Physical and Chemical Aging

In general, as the temperature is decreased, molecular motions decrease and the molecular structure becomes more tightly packed as indicated by the amount of free volume. Above the Tg, molecular reconfigurations to at- 

Notice that data at each aging time is related via a simple shift in log space. This shift factor is typically denoted as a,. The aging shift factor takes a particularly simple analytical form 

Because the material ages continuously, the creep tests performed as in Fig, 7.27 must be of short duration at each aging time so that the compliance result is representative of viscoelastic properties at that aging time. The individual curves and the shifted master curve (also called the momentary master curve) is typically well fit by a Kohlrausch stretched exponential function 

28 Long term data for PEEK and effective time theory prediction based on momentary master curve (based on short term creep experiments). (Data courtesy of R. D. Bradshaw, University of Louisville long term prediction using shift rate from Guo and Bradshaw (2007).) 

The aging superposition process can be combined with the TTSP provide more extensive information of the material response as a function of time, aging time and temperature. In the last two decades many have studied physical aging extensively. Representative references include Wong, et al., (1981), McKenna, (1989, 1994), and Crissman, et al., (1990). A discussion 

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Once in the atmosphere, OAs can also undergo a wide range of physical and chemical aging processes under atmospheric conditions. OA components can react with atmospheric photooxidants (e.g., OH, NO3, and 03), acids (e.g., H2S04 and HN03), water, and UV radiation, forming for instance more polar and hygroscopic products than the precursor material. These atmospheric transformation processes can also occur at the surface layers of BC or EC (Poschl, 2005). 

A comprehensive analytical model for predicting long term durability of resins and of fibre reinforced plastics (FRP) taking into account viscoelastic/viscoplastic creep, hygrothermal effects and the effects of physical and chemical aging on polymer response has been presented. An analytical tool consisting of a specialized test-bed finite element code, NOVA-3D, was used for the solution of complex stress analysis problems, including interactions between non-linear material constitutive behavior and environmental effects. 

Asphalt samples were heated and cooled down before the experiments began. Samples were used immediately after they cooled down and the reminder was discarded after each experiment was finished. Thus, the asphalt samples were free of physical and chemical age hardening. 

All asphalt samples were heated above 120°C. Experiments were performed immediately after the samples cooled down, to avoid physical and chemical age hardening. All asphaltene samples were isolated by pentane using the solvent fraction method (see Fig. 3). All the solvents, including mixtures, used in these experiments were reagent grade, and the solubility parameters of all solvents are listed in Table III. 

However, most frequently the decision whether or not to use a particular blend is based on the replacement calculations. These involve not only the simple material cost (expressed by Eq 1), but the total comparable cost of the materials, their forming and assembling, customer satisfaction, esthetics, service life-spans, ease of disposal or recycling, etc. In this approach, one of the most serious problems is to find reliable data on long-term blend performance, i.e., on the mechanisms and rates of physical and chemical aging, as well as weatherability. 

The effects of a number of environmental factors on viscoelastic material properties can be represented by a time shift and thus a shift factor. In Chapter 10, a time shift associated with stress nonlinearities, or a time-stress-superposition-principle (TSSP), is discussed in detail both from an analytical and an experimental point of view. A time scale shift associated with moisture (or a time-moisture-superposition-principle) is also discussed briefly in Chapter 10. Further, a time scale shift associated with several environmental variables simultaneously leading to a time scale shift surface is briefly mentioned. Other examples of possible time scale shifts associated with physical and chemical aging are discussed in a later section in this chapter. These cases where the shift factor relationships are known enables the constitutive law to be written similar to Eq. 7.53 with effective times defined as in Eq. 7.54 but with new shift factor functions. This approach is quite powerful and enables long-term predictions of viscoelastic response in changing environments. 

The subject of physical and chemical aging received a great deal of attention for applications related to the aerospace industry in the late 20 ... 

Crissman, J.M. and McKenna, Physical and Chemical Aging in PMMA and Their Effects on Creep and Creep Rupture Behavior , J. Poly. Sc., Part B Polymer Physics, Vol. 28, 1990, p. 1463-1473. 

Kuhn, H.H., Skontrop, A. and Wang, S.S., High Temperature Physical and Chemical Aging in Carbon Fiber Polyimide Composites Experiment and Theory in Recent Advances in Composite Materials, (S.R. White, H.T. Hahn, and W.T. Jones, Ed. s), ASME MD Vol. 56, 1995, p. 193-202. 

Parvatareddy, H., Dillard, J.G., McGrath, J.E. and Dillard, D.A., Environmental aging of the Ti-6Al-4V/FM-5 polyimide adhesive bonded system implications of physical and chemical aging on durability. J, Adhes. Sci. Technol., 12(6), 615-637 (1998). 

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