PTFE layers and methods of manufacturing
Thin PTFE layers are described having little or no node and fibril microstructure and methods of manufacturing PTFE layers are disclosed that allow for controllable permeability and porosity of the layers. In some embodiments, the PTFE layers may act as a barrier layer in an endovascular graft or other medical device.
1. A method of processing PTFE, comprising:
extruding a compounded PTFE resin through an extruder in a machine direction to form a PTFE ribbon extrudate;
calendering the PTFE ribbon extrudate in the machine direction to form a layer of PTFE having an upper surface, a lower surface and a longitudinal length in a machine direction of an extruder used to produce the layer of PTFE and having a width in a cross machine or transverse direction;
applying a liquid stretching agent in an excess amount to at least a portion of the upper surface or the lower surface of the layer of PTFE;
stretching the layer of PTFE in the transverse direction to the machine direction; and
maintaining the excess amount of the liquid stretching agent on the at least portion of the upper or the lower surface of the layer of PTFE during at least part of the stretching to fully saturate the layer of PTFE throughout its thickness with the liquid stretching agent during the stretching to form a stretched layer of PTFE that has:
(i) a closed cell microstructure with a plurality of interconnected high density regions of PTFE having no distinct fibrils interconnecting adjacent nodes when viewed at a magnification of 20,000 under a scanning electron microscope; and
(ii) a thickness from 0.00005 inch to 0.005 inch;
wherein the stretching is at a rate from 10 percent per second to 100 percent per second;
wherein the stretching is performed at a stretch ratio from 2:1 to 20:1; and
wherein stretching of the layer of PTFE is performed at a temperature of 80° F. to 100° F.
2. The method of claim 1 wherein the stretching is performed at a stretch ratio from 3:1 to 20:1.
3. The method of claim 1 wherein the stretching is performed at a stretch ratio from 3:1 to 12:1.
4. The method of claim 1 wherein the stretching is performed at a stretch ratio from 3:1 to 7.8:1.
5. The method of claim 1 wherein the step of applying the liquid stretching agent on the upper surface of the layer of PTFE further compromises:
pooling or puddling the upper surface of the layer of PTFE with the liquid stretching agent.
6. The method of claim 1 further comprising spreading the liquid stretching agent after application to the layer of PTFE with a skimming member disposed adjacent the upper or the lower surface of the layer of PTFE.
7. The method of claim 1 wherein the liquid stretching agent is applied to substantially all of the upper or the lower surface of the layer of PTFE prior to stretching.
8. The method of claim 1 wherein the stretched layer of PTFE, after sintering at a temperature from about 350° C. to about 380° C. for about 2 minutes to about 5 minutes, has substantially no fluid permeability with a Gurley Number of greater than 12 hours for 100 cc of air at a pressure of 12.4 cm of water.
9. The method of claim 1 wherein the stretched layer of PTFE has a limited fluid permeability with a Gurley Number of greater than 1,500 seconds for 100 cc of air at a pressure of 12.4 cm of water.
10. The method of claim 1 wherein the stretched layer of PTFE has a range of permeability from substantially no fluid permeability with a Gurley Number of greater than 12 hours for 100 cc of air at a pressure of 12.4 cm of water to a limited permeability with a Gurley Number of less than 1,500 seconds for 100 cc of air at a pressure of 12.4 cm of water.
11. The method of claim 1 wherein the liquid stretching agent is selected from the group consisting of naphtha, mineral spirits, MEK, toluene and alcohol.
12. The method of claim 1 wherein the liquid stretching agent is an isoparaffin stretching agent.
13. The method of claim 1 wherein the stretching of the layer comprises stretching the layer of PTFE in the direction transverse at a rate of 15 percent per second to 25 percent per second.
14. The method of claim 1 further comprising calendering the stretched layer of PTFE to densify and compress the layer of PTFE.
15. The method of claim 1 wherein the PTFE ribbon extrudate has a ribbon configuration and a thickness of 0.020 inch to 0.040 inch.
16. The method of claim 1 wherein the PTFE ribbon extrudate is calendered in the machine direction to a reduced thickness of 0.001 inch to 0.005 inch prior to stretching.
17. The method of claim 1 further comprising sintering the stretched layer of PTFE.
18. The method of claim 17 , wherein the step of sintering is at 350° C. to 380° C. from 2 to 5 minutes.