Excerpts

TECHNICAL REPORT 69-7-C14

THERMAL DEGRADATION OF POLYVINYLIDENE FLUORIDE AND POLYVINYL FLUORIDE BY OVEN PYROLYSIS

by
John T. Stapler
William J. Barnes
William E. C. Yelland
Materials Research Division

Project Reference IT024401A329
Series: C&W4-50

July 1968

Clothing and Organic Materials Laboratory
U. S. ARMY NATICK LABORATORIES
Natick, Massachusetts 01760

FOREWORD

This report is based on a series of in-house studies made during the past four years on the thermal degradation of two fluozopolymers, polyvinylidene fluoride and polyvinyl fluoride. These studies were carried out as part of the research on Thermal Protection, Task 04, of Project IT024401A329, Organic Materials Research for Army Materiel.

ABSTRACT

The purpose of this study was to determine differences in the major thermal degradation products of polyvinylidene fluoride (FVF2) and polyvinyl fluoride (PVF). Such differences might help explain the greater ability of PVF2 to attenuate radiant energy from high-intensity thermal sources. Investigated were degradation of the polymers by oven pyrolysis, separation of the degradation products into major fractions,elemental analyses of the fractions, and identification of a number of the components of the volatile fractions.

During pyrolysis, both PVF2 and PVF yielded hydrogen fluoride, complex mixtures of other volatile products, and residual char. The main differences were in the nature of the volatile products and the amounts of char formed. The volatile products from PVF2 consisted largely of highly fluorinated nonflaxmmable materials; those from PVF contained much less fluorine and were flammable. The amount of char formed from PVF2 was approximately twice as great as that formed from PVF. Although the amounts of hydrogen fluoride yielded by each polymer were large, differences between them were small and not considered significant.

It is inferred that the greater ability of PVF2 to attenuate energy from high-intensity thermal sources, as compared to that of PVF, may be attributed in part to its ability to produce nonflammable smoke. This would tend to scatter the radiant thermal energy and keep it from reaching the polymer surface, whereas the smoke from PVF would ignite and thus create an additional heat source. Another possible contributing factor to the greater energy-attenuating capability of PVF2 is its ability to form much larger amounts of char than PVF.

Degradation mechanisms are postulated to explain the formation of several compounds which were identified in the volatile products fromPVF2.

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INTRODUCTION

The military has a definite and pressing need to develop materials to protect personnel and equipment against the thermal effects of a nuclear weapon detonation. The thermal behavior of polymers which appear to have capabilities for attenuating such energy is therefore of interest to the U. S. Army. If the overall processes or mechanisms by which such polymers degrade thermally could be established and related to the original chemical structure of the polymers, it might then be feasible to design other polymer structures that would have better energy-attenuating characteristics than presently available materials…

V. SUMMARY OF THERMAL DEGRADATION PROCESSES OF PVF2 AND PVF DURING OVEN PYROLYSIS

During oven-pyrolysis of IFVF2, the predominant reaction appears tobe the elimination of HF, either by stripping it off the main polymer chain or by removal of H from one chair, and F from the chain next toit. In either case there is formed a large amount (40 percent b.w. of the original polymer) of a highly carbonaceous char containing small amounts of hydrogen and fluorine. Simultaneously, a very complicated mixture of highly fluorinated compounds is produced, including some of the original monomer. Some of these (amounting to 7 percent or less of the original polymer) condense at room temperature; others (less than3 percent of the original polymer) condense at -190’C, while a few(again less than 3 percent of the original polymer weight) do not condense even at -190C. Since the fluorine content of the volatile products is high, these products would be expected to be nonflammable.

VII. FUTURE WORK

No further investigation of fluoropolymers of the aliphatic PVF2-PVF type is planned for two reasons: (i) Since this type of polymer gives off large amounts of 1F during thermal degradation, an additional hazard to personnel would be created because of the extremely corrosive action of 1F on the skin and eyes; and (2) in the concomitant studies on screening polymers in the carbon arc-image furnace, several polymers(e.g., nitroso rubber, polytrifluorostyrene and zein) have been found that are measurably better energy attenuators than PVF2 and PVF. Therefore, it is felt that expenditure of further time and effort on these fluoropolymer studies is not warranted.

IX. REFERENCES

1. Barnes, William J. and William E. C. Yelland, ‘Degradation of Halopolymers by Exposure to High-Intensity Thermal Pulses,” U. S. Army Natick Laboratories, Materials Research and Engineering Report No. 65-4 (May 1965).

2. Prosser, R. A., J. T. Stapler and William E. C. Yelland, “A Pyrolysis Oven which Utilizes a Preheated Helium Stream,” Analytical Chemistry 39(6) 694-695 (1967).

3. Madorsky, S. L., “Thermal Degradation of Organic Polymers,”Chanter ii, p. 10, Interscience Publishers, 1960.

4. Prosser, R. A., unpublished report on the non-condensable degradation products of oven pyrolysis of polyvinylidene fluoride and polyvinyl fluoride.

5. Madorsky, S. L., ‘”Thermal Degradation of Organic Polymers,”Chapter V, Interscience Publishers, i960.

6. White, L. Jr., and 0. K. Rice, “The Thermal Reaction of Hexafluoroethane with Quartz,” J. Am. Chem. Soc. 69 267 (1947).

7. Madorsky, S. L., V. E. Hart, S. Straus, and V. A. Sedlak, ‘Thermal Degradation of Tetrafluoroethylene and Hydrofluoroethylene Polymers in a Vacuum,” J. of Research, Nat. Bur. Standards, 512461 (1953).

8. Butler, J. M., and L. E. Erbaugh, “Investigation of FluoroCompounds for Thermal Protection,” Final Summary Report, 19 April 1963, Monsanto Research Corporation. Contract DA19-129-QM-1773.

9. Pauling, Linus, “The Nature of the Chemical Bond,” Tables 3-5, p. 85, Cornell University Press, 3rd Edition, 1960.

10. Holmes, G. T., “‘Further Work on the Behavior of Multiple Cloths Exposed to High-Intensity Thermal Radiation”; Paper presented at 8th Commonwealth Defense Conference on Clothing and General Stores, Melbourne, Australia, 1965.

*Read the full report at http://fluoridealert.org/wp-content/uploads/incineration.pvf-pvf2.july-1968.pdf