The mechanical properties of woven tape all-polypropylene

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Composites: Part A 38 (2007) 147–161

The mechanical properties of woven tape all-polypropylene composites

B. Alcock a,*, N.O. Cabrera a,b, N.-M. Barkoula a, A.B. Spoelstra b,J. Loos b, T. Peijs a,b

a Department of Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, UKb Eindhoven Polymer Laboratories, Eindhoven University of Technology, PO BOX 513, 5600MB Eindhoven, Netherlands

Received 5 June 2005; received in revised form 14 December 2005; accepted 9 January 2006

Abstract

The creation of highly oriented, co-extruded polypropylene (PP) tapes allows the production of recyclable ‘‘all-polypropylene’’ (all-PP) composites, with a large temperature processing window (>30 �C) and a high volume fraction of highly oriented PP molecules(>90%). This paper describes all-PP composites made from woven tape fabrics and reports the tensile and compressive properties ofthese, with reference to composite processing conditions and compares these mechanical properties to those of commercial alternatives.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Recycling; A. Tape; D. Acoustic emission; A. Fabrics/textiles

1. Introduction

In a series of academic theses [1–3], composite materialsin which both the fibre and the matrix are based on poly-propylene (PP) have been described. The behaviour ofthese composites will be presented in a range of publica-tions. These so-called ‘‘all-PP’’ composites are designedto compete with traditional thermoplastic composites suchas glass fibre reinforced PP, so must possess comparable orsuperior mechanical properties. One of the main advanta-ges of all-PP composites is the enhanced recyclability whichis achieved by using the same polymer for both fibre andmatrix phase of the composite. Unlike glass fibre reinforcedPP (GFRPP) composites, all-PP composites can be entirelymelted down at the end of the product life for recycling intoPP feedstock. This enhanced recyclability is desirable tosatisfy new environmental legislation which is currentlytargeting high volume industries particularly the automo-tive industry [4].

The mechanical properties of PP can be improved in agiven direction by orienting the polymer molecules in the

1359-835X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesa.2006.01.003

* Corresponding author.E-mail address: [email protected] (B. Alcock).

direction of loading. Numerous routes have been describedto achieve this molecular orientation such as melt spinning,gel (solution) spinning, and solid state deformation [5].However, in these cases, orientation is achieved in onlyone direction and while this ‘‘self-reinforcement’’ is suitablefor fibre applications, it has limited potential to otherapplications that require load bearing in more than onedimension. Thus, to apply the improved mechanical prop-erties achieved through molecular orientation, alternative/additional routes must be devised to produce PP whichpossesses molecular orientation in two- or three-dimen-sional geometries. Reported routes include the roll drawingof PP sheets [6–8], flow-induced crystallisation during sheetextrusion [9,10], uniaxially oriented PP films weldedtogether in multidirectional plies [11], stacking of orientedPP fibres together with isotropic propylene–ethylenecopolymer films [12], and stacking biaxially oriented PPfilms, which have been co-extruded with a propylene–ethyl-ene copolymer film to facilitate bonding at temperaturesbelow the melting temperature of the films [13]. However,while roll drawing and flow induced crystallisation producemolecular orientation in two-dimensional geometries,molecular orientation is still limited to one direction: therolling, or extrusion direction. Bidirectional properties

mailto:[email protected]

Compactiontemperature

Compactionpressure

Minimum temperature toprovide consolidation

Maximum temperature toprevent total melting

Minimum pressurerequired to prevent

shrinkage

Maximum pressure toprevent plastic flow

Processing Window

Fig. 1. Temperature–pressure processing window for all-PP compositeconsolidation.

148 B. Alcock et al. / Composites: Part A 38 (2007) 147–161

can be achieved by welding alternatively rotated stacks ofuniaxially oriented sheets together, and tensile moduli of5 GPa and tensile strengths of 150 MPa have been reportedusing this method [11].

However, an alternative route to create reinforced poly-mer laminates by welding highly oriented PP fibre bundlesor PP tapes by careful heating has been devised at Univer-sity of Leeds, UK [14–19]. This so-called ‘‘hot compaction’’route provides an interesting alternative to achieve entirelyPP sheet structures with high volume fractions of reinforce-ment and hence high mechanical properties (a tensile mod-ulus of 5.1 GPa and a tensile strength of 182 MPa havebeen reported [20]). The main complication with this pro-cess route is that this process is limited by a small temper-ature processing window. Excessive temperature results inrelaxation and hence a loss of molecular orientation, whileinsufficient temperature leads to a poor interfacial bondingbetween fibres/tapes. This small temperature processingwindow reduces the versatility of this processing route.However, the temperature processing window is adequatefor carefully controlled consolidation of biaxially orientedlaminates (for example in a hot press or double belt press[21]), which can subsequently be thermoformed.

However, the drapability of unconsolidated fabricsmakes a fabric structure more desirable than consolidatedsheet precursors for forming processes. The focus of theresearch reported in this paper is to create all-PP laminateswith excellent bidirectional mechanical properties, a highvolume fraction of reinforcement and a large temperatureprocessing window which allows direct forming of three-dimensional geometries for unconsolidated fabric precur-sors. This can be achieved by consolidating fabrics wovenfrom a highly oriented, co-extruded PP tape. The tape iscomposed of three layers: a central highly oriented PPhomopolymer core, with a thin skin layer (5.5% tape thick-ness) of a propylene–ethylene copolymer on either side.After co-extrusion, the tape is highly drawn in a two stage,solid state drawing process to achieve high modulus andstrength by molecular orientation. The copolymer skinlayer possesses a melting temperature below that of thehomopolymer core, thus facilitating bonding of the tapesat temperatures below the melting temperature of the core.Thus, the fabric can be consolidated into a laminate with-out significant thermal relaxation of the highly oriented,high modulus homopolymer core [22].

The mechanical properties of any highly oriented poly-mer structure is dependant on microstructure and so ther-momechanical history [23,24]. The consolidation of theseco-extruded PP tapes into a coherent composite materialrequires heating, but highly oriented polymers experienceshrinkage forces in the drawn direction upon heating dueto thermal energy allowing relaxation to a higher entropy[25–27]. Thus, there is always a risk that shrinkage andrelaxation may occur during composite consolidation. Ifcomplete relaxation is allowed to occur, then the high ten-sile properties achieved during the tape drawing processwill be reduced and so not be transferred to the resulting

composite. In order to limit or prevent this relaxation, itis important to carefully define the parameters used to con-solidate a collection of tapes into a load bearing structure.The composite production parameters considered here aretemperature and pressure applied during consolidation,although the effect of temperature on the interfacial prop-erties of all-PP composites have been assessed [2] and willbe the subject of a subsequent paper. In order to make acoherent all-PP composite that possesses a strong interfa-cial strength and retains the high mechanical propertiesof the constituent tapes, the following processing condi-tions must be applied:

(i) Sufficient pressure to prevent shrinkage by lateralconstraining and encourage good interfacial contactfor bonding, but not enough to encourage flow ofhomopolymer core, which would result in a loss ofproperties.

(ii) Sufficient temperature to melt the copolymer layer toenable fibre bonding, but not so much as to encour-age shrinkage, relaxation or melting of the orientedhomopolymer phase.

This theoretical processing window is shown schemati-cally in Fig. 1, although conflicting processes complicatethe exact geometry of the window. As consolidation tem-perature is increased, viscosity of the copolymer decreasesand so a lower minimum pressure would be predicted foradequate consolidation. However, as temperature isincreased, shrinkage forces in the tapes increase [28] andso greater pressure is required to prevent tape shrinkage.As pressure is increased, the maximum applicable temper-ature decreases since at high pressures, flow and hencerelaxation, is encouraged, and so a combination of hightemperature and pressure aids relaxation by lateral flowof the composite in the mould.

Even though Fig. 1 presents a clearly defined processingwindow, in reality this is not the case. Within the process-ing window, mechanical properties are expected to vary as

Table 2Summary of all-PP laminates created for tensile testing

Specimen Compactiontemperature(�C)

Compactionpressure(MPa)

Production route

W140L 140 0.1 (Low) Stacked plies, vacuum ovenW140M 140 1.2 (Med.) Stacked plies, hot pressW140H 140 12.4 (High) Stacked plies, hot pressW160L 160 0.1 (Low) Stacked plies, vacuum ovenW160M 160 1.2 (Med.) Stacked plies, hot pressW160H 160 12.4 (High) Stacked plies, hot press

To Vacuum pump

Mould

Composite

Vacuum bag

Oven

Pressure applied byatmospheric pressure

Fig. 2. Schematic set-up for consolidation of all-PP composites in avacuum oven for consolidation at low pressure (vacuum bagging).

ure

PC

TC

B. Alcock et al. / Composites: Part A 38 (2007) 147–161 149

the microstructural mechanisms of shrinkage and lateralflow compete. By creating a range of composite specimenswith woven fibre architectures over a range of processingtemperatures and pressures, the effect on mechanical prop-erties can be established. A previous study [2,29] estab-lished that the temperature processing window ofunidirectional all-PP laminates exceeded 20 �C without asignificant effect on tensile modulus or strength of the lam-inates. The aim of this paper is to characterise bidirectionalall-PP composite laminates by investigating the tensile andcompressive strength and tensile modulus of all-PP com-posites made from consolidated woven tape fabrics, as afunction of compaction temperature and pressure.

2. Specimen preparation

The experimental tape used throughout this paper is aco-extruded three layer tape, with an A:B:A (copoly-mer:homopolymer:copolymer) structure, was manufac-tured at Lankhorst Indutech BV, the Netherlands. Thetape properties are summarised in Table 1. This tape wassubsequently woven into a balanced, plain weave fabricpossessing an areal density of �100 g m�2 by BW Indus-trial BV, the Netherlands.

Layers of this fabric are cut into square pieces measur-ing 180 mm · 180 mm, and stacked in a close fitting mouldup to a maximum depth of 7 mm. The mould is then sub-jected to heat and pressure to consolidate the fabric pliesinto a biaxially reinforced composite laminate. To establishthe effect of varying compaction pressure on the mechani-cal properties of all-PP composites, two different tech-niques are applied to compact the woven compositelaminates. For pressures in the range 1–15 MPa (�10–150 bar), a conventional hot press is used, while for con-solidation with a pressure of 0.1 MPa (1 bar), vacuumbagging technology is used. The nomenclature of specimensproduced together with processing route and conditions isshown in Table 2.

To consolidate all-PP composites in a hot press, the fab-ric plies are placed in a mould, which is positioned betweenthe platens of a 500 kN manual hot press equipped withelectronically controlled heating platens and water-cooledcooling platens. Due to the inaccuracy of pressure controlof the hot press at very low pressures, specimens com-pacted at 0.1 MPa (1 bar), are consolidated by vacuumbagging, rather than hot pressing. To manufacture vacuum

Table 1Mechanical properties of PP precursor tape for all-PP composites

Width [mm] 2.15Thickness [lm] 60Density, q [g cm�3] 0.732Draw ratio, k 17Composition [A:B:A] 5.5:89:5.5Tensile modulus [GPa] 15Tensile strength [MPa] 450Strain to failure [%] 7

bagged specimens, a mould is sealed in a vacuum bag con-nected to a vacuum pump that applies a negative pressureinside the vacuum bag, and so atmospheric pressure(�0.1 MPa) is used to compact the plates (see Fig. 2).The entire bag is placed inside a circulated hot air ovenand consolidation occurs.

The time–temperature and time–pressure profiles forboth methods are identical. It is important to note thatpressure is applied before heating starts and then remainsconstant during consolidation until the specimen has beencooled to a release temperature (approximately 40 �C). Thetime–temperature and time–pressure profiles for bothmethods are shown in Fig. 3. The specimen is placed in

t4

t3

t2

t1

Temperature

Tem

pera

t

Time

TR

Pressure

Pressure

Fig. 3. Time–temperature and time–pressure profiles during consolidationof all-PP composites.


the hot press or vacuum oven at the release temperature(TR) of 40 �C. At this temperature, there is no loss of ori-entation in the absence of applied pressure. It is importantto note that the compaction pressure (PC) is applied beforeheating starts and then remains constant during consolida-tion at the compaction temperature (TC) until the specimenhas been cooled to TR. Heating a specimen from TR to TC

(t2 � t1) takes approximately 10 min while cooling from TC

to TR (t4 � t3) is possible in approximately 5 min, althoughthese durations are specific to the equipment used. Theconsolidation time (t3 � t2) is kept constant at 10 min,while the entire cycle from insertion of the mould intothe hot press or vacuum oven, until removal of the consol-idated plate (t4 � t1) has a duration of approximately25 min. Table 3 summarises the parameters of time, tem-perature and pressure applied in the creation of the all-PP composites described in this paper. The application ofpressure acts to consolidate tapes, encourage copolymerflow and prevent shrinkage by lateral constraining.

All specimens for mechanical testing were cut from lam-inates consolidated as described above. Test specimenswere cut by using a razor blade, to the dimensions listedin Table 4. The dimension of the specimens was limitedin some cases by the dimensions of the mould used to man-ufacture the specimens. For the microscopy images pre-sented in Fig. 13, the all-PP composites are cut with aglass knife, after cooling by liquid nitrogen, using a LeicaRM2165 microtome. The images are captured using a ZeissStemi SV11 stereomicroscope equipped with a high resolu-tion, digital camera.

Table 3Consolidation parameters for the production of woven tape all-PPcomposites

Consolidation method Vacuum oven Hot press

Release temperature, TR [�C] 40 40Compaction pressure, PC [MPa] 1–15 0.1Heating time, t2 � t1 [min] 10 10Compaction time, t3 � t2 [min] 10 10Cooling time, t4 � t3 [min] 5 5

Table 4Test specimen dimensions for tensile and compressive testing of consol-idated all-PP laminates

Test Specimens Tapeorientation

Dimensionsa

Tension W140/160/L/M/H [0�/90�] 150 mm · 12 mm· 1 mm

Tension W140/160/L/M/H [±45�] 150 mm · 25 mm· 1 mm

Compression W140C/W160C [0�/90�] 15 mm · 5mm · 5 mm

a Dimensions given as length · width · thickness. Exact specimenthickness depends on processing parameters.

3. Mechanical testing of woven tape all-PP composites

Tensile strength and modulus were measured using anInstron 5584 tensile testing machine, equipped with a5 kN load cell and Merlin data acquisition software. Spec-imens were placed in specially designed composite gripswhich allow 10� free rotation to reduce the effect ofmoments in off-axis specimens. The gauge length of tensiletest samples was 120 mm and tests were performed at2 mm min�1 with a small preload (�1 N). To fully charac-terise the samples, two types of tensile tests were per-formed: tensile deformation at low strain to determinemoduli and Poisson’s ratio, and deformation to failure todetermine strength and strain to failure. To determine thetensile modulus for each specimen, strain was measuredby strain gauges placed in the direction of tensile loadingat low strains (<2.5%). The Young’s modulus was calcu-lated in all cases using a range of 0.05–0.2% strain; in allcases this proved to be a linear and reproducible regionwith very little deviation. To determine Poisson’s ratio,an additional strain gauge was placed at 90� to the direc-tion of loading. To determine the tensile strength and strainto failure of the specimens, high strain (>2.5%) data fromthe cross-head displacement was used to measure exten-sion. Five specimens were tested for each case to ensurereproducibility. Specimens which failed within, or veryclose to, the gripped region of the specimen were discarded.

Compressive tests, denoted by a ‘‘C’’ suffix in Table 4,were performed on end-loaded specimens manufacturedin the same way as tensile specimens, but cut from thicker(5 mm) plates. Compressive tests were performed using aHounsfield H25KS tensile testing machine configured forcompressive loading and equipped with a 2.5 kN compres-sion load cell and compression plates, in accordance withASTM D695-96 and as described elsewhere [2]. The speci-men geometry is shown in Table 4. Tests were performedby applying compressive loading at a rate of 1.3 mm min�1.The recorded load increases with compression until ulti-mately tends reaches a uniform stress. This plateau of con-stant stress is recorded as the compressive strength. Asbefore, five specimens were tested for each case to providean accurate measure of the compressive strength. Compres-sive strength tests were performed along one tape direction[0�/90�] of woven tape specimens. All tests reported in thispaper were performed at room temperature, but the tensileand flexural properties of all-PP composites at elevatedtemperatures have also been investigated [2] and will bepresented in a subsequent publication.

During tensile testing, acoustic emissions of the speci-mens during failure was also monitored using a 15 mm dif-ferential sensor, connected to a 40 dB 1220A preamp andAEWin data acquisition software manufactured by Physi-cal Acoustics Ltd, UK. An analogue filter was applied withupper and lower thresholds of 2 MHz and 100 kHz, respec-tively, and a gain of 60 dB. A high viscosity grease wasused to couple the acoustic emission sensor to thespecimen.

Table 5Compressive strength of unidirectional tape all-PP composites comparedto woven tape all-PP composites

Compactiontemperature

UD r11C

[MPa] [29]UD r22C

[MPa] [29]Woven r11C

[MPa]

140 �C 39.2 10.8 34.4160 �C 55.8 12.0 43.2Percentage difference 42% 12% 26%


4. Results and discussion

4.1. Compressive properties of woven tape all-PP composites

Table 5 shows the compressive strength of a woven tapecomposite, Wr11C, compacted at 140 and 160 �C. Previousresearch [2] revealed that there is no increase in the trans-verse compressive strength of the unidirectional compositewith increasing compaction temperature but there is an

Fig. 4. (Top) Optical micrograph of a cross-section of an end-loaded compreincreased magnification of buckled region showing tape buckling and delamin

increase in compressive strength in the longitudinal direc-tion. An increase can also be seen in the compressivestrength of the woven composite with increasing compac-tion temperature.

A photograph of a typical failed woven composite com-pression specimen is shown in Fig. 4. In these photos, thedark regions are tapes travelling out of the plane of thepaper while the light regions are tapes travelling left to rightin plane of the paper. The tape structure is clearly buckled,with some interfacial failure in the buckled region. Theincrease in compressive strength of the woven tape compos-ite with increasing compaction temperature can be attrib-uted to the increase in buckling stability of the tapes dueto and increase in interfacial adhesion with increasing com-paction temperature [2]. The interfacial properties of all-PPcomposites will be presented in more detail in a subsequentpaper. However, since failure is by buckling, the bendingstiffness must also contribute to the compressive strength.

Delamination

Buckling

Fibrillation (cohesive tape

failure)

ssion test specimen (W160C) which has failed by buckling, and (bottom)ation. (Compression loading direction is horizontal.)

High [0

O /90O ]

Med [0

O /90O ]

Low [0

O /90O ]

High [+

/-45O ]

Med

[+/-4

5O ]

Low

[+/-4

5O ]

0

40

80

120

160

200

240

Pressure[Orientation]

Tens

ile S

tren

gth

[MP

a]

Compaction Temperature 140 oC 160 oC

Fig. 5. Tensile strength of woven tape all-PP composite laminatesconsolidated at two different temperatures (140 and 160 �C) and threedifferent compaction pressures (0.1 MPa (Low), 1.2 MPa (Med.) and12.4 MPa (High)), showing a large effect of consolidation pressure forboth [0�/90�] and [±45�] loading directions, but a negligible effect ofconsolidation temperature in specimens loaded in the [0�/90�] direction.

High [0

O /90O ]

Med [0

O /90O ]

Low [0

O /90O ]

High [+

/-45O ]

Med

[+/-4

5O ]

Low [+

/-45O ]

0

1

2

3

4

5

6

7

8


Tens

ile M

odul

us [G

Pa]


Fig. 6. Tensile modulus of woven tape all-PP composite laminatesconsolidated at two different temperatures (140 and 160 �C) and threedifferent compaction pressures (0.1 MPa (Low), 1.2 MPa (Med.) and12.4 MPa (High)), showing a large effect of consolidation pressure forboth [0�/90�] and [±45�] loading directions, and only a small effect ofconsolidation temperature.


The bending stiffness of an all-PP woven fabric composite ismainly governed by the stiffness of the tapes in the longitu-dinal direction, and so the transverse tapes do not contrib-ute greatly to the bending stiffness during buckling. This issupported by considering that the increase of the compres-sive strength of the woven composite specimen with increas-ing compaction temperature is approximately half that ofthe increase of the unidirectional specimen with increasingcompaction temperature. This seems to be due to the 50%(see Table 5) volume fraction of tapes in the longitudinaldirection providing bending stiffness in the woven tape com-posite. A higher magnification of Fig. 4 shows that in thebuckled region of the failed specimen, some delaminationis seen with some fibrillation traversing the delaminations.It is likely then that the increase in compressive strengthwith increasing compaction temperature is due to theincrease in interfacial strength resulting from higher com-paction temperatures.

The drawing mechanism increases tensile properties ofpolymer structures by aligning the microstructure of thematerial in the drawing direction [30–32], however it isunlikely that any increase would be seen in compressiveproperties of tapes. The compressive strength seen here isvery close that of bulk PP (�50 MPa), and since tensiletests have shown that there is very little affect of processingtemperature in this region on the microstructure of the PPtapes, it is clear that no real benefit is gained by using thesehighly oriented PP’s rather than bulk PP for purely com-pressive applications. Even highly oriented polyethylenefibres which show a much higher tensile strength (3 GPa)exhibit a very low compressive strength (70 MPa) [33]. Thispoor compressive strength must be taken into accountwhen designing all-PP products which will experience com-pressive or flexural loads.

4.2. Tensile properties of woven tape all-PP composites

The tensile properties of woven tape composites are pre-sented in Figs. 5 and 6, which show tensile strength andmodulus of woven tape composites loaded at h = 0�/90�and h = ±45�, where h is defined as the angle betweenthe tape direction and the loading direction. From Fig. 5,the values obtained for processing temperatures of 140and 160 �C both show very similar strengths but ash = ±45�, a difference in strength between specimens withthese compaction temperatures is observed. As the process-ing temperature increases, the off-axis strength of thesecomposites increases. This can be expected because thestrength of off-axis composites is due to interfacial bondstrength between tapes resisting delamination and so tapepull-out; at h = 45�, failure occurs by intraply shearingand tape pull-out. As compaction temperature increases,the interfacial strength increases, and so superior loadtransfer increases the total strength of the compositestructure.

Fig. 7 shows photographs of tensile testing specimenswhich have failed by a variety of mechanisms. The two

specimens on the left of the photograph are [0�/90�] speci-mens compacted at 140 and 160 �C, and 0.1 and 1.2 MPa,respectively. The main difference between these two speci-mens is the improved interface as established earlier andthis clearly affects the failure mode of these two specimens.Specimen W140L shows large amounts of delaminationwith entire plies of fabric debonding from the compositetogether with only small fabric breakage along the lengthof the composite.

[0°/90°]W140L [0°/90°]W160M [±45°] W140L [±45°]W160H

Fig. 7. The range of failure modes of woven tensile specimens. Specimens consolidated at 140 �C/0.1 MPa and loaded at [0�/90�] show widespreaddelamination, whereas those processed at 160 �C/1.2 MPa and loaded at [0�/90�] show local fibrillation. Specimens consolidated at 140 �C/0.1 MPa andloaded at [±45�] show debonding and interlaminar shearing over the entire specimen, whereas those processed at 160 �C/12.4 MPa and loaded at [±45�]show much more localised damage.


Specimen W160M shows a completely different failuremode; a single failure is seen in the centre of the specimenlength with some very localised micro-fibrillation, but nocomplete tape pull-out. Since tests on unidirectional speci-mens showed a negligible decrease in tensile strength of thelongitudinal tapes with increasing compaction temperature[2,29], this change in failure mode must be due to increasedinterlaminar strength allowing local stresses to build up toreach the longitudinal tensile strength of the tapes.

A similar effect is seen in the case of woven fabric com-posites tested in the [±45�] direction. Unlike in the case ofloading W140M in the [0�/90�] direction, shear deforma-tion over the length of the specimen is clearly visible by areduction in specimen width following loading in the[±45�] direction. This reduction in width is accompaniedby an increase in specimen thickness caused by intraplyshearing (the so-called trellis effect [34]). Fig. 8 shows com-posite optical micrographs of the failure surfaces of (a)W140L and (b) W160H. Ultimately, as in the case of [0�/90�] loading, failure of W140L is by a tape pull-out mech-anism following rotation of the tapes to align nearer to the

loading direction, as can be seen at the failure location.Specimen W160H also shows a high degree of deformationover the whole specimen length due to tape shearing, but inthis case, tape pull-out is absent at the failure location.Instead, tape breakage occurs and the failure site is similarin appearance to the failure site of [0�/90�] specimen, with a‘‘brush-like’’ fracture surface showing extensive micro-fibrillation. In either case, tapes have rotated from 45� to27� to the loading direction.

The concept of intraply shearing and fibre pull-out isreinforced by Fig. 9, which shows the strain to failure ofwoven tape composites. At h = 0�/90�, the strain to failureis very low, approximating the strain to failure of a singletape, since this failure mode is dominated by tapes loadedin tension. As h increases, increasing amounts of shearingand tape debonding lead to a much greater apparent strainto failure of the composite system of �40%. The effect ofprocessing parameters is less clear, but specimens com-pacted at lower temperatures show a greater strain to fail-ure due to the lower interfacial strengths allowing greatertape pull-out and debonding.

Fig. 8. Optical micrographs showing tape debonding and interlaminar shear failure modes of woven all-PP composites compacted at 140 �C/0.1 MPa,compared to the localised failure of specimen consolidated at 160 �C/12.4 MPa and loaded at [±45�].

High [0

O /90O ]

Med [0

O /90O ]

Low [0

O /90O ]

High [+

/-45O ]

Med [+

/-45O ]

Low [+

/-45O ]

0

10

20

30

40

50


Stra

in to

Fai

lure

[%]


Fig. 9. Strain to failure of woven tape all-PP composite laminatesconsolidated at two different temperatures (140 and 160 �C) and threedifferent compaction pressures (0.1 MPa (Low), 1.2 MPa (Med.) and12.4 MPa (High)), showing a large increase in strain to failure inspecimens loaded at [±45�] due to tape debonding and interlaminarshearing compared to those loaded at [0�/90�].


The effect of processing conditions on the tensile modu-lus of woven tape all-PP composites is much smaller. Sincethe modulus depends on the microstructure of the tapes,whereas the tensile strength depends on a combination of

tape microstructure and composite macrostructure, it hasalready been shown for a unidirectional specimen thatthe modulus is largely unaffected by processing tempera-ture within this range [29]. However, increasing the appliedpressure during consolidation seems to increase the modu-lus of a composite loaded at h = 0�/90� and h = ±45�. Thegreatest modulus seen is for the specimen with the greatestapplied pressure and temperature, is 53% of E11 of the tape,and since the volume fraction of longitudinally orientedtapes in the woven fabric is 50% and the transverse modu-lus, E22, of tapes is relatively small (E22 = 1.5 GPa) [2,29],this agrees with the rule of mixtures.

If the woven composite is considered as two stacked uni-directional plies with 90� rotation between plies, the mod-ulus of the composite (Ecalc) can be predicted by usinglaminate theories [35] and compared to the experimentallymeasured values presented earlier. If the composite is con-sidered to be loaded in the [0�/90�] direction, then the mod-ulus can be calculated as follows. The modulus, Ec11, of theply containing tapes oriented in the 0� direction (parallel tothe direction of loading), is given by the rule of mixtures:

Ec11 ¼ Ef11V f þ Emð1� V fÞ ð1Þ

In which Ef11 is the tensile modulus of tapes in the tapedirection (Ef11 = 15 GPa), Vf is the volume fraction oftapes (Vf = 0.89) and Em is the tensile modulus of the ma-trix copolymer (Em � 1 GPa). This predicts a value for thetensile modulus of this ply, Ec11 = 13.46 GPa. The tensilemodulus, Ec22, of the ply containing tapes oriented in the

Table 6Efficiency factors, k1 and k2, comparing predicted and experimentallymeasured specific tensile moduli for all-PP composites consolidated at 140and 160 �C

Specimen EscðCalcÞ [GPa g cm3] Es

cðExpÞ[GPa g cm3] Efficiencyfactor

k1 k2

W140M 10.65 7.90 0.82 0.91W160M 9.55 7.41 0.82 0.95


90� direction (normal to the direction of loading) is pre-dicted by the Halpin–Tsai equation [35]:

Ec22 ¼ Em

1þ ngV f

1� gV f

� �ð2Þ

g ¼ ðEf22=EmÞ � 1

ðEf22=EmÞ þ nð3Þ

n ¼ 2ab

ð4Þ

where Ef22 is the transverse modulus of the tape (E

f22 = 1.5 GPa) and a and b are the width (a = 2.15 mm)and thickness (b = 65 lm) of the tape, respectively. It isimportant to note that since the tape is highly oriented inone direction, the tape itself is highly anisotropic and soEf22 6¼ Ef11. The high volume fraction of fibre, together withthe proximity of Ef22 and Em result in Ec22 being approxi-mately equal to Ef22, Ec22 = 1.45 GPa. Combining Ec11

and Ec22 predicts the tensile modulus of the combined lam-inates, Ec(Calc) = 7.45 GPa. However, due to the compac-tion process, varying compaction conditions alter thedensity of the composites by causing void closure and so af-fect apparent measurements. For this reason, the specifictensile properties must be compared since this takes into ac-count changes in density due to interlaminar void closure.The calculated specific modulus of the composite, Es

cðCalcÞ,i.e., normalised for composite density is given below:

EscðCalcÞ ¼

EcðCalcÞ

qð5Þ

where q is composite density. The density of the compositeprocessed at 140 �C and 1.2 MPa is 0.700 g cm�3 while thedensity of the composite processed at 160 �C and 1.2 MPais 0.780 g cm�3 (see Table 7). The calculated specific mod-uli of composites processed at 140 �C, Es

cðCalcÞ140, and160 �C, Es

cðCalcÞ160, are 10.65 GPa g cm3 and 9.55 GPa g cm3.

EscðExpÞ ¼ k1k2Es

cðCalcÞ ð6Þ

where k1 and k2 are efficiency factors which describe themajor discrepancies between the model and the experimen-tally measured values. k1 describes the loss of tensile mod-ulus caused by molecular relaxation which occurs in thehighly oriented homopolymer core during heating thestacked plies laminates in order to achieve bonding. k2 de-scribes the effect off-axis loading of tapes in the woven pliesdue to tape crimping in the woven structure [36,37]. Theuse of a thin tape geometry rather than a circular cross-sec-tion fibre geometry which is normally seen in conventionaltow or roving based composites, should reduce the degreeof crimping to maximise k2.

Both of these factors will reduce the tensile moduli seenexperimentally in composite laminates. Since the meltingtemperature of the copolymer skin layer is significantlylower than that of the homopolymer core, it would beexpected that k1 approaches 1, i.e., the loss of modulusdue to molecular relaxation is minimised. However, studiesrelating the tensile modulus of unidirectional all-PP com-posites with the tape precursor revealed a 18% loss in spe-

cific modulus due to molecular relaxation and tape densityincreases occurring during the consolidation process atboth 140 and 160 �C [2,29], therefore it is assumed thatk1 = 0.82. This value can be substituted in Eq. (6) to givek2 = 0.91 and 0.95 for compaction temperatures of 140and 160 �C, respectively (see Table 6). This effect is quitelow compared to that seen in conventional composites. A20% decrease in modulus due to fibre crimping has beenreported for carbon fibre reinforced epoxy composites[38]. The efficiency factor k2 increases with consolidationtemperature, suggesting a reduced crimping of tape proper-ties in the final composite consolidated at 160 �C.

In order to further evaluate the effect of the fabric crimpon the mechanical properties of all-PP composites, the the-oretical stiffness of the all-PP fabric was assessed by using atwo-dimensional finite element model using finite elementanalysis (FEA) software Abaqus v6.3. The model repre-sents the cross-section of a laminate based on multiple lay-ers of an all-PP plain weave fabric and consists of two unitcells of the fabric placed in parallel (see Fig. 10). In the fab-ric, the tape orientation is either parallel to the X direction(grey) or to the Z direction (black). The grey and the blacklayers represent only the homopolymer core of the co-extruded tape. The thickness of the copolymer skin wasset to 5 lm on both sides of the core. The model has alength X = 2.14 mm and a thickness Y = 0.260 mm. Thelength equals half the plain weave unit cell length (i.e.,the width of the tape) while the thickness equals four timesthe thickness of the tape. The following orthotropicproperties were considered for the core material:E11 = 15 GPa, E22 = 1.5 GPa, G12 = 0.8 GPa and m12 =0.36 while the copolymer skin was assumed isotropic(E = 1 GPa and m = 0.3). The longitudinal modulus ofthe tape (65 lm thick, E11 = 13 GPa) is then equivalentto the value reported previously and is discussed in a sepa-rate publication [2,29]. The region where the grey tape hasa 25� angle with the loading direction (X) is due to thecrimp of the fabric. This angle is realistic and correlatesto experimental observation. In the crimped region ofone fabric layer, no tape is parallel to the loading directionso that locally a low stiffness is expected based on the off-axis properties of the tape [2,29]. One fabric layer can thenbe assumed as a cross-ply element placed in series with ashort unidirectional off-axis element. If the fabric layerswere orderly stacked, the crimp regions would be alignedand only one fabric layer would be necessary to modelthe laminate. Here, two non-aligned layers of fabrics were

Fig. 10. Finite element model of a woven all-PP laminate. (Top) Geometry and material definition. (Bottom) Mesh and strain distribution, ex, when a totalstrain of 0.1% is applied along the X direction.

0.1 1 100.60

0.65

0.70

0.75

0.80

0.85

0.90

1.85

MP

a

0.35

MP

a

Den

sity

[g.c

m-3

]

Compaction Pressure [MPa]

UD140 UD160 W140 W160ρ

tape

Fig. 11. The effect of processing parameters on plate density of unidirec-tional tape plates compacted at 140 �C and 160 �C (UD140 and UD160)[29] and woven tape plates compacted at 140 �C and 160 �C (W140 andW160) compared to the density of the precursor tape, qtape. The woventape composites show an increase in density with increasing compactionpressure.


considered as a more realistic representation of a wovenlaminate. The distance between the crimp regions is maxi-mised and equals half the width of the tape. A total strainof 0.1% was applied to the model leading to a calculatedYoung’s modulus of 6.1 GPa. As this value compares wellwith the measured Young’s modulus (5.8 GPa), it can beconcluded that the properties of the oriented tape areretained. As expected the modulus of a woven fabric basedlaminate is lower than that of a non-crimp cross-ply lami-nate due to the low stiffness of the crimp regions. The dis-tribution of the strain, ex, reveals a locally higherdeformation in the crimp region (dark shades in Fig. 10).

Figs. 5 and 6 showed that the pressure applied duringcomposite consolidation greatly affects the tensile mechan-ical properties of the composites in the [0�/90�] direction.This observation will now be explained. It was observedduring specimen production that processing temperatureand pressure alter the density of the resulting composites.Density tests were performed on samples taken from thesame composite plates used for tensile testing, using theArchimedes principle. The relationship between the densityof the plates and the processing parameters is presented inFig. 11. It can be immediately seen that with increasingprocessing pressure, plate density increases. The effect ofincreasing compaction temperature is an increase in den-sity. The tape density, qtape, is also shown in Fig. 11, andis less than the density of isotropic polypropylene due tothe presence of micro-voids [2]. The woven tape specimens,W140 and W160, compacted at 0.1 MPa have a density lessthan the original tape. This can only be caused by the pres-ence of intertape voids, caused by the surface irregularity ofadjacent woven plies, due to tape crimping. In these speci-mens, consolidation is incomplete and poor interfacialproperties would be expected. With increasing compactionpressure, specimens compacted at both temperatures showan increase density, reaching and exceeding the originaltape density, qtape. Thus, with increasing pressure, therewill be a transition from a combined state of intertapevoids and voids within the tape, to solely voids within thetape. At the highest pressure applied here, 12.4 MPa, a den-

sity of �0.87 g cm�3 is achieved, which approaches thedensity of isotropic PP, �0.91 g cm�3. The copolymer skinlayer is highly viscous even above the melting temperatureand so requires some pressure to flow into the voids causedby the surface roughness due to the weave of the fabric.

From Fig. 11, the trends for compaction temperaturesof 140 and 160 �C have an approximately constant differ-ence suggesting that the density is predominantly depen-dant on compaction pressure, rather than temperature.An increased temperature will assist the filling of thesevoids by reducing the viscosity of the copolymer layerand encouraging copolymer flow, but compaction temper-ature in the range considered is not the dominant parame-ter. By assuming that filament wound unidirectionalcomposites possess a perfect, void free tape stackingarrangement, the effect of compaction pressure would notbe seen in unidirectional composites. Unidirectional speci-mens consolidated at 140 and 160 �C have very similar

0.1 1 104

5

6

7

8

9

10

11

Spe

cific

Ten

sile

Mod

ulus

[GP

a.g-1

cm3]

Compaction Pressure [MPa]

W160 E11

/ρ W140 E

11/ρ

50

100

150

200

250

300

W160 σ11

/ρ W140 σ

11/ρ

Specific T

ensile Strength [M

Pa.g

-1cm3]

Fig. 12. Specific tensile moduli and strengths of woven tape all-PPcomposite laminates consolidated at two different temperatures (140 and160 �C) and three different compaction pressures (0.1 MPa (Low),1.2 MPa (Med.) and 12.4 MPa (High)), showing an increase in specificmechanical properties with increasing consolidation pressure.


density, �7% greater than that of the tape [2,29]. The smalldifference in density between the tape and the unidirec-tional specimens is probably due some closure of voidswithin the tape during consolidation. The presence of inter-

140°C, 12.4MPa

Increasing Temperatu

140°C, 1.2MPa

140°C, 0.1MPa

Fig. 13. Cross-sections of woven tape composites compacted with increasing tintertape voids and degree of tape crimping.

tape voids in woven tape fabric must strongly affect themechanical properties of woven tape composites since thevoids can make no load bearing contribution to the com-posite structure in tension. However, in some compositesystems, the presence of intertape voids increases the strainto failure, and so the energy absorbed during failure of thecomposite, via a crack arresting mechanism [39]. Since ten-sile strength and modulus are inversely proportional to thecross-sectional area subjected to loading, the results pre-sented in Figs. 5 and 6 can be reassessed by normalisingthe mechanical properties for variations in density. Thissuggests that providing that molecular relaxation is pre-vented, tensile properties of all-PP composites increasewith increasing compaction pressure and temperature.The specific mechanical properties in the [0�/90�] directionas a function of compaction pressure are shown in Fig. 12.

Fig. 12 further reinforces the concept that tensile prop-erties in the [0�/90�] direction are not very sensitive tochanges in compaction temperature in the range 140–160 �C. It is likely that a set of optimum consolidationconditions will be seen with increasing compaction temper-ature and pressure, but ultimately these will encourageplastic flow, allowing molecular mobility and a loss ofmechanical properties. To fully understand the changes indensity seen in these composite plates, cross-sections of

160°C, 12.4MPa

Interply void

Increasing Pressure

re

160°C, 1.2MPa

160°C, 0.1MPa

emperature and pressure illustrating the effect of processing conditions on


woven tape composite plates were viewed using opticalmicroscopy. These images are presented in Fig. 13.Although some degree of crimping is seen in the compactedtape, the degree of crimping is reduced by the use of a thintape geometry reinforcement, rather than a traditionalglass fibre yarn or roving, which typically leads to a muchhigher degree of crimping and an associated decrease inmechanical performance of the composite [40].

In Fig. 13, light and dark areas are clearly visible in thecomposite cross-sections. The light areas are tapes travel-ling along the width of the specimen from left to right onthe page. The dark areas are tapes travelling out of theplane of the page. The reason for the differences in bright-ness between these tapes, is that the tapes oriented out ofthe plane of the paper are much rougher than those cutalong the plane of the paper, and so these tapes reflect lesslight and appear darker. The increase in compaction tem-perature from 140 �C (left-hand side) to 160 �C (right-handside) shows little affect, but increasing pressure (top tobottom of page) has a noticeable effect. Even though allcross-sections are presented at the same scale, the increasein density is clearly seen as the thickness of the laminatesdecreases noticeably with increasing pressure. Also seenin specimens compacted at 0.1 MPa is the conspicuouspresence of intertape voids, proving their existence. Theseintertape voids are also seen in specimens compacted at1.2 MPa at both temperatures but are absent from thosecompacted at 12.4 MPa and 160 �C. Therefore, it wouldseem that the optimum pressure to eliminate these intertapevoids is between 1.2 and 12.4 MPa, but also the exact valueof this ‘‘optimum’’ pressure may depend on compactiontemperature since following consolidation at 140 �C, even12.4 MPa is insufficient to totally close all the intertapevoids. This means that even though Fig. 11 showed thatthe composite plates processed at 140 and 160 �C wouldexceed the density of the tape if compacted at 0.35 and1.85 MPa, respectively, Fig. 13 shows that intertape voidsare still present in composites. The composite processedat 140 �C and 12.4 MPa has a density of �0.83 g cm�3,and so the presence of voids means that the density ofthe tape must be above this value; voids within the tapemust be closing before complete closure of intertape voids.This complicates a direct comparison between specificmodulus of tapes and specific modulus of composites.

The total closure of the intertape voids requires anexcess of molten copolymer to flow out from betweenadjacent tapes into the voids. However, the particularcopolymer selected for the skin layer was chosen to createa large enough temperature processing window as possible.The melting behaviour of the copolymer determines theminimum consolidation temperature during processingand the maximum application temperature of the all-PPcomposite product. The use of a copolymer with a lowermelting temperature would expand the temperature pro-cessing window to even lower temperatures, but wouldlimit the upper operating temperature of the composites;applying the composites in these higher temperatures

would lead to delamination and rapid composite failure,since the copolymer would soften and no longer be ableto facilitate load transfer between the tapes.

However, if the compaction temperature is too low, theviscosity of the copolymer melt may not be low enough toenable flow into these intertape voids. In this case the voidswill remain in the final composites. As well as affecting theapparent mechanical properties of all-PP composites, thepresence of these voids may have implications for applica-tions such as automotive in which the composites will be atrisk of fluid ingress. The specimen compacted at 160 �Cand 12.4 MPa shows total closure, flattening the compos-ites and forcing the tapes into alignment with the loadingdirection. This reduced crimping should definitely increasethe performance of the composite especially in view of theextremely poor off-axis properties of the tape, and isreflected in the increased k2 efficiency factor for compositesprocessed at higher temperatures, as described earlier.

A summary of the mechanical properties for woven tapefabric plates is presented in Table 7, with a comparison tocommercial glass reinforced PP composites: a typical glassmat reinforced PP (GMT, quadrant composites), a contin-uous, woven glass fibre reinforced PP (Twintex�, SaintGobain–Vetrotex) and a random flax mat reinforced PP(NMT) [41]. It is clear that woven all-PP composites cancompete with the GMT properties presented here, due tothe comparable mechanical properties together with theweight advantage of all-PP. The truly isotropic nature ofrandom short (glass or natural) fibre reinforced PP has tobe considered when comparing the apparently inferioroff-axis (h = ±45�) properties of woven tape all-PP com-posites. A variety of stacking arrangements (e.g., [0�/90�/±45�]s) can be applied to achieve quasi-isotropic propertiesin an all-PP laminate, if this is required.

Woven all-PP composites can compete with woven glassreinforced PP presented here in tensile strength applica-tions if density is taken into account, but cannot competein terms of tensile modulus due to the significantly greaterstiffness of glass fibres. While the ultimate mechanicalproperties of all-PP composites can be compared toGMT, the damage process during tensile deformation ofall-PP composites seems to be very much different. Thenumber of acoustic emission (AE) hits can be used to pro-vide valuable information about the onset of damage inglass fibre reinforced composites [42–44]. Fig. 14 showsthe AE hits per unit volume recorded during tensile defor-mation of all-PP composites consolidated at 140 and160 �C, compared to GMT and woven glass reinforcedPP. GMT and woven glass reinforced PP emit acousticevents from very low stresses due until failure. This isdue to the low strain to failure of glass fibres, combinedwith the relatively large scatter of glass fibre strength aswell as the poor interfacial strength giving a combinationof interfacial failure and fibre breakage. Conversely theall-PP composites reveal a much smaller number of AE hitsdue to the greater strain to failure of the PP tapes, togetherwith an excellent interfacial bonding. Also contributing to

0 50 100 150 200 250 300 3500

2

4

6

8

10

12

14

16

All-PP (140 oC) All-PP (160 oC) Woven Glass/PP GFRPP

Aco

ustic

Em

issi

on H

its

per

Uni

t Vol

ume

[mm

-3]

Stress [MPa]

Fig. 14. Acoustic emission of all-PP composites (compaction temperatureshown in brackets) loaded in tension to failure compared to twocommercial glass reinforced polypropylenes, indicating the excellentdamage resistance of well-compacted all-PP laminates.

Table 7Mechanical properties of all-PP woven tape laminates compared with commercially available alternatives (see also [45])

a GMT, quadrant composites.b Saint Gobain–Vetrotex.


this lower number of AE hits of all-PP composites com-pared to glass reinforced composites is the lower numberof larger reinforcing elements (tapes) in all-PP compositescompared to the high number of smaller, reinforcing ele-ments (glass fibres) present in either GMT or woven glassreinforced PP. The cross-sectional area of a PP tape(2.15 mm wide · 65 lm thick) is more than 500 times thatof a typical glass fibres (diameter 18 lm), and so the samevolume fraction of reinforcement is achieved with a muchgreater number of discrete glass fibres. Therefore, a muchlarger number of AE hits due to fibre failure in the GMTwould be expected, and this is seen in Fig. 14.

The difference in number of AE hits between all-PPspecimens compacted at 140 and 160 �C is indicative ofthe greater interfacial strength of all-PP composites consol-idated at higher temperatures, since Fig. 5 revealed that

these specimens have very similar tensile strengths. There-fore, the decreased number of AE hits seen for the speci-men compacted at 160 �C seems to be due to a reductionin AE hits caused by a reduced number of interfacialfailures.

5. Conclusions

It has been shown in this paper that the parametersapplied to consolidate all-PP composite specimens deter-mine the mechanical properties. Despite the high temper-atures involved during the consolidation process, the highstrength and stiffness of a highly oriented polymer tapecan be retained in a composite laminate, and all-PPcomposites agree with a rule of mixtures prediction ofmechanical performance, as shown in Table 6. Further-more, the tensile mechanical properties in the [0�/90�]direction are not sensitive to deviations in process temper-ature since mechanical properties proved approximatelyconstant within the temperature processing windowapplied (140–160 �C), while tensile mechanical propertiesin the [±45�] direction are sensitive to both consolidationtemperature and pressure. The mechanical properties mea-sured in this paper show that due to their high reinforce-ment fraction of around 90%, all-PP composites cancompete or outperform a popular type of glass fibre rein-forced PP (GMT), and shows only slightly inferior tensileproperties to a commercial woven glass reinforced PP,such as Twintex�.

The original reason for applying pressure during theconsolidation of woven tape all-PP composites was to pre-vent shrinkage and relaxation by lateral constraining. Sub-sequently, it became obvious that pressure is also requiredto provide good surface contact by making the opposingsurfaces, which are uneven due to the tape weave, meet.The compaction temperature and pressure can affect the


mechanical properties of these composites by altering boththe mesostructure of the composite (i.e., the structuralcomposition), and the microstructure (i.e., the molecularorientation) of the tape material, but by choosing suitableparameters, loss of mechanical properties can be mini-mised. Increasing compaction temperature increases theinterlaminar properties of all-PP composites [2], as indi-cated here by off-axis tensile testing together with acousticemission studies, and this will be presented in more detail ina subsequent paper. This study has clarified the relation-ship between these parameters and the mechanical perfor-mance of the composite although further study mayreveal absolute optimum processing conditions.

Acknowledgements

The authors would like to acknowledge the contributionof Fernando Eblagon for the FEA modelling presented inFig. 10. The co-extruded PP tapes used in this study werekindly supplied by Lankhorst Indutech BV, the Nether-lands, and woven into fabrics by BW Industrial BV, theNetherlands. This work was sponsored by the DutchGovernment’s Economy, Ecology and Technology (EET)programme for sustainable development, under grant num-ber EETK97104.

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Documents

The mechanical properties of woven tape all-polypropylene