A polytetrafluoroethylene porous film, which is prepared by stretching a semisintered polytetrafluoroethylene material and heating the stretched material at a temperature higher than the melting point of sintered polytetrafluoroethylene, has an area ratio of fibrils to nodes of from 99:1 to 75:25, an average fibril diameter of from 0.05 to 0.2 .mu.m, a largest node area of not larger than 2 .mu.m.sup.2 and an average pore size of from 0.2 to 0.5 .mu.m and achieves low pressure loss.
An air filter comprising a laminate of a non-woven fabric made up of polyolefin fibers or polyester fibers or a composite of the non-woven fabrics and a PTFE porous film, wherein the amount of the total gases generated from the air filter is 20 .mu.g/g by the analysis of gas chromatography. The amount of the gases generated from the air filter is greatly reduced by heat-treating the non-woven fabric at a temperature of from 80 to 130.degree. C. for from 5 minutes to 20 minutes before or after laminating with the PTFE porous film.
A nonwoven web is described that is a thin porous polytetrafluoroethylene membrane consisting essentially of a nonwoven web having a microstructure of substantially only microfibrils fused at crossover points. It is usually strong, is unusually thin, has unusually small pore sizes, but a very high air flow-through. It has a pore size between 0.05 and 0.4 micrometers; a bubble point between 10 and 60 psi; a pore size distribution value between 1.05 and 1.20; a ball burst strength between 0.9 and 17 pounds/force; an air flow of between 20 Frazier and 10 Gurley seconds; a thickness between 1.0-25.4 micrometer; and a fiber diameter ranging between 5 and 200 nm.
The split PTFE yarn and PTFE filament which are bulky and excellent in the feeling and have good intermingling property can be obtained and the cotton-like PTFE materials can be produced therefrom efficiently at a low percentage of dropped fibers. Further the filter cloth for dust collection which is obtained from those cotton-like materials has a high collection efficiency. The present invention can provide the split yarn having a network structure and obtained by splitting a uniaxially stretched PTFE film in the stretched direction with a needle blade roll; the filament having branches and obtained by cutting the network structure of the split yarn in a longitudinal direction; the methods of production thereof; the method of producing the cotton-like materials by cutting the split yarn or filament and then opening; and a filter cloth for dust collection obtained from the cotton-like materials.
A filter medium which has an average pore diameter of more than 0.5 .mu.m, a pressure loss of 2-50 mmH.sub.2 O determined when air is permeated therethrough at a velocity of flow of 5.3 cm/sec, and a PF value of 18-12, which is calculated in accordance with the following equation on the basis of this pressure loss and a collection efficiency using DOP of a grain size of 0.10-0.12 .mu.m. ##EQU1##
A closure member for use in closing vials that are subjected to lyophilization conditions is described where the member is a resilient stopper that has a plug movable within a passageway in the stopper. The plug is movable between a first raised venting position and second downwardly engaging, sealing position whereby fluid from the vial or container is precluded from flowing through the fluid passageway in the cap. The passageway has a venting medium filter covering it which allows passage of water vapor, but not bacteria.
