A high-resilience creep-resistant modified polytetrafluoroethylene sealing material and a method for preparing the same

Abstract

The application provides a high-resilience and creep-resistant modified polytetrafluoroethylene sealing material and a preparation method thereof, which is prepared by melt extrusion one-step forming of a hexafluoropropylene modified polytetrafluoroethylene resin as a polymer matrix, an internal lubricant and a reinforcing filler as modified components. The application uses hexafluoropropylene grafted modified polytetrafluoroethylene as the matrix, which fundamentally improves the resilience and creep resistance of the sealing material; the internal lubricant is introduced to improve the processability and dimensional stability of the sealing material, and the introduction and content control of a small amount of small-size reinforcing filler further improve the mechanical properties and creep resistance; the melt extrusion one-step forming of the sealing material plate is completed, the width of the gap of the flat seaming machine head is controlled to control the thickness of the formed plate, and the comprehensive control of the melt temperature, screw rotation speed and draft ratio improves the comprehensive performance and creep resistance of the sealing material, so that the sealing material can effectively improve the assembled size precision, creep resistance and service life when applied in the field of aerospace.

Description

TECHNICAL FIELD

[0001] The present application relates to a polytetrafluoroethylene-based sealing material, in particular to a hexafluoropropylene grafted modified polytetrafluoroethylene sealing material and a preparation method thereof, the material has good resilience and creep resistance, and is mainly used for manufacturing sealing elements of large-size mechanical parts in the field of aerospace, belonging to the field of composite materials and sealing. BACKGROUND

[0002] Polytetrafluoroethylene is a high-performance sealing material, which has many unique advantages, such as excellent chemical corrosion resistance, aging resistance, high temperature resistance and low friction coefficient, etc., and has been widely used in the field of aerospace. However, polytetrafluoroethylene itself has low yield strength and narrow elastic deformation section, which shows poor resilience, and even after composite modification, the resilience rate is usually below 40%, which leads to serious assembly deformation when polytetrafluoroethylene is used as a sealing material for large-size parts such as electric spindles and cylinders, causing size out-of-tolerance and even sealing failure. In addition, under the long-term action of load, the spiral molecular structure of polytetrafluoroethylene leads to easy slipping between molecules, so the creep is large, and the cold flow phenomenon is easy to occur. Therefore, the poor resilience and creep resistance of polytetrafluoroethylene are the key factors limiting its application in the sealing field, especially in large-size parts.

[0003] CN116082773A discloses a high-resilience, self-lubricating and wear-resistant polytetrafluoroethylene oil seal lip material and a preparation method thereof, which uses fibrous fillers, organic self-lubricating fillers and other multi-component fillers to improve the tensile resilience and friction and wear properties of polytetrafluoroethylene, and can meet the requirements of high-frequency jumping of rotating shafts. CN114458763A uses polyimide and polyphenyl ester as organic fillers, and combines flaky molybdenum disulfide and nano zinc oxide as inorganic fillers to obtain a polytetrafluoroethylene rotary sealing element for air compressors, which has good wear resistance and high creep resistance. Similar disclosed polytetrafluoroethylene-based composite materials and preparation methods include CN103756204A, CN103113697A, CN109608795A and CN102702653A. However, in the above-mentioned methods, polytetrafluoroethylene is used as the polymer matrix of the composite material, which limits the performance of the composite material to a great extent, and it is difficult to fundamentally improve the resilience and creep resistance of polytetrafluoroethylene. At the same time, the preparation method still uses traditional mold pressing and hot forming, which makes it difficult to realize continuous production and limits its application field. SUMMARY

[0004] The present application aims to solve the problems of low resilience, poor creep resistance and difficulty in realizing continuous production of existing polytetrafluoroethylene sealing materials, and provides a high-resilience and creep-resistant modified polytetrafluoroethylene sealing material and a preparation method thereof.

[0005] I. Preparation of modified polytetrafluoroethylene sealing material

[0006] The high-resilience creep-resistant modified polytetrafluoroethylene sealing material of the present application is prepared from the following raw materials and process:

[0007] The raw material ratio is as follows: 96-99.4 parts by volume of modified polytetrafluoroethylene resin, 0.1-1 part by volume of internal lubricant, and 0.5-3 parts by volume of reinforcing filler.

[0008] In the raw material components, the modified polytetrafluoroethylene resin is a copolymer of tetrafluoroethylene and hexafluoropropylene, which is polymerized at a temperature of 40-70℃ and a pressure of 6-12 MPa for 2-6 hours using tetrafluoroethylene and hexafluoropropylene as monomers and peroxymalonate as an initiator. The amount of monomers and initiator is as follows: 80-90 parts by volume of tetrafluoroethylene, 10-20 parts by volume of hexafluoropropylene, and 0.01-0.03 parts by volume of peroxymalonate.

[0009] The polytetrafluoroethylene resin has extremely high melt viscosity and can only be sintered into shape in a manner similar to powder metallurgy. After modification, the originally completely symmetrical and linear -CF2 molecular chain of polytetrafluoroethylene is broken, and a certain proportion of hexafluoropropylene (C3F6) is copolymerized, thereby reducing the melt viscosity of the polytetrafluoroethylene resin and enabling it to be compounded, blended, and extruded into shape by melt extrusion. Testing shows that the tensile strength of the modified polytetrafluoroethylene resin of the hexafluoropropylene grafted modified polytetrafluoroethylene resin is 25-35 MPa, and the melt mass flow rate is 5-15 g / 10 min; while the tensile strength of the unmodified polytetrafluoroethylene resin is 23-30 MPa, and the melt mass flow rate is 0.

[0010] The internal lubricant is one or more of calcium stearate, magnesium stearate, and talc, and the particle size of the internal lubricant is 20-500 μm. The reinforcing filler is one or more of titanium dioxide, aluminum oxide, zinc oxide, and calcium carbonate, and the particle size of the reinforcing filler is 5-200 nm.

[0011] The preparation process is as follows: after the modified polytetrafluoroethylene resin, internal lubricant, and reinforcing filler are thoroughly dried, they are placed in the feeder of an extruder, and the materials are thoroughly mixed and then melt-extruded at a temperature of 280-370℃ and a screw rotation speed of 5-80 rpm; the extruded material is drawn and cooled to obtain the modified polytetrafluoroethylene sealing material.

[0012] In the preparation process, the extruder is a single-screw extruder with a length-diameter ratio of 10-25 and a compression ratio of 2-7. The extruder is equipped with a flat seam head with a parallel gap of 0.05-5 mm. The drawing ratio of the extruded material is 5-60, and the cooling temperature is 120-150℃.

[0013] II. Properties of the modified polytetrafluoroethylene sealing material

[0014] 1. Tensile strength test

[0015] After punching the sample into dumbbell shape with 80 mm x 10 mm and different thickness, the test was carried out according to the standard GB / T 1040.2-2006, the gauge length of the sample was 25 mm, and the tensile rate was 50 mm / min. The experimental results showed that the tensile strength of the modified polytetrafluoroethylene sealing material was 20-35 MPa.

[0016] 2. Tensile resilience test

[0017] After punching the sample into a long strip shape with a length of 100 mm, a width of 10 mm and different thickness, the sample was fixed on the clamps of the tensile testing machine, and was directionally stretched by 5 mm (0), and was kept for 3 min. After the stress was removed, the length change value ΔL of the sample was measured. The tensile resilience was calculated according to the formula (L0-ΔL) / L0x100%. The experimental results showed that the tensile resilience of the modified polytetrafluoroethylene sealing material provided by the application was 50-85%. L L L L L 3. Friction and wear performance test

[0018] After punching the sample into a square sheet with a length of 50 mm, a width of 50 mm and different thickness, the UMT friction and wear testing machine was used to test the friction and wear performance of the polytetrafluoroethylene sealing material, and the test environment temperature was 23±5℃. The sealing material was fixed in the clamp as the lower test piece, and the upper test piece was a stainless steel ball with a hardness of HRC56 and a diameter of 5 mm. Under the conditions of a load of 10 N and a frequency of 5 Hz, reciprocating motion was carried out, the single stroke was 5 mm, and the total stroke was 500 m. The experimental results showed that the friction coefficient of the modified polytetrafluoroethylene sealing material provided by the application was 0.02-0.08, and the wear rate was (0.5-2) x 10-5mm / Nm. 3

[0019] 4. Creep performance test

[0020] After punching the sample into a square sheet with a length of 12.7 mm, a width of 12.7 mm and different thickness, a constant load of 556 N (3.45 MPa) was applied, and was kept for 100 h. The displacement sensor was used to record the height change ΔH of the sample. The creep rate was calculated according to the formula (H0-ΔH) / H0x100%. The experimental results showed that the creep rate of the modified polytetrafluoroethylene sealing material provided by the application was 15-30% (3.45 MPa, 100 h).

[0021] H H H H

[0022] ​​​​​​​​​​In summary, the present application uses hexafluoropropylene grafted polytetrafluoroethylene as a resin matrix to improve the resilience and creep resistance of the sealing material; by introducing an internal lubricant to improve the processability and dimensional stability of the sealing material; and by introducing a small amount of small-size reinforcing filler and controlling the content to further improve the mechanical properties and creep resistance. In the forming process, the sealing material plate is formed by one-step melt extrusion, the thickness of the formed plate is controlled by the gap width of the flat seam machine head, and the comprehensive performance and forming efficiency of the sealing material are improved by comprehensive control of the melting temperature, screw speed and draw ratio. The performance test results show that the mechanical properties, creep resistance, high resilience and wear resistance of the modified polytetrafluoroethylene sealing material are well improved, and the sealing material used in the aerospace field can effectively improve the dimensional accuracy, creep resistance and service life after assembly. DETAILED DESCRIPTION

[0023] The preparation and performance of the modified polytetrafluoroethylene sealing material of the present application are further described below in conjunction with examples. The materials and reagents used in the examples are commercially available.

[0024] Example 1

[0025] (1) 9.80 kg of modified polytetrafluoroethylene resin (containing 10% hexafluoropropylene, melt mass flow rate 10 g / min), 0.1 kg of talc (particle size 50 μm), 0.1 kg of titanium dioxide (particle size 50 nm) were thoroughly dried and placed in the feeder of the extruder;

[0026] (2) The mixture was melt extruded using a single screw extruder with a screw length-diameter ratio of 12 and a compression ratio of 3 at a temperature of 350℃ and a screw speed of 50 rpm. The parallel seam gap of the flat seam machine head used in the single screw extruder was 1 mm;

[0027] (3) The draw ratio was set to 20, and the modified polytetrafluoroethylene sealing material with a thickness of 1 mm, a width of 1 m and a length of about 45 m was obtained at a cooling temperature of 120℃. The performance indicators of the sealing material are shown in Table 1.

[0028] Example 2

[0029] (1) 9.9 kg of modified polytetrafluoroethylene resin (containing 12% hexafluoropropylene, melt mass flow rate 15 g / min), 0.05 kg of calcium stearate (particle size 80 μm), 0.05 kg of silicon dioxide (particle size 20 nm) were thoroughly dried and placed in the feeder of the extruder;

[0030] (2) The mixture was melt extruded using a single screw extruder with a screw length-diameter ratio of 20 and a compression ratio of 5 at a temperature of 355℃ and a screw speed of 30 rpm. The parallel seam gap of the flat seam machine head used in the single screw extruder was 5 mm;

[0031] (3) Set the draw ratio to 8, and cool at 140°C to obtain a white modified polytetrafluoroethylene sealing material with a thickness of 5 mm, a width of 1 m, and a length of about 10 m. The performance indicators of the sealing material are shown in Table 1.

[0032] Example 3

[0033] (1) 99.2 kg of modified polytetrafluoroethylene resin (containing 15% hexafluoropropylene, melt mass flow rate 9 g / min), 0.2 kg of magnesium stearate (particle size 200 μm), and 0.6 kg of silicon dioxide (particle size 50 nm) were thoroughly dried and then placed in the feeder of an extruder;

[0034] (2) The mixture was melt-extruded using a single-screw extruder with a screw length-diameter ratio of 20 and a compression ratio of 8, at a temperature of 330°C and a screw rotation speed of 45 rpm, and the parallel slit gap of the flat-knife head used in the single-screw extruder was 3 mm;

[0035] (3) The draw ratio was set to 50, and the material was cooled at 125°C to obtain a transparent modified polytetrafluoroethylene sealing material with a thickness of 3 mm, a width of 1 m, and a length of about 15 m. The performance indicators of the sealing material are shown in Table 1.

[0036]

[0037] As shown in Table 1, the modified polytetrafluoroethylene bearing lubricating material provided by the present application has a high tensile resilience, a low friction coefficient, a low wear rate, and a low creep rate.

[0038] Comparative Example 1

[0039] (1) 99.2 kg of modified polytetrafluoroethylene resin (containing 15% hexafluoropropylene, melt mass flow rate 9 g / min), 0.2 kg of magnesium stearate (particle size 200 μm), and 0.6 kg of silicon dioxide (particle size 50 nm) were thoroughly dried and then placed in the feeder of an extruder;

[0040] (2) The mixture was melt-extruded using a single-screw extruder with a screw length-diameter ratio of 20 and a compression ratio of 8, at a temperature of 330°C and a screw rotation speed of 45 rpm, and the parallel slit gap of the flat-knife head used in the single-screw extruder was 3 mm;

[0041] (3) The draw ratio was set to 50, and the material was cooled at 125°C to obtain a transparent modified polytetrafluoroethylene sealing material with a thickness of 3 mm, a width of 1 m, and a length of about 15 m. The performance indicators of the sealing material are shown in Table 1.

[0042] Comparative Example 2

[0043] (1) 99.2 kg of modified polytetrafluoroethylene resin (containing 15% hexafluoropropylene, melt mass flow rate 9 g / min), 0.8 kg of silicon dioxide (particle size 50 nm) were thoroughly dried and placed in the feeder of an extruder;

[0044] (2) The mixture was melt-extruded using a single-screw extruder with a screw length-diameter ratio of 20 and a compression ratio of 8 at a temperature of 330°C and a screw rotation speed of 45 rpm, and the parallel slit gap of the flat seam head used was 3 mm;

[0045] (3) The draw ratio was set to 50, and the polytetrafluoroethylene sealing material with a thickness of 3 mm, a width of 1 m and a length of about 15 m was obtained by cooling at 125°C. The performance indicators of the polytetrafluoroethylene bearing lubricating material are shown in Table 2.

[0046] Comparative Example 3

[0047] (1) 99.2 kg of modified polytetrafluoroethylene resin (containing 15% hexafluoropropylene, melt mass flow rate 9 g / min), 0.8 kg of magnesium stearate (particle size 200 μm) were thoroughly dried and placed in the feeder of an extruder;

[0048] (2) The mixture was melt-extruded using a single-screw extruder with a screw length-diameter ratio of 20 and a compression ratio of 8 at a temperature of 330°C and a screw rotation speed of 45 rpm, and the parallel slit gap of the flat seam head used was 3 mm;

[0049] (3) The draw ratio was set to 50, and the transparent modified polytetrafluoroethylene sealing material with a thickness of 3 mm, a width of 1 m and a length of about 15 m was obtained by cooling at 125°C. The performance indicators of the polytetrafluoroethylene bearing lubricating material are shown in Table 2.

[0050]

[0051] From the comparative examples, it can be seen that (1) when the content of hexafluoropropylene in the modified polytetrafluoroethylene resin is insufficient and the melt mass flow rate is not within the specified range, the tensile resilience of the composite material sharply decreases to 12%, and the creep rate increases to 85%; (2) without using internal lubricant, a large number of bubbles can be seen in the composite material, the tensile strength decreases to 17 MPa, and the tensile resilience decreases to 7%; (3) without adding reinforcing fillers, the friction coefficient of the composite material increases to 0.15, the wear rate increases to 2.5 x 10 -5 mm 3 / N·m, and the creep rate increases to 67%, all of which cannot simultaneously reach the performance range of the material described in the present application.

[0052] In each of the above examples and comparative examples, the amount of initiator used to prepare the modified polytetrafluoroethylene resin was 2% of the total amount of monomers, the polymerization reaction temperature was 60°C, the pressure was 10 MPa, and the polymerization reaction time was 4 h.

Claims

1. A high-resilience, creep-resistant modified polytetrafluoroethylene sealing material, prepared from the following raw materials and processes: Raw material ratio: by volume: 96-99.4 parts modified polytetrafluoroethylene resin, 0.1-1 parts internal lubricant, and 0.5-3 parts reinforcing filler; Preparation process: After the modified polytetrafluoroethylene resin, internal lubricant and reinforcing filler are fully dried, they are placed in the feeder of an extruder. The materials are fully mixed and melt-extruded at a temperature of 280~370℃ and a screw speed of 5~80rpm. The extruded material is stretched and cooled to obtain the modified polytetrafluoroethylene sealing material. The modified polytetrafluoroethylene resin is a copolymer of tetrafluoroethylene and hexafluoropropylene, obtained by polymerization reaction of tetrafluoroethylene and hexafluoropropylene as monomers and malonic acid peroxide as initiator for 2-6 hours under reaction conditions of 40-70℃ and 6-12MPa; the amount of monomers and initiator is as follows by volume: 80-90 parts of tetrafluoroethylene, 10-20 parts of hexafluoropropylene, and 0.01-0.03 parts of malonic acid peroxide initiator. The internal lubricant is one or more of calcium stearate, magnesium stearate, and talc; and the particle size of the internal lubricant is 20~500μm. The reinforcing filler is one or more of titanium dioxide, aluminum oxide, zinc oxide, and calcium carbonate, and the particle size of the reinforcing filler is 5~200nm.

2. The high-resilience, creep-resistant modified polytetrafluoroethylene sealing material as described in claim 1, characterized in that: In the manufacturing process, the extruder is a single screw with a length-to-diameter ratio of 10 to 25 and a compression ratio of 2 to 7.

3. The high-resilience, creep-resistant modified polytetrafluoroethylene sealing material as described in claim 2, characterized in that: The extruder is equipped with a flat-seam head, and the parallel seam gap is 0.05~5mm.

4. The high-resilience, creep-resistant modified polytetrafluoroethylene sealing material as described in claim 1, characterized in that: The extruded material has a draw ratio of 5 to 60.

5. The high-resilience, creep-resistant modified polytetrafluoroethylene sealing material as described in claim 1, characterized in that: In the preparation process, the cooling temperature is 120~150℃.

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