Carbon-dioxide-blown Expanded Polyamide Bead Foams with

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Materials and Interfaces

Carbon-dioxide-blown Expanded Polyamide Bead Foams with Bimodal Cell Structure Shu-Kai Yeh, Wei-Hsiang Liu, and Yen-Ming Huang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05195 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Industrial & Engineering Chemistry Research

0

Carbon-dioxide-blown Expanded Polyamide Bead Foams with Bimodal Cell Structure Shu-Kai Yeh1,2*, Wei-Hsiang Liu1, and Yen-Ming Huang1

1. Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, 10608

2. High Speed 3D Printing Research Center, National Taiwan University of Science and Technology, Taipei, Taiwan, 10608

KEYWORDS: polyamide 6, bead Foam, CO2, bimodal structure, nanocellular foam

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

ABSTRACT GRAPHIC


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2 ABSTRACT. Bead foam has received substantial attention in recent years because of its high expansion ratio and ease of formability. Currently, most commercially available bead foams are produced from commodity plastics, and not many engineering polymers are used for this. In this study, the potential of polyamide 6 (PA 6), a typical engineering polymer, for bead foam applications was investigated by batch foaming. PA 6 was blended with its own copolymer to create double melting peaks for foam sintering. Chain extenders such as styrene maleic anhydride and Joncryl ADR 4368 C were added to improve the melt strength. After foaming, a crystal structure with double melting peaks was formed. In addition, the foam possessed a bimodal/nanocellular structure, and the foamed beads could be sintered. This is the first report of bead foam manufacturing using engineering plastics without toxic solvents. These results provide insights for bead foam research.



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3 1.　INTRODUCTION

Foam materials provide the advantages of low density, light weight, thermal insulation, sound insulation, and superior mechanical properties. Therefore, they are widely used in our daily life, such as packaging, shoe midsoles, furniture, automobile, aerospace, transportation, construction, electronics, and biomedical materials. 1, 2 The recent progress in foam materials has widened their processing windows and applications for biodegradable and EMI shielding foam, making such materials even more attractive 3-6.

In recent years, bead foams or particle foams have attracted significant interest in traditional industries. The demand for them is growing rapidly owing to their high expansion ratio, light weight, and easy formability into different shapes. A good example is the application of bead foams in bumpers and door panels in the automobile industry. Foam materials play a critical role in these parts in that they absorb the impact energy as well as improve the fuel-efficiency of cars. The advancements in bead foam technology immediately satisfy a wide range of requirements in industry. There are two critical issues in bead foam production: (i) the easiness of foaming and (ii) to sinter foamed beads without significant foam collapse.7 Apparently, an understanding of the science underlying foaming and polymer sintering is critical in bead foam research. Foamed beads are generally sintered by steam chest molding. The beads are blown into a mold and sintered by the


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4 heat from steam. For amorphous polymers, fusion occurs when the steam temperature is higher than its glass transition temperature (Tg). With regard to crystallized polymers, polymers exhibiting double melting peaks are desirable for making bead foams. The bead foams were sintered at temperatures between the low and high melting temperatures (denoted as Tm-low and Tm-high, respectively). The crystals melting at Tm-low are responsible for fusion and sintering, whereas the unmolten crystals help maintain the foam structure.7 Polymers that are used to produce bead foams include polystyrene8, 9, polyethylene10, and polypropylene.11-14 Over the past decade, there have been significant advancements in bead foam technology. For example, expandable thermoplastic polyurethane for the midsoles of shoes15, and packaging materials from biodegradable polylactide16, 17

and its copolymer18 have been developed. These are revolutionary innovations that improve the

functionality of the product and lower the production cost. However, the materials listed above cannot endure temperatures over 150 °C. This limits the utilization of bead foam in high-end applications such as those in the automobile and aerospace industries. According to EU regulations, the carbon dioxide emissions of new car models must satisfy the annual carbon reduction target.19 Using lightweight materials is one of the most preferable strategies to satisfy the regulation. Foaming and the development of composite materials have presently become important research topics.



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5 At present, bead foam is the only established technology for producing low-density products with complex geometry.7 Producing bead foams that sustain high temperatures has become an urgent requirement for the industry. Various efforts have been undertaken for producing foam beads using engineering plastics such as polyamide20, 21, polyethylene terephthalate22, polybutylene terephthalate23, polycarbonate24, 25, polyimide26, and polyethersulfone27. Recently, BASF announced a prototype of polyethersulfone bead foam.28 The literatures regarding the production of bead foam using engineering plastics are all patents. The purpose of listing these documents is mainly to illustrate the industry’s strong interest in high temperature bead foams. Numerous new applications can be developed if commercial expandable polycarbonate or polyamide beads are made available. Bead foams are used inside cars as well as under their hoods.

Polyamide is an engineering polymer established for its resistance to high temperature solvents and wear. These characteristics caused polyamide to be used extensively in the automobile industry. Notwithstanding such remarkable properties of polyamide, only a few literatures on polyamide foam are available.20, 21, 29-36 Polyamide is a semi-crystalline polymer. The viscosity of the polymer melt changes abruptly because a large quantity of polymer crystals forms at the crystallization temperature. This phenomenon results in a low melting strength and narrow process window.


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6 Whereas there is highly limited information for producing PA bead foams, the patent by Kriha et al. provides valuable information.20 In this patent, PA 6 was blended with amorphous PA 6 and a chain extender to produce foam beads. The purpose of mixing PA 6/amorphous and PA 6/chain extender was to decrease the crystallinity and to improve the melt strength. Moreover, water and isopentane were applied as the co-blowing agents. This yielded PA 6 bead foams with an expansion ratio of 5–15. However, the application of water and isopentane as the co-blowing agents increased the complexity of equipment design; moreover, the flammability of isopentane also caused safety issues.

Chain extenders can be classified as condensation type and addition type based on their reaction with the polymers. Addition type chain extenders are widely used as they do not yield byproducts.30 Addition type chain extenders can be further subdivided into two categories based on their functionality. If the chain extender is bi-functional, the chains would extend linearly. Bisoxaline is a typical example. 37 The other chain extenders are oligomers with functionality larger than four.38 The end NH2 groups on PA 6 attach to the chain extenders and form a branched structure. Typical examples include styrene maleic anhydride (SMA) and styrene-acrylic oligomers such as Joncryl ADR 4368C (ADR) produced by BASF. Xu et al. compared the effect of bifunctional and oligomer chain extenders, whose structure is similar to ADR; they observed that oligomer chain



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7 extenders are more effective than bifunctional ones in increasing the viscosity of PA 6 and that the former significantly enhanced the foam structure of PA 6.30 Tazumi et al. established that adding SMA enhances the foaming capability of PA 6; they successfully prepared polyamide foam sheets by extrusion.39

As previously mentioned, double melting peaks is required for producing bead foams. To achieve this character, researchers have blended polymers with different melting points (Tm). 10 In this study, a co-polyamide of low melting point was blended with PA 6 to investigate the feasibility of producing PA 6 bead foam using the double melting peak characteristic. Furthermore, chain extenders were added to improve the foaming capability of the foam beads, and CO2 was used as the sole blowing agent. The effect of different oligomer-based chain extenders such as SMA and ADR were compared. The thermal properties of the bead foam were observed by differential scanning calorimetry (DSC), and the crystal structure of the PA 6 blends before and after foaming was investigated by XRD. Our results revealed the successful production of PA bead foams with double melting peaks. Adding chain extenders increased the foaming capability and significantly enhanced the surface quality of the foamed beads; moreover, the foamed beads could be successfully sintered without significant foam collapse. 2.　EXPERIMENTAL SECTION


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8 2.1. Materials Two polyamide 6 resins, TP6603 and TP8801, provided by Zig Sheng Industrial Co. Ltd, Taiwan, were used for this study. The melt volume rate (MVR) of TP6603 was 20–60 cm3/10 min (275 °C, 5 kg, ASTM D1238), and the polymer is referred to as PA 6. Meanwhile, TP8801 is a PA 6/PA 6, 6 copolymer. The MVR is 8–10 cm3/10 min. As it is a PA copolymer, it is named PA 6C. The Tm of PA 6 and PA 6C are 220–225 °C and 190–200 °C, respectively. Both the resins exhibited a solid density of 1.12 g/cm3. Two types of chain extenders, styrene maleic anhydride (SMA® 1000P) and multifunctional styrene-acrylic oligomers (Joncryl ADR 4368C), were purchased from Total Cray Valley and BASF, respectively. Both the additives are used as a chain extender to reduce polyamide 6 crystallinity. Table 1 presents the chemical structures of the chain extenders. The silicone heating oil, SHF-56, was purchased from Advanced Lubricants Technology Co. Ltd, Taiwan. The CO2 (purity: 99.9%), and the N2 (purity: 99 %) was purchased from Wanan Gas Co. Ltd, Taiwan.

2.2 Blending and chain extension reaction All the materials were pre-dried in an oven at 80 °C for 16 h to remove moisture. The samples were compounded using an Xplore MC-15 micro compounder (Xplore Instruments, Netherlands) under a nitrogen atmosphere. The barrel temperatures of the microcompounder were set at 230, 250, and 250 °C from the hopper to the die. The material was compounded for 5 min with a screw



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9 speed of 50 rpm. The weight percentage blending ratios (PA 6:PA 6C) were 20:80, 50:50, and 80:20. Moreover, 1-phr SMA or ADR were added to the optimal blends to reduce the crystallinity. The total weight of each formulation was maintained at 5 g per batch for compounding. The viscosity of the blends was recorded and analyzed by the Xplore data acquisition system with extended rheological software. After 2 min, the melt viscosity attained steady state, and the melt viscosity of the blends after 5 min of compounding is reported. For reference, 5 g of neat PA 6 and PA 6C was also processed, and the viscosity was recorded as the baseline.

2.3 Differential scanning calorimetry (DSC) The thermal behavior of the compounded PA 6 sample was determined by the heat–cool–heat experiment from 30 to 280 °C using a TA Instrument Discovery 250 DSC. The heating and cooling rates were both maintained at 10 °C/min.

2.4 Batch foam processing The PA 6 samples were foamed via a two-step batch foaming process. A sample of approximately 0.1 g was cut from the compounded sample to conduct the foaming experiments. In this system, a syringe pump (ISCO-260D) was connected to the foaming cells via a four-way union to deliver CO2 at a regulated pressure. Three high-pressure inline filters (Swagelok SS-2F-05) were used as the foaming cells. Each foaming cell was connected to a three-way ball valve for pressure release. The


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10 samples were saturated with 13.79 MPa (2000 psi) CO2 for 24 h. The saturation temperatures (Tsat) were set at 80, 100, or 120 °C. After saturation, the samples were foamed in a silicone oil bath at 200 °C or 205 °C for 3 min. Thereafter, the samples were quenched in an ice bath to maintain the foam structure. The density of the samples was determined by the Archimedes principle. Note that the saturation time and saturation pressure in this study was maintained constant in all the experiments. In the following paragraphs, the saturation time and saturation pressure are omitted to avoid repetition.

2.5 Scanning electron microscope (SEM) The morphology of the foamed samples was characterized using JEOL JSM-6390LV scanning electron microscope (SEM). The samples were cryo-fractured using liquid nitrogen for 5 min and sputter-coated with gold for observation.

2.6 X ray diffraction (XRD) The fracture surface of the solid and foamed samples was characterized by a Bruker D2 PHASER XRD instrument using a copper target (Kα, λ = 0.154 nm). The sample was cut down to a length of 5 mm and placed in an acrylic sample holder for testing. The scanning range was from 15° to 30°.



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11 Table 1 Chemical structures of chain extenders used in this work.38, 40 No.

Chemical structures

Chemical names

Styrene maleic anhydride

SMA

1000 powder

Multifunctional polymer

ADR

Joncryl ADR 4368C


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12 3.　RESULTS AND DISCUSSION 3.1 DSC analysis The melting point is the critical variable to determine the foaming and sintering temperatures. The melting temperatures of PA 6, PA 6C, and the PA 6 blends are determined through the DSC experiments using the results of the second heat process. The typical second heat DSC results of PA 6, PA 6C, and the PA 6 blends are shown in Figure 1. As shown in Figure 1, the melting points of PA 6 and PA 6C are consistent with the information provided by the supplier. Both the polyamides exhibited a principal melting peak and an indistinct shoulder peak, denoted as Tm-high and Tm-low, respectively. The temperature difference between Tm-high and Tm-low is denoted as ΔTm. In this study, a high ΔTm is preferred because it indicates a wide processing window. Khanna et al. investigated the effect of the cooling rate on the second heat melting temperature of PA 6. A shoulder peak was observed at a cooling rate of 10 °C/min, which is consistent with our observation.41 PA 6 is established to exhibit at least two crystal forms.42 The primary peak and shoulder peak are likely to result from the different crystal structure. Because the rates of crystallization of different crystals are likely to vary and they generally compete against each other, it is infeasible to determine the crystal composition of the melting peaks.42



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13 The goal of this study is to create PA bead foams. To achieve this, it is desirable for PA 6 to possess double melting peaks with limited crystallinity. In this study, PA 6 was blended with PA 6C in different ratios. The ratios were set at 20:80, 50:50, and 80:20. The DSC results of the PA 6/PA 6C blends are shown in Figure 1. Apparently, these blends exhibited distinct double melting peaks. With increasing amount of PA 6C, the main melting peak height decreased, and the shoulder peak became more apparent. Moreover, the main melting peak above 200 °C became dull, and the heights of the two peaks became comparable.

Figure 1. The DSC curves of PA 6 and PA 6C and PA 6/PA 6C blended with different ratios.


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14 Although the heat of fusion of PA 6C copolymer cannot be estimated, the heat of fusion of PA 6 crystals are generally considered as 230 J/g.43 With this assumption, the crystallinity of the PA blends can be estimated. These results, including Tm-high, Tm-low, ΔTm, Tg, and melt viscosity obtained from the microcompounder are listed in Table 2. Because only one Tg was observed for the blends, the two polymers are considered miscible.44 The crystallinity of the PA 6/PA 6C blends decreased as the amount of PA 6C increased. It is noteworthy that the crystallinity of the PA 6/PA 6C blends does not adhere to the rule of mixtures. In the case of equal weight mixing, the crystallinity decreased from 29.4% to 21.9%.

The principal melting temperature (Tm-high) of PA 6 and PA 6C are 221.96 °C and 195.22 °C, respectively. The difference between the two melting peaks is 26.74 °C. Blending PA 6C with PA 6 results in two melting peaks, and the difference between the two peaks (ΔTm) is less than 26.74 °C. This is partly because the two polymers are miscible and partly because the crystallization behaviors of the two polymers are different. In this study, the blend with 20 wt% PA 6 and 80 wt% PA 6C demonstrated the highest ΔTm. The melt viscosity of the blends is presented in Table 2. Blending PA 6 with PA 6C did not change the melt viscosity. However, the melt viscosity increased significantly with the addition of the chain extender; moreover, ADR is a more effective chain extender than SMA. The results are consistent with reference 30. Because the highest ΔTm indicates the largest



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15 processing window, the following experiments were conducted with a PA 6/PA 6C ratio of 20:80. The term “PA 6 blends” is used to represent the PA 6/PA 6C 20:80 blend. 24 Furthermore, PA 6/PA 6C blends with 1-phr SMA and 1-phr ADR are denoted as PA 6 blends/SMA and PA 6 blends/ADR, respectively.


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16 Table 2. DSC results of PA 6/PA 6C blends and PA 6 blends with chain extenders

Tm-low

Tm-high

ΔTm

ΔH (J/g)

Xc (%)

Melt Viscosity

PA 6

53.13 213.09

221.96

8.87

67.53

29.4

0.56

PA 6C

49.58 182.50

195.22

12.72

47.91

20.8

0.74

80:20 53.14 210.48

220.04

9.57

61.55

26.8

0.58

50:50 52.15 204.47

216.98

12.51

50.40

21.9

0.62

20:80 51.50 194.70

209.77

15.08

46.00

20.0

0.64

SMA

51.56 198.47

208.91

10.44

38.85

16.9

1.18

ADR

52.00 197.85

208.44

10.58

32.61

14.2

1.55

Name

PA 6/PA 6C

PA 6/PA 6C 20:80

ratio

Tg

(kPa∙s)

Although there have been some recent breakthroughs 45-47, it is generally considered challenging to foam crystalline polymers.48 The crystallization process is likely to cause a significant decrease in the melt strength. Thus, foaming a typical crystallized polymer such as PA 6 is challenging. In general, chain extenders are added to reduce the crystallinity and to increase the melt strength of the polymer. The abrupt decrease in the melt strength would be prevented. The first heat results of the PA 6 blends with and without chain extender is shown in Figure 2(a). Only one broad peak is observed. It could be because the compounded extrudates were quenched with ice water mixture and thus providing insufficient time for different crystals to form. The second DSC results of the PA 6/PA 6C blends with chain extenders are shown in Figure 2(b) and Table 2. As illustrated in Table 2, ADR was more effective in decreasing the crystallinity than SMA, whereas other thermal properties including Tg, Tm-high, Tm-low, and ΔTm were similar. However, adding chain extenders resulted in a



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17 decrease in ΔTm. This is because the bulky multifunctional groups on the chain extenders hindered the folding up of the polymer chains difficult. These phenomena either created more defects or changed the thickness of the crystals.49 The decrease in crystallinity made Tm indistinguishable, and two peaks started to merge. Therefore, ΔTm also decreased.

(a)

(b)

Figure 2. The 1st heat (a) and 2nd heat (b) DSC curves of PA6/PA6C blends with chain extenders

3.2 Foaming Results To investigate the foaming capability of PA 6 and its blends with chain extenders, the samples were foamed using the two-step foaming process. The samples were first saturated at 80, 100, 120 °C and then foamed at 190 °C or 200 °C for 3 min. The foaming temperature 190 °C was selected because PA 6C melted completely at 200°C, which is shown in Figure 1(a). Foaming PA 6C at 200 °C collapsed all the cells. The low crystallinity indicates that PA 6C could be an effective prospect for


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18 foaming. The expansion ratios of PA 6C saturated at 80, 100, and 120 °C and then foamed at 190 °C for 3 min were 4.1, 4.3, and 4 times, respectively. The first heat DSC graph of the foamed PA 6C is shown in Figure 3. Although a wide, indistinct shoulder melting peak is appeared, double melting peaks are not observed. Furthermore, the beads may not have been suitable for foam sintering.

Figure 3. First heat DSC graph of PA 6C foam made by different saturation temperature

Meanwhile, PA 6 blends with and without chain extender were foamed at 200 °C because it is the lowest temperature between the two melting peaks. The results are listed in Table 3. Among the three samples foamed, PA 6 blends showed the highest expansion ratio; however, blending PA 6 with PA 6C caused expansion ratio to fall below four times. In addition, adding a chain extender further lowered the expansion ratio of the PA 6 blends to less than three times, which is undesirable. In batch foaming, the expansion ratio generally increases with the foaming temperature



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19 (Tfoam). Once Tfoam reach the melting point, cell start to collapse, and the expansion ratio rapidly decreased.50 A similar principle may be applicable in this study. Because Tm-high of the PA 6 blends is approximately 209 °C, the highest Tfoam selected was 205 °C. The expansion ratio of the foam increased significantly. Note that the expansion ratio of the PA 6 blends without chain extender increased to five times. Apart from the increase in the expansion ratio, other noteworthy results are observed. Images of the samples saturated at 100 °C and foamed at 205 °C are shown in Figure 4. Although the expansion ratio of the PA 6 blends increased to five times, the sample turned yellow, and cracks were observed on the surface. However, the sample containing ADR remained intact, and its color remained unchanged. It is established that the thermal oxidation or hydrolysis of peptide bond on PA 6 generates acid end groups. 51 Joncryl ADR 4368C is known to react with the acidic end groups and increase the molecular weight to prevent degradation. Thus, the thermal stability of the polymers improved.52 In summary, adding a chain extender increases the expansion ratio to over four times without significant degradation.

Table 3. Expansion Ratio of PA 6 blends foams prepared by different Tfoam System

Tsat

Tfoam

PA 6 blends 200°C

PA 6 blends/SMA

205°C

200°C

205°C


PA 6 blends/ADR 200°C

205°C

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20 80

3.68

4.87

2.72

3.97

2.64

4.00

100

3.60

5.00

2.89

4.15

2.83

4.30

120

3.29

4.72

2.72

4.10

2.74

3.94

(a)

(b)

(c)

Figure 4. Appearance of expanded PA 6 blend beads saturated at 120°C and foamed at 205°C

Unlike other studies, an expansion ratio of less than five times was not prominent. However, these works involved the use of co-blowing agents to increase the expansion ratio. Kriha et al. applied a blend of 60% amorphous PA 6 with 40% PA 6 to reduce the crystallinity of PA blends, and 2-phr water with various amounts of isopentane as the co-blowing agents.20 The total loading level of the co-blowing agents was 3.3–4.5 phr. In our case, CO2 was applied as the only blowing agent. It was challenging to determine the loading level of CO2. When the samples were removed from the chamber, they were weighed to determine the blowing agent content. However, because the saturation temperature was over 80 °C, CO2 diffused out of the sample rapidly. It was challenging to determine the loading level of CO2 by measuring the weight gain. The loading level of the blowing



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21 agent was approximately 2 wt%, which is significantly lower than that applied in the reference.20 Thus, the expansion ratio could be limited.

The cell morphology of PA 6C foamed at 190 °C and PA 6 blends with and without chain extender foamed at 200 °C are shown in Figure 5. In all the three images, bimodal structures are observed. Blending PA 6 with PA 6C and adding a chain extender made the bimodal structure further apparent. The bimodal structure has always generated substantial interest among researchers. It is known to increase the mechanical properties and decrease the thermal conductivity of the foam.53, 54 An open porous bimodal structure is known to increase the damping effect and improve the acoustic absorption of the foam.55, 56

A good example of bimodal structure is shown in Figure 6. Microscopic pictures of 30x, 200x and 1000x magnification of PA 6 blends containing ADR are displayed. It is significantly challenging to count the cells and calculate the cell size and cell density. This is particularly true for the samples with chain extenders. Calculating the cell size and cell density using high magnification SEM pictures is likely to cause severe bias and provide an incorrect estimation of the cell size or cell density. It is also challenging to determine whether the cell structure is homogeneous. To address these issues, the cumulative cell size distribution curves were applied to represent the cell structure for discussion.57 At least 2,000 cells were counted for the cell size distribution. To satisfy these


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22 criteria, the cumulative cell size distribution curves were constructed using images of different magnifications. In these curves, cells of size less than 0.5 μm were counted as 0.5 μm cells. In the presence of the bimodal structure, it is inconvenient to determine whether the cell structure is macroscopically homogeneous. To address this problem, the cell structure uniformity was determined by classifying the 30x SEM image into center, zone A, and zone B, as shown in Figure 7(a).

(a)

(b)

(c)

Figure 5. Cell structures of PA 6C foamed at 190°C (a), PA 6 blends (b) and PA 6 blends/ADR (c) foamed at 200°C.

Figure 6. Typical cell structure of PA 6 blends/ADR saturated at 120°C and foamed at 200°C



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23 The cumulative cell size distribution curves of PA 6 blend foam are shown in Figures 7(b)-(d). Because the diffusion coefficient of CO2 is a function of the saturating temperature, it is suspected that a low saturating temperature of 80 °C is not likely to be adequate to saturate the sample.58 A reasonable approach is to compare the cell structures in the center and edge of the foamed beads. If the equilibrium state is attained and samples are completely saturated with CO2, the cell structure may be considered as uniform. As is evident from Figure 7(b), the samples saturated at 80 °C demonstrated a non-uniform structure. Cells in zone B were significantly smaller than those in zone A and center. Meanwhile, the cumulative cell size distribution curves of zone A, zone B, and center in the samples saturated at 100 and 120 °C overlapped to a significant extent. This indicates that the cell structure was macroscopically uniform and that the CO2 concentration is likely to have been uniform across the sample. The sample was completely saturated at Tsat of 100 and 120 °C; however, 24 h is not likely to be adequate for a sample saturated at 80 °C to attain equilibrium. Therefore, in the subsequent discussion, only samples saturated at 100 and 120 °C will be discussed. Note that approximately 20% of the cells in the PA 6 blends foamed at 100 and 120 °C is of size less than 0.5 μm. The size of these submicron cells are approximately 200–300 nm. Such a nanoporous structure has not been reported in the case of polyamide. It is highly noteworthy and likely to have new applications.2


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24 The cumulative cell size distribution curves of the samples saturated at 100 °C and 120 °C and foamed at 200 °C are shown in Figure 8. The expansion ratios (ER) are shown in the figures as reference. As expected, the curves in each figure overlap with each other. It indicates the homogeneity of the cell structure. Adding chain extenders did not change the macroscopic uniformity. The bimodal structure was further apparent in the presence of chain extenders. Without chain extender, over 99% of the cells were of size less than 20 μm. In the presence of chain extenders, cell sizes of approximately 40 μm were observed. Furthermore, the percentage of cells of size less than 0.5 μm was less than 10%. This is because the large cells occupied a larger area in the SEM image and significantly less sub-micron cells were counted. The cumulative cell size distribution curves of the samples foamed at 205 °C is shown in supporting information Figure S1. The curves still overlapped, although not to the extent as in the samples foamed at 200 °C. This was particularly true for samples saturated at 120 °C. Because 205 °C is close to the Tm-high of the polymer, the cell structure is likely to have been unstable during foaming.



25

(a)

Percentage (%)

100% 80%

ER: 3.68

60%

Center

40%

Zone A

20%

Zone B

0% 0

10

20

30

cell size (µm) (b)

40

50

100% Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80% ER: 3.60

60% 40%

Center

20%

Zone A Zone B

0% 0

10

20 30 cell size (µm) (c)

40


50

Page 27 of 73

26

Percentage (%)

100% 80% ER: 3.29

60% 40%

Center

20%

Zone A

0%

Zone B 0

10

20 30 cell size (µm) (d)

40

50

Figure 7. SEM images in (a) and the cumulative cell size distribution curves of PA 6 blends saturated at 80 °C (b), 100 °C (c), and 120 °C (d). The samples were foamed at 200 °C.

Tsat System

100°C

120°C 100%

Percentage (%)

SMA

Percentage (%)

100% 80%

ER: 2.89

60% 40%

Center

20%

Zone A

0

10

20

30

cell size (µm)

40

ER: 2.72

60% 40%

Center

20%

Zone A Zone B

0% 0

50

10

20

30

cell size (µm)

40

50

100%

Percentage (%)

100%

ADR

80%

Zone B

0%

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80%

ER: 2.83

60%

Center

40%

Zone A

20%

Zone B

0% 0

10

20

30

cell size (µm)

40

80%

ER: 2.74

60% 40%

Center

20%

Zone A Zone B

0% 50

0

10

20

30

cell size (µm)

40

50

Figure 8. The cumulative cell size distribution curves of sample containing chain extenders and foamed at 200°C.



27 Another noteworthy phenomenon of bimodal foam is observed in the cell size frequency distribution curve shown in Figures 9 with a resolution of 0.5 μm. The scale bar of cell size was intentionally set to highlight the cell size below 10 μm. After ADR or SMA was added, dual peaks at 1 μm and 2.5 μm were observed in Figure 9. The results imply that adding the chain extenders enlarged the small cells and created another set of cells of size 2.5 μm, through an unknown mechanism. Therefore, the prominent cell-sizes in the sample containing chain extenders are 1 μm, 2.5 μm, and 30–40 μm. We consider this to be the first report of this structure in literature. The rheological behavior of these samples is likely to be useful in explaining the phenomena; however, that is beyond the scope of this study.

30%

40%

PA 6 blends (200°C)

Percentage (%)

PA 6 blends/SMA (205°C)

20%

PA 6 blends (200°C)

35%


Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 73

PA 6 blends/ADR (200°C) PA 6 blends/ADR (205°C)

10%


30%


25%

PA 6 blends/ADR (200°C)

20%

PA 6 blends/ADR (205°C)

15% 10% 5%

0% 0.00

2.00

4.00

6.00

Cell size (µm)

8.00

10.00

0% 0.00

(a)

6.00

Cell size (µm)

10.00

(b)

Figure 9. Cell size distribution curve of samples saturated at 100°C (a) and 120°C (b) and foamed at 200°C or 205°C.


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28 The reason for the formation of the bimodal structure is an interesting and curious research topic for foam researchers. Most of the literatures that discussed how to create a bimodal cell structure were published after 2010. There still are a number of unknown factors. Regulating the nucleation and cell growth are the main strategies.59, 60 It can be regulated by blending polymers/additives to create new nucleation sites or to create solubility difference. These strategies can be further classified based on the underlying method as follows: (1) controlling pressure release rate, (2) aggregation of nucleation agents, (3) polymer blends, (4) copolymer, (5) co-blowing agents, (6) crystallization, and (7) external stresses. We briefly review these strategies and summarize the reasons why bimodal structures are formed, in this research. The earliest bimodal foam structure created by CO2 foaming technology was reported by Arora et al., in which the bimodal structure was created by two-step depressurization using different pressure release rates.61 Subsequently, researchers adopted this strategy to create bimodal foams62, 63 and measured the physical properties of the bimodal foams. 54, 64

Adding a nucleation agent is a common method for increasing the cell density. If the nucleation agent forms aggregates, there are insufficient nucleants. Homogeneous and heterogeneous nucleation could occur simultaneously, and both become significant. The result is a bimodal bubble size distribution.65 Because it is challenging to disperse nanoparticles, such a phenomenon is



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29 commonly observed when nanoparticles are used as the nucleation agents.66-69 Certain research groups also exploited the advantages of the nucleant size and geometry to create bimodal structure.70, 71 Rather than using nucleation agents, cell nucleation can be controlled by creating a solubility difference. This can be achieved by grafting copolymers with high CO2 affinity65, 72, blending polymers with different CO2 solubility60, 73-80, or using co-blowing agents with different solubility in the polymer matrix.81-83 Polymer crystallization is one of the most noteworthy factors that create bi-cellular structure. CO2 is likely to induce polymer crystallization, and the crystals are likely to function as nucleation sites.84, 85 The polymer crystals do not absorb CO2 and are likely to confine the cell growth. Foaming polymers create bimodal structure near their melting points.86-89 Finally, external stress such as ultrasound is likely to induce nucleation and create bimodal structure.90

In our case, the addition of chain extender is demonstrated to increase the melt strength of the polymer matrix; moreover, it is likely to function as a nucleation agent.91 The two factors likely to interact. In addition, polyamide is a crystalline polymer with Tfoam approximately equal to Tm, and PA 6C is a copolymer. All these issues could have caused the formation of the bimodal structure.

To realize how foaming affects the melting peak of the PA 6 blends, the foamed samples were characterized using DSC with a heating rate of 10 °C/min. The results are shown in Figure 10 and


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30 Table 4. As previously mentioned, for the solid PA 6 blends, double melting peaks appear during the second heat process; moreover, the ΔTms of the PA 6 blends, PA 6 blend/SMA, and PA 6 blend/ADR are 15.08, 10.44, and 10.58 °C, respectively. After the samples were foamed, ΔTm increased to approximately 20 °C. In addition, the crystallinity (Xc) of the foamed sample increased by at least 10%. The crystallinity of the unfoamed sample is provided in Table 2. The increase in crystallinity was anticipated. CO2 are known to induce polymer crystallization84, and the foaming process results in stress induced crystallization.92 The DSC results implies that foaming widened the processing window. The dramatic increase in crystallinity is likely to have improved the heat resistance and mechanical properties of the polymer matrix.

Figure 10. The 1st heat DSC curves of PA 6 beads saturated at 100°C and foamed at 200°C

Table 4. ΔTm and crystallinity of PA 6 blends with chain extenders prepared by different Tsat



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31 System

PA 6 blends

PA 6 blends/SMA

PA 6 blends/ADR

Tsat

ΔTm

Xc (%)

ΔTm

Xc (%)

ΔTm

Xc (%)

100

20.16

34.7

19.56

31.7

19.36

25.5

120

21.42

37.6

20.21

30.9

19.66

24.4

The DSC results revealed only the melting temperature, and the difference in crystal form could not be observed. Because polyamide 6 has at least two crystal forms and the melting temperature of the crystals are similar 42, DSC is not capable of distinguishing between the crystal forms. Another method for investigating the polymer crystallization behavior is XRD. Because the samples were quenched by ice water immediately after compounding, no significant peak was observed. The XRD results of the foamed samples are shown in Figure 11. All these samples exhibited apparent peaks at 2θ = 20° and 23.7°. These are the typical PA 6 α crystal peaks.42 The patterns of the PA 6 blends and PA 6 blends/SMA overlapped; however, adding ADR created another weak peak between the two α crystal peaks. An inconspicuous peak at approximately 21° was observed. According to the literature, it could be the γ peak or amorphous PA 6. Because ADR is a chain extender, it could retard the formation of the crystals, and thus, the crystallinity would be decreased. This rationale could be verified with the DSC data. The crystallinity of the PA 6 blends and PA 6 blends/SMA are over 30%. The addition of ADR decreased the crystallinity to 25.5%, which is likely to have resulted in the peak at approximately 21°. The above results also verified that ADR is a more effective chain


Page 33 of 73

32 extender. It helped improve the melt strength of the PA 6 blends and improve the expansion ratio of the foamed beads.

120%

PA 6 blends PA 6 blends/SMA

100%

relative intensity(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PA 6 blends/ADR 80% 60% 40% 20% 0% 15

16

17

18

19

20

21

22

23

2θ

(º)

24

25

26

27

28

29

30

Figure 11. XRD analysis results of PA 6 blends statured at 100°C and foamed at 200°C.

There are three likely reasons for increase in crystallinity. They are annealing42, CO2 saturation84, and foaming.92 In our case, the samples are saturated in CO2 at 100 °C. This implies that these factors cannot be separated. The DSC results of the PA 6 blends/ADR after compounding, annealing at 100 °C for 24 h, and saturation in CO2/annealing at 100 °C, are highly similar. All the results are similar to those in Figure 2(a). However, after foaming, double melting peaks were observed (Figure 10). To investigate these issues, the samples were analyzed using XRD; noteworthy phenomena were observed.



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33 The XRD results of the PA 6 blends/ADR are shown in Figure 12. No significant peak was observed for samples that were subjected to only compounding. After annealing and foaming, typical α crystal peaks were observed. Nevertheless, the sample annealed and saturated with CO2 exhibited a special crystal morphology. The α crystal peak at 20° vanished, and an indistinct peak was observed at 21°, which is the amorphous peak. It indicates that treating PA 6 with high pressure CO2 is likely to have changed the crystalline structure.

2

Figure 12. XRD analysis results of PA 6 blends annealing at 100°C with CO2, without CO2 and after foaming


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34 Finally, the foamed PA 6 blend/ADR beads were placed in a porous container and immersed in hot silicone oil at 200 °C for sintering the beads. The sintered product is shown in Figure 13(a). The sintered PA 6 bead foams could float on water. Because the relative density of PA 6 is higher than one, its floating on water indicated that the cells did not collapse during sintering and that the beads could be sintered. The optical microscopic images are shown in Figures 13(b) and 13(c). Figure 13(b) shows evidence of bead fusion. The contact surface between the beads is fused. On the other hand, the cells are deformed but not completely collapsed, and many cells can still be observed in Figure 13(c). Our results provide a preliminary study of producing PA 6 bead foams by blending PA 6 of different melting points with chain extenders.

(a)



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35

(b)

(c)

Figure 13. Schematic diagram of the sintering process. Sintered PA 6 bead foams floated on water.

4.　CONCLUSIONS

In summary, PA 6 bead foams were created by blending PA 6 and PA 6 copolymers. PA 6 foams with double melting peaks were successfully created by two-step foaming. Chain extenders such as SMA and ADR were added to improve the foaming capability. The results demonstrated that ADR is a more effective chain extender and addition of ADR is likely to retard foam degradation during foaming. The highest expansion ratio of the PA 6 beads without degradation was 4.3 times. Unlike other PA 6 foam studies, foaming PA 6 copolymers created a bimodal cell structure. Blending PA 6 and chain extenders made the bimodal structure more apparent, and a nanocellular structure was observed. Approximately 10–20% of the cells are of size less than 1 μm. Although the bimodal


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36 structure was present in the sample, the cell structure was macroscopically uniform; this was verified by overlaying the cumulative cell size distribution curves for the different regions of the foamed beads. This is the first time that bimodal structure and/or nanocellular structure in PA 6 foams are being reported. There are numerous likely reasons for the creation of the bimodal cell structures. In our case, the use of the copolymers and chain extenders and the foaming of the polymer at its melting point could be the reasons for the formation of the bimodal structure. Foaming the PA 6 blends created double melting peaks and increased ΔTm. It also increased the crystallinity of the PA 6 blends. On the other hand, adding ADR lowered the crystallinity of PA 6. These were verified by XRD analysis. The XRD results also demonstrated that treating samples with high pressure CO2 is likely to change the crystal morphology of PA 6. Finally, the foam could be sintered using hot silicone oil; moreover, the PA 6 blends foam did not collapse during foam sintering.

Supporting Information Cumulative cell size distribution curves of samples containing chain extenders. Samples were foamed at 205°C. The information is listed in Figure S1. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].



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37 ORCID Shu-Kai Yeh: 0000-0002-9131-5133 Notes The authors declare no competing financial interest. Acknowledgements This work was partly funded by contract numbers MOST 105-2622-E-011-015-CC3 from the Ministry of Science and Technology, Taiwan. This work was also partly funded by Zig Sheng Industrial Co. Taiwan. The authors wish to thank Dr. Yue-Tang Lin from Zig Sheng Innovation Research Center for his discussion and material supply.

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Abstract graphic 95x79mm (300 x 300 DPI)


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Figure 1. The DSC curves of PA 6 and PA 6C and PA 6/PA 6C blended with different ratios.



Figure 2. The 1st heat (a) DSC curves of PA6/PA6C blends with chain extenders


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Figure 2. The 2nd heat (b) DSC curves of PA6/PA6C blends with chain extenders



Figure 3. First heat DSC graph of PA 6C foam made by different saturation temperature 40x24mm (300 x 300 DPI)


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Figure 4. Appearance of expanded PA 6 blend beads saturated at 120°C and foamed at 205°C 54x40mm (300 x 300 DPI)





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Figure 5. Cell structures of PA 6C foamed at 190°C (a). 108x81mm (300 x 300 DPI)


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Figure 5. Cell structures of PA 6 blends (b) foamed at 200°C. 108x81mm (300 x 300 DPI)



Figure 5. Cell structures of PA 6 blends/ADR (c) foamed at 200°C. 108x81mm (300 x 300 DPI)


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Figure 6. Typical cell structure of PA 6 blends/ADR saturated at 120°C and foamed at 200°C 108x81mm (300 x 300 DPI)





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Figure 7. SEM images in (a)


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Figure 7. The cumulative cell size distribution curves of PA 6 blends saturated at 80 °C (b). The samples were foamed at 200 °C.



Figure 7.The cumulative cell size distribution curves of PA 6 blends saturated at 100 °C (c). The samples were foamed at 200 °C.


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Figure 7. The cumulative cell size distribution curves of PA 6 blends saturated at 120 °C (d). The samples were foamed at 200 °C.



Figure 8. The cumulative cell size distribution curves of sample containing chain extenders and foamed at 200°C. 105x57mm (300 x 300 DPI)


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Figure 9. Cell size distribution curve of samples saturated at 100°C (a) and foamed at 200°C or 205°C. 27x19mm (300 x 300 DPI)



Figure 10. The 1st heat DSC curves of PA 6 beads saturated at 100°C and foamed at 200°C 32x21mm (300 x 300 DPI)


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Figure 11. XRD analysis results of PA 6 blends statured at 100°C and foamed at 200°C. 38x25mm (300 x 300 DPI)



Figure 12. XRD analysis results of PA 6 blends annealing at 100°C with CO2, without CO2 and after foaming 40x32mm (300 x 300 DPI)


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Figure 13. Schematic diagram of the sintering process. Sintered PA 6 bead foams floated on water. 31x15mm (300 x 300 DPI)





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