Foamed morphology

EFFECT OF NUCLEATING AGENTS ON POLYPROPYLENE FOAM MORPHOLOGY... 

Keywords Blend Foam Morphology Compatibilization Multiphase Nanostructured... 

Influence of compatibilization on the foam morphology - despite the significant influence and relevance of compatibilization on the blend structure and interfacial behavior, only partly discussed for polyolefins/styrenics blends [37]... 

It should further be noted that the onset of continuity of the dispersed blend phase not only deteriorates the overall mobility of the SAN to form cellular structures, but also increases the average phase size of PPE and, thus, sterically hinders the incorporation into the cell walls. In particular, for the PPE/SAN 60/40 blend, showing some co-continuous features and solid-state characteristics at 180°C, the foamability is limited to such an extent that only local and less defined cell growth proceeds, leading to the highly inhomogeneous foam morphologies. 

The results demonstrate that, in contrast to blending of miscible polymers, which allows tailoring of both foam morphology and foam processing via controlling the... 

Fig. 18 Foam morphologies of PPE/SAN 60/40 blends as a function of the SBM content, as observed by SEM (foaming time 10 s, foaming temperature 160°C)...

Fig. 19 SEM and TEM micrograph showing the foam morphology and structure of the cell walls of foamed PPE/SAN/SBM 48/32/20 blends (PPE - dark, SAN - bright, PB of SBM - black, foam cell - white, foaming time 10 s, temperature 180°C)...

Fig. 27 Foam morphology of (PPE/PS)/SAN blends, as observed by scanning electron microscopy (foaming temperature 180°C, foaming time 10 s)...

Similar studies on foam morphology were reported by Williams et al.12,13 for polyimide foams with different densities or surface area also two different chemical formulations were used. Comparing foams with the same chemical composition, it was shown that no consistent correlation could be found between PHRR and foam density or open cell content while greater correlation is proved between the surface area and PHRR because they showed the same trend. Foams having the same density but different surface area and chemical composition show great variation of PHRR (up to 50%) both at 75 and 50kW/m2. 

A further elucidation of tte nature of foamed plastics requires an even more extensive use of methods of physics and physical chemistry of polymers. In this way, a quantitative estimation of the oligomeric specificity of foam morphology will be possibte and especially the relationship between chemical and supetmolecular organizations of walls and struts in gas-structure dements and the morphological parameters of foams could be established. 

According to Karapetyan the coefficient q is, in turn, related with the elastic wave propagation parameters in a cellular material aral foam morphology by the following formula ... 

These results reveal that- a plastic foam structure may be considered as a system of thin films and, therefore, support a model of plastic foam morphology namely a matrix system composed of thin polymeric films defining two groups of cells (macro- and microcells). Additional support in favor of this model of plastic foam structure is provided by the studies on the electric properties of plastic foams Among the numerous equations so far advanced for the calculation of the dielectric properties, the expressions which describe the dry foam structure by one of the limiting cases of a matrix system, namely a laminated dielectric structure with layers parallel to the force lines of the electrical field, agree best with the experiments >. 

Reid developed analytical tools for studing the cell size distribution from dissected sphere size data measured on a section surface. The first to introduce a general theory of cell size distribution in a solid body was Ceilings who calculated the macrostructural parameters of a number of real metal systems, On the basis of Ceilings method, Mihira et al. developed the principles of the statistical analysis of plastic foam morphology. 

In order to gain a more profound knowledge, the first problem must be a wider use of the concepts of polymer physics and physicochemistry. This would enable an evaluation of the specific polymeric features of plastic foam morphology, primarily the different structures and types of the submolecular organization of cell walls and struts, and thus allow to understand their effect on the microstructure of the foam density and the degree of orientation of foam cells. 

Initially several researchers measured the effective viscosity of bulk foam using rotational or capillary viscometers (1, 49) with the hope of applying their results to porous media. On the basis of the earlier discussion of foam morphology in porous media, such data are inappropriate (50). Interaction of elongated bubbles and pore-spanning lamellae with pore walls determines the effective viscosity of the flowing portion of foam. Such interactions are simply not mirrored in bulk foam viscometry. 

Microcellular materials exist in many forms. Methods for production of these materials are as varied as potential applications. This chapter reviews the technology of one class of microcellular materials, microcellular foams, which are sought for biomedical applications. Included is a description of several methods of foam production, foam morphologies, and present uses for microcellular foam materials. New methods of microcellular foam production and potential uses for the resultant foam materials are important to those interested in biomaterials and contemporary biomedical applications. It is for this reason that advances in microcellular foam formation are emphasized in the final section of this chapter. Increasingly, it is becoming evident that microcellular foams can be used effectively in many medical applications, particularly polymeric foam materials which are being investigated in this laboratory. For this reason, the focus of this chapter pertains to possible biomedical applications of polymeric microcellular foams. 

Lesser [88] studied the continuous extrusion of a range of high-melt-viscosity polymers using a single screw extruder with a temperature-controlled die. Control of the foam morphology (cell density and cell size distribution) of polytetra-fluoroethylene (PTFE), fluorinated ethylene propylene cofxtlymer (FEP), and... 

Kucheyev SO, Baumann TE, Cox CA, Wang YM, Satcher JH, Hamza AV, Bradby JE (2006) Nanoengineering mechanically robust aerogels via control of foam morphology. Appl Phys Lett 89 041911. 

Yao et al. [72] investigated PLA/PMMA blend foams at 75/25, 50/50, and 25/75 wt% compositions for medical applications such as cell growth. Factors such as solubility and carbon dioxide diffusivity played an important role in these applications. Gas solubility was studied using a microelectronic balance. Henry s law constant for CO2 in the blends increased with an increase in the PLA content of the blends. Foam morphologies of the blends as compared to neat polymers were investigated with respect to the effects of pressure and temperature. PLA/PMMA blends improved the foam morphologies compared to the neat polymers. More details about these blends can be obtained in Chapter 17. 

The pressure drop rate is correlated to the thermodynamic instability necessary to generate the nuclei and, mainly, affects foam morphology. The gas concentration is directly correlated to the availability of gas necessary for ceU growth, influencing the final density of the foamed plastic [72]. The pressure drop rate in the die (—dp/dt) is expressed as follows [24] ... 

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