1 Introduction

Microcellular foams form a category of foamed plastics characterized by cells of the order of 10 mm in size. Because of their novel properties,microcellular polymers have attracted great scientific interest and potential applications over the last two decades. Since the first attempt to produce microcellular materials using the solid-state batch process^[1],numerous experiments have been done in order to prepare^{[2, 3, 4, 5, 6, 7, 8, 9, 10, 11]} and characterize microcellular polymers in terms of mechanical properties such as tensile properties^{[12, 13, 14]},fracture toughness^{[13, 15, 16, 17]}, fatigue life^[18],and creep behavior^[19].

It has been proved by various experiments that nanoparticles have a high potential to produce microcellular polymeric foams with high cell density and improved mechanical properties. Among them,nanoclay particles are most commonly used in producing thermoplastic microcellular nanocomposite foams^{[20, 21, 22, 23, 24, 25, 26, 27, 28, 29]}.

Superior strength,stiffness,and toughness properties have earned acrylonitrile-butadienestyrene (ABS) a place as one of the most common engineering plastics. Despite its wide potential applications in diverse industries,ABS/clay nanocomposite has not yet been used for making microcellular foams.

Thus,the present study is conducted to produce microcellular ABS/clay nanocomposites by the solid-state batch foaming method. The objective is to investigate the effects of foaming parameters on the density,morphology,and flexural properties of the ABS and its nanocomposite foams. 2 Experimental study 2.1 Materials

General purpose ABS,SD-150 is obtained from Samsung Cheil Industries. Organically modified clay with methyl tallow bis-2-hydroxyethyl quaternary ammonium (cloisite 30B,southern clay products) is used to fabricate nanocomposites. Commercial grade carbon dioxide with the purity of 99.9% is used as the blowing agent. 2.2 Preparation of ABS/clay nanocomposites

ABS granules and nanoclay are dried at 80 ◦C for 24 h in a vacuum oven. ABS/clay nanocomposites are prepared using a co-rotating twin screw extruder,ZSK-25 with L/D = 40. The screw rotating speed is 60 r/min,and the barrel temperature ranges from 190 ◦C to 220 ◦C. Nanocomposite granules are then compression molded with a hydraulic hot press machine for 10 min at 215 ◦C using 15 MPa pressure. Flexural test specimens are prepared according to the standard test method,i.e.,ASTM D790. 2.3 Microcellular foaming

The compression-molded samples are dried at 80 ◦C for 24 h in a vacuum oven before foaming. The solid-state batch process is used for microcellular foaming of samples. In the first step,the samples are saturated in a pressurized CO2 chamber at the room temperature for 24 h. Then,they are immersed in a hot glycerin bath with stable temperature for 60 seconds. Different saturation pressures and foaming temperatures are used to control the density of foamed samples. The foamed samples are immediately quenched in alcohol bath to prevent cell deterioration. Various microcellular specimens are produced by controlling the process conditions including saturation pressure and foaming temperature. Table 1 lists the name of foamed samples for each foaming condition.

2.4 Microstructural characterization An A&D precision balance is utilized to determine bulk densities of the foamed samples based on the Archimedes’ buoyancy principle. To determine the cellular morphology of foamed samples,they are first dipped in the liquid nitrogen and then fractured to expose their cellular morphology. The fracture surfaces are then coated with gold vapor and scanned using a KYKYEM3200 scanning electron microscope (SEM). About 100 cells are used to evaluate cell size distribution for each sample. Since the probability of existence for cell with vanished dimension is zero,a log-normal probability distribution function (P_DF) is used to formulate the cell size distribution as follows: where D represents the cell diameter,and and indicate the mean value and standard deviation,respectively. 2.5 Mechanical testing

Three-point flexural tests are carried out on an instron testing machine with a 10 kN load cell and the crosshead speed of 5 mm/min at the room temperature. Three specimens are tested for each condition,and the average values are taken. The strain is measured based on the change in the displacement of the crosshead and dimensions of specimens. The flexural modulus is obtained from the steepest initial straight-line portion of the stress-strain curve. 3 Results

Figures 1 and 2 show the scanning electron micrographs of foamed ABS and its nanocomposites,respectively. In Fig. 2(a),no cell is created through the foaming process at the least pressure-and-temperature. This matter is related to the role of nanoclay particles in reducing the rate of gas diffusion in polymer as well as increasing its glass transition temperature.

According to Fig. 2(f),the nanocomposite sample foamed at the highest pressure and temperature,despite having a microcellular structure that contains large cavities which make it impossible to report a true value for its cell density (the number of bubbles that nucleated in each cubic centimetre of unfoamed specimen) and size distribution.

As expected,the integral solid (unfoamed) skins are created on both sides of all foamed samples due to escaping of the nucleating gas from the surface of saturated specimens before foaming. The solid skins of unfilled and nanocomposite samples can be distinguished in the scanning electron micrographs shown in Figs. 3 and 4,respectively.

The physical characteristics of foamed samples,including the total thickness of sample,the solid skin thickness,the bulk density,and the foam-core density,are evaluated in Table 2. Assuming a uniform thickness for the solid skin and a symmetric structure for the foamed samples,the mass density of foam core ρf is estimated based on the following equation:

where h,hs,ρ,and ρp are the total thickness,the solid skin thickness,the bulk density of the foamed specimen,and the bulk density of the unfoamed specimen,respectively.

The effect of the saturation pressure and the foaming temperature on the cell density of unreinforced and nanocomposite foamed samples is illustrated in Fig. 5. It can be understood that the nanocomposite samples have higher cell density in comparison with the neat samples foamed at the same conditions. This matter shows that the nanoparticles facilitate the almost instantaneous growth of cells by reducing their nucleation energy.

As observed,the largest cell density in polymer foams corresponds to the pressure of 6 MPa and the temperature of 60 ◦C,whereas the pressure of 5 MPa and the temperature of 60 ◦C yield the largest cell density for nanocomposite microcellular foams.

Figures 6(a) and 6(b) depict the contour plots of the relative density (the density of foam divided by the density of its matrix) as a function of the saturation pressure and the foaming temperature. As observed,the batch process can produce microcellular ABS foams with a wide range of relative densities. These figures illustrate that both the saturation pressure and the foaming temperature have direct effect on the expansion ratio of microcellular foams.

The probability distributions as well as the mean values (dmv) and the standard deviations (dsd) of cells diameters for unreinforced and nanocomposite foamed samples are shown in Figs. 7 and 8,respectively. As observed,the foams with nanocomposite matrix have a smaller mean cell size than those with neat ABS matrix. Also it is concluded that increasing temperature leads to increment of the cells’ mean size and standard deviation. Such effect is more apparent in nanocomposite foams.

The stress-strain curves in Figs. 9(a) and 9(b) show the flexural responses of unreinforced and nanocomposite foamed samples,respectively. Subsequently,Figs. 10(a) and 10(b) plot the variations of flexural modulus and flexural strength in terms of the relative density for neat polymeric microcellular samples as well as nanocomposite ones. As observed,all nanocomposite microcellular samples have more flexural moduli than unreinforced microcellular samples, except S12 which contains large cavities due to high foaming temperature. It is obvious that the relative density of microcellular foams has a direct impact on their flexural modulus and flexural strength. Although the flexural modulus of unreinforced samples is improved in the presence of nanoparticles,their strength is not significantly changed in comparison with the nanocomposite ones. By dividing the flexural modulus of each sample by its mass density,it is found that S10 has the maximum specific flexural modulus.

4 Conclusions

Microcellular ABS samples are prepared by the batch process at different pressure and temperature conditions. The effects of the nanoclay and foaming conditions on the foam morphology and its flexural modulus are investigated. The microstructural images observed by the SEM show that the saturation pressure and the foaming temperature are the main parameters controlling the foam density. It is observed that the nanoparticles can improve the quality of cellular structure at lower foaming temperatures by reducing the cell nucleation energy. The optimum conditions for achieving microcellular uniform foams with the largest cell density are found to be 6 MPa and 60 ◦C for neat ABS samples and 5 MPa and 60 ◦C for nanocomposite ones. The mechanical tests show that the flexural modulus of microcellular foams depends directly on their relative density. Also,S10 is found to have the maximum specific flexural modulus among foamed samples.