| Polymeric foams are used in a wide range of commercial applications because of their properties of high impact resistance, toughness, low dielectric constant, and good thermal insulation. The applications of polymeric foams decide not only by martial properties but also by foam structure, such as cell size, cell size distribution, cell density and open/close etc. From the whole process of bubble nucleation, growth, and solidification, cell density decides by nucleation, whose control factors include nucleation mode (homogeneous or heterogeneous nucleation), surface tension and pressure. The growth and solidification process strongly depend on materials'rheological properties. From bubble growth dynamics, viscoelastic properties are the main factors controlling foam structure. It is desirable that the viscosity is low during nucleation and early stage of bubble growth, increases gradually as the bubbles grow further, and becomes very high in the late stage of foaming to stabilize the foam structures. So it is necessary to understand the relationship between rheological properties and foam structure to better control of rheological properties and foam structures.Materials used to foam can be simply divided into two categories: single phase materials (amorphous polymer) and multi-component or multi-phase system (such as polymer blends, polymer with nucleation agents and crystalline polymer etc.). For single phase system, except for adjusting by foaming condition (such as saturation pressure, foaming temperature and pressure drop rate etc), an important method to tune foam structure is modification of polymers to get better rheological properties. The most popular modification methods include chain extending, long chain branching and crosslinking. It is just qualitatively described that increasing long chain branching level or molecular length will decrease bubble diameter. But this result is not enough to tune foam structure accurately. So it is necessary to quantitatively know the relationship among molecular structure (especially long chain branching), rheological properties and foam structure to control foam structure accurately. For multi-phase system, the rheological properties of matrix can be tuned by blending, adding nucleation agent, and changing crystallinity. For crystalline polymer, spherulites can act as heterogeneous nucleation agent. But the foaming agent can't enter crystal region, so the bubble can only exist in amorphous region and the bubbles can't distribute uniformly in the matrix. It is qualitatively known that the crystallinity will influence cell diameter, while the influence of spherulites size and density on foam structure is still unknown. So in this paper, supercritical carbon dioxide is used as foam agent to carry out detailed foaming experiments. Firstly, the influence of supercritical carbon dioxide on glass transition temperature, viscosity and crystal form was studied. Then, by free radical reaction, long chain branching (LCB) polystyrene (PS) and polypropylene (PP) with different rheological properties were prepared and LCB level was quantitatively characterized by rheology method. The foam structure was observed by scanning electron microscopy (SEM) and analyzed by image software. On the other hand, bubble growth process of amorphous polymer was simulated by combined bubble growth dynamics, nucleation theory and pom-pom model, and the relationship among molecular structure, rheological properties foam structure was established. For crystalline polymer, the ideal cell density was calculated by heterogeneous nucleation theory and the influence of spherulites density and nucleation mode on foam structure was found. The main contents and results of the research list below:1. As a plasticizer, supercritical carbon dioxide will increase system's free volume, decrease glass transition temperature and viscosity. In the foaming process, the viscosity of the system will increase with decreasing of carbon dioxide concentration, so it needs to know the variation or viscosity with carbon dioxide concentration. From the experimental results, it is found that glass transition temperature decreases linearly with increasing pressure. But for the limitation of experiment setup, the pressure in testing can't increase with freedom, Chow model can be used to predict the variation of glass transition temperature with carbon dioxide pressure. The experimental results are in accordance with the predictions. Combining Chow model with WLF equation, the changing of viscosity with carbon dioxide can be predicted further, and it is used in the simulation of bubble growth dynamics.2. Supercritical carbon dioxide will influence crystallization temperature, dynamics and crystal form. PP can simultaneously crystallize into three crystalline forms, namely, the most commonly observed monoclinic orαform, the hexagonal orβform and the orthorhombic orγform. In this part, the influence of LCB level, crystallization temperature and carbon dioxied pressure (or concentration) on the content ofγform crystal was studied. It is found that the content ofγform crystal increases with LCB level and carbon dioxide pressure. Carbon dioxide is the main factor to formγcrystal. The mechanism of formingγcrystal is explained from the arrangement of molecular chain. Because the arrangement of lelics inαandγcrystal lamellae is different, the number of chains that emerge from the lamellae surface (which means that the density of molecular chain on lamellae surface) will be different. This difference has a remarkable effect on the amorphous phase. Forαcrystal, only when the chains emerged from the lamellae surface fold back, the density of amorphous phase will be lower than that of crystalline phase. But forγcrystal, its density of molecular chain on lamellae surface is lower than that ofαcrystal, so only a few chains emerged fromγcrystal lamellae need to fold back. It can be said that any factor that suppressed chainfolding made the formation ofαcrystal difficult and promoted the formation of theγcrystal. LCB structure increases the chain folding energy and makes molecular chain fold back more hardly, so it will facilitate the formation ofγcrystal. Carbon dioxide increases the free volume of polymer, the chains emerged from lamellae have more space to arrange, which makes less chains fold back to lamellae. So the content ofγcrystal will increase with carbon dioxide pressure. With the synergistic effect of carbon dioxide and LCB structure, the content ofγcrystal in PP can be as high as 90% in total crystal.3. The influence of rheological properties, especially the melt strength, on the foam structure was carried out by the foaming experiments of PS. A series of LCB PS with LCB level range from 0.15 to 1.6 branching point/104 carbon atom was prepared by free radical reaction and their corresponding relaxation time were gotten from frequency sweep data. It is found that shear and elongational rheological properties are dependent on LCB structure, LCB samples show obvious strain hardening behavior and strain hardening coefficient increases with LCB level. From the foaming experiments, it is found that with increasing LCB level, cell diameter decreases, cell size distribution becomes narrower and cell density increases lightly. By strain hardening coefficient, the relationship between rheological properties and cell diameter can be established. The LCB PSs are the blends of linear PS and three-armed star PS and the length of arm is almost the same, so the changing of LCB level is because of the number of arm. When the arm length is close, the higher content of arms, the small the cell diameter is and the narrower the cell size distribution is. In order to know the influence of the length of arm and backbone on the foam structure, pom-pom used to describe the molecular dynamics of polymer, coupled with bubble growth dynamics and classical nucleation theory, the simulation on bubble nucleation and growth process is performed. The simulation results are in accordance with experimental results. It is found that the arm length had greater influence on the cell radius than the backbone length. Based on the experiment and simulation results, the relationship among rheological properties, molecular structure and foam structure can be established. The relationship can be used as a guidance to control the foam structure by designing and controlling the molecular structures and the corresponding rheological properties.4. The influence of crystal on foam structure was achieved by changing the rheological properties, nucleation mode, spherulits size and density of PP. By LCB modification of linear PP, its crystallization rate improves, spherulites size decreases and spherulits density increases greatly. For linear PP, cell diameter decreases with crystallization time. There are two reasons: on the one hand, the increasing of viscosity (because of crystallization) hinders bubble growth; on the other hand, the nucleation mode changes from homogeneous one to heterogeneous one. At the same time, heterogeneous nucleation increases the cell density. But because of the slow crystallization speed, big spherulites and small number of spherulites of linear PP, its cell size is bigger and cell density is not very high compared to that of LCB PP. After LCB modification, the viscosity and melt strength increase, which leads to smaller cell diameter. Cell diameter decreases with crystallization time and the bubbles will disappear when crystallization finishes. Cell density will increase first and then reach a plateau. At the earlier stage of crystallization, the crystal forms quickly, and it acts as the heterogeneous agent. The increasing trend of cell density is similar to that of spherulites density. After a period of crystallization, the spherulites density almost keeps constant while cell density still increases and even higher than spherulites density at last. This is because there is homogeneous and heterogeneous nucleation at the same time. For the heterogeneous nucleation of linear PP and LCB PP, the decreasing of nucleation barrier by different shperulites size is almost the same. The difference of spherulites density is the main reason for the difference of cell density of linear and LCB PP. By comparison of experimental and ideal cell density, it is found that the nucleation efficiency of spherulites in LCB PP is a little higher than that of linear PP. For cell size distribution, it is decided by rheological properties and crystal structure. It becomes narrower with increasing crystallization time, but LCB PP has narrower cell size distribution than linear PP. From the experiments, it can be concluded that the foam structure of crystalline polymer can be tuned by rheological properties and crystal structure.5. The above discussed experiments are all batch foaming process, and the polymers need modification before foaming, so the experimental period is long and the process is complicated. In order to improve efficiency, it is desirable to modification and foaming at the same time. By design of screw, die, and foaming condition, the continuous reactive foaming process is carried out successfully. By design of formulation and adding nucleation agent, the foam with small cell size and uniform distribution can be fabricated. This is a simple but effective method to extrusion foaming of low melt strength polymer.
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