Encapsulated felt

A sealing material of polytetrafluoroethylene felt is encapsulated by a polytetrafluoroethylene sheet coated with a thermoplastic fluoropolymer.

FIELD OF THE INVENTION 
This invention relates to an encapsulated felt for use as a sealant 
material. 
BACKGROUND OF THE INVENTION 
Felt materials have been used commonly in industrial applications. They 
possess good dimensional stability. They can be made with a wide variety 
of natural or synthetic fibers to withstand the mechanical, chemical and 
thermal requirements required by the application. 
Felts constructed with PTFE fibers possess superior chemical and thermal 
resistance and possess desirable mechanical properties, especially low 
coefficient of friction. 
Some applications using PTFE felt require a seal across a pressure 
differential. The seal requires resistance to solid or liquid penetration 
of the felt. Current felts are too porous and will allow particulate, 
liquid and gases to penetrate. 
The present invention attempts to retain the good physical, mechanical and 
chemical properties of PTFE felts while preventing penetration of liquids 
and solids through the felt. 
One use of felt sealing materials is in a rotating perforated drum for 
separating solid particulate from a liquid which needs to be sealed at the 
drum edges in order to prevent solids or liquids from being drawn in the 
stationary drive areas. 
This invention provides a novel material for sealing the area between such 
a rotating drum and the stationary drive housing. 
SUMMARY OF THE INVENTION 
A sealing material of polytetrafluoroethylene felt encapsulated in a tape 
of polytetrafluoroethylene having a layer of thermoplastic fluorinated 
polymer on one side of said tape, said tape having the thermoplastic 
fluorinated polymer adjacent the felt.

DETAILED DESCRIPTION OF THE INVENTION 
The felt used herein is made of short fibers, i.e. staple fibers, of 
polytetrafluoroethylene (PTFE). Preferably the PTFE will be porous PTFE 
and most preferably expanded porous PTFE. The felt can be prepared by the 
needle punching of staple fibers as described in Louterbach U.S. Pat. No. 
2,893,105 and the felt used in the present invention may sometimes be 
referred to herein as needle punched felt. 
The felts 10 (FIG. 1) are ordinarily 1/16 to 1/8 inch thick and they can be 
used as such in this invention or several layers of felts can be employed 
one on top of the other to form layers of any desired height, as for 
example, 1/8 inch, 3/8 inch or the like. The felts can be any width, but 
preferably are 1/2 inch to 40 inches wide or more; and any length, 
preferably 6 inches up to 10 feet to 50 feet or more. When several of the 
thinner felts are so layered, as in FIGS. 3a, 3b and 3c, the entire 
assembly is preferably needle punched on one side, or on both, as desired, 
to provide cohesiveness to the assembly. In one embodiment, 17 ply of felt 
sheets are used. In FIG. 3a, several layers of felt 10 are layered 
together. The felt layers may have a scrim added for increase strength, if 
desired. In FIG. 3b, scrim 14 is attached to one of the adjacent layers 
10. In FIG. 3c , scrim 14 is positioned in one of the layers 10. A scrim, 
if used, can be present adjacent each layer 10, or can be present in 
alternating or a few layers 10, or can be present, as shown, with only one 
layer 10. The scrim, when used, can be any strengthening scrim but 
preferably is made of woven PTFE fibers, and most preferably is made of 
woven porous expanded PTFE fibers. 
The polytetrafluoroethylene (PTFE) tape is a sheet of PTFE. As stated 
above, preferably, it is porous. By "porous" is meant that the sheet has 
continuous pores extending from one side of the membrane to the other. The 
sheet can be prepared by a number of different known processes. Porous 
sheets are preferably prepared by expanding polytetrafluoroethylene as 
described in U.S. Pat. Nos. 4,187,390; 4,110,392; and 3,953,566, to obtain 
expanded porous polytetrafluoroethylene. The pores are micropores formed 
by the nodes and fibrils of the expanded PTFE. 
The staple used in the felt can be made by slitting sheets into fibers and 
then chopping the resulting fibers into staple. 
To prepare the tape of PTFE, a sheet of polytetrafluoroethylene is 
laminated on one side with a film of a thermoplastic fluoropolymer, 
preferably a film of a copolymer of tetrafluoroethylene and 
hexafluoropropylene (Teflon.RTM. FEP fluoropolymer), using heat and 
pressure. Again, the sheet of PTFE is preferably porous PTFE, and most 
preferably expanded porous PTFE. 
The laminated sheet is wrapped securely around the felt assembly with the 
thermoplastic fluoropolymer side toward the felt. The wrapped felt is then 
heated to melt the thermoplastic fluoropolymer (270.degree. C. for Teflon 
FEP fluoropolymer) and bond it to the fibers of the felt. 
The encapsulated felt so prepared is impermeable to liquids and gases, and 
has a low coefficient of friction and a low coefficient of thermal 
expansion. It is conformable to curved surfaces and provides a strong, 
tough, assembly. The density of the assembly is between about 1 gm/cc and 
2.2 gm/cc. 
One use of the encapsulated felt of the invention is as a sealant material, 
especially to seal the inside of one stationary housing from the inside of 
an abutting rotating housing. Such a housing arrangement is depicted in 
FIG. 5. Stationary housing 20 contains drive elements (not shown) for 
driving rotating drum 21. Drum 21 contains a multiplicity of perforations 
22. Such an apparatus will have an entry means (not shown) for passing a 
particulate-laden liquid into contact with the drum 21 in the direction of 
the arrows in FIG. 5. The liquid passes through the perforations and can 
be withdrawn from the drum interior by exit means (not shown). 
FIG. 6 depicts a cutaway side view of the housing. Liquid with particulate 
passes in the direction shown by the solid line arrows. Unless the 
interface 25 between stationary housing 20 and rotating drum 21 is sealed, 
liquid containing particulate can flow through interface 25 as shown by 
the dotted line, into the interior of housing 20. To prevent such flow an 
encapsulated felt of the invention can be placed around the entire 
circumferential interface, as shown in FIGS. 7 and 8 where the 
encapsulated felt is shown as 26. The felt effectively seals off the 
interior of housing 20 from the interior of drum 21. 
The felt sealant is held in place by a vacuum in chamber 20 and by bolted 
spring 28. The felt provides an effective sliding seal. To enhance the 
sealing capability, vacuum ports are sometimes designed into the wall of 
chamber 20 under the felt to enhance sealing effectiveness. 
EXAMPLE 
An assembly of 17 layers of needle punched felt made of porous expanded 
PTFE staple fiber, each felt layer being about 40 mils thick, about 28 
inches wide and 30 feet long and each felt layer having in it a scrim of 
woven porous expanded PTFE fiber, was used. The assembly was needle 
punched to provide cohesiveness. 
A portion was cut into 2 inch wide by 6 inch long units. A sheet of 
expanded porous PTFE membrane, laminated on one side with a film of a 
copolymer of tetrafluoroethylene and hexafluoropropylene to make a 5 mil 
thick tape was spirally wrapped around the entire unit and each end sealed 
to obtain an encapsulated assembly. 
The assembly was tested for its impermeability to fluids by placing it in a 
test device which subjected all sides to water except the underside. The 
underside was subjected to a vacuum of 15 inches mercury. After 11/2 
hours, no water had penetrated through the assembly and into the vacuum 
area. In contrast, a 17 layer batt that was not encapsulated by the wrap 
of laminated sheet allowed water to pass through at a rate of 4 
gallons/foot.sup.2 /minute.