US3832411A - Method for the depolymerization of polytetrafluoroethylene - Google Patents

Abstract

High yield monomeric tetrafluoroethylene may be recovered from polytetrafluoroethylene and polytetrafluoroethylene composites by pyrolyzing the polymer in the presence of high temperature steam. The steam acts as a condensable carrier gas for the low molecular weight pyrolysis products and should be present in an amount such that the mole ratio of steam to pyrolysis products is at least one to one. The steam also acts as a means for heating the polymer to the necessary decomposition temperatures.

Description

United States Patent [191 Arkles et al.

m1 3,832,411 1451 Aug. 27, 1974 METHOD FOR THE DEPOLYMERIZATION OF POLY'IETRAFLUOROETHYLENE [75] Inventors: Barry C. Arkles, Malvem; Robert N.

Bonnett, West Chester, both of Pa.

[73] Assignee: Liquid Nitrogen Processing Corporation, Malvem, Pa.

[22] Filed: Feb. 2, 1971 [2]] Appl. N0.: 111,963

ZONE

A PYROLYSIS PRODUCTS 960,309 6/1964 Great Britain 260/6533 Primary ExaminerLeon Zitver Assistant ExaminerJoseph A. Boska Attorney, Agent, or Firm-Seidel, Gonda & Goldhammer [57] ABSTRACT High yield monomeric tetrafluoroethylene may be recovered from polytetrafluoroethylene and polytetrafluoroethylene composites by pyrolyzing the polymer in the presence of high temperature steam. The steam acts as a condensable carrier gas for the low molecular weight pyrolysis products and should be present in an amount such that the mole ratio of steam to pyrolysis products is at least one to one. The steam also acts as a means for heating the polymer to the necessary decomposition temperatures.

8 Claims, 2 Drawing Figures GASEOUS 54 PYROLYSIS 60 CONDENSED STEAM 63 CONDENSED PYROLYSIS PRODUCTS METHOD FOR THE DEPOLYMERIZATION OF POLYTETRAFLUOROETHYLENE The present invention relates to the production of low molecular weight fluorine-containing compounds by depolymerizing polymeric tetrafluoroethylene. More particularly, the invention is directed to a method of recovering monomeric tetrafluoroethylene from polymeric tetrafluoroethylene by pyrolysis using the controlled presence of high temperature steam.

The pyrolysis of polymeric tetrafluoroethylene (generally referred to as polytetrafluoroethylene, TFE fluorocarbon polymer or PTFE) under varying conditions, including in air and under vacuum, is well known in the art. See for example S. L. Madorsky, Thermal Degradation of Organic Polymers, pp. 130 et seq., Wiley (1964). The pyrolysis products are a mixture of many compounds including -TFE monomer, hexafluoropropene (C F octafluorocyclobutane (C F and other gaseous and liquid perfluorinated products having relatively low boiling points. When pyrolysis is conducted at near atmospheric pressures these products usually contain considerably less than 50 percent of the monomer, and consequently the mixture is of little commercial value.

Other attempts have been made to enhance the yield of monomer tetrafluoroethylene by decreasing residence time at pyrolytic temperatures by means of evacuating the pyrolysis zone to subatmospheric pressure. See for example US. Pat. No. 2,406,153 to E. E. Lewis. The latter patent discloses yields of monomeric tetrafluoroethylene as high as 85 percent based on the gaseous products alone when pressures were reduced below 150 millimeters of mercury. Although this represented a vast improvement in yield, the process cannot be satisfactorily employed for several reasons. Although it is not difficult to produce a vacuum in a vessel containing polytetrafluoroethylene, it is quite difficult to pyrolyze large quantities since the vessel must ordinarily be heated externally. The difficulty arises because: (1) materials capable of withstanding the high temperature and corrosive pyrolysis conditions generally have poor strength and rigidity at the high temperatures involved; (2) the use of vacuum precludes convective heat supply to the polymer and thus restricts the supply of en'- ergy necessary for pyrolysis to conduction and radiation from the surface of the vessel; and (3) polytetrafluoroethylene itself has very poor heat-transfer properties. The net result is that the maximum diameter of a vacuum-pyrolysis vessel is generally limited to about 10 inches, and the batch of polymer to be pyrolyzed is generally restricted to a relatively small weight. Furthermore, vacuum pyrolysis offers the probability of haz ardous introduction of air into the pyrolyzate stream. As a result, to the best of our knowledge no important commercial depolymerization of polytetrafluoroethylene in vacuum has been accomplished.

Scrap or waste polytetrafluoroethylene is an attractive source of monomer tetrafluoroethylene from several standpoints. In economic terms, the production of monomer tetrafluoroethylene by depolymerization of the polymer represents a substantial cost saving compared to the synthesis of the monomer from usual refrigerant gas sources. In process terms, a substantial difficulty encountered in producing polytetrafluoroethylene is the presence of by-product difluoroethylene and trifluoroethylene with the crude monomer tetrafluoroethylene made from CHCIF or CHF These impurities are generally removed by slow and costly methods. On the other hand, it is believed that monomer from the depolymerization of polytetrafluoroethylene has substantially reduced portions of these products.

Additionally, the depolymerization process is attractive from an environmental standpoint. Mechanical reprocessing of polytetrafluoroethylene is practiced only to a small extent due to the loss in mechanical properties and processability of the reprocessed polymer. Therefore, disposal of waste polymer is resulting increasingly in an accumulation of non-biodegradeable polytetrafluoroethylene. Thus, production of monomer tetrafluoroethylene from scrap polymer results in a beneficial recycling of material the accumulation of which is potentially disadvantageous to the environment.

Accordingly, it is an object of the present invention to develop a commercially feasible process for the production of tetrafluoroethylene monomer from polytetrafluoroethylene.

It is a further object of the present invention to produce high yields of tetrafluoroethylene monomer by the depolymerization of polytetrafluoroethylene.

It is a still further object of the present invention to produce good yields of other low molecular weight fluorine-containing compounds, such as commercially valuable hexafluoropropylene.

It is another object of the present invention to provide a method of depolymerizing polytetrafluoroethylene in any form without the necessity of using subatmospheric pressures.

It is still another object of the present invention to provide a method of recycling non-biodegradeable polytetrafluoroethylene whether it be in clean, contaminated or composite form.

Other objects will appear hereinafter.

The objects of this invention are achieved by heating polymeric tetrafluoroethylene to temperatures above the decomposition temperature of the polymer in the presence of high temperature steam under such conditions that the mole ratio of steam to off-gas decomposition products is at least one to one. The off-gas products are collected after condensing the steam.

For the purpose of illustrating the invention, there are shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIG. 1 is a simplified representation of an apparatus which may be used in the present invention according to Example 1.

FIG. 2 is a simplified representation of an apparatus which may be used for processing larger quantities according to the method of the present invention.

The process of the present invention depends in large part upon the role of steam as a diluent and carrier gas for the tetrafluoroethylene monomer, which is the predominant product of the decomposition of the polymer. In addition, the commercial feasibility of the present invention depends upon the fact that high temperature steam may conveniently be used to conduct sufficient heat to the polytetrafluoroethylene to decompose the material.

While thermal decomposition of polytetrafluoroethylene is observed as low as 450 F, the decomposition at this temperature'only proceeds at a rate on the order of percent per hour. It is not until considerably above the polymer first-order transition temperature (approximately 625 F) that substantial decomposition is observed. That is, the polytetrafluoroethylene should be heated to a temperature of between about 780 F and l,400 F, and preferably between about 800 F and I,IOO F. Good results may be achieved when the temperature of the steam impinging upon the polymer melt is between 950 F and l,800 F.

Although the specific temperature of the steam supplied to the polymer tetrafluoroethylene mass may be varied over a large range, the flow rate of steam required will be largely dependent on the temperature of the steam. This is due to the fact that the depolymerization rate is increased by increased interface temperature of the polymer under pyrolysis. Therefore, as the temperature is raised, the mass-rate of steam must be increased in order to adequately dilute the tetrafluoroethylene monomer which is forming at a more rapid rate, and thereby minimize the tendency of free-radical difluoromethylene groups to trimerize or polymerize. In other words, to maintain a high degree of TFE monomer yield, it is prudent to dilute free-radical interaction by maintaining an adequate steam flow rate. In general, the higher the temperature, the more steam required to maintain uniform yield.

An easy way to accomplish uniform yield is to maintain a constant mole ratio of steam to off-gas decomposition products. It has been found that a minimum mole ratio of steam to decomposition products of about four to one is required to maintain a high level of monomer yield. Mole ratios of as low as about one to one still produce good levels of monomer'yield as well as increased yields of other low molecular weight fluorinecontaining compounds. At lower mole ratios, monomer tetrafluoroethylene concentration is low, and the amount of solid sublimate as well as other decomposition by-products is increased. On the other hand, at extremely high mole ratios, it is possible to produce almost pure monomer tetrafluoroethylene. A mole ratio of steam to decomposition products of between about ten to one and forty to one is preferable. There is apparently no upper limit on the amount of steam which may be used or the maximum temperature of the steam. However, economic considerations render higher temperatures and amounts impractical.

The commercial depolymerization of polytetrafluoroethylene may be carried out in vessels of various configurations. A simplified representation of one such vessel is illustrated in FIG. 2 of the accompanying drawings.

The reaction vessel 10 should be made of a material which is able to withstand the high temperatures and corrosive byproducts of the pyrolysis reaction. An optimum material is platinum lined high temperature stainless steel. However, lnconel (a trademark for alloys of nickel and chromium) or Monel metal (a trademark for alloys of predominantly nickel and copper) may also be employed. It is possible to use stainless steel or iron, but these material are less desirable.

The high temperature steam used in the process of the present invention may be produced either outside or within the reaction vessel. Super-heated steam, steam or water enters the vessel 10 through inlet tube 12 at the side of the vessel near the bottom. Inlet tube 12 extends approximately to the center of the vessel.

The tube. may be made of stainless steel, for example.

In order to produce the steam or maintain or raise the temperature of the steam, the vessel 10 is provided with gas burners 14. In addition, the sides and top of the vessel are provided with suitable insulation 16.

The batch 18 of material. to be pyrolyzed is supported by a porous surface 20 which may be made of a heavy gaugescreen or a perforated metal plate. The porous surface 20 is in turn supported by a replaceable tripod 22, which is positioned over the steam inlet tube 12.

The screen or perforated metal plate which forms the porous surface 20 not only serves as a support for batch 18, but also conducts heat to the batch to further the decomposition. Since typically encountered PTFE melt does not usually flow until it reaches a temperature of about 850 F or more, flowing of the melt through the holes of the screen or perforated plate is not a major problem. In fact, the melt produces a stalactite effect, and the hotter steam'beneath the screen or plate decomposes the polymer before the drips of 'melt can hit the bottom of the vessel. Nevertheless, the holes in the screen or plate should be sufficiently small to prevent particles of the solid polymer from falling through. For example, two overlapped layers of 14 mesh screen have been found to be quite suitable, whereas 8 mesh screen or inch drilled holes in a steel plate were usually'too large.

It has also been found-preferable to cover or wrap the batch 18 with screen 24, which may suitably bemade of 14 mesh stainless steel. The screen 24 inhibits the blowing off of any solid raw-material with the steam and decomposition gases, as well as reducing sublimate formation. The reduction of sublimation is important since the condensation of the sublimate tends to block the exit gas lines. As already mentioned, sublimate formation may also be reduced or substantially eliminated by using higher mole ratios of steam.

After passing through .and around the porous supporting surface 20 and the batch 18, the steam carries the gaseous pyrolysis products away from the polymer mass and out of the vessel 10 through effluent gas outlet 26, which may suitably be stainless steel tubing. It is good practice to position the effluent outlet 26 below the vessel opening 28, since gasketing capable of withstanding the depolymerization temperatures and corrosive conditions (i.e., hydrofluoric acid) is generally not available. For example, in a vessel having a volume of 8 liters the efiluent gas outlet 26 may suitably be positioned about 2 inches above the batch 18 and 6 inches below the vessel opening 28. Also, the lid 30 of the vessel may be gasketed with polytetrafluoroethylene, provided enough of the heat supplied to the bottom of the vessel is removed through effluent outlet 26 to keep the gasket temperature below about 500 F.

In order to aid the control of the steam temperature, the vessel may be suitably provided with thermocouples 32, in wells 33 positioned in the lid 30 and the bottom 34 of vessel 10, for example, to determine the respective temperatures of the batch l8 and the steam below the batch. The vessel may also be provided with appropriate flowmeters (not shown) at the inlet tube 12 and/or outlet 26 to read the amounts of steam and pyrolyzate passing through the system.

Following the effluent gas outlet 26 is a heat exchanger and water drop-out container (not shown) for condensing the steam. This apparatus may be of any EXAMPLE I This example was performed in the apparatus which is illustrated in the simplified representation of FIG. 1 of the accompanying drawings. I

A 100 gram sample 40 of unfilled polytetrafluoroethylene resin was wrapped in stainless steel screening 42 and placed one foot from the end of a 3% foot long stainless steel tube 44 having a diameter of one inch. The 2 foot length of tubing 44 prior to the polytetrafluoroethylene sample 40 was wrapped with heating bands 46. The entire length of tube 44, except for the last 6 inches on the end 48 closest to the sample 40, was provided with insulation 50. The end 48 was reduced to A inch diameter stainless steel tubing 52 which ran for 3 feet, acting as an air condenser, which in turn was connected to a rubber tube 54 which led to a small filter flask 56 for dropping out the water 58 condensed from the steam. Filter flask 56 was provided with a product gas outlet 60 which led to a collection flask 62. Collection flask 62 was immersed in a dry ice-acetone bath 63 for condensation of the decomposition gases.

Steam entered end 64 of the reaction tube 44 after being boiled from a 2,000 milliliter flash (not shown). Over a one half hour period, 250 grams of steam were produced. A thermocouple (not shown) between the insulation and the outside of the tube indicated a temperature of 850 F at the site of the polymer. The mole ratio of steam to effluent gas products was approximately 60 to l, and the amount of material pyrolyzed was approximately 21 grams. Gas samples were taken from the gas outlet 60 between filter flask 56 and collection flask 62 and analyzed.

The analysis showed the effluent gas to contain 98 percent tetrafluoroethylene monomer (C F 1.25% C F and 0.75% C F,,. No Sublimate was produced.

EXAMPLE ll Steam at a temperature of 500 F was introduced at a rate of 7-l0 grams per minute to the bottom of an 8 liter stainless steel vessel similar to that shown in FIG. 2 of the drawings. The base of the vessel was heated by the gas burners to raise the steam temperature to about l,l00 F. The temperature at the bottom of the polymer was 950 F. Prior to the introduction of the steam, a 1 pound sample of polytetrafluoroethylene was loaded into the vessel. Under the action of the steam the resin was depolymerized over a one hour period and yielded an off gas concentration of 70 percent tetrafluoroethylene monomer.

EXAMPLE 111 One kilogram of unsintered scrap PT FE containing 25 percent fiber glass and pigmentation was pyrolyzed in the same vessel as Example ll with l,l00 F steam flowing at a rate of 12-15 grams per minute. Over a one half hour period, 250 grams of the material was pyrolyzed. The mole ratio of steam to off gas products was approximately 10 to l. The composition of the product gases was QB, 10% C F and 4% C F EXAMPLE IV A pyrolysis run was made in the same vessel as Example I! using sintered, unfilled scrap PT FE. Using l,l00 F steam at a rate of 15-20 grams per minute, a 500 gram sample of the scrap PTFE was completely pyrolyzed over a one hour period. The unrefined off gases were composed of percent tetrafluoroethylene monomer.

EXAMPLE V A l kilogram sample of PT F E material was pyrolyzed in the apparatus of Example II with l,l00 F steam flowing at 20-25 grams per minute. The PTF E material employed consisted of whole and broken sintered billets filled with bronze, carbon, molybdenum disulfide and some pigmentation. Over a minute period the sample was pyrolyzed completely and produced a monomer tetrafluoroethylene yield (based on gases) of 70-85 percent. Sublimate formation was reduced with thishigh steam flow to about 5 percent of the total solids.

EXAMPLE VI The pyrolysis vessel of Example II was charged with 7 pounds of flake reprocess grade (scrap and turnings of PTFE put through a grinder to make uniform size sintered particles) polytetrafiuoroethylene. Steam having a temperature ranging between 1,100 F and l,200 F was introduced at a rate of 35-40 grams per minute. The pressure measured at the bottom of the polymer melt was 5-8 psi during equilibrium pyrolysis conditions. The flow of off gases was measured after condensation of the steam, drying of the products and condensation of products heavier than TF E. The TF E gas was produced at about 2,200 cc per minute (approximately 10 grams per minute). This represented a mole ratio of steam to off gas products of approximately 20 to l. The analysis of the effluent gases indicated 90-95 percent monomer tetrafluoroethylene.

It will be readily understood by one of ordinary skill in the art that the above examples are only illustrative, and the exact process parameters can be varied widely. For example, the form of the apparatus may be varied in order to operate the process of the present invention in batches, semi-continuously, or continuously. Also, processes for the automatic feed of polytetrafluoroethylene and PTFE composites into the pyrolysis vessel are conceivable, as are processes for withdrawal of the non-decomposed inorganic fillers from the pyrolyzed composites.

The utility of the present invention is apparent when compared to previous processes which either yield extremely low concentrations of desirable pyrolysis products or have limited commercial success because of restricted capacity. The process of this invention offers a highly available feed stock for the production of tetrafluoroethylene polymer and fluorine containing compounds of various molecular weights. Furthermore, the invention can recycle scrap polytetrafluoroethylene in virtually any form, including scrap lathe-turnings and other machine scrap, unsintered or sintered tape and sheet trimmings, discarded finished parts, and other unfilled or composite forms. Polytetrafluoroethylene composites may include, for example, mixtures of PTFE with fiber glass, bronze, carbon and other inert materials.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

We claim:

1. A method of producing tetrafluoroethylene monomer by pyrolysis of polymeric tetrafluoroethylene, comprising the steps of heating polymeric tetrafluoroethylene to a temperature of at least about 780 F. at a pressure of at least atmospheric pressure and in the presence of high temperature steam, said steam being present in an amount such that the mole ratio of steam to pyrolysis products is at least about 4 to l, condensing the steam from the mixture of pyrolysis products of the polymeric tetrafluoroethylene and collecting the resulting pyrolysis products.

2. A method according to claim 1 wherein the polymeric tetrafluoroethylene is heated to a temperature of between about 780F. and 1,400F.

3. A method according to claim 1 wherein the polymeric tetrafluoroethylene is heated to a temperature of between about 800F. and 1,100F., and the mole ratio of steam to said pyrolysis products is between about 10 to l and 40 to l.

4. A method according to claim 1 wherein the polymeric tetrafluoroethylene is heated by steam having a temperature of at least 950F.

5. In a method for depolymerizing polymeric tetrafluoroethylene by pyrolysis, the improvement comprising heating polymeric tetrafluoroethylene at a pressure of at least atmospheric pressure and in the presence of high temperature steam sufficient to heat said polymeric tetrafluoroethylene to a temperature of at least about 780 F., said steam being present in an amount such that the mole ratio of steam to pyrolysis products is at least about 4 to l, condensing the steam from the mixture of pyrolysis products, and collecting the resulting pyrolysis products.

6. A method according to claim 5 wherein said steam has a temperature of between about 950F. and 1,800F.

7. A method according to claim 5 wherein the mole ratio of steam to said pyrolysis products is between about 10 to 1 and 40 to l.

8. A method according to claim 5 wherein said polymeric tetrafluoroethylene is recycled scrap containing fillers.

Claims (7)

1. 2. A method according to claim 1 wherein the polymeric tetrafluoroethylene is heated to a temperature of between about 780*F. and 1,400*F.
2. 3. A method according to claim 1 wherein the polymeric tetrafluoroethylene is heated to a temperature of between about 800*F. and 1,100*F., and the mole ratio of steam to said pyrolysis products is between about 10 to 1 and 40 to 1.
3. 4. A method according to claim 1 wherein the polymeric tetrafluoroethylene is heated by steam having a temperature of at least 950*F.
4. 5. In a method for depolymerizing polymeric tetrafluoroethylene by pyrolysis, the improvement comprising heating polymeric tetrafluoroethylene at a pressure of at least atmospheric pressure and in the presence of high temperature steam sufficient to heat said polymeric tetrafluoroethylene to a temperature of at least about 780* F., said steam being present in an amount such that the mole ratio of steam to pyrolysis products is at least about 4 to 1, condensing the steam froM the mixture of pyrolysis products, and collecting the resulting pyrolysis products.
5. 6. A method according to claim 5 wherein said steam has a temperature of between about 950*F. and 1,800*F.
6. 7. A method according to claim 5 wherein the mole ratio of steam to said pyrolysis products is between about 10 to 1 and 40 to 1.
7. 8. A method according to claim 5 wherein said polymeric tetrafluoroethylene is recycled scrap containing fillers.

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