2000,mechanical properties of natural fibres

Applied Composite Materials7: 403–414, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Mechanical Properties of Natural Fibre MatReinforced Thermoplastic

KRISTIINA OKSMANSICOMP AB (Swedish Institute of Composites), Box 271, SE-941 26, Piteå, Sweden

Abstract. The use of natural fibres instead of man made fibres, as reinforcements in thermoplastics,gives interesting alternatives for production of low cost and ecologically friendly composites. Inthis work different commercially available semi-finished natural fibre mat reinforced thermoplas-tics (NMT) composites have been studied. Mechanical properties and microstructure of differentNMT composites were investigated and compared to conventional GMT (glass fibre mat reinforcedthermoplastic) composites and pure polypropylene (PP). The study included also NMT compositesmanufacturing processing parameters as processing temperatures and pressure during compressionmoulding. The results showed that NMT composites have a high stiffness compared to pure polymerand the NMT with a high fibre content (50% by weight) showed even better stiffness than the GMT.The GMT composites had superior strength and impact properties compared to the NMT whichmight be due to the relatively low strength of the natural fibres but also to poor adhesion to the PPmatrix. The NMT materials showed a large dependence on direction and are therefore believed tohave more fibres oriented in one direction. The stronger direction (0◦) of the NMT was in somecases as much as 45% better than the 90◦ direction.

Key words: flax fibres, natural fibres, thermoplastics, tensile properties, impact properties, anisotropy,electron microscopy, morphology, compression moulding, processing.

1. Introduction

During the last few years there has been an increasing environmental conscious-ness, which has increased the interest to use natural fibres instead of man madefibres in composite materials. Therefore several manufacturers have started to pro-duce semi-finished NMT materials. The advantages using natural fibres instead ofglass fibres are low density, lower price, low abrasive wear and they are virtuallyavailable everywhere. Further the natural fibres are recyclable, biodegradable andcarbon dioxide neutral and can be energy recovered in an environmentally accept-able way [1–3]. The natural fibres normally involved in these kinds of materialscan be for example flax, and the matrix is usually polypropylene (PP). The com-mercially available NMT materials are nonwoven fibre mats melt impregnated bypolymers in the form of sheets but also like a nonwoven fibre mat where the flax andpolymer fibres are mixed together in the form of needle punched fibre mats. Duringprocessing the mats or sheets need to be heated at first to the melting temperatureof the polymer matrix, and then compression moulded to products.

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Several investigations have been made to study the potential of natural fibres asreinforcement in thermoplastics [1–6]. The results have shown that natural fibreshave potential to be used as reinforcement for plastics but they do not attain thestrength level of glass fibre reinforced plastics [2–4]. Mieck et al. [2, 3] showed thatwhen flax fibres were used as reinforcements in thermoplastics (PP) the moduluswas increased to the level of glass fibre reinforced thermoplastics. Heijenrath andPeijs [4] reported that NMT composites made by film stacking method resultedin composites with comparable stiffness to conventional GMT composites whilethe strength of NMTs was lower. It is well known that the stress transfer effi-ciency between natural fibres and synthetic polymers is poor due to incompatibilitybetween the polar and hydrophilic fibre and nonpolar and hydrophobic polymer[6–8]. Several investigations have been made to study the adhesion between naturalfibres and synthetic polymers and the results showed that the composite strengthand toughness are significantly improved when coupling agents are used [3, 5–8].

Possible applications for NMT composites can for example be door panels,headliners, sun visors, spare-tire covers, seat foundations, instrument panels inautomotive industry.

Non-automotive applications can be for example floor boards, boards for acous-tic damping, shipping crates, pallets and furniture.

The objective of this work was to investigate processing parameters, mechan-ical properties and morphology of different semi-finished NMT composites andcompare those with conventional GMT.

2. Experiments

2.1. MATERIALS

Materials used in this study are semi-finished NMT from different manufacturerswhere two different types of processing techniques have been used:

1. PP melt impregnated fibre mat, available as semi-finished product in the formof sheet.

2. Nonwoven of PP fibres and flax fibres, available as semi-finished product in theform of needle punched fibre mat.

The advantages of using melt impregnated NMT compared to needle punchedmats can be lower moisture uptake during storing and in applications due to com-plete penetration of the polymer, material flow during compression moulding, eas-ier handling and lower transportation cost. The disadvantages are lower fibre con-tent than in needle punched mats. Further, it is believed that the materials are notisotropic which results in different mechanical properties in different directions.The needle-punched mats have long fibres which are mostly oriented in one di-rection (machine direction) while the melt impregnated materials are expected to

MECHANICAL PROPERTIES OF NATURAL FIBRE MAT REINFORCED THERMOPLASTIC 405

Table I. Sample codes of the various NMT composites Material types, fibre fraction(% by weight), density and approximate price.

Materials Fabrication Fibre content by Density Price

and codes method weight (%)(a) (g/cm3) (DM/kg)

PP – – 0.90 –

Symalit (S) melt impregnated 25 1.00 5.4

Isosport (I) melt impregnated 37 1.04 4.7

Danflax (D) needle punched 50 1.16 3.5

Mühlmeier (M) needle punched 50 1.13 6.5

Symalit (GMT) melt impregnated 40 1.24 6.0

a Data from the manufacturer.

Figure 1. Processing steps of semi-finished NMT materials.

be more isotropic but the fibres are oriented due to the flow during compressionmoulding.

The composition of the studied materials is shown in Table I.Tested NMT composites were from Symalit AG, Switzerland; Isosport, Austria;

Danflax, Denmark and Mühlmeier GmbH, Germany. According to supplier data,the NMT materials are reinforced with flax fibres and the fibre content is varyingbetween 25–50 wt%. The GMT had a glass fibre content of 40 wt% and was meltimpregnated and supplied from Symalit GMT, Lenzburg, Switzerland. The densityof compression moulded materials was measured by using a standard test methodfor plastics (Accupyc 1330).

2.2. MANUFACTURING COMPOSITES

The processing of the semi-finished NMT materials is made through four steps(shown in Figure 1):

1. Cutting blanks of the sheets (or nonwoven fibre mats).2. Heating the blanks above PP melting temperature.3. Transport from oven to the press.4. Moulding of the heated materials by compression moulding.

Rectangular blanks with the width of 350 mm and the length of 450 mm wereheated in a combined hot air and infrared (IR) oven using a temperature range of200–220◦C and afterwards quickly moved to the conventional hydraulic compres-sion moulding press (Fjellman Press AB, Mariestad, Sweden, capacity of 3100 kN).

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The needle punched mats of PP and flax fibres were difficult to heat withoutburning the material and were therefore contact heated between thick aluminiumplates to the temperature of 195–200◦C. The temperature was controlled by a ther-mocouple placed between the nonwoven mats during heating. The materials werethen compression moulded. This kind of materials can also be heated by blowinghot air through the mats, which is a much quicker way to heat the material thancontact heating.

The pressure was between 8 and 12 MPa depending on the type of NMT. Meltimpregnated NMTs were compression moulded using lower pressure while theneedle punched mats need a higher pressure to get a uniform material. Mouldedcomposites had a size of 350× 450 mm and thickness between 2.6–5 mm. Themelt impregnated NMTs can flow during the moulding as GMT but the needlepunched NMT did not flow due to needle punched structure. All materials had aweight between 700–800 g.

2.3. MECHANICAL TESTING

Tensile testing of the specimens was performed on an Instron test machine (model8510) using a crosshead speed of 2 mm/min. Samples for the tensile testing werecut to the width of 30 mm and length of 250 mm. The thickness varied between 2.6to 5 mm. Izod impact testing was performed according to ASTM D 256 on a KarlFrank GmbH impact tester. Test samples for impact testing were machined with amilling cutter. At least 5 specimens of every composition were tested in 0◦ and 90◦directions. The stronger direction is called 0◦.

2.4. MICROSCOPY STUDY

Fractured surfaces of Izod impact specimens were sputter coated with platinumand examined using a Jeol JSM-5200 scanning electron microscope (SEM) at anacceleration voltage of 20 kV.

3. Results

3.1. COMPOSITE MECHANICAL PROPERTIES

The mechanical properties of the composites obtained from tensile and impact testsare summarised in Table II.

Typical tensile curves for tested materials are shown in Figure 2. The curvesof needle punched materials, Mühlmeier and Danflax follow each other quite welluntil the break of Danflax. The melt impregnated NMT shows similar curve shape.

The maximum tensile stress of the NMT composites is shown in Figure 3. Itcan be seen that it is increased with increased fibre content. The tensile stress isshown to be strongly dependent on the direction of the materials and all materi-als have higher stress in the 0◦ direction. GMT is less anisotropic compared to


Table II. Mechanical properties of different commercially available NMT composites,GMT and pure PP.a

Tensile properties Izod impact properties

Composite Dir. Strength Modulus Elongation Notched Unnotched

(MPa) (GPa) (%) - - - - - - - ( J/m) - - - - - -

PP 28.5± 0.6 1.5± 0.1 – 24± 1 553± 81

S 0◦ 30.8± 0.5 4.6± 0.6 1.2± 0.1 70± 6 129± 8

90◦ 21.3± 0.9 2.9± 0.3 1.4± 0.1 38± 0 66± 5

I 0◦ 34.7± 1.8 5.2± 0.4 1.2± 0.1 83± 6 149± 14

90◦ 26.1± 2.1 3.9± 0.9 1.4± 0.5 49± 8 100± 13

D 0◦ 40.2± 1.7 6.5± 0.4 1.4± 0.1 403± 23 751± 122

90◦ 35.3± 0.8 5.0± 0.5 2.2± 0.4 266± 14 450± 35

M 0◦ 56.4± 2.2 6.3± 0.9 1.9± 0.1 194± 7 386± 24

90◦ 31.1± 0.9 3.8± 0.4 2.2± 0.3 150± 9 265± 23

GMT 0◦ 77.2± 4.4 5.4± 0.3 1.8± 0.1 – –

90◦ 74.9± 7.5 5.1± 0.4 1.8± 0.1 410± 75 717± 91

a Standard deviations in parentheses.

Figure 2. Typical tensile test curves for the studied materials.

NMT. The melt impregnated NMTs have 25% to 30% better tensile stress in 0◦direction, Danflax about 10% and Mühlmeier about 45%. The tested materials didnot reach the level of GMT, which was expected due to higher strength of glassfibres, 3400 MPa, compared to flax fibres about 750–1000 MPa [2]. It should bementioned that the NMT with the fibre content of 50 wt% has a tensile strength of57 MPa, which is twice that of pure PP, 29 MPa.

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Figure 3. Tensile strength of different NMT composites compared to conventional GMT andpure PP.

Figure 4. The elongation at break of the NMT composites compared to the GMT and pure PP.

The elongation at break in Figure 4 shows that the melt impregnated NMT hasa lower elongation at break than needle punched NMTs and GMT. The maximumstrain is better in 90◦ direction for all NMT while the GMT have the same strainvalues in both directions.

Figure 5 shows that stiffness of the NMT composites with higher or at least thesame fibre content by weight as GMT is comparable with the glass fibre compos-ites, it is even higher for NMT with 50 wt% flax fibre. The modulus for the singleflax fibres is about 100 GPa, which is better than glass fibres [9]. The 0◦ direction isstiffer for all NMT material and seems to have especially strong effect for Symalitand Mühlmeier. Higher modulus in 0◦ direction indicates that the fibres are moreoriented in this direction.


Figure 5. Stiffness of different NMT composites compared to GMT and pure PP.

Composites impact properties are shown in Figure 6. Izod impact tests showedthat NMT composites have inferior impact properties compared to GMT exceptDanflax in 0◦ direction. In notched samples both NMT and GMT had better impactstrength than pure PP. Generally, the impact strength of NMT composites was im-proved with increased fibre content and 0◦ direction except in the case of unnotchedpure PP samples.

Tensile and impact strength indicates that the fibre-matrix adhesion is poor inthe NMT materials. The natural fibres can have wide variations in the quality butstrength and E-modulus are depending on the form of the fibres. Natural fibres areusually as fibre bundles where the individual fibres are bonded together with ligninand/or pectin. The fibre bundles have lower mechanical properties than individualfibres due to the low bonding strength between the individual fibres. We have verylittle information about the fibre quality in the composites used in this work. Forexample, the natural fibres in these kinds of technical applications may well be restproducts from textile industry or other low quality fibres.

3.2. COMPOSITE MORPHOLOGY

Scanning electron microscopy of fractured surfaces of different NMT and GMTcomposites were made to study the morphology and adhesion between the fibresand the matrix. Generally, the fracture surfaces of the NMT composites were notas sharp as the GMT fracture surfaces.

Figure 7(a) shows a typical fracture surface of NMT composites (S) with a fibrecontent of 25 wt%. It can be seen that there is a lot of fibre pull-outs and that thefibres are mostly as fibre bundles. Figure 7(b) shows a more detailed micrographof the composite structure. It can be seen that there appears to be poor adhesion

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(a)

(b)

Figure 6. Izod impact properties of (a) notched and (b) unnotched samples.

between the fibre bundles and the PP matrix. The surface of the fibre bundle is cleanand it is also possible to see where the fibres have been located before fracture.

Figure 8(a) shows an NMT composite (I) with 37 wt% fibre content and com-pared to Figure 7 it has higher fibre content. It is again possible to observe manyfibre pull-outs indicating poor adhesion. Compared to the micrographs in Fig-ure 7 there are more individual fibres in these composites. In (b) a more detailedmicrograph of the interface shows that the fibre surfaces are clean. At larger mag-nification there were voids visible between the fibre and matrix.


(a)

(b)

Figure 7. Scanning electron micrographs of the fracture surface of a NMT composite (S) with25 wt% flax fibre: (a) overview; (b) detail.

For the NMT composite with a fibre content of 50 wt% it was impossible tostudy the interface region between the matrix and the fibres. The fibres pull-outsare so long and so many that it was difficult to see any matrix.

Figure 9(a) shows that the fibre content (by volume) of the GMT is low com-pared to the NMT in Figure 8 even if the composite has almost the same fibrecontent by weight. The difference is explained by the higher glass fibre density,which is about 2.6 g/cm3 compared to natural fibres 1.4–1.5 g/cm3.

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(a)

(b)

Figure 8. Scanning electron micrographs of the fracture surface of NMT composites (I) with37 wt% flax fibre: (a) overview; (b) detail.

It is also possible to see that the adhesion between glass fibres and PP matrixis poor, there are a lot of long pull-outs, the interfacial region shows voids and thefibre surfaces are clean.

4. Conclusions

The main objective of this study was to investigate the mechanical properties andmorphology of four different commercially available semi-finished NMT compos-ites and compare these with conventional GMT and the pure polymer matrix.


(a)

(b)

Figure 9. SEM micrographs of the fracture surface of a GMT composite with 40 wt% glassfibre: (a) overview; (b) detail.

Processing parameters for melt impregnated semi finished NMT showed to besimilar to GMT materials, while the needle punched materials need to be contactheated and need longer heating time and also higher pressure during compressionmoulding. The material flow during compression moulding was good for meltimpregnated materials, needle punched materials did not show any flow.

The results of mechanical tests show that the stiffness of NMT composites iscomparable with conventional GMTs when the fibre content is higher or at leastas high as the glass fibre content by weight. But also that the NMT composites

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properties are highly anisotropic, 0◦ direction (being the stronger direction) showsa 20–40% higher tensile modulus than 90◦ direction.

The tensile strength of NMT materials is lower than the GMT, but it is increasedwith increased fibre content and it is expected to improve further if coupling agentsare used. Even the tensile stress is strongly dependent on the direction. For exampleMühlmeier NMT showed a tensile strength of about 57 MPa in 0◦ direction andonly 31 MPa in 90◦ direction.

Generally the NMT composites showed lower Izod impact strength comparedto GMT composites in both notched and unnotched samples except Danflax whichshowed impact properties comparable to GMT especially in 0◦ direction. Accord-ing to the material manufacturer this is because of the high fibre length.

Impact, tensile properties and the microscopy study indicated that there is poorfibre/matrix adhesion in all NMT composites.

These results show that NMT composites have a potential to be used instead ofconventional GMT in engineering applications where low weight, easily recyclableand environmental friendly materials are desirable. The NMT materials showed inmost cases somewhat lower prices compared to GMT but the material supplierscount on even lower prices in the future.

Acknowledgements

The author would like to thank Symalit AG, Switzerland, Molybon Agentur AB,Sweden, Danflax AS, Denmark and Lear Corporation, Sweden, Interior SystemsAB, Tanum, for supplied materials.

References

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3. Mieck, K.-P., Lützkendorf, R., and Reussmann, T., ‘Needle-Punched Hybrid Nonwovens ofFlax and PP Fibres – Textile Semiproducts for Manufacturing of Fibre Composites’,PolymerComp.17(6), 1996, 873–878.

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5. Hornsby, P. R., Hinrichen, E., and Taverdi, K., ‘Preparation and Properties of PolypropyleneComposites Reinforced with Wheat and Flax Straw Fibres’,J. Mater. Sci.32(4), 1997, 1009–1015.

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2000,mechanical properties of natural fibres

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