Wood Technology: Chemical Aspects

12 Chemical Changes in W o o d Associated with W o o d Fiberboard Manufacture H O W A R D A. S P A L T Department of Forestry, Southern Illinois University, Carbondale, Ill. 62901

Downloaded by MONASH UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.ch012

1

Over twenty b i l l i o n square feet (1/8-inch basis) of wood fiberboard are produced annually. Most of this footage goes into house and furniture construction as sheathing, acoustical t i l e , siding, paneling, underlayment, dustproofing, drawer bottoms, case backs, and doors. The industry had i t s beginning in 1914 with the development of insulation board by Muench (1). Mason's work on thermomechanical pulping (2, 3) and hot-press densification (4, 5) expanded the range of density and properties attainable with felted wood fibers. The classification of these products is given in Table I. Classification

Class Softboard (Insulation) Semirigid Rigid Hardboard Medium D e n s i t y High Density Special Densified

Table I o f Wood

Fiberboards

Density, lbs./ft.

Densified?

1.2-10 10-30

No No

3

30-50 50-75 84-90

Yes Yes Yes

M a n i p u l a t i o n o f raw m a t e r i a l and p r o c e s s ena b l e s f i b e r b o a r d s o f d i v e r s e p r o p e r t i e s t o be

1

The r e s e a r c h r e p o r t e d h e r e i n was conducted under the direction o f t h e a u t h o r when he was Manager o f t h e B a s i c R e s e a r c h S e c t i o n , Research C e n t e r , Masonite C o r p o r a t i o n , S t . C h a r l e s , Illinois.

193

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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WOOD TECHNOLOGY: CHEMICAL ASPECTS

produced. Low-density boards with heat-insulating or sound-absorbing qualities also possess sufficient strength for sheathing and dropped-ceiling applications. High-density boards possess machinability, embossability, and printability to take high-fidelity decorative designs in color and texture and to wear well as interior paneling, door skins, or furniture components. Medium-density boards combine a suitable level of machinability and embossability with high weatherability and coatability built in to perform well as exterior siding. Exterior applications require high durability of substrate and coating but are less demanding of fidelity in color and texture designs than are interior products. Raw materials for fiberboard manufacture come from several sources including whole wood, sawdust and m i l l residues, waste paper, agricultural wastes, and plant tissue other than woody stems. Additives usually enable attaining the desired properties but whole wood reduced to virgin fiberboard pulp often can be converted to board of impressive properties with l i t t l e or no additives. High lignin content of virgin fiberboard pulps is usually cited as the source of this superiority. Processes used to manufacture softboards and hardboards are basically similar and readily divisible into a furnish-preparation phase and a boardconversion phase. Hardboard processes differ in that board conversion uses pressure to densify the sheet whereas softboard processes do not. Both processes subject the felted sheets to high temperatures in the board-conver s ion phase. Hardboard processes use more severe conditions and effect more extensive physical and chemical changes; consequently, these processes offer more insight into chemical changes associated with board manufacture. The results reported in this paper are derived from studies of hardboard but they should apply equally to softboards and softboard processes. Furnish Preparation Processes Most virgin wood pulp used in fiberboard manufacture today is generated by thermomechanical pulping developed by William Mason in 1925 (2, 3). Few thermomechanical pulping processes existing today use the original Mason process now that more streamlined operations have evolved but a l l stem from Mason's development.


12.

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Wood

Fiberboard

Manufacture

195

Three major f u r n i s h p r e p a r a t i o n p r o c e s s e s ( F i g u r e 1) a r e now i n use today. These a r e the o r i g i n a l M a s o n i t e wet-form p r o c e s s , t h e wet form p r o c e s s u s i n g p r e s s u r i z e d r e f i n i n g , and t h e d r y form p r o c e s s . The M a s o n i t e Wet-Form P r o c e s s . Masonite Corpora t i o n was founded t o e x p l o i t M a s o n s p a t e n t s and s t i l l p r a c t i c e s the o r i g i n a l f u r n i s h - p r e p a r a t i o n process shown s c h e m a t i c a l l y i n F i g u r e 1. Cooking i s accomp l i s h e d by c h a r g i n g c h i p s i n t o a v e r t i c a l c y l i n d e r o u t f i t t e d w i t h a s l o t t e d p l a t e a t the bottom t h a t s u p p o r t s the c h i p s t a c k . Below the p l a t e i s an h y d r a u l i c a l l y - o p e r a t e d v a l v e t h a t p r o v i d e s passage to the atmosphere. Once f i l l e d w i t h c h i p s , the c y l i n d e r i s s e a l e d and p r e s s u r i z e d w i t h s a t u r a t e d steam a t p r e s s u r e s up t o 1000 p s i g . U s u a l l y p r e s s u r e s below 800 p s i g a r e used and t h e program o f steam p r e s s u r i z a t i o n v a r i e s . Steam p r e s s u r e may be r a i s e d s t e a d i l y u n t i l d i s c h a r g e t o the atmosphere a t t a r g e t p r e s s u r e . Or steam p r e s s u r e may be a d m i t t e d a t a p r e s c r i b e d r a t e u n t i l a target pressure i s attained, held f o r a prescribed time, and d i s c h a r g e d . Another v e r s i o n u s e s c o n t r o l l e d r a t e o f steam p r e s s u r i z a t i o n t o t a r g e t p r e s s u r e , h o l d at p r e s s u r e up t o 90 seconds, and r a i s e r a p i d l y t o a h i g h e r p r e s s u r e ( s h o o t i n g p r e s s u r e ) and d i s c h a r g e . R e g a r d l e s s which c o o k i n g program i s used, t h e c h i p s a r e permeated by t h e s a t u r a t e d steam and d e v e l o p h i g h i n t e r n a l steam p r e s s u r e s . A t the i n s t a n t o f v a l v e opening, the c o n t e n t s a r e f o r c e d by the h i g h p r e s s u r e d i f f e r e n t i a l through t h e s l o t t e d p l a t e s which have a p e r t u r e s t o o narrow f o r the c h i p s . E x t e n s i v e s i z e r e d u c t i o n o c c u r s by s h r e d d i n g f o l l o w e d by d e f i b r a t i o n e f f e c t e d by the sudden decompression. The r e s u l t o f t h i s s h o o t i n g o f c h i p s i s gun f i b e r which i s a m i x t u r e o f f i b e r , f i b e r b u n d l e s , and h a r d shives. A f t e r c o o k i n g and s h o o t i n g , the m i x t u r e i s s l u r r i e d i n water and r e f i n e d t o reduce the c o a r s e fraction. F o l l o w i n g the f i r s t (primary) r e f i n i n g , t h e s t o c k i s washed on c o u n t e r c u r r e n t , brown-stock washers t o remove s o l u b l e s g e n e r a t e d by the c o o k i n g . Secondary r e f i n i n g reduces the s t o c k t o t a r g e t f r e e ness and a f t e r a d d i t i v e s a r e b l e n d e d , t h e f u r n i s h i s ready f o r t h e f o r m i n g machine. A d d i t i v e s commonly used a r e a c i d - i n s o l u b l e p h e n o l i c r e s i n s and p e t r o latum waxes. M a c h i n a b i l i t y b e n e f i t s g r e a t l y even from low r e s i n add l e v e l s and water r e p e l l e n c y i n c r e a s e s s e n s i t i v e l y w i t h wax a d d i t i o n .


1



196

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

Wet Form P r o c e s s U s i n g P r e s s u r i z e d R e f i n i n g . Thermomechanical p u l p i n g has p r o g r e s s e d r a p i d l y s i n c e the advent o f the p r e s s u r i z e d r e f i n e r . Advances i n the knowledge o f c e l l w a l l s t r u c t u r e and c o m p o s i t i o n e n a b l e s u n d e r s t a n d i n g the workings o f the v e r s a t i l e thermomechanical p u l p i n g p r o c e s s . The r e s e a r c h o f Fergus e t a l . (6) r e v e a l s l i g n i n t o be c o n c e n t r a t e d i n the compound m i d d l e l a m e l l a and h o l o c e l l u l o s e t o be c o n c e n t r a t e d i n the secondary wall. L i g n i n can be s o f t e n e d a t t e m p e r a t u r e s between 130C and 1 9 0 C , t h e lower t e m p e r a t u r e s s u f f i c e at high moisture content ( 7 ) . S a t u r a t e d steam above 130C w i l l s o f t e n the l i g n i n - r i c h m i d d l e l a m e l l a as i t p e n e t r a t e s wood chips. The p r e s s u r i z e d r e f i n e r a p p l i e s shear t o wood c h i p s a t the same time t h a t t h e m i d d l e l a m e l l a i s s o f t e n e d by s a t u r a t e d steam. V e r y f i n e , a n a t o m i c a l l y whole f i b e r i s produced w i t h low m e c h a n i c a l energy consumption. F u r t h e r m o r e , low c o o k i n g steam p r e s s u r e s m i n i m i z e s o l u b l e s f o r m a t i o n and w e i g h t y i e l d s o f p u l p a r e h i g h . Koran's (8) s c a n n i n g e l e c t r o n p h o t o m i c r o g r a p h s o f thermomechanic a l h a r d b o a r d f i b e r r e v e a l c e l l c o r n e r r i d g e s on the f i b e r s u r f a c e which c o n f i r m s t h a t the zone o f s e p a r a t i o n i s the m i d d l e l a m e l l a . By e x t r a c t i n g the l i g n i n from the f i b e r s u r f a c e w i t h a c i d c h l o r i t e , Koran unc o v e r e d t h e random m i c r o f i b r i l l a r s t r u c t u r e o f t h e p r i m a r y w a l l which c o n f i r m s t h a t the e n t i r e c e l l wall structure i s intact. A t s a t u r a t e d steam t e m p e r a t u r e s below 130C, the m i d d l e l a m e l l a does n o t s o f t e n . Koran's (8) s c a n n i n g e l e c t r o n photomicrographs of p r e s s u r i z e d r e f i n e r p u l p produced a t temperatures s l i g h t l y below 1 3 0 C r e v e a l the S2 l a y e r s o f t h e c e l l w a l l . Simply by l o w e r i n g t h e c o o k i n g and r e f i n i n g steam p r e s s u r e , the zone o f f a i l u r e moves from the m i d d l e l a m e l l a t o t h e r e g i o n between the S j and S2 l a y e r s . Cons e q u e n t l y , t h e l i g n i n - r i c h p o r t i o n s o f t h e wood can be s e p a r a t e d as f i n e s from the secondary c e l l w a l l t o produce h o l o c e l l u l o s e - r i c h r e f i n e r groundwood. The e v i d e n c e s u p p o r t s the h y p o t h e s i s t h a t thermomechanical p u l p i n g e x p l o i t s the t h e r m o p l a s t i c c h a r a c t e r o f l i g n i n t o produce l i g n i n - r i c h f i b e r b o a r d f i b e r o r h o l o c e l l u l o s e - r i c h r e f i n e r groundwood f i b e r f o r papermaking. Once t h e f i b e r i s g e n e r a t e d , washing and b l e n d i n g o p e r a t i o n s complete t h e p r e p a r a t i o n o f f u r n i s h f o r the f o r m i n g machine.



12.

SPALT

Wood

Fiberboard

Manufacture

197

Dry Form P r o c e s s . The advent o f t h e p r e s s u r i z e d r e f i n e r l e d e v e n t u a l l y t o d r y - f o r m e d wood f i b e r b o a r d s because h i g h - c o n s i s t e n c y p u l p s can be produced. Green wood c h i p s c o n t a i n e q u a l p a r t s water and d r y wood m a t t e r and c a n be p r e s s u r e r e f i n e d i n s a t u r a t e d steam w i t h l i t t l e change i n m o i s t u r e c o n t e n t . F i b e r from the r e f i n e r a t f i f t y p e r c e n t c o n s i s t e n c y c a n be made f l u f f y and r e a d i l y s u s p e n d a b l e i n a i r . T h i s e n a b l e s vapor-phase d e w a t e r i n g i n h o t - a i r d r i e r s t o produce a f i n e , d r y f i b e r t h a t can be formed i n a i r and d r y pressed. Without l i q u i d - p h a s e d e w a t e r i n g , s o l u b l e s formed by t h e steam c o o k i n g o f c h i p s remain i n t h e f i b e r f u r n i s h g o i n g t o t h e board machine. B l e n d i n g o f a d d i t i v e s such as p h e n o l i c - r e s i n b i n d e r and wax i s u s u a l l y a c c o m p l i s h e d a t t h e p o i n t where t h e f i b e r i s d i s c h a r g e d from r e f i n e r p r e s s u r e t o t h e atmosphere. T h i s l o c a t i o n i n t h e blow l i n e p r o v i d e s t u r b u l e n t m i x i n g t o e f f e c t good a d d i t i v e d i s t r i b u t i o n b u t a l s o has t h e advantage o f o c c u r r i n g b e f o r e t h e blowers o r t h e windage r e f i n e r s used t o f l u f f the f i b e r . Clumps formed by poor a d d i t i v e d i s p e r s i o n c a n be broken up by f l u f f i n g b e f o r e h o t air drying. C h e m i c a l Changes i n Wood E f f e c t e d by F u r n i s h t i o n Processes

Prepara-

Wood, and t h e f i b e r o b t a i n e d from i t , i s a comp l e x arrangements o f polymers and macromolecules that are exceedingly d i f f i c u l t t o analyze chemically. But i t i s shown i n F i g u r e 1 t h a t wet p r o c e s s e s p r o duce two streams, p u l p from cooked f i b e r f o r b o a r d p r o d u c t i o n and a l i q u o r c o n t a i n i n g water s o l u b l e s e x t r a c t e d from t h e s t o c k . The l i q u o r i s e a s i e r t o a n a l y z e and i t s c o m p o s i t i o n may a f f o r d i n s i g h t i n t o the c h e m i c a l changes e f f e c t e d by f u r n i s h p r e p a r a t i o n . The r a t e o f h o t w a t e r - s o l u b l e s f o r m a t i o n as a f u n c t i o n o f c o o k i n g steam p r e s s u r e i s shown i n F i g u r e 2. Hardboard o p e r a t i o n s i n t h e p a s t have used steam p r e s s u r e s up t o 1000 p s i g a t which as much as f i f t e e n p e r c e n t o f t h e c e l l w a l l c a n be s o l u b i l i z e d i n s i x t y t o n i n e t y - s e c o n d cooks. Ext r a c t i o n o f t h e s e s o l u b l e s by t h e washers produces l i q u o r s o f pH 3.8-4.5 a t s o l i d s o f 3.5-4.0 p e r c e n t . Lower pH o c c u r s a t h i g h e r steam p r e s s u r e s . The sudden i n c r e a s e i n hot-water s o l u b l e s between 300 and 400 p s i g steam p r e s s u r e i s b e l i e v e d caused by c l e a v a g e o f a c e t y l groups on t h e h e m i c e l l u l o s e s , which g i v e s r i s e t o a c e t i c - a c i d - c a t a l y z e d h y d r o l y s i s of c e l l w a l l components.


WOOD

TECHNOLOGY:

C H E M I C A L

WET FORM (MASONITE) I COOK

REFINE

ELEVATED

ATMOSPHERIC

PRESSURE

PRESSURE

WASH

REFINE

BLEND

1

-j F O R M J L

n-

J

I—•H.O

WET FORM ι BLEND

REFINE ELEVATED

ATMOSPHERIC LIQUOR

PRESSURE


J

DRY FORM

PRESSURE

1 FORM l I

I 1

,Ο

ELEVATED

ATMOSPHERIC

PRESSURE

PRESSURE

Figure 1. Process schematics for the typical furnish preparation (defibering) processes used in wood fiberboard manufacture

12r

Steam Pressure — psig Figure 2. Hot-water extractability of wood chips as a function of saturated steam pressure used in the preheat segment of the digesting operation


ASPECTS


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Fiberboard

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When t h e e x t r a c t i s passed over an a n i o n exchange r e s i n , a p p r o x i m a t e l y 40-45 p e r c e n t o f t h e s o l i d s a r e r e t a i n e d on t h e r e s i n and t h e remainder passes through. Infrared absorption spectra of the two f r a c t i o n s a r e p r e s e n t e d i n F i g u r e 3. The neut r a l f r a c t i o n shows a b s o r p t i o n o f i n f r a r e d a t 3 m i c r o n s ( h y d r o x y l ) , 5.9 m i c r o n s ( a l d e h y d i c c a r b o n y l ) , and a t 9-10 m i c r o n s (C-O-C and C-O-H), w i t h l i t t l e a b s o r p t i o n a t 3.4 m i c r o n s (C-H s t r e t c h ) . This spectrum resembles t h a t o f t h e common s i m p l e s u g a r s when scanned as a m e l t between NaCl o p t i c s . The a c i d i c f r a c t i o n , which a d s o r b s t o t h e r e s i n and must be e l u t e d by a c i d i c e x t r a c t i o n , e x h i b i t s a d i f f e r e n t i n f r a r e d a b s o r p t i o n spectrum i n t h e 6-7 m i c r o n range. Sharp bands a t 6.3 and 6.6 m i c r o n s indicate absorption assigned to d i s t o r t i o n of the a r o m a t i c n u c l e i o c c u r r i n g i n l i g n i n breakdown products. C o n s i d e r i n g t h e l i g n o - h o l o c e l l u l o s i c composit i o n o f wood, i t i s n o t s u r p r i s i n g t h a t t h e l i q u o r i s d i v i s i b l e i n t o a carbohydrate (neutral) f r a c t i o n and an a r o m a t i c ( a c i d i c ) f r a c t i o n , t h e former o f h o l o c e l l u l o s i c o r i g i n and t h e l a t t e r o f l i g n i n o r i g i n . C a r e f u l a c i d h y d r o l y s i s o f t h e whole e x t r a c t from t h e washers produces a t h r e e - f o l d i n c r e a s e i n r e d u c i n g power ( F i g u r e 4 ) . When c a l c u l a t e d as t h e r e d u c i n g power o f g l u c o s e , t h e e x t r a c t s o l i d s as d i s c h a r g e d from t h e washers r u n 20 p e r c e n t r e d u c i n g sugar on t o t a l s o l i d s . A c i d h y d r o l y s i s a t 100C r a i s e s t h e r e d u c i n g sugar t o 60-65 p e r c e n t o f solids. I f this increase i s attributable exclusivel y t o r e d u c i n g sugar f o r m a t i o n , t h e o l i g o s a c c h a r i d e s i n t h e e x t r a c t have an average degree o f p o l y m e r i z a t i o n o f three anhydride u n i t s . This conclusion a g r e e s w i t h e a r l i e r e s t i m a t e s o f 450-550 grams/mole o b t a i n e d from measures o f b o i l i n g - p o i n t e l e v a t i o n . A t a r r y p r e c i p i t a t e e q u i v a l e n t t o 15 p e r c e n t o f l i q u o r s o l i d s forms on a c i d h y d r o l y s i s , a l s o . Crude a n a l y s e s o f t h i s m a t e r i a l i n d i c a t e a f u r f u r a l condensation product. G a s - l i q u i d chromatography o f t h e h y d r o l y z a t e a f t e r hydrogénation w i t h b o r o h y d r i d e and a c e t y l a t i o n with a c e t i c anhydride according t o the procedure o f Sawardeker, S l o n e k e r , and Jeanes (9) produces chromatograms w i t h s i x d i s t i n c t peaks. Five of the peaks o c c u r a t t h e same r e t e n t i o n times as t h e a l d i t o l a c e t a t e s o f t h e f i v e s u g a r s commonly found i n wood. These a r e t h e p e n t o s e s L - a r a b i n o s e and D - x y l o s e and t h e hexoses D-mannose, D - g a l a c t o s e , and D - g l u c o s e . The s i x t h sugar, which e l u t e s f i r s t



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2θί ni

I

2.5

3

I

I 4

I

I

1

5

5

WAVELENGTH

1

6

I

7

I 8

1 9

1

1

1

1

10

11

12

13

IN M I C R O N S

Figure 3. Top: infrared absorption spectrum of the neutral furnish extract fraction that passes through an anion-adsorption resin. Bottom: infrared absorption spectrum of the acid furnish extract fraction that adsorbs to the anion-adsorption resin.

lOOt

Hours of Reflux Figure 4. Reducing sugar content (Somogyi-Nelson Technique) of ex tract solids as a function of reflux time at several acid concentrations


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from t h e column, has t h e same r e t e n t i o n time as Lrhamnose. The p r e s e n c e o f 5-methyl f u r f u r a l i n t h e h y d r o l y z a t e i s s t r o n g e v i d e n c e o f a 6-deoxy hexose i n the e x t r a c t and o n l y L-rhamnose f i t s a l l the evidence. The r e l a t i v e p r o p o r t i o n s o f sugars i n t h e h y d r o l y z a t e s from hardwood l i q u o r s and from s o f t wood l i q u o r s a r e r e p o r t e d i n T a b l e I I . The mannose: g a l a c t o s e : g l u c o s e r a t i o i n softwood l i q u o r s i s t h e Table II D i s t r i b u t i o n o f Simple Sugars i n Hydrolyzates of M i l l Liquors

Sugar Rhamnose Arabinose Xylose Mannose Galactose Glucose Total

Sugar, P e r c e n t Hardwood

o f a l l Sugars Softwood Ί

5 77 7 4 5

5 25 40 13 15

same 3:1:1 r a t i o f o r softwood galactoglucomannans r e p o r t e d by T i m e l l (10). Mannose and g l u c o s e a r e present i n roughly equal q u a n t i t i e s i n e x t r a c t s from a l l hardwood cooks which r e f l e c t s t h e mannose: g l u c o s e r a t i o o f hardwood glucomannans between 1:1 t o 2:1 (10). The e v i d e n c e r e v e a l s t h a t t h e thermom e c h a n i c I T p u l p i n g p r o c e s s a t t a c k s o n l y t h e hemi c e l l u l o s e p o l y s a c c h a r i d e s and does not a f f e c t the c e l l u l o s e f r a c t i o n t o any d e g r e e . The n o n c a r b o h y d r a t e f r a c t i o n can i n d e e d be traced to l i g n i n source. G a s - l i q u i d chromatography of e t h e r e x t r a c t s o f t h e washer l i q u o r s i n d i c a t e s t h e p r e s e n c e o f guaiacol s y r i n g a l d e h y d e , v a n i l l i n , v a n i l l i c a c i d , s y r i n g i c a c i d , and hydroxybenzoic acid. When worked up s e q u e n t i a l l y w i t h sodium b i s u l f i t e , sodium b i c a r b o n a t e , and sodium h y d r o x i d e , t h e noncarbohydrate p o r t i o n d i v i d e s i n t o f r a c t i o n s r i c h i n a l d e h y d e groups, c a r b o x y l g r o u p s , and f r e e p h e n o l i c hydroxyl groups, a l l e x h i b i t i n g s t r o n g a r o m a t i c bands i n i n f r a r e d s p e c t r a ( F i g u r e 5 ) . A f o u r t h f r a c t i o n remains a f t e r e x t r a c t i o n by the o t h e r t h r e e r e a g e n t s w h i c h shows some h y d r o x y l and a l d e h y d i c f u n c t i o n a l i t y but mainly a l i p h a t i c hydro carbon s t r u c t u r e . This n e u t r a l f r a c t i o n i s d e r i v e d


202

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS


from t h e propane p o r t i o n o f t h e phenyl-propane b u i l d i n g block of l i g n i n . A l l of the evidence supports the hypothesis that c o o k i n g wood c h i p s w i t h s a t u r a t e d steam above 130C causes t h e l i g n i n - r i c h m i d d l e l a m e l l a t o f a i l when subjected to the shearing a c t i o n of the r e f i n e r . Coincident with thermoplastic separation of fibers i s t h e h y d r o l y t i c d e g r a d a t i o n o f h e m i c e l l u l o s e and l i g n i n f o r m i n g w a t e r - s o l u b l e o l i g o s a c c h a r i d e s and a r o m a t i c and a l i p h a t i c l i g n i n fragments. Board C o n v e r s i o n

Processes

C o n v e r s i o n o f f i b e r b o a r d f u r n i s h t o board beg i n s w i t h t h e f e l t i n g o p e r a t i o n . F i b e r s suspended i n a l i q u i d o r gaseous f l u i d a r e d e p o s i t e d on a moving w i r e and c o n c e n t r a t e d i n t o an i n t e r f e l t e d mat by t h e removal o f t h e suspending f l u i d . Applic a t i o n o f p r e s s u r e between r o l l s o r p l a t e n s i n t h e c o l d - p r e s s o p e r a t i o n f u r t h e r reduces i n t e r f i b e r v o i d space and e x p r e s s e s t h e f o r m i n g f l u i d . The c o n v e r s i o n p r o c e s s e s a r e shown s c h e m a t i c a l l y i n F i g u r e 6. S o f t b o a r d s o r i n s u l a t i o n boards a r e c o l d - p r e s s e d and h o t - a i r d r i e d t o s p e c i f i c g r a v i t i e s o f 0.2 t o 0.5. A i r temperatures o f 200250C a r e used t o e v a p o r a t e r e s i d u a l water. Hardboard p r o c e s s e s r e q u i r e h o t p r e s s i n g t o f u r t h e r d e n s i f y t h e s h e e t above s p e c i f i c g r a v i t y 0.5. Three d i f f e r e n t b o a r d - c o n v e r s i o n p r o c e s s e s are used: 1.

2.

3.

Wet/Wet - wet formed mats a t 50 p e r c e n t c o n s i s t e n c y a r e p r e s s e d between a h o t p l a t e and a s c r e e n t o form a smooth-ones i d e board. Wet/Dry - wet formed mats a r e h o t - a i r d r i e d t o anhydrous c o n d i t i o n and h o t p r e s s e d between two smooth p l a t e s t o form a smooth-two-sides b o a r d . Dry/Dry - d r y formed mats o f s i x p e r cent moisture content are pressed between two smooth p l a t e s t o form a smooth-two-sides b o a r d .

A l t h o u g h f o r m i n g and c o l d p r e s s o p e r a t i o n s d e n s i f y the i n t e r f e l t e d sheet, l i t t l e chemical change i s e f f e c t e d a t t h e temperatures used which a r e w e l l below t h e b o i l i n g p o i n t o f water. Major changes i n b o a r d p r o p e r t i e s and some changes i n


12.

SPALT

Wood

Fiberboard

Λ


203

Manufacture

HYDROXIDE

-ft

r

t

If

:

5 5

Wood Technology: Chemical Aspects

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