Formaldehyde Release from Wood Products - American Chemical

7 Urea-Formaldehyde Resins

Downloaded by UNIV OF MISSOURI COLUMBIA on November 27, 2013 | http://pubs.acs.org Publication Date: August 8, 1986 | doi: 10.1021/bk-1986-0316.ch007

William E. Johns and A. K. Dunker Wood Engineering Laboratory, Washington State University, Pullman, WA 99164-3020

Urea-formaldehyde resin solutions are shown to be do minated by physical associations rather than primary chemical bonding. These physical associations, or colloidal dispersions, are directly related to the thermodynamic balance of secondary bond formation be tween resin and solvent systems. Steric and entripic evaluations of molecule configuration have shown that linear urea-formaldehyde oligomers resemble polypep tides, and have the potential to form both β-sheets and π-helixs. While the exact configuration of the associations is not known, their presence has been confirmed by x-ray analysis, which shows that urea -formaldehyde resins are crystalline in solid form. I t seems o n l y f i t t i n g t h a t t h e most commonly used r e s i n i n t h e w o r l d today i s based on t h e f i r s t o r g a n i c compound t o be s y n t h e s i z e d e n t i r e l y from i n o r g a n i c m a t e r i a l s . Today u r e a - f o r m a l d e h y d e (UF) r e s i n s a r e produced a t t h e r a t e o f m i l l i o n s o f t o n s p e r y e a r . It i s inter e s t i n g t h a t t h i s most common o f s y n t h e t i c b i n d e r s i s one o f t h e most p u z z l i n g t o work w i t h and u n d e r s t a n d . T h i s paper w i l l r e v i e w some r e c e n t work on t h e n a t u r e o f UF r e s i n s from a somewhat d i f f e r e n t ap proach; that o f c o l l o i d a l d i s p e r s i o n s which a r e s i m i l a r t o another more commonly i n v e s t i g a t e d h i g h m o l e c u l a r w e i g h t polymer, p r o t e i n s . Discussion Urea-formaldehyde r e s i n , l i k e phenol-, o r f u r f u r y l a l c o h o l - f o r m a l d e hyde r e s i n s , i s t y p i c a l l y thought o f as r e s u l t i n g from s i m p l e conden s a t i o n c h e m i s t r y . The u l t i m a t e h a r d e n i n g o f t h e r e s i n i s thought t o be t h e r e s u l t o f t h e f o r m a t i o n o f a c r o s s - l i n k e d network b r o u g h t about by a c i d c a t a l y s i s . C u r r e n t r e v i e w s a r e a v a i l a b l e (1, .2) w h i c h d i s c u s s t h i s t r a d i t i o n a l p r e c e p t i o n o f UF r e s i n c h e m i s t r y . In many i n t e r e s t i n g ways, UF r e s i n s a r e d i f f e r e n t from o t h e r types of condensation polymers. While o t h e r l i q u i d r e s i n s a r e c l e a r , UF i s t y p i c a l l y w h i t e o r c l o u d y . H e a t i n g a r e s i n such a s p h e n o l 0097-6156/ 86/ 0316-O076S06.00/ 0 © 1986 American Chemical Society

In Formaldehyde Release from Wood Products; Meyer, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


7.

JOHNS A N D DUNKER

Urea-Formaldehyde

Resins

11

formaldehyde w i l l r e s u l t i n a slow, p r e d i c t a b l e i n c r e a s e i n v i s c o s i t y , w h i l e a UF w i l l remain v i r t u a l l y unchanged i n v i s c o s i t y u n t i l g e l l i n g , a t w h i c h time t h e r e s i n t u r n s a l m o s t i n s t a n t a n e o u s l y i n t o a solid. In t h e f o r m u l a t i o n o f r e s i n s such as p h e n o l - f o r m a l d e h y d e o r epoxy r e s i n s , s t o i c h i o m e t r i c r e q u i r e m e n t s c a l l f o r a 2 + : l mole r a t i o of r e a c t a n t s to achieve a h i g h c r o s s - l i n k d e n s i t y . UF r e s i n c a n be p r e p a r e d a t mole r a t i o s on t h e o r d e r o f 1:1.10 w i t h l i t t l e problem. D u r i n g t h e manufacture o f UF r e s i n w i t h a t y p i c a l cook, an extended a c i d h o l d w i l l r e s u l t i n a r e l a t i v e l y h i g h v i s c o s i t y . The a d d i t i o n o f d r y u r e a s o l i d s b o t h i n c r e a s e s t h e s o l i d c o n t e n t s and produces a s u b s t a n t i a l drop i n v i s c o s i t y . F i n a l l y , t h e o v e r a l l b e h a v i o r o f u r e a toward formaldehyde i s much d i f f e r e n t than i s t h e b e h a v i o r o f , f o r example, formaldehyde w i t h p h e n o l . M i x i n g p h e n o l and formaldehyde a t a r a t i o o f 4:1 i n an a c i d i c medium w i l l r e s u l t i n a r e a c t i o n o f i m p r e s s i v e v i g o r . A t r a t i o s o f 4:1 u r e a and formaldehyde a r e n o t c a p a b l e o f a d v a n c i n g under a c i d i c c o n d i t i o n s even w i t h t h e a p p l i c a t i o n o f h e a t . Ureaformaldehyde c o n c e n t r a t e , a s t a b l e m i x t u r e o f u r e a and formaldehyde a t a mole r a t i o o f 4.8:1 and c o n c e n t r a t i o n s o f as h i g h as 85% s o l i d s , i s a common m a t e r i a l o f commerce. These o b s e r v a t i o n s , t a k e n t o g e t h e r , a r e n o t c o n s i s t e n t w i t h the o r d e r l y f o r m a t i o n o f a u r e a - f o r m a l d e hyde c o n d e n s a t i o n polymer. In o r d e r t o more f u l l y e x p l a i n t h e n a t u r e o f t h e UF system, P r a t t and co-workers ( 3 ) , i n v e s t i g a t e d t h e p o t e n t i a l f o r e x p l a i n i n g UF r e s i n s as c o l l o i d a l d i s p e r s i o n s r a t h e r than o l i g o m e r i c s o l u t i o n s and found t h e r e s u l t s most i n t e r e s t i n g . P r a t t ' s model c o n s i d e r e d the i m p l i c a t i o n s o f c o l l o i d a l b e h a v i o r as r e s u l t i n g from t h e condens a t i o n o f u r e a and formaldehyde t o an o l i g o m e r . A t some p o i n t i n the c o u r s e o f a t y p i c a l r e s i n cook, t h i s o l i g o m e r would c o a l e s c e t o form a s t a b l e c o l l o i d a l p a r t i c l e . T h i s i n i t i a l c o a l e s c i n g would r e s u l t i n the f o r m a t i o n o f t h e clowdy UF, t y p i c a l i n l a r g e s c a l e manufacture. The c o n c e n t r a t i o n o f formaldehyde was c o n s i d e r e d i m p o r t a n t i n t h e f o r m a t i o n o f t h i s c o l l o i d a l system. An e x c e s s o f f o r m a l d e hyde was s u g g e s t e d as f o r m i n g a p r o t e c t i v e s h e a t h around t h e UF p a r t i c l e and thus s t a b i l i z e i t . H a r d e n i n g was a c c o m p l i s h e d by g r a d u a l l y consuming formaldehyde i n c o n t i n u i n g r e a c t i o n s o f u r e a and f o r maldehyde w i t h a c i d c a t a l y s t . A t some p o i n t , i t was s u g g e s t e d , t h e r e would be i n s u f f i c i e n t formaldehyde t o s t a b i l i z e t h e c o l l o i d a l p a r t i c l e and t h e system would harden by c o a l e s c i n g . I f t h e h a r d e n i n g o f a UF i s s i m p l y t h e c o a l e s c i n g o f a c o l l o i d , i t s h o u l d be p o s s i b l e t o see t h e c o l l o i d a l p a r t i c l e i n t h e hardened state. Scanning e l e c t r o n m i c r o p h o t o g r a p h s o f hardened UF polymer a r e shown i n F i g u r e s 1 and 2. F i g u r e 1 shows UF r e s i n c o l l e c t e d by p r e c i p i t a t i o n from a d i l u t e s o l u t i o n o f UF r e s i n . F i g u r e 2 shows a f r a c t u r e i n t e r f a c e o f a s o l i d UF p l u g c u r e d by a c i d c a t a l y s i s . Shown a r e s t r u c t u r e s o f a n o d u l a r n a t u r e v e r y s i m i l a r t o s i l i c a and c a r b o n c o l l o i d s ( and \p .

The c r o s s - h a t c h e d



7.

JOHNS AND DUNKER

Urea-Formaldehyde

Resins

83

F i g u r e 8. End view (a) and s i d e view (b) o f a H h e l i x . This i s a proposed model f o r a u r e a - f o r m a l d e h y d e r e s i n based on c o l l o i d a l considerations.

F i g u r e 9. S i d e view o f a 3-sheet. T h i s i s a p r o p o s e d model f o r a u r e a - f o r m a l d e h y d e r e s i n based on c o l l o i d a l c o n s i d e r a t i o n s .


F O R M A L D E H Y D E RELEASE FROM WOOD PRODUCTS

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Formaldehyde Release from Wood Products - American Chemical

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