Thermoplasticization of Cellulose and Wood by Graft

20 Thermoplasticization of Cellulose and Wood by Graft Copolymerization and Acylation

Downloaded by INDIANA UNIV BLOOMINGTON on May 9, 2015 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch020

NOBUO SHIRAISHI—Kyoto University, Department of Wood Science and Technology, Kyoto 606, Kyoto, Japan T U T O M U AOKI and MISATO NORIMOTO—Kyoto University, Wood Research Institute, Uji 611, Kyoto, Japan MASAKAZU OKUMURA—Harima Chemicals Inc., Kokogawa 675, Hyogo, Japan

A homogeneous grafting using an organic cellulose solvent as a reaction medium makes cellulose ther mally meltable, while heterogeneous graftings exam ined do not convert cellulose or wood into a melt able material. When thermally unmeltable acetylated wood as well as acetylated-propionylated wood with low propionyl contents are further chemically modified by grafting, the thermally meltable prop erties are rendered even by very low graft add-ons. These modified cellulose and wood with thermally meltable properties can be molded to form transpar ent sheets at adequate temperatures and pressing time not causing thermal degradation, under pres sures usually used for the compression molding of plastics.

Wood i s a t h e r m a l l y i n s e n s i t i v e m a t e r i a l . G e n e r a l l y accepted concepts that (a) c e l l u l o s e i s a c r y s t a l l i n e polymer, (b) l i g n i n has a three-dimensional network molecular s t r u c t u r e w i t h very h i g h molecular weights, and (c) chemical bondings are formed between wood components as are found in l i g n i n - c a r b o h y d r a t e complexes, lead us t o have a confidence that wood i s not a t h e r m a l l y m e l t a b l e m a t e r i a l . The annual r i n g s can still be seen in the cross s e c t i o n of c h a r c o a l obtained a f t e r heat treatment of wood above c a r b o n i z a t i o n temperatures. This demonstrates c l e a r l y that wood, by i t s own inherent n a t u r e , dose not melt w i t h heat treatment. Consequentl y , once wood i s ground i n t o powder w i t h the s i z e o f l e s s than 5 mm and l o s e s i t s f i b r o u s p r o p e r t i e s , i t can no longer be used as molded m a t e r i a l s o r boards without e f f e c t i v e use o f b i n d e r s , adhesives or s y n t h e t i c polymers. This i s q u i t e d i f f e r e n t from the prope r t i e s o f t h e r m o p l a s t i c s y n t h e t i c polymers which can be molded t o any shape even from the f i n e l y ground powder. I t i s then of i n t e r e s t t o give t h e r m o p l a s t i c p r o p e r t y t o wood or c e l l u l o s e in order t o o b t a i n s i m i l a r l y workable m a t e r i a l s as

©

0097-6156/82/0187-0321$8.00/0 1982 American Chemical Society

In Graft Copolymerization of Lignocellulosic Fibers; Hon, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS


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from f o s s i l resources. Though not so workable as petroleum- or coal-based p l a s t i c s , thermal p r o p e r t i e s of g r a f t e d c e l l u l o s i c p o l y mers have been r e p o r t e d . For example, Arthur -et a l . (1) enhanced t h e r m o e l a s t i c i t y of c e l l u l o s e by g r a f t i n g w i t h main i n t e r e s t of lowering the s o f t e n i n g temperature. On the other hand, Yoshimura (2), f o r i n s t a n c e , prepared c e l l u l o s e - b a s e d m a t e r i a l s w i t h t r a n s i t i o n temperatures higher than the s t a r t i n g c e l l u l o s e d e r i v a t i v e s through c r o s s l i n k i n g by g r a f t i n g . No r e p o r t s have been made on c o n v e r t i n g c e l l u l o s e or wood i n t o a t h e r m a l l y meltable m a t e r i a l by grafting. The present paper r e p o r t s novel methods to prepare t h e r m a l l y meltable c e l l u l o s e and wood-based m a t e r i a l s and t h e i r p r o p e r t i e s . Two methods are emphasized: (a) a g r a f t i n g and (b) a c y l a t i o n combined w i t h g r a f t i n g .

Experimental M a t e r i a l s . Commercial c o t t o n ( P a k i s t a n i cotton) cut i n t o 5 mm l e n g t h and Whatman c e l l u l o s e powder CF-11 was used as c e l l u l o s e samples a f t e r Soxhlet e x t r a c t i o n w i t h alcohol-benzene f o r 7 h. Wood meal (mainly 40 - 80 mesh) from Makanba ( B e t u l a Maximowicziana Regel) was used a f t e r washing w i t h c o l d water. Methyl methacrylate and styrene were p u r i f i e d by the c o n v e n t i o n a l methods. Other reagents used were a n a l y t i c a l reagent grade. P r e p a r a t i o n of C e l l u l o s e - M e t h y l Methacrylate G r a f t Copolymer. Cotton c e l l u l o s e (1 g) was allowed to stand w i t h 1 % aqueous s o l u t i o n of ammonium p e r s u l f a t e (O.5 g) f o r 30 min at room temperature Methyl methacrylate (MMA) (20 ml) and methanol (20 ml) were added to the mixture. A f t e r degassed, the f l a s k was sealed and shaken at 60 °C f o r 1 to 4 h. The g r a f t product was i s o l a t e d by pouring i n t o a l a r g e excess acetone f o l l o w e d by Soxhlet e x t r a c t i o n w i t h acetone f o r 48 - 72 h. P r e p a r a t i o n of C e l l u l o s e - S t y r e n e - S u l f u r Dioxide G r a f t Copolymer (Homogeneous G r a f t i n g ) . The procedure f o r p r e p a r a t i o n of c e l l u l o s e - s t y r e n e copolymer in a S0 -diethylamine (DEA)-dimethylsulfoxide (DMSO) medium was d e s c r i b e d in our previous paper ( 3 ) . The g r a f t i n g proceeds homogeneously throughout the r e a c t i o n . ?

P r e p a r a t i o n of Wood-Methyl Methacrylate G r a f t Copolymer. Wood Meal (O.5 g ) , an a c e t i c acid-sodium acetate b u f f e r s o l u t i o n w i t h pH 4.6 (5 m l ) , a f e r r o u s s u l f a t e aqueous s o l u t i o n (5 ml; FeSO^ 1x10 mole), MMA^(8 m l ) , and a hydrogène peroxide aqueous s o l u t i o n ( 5 ml H^O^ 1x10 mole) were mixed in t h i s order. A f t e r degassed, the f l a s k was sealed and shaken a t 50 °C f o r 1 to 8 h. The r e a c t i o n mixture was poured i n t o a l a r g e excess of methanol. To remove homo polymer, the product was then e x t r a c t e d w i t h acetone f o r 36 - 72 h.


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SHiRAiSHi E T A L .

Thermoplasticization of Cellulose

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P r e p a r a t i o n of Wood-Styrene-Sulfur Dioxide G r a f t Copolymer. Procedure f o r p r e p a r a t i o n of wood-styrene-sulfur d i o x i d e g r a f t copolymer was almost the same as described in our previous paper ( 3 ) , except that wood was g r a f t e d i n s t e a d of c e l l u l o s e and t h a t the r e a c t i o n proceed heterogeneously. A c e t y l a t i o n and Mixed A c e t y l a t i o n - P r o p i o n y l a t i o n of Wood. Wood meal (15 g) was p r e t r e a t e d w i t h an a c e t i c a c i d (26.3 ml)-acet i c anhydride (4.2 ml) mixture f o r overnight. A c y l a t i n g agents were precooled to about -25 °C and added to the p r e t r e a t e d mixture. The compositions of the a c y l a t i n g agents were a c e t i c anhydride (8,91 m l ) - a c e t i c a c i d (48 ml) f o r a c e t y l a t i o n , and a c e t i c anhydride (62.2 m l ) - p r o p i o n i c anhydride (36.5 m l ) - a c e t i c a c i d (48 ml) f o r the mix ed a c e t y l a t i o n - p r o p i o n y l a t i o n . P e r c h l o r i c a c i d (O.19 ml) was used as a c a t a l y s t and premixed w i t h the a c y l a t i n g agents. The r e a c t i o n mixture was g e n t l y s t i r r e d . The temperature of the mixture rose to 25 °C at r e a c t i o n time of about 1 h, a f t e r which the r e a c t i o n temperature was kept constant at 25, 35, or 45 °C. The t o t a l reac t i o n time was 6 h. When the r e a c t i o n was completed, p e r c h l o r i c a c i d was n e u t r a l i z e d by adding an a c e t i c a c i d s o l u t i o n of potassium carbonate (1.5 ml; K ° 3 ^ . mixture was then poured i n t o a l a r g e excess of d e i o n i z e d water or methanol, and the s o l i d product was repeatedly washed by water or methanol. C

g

T l i e

r

e

a

c

t

i

o

n

2

P r e p a r a t i o n of A c y l a t e d Wood-Styrene G r a f t Copolymer. To d r i ed a c e t y l a t e d wood or a c e t y l a t e d - p r o p i o n y l a t e d wood (2.5 g ) , p y r i dine (20 ml) and styrene (20 ml) were added. A f t e r passing through n i t r o g e n f o r 1 min, the f l a s k was stoppered, dipped s l o w l y in l i q u i d n i t r o g e n to f r e e z e the content, and sealed. The g r a f t i n g was achieved by i r r a d i a t i n g the mixture a t 2x10^ r/h w i t h γ-radiation from a c o b a l t 60 source to a d e f i n i t e t o t a l dose. The g r a f t prod uct w i t h the homopolymer was obtained by p r e c i p i t a t i n g in a l a r g e excess of methanol, washing by methanol, and d r y i n g . The product was, f u r t h e r , e x t r a c t e d w i t h benzene to remove the homopolymer. The dose r a t e was low enough not to cause the degradation of wood. Measurement of Thermal Softening and M e l t i n g . About 20 mg samples of both untreated and c h e m i c a l l y modified c e l l u l o s e and wood were used f o r t h i s measurement. Thermal s o f t e n i n g and m e l t i n g were observed as the c o l l a p s e of a column of powder under a constant pressure of 3 kg/cm in a heated g l a s s c a p i l l a r y tube. A thermomechanical analyzer (Sinku Riko Co. L t d . , TM 3000) was used as measuring d e v i c e . The measurement was conducted from 20 °C to 400 °C at a programmed h e a t i n g r a t e of 1 °C/min.

R e s u l t s and

Discussion

T h e r m o p l a s t i c i z a t i o n of C e l l u l o s e by Heterogeneous G r a f t Cop o l y m e r i z a t i o n . As the f i r s t t r i a l to o b t a i n a thermoplastic e e l -


324


l u l o s e g r a f t copolymer, a heterogeneous g r a f t i n g was examined. A c a t a l y z e d g r a f t i n g method u s i n g ammonium p e r s u l f a t e as the c a t a l y s t was employed f o r c o p o l y m e r i z i n g MMA to c o t t o n c e l l u l o s e . The r e s u l t s are shown in Table I . The r e a c t i o n proceeds r e a d i l y to a h i g h g r a f t add-on by t h i s method.


Table I .

Sample

Heterogeneous G r a f t Copolymerization of MMA onto C e l l u l o s e Using Ammonium P e r s u l f a t e as a C a t a l y s t

Polymerization Time (hr)

Weight Increase

(%)

Grafting E f f i c i e n c y (%)

PC-1

1

125

20.3

PC-2

2

178

16.4

PC-3

3

374

18.8

PC-4

4

413

18.5

Thermoplastic p r o p e r t i e s of these g r a f t e d c e l l u l o s e samples were examined and the r e s u l t s are shown in F i g s . 1 and 2. In F i g . 1, deformation of the sample (Δ) under a constant load at a pro grammed h e a t i n g r a t e versus temperature (T) i s shown. The curve f o r untreated c e l l u l o s e shows one s o f t e n i n g r e g i o n where the d e f o r mation occurs only to a Δ value of O.67. Other curves f o r the g r a f t e d m a t e r i a l s show two s p e c i f i c regions around 130 °C and 340 °C where the deformations occur s h a r p l y . The deformation of all the g r a f t e d c e l l u l o s e samples reaches the Δ value of 1.0, i n d i c a t i n g a complete f l o w of the m a t e r i a l . This phenomenon can be a s c r i b e d to e i t h e r m e l t i n g of all the components or t h e r m o p l a s t i c flow of the m a t e r i a l w i t h remaining s o l i d p a r t s . To i d e n t i f y the mechanism, the samples were submitted to h i g h pressure (50 kg/cm2) a t . around 300 °C to mold f i l m s of the g r a f t ed c e l l u l o s e and m o l d a b i l i t y of the composites was examined. Un a l t e r e d c e l l u l o s e f i b e r s were observed in the f i l m s (Photo. 1 ) . This can be i n t e r p r e t e d that the g r a f t e d c e l l u l o s e s which have more than 125 % polymer add-ons do not melt thoroughly, and the complete flow observed in the thermodiagram in F i g . 1 i s due to thermoplas t i c f l o w in which PMMA i s melted and c e l l u l o s e remain s o l i d . In F i g . 2, p l o t s of the deformation r a t e (dA/dT) a g a i n s t Τ f o r the samples in Table I are shown. Two d i s t i n c t peaks around 130 and 340 °C are in good accord w i t h the c o n c l u s i o n s drawn from F i g . 1. A subpeak appears about 300 °C. To i n t e r p r e t these peaks, thermomechanical behavior of homopolymethyl methacrylate (PMMA) w i t h d i f f e r e n t molecular weights in



325


SHiRAiSHi E T A L .

0

100

200

300

400

T(«c) Figure 1. Plots of the deformation vs. temperature. Key: C-O, untreated cellulose and PC-1—PC-4, cellulose-PMMA composites prepared by heterogeneous grafting (see Table I).




326

Figure 2. Plots of the deformation rate vs. temperature. Key: C-O, untreated cellulose and PC-1—PC-4, cellulose-PMMA composites prepared by hétérogèneous grafting (see Table I).


SHiRAiSHi E T A L .


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20.

Photo 1. Cellulose sample used. Top: A part of molded film from the cellulosePMMA composite, PC-3. Molding conditions: temperature 300°C.; time, 2 min; pressure, 50 kg/cm . Bottom: under a microscope. 2


328



4

U

the range 1 0 t o 1 0 were examined, and the r e s u l t s are shown in F i g . 3. A l l the curves show two regions w i t h considerable deformat i o n ; one a t about 130 °C which does not s h i f t w i t h molecular weight changes i s a s c r i b e d t o the glass-rubber t r a n s i t i o n of PMMA, and another in the temperature range 160 - 350 °C depends on the molecular weight o f PMMA. Independence of the glass-rubber t r a n s i t i o n temperature and dependence of apparent m e l t i n g temperatures on the molecular weight are recognized w i t h amorphous polymers. I t i s noted here that the term " m e l t i n g " means a flow o c c u r r i n g under a constant load (3 kg/cm^). The flow i s caused by whole molecular motions, in which the s l i p p i n g of the main chains plays a s i g n i f i c a n t r o l e and the lower the molecular weight o f the p o l y mer i s , the e a s i e r the s l i p p i n g of the main chains takes p l a c e . Thus, the second peak appearing in the range 160 - 350 °C i s concluded t o be due t o the m e l t i n g of the sample. Another f i n d i n g from F i g . 3 i s that PMMA e x t r a c t e d from the c e l l u l o s e s t r u c t u r e shows a m e l t i n g behavior a t the highest temperature (350 °C) among the PMMA samples. This i n d i c a t e s that the PMMA formed w i t h i n the c e l l u l o s e amorphous r e g i o n has a very high molecular weight. The F i g . 3 e x p l a i n s w e l l the i n t e r p r e t a t i o n of peaks in F i g . 2. The peak around 130 °C i s due t o the glass-rubber t r a n s i t i o n of PMMA and the one around 340 °C the m e l t i n g of PMMA. The thermal s o f t e n i n g peak f o r untreated c e l l u l o s e around 340 °C i s overlapping the m e l t i n g peak of PMMA f o r the g r a f t e d c e l l u l o s e . The wide envelope w i t h the sub-peak (ca. 300 °C) f o r the g r a f t e d c e l l u l o s e appearing between the two main peaks (150 - 320 °C) may be due to i n t e r a c t i o n between c e l l u l o s e and PMMA w i t h i n the composite. E f f e c t s of the i n t e r a c t i o n on the thermomechanical behavior have been discussed by v a r i o u s i n v e s t i g a t o r s (k - 9 ) . The g r a f t e d c e l l u l o s e samples prepared by the heterogeneous g r a f t i n g thus do not show m e l t i n g behavior r e g a r d l e s s o f t h e i r high polymer add-ons. This can be a t t r i b u t e d in p a r t t o the f a c t that no uniform i n t r o d u c t i o n o f g r a f t e d branches along the c e l l u l o s e main chain i s achieved by the heterogeneous g r a f t i n g method employed in t h i s r e p o r t ; the g r a f t i n g can not be expected t o occur in the c r y s t a l l i n e r e g i o n of c e l l u l o s e . To confirm t h i s p a r t l y , the g r a f t e d c e l l u l o s e w i t h d e c r y s t a l l i z e d c e l l u l o s e s t r u c t u r e were prepared by d i s s o l v i n g the heterogeneously-prepared composites in a non-aqueous c e l l u l o s e s o l v e n t , the paraformaldehyde (PF)-DMSO system, and regenerating them in methanol. This treatment a c t u a l l y r e s u l t e d in a permanent decryst a l l i z a t i o n o f c e l l u l o s e main chains, which was confirmed by X-ray d i f f r a c t o m e t o r y . That i s , the r e s u l t a n t products show almost comp l e t e l y d e c r y s t a l l i z e d X-ray diagrams. F i g u r e 4 shows thermomec h a n i c a l diagrams of the d e c r y s t a l l i z e d composites w i t h those f o r untreated c e l l u l o s e and d e c r y s t a l l i z e d c e l l u l o s e . The diagrams of untreated and d e c r y s t a l l i z e d c e l l u l o s e r e v e a l t h a t the d e c r y s t a l l i z a t i o n treatment lowers the thermal s o f t e n i n g peak by 70 °C. This i s considered t o be caused by d e s t r u c t i o n o f the c r y s t a l l i n e s t r u c -


SHIRAISHI

E T

AL.


329


20.

0

100

200

300

τ Ce)

400

Figure 3. Thermomechanical behavior of various homo-PMMA with different molecular weights. (PMMA is extracted from the cellulose structure of the cellulose-PMMA composite prepared by the heterogeneous grafting).




0

100

200

300

400

T(°c) Figure 4. Plots of the deformation rate vs. temperature for PC-Γ—PC-4', the de crystallized cellulose-PMMA composites; C -O', decrystallized cellulose; and C-O, untreated cellulose. Decrystallization was achieved by using a PF-DMSO solution.



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SHiRAiSHi E T A L .

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ture of c e l l u l o s e and l o o s e n i n g of the cohesive s t r u c t u r e of c e l l u l o s e . This change in the diagram of c e l l u l o s e by the treatment evidences t h a t the s e p a r a t i o n of the peak f o r c e l l u l o s e from one f o r PMMA at 340 °C i s r e a l i z e d by the d e c r y s t a l l i z a t i o n . Both the peaks appeared at the same temperature (340 °C) in F i g . 2. Although an a p p r e c i a b l e t h e r m o p l a s t i c i z a t i o n was r e a l i z e d by the d e c r y s t a l l i z a t i o n treatment, the treatment d i d not g i v e c e l l u l o s e g r a f t products which melt upon h e a t i n g . This l a c k of m e l t i n g was confirmed by m i c r o s c o p i c observations of f i l m s prepared w i t h the d e c r y s t a l l i z e d composites. T h e r m o p l a s t i c i z a t i o n of C e l l u l o s e by Homogeneous Copolymerizat i o n . A homogeneous g r a f t i n g was c a r r i e d out to prepare thermop l a s t i c i z e d c e l l u l o s e u s i n g a non-aqueous c e l l u l o s e s o l v e n t as a r e a c t i o n medium. C e l l u l o s e was f i r s t d i s s o l v e d in the SO2-DEA-DMSO s o l u t i o n and then styrene was g r a f t - c o p o l y m e r i z e d onto c e l l u l o s e by a c o b a l t 60 γ-ray mutual i r r a d i a t i o n method. This method was found to introduce u n i f o r m l y p o l y s u l f o n e branches w i t h low molecu l a r weight (Mn = 3.2 - 3.7 xlO^) along the c e l l u l o s e main c h a i n and the number of the branches introduced per u n i t c e l l u l o s e c h a i n was found to be l a r g e (6.4 - 10.6) ( 1 0 ) . This g r a f t i n g method gave p o l y s u l f o n e , because of the c o n d i t i o n s of p o l y m e r i z a t i o n in the presence of SO2. The r e s u l t s of the g r a f t i n g are shown in Table I I . T y p i c a l thermomechanical diagrams f o r the g r a f t e d products are shown in F i g . 5 together w i t h homo-polysulfone. Although the g r a f t e d c e l l u l o s e sample having a t r u e g r a f t i n g value of 2 1 . 0 % does not show a m e l t i n g behavior, the sample w i t h 104.6 % t r u e g r a f t i n g shows a m e l t i n g behavior very c l e a r l y . The sample w i t h 86.8 % true g r a f t i n g was a l s o found to be m e l t a b l e . The t r u e melt i n g f o r the l a t t e r two cases i s confirmed as f o l l o w s : (a) the sharp m e l t i n g occurs at 126 °C f o r the homo-polysulfone, which i s about 70 °C lower than that f o r the p o l y s u l f o n e - g r a f t e d c e l l u l o s e (more than a t l e a s t 86 % g r a f t i n g ) . This i m p l i e s t h a t the complete flow found f o r the g r a f t e d c e l l u l o s e i s not caused by a mere thermoplas t i c flow; (b) the molded f i l m s of the g r a f t e d products w i t h enough add-ons are homogeneous and t r a n s p a r e n t . The r e s u l t s obtained in d i c a t e that c e l l u l o s e can melt upon h e a t i n g when the true g r a f t i n g Table I I . Homogeneous G r a f t Copolymerization of Styrene onto C e l l u l o s e Using a S0 -DEA-DMS0 S o l u t i o n as a Reaction Medium 2

Sample


Weight Increase

(%)


SC-1

10

58.3

21.0

SC-2

60

111.4

86.8

SC-3

80

131.9

104.6




Figure 5. Thermomechanical diagrams of a polysulfone homopolymer and SC-1 and SC-3, the cellulose-polysulfone composites prepared by the homogeneous graft copolymerization of styrene onto cellulose (See Table II).


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exceeds a t l e a s t 86.8 %. The r e s u l t s are of i n t e r e s t because they show that c e l l u l o s e can be converted i n t o a t h e r m a l l y meltable ma t e r i a l even by g r a f t i n g , provided that the i n t r o d u c t i o n o f branch polymers i s achieved u n i f o r m l y w i t h h i g h d e n s i t y . T h e r m o p l a s t i c i z a t i o n of Wood by Heterogeneous G r a f t Copolym e r i z a t i o n (Redox I n i t i a t i o n ) . A redox g r a f t i n g method using f e r rous sulfate-hydrogene peroxide as the i n i t i a t o r was conducted t o copolymerize MMA t o wood meal. The r e s u l t s of g r a f t i n g are shown in Table I I I . Thermoplastic p r o p e r t i e s of these g r a f t e d wood samples a r e shown in F i g s . 6 and 7. F i g u r e 6 shows that the deformation o f g r a f t e d wood samples w i t h more than 101 % polymer add-on a t t a i n s a Δ value of 1.O. The deformation of the g r a f t e d samples having polymer add-ons l e s s than 86.8 %, however, does not reach the Δ value of 1.O. I t can be concluded that the complete f l o w i s r e a l i z e d w i t h the g r a f t e d wood having more than 100 % of weight in crease. I t was confirmed by microscopic observations o f the c o r r e sponding molded f i l m s that t h i s phenomenon, the complete f l o w , i s caused by a t h e r m o p l a s t i c flow. Another f i n d i n g obtained from F i g . 6 i s that the g r a f t i n g even w i t h a low degree of the polymer add-on can a l t e r the t h e r m o p l a s t i c p r o p e r t i e s of wood. Wood samples, though g r a f t e d only t o a lower degree than t h a t t o cause a t h e r m o p l a s t i c flow of the composite as a whole, have thermal s o f t e n i n g temperatures lower than untreated wood. The degree of the thermal s o f t e n i n g appearing a t lower than 300 °C becomes greater w i t h an i n c r e a s e in the polymer add-on. From F i g . 7, i t can be concluded that the e x i s t e n c e of s m a l l amounts of PMMA in the c e l l w a l l of wood s h i f t s the thermal s o f t e n i n g peak to a low temperature by about 60 °C without changing the p r o f i l e of the curve. As pointed out, t h i s has been i n t e r p r e t e d in terms of the i n t e r a c t i o n o f wood components w i t h PMMA (4^ - 9). For the composite w i t h more than PMMA content of 50 %, the peaks due t o t h e r m o p l a s t i c i t y of PMMA appear c l e a r l y , though o v e r l a p p i n g the thermal s o f t e n i n g curve f o r wood. A peak a t about 130 °C i s a t t r i b u t a b l e t o the glass-rubber t r a n s i t i o n o f PMMA and the other peak a t about 340 °C t o the m e l t i n g of PMMA. T h e r m o p l a s t i c i z a t i o n o f Wood by G r a f t Copolymerization in De c r y s t a l l i z e d S t a t e . We have reported that wood can e f f e c t i v e l y be d e c r y s t a l l i z e d without a w e i g h t - l o s s by t r e a t i n g w i t h a non-aqueous c e l l u l o s e s o l v e n t , the SO2-DEA-DMSO s o l u t i o n (11). Thus, use of the non-aqueous c e l l u l o s e s o l v e n t as a r e a c t i o n medium f o r the g r a f t - c o p o l y m e r i z a t i o n of monomers to wood was expected t o r e s u l t in products w i t h branch polymers more u n i f o r m l y d i s t r i b u t e d . The r e s u l t s obtained by the homogeneous g r a f t i n g of c e l l u l o s e (10) were expected t o support t h i s i d e a . From t h i s p o i n t of view, a g r a f t i n g of wood in a d e c r y s t a l l i z ed s t a t e was c a r r i e d out u s i n g the SO2-DEA-DMSO system as the reac t i o n medium. Wood meals were f i r s t contacted w i t h the c e l l u l o s e




334

Table I I I . Redox G r a f t Copolymerization of MMA onto Wood Using Ferrous S u l f a t e - Hydrogen Peroxide as an I n i t i a t o r

Sample


Weight Increase (%)


10.6

32.1

2

21.1

40.0

3

42.1

34.9

PW-4

4

55.3

22.7

PW-5

5

75.1

19.8

PW-6

6

86.8

13.0

PW-7

7

101

10.8

PW-8

8

132

PW-1 PW-2 PW-3

1

9.39



335


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0

100

200

300

400

T(°C)

Figure 6. Plots of the deformation vs. temperature for W-O, untreated wood an PW-2—PW-8, the wood-PMMA composites prepared by the redox grafting method (See Table 111).




336

Figure 7. Plots of the deformation rate vs. temperature for W-O, untreated wood and PW-2—PW-8, the wood-PMMA composites prepared by the redox grafting method (See Table 111).



20.

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337

s o l v e n t , and then styrene was added t o the mixture and g r a f t - c o polymerized. Although the c e l l u l o s e s o l v e n t was used as the medium in t h i s g r a f t i n g , the r e a c t i o n proceeded heterogeneously, because the p r e s ence of l i g n i n prevented complete d i s s o l u t i o n of wood. The presence of l i g n i n i s a l s o known t o r e t a r d the g r a f t - c o p o l y m e r i z a t i o n c h e m i c a l l y . These may e x p l a i n p a r t l y why the g r a f t i n g by t h i s system proceeded very s l o w l y compared w i t h the corresponding homogeneous g r a f t i n g of c e l l u l o s e as shown in F i g . 8. T y p i c a l thermomechanical diagrams f o r these g r a f t e d wood are shown in F i g s . 9 and 10. A complete flow i s not observed in F i g . 9, because the polymer add-ons are low. I t can be learned from F i g s . 9 and 10, however, that the thermal s o f t e n i n g temperature of wood i s lowered c o n s i d e r a b l y by the g r a f t i n g w i t h very low degrees of the polymer add-on (even l e s s than 10 % ) . In F i g . 10, i t i s found that curves f o r the g r a f t e d wood samp l e s show two peaks. Both of the peaks s h i f t t o lower temperature w i t h an i n c r e a s e in the polymer add-on. As shown in F i g . 5, c e l l u l o s e g r a f t e d w i t h p o l y s u l f o n e melts around 200 °C. The lower temperature peak f o r the g r a f t e d wood in F i g . 10 s h i f t s toward the temperature a t which the p o l y s u l f o n e - g r a f t e d c e l l u l o s e shows a comp l e t e f l o w and enlarges i t s peak height w i t h i n c r e a s i n g polymer add-ons. Hence, t h i s peak i s considered to be due t o the thermal behavior of the p o l y s u l f o n e g r a f t e d c e l l u l o s e w i t h i n the wood c e l l w a l l . The higher temperature peak f o r the g r a f t e d wood samples, a s c r i b a b l e t o the thermal s o f t e n i n g of u n a l t e r e d p a r t s of wood, also, not only s h i f t s to lower temperature but decreases i t s peak height w i t h an increase in the polymer add-ons. The l a t t e r i s exp l a i n a b l e by the gradual decrease in the u n a l t e r e d p a r t s of wood by g r a f t i n g . The former can be r e l a t e d t o the formation of meltable g r a f t polymers w i t h i n wood. Because the g r a f t products w i t h enough polymer add-ons can show m e l t i n g behavior a t a lower temperature range than the thermal s o f t e n i n g temperature of wood, the g r a f t e d products w i t h i n wood are considered t o a c t as an e x t e r n a l p l a s t i c i z e r t o lower the thermal s o f t e n i n g temperature of the una l t e r e d p a r t s of the g r a f t e d wood. Comparison of the r e s u l t s in F i g . 10 w i t h those in F i g . 7 makes i t c l e a r t h a t the d i f f e r e n c e s in monomer s p e c i e s , d i s t r i b u t i o n of g r a f t e d branches, and the degree of d e c r y s t a l l i z a t i o n o f wood r e s u l t in q u i t e d i f f e r e n t thermograms. T h e r m o p l a s t i c i z a t i o n of Wood by I n t r o d u c i n g Higher A l i p h a t i c A c y l Groups. As shown in the previous sections, wood i s not e a s i l y converted i n t o a t h e r m a l l y meltable m a t e r i a l by g r a f t i n g , although c e l l u l o s e i s found to be convertable t o the meltable m a t e r i a l by g r a f t i n g using a non-aqueous c e l l u l o s e s o l v e n t as the r e a c t i o n medium. I t should be noted t h a t in order to o b t a i n the t h e r m a l l y meltable wood the g r a f t i n g should be conducted so as to get products w i t h homogeneously introduced g r a f t - s i d e - c h a i n s and,at the same time,with high polymer add-ons. Considering these two f a c t o r s ,



338


Figure 8. Total dose-weight increase curves for the graft polymerization of styrene onto cellulose and wood using a S0 -DEA-DMSO solution as a reaction medium. 2


SHiRAiSHi E T A L .


339


20.

Τ (°C)

Figure 9. Plots of the deformation vs. temperature for W-O, untreated wood, and SW-1, -3, -5, and -6, the wood-polysulfone composites prepared by the graft copolymerization using a SO -DEA-DMSO solution as a reaction medium. The weight increases are SW-1, 7.9%; SW-3, 11.4%; SW-5, 14.9%; and SW-6, 33.0%. z



τ

Γ

200

300


340

0

100

400

TCO

Figure 10. Plots of the deformation rate vs. temperature for W-O, untreated wood and SW-2—SW-6, the wood-polysulfone composites prepared by the graft copolymerization using a S0 -DEA-DMSO solution as a reaction medium. The weight increases are SW-1, 7.9%; SW-2, 9.4%; SW-3, 11.4%; SW-4, 12.8 SW-5,14.9%; and SW-6. 33.0%. 2



20.

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341

the g r a f t i n g procedures are considered to be a r a t h e r d i f f i c u l t method f o r t h e r m o p l a s t i c i z a t i o n of wood compared w i t h other chemic a l m o d i f i c a t i o n such as a c y l a t i o n and e t h e r i f i c a t i o n . We have reported that when wood i s a c y l a t e d w i t h a s e r i e s of higher a l i p h a t i c a c i d c h l o r i d e , c a p r o y l to s t e a r o y l c h l o r i d e , in a non-aqueous c e l l u l o s e s o l v e n t , an ^ O ^ d i m e t h y l f o r m a m i d e (DMF)-pyr i d i n e s o l u t i o n , the r e s u l t a n t modified wood i s given thermally s e n s i t i v e p r o p e r t i e s showing c l e a r m e l t i n g behavior in t h e i r t h e r momechanical diagrams (12, 13). I n t h i s method, a very high degree of a c y l a t i o n was not always r e q u i r e d t o provide the thermally meltable property t o wood, and when almost one t h i r d of hydroxyl groups in wood were a c y l a t e d , the products became the t h e r m o p l a s t i c i z e d m a t e r i a l s . This suggested that the use of c e l l u l o s e s o l v e n t s in wood m o d i f i c a t i o n r e s u l t s in the uniform i n t r o d u c t i o n of c e r t a i n chemical groups along the c e l l u l o s e chain (14) I t was d e s i r a b l e , however, t o i n v e s t i g a t e the p o s s i b i l i t i e s of preparing t h e r m o p l a s t i c wood using more common r e a c t i o n procedures than the above method. I f wood can be converted i n t o a t h e r m o p l a s t i c m a t e r i a l by some simple and economical methods i t w i l l have a great v a l u e . As the f i r s t step, we have t r i e d t o a c y l a t e wood in a t r i f l u o r o a c e t i c anhydride (TFAA)-higher a l i p h a t i c a c i d system a t 30 or 50 °C (TFAA method) and in a higher a l i p h a t i c a c i d c h l o r i d e - p y r i dine-DMF system a t 100 °C ( C h l o r i d e method) (15). Both the methods r e s u l t e d in t h e r m a l l y meltable products. An example of the thermomechanical diagram f o r the products i s shown in F i g . 11. I n t h i s f i g u r e , the diagram f o r a l a u r o y l a t e d wood sample prepared by the TFAA method i s compared w i t h that f o r u n t r a t e d wood. The l a u r o y l ated wood shows thermal behavior w i t h a sharp drop caused by comp l e t e flow of the sample at 195 °C. In order t o examine whether or not the flow i s a s c r i b e d to m e l t i n g of the l a u r o y l a t e d wood, we t r i e d to mold sheet from i t by h o t - p r e s s i n g a t 140 °C under a pressure of about 150 Kg/cm. Transparent sheets were molded from l a u r o y l a t e d wood meals as shown in Photo. 2. This r e s u l t i n d i c a t e s c l e a r l y that the flow behavior observed in the thermomechanical diagram i s a t t r i b u t a b l e to melti n g . The molding temperature used, however, i s c o n s i d e r a b l y lower than the m e l t i n g temperature found in F i g . 11. On t h i s temperature d i f f e r e n c e , we have r e c e n t l y reported t h a t the apparent m e l t i n g temperature of a c y l a t e d wood sample i s lowered w i t h an i n c r e a s e in the a p p l i e d pressure (16); amorphous polymers l i k e e s t e r i f i e d wood w i t h higher a l i p h a t i c a c y l groups do not have m e l t i n g p o i n t s being d i s t i n c t l y d e f i n a b l e , but have apparent m e l t i n g temperatures (flow temperatures). The flow temperature v a r i e s w i t h measuring c o n d i t i o n s such as the pressure a p p l i e d and the h e a t i n g r a t e . The apparent m e l t i n g temperature obtained f o r v a r i o u s a c y l a t ed wood prepared by both the TFAA and the C h l o r i d e methods by using the thermomechanical analyzer under a pressure of 3 Kg/cm^ are shown in Table IV. The a c y l a t e d wood samples prepared by the TFAA method show somewhat lower apparent m e l t i n g temperatures compared w i t h those prepared by the C h l o r i d e method.




342



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343

Photo 2. Lauroylated wood meals, a; and a sheet prepared from the lauroylated wood meals by compression molding, b. Molding conditions: temperature, 140°C.; time, 2 min; pressure, 150 kg/cm . 2

Table IV. Apparent M e l t i n g Temperature of Various Higher A l i p h a t i c A c i d E s t e r s of Wood Prepared by the TFAA or the C h l o r i d e Method MELTING TEMP. (°C) Sample (ACYL)

TFAA

CHLORIDE

BUTYRYL

300

310

VALEROYL

235

305

CAPROYL

250

260

CAPRYLYL

210

245

CAPLYL

205

290

LAUROYL

195

240

MYRISTOYL

200

-

PALMITOYL

195

295

-

220

STEALOYL



344


T h e r m o p l a s t i c i z a t i o n of Wood by Introducing Lower A l i p h a t i c A c y l Groups Followed by G r a f t i n g . T h e r m o p l a s t i c i t y of a c e t y l a t e d wood i s found to be dependent on the method of p r e p a r a t i o n and the degree of s u b s t i t u t i o n (15, 17). A c e t y l a t e d wood samples prepared by the TFAA method showed a c l e a r m e l t i n g phenomenon a t 320 °C under a pressure of 3 Kg/cm^ (15). Wood samples a c e t y l a t e d by the C h l o r i d e method or by a method u t i l i z i n g the a c e t i c anhydride-pyr i d i n e or triethylamine-DMF system (25 °C) d i d not undergo complete f l o w , w h i l e a considerable t h e r m o p l a s t i c i t y was provided (15). A l though wood samples f u l l y a c e t y l a t e d by a procedure u t i l i z i n g the a c e t i c a n h y d r i d e - a c e t i c a c i d - s u l f u r i c a c i d system d i d not show c l e a r m e l t i n g , t h e i r p a r t i a l l y s a p o n i f i e d samples gave thermomec h a n i c a l diagrams w i t h a sharp drop corresponding t o complete flow (17). A c e t y l a t e d wood samples prepared by a procedure u s i n g the a c e t i c a n h y d r i d e - a c e t i c a c i d - p e r c h l o r i c a c i d system d i d not show c l e a r m e l t i n g , e i t h e r . Thermal p r o p e r t i e s o f the a c e t y l a t e d wood were enhanced by mixed e s t e r i f i c a t i o n w i t h other a c y l groups. That i s , wood e s t e r s c o n t a i n i n g e i t h e r p r o p i o n y l or b u t y r y l groups in a d d i t i o n t o a c e t y l revealed meltable p r o p e r t i e s , if the mixing r a t i o was appropriate ( 1 7 ) . With these r e s u l t s in mind, we t r i e d t o enhance the thermal p r o p e r t i e s of the e s t e r i f i e d wood samples w i t h lower a l i p h a t i c a c y l groups by g r a f t i n g . I n the f i r s t t r i a l , wood samples a c y l a t e d w i t h the a c e t i c anhydride-propionic a n h y d r i d e - a c e t i c a c i d - p e r c h l o r i c a c i d system were g r a f t e d w i t h styrene. The thermomechanical d i a grams of the products are shown in F i g . 12 w i t h that of the s t a r t i n g m a t e r i a l . I n t h i s case, the thermomechanical behavior was examined w i t h the g r a f t products without e x t r a c t i o n of the homopolymer. This i s based on an i d e a that the g r a f t i n g can be compared w i t h the corresponding polymer blend in o b t a i n i n g molded sheets or f i l m s w i t h good q u a l i t i e s . I t was a l s o found in t h i s experiment that the e x t r a c t i o n of the homopolymer d i d not change the meltable p r o p e r t i e s of the a c y l a t e d - g r a f t e d products, g i v i n g s i m i l a r r e s u l t s as shown in F i g . 12 f o r the non-extracted m a t e r i a l . I t i s Learned from the f i g u r e that the g r a f t i n g can convert the unmeltable a c e t y l a t e d - p r o p i o n y l a t e d wood sample i n t o meltable m a t e r i a l s , and the apparent m e l t i n g temperature decreases w i t h an i n c r e a s e in the amounts of polymer depositee. The f u r t h e r s t r i k i n g f i n d i n g i s that the degree of g r a f t i n g enough to cause such a d r a s t i c change in the t h e r m o p l a s t i c property of the e s t e r i f i e d wood i s very s m a l l . Even the g r a f t products w i t h t o t a l weight increase of l e s s than 10 %, which are prepared by i r r a d i a t i o n t o a t o t a l dose of l e s s than O.2 Mrad, behave as t h e r m a l l y meltable materials. Secondly, t h e r m a l l y unmeltable a c e t y l a t e d wood was t r i e d t o be converted t o a meltable m a t e r i a l . Three kinds o f a c e t y l a t e d wood samples, which has been prepared by the a c e t i c anhydride-acet i c a c i d - p e r c h l o r i c a c i d system a t d i f f e r e n t temperatures of 25, 35 and 45 °C., were all converted i n t o thermally meltable m a t e r i a l s by the g r a f t i n g . An example i s shown in F i g . 13. I n t h i s f i g u r e ,


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20.

T(°c) Figure 12. Thermomechanical behavior of an acetylated-propionylated wood sample and the acetylated-propionylated wood-polystyrene composites prepared by the y-ray induced graft copolymerizationina pyridine medium. Numerical values on the curves represent the total dose of the irradiated y-ray. Key (total irradiation, resultant weight increase based on the weight of wood): A,O.1Mrad, 4.2%; •,O.5Mrad, 12.2%; Φ, 1.9 Mrad, 49.1%; Δ, 2.4 Mrad, 66.0%; and |, 3.4 Mrad, 87.1%.


346



0

100

200

Τ

300

400

CO

Figure 13. Thermomechanical behavior of O, the acetylated wood and Φ, the acetylated wood-polystyrene composite prepared by the y-ray induced graft copolymerizationina pyridine medium. Conditions: total dose, 2 Mrad; resultant weight increase, 76.7%.



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347

a thermomechanical diagram f o r the g r a f t e d product of a c e t y l a t e d wood i s compared w i t h that f o r the corresponding a c e t y l a t e d wood. I t i s c l e a r l y seen that the g r a f t i n g i s e f f e c t i v e f o r preparing a thermally meltable m a t e r i a l from the a c e t y l a t e d wood. Although i r r a d i a t i o n to a t o t a l dose of 2 Mrad was a p p l i e d f o r the g r a f t i n g in t h i s case, the low degree of i r r a d i a t i o n as low as 1 Mrad or l e s s has almost the same e f f e c t f o r the t h e r m o p l a s t i c i z a t i o n of a c e t y l a t e d wood. These r e s u l t s obtained can be i n t e r p r e t e d , a t l e a s t , in terms of the e f f e c t of e x t e r n a l p l a s t i c i z a t i o n caused by the d e p o s i t i o n of p o l y s t y r e n e w i t h i n wood c e l l w a l l in a d d i t i o n t o the i n s u f f i c i ent i n t e r n a l p l a s t i c i z a t i o n p r e v i o u s l y provided by the a c y l a t i o n . The above r e s u l t s shown in F i g s . 12 and 13 can be discussed in connection w i t h the a p p l i c a t i o n of the t h e r m o p l a s t i c i z e d wood. The t h e r m o p l a s t i c i z e d wood can be used as m a t e r i a l f o r molding, and as one way of u t i l i z a t i o n , can be used as blend composites w i t h s y n t h e t i c polymers. I f t h i s b l e n d i n g i s made by g r a f t i n g as shown above, two b e n e f i t s can a t l e a s t be pointed out: (a) the t h e r m o p l a s t i c i t y of wood m a t e r i a l s i s enhanced. (Better r e s u l t s can be obtained w i t h e s t e r i f i e d wood.) (b) the c o m p a t i b i l i t y of the p l a s t i c i z e d wood w i t h s y n t h e t i c polymers increases by the g r a f t i n g . These f a c t o r s are considered t o be advantageous f o r prep a r i n g molded composites w i t h e x c e l l e n t f i n a l p r o p e r t i e s . Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Arthur, J. C. Jr.; Blouin, F. A. Am. Dyestuff Reptr. 1962, 51, 1024. Yoshimura, S. Sen-i Gakkaishi 1965, 21, 358. Tsuzuki, M.; Hagiwara, I.; Shiraishi, N.; Yokota, T. J. Appl. Polymer Sci. 1980, 25, 2721. Mizumachi, H.; Kamidozono, M. Holzforschung 1975, 29, 229. Tadokoro, K.; Sadoh, T.; Nakato, K. Mokuzai Gakkaishi 1976, 22, 309. Handa, T.; Yoshizawa, S.; Ikeda, Y.; Saito, M. Kobunshi Ronbunshū 1976, 33, 147. Okumura, M.; Shiraishi, N.; Sadoh, T.; Yokota, T. J. Soc. Mate r i a l s Sci. Japan 1977, 26, 465. Kawakami, H.; Shiraishi, N.; Yokota, T. Mokuzai Gakkaishi 1977, 23, 143. Okumura, M.; Asο, Κ.; Shiraishi, Ν.; Yokota, T. Holzforschung 1980, 34, 23. Tsuzuki, M.; Hagiwara, I.; Shiraishi, N.; Yokota, T. J. Appl. Polymer Sci., 1980, 25, 2909. Shiraishi, N.; Sato, S.; Yokota, T. Mokuzai Gakkaishi 1975, 21, 297. Shiraishi, N.; Matsunaga, T.; Yokota, T. J. Appl. Polymer Sci., 1979, 24, 2361. Funakoshi, H.; Shiraishi, N.; Norimoto, M.; Aoki, T.; Hayashi, S.; Yokota, T. Holzforschung, 1979, 33, 159.


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14. Shiraishi, N.; Okumura, M.; Yokota, T. Mokuzai Gakkaishi, 1976, 22, 232. 15. Shiraishi, N.; Tsubouchi, K.; Matsunaga, T.; Yokota, T.; Aoki, T. Abst. Papers Presented at the 30th National Meeting, Japan Wood Res.Soc.,Kyoto, 1980, p. 34. 16. Aoki, T.; Shiraishi, N.; Tanahashi, M.; Yokota, T.; Yamada, T. Wood Research and Technical Note, 1980, No. 15, 61. 17. Shiraishi, N.; Fukuhara, K.; Tsubouchi, K.; Yokota, T.; Aoki, T. Abst. Papers Presented at the 31st National Meeting, Japan Wood Res. Soc., Tokyo, 1981, p. 263.

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January 28,

1982.


Thermoplasticization of Cellulose and Wood by Graft

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