Long-Term Exposure to Temperature

Behavior under conditions of long-term exposure to temperature cannot be ascertained in short-term mechanical or thermo-mechanical tests, because time-dependent aging and relaxation processes are involved. 

As set out in DIN EN ISO 2578, aging can be expressed in terms of temperature indices (Tl). Selected properties are examined and compared before and after test specimens are kept under elevated temperatures in heating chambers. If there are other mechanical strains or deformations that still apply, the behavior of the plastic can only be gauged on the basis of creep tests and stress relaxation tests [8]. 

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Heat resistant fibres, however, are those having chemical structures that are little changed physically or chemically by temperatures above the 200°C, and even the 300°C, levels and, in the case of ceramic fibres, above 1000°C. For textiles used in high temperature industrial processes, such as gas and liquid filtration, long term exposure to temperatures of about 100°C is often required, but not all these fibres are used in thermally protective applications. However, in long term exposure thermally protective applications, we need to be able to define maximum service fife temperatures, and these are listed in Table 4.2 for selected heat resistant fibres. 

Lithium-ion batteries are electrochemical systems whose processes are related to temperature higher temperatures accelerate side reactions which cause a reduction of battery capacity or an increase in battery resistance. Long-term exposure to temperature values above 32 °C should be minimized to meet a 10-year durability target (see Figure 8.16). Each specific battery chemistry has a different sensitivity, but the general rule applies to all systems. 

This chapter is concerned with the short-term mechanical properties — moduli and strengths — of glass, aramid and carbon fibres in a thermosetting resin matrix. A little information on reinforced thermoplastic matrix systems is also included. The data mainly refer to the room temperature properties of 55-65 v/o fibre, unidirectional, systems. The effects of the variation in fibre volume loading, method of test and instantaneous and long term exposure to temperature are briefly mentioned. Longitudinal properties tend to be fibre dominated, and so are compressive properties to some extent for glass and carbon fibres. The anisotropy of unidirectional materials is noticeable. 

The possible conversion of carbides to graphite during long-term exposure to temperatures above 427 °C (800 °F) of carbon steels, plain nickel steel, carbon-manganese steel, manganese-vanadinm steel, and carbon-silicon steel. 

Long-term exposure to high temperature is best described by the Underwriters Laboratory (UL) temperature index. The UL temperature index is ... 

Degradation products of LDPE/(BHT, Chimassorb 944) after long-term exposure to compost, water and air (chemical hydrolysis at pH 5 and pH 7) at room temperature were examined by GC-MS [277] the structural changes in the LDPE film were monitored by DSC and SEC. Among the 79 low-MW degradation products identified by GC-MS the main components were... 

The UL temperature indexes for a variety of engineering plastics are in Table 4. Glass-filled PPS compounds exhibit a high UL temperature index, which indicates excellent retention of properties for long-term exposure to high temperature. Other fillers, eg, mineral (talc), may also be used. ... 

The principal problem hindering the development of commercial systems for organic/organic separations is the lack of membranes and modules able to withstand long-term exposure to organic compounds at the elevated temperatures required for pervaporation. Membrane and module stability problems are not... 

Creep is the term used to describe the tendency of a material to move or to deform permanently to relieve stresses. Material deformation occurs as a result of long term exposure to levels of stress (physics) that are below the yield strength or ultimate strength of the material. Creep is more severe in materials that are subjected to heat for long periods and near melting point. Creep is often observed in glasses. Creep is a monotonically increasing function of temperature. 

The catalyst must be positioned where its operating temperature is reached quickly but long term exposure to high temperatures, >900 C, must be avoided. The position where suitable conditions can be met is at the inlet of the convection section. A schematic drawing of the boiler is shown in Figure 1 after the described modifications and the integration of the catalyst. 

Long-term exposure to cyfluthrin has been reported to cause diarrhea, reduced body temperature, and weight loss in laboratory animals. A 24 month chronic... 

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