RU2304592C1 - Method of the radiation-chemical modification of the polytetrafluorehylene and the substance produced on its basis - Google Patents

Abstract

FIELD: chemical industry; radiation-chemical methods of the production of the polymeric materials with the improved complex of properties.

SUBSTANCE: the invention is pertaining to the field of the radiation-chemical methods of production of the polymeric materials with the improved complex of properties. The invention presents the description of the method of the radiation-chemical modification of the polytetrafluorethylene providing for treatment of the articles made out of the polytetrafluorethylene with the gamma-rays exceeding the temperature of the fusion point of the crystalline phase differing that the block and the film articles are subjected to this radiation and this radiation is conducted at presence of the molecular oxygen, at the oxygen tension of 10² - 1 mmHg. The invention also presents the description of the substance produced by the above indicated method. The technical result of the invention is production of the substance possessing the high indexes of the wear-resistance and creepage.

EFFECT: the invention ensures production of the substance possessing the high indexes of the wear-resistance and creepage.

3 cl, 3 tbl, 7 dwg, 2 ex

Description

The invention relates to the field of radiation-chemical technologies for producing polymeric materials with an improved set of properties, in particular antifriction and sealing materials based on polytetrafluoroethylene, and can find application in general and chemical engineering.

Polytetrafluoroethylene (PTFE) refers to polymers of mass consumption due to its unique complex of physico-chemical properties. PTFE is used for products of structural, electrical, heat and tribotechnical purposes in almost all industries, on the basis of which a wide range of composite materials and copolymers with other hydrocarbon and fluorine compounds has been developed. High performance characteristics of this polymer are provided by its chemical and biological inertness, heat resistance, thermal, dielectric and antifriction properties.

The problem that is encountered during the operation of fluoroplastic products is related to its cold flow (high creep under static load, both at room temperature and in the region of low temperatures, up to minus 90 ° С) and low wear resistance. To address these shortcomings in world practice, two main approaches were used: a physical method for the modification of THFFE based on the creation of composite materials (graphite-, metal-, fiber-filled fluoroplastics) and a chemical modification method based on the creation of polytetrafluoroethylene copolymers with other polymer systems. Both methods made it possible to obtain a set of new materials with improved characteristics, but at the same time all of them are significantly inferior to PTFE in terms of a set of properties. Compositions with dispersed filler F4K20, F4K15M15, etc. have from 500 to 1000 times higher wear resistance, but at the same time they retain high creep, deteriorate physical and mechanical characteristics, lose chemical resistance, in turn copolymers (F-4MB, F- 40, F-42) have improved physical and mechanical characteristics, but at the same time they lose to PTFE in terms of heat resistance, antifriction and dielectric properties.

To increase the wear resistance of PTFE, radiation-chemical approaches were used. Shown [Istomin NP, Semenov AP Antifriction properties of composite materials based on fluoropolymers. M .: 1981] that irradiation of PTFE with gamma rays at room temperature in air at atmospheric pressure leads to a decrease in mass wear by about 20 times at an absorbed dose of 500-600 kGy. However, the presence of molecular oxygen in the air sensitizes the chain radiation-oxidative degradation of PTFE; therefore, the described treatment led to a sharp decrease in strength, up to the destruction of samples at doses of 1000 kGy.

The work [RF Patent No. 2207351 C2, C 08 J 5/16, C08J 7/18, C08L 27/18, 2003] describes a method for producing wear-resistant PTFE by irradiation with gamma rays in the absence of oxygen in acetylene at 20-100 ° C, pressure 0.01-0.15 MPa and an absorbed dose of 30-60 kGy. The effect of increasing wear resistance, compared with the original PTFE, amounted to several thousand times. The disadvantage of this method is the appearance in the volume of PTFE hydrocarbon modifier, which reduces the chemical and thermal stability of the polymer. In addition, the physical and mechanical characteristics are significantly impaired, since irradiation at 20-100 ° C, regardless of the composition of the medium, is accompanied by a sufficiently high yield of polymer chain breaks, which is not acceptable in practice.

Radiation modification was used to improve the performance of PTFE as a sealing material. In the work [A.S. No. 1642730 A1, C08J 3/28, 1999, BI No. 17] describes a method for the radiation treatment of PTFE sealing products in an oxygen-free environment. Products were irradiated at a temperature of 50-55 ° C in an inert medium to an absorbed dose of 8 kGy. As a result of processing, the life of sealing devices was increased several dozen times while maintaining other physical and mechanical characteristics.

In the last ten years, the number of publications on the study of radiation-chemical processes during irradiation of PTFE in a melt state has increased significantly. The surge of interest in these studies is associated with the discovery of an unusual phenomenon: an increase in the elastic modulus, yield strength, and radiation resistance of PTFE after its irradiation in an oxygen-free medium above the crystallite melting point (327 ° C) [Oshima A., Tabata Y, Kudoh H., Seguchi T .Rad. Phys. Chem. 1995. V. 45. No. 2. P.269]. A method for the radiation modification of PTFE above the melting point is described in [United States Patent: No. 5,444,103. C08F 2/46; C08J 3/28; 1995]. This method is closest to that proposed in this application. Let's consider it in more detail.

The fundamental requirement of the method is that the irradiation of the samples is carried out in a medium free from the presence of molecular oxygen. It is well known [Fluoropolymers. / Ed. L. Walla: Per. from English / Ed. I.L. Knunyantsa and V.A. Ponomarenko. M .: Mir, 1975], that in the presence of oxygen, the radiation-chemical yield of chain breaks in PTFE increases several tens of times. Thus, the radiolysis of PTFE in air at atmospheric pressure and room temperature leads to a decrease in strength by half at a dose of 1-3 kGy, which is extremely low from the point of view of radiation chemistry. With increasing temperature, the destruction efficiency increases even more. Therefore, the requirement of the absence of oxygen during high-temperature radiation treatment seems quite natural. Moreover, the method was based on the idea of radiation-chemical crosslinking of PTFE - the opposite process of polymer degradation.

Another distinctive feature of the prototype is the modification of samples only in the form of films (up to 500 μm), block products were not affected. For irradiation of films, electron accelerators are usually used, which allow creating a high electron flux density and, accordingly, the absorbed dose rate (in the prototype, an accelerator with an electron energy of 5 MeV was used). However, this leads to a limitation on the thickness of the material layer being modified due to the limited range of electrons in the substance. Radiation processing of materials with a characteristic size of 10 mm and above requires the use of gamma radiation sources with high penetrating ability and capable of ensuring a uniform distribution of properties over the volume of the modified product. In turn, the use of gamma sources is limited due to the low radiation intensity and, accordingly, the absorbed dose rate. For typical gamma installations using the ⁶⁰ Co isotope (average quantum energy 1.25 MeV), the dose rate is 2–3 orders of magnitude lower (usually 1–10 kGy / h) than for electron accelerators. Therefore, when the efficiency of radiation modification is not too high and large absorbed doses are required, their use is excessively costly due to the need to maintain technological conditions for a long time.

Based on the ideas about crosslinking of polymer chains in PTFE developed in the prototype, and taking into account the data of the same authors [Katoh E., Sugisawa H., Oshima A., Tabata Y., Seguchi T., Yamazaki T. // Rad. Phys. Chem. 1999. V. 54. No. 2. P.165], according to which the crosslinking yield at 1000 kGy is not high 0.2 / 100 eV and still decreases with decreasing dose (at 500 kGy crosslinking was not detected at all!), We can conclude that the use of gamma radiation for radiation-chemical Cross-linking of PTFE in the melt is impractical and the corresponding technology for modifying block products is unpromising (the possibility of radiation processing by the electron beam of the surface layers of block products requires separate consideration and is not considered here). This explains the main attention of researchers to films and the use of electron accelerators, which allowed their systematic study in a wide range of absorbed doses of 500-10000 kGy, which are practically not available for gamma rays.

The technical problem solved in this application is to develop a method for the radiation treatment of PTFE that meets the following requirements: 1) the ability to modify block and film products of PTFE, 2) the absence of low molecular weight hydrocarbon sensitizers, 3) to achieve a significant increase in wear resistance and reduce creep in doses acceptable for gamma sources (less than 500 kGy), 4) ensuring a uniform distribution of properties throughout the volume.

The technical result of the solution of this problem is to obtain a new material with higher indicators of wear resistance and creep, not only in relation to the starting material, but also to the best antifriction compositions (F4K20, F4K15M2, Fluvis and others).

The problem is solved by radiation processing of PTFE products with gamma rays near the melting point of the crystalline phase in an inert medium containing a controlled admixture of molecular oxygen.

The invention consists in the following. The PTFE product is placed in a vacuum heat chamber and pumped out to a residual pressure of 10 ^-3 -10 ^-5 mm Hg. The chamber is filled with molecular oxygen to a pressure of 10 ^-2 -1 mm Hg. and inert gas to atmospheric pressure. After that, the product is heated to a temperature above the crystallite melting point and irradiated with a gamma radiation source with an average quantum energy of 1.25 MeV at an absorbed dose rate of 1-5 Gy / s to an absorbed dose of not more than 200 kGy. After irradiation, the sample is cooled in a heat chamber to room temperature.

Carrying out processing in the described mode made it possible to obtain a material having, in comparison with PTFE, from 10 ³ to 10 ⁴ times higher wear resistance, from 20 to 10 ² times lower creep, 30-50% higher modulus of elasticity and yield strength, as well as a low coefficient of friction at the level of the initial PTFE. To date, there has been no information in the literature on materials possessing a complex of the listed characteristics. Such a significant change in characteristics achieved by affecting the material, and not by preparing the composition, indicates a qualitative change in its structure.

The wear rate of the best compositions based on PTFE with identical tests is 1-2 mg / hour. Modified PTFE has a wear resistance below this level in the range of molecular oxygen pressures of 10 ^-2 -1 mm Hg (Example 1, figure 1). At a pressure near 10 ^-1 mm Hg the optimal result is achieved: the wear resistance is 10 ⁴ times higher than for unmodified PTFE, and 10-20 times higher than for compositions (Example 1, table 1, figure 1), at pressures of 10 ^-2 and 1 mm RT. Art. these indicators are at the level of: 1600 and 3000 times in relation to the starting material, and 2-4 and 3-7 times in relation to the compositions. With a decrease in pressure to 10 ^-3 mm Hg and magnification over 1 mmHg wear rate increases sharply. Thus, outside the selected pressure range, a material with the declared characteristics is not formed. In the same pressure range, a minimum creep of the modified PTFE is observed: from 20 to 100 times lower than that of the starting material (Example 2, FIGS. 2-4).

An increase in the absorbed dose to 300 kGy does not lead to a significant change in properties, while its decrease to 100 kGy significantly increases the wear rate (Example 1, Table 1). The optimal result from the point of view of the efficiency of radiation modification is achieved at 200 kGy.

Another evidence of a qualitative structural change in the modified PTFE is a change in the plastic deformation mechanism, as indicated by tensile diagrams (Example 2, FIG. 5). The starting material is characterized by a high contribution of inelastic deformation already at the initial portion of the tensile diagrams, while the sample elongates uniformly along the entire length without the formation of a neck, which indicates the uniform stretching of polymer chains. Modified PTFE is deformed with the formation of a neck and, in contrast to the initial one, has a straight section in the initial section of the diagram. The latter is explained by an increase in intermolecular bonding of polymer chains and the elastic nature of the deformation. The main physical and mechanical characteristics of the modified PTFE are presented in table 2 (Example 2). It is seen that the obtained material has a high modulus of elasticity and yield strength.

Note that the absorbed dose rate at which PTFE was modified was 2–3 Gy / s (Examples 1, 2). It can be expected that the expansion of this interval to 1-5 Gy / s will not lead to a change in the result. The effects of dose rates associated with the competition of the diffusion and kinetic stages of radiation-chemical reactions are manifested and are significant when there are differences in dose rates of whole orders of magnitude. In our case, a decrease in the dose rate to 1 Gy / s or an increase to 5 Gy / s only affects the total time needed to set the required dose.

Unlike the prototype, which is based on the idea of crosslinking polymer chains, the proposed method is based on modifying the chemical structure of the polymer chain at the stage of irradiation of the polymer in the melt and the subsequent change in the supramolecular structure that occurs when the polymer is cooled to room temperature after the irradiation is stopped.

The main structural feature of the obtained material is the formation in it of systems of polyconjugated π bonds, the existence of which is unambiguously indicated by the acquired property of fluorescence in the visible part of the spectrum.

The initial PTFE does not have absorption in the spectral region (200-800 nm) and does not fluoresce when excited by UV light in the wavelength range of 200-400 nm. Modified PTFE has a bright fluorescence in the visible part of the spectrum (Example 2, table 3, Fig.6). Radiation treatment at pressures above 1 and below 10 ^-2 mm Hg leads to a sharp weakening of it.

The fluorescent centers arising in the modified PTFE are polyene structures with a different number of conjugated double bonds. The revealed structures do not exhibit phosphorescence when the samples are cooled to 77 K, which is a characteristic feature of the conjugated structures of the chain structure - (CF = CF) _n -. Cyclic structures with a π-system possess both fluorescence and phosphorescence [Nurmukhametov R.N. Absorption and luminescence of aromatic compounds. M: Chemistry, 1971]. The maxima of the polyene absorption bands shift to the region of large wavelengths as the number n increases [Stern E., Timmons K. Electron absorption spectroscopy in organic chemistry. M .: Mir, 1974]. The calculation shows that the fluoropolyenes in the polymer chain of modified PTFE (with the number of monomer units m≤5 × 10 ⁴ ) are characterized by values n = 4-11 and, accordingly, m / n≥10 ³ . If we assume that the value of the extinction coefficient for them is close to those for alkylpolyenes 80000-140000 (l mol ^-1 cm ^-1 ), we can estimate their concentration and radiation-chemical yield - (1-5) × 10 ¹⁸ cm ^-3 and ( 0.1-0.5) / 100 eV. As far as we know, this is the first case of the discovery and direct evidence of the formation of polyenes in PTFE.

The relative content of conjugated structures in the modified PTFE does not exceed 0.1%, which cannot significantly affect its chemical resistance, dielectric, and other characteristics. At the same time, the reason for the qualitative change in wear resistance and creep, apparently, should be attributed to the morphological factor, which contributes to the strengthening of the effect. It is known [Fluoropolymers. / Ed. Walla L.: Per. from English / Under. ed. Knunyantsa I.L., Ponomarenko V.A. M: Mir, 1975] that the viscoelastic properties of PTFE improve after quenching (rapid cooling from the melt), leading to a decrease in crystallinity (by 20-25%). In the modified PTFE, the degree of crystallinity also decreases by 35–40% and the average crystallite size decreases by at least 2 times, since the optical transparency is significantly increased (the initial PTFE is white due to diffuse light scattering, the modified PTFE in a thin layer is almost transparent and the light does not scatter). Thus, we can conclude that both effects are of the same nature, associated with a general increase in the entropy of the supramolecular structure of PTFE and the formation of a network of physical link nodes in disordered regions due to an increase in conformational mobility and a decrease in the rigidity of the polymer chain. A direct indication of the latter is the change in the nature of the tensile diagrams noted above (Fig. 5).

The invention is illustrated by the following examples.

EXAMPLE 1

Samples of polytetrafluoroethylene were subjected to radiation modification in the form of standard blocks with a diameter of 30 to 110 mm and a height of 50 to 120 mm of the fluoroplast-4 grade with a molecular weight of (5-10) × 10 ⁶ according to TU 6-05-810-88.

Samples were placed in a vacuum heat chamber and pumped out to a residual pressure of 10 ^-3 -10 ^-5 mm Hg. Then molecular oxygen was injected to the specified pressure values (10 ^-3 , 10 ^-2 , 10 ^-1 , 1, 10 and 100 mm Hg) and inert gas to atmospheric pressure. The chamber with the samples was irradiated with gamma rays with an average energy of 1.25 MeV at a source of ⁶⁰ Co at an absorbed dose rate of 2-3 Gy / s to an absorbed dose of 200 kGy. Samples were irradiated at a temperature above the melting point of 327 ° C.

After irradiation, the friction coefficient and the wear rate on the UMT-2168 friction machine were measured according to the “finger-disk” scheme at a load of 2.5 MPa and a linear sliding velocity of the sample relative to the fixed disk of 1 m / s. The samples were tested in the form of cylinders with a diameter of 10 and a height of 20 mm, made by machining modified blocks. Three samples, modified under identical conditions, were tested simultaneously. The counterbody is a steel disk made of St 45 steel with a hardness of HR C 45-50 and a roughness of Ra = 0.66-0.85 microns. After grinding for 20 minutes, the samples were weighed, after which they were abraded for 3-5 hours. The mass loss in a single experiment was determined by the results of averaging measurements on three samples. The average value of mass loss for each experimental point corresponding to a certain combination of modification conditions is determined by the results of at least 3 experiments. The error in determining the mass loss in the range of less than 1.0 mg / hour did not exceed ± 0.1 mg / hour. The friction path was 10,000-15,000. The test results are shown in table 1.

EXAMPLE 2

Modified samples of a standard PTFE film (GOST 24222) 0.1 thick, 60 wide and 100 mm long. Radiation-chemical modification of the samples was carried out similarly to that described in Example 1. Film samples for testing were made in the form of strips with a length of 30 and a width of 5 mm Labels were applied to the samples, the displacement of which determined the dependence of the absolute value of deformation on time. The accuracy of determining the strain was ± 0.01 mm. The static load was set equal to 11 MPa. Creep curves of modified PTFE films are shown in FIGS. 2-4.

The mechanical characteristics of the films were determined on the samples in the form of blades with a working part size of 3 × 10 mm. The blades were cut with a punch knife and tested at room temperature of 20-22 ° C for tension at a constant speed of 10 mm / min on an Instron testing machine. During the drawing process, digital and graphical recording of the tensile diagram was carried out. The position of the yield strength was determined by the inflection point on the stress-strain curve (σ-ε), after which the slope of the tangent to the curve changed from positive to negative and the stretching stress began to decrease. The elastic modulus E _{cfr was} determined by the maximum slope of the tangent to the initial portion of the tensile diagram. Since the yield strength is not explicitly observed in the diagrams for the initial sample and the deformation develops without the formation of a “neck”, we used the value at the intersection of the straight lines drawn near the clearly defined kink in the tensile curves in the strain region equal to 5-10% (see Fig. 5). Physico-mechanical characteristics of the modified PTFE films are presented in table 2.

Spectral studies of the films were carried out at room temperature on Perkin-Elmer instruments. The emission and excitation spectra of luminescence were recorded on a spectrofluorimeter (model LS-5). To reduce the distorting effect of scattered light when measuring the fluorescence spectra and excitation of fluorescence, the studied samples were immersed in distilled water, which served as an immersion medium. In addition to this, filters were used, which made it possible to more completely eliminate the effect of the scattering factor. Fluorescence spectra are corrected for the spectral sensitivity of the instrument. Data on the spectral-luminescent properties of the modified PTFE, depending on the conditions of the radiation treatment are presented in table 3 and Fig.6, 7.

Table 1
The effect of radiation-chemical modification of PTFE on the wear rate ^*) Sample Absorbed Dose, kGy The oxygen pressure in the reaction chamber, mm Hg The intensity of the average mass wear, mg / hour Coefficient of friction Source 0 - 900 0.08 one 10 ^-3 2.12 0.09 2 10 ^-2 0.42 0.10 3 200 10 ^-1 0.13 0.09 four one 0.30 0.08 5 10 46.0 0.09 6 one hundred 345 0.09 7 300 10 ^-1 0.10 0.09 8 one hundred 10 ^-1 17.2 0.11 Note:
^*) The wear rate was determined in the finger-disk kinematic diagram at a load of 2.5 MPa, a sliding speed of 1 m / s and a friction path of 10,000-15,000 m.

table 2 Physico-mechanical characteristics of modified PTFE Sample Dose, kGy The oxygen pressure in the reaction chamber, mm Hg Modulus of elasticity, MPa Yield Strength, MPa Source 0 - 250 10 Modified 200 10 ^-2 -1 330-360 14-16 Table 3 The effect of radiation-chemical modification of PTFE on the nature of fluorescence Sample Absorbed Dose, kGy The oxygen pressure in the reaction chamber, mm Hg The nature of fluorescence Source 0 - no one 10 ^-3 very weak 2 10 ^-1 intense 3 200 one intense four one hundred very weak

Claims (3)

1. The method of radiation-chemical modification of polytetrafluoroethylene, which consists in processing products from polytetrafluoroethylene with gamma rays above the melting point of the crystalline phase, characterized in that the block and film products are irradiated and irradiated in the presence of molecular oxygen, at an oxygen pressure of 10 ^-2 - 1 mmHg. 2. The method according to claim 1, characterized in that the PTFE products are irradiated with gamma rays with an average energy of 1.25 MeV at an oxygen pressure of 10 ^-2 - 1 mm Hg, absorbed dose rate of 1-5 Gy / s, absorbed dose no more than 200 kGy. 3. The material obtained by the method according to claims 1 and 2 and characterized by the structure - (CF ₂ -CF ₂ ) _m - (CF = CF) _n -, where n = 4-11 is the number of units with conjugated double bonds, m is the number of monomer units of the polymer chain, m / n≥10 ³ , and having intense fluorescence with a maximum in the wavelength range of 450-550 nm, optical transmission in the visible region up to 90%, a friction coefficient of 0.08-0.10, a wear rate of 0.1-0.4 mg / h and creep of not more than 5% (at a load of 12 MPa per 100 hours).

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