Thermal Plasticization of Lignocellulosics for Composites - ACS

Chapter 8

Thermal Plasticization of Lignocellulosics for Composites Hideaki Matsuda

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Research Laboratory, Okura Industrial Company, Ltd., 1515 Nakatsu-cho, Marugame, Kagawa-ken 763, Japan

Chemical modifications of wood with dicarboxylic acid anhydrides and epoxides are useful methods for obtaining plasticized crosslinked woods. By addition reaction of the anhydrides with hydroxyl groups of wood, esterified woods bearing carboxyl groups are obtained. When the esterified woods are allowed to react with bisepoxide under hot-pressing, crosslinking and plasticization of wood components occur simultaneously, to give reddish brown, yellowish brown, or blackish brown plasticized crosslinked wood boards. Further, polymerizable oligoester chains can be introduced into wood by oligoesterification reactions of wood with the anhydrides and polymerizable monoepoxides. Products of this reaction consist of the oligoesterified woods bearing polymerizable oligoester chains and viscous liquids consisting mainly of polymerizable free oligoesters not linked with the wood matrix. The products, when subjected to hot-pressing, give plasticized crosslinked wood boards. In this case, the free oligoesters work as a plasticizer for the wood components and are combined, by the crosslinking, with the oligoesterified woods, resulting in the formation of the network structure. These boards exhibit outstanding properties depending on the anhydride and the epoxide.

0097-6156/92/0476-0098$06.00/0 © 1992 American Chemical Society

In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

8. MATSUDA Effective

Thermal Plasticization of Lignocellulosics for Composites utilization

of unused

woods

such

as twiggy

99

woods and

p e r i o d i c a l l y thinned woods, and of wood meal, chips, etc., by-produced

as i n d u s t r i a l wastes, has not yet been practiced

development Therefore,

of more e f f e c t i v e u t i l i z a t i o n active

investigations

have

been

processes carried

well and

i s desired. out f o r t h i s

purpose. However, i t was not attempted whole, i s chemically

until


that

wood, as a

modified, with a view to providing

not observed f o r the o r i g i n a l wood ( U . recently reported

recently

properties

S h i r a i s h i et a l . (2-8) have

that wood becomes thermally

meltable by e s t e r i f i -

cation such as lauroylation and stearoylation, and also by e t h e r i f i cation such as benzylation.

In t h i s case, the thermal f l u i d i t y be-

comes higher as the carbon number of the acylation agent Generally,

lignin

i s belived

increases.

to be a three dimensional,

phenolic

molecule of complex structure and ultrahigh molecular weight. However, from the fact that the chemically

modified woods are thermally

meltable, S h i r a i s h i says that

might also

highly

branched,

lignin

l i n e a r high polymer with

large

be regarded as a branches

(7).

Of

other studies on chemical modification of wood by using i t s hydroxyl groups, those on the improvement of wood with isocyanates recently became actual (9,10). However, there was l i t t l e work on the method f o r e f f i c i e n t l y i n troducing

active functional groups into wood, and also on u t i l i z a -

tion of the obtained functional group-bearing woods. Recently, reaction of commercially e a s i l y available

dicarboxylic

acid anhydrides with hydroxyl groups of wood was investigated. I t was found that carboxyl

group-bearing e s t e r i f i e d woods could

effi-

c i e n t l y be obtained by the addition reaction ( e s t e r i f i c a t i o n ) of the wood with the anhydrides, as shown by Scheme 1 ( 1J_, 12).

Wood-OH +

properties tion

^0 CO

of the e s t e r i f i e d

of novel

wood-based

+

Scheme 1

woods were investigated

polymers was attempted

e s t e r i f i c a t i o n using the introduced series of studies,

Wood-00C-R-C00H

active carboxyl

Further,

and prepara-

by the addition groups.

In t h i s

i t was found that novel p l a s t i c i z e d crosslinked

woods having properties not observed f o r the o r i g i n a l wood could be obtained.

That i s , novel advantageous methods f o r thermal p l a s t i c i -


100

MATERIALS AND CHEMICALS FROM BIOMASS

zation of l i g n o c e l l u l o s i c s f o r composites

were found. This chapter

reviews interesting results obtained i n this series of studies. Introduction of Carboxyl Groups into Wood E s t e r i f i c a t i o n Reaction. ic

acid

chlorides,

The acylation of wood with higher aliphat-

from

caproyl

to stearoyl

chloride,

proceeds

e a s i l y i n a nonaqueous c e l l u l o s e solvent (N 0 -N,N-dimethylformamide d. 4 Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch008

o

(DMF) solvent) (£,5). Further, the reaction can be conducted t r i f l u o r o a c e t i c anhydride

in a

(TFAA) - higher a l i p h a t i c acid system at

30°C or 50°C (TFAA method) or i n a higher a l i p h a t i c acid chloride pyridine - DMF system at 100°C (chloride method) (13). In DMF or dimethyl sulfoxide,

which have a high swelling a b i l i t y

for wood, the e s t e r i f i c a t i o n reaction of Scheme 1 proceeds temperature,

at room

and the anhydrides add to the wood by ring-opening of

the anhydride group, giving e s t e r i f i e d woods bearing pendant carboxy l groups (1^1). In this case, wood meal i s used as the o r i g i n a l wood sample. Maleic anhydride (MA), phthalic anhydride (PA), and succinic anhydride

(SA) are the anhydrides

used.

After

the reaction, the

products are obtained by washing with acetone

and water, and then

subjecting to Soxhlet extraction with acetone.

The e s t e r i f i e d woods

thus obtained are denoted by W*MA, W*PA,

and W*SA,

respectively,

depending on the anhydride used. However, the acids such as maleic acid, phthalic acid, and succinic acid do not react with the wood. The

degree of e s t e r i f i c a t i o n can be evaluated by weight increase,

acid value, and saponification value of product ( 1_1, 1JÎ, 14). When the carboxyl

group

of monoester

derived

from

the anhydride

further

reacts with remaining hydroxyl group of wood, the monoester converts into diester. The content of the monoester i s obtained from the acid value, and the content of the diester from

the difference between

the acid value and the saponification value. Throughout this chapter sum of the monoester content

and the diester content w i l l

be r e -

ferred to as ester content. In

esterified

woods prepared

i n a solvent, the ester content

agrees well with the monoester content, indicating dride

has added

to the wood

i n the form

that the anhy-

of monoester.

infrared (IR) spectra of the e s t e r i f i e d woods,

In

the

they exhibit a sharp 1

absorption band i n the range of 1,720 - 1,750 cm" , attributable to -C00H and -C00-.

-1

In addition, W-MA shows a peak at 1,640 cm , due

to -CH=CH-. It i s noteworthy that, even i n the absence of solvent, the ester-


8.

MATSUDA

Thermal Plasticization of Lignocellulosics for Composites

i f i c a t i o n reaction proceeds easily at high temperatures (12). 1 shows the effect

of reaction temperature on weight

101 Figure

increase of

e s t e r i f i e d wood meal for the reaction of wood meal with SA

without

solvent. The reaction was conducted by heating the mixture of 2 g of wood meal and 70 g of SA with s t i r r i n g . The

esterificatin

reaction without

The reaction time was 3 hr.

catalyst

begins

to occur

above

ca. 60°C; above ca. 80 °C, the progress of the reaction becomes re markable.

Interestingly, the

reaction


melting point (120°C) of the SA,

proceeds

even

below

the

that i s , i n the s o l i d state of SA.

This i s very advantageous from the i n d u s t r i a l

standpoint.

The presence of a catalyst such as Na^CO^ accelerates the e s t e r i f i c a t i o n reaction further. For example, the e s t e r i f i e d wood obtained e

at 160 C i n the presence of Na CO

shows a very high weight increase

C. ό of

118

%.

This value corresponds

to ca. 12 moles of SA/1000 g of

wood. The those and

esterification

reactions without

solvent, as compared

i n the presence of a solvent, are i n d u s t r i a l l y

give e s t e r i f i e d

(12).

Therefore,

woods with a wide range of monoester

the e s t e r i f i e d

woods prepared

with

advantageous

without

contents

solvent are

described i n the following sections. It has

been found

that an increase i n the ester content

e s t e r i f i e d woods leads to decreases

i n the

i n hygroscopicity and i n i n i t i a l

weight loss temperature ( 1_5). Thermal P l a s t i c i t y of E s t e r i f i e d Woods. by the TFAA method and

The acylated woods prepared

the chloride method are thermally

meltable

(4,5). For example, the lauroylated wood can be molded into trans parent MPa.

sheet by hot-pressing at 140°C under a pressure of ca.

Further,

15.0

Morita and Sakata recently reported that a chemically

modified wood by cyanoethylation exhibits thermal flow and that the thermal f l u i d i t y and the s o l u b i l i t y i n organic solvent of the cyanoethylated wood are considerably improved by chlorination (16-18). The

carboxyl

group-bearing

esterified

into yellowish or reddish brown, at 160°C, 55.9 MPa

for 10 min

plasticized

(19).

tures, the f l u i d i t y decreases.

wood meal can

of W'SA

sheets by hot-pressing

Thus, the p l a s t i c i t y

2

ester

3

i s lower to the

Table I shows the thermal

prepared at various reaction temperatures,

of Na C0 .

plasticity e

agrees well with

than

intro

i n the presence

When the reaction temperature i s below ca. 100 C,

content

molded

At lower pressures and tempera

that of the lauroylated wood. This i s considered due duced polar carboxyl groups.

be

the monoester content,

the

indicating



102


Reaction temperature (*C) Fig.

1

Effect of reaction

esterified

wood meal

temperature on weight

f o r reaction

(70 g) without a catalyst (O)

o

r

i

n

of wood meal t

n

e

increase of

(2 g) and SA

presence of a catalyst

(Na C0 : 0.2 g) ( # ) . 2

3


8. MATSUDA


103

that SA has added to the wood in the form of monoester, that i s , carboxypropionyl group

However, above ca. 100°C not only the mono-

Table I. Thermal P l a s t i c i t y of E s t e r i f i e d Woods Based on SA Wood Meal E s t e r i f i e d


Reaction Temperature (°C)

Ester Content

0

Monoester b Content

b

Λ

(%)

Thermal p l a s t i c i t y

(%)

60

9.0

9.,0

80

14.8

15,,6

100

28.5

24,.5

Good

120

60.8

51,,0

Good

140

86.4

69.,4

Good

160

105.2

84.,1

Good

180

99.3

75..7

Poor

200

90.6

67. 5

Poor

Poor Intermediate

Obtained by reaction of 2 g wood meal with 70 g SA i n the pres ence of 0.2 g of Na CO . Reaction time = 3 hr. b Values reported i n r e f . 19. Hot-press condition = 160 C, 55.9 MPa , 10 min. Sheet thickness = 0.5 mm. c

e

ester content but also the diester content shows a tendency crease with increase i n reaction

to i n

temperature.

An increase i n the monoester content results i n increased thermal plasticity.

However,

as the reaction

temperature

p l a s t i c i t y decreases at a given temperature, crease i n the diester content,

increases, the

probably due to the i n

leading to a decrease i n the thermal

f l u i d i t y by c r o s s l i n k i n g . Addition Reaction of Carboxyl Group-Bearing Addition

Reaction with

Epoxides.

E s t e r i f i e d Woods

The carboxyl groups introduced

into the wood are reactive with epoxide groups.

When the e s t e r i f i e d

woods are subjected to the addition reaction with phenyl

glycidyl


104


ether (PGE),

the epoxide group i n PGE adds to the carboxyl group i n

the e s t e r i f i e d wood to form an ester linkage, as shown by Scheme 2 (20).

In this

case,

other epoxides

such

as a l l y l

glycidyl

ether

Wood-00C-R-C00H + CH — CH-CHgO-^^ — • Wood-OOC-R-COOCHgCHCHgO-^^ \ ^

OH


Scheme 2 (AGE) and g l y c i d y l methacrylate (GMA) also react with the e s t e r i f i e d woods (21). The epoxide-adducted

e s t e r i f i e d woods also can be molded by the

hot-pressing into reddish brown, yellowish brown, or yellow p l a s t i cized sheets (22).

Furthermore,

the addition of the epoxide results

in an improvement i n the moisture resistance of the molded sheets. Alternately Adding

E s t e r i f i c a t i o n Reactions.

When the epoxide-ad

ducted e s t e r i f i e d woods are further allowed to react with the anhy dride and the epoxide at high temperatures, t e r i f ication

alternately adding es

reactions occur, to produce o l i g o e s t e r i f i e d woods, as

shown by Scheme 3 (20).

Wood-OOC-R-COOCH CHCH 0-^^ 2

2

+ η R^

I

+ η CH—CH-CHgO-^^

co

OH

\

/ 0

I

Wood-OOC-R-COOCHgCH—f-OOC-R-COOCHgCH-J-jj—OH

Scheme 3

Crosslinking Reaction of E s t e r i f i e d Woods with Bisepoxide Crosslinking Reaction Accompanying P l a s t i c i z a t i o n of Wood. linking cidyl

ether

(BADG) having

high temperatures to

Cross-

reactions of the e s t e r i f i e d woods with bisphenol A d i g l y two epoxide

groups proceed

smoothly at

(23). In this case, the epoxide groups i n BADG add

the carboxyl groups

i n the e s t e r i f i e d

woods to produce ester

linkages, resulting i n a formation of crosslinks between the e s t e r i woods v i a BADG.

Further, under higher pressures, the wood

components p l a s t i c i z e

fied

to give reddish brown, yellowish brown, or


8. MATSUDA


blackish brown, crosslinked wood boards whose surfaces glossy,

and p l a s t i c - l i k e . The following conditions

to be suitable f o r the preparation

105

are smooth,

have been found

of p l a s t i c i z e d crosslinked wood

e

boards: f i r s t step, 150 - 170 C, 1.80 MPa, 10 - 40 min; second step, e

180 - 190 C, 18.0 - 27.0 MPa, 20 - 60 min. Properties

of P l a s t i c i z e d

Crosslinked

Wood Boards.

Plasticized

crosslinked wood boards of various wood contents can be prepared as Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch008

follows. F i r s t , e s t e r i f i e d wood meal with a desired wood content and BADG are blended so that acid value and epoxide value of the system might

become

hot-pressing. desirable

equal,

and then

the system

i s subjected

to the

Wood contents (true wood contents) of 60 - 70 % are

to obtain

plasticized

crosslinked

wood boads with

high

water resistance. Table II shows the physical and other properties of the p l a s t i cized crosslinked wood boards based on various e s t e r i f i e d woods. The

Table I I . Physical and Other Properties of P l a s t i c i z e d

Crosslinked

Wood Boards Physical and Other Properties

W-(25.7)MA b

W- (25.1)SA b

- BADG

• BADG

W-(30.4)PA b

- BADG

BADG

BADG

-PA

C

-DSA

WC

{%)

59

62

FS

(MPa )

86.8

70.5

77.6

93.1

110.3

CS

(MPa )

178.0

183.5

198.7

73.1

151.6

(J/m)

16

15

16

RH (M scale)

93

95

116

81

114

116

IS

e

HDT ( C) WA

60

(%)

0.81

0.50

0.35

THSW (%)

0.76

0.41

0.25

LSW

0.04

0.02

0.01

{%)

WC = wood content; FS == f l e x u r a l strength;

a

strength; distortion

IS = impact strength; temperature;

WA

110-152

66-70

CS = compressive

RH = Rockwell hardness; HDT = heat

= water

absorption;

THSW =

thickness

swelling; LSW = l i n e a r swelling. b

C

0

Adapted from ref. 23.

Q Adapted from ref. 24. DSA = Dodecylsuccinic

anhydride.


106


wood contents are ca. 60 %.

Generally, these boards exhibit proper

t i e s which are much superior to those of usual woody boards such as f i b e r boards and p a r t i c l e boards. The board based on MA exhibits the highest

flexural

strength

of

86.8

MPa.

strength, hardness, and heat d i s t o r t i o n

Meanwhile,

temperature

compressive

(HDT) are the

highest i n the board based on PA and are 198.7 MPa, 116, and 116°C, respectively.

It i s c h a r a c t e r i s t i c of these p l a s t i c i z e d crosslinked

wood boards that they show very high compressive strength.

In addi


tion, the ΡΑ-based wood board has higher water resistance than the MA and SA-based ones. This i s considered to be attributable to the high hydrophobic!ty of the phenyl ring. Furthermore,

i n Table II are also

shown the properties of the

representative cured resins of the usual BADG - anhydride (24).

The above p l a s t i c i z e d

crosslinked

wood

boards

systems

are greatly

superior to these cured resins i n compressive strength, although the former are i n f e r i o r to the l a t t e r i n f l e x u r a l strength. Introduction of Polymerizable Oligoester Chains into Wood and Crosslinking Q l i g o e s t e r i f i c a t i o n Reaction.

Polymerizable oligoester chains can

be introduced into wood by the o l i g o e s t e r i f i c a t i o n reaction of wood with the anhydrides and the epoxides such as AGE or GMA, as shown by Scheme 4 (25,26).

This route i s an extention of the reaction of

Wood-0H • X R!

;0 + Y CH =C-R"-CH—CH 2 ν / 2 0 0

Scheme 4

WoodH-00C-R-C00CH—CH^j-0H

R = -CH=CH-, (ξ)}

Scheme 3.

; R» = H-, C H ^ ; R" = -CHgOCHg-, -COOCH^

The reaction of Scheme 4 without the process of i s o l a t i n g

the intermediate e s t e r i f i e d wood i s advantageous

from the i n d u s t r i a l


8. MATSUDA


standpoint.

107

The o l i g o e s t e r i f i e d woods thus obtained are of interest

in that they w i l l be crosslinked at high temperatures and under high pressures, accompanying p l a s t i c i z a t i o n of the wood components, to give p l a s t i c i z e d crosslinked woods. In this type of o l i g o e s t e r i f i c a t i o n , the following main reactions of the functional groups are considered:


/

-OH • R -C00H + X-CH—CH \ / 0 2

/

C

0

0

0

\

^0 CO

* -00C-R-C00H

— * -C00CH CH-X \ OH o 2

(1)

(mother isomer)

(2)

\

R

X) • X-CH—CH

^ First,

C

• -0C-R-C00CH CH0-

(3)

o

2

V

2

i

the reaction of hydroxyl groups of wood with acid anhy-

dride group, that i s , reaction 1 should occur, to produce e s t e r i f i e d wood

bearing

carboxyl

groups.

The carboxyl

groups

are s t a r t i n g

points for chain extension. That i s , to the carboxyl group thus produced, the epoxide group adds to form a new hydroxyl group, as shown by reaction 2. with the acid

Next, the reaction of the hydroxyl group so produced anhydride group, that

i s , again reaction

1 occurs.

Thus, these addition reactions are considered to repeat alternately, leading

to the formation

Scheme 4.

Meanwhile, since

of o l i g o e s t e r i f i e d woods, as shown by the concentration

groups i s high i n the i n i t i a l

of the acid anhydride

stages of the reaction, the reaction

of the acid anhydride group with the epoxide group (27),

that i s ,

reaction 3 i s considered possible. However, t h i s reaction results i n s t r u c t u r a l l y analogous free oligoester chains which are not linked with

the wood matrix.

In addition,

active hydrogens contained i n

trace amounts of impurities e x i s t i n g i n the reaction mixture would initiate

the alternately adding e s t e r i f i c a t i o n reactions.

Also i n

this case, s i m i l a r free oligoester chains are formed. Figure 2 shows, as a t y p i c a l example, the o l i g o e s t e r i f i c a t i o n reaction of wood with PA and GMA. reaction

e

at 150 C, GMA

After 1 hr of the i n i t i a l wood - PA

was gradually

added over 15 min at 90 °C.

Then, the mixture was s t i r r e d further at 90°C. The reaction proceeds rapidly groups

during

the addition.

and further

The acid values are due to carboxyl

to those produced

by hydrolysis

of anhydride


108


groups i n the determination. The NA acid values determined

i n non

aqueous medium are due to the carboxyl and the anhydride groups. The difference between the acid value and the NA acid value corresponds to concentrations of the anhydride groups. With increase i n reaction time, acid value and epoxide nearly

maximum conversions

value decrease

attainable

after

and conversions 7 - 8

reach

hr. Anhydride

groups are almost consumed after ca. 6 hr, indicatinng that the pro gress of the reaction i n the l a t t e r stages i s due mainly to reaction Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch008

2. The decrease of epoxide

value i s a l i t t l e

greater than that of

acid value, even with a s l i g h t excess of GMA over PA i n feed. This shows that e t h e r i f i c a t i o n under the c a t a l y t i c

of epoxide

occurred

to a s l i g h t

degree

influence of the anhydride and carboxylic acid

(28). PA i s f a i r l y reactive at 90°C, probably because GMA works as a solvent

f o r PA i n the system of the o l i g o e s t e r i f i c a t i o n .

o l i g o e s t e r i f i c a t i o n reactions with MA, temperatures have been found to give a convenient

For the

of 110 - 120*C

rate (25), (Matsuda, Η. , Okura

Industrial Co., Ltd., Marugame, unpublished data). By varying the feed weight r a t i o , reaction systems with various wood contents

are obtained.

Products

of the reaction

consist of

acetone-insoluble and soluble parts (25,26). The insoluble parts are o l i g o e s t e r i f i e d woods bearing polymerizable oligoester chains. The soluble parts which are viscous l i q u i d s consist mainly of polymer izable free oligoesters not linked with the wood matrix.

The forma

tion of the free oligoesters i s considered due to the side reactions described above. Effects of wood content i n feed on acetone-soluble and insoluble parts, and weight increase of the insoluble part f o r the products were investigated

f o r various systems

(25,26),(Matsuda, Η. , Okura

Industrial Co., Ltd., Marugame, unpublished data) and are summarized in Figure 3.

The soluble part shows a tendency to decrease with i n

crease i n the wood content. However, a reverse trend i s observed f o r the insoluble part. Meanwhile, the insoluble parts show weight i n creases

(based on o r i g i n a l wood) of ca. 5 - 65 %

9

which

decrease

with increase i n the wood content. The weight increases are due to oligoester chains linked with the wood matrix.

In addition, the i n

soluble and soluble parts exhibit s l i g h t residual acid values, i n d i cating that they contain small amounts of residual terminal carboxyl groups. series, gible

In the case of the wood - PA - GMA

and wood - MA - GMA

residual epoxide values of the soluble parts are not n e g l i

(26),(Matsuda,

H., Okura Industrial Co., Ltd., Marugame, un

published data). The soluble parts are a mixture of free oligoesters (predominantly)

and small amounts of unreacted

GMA

and dissolved



8, MATSUDA


109

Reaction t i m e ( h r ) Fig. 2 Oligoesterification reaction of wood with PA and GMA. Feed weight ratio of wood : PA : GMA = 100 : 29.7 : 37.0. Mole ratio of PA : GMA = 1 : 1.3. (Ο), (φ), and (#) are initial acid value, NA acid value, and epoxide value, respectively, which were calculated on the assumption that wood, PA, and GMA were mixed at a time at the beginning of the reaction.

Wood content (7.)

Fig. 3 Effect of wood content in feed on acetone-soluble part, insoluble part, and weight increase of insoluble part for products of oligoesterification reaction of wood with MA or PA and A G E or G M A (O) wood-MA-AGE series; (φ) wood-PA-GMA series; and ( φ ) wood-MA-GMA series.


110


oligoesterified

wood components. GMA

seems to have higher solvent

power f o r wood components than AGE. Crosslinking

Accompanying

Plasticization

parts, that i s , the o l i g o e s t e r i f ied

of Wood.

The insoluble

woods do not show good thermo

p l a s t i c properties. S i m i l a r l y , i n the case of the wood - MA - AGE series, when the products which have not been separated into the i n soluble and the soluble parts,

that

i s , the o l i g o e s t e r i f ied

wood-


containing mixtures are subjected to hot-pressing, a great part of the soluble part exudes from the system,

and s u f f i c i e n t p l a s t i c i z a

tion of the wood components i s not observed. of

Meanwhile, i n the case

the mixtures into which a c a t a l y t i c amount of dicumyl peroxide

(DCP) has been added, both at high temperatures and under high pres sures, the wood components p l a s t i c i z e to give reddish or yellowish brown, crosslinked wood boards whose surfaces are smooth, glossy, and p l a s t i c - l i k e

(25). In this case, the exudation of the soluble

parts i s not observed, are

indicating

that the free oligoester chains

combined, by the crosslinking, with the o l i g o e s t e r i f i e d woods,

resulting i n the formation of the network structure.

I t i s advanta

geous that the free oligoesters which are hardening work as a plast i c i z e r f o r the wood components.

The crosslinking i s due largely to

polymerization of the a l l y l i c double bonds;

however, also, copolym-

e r i z a t i o n of the a l l y l i c and the maleate double bonds would occur to some extent (27,29). The p l a s t i c i z a t i o n of the wood components i s difficult

f o r the products from wood - PA - AGE system (Matsuda, H.,

Okura Industrial Co., Ltd., Marugame, unpublished data). I t i s known that

mere a l l y l a t i o n

thermally meltable. boxymethylated

or carboxymethylation

does not render wood

However, by blending the a l l y l a t e d wood or car-

wood with appropriate synthetic polymers

or low mo

lecular weight p l a s t i c i z e r s such as dimethyl phthalate or resorcino l , the wood components become thermally meltable (30). On the other hand, i n the case of the products of the wood - PA GMA

and wood - MA

- GMA

under the hot-pressing, even i n the absence Industrial

series,

the wood components

plasticize,

to give p l a s t i c i z e d crosslinked wood boards

of r a d i c a l

initiator

Co., Ltd., Marugame,

(26),(Matsuda,

unpublished

Η. , Okura

data). Methyl

acrylate i s known to thermally polymerize even without r a d i c a l tiator

ini

(3_1). Also, the methacrylate double bonds i n the oligoester

chains polymerize at high temperatures This i s i n d u s t r i a l l y favorable. allyl

meth-

group,

without

radical

initiator.

Meanwhile, i n polymerization of the

the existence of radical i n i t i a t o r i s absolutely nec

essary because of degradative chain transfer (32).


8. MATSUDA


Properties of P l a s t i c i z e d Crosslinked Wood Boards.

111

I t i s known that

films obtained by the thermal p l a s t i c i z a t i o n of acetylated-butylated woods have t e n s i l e strengths of 41.0 MPa and benzylated wood films from

a l l y l a t e d wood - poly-

ethylene and a l l y l a t e d wood - polypropylene

27.7

- 40.6 MPa (33). Further, films

( 1 : 2 ) blends exhibit

t e n s i l e strengths of 92.2 and 159.0 MPa, respectively and a l l y l a t e d wood - resorcinol formaldehyde f i l m 87.3 MPa (30). Table III shows the physical and other properties of the p l a s t i Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch008

cized

crosslinked

wood

boards

obtained

from

the o l i g o e s t e r i f i e d

Table I I I . Physical and Other Properties of P l a s t i c i z e d Crosslinked Wood Boards from Products of Wood - MA - AGE Series Wood content (%) of P l a s t i c i z e d Crosslinked 3

Physical and

Properties

Wood Board*

0

e

HDT ( C)

45 >220

TS (MPa)

35. 7

FS (MPa)

77. 6

IS (J/m)

50

55

>220

211

37.1 79.7

60 209

43.,8 81.,1

65

70

75

191

179

165

46. 8

49.6

48.4

39.2

77. 1

79.7

82.8

76.9

14

13

14

13

15

13

14

RH (M Scale)

112

112

112

109

109

106

104

CS (MPa)

227. 9

220.1

216. 6

215. 5

192.5

180.3

158.3

WA (%)

1.32

1.45

1..59

1.98

2.67

2.65

THSW (%)

0.95

1.08

1.32

1.89

2.90

2.84

3.87

LSW (%)

0.24

0.23

0.21

0.20

0.23

a

0.23

0.22

3.37

HDT, FS, IS, RH, CS, WA, THSW, and LSW are the same as i n Table

I I . TS = t e n s i l e strength. b

e

Obtained by hot-pressing at 150 C, 27.0 MPa, 30 min.

wood-containing

mixtures

of the wood - MA - AGE series

exhibit HDT above 165°C, which increase with decrease

(25). They i n the wood

content. I t should be noted that the HDT values are above 220°C at wood contents of 45 - 50 %.

Tensile strength ranges from ca. 36.0

to ca. 50.0 MPa, showing a peak at a wood content of 65 %.

Flexural

and

content,

impact

strengths are l i t t l e

influenced by the wood


112


remaining around 80.0 MPa and around 14 J/m, respectively. hardness shows values i n the range of 104 - 112, to decrease with increase i n the wood content.

Rockwell

and had a tendency

Compressive strength

increases with decrease i n the wood content and, at wood contents of 45 - 60

exhibits very high values of ca. 216 - 228 MPa.

As f o r water resistance, linear swelling i s not affected by the wood content, showing almost constant values of ca. 0.2 %;

however,

water absorption and thickness swelling range from ca. 1.3 to ca. Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch008

3.4 %

t

and from ca. 0.9 to ca. 3.9 %

f

respectively, and increase

with increase i n the wood content. On the other hand, the p l a s t i c i z e d crosslinked wood boards of the wood - PA - GMA

series exhibit

strength ( - ca. 69 MPa),

outstanding properties

in tensile

f l e x u r a l strength (ca. 88 - ca. 100 MPa),

and Rockwell hardness (ca. 120) (26).

The boards of the wood

-MA-

GMA series show excellent properties i n HDT, t e n s i l e and compressive strengths, which are superior to those of the other series

(Matsuda,

H., Okura Industrial Co., Ltd., Marugame, unpublished data). The above crosslinkable mixtures are able to give, by compression molding or i n j e c t i o n molding, various types of p l a s t i c i z e d

cross-

linked wood samples as shown i n Figure 4 (34). Summary As described above,

the p l a s t i c i z e d crosslinked woods having prop-

e r t i e s not observed f o r the o r i g i n a l wood can be obtained from the carboxyl group-bearing e s t e r i f i e d woods or the o l i g o e s t e r i f i e d woods bearing the polymerizable double bonds i n their oligoester chains by the crosslinking

reactions

accompanying

the p l a s t i c i z a t i o n

of the

wood components. The chemical modifications of wood with the anhydrides and the epoxides are i n d u s t r i a l l y advantageous,

because the

addition reactions, such as hydroxyl - anhydride reaction and carboxyl - epoxide reaction, are u t i l i z e d , which produce no by-products to be removed from the system, and because no solvent i s needed. However,further research work i s needed

f o r commercializing the

p l a s t i c i z e d crosslinked woods. From the economical viewpoint, production

by extrusion

geous.

In this case, further higher thermal f l u i d i t y of the mixture

is

required.

solvents,

or i n j e c t i o n

For this

steam

purpose,

explosion,

molding would

be more advanta-

pretreatment of wood by suitable

e t c . would

be e f f e c t i v e .

Furthermore,

investigations on resistances against weathering and biodeterioration are necessary. These are i n progress.



8. MATSUDA

Fig. 4


P l a s t i c i z e d crosslinked wood samples. (Reproduced from

ref. 34. Copyright

1990

American Chemical Society)

Literature Cited 1. Shiraishi, N. Nihon Setchaku Kyokaishi 1977, 13, 49. 2. Kawakami, H.; Shiraishi, N.; Yokota, T. Mokuzai Gakkaishi 1977, 23, 143. 3. Shiraishi, N.; Matsunaga, T.; Yokota, T. J. Appl. Polym. Sci. 1979, 24, 2347. 4. Shiraishi, N.; Matsunaga, T.; Yokota, T. J. Appl. Polym. Sci. 1979, 24, 2361. 5. Funakoshi, H.; Shiraishi, N.; Norimoto, M; Aoki, T.; Hayashi, S.; Yokota, T. Holzforschung 1979, 33, 159. 6. Shiraishi, Ν. Mokuzai Kogyo 1980, 35, 150. 7. Shiraishi, N. Mokuzai Kogyo 1980, 35, 200. 8. Shiraishi, Ν.; Aoki, T.; Norimoto, M.; Okumura, M. Chemtech 1983, 6, 366. 9. Steiner, P. R.; Chow, S.; Vagda, S. Forest Prod. J. 1980, 30, 21.


113


114


10. Rowell, R. M.; E l l i s , W. D. in "Urethane Chemistry and Applica tions"; Edwards, Kenneth Ν., Ed.; ACS SYMPOSIUM SERIES No.172; ACS: Washington, D. C . , 1981; 263-284. 11. Matsuda, H . ; Ueda, M.; Hara, M. Mokuzai Gakkaishi 1984, 30, 735. 12. Matsuda, H.; Ueda, M.; Murakami, Κ. Mokuzai Gakkaishi 1984, 30, 1003. 13. Shiraishi, Ν.; Tsubouchi, K.; Matsunaga, T . ; Yokota, T . ; Aoki, T. Proc. - Annu. 30th Meet. Japan Wood Research S o c , 1980, 34. 14. Matsuda, H. Wood Sci. Technol. 1987, 21, 75 15. Matsuda, H.; Ueda, M. Mokuzai Gakkaishi 1985, 31, 103. 16. Morita, M.; Sakata, I. J . Appl. Polym. Sci. 1986, 31, 831. 17. Morita, M.; Shigematsu, M.; Sakata, I. Cellulose Chem. Technol., 1987, 21, 255. 18. Morita, M.; Sakata, I. Mokuzai Gakkaishi 1988, 34, 917. 19. Matsuda, H.; Ueda, M. Mokuzai Gakkaishi 1985, 31, 215. 20. Matsuda, H.; Ueda, M. Mokuzai Gakkaishi 1985, 31, 267. 21. Matsuda, H.; Ueda, M. Mokuzai Gakkaishi 1985, 31, 468. 22. Matsuda, H.; Ueda, M. Mokuzai Gakkaishi 1985, 31, 579. 23. Matsuda, H.; Ueda, M. Mokuzai Gakkaishi 1985, 31, 903. 24. "Kobunshi Kako Bessatsu 9: Epoxy Resins"; Saeki, Κ., Ed.; Kobunshi Kankokai: Kyoto, 1973; Vol. 22. 25. Matsuda, H.; Ueda, M.; Mori, H. Wood Sci. Technol. 1988, 22, 21. 26. Matsuda, H.; Ueda, M.; Mori, H. Wood Sci. Technol. 1988, 22, 335. 27. Fischer, R. F. J . Appl. Polym. Sci. 1963, 7, 1451. 28. Fisch, W.; Hofmann, W. J . Polym. Sci. 1954, 12, 497. 29. Urushido, K.; Matsumoto, Α.; Oiwa, M. J . Polym. Sci. Polym. Chem. Ed. 1980, 16, 1081. 30. Shiraishi, N.; Goda, K. Mokuzai Kogyo 1984, 39, 329. 31. Tani, H. "Synthesized high molecular compounds"; Kotake, M., Ed.; Daiyukikagaku; Asakura Shoten: Tokyo, 1958; Vol. 22; 41. 32. Bamford, C. H.; Barb, W. G. ; Jenkins, A. D.; Onyon, P. F. "The kinetics of vinyl polymerization by radical mechanisms"; Butterworths Scientific Publications: London, 1958; 46. 33. Shiraishi, N. "Advanced Technology and Perspective of Wood Chemicals"; R & D Report No.40; CMC: Tokyo, 1983; 271-285. 34. Worthy, W. C & EN, 1990, January 15, 19. RECEIVED March 20, 1991


Thermal Plasticization of Lignocellulosics for Composites - ACS

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