CN112880460A

CN112880460A - PTFE double-layer composite pipe, preparation method thereof and heat exchanger

Info

Publication number: CN112880460A
Application number: CN202110023960.XA
Authority: CN
Inventors: 宋朋泽
Original assignee: Individual
Current assignee: Individual
Priority date: 2021-01-08
Filing date: 2021-01-08
Publication date: 2021-06-01
Anticipated expiration: 2041-01-08
Also published as: CN112880460B

Abstract

The invention provides a PTFE double-layer composite pipe, which comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe compounded on the surface of the inorganic fiber net; the inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE inner pipe; the pretreated inorganic fiber is an inorganic fiber which is soaked in a polytetrafluoroethylene solution with the mass concentration of 50-70%. The invention adopts the inorganic fiber net which is specially pretreated to form the net-shaped mosaic structure between the inner layer and the outer layer of the PTFE pipeline, thereby effectively enhancing the compression resistance and the dimensional stability of the PTFE pipeline wall material. The invention also provides a preparation method of the PTFE double-layer composite pipe and a heat exchanger.

Description

PTFE double-layer composite pipe, preparation method thereof and heat exchanger

Technical Field

The invention belongs to the technical field of organic polymer materials, and particularly relates to a PTFE double-layer composite tube, a preparation method thereof and a heat exchanger.

Background

The tubular heat exchanger is equipment commonly used in petroleum, chemical industry, metallurgy, electric power, food, medicine and other industries, has two functions of heat exchange and cooling, and the metal heat exchanger in the leading position often causes rapid reduction of heat exchange effect due to various corrosive media, so that the service life of the equipment is greatly shortened, great hidden dangers can appear in production safety, and the research of novel non-metal heat exchange equipment has great practical significance. The non-metal heat exchangers widely applied to the market at present comprise a graphite heat exchanger, a silicon carbide heat exchanger and various plastic heat exchangers, wherein the fluoroplastic heat exchanger is heat exchange equipment with the most excellent comprehensive performance in the application of the plastic heat exchanger.

The fluoroplastic is a paraffin polymer with partial or all hydrogen replaced by fluorine, has very stable chemical properties, and is more easily accepted by many fields compared with the defects of large volume, frangibility and the like of heat exchangers made of nonmetallic materials such as graphite, ceramic and the like. As fluoroplastics, Polytetrafluoroethylene (PTFE), Fluorinated Ethylene Propylene (FEP), soluble Polytetrafluoroethylene (PFA), Polychlorotrifluoroethylene (PCTFE) and the like are widely used.

The PTFE (polytetrafluoroethylene) pipe has the advantages of strong chemical stability, aging resistance, non-stickiness, non-inflammability and the like, is suitable for conveying strong acid, strong alkali and various corrosive media compared with other pipeline materials, and can reach the use temperature range of-180-260 ℃ under the normal pressure state. In addition, the PTFE has a small friction coefficient and a self-lubricating effect, and can greatly reduce the conveying resistance of pipeline media. The properties of the PTFE pipe are materials which cannot be replaced by other material pipelines, so the PTFE pipe is widely used in many fields such as petroleum, chemical industry, food, national defense industry, advanced science and technology, medicine and the like. However, in practical applications, the PTFE tube still has the problems of poor antiknock capability, poor dimensional stability, and the like, and particularly in some fields requiring higher service pressure, further improvement of the pressure resistance, tensile strength, and the like is needed.

Disclosure of Invention

The invention aims to provide a PTFE double-layer composite tube, a preparation method thereof and a heat exchanger.

The invention provides a PTFE double-layer composite pipe, which comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe compounded on the surface of the inorganic fiber net;

the inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE inner pipe;

the inorganic fiber is one or more of glass fiber, special glass fiber, basalt fiber, perlite fiber, high silica fiber and high silica modified fiber; the pretreated inorganic fibers are inorganic fibers subjected to dipping treatment in a polytetrafluoroethylene solution with the mass concentration of 50-70%;

the diameter of the inorganic fiber is 3.5-6 μm.

Preferably, the weaving density of the inorganic fiber net is 1-30 pieces/cm.

Preferably, the thickness of the inorganic fiber net is 0.1-0.3 mm.

Preferably, the total thickness of the PTFE double-layer composite pipe is 0.4-2.0 mm.

The invention provides a preparation method of a PTFE double-layer composite pipe, which comprises the following steps:

A) dipping inorganic fibers in a polytetrafluoroethylene solution with the mass concentration of 50-70% to obtain pretreated inorganic fibers;

B) sealing one end of the PTFE inner tube, inputting compressed air into the other end of the PTFE inner tube to enable the PTFE inner tube to be always in an inflated state, and then winding and weaving inorganic fibers on the outer surface of the PTFE inner tube to obtain the PTFE inner tube covered with an inorganic fiber net;

the inorganic fiber is one or more of glass fiber, special glass fiber, basalt fiber, perlite fiber, high silica fiber and high silica modified fiber;

the diameter of the inorganic fiber is 3.5-6 μm;

C) and sequentially performing nesting compounding, oil removal and sintering molding on the PTFE outer pipe and the PTFE inner pipe covered with the inorganic fiber net to obtain the PTFE double-layer composite pipe.

Preferably, the speed of impregnation in step A) is 2 to 5 m/min.

Preferably, the pressure of the compressed air in the step B) is 0.35-0.85 kPa.

Preferably, the pretreated inorganic fibers are subjected to pre-sintering treatment and then woven on the surface of the PTFE inner tube;

the pre-sintering temperature is 300-600 ℃;

the pre-sintering time is 5-20 min.

Preferably, the stretching ratio in the step C) is 2-10.

The invention provides a heat exchanger which comprises a heat exchange tube, wherein the heat exchange tube is the PTFE double-layer composite tube.

The invention provides a PTFE double-layer composite pipe, which comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe compounded on the surface of the inorganic fiber net; the inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE inner pipe; the inorganic fiber is one or more of glass fiber, special glass fiber, basalt fiber, perlite fiber, high silica fiber and high silica modified fiber; the pretreated inorganic fibers are inorganic fibers subjected to dipping treatment in a polytetrafluoroethylene solution with the mass concentration of 50-70%; the diameter of the inorganic fiber is 3.5-6 μm. The invention adopts the inorganic fiber net which is specially pretreated to form the net-shaped mosaic structure between the inner layer and the outer layer of the PTFE pipeline, has better fusion with the PTFE pipe, high combination fastness, effectively enhances the compression resistance and the dimensional stability of the PTFE pipe wall material, obviously improves the bursting pressure of the PTFE composite pipe compared with a pure PTFE pipe, and ensures that the composite pipe is more suitable for being used in high temperature, high pressure and certain strong corrosive environments.

In addition, the invention also provides a preparation method of the PTFE double-layer composite pipe, compressed air is filled into the PTFE inner pipe, so that the pipe keeps a full expansion state when in use, the problems of loose fiber winding and infirm combination with the pipe and deformation of the PTFE pipe caused by over tight fiber winding are effectively solved, and the surface fiber winding of the pipe is uniform and the tension is consistent.

Detailed Description

the diameter of the inorganic fiber is 3.5-6 μm.

The PTFE double-layer composite tube is composed of a three-layer nested composite structure, and sequentially comprises a PTFE inner tube, an inorganic fiber net and a PTFE outer tube, wherein the inorganic fiber net is a reinforced core layer of the composite tube.

In the present invention, the PTFE inner tube is made of PTFE, and the thickness of the PTFE inner tube is preferably 0.2 to 0.5mm, and specifically, in an embodiment of the present invention, may be 0.25mm, 0.3mm, or 0.4 mm. The outer diameter of the PTFE inner pipe is not particularly limited.

In the present invention, the PTFE outer tube is made of PTFE, and the thickness of the PTFE outer tube is preferably 0.2 to 0.5mm, and specifically, in an embodiment of the present invention, may be 0.25mm, 0.3mm, or 0.4 mm. Preferably, the outer PTFE tube is of the same thickness as the inner PTFE tube. The inner diameter and the outer diameter of the PTFE outer pipe are not particularly limited, and the PTFE outer pipe can be matched with the inner pipe and the inorganic fiber net to form a good nested composite structure.

In the invention, the inorganic fiber net is wound and woven on the outer surface of the PTFE inner pipe and is compounded with the inner wall of the PTFE outer pipe to form a three-layer nested composite structure.

The inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE, and the inorganic fibers are preferably one or more of glass fibers, special glass fibers, basalt fibers, perlite fibers, high silica fibers and high silica modified fibers; the inorganic fiber pretreatment refers to the step of soaking the inorganic fiber in a polytetrafluoroethylene solution, wherein the polytetrafluoroethylene solution is an emulsion prepared from polytetrafluoroethylene and water, and the mass concentration of the polytetrafluoroethylene solution is preferably 50-70%, more preferably 55-65%, and most preferably 60%; the diameter of the fiber is preferably 3.5-6 mu m, namely, the fiber is preferably woven by monofilament fine denier fiber, the invention preferably adopts cross weaving, longitudinal and transverse grid weaving or spiral winding weaving along the pipe wall, and more preferably adopts longitudinal and transverse grid weaving; the weaving density is preferably 1-30 pieces/cm, more preferably 5-25 pieces/cm, and specifically, in the embodiment of the invention, the weaving density may be 6 pieces/cm, 10 pieces/cm or 20 pieces/cm.

The thickness of the inorganic fiber web is preferably 0.1 to 3mm, more preferably 0.1 to 2mm, and most preferably 0.2 to 1mm, and specifically, in the embodiment of the present invention, may be 0.1mm or 0.2 mm.

In the invention, the overall thickness of the PTFE double-layer composite pipe is preferably 0.4-2.0 mm, more preferably 0.6-1.0 mm, and specifically, in the embodiment of the invention, the thickness can be 0.6mm, 0.8mm or 1.0 mm.

The invention also provides a preparation method of the PTFE double-layer composite pipe, which comprises the following steps:

the diameter of the inorganic fiber is 3.5-6 μm;

In the present invention, the kind, amount and source of the polytetrafluoroethylene solution are the same as those of the polytetrafluoroethylene, and the description of the present invention is omitted.

In the invention, the dipping speed of the inorganic fiber dipping treatment is preferably 2-5m/min, and more preferably 3-4 m/min.

The invention preferably carries out sintering treatment on the pretreated inorganic fiber, and aims to remove the residues of the impregnating compound on the surface of the fiber and ensure that the inorganic fiber braided layer and the PTFE pipe are better and more firmly combined. The pre-sintering temperature is preferably 300-600 ℃, more preferably 300-450 ℃, and specifically, in the embodiment of the invention, the pre-sintering temperature can be 300 ℃, 400 ℃ or 450 ℃; the pre-sintering time is preferably 8-12 min.

After the sintered inorganic fiber is obtained, one end of the PTFE inner tube is closed, compressed air is input into the other end of the PTFE inner tube, the PTFE inner tube is always kept in an inflated state, and then the inorganic fiber is wound and woven on the outer surface of the PTFE inner tube, so that the PTFE inner tube covered with the inorganic fiber net is obtained.

In the present invention, the pressure of the compressed air charged into the PTFE inner tube is preferably 0.35 to 0.85kPa, more preferably 0.5 to 0.7kPa, and most preferably 0.5 to 0.6 kPa.

The invention preferably prepares the PTFE inner pipe according to the following steps,

and mixing polytetrafluoroethylene resin and an auxiliary agent, standing, and sequentially performing prepressing molding, pushing molding and sintering molding to obtain the PTFE inner tube.

In the invention, the auxiliary agent preferably comprises aviation kerosene and a silane coupling agent, the aviation kerosene is preferably high-flash-point aviation kerosene, and the mass ratio of the aviation kerosene to the polytetrafluoroethylene resin is preferably (15-35): 100, more preferably (20-30): 100, and most preferably (25-30): 100, respectively;

the silane coupling agent is preferably one or more of vinyl trimethoxy silane, vinyl triethoxy silane, gamma-glycidoxypropyl trimethoxy silane and gamma-aminopropyl triethoxy silane; the mass ratio of the silane coupling agent to the polytetrafluoroethylene resin is preferably (0.1-1.5): 100, more preferably (0.5 to 1): 100.

in the present invention, the addition of the auxiliary agent is more advantageous in promoting the bonding between the PTFE inner tube and the PTFE outer tube and the inorganic fiber web.

According to the invention, the polytetrafluoroethylene resin and the auxiliary agent are mixed, and the uniform distribution of the polytetrafluoroethylene resin and the auxiliary agent is realized through the dual actions of the stirrer and the rotation of the container, so that the subsequent processing is facilitated;

after the mixing is finished, the obtained mixture is subjected to standing and material standing so as to promote the fusion of the polytetrafluoroethylene raw material and the auxiliary agent. The temperature of the standing is preferably a certain constant temperature between 22 and 26 ℃, and more preferably 25 +/-1 ℃; the standing time is preferably 8-16 hours, and more preferably 10-15 hours.

After the standing is finished, the mixture after standing is subjected to pre-pressing forming, the pressure of the pre-pressing forming is preferably 0.35-0.40 MPa, and a pre-pressing machine is preferably adopted for pre-pressing.

After the prepressing is finished, extruding the material blank subjected to the prepressing forming by pushing to prepare the PTFE inner tube with a uniform structure, wherein the pressure of the pushing forming is preferably 0.40-0.45 MPa;

after the extrusion molding, the molded PTFE inner tube is sintered and molded to obtain the PTFE inner tube, wherein the sintering and molding temperature is preferably 325-355 ℃, and more preferably 330-340 ℃; the sintering and shaping time is preferably 10-20 min, and specifically, in the embodiment of the invention, the sintering and shaping time can be 10min, 15min or 20 min.

The preparation method of the PTFE outer pipe is consistent with that of the PTFE inner pipe, and different extrusion pipe diameters can be adjusted, so that the invention is not repeated herein.

After the PTFE inner pipe and the PTFE outer pipe are obtained, the inorganic fibers are wound and woven on the outer surface of the PTFE inner pipe, and then the inorganic fibers and the PTFE outer pipe are subjected to nested compounding, oil removal and sintering molding to obtain the PTFE double-layer composite pipe.

In the present invention, the kind of the inorganic fiber, the knitting method and the knitting density of the inorganic fiber are the same as those of the inorganic fiber, the knitting method and the knitting density of the inorganic fiber, which are described above, and thus, detailed description thereof is omitted.

The nested composite process of the invention is to combine the inner pipe and the outer pipe together through the operation of vacuum suction, and the vacuum degree is preferably-0.01 to-0.04 MPa.

And introducing the nested and compounded composite pipe into an oil removal system to remove oil and auxiliary oil, wherein the oil removal process is an oil removal technology known by persons skilled in the art and is not described herein again.

After oil removal, sintering and shaping are carried out on the composite pipe, wherein the sintering and shaping temperature is preferably 325-355 ℃, more preferably 330-340 ℃, and specifically, in the embodiment of the invention, 330 ℃, 340 ℃ or 355 ℃; the sintering and shaping time is preferably 5-30 min, and more preferably 9-12 min.

The invention also provides a heat exchanger which comprises the heat exchange tube, wherein the heat exchange tube is the PTFE double-layer composite tube.

In order to further illustrate the present invention, the following will describe the PTFE double-layer composite tube, the preparation method thereof and the heat exchanger in detail with reference to the examples, but the scope of the present invention should not be construed as being limited thereto.

Example 1

1000g of polytetrafluoroethylene resin, 150g of aviation kerosene and 1g of vinyl trimethoxy silane are rotated and doubly acted by a stirrer and a container to realize the uniform distribution of the polytetrafluoroethylene resin and the auxiliary agent, so as to be beneficial to subsequent processing. The evenly mixed raw materials are kept stand for 12 hours at the constant temperature of 25 +/-1 ℃ to promote the fusion of the polytetrafluoroethylene raw materials and the auxiliary agent.

After standing and material standing, the premix is put into a prepress for pressing, and the pressure value is preferably 0.35MPa, so that a blank is prepared.

The preform obtained by the preliminary press molding was introduced into a pushing press machine, and was subjected to pushing extrusion molding under a pressure of 0.40MPa to obtain an inner layer PTFE tube having a uniform structure and a wall thickness of 0.25 mm.

And (3) conveying the inner layer PTFE pipe into a sintering machine, and sintering and shaping at 330 +/-2 ℃ for 20 min.

Dipping the fine-denier special glass fiber in 60% polytetrafluoroethylene solution for 2m/min, and then pre-sintering at 300 ℃ to remove the impregnating compound residues on the surface of the fiber. The fine-denier special glass fiber winding is woven on the surface of the sintered inner-layer PTFE tube in a criss-cross mode through a warp knitting machine, the weaving density is 10 pieces/cm, the winding thickness is 0.1mm, in the weaving process, one end of the inner-layer PTFE tube is closed, and compressed air with the air pressure of 0.5kPa is input into the other end of the inner-layer PTFE tube, so that the PTFE inner tube is always kept in a full expansion state.

The outer layer PTFE pipe is manufactured into the outer layer PTFE pipe with the wall thickness of 0.25mm through the same manufacturing procedures as the inner layer PTFE pipe. The inner-layer PTFE pipe and the outer-layer PTFE pipe, the surfaces of which are woven and wound with the fine-denier special glass fiber, are nested and compounded, the PTFE composite pipe after composite processing is introduced into an oil removal system to remove auxiliary oil, and the PTFE composite pipe enters a sintering machine after oil removal and is sintered and shaped at the temperature of 330 +/-2 ℃. Through the nesting composite processing and the sintering shaping, the inner layer PTFE pipe, the fine denier special glass fiber winding braid layer and the outer layer PTFE pipe are tightly combined together to manufacture the high-strength special glass fiber/PTFE composite pipe with the wall thickness of 0.6 mm. And finally, rolling, inspecting, packaging and warehousing.

The pressure resistance of the PTFE composite pipe obtained in example 1 is shown in table 1,

TABLE 1 pressure resistance of PTFE composite pipe obtained in example 1

Example 2

1000g of polytetrafluoroethylene resin, 280g of aviation kerosene and 6g of vinyl triethoxysilane are subjected to double actions of rotation of a stirrer and a container to realize uniform distribution of the polytetrafluoroethylene resin and the auxiliary agent, so that subsequent processing is facilitated. The evenly mixed raw materials are kept stand for 16 hours at a constant temperature of 22 +/-1 ℃ to promote the fusion of the polytetrafluoroethylene raw materials and the auxiliary agent.

After the premix is stood for material standing, the premix is put into a pre-press to be pressed into a material blank under the pressure of 0.38 MPa.

The pre-pressed blank is introduced into a pushing press machine, and is extruded and molded under the pressure of 0.43MPa to prepare the inner layer PTFE pipe with uniform structure and 0.3mm wall thickness.

And (3) conveying the inner layer PTFE pipe into a sintering machine, and sintering and shaping at 340 +/-2 ℃ for 15 min.

Dipping fine denier basalt fiber in 60% polytetrafluoroethylene solution for 3m/min, and then presintering at 400 ℃ to remove the impregnating compound residues on the fiber surface. Fine denier basalt fiber is woven on the surface of the inner-layer PTFE pipe which is formed by sintering through a warp knitting machine, the weaving density is 6 pieces/cm, the thickness is 0.2mm, in the weaving process, one end of the inner-layer PTFE pipe is closed, and compressed air with the air pressure of 0.6kPa is input into the other end of the inner-layer PTFE pipe, so that the PTFE inner pipe is always kept in a full expansion state.

The outer layer PTFE pipe is manufactured into the outer layer PTFE pipe with the wall thickness of 0.3mm through the same manufacturing procedures as the inner layer PTFE pipe. The inner layer PTFE pipe and the outer layer PTFE pipe which are woven and wound with fine denier basalt fiber on the surface are nested and compounded, the PTFE composite pipe after composite processing is introduced into an oil removing system to remove auxiliary oil, and the PTFE composite pipe enters a sintering machine after oil removal and is sintered and shaped at the temperature of 340 +/-2 ℃. Through nesting composite processing and sintering shaping, the inner layer PTFE pipe, the fine denier basalt fiber winding braid layer and the outer layer PTFE pipe are tightly combined together to prepare the high-strength basalt fiber/PTFE composite pipe with the wall thickness of 0.8 mm. And finally, rolling, inspecting, packaging and warehousing.

The pressure resistance of the PTFE composite pipe obtained in example 2 is shown in table 2,

TABLE 2 pressure resistance of PTFE composite pipe obtained in example 2

Example 3

1000g of polytetrafluoroethylene resin, 350g of aviation kerosene and 10g of gamma-glycidoxypropyltrimethoxysilane are subjected to double actions of rotation of a stirrer and a container to realize uniform distribution of the polytetrafluoroethylene resin and an auxiliary agent, so that subsequent processing is facilitated. The evenly mixed raw materials are kept stand for 8 hours at a constant temperature of 26 plus or minus 1 ℃ to promote the fusion of the polytetrafluoroethylene raw materials and the auxiliary agent.

After the premix is stood for material standing, the premix is put into a prepress and is pressed into a material blank under the pressure of 0.4 MPa.

And introducing the blank subjected to the pre-pressing molding into a pushing press, and performing pushing extrusion molding under the pressure of 0.45MPa to prepare the inner-layer PTFE pipe with a uniform structure and a wall thickness of 0.4 mm.

And (3) conveying the inner layer PTFE tube into a sintering machine, and sintering and shaping at 355 +/-2 ℃ for 10 min.

Soaking fine denier high silica (modified) fiber in 60% polytetrafluoroethylene solution for 4m/min, and then pre-sintering at 450 ℃ to remove the impregnating compound residues on the fiber surface. The surface of the sintered inner-layer PTFE tube is criss-cross woven and wound with fine denier high silica (modified) fibers through a warp knitting machine, the weaving density is 20 pieces/cm, the thickness is 0.2mm, one end of the inner-layer PTFE tube is closed in the weaving process, and compressed air with the air pressure of 0.55kPa is input into the other end of the inner-layer PTFE tube, so that the PTFE inner tube is always kept in a full expansion state.

The outer layer PTFE pipe is manufactured into the outer layer PTFE pipe with the wall thickness of 0.4mm through the same manufacturing procedures as the inner layer PTFE pipe. The inner layer PTFE pipe and the outer layer PTFE pipe which are woven and wound with fine denier high silica (modified) fibers on the surface are nested and compounded, the PTFE composite pipe after composite processing is introduced into an oil removal system to remove auxiliary oil, and the PTFE composite pipe enters a sintering machine after oil removal and is sintered and shaped at the temperature of 355 +/-2 ℃. Through nesting, composite processing and sintering and shaping, the inner layer PTFE tube, the fine denier high silica (modified) fiber winding braid layer and the outer layer PTFE tube are tightly combined together to prepare the high strength high silica (modified) fiber/PTFE composite tube with the wall thickness of 1 mm. And finally, rolling, inspecting, packaging and warehousing.

The pressure resistance of the PTFE composite pipe obtained in example 3 is shown in table 3,

TABLE 3 pressure resistance of PTFE composite pipe obtained in example 3

Comparative example 1

A PTFE composite pipe was produced by the production method in example 1, except that only a polytetrafluoroethylene resin was used as the raw material for the inner PTFE pipe and the outer PTFE pipe in this comparative example, and aviation kerosene and a silane coupling agent aid were not used.

The pressure resistance of the obtained PTFE composite pipe is shown in table 4.

TABLE 4 pressure resistance of PTFE composite pipe obtained in comparative example 1

Comparative example 2

A PTFE composite pipe was produced according to the production method in example 1, except that the inorganic fibers in this comparative example were used directly for weaving an inorganic fiber web without impregnation pretreatment.

The pressure resistance of the obtained PTFE composite pipe is shown in table 5.

TABLE 5 pressure resistance of PTFE composite pipe obtained in comparative example 2

TABLE 6 comparison of the properties of PTFE composite tubes and neat PTFE tubes in example 3 and comparative examples 1-2

Performance of	Unit of	Pure PTFE	Comparative example 1	Comparative example 2	Example 3
						Specific gravity of	—	2.16	2.19	2.18	2.18
Molding shrinkage ratio	％	4.91	4.12	4.09	3.55
						Tensile strength	MPa	17.55	20.44	21.73	35.50
Elongation at break	％	352	228	235	186
						Flexural modulus	MPa	5977	6342	6860	7870
Notched impact strength	kgf·cm/cm²	16.32	18.31	19.22	23.22
						Melting point temperature	℃	327	327	327	327
Long term heat resistance temperature	℃	260	260	260	260
						Heat distortion temperature (4.6 kg/cm)²)	℃	121	133	135	180
Dielectric constant	-	2.11	2.13	2.27	2.06

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. A PTFE double-layer composite pipe comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe compounded on the surface of the inorganic fiber net;

the diameter of the inorganic fiber is 3.5-6 μm.

2. The PTFE composite tube of claim 1, wherein the inorganic fiber mesh has a woven density of 1 to 30 pieces/cm.

3. The PTFE double-layer composite pipe according to claim 1, wherein the inorganic fiber net has a thickness of 0.1 to 0.3 mm.

4. The PTFE double-layer composite tube according to claim 3, wherein the PTFE double-layer composite tube has an overall thickness of 0.4-2.0 mm.

5. A preparation method of a PTFE double-layer composite pipe comprises the following steps:

the diameter of the inorganic fiber is 3.5-6 μm;

6. The method as claimed in claim 5, wherein the speed of the impregnation in step A) is 2 to 5 m/min.

7. The method according to claim 5, wherein the compressed air pressure in the step B) is 0.35 to 0.85 kPa.

8. The method according to claim 5, wherein the pretreated inorganic fiber is subjected to a presintering treatment and then woven on the inner tube surface of the PTFE tube;

the pre-sintering temperature is 300-600 ℃;

the pre-sintering time is 5-20 min.

9. The method according to claim 5, wherein the draw ratio in the step C) is 2 to 10.

10. A heat exchanger comprises a heat exchange tube, wherein the heat exchange tube is a PTFE double-layer composite tube as defined in any one of claims 1 to 4 or a PTFE double-layer composite tube prepared by the preparation method as defined in any one of claims 5 to 9.