GB2026935A - Porous threads - Google Patents

Abstract

A mixture comprising at least two components of which one is a fusible polymer and the other is a liquid which is inert towards the polymer, the two components being such as to form a binary system having in the liquid state, a region in which they are totally miscible and a region in which there is a discontinuity in mixing, is extruded at above the critical temperature of the binary system into a bath of the inert liquid component having a temperature below the critical temperature and the thread structure formed is solidified. The inert liquid may be washed out by a solvent eg acetone. The fusible polymer may be polypropylene and the inert liquid N, N-bis-(2-hydroxyethyl)- hexadecylamine.

Description

SPECIFICATION Porous threads The present invention relates to porous threads, a method of producing them and their use, in particular, as a support substance for active ingredients.

It has already been known for a long time to produce porous threads. In many cases, these threads are designated as foam threads.

A large number of methods for obtaining threads having a porous structure is already known. Thus, East German Patent No. 103 375 describes a method in which a polymer melt consisting of one or more components, preferably components which are incompatible with each other, is extruded with the addition of physical and/or chemical blowing agents through one or more rows of holes in spinnerets with round, round and hollow or profiled and hollow spinneret holes. These threads can be deposited so as to form non-woven fabrics by suitable processing.

The threads with cavities known hitherto have various disadvantages, and several methods thus lead to threads which only have cavities in the interior and which have a completely closed surface or possible a surface having a slight permeability. Other threads whose surfaces exhibit opening are, however, produced with a surface which is rough, thus rendering them unsuitable for many uses.

There is thus a need for improved methods of production, in particular to provide methods by which the spinning composition to be extruded can be produced simply and which operate without complicated spinning baths. There is also a need for improved threads which are porous, having openings on their surface but nevertheless have a smooth surface structure.

An object of the invention is therefore to provide a method by which polymers can be shaped in a simple manner in an extrudable spinning composition and which simultaneously allows extrusion and allows the extruded material to be solidified without complicated spinning techniques and spinning baths having to be used. Another object of the invention is to provide a method which allows threads having adjustable porosity to be obtained merely by varying the operating parameters. An object of the invention is also to provide porous threads which have a desirable open surface relative to the known foam threads. A further object of the invention is to pro videthreadswhich can be used in the textile field as well as in industrial and other spheres and which are particularly suitable as a support substrate forcer- tain active ingredients.

According to the present invention there is provided a method of producing porous threads in which a homogeneous mixture is extruded, the homogeneous mixture comprising at least two components, of which one component is a fusible polymer and the other component is a liquid which is inert towards the polymer as (hereinafter defined), the two components forming a binary system, which has, in the liquid state, a region in which the components are completely miscible and a region in which there is a discontinuity in, the homogeneous mixture being extruded at a temperature above the critical temperature of the binary system in to a bath containing said other inert liquid component of the homogeneous mixture and which is at a temperature below said critical temperature, and the thread structure so formed is solidified.

The fibre structure formed can be washed out using a solvent after solidification, acetone being particularly useful for this purpose.

It is advantageous if an air gap is included between the outlet surface of the extrusion tool and the surface of the bath. This air gap can be heated.

It is also possible to extrude the homogeneous mixture directly into the bath.

A bath having a staggered temperature is used in a particular embodiment of the method according to the invention. In that embodiment, the bath can comprise one or more parts which have a temperature gradient in such a way that the temperature increases continuously from the beginning of the spinning bath to the outlet end.

It is also possible to use two or more separate baths which each have a different temperature.

It has proved advantageous if the bath is at a temperature which is at least 100"C lower than the critical temperature of the binary composition used.

Homogeneous mixtures of from 10 to 90% by weight of polymer and from 90 to 100/o by weight of inert liquid can be extruded according to the invention.

The homogeneous mixture can also initially be extruded into a spinning tube connected upstream of the bath and filled with the bath liquid.

Polypropylene is preferably used as polymer and NN - bis - (2 - hydroxyethyl) - hexadecylamine is preferably used as inert liquid.

The two components, namely the molten polymer and the inert liquid, are preferably mixed continuously prior to extrusion, in which process it is advantageous for mixing only to take place immediately prior to extrusion. The mixture can still be homogenised prior to extrusion. A pin mixer is particularly suitable for mixing purposes.

According to another aspect of the present invention there is provided porous threads comprising a synthetic polymer having from 10 to 90 /O by volume of pores which communicate with each other and a smooth surface structure having open pores, wherein the proportion of openings in the surface thereof amounts to from 10 to 900/c. Preferably the apparent density of the threads lies between about 10 to 90 /O of the actual density of the polymer used.

The threads can be used as a support substrate, i.e. the threads can be impregnated with specific substances which are given off at a subsequent moment. They can also be used as insulating material.

It is possible to use, in particular, fibre-forming macromolecularsubstances which are known per se, particularly synthetic polymers which are obtained, for example, by polymerisation, polyaddition or polycondensation, for carrying out the method according to the invention and for the production of threads according to the invention. The polymer must be fusible, i.e., it must be able to pass into the liquid state without decomposing and form a binary system with a liquid which is inert towards it, which system having a range of complete miscibil ity in the liquid state and also a range in the liquid state with mixing gaps.

Systems of this type have a phase diagram forthe liquid state of, for example, the type shown for the aniline/hexane system on page 724 of the Text Book of physical Chemistry by S. Glasstone, Macmillian and Co. Ltd, St. Martins' Street, London, 1953. This diagram indicates complete miscibility for the two components above the arched curve. Two liquid phases are in equilibrium with each other beneath the curve.

It is not absolutely essential for carrying out the invention for each of the two components in the two-phase range still to exhibit considerably solubility relative to the other components, as is the case in the above-mentioned graph. In many cases, it is suf ficientto have marginal solubility in the liquid twophase domain. It is however important for the two components in the liquid state to form two liquid phases next to each other. In this respect, the systems which can be used according to the invention differ from those systems in which the dissolved polymer is precipitated directly as a solid when the temperature is lower without firstly passing through the liquid state during the cooling treatment.

Conventional fusible polymers can be used within the scope of the invention, such as, for example the polymers obtained by polymerisation, such as, polyethylene, polypropylene, polyvinylchloride, polyacrylates, polycaprolactam as well as corresponding copolymers among others; polycondensation polymers, such as, polyethylene terephthalate, polybutylene terephthalate, polyamide 6,6 polyphenylene oxide, and poly addition polymers, such as, polyurethanes and polyureas.

All those liquids which form a binary system of the above-mentioned type with the polymer in the liquid state are basically suitable as inert liquid within the scope of the invention. By the phrase "inert towards the polymer" as used herein, is meant that the liquid does not cause considerable decomposition of the polymer within a short period or does not react with the polymer itself.

Although the above-mentioned phase diagram of the state of the aniline/hexane system shows the ratios for a binary mixture which consists of two uniform substances which are themselves essentially pure, the term binary system should not be applied strictly, in the context of the invention, to mixtures of only two pure uniform substances. The average skilled man knows that a polymer substance is composed of a plurality of molecules of differing molecular weight, and these polymers with a cor- responding molecular weight distribution are therefore to be considered within the scope of the invention as one component, and the same applies to mixed polymers.Under certain circumstances, polymer mixtures can even behave as a uniform component and form a single phase mixture with an inert solvent and separate only into two liquid phases below a critical temperature. It is preferable, however, to use only one polymer. The liquid need not necessarily be completely pure and represent a completely uniform substance. In many cases, it does not matter if relatively small quantities of impurities and possibly also proportions of homogeneous compounds of the type which are necessitated by industrial production are added.

In order to carry out the method in practice, a homogeneous mixture is produced from the two components at the required temperatures. This can be effected by mixing the inert liquids with the comminuted polymer heating ittosuitabletempera- tures, ensuring that mixing is sufficiently thorough.

Anothersuitalble method involves bringing the two componentsto the required temperature sepa- rately and mixing the two components togetheFcoii- tinuously in the desired proportion just before extrusion. This mixing can be carried out in a pin mixer which is advantageously arranged between the metering pumps for the individual components and the spinning pump. Subsequent homogenisation may be advisable.

In many cases, it is advisable to aerate the homogeneous mixture by applying a suitablevac- uum before extrusion.

The ratio of the polymer to inert liquid in the spinning composition can be varied within wide limits.

The internal pore volume and also the surface texture as well as the number of open pores on the surfaces of the hollow threads obtained, can be controlled to a large extent by adjusting the ratio of polymer to inert liquid. Porous threads which are suitable for a very wide range of applications can thus be obtained.

It is generally sufficient for the temperature of the homogeneous mixture prior to extrusion to lie only a few degrees above the critical temperature or above the disintegration temperature, depending on the respective composition.

Interesting effects with regard to the texture of the threads obtained can however also be achieved by increasing the difference between the temperature of the homogeneous mixture to be extruded and the critical temperature.

The homogeneous spinning composition is then extruded into a bath containing the inert liquid of the extruded component mixture and having a temperature below the critical temperature. The bath preferably consists completely or mainly of the inert liquid which is also present in the extruded mixture. The temperature of the bath lies below the critical temperature of the mixture used, i.e. below the temperature above which the two components can be mixed together completely homogeneously. The temperature of the bath preferably lies at least 100"C below the critical temperature of the mixture used.

The temperature can also be sufficiently low to progress in the range in which a solid phase occurs according to the phase diagram relating to the binary system.

Once the temperature of the bath is sufficiently high that it continues progressing in the liquid two phase region, then it is necessary to solidify the thread structure formed as soon as possible, which can be effected by reducing the temperature accord ingly after a certain interval inside the bath.

It is important for the extruded mixture still to be single-phase before it enters the bath, i.e. for substantially no further disintegration into two phases to take place.

It has proven advantageous to arrange in front, upstream end of the bath in certain cases a spinning tube which is also filled with the bath liquid and the end of which is submerged in the spinning bath. The spinning tube can have a conventional spinning funnel at its inlet opening, and the tube can be curved at its lower end in order to make it easier to take off the thread through the bath.

The spinning tube can be filled by means of a level container surrounding the spinning tube as a result of overflow into the spinning tube. In order to fill the spinning tube completely and to maintain the level in it, it is necessary to supply more bath liquid to this level container from a main reservoirthan flows out through the spinning tube. The excess bath liquid can be recirculated via a second overflow on the level container into the main reservoir. The main reservoir and level container can be controlled thermostatically.

Once it has passed from the spinning bath, the thread can be washed out with a suitable extraction agent. A number of solvents such as, for example, acetone, cyclohexanone and ethanol as well as mixtures of these liquids, are suitable for extraction.

In some cases, it is not necessary to wash out the thread, particularly when the actual inert liquid used is to impart to the thread extra properties which are designed for its subsequent application or is even to perform a function. Thus, for example, it is possible to use liquids which exert an anti-static effect on the thread or which act as lubricants. It has proven advantageous for a number of applications to maintain an air gap between the outlet surface of the extrusion tool, i.e. the outlet surface, for example, of a corresponding thread nozzle and the surface of the bath. By varying the air gap, it is possible to influ ence the structure of the thread obtained, and in particular, it surface.

It has been found that the number of open pores can be reduced by extending the air gap and can be increased by shortening it. The diameter of the pores also decreases as the air gap increases.

The air gap can be heated, preferably to a temperature above the critical temperature of the extruded mixture.

The air gap is generally at least 1 mm wide and can assume a length of up to about 10 cm depending on the operating conditions. It is important to prevent any disintegration or at least any noticeable disintegration into two liquid phases in the air gap before entry into the bath. As stated, this can be controlled by the shortness of the interval or by heating, but it is however also possible to counteract premature disintegration by increasing outlet speed at the nozzle.

In a particular embodiment of the method according to the invention, however, the homogeneous mixture is extruded directly into the bath, open pores having a maximum diameter being formed at the surface.

Owing to their particular surface structure and the structure inside the threads, they are also eminently suitable as a substrate for certain substances. Thus, the threads can be impregnated with anti-static agents which are already in use as an inert liquid during the spinning process or the agent can be introduced into the thread structure only subsequently by treating it, for example, by impregnation, after production of the thread.

In this way, it is possible to obtain compositions having a long term effect, which slowly give off the absorbed active ingredient again. Conversely, the threads can also be used for the adsorption of substances.

The threads can also be used in a wide variety of textile applications, for example, they can be processed into sheets, such as, woven or non-woven fabrics. Another field of application is insulation, such as, for example, thermal insulation and sound-proofing.

The pores in the threads according to the invention can be of various shapes. Thus, they can be rounded or elongated and can be interconnected, in some cases by small connecting passages and in other cases due to them passing directly into each other. Even in threads which have been obtained from mixtures having a polymer content of about 300/c, the polymer can be the matrix, in which the individual pores are distributed and form fairly discrete, but interconnected cavities. Conversely, it is also possible to produce structures in which the cavities form the matrix as with non-woven fabrics and the polymer substance is arranged in an almost fibrillar configuration. The transitions between these two structures are not strongly defined and are thus partly mixed.The structural forms can also be influenced by other operating parameters such as, for example, take-off speed, cooling speed, draft below the nozzle.

An apparatus which is suitable for the production of the threads according to the invention is illustrated in more detail in the accompanying drawing.

The numeral 1 refers to a container which can be thermostatically controlled, from which inert liquid is metered by means of a double piston pump 3 and another heater 4 into the mixer 8. The heater 2 provides preliminary heating. Polypropylene passes from the chip container 5 via an extruder 6 and a gear pump 7 into the mixer8, from which a nozzle 10 is supplied via a gear pump 9. The issuing mass passes via an air gap into a spinning tube 12 which is provided with a spinning funnel 11 and is provided .with inert liquid via a level container 13 from the main reservoir 14. The spinning tube has a curve at its lower end and the threads are conveyed to a winder 16 after leaving a bath 15.

The invention is illustrated in more detail by the following Example: Polypropylene chips are melted in an extruder and metered into a pin mixer using a gear pump.

Liquid NN - bis - (2 - hydroxyethyl) - hexadecylamine at a temperature of 40"C is simultaneously pumped from a storage container which can be heated, via a double piston pump into an electrically heated heater and is conveyed from there into the mixer at a temperature of about 1500C. A pin mixer is used as a mixer.

After homogenisation of the two components, the melt is pressed through a spinneret by a metering pump and extruded into a spinning bath which con sists of pure NN - bis - (2 - hydroxyethyl) - hex adecylamine and has a temperature of 50"C.

Once the thread has passed through the spinning bath, having a length of 50 cm, the thread obtained is extracted over ethanol and dried. A porous thread having excellent properties and a particularly good surface structure is obtained.

Claims (22)

1. A method of producing porous threads, in which a homogeneous mixture is extended, the homogeneous mixture comprising at least two components, of which one component is a fusible polymer and the other component is a liquid which is inert towards the polymer (as herein defined), the two components forming a binary system which has in the liquid state a region in which the components are completely miscible and a region in which there is a discontinuity in mixing, homogeneous mixture being extended at a temperature above the critical temperature of the binary system into a bath containing said other, inert liquid component of the homogeneous mixture and which is at a temperature below said critical temperature, and the thread structure so formed is solidified.

2. A method according to claim 1, wherein the thread structure formed is washed-out with a solvent after solidification.

3. A method according to claim 2, wherein said solvent comprises acetone.

4. A method according to any of claims 1,2 or 3, wherein the mixture is extruded by means of an extrusion tool, an air gap existing between the outlet surface of the extrusion tool and the surface of the bath.

5. A method according to claim 4, wherein the air gap is heated.

6. A method according to any of claims 1 to 3 wherein the homogeneous mixture is extruded directly into the bath.

7. A method according to any of claims 1 to 6 wherein the temperature of the bath lies at least 100"C below the critical temperature of the binary system.

8. A method according to any of claims 1 to 7 wherein the homogeneous mixture is initially extruded into a spinning tube arranged upstream of the bath and filled with the inert liquid in the bath.

9. A method according to any of claims 1 to 8 wherein said fusible polymer comprises polypropylene.

10. A method according to any of claims 1 to 9 wherein the inert liquid comprises NN - bis - (2 hydroxyethyl) - hexadecylamine.

11. A method according to any of claims 1 to 10 wherein the temperature in the bath is staggered.

12. A method according to any of claims 1 toll wherein the homogeneous mixture consists of from 10 to 90 /O by weight of polymer and from 90 to 10% by weight of inert liquid.

13. A method according to any of claims 1 to 12 wherein the two components are mixed continu ously before extrusion.

14. A method according to claims 13, wherein a pin mixer is used for mixing purposes.

15. A method of producing porous threads, substantially as herein described with reference to the accompanying drawing.

16. Porous threads when produced by a method according to any of claims 1 to 14.

17. Porous threads comprising a synthetic polymer having from lOto 90 /O by volume of pores which communicate with each other and a smooth surface structure having open pores, wherein the proportion of openings in the surface thereof amounts to from lOto 900/c.

18. Porous threads according to the claim 17, wherein an apparent density of the threads is from 10 to 90 /O of the actual density of the polymers

19. Use of the porous threads according to any one of claims 16, 17 or 18 as support substrate.

20. Use of the porous threads according to any one of claims 16, 17 or 18 as insulating material.

21. Asupportsubstrate comprising porous threads according to any one of claims 15, 16 or 17.

22. An insulating material comprising porous thread according to any one of claims 15, 16 or 17.