Valves used in industrial environments often have rigorous performance standards. In the past, packings of polytetrafluoroethylene and other fluoropolymers have been used which were considered to seal valves satisfactorily. While packings were previously considered satisfactory with leakages of 10,000 pp , such levels have now been found to be unacceptable for many applications. Desired emission levels for many industrial applications are below 500 ppm, and levels of less than 10 ppm for particularly toxic and carcinogenic materials would be desirable, particularly for those materials for which emission or handling standards have been established by various governmental agencies.

Current requirements for valve sealing have created a need for packing systems that exceed earlier performance requirements in a cost-effective manner.

Summary of the Invention

The present invention provides a valve packing which can reduce valve emissions to levels previously unattainable without precision metal bellows components.

Specifically, the present invention provides a valve packing comprising at least three rings, including

(a) a male adapter,

(b) a female adapter, and

(c) at least one perfluoroelastomer chevron seal ring positioned between the male adapter and the female adapter.

Brief Description of the Drawings Figure 1 is a cross-sectional view of a valve packing of the present invention.

Figure 2 is a perspective view of a preferred valve packing of the present invention. Detailed Description of the Invention The valve packings of the present invention are composed of a plurality of individual rings, as can be seen in cross-sectional illustration in Figures ι and 2. There, male adapter 1 is at the upstream end of the packing, and female adapter 2 is at the atmospheric, or downstream end of the packing. Perfluoroelastomeric seal rings 3 are in a chevron or "V" configuration, fitting with ridge 4 of the male adapter and groove 5 of the female adapter.

When two or more seal rings are used in the packings of the present invention, the seal rings are separated by spacer rings 6. These spacer or backup rings can be prepared from the same or different materials as the male and female adapters. The particular material should be adapted to the conditions of use that the packing will experience. High temperature polymeric resins, that is, those stable at temperatures of at least about 500"F, are preferred, such as substantially non-elastomeric fluoropolymers or polyetheretherketones.

Fluoropolymers which can be used for this component include polytetrafluoroethylenes, such as those commercially available from the Du Pont Company as Teflon ^® PTFE fluoropolymer resins; melt-processable copolymers of tetrafluoroethylene and hexafluoropropylene such as those commercially available from the Du Pont Company as T'eflon ^® FEP fluoropolymer; and clear, thermoplastic fluoropolymers such as those tetrafluoroethylene/fluorovinylether

copolymers commercially available from the Du Pont Company as Teflon ^® PFA. Still other fluoropolymers which can be used effectively in the present invention are those modified copolymers of ethylene and tetrafluoroethylene commercially available from the Du Pont Company as Tefzel ^® fluoropolymers. Polyetheretherketones which can be used include those commercially available from ICI as PEEK.

Perfluoroelastomers which can be used in the preparation of the seal rings of the present invention include, for example, those perfluoroelastomers described in Breazeale, U.S. Patent 4,281,092. Perfluoroelastomers of tetrafluoroethylene and perfluoro (methyl vinyl ether) are commercially available from the Du Pont Company as Kalrez ^® perfluoroelastomers. These perfluoroelastomers generally have a termonomer to facilitate curing, such

CF ₂ =CF-0-CF ₂ CF(CF ₃ )OCF ₂ CF ₂ CN, and CF ₂ =CH ₂ . Other perfluoroelastomers which can be used are those copolymers of tetrafluoroethylene and a mixture of perfluoromethylvinyl ether and higher molecular vinyl ether, and having a cure site monomer derived from perfluoro alkyl diiodide. Those copolymers are commercially available from Daikin

Kogyo Co., Ltd. as Perfluor ^® perfluoroelasto er, and marketed by Green, Tweed as Chemraz ^® perfluoroelastomer. Still another perfluoroelastomer which can be used in the present invention is that commercially available from NOK-Freudenberg as Si raz per luoroelastomer.

For those applications in which temperatures greater than about 400 ^* F will be encountered, Kalrez ^® perfluoroelastomers exhibit exceptional stability, and better long term performance than other commercial

perfluoroelastomers. These materials are accordingly preferred.

The configuration of the present packings will, of course, vary with the particular stem which is to be sealed and the associated cavity. Typically, the valve stems to be sealed have an outer diameter of 1/4 to 1 1/2 inch, which corresponds to the inner diameter of the packing. The diameter of the cavity in which the stem is located, corresponding to the outer diameter of the packing, is generally about from 1 1/2 to 2 times the diameter of the stem.

The packings of the present invention provide a marked improvement in sealing performance over standard fluoropolymer or flexible graphite packings that have heretofore been used. This is particularly advantageous in industrial chemical equipment, where the emissions of toxic or corrosive fluids such as benzene, butadiene and other carcinogenic or toxic substances can be reduced to less than 500 ppm. In many cases, emissions can be reduced to less than 10 ppm or substantially eliminated.

The present invention is further illustrated by the following specific examples, in which parts and percentages are by weight unless otherwise indicated. Examples 1-3

In Examples 1-3, valve packings were assembled substantially as illustrated in Figure 1, to fit valves having a 3/8-inch stem. In each packing, the male and female adapters, as well as the spacer, were prepared from Teflon ^® PTFE fluoroelastomer, and the two chevron seal rings were prepared from Kalrez ^® 3018 perfluoroelastomer compound. The outer diameter of the packing components was 0.873 inch. The spacer and the perfluoroelastomer seals were each 0.274 inch

thick, the male adapter was 0.184 inch thick, and the female adapter was 0.242 inch thick.

The packings were assembled in valves controlling benzene, butadiene, and butadiene, in Examples 1-3, respectively. The pressure of the systems was maintained at less than about 200 psi in these Examples. The temperatures were less than 350"F. Emission levels were checked for 9 separate readings over an 8 week period, and found to average the following readings:

Example 1 - Benzene - 5 parts per million Example 2 - Butadiene - 7 parts per million Example 3 - Butadiene - 19 parts per million