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Bimodal Microcellular Morphology Evaluation in ABS-Foamed Composites Developed Using Step-Wise Depressurization Foaming Process.

INTRODUCTION

Polymeric foams with bimodal cellular microstructures possess numerous advantages when compared with its unimodal cellular counterparts owing to its peculiar cell morphologies. Such as the combination of both the small and large cells. It is widely reported that, in bimodal cellular structures, the large cells could reduce the overall foam density, whereas small cells assist in enhancing the mechanical and thermal insulation properties [1-3], pertaining to which the bimodal cellular structures have been utilized extensively in various diverse applications throughout the chemical industries, packaging sector, and construction applications.

The bimodal cellular structures have been developed by inducing two different nucleation phenomena or mechanism that could be achieved by following methods, which are co-blowing agent method [1-4], polymer blending system method [5-12], two-step depressurization method [13-24], several steps depressurization [25], cooling and depressurization method [26], cooling and two-step depressurization [27], varying temperature mode (VTM) [28], multiple soaking temperature [29], and ultrasound cavitation exposure [30].

In the co-blowing agent method, two different blowing agents prompt the two different nucleating mechanism that leads to development of bicellular structures in the foamed samples. Gendron et al. [31] explained the significance of the blowing agent and the level of plasticization of physical blowing agents. In this method, generally, water were used as a co-blowing agent. It could aid to achieve lower foam densities by enlarging the cell size in the extrusion foaming process. Likewise, Lee et al. [1] used water, n-butane, and precipitated silica as co-blowing agent found that by varying the ratio of n-butane/water/silica, the bicellular structures could be well controlled and also reported that the bimodal cellular structures possess excellent heat insulation property. Nistor et al. [4] studied the effects of n-pentane (nC5) and cyclopentane (cC5) as a co-blowing agent on development of bimodal morphologies. And also found that, they were reduced the solid skin, acted as plasticizers, and both porosity and cell diameters were increased with their concentration.

In polymer blend method, when two immiscible polymers were compounded, two very distinct types of blend phases (dispersed and co-continuous) developed, because of its heterogeneous nature. As a result, different diffusion and permeation behavior of the blowing agent in the two phases of polymer blends was observed. This eventually affected the cell nucleation process and resulted in development of bimodal microstructures. The crucial factors that govern the cell morphologies were reported to be the composition, processing conditions, interfacial tension, and rheological properties of the components [32-34]. In this method, open-cell structures could be developed when blend forms a co-continuous morphology and its polymer melt strength should be very low then the interconnected cell walls will rupture, thus open cells could be generated [5]. Further author investigated the cell morphologies of several polypropylene/low-density polyethylene (PP/LDPE) blends and found that an interconnected cell structure could be fabricated in the co-continuous 50/50 (PP/LDPE), particularly when PP phase could be foamed. Tang et al. [6] observed that PP foams exhibited a bimodal type of cellular structure at foaming temperature 93[degrees]C and 94.5 [degrees]C and also the uniform cell structures were observed at foaming temperature 97.5[degrees]C and 99[degrees]C. Doroudiani et al. [35] reported that the uniform cells with fine cell size were obtained in low-crystalline polymers and nonuniform cell structure were obtained in the high-crystalline polymer. Generally, in the crystalline regions, the growth of the nucleated bubbles was limited due to high matrix stiffness [36], which restricts the expansion of cell. Further, the crystal regions could work as nucleating agent and induces more nucleating sites during the foaming process [37, 38] and the larger cells were normally generated in the amorphous regions owing to its low matrix stiffness, thus the combination of two different crystallinity has been lead to development of bimodal type of cellular structures. Kohlhoff et al. [7] developed bimodal cell distribution via creating a difference in elasticity by adding crosslinking agent as methyl methacrylate (MMA), which leads to induce a significant difference in the nucleation behavior of each blend component. Here, the smaller cells are ranging from 10 to 30 [micro]m in diameter and large cells ranging from 200 to 400 [micro]m in diameter were reported. Salerno et al. [11] developed the bimodal open microporous scaffolds by overnight soaking poly([??]-caprolactone/thermoplastic zein-hydroxyapatite (PCL/TZ and PCL/TZ-HA) foams in the distilled water at 37[degrees]C. These phenomena promoted the leaching of the polyethylene glycol) (PEG) from thermoplastic zein (TZ) phase and initiated the cell formation within the scaffolds leading to bimodal type microstructure. Duan et al. [8] reported that the organic montmorillonite (OMMT) acts as heterogeneous nucleation agent and it has been reduced the cell size and also the addition of stearic acid (SA) has been changed the microscopic dispersion of OMMT. The combined effect of these could developed a bimodal type of cellular structures. Similarly, Wang et al. [9] reported that a large difference in cell nucleation was necessary for developing bimodal cell morphologies in the binary polymer blends and found that the large cells were developed in polystyrene (PS) phase and small cells were developed in polypropylene (PP) phase. Bernardo et al. [10] proposed a model to predict thermal conductivity (conduction mechanism) of a bimodal cell structures with a combination of both micro and nanocellular cell population and found that the conductivity of a bimodal system depends on the volume fraction of nanometric cells.

The two-step depressurization method has been reported to create a difference in nucleation mechanism by releasing pressure in two-steps during solid-state batch foaming process. Bao et al. [14] reported that the holding stage between the two depressurization steps plays a vital role in governing of bimodal cellular structures. During the holding stage, the larger cells further increase in size and smaller cells further decrease in size up to complete collapse [14], due to the gas diffusion phenomena from smaller cells to larger cells. Bao et al. [15] found that, for better mechanical properties, the volume fraction of the larger cells should be in the range of 25-32% and simultaneously the cell size of larger ones smaller than 25 pm. Salerno et al. [16] reported that the thermal history, depressurization profile, and the intermediate pressures significantly affect the attributes of the porous structures. Highly interconnected porous bimodal scaffolds were reported to be developed for tissue engineering applications by quenching in liquid nitrogen along with two-step depressurization method [17]. Likewise, Ma et al. [18] reported that, bimodal cellular structures have been developed by two-step solid-state batch foaming process in which they found that the significant parameters, which influence morphologies, are a degree of depressurization and the foaming temperature. Further, author also reported that the bimodal cellular structures exhibited elevated tensile, compressive, and dynamic mechanical properties when compared with unimodal cellular structures [19]. Yu et al. [20] reported that polylactic acid (PLA) was immiscible with the poly(butylenes succinate) (PBS) and it was dispersed with various concentrations and found that the presence of PBS reduced the viscosity of polymer blend. During the foaming process, PLA/PBS interfaces act as cell nucleating sites and due to low melt strength of PBS, the interconnected cell walls collapsed and thus open cell structures were achieved. Subsequently, the two-step depressurization method was incorporated during the foaming process thereby obtained bimodal cell structures with up to 97% of open cell content. Wang et al. [21] reported that the complex cellular structures (CCS) of PS and polystyrene/poly(ethylene terephthalate glycolmodified) (PS/PETG) were developed by two-step depressurization method. The degree of first-step depressurization and holding time was two governing parameters to regulate the cell morphologies. A large difference in the degree of first-step depressurization was reported to be difficult to achieve with CCS. Also the longer holding times was found to increase the cell size of former formed cells [21]. Li et al. [22] reported that the rise in the degree of first depressurization could increase the density of the final larger cells but an insignificant effect on final smaller cells. However, extending the holding time at intermediate pressure could reduce the density of larger cells. Chen et al. [23] reported that, extended soaking time leads to produce large pores, however extending holding time decrease the density of large pores by increasing the mean pore size. Sierra et al. [25] developed hierarchical porosity in ScC[O.sub.2] condition at a pressure of >100 bar and <300 bar, along with saturation temperature being lesser than its melting temperature ([T.sub.m]) following the multiple step depressurization technique. The hierarchical structure possesses two types of cell sizes in the range of 300-500 pm and 15-50 [micro]m, and interestingly the interconnected pores were obtained in the range of 50-90 pm of membrane thickness [25].

Liu et al. [26] reported that in cooling and depressurization method initially specimens were saturated at above its foaming temperature, during this period, first nucleation was induced due to acute depression of C[O.sub.2] solubility in the isotactic polypropylene (iPP) matrix. Subsequently, followed by sudden depressurization results in development of bimodal type of cell structures by enlarging existing pores as well as generating a small new pores. Cooling and two-step depressurization method was comprised of cooling, depressurization and followed by two-step depressurization technique. Wang et al. [27] reported that by adding dicumyl peroxide (DCP) in high-density polyethylene (HDPE) as a crosslinking agent, its viscosity improved. Furthermore, the presence of carbon nanotubes (CNT) acted as a nucleation agent that enhanced the nucleation efficiency hence developed high-cell density porous foams [27]. Xu et al. [28] reported that at supercritical C[O.sub.2] saturated condition, the rise in temperature initiates the first nucleation, which results in development of larger cells, followed by sudden depressurization triggering the second nucleation to develop small cells; hence, bimodal structures were developed. Huang et al. [29] reported that bimodal cellular structures were developed by multiple soaking temperature (MST) method followed by one-step depressurization. The MST technique assists to induce two different nuclei, which results in the development of a bimodal type of cellular structures. In this technique, soaking time and soaking temperature are key parameters to govern the cell morphologies. Gandhi et al. [30] reported that ultrasound excitations with sonication frequency of 45 KHz developed bimodal type of microcellular I morphologies in ABS microcellular foam system. The literature is evident that the difference in nucleation mechanism can successfully produce bimodal type of cellular structures in microcellular foaming process. In this article, various crucial process parameters in batch microcellular foaming process have been studied extensively to understand its effect on the development of bimodal and multimodal type of microstructure.

EXPERIMENTAL

Materials

Acrylonitrile-butadiene-styrene (ABS) HI 121 was used as the base polymer for this study (Make: LG Chem) having density of 1.04 g/cc and glass-transition temperature ([T.sub.g]) of approximately 105[degrees]C. Industrial graded 99% purity C[O.sub.2] was used as physical blowing agent, which was supplied in a standard cylinder (60 bar pressure).

To prepare sheets, pellets was dried at 60[degrees]C for 8 h in an hot air oven and then processed into the sheet with dimension of 180 mm * 120 mm * 1.85 mm by utilizing compression molding setup. During this process, mold temperature maintained at 220[degrees]C, time 7 min and fore was used 1,510 N. From these sheets, samples were prepared with dimension of 30 mm * 20 mm * 1.85 mm for experimentation (Table 1).

Foaming Methodology

Microcellular foaming methodology comprised two stages, namely the saturation and the foaming stages. Samples were saturated in a high-pressure autoclave with predetermined saturation parameters, which are depicted in Tables 2-6. These samples were subsequently depressurized in multiple stages. Post depressurization, these saturated samples were immersed into the 130[degrees]C hot glycerin bath for 30 s in order to induce thermodynamic instability. To restrict the bubble growth and to provide stability to foam, the samples were quenched in water maintained at room temperature for 10 s. Throughout the experimentation, samples were saturated at room temperature and the desorption time was maintained approximately 3 min. Further, the depressurization of C[O.sub.2] gas from the vessel was pursued at 10 bar/s for the entire experimentation. The schematic of the foaming methodology was depicted in Fig. 1.

CHARACTERIZATION

Sorption

Sorption characterization was investigated for the saturated samples at various saturation parameters and its range (Table 2). The average time lapse between the depressurization and foaming process was maintained approximately 3 min. The weight of the samples was measured using a weighing balance (Make: Wensar-MAB-220) with an accuracy of 0.1 mg. Further, the percentage of weight gain by specimens were measured by the gravimetric method as described by Nalawade et al. [39]

Percentage of gas absorbed (wt%) = [W.sub.f]/[W.sub.0]/[W.sub.0] (1)

where [W.sub.f] refers to the weight of the specimen after saturation and [W.sub.0] refers to the weight of the specimen before saturation.

Density

The density of the bimodal cellular structured ABS samples was measured according to ASTM D 792 standard. Samples weights were measured in air and in distilled water by utilizing the Wensar density measurement equipment. The density of distilled water was considered to be 0.9975 g/cc at room temperature. The density of ABS bimodal cellular structures was determined by using the following Eq. 2 [40].

[[rho].sub.f] = a/a-b [[rho].sub.water] (2)

where [[rho].sub.f] is the density of foamed polymer, a is the weight in air, b is the weight in standard liquid, and [[rho].sub.water] is the density of standard liquid.

Expansion Ratio

Expansion ratio ([DELTA]) is defined as the ratio of density of virgin polymer to foamed polymer samples. The expansion ratio of bimodal ABS cellular microstructure of foamed samples was calculated by using Eq. 3 [40].

[DELTA] = [[rho].sub.n]/[[rho].sub.f] (3)

where [[rho].sub.f] is the density of foamed polymer and [[rho].sub.n] is the density of neat polymer.

Cell Morphology

Cell morphologies are defined as the arrangement of cellular microstructures in the foamed polymer samples. The cell morphologies were evaluated by utilizing scanning electron microscopy (SEM), in Hitachi SU 3500N. The foamed samples were cryogenically fractured by immersing into liquid nitrogen for 5 min and was subsequently brittle fractured. Then fractured surface was the gold sputter coated using a (Make: Hitachi MC1000) Ion Sputter Coater. Image analysis of the SEM micrograph, performed by utilizing the image-j software in order to determine the average cell size, cell size distribution, and cell density.

Cell Density

The total number of cells that are presented per cubic centimeter volume of foamed samples is defined as cell density. The cell density of ABS bimodal cellular structure foamed samples was determined using SEM micrograph using Eq. 4 [41].

[N.sub.o] = [(n/A).sup.3/2] (4)

where [N.sub.o] is the cell density, n is the total number of cells in micrograph, and A is the area of micrograph in [cm.sup.2].

RESULTS AND DISCUSSION

In this article, bimodal and multimodal cellular microstructures have been developed by utilizing stepwise depressurization technique in the solid-sate batch foaming process. This study involves multiple depressurization steps that are ranging from a single step to four steps. For the development of bimodal and multimodal cellular microstructure, process parameters plays a crucial role and these parameters includes saturation pressure, holding pressure, holding time, and holding steps. The significance of these process parameter on the cell morphologies and its characterization was investigated in detail. For better comprehension to reader, each processing condition was designated by Greek letters [alpha], [beta], [gamma], and [delta] and followed by a subscript, which are explained in Tables 2-6.

Significance of Saturation Pressure on the Cell Morphology and its Characteristics

Table 2 shows the designation of samples for the significance of saturation pressure, in which saturation pressure [P.sub.1] has been varied from 20 bar to 50 bar with an interval of 10 bar pressure. Further, all other process parameters which are holding pressure [P.sub.2] = 10 bar, saturation time [H.sub.1] = 20 h, holding time [H.sub.2] = 4 h, and depressurization rate 10 bar/s was maintained constant. Figure 2 depicts the pressure versus time profile for the significance of saturation pressure.

Figure 3 shows the effect of saturation pressure on the percentage of weight gain of physical blowing agent and the expansion ratio of foams. It was found that an increase in saturation pressure result in increase in the percentage weight gain and the expansion ratio. The average percentage of weight gain and average expansion ratio ranging from 2.8% to 6.4% and 6.7 to 11.1, respectively, within the range of sample designation [[alpha].sub.1]-[[delta].sub.1]. The amount of physical blowing agent (C[O.sub.2]) in the polymer matrix was influenced the expansion ratio of microcellular foams.

Figure 4 depicts, the significance of saturation pressure on cell morphologies with magnification 500x and 1,000x sample with designation [[alpha].sub.1] (a, b), [[beta].sub.1] (c, d), [[gamma].sub.1] (e, f), and [[delta].sub.1] (g, h), respectively. Sample with designation [[alpha].sub.1] (a, b) shows scanning electron micrographs which depict the uniform cellular microstructure, that is, no larger bubbles are absent and foam contains only smaller bubbles. Since at sample with designation [[alpha].sub.1], the pressure difference between the saturation pressure and holding pressure is only 10 bar, which is not sufficient to develop bimodal cellular microstructure, therefore only unimodal foamed microstructure was obtained. Further, sample with designations [[beta].sub.1] (c, d), [[gamma].sub.1] (e, f), and [[delta].sub.1] (g, h) clearly shows the bimodal cellular microstructure (i.e., the larger bubbles are surrounded by smaller bubbles) in the corresponding SEM micrographs. The increase in saturation pressure results in decrease in the size of smaller bubbles, while the larger bubbles are found to be insignificantly affected, which is evident in the SEM micrographs of sample with designation [[beta].sub.1] (c, d), [[gamma].sub.1] (e, f), and [[delta].sub.1] (g, h) are evident for that.

Further, Fig. 5a-d shows the cell size distribution plots of sample with designation [[alpha].sub.1], [[beta].sub.1], [[gamma].sub.1], and [[delta].sub.1], respectively. Cell size distribution plots also supports that the increasing saturation pressure results in decrease in the size of smaller bubbles. Sample with designation [[beta].sub.1], [[gamma].sub.1], and [[delta].sub.1], the percentage of cells frequency around 4 [micro]m was increased as saturation pressure increased.

Figure 6 shows the effect of saturation pressure on average cell size and cell density. It was once again observed that increase in the saturation pressure, reduce in the average cell size, and increase in the average cell density. At 20 bar saturation pressure, the average cell size was approximately 10 times larger than at 50 bar saturation pressure. This is because of less pressure difference of the saturation pressure to holding pressure, holding pressure to ambient pressure. The average cell size and average cell density were found be in the range of approximately 51.5-4.3 [micro]m and 4.9 * [10.sup.6] cell/ [cm.sup.3] to 9.1 * [10.sup.9] cell/[cm.sup.3], respectively, for sample designation [[alpha].sub.1]-[[delta].sub.1].

Influence of Holding Pressure on the Cell Morphology and its Characteristics

Table 3 shows designation of samples for the influence of holding pressure, in which saturation pressure [P.sub.1], saturation time [H.sub.1], and holding time [H.sub.2] have been maintained constant 50 bar, 20 h, and 04 h, respectively. Also, holding pressure [P.sub.2] varied from 10 bar to 40 bar with an interval of 10 bar. Further, the depressurization rate was maintained constant 10 bar/s for all the conditions (sample designation [[alpha].sub.2]-[[delta].sub.2]). Figure 7 depicts the pressure versus time profile for the influence of holding pressure.

Figure 8 shows the effect of holding pressure on the percentage of weight gain of physical blowing agent and the expansion ratio of foams. It shows that with an increase in holding pressure, an increase in the percentage of weight gain and the expansion ratio were observed. The average percentage of weight gain and average expansion ratio ranging from 6.4 to 8.5% and 11.1 to 14.6%, respectively, within the range of sample designation [[alpha].sub.2]-[[delta].sub.2].

Figure 9 depicts the effect of holding pressure on microstructure with magnification 500x and l,000x, most of the SEM micrographs depicts that the large bubbles are surrounded by small bubbles. By observing SEM micrographs of sample with designation [[alpha].sub.2], [[beta].sub.2], [[gamma].sub.2], and [[delta].sub.2], the size of larger bubbles is higher in sample designation [[alpha].sub.2], [[delta].sub.2] than the sample with designation [[beta].sub.2], [[gamma].sub.2]. The difference in pressure is more (either saturation pressure to holding or holding pressure to ambient pressure), for instance, at sample with designation [[alpha].sub.2] the pressure difference of saturation pressure to holding pressure is 10 bar only but holding pressure to ambient pressure is 40 bar. Likewise, at sample with designation [[delta].sub.2], the pressure difference between saturation pressures to holding pressure is 40 bar but holding pressure to ambient pressure is 10 bar only. Similarly, at sample with designation [[beta].sub.2], [[gamma].sub.2], these pressure differences are lesser than the sample with designation [[alpha].sub.2], [[delta].sub.2]. The increase in holding pressure results in enlarging the size of smaller bubbles, which are surrounded by larger bubbles, even the SEM micrographs (Fig. 9) and cell size distribution plots (Fig. 10) are more evident. Due to this effect, the average cell size increased and the average cell density was found to decrease. In the sample with designation [[delta].sub.2], the average cell size was approximately 1.8 times larger than the average cell size when compared with the sample with designation [[alpha].sub.2]. Figure 11 shows the effect of holding pressure on average cell size and average cell density. The average cell size and average cell density were found to be in the range of approximately 4.3-7.9 [micro]m and 9.1 * [10.sup.9] cell/[cm.sup.3] to 1 * [10.sup.9] cell/[cm.sup.3], respectively, for sample designation [[alpha].sub.2]-[[delta].sub.2].

Effect of Holding Time on the Cell Morphology and its Characteristics

Table 4 shows designation of samples for the influence of holding time, in which saturation pressure Pi, holding pressure [P.sub.2] maintained constant 50 bar, 20 bar, respectively. Further, saturation time [H.sub.1] varied from 20 to 08 h with an interval of 04 h and holding time [H.sub.2] varied from 04 h to 16 h with an interval of 04 h. The first and second step depressurization rate was maintained constant 10 bar/s. Figure 12 depicts the pressure versus time profile for the effect of holding pressure.

Figure 13 shows the effect of holding time on the percentage of weight gain of physical blowing agent and the expansion ratio of foams. It shows that with the increase in holding time, the percentage of weight gain and foam expansion ratio decrease. With up to 8 h of holding time (sample with designation [[alpha].sub.3], [[beta].sub.3]), there is no significant difference in both percentages of weight gain and expansion ratio, beyond that percentage of weight and expansion ratio dropped due to the diffusion back of absorbed gases in the polymer matrix. To the best of authors understand, these phenomena are due to the pressure difference between the saturation pressure and holding pressure. The average percentage of weight gain and average expansion ratio were found to be in the range of 7.2-6.2% and 12.5-10.7, respectively, for sample with designation [[alpha].sub.3]-[[delta].sub.3].

Figure 14 depicts the effect of holding time on microstructure with magnification 500x and 1,000x. By observing SEM micrographs, except sample designation [[delta].sub.3], all other sample designation [[alpha].sub.3], [[beta].sub.3], and [[gamma].sub.3] shows clear bimodal microstructure. The longer holding time results in reduction of size in the larger bubbles, due to diffusion of the absorbed gases. This results in lower the number of larger cells thereby reducing the overall average cell size and the cell density was found to be increased. The explanation of same to be evidently seen in Fig. 15 which depicts the cell size distribution of sample designation [[alpha].sub.3], [[beta].sub.3], [[gamma].sub.3], and [[delta].sub.3]. Figure 16 shows the effect of holding time on average cell size and cell density. It can be observed that, in the sample with designation [[alpha].sub.3], [[beta].sub.3] (4 and 8 h holding time), there is no significant change in the average cell size and cell density. The average cell size and average cell density were found to be in the range of approximately 5.6-4.8 [micro]m and 4.5 * [10.sup.9] cell/[cm.sup.3] to 1 * [10.sup.9] cell/[cm.sup.3], respectively, for sample designation [[alpha].sub.3]-[[delta].sub.3].

Effect of Number of Holding Steps on the Cell Morphology with Variable Saturation Time and its Characteristics

The vast literature available on bimodal microcellular foams is limited to one or two holding steps; however, in this article, more than 02 steps were investigated. Table 5 shows the designation of samples for multiple holding steps at variable saturation time. Saturation pressure maintained constant [P.sub.1], saturation time [H.sub.1] has been varied from 20 to 5 h with an interval of 5 h each from the second step to the fourth step. Further holding pressures from [P.sub.2] to [P.sub.4] was reduced from 30 to 10 bar with an interval of 10 bar and holding time [H.sub.2]-[H.sub.4] reduced from 15 to 5 h with an interval of 5 h, respectively, from the second step to the fourth step. Here, the total sorption time was maintained constant (20 h), that is, saturation time [H.sub.1] plus holding time ([H.sub.2] + [H.sub.3] + [H.sub.4]). Depressurization rate was maintained 10 bar/s in the all conditions. Figure 17 depicts the pressure versus time profile for the effect of holding steps variable saturation time.

Figure 18 depicts the effect of holding steps on the percentage of weight gain of physical blowing agent and on expansion ratio of foams. The plot shows that by increasing the holding steps (sample with designation [[alpha].sub.4]-[[delta].sub.4]) decrement was observed in the percentage of weight gain and in the expansion ratio. Up on comparison of samples from the first step ([[alpha].sub.4]) to the fourth step ([[delta].sub.4]), the weight gain and the expansion ratio in the samples from fourth step were found to be approximately half of the first step. Further, single-step ([[alpha].sub.4]) and two-step ([[beta].sub.4]) depressurization depicts less difference in its weight gain and expansion ratio, whereas the third ([[gamma].sub.4]) and fourth step ([[delta].sub.4]) depressurization was found to have significant difference. The increase in holding steps leads to diffusion of the absorbed gases from the polymer matrix. The average percentage of weight gain and average expansion ratio were found to be in the range of 7.9-3.9% and 13.1-7.2, respectively, from the first step to the fourth step ([[alpha].sub.4]-[[delta].sub.4]).

Figure 19 depicts the effect of holding steps with variable saturation pressure on microstructure at magnification 500x and 1,000x. It can be observed that, the single-step depressurization (sample with designation [[alpha].sub.4]) could not develop bimodal structures, whereas two-step (sample with designation [[beta].sub.4]) was found to be development of bimodal. Interestingly, the third and fourth step (sample with designation [[gamma].sub.4], [[delta].sub.4]) depressurization developed multimodal type of microstructure. Figure 19e-h SEM micrographs of sample with designation [[gamma].sub.4], [[delta].sub.4] clearly shows a multimodal type of microstructures, which comprise of different cell sizes including small, medium, and large when compared simultaneously. Figure 20 shows cell size distribution for various holding steps ([[alpha].sub.4]-[[delta].sub.4]). As the holding steps increased, the size of both smaller and larger bubbles size was found to be reduced, which results in a decrease in the average cell size and increase in the average cell density. The average cell size and average cell density were found to be in the range of approximately 79.7-4.7 [micro]m and 2.1 * [10.sup.6] cell/[cm.sup.3] to 6.8 * [10.sup.9] cell/[cm.sup.3], respectively, from the first step to fourth step ([[alpha].sub.4]-[[delta].sub.4]) (Fig. 21).

Effect of Number of Holding Steps on the Cell Morphology with Constant Saturation Time and its Characteristics

Table 5 shows the designation of samples for multiple holding steps at variable saturation time. [P.sub.1] and [H.sub.1] indicate saturation pressure and saturation time, respectively, both maintained constant for all the steps. Further, holding pressures from [P.sub.2] to [P.sub.4] was reduced from 30 to 10 bar with an interval of 10 bar, holding time from [H.sub.2] to [H.sub.4] was maintained 5 h each, respectively, from the second step (sample with designation [[beta].sub.5]) to the fourth step (sample with designation [[delta].sub.5]). Here, the total sorption time was varied from 20 to 35 h with an increment interval of 5 h for each step, that is. saturation time [H.sub.1] plus holding time ([H.sub.2] + [H.sub.3] + [H.sub.4]). Depressurization rate was maintained at 10 bar/s in the all foaming experiments. Figure 22 depicts the pressure versus time profile for the effect of constant saturation time.

Figure 23 shows the effect of holding steps with constant saturation pressure on the percentage of weight gain of physical blowing agent and an expansion ratio of foams. The plot shows that by increasing the holding steps (sample with designation [[alpha].sub.5], [[beta].sub.5], [[gamma].sub.5], and [[delta].sub.5] respectively), it results in decrease in the percentage of weight gain and expansion ratio. When compared the multiple holding steps at constant saturation time with multiple holding steps at variable saturation time, that is, the sample designation [[alpha].sub.4], [[beta].sub.4], [[gamma].sub.4], and [[delta].sub.4] with sample designation [[alpha].sub.5], [[beta].sub.5], [[gamma].sub.5], and [[delta].sub.5] respectively, [[alpha].sub.4] and [[alpha].sub.5] were found to be same because they both are processed at the same conditions. But sample designation [[beta].sub.5], [[gamma].sub.5], and [[delta].sub.4] are shown a slightly higher percentage of weight gain and expansion ratio than the sample designation [[beta].sub.4], [[gamma].sub.4], and [[delta].sub.4] because sorption time (saturation time + holding time) was more. The average percentage of weight gain and average expansion ratio was found to in the range of 7.9-5.8% and 13.1-8.5, respectively, from the first step to the fourth step (sample with designation [[alpha].sub.5]- [[delta].sub.5]).

Figure 24 shows the SEM micrographs for the effect of holding steps with constant saturation pressure on microstructure with magnification 500x and 1,000x. Further, in Fig. 24, it can be observed that the single-step depressurization (sample with designation [[alpha].sub.5]) could not develop bimodal structures, whereas the two-step depressurization (sample with designation [[beta].sub.5]) was found to be development of bimodal type of microstructure. It is also observed that, in third and fourth step (sample with designation [[gamma].sub.5] and [[delta].sub.5]), depressurization was fund to be development of multimodal type of microstructure. The explanation of same can be evidently seen in Fig. 25 which shows cell size distribution for various holding steps ([[alpha].sub.5]- [[delta].sub.5]). As the holding steps increased, the size of both smaller and larger bubbles size was found to be reduced, which results in a decrease in average cell size and increases the average cell density. The average cell size and average cell density were found to be in the range of approximately 79.7-4.1 [micro]m and 2.1 * [10.sup.6] cell/[cm.sup.3] to 9.0 * [10.sup.9] cell/[cm.sup.3] respectively from the first step to fourth step ([[alpha].sub.5]-[[delta].sub.5]) which is depicts in Fig. 26.

CONCLUSIONS

Bimodal or multimodal microstructure microcellular ABS foams were successfully fabricated by solid-state batch foaming process by multiple distinct stepwise depressurization technique. This article focused to study the significance of crucial process parameters, which include saturation pressure, holding pressure and holding time, and effect of distinct depressurization steps. As the increase in both saturation pressure and holding pressure results in increase of both the percentage of weight gain and expansion ratio, it also decreases the average cell size and increases the average cell density. However, increase in holding pressure increase the average cell size and decreases the average cell density. Likewise, an increase in holding time reduces the percentage of weight gain and expansion ratio, also reduces average cell size thereby increase average cell density. Further, the longer the holding time results in developing smaller bubbles in size by diffusion back phenomena of absorbed gases from the polymer matrix, this results in lower the quantity of larger pores. The larger pressure difference (saturation to holding or holding to ambient) leads to develop larger bubbles in its size. Finally, concluding that two-step depressurization could develop bimodal type of microstructure and multistep depressurization could develop multimodal type of microstructure. Also, by regulating crucial process parameters, the volume fraction of smaller bubbles to larger bubbles vice versa can be controlled.

ACKNOWLEDGMENT

Financial assistance has been received from Department of Science and Technology (DST), Government of India under the project titled "Development of Microcellular & Nanocellular 3D Printing Process to Manufacturing Acrylonitrile Butadiene Styrene Foamed Products" with sanction order DST/TDT/AMT/2017/092 (G) August 08, 2018.

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Radhakrishna G, (1,2) Rupesh Dugad, (1,2) Abhishek Gandhi (iD)(1,2)

(1) CIPET: School for Advanced Research in Polymers (SARP) - APDDRL - Bengaluru, #488-B, 4th Floor, Block-2, KIADB Building, 14th Cross, Peenya 2nd Stage, Bengaluru 560058, Karnataka, India

(2) CIPET: School for Advanced Research in Polymers (SARP) - LARPM - Bhubaneswar, B-25, CNi Complex, Bhubaneswar 751024, Odisha, India

Correspondence to: A. Gandhi; e-mail: abhishekgandhi@cipet.gov.in and dr. abhishekgandhi@gmail.com Contract grant sponsor: Department of Science and Technology. DOI 10.1002/pen.25265

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Schematic of foaming methodology. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. Pressure versus time profile for effect of saturation pressure.

Caption: FIG. 3. Effect of saturation pressure on percentage of weight gain and expansion ratio.

Caption: FIG. 4. Effect of saturation pressure on microstructure; (a) sample designation [[alpha].sub.1], 500x, (b) sample designation [[alpha].sub.1] 1.000x, (c) sample designation [[beta].sub.1], 500x, (d) sample designation [[beta].sub.1], 1.000x, (e) sample[ [[gamma].sub.1], 500x, (f) sample designation [[gamma].sub.1] 1.000x, (g) sample designation [[delta].sub.1], 500x, (h) sample designation [[delta].sub.1], 1.000x.

Caption: FIG. 5. Cell size distribution at various saturation pressures; (a) sample designation [[alpha].sub.1] (b) sample designation [[beta].sub.1] (c) sample designation [[gamma].sub.1], and (d) sample designation [[delta].sub.1].

Caption: FIG. 6. Effect of saturation pressure on average cell size and cell density.

Caption: FIG. 7. Pressure versus time profile for effect of holding pressure.

Caption: FIG. 8. Effect of holding pressure on weight gain and expansion ratio.

Caption: FIG. 9. Effect of holding pressure on microstructure; (a) sample designation [[alpha].sub.2], 500x, (b) sample designation [[alpha].sub.2], 1,000x, (c) sample designation [[beta].sub.2], 500x, (d) sample designation [[beta].sub.2], l,000x, (e) sample designation [[gamma].sub.2], 500x, (f) sample designation [[gamma].sub.2], 1,000x, (g) sample designation [[delta].sub.2], 500x, (h) sample designation [[delta].sub.2], 1.000x.

Caption: FIG. 10. Cell size distribution at various holding pressures; (a) sample designation [[alpha].sub.2], (b) sample designation [[beta].sub.2], (c) sample designation [[gamma].sub.2], (d) sample designation [[delta].sub.2].

Caption: FIG. 11. Effect of holding pressure on average cell size and cell density.

Caption: FIG. 12. Pressure versus time profile for effect of holding time.

Caption: FIG. 13. Effect of holding time on weight gain and expansion ratio.

Caption: FIG. 14. Effect of holding time on microstructure; (a) sample designation [[alpha].sub.3], 500x, (b) sample designation [[alpha].sub.3], 1.000x, (c) sample designation [[beta].sub.3], 500x, (d) sample designation [[beta].sub.3], 1.000x, (e) sample designation [[gamma].sub.3], 500x, (f) sample designation [[gamma].sub.3], 1.000x, (g) sample designation [[delta].sub.3], 500x, (h) sample designation [[delta].sub.3], 1.000x.

Caption: FIG. 15. Cell size distribution at various holding time; (a) sample designation [[alpha].sub.3], (b) sample designation [[beta].sub.3], (c) sample designation [[gamma].sub.3], and (d) sample designation [[delta].sub.3].

Caption: FIG. 16. Effect of holding time on average cell size and cell density.

Caption: FIG. 17. Pressure versus time profile for effect of variable saturation time.

Caption: FIG. 18. Effect of holding steps with variable saturation pressure on weight gain and expansion ratio.

Caption: FIG. 19. Effect of holding steps with variable saturation pressure on microstructure; (a) sample designation [[alpha].sub.4], 500x, (b) sample designation [[alpha].sub.4], 1,000x, (c) sample designation [[beta].sub.4], 500x, (d) sample designation [[beta].sub.4], 1,000x, (e) sample designation [[gamma].sub.4], 500x, (f) sample designation [[gamma].sub.4], 1,000x, (g) sample designation [[delta].sub.4], 500x, (h) sample designation [[delta].sub.4], l,000x.

Caption: FIG. 20. Cell size distribution at various holding steps with variable saturation pressure; (a) sample designation [[alpha].sub.4], (b) sample designation [[beta].sub.4], (c) sample designation [[gamma].sub.4], (d) sample designation [[delta].sub.4].

Caption: FIG. 21. Effect of holding steps with variable saturation pressure on average cell size and cell density.

Caption: FIG. 22. Pressure versus time profile for effect of constant saturation pressure.

Caption: FIG. 23. Effect of holding steps with constant saturation pressure on weight gain and expansion ratio.

Caption: FIG. 24. Effect of holding steps with constant saturation pressure on microstructure; (a) sample designation [[alpha].sub.5], 500x, (b) sample designation [[alpha].sub.5], 1.000x, (c) sample designation [[beta].sub.2], 500x, (d) sample designation [[beta].sub.5], 1.000x, (e) sample designation [[gamma].sub.5], 500x, (f) sample designation [[gamma].sub.5], 1.000x, (g) sample designation [[delta].sub.5], 500x, (h) sample designation [[delta].sub.5], 1.000x.

Caption: FIG. 25. Cell size distribution at various holding steps with constant saturation pressure; (a) sample designation [[alpha].sub.5], (b) sample designation [[beta].sub.5], (c) sample designation [[gamma].sub.5], (d) sample designation [[delta].sub.5].

Caption: FIG. 26. Effect of holding steps with constant saturation pressure on average cell size and cell density.
TABLE 1. Literature review on manufacturing of
microcellular bimodal foam microstructure.

SL.       Author        Base      Study parameters
No        (year)       polymer

1        Arora et        PS         Depressurization rate and
        al. (1998)                           profile

2         Lee et         PS          Die temperature, silica
        al. (2009)                  content, and blowing agent

3        Jiang et      PP/LDPE         Effect of blend and
        al. (2009)                         morphologies

4        Zhang et        PS           Effect of particulates
        al. (2011)

5       Tang et al.    PP/PLA        Foaming temperature and
          (2011)                       pressure drop rate.

6        Kohlhoff     PMMA /PS    Rheology, crosslinking agent,
          et al.                     rate of depressurization

          (2011)

7        D C Li et       iPP              Non-isothermal
        al. (2011)                   crystallization behavior

8       Bao et al.       PS          Saturation temperature,
          (2011)                      Degree & Rate of dep.

9       Salerno et    PCL, PCL           Thermal history,
        al. (2011)     with HA       depressurization profile

10      Salerno et     PCL/TZ        Compression and in vitro
        al. (2011)      & HA                 behavior

11       Zhang et        PS           Thermal and mechanical
        al. (2012)                         performance

12      Salerno et     PCL and     Thermal characterization and
        al. (2013)     PCL-HA           process parameters

13        Duan et     EVAJ SA/      Morphologies and physical
        al. (2014)      OMMT                properties

14        Bao et         PC       Tensile and impact properties
        al. (2014)

15       Ma et al.       PC          Foaming temperature and
          (2014)                        tensile properties

16       Sierra et       PCL        Depressurization rate and
        al. (2014)                           profiles

17       Gandhi et       ABS          Ultrasonic excitations
        al. (2014)

18       Ma et al.       PC           Mechanical, dielectric
          (2015)                            properties

19       Huang et        PLA       Multiple soaking temperature
        al. (2015)                          mechanism

20      Peng Yu et    PLA, PBS       Viscoelastic properties,
        al. (2015)                       foaming behavior

21        Wang et       PP/PS       Pressure drop, nucleating
        al. (2016)                            agent

22       LQ Xu et        PS         Variable temperature mode
        al. (2016)

23        Wang et     PS, PETG       Morphology and rheology
        al. (2016)

24       Nistor et       PS            Bulk porosity, cell
        al. (2017)                      size distribution

25        Wang et       HDPE,          Torque, rheological
        al. (2017)    DCP, CNT              properties

26      Cong Li et       PS           Holding stage, process
        al. (2018)                          parameter

21        Chen et        PCL        Holding time, intermediate
        al. (2019)                           pressure

28       Bernardo       PMMA/         Compression, fracture
          et al.      sepiolite              behavior
          (2019)

29       Bernardo     PMMA/MAM        Model of heat transfer
          et al.
          (2019)

30       Gandhi et       ABS          Initial, intermediate
        al. (2019)                  pressure, and holding time

SL.     Method used        Outcome of study                  Ref
No

1           Two-step         Bimodal cellular foams have     [13]
        depressurization      been produced by reducing
                               the pressure in stages

2       Co-blowing agent    The bicellular structure can     [1]
                            be controlled by changing the
                           ratio of n-butane/water/silica
                             content and die temperature

3       Polymer blending    Interconnected cell structure    [5]
                           generated in the co-continuous
                                     50/50 blend

4       Co-blowing agent       Exhibits a bimodal cell       [2]
                              structure when two carbon
                              particles and two blowing
                                     agents were

5       Polymer blending     PP foams exhibited bimodal      [6]
                               cell structure at lower
                                  temperatures (93
                                 and 94.5[degrees]C)

6       Polymer blending   Variance in elasticity delayed    [7]
                               nucleation in PMMA that
                               leads to developing the
                              bimodal cell distribution

7         Cooling and         Over saturated C[O.sub.2]      [26]
        depressurization   would accelerate the growth of
                           existing bubbles and generates
                                  new small bubbles

8           Two-step          The holding stage plays a      [14]
        depressurization     crucial role in controlling
                             the bimodal cell structure.

9           Two-step        Intermediate pressure plays a    [16]
        depressurization         key role to develop
                            homogeneous bimodal scaffolds

10       Polymer blend         Develop bimodal porous        [11]
                                 scaffolds for bone
                              regeneration applications

11      Co-blowing agent        Superior compressive         [3]
                            properties, marginal thermal
                               insulation performance

12          Two-step            Highly interconnected        [17]
        depressurization      porous bimodal scaffolds
                            developed for TE applications

13      Polymer blending       Effect of heterogeneous       [8]
                             nucleation agent (OMMT) and
                             microscopic dispersion (SA)
                              fabricates bimodal cells

14         Two-step             For better mechanical        [15]
        depressurization    properties volume fraction of
                             large cells 25-32%, larger
                             cell size should be smaller
                                    than [micro]m

15          Two-step         Bimodal foams have superior     [18]
        depressurization       tensile properties than
                                   unimodal foams

16       Several steps          Obtained hierarchical        [25]
                             structure with two types of
                                      cell size

17         Ultrasound          At sonication frequency       [30)
           assistance       45 KHz, bimodal microcellular
                              morphology was developed

18          Two-step       Bimodal foams showed elevated     [19]
        depressurization       dynamic mechanical and
                               compressive properties

19          MST and         Soaking time and temperatures    [29]
        depressurization      play a vital role in the
                           formation of bimodal structure

20          Two-step        Bimodal cell structure could     [20]
        depressurization      be up to 97% opening rate

21      Polymer blending    The large difference in cell     [9]
                               nucleation able to form
                               bimodal cell structures

22          VTM and           Varying temperature modes      [28]
        depressurization      develop bimodal cellular
                                     structures

23          Two-step          Degree of the first-step       [21]
        depressurization    depressurization and holding
                           time controls the morphologies
                                       of CCS

24      Co-blowing agent    nC5, cC5 agents increase foam    [4]
                              porosity and can induce a
                                 bi-modal cell size
                                    distribution

25        Cooling and           The developed bimodal        [27]
            two-step         structure obtained with an
        depressurization        expansion ratio of 9

26          Two-step          Increase in intermediate       [22]
        depressurization    pressure enhances the density
                                  of smaller cells

21          Two-step           Elongating holding time       [23]
        depressurization       increased mean size and
                           decreased the density of large
                                        pores

28       Polymer blend      Mechanical properties of PMMA    [12]
                                foams with a bimodal
                            nanometric cells (300-500 nm)
                                  are more complex

29       Polymer blend         The conductivity of the       [10]
                            bimodal system depends on the
                            volume fraction of nanometric
                                        cells

30          Two-step          Intermediate pressure has      [24]
        depressurization         significant role in
                             controlling the cell sizes

TABLE 2. Designation of samples for significance of
saturation pressure.

S. no        Sample        [P.sub.1]   [H.sub.1]
           designation       (bar)        (h)

1        [[alpha].sub.1]      20          20
2        [[beta].sub.1]       30          20
3        [[gamma].sub.1]      40          20
4        [[delta].sub.1]      50          20

S. no    [P.sub.1]   [H.sub.2]
           (bar)        (h)

1           10          04
2           10          04
3           10          04
4           10          04

TABLE 3. Designation of samples for influence of holding pressure.

S. no        Sample        [P.sub.1]   [H.sub.1]
           designation       (bar)        (h)

1        [[alpha].sub.1]      50          20
2        [[beta].sub.1]       50          20
3        [[gamma].sub.1]      50          20
4        [[delta].sub.1]      50          20

S. no    [P.sub.1]   [H.sub.2]
           (bar)        (h)

1           10          04
2           20          04
3           30          04
4           40          04

TABLE 4. Designation of samples for influence of holding time.

S. no        Sample        [P.sub.1]   [H.sub.1]
           designation       (bar)        (h)

1        [[alpha].sub.1]      50          20
2        [[beta].sub.1]       50          16
3        [[gamma].sub.1]      50          12
4        [[delta].sub.1]      50          08

S. no    [P.sub.1]   [H.sub.2]
           (bar)        (h)

1           20          04
2           20          OS
3           20          12
4           20          16

TABLE 5. Designation of samples for multiple holding steps
at variable saturation time.

S.      Designation     [P.sub.1]   [H.sub.1]   [P.sub.2]   [H.sub.2]
no                        (bar)        (h)        (bar)        (h)

1     [[alpha].sub.4]      40          20          --          -2
[[beta].sub.4]       40          15          30           5
3     [[gamma].sub.4]      40          10          30           5
4     [[delta].sub.4]      40                      30           5

S.    [P.sub.3]   [H.sub.3]   [P.sub.4]   [H.sub.4]
no      (bar)        (h)        (bar)        (h)

1        --          --           _          -2
--          --          --          -3
20           5          --          -4
20           5          10           5

TABLE 6. Designation of samples for multiple holding steps
at constant saturation time.

S.      Designation     P1 (bar)   H1 (h)   P2 (bar)   H2 (h)
no

1     [[alpha].sub.4]      40        20        --        -2
[[beta].sub.4]       40        20        30        5
3     [[gamma].sub.4]      40        20        30        5
4     [[delta].sub.4]      40                  30        5

S.    P3 (bar)   H3 (h)   P4 (bar)   H4 (h)
no

1        --        --        --        -2
--        --        --        -3
20        5         --        -4
20        5         10        5
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Author:G, Radhakrishna; Dugad, Rupesh; Gandhi, Abhishek
Publication:Polymer Engineering and Science
Date:Jan 1, 2020
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