Flexure ring

For many applications low-temperature flexibility of the plasticized composition is also important. Plasticizers of low viscosity and low viscosity-temperature gradient are usually effective at low temperature. There is also a close relationship betv/een rate of oil extraction and low-temperature flexibility plasticizers effective at low temperature are usually rather readily extracted from the resin. Plasticizers containing linear alkyl chains are generally more effective at low temperature than those containing rings. Low-temperature performance is evaluated by measuremen t of stiffness in flexure or torsion or by measurement of second-order transition point, brittle point or peak dielectric loss factor. 

A specific example of this can be found in the evaluation of composites for heat exchanger applications.13 In this case, the heat exchanger design calls for a tubular construction which will be pressurized. Under these conditions, a flexural stress will be present in service, and consequently, a C-ring test specimen configuration provides a reasonable way to examine the properties of the composite. 

Ho et al (1996) examined polyol or polysiloxane thermoplastic polyurethanes (TPUs) as modifiers in cresol-formaldehye novolac epoxy resins cured with phenolic novolac resin for computer-chip encapsulation. A stable sea-island dispersion of TPU particles was achieved by the epoxy ring-opening with isocyanate groups of the urethane prepolymer to form an oxazolidone. The flexural modulus was reduced by addition of TPU and also the Tg was increased due to the rigid oxazolidone structure. Mayadunne et al (1999) extended this work to a series of phenol- and naphthol-based aralkyl epoxy resins. 

Besides cap formation, defects of individual tubes may influence the MWNT structure in other ways too. The simultaneous existence of five- and seven-mem-bered rings, for example, makes a bent nanotube (Figure 3.12). In multiwalled systems, these so-called elbow structures are found in between encapsulated regions. The flexure thus is confined to a few graphene layers. 

It is well known that polymers with condensed imide rings exhibit high thermal stability. Properties of the imide ring containing ACEC LXII (Scheme 78), cured with amines and with an acid anhydride, are given in Table 20. Acid anhydride cured epoxyimides reach the heat resistance of 190 °C, temperature of the start of decomposition above 265 °C and flexural strength above 70 MPa. 

Biaxial flexure is another approach that uses a bending configuration. In this test, the specimen is supported on a ring or three balls and the load is applied in the center of the opposite face. The specimens for this test are often circular plates but rectangular plates can also be utilized. 

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