Different Types of Sealants

For planar SOFC stacks, four different sealing sections are given  

The seals should be reliable to obtain long-term performance goals. The SOFC consists of 10-15-pm thin electrode-electrolyte delicate assembly, which requires the seal separating the cathode from the anode with minimum stress transmission. Basically, three different sealing approaches are given for SOFC which are shown in Fig. 5.4 and explained in the following section. 

All sealing techniques have offered wide advantages along with certain disadvantages. 

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The choice of the correct depth-width ratio for each type of sealant minimizes local stresses, which could promote premature failure of the sealant. The recommended depth width ratios for different types of sealant are plastic 3 1 to 1 1, phaso-elastic 2 1 to 1 1,... 

Perhaps the biggest thrust for the development of high performance polymers over the next 10 years will be in the aerospace industry where materials will be required for a fleet of high speed civil transports (supersonic transports). At a speed of Mach 2.4, an aircraft surface temperature of about 150 to 180°C will be generated. The life requirement of materials at these temperatures will be about 60000 hours. Many different types of materials such as adhesives, composite matrices, fuel tank sealants, finishes and windows will be needed. These materials must exhibit a favorable combination of processability, performance and price. The potential market for these materials total several billions of US dollars. 

As already mentioned, one-component silicone sealants cure by moisture pick-up from the atmosphere to form permanently elastic rubber. There are different types of curing systems for one-part silicone sealants (Table 6.5). However, the most commonly available systems are the acetoxy and neutral curing systems. 

Different types of joints are associated with silicone sealants. The curing system and modulus of a sealant are important factors for the joint application type. Figure 6.2 illustrates the type of joints and Table 6.7 lists suitable products for use with each type of joint. 

In this type of sealant, the sealant consists of two parts, one containing polymer (also called the base) and a separate component containing the curing agent. These are combined by a mechanical mixer prior to application. Proper mixing is attained in approximately five minutes. Incomplete mixing is indicated by the appearance of streaks in the mix (if the two components are of different colour). At ambient temperatures the pot-life is about three hours. The pot-life decreases with increase in temperature. It is possible to increase or decrease the pot-life by incorporation of an accelerator or a retarder, respectively. At ambient temperatures the sealant cures to form a rubber-like solid. 

The introduction of different types of reactive groups on the polydimethylsiloxane backbone enables crosslinking reactions to take place. The silicones can be applied as coatings on a variety of substrates, or as sealants. Elastomeric products such as seals, tubing, or other shaped products can be made by extrusion or molding processes. 

The time dependence of the strain at break is very different for the two types of sealant. The break points of the silicone data can be fitted by a straight line, and confidence limits at various levels can be drawn on the plot (Figure 11). The break points of the two-part polysulfide sealant form a broad band, the upper and lower limits of which are drawn qualitatively. The upper limit is better defined than the lower one (as for the failure envelope). Because of the difference between the plots for silicone and polysulfide sealants the further simplification of characterization is different for the two types of sealant. 

One-part silicone systems are ready to use, require no mixing, present no pot-life problem, and are generally the least expensive. Conventional one-part adhesive/sealants are available with two different types of cure systems acid and non-acid cure. Both require moisture from the atmosphere to cure. The acid-curing type has the greatest unprimed adhesion and the longest shelf life. The non-acid-curing type is effective when the acetic acid released by the cure reaction may cause corrosion, or be otherwise objectionable. 

When calculating the design width of joints in a building, it is essential to take account of the tolerances on the relevant dimensions of the components and the accuracy in placing them that is likely to be achieved on site. For these reasons, there may, in practice, be considerable variation in the widths of a number of ostensibly identical joints. The joint width for the design must ensure that in no joint will the sealant be subjected to a level of compression or extension that, when expressed as a proportion of the achieved joint width, exceeds its movement capability. (See the Selection of joint sealants, where Table 1 gives maximum movement permitted for each major type of sealant.) However, considerable variation in formulation and properties may occur between different brands of the same chemical type, which may affect this value. The maximum tolerable joint movement quoted by the manufacturer should be used in joint design calculations. 

There are also preformed sealants to consider within the ambient type of sealants that can be applied on-site. They are pre-moulded and are fabricated from a range of materials (synthetic rubber, PVC) with different shapes (ribbons, tapes, beads, or extruded shapes), and are mainly used for glazing applications. 

This technique is used with all kinds of sealants and putties, the viscosity should range from 200,000 to 1 million mPas. The gun can extrude a bead of product of diameters ranging from 2 to 12 mm. Fig. 12 shows the different types of guns. Guns may be hand operated or operated by air pressure, as shown in this figure. There are different sizes of cartridges ... 

A number of different types of end-products, molded in treated rubber/ polyurethane, have been developed and commercialized. These include rollers, wheels, flexible foams, molded goods, coatings, adhesives and sealants. It is obvious that this is just the beginning and that a large number of additional uses will be developed in the future. 

A different type of butyl mastic with a much lower viscosity is used by the canning and closure industry where highly automated application equipment applies a thin bead of sealant on metal can ends. Here the good FDA regulatory status and chemical resistance of butyl are valued. 

In common with most polymeric materials, adhesives and sealants are sensitive to the rate of applied force. There are several different types of impact test for adhesives. The most common test (similar in application to the Izod impact test for plastic materials) has been withdrawn without replacement however, the test was not widely used in a production situation due to the difficulty in achieving reproducible results. A more informative test for impact is the cleavage test in which the test specimen is subject to an impact force through a wedge (refer to Table 24). This can be initiated by a pendulum impact machine with an impact energy range of 50-300 J and an impact speed of 3-5.5 m/s. 

Complete and imiform dispersion of the ingredients in an adhesive or sealant is eritieal to product performance. The ehoiee of mixer, from among the many available, depends on the viscosity of the produet, any special needs (e.g. vaeuum) and how the mixed product is removed form the mixer to the filling line. Table 5 shows the viscosity of everyday items (Ross, 1989) in comparison to typical viscosities for several different types of adhesives and sealants (Flick, 1988). 

As seen so far, the most descriptive classiftcation method for adhesives and sealants is by physicochemical nature of the base resin. Thus, methods of reaction for different types of adhesives and sealants can also be summarized as follows ... 

The stability of other polymer and gel electrolytes based on PEO and PVDF derivatives have also been investigated. So far, most studies involving the stability of polymer electrolyte-based DSSC show the benefit of the replacement of the liquid component. The positive effect after longterm operation is more evident in the Jsc parameter. Besides, the use of the polymer electrolyte allows for easier assembly while different types of materials can be used as sealants. 

The base polymer for this t5q>e of sealant exists in the form of an emulsion of micron- and submicron-sized partides of the polymer suspended in water. The base polymer formed by free radical polymerization may be a homopolymer of an acrylic monomer but is more likely to be a copolymer of a number of different monomers chosen to provide the correct balance of properties. The polymer latex has to be made more permanent and therefore a nonionic surfactant such as a nonyl phenol/ polyethylene oxide is added to help stabilize the emulsion. Other additives to the sealant formulation include plastid-zers, fillers, solvents, and silanes. A plasticizer is added to the formulation in order to improve upon or maintain the flexibility of the sealant. Solvents (usually a small amount) are added to improve the tooling of the sealant after it is applied. In addition, a solvent could be a material such as ethylene or propylene glycol which can improve the resistance of the packaged sealant to temperatures below freezing. The most widely used filler for this type of sealant is calcium carbonate. Silanes are often added to acrylics to improve the wet adhesion of the sealant to glass. Other additives include antimildew agents (for tub and tile applications) and clay for rheological control. 

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