high-density polyethylene blend: Cell structure and tensile property

Author’s Accepted Manuscript Supercritical CO2 foaming of radiation crosslinked polypropylene/high-density polyethylene blend: cell structure and tensile property Chenguang Yang, Zhe Xing, Mingxing Zhang, Quan Zhao, Mouhua Wang, Guozhong Wu www.elsevier.com/locate/radphyschem

PII: DOI: Reference:

S0969-806X(17)30230-X http://dx.doi.org/10.1016/j.radphyschem.2017.07.028 RPC7605

To appear in: Radiation Physics and Chemistry Received date: 22 February 2017 Revised date: 30 June 2017 Accepted date: 31 July 2017 Cite this article as: Chenguang Yang, Zhe Xing, Mingxing Zhang, Quan Zhao, Mouhua Wang and Guozhong Wu, Supercritical CO 2 foaming of radiation crosslinked polypropylene/high-density polyethylene blend: cell structure and tensile property, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2017.07.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Supercritical

CO2

foaming

of

radiation

crosslinked

polypropylene/high-density polyethylene blend: cell structure and tensile property Chenguang Yanga,b,c, Zhe Xinga, Mingxing Zhanga, Quan Zhaoa,Mouhua Wanga, Guozhong Wua,b,* a

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Jialuo Road 2019, Jiading, Shanghai 201800, China

b

School of Physical Science and Technology, ShanghaiTech University, Shanghai, 200031, China

c

University of China Academy of Sciences, Beijing, 100049, China

*

Corresponding author. P.O. Box 800-204, Shanghai 201800, China. Tel. +86-21-39194531. Fax.

+86-21-39195118. [email protected]

Abstract A blend of isotactic polypropylene (PP) with high-density polyethylene (HDPE) in different PP/HDPE ratios was irradiated by γ-ray to induce cross-linking and then foamed using supercritical carbon dioxide (scCO2) as a blowing agent. Radiation effect on the melting point and crystallinity were analyzed in detail. The average cell diameter and cell density were compared for PP/HDPE foams prepared under different conditions. The optimum absorbed dose for the scCO2 foaming of PP/HDPE in terms of foaming ability and cell structure was 20 kGy. Tensile measurements showed that the elongation at break and tensile strength at break of the crosslinked PP/HDPE foams were higher than the non-crosslinked ones. Of particular interest was the increase in the foaming temperature window from 4 ℃ for pristine PP to 8–12 ℃ for the radiation crosslinked PP/HDPE blends. This implies much easier handling of scCO2 foaming of crosslinked PP with the addition of HDPE.

1

Keywords radiation cross-linking; polypropylene; supercritical carbon dioxide; microcellular foams; tensile property. 1. Introduction Polypropylene (PP) porous materials with micro-grade pores have attracted much attention in many areas because of their excellent performance (Acebo et al., 2014; Hochleitner et al., 2014; Mori et al., 2009; Moutsatsou et al., 2015; Nofar et al., 2015; Pientka et al., 2009; Sai et al., 2013), for instance, higher temperature resistance and better mechanical strength than other polyolefins. The foaming of PP using supercritical carbon dioxide (scCO2) as a green physical blowing agent has attracted much interest, as this method has been utilized in the foaming of many other polymers (Ameli et al., 2015; Keshtkar et al., 2014; Nofar et al., 2015; Nofar and Park, 2014). However, compared to polystyrene, a continuous industrial fabrication of PP foam using scCO2 is difficult to realize because of the narrow foaming temperature window. Also, the melt strength of PP is relatively too low to form a well-defined cellular structure. PP with branching or crosslinked structures has a higher melt strength than its linear counterparts (Bouhelal et al., 2007), which is beneficial for scCO2 foaming. Radiation cross-linking can also significantly improve the melt strength of PP, the addition of multi-functional cross-linking agent or cross-linkable polymer is normally required to avoid the decomposition of PP upon irradiation (Bouhelal et al., 2007; Kubo et al., 1997). Some studies have reported using CO2 as the blowing agent to prepare PP foam materials (Antunes et al., 2012; Rizvi and Park, 2014). In most cases, PP can be foamed by improving its melt strength. Blending PP with other polymers or inorganic 2

substances can also improve its foaming ability (Yousefian and Rodrigue, 2015). Herein, we report the formation of a well-defined cell structure by the foaming of radiation crosslinked polypropylene/high-density polyethylene (HDPE). The blending of PP with HDPE also improved the swelling of CO2 into the PP domains. The gel content and crystallization behavior of the irradiated PP/HDPE blends were investigated. The cell morphology and tensile property of the crosslinked PP/HDPE foam was greatly improved as compared to the pristine PP foam. The foaming temperature window for radiation crosslinked PP/HDPE was expanded to 8–12 ℃, which was very useful for scCO2 foaming of PP in a potential industrial operation. 2. Experimental 2.1 Materials Polypropylene T30s (pellets, isotactic, homo-polymer) with a density of 0.91 g·cm−3, a melt flow index of 3.0 g/10 min at 230 °C, and a weight of 2.16 kg was purchased from Sinopec Shanghai Chemical Co (Khonakdar et al., 2006). High-density polyethylene 8010 (pellets) with a density of 0.956 g·cm−3, a melt flow index of 1.0 g/10 min at 230 °C, and a weight of 2.16 kg was purchased from Taiwan Plastic Group Co. Carbon dioxide with a purity of 99.5% was supplied by Xiangkun Special Gases of Shanghai. Xylene with a purity of 99.6% was purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2 Sample preparation PP/HDPE blends were prepared with weight ratios of 95/5, 90/10, and 85/15. The blended pellets were mixed at 180 °C using a mixer (Thermo Haake PolyDrive, Tianjin). The mixtures were then hot pressed at 195 °C and 10 MPa for 15 min and 3

cooled to ambient temperature. The as-obtained PP sheets were then irradiated with Co-60 γ-rays (Shanghai Institute of Applied Physics) under a nitrogen atmosphere at room temperature. The dose rate was 0.114 kGy/min. 2.3 Gel content measurement The gel content of the samples was measured using Soxhlet extraction. Xylene was used as a solvent and the extraction cycle was performed at the boiling point of xylene (140 °C) for 72 h (Yao et al., 2009). The solid remainder was dried to a constant weight at 60 °C and then reweighed. The gel content was calculated using the following equation: M

Gel content (%)= M1 ×100% 0

(1)

where M0 is the initial weight of PP and M1 is the weight of the insoluble part. 2.4 Melting point and crystallization The melting points and crystallinity of the samples were determined by A NETZSCH STA 449 F3 Jupiter differential scanning calorimeter (DSC) equipped with a data station for scanning the melting transitions of the γ-irradiated samples (10–15 mg) in aluminum pans. The crystallinities of PP and HDPE were calculated as follows: The pure polymers, ΔH

XPP (%)= ΔH m,PP ×100 % m0,PP

ΔH

XHDPE (%)= ΔH m,HDPE ×100 % m0,HDPE

(2) (3)

Each component in the blend, ΔHm,PP

XPP (%)= ΔH

m0,PP

(1-x)

×100 % 4

(4)

XHDPE (%)=

ΔHm,HDPE ΔHm0,HDPE x

×100 %

(5)

The total sample, Xtotal (%)=(1-x)(XPP )+(x)(XHDPE )

(6)

where XPP and XHDPE are the crystallinity percentages of PP and HDPE, respectively, and Xtotal is the crystallinity percentage of the total sample. ΔHm,PP and ΔHm,HDPE are the melting enthalpies of the PP and HDPE samples, respectively, which were measured in the heating experiments. ΔHm0,PP and ΔHm0,HDPE are the theoretical enthalpies of 100% crystalline PP (209 J/g) and HDPE (293 J/g) (Rachtanapun et al., 2004; Wunderlich, 1973), respectively, and x is the weight fraction of HDPE in the blend. 2.5 Molecular Weight Characterization The molecular weight distribution measurement was performed using an Agilent gel permeation chromatography (GPC) PL-GPC 220, (Trichlorobenzene system, PL gel Olexis (300×7.5mm)) at 150 °C. 2.6 Foaming process The foaming of the PP/HDPE sheets was conducted in a high-pressure reactor by using scCO2 as the blowing agent. The supercritical experimental apparatus was schematically depicted in a previous paper (Huang et al., 2007; Xing et al., 2008; Yang et al., 2016; Yang et al., 2017). In this study, the saturation time of CO2 was 90 min and the diffused CO2 in the melted polymer reached a balance at 20 MPa . The preparation of the PP/HDPE foam is shown in Fig. 1.

5

Fig. 1. Preparation of microcellular crosslinked PP/HDPE foam by scCO2 method. 2.7 Morphology of foam A Zeiss MERLIN Compact 14184 scanning electron microscope (SEM) was used to observe the cell morphologies of the foamed PP samples. The foaming samples were immersed in liquid nitrogen for 2 min, fractured at the liquid nitrogen temperature, and then mounted on stubs. The fractured surfaces were sputter coated with a 10-nm-thick gold layer. Foams with uniform cell morphology were also characterized in terms of cell density and average cell size. Image Pro-plus was used to analyze the SEM photographs. The average diameters of all the cells in the micrographs, 𝐷, were calculated using Eq. (7). D=

∑ di ni ∑ ni

(7)

where ni is the number of cells with a perimeter-equivalent diameter of di . The cell density (Nf ) was determined by the number of cells per unit volume of the foam, calculated using Eq. (8). Nf =(

nM2 A

3⁄2

)

(8)

where n, M, and A are the number of cells in the micrograph, the magnification of 6

the micrograph, and the area of the micrograph (cm2), respectively. 2.8 Tensile testing Tensile tests of the PP and PP/HDPE foams were carried out using a universal testing machine (Instron 5943). In each group, three samples were prepared at the same condition. The tensile data were collected by measuring the same sample for 3 times. The foam samples were cut into 2 mm × 4 mm × 22 mm and the specimens were measured at room temperature in accordance with ASTM D-638 at a speed of 50 mm/min (Bao et al., 2016). 3. Results and discussion 3.1 Gel content analyses

Fig. 2. Effect of absorbed dose on the gel content of the pristine PP, pristine HDPE and PP/HDPE blends.

Fig. 2 shows the changes of gel content of the blended samples at different doses. The gel content of PP/HDPE increased as the dose increased up to 60 kGy. The gel content of PP/HDPE increased quickly with increasing dose from 0 to 20 kGy. The formation of gel in the blend indicates cross-linking by irradiation. In addition, the 7

possible reaction and interactions of the irradiated PP and HDPE macromolecules likely occur in the PP-HDPE interface, and PP-graft-PE copolymers may be also formed (Fel et al., 2016), which could greatly increase the gel content of the PP/HDPE blends. The processes of cross-linking and free radical formation prevail in the PP/HDPE mixture as compared to degradation at low doses (Bouhelal et al., 2007; Magerramov and Dashdamirov, 2005). Radiation induced intramolecular and intermolecular cross-linking owing to the formation of free radicals in the macromolecules (Magerramov and Dashdamirov, 2005). Cross-linking may have also been caused by the chain folding of PP and HDPE and the combination of free radicals in the amorphous region. The crosslinked segments in the mixed polymer led to an increase in the melt strength of the PP/HDPE blends, favoring scCO2 foaming. The radiation cross-linking process may have followed three steps. First, free radicals were produced in the PP and HDPE phases. Then, the recombination of free radicals and the presence of small numbers of junctions induced macromolecular chain folding and intertwining. Finally, a crosslinked network structure formed, which was beneficial to the foaming of PP/HDPE. Micrographs of the gels were obtained using SEM, and that of the PP90/HDPE10 gel obtained after a dose of 20 kGy is shown in Fig. 3. It could be seen that the gel has a tangled and entwined mesh structure, and many macromolecular chains groups intertwined together. It was also confirmed that there was a lot of gel in the irradiated samples. The formation of mesh structure is mainly caused by cross-linking, entanglement, and intertwining between macromolecules in the blend. 8

Fig. 3. (a) Low- and (b) high-magnification micrographs of the gel structure of PP90/HDPE10 after irradiation at a dose of 20 kGy.

3.2 Radiation effect on melting point and crystallinity The radiation effects on the PP/HDPE blends of different weight ratios were investigated in terms of the melting point, heat of fusion, and crystallinity of PP and HDPE. The samples were heated from room temperature to 200 °C at a scanning rate of 10 °C/min under a nitrogen flow (20 mL/min). The thermal properties, such as melting point (Tm), melting enthalpy (Hm) and crystallinity (X), are shown in Table 1. All these data are collected in the first scan. The melting point of PP decreased with increasing dose, and there was a very slight variation in the melting point of HDPE over the dose range 0–40 kGy. The decrease in the melting point of PP was due to the radiation degradation of the crystalline area of PP, as well as to the imperfection of polymeric chains in the crystalline region, which led to a decrease in the melting point after irradiation (Badr et al., 2000). The scission of macromolecular chains led to a higher mobility of the PP phase, hence less heat fusion was required to melt the PP phase. The molecular weight distribution curves of pristine PP and irradiated PP (40 kGy) are shown in Fig. 4. It is clearly seen that the molecular weight of pristine PP decreased after irradiation at a dose of 40 kGy. Upon irradiation, scission and cross-linking occurred simultaneously in the PP and HDPE phases. The 9

radiation-induced combination of free radicals resulted in cross-linking in the amorphous region. There was a slight change in the total crystallinity of the irradiated blends, implying degradation after irradiation. The degradation might both happened in the amorphous and crystalline regions. The degradation in the crystalline regions led to a decrease in crystallinity.

Fig. 4. Molecular weight distribution curves of pristine PP and irradiated PP (40 kGy).

Table 1 Effects of radiation on the melting point (Tm), heat fusion (Hm), and percentage crystallinity (X) of the PP/HDPE blends with different doses. Absorbed

Sample

HDPE

PP Total X (%)

dose Tm (℃)

Hm (J/g)

Xc (%)

Tm (℃)

Hm (J/g)

Xc (%)

in blends

0

/

/

/

167.9

98.2

/

47

0

132.7

12.4

84.6

163.7

84.1

42.3

44

20

132.1

12.3

83.9

163.4

79.9

40.2

42

40

132.2

10.1

69.0

161.8

79.6

40.1

41

60

132.5

12.1

82.5

162.1

76.9

38.7

41

0

132.3

22.1

75.4

163.4

74.8

39.7

43

20

132.5

22.5

76.8

162.9

77.3

41.1

44

40

131.8

20.6

70.2

161.2

74.8

39.8

42

60

132.2

23.5

80.2

161.1

72.7

38.6

42

0

132.7

28.3

64.5

163.7

70.8

39.9

43

20

132.5

29.5

67.2

162.7

69.8

39.3

43

40

132.5

32.7

74.5

161.9

67.7

38.1

43

(kGy) PP

PP95/HDPE5

PP90/HDPE10

PP85/HDPE15

10

60

132.3

27.9

63.6

161.2

70.7

39.8

43

3.3 Morphology of PP/HDPE foams 3.3.1 Cell morphology of non-crosslinked PP/HDPE foams The cell morphology of the non-irradiated PP/HDPE foams prepared at 146 ℃ and 20 MPa is shown in Fig. 5. The cells of pristine PP were merged and destroyed, whereas a better cell morphology was observed for the PP95/HDPE5, PP90/HDPE10, and PP85/HDPE15 foams. The cell size was smaller and the cell density was higher than the pristine PP foam. In addition, more geometric cells were formed in the PP95/HDPE5 blends foam. We conclude that the addition of HDPE to PP improved the foaming ability of the blends. The stress-strain curves of the non-crosslinked PP/HDPE foams with different ratios are shown in Fig. 8 (a). Much higher tensile strength at break and elongation at break are observed for PP/HDPE foam as compared to the pristine PP foam. For the PP/HDPE foam, the tensile strength increases from 11.4 MPa to about 12.5 MPa and the elongation at break increases with the content of HDPE, implying the improved melt strength of PP/HDPE blend via the addition of HDPE and forming a better cellular structure. The interaction between PP and HDPE macromolecules may result in an increased melt strength of the blends.

11

Fig. 5. SEM micrographs for PP/HDPE blend foams prepared at 146 °C and 20 MPa: (a) pristine PP, (b) PP95/HDPE5, (c) PP90/HDPE10, and (d) PP85/HDPE15.

HDPE may improve the dissolution of CO2 into the melted mixture and promote nucleation among the different phases of the blend.

The schematic of the

morphological evolution of CO2 swelling in the PP/HDPE blends is shown in Fig. S1. As CO2 continuously diffuses into the PP and HDPE phases, CO2 may first dissolve into the interface of the two phases and especially into the HDPE domains as the temperature increases. The CO2 areas start to increase in size as more and more CO2 diffuses into the blends. Adjacent CO2 regions connect with each other, which results in a significant increase in the sizes of the CO2 regions. Once CO2 saturation reaches equilibrium in the HDPE domains, the dissolved CO2 can easily expand to the PP phase until the whole blend is fully swollen. 3.3.2 Cell morphology of crosslinked PP/HDPE foams The effect of absorbed dose on the cell morphology of the PP/HDPE foams is 12

shown in Fig. 6. The cellular structure of the PP/HDPE foams was greatly improved after an irradiation dose of 20 kGy. The cell morphology of the PP/HDPE foams is more uniform and the cells have thicker walls as compared to the pristine PP foam shown in Fig. 5(a). This can be explained by the gel formation in PP/HDPE as a result of radiation cross-linking, which is conducive to the formation of PP/HDPE foams with good cell morphology. The interaction between irradiated PP and HDPE macromolecules and the combinations of macroradicals between PP and PE chains may occur, which could improve the melt strength of the blend. However, the cell morphologies of the PP/HDPE foams became slightly worse when the dose was increased from 20 to 60 kGy, as can be seen in Fig. 6. The cells became collapse and rupture as the absorbed dose increase, which could reduce mechanical properties of the foam. This is likely attributed to the serious degradation of PP in the blend at higher doses, resulting in the decreased melt strength of the PP/HDPE blends (Vilaplana et al., 2004), which is infavorable for the expansion of PP/HDPE.

13

Fig. 6. SEM micrographs for PP/HDPE foams produced at 146 °C and 20 MPa at different doses.

Fig. 7 shows the effect of dose on the cell size and cell density of the PP/HDPE foams whose cell morphology micrographs are shown in Fig. 6. It can be seen in Fig. 7(a) that the average diameter of PP95/HDPE5 after irradiation tends to decrease as compared to non-irradiated PP/HDPE foam. These results indicate that the intersecting mesh structure of the irradiated PP/HDPE blends improved the ability of the melted polymer to bear the pressure of CO2. Therefore, the CO2 escaped from the melting polymer, resulting in a thicker cell wall. The cell size of the PP85/HDPE15 foam increased as the dose increased from 0 to 60 kGy because the foam temperature (146 ℃) facilitated foaming of PP85/HDPE15 with its higher amount of HDPE as compared to PP95/HDPE5. The foaming temperature was much higher than the melting point of HDPE, resulting in a slight increase in the mobility of the molecular chains as the temperature increased, resulting in a larger cell size. The cell density of 14

the PP/HDPE foams is shown in Fig. 7(b). There is opposite relationship between the cell size and cell density of the sample foams as the dose from 20 to 60 kGy. A decrease in the cell densities of the foams was observed as the dose increased from 20 to 60 kGy because the cells of the PP/HDPE foams merged owing to the increased degradation at those doses. The degradation negatively affected the foaming of PP by destroying the molecular structure, which hindered the formation of cells (Xing et al., 2008). The cell density of the PP/HDPE foams at an absorbed dose of 20 kGy was higher than that of the non-irradiated PP/HDPE, which was approximately 4.5 × 106 cells/cm3. The reason for the higher cell density may be that the cross-linking junctions and interfaces of the two phases increased the nucleation of PP/HDPE. A lower cell size and higher cell density is the desirable cell structure for a polymer foam as it shows better properties. Therefore, we believe that 20 kGy is the best absorbed dose for blends of PP/HDPE. The obtained crosslinked PP/HDPE foam with well-defined closed cells can be applied to many more fields as compared to the polymer foam that is not crosslinked and exhibits uneven and open cells (Fang et al., 2016; Kong et al., 2016; Sahagun et al., 2006).

Fig. 7. Effect of absorbed dose on the (a) cell diameter and (b) cell density of the 15

PP/HDPE foams produced under the same conditions as those shown in Fig. 6.

Fig. 8 (b), (c) and (d) display the stress-strain curves of PP/HDPE foams with a dose from 0 to 60 kGy. Higher tensile strength at break and elongation at break of the crosslinked PP95/HDPE5 foam are clearly observed, compared to the pristine PP and non-crosslinked PP/HDPE foams, owing to the enhanced mechanical strength by radiation cross-linking. However, there is an optimum gel content for PP/HDPE blends to form well-defined cellular structure. Higher gel content may lead to the higher rigidness of polymer blend, which is bad for the scCO2 foaming, indicating a weak tensile strength. In addition, the elongation at break of crosslinked PP95/HDPE5 foam is larger than the crosslinked PP90/HDPE10 and crosslinked PP85/HDPE15 foams. It may be due to a smaller cell size and a larger cell density shows a better tensile strength and the PP/HDPE foam material possess good mechanical properties at the optimum absorbed dose (Bao et al., 2016).

Fig. 8. Strain-stress curves of the PP/HDPE foams prepared at 146 ℃ and 20 MPa: (a) non-irradiated; and (b) PP95/HDPE5, (c) PP90/HDPE10, (d) PP85/HDPE15 at a dose 16

from 0 to 60 kGy.

3.4 Radiation effects on the foaming temperature window of the PP/HDPE blends The foaming temperature window of PP/HDPE blends irradiated at 20 kGy was investigated at the predefined pressure of 20 MPa. The cellular structures of the PP/HDPE foams obtained at different foaming temperatures are shown in Fig. 9. It is seen that PP/HDPE can be foamed at temperatures of 142–158 ℃. And there is a relative good cell morphology for the foams in Fig. 9 (see the blue box). The blue box shows the suitable foaming temperature window of the PP/HDPE blend. The red dotted box shows the sample foam obtained at the controllable foaming temperature window in Fig. 9. The PP95/HDPE5 samples have a larger foaming temperature window (8 ℃ - 12 ℃), which is useful for the scCO2 foaming of PP as compared to the narrow 4 ℃ foaming temperature for pristine PP. The increase of the foaming temperature window was due to the cross-linking and macromolecular interaction, which improved the melt strength after irradiation. A good cellular structure of the ratio PP95/HDPE5 sample foam obtained at different temperatures showed a better tensile strength in Fig. 11.

Fig. 9. SEM micrographs for PP/HDPE blended foams produced at 20 MPa and different foaming 17

temperatures after a 20 kGy dose.

The effects of foaming temperature on the average cell diameter and cell density of the PP/HDPE blended foams are shown in Fig. 10. The average diameter of the PP/HDPE blended foam increases greatly and the cell density decreases as the foaming temperature increases from 146 to 158 ℃. This is consistent with the classical foam results (Leung et al., 2006; Park et al., 1995; Xu et al., 2003). The average diameter of the irradiated PP/HDPE foam is lower and the cell density is higher than the average diameter and cell density of the pristine PP foam, which is approximately 100 μm and 1.7 × 106 cells/cm3, respectively, which we can see the cell micrograph in Fig. 5. In this work, the appropriate temperature window for foaming is from 146 to 154 ℃, which allows us to obtain PP/HDPE foam with a smaller cell size and larger cell density and larger foaming temperature window is an important impetus to the industrialization of PP foam materials.

Fig. 10. Effects of the foaming temperature on the cell morphology of the PP/HDPE blended foams after a 20 kGy dose in terms of (a) average diameter and (b) cell density.

The stress-strain curves of the PP95/HDPE5 foams (dose: 20 kGy) prepared at 20 MPa and different temperatures are shown in Fig. 11, and the SEM micrographs of the ratio sample foam are shown in Fig. 9. Relative larger elongation at break of the 18

PP95/HDPE5 foam is observed at the foaming temperature of 146 and 150 ℃, which is considered as the optimum foaming temperature. Foaming at the temperature lower or higher than 146-150 ℃ may result in a non-uniform cell structure and relative poor mechanical property of PP/HDPE foam.

Fig. 11. Strain-stress curves of the foamed PP95/HDPE5 blends (20 kGy) at 20 MPa and 142 ℃, 146 ℃, 150 ℃, 154 ℃ and 158 ℃.

4. Conclusion The foaming behavior of irradiated PP/HDPE blends with scCO2 as the blowing agent was investigated in detail. The effects of irradiation on the blends were characterized in terms of the variation of the gel content, melting point, and crystallinity. Foams of PP with different weight ratios of HDPE irradiated at different doses were prepared under the conditions of 20 MPa and 146 ℃, and the cells structures were analyzed and compared. The existence of an HDPE phase was particularly beneficial for the swelling of CO2 in the blends. The results of the tensile show that the tensile strength and elongation at break of the crosslinked PP/HDPE foam are higher than the pristine PP. And small cells size and high cell density show a higher elongation at break. Moreover, radiation cross-linking in the HDPE phase 19

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Highlights 

HDPE was added to improve the scCO2 foaming of polypropylene;

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Radiation cross-linking was employed to improve the scCO2 foaming of PP/HDPE blend;

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Tensile strength of irradiated PP/HDPE was improved compared to un-treated PP foam;

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Larger foaming temperature window (12 ℃) was achieved for scCO2 foaming of PP/HDPE.

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Graphical Abstract

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