Solvent blends solubilities

Computerized optimization using the three-parameter description of solvent interaction can facihtate the solvent blend formulation process because numerous possibihties can be examined quickly and easily and other properties can also be considered. This approach is based on the premise that solvent blends with the same solvency and other properties have the same performance characteristics. Eor many solutes, the lowest cost-effective solvent blends have solvency that is at the border between adequate and inadequate solvency. In practice, this usually means that a solvent blend should contain the maximum amount of hydrocarbon the solute can tolerate while still remaining soluble. 

Cloud point. Measures the solubility/compatibility of a resin with solvents. The value reported is the temperature at which a specific mixture of a resin and a solvent or solvents blend gives a cloudy appearance, having been cooled from a temperature at which the mixture was clear. Commonly, a test tube of a given diameter is used and the temperature is noted when the lower end of the thermometer, placed at the bottom of the tube, disappears. Resins with cloud points below 0°C are commonly regarded as soluble and cloud points greater than 10°C indicate poor solubility/compatibility. White spirit with various aromatic contents is a widely used solvent in the determination of cloud point, but other solvents or solvents mixtures are also used. 

The use of the solubility envelope, together with the volumetric additivity rule for calculating solubility parameters of solvent blend and the solvent evaporation model described previously, allows an approximate assessment whether phase separation will take place or not during solvent evaporation. 

The most widespread use of the solubility parameter has been in the formulation of surface coatings. Single solvents are rarely used because the requirements for evaporation rates, safety, solvency, and so on generally mean that a solvent blend... 

Table 1 list solvents and cosolvents used in parenteral products. Water for injection is the most common solvent but may be combined or substituted with a cosolvent to improve the solubility or stability of drugs.f The dielectric constant and solubility parameters are among the most common polarity indices used for solvent blending.f Ethanol and propylene glycol are used either alone or in combination with other... 

A linear programming technique is described which selects mixed solvents based on specifications of the ffo, 8p, end SH solubility parameters, evaporation rates, and other significant parameters of a solvent blend. Suggestions are made for setting the various restrictions and for setting procedures of data processing. For simpler cases of solvent selection, recourse to a computer is not necessary. Use of a solvent improvement cost factor, 8 J cost, then leads to optimum formulations since one can determine which solvents increase solubility at least cost. 8 is given by y/ 8P2 -(- 8H2. 

The selection of solvents and solvent blends for use in coatings and inks is based upon solubility/viscosity characteristics and application/performance properties. Published solubility parameters and hydrogen bonding indexes are used to construct two-dimensional solubility maps. Methodology is described, and illustrations are shown. Data are provided on evaporation times of neat solvents, viscosities and dry times of polymer solutions, electrostatic characteristics of solvents, and on selected solvent blend recommendations for several polymers. Unpublished test methods for flow testing and for substrate testing are provided. Combination of the results from these areas provides a viable method for practical solvent blend selection this approach is faster than random trial-and-error and can result in superior, formulated solvent blends. 

In choosing a solvent blend, its effect on a soluble substrate is an important consideration. An example which illustrates this is the application of a vinyl copolymer ink to a low-molecular-weight polyvinyl chloride (PVC) film. The Commercial Solvents Corp. test for determining film substrate attack is given in Appendix B. This test method could be combined with an Instron to provide more exact detail on film strength loss. As shown by the data in Table VIII ... 

The solubility map does not indicate evaporation rate. Evaporation rate depends on many factors including solvent vapor pressure, hydrogen bonding, solvent-resin forces, solvent molecule shape, and polar forces among the constituents of the solution but solvent blends at the border of immiscibility evaporate more rapidly than blends of true solvents which fall at the center of the soluble area of a solubility map. This is die result of all the factors that influence evaporation. 

It is useful to formulate solvents using solubility diagrams based on the volume composition of the blends in place of calculating the physical chemical constants for the composites. Ternary diagrams can be readily plotted on three-component solvent blends. Blends containing more than three materials can be plotted on these same diagrams providing the ratio of several of the materials is held constant and the result is only three variables. 

Solubility work shown in Figure 6 shows that solution can be achieved by two non-solvents with the aid of a true solvent, e.g., 55/40/5 vol % hexane/methanol/toluene or acetone. Toluene or acetone is the only true solvent in the blend. Paints were made with the following two solvent combinations to demonstrate the value of a nearly immiscible solvent blend ... 

Following the technique of solvent selection by solubility maps and ternary solubility diagrams, the coatings formulator can adapt solvent blends for epoxy resins to obtain lower viscosities and improved drying rates. It is obvious that lowering the solvent cost, conforming to air pollu-... 

Rocklin and Bonner (65) developed a computer method that predicts solvent balance and evaporation times of water-solvent blends at any humidity with any number of water-soluble organic solvents. The method also can be used for regular water-free solvent blends but Ignores humidity. Key considerations of the method are the following it uses the UNIFAC method for calculating activity coefficients it computes the actual evaporation temperature on the filter paper substrate it calculates evaporation rates at the calculated temperature by using the activity coefficients at that temperature humidity is accommodated by applying a correction factor to the water evaporation rate. Experimental data on several systems verified the computer calculations. 

Three solution parameters that are used to describe solvency of the solvent blend in fundamental terms solubility parameter, fractional polarity, and hydrogen bonding index number (85). 

Hansen (86) has modified the method of Nelson, et al. by incorporating solubility parameters for the three component forces in the solubility parameter, viz, dispersion, polar, and hydrogen bonding forces. The solvent selection procedure was designed for use by plant laboratories on a time sharing terminal. The enlistment of the computer in the selection of solvent blends has been a boon to the formulator, but the use of older methods is still very useful. 

During the extraction of otganic species, it may be desirable to modify the solvent. An inert paraffinic compound or mixture may be blended with a suitable modifier (e.g., a species that hydrogen bonds) to enhance the solvent properties. Such properties might include viscosity, density, surface tension, or attraction for the solute. In these cases, the mutual solubility curve may appear as in Fig. 7.2-4 when the solvent mixture is plotted at one vertex. Reasons for solvent blending may include improved solvent selectivity, interiacial tension, reduced solvent phase viscosity, and increased density differences between the two phases. A solvent that forms stable etnulstotte when mixed with the diluent phase, for example, may be suitable for use when it is modified with a suitable inert paraffinic material. 

Both examples Illustrate that solvent blends can be used to extract mesophase forming fractions from the A-240. Figure 9 suggests several other solvents may form successful blends. Solvents normally inappropriate should be blended to shift the solubility parameters of the mixture closer to the edge of the enhanced solubility region where the blend can extract a pitch fraction capable of complete coalescence. Solvent blends can also be used to moderate solvent costs without impairing solvent effectiveness. 

Easy to use, applied as received, water soluble solvent blend requiring water rinse and heated dry... 

Solubility parameters of solvent blends are calculated using the linear relationship ... 

Acrylic resins are soluble in a broad range of solvents with the exception of aliphatic hydrocarbons. Usually a combination of a good and a cheap solvent is used for thermoplastic acrylics, e.g. ketone/aromatic hydrocarbon 1/1 or 2/1. For convertible stoving resins xylene/glycolether (ester) combinations are suitable. Small proportions of alcohol are used to improve solvent blend performance. 

One of the main differences of electrodeposition paints with conventional water soluble paints is their lower solids and thus solvent content. A typical binder content is around 10%w, the amount of solvent approximately 5%. The rest, apart from pigmentation, is water. The influence of solvent in the early stages of binder/paint formulation is very similar to the effects described for conventional aqueous paints which is also started from an approx. 70% solids binder solution in coupling solvent(s). The choice of the solvent (blend) is, however, less influenced by its evaporation characteristics as the deposited paint film does not contain much water and is stoved after application. Of more importance are paint stability and electrical properties (conductivity, rupture voltage). 

Where such a program is available, in principle the following steps have to be taken. For the design of a completely new solvent system, the solubility sphere within a three-dimensional solubility parameter system should either be known or has to be constructed for the solute in question. As a simplification, a solubility map described in the chapter on solvent power (Figure 2.10) may be used [10]. For many polymeric materials these data already exist for the Nelson, Hemwall and Edwards or the Hansen solubility parameter concepts. Alternatively as described in section 2.2 a sphere or map can be constructed. Once the area of solubility is known, suitable solvent blends can be designed with solubility parameters falling within this area. When one has to choose one of the above concepts it should be noted that the idea of a sphere of... 

With these solubility parameters in hand, further optimisation can be achieved with help of the theories on viscosity, evaporation rate and flashpoint of solvent blends. 

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