Sealants high-temperature types

The original intent of this project was to identify and/or perfect vendor-supplied, state-of-the-art hydro-phobic polymer sealants, such that the end product could meet military criteria for hermeticity in hybrid microelectronic devices. Solubility and diffusivity constants and permeabilities for several types of hydrophobic polymers were determined. Data from the laboratory and theory from literature both indicate that moisture impermeable polymers are not attainable, at least to meet military standards. The project objective was then redirected towards finding a means of passivating the surface of a microelectronic device against moisture. It was found that interface modification, i.e., passivation of the chemically reactive surface species, provides significant protection in high temperature and humidity environments. Plasma polymerization of hexamethyIdi-silazane (HMDS) afforded phenomenal protection to moisture sensitive thin film nichrome resistors even under the most severe test conditions. 

The two-part sihcone adhesive/sealants do not require moisture to cure and produce a superior deep-section cure. Two types are available addition-cure and condensation-cure. Addition of curing produces no by-products, can be heat-accelerated, produces negligible shrinkage, and provides the best high-temperature resistance of all silicone adhesives. Condensation-cure silicones are not easily inhibited and can be used on a greater variety of materials. ... 

Dow Coming Corp. offers an improved silicone adhesive/sealant for high-temperature use. This is a one-part, non-slumping paste that cures to a tough, mbber sohd at room temperature on exposure to water vapor in the air. This material is said to perform at temperatures ranging from -65 °C to 260°C for continuous operation, and to 316°C for intermittent exposure. This material will meet the requirements of M1L-A-46106A (2), Type 1 (see http //mil-spec-industries.com). The adhesive/sealant is acid-cured and acetic acid is evolved during cure. ... 

Silicone adhesives have very good thermal stability, but low strength. Their primary application is in non-structural uses, such as high-temperature pressure-sensitive tapes. Attempts have been made to incorporate silicones in other adhesives, such as epoxies and phenolics, but long cure times and low strength have limited their use. The maximum service temperature for silicone adhesive/sealants is 260°C for continuous operation and up to 316°C for intermittent exposure, depending on the type used. 

Rosins - These are abietie aeid resins obtained from wood sourees. There are several different types wood rosins obtained from the naptha extraetion of pine stumps, gum rosins obtained from the destruetive distillation of turpentine, and tall oil rosins obtained by the destruetive steam distillation of tall oil from the manufacture of paper. Rosins are widely used in the adhesive and sealant industry. In hot melts they oxidize and diseolor easily at high temperatures due to the imsaturation in the resin. Hydrogenated grades are available which have much better thermal stability. 

Abstract In this chapter, classification of adhesive and sealant materials is presented. For this purpose, various categories are considered depending on the polymer base (i.e., natural or synthetic), functionality in the polymer backbone (i.e., thermoplastic or thermoset), physical forms (i.e., one or multiple components, films), chemical families (i.e., epoxy, silicon), functional types (i.e., structural, hot melt, pressure sensitive, water-base, ultraviolet/ electron beam cured, conductive, etc.), and methods of application. The classification covers high-temperature adhesives, sealants, conductive adhesives, nanocomposite adhesives, primers, solvent-activated adhesives, water-activated adhesives, and hybrid adhesives. 

O Section 12.2 of this chapter classifies adhesives according to functional types including, structural, hot-melt, pressure sensitive, water-base, ultraviolet/electron beam cured, conductive, high-temperature, and sealant type. 

The cure rate of a sihcone sealant is dependent on the reactivity of the cross-linker, catalyst type, catalyst level, the diffusion of moisture into the sealant, and the diffusion of the leaving group out of the sealant. For one-part sealants, moisture diffusion is the controlling step and causes a cured skin to form on the exposed sealant surface and progress inward. The diffusion of moisture is highly dependent on the temperature and relative humidity conditions. 

Silicone They have excellent heat resistance up to 260°C (500°F), chemical resistance, good electricals, compatible with human body tissues, etc. and a high cost. There are the room temperature vulcanizing (RTV) types that cure and cross-link at ambient temperatures, catalyzed by moisture in the air. It is a good sealant and excellent for making flexible molds for casting. It is widely used for human implants. 

The cylindrical-type sealing process for strip packaging does not usually have a cooling cycle or quench surface, hence any pull on the seal ply will tend to weaken the seal while the sealant is still pliable. Therefore, it is desirable that the sealant have good hot tack or a high melt viscosity at processing temperatures. Hot tack is generally accepted to mean the adhesion quality of the sealant from the moment it leaves the seal head and the time it returns to its set or permanent condition. 

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. 

A sealant, such as Apiezon W, is a good example of a material that was designed for an extremely limited application. Apiezon W is a hard black wax that needs to be heated to 80°-90°C before it is soft enough to apply to the members you wish to join. At room temperatures it is hard and has no lubrication abilities whatsoever and is therefore not usable for a stopcock. It has a relatively high vapor pressure (10 3 torr), so it cannot be used for most vacuum work. However, at temperatures of about 100° to 150°C, it becomes very thin like hot honey and can easily be applied to joint members. Therefore, if you have a standard taper joint that will not be in a high-heat or ultrahigh-vacuum environment, there is no need to separate often, and you don t want it to accidentally separate, Apiezon W is the type of sealant you need. 

The properties of high performance building sealants have been studied as a function of four variables stress, strain, time and temperature, using tensile tests. By time-temperature superposition the number of variables can be reduced to three and the material properties characterized in a three-dimensional coordinate system. For sealants the projection of the failure points to this system in the log strain versus log time plane is sufficient for characterization. Further simplification can be made depending on the properties of the various types of sealant. 

Silicones are available in one- and multicomponent forms. The one-component types are commercially the most important and will be the focus of most of this discussion. These products, which generally cure by reaction with atmospheric moisture, are called RTV (room-temperature vulcanizing) sealants or adhesives. The surface cure rate of these products is a function of the cure system, but the rate of cure in depth depends on the ability to transmit water vapor through the mass of sealant. Silicones are highly permeable to moisture vapor, and generally the one-component types cure at a rate of about 0.3 cm/day. Due to this high vapor permeability, the one-component silicones typically cure faster than do their nonsilicone counterparts. 

ANSWER The usual type of sealant for this application is a silicone, polyurethane or a silyl-terminated polyether sealant. However, 1 would also suggest that you consider a flexible two-component epoxy adhesive or sealant. Concrete does not expand or contract very much with temperature changes, and you do not really need a sealant with high flexibility. 

In the field of aerospace applications, the reliability of materials under extreme exposure conditions is of prime importance. The high- and low-temperature properties of the fluoroelastomers have permitted them to give reliable performance in a number of aircraft and missile components, specifically manifold gaskets, coated fabrics, firewall seals, heat-shrinkable tubing and fittings for wire and cable, mastic adhesive sealants, protective coatings, and numerous types of O-ring seals. 

Silicone adhesives cure without the application of heat or pressure to form permanently flexible silicone rubber. The rubber remains flexible despite the exposure to high or low temperatures, weather, moisture, oxygen, ozone, or UV radiation. This makes them useful for joining and sealing joints in which considerable movement can be expected, such as intermediate layers between plastics and other materials of construction (e.g., acryUc glazing). Several types of silicone adhesives/sealants are available, including one-part and two-part systems. 

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