Evaporation rate blends

The open tack time of the CR adhesives partially depends on the evaporation rate of the solvent blend. If a solvent evaporates slowly, the CR adhesive will retain tack longer, whereas if the solvent evaporates quickly, the cohesive strength will develop more rapidly. According to Table 13, addition of small amounts of xylene (generally lower than 5%) will increase the open time of CR adhesives. 

Solvents have been classified on various arbitrary bases (1) boiling point, (2) evaporation rate, (3) polarity, (4) industrial applications, (5) chemical composition, (6) proton donor and proton acceptor relationships, and (7) behavior toward a dye, Magdala Red, Thus on the basis of industrial application one can classify solvents as those for (1) acctyl-ccliulosc, (2) pyroxylin, 13) resins and lubber, (4) cellulose ether, (5) chlorinated rubber, (6) synthetic resins, and (7) solvents and blending agents for cellulose ester lacquers. Solvents classified according to chemical composition are noted below. 

Fig. 6.1 (a) Electron micrograph showing micellar aggregates in a blend of a PS-PMMA diblock (A/w = 175kgmol1, /ps0.53) in a blend with PS homopolynier (A/w = 95 kg mol 1) with a copolymer volume fraction c = 0.3, at room temperature (Lowenhaupt and Hellmann 1991). (b) Enlargement of a micellar aggregate. R denotes the solvent evaporation rate. 

Solutions made from this solvent blend were found to provide good leveling and flow properties and resulted in a practical evaporation rate.2... 

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... 

To investigate the contribution of evaporation rate on the deposit morphologies on germanium discs, an experiment was conducted at a low evaporation rate using a polyethylene-polypropylene blend on one... 

Figure 12. Photomicrograph of polyethylene-polypropylene blend deposited at a low evaporation rate on a germanium disc half-coated with carbon. The top half of the photo corresponds to the noncoated surface and the bottom half to the carbon-coated surface.

For the water component of the blend, the evaporation rate term in Equation 1 is multiplied by the linear humidity correction factor (l-RH/100) as follows ... 

Several years ago, an extended experimental program was carried out by G. M. Sletmoe (Shell Development Co.) geared towards helping the coatings formulator and pressmen to utilize solvent blends effectively. These studies involved determination of solvent evaporation rates for both neat solvent blends and alkyd resin solutions. Sletmoe also determined isotherms showing solvent activity in resin solutions through isopiestic measurements to relate evaporation rates to equilibrium solvent partial pressures. Some of these data have been published (2, 3, 4) with guideline observations and rules for their application. 

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. 

Here vt is the volume fraction of the Tth component of the blend, and the summation is over all of the j solvents present. The various PN may be evaporation rate, solubility parameters, content of aromatic compounds, or whatever the formulator desires to control in his formulation. The results will be as accurate as the ability of the linear equations to represent the true average properties. 

A problem can arise if a hydrocarbon non-solvent having an evaporation rate lower than that of the average solvent blend is included in the formulation. To eliminate this possibility of eventual precipitation of the polymer, one should not consider these solvents as potential ingredients in the formula or else specify a given amount of still less volatile, good solvent. 

The properties of the optimized solvent blend generally meet the restrictions of minimum solvency and either a lower or upper limit on average evaporation rate. The use of two equations to place upper and lower bounds on the evaporation rate is recommended. If the evaporation rates are too restrictive, the computer tends to select solvents which are more expensive and of better quality. It is possible to set such stringent or unreasonable restrictions that no solution is acceptable to the computer. It is instructive at times to eliminate the evaporation rate restrictions altogether evaporation rate can be controlled by the solvents the computer is allowed to consider. Our solvent selection programs have operated satisfactorily with the relative evaporation rate system based on n-butyl acetate as 100 although this is an arbitrary choice which has not been evaluated by comparison. 

Even though the relative evaporation rate of the recommended blend is considerably faster than the standard, flow was greatly improved. The improved flow eliminated voids in the print and improved print quality. 

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. 

Formulation of solvent combinations for epoxy resin coatings is based on blends of solvents as composites with (8) and (p) within the solubility area shown in Figure 1. Evaporation rate, flash point, air pollution restrictions (if appropriate), and cost are among the other considerations in solvent selection. 

Evaporation Rate of Solvents Evaporation of Solvent-Water Blends Flash Point of Solvents Air Quality Regulations and Solvents Formulation of Solvent Blends Solvents for High-Solids Coatings... 

Evaporation rate of solvent and drying of coating can be regulated easily by choice of solvents in solvent blends. Proper evaporation rate helps to give smooth films and to minimize sagging, running, and popping of paint. 

Some factors that influence the rate of evaporation of a solvent include temperature, flow of air over sample, vapor pressure of the solvent, latent heat, specific heat, and molecular weight (53). Galstaun (55) in 1950 reported a thorough study of the evaporation of some hydrocarbon solvents and developed equations for evaporation rates as related to several factors including temperature drop of liquid as it evaporates. Sletmoe (56) developed equations for the evaporation of neat solvent blends. The total rate of evaporation was proposed to be equal to the sum of the rates for the individual solvent components ... 

Evaporation of Solvent-Water Blends. The evaporation of blends of solvents and water presents several problems, the most prominent of which is the fact that the relative rates of evaporation of solvents and water in a given blend vary greatly under conditions of varying humidity. Under dry conditions the water evaporates relatively rapidly, but under high humidity the water evaporates relatively slowly in comparison to solvent. Usually it is desired that the water and solvent evaporate at rates such that the residual liquid remains constant in composition (perfect solvent balance). Often, enrichment of solvent in the residual liquid is desired, but depletion of solvent in the residual liquid might lead to resin kick-out or loss of ability of the resin to coalesce into a smooth, continuous film. One means of minimizing premature loss of... 

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. 

---