Influence of the dispersion of nanoclays on the cellular structure of foams based on polystyrene

ARTICLE Influence of the dispersion of Nanoclays on the cellular structure of foams based on polystyrene Alberto Ballesteros 1 | Ester Laguna-Gutierrez 2 | Paula Cimavilla-Roman 1 | Maria Luisa Puertas 3 | Antonio Esteban-Cubillo 3 | Julio Santaren 3 | Miguel Angel Rodriguez-Perez 1 1 Cellular Materials Laboratory (CellMat), Universidad de Valladolid, Valladolid, Spain 2 CellMat Technologies S.L., Valladolid, Spain 3 Tolsa SA, Madrid, Spain Correspondence Alberto Ballesteros, Cellular Materials Laboratory (CellMat), Universidad de Valladolid, Valladolid 47011, Spain. Email: [email protected], albalag@ fmc.uva.es Funding information Consejería de Educaci on, Junta de Castilla yLe  on, Grant/Award Number: VA202P20; Ente publico regional de la energia de castilla y leon; Ministerio de Ciencia, Innovaci on y Universidades, Grant/Award Number: RTI2018-098749-B-I0 Abstract In the present work blends of polystyrene (PS) with sepiolites have been produced using a melt extrusion process. The dispersion degree of the sepiolites in the PS has been analyzed by dynamic shear rheology and X-ray microcomputed tomography. Sepiolites treated with quaternary ammonium salts (O-QASEP) are better dispersed in the PS matrix than natural sepiolites (N-SEP) or sepiolites organo-modified with silane groups (O-SGSEP). A percolated network is obtained when using 6.0 wt% of O-QASEP, 8.0 wt% of N-SEP and 10.0 wt% of O-SGSEP. It has been shown that multiple extrusion processes have a negative effect on the polymer architecture. They produce a reduction in the length of the polymeric chains, and they do not lead to a better dispersion of the particles in the polymer matrix. Foams have been produced using a gas dissolution foaming process, where a strong effect of the dispersion degree on the cellular structure of the different foams was found. The effects on the cellular structure obtained by using different types of sepiolites, different contents of sepiolites and different extrusion conditions have been analyzed. The foams produced with the formulations containing O-QASEP present the lowest cell size and the most homogeneous cellular structures. KEYWORDS composites, foams, nanoparticles, nanowires and nanocrystals, polystyrene, rheology 1|INTRODUCTION Polystyrene (PS) foams are the second largest component of the foam market, after polyurethane (PU) foams, thanks to their low thermal conductivity, lightweight, high compressive strength, high resistance to moisture and medium to high tensile strength. 1,2 Among the different varieties of PS foams expanded PS (EPS) or extruded PS (XPS) are the used as thermal insulators. The possibility of using CO 2 as the primary blowing agent to produce XPS foams, replacing ozone-depleting blowing agents (fluorocarbons or chlorofluorocarbons), has conferred PS foams a privileged place in the thermal insulation market. 2 For XPS foams with densities around 30 kg/m 3 the thermal conductivity varies between 33 and 35 mW/mK, which is still higher than the thermal conductivity of Received: 26 April 2021 Revised: 20 June 2021 Accepted: 28 June 2021 DOI: 10.1002/app.51373 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2021 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC. J Appl Polym Sci. 2021;e51373. wileyonlinelibrary.com/journal/app 1of19 https://doi.org/10.1002/app.51373 rigid PU foams (around 23–26 mW/mK, for the same density). 3 A strategy to improve the thermal, electrical and mechanical properties of foams consists of incorporating nanoparticles into the polymer matrix. 4–6 The blending of polymeric foams and functional nanoparticles generates a class of materials known as cellular nanocomposites. Cellular nanocomposites combine the advantages of having a cellular structure with the multifunctional effects provided by the nano-particles. 7 On the one hand, particles have an important effect on the microscale properties, improving characteristics of the cellular structure and consequently, they also have a macroscopic effect on some physical properties like thermal or mechanical ones. 5,8 Nucleation can be modified by the presence of a small amount of well-dispersed nanoparticles leading to lower cell sizes and higher cell nucleation densities. 9 Furthermore, nanoparticles can also modify the extensional rheological properties of the polymer matrix, which have an important effect on the degeneration mechanisms and, as a consequence, on the cell size, cellular structure homogeneity and foam density. 10–12 Chen et al. demonstrated that foams with regular and homogeneous cellular structures present improved mechanical properties. 13 Moreover, it is known that thermal conductivity strongly dependsonsomeparametersofthecellularstructuresuch as cell size or porosity. 11 Therefore, nanoparticles could change the thermal aspects of the foams dramatically by decreasing considerably their thermal conductivity. 12 The effects of incorporating nanoparticles into PS foams have been analyzed by several authors. Zhang et al. showed that the thermal insulation performance of PS foams was improved when introducing activated carbon nanoparticles. 14 Han et al. showed that nanoclays in a PS matrix it was possible to produce foams with lower cell sizes and higher cell densities. These cellular composites exhibited higher tensile modulus, better fire retardance, and better barrier properties than the pure PS foam. 15 Kaynak et al. demonstrated that the inclusion of just 5% of montmorillonites combined with some usual phosphate flame retardants produced a synergistic effect leading to a reduction of the flammability of the polymer. 16 Finally, Shen et al. claimed that the incorporation of carbon nanofibers to PS foams created a protective layer around the cell walls that resulted in the enhancement of foam strength. 17 However, research efforts are still necessary to thoroughly understand the role of nanoparticles and the importance of having a suitable dispersion to obtain the desired properties. Only when the dispersion degree of the particles in the polymeric matrix is optimal, it is possible to increase the cell density and reduce the cell diameter, with respect to the pure polymeric foams. 18 Due to the role that nano-particles play in the microscopic and macroscopic properties it is not difficult to understand why they have aroused a large interest in the scientific community. 19–22 Most of the papers dealing with nano-particles are based on the use of particles with spherical or layered morphologies. 13 For example, montmorillonites, silica or nano-porous silica particles have been widely used in PS foams. 23–25 However, it is not simple to find in literature studies analyzing the effects that needle-like shape particles, like sepiolites, have on the final structure of foams. 21,26–29 In fact, only one conference paper has been found. In this work, Notario et al. reported that it was possible to reduce, in a 60%, the cell size of PS foams by adding 0.5 wt% of sepiolites. 30 However, this paper does not provide a systematic study of how the dispersion degree of these particles affects the cellular structure of the foams. Among all the needle-like shape particles available, sepiolites have been selected for this research work due to several reasons. On the one hand, sepiolites (Si 12 Mg 8 O 30 (OH) 4 (OH 2 ) 4 8H 2 O), which are natural fibrous clays, present outstanding sorption and rheological properties. 31,32 On the other hand, the structural characteristics of sepiolites favor their dispersibility in the polymer matrix. The structure of sepiolites consists of blocks of two tetrahedral silica sheets sandwiching an octahedral sheet of magnesium oxide-hydroxide (more information about the structure of the sepiolites can be found in the work of Tian et al. 33 ).The dimensions of the cross-section tunnels are about 0.36 nm 1.1 nm. The discontinuity of the silica sheets allows the presence of a significant number of silanol (Si–OH) groups on the surface of the particles. The existence of silanol groups can enhance the interfacial interaction between the nanoparticles and the polymer and therefore, they could help to improve the dispersion of the sepiolites in the polymeric matrix. Finally, due to their high aspect ratio (thicknesses in the nanometric scale, between 20 and 30 nm, average particle length ranging among 1 and 2 μm and large surfaces areas 300 m 2 / g), sepiolites are very interesting particles for being used as cell nucleating agents in thermoplastic foams. Bernardo et al. concluded that when incorporating 1.5 wt% of sepiolites organomodified with quaternary ammonium salts in a matrix of polymethyl methacrylate (PMMA) the cell size decreases by a factor of five and the cell nucleation density increasesinafactorof160. 21 Additionally, the inclusion of sepiolites can modify several properties of the polymer matrix like the behavior of the material against fire and its mechanical and thermal properties. 34–40 The present work aims to analyze the dispersion of different types of sepiolites in a PS matrix as well as the effect of the dispersion degree on the cellular structure of PS foams produced by a gas dissolution foaming process. This 2of19 BALLESTEROS ET AL. systematic study has been performed by evaluating, on the one hand, the importance of modifying the surface of sepiolites to ensure a proper interaction between the particles and polymer. For this purpose, natural sepiolites and sepiolites organo-modified with quaternary ammonium salts and silanol groups have been considered. The selection of quaternary ammonium salts and silanol groups was performed considering the results obtained in previous works in which both treatments were used to improve the interaction between these particles and a polymer matrix. 21,41–43 On the other hand, composites containing different amounts of sepiolites have been produced to evaluate the influence of the content of sepiolites on the dispersion degree and to determine the percolation threshold. Finally, different extrusion conditions have been used to produce the PS based composites. In particular, the number of extrusion cycles has been modified. Finally, the composites produced with 6 wt% of sepiolites have been foamed by the gas dissolution foaming process. The cellular structure of these foamed samples has been analyzed and the obtained results have been related to the results obtained after the dispersion analysis. 2|EXPERIMENTAL 2.1 |Materials A commercial PS, recommended for foam applications, (INEOS, Styrolution PS153F) with a melt flow index of 7.5 g/10 min (200C/5 kg) and a glass transition temperature (Tg) of 102C was used as polymer matrix. Three kinds of sepiolites, kindly supplied by Tolsa S.A. (Madrid, Spain), were used in this work. They can be distinguished between non-organically modified sepiolites (labeled as N-SEP), sepiolites organically modified with quaternary ammonium salts (O-QASEP) and sepiolites organically modified with silanol groups (O-SGSEP). An antioxidant (BASF, Irganox 1010) was also used to reduce thermal degradation during the extrusion stage. 2.2 |Production process A wet milling process was used to obtain separated sepiolites from the starting sheaf-form in which particles appear naturally. The procedure of the wet milling and surface treatment of the particles was performed as it was described in a previous work. 43 Before the extrusion process, the materials were dried in a vacuum oven (Mod. VacioTem TV, P-Selecta) at 70Cfor 4 h, in the case of pure PS, and at 80Cfor8hinthecase of the different types of sepiolites. The mixing of the polymer with the sepiolites was carried out in a twin-screw extruder (Collin ZK 25 T with L/D of 24) following a temperature profile that goes from 145 to 185C(atthedieof the extruder) and with a screw rate of 50 rpm. Some amount of this material was re-extruded, once again, under the same conditions, to analyze how the number of extrusion cycles affects the dispersion of the sepiolites in the PS matrix. The pure PS was also submitted to different extrusion cycles to analyze how the rheological behavior of the polymer is affected by the extrusion process. The different formulations produced in this work are shown in Table 1. Formulations containing 2, 6, 8, and 10 wt% of the different types of sepiolites were fabricated to evaluate the effects of changing the content of the particles in their dispersibility. The different formulations were later thermoformed in a hot-cold plate press to obtain materials with the desired shape and size. The compression molding process was carried out at a temperature of 235C and at a pressure of 27 bars. The foaming of the samples was carried out using the solid-state gas dissolution foaming method. 44 For this purpose, a high-pressure vessel (model PARR 4681), provided by Parr Instrument Company, with a capacity of 1 L and capable of operating at a maximum temperature of 350C and at a maximum pressure of 41 MPa was used. The reactor is equipped with a pressure pump controller (model SFT- 10), provided by Supercritical Fluid Technologies Inc., which is controlled automatically to keep the pressure on the desired values. The pressure vessel is also equipped with a clamp heater of 1500 W where the temperature is controlled via a CAL 3300 (from CAL controls) temperature controller. The foaming process was performed in two steps. 45 Samples were firstly introduced in the pressure vessel at 8 MPa of CO 2 pressure for the saturation stage. The saturation temperature was 40C and the saturation time was 24 h. It was experimentally proved that this time is enough to achieve full saturation of CO 2 in PS at the conditions under study. After saturation, the pressure was abruptly released. Finally, for the foaming stage, samples were removed from the pressure vessel and introduced in a thermostatic silicon bath at 120C for 1 min. The time between the release of the pressureandtheimmersioninthethermalbathwas2min. Once the materials were expanded, they were cooled down in water to stabilize their cellular structure. 2.3 |Characterization 2.3.1 | Dynamic shear measurements A shear stress-controlled rheometer (AR 2000 EX from TA Instruments) was used to measure the dispersion degree of the different formulations. Dynamic shear measurements were conducted at a temperature of 220C, BALLESTEROS ET AL.3of19 under a nitrogen atmosphere, and using 25 mm diameter parallel plates. A fixed gap of 1 mm was selected to perform the rheological measurements. First a strain sweep test, at a fixed dynamic frequency (1 rad s 1 ), was performed to determine the linear viscoelastic regime of the different nanocomposites. Later, a time sweep was performed to recover the initial state of the particle network, which was partially deformed when the sample was loaded in the rheometer. The duration of the time sweep varied between 360 and 600 s, depending on the material. Finally, the frequency sweep step was performed, in a range of angular frequencies varying between 0.01 and 100 rad s 1 . From these measurements, four parameters were analyzed: dynamic shear viscosity (η jj ), storage modulus ( G0ωðÞð), loss modulus (G00 ωðÞ)) and crossover frequency (ωxÞ. Furthermore, the Mark- Houwink Sakurada equation, which relates the complex viscosity with the molecular weight, was used to estimate the molecular weight of the polymer matrix. 46 2.3.2 | X-ray micro-computed tomography The set-up employed to perform the micro-computed tomography experiments consisted of a micro-focus conebeam X-ray source L10101 from Hamamatsu (spot size: 5μm, voltage: 20–100 kV, current: 0–200 μA) with a maximum output power of 20 W and a high sensitivity flat panel detector C7940DK-02 also from Hamamatsu (2240 2344 pixels, 50 μm of pixel size). In addition, a rotation stage was mounted on a linear stage which enables movement between the source and detector and permits varying the magnification factor. 47 The linear stage was placed in a position so that the magnification value was 20, leading to a pixel size of 2.5 μm. A tube voltage of 55 kV and a current of 170 μA were selected for the measurements. The detector exposure time was 1000 ms and the rotation step was 0.3.Toenhancethecontrast in the reconstructed images, each projection was the result of integrating three consecutive images. Once all the projections were acquired, the reconstruction process of the tomogram was carried out using Octopus, server/client reconstruction package. 48 Due to the limited spatial resolution of the tomographic system (2.5 μm), the reconstructed slices can be used to analyze the number of agglomerates of particles present in the solid PS nanocomposites. The isolated sepiolites present sizes that are between 1 and 2 μm and with this X-ray experiment only particles with sizes higher than this 2.5 μm can be detected. Therefore, the particles observed in this experiment are agglomerations of the primary sepiolites particles. This method to determine the agglomerates of sepiolites has been applied in other works, like the one performed by Bernardo et al. 49 To calculate the number of agglomerates of particles, a tomographic volume of 1.25 2.50 3.50 mm 3 has been considered. Firstly, two consecutive 3D filters have been applied in the reconstructed slices. In addition, a 3D median filter (two pixels of radius) has been applied to remove noise from the images and later, a 3D maximum filter (one pixel of radius) has been computed to enhance the particles gray level intensity. Then, the particles have been binarized by means of a thresholding process based on the different level of absorption between polymer and fillers. The percentage of agglomerates in the sample has been calculated according to (Equation (1)), by TABLE 1 Formulations produced and characterized during the present study Sample name Content of polymer (wt%) Content of Sepiolites (wt%) Content of antioxidant (wt%) Pure PS 99.5 0.0 0.5 PS +2% N-SEP 97.5 2.0 0.5 PS +2% O-QASEP 97.5 2.0 0.5 PS +2% O-SGSEP 97.5 2.0 0.5 PS +6% N-SEP 93.5 6.0 0.5 PS +6% O-QASEP 93.5 6.0 0.5 PS +6% O-SGSEP 93.5 6.0 0.5 PS +8% N-SEP 91.5 8.0 0.5 PS +8% O-QASEP 91.5 8.0 0.5 PS +8% O-SGSEP 91.5 8.0 0.5 PS +10% N-SEP 89.5 10.0 0.5 PS +10% O-QASEP 89.5 10.0 0.5 PS +10% O-SGSEP 89.5 10.0 0.5 Note: All formulations have been subjected to one and two extrusion cycles. 4of19 BALLESTEROS ET AL. measuring the volume fraction occupied by the agglomerates (%Vsep) and considering the real mass fraction of particles in the sample (%msep). Agglomerates %ðÞ¼ ρsep%Vsep ρsolid %msep ,ð1Þ where ρsep is the theoretical density of the sepiolites (2.1 g/cm 3 ) and ρsolid represents the density of the solid nanocomposite, which was measured by gas pycnometry. 2.3.3 | Gas uptake The gravimetric method has been used to measure the gas absorbed during the saturation stage. By using this method, the gas absorbed is obtained directly from determining the weight gain by the polymer sample during the sorption stage. It consists of calculating the ratio between the difference in the sample weight before and after the saturation process and the initial weight of the composite. This ratio provides the percentage of the CO 2 absorbed by the composite, as can be seen in (Equation (2)). CO2 Uptake ¼WsW0 W0 ,ð2Þ where Wsis the weight after saturation and W0is the initial weight. 2.3.4 | Density The density of the solid precursors was analyzed using a gas pycnometer (Accupyc II 1340 from Micromeritics). The density of the cellular materials, which were produced using the gas dissolution foaming process, was determined by the geometric method, that is, dividing the corresponding mass of each specimen by its geometric volume (ASTM standard D1622-08). The foamed samples used to determine the density have a cylindrical shape with a diameter of 7.5 cm and a thickness of 6 mm. The following equations (Equations (3) and (4)) show the way of determining the relative density (ρrÞand expansion ratio (E). Relative density is defined as the ratio between the density of the foam (ρfoam) and the density of the bulk solid material (ρsolid). On the other hand, the expansion ratio is defined as the inverse of the relative density. ρr¼ρfoam ρsolid ,ð3Þ E¼1 ρr :ð4Þ 2.3.5 | Open cell content To evaluate the open cell content of the samples, according to the Standard ASTM D6226-10, see (Equation (5)), a gas pycnometer Accupyc II 1340 from Micromeritics was used. OC %ðÞ¼100 vgeometric vpycnometer p:Vgeometric  ,ð5Þ where vgeometric is the geometric volume of the sample, vpycnometer is the volume of the sample obtained with the pycnometer and pis the porosity calculated as 1ρfoam ρsolid  . 2.3.6 | Cellular structure The structure of the cellular materials, produced by gas dissolution foaming, was analyzed with a scanning electron microscope (SEM) (Jeol, Mod. JSM-820). Parameters such as the average cell size (Φ), and the cell nucleation density (N0Þwere analyzed with an image processing tool based on the software Fiji/Image J. 50 More than 100 cells of different regions of each cellular material have been considered to determine these parameters. The average cell size is defined as it is indicated in (Equation (6)). ϕ¼X n i¼1 ϕi n¼X n i¼1 cf 2nϕi xþϕi y  ,ð6Þ where nis the total number of cells counted in the image, ϕiis the three-dimensional value of the cell size for and specific cell. ϕi xand ϕi yare the chord lengths of the cells in the directions x and y, respectively and cf is a correction factor used to convert the two-dimensional value of the cell size to a three-dimensional value. As it was indicated in the work of Pinto et al. a value of 1.273, has been selected for the mentioned correction factor. 50 The cell nucleation density N0 ðÞ, defined as the number of cells per unit volume of the solid, was obtained using the Kumar's theoretical approximation represented in (Equation (7)). 45 In this formula Nvis the cell density, defined as the number of cells per cubic centimeter of the foamed material, and ρris the relative density (see Equation (3)). BALLESTEROS ET AL.5of19 N0¼Nv ρr :ð7Þ In this work, the cellular materials containing sepiolites present a bimodal structure showing small cells, combined with large cells, those with a size higher than 200 μm. Although, the number of large cells is lower than the number of small cells, the volume occupied by the large cells is not negligible. For this reason, to quantify the observed bimodality the relative volume fraction occupied by the small cells is defined as it is indicated in (Equation (8)). vs¼100AtAl At ,ð8Þ where Alis the observed area occupied by the large cells in the SEM images and Atthe total area of the image. In the pure PS foams, which do not present a bimodal structure, this parameter was not calculated, and it was considered as zero. Finally, the ratio between the standard deviation of the cell size (SD) and the average value of the cell size (SD=ΦÞ, allows analyzing the homogeneity of the cellular structure. Low values of this parameter are related to a homogeneous cellular structure with a narrow cell size distribution. Only the small cells were considered for the determination of the different parameters: cell size, cell nucleation density and SD=Φ. 3|RESULTS 3.1 |Analysis of the dispersion degree by shear rheology The values of the zero-shear viscosity, the slopes of the storage modulus and loss modulus and the number of crossover points between the curves corresponding to the loss and storage moduli have been analyzed for the different formulations. 3.1.1 | Effects of the type of particles In this section, the study is focused on materials produced with different types of sepiolites: N-SEP, O-QASEP, and O-SGSEP. Therefore, for comparative purposes, the content ofparticleshasbeenfixed(6wt%)aswellastheproduction conditions (one single extrusion cycle). Figure 1 shows the behavior of the complex viscosity (η jj ), the storage modulus G0ωðÞð), and the loss modulus (G00 ωðÞ) as a function of the angular frequency for the pure PS and for the three types of composites. Figure 1(a) shows an increase in the complex viscosity values of the formulations containing sepiolites with respect to the pure PS. This increment is chiefly remarkable in the formulation containing O-QASEP. Moreover, in this composite the Newtonian plateau, observed with the other materials at low frequencies, disappears and the behavior corresponds to that of a non-Newtonian power law material. For the materials presenting a Newtonian-plateau, the zero-shear viscosity (η0) is determined as the value of the viscosity in the Newtonian-plateau. The results of zero-shear viscosity are collected in Table 2. Results indicate that the composites present a higher zero-shear viscosity than the pure PS. Storage modulus (G0ωðÞÞis altered by variations in the molecular structure of polymers. At low frequencies, in the area called terminal region, where the longest relaxation times play a major role, the storage modulus is usually proportional to the square of the frequency G0/ω2. 51 To quantify the changes in the storage modulus, the slopes of the G0curves in the terminal region (between 0.01 and 0.1 rad/s) were measured. The obtained results are collected in Table 2. For the pristine polymer, it is expected that the slope of G0presents a value close to 2. As soon as the density of particles increases, the slope should approach to 1. Finally, when the formulations are reaching the percolation state or they are completely percolated, the slope of G0should present values approaching to 0. 52 It is important to mention that the density of particles can increase due to two main reasons. On the one hand, the density of particles increases if the content of particles introduced in the polymeric matrix increases. On the other hand, the density of particles increases if the content of particles is fixed but the particles are dispersed in a more efficient way. Therefore, in systems in which the number of particles is fixed, a reduction of the slope of G0is related with an increment of the dispersion degree. Figure 1(b) shows the differences between the curves of the formulations containing particles and that of the pure polymer. These differences are mainly remarkable in the slopes of the curves in the terminal region, which are clearly reduced in the systems containing particles. A reduction of 29% of the slope of G0 is achieved when adding 6 wt% of N-SEP. Meanwhile, a reduction of the slopes of G0of 62% and 26% is obtained when adding 6 wt% of O-QASEP and 6 wt% of O-SGSEP, respectively. Furthermore, as it is indicated in Table 2, the slope of G0corresponding to the composite containing 6 wt% of O-QASEP particles presents a value close to 0, which indicates that, for this kind of sepiolites, this content is close to the percolation threshold. 6of19 BALLESTEROS ET AL. On the other hand, the loss modulus, G00 ω ðÞ , of a pure polymer should be proportional to the frequency in the terminal region G00 /ω. 51 The effect of the type of particles in the loss modulus is shown in Figure 2(c). The slopes of G00 in the terminal region have been also calculated and the obtained values are shown in Table 2. The slopes of G00 for the systems containing N-SEP or O-SGSEP are like that of the pure polymer. However, the value for the material containing O-QASEP is much lower. A reduction of 21% is reported indicating, once again, a much better dispersion of this type of particles in the PS matrix. The crossover frequency ωx ðÞhas been also measured (see Table 2). This parameter is defined as the frequency at which the storage modulus G0ωðÞand the loss modulus G00 ωðÞintersect. The presence of one single crossover FIGURE 1 Viscoelastic properties for the pure PS and the different composites produced with a fixed content of sepiolites (6 wt%). All formulations were produced using one single extrusion cycle. (a) Complex viscosity versus angular frequency. (b) Storage modulus versus angular frequency. (c) Loss modulus versus angular frequency. Common slopes values for a pure polymer are also shown in (b, c) [Color figure can be viewed at wileyonlinelibrary.com] TABLE 2 Linear viscoelastic properties of the pure PS and the different composites containing 6 wt% of different types of sepiolites, all of them subjected to a single extrusion process Sample name Zero shear viscosity, η0(Pa s) Slope of G0(Pa s) Slope of G00 (Pa s) Crossover frequency, ωx(rad/s) Pure PS 2905 1.78 0.96 42.29 PS +6% N-SEP 3839 1.27 0.95 41.18 PS +6% O-QASEP Non-Newtonian 0.67 0.75 3.67/28.67 PS +6% O-SGSEP 3498 1.32 0.92 42.16 BALLESTEROS ET AL.7of19 point indicates that the material is not already percolated. 52 On the other hand, the presence of two crossover points indicates that the density of particles is close to the percolation threshold. Finally, a spectrum in which no crossover points appear implies that the density of particles is higher than the percolation threshold. 52 The data collected in Table 2 show that the composites produced with 6 wt% of N-SEP and with 6 wt% of O-SGSEP only show a single crossover point, which indicates that in these materials a network structure has not been formed. Meanwhile, for the material produced with 6 wt% of O-QASEP two crossover points have been obtained, which indicates that this composite presents a percolated structure. Furthermore, the crossover frequencies of the storage and loss modulus curves shift to lower frequencies thanks to the inclusion of particles. This behavior indicates that the composites remain in a solid-like behavior for a wider region of frequencies. In other words, the dynamic oscillatory shear measurements indicate that the incorporation of sepiolites to a PS matrix modifies the long-time relaxation behavior of the polymer by increasing the relaxation time due to the formation of a threedimensional network structure. 53 Kotsilkova et al. suggested in their work that longer relaxation times indicate that the structure of the nanocomposite is creating a significant energetic barrier against the molecular motion during the shear flow. 54 FIGURE 2 Viscoelastic properties for the pure PS and the blends of the polymer with different contents of N-SEP and O-QASEP. All the formulations were produced with one extrusion cycle. (a) Complex viscosity versus angular frequency. (b) Storage modulus versus angular frequency. (c) Loss modulus versus angular frequency. Common slopes values for a pure polymer are also shown in figures (b) and (c) [Color figure can be viewed at wileyonlinelibrary.com] 8of19 BALLESTEROS ET AL. With all the information collected it is possible to conclude that only the formulation containing 6 wt% of O-QASEP is already percolated. Moreover, the rheological results also indicate that O-QASEP presents a much better dispersion degree. On the other hand, the formulations containing N-SEP and O-SGSEP exhibit a rheological behavior that is like that of the pure PS matrix, which indicates a poor dispersion of these particles in the polymer matrix. 3.1.2 | Effect of the content of particles This section analyzes the effects on the rheological behavior produced by changing the content of particles. The percentages of particles introduced in the polymer are the following ones: 2, 6, 8, and 10 wt%. All the data showed in this section belongs to blends produced in a single extrusion process. Figure 2(a) shows the effect of changing the content of particles on the complex viscosity. Only the particles which lead to the most promising results have been included in this figure, On the other hand, in Table 3, the data corresponding to the three types of particles are reported: N-SEP, O-QASEP, and O-SGSEP. For blends containing N-SEP, the complex viscosity slightly increases as the content of particles increases. For example, the zero-shear viscosity increases 0.3% and 32%, with respect to the pure PS, when incorporating 2 and 6 wt%ofN-SEP,respectively. However,thepresenceofanon-Newtonian power law behavior in the terminal region is only observed when high amounts of particles are introduced in the system (8 and 10 wt%). On the other hand, the inclusion of O-QASEP, in percentages similar or higher than 6 wt%, leads to a significant modification of the structure of the material. Finally, with the O-SGSEP particles the viscosity increases 0.2%, 20%, and 62%, compared to the neat material, when introducing 2, 6, and 8 wt% of particles, respectively. Therefore, it is necessary to incorporate contents close to 10 wt% to create a percolated network when using these O-SGSEP particles. Due to the change perceived from a Newtonian to a non-Newtonian power law regime in the terminal region of complex viscosity and with the data collected in Table 3, it is possible to assert that the percolation state is reached when incorporating 6 wt% of O-QASEP, 8 wt% of N-SEP and 10 wt% of O-SGSEP. Both the storage modulus, G0ωðÞ, and loss modulus, G00 ωðÞ, increase when the content of particles increases, as it can be seen in Figure 2(b,c), respectively. From the values of the slopes of G0and G00, collected in Table 3, it is possible to conclude that the slopes decrease when the content of particles increases. For instance, a reduction of the slopes of G0of 53%, 62%, 91% and 97%, with respect to the pure PS, is reached for the formulations containing 2, 6, 8, and 10 wt% of O-QASEP, respectively. Furthermore, the slopes of the two moduli, in the terminal region, of the samples presenting a Newtonian plateau (2 and 6 wt% of N-SEP, 2 wt% of O-QASEP and 2, 6, and 8 wt% of O-SGSEP) are closed to 1. Moreover, the storage modulus and loss modulus of these last formulations intersect in a single crossover point, which indicates that in these materials a percolated network structure has not been formed. On the contrary, the formulations containing contents similar or higher than 6 wt% of O- QASEP, 8 wt% of N-SEP and 10 wt% of O-SGSEP present slopes of G0and G00that approach to 0. The theoretical percolation threshold (ϕperÞhas been determined. To obtain this parameter the following (Equation (9)) has been used. 55 G0¼Cϕcϕper  n,ð9Þ where Cis a constant, nis a power law exponent, ϕcis the clay volume fraction, ϕper is the percolation threshold volume fraction and G0is the value of the storage modulus at low frequencies. The value of ϕper is obtained by fitting to a linear regression the curve log G0 ðÞversus log ϕϕper  . This procedure is done for different values of ϕper. This equation can be used only in the proximities of the percolation threshold. 56 Finally, the value of ϕper for which the best fit is obtained is considered as the percolation threshold of the composite. In the system that contains O-QASEP particles a value of the percolation threshold of 4.0 wt% was obtained. This value was 7.5 wt% for the system containing N-SEP and 9.0 wt% for the system containing O-SGSEP. Finally, it is essential to mention that the crossover points between G0and G00change from 1 single point to 2 points or even to 0 points depending on the type and content of particles. Table 3 summarizes this behavior. The formulations containing 2 and 6 wt% of N-SEP, 2 wt% of O-QASEP and 2, 6, and 8 wt% of O-SGSEP present only a single crossover point, which indicates that in these formulations the percolation threshold is not reached. Meanwhile, the formulations containing 8 wt% of N-SEP, 6 wt% of O-QASEP and 10 wt% of O-SGSEP present two crossover points and hence, it can be concluded that they are close to the percolation threshold. Finally, the materials containing 10 wt% of N-SEP and 8 and 10 wt% of O-QASEP do not present any crossover point, which indicates that these contents of particles are higher than the percolation threshold. Furthermore, when the particles are introduced in the polymer it is also possible to observe a shifting effect of the crossover point to lower frequencies. It is remarkable the shifting BALLESTEROS ET AL.9of19 between the dispersion degree of the particles in the polymer matrix and their effectivity as nucleating agents. The cell size of the composites subjected to only one extrusion cycle is reduced in a 78%, 85%, and 72%, with respect to that of the polymer matrix, when using N-SEP, O-QASEP, and O-SGSEP, respectively. In the case of the composites subjected to two extrusion cycles, the cell size is reduced in a 71%, 78%, and 68%, with respect to that of the polymer matrix, when using N-SEP, O-QASEP, and O-SGSEP, respectively. In all the cases the cell size is FIGURE 7 Scanning electron microscope images of the foamed samples produced with the pure PS and with the composites containing the different types of sepiolites [Color figure can be viewed at wileyonlinelibrary.com] 16 of 19 BALLESTEROS ET AL. higher in the samples subjected to two extrusion cycles than in the samples subjected to one extrusion cycle. This result confirms that the increment of the extrusion cycles does not improve the dispersion of the particles and therefore, the cellular structure is not improved. As the density of these materials is very similar, there is a relationship between cell size and cell nucleation density. The higher the cell size the lower the cell nucleation density. The highest value of cell nucleation density is detected in the foam containing 6 wt% of O-QASEP subjected to one extrusion cycle. The cell nucleation density of this material is 80 times higher than that of the foam produced with the pure PS. When analyzing the relationships between the dispersion degree of the particles in the PS matrix and the characteristics of the cellular structure it is possible to conclude that the samples with the lowest percentage of aggregates, those produced with the O-QASEP, present the lowest cell sizes and the highest cell nucleation densities. This fact could indicate that the main nucleation of the small cells is related to the isolated sepiolites rather than with the agglomerations of particles. In Figure 8(a) TABLE 7 Cell size, cell nucleation density, volume fraction of large cells, and SD/ϕof the pure PS and the composites containing 6 wt% of the different sepiolites, subjected to one and two extrusion cycles Sample name Cell size (μm) Cell nucleation density (nuclei/cm 3 ) Volumetric fraction of large cells SD=ϕ Pure PS one extrusion cycle 88.40 ± 28.12 (4.82 ± 0.5) 106 0.00 0.31 Pure PS two extrusion cycles 101.15 ± 34.27 (4.74 ± 0.2) 106 0.00 0.33 PS +6% N-SEP one extrusion cycle 19.14 ± 5.06 (3.73 ± 0.4) 108 0.21 0.26 PS +6% N-SEP two extrusion cycle 29.32 ± 8.09 (3.06 ± 0.4) 108 0.18 0.30 PS +6%O-QASEP one extrusion cycle 13.01 ± 2.69 (3.85 ± 0.1) 108 0.18 0.20 PS +6%O-QASEP two extrusion cycle 22.18 ± 4.79 (3.61 ± 0.2) 108 0.15 0.21 PS +6%O-SGSEP one extrusion cycle 24.88 ± 4.65 (3.43 ± 0.7) 108 0.24 0.18 PS +6%O-SGSEP two extrusion cycle 32.64 ± 7.41 (2.86 ± 0.9) 108 0.20 0.20 FIGURE 8 (a) Relation between the cell size and the percentage of agglomeration for formulations containing 6 wt% of sepiolites subjected to a single extrusion process. (b) Relation between the fraction of large cells and the percentage of agglomeration for materials containing 6 wt% of sepiolites subjected to a single extrusion process BALLESTEROS ET AL.17 of 19 it is possible to see the relation between the cell size and the percentage of agglomeration for the formulations that contain 6 wt% of sepiolites subjected to a single extrusion process. Regarding the data corresponding to the volumetric fraction of small cells depicted in Table 7, it is possible to see how samples with O-QASEP present higher percentages of small cells than the other cellular composites. This fact could be related with the better dispersibility observed of this kind of particles in the PS matrix. In other words, the bimodal behavior that appears with the sepiolites depends strongly on the agglomeration of these nanoclays. This behavior is confirmed with the Figure 8 (b), which represents the relation between the presence of large cells and the percentage of agglomeration. The higher the agglomeration ratio the higher the number of large cells that appear in the cellular structure. Finally, the ratio of the SD divided by the cell size (ϕ) gives information about the homogeneity of the cellular structure. The lower the value of SD=ϕthe more homogeneous the cellular structure. Results indicate that, when the largest cells are not considered, the structures of the foams containing sepiolites are more homogeneous than the structure of the foams produced with the pure PS. In addition, the foams produced with the formulations subjected to one extrusion cycle are also more homogeneous than the foams produced with the formulations subjected to two extrusion cycles, which agrees with the results obtained in the rheological study. 4|CONCLUSIONS Blends of PS with sepiolites have been prepared and characterized. The effects on the dispersion degree of the particles in the PS matrix associated to modify the type of particles (N-SEP, O-QASEP, O-SGSEP), the content of particles (2, 6, 8, and 10 wt%) and the process conditions (one and two extrusion cycles) have been analyzed in this research by using two different techniques: dynamic shear rheology and X-ray micro-computed tomography. Shear dynamic rheology results show that the O-QASEP particles are the ones with the best dispersibility. The increment detected in the complex viscosity values of the formulations containing O-QASEP and the reductions observed in the values of the slopes of G0and G00 curves are more pronounced than those observed for the other particles. Furthermore, rheological results indicate that in the formulations containing 6 wt% of O-QASEP a percolated structure has been formed; whereas it is necessary to incorporate 8 wt% of N-SEP and 10 wt% of O-SGSEP to achieve the percolation state. Moreover, the conclusions reached with the shear dynamic rheology measurements are corroborated with the X-ray micro-tomography results. An increase of the number of extrusion cycles lead to a deterioration of the molecular structure of the PS matrix due to the high shear forces that are produced during the extrusion process. Furthermore, by increasing the number of extrusion cycles it is not possible to achieve a better dispersion of the particles in the polymer matrix. Finally, the pure PS and the composites containing 6 wt% of sepiolites have been foamed and their cellular structure has been characterized. Results indicate that sepiolites are strong nucleating agents for PS foams Cell sizes were reduced up to 80% and cell densities were increased by 80 times. Moreover, the dispersion degree of the particles in the PS has an important effect on the cellular structure characteristics. The formulation with the best dispersion of particles (6 wt% O-QASEP) presents the lowest cell size, the highest cell density, and a very homogeneous cellular structure. In addition, the foams produced with the formulations subjected to two extrusion cycles present worse cellular structures than their counterparts subjected only to one extrusion cycle. ACKNOWLEDGMENTS Financial assistance from the Junta of Castile and Leon (VA202P20) and Spanish Ministry of Science, Innovation and Universities (RTI2018-098749-B-I00) and the “Ente Público Regional de la Energía de Castilla y Le on” (EREN) are gratefully acknowledged. 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