Resistance, thermal

Several formulations of elevated-temperature curing epoxy adhesives have been developed with improved thermal resistance and greater toughness. The next section describes the processes and materials that can be used to achieve moderately better heat resistance and toughness. However, formulations with the optimum temperature resistance are discussed in Chap. 15, and tougheners are described in Chap. 8. 

A general-purpose two-component adhesive that will provide high tensile shear strength up to 150°C is described in Table 12.11. The base epoxy resin in this formulation is a mixture of an epoxy novolac and a liquid DGEBA epoxy resin. 

Benzoquinone tetracarboxylic acid dianhydride (BTDA) has been found to provide epoxy adhesives with excellent high-temperature properties, in both the short and long terms. The formulation described in Table 12.12 provides good resistance to 260°C. This two-part adhesive can be cured 2 h at 200°C. The disadvantage of BTDA is that relatively high cure temperatures are required that result in a high degree of internal stress within the bond line. 

TABLE 12.11 High-Temperature Epoxy Adhesive Utilizing Epoxy Novolac Resin14 

PMDA or trimellitic anhydride has also been shown to provide epoxy adhesive formulations with high-temperature properties. Table 12.13 shows the elevated-temperature tensile shear strength of an epoxy adhesive cured with 4 pph of PMDA. Another specialized formulation employing PMDA was found to provide high shear strength when tested at 260°C even after aging 1000 h at 260°C.15 

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Styrene—acrylonitrile (SAN) copolymers [9003-54-7] have superior properties to polystyrene in the areas of toughness, rigidity, and chemical and thermal resistance (2), and, consequendy, many commercial appHcations for them have developed. These optically clear materials containing between 15 and 35% AN can be readily processed by extmsion and injection mol ding, but they lack real impact resistance. 

Thermal Resistance and Flammability. Thermal analysis of PVA filament yam shows an endothermic curve that starts rising at around 220°C the endothermic peak (melting point) is 240°C, varying afitde depending on manufacture conditions. When exposed to temperatures exceeding 220°C, the fiber properties change irreversibly. 

Eoamed polystyrene sheet has exceUent strength, thermal resistance, formabUity, and shock resistance, as weU as low density. It is widely known for its use in beverage cups, food containers, building insulation panels, and shock absorbent packaging. Polystyrene products can be recycled if suitable coUection methods are estabUshed. Eoamed polystyrene sheet can also be easily therm oformed (see Styrene plastics). 

A number of thermally stable polymers have been synthesized, but in general the types of stmctures that impart thermal resistance also result in poor processing characteristics. Attempts to overcome this problem have largely been concentrated on the incorporation of flexible groups into the backbone or the attachment of stable pendent groups. Among the class of polymers claimed to be thermally stable only a few have achieved technological importance, some of which are polyamides, polyimides, polyquin oxalines, polyquinolines, and polybenzimidazoles. Of these, polyimides have been the most widely explored. 

Fig. 2. Change in insulation value with time for insulation of various thicknesses. SHce thickness = 2.5-6 mm and R ate defined as thermal resistance at...

An alternative method known as slicing and scaling has been developed (23,24). In this, the rate of diffusion is determined on a thin specimen (6—10 mm thick) and a scaling factor S used to relate the results to a thick specimen. For a material satisfying the requirements of a constant diffusion and constant initial pressure,, the same ratio of time thickness provides the same values of p and %. Thus the thermal resistance of a specimen of thickness at time can be obtained by conditioning a specimen of thickness over a time given by... 

Optics. Good optical properties and low thermal resistance make poly(methyl methacrylate) polymers well suited for use as plastic optical fibers. The manufacturing methods and optical properties of the fibers have been reviewed (124) (see Fiber optics). Methods for the preparation of Fresnel lenses and a Fresnel lens film have been reported (125,126). Compositions and methods for the industrial production of cast plastic eyeglass lenses are available (127). 

Phosphatase Test. The phosphatase [9001-78-9] test is a chemical method for measuring the efficiency of pasteurization. AH raw milk contains phosphatase and the thermal resistance of this enzyme is greater than that of pathogens over the range of time and temperature of heat treatments recognized for proper pasteurization. Phosphatase tests are based on the principle that alkaline phosphatase is able, under proper conditions of temperature and pH, to Hberate phenol [108-95-2] from a disodium phenyl phosphate substrate. The amount of Hberated phenol, which is proportional to the amount of enzyme present, is determined by the reaction of Hberated phenol with 2,6-dichloroquinone chloroimide and colorimetric measurement of the indophenol blue formed. Under-pasteurization as well as contamination of a properly pasteurized product with raw milk can be detected by this test. 

Carbon-Fiber Composites. Cured laminates of phenoHc resins and carbon-fiber reinforcement provide superior flammabiHty resistance and thermal resistance compared to unsaturated polyester and epoxy. Table 15 shows the dependence of flexural strength and modulus on phenoHc—carbon-fiber composites at 30—40% phenoHc resin (91). These composites also exhibit long-term elevated temperature stabiHty up to 230°C. 

From bolometer theory (1) the change in film temperature is proportional to the absorbed power and thermal resistance (inverse of the thermal conductance) and is given by the following ... 

A guarded hot-plate method, ASTM D1518, is used to measure the rate of heat transfer over time from a warm metal plate. The fabric is placed on the constant temperature plate and covered by a second metal plate. After the temperature of the second plate has been allowed to equiUbrate, the thermal transmittance is calculated based on the temperature difference between the two plates and the energy required to maintain the temperature of the bottom plate. The units for thermal transmittance are W/m -K. Thermal resistance is the reciprocal of thermal conductivity (or transmittance). Thermal resistance is often reported as a do value, defined as the insulation required to keep a resting person comfortable at 21°C with air movement of 0.1 m/s. Thermal resistance in m -K/W can be converted to do by multiplying by 0.1548 (121). 

Expressed as percentages of total annual synthetic mbber consumption worldwide, EPM and EPDM have increased from 0% in 1964 to 10.6% in 1990, as shown in Table 4. Contrary to the general-purpose elastomers such as NR, SBR, and BR, EPM and EPDM still show a steady growth over the years. Part of this growth stiU comes from replacement of these commodity mbbers by virtue of thek better o2one and thermal resistance. 

Mihtary interest in the development of fuel and thermal resistant elastomers for low temperature service created a need for fluorinated elastomers. In the early 1950s, the M. W. Kellogg Co. in a joint project with the U.S. Army Quartermaster Corps, and 3M in a joint project with the U.S. Air Force, developed two commercial fluorocarbon elastomers. The copolymers of vinyUdene fluoride, CF2=CH2, and chlorotrifluoroethylene, CF2=CFC1, became available from Kellogg in 1955 under the trademark of Kel-F (1-3) (see Fluorine compounds, ORGANic-POLYcm.OROTRiFLUOROETHYLENE Poly(vinylidene) fluoride). In 1956, 3M introduced a polymer based on poly(l,l-dihydroperfluorobutyl acrylate) trademarked 3M Brand Fluorombber 1F4 (4). The poor balance of acid, steam, and heat resistance of the latter elastomer limited its commercial use. 

N. Miyoshi and co-workers. Development of Thermal Resistant Three-Way Catalyst, SAE 891970, Society of Automotive Engineers, Warrendale, Pa.,... 

R Thermal resistance, equals x/kA, 1/UA, l/hA Ri, Ro, R l, R for thermal resistance of sections 1, 2, 3, and n of a composite body Rj for sum of individual resistances of several resistances in series or parallel R -, and for dirt or scale resistance on inner and outer surface respectively Ratio of total outside surface of finned tube to area of tube having same root diameter (s-K)/J (h- F)/Btu... 

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