F. Gugumus

Effect of temperature on the lifetime of stabilized and unstabilized PP films

Unstabilized and phenolic antioxidant stabilized PP films were aged in draft air ovens at temperatures between 150 and 40°C. The Arrhenius plots of the failure times reveal quite distinctive features. Thus, unstabilized PP films show a marked downward curvature of the plot at temperatures below 80°C. The role of Ti catalyst residues in this behavior is discussed.Phenolic antioxidant stabilized films show a similar bent in the Arrhenius plot at significantly higher temperatures. Furthermore, these plots can be superposed onto the curve obtained for the unstabilized film by a shift parallel to the reciprocal absolute temperature axis. The numerical value of the shift is a function of the chemical nature of the phenolic antioxidant and of its concentration. Remarkably, the phenol stabilized films show a second curvature in the Arrhenius plot at temperatures below 80°C. This second curvature is an upward bent so that the Arrhenius plot becomes a typical sigmoid. This low temperature curvature depends also on the nature of the phenolic antioxidant and its concentration. It is attributed to some kind of complexation of the Ti catalyst residues by the phenols. Comparison with PP films stabilized with Hindered Amine Stabilizers (HAS) and with PE-LD films lends additional support to the conclusions concerning the role of transition metals.

Thermooxidative degradation of polyolefins in the solid state—6. Kinetics of thermal oxidation of polypropylene

Thermal oxidation of polypropylene films at 80°C was examined as a function of aging time, sample thickness, polymer grade and sample preparation. The experimental kinetics of functional group formation do not correspond to clear power laws as claimed in the literature. After the end of the induction period, the kinetics conform rather to an autoaccelerated, exponential-type increase in the early stages of oxidation. This is followed by a linear increase of the concentration of carbonyl groups in more advanced stages of oxidation. It is found that formal homogeneous kinetics accounts for some aspects of the experimental results with PP.

Formation of ester functional groups in oxidizing polymers

Abstract The possibility for the condensation reaction between carboxylic acids and alcohols to proceed in oxidizing polymers has been examined. The reaction proceeds readily with model compounds in low-density polyethylene, in the absence of any mineral acid as a catalyst. The temperature range for the study was between 130 and 190°C. The activation energy found for the reaction, 14–15 kcal/mole is comparable with the values found in solution experiments with low molecular mass solvents. The results show that formation of esters through reaction of carboxylic acids with alcohols can contribute significantly if not overwhelmingly to the overall ester group concentration found in oxidized polyolefins. This will be particularly important on heterogeneous oxidation if high molecular mass acids and alcohols are formed in proximity. Since these compounds cannot diffuse apart from each other, the probability for reaction will be enhanced. ©

Thermooxidative degradation of polyolefins in the solid state—7. Effect of sample thickness and heterogeneous oxidation kinetics for polypropylene

Thermal oxidation of PP films at 80°C was examined as a function of aging time, sample thickness, polymer grade and sample preparation. The variation of the formation of oxidation products with sample thickness shows that the good fit of the data deduced from formal homogeneous kinetics is quite fortuitous. The heterogeneous oxidation model developed initially for PE-LD and based on rapid oxidation of individual amorphous domains, followed by spreading of this oxidation to nearby domains, is applied successfully to PP. The heterogeneous kinetics based on this model fit well the experimental kinetics of the oxidation products. Therefore, the model seems to be valid for polyolefins in general.

Thermolysis of polyethylene hydroperoxides in the melt3. Experimental kinetics of product formation

Thermolysis of polyethylene hydroperoxides restricted to the initial stages can be readily understood. The same is true for the oxidation products formed on thermolysis. These products are essentially the same as those formed on processing in mixers open to air. They are mainly alcohols and ketones, in-the chain ketones and methyl-ketones. Carboxylic acids and aldehydes are also formed in a small amount. The same is valid for trans-vinylene and γ-lactone groups. The kinetics of oxidation product formation in the initial stages are clearly related to the corresponding kinetics for hydroperoxide decomposition. However, the various products show significant differences in this respect. Thus, thermolysis of hydroperoxides in the initial stages consists of direct hydroperoxide decomposition and free radical induced hydroperoxide decomposition. Formation of carbonyl groups corresponds also to such a process. This means that carbonyl groups result from both direct and induced hydroperoxide decomposition. However, alcohol groups are formed essentially in reactions following direct hydroperoxide decomposition according to the pseudo-monomolecular reaction. Formation of γ-lactones does not proceed according to the same general scheme. Its rate is increasing considerably with the initial oxidation of the samples. Hence, it looks as if it were formed out of products accumulating in the oxidizing polyethylene melt.

Light stabilization of metallocene polyolefins

Ultraviolet (UV) stabilization of metallocene polyolefins is examined from a practical and theoretical point of view. First of all, the possibility of using existing stabilizers and stabilizer systems is investigated. To this avail, the compatibility of the stabilizers needs to be checked at the concentrations most commonly used. It is found that, in this respect, there is no fundamental difference between conventional high-pressure polyethylene (PE-LD), linear low-density polyethylene (PE-LLD) and metallocene polyethylene (m-PE). With polypropylene (PP), stabilizer compatibility is generally not so much a problem as with polyethylene (PE). The assessment of the performance of light stabilizers is performed with compression molded films. The results so far available show that the UV absorbers and hindered amine light stabilizers (HALS) examined confer comparable UV stability to PE-LD, PE-LLD and m-PE. Similar results are observed with PP. It is found that the lifetimes obtained on artificial weathering of m-PP and third generation PP are comparable. Furthermore, the synergism between two polymeric HALS that has been observed previously with second and third generation PP is also found with metallocene polyolefins. The experimental laws relating the performance of the HALS to their concentration are deduced from the preliminary data available. It is found that they are the same as those found previously for PP and PE. With PP, UV stability increases linearly with HALS concentration as long as the last is not too high. With m-PE as with PE-LLD, the performance is directly proportional to the square root of the HALS concentration. From these and additional results it is concluded that the degradation and stabilization mechanisms with m-polyolefins are essentially the same as those developed for the previous types of polyethylene and polypropylene.

Novel role for tropospheric ozone in initiation of autoxidation

The views on initiation of autoxidation of organic materials need considerable revision. In the past 50 years, autoxidation of organic materials has been attributed to chain reactions initiated by the decomposition of hydroperoxides contained in the substrate. It is found that, initiation of polyolefin oxidation is caused much more by the ozone contained in the atmosphere. The accumulated ozone exposure is determining the length of the induction period and, therefore, to a large extent the overall thermal oxidative lifetime of polymers. It is assumed that it is also valid for other organic materials including biologically important molecules.

Critical antioxidant concentrations in polymer oxidation-III. Application to lifetime prediction

Abstract Lifetime predictions based on the ‘critical antioxidant concentration’ are misleading. This results not only from the fact that there is no ‘critical antioxidant concentration’ for fundamental reasons. The basic assumption used in numerous publications over the past 35 years, i.e. first-order decrease of the antioxidant concentration with aging time, has even more drawbacks. An improved version, which assumes the existence of a minimal effect or no-effect antioxidant concentration, can explain the results with phenolic antioxidants stabilized polyolefins. A completely independent lifetime prediction method based on the polymer oxidation level is much more broadly applicable. It is not only valid with phenolic and aromatic amine antioxidants but also accounts for the effect of Hindered Amine Stabilizers on thermal oxidative aging. Furthermore, the oxidation level is useful for the prediction of lifetime under photo-oxidative conditions.

Critical antioxidant concentrations in polymer oxidation—II. Experimental ‘proofs’∗

Abstract Experiments thought to prove the existence of critical antioxidant concentrations are examined. In this respect, a distinction is made between polymer melts and polymers in the solid state. In polymer melts it is hard to find any indication of a ‘no-effect’ antioxidant concentration. It is possible to simulate such effects by preoxidation of the polymer or through the use of high initiation rates. With polymers in the solid state, there is an additional possibility caused by the necessity of having at least one antioxidant molecule in every amorphous domain of semi-crystalline polymers. This is a consequence of the heterogeneous nature of the polymers and is not related to any ‘critical’ conditions. The majority of the ‘proofs’ of the ‘critical antioxidant concentration’ are thought to be caused by artifacts introduced with the representation of the induction times as a function of the logarithm of the antioxidant concentration.

Critical antioxidant concentrations in polymer oxidation—I. Fundamental aspects☆

Abstract Different theoretical derivations of the ‘critical antioxidant concentration’ in inhibited polymer oxidation are reviewed and discussed in detail. It is shown that they are not valid. There are no critical phenomena in oxidizing polymers in the absence of antioxidants. There are no critical conditions in the presence of inhibitors. There is no critical antioxidant concentration.