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Selecting the Right Material for Medical Seals

Medical Device & Diagnostic Industry Magazine MDDI Article Index   Choosing the optimal elastomer for an application involves prioritizing the required performance properties. Barry Chadwick and Chris Toto

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

Choosing the optimal elastomer for an application involves prioritizing the required performance properties.

Barry Chadwick and Chris Toto

Just as a chain is only as strong as its weakest link, the reliability of a fluid-control system is only as good as the seals or gaskets between the components. The increased demand for advanced medical procedures and treatments has placed rigorous new requirements on systems and devices. Today's analytic and clinical devices are being exposed to a wider range of chemical and temperature environments than ever before, during both analysis and sterilization processes. Furthermore, the move towards increased analytic power with less sample volume has driven down the size of system components.

Seals and gaskets play a significant role in determining the reliability of a fluid-control system.

These trends have resulted in smaller, less physically robust components exposed to more intense environments, and engineers and designers have been redesigning primary system components using construction materials with higher-performance properties. More recently, the focus has shifted to the seals and gaskets connecting these components, with traditional elastomers being challenged in their ability to provide adequate reliability in such applications. And while many medical designers and engineers are familiar with the polymers and compounds used to design primary system components, fewer have an equivalent depth of experience in elastomeric materials for seals and gaskets that can meet the challenging demands of today's medical industry.

Advancements in material and manufacturing technologies have endowed products with much greater resistance to chemicals and temperature extremes. Glassware and stainless steel remain viable options for many aggressive environments. Additionally, fluoropolymers, such as PFA, are now available in grades that can be processed in a wide variety of configurations, including tubing, housings, and fittings. Heightened mechanical properties are available from high-performance thermoplastics, such as polyketones, which exhibit excellent chemical resistance with minimal attrition and are now widely used in numerous components for specialized applications, such as chromatography fittings and tubing.

These new materials have afforded engineers the design freedom necessary to develop smaller, more complex, and more robust systems. As a result, seals and gaskets must be equally aggressive in resisting increasingly demanding chemical and environmental conditions. Traditionally, the trade-off between the mechanical performance of a seal and its environmental resistance was considerable: the materials that offered the best mechanical seal performance offered less-impressive chemical and temperature resistance. While this is still the case to some extent, the trade-off is less significant given advancements in elastomeric polymers and attendant compounding technologies.

Medical designers and engineers should consider several parameters when selecting an appropriate seal material. While not all-inclusive for all applications, the following criteria are among the key considerations for medical applications.

  • Fluid Compatibility. Me-dia incompatibility can cause swelling, material hardness changes, and physical property deterioration.
  • Temperature Range. Different elastomers vary in their ability to withstand high and low temperatures.
  • Mechanical Requirements. High strength coupled with high elongation are desirable for dynamic seals. Diaphragm seals, which have low durometers , are often used when low friction is important. By contrast, higher-durometer materials will resist extrusion in applications involving high pressure, without the necessity for antiextrusion rings.

    The starting point in selecting the proper material for a medical seal is the identification of the range within the above parameters to which the seal will be exposed. By comparing these parameters to the capabilities of the seal materials available, the scope of candidates can be significantly narrowed. In many cases, the process is one of eliminating candidate materials that will not work. Often, several viable candidates may remain as suitable contenders for the application. In these cases, other parameters, as well as nonperformance factors such as safety, cost, or availability, may drive the final selection.

    Dome valve seals manage fluid flow in this advanced hematology system.

    The designer should approach the selection process in two stages. First is the selection of the elastomer "family," followed by a more narrow selection of the specific compound within the family. There are a number of common polymers used for seals and gaskets. Within each elastomer family are a variety of compounds to meet specific characteristics such as durometer, modulus, compression set, temperature, fluid compatibility, and so forth.

    Selecting the right elastomer and seal design for a particular application can be a difficult task. Often, there is no ideal solution; rather, an optimal solution can be found by prioritizing the required performance properties. The highest-priority parameters should drive the selection process. For example, if chemical resistance is the most critical parameter, this should be used to narrow the selection. Even though a candidate material may have a relatively low rating in less-critical parameters such as certain mechanical properties, these may be adequate for the application.

    The compatibility of a seal with the fluid with which it is in contact is often high on the list of priorities. Swell is the most common byproduct of chemical incompatibility in seals. Excessive swell can lead to extrusion of the seal through the opening in the mating surface. Even if the seal does not extrude out, expansion of the seal can place pressure on the mating surfaces, causing misalignment and possible leakage, which could lead to system failure. In situations in which the seal is mounted in a single plastic component, excessive swell can result in cracking of the component.

    The specific fluids and chemicals used in the application must be identified and compared to resistance charts supplied by manufacturers. However, this effort can be a challenge without a general guide such as Table I to narrow the research.

    Elastomer Abbreviation Acids Caustics Amines Aldehydes Hot Water/
    Steam
    Perfluoro-
    elastomer
    FFKM 1 1 1 1 1–2a
    Fluoro-
    elastomer
    FKM 1 4 3 3 2–3
    Ethylene propylene EPDM 1–2 2 1 1 1
    Nitrile NBR 4 1–2 4 4 3
    Neoprene CR 4 3–4 2 3 3
    Polyurethane AU 4 1–2 4 3–4 4
    Silicone SI 2–3 1 1 1 3
    1 = Excellent   2 = Good   3 = Fair   4 = Poor
    aPrimarily peroxide-cured perfluoroelastomers.

    Table I. General chemical resistance of elastomers.

    The most chemically resistant elastomers are perfluoroelastomers, which combine the broad, almost universal chemical resistance of PTFE with the resilience of elastomers. When coupled with precision seal design, perfluoroelastomer parts represent the ultimate in high-technology elastomeric seals.

    Although medical equipment and instrumentation are not often used in extreme temperature environments, some seals and gaskets may be exposed to significant temperature ranges. This can be the case in applications in which fluids are heated or cooled, when parts are in proximity to other heated or chilled components, or when parts are subject to sterilization temperatures. Elevated temperatures beyond the recommended limit of the seal material can result in extrusion of the component, compression set, or degradation. In these cases, the seal is highly susceptible to failure in subsequent uses.

    As with fluid compatibility, specific temperatures must be defined and compared to the limits of the materials. Generally, polymer families have effective temperature ranges that can facilitate the selection (see Table II).

    Polymer Minimum
    Temperature (°F)
    Maximum
    Temperature (°F)
    Perfluoroelastomer –20 500
    Fluoroelastomer –22 450
    Ethylene propylene –65 300
    Nitrile –75 250
    Neoprene –65 250
    Polyurethane –65 225
    Silicone –150 500

    Table II. General use-temperature ranges for elastomers.

    The mechanical performance of seals involves a broad range of properties including service pressure, compression, abrasion resistance, and tear resistance. These factors can be substantially influenced by the design of the seal. This said, medical equipment designers can narrow the scope of appropriate materials through general knowledge of the mechanical requirements (see Table III). Selection of the optimal seal material will require integration of the candidate elastomer with the seal design, and a careful consideration of the expected conditions.

    Polymer Abrasion Resistance Compression
    Set
    Resistance
    Tear Resistance Ultimate Elongation Tensile Strength
    Perfluoro-
    elastomer
    4 1–2 3–4 3 1–2
    Fluoro-
    elastomer
    2 1–2 3 3 1–2
    Ethylene propylene 1–2 1–2 1–2 3 1–2
    Nitrile 2 1–2 2–3 2 1–2
    Neoprene 2 3 2–3 1 2
    Poly-
    urethane
    1 3 1–2 1 1
    Silicone 4 1–2 4 1 4
    1 = Excellent   2 = Good   3 = Fair   4 = Poor

    Table III. General mechanical performance of elastomers.

    The number of potential seal materials may be greatly reduced if the application mandates that the material meet regulatory requirements, including FDA or USP compliance. Because the range of compounds available within elastomer families is so broad, many suppliers generally test only specific grades for compliance to industry standards. In such cases, designers and engineers can often narrow their selection process to an elastomer family first, and then discuss the availability and use of FDA- or USP-compliant compounds with suppliers that provide seals from the identified family.

    Perfluoroelastomers are becoming the materials of choice for many medical applications by virtue of their combination of environmental-resistance properties. In addition to the highest temperature resistance, these materials have the broadest range of chemical resistance of any elastomeric material (see Table IV). Certain perfluoroelastomer compounds have been tested thoroughly by their suppliers and are available in USP Class VI–compliant grades.

    Thermal Stability
    (70 hr at 450°F)
    Tensile (% change) –9
    Elongation (% change) +40
    Hardness (pts. change) –4
    Steam Resistance
    (volume swell, %)
    375°F, 30 days 2
    300°F, 30 days <1
    Temperature (°F)
    Estimated Service
    Low-temperature
    limit (°F)
    –20
    High-temperature
    limit (°F)
    450
    Miscellaneous Properties
    Specific gravity (D) 2.0
    Pyrolis initiation (C) Approx. 400
    Specific heat (cal/g C) 0.2
    Thermal conductivity
    (cal/cm sec °C)
    0.2
    Glass transition
    temp. (°C)
    –19
    Gerhman torsion
    test T50 (°C)
    –21
    Impact resilience (%) 12
    Tabor abrasion CS-17,
    1000 g (mg)
    2
    Flammability (oxygen
    index) (%)
    >95
    Volume resistance
    ((omega)-cm)
    1.4 x 10 17
    Dielectric constant
    (23°C, 10 3 Hz)
    2.4
    Dissipation factor 2 x 10 –3
    Dielectric breakdown
    strength (kV/–0.15 mm)
    7.0
    Refractive index (raw
    rubber) ND (23°C)
    1.25 x 10 –4
    125–230°C 2.15 x 10 –4

    Table IV. General properties of perfluoroelastomers.

    The unique environmental resistance of perfluoroelastomers results from the chemistry of the material. Base perfluoroelastomers are polymers of three or more monomers, in which all hydrogen positions have been replaced by fluorine (Figure 1). The outstanding resistance of perfluoroelastomer vulcanates to most chemicals and solvents—as well as to heat— derives from this state of complete fluorination. The principal monomer in perfluoroelastomers is tetrafluoroethylene, or TFE. The second and third perfluorinated monomers are unique to specific supplier formulations, and together confer the material's particular balance of properties, including its degree of chemical and temperature resistance, and its mechanical characteristics.

    The reliability of medical devices and equipment that incorporate fluid-control systems depends in large part on the seals between the components. With today's advancements in medical procedures, these seal components are being exposed to a wider range of chemicals and environments. Furthermore, demands for smaller components have placed more stringent requirements on both materials of construction and methods of fabrication.

    Figure 1. Schematic of molecular structure of perfluoroelastomers. (F = fluorinated groups, C = principal monomer [tetrofluoroethylene], A and B = secondary and tertiary perfluorinated monomers.)

    Of the compounds used for medical seals, perfluoroelastomers have the broadest range of chemical resistance of any elastomeric material and are readily formable into O-rings, gaskets, and other standard and custom seal configurations. Variations in polymer composition, coupled with compounding differences, allow properties to be tailored to meet specific application requirements, including chemical resistance, temperature resistance, and mechanical properties.

    Barry Chadwick is medical product manager and Chris Toto is fluid-handling engineering manager for Greene, Tweed & Co. (Kulpsville, PA; http://www.gtweed.com), a multinational manufacturer of specialty seals and components for the medical, semiconductor, and aerospace industries.


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    Copyright ©2000 Medical Device & Diagnostic Industry
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