Critical Effects of Polyethylene Addition on the ... - ACS Publications

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Critical Effects of Polyethylene Addition on the Autoclave Foaming Behavior of Polypropylene and the Melting Behavior of Polypropylene Foams Blown with n‑Pentane and CO2 Xiaoqin Lan,†,‡ Wentao Zhai,*,† and Wenge Zheng*,† †

Ningbo Key Lab of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang Province 315201, China ‡ The School of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang Province 315211, China ABSTRACT: Polypropylene (PP) bead foams were prepared by autoclave foaming using n-pentane and CO2 as the blowing agents. PP foams blown with n-pentane had foaming temperatures of 92−96 °C, expansion ratios of 10−50 times, and a signal Tm at 150.1 °C, while PP foams blown with CO2 had foaming temperatures of 151−153 °C, expansion ratios of 8−20 times, and dual melting peaks at 164.0 and 140.9 °C. Polyethylene (PE) addition was used to improve the foaming behavior of PP and to induce the formation of dual and multiple Tm in PP/PE foams. A differential scanning calorimetry procedure was carried out to simulate the steam-chest molding of bead foams. Interbead bonding was found to be determined by the heat of fusion of Tmc (crystal melting of the newly formed crystals during fast cooling), ΔHmc. Recrystallization of the PE component contributed to the increase of ΔHmc, which potentially improved interbead bonding of the molded PP/PE bead foams.

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INTRODUCTION

an increase in the molding temperature increased the mechanical properties of the molded EPS and EPP.10,11 For semicrystalline EPP bead foams, the molding temperature should be high enough to melt the partial crystals with the aim of softening and fusing the surface of bead foams, but the molding temperature cannot be higher than the melting peak of the bead foams; otherwise, the cell structure will be destructed. Therefore, a common way of generating good bead foam bonding is to develop two melting peaks in EPP, where the higher melting peak is to maintain the cell structure and the lower one is beneficial to the surface fusion of bead foams. It has been shown that the blending presents a significant effect on the differential scanning calorimetry (DSC) curve behavior of polymeric materials. For example, the addition of about 25 wt % LDPE to LLDPE promoted dual peaks in DSC curves,12,13 and the addition of about 40 wt % EVA to LLDPE promoted dual peaks.14 Behravesh et al. did not observe the dual peaks in the LLDPE/HDPE (20 wt %), LLDPE/LDPE (20 wt %), and LLDPE/HDPE (10 wt %)/LDPE (10 wt %) blends. After autoclave foaming, however, they found dual and multiple melting peaks for the blend foams blown with CO2 at some temperatures.15 Researchers observed that PP foam blown with n-pentane had a single melting peak and presented dual melting peaks when it was blown with CO2. Considering the different PP resins, it is difficult to figure out the reasons why the dual melting peaks appear. In this study, one PP resin was selected, and autoclave foaming was carried out using n-pentane and CO2 as the physical blowing agents. The foaming behavior of the PP resin and the melting behavior of PP foams were

Polymeric foams exhibit many advantages over bulk materials such as lower density, higher specific strength, and excellent energy absorption. These unique properties enable polymeric foams to be used effectively in packaging, acoustic absorbents, and automotive parts, as well as in sporting equipment and construction industries.1,2 The preparation of high-performance polymeric foams has drawn a great deal of interest in recent decades.3 Expanded polystyrene (EPS), expanded polyethylene (EPE), and expanded polypropylene (EPP) are three kinds of most popularly used moldable bead foams. Among them, EPS has the lowest price and the highest market share. For the purpose of environment protection, however, the usage of EPS has gradually been forbidden in many countries.4 On the other hand, EPP has become one of the fastest-growing products used in the automotive industry as well as in transport packaging because of its capacity to withstand repeated impact and compression. Steam-chest molding is a commercial method used to manufacture bead foam products, and the details have been well described by many researchers.5−7 During the molding process, steam works as a heating medium to soften and fuse the surface of the bead foams, which enables amorphous polymer chains to self-diffuse at the interbead interfaces. After the cooling process is applied, the self-diffused polymer chains solidify or crystallize, and interbead bonding is generated. 8,9 Compared to amorphous EPS bead foam, semicrystallize EPP bead foam exhibits much higher interbead bonding because the crystal formation of the self-diffused chains supplies additional bonding.10 In order to maximize the bonding force among bead foams, it is important to generate a large amount of polymer chains that have high self-diffusion ability at the interfaces of bead foams and have a high degree of crystallinity during solidification. The increased molding temperature seems to increase the mobility of the polymer chains. As was reported, © 2013 American Chemical Society

Received: Revised: Accepted: Published: 5655

October 23, 2012 March 4, 2013 March 26, 2013 March 26, 2013 dx.doi.org/10.1021/ie302899m | Ind. Eng. Chem. Res. 2013, 52, 5655−5665

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic of the autoclave foaming system.

autoclave, water was used as an aqueous dispersion medium, and n-pentane and CO2 were used as the physical blowing agents. The pellets were dispersed in liquid with the assistance of a mechanical agitator and were impregnated with a foaming agent at elevated temperatures. After the scheduled saturation time was reached, the pressure was quenched to atmospheric pressure, and the expanded bead foams were injected into a big container. The die geometry is critical to maintaining a high enough pressure inside the chamber to prevent prefoaming of the gas-impregnated beads, and a shut-off valve was selected to fulfill the task. n-Pentane and CO2 are two typical physical blowing agents to foam PP micropellets.16−19 During the saturation process, the saturation pressures of autoclave systems were set at about 1.0 and 3.0 MPa, when n-pentane and CO2 were used as the blowing agents, respectively. DSC Analysis. The melting peak (Tm) of PP resins and foams was determined by the DSC instrument (Mettler Toledo) calibrated with indium. During DSC tests, a thermal scanning from 25 to 200 °C with a heating rate of 10 °C/min was applied. In order to remove the thermal history of PP and PP foams, a DSC procedure of heating/cooling/heating with 10 °C/min and isothermal treatment at 200 °C for 10 min was carried out. The steam-chest molding process consists of three processes, i.e., fast heating, high temperature treatment, and fast cooling. The total time for processing was about 3 min. A DSC simulation was carried out to investigate the evolution of Tm in PP bead foam during stream-chest molding. A detailed description of this simulation has been presented in our previous researches.7,10 Specifically, the samples were heated with a heating rate of 60 °C/min from 25 °C to a high

investigated. Three kinds of PE, i.e., LDPE, LLDPE, and HDPE, with lower melting peak than PP were added to induce dual or multiple peaks in PP/PE foams. The effect of PE addition on the foaming behavior was discussed. A DSC simulation was carried out to describe evolution of the melting behavior of PP and PP/PE foams during steam-chest molding, in which a process of a fast heating, isothermal treatment, and a fast cooling was included. As verified in our previous study,7 interbead bonding was determined by the newly formed crystals during the cooling process. The critical effects of the types of blowing agents and PE addition on the heat fusion of new crystals were summarized.

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EXPERIMENTAL SECTION Materials and Micropellets Preparation. A PP random copolymer, with a melt flow rate of 8 g/10 min (M800E), was provided by Shanghai Petrochemical Co., China. LDPE (2220H), HDPE (HD5502FA), and LLDPE (LL0220KJ) were obtained from BASF-YPC and Shanghai Secco Petrochemical Co., Ltd., respectively. CO2 with a purity of 99.9% (Wanli Gas Co., Ningbo, China) and n-pentane with a purity of 95% (XinLongXin Chemical Co., Ningbo, China) were used as the physical blowing agents. A laboratory counter-rotating twin-screw extruder was used to pelletize the PP resin and PP/PE blends into small beads with a size of 1 mm. For PP/PE blends, three kinds of PE, i.e., LDPE, HDPE, and LLDPE, were used, and a PE loading of 10 wt % was fixed. Foaming Process. Two autoclave foaming systems with a volume of 50 L were self-designed and used for PP bead foaming. Figure 1 indicates the schematic of the autoclave foaming system. Around 1 kg of micropellets was put into the 5656

dx.doi.org/10.1021/ie302899m | Ind. Eng. Chem. Res. 2013, 52, 5655−5665


Article

temperature of about 110−165 °C and then were treated isothermally for 3 min. When the isothermal treatment time was reached, the samples were cooled to 25 °C again with a cooling rate of 60 °C/min. Afterward, the samples were heated to 200 °C at 10 °C/min to check the previous thermal history, which included the thermal history of fast heating, isothermal treatment, and fast cooling processes. Foam Characterization. The mass densities of the samples (ρ and ρf) before and after foaming were measured by the water displacement method in accordance with ASTM D792, and the apparent density of the foams was measured based on ASTM 1895. ϕ is the volume expansion ratio of the polymer foam, which can be calculated using eq 1 as follows: ρ ϕ= ρf (1)

PP foams using n-pentane showing much higher foam expansion. Another effect of gas sorption is to reduce the Tm value of the semicrystalline polymer.23 n-Pentane had a high gas solubility in a PP matrix, which tended to decrease the Tm value of PP significantly. As a consequence, the PP bead could foam at 92−96 °C, which was much lower than the Tm value of the PP resin, i.e., 150.5 °C. When a high foaming temperature was applied, it was found that a cotton-shaped powder was obtained, possibly resulting from the melting and deformation of PP beads within the autoclave. In the case of using CO2 as a blowing agent, a foaming temperature of 150−153 °C was required possibly because of a slight reduction of Tm of PP at this condition. Figure 2 shows the DSC thermograms of PP and PP foams blown with n-pentane at 95 °C and CO2 at 151 °C. For the PP

The morphology of the foamed PP and PP/PE beads was analyzed by a TM-1000 scanning electron microscope. The samples were freeze-fractured in liquid nitrogen and sputtercoated with gold. Both the cell size and cell diameter were the average of at least 100 cells on the scanning electron microscopy (SEM) micrographs. The cell density (N0), the number of cells per unit cubic centimeter of solid sample, was determined by eq 2 as

⎡ nM2 ⎤3/2 N0 = ⎢ ⎥ ϕ ⎣ A ⎦

(2)

where n is the number of cells in the SEM micrograph, M is the magnification factor, and A is the area of the micrograph (in cm2).

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RESULTS AND DISCUSSION Foaming Character of PP Blown with Pentane and CO2. Table 1 summarizes the foaming character of PP, which

Figure 2. DSC curves of PP beads and foams blown with n-pentane and CO2. The heating and cooling rates were 10 °C/min.

Table 1. Foaming Character of PP Blown with n-Pentane and CO2 blowing agent

foaming temperature, °C

expansion ratio

cell density, cells/ cm3

n-pentane CO2

92−96 150−153

10−50 8−20

105−108 107−109

resin, only one broad Tm was observed at 150.5 °C at the first heating. When the thermal history was removed, the PP resin had a sharp Tm and a broad shoulder peak was seen at low temperature. For the PP foam prepared using n-pentane, only one Tm value was observed at 150.1 °C at the first heating. Compared to the PP bead, however, the Tm value of the PP foam was much sharper, suggesting that the distribution of crystal domains in the PP foam might reduce obviously. When the thermal history was removed, a single Tm value at 150.5 °C was observed. For the PP foam prepared using CO2, however, two Tm values were found at 140.9 and 164.0 °C at the first heating. A similar phenomenon has been observed in other papers.24,25 After removal of the foaming history, the melted PP foam only presented one Tm value at 148.2 °C, which was similar to the melting behavior of the PP bead and foam prepared using n-pentane. The origin of dual melting behavior of PP has been widely investigated. The chain structure, the coexistence of multiple crystal phase, the lamellae thickening process, and the melting of different types of spherules are generally thought of as possible reasons to explain the phenomenon.26 Kim et al. investigated the influence of the isothermal crystallization temperature on the melting behavior of PP.27 They found that the dual melting peaks could be obtained at crystallization temperatures ranging from 110 to 140 °C. For PP crystallized below 125 °C, the single melting peak split into two peaks, while the preexisting crystal fraction having different Tm values

includes the information of the foaming temperature, expansion ratio, and cell density. In the case of using n-pentane as the blowing agent, PP exhibited low foaming temperatures of 92− 96 °C, and the prepared PP foams possessed high expansion ratios of 10−50 times and low cell densities of 105−108 cells/ cm3. In the case of using CO2 as the blowing agent, however, the foaming temperatures of PP increased up to 150−153 °C, the expansion ratios of PP foams decreased to 8−20 times, and the cell densities increased up to 107−109 cells/cm3. These results demonstrated that PP beads blown with n-pentane and CO2 presented very different foaming behaviors. The gas solubility has been verified as a critical parameter to affect the foaming temperature and expansion ratio of polymeric foams.20 It is well accepted that PP has much higher gas solubility in n-pentane relative to CO2, i.e., 3−521,22 vs 20 wt%,16 at pressures of 1−3 MPa and temperatures of 100−160 °C. In general, a blowing agent with high gas solubility facilitates cell growth and foam expansion. n-Pentane had a higher gas solubility in a PP matrix than CO2, which led to the 5657



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Figure 3. Phase morphologies of PP (a), PP/LDPE (b), PP/LLDPE (c), and PP/HDPE (d) blends.

separated PE phases with size 2−5 μm were well dispersed in a PP matrix, and a similar phenomenon has been widely observed by many researchers.29 It is indicated that PP and PE were not well miscible but similar chain structures were benefitical to reduce the phase size of PE. The melting behavior of PP and PP/PE blends is shown in Figure 4, and the Tm values of these samples are summarized in

was obtained for PP crystallized above 125 °C. A similar phenomenon was observed in this study, here, PP and PP/PE foams blown with n-pentane only had a single Tm, but the isothermal treatment at 150 °C for 3 min (carried out in below) could induce the formation of a new Tm at 161 °C and that at 110−145 and 155−165 °C failed to induce the formation of Tm at 161 °C. As shown in Table 1, PP beads had very different foaming temperatures, i.e., 92−96 vs 150−153 °C, when they were blown with n-pentane and CO2, respectively. We speculated that the difference in the foaming temperature might be a key reason for the different thermal behaviors of PP foams. The crystal domains associated with Tmhigh might result from recrystallization of the less perfect crystals during the CO2 saturation process.28 In summary, the difference in the gas solubility led to very different foaming characters of PP beads. The interesting information was that dual Tm values could be generated in PP foam when CO2 was used, while only a single Tm value could be formed in PP foam using n-pentene. According to our previous research,7,10 the formation of dual Tm values is critical to generating strong interbead bonding during steam-chest molding. The PP foam prepared with n-pentane possessed a high foam expansion ratio and mild foaming conditions, i.e., low foaming temperature and pressure. It is promising to induce dual Tm values in these foams in order to improve the molding behavior of EPP. In this study, we blended three kinds of PE with low Tm, i.e., LDPE, HDPE, and LLDPE, into a PP matrix. The blended component might be helpful to generate dual Tm values for the PP foam. Moreover, the influence of PE addition on the foaming behavior of PP was investigated. Phase Morphology and Thermal Behavior of PP/PE Blends. PP and PP/PE dry blends were extruded by a twin extrusion system. Their phase morphologies were observed by SEM, and the results are shown in Figure 3. It is seen that

Figure 4. DSC curves of PP, LDPE, LLDPE, HDPE, and PP/PE blends.

Table 2. All four polyolefin resins had a single Tm, which was 150.5 °C for PP, 109.7 °C for LDPE, 126.2 °C for LLDPE, and 132.8 °C for HDPE. After the PE component was blended into PP, it was found that the Tm value of the PP component did not show any visible change compared to that of the PP bead, while the Tm value of the PE component decreased slightly, i.e., 109.7 vs 108.8 °C for LDPE, 126.2 vs 123.8 °C for LDPE, and 132.8 5658



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foam prepared at 92 °C possessed tiny cells with a size of ∼30 μm, large cells with a size of ∼120 μm, a thick cell wall, and a low foam density of 73 g/L. With the addition of 10 wt % LDPE, however, the cell size of the PP/LDPE foam dramatically increased up to 100−300 μm and the cell wall was very thin (in the range of 10 μm), which accompanied with an obvious foam density reduction, i.e., 22 g/L. For PP/LLDPE and PP/HDPE foams, they also presented an improved cell structure and a reduced foam density compared to that of the PP foam, i.e., 29 and 44 g/L vs 73 g/L. These results demonstrated that the introduced PE components improved the foaming behavior of PP. The PP foam prepared at 95 °C exhibited a well-defined cell structure, a large cell size, a thin cell wall, and a low foam density of 15 g/L. The addition of PE did not obviously change the cell morphology and foam density of the PP foam, i.e., 13 and 14 g/L vs 15 g/L. PP with a linear chain structure has a low melt strength, and the PP foam prepared by continuous extrusion foaming usually presents poor cell morphology, such as large cell cells, low cell density, and nonuniform cell distribution.30−33 When autoclave foaming technology was applied, however, a PP foam with a well-defined cell structure and extremely low foam density could be achieved.16,19 Researchers have found that crystal domains generated by an induced or preexisting gas could improve the cell morphology of foams by increasing the melt strength of the polymer.34,35 For a semicrystalline polymer with too high crystallinity, however, a polymer matrix with high stiffness tends to spoil the cell growth and foam expansion.36 The foaming temperature seemed to significantly affect the stiffness of the PP matrix, specifically the crystallinity of the PP matrix. The PP matrix had low stiffness at high foaming temperature, which facilitated foam expansion. At a foaming temperature higher than 95 °C, however, no regular PP bead foams could be obtained. The possible reason was that PP

Table 2. Melting Peaks of PP and PP/PE Beads and Foams melting peak (°C) samples PP PP/LDPE LDPE PP/LLDPE LLDPE PP/HDPE HDPE

PP LDPE PP LLDPE PP HDPE

beads 150.5 151.0 108.8 109.7 150.2 123.8 126.2 149.8 129.7 132.8

foams blown with n-pentanea

foams blown with CO2b

149.8 149.0 104.5

150.1 149.3 104.2

164.0/140.9 163.3/140.3 105.8

148.4 120.2

149.2 121.1

163.0/141.2 121.8

149.8 128.5

149.2 127.0

163.5/140.8 126.7

The foaming temperature was 92 and 95 °C, respectively. bThe foaming temperature was 151 °C. a

vs 129.7 °C for HDPE. This phenomenon suggested that the blending might slightly affect the crystallization behavior of PE. Autoclave Preparation of PP and PP/PE Foams. Two self-designed autoclave foaming systems with a volume of 50 L were used to prepare PP and PP/PE foams. In the case of using n-pentane as a blowing agent, the saturation pressure was built by the saturated vapor of n-pentane and water,16 and the achieved equilibrium pressure was about 1 MPa. In the case of using CO2 as a blowing agent, the saturation pressure could easily be adjusted by the injected CO2, and the saturation pressure was fixed at about 3 MPa in this study for a comprehensive consideration. PP and PP/PE Foams Blown with n-Pentane. Autoclave foaming experiments were conducted on PP and PP/PE blends using n-pentane. Figures 5 and 6 show the cell morphology of the as-prepared foams at 92 and 95 °C, respectively. The PP

Figure 5. SEM micrographs of PP (a), PP/LDPE (b), PP/LLDPE (c), and PP/HDPE (d) foams blown with n-pentane at 92 °C. 5659



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Figure 6. SEM micrographs of PP (a), PP/LDPE (b), PP/LLDPE (c), and PP/HDPE (d) foams blown with n-pentane at 95 °C.

melting peaks at 104.5, 120.2, and 128.5 °C were associated with crystal melting of the LDPE, LLDPE, and HDPE components, respectively. Moreover, no obvious influence of the foaming temperature on the melting behavior of PP/PE foams was observed. When the melt behaviors of PP/PE resins and foams were compared, it was found that the foaming process did not affect the Tm value of the PP component but slightly decreased the Tm value of the PE components from 108.8 to 104.5 °C for LDPE, from 123.8 to 120.2 °C for LLDPE, and from 129.7 to 128.5 °C for HDPE, respectively. Furthermore, compared to PP/PE beads, PP/PE foams had broad melting region pointed out by red arrows, as shown in Figure 7. PP and PP/PE Foams Blown with CO2. Figure 8 shows the cell morphology of PP and PP/PE foams prepared at 151 °C. The PP foam had an obvious broad cell distribution and a small number of large cells with sizes of 200−300 μm. With the introduction of PE, however, a uniform cell distribution was achieved in the three PP/PE blend foams. In addition, as shown in Table 3, PE addition was also beneficial to the reduction in the foam density from 90 g/L for the PP foam to 85 g/L for the PP/LDPE foam, to 70 g/L for the PP/LLDPE foam, and to 81 g/L for the PP/HDPE foam. Figure 9 shows the thermal behavior of PP and PP/PE foams blown with CO2 at 151 °C, and the information of Tm is summarized in Table 2. The PP foam had a dual melting peak; the high melting peak, Tmhigh, was 164.0 °C, and the low melting peak, Tmlow, was 140.9 °C. PP/PE blend foams had three melting peaks, Tmhigh, Tmlow, and TmPE, that were associated with crystal melting of the PP and PE components, respectively. It was found that the foaming process did not affect the Tm value of the PP component but slightly decreased the Tm value of the PE components from 108.8 to 105.8 °C for LDPE, from 123.8 to 121.8 °C for LLDPE, and from 129.7 to 126.7 °C for HDPE.

beads had already deformed during the saturation process at high foaming temperature. PE addition was verified to improve the foaming behavior of PP, especially at low foaming temperature. A similar phenomenon has been widely observed in PE/PP foaming systems by other researchers.37,38 The enhanced heterogeneous nucleation induced by the phase-separated PE components might be the main reason. The melting behavior of PP and PP/PE foams prepared at 92 and 95 °C is shown in Figure 7, and the information of Tm is summarized in Table 2. The PP foam had a single Tm value at 149.8 °C, and the foaming temperature did not affect the Tm value of the PP foam obviously. As expected, the PP/PE foam presented two Tm values; the melting peak at 149.0 °C was associated with crystal melting of the PP component, and the

Figure 7. DSC thermograms of PP and PP/PE foams blown with npentane at different foaming temperatures. 5660



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Figure 8. SEM micrographs of PP (a), PP/LDPE (b), PP/LLDPE (c), and PP/HDPE (d) foams blown with CO2 at 151 °C.

this study, following the procedure suggested in the Experimental Section,7 an isothermal treatment was carried out using DSC to simulate the melting behavior evolution of PP and PP/HDPE blend foams during the steam-chest molding process. Figure 10 shows the DSC simulation results of PP and PP/ HDPE foams at different isothermal treatment temperatures. The melting behavior evolution of PP foam blown with npentane during DSC simulation is shown in Figure 10A. At isothermal treatment temperatures of 110−140 °C, the Tm value of the PP foam at 149.6 °C did not change. On the other hand, it was noted that there was a small melting peak additionally generated at the temperature below the Tm value. This peak was about 7 °C higher than the treatment temperature and tended to increase linearly with an increase of the treatment temperature. This melting peak was named Tmi, which was created by the melting of crystals that had possibly been induced by the fast heating and the isothermal treatment that followed.7 At isothermal temperatures of 145 and 150 °C, the original crystals in PP foams became prefect, and the newly formed Tmi values increased slightly up to 153.5 and 161.0 °C, respectively. Furthermore, at a treatment temperature of 150 °C, the Tm value at 147.7 °C was observed, possibly resulting from the partial melting of the PP foam. At isothermal temperatures of 155−165 °C, the original crystals of PP foams were melted completely, and a single Tm value was observed at 149.2 °C, resulting from the melting of the formed crystals during the cooling process. Figure 10B shows the melting behavior evolution of PP/PE foams blown with npentane during DSC simulation. It is seen that the evolution of Tm and Tmi values of PP/PE foams was very similar to that of the PP foam, except for the presence of TmPE and TmcPE (the melting of PE′ crystals formed during the cooling process) at low temperature. In addition, at a treatment temperature of 125

Table 3. Apparent Density of PP and PP/PE Foams Blown with n-Pentane and CO2 apparent density (g/L) blown with n-pentane

blown with CO2

foams

92 °C

95 °C

151 °C

PP PP/LDPE PP/LLDPE PP/HDPE

73 22 29 44

15 13 14 13

90 85 70 81

Figure 9. DSC thermograms of PP and PP/PE foams blown with CO2 at 151 °C.

Evolution of the Melting Peaks of PP and PP/PE Foams during DSC Simulation. During the steam-chest molding process of EPP, steam works as a heating medium to soften and fuse the beads and induces the formation of bonding between bead foams during the cooling process that follows. In 5661



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Figure 10. DSC simulation results of PP (A and C) and PP/HDPE (B and D) foams at different treatment temperatures. PP and PP/HDPE foams were prepared by blowing with n-pentane at 95 °C (A and B) and CO2 at 151 °C (C and D).

°C, the shape of TmPE was different from others, possible because of the coupling effect of TmPE and Tmi. Figure 10C shows the melting behavior evolution of PP foams blown with CO2 during DSC simulation. The Tmhigh value of the PP foam at 163.3 °C and the Tmlow value at 140.5 °C did not change with the isothermal treatment temperatures at 110−160 and 110−125 °C. At higher treatment temperatures of 135−150 °C, Tmlow disappeared, and a new melting peak, Tmi, was found,7 which increased gradually with the treatment temperature. At treatment temperatures of 150−165 °C, a new melting peak at 140.0−148.7 °C, which was much lower than the treatment temperature, was found in DSC curves. This peak resulted from the melting of crystals formed during the cooling process and is coded as Tmc. Figure 10D shows the melting behavior evolution of PP/PE foams blown with CO2 during DSC simulation. The melting behavior of the PP component, i.e., Tmhigh and Tmlow, in PP/PE foams was very similar to that of PP foams. Because of the presence of TmPE/ TmcPE, Tmi, and Tmlow, the coupling effect of these peaks makes DSC curves very complex. During DSC simulation, for PP and PP/PE bead foams blown with n-pentane, the possible present crystal melting in DSC curves included the original crystal melting Tm and TmPE (which were from the crystal melting of the PP and PE components), the heating-induced crystal melting Tmi, and the cooling-induced crystal melting Tmc and TmcPE (which resulted from the DSC simulation process). For PP and PP/PE bead foams blown with CO2, the possible crystal melting values were as follows: Tmhigh, Tmlow, and TmPE, which were due to the

crystal melting of the PP and PE components; Tmi, which was induced by the crystal melting formed during the heating process; Tmc and TmcPE, which were from the crystal melting formed during the cooling process. Figure 11 summarizes the melting behavior of PP and PP/PE foams at different isothermal treatment temperatures. It is seen from Figure 11A,B that the newly formed Tmi during the heating process in PP and PP/PE foams increased linearly, and about 7 °C higher than the isothermal treatment temperature. On the left side of Tmi, the original crystals of the PP and PE components did melt, while on the right side of Tmi, the original crystals of PP and PP/PE foams melted, and new crystals were formed during the cooling process. For PP foams blown with n-pentane, only a very few crystals with low perfection in foams could melt, and the melted crystals might recrystallize during the cooling process. At a treatment temperature higher than Tm, all original crystals melted, and the cell structure of foams could be destructed. For PP foams blown with CO2, the crystal melting associated with Tmlow contributed to the formation of new crystals during the cooling process, and the crystals associated with Tmhigh could stabilize the cell structure.7,10,15 The TmPE value of the HDPE component was much lower than Tm or Tmhigh and Tmlow of the PP component; the crystal domain tended to melt during the heating process and then recrystallized during the cooling process. It should be pointed out here that, similar to TmHDPE/ TmHDPE, the TmLDPE/TmcLDPE and TmLLDPE/TmcLLDPE values did not change with treatment temperatures obviously during the 5662



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Figure 13 summarizes the heat of fusion of the newly formed crystals in PP and PP/HDPE foams, ΔHmc, at different isothermal treatment temperatures. PP foams blown with npentane had lower ΔHmc compared to those blown with CO2, i.e., 7.2−35.7 vs 21.5−94.6 J/g. This suggested that, for the PP bead foam prepared using CO2, many more crystals could be generated during the cooling process at the bead foam interfaces, which was beneficial for the formation of strong interbead bonding. The addition of the HDPE component increased ΔHmc from 7.2−35.7 to 12.1−64.6 J/g for PP foams blown with n-pentane and from 21.5−94.6 to 67.4−132.1 J/g for PP foams blown with CO2. These results demonstrated that HDPE addition facilitated recrystallization of the self-diffused amorphous polymer chains at the interfaces, which was good for the formation of strong interbead bonding. It should be pointed out that only the recrystallization of HDPE at the interface of bead foams could contribute to the enhancement of interbead bonding. As shown in Figure 3, most of PE components dispersed inside the bead foams, and they might not easily migrate to the surface of bead foams because of its long molecular chain during the steam-chest molding process. Therefore, the actual contribution of PE addition on the interbead bonding of PP bead foams might be much less than the results of DSC simulation, and our further study will answer this question.

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CONCLUSIONS In this study, two autoclave foaming systems with a volume of 50 L were self-designed and used for producing PP and PP/PE bead foams with n-pentane and CO2 as physical blowing agents. PP foams blown with n-pentane exhibited a lower foaming temperature, a higher expansion ratio, and a lower cell density relative to those blown with CO2 because of the different gas solubilities of the blowing agents. The melting behavior of the PP foams was investigated by DSC analysis; PP foams blown with n-pentane had a signal melting peak, while PP blown with CO2 had dual Tm values. PE components enhanced the foaming behavior of PP by inducing heterogeneous nucleation, significantly improving the foaming behavior of PP, especially at low foaming temperature. In addition, PE addition generated the dual and multiple Tm values in PP/PE foams. A DSC simulation was carried out to show the evolution of Tm during the steam-chest molding process. An induced Tmi was observed in the DSC curve, which was about 7 °C higher than the treatment temperature, and tended to increase linearly with an increase of the treatment temperature. On the right side of Tmi, the original crystals of PP and PP/PE foams were melted or partially melted, and new crystals were formed during the cooling process, which was associated with the crystal melting of Tmc and TmcPE. The heat of fusion of both Tmc and TmcPE was ΔHmc. The advantage of dual Tm values was that the crystals associated with Tmhigh could stabilize the cell structure and the crystals associated with Tmlow contribute to the softening and fusing of the surface of bead foams. PP foams blown with CO2 had dual Tmc values and a high ΔHmc, which was beneficial to the formation of stronger interbead bonding. The PE component with low TmPE was easy to recrystallize during the cooling process, and the increase in ΔHmc facilitated the generation of strong interbead bonding for the molded PP/PE bead foams.

Figure 11. Effect of isothermal treatment temperatures on the melting point of PP and PP/HDPE foams blown with n-pentane (A) and CO2 (B).

DSC simulation process. We did not include the simulation results of LLDPE and LDPE to avoid the overlong manuscript. Effects of the Type of Blowing Agent and PE Addition on ΔHmc. As presented in our previous study,10 during the steam-chest molding process, a high temperature tends to melt crystals that are not stable enough during the heating process, and the melted amorphous polymer chains have the ability to self-diffuse between bead foams. During the cooling process, the amorphous polymer chain recrystallizes, which facilitates the formation of strong interbead bonding. Therefore, the bonding force was potentially determined by the amount of recrystallized regions. In this section, the amount of the recrystallized part was calculated, and the effects of the type of blowing agent and HDPE addition on the heat of fusion, ΔHmc, of the newly formed crystals were discussed. The recrystallized part during the cooling process was estimated roughly based on the melting regions of the DSC curve at a temperature lower than the isothermal treatment temperature, which is marked in the DSC curves. Figure 12 shows the crystal melting of the newly formed crystals during the cooling process. PP foams blown with n-pentane did not have Tmc but just had a broad melting region in the DSC curve, as pointed out by arrows. However, for PP foams blown with CO2, we did found a Tmc value at treatment temperatures of 145 and 150 °C. In addition, the isothermal treatment temperature of the former was much lower than that of the latter, resulting from its low Tm, 149.3 vs 163.2 °C. This indicated that the PP foam blown with n-pentane possessed a low steam molding temperature. HDPE addition presented an obvious contribution on the newly formed crystals, as shown in Figure 12B,D. 5663



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Figure 12. Separation of the newly formed crystals of PP (A and C) and PP/HDPE (B and D) foams during the cooling process. Foams were prepared by blowing with n-pentane (A and B) and CO2 (C and D).

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful to the National Natural Science Foundation of China (Grant 51003115) and the Ningbo Natural Science Foundation (Grant 2011A610118) for their financial support of this study.

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Figure 13. Effect of the isothermal treatment temperature on the heat of fusion of PP and PP/HDPE foams.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0574 8668 5256. Fax: +86 0574 8668 5186. E-mail: [email protected] (W.T.Z.), [email protected] (W.G.Z.). Notes

The authors declare no competing financial interest. 5664



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