A novel technique in the foaming process of EPDM/PP via microwave radiation: the effect of blend compatibilization and additive encapsulation

Abstract

Uniform voids, greater cell sizes, and their better distribution in the foaming of EPDM/PP could be achieved using microwave radiation with the peerless approach of employing encapsulated additives. The introduction of PP-g-MA as a compatibilizer proved to be an effective way to achieve better dispersion of PP droplets in the matrix of EPDM, and also to improve the elongational viscosity at higher extension rates. The ultimate foam density was lowered from 0.71 to about 0.59 g cm⁻³ as a result of applying foaming agents, along with iron oxide particles, encapsulated by urea–formaldehyde polymer at a microwave oven power of 900 W. It is of vital significance that the mean cell sizes from this technique reached 435 microns, compared to 277 microns attained in conventional methods.

---

1. Introduction

Casting a glance over the past two decades of foaming processes for materials may reveal the importance of various cellular plastics, yet the procedures applied to achieve a satisfactory polymeric foam are of particular interest. Owing to their good mechanical and thermal properties, as well as low density, plastic foams, either flexible or rigid types, have been attracting much attention in such applications¹ as furniture, transportation, appliances, sporting goods, gasketing, and the like. It is not surprising to find that, even with a fixed polymer/gas system and the same foaming method, the amount of voids, their dispersion and distribution, void inter-connection, and foam density have a profound impact upon properties and thereby applications.² In this respect, among common foaming technologies, including extrusion,³ molded beads, injection molding,⁴ and reactive injection, the radiation method showed amazing benefits in managing the aforementioned parameters utilized to control the foam microstructure with respect to its properties. Contrary to conventional heating, with microwave (μν) radiation technology,^5–7 the energy introduced into the material is transferred through the material surface and does not flow as heat flux. As a result, the rapid and uniform heating of both small and large complex shapes could be affordable.⁸

Isotactic polypropylene (PP) is a commodity plastic with a wide range of applications due to its excellent cost/properties ratio. However, PP has poor foamability because of its high crystallinity and low melt strength,⁹ as a result of its linear molecular structure. Modification with thermoplastic olefin elastomers,¹⁰ such as EPDM, is a simple, economical, and commonly used method to improve its toughness and impact behavior and achieve better foam processability. Regarding this, photo-grafting surface modification of PP/PP–MA/modified waste rubber composites was performed using UV radiation,¹¹ and it was found that the improved mechanical properties are due to increased compatibility through a chemical reaction between the maleic anhydride-grafted PP with the modified rubber powder. Wong et al.,¹² concerning the effects of processing parameters on the cellular morphologies and mechanical properties of foamed PP/ethylene–octene copolymer thermoplastic polyolefin, prepared using a two-stage batch process method, showed that the cell density increased with foaming temperature, but decreased with increasing foaming time. Zhang et al.¹³ investigated the microcellular foaming of PE/PP blends, which were prepared using an internal mixer and foamed with the aid of a plane sulfuration machine, and reported that the microcellular structure was strongly related to the crystallinity and melt strength. Also, Bledzki and his research team¹⁴ suggested that the injection moulding method yielded the best performance in terms of properties, and especially in terms of cell size, shape, and distribution, compared to extrusion and compression moulding of microcellular foams of wood–fibre-reinforced PP composites. The compatibilizing effect of PP-g-MA on polymer/gas mixtures, which were foamed through an injection method, was shown by Santoni et al.;¹⁵ this allowed the achievement of larger void fraction values, a higher cell density, a finer microstructure, and a substantially improved surface quality. Another study by Nemoto et al.¹⁶ revealed that, to control the bubble size in a microcellular foam of PP/propylene ethylene copolymer (PER) provided by a batch foaming process, reducing the size of the dispersed phase is inevitable. A number of researchers^17–20 have been working on the microcellular foaming of PP/waste rubber powder composites and have shown that the effect of a compatibilizer and heating time, as well as the amount of blowing agent, are the most crucial parameters in controlling the cell morphology and hence the cell density. The presence of organoclay nanoparticles was also examined by Keramati et al.²¹ in the foaming of PP/EPDM, using a batch-mixing process in a home-made autoclave. They showed that an improved foam structure was achieved by adding only small amounts of nanoclays to the blend.

In the present work, the novel technique of microwave (μν) irradiation was applied in order to prepare EPDM/PP cellular foams, combined with the new approach of additive encapsulation for better heat absorption within the sample. Then, the compatibilizing effect of PP-g-MA was investigated by SEM and elongational rheometry. Finally, μν irradiation was employed at 600 and 900 W to study the ultimate foam density over various time spans.

2. Experimental

2.1 Materials

A foam-grade ethylene-propylene-diene monomer (EPT 8030 M) containing 9.5% diene monomer was supplied by Mitsui Chemical. Also, polypropylene (random copolymer pipe grade RP2400), having a melt flow index (MFI) of 0.25 g/10 minutes at 230 °C and a solid density of 0.90 g cm⁻³, was provided by Korea Petrochemical. PP-g-MA (MD-353D) as compatibilizer was obtained from Fusabond with an MFI of 23 g/10 min and a melting point of about 136 °C. The blowing agent used was azodicarbonamide (ADA, decomposition temperature 180–214 °C – Fig. 1), manufactured by Kumyang Corporation, Korea. Also, amorphous iron oxide particles (Bayferrox Brown 686 – Germany), with a density of 4.8 g ml⁻¹, were used for better microwave absorbance (their FTIR spectra are available in Fig. 2a). Urea (U) and paraformaldehyde (PF), used as shell materials, and triethanolamine (TEA), used to control the pH of the solution, were purchased from Merck Company. Sulfuric acid solution (10 wt%) was prepared in our laboratory to adjust the pH in the final stage of the process.

Fig. 1 DSC characterization of azodicarbonamide as foaming agent at a rate of 10 °C min⁻¹.

Fig. 2 FTIR spectra of (a) iron oxide particles (b) EI and (c) EI + ADA.

2.2 Preparation of samples

All the EPDM, PP, and PP-g-MA materials were blended using a batch process in a Brabender internal mixer with a rotor speed of 60 rpm at 160 °C. The foaming agent and iron oxide were then added to the mixture once the torque started to decrease and the viscosity of the materials dropped. After that, mixing was continued for 3 additional minutes until a homogeneous mixture was obtained.

The compositions of blends with the corresponding sample designations are presented in Table 1. It can be seen that the EPDM/PP blend ratios were stipulated to be constant throughout all the samples. The preparation of the samples was achieved via two distinct procedures. In the first procedure, the foaming agent, along with iron oxide particles, was directly added to the blend mixture, while in the second one, both the foaming agent and the iron oxide particles were encapsulated separately in the presence of a urea–formaldehyde polymer coating and then added to the mixtures.

Table 1 Primary compositions of prepared samples

Sample	EPDM (phr)	PP (phr)	PP-g-MA (phr)	Foaming agent (phr)	Iron oxide (phr)
1	100	40	5	8	5
1E	100	40	5	15

---

As regards the preparation of the microcapsules, all the reactions were carried out in a 250 ml three-necked round-bottomed flask connected to a reflux condenser and equipped with a mechanical stirrer. The foaming agent, along with iron oxide particles, was encapsulated by urea–formaldehyde (U–F) polymer, according to the following two-step process:

Firstly, the U–F pre-polymer solution was prepared as follows: at room temperature (20–25 °C), 1.89 g of paraformaldehyde and 120 ml of deionized water were mixed and the pH of the mixture was adjusted to 8–9 with TEA. The mixture was heated up to a temperature of 60–65 °C and, after depolymerization and dissolving of paraformaldehyde, the solution of 2.6 g of urea in 30 ml of deionized water was added and the temperature was kept at 60–65 °C for 1 h. Finally, the obtained solution was cooled to room temperature. In the second stage, 6 g of iron oxide and 14 g of foaming agent were added to the prepared pre-polymer solution, and the pH of the mixture was adjusted to 3–4 with 10 wt% sulfuric acid solution before heating to a temperature of 60–65 °C. After 10 h, the reaction was ended. The obtained suspension of microcapsules was cooled to room temperature. The sediment was washed with a large amount of deionized water, filtered and air-dried for 48 h.^22,23

Also, a microwave oven (Samsung Model SAM 11) with an approximate power of 900 W was used for the irradiation process, and the various processing conditions for the μν irradiation of samples are presented in Table 2.

Table 2 Various processing conditions for the μν irradiation of samples

Sample	Process
	μν power (W)	Time of μν exposure (min)
1/1	600	10
2/1		12
3/1		14
4/1	900	10
5/1		12
6/1		14
1/1E	600	10
2/1E		12
3/1E		14
4/1E	900	10
5/1E		12
6/2E		14

---

3. Characterization

3.1 FTIR spectroscopy

The chemical functional groups of the particles were characterized using Fourier transform infrared spectroscopy (FTIR), Bruker, Equinox-55 (Germany). Sample preparation was performed after encapsulation, drying at 50 °C and preparing KBr pellets.

3.2 TGA analysis

Thermogravimetric analysis (TGA) was carried out under an N₂ atmosphere at a heating rate of 10 °C min⁻¹ with a Mettler-Toledo TGA/DSC1 (Swiss). Samples were heated from ambient temperature to 700 °C.

3.3 SEM measurements

The cell morphologies of the EPDM/PP foams and synthesized particles were visualized using a scanning electron microscope, Tescan Vega II (Czech Republic). The samples were immersed in liquid nitrogen for 3–5 min and then fractured. Before the SEM observations, the blends, as well as the dried particles, were placed on a sample holder and evacuated under vacuum, and a layer of gold was deposited under flushing with argon, using an EMITECH K450x sputter-coater (England).

3.4 Extensional rheometry

The samples were also rheologically characterized in simple extension using an SER Universal Testing Platform from Xpansion Instruments, which was used with an MCR 301 rheometer and a CTD450 convection oven (Anton Paar). The 15 × 1.0 × 0.5 mm³ samples were prepared in a hot press at a temperature of 150 °C. The extensional viscosity was measured at Hencky strain rates of 0 to 8 s⁻¹ at 210 °C.

3.5 Density determination

The density of foams was determined using the immersion method according to ISO 1183-1 standard, with the aid of an BX220A Precisa apparatus.

4. Results and discussion

Using the aforementioned procedure of preparation of microcapsules, two types of particles were prepared: encapsulated iron oxide (EI), using urea formaldehyde resins without and in the presence of azodicarbonamide (EI + ADA). The EI particles synthesized in the first step were not utilized in the formulation, yet were prepared to ensure that iron oxide particles are definitely present inside the microcapsules. FTIR spectroscopy, in transmittance mode, was used for the characterization of the functional groups of the particles. The absorbance spectra of iron oxide, EI, and EI + ADA are shown in Fig. 2 (Fig. 2a, b and c, respectively). In the FTIR spectra of the particles, the following signals were distinguished: (1) two absorption bands occurred near 578 and 470 cm⁻¹, which are related to the Fe–O band of iron oxide (Fig. 2a). (2) The peaks revealed could be attributed to the characteristic functional groups of the urea formaldehyde resin, such as C–O–C at 1033, amides I, II, and C=O at 1650–1550 cm⁻¹, and CH₂OH, CH₃, and C–N at 1400–1360 cm⁻¹ (Fig. 2b). (3) The characteristic absorption bands of ADA observed at 1114 cm⁻¹ and 1330 cm⁻¹ are assigned to the deformation bands of NH₂, the N–C stretching vibration, and the N–C=O bending vibration. The band at 1731 cm⁻¹ is assigned to a superposition of the stretching vibrations of C=O and N–C, which are more strongly infrared-active (Fig. 2c).

The iron oxide content and thermal stability of the synthesized particles can be studied by TGA. Fig. 3a and b represent the TGA (and DTG) results of the synthesized particles (EI and EI + ADA). As is seen (Fig. 3a), the initial weight loss of 2.7% (up to 250 °C) is due to the evaporation of physically adsorbed water and the subsequent weight loss in the range 250–300 °C is the result of thermal degradation of the urea–formaldehyde coating of iron oxide particles.²⁴ Furthermore, the iron oxide content of the EI particles after degradation of the polymer matrix is about 73 wt%. Similar results can be observed by repeating this experiment on EI + ADA particles (Fig. 3b), with the difference that the sharp weight loss of the sample at a temperature of about 225 °C could be due to thermal degradation of ADA in the EI + ADA particles. In this case, the iron oxide content of the particles (EI + ADA particles) was determined to be about 29 wt%.

Fig. 3 TGA-DTG thermograms of (a) EI and (b) EI + ADA in N₂ atmosphere, at a heating rate of 10 °C min⁻¹.

The morphology of the synthesized particles was studied by SEM (Fig. 4a and b). SEM micrographs of EI + ADA particles indicate that: (1) the bare iron oxide particles are not spherical²⁵ and have irregular shapes with harsh surfaces. The synthesized particles are not spherical but the surface of them is smooth. (2) According to the data sheet provided by the manufacturer of powder Bayferrox Brown 686, the particle size is predominantly less than 0.6 μm, but the size of the synthesized particles (EI + ADA) is less than 50 μm. This suggests that the iron oxide particles are mainly located inside the polymer phase, and it is confirmed that none of the iron oxide particles exist separately along with EI + ADA particles.

Fig. 4 SEM micrograph of (a) bare iron oxide and (b) EI + ADA particles.

To achieve a compatible blend of PP and EPDM, PP-g-MA was utilized as a compatibilizer. As a matter of fact, maleic anhydride groups are extensively used to compatibilize the polymer blends due to their facile reaction with numerous polymers within the conventional processing temperature ranges.¹¹ The SEM micrographs of EPDM/PP and EPDM/PP/PP-g-MA blends are featured in Fig. 5. When comparing parts a and b, it can be seen that the presence of MA brought about a drastic decrease in the diameter of the dispersed phase by virtue of reduced interfacial tension between PP and EPDM. The size of finer dispersed particles of PP represents an increased interfacial area with the EPDM matrix as a result of better compatibility.

Fig. 5 SEM images of EPDM/PP (a) before and (b) after the addition of PP-g-MA as compatibilizer.

Achieving a desirable foam product entails approaching a shear thickening behavior, rather than a shear thinning behavior, under applied extensional stresses. In the case of shear thinning behavior, no cell stability would be observed and cell coalescence occurs during structural evolution.

Regarding the fact that no strain hardening or tension thickening behavior could be observed for PP while in the elongational field, its blend with EPDM shows strain-hardening behavior above moderate extension rates. Adding 5 phr of PP-g-MA as a compatibilizer can reduce interfacial tension between the two phases, hence at almost every extension rate, tension thickening behavior is discernible. The addition of PP-g-MA to the EPDM/PP blend drastically increases the strain hardening behavior up to 5 s⁻¹ and then the elongational viscosity continues to rise gradually. Finally, at higher extension rates, the variation in viscosity levels off to a constant value. It is worth mentioning that this typical behavior is of vital importance while foam processing in the vicinity of 210 °C. The improved melt elongational viscosity of the samples, including the compatibilizer, would help to boost the gas-releasing mechanism within cells and, as a result, cell coalescence is decreased to a large extent. A higher polymer elongational viscosity or its melt strengthening during uniaxial and biaxial stretching processes has a crucial role in cell stability during cell evolution. The curve of elongational viscosity versus various extension rates is plotted in Fig. 6, before and after the addition of compatibilizer.

Fig. 6 Elongational viscosity versus various extension rates for EPDM/PP blend, before and after the addition of PP-g-MA as compatibilizer at 210 °C.

The variation in tensile stress versus time for both EPDM/PP and PP/PP-g-MA/EPDM is depicted in Fig. 7. The ultimate tensile stress, which is associated with melt break up, can be considered as a measure of polymer melt strength. Bearing this in mind, a careful look at Fig. 7 reveals that samples containing compatibilizer broke at a tensile stress of 60 kPa, whereas the ultimate tensile stress for a sample without any compatibilizer is approximately 26.8 kPa. On the strength of these remarkable differences, one can conclude that the presence of compatibilizer renders it possible for a polymer melt to withstand greater tensile stresses and, within the foam processing temperature, much higher cell stability is also obtained. It is also believed that both expandability and foaming agent diffusion is notably enhanced during the process when compared to samples with compatibilizer.

Fig. 7 Tensile stress versus time at a constant Hencky strain of 0.2 s⁻¹ for EPDM/PP blend before and after the addition of PP-g-MA as compatibilizer at 210 °C.

The mechanism of heat absorption in samples takes place via μν radiation beams and, with the aid of iron oxide particles, the foam product could be achieved. Polymeric resins, mostly elastomers, possess poor dielectric damping factors and, as a result, are not capable of absorbing microwave energy;^26,27 therefore, the incorporation of μν-absorbing materials to attain a uniform heat gradient is inevitable. It is noteworthy to say that the type and amount of absorbing material, as well as its distribution within the polymer matrix, also play a crucial role. Magnetic materials, such as iron oxide, use the applied electromagnetic energy at the expense of atomic alignment, through which they become capable of absorbing microwave energy.²⁵ They function to alter the magnetic permeability of the pristine material and, as a result, the absorbed energy causes the generated heat to be transferred to foaming-agent particles.²⁸ Consequently, the ADA particles decompose and the released gaseous products lead to formation of cells within the polymer matrix, on condition that the generated heat is high enough to decompose the foaming agents. The varying densities of different cells stem from the broad decomposition temperature of ADA, starting from 180 °C and rising to 220 °C, which also affects the cell type.

As can be seen in Fig. 8, which shows density variations over time of μν exposure, the changes in density over time had a slight descending trend and no considerable changes were found at a μν-power of 600 W. This is while, at a μν-power of 900 W, the density was remarkably decreased for a longer period of time-exposure. Iron oxide particles as wave absorbents allow better heat transfer through the polymer matrix and, furthermore, this heat eventually decomposes ADA and then a foam cell is formed. The greater the heat transferred, the sooner the foam will be formed. At a μν-power of 900 W, which is the highest power applied, not only was a satisfactory foam achieved, but the foam was also degraded and collapsed. This can be attributed to the incomplete dispersion of iron oxide nanoparticles, hence a thermal concentration occurred and the polymer matrix could not withstand the accumulated heat at the particle agglomeration regions. As a result, the polymer melt cannot be drawn and cell collapse takes place. Owing to their lower affinity with the polymer matrix, the polar iron oxide particles should be incorporated along with another wave absorbent material to avoid such a failure and to ensure that wave absorbance is reached within a shorter processing time.²⁹

Fig. 8 Plot of density variation versus the time of μν exposure for sample no. 1 at 600 and 900 W.

In this regard, to achieve a suitable foam of lower density, the wave absorbance through iron oxide particles within the polymer matrix must be enhanced. In doing so, we discovered a novel technique that attempts to reduce the distances between iron oxide and ADA particles. As these particles approach one another, the heat would transfer across a smaller distance, and then the ADA particles would decompose within shorter time intervals, which could lead to better formation of uniform foam cells. In order to conduct this procedure, these two particles were encapsulated by a urea–formaldehyde coating, compatible with the polymer matrix, and then the attained materials were mixed with the samples in a predetermined weight percent according to Table 1.

Comparing Fig. 8 with Fig. 9, which demonstrates the foam density of samples processed using the aforementioned technique, we notice that a greater decrease in the density of sample 1E stems from effective dispersion of encapsulated iron oxide and ADA particles, which consequently leads to the formation of uniform cells.

Fig. 9 Plot of density variation versus the time of μν exposure for sample 1E at 600 and 900 W.

Fig. 10 reveals the SEM micrographs of the fractured surface of the foam samples irradiated at a power of 900 W for 12 min. When compared to sample 1, it is obvious that the encapsulation technique applied in sample 1E resulted in a larger cell size and an effective uniformity of cells within the polymer matrix. As recorded in Table 3, in the first procedure, the calculated mean cell sizes of sample 1 are approximately 277 microns, while in the second procedure, i.e. the encapsulation technique, the mean cell sizes reach 435 microns. The increased cell sizes are commensurate with the reduction in the number of cells per unit surface, hence the cell density and the ultimate foam density would decrease. With this in mind, the resultant reduction in the ultimate foam density from the encapsulation technique reported in Fig. 9 is then approved, based on the SEM micrographs in Fig. 10. Using the encapsulation technique at a μν-power of 900 W, the lowest ultimate foam density obtained was about 0.59 g cm⁻³.

Fig. 10 SEM micrographs of fractured surface of sample (a) 1 and (b) 1E, irradiated at a μν-power of 900 W for 12 min.

Table 3 Comparison between cellular characteristics of samples 1 and 1E, irradiated at a μν-power of 900 W for 12 min

Sample	Mean cell size (μ)	Cell density (×10^−9)	Final foam density (g cm^−3)
1	277	8.5	0.73
1E	435	1.3	0.61

---

Fig. 11 demonstrates the cell size distribution in sample 1, irradiated at a μν-power of 900 W. Contrary to sample 1E, which is shown in Fig. 12, the cell size distribution in sample 1 is not satisfactorily uniform, and it can be noticed that, due to encapsulation of ADA and foaming-agent particles, a broad distribution is obtained.

Fig. 11 Cell size distribution in sample 1, irradiated at a μν-power of 900 W.

Fig. 12 Cell size distribution in sample 1E, irradiated at a μν-power of 900 W.

5. Conclusion

The scope of this work was to examine the effect of PP-g-MA as compatibilizer on the foamability of EPDM/PP blends and, furthermore, to consider the novel technique of encapsulation of iron oxide and ADA particles to be incorporated into the blend matrix. The addition of PP-g-MA led to better dispersion of PP droplets, and the compatibility of the two blends was enhanced due to reduced interfacial adhesion between PP and EPDM. Additionally, the enhanced compatibility improved the elongational viscosity at higher extension rates and its momentous role in the effective foamability of the blend was approved. It was explained that the encapsulation of foaming agents with iron oxide particles in the presence of urea–formaldehyde coatings efficiently lowers the ultimate foam density from 0.71 to about 0.59 g cm⁻³ at a μν power of 900 W. The results from SEM analyses show that the cell size distribution was largely uniform and that the formed cells were greater in size when the encapsulation technique was implemented. It was also concluded that the mean cell sizes from this technique reached 435 microns, rather than 277 microns.

References

1. A. H. Landrock, Handbook of Plastic Foams: Types, Properties, Manufacture and Applications, Elsevier Science, 1995.
2. S. T. Lee and C. B. Park, Foam Extrusion: Principles and Practice, CRC Press, 2nd edn, 2014.
3. S. G. Kim, C. B. Park and M. Sain, Foamability of thermoplastic vulcanizates blown with various physical blowing agents, J. Cell. Plast., 2008, 44(1), 53–67.
4. K. Rogers, M. Thompson and A. Hrymak, Mechanical Properties Associated with the Interface of a Co-injection Molded Structural TPO Foam, SAE Tech. Pap. Ser., 2008, 0148–7191.
5. C. A. Calles-Arriaga, J. López-Hernández, M. Hernández-Ordoñez, R. A. Echavarría-Solís and V. M. Ovando-Medina, Thermal characterization of microwave assisted foaming of expandable polystyrene, Ing., Invest. Tecnol., 2016, 17(1), 15–21.
6. R. Paul, A. A. Voevodin, D. Zemlyanov, A. K. Roy and T. S. Fisher, Microwave-Assisted Surface Synthesis of a Boron–Carbon–Nitrogen Foam and its Desorption Enthalpy, Adv. Funct. Mater., 2012, 22(17), 3682–3690.
7. J.-M. Feng, W.-K. Wang, W. Yang, B.-H. Xie and M.-B. Yang, Structure and properties of radiation cross-linked polypropylene foam, Polym.-Plast. Technol. Eng., 2011, 50(10), 1027–1034.
8. M. M. Alavi Nikje and M. Nikrah, Microwave-assisted glycolysis of polyurethane cold cure foam wastes from automotive seats in “split-phase” condition, Polym.-Plast. Technol. Eng., 2007, 46(4), 409–415.
9. F. Gunkel, A. Spörrer, G. Lim, D. Bangarusampath and V. Altstädt, Understanding melt rheology and foamability of polypropylene-based TPO blends, J. Cell. Plast., 2008, 44(4), 307–325.
10. M. Keramati, I. Ghasemi, M. Karrabi and H. Azizi, Microcellular foaming of PP/EPDM/organoclay nanocomposites: the effect of the distribution of nanoclay on foam morphology, Polym. J., 2012, 44(5), 433–438.
11. A. Shanmugharaj, J. K. Kim and S. H. Ryu, UV surface modification of waste tire powder: characterization and its influence on the properties of polypropylene/waste powder composites, Polym. Test., 2005, 24(6), 739–745.
12. S. Wong, H. E. Naguib and C. B. Park, Effect of processing parameters on the cellular morphology and mechanical properties of thermoplastic polyolefin (TPO) microcellular foams, Adv. Polym. Technol., 2007, 26(4), 232–246.
13. P. Zhang, N. Q. Zhou, Q. F. Wu, M. Y. Wang and X. F. Peng, Microcellular foaming of PE/PP blends, J. Appl. Polym. Sci., 2007, 104(6), 4149–4159.
14. A. Bledzki and O. Faruk, Microcellular wood fibre reinforced PP composites: a comparative study between extrusion, injection moulding and compression moulding, Int. Polym. Process., 2006, 21(3), 256–262.
15. A. Santoni, M.-C. Guo, M.-C. Heuzey and P. Carreau, Surface defects of TPO injected foam parts for automotive applications, Int. Polym. Process., 2007, 22(2), 204–212.
16. T. Nemoto, J. Takagi and M. Ohshima, Control of Bubble Size and Location in Nano-/Microscale Cellular Poly(propylene)/Rubber Blend Foams, Macromol. Mater. Eng., 2008, 293(7), 574–580.
17. Z. X. Zhang, V. Sridhar and J. K. Kim, Polypropylene–waste ground rubber tire powder microcellular composites: Effect of processing variables on morphology and physico-mechanical properties, Polym. Compos., 2008, 29(11), 1276–1284.
18. Z. X. Zhang, J. l. Fan, K. Pal, J. K. Kim and Z. X. Xin, Influence of compatibilizers and processing temperature on microcellular injection-molded polypropylene/(waste tire powder) composites, J. Vinyl Addit. Technol., 2011, 17(4), 254–259.
19. P. Mahallati, Optimization of Thermoplastic Elastomer Foams Based on PP and Recycled Rubber, Doctoral thesis, Laval University, 2015.
20. P. Mahallati and D. Rodrigue, Effect of feeding strategy on the mechanical properties of PP/recycled EPDM/PP-g-MA blends, Int. Polym. Process., 2014, 29(2), 280–286.
21. M. Keramati, I. Ghasemi, M. Karrabi and H. Azizi, Production of microcellular foam based on PP/EPDM: The effects of processing parameters and nanoclay using response surface methodology, e-Polym., 2012, 12(1), 612–628.
22. L. Yuan, G. Liang, J. Xie, L. Li and J. Guo, Preparation and characterization of poly(urea–formaldehyde) microcapsules filled with epoxy resins, Polymer, 2006, 47(15), 5338–5349.
23. L. Yuan, A. Gu and G. Liang, Preparation and properties of poly(urea–formaldehyde) microcapsules filled with epoxy resins, Mater. Chem. Phys., 2008, 110(2), 417–425.
24. T. Zorba, E. Papadopoulou, A. Hatjiissaak, K. M. Paraskevopoulos and K. Chrissafis, Urea–formaldehyde resins characterized by thermal analysis and FTIR method, J. Therm. Anal. Calorim., 2008, 92(1), 29–33.
25. R. M. Cornell and U. Schwertmann, The iron oxides: structure, properties, reactions, occurrences and uses, John Wiley & Sons, 2003.
26. S. O. Movahed, A. Ansarifar, G. Zohuri, N. Ghaneie and Y. Kermany, Devulcanization of ethylene-propylene-diene waste rubber by microwaves and chemical agents, J. Elastomers Plast., 2016, 48(2), 122–144.
27. A. A. Al-Ghamdi, O. A. Al-Hartomy, F. R. Al-Solamy, N. Dishovsky, M. Mihaylov, P. Malinova and N. Atanasov, Dielectric and microwave properties of elastomer composites loaded with carbon–silica hybrid fillers, J. Appl. Polym. Sci., 2016, 133(7) DOI:10.1002/APP.42978.
28. T. Wu, Y. Liu, X. Zeng, T. Cui, Y. Zhao, Y. Li and G. Tong, Facile Hydrothermal Synthesis of Fe₃O₄/C Core–Shell Nanorings for Efficient Low-Frequency Microwave Absorption, ACS Appl. Mater. Interfaces, 2016, 8(11), 7370–7380.
29. M. Zong, Y. Huang, Y. Zhao, X. Sun, C. Qu, D. Luo and J. Zheng, Facile preparation, high microwave absorption and microwave absorbing mechanism of RGO–Fe₃O₄ composites, RSC Adv., 2013, 3(45), 23638–23648.

---