c a l reaction w i t h P F resins. F o r example, lignin, w h i c h c o n s t i t u t e s a b o u t 2 0 - 2 5 % o f w o o d , is p h e n o l i c i n s t r u c t u r e , a n d c a n c o r e a c t w i t h P F a d h e s i v e s (13). F u r t h e r m o r e , r e s i n v i s c o s i t y is l o w e n o u g h t o e n a b l e p e n e t r a t i o n o f m i c r o p o r e s o f w o o d , a n d cause m e c h a n i c a l anchorage of the adhesive. Finally, the cohe sive s t r e n g t h o f t h e r e s i n surpasses that o f w o o d . A l l of these factors c o n t r i b u t e to t h e s t r e n g t h o f t h e a d h e s i v e l y b o n d e d w o o d p r o d u c t s . I n p r e p a r i n g d r y films, s p e c i a l p a p e r is i m p r e g n a t e d w i t h a r e s o l e s o l u t i o n a n d d r i e d . T h e d r y film f o r m h a s l o n g s t o r a g e l i f e , u p t o 1 2 m o n t h s a n d is p a r t i c u l a r l y w e l l a d a p t e d t o g l u i n g t h i n a n d c r o t c h veneers because p r o b l e m s of c o n t r o l l i n g spread are avoided. T h e m o i s t u r e n e e d e d to e n a b l e t h e r e s o l e to b e c o m e a l i q u i d u n d e r a p p l i e d p r e s s u r e h a s to c o m e f r o m t h e w o o d ; t h e r e f o r e , t h e m o i s t u r e content should be controlled carefully and maintained above 6 % , preferably i n the range of 8 - 1 2 % d e p e n d i n g o n the species g l u e d a n d t h e v e n e e r t h i c k n e s s . L i q u i d r e s o l e s a r e p r e p a r e d as a q u e o u s o r a l c o h o l i c s o l u t i o n s o f t h e P F r e s i n . R e s i n p o w d e r s , p r e f e r r e d for l o n g s t o r a g e l i f e , c a n b e d i s s o l v e d at t h e t i m e o f a p p l i c a t i o n to y i e l d l i q u i d


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adhesives. T h e hot-setting p h e n o l i c adhesives are u s e d mostly for b o n d i n g e x t e r i o r s o f t w o o d p l y w o o d a n d r e q u i r e p r e s s i n g at 1 3 0 - 1 5 0 ° C f o r a f e w m i n u t e s . U s u a l l y a f i l l e r s u c h as c o c o n u t o r w a l n u t s h e l l f l o u r is u s e d t o a d j u s t w e t t i n g , a v o i d e x c e s s i v e p e n e t r a t i o n , a n d t o obtain u n i f o r m j o i n t thickness. T h e fillers r e d u c e cost a n d , w h e n u s e d in reasonable proportions, the brittleness of the adhesive joint. A s i n d i c a t e d p r e v i o u s l y , a s i g n i f i c a n t f e a t u r e o f t h e s e r e s i n s is the very l o w content ( R N H + C 0 > (ΧΠ) R-NH R-NH-C(0)-NH-R' (XIII) = C= 0/R'NH-C(0)-NH-R" R'-N(C(0)NHR)-C(0)NHR (XIV) R'NHC(Q)-OR R'-N(C(Q)NHR)-C(Q)QR (XV) ο II 2R-N = C = Q R C R 2

2

2

2

R-N

v

Ν


Ν

Ο

Ο R (XVI)

Scheme 2. Typical reactions of isocyanates leading to the formation of substituted carbamic acid (XII), substituted urea (XIII), biuret (XIV), allophanate (XV), and isocyanurate (XVI). merization, and produces excellent adhesion (26). In the case of wood surface, the excellent wetting achieved by isocyanates leads to i m mediate penetration of exposed cells by the isocyanate adhesive. The wetting a n d permeation of polymeric isocyanates mixed with a small amount of dye were observed in optical and scanning electron micrographs (27). T h e ready formation of an interphase by the p e r meation of p o l y m e r i c isocyanates has important consequences for m e chanical anchorage of adhesive as well as for the fracture behavior of the b o n d e d wood composite. T a n n i n - B a s e d Adhesives. C o n d e n s e d t a n n i n s — o b t a i n e d as extracts of the barks of trees such as wattle, hemlock, or p i n e — consist of flavonoid units that have undergone varying degrees of condensation. T h e phenolic rings of tannins are important to adhesive chemistry because they have reactive sites available for condensation with formaldehyde. T h e main polyphenolic pattern is represented by f l a v o n o i d analogs based on r e s o r c i n o l - p y r o g a l l o l , r e s o r c i n o l - c a t echol, p h l o r o g l u c i n o l - p y r o g a l l o l , and p h l o r o g l u c i n o l - c a t e c h o l rings shown in F i g u r e 3. Wattle extracts, rich in the resorcinolic tannins, have b e e n exploited more commercially than tannins of the phloroglucinolic type obtained from pine trees. The major effort in understanding plex derivatives in attempts to use t h e m is exemplified by the work of Pizzi (10, the composition of tannin extracts and

the chemistry of these c o m most effectively in adhesives 11, 28). A detailed study of their reaction characteristics


340



OH

OH

OH XIX

XX

Figure 3. Polyphenols patterns occurring in tannins. Key: X V I I . resor cinol A-pyrogallol Β; XVIII, resorcinol A-catechol Β; X I X , phloroglu cinol A-pyrogallol B; and X X , phloroglucinol A-catechol Β ring systems. a n d m a c r o m o l e c u l a r s t r u c t u r e (10,

11, 28) has l e d t o t h e f o r m u l a t i o n

of processes for adhesives f r o m t a n n i n extracts that m a y be

commer

cially useful. T h e basic reactions w e r e discussed i n the section on P F resins; experimental details may be found i n References

10,

11,

a n d 28. Tannins are most useful w h e n a p p l i e d i n c o m b i n a t i o n w i t h s y n t h e t i c resins. I n a d d i t i o n to the c r o s s - l i n k i n g r e a c t i o n u t i l i z i n g the phenolic rings, the reaction of phenolic hydroxyls w i t h

isocyanate

groups can b r i n g about additional cross-linking and improve the re sults considerably. E x t e r i o r - g r a d e p a r t i c l e b o a r d adhesives have b e e n made by combinations of tannins w i t h commercial d i p h e n y l m e t h a n e d i i s o c y a n a t e a n d f o r m a l d e h y d e (10,

28,

29).

T h e i s o c y a n a t e g r o u p is

d e a c t i v a t e d b y w a t e r less r e a d i l y i n t h e p r e s e n c e o f a p h e n o l ; t h e r e fore, the reaction of isocyanate w i t h a w a t e r solution of t a n n i n extract b e c o m e s q u i t e p r a c t i c a b l e (10).

C o n t i n u e d research on using tannins

for a d h e s i v e s c a n f u r t h e r t h e goal of s u b s t i t u t i n g p l a n t c h e m i c a l s for expensive petrochemical intermediates. Thermoplastic Adhesives.

T h e p o l y m e r adhesives

described

i n t h e f o l l o w i n g s e c t i o n s a r e n o t u s e d as e x t e n s i v e l y w i t h w o o d as the thermosetting adhesives discussed earlier. H o w e v e r , they do i l lustrate m a n y i n t e r e s t i n g p r i n c i p l e s of the c h e m i s t r y of adhesion through organic polymers.


9.

SUBRAMANIAN


HOT-MELT

341

Chemistry of Adhesion ADHESIVES.

Thermoplastic polymers,

it m a y b e

re

c a l l e d , a r e e s s e n t i a l l y l i n e a r p o l y m e r s that soften a n d f l o w w h e n heated and solidify o n cooling. Therefore, m a n y of t h e m are suitable f o r u s e as h o t - m e l t a d h e s i v e s . B e c a u s e t h e c o o l i n g o c c u r s q u i c k l y a n d t h e r e i s n o c u r i n g r e a c t i o n i n v o l v e d , fast a s s e m b l y o n a m a s s s c a l e , as i n p a p e r b a c k b o o k b i n d i n g , is p o s s i b l e . S o m e p r o b l e m s t h a t f o l l o w s o l v e n t u s e a r e a v o i d e d b e c a u s e n o s o l v e n t is r e q u i r e d . O n e s u c h p r o b l e m is s o l v e n t p o l l u t i o n . A n o t h e r is s w e l l i n g b y t h e s o l v e n t , like swelling o f w o o d b y water-based systems. H o w e v e r , because hotm e l t adhesives a r e b a s e d o n p o l y m e r s that soften o n h e a t i n g , t h e joints m a d e w i t h h o t - m e l t adhesives are susceptible to softening w h e n e v e r they are exposed to sufficiently h i g h temperature. L o w m o l e c u l a r w e i g h t p o l y e t h y l e n e , ( C H - C H ) , is u s e d w i d e l y as a h o t - m e l t a d h e s i v e . T o i m p r o v e t h e a d h e s i v e p r o p e r t i e s o f t h i s h y d r o c a r b o n p o l y m e r , e s p e c i a l l y t o p o l a r s u b s t r a t e s , e t h y l e n e is c o p o l y m e r i z e d w i t h v i n y l acetate (VAc), w h i c h introduces t h e polar acetate g r o u p s i n t o t h e p o l y m e r , to m a k e e t h y l e n e - v i n y l acetate c o polymers ( E V A c ) , 4 0 Η - Ο Η ) - [ Ο Η 2 - 0 ( Ο Ο Ο Ο Η ) Η ^ . These copoly m e r s a r e a v e r s a t i l e class o f h o t - m e l t adhesives w h o s e p r o p e r t i e s can b e m o d i f i e d o v e r a w i d e r a n g e b y v a r y i n g t h e p r o p o r t i o n o f V A c c o m o n o m e r f r o m 1 8 t o 4 0 % (8). T h e y a r e r a n d o m c o p o l y m e r s , w h o s e m o l e c u l a r w e i g h t c a n b e v a r i e d to a l t e r f l o w p r o p e r t i e s . T h e ease o f f l o w , a n i m p o r t a n t f a c t o r i n w e t t i n g , is m e a s u r e d b y t h e m e l t i n d e x , w h i c h is t h e w e i g h t o f p o l y m e r f l o w i n g t h r o u g h a g i v e n o r i f i c e i n 1 0 m i n at a fixed t e m p e r a t u r e a n d p r e s s u r e p r e s c r i b e d b y A S T M s t a n d a r d s (30). T h e h i g h e r t h e m o l e c u l a r w e i g h t o f t h e p o l y m e r , t h e l o w e r the m e l t index. C o p o l y m e r s w i t h m e l t indices v a r y i n g from 2 to 200, s u c h as E V A c c o p o l y m e r s , a n d e t h y l e n e - a c r y l i c a c i d , a r e e m p l o y e d in various applications. 2

2

2

χ

n

3

M o r e w i d e l y u s e d i n w o o d b o n d i n g are h o t - m e l t adhesives based o n p o l y a m i d e s . A p o l y a m i d e is f o r m e d w h e n a m o n o m e r c a r r y i n g t w o a m i n e g r o u p s ( - N H ) is r e a c t e d w i t h a n o t h e r c a r r y i n g t w o c a r b o x y l i c a c i d g r o u p s ( - C O O H ) . E a c h a m i n e g r o u p , at e i t h e r e n d o f t h e m o l ecule, can condense with a carboxylic acid group of the other m o n o m e r to form an amide link ( - C O N H - ) w i t h the elimination of a water molecule: 2

- N H

2

+ H O O C -

^ >

- N H C O -

By such step wise condensation o f the a m i n e a n d carboxylic groups o f m a n y m o n o m e r m o l e c u l e s , a l o n g c h a i n p o l y a m i d e , k n o w n as n y l o n XXI is f o r m e d .


342

THE

H N(CH ) NH 2

2

6

CHEMISTRY O F SOLID WOOD

2

+ HOOC(CH ) COOH 2

4

(NH-(CH ) -NHCO(CH ) -CO-) 2

6

2

4

n


(XXI) T h e p o l a r i t y o f p o l y a m i d e s t h a t a r e u s e d as t h e r m o p l a s t i c , h o t - m e l t adhesives can be v a r i e d b y altering the proportion of hydrocarbon ( - C H 2 - ) and amide ( - C O N H - ) groups i n - C O C C H ^ C O N H - C C H ^ N H - . T h e y a r e m o r e e x p e n s i v e t h a n E V A c c o p o l y m e r s , b u t set m o r e r a p i d l y , a n d a r e p r e f e r r e d as e d g e v e n e e r a d h e s i v e s i n t h e f u r n i t u r e industry. ADHESIVE EMULSIONS. Thermoplastic, synthetic polymers can be p r e p a r e d as e m u l s i o n s f o r u s e as a d h e s i v e s . F o r e x a m p l e , w h i l e E V A c h o t - m e l t a d h e s i v e s d e s c r i b e d i n t h e p r e v i o u s s e c t i o n c o n t a i n less t h a n 4 0 % V A c , w h e n t h e c o n t e n t o f V A c i n t h e c o p o l y m e r is i n c r e a s e d to 6 0 % , a n d t h e c o p o l y m e r is p r e p a r e d i n t h e f o r m o f a q u e o u s e m u l s i o n s , a v e r y u s e f u l a n d v e r s a t i l e a d h e s i v e p o l y m e r is o b t a i n e d . A l t h o u g h t h e V A c h o m o p o l y m e r , p o l y v i n y l a c e t a t e ) , is a b r i t t l e s o l i d , w i t h a T = 28 °C, the ethylene units present i n the E V A c c o p o l y m e r act as a n internal plasticizer, a n d l o w e r the T to b e l o w r o o m t e m perature. T h e plasticization results from the reduction of interchain interaction of the V A c p o l y m e r chains b y the ethylene units inter spersed a m o n g the strongly interacting V A c units. T h i s reduction of the T has i m p o r t a n t c o n s e q u e n c e s b e c a u s e the f o r m a t i o n o f a f l e x i b l e a d h e s i v e f i l m f r o m the e m u l s i o n d e p e n d s u p o n the T o f the p o l y m e r . g

g

g

R

T h i s e m u l s i o n is n o t a l i q u i d - l i q u i d s y s t e m , b u t is a n a q u e o u s dispersion of solid p o l y m e r particles. Therefore, if the T were above r o o m t e m p e r a t u r e (at w h i c h t h e e m u l s i o n is a p p l i e d ) , t h e p o l y m e r segments, l a c k i n g s e g m e n t a l m o b i l i t y , w o u l d not diffuse r e a d i l y f r o m o n e p a r t i c l e o f t h e e m u l s i o n i n t o a n o t h e r after e v a p o r a t i o n o f the w a t e r m e d i u m i n w h i c h t h e c o p o l y m e r e m u l s i o n is p r e p a r e d . T h e r e s u l t w o u l d b e a p o w d e r y film. C o n v e r s e l y , w h e n t h e Τ is r e d u c e d b e l o w r o o m t e m p e r a t u r e , segmental m o b i l i t y i n the c o p o l y m e r leads to diffusion a n d f o r m a t i o n o f a f l e x i b l e , s t r o n g a d h e s i v e f i l m f r o m the l a t e x b y c o a l e s c e n c e o f t h e e m u l s i f i e d p a r t i c l e s d u r i n g d r y i n g at r o o m temperature. g

P o l y v i n y l acetate) ( P V A c ) , { C H - C H ( O C O C H ) » , is also p r e p a r e d as an e m u l s i o n for a d h e s i v e a p p l i c a t i o n s , a n d is f a m i l i a r to users as white glue. A s m e n t i o n e d a l r e a d y , t h e T o f P V A c is a b o v e r o o m t e m p e r a t u r e , w h i c h m a k e s t h e p o l y m e r r i g i d a n d b r i t t l e at r o o m t e m p e r a t u r e . F o r a d h e s i v e a p p l i c a t i o n , t h e r e f o r e , a n external plas ticizer, s u c h as d i b u t y l p h t h a l a t e , is a d d e d t o l o w e r t h e T below r o o m t e m p e r a t u r e a n d to facilitate f i l m f o r m a t i o n f r o m e m u l s i o n s . 2

3

T

g

g


9.

SUBRAMANIAN

343



T h e f u n c t i o n o f a d d e d p l a s t i c i z e r m o l e c u l e s is t o r e d u c e t h e i n t e r chain interaction of the V A c p o l y m e r chains a n d facilitate the m o b i l i t y of the p o l y m e r segments. P V A c e m u l s i o n s h a v e e x c e l l e n t a d h e s i o n to c e l l u l o s i c m a t e r i a l s a n d find e x t e n s i v e u s e i n b o n d i n g p a p e r a n d i n w o o d a s s e m b l y i n a w i d e v a r i e t y of p r o d u c t s — p a p e r bags, m i l k cartons, envelopes, b o o k s , p e n c i l s , e t c . H i g h b o n d s t r e n g t h , fast set, a n d c o l o r l e s s g l u e lines c o m b i n e d w i t h ease o f a p p l i c a t i o n are advantages of P V A c e m u l sions i n w o o d b o n d i n g . A s m e n t i o n e d earlier, the p o l y m e r e m u l s i o n is a n a q u e o u s d i s p e r s i o n o f s w o l l e n s o l i d p a r t i c l e s . O n e o b v i o u s a d v a n t a g e o f a n e m u l s i o n is t h e u s e o f w a t e r as t h e m e d i u m f r o m w h i c h t h e p o l y m e r is a p p l i e d . T h e o t h e r a d v a n t a g e is t h a t t h e e m u l s i o n e n a b l e s t h e Solids c o n t e n t o f t h e p o l y m e r a d h e s i v e to b e h i g h e r t h a n in s o l u t i o n . B e c a u s e the viscosity of a p o l y m e r solution rises r a p i d l y w i t h the molecular weight a n d concentration of polymer, the re q u i r e d c o n c e n t r a t i o n m a y r e d u c e f l u i d i t y a n d d e c r e a s e t h e ease o f application of the adhesive. H o w e v e r , the high molecular weight of t h e p o l y m e r h a s l i t t l e effect o n t h e v i s c o s i t y o f t h e e m u l s i o n . T h e r e fore an aqueous d i s p e r s i o n w i t h a h i g h ratio of h i g h m o l e c u l a r w e i g h t solids to v i s c o s i t y c a n b e p r e p a r e d a n d a p p l i e d to p r o d u c e a d h e s i v e films o f d e s i r a b l e s t r e n g t h a n d t h i c k n e s s . T h e l o w e r i n g of the T of P V A c does result i n some creep of the a d h e s i v e b o n d . I n o r d e r to p r e v e n t s l i d i n g o f m a c r o m o l e c u l e s , w h i c h p r o d u c e s c r e e p , V A c is c o p o l y m e r i z e d w i t h s m a l l a m o u n t s o f c o m o n o m e r s that can b e c r o s s - l i n k e d d u r i n g c u r i n g of the adhesive. T h u s , a s i g n i f i c a n t r e d u c t i o n i n c r e e p is a c h i e v e d b y c r o s s - l i n k i n g t h e p o l y m e r chains a n d r e d u c i n g t h e i r s u s c e p t i b i l i t y to d i s p l a c e m e n t r e l ative to e a c h o t h e r . S u c h c u r a b l e P V A c e m u l s i o n s are u s e d i n t h e b o n d i n g of n o n w o v e n fabrics, i . e . , to i m p a r t w a s h a n d d r y - c l e a n r e sistance. C o p o l y m e r i z a t i o n of V A c w i t h a flexible c o m o n o m e r results i n t a c k y , soft, l o w - T c o p o l y m e r e m u l s i o n s s u i t e d f o r p r e s s u r e s e n sitive adhesives. g

g

P V A c is also the p r e c u r s o r to a f e w other types o f adhesives. F o r e x a m p l e , p o l y v i n y l alcohol) ( P V A ) , - [ C H - C H ( O H ) , is f o r m e d b y the h y d r o l y s i s o f P V A c . L o w m o l e c u l a r w e i g h t grades o f P V A a r e u s e d i n P V A c e m u l s i o n s to i m p r o v e c o l l o i d a l s t a b i l i t y o f t h e e m u l s i o n s , i . e . , as a p r o t e c t i v e c o l l o i d . H i g h e r m o l e c u l a r w e i g h t g r a d e s , o n l y p a r t i a l l y h y d r o l y z e d , m o d i f y o t h e r p r o p e r t i e s s u c h as a d h e s i o n , v i s c o s i t y , a n d film f o r m a t i o n . 2

T n

W i t h its h i g h h y d r o x y l c o n t e n t a n d e x c e l l e n t b i n d i n g capacity, P V A is u s e d w i d e l y as a w a t e r - s o l u b l e a d h e s i v e w i t h e x c e l l e n t a d h e sion to p a p e r , a n d n a t u r a l a n d s y n t h e t i c fibers. W h e n the - O H groups i n partially h y d r o l y z e d P V A c are reacted w i t h aldehydes, a c e t a l u n i t s a r e f o r m e d so t h a t t h e p o l y m e r n o w c o n t a i n s a c e t a l


344

THE CHEMISTRY O F SOLID WOOD

groups i n a d d i t i o n to t h e a l c o h o l a n d acetate units already present ( R e a c t i o n 2).

• - - C H — CH — C H — CH — C H — C H — C H — 2

2

2

CH-

2

I OJH

OH

HJO

0

1

-oII

-H 0 2

C=0

I

CH,

C — H

I


R C H — CH — C H — C H f f C H — C H - f f C R 0

0

\

0

'

0

\ / R

c

/

0

I

OH

CH - +

I 0

1

/

(2)

CO

i

\

CH

3

H ALCOHOL

ACETATE

ACETAL

Poly(vinyl acetals) c a n b e a p p l i e d f r o m s o l u t i o n i n o r g a n i c s o l v e n t s o r as h o t - m e l t a d h e s i v e s . T h e p r o p e r t i e s o f t h e s e p o l y m e r s depend on their molecular weights, on the degree of hydrolysis of the acetate, a n d o n t h e type a n d p r o p o r t i o n of acetal units. Polyvinyl butyral) a n d p o l y ( v i n y l formal) f o r m e d b y reaction w i t h b u t y r a l d e h y d e a n d f o r m a l d e h y d e , respectively, are examples of such acetal r e s i n s . P o l y ( v i n y l acetals) a r e c a p a b l e o f c r o s s - l i n k i n g b y h e a t o r m i n eral acids. I n a d d i t i o n , t h e residual h y d r o x y l groups can condense with methylol derivatives i n phenol - formaldehyde a n d m e l a m i n e f o r m a l d e h y d e resin systems, w i t h isocyanates, a n d w i t h epoxy resins. T h e r e f o r e , i t is n o m e r e c o i n c i d e n c e that p o l y v i n y l formal), c u r e d w i t h p h e n o l i c r e s i n s , w a s t h e first m a t e r i a l t o f i n d m a j o r s t r u c t u r a l adhesive use with metals. L i k e w i s e , many commercial adhesives b a s e d o n p o l y v i n y l acetals) i n v o l v e c u r i n g w i t h t h e r m o s e t t i n g r e s i n s , mainly, p h e n o l i c s ; t h e t y p e s o f a c e t a l , a n d t h e r a t i o s o f t h e t w o c o m ponents i n the adhesive provide a range of performance. T h e polyvinyl a c e t a l ) - P F resin adhesive systems are excellent for w o o d b o n d i n g (3). P o l y v i n y l acetals) a r e a l s o c o m p o u n d e d w i t h a p p r o p r i a t e p l a s t i c i z e r a n d o t h e r a d d i t i v e s f o r u s e as h o t - m e l t a d h e s i v e s . A s h o t - m e l t a d h e s i v e s , t h e y p r o v i d e m o d e r a t e s t r e n g t h as s t r u c t u r a l a d h e s i v e s f o r b o n d i n g w o o d to m e t a l . P o l y ( v i n y l b u t y r a l ) is w e l l k n o w n i n i t s a p p l i c a t i o n as t h e i n t e r -


9.

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345

l a y e r i n l a m i n a t e d a u t o m o t i v e safety g l a s s . L a m i n a t e d a r c h i t e c t u r a l glass is m a d e s i m i l a r l y w i t h p o l y ( v i n y l b u t y r a l ) , w i t h c o n t r o l l e d t r a n s m i s s i o n o f l i g h t a n d h e a t , t o p r o v i d e a e s t h e t i c a p p e a l as w e l l as r e d u c e glare, heat loss, a n d U V - l i g h t t r a n s m i s s i o n . M u l t i p l e laminates are useful i n m a k i n g transparent bulletproof shields. A s a component of washcoats a n d sealers, p o l y ( v i n y l butyral) finds use i n w o o d finishing.


Acidity of Wood T h e m a r k e d a c i d i t y o f s o m e s p e c i e s o f w o o d s u c h as o a k is w e l l k n o w n . Before c o m p l e t i n g the discussion of the c h e m i s t r y of adhesion t o w o o d , i t is n e c e s s a r y t o c o n s i d e r t h e i n t e r a c t i o n b e t w e e n w o o d a c i d i t y a n d a d h e s i v e c h e m i s t r y . It was p o i n t e d out e a r l i e r that the b o n d s t r e n g t h o f r e s o r c i n o l i c a d h e s i v e s to o a k s u r f a c e s w a s s i g n i f i c a n t l y r e d u c e d b y t h e a c i d i t y o f t h e l a t t e r . T h a t t h i s is a g e n e r a l p h e n o m e n o n is a p p a r e n t f r o m s e v e r a l s t u d i e s o f t h e c h e m i c a l r e a c tivity of p h e n o l i c a n d u r e a resins i n the presence of several species o f w o o d (31, 32). T h e c u r i n g r e a c t i o n s o f t h e r e s i n s c o u l d b e r e t a r d e d b y s o m e w o o d s a n d o t h e r s h a d h a r d l y a n y effect. It has b e e n s u g gested that s o m e l o w m o l e c u l a r w e i g h t substances migrate f r o m the w o o d i n t o the a d h e s i v e p h a s e to r e t a r d t h e c u r i n g reaction i n s o m e cases. T h e r e h a v e b e e n m a n y a t t e m p t s t o i n v e s t i g a t e t h e effect o f e x t r a c t i v e s o n c u r e c h e m i s t r y a n d b o n d i n g t o w o o d (33-35). F o r ex a m p l e , t h e e f f e c t o f e x t r a c t i v e s f r o m p r e s s u r e - r e f i n e d h a r d w o o d fiber o n u r e a - f o r m a l d e h y d e r e s i n w a s s t u d i e d (34, 35) a n d i t w a s f o u n d t h a t t h e e t h a n o l - s o l u b l e e x t r a c t i v e s d e c r e a s e d t h e g e l t i m e as m u c h as 4 1 % , a n d t h e s e q u e n t i a l l y e x t r a c t e d w a t e r - s o l u b l e e x t r a c t i v e s i n creased the g e l t i m e i n excess of 6 5 % . T h e r e was little correlation between the extractive content and gel time; however, an e m p i r i c a l relation b e t w e e n the p H of the extractives a n d the gel t i m e was o b s e r v e d (35). T h e e f f e c t o f s e v e r a l s p e c i e s o f w o o d o n t h e g e l t i m e o f u r e a - f o r m a l d e h y d e r e s i n h a v e a l s o b e e n s t u d i e d (36). I n t h e s e studies the gel t i m e was correlated w i t h the p H a n d acid buffering capacity of the extract. H o w e v e r , i t a p p e a r s t h a t b e t t e r c o r r e l a t i o n w i t h g e l t i m e s is obtained w h e n the amounts of extractable and unextractable w o o d a c i d s a r e c o n s i d e r e d s e p a r a t e l y (37). A l t h o u g h s e v e r a l p r o c e d u r e s d o exist i n the l i t e r a t u r e for t h e d e t e r m i n a t i o n of a c i d c o n t e n t i n w o o d , m a n y of these i n v o l v e extraction of w o o d w i t h various solvents, a n d they estimate the acid content i n the extractives only. I n contrast, w e d e v e l o p e d a p r o c e d u r e that acknowledges the presence of soluble as w e l l as i n s o l u b l e a c i d s i n w o o d . T h e m e t h o d is b a s e d o n t h e r e a c t i o n o f w o o d acids w i t h a q u e o u s s o d i u m acetate to l i b e r a t e an e q u i v alent a m o u n t of acetic acid. S u b s e q u e n t p H titration gives the total a c i d content. T h e soluble acids are d e t e r m i n e d b y w a t e r extraction


346

T H E CHEMISTRY OF SOLID W O O D

a n d titration. Subtraction of soluble acids from total acid content gives the insoluble or b o u n d acid i n wood. T h e reaction of the b o u n d , i.e., unex tractable, w o o d - carboxylic acid can be w r i t t e n i n a simplified m a n n e r as f o l l o w s : W o o d ^ C O O H + C H - C O O N a -> W o o d ' W C O O N a + C H -


3

3

COOH

O u r estimates of the acid contents of heartwoods and sapwoods of r e d oak, h i c k o r y , s o u t h e r n p i n e , w h i t e fir, a n d D o u g l a s - f i r are g i v e n i n Table I. T h e a c i d contents w e r e correlated w i t h the gel times o f u r e a - f o r m a l d e h y d e r e s i n s (36) i n c o n t a c t w i t h t h e s a m e s a m p l e lots of the different species of w o o d . T h e correlations are p r e s e n t e d i n T a b l e I I . I t is s e e n f r o m T a b l e I I t h a t t h e b e s t c o r r e l a t i o n o f g e l t i m e s is o b t a i n e d w i t h i n s o l u b l e a c i d c o n t e n t f o r e a c h o f t h e e m p i r i c a l fits t h a t w e r e t r i e d . S i m i l a r l y , t h e p H o f t h e s o d i u m a c e t a t e e x t r a c t s showed better correlations w i t h the gel times than the p H of the w a t e r e x t r a c t s (37). T h e unextractable a c i d i n w o o d plays a major role i n the catalysis of the u r e a - f o r m a l d e h y d e polycondensation reaction. T h e signifi c a n c e o f this i n d i c a t i o n m u s t b e v i e w e d i n contrast to p r e v i o u s i n vestigations w h i c h have a t t e m p t e d to correlate p r o p e r t i e s of w o o d w i t h t h e p r o p e r t i e s o r a m o u n t s o f e x t r a c t i v e s . It w o u l d n o t b e p r u d e n t t o g e n e r a l i z e r e g a r d i n g t h e effect o f u n e x t r a c t e d a c i d s b e c a u s e only seven species w e r e studied. H o w e v e r , i n future studies these o b s e r v a t i o n s m a y b e f o u n d to b e g e n e r a l l y t r u e for most, i f not a l l , species. T h e m a r k e d i n f l u e n c e of b o u n d acids i n w o o d o n the c u r i n g T a b l e I. A c i d C o n t e n t i n W o o d s Species of Wood R e d oak Heartwood Sapwood Hickory Heartwood Sapwood Southern pine W h i t e fir Sapwood Douglas-fir

Acid

Content

Water-Soluble

(meq/100 Total

a

g

wood)

Gel

Water-Insoluble

h

Time (min)

0

0.118 1.845

2.577 4.183

2.459 2.338

19.00 19.18

1.651 1.982

2.869 3.937

1.218 1.955

25.00 15.00

1.390

4.824

3.434

12.08

0.588 1.930

2.071 9.130

1.483 7.200

16.38 7.75

B y s o d i u m acetate extraction B y difference Values r e p o r t e d i n Ref. 36 for u r e a - f o r m a l d e h y d e reaction. ( R e p r o d u c e d w i t h p e r m i s s i o n from R e f 37. C o p y r i g h t 1983, S p r i n g e r - V e r l a g . )

a

h

c


9.

SUBRAMANIAN


347

Table I I . E m p i r i c a l Correlations Between A c i d Content and G e l T i m e of U r e a - F o r m a l d e h y d e (36) Empirical

Correlation

0

Linear: y = a + b · χ Soluble Total Insoluble Exponential: y = a · e Soluble Total Insoluble L o g a r i t h m i c : y = a + b · In χ Soluble Total Insoluble Power: y = a · x Soluble Total Insoluble

r

a

b

-0.219 -0.770 -0.816

18.608 23.948 22.691

-1.670 -1.799 -2.213

-0.288 -0.863 -0.899

18.936 27.681 24.936

-0.150 -0.138 -0.167

-0.220 -0.727 -0.851

16.361 27.304 23.464

-1.171 -8.238 -8.018

-0.277 -0.805 -0.895

15.470 35.434 25.799

-0.101 -0.624 -0.578


bx

b

T h e acid content (x) is in milliequivalents p e r 100 g w o o d , a n d gel time (y) is i n minutes. ( R e p r o d u c e d w i t h p e r m i s s i o n from R e f 37. C o p y r i g h t 1983, S p r i n g e r - V e r l a g . ) 0

r e a c t i o n of u r e a - f o r m a l d e h y d e resins has s t r o n g i m p l i c a t i o n s for the adhesive b o n d i n g of w o o d . It m u s t not be c o n s t r u e d , h o w e v e r , that t h e p H v a l u e a n d b u f f e r i n g c a p a c i t y o f w o o d are t h e o n l y factors that affect i t s b o n d i n g . F u r t h e r experiments have s h o w n the applicability of this ana l y t i c a l t e c h n i q u e to t h e d e t e r m i n a t i o n o f b o u n d a c i d g e n e r a t e d b y HN0 t r e a t m e n t o f w o o d (38). T h u s t h e k i n e t i c s o f t h e r e a c t i o n c a n be f o l l o w e d . T h e type of a c i d groups generated are carboxylic acids. T h e i r ability to initiate the i n situ p o l y m e r i z a t i o n of furfuryl alcohol i n w o o d has b e e n o b s e r v e d . D e t a i l e d s t u d i e s o f t h e r e a c t i o n s o f w o o d a c i d g r o u p s w i t h a d h e s i v e systems a n d t h e i r i m p l i c a t i o n for a d h e s i v e b o n d i n g of w o o d are subjects d e s e r v i n g of c o n t i n u e d investigation. 3

Literature Cited 1. Kaelble, D. H . "Physical Chemistry of Adhesion"; Wiley-Interscience: New York, 1971. 2. Wake, W. C. "Adhesion and the Formulation of Adhesives"; Applied Sci ence Publishers Ltd.: London, 1976; 2nd ed., 1982. 3. Houwink, R.; Salomon, G. "Adhesion and Adhesives," Vol. 1; "Adhesives," 2nd ed.; Elsevier Publishing Company: New York, 1965. 4. Blomquist, R. F., E d . "Adhesive Bonding of Wood and Other Structural Materials"; Educational Module for Materials Science and Engineering ( E M M S E ) , The Pennsylvania State University, University Park, PA, 1983.



348

THE CHEMISTRY OF SOLID WOOD

5. "Adhesive Bonding of Wood," Technical Bull. No. 1512, U.S. Department of Agriculture, Forest Service, 1975. 6. Kinloch, A. J. J. Mater. Sci. 1980, 15, 2141. 7. Kinloch, A. J. J. Mater. Sci. 1982, 17, 617. 8. Skeist, Irving "Handbook of Adhesives," 2nd ed.; Van Nostrand Reinhold: New York, 1977. 9. Knop, Α.; Scheib, W. "Chemistry and Application of Phenolic Resins"; Springer-Verlag: New York, 1979. 10. Pizzi, A. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 359. 11. Pizzi, A. J. Macromol. Sci., Rev. Macromol. Chem. 1980, C18 (2), 247. 12. Peer, H . G. Rec. Trav. Chim. 1960, 79, 825. 13. Marton, J.; Marton, T.; Falkehag, S. I.; Adler, E . in "Lignin Structure and Reactions," Marton, J., Ed.; A C S A D V A N C E S IN C H E M I S T R Y SERIES No. 59, ACS: Washington, D . C . , 1966; 125. 14. Ebewele, R. O.; River, B. H . ; Koutsky, J. A. J. Adhes. 1982, 14, 189. 15. Ebewele, R. O . ; River, Β. H . ; Koutsky, J. A. Wood Fiber 1979, 11, 197. 16. Ebewele, R. O.; River, Β. H . ; Koutsky, J. A. Wood Fiber 1980, 12, 40. 17. Ebewele, R. O. "The Fracture Mechanics Approach to the Assessment of Adhesive Joint Performance in Bonded Wood Products," Ph.D. thesis, Univ. of Wisconsin: Madison, WI, 1980. 18. Meyer, Beat "Urea Formaldehyde Resins"; Addison-Wesley Publishing Company: New York, 1979. 19. Myers, G. For. Prod. J. 1983, 33 (5), 27. 20. Myers, G. For. Prod. J. in press. 21. Nestler, F. H . Max "The Formaldehyde Problem in Wood-Based Prod ucts—An Annotated Bibliography"; USDA Forest Service General Tech nical Report 1977, F P L - 8 . 22. Rowell, R. M . ; Ellis, W. D . in "Urethane Chemistry and Applications"; Edwards, Κ. N . , E d . ; ACS SYMPOSIUM SERIES No. 172; ACS: Wash ington, D . C . , 1981, 263. 23. Rowell, R. M . ; Ellis, W. D. Wood Sci. 1979, 12, 52. 24. Frink, J. W.; Sachs, H . I. in "Urethane Chemistry and Applications"; E d wards, Κ. N . , E d . , ACS SYMPOSIUM SERIES No. 172; ACS. Wash ington D . C . , 1981, 285. 25. McLaughlin, Α.; Alberino, L . M . ; Farrissey, W. J . ; Waszeciak, D . P. in "Proc. Symposium on Wood Adhesives—Research, Applications and Needs," Madison, WI, Sept. 23-25, 1980, p. 112. 26. Schallenberger, C. S. in "Handbook of Adhesives"; Skeist, I., E d . ; 2nd ed.; Van Nostrand Reinhold: New York, 1977; Chap. 7. 27. Johns, W. E.; Plagemann, W., paper presented at the Annual Adhesion Society Symposium, Savannah, GA, Feb. 20-23, 1983. 28. Pizzi, A. Holz Roh- Werkst. 1982, 40, 293. 29. Pizzi, A. J. Macromol. Sci., Chem. 1981, A16, 1243. 30. A S T M , Standard D1238-70, "Measuring Flow Rates of Thermoplastics by Extrusion Plastometer," 1970. 31. Mizumachi, H . Wood Sci. 1973, 6 (1), 14. 32. Mizumachi, H . ; Morita, H . Wood Sci. 1975, 7 (3), 256. 33. Jain, N. C . ; Gupta, R. C . ; Chauhan, B. R. S. Holzforsch. Holzverwert. 1974, 26 (6), 129. 34. Albritton, R. O.; Short, P. H . For. Prod. J. 1979, 29 (2), 40. 35. Slay, J. R.; Short, P. H . ; Wright, D. C. For. Prod. J. 1980, 30 (3), 22. 36. Johns, W. E.; Niazi Κ. E . Wood Fiber 1981, 12 (4), 255. 37. Subramanian, R. V.; Somasekharan, Κ. N . ; Johns, W. E . Holzforschung 1983, 37, 117. 38. Subramanian, R. V.; Balaba, W. M.; Somasekharan, Κ. N. J. Adhes. 1982, 14, 295. RECEIVED

for review May 19, 1983.

ACCEPTED

August 22,

1983.


The Chemistry of Solid Wood - American Chemical Society

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