Hydrophilic Polymers - American Chemical Society

7

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch007

Biodegradable Plastics Derived from Cellulose Fiber and Chitosan M. Nishiyama, J. Hosokawa, K. Yoshihara, T. Kubo, H . Kabeya, T. Endo, and R. Kitagawa Shikoku National Industrial Research Institute, 2217-14 Hayashi-cho, Takamatu, 761-03 Japan

RECENTLY, ENVIRONMENTAL POLLUTION CAUSED BY WASTE plastics has become a global problem. One countermeasure has been the development of biodegradable plastics that are decomposed within soil, and some biodegradable plastics have been proposed. Such biodegradable plastics must have nearly the same strength as ordinary plastics, must decompose in soil after use and not cause environmental pollution, must be controllable for the period of decomposition, and must be producible at a low cost. Natural polymers such as cellulose are decomposed by microorganisms in soil, and the decomposed materials do not pollute the environment. Judging from their decomposability, natural polymers, which exist in nature abundantly, should be excellent biodegradable plastics. We developed novel biodegradable plastics by making natural polymers into composites. The materials are finely divided cellulose and chitosan, which is produced by the deacetylation of chitin. The novel biodegradable plastics that we developed and their applications are reported here.

Biodegradable Film Derived from Cellulose-Chitosan Cellulose is the major component of the cell walls of plants and the most abundant polymer on earth. Chitin is contained in the shells of 0065-2393/96/0248-0113$ 12.00/0 © 1996 American Chemical Society

In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

114

HYDROPHILIC POLYMERS


H

CHjOH

OH

CH,OH

H

H

NHCOCH,

OH

CH,OH

Chitin

CH,OH

Η

ΝΗ,

CH OH 2

Chitosan Figure 1. Chemical structures of cellulose, chitin, and chitosan. shrimp and crabs and the cell walls of fungi and insects; the global production of chitin is believed to be second only to that of cellulose. As shown in Figure 1, the structure of chitin is similar to that of cellu lose; the chitin molecule is a linear natural polymer that corresponds to cellulose in which C-2 is replaced by an acetylamino group. Chitin is converted to chitosan by deacetylation with concentrated alkali. Both chitin and chitosan have various functions such as biological com patibility, antibiotic activity, and film-forming capability that are at tractive, and chitin and chitosan are being studied for use in medical and industrial materials and in foods. Cellulose suspended in water becomes anionic at the surface from the actions of hydroxy 1 groups and a trace amount of carboxyl groups. Chitosan, on the other hand, is insoluble in water, and its salts, such as those with acetic acid, become water-soluble and cationic. Chitosan


7.

NISHIYAMA ET AL.

Biodegradable

115

Plastics

has good affinity for cellulose because of the similarity i n their struc tures. T a k i n g advantage of these characteristics, w e d e v e l o p e d b i o d e gradable plastics d e r i v e d from cellulose a n d chitosan. Materials. T h e raw material cellulose was obtained as M i c r o F i b r i l C e l l u l o s e ( M F C - 1 0 0 ) from D a i c e l C h e m i c a l Industries L t d . (81, Kasumigaseki 3-ehome, C h i y o d a - k u , T o k y o 100, Japan) a n d selected from samples w i t h diameters of 0.1 μπι or less, lengths of 1 0 0 - 5 0 0 μτη, a n d surface areas o f 2 0 0 m /g. T h e raw material chitosan h a d 99.8% deacetylation and was p r o d u c e d by K a t o k i c h i C o . , L t d . (2-35, Sanbonmatsu 4-chome, K a n n o n j i 768, Japan), from s h r i m p s h e l l .


2

F i l m Formation. T h e composite c e l l u l o s e - c h i t o s a n film was prepared b y f o l l o w i n g the process o u t l i n e d i n F i g u r e 2. A solution of chitosan i n acetic a c i d was b l e n d e d w i t h an aqueous d i s p e r s i o n of the M i c r o F i b r i l C e l l u l o s e ; after the generated foam had b e e n r e m o v e d , the b l e n d was spread over a flat plate. Thereafter, d r y i n g and thermal treatment were a p p l i e d to produce a translucent film. T h e r m a l d r y i n g was necessary to make both materials into a water-resistant composite film of h i g h strength (1,2).

Water Suspension of Fine Cellulose

Acetic Acid Solu t i o n of Chitosan

(Plasticizer)

Mixing

Degassing

Casting

Drying 70TJ.15 hr

Heat Τι:eatment hr 70~19(

Composite Film

Figure 2. Method of molding biodegradable

film.


116


Properties of the Composite F i l m . How the amount of chitosan influences the tensile strength of the film is shown in Figure 3. In dry conditions, the strength increases as the chitosan content increases; strength reaches about 1000 kg/cm when the chitosan content is 5 % or more. This level is several times as strong as commercial polyethylene films. Although finely divided cellulose alone or chitosan alone is very weak in wet conditions and is difficult to mold, the maximum wet strength of the composite film occurred when chitosan was present (at 10-20%). The 1 0 - 2 0 % content makes the film economically reasonable despite the relatively higher price of chitosan compared to that of cellulose. The development of the higher strength in the composite may be understood from several standpoints: (1) The affinity between cellulose and chitosan is excellent, and cross-links are formed by the amino groups in chitosan and the carboxyl and carbonyl groups on the surface of cellulose; (2) chitosan salts are converted to water-insoluble amine types; and (3) self-condensation of chitosan occurs. This composite film is not flexible. Addition of plasticizers to im-


2

Chitosan Content {%) Figure 3. Effect of chitosan content on tensile strength of a composite film.



7.

NISHIYAMA ET AL. Biodegradable

Ο

10

20

Plastics

30

117

40

Chitosan Content (%) Figure 4. Tensile strength of a composite film containing 75% glycerol. prove flexibility was studied. Figure 4 shows the results when 75 parts of glycerol is added to 100 parts of cellulose and the mixture is blended with chitosan. Although the strength decreases to some extent, a film with good flexibility is obtained. Thermal analysis of this composite film did not show any notice able endothermic or exothermic peak up to 280 °C; how the thermal treatment temperature influences the composite was examined. A l though no substantial influence on tensile strength was revealed, the degree of swelling with water decreased as the thermal treatment tem perature rose. These facts suggest an increase in cross-linkings in the composite film (1 ). Thus simple procedures such as thermal treatment cause cross-links to form. More cross-links are effective in decreasing the hydrophilic property, preventing attacks by microorganisms and enzymes, and extending the biodégradation period.

Accelerated Degradation Test of Biodegradable Film Although no standard degradation test for biodegradable plastics has been established, the degradation period is practically important, be-



118


cause this composite film is expected to biodegrade completely i n soil w i t h i n several months. W e adapted an accelerated biodégradation test u s i n g f u n g i that degrade the cellulose a n d chitosan constituents of the film (1). C h e m i c a l analysis o f the composite film i n the early stages of degradation p r o v e d that chitosan is degraded preferentially. I n searching for microorganisms that degrade chitosan, w e f o u n d a chitosan-degrading bacterium i n soils i n various places i n Japan (3). T h i s bacillus has a single f l a g e l l u m and was i d e n t i f i e d as b e l o n g i n g to the genus Pseudomonas. It was named Pseudomonas sp. H - 1 4 (called H - 1 4 for short). H - 1 4 grows o n chitosan as the o n l y carbon a n d o n l y nitrogen source a n d does not use cellulose for nutrition. T h e early stage o f degradation of the composite film is thus probably brought about b y chitosanase.

Accelerated Degradation Test Method.

I n an accelerated

degradation test, a pure culture o f H - 1 4 was transplanted u n d e r the conditions s h o w n i n F i g u r e 5, a n d the culture was c o n t i n u e d u n d e r vibrating conditions u n t i l the film was finely d i v i d e d ; the time from transplantation to fine d i v i s i o n was recorded. Glass beads p r o v i d e d

A porous stopper

Sterilization w i t h steam at 120 °C

A glass tube

A 5 - m L medium! chitosan pH6 Test pieces

Inoculation of H-14 on the m e d i u m

Incubation at 28 °C w i t h 300 cpm

Glass beads 0.3 g 6-8 mesh

Figure 5. Accelerated degradation test.


7.

NISHIYAMA ET AL. Biodegradable Plastics

119


light shocks to the film, and chitosan was a d d e d to h e l p H-14 m u l t i p l y and to promote chitosanase secretion. T h r e e test pieces per test tube were used. Basic films o f the composite f i l m d e r i v e d from cellulose and chitosan collapsed into f i n e l y d i v i d e d pieces w i t h i n about 3 to 4 days. T h i s accelerated test made it possible to observe degradation occurring 10 times as fast as the actual rate i n nature. Furthermore, degradation was also observed w h e n the accelerated degradation test used a commercial cellulase. I n the absence of H - 1 4 or cellulase, n o film degradation was observed.

Biodégradation of Composite Film.

T h e decomposition pe-

r i o d varies d e p e n d i n g o n composite p r o d u c t i o n conditions a n d the b l e n d i n g ratio o f the raw materials; some test results are s h o w n i n F i g u r e 6. Higher-temperature thermal treatment a n d more oxidative groups i n the cellulose increase the time u n t i l degradation i n i t i a t i o n because increased cross-linking i n the composite film makes i t d i f f i cult for microorganisms and enzymes to attack the film. T h e influence of carboxyl a n d carbonyl groups i n the raw material cellulose o n the degradation p e r i o d was studied i n detail (Figure 7). Results indicate that the carbonyl groups have a strong correlation w i t h the degradation p e r i o d b u t the carboxyl groups d o not. T h e s e facts suggest that the carbonyl groups i n cellulose a n d the amino groups i n chitosan form Schiff bases that cross-link (2). T h e degradation p e r i o d may be controllable b y m a n i p u l a t i o n of these characteristics.

Joint Research T h e s e materials, u n l i k e p e t r o l e u m - d e r i v e d plastics, are not thermoplastic and therefore cannot b e processed b y conventional machines for plastic m o l d i n g or processing. T o overcome this p r o b l e m , n e w fabricating apparatus and commercialization are b e i n g d e v e l o p e d j o i n d y w i t h private enterprises. T h i s work is s u m m a r i z e d b e l o w . Biodegradable Films. T w o private companies participated i n joint research o n f o r m i n g films. O k u r a Industrial C o . , L t d . (1515, N a katsu-cho, M a r u g a m e , 763 Japan), succeeded i n the continuous production o f films i n a r o l l e d form b y u s i n g a n e w l y d e v e l o p e d test plant m a c h i n e ; the test products i n c l u d e m o l d e d materials such as trays. M o d i f i c a t i o n o f the machine to permit commercialization, i m p r o v e ment o f film formation, a n d reduction o f manufacturing cost is n o w u n d e r way. A i c e l l o C h e m i c a l C o . , L t d . (45 K o s h i k a w a , I s h i m a k i h o n m a c h i , T o y o h a s h i , 441-11 Japan), is i n v o l v e d i n a 3-year project, schedu l e d to be c o m p l e t e d i n 1995, to d e v e l o p a film 500 m l o n g a n d 1 m


120


h

*

*

G75 Ch20

7

S


'S 20 G75 Ch20

•Η

15

/ /

/

1

/ ι

10 '

-

Π

Ο

2

"8 •Η

-«

Ή

~*—71

Carboxyl Groups 30 40 50

u

I

1 70

1

1

100 130

1

1

160 190

I .

30

40

I

J

50

I,

60

ο

I

70

Figure 6. Period of degradation of a composite film by H-14 and cellu lase. w i d e . T h i s work has b e e n selected as a d e v e l o p m e n t project b y the Research D e v e l o p m e n t Corporation of Japan. Biodegradable Nonwoven Fabrics. K a n a i Juyo K o g y o C o . , L t d . ( 2 - 9 , D o j i m a 1-chôme K i t a k u , Osaka, 530 Japan), d e v e l o p e d a process for u s i n g these materials as a b i n d e r for dry n o n w o v e n fabrics. As s h o w n i n F i g u r e 8, n o n w o v e n fabrics thus manufactured that contain a natural fiber such as cotton or rayon are biodegradable a n d may be p r o d u c e d w i t h o n l y a small additional cost because production w i t h existing e q u i p m e n t is possible a n d the amount o f b i n d e r r e q u i r e d is


7.

NISHIYAMA ET AL. Biodegradable

Plastics

Γ 20

Carbonyl Groups 8-15nmiol/kgr-cel l u .


15

Carboxyl Groups 91-116imnol/kgrcellu.

10

o Ή •H

& Οι I

0

50 100 Carboxyl Groups i n Cellulose (mmol/kg)

0

50 100 Carbonyl Groups In Cellulose (mmol/kg)

Figure 7. Period of degradation of a composite film by H-14 and cellu tase.

r

Binder

Rayon or

r

r

Needle Punch

Figure 8. Cross section of biodegradable nonwoven fabric.


122

HYDROPHILIC POLYMERS Table I. Properties of Biodegradable Nonwoven Fabrics

Property

BD-30D

BD-100S

BD-30W

Fiber Structure

100% rayon cross dipping

100% rayon cross needle punch

30 0.2

spray 100 0.8

100% rayon cross water needle punch dipping 32 0.2

0.6 0.3

1.6 1.8

4.5 2.2

0.1 0.1 54

0.8 0.7 210

0.1 0.2 53

Weight (g/m ) Thickness (mm) Tensile strength (kg/5 cm) MD CD Tear strength (kg/5 cm) MD CD Wet/dry strength (%)


2

small. S u c h n o n w o v e n fabrics, w h e n b u r i e d i n s o i l , completely d e graded i n 1 m o n t h i n summer a n d 2 months i n winter. T h e degradation p e r i o d for these n o n w o v e n fabrics is controllable: Fabrics that degrade i n 1 w e e k or i n 1 to 3 years have b e e n d e v e l o p e d . T h e s e fabrics have excellent porosity, permeability, and water-resisting properties a n d can be fabricated. Various biodegradable n o n w o v e n fabrics w i t h different characteristics have also b e e n d e v e l o p e d (Table I). Pots a n d sheets for gardening, sanitary materials such as paper diapers, p a c k i n g materials, a n d w o u n d dressings, among others, are targets of the a p p l i cations. Biodegradable Foams. N i s h i k a w a R u b b e r C o . , L t d . (2-2-8, Misasa-machi, N i s h i - k u , H i r o s h i m a , 733 Japan), d e v e l o p e d biodegradTable II. Properties of Biodegradable Foams Property Apparent density (g/cm ) Tensile strength (kg/cm ) Elongation (%) Water-absorbing capacity (%) Water-holding capacity (%) Calorific value (cal/g) Hardness, degree (Asker F) Biodegradable time (days ) 3

2

a

a

Opencell foams

Closedcell foams

0.02-0.05 0.5-10 15-40 1000-4000 50-400 4150 5-70 4-6

0.1-0.3 5-20 10-20 50-200 10-50 4010 60-80 9

Acceleration test, SNIRI Method.


Film 1.3 1000 10

15-18

7. NISHIYAMA ET AL. Biodegradable

Plastics

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able open-cell and closed-cell foams by using materials derived from cellulose and chitosan (Table II). The open-cell foams are flexible, like a cosmetic puff, and are highly water-absorbing, being able to absorb several orders of magnitude more water than typical foams can. The closed-cell foams are like a honeycomb and have nearly the same hardness and strength as conventional plastic foams. Their excellent characteristics of lightness, thermal insulation, permeability, and water absorption are put to use in agricultural and industrial applications. Their biological adaptability is also useful for new applications.

Conclusions The most difficult problem to overcome for cellulose-chitosanderived biodegradable plastics was processing, because these materials are not thermoplastic. However, this problem has been solved, and biodegradable plastics in various forms are being commercially produced. Biodegradable plastics degrade to carbon dioxide and water and may turn to compost in the process. Compost is a valuable carbon source for microorganisms in soil and seems to aid in activating soil. Cost reduction and quality improvement of the composite remain subjects for future attention. We believe that biodegradable plastics that return to nature when discarded not only solve waste problems but also may serve as novel functional materials.

References 1. Hosokawa, J.; Nishiyama, M.; Yoshihara, K.; Kubo, T. Ind. Eng. Chem. Res. 1990, 29, 800-805.

2. Hosokawa, J.; Nishiyama, M.; Yoshihara, K.; Kubo, T.; Terabe, A . Ind. Eng. Chem. Res. 1991, 30, 788-792.

3. Yoshihara, K.; Hosokawa, J.; Kubo, T.; Nishiyama, M. Agric. Biol. Chem. 1990, 54, 3341-3343.

RECEIVED for review April 16, 1994. ACCEPTED revised manuscript April 7, 1995.


Hydrophilic Polymers - American Chemical Society

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