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Characteristics of good stabilizers and methods of evaluation.

INTRODUCTION

In part I of this paper the importance of eliminating circumferential cracks due to oxidative cracking in telephone insulation was explained. That paper dealt with the theory of stabilization and major factors which contribute to cracking. This paper continues the review of the subject with a presentation of the desirable characteristics in a good stabilizer, and the methods which have been used to study the problem of cracking.

Figure 1 is a cross section of a foam-skin insulated copper conductor where both foam and solid skin are a high density polyethylene, HDPE, insulation. The degree of expansion in the foam is 40 to 50 percent. This particular type of insulation has been found to be more susceptible to field failure cracks of the type shown in Fig. 2 than a solid HDPE would be.

Having completed the review of the stabilization, we will explain the approach we have chosen to take in trying to gain a fundamental understanding of stabilization. Our ultimate objective is to gain customer confidence that we can meet the Bellcore thermal stability requirement in Appendix B of TR-TSY 000421 Issue 02 Revision 1 dated March 1989 (1).

CHARACTERISTICS OF A GOOD STABILIZATION SYSTEM

Several factors are worthy of note when choosing the stabilization system for a polyolefin: color, melt temperature, solubility, resistance to extraction, migration or mobility, phase state, structural arrangement, energy considerations, and thermodynamic potential. These characteristics will be examined in turn.

Discoloration

A translucent white polymer like high density polyethylene must remain colorless after the addition of a stabilizer package. The polymer must also stay colorless after thermal aging at service temperature (2, 3). This eliminates aromatic amines which tend to yellow.

Melt Temperature

Antioxidants must be incorporated into the polymer melt by the manufacturer and must melt at a temperature less than the compounding temperature of the high density polyethylene. On the other hand, the melt temperature must be high enough to prevent volatilization during extrusion or in service (3).

Solubility

It has been found that most if not all stabilizers, even at the typically low concentration levels of 0.1 to 0.2% by weight will have greater solubility in the amorphous regions of HDPE than in the crystalline regions (4).

Resistance to Extraction

An effective stabilization system must remain in the polymer and not be extracted into the surrounding medium. In filled cables, some stabilizers seem to dissolve into the petroleum based filling compounds used for waterproofing. This naturally leads to reduced stabilization of the HDPE insulation as stabilizers are extracted into the cable filler (2, 5).

Migration or Mobility

There exist two schools of thought which are somewhat at odds with regard to migration. Some feel that a large bulky molecule with a low diffusion rate which is well dispersed within the HDPE matrix is desired (4). This seems quite reasonable, but others point out that some diffusion or migration is desirable to move antioxidant molecules to sites of oxidation such as the surface where free radical formation is taking place and must be stopped immediately (2).

Phase State

The physical form of the stabilizer molecules seems to be as important as that of the HDPE (4). Amorphous antioxidants seem to be more soluble than crystalline structures and, thus, impart greater stability to the polymer.

Structural Arrangement

If antioxidant efficiency is measured, it can be shown that alkoxyl and alkyl groups substituted in the para position on phenol compounds are more effective than those on meta or ortho positions as long as the groups are electron donors. The opposite is true if they are electron acceptors (6).

Energy Considerations

It has been demonstrated that for polycyclic phenols, the energy of dissociation of the most loosely bound hydrogen atom in the compound is inversely proportional to the efficiency of the antioxidant.

Thermodynamic Potential

It is known from thermodynamics that when entropy terms are neglected the heat of dissociation of an antioxidant may be expressed in terms of oxidation reduction potential as:

[Delta] H = NFE (1)

where H is heat of dissociation, N is the number of moles dissociated, F is Faraday's constant, and E is the potential for dissociation. The higher the dissociation potential, the higher will be the stabilizer efficiency (6).

METHODS OF EVALUATION EMPLOYED TO DATE

With such a wide range of antioxidants and metal deactivators to choose from, it is necessary to have a way of comparing their effectiveness in HDPE. The literature lists the following procedures that are currently in use with varying degrees of success: pedestal testing, oven aging, oxygen uptake, testing under stress, melt flow testing, thermal analysis by DSC, thermal analysis by TGA, color change, torque rheometry, and aged elongation. These will be briefly examined in turn.

Pedestal Testing

Pedestal testing was devised in the early 1970's. It consists of taking a standard pedestal (perhaps better known outside the telecommunications industry as a distribution terminal) and controlling the temperature within by applying power to a heating mantle. The polyolefin insulated conductors within the pedestal are exposed to test temperatures as shown on the next page, depending on the duration of the test:
Test Temperature, [degree] C Test Duration, Days


110 45
100 160
90 365


Figure 3 shows a cutaway view of the experimental setup. At intervals of several days, a wire is removed and twisted around a mandrel whose diameter is equal to that of the insulated conductor or observed by a technician if the method calls for the wires to be twisted before being placed in the pedestal. A 5x eyepiece is used to inspect the wires for cracks of the type shown earlier in Fig. 2. Cracking is evidence of thermal degradation.

This test is widely accepted throughout the industry as a relatively low temperature aging test which may predict long term field failures. The test temperatures are above what one would expect in the desert, and the temperatures are continuous rather than varying throughout the day, but the test at its longest is only 1 year, where a field test could take much longer.

Oven aging

Oven aging is another widely accepted test method. The insulated wires can be more conveniently placed in an over and ovens are readily available to all laboratories. Temperature is uniform to within 1 [degree] C in a forced air oven meeting the requirements of ASTM E 145 type IIB. Wires can be exposed to any temperature for any duration desired. The test again involves looking for cracks in insulation of a wire which has been wrapped around a mandrel.

Oxygen Uptake

Oxygen uptake has been used by several authors and appears to have good correlation with field data (7-9). The test consists of exposing the stabilized polyolefin to oxygen in a manometer at elevated temperature and measuring the volume of oxygen absorbed. The rate of oxygen absorbed is a measure of oxidation rate. Samples with high oxygen absorption degrade faster than those with low oxygen absorption.

Testing Under Stress

Observation that failures in the pedestal test and in actual field failures always occur at a bend or loop in the wire,(5) led to efforts to determine the effect of stress on polyethylene stability. This required a device to apply a given stress to the polymer while it was being oven aged. This procedure is described in several papers (4, 10, 11).

[ILLUSTRATION OMITTED]

The conclusion seems to be that stress accelerates thermal degradation. They make the interesting point that two mechanisms work to retard degradation, but that one other is dominant enough to make them negligible. The density of the amorphous phase increases and the quantity of crystalline polymer increases as stress is applied. One would expect these changes to retard oxidation, but measurements of carbonyl formation indicate that the opposite is true. They explain this by suggesting that the kinetics of the reaction play a more important role than diffusion or mobility of the stabilization system.

Melt Flow Testing

As discussed earlier, thermal degradation may proceed by either chain splitting or chain branching. The standard melt flow test of ASTM D 1238 Procedure A Condition E is sensitive to molecular weight changes. For a given set of conditions the amount of polymer exiting a 0.083 inch diameter orifice will increase as molecular weight decreases. This offers a clue as to whether degradation is taking place, and whether it is predominantly by branching or chain scission (7).

Thermal Analysis by DSC

Differential scanning calorimetry (DSC) testing has been popular since the late 1970's as the most rapid method of comparing stabilization systems. Several thermal analyzers are now on the market, but they all share the ability to measure heat evolution as a function of time when a small sample (approximately 50 mg.) is exposed to a constant flow of oxygen at about 200 [degrees] C. Typical times to failure are 15 min to 150 min, so the test is quick. It has now become a standard ASTM procedure designated ASTM D 3895. The convenience of this test makes it very widely used. Details of the method can be found in the ASTM or any of several papers (12, 13, 5).

Unfortunately, there may be two significant drawbacks to DSC testing. The test temperature of 200 [degrees] C., while providing an accelerated test, is so high that we are testing not solid polymer, but molten material. This means that the reaction kinetics associated with chain branching are artificially high. It also means that the effects of solubility and crystalline versus amorphous structure of both antioxidant and polymer as described in several papers is ignored (14-16).

Thermal Analysis by TGA

Thermal gravimetric analysis (TGA) is performed with the same type of equipment as DSC testing, but operates on a different principle. Instead of plotting heat evolution as a function of time, the instrument plots weight loss of a small (50 mg) sample of polymer as a function of increasing temperature. This is an effective way of screening polymers whose volatility is so high that they will "bloom" to the sample surface at relatively low temperatures and become useless as stabilizers (7).

Color Change

It has already been noted that a color change in a translucent white polyolefin specimen may indicate that degradation is taking place. The oven aging test can be effective in testing for color changes as many oven are available with viewing windows so periodic observations can be made without removing samples. The aromatic amines have been noted to discolor polyolefins prematurely. Since most polyolefins are colored with a pigment system of some type, discoloration may not be easily detected.

Torque Rheometry

The torque rheometer equipment currently on the market is particularly suited to testing stabilizers intended to get a particular polyolefin through the extrusion equipment without degradation (7). It consists of a 45 cc bowl with a dual bladed high intensity fluxing head which rotates in the polymer melt at a constant speed. An attached chart recorder plots the torque in meter-grams required to maintain the constant rotor speed. As the polymer chains break down (molecular weight decreases) the torque required to drive the mixing blades drops. The initial drop in torque is recorded as the time to onset of thermal degradation. The test temperature is usually chosen to duplicate extruder head temperatures.

This technique suffers from the same problems that were brought up under thermal analysis testing and should probably not be used to predict time to field failure. Another obvious problem is that obtaining a 45 cc sample from an insulated wire would require several feet of wire.

Aged Elongation

Although tensile strength is not strongly dependent on aging, elongation is. Good results have been obtained by comparing stabilization systems as well as determining levels of performance to be expected from aged polymers with this technique (17). The test is a simple one. Tensile dumbells are prepared from the polymers of interest and aged in ovens at specific temperatures for the intervals of interest. Low elongation values are indicative of polymer degradation.

REFERENCES

1. Bellcore, TR-TSY 000421 Issue 02, Rev. 01, App. B, March 1989.

2. R. Gachter and H. Muller, Plastics Additives Handbook p. 1, Hanser Publishers (1984).

3. A. G. Sirota, Polyolefins Modification of Structure and Properties, pp. 6-8, Keter Press, Jerusalem.

4. J. Kresta, Polymer Additives, p. 103, Plenum Press, New York (1984).

5. D. D. O'Rell, Wire and Cable Proc., 2331 (1975).

6. L. Reich, Autoxidation of Hydrocarbons and Polyolefins, Marcel Dekker Inc., New York (1969).

7. N. N. Blinov, A. A. Popov, N. N. Kornova, and G. Ve. Zaikov, Polym. Sci. USSR, 27.6, 1311 (1985).

8. D. L. Allara and C. W. White, Stab. Degrad. Polym., 273 (1973).

9. D. L. Allara and M. G. Chan, J. Polym. Sci., 14, 1857 (1976).

10. M. G. Chan, Polym. Eng. Sci., 14, 12 (1974).

11. M. G. Chan, J. Coll. Interf. Sci., 47, 697 (1974).

12. W. L. Hawkins, M. G. Chan, and G. L. Link, Polym. Eng. Sci., 11, 377 (1971).

13. Y. Suzuoki, K. Yasuda, T. Mizutani, and M. Ieda, Jap. J. Appl. Phys. 16, 81339 (1977).

14. M. G. Chan, H. M. Gilroy, L. Johnson, and W. W. Martin, Proc. 27th Intl. Wire and Cable Symp., 99 (1978).

15. J. Peeling and D. T. Clark, J. Polym. Sci., 21, 2047 (1983).

16. H. V. Boenig, Polyolefins: Structure and Properties, p. 234, Elsevier Publishing Co., New York (1966).

17. G. L. Wilkes, Encyclopedia of Materials Science and Engineering, Vol. 5, 3348.
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Title Annotation:Review of Stabilization of Polyolefin Insulated Conductors, part 2
Author:Hendrickson, L.; Connole, Kent B.
Publication:Polymer Engineering and Science
Date:Jan 1, 1995
Words:2239
Previous Article:Theory and factors affecting stability.
Next Article:Screw optimization of a co-rotating twin-screw extruder for a binary immiscible blend.
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