The effects of gas counter pressure and mold temperature variation on the surface quality and morphology of the microcellular polystyrene foams

In this study, we developed a foaming control system using the Gas Counter Pressure (GCP) combined with mold temperature control during the microcellular injection molding (MuCell) process and investigated its influence on the parts' surface quality and foams structures. The results revealed that under GCP control alone when GCP is greater than 10 MPa, part surface roughness for transparent polystyrene (PS) improved by 90%. When GCP increased, the skin thickness also increased, the weight reduction decreased and the average cell size reduced to about 30 μm. For black PS parts, when GCP is greater than 10 MPa, the part gloss reaches the same value as that molded by conventional injection molding. By increasing gas holding time, the cell density decreased and the cell size distribution became more uniform. The increase in amount of supercritical fluid foaming agent also increased the cell density. Applying mold temperature control alone with temperature in the range of 90–120°C (near T_g), the surface roughness improved by 65%. Increasing mold temperature decreased the skin thickness; however, the cell size distribution became significantly nonuniform. It was found that thin skin, small and uniform cell size as well as good surface quality can be achieved efficiently by simultaneous combining of GCP and mold temperature control. The proposed innovative approach may lead to a significant improvement and a more broad application for MuCell process. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

微孔泡沫注射成型

概述了多种可行的注射成型微孔泡沫新方法，各种加工方法各有特色，工艺设计重点应在热力学、模具几何结构以及充模阶段。     

Low-density, microcellular polystyrene foams

Numerous applications have been identified for low-density, microcellular, polymeric foams. In this paper the authors describe a general technique to produce foams of this type with organic-soluble polymers, in particular polystyrene. Open-celled polystyrene foams have been developed with densities of 0.02–0.2 g cm⁻³ and uniform cell sizes of 1–20 μm. By using well-characterized polymers the authors have related form morphology to the phase diagram of the polymer/solvent system employed.     

A model for the unfoamed skin on microcellular foams

In microcellular plastics, an unfoamed skin that is integral with the foamed core can be created by allowing the nucleating gas to diffuse from the surfaces of a gas saturated specimen prior to foaming. In this paper, a semi-empirical model is proposed that predicts the skin thickness variation in microcellular foams as a function of gas desorption time. The model shows good agreement with experimental results on the polycarbonate–carbon dioxide system.     

Generation of microcellular polymeric foams using supercritical carbon dioxide. I: Effect of pressure and temperature on nucleation

Supercritical carbon dioxide is known to swell and plasticize poly(methyl methacrylate), PMMA, dramatically. We have employed a pressure quench in a CO₂-swollen PMMA sample to generate a microcellular core structure encased by a nonporous skin. Further, we have demonstrated that classical nucleation theory can be used to model the effects of saturation pressure, temperature, and time on the cell density of the porous materials, provided that the effects of the CO₂-diluent on the surface tension of PMMA are adequately taken into account. This is because our system is in a homogeneous liquid state at our operating conditions because of the plasticization. Both model predictions and data indicate that cell density rises sharply at a saturation pressure of approximately 14 MPa (at 40°C), leveling out above 27 MPa. By contrast, the effect of temperature on cell density in the range 40°C to 80°C is minimal.     

High temperature morphology and stability of nanoporous Ag foams

Nanoporous silver foams (75 % porosity) synthesized by dealloying Ag₂₅Al₇₅ using two different electrolytes were heat treated in either argon or vacuum in order to induce changes in the morphology and ligament size. Several techniques including scanning electron microscopy, Raman spectroscopy, and X-ray powder diffraction are utilized to study the elevated temperature morphology and oxide formation. The ligament sizes increase with heat treating temperature, except for the sample heat treated in vacuum at 600 °C. Results show no significant silver oxide formation for any of the foams.     

Fabrication of microcellular polymer/graphene nanocomposite foams

Graphene has recently gained revolutionary aspirations because of its remarkable electronic, thermal and mechanical properties. These unique properties make it promising for preparing multifunctional nanocomposites. In recent years, polymer foams based on graphene have also received increasing attention in both the scientific and industrial communities. This review presents an overview of polymer/graphene nanocomposite foams discussing the production of graphene, the polymer functionalization of graphene and different polymer/graphene foams with different properties. One of the most promising avenues is to fabricate tough and lightweight materials with superior electrical and electromagnetic interference shielding properties. Copyright © 2012 Society of Chemical Industry     

微孔泡沫塑料性能优化方案

随着热塑性微孔泡沫塑料的应用越来越普及(例如TreXd公司的MuCeu微孔泡沫塑料),人们越来越需要知道怎样的泡沫结构才能达到泡沫材料的性能最优化.早期的研究阐述了加工工艺和泡孔结构之间的关系,但没有说明泡孔结构和泡沫材料力学性能之间的关系.后者的答案最终将使生产企业能够定制工艺,以满足最终用途所需.     

Effect of the crystallinity and morphology on the microcellular foam structure of semicrystalline polymers

Microcellular foam processing of polymers requires a nucleated cell density greater than 10⁹ cells/cm³ so that the fully grown cells are smaller than 10 μm. A microcellular foam can be developed by first saturating a polymer sample with a volatile blowing agent, followed by rapidly decreasing its solubility in the polymer. In general, the cellular structure of semicrystalline polymer foams is difficult to control, compared to that of amorphous polymer foams. Since the gas does not dissolve in the crystallites (1), the polymer/gas solution formed during the microcellular processing is nonuniform. Moreover, the bubble nucleation is nonhomogeneous because of the heterogeneous nature of the semicrystalline polymer. In this paper, the effects of the crystallinity and morphology of semicrystalline polymers on the microcellular foam processing and on the cellular structure of products are investigated. First, polymer specimens with various crystallinities and morphologies were prepared by varying the cooling rate of the polymer melt. Then, the solubility and diffusivity of the blowing agent in and through specimens were studied. The specimens with differing crystallinities and morphologies were foamed and their cellular structures were compared. The experimental results agree with theoretical predictions, indicating that the crystallinity and morphology of semicrystalline polymers exert a strong influence on the foam processing and the structure of the product.     

凝固条件对PAN初生纤维微孔结构形态的影响

采用一维多取向小角X射线散射研究了聚丙烯腈细流在凝固过程中,凝固浴温度、浓度以及喷丝头拉伸比对初生纤维中微孔结构形态的影响。结果表明,凝固温度由30％提高到60℃时,微孔沿纤维轴取向增强,微孔尺寸减小,但微孔数量增加;凝固浴质量分数由67．5％升高到76．0％,微孔数量减少,但微孔尺寸变大,微孔沿纤维轴取向减弱;喷丝头拉伸比为-36．7％-10％时,微孔尺寸和取向角都增大。在凝固浴温度为52-55℃,凝固浴质量分数为70％,选择负拉伸能得到性能优异的初生纤维。     

The elastic and plastic mechanical responses of microcellular foams

Microcellular foams were prepared by the thermally induced phase-separation technique, which yields materials having very small cell dimensions (0.1–20μm). The polymers employed were isotactic polystyrene, polyacrylonitrile, poly(4-methyl-1-pentene), polyurethane, and Lycra® and the resulting foams all had densities in the range 0.04–0.27 g cm^?3. Values of Young's modulus and the collapse stress for these foams were measured and compared with predictions for conventional foams containing defects. Also investigated were plastic deformations, some time-dependent behavior, and Poisson's ratio. © 1993 John Wiley & Sons, Inc.     

Constitutive modeling for mechanical behavior of PMMA microcellular foams

Constitutive equations for nonlinear tensile behavior of PMMA foams were studied. Five viscoelastic models composed of elastic and viscous components were accounted for the modeling of the constitutive equations. The developed constitutive equations are expressed in terms of material properties and foam properties such as strain, strain rate, elastic modulus, relative density of foam, and relaxation time constant. It was found that the stress-strain behaviors by Generalized Maxwell model, Three Element model and Burgers model could be described by the constitutive equation obtained from the Maxwell model. For the verification of the constitutive model, poly(methyl methacrylate) (PMMA) microcellular foams were manufactured using batch process method, and then uniaxial tensile tests were performed. The stress-strain curves by experiment were compared with the theoretical results by the constitutive equation. It was demonstrated that nonlinear tensile stress-strain behaviors of PMMA foams were well described by the constitutive equation.     

Fabrication of microcellular polylactide/modified silica nanocomposite foams

Microcellular polylactide (PLA)/modified-silica (m-silica) nanocomposite foams were prepared in a batch process using supercritical carbon dioxide as physical blowing agent. To enhance the dispersion in the PLA matrix, silica nanoparticles were modified by dodecyltrichlorosilane) and were melt compounded with PLA using a twin-screw extruder. PLA/m-silica nanocomposites with m-silica contents of 0.5, 1, 2, 3, and 5 wt % were obtained. The incorporation of m-silica nanoparticles in PLA enhanced the thermal and mechanical properties of PLA. The resultant foams were observed by scanning electron microscopy (SEM) and average cell diameter and cell density were calculated using SEM micrographs. The incorporation of m-silica nanoparticles into the PLA matrix had the effect of decreasing the cell diameter and increasing the cell uniformity and cell density. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48616.     

Cell morphology and mechanical properties of microcellular mucell® injection molded polyetherimide and polyetherimide/fillers composite foams

Microcellular polyetherimide (PEI) foams were prepared by microcellular injection molding using supercritical nitrogen (SC‐N₂) as foaming agent. The effects of four different processing parameters including shot size, injection speed, SC‐N₂ content, and mold temperature on cell morphology and material properties were studied. Meanwhile, multiwalled carbon nanotube (MWCNT), nano‐montmorillonoid (NMMT), and talcum powder (Talc) were introduced into PEI matrix as heterogeneous nucleation agents in order to further improve the cell morphology and mechanical properties of microcellular PEI foams. The results showed that the processing parameters had great influence on cell morphology. The lowest cell size can reach to 18.2 μm by optimizing the parameters of microcellular injection molding. Moreover, MWCNT can remarkably improve the cell morphology of microcellular PEI foams. It was worth mentioning that when the MWCNT content was 1 wt %, the microcellular PEI/MWCNT foams displayed optimum mechanical properties and the cell size decreased by 28.3% compared with microcellular PEI foams prepared by the same processing parameters. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4171–4181, 2013     

Generation of microcellular biodegradable polycaprolactone foams in supercritical carbon dioxide

Microcellular foaming of low‐T_g biodegradable and biocompatible polycaprolactone (PCL) in supercritical CO₂ has been studied. The purpose is to apply microcellular materials to drug containers and medical materials for artificial skin or bone. Effects of a series of variable factors on the foam structures, such as saturation temperature, saturation pressure, saturation time, and depressurization time were studied. The experimental results indicate that, while keeping other variables unchanged, higher saturation temperature leads to reduced bulk densities and different saturation pressures result in different nucleation processes. In addition, saturation time has a profound effect on the structure of the product. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 593–597, 2004     

Tesile toughness of microcellular foams of polystyrene,styrene-acrylonitrile copolymer,and polycarbonate,and the effect of dissolved gas on the tensile toughness of the same polymer matrices and microcellular foams

This paper reports on the tensile properties of microcellular foams of three different thermoplastics, since there have been several reports in the literature, but with indefinite conclusions so far, that microbubbles act in a manner similar to rubber particles in toughening thermoplastics. Polystyrene (PS), styreneacrylonitrile copolymer (SAN), and polycarbonate (PC), were selected based on their different intrinsic ductilities. The gas supersaturation technique was used to generate samples with microbubbles. The effect of the presence of microbubbles inside the polymer matrix was separated from the effects of the pressure and thermal history experienced by the samples. Nitrogen gas dissolved into PS, and to a lesser extent into SAN, caused and increase of the tensile toughness, but this increased decayed with time as nitrogen gas diffused out of the samples. Furthermore, microcellularly foamed PS samples showed some limited improvement in terms of tensile toughness after all the nitrogen gas diffused out. SAN and PC showed deterioration of the tensile toughness in the presence of microbubbles.     

Impact behavior of microcellular foams of polystyrene and styrene-acrylonitrile copolymer,and single-edge-notched tensile toughness of microcellular foams of polystyrene,styrene-acrylonitrile copolymer,and palycarbonate

In the first part of this series of papers, the tensile properties of microcellular foams of polystyrene (PS), styrene-acrylonitrile copolymer (SAN), and polycarbonate (PC) were reported. In this part, the impact properties of unnotched, sharply and bluntly notched samples of microcellularly foamed PS and SAN samples were studied. Furthermore, the effects of the sharpness of the notch as well as of the speed of the test were studied by comparing the impact tests with the single-edge-notched (SEN) tensile tests, which were carried out for the PS, SAN, and PC sample. Some limited improvement in impact and SEN tensile properties was exhibited in some experimental conditions. The impact properties of microcellularly foamed PC samples are reported elsewhere.     

Heterogeneous nucleation uniformizing cell size distribution in microcellular nanocomposites foams

Microcellular polycarbonate/nano-silica nanocomposites (PCSN) were prepared by temperature rising process using supercritical CO₂ as the blowing agent. Neat PC foam showed a quite broad distribution of cell sizes. Under the same foaming conditions, the addition of nano-silica resulted in PCSN foams having uniform cell size distribution, reduced cell size of 0.3-0.5 μm and increased cell density of 10¹¹-10¹³ cells/cm³. The underlying nucleation mechanism was semi-quantitatively analyzed by the classical nucleation theory. The results indicate that the energy-barrier for heterogeneous nucleation was three orders of magnitude lower than that of homogeneous one. The heterogeneous nucleation of nano-silica aggregates dramatically increased the nucleation rate, decreased the nucleation time interval, and hence facilitated the almost instantaneous growth of cell size. Combined with the well-dispersed nucleation sites, resulted from the uniform dispersion of nano-silica aggregates, the narrow-distributed cell size was obtained in PCSN foams.     

Compressive behavior of microcellular polystyrene foams processed in supercritical carbon dioxide

Microcellular polystyrene foams have been prepared using supercritical carbon dioxide as the foaming agent. The cellular structures resulting from this process have been shown to have a significant effect on the corresponding mechanical properties of the foams. Compression tests were performed on highly expanded foams having oriented, anisotropic cells. For these materials an anisotropic foam model can be used to predict the effect of cell size and shape on the compressive yield stress. Beyond yield, the foams deformed heterogeneously under a constant stress. Microstructural investigations of the heterogeneous deformation indicate that the dominant mechanisms are progressive microcellular collapse followed by foam densification. The phenomenon is compared to the development of a stable neck commonly observed in polymers subjected to uniaxial tension, and a model that describes the densification process is formulated from simple energy balance considerations.     

Compressive response of PMMA microcellular foams at low and high strain rates

Microcellular foams are widely applied in various applications in both civil and military applications for barriers and energy absorption materials. Poly(methyl methacrylate) microcellular foams were fabricated via supercritical foaming method. Field emission scanning electron microscopy, differential scanning calorimetry, and mechanical test machine were used to visualize the foam structure and test the quasi‐static compression properties. Moreover, Split Hopkinson Bar (SHPB) setups were adopted to explore the dynamic compression properties. The experimental results show that the microcellular foams have homogeneous cell size distribution and exhibit superior compressive behavior at both quasi‐static and high strain rates. The mechanical properties depend on both foam density and strain rate. Strain rate effects are clearly observed. At quasi‐static strain rate and 7500 S^?1 regime, cell wall bucking and folding are the main failure mechanism. However, at high strain rate regime, softening phenomenon is observed. By roughly calculating the energy absorbed and the temperature rise, the temperature of the foams will rise up to as high as 130 °C after conducting high strain rate compression, and it is postulated that the generated heat will destroy the cell structure of the foams. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46044.