FI78715C - FOERFARANDE FOER FRAMSTAELLNING AV POROEST MATERIAL BESTAOENDE VAESENTLIGEN AV PTFE-POLYMER. - Google Patents

Description

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The present invention relates to a process for producing porous polytetrafluoroethylene (hereinafter "PTFE") materials according to the preamble of claim 1, in which a combination of a high-strength and coarse microstructure is useful. Products made from these materials are very well suited for medical use.

The products of this invention are based on paste-formed PTFE products. Paste extrusion or paste molding techniques are prior art and involve mixing a coagulated dispersion of PTFE resin and a liquid lubricant and forcing the mixture through a die or machining the otherwise lubricated mixture to form a cohesive shaped product. The lubricant is then removed, usually by drying, to produce a porous non-sintered PTFE product having a density of usually 1.4 to 1.7 g / cm 2. Such densities correspond to 39% ··· 26% η porosities. At this point, the temperature of the product can be raised above its crystalline melting point, about 345 ° C, to sinter it, whereby the porous material coalesces to form a non-porous sintered product.

Alternatively, the unsintered product can be made porous and stronger by stretching according to the methods of U.S. Patent 3,953,566. After stretching, the stretched product can be kept fixed and heat-treated above the crystalline melting point. In this case, the product remains porous and when it cools, a strong porous PTFE product is obtained. Hereinafter, the term "sintering" is used without distinction in connection with the process step of raising the temperature of the unsintered product above its crystalline melting point.

U.S. Patent No. 3,953,566 discloses a process for making all types of 2,78715 types of microporous stretched PTFE, such as films, tubes, rods, and continuous fibers. The products are covered by U.S. Patent 4,187,390. The 5 · microstructure of these products consists of nodes connected by fibrils.

A key element of the method of U.S. Patent 3,953,566 is rapid PTFE stretching. The rapid stretching allows the unsintered product to be stretched much further than has been possible in the past while making the PTFE stronger at the same time. Rapid stretching also produces a microstructure that is very fine and has, for example, a very small effective pore size. U.S. Patent 3,962,153 discloses a very large number of stretched products, with stretching amounts exceeding 50 times the original length. The products of both 4,187,390 and 3,962,153 patents have relatively high basic tensile strengths (see discussion of "basic tensile strengths" and the relationship to product tensile strength and density in U.S. Pat. No. 3,953,566 in column 3, lines 28-43).

To calculate the basic tensile strength of a porous sample, the maximum force required to break the sample is divided by the cross-sectional area of the porous sample, and then this amount is multiplied by the density of the PTFE polymer component divided by the density of the porous sample. The density of a PTFE whose temperature has never been raised above its crystalline melting point is 2.30 g / cm 2, while that of a sintered or raised temperature above its crystalline melting point can be about 2.0 g / cm 2 - less than 2.30 g / cm 2. To calculate the basic tensile strength in the following examples, we have used a PTFE polymer density of 2.20 g / enr for products raised above the crystalline melting point and a density of 2.30 g / cnr1 for products not raised above this.

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When unsintered products are stretched at lower speeds, either limited elongation due to material breakage occurs, or weak materials are obtained. The microstructures of these weak materials are rougher 5 than the microstructures of products that have been stretched by the same amount but at faster speeds. The term "coarse" is used to indicate that the nodes are larger, the fibrils smaller, and the effective pore size larger. Such coarse microstructures would still be usable if they were strong rather than weak.

According to the present invention, there is provided a porous material consisting essentially of a PTFE polymer having fibrils interconnected in the microstructure of the material, the material being characterized in that the average matrix tensile strength of the material is greater than 103.5 MPa (15,000 psi) when measured in at least one direction. and its average node height / width ratio is greater than 3 *

Preferably, the matrix tensile strength is greater than 276 20 MPa (40,000 psi), the average node height / width ratio is greater than 5, the average fibril length is greater than 15 μm, particularly preferably greater than 50 μιη.

The invention will now be examined in more detail by way of example with the aid of the accompanying drawings.

Figure 1 is a schematic representation of the microstructure of a PTFE material according to the invention.

Figure 2 is a photomicrograph of PTFE material.

Fig. 3 is a diagram showing a characteristic range of roughness index and basic tensile strength obtained by compaction before stretching, and a characteristic range when compaction has not been performed before stretching.

Figure 4 is a photomicrograph of a surface of a prior art PTFE material stretched in one direction.

Fig. 5 is a photomicrograph of the surface of a PTFE material 35 of the present invention stretched "78715 in one direction;

Figure 6 is a photomicrograph of a cross section of a prior art PTFE material according to the figure;

Figure 7 is a photomicrograph of a cross-section of the PTFE-5 material of Figure 5;

Figure 8 is a photomicrograph of the surface of a prior art PTFE material stretched biaxially;

Fig. 9 is a photomicrograph of the surface of a PTFE material according to the present invention 10 stretched biaxially;

Figure 10 is a schematic cross-section of the type of sealing nozzle used in Example 3;

Figures HA and 11B are light micrographs of his-15 histological sections of fibers made in accordance with the new invention and the prior art, respectively, showing collagen growth;

Figures 12A and 12B are photomicrographs of the fibers of Figures 11A and 11B, respectively, and Figures 13A and 13B are photomicrographs of other fibers made in accordance with the present invention and the prior art, respectively.

A fully compacted unsintered PTFE product is one that has no void space, and such a product has a density of 2.30 g / cm 2. When stretched under the same conditions, it is found that products which have been compacted close to this limit before stretching have clearly higher structures than products which have not been compacted. There is a growing effect with increasing compaction. The largest seals produce the clearest effect. In order to achieve maximum sealing, it is necessary to subject the sealed product to compressive forces until the entire pore space is closed. The increasing compressive force at fixed temperature accelerates the compaction rate as expected. At a given compressive force of 5,78715, compaction occurs faster at higher temperatures of 300 ° C than at lower temperatures. Less force is required to achieve sealing at higher temperatures. Higher temperatures therefore facilitate sealing because less time and / or less compressive force is required. However, under otherwise identical conditions, it is seen that equivalent stretched products are obtained regardless of whether the compaction takes place at low temperatures or at high temperatures as long as equivalent seals are obtained. It is seen that the only significant variable is the actual compaction achieved as measured by the density of the compacted product before stretching.

15 The experiments presented here show that when compaction conditions are used which cause the material to sinter, the material cannot be stretched uniformly. Partial sintering is known to occur at less than 3-5 ° C. The conditions which cause the sintering thus indicate an upper usable limit of the compaction temperature.

Sealing can be performed using presses, nozzles or calendering machines. The use of calendering machines to seal dry PTFE allows the production of long films.

The preferred conditions for nozzle compaction appear to cause material to be drawn through the nozzle at relatively low speeds. The force produced to pull the material 30 through the die can cause the material to stretch as it passes through the die. Lower speeds require less force to pull the material through the die, resulting in less elongation of the material. It is seen that it is desirable to minimize 35 stretches from the die. Stretching is better controlled in process steps specifically designed to stretch the material.

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A number of processing steps may be performed prior to compaction, such as calendering with lubricant and stretching with or without lubricant. These steps can increase the strength of the final product, 5 but again no such preferred methods have been found. It is further preferred that the material is not completely compacted prior to stretching. It is believed that sealing can be accomplished by applying compressive forces in one or all directions, and that stretching can then be performed in any or all directions to achieve the benefits of the present invention.

Although fibril lengths and knot dimensions are particularly useful features for identifying coarse microstructures, they present a few problems in quantification. This is because there is a division of node sizes and a division of fibril lengths in any given microstructure. Slightly different microstructures are also obtained depending on whether the product is stretched uniaxially, biaxially or is periodically stretched first in one direction and then in the other. Figure 1 shows an idealized drawing of a knot-fibril structure when the membrane is uniaxially stretched. The actual electron micrograph of this structure is shown in Figure 2 at 198x magnification.

The products of the present invention have larger knots and longer fibrils than prior art materials with similar basic tensile strength. The four characteristic dimensions of the microstructure are: node height, node width, node length, and fibril lip length. See. Figure 1 shows the determination of these dimensions of nodes 2 and fibrils 1 7 78715 for uniaxially stretched films. Fibril length 3 and knot width h are measured in the direction of stretching. The knot length 6 is measured in the width direction of the film; i.e. perpendicular to the direction of stretching 5, in the plane of stretching. The knot height 5 is measured in the film thickness direction; i.e. perpendicular to the stretching plane. The difference in knot width and knot length is not obvious for films stretched in more than one direction because fib-10 coils can be oriented in many directions and knots can be the same size in more than one direction. In this case, the knot width is determined by the knot dimension in the same direction as the longest fibrils, in the stretch plane. Knot height is measured in the film thickness-15 direction; i.e. perpendicular to the stretching plane. The difference between knot length and knot height is not clear for products with a symmetrically shaped cross-section, such as round bars, fibers, and products with a square cross-section.

20 In this case, the knot height and the knot length are said to be the same dimension, which is called the "knot height", and this dimension is measured in a direction perpendicular to the stretch.

A combination of two microstructural dimensions and strength measurements in the strongest direction can be used to distinguish the products of this invention and the prior art products. The ratio of the average knot height to the average knot width in addition to the average basic tensile strength 30 in the strongest direction is useful for characterizing the products of the present invention.

For materials that are biaxially stretched or that are first stretched in one direction and then in the other, it is difficult to accurately quantify the geometry of the knot-fibril structure. Materials stretched in more than one direction have a larger distribution of microstructural dimensions. For this reason, roughness has also been determined in the form of other properties, and in particular the bubble point (EBP) of ethanol, which is a measure of the maximum pore size of a test sample (see ASTM 5 F316-80). More specifically, EBP is the minimum pressure required to force air through the ethanol-impregnated product of the invention. A slight increase in pressure should produce continuous flows of bubbles at many points. Thus, the measurements are not affected by artifacts when the material 10 has plug holes. The bubble point of ethanol is inversely proportional to the pore size: The lower the EBP value, the larger the pore size, or expressed in accordance with this application, the coarser the structure. It is believed that EBP can be assumed to be independent of the length of the distance the air passes through the 15 products. In other words, it is believed that EBP can be used to characterize pore size in a way that is not impossibly dependent on the dimensions of the product tested.

In a product having first-direction first fibrils perpendicular to the second-direction second fibrils, it is desirable that the EBP be less than 20.7 kPa (3.0 psi) (e.g., about 5.3 kPa , i.e. 1.2 psi), wherein the ratio of the tensile strength of the first and second directions is 0.5 to 2.0, preferably 0.67 to 1.5.

25 Another indication of a rough structure is the relatively low resistance to air passage (Gurley number). The Gurley number is defined as the time in seconds when 100 cm 3 of air flows through a material surface of 6.5 cm 2 at a pressure of 1.2 kPa (12.4 cm water column) across the material. See. ASTM D-726-58 30 Method for Measuring Gurley Number.

In order to provide a basis for comparing the roughness of products that have been compacted to different densities and then stretched, the "roughness index" is defined herein as the density of the stretched porous product divided by the EBP of that product. Density is an indication of total pore volume.

If two products have the same density, the product with the lower EBP is said to be coarser. If two 9,78715 products have the same pore size, the product with the higher density is said to be coarser. Thus, the roughness index is directly proportional to density and inversely proportional to EBPrhe. The increase in roughness is indicated by the increase in roughness index.

The introduction of density in combination with EBPr provides a means for comparing prior art products to the products of the present invention over a wide range of basic tensile strengths.

Sintering of the attached stretched product does not lower the EBPr of the product, and usually increases the roughness index.

10 However, in some cases, the roughness index does not increase due to sintering because the density of the product may decrease during sintering.

Figure 3 shows a diagram of the variables, roughness index and matrix tensile strength. Products that have not been available to date are made by the present invention and have a matrix strength greater than or equal to about 20.7 MPa (3 · 000 psi) and a corresponding roughness index greater than or equal to than the value which is on the line joining the points A, B, C and D. The coordinates of these points are as follows: Point A [20.7 MPa (3,000 psi), 58 g / cm3 / MPa (0.40 (g / cm3) / psi)], point B [82.2 MPa (12,000 psi)] psi), 58 g / cm 3 / MPa (0.40 (g / cm 3) / psi)] t point C [110.4 MPa (16,000 psi), 29 g / cm 3 / MPa (0.20 (g / cm 3) / psi)] and point D [172.5 MPa (25,000 psi), 29 g / cm 3 / MPa (0.20 (g / cm 3) / 25 psi)]. The roughness indices of points C and D may alternatively be 33.3 or 58. On the other hand, the roughness indices of points A, B, C and D may alternatively be: 62.25 g / cm3 / MPa (0.45 (g / cm3) / psi) .

No examples are shown for films that would have been stretched more than 4: 1 in one direction.

Higher stretch ratios generally result in products with higher tensile strengths, as disclosed in U.S. Patent No. 3,953,566. There is no evidence that the films of the present invention could still be stretched to obtain a higher strength, while still retaining coarser structures than prior art films of the same length. It is assumed that treatment of the films with higher stretch ratios will certainly produce films of the present invention having basic tensile strengths in excess of 172.5 MPa.

Items in the region corresponding to the present invention were derived from the data presented in the following examples. EBP and basic tensile strength measurements were performed after sintering of the fixed, stretched products. The sintering conditions are shown in the examples. The basic tensile strength value used was the value corresponding to the strongest direction of the material. This representation of roughness and strength is useful for characterizing materials 15 stretched in one or more directions before or after sintering.

The representation of roughness index and strength in Figure 5 is characteristic of filler-free porous PTFE products. Porous PTFE products can be coated with fillers such as asbestos, carbon black, pigments, and mica, as disclosed in U.S. Patents 3,953,666 and 4,096,227. The products of the present invention can likewise be compacted with fillers. However, the presence of filler may affect the measurement of the roughness index because EBP is a function of the surface tension of the porous product and the filler can affect the surface tension of the product.

The products of the present invention can thus be characterized in many ways. Either the roughness index or the ratio of knot height to knot width can be used in conjunction with the basic tensile strength in the strongest direction to describe the same products of the invention. K. O. the roughness index and the ratio of node height to node width are not independent parameters; 35 both describe the structure of the products of the present invention. The roughness index is particularly useful for describing the structure of thin films in which the maximum knot height is limited by the thickness of the film. The ratio of node height to node width is particularly useful for describing the structure of products that are too small to allow EBP measurement. In many cases, either of these parameters can be used to describe the structure of the same products.

Films of the present invention having the same strength 10 in the perpendicular direction can be distinguished from prior art films by showing the basic tensile strength in the perpendicular directions and EBP. This characterization is characteristic of sintered films having a ratio of basic tensile strengths in the perpendicular directions of 15 to 2.5, with the basic tensile strength in the weaker direction being greater than or equal to about 20.7 MPa. The films of this invention that satisfy these strength requirements have an EBP of less than or equal to about 27.6 MPa.

Non-sintered products extruded from the preferred resins can be stretched more and more uniformly to provide stronger stretched products than non-sintered products extruded from less suitable resins. Preferred resins are highly crystalline (such as Fluon CD 123, manufactured by ICI-Fluon is a trademark), but other resins may be used in the practice of this invention (see U.S. Patents 4,016,345 and 4,159,370).

Methods that appear to cause the non-sintered product to compress but do not provide compaction can produce results that are inconsistent with the methods of this invention. For example, U.S. Patent 4,250,138 discloses a drawing step which appears to correspond to the method set forth in Example 3 herein.

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However, the opposite effect is achieved; i.e. finer structures are obtained, as shown by increasing EBP. U.S. Patents 4,248,924 and 4,277,429 disclose a method of applying compressive forces to a film that appears to correspond to the sealing step disclosed herein. Again, the opposite effect is obtained; i.e. the prior art process reduces the pore size of one side of the film relative to the other side.

The conditions under which the compacted product is stretched 10 greatly affect the microstructure obtained. Higher stretching rates gradually produce finer microstructures, and the same qualitative interaction between stretching speed and stretching temperature occurs as disclosed in U.S. Patent 3,953,566. Thus, sealed non-sintered products can be stretched under conditions that produce products similar to prior art products, as disclosed, for example, in U.S. Patent Nos. 4,187,390 and 3,962,153. The method of the present invention can also produce products with properties not previously available. Attempts have been made to identify these latter materials in particular by the values of the parameters set out in the claims.

Experiments comprising the following examples show that for otherwise identical processing conditions, the addition of a compaction step produces coarser products compared to prior art products with comparable strength. Roughness was characterized by permeability, maximum pore size, and dimensions of sol-30 mu and fibrils. Materials prepared by the compaction step were considered to have knots passing through the thickness of the product (i.e., in a direction perpendicular to the direction (s) of stretching). This film or strip structure can provide 35 higher shell strength and / or higher tensile strength in the thickness direction and / or higher compressive strength in the thickness direction.

The biaxially stretched films of the present invention can be used in surgical reinforcement members. The uniaxially stretched fibers of the present invention can be used as sutures and ligatures. These products are both strong and have a rough microstructure. Coarse microstructure is desirable in medical applications because they allow cell growth and attachment to body tissue. The films of the present invention can be used in the manufacture of coaxial cables because they are both crush resistant and porous. The films 15 of the present invention can also be used in applications where tensile strength in the thickness direction is required.

The following examples, which illustrate the methods and products of the present invention, are illustrative only and are not intended to limit the shortcomings of the invention.

EXAMPLE I

Films uniaxially stretched with 25 PTFE resin (Fluon CD123, ICI) were paste extruded as a film extrudate and calendered. The calendered film was then dried to remove the extrusion aid. The properties of the dry, calendered film were as follows: thickness about 0.04 cm, density about 1.6 g / cm 2, basic tensile strength in the extrusion direction 11 MPa, and basic tensile strength in the transverse (width) direction 4, 14 MPa. The dry calendered extrudate was cut into approximately 11.4 cm models.

35 A few of the models were then compacted by pressing in a Carver press that could be heated; The remaining models were left uncompacted at a density level of 1.6 g / cm 2 for control of the tests. Measuring plates were used between the flat pressure plates (and next to the models) to control the density 5 by allowing sealing only to predetermined thicknesses. In a few cases, the gauge plates were thinner than the thickness calculated to produce the desired density. These thinner slabs were needed because a few samples would regain some of their thickness after removing the compressive forces. The range of densities was studied from 1.6 g / cm 2 ("control" uncompacted) to values approaching the maximum achievable density of 2.3 g / cm 2. Condensations were performed at temperatures starting at ambient temperature (22 ° C) and ending at slightly above 300 ° C. The times taken to reach the desired sealing temperature and the times taken to achieve the desired seal at these temperatures were taken into account. The "control" bodies were subjected to the same temperature and time conditions as used for sealing. For convenience, two film samples were stacked together with a Kapton (R.T.M.) Polyimide film (DuPont) sheet between them 2 so that two 11.4 cm film samples could be folded simultaneously.

The following steps were used to seal the dry PTFE film: 1. Carver press plates were heated to the specified temperature; 2. The film was fitted between two flat steel plates together with a Kapton polyimide film which acts as a release agent; 3. The measuring plates were placed on the perimeter of the plate. (Measuring plates not used for maximum density sealing); 4. The plates and the film between them were placed inside the press; i5 7871 5 5. The mounting plates were closed until contact was made; 6. The steel plates were heated to the desired temperature for sealing; 5 7. The pressure was applied and both steel plates were slowly brought into contact with the thickness gauge plates (or with the model if no gauge plates were used); 8. The pressure was maintained long enough to achieve the desired densities; 9. The pressure was released; 10. The materials were compacted at higher than ambient temperatures and cooled in water upon removal from the press.

2 15 11.4 cm samples were weighed before the compaction step. Thickness measurements were performed from four angles, 2.54 cm from each edge, and averaged from these four readings. Density was calculated by dividing the weight of the models in the range of 20 to 20 times the average thickness. This procedure produces the nominal density of the sample because the thickness of the sample varied due to local inconsistencies.

The materials (compacted and uncompressed) were then stretched with a pantograph in the longitudinal direction (i.e., the primary direction of both extrusion and calendering) to perform stretching. The pantograph used was able to stretch 11.4 cm film samples to a size of 10 x 41 cm in uniaxial stretching. (An additional 0.64 cm in length was required for 30 specimens on each side to attach the material to the machine). The 11.4 cm film was fastened on each side with 13 drive brackets that could be moved apart uniformly by a scissor mechanism at a constant speed to stretch the film. Membrane 35 was heated to the desired temperature for stretching with ie 7871 5 2 heating plates directly below the 11 μm samples.

The stretching conditions were:

Temperature: about 300 ° C

5 Stretch ratio: 4: 1 (300 # increase in length)

Stretch speed: about 400 # / sec.

Determined by dividing the percentage change in length by the duration of the stretch)

The stretched samples were prevented from shrinking by si-10 placement in a pin frame and immersed at 370 ° C.

to a salt bath for about 20 seconds at which time they were sintered.

The temperature did not appear to significantly affect the sealing process. For this reason, the data shown in Table 1 are the average values of the measurements obtained for the given densities, regardless of the sealing temperature.

All basal tensile strength, fibril length, and knot width data are presented for measurements made in the direction of stretching (which is also the direction of extrusion-20 crossing and calendering). Fracture forces were measured using samples with a measuring length of 2.5 em; the crosshead speeds of the tensile tester were 25 cm per minute. The density before stretching is given as a single number and is the nominal value mentioned above. Var-25 blue densities after compaction varied due to experimental variability and unavoidable small measurement errors. Thus, individual measurements for 1.63 g / cm 2 materials ranged from 1.60 to 1.64 g / cm 2. Individual measurements for 1.83 g / cm 2 materials 30 were 1.75 "to 1.85 g / cm 2. Individual measurements for 2.01 g / cm 2 materials ranged from 1.97 to 2.04 g / cm 2. The individual measurements for 2.27 g / cm 2 materials ranged from 2.19 to 2.35 g / cm 2, therefore the nominal range of 2.27 g / cm 2 includes the maximum achievable weights.

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Figures 4 and 5 show scanning electron micrographs of the surfaces of the final samples (stretched and sintered) which were not previously compacted (nominal density 1.63 g / cm 2) and which were previously compacted to 2.27 g / cm 2 (nominal density). ) before stretching.

These two micrographs of the right and left sides of magnifications (Figures 4 and 5) are about 155 and 1550. These micrographs show easy distinction between "coarseness" due to the sealing effect. Figures 6 and 710 show scanning electron micrographs of cross-sections of the same two final samples which had not been previously compacted and which had been previously compacted (2.27 g / cm 2), respectively. Figure 6 micrograph of the left and right sides of about 15 magnifications and 152 magnifications, respectively, 1520. The micrograph of Figure 7, the left and right sides are about 147 and 1 * 170. Again, the difference in roughness is clear. These micrographs also show the difference in node height through the cross section. Not only do the material of the present invention have higher knot heights compared to uncompressed materials, but a significant number of knots are seen to pass completely through the cross section as opposed to uncompacted material. These micrographs represent all non-dense control materials (1.63 g / cm 2) and materials compacted to a density level of 2.27 g / cm 2 regardless of the compaction temperature. The difference in roughness seen in Figures 4 and 5 and Figures 6 and 7 is reflected in the fibril length and knot width measurements shown in Table 1. The materials prepared in accordance with the present invention had longer mean fibril lengths and wider mean, knot widths. with materials that had not been compacted prior to stretching but had been treated in a similar manner. Equally important, the data in Table 1 indicate that the average basic tensile strength in the stretching direction for all materials that were sealed prior to stretching is at least of the same order of magnitude as for uncompacted control materials. The combination of long fib-5 grain lengths, wide knot widths, and high basic tensile strengths relative to previous PTPE materials obtained by conventional processes is surprising.

Returning to Figure 1, fibril lengths and knot widths (in the stretching direction) were measured from scanning electron micrographs of cross-sections of stretched, sintered materials to determine relative roughness based on microstructural dimensions. Fibril length and knot width measurements were performed using scanning electron micrographs magnified 150-fold and doubled 10-fold (approximately 1500 x original), and the following steps were performed: 1. SEM images were marked with two lines spaced approximately 24 mm apart. using a plexiglass device.

2. Fibril lengths were determined using dividers to measure the distance between nodes along the outer edge of the line from the upper left corner of the image in the first distinct node spacing. The divisors were then placed on a scale, the magnification factor was already determined and the lines were read into the nearest semi-micron, and the values were plotted.

This procedure was repeated for subsequent consecutive node intervals and each measurement was recorded.

30 3. The procedure was repeated to measure node widths instead of inter-node spacing and the data were recorded.

Examination of this data shows that materials that had been compacted to the maximum degree, i.e.

35 2.27 g / cm 2 (and subsequently stretched and sintered) i9 7871 5 were significantly coarser than other materials, as seen from longer fibrils and wider knots. The data in Table 1 are typical and show stretched materials that were compacted less than 2.27 g / cm 2 prior to stretching and had longer fibril lengths and wider knot plates than controls, but 2.27 g / cm 2 materials have clearly coarser structures without significant loss of basic tensile strength.

10 Gurley number data can characterize the roughness of these materials. Lower values of this parameter indicate higher permeability of the structures. Permeability and therefore Gurley number measurements are strongly dependent on the length of the journey.

15 However, the use of the Gurley number is an appropriate tool to compare the products presented in this example, as the materials were treated from identical except for the compaction step. Data on the bubble point (EBP) of ethanol can also characterize the roughness of the material. Lower values of this parameter indicate a larger maximum pore size of the structures. Higher permeability as well as larger pore size quantify greater roughness. The data in Table 1 show that materials compacted to 1.83 -25 to 2.01 g / cm when stretched show corresponding parameter lower values than the 1.63 g / cm 2 control materials, and that 2, Materials of 27 g / cm have significantly higher permeability and larger pore sizes than control materials of 1.63 g / cm. The final 30 materials compacted to 2.27 g / cm 2 had significantly lower values for these parameters than the less compacted materials.

The crushability test data in Table 1 show the macroscopic manifestation of the coarse microstructure obtained in accordance with the present invention. In this test 78715, the samples were placed under tensile loading using a force of 0.22 kg on the material in the stretching direction. A thickness measurement was performed that determined the initial thickness. Next, a weight of 0.5 kg with an area of 2 0.077 cm was applied to the sample for 0.5 minutes, and the resulting thickness was recorded while the weight was still applied. Percent crushing or fracture-to-crushing was defined as (tc) (100%) / t, where "t" is the initial thickness and "cM is the thickness under load. A crushing of less than 10 # is desirable, preferably it should be less than 8 #, particularly preferably less than 5% .1 Again, the most notable feature of this data is the difference in crushability between materials compacted to about 2.27 g / cm 2. , although materials compacted at lower densities did not show improved resistance to crushing compared to non-compacted materials, and materials compacted to about 2.27 g / cm 2 showed significantly higher crush resistance as seen from lower crushability.

Test data show that by compacting the dry calendered extrudate 25 to about 2.27 gm / cc or more (i.e., at the highest densities) prior to stretching, Dii greatly enhanced the effect on the "roughness" of stretched, heat-treated PTFE materials without reducing the basic tensile strength.

30 The other samples were then treated in essentially the same manner to investigate the applicability of higher compaction temperatures. The same density ranges before stretching, as used in the above experiments, were examined for higher compaction temperatures.

35 Similar results were not obtained with materials subjected to elevated sealing temperatures. Many of the final samples were roughly non-uniform in appearance, unlike any of the final samples that had not been subjected to these elevated temperatures prior to stretching. A few samples, concentrated under identical conditions but not diluted, were subjected to differential screening calorimetry analyzes. The identification of the reduced heat of melting for these materials compared to the untreated resin showed that the samples were Sint-cured to some extent. Unintentional sintering is considered to be the cause of the temperature non-uniformity across the plate. However, an important finding is that partially or fully sintered materials, whether compacted or not, cannot be stretched to produce uniform final materials at the above-mentioned stretching conditions.

The following conclusions can be drawn from these tests: 1. The compaction-stretch process produces materials with very high strength, coarse microstructure when the extrudate is compacted to 2.27 g / cm 2. The density of 2.27 g / cm 2 refers to the range of densities actually obtained. The maximum achievable density belongs to this field.

2. The inclusion of "dry" - 25 sealing steps of any degree of seal (i.e., lubricant removed from the extrudate) prior to stretching does not compromise the basic tensile strength of the stretched material.

3. Concentration of the dry extrudate to a density of 2.27 g / cm 2 prior to stretching produces a stretched material having a coarse structure quantified by EBP, Gurley number, knot width and fibril length measurements. By comparison, compaction to lower densities or non-compaction prior to stretching produces a stretched material with a finer structure.

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Compression of the dry extrudate to a density of 2.27 g / cm 2 prior to stretching produces a more fracture-resistant stretched material than if the dry extrudate is compacted with less or no ring.

5. The degree of compaction (as quantified by density measurements) has a well-emphasized effect on the properties of the stretched material. The degree of compaction essentially describes a characteristic feature of the compaction process, provided that the material is not sintered.

6. The effect of temperature is to act as a catalyst for the process. Less time is required to achieve the desired density in the compaction step at higher compaction temperatures. A higher sealing temperature may allow lower compressive forces to be used to achieve sealing.

7. The preferred sealing conditions are those that do not cause sintering of the dry extrudate.

TABLE 1; FEATURES OF FINAL MODELS **

Density before stretching 1.63 ± 0.01g / cm5 * 1.83 + 0.02g / cm5 2.01 ± 0.02g / cm5 2.2710.05g / cm5 23 7871 5 5 Thickness (cm) 0.030210, 0005 0.03 ± 0.0005 0.029 ± 0.0005 0.284 * 0.0013

Density (g / cnP) 0.56 ± 0.02 0.57 * 0.2 0.59 * 0.02 0.58 * 0.04 10

Basic tensile strength (MPa) 107.6 + 4.8 108.3 ± 6.3 109.7 * 4.1 113.9 * 0.69

Fiber length 15 (μ) 4 * 1 5 * 1 5 * 1 23 * 4

Node width (μ) 3 * 1 3 * 1 4 * 1 15 * 3 20

EBP

(kPa) 5413.4 47.6 * 2.8 44.214.1 17.3 * 6.9

Gurley number 25 (s) 27.5 * 3.8 23.8 * 3.5 19.4 * 3.5 6.5 * 2.4

Crushability {%) 15 * 1 14 * 2 14 * 2 9 * 2 30 Roughness index (g / cm3 / MPA) 10.1 11.6 13 33.3 ♦♦ All values are rounded. The values shown are the means i standard deviation calculated from the means of each of the 35 models produced at each density level before stretching. ♦ Check without sealing step.

2H 7871 5

EXAMPLE II

Films stretched biaxially 2

Four 11.7 cm samples were prepared from a film of the type shown in the first paragraph of Example I 5 and stretched on a pantograph. In this case, three samples were concentrated in a Carver press at a temperature of about 300 ° C and a fourth sample was subjected to the same thermal conditions but not compacted. The unstoppable Tii-10 material served as a control. The materials were compacted in substantially the same manner as described in the third paragraph of Example I.

All four samples were simultaneously stretched in two directions at right angles to the others in a pantograph (shown in Example I), 100% in each direction. Thus, the area of the stretched film was four times larger than the area of the original film. The film temperature was about 300 ° C at the beginning of the stretch. A stretching speed of about 130¾ per second was used in both directions. The stretch rate was determined by dividing the percentage change in length by the duration of the stretch. (The pantograph clamps moved apart at a constant speed). Shrinkage of the stretched samples was prevented by placing them in a pin frame, removed from the fasteners of the 25 pantographs, and immersed in a 370 ° C salt bath for approximately 20 seconds, at which time the samples Sint-cracked. The models were then cooled in water to produce final models.

The data in Table 2 demonstrate the effects of this invention. Figures 8 and 9 show scanning corona micrographs of the surfaces of the control material (1.6 g / cm 2) and the surface of the material compacted to 2.25 g / cm 2, respectively. Figures 8 and 9 mikroku-on of the left and right side magnifications are about 35 to 150, and 1500, respectively, is clearly seen in the material 7871 25 5

The relative roughness was concentrated to 2.25 g / cm 2. These figures show the structural differences caused by the invention, which produce a difference in the bubble points of ethanol, as shown in Table 2. The photomicrograph of Figure 9 shows the structure that arose when the compaction step was included. However, the final material is not completely uniform and a few areas are not considered to be as rough in terms of microstructural dimensions as other areas of the same material. This inconsistency is due to local inconsistencies during compaction.

The data in Table 2 show that the material that was compacted the most before stretching was much more resistant to crushing than the materials that were compacted less or not at all. Four additional samples were prepared from the same raw materials using the same processes to further investigate the present advantages in terms of crush resistance. The same area of densities before stretching was studied. These specimens were not Sint-20 strapped after stretching as shown in Table 2.

Data for these materials, which were not sintered, are shown in Table 3. The crushability of stretched materials with pre-stretch densities of 1.63, 1.89, 2.06, and 2.29 g / cm 2 was 30.1, 19 , 7, 10.2 and $ 3.6, respectively, indicating that the materials with the highest compaction produced the most crush-resistant end products. A comparison of the data for sintered and sintered tower materials that had not been compacted shows that sintering reduces the crushability of the compacted-30 tower materials (from $ 30 to $ 14.6 in this case). The material that was most compacted but not sintered was still much more resistant to crushing (crushing $ 3.6) than the uncompacted material that was sintered (crushing $ 14.6).

26 7871 5

Fracture forces were measured using models with a measuring length of 2.54 cm; the crosshead speed of the tensile tester was 25.6 cm per minute. The longitudinal direction is the primary direction of extrusion and calendering. The transverse direction of the crosses-5 is perpendicular to the longitudinal direction, in the plane of stretching.

27 7871 5 TABLE 2: CHARACTERISTICS OF FINAL SAMPLES1 Density before stretching 1.6 g g / cm 2 * 1.83 g / cm 3 * 2.02 g / cm 2 1.23 g / cm 2 Thickness (cm) 0.0282 0.0277 0.0267 0.0310

Density (g / cm3) 0.57 0.54 0.65 0.54 ® Bubble point (kPa) of ethanol 67j6 48.3 24.8 8.28

Longitudinal basic tensile strength (MPa) 69.7 57.3 48.3 44.9

Transverse basic-20 tensile strength (MPa) 70.4 77.3 59.4 44.2

Roughness index (g / cm3 / MPa) 8.7 11.6 26.1 65.2 25

Crushability {%) 14.6 17.0 16.6 4.2

All values are rounded.

30 * Check without sealing step.

28 7871 5 TABLE 3: CHARACTERISTICS OF FINAL MODELS WHICH HAVE NOT SINTERED AFTER STRETCHING 1 Density before stretching 5 1.63 g / cm ^ 1.89 g / cm ^ 2.06 g / cnP 2.29 g / cm3

Thickness (cm) 0.0371 0.0330 0.0305 0.0295

Density (g / cm3) 0.58 0.63 0.6l 0.72 10

Bubble point of ethanol (kPa) 98.0 55.9 33.1 21.4 15 Longitudinal basic tensile strength (MPa) 30.4 29.7 33.1 26.2

Transverse basic-20 tensile strength (MPa) 16.6 15.2 19.3 16.6

Roughness index (g / cm 3 / MPa) 5.8 11.6 18.8 33.3: 25

Crushability {%) 30.1 19.7 10.2 3.6

All values are rounded.

30 * Check without sealing step.

29 7871 5

EXAMPLE III

Fibers stretched uniaxially

5 PART A

Part A shows the effect that the sealing nozzle may have on the microstructure of the uniaxially stretched fiber. The treatment of the two fibers finished as described herein was adjusted to obtain materials with the same diameters, densities, and basic tensile strengths.

The PTFE dispersion powder ("Fluon CD-123" resin, manufactured by ICI America) was mixed with 130 cm 2 of odorless "Isopar M" solvent (manufactured by Exxon Corporation) per 15 pounds of PTFE, compressed into a pellet and extruded with a diameter of 0, 27 cm of fiber in a piston extruder with a 95: 1 reduction ratio in the cross-sectional area from pellet to extruded fiber.

Isopar M was evaporated from 20 samples of extruded fiber. This sample had a density of about 1.49 g / cm 2 and a basic tensile strength of about 6.2 MPa.

Further, the extruded fiber containing Isopar M solvent was immersed in a container of Isopar M at 60 ° C and stretched nine times (800/6) between the drawing rolls 25 at a production rate of 26.3 n / min). Their traction rollers had a diameter of about 7.1 cm and a distance from the center to the center of about 11.4 cm. The fiber diameter was reduced from about 0.269 cm to about 0.099 cm by this stretch. Isopar M was removed from this stretched material. The density of the stretched fiber was about 1.3 g / cvn? and the basic tensile strength was about 37.3 MPa.

The stretched fiber from which Isopar M had been removed was then drawn through a round sealing nozzle heated to 300 ° C. The production speed of the material leaving the nozzle was about 2.2 m / min.

jo 7871 5

The orifice of the nozzle narrowed at an angle of 10 ° from about 0.013 cm in diameter to 0.076 cm in diameter and then was constant between about 0.076 cm.

0. A nozzle diameter of 76 cm was selected based on two conditions: 1. It was desirable to compact the stretched fiber to about 2.2 g / cm 2.

* 2. There was no weight / meter change in the stretched bar when it was sealed.

Using these conditions, the die diameter was calculated to correspond to the reduction in cross-sectional area required to increase the density of the stretched rod to about 2.2 g / cm 2. In the specific case of A-16 fiber, this calculation was performed as follows: 15 2 P- 1/2 D2 = (D1 x 57> 2 1 3 1/2 = (0.099 ^ x 20 = 0.076 cm = initial diameter of the stretched bar (cm) D „= nozzle diameter (cm) 25 P1 = initial density of the stretched bar (g / cm)

-L X

P2 = nominal value of hollow PTFE 2.2 (g / cmr).

Removing a piece of fiber from the die by stopping the sealing process and pulling the material back through the die inlet showed that when the stretched fiber was pulled through the sealing die, a translucent segment characteristic of PTFE having a density of about 2.2 g / cm 2 developed. This segment corresponded to a 0.076 cm gap in the nozzle immediately following 35 angular displacements of 10 ° (see Figure 10).

3i 7871 5

However, when the material exited the die, it again developed a white appearance characteristic of PTFE having a density of less than 2.2 g / cm 2. This is because the force required to pull the stretched fiber through the die is sufficient to stretch the material slightly after it exits the die. This was confirmed by measuring the weight of the material / meter before and after the die. A decrease in weight / meter was observed in the material after the nozzle, indicating where stretching occurred. The following research shows that diameters both above and below 0.076 cm of the nozzle can also bring about the desired change in microstructure. Important in selecting the diameter of the die is that it changes the cross-sectional area of the stretched rod so that the density of the material in the die is greater than or equal to about 2.0 g / cm. There is an increasing effect on the structure as the seal increases.

The stretched fiber drawn through the die was then heated in a 300 ° C oven and further stretched sevenfold ($ 600) from an initial length of about 18.3 cm at a very rapid speed of 11.3 m / min.

Finally, the fiber was prevented from shrinking and heated in a 367 ° C oven for 30 seconds.

As shown in Table 4, the fiber from the final heat treatment (A-16) had a density of about 0.4 g / cm 2, a diameter of about 0.56 cm, and a basic tensile strength of 338 MPa. The structure clearly consisted of nodules of solid PTFE joined by fibrils. The average fibril length was about 120 microns, the average knot width was about 17 microns (measured in the direction of stretch), and the average knot height was about 102 microns (measured perpendicular to the direction of stretch). The fiber was subjected to an overall stretch ratio of 79: 1 from the extrusion-35 step. This was calculated by dividing the weight / meter of dry fiber extrudate by the weight / meter of finished fiber.

32 7871 The other end of the length of the A-16 fiber was heated, sealed, and then drawn onto a surgical needle with a constant diameter of 0.056 cm to form a prototype suture by matching the diameters of the needle and thread 5. This combination of needle and thread is not currently available on the market and has the untapped advantage of reducing the leakage of the suture row in the vascular connective arms. This material was knitted into guinea pig tissue and the results were checked after 30 days. Fibroblast cells had invaded the structure of the suture and abundant collagen had formed throughout the internal structure of the suture (see Figure 11A). Sutuu-ra also sank well into the tissue. These properties, combined with the strength of the material and the ease of handling, made it a useful suture.

The second material (3-1-3) was prepared using a process similar to that described above, with the important difference that the stretched fiber was not drawn through the sealing die. Minor processing changes 20 were necessary to achieve this equivalence. In particular, the extruded fiber had a diameter of about 0.299 cm, a basic tensile strength of about 8.3 MPa, and underwent a 51: 1 total elongation in the process. Table 4 shows that the diameter, basic tensile strength 25 and density of this material were almost identical to the material drawn through the die. When implanted in a guinea pig, this material (3-1-3) allowed only minimal collagen permeation (see Figure 11B).

As shown in the figures of Figures 12A and 12B and the data in Table 4, these materials have very different microstructures. The A-16 material had much longer fibril lengths and knots where its height / width (H / W) ratio was substantially higher than that of the uncompacted (3-1-3) material.

35 For materials with basic tensile strengths greater than about 103.5 MPa, this knot ratio is unique. In the past, only materials with a basic tensile strength of less than about 103.5 MPa have had a knot H / W ratio greater than or equal to 3 · Conversely, when prior art basic tensile strengths exceed 103.5 MPa, the H / W ratio drops below 3.

The only materials with basic tensile strengths greater than or equal to 103.5 MPa and a knot H / W ratio greater than or equal to 3 are those materials that are subjected to a compaction step prior to stretching, whereby compaction increases the specific gravity of the material; or equal to 2.0 g / cm 2.

It is seen from this example that compaction performed prior to final stretching can produce fibers with longer fibril lengths than would be achieved with the same amounts of stretching in a process that does not include a compaction step.

PART B

The following examples further illustrate the effect of the 20 sealing nozzles on the microstructure. No attempt was made to match the properties of the finished fibers as in Part A. The final stretching of both materials presented herein was adjusted so that they were subjected to equal stretching rates from the die-25 crosslinking step. This was done to study the effects of compaction on similarly stretched pieces of material from the same extrusion batch.

The PTFE dispersion powder (I'Fluon CD-123, L resin $ from ICI America) was mixed with 287 cm 2 of odorless 30 "Isopar M" solvent (manufactured by Exxon Corporation) for each kilogram of PTFE, compressed into a pellet and extruded. -pressed into a 0.27 * 1 cm diameter rod in a piston extruder with a 153: 1 reduction ratio in cross-sectional area from pellet to extruded fiber.

3¾ 7871 5

Isopar M was evaporated from a sample of extruded fiber. This sample had a density of 1.48 g / cm 2 and a basic tensile strength of about 11.7 MPa.

The extruded fiber, which still contained Isopar-5 M solvent, was immersed in Isopar M. to a tank at 60 ° C and stretched seven times (600¾) between the drive rollers at a production speed of about 17.6 m / min. These traction rollers had a diameter of about 7.1 cm and a distance from the center to the center of about 11.4 cm. The fiber diameter was reduced from about 0.274 cm to about 0.125 cm by this stretch. Isopar M was removed from this stretched material. The stretched fiber had a density of 1.02 g / cm 2 and a basic tensile strength of about 54.5 MPa.

At this point, the stretched fiber was divided into two 15 separate sections for further processing. Part 661 was pulled through the sealing nozzle, while at part 665 this was omitted.

The stretched fiber (part 661) from which Isopar M had been removed was then drawn through a circular sealing nozzle 20 heated to 300 ° C. The production speed of the material coming from the die was about 0.58 m / min. The orifice of the nozzle narrowed at an angle of 10 ° from a diameter of about 0.19 cm to a diameter of 0.066 cm and then was constant at an intermediate length of about 0.066 cm.

The stretched fiber (part 661) drawn through the nozzle was then heated to 300 ° C and further stretched 4.5 times (350¾) between the draw rolls at a production rate of about 3.96 m / min.

The diameter of these traction rollers was about 7.1 cm and the distance from the center to the center was about 6 cm.

The stretched fiber (part 665), which had not been drawn through the die, was heated in a 300 ° C oven and further stretched eight times (700¾) through the same traction roller using a production speed of about 3.5 m / min.

35 7871 5

Finally, both rods (fibers) were prevented from shrinking and heated in a 362 ° C oven for 60 seconds.

As shown in the figures of Figures 13A and 13B and the data in Table 5, the two types of material 5 had very different microstructures. Part 661 had much longer fibril lengths and knots with a substantially higher H / W ratio than the uncompacted (part 665) material. This example clearly shows that fibers that were compacted before the final stretch had much longer fibril lengths than uncompressed fibers when both materials were subjected to similar amounts of stretch.

These examples, Parts A and B, show that compacting a stretched fiber using a sealing nozzle can produce a very strong material with a unique microstructure when further stretched. An important aspect of the invention is that the stretched fiber was compacted to at least 2.0 g / cm 2 prior to further stretching.

56 7 8 7 1 5

TABLE A

Properties of the finished fiber 5 A - 16. 3-1-3 (with nozzle) (without nozzle)

Density (g / cm 2) 0.4 0.5

Basic tension (MPa) 338 338 10 Diameter (cm) 0.056 0.056

Node width (μ) 17 9

Node height (μ) 102 16 H / W ratio 6 1.8

Fibril length (μ) 120 32 15 TOTAL EXTENSION RATIO 79: 1 52: 1 TISSUE INGREDIENT collagen minimal throughout collagen interstitial infiltration in 30 days 30 37 207 78715 TABLE 5

Properties of the finished fiber 5 661 665 (with nozzle) (without nozzle)

Density (g / cm 2) 0.6 0.5

Basic draw (MPa) 379.5 44l, 6 10 Diameter (cm) 0.059 0.064

Node width (μ) 11 6

Node height (μ) 79 3 H / W ratio 7.2 0.5

Fibril length 74 16 15 TOTAL EXTENSION RATIO 58: 1 57: 1

To achieve the densities shown above, the volumes of material were calculated from the diameter and length measurements, and this volume was divided by the weight of the material.

20 Density calculations are to two decimal places.

The basic tensile strengths were calculated as described above and are to one decimal place. Diameters were calculated using a non-contact laser micrometer. The values shown represent the average through-25 length of the material over a distance of several decimeters and are to four decimal places.

Images at 200x magnification were used to obtain knot widths, knot heights, and fibril lengths. Images were taken with a scanning electron microscope and 30 Nikon Biophot devices (Brightfield Microscope).

Measurements were performed with a millimeter measuring harp and then converted to microns. Measurements were selected (4-5 measurements per image for a given type of material) by randomly drawing two horizontal 35 lines on each image approximately 2.54 cm apart. Five consecutive measurements were then taken from the left edge of the road. After 20 measurements, averages were calculated. The values of knot width, knot height, and fibril length are to one decimal place. The total stretch ratio was calculated by dividing the weight / meter of dried fiber-5 extrudate by the weight / meter of finished fiber. The calculated ratios are to one decimal place.

Although the method used in the invention increases the density of the dry extrudate to at least 10 to 1.75 g / cm 2, the following claims define the porous material using other attributes. For this reason, not every product prepared according to the presented method necessarily falls within the scope of the requirements.

Claims (15)

A process for producing a porous material of polytetrafluoroethylene polymer, which material typically has relatively large nodes, which are interconnected by relatively long fibrils, and exhibits a relatively high tensile strength, which material is made from paste-extruded polytetrafluoroethylene extrudate, from whatever auxiliary extrusion lubricant has been removed, characterized in that (a) the density of the dry extrudate is increased to at least 1.75 g / cm crystalline melting temperature. 2. A method according to claim 1, characterized in that in said densification step the density of the dry extrudate is increased to more than 2.0 g / cm 3. Method according to claim 1, characterized in that in said densification step the density of the dry extrudate is increased to more than 2.2 g / cm 2. 4. A method according to claim 1, characterized in that in said densification step the density of the dry extrudate increases to about its maximum value. Process according to claim 1, characterized in that the densification step is carried out by densifying the dry extrudate in a press. 6. A method according to claim 1, characterized in that the densification step is performed by calendering the dry extrudate. 7. A method according to claim 1, characterized in that the densification step is carried out by drawing the dry extrudate through a densification nozzle. Method according to any of claims 1,5,6 and 7, characterized in that the densified dry extrudate is uniaxially stretched. Method according to any of claims 1.5 and