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Foaming Behaviour and Compressive Properties of Microcellular Nanostructured Polystyrene

Foaming Behaviour and Compressive Properties of Microcellular Nanostructured Polystyrene Jose Antonio Reglero Ruiz1*, Philippe Viot2 and Michel Dumon1* 1

Laboratoire de Chimie de Polymères Organiques (LCPO), IPB. 16 Avenue Pey-Berland, Université Bordeaux 1, 33607 Pessac, Cedex, France 2 Laboratoire Matériaux Endommagement Fiabilité (LAMEFIP), ENSAM. Esplanade de Art et Métiers 33405 Talence, Cedex, France Received: 26 October 2009 Accepted: 24 November 2009

ABSTRACT A batch foaming process has been employed to obtain microcellular materials from polystyrene plus a SBM copolymer (polystyrene-co-1,4-polybutadiene-copoly(methyl methacrylate). In the first part of the process, raw materials were mixed and extruded in a proportion 90:10 to obtain the precursor materials, leading to a nanostructured assembly in which SBM self-organizes in the polystyrene matrix. In a second stage, foaming was carried out by means of supercritical CO2 in a single step-process. Foamed samples were produced using a technique based on the saturation of the polymer under scCO2, and final properties were controlled by varying the temperature. The swelling in scCO2 was performed at 300 bar during 16 h, and subsequently releasing the gas with a fixed depressurization rate of 60 bar/min. Temperature was varied from 30 °C to 80 °C, leading to densities from 1.0 g/cm3 to 0.5 g/cm3 and cell sizes from 2 micron to 100 micron. In this part of the work, a comparison between foaming behaviour of neat PS and a nanostructured PS+SBM blend is reported, investigating the role of the nanostructured phase as nucleating agents for microcellular foaming. Finally, low rate compression tests were carried out, analyzing the dependence of mechanical parameters such as elastic modulus, yield stress and densification strain with density.

1. INTRODUCTION During the last few years, the use of supercritical carbon dioxide as a medium for polymer synthesis and for polymer processing has increased greatly(1). * Corresponding authors : Tel: +33(0)540008486; Fax: +33(0)540008487; Email address: [email protected] ©

Smithers Rapra Technology, 2009

Cellular Polymers, Vol. 28, No. 6, 2009

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Jose Antonio Reglero Ruiz, Philippe Viot and Michel Dumon

The unique properties associated with supercritical fluids, and especially with supercritical CO2, due to its chemical, environmental and economical advantages, have been explored in fields such like organic synthesis, catalysis and polymer science(2,3). In this work, we present the use of supercritical carbon dioxide as a foaming agent for non-reactive processing of microcellular polymers. Cellular materials can be classified in terms of the average cell size. In this context, cellular materials have and average cell size in the range of 100 μm up to several millimetres, whereas microcellular materials present average cell sizes in the range of 1-10 μm, with cell densities on the order of 109-1015 cells/ cm3, and it is well known that inert gases such as CO2 can be used to foam amorphous polymeric materials in the microcellular range(4,5). There are two well-known processes to produce microcellular materials using supercritical CO2. In the first process, denominated single-step process, the polymer is saturated with CO2 in the supercritical regime, during a fixed time and temperature. After saturation, the sample is depressurised to atmospheric pressure at a fixed depressurization rate, taking advantage of the swelling and plasticization of the polymer, which reduces the glass transition temperature, allowing the gas expansion. In the second process, a two-step process, the polymer is saturated with the supercritical CO2 at high pressure and low temperature. Next, the polymer/gas mixture is quenched into a supersaturated state by reducing drastically the pressure to atmospheric pressure. Finally, after removing the sample from the vessel, polymer is foamed by heating to a temperature above the glass transition temperature in a hot bath, leading to nucleation and cell growth. A brief literature review shows a great number of investigations related to microcellular polystyrene. For example, the addition of organic nanoclays and nano CaCO3 has been employed to produce microcellular injected polystyrene in industrial applications (continuous process)(6,7,8). The effect of processing parameters on polystyrene cell morphology, such as saturation pressure, has been studied by Gao et al.(9), whereas low density open cell microcellular foams, based on polystyrene, have been also investigated deeply by Aubert et al.(10). Regarding the microcellular batch processes, a great number of works have been presented recently investigating the foaming behaviour of polystyrene. For example, Reverchon et al.(11) present a study of the effect of processing parameters in the cellular structure of PS, varying foaming temperatures from 55 to 125 °C and keeping saturation pressure to 230 bar a with constant depressurization time of 20 s, in a two-step batch process. A wide investigation is carried out 364



to analyse the effects on the microstructure, but few mechanical properties are reported. Arora et al.(12), present an investigation of the compression properties of microcellular polystyrene foams, produced in one step process, at 100 °C, at a similar density of 0.3 g/cm3, varying saturation pressure. An exhaustive investigation was also presented by Lesser et al.(13), analyzing the effect of process parameters on microstructure, in a one step process varying foaming temperature, saturation pressure and depressurization rate. Density of samples produced could be varied between 1.0 g/cm3 to 0.6 g/cm3. In a similar way, Tsivintzelis et al.(14) investigate the effect on microstructure of several processing parameters, such as foaming temperature, foaming pressure and depressurization rate, in a one step process, but there is a lack of investigation in the mechanical properties of the materials. Finally, it is important to remark that other lines of investigation involving the use of polystyrene have been carried out to produce microcellular materials. For example, Otsuka et al.(15) present the foaming of thin films of PS-PMMA copolymers, obtaining nanocellular structures by chemical removing of PMMA, but not mechanical properties are considered. A one-step process is employed, with temperatures of 20 °C or 40 °C, and depressurization rates of 10 bar/min or 50 bar/min. To conclude, Jacobs et al.(16) showed a parametric study of microcellular foams based on Styrene-Methyl-Methacrylate copolymers (SMMA), with a morphology analysis, but without studying of mechanical properties. Investigation was carried out following a one step process, varying the foaming temperature, with low density foams, from 300 kg/m3 to 50 kg/m3. In conclusion, the number of investigations and publications concerning the study of microcellular polystyrene has increased greatly in the previous years; however, there is a lack of research in the study of mechanical characteristics, especially compression properties, in microcellular polystyrene samples with different densities. In our work, a new approach to elaborate foams is presented. Firstly, the use of a nanostructured copolymer (SBM) as an additive is presented, comparing the foaming behaviour of the obtained blend to neat PS and demonstrating its potential use as a nucleating agent to improve the foaming process. And secondly, a complete study of the compression mechanical properties of the microcellular foams with different densities is showed, analysing the relation between the microstructure obtained and several mechanical parameters.


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2. EXPERIMENTAL 2.1 Materials Polystyrene was kindly supplied by Total Petrochemicals (Courbevoie, France) in the form of pellets. Triblock copolymer SBM (polystyrene-co-1,4polybutadiene-co-poly(methyl methacrylate) was supplied by Arkema (Lacq, France) in the form of pellets. Fabrication and properties of SBM can be found elsewhere(17,18,19). Both materials were dried in vacuum (680 mm Hg), at 80 °C during 4 hours before processing. Mixing and extrusion were carried out using a Scamex CE02 extruder, with a temperature profile from 165 °C to 225 °C. All the samples were fabricated containing 10% wt. of SBM and 90% wt. of polystyrene. On the other hand, carbon dioxide (99.9%) was obtained from Air Liquide (Cergy Pontoise, France). Main characteristics of the blend were as follows: bulk density ρs of 1.05 g/cm3, Tg was about 98 °C (measured in a DSCQ100 equipment, using a heating cycle from -50 °C to 200 °C at 5 °C/min). Due to the low content of SBM triblock copolymer, (10% wt.) the glass transition temperature of the SBM was undetectable. In addition, Mn was 30000 and Mw was 35000 g/mol, determined in a PL-GPC 50 Plus device using THF (Tetrahydrofuran) as a solvent with a concentration of 3 mg/ml with UV detection at 254 nm. Figure 1 shows a TEM image of the nanostructured sample fabricated, using a Jeol 2000 fx equipment, with 118 μmA of intensity and 10-6 tor of vacuum. The sample was treated with OsO4 to perform the observation. In the (a)

(b)

Figure 1. TEM images of the PS+SBM (90:10) blend, showing the nanostructured SBM copolymer embedded in the polystyrene matrix. Bar scales 100 nm (Figure 1a), 20 nm (Figure 1b)

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nanostructuration, particles of 20 nm are dispersed in the PS matrix, with a morphology consisting of a core of PMMA surrounded by a shell of PB. 2.2 CO2 Absorption and Depression of the Glass Transition Temperature It is well known that as gases solve into polymers matrices, many properties are changed. One of these properties is glass transition temperature. In general, dissolving gases into polymer matrices leads to a plasticization of the material, lowering the glass transition temperature. For a thermodynamic point of view, there are two factors which determine the good interaction between the plasticizer and the polymer: first, the intermolecular forces between the polymer and the plasticizer, and second, the molecular size of the diluent. As a first approximation, the Tg depression is a function of the gas concentration in the polymer. When the diluent concentration increases, glass transition temperature decreases. This occurs because the interchain distance, the mobility of the polymer segments and so the free volume of the system increase as well the plasticizer concentration. There are other theoretical interpretations that take into account non-dimensional parameters, such as number of lattice sites, monomer molecular weight and transition isobaric heat capacity increment of the polymer. This is why a complete knowledge of the influence of the diluent in the Tg, which results from the sorption of the supercritical CO2 at elevated pressures, is essential for determining the optimal conditions for the microcellular foaming processing, especially foaming temperature. There are a wide number of investigations related to the plasticization effect in polymers. Kweeder et al.(20) presented a study of nucleation processes in microcellular polystyrene foam, varying different parameters such as saturation pressure and temperature. Determination of glass transition behaviour with compressed fluid diluents has been presented by several authors. For example, Condo et al.(21) showed the first experimental data of glass transition behaviour with compressed fluid diluents in several polymers, employing theoretical models under thermodynamical assumptions to predict the glass transition depression in a wide range of polymers. Zhang et al.(22,23) studied the glass transition temperature of several gas-polymer systems, employing a high pressure DSC up to 100 bar. Focusing on polystyrene, Alessi et al.(24) presented a study of the plasticisation effect in polystyrene by high pressure partition chromatography, comparing the Cellular Polymers, Vol. 28, No. 6, 2009

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experimental results with data available in the literature. Results showed how Tg is reduced 40 °C at saturation pressures of 80 bar. In a similar way, Hilic et al.(25) described a study of the solubility of nitrogen and carbon dioxide in polystyrene and the associated polymer swelling, by measuring the change of the volume in the polymer due to the absorption process, leading to analogous results for the glass transition depression. Theoretical models to predict the depression of the glass transition temperature has been proposed. First approximation was given by Chow(26), which is based on a molecular approach. To correlate the model with some experimental data, a complete study of the polystyrene plasticization under different diluents was presented. The expression proposed by Chow is: T ln g = {(1 ) ln (1 ) + ln } Tg0

(1)

where, =

Mp

w zR ; = zM d 1 w M p Cp

(2)

In previous equation, θ and β are non-dimensional parameters, whereas Tg is the glass transition temperature of the polymer/gas system, Tg0 is the glass transition temperature of pure polymer, Mp is the average molecular weight of the polymer, Md is the average molecular weight of the diluent, ΔCp is the excess transition isobaric specific heat of the polymer, z is the lattice coordination number, w is the weight fraction of the diluent and R is the gas constant. However, equation (1) was not stated for predicting the glass transition temperature for a microcellular foaming process, although, since there was no specific and appropriate formula for predicting this phenomena, this equation has been employed for this purpose in our investigations. More recently, another expression has been developed by Hwang et al.(27). In his investigation, the relationship between gas absorption and the glass transition temperature in a batch microcellular process is analysed, measuring the change in the elasticity modulus induced near the glass transition temperature. Hwang compared experimental data with Chow’s model, proposing a new expression as a function of weight gain ratio during the saturation process: 1/3 1/4 Tg = Tg0 exp ( M p ) () w

368

(3)



where α is a material constant determined by both polymer and gas and ρ is the specific density of the polymer. The rest of the parameters are defined previously. Predictions of both models can be easily calculated, assuming different lattice coordination numbers (z=1, z=2) and considering α=0.7, in a PS-CO2 system(27). The rest of the experimental parameters employed in the equations have been determined experimentally: Tg0=371 K is measured from a DSC thermogram, ΔCp=0.15 J/g K is also determined from the DSC curve, the specific gravity of the polymer ρ=1.05 g/cm3 is measured using a high precision balance, molecular weight of the polymer Mp=30000 g/mol (chosen as Mw), and finally molecular weight of diluent is Md = 44 g/mol. Figure 2 shows the glass transition depression as a function of weight fraction of carbon dioxide, for both theoretical expressions. As it can be seen in Figure 2, predictions of both models differ greatly, especially at weight fractions of CO2 above 3%. Chow’s model predictions depends on the values for coordination number z, predicting rather a low plasticization effect for z=1. However, no great differences are appreciable between predictions with different coordination numbers. For a given fixed weight ratio, (e.g. w=11%, which will be the measured gas absorption of our PS system, see results section), Hwang model predicts a depressed value of Tg1 of 35 °C. On the other hand, Chow’s model predictions depend on coordination

Figure 2. Theoretical predictions of the Tg depression due to plasticization effect in our PS


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number z. For z=2, a predicted value of Tg2=65 °C is given by Figure 2, and for z=1, the depressed Tg3 is 75 °C. Taking into account these theoretical predictions, it is possible to estimate the depressed glass transition temperature in our experiments, by measuring the weight fraction of carbon dioxide at a given pressure, and to determine which model presents a better agreement according to the experimental results. 2.3 Foaming Set-up and Characterization 2.3.1 Sample Preparation Pellets were injected into bars (50x15 mm2) with 3 mm thickness, using a small scale injection moulding machine developed by DSM Xplore. The working temperature was fixed at 250 °C, whereas mould temperature was 60 °C. The injection pressure was fixed at 12 bar. Nearly transparent bulk samples were obtained showing a good surface appearance with no presence of air bubbles inside the bar. 2.3.2 Microcellular Foaming Gas saturation was carried out in a high pressure reactor provided by TOP industry, with a capacity of 300 cm3 and able to operate at maximum temperature of 250 °C and 400 bar. The reactor was equipped with a pressure pump controller provided by Teledyne ISCO, and controlled automatically to keep the temperature and pressure at the desired values. Microcellular foaming was induced in the vessel by a depressurization at a fixed rate of 60 bar/min. 2.3.3 Density Measurements Foam density ρf was determined by water-displacement method, based on Archimedes principle. It is important to notice that due to the closed cell structure of the foam samples, there was not uptake of water by the samples during measurements. At least three measurements were carried out for each sample produced. 2.3.4 SEM Observations Cellular structure was analysed by means of scanning electron microscopy (model HITACHI S-3000N). For the preparation of the samples, foams were frozen in liquid nitrogen and fractured to assure that the microstructure was not plastically damaged. For these observations, surfaces were coated with gold using a sputter coater (model EMSCOPE SC 500), in argon atmosphere. The 370



micrographs obtained were analyzed to calculate cell sizes and to detect the presence of the solid outer skin. An estimation of the cell size φ was obtained from direct observations, using a minimum of 100 cells in each calculation(23), whereas cell density NC (cells/cm3) was calculated from the equation (4): 6 1 f s N C 10 3 3

( )

(4)

2.3.5 Compression Tests To perform compression tests, cylindrical samples of 9 mm diameter were machined from the foamed bars. The thickness of the samples varied with foam density in the range [3.5-5] mm. All the compression tests were carried out at room temperature using a universal test machine (Zwick, model Z250) with a load cell of 10KN at a constant crosshead velocity of 0.5 mm/min. At least, three samples of each batch were tested reporting the average data in the present work. Raw data obtained was force and displacement, selecting the test parameters in order to reach 55% of total deformation of the sample in each test. Main mechanical parameters, Young modulus, yield stress, densification strain and density of energy absorbed were determined. A typical stress-strain curve from a cellular material is presented in Figure 3.

Figure 3. Typical stress-strain curves obtained in compression mechanical tests for closed cell cellular materials showing the three main different regimes, vertical bold black line stands for the upper limit of calculation of W50


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As it can be seen, the typical mechanical behaviour of a cellular material includes the initial linear zone, between ε0 and ε1, the plateau regime, between ε1 and ε2, in which deformation occurs with no significant increase of stress and finally the densification zone, from ε2 to the end of the test, where cells begin to collapse. W50 is the energy density absorbed during the compression test up to 50% strain.

3. RESULTS AND DISCUSSION 3.1 Absorption of CO2 into PS+SBM Blends The absorption kinetics of CO2 in PS+SBM system at the given conditions (300 bar, 40 °C) was measured experimentally. Experimental data was obtained removing the samples and immediately transferring them to a high precision balance to record the increase in weight due to gas absorption. It is known that the mass uptake of gases into polymers, as a function of time28, is described as follows: M t / M = 4 ( D / )

0.5

(t

0.5

/ L)

(5)

where Mt / M∞ is the relation of the total amount of gas that has diffused into the polymer at a time t divided by the total amount of gas diffused at a infinite time, D represents the diffusivity of the gas into the polymer, measured in cm2/s, and finally L represents the total thickness of the sample. A simple representation of Mt as a function of (t0.5/L) is presented in Figure 4. After an initial zone, diffusivity D can be estimated by fitting the linear part of the curve to a straight line with slope 4(D/π)0.5. In the last part of the curve, saturation is reached and the maximum quantity of gas has been absorbed by the polymer. In our case, saturation time is reached at about 10 h, under 300b of gas pressure, for samples with 3 mm thickness, m≈3.2 g. From the data of the presented Figure 4, a diffusivity value of D ≈ 2,183×10-6 cm2/s was calculated. However, it is important to notice that this one-dimensional diffusion is calculated assuming that thickness is much more smaller that other dimensions, and must be considered as an estimation, as D is a function of gas concentration and it changes during the experiment. Moreover, to assure the maximum quantity of gas absorbed in the PS+SBM, an over estimated saturation time was set at 16 h, which leads to percentages of about 11% wt. CO2 absorbed in the sample (340 mg of gas absorbed considering 3.2 g of each unfoamed sample, approximately). The max% of CO2 absorbed and the diffusivity coefficient are consistent with those published in literature(21,24). It 372



Figure 4. Absorption of CO2 into PS+SBM during saturation at 300 b and 40 °C

is clear that thickness is the key parameter for selecting saturation time, and samples with lower thickness can be used to reduce the saturation time. On the other hand, the thickness of the foamed samples must be enough to perform mechanical test, assuring that a representative number of cells are compressed during the process. It is important to notice that sorption experiments were performed at different temperatures, presenting in this work only the curve corresponding to 40 °C. As expected(34), values of weight ratio and saturation time decrease slightly with temperature. However, no great differences in the maximum weight ratio of carbon dioxide were found in PS/SBM systems. Therefore also the Tg of all swollen blends lies in the same order of magnitude. On the other hand, diffusivity values D increase slightly with temperature. Table 1 presents the values of weight ratio and diffusivity D for the different experiments performed. Table 1. Parameters of absorption of CO2 into PS+SBM at different saturation temperatures (saturation pressure fixed at 300 b) Saturation Temperature (°C) 30 40 50 60 70 80

M∞ CO2 (mg)

Weight ratio (%)

Diffusivity (cm2/s)

351 346 343 341 335 321

10,96 10,81 10,71 10,65 10,46 10,03

2,183×10-6 2,234×10-6 2,148×10-6 2,212×10-6 2,265×10-6 2,320×10-6


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3.2. Foaming Experiments and Density of Materials Foaming experiments were performed in a single-step batch process. First, samples were saturated with supercritical CO2 at a maintained pressure of 300 bar during 16 h. Such a saturation time was selected in order to assure the complete dissolution of CO2 in the polymer, taking into account the sorption curve presented in figure 4. Saturation temperatures varied between 30 °C and 80 °C. After the saturation process, foaming was carried out by depressurization at a constant rate of 60 b/min. During the foaming process, due to the pressure decrease, temperature inside the reactor was reduced by nearly 10 °C in all case. This reduction is only dependant on the rate of depressurization, and higher depressurization rates lead to greater temperature reductions. Controlling this reduction is the key parameter to estimate the real foaming temperature and the plasticization state of the sample during the process. In our work, foaming temperatures were 10 °C below saturation temperature for all the experiments carried out. A total number of six experiments were performed, saturating the samples at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C and 80 °C. Samples were removed from the vessel after reaching atmospheric pressure and kept 30 min before performing the density measurements. Microcellular foams presented a white opaque colour and a good external appearance. Several characteristics were determined, such as density, cellular structure and mechanical properties. Figure 5 presents the results for the density measurements. As explained before, at least three determinations were carried out in each sample, and average values are represented, with a deviation of 5%.

Figure 5. Foam density as a function of saturation temperature and corresponding aspects of two samples (below or above their plasticized Tg) 374



From the previous data, it is clear that density decreases with saturation (/foaming) temperature, and values vary between 0.99 g/cm3, for a foaming temperature of 30 °C, and 0.48 g/cm3 in samples produced at 80 °C. At higher temperatures, cells are allowed to grow more easily, which derives in a lower density. Furthermore the difference between the foam density at nearby temperatures of 40 °C and 50 °C is remarkable, (density decreases from 0.92 g/ cm3 to 0.67 g/cm3) which is due to the effect of the depressed glass transition of the system and the transition from glass to rubbery states. To assure this assumption, it is included in Figure 5 a photograph of the samples produced at 40 °C and 50 °C, showing different aspects between both of them. It is clear that a plasticization effect occurs at temperature between 40 °C and 50 °C and sample becomes rubbery. Taking into account the experimental value of weight ratio w=11% (Table 1), and looking at the predictions of theoretical models of Cha Yoon and Hwang (Figure 2), Hwang model predicts a value of depressed Tg1 of 35 °C, whereas, Chow’s model predictions are Tg2=65 °C and Tg3=75 °C. In our work, plasticization and the value of experimental Tg are correlated to the change of foaming behaviour at a temperature around 40 °C, i.e. 10 °C below the saturation temperatures showed in Figure 5. Having a look at the theoretical models exposed previously, it can be assumed that predictions of Hwang’s model accounts for the experimental value of Tg. Therefore by comparing the evolution of foaming behaviour with temperature to the prediction of depressed glass transition of this system, this temperature can be estimated at a temperature of about 40 °C, which indicates a decrease about 50 °C with respect to glass transition temperature of the unfoamed material (Tg0 ≈ 98 °C). The systems are quenched from the same initial swollen state but allowing this system different times (growth of cells) to the glassy state. 3.3. Microstructural Characterization 3.3.1 PS+SBM Samples Several SEM micrographs showing the cellular structure of the foams are presented in Figure 6. Cell size is lower at minimum foaming temperatures (2 μm, Figure 6a), whereas increasing foaming temperatures leads to greater cell sizes, (20 μm, Figure 6b and 100 μm, Figure 6c). It is clear that increasing foaming temperature allows nucleated cells more time to develop, as it was reflected in the density values presented previously in Figure 5. Moreover, all the microstructures presented are homogeneous, which indicated that effects of coalescence can be neglected, even at the highest foaming temperatures. All the structures obtained are closed-cell. Microstructural parameters such as cell density, calculated from equation 4, average cell size and foam density are summarized in Table 2. Cellular Polymers, Vol. 28, No. 6, 2009

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In addition, Figure 6d presents a SEM micrograph showing the presence of an outer solid skin. To assure a foam-only determination of density and compression properties of the cellular structure, the skin was removed mechanically. The thickness of the closed skin was about varied between 50 μm and 200 μm in all the samples produced, but the ratio between the total sample foam thickness Table 2. Comparison between microstructural parameters of the neat PS and PS+SBM samples Average cell size Cell density Foam density (μm) (cells/cm3) (g/cm3) Saturation From From From From From From Temperature Neat PS PS+SBM Neat PS PS+SBM Neat PS PS+SBM (°C) 30 1,01 0,99 10 2 2,61×1011 1,37×1013 40 0,97 0,92 15 5 9,67×1010 1,68×1012 50 0,83 0,67 20 20 6,93×1010 8,51×1010 60 0,75 0,62 60 40 3,17×109 1,20×1010 70 0,65 0,54 80 50 1,65×109 7,31×109 80 0,54 0,48 100 60 1,02×109 1,01×109

(a)

(b)

(c)

Figure 6. SEM micrographs of the PS+SBM microcellular samples produced; a) 30 °C, bar scale 20 μm b) 50 °C, bar scale 200 μm c) 80 °C, bar scale 300 μm d) Solid outer skin, 50 °C, bar scale 200 μm

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and the thickness of the skin remains constant (about 4%), which indicates that this characteristic should be only dependant on the depressurization rate, being originated for the diffusion of CO2, which is not able to nucleate and growth on the edges. 3.3.2 Comparison of Microstructure Between neat PS Samples and PS+SBM Samples The effect of the addition of 10 wt.% SBM triblock copolymer is significant. Indeed a similar series of foaming experiments was carried out in neat polystyrene, in order to compare the microcellular structure of the sample obtained with the PS+SBM foams. Experiments were carried out in the same saturation conditions, 300b during 16 h at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C and 80 °C, with 60 bar/min of depressurization rate. Figure 7 presents three SEM micrographs of the PS cellular structure obtained, for the samples fabricated at 30 °C, 50 °C and 80 °C. Microstructure in the neat PS samples differs greatly from the microstructure observed in PS+SBM samples (Figure 6). In this case, the number of nucleated cells is much lower, as it is showed in all the micrographs presented. Moreover, at high saturation temperatures (80 °C, Figure 7c), the microstructure of the (a)

(b)

(c)

Figure 7. SEM micrographs of neat PS microcellular samples foamed at; a) 30 °C, bar scale 100 μm b) 50 °C, bar scale 50 μm c) 80 °C, bar scale 200 μm


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neat PS samples is inhomogeneous, with the presence of big cells (about 100 micron of diameter) surrounded by small cells (about 10 micron). As a conclusion, it can be said that nanostructured SBM particles do play a role as nucleating agents, increasing the number of the nucleated sites, thus leading to higher cell densities. Other foaming parameters, such as the quantity of CO2 absorbed during saturation process are also affected. In the neat PS samples, the percentage of CO2 absorbed is 6%, instead of 12% for the PS+SBM samples. For these reasons, it is clear that a low addition of SBM particles to neat PS (10% wt.), results in an improvement of the microcellular foaming process. Therefore, the overall material density is changed since the SBM does provide an addition of CO2 philic additive. Table 2 presents a comparison of the main microstructural parameters (foam density, average cell size and cell density), determined for both materials. 3.4 Mechanical Properties of PS/SBM Blends Stress-Strain curves are presented in Figure 8. Results are divided in two different groups. In Figure 8a results of samples foamed at 30 °C, 40 °C and 50 °C are presented, together with the stress-strain curve of the unfoamed PS+SBM. In Figure 8b are shown the results of microcellular materials obtained at temperatures of 60 °C, 70 °C and 80 °C respectively. From the data observed in Figure 8, it is clear stress-strain curves show the typical behaviour of a cellular material, including the initial linear zone, the plateau regime, in which deformation occurs with no significant increase of stress and finally the densification zone, where cells begin to collapse (see Figure 3). Moreover, increasing foaming temperature time is reflected in the stress-strain curve appearance, as a consequence of the decrease in the density. In both Figures 8a and 8b, it can be noticed that foams with lower density presents the beginning of the plateau regime at lower stresses, and this tendency is clearly maintained for all the samples produced. Moreover, samples with lower densities show a more defined plateau regime, (see Figure 8b), comparing, for example, materials produced at 30 °C and 70 °C. Several mechanical parameters were obtained directly from the stress-strain data, namely the elasticity modulus E, yield stress σy and densification strain εD, and are represented in Table 3, together with elasticity modulus of the solid material.

378



(a)

(b)

Figure 8. Stress stain curves obtained in compression tests; a) PS+SBM samples obtained at 30 °C, 40 °C and 50 °C: b) PS+SBM samples obtained at 60 °C, 70 °C and 80 °C

Data presented in Table 3 shows that elasticity modulus is lower, in all the samples fabricated, that the elasticity modulus from the solid material (measured in the same conditions). In the microcellular samples fabricated, it is showed that values of yield stress and elasticity modulus are lower as decreasing density, as expected. Values vary between 36.47 MPa to 9.74 MPa, in the case of yield stress, and between 2.79 GPa to 0.46 GPa in the case of


379


Table 3. Mechanical parameters of the samples fabricated together with elasticity modulus and yield stress of the solid material Average εD E σy Saturation Foam density cell size (%) (GPa) (MPa) Temperature (g/cm3) (μm) (°C) Solid Material 1,05 --3,13 65,15 30 40 50 60 70 80

0,99 0,92 0,67 0,62 0,54 0,48

2 5 20 40 50 100

37,78 45,12 48,44 44,99 51,88 59,09

2,79 1,57 1,24 1,02 0,81 0,46

36,47 49,06 26,4 19,53 12,17 9,74

elasticity modulus. Only sample produced at 40 °C shows a higher value of yield stress (49.06 MPa), but it can be due to inhomogeneities in the cellular structure not reflected in the foam density value. In order to carry out a more exhaustive analysis of the compression properties of the foams produced, a theoretical approach was applied. Mechanical properties of cellular materials have been widely study by Gibson(29), Rusch(30,31,32) or Avalle(33), but there is not an simple and unique model to correlate parameters such as elastic modulus or yield stress to microstructural parameters (average cell size, wall thickness, etc.) In the classical model of Gibson, the effect of the density on the mechanical parameters is based on the micromechanical deformation mechanisms of the cell structure. Gibson proposed a series of experimental relations between the most important mechanical parameters and the relative density ρr of the material, for closed cell materials. Some of these expressions are as follows: E k f ES S

y y,s

2

k f S

(6) 3/2

'

D = 1 k '' f S

380

(7)

(8)



where ρr is defined as the ratio ρf/ρs. Equations (6) and (7) are derived theoretically from the micromechanical structure, but they are focused in low density foams. In both cases, k and k’ parameters must be fitted from the experimental data. On the contrary, expression 8 is not derived from the micromechanical mechanism, and it is defined directly from an experimental approach, with a different parameter k’’. Figure 9 presents the evolution of E, σy and εD and the best experimental fit according to previous equations. As it can be seen in Figures 9a and 9b, correlation between Gibson model and experimental values shows a good agreement only for densities below 0.9 g cm3. (a)

(b)

Figure 9. Correlation between Gibson model and mechanical parameters; a) Elastic modulus E: b) Yield stress σy (σy,s= 70 MPa)


381


(c)

Figure 9. Correlation between Gibson model and mechanical parameters; c) Densification strain εD

For example, in Figure 9a, experimental values of elasticity modulus are compared with exponential fit, using a value of k=0.7 as a fitting parameter, and only microcellular samples with higher density (ρ>0.9 g/cm3) are not in good agreement with the model. In addition, the correlation between experimental yield stress and theoretical model shows a similar behaviour, using a value of k’=0.7 (Figure 9b). Also in this case, a deviation from the theoretical model is found at high densities (ρ>0.9 g/cm3). In both cases, this can be related to the limitations of the model purposed, which specially focused in low density foams (densities between 0.1 g/cm3 and 0.8 g/cm3, approximately). Finally, in Figure 9c, densification strain presents greater differences at lower densities, (ρ>0.7 g/cm3), because in these materials to define the end of the plateau stress and the beginning of the densification can be more complicated. Also the fitting parameter employed is k’’ = 0.9, a different value that k and k’. It is important to remark that this fact is in good agreement with the assumptions of the model, in which k and k’ are related to the microstructure of the foam, and k’’ is only a fitting parameter. For this reason, the homogeneity of the structure presented in the previous SEM micrographs (Figure 6) can be related to the common value of k and k’.

4. CONCLUSIONS A brief list of conclusions are detailed as follows:

382



1. Blending by extrusion of neat polystyrene PS and a triblock copolymer SBM (polystyrene-co-1,4-polybutadiene-co-poly (methyl methacrylate) was carried out to produce raw material. Processing conditions and fabrication process led to homogeneous materials, showing a nanostructured assembly between the PS matrix and the SBM additive. 2. In a second step, a collection of microcellular PS+SBM samples was fabricated following a batch process, using supercritical carbon dioxide as a foaming agent. Different temperatures were employed, from 30 °C to 80 °C, resulting in a range of densities from 0.99 g/cm3 to 0.48 g/cm3 and cell sizes from 2 μm to 100 μm. A comparison between foaming behaviour of neat PS and PS+SBM has been analysed, demonstrating the role of the nanostructured SBM particles as a nucleating agent. Employing a small amount of SBM (10 % wt.), cell density increases from 2.6×1011 cells/cm3 to 1.3×1013 cells/cm3, whereas average cell size decreases from 10 μm to 2 μm. 3. In addition, low velocity compression tests have been performed, showing the density dependence of several mechanical parameters, namely elasticity modulus, yield stress and densification strain, for which a theoretical correlation based on Gibson approach has been employed, finding a good agreement between experimental results and theoretical predictions for densities below 0.9 g/cm3. Small deviations were found at higher densities in the case of elasticity modulus and yield stress, using a similar fitting parameter of k,k’=0.7, limitations which can be due to the validity range of the model employed, focused in low density foams. On the other hand, densification strain presents greater differences at lower densities, because the difficult to define the end of the plateau stress and the beginning of the densification zone. In this later case, the fitting parameter utilised was k’’=0.9.

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