The University of Manchester Research

Natural fibre thermoplastic tapes to enhance reinforcingeffects in composite structuresDOI:10.1016/j.compositesa.2020.105822

Document VersionAccepted author manuscript

Link to publication record in Manchester Research Explorer

Citation for published version (APA):Akonda, M. H., Shah, D. U., & Gong, R. H. (2020). Natural fibre thermoplastic tapes to enhance reinforcing effectsin composite structures. Composites Part A: Applied Science and Manufacturing, 131, 105822.https://doi.org/10.1016/j.compositesa.2020.105822

Published in:Composites Part A: Applied Science and Manufacturing

Citing this paperPlease note that where the full-text provided on Manchester Research Explorer is the Author Accepted Manuscriptor Proof version this may differ from the final Published version. If citing, it is advised that you check and use thepublisher's definitive version.

General rightsCopyright and moral rights for the publications made accessible in the Research Explorer are retained by theauthors and/or other copyright owners and it is a condition of accessing publications that users recognise andabide by the legal requirements associated with these rights.

Takedown policyIf you believe that this document breaches copyright please refer to the University of Manchester’s TakedownProcedures [http://man.ac.uk/04Y6Bo] or contact uml.scholarlycommunications@manchester.ac.uk providingrelevant details, so we can investigate your claim.

Download date:10. Dec. 2021

1

Natural Fibre Thermoplastic Tapes to Enhance Reinforcing Effects in

Composite Structures

M. H. Akondaa, D. U. Shahb, R.H. Gonga

a Department of Materials, University of Manchester, Manchester, UK.

b Centre for Natural Material Innovation, Dept. of Architecture, University of Cambridge,

UK.

Corresponding Author. Email: mahmudul.akonda@manchester.ac.uk / mhakonda@yahoo.com

Abstract Semi-consolidated thermoplastic tapes were produced by spreading flax and polypropylene

matrix fibres using a newly developed technology. This lightweight tape was structurally

stable and contained 38% flax fibres by volume. The tapes were processed in unidirectional

and woven fabric format for composite fabrication. We found that the flax/PP tape-based

composites had 60-110% higher flexural modulus and 35-65% higher tensile modulus

compared to flax/PP yarn based thermoplastics. Thermoanalytical results showed that the

heating conditions used in the tape-making process did not degrade the flax fibres and PP

matrix. We conclude that such semi-consolidated flax/PP tapes enable the achievement of

properties not seen before for yarn-based composites, and therefore are an important step

forward in optimising the reinforcing effect of natural fibres in composite applications.

Key words:

A. Natural fibres, B. Thermal properties, D. Mechanical properties, E. Tape placement

2

1. Introduction

Synthetic fibres, such as glass, aramid and carbon, dominate the fibre-reinforced composite

markets. However, these fibres are not biodegradable nor energy recoverable, and at the end

of their usable life exacerbate the waste and disposal challenge. Therefore, in the past few

years much research has focused on the development of light-weight biocomposites

comprising natural fibres. The use of short wood based fibres (fibre length > 5 mm) has

become popular but is limited to non-structural applications [1], whereas longer bast fibres

(fibre length 50-120 mm) such as flax, jute, hemp, sisal and kenaf are attracting strong

interest for many structural end uses. Such fibres may be used to reinforce thermosetting

resins (e.g. epoxy, phenolic, bio-derived resins), common thermoplastic matrices (e.g.

polypropylene, polyamide), or biodegradable thermoplastic matrices (e.g. polylactic acid) [2-

3]. Employing natural fibres in fibre reinforced composites has many notable advantages over

synthetic fibres, such as lower weight, lower cost and lower carbon footprint [4]. Natural

fibre–reinforced composites have therefore received much attention in recent years from

composite manufactures and end users in various sectors, including construction [5] and

automobile [6], with demand growing at 20-25% year on year [7].

Generally, long fibres provide better mechanical properties in composite structures, and

therefore plant fibres such as flax, jute, kenaf, sisal and hemp have been extensively

investigated [8-13]. Bast fibres such as flax and jute (densities from 1.4-1.5 g/cm3) have

gained stronger interest, particularly as a low-density replacement of glass fibres (density of

glass fibre ̴ 2.6 g/cm3) in order to produce lighter weight composite parts for structural

applications [14-15].

3

An effective way of utilising these longer natural fibres as reinforcing elements is to convert

them into spun yarns [16] which can then be easily transformed into composite preforms such

as woven fabrics. Spun yarns generally have a twisted structure; the fibres are held together

by fibre to fibre friction, resulting from the compacting forces generated by the twist helix

during spinning [16]. Reportedly, the helical configuration of fibres in twisted yarns lowers

the yarn modulus and strength, and this is reflected in the mechanical properties of resulting

composite structures [17-20]. A further disadvantage of twisted yarn structures is poor resin

penetration into the core of the yarn [21-23]. These two negative effects indicate that the

twisted yarn structure is not ideal for fibre reinforced composites.

An alternative structure is that of a twist-less wrap-spun yarn [24-25], where the reinforcing

fibres are assembled aligned and parallel to each other, and a synthetic filament is spirally

wrapped around the assembly of the reinforcing fibres, binding them together to give the yarn

integrity. The filament is usually made of the same polymer as the thermoplastic matrix to be

reinforced, or of a soluble polymer if a thermoset matrix is to be used. Since filament binding

replaces twist in the wrap spun yarn structure, better utilisation of initial fibre modulus and

strength is possible [24-25]. However, a disadvantage of the wrapped structure is that the

assembly of the fibres become crimped or wavy owing to the tension of the wrapping thread

[26]. Therefore, full utilisation of the natural fibre mechanical properties is still not be

achieved [26]. The challenge then is to give coherence to the assembly of reinforcing fibres in

such way that their alignment and parallelism are retained in the woven fabric or prepreg.

‘Twistless’ wet-spun rovings of flax, with virtually zero-twist - really around ca 20 turns per

meter - have also been developed in industry. Composites with high modulus and strength are

possible with these, as shown by the work of Shah et al. [27,22]. However, as the rovings are

4

not ‘compact’, the resulting fabrics tend to be ‘lofty’ and therefore low-pressure composite

processes, such as vacuum infusion and resin transfer moulding, limit the achievable fibre

volume fraction of the composites to ca 30-35% [27, 28]. More recently, pre-consolidated

unidirectional tapes from 100% flax yarns, where PLA powder binder was applied to keep

the yarns together in tape form have been developed by another researcher [29]. However,

optimum mechanical properties of the yarn-tape composites cannot be achieved due to the

twisted and wavy structure of the yarns utilised.

In this work, a new approach for the conversion of natural fibres into a non-yarn based tape

preform was investigated. The flax reinforcing fibres were intermingled with thermoplastic

polypropylene matrix fibres, and then passed through a unit with heaters and calendar rollers

to convert it into a tape/sheet. The latter stages melt the thermoplastic fibres to the reinforcing

fibres to give the tape sufficient strength for handling and subsequent processing into a

composite. In forming the composite, the preform would be constructed from the tape and

subsequently hot-pressed to melt the thermoplastic fibres to become the matrix. The novelty

of this approach is the flat construction of tape, in which the reinforcing fibres are straight

and not wavy like yarn structures. A range of semi-consolidated tapes in different widths

were produced, and the suitability of the tapes for hot-press moulding was evaluated. Both

unidirectional and biaxial woven preforms were produced. The performance of the moulded

tape composites was compared to composites from flax yarn based preforms produced at the

same fibre content.

5

2. Materials

2.1. Flax fibres

Commercial grade flax fibres were supplied by Roctool Ltd. (Belgium). The datasheet

density of the flax fibre was 1.50 g/cm3. The properties of flax fibres are given in Table 1.

2.2. Matrix fibres

Staple polypropylene (PP) fibres (100.0 ± 0.5mm long) with density of 0.91 g/cm3, used as

matrix in this work, were sourced from Asota GmbH (Austria). The properties of staple PP

matrix fibres are given in Table 1.

2.3. Yarn-based preforms: Flax/PP hybrid yarn and yarn woven fabric

Flax/PP hybrid wrap spun yarn (50% flax by weight, 38% by volume) with linear density of

250±5 tex, and yarn woven fabric (YWF) made of the flax/PP yarns (2x1 twill fabric, 230±50

gsm) (see Figure 1a), were sourced from Composite Evolution (UK).

2.4. Tape-based preforms: Flax/PP semi-consolidated thermoplastic tape prepreg

Semi-consolidated flax/PP unidirectional tape (TUD) and tape woven fabric (TWF) (see

Figure 1b and 1c) was produced in this work by using a newly developed technology at the

same fibre volume fraction as the yarn-based preform composites.

3. Experimental work

3.1. Characterisation of flax reinforcement fibres and PP matrix fibres

The mean length of flax fibres was measured by an Almeter and fibre diameter was measured

by using an optical microscopy technique [30]. The tensile properties of single flax fibres

were measured at 30 mm gauge length at 1mm/min in accordance with EN ISO 53812.

6

The melting point and crystallinity of the PP fibres was determined by differential scanning

calorimetry (DSC). The melt flow index (MFI) of the matrix was also measured according to

ASTM D1238 standard. The tensile and flexural properties of the PP matrix were determined

by testing PP plaques, in accordance with BS EN ISO 527-1:1996 for tensile and ISO

1782003 for flexural, respectively. Plaques were made from 100% PP fibres by hot press

moulding at 185o C for 10 minutes and 20 bar (2 MPa) pressure.

3.2. Production of semi-consolidated flax/PP tape

Slivers of commingled flax and PP fibres, 50:50 w/w were produced by a conventional

carding process, and subsequently thermally treated by a specially developed process (see

Figure 2) to convert them into flexible tape materials. The latter process aligns the flax fibres

unidirectionally with the axis of the resulting tape (i.e. machine direction). This is achieved

by assembling several slivers in parallel and attenuating them using the mechanical action

known as drafting, effected by a set of rollers, as illustrated in Figure 2. Each successive pair

of rollers operates with a surface speed slightly faster than the preceding pair. During

drafting, the frictional contact between fibres induces the localised forces required to

orientate the flax fibres unidirectionally. In addition to fibre alignment, drafting thins and

spreads the slivers into a thin sheet of fibres. On leaving the drafting stage the thin sheet of

fibres are heated just above the melting point of the PP and drafted again for further

alignment of the flax fibres. A pressure of 2 bar (0.2 MPa) is then applied to the material by a

pair of pressurised rollers to the flax/PP sheet, melding the PP fibres to the flax to produce a

semi-consolidated tape (partial consolidate tape). Partially consolidated wider tape (90 mm

width) (see Figure 1b), was produced by heating the fibre blends at different temperatures of

180, 200 and 220oC, respectively (Table 2). Then the wide sheet was slit to narrower width

7

(18 mm), which was used for property measurement of the tapes. The 18 mm tapes were also

used to produce tape woven fabric (1x1 weave) by using the tapes in both warp and weft

directions. The details of the tapes produced in this experiment are given in Table 2.

3.3. Characterisation of semi-consolidated tape

The thicknesses, linear densities and weight variation of the tape was determined by cut and

weight method. Scanning electron microscopy (SEM) was used to observe the fibre blend

performance of the tapes. The orientation of flax fibres in tape was also determined by image

analysis using Image Pro+ software [31]. To do so, thin, semi-consolidated fibre-web

samples were collected, photographic images were taken of each sample with a digital

camera, the images then used to measure fibre orientation (see Figure 3). A total of 10

samples were analysed involving 500 flax fibre positions, their angles relative to the tape axis

(i.e. the machine direction) were measured. The tensile strength and modulus of the tapes

were also measured in accordance with BS EN ISO 527-1:1996 standard.

3.4. Thermal degradation studies of flax and PP matrix of the tapes

Thermal gravimetric analysis (TGA) was carried out for both flax and PP matrix fibres

individually. The flax and PP fibres were treated first at 180o, 200o and 220o C under two hot

plates with 2 bar pressure to follow the same processing parameters (temperature, heating time

etc.) used during tape making process. Then TGA was carried out on the treated samples to

investigate their thermal degradation. For simultaneous differential thermal /thermogravimetric

analysis (DTA/TGA), an STD2606 (TA Instruments) thermal analyser was used under

flowing air (100 ml / min) and at a heating rate of 10o C min-1 [32]. Sample masses of 10 ±

0.5 mg were used for each sample.

8

3.5. Composite fabrications

Six layers of individual forms of UD yarn, yarn woven fabric (2/1 twill fabric), UD tape and

tape woven fabric (1/1 plain weave) were used to fabricate thermoplastic composites (see

Figure 4). For unidirectional composites, yarn and tape were wound onto a steel frame (250

mm x 250 mm) unidirectionally to produce yarn UD composite (YUDC) and tape UD

composite (TUDC) laminates. For woven fabric composites, 6 layers of yarn fabrics and 6

layers of tape fabrics were taken. These assembles were subsequently compressed and

consolidated using hot compression moulding at 185C under 10 bar (1 MPa) pressure for 15

minutes dwell time. The laminated composite panels were then cooled at a rate of

15oC/minute to room temperature and measured to be 2.20 ± 0.03 mm in thickness. The

details of the composite panels fabricated in this work are given in Table 3.

3.6. Composite density and fibre volume fraction

The density of each composite specimen was calculated from its weight, measured on

precision balance, and volume was calculated. To determine the fibre volume fraction,

representative specimens taken from the yarn and tape composite panel were immersed in O-

xyline solvent at 150o C for 3 hours to completely dissolve the PP matrix to follow ASTM

D5492 standard. The flax fibre residues were washed, dried at 110o C for 12 hours, and

weighed. The calculated mass loss was assumed to be the resin content [33]. With the matrix

and fibre volume fractions, the void content in the composites was also calculated.

3.7. Mechanical testing of composites

Five test specimens (250 mm x 25 mm) were cut from the tape- and yarn-based composite

panels for tensile measurements (BS EN ISO 527-1:1996), and a further five (60 mm x 15

mm, 40 mm span length) for flexural properties (ISO 14125). The tensile and flexural

9

strengths and tangent moduli at 0.2% strain of specimens were also determined from their

individual stress-strain curves.

4. Results and discussion

As indicated in Table 1, the flax fibres had an average length of 90 ± 5 mm, linear density of

7.0 ± 1.0 dtex and mean diameter of 24 ± 1 μm. The average ultimate tensile strength (UTS)

and modulus for the flax fibres were found to be 560 ± 15 MPa and 50 ± 5 GPa, respectively.

The mean length and diameter of the flax fibres were similar to PP matrix fibres (see Table

1), ensuring good commingling of their blends to be obtained by the carding process. Table 1

also shows the selected PP matrix fibre to have a melting point of 164oC, crystallinity of

60.0% and melt flow index of 48g/10min. The tensile strength and modulus of PP matrix

were found to be 30 ± 3 MPa and modulus 2.0 ± 0.5 GPa and the flexural strength and

modulus were found to be 65 ± 2 MPa and 2.0 ± 0.5 GPa, respectively.

The semi-consolidated flax/PP matrix hybrid tape was produced in this work by melting the

PP matrix fibres and pressing the assembly to hold the reinforcing fibres in a tape/sheet form.

Using Eq. 1, the contact dwell time in second (CDT) for a unit length of fibre blends would

be in contact with the pressurised roller during tape making process, was calculated for

different roller-surface speeds and temperatures used (see Figure 2).

V

TTTTD

speedDelivery

areaContactCDTtimedwellContact

211)( Eq.1

where, T = distance between the rollers (0 mm), T1= thickness of feed material (10 mm), T2 =

thickness of deliver tape (0.6 mm), D1 = diameter of the top roller (60 mm), V = delivery

speed, m/second.

10

Figure 5a shows that the contact dwell time (CDT) decreases with the increased delivery

(feed) speed. The corresponding tape strengths (breaking load) with increased speed were

measured to be 260N for 7.6 CDT and 150N for 5.07 CDT (see Figure 5b). Evidently, the

matrix impregnation, which played the major role for strength of the tapes, was directly

related to the heat dwell time (heat penetration time; HPT) of the fibre blend as they pass

though the heating unit. Using Eq. 2, the HPT was found to have decreased from 12.5 second

to 8.3 second as the production speed increased from 4 m/sec to 6 m/sec.

HPT = S/V Eq. 2

where, S is the width (50 mm) of the heating unit (heating length of materials), V = material

delivery speed, m/second, i.e. T = 50 / 4 = 12.5 second, when V was 4 m/second.

Figure 6 shows the effect of heating zone temperature on the mechanical properties of the

semi-consolidated tapes. As would be expected, for set HPT and CDT, there is a direct

relationship between zone temperature and the resulting tape strength and modulus. The

strongest tape was therefore produced using the highest zone temperature and the lowest

pressurised-roller speed to obtain longest HPT and CDT. It is also found that that the highest

tensile stress (45 MPa) (see Figure 6a) and modulus (1.7GPa) (see Figure 6b) was found for

the tape specimen produced at 220oC temperature, where the lowest values (25MPa and 1.0

GPa) were found for the tape specimen produced at 180oC, respectively (see Figure 6a). But

good values for mechanical properties of thin semi-consolidated tapes were due to uniform

blending of flax fibres with PP matrix fibres. As would be expected the longer dwell time

gives better semi-molten or molten PP matrix through the flax/PP blend assembly. The PP

fibre dissolution test results of hybrid tapes confirmed the Flax and PP fibres blend ratio to be

50/50 by weight indicating that no loss of material had occurred during processing, and that

11

there was a sufficient number of PP fibres per unit length for the molten polymer to

subsequently fully encapsulate the flax fibres.

The thickness of the semi-consolidated tape (18 mm width) produced at 200o C is quite

constant along the tape length – mean thickness 0.6 mm, with coefficient of variation of

1.7%. Similarly, the mass per unit length of the tape was found to be fairly constant at 330

gsm, with coefficient of variation of 1.3%. The linear densities were found to be the same

(330 gsm) for all production temperature (see Table 2), which indicated commingling of the

fibres and uniform spreading of the fibre blends during tape making process. The dissolution

test showed the tape specimens also contained same amount of flax fibres (50% by weight,

38% by volume) (see Table 2). The result for fibre orientation measurement obtained from

optical image analysis, presented in Figure 3, shows over 95% flax fibres are oriented in the

machine direction (0o direction) in the tape. The flat image of the tape (see Figure 1b, 1c) also

indicated that crimp or waviness /undulations of flax fibres were removed by drafting and the

fibres remained straight in the produced sheet by stabilizing effect of the molten PP matrix

fibres.

Figure 7 shows photomicrographs of the semi-consolidated tape. In Figure 7a, fibre surfaces

and voids are clearly evident on the surface of the tape since the tapes are not fully

consolidated. As seen through the cross-section in Figure 7b, voids were almost eliminated

with the increasing degree of consolidation, i.e. with respect to increase in CDT for fully

consolidated tapes. SEM images in Figure 7b also show that the flax fibres were well

distributed in the matrix.

The DTA behaviour of both flax and PP matrix fibres of the tapes processed at different

temperatures are shown in Figure 8a-b, whereas, the TGA graphs are shown in Figure 8c-d. It

12

is seen that all flax fibre samples, standard and those exposed to high temperature processing,

show one exothermic peak. For standard flax sample, the DTA peak is at 355 oC, which

represents the decomposition temperature of cellulose. There is another small exothermic

peak at about 420oC, which may be due to oxidation of the char. For other flax samples,

exposed at 180, 200 and 220 oC for 8 seconds during tape making process, also have DTA

peak maxima at 355 oC (see Figure 8a). The mass losses calculated from their TGA curves

between 20-200 oC (Figure 8c) is found to be around 8 wt% for all samples, which is

accounted for by the loss of water and other processing lubricant aids and volatiles from the

fibres. The mass losses between 200-355 oC are also found to be the same (77wt%) for flax

samples, which indicates that the 8 second high temperature exposure time during tape

processing does not degrade the flax fibres. For PP matrix, one endothermic peak was found

at around 165 oC due to the melting, followed by three exothermic peaks - one in the range of

280-320oC, another around 380-390oC, and a third around 410-430oC – due to

depolymerisation and decomposition in air [32]. For PP matrices processed at the different

temperatures, the endothermic peak was found at a similar temperature to the standard PP

matrix (see Figure 8b), but the exothermic peaks showed much variability, typically shifting

to a higher temperature. The TGA curves of PP in Figure 8d show only two main mass loss

stages. As melting is a phase change, no mass loss should be observed before 220oC. From

220 to 410oC, mass loss of over 90% (up to 100%) is observed for the PP samples. The DTA

observations reveal some discernible differences in thermal behaviour of PP matrix fibres that

are processed at 180, 200 and 220 oC.

Figure 9 depicts the typical stress-strain curves of tape UD composites (TUDC) under tensile

and flexural loading. The results obtained from the tests are presented in Table 4. The mean

tensile and flexural strength obtained was 125 MPa and 132 MPa for the TUDC (see Table

13

4). Both strength values of TUDC are 4-5% higher than that of the yarn-based unidirectional

composites (YUDC) (see Figure 10a). Notably, significantly higher tensile (37%) and

flexural (57%) moduli were obtained for TUDC compared to YUDC (see Figure 10a). From

the trajectory of the curves of TUDC (see Figure 9) delamination did not occur during load

up fracture, which suggests that good fibre-matrix bonding was achieved. Similar trends were

also found for TWFC laminates, where both tensile and flexural strength were 5-6% and

moduli were 62-111% higher than that of YWFC laminates (see Figure 10a). Interestingly, it

was seen that the failure elongation of both tape composites (TUDC and TWFC) were 50%

lower than that of both yarn based laminates (YUDC and YWFC) under tensile and flexural

loading (see Figure 10b). This may be due to the flat structure of the tape, where the flax

fibres were placed in fully straightened position along the tape length. As the tapes are flat

structurally, there is negligible crimp in the intersection of the warps and wefts in the fabric

(see figure 1c). On the other hand, the yarn structure has circular cross-sectional shape (non-

flat) and has a wavy structure. Therefore, the yarns (warp and weft) in the woven fabric

(YWF) have crimp, which may straighten when stress is applied, leading to higher failure

strains for both yarn based composites.

The composite densities of the both tape composites (TUDC and TWFC) were found to be

1.11g/cm3 (see Table 4). Around half the fraction of voids was found in tape-based

composites (2-3 v%) in comparison to yarn-based composites (4-6 v%) (see Table 4). This

may be attributed to better impregnation in the former due to the high degree of commingling

of the flax fibres and PP matrix fibres achieved in the tape. This in turn provided better

mechanical properties, particularly modulus, of the tape-based composites in comparison to

the yarn-based composites, although they contained the same flax volume fraction. Moreover,

the higher degree of fibre orientation in the prepregs also contributed to the better mechanical

properties of the tape composites compared to yarn-based thermoplastics.

14

5. Conclusion

Semi-consolidated flax/PP thermoplastic tapes produced in this work is a new development in

natural fibre-based thermoplastic preforms. The low areal density flax/PP hybrid

thermoplastic tapes comprised 38% flax by volume (50% by weight) and had sufficient

handling strength for further downstream processes. The thermal analytical studies show that

exposing the flax fibres and PP matrix to high temperatures (180-220oC) during the

controlled tape making process did not degrade them. The unidirectional and biaxial woven

composites fabricated from these semi-consolidated tapes showed substantially higher

mechanical stiffness compared to yarn-based composite laminates. This is likely due to

improved fibre alignment/orientation and reduction in crimp and waviness, as well as

improved impregnation and reduced porosity. We conclude that such thermoplastic tape-

based preforms offer opportunities to optimise the reinforcement effect of natural fibres in

thermoplastic composite structures. These tape-based composites with good finish and

reasonable mechanical properties may find suitable applications in the automotive sector

(which prefers thermoplastics), as well as for consumer goods and sporting goods.

Acknowledgement

The authors would like to acknowledge Tisatec Advanced Textile Materials (UK), Composite

Evolution (UK), Oxeon AB (Sweden) for their supports.

15

References

1. Bledzki AK, Gassan J. Natural fibre reinforced plastics. N.P. Cheremisinof Ed.

Handbook of engineering polymeric materials, Marcel Dekker, Inc., New York, p787-

809, 1997.

2. Eichhorn SJ, Baillie CA, Zafeiropoulos N, Mwaikambo LY,Ansell MP, Dufresne A,

Entwistle KM, Herrera-Franco PJ, Escamilla GC, Groom L, Hughes M, Hill C, Rials T,

Wild PM. Review Current International Research Into Cellulosic Fibres and

Composites. Journal of Material Science and Technology 2001, 36, 2107-2131.

3. Graupner N, Herrmann AS, Mussig J. Natural and man-made cellulose fibre-reinforced

poly(lactic acid) (PLA) composites: An overview about mechanical characteristics and

application areas. Composites Part A: Applied Science and Manufacturing, Volume 40

(6-7), p810-821, 2009.

4. Joshi SV, Drzal LT, Mohanty AK, Arora S. Are Natural Fiber Composites

Environmentally Superior to Glass Fiber Reinforced Composites?. Composite Part A, 35,

2004, p371-376.

5. Drzal LT, Mohanty AK, Burgueno R, and Misra M. Biobased Structural composite

Materials for Housing and Infrastructure Applications: Opportunities and Challenges.

The NSF- PATH Housing Research Agenda Workshop. February 12-14, 2004.

6. Karus M, Ortmann S, Vogt GD. Use of Natural Fibres in Composites in the German

Automotive Production 1996 till 2003. Nova-Institute, September 2004.

7. Mohanty AK, Misra M, Drzal LT. Sustainable Bio-Composites From Renewable

Resources: Opportunities and Challenges in the Green Materials World. Journal of

Polymers and the Environment 2002. Accepted June 30, Vol 10, Issue 1, 19-26.

8. Kestur GS., Gregorio GA, Fernando W. Biodegradable composites based on

lignocellulosic fibers—an overview. Progress in Polymer Science. Vol. 34, p928-1021,

2009.

9. Massimo B, Elisa Z, Mariastella S. Flax Fibre-Polyester Composites. Composites Part

A, 2004, 35, 703-710.

10. Cabral H, Cisneros M, Kenny JM, Vazquez A, Bernal CR. Structure properties

Relationship of Short Jute Fibre-reinforced Polypropylene Composites. Journal of

Composite Materials 2005, 39(1), 51-65.

16

11. Feng D, Caulfield DF, Sanadi AR. Effect of Compatibilizer on the Structure-Property

Relationships of Kenaf-Fiber/Polypropylene Composites. Polymer Composites 2001, 22,

4, 506-517.

12. Jayaraman K. Manufactoring of Sisal-Polypropylene Composites with Minimum Fibre

Degradation. Composites Science Technology 2003, 367-374.

13. Madsen B, Hoffmeyer P, Thomsen AB, Lilholt H. Hemp Yarn Reinforced Composites-I,

Yarn Characteristics. Composite Part A, 2007, 38, 2194-203.

14. Olesen PO. Perspectives on the Performance of Natural Plant Fibres, The Royal

Veterinary and Agricultural University, Taastrup, Denmark. Conference 27 – 28 May,

1999, Copenhagen, Denmark.

15. Lee BH, Kim HJ, Yu WR. Fabrication of long and discontinuous natural fibre reinforced

polypropylene biocomposites and their mechanical properties. Fibre and Polymers, Vol.

10, p83-90, 2009.

16. Lawrence CA. Fundamentals of Spun Yarn Technology, pub. CRC press LLC,

Florida, 2003.

17. Lauke B, Bunzel U, Schneider K. Effect of Hybrid Yarn Structure on the Delimitation

Behaviour of Thermoplastic Composites. Composite Part A, 29 A, 1998, 1397-1409.

18. Ku H, Wang H, Pattarachaiyakoop N, Trada MA. Review on the tensile properties of

natural fibre reinforced polymer composites. Composites Part B: Engineering, Volume

42 (4), p856-873, 2011.

19. Shah D, Schubel PJ, Clifford MJ. Modelling the effect of yarn twist on the tensile

strength of unidirectional plant fibre yarn composites. Journal of Composite Materials,

2013. 47(4): p. 425-436.

20. Shah D. Developing plant fibre composites for structural applications by optimising

composite parameters: a critical review. Journal of Materials Science, 2013. 48(18): p.

6083-6107.

21. Goutianos S, Peijs T. The optimisation of flax fibre yarns for the development of high-

performance natural fibre composites. Advanced Composites Letters, 2003. 12(6): p.

237-241.

22. Shah D, Schubel PJ, Clifford MJ, Licence P. Mechanical property characterization of

aligned plant yarn reinforced thermoset matrix composites manufactured via vacuum

infusion. Polymer-Plastics Technology and Engineering, 2014. 53: p. 239-253.

23. Loan TT. MSc thesis on Investigation on Jute Fibres and Their Composites Based on

Polypropylene and Epoxy Matrices. Dresden University of Technology, 2006.

17

24. Zhang L, Miao M. Commingled Natural Fibre/Polypropylene Wrap Spun Yarns for

Structured Thermoplastic Composites. Comp. Sci. and Tech., 2010, p130-135.

25. Khondker OA, Ishiaku US, Nakai A, Hamada H. A novel processing technique for

thermoplastic manufacturing of unidirectional composites reinforced with jute yarns.

Comp. Part A, 37, p2274-2284, 2006.

26. Akonda MH, Weager BM. Aligned Natural fibre Thermoplastic Tape for Light weight

Composite Structures, 4th Int.Con. on Sust. Mat. Polym. And Comp. (ECOCOMP), UK,

2011.

27. Shah D, Schubel PJ, Clifford MJ. Can flax replace E-glass in structural composites? A

small wind turbine blade case study. Composites Part B: Engineering, 2013. 52: p. 172-

181.

28. Akonda MH, Weager BM, Ohlsson F. High-performance natural fibre tape and spread

tow fabric. JEC composites, vol. 71, March, 2012.

29. McGregor OPL., Duhovic M., Somashekar A A, Bhattacharyya D. Pre-impregnated

natural fibre-thermoplastic composite tape manufacture using a novel process.

Composites Part A: Applied Science and Manufacturing, vol.101, 59-71, October 2017.

30. Garkhail SG. Composites based on natural fibres and thermoplastic matrices. PhD

Thesis. Queen Marry College, University of London, 2001.

31. Kim KW, Lee KH, Lee BS, Ho HS, Oh SJ, Kim HY. Effects of Blend Ratio and

Heat Treatment on the Properties of the Electrospun Poly(ethylene terephthalate)

Nonwovens. Fibres and Polymers, 6, 2005.

32. Akonda MH, Kandola BK, Horrocks AR. Effect of alkali and ultraviolet ageing on

physical, and mechanical properties of fibres for potential use as reinforcing elements in

glass/silicate composites. Polymer Advanced Tech, vol. 23 (11), 2011.

33. Lilholt H., Bjerre, AB. Composites based on jute-fibres and polypropylene matrix, their

fabrication and characterisation. Proceedings of the 18th Risø International Symposium

on Materials Science: Polymeric Composites – Expanding the limits, Risø National

laboratory, Denmark, 1997, pp. 411-423.

18

List of tables and figures

Table 1. Physical properties of flax and PP fibres

Table 2. Specifications of semi-consolidated flax/PP thermoplastic tapes

*Wider tapes produced, were slit in to18 mm tapes for characterisation and easy to use in tape

winding unit and tape weaving machine

Properties Flax fibre PP fibre PP matrix (100% PP plaque)

Fibre diameter (µm) 24.0 ± 1.0 20.0± 1.0 - Mean fibre length (mm) 90.0 ± 5.0 100.0± 0.5 - Linear density (dtex) 7.0±1 3.3±0.2 - Melting point, (oC) 164.0 Crystallinity, % 60.0 Melt Flow Index (g/10min) 48.0 Tensile strength, MPa 560.0 ±15.0 - 30.0±3.0 Tensile modulus, GPa 50.0±5.0 - 2.0±0.5 Flexural strength, MPa - - 65.0±2.0 Flexural modulus, GPa - - 2.0±0.5

Tape ID Flax/PP ratio, w/w

Production temperature , oC

Production pressure, bar

Tape width, mm

Tape thickness, mm

Areal density, gsm

Tape 1 50:50 180 2 18 0.6 ± 0.20 330 ± 30 Tape 2 50:50 200 2 18 0.6 ± 0.20 330 ± 30 Tape 3 50:50 220 2 18 0.6 ± 0.20 330 ± 30

19

Table 3. Details of the flax/PP hybrid prepregs and fabricated composites

Table 4. Comparison of the mechanical properties of tape composites with yarn based

structures

Composite

ID

Flax

volume

fraction, %

Tensile

Strength,

MPa

Flexural

strength,

MPa

Tensile

modulus,

GPa

Flexural

modulus,

GPa

Void,

%

Composite

Density

g/cm3

YUDC 38.2±0.5 120.0±2 124.0±5 19.0±1 14.0±1 4-5 1.11

TUDC 38.5±0.5 125.0±3 132.0±4 26.0±1 22.0±2 2-3 1.11

YWFC 37.5±0.5 45.0±3 79.0±3 8.0±1 4.5±1 5-6 1.11

TWFC 38.5±0.5 46.0±3 82.0±4 13.0±1 9.5±1 2-3 1.11

Flax/PP blended prepregs Forms of used Composite

ID

Composite

thickness, mm

Flax fibre weight

fraction, %

Spun yarn Unidirectional YUDC 2.2±0.1 50.5

Semi-consolidated tape Unidirectional TUDC 2.1±0.2 50.0

Yarn woven fabric Woven YWFC 2.3±0.2 49.5

Tape woven fabric Woven TWFC 2.2±0.2 50.0

20

Figure 1. Photographic images of (a) woven fabric of flax/PP hybrid yarns (YWF), (b) semi-

consolidated flax/PP UD tape (TUD) and (c) tape woven fabric (TWF).

Figure 2. Schematic diagram of the semi-consolidated tape making unit and process

21

(a)

(b)

Figure 3. (a) Image of flax fibres in blended sample and (b) the measured fibre orientation in

the unidirectional tape.

(a) (b)

Figure 4. Photgraphic images of (a) yarn UD (YUDC) and (b) tape woven fabric (TWFC)

composite laminates produced using hot pressed moulding

22

(a) (b)

Figure 5. (a) Contact dwell times relation to the feed speed and (b) tape strength (18 mm

width) relation to the contact dwell time.

(a) (b)

Figure 6. (a) Strength and (b) modulus of the semi-consolidated tapes (18 mm width)

produced at different temperatures.

0

2

4

6

8

4 5 6

Con

tact

dw

ell t

ime,

s

Feed speed, m/s

Feed speed, m/s

Contact dwell time, s

0

100

200

300

7.6 6.08 5.07

Tap

e st

reng

th, N

Contact dwell time, (s)

0

20

40

60

0

100

200

300

180 200 220

Ten

sile

str

engt

h, M

Pa

Bre

akin

g lo

ad, N

Heating temp. (oC)

Breaking load, N

Tensile strength, MPa0.5

1

1.5

2

180 200 220

Ten

sile

mod

ulus

, GP

a

Heating temp. (oC)

23

(a) (b)

Figure 7. SEM images showing (a) flax fibres and voids in semi-consolidated tape and (b)

flax fibre distribution in the fully consolidated tape

(a) (b)

(c) (d)

Figure 8. (a-b) DTA peak maxima and (c-d) TGA (wt. loss%) of PP and flax in air

24

(a) (b)

Figure 9. Stress-strain curves of tape UD composites under (a) tensile and (b) three-point

flexural loading.

(a) (b)

Figure 10. (a) Increased tensile and flexural strength and (b) elongation % under tensile and flexural test of the tape composite compared to yarn-based composites.

0

10

20

30

40

50

60

70

TUDC/YUDC TWFC/YWFC

Val

ue in

crea

sed,

%

Tensile Strength (MPa)Flexural Strength (MPa)Tensile Modulus (GPa)Flexural Modulus (GPa)

0.0%

1.0%

2.0%

3.0%

4.0%

YUDC TUDC YWFC TWFC

Tensile Elongation(%)

Flexural Elongation(%)