SOLUBILITY PARAMETER CONCEPTS - A NEW LOOK*

1 Paper No. 36 SOLUBILITY PARAMETER CONCEPTS -A NEW LOOK* Daniel L. Hertz, Eastern, Inc. +Red Bank, NJ 07701-0519* Presented at a meeting of the Rubber Division, American ChemicalSociety, Mexico City, May 9-12, 1989+ Contribution No. 36 2 INTRODUCTIONThe SOLUBILITY PARAMETER concept , on first discovery appears tooffer a rationalized approach for a simplified elastomer selection invarious solvents. The original definitive book, " SOLUBILITY ofNonelectrolytes"' by the title gives the first clue that the problem ismore complex than the simple "likes dissolves likes" approach.

2 We livein a world of aqueous and nonaqueous electrolytes. Barton's recentwork, "Handbook of SOLUBILITY parameters and Other Cohesion parameters 2 is a good starting point for a serious study. Kamlet et a13offer somenewer thoughts on SOLUBILITY including more recent references. Jensen4 in a recent chapter correctly notes that most of the solubilityparameter CONCEPTS use the historic "similarity matching" of propertiesrather than the more appropriate "complementary matching" ofproperties. The former approach still pervades "as a result of aninappropriate extension of the dispersion-only arguments used in theoriginal theory of regular solutions.

3 " Consider now, three examples ofa mixed-solvent mixtures are more often the rule rather than theexception; non-latex paints, lacquers, gasohol. Less obvious is highpressure sour gas (a mixture of methane, carbon dioxide and hydrogensulfide). Consider first, a typical high molecular weight polymersuitable as a paint, lacquer or ink. The following solvents might beutilized to create a solution-based product: 3a. Theta solvent (one that dissolves the elastomer withthe solvated macromolecule size corresponding to it'sunperturbed [bulk] dimension).

4 This solvent category isnecessary to develop high % solids while maintaining Diluent solvent (one compatible with the theta solvent butincompatible with the polymer).c. Viscosity control solvent (a high volatility solvent toreduce overall viscosity for spray painting - typicallyevaporates before the paint or lacquer coats the object).d. Evaporation rate solvent - a low molecular weight, lowvolatility film-forming (leveling) solvent that allowssolvents "a" and "b" to evaporate without forming a hard filmfirst.

5 This is necessary to develop a glossy Plasticizer solvent - develops flexibility in the typical range of solvents in a polymer based lacquersystem could be:A. tetrahydrofuran (theta solvent)B. ketone (diluent)C. aromatic hydrocarbon (viscosity control)D. glycol ether (evaporation rate control)E. phthalate ester plasticizer (flexibilizer)Gasoline-alcohol mixtures, -the second example, are "nonidealsolutions" as indicated by the increase in both volume and bulk 4temperature on mixing. To prevent phase separation during temperatureshifts, a third component such as MTBE (methyl tertbutyl ether) orhigher molecular weight alcohols must be added to couple the twophases.

6 Simplistically the -OH groups have a strong physical attractionto the alcohol and the -R groups are soluble in the gasoline. Thus agasoline-alcohol mixture typically contains:a. hydrocarbon(s)b. alcoholc. ether or higher alcoholThe third case, sour gas is again different, the elevated pressure andtemperature increase the density of the C02and H2S so they have themobility of a gas but the solvent power of liquids, each totallydissimilar in their SOLUBILITY response. The gases are best illustratedby Figure 1 in a molecular thermodynamic sense as:a.

7 Methane (octopole)b. carbon dioxide (quadrupole)c. hydrogen sulfide (dipole)RESULTSH ansen and Beerbower5give a brief review suitable for a basicexplanation of the "similarity matching" concept of solubilityparameters. It is less than adequate to develop a reasonableunderstanding of the SOLUBILITY problem as defined in the first examplenoted in the "Experimental" discussion. This equation is simply stating that the heat of vaporization Hv less thevolume work (-RT, where R=gas constant, T=absolute temperature) is theestimate of energy to maintain the liquid state 5 The second example, gasoline-methanol is totally incompatible with theHansen-Beerbower approach.

8 This was noted by Hertz6wherein the effectof gasoline, methanol and their mixtures on a 38% ACN nitrile elastomeris tabulated in Figure third example, -sour gas/elastomer interactions, is based onoriginal data from Ender7. His data, plotted by Hertz8in Table Iindicates a substantially greater swelling than predicted inpropylene-TFE elastomer (AFLAS -ASAHI-3M), as compared to ethylene-propylene elastomer and the perfluoroelastomer (KALREZ -Dupont).Subsequent discussions with Jensen9shed new thoughts on this swellinganomaly.

9 This concept was outlined by Hertz10using Dr. Jensen's majorcontribution of Lewis acid-base considerations as a logical , we will review SOLUBILITY PARAMETER CONCEPTS and commenton them. The basic equation of Hildebrand and Scott designated theenergy of vaporization as thecohesive energy density(ced) and it'ssquare root as thesolubility PARAMETER , ( )thus, 6orcohesive this value by molar volume V corrects fordensity leading to the termcohesive energy values arereported in MPa1/2units. This equation is only suitable for vaporsobeying the ideal gas law ie: nonpolar fluids (nonelectrolytes).

10 An area of somewhat generalized agreement was the considerationthat polar (aqueous and nonaqueous electrolytes) fluids had three majorintermolecular forces to consider:Dispersion (London) forces "D"-common in all cohesive energyHydrogen bonding "H", now referred to as H-bondingDipole moment "P", a measure of thepolar(electrostatic)aspect of a Hansen modification, utilizing these three intermolecularattractions,was intended for polar fluids by assigning a partial solubilityparameter equal to the square root of the corresponding partial cohesiveenergy density.