Sealing glass thermal properties

Thermal Properties. Many commercial glass-ceramics have capitalized on thek superior thermal properties, particularly low or zero thermal expansion coupled with high thermal stabiUty and thermal shock resistance properties that are not readily achievable in glasses or ceramics. Linear thermal expansion coefficients ranging from —60 to 200 x 10 j° C can be obtained. Near-zero expansion materials are used in apphcations such as telescope mirror blanks, cookware, and stove cooktops, while high expansion frits are used for sealing metals. 

Another benefit of copolyermization of ethylene is incorporation of appropriate polar groups such as carboxylic acid groups or salt groups in the ionomers. Du Pont is still the only producer of the Surlyn-type of polyethylene ionomers. However, several companies produce copolymers of ethylene with acrylic acid (EAA). Such copolymers offer improved thermal properties with retention of clarity, improved heat-seal strength, and impact strength, as well as improved adhesion to a variety of substances, such as aluminum foil, glass fibers, and mineral fibers. 

As with glasses, the properties — most notably the thermal expansion coefficients — of glass-ceramics can be controlled by adjusting the composition. In many applications, such as glass-metal seals and the joining of materials, it is very important to match the thermal expansion coefficients to avoid the generation of thermal stresses. 

Differential scanning calorimetry directly measures the heat flow to a sample as a function of temperature. A sample of the material weighing 5 to 10 g is placed on a sample pan and heated in a time- and temperature-controlled manner. The temperature usually is increased linearly at a predetermined rate. DSC is used to determine specific heats (Fig. 10-11), glass transition temperatures (Fig. 10-12), melting points (Fig. 10-13) and melting profiles, percent crystallinity, degree of cure, purity, thermal properties of heat-seal packaging and hot-melt adhesives, effectiveness of plasticizers, effects of additives and fillers (Fig. 10-14), and thermal history. 

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