Isocyanate Binders for Wood Composite Boards - ACS Symposium

20 Isocyanate Binders for Wood Composite Boards J. W. FRINK

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Mobay Chemical Corporation, Pittsburgh, PA 15205 H. I. SACHS Bayer AG, Leverkusen, West Germany

This paper is intended to present the history and current development status of a relatively new application for commercial ly produced polymeric MDI and to demonstrate its potential. The monomer upon which polymeric MDI is based is diphenylmethane diisocyanate:

In polymeric MDI's the 4-4'isomer usually predominates but varying amounts of 2-4' and some 2-2' are also present. The polymerics are represented by the generalized structure:

Commercial polymeric MDI's consist of a mixture of molecules of varying n values with small amounts of high molecular weight species present with n as large as 8. Isocyanates are very reactive toward compounds containing labile hydrogen atoms, forming addition compounds with the hydrogen donors. One such reaction, with hydroxyl-bearing compounds to form carbamates, or urethanes,

0097-6156/81/0172-0285$05.25/0 © 1981 American Chemical Society

Edwards et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

286

URETHANE CHEMISTRY AND

APPLICATIONS

is the basis for the commercial use of isocyanates. Polyisocyanates, such as polymeric MDI, are reacted with polyfunctional alcohols, or polyols, to produce polyurethanes. Wood contains a multiplicity of hydroxyl-bearing constituents such as cellulose, hemi-eellulose and lignin. Rowell and E l l i s (1) have published evidence that isocyanates can react chemically with wood to produce urethane linkages. Hartman (2) has prepared r i g i d polyurethane foams by mixing polymeric MDI w i t h c a t a l y s t s , s u r f a c t a n t , blowing agent and ground Douglas F i r o r Ponderosa Pine bark. The bark served as the s o l e p o l y o l i n the system. The i n i t i a l development o f polymeric MDI as a p a r t i c l e b o a r d b i n d e r predates the above work but i t i s , we b e l i e v e , based upon the same chemistry. E x t e r i o r o r s t r u c t u r a l p a r t i c l e b o a r d has been manufactured i n North America and i n Europe f o r years w i t h predom i n a n t l y phenol-formaldehyde b i n d e r s and, i n a few cases, w i t h melamine-modified urea-formaldehyde b i n d e r s . The f a m i l i a r ureaformaldehyde r e s i n s are s u b j e c t t o h y d r o l y s i s and are thus s u i t able f o r i n t e r i o r board o n l y . Proposed Chemistry o f Isocyanate Bonding Most i n v e s t i g a t o r s agree t h a t the s t r e n g t h and d u r a b i l i t y o f isocyanate-bound wood panels are due t o the chemical r e a c t i o n o f the isocyanate group w i t h wood hydroxyls as i l l u s t r a t e d by the above equation. Thus, the m u l t i f u n c t i o n a l isocyanate molecule forms a c h e m i c a l l y bonded b r i d g e between two or more adjacent wood p a r t i c l e s . T h i s r e a c t i o n i s o n l y one o f s e v e r a l i n v o l v i n g i s o cyanates t h a t can and probably do occur i n a hot press d u r i n g formation o f p a r t i c l e b o a r d s when isocyanate b i n d e r i s used. A very important r e a c t i o n i s t h a t o f isocyanate w i t h water t o pro duce a very unstable carbamic a c i d which immediately decomposes t o form a primary amine and COg: H

R-N

=C=0+H 0 2

A A

0

ι

i l

> R - Ν - C - OH

R - NH^ + CO^

The r e s u l t a n t amine r a p i d l y consumes another e q u i v a l e n t o r i s o cyanate t o form a symmetrical d i s u b s t i t u t e d urea:

f

R - NH + R N = C = 0 2

Η

0

I

II

Η ι

>R - N - C - N - R

1

By c o l l e c t i n g and measuring the CO2 evolved from a l a b o r a t o r y p a r t i c l e b o a r d p r e s s , Wittman (3.) has c a l c u l a t e d t h a t l/k t o 1/3 o f the i s o c y a n a t e groups p r e s e n t , depending upon wood moisture con t e n t and b i n d e r l e v e l , are consumed i n the water r e a c t i o n . T h i s means t h a t a l i k e amount o f isocyanate must r e a c t w i t h the r e s u l t a n t amine t o form s u b s t i t u t e d ureas. Since at l e a s t 50%, and l i k e l y more, o f the isocyanate i s apparently consumed by the water r e a c t i o n , chemical bonding through urethane l i n k a g e s appears t o be


20.

FRINK AND SACHS

287

Isocyanate Binders

e f f e c t i v e a t r a t h e r low l e v e l s . Loss o f some isocyanate m o i e t i e s through urea formation i s not n e c e s s a r i l y harmful t o board prop e r t i e s . Urea formation i s probably the p r i n c i p l e c r o s s - l i n k i n g mechanism i n the polymer m a t r i x t h a t i s formed between wood particles. Another p o s s i b l e c r o s s - l i n k i n g r e a c t i o n i s isocyanurate» o r isocyanate t r i m e r , formation: 0 II

3 R - Ν = C = 0

>

Ν

ι

The r e a c t i o n i s known t o occur a t e l e v a t e d temperatures and c o u l d occur d u r i n g hot p r e s s i n g depending on the s t o i c h i o m e t r y and s t e r i c environment o f the system. The isocyanate-urethane r e a c t i o n t o form a l l o p h a n a t e s , 0

H O R-N=C = 0 + R

I f

II

ii

N-C-O-R

>R

f

- N - C - O - R ι

C = 0 W

- Η

R and the i s o c y a n a t e - s u b s t i t u t e d urea r e a c t i o n t o form b i u r e t s , Η 0 Η 0 Η R-N

= C = 0 + R

I t

II

H

ί ff

- Ν - C - Ν - R

> R

!

I

-N-C-N-R" ι

C = 0 Ν - Η ι

R are other p o s s i b l e c r o s s - l i n k i n g mechanisms. I t i s not known whether these r e a c t i o n s take p l a c e during p a r t i c l e b o a r d formation. In summary, i t can be p o s t u l a t e d t h a t chemical bonding o f the isocyanate t o wood occurs through urethane l i n k a g e s w h i l e other isocyanate groups on the same molecule can c r o s s - l i n k w i t h other i s ο cyanat e-woo d l i g a n d s through urea o r p o s s i b l e i s o c y a n urate l i n k a g e s . F i g u r e 1 shows t h i s concept s c h e m a t i c a l l y . One t h r e e - f u n c t i o n a l isocyanate molecule i s shown b i n d i n g t o a wood h y d r o x y l and c r o s s - l i n k i n g w i t h t h r e e other isocyanate-wood l i g a n d s through a urea and an isocyanurate s t r u c t u r e . I t must be emphasized t h a t t h i s i s o n l y a s i m p l i f i e d r e p r e s e n t a t i o n o f some of the c r o s s - l i n k i n g mechanisms t h a t c o u l d take p l a c e .


288

URETHANE CHEMISTRY AND APPLICATIONS

Figure 1. Cross-linking concept of polymeric MDI wood matrix.


20.

FRiNK AND

SACHS

Isocyanate Binders

289

History Development o f isocyanate binders f o r p a r t i c l e b o a r d began i n Europe i n the l a t e 1 9 6 θ s . The development gained momentum i n the early J 0 s . Deppe and Ernst (h) were among the f i r s t t o r e p o r t the achievement o f V^QO (West German e x t e r i o r s t r u c t u r a l grade) boards s u i t a b l e f o r b u i l d i n g c o n s t r u c t i o n u s i n g polymeric MDI as the b i n d e r . They demonstrated s t r e n g t h values at l e a s t e q u i v a l e n t to phenolic-bound boards and, i n p a r t i c u l a r , found the i s o c y a n a t e bound boards t o be more hydrophobic as determined by the West German t e s t method. The f i r s t commercial production o f an i s o c y a n a t e - c o n t a i n i n g p a r t i c l e b o a r d was achieved by Deutsche Novopan o f West Germany. They commercialized a product c a l l e d Phenapan-V-100-Iso-Spanplatte which c o n s i s t s o f a polymeric MDI-bound core w i t h phenolic-bound face l a y e r s . This c o n f i g u r a t i o n accommodated one o f the major problem areas c h a r a c t e r i s t i c o f isocyanate binders - namely adhe s i o n t o metal surfaces during p r e s s i n g (k 5)· Ernst ( 6 ) describes Phenapan-V-100-Iso-Spanplatte as a t e c h n o l o g i c a l advance vs. the previous p h e n o l i c V-100 boards and c i t e s moisture r e s i s t a n c e and b e t t e r preformance under permanent l o a d i n outdoor c o n d i t i o n s among i t s advantages. P i o n e e r i n g work i n the development of an isocyanate-bound p a r t i c l e b o a r d i n the U.S. has been conducted by the E l l i n g s o n Lumber Company. This work has r e s u l t e d i n a p r o p r i e t a r y process f o r bonding c e l l u l o s i c m a t e r i a l s w i t h polymeric MDI t o produce a m u l t i p l e - p l y s t r u c t u r e panel ( J , _8, 9)· Advantages c i t e d f o r t h i s process i n c l u d e a t o l e r a n c e f o r up t o 22% moisture i n the wood raw m a t e r i a l without p r e d r y i n g and the a b i l i t y t o a l s o i n c l u d e s i g n i f i c a n t q u a n t i t i e s o f bark and needles. As a r e s u l t o f t h i s work, E l l i n g s o n has become the f i r s t , and at p r e s e n t , the o n l y commercial producer o f a wood panel product u t i l i z i n g isocyanate b i n d e r i n North America. This product, c a l l e d Elcoboard ( 1 0 , 11) c o n s i s t s of an isocyanate-bound saw m i l l waste core and two surface veneers o f v a r y i n g grades. Elcoboard d i f f e r s from s i m i l a r composite panels not o n l y i n the use o f i s o cyanate, but a l s o i n t h a t the e n t i r e composite i s pressed i n one step. Elcoboard has gained I n t e r n a t i o n a l Conference of B u i l d i n g O f f i c i a l s (ICBO) approval as an e x t e r i o r grade plywood s u b s t i t u t e in building applications. The West German parent o f Mobay Chemical Corporation was very a c t i v e , along w i t h Deutsche Novopan, i n the development o f i s o cyanate as a p a r t i c l e b o a r d b i n d e r i n Europe. The work o f Sachs ( 1 2 , 13» lh) and o f Deppe (k 5.), along w i t h the aforementioned commercial developments, has provided a background f o r i n t e r e s t i n isocyanate b i n d e r s i n North America. This i n t e r e s t i s beginning to be r e f l e c t e d i n the l i t e r a t u r e . For example, Hse ( 1 5 ) has p u b l i s h e d papers on the development o f both plywood and f l a k e b o a r d adhesives which combine isocyanate and p h e n o l i c r e s i n s . In the former case, he claims adequate bonding w i t h wetter veneers than can be used w i t h c o n v e n t i o n a l p h e n o l i c adhesive. In the case o f !

f

f

9

1

9


290

URETHANE

CHEMISTRY

AND

APPLICATIONS

southern hardwood f l a k e b o a r d , Hse has found a s u p e r i o r performance vs. p h e n o l i c r e s i n a t high f l a k e moisture content, low b i n d e r cont e n t and low panel d e n s i t y . He has r e c e n t l y been awarded a U.S. patent on t h i s development (l6). During t h e past two y e a r s , papers have been given a t t h e annual Washington S t a t e U n i v e r s i t y Symposium on P a r t i c l e b o a r d by Udvardy (17) > Wilson (l8) and Johns (l£). T h e i r l a b o r a t o r y s t u d i e s along w i t h some o f t h e others c i t e d i n t h i s paper have shown t h a t a number o f advantages are p o s s i b l e when u s i n g polymeric MDI, as compared t o c o n v e n t i o n a l p h e n o l i c r e s i n s , f o r b i n d i n g wood composi t e panels i n c l u d i n g p a r t i c l e b o a r d s , flakeboards and waferboards. Mob ay has a l s o developed data which c o n t r i b u t e s t o t h e c u r r e n t s t a t e o f t h e technology, p a r t i a l l y i n j o i n t research programs at recognized wood research l a b o r a t o r i e s . There are two major disadvantages o f t e n c i t e d i n t h e use o f isocyanate wood b i n d e r s . The f i r s t i s a h i g h e r raw m a t e r i a l cost as compared t o c o n v e n t i o n a l b i n d e r s and t h e second i s a tendency t o adhere t o metal t r a n s f e r p l a t e s o r press p l a t e n s . The l a t t e r problem has been s o l v e d on a commercial s c a l e by producing a m u l t i l a y e r product w i t h s o l i d veneers o r phenolic-bound p a r t i c l e s as face l a y e r s . A l l isocyanate-bound non-veneered board can be made by t r e a t i n g t h e metal surfaces w i t h a r e l e a s e m a t e r i a l o r , more r e c e n t l y , using a s e l f - r e l e a s a b l e isocyanate. The i n c r e a s e d c o s t s represented by t h e above can be o f f s e t o n l y i f p r o c e s s i n g advantages a r e p o s s i b l e w i t h isocyanates t h a t are not p o s s i b l e w i t h competing b i n d e r s . I t i s t h e purpose o f t h i s paper t o i l l u s t r a t e some o f these advantages f o r t h e product i o n o f e x t e r i o r grade wood composite panels. Red Oak Flakeboard A j o i n t program i n v o l v i n g Mobay, Purdue U n i v e r s i t y and t h e U.S. F o r e s t Products Laboratory (USFPL) has been documented as p a r t o f a Purdue U n i v e r s i t y Research B u l l e t i n authored by Hunt, et.al. (20). The wood raw m a t e r i a l used was r e d oak f l a k e s . Red oak, an abundant species i n t h e North C e n t r a l U n i t e d S t a t e s , i s a r e l a t i v e l y dense hardwood, known t o be r a t h e r d i f f i c u l t t o b i n d . The u l t i m a t e goal was t h e development o f a r o o f decking panel which r e q u i r e d a 1-1/8 i n c h (29mm) board, c o n s i d e r a b l y t h i c k e r than t y p i c a l f l a k e b o a r d products. An e x t e r i o r grade b i n d e r had t o be used but c o n v e n t i o n a l aqueous p h e n o l i c s are r e l a t i v e l y slow c u r i n g r e s i n s . Polymeric MDI was evaluated i n an attempt t o achieve t h e necessary e x t e r i o r d u r a b i l i t y without t h e e x c e s s i v e l y l o n g press times otherwise needed f o r c u r i n g such t h i c k boards. Procedure. A d e s c r i p t i o n o f t h e type o f panels made i s given i n Table I . Both t h e core and face f l a k e s as r e c e i v e d from USFPL were m i l l e d i n a Condux hammer m i l l so t h a t f l a k e geometry was constant throughout t h e study. Table I I gives t h e p r o c e s s i n g


FRINK AND SACHS

lsocyanate

Binders

Table I Panel C h a r a c t e r i s t i c s - Red Oak Flakeboard

Type:

3 - l a y e r l a b f l a k e b o a r d , hand formed

Dimensions:

U60xU60x299mm ( l 8 - l / 8 " x l 8 - l / 8 " x l - l / 8 " ) 750 Kg/m

3

Target Density: Raw M a t e r i a l :

( U 6 . 9 PCF)

Red oak f l a k e s - 65% by wt. i n core 17.5% by wt. i n ea.face

Table I I Constants For A l l Panels - Red Oak Flakeboard

1

Flake Moisture C o n t e n t :

P a r a f f i n Wax:

13% 11%

Face l a y e r s - 5% s o l i d s on 0D wood wt Core l a y e r - 6% s o l i d s on 0D wood wt

1T6°C (3*+9°F)

Press C l o s i n g Time: Open Time:

-

1% s o l i d s based on oven-dry wood "Wt. added as kj% aqueous emulsion

Binder Content:

Press Temp.:

Face l a y e r s Core l a y e r

U0 sec. t o stops

1 5 - 3 5 min. blender t o press

"Water added t o f l a k e s t o equal these l e v e l s when isocyanate used


292


parameters t h a t were constant f o r a l p a n e l s . Binder and p a r a f f i n wax were i n t r o d u c e d t o the f l a k e s through an a t o m i z i n g n o z z l e i n a D r a i s FSP 80 discontinuous l a b o r a t o r y b l e n d e r . The two b i n d e r s evaluated were: 1) Aqueous phenol-formaldehyde r e s i n s o l u t i o n (h3>5% s o l i d s ) 2) Desmodur PU-1520A 20 polymeric MDI (Bayer AG, West Germany) F i g u r e 2 i s a photograph o f a s e c t i o n o f one o f the panels bound w i t h Desmodur PU-1520A 20. T e s t i n g . Two panels were made f o r each experimental p o i n t and p h y s i c a l p r o p e r t i e s measured f o r each a c c o r d i n g t o the app r o p r i a t e West German "Deutschen N o r m v o r s c h r i f t e n " (DIN) standards. The p r i n c i p l e d i f f e r e n c e between these and the ASTM D - 1 0 3 7 - 7 2 ("Standard Methods o f E v a l u a t i n g the P r o p e r t i e s o f Wood-Base F i b e r and P a r t i c l e Panel M a t e r i a l s " ) procedures i s i n the d u r a b i l i t y t e s t . In the German procedure t h i s i s determined by measuring the i n t e r n a l bond ( t e n s i l e s t r e s s a p p l i e d i n the t h i c k n e s s d i r e c t i o n t o f a i l u r e ) on wet specimens a f t e r b o i l i n g two hours i n water. T h i s i s c a l l e d the V100 t e s t . Other p r o p e r t i e s measured were dry i n t e r n a l bond ( V 2 0 ) , bending modulus o f r u p t u r e (MOR) and e l a s t i c i t y (MOE), d e n s i t y and t h i c k n e s s s w e l l , the l a t t e r a f t e r 2 and 2k hour water soaks. The p a t t e r n used f o r c u t t i n g t e s t specimens i s shown i n F i g u r e 3. The p r o p e r t y values g i v e n i n t h i s r e p o r t are mean values o f the number o f specimens i n d i c a t e d times two s i n c e each board was d u p l i c a t e d . D e n s i t i e s were measured on a l l t h i c k n e s s s w e l l , i n t e r n a l bond (IB) and bending modulus specimens b e f o r e those r e s p e c t i v e t e s t s were run. In a d d i t i o n , f o u r panels ( r e p r e s e n t i n g two experimental p o i n t s ) were submitted t o Purdue U n i v e r s i t y f o r l i n e a r expansion t e s t i n g . Two specimens were cut from each board at r i g h t angles t o each o t h e r . A f t e r c o n d i t i o n i n g at 80°F (27°C) and 50% r e l a t i v e h u m i d i t y , the samples were brought t o constant weight i n an 80 F (27 C) and 90% r e l a t i v e humidity environment and l i n e a r expansion measured over a 10 i n c h (25^mm) gauge l e n g t h . V a r i a b l e L e v e l s . The v a r i a b l e s t u d i e d i n the work r e p o r t e d here was press t i m e , s i n c e we were seeking t o produce a f a s t e r c u r i n g p a n e l . Each b i n d e r was e v a l u a t e d at t h r e e press time l e v e l s - 0 . 2 , 0 . 3 and 0 . 5 seconds per mm p a n e l t h i c k n e s s . For the nominal 29mm t h i c k n e s s , t h i s r e s u l t e d i n design press times o f 5 · 8 , 8.7 and Ik.5 minutes. R e s u l t s . P h y s i c a l p r o p e r t y values f o r the panels produced are given i n Table I I I . A more r a p i d c u r i n g r a t e w i t h the Desmodur PU-1520A 20 i s s t r o n g l y i n d i c a t e d by the modulus o f r u p t u r e and i n t e r n a l bond r e l a t i o n s h i p s as i l l u s t r a t e d by F i g u r e s h and 5 , r e s p e c t i v e l y . In both i n s t a n c e s i t appears t h a t the p h e n o l i c r e s i n has not t o t a l l y cured at 8 . 7 minutes press time and perhaps not at l U . 5 · The i s o c y a n a t e , on t h e other hand, has developed e s s e n t i a l l y f u l l MOR and IB values at 5-8 minutes. The IB i n


20.

FRINK AND SACHS

Figure 2.

Isocyanate Binders

293

Red oakflakeboardwith Desmodur PU-1520A 20 binder.


294


mm

SO

so

so

25 SO

MIASM!t MINTS IN It. II.

DKY WIT

SO

so

mm

(V20)i 2, 4,4, 20, 24, 26, 2 · , 30 (VIOO) t 3, S. 7. 19. 21, 23, 27, 29, 31 lINGfM 290mm : 1. · , 2S

MOISTUm ANALYSIS: 9,13,16, 16 THKKNtSS SWIUI 2 hrm. 24 hrt. ι 10.11, 12,14, IS, 17

Figure 3.

Cutting pattern for test specimens.



1U.5

PF

8.7

1U.5

PU1520A 20

Desmodur P U 1 5 2 0 A 20

Desmodur

PU1520A 20

5.8

8.7

PF

Desmodur

5.8

P r e s s Time Min.

PP

Binder

h6.6

h6.9

1+7.0

7107

7208

7136

70U9

6613

1+6.8

U6.T

h9ke

Modulus of psi

2

Density PCF

of

R e d Oak

III

725

756

7l43

569

658

703

1.3

1.5

1.3

5.3

5.1

7.14

Thickness 2 Hrs.

Flakeboards

Modulus of E l a s t i c i t y p s i χ 1θ3

Properties

Rupture

Physical

Table

5.7

5.7

5Λ

lh.5

Ih.h

16.8

Swell, % 2h H r s .

261+

268

260

162

129

81+

Internal Dry

89

93

95

18

ll+

11+

Bond, 1 Wet

296

URETHANE CHEMISTRY AND APPLICATIONS MOR, psi Desmodur PU-1520A 20

6,900

/PF

6,100

5,300

•

Figure 4.

ι

i

5

10 Press Time, min.

15

Modulus of rupture vs. press time for red oak flakeboard.

IB, psi

Desmodur PU-1520A 20-Dry

240 -

200

-

160

120 ^

Desmodur PU-1520A 20-Wet

80

40 PF-Wet 5

I

l

10 Press Time, min.

15

FigureEdwards 5. Internal bonds vs. press time for red oak flakeboard. et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

FRINK AND SACHS

297

Isocyanate Binders

p a r t i c u l a r i s a good i n d i c a t o r o f degree o f cure s i n c e i t i s a core p r o p e r t y , whereas MOR, which measures bending s t r e n g t h , i s h i g h l y r e l a t e d t o surface c h a r a c t e r i s t i c s . A comparison o f t h e d u r a b i l i t y o f t h e adhesive bonds formed by t h e two b i n d e r s i s p r o v i d e d by t h e wet i n t e r n a l bond and by t h e t h i c k n e s s s w e l l values shown g r a p h i c a l l y i n F i g u r e s 5 and 6 , r e s p e c t i v e l y . The former, measured a f t e r a 2 hour water b o i l , are s t r i k i n g l y s u p e r i o r f o r t h e Desmodur PU-1520A 20 vs. t h e p h e n o l i c b i n d e r at a l l press times. The same can be s a i d f o r t h i c k n e s s s w e l l a f t e r 2 and 2k hour water soaks. I t seems safe t o say t h a t a 50% r e d u c t i o n i n press time i s p o s s i b l e w i t h t h e isocyanates vs. t h e p h e n o l i c r e s i n w h i l e maint a i n i n g e q u i v a l e n t bending s t r e n g t h and v a s t l y improved wet and dry i n t e r n a l bond. This i s f u r t h e r shown, although perhaps not q u i t e so d r a m a t i c a l l y , by t h e dimensional s t a b i l i t y data g i v e n i n Table IV. Mixed Hardwood Flakeboard Among other hardwoods indigenous t o t h e North C e n t r a l U n i t e d States a r e maple, b i r c h and aspen. These species have l i t t l e commercial value but a r e found i n areas c l o s e t o major markets. Mobay sponsored a study, conducted by t h e I n s t i t u t e o f Wood Research a t Michigan T e c h n o l o g i c a l U n i v e r s i t y , t o determine t h e e f f e c t s o f p r o c e s s i n g v a r i a b l e s on isocyanate-bound f l a k e b o a r d prepared from a mixture o f these s p e c i e s and t o compare r e s u l t a n t panel p r o p e r t i e s w i t h those o f a standard panel prepared w i t h a commercial aqueous p h e n o l i c r e s i n under t y p i c a l p r o c e s s i n g c o n d i t i o n s f o r t h a t type o f r e s i n . Procedure. Panel c h a r a c t e r i s t i c s a r e given i n Table V. Table V I gives t h e p r o c e s s i n g parameters f o r t h e standard panels. Binders and p a r a f f i n wax were i n t r o d u c e d i n t o t h e blender by s p r a y i n g . The b i n d e r s employed i n t h i s study were an aqueous phenol-formaldehyde r e s i n and Mobay s Mondur MR polymeric MDI. T

T e s t i n g . Three r e p l i c a t e panels were made f o r each e x p e r i mental p o i n t and p r o p e r t i e s were measured, i n g e n e r a l , a c c o r d i n g t o ASTM D-103T-72. One s l i g h t d e v i a t i o n was i n t h e d u r a b i l i t y , or a c c e l e r a t e d aging t e s t . The ASTM method c a l l s f o r t h e f o l l o w i n g : - Immerse i n water § 120°F (k9°C) f o r 1 hour - Spray w i t h steam and water vapor a t 200°F (93 C) f o r 3hours - Store a t 10°F (-12°C) f o r 20 hours - Heat i n d r y a i r a t 210°F (100°C) f o r 3 hours - Spray w i t h steam and water vapor a t 200 F (93 C) f o r 3hours - Heat i n d r y a i r a t 210°F (100 C) f o r 18 hours The c y c l e i s run s i x t i m e s . Since IWR s u n i t r e q u i r e d some time f o r steam g e n e r a t i o n , t h e f r e e z i n g phase and l o n g d r y phase were each reduced by one hour so t h a t t h e t o t a l c y c l e c o u l d be completed i n kQ hours. f


298


16 PF-24 nr. Soak 14 12

I

10

CO

Desmodur 1520A 20-24 hr. Soak PF-2 hr. Soak

Desmodur 1520A 20-2 hr. Soak

6

Figure 6.

8

10 12 Press Time, min.

14

16

Thickness swell of red oak flakeboards.

Table IV Dimensional S t a b i l i t y :

Press Time Min.

Binder PF

50 t o 90% R.H.

Water Absorption %

Thickness Swell %

Linear Expansion %

ik.5

8.8

6.8

0.20

5.8

6.k

k.k

0.1k

Desmodur PU-1520A 20


20.

FRiNK

AND SACHS

299

Isocyanate Binders

Table V Panel C h a r a c t e r i s t i c s - Mixed Hardwood Flakeboard

Type:

Single layer

Dimensions :

U60xU60xllmm ( l 8 - l / 8 " x l 8 - l / 8 " x 7 / l 6 " )

Target D e n s i t y : 720 Kg/m Raw M a t e r i a l : 5 2 . 5 ? maple, 1 7 - 5 ? b i r c h , 3 0 . 0 ? aspen f l a k e s ( 2 . 0 " χ 0.U" t o 1.2" χ 0 . 0 2 5 " )

Table VI P r o c e s s i n g Parameters For Standard Panels Mixed Hardwood Flakeboard

F l a k e M o i s t u r e Content Paraffin Binder

-

10% 1% s o l i d s based on O.D.

Wax

5% s o l i d s on O.D.

Content

wood wt.

wood wt.

190°C (375°F) f o r p h e n o l i c l63°C (325°F) f o r i s o c y a n a t e

Press Temperatue

Press C l o s i n g Time

-

1 min. t o stops

T o t a l Press Time

-

7 min. f o r p h e n o l i c 5 min. f o r i s o c y a n a t e


300


APPLICATIONS

Thickness s w e l l measurements were made on 2 χ 12 χ T/l6" (51x 305 x 11mm) specimens a f t e r c o n d i t i o n i n g at 21°C (TO F) and 90% r e l a t i v e humidity f o r two months. Since bending s t r e n g t h values were a f f e c t e d by specimen den s i t y , and as the d e n s i t i e s v a r i e d somewhat (from k3-b6 PCF [688736 Kg/m3]), these values were adjusted t o correspond t o the t a r g e t d e n s i t y . N o r m a l i z a t i o n f a c t o r s used were based on e a r l i e r work w i t h these wood s p e c i e s . The a d j u s t e d MOR a f t e r a c c e l e r a t e d aging (AA/MOR) was c a l c u l a t e d by a p p l y i n g the percent r e t e n t i o n o f the o r i g i n a l MOR t o the o r i g i n a l a d j u s t e d MOR. C o n s i d e r i n g the number o f t e s t specimens cut from each of the t h r e e r e p l i c a t e panels f o r each experimental p o i n t , the t h i c k n e s s s w e l l data are averages o f t h r e e specimens; a l l other p r o p e r t i e s are averages o f n i n e . V a r i a b l e L e v e l s . In a d d i t i o n t o the standard panels as de s c r i b e d above, panels were made w i t h the Mondur MR w i t h v a r i a t i o n s from standard c o n d i t i o n s as shown i n Table V I I . R e s u l t s . P h y s i c a l p r o p e r t i e s f o r the standard, or c o n t r o l , panels are given i n Table V I I I . Although the Mondur MR panels were cured two minutes l e s s and at 50°F (28°C) lower press temperature, o r i g i n a l s t r e n g t h p r o p e r t i e s and t h i c k n e s s s w e l l values are c l e a r l y s u p e r i o r t o those o f the phenolic-bound panels. There d i d appear t o be a s l i g h t l a g i n modulus o f rupture r e t e n t i o n a f t e r a c c e l e r ated aging. Binder L e v e l . Mondur MR-bound panels were made at 3 and k% b i n d e r l e v e l s w h i l e m a i n t a i n i n g a l l other parameters at the con t r o l l e v e l s . The r e s u l t i n g p r o p e r t y values are given i n Table IX. The 5? b i n d e r column represents the c o n t r o l board values f o r each of the b i n d e r s and i s i n c l u d e d f o r comparative purposes. Modulus o f r u p t u r e v a l u e s , both o r i g i n a l and a f t e r a c c e l e r a t e d a g i n g , are shown g r a p h i c a l l y i n F i g u r e 7 · I t i s s u r p r i s i n g t o note t h a t bend i n g s t r e n g t h values are h i g h e r at h% than at 5? Mondur MR l e v e l . D u r a b i l i t y o f the MR boards a l s o seems best a t the i n t e r m e d i a t e b i n d e r l e v e l as i n d i c a t e d not o n l y by the M0R s a f t e r a c c e l e r a t e d a g i n g , but a l s o by the t h i c k n e s s s w e l l values i l l u s t r a t e d i n F i g ure 8. D u r a b i l i t y o f the i s o c y a n a t e boards at k% b i n d e r exceeds t h a t o f the p h e n o l i c c o n t r o l boards. Values f o r the l a t t e r are shown as s i n g l e p o i n t s i n F i g u r e s 7 and 8. At the 3% l e v e l the Mondur MR boards r e t a i n too l i t t l e MOR a f t e r a c c e l e r a t e d a g i n g , although t h i s may not be the case at a h i g h e r press temperature. f

F l a k e M o i s t u r e Content. Panels were made w i t h Mondur MR i n which the moisture content o f the f l a k e s was a d j u s t e d t o 20% i n t o t h e p r e s s . I t i s w e l l known t h a t panels cannot be made w i t h p h e n o l i c r e s i n s at such a h i g h moisture content, s i n c e steam p r e s sure blows them apart upon opening o f the p r e s s . Even t o a t t a i n 10? moisture content i n t o the p r e s s , raw m a t e r i a l must be d r i e d t o


FRINK AND SACHS

Isocyanate Binders

Table V I I V a r i a b l e Levels S t u d i e d With Mondur MR Mixed Hardwood Flakeboard

Binder L e v e l

-

3? k% 5?

F l a k e Moisture Content

-

10? 20?

T o t a l Press Time

-

3 minutes 5 minutes

Table V I I I Average Property Values For C o n t r o l Mixed Hardwood Panels

1

Property

PF""

Mondur MR'

52U5

53^0

Modulus o f E l a s t i c i t y * (psi χ 10 )

675

721

I n t e r n a l Bond ( p s i )

138

286

Aged MOR* ( p s i )

3^72

3028

Thickness S w e l l (?)

20.33

17.08

Modulus o f Rupture* ( p s i )

3

*Adjusted t o U5 p c f Press time o f 7 min. w i t h 375 F press temperature P r e s s time o f 5 min. w i t h 325°F press temperature 2


U R E T H A N E CHEMISTRY A N D APPLICATIONS

302

Table IX E f f e c t Of Binder L e v e l On Mixed Hardwood Flakeboard P r o p e r t i e s

3?

Binder

Property

MRp PF MR PF MR PF MR PF MR PF

Modulus o f Rputure* ( p s i ] Modulus o f E l a s t i c i t y * ( p s i χ 103) I n t e r n a l Bond Aged MOR* ( p s i ) Thickness Swell (?)

Binder L e v e l k% 5%

5223

6673

696

867

—

226

—

—

233

—

1917

1+691

18.95

16.58

— —

*Adjusted t o k5 PCF Press time o f 5 min. with 325°F press P r e s s time o f 7 min. with 375 F press 1 and 2 at 1 0 ? MC l e v e l

—

— —

53UO 52h5 721 675 286 138 3028 3^72 17.08 20.33

o

2

temperature temperature

7,000 Original 6,000

2

(PF-Orig.)o

§-5,000 Κ -δ

m 3

f 4,000

After Accelerated Aging

3,000

2,000

Mondur MR Figure 7.

Modulus of rupture vs. Mondur MR level for mixed hardwood flak boards.


20.

FRiNK AND SACHS

Figure 8.

Isocyanate Binders

303

Internal bond and thickness swell vs. % Mondur MR for mixed hardwood flakeboards.


304


a l e v e l w e l l below t h a t i f aqueous p h e n o l i c r e s i n s are used. I f panels can be made w i t h isocyanates at 20% moisture content i n t o the p r e s s , the m a t e r i a l need not be d r i e d below 19% (assuming 1% water added w i t h the p a r a f f i n wax), s i n c e no a d d i t i o n a l water i s added w i t h the b i n d e r . P r o p e r t i e s of the panels made w i t h the h i g h moisture raw m a t e r i a l are compared w i t h those o f t h e c o n t r o l boards i n Table X. Bending s t r e n g t h s are somewhat h i g h e r and i n t e r n a l bonds are lower at 20% MC. This i s t o be expected s i n c e the p l a s t i c i z i n g e f f e c t o f the excess water on the wood f l a k e s tends t o i n c r e a s e t h e den s i t y g r a d i e n t between the core and s u r f a c e o f t h e board, weakening core p r o p e r t i e s and i n c r e a s i n g s u r f a c e - r e l a t e d p r o p e r t i e s . T h i s can be c o r r e c t e d by l e n g t h e n i n g the press c l o s i n g time. That wasn't done here s i n c e i n t e r n a l bonds are s t i l l c o n s i d e r a b l y h i g h er than those o f the p h e n o l i c c o n t r o l panels. MOR a f t e r a c c e l e r a t e d aging i n c r e a s e s s l i g h t l y i n going from 10 t o 20% f l a k e moisture content. The dramatic improvement i n t h i c k n e s s s w e l l at the h i g h e r moisture content i s unexplained. The data show the f e a s i b i l i t y o f making good q u a l i t y f l a k e board w i t h Mondur MR at f l a k e moisture content l e v e l s as h i g h as 20%. Press Time. P r o p e r t i e s o f the Mondur MR-bound boards pressed o n l y 3 minutes vs. those o f the c o n t r o l boards are given i n Table X I . The i n t e r n a l bond s t r e n g t h o f the Mondur MR boards d i d d r o p as expected, although even at 3 minutes press time at 325 F ( l 6 3 ° C ) , i t i s h i g h e r than t h a t o f the phenolic-bound board cured Τ minutes at 375 F (191 C). The s u r p r i s e comes i n the bending s t r e n g t h and d u r a b i l i t y p r o p e r t i e s where the performance a c t u a l l y appears b e t t e r at the s h o r t e r press time. To summarize the mixed hardwood f l a k e b o a r d study, i t can be s a i d t h a t good q u a l i t y panels were made w i t h Mondur MR at reduced press t i m e , temperature and b i n d e r l e v e l s and i n c r e a s e d f l a k e moisture content v s . the commercial p h e n o l i c r e s i n . a

Aspen Waferboard Another parameter i n which savings can be made w i t h the use o f polymeric MDI b i n d e r i s panel d e n s i t y . Although d e n s i t y v a r i a t i o n s were not s t u d i e d i n e i t h e r o f the two above programs, they were i n a program Mobay sponsored at t h e former E a s t e r n Canadian F o r e s t Products L a b o r a t o r y , now F o r i n t e k Canada Corp. - E a s t e r n F o r e s t Products Laboratory. The work was done w i t h 3 l a y e r aspen waferboard w i t h cores bound w i t h Mondur E - U U l , another Mobay polymeric MDI, and the faces w i t h a s p r a y - d r i e d powdered p h e n o l i c novolac r e s i n . C o n t r o l s were homogeneous boards bound e n t i r e l y w i t h the novolac. Although homogeneous boards bound e n t i r e l y w i t h Mondur E - U U l were not i n c l u d e d i n t h i s study, one such board i s shown i n F i g u r e 9 t o i l l u s t r a t e the appearance o f aspen waferboard. Note the much wider wafers as compared t o the r e d oak f l a k e s shown earlier.


20.

FRINK AND SACHS

305

Isocyanate Binders Table X

E f f e c t Of F l a k e Moisture Content On Mixed Hardwood Flakeboard P r o p e r t i e s

Moisture Content 20? 10?

Binder

Property

531+0 52U5 721 675 286 138 3028 3^72 17.08 20.33

1

m

Modulus o f Rupture* ( p s i )

PF MR PF MR PF MR PF MR PF

Modulus o f E l a s t i c i t y * ( p s i χ 1θ3) I n t e r n a l Bond ( p s i ) Aged MOR* ( p s i ) Thickness S w e l l ( ? )

*Adjusted t o 1+5 PCF i p r e s s time o f 5 min. w i t h 325°F press P r e s s time o f 7 min. w i t h 375°F press 1 and 2 a t 5? b i n d e r l e v e l

5785

— 7^9

— 215 —

3U83

— 8.91 —

temperature temperature

2

Table X I E f f e c t o f Press Time on Mixed Hardwood Flakeboard P r o p e r t i e s

Property

Binder

Modulus o f Rupture* ( p s i ) Modulus o f E l a s t i c i t y * (psi χ 1 0 ) I n t e r n a l Bond ( p s i ) 3

Aged MOR* ( p s i ) Thickness S w e l l ( ? )

MRp PF MR PF MR PF MR PF MR PF

7 Min.

52U5

—

675

— 138

—

3UT2

— 20.33

Press Time 3 Min. 5 Min. 53^0

5950

— 721

— 733

— 3028

— 1+230

17.08

15.71+

— 286 — —

*Adjusted t o 1+5 PCF l a t 325°F press temperature a t 375°F press temperature 1 and 2 at 1 0 ? M.C and 5? b i n d e r l e v e l s 2


—

167

—

—

306


APPLICATIONS

Subsequent t o p u b l i c a t i o n o f the o r i g i n a l work done by Udvardy ( I T ) , a m u l t i p l e c o r r e l a t i o n a n a l y s i s o f a l l r e p l i c a t e s o f the data p o i n t s was conducted at Mobay. A set o f r e g r e s s i o n equations was generated f o r each b i n d e r s t u d i e d . Each equation p r e d i c t e d one o f the board p r o p e r t i e s i n terms o f a number o f proc e s s i n g v a r i a b l e s i n c l u d i n g panel d e n s i t y . C o r r e l a t i o n c o e f f i c i e n t s were 0 . 8 f o r i n t e r n a l bonds f o r the Mondur E-UUl. panels w i t h a l l others b e i n g 0 . 9 or g r e a t e r . Some o f the p r e d i c t i v e p r o p e r t y equations were p l o t t e d v s . panel d e n s i t y f o r each b i n d e r w i t h b i n d e r l e v e l and press time h e l d constant at 2? and 5 minutes, r e s p e c t i v e l y . F i g u r e 10 shows t h a t comparable MOR s are a t t a i n a b l e w i t h the Mondur E-Ul+1 at about a 10? lower d e n s i t y than w i t h the p h e n o l i c . Aged MOR ( F i g ure 11) determined i n t h i s study by MOR measurement a f t e r a two hour water b o i l (Canadian Standard CSA 0188-0MT8) i n d i c a t e s the p o s s i b i l i t y of a savings o f about T . 5 - 1 0 ? i n d e n s i t y . F i n a l l y , F i g u r e 12 shows t h a t comparable i n t e r n a l bonds can be achieved by the isocyanate at up t o 25? lower d e n s i t y vs. the p h e n o l i c r e s i n . 1

Summary The purpose o f t h i s p r e s e n t a t i o n has been t o i l l u s t r a t e some advantages t h a t are p o s s i b l e i n the use o f polymeric MDI as a b i n d e r f o r e x t e r i o r grade wood composite panels. The data g i v e n have i n d i c a t e d the p o t e n t i a l f o r savings i n press t i m e , press temperature, f l a k e moisture content, r e s i n l e v e l and panel d e n s i t y (as compared t o c o n v e n t i o n a l phenol-formaldehyde b i n d e r s ) i n s e v e r a l types o f such panels. The panel producer must o p t i m i z e h i s p r o c e s s i n g parameters t o take f u l l advantage o f the savings i s o c y a n a t e s can o f f e r . The answer as t o what the optimum c o n d i t i o n s are must be determined on a case-by-case b a s i s . To date these o p t i m i z e d c o n d i t i o n s f o r polymeric MDI have been found f a v o r a b l e versus those o f a l t e r n a t e b i n d e r s by s e v e r a l European and at l e a s t one U.S. panel producer. Acknowledgement We wish t o thank Dr. D a r r e l l D. N i c h o l a s , Mr. Roy D. Adams and Ms. Susan Mateer o f the I n s t i t u t e o f Wood Research at Michigan T e c h n o l o g i c a l U n i v e r s i t y f o r t h e i r work i n conducting the mixed hardwood f l a k e b o a r d experimental program. We a l s o wish t o thank Dr. M i c h a e l 0 . Hunt of Purdue U n i v e r s i t y and Dr. W i l l i a m F. Lehmann o f Weyerhaeuser C o r p o r a t i o n f o r t h e i r h e l p i n the r e d oak f l a k e board work and Mr. Otto G. Udvardy o f Borden Chemical f o r the aspen waferboard study. F i n a l l y , we would l i k e t o thank Dr. Ronald T a y l o r o f Mobay Chemical C o r p o r a t i o n f o r h i s c o n s i d e r a b l e advice and h e l p w i t h the m u l t i p l e c o r r e l a t i o n a n a l y s i s .


20.

FRiNK AND SACHS

lsocyanate

Binders


307

Figure 11. Predicted aged modulus of rupture vs. nominal density-aspen wafer

Figure 12. Predicted internal bond vs. nominal density-aspen waferboard Edwards et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

FRINK AND SACHS

Isocyanate Binders

309

Literature Cited 1. Rowell, R.M. and Ellis, W.D., Wood Science, (1979) 12 (1) 2. Hartman, S., Wood Technol.: Chem. Aspects Symp., 1976 (1977), 43 3. Wittman, O., Holz als Roh-und Werkstoff, (1976) 34 (11) 4. Deppe, H.-J. and Ernst, Κ., Holz als Roh-und Werkstoff (1971), 29 (2) 5. Deppe, H.-J., Proc. of Washington State University Symposium on Particleboard, (1977), 11 6. Ernst, K., Holz-Zentralblatt, (1975) 122 7. Shoemaker, P.D. and McQueary, H.O., U.S. Patent No. 3,919,017 (1975) 8. Shoemaker, P.D. and McQueary, H.O., U.S. Patent No. 3,930,110 (1975) 9. Shoemaker, P.D. and McQueary, H.O., U.S. Patent No. 4,946,952 (1977) 10. Ellingson, G.P., Proc. of Washington State University Symposi um on Particleboard (1977), 11 11. Braun, F.W. and Ellingson, G.P., ibid, (1979) 13 12. Loew, G. and Sachs, H.I., ibid, (1977) 11 13. Sachs, H.I., Holz-Zentralblatt, (1977), 103 (20) 14. Sachs, H.I., ibid (1977), 103 (25) 15. Hse, C.-Y., Proc. of Complete Tree Utilization of Southern Pine Symposium, (1978) 16. Hse, C.-Y., U.S. Patent No. 4,209,433, (1980) 17. Udvardy, O.G., Proc. of Washington State University Symposium on Particleboard, (1979), 13 18. Wilson, J.B., ibid, (1980), 14 19. Johns, W.E., ibid, (1980), 14 20. Hunt, M.O., Hoover, W.L., Fergus, D.A., Lehmann, W.F., and McNatt, J.D., Purdue University Agricultural Experiment Station Research Bulletin 954, (1978) RECEIVED April 30, 1981.


Isocyanate Binders for Wood Composite Boards - ACS Symposium

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