Magnetic leathers

Abstract

Leather, a stable chromium–collagen matrix, is insensitive to magnets although it is paramagnetic. Here we show that the coating of the leather surface with iron oxide nanoparticles can provide magnetic leathers showing significant responses to permanent magnets. Iron oxide nanoparticles, synthesized using a simple co-precipitation technique, predominantly comprised magnetite (Fe₃O₄) with a particle size around 16 nm as revealed through X-ray diffraction and scanning electron microscopic analyses. Ethanol dispersed nanoparticles were mixed with commercial leather finishing dispersion and coated on the leather surface. Vibrating sample magnetometric measurements show that the resulting ferromagnetic effect is comparable to the leathers coated with commercial magnetic pigments. Surface coating of leathers with magnetic nanoparticles did not affect any of the physical properties of the leather. We further demonstrate that the prepared magnetic leathers have potential for advanced applications including adhesive-free wall tiling and energy harvesting from human motions.

---

1. Introduction

Leather finds several applications from wallets to wall coverings due to its excellent physical properties.¹ However, the use of leather in advanced applications such as smart/interactive clothing, electromagnetic interference (EMI) shielding, intelligent garments, adhesive-free wall covering, etc. is limited since it does not possess functional properties such as electrical conductivity, magnetism, etc.^2,3 The building block of leather, collagen, and most of the natural fiber based fabrics are diamagnetic and do not respond to a magnetic field effectively.² Although commercially available chromium tanned leather matrix exhibits very weak paramagnetic properties (0.05 emu g⁻¹), it is not sufficient to fulfill the required advanced application needs.⁴

Magnetically responsive textiles and fabrics are known in the literature.^5,6 Such materials have found a wide variety of uses including functional or smart products and floor or wall covering or tiles. Although attempts have been made to incorporate magnetic properties in collagen fibers,^2,7,8 currently there are no effective techniques available for large leather sheets. However, physical methods do exist for supplementing magnetic properties in natural and synthetic or artificial leathers. Such magnetic leathers are commercially prepared by attaching a magnetic substrate with leather panels employing adhesives and used widely as magnetic leather tiles for interior decoration applications such as wall and floor tiling.^9,10 Therefore, the incorporation of magnetic properties in leather and allied materials will open up avenues for new and advanced areas of application.

Although various magnetic materials available in nature (Fe, Co and Ni), iron oxides form eminent class from the view point of cost, availability, magnetism and toxicity.^11–13 Among several types of magnetic iron oxide nanoparticles (IONP), magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) are very promising and popular materials for variety of applications.¹⁴ The most common procedure for obtaining Fe₃O₄ or γ-Fe₂O₃ is by co-precipitation of mixture of ferric and ferrous ions in a 2 : 1 molar ratio.^13,14 The size and shape of the IONP depend on various parameters such as the type of iron salts, ferric to ferrous ion ratio, pH, temperature, ionic strength and other reaction conditions.¹⁴ Recently, we reported the synthesis of iron oxide nanoparticles using co-precipitation technique for biomedical¹⁵ and biotechnological⁸ applications.

Herein, we describe a simple protocol to prepare magnetic leathers by surface coating of iron oxide nanoparticles. Iron oxide nanoparticles, synthesized by simple co-precipitation technique as well as industrially available commercial magnetic pigment (CMP), were coated on to the leather surface using conventional finishing technique. The treated leathers were examined for their magnetic properties as well as physical characteristics.

2. Experimental

2.1. Materials

Ferric chloride (FeCl₃) was purchased from M/s SD Fine-Chem, Mumbai, India. Ferrous sulphate (FeSO₄·7H₂O) and acetone were purchased from M/s Merck, Mumbai, India. Ammonium hydroxide (NH₄OH) was procured from M/s Sisco Research Lab, Mumbai, India. Commercial magnetic pigment was procured from M/s BASF, India (Table S1†). Cow crust black colored leathers of Indian origin with fairly uniform size of 1.11 ± 0.19 m² and thickness of 1.2 ± 0.1 mm were procured from a tannery in Chennai, India. Leather finishing (surface coating) chemicals namely protein binder (BI-1352), resin binder (RA-2312), PU (WT-2564), feel modifier (HM-145), wax filler (FI-50), black pigment (PP-18-032) and nitrocellulose lacquer (LW-65-000) used in this study were of commercial grade and procured from M/s Stahl India Pvt. Ltd., Chennai. The technical details of chosen commercial leather finishing chemicals are shown in Table S2.†

2.2. Synthesis of iron oxide nanoparticles

The magnetic iron oxide nanoparticles were synthesized by co-precipitation of the FeCl₃ and FeSO₄ solutions according to our previously reported procedure.¹⁵ Briefly, 750 ml of 0.1 M FeCl₃ and 750 ml of 0.05 M FeSO₄·7H₂O were mixed and stirred for 30 min at 30 °C. Then, NH₄OH was added drop wise into the solution to adjust the pH to 10. A black color precipitate was formed indicating the formation of nanoparticles. The precipitate was washed with acetone for 3 times. Finally, the collected precipitate was dried at 55 °C and then powdered to obtain iron oxide nanoparticles.

2.3. Characterization of iron oxide nanoparticles

The prepared iron oxide nanoparticles were characterized using Rigaku Miniflex (II) desktop X-ray diffractometer (XRD, Ni filtered CuK_α radiation with λ = 0.15418 nm) to identify the crystallographic structure of IONP. The samples were analyzed in the 2θ range of 10–80° in a step of 0.02° and with a counting time of 10 s per step. Room temperature magnetic properties of the as-synthesized IONP were measured using the vibrating sample magnetometer (VSM, Lakeshore, 7410 model). The morphology and size of the IONP were analyzed using high resolution scanning electron microscope (HRSEM, Quanta 200 series, FEI company).

2.4. Preparation of magnetic leathers

A conventional resin finishing system was adopted for control and experimental leathers as described below. A 2 ± 0.2 sq. ft. cow crust black colored leather piece with 1.2 ± 0.1 mm thickness was used for each control and experimental trials. For the control leather, the bottom coat solution comprised of 25 g protein binder, 5 g resin binder, 5 g PU, 5 g feel modifier, 10 g wax filler, 4 g black pigment and 55 ml water. This solution was applied as cross coats on the leather grain surface four times with drying in between the cross coats. This was followed by plain plating at 80 °C and 50 Pa. Top coat solution comprised of 40 g nitrocellulose lacquer, 2 g wax filler and 60 ml water. This solution was applied as cross coats on the leather grain surface three times with drying in between the cross coats. This was followed by plain plating at 100 °C and 100 Pa. Similar finishing methodology was adopted for the experimental leathers except that the CMP and synthesized IONP were added in the bottom coat solution as described here. CMP and IONP particles at a dosage of 10% (based on the weight of the leather) were separately dispersed in ethanol under sonication for 15 min. The dispersed solution was added separately to the bottom coat solution along with other ingredients and applied on the respective experimental leathers (CMP and IONP). The incorporation of ethanol-dispersed IONP and CMP nanoparticles with the bottom coat solution is shown in Fig. S1.† It can be seen that the IONP and CMP nanoparticles are well dispersed in ethanol after sonication and further mixing with bottom coat solution did not result in any agglomeration. The prepared control and experimental leathers were characterized as detailed below.

2.5. Characterization of magnetic leathers

The prepared control, IONP and CMP leathers were characterized using Rigaku Miniflex (II) desktop X-ray diffractometer. The surface and cross section morphology of control, IONP and CMP leathers were analyzed using a high resolution scanning electron microscope (HRSEM, Quanta 200 series, FEI company). Elemental composition of control, IONP and CMP leathers was also investigated through energy dispersive X-ray analysis (EDX) attached with the HRSEM equipment. The magnetic property of the treated and un-treated leather samples were measured using vibrating sample magnetometer (Lakeshore, 7410 model) at room temperature. The rub fastness properties for the prepared control, IONP and CMP coated leathers were tested at dry and wet condition according to the reciprocating method (ISO 11640).¹⁶ The colour fastness to water for the leather samples were tested according to ISO 11642.¹⁷ Finish adhesion test for the leather samples at dry and wet condition was carried out using sole bond test method (ISO 11644).¹⁸ Tensile strength and elongation of the coated leathers were tested according to ISO 3376:2002 (ref. 19) using an Instron Universal testing machine model 4501 at an elongation rate of 100 mm min⁻¹. Heat resistance test on contact with a hot surface was also carried out for the samples using SATRA TM 49:1995 method.²⁰ Flexing resistance of the leather samples in wet and dry condition were studied according to ISO 5402.²¹

3. Results and discussion

3.1. Characterization of iron oxide nanoparticles

XRD pattern of the as-synthesized iron oxide nanoparticles is shown in Fig. 1a. The positions of all the indexed diffraction peaks namely (220), (311), (400), (422), (511) and (440) match well with those of cubic Fe₃O₄ (magnetite, JCPDS no. 85-1436). The appearance of slightly broad peaks indicates the nanocrystalline nature of Fe₃O₄ particles. The absence of other major phases in the XRD pattern indicates the high purity of the obtained product. The average crystallite size was calculated based on the width of the strongest diffraction line (311) using the Debye–Scherrer's formula and the value was found to be 15.3 nm.

Fig. 1 (a) XRD, (b) HRSEM and (c) VSM data of as-synthesized iron oxide nanoparticles. Inset of (c) shows the attraction of the particles towards the wall of the container due to the magnetic field generated through permanent magnets.

High resolution scanning electron microscopic image of the as-synthesized Fe₃O₄ nanoparticles is shown in Fig. 1b. The prepared nanoparticles seem to be spherical and agglomerated. Agglomeration of particles could be due to the absence of surface coating, which results in the increased attractive force between the nanoparticles owing to the increase in the large surface area to volume ratio.²² In order to reduce the surface energy, the nanoparticles are agglomerated. The average particle size of the Fe₃O₄ nanoparticles is found to be 16.6 ± 0.4 nm (Fig. S2†). This value is in close agreement with the size calculated from XRD data.

Room temperature magnetization curve of the as-synthesized Fe₃O₄ nanoparticles is shown in Fig. 1c. It is seen that the particles exhibit a ferromagnetic like hysteresis curve with a saturation magnetization of 59 emu g⁻¹, which is roughly a half of the value for bulk magnetite.¹⁴ The low magnetization value observed for the as-synthesized Fe₃O₄ nanoparticles in comparison to the bulk magnetite is predominantly due to the nano size. Further, they display a perfect saturation magnetization even at fairly lower fields of <1 T with almost similar magnetic moment. The as-synthesized Fe₃O₄ nanoparticles show minimal coercivity and remanence of 6.4 G and 0.47 emu g⁻¹, respectively (Fig. S3†), which indicates that the particles are relatively ferromagnetic falling slightly above their superparamagnetic critical regime. The observed coercivity may be due to the magnetic interactions between the particles since the particles have not been isolated in a matrix or fluid.²³

3.2. Characterization of magnetic leathers

XRD patterns of IONP and CMP coated leathers are shown in Fig. 2. It is seen that both IONP and CMP coated leathers possess major diffraction peaks corresponding to cubic Fe₃O₄ (JCPDS no. 85-1436) and in particular (311) peak. This also indicates that the chosen commercial magnetic pigment is predominantly comprised of cubic Fe₃O₄ (Fig. 2b). The appearance of a small hump between 2θ = 20° and 30° indicates the amorphous nature of collagen, the building block of leather. It should be noted that the control leather after surface coating without any IONP or CMP did not show any peaks corresponding to cubic Fe₃O₄ (Fig. S4†). It exhibits slightly broad peak centered around 21° showing its amorphous nature.

Fig. 2 X-ray diffraction pattern of (a) IONP and (b) CMP coated leathers.

The HRSEM micrographs and EDX spectra of the control, IONP and CMP coated leathers are shown in Fig. 3. The micrograph of the control leather shows fairly smooth surface with the distribution of carbon black pigment particles (Fig. 3a). Whereas the IONP and CMP treated leathers show relatively rougher surface with uniform distribution of particles on the surface of the leathers (Fig. 3b and c). Higher magnification images show that the IONP and CMP particles are well distributed on the surface of the leathers (Fig. S5†). The EDX spectrum of the control leather shows the presence of silica along with other trace elements, which may arise from the silicone based feel modifier incorporated in the surface coating solution (Fig. 3d). On the other hand, IONP and CMP coated leathers show the presence of iron signifying the distribution of iron oxide nanoparticles, along with the elements present in the surface coating solution (Fig. 3e and f). The cross sectional morphology of control, IONP and CMP coated leathers exhibits coating thickness of 18.3 ± 0.2, 25.6 ± 0.8 and 20.2 ± 0.1 μm, respectively (Fig. 3g–i). The slight increase in the coating thickness for the IONP and CMP treated leathers in comparison to the control leathers may be due to the presence of iron oxide nanoparticles.

Fig. 3 High resolution scanning electron microscopic images showing the surface morphology of (a) control, (b) IONP and (c) CMP coated leathers; EDX spectra showing the elemental composition of (d) control, (e) IONP and (f) CMP coated leathers; cross sectional images showing the coated layers of (g) control, (h) IONP and (i) CMP treated leathers.

Room temperature magnetization curves of control, IONP and CMP coated leathers are shown in Fig. 4. As can be seen, the control leathers finished without any IONP or CMP particles exhibit a paramagnetic behavior mainly due to the chromium(III) species complexed with collagen molecules (Fig. 4a). However, both IONP and CMP coated leathers exhibit a ferromagnetic behavior with saturation magnetization and magnetic moment of 0.6 and 1.0 emu g⁻¹, respectively. To confirm the magnetic properties of the IONP and CMP coated leathers, we analyzed the response of the leathers against permanent magnets. Digital images of control, IONP and CMP coated leathers under the influence of magnetic field generated using permanent magnets (∼1500 Oe) are shown in Fig. 5. It is seen that the control leather did not respond to the magnetic field created by the permanent magnets. However, both IONP and CMP coated leathers are attracted towards the permanent magnets and lifted against the gravitational force (Fig. 5b and c), as also can be seen in Movie S1.† These results confirm that it is possible to prepare magnetic leathers with sufficient response to permanent magnets by simple application of IONP or CMP particles in conventional leather finishing technique.

Fig. 4 Room temperature M(H) curve of (a) control, (b) IONP and (c) CMP coated leathers probed using a vibrating sample magnetometer.

Fig. 5 Digital images of (a) control, (b) IONP and (c) CMP coated leathers under the influence of magnetic field generated using permanent magnets (∼1500 Oe).

3.3. Physical properties of magnetic leathers

Although it is possible to prepare magnetic leathers, it is of paramount importance to verify the physical properties of the treated leathers since the application of IONP or CMP particles on the leather surface may negatively influence some of the properties. Dry and wet rub fastness of the treated leathers are shown in Table 1. In general, all the control, IONP and CMP coated leathers display better dry rub fastness in comparison to wet rub fastness, which may be due to the solubilization of some of the finishing chemicals in wet condition. It is seen that both dry and wet rub fastness of the IONP coated leathers is comparable to the control leathers. On the other hand, the CMP coated leathers exhibit slightly lower dry and wet rub fastness.

Table 1 Dry and wet rub fastness of the control and IONP and CMP coated leathers examined by reciprocating method

Sample	Control		IONP		CMP
	Leather	Felt	Leather	Felt	Leather	Felt
Dry (150 cycles)	4	4/5	4	4/5	3/4	4
Wet (50 cycles)	3/4	4	3/4	4	3	3/4

---

Treated and control leathers were analyzed for color fastness to water and the results are shown in Table 2. As can be seen, both IONP and CMP coated leathers exhibit comparable color fastness (to water) to that of control leathers irrespective of variation in substrates. Treated leathers were analyzed for finish adhesion (sole bond test) and the results are shown in Table 3. In general, all the control, IONP and CMP coated leathers show better peel strength and finish adhesion in dry condition in comparison to wet condition. This could be due to the swelling and peeling of finish layers in wet condition. Both IONP and CMP coated leathers exhibit comparable peel strength and in turn finish adhesion in comparison to control leathers in dry and wet conditions. There was no failure observed. Tensile properties of the treated leathers are shown in Table 4. As can be seen, tensile strength and percentage elongation at break values of IONP and CMP coated leathers are comparable to that of control leathers.

Table 2 Color fastness of the control and IONP and CMP coated leathers to water

Color fastness	Control	IONP	CMP
Cellulose acetate	4	4	4/5
Bleached cotton	4	4	4
Spun nylon	4/5	4	4
Spun polyester	4/5	4/5	4/5
Spun acrylic	4/5	4/5	4/5
Worsted spun wool	4/5	4/5	4/5

---

Table 3 Finish adhesion of the control and IONP and CMP coated leathers examined using sole bond test

Sample	Condition	Maximum load (N)	Peel strength (N mm^−1)	Type of failure
Control	Dry	16.1 ± 4.6	1.3 ± 0.4	Nil
	Wet	13.5 ± 3.3	0.9 ± 0.02	Nil
IONP	Dry	15.2 ± 0.4	1.2 ± 0.4	Nil
	Wet	11.9 ± 4.3	1.1 ± 0.5	Nil
CMP	Dry	20.4 ± 3.8	1.8 ± 0.4	Nil
	Wet	12.9 ± 2.1	0.9 ± 0.1	Nil

---

Table 4 Tensile properties of the control, IONP and CMP coated leathers

Sample	Maximum load (N)	Tensile strength (N mm^−2)	Elongation at break (%)
Control	270.5 ± 22.7	21.3 ± 4.9	54.9 ± 8.5
IONP	291.3 ± 23.9	20.8 ± 3.8	57.2 ± 2.5
CMP	292.5 ± 27.7	22.4 ± 4.3	53.2 ± 3.5

---

Heat resistance of control, IONP and CMP coated leathers as a function of grey scale grading is shown in Fig. 6. It is seen that both IONP and CMP coated leathers exhibit comparable heat resistance to that of control leathers over a temperature range of 100 to 225 °C. However, CMP coated leathers display slightly low heat resistance at 250 °C in comparison to control and IONP coated leathers. At higher temperature (300 °C), all the control, IONP and CMP coated leathers display poor heat resistance as evidenced by inferior grey scale grading.

Fig. 6 Heat resistance as a function of grey scale grading of control, IONP and CMP coated leathers.

Flexing resistance of control, IONP and CMP coated leathers is given in Tables S3 to S5.† As can be seen, both IONP and CMP coated leathers possess superior flexing resistance in comparison to the control leathers. In particular, CMP coated leathers exhibit better resistance to flexing in dry condition up to 100 000 cycles while it is 90 000 cycles for IONP coated leathers. In wet condition, both IONP and CMP coated leathers exhibit only slight creasing while control leathers show pipiness (separation of layers within the leather matrix) at the completion of 10 000 cycles. These results show that the application of IONP and CMP particles using conventional finishing chemicals and system does not affect the color and rub fastness, finish adhesion on the leather surface, heat and flexing resistance as well as tensile properties of treated leathers.

3.4. Demonstration of magnetic leathers for electromagnetism and magnetic leather tiles applications

In 1820, Hans Christian Orsted showed that an electric current flowing in a wire produces its own magnetic field. An electromagnet is an iron core wound by copper wire which produces the magnetic field when current passes through it. In order to demonstrate the usefulness of developed magnetic leathers for electromagnetism related applications, we conducted a small experiment by constructing an electromagnet. A small iron screw was wound by an insulated (transparent polymer coating) copper wire and the ends of copper wire were connected to a 9 V battery as shown in Movie S2.† Small pieces from the finished grain surface of the control and experimental leathers were sheared and placed on a paper. The copper wire wound iron screw was moved on the small shaved pieces after switching on the battery. As can be seen, there is no response in the control leather pieces; whereas the experimental leather pieces were attracted towards the iron screw due to the magnetic field generated in the iron screw upon current flowing through the copper wire. These results suggest that the magnetic leathers produced in this study have potential for applications such as electromagnetic generator for energy harvesting from human motion.^24–26

Another major area of application of magnetic leathers is adhesive-free leather tiles for interior decoration. The prepared IONP and CMP coated leathers with the size of 1.5 × 1.5 cm² were placed on a white art paper (0.2 mm thick), which was positioned behind a commercial circular permanent magnet (∼500 Oe). Digital photographic images of the IONP and CMP treated leathers demonstrating their ability for adhesive-free wall tiling are given in Fig. 7. It can be seen that the IONP and CMP coated leathers are standing freely on the art paper without using any adhesive/glues, even with the influence of low magnetic field (∼500 Oe) generated by a commercial magnet. It should be noted that the backside (flesh) of the IONP and CMP coated leathers are attracted to the magnet although magnetic coating was carried out only on the grain side of the leathers having a thickness of 1.2 ± 0.1 mm. Hence, the results demonstrate that the prepared IONP and CMP coated leathers are useful for a range of advanced applications.

Fig. 7 Digital images showing the magnetic leather tiles application in front and side view for (a and b) IONP and (c and d) CMP treated leathers. A commercial circular permanent magnet with a magnetic field of ∼500 Oe was used in the experiment as shown.

4. Conclusion

Magnetic leathers were prepared using as-synthesized iron oxide nanoparticles and commercially available magnetic pigments with the combination of commercial leather finishing chemicals through surface coating. X-ray diffraction patterns of the IONP and CMP coated leathers confirm the presence of Fe₃O₄. Room temperature VSM data reveal that both IONP and CMP coated magnetic leathers exhibit a ferromagnetic behavior with saturation magnetization of 0.6 and 1.0 emu g⁻¹, respectively. Prepared magnetic leathers display significant response towards permanent magnets (∼1500 Oe). Incorporation of IONP or CMP particles does not affect the physical properties of the leather such as color fastness to water, rub fastness, finish adhesion, flexing resistance, tensile strength and heat resistance. Further, simple experiments demonstrate the attraction of magnetic leathers towards an electromagnet as well as wall tiling. Hence, the results suggest that the prepared IONP and CMP coated magnetic leathers can be used for advanced applications such as smart/interactive clothing, electromagnetic interference shielding, adhesive-free wall covering and energy harvesting.

Author contributions

P. T. and R. M. contributed equally to the manuscript. P. T. and K. K. involved in the project planning. All authors have given approval to the final version of the manuscript.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

Authors thank CSIR for providing financial support under XII plan project “S&T Revolution in Leather with a Green Touch” (STRAIT-CSC0201) project scheme. Authors also thank Ms. U. Sakthipriyadharshini for carrying out some of the experiments. CSIR-CLRI Communication No. 1157.

Notes and references

1. P. Thanikaivelan, A. Ashok and B. Chandrasekaran, J. Am. Leather Chem. Assoc., 2001, 96, 23.
2. M. Ashokkumar, K. M. Sumukh, R. Murali, N. T. Narayanan, P. M. Ajayan and P. Thanikaivelan, Carbon, 2012, 50, 5574.
3. F. Kabat, US Pat., 2127034, 1938.
4. M. Ashokkumar, N. T. Narayanan, B. K. Gupta, A. L. M. Reddy, A. P. Singh, S. K. Dhawan, B. Chandrasekaran, D. Rawat, S. Talapatra, P. M. Ajayan and P. Thanikaivelan, ACS Sustainable Chem. Eng., 2013, 1, 619.
5. C. Daoquan, Y. Kelu, Y. Mei, H. Peng and Z. Xingping, CN Patent 101235594, 2010.
6. H. Jie and M. Town, CN Patent 203049165, 2013.
7. P. Thanikaivelan, N. T. Narayanan, B. K. Pradhan and P. M. Ajayan, Sci. Rep., 2012, 2, 230.
8. C. Alliraja, J. R. Rao and P. Thanikaivelan, RSC Adv., 2015, 5, 20939.
9. S. T. Manchee, US Pat., 20050276982, 2005.
10. A. R. Esposito, US Pat., 20120000156, 2008.
11. J. Tucek, K. C. Kemp, K. S. Kim and R. Zboril, ACS Nano, 2014, 8, 7571.
12. C. Corot, P. Robert, J.-M. Idée and M. Port, Adv. Drug Delivery Rev., 2006, 58, 1471.
13. S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. Vander Elst and R. N. Muller, Chem. Rev., 2008, 108, 2064.
14. W. Wu, Q. He and C. Jiang, Nanoscale Res. Lett., 2008, 3, 397.
15. R. Murali, P. Vidhya and P. Thanikaivelan, Carbohydr. Polym., 2014, 110, 440.
16. ISO 11640:1993, Leather. Tests for colour fastness. Colour fastness to cycles of to-and-fro rubbing, International Organization for Standardization, Geneva, 1993.
17. ISO 11642:2012, Leather. Tests for colour fastness. Colour fastness to water, International Organization for Standardization, Geneva, 2012.
18. ISO 11644:2009, Leather. Test for adhesion of finish, International Organization for Standardization, Geneva, 2009.
19. ISO 3376:2002, Leather. Physical and mechanical tests. Determination of tensile strength and percentage extension, International Organization for Standardization, Geneva, 2002.
20. SATRA TM 49:1995, Resistance to damage due to contact with a hot surface, Shoes and Allied Trades Research Association, United Kingdom, 1995.
21. ISO 5402:2002, Leather. Physical and mechanical tests. Determination of flex resistance by flexometer method, International Organization for Standardization, Geneva, 2002.
22. D. Kim, Y. Zhang, W. Voit, K. Rao and M. Muhammed, J. Magn. Magn. Mater., 2001, 225, 30.
23. M. Morales, S. Veintemillas-Verdaguer, M. Montero, C. Serna, A. Roig, L. Casas, B. Martinez and F. Sandiumenge, Chem. Mater., 1999, 11, 3058.
24. G. Zhu, Z.-H. Lin, Q. Jing, P. Bai, C. Pan, Y. Yang, Y. Zhou and Z. L. Wang, Nano Lett., 2013, 13, 847.
25. P. Bai, G. Zhu, Z.-H. Lin, Q. Jing, J. Chen, G. Zhang, J. Ma and Z. L. Wang, ACS Nano, 2013, 7, 3713.
26. Y. Hu, J. Yang, S. Niu, W. Wu and Z. L. Wang, ACS Nano, 2014, 8, 7442.

---

Footnote

† Electronic supplementary information (ESI) available: Further information regarding magnetic properties of commercial magnetic pigment, technical details of commercial leather finishing (surface coating) chemicals, images showing the distribution of IONP and CMP particles with the bottom coat solution, HRSEM image and coercivity of as-synthesized iron oxide nanoparticles, XRD of control leather, flexing resistance of control and treated leathers and movies showing magnetic and electromagnetic response of control and treated leathers. See DOI: 10.1039/c5ra21909d

---