Stabilization of polypropylene

Table 3. Stabilization of Polypropylene Fiber by Polymeric HALS ...

Both thermooxidation and photooxidation of polyolefins can be prevented by using the same antioxidants as those employed for the stabilization of polypropylene, ie, alkylated phenols, polyphenols, thioesters, and organic phosphites in the amount of 0.2—0.5% (22,25). 

Certain metal salts effectively reduce the photoactivity of titanium dioxide pigments. Combination of these salts with an appropriate antioxidant and/or ultraviolet stabilizer provided highly efficient stabilization of polypropylene. The deactivation/ stabilization performance of the metal salts is adequately explained on the basis of their decomposition of hydrogen peroxide at the pigment surface and by annihilation of positive holes in the pigment crystal lattice. 

L. Rychla and J. Rychly, New concepts in chemiluminescence at the evaluation of thermooxidative stability of polypropylene from isothermal and non-isothermal experiments. In A. Jimenez and G.E. Zaikov (Eds.), Polymer Analysis and Degradation, Nova Science Publishers, New York, 2000 p. 124. 

The above reaction (Equation 4.25) would reduce the rate of volatilization of polypropylene by reducing the concentration of unsaturated chain ends which act as initiating structures.56 As an alternative or in addition, a stabilization mechanism based on the reported formation of metallic bismuth can be proposed.53 57 Stabilization of polypropylene by metal compounds is in agreement with activity of several metal compounds as radical catalyst/inhibitor depending on metal concentration and/or temperature of the system.58... 

Observed differences in the influence of the nature of the substituent R2 on activity during the stabilization of polypropylene against y-irradia-tion and oxygen must be ascribed to differences in the mechanism of both... 

Mechanisms of Degradation Inhibitors. The degradation mechanisms described above have no little effect on end-use stability and processing stability of polypropylene. Indeed, without stabilizers and antioxidants to inhibit such degradation reactions, polypropylene would have limited usefulness as a structural plastic if any usefulness at all. 

Figure 1. Processing stability of polypropylene, PPP vs. aryl phosphite...

Figure 2. Processing stability of polypropylene in a Brabender torque...

Figure 3. Processing stability of polypropylene on large-scale equipment...

Table V. Ultraviolet Stability of Polypropylene Films with Various Ultra violet-Absorber Formulations...

Some of the test methods being used to measure the processing stability of polypropylene include melt flow drift measurements at elevated temperatures using an extrusion plastometer (melt indexer), melt viscosity retention measurements using a torque rheometer, retention of melt flow after repeated extrusions, and injection molded spiral test measured by the flow in inches at various temperatures and the retention of melt flow of the injected spirals. The nine commercial resins were evaluated by these methods. 

Influence of UV Irradiation on the Stability of Polypropylene and Blends of Polypropylene with Polymethyl methacrylate... 

Polyolefins - An Arrhenius model has been developed for lifetime prediction of the light stability of polypropylene . Photooxidation processes in blends of polypropylene with poly(butylene terephthalate) (PBT) are complicated by the thermal sensitivity of the polypropylene and the screening effect of the terephthalate ester . This effect is shown in Scheme 1. 

Henman, T.J. Melt stabilization of polypropylene. In Developments in Polymer Stabilisation Scott, G., Ed. Applied Science London, 1979 Vol. 1, 39-99. 

It can be postulated that three factors may play a role in stabilization in the presence of fillers. One factor is the discussed absorption which immobilizes stabilizer to the extent that it cannot perform its function. The second is its desorption capability, which, if it exists, may enhance the performance of stabilizer due to its better retention. However, it should be mentioned that photochemical changes occur at very short time scales therefore, there must always be a sufficient concentration of free (not absorbed) stabilizer to react with radicals. The third factor is the effect of filler on structure and related stability of stabilizer. The reduced stability of polypropylene in the presence of some stabilizers in Table 13.4 can be explained only by the formation of degradation products. It... 

Metal oxide semiconductor chemical sensors in combination with MDA have been shown to be useful to estimate the oxidative stability of polypropylene during processing instead of traditional melt flow index analysis (50). An array of sensors was used to receive a detailed analysis of volatiles. At quality measurements of different poly(butylene adipate)s the use of indicator products has been proven better than analyses of the decrease in molecular weight or mass loss for early degradation detection. Adipic acid, quantified using gas chromatography, was then used as the indicator product [51]. 

Figure 7. Light stability of polypropylene tapes (Florida) KLY to 50% retention of tensile strength.

Piperazine HALS 2 has a positive effect on the processing stability of polypropylene when used in conjunction with a phenolic antioxidant ... 

TUCKER AND SUSI Stabilization of Polypropylene Multifilaments Table II. PP Light Stability versus Thickness... 

Another important attribute of a HALS is its effect on the thermo-oxidative stability of polypropylene multifilaments. This property is important in certain end-use applications where elevated temperature over a period of time is experienced, such as in automobile rear shelf fabrics. The excellent performance of the oligomeric HALS 4 and 6, as determined by 120°C oven aging, is shown in Figure 2. The superior activity of these products may be due not only to their low volatility but also to the presence of a triazine moiety in the structure, which appears to have a positive effect on the thermo-oxidative stability of polypropylene. 

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