Polypropylene oven ageing

To evaluate antioxidants for polypropylene, various test methods have been developed. Gordon correlates these methods, and Gysling suggests a modified oven-aging technique and shows antioxidant synergist combinations. 

By using very thin samples, the oven aging of polypropylene can be appreciably accelerated. Specimens of definite and reproducible shape and thickness were made by using a microtome. The first phase of the work evaluated the influence of sample thickness from 0.2 to 12 mils on oven life at different temperatures. In the second phase, this modified technique was used to study the effectiveness of three anti-oxidant/DLTDP systems as thermal stabilizers for polypropylene. The results obtained over a wide spectrum of antioxidant/DLTDP combinations are shown in three-dimensional graphs. 

Figure 4. Oven aging of polypropylene samples stabilized with 0.1% Antioxidant A (—) and B (- -). Logarithmic scale...

Oven aging of polypropylene plaques was done in an 125°C oven with 1" x 1" plaques in triplicate strung on aluminum wire. Degradation is the time for samples to crumble and fall from the wire. 

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. 

Table I. The evaluation of polypropylene thermal oxidative stability by the chemiluminescence and oven aging methods at 150°C...

Table IV. Oven Aging of Polypropylene in the Presence of Antioxidants...

The origin of the talc ore used in polypropylene will have a significant influence on the color and long-term oven aging performance of the composite. Numerous tests have been performed to determine the variables in talc that contribute to this difference. It appears that talc surface chemistry can lead to talc-polymer and talc-stabilizer interactions that can cause discoloration and depolymerization under long-term heat (150-160°C) aging. High iron content in the talc can also adversely influence the level of performance. 

Fig. 5.4. Change of the molecular mass distribution after oven ageing (t ageing time) at 130 °C in an oxygen atmosphere, po2 = 760 nun Hg. a polypropylene, the distribntion is not only shifted towards low molecular masses, but it also becomes narrower. b polyethylene, the distribution is shifted towards low molecular masses, the width changes only shghtly. Soiuce (Emanuel and Buchachenko 1987)...

Carbonyl absorption, change in moleoular weight of polypropylene after multiple processing, oven aging, and UV loading [38]... 

Change in molecular weight and molecular weight distribution of unstabilized polypropylene after oven aging at I35°C [38]... 

Figure 2.3 Problems in the evaluation of stabilizers using oxidative induotion times Left Influenoe of phosphide stabilizers on oxidative induction times following oven aging of polypropylene at 150°C[182J...

Dielectric loss factor of isotactic polypropylene films without antioxidant as a function of oven aging time at 90 °C [763]... 

Wyzgosky MG. Effect of oven ageing on polypropylene. J Appl Polym Sci 1981 26 1689-704. 

Glass RD, Valange BM. Antioxidant crossover effect in oven aging of polypropylene. Polym Degrad Stab 1988 20(3 ) 355-63. 

In addition some samples were reformed by pressing into sheets small samples were then cut for ageing in air drculated ovens at various temperatures, between 120 and 150°C, both in and out of contact with copper. These oven ageing trials were conducted in order to validate the extrapolation to higher temperatures so that future material selection based solely on DSC measurement of OIT could be vahdated. The time to failure in the oven ageing trials was assessed by a simple three-point bending test, since it was found that the polypropylene lost aU mechanical integrity rather rapidly once the oxidation process was under way. The time to failure was reported as OIT at the lower temperatures and these results incorporated into the Arrhenius plots. 

Polypropylene has been used for many years in hot water applications with varying degrees of success. Failures in one instance were traced to a stabilizer package that inadequately protected the polymer in a hot, chlorinated water environment. This same stabilizer system performed extremely well in oven aging tests. The current study involved examination and analysis of failed samples from the field, along with laboratory aging studies designed to replicate the field failures and compare the performance of the stabilizer system in different environments. 

Note of the author This procednre is aimed at estimating the resistance of polypropylene, in molded form, to accelerated aging by heat in the presence of oxygen from air using an airflow oven. Technically, it can apparently be applied to polypropylene-based composite materials without any changes in the procedure. In case of HDPE-based composites, the test should be conducted at temperatures lower than 150°C, and apparently, not higher than 120°C. Also, in a modified procedure, an extrnded specimen of a composite material can be employed. 

Fig. 5.5. Arrhenius diagram for ageing of polypropylene samples stabilised using different stabilisers in an oven with air exchange. Days to brittleness have been plotted against the reciprocal value of ageing temperature. A un-stabihsed polypropylene, stabilised using 0.05 % AO-1, stabilised using 0.05 % AO-2. Source (Gugumus 1999)...

Fig. 15.2. Thermally degraded polypropylene samples. Left macroscopic aspect, showing the interior of a water pipe degraded in hot water (six months, 90°C). Right microscopic (SEM) view of an HAS-stabilized sample thermally aged in an oven (54 days at 135°C [11]).

Accelerated polypropylene aging in a convection oven caused by the presence of clearly recognizable metallic contamination (caused during processing) in the center of the PP pest ... 

Influence of recycling on aging properties (120 °C) of polypropylene as a function of quality of the recycled material recycled material = polypropylene aged in a convection oven (new material) recycled material w/o optical changes recycled material, oxidized discolorations recycled material, strongly oxidized = polypropylene pest... 

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