Blend aging

Figure 4 Cls, Ols, Si2s, and Si2p peaks of (a) unaged 50 50 silicone-EPDM blend (b) 50 50 silicone-EPDM blend aged for 9 h at 175°C (c) silicone rubber aged for 9 h at 175°C and (d) EPDM aged for 9 h at 175°C.

Blending.—Aged brandies are very often blended, since they may vary in characteristics according to source of raw materials, district of production, and year of vintage. Blending has been found necessary to produce a product of uniform characteristics year after year. As in whiskey blending, cheapening may also be a desideratum. 

On the contrary, Contat-Rodrigo et al." proposed USE as a better method in terms of reproducibility and extraction efficiency. They studied the extraction of degradation products from degradable polyolefin blends aged in soil. Higher amounts of certain products (e.g., carboxylic acids) were extracted by USE than by MAE. 

Contat-Rodrigo, L., Haider, N., Ribes-Greus, A., and Karlsson, S., Ultrasonication and microwave-assisted extraction of degradation products from degradable polyolefin blends aged in soil, J. Appl. Polym. Set, 79, 1101-1112, 2000. 

Figure 14.5. DSC thermograms for aged polymer blends (a) polyvinylchloride/poly isopropyl methacrylate, immiscible blend, aged at a temperature of 60°C, and (b) polyvinyl chloride/polymethylmethacrylate, miscible blend, aged at 80°C. Time of aging, t in hours, is shown alongside each curve. Broken lines represent the un-aged samples for comparison.

A more comprehensive study of this system has been carried out by Cowie and Ferguson [1991]. These authors followed the enthalpy and stress relaxation of a series of PMMA/SAN blends with SAN compositions spanning the miscibility window, i.e., from 13.3 to 30 wt% AN. It was found that the blends relaxed faster than either of the components, when aging temperatures were (Tg - T ) = 10°C, but that this was no longer the case at (Tg - T ) = 20°C where blend aging was intermediate to both components. The data were analyzed using both the P-M and C-F approaches and examples of AH(t, T ) vs. log t plots are shown in Figure 14.6. The C-F model... 

Figure 6.8 Amount of crystalline form I (mi,) in PB-l/HOCP 70/30 blend, aged at 40°C, as a function of the disappearing traction of the crystalline form II (mo — mn,).

Figure 5.8. Mercaptide-polystyrene blends aged for a long time (a) AgSCijHjs-polystyrene aged for 8 weeks at 25°C, and (b) Pb(SCi2H25)2-polystyrene aged for 12 weeks at 25°C.

The rate at which blends age is expected to vary with the types of component involved. Blends of PS and PPE, where the components both have relatively rigid chain structures, have been studied [Elliot, 1990 Oudhuis and ten Brinke, 1992], In this case, while the amount of enthalpy relaxation in the blends was lower than that for either component, no evidence for faster relaxation of the component with the lower Tg value (PS) was found. The (E<,)values calculated from the C-F enthalpic aging are shown in Table 9.3 and tend to support this conclusion. There is a steady decrease in Eg) for PPE through the blend compositions to PS, with no apparent domination by either component in an approximately equimolar blend composition. 

The first comprehensive study of physical aging in a miscible blend system using enthalpy relaxation was reported by Cowie and Ferguson (1989) who followed the enthalpic relaxatirm in a series of blends of PS and poly(vinyl methyl ether), PVME. ComparisOTi of the blend behavior with that of the two components by analyzing the data oti the basis of both the P-M and C-F models led to the conclusions that the blends aged more slowly than PVME when aging was carried... 

---