Supercritical modification

Supercritical modification of polymers was studied by several scientists to improve or change the properties of polymers. Polymers can either be chemically or physically modified. Examples of chemical modifications are the functionalization of polymers (grafting) or a chemical reaction of the functional groups of polymers to obtain new materials [38, 39]. Examples of physical modifications are the preparation of polymer blends, impregnation of polymers with additives [46], or foaming of polymers [59-61]. Another studied topic of polymer modification and impregnation is the supercritical dyeing of polymer fibers [40, 41). 

The aim of this chapter is to present the scope and limitations of the chemical modification of (swollen) polymer particles in supercritical and subcritical CO2. The supercritical modification technique will be critically evaluated as a promising alternative for the more conventional melt and solution modification techniques. In this respect, both effectiveness and economical and environmental issues play a role. 

As reported for the PA-6 samples whose amine end groups had been blocked with SA, the molecular weights (M ) before and after modification with 1,2-EB proved to be the same within experimental error. Moreover, the melt stability of these modified PA-6 samples proved to be virtually invariant after a treatment for 30 min at 260 °C. Consequently, the supercritical modification of PA-6 with 1,2-epoxybutane also results in a polyamide film grade with an enhanced melt stability. 

For this reason, the fact that for the modification with diketene at 75 °C and 6 MPa for 24 h a low concentration of amine end groups is found (i.e. 8 mmol kg , which is in the same order of magnitude as the 11 mmol kg obtained under supercritical conditions at 100°C/10 MPa, see earlier) is ascribed to both the very high reactivity of the diketene and the concentration effect. Interestingly, after 4 h the supercritical modification with diketene at 100 °C and 10 MPa is more... 

Polymer miscibility, enhancing, 14 476 Polymer modification, supercritical fluid impregnation in, 24 20 Polymer networks, interpenetrating, 79 834 20 327... 

The use of a polyfunctional catalyst could enhance the life of the catalyst. A clear example is the use of H3PWi2O40-SO4 /ZrO2 mixtures for isobutane/ butenes alkylation (Table 13.4-). However, modifications of the t) pe of reactor could also favor extended catalyst longevity." During the last few years, other alternatives have been proposed that favor a better catalyst regeneration and/or lower catalyst deactivation the use of supercritical isobutene regeneration or dense-C02 enhanced the reaction media. ... 

For systems where the catalyst is required in the CO2 phase a modification of the hgand periphery to increase the solubihty in the supercritical medium is usually necessary. This has to be mostly done via introduction of perfluorinated tags ( ponytails ) which causes expensive and/or sometimes difficult synthetic operations. 

The major drawback of this reaction system is the high energy and equipment costs due to the use of high pressures. In addition, the use of supercritical carbon dioxide can have adverse effects on enzymes, for example, by decreasing the pH of the microenvironment of the enzyme, by the formation of carbamates owing to covalent modification of free amino groups at the surface of the protein and by deactivation during pressurisation-depressurisation cycles [4]. 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

In the present work, three approaches supercritical drying, templating with surfactants of quaternary ammonium salts and a two-step modification are applied to the sol pillared clay to tailor the pore structure. Mechanisms involved in these processes are discussed in detail. 

BRITE/Euram Project Polymerization and Polymer Modification in Supercritical Fluids (1997 to 2000). 

A gaseous pure component can be defined as supercritical when its state is determined by values of temperature T and pressure P that are above its critical parameters (Tc and Pc). In the proximity of its critical point, a pure supercritical fluid (or a dense gas as it is alternatively known) has a very high isothermal compressibility, and this makes possible to change significantly the density of the fluid with relatively limited modifications of T and P. On the other hand, it has been shown that the thermodynamic and transport properties of supercritical fluids can be tuned simply by changing the density of the medium. This is particularly interesting for... 

We have demonstrated in this paper that two and four samples can be extracted in parallel with supercritical carbon dioxide without significant impact on data quality. Modifications made to an off-line extractor involved addition of a multiport manifold for the distribution of supercritical fluid to four extraction vessels and of a 12-port, two-way switching valve that allowed collection of two fractions per sample in unattended operation. The only limitation that we have experienced with the four-vessel extraction system was in the duration of the extraction. When working with 2-mL extraction vessels and 50-/zm restrictors, and using the pressure/temperature conditions mentioned above, the 250-mL syringe pump allows us a maximum extraction time of 60 min. During this time, two 30-min fractions can be collected with the present arrangement. 

Supercritical water represents a potentially important component of sonochemistry, in addition to the free-radical reactions and thermal/pyrolytic effects. Because the reaction occurs at or close to the bubble/water interface, compounds more hydrophobic than p-NPA are expected to exhibit even higher hydrolysis rate enhancements. Finally, the existence of the supercritical phase in an ultrasonically irradiated solution suggests a modification of the conventional view of the reactive area at the cavitation site. This region is normally considered to consist of two discrete phases a high-temperature, low-density gas phase and a more condensed, lower temperature liquid shell. 

Fly ash, as it is a large-volume industrial waste, is both cheap and abundant, so that there is an economic incentive to use fly-ash-modified cements. In addition, C02 is also produced as a waste by-product of industrial processes (power generation, cement manufacture, etc.), and its permanent sequestration into cement is an added environmental benefit. A fully carbonated Portland cement permanently sequesters about 130 L of C02 per kilogram of cement. Figure 15.8 shows the structural and chemical modifications produced in cemented fly ash microspheres as a result of the supercritical C02 treatment. As is the case with fly ash, kiln dusts are primarily siliceous, so that the same benefits can be derived from their use as modifiers in immobilization and S/S matrices. 

In a DTA study of 14 anthraquinone dyes, most had high flash points (225—335°C) and ignition points (320—375°C). Purpurin dianilide [107528-40-5] was exceptional with the much lower values of 110 and 155°C, respectively [1]. A similar study of indigo type dyes and vat solubilised modifications is reported. The basic dyes decompose over 350°C, destabilised to around 200°C for solubilised dyes. The relation between functional groups, structure and flammability is discussed [2]. Sulfonyl azides have been employed for attachment of reactive dyes, it is claimed they are safer used in supercritical carbon dioxide than in water [3]. 

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