IE42330B1

IE42330B1 - Process for inducting the formation of microcavities in solid polymeric materials

Info

Publication number: IE42330B1
Application number: IE778/75A
Authority: IE
Original assignee: Consiglio Nazionale Ricerche
Priority date: 1974-04-08
Filing date: 1975-04-08
Publication date: 1980-07-16
Also published as: IT1018499B; LU72232A1; FR2266716B3; NL7503891A; DE2514860A1; IL47038A; IE42330L; FR2266716A1; JPS50148464A; BE827640A; GB1507345A; IL47038A0

Description

The present invention relates to processes for inducing the formation of microcavities in solid polymeric materials.

According to one aspect of the present invention there is provided a process for inducing the formation of microcavities in solid polymeric materials so as to transform said polymeric materials, which are themselves not permeable to liquids or gases, into permeable polymeric products, said process comprising forming a film of an amorphous polymeric material which is acrylonitrile-butadiene-styrene copolymer, polycarbonate or polystyrene and subjecting said film to tensile stress at a temperature not greater than the glass transition temperature of the polymeric material to induce the formation of microcavities and to produce a polymeric product which is permeable to liquids and gases.

The polymers may be subjected to attack b'·' nt least one solvent capable of inducing localised stress patterns.

Preferably, the tensile stress on the polymeric material is applied by mounting it on a support the dimensions of which are different from those of the polymeric material in its natural unstretched condition.

If acrylonitrile-butadiene-styrene copolymer (ABS), polycarbonate (PC), or polystyrene (PS) are subjected to tension at a temperature, in the environment in which the operation is carried out, less than the glass transition temperature of said materials, microcavities (crazes) form -242330 which tend to originate in proximity to all those zones in which, for various reasons there is a concentration of forces .

J These microcavities or crazes, which hitherto constituted a defect and led to limitations in the use of the aforementioned material are here utilised positively in that they are suitable for transforming the materials, which are themselves impermeable to fluids, into permeable or semipermeable materials.

Finely divided inert or filler materials may be included as microinclusions in the polymeric materials for the purpose of making the formation of microcavities more uniform and for controlling the termination phenomenon of the microcavities. Microscope analysis has indicated that when such a finely divided filler material is used practically every microcavity terminates at a discontinuity; that is an interface between the polymeric material and a particle of the filler material.

By means of the treatments indicated above, microcavities may be obtained, the diameters of which are dependent on the type of polymeric material used, (usually lying between 20 and 3000A), and the length of which are dependent on the applied force and percentage of microinclusions.

The presence of microinclusions (for example glass microspheres) or crystalline hydrated aluminium silicates), which in themselves are not essential for creating the desired permeability -342330 is useful for improving the mechanical properties of the treated polymer (for example by increasing the modulus ' of elasticity or reducing the coefficient of thermal expansion) and also for controlling the formation and distribution of the microcavities in it.

The preparation of the basic material, which as stated consists preferably of a thermosplastic polymer membrane, can be carried out by known methods such as lamination or extrusion, or rapid evaporation of a solution of the polymer in known solvents spread on a perfectly flat support, then immersing the support in water.

Permeable membranes obtained by the process of the invention may be advantageously employed in numerous separation processes presently used in various fields, ’such as desalination, purification of polluted waters, separation operations in the food industry, in bioengineering etc. Furthermore, particularly if constructed in tubular form, they have wide possible application in all those processes in which the distribution of liquid or gas in as fine and homogeneous a form as possible is necessary, such as the distribution of water in soil, oxygenation and/or chlorination of liquids etc.

The invention will be more particularly described below in the following non-limiting examples, in which ABS, PC, and PS are used as starting polymeric materials, and glass microspheres having diameters in the region of 34-44 microns, or hydrated crystalline aluminium silicates with ion exchange properties, are used as the inert material. -44 2330 EXAMPLE 1 300 g of ABS in granulated form are poured on to mixer rollers heated to about 180°C and, when the polymer has formed a uniform film, 100 g of glass microspheres having a diameter of about 34 - 44 microns are added, the microspheres corresponding to 12% of the total volume of the mixture. After mixing for 20 minutes, the film is removed still hot from the rollers and subjected to tensile stress, up to 1800 lb/ft if working at 2 ambient temperatures or up to 400 lb/ft if working at a temperature close to the glass transition temperature (100°C) of ABS, which is sufficient to induce the required microcavities in the film structure.

Water permeability measurements indicate that while the material in its non-deformed state has no permeability either to liquids or to gases, the same material after the mechanical treatment has a 3 2 permeability of 1 X 10 cm /sec cm under an effective pressure of 36 atm.

EXAMPLE 2 g of ABS (glass transition temperature about 100°C) and 10 g of glass microspheres having a diameter not exceeding 44 microns are dissolved in a solvent consisting of 200 cc of methylene chloride and 300 cc of methylethylketone. The solution so obtained is spread at ambient -542330 temperature over an extremely flat surface for example a sheet of glass, and after a short period of evaporation time the glass sheet is immersed in water. Tensile stress is then applied to the film so obtained, as stated in example 1. Permeability measurements give resultsanalogous to those indicated in example 1.

EXAMPLE 3 Polystyrene membranes were prepared from a solution of 10 g of polystyrene (glass transition temperature about 100°C), 90 Cc of methylethylketone and 60 cc of methylene chloride by the method described in example 2. These membranes were prepared both with microinclusions as the inert material (approximately 37% by volume) and without microinclusions. Such membranes show no perme15 ability when an effective pressure difference of 6 atm is applied across them nor do they show any fissures under the microscope. The same membranes under an effective pressure difference of 40 atm, which effects the mechanical stressing of the membrane, show however a significant permeability to water ( 5 X 10 cm /sec cm for the membrane without microinclusions) at 20°C. Membranes stressed in this way show?microfissures under the microscope.

EXAMPLE 4 Polycarbonate membranes were prepared from a solution of 10 g of polycarbonate (glass transition temperature -642330 about 120°C), 100 cc of tetrahydrofuran and 100 cc of ethylenedioxan by the method described in example 2, and tensile stress is applied as in example 1. These membranes were prepared both with microinclusions (approximately 10% by volume) and without microinclusions. Membranes of 20 micron thickness have no permeability under an effective stressing pressure of 40 atm at 20°C.

However if this test is carried out at 70°C, the same -4 3 membranes have a permeability in the order of 2 X 10 cm / o sec cm . In the latter case, microfissures are observed under the microscope.

In the above examples, it should be noted that if the temperature T is increased the permeability increases when TTg it tends towards zero, where Tg is the glass transition temperature. The microfissures dissappear when T>Tg.

Claims

1. WHAT WE CLAIM IS:1. A process for inducing the formation of microcavities in solid polymeric materials so as to transform said polymeric materials, which are themselves not permeable 5 to liquids or gases, into permeable polymeric products, said process comprising forming a film of an amorphous polymeric material which is acrylonitrile-butadiene-styrene copolymer, polycabonate or polystyrene and subjecting said film to tensile stress at a temperature not greater than the 10 glass transition temperature of the polymeric material to induce the formation of microcavities and to produce a polymeric product which is permeable to liquids and gases.

2. A process, as claimed in claim 1 wherein finely divided inert materials are included in the polymer. 15

3. A process as claimed in claim 2, wherein glass microspheres are included in the polymer.

4. A process as claimed in any preceding claim, wherein finely divided fillers are included in the polymer.

5. A process as claimed in claim 1 and 4, wherein a 20 crystalline hydrated aluminium silicate is included in the polymer. -842330

6. A process as claimed in any preceding claim wherein the tensile stress on the polymeric material is applied by mounting it on a support the dimensions of which are different from those of the polymeric material in 5 its natural unstretched condition.

7. A process as claimed in claim 1 and substantially as hereinbefore described with reference to any one of examples 1 to 4.

8. A polymeric product obtained by a process as io claimed in any preceding claims.