No89 May 2014 / jec composites magazine 73

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Flax/Acrodur® sandwich panel: an innovative eco-material for automotive applications

Currently, the automotive industry is greatly marked by the growing use of natural fibres. In particular, flax fi-bres are widely used as an eco-friendly reinforcement in various applications, combining higher specific proper-ties than glass fibres with interesting cost and weight reductions [1] [2] [3]. In this context and according to new European environmental regulations, OEMs are looking for new alternatives in order to decrease the fuel con-sumption of vehicles.

To fulfil customer requirements, a new composite material named Flaxpreg (Figure 1) was

developed by a consortium composed of Faurecia, PSA, LINEO NV and the University of Reims Champagne-Ardenne. This new bio-based sand-wich material shows an extremely low weight (2kg/m²) and high mechanical properties. Its skins are made of three

layers of an innovative flax fibre rein-forcement impregnated with Acrodur® acrylic thermoset resin, stacked in a privileged direction according to the applied stress fields. Due to the use of flax fibres, this sandwich material has an excellent stiffness/weight ratio and a lower environmental impact. Acrodur® is a thermoset resin from the BASF Company essentially developed for natural fibres, which is widely used in the industry due to its excellent processability and good crosslinking properties, as well as its good thermo-mechanical properties when cured. A cardboard honeycomb is incorpo-rated between the skins to provide a very good bending stiffness and high strength in the overall panel. This paper aims to determine the process-ing parameters of the sandwich skins in order to optimize the quality and properties of the final products. The mechanical characteristics of the flax laminates are governed by the proper-

ties of both the fibres and the resin matrix. In this paper, only the specific tensile properties of flax tape speci-mens and the Young’s modulus of de-rived flax/Acrodur® biocomposite are investigated, polyester acrylic resins being largely used and well-known in the industry. To this end, an automo-tive part prototype was prepared and subjected to flexural tests according to the customer’s specifications. Its mechanical behaviour was then mod-elled with the ABAQUS finite element analysis software.

Raw materialsFlax tape: a green reinforcementThe new bio-based flax/Acrodur® prepreg uses innovative, patented flax tapes [4] (Figure 2) composed of weight-controlled technical flax fibres arranged unidirectionally without any weft spinning (as observed in Figure 1). To maintain parallel cohesion of the fibres, a new process based on re-

M. Khalfallah, PhD student (1),

V. Marcel, Innovation Engineer, Expert (1),

A. Duval, Acoustic & Innovation Manager, AST (1),

B. Abbes, Associate Professor (2),

F. Abbes, Associate Professor (2),

Y.Q. Guo, Associate Professor (2),

F. Vanfleteren, Managing Director (3),

F. Rousseau, Innovation project leader (4),

(1) Faurecia, (2) University of Reims Champagne-

Ardenne, (3) Lineo, (4) PSA Peugeot Citroën

By

Fig. 1: The Flaxpreg sandwich panel

the damage sustained during pro-cessing and thus have considerable scatter [7]. Therefore, a simple and efficient method is needed to evalu-ate the specific modulus of flax tape. Conventional test methods are no longer relevant for the mechanical characterization of flax tape due to its structure and section discontinuity. To circumvent this issue, a new char-acterization protocol was established, mainly based on a statistical study, by analyzing the specific modulus (E/ρ) for each weight per unit area. In order to define the best test sample size for good repeatability of this modulus, different sample widths and lengths were investigated (Figure 4). The specific modulus value (E/ρ) is related to the flax tape density, which reflects not only the physical char-acteristics but also the quality of the tape. Quality is conditioned by several factors (fibre dispersion, fibre and

activating the pectic cement by spray-ing a water mist over the surface of the fibre bundles is used. At first, flax is scutched: the bundles forming the flax stems are decorticated and sepa-rated, then the remaining shives and woody core of the stem are removed. Secondly, the scutched flax is hackled to obtain long technical flax fibres. These fibres are collected without any particular selection and transported to the flax tape processing line where the bundles are stretched, maintained in a unidirectional direction and sprayed with a water mist. This guarantees the cohesion of the flax tapes by binding all the fibres. Then, the sprayed flax tapes are dried in-line by an infrared oven. Finally, the reinforcements are air-cooled and ready for impregnation and packaging. As shown in Figure 2, the semi-finished products are completely free of twisting, crimping or weft spinning. Flax tape weights per unit area ranging from 50g/m² to 200g/m² are available.

BASF Acrodur® resinA commercial polyester acrylic resin was chosen as a relevant material for a real industrial automotive process.

This resin offers good mechani-cal properties, a short crosslinking time, easy handling and cleaning, combined with a good “open” time. It is produced by dispersing a modi-fied polycarboxylic acid and a polyol (cross-linking agent) to create a thermoset material (Figure 3). This formaldehyde-free binder is suitable for wood and vegetable fibres such as flax, sisal or jute. The original water content is 50% but the resin can be diluted for easier impregnation. Crosslinking at 160-180°C takes less than two minutes.

Dry flax tape characterizationThe properties of the flax tape rein-forcement are influenced by several processing factors such as the pres-ence of tangle areas due to the flax fibre length and content, organization and distribution over the tape width. Moreover, it has been shown in lit-erature that the characteristics of flax fibres also depend on their variable morphology, biochemical composi-tion and biodegradability, and on the presence and location of defects in the stem [4] [5] [6]. In addition, their mechanical properties are affected by

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74 jec composites magazine / No89 May 2014

Fig. 2: The LINEO flax tape (Flaxtape110) used as composite reinforcement

Fig. 4: Flax tape reinforcement specimens

Fig. 3: Crosslinking reaction of Acrodur®

Tab. 1: Properties of BASF Acrodur® resin

Viscosity at 23°C [approx., mPa S]

Density at 20°C [approx., g/cm3]

Miscibility with water

Solid content [approx.,%]

pH [approx.,%]

150-300

1.2

Miscible in all proportions

50

3.5

Typical valuesProperties

Unidirectional Flax/Acrodur® composites

Preparation and characterization of composite samples Unidirectional (UD) prepregs were prepared using the com-

pression moulding process (3 min,

200 bars, 190°C), with two impreg-nation techniques such as spraying (expected for industrial production) and roll coating (used for prototyp-ing) (Figure 6). Four parameters were investigated: impregnation over one or both sides, variation of resin concentration, resin weight in the prepreg and drying cycle. A wetting agent is commonly applied on the flax tape to modify its surface ten-sion and enhance its impregnability before adding the resin. The mois-ture content of the prepregs plays a key role. Due to the high curing temperature, vapour is released and may create blisters and voids in the final composite. An optimal fibre/resin ratio and process (wedge) were identified to avoid blisters and thus control the quality of the laminates in terms of density, visual aspect and mechanical properties (Figure 7). Tensile tests were carried out on laminate samples following the ISO 527 standard.

The experimental setup used an Instron 33R 4204 testing machine equipped with a 50kN load cell and coupled to a stereovision apparatus for the experimental kin-ematics full-field measurements (Figure 8).

water content, fibre origin and storage conditions) that strongly influence the repeatability of the measurement.

The specific modulus (E/ρ) can be obtained with the following formula:E/ρ= k L²/mwhere E is the tensile modulus and k the slope of the load-displacement curve. L, m and ρ are the initial length, mass and density of the sam-ple, respectively. Finally, the 25mm x 200mm specimen is the optimal representative element of the flax tape to check its quality and interpret the behaviour of the derived bio-compos-ite (Figure 5).

Effect of impregnation, density and fibre content on the Young’ modu-lus The results show that the tensile mod-ulus increases proportionally with the density (Figure 9 (b)). For optimal densities close to 0.9 ± 0.04g/cm3, the tensile modulus values reach 18 ±1GPa (calculated on the slope over 0.2% of strain). The density cannot be increased beyond 0.9g/cm3 because the over-compacting leads to delami-nation and blistering. This could be related to the water contained in flax fibres and released during the Acro-dur® crosslinking reaction. Manual impregnation of the flax tape using the roll coating method results in high prepreg quality in terms of matrix distribution over the rein-forcement. Microscopic observations show that the Acrodur® resin fills the

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Fig. 5: Specific modulus of flax tape. Each point of the graph is the average of 20 samples; the 25mm x 200mm sample shows good repeatability

Fig. 7: Correlation between wedge thickness and resin weight fraction

Fig. 9: (a) Effect of impregnation methods on the elastic modulus, (b) relation between the elastic modulus of Flaxpreg samples and density

Fig. 6: Compression moulding process

Fig. 8: ArAMIS system

The DMA results are illustrated in Figure 11. At lower temperatures (-40°C to 25°C), the mechanical properties of the composite were not affected. The increase in temperature enhanced the storage modulus of the composite by evaporation of the con-tained moisture. The glass transition temperature (close to 75°C) can be obtained from both the loss modulus and tanδ curves. It is associated to the crosslinking density of the composite. The higher storage modulus above Tg reflects the higher performance level of the composite at high tempera-ture. Tan δ also shows the damping capacity of this green composite – a sensible change in the damping factor is observed when the temperature increases. This factor is close to 1.9% and 2.6% respectively at 25°C and 80°C, which is excellent (as compared to 0.10-0.15% for common glass/epoxy or carbon/epoxy composites).

Flexural test on a prototype automotive part Considering the optimal process pa-rameters, a Flaxpreg sandwich panel was prepared in a one-shot process

the variable properties of the flax fibres and the prototype impregnation. Thermo-mechanical behaviour of the UD flax/Acrodur® composite To assess the load-bearing capability of the flax tape reinforced Acrodur composite, it is crucial to measure the storage modulus, which is very close to the flexural modulus as measured by ASTM D-790 [9]. A dynamic me-chanical analysis was carried out with a Q800 DMA thermal analysis instru-ment, using a dual cantilever bending mode and a liquid nitrogen cooling system. UD flax/Acrodur samples (55wt. % of fibres) were evaluated at temperatures ranging from -40°C to 160°C, at a 3°/min heating rate and a 1Hz frequency (heating to max. 160° to avoid composite degradation). The storage modulus, loss modulus and mechanical damping factor (tan δ) were measured.

Young’s modulus is proportional to the fibre volume frac-tion (Figure 10) and this finding is highly related to the law of mixtures (Ec = Vf Ef + Vm Em). The scattered values result from the specimen’s structural hetero-geneity caused by

micro-pores, flows between the fibres and increases the specific surface area between matrix and reinforcement. Hence, the Acrodur® matrix shows good adhesion to cellulose fibres [8]. It could improve the mechanical properties of the final laminates by enhancing the stress transfer between the flax and the matrix. Based on this knowledge, in order to check the ef-ficiency of industrial impregnation, it is crucial to evaluate the properties that can be achieved with industrial prepregs and compare the Young’s modulus and densities obtained with both impregnation methods. Figures 9 (a) and (b) show that the characteris-tics obtained with both processes are almost similar and superimposed. This means that the industrial prepregs are highly optimized. The influence of the fibre content on the mechanical properties has been highlighted. The results show that the

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76 jec composites magazine / No89 May 2014

Fig. 11: Viscoelastic properties of Flaxpreg as function of temperature

Fig. 12: (a) Flaxpreg panel subjected to mechanical tests, (b) prototype automotive part

Fig. 10: Young’s modulus as function of fibres volume fraction

a

b

[0/90/0], dimensions and orthotrop-ic planes (local coordination system). The properties attributed to the sections of the skins and the spacer cardboard paper were determined experimentally.

- Simulation test with a homogenized honeycomb spacer: in this case, the composite laminate parts were repre-sented by shell elements too but the honeycomb spacer was replaced by a homogenous deformable solid whose properties were calculated by the developed analytical method.

The resulting deflections are summa-rized in Table 2 below, which shows that the homogenous model devel-oped gives accurate results compared to both numerical and experimental results. According to the specifica-tions, the model can be used to simu-late several mechanical tests such as compression and flexural tests using a simplified structure with little CPU time. Figure 14 presents the iso-values of vertical displacement recorded dur-ing the simulation of the flexural tests.

and used to produce a real automo-tive trunk load floor (Figure 11). According to the customer’s specifica-tions, this luggage board was sub-jected to flexural and creeping tests. The results show that the Flaxpreg biomaterial can withstand 100kg for 4 hours at ambient temperature, and 2 hours at high temperature, thanks to the high thermal stability of Acrodur and the good specific mechanical properties of the almost continuous flax fibres. At high temperature, the measured deflection is close to 13.5 mm, a value in accordance with the specifications.

Flaxpreg behaviour modelling To simulate the flexural behaviour of automotive parts, a finite ele-ment analysis of the Flaxpreg sand-wich panel was conducted using the ABAQUS software. The difficulty was the high computation time due to the complex 3D geometry of honeycomb cardboard. Therefore, an analytical model was developed to replace the spacer structure by a homogenous solid part with equivalent mechani-cal behaviour (Figure 13). To check its efficiency in terms of behaviour and deflection, two simulations were conducted and compared to the real flexural test:

- Simulation test with a fully discre-tized honeycomb spacer: the skins and the 3D spacer parts were gener-ated using shell elements according to the laminates’ stacking sequence

ConclusionsThe Flaxpreg project follows the triple objective of drastically reduc-ing weight, using renewable resources and being able to process the result-ing composite with cycle times and material costs meeting the constraints of automotive mass production. These first results show that the flax tape reinforced Acrodur® biocomposite has the potential to be integrated in automotive applications thanks to its high stiffness/weight ratio and its environmental advantages. The characterization tests provide good thermo-mechanical properties that remain almost unchanged over a wide

No89 May 2014 / jec composites magazine 77

Fig. 13: Geometry of a Flaxpreg sandwich panel studied with finite element simulation

Tab.2 : Deflections obtained with simulations and experimental tests

3) Experimental measurement

1) Simulation with fully discretized honeycomb

2) Simulation with homogenized honeycomb

13.5

13

14.5

Measured deflections

(mm)Tests

Fig. 14: ISO- values of vertical displacement (a) simulation 1, (b) simulation 2

ing, vol. 38, pp. 1912-1921, 2007. [6] M. Hughes, Defects in natural fibres: their origin, characteristics and implica-

tions for natural fibre-reinforced compos-ites, Anniversary Review, pp. 599-609,

2012. [7] E. Sparninš, Mechanical properties of flax fibers and their composites, Depart-ment of Applied Physics and Mechanical

Engineering: Division of Polymer Engineer-ing , Luleå University of Technology, 2006.

[8] Q. G. S. Q. S. Kaiwen Liang, Kenaf Fiber/Soy Protein Based Biocomposites

Modified with Poly(carboxylicacid) Resin, Applied Polymer Science, pp. 1213-1218,

2012. [9] S. D. D. B. B. C. M. A. K. Saha, Study of

Jute Fiber Reinforced Polyester Compos-ites by dynamic mechanical analysis, Applied Polymer Science , vol. 71, pp.

1505-1513, 1999.

Physical Structure of Flax Fibres on the Mechanical Properties of Flax Fibre Rein-

forced Polypropylene Composites, Applied Composite Materials, vol. 7, pp. 387-402,

2000. [3] H. L. B. K. M. M. J. A. van den Oever,

Flax fibre physical structure and its effect on composite properties: Impact strength

and thermo-mechanical properties, Die Angewandte Makromolekulare Chemie,

vol. 272, pp. 71-76, 1999. [4] C. B. Lamy, Influence of fiber defects on the stiffness properties of flax fibers-

epoxy composite materials, Journal of Materials Science Letters, p. 1211-1213,

2002. [5] K. Charlet, Characteristics of Hermès flax fibres as a function of their location

in the stem and properties of the derived unidirectional composites, Composites

Part A: Applied Science and Manufactur-

temperature range. Furthermore, Flaxpreg presents a low density which offers a good trade-off for semi-struc-tural automotive applications. The use of a thermoset resin blended with long flax fibres (dm) confirms that a potential weight reduction of 35% to 50% can be achieved at the same level of performance compared to the Baypreg technology. n

More information:www.

[1] J. Zhu, Recent Development of Flax Fibres and Their Reinforced Composites Based on Different Polymeric Matrices, Materials, vol. 6, pp. 5171-5198 , 2013.

[2] M. v. d. Oeven, Influence of the

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