Matrix melt impregnation

Matrix melt impregnation Processing window can be large by selecting very different polymers Can be applied to commodity commercial polymer fibres or fabrics Vf limited by the need for a matrix flow path Difficult to achieve good matrix distribution Usually uses more than one type of polymer... 

The main advantage of this technique is that the introduction of the thermoplastic matrix occurs via simple melting. This has led to the development of various types of melt impregnation techniques, which are surveyed below. 

Whiskers can be incorporated into the metallic matrix using a number of compositeprocessing techniques. Melt infiltration is a common technique used for the production of SiC whisker-aluminum matrix MMCs. In one version of the infiltration technique, the whiskers are blended with binders to form a thick slurry, which is poured into a cavity and vacuum-molded to form a pre-impregnation body, or pre-preg, of the desired shape. The cured slurry is then fired at elevated temperature to remove moisture and binders. After firing, the preform consists of a partially bonded collection of interlocked whiskers that have a very open structure that is ideal for molten metal penetration. The whisker preform is heated to promote easy metal flow, or infiltration, which is usually performed at low pressures. The infiltration process can be done in air, but is usually performed in vacuum. 

One of the potential ways how to improve CNT dispersion in polymer matrixes is in-situ polymerization of monomers in presence of nanotubes. Monomers have very small shear viscosity in orders of about lO -lO"3 Pa.s, compared to relatively high viscosity of polymer melts, 103-106 Pa.s. This low viscosity helps to better impregnation and wetting of CNT material, which leads to more efficient dispersion and debundling of the nanotubes aggregates, especially when ultrasound is used as a dispersing agent. 

SC CO2 used as a carrier of drug molecules into a polymer matrix has a number of advantages such as the plasticizing ability of CO2 (based on specific interactions between CO2 and polymer moieties), which both enhances the diffusion rates of drug molecules into the polymer and facilitates solvent removal. Polymer plasticization is accompanied by the swelling of the polymer matrix, with a concomitant increase in the free volume of the polymer. Moreover, SC CO2 can reduce the melting temperature of semicrystalline polymers. These effects are crucial to the impregnation and modification of polymeric materials. 

Compared with conventional techniques for fabricating ceramic-matrix composites, such as hot pressing (HP), reactive melt infiltration (RMI) and polymer impregnation and pyrolysis (PIP), CVI techniques have distinct advantages, which can be summarised as follows [8, 9] ... 

Silicon carbide (SiC) matrix composites have been fabricated by chemical vapor infiltration (CVl), polymer impregnation and pyrolysis (PIP), and reaction sintering (RS). The RS process can be recognized as an attractive technique, because it offers a high density and good thermal conductivity, compared to those of CVl and PIP process. In general, the fabrication of fiber reinforced SiC matrix composites by reaction sintering involves melt infiltration (Ml) or liquid silicon infiltration (LSI). However, the fabrication of continuous fiber reinforced SiC matrix composites by RS focused in melt infiltration (Ml) such as liquid silicon infiltration (LSl) Vapor silicon infiltration was rarely used for SiC matrix composites. 

This technique is based on the impregnation of porous carbon/carbon composites by molten silicon and the reaction of the metallic melt with the solid matrix carbon to silicon... 

The composites included in Table 6.3 were prepared by different methods, indicated in the first column by impregnation (I), recast (R), sol-gel reaction (SG), in-situ crystallization (ISC), melt and extrusion (ME), and in-situ polymerization (ISP). A half of the membranes in Table 6.3 were prepared by the recast procedure, and the selectivity and maximum power density was calculated with reference to the Nafion recast membrane. For the rest of the membranes the Nafion matrix (112, 115 or 117) is indicated. 

This Tg depression enables the processing of polymeric systems that were previously not possible by conventional methods. If one wanted to impregnate a polymer with an active component, the selection of active components would be dictated by the Tg of the host matrix in terms of a requirement for thermal sta-bihty of the component at temperatures of at least the Tg. With the use of high-pressure CO2 and the consequential reduction of the Tg, the selection of active components is broadened to include components that would have suffered from thermal degradation via traditional melt processing. This ability of high-pressure or supercritical CO2 to plasticize the matrix is at the heart of aU applications described in the following sections. 

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