Compounding and mechanical properties of biodegradablehemp fibre composites

Andreas Keller*

Swiss Federal Research Station for Agricultural Economics and Engineering (FAT), CH-8356 Taenikon b. Aadorf, Switzerland

Accepted 21 February 2003

Abstract

Up to now the reinforcing potential of hemp fibres has not been exhausted, as the fibres are bundled and, therefore, a homogenousdistribution of fibres and matrix has not been possible. In the present study the fibre bundles used for the composites were degummedby means of biological processes and steam explosion. The degummed fibres, separated into single cells, were integrated into the

brittle poly (3-hydroxybutyrate-co-hydroxyvalerate) matrix and into the ductile co-polyester amide matrix by means of a co-rotatingtwinscrew extruder. Composites with a fibre volume fraction of up to 42% could be achieved. The tensile strength of the ductilematerial was almost doubled by the reinforcement with 27% of fibres to 30 MPa, the E-modulus was quadrupled to 3.5 GPa. No

improvement of the tensile strength of the brittle matrix could be achieved, whereas its E-modulus was increased up to 6 GPa. As thecomposite behaviour is determined by the matrix, the fibre reinforcement is accompanied by a reduction of the impact strength.# 2003 Elsevier Science Ltd. All rights reserved.

Keywords: A. Polymer-matrix composites (PMCs); A. Short-fibre composites; B. Mechanical properties; B. Stress/strain curves; E. Injection moulding

1. Introduction

Plants contain elements which fulfil a load bearingfunction. These elements frequently take the form offibre composites. Thus, for instance, wood or grassstalks consist of cellulose fibres embedded in a matrix oflignin, pectin and hemicellulose. Many of these naturallyoccurring composites display mechanical properties whichpermit their use in technical areas. Low plasticity, how-ever, limits their application in designing components ofcomplex geometry. This drawback may be circumventedby extracting natural fibres from the plant using mechan-ical processes and joining them with natural or syntheticpolymers to form new composites suitable for industrialprocessing. Natural nonwoven fibres, usually impregnatedwith polypropylene as a matrix in present-day applica-tions, are already used to manufacture mouldings [1].Compared with composites from glass fibre poly-propylene, however, natural fibre composites are of lowstrength and impact strength, which is a disadvantage instructural applications. This is because of the low adhesionbetween fibre and matrix and because of the flaws intro-

duced by fibre bundles [2–16]. These fibre bundles can beseparated into biological fibre cells [16–18]. Several chemi-cal, enzymatic and physical degumming methods havebeen developed for that purpose [18–21].If biodegradable polymers are reinforced with

degummed fibres, these materials have sufficientmechanical properties to be used as lightweight con-struction materials. In particular bast fibres, such ashemp or flax, are suitable for polymer reinforcementwithout the loss of the materials’ biodegradability. Theirweight specific mechanical properties are comparable tothose of glass fibres [22–25]. The present study focuseson the production of injection moulded materials withhemp fibres broken down into single cells. It also makessuggestions for optimising the mechanical properties.

2. Materials and methods

2.1. Fibres

Fibre composites were produced using hemp fibresseparated by the steam explosion process [26,27] (DDA)on the one hand and hemp fibres degummed by thebiological separation process [21,28] (BIA) on the other.

0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0266-3538(03)00102-7

Composites Science and Technology 63 (2003) 1307–1316

www.elsevier.com/locate/compscitech

* Tel.: +41-52-368-31-31; fax: +41-365-11-90.

E-mail address: gabiandi@bluewin.ch (A. Keller).

The properties of both types of fibre are compared inTable 1. To determine fibre length distribution the fibreswere arranged end to end in a FL 101 Fibroliner andmeasured with an AL 100 Almeter. As length distribu-tion was highly asymmetrical, the medians were giveninstead of the mean values (Fig. 1).The fibre diameters were ascertained by scanning

electron microscope analysis (Fig. 2). Some fibre bun-dles were still present in the case of both the DDA fibresand the BIA fibres. However, their gum substances weredissolved (DDA) or degraded (BIA) to such an extentthat the bundles disintegrated into single fibre cells underthe shear stress in the compounding extruder and in theinjection moulding machine. Unlike the isolated bundlesof these degummed fibres, the bundles of fibres extractedby purely mechanical means did not disintegrate in thecompounding and injection moulding process, givingmarkedly poorer mechanical properties [16].

2.2. Polymers

Two different biodegradable matrix systems were used.A statistic co-polyesteramide (PEA) of petrochemicalorigin (‘‘BAK 2195’’, Bayer AG) and a poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV) fermen-

tatively produced by Monsanto Europe SA (‘‘BiopolD400 GN’’). Table 2 shows their most important prop-erties by comparison with polypropylene.

2.3. Compounding

Natural cellulose fibres with their hydroxyl groups arehydrophilic. They absorb between 8 and 13% waterfrom the atmospheric air [25]. To prevent gas formationduring the compounding process, the fibres and poly-mers were dried under vacuum (p=100 mbar) for 16 hat 60 �C prior to treatment. The polymer chains of thebiodegradable polyesters used can be split under unfa-vourable compounding conditions such as high localtemperatures or pressures and high shear. The organicacids produced by splitting the ester groups in turnsupport the hydrolytic degradation of the cellulosefibres [29,30]. The risk of such fibre degradation canalso be minimised by prior drying of the fibres.Compounding of the injection moulded granulate was

carried out on a Brabender PL 2100-6 co-rotating twin-screw extruder. This extruder had a screw diameter of25 mm with an L/D ratio of 22. A circular nozzle 6 mmin diameter was used for extrusion. The extruded strandwas cooled in the air and then granulated in a cuttingmill (H. Dreher GmbH & Co. KG, model 915/20 L).Because of the irregular fibre feed, manual metering

of the tangled fibres not only produced non-homo-genous granulate with poorly wetted fibres, but alsocreated blockages in the nozzle inlet area, particularly athigher fibre volume levels. The fibres were thereforeeither spun into a thread with a specific mass of 2 g/m(BIA) or transferred into fleeces (1.7 g/m) (DDA). Inthis form the fibres were automatically drawn throughthe fibre metering orifice by means of the rotatingscrews. It was then possible to adjust the fibre contentusing polymer granulate metering and the rotational

Table 1

Comparison of hemp fibres from steam explosion separation (DDA)

and biological separation (BIA) [21,24,36,37]

DDA

BIA

Median fibre length (quartiles) (mm)

8.0 (10.4; 6.6) 15.2 (22.6; 8.2)

Mean fibre diameter (mm)

23�6 25�8

Gum fraction (%)

15.85 17.85

Tensile strength (MPa)

900 534�150

Tensile modulus (GPa)

30–90 30–90

Elongation at break (%)

1.6–3.5 –

Fig. 1. Fibre length distribution of DDA hemp and BIA hemp. With a closer fibre length distribution the DDA fibres are markedly shorter (median

8.00 mm) than BIA fibres (15.2 mm), where a higher proportion of fibres are over 25 mm in length.

1308 A. Keller / Composites Science and Technology 63 (2003) 1307–1316

speed of the screws. It was also possible to draw in twothreads or strips simultaneously in order to achieve highfibre levels at low rotational speeds. However the speedof rotation was not allowed to fall below 50 min�1, asthe risk of blockage in the nozzle inlet area increases asthe speed of rotation decreases. The screw configurationand fibre intake location were optimised to minimisefibre damage during the compounding process. Figs. 3and 4 show the different screw configurations designedfor the two types of fibre. Screw configuration A com-prises merely a fibre compounding length of 112.5 mmwith no kneading elements. In this way it was possibleto homogeneously introduce DDA fibres with a median

fibre length of 8.0 mm into the matrix. The BIA fibres,which were longer than the DDA fibres, could not becompounded using screw configuration A, as they twis-ted together during the process. This resulted in insuffi-cient fibre wetting and blockages in the nozzle inlet. Thegreater length (15.2 mm) of the BIA fibres comparedwith the DDA fibres (8.0 mm) played a crucial part intheir tendency to twist. Twisting was not due to the factthat the fibres were present as thread, because the sameproblems occurred when loose fibres were meteredmanually. The first measure was to move the fibre feedback to increase the compounding length, but this didnot significantly improve compounding behaviour.Screw configuration B functioned only when an addi-tional kneading element was inserted into the fibrecompounding section (Fig. 4).The temperature pattern in the extruder’s five con-

trollable zones (Table 3) was adjusted so that there wasa homogeneous melt at the fibre feed orifice and thepolymer matrix suffered the minimum possible thermaldamage.

2.4. Injection Moulding

The compounded granulates were extruded in theform of tensile test specimens with a cross-sectional area

Table 2

Properties of the biodegradable polymers BAK 2195 (PEA) and Bio-

pol D400 GN (PHBV) (polypropylene is shown as a comparison)

BAK 2195

(PEA)

Biopol D400 GN

(PHBV)

Polypropylen

(PP)

Glass transition Tg (�C)

– �1 �10

Melting point (�C)

175 153 161

Cristallinity (%)

0 58 70

Density (kg/m3)

1180 1250 900

Tensile strength (MPa)

16.4�0.2 25.9�1.3 38

Tensile modulus (MPa)

418�51 2382�85 1700

Elongation at break (%)

102�17 1.4�0.1 400

Fig. 2. The fibre diameters of the DDA fibres (left) and the BIA fibres (centre) vary between 10 and 45 mm with a mean value of 24 mm. Isolated fibre

bundles are still retained after fibre separation. An enlarged image of such a fibre bundle is shown on the right (BIA fibres).

Fig. 3. Screw configuration A, optimised for compounding DDA hemp fibres.

A. Keller / Composites Science and Technology 63 (2003) 1307–1316 1309

of 5 mm � 3 mm on a Ferromatik Milacron K40/80injection moulding machine with a K30 injection unit.The screw diameter was 25 mm with an L/D ratio of 24.The screw compression ratio was 2.03. The processparameters are summarised in Table 4.The fibre lengths and their distribution in the compo-

sites were determined after dissolving the samples inmetacresol (PEA) and trichloromethane (PHBV) atroom temperature [16]. The fibre mass fraction Xfibre ofthe specimens was calculated using the nitrogen masscontent XN measured by means of elementary analysis(LECO CHN-900) according to Eq. (1):

Xfibre ¼XNcomposite � XNmatrix

XNfiber � XNmatrixð1Þ

At known fibre and matrix densities � the fibre masscontent Mf could be converted into the fibre volumefraction Vf (�Fibre=1500 kg/m3) according to (2):

Vf ¼Mf � �Matrix

Mf � �Matrix þ 1�Mf

� �� �Fibre

ð2Þ

Fibre volume fractions between 0 and 0.42 wereobtained.

2.5. Tensile testing

Tensile testing of the extruded test specimens wascarried out on a ‘‘1456 Zwick Universal TestingMachine’’ at a test speed of 2 mm min�1 following 1week’’s storage at room temperature and 65% relativeatmospheric humidity. The gauge length was 45.5 mm,the measurement length for determining elongation 20mm. According to Standard ‘‘ASTM D 3009/D3039M—93’’ the modulus of elasticity for fibre compo-sites with an elongation at break above 1.2% is calcu-lated as the slope of the stress–strain curve at anelongation between 0.1% and 0.6%. For materialswhere elongation at break is less than 1.2%, the mod-ulus of elasticity must be calculated at an elongationbetween 0.1% and 0.3%. The materials examined in thisstudy had elongations at break partly in excess of 1.2%and partly below 1.2% and demonstrated non-linearbehaviour even at low elongations. To counter this non-linearity and to determine the moduli of elasticity inde-pendently of elongation at break, an initial E0-moduluswas introduced according to [31]. This could be com-pared directly with the modulus of elasticity from modelcalculations predicting the modulus at elongationsaround 0%.

�=" ¼ E0 � 1�D1 � "ð Þ ð3Þ

The initial E0-modulus was determined by linearregression of the measuring points from the tensile testaccording to Eq. (3). The measuring points at elonga-tions in the vicinity of elongation at break deviatedgreatly from the regression lines. The same applied tomeasuring points below 1% elongation, as these werestrongly influenced by the initial tension of 40 Nrequired for technical reasons. Measuring points wherethe �/" values deviated from the regression straight lines

Fig. 4. Screw configuration B, optimised for compounding BIA hemp fibres.

Table 3

Compounding parameters for the extruded granulate produced

Polymer

Fibre Fibre

fraction

(% vol.)

Screw design/

screw speed

(min�1)

Zone temperatures

(g min�1)

T1 T2 T3 T4 T5

PEA

37 DDA 6 A / 609>>

190

PEA

30 DDA 15 >>>>A / 54

PEA

20 DDA 20 =

A / 60

185 180 180 180

PEA

18 DDA 27 >>>A / 82

PEA

13 DDA 42 >>>;A / 90

PEA

20 BIA 30 B / 150

PHBV

20 BIA 30 B / 150 195 190 180 165 165

Table 4

Extrusion parameters for the production of the tensile test specimens

Polymer

Fibre Vf

(% vol.)

Zone temperatures

pE(bar)

pN(bar)

TW

(�C)

tK(s)

T1

T2 T3 T4

PEA

– 0 140 160 170 180 40 35 25 8

PEA

DDA 6 140 160 170 180 28 31 25 10

PEA

DDA 15 140 160 170 180 28 31 25 10

PEA

DDA 20 140 160 170 190 29 31 25 12

PEA

DDA 27 140 160 170 195 29 31 25 12

PEA

DDA 42 140 160 180 210 50 55 25 12

PEA

BIA 30 140 160 170 190 30 40 40 11

PHBV

– 0 140 160 170 175 40 45 60 10

PHBV

BIA 30 140 160 170 175 10 15 60 10

Vf=fibre volume content, pE=injection pressure, PN=dwell pressure,

tK=cooling time.

1310 A. Keller / Composites Science and Technology 63 (2003) 1307–1316

by over 20 MPa were therefore iteratively removed forthe E0 calculation.

2.6. Impact strength

Impact strength was determined directly on theunnotched injection moulded tensile test specimens usinga Zwick pendulum impact testing machine. The supportwidth was 30 mm, pendulum length 230 mm and pendu-lum energy 4.0 J, with the direction of impact vertical tothe breadth (5 mm) of the tensile test specimen.

3. Results and discussion

3.1. Fibre length distribution

Fibre length distribution in the injection mouldedtensile test specimens barely deviated from that of thegranulates (Fig. 5). The injection moulding processtherefore caused no further shortening of the fibres.Table 5 summarises the geometric fibre data.The DDA fibres in the composite were of similar dia-

meter but were 4 times as long as the BIA fibres andtherefore had a significantly higher aspect ratio than theBIA fibres.Fig. 6 shows the resultant fibre length distribution of

the granulates for the different materials compoundedwith steam explosion separated hemp.Where the screw configuration was the same it was

found that the resultant fibre length distribution in thegranulate was independent of the fibre volume content.The main proportion of the fibres compounded with thePEA matrix had lengths of between 0.3 and 0.7 mm.These granulates had a median fibre length of 0.53 mm.

In screw configuration B the fibres were subjected togreater mechanical stress than in configuration A, and,because of the longer dwell time, to greater thermalstress. The granulates compounded with screw con-figuration B therefore also had shorter fibre lengths of0.11 mm (median) compared with 0.53 mm for the fibrescompounded using screw configuration A (Fig. 7).Analysis of the fibre lengths in the granulates showedthat it was advantageous to feed the compoundingextruder with fibres below 10 mm in length. Longerfibres required more stringent compounding conditionsif they were to be satisfactorily dispersed. The highershear stresses thus applied shortened the fibres into asmaller resultant length than the fed shorter fibres com-pounded under milder conditions.

3.2. Stress/strain curves

The diagrams show non-linear matrix-dominatedbehaviour of the composites (Fig. 8). It is known thatfibre ends in the composite cause increased levels ofelongation in the fibre–matrix interface of adjacentfibres. These can produce breakdown at the interface[32] when there is insufficient fibre–matrix adhesion andmatrix elongation at break. The greater the fibre contentand the shorter the fibres, the greater the number of

Fig. 5. The fibre length distribution of the fibres in the tensile test specimens coincided with that of the granulate. The fibres underwent no further

shortening in the injection moulding process. The histograms are based on the measurement of approx. 2000 fibres of PEA–DDA composites of

varying fibre content in each case.

Table 5

Fibre lengths and diameters of both fibre types (DDA and BIA) in the

extruded tensile test specimens

Fibre

Median fibre length

(quartile) (mm)

Ø Fibre diameter

(mm)

Aspect

ratio

DDA

0.53 (0.38; 0.82) 23�6 23

BIA

0.10 (0.03; 0.24) 25�8 4

A. Keller / Composites Science and Technology 63 (2003) 1307–1316 1311

fibre ends present to introduce such flaws. So on theone hand the fibres have a reinforcing effect and onthe other hand they have a fracture-generating effectat the interface of adjacent fibres. In the PEA–DDAcomposites studied the fibres had a reinforcing effectup to a fibre content of 30% vol. At higher fibrelevels this effect again decreased, as there the fracture-generating effect of the fibres increased considerably(Fig. 9).It was possible to increase the tensile strength of the

PEA materials by reinforcement with 27% vol. of DDAfibres from 16.4 to 29.4 MPa.It was also possible to measure the described effect of

fibre length on the mechanical properties. With a fibrecontent of 25% vol. the BIA fibres in the composite,one quarter the length of the DDA fibres, showed a

reinforcing effect comparable to the DDA fibres with afibre content of 15% vol. At 2.3%, however, the elon-gation at break of the PEA–BIA composite was lessthan half that of the PEA–DDA composite of the samestrength. The elongation at break of the matrix had acrucial influence on the tolerance of the matrix to strainconcentrations induced by the fibres. This is shown bythe fact that it was not possible to increase the tensilestrength of the PHBV matrix by means of fibre com-pounding, unlike that of the PEA matrix ("PHBV=1.4%,"PEA=102%).As the E0-modulus is measured in a low elongation

range long before material damage occurres, it does notreact sensitively to fibre length and induced stresses. Themeasurements showed a linear rise of the E0-moduluswith fibre content (Fig. 10).

Fig. 6. Fibre length distribution for granulates compounded using screw configuration A. There was no evidence that the fibre length distribution

had been influenced by the fibre volume content.

Fig. 7. The BIA fibres, which had to be compounded using the harsher screw configuration B, were shortened more than the DDA fibres com-

pounded using fibre preserving screw configuration A.

1312 A. Keller / Composites Science and Technology 63 (2003) 1307–1316

3.3. Impact strength

The impact strength of a short-fibre composite isdetermined by the energy required for the plastic defor-mation of matrix and fibres, for the fracture of matrix

and fibres, for fibre-matrix debonding in the interfaceand for overcoming the friction following the debond-ing during pull-out [33,34]. The fracture patterns of thePEA–DDA composites and of the PHBV–BIA compo-sites showed fibres with no adhering matrix fragments

Fig. 8. Stress/strain diagrams for the composites produced. Even where fibre content was higher the non-linear behaviour of the matrix remained

dominant. PEA: co-polyesteramide, PHBV: poly(3hydroxybutyrate-co-hydroxyvalerate), DDA: hemp fibres from steam explosion separation, BIA:

hemp fibres from biological separation.

Fig. 9. The strength of the PEA materials (high elongation at break) could almost be doubled by compounding with hemp fibres. The strength of the

PHBV materials (low elongation at break) could not be increased because of the stress concentrations introduced by the fibre ends.

A. Keller / Composites Science and Technology 63 (2003) 1307–1316 1313

Fig. 10. The E0-modulus showed a linear rise with fibre content.

Fig. 11. Fracture surfaces of the PEA with 6% vol. DDA fibres (left) and of the PHBV with 32% vol. BIA fibres (right). Unlike the brittle PHBV

matrix, the ductile PEA matrix when fracture forms zones of plastic deformation which leave a halo around the fibres. In the PHBV–BIA composite

there are cracks in the matrix which do not lie in the fracture plane. The fibres were partially fibrillated in both composites.

Fig. 12. The impact strength of the PEA–DDA composites decreased as the fibre content increased. That of the PEA–BIA composite, where the

fibre content was comparable, was only marginally lower despite the BIA fibres being shorter than the DDA fibres. The composite with the brittle

PHBV matrix had the lowest impact strength.

1314 A. Keller / Composites Science and Technology 63 (2003) 1307–1316

on the surface (Fig. 11). The fracture ran along theinterface between fibre and matrix, indicating low fibre–matrix adhesion [35]. Debonding energy was thereforelow and fibre pull-out prevailed over fibre fracture. Thebrittle hemp fibres show practically no plastic deforma-tion. The fracture energy of the systems investigated wastherefore determined mainly by the plastic deformationof the matrix and by the pull-out energy.The elongation at break of the matrices used, PEA

(102%) and PHBV (1.4%), were very different. Theytherefore also showed a different fracture pattern in thecomposite. The PEA matrix, which had zones of plasticdeformation after fracture, was able to absorb morefracture energy than the low-deformation breakingPHBV matrix. This was expressed in the PEA compo-site having double the impact strength of the PHBVcomposite with the same fibre content (Fig. 12). Theimpact strength of the PEA–BIA composite was onlymarginally less than that of the PEA–DDA compositewith comparable fibre content, despite the fact that theBIA fibres in the composite were one quarter the lengthof the DDA fibres. This indicates that the pull-outenergy, which rises as fibre length increases, plays only asubordinate role to plastic deformation energy in thePEA composites.

4. Conclusions

4.1. Compounding

The compounding of degummed hemp fibres withbiodegradable thermoplastic polymers is possible ontraditional twin-screw extruders up to a fibre volumecontent of 50%. Degummed bast fibres do not free flow,so they cannot be metered using conventional devices.As fleece or spun yarns they can automatically be fedthrough the fibre metering orifice by rotation of thescrew. The development of a metering unit for non-pourable fibres would be beneficial for the industrialimplementation of the process, as this would avoid theexpense of producing fleece or yarn. It would also besimpler to set the metering quantity independently ofscrew rotation. To preserve the fibres during com-pounding, the compounding distance must be as shortas possible and must exclude kneading elements. If thefibres fed are shorter than 10 mm, a homogenous fibredistribution in the extrudate can be achieved using afibre compounding length of only 113 mm (4.5 D)without kneading elements.

4.2. Mechanical properties

In the composites with a PHBV matrix it was notpossible to achieve any increase in strength by fibrecompounding, as the stress/strain concentrations

induced by fibre ends were not absorbed by the brittlematrix. In PHBV only the modulus of elasticity could beincreased by short-fibre reinforcement, though this wasat the expense of impact strength. The ductile PEAmatrix was more tolerant of the stress/strain concentra-tions introduced by the fibres. The E0-modulus attainedwith PEA–DDA composites exceeded that of non-rein-forced polypropylene. The strength achieved with PEA-DDA composites was 23% below that of poly-propylene. The strength of these degradable materialscould be further increased if the fibres in the componentwere longer than the measured 0.53 mm. The com-pounding process would have to be changed to achievethis. The pultrusion of lightly twisted bast fibre yarns ordirect compounding in the injection moulding machinewould be feasible.

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