Stabilization of Polymers and Stabilizer Processes

17 A Modified Oven-Aging Technique for

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Studying Polymer Antioxidant Systems H . GYSLING J. R. Geigy S. Α., P. O. Box 4000 Basel 21, Switzerland

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 influ ence 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 polypro pylene. The results obtained over a wide spectrum of antioxidant/DLTDP combinations are shown in three -dimensional graphs.

Speeding up the tests that help predict the effectiveness of antioxidants ^ for polymers is a crucial problem for those involved i n developing new stabilizers or evaluating new polymer formulations. Numerous methods have already been proposed to accelerate aging. Among these are pre-exposure of the samples to high energy radiation or addition of copper as a catalyst to promote oxidation (5). Such methods often work only for a specific formulation and depend strongly on the chemical nature of the stabilizer; hence, they cannot be used in a more general evaluation of polymers. Among the numerous well-known aging methods, such as the oxygenuptake test, the U-tube test, the use of differential thermal analyses to indicate the oxidation exotherm, oven aging proved to be one of the most versatile methods. Not only can many specimens of any size and shape be tested simultaneously, but samples can also be withdrawn at convenient intervals for inspection. 239

Platzer; Stabilization of Polymers and Stabilizer Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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Experimental The reproducibility and precision of the oven-aging test have already been thoroughly analyzed and discussed by Forsman ( 4 ) , whose analysis has been corroborated b y our early work. W i t h respect to sample thick ness (2) we have carried this study further i n an attempt to provide an improved method. T o achieve good reproducibility, we relied on his analysis of most of the other critical parameters.

Figure 1.

Use of a microtome to cut thin samples

Figure 2. Specimens sandwiched between steel screens The temperature within our oven was controlled b y a multielement air heater and proportional energy input control. Fluctuations were within ± 0 . 5 ° C ; and the samples were mounted on a rotating rack. T o accelerate the aging, very thin specimens were used. In fact, the first goal


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Modified Oven Aging

Figure 3.

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Mounted specimens in a forceddraft oven

of our work was to study the influence of thickness on the oven life of polypropylene samples. Thin specimens prepared in the usual way by pressing or calendering i n laboratory equipment vary i n thickness. This difficulty has been overcome by using a microtome. Samples of uniform shape and thickness were prepared as follows. The polymer formulations were processed under a nitrogen blanket in a Brabender plastograph at 200°C. and pressed into 40-mil sheets. Bundles of rectangular small plates cut from such sheets, four or five at a time, were cut into sections of definite thickness from 0.2 to 4 mils, using a heavy microtome equipped with a d-type knife. F o r the aging test, approximately 80 to 100 cuttings were sandwiched between two steel screens, which were then mounted on a rotating rack i n a forceddraft oven (Figures 1, 2, 3 ) . Since microscopic inspection ( 6 0 χ ) of the sections during aging revealed no catalytic effect from the stainless steel wire, and samples sandwiched between aluminum wire-screens (which cannot be cleaned as easily as stainless steel) gave identical results, stainless steel was used in a l l our tests. The onset of embrittlement is easily recognized and is usually preceded by a reddish fluorescence revealed under a black light. A relatively short time (compared with the total oven life) elapsed between the first signs of degradation and mechanical disintegration. Since the various methods of estimating failure showed that weight loss, carbonyl-formation (1 ), or solution viscosity d i d not change markedly until visual signs of degradation occurred (6), mechanical degradation upon tapping the sample ( screen ) was taken as the point of failure. The single data obtained, showed poor precision only at the very short oven lives produced by poor stabilization but better precision with samples of longer oven life. However, the over-all picture of stabilizer performance as obtained from all these tests is not significantly influenced by devia tions of individual results. This is especially evident i n Figures 8-13.


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STABILIZATION OF POLYMERS A N D STABILIZER PROCESSES

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Figure 4. Oven aging of polypropylene samples stabilized with 0.1% Antioxidant A (—) and Β (--). Logarithmic scale To demonstrate the influence of sample thickness on oven life, two formulations, based on the same batch of resin were tested as described above. One was stabilized with 0.1% of Antioxidant A and the other with 0. 1 % of Antioxidant B. The samples had a uniform thickness of 0.2, 0.4, 1, 2, 4, 12, and 40 mils. Specimens of 12 and 40 mils were made by com pression molding. Oven aging was run at 120°, 135°, and 147°C. The results are given i n Figure 4. Results and Discussion Oven life and sample thickness are both given on a logarithmic scale to cover the whole range of test conditions. Both formulations yield results which can be interpreted by almost parallel curves, when the temperature is changed from 147° to 135° to 120 °C. However, the two formulations do not respond to changes in the specimen thickness to the same degree. The significant difference is somewhat obscured by the logarithmic presentation of Figure 4, but is easily recognized i n the linear presentation of Figure 5. The oven life of Antioxidant A increases almost i n proportion to the thickness of the sample over the entire range, 0.2-40 mils; that of


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Modified Oven Aging

B, however, rises steeply at low thickness but flattens off strongly above ca. 4 mils. To demonstrate the influence of both parameters—specimen thickness and temperature—on oven life, the data from Figure 5 have been replotted three dimensionally. The resulting chart, Figure 6, is characteristic of Antioxidant A and shows how the oven life increases almost linearly with thickness and the reciprocal of the temperature. 5000-1

Figure 5.

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Linear plot of data from Figure 4

A representation such as Figure 6 allows the test results to be extrapolated over a limited temperature range. However, it is often desirable to extrapolate to much lower temperatures to obtain an indi cation of the service life of a new formulation. Arrhenius plots may be used as a first approximation. It must be emphasized that the straight lines from oven aging tests at only two temperature levels are a simplifi cation of the results obtained i n a more thorough evaluation. In our study, slightly but significantly curved lines were always obtained, whenever the oven-aging tests were carried out at more than


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STABILIZATION OF POLYMERS A N D STABILIZER PROCESSES

two temperature levels. Figure 7 illustrates the results with the Anti oxidants A and Β at 9 0 ° , 120°, 135°, and 147°C. The extrapolation (dotted lines) of the two results at 120° and 147°C. to lower temperatures ( 9 0 ° C . ) leads to highly exaggerated values compared with the experi mentally determined oven life of the material. As Figure 7 also shows, an increase i n the thickness of the test specimen displaces the lines i n a parallel direction, whereas a change of the stabilizer system—e.g., from A to B, causes a definite change i n the slope, especially at higher temperatures.

Figure 6. Three-dimensional plot of the influence of specimen thickness and temperature on oven life (data from Figure 5) That the composition of the resin has a significant influence upon the temperature gradient of the aging rate was also shown i n a study of pigmented polypropylene by Fitton and Taylor ( 3 ) . Their data show


Figure 8.

Oven life vs. (DLTDP + Antioxidant A) concentration for 40-mil samples at 147°C.


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clearly that the temperature gradient of aging depends strongly on the stabilizer system and pigments used, especially at relatively high test temperatures. A n interpretation of their result as an Arrhenius diagram would lead to extremely bent lines.

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Such findings do not agree with earlier results on oxygen-uptake tests, where almost ideal straight Arrhenius plots were obtained ( I , 2, 5) and where the slope of the line—e.g., the apparent activation energy (ca. 25


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Modified Oven Aging

Figure 12. Oven life vs. (DLTDP + Antioxidant B) concentration for 1-mil samples at 120°C.


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kcal.)—was found to be independent of the type of polyolefin and the type of stabilizer used. One reason for the deviation of the results based on oven aging from such ideal performance certainly arises from the fact that mechanical degradation rather than thermodyhamically meaningful reaction rates was the criterion. Such contradictory results obtained by different aging methods sug gest that one should use Arrhenius extrapolations to predict service life only with great reservation. Obviously aging tests should be run over a wide spectrum of temperatures and as near as possible to the anticipated end-use temperature of the resin. One way to shorten the duration of the test at lower temperatures is to use thin specimens, such as micro tome cuttings. Figures 5 and 6 show that acceleration by an order of magnitude may be obtained.

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Figure 13. Oven life vs. (DLTDP + Antioxidant C) con centration for 1.5-mil samples at 120°C. In another series of tests the effectiveness of the phenolic Antioxi dants A and Β i n combination with D L T D P (dilauryl thiodipropionate) was evaluated over a wide range of concentrations at 147° and 120 °C. The results are shown i n Figures 8-13, where the oven life is plotted as a function of stabilizer composition. Figure 8 is an evaluation of normal 40-mil specimens tested at 147 °C. It is obvious that no synergism between the antioxidants A or Β and the D L T D P exists. Both the phenol and the


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D L T D P make an individual contribution, depending on their concentra tion, to the total stability of the sample. Figure 9 shows the drastic but proportional reduction of oven life when 1-mil samples were used. Figure 10 shows the results obtained with 1-mil specimens at 120 °C. The oven life increases steeply with increasing phenolic antioxidant concentration from 0-0.2%, flattening off above 0.3%. The effect of D L T D P is less noticeable than in Figure 8. A n analogous evaluation was carried out with Antioxidant B, but for the sake of time, the tests were restricted to 1-mil specimens. Figures 11 and 12 are characteristic of the excellent performance of this high molecular weight antioxidant. Again no synergism was found. Finally, Figure 13 refers to a series of antioxidant/DLTDP combi nations (tested as 1.5-mil cuttings at 1 2 0 ° C ) , which showed significant synergism. The shape of the figure differs appreciably from that of foregoing combinations. Literature Cited

(1) (2) (3) (4) (5) (6)

Beachell, H. C., Beck, D. L., J. Polymer Sci. A3, 457 (1965). Boss, C. R., Chien, J. C. W., Proc. PIA Conf. (Nov. 1965). Fitton, S. L., Taylor, W., Plastics 1966, 1139. Forsman, J. P., S.P.E. (Soc. Plastics Eng.) Tech. Papers 10, 1 (1964). Hansen, R. H. et al., J. Polymer Sci. A2, 587 (1964). Oswald, H. J., Turi, E., Polymer Eng. Sci. 1965, 152.

RECEIVED June 19, 1967.


Stabilization of Polymers and Stabilizer Processes

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