Formaldehyde Release from Wood Products - American Chemical

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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.

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

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

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

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

7.

JOHNS AND DUNKER

Urea-Formaldehyde

Resins

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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 .

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

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

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