Polypropylene oxygen uptake

The synergistic effect of a hydroperoxide decomposer, eg, dilauryl thiodipropionate [123-28-4] (34), and a radical scavenger, eg, tetrakis[methylene(3,5-di-/ f2 butyl-4-hydroxyhydrocinnamate)]methane (9), ia protecting polypropylene duting an oxygen-uptake test at 140°C is shown ia Table 3. 

Polypropylene differs from polyethylene in its chemical reactivity because of the presence of tertiary carbon atoms occurring alternately on the chain backbone. Of particular significance is the susceptibility of the polymer to oxidation at elevated temperatures. Some estimate of the difference between the two polymers can be obtained from Figure 1J.7, which compares- the rates of oxygen uptake of eaeh polymer at 93°C. Substantial improvements can be made by the inclusion of antioxidants and such additives are used in all commercial compounds. Whereas polyethylene cross-links on oxidation, polypropylene degrades to form lower molecular weight products. Similar effects are noted... 

Fig. 7. Plots of oxygen uptake against time [333] (a) linear, polymers that show no induction period but absorb oxygen at a relatively constant rate (polymethylmethacrylate, polystyrene, polycarbonate) (b) autoretardant, polymers that exhibit no induction period but initially absorb oxygen at a relatively rapid rate, followed by a slower steady rate (polyethylene, polypropylene, nylons) (c) polymers that display autocatalytic behaviour (the modified acrylics, acrylonitrile—butadiene—styrene copolymer) (d) polymers that can be considered a combination of autocatalytic and autoretardant, (c) and (d) can be considered as autocatalytic, since the processes usually become autoretardant in the later stages of oxidation.

Effects of a series of transition metal stearates, the concentration of the copper stearate, the solvent, various additives, and other factors on the thermal oxidation of polypropylene were studied in trichlorobenzene solution. The mechanism of copper catalysis is discussed. The order of decreasing catalytic activity of the metal stearates was Cu > Mn > Fe > Cr > Al Ni Co control Ti >> Zn >> V. The addition of propionic acid to the solvent accelerated the oxidation of the polymer. The presence of the copper leveled off oxygen uptake of the polymer after a certain time. The amount of oxygen absorbed decreased with increasing concentration of the copper, and at higher concentration (7.9 X 10 3M) the polymer oxidation was inhibited. 

Effect of Metal Stearates. The oxygen uptake curves of isotactic and atactic polypropylene in the presence of transition metal stearates in trichlorobenzene are shown in Figures 1 and 2, respectively. The order of decreasing catalytic effect of the metal stearates at the early stage of the oxidation of the polymers is for isotactic PP, Cu > Mn > Fe > Cr > Co > Ni > Ti > control > A1 > > Zn > > V and for atactic PP, Cu > Mn> Fe > Cr > Ah Ni Co control Ti >> Zn >> V. The order of the catalytic effect of the metals is quite different from that in bulk reported previously (16). In particular, V-stearate inhibits the thermal oxidation, and Co-stearate is not as effective as in bulk. In the presence of effective metal stearates such as Cu and Fe, the oxygen uptake levels off after a certain time. However, the amount of oxygen absorbed in the isotactic polypropylene is higher than that in the atactic polypropylene. 

Effect of Anions in Copper Compounds. Since the copper stearate was the most effective catalyst among the transition metal stearates for the early stage of the thermal oxidation of polypropylene, the effect of anions in copper compounds on the thermal oxidation of atactic polypropylene was examined. The oxygen uptake curves of the polymer in the presence of various copper compounds (acetate, propionate, butylate, stearate, laurate, polyacrylate, and cupric oxide) are shown in Figure 4. In the absence of the copper compounds, oxygen uptake of the polymer increases linearly with time. In the presence of copper compounds of fatty acids (acetate, propionate, butyrate, laurate, and stearate), the oxygen uptake of the polymer levels off at ca. 25-30 O2 mL/g polymer after... 

Effect of Copper Stearate Concentration. As mentioned above, the thermal oxidation of the polymer catalyzed by copper compound (7.9 X 10 4M) proceeds very rapidly at the early stage and shows the leveling off of the oxygen uptake. Thus, the effect of the concentration of the copper stearate on the oxidation of the polymer was examined. The oxygen uptake curves of the thermal oxidation of the polymer in the presence of 3.2 X 10 5Af-7.9 X 10 3M copper stearate are shown in Figure 5. In this figure, the thermal oxidation of the neat polypropylene proceeds almost linearly. However, the copper stearate-catalyzed thermal oxidation of the polymer is affected remarkably by the copper concentration. In the presence of less than 3.2 X 10 3M copper stearate, the... 

Effect of Propionic Acid in Solvent. The thermal oxidation of the polypropylene in mixed solvent of trichlorobenzene and propionic acid was carried out, and the oxygen uptake curves obtained are shown in... 

Effect of Various Additives. From the results mentioned above, it is evident that the copper stearate is converted to inhibitors by the interactions of oxidation products, and the oxygen uptake appears to level oflF after a certain time. Thus, the effects of model compounds of oxidation products (n-octylaldehyde, n-octanoic acid, 2-octanone, n-octanol, and n-octanoic aCid methyl ester), tert-hutyl hydroperoxide, and di-ter -butyl peroxide on the thermal oxidation of the polypropylene in trichlorobenzene were examined. 

Oxidation of polypropylene describes the kinetic curve of oxygen absorption, which has an S-shape (fig.l). This curve is characterized by an induction period of self-acceleration and deceleration of oxidation in a deep stage of the process. A typical kinetic curve of oxygen uptake for isotactic PP is shown in Figure 1. For comparison, the kinetic curve for polyethylene (PEHD). The kinetic equation represents the dependence of the amount of... 

In liquid-phase oxidation of hydrocarbons the rate of oxygen uptake equals the rate of accumulation of hydroperoxide. The yield ROOH p>er mole of absorbed oxygen a=l. In the autoxidation of solid polymer a much less than unity. For isotactic polypropylene. 

Figure 10-1. Typical oxygen uptake by an unstabilized sample of polypropylene at 120 °C. The reaction is au-toaccelerating. Extrapolation of the approximately linear part, as indicated by the dotted line, gives the induction period and the polymer typically loses its useful mechanical properties at this point. The amount of oxygen absorbed is low, corresponding to less than one molecule for every 100-150 polymer repeat units.

Other possible simultaneous techniques in combination with CL are /zFTIR, FTIES and oxygen uptake. Simultaneous FTIES-CL analysis (both emission spectroscopies) has been used to evaluate oxidation models for polypropylene [598]. In a further instrumental advancement, imaging chemiluminescence (ICL) has become available cfr. Chp. 5.6.4.1). Early stages of polymer oxidation can also be studied using 02 exposure and ToF-SIMS analysis [599,600]. 

The photo-oxidation of polypropylene is characterized by an induction period in which oxygen uptake (whether measured directly or by carbonyl index) occurs at a very low rate, followed by autoacceleration to a steady rate of oxidation. Measurements of mechanical properties and polymer molecular weight [2187] have shown that extensive degradation occurs in the induction period and the useful life extends little beyond this point [1443]. The surface of photo-oxidized polypropylene is highly degraded to a depth of 0.5 ym during the induction period, leading to microcracks and loss of mechanical properties [390]. 

Figure 10.38 shows examples of oxygen uptake curves for polypropylene, obtained with the oxygen pressure monitor represented in Figure 10.37. 

Fig. 10.38. Typical oxygen uptake curves for polypropylene films [836]. (Reproduced with permission from the Society of Chemical Industry.)...

As for container material, polypropylene containers are as satisfactory as glass ones in compound stability and recovery. Since water is more harmful than oxygen to compound degradation and the hygroscopic nature of DMSO favors water uptake, humidity control is a key parameter to ensure compound storage stability. 

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