Tannin-Induced Formaldehyde Release Depression in Urea

15 Tannin-Induced Formaldehyde Release Depression in Urea-Formaldehyde Particleboard

Downloaded by UNIV OF LEEDS on May 17, 2015 | http://pubs.acs.org Publication Date: August 8, 1986 | doi: 10.1021/bk-1986-0316.ch015

F. A. Cameron and A. Pizzi National Timber Research Institute, Council for Scientific and Industrial Research, P. O. Box 395, Pretoria, South Africa Addition of tannin extract to UF resins in particleboard appear to decrease HCHO-emission over periods of time proportional to the amount of tannin added. The addition of tannin extract appears only to be a "stop-gap" short-to medium-term measure because, once the capability of the tannin to absorb and react with HCHO fumes slowly released by the board has been exhausted, the board revert to emissions similar to those of the UF controls. The emission of formaldehyde fumes from particleboard manufactured using urea-formaldehyde resins, and its decrease, have now been topics of interest in the timber and wood adhesives industry for a long time. Many solutions, some very effective, to this problem have already been advanced by many authors. In this brief article we do not pretend to present yet another successful or less successful method to control HCHO emission but to show the decrease in the amount of formaldehyde emitted by UF-bonded particleboard, over a period of time, to which tannin extract has been added in small amounts. Tannin extract is an inexpensive commodity in Southern Africa as well as in many other countries in the southern emisphere such as Brazil, Argentina and New Zealand. The method presented, if not completely effective may be an inexpensive system of control of HCHO emission over a limited period of time. Experimental Duplicate 12 mm thick three layers particleboard 600 mm x 300 mm in dimensions were prepared in the laboratory using 7 % UF resin solids total on oven dry pine wood chips. The glue mix used was as follows: UF resin 64 % solids Water NHC1 NH 25 % solution 4

3

0 100 0 50 1,6 3«8

parts parts parts parts

by by by by

mass mass mass mass

0097-6156/ 86/ 0316-0198$06.00/ 0 © 1986 American Chemical Society

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

15.

C A M E R O N A N D PIZZI

Tannin-Induced

Formaldehyde

Release Depression

199

No wax e m u l s i o n was added t o t h e b o a r d t o a v o i d t h e i n t r o d u c t i o n o f another f a c t o r t h a t c o u l d have l i m i t e d formaldehyde e m i s s i o n . To t h i s g l u e mix were added 2 %, 5 % and 10 % UF r e s i n s o l i d s by mass o f commercial mimosa ( w a t t l e , A c a c i a m e a r n s i i f o r m e r l y m o l l i s s i m a ) b a r k e x t r a c t , a commercial f l a v o n o i d - t y p e t a n n i n e x t r a c t . In another s e r i e s o f p a n e l s i n s t e a d 10 %, 20 % and 50 % on UF r e s i n s o l i d s , by mass, o f t h e same e x t r a c t i n s p r a y - d r i e d powder form were added, d i r e c t l y t o t h e wood c h i p s i n t h e g l u e b l e n d e r d u r i n g s p r a y i n g w i t h t h e UF g l u e mix. The boards were p r e s s e d a t 25 kg/cm , 170 °C f o r 7 i minutes w i t h a t o t a l c y c l e o f 2 minutes + 2\ minutes + 3 minutes. One month a f t e r p r e s s i n g , t h e boards were c u t and t r i p l i c a t e samples f o r each d u p l i c a t e b o a r d tested according to the dessicator method, u s i n g P u r p a l d s o l u t i o n and a c o l o r i m e t e r , f o r formaldehyde e m i s s i o n o v e r a p e r i o d o f 24 h o u r s and 30 minutes P u r p a l d development. A f t e r t h i s i n i t i a l assessment t h e samples were p l a c e d i n a l a b o r a t o r y f a n - e x h a u s t oven a t a temperature o f 50 °C t o a c c e l e r a t e t h e t e s t f o r a p e r i o d o f two months. The samples were t e s t e d a t r e g u l a r i n t e r v a l s o f t h r e e weeks o v e r t h e two months p e r i o d . The formaldehyde e m i s s i o n r e s u l t s o b t a i n e d a r e shown i n T a b l e I ,


2

A f u r t h e r experiment was c a r r i e d o u t . I n d u s t r i a l boards i n which 1 5 % t a n n i n e x t r a c t was added i n t h e g l u e - b l e n d e r (1*5 % on UF r e s i n s o l i d s ) were p r e s s e d a t 160 °C, 5 i m i n u t e s , 25 kg/cm , 9 % UF s o l i d s s u r f a c e s , 5\ % UF s o l i d s i n c o r e . T h i c k n e s s was o f 18 mm f i n i s h e d b o a r d . Average d e n s i t y was o f 0.670 g/cm . A s e t o f UF c o n t r o l s was p r e s s e d under t h e same c o n d i t i o n s . The r e s u l t s o b t a i n e d f o r formaldehyde r e l e a s e a r e shown i n F i g u r e 1 e x p r e s s e d as #

2

3

A formaldehyde A time i n f u n c t i o n o f time ( i n h o u r s ) u s i n g t h e d i n a m i c f l o w method. Discussion I t i s e v i d e n t from t h e l a b o r a t o r y e x p e r i m e n t s t h a t a d d i t i o n o f t a n n i n e x t r a c t t o t h e UF g l u e mix does n o t improve t h e l o n g - t e r m e m i s s i o n o f HCHO from t h e b o a r d u n l e s s as much as 10 % t a n n i n e x t r a c t i s added. T h i s may be a s c r i b e d t o t h e f a c t t h a t t a n n i n a v a i l a b l e t o -CH2OH groups o f t h e UF r e s i n i n t h e g l u e mix r a p i d l y r e a c t w i t h them and thus cannot f u n c t i o n as a scavenger o f HCHO vapour a f t e r t h e board has been p r e s s e d . The 10 % l e v e l i s a l s o n o t t o o c e r t a i n as the amount r e q u i r e d may v a r y w i t h p r e s s i n g t e m p e r a t u r e , p r e s s i n g m o i s t u r e , m o i s t u r e i n t h e environment a f t e r p r e s s i n g , e t c . I t i s i n t e r e s t i n g t o n o t e t h a t a f t e r one month a t ambient temperature t h e boards w i t h t a n n i n e x t r a c t added t o t h e g l u e mix a l l p r e s e n t lower e m i s s i o n t h a n t h e UF c o n t r o l . However, t h i s e f f e c t s h o u l d n o t l a s t l o n g , even a t ambient t e m p e r a t u r e , as shown from t h e r e s u l t s o f t h e 50 % a c c e l e r a t e d t e s t . More i n t e r e s t i n g a r e t h e c a s e s i n which t h e t a n n i n e x t r a c t was added t o t h e c h i p s r a t h e r t h a n t o t h e g l u e mix. The e f f e c t h e r e i s a l s o a d e p r e s s e d formaldehyde e m i s s i o n . The e f f e c t appears a l s o t o l a s t much l o n g e r due t o t h e h i g h e r amount o f t a n n i n added. ( I t must be borne i n mind t h a t pure t a n n i n - f o r m a l d e h y d e commercial boards



Case No. 1 2 3 4 5 6 7

UF UF UF UF UF UF UF

I.

Emission

3

Boards ave density (g/cm ) 0.600 0.570 0.613 0.570 0.611 0.560 0.551

Extract

Boards ave I.B. (MPa) 0.46 0.33 0.42 0.45 0,47 0,52 0.43

R e s u l t s o f Boards " S p i k e d " w i t h T a n n i n

HCHO e m i s s i o n (x 10 °) A c c e l e r a t e d t e s t a t 50 °C a f t e r 1 month a f t e r 6 3 9 manufacture weeks weeks weeks 1,21 5.1 15 J 0.71 1.82 5.5 10.8 7.0 12.6 1.85 0.84 0.60 5.4 1.16 11.0 9.2 5,'6 1.16 0.65 3,8 0.76 0.57 11.5 0,71 0.57 9.9 1.9

Formaldehyde

Board type control + 2 % t a n n i n i n g l u e mix + 5 % t a n n i n i n g l u e mix + 10 % t a n n i n i n g l u e mix + 10 % t a n n i n powder + 20 % t a n n i n powder + 50 % t a n n i n powder

Table


(%) 24.9 26,5 19.5 20.3 19.5 19-1 16.7

Boards ave 24h s w e l l i n g

a C o

73 O

"0

o a

1

0

73

Tl

tn

>

m r m

73

m

a

x >

Tl

O 73


15.

C A M E R O N A N D PIZZI

1 50

Tannin-Induced

1 100

Formaldehyde

1 150

Release Depression

1 200

1 250

201

r 300

TIME (HOURS) Figure

1 . D i f f e r e n t i a l p l o t formaldehyde

manufactured i n South A f r i c a , p r e s e n t The i n d u s t r i a l experiment (see F i g u r e i n t e r e s t , namely: 1.

2.

release

e m i s s i o n s lower than 0 . 0 1 ppm). 1 ) shows two r e s u l t s o f

t h a t an a d d i t i o n o f 1 . 5 % causes an i n i t i a l d e c r e a s e i n t h e amount o f HCHO e m i s s i o n b u t t h a t a f t e r one week t h e s c a v e n g i n g a b i l i t y o f t h e s m a l l amount o f t a n n i n has been e x h a u s t e d , and t h a t f o r some r e a s o n unknown t o t h e a u t h o r s , t h e amount o f HCHO r e l e a s e d i n b o t h UF and UF + t a n n i n e x t r a c t b o a r d s a b a t e s 7 0 t o 7 5 hours a f t e r m a n u f a c t u r e .

The f i r s t p o i n t i n d i c a t e c l e a r l y t h a t t h e a d d i t i o n o f t a n n i n i s o n l y a " s t o p - g a p " measure t o d e c r e a s e HCHO-emission from UF-bonded p a r t i c l e b o a r d as t h e p e r i o d o f lowered HCHO e m i s s i o n i s d i r e c t l y p r o p o r t i o n a l t o t h e amount o f t a n n i n e x t r a c t (or b e t t e r o f p h e n o l i c matter i n the t a n n i n e x t r a c t = 8 0 %) added. Once t h e t a n n i n h a s , over a p e r i o d o f time a l l r e a c t e d w i t h t h e HCHO s l o w l y r e l e a s e d , the board w i l l r e v e r t t o t h e same l e v e l s o f e m i s s i o n s which would have been o b t a i n e d w i t h o u t t a n n i n a d d i t i o n . F u r t h e r m o r e , t a n n i n e x t r a c t a d d i t i o n s o f t h e o r d e r o f 1 0 % t o 5 0 % a r e n e c e s s a r y f o r l o n g e r term effect. However, n o t w i t h s t a n d i n g the f a c t that a d d i t i o n o f tannin i s o n l y a s h o r t t o medium term measure i t may w e l l c o n s t i t u t e a s o l u t i o n f o r UF b o a r d s which a r e used f o r o n l y a l i m i t e d p e r i o d o f t i m e , such as i n temporary b u i l d i n g s . The o t h e r p h y s i c a l p r o p e r t i e s o f t h e boards so produced a r e a c t u a l l y s l i g h t l y improved by t h e a d d i t i o n o f t a n n i n (as e x p e c t e d , see T a b l e 1 ) . Small a d d i t i o n s o f t a n n i n s may a l s o be u s e d , however, t o d e c r e a s e HCHO e m i s s i o n i n t h e f a c t o r y d u r i n g board p r e s s i n g . R E C E I V E D J a n u a r y 14, 1 9 8 6


Tannin-Induced Formaldehyde Release Depression in Urea

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