Surface modification of ferromagnetics for polymer composites

Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 95–99

Surface modification of ferromagnetics for polymer composites I.A. Polunina∗ , V.I. Roldughin, G.S. Matrosova, S.A. Sosnina, K.E. Polunin Institute of Physical Chemistry, Russian Academy of Sciences, Leninskii Prospect, 31, Moscow 119991, Russia Received 25 July 2003; accepted 28 November 2003 Available online 2 April 2004

Abstract The adsorption properties of highly disperse powders of metallic nickel and ␥-Fe2 O3 used as a ferromagnetic filler in polymeric composites are investigated. It studied the adsorption of model compounds (octadecyl amine (ODA) and stearic acid (SA)) from an organic medium on ferromagnetic powders. It was established that there are approximately equal quantity of active acidic and basic sites at the ␥-Fe2 O3 surface available for the chemisorption of these surfactants. The adsorption of octadecylamine and octadecyl alcohol (ODCA) on nanodisperse nickel proceeds reversibly, whereas the adsorption of stearic acid is irreversible, thereby leading to the formation of dense modifying layers. The study of the physicochemical and magnetic properties of the magnetorheological suspensions in a VAGH solution (copolymer of vinyl chloride with vinyl acetate and maleic acid) allows us to establish that the use of nickel makes it possible to reduce the viscosity of ␥-Fe2 O3 organosuspensions and hence, to increase the degree of polymer filling with a solid disperse phase while conserving the desired performance properties of magnetic materials. © 2004 Elsevier B.V. All rights reserved. Keywords: Magnetorheological suspension; ␥-Fe2 O3 ; Nanodisperse Ni

1. Introduction Surface properties of ferromagnetic fillers in polymeric suspensions play an important role because they determine the character of interaction with the polymeric medium, the dispersion of a filler to the size of individual particles, and the retention of system stability during the production of a magnetic coating. Magnetic polymeric suspensions are known to contain ␥-Fe2 O3 powder as the main magnetic carrier. A second filler is frequently added to these suspensions in order to control their rheological properties and to impart specific properties to magnetic polymer compositions, for example antifriction (MoS2 , Al2 O3 ) or antistatic (carbon black) properties [1]. Usually, the concentration of the second filler, which is not ferromagnetic, comprises no more than 5% of the mass of the magnetic carrier ␥-Fe2 O3 , in order to ensure that the magnetic properties of compositions remain constant. If a ferromagnetic is used as the second filler, its quantity in the composition of a dispersed phase may be considerably higher. Therefore, it is of both scientific and practical interest to study the properties of such ∗

Corresponding author. E-mail address: [email protected] (I.A. Polunina).

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dispersions, especially since such studies have not been conducted until now. For this purpose, we used ␥-Fe2 O3 powder (as the basic magnetic carrier) and a nanosized metallic nickel. Nickel is one of the first ferromagnetic materials used to develop the magnetic recording carriers. At present, highly disperse powders produced from the alloys of iron with cobalt and nickel are still used for this purpose, both as a purely metallic coating and as a suspension in the polymeric medium [1,2]. In the latter case, in addition to the magnetic properties of the filler, its surface properties play an important role. Therefore, first of all, we studied the adsorption properties of the ferromagnetic powders. These properties are primarily due to the chemical nature of ␥-Fe2 O3 and nickel; however, they may be noticeably changed in the presence of impurities and ions contaminating the surface of the compounds in the course of the preparation, purification, and processing [2,3]. The active sites at the surfaces of ␥-Fe2 O3 and Ni are coordination-unsaturated Fe3+ and Ni2+ ions, as well as O2− and OH− ions formed as a result of the dissociation chemisorption of oxygen and water at the freshly formed their surfaces. In accordance with [4,5], the surface OH groups of ␥-Fe2 O3 reveal amphoteric and weakly basic properties, the surface OH groups of Ni reveal

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basic properties. Owing to the presence of two electrons on the external 4s-orbital and an incomplete occupancy of the 3d-orbital, atoms of Fe and Ni can participate in chemical reactions as electron donors and acceptors. To study the surface chemistry of ferromagnetic fillers, we studied the adsorption from an organic medium of model compounds (surfactants with various functional groups). This allows us to simulate the interaction of ferromagnetics with dispersants and polymers under real conditions of the fabrication of magnetic polymer compositions [6,7].

2. Experimental We used the commercial product, ␥-Fe2 O3 powder PMA as the basic magnetic carrier; it consisted of needle particles 0.2–0.6 ␮m in length. Specific surface area of dispersed ␥-Fe2 O3 determined by the low-temperature adsorption of nitrogen (the BET method) is equal to 14.5 m2 g−1 . Prior to adsorption studies, the adsorbent was cleansed of impurities with a hot solvent, dried at 150 ◦ C, and cooled in a desiccator. The sample of ultradispersed powdered Ni synthesized by the plasma evaporation of a metal with the subsequent condensation of particles in the inert gaseous mixture [8] was used as an object for this study. The X-ray powder diffraction (XRD) pattern of synthetic Ni (Fig. 1) showed good agreement with that of cubic form of Ni (a = 3.523 ± 0.001 Å). Experiments were performed with Dron-3 XRD diffractometer, Russia, using Cu K␣ radiation: 1.5418 Å. Only the diffraction lines of fcc crystalline Ni (99%) and NiO (1%) are observed. The crystallite particle size was estimated from the width of Ni (1 1 1) peak using the Debye–Scherrer method and instrumental broadening of 0.3◦ (2θ). Close examination of this sample by scanning electron microscopy showed only the nearly spherical Ni particles. According to XRD and electron microscopy data, its particle size varies from 25 to 50 nm; larger particles are the aggregates of smaller particles. Specific surface area of dispersed Ni determined by the

16000

+ Ni, fcc o NiO

+

14000 12000

I, a.u.

10000 8000 6000

+

4000

+ o

2000

o

+

low-temperature adsorption of nitrogen (the BET method) is equal to 12.5 m2 g−1 . Chemically pure model compounds (stearic acid (SA), octadecyl amine (ODA), and octadecyl alcohol (ODCA)) were studied as adsorbates. The adsorption of surfactants from toluene was performed by continuous stirring (at room temperature) of the powders dehydrated at 150 ◦ C in 1% toluene surfactant solutions until the adsorption equilibrium was established. This solvent is weakly adsorbed on the ferromagnetics and does not compete with the surfactant adsorption. The preliminarily investigated kinetics of the surfactant adsorption on nickel enabled us to establish the time of achieving adsorption equilibrium: 12 h for ODA and ODCA, and 14 h for SA. The initial and equilibrium concentrations of surfactants were determined gravimetrically from the dry residue in the sample. The modified powders were treated for 24 h with boiling toluene in a Soxhlet apparatus, which allows us to estimate the amount of surfactant molecules that were reversibly or irreversibly bonded to the surface [4]. In order to prepare magnetic suspensions, the ␥-Fe2 O3 powder was mixed in different weight proportions with the powdered nickel (8:1 or 2:1) and dispersed in a ball mill in a 15% VAGH (copolymer of vinyl chloride with vinyl acetate and methacrylic acid, containing 2.5% of OH groups) solution in a mixture of ethyl and butyl acetates (1:1) with the addition of a surfactant. The thus prepared suspension contained 42–43% of nonvolatile components (at a constant powder-to-polymer weight ratio equal to 3:1). The dispersion process was controlled by the changes in the sizes of the powder particle aggregates in the suspension being prepared, the aggregate sizes being determined with an optical microscope in a thin sample layer deposited onto glass. The prepared system contains no aggregates of particles more than 2 ␮m long. The viscosity of the studied organosuspensions with the same degree of volume filling (ϕ = 7%) was determined on a Reotest-2 instrument at 22 ◦ C. The shear stress τ r (Pa) was measured at the varying shear rate Dr within a range from 1.5 to 1300 s−1 . The viscosity value was calculated by the formula: ηr = 100τr /Dr (mPa s). The magnetic characteristics of pastes were measured using an F 5063 ferrometer and a TR 9802/A ferrotester. The sample to be tested was placed into a measuring coil, and the parameters of the hysteresis loop of the sample and the reference standard were compared. The following characteristics were determined: coercive force Hc (kA m−1 ), magnetic flux Φs (Wb), saturation magnetization Bs residual magnetization Br (T), and the squareness ratio of the hysteresis loop Kn = Br /Bs . All measurements were carried out at 20±5 ◦ C and a relative humidity of 50–80%.

+

o

0 0

20

40

60

80

100

120

2 Θ, degree

Fig. 1. XRD pattern of synthetic nanosized Ni powder (Dron-3, Cu K␣).

3. Results and discussion Fig. 2 presents the adsorption isotherms of ODA, ODCA, and SA for a highly disperse nickel. The adsorption of SA

I.A. Polunina et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 95–99

Fig. 2. Isotherms of the surfactant adsorption from toluene on highly dispersed nickel: (1) SA, (2) ODA, and (3) ODCA.

is described by a steep Langmuir isotherm, which rapidly reaches the saturation (curve 1); the adsorptive capacity of the saturated monolayer am is equal to 9.8 × 10−2 mol kg−1 . Assuming that the area per SA molecule in a dense monolayer ω = 20.5 Å2 [9] and using formula Ssp = am ωNA (where NA is the Avogadro number), we calculated the specific surface area of the adsorbent, which was equal to 12.4× 103 m2 kg−1 . The same value was obtained in calculating the mean value of Ssp from the particle sizes of nickel powder under the assumption that the particles were spherical and nonporous [9]. Thus, a dense monolayer is formed during the adsorption of SA; the monolayer completely screens the surface, where the SA molecules are oriented vertically. As a result of the SA desorption in treating the modified powder with a hot solvent in the Soxhiet apparatus, less than one-third of a monolayer (3.3 × 10−2 mol kg−1 ) of the strongly adsorbed acid molecules remains on the nickel surface; according to [10], SA molecules are chemically bonded to the surface OH groups, thereby forming salt like compounds. The adsorption isotherm for ODA on nickel (Fig. 2, curve 2) runs lower than that for SA. The adsorptive capacity of the ODA saturated monolayer is equal to 5.8 × 10−2 mol kg−1 , which corresponds to the modification of approximately 60% of the nickel surface. In such a monolayer, according to our calculations, one ODA molecule occupies an area of 35 Å2 , i.e. it is oriented almost vertically with respect to the surface. The adsorption layer of ODA on nickel is completely destroyed as a result of a prolonged treatment of the modified adsorbent with a hot solvent. Thus, the bond of amine molecules with the active sites is not very strong, and the adsorption of ODA molecules is completely reversible. The adsorption isotherm for ODCA (Fig. 2, curve 3) has the step-wise pattern with three inflection points. The Langmuir equation may arbitrarily be applied only to the first part of such an isotherm. In this case, the horizontal plateau corresponds to the formation of a mono-layer with the capacity of 2.1 × 10−2 mol kg−1 , and to the modification of almost 20% of the surface. In such a monolayer, the ODCA molecules are oriented virtually parallel to the surface (the calculated value of ω = 100 Å2 ) [9]. The next region of the adsorption isotherm resembles a plateau; it begins after the

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third inflection point corresponding to the adsorption value, which is approximately equal to 0.1 mol kg−1 . This value is almost five times larger than the adsorptive capacity of the primary ODCA monolayer and is close to that of the dense SA monolayer. Thus, when passing over from the first plateau to the second one, the horizontal orientation of the ODCA molecules on the nickel surface becomes vertical. The adsorption of the ODCA molecules becomes polymolecular as the concentration of the surfactant solution further increases. Previously, the isotherms shaped this way (i.e. with several plateaus and inflection points) were observed for the adsorption of different aliphatic alcohols and sodium maleate on porous nickel [5,10]. Apparently, the studied nickel particles also possess a certain degree of porosity, thus, distorting the results of the calculations of adsorption characteristics performed above. An analogous step-wise pattern of the adsorption isotherm could also be expected for ODA, whose molecules are reversibly adsorbed and have the same sizes as the ODCA molecules. However, due to the rather low solubility of ODA in toluene (up to 0.3% at 20 ◦ C), we failed to study its isotherm in the region of high concentrations. In the region of low concentrations, the adsorption of ODA is twice as high as that of ODCA. Taking into account the electronodonor properties of amine, it may be assumed that specific adsorption of ODA occurs on nickel, due to the formation of a coordination bond between its amino group and the surface Ni2+ ions. The modification of the nickel surface with surfactants noticeably changes the kinetics of sedimentation of nickel suspensions in toluene: the times of the complete sedimentation of powders that were to the largest extent modified with ODCA, SA, and ODA were by factors of 20,15, and 12, respectively, longer than those of the nonmodified nickel. This allows us to use the surfactants being investigated for the stabilization of nickel organosuspensions. Moreover, the results obtained allow us to predict the composition of polymers actively interacting with this ferromagnetic filler. The adsorption properties of ␥-Fe2 O3 , PMA grade, we studied earlier [11]. It was shown that SA and ODA are adsorbed by this filler irreversibly. The study of the IR absorption spectra of the modified ␥-Fe2 O3 samples allowed us to establish that the molecules of both surfactants form chemical bonds with the oxide surface [12]. The adsorption isotherms of the surfactants on ␥-Fe2 O3 and Ni are compared in Fig. 3. Table 1 lists data on the limiting adsorption of test molecules (SA and ODA) determined from the beginning of Table 1 Adsorption of surfactants (A × 106 mol m−2 ) on the ferromagnetic fillers Number

1 2

Adsorbent

␥-Fe2 O3 Ni

Stearic acid

Octadecyl amine

A

θ (%)

Ach

A

θ (%)

Ach

7.5 7.8

96 100

3.6 2.6

5.7 4.6

74 60

3.7 0

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A×106, mole/m2

8 6 4 1 2 3 4

2

0

0,5

1,0

1,5

2,0

2

Cr ×10 , mole/l

Fig. 3. Isotherms of the surfactant adsorption from toluene on ␥-Fe2 O3 and Ni: SA (1, 2), ODA (3, 4).

horizontal plateau of adsorption isotherms on ␥-Fe2 O3 and nickel from toluene. Surfactant adsorption A was calculated as the difference between the gravimetric concentrations of the initial and equilibrium solutions. The value of irreversible adsorption Ach was determined by the difference between the amounts of adsorbed and desorbed substance. The degrees of surface coverage with their molecules were calculated with formula θ = 100%ωo /ω. Thus, on the surface of the ␥-Fe2 O3 powder, there are approximately equal numbers of active sites that chemically interact with surfactant amino and carboxy groups, while active sites on the nickel surface are available for the chemisorption of carboxy lie acids. In order to obtain stable organosuspensions containing both these fillers, it would be the most effective practice to use acidic dispersants and polymers. In this work, we used the VAGH carboxyl-containing copolymer and two dispersants: oleic acid and ethoxylated ether of alkylphosphoric acid (KD-6). Owing to the presence of polar groups in the hydrocarbon radical, the KD-6 preparation ensures good wetting of the solid phase by a polar dispersion medium [11]. Results of the measurement of the magnetic properties of organosuspensions of pure ␥-Fe2 O3 and its mixtures with nickel in a VAGH solution in a mixture of alkyl acetates are listed in Table 2. Large values of the Kn and Φs parameters prove that all the investigated suspensions are well dispersed. The substitution of nickel for approximately 33% of ␥-Fe2 O3 (sample no. 3) contributes to some increase in the magnetic flux Φs , and saturation magnetization Bs which reflects the ability of ferromagnetic particles to be oriented

Table 2 Magnetic properties of the suspensions of ferromagnetics in the VAGH solution Number

1 2 3

Composition of dispersed phase

␥-Fe2 O3 ␥-Fe2 O3 + Ni (8:1) ␥-Fe2 O3 + Ni (2:1)

Magnetic properties Hc

Φs

Bs

Kn

26.1 26.0 24.6

21.3 21.0 22.0

324 324 330

0.86 0.85 0.85

Fig. 4. Rheological curves for the suspensions of ferromagnetic powders in 15% VAGH solution in a mixture (1:1) of ethyl and butyl acetates: (1) ␥-Fe2 O3 ; (2) γ-Fe2 O3 + Ni (8:1); and (3) γ-Fe2 O3 + Ni (2:1).

in the dispersion medium. This may be explained by an improvement in the dispersion composition of a suspension; because of this, the number of free stabilized ferromagnetic particles per unit volume of a system increases. On the other hand, this may cause a decrease in the viscosity of the system and change the type of its flow [13]. As was shown in [14], the extent of particle disaggregation may be judged by the changes in the pattern of suspension viscosity under the effect of an applied load, depending on various factors such as the nature and concentration of surfactant, polymer, solvent, or, as in our case, of the second filler. Fig. 4 shows the rheological curves of suspensions of the studied ferromagnetic powders in a 15% VAGH solution in a mixture of alkyl acetates. The viscosity of the system filled with pure ␥-Fe2 O3 (curve 1) gradually decreases as the shear rate increases. According to [15], such a pattern of rheological curves is typical of the flow of structured colloidal systems. As is known [13], the ␥-Fe2 O3 suspensions are unstable with respect to aggregation because of a strong dipole interaction of particles; they have a tendency to structurize at a sufficient concentration of the solid phase. The particles of ␥-Fe2 O3 , PMA grade, are also anisometric, and, according to electron microscopy data, they have the shape of ellipses with the axes ratio varying from 1:6 to 1:9. The interaction between such ferromagnetic particles is due to the variety of forces, among which, at least, magnetic and dispersion forces should be taken into account. The primary aggregation of the ␥-Fe2 O3 particles occurs due to magnetic forces. The dispersion forces are responsible for the secondary aggregation, i.e. coagulation structuring in the systems filled with anisometric particles. As is seen from Fig. 4, the partial substitution of much smaller spherical nickel particles for elongated ␥-Fe2 O3 particles (with the degree of the volume filling of the suspension being constant) leads to a decrease in the viscosity of the system. This decrease in viscosity becomes greater the higher the nickel content in the system and the higher the shear rate (curves 2 and 3 for the systems containing 11 and

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33% of nickel, respectively). The discovered effect is presumably due to the known effect of particle shape on the viscosity and structuring of disperse systems [15]. Considerable changes in the rheological properties and dispersity of the ␥-Fe2 O3 (PMA-grade) suspension was observed upon the addition of nonmagnetic particles of highly dispersed carbon black [9]. This was explained by a decrease in the binding forces between the magnetic particles of ␥-Fe2 O3 and an increase in the suspension fluidity. However, on adding a coarse Al2 O3 powder to the same system, the fluidity of the system decreases and its magnetic properties deteriorated. Such a change in the rheological properties and dispersity of suspensions is characteristic of the systems containing two fillers differing from each other in particle size and shape. The electron microscopy studies of a binary suspension of titanium dioxide (rav = 10 ␮m) and microbarite (rav = 1 ␮m) in the pentaphthalic resin enabled the author [7] to obtain color photographs of the coagulation structure of a binary dispersed phase. On the micrographs taken in different, arbitrarily selected colors for different fillers, we observe distinct TiO2 particles that were peptized to a maximal degree, and the particles of highly dispersed barite incorporated between the TiO2 particles. Such a method of improving the disperse composition and stabilizing ferromagnetic particles seems to be very useful because it allows us to achieve optimal magnetic properties avoiding the use of an excess of low-molecular surfaceactive dispersants. It is known [7] that the optimal amount of surfactant required for the complete peptization and stabilization of particles in the disperse system is usually determined by the maximum at the dependence of the shear stress on the surfactant concentration (Csurf ) for model suspensions. In the case of magnetic dispersions, the maximum magnetic characteristics are achieved at the Csurf values much higher than the concentration corresponding to the maximum shear stress of the model system [7]. This is due to the necessity of creating a system consisting of separate, well-stabilized ferromagnetic particles. In practice, however, an excess of surfactant frequently hampers the curing of polymeric compositions and decreases their physicomechanical characteristics. Consequently, the observed effect of decreasing the viscosity of magnetic compositions containing ␥-Fe2 O3 and

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highly dispersed nickel makes it possible to increase the degree of polymer filling with ferromagnetic powders and to retain high magnetic characteristics of the materials. In turn, this allows us to considerably improve the magnetic properties of composites and retain their optimal technological and service properties.

Acknowledgements We are grateful to A.N. Strelezky for help with X-ray analysis of the Ni powder. This work was supported by the Russian Foundation for basic Research, project no. 03-03-32513.

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