Physical aging process

The short-term behavior of plastics under thermal loading is determined by their softening behavior and physical aging processes, whereas long-term behavior is mainly dominated by chemical degradation. 

Softening behavior plays a decisive role mainly under high thermal loads (such as when plastics substrates are welded) and is determined by the glass transition temperature in amorphous and by the melting temperature in semi-crystalline 

The softening behavior is characterized by the heat distortion temperature. This is the temperature at which a test specimen deforms up to a limit value under a particular external load [15]. The determination of heat distortion temperatures is performed according to various processes such as Vicat (ISO 306 [16]) or ISO 75 [17]. These processes involve one-point, fast determination methods. The various 

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Thermal annealing at 140 °C in an inert dark atmosphere resulted in decreases in strength, ductility, and toughness, as seen in Fig. 3 and Table 1. These changes are attributed to physical aging processes occuring in the glassy polymer. As an additional... 

Figure 26 shows the thermal expansion behavior of requenched epoxies. Upon reaging, the densification process was again measurable. Data shown in Figure 26, therefore, supports the notion that physical aging processes are thermoreversible. 

A convenient way to analyze physical aging processes (structural recovery) is to use a relaxation function < t) defined as... 

Hnally we discuss relaxation or physical aging processes with relaxing free volume functions and their distributions as the determinants. Based on stochastic formulations,simulations of the coding process from melt to glass yidd as a functicm of cooling rate and most importantly, of the diaracteristic interaction parameters of the polymor. 

Physical aging processes involve changes in the structure or molecular order state ... 

Kozlov, G. V. Beloshenko, V. A. Lipatov, Yu. S. The fractal treatment of cross-linked polymers physical aging process. Ukrainian Chemical Journal, 1998,64(3), 56-59. 

As the annealing temperatures T drop further away from Tg, the aging process slows down and the time scales involved become quite long. Consequently many studies are carried out under thermally accelerated conditions. The relaxation of the enthalpy and volume of the glass are convenient parameters to follow when monitoring the physical aging process, as are the time-dependent small strain mechanical properties. Spectroscopic and scattering methods can also be employed... 

The majority of models developed to describe the physical aging process are based on the idea of free volume in the polymer. Some only attempt to model the... 

The C-F model has been extensively used to compare the aging behavior of polymer blends to that of the corresponding homopolymers, because of its simplicity. However, it should be noted that this model does not adequately describe all of the phenomenology associated with the glass transition, namely, its nonlinear character. The consequences of this when describing the physical aging process have been recently discussed in the literature (Li and Simon 2006 Hutchinson and Kumar 2002). 

The decrease in volume that accompanies the physical aging process can be related to a change of the distribution of free volume holes. Since the size and concentration of free volume holes in amorphous polymers is closely linked to the thermal expansion, direct measurements of the free volume can be used to monitor the aging processes. 

For the tyrosine-derived polycarbonates tested, the enthalpy relaxation process was not sensitive to the length of the pendent chain. This obser ation suggests that structural relaxation in these polymers is limited by backbone flexibility, and that the fraction of free volume in these polymers is not the limiting factor for polymer mobility. Furthermore, since the enthalpy relaxation time is short at aging temperatures of Tg-15°C, a few hours of storage at that temperature will be sufficient to bring the physical aging process to completion. The results obtained by dynamic... 

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