Plasticizers, solubility parameters

Further analysis of the above results may be obtained by studying the solubility parameters of the polymers and plasticizers. Solubility parameters provide a measure of the extent of Interaction possible between chemical species. To determine the solubility parameters, the structure of the smallest repeat unit was considered and the contribution of each atomic group to the total energy of vaporization and molar volume was summed over the molecular structure. The ratio Is calculated as the cohesive energy. The calculated solubility parameters of the polymers and plasticizers are shown In Table II. 

The Hildebrand Solubility Parameter. This parameter, 4 can be estimated (10) based on data for a set of additive constants, E, for the more common groups ia organic molecules to account for the observed magnitude of the solubiHty parameter d = EE/V where Erepresents molar volume. SolubiHty parameters can be used to classify plasticizers of a given family ia terms of their compatibihty with PVC, but they are of limited use for comparing plasticizers of differeat families, eg, phthalates with adipates. 

When more than routine water resistance is required, a copolymer vinyl acetate emulsion can be used. The plasticizing comonomer in the polymer particles increases their intrinsic coalescing ability thus, they can coalesce more readily than homopolymer particles to a film that has a higher resistance to water. This resistance to water does not extend to the organic solvents, however, which are better resisted by homopolymer films. The soft copolymers have lower solubility parameters than homopolymers and are more readily attacked by solvents of low polarity, eg, hydrocarbons. 

The conventionally covalently cross-linked rubbers and plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar solubility parameter, the degree of swelling decreasing with increase in cross-link density. The solution properties of the thermoelastomers which are two-phase materials are much more complex, depending on whether or not the rubber phase and the resin domains are dissolved by the solvent. 

Plasticizers reduce hardness, enhance tack and reduce cost in rubber base adhesive formulations. A plasticizer must be easily miscible and highly compatible with other ingredients in the formulations and with the surfaces to which the adhesive is applied. The compatibility and miscibility of plasticizers can be estimated from the solubility parameter values. Most of plasticizers have solubility parameters ranging between 8.5 and 10.5 hildebrands. However, the high miscibility and compatibility also lead to easier diffusion of the plasticizer to the surface, decreasing the adhesion properties. Therefore, plasticizers should be carefully selected and generally combinations of two or more of them are used. 

The morphology of a typical urethane adhesive was previously shown in Fig. 3. The continuous phase usually comprises the largest part of the adhesive, and the adhesion characteristics of the urethane are usually controlled by this phase. From a chemical standpoint, this continuous phase is usually comprised of the polyol and the small amount of isocyanate needed to react the polyol chain ends. A wide variety of polyols is commercially available. A few of the polyols most commonly used in urethane adhesives are shown in Table 2. As a first approximation, assuming a properly prepared bonding surface, it is wise to try to match the solubility parameters of the continuous phase with that of the substrate to be bonded. The adhesion properties of the urethane are controlled to a great extent by the continuous phase. Adhesion to medium polarity plastics, such as... 

In the high concentration regime, our SCP is different from a typical SCP observed in Case II diffusion. Specifically, our SCP lacks the sharp solvent front(fig.8). The abrupt increase in solvent concentration normally observed is due to the long relaxation time of the polymer chain in response to solvent plasticization. Then, the absence of this feature points to a very rapid relaxation of PMMA chains by MEK. This is probably due to a good match in the solubility parameters of PMMA and MEK ( =9.3 for both). 

Finally, the solubility parameter of the adhesive and the substrate must be close. Without getting too teehnieal, the solubility parameter is a rough estimate of polarity. The old saying like dissolves like can be extended to like bonds like. More aeeurately, the solubility parameter is the ealeulated potential energy of 1 em of material for eommon solvents. Polymers are assigned solubility parameters of solvents in which they are soluble. Table 19.3 lists solubility parameters for various solvents and polymers. As an example of how to use this table, butadiene-acrylonitrile rubber with 6= 9.5 bonds natural rubber (6= V.9-8.3) to phenolic plastics (6= 11.5). Note that its solubility parameter is between that of the two substrates. 

Plasticizers help the flexibility of polymers and are chosen so that they do not dissolve the polymer but rather allow segmental mobility. Through experience, it is found that the solubility parameter differences between the plasticizer and polymer should be less than 1.8 H for there to be compatibility between the plasticizer and polymer. 

Compatibility is the ability to mix together without forming different phases. A good approach to measure the likelihood of compatibility is solubility parameters (Section 3.2). Plasticizers with solubility parameters and type of bonding similar to those of polymers are more apt to be compatible than when the solubility parameters are different. The solubility parameter for PVC (9.66) is near that of good plasticizers such as DOP (8.85). Table 15.2 contains solubility parameters for typical plasticizers. 

Plasticizers are compounds which increase the flexibility and process -bility of polymers. It has been postulated that the added plasticizer reduces the intermolecular forces in PVC and increases the free volume. Effective plasticizers, like effective solvents, have solubility parameters within 1.8 H (Hildebrand units) of that of the polymer. 

Rigid PVC has been available commercially for over half a century, but the unstabilized polymer could not be molded or extruded until plasticizers were incorporated into it in the 1930s. The solubility parameters of commercial PVC plasticizers such as dioctylphthalate (DOP) are similar to those of PVC... 

Intractable polymers, such as polyvinyl chloride (PVC), may be flexibilized, to some extent, by the formation of copolymers, such as the copolymers of vinyl chloride and vinyl acetate or octyl acrylate, or by the addition of nonvolatile low-molecular-weight compounds (plasticizers) having solubility parameters similar to those of the polymer. Thus PVC is plasticized by the addition of dioctyl phthalate. The flexibility of these products is proportional to the amount of plasticizer added. Copolymers, such as the vinyl chloride-vinyl acetate copolymer, require less plasticizer to obtain the same degree of flexibility. 

Hansen, C. M. 1967. The three dimensional solubility parameter. Key to paint component af nities I. Solvents, plasticizers, polymers, and resiials.PaintTechnol39 104-117. 

Unformulated poly (vinyl acetal) resins form hard, unpliable materials which are difficult to process without using solvents or plasticizers. The solubility parameter ranges for some commercially available PVF and PVB resins are listed in Table 1. Plasticizers not only aid resin processing but also lower the glass-transition temperature, T, and can profoundly change other physical properties of the resins. For example, the mechanical glass-transition... 

Table 4-1 Solubility parameters 8 for several plastics and solvents. (Barton, 1983).

Ethyl acetate is the most soluble solvent in the plastic based on having the smallest A8 with LDPE. Note that the solubility of these solvents in OPP will not differ greatly from LDPE based on the similar average solubility parameters between OPP and LDPE (17.8 vs. 16.5). 

Kambour et al. performed extensive studies on the mechanisms of plasticization [18-25]. The correlation observed between the critical strain to craze and the extent of the glass-transition temperature (Tg) depression speaks strongly in favor of a mechanism of easier chain motion and hence easier void formation. In various studies on polycarbonate [19,24], polyphenylene oxide [20], polysulfone [21], polystyrene [22], and polyetherimide [25], Kambour and coauthors showed that the absorption of solvent and accompanying reduction in the polymer s glass-transition temperature could be correlated with a propensity for stress cracking. The experiments, performed over a wide range of polymer-solvent systems, allowed Kambour to observe that the critical strain to craze or crack was least in those systems where the polymer and the solvent had similar solubility values. The Hildebrand solubility parameter S [26] is defined as... 

Hansen CM, Just L (2001) Prediction of environmental stress cracking in plastics with Hansen solubility parameters. Ind Eng Chem Res 40( 1) 21—25... 

Hoy KL (1975) Tables of solubility parameters, 3rd edn, Union Carbide Corp, Chemicals and plastics research and development dept, South Charleston, WV... 

Some degree of solvency of the plasticizer for the host polymer is essential for plasticization. Not surprisingly, a match of solubility parameters of the plasticizer and polymer is often a necessary but not a sufficient condition for compatibility. In the case of PVC, the dielectric constants of the plasticizer should also be near that of the polymer. 

Hansen, C.M. and Just, L. (2001) Prediction of Environmental Stress Cracking in Plastics with Hansen Solubility Parameters, Ind. Eng. Chem. Res. 40,21-25. 

When two dissimilar plastic foams are to be joined, which is rarely done, adhesive bonding is generally preferable because of solvent and polymer incompatibility problems. Solvents used to cement plastics should be chosen with approximately the same solubility parameter (5) as the plastic to be bonded. The solubility parameter is the square root of the cohesive energy density (CED) of the liquid solvent or polymer. CEDs of organic chemicals are primarily derived from the heat of vaporization and molecular volume of the molecules, and are expressed as calories per cubic centimeter (cal/cm ). Literature sources provide data on 6 s of a number of plastics and resins (2) (3) (4). 

Inverse Gas Chromatography (IGC) has been used to measure solubility parameters for three polymers at 25° C using the method of Guillet and DiPaola-Baranyi. The linear relationship noted with other polymers was found and the results add further credance to the method. Solubility parameters have also been calculated for six small molecule involatile compounds of the type use as plasticizers. The original method did not yield values in good agreement with literature results but estimation of the different contributions to the solution interactions allowed calculation of more meaningful values. 

A frequent use of solubility parameters is the prediction of compatibility of blends of polymers with additives such as plasticizers used to modify the polymer properties. Plasticizers are generally involatile organic molecules such as dialkyl phthalates. Thus it was of interest to determine the usefulness of ICC method for estimating the solubility parameters of these compounds. 

Table II. Solubility Parameters of Various Components and Plasticizers...

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