CA1099469A - Sheet of precipitated polymer material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A paper with textile-like characteristics is provided which is expected to be useful for sanitary, medical, surgical and veterinarian purposes. The sheet consists of at least two layers, each consisting of a polymer precipitate in the form of a three dimensional reticulate fibrous structure with fibre dimensions in range from below 1 micron to a a few microns, each having a predominant direction of polymer grain.
The direction of polymer grain in the different layers criss-cross one another.

Description

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My copending Canadian application Serial No. 245,593, filed February 12, 1976 discloses methods of extruding sheet material from polymers containing a solvent or swelling agent, in which methods the dissolved or swollen material is stretched, whereupon the solvent or swelling agent is caused to segregate in the polymer during solidification of the latter.
By use of these methods I have now found an interesting sheet structure, with paper to textile-like characteristics, and suitable among other applications for sanitary purposes, medical, surgical and veterinarian purposes and the like.
The sheet according to the present invention comprises two or more layers each consisting of a polymer precipitate in form of a 3-dimensional reticulate fibrous structure with fibre dimensions from below 1 micron to a few microns, each with a predominant direction of polymer grain, and the said directions in different layers criss-crossing one another.
Specific examples of the present invention will now be described together with a suitable apparatus for making the invention with reference to the accompanying drawings which appear also in copending Canadian patent --application Serial No. 245,593, in which drawings:
Figure 1 is a schematic, perspective view illustrating a method of and apparatus for making an extruded product;
Figure 2 is a schematic, perspective view, partly in section, illustrating another embodiment of method and apparatus for making an extruded product; and Figure 3 is a schematic, perspective view, partly in section, illustrating yet another embodiment of method and apparatus for making an extruded product.
The apparatus shown in Figure 3 is, with minor modification described below, most suitable for making the new product of the present invention but firstly, the apparatus of Figures 1 and 2 will be described because the apparatus of Figure 3 represents a development of that of Figure 1 and, in particular Figure 2.

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1(~99469 The apparatus of Figure 1 comprises a fixed mandrel 1 that serves as the support for three endless conveyor belts 2 that mainly cover the surface of the mandrel and that serve as a collector. After cooling of the film extruded onto the belts it is cut up and rolled on bobbins 4 and 5.
The apparatus shown also comprises a composite annular die 3 formed of two counter-rotating annular dies 6 and 7. Each die has extrusion passages each leading inwardly from an inlet to an outlet. Each inlet is fed by fluid material pressure free by supply extruders of which only two, 8 and 9, are shown. Streams emerging from two other extruders (not shown) are indicated by 10 and 11. Rollers 12 press the fluid polymer or other material into circular inlet grooves 13 which can conveniently be heated by hot air, e.g. from within the annulus. The rollers may be substituted by scrapers heated e.g. by a cycloterm.
The extrusion-die 3 may be heated from only one location as the rotation of the die will distribute the heat. Induction heating may be used, and the temperature may be controlled by pyrometers.
Each inlet groove 13 is connected with one or several exit orifices (not shown). The extrusion-die 3 can be fed from fewer or more extruders and the die could consist of one, two or more rotating or counter-rotating single dies, optionally with fixed extrusion dies in between. Thus in the invention although it is essential for one annular die to rotate fast as described, there may also be one or more non rotating annular dies.
A pronounced advantage of combining several single dies around one mandrel is that the layers of a composite sheet may be extruded successively over each other but relatively independent of one another so that each layer may be treated individually e.g. as to heat. The exit from the extrusion passage through the die can be in the form of a circular exit slot or slots or of one or more orifices or a circular row of orifices. The exit or exits can be situated on the inside surface of the annular extruder die, which is convenient for fast rotation of the die or, at the end surface of the die, which may be convenient when the die is a supplementary die that will not

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rotate or will rotate only slowly, but preferably is near the interface between the inner and end surfaces since the conditions for simultaneously controlling and cooling the polymer stream at this point are optimal.
While it is expected that the rolling or scraping action described in connection with Figure 1 is especially efficient in connection with material of particularly low fluidity, e.g. very high molecular weight poly-mers, the extrusion die of Figure 2 is preferred in many other cases due to its simplicity. The die is fed with a pressure free stream or strand from ~-an extruder outlet 14 and consists of two unconnected parts 15 and 16 which define a conduit consisting of the inlet orifice 17 leading directly to an exit orifice 18. ~There may however conveniently be a longer passageway between the inlet and exit comprising one or several widened chambers for further improvement of the distribution).
The two parts 15 and 16 are held in position and in the proper distance from each other through external bearings and are driven at differ-ent velocities through gear wheels (for details regarding the arrangement of bearings and gear wheels: see Figure 3). The different velocities are indicated by the two arrows of different length. In order to achieve an efficient pumping action, the walls of the inlet orifice can be supplied with suitable vanes 19 which here are only shown on one of the parts. However a sufficient pumping action can often be obtained without such vanes or corrugations due to the known tendency in viscoelastic material to drag inward-ly when applied between discs which counterrotate (the Weissenberger effect.) At the same time as the two parts 15 and 16 move relative to each other, it is essential that the material fed into the die is extruded from the exit 18 with a strong rotation in order to become properly distributed.
The arrows indicate that in the drawing both parts rotate in the same direc-tion but at different velocities. However one part may be stationary or the two parts may even be rotated in opposite directions, provided the difference in their numerical velocities is sufficiently large that the material has, overall, a strong rotation in one direction.

~(~99~69 Also in this embodiment the heating of the die can be by induction, but due to the simplicity and compactness of the construction it is even possible to use open flames.
The exit orifice can be a plain circular slot extruding a tubular film - as indicated at 18 - or alternatively it can be supplied with corru-gations - as indicated at 20 adapted to extrude a circular array of filaments.
When the distance from the die exit to the collecting mandrel 21 is short, the risk of breaking such fibres is greatly reduced, and a layer of fine fibres can be produced even from a rather unevenly corrugated exit slot.
The collecting and forwarding mandrel 21 is of toroidal shape and is supported and continuously driven in the direction of the arrow 22 by means of a series of driven wheels of which one 23 is shown.
In order to facilitate the support and drive, the inner part of the toroidal mandrel is supplied with a deep narrow groove 24 with which the wheels fit. The wheels 23 may conveniently be gear-wheels fitting with a gearing in the groove of the mandrel.
j The invention is very suitable for materials which require a relatively complicated or prolongated treatment, e.g. coagulation of dissolved polymer~ or other chemical treatment. Such treatment is indicated by the circular airless spray 25 from which e.g. a solution for coagulation can be sprayed onto the material. Similarly there can be special heating and/or cooling means and/or irradiation means in conjunction with the mandrel.
Before being stripped off from the mandrel, the material is cut, conveniently at the place of the groove 24, as shown by the rotating knife 26. There can also be scrapers or the like (not shown) to remove extruded material.
Since this apparatus does not require complex pressure seals the die can easily be manufactured with a relatively large diameter, e.g. 1-2 m.
The toroidal mandrel can conveniently have a main diameter 5-20 times that of the inner diameter of the die, and should in practice be assembled from several preferably hollow sections.

In Figure 3 there is fed to the annular die simultaneously a pres-,;
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A, ~399469 sure free feed of polymer from the extruder outlet 14 and a web 32 ofstaple fibres which have a melting point higher than the processing temper-ature of the die~ Suitable fibres are inorganic fibres such as glass, asbestos or rockwool. The feed is shown taking place by means of a conveyor belt 33 but could also be by any other means. The die is shown without any internal vanes or corrugations, i.e. the extrusion pressure is set-up entire-ly by the Weissenberger effect. In fact, a slight corrugation is generally preferable, while strong shear forces may cause excessive breaking of the fibres. It should be noted that a direct feed of fibres into the die is sometimes highly preferable compared with a prior admixture of the fibres to the polymer and common feeding through 14 since it results in a more even feed and much less fibre breakage, and permits much higher fibre con-tents to be used.
The mixing of fibres and fluid polymer takes place partly at the feed and partly by the shear exerted during the passage towards the circular exit slot 18. The collector is a flat sheet 35 that is folded to a tubular shape around the mandrel 34 which supports the sheet. The sheet 35 may be produced e.g. from a flat die in line with the rotating extrusion or may be produced beforehand. The sheet collector moves while the supporting mandrel is stationary.
For the sake of clarity a space is shown between the mandrel 34 and the folded sheet 35, but of course the sheet is in a close sliding fit over the mandrel. The sheet 35 is pulled through the extrusion die over the mandrel as indicated by the arrow 35. When the polymer film in molten state leaves the rotating exit slot 18 it is strongly melt-stretched (attenuated) and the fibres are aligned in the direction of attenuation, and it is collect-ed and held by the folded sheet 35 because of the elastic retention in the attenuated polymer. Thus it is wound around the folded sheet and forwarded along with it, resulting in a strongly helical grain as indicated by the broken line 37.
The forward movement of the sheet 35 is established by conveyor 1~99~6~

belts 38. The mandrel is preferably supplied with cooling means ~not shown).
The contraction of the extruded material can conveniently be matched by a gradual reduction of the diameter of the mandrel.
The conveyor sheet 35 may be formed of the same polymer as the material that is being extruded onto it, but if necessary an adhesive can be applied to ensure that the collector sheet will be laminated with the ex-truded material to form a layer in the final product.
The product so manufactured is a tube or pipe having transverse reinforcement, and its strength characteristics can be improved still further by the use of two extrusion dies ~see Figure 1) and/or by inclusion of longi-tudinally arranged fibres in the collector sheet.
The feature of laminating the collector with the extruded material is neither restricted to the application in conjunction with a feed of higher melting fibres to the rotating die, nor to the production of reinforced pipes or tubes, but has many other applications in connection with consolidation or reinforcement of rotatingly extruded material.
The above description is taken with appropriate editing from application Serial No. 245,593. As described, the apparatus of Figure 3 includes a conveyor belt 33 for providing a direct feed of fibres, the die is shown without any internal vanes or corrugation and no cooling means for the mandrel are specifically shown. In order to be suitable for the produc-tion of the present invention, the apparatus of Figure 3 is modified slightly.
In particular, the conveyor belt 33 is dispensed with since the direct fibre feed is not used to produce the new product. Moreover, the die is preferably formed with vanes to increase the pumping effect and the mandrel should be cooled from the inside.
A specific example of material used to obtain the product of the present invention is a 50% solution of high density polyethylene in xylene (density of the solid polymer: 0.96, melt index: 0.2 according to the ASTM melt index specification, condition L).
The inner diameter of the die is 300 mm and the extrusion temper-1~994651 ature 120C. Two such dies arranged adjacent each other around the same mandrel should be used, the dies rotating in opposite directions to obtain a criss-crossing fibre grain.
The conveying sheet, forwarded over a fixed mandrel of diameter 280 mm, can conveniently be a nylon or polyethyleneterephthalate film taken from one bobbin, wound around the mandrel, brought back to flat shape after use, and finally collected on another bobbin for re-use.
The polyethylene collected will hereby precipitate as a 3-dimension-al net-like substructure consisting of micro-fibres. A part of the xylene will bleed-out, while most of the rest remains between the fibres as a distinct phase.
One extrusion die lays-up the polymer in a left-handed helical grain, and the other in a right-handed helical grain. Angles of about 70 with the mandrel axis are suitable.
After stripping-off from the conveyor film, the fibrous sheet should be stretched longitudinally, e.g. until the two directions of grain become essentially perpendicular to each other, taken as an average (the fibres will become partly randomized by the stretching). The sheet is hereby made more porous and flexible. The remaining xylene is finally removed by - 20 vacuum-drying.
The invention is not limited to polyolefins. As another example a solution of a polyamide, e.g. nylon 6 or 666, can conveniently be used.
The coagulation can in this case be carried out by diluting and/or removing the solvent or swelling agent, which may e.g. be formic acid or phenol, by means of water.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A sheet comprising two or more layers each consisting of a polymer precipitated from a solution, which precipitate is in the form of a 3-dimensional reticulate fibrous structure with fibre dimensions from between approximately 1 micron and a few microns, each with a predominant direction of polymer grain, and the said directions in different layers criss-crossing one another.