properties and modification methods for vegetable fibers for natural fiber composites

8/8/2019 Properties and Modification Methods for Vegetable Fibers for Natural Fiber Composites

1/8

Properties and M odification Methods for Vegetable Fibersfor Natural Fiber Composites

A. K. BLEDZKI,' S. R E I H M A N E ? and J. GASSAN',*

' Institut f u r Werkstofftechnik, University Kassel, Kassel, Germany; 21nstitute of Polymer Materials, T U Riga, Riga, Latvia

SYNOPSIS

Studies on struc ture a nd properties of natural vegetable fibers (N VF) show th at compositesmade of NVF combine good mechan ical properties with a low specific mass. Th e high levelof moisture absorption by t he fiber, its poor wettability, as well as th e insufficient adhesionbetween untreated fibers and the polymer matrix lead to debonding with age. To buildcomposites with high mecha nical properties, therefo re, a surface modification of the fibers

is necessary. Th e existin g physical an d chem ical NVF modification methods-e.g., plasm atreatment or graft copolymerization-which are used for th e develo pment of NVF-polymercomposite propertie s is discussed. It is shown th at modified cellulose fiber-polymer inte r-action m echani sms are complex an d specific to every definite system. By using an couplingagent, like silanes or stearin acid, the Young's modulus and th e tensile strength increases,dependent on the resin, until 50%. Simultaneously, the moisture absorption of th e com-posites decreases for about 60%. With other surface modifications, similar results ar e ob-tained. 0 996 John Wiley & Sons, In c

INTRO DUCT10 N

Fiber-reinforced plastics are structural composites,

in which a polymer matrix is combined with fibers,fabric cuttings, or filling webs. In conventional com-posites, the fibers are made of glass, aramid, or car-bon. T he advantage of fiber-reinforced plastics re-sults from their structure. Th e controlled struc turein which the fibers are lying in the polymer matrixincreases the strength and t he stiffness of the com-posite. A t present, natural vegetable fibers (NVF)are used in composites, where high strength andstiffness are not of first priority. N V F reduce themass of th e composite, because they have a low den -sity. Their production is economical, with low re-quirements on equipment, and they can easily be

recycled.Conventional fibers (glass, aramid, carbon, etc.)

can be produced with a definite range of properties.The characteristic properties of NVF vary consid-erably. This depends on whether t he fibers are taken

Journal of Applied Polymer Science, Vol. 59, 1329-1336 (1996)0 1996 John Wiley & Sons, Inc. CCC 0021-8995/96/081329-08

from the plant stems or leafs,' the quality of theplants location, and on the pre ~on dit ioni ng. ~.~

Depending on their origin, vegetable fibers canbe grouped into seed, bast, leaf, or fruit qualities.Ba st an d leaf qualities are so-called hard fibers ( e.g.,flax, jute, and ramie) 1,5-7 and are the most used ones.In practically all cases, cellulose is the main com-ponent of vegetable fibers (Table I ) .

Th e elementary u nit of a cellulose macromolecule(Fig. 1 is anhydro-&glucose, which contain s threealcohol hydroxyls ( - H ) . Thes e hydroxyls formhydrogen bonds inside the macromolecule itself ( in -tramolecular ) and between other cellulose macro-molecules (inte rmolecular ) as well as with hydroxylgroups from t he air. There fore, all vegetable fibersare of a hydrophil nature; their moisture contentreaches 8-12.6% (Table I ) .

An important characteristic of vegetable fibers istheir degree of polymerization (DP) . The cellulose mol-ecules of each fiber differ in their D P and, consequently,the fiber is a complex mixture of polymer homolog(C&1005)n. Bast fibers commonly show the highest DPamong all the NVF (approximately 10,000).6

The fibrils of the cellulose macromolecules formspirals along th e fiber axis.8 Th e strength and stiff-

1329


2/8

1330 BLEDZKI, REIHMANE, AND GASSAN

Table I Chemical Composition and Structure P arameters of Vegetable Fibers

Fiber

JuteFlaxHempRamieSisalCoir

Cellulose Cell Leng th(W t %) (mm)

Spiral Angle(Ded

Allied Substances(Wt % )

Hemi-cellulose Pectins Lignin Wax

MoistureContent(W t % ) Refs.

61.0 2.371.0 20.074.4 23.068.6 154.078.0 2.243.0 0.8

8.010.06.27.5

20.045.0

20.4 0.2 13.0 0.518.6 2.3 2.2 1.717.9 0.9 3.7 0.813.1 1.9 0.6 0.310.0 - 8.0 2.0

0.3 4.0 45.0 -

12.6 8, 910.0 8, 910.8 8, 9

8.0 8, 1011.0 8, 11

8.0 8, 11

ness of hemp, ramie, a nd jute correlates with theangle between the axis and the fibril of the fiber.The smaller this angle is, the higher are the me-chanical properties (Ta ble 11). Th e cellulose of N V Fcontains different natural substances (Table 1) . Themost importan t of them are lignin and several waxes.Th e distinct cells of hard NVF are bonded togetherby lignin ( or lignin/hemicellulose) , acting as a ce-menting material.15 Th e lignin content of th e bas tfibers influences its structure,6 ts andmorphology." T he waxy substances of vegetable fi -bers can be eliminated by extraction with organicsolvents. They are the cause of the fiber's wettabilityand adhesion characteristic^.'^*'^

Comparative values of cellulose and conventionalreinforcing fibers (Table 11) show the high att ractionfor composites made of ramie, flax, and jute fibers.Their density values make it possible to produce

composites which combine good mechanical prop-erties with a low specific mass.

NVF COMPOSITES

The properties of composites depend on those of th eindividual components and on their interfacialcompatibility. The adhesion between fiber and ma-trix is obtained by the mechanical anchoring of th efiber ends into the matrix. Polymers for which thechemical bonding with reinforcements made of cel-lulose is known are methylolfunctional polymers,

such as phenol-formaldehyde resins.lg Methylolgroups react with the hydroxyl groups of the cellu-lose to form stable ether linkages. So , a high com-patibility in the system cellulose and polymer isachieved. Unfortunately, this is the only example ofa high compatible cellulose-polymer compound. Inother cases, the absorption of moisture by untreatedfibers, poor wettability, and insufficient adhesion

between polymer matrix and fiber leads in time todeb~nding . ' ,~ itho ut effective wetting of the fiber,strong interfacial adhesion cannot exist. The lackof interfacial interactions leads to internal strains,"porosity, an d environmental degradation.21,22 hewettability of the fiber depends on the viscosity ofthe polymer and the surface tension of both mate-rials. Th e surface tension of the polymer must be a slow a s possible,23 t least l o ~ e r ' ~ , ~ ~han the surfacetension of the fiber. There are different methods ofmodification to change the surface energy of thefiber25 nd the polymer. The fibers' minimum valueof surface tension must be 43 din/c m in the case ofepoxide ( E P ) an d 35 din/cm in the case of unsat-urated polyester ( U P ) resins.24

Therefore, the modification of the fiber or/andthe polymer matrix is a key area of research a t pres-ent to obtain optimum fiber matrix properties.

PHYSICAL METHODS OF NVFMODIFICATION

Reinforcing fibers can be modified by physical andchemical methods. Physical methods, such asstretching, calendering, 26,27 therm~trea tment ,~ ndth e production of hybrid yarns28,29 o no t changethe chemical composition of the fibers. Physicaltreatments change structural and surface propertiesof the fiber and thereby influence the mechanical

bondings in the matrix.Electric discharge (corona, cold plasm a) is an -

other way of physical treatment. Corona treatmentis one of the most interes ting techniques for surfaceoxidation activation. Th is process changes th e sur -face energy of the cellulose fibers,25 and in the caseof wood, surface activation increases the a mount ofaldehyde groups.30


3/8

NATURAL VEGETABLE FIBER COMPOSITES 133

Figure 1 Macromolecules of cellulose.12

The same effects are reached by cold plasmatreatment. Depending on the type a nd natu re of th eused gases, a variety of surface modifications can beachieved. Surface crosslinkings could be introduced,surface energy could be increased or decreased, andreactive free radicalsz5 an d groups3 could be pro-duced.

Electrical discharge methods are known3 to be

very effective for nonactive polymer substratessuch as polystyrene, polyethylene, an d polypropyl-ene. They are successfully used for cellulose fibermodification to decrease the melt viscosity of cel-lulose-polyethylene composites33 and t o improvemechanical properties of cellulose-polypropylenec0mposites.2~

An old method of cellulose fiber modification ism e r c e r i z a t i ~ n ~ ~ ~ ~ * ~ ~ , ~ ~ ;t has been widely used oncotton textiles. Mercerization is an alkali treatme ntof cellulose fibers; it depends on th e type and con-centration of the alkalic solution, its temperature,time of treatment, tension of the material, as well

as on the additive^.'^.^^ A t present, there is a ten-dency to use mercerization for NVF as well. Optimalconditions of mercerization ensure th e improvementof the tensile proper tie^'^'^*'^^^^and absorption char-

ac t e r i s t i c~ , * ~ ,~~hich are important in the com-posing process.

CHEMICAL METHODS OF MODIFICATIONFOR NVF

Strongly polarized cellulose fibersz3 nhe rently arelittle compatible with hydrophobicWhen two materials are incompatible, it is oftenpossible to bring about compatibility by introducinga third material that has properties intermediatebetween those of the other two. There are severalmechanisms l 9 of coupling in materials:

Weak boundary layers-coupling agents elim-inate weak boundary layers.

Deformable layers-coupling agents produce atough, flexible layer.

Rest rained layers-coupling agents develop ahighly crosslinked interphase region with amodulus intermediate between th at of th e sub-stra te and t he polymer.

Table I1 Comparative Values of Cellulose and with Conven tional Reinforcing F ibers

Density Elongation to Break Tensil e Stre ngth Youngs Modulus CostFiber (g/cm3) (% ) ( M W (GPa) ($/kg1 Refs.

JuteFlaxHempRamieSisalCoir

E-glassS-glassAramid (normal type)Carbon (high tensile strength)

1.451.50

1.501.451.15

2.502.501.401.70

-1.52.41.61.22.0

15.0

2.52.83.3-3.71.4-1.8

5501100

690870640140

2000-35004570

40003000-3 150

13100

12815

5

7086

-

63-67230-240

0.3 1, 81 , 8

8

--- 8, 130.36 1, 80.25 1, 8

3.25 1 4

1 4> 16 1 4

- 1, 14-


4/8


The development of a definitive theory for th emechanism of bonding by coupling agents in com-posites is a complex problem. The main chemicalbonding theory alone is not sufficient. So, consid-eration of other concepts appears to be necessary,which include the morphology of t he interphase, t heacid-base reactions a t the interface, surface energy,and the wetting phenomena.

I I IN- H N -H N- H

The Change of Surface Tension

Th e surface energy of fibers is closely related to t hehydrophility of the fiber.23 Some investigations areconcerned with methods to decrease hydrophility.Th e modification of wood cellulose fibers with stearicacid3 hydrophobizes those fibers and improves theirdispersion in polypropylene. As can be observed injute-reinforced unsa turated polyester resin compos-ites, treatment with polyvinylacetate increases themechanical properties and water repellence.

Silane coupling agents may contribute hydro-philic properties to the interface, especially whenamino-functional silanes, such as epoxies and ure-tha nes silanes, are used as primers for reactive poly-mers. The primer may supply much more aminefunctionality t ha n can possibly react with th e resin

-c n - H ~ CH - u Z - C H - C H ~P O YS T Y R EN E- -

Figure 2lulose-P MPPIC-P S interface area.36

Hypothetical chemical structure of the cel-

Wettability-coupling agents improve the wet-ting between polymer and substrate (criticalsurface tension fac tor) .

Chemical bonding-coupling agents form co-valent bonds with both ma terials.

Acid-base effect-coupling agen ts alte r theacidity of the subst rate surface.

Table 111 Charac teristics of Rep resentative Commercial Silane Coupling Agents

Critical SurfaceTension of Glass

OSi-Specialties with Silan e Applied PolymersGerma ny GmbH Trea tmen t (Abbreviat ions According

Chemical Structure Product (dyn ekm ) ASTM1600)Organofunctional

Group

Vinyl

Vinyl

CH,=CHSi (OCH,),

CH,=CHSi (OC,H,),

A-171

A-I51

25.0

30.0

U P

UP, PE , PP , DAP,EPDM, EP M

E Phloropropy l CICH,CH,CH,Si iOCH,,) , A-143 40.5

0/ \

CH2CHCH,0(CH &3i (OCH,),

CH3I

CHz=C-COO(CH,),Si (OCH&

EPOXY A-187 38.5-42.5 EP , PA, PC,PF,PVC, PUR

A-174 28.0 UP, PE, PP, DAP,EPDA, EPM

U P, PA, PC, PUR,MF, PF, P I , MPF

All polymers

PS, addition t o aminesi lanes

EP, PUR, SBR,EPDM

Methacrylate

Pr imary amine

Diamine

Cationic styryl

Phenyl

A-1100

A-1120 33.55.0 I-

40.0

Mercapto A-189 41.0


5/8


ASIOR'I

or on OH

SUBSlFIAT E

A

1R A

no - 8 - 0 -

a a 0H H H H H H

0

fabrics with polymer matrices compatible with thepolymer. For th is purpose, polymer s o l u t i ~ n s ~ ~ , ~ ~rdispersions4' of low viscosity are used. For a numberof interesting polymers, the lack of solvents limitsthe use of the method of im pr eg na ti ~n .~ ~ hen cel-lulose fibers are impregnated with a butylbenzyl

phtha late plasticized poly (vinyl chloride) ( PV C)dispersion, excellent partitions can be achieved. Inpolystyrene ( P S ) his significantly lowers the vis-cosity of the compound and t he plasticator and re-sults in a co-solvent action for both PS and PVC.40

Chemical Coupling

An important chemical modification method is thechemical coupling method. This method improvesth e interfacial adhesion. Th e fiber surface is treatedwith a compound that forms a bridge of chemicalbonds between the fiber and matrix.

BOND FORMATION S'BSTAATE Graft Copolymerization

R

R l'A

I I

I Ina - s - Q - g -0 - g- OH

4 .. HH -. 0

I

0

I

R

R l'A

I I

I Ina - s - Q - g -0 - g- OH

4 .. HH -. 0

I

0

I

Figure 3with hydroxyl groups contai ning th e surface.lg

Scheme of the alkoxysilanes bond formation

at the interphase. Those amines which could notreact are hydrophilic and therefore responsible for

the poor water resistance of the bonds. An effectiveway to use hydrophilic silanes is to blend them withhydrophobic silanes such as phenyltrimethoxysilane.Mixed siloxane primers also have an improved the r-mal stability, which is typical for aromatic sili-cones."

Impregnation of Fibers

An effective method of NVF chemical modificationis graf t copolymerization.8p'2 Th is reaction is in iti-ated by free radicals of the cellulose molecule. Th ecellulose is treated with an aqueous solution withselected ions and is exposed to a high-energy radia-tion. Then, the cellulose molecule cracks and radi-cals are formed. Afterward, the radical sites of thecellulose are treated with a suitable solution (com-patible with th e polymer m at ri x) , e.g., vinyl mono-mer, 2 acrylonitrile,' methyl methacrylate, 41 an d

p o l y ~ t y r e n e . ~ ~ he resulting copolymer possessproperties characteristic of both fibrous cellulose an dgrafted polymer.

For example, the tr eatment of cellulose fibers withhot polypropylene-maleic anhydride ( MAH-PP )copolymers provides covalent bonds across the in-terface.20 Th e mechanism of reaction can be dividedinto two steps:

A bette r combina tion of fiber an d polymer is Activation of th e copolymer by heat ing ( tachieved by an impregnat i~n~~ f the reinforcing = 17OoC) (before fiber treatment):

P P

0 C H A I f 1

- -A

110 - - - c +

0

,C--

' c - c - c

11 H I

0 ' I HZ iI I H I


6/8


7/8


The reduction of the water absorbtion of cellulose

fibers and their composites treate d with triazine de-rivates is explained by 7*44

Reducing the number of cellulose hydroxylgroups which are available for water uptake,Reducing the hydrophility of the fiber's surface,andRes traint of the swelling of the fiber, by creatinga crosslinked network due to covalent bonding,between th e matrix and fiber.

Organosilanes as Coupling Agents

Organosilanes are the main group of coupling agentsin glass-reinforced polymers. They have been de-veloped to couple virtually any polymer to t he m in-erals, which are used in reinforced c~mposites.'~Most of the silane coupling agents can be repre-sented by the following formula:

with n = 0-3; OR', the hydrolyzable alkoxy group;and R , he functional organic group.

The organofunctional group ( R ) n th e coupling

agent causes the reaction with the polymer. Thiscould be a copolymerization and/or the formationof an interpene trating network. Th is curing reactionof a silane-treated substrate enhances th e wettingby the resin (Table 111).

The general mechanism of how alkoxysilanesform bonds with the fiber surface which containshydroxyl groups is depicted in Figure 3. Alkoxysi-

lanes undergo hydrolysis, condensation (catalystsfor alkoxysilane hydrolysis are usually cata lysts forcondensation), an d t he bond-formation stage, underbase- as well as under acid-catalyzed mechanisms.In addition to these reactions of silanols with hy-droxyls of t he fiber surface, the formation of poly-

siloxane structures also can take place.Analogous to glass fibers, silanes are used as cou-pling agents for natural fiber-polymer composites.For example, the treatment of wood fibers withproduct A- 175 improves woo ds dimensional stabil-it^.^^ In cont rast, a decrease of mechanical properties

was observed for coir-UP composites afte r a fibermodification with dichloromethylvinylsilane.46 Thetre atme nt of mercerized sisal fiber with aminosilane1100 (Ref. 16 ) before forming sisal-epoxy compos-ites markedly improves the moisture resistance ofthe composite. These examples show tha t theoriesused for the silane treatment of N V F are not perfect

and are contradictory; therefore, further studies arenecessary.

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.9.

10.

11.

12.

13.

14.

E. T. N. Bisanda and M. P. Ansell, J. Mater. Sci., 27 ,

B. C. Barkakaty, J . Appl. Polym. Sci., 20,2921-2940( 1976).K. K. Chawla and A. C. Bastos, Znt. Conf. Mech. Beh.Mat., 3, 191-196 (1979).

P. K. Ray, A. C. Chakravarty, and S. B. Bandyopa-dhyay, J. Appl. Polym. Sci., 20,1765-1767 (1976).H. Wells, D. H. Bowen, I. Macphail, and P. K . Pal,in 35th Annual Technical Conference, 1980, Section

L. M. Lewin and E. M. Pearce, Handbook of FiberScience and Technology: Volum e ZV Fiber Chemistry,Marcel Dekker, New York, 1985.P. Zadorecki an d P. Flodin, J. ppl. Polym. Sci., 31,

S. C. 0. Ugbolue, Tex t. Znst., 20( ) , 1-43 ( 1990).M. S. Kucharyov, Tekst. Prom., 8-9,23-25 (1993).E. W. Wuppertal, Die Tex tilen Rohstoffe (N at ur undChemiefasern), Dr. Spohr-Verlag/Deutscher Fach-

verlag, Frankfur t, 1981.D. S. Varma, M. Varma, and I. K. Varma, Text. Res.Znst., 54 ( 12 ), 821-832 ( 1984).J . I. Kroschwitz, Polymers: Fibers and Textiles, Wiley,New York, 1990.S. H. Zeronian, J. Appl. Polym. Sci., 47 , 445-461( 1991).H. Saechtling, International Plastics Handbook, Han-ser, Munich, 1987.

1690-1700 (1992).

1-F, pp. 1-7.

1699-1707 (1986).


8/8


15. M. K. Sridhar, G. Basavarajappa, S. G. Kasturi, andN. Balasubramanian, Ind. J . Text. Rex , 7 9 ) , 87-92(1982).

16. E. T. N. Bisanda an d M. P. Ansell, Comp. Sci. Tec h-nol., 165-178 (1991).

17. V. V. Safonov, Treatment of Textile Materials, Leg-prombitiz dat, Moscow, 1991.

18. S. H. Zeronian, H. Kawabata, and K. W. Alger, Text.Res. Znst., 60 (3 ), 179-183 (1990) .

19. K . L. Mittal, Silanes and Other Coupling Agents, VS PBV, Netherlands, 1992.

20 . J. M. Felix and P. Gatenholm, J. Appl. Polym. Sci.,

21. S. K. Pal, D. Mukhopadhyay, S. . Sanyal, and R. N.Mukherjee, J . Appl. Polym. Sci., 35,973-985 ( 1988).

22. L. Hua, P. Flodin, and T. Ronnhult, Polym. Comp.,8 ( 3 ) , 203-207 (1987).

23. B. S. Westerlind and J. C. Berg, J . Appl. Polym . Sci.,

24. E. P. Plueddemann, Interfaces in Polymer Matrix

25. M. N. Belgacem, P. Bataille, an d S. Sapieha, J . Appl.

26. M. A. Semsarzadeh, Polym. Comp., 7( 2) , 23-25

27. M. A. Semsarzadeh , A. R. Lotfali, a nd H. Mirzadeh,

28. A. N. Shan and S. C. Lakkard, Fibre Sci. Technol.,

29. B. Wulfhorst, G. Tetzlaff, and R. Kaldenhoff, Technol.

30. I. Sakata, M. Morita, N. Tsuruta, and K. Morita, J .

42,609-620 ( 1991).

36,523-534 ( 1988).

Composites, Academic Press, New York, 1974.

Polym. Sci., 53,379-385 (1994).

( 1986).

Polym. Comp., 5 ( 2) , 141-142 (1984) .

15,41-46 (1981).

Text., 3 5 ( 3 ) , S . 10-11 (1992).

Appl. Polym. Sci., 49 , 1251-1258 (1993).

31. Q. Wang, S. Kaliaguine, and A. Ait-Kadi, J. Appl.

32. S . Goa and Y. Zeng, J . Appl. Polym. Sci., 47 , 2065-

33. S. Dong, S. Sapieha, and H. P. Schreiber, Polym. Eng.

34. T. P. Nevell and S. H. Zeronian, Cellulose Chemistr y

35. M. J. Schick, Surface Characteristics of Fibers and

36. D. Maldas, B. V. Kokta, and C. Daneaulf, J . Appl.

37. R. G. Raj, B. V. Kokta, F. Dembele, and B. Sansch a-

38. W. GeGner, Chemiefasernl Tex tili d. (I d.-T ext .), 391

39. A. C. Khazanchi, M. Saxena, and T. C. Rao, Text.

40. P. Gatenholm, H. Bertilsson, and A. Mathiasson, J .

41. P. Ghosh, S. Biswas, and C. Datta , J . Muter. Sci., 24,

42. L. Hua, P. Zadorecki, and P. Flodin, Polym. Comp.,

43. D. Maldas, B. V. Kok ta, and C. Daneault, J . Vinyl

44. P. Zadorecki and T. Ronnhult, J . Polym. Sci. Part A

45. M. H. Schneider and K. I. Brebner, Wood Sci. Tech-

Polym. Sci., 48 , 121-136 (1993).

2071 ( 1993).

Sci., 32 , 1734-1739 (1992).

and I ts Applications, Wiley, New York, 1985.

Textiles. Part ZI, Marcel Dekker, New York, 1977.

Polym. Sci., 37 , 751-775 (1989).

grain, J . Appl. Polym. Sci., 38 , 1987-1996 (1989).

9 1 ( 7 / 8 ) , S. 85-187 (1989).

Comp. Build. Constr., 69-76 ( 1990).

Appl. Polym. Sci., 49,197-208 ( 1993).

205-212 (1989).

8 ( 3 ) , 199-202 (1987).

Technol., 11 2 ) , 90-99 ( 1989).

Polym. Chem., 24 , 737-745 ( 1986).

nol., 19,67-73 (1985).

Received November 23, 1994Accepted August 18, 1995

properties and modification methods for vegetable fibers for natural fiber composites

Documents