Method of producing porous rubber sheeting

This invention provides a method for producing porous elastomeric sheet material, wherein ungelled elastomer liquid material is admixed with hollow spherical filler particles and then spread to form a sheet, which is then dried and gelled. The dried and gelled sheet of filled elastomer is subjected to rigorous biaxial stretching, wherein the elastomeric material surrounding at least some of the particles is stretched and ultimately broken by the rigorous stretching, to form a porous sheet material.

A method of producing porous articles of elastomeric material, particularly 
porous latex rubber sheeting, in which a plurality of hollow spherical 
filler members of hard inert material are incorporated in a mix of 
elastomeric material which is spread to form a sheet and is allowed to gel 
and dry after which the dried sheet is subjected to rigorous multiple 
stretching thereby to produce a sheet with permanent porosity. 
The mix may be of natural latex or of a synthetic latex such as 
polychloroprene or acrylonitrile-butadiene co-polymer.

DESCRIPTION OF A PREFERRED EMBODIMENT 
A dispersion consisting of 35% (by weight) of hollow spherical filler 
products in water is first prepared. The filler particles are preferably 
hollow spheres of the order of 50 microns or less in size. Such filler 
particles are known and are, for example, supplied under the name "Fillite 
300/7" by a firm called Fillite (Runcorn) Limited. The composition of a 
suitable dispersion is as follows: 
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Fillite 300/7 35.00 
Dispersol (a sodium salt of 
a disulphonic acid) 
0.35 
20* Potassium Oleate 
0.85 
Reversol DH461 (Polyacrylate 
thickner of Revertex Ltd) 
1.60 
Water 62.20 
100.00 
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The dispersion is incorporated in a latex rubber mix. This latex mist may 
have the following composition: 
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Prevulcanised natural latex 
(MR Revultex of Revertex Ltd) 
167.00 
50% Zinc Oxide dispersion 
2.00 
50% Sulphur dispersion 
0.20 
50% Zinc diethyldithiocarbamate 
dispersion 0.20 
35% Fillite dispersion 
100.00 
25% Polysiloxane heat-gel 
sensitiser (Coagulant WS of 
Bayer U.K. Ltd) 6.00 
Black pigment dispersion (Irgalite 
Black CP V1 Paste of Ciba-Geigy 
5.00 
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After the dispersion has been incorporated in the latex mix, the mixture is 
spread to sheet form and caused to gel by the application of heat and 
dried. The dried sheet will then be filled with minute cavities each 
defined by one of the hollow filler particles, the particles being bound 
by adhesion to the rubber. These cavities will normally be very close 
together, the closeness of the cavities depending on the portion of filler 
particles incorporated into the mix. 
The dried sheet is then subjected to rigorous stretching. Such stretching 
elongates the cavities and causes the rubber to break away from the 
outside of the filler spheres. The thin membranes extending between the 
multiplicity of elongated cavities rupture under the strain thereby to 
produce a sheet of permanent porosity. 
The sheet is conveniently stretched by a machine as illustrated in the 
drawing. 
The machine comprises a lower plate 1 perforated by holes of about 0.65" 
diameter. These holes are uniformly spaced in and extend over an area of 
210 square inches. The holes are as close together as is practical. An 
upper plate 2 is located above in plate 1 and carries pins 3 about 11/2" 
long .times. 1/2" diameter. The pins have spherical ends and the pin 
centres match the centres of the holes in plate 1 so that they can be 
entered in the holes. The plates 1 and 2 are mounted on beams 4 and 5 
respectively. The beams are located by guide pins 6. The beam 5 is 
attached to connecting rods 7 and thence to crank shafts 8 which allow 
vertical movement of about 1". The crankshafts are driven by a geared 
electric motor. The bottom beam 4 which carries the plate 1 is adjustable 
up and down so that the amount of penetration of the pins 3 into the holes 
can be varied. 
Rubber sheeting is drawn through the machine and between the plates 1, 2 
from a roll R1 by rotation of two driven rollers 9 on which is a roll R2 
of processed rubber. The roll R1 of rubber to be processed rests on 
rollers 10. The rollers 10 are not driven and are slightly braked to 
create a small tension. 
When the upper plate 2 descends, the pins 3 press on the sheeting and 
stretch it over the pins and into the holes in the lower plate 1. The 
degree of stretch depends on the amount by which the pins 3 are allowed to 
penetrate the holes. 
Conveniently, the sheeting is advanced between the plates at a rate of 3 
feet per minute and the sheeting is pressed into the holes by the pins 180 
times per minute. 
As an alternative to stretching the sheeting as hereinbefore described it 
is possible to produce the desired porosity by the application of pressure 
to sheeting stretched over a frame and supported on a mesh structure. Such 
pressure may be liquid pressure, positive air pressure or, more 
conveniently, negative air pressure, i.e. suction. 
The stretching operation will cause some of the hollow filler spheres to be 
visibly removed from the sheet. The hollow filler spheres nearest the 
surface of the sheets will obviously be most likely to be removed but 
examination of such a sheet under a microscope will show that even after 
stretching and sucking water through a sheet more than one hundred times, 
a large proportion of filler spheres remain in the sheeting. 
Experiments have shown that sheeting 0.012 inch thick is porous to water if 
it contains 25 parts or more, by weight, of filler spheres per 100 parts 
of rubber. With a mix in which there are 15 parts, by weight, or more of 
filler spheres of less than 50 microns diameter per 100 parts of rubber 
the sheet is porous to air but in sheets with less than 15 parts (by 
weight) per 100 parts of rubber, the porosity of the sheet is gradually 
reduced to 0. It has been found that where the proportion of hollow filler 
spheres is 35 parts (by weight) per 100 parts of rubber, a sheet with very 
good porosity to water is obtained. 
If larger filler spheres, for example of 60-100 microns diameter, are used 
the sheeting produced has visibly greater porosity. 
The filler spheres are dispersed in water in order to facilitate addition 
to the latex mix. The spheres have a low specific gravity which causes the 
spheres to rise to the surface of the dispersion, but they can easily be 
stirred back in. Similarly, the spheres tend to rise to the surface of the 
latex mix when the mix is standing but can be easily redispersed. It is 
necessary periodically to stir the mix while it is being fed to a 
spreading machine which can be as described in the specification of U.K. 
Pat. No. 1,326,541. 
The invention is also applicable to the production of porous Neoprene 
sheeting. A suitable dispersion for use in this case is as follows: 
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Neoprene 671 Latex(DuPont U.K. Ltd) 
173.00 
20% Emulvin W* solution 
6.00 dissolve Coagulant 
Coagulant WS* 1.50 the Emulvin solution 
50% Sulphur dispersion 
2.00 
50% Zinc diethyl-dithiocarbamate - 
4.00 
50% Zinc Oxide dispersion 
10.00 
33% Antioxidant 425 - . - 
6.00 (2.2'-methylene bis(4 
ethyl - 6 tertiary 
butyl phenol) 
35% Fillite "1" dispersion 
100.00 
Ingralite CPV1 Paste 
7.60 (Ciba-Geigy black 
pigment dispersion) 
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*Trade Marks of Bayer(U.K.) Ltd. Emulvin W is an aromatic polyglycol ethe 
- nonionic emulsifying agent. 
1 Fillite is a glass-hard inert silicate in the form of high strength 
hollow spheres of the order of 50 microns in diameter.