Specialty TPEs: Fending Off Fluids
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.
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.
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