FR2559492A1 - Polymer foams and processes for obtaining them - Google Patents

Abstract

These polymer foams are characterised chiefly in that the blowing agent employed is a solid material which can be extracted by dissolving. When the blowing agent is an alkali or alkaline-earth metal salt such as sodium chloride, the extraction is performed by washing with hot water followed by oven drying. The compound is obtained by mixing powders or by aqueous dispersion of the polymer saturated with blowing agent. Chief applications: porous insulators of low dielectric constant, flexible insulating coatings, artificial veins and arteries, clothing which is imperveous to liquids and permeable to air.

Description

 The present invention relates to a process for obtaining foams of polymeric materials or expanded plastic materials or material: cellular plastics.

 In general, the introduction of gaseous molecules into a polymeric material has the effect, in terms of physical properties, a decrease in density resulting in an increase in the permeability to gases and liquids; in terms of mechanical properties, an improvement in flexibility and flexibility; in terms of thermal properties, an improvement in the insulating qualities of the material and a reduction in flammability; in terms of electrical properties, a decrease in the dielectric constant and the loss angle (especially in the case of open structures). The presence of gases in this type of material introduces, on the other hand, little or no variation in the chemical properties.

 It is thus seen that the introduction of gas into a plastic material has the effect of improving a number of properties of this material.

This method has been used for several years to prepare many materials used in very varied fields
- the chemical industry t filtration and ultrafiltration membranes, gas diffusion materials, gland gaskets, diaphragms ...

 - building and industries derived from insulation materials, resilient coatings ...

 - the medical sector i artificial veins and arteries ...

 - the cable industry t porous insulators with low dielectric constant ...

 - Textile industries: liquid impermeable and breathable clothing.

 The preparation of cellular plastics, although it may be sorted in certain particular cases (polyethylene, polystyrene, polyurethane, polyvinyl chloride, tetrafluoroethylene copolymer, perfluoropropene, etc.) poses from a general point of view many problems, and each subject brings to the technician a specific problem to be solved (expansion agent adapted to the material).

There are several processes for obtaining cellular polymeric materials
introduction during the operation of a gas under pressure (for example perfluoropropene, perfluoroethylene (FEP) copolymers, expanded at the time of extrusion with FREON 22)
- Introduction into the material, before the implementation, of a chemical or pore-forming agent, which decomposes giving gases at the moment, ie essentially at the temperature of implementation (for example expansion of polychloride) vinyl, polyolefins, ABS with azodicarbonamide).

 In these different processes, the general idea is to create in the material moment of its implementation gases which will remain prisoners, in the case of closed cellular structures, or which will be replaced by air, in the case of structures open cell phones.

There are currently two methods for obtaining polytetrafluoroethylene (PTFX) cells, which are described in the patent applications.
US Pat. No. 3,962,153 of June 8, 1976 (CORE) relating to an expansion process by mechanical stretching of PTFE

 - FR.A-83.15694 of 29 September 1983 concerning a process of expansion by chemical agents.

 These methods, however, make it possible to obtain only parasites which do not exceed 70%.

 It is not currently known a process for expanding a material after its implementation, rather generally.

 The process according to the invention aims to remedy the aforementioned drawbacks and to obtain porosities that can reach or even exceed 90 k in certain materials after their implementation, in a very general way, since it does not take or little account of the conditions of implementation t pressure, temperature, etc.

materials, and that, initially, the judicious choice of the pore-forming agent allows the expansion of a wide range of material.

This process mainly involves, as a porous agent, a solid material having the following characteristics:
chemically compatible with the plastic or the polymer that is used.

 - Resistant in the conditions of implementation of the plastic or polymer (temperature, pressure, flow, etc ...).

 - Extractable, ie soluble in a solvent that does not alter the properties of the expanded material.

 This blowing agent can belong to different families of chemical compounds such as, for example, organic carboxylic acids and their salts, alcohols or polyols, amides, amines, ofitones, esters. .., mineral or organic salts.

The list of these products is not exhaustive because it suffices that the pore-producing agent that one wishes to use is solid and has the characteristics described above.

 This blowing agent is introduced into the plastic material or the polymer by a suitable method, before the use of this material or this polymer. The compound thus prepared can then be extruded, calendered, injected, molded, thermoformed. by techniques very close to those currently known to those skilled in the art.

 The finished product is then extracted with a suitable solvent to remove the soluble pore-forming agent without altering the chemical structure or altering the properties of the plastic or base polymer.

 After drying, it is easy to obtain po reux materials whose porosity can reach values greater than 90%.

This method is applicable, in principle, to all polymeric materials currently known and more particularly in the case where the conventional methods of expansion are not suitable or do not lead to interesting results.
For example, this process is particularly well suited for obtaining polytetrafluoroethylene (PTFE) foams.

 In this case, the blowing agent used is a salt of alkali or alkaline earth metals and preferably, for economic reasons, sodium chloroze.

 This inert compound, with respect to the PTPE, perfectly fulfills the conditions imposed by the process since it withstands the erasure times and the operating pressures of this polymer and is easily extractable with water, a solvent which is also inert with respect to of the polymer.

 The examples detailed below will clearly demonstrate to one skilled in the art how easy the process is to work and the outstanding results obtained.

The desired porosity or density can be easily calculated before the elaboration of the material by knowing the specific densities of the materials used as well as the volumes respectively occupied by each of them: Porosity% = (I - ####) x 100 V = volume occupied by PTFE
V '= volume occupied by the
sodium chloride
The porosity Q density and, for example, the dielectric constant of the foamed material can also be connected by means of the formulas and diagrams given in the accompanying figures showing:
FIG. 1 shows the formula giving the porosity P as a function of the density d of the expanded material, and its graphical representation for PTFE, whose density d = 2.2.

FIG. 2 is the formula giving the dielectric constant of the expanded material as a function of the porosity P, and its graphical representation for PTFE whose dielectric constant, I
- The figure T s the formula giving the dielectric constant of the expanded material as a function of its density d and the dielectric constant # of the unexpanded material, and its graphical representation for PTFE whose dielectric constant # = 2.1
From a general point of view, the implementation of the process according to the invention for obtaining cellular PTFE is as follows:
The PTFE powder to be molded or extruded is sieved to a particle size suitable for the intended application.

This particle size is generally related to that of sodium chloride
The same procedure is followed for sodium chloride. In some cases, it will be preferable to remove the small diameter particles for other applications. On the contrary, it is preferable to use the fines.

 The P.T.F.E. powders were then weighed separately. and sodium chloride so that the respective masses m and m 'correspond to the desired porosity for the material.

For example, for powders with a particle size of less than 600 microns, the following approximate formula can be used:
P = 220 m P = porosity in%
1, 26 m + 2.2 m '= mass of TTP

 m '= mass of sodium chloride.

 Powders of P.T.F.E. and sodium chloride are then intimately mixed and, in the case of powders to be extruded, is carried out after mixing with a lubricant addition.

The compounds thus prepared can then be, depending on the case, compression molded or extruded.
The preforms, tubes, rods, sections ... thus produced are then brought to a temperature at least greater than 3270 C, which corresponds to the PTFE sintering temperature.

 The materials thus prepared are then subjected to washing with hot water, the purpose of which is to remove the sodium chloride.

The progress of sodium chloride extraction can then be monitored by conventional methods for determining the presence of chloride ions in residual waters.
When all the salt has been removed, the materials are then dried in an oven at temperatures ranging from 300 C to 2600 C.

 The porous materials obtained have an open cell structure. The size of the cells is determined by the granulometeite of the blowing agent used. It may be noted that the anime type of process is applicable to aqueous dispersions.

 It is thus possible to obtain materials with miorooellular structures or, conversely, open-cell materials of large sizes, depending on the type of applications that are used.

 When very fine porosity is required, a conventional dispersion based process of P.T.F.E. If it is a question of coating an object, it is sufficient to dip it into the dispersion, then to sinter it after having withdrawn it, so as to freeze the P.T.F.E .; it is then sufficient to proceed, as indicated above, to the extraction of the po rogne agent. The dimension of the layers thus deposited can be reduced to about 1/10 of a millimeter in thickness.

 The materials obtained possess, whatever their porosity, the chemical and thermal properties of P.T.F.E. The electrical properties, on the other hand, are improved very sensitively since the dielectric constant can be divided by factors up to 2 and the loss angle by 10.

Due to their exceptional properties, these materials can be used in many fields
because of their great chemical inertness and the manufacture of filtration and ultrafiltration membranes, materials intended for gaseous diffusion, diaphragm seals, etc.

 - because of their thermal and mechanical properties for the manufacture of soft coatings, insulation materials, special joints, etc ...

 - because of their compatibility with biological systems (biocompatibility) for the manufacture of veins and arteries, implants and inserts.

 because of their electrical properties and for the manufacture of porous insulators with a low dielectric constant intended, for example, for the production of low loss microwave coaxial cables.

The losses, in a microwave coaxial cable are, in fact, directly proportional to the square root of the dielectric constant and the angle of loss or tangent
The reduction of the dielectric constant and the loss angle of the insulator therefore leads to a significant reduction in the losses in a coaxial cable, and all the more so since the frequency of use is high. This is why the addition of air (dielectric constant: # = 1, loss angle tg "'''o) in PTFL (dielectric constant # = 2.1, loss angle tg # = 2.10 -4, can significantly improve the electrical properties of the latter and thus allows the manufacture of high performance microwave cables.

By way of example, it is possible, thanks to the method of the invention, to manufacture cellular PTFE insulators having dielectric constants of the order of I, I and loss angles of the order of
EXAMPLE I Preparation of an Extruded PTFE / Sodium Chloride Mixture and Conductive Coating by Extrusion

 The preparation of P.T.F.E./NaCl compound is preferably carried out in a clean and dry place, maintained at a temperature of less than 180 ° C. The powder of P.T.F.E. The extrusion is sieved to a maximum particle size of 400 microns. Sodium chloride is screened separately at a particle size of about 600 microns.

 The two powders are laid separately and their respective masses are calculated so as to obtain the desired final porosity.

 For example, for a porosity of 75% and for 1 kg of total mixture, 360 g of P.T.F.E. and 640 g of Nazi.

 The powder of P.T.F.E. and the sodium chloride powder is then poured successively into a cylindrical glass vessel and then mixed on a roller mixer for 5 minutes. The mixture is then homogenized by three successive sieves through a 600 micron sieve.

 140 g of a lubricant based on isoparaffinic compounds of low molecular weight are then added to the mixture. The mixture is then stirred as before for 10 minutes and sieved again three times. After progressive heating above I80 C, the compound is then ready for use.

 A cylindrical preform is then conventionally produced by compressing the compound in a tube of the diameter of the extruder barrel. This cylindrical preform has a caviez also cylindrical in its center, so as to allow the passage of the conductor. The pressures applied and the time required to produce the preforms depend essentially on the outside diameters of these preforms as well as on the exact composition of the compounds.

 The pressures vary between 2 and 15 bar, the preforming times between 5 and 60 minutes.

 In the particular case envisaged above, for a diameter of 30 mm, the pressure and the preforming time are respectively 7 bars and 30 minutes.

 The preform once demolded is introduced into a special extruder for the extrusion of P.T.F.E.

 The diameter of the cylinder that is used is directly darken the diameter of the conductor that one wishes to isolate and the diameter of the electric di-which one wishes to obtain, that is to say in reality, the reduction ratio usually used for the compound that is extruded.

 This reduction ratio, well known to those skilled in the art, sarie considerable manner depending on the composition of the compound. In general, for porosities ranging between 30 and 95%, this ratio varies between about 10 and about 1000.

 In the particular case envisaged above, it is possible to use indifferently 20 or 30 mm cylinder extruders; this for respective diameters of conductors and dielectrics varying between 0.80 and 1.50 mm on the one hand, for the conductor, and 2.5 and 4.5 mm on the other hand for the dielectric.

 The extrusion pressures naturally depend on the diameter of the cylinder used, the diameter of the nozzles and, therefore, the reduction ratio. These pressures also depend on the level of lubricant used. In general, they vary between 10 and 200 bar.

 In the particular case envisaged above, the extrusion pressures vary for a cylinder 20 mm, depending on the lubrication rate, between 20 and 70 bar and for a cylinder> 30 ml, between 40 and 200 bar.

 The conductor coated with its dielectric passes, at the output of the extruder successively in an evaporation oven (temperature between 150 and 200 C) a pre-sintering furnace (200 <t <300 C) and a sintering furnace (330 < t <370 C).

 The temperatures of the various furnaces are obviously adjusted according to the speed of extrusion.

 The sintered cable is cooled, then subjected to a process of extraction of sodium chloride which can be either a continuous extraction process by hot water against the current, or a process of extraction by successive baths in an autoolave under pressure.

 Advancement of "desalination" can be controlled by chemical clarification of the presence of chloride ions in the residual waters.

 After removing all traces of sodium chloride, the cable is oven-dried at temperatures which can vary between 50 and 2500 C. The drying time obviously depends on the temperature and the mass of P.T.F.E. cell to dry.

 In the particular case described above a preform kg kg of compound extruded in a 30 mm cylinder on a conductor # = 1.50 mm, can give a dielectric 4 mm in diameter that can prepare 30 meters of insulated cable.

 In this case, the extrusion pressure is close to I80 bar for a twist speed of Im / min.

 The desalting operation is carried out either continuously, or against current, or in four successive baths of 8 liters of demineralized water in an autoclave of 15 liters and this at temperatures of the order of 1000 C.

 After drying for 8 hours at 800 C, the cable is checked for porosity.

Control methods may involve measuring the density or measuring the electrical constant
In the example cited above, the dielectric-obtained constant is # = 1.24 and the density d = 0.56.

 Example 2 Preparation of a rod or profile or sheath by extrusion of a compound P.T.F.E./ sodium chloride.

 The compound is prepared as before (Example I). For a porosity of 90%, for example, and for 1 kg total mixing, 162.5 g of powder of P.T.F.E. and 837.5 g of sodium chloride. After mixing and sieving, I60 g of lubricant is added to the mixture. The operation is then carried out as above, except that the mixture is extruded without a conductor through a filigree of appropriate shape.

 It is thus possible to make rods, ducts and profiles of all kinds.

 The extraction of sodium chloride after sintering the materials in a ventilated oven is carried out as above.

 The measurement of the density (d - 0.2I) makes it possible to check the homogeneity of the starting compound.

Example 3 s Making a ribbon from a compound
PTFE / sodium chloride0
Obtaining PTFE foam tapes is a simple operation.

 From the manufacture of rods described in Example 2, at the extruder outlet, the rods are rolled to suitable thicknesses and then sintered in a tunnel oven. The ribbons obtained are extracted, as before, by desalting and then drying.

 These ribbons may subsequently be ribboned onto conductors to provide insulators with low electrical constant.

Example 4 Manufacture of Foam PTFE Sheets
In this case, the preparation of the PTFE / sodium chloride compound differs slightly from that mentioned in Example I, in that, on the one hand, the mixture does not necessarily require the presence of a lubricating agent and that on the other hand, the special powder for extrusion can be replaced by PTFE powder

to mold.

 After mixing and sieving the compound (calculated to obtain the desired final porosity), the powder is placed in a mold of suitable shape, and then compressed at pressures ranging between 100 and 350 bar.

 This operation is followed by sintering of the preform at temperatures that vary according to the thickness of the plates, but which must be greater than 340 ° C.

 As before, after cooling, the sodium chloride is removed by washing with water.

 EXAMPLE 5 The fabrications described in the preceding examples are possible with other porogenic agents than sodium chloride. In particular, it is possible to use chlorides, fluorides or bromides of alkali or alkaline earth metals, or combinations thereof.

 EXAMPLE 6 To manufacture the materials described above, it is also possible to use other porogenic agents than inorganic salts provided that these agents are compatible with PTFB, withstand the processing temperatures of the materials and are soluble in inert solvents. screw to PTF3.

 Example 7 In the above examples, P.T.F.E. can be replaced by any other polymer compatible with sodium chloride, the inorganic salts described in Example 5 or other blowing agents, as indicated in Example 6.

Claims (9)

 I. Foams of polymeric materials and process for obtaining them, characterized in that the blowing agent used is a solid material extractable by dissolving after use.  Foams of polymeric materials and process for preparing them according to claim 1, characterized in that the blowing agent used is an alkali metal or alkaline earth metal salt.  3. Foams of polymeric materials and process for obtaining them, according to claim 2, characterized in that the salt used is sodium chloride.  4. Foams of polymeric materials and process for obtaining them, according to claims 1 and 2, characterized in that the solvent used to extract the pore-forming agent is water.  5. Moussea of polymeric materials and process for obtaining them, according to claim 1, characterized in that the solid material used as pore-forming agent and the polymeric material are reduced to a powder of appropriate particle size, and then mixed, in proportions corresponding to the sought porosity, to form a compound.  6. Foams of polymeric materials and process for obtaining them, according to claim 5, characterized in that the compound is extruded, calendered, injected, molded or thermoformed.  7. Foams of polymeric materials and process for obtaining them according to claim 5, characterized in that the mixing is carried out by means of an aqueous dispersion saturated with porogenic agent, which may be deposited on a support by dipping or spray.  8. Foams of polymeric materials and process for obtaining them, according to claims 1 and 4, characterized in that the porogenic material is removed by washing with hot water.  9. Foams of polymeric materials and process for obtaining them, according to claim 1, characterized in that they are dried in an oven at a temperature ranging from 300 ° C. to 2600 ° C., after extraction of the pore-forming agent.  IO. Foams of polymeric materials and process for obtaining them, according to revendlations I, 3 and 4, characterized in that removal of the soluble pore-forming agent is controlled by searching for the presence of chloride ions in the waste water.