IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012
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Polymeric Composite Foams With Properties Controlled by the Magnetic Field Daniele Davino , Pasquale Mei , Luigi Sorrentino , and Ciro Visone Dipartimento di Ingegneria, Universitàa degli Studi del Sannio, I-82100 Benevento, Italy Insitute of Composite and Biomedical Materials, CNR P.le E. Fermi, 1 80055 Portici (NA), Italy Polymeric foams with embedded magnetic particles is a new class of lightweight systems that could lead to relevant industrial applications both for their enhanced directional passive mechanical properties and for their sensitivity to magnetic field that can be used to control the mechanical behavior. This paper is aimed to present the magnetomechanical properties of this novel class of functionalized lightweight composite materials, based on polymeric matrices and with suitable distributions of magnetosensitive micro-sized particles. The result of the ad hoc magnetic field application during the foaming process is a tailored degree of both structural and functional anisotropy without affecting the morphology of the cells, and the optimization of the reinforcing efficiency along one direction. The induced particle distribution can be then utilized for a real-time control of structural and functional properties under working conditions. Index Terms—Composite materials, magnetomechanical effects, polymer foams.
I. INTRODUCTION
I
N the framework of multifunctional materials, polymeric foams with embedded aligned magnetic particles is a new class of lightweight systems that could lead to relevant industrial applications both for their enhanced directional passive mechanical properties and for their sensitivity to an external magnetic field (MF) that can be used to modulate the mechanical behavior. The concept of magnetorheological (MR) foams is not new. But, until now MR foams have been considered only as an hosting porous material for MR fluids inside it [1], [2]. The new approach is based on the production of materials having magnetic particles directly embedded in the porous matrix during the foaming process. The structural and/or functional behavior are, as a consequence, not dependent on the particular behavior of any MR fluids inside the porous structure and have the great advantage to be lighter than conventional systems. Materials having similar weight and structure are elastic ferromagnetic composites made by using an elastic porous matrix of a silicone rubber with dispersed iron particles inside [3], but no alignment of the iron particles was induced. This paper is aimed to present the magnetomechanical properties of these new materials [4], [5]. A spatial distribution of magnetosensitive micro-sized particles was induced during the foaming process of filled matrices, giving distinctive structural and functional properties, otherwise only obtainable by means of complex procedures. The application of a MF in suitable processing conditions can induce, within struts and walls of foam cells, a controlled alignment of the magnetic particles, to form columnar structures, as represented in the optical micrograph of Fig. 1. The result of the ad hoc MF control is a tailored degree of both structural and functional anisotropy without affecting the morphology of the cells and the optimization of the reinforcing efficiency along one direction [4]. The induced particle distribution can be then utilized for a real-time control of structural and Manuscript received March 02, 2012; revised April 28, 2012; accepted April 30, 2012. Date of current version October 19, 2012. Corresponding author: D. Davino (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2012.2198634
functional properties under working conditions. Similar behaviors have been theoretical analyzed in [6], and experienced in [7]–[9] for solid magnetoelastomers. It is possible, for example, to increase the elastic modulus of the polyurethane foams along the direction of the aligned particles up to about a factor seven with respect to the unfilled foam. Furthermore, a clear active magnetoelastic behavior is experienced when the foam is kept under a time-variable MF, with an amplitude much lower than the one used in solid magnetoelastomers, such as the MFs higher than 0.5 T used in [10]. This behavior may be exploited in transducers, alternatively considering structural or magnetic parameters as input or output of the material. Preliminary results on the proposed foamed systems indicate that they should be further developed and optimized for a wide range of applications. In the paper, the magnetomechanical properties are presented together with a phenomenological interpretation of them. In perspective, this technology can be used in relevant applications through the appropriate selection of materials, processing conditions and MF configurations such as, for example, in peristaltic micropumps for fluid microdisplacements. Finally, it is worth to note that the high potential of the lightweight structures proposed resides in the coupling of: 1) the reduction of the total raw materials needed to produce the advanced composite systems [11]; 2) a design that exploit the functional and/or structural anisotropy, mimicking the Nature’s approach noticeble in lots of natural structures, such as wood or bone, where structural properties (orthotropic or gradient mechanical properties, respectively) are combined with functional ones (for example sap dreinage or blood transportation through vessels, respectively); [12], [13]; and 3) by using a production technology ready to be applied to industrial scale processes. II. EXPERIMENTAL Open cell polyurethane (PU) foams were prepared by using polyalcohol (Bayfit 12308/CO) and diisocyanate (Desmodur M-54) components, both supplied by BaySistems Italia (Italy). Iron micro-particles were purchased from Sigma-Aldrich (particle size lower than 44 ). Iron particles were mixed with the polyalcohol in a flask for 5 min at 2000 RPM to obtain an homogeneous particle dispersion. Diisocyanate was then added
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Fig. 2. (Up) Experimental setup utilized to measure the Young’s modulus and the magnetomechanical behavior. (Down) schematics of the MF and applied stress directions with respect to the magnetic particles orientation and to the test (preparation or characterization).
Fig. 1. Optical microscope photographs with a 500 scale. Legend: A) 15% N.O.; B) 5% O.; C) 10% O.; D) 15% O.; E) 20% O.; F) 25% O.
to the Fe/Polyalcohol mixture and intimately mixed before being injected in the aluminium mould. The reaction between polyalchol and diisocyanate induced the production of carbon dioxide responsible for the cells growth during the polymer reticulation. Oriented samples were produced by putting two sets of permanent magnets on the opposite side of the mould in the thickness direction after the mixture pouring. MF induced an alignment of microparticles in the polymeric matrix during the cell growth without affecting the development of the foam morphology. Five weight percentages (from 5 to 25) of iron particles were used to investigate the effect of particle content. Two series of foam samples were prepared: 1) without the MF application; and 2) with the MF application along the vertical direction, parallel to the foam growth direction. Foams morphologies were analyzed by using optical microscope (PlanApo MZ16, from Leica MicroSystems, Germany). The mechanical compressive and magnetomechanical behavior have been analysed by the same experimental setup (see Fig. 2). In particular, a quasistatic mechanical characterization (without MF) and a functionalized magnetomechanical characterization employing a step-like MF have been performed. The experimental setup was made by a traction-compression test machine, an electromagnet and a brass rod. The electromagnet has been used to generate a MF in the magnetomechanical tests and as a simple mechanical support in both characterizations. It was able to generate MF up to 170 kA/m with 3 A current, with an useful sample area of . A traction-compression test machine, equipped with a 100 N load cell, applies the compressive load through a mechanical arm made by a brass rod, to avoid perturbation of the MF force lines in
the airgap. The mechanical compressive behavior has been analyzed by setting the cross-head speed of the traction-compression test machine to 10 mm/min. Compression tests have been performed on samples, cut from plates produced with the mould. A. Measuring Procedure—Young’s Modulus The elastic modulus (Young’s modulus) has been calculated through a specific procedure, because it has been necessary to take into account the Mullins effect [14], the post-compression relaxation and the nonlinear stress-strain characteristic. As a result, Young’s modulus has been calculated in a specific linear range (0.04–0.06 mm/mm) and after six compression cycles. In this range, an average of Young’s modulus was calculated and reported. B. Measuring Procedure—Step-Like MF Tests The foams have been subjected to a variable MF (square waves), after setting the strain at different constant values, and they presented a change in the stress response immediately after each inversion of MF. This stress response has been detected in all samples: without iron, with randomly dispersed iron and with oriented iron particles. The adopted procedure, settled to take into accounts all previous issues, is the following: • The stress and deformation curves versus time have been acquired at 30 pt/s, over the test time interval (about 120 s). • The stress response variations, , have been calculated as differences between the minimum and maximum of each peak, see Figs. 5 and 6. Then, the values have been averaged over all the peaks of each curve:
DAVINO et al.: POLYMERIC COMPOSITE FOAMS WITH PROPERTIES CONTROLLED BY THE MAGNETIC FIELD
• The measurement error has been evaluated by considering two reference cases with no magnetic fillers and, then, with known zero stress responses. Indeed, the 0% PU has been used together with a foamed PE sample, produced with a different foaming process, to verify the accuracy of the mechanical testing facility and to evaluate the variance of measures without particles in order to establish the error threshold coming from the developed setup. • The average stress responses of 0% PU and PE have been compared with average stress responses of all different foams. • The average stress responses of randomly filled foam samples have been lower than average stress response of 0% PU and PE samples. While average stress responses of oriented foams have been higher than that of 0% PU and PE samples (see Fig. 7). Average stress responses of oriented foams were considered significative, while responses of randomly filled samples were considered equivalent to that of the references cases, and as a consequence, these systems did not show any relevant dynamic response.
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Fig. 3. Young’s modulus at different iron weight percents for both oriented (O.) and nonoriented (N.O.) particles. The Young’s modulus of the unfilled 0% PU is 82 kPa.
III. RESULTS AND DISCUSSION A. Particle Morphology in Reinforced Samples The use of the MF induced an alignment of the iron particles which resulted in the fibrillar rearrangement of the filler as shown in Fig. 1. Iron particles were prevalently present in vertical struts inducing an high structural anisotropy, whose effects will be evidenced by the mechanical characterization. On the contrary, samples produced without the MF exhibited a random distribution of the filler in the polymeric matrix. Foam samples were produced by keeping constant the mass of the polymer in the mould. The amount of iron particles influenced the final morphology of the cellular structures at the highest filler loading used (25 wt% by weight of iron particles). Indeed, as the weight percentage of iron particles raised, a lower amount of polymer was available to build up the cellular structure. As an example, in the samples containing 25 wt% of iron particles the polymer amount was 80% of that of the unfilled PU sample: both mean cell diameter and porosity increased [15], and the mechanical response of the foam decreased (see Fig. 3). It is interesting to note that the lengths of the fibrillar structures were directly related to the particle content, as evident from the comparison of the different samples, and that fibrils were longer than the mean cells diameters, resulting in an effective structural reinforcement through adjacent cells, as discussed in [5]. B. Young’s Modulus Characterization The addition of iron particles reinforced the foams structure increasing both elastic modulus (see Fig. 3) and yield stress. Samples filled with randomly dispersed particles showed an increase of the overall mechanical response when compared to the neat 0% PU, even if the particle content in randomly dispersed samples did not show a significative effect on the
Fig. 4. Sketch of the applied magnetic field for the magneto-mechanical tests. , . The typical rise-fall The step-like MF field had times were about 400 ms.
elastic response (N.O. red marks). Reinforced samples produced by aligning the iron particles exhibited higher structural responses, growing with the filler content, with respect to samples reinforced with randomly dispersed particles (see Fig. 3). The highest elastic compressive modulus was exhibited in the 20 wt% sample, with almost a factor seven with respect to the unfilled (0%) PU foam. At higher filler content mechanical properties dropped due to the worsening of the cellular morphology [15]. Indeed, in that case, there is the interference of the high amount of particles with the PU reticulation reaction. C. Step-Like MF Tests Samples with randomly dispersed (N.O.) and aligned (O.) particles were tested in compression by applying a step-like MF repetition time, as sketched in Fig. 4, keeping with constant the applied strain at 5%, 10%, and 15%. As evident in Figs. 5 and 6, each inversion of the MF induced a sharp reduction of the cellular structure stiffness (stress reduction, as shown in the insets of Figs. 5 and 6). This phenomenon resulted to be strong in samples with aligned particles (and linearly dependent on the particle loading) and weak (within the measurement error) for randomly filled samples with the same amount of particles (see Fig. 7). This clearly means that the stiffness of the cellular structure can be changed by applying the MF thanks to the presence of aligned iron particles along the MF lines.
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Fig. 5. Stress-Time Curve, of PU(5 wt%) with oriented particles, after a 5% Strain deformation and with applied MF of Fig. 4.
Fig. 7. Average variation of stresses, , for some samples (15 wt%, 20 wt%, 25 wt%) with oriented and nonoriented magnetic particles, as a function of the gray dash–dot lines, reapplied strain. The 0% PU and PE curves ( and spectively) are used as reference measurement errors.
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Fig. 6. Stress-time curve, of PU(20 wt%) with oriented particles, after a 5% Strain deformation and with applied MF of Fig. 4.
IV. CONCLUSION In this paper the mechanical and magnetomechanical behaviors of polymeric composite foams have been reported. The foams were prepared with a special mould where it was possible to apply a constant magnetic field during the foaming process, able to align reinforcing micrometric iron particles in fibrillars. The Young’s modulus of the foams resulted increased by changing the iron weight percent, up to a factor seven for the 20 wt% filled system. The foams were also able to respond to applied magnetic fields variations (with field lines aligned to the fibrillar structures) resulting in a significant effect on the stress response. This result has been already observed for solid magnetoelastomers at higher fields (hundreds of kA/m) [10] but never for magnetofoams, which showed stiffness variations at magnetic fields below 200 kA/m. In perspective, this new type of light weight magnetoelastic material, suitably developed, can be interesting for new applications and devices.
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