UP techniques

An interest has been developed in the use of vanadium naphthenates as accelerators. In 1956 the author found that if MEKP was added to a polyester resin containing vanadium naphthenate the resin set almost immediately, that is, while the peroxide was still being stirred in. Whereas this effect was quite reproducible with the sample of naphthenate used, subsequent workers have not always obtained the same result. It would thus appear that the curing characteristics are very dependent on the particular grade of resin and of vanadium naphthenate used. It was also observed by the author that the gelation rate did not always increase with increased temperature or accelerator concentration and in some instances there was a retardation. Subsequent workers have found that whilst the behaviour of the naphthenate varies according to such factors as the resin and catalyst used, certain vanadium systems are of value where a high productivity in hand lay-up techniques is desired. 

The major process today is the hand lay-up technique in which resin is stippled and rolled into the glass mat (or cloth) by hand. Moulds are easy to fabricate and large structures my be made at little cost. 

For mass production purposes matched metal moulding techniques are employed. Here the preform or mat is placed in a heated mould and the resin poured on. The press is closed and light pressure ( 501bf/in ) applied. Curing schedules are usually about three minutes at 120°C. It is possible to produce laminates using less resin with pressure moulding than with hand lay-up techniques and this results in better mechanical properties. 

The laminates are employed mainly where an intermediate degree of heat stability is required which does not Justify the use of the more expensive silicone and other laminates considered in Chapter 29. They have additional advantages over the silicones in their ease of forming by wet lay-up techniques and the greater strength of the laminates. 

On-line LC-GC has frequently been used as a clean-up technique for the analysis of trace levels of contaminants (pesticides, plasticizers, dyestuffs and toxic organic chemicals) in water and food products. Several different approaches have been proposed for the analysis of contaminants by on-line LC-GC. Since pesticide residues occur at low concentration in water, soil or food, extraction and concentration is needed before GC analysis is carried out. 

Scale-up techniques for using the results of pilot plant or bench scale test w ork to establish the equivalent process results for a commercial or large scale plant mixing system design require careful specialized considerations and usually are best handled by the mixer manufacturer s specialist. The methods to accomplish scale-up will vary considerably, depending on whether the actual operation is one of blending, chemical reaction tvith product concentrations, gas dispersions, heat transfer, solids suspensions, or others. 

The choice of scale-up technique depends on the particular system. As a general guide, constant tip speed is used where suspended solids are involved, where heat is transferred to a coil or jacket, and for miscible liquids. Constant power per unit volume is used with immiscible liquids, emulsions, pastes and gas-liquid systems. Constant tip speed seems more appropriate in this case, and hence the rotor speed should be 0.66 Hz. The... 

The UPS technique (ultra violet photoelectron spectroscopy), by measuring the cutoff energy of secondary electrons.56,57... 

Principles and Characteristics Solid-phase extraction (SPE) is a very popular sample preparation and clean-up technique. In SPE solutes are extracted from a liquid (or gaseous) phase into a solid phase. Substances that have been extracted by the solid particles can be removed by washing with an appropriate liquid eluent. Usually, the volume of solvent needed for complete elution of the analytes is much smaller (typically < 1 mL) than the original sample volume. A concentration of the analytes is thus achieved. 

There are a large number of extraction and clean-up techniques. Some of the more common ones are outlined in Table 4.5. The technique selected will depend on the nature of the sample and of the analyte. 

The solvent extraction of chlorinated pesticide residues from soil is often achieved by using mixtures of solvents such as hexane-isopropanol or hexane acetone, but can be unsatisfactory owing to the emulsification problems [2, 3] or, with hexane-isopropanol, poor recovery [2, 4], Acetone extraction of soil is efficient [4, 5] but problems can arise from large amounts of coextracted material unless an efficient clean-up technique [6] is used prior to analysis by gas chromatography. 

Normally an extraction technique is selected to give the highest recovery for a wide range of pollutants. Therefore, the extract will most likely contain a high proportion of co-extracted material. Many of the clean-up techniques have been tailored into a series of multi-residue schemes in order to maximize the use of each sample [189,402,453,454,478-481]. This is of particular value when the maximum amount of chemical information is required for each sample. 

Gross contamination can overload the HPLC or GC columns with obvious and usually rapid deterioration of chromatographic performance. This can occur with so called rapid techniques where the detector is used as a filter, e.g., selected ion monitoring (SIM) MS, or where the clean-up method has been overloaded (e.g., excess of lipid). This problem can be overcome by using and monitoring more selective clean-up techniques. 

These problems are overcome by applying a tailored LC separation prior to the final determination and having a built-in feedback to monitor the success of the separation or to give a warning of any failure. The following are the most commonly used clean-up techniques in organic analysis of environmental pollutants. 

Modem extraction and clean-up techniques, such as pressurised liquid extraction and microwave-assisted extraction, have almost not applied to the analysis of PFCs yet. Llorca et al. [49] reported the development and application of a PEE method for PFCs determination in fish. This technique provided rapid and accurately clean extracts for sensitive analysis. 

What is the advantage of using BMC and SMC over hand lay-up techniques such as those used to make boat hulls ... 

Walters SM. 1990. Clean-up techniques for pesticides in fatty foods. Anal Chim Acta 236 77-82. 

V-Nitrosodiethanolamine has been found in many complex matrices such as cutting and grinding fluids and cosmetics. Analysis for V-nitrosodiethanolamine is complicated by the matrix and a clean-up technique with derivatization is typically required before quantitation of the analyte to achieve adequate sensitivity and selectivity. Ammonium sulfamate may be added to the sample to prevent the artifactual formation ofV-nitrosamines. Derivatives of V-nitrosodiethanolamine have been prepared by acylation, trifluoroacylation, trimethylsilylation and methylation. The derivatives have been analysed by gas chromatography using flame ionization and mass spectro-metric detectors (Occupational Safety and Health Administration, 1990). 

Thermoforming is faster and higher fiber contents can be achieved than they can for the spray-up technique. Moreover, if continuous strand mat is used in thermoforming, it can be combined with patches of directional reinforcement. The major drawback with thermoforming is the relatively high level of waste material. 

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