CN116870716A

CN116870716A - PTFE porous membrane with low-orientation-degree pore structure and preparation process thereof

Info

Publication number: CN116870716A
Application number: CN202311080880.3A
Authority: CN
Inventors: 贾建东; 相黎超
Original assignee: Hangzhou Cobetter Filtration Equipment Co Ltd
Current assignee: Hangzhou Cobetter Filtration Equipment Co Ltd
Priority date: 2023-08-25
Filing date: 2023-08-25
Publication date: 2023-10-13

Abstract

The application relates to a PTFE porous membrane with a low-orientation pore structure and a preparation process thereof, wherein the porous membrane comprises a membrane main body, the surface of the membrane main body is provided with surface fibers and blocky surface nodes, long strip-shaped holes are formed between adjacent surface fibers and are distributed in a radial manner, and the fiber structures are crossly laminated to form a communicated flow path in the thickness direction of the membrane main body; the thickness of the porous main body is 3-40 mu m; reinforcing fibers and distribution fibers, wherein the reinforcing fibers account for 10-40% of all surface fibers; the ventilation rate of the ventilation film is not lower than 2 x 10 ⁴ . The porous membrane has a radial distributed long-strip-shaped pore structure, and is matched with the block-shaped surface nodes and the reinforced fibers, so that the porous membrane has higher air permeability; and the high tortuosity caused by the radial distribution pore structure is formed by the matching of coarse and fine fibersThe three-dimensional network structure of the porous membrane ensures that the porous membrane has higher ventilation and waterproof effects.

Description

PTFE porous membrane with low-orientation-degree pore structure and preparation process thereof

Technical Field

The application relates to the field of microporous membranes, in particular to a PTFE porous membrane with a low-orientation-degree pore structure and a preparation process thereof.

Background

The special molecular structure of Polytetrafluoroethylene (PTFE) ensures that the PTFE has outstanding chemical stability, extremely strong high and low temperature resistance, good non-stick property, lubricity, excellent electrical insulation, aging resistance and the like. Based on the advantages, the PTFE amount accounts for a relatively high total amount of fluoroplastic, and is widely applied to the fields of petroleum, chemical industry, textile and the like.

In addition, PTFE is also used for preparing microporous membrane products, and PTFE microporous membranes are widely applied to the fields of microelectronics and semiconductors, and are also widely applied to the fields of waterproof and ventilation of various electronic components, automobiles, electric appliances and the like. Because the molecular acting force of the PTFE material is small, the ribbon-shaped crystal structure can be pulled out to form a fibrous structure by only needing small drawing force, and therefore, the most common preparation method of the PTFE microporous membrane is a drawing method.

The preparation method of PTFE microporous membrane is disclosed in U.S. Pat. No. 3,959,566 and U.S. Pat. No. 5, 4187390, wherein a film-forming liquid is obtained by blending PTFE dispersion resin with a lubricant (such as kerosene), the paste film-forming liquid is extruded and then the lubricant is removed, and then the PTFE microporous membrane is obtained by unidirectional or bidirectional stretching at a certain temperature (lower than the melting point of PTFE).

A large application area of the current PTFE microporous membrane is as a waterproof breathable membrane for outdoor use equipment, which requires a PTFE membrane with high breathable capacity and good waterproof effect. For example, when equipment such as a battery pack, a communication tower, a lamp and the like of a new energy automobile is used, a large amount of heat is generated by electronic components and the like in the equipment, so that the internal air pressure of the equipment is quickly increased, and in order to avoid the influence of high pressure on the operation of the electronic components and the unnecessary safety risk, the internal high-pressure air is required to be quickly discharged through an air-permeable membrane. When acoustic devices such as headphones and headsets are used, the internal vibrating diaphragm rapidly and greatly vibrates, so that the air pressure in the acoustic device rapidly and drastically changes, and if the air permeability of the PTFE film is insufficient, the air pressure changing in the interior limits the movement of the vibrating diaphragm to influence the audio curve, so that the audio distortion is caused, which is unacceptable for acoustic devices requiring low distortion. On the basis that the breathable film has large air permeability, whether the breathable film is used for outdoor equipment such as a battery pack, a communication tower and a lamp or acoustic equipment such as an earphone and a headset with outdoor use requirements, the breathable film is required to further have higher waterproof capability so as to prevent external liquid from penetrating the breathable film and entering the equipment to cause damage. However, it is not simple to obtain a PTFE film having both high waterproof performance and high air permeability.

In the patent application document with the application publication number of CN112717729A filed by Hangzhou greeng filter equipment, the utility model discloses a PTFE porous membrane, a preparation method and application thereof, wherein the membrane section parallel to the membrane thickness direction comprises primary nodes, the primary nodes are of a granular structure, a plurality of primary nodes are mutually stacked to form node points, the adjacent node points are connected through first fibers, and the primary nodes on the same node point are connected through second fibers; the nodes extend in a uniform direction on the outer surface of the film, the average width of the nodes is 1.3-4.3 μm, and the length of the nodes is at least 50 μm.

The PTFE porous membrane disclosed in the above patent has a relatively remarkable fibril-long node structure, the surface of which is composed of long nodes which are approximately parallel and have a length of at least 50 μm, and fibers connecting adjacent long nodes, and the pore structure between adjacent fibers is a pore structure of the surface, which exhibits a relatively high degree of orientation.

The patent application publication No. CN112717728A filed by the company also describes a PTFE macroporous film with long nodes and fibrils on the surface, and the surface pore structure of the PTFE macroporous film also has higher orientation degree. The PTFE porous membrane with the highly oriented pore structure has the advantages of larger flow rate, high filtering speed and good air permeability, because the pore structure with higher orientation degree ensures that the contact ratio of the pore structure of the PTFE membrane in the thickness direction is higher, and when the feed liquid flows through the pores with no lamination but higher contact ratio in the thickness direction, the actual flow path length and the membrane thickness are smaller, so that the PTFE porous membrane with higher water permeability and higher air permeability are obtained. Thus, PTFE membranes with highly oriented macropores have too high a water permeability that makes them unsuitable for breathable applications where water resistance is a requirement, and would likely result in damage to the device once the outside liquid has permeated and entered the interior of the device.

If the PTFE membrane is to be used in conditions where waterproofing and ventilation are required, the waterproofing capacity of the PTFE membrane is to be increased to ensure a low risk of water leakage. At present, common practices such as improving the density of the PTFE membrane, increasing the thickness of the PTFE membrane and the like can improve the waterproof performance of the PTFE membrane by utilizing the pore structure with smaller size of the PTFE membrane and the hydrophobic performance of the PTFE material. However, increased density and increased thickness of PTFE films would be expected to result in reduced air permeability, which is unacceptable for applications requiring large air permeability.

Obviously, the achievement of a PTFE membrane having both high air permeability and high water resistance is a current urgent need for but difficult problem to solve.

Disclosure of Invention

The application provides a PTFE porous membrane with a low-orientation pore structure and a preparation process thereof, wherein the surface of the PTFE porous membrane is provided with radial distributed long-strip holes, and the PTFE porous membrane has high air permeability by reducing air resistance of the pore structure on the long diameter of the PTFE porous membrane, and the lamination of the pore structure in the thickness direction brings higher waterproof capability; the surface fibers are provided with reinforcing fibers with larger diameters and the ratio of 10-40%, and distribution fibers with smaller diameters and the distribution fibers are distributed among the reinforcing fibers, and the coarse fibers and the fine fibers are matched to form a hydrophobic network structure with higher water pressure resistance, so that the waterproof performance of the porous membrane can be further improved; the surface nodes of the block shape have small air resistance compared with the long node structure and do not prevent the gas from automatically adjusting to the optimal path of the flowing in the membrane, so that the porous membrane can have large air permeability.

In a first aspect, the present application provides a porous PTFE membrane having a pore structure with a low degree of orientation, which adopts the following technical scheme:

a PTFE porous membrane having a low degree of orientation pore structure, comprising a membrane body comprising a first outer surface and a second outer surface, each of the first outer surface and the second outer surface comprising surface fibers and bulk surface nodes, the surface fibers being connected to the surface nodes or to adjacent surface fibers, interstices between adjacent surface fibers forming elongated pores;

the holes are distributed radially around the surface nodes, and fiber structures are crossly laminated in the thickness direction of the film main body to form communicated flow paths;

the thickness of the porous main body is 3-40 mu m;

the average diameter of the surface fibers is Z, the surface fibers are provided with reinforcing fibers with diameters not smaller than 1.2 times of Z and distribution fibers with diameters not larger than 0.6 times of Z, and the reinforcing fibers account for 10-40% of all the surface fibers;

the air permeable film has an air permeability of not less than 2×10 ⁴ ml/min/cm ² @7kpa。

Optionally, the thickness of the film body is 5-35 μm; further optionally, the thickness of the film body is 5-30 μm; still further alternatively, the film body has a thickness of 5 to 20 μm.

Optionally, the breathable film has a breathable rate of not less than 5×10 ⁴ ml/min/cm ² 7kpa; further alternatively, the breathable film has a breathable rate of not less than 8×10 ⁴ ml/min/cm ² 7kpa; the air permeable film has an air permeability of not less than 10×10 ⁴ ml/min/cm ² @7kpa。

Through adopting above-mentioned technical scheme, to having outdoor waterproof operation requirement and inside and outside atmospheric pressure need the application scene of quick balance, for example new energy automobile's battery package, basic station, lamps and lanterns and low distortion acoustic equipment etc. the porous membrane of PTFE must have higher ventilation to guarantee the quick balance of the inside and outside atmospheric pressure of equipment, in addition, still demand the porous membrane of PTFE has good waterproof performance, in order to prevent outside liquid water to enter into the damage that causes electronic components in the equipment inside under the outdoor application scene. However, it is difficult to achieve both high air permeability and high water resistance, since in order to achieve high air permeability, it is expected that a decrease in water resistance is brought about, and vice versa, whether the film thickness is reduced or the porosity of the film is increased.

For example, the current long node-fibril structure PTFE membrane often has difficulty in achieving both high air permeability and high water resistance, as disclosed in application publication No. CN112717728A, has a pore structure with a high degree of apparent orientation, and since the PTFE porous membrane is made by a stretching method, it has a small difference in the thickness direction and is in a substantially symmetrical structure, and therefore, the highly oriented pore structure exists not only on the surface but also inside the membrane. That is, in the thickness direction of the membrane body, the long diameters of the pore structures all extend in approximately the same direction, which means that the pore structures have a large overlap in the thickness direction, and in order to secure a high air permeation amount, it is necessary to secure the membrane body with a pore structure of a large size and/or a small membrane thickness, which all result in a decrease in the waterproof ability of the porous membrane.

Based on the above knowledge, the inventors of the present application have unexpectedly found that the air permeability of a PTFE porous membrane does not necessarily increase with an increase in film thickness, unlike what is generally considered to be that a membrane having a large thickness should not be used in order to obtain a high air permeability (e.g., greater than 3 μm, even greater than 5 μm, greater than 10 μm). In particular, when the thickness of the film body is 3-40 μm and the surface of the film body is provided with radial strip-shaped holes, and the film surface forms a three-dimensional network structure with higher integrity and stronger self-supporting capability of reinforcing bulk surface nodes through reinforcing fibers and distribution fibers, the PTFE porous film can still have the thickness of not less than 2 multiplied by 10 ⁴ ml/min/cm ² The porous film of the present application has a high air permeability of @7kPa, and further, has a good waterproof effect, so that the porous film of the present application has both high waterproof property and high air permeability, which are considered to be difficult to achieve.

Wherein, when the gas flows inside the porous membrane, the gas is mainly subjected to the resistance of the surface of the solid part (such as a fiber structure and a node structure) in the membrane, so that the higher the porosity of the membrane, the less the solid part, the lower the resistance to the gas flow is, which is generally considered to be; the smaller the film thickness, the lower the resistance of the solid part of the gas flow is naturally, so that the porous film with high porosity and low thickness naturally has higher air permeability. However, by incorporating the long-strip-like pore structure in combination with the block-like surface node structure (instead of the long node structure) in the present application, the porous film can have a film thickness of 3 to 40 μm and still have a thickness of not less than 2X 10 ⁴ ml/min/cm ² High permeability at 7 kpa.

This is probably because the elongated holes in the present application mean that there is no solid structure forming the gas flow resistance in the long diameter direction of the hole structure, and although the gas is still unavoidable to be subjected to the resistance of the fiber structure on both sides and the node structure, its low resistance in the long diameter direction of the elongated holes causes the gas to be subjected to significantly lower gas resistance.

More importantly, the nodes are also an important component of the solid structure in the membrane and are an important source of gas flow resistance, the node structure in the application is not a conventional long node structure, but a block node structure, the block node structure means that part of the long nodes are converted into a fiber structure, the pore structure formed between the fiber structures obviously has lower gas resistance compared with the long node structure of a pure entity, and the less solid structure also tends to mean relatively higher porosity so as to ensure low gas resistance. In addition, the long node structure often exists on the surface and extends into the membrane (as explicitly shown in the drawing of application publication No. CN 112717728A), so that the membrane structure is divided into a plurality of mutually independent areas, when gas flows in the membrane, the gas is difficult to flow between the mutually independent areas, the limitation on the gas flow path is large, and the gas flow path is difficult to optimize by itself.

Compared with a long node structure, the block-shaped nodes in the application extend into the membrane as well, but the block-shaped nodes cannot separate the areas in the membrane, so that when gas flows in the membrane, the gas is only limited by the resistance of the surfaces of the nodes and is not excessively limited by the flow of the nodes, the mutual flow resistance of the gas in the areas in the membrane main body is greatly reduced, the limitation of the paths when the gas flows is greatly reduced, and the gas tends to form a low-gas-resistance flow path when the gas flows through the membrane main body so as to further improve the ventilation rate. Therefore, the long-strip-shaped pore structure and the block-shaped node structure have low air resistance, and can optimize the air flow path, so that the porous membrane of the application has the thickness of not less than 2 multiplied by 10 on the basis of 3-40 mu m ⁴ ml/min/cm ² High permeability at 7 kpa.

In the present application, the surface of the porous film is observed to have a significant radial pore distribution, which means that the longitudinal direction of the pores is directed at various angles (low degree of orientation) instead of being directed in approximately the same direction (high degree of orientation), on the basis of ensuring high air permeability of the porous film.

The radial holes mean that the fiber structures in each plane in the thickness direction are arranged in a low orientation mode, all layers of fibers are mutually supported when being pressed, and the possibility of water seepage caused by excessive deformation of the membrane structure when being pressed by water can be greatly reduced by matching with the surface node structure with the local strength reinforcing effect. In addition, in the three-dimensional network structure formed by the coarse and fine fiber structures matched with the block nodes, the pore structures among the adjacent coarse fibers form channels for gas to flow, and the fine fiber structures among the coarse fibers are smaller in size and smaller in gas flow resistance, but the porous membrane with the strip-shaped pore structures has the advantages that the fine fiber structures among the coarse fibers greatly improve the repulsive force to liquid water through the hydrophobicity of the porous membrane, so that the coarse fibers matched with the fine fibers can form size exclusion and hydrophobic exclusion effects to liquid water, and only form size exclusion effects to gas. At this time, the gas relatively easily passes through the pore structure between the fibers, while the liquid water does not easily pass through the three-dimensional network structure having a relatively complete hydrophobic repulsive force. In addition, stacking of radial holes in the thickness direction means that the aperture structure has a lower overlap in the thickness direction, resulting in a relatively higher tortuosity, the improvement of which has less impact on gas flow under the action of the block nodes, while larger size exclusion fits a more complete hydrophobic three-dimensional network structure, meaning a significantly improved waterproof ability.

It should be noted that radial holes means that fiber structures forming holes are also cross-layered and supported with each other in the thickness direction, and the reinforcing fibers with a 10-40% ratio are used to reinforce the three-dimensional network structure in cooperation with the specific block surface nodes of the present application, so that the porous membrane of the present application has high mechanical strength even without long node structures with large size and high reinforcing effect. The high mechanical strength means that the porous film is less likely to collapse in pore structure at higher air pressure, and therefore, even though the PTFE film having a radial pore structure in the present application has a longer actual flow path, it has a lower air resistance and a higher air permeation amount.

For the current common PTFE membrane with a long fiber-original node structure, the hole structure with higher orientation degree leads the coincidence ratio of the hole structure of the PTFE membrane to be higher in the thickness direction, and when the feed liquid flows through the holes with no lamination but higher coincidence ratio in the thickness direction, the actual flow path length and the film thickness are smaller, so that the PTFE membrane with higher air permeability also has higher water permeability, and the waterproof effect is poorer. Thus, PTFE membranes having highly oriented pore structures have too high a water permeability that makes them unsuitable for breathable applications where water resistance is a requirement, and would likely result in damage to the device once the outside fluid has penetrated and entered the interior of the device.

In summary, the PTFE membrane of the present application has a radially distributed elongated pore structure, and, in combination with the block-shaped surface nodes and the reinforcing fibers, can provide the porous membrane with a lower gas flow path for gas resistance and a higher resistance to pore collapse, thereby having a higher air permeability; the high tortuosity caused by the low overlapping ratio of the radial distributed long strip holes in the thickness direction and the three-dimensional network structure formed by the cooperation of the thick fiber and the thin fiber, which is hydrophobic and not hydrophobic, ensure that the porous membrane has good waterproof effect even if the porous membrane has high ventilation quantity.

It is understood that the ventilation amount is not less than 2X 10 ⁴ ml/min/cm ² By @7kPa is meant that the PTFE porous membrane is capable of passing an amount of gas of not less than 2X 10 per square centimeter of membrane per minute at a gas pressure of 7kPa ⁴ ml. The testing method comprises the following steps: the effective ventilation area is Scm ² The diaphragm sample is stuck on the surface of a stainless steel tool with an opening (the diameter of the opening is 1.0 mm), and the other end of the stainless steel tool is connected with a gas pressure sensor, a gas flow sensor, a gas pressure regulating valve and a gas source. Regulating the gas pressure regulating valve to enable the indication number of the gas pressure sensor to reach 7kPa, and reading Q mL/min on the gas flow sensor, wherein the ventilation rate of the waterproof ventilation membrane assembly is Q/Sml/min/cm ² @7kPa。

The measurement mode of various surface morphology parameters (such as thickness, fiber diameter, aperture, hole area rate and the like) of the porous membrane can be realized by using a scanning electron microscope to characterize the morphology of the membrane structure, and then computer software is utilized(e.g., matlab, NIS-Elements, etc.) or manually, and corresponding calculations are performed. In practice, the surface (or cross section) of the membrane can be characterized by electron microscopy to obtain corresponding SEM image, and selecting a certain area such as 10000 μm ² (100 mu m multiplied by 100 mu m), the specific area is determined according to the actual situation, the corresponding computer software or manual measurement is used for measuring the pore diameters, fiber diameters and other morphological parameters of all the holes on the area, and then calculation is carried out to obtain the average pore diameter, average fiber diameter and other morphological parameters of the area; of course, the person skilled in the art can also obtain the above parameters by other measuring means, which are only used as reference.

Optionally, the porous body has a relative deviation of the breadth tensile strength from the film to tensile strength of no more than 60%; the soapy water permeation time of the PTFE porous membrane is not less than 10min.

By adopting the technical scheme, the PTFE film with the conventional long node-fibril structure has high orientation degree due to the fiber and pore structure, the mechanical property of the PTFE film also has obvious orientation degree, and the tensile strength in the width direction and the tensile strength in the film direction of the PTFE film tend to have large difference. The application reduces the orientation degree of the porous membrane pores and fibers by controlling the pore structure of the porous membrane to be radial distribution. And when the relative deviation between the breadth tensile strength of the porous membrane and the tensile strength of the membrane is not more than 60%, the waterproof effect of the PTFE porous membrane is further improved, and on the basis of ensuring high air permeability, the soapy water permeation time is not less than 10min.

This is probably because, when the relative deviation of the width tensile strength of the porous film and the film tensile strength is not more than 60%, it is explained that the degree of orientation of the pore structure and the fiber structure of the porous film is low and that the porous film has a better cross-supporting effect in the thickness direction, ensuring that the porous film has good resistance to pore collapse even if the porosity is high, in cooperation with the block-shaped node structure and the reinforcing fiber, so that the porous film of the present application has very good air permeability under the action of a higher gas pressure to ensure a high air permeability of the porous film.

If the relative deviation between the width tensile strength of the porous membrane and the tensile strength of the membrane is greater than 60%, the fibrous structure and the pore structure have higher orientation degree, the cross supporting effect of the fibrous structure in the thickness direction is poor, and even if the local reinforcement of block nodes and the structural reinforcement of the reinforced fibers exist, the three-dimensional network structure is still likely to generate certain pore collapse phenomenon under the action of higher air pressure, so that the air permeability is reduced. In addition, the higher degree of pore orientation also means that the tortuosity of the internal flow passage of the porous membrane is reduced, and the formed hydrophobic three-dimensional network structure has more water flow passages and is more prone to water leakage.

It is understood that the relative deviation refers to the ratio of absolute deviation to average value, and the relative deviation of the tensile strength of the width and the tensile strength of the film in the direction of the width of the porous film and the tensile strength of the film in the direction of the length of the porous film are respectively tested by a universal tensile tester, the average value of the tensile strength of the width of the porous film and the tensile strength of the film in the direction of the length of the porous film is calculated, and the relative deviation of the tensile strength of the width of the porous film, the tensile strength of the film in the direction of the film and the average value of the tensile strength of the film and the film is further calculated.

And the soapy water permeation time is not less than 10min, namely, the PTFE porous membrane is stuck to the jig, soaked in the soapy water with the concentration of 0.1g/L, placed in a roller to shake for at least 10min, and no water enters the jig. The soapy water is easier to wet and penetrate the porous membrane than ordinary deionized water, and therefore the soapy water penetration time can be used to characterize the water resistance of the porous membrane.

Optionally, the distribution fibers are dispersed among the reinforcing fibers, and the distribution fibers account for 10-60% of all surface fibers.

By adopting the technical scheme, the reinforced fiber is matched with the mutual cross support of the fiber in the thickness direction through the strong support capacity of the reinforced fiber, so that the collapse of the holes of the porous membrane under the action of high air pressure can be reduced, and the high air permeability is ensured. However, in order to ensure high air permeability of the porous membrane, the pore structure size of the porous membrane should not be too small, so that the hydrophobic three-dimensional network structure formed by the simple reinforcing fibers tends to have high porosity, and many hydrophobic weak points exist, so that it is difficult to ensure good waterproof effect. The distributed fiber structures with smaller diameters are dispersed among the reinforced fibers and form a hydrophobic three-dimensional network structure with higher hydrophobic integrity together with the reinforced fibers, so that high waterproof performance is ensured; further, although the three-dimensional hydrophobic network formed by the reinforcing fibers and the distribution fibers has fewer hydrophobic weak points, the distribution fibers have smaller fiber diameters, and the hydrophobic property does not play a role in expelling gas, so that the three-dimensional hydrophobic network has a small gas resistance rise although the waterproof property is remarkably improved.

If the distribution fiber is relatively low (e.g., less than 10%), it is indicated that there are a large number of medium-sized fiber structures (smaller in diameter than the reinforcing fibers but larger than the distribution fibers) in the surface fibers, and this medium-sized fiber structure has better mechanical strength and better hydrophobic effect, but a larger size also tends to mean a larger gas resistance, thereby affecting the air permeability of the porous membrane. If the distribution fiber accounts for a relatively high percentage (e.g. less than 60%), it means that there are a large number of distribution fibers with smaller size in the surface fibers, and the excessively small size of the distribution fibers means that the three-dimensional network structure formed by the distribution fibers is relatively compact, but has poor mechanical strength, under the action of relatively large gas pressure, once the three-dimensional network structure is compressed, the pore structure collapses, which results in the decrease of ventilation, and the damage of the pore structure in a partial area easily results in the water seepage phenomenon. Therefore, by further controlling the ratio of the distribution fibers, the porous film can be made to have a further preferable air permeation amount and water-repellent performance.

Optionally, the length-diameter ratio of the reinforcing fiber is 60-150, and the length-diameter ratio of the distribution fiber is 450-650.

By adopting the technical scheme, the reinforcing fiber has a great influence on the pressure resistance of the three-dimensional network structure of the porous membrane, and the length-diameter ratio of the reinforcing fiber is further controlled to be 60-150 on the basis that the ratio of the reinforcing fiber is 10-40% and the block node reinforcement exists, so that the porous membrane can be ensured to have high ventilation and high pressure resistance.

If the aspect ratio of the reinforcing fiber exceeds 150, although a longer aspect ratio tends to mean that the size of the pores formed by the reinforcing fiber in the long diameter direction is larger and the fiber diameter is smaller, and although the porous film has a lower gas flow resistance, an excessively large size of the pores in the long diameter direction means that the self-supporting property of the formed three-dimensional network structure is poor, and even if there is reinforcement of the fiber mutual support and the block-shaped node in the thickness direction, it is difficult to ensure that the porous film has a good pressure-resistant property. In addition, even if a low overlap in thickness is formed by radial hole distribution in a hole structure having a large long diameter size, the tortuosity of the flow path may be low, resulting in a decrease in the waterproof effect of the porous membrane. And the collapse of the pore structure of the porous membrane after being pressed can affect the ventilation quantity of the porous membrane, and the local pore structure damage can also improve the possibility of water leakage.

If the aspect ratio of the reinforcing fiber is less than 60, it means that the size of the holes formed by the reinforcing fiber in the longitudinal direction is small and the fiber diameter is large, which ensures good pressure resistance and water resistance of the porous membrane, but the too small size of the holes in the longitudinal direction and the large size of the fibers, and the low overlap ratio of the fibers and the pore structure in the thickness direction due to the radial distribution of the holes, all cause too high gas resistance of the porous membrane, so that the ventilation amount of the porous membrane is low.

The distributed fibers dispersed among the reinforcing fibers can form a more complete hydrophobic three-dimensional network with the reinforcing fibers and the rest of the surface fibers on the premise of only slightly improving the gas resistance, thereby improving the water resistance of the porous membrane on the basis of ensuring high gas permeability. However, when the aspect ratio of the manufactured reinforcing fibers is in the range of 60 to 150, if the aspect ratio of the distribution fibers exceeds 650, it is explained that the diameter of the distribution fibers is too small, and a higher air permeability can be obtained by making the resistance against gas smaller, however, in the case of the distribution fibers having a smaller diameter, the improvement of air permeability is limited even if the diameter is further reduced, but the hydrophobic property of the distribution fibers having too small diameter is insufficient, and it is difficult to ensure the hydrophobic integrity of the three-dimensional hydrophobic network formed, and water is liable to leak from the hydrophobic weak points. If the aspect ratio of the distribution fibers is less than 450, it means that the diameter of the distribution fibers dispersed among the reinforcement fibers is too large, the distribution fibers with larger diameter and the rest of surface fibers with larger size form a more compact three-dimensional network structure, and the porous membrane has too high gas resistance due to the fact that the porous membrane has a specific radial distribution pore structure, and the pore structures are mutually laminated in the thickness direction.

Optionally, the surface nodes have a distribution density of (10-30) pieces/10000 μm ² The surface nodes have an area of not less than 150 μm ² Node aggregates and areas of not more than 50 μm ² The number of node aggregates is less than the number of scatter nodes.

By adopting the technical scheme, in the long node-fibril structure with obvious long diameter which is common at present, the long node structure has better reinforcing effect on the three-dimensional network structure of the porous membrane, but the high resistance of a large-volume solid part to gas and the limitation of the gas flow path caused by dividing the membrane structure into multiple regions can lead to the reduction of the gas flux of the porous membrane. The porous membrane of the application has special node aggregate and dispersed node structure, and has larger size and area of not less than 150 μm although both can produce reinforcing effect on the three-dimensional network structure ² Clearly has a node aggregate of not more than 50 μm compared to the area ² The dispersed nodes of (a) have a stronger reinforcing effect, however, on the basis of the better reinforcing effect brought by the node aggregate, larger gas resistance is also often caused. Therefore, the number of the control node aggregates is smaller than that of the dispersed nodes, and the three-dimensional network structure is reinforced by matching the node aggregates with small number, large size and good reinforcing effect with the dispersed node structures with poor reinforcing effect, large number, small size and small air resistance, and the porous membrane is ensured not to easily collapse and damage the pore structure under the action of higher pressure by matching the fiber structures which are mutually crossed and supported in the thickness direction.

The distribution density of the surface nodes is controlled to be (10-30) per 10000 mu m ² Ensure that the distribution density of node structures in the three-dimensional network structure of the porous membrane is not too low (not less than 10/10000 μm) ² ) Through the mutual intersection and support of the local reinforcement effect of the node structure and the fiber structure in the thickness direction, the porous membrane is ensured not to be easy to generate pore knots under the action of higher gas pressureCollapse and destruction of the structure, reducing the possibility of air permeability reduction and water seepage; of course, the distribution density of the node structure in the three-dimensional network structure of the porous membrane is not preferably too high (not more than 30 pieces/10000 μm ² ) So as to prevent the node structure which is a solid part and has large size from forming excessive gas resistance. That is, by controlling the distribution density of the surface nodes and controlling the node aggregate with larger size in the surface nodes to occupy less amount, and matching with the radial distributed fiber structure, the porous membrane can obtain higher compressive strength and still have higher ventilation capacity.

Optionally, the node aggregate has a distribution density of (0.2-2) pieces/10000 μm ² The distribution density of the dispersed nodes is (5-25) per 10000 μm ² 。

By adopting the technical scheme, the control area is not smaller than 150 mu m ² The distribution density of the node aggregates of (2) is not less than 0.2/10000 μm ² The node aggregate can be ensured to have good reinforcing effect on the regional three-dimensional network structure; and the distribution density of the control node aggregate is not more than 2/10000 μm ² It can be ensured that the large volume of node aggregates has no excessive influence on the gas resistance. Control area is not more than 50 mu m ² The distribution density of the dispersed nodes of (2) is not less than 5/10000 μm ² The distribution nodes with relatively poor reinforcing effect can be ensured to form better reinforcing effect through relatively more distribution nodes; and the control area is not more than 50 mu m ² The distribution density of the dispersed nodes of (a) is not more than 25/10000 μm ² On the basis of ensuring a good reinforcing effect, the possibility that the dispersed nodes with larger size than the fiber structure form excessive gas resistance can be reduced.

In summary, the node aggregate with good reinforcing effect but large air resistance needs to be controlled to be smaller in number, the dispersing nodes with poor reinforcing effect but small air resistance need to be controlled to be relatively larger in number, the node aggregate and the dispersing nodes cooperate to form good reinforcing effect, and the node aggregate and the dispersing nodes cooperate with each other to form a fiber structure distributed in a radial manner to have a mutual supporting effect in the thickness direction, so that the porous membrane can be ensured to have good compressive strength and higher air permeability.

Optionally, aThe surface fibers comprise longitudinal fibers and transverse fibers, the angle between the longitudinal fibers and the longitudinal stretching direction is not more than 55 degrees, the angle between the transverse fibers and the transverse stretching direction is less than 35 degrees, and the SEM measured average length of the longitudinal fibers is larger than the SEM measured average length of the transverse fibers; the breathable film has a breathable rate of not more than 25X 10 ⁴ ml/min/cm ² @7kpa。

By adopting the technical scheme, the holes of the porous membrane are distributed in a fiber radial mode, and the hole structure is surrounded by the fiber structure, so that the fiber structure is distributed in a fiber radial mode similar to the hole structure. The radially distributed fibrous structure may be divided into transverse fibers and longitudinal fibers, which cross each other and are supported by each other in the thickness direction, thereby improving the pressure resistance of the porous membrane. In addition, the longitudinal fibers refer to surface fibers having an angle of not more than 55 ° with the longitudinal stretching direction, which refers to the film running direction (film length direction) at the time of production; the transverse fibers are surface fibers with an included angle of less than 35 degrees with the transverse stretching direction, and the transverse stretching direction is perpendicular to the film running direction (film width direction) during production.

In the application, by controlling the length of the longitudinal fibers to be longer than that of the transverse fibers, a long-diameter hole structure with a larger length can be formed by the longer longitudinal fibers, so that high ventilation can be ensured, and the shorter transverse fibers form a hole structure with a shorter length, but can form good support on the longitudinal fibers in the thickness direction, so that the pressure resistance of the hole structure formed by the longitudinal fibers with weaker self-supporting capability due to the larger length is improved. And the pore structure formed by the transverse fibers has a significantly larger ventilation effect than the long node structure (pure entity) which is common at present, although the long diameter is shorter. Therefore, the transverse fibers have the effect of forming holes to improve the ventilation capacity, and also have the effect of forming longitudinal fibers with poor self-supporting performance to improve the pressure resistance of the porous membrane, so that the porous membrane has both higher ventilation capacity and higher pressure resistance.

Optionally, the average spacing between adjacent longitudinal fibers is 2.5 to 7.5 μm; the average spacing between adjacent transverse fibers is 0.5-3 μm.

By adopting the technical scheme, the distance between the adjacent longitudinal fibers reflects the short-diameter length of the hole structure formed by the longitudinal fibers to a certain extent, the distance between the transverse fibers reflects the short-diameter length of the hole structure formed by the transverse fibers to a certain extent, and the short-diameter length of the hole structure has important influence on the gas resistance and the waterproof performance of the hole structure.

When the average spacing of the longitudinal fibers is less than 2.5 μm and/or the average spacing of the transverse fibers is less than 0.5 μm, it is indicated that the spacing between the transverse and longitudinal fibers is small, and the adjacent transverse and longitudinal fibers form an elongated hole structure, the hole structure has higher density, and although a hydrophobic three-dimensional network structure with higher integrity can be formed, the water-proof effect is better, too high density also means larger gas resistance, and thus the ventilation rate is low. When the average distance between the longitudinal fibers is larger than 7.5 μm and/or the average distance between the transverse fibers is larger than 3 μm, the distance between the transverse fibers and the longitudinal fibers is larger, which means that the hole structure formed by the longitudinal fibers has larger long diameter but larger size of short diameter, and for the hole structure formed by the longitudinal fibers, the hole structure has lower air resistance and larger air permeability, but the formed hydrophobic network has poorer integrity, and water is still subjected to the hydrophobic acting force of the fibers at two sides of the hole and the node structure, but water seepage problem is easy to occur under certain water pressure acting force; for the pore structure formed by the transverse fibers, this means that the supporting ability of the transverse fibers themselves is poor, and it is difficult to form good support for the longitudinal fibers adjacent in the thickness direction, resulting in a decrease in the pressure resistance of the porous membrane, and the probability of pore collapse and destruction under a large pressure is greater.

Optionally, the ratio of the number of the longitudinal fibers to the number of the transverse fibers is 1.5 to 6; the ratio of the SEM measured average length of the longitudinal fibers to the SEM measured average length of the transverse fibers is 1.5 to 4.5.

By adopting the technical scheme, if the ratio of the number of the longitudinal fibers to the number of the transverse fibers is more than 6, the longitudinal fibers in the surface fibers are higher, and the longitudinal fibers have certain orientation, so that the longitudinal fibers have higher orientation degree, which leads to insufficient tortuosity of the formed flow path and easy water seepage; and too few transverse fibers are difficult to form good support for longitudinal fibers with relatively poor self-supporting performance, so that the porous membrane is easy to collapse or damage due to compression, and the air permeability is reduced, and the water seepage phenomenon is generated. If the ratio of the number of the longitudinal fibers to the number of the transverse fibers is less than 1.5, the ratio of the longitudinal fibers with larger length in the surface fibers is not high, the ratio of the transverse fibers with smaller length is not low, and the transverse fibers with higher ratio can form good support for the longitudinal fibers, but the hole structures formed by the transverse fibers are shorter in length and diameter and poorer in ventilation quantity; although the pore structure formed by the longitudinal fibers has a larger long diameter and thus a larger air permeability, the longitudinal fibers occupy a relatively low proportion, and it is difficult to ensure a porous film having a higher air permeability.

On the basis of the number ratio of the longitudinal fibers to the transverse fibers being 1.5-6, the length ratio of the longitudinal fibers to the transverse fibers is further controlled to be 1.5-4.5, on the one hand, the longitudinal fibers have enough length to ensure the ventilation capacity of the porous membrane, and on the other hand, the transverse fibers are ensured not to be too long to influence the support of the longitudinal fibers on the basis of supplementing the ventilation capacity of the transverse fibers. When the ratio of the two lengths is 1.5-4.5, the longer longitudinal fibers form a hole structure with higher ventilation, the shorter transverse fibers form a hole structure with certain supplementary ventilation, the self-supporting performance is stronger, and the longitudinal fibers with weaker self-supporting performance can be well supported.

Optionally, the gaps between adjacent longitudinal fibers are longitudinal holes, the gaps between adjacent transverse fibers are transverse holes, the SEM measurement long diameter of the longitudinal holes is larger than that of the transverse holes, the SEM measurement long diameter of the longitudinal holes is 60-130 μm, and the average long diameter of the transverse holes is 25-65 μm.

By adopting the technical scheme, the holes formed by surrounding the longitudinal fibers are longitudinal holes, the holes formed by surrounding the transverse fibers are transverse holes, and the length of the longitudinal holes is larger than that of the transverse holes. The longitudinal holes with the length of 60-130 mu m can ensure that the porous membrane has good ventilation, and the transverse holes with the length of 25-65 mu m can further compensate the ventilation, so that the porous membrane has good ventilation, and can form a support for longitudinal fibers with poor self-supporting performance through stronger self-supporting capability, thereby ensuring that the porous membrane has good pressure resistance even though the porous membrane has higher ventilation, and further reducing the possibility of water seepage caused by damage of a pore structure when resisting higher water pressure.

Optionally, the water contact angle of the first outer surface and the second outer surface is 110-150 degrees, and the surface density of the porous main body is 1.2-7.5 g/m ² 。

By adopting the technical scheme, the water contact angle shows the water drainage resistance of the porous membrane hydrophobic three-dimensional network structure to a certain extent, and the porous membrane structure with high integrity is formed by matching the radially distributed pore structure with the reinforced fiber, the distributed fiber and the block node structure. The water contact angle of the surface of the porous membrane is not lower than 110 degrees, so that a good waterproof effect of the porous membrane can be ensured, the water contact angle of the surface of the porous membrane is not higher than 150 degrees, and the problem that the air permeability of the porous membrane is reduced due to too compact hydrophobic three-dimensional network structure of the porous membrane can be avoided.

The surface density of the porous main body shows the compactness of the porous membrane hydrophobic three-dimensional network structure to a certain extent, and the higher the compactness is, the better the waterproof capability is, but the lower the ventilation is, and vice versa. The porous main body is controlled by special fiber and node structures, and the porous main body has good ventilation and waterproof performances.

In a second aspect, the application provides a process for preparing a porous PTFE membrane with a low-orientation pore structure, which adopts the following technical scheme:

A preparation process of a PTFE porous membrane with a low-orientation pore structure comprises the following process steps:

s1, mixing and swelling, namely uniformly mixing film-forming resin and auxiliary oil, and swelling for 8-24 hours at the temperature of 30-60 ℃ to obtain a pasty mixture, wherein the film-forming resin is prepared from low-crystallinity PTFE resin with the crystallinity of 90-93% and high-crystallinity PTFE resin with the crystallinity of not less than 94% according to the mass ratio of 1: (1-4) mixing to obtain;

s2, pre-forming a film, and extruding the pasty mixture to form an oil-containing base band with the thickness of 0.15-0.45 mm;

s3, degreasing, namely heating the oil-containing baseband to remove auxiliary oil to obtain an oil-free baseband;

s4, longitudinally stretching, namely placing the oilless base band in an environment of 200-300 ℃ to carry out longitudinal stretching, wherein the stretching multiple is 8-20 times, and obtaining a unidirectional stretching film;

s5, heat treatment, namely, placing the unidirectional stretching film at the temperature of 300-360 ℃ for heat treatment for 0.5-2 min to obtain an unwrapped film;

s6, transversely stretching the unwrapped film to obtain a biaxially stretched film, wherein the transverse stretching multiple is 1.2-2 times of the longitudinal stretching multiple, and the stretching temperature is 20-60 ℃ lower than the longitudinal stretching temperature;

s7, heat setting, namely, heat setting the biaxially oriented film at the temperature of 330-390 ℃ for 1-10 min.

By adopting the above technical scheme, it is considered that in order to ensure film forming performance and strength of the film, a film forming resin having a low crystallinity should not be used, because the film forming resin having a low crystallinity tends to have poor mechanical strength itself, and it is difficult to ensure mechanical strength of the produced porous film. However, the inventors of the present application have unexpectedly found that, compared to using a single high-crystallinity PTFE resin as the film-forming resin, a high-low-crystallinity PTFE mixed resin is used as the film-forming resin, and the crystallinity of the low-crystallinity PTFE resin is controlled to be 90 to 93%, and the crystallinity of the high-crystallinity PTFE resin is controlled to be not less than 94%, the mass ratio of the two being 1: (1-4) and matching with the subsequent special biaxial stretching technology, the prepared porous membrane has higher strength, higher ventilation and good waterproof effect.

This may be due to the poor fiber forming properties of low crystallinity PTFE resins compared to high crystallinity PTFE resins, which are more prone to form blocky nodes and thicker reinforcing fibers when subjected to stretching forces; the reinforcing effect of reinforcing fibers and block nodes is superior to that of the porous membrane with special radial holes and fiber distribution, and the effect of the fiber strength improvement on the strength of the porous membrane caused by the high-crystallinity PTFE resin is better than that of the porous membrane. By controlling the crystallinity of the low-crystallinity PTFE resin to be not low (e.g., less than 90%) and the ratio to be not high (e.g., not more than 50 wt%), it is possible to ensure that even the crystallinity of the low-crystallinity PTFE resin has a certain fiber forming property so as not to form too many, too coarse fiber structures and block nodes, and thus the ratio of the formed reinforcing fiber and block node structures is not high, ensuring that the produced porous film has a high air permeability on the basis of a waterproof effect and mechanical properties. In addition, the crystallinity of the low-crystallinity PTFE resin is controlled to be not higher (such as higher than 93%) and the ratio of the low-crystallinity PTFE resin to be not lower (such as lower than 25 wt%), so that the low-crystallinity PTFE resin has certain fiber forming property but not good fiber forming property, a finer fiber structure is prevented from being generated when the low-crystallinity PTFE resin is biaxially stretched, the surface of the porous film is ensured to have a required blocky node and a reinforced fiber structure with the number of 10-40%, and the porous film is ensured to have good pressure resistance and waterproof effect on the basis of higher air permeability by matching with radially distributed holes and fiber structures.

In addition, a specific biaxial stretching process is also required to be matched on the basis of the PTFE mixed resin with high and low crystallinity as a film forming resin, so that the porous film with high water resistance and high air permeability can be ensured. Namely, longitudinal fibers in the porous film are formed by longitudinal stretching; performing heat treatment after the longitudinal stretching is finished to remove stress concentration and molecular chain disentanglement caused in the longitudinal stretching process; then a low-temperature transverse stretching process is adopted to stretch to obtain transverse fibers and pull the fibers of the micro-parallel yarns during heat treatment; the porous membrane has a radially distributed pore structure.

It should be noted that the application does not adopt the high temperature hot drawing process which is generally considered to be better at present, but adopts the special high Wen Zongla +high temperature pyrolysis winding+low temperature transverse drawing biaxial drawing process. It is generally believed that for stretch-formed films, the film tends to soften at higher temperatures, the transfer of tensile stresses within the film is faster, the molecular chains tend to disentangle, and naturally to be drawn out into fibers, thereby forming the desired fiber structure. However, since a certain amount of low-crystallinity PTFE resin with crystallinity of 90-93% is added in the application, the low-crystallinity PTFE resin has better toughness, is easier to soften at higher temperature and has lower elastic modulus at high temperature. This results in a membrane having a relatively high degree of porosity due to the longitudinal stretching of the membrane, and a relatively low tensile stress resulting in a relatively high strain in the membrane with a relatively high degree of porosity and a relatively low modulus of elasticity due to the high temperature, and the presence of the low crystallinity PTFE resin results in a significantly greater increase in strain than the strain change resulting from the reduced tensile strength, for the system of the present application wherein the film is a film-forming resin. Because the tensile stress is mainly used for causing deformation of the membrane and is not used for stretching and forming fibers, the phenomenon that the fiber is easily formed by the abnormal low-temperature transverse drawing (the transverse drawing temperature is 20-60 ℃ lower than the longitudinal drawing temperature) occurs for the special film forming system of the application. At lower temperature, the elastic modulus of the membrane is higher, and the tensile stress is mainly used for stretching and forming fibers, so that the membrane is easier to form fibers, and the temperature is not too low (only lower than the longitudinal stretching temperature by 20-60 ℃), so that the stress transmission and molecular chain disentangling capacity are still stronger at the moment, and the membrane still has better fiber forming capacity when being pulled by Wen Heng.

Of course, the decrease of the transverse stretching temperature inevitably leads to the problems of slow stress transmission and difficult entanglement of molecular chains, and in order to solve the problems of difficult entanglement of molecular chains during low-temperature transverse stretching, the membrane is subjected to high-temperature heat treatment after longitudinal stretching and before transverse stretching without applying external stretching force, so that the stress concentration in the membrane during longitudinal stretching can be eliminated (the stress is easier to uniformly and quickly transmit during low-temperature transverse stretching), and the molecular chains in the membrane are promoted to be disentangled. Of course, the heat treatment time is not too long, and the heat treatment temperature is not too high, so that the fiber structure generated during longitudinal stretching is prevented from being excessively melted and combined (2 or even a plurality of fibers are fused into 1 fiber), a fiber structure with local oversized is formed, and the ventilation amount of the prepared porous membrane is influenced; in addition, excessive heat treatment can also lead to the diaphragm intensity to be improved before low-temperature horizontal drawing, and the fiber structure with higher intensity is difficult to transfer stress, so that the diaphragm is difficult to stretch into fibers during low-temperature horizontal drawing, and the stress is too concentrated, so that the fiber breakage is easier to occur during low-temperature horizontal drawing, and the structure of the diaphragm hole is damaged.

It should be noted that, by controlling the stretching ratio of the longitudinal stretching to 8 to 20 times, a sufficient number of fiber structures can be obtained by stretching in the longitudinal stretching (sufficient stretching can ensure that the porous film has a desired distributed fiber structure), but by performing the heat treatment after the longitudinal stretching, a micro-drawing phenomenon of part of the fibers is unavoidable, and at this time, the fiber drawing phenomenon will cause a stress concentration phenomenon at low temperature transverse stretching. By controlling the stretching ratio of the transverse stretching to be 1.2 to 2 times the longitudinal stretching ratio, it is possible to redraw the fiber partially lightly textured during the heat treatment (made of a low crystallinity PTFE resin as a whole, hardly pulled, only the fiber slightly textured during the short-time heat treatment can be pulled), and stretch the long node structure into a block node and fiber structure, thereby making the produced porous film have a remarkable radial pore structure.

In summary, the application can ensure that the prepared porous membrane has a required radial distribution pore structure and 10-40% of reinforced fiber and blocky surface nodes by selecting a mixed film-forming resin system of PTFE resin with high and low crystallinity and matching with a special high Wen Zongla +high-temperature pyrolysis winding+low-temperature transverse drawing biaxial stretching process, thereby ensuring that the porous membrane has high air permeability and high waterproof performance.

It can be understood that the high-low crystallinity PTFE resin of the present application can be obtained by directly purchasing a resin with a desired degree of crystallinity, or by purchasing a resin with a certain degree of crystallinity and then subjecting the resin to a heat treatment or the like to control the degree of crystallinity of the resin, thereby obtaining a resin with a desired degree of crystallinity.

Optionally, in the step S1, the solid content of the pasty mixture is 70% -85%; the auxiliary oil is at least one of petroleum ether, solvent naphtha and aviation kerosene.

Optionally, in the step S4, the longitudinal stretching is performed in 4 to 20 steps;

in the step S7, the biaxially oriented film is subjected to micro stretching with a stretching multiple of 1.1-1.5 during heat setting, and a discontinuous stretching process is adopted, specifically, the stretching force is removed and kept for 10S every 20S of stretching, and the finished PTFE film is obtained after heat setting.

By adopting the technical scheme, the density of the membrane is high during longitudinal stretching, and if the membrane is rapidly pulled longitudinally, the tensile stress is likely to be difficult to transfer in time, so that the uniformity of the pore structure obtained by stretching is reduced, and the defect of fiber breakage in a partial region is caused.

Furthermore, in the heat setting process after the two-way stretching is finished, the micro stretching with the stretching multiple of 1.1-1.5 is introduced, a specific intermittent stretching process is adopted, a PTFE resin system with high and low crystallinity is cooperated, and a special two-way stretching process is adopted, so that the porous membrane can be further ensured to obtain the required supporting fiber and block nodes. This is probably due to the fact that when biaxially stretched films are heat-set at a relatively high temperature, they have a relatively high ductility, and at this time, a relatively high strain can be obtained by applying a small multiple of stretching, so that part of the adjacent fiber structures are in contact with each other, even fused, and a partial doubling phenomenon occurs, thereby obtaining the desired thicker reinforcing fibers. It should be noted that if the stretching ratio is too high or a continuous stretching process is adopted during heat setting, the fiber structure is likely to be excessively fused, so that an oversized fiber structure is generated, and instead, larger air resistance is generated, so that the ventilation is affected; and the pore structure with oversized size formed after the fibers are combined also becomes a hydrophobic weak point in the hydrophobic three-dimensional network structure, so that the waterproof performance is reduced.

Optionally, the step S2 specifically includes the following process steps:

s21, preforming, namely preforming the pasty mixture at the temperature of 30-45 ℃ to obtain a cylindrical preformed blank;

s22, carrying out primary rolling, namely extruding and rolling the preformed blank body to form a sheet with the thickness of 2-5 mm, wherein the temperature of the primary rolling is 35-55 ℃;

s23, secondary rolling, namely rolling the sheet material into a base band with the thickness of 0.15-0.45 mm, wherein the temperature of the secondary rolling is 50-80 ℃ and the temperature of the secondary rolling is not lower than that of the primary rolling.

Optionally, in the step S6, when the heat treatment temperature is reduced to the transverse stretching temperature, the temperature reduction speed is 3-15 ℃/10min, and heat preservation is required for 10min every 50 ℃ of temperature reduction.

By adopting the technical scheme, the special low-temperature transverse drawing process is adopted, so that the problems of stress concentration and difficult disentanglement of molecular chains are more easily generated by low-temperature drawing. In order to ensure that the film is not defective due to stress concentration during low-temperature transverse stretching, is not difficult to form fibers due to excessive entanglement of molecular chains, local stress concentration generated during longitudinal stretching is eliminated as much as possible during heat treatment, and the molecular weight is properly disentangled. The temperature reduction speed after heat treatment is controlled not to be too high (not higher than 15 ℃/10 min), and a special temperature reduction and heat preservation process is adopted, so that concentrated stress in the longitudinally stretched film can be fully relaxed in time, and the entangled molecular weight can be fully unwound in time.

Of course, the cooling rate after heat treatment is not too slow (lower than 3 ℃/10 min), because the special high-low crystallinity PTFE mixed resin system is adopted in the application, and the cooling rate has a certain influence on the crystallinity of the film-forming resin, and too slow crystallization rate is likely to lead to the increase of the crystallinity of the low-crystallinity PTFE resin with the crystallinity not being too low, thereby leading to the rapid increase of the fiber forming performance and being difficult to generate the required reinforced fiber and blocky node structure.

In summary, the present application includes at least one of the following beneficial technical effects:

1. the porous membrane of the application can lead the porous membrane to have a gas flow path with lower air resistance and higher pore collapse resistance by introducing a strip-shaped pore structure distributed in a radial way and matching with block surface nodes and reinforcing fibers, and can lead the porous membrane with the thickness of 3-40 mu m to have the thickness of not less than 2 multiplied by 10 ⁴ ml/min/cm ² Higher ventilation at 7 kPa; the reinforced fibers with larger diameters and the distributed fibers with smaller diameters form a hydrophobic three-dimensional network structure with higher integrity, so that the porous membrane has better hydrophobic performance, and further, the radial distributed fiber structure is crossly supported in the thickness direction to be matched with block nodes and the regional reinforcing effect of the reinforced fibers, so that the porous membrane can resist higher water pressure, and therefore, the porous membrane not only has higher air permeability, but also has good waterproof performance;

2. On the basis of introducing a radial distributed long strip-shaped pore structure, the relative deviation between the width tensile strength of the porous membrane and the tensile strength of the membrane is not more than 60%, so that the porous membrane has more selective waterproofness on the basis of higher ventilation quantity, and the soapy water permeation time is not less than 10min;

3. by further controlling the surface morphology such as the length-diameter ratio of the reinforcing fibers, the length-diameter ratio of the distribution fibers, the number proportion of the distribution fibers, the density of the surface nodes and the like, the porous membrane can have more preferable high ventilation and high waterproof effects;

4. the preparation process of the application introduces low crystallinity PTFE resin into film forming resin to form high and low crystallinity PTFE mixed resin, and combines special high Wen Zongla +high temperature pyrolysis winding+low temperature transverse drawing biaxial stretching process, thus ensuring that the prepared porous film has the required radial distribution pore structure and 10-40% of reinforced fiber and blocky surface nodes, thereby ensuring that the porous film has high air permeability and high waterproof performance.

Drawings

FIG. 1 is a scanning electron micrograph of one side surface of a porous film prepared in example 1 of the present application at 100X magnification.

Fig. 2 is a further enlarged view of fig. 1 to show the structure of the body fiber and body nodes, at 500 x magnification.

Fig. 3 is a further enlarged view of fig. 2 to show the structure of the host fiber and the host node, at 1000 x magnification.

FIG. 4 is a scanning electron micrograph of one side surface of a porous film prepared in example 4 of the present application at 100X magnification.

Fig. 5 is a further enlarged view of fig. 4 to show the structure of the body fiber and body nodes, at 500 x magnification.

Fig. 6 is a further enlarged view of fig. 5 to show the structure of the body fiber and body nodes, at 1000 x magnification.

Detailed Description

The present application will be described in further detail with reference to fig. 1 to 6.

The embodiment of the application discloses a PTFE porous membrane with a low-orientation pore structure and a preparation process thereof.

Example 1

The embodiment discloses a PTFE porous membrane with a low orientation degree pore structure, and the preparation process comprises the following process steps:

s1, swelling a mixed material, uniformly mixing film-forming resin and auxiliary oil, and swelling for 16 hours at the temperature of 45 ℃ to obtain a pasty mixture with the solid content of 75 wt%; wherein the film-forming resin is prepared from a low-crystallinity PTFE resin with a crystallinity of 91.4% and a high-crystallinity PTFE resin with a crystallinity of 95.3% according to a mass ratio of 1:2, and the auxiliary oil is aviation kerosene.

S2, prefilming, specifically comprising the following process steps:

s21, preforming, namely preforming the swollen pasty mixture obtained in the step S1 into a preformed blank body which is assembled with the preform body at the temperature of 35 ℃;

s22, primary rolling, namely, performing primary rolling on the preformed blank body to form a sheet with the thickness of 2.5mm, wherein the temperature of the primary rolling is 45 ℃;

s23, secondary rolling, namely rolling the sheet material into a base band with the thickness of 0.25mm, wherein the temperature of the secondary rolling is 65 ℃.

And S3, removing oil, and heating the oil-containing baseband to 205 ℃ to remove auxiliary oil, so as to obtain the oil-free baseband.

S4, longitudinally stretching, namely, longitudinally stretching the oilless base band in the environment of 250 ℃ with the stretching multiple of 12 times, wherein the stretching is performed in 6 steps, and the single stretching is 2 times to obtain the unidirectional stretching film.

S5, heat treatment, namely, heat treating the unidirectional stretching film at the temperature of 330 ℃ for 1min to obtain the disentangled film.

S6, transversely stretching the unwrapped film to obtain a biaxially stretched film, wherein the transverse stretching multiple is 1.6 times of the longitudinal stretching multiple, namely 19.2 times of the transverse stretching multiple, and the stretching temperature is 35 ℃ lower than the longitudinal stretching temperature, namely 215 ℃; and when the disentangled film is cooled from 330 ℃ to 215 ℃, the cooling speed is 7 ℃/10min, and the temperature needs to be kept for 10min every 50 ℃ of cooling.

S7, heat setting, namely heat setting the biaxially oriented film at the temperature of 350 ℃ for 4min; and (3) carrying out micro stretching with the stretching multiple of 1.25 on the biaxially oriented film at the thermal timing, adopting a discontinuous stretching process, specifically removing the stretching force for 10s every 20s of stretching, and carrying out heat setting to obtain the finished PTFE porous film.

Examples 2 to 7

Examples 2 to 7 differ from example 1 mainly in the film formation system and the adjustment of the film formation process.

The difference between example 2 and example 1 is mainly that by controlling the biaxial stretching process, a longitudinal stretching process with a very large stretching multiple and a transverse stretching process with a small stretching multiple (1.2 times of the longitudinal stretching multiple) are adopted, and the heat preservation operation is not performed when the temperature is reduced after the heat treatment is matched, so that the fiber forming difficulty in transverse stretching is improved, the stretched porous film has a pore structure with a relatively large orientation degree, and the other process parameters are shown in table 1 in detail.

Example 3 differs from example 1 mainly in that a relatively high longitudinal stretching ratio and a relatively high transverse stretching ratio are used to give a porous film having a distributed fibrous structure with a relatively high diameter, and the remaining process parameters are detailed in table 1.

Example 4 differs from example 1 mainly in that the film-forming resin has a higher crystallinity, either a high crystallinity PTFE resin or a low crystallinity PTFE resin, and the high crystallinity PTFE resin is present in a relatively large amount in the film-forming resin, resulting in good fiber-forming properties of the film-forming resin. When the porous membrane is matched with biaxial stretching, a longitudinal stretching process with a higher multiple and a transverse stretching process with a higher multiple are adopted, so that the stretched porous membrane not only has distributed fibers with a higher proportion, but also has a small overall size and a small number of node aggregates, and the rest of process parameters are shown in Table 1 in detail.

Example 5 differs from example 1 primarily in that by controlling the film-forming resin to have a relatively low crystallinity and a relatively high duty cycle for the low crystallinity PTFE resin, the high crystallinity PTFE resin also has a relatively low crystallinity and a relatively low duty cycle; and the fiber forming difficulty in transverse stretching is improved by controlling the higher longitudinal stretching multiple, the lower transverse stretching multiple and the higher transverse stretching temperature in the process of biaxial stretching, so that the number of block nodes of the prepared porous film is smaller, but the number of node aggregates with larger size is larger, and the other technological parameters are shown in Table 1 in detail.

Example 6 is different from example 1 mainly in that the difference between the number of longitudinal fibers and the number of transverse fibers, the length and the like of the produced porous film is small by controlling the lower longitudinal stretching ratio and the higher transverse stretching ratio in the biaxial stretching, and the other process parameters are shown in table 1 in detail.

Example 7 differs from example 1 primarily in that by controlling the film-forming resin to have a relatively low crystallinity and a relatively high duty cycle for the low crystallinity PTFE resin, the high crystallinity PTFE resin also has a relatively low crystallinity and a relatively low duty cycle; and the longitudinal stretching process with lower multiple and the transverse stretching process with lower multiple are adopted in the process of controlling the biaxial stretching, and the fiber is promoted to be doubled in cooperation with the higher heat setting temperature, heat setting time and heat setting stretching multiple in the heat setting process, so that the prepared porous film has reinforced fiber with higher proportion and distributed fiber with lower proportion, and the other technological parameters are shown in Table 1 in detail.

Comparative example

Comparative example 1

The main difference between comparative example 1 and each example is that only the high crystallinity PTFE resin is used in the film forming resin, and the prior more common two-way stretching process is adopted, wherein the heat treatment is not carried out after the longitudinal stretching, and the higher stretching temperature is adopted during the transverse stretching; and during the heat setting treatment, no micro stretching is performed.

Comparative example 2

The main difference between comparative example 2 and each example is that although the film-forming resin has a high crystallinity PTFE resin and a low crystallinity PTFE resin, the content of the high crystallinity PTFE resin is relatively low (low high crystallinity PTFE resin mass ratio 1:0.5); in addition, a common biaxial stretching process is adopted at present, wherein heat treatment is not carried out after longitudinal stretching, and a higher stretching temperature is adopted during transverse stretching.

Comparative example 3

The main difference between comparative example 3 and each example is that although the film-forming resin has a high crystallinity PTFE resin and a low crystallinity PTFE resin, a higher stretching ratio in the machine direction and a lower stretching ratio in the transverse direction (stretching ratio is only 0.5 times the stretching ratio in the machine direction) are used in the biaxial stretching, and a higher stretching temperature is used in the transverse direction and no heat treatment is performed after the longitudinal stretching.

TABLE 1 film formation System and film formation Process parameters for examples and comparative examples

Performance detection method

1. Waterproof property

1.1 soapy Water permeation time

The PTFE porous membrane prepared in each example or comparative example is taken as a sample, stuck to a jig, soaked in soapy water with the concentration of 0.1g/L, placed in a roller to shake for a certain time, the water inflow condition in the jig is observed, and if no water inflow phenomenon exists in the jig after the shaking time, the soapy water permeation time of the sample is considered to be longer than the time. If no water enters the jig after shaking for 10min, the soapy water permeation time of the sample is considered to be more than 10min. It should be noted that, for the sample with the soapy water permeation time greater than 10min, whether the sample has higher soapy water permeation time is not tested any more, and for the sample with the soapy water permeation time lower than 10min, the soapy water permeation condition under lower shaking time is tested further, and the single reduction of shaking time is 1min, namely, the water inlet condition in the jig is tested when the shaking time of the sample is 9min and 8min … … min in sequence, if the soapy water permeation time of the sample is lower than 5min, the waterproof effect of the sample is considered to be insufficient, and the soapy water permeation time test with lower shaking time is not performed any more.

1.2 Water pressure resistance test

The method B (high water pressure method) of the waterproof test method specified by JISL1092 is used for proper adjustment, the assembly is attached to the surface of a tool with a water outlet hole with the diameter of 1mm, and the annular glue part of the assembly is pressed by a clamp; and (3) keeping for 30min under the action of corresponding water pressure, and then observing whether the front and the side of the assembly leak water. If no water leakage exists, the judgment component can pass the water pressure resistance test under the corresponding water pressure of 30 min.

2. Relative deviation of intensity

The tensile strength of the porous film in the width direction and the tensile strength of the porous film in the length direction (average value is obtained by 3 times of testing) are respectively tested by a universal tensile tester, the average value of the tensile strength and the tensile strength of the porous film in the width direction is calculated, and the relative deviation of the tensile strength of the porous film in the length direction and the average value of the tensile strength and the tensile strength of the porous film in the length direction is further calculated.

The morphology parameters and performance parameters of each example and comparative example are detailed in Table 2.

Table 2 morphology parameters and performance parameters of the examples, comparative examples

Summary

By comparing the technical schemes and performance parameters of examples 1-7 and comparative examples 1-3, the porous membrane has good waterproof performance by controlling the pore structure with lower orientation degree (radially distributed pore structure) to match with the reinforcing fiber and the distribution fiber with a certain quantity of proportion, so that the hydrophobic three-dimensional network structure with higher integrity and higher pressure resistance is formed; the three-dimensional network structure with good pressure resistance is reinforced through the block nodes, the possibility of hole collapse of the porous membrane under the action of higher air pressure is reduced, and the porous membrane has higher air permeability on the basis of good waterproof performance by matching with the low air resistance of the long-strip holes and the block nodes.

The porous film prepared in comparative example 1, because of the high crystallinity PTFE resin alone, in combination with the higher biaxially stretching ratio, will produce a three-dimensional network structure with higher density and smaller fiber diameter, the reinforcing fibers in the three-dimensional network structure being relatively low and almost no node structure, therefore, the three-dimensional network structure of the porous film has poor pressure resistance, and such a film structure has a lower water pressure resistance performance as well as lower air permeability because collapse or damage of the pore structure is more likely to occur at higher air pressure in the case of similar thickness (as in example 6).

The porous membrane prepared in comparative example 2 uses high and low crystallinity PTFE resin as the film forming resin, but the low crystallinity PTFE resin in the film forming resin occupies a relatively high proportion, and a relatively common biaxial stretching process is adopted, and the film forming resin is not subjected to heat treatment, the temperature is relatively high during transverse stretching, the fiber forming difficulty is high during high-temperature transverse stretching, the number of produced transverse fibers is relatively small, the pore structure of the prepared porous membrane has relatively high orientation degree, and a structure similar to long node-fibril is still produced. Such a film structure, in the case of an approximate thickness (as in example 1), has only a slightly lower air permeability, but its waterproof performance is clearly inferior.

The porous film prepared in comparative example 3 uses a high-low crystallinity PTFE resin as the film-forming resin, but the low crystallinity PTFE resin in the film-forming resin occupies a relatively high proportion, and a transverse stretching process with a relatively low multiple and a relatively high transverse stretching process are adopted, so that the fiber-forming difficulty is high during transverse stretching, the number of produced transverse fibers is relatively small, the pore structure of the prepared porous film has relatively high orientation degree, and a structure similar to long node-fibril is still produced. Such a film structure has significantly lower air permeability and water repellency at similar thicknesses (as in example 6).

The present embodiment is only for explanation of the present application and is not to be construed as limiting the present application, and modifications to the present embodiment, which may not creatively contribute to the present application as required by those skilled in the art after reading the present specification, are all protected by patent laws within the scope of claims of the present application.

Claims

1. A PTFE porous membrane having a low degree of orientation pore structure, characterized by: the film comprises a film body, wherein the film body comprises a first outer surface and a second outer surface, the first outer surface and the second outer surface comprise surface fibers and massive surface nodes, the surface fibers are connected with the surface nodes or connected with adjacent surface fibers, and gaps between the adjacent surface fibers form long strip-shaped holes;

the thickness of the porous main body is 3-40 mu m;

2. The porous PTFE membrane with a low degree of orientation pore structure according to claim 1, wherein: the relative deviation between the breadth tensile strength of the porous main body and the film tensile strength is not more than 60%; the soapy water permeation time of the PTFE porous membrane is not less than 10min.

3. The porous PTFE membrane with a low degree of orientation pore structure according to claim 1, wherein: the distribution fibers are dispersed among the reinforcing fibers, and the distribution fibers account for 10-60% of all surface fibers.

4. The porous PTFE membrane with a low degree of orientation pore structure according to claim 1, wherein: the length-diameter ratio of the reinforcing fiber is 60-150, and the length-diameter ratio of the distribution fiber is 450-650.

5. The porous PTFE membrane with a low degree of orientation pore structure according to claim 1, wherein: the distribution density of the surface nodes is (10-30) per 10000 mu m ² The surface nodes have an area of not less than 150 μm ² Node aggregates and areas of not more than 50 μm ² The number of node aggregates is less than the number of scatter nodes.

6. The porous PTFE membrane with a low degree of orientation pore structure of claim 5, wherein: the distribution density of the node aggregate is (0.2-2) per 10000 mu m ² The distribution density of the dispersed nodes is (5-25) per 10000 μm ² 。

7. The porous PTFE membrane with a low degree of orientation pore structure according to claim 1, wherein: the surface fibers comprise longitudinal fibers and transverse fibers, wherein the included angle between the longitudinal fibers and the longitudinal stretching direction is not more than 55 degrees, the included angle between the transverse fibers and the transverse stretching direction is less than 35 degrees, and the longitudinal direction is the same as the transverse directionThe SEM measured average length of the fibers is greater than the SEM measured average length of the transverse fibers; the breathable film has a breathable rate of not more than 25X 10 ⁴ ml/min/cm ² @7kpa。

8. The porous PTFE membrane with a low degree of orientation pore structure of claim 7, wherein: the average spacing between adjacent longitudinal fibers is 2.5-7.5 mu m; the average spacing between adjacent transverse fibers is 0.5-3 μm.

9. The porous PTFE membrane with a low degree of orientation pore structure of claim 7, wherein: the ratio of the number of the longitudinal fibers to the number of the transverse fibers is 1.5 to 6; the ratio of the SEM measured average length of the longitudinal fibers to the SEM measured average length of the transverse fibers is 1.5 to 4.5.

10. The porous PTFE membrane with a low degree of orientation pore structure of claim 7, wherein: the gaps between the adjacent longitudinal fibers are longitudinal holes, the gaps between the adjacent transverse fibers are transverse holes, the SEM measurement long diameter of each longitudinal hole is larger than that of each transverse hole, the SEM measurement long diameter of each longitudinal hole is 60-130 mu m, and the average long diameter of each transverse hole is 25-65 mu m.

11. The porous PTFE membrane with a low degree of orientation pore structure according to claim 1, wherein: the water contact angles of the first outer surface and the second outer surface are 110-150 degrees, and the surface density of the porous main body is 1.2-7.5 g/m ² 。

12. The process for producing a porous PTFE membrane having a low-orientation pore structure according to claim 1 to 11, characterized in that: the method comprises the following process steps:

13. The process for producing a porous PTFE membrane having a low-orientation pore structure according to claim 12, wherein: in the step S1, the solid content of the pasty mixture is 70% -85%; the auxiliary oil is at least one of petroleum ether, solvent naphtha and aviation kerosene.

14. The process for producing a porous PTFE membrane having a low-orientation pore structure according to claim 12, wherein: in the step S4, longitudinal stretching is performed in 4-20 steps;

15. The process for producing a porous PTFE membrane having a low-orientation pore structure according to claim 12, wherein: the step S2 specifically comprises the following process steps:

16. The process for producing a porous PTFE membrane having a low-orientation pore structure according to claim 12, wherein: in the step S6, when the heat treatment temperature is reduced to the transverse stretching temperature, the temperature reduction speed is 3-15 ℃/10min, and the heat preservation is required to be carried out for 10min when the temperature is reduced to 50 ℃.