Study of Vibration Field on Microcellular Foaming in Nucleating Agents

A novel microcellular processing experimental setup was developed in which various nucleation agents were added to foaming material (Polystyrene). The effect of the nucleation agents on cell morphology of foam plastics was studied in the vibration field. The effects of the processing conditions such as nucleating agents, particle size of the nucleating agents, content of the nucleating agents, and the vibration parameters on polystyrene cell morphology were investigated. The cell morphology was analyzed with a scanning electron microscope. The results showed that the samples blown with nucleating agents, in general, had larger cell density and much smaller cell size than the ones without nucleating agents. It is believed that the smaller nucleating agents will be helpful for the larger bubble densities and the smaller cell size. There is the optimum content range for the maximum bubble densities and the minimum average cell diameter in foaming processing with nucleating agents. In the range of vibration parameter, the cell density increases with an increase of frequency and amplitude, and the cell size is the reverse.     

Microcellular PVC

Novel microcellular PVC foams with a very homogenous cell distribution and cell densities ranging from 10⁷ to 10⁹ cells/cm³ have been created using carbon dioxide as the nucleating gas. Microcellular foams with relative densities (density of foam divided by the density of unfoamed polymer) ranging from 0.15 to 0.94 have been produced. It was found that the bubble nucleation density has and Arrhenius-type dependence on temperature, while the average bubble diameter is relatively independent of the foaming temperature. A majority of the cell growth was found to occur in the early stages of foaming.     

Ideal surface geometries of nucleating agents to enhance cell nucleation in polymeric foaming processes

Ideal nucleating agents are expected to improve the cell morphology of plastics foams (i.e., increasing the cell density, reducing the cell size, and narrowing cell-size distribution) by providing heterogeneous nucleation sites. A nucleating agent's surface geometry is one factor that governs its nucleating power. This paper discusses the surface geometry of an ideal nucleating agent. On the basis of computer simulations of a batch foaming process using polystyrene and carbon dioxide, we found that nucleating agents having numerous crevices with small semiconical angles are the most desirable for polymeric foaming processes. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Foam processing and cellular structure of polylactide-based nanocomposites

Via a batch process in an autoclave, the foam processing of neat polylactide (PLA) and two different types of PLA-based nanocomposites (PLACNs) has been conducted using supercritical carbon dioxide (CO₂) as a foaming agent. The cellular structures obtained from various ranges of foaming temperature-CO₂ pressure were investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The incorporation with nano-clay induced heterogeneous nucleation because of a lower activation energy barrier compared with homogeneous nucleation as revealed by the characterization of the interfacial tension between bubble and matrix. The grown cells having diameter of ∼200 nm were localized along the dispersed nano-clay particles in the cell wall. The dispersed nano-clay particles acted as nucleating sites for cell formation and the cell growth occurs on the surfaces of the clays. The PLACNs provided excellent nanocomposite foams having high cell density from microcellular to nanocellular.     

A highly effective reactive liquid crystal for the improved β‐nucleation of isotactic polypropylene

We report here the synthesis and characterization of a reactive liquid crystal (RLC) as a novel polymeric nucleating agent for the promotion of the nucleation efficiency of isotactic polypropylene (iPP). The RLC was synthesized by an in‐situ photo‐polymerization and was then grafted onto the molecular chain of iPP by the reactive blending. The phase transition and crystalline morphologies of RLC‐iPP in the β‐nucleation were studied. It is found that the nucleation efficiency of β‐crystals of iPP can be increased to 42% with a very small amount of RLC grafting, which is much higher than the reported nucleation efficiency of polymeric nucleating agents up to now (23%). In addition, we found that the nucleation efficiency of β‐crystals is strongly related to the concentration of RLC for the reactive blending. The nucleation efficiency was decreased from 42% to about 17% with the increase of RLC concentration from 0.5% to 4%. We propose a possible nucleation mechanism for this interesting phenomenon. It is expected that this new β‐nucleation RLC will have potential industrial applications in the future. POLYM. ENG. SCI., 54:2112–2120, 2014. © 2013 Society of Plastics Engineers     

The heterogeneous nucleation of microcellular foams assisted by the survival of microvoids in polymers containing low glass transition particles. Part I: Mathematical modeling and numerical simulation

The existing models based on classical nucleation theory are not able to explain satisfactorily the nucleation phenomenon of microcellular foams in thermoplastics. Here, we extend the analysis of Kweeder (24), who developed a new model that considers the presence of microvoids, resulting from the thermal processing history of the polymer, as potential nucleation sites. The nucleation model “concentrates” on the stresses and thus void formations in the rubber particles. Since these are pre-existing microvoids, bubble nucleation depends on the survival of these voids to grow rather than the formation of a new phase as modeled by classical nucleation theory. The population of viable microvoids with a sufficiently large radius to survive and overcome surface and elastic forces has been modeled to yield the cell density. A log-normal distribution, which relates to the rubber particle size, has been used to model the distribution of microvoids in the polymer composite material. The model depends on various process parameters such as saturation pressure, foaming temperature, concentration of nucleating agents, solubility of the blowing agent in the polymer, and the modulus. High impact polystyrene (HIPS) was added to polystyrene to obtain polymers with different concentrations of rubber gel particles, the nucleating agent, and used here for this study.     

Synthesis of polystyrene-carbon nanofibers nanocomposite foams

In this study, the use of carbon nanofibers (CNFs) as nucleating agents to produce polystyrene nanocomposite foams was demonstrated. With the addition of CNFs, microcellular foams with uniform cell size distributions were obtained. Compared to nanoclay and single-walled carbon nanotubes (SWCNTs), CNFs exhibit substantially higher nucleation efficiency in the foaming process. The underlying mechanism is semi-quantitatively explained by the classical nucleation theory. The homogeneous fiber distribution and favorable surface and geometrical characteristics of CNFs make them ideal nucleating agents.     

聚丙烯挤出发泡中的关键技术——发泡体系的性能和发泡机理研究

从聚丙烯挤出发泡体系的性能包括聚丙烯熔体的黏弹性、发泡剂的溶解度和扩散系数、聚丙烯的结晶行为和成核剂的性能以及聚丙烯挤出发泡的气泡成核机理和气泡增长机理系统介绍了聚丙烯挤出发泡中的一些关键技术。研究表明：具有显著应变硬化行为和高熔体强度的长链支化聚丙烯是获得优质PP发泡材料的前提；发泡剂的溶解度和扩散系数、聚丙烯的结晶行为和成核剂的种类和性能对发泡材料的泡孔密度、泡孔尺寸和泡孔尺寸分布有显著影响；气泡成核和气泡增长机理对于聚丙烯挤出发泡的配方设计、工艺确定和设备选型具有极其重要的意义。     

Probing structure–heterogeneous nucleation efficiency relationship of mesoporous particles in polylactic acid microcellular foaming by supercritical carbon dioxide

Heterogeneous nucleating agents that can improve cell morphology of polymer foams have been studied extensively, however, the exact relationship among particle structure, surface modification and nucleating efficiency still remain elusive. Previously, we demonstrated that mesoporous structure benefited nucleating efficiency by comparing solid silica particles and mesoporous particles (MCM-41). Herein, the feasibility of using another type of mesoporous particles, namely, SBA-15 as a nucleating agent for polymer foaming was investigated, but surface modification issue was emphasized. Results reveal that SBA-15 particles show excellent nucleation performance on polylactic acid foaming, and such nucleating effect are dependent on the surface modification. The surface modification using fluorinated silane significantly decreases nucleation energy barrier, and thus shows the highest nucleation efficiency. This work provides a comprehensive insight into structure–nucleating efficiency relationship for mesoporous particles, and highlights the importance of surface modification as a structural factor to optimize the design of effective nucleating agents for polymer foaming.     

Gas sorption and transport in syndiotactic polystyrene with nanoporous crystalline phase

In this contribution we analyse sorption and transport of several gases in semicrystalline syndiotactic polystyrene with nanoporous crystalline δ form. Investigation was performed on amorphous samples and on samples characterized by different degrees of crystallinity. Sorption isotherms of carbon dioxide, nitrogen and oxygen in the crystalline phase have been determined starting from experimental results obtained for semicrystalline and amorphous samples. Corresponding isosteric heats of sorption were evaluated for the crystalline and amorphous phase. Permeation tests were also performed to gather information on mass transport properties of semicrystalline samples, evaluating average diffusivities of carbon dioxide and oxygen, in the limit of small concentrations as function of degree of crystallinity.     

热固性聚氨酯泡沫成核机理的研究

用自制的一套在线显微观测系统研究了热固性聚氨酯泡沫合成初期的气泡成核机理,探讨了聚氨酯发泡过程中搅拌速率(剪切力)、固体成核剂及反应前体系中溶解的气体量等工艺参数对气泡成核过程的影响。通过研究发现,热固性聚氨酯泡沫的成核机理为空气分散成核,在反应原料中添加固体成核剂、增加反应前体系中溶解的气体量以及提高物料的搅拌速率等都可以在一定程度上促进成核。     

A microcellular processing study of poly(ethylene terephthalate) in the amorphous and semicrystalline states. Part I: Microcell nucleation

Microcellular semicrystalline polymers such as poly(ethylene terephthalate) show great promise for engineering applications because of their unique properties, particularly at higher densities. Recent studies reveal some high density microcellular polymers have longer fatigue lives and/or equal strengths to the neat polymer. Relatively few microcellular processing studies of semicrystalline polymers have been presented. In general, semicrystalline polymers are relatively difficult to microcellular process compared to amorphous polymers. In this paper and a companion paper, the microcellular processing of poly(ethylene terephthalate) in the amorphous and semicrystalline states is studied in order to quantify the processing differences. The microcellular processing steps addressed in this paper include gas/polymer solution formation and microvoid nucleation. Particular emphasis is given to microvoid nucleation comparing the processing characteristics of semicrystalline and amorphous materials. Moreover, this study identifies a number of critical process parameters. In general, the semicrystalline materials exhibit ten to one thousand times higher cell nucleation densities compared with the amorphous materials, resulting from heterogeneous nucleation contributions. The amorphous materials show a strong dependence on cell density, while the semicrystalline materials show a weaker dependence. Moreover, classical nucleation theory is not adequate to quantitatively predict the effects of saturation pressure on cell nucleation for either the amorphous or semicrystalline polyesters. Both the semicrystalline and amorphous materials exhibit constant nucleation cell densities with increasing foaming time. Foaming temperatures near the glass transition are found to influence the cell density of the amorphous polyesters, indicating some degree of thermally activated nucleation. Furthermore, classical nucleation theory is not adequate to predict the cell density dependence on foaming temperature. Similar to the amorphous polyesters above the glass transition temperature, nucleation in the semicrystalline materials is found to be independent of the foaming temperature.     

The influence of carbon nanotube aspect ratio on the foam morphology of MWNT/PMMA nanocomposite foams

Polymer nanocomposite foams, products from the foaming of polymer nanocomposites, have received increasing attention in both the scientific and industrial communities. Nanocomposite foams filled with carbon nanofibers or carbon nanotubes with high electrical conductivity, enhanced mechanical properties, and low density are potential effective electromagnetic interference (EMI) shielding materials. The EMI shielding efficiency depends on the electrical conductivity and bubble density, which in turn, depend on the properties of the filler. In the current study, multi walled carbon nanotubes (MWNT) with controlled aspect ratio were used to alter the bubble density in MWNT/poly(methyl methacrylate) (PMMA) nanocomposites. It was found that the nanocomposite foams filled with shorter MWNT had higher bubble density under the same foaming conditions and MWNT concentration. Both the ends and sidewalls of carbon nanotubes can act as heterogeneous bubble nucleation sites, but the ends are more effective compared to the sidewalls. Shorter nanotubes provide more ends at constant MWNT concentration compared to long nanotubes. As a result, the difference in the foam morphology, particularly the bubble density, is due to the difference in the number of effective bubble nucleation sites.     

Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process

Use of supercritical carbon dioxide (scCO₂) as a blowing agent to generate microcellular polymer foams (MPFs) has recently received considerable attention due to environmental concerns associated with conventional organic blowing agents. While such foams derived from amorphous thermoplastics have been previously realized, semicrystalline MPFs have not yet been produced in a continuous scCO₂ process. This work describes the foaming of highly crystalline poly(vinylidene fluoride) (PVDF) and its blends with amorphous polymers during extrusion. Foams composed of neat PVDF and immiscible blends of PVDF with polystyrene exhibit poor cell characteristics, whereas miscible blends of PVDF with poly(methyl methacrylate) (PMMA) yield foams possessing vastly improved morphologies. The results reported herein illustrate the effects of blend composition and scCO₂ solubility on PVDF/PMMA melt viscosity, which decreases markedly with increasing PMMA content and scCO₂ concentration. Morphological characterization of microcellular PVDF/PMMA foams reveals that the cell density increases as the PMMA fraction is increased and the foaming temperature is decreased. This study confirms that novel MPFs derived continuously from semicrystalline polymers in the presence of scCO₂ can be achieved through judicious polymer blending.     

Visual observation and numerical studies of N2 vs. CO2 foaming behavior in core‐back foam injection molding

We investigated , by visual observation and numerical calculations , the foaming behavior of polypropylene within a foam injection mold cavity with the environmentally benign physical blowing agents nitrogen (N₂) and carbon dioxide (CO₂) . An 85‐ton core‐back injection‐molding machine with temperature and pressure monitoring systems as well as a high‐pressure view cell was used for the investigation . The experiments showed a prominent difference in bubble nucleation and growth between N₂ and CO₂ injection foaming . Even when the weight concentration of N₂ dissolved in polymer was one‐third that of CO₂ , N₂ injection foaming provided a bubble number density that was 30 times larger and a bubble size that was one‐third smaller compared to CO₂ injection foaming . Classical bubble nucleation and growth models developed for batch foaming were employed to analyze these experimental results . The models reasonably explained the differences in injection foaming behavior between N₂ and CO₂ . It was clearly demonstrated by both experiments and numerical calculations that N₂ provides a higher number of bubbles with a smaller bubble size in foam injection molding compared to CO₂ as a result of the lower solubility of N₂ in the polymer and the larger degree of super‐saturation . POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers     

Gas transport behavior of semicrystalline syndiotactic polystyrene containing α and β crystalline forms

Solid-state structure of syndiotactic polystyrene (s-PS) after crystallization from the melt and the glassy state was examined by differential scanning calorimetry, density and X-ray diffraction analysis (WAXS). It was possible to prepare semicrystalline s-PS containing pure α (α″ or α′ modifications) and pure β crystalline forms with different crystallinities (0-40%). The measurements confirmed the low density of both crystalline forms, which in the case of α crystalline form was smaller and in the case of β crystalline form was slightly larger than the density of the glassy amorphous s-PS. Oxygen and carbon dioxide gas permeability, diffusion and solubility of semicrystalline s-PS containing different crystalline forms were studied as a function of crystallinity. These measurements confirmed that more dense β crystalline form was impermeable for the transport of small gas molecules while less dense α crystalline form was highly gas permeable. Unusual gas transport behavior of the α crystalline form was attributed to porous crystalline structure containing the nanochannels. Despite the porous structure, α crystalline form showed very low oxygen and carbon dioxide solubility compared to gas solubility in the amorphous phase. The proposed diffusion model explained the characteristic features of the gas permeation behavior for chemically ‘inert’ small gas molecules in the permeation medium consisting of glassy amorphous polymer with dispersed porous crystalline phase containing the nanochannels. A new relaxation process with the maximum at about −35 °C was observed in semicrystalline s-PS containing the α crystals and was associated with the porous structure of this crystalline form.     

Visual observation and numerical studies of polymer foaming behavior of polypropylene/carbon dioxide system in a core‐back injection molding process

Thermoplastic foaming within a mold cavity was visualized as it was conducted in an 85‐ton core‐back injection‐molding machine. The core‐back molding process moved a section of the mold just after injecting a molten polymer into the cavity, quickly reducing the pressure to enhance the bubble nucleation. The foaming behavior during core‐back was observed directly through the glass windows of the mold. In the experiments, impact copolymer polypropylene was foamed with carbon dioxide. The effects of the gas concentration and the core‐back rate on bubble nucleation and growth were investigated. It was experimentally confirmed that the bubbles disappeared when the cavity was fully packed and that bubble nucleation occurred when the mold plate was moved and the cavity pressure dropped. Faster core‐back rates and higher gas concentrations increased the number of bubbles while decreasing their size. To analyze the experimental results, a bubble nucleation and growth model was employed that was based on batch foaming. The numerical results were a reasonable representation of the experiments, and this study demonstrated the applicability of the conventional free foaming model to the industrial core‐back molding process. Many aspects of the foaming in the core‐back molding aresimilar to the behaviors observed by batch foaming. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

成核剂对聚氨酯泡沫成核性能的影响

主要研究了聚氨酯发泡过程中成核剂的添加量、颗粒大小及颗粒的表面状态对气泡成核过程的影响。结果表明:成核剂的加入可有效地促进气泡成核,降低泡孔尺寸;大尺寸颗粒的成核效果要好于小尺寸颗粒;成核剂的颗粒状态对气泡成核的影响不明显。     

成核剂对软质PVC鞋底发泡材料的影响

讨论了4种成核剂二氧化钛、微细重质碳酸钙、纳米级超细碳酸钙和滑石粉对软质PVC鞋底发泡材料性能的影响。结果表明:成核剂TiO2、微细重质碳酸钙、纳米级超细碳酸钙能有效降低材料的密度,而滑石粉增大了材料的密度;成核剂TiO2,微细重质碳酸钙,纳米级超细碳酸钙用量分别为2phr、2phr、1phr时,获得的鞋底发泡材料性能优异;粒径越小的成核剂,获得材料的密度越小、泡孔越均匀细密。     

Morphology modulation of gas‐foamed,micrometric, hollow polystyrene particles

In this article, we reports the effects of the processing conditions on the morphological and hollow attributes of polystyrene micrometric hollow particles produced by the use of a recently developed technique based on the gas foaming of spherical, dense particles. By modulating the foaming temperature and saturation pressure, we produced hollow particles with different attributes in terms of hollow dimensions, eccentricity, and open–close features. The results from these small systems were compared, and we found agreement with what is typically observed in bulk polymeric foaming, for example, an increase in the foaming efficiency with saturation pressure and the nonmonotonic effect of temperature. Furthermore, we observed an increase in the hollow number when using nucleating agents with respect to the neat polymer and when using nitrogen with respect to carbon dioxide as the blowing agent. The effects of particle manipulation before foaming to achieve hollow elongated or distorted particles are also reported. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44236.