Dissolution/reprecipitation

MODEL STUDY FOR THE RECOVERY OF POLYAMIDES USING THE DISSOLUTION/ REPRECIPITATION TECHNIQUE Papaspyrides C D Kartalis C N Athens,National Technical University... 

Dissolution/reprecipitation processes were evaluated for the recycling of poly-epsilon-caprolactam (PA6) and polyhexamethyleneadipamide (PA66). The process involved solution of the polyamide in an appropriate solvent, precipitation by the addition of a non-solvent, and recovery of the polymer by washing and drying. Dimethylsulphoxide was used as the solvent for PA6, and formic acid for PA66, and methylethylketone was used as the non-solvent for both polymers. The recycled polymers were evaluated by determination of molecular weight, crystallinity and grain size. Excellent recoveries were achieved, with no deterioration in the polymer properties. 33 refs. 

Applications Dissolution/reprecipitation is claimed to be the most widespread approach to polymer/additive analysis [603], but recent round-robins cast some doubt on this statement. Dissolution appears to be practised much less than LSEs. However, in cases where exhaustive extraction is difficult, e.g. for polyolefins containing high-MW (polymeric) additives, a dissolution/precipitation method is preferred. 

In an industrial application dissolution/reprecipitation technology is used to separate and recover nylon from carpet waste [636]. Carpets are generally composed of three primary polymer components, namely polypropylene (backing), SBR latex (binding) and nylon (face fibres), and calcium carbonate filler. The process involves selective dissolution of nylon (typically constituting more than 50wt% of carpet polymer mass) with an 88 wt % liquid formic acid solution and recovery of nylon powder with scCC>2 antisolvent precipitation at high pressure. Papaspyrides and Kartalis [637] used dimethylsulfoxide as a solvent for PA6 and formic acid for PA6.6, and methylethylketone as the nonsolvent for both polymers. 

Time-consuming sample preparation (by extraction or dissolution/reprecipitation)... 

HPLC methods of determining the amounts of different additives in polymeric materials are preceded by an extraction process or dissolution of the polymer matrix. Although extraction-HPLC is often observed to be superior to the traditional spectroscopic techniques (UV and IR) in analysing additives, it is frequently difficult to obtain reproducible results in view of the variability of the extraction yield. On the other hand, it is equally difficult to obtain quantitative data in the dissolution/reprecipitation-HPLC method because of entrapment of analytes in the polymer precipitate and the potential for high absorption of the additives on the polymer surface. 

It has been well established that Pt can dissolve under oxidizing conditions, although the exact manner of how the species formed is a matter of debate at present. The formation of Pt crystallites in the membrane (or at the anode if no H2 is present) would indicate that micrometer transport of soluble Pt occurs. However, careful analysis of the Pt particle size distributions in the cathode after testing suggested that purely Ostwald ripening could not explain the observed distributions. Therefore, at present, it is concluded that a mixture of Pt dissolution/reprecipitation and Pt particle coalescence is responsible for Pt ECA loss. 

In the presence of a fluid phase, coupled dissolution - reprecipitation is known to be a much more effective process than diffusion. This has been first demonstrated experimentally by O Neil and Taylor (1967) and later re-emphasized by Cole (2000) and Fiebig and Hoefs (2002). 

Feroxyhyte Goethite Dissolution/reprecipitation Alkaline solution... 

Substituted magnetite Dissolution/reprecipitation Alkaline solution with M ... 

Hematite Magnetite Reduction Reduction-dissolution reprecipitation Reducing gas Alkaline solution with N2H4... 

Nitschmann, 1938 van Oosterhout, 1967 Bechine et al., 1982). The process involves a dissolution-reprecipitation mechanism and is promoted by the presence of Fe ions which assist dissolution of lepidocrocite (see Chap. 12) the level of Fe may be increased by addition of metallic iron to the system. 

Under hydrothermal conditions (150-180 °C) maghemite transforms to hematite via solution probably by a dissolution/reprecipitation mechanism (Swaddle Olt-mann, 1980 Blesa Matijevic, 1989). In water, the small, cubic crystals of maghemite were replaced by much larger hematite rhombohedra (up to 0.3 Lim across). Large hematite plates up to 5 Lim across were produced in KOH. The reaction conditions influenced both the extent of nucleation and crystal morphology. The transformation curve was sigmoidal and the kinetic data in water and in KOH fitted a first order, random nucleation model (Avrami-Erofejev), i.e. 

Low-temperature exchange reactions have been described forfluorhydroxyapatite solid solutions [115,130,131], They generally occur in aqueous media and in most instances involve a dissolution-reprecipitation mechanism. Such reactions may be used to partly or totally modify the surface composition of ceramics or coatings. In order to observe such reactions, the resulting apatites should be less soluble than the starting compounds in the solution conditions [132], This is the case, for example, with fluoride uptake by HA. 

H. Zheng, K.K. Chittur, W.R. Lacefield, Dissolution/reprecipitation of calcium phosphate thin films produced by ion beam sputter deposition technique. Biomaterials 20(1999) 443- 51. 

The classical dissolution/reprecipitation process could explain this phenomenon. However, under the experimental conditions used, the F catalysed redistribution process is an alternative process. It is initiated by the coordination of F at silicon, inducing a catalytic... 

---