Sealing elastomeric

Vascular access ports typically consist of a self-sealing siUcone septum within a rigid housing which is attached to a radiopaque catheter (see Radiopaques). The catheter must be fabricated from a low modulus elastomeric polymer capable of interfacing with both soft tissue and the cardiovascular environment. A low modulus polyurethane-based elastomer is preferred to ensure minimal trauma to the fragile vein. 

Compatibility and Corrosion. Gas turbine fuels must be compatible with the elastomeric materials and metals used in fuel systems. Elastomers are used for O-rings, seals, and hoses as well as pump parts and tank coatings. Polymers tend to swell and to improve their sealing abiUty when in contact with aromatics, but degree of swell is a function of both elastomer-type and aromatic molecular weight. Rubbers can also be attacked by peroxides that form in fuels that are not properly inhibited (see Elastomers, synthetic Rubber, natural). 

Joining NonmetaUic Pipe Thermoplastic piping may be joined by a qiiahfied hot-gas welding procedure, a qnalined solvent-cement procedure, or by a qualified heat-fusion procedure. The general welding and heat-fiision procedures are described in ASTM D-2657 and solvent-cement procedures in ASTM D-2855. Two other techniques, for flared joints and elastomeric-sealed joints, are described in ASTM D-3140 and D-3139, respectively. 

Quite often problems arise when instruments for normal seiwice are subjected to low temperature use. Since some metals become brittle at low temperatures, the instrument hteraUy falls apart. Elastomeric gaskets and seals contract faster with decreasing temperatures than the surrounding metal parts, and the seal often is lost. Even hermetically sealed instruments can develop pin holes or small cracks to permit ciyogenic liqmds to enter these cases with time. Warming the instrument causes the trapped hquid to vaporize, sometimes generating excessive gas pressure and failure of the case. 

The outside diameter of the lip. seal fits and seats into the housing bore (transmission or pump). The inside diameter, with the elastomeric lip, rides onto the spinning shaft (whether vehicular drive shaft or pump shaft). 

Availability - It s the only elastomeric seal form easily available in most cities and towns. 

Vacuum Service - The o-ring is the only common popular elastomeric form that seals in both directions. 

Although the primary function of sealants is to seal, adhesion promoters are often added, which allows them to adhere to the adjoining base materials. It is therefore sometimes difficult to distinguish between an adhesive and a sealant. For example, structural silicone adhesives are used in the building construction industry owing to their sealing, adhesive, elastomeric properties, and their resistance to harsh environmental conditions [67,70,77]. 

HEPA filters consist of a filter pack sealed into a case. The filter pack or core is made by pleating a continuous web of fiberglass paper back and forth over corrugated separators. The filter pack is sealed into a full-depth wood or steel casing using a elastomeric sealant. Gasketing is a critical item to ensure that the filter passes the air leakage tests. 

The manufacture of elastomeric radial lip seals centers around the molding operation. Figure 2 [1] illustrates the manufacturing flow diagram for the two primary processes used compression molding and injection molding. 

The choice of process is generally based on economic issues such as volume of seals to be produced elastomeric compound, cost and waste and product design. 

During the shaft seal molding process, the elastomeric compound has relatively large distances to travel/ flow in the mold, particularly when using shelf loaded compression molding (Fig. 17a) and injection molding (Fig. 17b). History has shown that without the use of a vacuum, either applied directly through the mold or... 

Duct mastic has replaced duct tape as the most effective sealing material. Mastic is a fibrous, elastomeric compound that permanently seals duct connections and seams. In addition, the boot connections between ducts and floors and between walls and ceilings should be fully caulked and/or foamed airtight. Lastly, air handlers that house the... 

Fig. 4-2(o) Example of using elastomeric bell-and-spigot seals. 

Gillen, K.T., M. Celina, and R. Bernstein, Validation of improved methods for predicting long-term elastomeric seal lifetimes from compression stress-relaxation and oxygen consumption techniques. Polymer Degradation and Stability, 82(1), 25-35, 2003. 

Compression set and durometer hardness are also important mechanical properties. Compression set is defined as the amount by which an elastomer fails to return to its original thickness after being subjected to a standard compressive load or deflection for a specified time at a specified temperature. A low percent compression set typifies a more compression resistant elastomeric formulation. Compression set of a closure on a sealed vial is a factor in maintaining the sterility and potency of the drug itself. 

The ideal elastomeric closure is nonreactive physically and chemically, a complete barrier to vapor/gas permeation, easily penetrable via needle or spike, re-sealable, resistant to coring and fragmentation, and maintains package integrity at the seal surface. 

Paralleling the corrosion problem is one involving compatibility of any well fluid with nonmetallic materials used in well completion apparatus. All injection wells and many producing wells are equipped with packers to isolate the casing annulus from the high temperature, pressure, and salinity characteristic of the petroleum reservoir environment. Conventional packers, as well as other well tools, utilize elastomeric materials to mechanically seal appropriate locations. 

Sealants. Gaskets, and Beads. All gaskets or beads are required to be at least 3/8 inch wide with a Shore "A" durometer hardness of 50 and conform to ASTM Specification C509-84 (Cellular Elastomeric Preformed Gasket and Sealing Material). 

ISO 13844 2000 Plastics piping systems - Elastomeric-sealing-ring-type socket joints of unplasticized poly(vinyl chloride) (PVC-U) for use with PVC-U pipes - Test method for leaktightness under negative pressure... 

ISO 23711 2003 Elastomeric seals - Requirements for materials for pipe joint seals used in water and drainage applications - Thermoplastic elastomers ISO 14910-1 1997 Plastics - Thermoplastic polyester/ester and polyether/ester elastomers for moulding and extrusion - Part 1 Designation system and basis for specifications ISO 14910-2 1997 Plastics - Thermoplastic polyester/ester and polyether/ester elastomers for moulding and extrusion - Part 2 Preparation of test specimens and determination of properties... 

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