influence of natural fibres on the mechanical properties of biodegradable polymers

Industrial Crops and Products 8 (1998) 105–112

Influence of natural fibres on the mechanical properties ofbiodegradable polymers

Martina Wollerdorfer a,*, Herbert Bader b

a Institute for Agrobiotechnology, Department of Natural Materials and Packaging Technology, Konrad Lorenz Straße 20,3430 Tulln, Austria

b Fraunhofer Institute for Food Technology and Packaging, Department of Materials De6elopment, Giggenhauserstraße 35,85354 Freising, Germany

Received 17 July 1996; accepted 3 October 1997

Abstract

Fibre reinforced plastics are used whenever there is the need for very high mechanical properties combined with lowweight. In that respect natural fibres are of basic interest since they not only have the functional capability tosubstitute the widely used glass fibres but they also have advantages from the point of view of weight andfibre–matrix adhesion, specifically with polar matrix materials. They have good possibilities in waste management dueto their biodegradability on the one hand and their much lower production of ash during incineration on the other.The influence of plant fibres such as flax, jute, ramie, oil palm fibres and fibres made from regenerated cellulose onthe mechanical properties of biodegradable polymers was investigated using thermoplasts like polyesters, polysaccha-rides and blends of thermoplastic starch. The composites were produced by extrusion compounding with a co-rotatingtwin screw extruder. The pellets obtained were further processed into tensile test bars by injection moulding.Depending on the kind of polymer, a fibre content of 20–35% could be achieved. Generally a considerable tensilestrength improvement of polyesters could not be observed. However the chemical similarity of polysaccharides andplant fibres, which consist mainly of cellulose, resulted in an increased tensile strength of the reinforced polymers. Forreinforced thermoplastic wheat starch, it was four times better (37 N/mm2) than without fibres. The reinforcement ofcellulose diacetate and starch blends caused a stress increase of 52% (55 N/mm2) and 64% (25 N/mm2), respectively.© 1998 Elsevier Science B.V. All rights reserved.

Keywords: Fibre reinforcement; Composite material; Plant fibres; Biodegradable polymers

* Corresponding author. Tel: +43 2272 66280306; fax: +43 2272 66280303; e-mail: [email protected]

0926-6690/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved.

PII S0926-6690(97)10015-2

M. Wollerdorfer, H. Bader / Industrial Crops and Products 8 (1998) 105–112106

1. Introduction

Fibre reinforced polymers, specifically withglass fibres, have gained importance in technicalapplications such as the automotive sector,where high mechanical properties and dimen-sional stability have to be combined with lowweight. During the last few years many effortshave been made to investigate the suitability ofnatural fibres as a reinforcing component forthermoplastic and injection mouldable materialsbecause of their low density and ecological ad-vantages. They open up further possibilities inwaste management as they are biodegradableand therefore can lead to highly functional com-posite materials if used in combination withbiodegradable thermoplastic polymers.

The current biodegradable polymers may bedivided into synthetic and natural polymers,while the latter are classified into those of plantand microbial origin (Endres and Pries, 1995).The degradation of such polymers includes thedisintegration into their monomers. Thereforeunstable and hydrolyzable linkages are required,where chemical, biological or photochemical re-actions take place. Some polyesters (e.g. poly-b-hydroxybutyrate) and polysaccharides likethermoplastic starch or modified cellulose arewell suited for that purpose. On the one hand,water solubility raises the degradability, but onthe other, in most cases a water-resistant mate-rial should be obtained for most applications.This problem can be avoided by blending suchsoluble polymers like thermoplastic starch withinsoluble polyesters which additionally showvery good manufacturing properties (Fritz et al.,1995).

As these materials have limited mechanicalproperties for several applications, an increaseof tensile strength can be achieved by com-pounding them with fibres provided that thefibres show a higher tensile strength andYoung’s modulus and a lower elongation thanthe matrix (Ehrenstein, 1992). The quality of afibre reinforced composite depends considerablyon the fibre–matrix interface because only awell formed interface allows stress transfer fromthe matrix to the fibre. The extent of the adhe-

sion between fibre and matrix can be describedby the critical fibre length resulting from thebalance of interface shear force and normalforce in the fibre. Sufficient adhesion, low fibrediameter and high tensile strength allow shortcritical fibre lengths. The maximum tensilestrength of fibres can only be exploited if theycan be stressed until they break without beingpulled out. The critical fibre length can be esti-mated by the surface of fracture since the aver-age pull out length of a fibre cannot be longerthan half of the critical fibre length. This is im-portant during extrusion compounding and in-jection moulding when fibres are shortenedconsiderably. The tensile strength of the com-posite is proportional to the volume fraction offibres assuming that the orientation of the fibresand the direction of the applied stress are paral-lel (Voss, 1987; Gachter and Muller, 1989).

Because of their static function in plant tis-sues, bast (e.g. flax, ramie, jute or hemp), leaf(e.g. sisal) and fruit fibres (e.g. cotton) are alsoexpected to act as reinforcing components incomposite materials. Information concerning themechanical properties of fibres can be found inthe literature. The tensile strength of flax rangesfrom 254–390 N/mm2 (Folster and Michaeli,1993) to 1100 N/mm2 (Flemming et al., 1995),that of jute varies between 187 N/mm2 (Hinrich-sen, 1991) and 533 N/mm2 (Flemming et al.,1995). Ramie has a very high tensile strength at850–900 N/mm2 (Hock, 1995). There are severaladvantages of plant fibres in comparison to cus-tomary glass or carbon fibres. Besides their lowdensity they reduce the abrasion of screw andbarrel and the energy input for distributingthem in the polymer melt. The low sensibilityagainst mechanical treatment results in longerfibres. Finally the biodegradability of this renew-able raw material is important (Kohler andWedler, 1994; Mieck and Reußmann, 1995).While processing plant fibres, their thermal sta-bility is limited to temperatures below 200°C.One problem that arises is the high variation ofmechanical properties due to climatic and grow-ing conditions, the different preparation meth-ods and water adsorption (Flemming et al.,1995).

M. Wollerdorfer, H. Bader / Industrial Crops and Products 8 (1998) 105–112 107

2. Materials and methods

2.1. Matrix polymers and fibres

Available polyesters, polysaccharides andblends of thermoplastic starch were selected forthe experiments. Polybutylene succinate adipatecopolymer is a biodegradable, aliphatic polyester(Bionolle®, type 3020, Showa Highpolymer,Tokyo). The poly-b-hydroxybutyrate copolyester(Biopol® D300G) was obtained from Zeneca BioProducts (Billingham, UK). Cellulose diacetate(Biocell® 163) was supplied by Tubize Plastics(Groupe Rhone-Poulenc, Belgium). For the pro-duction of thermoplastic starch (TPS), wheatstarch (Definol WN) from Crespel and Deiters(Ibbenburen, Germany) was plastified with sor-bitol (DHW Deutsche Hydrierwerke Rodleben,Roßlau/Elbe, Germany) and glycerol (Fauth,Mannheim, Germany). Commercially availablethermoplastic starch blends used in this studywere Bioplast GS 902 (Biotec, Emmerich, Ger-many), a blend consisting of potato starch,modified cellulose and synthetic polymers, andMater-Bi®, type ZI01U (Montedison Deutsch-land, Eschborn, Germany). This blend consists ofcorn starch and a biodegradable polyester. Afurther starch blend was produced by compound-ing thermoplastic wheat starch with 40% poly(e-)caprolactone (PCL) of the type Tone® PolymerP-787 from Union Carbide Benelux (Antwerp,Belgium).

Flax and jute were delivered as long fibres fromFuessener Textil (Fuessen im Allgau, Germany)Jute-Spinnerei and Weberei Bremen (Delmen-horst, Germany), respectively. The ramie fibreswith a length of 12 mm were obtained fromFischer Doticon (Switzerland). Oil palm fibres area by-product of the palm oil industry. They weresupplied by the Palm Oil Research Institute ofMalaysia. Long fibres made from regenerated cel-lulose were delivered from Lotteraner Wustner(Mellau, Austria).

As resins are usually used as bonding agents forglues and varnishes they also seemed to be suitedto improve the fibre–matrix interface. The ap-plied resins based on colophony (Kolophonium)and maleinate-modified colophony (Rokramar

3030 and Rokramar VP 1681) were obtained fromRobert Kraemer (Delmenhorst, Germany).

2.2. Compounding of thermoplastic starch (TPS)

During the high-temperature short-term reac-tion in an extruder, partly crystalline starch gran-ules can be converted into a homogeneous plasticmatrix. The degree of conversion is determined bythe mechanical shear, the thermal stress as afunction of temperature and time and the contentof water and other plasticizers for adjusting theglass transition temperature (Wiedmann and Stro-bel, 1991).

The plastification of starch was carried out witha co-rotating twin screw extruder (Leistritz, typeLSM 30.34, Germany) at 115°C and 50 rev./min.The screws with a diameter of 34 mm and a L/Dratio of 26.47 were equipped with kneading ele-ments. To prevent the plastified starch from ex-panding at the nozzles, surplus steam wasremoved with a degassing zone operating at atmo-spheric pressure. After leaving the nozzles with adiameter of 2 mm, the extrudate was pelletized.

Investigations have shown that the thermalstress can be reduced by increasing the watercontent of starch to 22%—its maximal equi-librium moisture content. Glycerol (86% in water)and sorbitol were identified as the most suitableplasticizers (Utz et al., 1994). The mixing ratio ofsorbitol and moist starch was 1:6.4. About 13%glycerol was added to get a well disintegratedthermoplast.

2.3. Fibre compounding

For compounding the fibre reinforced com-posites, the same extruder was equipped with a4-mm nozzle and a new screw configuration. Thekneading elements were substituted by transportelements to minimize the shearing of the fibres.Because of the low bulk density an automaticdosing of the fibres was impossible so the fibreshad to be fed manually. The resins were pulver-ized and mixed with the polymer pellets. Thetemperature profile along the extruder barrel wasadjusted according to the requirements of thedifferent polymers. The screw rotation speed was50 rev./min.


Table 1Processed polymer-fibre composites with the fibre content in % (w/w)

Ramie Jute Oil palm fibre Cellulose fibreFlax

Polyesters25 252525Bionolle® 25

25aBiopol® 25

Polysaccharides25Biocell® 15/25/35 25

TPS 10/15a/20 15

Starch blends15Bioplast® 15/25/35

Mater-Bi® 15/20/25 1515TPS/PCL 15

a These compounds have also been alloyed with resins.

The pellets were processed into tensile speci-mens according to DIN 53455 by injection mould-ing (Boy 50M, Germany). The followingparameters were adjusted for the various compo-sitions: temperature profiles, injection and coolingtime, mould temperature and pressure.

Table 1 shows the processed compounds withtheir fibre contents in % (w/w). These combina-tions of polymers and fibres were investigatedregarding their compatibility and reinforcing ef-fect. This work concentrated particularly on ther-moplastic starch and its blends, as these arerenewable raw materials, and on flax as a Eu-ropean fibre plant. In most cases the maximumfibre content was restricted by insufficient distri-bution in the matrix or the limited extruder per-formance. The increasing fibre contents in somecompounds should demonstrate the correlationbetween fibre content and tensile strength.

2.4. Analytical methods

According to DIN 53455, the tensile strengthwas tested on 10 specimens after conditioning for3 days at 23°C and 50% relative humidity. Thetesting machine (Schenk Trebel, type RM 50 kN,Germany) was operated with a velocity of 100mm/min.

The fibre length was analyzed in some selectedreinforced pellets and injection moulded speci-mens to investigate the effect of fibre content,processing step and matrix polymer. According to

the method suggested by Thieltges (1991), thematrix polymer was dissolved in suitable solventsover several days at a temperature of 70°C: waterfor TPS, acetone for Biocell and ethylacetate forBionolle. The fiber suspension produced was dis-tributed on an object slide of a microscope. Com-puter aided measurements were carried out on 150fibres of each sample.

3. Results and discussion

3.1. Mechanical properties

Figs. 1–7 demonstrate the tensile strengths ob-tained for several fibre reinforced compoundscompared to the pure matrix.

The tensile strength of Bionolle compounds in-creased only in combination with ramie. An

Fig. 1. Tensile strength of Bionolle®–fibre compounds.


Fig. 2. Tensile strength of Biopol®–fibre compounds. Fig. 4. Tensile strength of TPS–fibre compounds.

amount containing 25% fibres could be processedquite well. With oil palm fibres, the tensilestrength decreased, basically because of the highcontent of impurities leading to fractures in thematrix. Apart from the oil palm fibre compound,the Young’s modulus of about 460 N/mm2 couldat least be doubled. The elongation was consider-ably reduced from 274% to about 5%.

The mechanical properties of Biopol com-pounds behaved in a similar manner. Fig. 2 showsthat the application of the resin Rokramar 3030gave only a negligible improvement.

Compared with a fibre content of 25%, anincrease up to 35% did not improve the tensilestrength of the Biocell 163 composite due to in-sufficient incorporation of this high fibre amount(Fig. 3). The elongation of 9.1% was reduced tovalues between 3.0 and 4.2%. Also in this case, theinitial Young’s modulus of about 1500 N/mm2

increased by 80%. The high processing tempera-tures of Biocell 163, ranging from 180 to 200°C,caused thermal stress on the fibres which becameapparent in combustion smell.

Remarkable results were achieved by com-pounding thermoplastic starch with plant fibres. Itwas possible to increase the initial tensile strengthby four times up to nearly 37 N/mm2. Fig. 4demonstrates the correlation between the contentof flax fibres and the tensile strength. The elonga-tion was reduced from 45% to about 1.3–2.0%.The Young’s modulus of thermoplastic starch(420 N/mm2) behaved analogous to the tensilestrength as a consequence of compounding withfibres. A higher fibre content could not be ob-tained because of the great increase in the process-ing viscosity. Two other kinds of resins weretested as Rokramar 3030 was too sticky for pro-cessing fibre composites with thermoplasticstarch. Both resulted in a significant improve-ment. The tensile strength of compounds with15% flax and 1% resin was the same as with 20%fibres.

Very different results were obtained by the fibrereinforced starch blends. The independence of thefibre content with respect to the missing reinforce-

Fig. 3. Tensile strength of Biocell®–fibre compounds. Fig. 5. Tensile strength of Bioplast®–fibre compounds.


Fig. 6. Tensile strength of Mater-Bi®–fibre compounds.

Fig. 8. Fibre length distribution in TPS–compound with 10%flax.

ment of Bioplast compounds was probably a re-sult of incompletely disintegrated starch grains(Fig. 5). The tensile strength of Mater-Bi risesconsiderably as an effect of compounding but asFig. 6 shows there is no obvious increase with aflax content greater than 15%. While the elonga-tion decreased a little, the Young’s modulus rosefrom 540 N/mm2 to about 1400 N/mm2. Apartfrom the missing reinforcement by ramie, theTPS/PCL compounds have similar properties tothose based on Mater-Bi.

3.2. Fibre length studies

Some compounds with either good or missingreinforcing effects were selected to study the dis-tribution of the fibre length. Figs. 8–10 demon-strate the influence of the processing stepsextrusion and injection moulding and the fibrecontent on the resulting fibre length in TPS–flaxcompounds. The peak becomes narrower andshifts to fibre lengths of 101–200 mm with in-

creasing fibre content. But there are hardly anyfibres which are shorter than 100 mm. The fibresare also shortened by injection moulding. Buteven these short fibres provide a considerablereinforcing effect. Scanning electron microscopy(SEM) images show a smooth fracture surface,indicating that no fibres have been pulled out.Therefore extremely short critical fibre lengths canbe assumed for this compound.

The Biocell–flax compound has a similar distri-bution but still with a peak maximum at 201–300mm (Fig. 11). The SEM image of the fracturesurface (Fig. 12) demonstrates that the criticalfibre length of this compound is about 150 mm,but the residual fibres longer than 150 mm aresufficient to increase the tensile strength.

Fig. 9. Fibre length distribution in TPS–compound with 15%flax.Fig. 7. Tensile strength of TPS/PCL–fibre compounds.


Fig. 10. Fibre length distribution in TPS compound with 20%flax.

Fig. 12. Fracture surface of Biocell®–flax compound.For the Bionolle–flax compound, the wide dis-

tribution with quite long fibres (Fig. 13) mayresult from the low melt viscosity of Bionolle andthereby the low mechanical stress during the com-pounding process. But nevertheless no reinforce-ment is present as the critical fibre length of a fewhundred mm (Fig. 14) cannot be obtained by themajor part of the fibres.

4. Conclusion

From the material side, the fibre content islimited by insufficient compatibility or high vis-cosity on the one hand, and on the other by thedrastic shortening of fibres during processing. It

can be concluded that the fibre length distributiondepends in a very complex manner on the kind ofmatrix polymer and its rheological properties, thefibre content and the resulting interactions. Theresults show that polyesters are not very wellsuited as matrix polymers for native natural fibresas a considerable increase of tensile strength wasnot observed. On the other hand, the polysaccha-rides and most likely starch blends appear to becompatible matrices. The reasons are due to theremarkable intrinsic adhesion of the fibre–matrixinterface caused by the chemical similarity of suchthermoplasts and the plant fibres.

Fig. 13. Fibre length distribution in Bionolle®–compoundwith 25% flax.

Fig. 11. Fibre length distribution in Biocell®–compound with25% flax.


Fig. 14. Fracture surface of Bionolle®–flax compound.

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Gachter, R., Muller, H., 1989. Taschenbuch der Kunststoff-Additive, 3rd edn. Carl Hanser, Munich.

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Endres, H.-J., Pries, A., 1995. Mechanische Eigenschaftenstarkegefullter Polymerverbunde. Starch/Starke 47, 384–393.

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influence of natural fibres on the mechanical properties of biodegradable polymers

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