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。     

Conventional and nanometric nucleating agents in poly(ϵ‐caprolactone) foaming: Crystals vs. bubbles nucleation

The aim of this article was to investigate the nucleating ability of different nucleating agents for the foaming of poly(ε‐caprolactone), a biodegradable, semicrystalline polymer. In particular, the efficiency of the nucleating agent in inducing the formation of the gaseous phase has been compared to the efficiency in inducing the formation of the crystalline phase. In effect, in foaming of semicrystalline polymers, bubble nucleation and crystal nucleation are concurrent and somehow interacting phenomena. Here, these two aspects have been evidenced and clarified. Foams were prepared by using a batch process with the pressure quench method, with nitrogen and carbon dioxide as the blowing agents. Conventional and novel nucleating agents were used: talc has been compared to several novel nanometric particles of different geometries and dimensions, such as titanium dioxide and alumina powders, exfoliated and intercalated clays, and carbon nanotubes. Foam densities and morphologies, in terms of number of cells per initial unit volume, were measured and found to depend both on crystalline phase nucleation and gaseous phase nucleation. In fact, the different nucleating agents, depending on shape, dimension, and surface functionalization, selectively nucleated the crystallites and/or the bubbles, affecting, respectively, bubble growth (and, hence, final foam density) and bubble nucleation (and, hence, cell number density—morphology). POLYM. ENG. SCI., 2008. © 2007 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。     

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     

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.     

振动力场对含有成核剂的微孔塑料泡孔结构的影响

在微孔塑料成型加工过程中,成核剂的加入可以增加泡孔成核点数量,从而增加泡孔密度,改善泡孔的结构,但如果纳米级成核剂在发泡过程中分散性不好,会使泡孔分布不均匀,影响泡孔质量。在实验中,对含有成核剂(纳米碳酸钙)的发泡材料(聚苯乙烯)施加振动场,通过比较发现,施加振动后,纳米级成核剂的分散效果明显改善,颗粒分布变得更均匀,从而使得泡孔分布更均匀,改善了泡孔的结构,提高了微孔塑料泡孔结构质量。     

成核剂对聚丙烯釜压发泡的影响

用超临界二氧化碳（CO2）釜压发泡的方法,研究了成核剂类型、成核剂粒径以及成核剂添加量对聚丙烯（PP）发泡材料泡孔结构的影响。结果表明,用碳酸钙（CaCO3）作成核剂时PP泡沫的泡孔完整性高,泡孔尺寸分布均匀,且发泡倍率比添加蒙脱土及滑石粉时的要大;成核剂粒子粒径越小,体系的成核点越多,发泡时产生的气泡核越多,所得到的PP泡沫的泡孔密度越大,但是由于纳米碳酸钙（nano-CaCO3）更容易出现团聚现象,直接导致最终发泡制品产生泡孔破裂以及发泡倍率的降低;成核剂CaCO3的添加量为3份时,与添加1份和5份相比,可得到发泡倍率更高,泡孔密度更大的PP泡沫。     

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     

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.     

Influence of the viscosity of poly(methyl methacrylate) on the cellular structure of nanocellular materials

Three different grades of poly(methyl methacrylate) (PMMA) with different rheological properties are used for the production of nanocellular materials using gas dissolution foaming. The influences of both the viscosity of the different polymers and the processing parameters on the final cellular structure are studied using a wide range of saturation and foaming conditions. Foaming conditions affect similarly all cellular materials. It is found that an increase of the foaming temperature results in less dense nanocellular materials, with higher cell nucleation densities. In addition, it is demonstrated that a lower viscosity leads to cellular polymers with a lower relative density but larger cell sizes and smaller cell nucleation densities, these differences being more noticeable for the conditions in which low solubilities are reached. It is possible to produce nanocellular materials with relative densities of 0.24 combined with cell sizes of 75 nm and cell nucleation densities of 10¹⁵ nuclei cm^?3 using the PMMA with the lowest viscosity. In contrast, minimum cell sizes of around 14 nm and maximum cell nucleation densities of 3.5 × 10¹⁶ nuclei cm^?3 with relative densities of 0.4 are obtained with the most viscous one. © 2019 Society of Chemical Industry     

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.     

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。     

SBA-15形貌对聚乳酸微孔发泡的影响

文章采用水热法合成了球状、棒状与环状三种形貌的SBA-15无机介孔粒子,利用含氟硅烷对粒子进行亲二氧化碳修饰后,用于聚乳酸超临界二氧化碳(scCO2)微孔发泡,探索了粒子形貌对聚乳酸发泡材料的泡孔结构的影响。TG,BET和SEM等分析表明,不同粒子形貌的介孔粒子具有不同的硅烷接枝率,在聚乳酸微孔发泡材料中异相成核作用也不同,就泡孔尺寸和泡孔密度来讲,环状SBA-15粒子对聚乳酸发泡的异相成核作用最好,表明介孔粒子有望成为新型的微孔发泡成核剂。     

Polypropylene‐dispersed domain as potential nucleating agent in PS and PMMA solid‐state foaming

The potential of using dispersive domains in a polymer blend as a bubble nucleating agent was investigated by exploiting its high dispersibility in a matrix polymer in the molten state and its immiscibility in the solid state. In this experiments, polypropylene (PP) was used as the nucleating agent in polystyrene (PS) and poly(methyl methacrylate) (PMMA) foams at the weight fraction of 10, 20, and 30 wt %. PP creates highly dispersed domains in PS and PMMA matrices during the extrusion processing. The high diffusivity of the physical foaming agent, i.e., CO₂ in PP, and the high interfacial tension of PP with PS and PMMA could be beneficial for providing preferential bubble nucleation sites. The experimental results of the pressure quench solid‐state foaming of PS/PP and PMMA/PP blends verified that the dispersed PP could successfully increase the cell density over 10⁶ cells/cm³ for PS/PP and 10⁷ cells/cm³ for PMMA/PP blend and reduce the cell size to 24 μm for PS/PP and 9 μm for PMMA/PP blends foams. The higher interfacial tension between PP and the matrix polymer created a unique cell morphology where dispersed PP particles were trapped inside cells in the foam. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structure and dielectric properties of polyimide/silica nanocomposite nanofoam prepared by solid‐state foaming

In this article, polyimide (PI)/silica nanocomposite nanofoams were prepared by solid‐state foaming using supercritical CO₂ as foaming agent. To control the cell size and morphology of the PI/silica foam, the silica nanoparticles as nucleating agent were in situ formation from TEOS via sol‐gel process, which make the silica nanoparticles homogeneously dispersed in PI matrix. The resulting PI/silica nanocomposite nanofoams were characterized by scanning electron microscopy (SEM), the image analysis system attached to the SEM and dielectric properties measurements. In PI/silica nanocomposite nanofoams, one type of novel morphology was shown that each cell contained one silica nanoparticle and many smaller holes about 20–50 nm uniformly located in the cell wall. This special structure could visually prove that the nucleation sites during foaming were formed on the surface of nucleating agents. Compared with those of neat PI foam, the cell size of PI/silica nanocomposite nanofoams was smaller and its distribution was narrower. The dielectric constant of PI/silica nanocomposite nanofoams was decreased because of the incorporation of the air voids into the PI/silica nanofoams. While the porosity of PI/silica nanocomposite nanofoam film was 0.45, the dielectric constant of the film (at 1 MHz) was reduced from 3.8 to about 2.6. Furthermore, the dielectric constant of PI/silica nanofoam films remained stable across the frequency range of 1×10²～1×10⁷ H_Z. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42355.     

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.     

Foaming of rigid PVC/wood‐flour composites through a continuous extrusion process

The effects of chemical foaming agent (CFA) types (endothermic versus exothermic) and concentrations as well as the influence of all‐acrylic processing aid on the density and cell morphology of extrusion‐foamed neat rigid PVC and rigid PVC/wood‐flour composites were studied. Regardless of the CFA type, the density reduction of foamed rigid PVC/wood‐flour composites was not influenced by the CFA content. The cell size, however, was affected by the CFA type, independent of CFA content. Exothermic foaming agent produced foamed samples with smaller average cell sizes compared to those of endothermic counterparts. The experimental results indicate that the addition of an all‐acrylic processing aid in the formulation of rigid PVC/wood‐flour composite foams provides not only the ability to achieve density comparable to that achieved in the neat rigid PVC foams, but also the potential of producing rigid PVC/wood‐flour composite foams without using any chemical foaming agents.     

Investigation of a strategy for well controlled inducement of microcellular and nanocellular morphologies in polymers

This study is an effort to modify conventional batch processes to be able to produce polymeric foams with high cell density and small cell size, which cannot be reached by conventional batch foaming processes. This has been attained by controlling the foaming temperature and controlled stabilization of the cellular structure. The method was tested for both with and without addition of nanosized particles in polymeric matrix. The desired morphologies were obtained using a novel apparatus with the capability of instantaneous pressure drop and controlling stabilization of the foam structure. The design of the said apparatus was based on the idea that in a foaming process, nucleation is the predominant mechanism that determines the final foam structure. The produced foam products have uniform structures without any unfoamed skin. Results show that the control of the foaming temperature and the cell stabilization are the predominant factors in adjustment of the final foam morphology. A wide range of microcellular structures with cell densities between 10⁷ and 10¹² bubbles/cm³ and average cell sizes of 500 nm–20 μm were produced. Foaming of polystyrene‐nano‐silica nano‐composites with the same method showed that nanoparticles act as nucleating agent and increase the cell density in the final foam products compared with that of neat polystyrene. POLYM. ENG. SCI., 50:1558–1570, 2010. © 2010 Society of Plastics Engineers     

Effect of die temperature on the morphology of microcellular foams

A study on the extrusion of microcellular polystyrene foams at different foaming temperatures was carried out using CO₂ as the foaming agent. The contraction flow in the extrusion die was simulated with FLUENT computational fluid dynamics code at two temperatures (150°C and 175°C) to predict pressure and temperature profiles in the die. The location of nucleation onset was determined based on the pressure profile and equilibrium solubility. The relative importance of pressure and temperature in determining the nucleation rate was compared using calculations based on classical homogeneous nucleation theory. Experimentally, the effects of die temperature (i.e., the foaming temperature) on the pressure profile in the die, cell size, cell density, and cell morphology were investigated at different screw rotation speeds (10 ～ 30 rpm). Experimental results were compared with simulations to gain insight into the foaming process. Although the foaming temperature was found to be less significant than the pressure drop or the pressure drop rate in deciding the cell size and cell density, it affects the cell morphology dramatically. Open and closed cell structures can be generated by changing the foaming temperature. Microcellular foams of PS (with cell sizes smaller than 10 μm and cell densities greater than 10 cells/cm³) are created experimentally when the die temperature is 160°C, the pressure drop through the die is greater than 16 MPa, and the pressure drop rate is higher than 10⁹ Pa/sec.     

Preparation and characterization of biodegradable foams from calcium carbonate reinforced poly(propylene carbonate) composites

Biodegradable foams were successfully prepared from calcium carbonate reinforced poly(propylene carbonate) (PPC/CaCO₃) composites using chemical foaming agents. The incorporation of inexpensive CaCO₃ into PPC provided a practical way to produce completely biodegradable and cost‐competitive composite foams with densities ranging from 0.05 to 0.93 g/cm³. The effects of foaming temperature, foaming time and CaCO₃ content on the fraction void, cell structure and compression property of the composite foams were investigated. We found that the fraction void was strongly dependent on the foaming conditions. Morphological examination of PPC/CaCO₃ composite foams revealed that the average cell size increased with increasing both the foaming temperature and the foaming time, whereas the cell density decreased with these increases. Nevertheless, the CaCO₃ content showed opposite changing tendency for the average cell size and the cell density because of the heterogeneous nucleation. Finally the introduction of CaCO₃ enhanced the compressive strength of the composite foams dramatically, which was associated with well‐developed cell morphology. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5240–5247, 2006