ARTICLE pubs.acs.org/IECR
Steam-Chest Molding of Expanded Polypropylene Foams. 2. Mechanism of Interbead Bonding Wentao Zhai,†,‡ Young-Wook Kim,‡,§ Dong Won Jung,‡,|| and Chul B. Park*,‡ †
)
Ningbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of Sciences, Ningbo, Zhejinag Province 315201, China ‡ Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario M5S 3G8, Canada § Department of Materials Science and Engineering, The University of Seoul, 90 Jeonnong-dong, Dongdaemoon-ku, Seoul 130-743, Republic of Korea Department of Mechanical Engineering, Cheju National University, 690-756 Jeju-do, Republic of Korea ABSTRACT: Expanded polypropylene (EPP) bead foams were processed using steam-chest molding. Interbead bonds formed during processing and was characterized by observing both the product surface and the fracture surfaces using scanning electron microscopy (SEM) and by measuring the tensile properties of specimens. It was found that both the degree of interbead bonding and the tensile strength had a direct relationship to the steam pressure/temperature and that high steam temperature enhanced the crystallization behavior of EPP beads during the cooling process. A formation mechanism of interbead bonding during processing was proposed based on the crystal melting behaviors, as discussed in the first article of this series (Zhai et al. Ind. Eng. Chem. Res. 2010, 49, 9822). Possible parameters influencing the formation of interbead bonding, namely, steam temperature/pressure and bead foam expansion ratio, were investigated in this study. The effect of postannealing of the EPP products on the shrinkage and crystallinity of the molded EPP products was also addressed.
’ INTRODUCTION Expanded polymeric bead foams are widely used in packaging, as well as thermal and sound insulation applications.1,2 With the successful commercialization of expanded polyethylene (EPE) and expanded polypropylene (EPP), polymeric bead foams have been moving into more advanced applications in areas such as automotive production.3 For all applications, the physical and mechanical properties of bead foams are influenced mainly by interbead bonding, because the bead boundaries usually develop into fracture paths when a force is applied.4 Interbead bonding is highly dependent on molding conditions, and interbead bonding management is essential for quality control.5 Steam-chest molding is a commercialized method used to manufacture molded bead foam products, where high-temperature steam works as an effective heating medium.6 Expanded polystyrene (EPS) became a major concern in the past because of its wide usage. A formation mechanism for interbead bonding in EPS processing was proposed by Rossacci et al.5 They believed that interbead bonding development involved the diffusion of polymer chains across interbead regions. They also thought that the subsequent physical entanglement of the chains, along with any variables that could affect the polymer chain diffusion process, tended to influence the mechanical properties of the EPS sample. It was reported that interbead bonding normally increased with the molding pressure and time5,7 and that wellformed interbead bonding could improve the tensile and comprehensive properties and fracture toughness.8 However, if beads are steamed for too long, their cell structure might collapse.9 The molecular weight of EPS is a possible variable affecting interbead r 2011 American Chemical Society
bonding. EPS with high molecular weight was found to weaken interbead bonding because chain diffusion at the junction of the beads becomes difficult.5 Bead size is another important variable that influences interbead bonding.10 For example, in the production of EPS coffee cups, at least three beads through its thickness are needed. If beads that are too large are used, the cups might leak.11 Moreover, the tensile properties, compressive behaviors, and energy absorption capabilities of EPS foams have also been widely investigated.5,12,13 Currently, there is increasing interest in investigating the processing behavior and mechanical properties of EPP,1418 because it has a higher service temperature and better mechanical properties than EPS. By using a numerical simulation method, Nakai et al. investigated two fundamental aspects of the steamchest molding process: evaporation and condensation of steam and heat conduction.19 Mills et al.20 reported on the various mechanical properties of EPP, such as uniaxial compression, Poisson’s ratio, and indentation resistance, and then modeled these properties using different methods. In other studies,21,22 Mills et al. analyzed the compression properties of EPP and discussed the relationship between the creep rate, gas escape, and buckling rate of cell structure. EPP is fabricated based on a semicrystalline PP resin; the melting behavior of EPP beads during the steam-chest molding Received: August 20, 2010 Accepted: March 11, 2011 Revised: February 16, 2011 Published: March 25, 2011 5523
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process is much more complicated than for EPS. To the best of our knowledge, however, the evolution of the thermal behavior of EPP during processing has not yet been investigated. Meanwhile, the formation mechanism of interbead bonding in EPP processing remains unclear. In a separate work that was part 1 of this study, we simulated the melting behavior of EPP during processing using differential scanning calorimetry (DSC) and analyzed the evolution of melting peaks and crystallinity with steam pressure.23 Some fundamental issues of the steam-chest molding process such as temperature gradient in the mold cavity and the actual temperature during processing were elucidated. For this second part of the study, three kinds of EPP were selected, and molded EPP samples were produced at four different steam pressures. Interbead bonding was characterized by scanning electron microscopy (SEM) observations, as well as by the measurement of tensile strength. Based on the DSC simulation results, the origin of interbead bonding during processing is discussed, along with the possible parameters that influence it.
temperature, as described in Table 3. Unless otherwise indicated, this pressure was then used to represent each test condition in this article. After steam-chest molding, the molded EPP30 and EPP45 samples shrank significantly under atmospheric pressure, whereas the molded EPP15 sample did not exhibit any obvious shrinkage.24 To prevent shrinkage, within 3 min after ejection, the molded EPP30 and EPP45 samples were transferred to an oven that was kept at 80 °C and were then annealed in the oven for 4 h. After being annealed, these samples were removed from the oven for characterization. The EPP15 sample did not require annealing. Characterization. The thermal history of molded EPP samples was analyzed by DSC (Q2000, TA Instruments), calibrated against characterized indium, where a temperature ramp process from 20 to 200 °C at a heating rate of 10 °C/min was carried out. The degree of crystallinity was calculated from the integration of the DSC melting peaks by using 290 J/g as the heat of fusion (ΔHm) of 100% crystallized PP.25 Rectangular specimens were prepared from the edge region of the molded EPP samples, as indicated in Figure 1. Typical dimensions of the specimens were as follows: thickness = 14 mm, width = 19 mm, and height = 155 mm. Tensile strengths of the specimens were measured using a Micro tester (Instron 5848) at a crosshead speed of 5 mm/min. At least five specimens were tested under each condition. The morphologies of the molded EPP samples were observed by SEM (JEOL JMS 6060). For the molded EPP samples, the product surface; the cut surface, where the product was cut directly by a sharp knife; and the fractured surface were obtained after the tensile test had been performed. Both cell size and cell density were determined from the SEM micrographs. The cell density (N0), that is, the number of cells per cubic centimeter of unfoamed polymer, was determined as
’ EXPERIMENTAL SECTION Materials. Three kinds of elliptically shaped EPP beads, namely, ARPRO 5446, ARPRO 5425, and ARPRO 5415, were supplied by JSP (Tokyo, Japan). Characteristics of the EPP studied, such as expansion ratio, bulk density, and shape size, are listed in Table 1. Characteristics of the EPP cell morphology, such as cell size, cell wall, and skin thickness, are listed in Table 2. The EPP beads are denoted herein as EPP15, EPP30, and EPP45, respectively, according to their expansion ratios. Steam-Chest Molding Setup and Procedure. Laboratoryscale steam-chest molding equipment (DABO Precision, Incheon, Korea) was installed by connecting the pipes for high-pressure steam, cooling water, and compressed air. The dimensions of the mold cavity were 30 cm 30 cm 10 cm. The basic steam-chest molding process includes the following: (1) bead filling in the mold cavity, (2) steam injection from the fixed mold, (3) steam injection from the moving mold, (4) steam injection from both sides of the mold, (5) depressurization, (6) water cooling, (7) vacuuming to remove remnant water, and (8) mold opening and ejection. As was shown in part 1 of this study, the injected steam pressure was controlled by changing the following pressures during EPP processing: the fixed mold pressure, the moving mold pressure, and the applied steam pressure.23 The unit of steam pressure/gauge pressure used in this study is the relative pressure in bars, which is 1 bar lower than the absolute pressure. Based on part 1 of this study,23 the maximum steam pressure was used to describe the actual steam
N0 ¼
sample
bulk density
!3=2 φ
ð1Þ
Figure 1. Schematic of specimen preparation for tensile tests. Five samples with the same location were used.
Table 3. Steam Pressures and Their Corresponding Steam Temperatures
Table 1. Characteristics of EPP Beads expansion
nM 2 A
steam pressure (bar)
steam temperature (°C)
sample
grade
ratio
size (mm)
(g/L)
2
133.5
EPP15
ARPRO 5446
15
2.51/2.82
60.9
2.5
138.9
EPP30 EPP45
ARPRO 5425 ARPRO 5415
30 45
2.80/3.32 3.07/4.27
31.3 20.9
3.0 3.9
143.6 151.9
Table 2. Characteristics of EPP Cell Morphology sample
cell wall thickness (μm)
skin thickness (μm)
cell size (μm)
cell density (cells/cm3)
EPP15 EPP30
∼14.2 ∼4.7
∼20.1 ∼7.5
∼210.5 ∼335.2
1.6 106 1.8 106
EPP45
∼2.8
∼4.2
∼409.2
1.7 106
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Figure 2. DSC thermograms of the molded EPP30 samples (a) without and (b) with the annealing process. L15 were the five locations across the molded EPP sample with the same distance in the thickness direction. The EPP30 samples were prepared with a steam pressure of 3.9 bar.
where n is the number of cells in the SEM micrograph, M is the magnification factor, A is the area of the micrograph (in cm2), and φ is the volume expansion ratio of the polymer foam.
’ RESULTS AND DISCUSSION Effect of Annealing on the Thermal Behaviors of Molded EPP Samples. During the steam-chest molding process, steam
can penetrate into the bead foams. Once cooling is applied at the end of processing, the high-temperature steam tends to condense in the cell structures where it forms a vacuum state. Because of the characteristics of the close cell structure, air cannot penetrate into the foam within a short period of time, resulting in a dramatic decrease in the internal pressure of the foams. Consequently, molded EPP samples tend to shrink after removal from the mold. An annealing process is usually carried out at a high temperature to prevent shrinkage by enhancing the diffusion rates of steam and air.24 In this study, the molded EPP30 and EPP45 samples exhibited serious shrinkage, and these samples were annealed at 80 °C for 4 h. For the molded EPP15 sample, however, no obvious shrinkage was observed, and no further annealing process was applied. As indicated in Table 2, the cell wall of the EPP15 bead was 3.0 times thicker than that of the EPP30 bead and 5.1 times thicker than that of the EPP45 bead. The skin of the EPP15 bead was 2.7 times thicker than that of the EPP30 bead and 4.7 times thicker than that of the EPP45 bead. It is believed that the thick cell wall and foam skin of the EPP15 bead could resist the compression force resulting from the pressure difference that arose during the cooling process. As mentioned in part 1 of this study,23 the melting behaviors of the EPP bead foams were highly sensitive to the treatment temperature. The effect of the annealing process on the melting behavior of the molded EPP samples was investigated. Figure 2 shows the DSC thermograms of the EPP30 sample produced at a steam pressure of 3.9 bar both with and without annealing. It can be seen that the EPP30 sample without annealing had three melting peaks from high to low temperature, denoted Tmhigh, Tmi, and Tmc, respectively. It was verified that Tmhigh was the original high melting peak of the bead foam, that Tmi was the induced melting peak resulting from the heating process during bead foam processing, and that Tmc was the melting peak of crystals formed during the cooling process.23 Basically, Tmhigh is
used to maintain the cell structure of the EPP bead foam and does not change with processing conditions. However, its absence under some processing conditions means that the cell structure has been destroyed. L15 were the five locations across the molded EPP sample having the same distance in the thickness direction that were used to investigate the effect of the sample thickness on the EPP thermal history. It was observed that the melting behaviors of the EPP sample at these five locations were quite similar regardless of whether the annealing process was applied. These phenomena indicate that the temperature gradient in the mold during processing or for the molded samples in the oven during the annealing process was negligible.23 A main difference in the DSC thermograms of the molded EPP30 sample both with and without annealing was the presence of a slight melting peak at a temperature of about 86.5 °C, which was about 6.5 °C higher than the annealing temperature. Based on the DSC simulation results reported in part 1 of this study,23 it is believed that the melting peak should be a new induced melting peak resulting from the annealing process. It was further found that the three melting peaks at Tmc, Tmi, and Tmhigh at 140160 °C in the DSC curve were not affected by the annealing process. Figure 3 summarizes the crystallinity change in the molded EPP samples with annealing and with the expansion ratio of the EPP beads. The error bars show the crystallinity fluctuations at the five locations. The steam temperature was obtained according to the corresponding steam pressure.26 As shown in Figure 3a, the crystallinity percentages of the molded EPP30 samples without annealing were 23.1%, 22.7%, 23.5%, and 24.7% at gauge pressures of 2.0, 2.5, 3.0, and 3.9 bar, respectively. With annealing, however, the crystallinities of the samples became 25.2%, 24.5%, 25.6%, and 27.4%, respectively, indicating that annealing tended to slightly increase the crystallinity of the molded EPP30 samples. The crystallinity of the EPP30 samples exhibited an obvious steam temperature dependency, and an increase in steam temperature gradually increased the crystallinity. This phenomenon is very similar to the DSC simulation results, where an increased treatment temperature was found to increase the crystallinity at treatment temperatures higher than 135 °C, because of the enhanced crystallization behavior of EPP during the cooling process.23 Figure 3b shows the crystallinity of molded EPP15 samples without annealing and molded EPP30 5525
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Figure 3. (a) Crystallinity changes of the molded EPP samples with the annealing process and (b) expansion ratio of EPP beads.
Figure 4. (a) Separation of the crystallinity contribution of each melting peak, with shaded areas indicating the crystal melting of Tmc, whose crystallinity was formed during the cooling process. (b) Effect of steam temperature on the crystallinity associated with Tmc.
and EPP45 samples with annealing. EPP30 and EPP45 had very similar crystallinities, both of which were modestly higher than that of EPP15. It was noted that the annealing process tended to slightly increase the crystallinity of the samples. Therefore, the crystallinities of all three EPP samples might be very similar if the same annealing process were applied to each of them. The crystallinity contributions of each melting peak were separated as indicated in Figure 4a. The contribution of the cooling process to crystallinity was estimated roughly, and the values are shown in Figure 4b. The error bars show the crystallinity fluctuations at the five measured locations. At 2.0 bar, the contribution of the cooling process to crystallinity was about 3.8%, which means that very few crystal domains formed during the cooling process and that they comprised only 15% of the total crystallinity of the final EPP samples. An increase in steam pressure tended to increase the contribution of the cooling process to the crystallinity to 7.3% (i.e., about 30% of the total crystallinity). At a higher steam pressure, the contribution of the cooling process to the crystallinity increased dramatically to 15.7% at 3.0 bar (i.e., about 63% of the total crystallinity), and then to 20.1% at 3.9 bar (i.e., about 75% of the total crystallinity).
Based on the DSC simulation results and the effect of the annealing process, it is known that the melting of the low-melt region during heating decreased the crystallinity whereas the occurrence of crystallization during the cooling and annealing processes increased the crystallinity of the final EPP samples. On the other hand, as indicated in Figure 4, increased steam pressure/temperature facilitated increased crystallinity in the EPP samples. Therefore, it is believed that increased steam temperature not only tended to melt some original crystals, but also induced more crystal formation during the cooling process. Effect of Steam Pressure on Interbead Bonding. When a bead foam product is broken into pieces by force, bead boundaries are potential fracture paths,4 and interbead bonding tends to determine the mechanical properties of the bead product.5 This section describes the observation of interbead bonding by SEM and its further assessment by application of the tensile test to specimens. Molded EPP15, EPP30, and EPP45 samples produced under different conditions were cut directly with a sharp knife. SEM micrographs of the cut surfaces of the samples are shown in 5526
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Figure 5. SEM micrographs of the cut surfaces of molded EPP15, EPP30, and EPP45 samples produced at different steam pressures.
Figure 5. At a steam pressure of 2.0 bar, the EPP15 and EPP30 beads almost kept their original elliptical shapes, and large volumes of interbead voids were observed, as indicated by the arrows. The SEM micrograph of the EPP45 beads is not shown here because the molded sample was already broken after being pushed out of the mold. At 2.5 bar, the shape of the EPP beads deformed more, and most of the bead surfaces touched each other, resulting in a significant decrease in the interbead void fraction. At 3.0 bar, the bead surfaces accommodated each other very well, and the interbead void fraction decreased dramatically. At a higher steam pressure of 3.9 bar, the interfaces between the beads became fuzzy, and interbead voids were not observed under the present magnification. It is noted that the interbead properties for all three kinds of the EPP beads exhibited similar dependencies on the steam pressure, even though they had different original bead sizes and expansion ratios. The interbead properties of the EPP product surfaces are shown in Figure 6. Similarly to the cut surface of the EPP samples, the surface of the EPP products exhibited less deformation at lower steam temperatures and more deformation at higher steam temperatures. These results indicate that the increased steam pressure seriously deformed the beads’ shapes, resulting in an increase of the contact area among the beads and a decrease or removal of the interbead voids. Considering that the same amounts of EPP
beads were injected into the mold for each kind of material, the deformation of the bead shape demonstrates that bead expansion occurred during the processing and that a higher steam temperature led to a higher degree of bead expansion. The interbead properties of the foams were further investigated using the fracture surfaces of the molded EPP samples after the tensile test, as shown in Figure 7. At 2.0 bar for EPP15 and EPP30 and at 2.5 bar for EPP45, the fracture mode was almost 100% interbead fracture, and consequently, bead surfaces and interbead voids were observed in the SEM micrographs. Furthermore, the EPP beads exhibited only very slight deformities. At 2.5 bar for EPP15 and EPP30 and at 3.0 bar for EPP45, the samples still showed interbead fracture behavior, but the bead shape clearly changed from the original elliptical shape to a polyhedral shape. Meanwhile, more bead surfaces were touched by others. At a higher steam pressure, the EPP samples exhibited mostly intrabead fracture, where cracks ran not only along intrabead boundaries but also across the cell structures of some broken beads. This phenomenon demonstrates that the degree of interbead bonding increased dramatically with increasing steam pressure. Interbead bonding was also assessed by the tensile test, and the tensile strengths of the samples are shown in Figure 8. The tensile strength of EPP15 gradually increased from 0.14 MPa at 2.0 bar 5527
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Figure 6. SEM micrographs of the surfaces of molded EPP15, EPP30, and EPP45 samples produced at different steam pressures.
to 0.41 MPa at 2.5 bar and then significantly increased to 1.02 MPa at 3.0 bar, indicating an increased interbead bonding force at high steam pressure. At a higher steam pressure of 3.9 bar, the tendency of the tensile strength to increase with the steam pressure diminished, and a maximal tensile strength value of 1.21 MPa was obtained under these conditions. Similarly to EPP15, EPP30 exhibited an increase in tensile strength with increasing steam pressure from 0.04 MPa at 2.0 bar to 0.14 MPa at 2.5 bar, 0.43 MPa at 3.0 bar, and 0.63 MPa at 3.9 bar. EPP45 showed a very similar steam pressure dependency on tensile strength; specifically, the tensile strength increased from 0.09 MPa at 2.5 bar to 0.21 MPa at 3.0 bar and to 0.41 MPa at 3.9 bar. According to the observations of the fracture surfaces and the corresponding tensile strengths, it is believed that interbead bonding has a direct correlation with the tensile strength of the sample and that stronger interbead bonding leads to a higher tensile strength value. The tensile strength of the EPP samples exhibited an obvious dependency on the steam pressure/temperature. An increase in steam pressure increased the tensile strength of the EPP samples because of their stronger interbead bonding, as shown in Figures 57. A similar phenomenon was observed in EPS bead processing, where a high tensile strength and a high degree of interface fusion were obtained at high molding pressures.5 Bead size also has an effect on the tensile
strength of the samples. As shown in Figure 8, the tensile strength decreased with increasing bead expansion ratio at an equivalent steam pressure; for example, the tensile strengths of EPP15 and EPP45 after molding at 3.9 bar were 1.21 and 0.41 MPa, respectively. Formation Mechanism of Interbead Bonding during EPP Bead Processing. During bead foam processing, high-temperature steam penetrates and condenses inside the beads. When depressurization is applied to the mold cavity, the water in the cell structures gasifies, and the internal pressure of the beads increases, causing a slight expansion of the softened beads. Consequently, flat regions develop between the beads, and a welding process starts. In this section, we propose a formation mechanism for interbead bonding during EPP bead processing based on the DSC simulation results and the SEM fracture surface observations. It is known that steam pressure is associated with steam temperature. During bead foam processing, high-temperature steam can melt the original crystals, and this decrease in crystallinity softens the polymer matrix. Meanwhile, high-temperature steam (i.e., high pressure) leads to high internal pressure inside the bead foam as depressurization is applied. Consequently, at a high steam temperatures, bead foams tend to be highly expanded. Because of the fixed volume of the mold cavity, the expanded 5528
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Figure 7. SEM micrographs of the fracture surfaces of molded EPP15, EPP30, and EPP45 samples produced at different steam pressures after a tensile test.
Figure 8. Tensile strengths of molded EPP15, EPP30, and EPP45 samples produced at different steam pressures.
beads could impinge upon one another and, thereby, develop a polyhedral shape. As indicated in Figures 5 and 6, more bead shape deformities occurred to match the shapes of the other beads at high steam pressures, which led to a gradual decrease in
interbead void areas. Once interbead contact occurred, the molecular chains that were in an amorphous state with high mobility became free to diffuse across the interbead area. High steam temperatures could melt large amounts of previous crystals 5529
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Industrial & Engineering Chemistry Research in the beads, leading to large quantities of amorphous regions and thus facilitating this kind of self-diffusion. Afterward, a cooling process was applied to stabilize the molded bead samples. Based on the DSC simulation results, as shown in part 1 of this study,23 crystallization tended to occur during the cooling process when the treatment temperature was higher than 130135 °C, and its corresponding contribution to the final crystallinity increased with the treatment temperature, as indicated in Figure 4. The development of crystal domains with high crystallinity in the interbead areas significantly increased the interbead bonding force, resulting in an obvious increase in the tensile strength of the EPP samples at high steam temperatures. Therefore, we believe that the occurrence of crystallization in the interbead area is possibly a mechanism for interbead bond formation during EPP processing and that the corresponding crystallinity is obviously affected by the steam temperature. The quality of bonding among polymer beads was investigated by Bellehumeur et al. using a fused deposition modeling process,27 where sintering experiments were carried out to evaluate the dynamics of bond formation between polymer beads. In that study, the information on molecular mobility at the interface was used to characterize the bonding strength. The results demonstrated that the cooling conditions had strong repercussions on the interbead bonding and, thereby, on the mechanical properties of final product. Even though two processing methods, polymer bead sintering and bead foam steam-chest molding, possibly had obvious differences in heating medium and thermal transfer efficiency, the cooling conditions exhibited a significant effect on the interbead bonding force. In EPS processing with steam-chest molding, it was also observed that increased steam tended to increase the tensile strength of the EPS sample.5 However, the mechanism of interbead bonding between EPS beads is very different from that of EPP beads. PS is an amorphous polymer resin. Therefore, once the steam temperature is higher than the glass transition temperature, the molecular chains can diffuse into the interbead area, and the resulting chain entanglements enhance interbead bonding.5 Therefore, the quality of the bonding formed between individual EPS beads depends on the molecular diffusion and randomization at the interface. In EPP processing, however, the formed crystal domains contribute to interbead bonding, where most of the molecular chains are anchored in crystals by strong chain interactions. It is believed that the interbead bonds formed during the crystallization process are much stronger than those formed from physical chain entanglement, which results in a much higher tensile strength of EPP compared to EPS. For example, the maximum tensile strength of the EPS beads was about 0.44 MPa,5 which was much lower than the 1.21 MPa tensile strength value obtained for the EPP15 beads in this study. The better mechanical properties of EPP beads have led to their popularity in many advanced applications. Bead size was another important parameter affecting the interbead bonding of molded EPP samples. As shown in Figure 8, it was found that the tensile strengths of the EPP15 beads were 1.933.02 times higher than those of the EPP30 beads and 2.954.51 times higher than those of the EPP45 beads. There are two possible reasons for this phenomenon. As indicated in Table 1, bead foams have different sizes that inversely follow the order of the bead expansion ratio: EPP15 < EPP30 < EPP45. During bead processing, the mold cavity was fixed, and a smaller sample size meant a larger surface area, so that more of the surface area of each bead could touch other beads under the same
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processing conditions, resulting in the formation of more interbead bonding regions per unit volume. Alternatively, the presence of an intrabead fracture structure suggests that the crack path ran not only through the bead boundaries but also across the beads’ cell structures. Therefore, the mechanical properties of the beads also affected the tensile strengths of the EPP samples. As indicated in Table 2, EPP15 has a thicker cell wall and skin than the other EPP samples, and its higher mechanical properties contributed to the high tensile strengths of the final EPP samples. Thus, the observed higher strength of molded EPP15 can be attributed to the greater cell wall and skin thicknesses of its beads (see Table 2) and to the greater interbead contact area per unit volume because of its smaller bead size (see Table 1), compared with those of the other samples.
’ CONCLUSIONS In this study, three EPP bead foams with different sizes and expansion ratios were selected, and steam-chest molding was used to prepare the molded EPP samples. The molded EPP samples exhibited obvious shrinkage after removal from the mold cavity, depending on their cell wall and skin thicknesses. A 4-hlong postannealing process at 80 °C after steam-chest molding was effective in preventing shrinkage of the molded EPP30 and EPP45 samples. This process induced a low melting peak of about 86.5 °C and increased the crystallinity of the samples slightly. A new crystal melting area and the further formation of a melting peak at 141145 °C resulting from the cooling process was observed in the DSC curves. The corresponding contribution of the cooling process to the crystallinity increased dramatically with the steam temperature. SEM micrographs of cut and fractured surfaces of the molded EPP samples indicated that the interbead contact area and the deformation degree of each bead obviously increased with increasing steam temperature. Further, the facture mode changed from interbead fracture at low steam pressures to intrabead fracture at high pressures. The tensile strength increased continuously with as the steam pressure was increased from 2.0 bar to 3.9 bar. A possible mechanism of interbead bond formation was proposed based on the DSC results and the SEM observations of the fracture surfaces: The application of high-temperature steam melts the crystalline domains at the interbead areas, and the molecular chains in the amorphous regions become free to diffuse across the interbead area. Once a cooling process is started, crystallization takes place at steam temperatures of higher than 130135 °C and strengthens interbead bonding. An increase in steam temperature dramatically increases the contribution of the cooling process to the crystallinity and thus increases interbead bonding, as evidenced by the improvement in tensile strength. The tensile strengths of molded EPP samples are influenced by the expansion ratios of the original bead foams because smaller beads form larger interbead contact areas and have thick cell walls. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors are grateful to the National Natural Science Foundation of China (Grant 51003115), the Consortium of 5530
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Industrial & Engineering Chemistry Research Cellular and Micro-Cellular Plastics (CCMCP), and The Korea Research Foundation (KRF-2009-013-D00050) for their financial support of this project.
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