1. Introduction

There is currently interest in incorporating inorganic whiskers into various materials because of their effects on enhancing their mechanical properties and thermal stability. Whiskers exhibit fiber-shaped single crystals, super-high tensile strength and a high melting point owing to their ideal microstructure, which is nearly free from internal flaws [1, 2]. In particular, one-dimensional 513 MHSH whiskers (5Mg(OH)₂·MgSO₄·3H₂O, abbreviated 513 MHSH) has attracted attention due to its practical applicability including use in resins, rubber, fillers and reinforcement [3-5].

Various compositions of MHSH, xMg(OH)₂·yMgSO₄· zH₂O (x-y-z phase; 513 MHSH, 512 MHSH, 511 MHSH) have been prepared by varying of the molar ratio of Mg(OH)₂, MgSO₄, and H₂O [6, 7]. 513 MHSH whiskers meet the fundamental requirements of a flame retardant (FR) additive such as magnesium hydroxide (Mg(OH)₂), according to a flame retardant mechanism based on physical effects governing the endothermic processes [8, 9] 513 MHSH whiskers can be obtained by using various starting materials such as MgO, MgSO₄ and MgCl₂ or Mg(OH)₂ [10-12]. In a high concentration of OH⁻, it is difficult to produce high-quality MHSH using a comparatively simple synthesis technology. Mg(OH)₂ impurities are formed in high concentrations of OH⁻ and in the interaction between Mg²⁺ and NH₄ ⁺. Mg(OH)₂ molecules usually aggregate with each other, leading to a poor crystalline structure that strongly limits practical applications [13, 14].

A hydrothermal method was hard to control the purity and it need to complicated process with high pressure/ temperature. Therefore, to obtain high quality 513 MHSH whisker, our groups studied that the synthesis and morphologies of MHSH using ambient condition using only a single precursor of MgSO₄ instead of double precursor in a hydrothermal system [13, 14]. Therefore, simple reaction system with low pressure were need.

In order to enhance the property of the whiskers, a silica coating can be applied. Among the various oxide shells available, silica shells (SiO2) are a very common and useful class of materials offering easy surface modification [15, 16].

In this work, we synthesized high quality 513 MHSH whiskers in the ambient conditions with single source or MgSO₄·7H₂O instead of dual materials of MgSO₄ and MgO without applying high pressure. Also, we adopted a silica coating approaches to enhance the mechanical and thermal stability of the 513 MHSH whiskers. Physical properties of the silica-coated 513 MHSH whiskers were characterized by an abrasion test, thermogravimetric analysis (TGA) and transmission electron microscopy (TEM).

2. Experimental

2.1. Synthesis of 5Mg(OH)₂·MgSO₄·3H₂O whisker

Magnesium sulfate (MgSO₄·7H₂O, 98%, Daejung Chem., Korea) and ammonium hydroxide (NH₄OH, 28%, Daejung Chem., Korea) were used without further purification as starting materials. An aqueous solution of the precursors systems was prepared with appropriates quantities of reagents in distilled water using different synthetic condition to obtain 513 MHSH whiskers. MgSO₄·7H₂O in a desired concentration (0.5~4.0 mol) was dissolved in deionized water (50 mL) and subjected to ultra-sonication for 10 min. The solution was maintained at 110ºC for 6 hours under reflux in a round bottom flask and then naturally cooled to room temperature. A schematic illustration of the reaction set up is shown in Fig. 1. Table 1 shows the detailed reaction conditions of the synthetic parameters. In route I, whisker type MHSH was prepared by changing the MgSO₄ concentration such as [Mg²⁺]/[OH⁻] = 1.2 fixed concentration and amount NH₄OH. In route II, the length and width of 513 MHSH whiskers is controlled depending of the concentration of MgSO₄ and NH₄OH and amount of NH₄OH is twice.

Fig. 1

Synthetic scheme of the 5Mg(OH)2·MgSO4·3H₂O whiskers.

Table 1

Synthetic parameters for controlling the length and width of the products

The obtained products were centrifuged, washed with deionized water and dried in air at 80ºC for one day. The morphologies and structures were then examined with a scanning electron microscope (SEM, Model JSM-6390, JEOL, Japan) and with powder X-ray diffraction (XRD, Model D/Max 2500, Rigaku, Japan). A thermo-gravimetric analyzer (TGA, Model SDT Q600, TA instruments, USA) was used to identify the thermal behavior of the product.

2.2. Silica coating

The silica-coated MHSH whiskers were obtained by modifying the process reported in the literature [15, 16]. The MHSH (1.0 g) was re-dispersed in a mixed solution of ethanol (50 mL) and DI water (50 mL) under ultrasonication for 5 min. The NH₄OH (28%) solution (1 mL) was added to the solution and stirring was applied for 15 min. Tetraethylorthosilicate (TEOS, 98%, Sigma Aldrich; 1 mL) was introduced to the solution at room temperature. Core-shell whisker could be obtained in 24 h. The resulting 513 MHSH whiskers were collected by centrifuge and washed several times with pure ethanol and dried in vacuum dry oven at 80ºC.

And then, silica-coated 513 MHSH whiskers were characterized by a fade test and an abrasion test in order to determine their mechanical properties. Two different brake pad named Mg fiber non-added pad and Mg fiber addition pad. The friction test of brake pad is modified JASO C406-P1. The test conditions are braking number 15 times, deceleration 0.45 h and baking interval 35 seconds.

3. Results and Discussion

Fig. 2 show the SEM images and XRD patterns of the products obtained at different mole ratios of starting materials such as MgSO₄·7H₂O and NH₄OH catalysis. The molar ratios of MgSO₄·7H₂O/NH₄OH are in a range of 0.2~1.5. When the ratio of [Mg²⁺]/[OH⁻] is 0.2, a large amount of hexagonal Mg(OH)₂ was formed, for which the peaks were indexed by XRD (JCPDS No. 98-000- 0021).(Fig. 2a, 2d) In Fig. 2(a), the precipitant was composed of aggregate Mg(OH)₂ particles. In previous studies, we obtained high purity MHSH depending on the increased concentration of MgSO₄ in a hydrothermal system [13, 14]. When the molar ratio of MgSO₄ and MgO was 7:1, length exceeding 30 μm was obtained by a hydrothermal reaction [14]. As shown in Figs. 2b~2c, 513 MHSH whisker diameters ranged from 1 μm to 2 μm and lengths typically exceeded 10 μm (Fig. 2c, 2d). All diffraction peaks can be indexed with respects to the orthorhombic structure of 513 MHSH whiskers (JCPDS No. 00-07-0415) (Fig. 2d).

Fig. 2

SEM images of the prepared products synthesized at different ratios of starting materials ratio ([Mg²⁺]/[OH−]); (a) MgSO₄:NH₄OH=0.5M:2.6M, (b) MgSO₄:NH₄OH=3.0M:2.6M and (c) MgSO₄:NH₄OH=4.0M:2.6M. (d) XRD patterns of the asprepared products synthesized at different molar ratio of starting materials. (Route I in Fig. 1 and Table 1)

Concentration of MgSO₄ is high in low levels the NH₄OH, still remain of MgSO₄·7H₂O were indexed by XRD. (Fig. 2d) The remained MgSO₄·7H₂O can be removed the filter washing. In particular, high quality and yield of 513 MHSH whiskers was obtained when the ratio of [Mg²⁺]/[OH⁻] was 1.2. The pH value of the reaction solution exerts a considerable effect on the dissolution equilibrium of the Mg(OH)₂, there by controlling the concentration of Mg²⁺ ions and MgOH⁺ ions in solution [12].

We found that NH₄OH effect in high concentration of MgSO₄ is closely related to the morphology of 513 MHSH whiskers. Fig. 3 shows SEM images the as-prepared products synthesized under different synthetic conditions such as concentration of MgSO₄ and NH₄OH. The molar ratio of [Mg²⁺]/[OH⁻] as starting materials is in a range of 1.0~2.0. Although the same molar ratio of Fig. 3a and Fig 3c, the concentration of starting material is higher in Fig. 3c. By increasing the amount of NH₄OH catalysis up to 10 mL, the remaining 513 MHSH whiskers had even greater length. (Fig. 3a) Comparing Fig. 3a with Fig. 2c, the amount of magnesium sulfate in the products disappeared and single phase 513 MHSH whiskers were obtained.

Fig. 3

SEM images of the as-prepared products synthesized under different synthetic condition of MgSO₄ and NH₄OH concentration. (a) MgSO₄:NH₄OH = 4M:2.6M, (b) MgSO₄:NH₄OH = 4M:4.0M, (c) MgSO₄:NH₄OH = 6.0M:4.0M, (d) MgSO₄: NH₄OH = 8.0M:4.0M. (Route II in Fig. 1 and Table 1)

On the other hand, length and diameter of 513 MHSH whiskers is short and thick when high concentration of MgSO₄ and NH₄OH (Fig. 3b~3d). The concentration of MgSO₄ also accounts for the growth mechanism of the 513 MHSH splinters. 513 MHSH whiskers crystals display morphology with diameters ranging from 2 μm to 6 μm and length typically exceeding 10 μm. According to Yan research, 513 MHSH whiskers is composed of fragments of Mg(OH)₆ ⁴⁻ and SO₄ ²⁻ ions, while the hydrogen atoms link the neighboring helical Mg-O-S chains via hydrogen bonds. Along the b-axis, the Mg(OH)₆ ⁴⁻ fragments are compactly connected by the strong Mg-O bonds, while the total bond strength is stronger than those along both the c and a axes [12]. Also, we found that 513 MHSH whiskers was changed splinters when high concentration of MgSO₄ and NH₄OH.

To study thermal behavior of the 513 MHSH whiskers, we adopted a silica coating approaches. Fig. 4 shows a TEM image of silica-coated MHSH whiskers and TGDTA data of non-coated and silica-coated MHSH. Fig. 4a shows TEM images of the as-synthesized silica-coated 513 MHSH whiskers. The SiO2 layer is obtained separately in corresponding solution by hydrolysis via the stöber methods [15, 16]. High magnification TEM images confirm that each silica shell contains a 513 MHSH whiskers. The SiO2 shell were about 10 and 20 nm in thickness. Figs. 4b and 4c shows the thermal analysis curves for thermal gravimetry (TGA) and differential thermal gravimetry (DTG). The thermal analysis of 513 MHSH whiskers was carried out to understand the mechanism of dehydration and desulfuration. The dehydration of hydrous 513 MHSH whiskers has been studied by a thermal analysis to control the transition from 513 MHSH whisker to anhydrous MgO whiskers. The thermal decomposition of whiskers can be divided into four stages. The first weight loss at around 150ºC was due to the evaporation of absorbed water. The second and third weight losses at 250 - 300ºC, 370 - 400ºC were due to removal of 3H₂O and 5H₂O, respectively. In particular, water evaporation of SiO2-coated MHSH samples is higher than that of non-coated MHSH samples above 17ºC. The weight loss up to 900ºC was caused by SO3. Finally, 513 whiskers decomposed gradually and were converted to MgO whiskers after being heated in air at temperature up to 1050ºC [9, 10]. It is believed that the degradation processes can be described by the following reactions ~ (1) - (3):

Fig. 4

(a) TEM image of silica-coated 513 MHSH whisker. TG-DTA data of (b) 513 MHSH whiskers and (c) SiO2-coated MHSH whisker.

(1)

5Mg(OH)2⋅MgSO4⋅3H2O → 5Mg(OH)2⋅MgSO4+0.5H2O(≈260 to 360°C)

(2)

5Mg(OH)2⋅MgSO4+0.5H2O →MgSO4+5MgO(≈360 to 800°C)

(3)

MgSO4 + 5MgO→6MgO + SO3(↑)(above 800°C)

Maybe, the endothermic decomposition, the accompanying release of water vapor and the generation of silicacoated MHSH whisker residue at the temperature that the 513 MHSH whiskers themselves degrade, correspond with the flame retardant mechanism that is based on the physical effects governing the endothermic processes.

513 MHSH whiskers were characterized by a fade test and an abrasion test in order to determine their mechanical properties. (Fig. 5) Two different brake pad named Mg fiber non-added pad and Mg fiber addition pad. The friction test of brake pad is modified JASO C406-P1. Fig. 5a shows the fade test of the brake pad measured in braking number 15 times, deceleration 0.45 g, and a braking interval of 35 seconds. The initial temperature (T_i) is compared to final the temperature (T_f), and the raised temperature of the pad with 513 MHSH whiskers filler is lower than that of the Mg fiber non-added pad. Also, the minimum coefficient of friction was confirmed as 0.36 for the Mg fiber addition pad, which is higher than that (0.31) of the Mg fiber non-added pad. In Fig. 5a, Mg fiber addition pad showed higher friction coefficients than the Mg fiber non-added pad. This is attributed to the 513 MHSH whiskers filler generating H₂O during heat degradation with increasing temperature because the surface temperature of the friction pad is lower. Fig. 5b shows the relationship between the amount of pad wear and filler. As shown in Fig. 5b, the wear resistance of the pad is enhanced by addition of the fibrous 513 MHSH whiskers filler because the strength of the friction material is increased and it acts as reinforcement fiber. In other words, we confirmed that the 513 MHSH whiskers filler improved the fade resistance and wear resistance of the friction pad.

Fig. 5

The friction data of brake pad under JASO C406-P1. (a) Fade test, (b) wear amount test.

4. Conclusions

We synthesized 513 MHSH whiskers using a reflux system by the reaction between MgSO₄·7H₂O and NH₄OH in an aqueous solution. Morphologies of 513 MHSH whiskers were controlled through a variety of parameters including the molar ratio of the starting materials such as MgSO₄/NH₄OH. High quality 513 MHSH whiskers were formed from 3.0 M MgSO₄ and 2.6 M NH₄OH after reflux treatment at 110ºC for 6 h. The water evaporation temperature of silica-coated 513 MHSH whiskers (above 17ºC) is higher than that of non-coated 513 MHSH whiskers The fade resistance and the wear resistance of the brake friction pad are increased by the addition of 513 MHSH whiskers whisker filler. We confirmed the silicacoated 513 MHSH whiskers improved the dehydration temperature and the coefficient of friction on the basis of its thermal behavior and physical properties. It is anticipated that 513 MHSH whiskers will find applications as fillers in flame and friction reinforcement for brake pads.

Acknowledgements

Acknowledgement

This work was supported by the Industrial technology innovation program (10080275) funded by the Ministry of Trade, Industry and Energy (MOTIE), Republic of Korea.

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Figure & Data

References

Citations

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