Adhesives from Renewable Resources - ACS Publications - American

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Chapter 19 Research vs. Industrial Practice with Tannin-Based Adhesives Antonio Pizzi National Timber Research Institute Council for Scientific and Industrial Research P.O. Box 395, Pretoria, 0001 Republic of South Africa

Problems and facts that "in the author's personal experience" arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the "credibility gap" between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are: consistency of tannins, extracts and adhesives properties due to the natural raw material variability; formulation in cold-setting adhesives; and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. C o m p a r i n g research results o b t a i n e d i n a clean, a l m o s t c l i n i c a l , l a b o r a t o r y s i t u a t i o n w i t h i n d u s t r i a l a p p l i c a t i o n realities is always a n i n t e r e s t i n g exercise. It is interesting because good correspondence between the two s i t u a t i o n s does a n d must exist b u t often i t is far f r o m satisfactory or even o u t r i g h t p o o r . W a t t l e ( m i m o s a e x t r a c t ) t a n n i n adhesives have been i n d u s t r i a l l y p r o d u c e d a n d used i n S o u t h A f r i c a for m a n y years. F r o m t h e first, m o s t l y unsuccessful, a t t e m p t s i n 1968 has evolved the consistent m a n u f a c t u r e , i n d u s t r i a l a p p l i c a t i o n , 0097-6156/89A)385-0254$06.00A) · 1989 American Chemical Society

Hemingway et al.; Adhesives from Renewable Resources ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Research vs. Industrial Practice

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a n d e x p o r t of exterior-grade p a r t i c l e b o a r d adhesives at the e n d of 1971, 1972, a n d 1973, respectively; of c o l d - s e t t i n g l a m i n a t i n g a n d finger-jointing adhesives i n 1973; of phenolic-fortified p l y w o o d adhesives i n 1973-1974; of z i n c acetatec a t a l y z e d p l y w o o d f o r m u l a t i o n s i n 1975; of U F - f o r t i f i e d p l y w o o d resins i n 19761977, a n d of " h o n e y m o o n " fast-setting adhesives for finger-joints i n 1982-83.(1). W a t t l e t a n n i n resins are also used t o m a n u f a c t u r e other resins, such as foams c o m p a r a b l e to phenolics, as w a t e r p r o o f i n g a d d i t i v e s , a n d binders for c o r r u g a t e d c a r d b o a r d or charcoal briquettes. T h i s discussion, however, deals o n l y w i t h p a r t i c l e b o a r d , p l y w o o d , g l u l a m , a n d finger-jointing exterior-grade w o o d adhesives. F o r m u l a t i o n s of the adhesives w i l l be m e n t i o n e d " a d hoc," i f at a l l necessary, as they have a l r e a d y been extensively discussed i n articles a n d reviews i n the relevant l i t e r a t u r e . ( i ) Tannin Extract Adhesive

Consistency

O n e of the first objections t o t a n n i n adhesives advanced b y i n d u s t r i a l m a n u facturers regards the consistency of the raw m a t e r i a l , hence of the resins a n d adhesives p r o d u c e d f r o m i t . T h e consistency a n d c o m p o s i t i o n of t a n n i n e x t r a c t s do v a r y considerably w i t h tree l o c a t i o n , season, tree age, c l i m a t i c c o n d i t i o n s , tree species, a n d e x t r a c t i o n process. T h e c o m b i n a t i o n of a l l these factors can indeed cause a considerable v a r i a t i o n i n the c o m p o s i t i o n of the r a w m a t e r i a l i f one takes as a c o m p a r a t i v e s t a n d a r d other i n d u s t r i a l s y n t h e t i c raw m a t e r i a l s . ( A f t e r a l l , the tree does not p r o d u c e t a n n i n specifically t o allow us t o p r o d u c e adhesives!) T h i s o b j e c t i o n is generally difficult t o counteract b y researchers. If y o u have not done f a i r l y extensive i n d u s t r i a l r u n s , no i n d u s t r i a l i s t is l i k e l y t o believe your desperate pleas t h a t , after a l l , the r a w m a t e r i a l or the finished p r o d u c t is consistent enough not to cause p r o b l e m s i n the f a c t o r y or p r o m p t c l a i m s against the m a n u f a c t u r e r . T h e r e is t h e n a c r e d i b i l i t y gap between research a n d i n d u s t r i a l a p p l i c a t i o n i n t h i s context t h a t is always difficult to fill i n t h i s first step t o w a r d i n d u s t r i a l a p p l i c a t i o n of t a n n i n adhesives. It is, i n m y experience, the first of the two greatest obstacles anyone w i l l find w h e n m o v i n g t o w a r d i n d u s t r i a l i z a t i o n of research results, the second one b e i n g : " T a n n i n a d hesives? W h a t ' s t a n n i n ? " "Hence: W h a t we're d e s c r i b i n g is m a r k e t resistance by unaware people." B o t h these p r o b l e m s are e m o t i o n a l , not t e c h n i c a l , a n d thus difficult t o deal w i t h . A s a c o n s o l a t i o n , a n i n d u s t r i a l s t u d y c a r r i e d o u t i n 1979 o n the viscosity of a series of i n d u s t r i a l batches of t a n n i n e x t r a c t a n d of p a r t i c l e b o a r d adhesive prepared f r o m i t appeared as related i n the discussions t h a t follow. T h e experience of t a n n i n e x t r a c t m a n u f a c t u r e r s , a n d of companies d e a l i n g w i t h t a n n i n adhesives is even more extensive, b u t results are s i m i l a r t o those i l l u s t r a t e d i n F i g u r e 1. T h u s , the first feedback f r o m any i n d u s t r i a l a p p l i c a t i o n is t h a t the f o r m u l a t i o n s must s m o o t h a n d reduce the greater v a r i a b i l i t y of the n a t u r a l r a w m a t e r i a l . It is a m a z i n g , b u t i n a l l the articles presented by a l l the research groups, t h i s p o i n t is o n l y h i n t e d a t , or not m e n t i o n e d at a l l ! M a n y



1600

ω

Ο

>· Ι α) Ο

200.

400-

600-

800 ^

1000J

12001

Ο 1400

ω

1800J

2000

BATCHES

made w i t h them.

F i g u r e 1. C o n s i s t e n c y i n the v i s c o s i t y o f t a n n i n e x t r a c t s o l u t i o n s a n d adhesives

INDUSTRIAL

TANNIN ADHESIVE

TANNIN EXTRACT



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257

i n n o v a t i v e adhesive f o r m u l a t i o n s have been presented, b u t i f t h i s p o i n t is not t a k e n i n t o account, one can forget even a r r i v i n g o n the floor o f a f a c t o r y ! I m u s t p o i n t o u t t h a t no l a b o r a t o r y s t u d y of t h i s k i n d , n a t u r a l l y , w i l l be sufficient to convince m a n u f a c t u r e r s , b u t also t h a t m a n y difficulties m u s t be overcome to c a r r y such a s t u d y out p r o p e r l y . W h e r e large-scale afforestation a n d i n d u s t r i a l e x t r a c t i o n of t a n n i n is p r a c t i c e d , at least a c e r t a i n a m o u n t of c o n t r o l is exercised o n the consistency of the p r o d u c t . If the source of t a n n i n is not a n established i n d u s t r i a l one, one m a y indeed have a n u n s u r m o u n t a b l e p r o b l e m at h a n d . W h e r e Lies the Secret to A p p l i e d

Success?

W h e r e does the secret of t e c h n i c a l i n d u s t r i a l success w i t h t a n n i n adhesives lie? It varies: for c e r t a i n adhesives, i t is i n the f o r m u l a t i o n of the adhesives o n l y ; for others, i t lies i n the a p p l i c a t i o n c o n d i t i o n s a n d techniques o n l y ; for m o s t , success requires a g o o d balance of b o t h . W h i l e excellent correspondence between l a b o r a t o r y a n d p l a n t results c a n often be o b t a i n e d i n adhesives t h a t d e p e n d for success o n resin f o r m u l a t i o n , t h i s is u s u a l l y not the case w h e n success is more dependent o n a p p l i c a t i o n techniques a n d c o n d i t i o n s ( T a b l e I).

T a b l e I. A p p r o x i m a t e I n d u s t r i a l Success D e p e n d e n t o n Formulation and A p p l i c a t i o n by Adhesive T y p e Adhesive Application

Critical Aspect

C o l d - s e t adhesives:

8 0 % o f success dependent u p o n adhesive f o r m u l a t i o n

P a r t i c l e b o a r d adhesives:

7 0 % success dependent u p o n application conditions and techniques

P l y w o o d adhesives:

5 0 / 5 0 dependent u p o n formulation and application

A n adhesive c a n be considered successful if, w h e n used consistently i n i n d u s t r i a l p r a c t i c e , i t provides p r o d u c t s whose performance is c o m p a r a b l e or s u p e r i o r t o e x i s t i n g p r o d u c t s at a lower effective cost a n d w i t h fewer or c o m p a r a b l e h a n d l i n g p r o b l e m s . L e t ' s now a n a l y z e ( w i t h examples) two cases r e l a t e d t o T a b l e I. Cold-Setting Glulam Finger-Jointing Adhesives In c o l d - s e t t i n g adhesives, the c o m b i n a t i o n of resorcinol w i t h the t a n n i n i n some f o r m , be i t a d d e d or p a r t l y generated f r o m the t a n n i n itself, hides a m u l t i t u d e


258

ADHESIVES F R O M RENEWABLE RESOURCES

of sins. It is easy, f u r t h e r m o r e , to replicate i n d u s t r i a l a p p l i c a t i o n c o n d i t i o n s for such adhesives i n the l a b o r a t o r y . A s regards c o r r e l a t i o n between i n d u s t r i a l a n d research results, the i m p o r t a n t p o i n t s t o w a t c h are discussed below.


I n R e s i n M a n u f a c t u r e . P r o p e r c o n t r o l of the e x o t h e r m i c r e a c t i o n of f o r m a l d e hyde w i t h the t a n n i n / r e s o r c i n o l m i x t u r e s is essential. F u l l water c o o l i n g s h o u l d be a p p l i e d a l r e a d y at temperatures as low as 25 t o 30 ° C , otherwise, w i t h o u t i t , a 1-ton i n d u s t r i a l b a t c h can sometimes pass f r o m a m b i e n t to 80 ° C i n less t h a n 2 m i n u t e s ! I have seen the stainless steel door of a r e a c t i o n vessel fly 20 meters i n t o the a i r as a consequence of t h i s m i s t a k e , a n d the o p e r a t o r was l u c k y ! T h e vessel m a y easily explode or the r e a c t i o n m i x t u r e b o i l over t h r o u g h the condenser, i f the r e a c t i o n is not p r o p e r l y c o n t r o l l e d at i t s b e g i n n i n g s . A p e c u l i a r p h e n o m e n o n is the slow b u t continuous increase i n viscosity of a b a t c h once d r u m m e d . It is s o m e t h i n g y o u w i l l not notice easily i n l a b o r a t o r y batches. It is counteracted b y m i x i n g some water w i t h the b a t c h after some p e r i o d o f t i m e . T h e p h e n o m e n o n is m a r k e d , the viscosity c a n go u p m o r e t h a n 5 0 % , at constant solids over a p e r i o d of t i m e , b y some process of a g g r e g a t i o n . G e n e r a l l y , vigorous m e c h a n i c a l m i x i n g w i t h a d d i t i o n of 0.5 to 1 k g water o n 4 to 5 tons l i q u i d resin is enough to restabilize the b a t c h . T h e o n l y h y p o t h e s i s I c a n advance is t h a t t h i s p e c u l i a r b e h a v i o r is due t o the t h i x o t r o p i c n a t u r e of the r e s i n . I n A d h e s i v e A p p l i c a t i o n . T a n n i n c o l d - s e t t i n g adhesives have a tendency t o d r y o u t m o r e r a p i d l y t h a n s y n t h e t i c c o l d - s e t t i n g adhesives after a p p l i c a t i o n to the w o o d surface. T h i s m a y become a serious p r o b l e m as i t l i m i t s the assembly t i m e of g l u l a m t o shorter times t h a n P R F ' s . A l t h o u g h several t r i c k s , such as a d d i t i o n of glycols, have i m p r o v e d the s i t u a t i o n so c o n s i d e r a b l y t h a t u n d e r m a n y a p p l i c a t i o n c o n d i t i o n s there is p r a c t i c a l l y no difference between t a n n i n and p h e n o l i c cold-sets, under p a r t i c u l a r l y h a r s h c o n d i t i o n s , h i g h t e m p e r a t u r e a n d h i g h speed of a i r c i r c u l a t i o n , the p r o b l e m m a y s t i l l become evident i n g l u l a m plants t h a t are slow i n assembly; t h i s m a y cause a d r o p i n the performance of the b o n d e d j o i n t . T h e p r o b l e m of d r y - o u t has well-defined t h e o r e t i c a l causes. W e have recently f o u n d (by c o n f o r m a t i o n a l analysis means) t h a t w h i l e the average energy of i n t e r a c t i o n of Ρ F dimers w i t h cellulose is higher t h a n the average energy of s o r p t i o n of the water m o n o l a y e r , the average energy of i n t e r a c t i o n of flavonoid t a n n i n d i m e r s w i t h cellulose is lower due to steric factors. T h i s means t h a t w h i l e P F resins are likely to r e t a i n water longer w h e n a p p l i e d i n l i q u i d f o r m t o a lignocellulosic surface, T R F resins are n o t , hence t h e i r d r y - o u t characteristics. C C A preservatives interfere w i t h the c u r i n g of any phenolic adhesive, b u t p a r t i c u l a r l y so w i t h t a n n i n cold-sets. T h i s is due to the higher c a p a b i l i t y of c o m p l e x a t i o n C u a n d C r w i t h o r t h o - d i p h e n o l s (such as the catecholic a n d p y r o g a l l o l i c Β rings of flavonoids). T h e p r o b l e m is not grave under n o r m a l a m b i e n t g l u i n g c o n d i t i o n s , b u t i t becomes more evident at a m b i e n t t e m p e r a t u r e s of 30 ° C or h i g h e r . T h e p r o b l e m , of course, lies w i t h C C A as a w o o d preservative r a t h e r


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t h a n w i t h the t a n n i n adhesive. A p p r o p r i a t e a p p l i c a t i o n c o n d i t i o n s are c e r t a i n t o be developed i n the future to further m i n i m i z e or e l i m i n a t e t h i s p r o b l e m . A s regards correspondence of l a b o r a t o r y results w i t h i n d u s t r i a l m a n u f a c t u r e a n d g l u i n g , the i n d u s t r i a l l y p r o d u c e d batches are generally a n d consistently bet ter performers t h a n l a b o r a t o r y ones, due t o better r e a c t i o n controls. S t r e n g t h


of g l u e d j o i n t s is c o m p a r a b l e . If y o u have a g o o d f o r m u l a t i o n i n the l a b o r a t o r y , y o u c a n be sure the i n d u s t r i a l batches w i l l also be g o o d . Particleboard Adhesives In t a n n i n - b a s e d p a r t i c l e b o a r d adhesives, the a p p l i c a t i o n techniques a n d c o n d i t i o n s o u t w e i g h the i m p o r t a n c e of the f o r m u l a t i o n : t h i s does not m e a n t h a t the f o r m u l a t i o n of the adhesive is not i m p o r t a n t . It means o n l y t h a t m a n y m o r e usable adhesive f o r m u l a t i o n s can be f o u n d . F o r c e r t a i n , some are g o o d a n d some m a y be not so g o o d , b u t several c e r t a i n l y p r o d u c e acceptable b o a r d s . T h e variety of f o r m u l a t i o n s is great, for instance: 1) u n m o d i f i e d t a n n i n e x t r a c t s w i t h adjusted p H only, 2) unfortified t a n n i n extracts i n w h i c h s i m p l e c h e m i c a l t r e a t m e n t s have reduced the viscosity a n d i m p r o v e d b o n d s t r e n g t h a n d water resistance, 3) f o r m u l a t i o n s i n w h i c h the t a n n i n e x t r a c t is fortified w i t h a v a r i e t y of s y n t h e t i c p o l y m e r s such as s m a l l percentages of P F , P R F , U F resins or even w i t h diisocyanates, 4) m e t a l - c a t a l y z e d t a n n i n e x t r a c t f o r m u l a t i o n s , or even 5) an o l d A u s t r a l i a n f o r m u l a t i o n (now somewhat expensive) t h a t was used c o m m e r c i a l l y for m a n y years a n d consisted of j u s t a d d i n g 5 % resorcinol c h e m i c a l to the t a n n i n e x t r a c t a n d t o adjust the p H . However g o o d the f o r m u l a t i o n , a n d there are some r e a l l y g o o d ones a r o u n d , none of t h e m w i l l w o r k i f the a p p l i c a t i o n c o n d i t i o n s are not correct, a n d g o o d a p p l i c a t i o n c o n d i t i o n s for t a n n i n adhesives are often very different f r o m those for Ρ F resins. T h e most i m p o r t a n t factors d e t e r m i n i n g the a p p l i c a t i o n c o n d i t i o n s (there are others t o o , of course) are the percentage m o i s t u r e content of the glued p a r t i c l e s , the rate of pressing, the m e t h o d of a p p l i c a t i o n of the glue m i x to the w o o d p a r t i c l e s , a n d the percentage of adhesive solids o n w o o d . P e r c e n t a g e M o i s t u r e C o n t e n t ( M C ) a n d P r e s s i n g R a t e . T h e s e two factors are related i n m a n y ways, b u t t h e i r r e l a t i o n s h i p acquires a n a s t o n i s h i n g new d i m e n s i o n w i t h t a n n i n adhesives. M a n y p a r t i c l e b o a r d adhesives appear t o require a n a r r o w range of M C to f u n c t i o n properly. If the pressing t i m e is slow 7 t o 7 - 1 / 2 m i n u t e s for a 1 2 - m m b o a r d ) , the properties of the t a n n i n - b o n d e d b o a r d (exterior grade) i m p r o v e w i t h i n c r e a s i n g M C . I m e a n t h a t a b o a r d i n w h i c h the g l u e d w o o d chips h a d a M C of 3 0 % is better t h a n t h a t p r e p a r e d at M C of 2 5 % , w h i c h is better t h a n the one prepared at M C of 2 0 % ! W e are t a l k i n g here of m o i s t u r e ranges t h a t c o u l d be considered as a h a l l u c i n a t i o n for any o t h e r adhesive! T h i s is p a r t i c u l a r l y true i n the l a b o r a t o r y , b u t even i n i n d u s t r i a l c o n d i t i o n s , I have seen boards pressed at 2 8 % M C t h a t d i d not blister at a l l at press-opening. T h i s is a characteristic of w a t t l e t a n n i n - b a s e d p a r t i c l e b o a r d : they never r e a l l y b l i s t e r . F o r a l l p r a c t i c a l purposes, the M C a n d


260



pressing t i m e s used for b o t h three-layer, m u l t i d a y l i g h t processes a n d continuous belt c o n t i n u o u s l y chip-size-graded single d a y l i g h t processes differ considerably. A n a p p r o x i m a t e c o m p a r i s o n of i n d u s t r i a l c o n d i t i o n s is s h o w n i n T a b l e I I . It is noticeable f r o m T a b l e II t h a t pressing t i m e s of t a n n i n - b a s e d b o a r d s c a n be very fast, faster t h a n for U F - b o n d e d p a r t i c l e b o a r d ! ( Y o u c a n achieve t h i s f a i r l y easily i n a n i n d u s t r i a l p l a n t , b u t I have never seen i t achieved i n a l a b o r a t o r y press!) T h i s is a considerable advantage t h a t , c o u p l e d w i t h the fact t h a t a higher a m o u n t of lower cost t a n n i n adhesive t h a n P F resin (higher cost) m u s t be used, renders t a n n i n adhesives a n e c o n o m i c a l l y e x c i t i n g a l t e r n a t i v e to P F resins. T h e reason for the need of higher resin content w h e n u s i n g t a n n i n adhesives ( 1 0 - 1 2 % o n w o o d i n s t e a d of 8% for P F resins) is m o s t l y due t o the presence of sugar a n d h y d r o c o l l o i d g u m s i n t a n n i n e x t r a c t s ( F i g u r e s 2 a n d 3).

T a b l e I I . C o m p a r i s o n of A p p r o x i m a t e I n d u s t r i a l C o n d i t i o n s of A p p l i c a t i o n for P a r t i c l e b o a r d B o n d e d w i t h W a t t l e T a n n i n Adhesives

Content

Glued Core Chips Moisture Content

(sec/mm)

(%)

(%)

(°C)

T h r e e layers multidaylight

35

23-25

18

160

T h r e e layers multidaylight

17.5

18-22

10

160

G r a d e d single daylight

6

14-17

10-14

190

G l u e d Surface Pressing

Chips

Time

Moisture

Pressing Temperature

It is q u i t e likely t h a t h i g h spread rates w o u l d not be needed w i t h pure t a n n i n s f r o m w h i c h the h i g h a m o u n t s of sugars a n d g u m s have been e l i m i n a t e d . P u r i f i c a t i o n may, however, be a n expensive exercise i f c a r r i e d o u t o n i n d u s t r i a l scale. G l u e M i x A p p l i c a t i o n t o t h e W o o d P a r t i c l e s . T h e techniques used t o o b t a i n fast pressing t i m e s w h e n u s i n g t a n n i n adhesives v a r y c o n s i d e r a b l y a c c o r d i n g to the t y p e o f adhesive blender a v a i l a b l e . A d h e s i v e blenders, i n w h i c h the w o o d particles are resinated, c a n be d i v i d e d as regards the a p p l i c a t i o n of t a n n i n - b a s e d adhesives i n t o two b r o a d classes: 1) s p r a y i n g blenders, i.e., blenders i n w h i c h the l i q u i d adhesive is sprayed o n t o the w o o d particles b y means of compressed a i r operated s p r a y i n g nozzles; a n d 2) s p r e a d i n g a n d s p r a y i n g blenders i n w h i c h the l i q u i d adhesive is spread a n d / o r sprayed by p u m p i n g the l i q u i d at h i g h pressure



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STRENGTH DRY 24 4000 H

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80 70 60 50 Phenolic glue (solid) 20 30 40 50 Gum Arabic (solids) GLUE: GUM ARABIC RATIO

F i g u r e 2. Effect o f g u m a r a b i c o n b o n d s t r e n g t h a n d w o o d f a i l u r e o f p h e n o l formaldehyde bonded wood.



262

ADHESIVES F R O M RENEWABLE

STRENGTH

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Θ0 70 60 20 30 40 GLUE: GLUCOSE

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F i g u r e 3. Effect o f glucose o n b o n d s t r e n g t h a n d w o o d f a i l u r e o f p h e n o l formaldehyde bonded wood.


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t h r o u g h nozzles, i.e., D r a i s T u r b o p l a n blenders. B o t h types of blenders are able to p r o d u c e g o o d t a n n i n - b o n d e d p a r t i c l e b o a r d u s i n g fast pressing t i m e s w h e n


the a p p r o p r i a t e technique is used. I n the use of s p r a y i n g blenders, the f o l l o w i n g technique can be successfully a p p l i e d . A l l the p a r a f o r m a l d e h y d e powder hardener a n d 3 0 % of the t a n n i n adhesive s p r a y - d r i e d powder is removed f r o m the l i q u i d glue m i x . T h e p a r a f o r m a l d e hyde a n d w a t t l e adhesive powder are t h e n a d d e d t o the w o o d chips j u s t before the adhesive blender where the l i q u i d glue m i x is sprayed o n t o the w o o d p a r t i cles. C o r e m a t e r i a l M C s of 10 t o 1 2 % a n d face m a t e r i a l M C s o f 20 t o 2 2 % c a n t h e n easily be achieved i n spite of the h i g h viscosity of t a n n i n s o l u t i o n s . T h e percentage of p a r a f o r m a l d e h y d e used s h o u l d be 1 4 % based o n w a t t l e e x t r a c t solids. T h e adhesive p H s h o u l d be, i n o p t i m a l cases, 6.5 t o 6.7 for face m a t e r i a l a n d 6.9 t o 7.3 for core m a t e r i a l . T h e percentage of resin solids o n d r y w o o d s h o u l d be 1 1 % for core m a t e r i a l a n d 14 to 1 8 % for face m a t e r i a l . I n the use of s p r e a d i n g a n d s p r a y i n g blenders, as i n the case o f D r a i s T u r b o p l a n blenders, the f o l l o w i n g technique c a n i n s t e a d be successfully a p p l i e d . T h e w a t t l e - b a s e d adhesive is d i l u t e d w i t h water to a p p r o x i m a t e l y 3 0 % solids content a n d b l e n d e d to the chips i n the glue blender. A l l the adhesive is a p p l i e d as a l i q u i d glue m i x . A f t e r r e s i n a t i o n , the w o o d p a r t i c l e s have a very h i g h M C , as h i g h or higher t h a n 3 0 % . Before r e a c h i n g the b o a r d - f o r m i n g s t a t i o n , the w o o d p a r t i c l e s are passed t h r o u g h a forced a i r countercurrent p r e d r i e r ( t h a t i n m a n y i n d u s t r i a l systems, is n o t h i n g other t h a n the a i r flow chips conveyor f r o m the g l u i n g s t a t i o n to the s p r e a d i n g s t a t i o n o n the l i n e , to w h i c h a heat exchanger has been fitted) at a t e m p e r a t u r e v a r i a b l e between 70 a n d 90 ° C . T h e m a x i m u m p e r i o d for the resinated w o o d particles to r e m a i n i n the predrier is a p p r o x i m a t e l y 4 seconds. D u r i n g the t i m e of their h e a t i n g i n the p r e d r i e r , t h e i r M C decreases to the low values required for fast pressing t i m e s , n a m e l y 10 to 1 4 % for core particles a n d 16 to 2 2 % for face particles. T h e a i r t e m p e r a t u r e i n the predrier as w e l l as the flow rate of the resinated w o o d p a r t i c l e s t h r o u g h i t m u s t be regulated so as t o o b t a i n the r e q u i r e d M C . T h e short h e a t i n g t i m e of the resinated particles i n the predrier does not i m p a i r the performance of the adhesive. A l s o i n t h i s s y s t e m , the p a r a f o r m a l d e h y d e powder hardener m u s t be a d d e d i n powder f o r m b y means of a screw conveyor t o the p a r t i c l e s before they are r e s i n a t e d . W h i l e the second s y s t e m described can be a p p l i e d to a l l types of blenders, the first cannot be a p p l i e d t o the second t y p e of blender described. I n the case o f a D r a i s T u r b o p l a n m a c h i n e , the first s y s t e m can produce g o o d exterior p a r t i c l e b o a r d i n runs no longer t h a n 1 h o u r as the m a c h i n e is s l o w l y choked by a s o l i d r i n g c o m p o s e d of resin a n d the finer w o o d p a r t i c l e s , h a r d e n e d b y the p a r a f o r m a l d e h y d e present a n d the heat of p a r t i c l e f r i c t i o n . T h i s c h o k i n g r i n g of b a k e l i t e - t y p e consistency forms i n the first h a l f of the g l u i n g blender a n d o n the inner w a l l of i t s casing. Its d i r e c t i o n of g r o w t h is f r o m the casing w a l l t o w a r d


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the c e n t r a l axis of the blender. These p r o b l e m s are not likely t o be encountered i f t a k e n care of at the t i m e research at the l a b o r a t o r y level is t a k i n g place. L o n g t e r m Performance of Exterior-Grade T a n n i n - B o n d e d


Particleboard I n n u m e r a b l e d a t a exist i n the relevant l i t e r a t u r e f r o m m a n y accelerated l a b o r a t o r y tests o n p a r t i c l e b o a r d b o n d e d w i t h w a t t l e t a n n i n adhesives. However, since a p i c t u r e is w o r t h a t h o u s a n d words, a n d a field test is w o r t h a t h o u s a n d accelerated ones, i n the f o l l o w i n g p h o t o g r a p h s ( F i g u r e 4 ) , c o m m e r c i a l w a t t l e - t a n n i n b o n d e d , M U F - b o n d e d a n d P F - b o n d e d 1 2 - m m - t h i c k p a r t i c l e b o a r d , exposed t o the elements for a p e r i o d of 15 years i n our t e s t i n g g r o u n d i n P r e t o r i a , are s h o w n . It is easy t o see w h y M U F - a n d P F - b o n d e d p a r t i c l e b o a r d s have become e x t i n c t species i n S o u t h A f r i c a for m a n y years! F o r a l m o s t 18 years t a n n i n b o n d e d p a r t i c l e b o a r d s have been used extensively as the exterior c l a d d i n g of houses, a n d for r o a d signs i n aggressive c l i m a t i c c o n d i t i o n . T h e s e p i c t u r e s c a n easily be used as examples t h a t i f adhesives are s o l d m a i n l y o n price, q u a l i t y is i n s t e a d w h a t sells exterior b o a r d s . C o m m e r c i a l Use of T a n n i n Adhesives C o u n t r i e s where t a n n i n adhesives have f o u n d c o m m e r c i a l a p p l i c a t i o n a n d are c o m m e r c i a l l y used now or have been c o m m e r c i a l l y used at some t i m e i n the past are s h o w n i n the a c c o m p a n y i n g m a p ( F i g u r e 5). It is i m p o r t a n t t o recognize t h a t w a t t l e t a n n i n - b a s e d adhesives are now i t e m s of w o r l d c o m m e r c e a n d not restricted t o use i n S o u t h A f r i c a . T h e g r o w i n g use of w a t t l e t a n n i n based adhesives has been b u i l t o n a s o u n d b a c k g r o u n d of u n d e r s t a n d i n g o f t h e i r f u n d a m e n t a l c h e m i s t r y a n d a relentless effort to use t h i s u n d e r s t a n d i n g i n r a t i o n a l approaches to adhesive f o r m u l a t i o n . W a t t l e t a n n i n - b a s e d adhesives are e x a m p l e s of successful transfer of technology f r o m the l a b o r a t o r y t o i n d u s t r i a l practice. Conclusions I n c o n c l u s i o n , i t is w o r t h w h i l e to note t h a t i n 1970-71, w h e n I s t a r t e d to be i n v o l v e d w i t h t a n n i n adhesives, there were no m o r e t h a n three l a b o r a t o r i e s , a c a d e m i c or i n d u s t r i a l , c o n d u c t i n g research o n t a n n i n adhesives. B y 1986, I have h a d direct or i n d i r e c t contact w i t h 42 different l a b o r a t o r i e s i n m a n y c o u n tries t h a t have c a r r i e d o u t at some t i m e , or are now c a r r y i n g o u t , research o n these b o n d i n g m a t e r i a l s . T h e increase i n i n d u s t r i a l usage has also been fast b u t for v a r i o u s reasons has lagged b e h i n d the t e c h n i c a l a n d scientific interest. However, after p r a c t i c a l l y zero c o n s u m p t i o n i n 1970, i n d i c a t i o n s are t h a t a b o u t 12,000 tons of exterior-grade t a n n i n resin solids per year are now p r o d u c e d a n d c o n s u m e d i n the t i m b e r i n d u s t r y based o n reliable i n f o r m a t i o n f r o m several



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F i g u r e 4. P h o t o g r a p h i c c o m p a r i s o n of 12 m m t h i c k exterior grade c o m m e r c i a l p a r t i c l e b o a r d s m a n u f a c t u r e d i n the same i n d u s t r i a l p l a n t a n d b o n d e d w i t h M U F , w a t t l e t a n n i n - f o r m a d e h y d e , a n d P F resins T h e panels were exposed at the C S I R t e s t i n g site (4,500 feet above sea level) at P r e t o r i a , S o u t h A f r i c a for 15 years. N o t e the extensive degeneration of M U F b o a r d ( c o m m e r i c a l l y i m p o r t e d M U F resins f r o m W e s t G e r m a n y ) . A l s o note the more m a r k e d edge a n d surface degeneration of the P F i n c o m p a r i s o n to the T F - b o n d e d b o a r d s .



F i g u r e 5. W o r l d p r o d u c t i o n a n d use of w a t t l e t a n n i n - b a s e d adhesives.

importers of tannin extracts for inclusion as minor components i n phenolic adhesives.

importers (at some time) of products bonded with tannin adhesives.

producers and/or consumers (at some time) of tannin adhesives.



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countries. These values do not i n c l u d e countries k n o w n to use these m a t e r i a l s b u t whose c o n s u m p t i o n d a t a are u n k n o w n . It is easy t o see t h e n , t h a t w h i l e the increase i n p r o d u c t i o n a n d c o n s u m p t i o n of these m a t e r i a l s has been r a p i d , considerable scope for e x p a n s i o n s t i l l exists. B r i d g i n g the gap between new re sults o b t a i n e d i n the l a b o r a t o r y a n d i n d u s t r i a l a p p l i c a t i o n of t h e m is never a n easy m a t t e r ; i t is essential, however, i f the m u c h - p u b l i c i z e d search for n o n - o i l derived adhesives is t o be a r e a l l y serious u n d e r t a k i n g a n d not j u s t l i p service. T h i s chapter has related a few of the m o r e i m p o r t a n t p r o b l e m s observed b y the a u t h o r i n t r a n s l a t i n g l a b o r a t o r y results to i n d u s t r i a l p r a c t i c e . It is h o p e d t h a t it w i l l help others i n t r a n s l a t i n g t h e i r l a b o r a t o r y findings to usable i n d u s t r i a l p r o d u c t s for a class of w o o d adhesives based o n n o n - o i l - d e r i v e d raw m a t e r i a l s . Literature Cited 1. Pizzi, Α., 1983, Wood Adhesives: Chemistry and Technology, Marcel Dekker, New York, 364 pp. RECEIVED September 13, 1988


Adhesives from Renewable Resources - ACS Publications - American

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