PVC微孔塑料加工工艺与泡孔结构及耐热性关系研究

以超临界CO2为发泡剂,用动态发泡实验装置在不同的发泡温度、气体饱和压力及振动频率和振幅等加工工艺条件下制备了聚氯乙烯(PVC)微孔塑料。研究发现,发泡温度存在着一个最佳温度范围,使得泡孔密度最大、泡孔尺寸最小;气体饱和压力越大,泡孔结构越好;当剪切速率较低时,在发泡过程中施加强振动作用能显著提高泡孔密度,减小泡孔尺寸,当剪切速率较高时,施加较弱的振动作用即可改善泡孔形态,而施加较强的振动作用可能会产生较大的剪切热和脉动剪切应力,从而破坏泡孔结构;PVC微孔塑料的维卡软化温度与泡孔结构有着密切的关系,泡孔密度越大、泡孔尺寸越小,维卡软化温度就越高。     

Optimized Polystyrene Cell Morphology by Orthogonal Superposition of Oscillatory Shear

A novel dynamic microcellular foaming technology is presented in the form of mechanical vibration, forming an oscillatory shear orthogonal superposition upon steady shear flow. The effects of steady and oscillatory shear on polystyrene foam processing were investigated with a novel dynamic foam processing simulator. Cell morphology was analyzed by using a scanning electron microscope (SEM). The results show that the oscillatory shear has a significant influence on the cell morphology. A fine cell structure with nearly spherical cell shape is nucleated with properly arranged rotor speed and vibration parameters. At the same time, cell density increases remarkably with the introduction of vibration.     

振动作用对PS和PVC微孔塑料泡孔结构影响的研究

以超临界CO2为发泡剂，用动态发泡实验装置制备了PS和PVC微孔塑料，通过扫描电镜照片观察和研究了振动作用对PS和PVC微孔塑料泡孔结构的影响。结果表明，当剪切速率较低时，在PS发泡过程中施加较弱的振动作用即可显著提高泡孔密度，减小泡孔直径；而在PVC发泡过程中，只有施加相对较强的振动作用才能达到同样的效果。当剪切速率较高时，不论何种发泡体系，施加较弱的振动作用可以改善泡孔的形态；而施加较强的振动作用可能会产生较大的剪切热和脉动剪切应力，从而破坏泡沫的微孔结构。     

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。     

剪切对PP/HDPE共混体系微孔发泡成型的影响

在熔体流动方向垂直叠加一个轴向脉动剪切,研究转子转动产生的剪切和轴向脉动剪切对PP/HDPE共混体系微孔发泡成型的影响。研究结果表明,随着转子转速增加,泡孔尺寸减小,气泡成核密度增大。但是转子转速过快,泡孔沿剪切的方向被拉长,泡孔取向严重,泡体质量变差。在熔体流动方向垂直叠加一个轴向脉动剪切以后,泡孔的取向现象减小,泡孔逐渐趋于圆形,泡孔结构得到改善。而且随着振动振幅和频率的增加,泡孔直径减小,泡孔密度增加。     

微孔发泡过程中脉动剪切对聚合物/超临界 CO2均相体系形成的影响

提出一种聚合物／超临界CO2均相体系快速形成的新方法——将振动力场引人到发泡过程，在简单剪切的基础上垂直叠加一个脉动剪切场，考察了该方法对均相体系形成的影响。实验材料用聚苯乙烯(PS)，用自行设计的电磁动态发泡模拟机，了不同剪切速率、振动力场及混合时间对聚合物／超临界CO2均相体系形成的影响。结果表明，随着剪切速率的增加，聚合物和气体的混合程度明显增强；而振动力场的引入进一步增强了剪切混合的效果，使聚合物／超临界CO2均相体系能够在较短的时间内形成。     

动态发泡工艺参数对PS微孔塑料泡孔结构的影响

以超临界CO2为发泡剂,用振动诱导发泡模拟装置研究了微孔塑料动态成型过程中气体饱和压力、压力释放速率、温度、气体饱和时间、稳态剪切速率、振动等工艺参数对聚苯乙烯（PS）微孔塑料泡孔结构的影响。研究发现,PS微孔塑料试样的泡孔结构随着气体饱和压力和压力释放速率的提高而得到改善,而温度、气体饱和时间、稳态剪切速率则存在一个最佳的操作范围,在此范围内制得的PS微孔塑料试样泡孔密度最大,泡孔尺寸最小。在稳态剪切速率一定的情况下,通过施加振动可以进一步改善泡孔结构．     

Effect of dynamic shear on the microcellular foaming of polypropylene/high‐density polyethylene blends

Dynamic shear in the axial direction of a rotor was vertically superposed on the melt flow direction, and its effects on the shear rate and melt strength were investigated theoretically. Polypropylene/high‐density polyethylene blends were microcellularly foamed with different vibration parameters. The experimental results were compared with those of a theoretical analysis, and the effects of dynamic shear on the foamability and ultimate cell structure were analyzed in detail. The theoretical results showed that the shear rate and melt strength increased with an increase in the vibration amplitude and frequency. The enhanced melt strength could effectively restrict cell growth, prevent cell rupture, and improve foamability. The experimental results showed that the cell orientation decreased and the cell structure was improved when axial dynamic shear induced by rotor vibrations was superposed on the melt flow direction. Furthermore, the cell diameter decreased and the cell density increased with increases in the vibration amplitude and frequency. The experimental results were very consistent with the theoretical analysis. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

振动共混对PC/PP多重结晶行为的影响

在传统转矩流变仪上叠加机械振动帛成了振动共混流变仪，叠加机械振动的流动影响共混物的相形态，因此影响了PC／PP的多重结晶行为。DSC和偏光显微联用分析发现，所有峰对应的结晶都是异相成核，共混频率或振幅增高，减小分散相相畴尺寸，PP粒子的异相核会向PC迁移，所以制备的共混物的低温结晶峰越强，而高温结晶峰减弱，共混振幅很大时，中等尺寸粒子界面附生非均相成核等效增加了较高温度成核率，结晶温度较高。     

Shear stress nucleation in microcellular foaming process

The effect of shear stress on the foaming process has been studied using the Foaming Process Simulator developed previously. The polymer samples were saturated with gas in the test chamber. A rotor was used to apply shear stress to the polymer samples. Foams were obtained by releasing the pressure quickly. Polystyrene, filled and unfilled, was used as the material. The cell density was analyzed with a scanning electron microscope. It was found that the cell density was significantly increased by introducing shear stress. The higher the shear stress, the more significant the effect. A cell stretch model has been developed to explain the cell nucleation enhancement with shear stress. The nucleation sites are stretched under the shear stress. The stretched nuclei are much easier to expand for cell formation owing to their larger surface areas and non‐spherical shapes. The model prediction shows the same tendency of the effect of shear stress observed in the experiment. The key issue with shear stress nucleation is the transformation of mechanical shear energy into surface energy.     

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.     

Morphology Evolution of the Microcellular Polymethyl Methacrylate with Supercritical CO2: Effects of Shear Stress and Orthogonal Vibration

This study presents a foaming system that was developed to generate shear stress as well as oscillatory mechanical vibration in an orthogonal direction. The microcellular processes of PMMA specimens blown with ScCO₂ have been conducted to find the optimal processing parameters under the static conditions as well as dynamic conditions. It is evident that the cells are drawn into elliptical geometries sheared circumferential by the rotor rotation in isolation. However, the orthogonal pulsatile axial vibration superimposed on the polymer melt synchronously drives the oriented and elongated cells to isotropy as well as a more homogenous cells distribution and planar structure.     

Foaming of Homogeneous Polypropylene and Ethylene-Polypropylene Block Copolymer Using Supercritical Carbon Dioxide

A new process was used to foam homogeneous polypropylene (HPP) and ethylene-polypropylene block copolymer (CPP). Many different foaming behaviors of these two kinds of PP were observed. The HPP and CPP were characterized by Differential Scanning Calorimetry (DSC), and Capillary Rheometry. We find that the melt shear viscosity of CPP is more sensitive to the temperature variation than that of HPP, thus leading to larger change of cell diameter of CPP with foaming temperature. Cell size of CPP is the result of competition between cell nucleation and cell growth. When the saturation pressure is lower or higher than 25 MPa, cell nucleation or cell growth plays a dominant role, which leads to the increase or decrease of cell size, respectively. Because of its low melting temperature and crystallinity, cell diameter of CPP increases with the infiltration temperature increasing, which is opposite to those of HPP. And at the foaming temperature of 152°C or 158°C, cell diameter of CPP increases or decreases with an increase in foaming time, while that of HPP decreases with foaming time increasing at both foaming temperatures.     

Effect of Processing Parameters and Vibrating Field on Poly(Vinyl Chloride) Microcellular Foam Morphology

The effects of processing parameters such as processing pressure, temperature, mixing time and rotor speed on polyvinyl chloride foams were investigated by using a novel microcellular foaming setup. The experimental results show that a proper temperature and a high pressure can promote CO₂ dissolving in polymer, which makes cell density increase and cell size decrease. Increasing mixing time and rotor speed also promote CO₂ dissolving in PVC and speed up forming single-phase polymer/CO₂ solution. The effects of oscillatory shear on polyvinyl chloride cell morphology were also studied. The combined shear improves the mixing, and thus shortens the time needed for homogeneous polymer/supercritical CO₂ solution formation. Foamed samples with the cell density of 1.0 × 10⁷–3.5 × 10⁸ cells/cm³, average cell size of 15–60 µm and bulk density of 0.6–0.87 g/cm³ had been produced.     

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     

Solid‐state microcellular polycarbonate foams. I. The steady‐state process space using subcritical carbon dioxide

The process parameters for production of solid‐state microcellular polycarbonate using subcritical CO₂ were explored. Sufficiently long foaming times were used to produce foams, where cell growth had completed, resulting in steady‐state structures. A wide range of foaming temperatures and saturation pressures below the critical pressure of CO₂ were investigated, establishing the steady state process space for this polymer–gas system. Processing conditions are presented that produce polycarbonate foams where both the foam density and the average cell size can be controlled. The process space showed that we could produce foams at a constant density, while varying the cell size by and order of magnitude. At a relative density of 0.5, the average cell size could be varied from 4 to 40 μm. The ability to produce such a family of foams opens the possibility to explore the effect of microstructure, like cell size on the properties of cellular materials. It was found that the minimum foaming temperature for a given concentration of CO₂, determined from the process space, agrees well with the predicted glass transition temperature of the gas–polymer solution. A characterization of the average cell size, cell size distribution, and cell nucleation density for this system is also reported. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

An extrusion die with rollers for foaming polymers

Operation of a flat slot die with rollers for the extrusion foaming of polymers, which has been originally designed by Benkreira et al. (Int. Polym. Proc., 19, 111 (2004)), is considered. The rotation of rollers makes it possible to independently control shear rate inside the die, in the bubble nucleation zone, thus influencing the cell density. In experimental studies of low‐density polyethylene foaming, the influence of the roller rotation speed on the cell density at variable isobutane and talc concentrations and die outlet area has been determined. Based on the fluctuational nucleation theory, a simple model is proposed for estimating the number density of supercritical nuclei formed at the nucleation stage and evaluating the cell density in the foam with allowance for bubble coalescence effects. POLYM. ENG. SCI., 54:96–109, 2014. © 2013 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.     

A basic population balance model for fluid bed spray granulation

A basic population balance approach is developed for a granulation process in a fluid bed spray granulator. The particle size distribution predicted by the model is confirmed by plant data. Hence this model is considered to be useful to describe and optimize an industrial process. The model depends on a limited number of parameters (most of these factors can be measured or are known): the spray volume flux, the nucleation fraction (the fraction of the spray volume flux which leads to new particles formed), the nucleation particle diameter, the product withdrawal threshold diameter, and the product withdrawal rate. Analysis of the model reveals a steady-state constraint; a steady state does not exist if the nucleation fraction is too large. For cases where the steady state does exist, the steady-state particle size distribution is solved analytically. A numerical implementation of the model is used to illustrate the transient evolution of the process. The steady-state solution appears to be stable for a constant nucleation fraction. However, if the nucleation fraction depends on the bed height the steady state can be unstable. Such a situation may occur if the spray inlet is near the height of the bed surface. Instead of convergence towards a steady state, the transient solution displays ongoing oscillatory behavior with an oscillation period of a number of hours. A linear stability analysis is performed to confirm the findings on the stability of the steady state.     

Effects of the shear flow on a homogeneous polymeric reaction

The reaction kinetics of the postamidation of polyamide 6 in the melt was studied by a new rheological method, and the reaction rate constant was calculated to be about 0.087 kg/mol min at 250°C. To study the effects of the shear flow on a homogeneous polymeric reaction, a more practical technique was used to select the data that responded only to the reaction without the effect of different shear flows. The experimental results showed that the reaction rate constant decreased with an increase in the shear rate. Moreover, the decreases in the rates were different under different shear modes: the reaction rate constant decreased slightly under oscillatory shear but strongly under steady shear. Possible reasons for these phenomena were examined. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3056–3061, 2006