Flushing with Solvents

the pot life is limited, it is necessary to flush the mixing head from time to time to eliminate any mix which could start to gel or harden. 

Flushing is done by sending solvents under high pressixre into the equipment (refer to Fig. 81). If the equipment has some recessed places, it may be necessary to dismantle the equipment in order to clean it completely. 

For high-performance bonds, it is advisable or even mandatory to include several control systems  

A microprocessor may be added to monitor all these information and stop the machine in case of problem. 

Examples of Complete Systems for Metering, Mixing and Dispensing Two-Component Adhesives/Sealants 

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Figure 6 (a) Stainless steel column is filled with polymerization mixture and capped at both ends, (b) Column is incubated in a water bath to allow polymerization to occur, (c) Column is flushed with solvent to remove print molecule, (d) Column is ready to use. (Reproduced with permission from Ref. 43.)... 

Samples can be injected automatically with mechanical devices that are often placed on top of gas chromatographs. These autosamplers mimic the human injection process just described using syringes. After flushing with solvent, they draw up the required sample several times from a sealed vial and then inject a fixed volume into the standard GC inlet. Autosamplers consist of a tray which holds a large number of samples, standards, and wash solvents, all of which are rotated into position under the syringe as needed. They can run unattended and thus allow many samples to be run overnight. Autosamplers provide better precision than manual injection— typically 0.2% relative standard deviation (RSD). 

Columns are commonly made of stainless steel. The stationary phase is retained by porous, stainless steel frits at both ends of the column. The entrance frit acts as a filter for particles and helps distribute liquid evenly over the column diameter. Samples should be passed through a 0.5- to 2-pim filter prior to injection to prevent contaminating the column with particles, plugging tubing, and damaging the pump. Deterioration of peak shape is a common indication of clogging of the frit, which can be reverse-flushed with solvent or replaced. 

The solvent blank should not produce a significant TIC peak. If a small peak or transient rise in the baseline is observed, then the spectrum within this peak should be controlled for the presence of characteristic m/z values. Special attention should be given to m/z values that can be associated with contaminants from previous series of samples. If the frit in front of the transfer line is replaced regularly and the LC-system is flushed with solvent between large sample series, then such contamination should not represent a problem. One should, however, be aware that precipitation and deposits might occur, especially if new and unknown samples are analyzed. Possible locations of such deposits could be not only in the frit, but also in the needle seat of the injector. 

In the second type of experiment the vesicle with encapsulated solute is placed in an ultraflltration cell which is continuously flushed with solvent at a rate vi (in mL/min) (162,163)... 

Desorption of an adsorbed polymer by flushing with solvent proceeds particularly slowly and, in practice, it is impossible to desorb a significant amount in this way. However, we have seen that desorption of a polymer can be accomplished under the influence of an external shear force. In addition, desorption may be caused by competition with a polymeric or a monomeric displacer. In this section we will discuss exchange by a chemically identical polymer or by a polymer differing in size or chemical composition. 

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