Effect of Diffusion Barriers on Formaldehyde Emissions from

16 Effect of Diffusion Barriers on Formaldehyde Emissions from Particleboard

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 13, 2015 | http://pubs.acs.org Publication Date: August 8, 1986 | doi: 10.1021/bk-1986-0316.ch016

Per Hanetho Dyno Industrier A.S., Lillestrøm Fabrikker, P. O. Box 160, N-2001, Lillestrøm, Norway

After a discussion of mechanisms for the liberation and subsequent emission of formaldehyde from particleboard, methods to assess the extent of these processes are described. Data are presented for the formaldehyde emission from particleboard with various surface treatments. These data were obtained by a laboratory method and by large climate chamber measurements and show that some of the surface treatments studied constitute very efficient diffusion barriers and considerably reduce the formaldehyde emission rate.

In t h i s p r e s e n t a t i o n t h e term " d i f f u s i o n b a r r i e r " w i l l be u s e d f o r f i n i s h e s o r o v e r l a y s f o r p a r t i c l e b o a r d t h a t increase the d i f f u s i o n resistance of the particleboard s u r f a c e , t h u s r e t a r d i n g t h e r a t e o f mass t r a n s f e r (formaldehyde emission) from t h e board t o t h e surrounding a i r . Sources o f Formaldehyde

i n Particleboard

Formaldehyde i s l i b e r a t e d d u r i n g t h e c o n d e n s a t i o n r e a c t i o n s t h a t t a k e p l a c e when t h e u r e a f o r m a l d e h y d e r e s i n b i n d e r i n p a r t i c l e b o a r d i s c u r e d by h o t p r e s s i n g . Some o f t h i s f o r m a l d e h y d e i s r e t a i n e d i n t h e b o a r d a n d i s a v a i l a b l e f o r subsequent emission t o t h e surroundings. In theory there are s e v e r a l p o s s i b l e s t a t e s i n w h i c h t h i s r e t a i n e d f o r m a l d e h y d e may e x i s t , v i z . : - as monomeric f o r m a l d e h y d e e n t r a p p e d i n v o i d s o r a d s o r b e d t o t h e wood - as monomeric f o r m a l d e h y d e h y d r o g e n - b o n d e d t o t h e wood 0097-6156/86/0316-0202$06.00/0 © 1986 A m e r i c a n C h e m i c a l Society

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


16.

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Effect of Diffusion

Barriers on Formaldehyde

Emissions

203

- as p o l y m e r i c ( s o l i d ) f o r m a l d e h y d e - as l o o s e l y bound f o r m a l d e h y d e , e.g. m e t h y l o l end g r o u p s on t h e r e s i n c h a i n , w h i c h r e a d i l y s p l i t s o f f i n hydrolytic reactions. So f a r no one has b e e n a b l e t o d e m o n s t r a t e beyond d o u b t i n w h i c h o f t h e above s t a t e s t h e f o r m a l d e h y d e actually exists. However, a t t h e 4 t h A n n u a l I n t e r n a t i o n a l Symposium on A d h e s i o n and A d h e s i v e s f o r S t r u c t u r a l M a t e r i a l s i n P u l l m a n , WA, September 1984, George Myers p r e s e n t e d a p a p e r c o n c l u d i n g t h a t "most o f t h e f o r m a l d e hyde i n a b o a r d i s c h e m i c a l l y , n o t p h y s i c a l l y bonded t o r e s i n , t o wood, t o i t s e l f as a p o l y m e r , o r t o ammonia" (1). He a l s o c l a i m e d t h a t a l l t h o s e f o r m a l d e h y d e s t a t e s a r e p o t e n t i a l l y h y d r o l y z a b l e , and t h e more m o i s t u r e s e n s i t i v e o f them, i n h i s o p i n i o n , u n d o b t e d l y a c t as sources of a board's emitted formaldehyde. I t i s , howe v e r , n o t p o s s i b l e t o d i s t i n g u i s h between f o r m a l d e h y d e produced from the v a r i o u s s t a t e s . Some a u t h o r s c l a i m t h a t s u b s e q u e n t h y d r o l y s i s o f the r e s i n i t s e l f a l s o c o n t r i b u t e s t o t h e formaldehyde emission. T h i s i s n o t l i k e l y , among o t h e r t h i n g s b e c a u s e t h e f o r m a l d e h y d e e m i s s i o n i s n o t a c c o m p a n i e d by t h e bond d e t e r i o r a t i o n and s t r e n g t h l o s s t h a t w o u l d be the r e s u l t o f r e s i n h y d r o l y s i s . D u r i n g t h e m a n u f a c t u r e (hot p r e s s i n g ) o f t h e p a r t i c l e b o a r d the formaldehyde i s c o n c e n t r a t e d i n the core of the board. T e s t s r u n on l a b o r a t o r y made p a r t i c l e b o a r d w i t h t h e same b i n d e r l e v e l t h r o u g h o u t t h e b o a r d , have shown a b o u t 75% h i g h e r c o n t e n t o f e x t r a c t a b l e f o r m a l d e h y d e i n t h e c o r e t h a n i n t h e f a c e (2^) . E m i s s i o n t e s t s i n d i c a t e an e v e n g r e a t e r d i f f e r e n c e between t h e two l a y e r s o f t h e b o a r d . The c o n c e n t r a t i o n g r a d i e n t t h a t e x i s t s between t h e c o r e and t h e f a c e , l e a d s t o a m i g r a t i o n o f f o r m a l d e h y d e to the s u r f a c e of the p a r t i c l e b o a r d . From t h e s u r f a c e l a y e r i t i s r e l e a s e d t o the s u r r o u n d i n g a i r . Formaldehyde

Emission

The c o n c e n t r a t i o n o f f o r m a l d e h y d e i n t h e a i r o f a room c o n t a i n i n g p a r t i c l e b o a r d s , w i l l depend on t h e c o n t e n t o f f o r m a l d e h y d e i n t h e b o a r d s and on t h e r a t e o f i t s r e lease. The f o r m a l d e h y d e c o n t e n t o f a p a r t i c l e b o a r d i s d e t e r m i n e d by t h e b i n d e r u s e d t o m a n u f a c t u r e t h e b o a r d and a number o f p r o d u c t i o n p a r a m e t e r s . The r e l e a s e r a t e i s a f f e c t e d by t h e t e m p e r a t u r e and t h e r e l a t i v e h u m i d i t y o f t h e s u r r o u n d i n g a i r , b u t a l s o by some o f t h e p h y s i c a l p r o p e r t i e s of the board. The most i m p o r t a n t one p r o b a b l y i s the d i f f u s i o n r e s i s t a n c e o f the s u r f a c e l a y e r , w h i c h may be e x p r e s s e d by means o f a mass t r a n s f e r coefficient. A. B e r g e e t a l . (3) and J . J . H o e t j e r (4_) have d e v e l o p e d models f o r the formaldehyde e m i s s i o n from p a r t i c l e b o a r d w h i c h c a n be p r e s e n t e d as f o l l o w s :


204

FORMALDEHYDE RELEASE FROM WOOD PRODUCTS

c = s

—

2

—

n +k

where C = steady s t a t e formaldehyde c o n c e n t r a t i o n o f the a i r i n a v e n t i l a t e d s y s t e m , mg/m^ C* = e q u i l i b r i u m f o r m a l d e h y d e c o n c e n t r a t i o n o f t h e a i r i n a n u n v e n t i l a t e d s y s t e m , mg/m kg = mass t r a n s f e r c o e f f i c i e n t , m/h = p a r t i c l e b o a r d l o a d i n g , m2/m^, a n d n = v e n t i l a t i o n rate, h ~ l . g


3

I f t h e mass t r a n s f e r c o e f f i c i e n t i s s u f f i c i e n t l y low, t h e e m i s s i o n w i l l be s o s l o w t h a t t h e v e n t i l a t i o n can manage t o remove t h e f o r m a l d e h y d e a t a l m o s t t h e same r a t e as i t i s l i b e r a t e d , r e s u l t i n g i n a v e r y low formaldehyde c o n c e n t r a t i o n i n the a i r . This p r e s e n t a t i o n d e a l s w i t h what can be a c h i e v e d i n t e r m s o f r e d u c e d mass t r a n s f e r c o e f f i c i e n t a n d e m i s s i o n r a t e by a p p l y i n g some s o r t o f d i f f u s i o n b a r r i e r t o the s u r f a c e o f the particleboard. The d i f f u s i o n b a r r i e r s s t u d i e d c o m p r i s e o v e r l a y s o r s u r f a c e f i n i s h e s commonly a p p l i e d when p a r t i c l e b o a r d i s used as a b u i l d i n g m a t e r i a l , such as w a l l paper, p a i n t i n g and f l o o r c o v e r i n g , but even o v e r l a y s t h a t a r e u s e d by t h e f u r n i t u r e a n d j o i n e r y i n d u s t r i e s , s u c h a s v e n e e r s , melamine f a c i n g a n d r e s i n s a t u r a t e d paper f o i l s ( f i n i s h f o i l s ) . Test

Methods f o r F o r m a l d e h y d e C o n t e n t a n d E m i s s i o n

A l a r g e number o f t e s t methods have been i n t r o d u c e d f o r the d e t e r m i n a t i o n o f the tendency o f p a r t i c l e b o a r d t o r e l e a s e formaldehyde. Some a r e a n a l y t i c a l methods f o r t h e c o n t e n t o f f o r m a l d e h y d e i n t h e b o a r d , some a r e e m i s s i o n t e s t s , a n d some a r e c o m b i n a t i o n s o f t h e two types. I t seems t o be g e n e r a l l y a c c e p t e d t h a t t h e e m i s s i o n t e s t s a r e t h e more m e a n i n g f u l o n e s , among o t h e r t h i n g s b e c a u s e most f o r m a l d e h v d e r e g u l a t i o n s l i m i t the p e r m i s s i b l e c o n t e n t o f formaldehyde i n the a i r r a t h e r than i n the p a r t i c l e b o a r d . I t i s i m p o r t a n t t o d i s t i n g u i s h between t h o s e e m i s s i o n t e s t s t h a t measure t h e e m i s s i o n i n a c l o s e d , o r u n v e n t i l a t e d , s y s t e m and t h o s e t h a t measure i n a v e n t i l a t e d system. I f a p a r t i c l e b o a r d i s kept i n an u n v e n t i l a t e d system, the formaldehyde c o n c e n t r a t i o n w i l l i n c r e a s e u n t i l i t l e v e l s o f f a t an e q u i l i b r i u m concentrat i o n which w i l l depend on the formaldehyde c o n t e n t o f the board under t e s t , the temperature and the r e l a t i v e humidity. The p a r t i c l e b o a r d l o a d i n g , o n t h e o t h e r hand, w i l l not i n f l u e n c e the e q u i l i b r i u m c o n c e n t r a t i o n , j u s t the time i t t a k e s t o r e a c h i t . The time t o r e a c h the e q u i l i b r i u m c o n c e n t r a t i o n i s a l s o i n f l u e n c e d b y t h e mass



16.

HANETHO

Effect of Diffusion

Barriers on Formaldehyde

Emissions

205

t r a n s f e r c o e f f i c i e n t , o r i n o t h e r words by t h e d i f f u s i v i t y of the s u r f a c e l a y e r of the board. I n a v e n t i l a t e d s y s t e m t h e e x h a u s t a i r w i l l remove some o f t h e e m i t t e d f o r m a l d e h y d e , and a s t e a d y s t a t e c o n c e n t r a t i o n w i l l be e s t a b l i s h e d . The s t e a d y s t a t e c o n c e n t r a t i o n w i l l be l o w e r t h a n t h e e q u i l i b r i u m c o n centration. How much l o w e r , w i l l depend on t h e v e n t i l a t i o n r a t e , t h e p a r t i c l e b o a r d l o a d i n g and t h e mass t r a n s fer coefficient. Dyno has c o n t r i b u t e d t o t h e d e v e l o p m e n t o f a method, named t h e B e l l method, f o r t h e q u a n t i t a t i v e d e t e r m i n a t i o n o f the formaldehyde e m i s s i o n from a p a n e l s u r f a c e (5). A g l a s s f l a s k or b e l l having a plane flange i s p l a c e d on t h e s u r f a c e t o be m e a s u r e d . A tight sealing b e t w e e n t h e f l a n g e o f t h e b e l l and t h e p a n e l s u r f a c e i s very important. The a i r can be k e p t i n c i r c u l a t i o n by means o f a membrane pump, pumping a b o u t 2 l i t e r s p e r m i n u t e i n a c l o s e d l o o p , w h i c h a l s o c o n t a i n s a gas burette. A f t e r a predetermined time the formaldehyde c o n c e n t r a t i o n o f t h e a i r i n t h e gas b u r e t t e i s d e t e r m i n e d by a s e n s i t i v e a n a l y t i c a l method. The B e l l method can be u s e d t o d e t e r m i n e t h e e q u i l i b r i u m c o n c e n t r a t i o n o f f o r m a l d e h y d e , C* i n t h e model above. When t h e f o r m a l d e h y d e c o n c e n t r a t i o n i n t h e B e l l system i s p l o t t e d a g a i n s t time, the i n i t i a l s l o p e of t h e r e s u l t i n g c u r v e can be u s e d t o d e t e r m i n e t h e mass t r a n s f e r c o e f f i c i e n t , kg i n t h e same m o d e l . T h u s , a l t h o u g h t h e r e i s no a i r exchange between t h e g l a s s b e l l and t h e s u r r o u n d i n g s , t h e B e l l method c a n be used t o p r o v i d e d a t a t o c a l c u l a t e the steady s t a t e conc e n t r a t i o n i n a v e n t i l a t e d system. Experimental

Work

The o b j e c t i v e o f o u r work was t o d e t e r m i n e t h e e f f e c t o f some common s u r f a c e f i n i s h e s and o v e r l a y s on t h e f o r m aldehyde e m i s s i o n from p a r t i c l e b o a r d . F i n i s h e s used i n t h e b u i l d i n g t r a d e as w e l l as s u c h u s e d i n t h e f u r n i t u r e and j o i n e r y i n d u s t r i e s were s t u d i e d . The p r o j e c t p l a n i n v o l v e d t h e use o f t h e B e l l method t o d e t e r m i n e t h e e q u i l i b r i u m c o n c e n t r a t i o n and mass t r a n s f e r c o e f f i c i e n t f o r a number o f p a r t i c l e b o a r d samples w i t h d i f f e r e n t s u r f a c e f i n i s h e s and o v e r l a y s . The e q u i l i b r i u m c o n c e n t r a t i o n and t h e mass t r a n s f e r c o e f f i c i e n t were t h e n u s e d t o c a l c u l a t e t h e s t e a d y s t a t e c o n c e n t r a t i o n i n a system w i t h a i r exchange w i t h the s u r r o u n d i n g s , u s i n g t h e model p r e s e n t e d a b o v e . Tests i n a 2 4 m3 c l i m a t e chamber, i n w h i c h t e m p e r a t u r e , r e l a t i v e h u m i d i t y and v e n t i l a t i o n r a t e c o u l d be v a r i e d , were r u n t o c h e c k t h e agreement between t h e c a l c u l a t e d and measured v a l u e s . E v e n i f i t w o u l d have been h i g h l y d e s i r a b l e t o combine t h e f o r m a l d e h y d e measurements w i t h d e t e r m i n a t i o n s o f t h e d i f f u s i v i t y o f t h e v a r i o u s o v e r l a y s and



206


f i n i s h e s , we had t o r e f r a i n f r o m t h i s . We h a v e , however, a s e m i - q u a n t i t a t i v e conception of the d i f f u s i v i t y of the c o a t i n g s and f i n i s h e s u s e d . We know f o r i n s t a n c e t h a t t h e d i s p e r s i o n p a i n t has a v a p o u r p e r m e a b i l i t y a t l e a s t t w i c e as h i g h as t h e a l k y d p a i n t . Also, v i n y l surfaced w a l l p a p e r has a l o w e r d i f f u s i v i t y t h a n n o r m a l w a l l p a p e r , and t h e h e a v i e r v i n y l m a t e r i a l s and p a p e r p l a s t i c l a m i n a t e s a r e g e n e r a l l y c o n s i d e r e d as b e i n g a l m o s t impermeable . A s u r f a c e f i n i s h o r an o v e r l a y may: 1. A f f e c t t h e e q u i l i b r i u m c o n c e n t r a t i o n i n an u n v e n t i l a t e d s y s t e m , C*. A coating containing a formaldehyde s c a v e n g e r w o u l d a c t by b i n d i n g f o r m a l d e h y d e , thus r e d u c i n g the e q u i l i b r i u m c o n c e n t r a t i o n . On t h e o t h e r hand some s u r f a c e f i n i s h e s w i l l i n t r o d u c e e x t r a f o r m a l d e h y d e , and may t h u s i n c r e a s e C*. 2. Reduce t h e mass t r a n s f e r c o e f f i c i e n t , kg, i . e . t h e r a t e of formaldehyde t r a n s f e r from the p a r t i c l e b o a r d s u r f a c e i n t o t h e room a i r , w i t h o u t C* b e i n g a f f e c t e d . T h i s mechanism i s l i k e l y f o r c o a t i n g s and o v e r l a y s which present a p h y s i c a l r e s t r i c t i o n to the formaldehyde d i f f u s i o n , b u t do n o t r e a c t w i t h f o r m a l d e h y d e . 3. A f f e c t b o t h C* and kg. T h i s w o u l d be t h e c a s e f o r finish foils. T h e s e a r e u r e a o r melamine r e s i n s a t u r a t e d p a p e r f o i l s w h i c h a r e bonded t o t h e p a n e l with urea adhesive. A n o t h e r example i s a c i d - c u r i n g l a c q u e r s w h i c h c o n t a i n f o r m a l d e h y d e and, a t l e a s t f o r a l i m i t e d p e r i o d of time, s u b s t a n t i a l l y i n c r e a s e the e m i s s i o n p o t e n t i a l , b u t a t t h e same t i m e i s an e f f i c i e n t d i f f u s i o n b a r r i e r f o r the formaldehyde from the p a r t i c l e b o a r d underneath. I t would l e a d too f a r here t o d e s c r i b e i n d e t a i l the v a r i o u s s u r f a c e treatments s t u d i e d . Information a b o u t t y p e o f m a t e r i a l , a p p l i c a t i o n methods, a d h e s i v e t y p e s , e t c . , i s , however, a v a i l a b l e . Results T a b l e I shows t h e r e s u l t s o b t a i n e d w i t h t h a t a r e common i n t h e b u i l d i n g t r a d e . D i s c u s s i o n of the

surface

finishes

Results

I t s h o u l d be e m p h a s i z e d t h a t t h e v a l u e s p r e s e n t e d apply t o t h e p a r t i c u l a r m a t e r i a l s t h a t we s t u d i e d , and t h a t t h e a b s o l u t e v a l u e s c a n n o t be c o n s i d e r e d as g e n e r a l l y v a l i d . We b e l i e v e , however, t h a t t h e y c a n s e r v e t o i l l u s t r a t e the r e l a t i v e r e d u c t i o n s i n formaldehyde e m i s s i o n t h a t can be a c h i e v e d .


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Table I

Type o f f i n i s h

Mass t r a n s f e r coefficient k , m/h 0.65 0.18 0.23 0.24 0.11


q

None (reference) Alkyd paint Latex paint Wall paper V i n y l w a l l paper Needle f e l t carpeting Cushion f l o o r C a r p e t i n g w/foam backing

Equilibrium c o n e . , 20OC, C*, mg/m3 2.18 0.25 1.98 1.88 0.39

Steady s t a t e cone.*), C , mg/m3 C a l c . Measured 1.70 1.69 0.11 0.10 1.37 0. 97 1.67 0. 93 0.27 0.11 s

0.04 0.04

0.60 0.40

0.065 0.045

0.06

0.50

0.088

*) A t 2 2 ° C , 60% R.H., v e n t i l a t i o n r a t e 0.5 h ^, p a r t i c l e b o a r d l o a d i n g 1.6 m /m^ 2

T a b l e II g i v e s t h e r e s u l t s o b t a i n e d w i t h o v e r l a y s t h a t a r e commonly u s e d b y t h e f u r n i t u r e a n d j o i n e r y industries.

Table II

Type o f f i n i s h

Mass t r a n s f e r coefficient k , m/h 0.40 q

None (reference) Melamine f a c e d (short cycle) Paper p l a s t i c laminate Finish f o i l , 100 g/m Finish foil, 50 g/m2 V e n e e r (0.9 mm t e a k f a c e 1.2 mm p i n e back) 2

Equilibrium c o n e . , 20°C, C*, mg/m3 1.06

Calculated steady s t a t e cone.*) C , mg/m3 r

0.60

0.06

1.55

0.25

0.06

1.19

0.19

0.10

2.01

0.49

0.10

2.80

0.69

0.16

0. 98

0.33

*) A t 22°C, 60% R.H., v e n t i l a t i o n r a t e b o a r d l o a d i n g 1.6 m /m3

s

1

0.5 h " , p a r t i c l e -

2


208



The f i r s t two measured values are i n excellent agreement with the corresponding calculated values, whereas for the remaining values the agreement is not equally good. The most l i k e l y reason for this is inaccuracies in determining the mass transfer coefficients . The finishes in table 2 were not tested i n the c l i mate chamber, because the necessary equipment for the controlled application of them to f u l l - s i z e particleboards was not available. Conclusion Finishing or overlaying particleboard can be an efficient way to reduce the formaldehyde concentration of the a i r in rooms where particleboards are used e.g. as building panels or in furniture. Our work shows that a l l the finishes and overlays that we have tested, reduce the mass transfer coefficient and lower the rate of formaldehyde emission. Some of the overlays that are common in the woodworking industries involve the use of a formaldehydebased adhesive. In such cases the adhesive can increase the emission potential so that, at least for a period of time, some of the gain due to a reduced mass transfer coefficient is lost. Literature Cited 1. Myers, George. Unpublished data. 2. Hanetho, P. Proc. 12th Symp. on Particleboard, Washington State University, Pullman, WA, 1978, 275-286. 3. Berge, A . ; Mellegaard, B.; Hanetho, P.; Ormstad, E.B. Holz Roh- u. Werkst., 1980, 38, 251-255. 4. Hoetjer, J.J. Holz-Zbl., 1978, 120, 1836-1838. 5. Berge, A . ; Mellegaard, B. For. Prod. J . , 1979, 29 (1), 21-25. RECEIVED January

14, 1986


Effect of Diffusion Barriers on Formaldehyde Emissions from

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