Method for producing a flat porous product

The invention relates to a method for producing a flat porous product, in which a solution of an elastomeric material is deposited on a temporary substrate and the elastomeric material is coagulated on the substrate from the solution with the aid of a nonsolvent, wherein the surface of the substrate has an adhesion to the coagulating elastomer which is such that, after coagulation, the elastomeric flat product can be removed from the substrate without damage and the side (macroporous zone or bottom layer) of the elastomeric flat product facing the substrate possesses pores having a diameter in the region of 20-200 .mu.m and has a porosity of more than 75%, a flat product which can be obtained according to this method for use as wound covering material and a method for treating wounds therewith.

FIELD OF THE INVENTION 
The invention relates to a method for producing a flat porous product, in 
which a solution of an elastomeric material is deposited on a temporary 
substrate and the elastomeric material is coagulated on the substrate from 
the solution with the aid of a nonsolvent. 
BACKGROUND OF THE INVENTION 
In membrane technology, porous films are made from polymer solutions by 
spreading a thin layer of polymer solution on a suitable flat bottom layer 
(substrate), for example a glass plate. The bottom layer is then immersed 
together with the polymer solution in a nonsolvent for the polymer. The 
polymer solution will separate (precipitate), and this results in a 
particular porous structure in the polymeric material which has then 
solidified. This process is termed coagulation. After coagulation, the 
membrane can be removed from the glass plate and used as is. This method 
is in principle much used for producing reverse osmosis and 
ultrafiltration membranes, in particular of nonelastomeric polymers. 
A specific membrane having a strongly asymmetrical structure and suitable 
for use as a wound covering material (artificial skin) is described in the 
Dutch Patent Application NL-A-8801741, which corresponds to a considerable 
extent to EP-A-0,351,016 (publication date Jan. 17, 1990). The wound 
covering material has a gradient in the pore size distribution viewed in 
the cross section of the material, and this implies that there is a top 
layer which is in contact with the environment and possesses pores of less 
than 0.5 .mu.m, while the side which is in contact with the damaged skin 
(bottom layer) possesses relatively large pores in the region of 20-200 
.mu.m. The wound covering material is produced by a method in which the 
starting point is a polymer solution (preferably polyurethanes) to which 
particles (for example salt particles) have been added. These solid 
particles have a twofold purpose. Firstly, they serve to prevent shrinkage 
phenomena which occur during and shortly after the coagulation process; 
secondly they serve as so-called pore formers. The relatively large pores 
in the bottom layer are obtained by adding particles with the correct 
dimensions to the polymer solution. These particles which are washed out, 
after coagulation of the polymer solution, using a solvent suitable for 
said particles, therefore leave behind pores having a particular size and, 
in addition, prevent shrinkage phenomena in the precipitating polymer 
solution. It will be clear that the solvent used for washing out has to be 
an agent other than the coagulating medium for the polymer solution so 
that the particles are not already washed out in the coagulating medium. 
The polymer therefore has first to be precipitated and then the salt 
particles still present have to be washed out. Suitable particles are, for 
example, crystals of the salt sodium citrate. The thin top layer having 
the small pores present therein can be obtained from a similar solution 
(but now without salt crystals). 
The method described is very time-consuming and is only suitable for 
obtaining small surfaces (a few cm.sup.2) of the wound covering described 
on a laboratory scale. The very thin top layer (thickness 0.01-0.2 mm) is 
very difficult to apply, while the use of two different nonsolvents also 
presents very many production problems. 
The use of elastomeric materials for producing imitation leather is also 
known from the literature. An important property of imitation leather is 
the permeability to water vapour, the material nevertheless having to have 
the necessary mechanical strength. Imitation leather can also be made by 
starting from a polymer solution and a coagulation process in which a 
product is eventually obtained which is composed of two layers: a thin 
layer having small pores is applied to a porous carrier and eventually 
forms an integral whole with said porous carrier. The pore size in the 
porous carrier layer is in general not larger than a few micrometers, 
while the pore size in the top layer is not described. It will be clear 
that, in view of the applications of imitation leather, the microporous 
top layer has to have a very good and lasting adherence to the porous 
substrate layer. 
The problem underlying the present invention was to provide a method for 
producing a flat porous product which is suitable, for example, as wound 
covering material, which method can in principle be carried out in one 
step, the use of two different nonsolvents being avoided and the desired 
pore structure nevertheless being obtained. 
This problem is solved according to the invention. 
SUMMARY OF THE INVENTION 
The invention relates to a method for producing a flat porous product, in 
which a solution of an elastomeric material is deposited on a temporary 
substrate and the elastomeric material is coagulated on the substrate from 
the solution with the aid of a nonsolvent, wherein the surface of the 
substrate has an adhesion to the coagulating elastomer which is such that, 
after coagulation, the elastomeric flat product can be removed from the 
substrate without damage and the side (macroporous zone or bottom layer) 
of the elastomeric sheet facing the substrate possesses pores having a 
diameter in the region of 20-200 .mu.m and has a porosity of more than 75% 
.

DETAILED DESCRIPTION OF THE INVENTION 
In the method according to the invention, smaller pores, for example pores 
having a diameter of 0.1-25 .mu.m, preferably 1-20 .mu.m, may be present 
in the walls of the pores having a diameter of 20-200 .mu.m. 
In order to obtain the desired pore structure in the bottom layer of the 
flat product, the use of a solution having a relatively low polymer 
concentration is necessary. In general, the elastomer concentration of the 
solution should not exceed 18% by weight, referred to the total solution. 
Preferably, the elastomer concentration is even lower than 10% by weight. 
The invention is based on the principle that an elastomeric sheet having 
desired pore sizes can be obtained by spreading a dilute polymer solution 
on a substrate layer which has a porosity or surface roughness such that 
shrinkage of the polymer during or after coagulation is prevented, in 
particular in a manner such that the intended structure is obtained. After 
coagulation and possible post-treatments, it is possible to remove the 
elastomeric flat product once produced from the temporary carrier layer 
(substrate) used, without damaging the elastomeric flat product. According 
to the method of the present application, it is possible to produce the 
wound covering materials described in NL-A-8801741 or EP-A-0,351,016 by a 
one-step process. It is also still possible, however, to produce a wound 
covering composed of more than one layer by the method according to the 
invention. 
With respect to the temporary substrate used in the method according to the 
invention, two parameters play an important part. 
The physical or chemical structure of the material. The structure of the 
temporary carrier is such that the usual shrinkage during the coagulation 
of the elastomeric polymer solution is prevented or counteracted. 
The permeability of the material to the coagulant. 
An important aspect of the present invention is the fact that the desired 
porous structure (including the so-called micropores in the walls of the 
pores of the macroporous zone or bottom layer) cannot be obtained if the 
separation of the bottom layer is started by the coagulating medium which 
reaches said bottom layer via the temporary support layer. The desired 
situation is therefore that the separation is initiated in the bottom 
layer by the coagulating medium which has travelled through the polymeric 
film, originating from the top of the deposited polymeric film. 
In the method according to the invention, it is preferable to use water as 
nonsolvent or coagulating medium. 
A very important advantage of the invention is that a flat porous product 
is obtained which detaches from the temporary carrier in a manner such 
that the unshrunken structure initially present remains unchanged and the 
product can be used as a wound covering material. It is important that, 
during the coagulation and possible post-treatment, the coagulated 
polymeric layer continues to adhere well to the temporary carrier layer or 
support layer. 
In the method according to the invention, the substrate used can be a 
material which yields the intended structure of the elastomeric flat 
product. In particular a woven or nonwoven material of cellulose, 
polypropene, polyethene and/or polyester is used as substrate. It is also 
possible, however, to use as substrate an impermeable sheet or plate whose 
surface has been roughened, for example by means of a corona treatment. 
It is not readily possible to define unambiguously the nature of the 
surface of the substrate which is used in the method according to the 
invention. Nevertheless, the adhesion of the elastomeric flat product to 
the substrate can be quantified in various ways. For porous support 
layers, use can be made of the weight per m.sup.2 since the amount of 
fibrous material per unit surface area can be regarded as a measure of the 
porosity. In the case of a nonwoven polyester material, reference may be 
made here to the materials Viledon F0 2402 and Viledon F0 2406 (cf. the 
examples). The adhesion of the elastomeric flat product (for example 
Pellethane 2363) to Viledon F0 2402 is too strong and to Viledon F0 2406 
it is good. Viledon F0 2402--weight 85 g/m.sup.2, permeability to air 250 
dm.sup.3 /s.m.sup.2 at 2 mbar. Viledon F0 2406--weight 180 g/m.sup.2, 
permeability to air 10 dm.sup.3 /s.m.sup.2 at 2 mbar. 
Another and more general way of quantifying can be borrowed from the 
adhesive tape industry. The following standard procedure may be used: 
The elastomeric flat product projects 10 mm at one side on a strip of the 
support layer having a length of 250 mm with an elastomeric flat product 
applied to it. The width of the test strip is 25 mm. The condition of the 
elastomeric flat product is such that it can be used as a wound covering 
material. The temperature is 21.degree.-25.degree. C. and the relative 
atmospheric humidity 25-55%. The wound covering is removed gradually. The 
force with which this is done must be constant and not less than 1N over a 
length of 125 mm. The force must not, however, exceed a value of 30N. 
The elastomeric flat product produced by the method of the invention 
preferably possesses pores having a diameter of not more than 0.7 .mu.m, 
preferably 0.1-0.5 .mu.m, at the side (microporous zone, top layer) facing 
away from the substrate. These pores in the top layer of the elastomeric 
flat product can be formed by bringing the elastomeric flat product into 
contact with vapour of a nonsolvent, preferably water, before coagulation. 
Surprisingly, it has been found that adding a hydrophilic polymer to the 
solution of the elastomeric material yields an elastomeric flat product 
which is more hydrophilic than can be expected on the basis of the 
properties of the starting materials. As hydrophilic polymers, mention may 
be made here of polyacrylic acid, polyvinyl alcohol, polyvinyl acetate, 
polyvinylpyrrolidone, polyethylene glycol, polyvinylpyridine, 
polyethyleneimine etc. Excellent results are obtained, however, if 
polyvinylpyrrolidone is used as a hydrophilic polymer. 
The amount of hydrophilic polymer depends strongly on the concentration of 
the elastomer in the solution. In general, the amount of the hydrophilic 
polymer will be 1-20% by weight, referred to the elastomer. 
For the use as a wound covering material, the hydrophilic nature of the 
elastomer has advantages in relation to the water transport from the 
damaged skin covered (wound), and the acceptance of the wound covering 
material by the wound, and in relation to avoiding adsorption of, for 
example, protein which could block the small pores in the top layer. 
The invention also relates to a flat product which can be obtained by the 
method described above and is suitable for use as a wound covering 
material which may have a thickness, for example, of 10-500 .mu.m. 
The invention also relates to the use of such a flat product in the field 
of pharmacy, medicine or cosmetics. Thus, the invention also relates to a 
method for treating wounds which is characterised in that a flat product 
which can be obtained by the method of the invention is used in the 
process. 
In the following examples, which must not be interpreted as restrictive, 
the invention is explained in greater detail. 
COMATIVE EXAMPLE 1 
A solution of 10 g of polyether urethane (Pellethane 2363; DOW Chemical) in 
100 g of N-methylpyrrolidone to which 1 g of lithium chloride (which is 
used to increase the solubility of the polymer) has been added was spread 
on a clean glass plate having a thickness of 0.4 mm using a so-called 
"doctor knife". The glass plate with the polymer solution on it was then 
immersed in a water bath (temperature 45.degree. C.). The film produced 
separated from the glass plate even during the coagulation of the polymer 
solution and exhibited a shrinkage exceeding 200%, No. 20-200 .mu.m pores 
could be detected in the bottom layer. The elastomeric flat product 
shrivelled up completely during drying after complete coagulation, and 
this also indicates unduly small pores in the bottom layer. 
EXAMPLE I 
The solution prepared in accordance with Comparative Example 1 was spread 
on a nonwoven polyester support layer (Viledon FO 2406) in the same 
thickness and was then immersed in a water bath (temperature 45.degree. 
C.). The elastomeric flat product now continued to adhere to the porous 
nonwoven during and after coagulation and exhibited no shrinkage. After 
complete coagulation and removal of the support layer, it was possible to 
dry the elastomeric flat product normally and 20-200 .mu.m pores were 
found in the bottom layer. After examination with the scanning electron 
microscope (SEM examination) it was also found that the walls of the large 
pores possessed still smaller pores which are able to serve as adhesion 
points. 
COMATIVE EXAMPLE 2 
A solution from Comparative Example 1 was spread on another type (more 
open; see explanation in the text) of polyester nonwoven (Viledon F 2402) 
and was processed further in the same way. After complete coagulation, it 
was found that the elastomeric flat product was very difficult to remove 
from the support layer without damaging it. In addition, after SEM 
examination it was found that there were few to no large pores (20-200 
.mu.m) present in the bottom layer. These effects are attributed to the 
fact that, in the first place, the polymer solution can penetrate too far 
into the porous nonwoven (result-unduly strong adhesion!) and that water 
(coagulant) also penetrates from the rear side at the same time, as a 
result of which coagulation can also take place starting from the rear 
side, with the result that the structure desired for a wound covering is 
not obtained. 
EXAMPLE II 
A polymer solution containing 10 g of polyether urethane (Pellethane 2363), 
100 g of N-methylpyrrolidone and 3.3 g of polyvinylpyrrolidone was spread 
on a polyester nonwoven (Viledon FO 2406) and passed in the course of 10 
seconds through a zone of water vapour saturated at 45.degree. C. and then 
immersed in a 45.degree. C. water bath. After coagulation, it was found 
that a wound covering had been produced which had a thin top layer 
possessing approximately 0.1-0.5 .mu.m pores and a bottom layer possessing 
20-200 .mu.m pores. After rinsing out and drying, this elastomeric flat 
product was found to be markedly more hydrophilic than a specimen made 
without adding PVP to the polymer solution. Such an experiment carried out 
on a polypropylene nonwoven (AWA 17) revealed corresponding results. 
COMATIVE EXAMPLE 3 
A polymer solution prepared in accordance with Example II was spread on a 
smooth, nonporous polypropylene sheet and processed further identically to 
Example II. Even during the coagulation the elastomeric flat product 
separated from the sheet and a large shrinkage occurred, with the result 
that the resultant material could no longer be used as an elastomeric flat 
product. 
EXAMPLE III 
A solution in accordance with Example II was spread on a nonporous 
polypropylene sheet having a surface which had been modified by corona 
treatment. No shrinkage now occurred and the resultant artificial skin had 
the same properties as that of Example II.