Softening behaviour

The softening behaviour of a thermoplastic material depends to a large extent on the flexibility of the chain and the ability to crystallise. Significant cross-linking of a reasonably stiff-chained polymer will lead to material that is unlikely to soften below its decomposition temperature. Intermediate to the linear and cross-linked polymers are various ladder polymers in which the polymer molecule consists of a pair of more-or-less parallel chains bridged in a manner analogous to the rungs of a ladder. 

The properties of a material must dictate the applications in which it will best perform its intended use. All materials made to date with polymerized sulphur show time-dependent stress-strain behaviour. The reversion to the brittle behaviour of orthorhombic sulphur is inevitable as the sulphur transforms from the metastable polymeric forms to the thermodynamically stable crystalline structure. The time-span involved of at most 15 months (to date) would indicate that no such materials should be used in applications dependent on the strain softening behaviour. Design should not be based on the stress-strain relationships observed at an age of a few days. Since the strength of these materials is maintained, however, uses based on strength as the only mechanical criterion would be reasonable. 

Straining the specimen beyond ey, a decrease of true stress is observed this is the strain softening behaviour. 

However, when considering temperature or strain dependencies of strain softening, the absolute stress difference (ay - apf) does not allow one to discriminate between a change of the absolute values of ay and crpf and an intrinsic effect on the strain softening behaviour. 

Though SSA was used in our previous papers on this topic, hereafter the strain softening behaviour will be discussed in considering preferentially nSSA as descriptor. 

As the nSSA curves, as a function of temperature, for the various xTy 11 copolyamides (Fig. 92) have quite a similar shape to those obtained for BPA-PC and TMBPA-PC (Sects. 4.1.1.3 and 4.1.2.3, respectively), the discussion will be based on the mechanism proposed for the strain softening behaviour of BPA-PC (Sect. 4.1.3) and will emphasise the intermolecular cooperativity of the secondary transition motions. 

At elevated temperatures al polymers soften, dependent on their glass-rubber transition points, Tg, and/of their melting points, Tm. These temperatures limit the practical use of plastics. To characterize the softening behaviour, in practice various types of standard tests are being carried out, resulting in values for the softening temperature , defined in different ways. The values mostly used are the ISO Heat Deflection Temperature (HDT) and the Vicat Softening Temperature (VST or... 

In this picture the dependence of on I is schematically sketched for three polymers, largely differing in softening behaviour ... 

Fracture tests on notched bulk adhesive specimen have been made in three point bending to observe the real mechanical behaviour of the resin [16]. The Young s modulus is estimated to be equal to 96 10 MPa and fracture energy measured reaches 3900 600 J/m and the PU resin displayed softening behaviour. 

Casting Oxazolidone Resin Films. To explore further the structural property relationships in epoxy resins cured with isocyanate, a series of films were cast, and then IR absorption spectra and softening behaviour examined. 

These tests relate to the temperature-dependence of the properties of a polymeric material. Common tests include softening behaviour (particularly of thermoplastics) and low-temperature flexibility, for example Vicat softening point, heat-deflection temperature and Gehmann, Clash and Berg apparatus testing. 

At lower additions of plasticizer (up to 10 phr), the expected softening behaviour is reversed and the compound increases in hardness and Tg. This anti-plasticization has been explained by increased crystallinity facilitated by plasticizer. 

RILEM TC 148-SCC, Test method for the measurement of the strain-softening behaviour of concrete under uniaxialcompression . Mater. Struct (RILEM).33,2000, 347-351. 

Yam] Yamada, S., Kato, Y., Effect of Cooling Rate of Rapid Solidification on Softening Behaviour of Al-8%Fe-2%Mo Alloy , J. Jap. Inst. Light Met, 46(1), 21-26 (1996) (Experimental, Meehan. Prop., 7)... 

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