Shayesteh Peydaa,
Jalil Morshedian*a,
Milad Karbalaei-Bagherb,
Habibollah Baharvandc and
Mohammad Taghi Khorasanid
aDepartment of Polymer Processing, Iran Polymer and Petrochemical Institute, Tehran, Iran. E-mail: J.Morshedian@ippi.ac.ir; shayeste.peyda@yahoo.com
bDepartment of Polymer Engineering and Color Technology, Amirkabir University of Technology, Mahshahr, Iran. E-mail: M.Karbalaei@aut.ac.ir
cDepartment of Polymer Science, Iran Polymer & Petrochemical Institute, Tehran, Iran. E-mail: H.Baharvand@ippi.ac.ir
dDepartment of Biomaterials, Iran Polymer and Petrochemical Institute, Tehran, Iran. E-mail: M.Khorasani@ippi.ac.ir
First published on 11th August 2016
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.
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 researchers17–20 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.
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.
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.
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 |
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%.
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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.
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.
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.
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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.
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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
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. 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.
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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. |
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.
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