Damping of thermoset and thermoplastic flax fibre composites

Accepted Manuscript Damping of thermoset and thermoplastic flax fibre composites F. Duc, P.E. Bourban, C.J.G. Plummer, J.-A.E. Månson PII: DOI: Reference:

S1359-835X(14)00118-3 http://dx.doi.org/10.1016/j.compositesa.2014.04.016 JCOMA 3607

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

29 July 2013 14 April 2014 21 April 2014

Please cite this article as: Duc, F., Bourban, P.E., Plummer, C.J.G., Månson, J.-A.E., Damping of thermoset and thermoplastic flax fibre composites, Composites: Part A (2014), doi: http://dx.doi.org/10.1016/j.compositesa. 2014.04.016

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Damping of thermoset and thermoplastic flax fibre composites F. Duca , P.E. Bourban∗,a , C. J. G. Plummera , J.-A.E. M˚ ansona a

Laboratoire de Technologie des Composites et Polym`eres (LTC), Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Station 12, CH-1015 Lausanne, Switzerland

Abstract The mechanical and damping properties of unidirectional (UD) and twill 2/2 flax fibre (FF) reinforced thermoset (epoxy) and thermoplastic (polypropylene (PP) and polylactic acid (PLA)) composites containing 40 vol% of fibres have been compared with those of carbon (CF) and glass (GF) fibre reinforced epoxy composites. Thanks to the relatively low density of the FF, the specific mechanical properties of the UD FF based composites were comparable with those of the GF epoxy composites. The composites reinforced with FF also showed improved damping as reflected by dynamic mechanical analysis with respect to composites reinforced with synthetic CF and GF. For example, the addition of UD FF to epoxy led to an approximately 100 % increase in loss factor with respect to both the matrix and GF reinforced epoxy. FF/PP showed the highest damping at 25 ◦ C and 1 Hz of all the composites investigated (tanδ = 0.033). However the best compromise between stiffness and damping was obtained with FF reinforced semicrystalline PLA. Key words: ∗

Corresponding author. Tel.: +41 21 693 58 06; fax: +41 21 693 58 80 Email address: [email protected] (P.E. Bourban)

Preprint submitted to Composites Part A

April 26, 2014

Natural fibres, A. Polymer-matrix composites (PMCs), B. Mechanical properties, B. Vibration, D. Mechanical testing 1. Introduction The use of natural fibres (NF) dates back to at least 8000 BC, at which time linen and hemp fabrics are known to have existed. These and many other types of NF have since been widely used for clothes, ropes, canvas, paper, pottery, etc. Moreover, the potential of plant fibres as reinforcements for composite materials was recognized as long as 3000 years ago, when straw reinforced clay was first used by the Egyptians as a building material [1]. Owing to the improved performance and reduced price of technical fibres (glass, aramid, carbon etc.), the use of the NF in composites had almost ceased by the middle of the 20th century [1, 2]. However, increased environmental awareness, dwindling non-renewable raw materials and a growing global waste problem have led to a renaissance in NF from sustainable resources, and stimulated interest in so-called biocomposites. NF may be classified according to their origins, i.e. whether they are derived from plants, animals or minerals [3]. Of these, plant fibres (mainly bast and leaf types) are the most widely used as reinforcements in biocomposites. Such fibres are themselves composite materials, being made up of an amorphous lignin and/or hemicellulose matrix reinforced with crystalline cellulose microfibrils [3]. Certain types of fibre also contain small quantities of pectin. The properties of each constituent contribute to the overall properties of the fibre. Plant-based NF typically have poorer mechanical properties than synthetic fibres [3, 4]. However, owing to their lower density, their specific properties 2

may be comparable to, or even better than those of e.g. glass fibres (GF). This represents one of the main advantages of NF composites in applications for which property requirements include weight reduction. NF have other advantages, however. They are not only derived from low cost renewable raw materials but are also biodegradable, CO2 neutral and nonabrasive. They cause less dermal and respiratory irritation than synthetic fibres, and enhance energy recovery during their end-of-life treatment by incineration. The intrinsic damping properties of NF are also of key interest, notably in sports equipment [5], where a good balance between damping and stiffness is important for providing the athlete with optimum feel and control. As a consequence, tennis rackets, bicycle frames and skis incorporating NF composites are now entering the market. Flax fibres (FF) are currently the most common NF for composite applications. However only 6% of annual FF production is used in composites [5]. In industry, FF are typically used with epoxy (EP) resins, and EP reinforced with FF or chemically treated FF is also widely described in the literature [6–15]. For example, van de Weyenberg et al. report a Young’s modulus (E) and a tensile strength (σmax ) of 28 GPa and 133 MPa respectively for an unidirectional (UD) EP composite containing 40 vol% FF. As ecological impact becomes increasingly important in the development of new materials, thermoplastic and even bio-based matrices combined with NF are also gaining interest. Moreover, thermoplastic matrices may also show improved damping behaviour with respect to EP [16]. Polypropylene (PP) is currently the thermoplastic matrix most widely used with NF [17–23]. However, polylactide (PLA) has also recently been combined with various NF, including jute, FF,

3

hemp, bamboo, wood flour and wood fibres [24–28]. While the damping of NF composites is of increasing interest in industry, to the authors’ knowledge, there has so far been little detailed comparison of the damping performance of NF composites with composites based on synthetic fibres. There are many different definitions and ways of measuring damping. For example, the loss factor, the quality factor, the specific damping capacity, the logarithmic decrement or the damping ratio all provide information on damping properties, and indeed are explicitly linked at low damping levels [29]. Dynamic mechanical analysis (DMA) is a particularly convenient way to measure the loss factor and hence assess the damping performance of composites. Talib et al. [30] studied the dynamic mechanical properties at 1 Hz of 0 to 60 wt% randomly oriented kenaf fibre reinforced PLA composites. The damping peak (tan δ) of composites containing more than 50 wt% kenaf fibres showed a decrease in amplitude with respect to that for neat PLA. However the amplitude of the tan δ peak increased sharply at low fibre contents. Wielage et al. [18] considered the dynamic mechanical properties of FF, hemp and GF reinforced PP composites. They again found that the loss factor decreases as the fibre content is increased. Even so, at a fibre content of 30 wt%, the loss factor was significantly higher for FF than for GF. The aim of the present study is to quantitatively determine and directly compare the damping properties of thermoset and thermoplastic composites reinforced with UD and twill 2/2 (TW) FF fabric. The properties measured for the FF/EP composites will first be compared with those of the currently most widespread composite materials, i.e. carbon fibre-reinforced EP (CF/EP) and GF/EP. A discussion will then be given of the behaviour of FF-based

4

thermoplastic composites, with the aim of assessing the effect of the matrix on the damping properties. The main objective is to quantitatively determine the damping performance of NF composites and thus to identify the damping mechanisms induced by NF. 2. Experimental details 2.1. Fibres and polymer matrices 2.1.1. Carbon fibres UD and TW woven CF fabrics were purchased from Swiss-Composite in Fraubrunnen, Switzerland. The fibre average weight (FAW) of the UD and TW fabrics were 270 g/m2 and 200 g/m2 respectively. 2.1.2. Glass fibres UD and TW woven GF fabrics were purchased from Swiss-Composite. The FAW of the UD and TW fabrics were 220 g/m2 and 280 g/m2 respectively. 2.1.3. Flax fibres UD and TW woven FF fabrics were purchased from LINEO, in Meulebeke, Belgium. The yarns used to produce the TW fabric consisted of FF assembled by torsion, which introduces significant crimp. The fibres were neither treated nor sized to modify their original surface conditions. The two only fabrics containing fibres and yarns pre-coated with epoxy were the FlaxPly-E UD and TW used for the FF/EP composites. The FAW of the UD FlaxPly-E and TW FlaxPly-E were 180 g/m2 and 300 g/m2 respectively.

5

Dry FF woven fabrics (FlaxDry) (UD and TW) were used for the thermoplastic based FF composites. The FAW of the UD FlaxDry and TW FlaxDry were 180 g/m2 and 300 g/m2 respectively. In order to reduce moisture and thus formation of defects in the composite parts, the fibres were always dried for 12 hours at 60 ◦ C before processing. 2.1.4. Epoxy matrix The EP resin L-235, purchased from Swiss-Composite, was used as a thermoset resin. The hardener was Epoxy-H¨arter 236 from the same company. Its density was 0.99 g/cm3 . E and elongation at break were 3.5 GPa and 1.4 % respectively. 2.1.5. Polypropylene matrix In order to ensure good impregnation of the woven FF fabrics, a low viscosity PP homopolymer, Molpen HP500V from LyondellBasell industries, was chosen as a thermoplastic resin. Its density was 0.910 [g/cm3 ]. E and elongation at break were 2.0 GPa and 20 % respectively. 2.1.6. Polylactide matrix To investigate the performance of novel sustainable ”green composites” based on NF and biopolymers, two different grades of PLA from NatureWorks LLC (USA) were selected for their processing properties: (i) PLA 2002D (PLA2), a semicrystalline grade, with a density of 1.24 g/cm3 , and E and elongation at break of 3.6 GPa and 5.5 %, respectively. (ii) PLA 4032D (PLA4), a less crystalline grade, with a density of 1.24 g/cm3 , and E and elongation at break of 3.6 GPa and 7 %, respectively.

6

Following previous work [31], the hygroscopic PLA granules were dried in a vacuum oven at 35 ◦ C for one day and then overnight at 65 ◦ C, with a vacuum of at least 200 mbar, so as to limit the moisture content to below 250 ppm, and hence prevent degradation during subsequent processing. The thermal properties of the polymers were determined by Differential Scanning Calorimetry (DSC) temperature scans at a rate of 10 ◦ C/min. The glass transition temperature (Tg ) (from cooling scans) and, where relevant, the melting temperature (Tm ) (from heating scans) are given in table 1. Table 1: Tg (from cooling scans) and Tm (from heating scans) measured by DSC at heating and cooling rate of 10 ◦ C/min for the different matrices. Materials Tg [◦ C] Tm [◦ C] EP 77.4 − PLA2 47.0 154.8 PLA4 50.7 171.2 PP (≈−10) 170.9

2.2. Composite processing 2.2.1. Resin Transfer Molding (RTM) RTM was used to produce CF, GF and FF/EP composite plates. A volume fibre fraction of 40 % was used throughout. The final dimensions of the plates were 260×260×2.5 mm3 . Prior to impregnation, the mold and the resin were preheated to 60 ◦ C in order to reduce the resin viscosity. A vacuum pressure of 0.8 bar and a constant injection pressure of 0.7 bar were applied during impregnation. The plates were then cured in the mould at 40 ◦ C for twelve hours.

7

2.2.2. Compression molding Compression moulding (CM) was used to produce 140×60×2.5 mm3 rectangular FF/PP and FF/PLA plates, again with a fibre volume fraction of 40 % throughout. PP and PLA sheets of 1 mm thick were prepared using a Twin Screw Prism extruder, with the temperatures of the four heating zones (in the direction of extrusion) set to 160 ◦ C, 160 ◦ C, 150 ◦ C and 150 ◦

C, and 190 ◦ C, 190 ◦ C, 180 ◦ C and 180 ◦ C for PP and PLA respectively.

The sheets were stacked with interleaved woven FF fabrics and consolidated using a Fontijne press. In order to prevent degradation of the FF [32], the processing cycle was adapted to minimize exposure of the fibres to elevated temperatures. The mould was first preheated to 180 ◦ C for one hour to obtain an homogeneous temperature distribution. Then the FF/polymer stack was placed in the mold. A pressure of 5 bar was applied for five minutes at 180 ◦

C. The mold was then cooled under pressure to room temperature over 10

minutes. Table 2 summarizes the different composites produced for this study and their thermal properties and density are given in table 3. The densities were generally similar for the UD and TW composites. 2.3. Mechanical properties The mechanical properties of the polymers and composites were determined from tensile tests at 1 mm·min−1 using a screw driven tensile test machine from UTS Testsysteme GmbH, equipped with a 100 kN load cell and extensometers and following the norms ASTM D638 and ASTM D3039 for polymers and composites respectively. All the composite specimens had a width of 20 mm and a thickness of approximately 2.5 mm. The length 8

Table 2: The Fibres CF CF GF GF FF FF FF FF FF FF FF FF

different composites and abbreviations used in this study. Matrices Fabrics Process Abbreviations EP UD RTM CF EP UD EP TW RTM CF EP TW EP UD RTM GF EP UD EP TW RTM GF EP TW EP UD RTM FF EP UD EP TW RTM FF EP TW PP UD CM FF PP UD PP TW CM FF PP TW PLA2 UD CM FF PLA2 UD PLA2 TW CM FF PLA2 TW PLA4 UD CM FF PLA4 UD PLA4 TW CM FF PLA4 TW

Table 3: Tg (from cooling scans) and Tm (from heating scans) measured at heating and cooling rate of 10◦ C/min and density for the composites. Materials Tg [◦ C] Tm [◦ C] Density [g/cm3 ] CF EP 73.2 − 1.38 GF EP 70.5 − 1.73 FF EP 68.3 − 1.21 FF PLA2 46.9 148.1 1.29 FF PLA4 48 167 1.33 FF PP (≈−10) 166.3 1.10

9

of the EP-based samples was 240 mm. The clamped length was 50 mm (at either end) and the gage length was 60 mm. The corresponding values for the thermoplastic-based specimens were 130 mm for the length, 30 mm for the clamped length and 40 mm for gage length. Three specimens of each material were tested. E was determined by linear extrapolation of the stress-strain curve between 0.05 and 0.15 % of strain. 2.4. Damping properties DMA (Q800 from TA Instruments) was used in the single cantilever mode to characterize the damping behaviour of the composites under flexural conditions of sollicitation. Specimens of 35 mm in length (in the direction of the fibres for the UD composites), 10 mm in width and approximately 2.5 mm in thickness were used throughout. Temperature sweep tests (TS) were made to evaluate the damping behaviour of the composites at specific frequencies. Each composite was tested at three different frequencies (0.1 Hz, 1 Hz and 100 Hz). At each frequency, the composites were heated from -40 ◦

C to 120 ◦ C at 2 ◦ C/min at a constant deformation of 0.01 %. A soak time

of 2 minutes was applied at -40 ◦ C and the preload force was set to 0.0001N. All tests were repeated twice with different samples. 3. Results and discussion 3.1. Mechanical properties 3.1.1. Stress-strain curves Figure 1 shows typical stress-strain curves for the neat EP resin and the CF EP UD, GF EP UD and FF EP UD composites. The CF EP UD and 10

the GF EP UD showed the highest σmax , while the FF EP UD showed a slightly higher elongation at break than the former, albeit less than that for the neat EP. 1400 CF_EP_UD 1200

Stress [MPa]

1000 800 600 GF_EP_UD 400 FF_EP_UD 200 EP 2

1.5

1

0.5

0

0

Strain [%]

Figure 1: Stress-strain curves obtained from tensile tests on unreinforced EP and CF EP UD, GF EP UD and FF EP UD composites. All the EP UD composites showed characteristic stress-strain curves with an initial linear elastic regime followed by a point of inflection and a non-linear regime at high strains, as is seen most clearly for CF EP UD in Figure 1. This transition occured at about 0.6 % strain for CF EP UD and GF EP UD and at about 0.2 % for FF EP UD. The form of the curves is explained by the nature of the reinforcement. As pointed out in section 2.1.3, the FF yarns in the FF fabrics are assembled by torsion. The axial stiffness of the fibre yarns depends strongly on the compression and friction between the twisted fibres. With increasing axial load, the fibres may slip within the yarns, decreasing the effective stiffness.

11

3.1.2. Mechanical properties E and σmax of the polymers and composites are given in table 4, and the specific moduli (Es ) and strains at break of the composites are shown in figure 2(a) to (d).

TW composites UD composites

Polymers

Table 4: E and σmax of the unreinforced polymers and the UD and TW composites. The standard deviations are given in brackets (Std). Materials E (Std) σmax (Std) [GPa] [MPa] EP 3.5 (0.3) 43.3 (14.0) PP 2.0 (0.3) 28.0 (6.5) PLA2 3.6 (0.1) 56.3 (0.2) PLA4 3.6 (0.1) 65.8 (0.1) CF EP 101.2 (6.6) 1207.7 (47.72) GF EP 35.4 (1.7) 514.2 (83.7) FF EP 20.2 (2.0) 258.8 (3.1) FF PP 17.4 (3.7) 215.4 (23.4) FF PLA2 18.2 (2.5) 240.0 (13.4) FF PLA4 18.3 (0.7) 234.8 (19.7) CF EP 28.4 (0.3) 607.7 (28.6) GF EP 18.3 (2.9) 245.5 (7.4) FF EP 7.9 (0.7) 85.1 (5.5) FF PP 5.8 (0.3) 64.5 (2.0) FF PLA2 8.8 (0.6) 85.8 (5.3) FF PLA4 8.8 (0.1) 79.0 (0.1) The highest values of E, σmax and Es were obtained for the CF reinforced composites. Indeed, the specific mechanical properties of the CF reinforced composites were also considerably better than those of the FF reinforced composites. As expected, E and Es were generally higher for the UD composites than for the TW composites, owing to the higher degree of fibre orientation in the former, but the TW composites showed relatively high strains at break. 12

UD composites

73.6 (4.8)

UD composites

2

3

Specific Young's modulus [GPa cm /g]

30

Strain at break [%]

25 20 15 10

1.5

1

0.5 5 Epoxy 0

CF

PP

GF

PLA2 FF

Epoxy

PLA4

0

CF

(a)

PLA2 FF

PLA4

PLA2

PLA4

(b) 5 TW composites

14 20.47 (0.22)

TW composites

3

Specific Young's modulus [GPa cm /g]

PP

GF

4

Strain at break [%]

12 10 8 6 4

3

2

1 2 0

Epoxy CF

PP

GF

PLA2 FF

Epoxy

PLA4

0

(c)

PP

GF

FF

(d)

Figure 2: Specific Young’s moduli and strains at break of UD ((a) and (b)) and TW ((c) and (d)) composites.

13

The absolute values of E and σmax obtained for the FF EP UD composites were similar to those reported by Weyenberg et al. [11] and Bensadoun et al. [33] for a FF EP UD composite with the same fibre content, bearing in mind that there may be differences in the characteristics of the bundled fibres used in each case [5]. They were also substantially lower than the values for the GF EP UD composites. However, Es was only 18 % lower in FF EP UD than in GF EP UD. In view of the experimental scatter, the stiffness obtained with FF was considered to be comparable with that obtained with GF. As seen from table 4, EP, PLA2 and PLA4 showed significantly higher E and σmax than PP. Addition of 40 vol% of FF UD nevertheless resulted in similar specific stiffness in the fibre direction, again, in view of the experimental scatter. Given the relatively low E of the PP matrix, the relative increase in the Es was much greater upon addition of FF for PP composite (636 %) than for the other polymers (about 375 %). The TW composites showed somewhat contrasting behaviour. E and σmax for FF EP TW were much lower than those for GF EP TW and the corresponding decrease in Es was 38 %. This trend was seen in all the TW composites and may be explained by the higher crimp in the FF TW woven fabrics than in the GF TW and FF UD woven fabrics. Verpoest et al. [5], have demonstrated that a higher crimp in twisted yarns may result in poorer composite mechanical properties owing to the greater misorientation within the yarns. On the other hand, the crimp effect may also contribute to the higher strain at break obtained with FF TW fabrics. The larger contribution of the matrix strain in TW composites also explains the higher strains at break of the thermoplastic composites.

14

Of the FF reinforced thermoplastic composites, those based on PLA2 gave the best results, showing similar or even improved properties with respect to the FF EP based composites, while the more amorphous PLA (PLA4) gave somewhat lower σmax . However, the lowest values of E and σmax were obtained with the PP composites, presumably reflecting the lower stiffness and tensile strength of the PP matrix. The FF PLA2 UD composites showed E and σmax that were respectively 5 % and 11 % higher than for the FF PP UD composites. As a sustainable biocomposite, FF PLA therefore has competitive mechanical performance. 3.2. Damping properties In this section representative results from DMA temperature sweep tests at different frequencies on FF PP TW are first considered. The results obtained at 1 Hz will then be used to compare the different materials. Temperature sweep (TS) test. The evolution of the storage modulus, loss modulus and loss factor with temperature at three different frequencies is shown in figure 3 for FF PP TW. These results are consistent with those obtained by Wielage et al. [18]. The peaks in the loss modulus and loss factor at around 0 ◦ C shifted to higher temperatures as the frequency increased as expected for a classical viscoelastic response. This peak corresponds to the onset of long range cooperative mobility in the amorphous phase, resulting in a decrease in the storage modulus and an increase in the loss factor and loss modulus. At temperatures above this transition, the damping properties were dominated by the matrix, resulting in increased damping at low frequencies, for which more relaxation process are activated at a given temperature.

15

However at temperatures below this transition the trend was reversed, with an increase in energy dissipation with frequency, suggesting the fibres and/or fibre/matrix interactions to have greater influence on the damping properties. For each polymer, this transition temperature was defined as TT rans and correspond to the temperature of the loss factor peak just below the Tg of the amorphous phase measured in DMA temperature scans at 1 Hz. 4

Storage modulus

FF_PP_TW

0.16 0.12

0.1 Hz 1 Hz 100 Hz

0.08

Loss factor

1000

0.04 0

Loss modulus

-0.04 150

100

50

0

100 -50

Loss factor

Storage (E'), loss (E'') modulus [MPa]

10

Temperature [°C]

Figure 3: Evolution of the storage modulus, loss modulus and loss factor of FF PP TW with temperature at three different frequencies.

Epoxy based composites at 1Hz. The storage moduli and loss factors for the UD and TW EP based composites are shown in figure 4(a) and (b) respectively. As seen in figure 4(a), on adding 40 vol% UD fibres to the EP matrix, the storage modulus generally increased, and the intensity of the loss factor peak decreased. Moreover TT rans , which was characterized by a sharp drop in storage modulus and a peak in loss factor, was higher in the composites than in the matrix. This may be due to a higher degree of matrix cure in the presence of fibres. That the EP may not have been completely cured after

16

processing in the absence of fibres is consistent with the sharp increase in the storage modulus with temperature above TT rans (figure 4(a)). Consistent with the results of the tensile tests, at temperatures below TT rans , the storage moduli of FF EP UD and FF EP TW were lower than those of the respective GF and CF reinforced EP, implying the properties of the fibres to dominate the response under these conditions. Above TT rans , better adhesion between the FF and EP than between CF and EP or GF and EP, owing to the rougher surface of the FF, may have contributed to the observed behaviour [1]. The consequently limited sliding between FF and the matrix would account for both its relatively high storage modulus above TT rans and the reduced intensity of the loss factor peak. The importance of the fibre/matrix sliding on the damping properties has been demonstrated by Khan et al. [34], who showed that increased sliding obtained by incorporating CF nanotubes in CF reinforced EP composites, increased damping, even at temperatures below TT rans . Thermoplastic based composites at 1Hz. The storage moduli and loss factors at 1 Hz are shown as a function of temperature for all the thermoplasticbased composites in figures 5 and 6. FF PP UD showed markedly different behaviour to FF EP UD, the sharp drop in storage modulus characteristic of the glassy/rubbery transition in the EP composites being absent in the PPbased composites owing to its relatively high degree of crystallinity. Instead, this transition marks a transition to a regime in which the storage modulus decreases gradually with increasing temperature. It follows that at high temperature. FF PP UD showed a higher storage modulus than FF EP UD. The storage modulus of FF PLA composites showed a glassy plateau at 17

UD composites 3

4

10

1000

2 CF_EP GF_EP

100

1.5

FF_EP EP

Loss factor

Storage modulus [MPa]

2.5 Storage modulus

1

10 0.5 Loss factor 150

50

100

0 0

-50

1 Temperature [°C]

(a) TW composites 1.6

4

1.4 Storage modulus

1.2

1000

100

1

CF_EP GF_EP FF_EP

0.8 0.6 0.4

10

0.2

Loss factor 150

100

50

0 0

1 -50

Loss factor

Storage modulus [MPa]

10

Temperature [°C]

(b)

Figure 4: Storage modulus and loss factor at 1 Hz as a function of temperature for (a) UD and (b) TW EP based composites.

18

low temperature followed by a sharp drop at TT rans as with FF EP UD. However, on further increasing the temperature, the storage modulus increased, presumably owing to crystallization during the scan. UD composites 0.2 4

0.15 1000

Storage modulus 0.1

FF_PP PP 100

Loss factor

Storage modulus [MPa]

10

0.05 Loss factor

150

50

100

0 0

-50

10

Temperature [°C]

(a) UD composites 5

4

10

Storage modulus

4

100 10

3

FF_PLA2 FF_PLA4

1

PLA2

2

PLA4

Loss factor

Storage modulus [MPa]

1000

0.1 1 0.01 Loss factor 150

100

50

0 0

-50

0.001

Temperature [°C]

(b)

Figure 5: Storage modulus and loss factor at 1 Hz of UD (a) PP and (b) PLA based composites. Consistent with the results of the tensile tests, the TW composites showed lower storage moduli than the UD composites (figure 6). The main loss factor peak was also generally weaker for the TW composites, owing to a decrease 19

UD composites 4

1.4 1.2

Storage modulus 1000

1 FF_EP 0.8

FF_PP 100

FF_PLA2

0.6

FF_PLA4

Loss factor

Storage modulus [MPa]

10

0.4

10

0.2 Loss factor 150

100

50

0 0

-50

1

Temperature [°C]

(a) 10

4

TW composites 0.8 0.7 0.6

1000

0.5

FF_EP FF_PP

0.4

FF_PLA2 100

0.3

FF_PLA4

Loss factor

Storage modulus [MPa]

Storage modulus

0.2 0.1

Loss factor 150

100

50

0 0

-50

10

Temperature [°C]

(b)

Figure 6: Storage modulus and loss factor at 1 Hz of (a) UD and (b) TW FF based composites.

20

in the fibre/matrix sliding length. In a TW weave, the distance between the intersections between warp and weft yarns limits the length available for fibre/matrix sliding. 0.04

Loss factor at 1Hz and 25°C

0.035 0.03 0.025 0.02 0.015 0.01 0.005 0

Epoxy

PP

PLA2

PLA4

(a) 0.035

0.035

UD composites

0.025 0.02 0.015 0.01 0.005 0

TW composites

0.03

Loss factor at 1Hz and 25°C

Loss factor at 1Hz and 25°C

0.03

Epoxy CF

PP PLA2 FF

GF

0.025 0.02 0.015 0.01 0.005

PLA4 0

(b)

CF

Epoxy GF

PP PLA2 PLA4 FF

(c)

Figure 7: Loss factor at 1 Hz and 25 ◦ C of (a) the polymer matrices, (b) UD and (c) TW composites.

All composites at 1Hz and 25 ◦ C. Figure 7 summarizes the loss factor at 1 Hz and 25 ◦ C for all the polymers and composites, allowing quantitative comparison and illustrating the influence of the matrix properties on the composite damping performance. At 25 ◦ C, all the polymers are in their glassy state, with the exception of PP. 21

A marked increase in damping when using FF instead of CF or GF may be inferred from figure 7(b) and (c). However, while the evolution of the storage modulus was dominated by the fibre type, that of the loss factor also depended strongly on the matrix. Thus, the addition of FF to PP resulted in a decrease in loss factor, indicating the damping properties of the matrix to be superior to those of the fibres. On the other hand, addition of FF to EP increased damping significantly. Thus, at 25 ◦ C, the loss factors for FF EP UD and FF EP TW were 117 % and 232 % greater than for GF EP UD and GF EP TW, respectively. Similarly, when FF was used instead of CF in the UD EP composites, the loss factor increased by 201 %. Moreover, unlike CF and GF, addition of FF to the EP increased the damping with respect to that in the neat EP. The loss factor increased from 0.015 to about 0.030. This improvement is thought to be directly linked to the FF architecture. NF used for composite applications are generally made up of yarns of elementary fibres, each being composed of cell walls in which rigid cellulose microfibrills are embedded in a soft lignin and hemi-cellulose matrix. These cell walls consist of several layers differing in composition, the ratio between cellulose and lignin/hemicellulose, and the orientation of the cellulose microfibrils. The resulting structure promotes dissipation of energy through friction between cellulose and hemicellulose in each cell wall, called intra-cell wall friction, and friction between the cell wall, called inter-cell wall friction. These friction mechanisms increase the intrinsic damping with respect to that obtained with synthetic fibres. The use of FF yarns as a reinforcement in composites may also contribute to damping through friction between the fibres within the yarns, called intra-yarn friction, and friction

22

between the yarns, called inter-yarn friction. At 25 ◦ C, PLA is below its glass transition temperature and hence has properties similar to EP. However, addition of FF resulted in little change in the loss factor for UD composites. This might be accounted for by weaker interactions between PLA and FF than between EP and FF, such that less load is transmitted to the fibres during deformation, reducing the contribution of the internal friction in the fibres to the overall damping response. In the case of TW composites, weaker adhesion may also promote energy dissipation through inter-yarn friction, increasing the loss factor at 25 ◦ C of FF PLA2 TW and FF PLA4 TW with respect to that in FF PLA2 UD and FF PLA4 UD respectively. With better adhesion between the fibres and the matrix, the fibre/matrix sliding effect dominates as already discussed in the context of figure 4 and figure 6. In this case, the loss factor was higher for FF EP UD than for FF EP TW. The comparison of the loss factors obtained at 25◦ C and at TT rans for UD EP and PLA based composites at a frequency of 1 Hz, shown in figure 8, highlights the different mechanisms that contribute to the damping behaviour of the composites. At room temperature the nature of the fibres and the quality of the interface dominated. Thus, the use of FF instead of GF in EP composites increased damping, owing to the intrinsic nature of the FF and the yarns, although the specific mechanical properties remained similar (figure 2). On the other hand, the FF PLA2 UD biocomposite showed similar damping to the synthetic GF EP UD composite. The limited damping in this case may be due to limited adhesion between FF and PLA2, as already discussed in section 3.2, resulting in poor load transfer between the matrix and the

23

fibres. Intra-fibre damping phenomena are thus less strongly activated. At TT rans , the matrix properties became dominant. Even so, the addition of FF instead of GF to EP reduces the loss factor, because the rougher surface of the FF decreases friction and sliding between the matrix and the fibres.

0.03

UD composites

T=25°C T=T

2.5

Trans

2

0.025 0.02

1.5

0.015 1 0.01

0

0.5 GF Epoxy

FF

Trans

0.005

Loss factor at 1 Hz and T

Loss factor at 1 Hz and 25°C

0.035

FF PLA2

0

Figure 8: Loss factor values obtained at 25 ◦ C and TT rans for UD EP and PLA based composites

4. Conclusion At a fixed volume fraction of unidirectional fibres, the low density of flax resulted in specific mechanical properties in epoxy, polypropylene and polylactide composites that are comparable with those of glass fibre reinforced epoxy composites. The damping properties obtained with the flax fibre reinforced composites were also generally better at around room temperature than those of the carbon and glass fibre reinforced composites. Indeed at 1 Hz and 25 ◦ C, addition of unidirectional flax fibre to epoxy led to an approximately 100 % increase in loss factor with respect to both the matrix and GF reinforced epoxy. 24

However the best compromise between stiffness and damping was obtained with flax fibre reinforced semicrystalline PLA, which also has the advantage of being produced entirely from renewable resources and biodegradability. With regard to the underlying mechanisms, complex multiscale phenomena are known to control the tanδ of composite materials. Thus, the nature of the matrix, the stiffness increases due to the fibres, the textile architecture and the yarn lengths available for fibre sliding and friction have all been identified in this study to be important considerations for damping. The next step towards a better understanding of these phenomena will be to investigate in more detail the role of the interface quality, as well as intra-yarn and intra-fibre damping phenomena. At the same time the work will be extended to other types of test and vibration conditions, in order to gain insight into the damping performance of natural fibre composites under conditions representative of real applications. 5. Acknowledgments The authors acknowledge Thomas Chenal and Lara Arietano for their contribution to this work, and Dr. Christian Neagu and Dr. Yves Leterrier for fruitful discussions and advice.

25

References

References [1] Rowell, R.M.. Properties and performance of natural-fibre composites; chap. Natural fibres: types and properties. Woodhead Publishing Cambridge; 2008, p. 3–66. [2] Bledzki, A.K., Gassan, J.. Composites reinforced with cellulose based fibres. PROGRESS IN POLYMER SCIENCE 1999;24(2):221–274. [3] Bismarck, A., Mishra, S., Lampke, T.. Natural Fibres, Biopolymers, and Biocomposites; chap. Plant Fibers as Reinforcement for Green Composites. CRC Press: Boca Raton; 2005, p. 37–108. [4] Mohanty, A.K., Misra, M., Hinrichsen, G.. Biofibres, biodegradable polymers and biocomposites: An overview. MACROMOLECULAR MATERIALS AND ENGINEERING 2000;276(3-4):1–24. [5] Verpoest, I., Baets, J., Acker, J.V., Lilholt, H., Hugues, M., Baley, C., et al. Flax and Hemp fibres: a natural solution for the composite industry. JEC composites; 2012. [6] Bledzki, A.K., Gassan, J., Zhang, W.Y.. Impact properties of natural fiber-reinforced epoxy foams. JOURNAL OF CELLULAR PLASTICS 1999;35(6):550–+. [7] Hepworth, D.G., Vincent, J.F.V., Jeronimidis, G., Bruce, D.M.. The penetration of epoxy resin into plant fibre cell walls increases the stiffness of plant fibre composites. COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 2000;31(6):599–601. [8] Van de Weyenberg, I., Ivens, J., De Coster, A., Kino, B., Baetens, E., Verpoest, I.. Influence of processing and chemical treatment of flax fibres on their composites. COMPOSITES SCIENCE AND TECHNOLOGY 2003;63(9):1241–1246. doi:10.1016/S0266-3538(03)00093-9. [9] Bledzki, A.K., Fink, H.P., Specht, K.. Unidirectional hemp and flax ep- and pp-composites: Influence of defined fiber treatments. JOURNAL OF APPLIED POLYMER SCIENCE 2004;93(5):2150–2156. doi:10.1002/app.20712. [10] Bos, H.L., Molenveld, K., Teunissen, W., van Wingerde, A.M., van Delft, D.R.V.. Compressive behaviour of unidirectional flax fibre reinforced composites. JOURNAL OF MATERIALS SCIENCE 2004;39(6):2159–2168.

26

[11] Van de Weyenberg, I., Truong, T.C., Vangrimde, B., Verpoest, I.. Improving the properties of ud flax fibre reinforced composites by applying an alkaline fibre treatment. COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 2006;37(9):1368–1376. doi:10.1016/j.compositesa.2005.08.016. [12] Hughes, M., Carpenter, J., Hill, C.. Deformation and fracture behaviour of flax fibre reinforced thermosetting polymer matrix composites. JOURNAL OF MATERIALS SCIENCE 2007;42(7):2499–2511. doi:10.1007/s10853-006-1027-2. [13] Santulli, C.. Impact properties of glass/plant fibre hybrid laminates. JOURNAL OF MATERIALS SCIENCE 2007;42(11):3699–3707. doi:10.1007/s10853-006-0662-y. [14] Newman, R.H.. Auto-accelerative water damage in an epoxy composite reinforced with plain-weave flax fabric. COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 2009;40(10):1615–1620. doi:10.1016/j.compositesa.2009.07.010. [15] Di Bella, G., Fiore, V., Valenza, A.. Effect of areal weight and chemical treatment on the mechanical properties of bidirectional flax fabrics reinforced composites. MATERIALS and DESIGN 2010;31(9):4098–4103. doi:10.1016/j.matdes.2010.04.050. [16] Chung, D.D.L.. Materials for vibration damping. JOURNAL OF MATERIALS SCIENCE 2001;36(24):5733–5737. [17] Karnani, R., Krishnan, M., Narayan, R.. Biofiber-reinforced polypropylene composites. POLYMER ENGINEERING AND SCIENCE 1997;37(2):476–483. [18] Wielage, B., Lampke, T., Utschick, H., Soergel, F.. Processing of natural-fibre reinforced polymers and the resulting dynamic-mechanical properties. JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 2003;139(1-3):140–146. doi:10.1016/S09240136(03)00195-X. [19] Arbelaiz, A., Fernandez, B., Cantero, G., Llano-Ponte, R., Valea, A., Mondragon, I.. Mechanical properties of flax fibre/polypropylene composites. influence of fibre/matrix modification and glass fibre hybridization. COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 2005;36(12):1637–1644. doi:10.1016/j.compostiesa.2005.03.021. [20] Angelov, I., Wiedmer, S., Evstatiev, M., Friedrich, K., Mennig, G.. Pultrusion of a flax/polypropylene yarn. COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 2007;38(5):1431–1438. doi:10.1016/j.compositesa.2006.01.024. [21] Bledzki, A.K., Mamun, A.A., Lucka-Gabor, M., Gutowski, V.S.. The effects of acetylation on properties of flax fibre and its polypropylene composites. EXPRESS POLYMER LETTERS 2008;2(6):413–422. doi:10.3144/expresspolymlett.2008.50. [22] John, M.J., Anandjiwala, R.D.. Chemical modification of flax reinforced polypropylene composites. COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 2009;40(4):442–448. doi:10.1016/j.compositesa.2009.01.007.

27

[23] Zhang, L., Miao, M.. Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites. COMPOSITES SCIENCE AND TECHNOLOGY 2010;70(1):130–135. doi:10.1016/j.compscitech.2009.09.016. [24] Oksman, K., Skrifvars, M., Selin, J.F.. Natural fibres as reinforcement in polylactic acid (pla) composites. COMPOSITES SCIENCE AND TECHNOLOGY 2003;63(9):1317–1324. doi:10.1016/S0266-3538(03)00103-9. [25] Huda, M.S., Drzal, L.T., Misra, M., Mohanty, A.K.. Wood-fiber-reinforced poly(lactic acid) composites: Evaluation of the physicomechanical and morphological properties. JOURNAL OF APPLIED POLYMER SCIENCE 2006;102(5):4856–4869. doi:10.1002/app.24829. [26] Mathew, A.P., Oksman, K., Sain, M.. The effect of morphology and chemical characteristics of cellulose reinforcements on the crystallinity of polylactic acid. JOURNAL OF APPLIED POLYMER SCIENCE 2006;101(1):300–310. doi:10.1002/app.23346. [27] Bledzki, A.K., Jaszkiewicz, A., Scherzer, D.. Mechanical properties of pla composites with man-made cellulose and abaca fibres. COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 2009;40(4):404–412. doi: 10.1016/j.compositesa.2009.01.002. [28] Bogren, K.M., Gamstedt, E.K., Neagu, R.C., Akerholm, M., Lindstrom, M.. Dynamic-mechanical properties of wood-fiber reinforced polylactide: Experimental characterization and micromechanical modeling. JOURNAL OF THERMOPLASTIC COMPOSITE MATERIALS 2006;19(6):613–637. doi:10.1177/0892705706067480. [29] Kinra, V.K., Wolfenden, A.. M3D: Mechanics and Mechanisms of Material Damping. Baltimore: ASTM; 1992. [30] Talib, R.A., Tawakkal, I.S.M.A., Abdan, K.. The influence of mercerised kenaf fibres reinforced polylactic acid composites on dynamic mechanical analysis. COMPOSITE SCIENCE AND TECHNOLOGY 2011;471-472:815–820. [31] Boissard, C.. Processing of sustainable cellular biocomposites. Ph.D. thesis; Ecole Polytechnique F´ed´erale de Lausanne; 2012. [32] Wielage, B., Lampke, T., Marx, G., Nestler, K., Starke, D.. Thermogravimetric and differential scanning calorimetric analysis of natural fibres and polypropylene. THERMOCHIMICA ACTA 1999;337(1-2):169–177. doi:10.1016/S0040-6031(99)001616. [33] Bensadoun, F., Depuydt, D., Baets, J., Vuure, A.W.V., Verpoest, I.. Biinfluence of fibre architecture on impact and fatigue behaviour of flax fibre-based composites. 17th International Conference on Composite Materials, ICCM-19, July 29-August 2 2013, Montreal, Canada 2013;.

28

[34] Khan, S.U., Li, C.Y., Siddiqui, N.A., Kim, J.K.. Vibration damping characteristics of carbon fiber-reinforced composites containing multi-walled carbon nanotubes. COMPOSITES SCIENCE AND TECHNOLOGY 2011;71(12):1486–1494. doi:10.1016/j.compscitech.2011.03.022.

29