dispersion can be observed. It also noticed there is no tendency to form agglomerates in the present RFCs even at high loading level. These unusual behaviors can be attributed to the nature of ferrite particles and the degree of mixing, which are critical factors in controlling the physical properties of a polymer-bonded magnet [11].

3.2. IR Spectroscopy

Infrared studies indicated the presence of two strong absorption bands ν₁ and ν₂ which are found in the expected range as shown in Figure 2. Assignment of bands is given in Table 2. The high frequency band ν₁ is in the range 590 to 600 cm^–1 and the lower one ν₂ in the rang 438 to 446 cm^–1. The change in the band position is due to the change in the Fe³⁺-O^2– internuclear distance of the octahedral and tetrahedral sites. Waldron [12] and Hafner [13] attributed the ν₁ to the intrinsic vibration of the tetrahedral groups, whereas ν₂ is due to is due to octahedral groups. A small band ν₃ in the range of 334 to 342 cm^–1 is assigned to the divalent octahedral metal ions—oxygen ion complex [14]. The appearance of the band ν₄ in the range 292 - 298 cm^–1 depends on the mass of the divalent tetrahedral cation and is assigned to the lattice vibration [15]. It is also observed that, ν₁ shifts towards higher frequency side with increasing particle size. The reason for this could be attributed to shifting of Fe³⁺ ions toward oxygen ions with increasing the particle size, then the average bond length increases.

3.3. Differential Scanning Calorimetry and Thermodynamic Parameters

DSC provides information concerning the enthalpy change accompanying a physical or chemical event, within a material. The glass transition is a second order event, which manifests itself in a DSC curve as a step changing corresponding to the change in the heat capacity of the system [16].

Figure 3 shows DSC curves measured from natural rubber-barium ferrite composites at heating rate 10˚C/min. The curves exhibited changes in the base line in the temperature range (–59˚C - 58˚C) which are attributed to the

glass transition temperatures of the investigated samples. The results of the glass transition temperatures are summarized in Table 3. The variation of glass transition temperature with ferrite loading is small (≈–1˚C). This

small variation could be associated with the imposed restriction due to the interaction between the filler and the rubber matrix [17]. The present T_g behavior is exactly coincide with the results which obtained from the dynamic properties of RFCs [18]. The activation energy of glass transition can be achieved by using the Kissinger formula, which was initially derived for the crystallization process but it is suggested as being valid for the glass transition [19,20]. This formula has the following form:

(1)

where R is the universal gas constant, β is heating rate, T_g is the glass transition temperature, E_g is the glass transition activation energy, and A is the pre-exponential factor. The change of the entropy (∆S^#) may be calculated according to the relation [21,22]:

(2)

where A is the pre-exponential factor obtained from the Kissinger method, e is the Neper number, χ is transition factor, k_B is the Boltzmann constant, h is the Plank constant. The change of enthalpy (∆H^#) may be obtained according to:

(3)

where E^# is the activation energy. The change of Gibbs free energy ∆G^# for the glass transition process can be calculated using the well known thermodynamic relation:

(4)

The variation of glass transition temperature with heating rate (β = 10, 20, 30, 40˚C/min) is represented in Figure 4 for NR and 90 phr loading samples (for sake of brevity). The values of E_g can be estimated from the plot of versus 1/T_g relation (Figure 5) whose slope is –E_g/R.

The values of the activation parameters are summarized in Table 3. The activation energy takes values between (53 - 110 kJ/mol). The higher values of activation energy can be attributed to the adhesion between the ferrite particles and NR matrix [10] as a result of increasing ferrite loading up to 160 phr. After this ferrite loading the dilution effect occurs as a result of the diminishing volume fraction of the polymer in the composites. The values of ∆S^# for glass transition are positive indicating that the corresponding activated composites have higher disorderness than the initial state. The positive values of ∆G^# indicated that glass transition process is not spontaneous at room temperature. The larger ∆G^#, the harder transition process. The endothermic peaks in DSC curves agree well with the positive sign of the activation enthalpy. The larger values of ∆H^# means more energy needed for transition.

3.4. Thermal Decomposition Characteristics and Thermal Stability

Figure 6 shows the Thermogravimetric (TG) and its derivative DTG curves for natural rubber-barium ferrite composites. The decomposition step as well as percentage weight loss are given in Table 4. TG and DTG curves of all samples have shown the same general shape, suggesting that the decomposition mechanisms are simi-

lar. All samples studied exhibited an initial small mass loss attributed to the elimination of volatile components such as water, and ammonia residue [23]. All samples are stable up to 300˚C. It can be seen that the TG curves

have only one large plateau and the DTG curves have only one degradation peak, indicating that thermal degradation of samples is mostly one stage process. The decomposition ranged from approximately 300˚C - 400˚C, with mass loss ranging from 52.17% - 19.43%, and this is assigned to thermal decomposition of natural rubber into monomers, dimers, trimers, etc.

The temperature of the maximum loss rate or peak temperature of the DTG curves ranges from 379˚C - 387˚C, and the decreasing of weight loss is due to the increase of non-volatile filler content in the composite samples.

3.5. Kinetics of Thermal Degradation

It has been reported that above 300˚C, the volatilization of natural rubber becomes more rapid and is substantially complete at temperature near 400˚C. In cis-1,4-polyisoprene vulcanizates, the transitions which occur above 300˚C have been found to be of greater significance because of a strong correlation between the thermal properties and the elastomer combustibility [24]. Therefore, the kinetic parameters have been evaluated only for the major decomposition observed between 300˚C and 400˚C. The kinetic parameters are calculated using Coates and Redfern method [25]:

(5)

where α is fraction of decomposition, is the activation energy and other symbols as defined before. Therefore, plotting against 1/T according to Equation (5) should give a straight line whose slope is directly proportional to the activation energy ().

The activation entropy ∆S^# [26]:

(6)

The activation enthalpy ∆H^#, and the Gibbs free energy ∆G^# were calculated using the following Equations (3) and (4) with replacing T_g by T_p (T_p is the peak temperature of DTG). The entropy ∆S^# gives information about the degree of disorder of the system, the enthalpy ∆H^# gives information about the total thermal motion and Gibbs or free energy (∆G^#) gives information about the stability of the system.

Figure 7 represents the Coats-Redfern plots of the decomposition step for NR and 90 phr loading samples

(for sake of brevity). The calculated kinetic parameters are given in Table 5. The higher values of activation energies show an improved resistance to degradation process.

4. Conclusion

Natural rubber-barium ferrite composites have been prepared. Variation of particle sizes, distribution, and the homogeneous dispersion can be observed Via SEM. Also there is no tendency to form agglomerates in the present composite system even at high loading level. Infrared studies indicated the presence of two strong absorption bands ν₁ and ν₂. The change in the band position is due to the change in the Fe³⁺-O^2– internuclear distance of the octahedral and tetrahedral sites. There is small variation of the glass transition temperatures under increasing of ferrite loading. The thermodynamic parameters values indicated that the glass transition process is more disorderrness, not spontaneous or harder process, more energy

needed to occur. Increasing of ferrite loading enhance the thermal stability of natural rubber.

REFERENCES

NOTES

^*Corresponding author.