Insulation bonding

Bond Properties. The strength of the propellant-to-liner-to-insulation bond must be sufficient to maintain its integrity under the stresses mentioned above. 

We shall use the same machinery as that developed above in the case of the lattice fuse model. In order to mimic the insulator part, one takes capacitors with a given value Ci as insulating bonds. For the conducting bonds, one takes very large capacitor Cq such that Co Ci (Beale and Duxbury 1988). By this choice the voltage across a Cq capacitor is zero, as within the conductor the field must be zero, p is the fraction of Cq capacitors which are now seen as defects. Of course, the other possibility is to take... 

The qualitative variation with time of temperature, strain, and stress through the thickness of foam insulation bonded to an aluminum tank surface is shown in Fig. 3. At time zero, t, there is a step change in temperature in the aluminum plate from ambient temperature to LF2 temperature, as shown in Fig. 3a. At some short time later, t, there is a very steep temperature gradient existing in a very thin layer of foam immediately adjacent to the aluminum. As time progresses, the temperature gradient tends to flatten out. 

The magnitude of the tensile stresses is determined primarily by the thermal contraction mismatch between the aluminum and the foam however, the length, width, and thickness of the insulation influences the level of the compressive stresses. For 15.2 cm (6 in) thick insulation, as shown in Fig. 4, edge effects significantly reduce compressive stresses for smaller pieces of insulation. [The stresses in Figs. 4 and 5 are based on polymethacryli-mide insulation bonded on an aluminum tank at 20 K (-424°F) with a maximum insulation exterior temperature of 317 K (110°F)]. For example, a 0.3 m (1 ft) square insulation sample experiences a maximum compressive stress that is approximately 1/3 the stress in a 1.83 m (6 ft) square piece of insulation. The latter approaches the stress level for an infinite slab of insulation. 

An especially severe case of thermal stresses in expanded plastic insulation occurs when the insulation is bonded to a more rigid member of the structure, A theoretical analysis of thermal stresses has been made for cylindrical geometry in which the boundary conditions approximate the case of insulation bonded to the inner surface of the warm outer shell of a low temperature storage vessel, From this analysis a prediction of the low temperature performance of such insulations can be made if they are isotropic and if their mechanical properties are known. The properties that must be known as functions of temperature are the modulus of elasticity in tension and Poisson s ratio. In addition to these properties, the tensile strength and the modulus of rigidity have been obtained at selected temperatures down to 20 K for two densities of expanded polystyrene and an expanded epoxy resin. 

Construction Gypsum board joint cement Concrete Paper and film lamination Vinyl flooring Wood doors Glass fiber insulation Bonding facing to fibers... 

Ariane 5 cryogenic tank - thermal insulation bonding (CRYOSPACE image)... 

With suitable bonding techniques, the metal parts can be joined vacuum tight to the ceramic insulator in automated brazing procedures. 

Specifications, Standards, Quality Control, and Health and Safety Factors. Formerly, there was an Insulation Board Institute representing the insulation board industry, but the decline in the market and number of producers has led to its demise. Currently (ca 1997), the industry is represented by the American Hardboard Association (AHA). Specifications and standards are found in American National Standards Institute (ANSI) standard for CellulosicFiberboard (7). The standard includes descriptions of the various types and classes of ftberboard, as well as requirements for physical and dimensional stabiUty properties. QuaUty control tests are limited to a few basic strength and stabiUty tests, including bending strength, bond strength, and moisture resistance. 

Heatshield thickness and weight requirements are determined using a thermal prediction model based on measured thermophysical properties. The models typically include transient heat conduction, surface ablation, and charring in a heatshield having multiple sublayers such as bond, insulation, and substmcture. These models can then be employed for any specific heating environment to determine material thickness requirements and to identify the lightest heatshield materials. 

Propellants cast into rockets are commonly case-bonded to the motors to achieve maximum volumetric loading density. The interior of the motor is thoroughly cleaned, coated using an insulating material, and then lined with a composition to which the propellant binder adheres under the environmental stresses of the system. The insulation material is generally a mbber-type composition, filled with siUca, titanium dioxide, or potassium titanate. SiUca-filled nitrate mbber and vulcanizable ethylene—propylene mbber have been used. The liner generally consists of the same base polymer as is used in the propellant. It is usually appHed in a thin layer, and may be partially or fully cured before the propellant is poured into the rocket. 

As a tme thermoplastic, FEP copolymer can be melt-processed by extmsion and compression, injection, and blow molding. Films can be heat-bonded and sealed, vacuum-formed, and laminated to various substrates. Chemical inertness and corrosion resistance make FEP highly suitable for chemical services its dielectric and insulating properties favor it for electrical and electronic service and its low frictional properties, mechanical toughness, thermal stabiUty, and nonstick quaUty make it highly suitable for bearings and seals, high temperature components, and nonstick surfaces. 

Most medium voltage cables are made with insulation shield layers that are bonded but easily stripped from the insulation in order to avoid pockets of air at the interface and at the same time to allow easy field handling for termination and splicing (during installation). 

The cables designed for use at voltages over 49 kV require that the conductor and insulation shields be firmly bonded to the insulation in order to avoid any possibiUty of generating corona at interfaces strippable insulation shields are not accepted. The A ETC specifications for cables rated for 59—138 kV require a volume resistivity of one order of magnitude lower than for the medium voltage cables. 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

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