Specialty TPEs: Fending Off Fluids

How thermoplastic elastomers function in fluid-resistant applications.

Editor’s Note: The following is an excerpt of a white paper entitled, “Specialty Thermoplastic Elastomers for Fluid-Resistant Applications.” For detailed data and findings on this study, please refer to the contact information at the end of this excerpt.

As a rule, rigid thermoplastics only have useful properties at temperatures below either their glass transition temperatures or their crystalline melting points. The extremely dense frozen crystalline melt or glass structure of rigid plastics effectively prevents penetration by most fluids that would otherwise solvate these materials. Hence, ordinary high-density polyethylene is typically used for gasoline and oil containers and isotactic polypropylene can be used for chemical beakers. With the exception of some very aggressive organic solvents, those specifying most rigid plastics for applications, which may involve fluid exposure, can often safely ignore fluid resistance as a specified requirement.

Thermoset rubbers are largely amorphous homo-polymers or random copolymers. They only function as rubbers above their glass transition temperatures. Thermoplastic elastomers are similar to their thermoset counterparts in that they contain large amorphous rubbery components, which are the soft segments or phases of block copolymer and two-phase systems, respectively. These non-crystalline elastomers and elastomeric components are much more vulnerable to fluid penetration and attack than crystalline, rigid plastics.

Failure to consider fluid resistance when specifying a TPE for a fluid-contact application can have serious consequences. The TPE part having a perfect match to the required appearance and initial physical properties, quickly and dramatically may sustain unacceptable changes in dimensions and physical properties after fluid exposure, leading to total part failure.

Theories and Definitions
Fluid resistance is often grouped with chemical resistance. This article focuses on the physical effects of various fluids on the properties of TPEs. No intentional chemical degradation is involved. The most end-use significant of these effects include volume change and related changes in hardness and stress/stain properties. Fluid resistance is strongly related to solubility, with classes of fluids having commensurately greater effects on chemically similar polymers.

Most TPEs derive their rubber properties from a combination of one or more hard plastic phases with one or more soft elastic phases. Only one of those components need be vulnerable to a particular fluid to render it unsuitable for service. For example, for thermoplastic vulcanizates (TPVs), which are typically comprised of a crystalline, isotactic polypropylene hard phase, and a highly crosslinked, amorphous, thermoset rubber soft phase, the latter phase normally determines fluid resistance. In block copolymer TPEs, such as thermoplastic polyurethane (TPU) and hydrogenated styreneolefin-styrene tri-block TPEs (HSOS), the amorphous olefinic soft segment usually is the more vulnerable component.

In fully compounded TPEs, the presence of certain compounding ingredients also may impact resistance to specific fluids. For example, highly water-soluble inorganic additives and hygroscopic fillers may influence dimensional change in aqueous fluids. Conversely, organic-fluid-soluble plasticizers may be extracted and/or replaced by some non-aqueous immersion fluids.

There are a number of possibilities for physical interaction with immersion fluid. Where the TPE has very high resistance to swelling in the immersion fluid, virtually all of the soluble plasticizer may be extracted, resulting in significant shrinkage of the TPE specimen and attendant increases in stiffness and hardness.

In cases where the TPE has a tendency to swell in the immersion fluid, there are two options. One applies when the plasticizer has nearly equal solubility in the TPE and the immersion fluid. An equilibrium may be established, wherein some of the immersion fluid is exchanged for some of the extracted plasticizer. Where the plasticizer is more soluble in the immersion fluid and is completely replaced by it in the TPE.

The latter two cases will typically show increases in volume after immersion, however, the resultant effects on changes in physical properties will depend upon the relative ability of the plasticizer and the immersion fluid to plasticize the TPE.

This image demonstrates the swelling and shrinking of specialty TPEs and other materials.

Application of Equipment and Processes
The scope of our full study determined the effects of total immersion on the original physical properties of nineteen commercial and developmental thermoplastic elastomers in eight aqueous and petroleum-based fluids, all of which are routinely encountered in typical service environments.

After completing separate determinations of the effects of immersion time and of immersion temperature on the physical effects of selected TPEs in IRM 903 Oil, 70 hours was selected as the immersion time to generate the bulk of our data. This time interval permitted two sets of immersion tests per workweek, with results entirely consistent with longer time exposures. The 70-hour immersion time is also in agreement with that of the ASTM D-2000 system for establishing the fluid resistance of commercial thermoset rubbers. An immersion temperature of 100°C was chosen as the best compromise to accelerate results for all nonvolatile fluids, considering the wide range of heat resistance among the TPEs tested. Specimens were immersed in the highly volatile and flammable fuels at 23°C.

The process used to evaluate TPE fluid resistance followed procedures set forth in ASTM D-471, “Standard Test Method for Rubber Property — Effect of Liquids.” ASTM D-471 specifies test specimen dimensions, the conditions of immersion and the methodology for preparing the immersed specimens for subsequent measurement of changes in volume, mass, 100 percent modulus, tensile strength, ultimate elongation and hardness, all performed at 23°C and 50 percent R.H. on specimens cooled to 23°C.

The full study generated far more data than can be addressed. To capture the essence of this work, the list of TPEs was pared down to 10, the list of fluids to six, and the monitored properties to four.

Interpretation of Data
The significance of fluid-contact induced physical property changes depends a great deal on the exact exposure conditions of the intended fluid-contact application. Thus, the results seen in our immersion tests may not be valid for applications involving brief external contact with highly volatile fluids, because the fluid may simply evaporate without causing significant changes in dimensions or other physical properties.

However, our results would apply to applications involving similar contact with non-volatile fluids, wherein those TPEs, which were seriously softened and weakened in our study, should be avoided. This category would include ergonomic grips and other overmoldings encountering frequent contact with fuels and lubricants, found on factory equipment, mechanics power-tools, and all gasoline-powered tools and portable devices.

Clearly, applications for which dimensional change in contact with a specific fluid is critical to functionality, such as physically constrained compression seals, volume change is the single most important consideration. For