Elastomers, solubility parameters

Description of samples tested, specific test methods used, exposure medium notes, solubility parameters, and other important details are provided. Emphasis is on providing all relevant information so the most informed conclusions and decisions can be made by the user. Over 60,000 individual entries (specific tests) are covered in the database. Classes of materials covered include thermosets, thermosetting elastomers, thermoplastics, and thermoplastic elastomers. Approximately 700 different trade name and grade combinations representing over 130 families of materials are included. Over 3300 exposure environments are represented. 

Individual liquids and elastomers each possess their own solubility parameter, 5. This is a thermodynamic property which is related to the energy of attraction between molecules. In its simplest form, an elastomer will possess a drive to absorb a liquid of similar 8, and be swollen by it. As the difference between the solubility parameter values of species increases, so their affinity for each other decreases. The commonest units for 8 in the literature are (cal cm ) / to convert values thus to MPa, multiply by 2.05. 

Intermediate liquid 8 values are obtained by mixing liquids of known solubility parameter SPS makes use of this. The 8 value of the mixture is equal to the volume-weighted sum of the individual component liquid 8 values. Thus, the mass uptake of a miscible liquid mixture by an elastomer may be very much greater than the swelling which would occur in the presence of either one of the constituent liquids alone. The mixture could of course comprise more than two liquid components, and an analogous situation would apply MERL have applied this approach for the offshore oil-production industry to allow realistic hydrocarbon model oils to be developed,basically by mixing one simple aliphatic (paraffinic) hydrocarbon, one naphthenic, and one aromatic to proportions that meet two criteria, namely, that... 

FIGURE 23.4 Solubility parameter spectra for elastomers ethylene propylene, nitrile (at 22% and 38% acrylonitrile content) and tetrafluoroethylene propylene copolymer. 

Campion and Morgan developed the technique of reverse solubility parameter spectroscopy (RSPS)" to determine the 8 value for a liquid (e.g., an oU), from a series of swelling measurements using a range of elastomers of known 8. In this way, 8 for cmde Brent oU from the North Sea has been found to be 8.2 (cal cm ). ... 

A small selection of solubility parameter values in (cal cm ) for elastomers and liquids are... 

The horizontal portion of the plot in Figure 23.6 represents the equilibrium mass uptake—the amount absorbed at equilibrium whose magnitude is influenced by the solubility parameter 8 (Section 23.4.3.1). In reality, coefficient D quite often varies with concentration, so that the overall plot takes on a sigmoid shape (see Section 23.4.4.3), although a horizontal portion is still usually achieved eventually if it is not, the elastomer is possibly a two-phase material (a blend), with one phase much slower at absorbing the incoming liquid. 

Following this, elastomers can be swollen by some high-pressure gases (especially CO2) as the densities of these gases approach liquid-like levels, at appropriate temperatures they become supercritical fluids which possess a solubility parameter magnitudes that, however, are highly dependent on temperature and pressure... 

Table VII summarizes the solubility parameters of MEK, the CTBN elastomers, and an epoxy resin of similar EEW.

These highly amorphous elastomers have relatively low Tt values (—73 C) and tend to crystallize when stretched. The cold flow of these thermoplastic polymers is reduced when they are crosslinked (vulcanized) with a small amount (2%) of sulfur. Since these polymers of isoprene have a solubility parameter of 8.0 H, they are resistant to polar solvents but are soluble in many aliphatic and aromatic hydrocarbon solvents. The cross-linked derivatives swell but do not dissolve in these solvents. 

PMA is a tough leathery resin with a low Tg and a solubility parameter of 10.5 H. In polymers of alkyl acrylates the solubility parameter decreases as the size of the alkyl group increases. The flexibility also increases with the size of the pendant groups but because of side chain crystallization this tendency is reversed when the alkyl group has more than ten carbon atoms. Polyalkyi acrylates are readily hydrolyzed by alkalis to produce salts of polyacrylic acid. The copolymer of ethyl acrylate (95%) and chloroethyl vinyl ether (5%) is a commercial oil-resistant elastomer. 

Polymers with solubility parameters differing from those of the solvent by at least 2.0 H, will not dissolve in the solvent at room temperature. Thus although unvulcanized natural rubber (NR), unvulcanized styrene-butadiene elastomer (SBR), unvulcanized butyl rubber, and EPDM dissolve in gasoline or benzene, the vulcanized (cross-linked) polymers are swollen but will not dissolve due to the presence of the crosslinks. 

The incorporation of polar groups in unvulcanized polymers reduces their solubility in benzene. Thus the copolymer of acrylonitrile and butadiene (NBR), polychlorobutadiene (Neoprene), and fluorinated EP (the copolymer of ethylene and propylene) are less soluble in benzene and lubricating oils than the previously cited elastomers. Likewise, silicones and phosphazene elastomers, as well as elastomeric polyfluorocarbons, are insoluble in many oils and aromatic hydrocarbons because of their extremely low solubility parameters (silicons 7-8 H polytetrafluoroethylene 6.2 benzene 9.2 toluene 8.9 pine oil P.6). 

Table I lists the final results of solvent-swelling conditions which resulted in selecting 2,2,4-trimethylpentane and styrene at —25°C for a differential solvent pair. The table also includes the published values for the solubility parameters (a/CED) of the elastomers and the solvents. This table indicates that for the elastomer systems Cl-butyl-cts-polybutadiene or Cl-butyl-SBR excellent differentiation can be obtained.

It is known that most mixtures of elastomers and resinous copolymers are rather polydisperse, their incompatibility giving rise to poor interfacial adhesion. Preferably, a resinous monomer or monomers are grafted to the rubber, and they act as a link between the rubber and the resin phases. As Rosen (13) has pointed out, one means of predicting the affinities of polymer pairs from measurable properties is through the use of the solubility parameter, and this has proved useful in this study. The solubility parameter concept was originally derived from the thermody-... 

The acrylic elastomers were graft copolymerized and either investigated in that form or blended with resins having matching refractive indices and suitable solubility parameters. Compositional homogeneity was maintained using techniques similar to those outlined in the first paper (3). 

Another application would be to minimize the swelling of a cross-linked elastomer in contact with a solvent. In this case, of course, one would be looking for a polymer giving the largest mismatch with the solubility parameters of the solvent to which the elastomer was to be exposed. 

The nature of the polymer slightly affects the solubility and is probably related to the solubility parameter of the polymer. For amorphous elastomers without strong polar groups (and even for amorphous polymers in general ) Eq. (18.12c) may be used as a first approximation (with an accuracy of 0.25). 

Prior to this discovery, in 1954 Silberberg and Kuhn (62) were first to study the polymer-in-polymer emulsion containing ethylcellulose and polystyrene in a nonaqueous solvent, benzene. The mechanisms of polymer emulsification, demixing, and phase reversal were studied. Wetzel and Hocks discovery would then equate the pressure-sensitive adhesive to a polymer-polymer emulsion instead of a polymer-polymer suspension. Since the interface is liquid-liquid, the adhesion then becomes one type of R-R adhesion (35, 36). According to our previous discussion, diffusion is not operative unless both resin and rubber have an identical solubility parameter. The major interfacial interaction is physical adsorption, which, in turn, determines adhesion. Our previous work on the wettability of elastomers (37, 38) can help predict adhesion results. Detailed studies on the function of tackifiers have been made by Wetzel and Alexander (69), and by Hock (20, 21), and therefore the subject requires no further elaboration. 

Wettability of Elastomers and Copolymers. The wettability of elastomers (37, 38) in terms of critical surface tension was reported previously. The elastomers commonly used for the reinforcement of brittle polymers are polybutadiene, styrene-butadiene random and block copolymers, and butadiene-acrylonitrile rubber. Critical surface tensions for several typical elastomers are 31 dyne/cm. for "Diene rubber, 33 dyne/cm. for both GR-S1006 rubber and styrene-butadiene block copolymer (25 75) and 37 dyne/cm. for butadiene-acrylonitrile rubber, ( Paracril BJLT nitrile rubber). The copolymerization of butadiene with a relatively polar monomer—e.g., styrene or acrylonitrile—generally results in an increase in critical surface tension. The increase in polarity is also reflected in the increase in the solubility parameter (34,39, 40) and in the increase of glass temperature (40). We also noted a similar increase in critical surface tensions of styrene-acrylonitrile copolymers with the... 

Critical surface tensions of functional polymers were experimentally determined. This set of data and the data on elastomers obtained previously were used to elucidate the proposed solubility parameter-surface tension relationship and the proposed parachor-surface tension relationship. The results show that the former has a higher correlation coefficient than the latter. The correlation coefficients, including three highly hydrogen-bonded polymers, are 0.731 for the former and 0.299 for the latter. Otherwise, they are 0.762 for the former and 0.178 for the latter. For the size of samples examined, we can conclude that the proposed solubility parameter-relationship is more effective than the proposed parachor-relationship in calculating critical surface tension of a polymer. 

Blends of elastomers are routinely used to improve processability of unvulcanized rubbers and mechanical properties of vulcanizates like automobile tires. Thus, cis-1,4-polybutdiene improves the wear resistance of natural rubber or SBR tire treads. Such blends consist of micron-sized domains. Blending is facilitated if the elastomers have similar solubility parameters and viscosities. If the vulcanizing formulation cures all components at about the same rate the cross-linked networks will be interpenetrated. Many phenolic-based adhesives are blends with other polymers. The phenolic resins grow in molecular weight and cross-link, and may react with the other polymers if these have the appropriate functionalities. As a result, the cured adhesive is likely to contain interpenetrating networks. 

The release of lipophilic steroids from silicone elastomer matrices is dependent on the cross linking density of the polymer and the content of filler, but also on the lipophilicity of the dmg. A relationship between the solubility parameters of a number of dmgs and their release rate is shown in Fig. 8.27. 

The presence of a higher aromatic content In the gasoline resulted In Increased swell and hence Increased deterioration of tensile properties of elastomers exposed to the gasoline and Its mixtures. Addition of benzene to Increase the aromatic content resulted In slightly more detrimental effects on nitrile elastomers than the addition of toluene. The data on all elastomers can be explained In terms of the solubility parameter concept. 

Qualitatively MTBE Is estimated to have an overall solubility parameter value close to that of Indolene, but has higher polar and hydrogen bonding forces. As a result polar polymers such aa fluorocarbon, epichlorohydrin homopolymer and chlorosulfonated polyethylene tend to swell to a greater extent In MTBE rich mixtures, while nonpolar EPDM elastomer swells to a lesser extent In these mixtures. The very large swell of the fluorocarbon In MTBE Is not surprising since other ethers such as diethyl ether and dioxane are known to swell the fluorocarbon to a large extent [3]. 

Indolene mixtures have been adequately explained In terms of the solubility parameter concept [2]. It was found that with the exception of the fluorocarbon elastomer the overall solubility parameter of elastomers was equal to that of the methanol/Indolene mixture In which maximum swell of the elastomer was measured. 

Ethanol has an overall solubility parameter (4] of 13.0 (cal/cc). The contributions of the London dispersion (nonpolar) forces, polar forces and hydrogen bonding forces to the overall solubility parameter are 6 =7,5, 6 =4.3 and jj=10.9 (cal/cc) respectively. The overall solubility parameter of ethanol Is slightly lower In value than the solubility parameter of methanol mainly due to smaller contributions by the polar forces. The 6p for ethanol ts 4.3 as compared to 6.0 (cal/cc) for methanol. The swell behavior of most elastomers In ethanol resembles that In methanol. In order to examine tdiether or not the swell data In ethanol obtained for the various polymers (Table III) can be explained In terms of the solubility parameter concept we compared established solubility parameters of the elastomers with those calculated from maximum swell data. The solubility parameters of nine of the elastomers Investigated have been reported In the literature [3-6] and are shown in column 2 of Table VII. The next column of the Table Is generated from the present data and lists the concentrations of ethanol In the... 

Coroparlslon of Solubility Parameters of Elastomers With Solubility Parameters of Ethanol/Gasollne Mixtures at Maximum Swell... 

Figure 5. Solubility parameters of elastomers compared to solubility parameters of ethanol Indolene at maximum swell.

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