Oven-aging

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

Antioxidants have been shown to improve oxidative stabiHty substantially (36,37). The use of mbber-bound stabilizers to permit concentration of the additive in the mbber phase has been reported (38—40). The partitioning behavior of various conventional stabilizers between the mbber and thermoplastic phases in model ABS systems has been described and shown to correlate with solubiHty parameter values (41). Pigments can adversely affect oxidative stabiHty (32). Test methods for assessing thermal oxidative stabiHty include oxygen absorption (31,32,42), thermal analysis (43,44), oven aging (34,45,46), and chemiluminescence (47,48). 

In general, one day of oven aging at 70°C corresponds to one year of natural or shelf aging (a minimum requirement for mbber products), whereas the oxygen and air bomb methods are more drastic. By varyiag the amounts and types or combiaations of antioxidants the relative effectiveness of these materials against normal oxygen deterioration can be determined. 

Polyesters and polycarbonate polyols show improved resistance to oxidative attack, compared with that of the polyethers. Stress relation studies run at 130°C, comparing a urethane based on a poly(oxypropylene) polyol and a urethane based on poly(butane adipate) polyol show that, after 60 h, the urethane based on PPG lost most of its strength, while the polyester retained most of its strength [83], Urethanes made from poly(butadiene) polyols are also susceptible to oxidation, but they show good resistance to air-oven aging with antioxidants present (see p. 290 in [45],... 

Figure 13 Percent weight loss versus oven aging time for...

This chapter will explain the setup and methodology used for the field retrieval study. The physical and chemical analysis techniques used to study the oxidation of the tire rubber, along with the data analysis developed to interpret the results, will also be explained. Then, the development of an oven-aging protocol that attempts to reproduce the mechanism and rate of tire field aging will be described. 

Being cut from tires, the rubber samples in this work cannot always be studied using standard test procedures. The specific techniques used to measure rubber aging have been described in detail elsewhere and are summarized below [2]. The same techniques have been used to evaluate rubber aging in both field and the laboratory oven-aging studies. For reference. Figure 34.2 shows a diagram of the internal components of a radial-ply tire. 

Tires were mounted and inflated to the maximum pressure listed on the sidewall prior to oven aging using either air or a 50/50 blend of N2/O2. In the case of tires inflated with the 50/50 blend of N2/O2, the atmospheric air present was not purged the blend was added on top of it yielding a tire cavity concentration of approximately 41%-44% O2. Two of the tires in this study (SUV/Minivan-A and SUV/Minivan-C) were aged at 50°C, 60°C, and 70°C for up to eight weeks, while the tires and... 

FIGURE 34.22 Log (strain ratio) versus log (modulus) for field and oven-aged SUV/Minivan-A tires. Both sets are linear indicating oxidative aging, but the slopes are different indicating a change in composition of the wedge rubber. 

Figure 5 Left thermo-oxidative carbonyl index vs oven ageing times at 90°C. Right photo-oxidative carbonyl index vs irradiation time after exposure to UV radiation (300-800 nm wavelength). In each case, the carbonyl index is based on the intensity of the mid-infrared absorption at 1,720 cm-1 normalised against sample film thickness, d, which was 290 gm. Reprinted from Corrales et al. [13]. Copyright 2002, with permission from Elsevier. 

Figure 8 Hydroperoxide formation in LDPE granules during oven ageing at 100°C. Reprinted from Luzuriaga et al. [18]. Copyright 2006, with permission from Elsevier.

As discussed earlier under Section 2.3, Carbonyl index, in one relatively recent comparison of the photo-oxidative and thermal (oven-aged) degradation behaviour of different polyethylenes, additive free grades of a metallocene (mPE), an HDPE and a linear low-density PE (LLDPE) were analysed by a combination of mid-IR spectroscopy, TGA and CL [13]. The mid-IR... 

Figure 36 TOF-SIMS 160 and lsO peaks after 0,144 and 288 h oven ageing at 90°C. Reprinted from Moller et al. [102]. Copyright 2003, with permission of Elsevier.

The American standard ASTM D-2307 [12] provides a test method for measuring Relative thermal endurance properties of film insulated magnet wire . The ageing philosophy is similar to that required by IEC 60216 [9] and is based on the Arrhenius ageing model. Oven ageing is carried out at three specific temperatures. The samples are twisted wire... 

The temperature index is the maximum temperature that causes a 50% decay of the studied characteristics in the very long term. It is derived from long-term oven-ageing test runs. The UL Temperature Index depends on ... 

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