RU2839734C1 - Method of disperse polymer electrets producing - Google Patents

Abstract

FIELD: performing operations.

SUBSTANCE: invention relates to a method of producing dispersed polymer electrets for various engineering fields as sources of an external electric field, particularly dispersed polymer electrets can be used in medicine for polarizing substances and activating regenerative processes in tissues. Method comprises the steps of processing polymer materials, at which their grinding is carried out to obtain polymer particles with surface defects by crushing and sending them for washing with an aqueous solution of detergents, after which washed polymer particles with surface defects are subjected to chemical activation and delivered for drying, after which chemically activated polymer particles are subjected to electrisation in electrostatic field of corona discharge, so that polymer particles surface defects obtained in the process of crushing and treatment with a chemical activator acquire the role of cells and accumulate electric charges, wherein the process of chemical activation involves the use of a chemical activator, which provides a possibility of increasing the depth of already existing surface defects and the formation of new defects on the surface of a polymer particle without its destruction.

EFFECT: high charge density of the polymer electret.

4 cl, 9 dwg, 4 tbl, 6 ex

Description

Field of technology

The proposed invention relates to a method for producing dispersed polymer electrets for various fields of technology as sources of an external electric field, in particular, dispersed polymer electrets can be used in medicine for polarizing substances and activating regenerative processes in tissues.

State of the art

A technical solution is known from the prior art (patent for invention of the Russian Federation 2477540, published on 10.03.2013), which relates to a method for producing film electrets, and can be used, for example, in the production of bipolar electret microphones and a new class of piezoelectric sensors based on laminated electret films with a giant piezoelectric modulus (up to 1000 nC/N). The technical result is an increase in the value and stability of the surface density of the positive charge in film fluoropolymers. The technical result is achieved by applying a discrete layer to the surface of the fluoropolymer film, consisting of nanosized aggregates of titanium-containing nanostructures isolated from each other. The application of nanosized aggregates of titanium-containing nanostructures is carried out using chemical nanotechnology based on the principles of the molecular layering method. In a flow-type reactor, the polymer surface is treated with titanium tetrachloride vapors. As a result, titanium-containing nanostructures of the O-Ti(Cl) ₃ type are grafted to the carbon chain of surface macromolecules due to exchange chemical reactions. Aggregates of such nanostructures have a rounded shape with a diameter of 80-200 nm in plan and a thickness of about 20-30 nm. Titanium-containing nanostructures are deep traps for the positive charge and, in addition, significantly reduce molecular mobility, which leads to an increase in the stability of the surface density of the positive charge imparted to the fluoropolymer during electretization.

However, this method does not allow obtaining dispersed polymer electrets, which have a larger surface area for the formation of new energy centers during processing and, accordingly, the known method does not allow obtaining electrets with a high charge density.

Disclosure of invention

First of all, it should be noted that within the framework of this application, electrets are understood as electrical analogues of permanent magnets, the physics of whose operation is based on fairly complex phenomena underlying the electret state of dielectrics. Electrets belong to the class of active dielectrics and are materials capable of maintaining residual polarization for a long time (months and even years) after removal of external influence and generating a relatively strong (up to 106 V/m) quasi-static (slowly changing over time) electric field. Thus, the electret effect is the ability of dielectrics to accumulate and retain electric charges for a long time.

In this technical solution, polymers are chosen as the material for obtaining electrets.

According to the elemental composition, the existing polymer electrets are divided into two categories: fluorinated polymers and non-fluorinated polymers. Fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), perfluoro(1-butenyl vinyl ether) polymer, polyvinylidene fluoride (PVDF), etc. are classified as fluorinated polymer electrets. Non-fluorinated polymers include Kapton, Parylene, epoxides or polyethers, etc.

Today, there are film and fibrous forms of products made of polymer electrets. Polymers based on vinylidene fluoride have the highest piezo- and pyroelectric activity. Thus, a piezoelectric film made of polyvinylidene fluoride (PVDF), as a converter of mechanical energy into electrical energy, surpasses quartz and significantly ceramics in sensitivity. In addition, a piezoelectric film can be given almost any shape without any loss of its activity, it has good mechanical strength and impact resistance. It is also known that the stability of the potential of fiber reinforced plastics based on polyethylene and polypropylene is better than that of films made of the same materials. This has led to the widespread use of fiber reinforced plastics in the creation of filters. It is assumed that the high stability of the potential of fiber reinforced plastics is due to the presence of non-stoichiometric defects in them (carbonyl groups and unsaturated links).

However, electrets made from polymeric materials in film and fibrous forms have a disadvantage - they quickly lose their captured charges, i.e. such electrets do not have a stable surface charge and, as a consequence, have a low charge density, limited in the values of surface charge density.

To solve the problem, namely, to increase the stability of the surface charges of polymer electrets, various types of effects on the surface structure of polymers are used to develop microrelief (create surface defects in the polymer structure). The resulting defects play the role of so-called "traps" or charge cells.

Cells or charge "traps" are electrically active defects of the material capable of capturing external charges as a result of the impact of energy factors: radiation, mechanical impacts, constant electric field, electron beam, corona discharge, etc. The most common technologies for creating electrets are thermal polarization and corona discharge.

The nature of surface defects in the case of high-molecular compounds depends on the kinetic features of the crystallization process of polymer structures, as well as on the nature of the physical and chemical effects on the already formed polymer chain. The following types of defects are possible in polymers: ends of a macromolecule, loops, other supramolecular structures and point defects, shear dislocations.

Thus, the dielectric passes into an electret state as a result of the influence of energy factors and the subsequent capture of injected charges by existing defects (traps) in the polymer structure, as well as polarization (in the case of polar substances).

On the one hand, traps not only capture and hold injected charges, but at the same time are sources of charge leakage.

In addition, molecular relaxation processes of various natures may occur in polymers depending on the type of acquired charges. The temperature of application of electrets affects the movement of molecular groups and segments of the main chain. The higher the temperature, the more intense and deeper the mobility, which affects and induces the rearrangement of the main chain of polymers, which accordingly affects the relaxation time: the movement of molecular groups stimulates the relaxation of injected charges, the rearrangement of the main chain - the relaxation of spatial charges.

Therefore, an important factor for increasing the stability and density of the charge is the depth of the cells. It is the depth of the cells that has a direct effect on the relaxation time of the charges. The deeper the defect, the greater the opportunity for the charge to penetrate into the deeper layers of the polymer. Thus, the greater the distance the charge must travel to release it from the trap (i.e., the relaxation time increases).

In connection with the above, the aim of the invention is to develop a method for producing chemically activated dispersed polymer electret particles with a high charge density compared to the magnitude of the charge acquired by film polymer electrets in the electrostatic field of a corona discharge.

The technical result that the proposed technical solution is aimed at achieving consists of increasing the charge density of the polymer electret.

This technical result is achieved by using the proposed method for obtaining dispersed polymer electrets, including stages of processing polymeric materials, in which they are crushed to obtain polymer particles with surface defects by crushing and sent for washing with an aqueous solution of detergents, after which the washed polymer particles with surface defects are subjected to chemical activation and sent for drying, after which the chemically activated polymer particles are subjected to electrification in an electrostatic field of a corona discharge so that the surface defects of the polymer particles obtained in the process of crushing and processing with a chemical activator acquire the role of cells (traps) and accumulate electric charges, and the process of chemical activation consists in using a chemical activator, which provides the possibility of increasing the depth of already existing surface defects and the formation of new defects on the surface of a polymer particle without its destruction.

Chemically activated polymer particles subjected to electrification in the electrostatic field of a corona discharge are heated to 60° C.

The chemical activator is an aqueous solution of acid or an aqueous solution of alkali or an aqueous solution of surfactants, which provide the ability to increase the depth of existing surface defects and form new defects on the surface of a polymer particle without destroying it.

The chemical activator is an aqueous solution of sulfuric acid or sodium hydroxide or alkylbenzene sulfonic acid.

Brief description of the drawings

Fig. 1. Micrograph of the surface of crushed polypropylene before activation with a chemical reagent (chemical activator).

Fig. 2. Micrograph of the surface of crushed polypropylene activated by a solution of H ₂ SO ₄ .

Fig. 3. Micrograph of the surface of crushed polypropylene activated by NaOH solution.

Fig. 4. Micrograph of the surface of crushed polypropylene activated by ABSK solution.

Fig. 5. Micrograph of the surface of crushed polyethylene terephthalate before activation with a chemical reagent (chemical activator).

Fig. 6 Micrograph of the surface of crushed polyethylene terephthalate activated by an ABS solution.

Fig. 7 Micrograph of the surface of crushed polyethylene terephthalate activated with NaOH solution.

Fig. 8. Technological scheme for implementing the method for producing dispersed polymer electrets.

Fig. 9. Design of electrostatic drum activator.

Implementation of the invention

The starting material for obtaining electrets are polymers. As indicated above, both fluorine-containing polymers (e.g., polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), fluorinated ethylene propylene (FEP), polychlorotrifluoroethylene, polyethylenetetrafluoroethylene, etc.) and non-fluorine-containing polymers (polypropylene (PP), polyethylene, polyethylene terephthalate, polystyrene, etc.) are used as polymers. The choice of one or another type of polymer is determined by the area of use of the electrets.

In order for a dielectric, and in this case a polymer, to be able to transition to an electret state, it must be exposed to energy factors, such as electrification in a corona discharge.

Electrification in a corona discharge is the most technologically advanced method of obtaining electrets. Comparative studies of the stability of polymer electrets obtained by various methods have shown that in terms of such a parameter as the relaxation time of the surface potential, corona electrets are not inferior, and in most cases surpass other types of electrets.

The validity of this statement is especially evident in the production of electrets from non-polar polymers such as PTFE, tetrafluoroethylene-hexafluoropropylene copolymer, polypropylene, polyethylene, etc.

Thin wires, needles or blades can be used as corona electrodes to obtain corona electrets.

According to the proposed method, in order to enhance the effect of the charge, i.e. to increase the possible charge density that the electret can acquire and the relaxation time, it is necessary to increase the contact area of the electret material with the corona electrode. In order to increase the contact area, as well as the depth of the existing cells and the emergence of new energy centers uniformly distributed over the entire surface, it is necessary to act on the structure of the polymers in advance by physical and chemical methods.

Previously, the state of the art did not consider the method of electrification of polymer particles in the electrostatic field of a corona discharge. The use of this method made it possible to obtain dispersed electret polymer particles of various sizes (for example: 0.5 - 5 mm, and the size as a whole depends on the capabilities of the equipment on which the production of electrets is carried out and, accordingly, the particle size may have other values) due to the simplicity and convenience of the technological process, the relatively short time of passage of particles through the electrostatic field.

As an example, we can give a calculation with particles that have a spherical or nearly spherical shape. Thus, if we arrange spherical crushed polymer particles of 0.1 cm in size on an area of 100 cm ² , then there will be 100 such particles per 10 cm, and accordingly, there will be 10,000 crushed particles per area of 100 cm ² . With the area of a sphere: S (sphere) = 4πR ² = 4⋅3.14⋅0.01 = 0.1256 cm ² , the contact area of spherical dispersed polymer particles S _to 10,000 * 0.1256 cm ² = 1256 cm ² .

However, the calculation was carried out taking into account the defects on the surface of the crushed particles that arise during crushing with subsequent chemical activation, which increases the surface area compared to a sphere. In these calculations, the Bussroyd coefficient is taken into account - the deviation of the surface from a spherical shape. The smaller the equivalent radius of the particles, the greater the value of the Bussroyd coefficient, which depends on the volume and surface area of the particles. Under equal activation conditions, particles with a smaller radius develop a degree of non-sphericity more actively.

Accordingly, during corona charging of dispersed polymer particles in an electrostatic drum separator, the contact area with the corona electrode increases by more than 1256 times, compared to a polymer film charged by a corona discharge. The data of these calculations are confirmed by the results of the analysis of microphotographs of the surface of crushed polymers activated by surfactant solutions, presented in Figures 2-7.

Table 1 presents the main electrical properties of polymers, which provide a general picture of the differences between polymer types. These characteristics are a kind of support point for selecting the main parameters of the processes of activation and electrification of particle surfaces.

Table 1.

Properties Specific volumetric electrical
resistance, Ohm⋅cm Surface resistance, Ohm Dielectric constant at 10 ⁶ Hz Electrical strength, mV/m High-density polyethylene (HDPE) 10 ¹⁷ -10 ¹⁸ 10 ¹⁵ 2.2-2.3 50±5
(1 mm) Low density polyethylene (HDPE) 10 ¹⁷ -10 ¹⁸ 10 ¹⁵ 2.2-2.4 50±5 Polypropylene (PP) 10 ¹⁷ - 2.0-2.1 150±30 Polyvinyl chloride (PVC) 10 ¹⁵ -10 ¹⁷ 10 ¹³ -10 ¹⁴ 2.9-3.0 40±5 Polystyrene (PS) 10 ¹⁶ -10 ¹⁷ 10 ¹⁵ -10 ¹⁷ 2.4-2.7 100±40 Polyethylene terephthalate (PET)
(film, degree of crystallinity 60%) 10 ¹⁶ -10 ¹⁷ 10 ¹⁴ 2.9-3.0 300
(25 mm) Polycarbonate amorphous film
(crystallinity 26%) 1.5-6⋅10 ¹⁶
1-2⋅10 ¹⁷ 1.5-8⋅10 ¹⁵ 2.96
2.93 20±1 (2 mm)
150-200 (10 microns) PU 10 ⁹ - 10 ¹² - 4.5-4.8; 20-25;

According to electrical resistance, polymeric materials can be divided into conductors (<10 ³ Ohm⋅cm), antistatic materials (from 10 ³ to 10 ⁹ Ohm⋅cm) and insulators (>10 ⁹ Ohm⋅cm). Specific volume resistance (p _v ) is the ratio of the electric field strength to the current density passing through the volume of the material sample, specific surface resistance (p _s ) is the ratio of the electric field strength to the current per unit width of the surface of the material sample:

where E is the electric field strength (V/m); i _v is the current density,

where E is the electric field strength (V/m); i _s is the surface current density.

Knowing the specific volume and surface resistances of materials, it is possible to estimate the current density that each type of polymer can pass through its volume or surface at a certain voltage.

The permittivity characterizes the behavior of a dielectric in electric fields, namely the increase in the capacitance of a capacitor (C) when a dielectric is introduced into it compared to the capacitance of the same capacitor with a vacuum (C ₀ ): ε = C/C ₀ .

Thus, this parameter, reflecting the behavior of the polymer dielectric in an alternating electric field, also determines the polarity or non-polarity of the dielectric, which provides knowledge about the nature of the effect of the electric field on the polymer.

Analyzing the dielectric characteristics of polyolefins, it was concluded that LDPE and PP are non-polar polymers, HDPE, PVC, PS, PETF, PC belong to the group of low-polar polymers, PA is a polar polymer, PU is highly polar. These characteristics determined that the most suitable method for creating an electret is preliminary physicochemical surface treatment in order to create defects in the form of cracks and free bonds without violating the volumetric structure in order to avoid losing the main qualities and properties.

In the case of mechanical action in the form of crushing, to assess the energy that must be applied to break the bond and form charge cells, a comparison is made of the tabular values of the molecular bond energy in Table 2.

Table 2.

No. Polymer Cohesive energy, E _cog kJ/mol Average molecular bond energy, kJ/mol 1 Polyethylene terephthalate (PET) 43.6 487 2 Low density polyethylene (HDPE) 9.6 350 3 High-density polyethylene (HDPE) 7.9 350 4 Polypropylene (PP) 14.9 350 5 Polyurethane (PU) 36.62 487 6 Polystyrene (PS) 22.9 487

The values of the impact force (stress) required to destroy the polymer material and obtain effective cell crack sizes can be recalculated into the crusher parameter - the rotor circumferential speed using the well-known formula of classical impact theory

where ρ is in kg/ ^m3 , D _св is the average weighted size of the source material, m.

To estimate the energy used for destruction (E _dr - J), the formula of A.I. Kosarev is applicable:

where k ₁ = 0.9-0.95 is a coefficient taking into account the process conditions, m _k is the mass of the particle, kg.

Thus, initially the structure of polymers is affected by a physical method - the polymer is crushed to obtain polymer particles with surface defects by crushing. Any device known to a specialist from the state of the art and used for crushing polymer materials can be used for crushing, for example, a rotary knife crusher of the RM 120M type. Crushing is an important stage of the proposed method, since it initiates mechanical destruction of the surface layers of the polymer - the formation of surface defects in the polymer structure, which will subsequently act as cells for capturing the charge.

Next, the crushed polymer particles are sent for washing with an aqueous solution of detergent. Any detergent known to a specialist from the state of the art and used to clean the surface of solid polymer particles from contamination can be used as a detergent.

Washed polymer particles with already surface defects formed after the crushing stage are subjected to chemical activation. Chemical activation is carried out using a chemical activator. The chemical activator is selected so that it provides the possibility of increasing the depth of already existing surface defects and forming new defects on the surface of the polymer particle without its destruction. In this case, destruction is understood as the complete destruction of the particle with the loss of the volume structure that could ensure the retention of electric charges. To carry out the chemical activation process, any device known to a specialist from the prior art (for example, a mixer type) can be used, ensuring full contact of the surface of the polymer particles with the chemical activator. Thus, for example, a pure polymer mixture of particles is placed in a container with a magnetic stirrer, to which a chemical activator is added. The process of mixing the particles can last, for example: 5, 10, 15, 25, 50 minutes.

The chemical activator is an aqueous solution of acid or an aqueous solution of alkali or an aqueous solution of surfactant.

For example, an acidic destructor - an aqueous solution of sulfuric acid, an alkaline destructor - an aqueous solution of alkali (for example, caustic soda or caustic potassium), as well as aqueous solutions of surfactants (for example, alkylbenzene sulfonic acid, sodium alkylbenzene sulfonate) can be selected as a chemical activator. The concentrations of the specified solutions can be selected as 1 mol/l; 1.5 mol/l; 2 mol/l.

The chemical activator is selected empirically, i.e. their efficiency is compared with respect to a certain type of polymer. Activation depends on the type of polymer, namely its chemical structure, molecular weight, dielectric properties, as well as the type of activator itself, temperature and time of polymer processing. For a specific polymer, a chemical activator is selected one at a time to compare the efficiency. The efficiency is assessed by the possibility of increasing the depth of existing surface defects and forming new defects on the surface of a polymer particle without destroying it. The study used both alkaline and acidic destructors, which showed approximately equal values of the cell size, however, when processing a polymer mixture with sodium and potassium hydroxide solutions, as well as sulfuric acid, it is necessary to take into account the degree of degradation of polymers, especially PET polymers. Therefore, when developing a technology, the choice of a destructor should depend on environmental, economic factors, as well as the final characteristics of secondary raw materials.

As an example, it can be noted that a number of chemical activators were determined in the course of experiments for each of the studied polymer electrets. As was indicated above, such chemical activators can be an aqueous solution of sulfuric acid or an aqueous solution of alkali or aqueous solutions of surfactants. Table 3 provides an example of the main indicators of some activator solutions (there may be other chemical activators).

Table 3

No. Name Classification Chemical formula Molecular weight, g/mol 1 Sulfuric acid hh _{H2SO4 ​} 98,079 2 Sodium hydroxide hh NaOH 39,997 3 Potassium hydroxide hh KOH 56,106 4 Alkyl benzene sulfonic acid (ABSA) Brand A RC ₆ H ₄ SO ₃ H, where
R = C ₁₂ H ₂₅ - C ₁₄ H ₂₉ 319-326 5 Sodium alkyl benzene sulfonate (Sodium ABS) Bleached RC ₆ H ₄ NaO ₃ S, where
R = C ₁₄ H ₂₉ - C ₁₈ H ₃₇ 320-330

The depth of defects formed as a result of chemical action on the surfaces of the studied polymers is in the range from 1.7 µm to 7 µm, when exposed to such reagents as aqueous solutions of surfactants: alkylbenzene sulfonic acid and sodium alkylbenzene sulfonate, aqueous solutions of sodium and potassium hydroxide, as well as an aqueous solution of sulfuric acid, for 5 - 60 minutes at a temperature of 21 °C to 65 °C.

The size of the fractions of activated polymer particles can be regulated using laboratory sieves with cell sizes of, for example: 1, 1.5, 2, 2.5 and 3 mm.

The obtained polymer particles, physically and chemically activated, are sent for drying. Any device (dryer) known to a specialist from the state of the art can be used for the drying process. Drying can vary in different temperature ranges taking into account the drying time, for example, at a temperature of 75 °C in a drying cabinet the process lasts for 6 hours.

The dried polymer particles are subjected to electrification in the electrostatic field of the corona discharge. As a result, the surface defects of the polymer particles acquire the role of cells or traps and accumulate electric charges. We obtain dispersed polymer electrets (corona electrets).

In the proposed method, a mixture of polymer particles can be used as the starting material.

From the point of view of polymer mixture separation technology, crushing is a preliminary stage, since the sizes of surface cracks-traps are stochastic in nature, whereas chemical destruction is a controlled process that allows for a targeted change in the size and area of surface defects of polymer particles, depending on the given selectivity of the process.

Crushing initiates mechanical destruction of the surface layers of the polymer, which subsequently intensifies the process of chemical activation of polymer particles, allowing molecules of chemical activators to adsorb on the surface of polymers by physical and chemical mechanisms.

The data of the experiments on activation of polymeric materials, namely the non-polar polymer LDPE, the group of low-polar polymers - HDPE, PP, PS, polar polymers PVC, PETF, show that the most rational technology for creating an electret from a mixture of polar and non-polar polymers also includes the stages of mechanical cracking and subsequent chemical treatment in order to create defects on the surface. The resulting defects then play the role of "traps" for capturing the injected charge from the electrostatic field of the corona discharge. For simplicity of the technological process, one can strive to minimize the reagents used, namely, to select a universal activator that will modify the surface of all types of polymers in the mixture at once. Such activators can be solutions of surfactants with an acidic and alkaline environment, namely the surfactants under study - alkylbenzene sulfonic acid (ABS), sodium salt of alkylbenzene sulfonic acid (ABS Na), etc.

To implement the stages of the proposed method, a corresponding system for obtaining polymer dispersed electrets is used, which can be performed as follows.

The system is shown in Figure 8 and comprises a hopper (1) with a feedstock, at least three belt conveyors ((2.1, 2.2, 2.3, 2.4), a rotary crusher (3), a reactor with a stirrer for producing an aqueous solution of detergents (4), a reactor with a stirrer for producing an aqueous solution of activator (5), at least four dispensers, one of which is a detergent dispenser (6.1), the second a chemical activator dispenser (6.2), the third a detergent aqueous solution dispenser (6.3), the fourth a chemical activator aqueous solution dispenser (6.4), at least two water tanks (7.1, 7.2), a friction screw washer (8), a filter (9), a screw mechanical activator (10), a drum-type dryer (11), an electrostatic drum activator (12), a classifier of separated materials (13).

The process of obtaining dispersed polymer electrets using the above system is carried out as follows.

The initial polymer material (raw material) in the form of polymer blanks, polymer fractions, etc. is fed into the hopper (1). From the hopper (1), the polymer material is directed to the rotary crusher (3) using a belt conveyor (2.1). In the rotary crusher (3), the polymer material is crushed and ground until the corresponding particles are obtained. The particle size is determined based on the final requirements for the product obtained and the capabilities of the process line equipment (for example: 0.5 - 5 mm). Then, the crushed particles are directed to the friction screw washer (8) using a conveyor (2.2). The particles are washed by feeding the friction screw washer (8) with the appropriate aqueous solution of detergents, which is formed in the reactor (4) with a mixer to obtain an aqueous solution of detergents. In order to prepare an aqueous solution of detergents, water is first fed into the reactor (4) from the water tank (7.1) and detergents from the dispenser (6.1). Then, the aqueous solution of detergents is fed by the dispenser (6.3) to the friction auger washer (8), the wastewater from which is sent to the filter (9), and the crushed washed particles are sent to the auger mechanical activator (10). The auger mechanical activator (10) is designed to ensure the withdrawal of the liquid fraction for cleaning (or disposal) and chemical activation of the crushed particles by feeding the chemical activator from the dispenser (6.4) to the auger mechanical activator (10). The chemical activator is fed to the dispenser (6.4) from the reactor (5), in which the chemical activator is prepared by feeding the main chemical component of the activator from the dispenser (6.2) and water from the reservoir (7.2) to the said reactor. Then, the chemically activated polymer particles from the screw mechanical activator (10) are sent to a drum-type dryer (11), from where, after the drying process, the particles are sent by means of a conveyor (2.3) to a device for electrifying the particles in an electrostatic field of a corona discharge. An electrostatic drum activator (12) can be used as such a device.

In the electrostatic drum activator (12), the particles are electrified in the electrostatic field of the corona discharge. The process of charging the particles includes the final separation stage, at which the particles are separated by size by a classifier of separated materials. This classifier belongs to the class of electric separators, which are characterized by low energy consumption, do not use industrial water, do not pollute the air, and are fully automated and controllable. They consist of three main parts: a charger (electrifier); a separator; a high-voltage unit. The basic diagram of the drum activator (12), which contains a classifier, is shown in Fig. 9.

In devices of this type, the method of separation by electrical conductivity is used for mixtures whose particles differ in the value and nature of conductivity. In this process, the contrast of the values of surface charges plays a role. In this case, charging occurs upon contact with the electrode. With a large range of charges formed, qualitative separation into fractions occurs. When separating by electrical conductivity, the process can be influenced to a greater extent by contact electrification and to a lesser extent by ponderomotive forces arising from the difference in the permittivity of the separated particles and the air environment.

The main design elements of the electrostatic drum activator (12) are: feeder (12.1), metal drum (precipitation electrode) (12.2); particle charging area (12.3); corona electrode (12.4); deflecting electrode (12.5); scraper (12.6); sectioned receiving bin with dampers (cut-off valves) (12.7). Position (13) designates the electrets obtained by the claimed method. The main parameter of the electrostatic separator: dU = 20-50 kV.

Further, for a more detailed consideration of the proposed method, examples of the implementation of the method for obtaining dispersed polymeric materials are given.

Example 1

Polypropylene (PP) was chosen as the polymer material. Polypropylene first undergoes a stage of crushing and washing with detergent. Thus, crushing can be carried out on a rotary knife crusher of the RM 120M type. Fig. 1 shows a microphotograph of the surface of crushed polypropylene before activation with a chemical reagent (chemical activator). Microphotographs of the surface of activated polymers before and after treatment were obtained by scanning electron microscopy (SEM) using a Jeol JSM-7100F microscope at a magnification of 10,000 and 30,000 times.

To perform the preliminary stage of washing polymer particles, solutions with a concentration of 1.5 wt.% can be prepared. For this purpose, as an example, the required amount of detergent was dissolved in 1 liter of distilled water while stirring with a magnetic stirrer in a vertical container at a rotation speed of 100 rpm for about 2-3 minutes. The detergent can be a technical detergent of the Neolas A brand with a surfactant content of up to 25 wt.%.

The obtained crushed particles are washed in a detergent solution, after which the pure polymer mixture of particles is placed in a container, for example, with a magnetic stirrer, to which a chemical activator is added - an aqueous solution of H ₂ SO ₄ with a concentration of 1.5 mol / l. The process of mixing the particles lasts 15 minutes at a temperature of T = 60 ° C. The depth of the defects formed after the chemical action of an aqueous solution of H ₂ SO ₄ on the surface of the studied polymers based on polypropylene (PP) is 4 μm.

The state of the surface of polypropylene particles after their treatment with a chemical activator is shown in Fig. 2. The pictures were taken at an angle of 15 degrees. This made it possible to estimate the contribution of chemical activation to the development of surface defects, which subsequently play the role of "traps" for injected charges under the action of the electrostatic field of the corona discharge. Thus, the presented microphotographs of the surface of polymer particles show that the surface relief is non-uniform, rough, there are microcracks caused by mechanical destruction due to particle grinding.

The size of the fractions of activated polymer particles can be regulated using laboratory sieves with cell sizes of, for example: 1, 1.5, 2, 2.5 and 3 mm.

The mixture is then dried at a temperature that can vary within different limits, taking into account the drying time; for example, at a temperature of 75 °C in a drying cabinet, the process lasts for 6 hours.

The dried activated polypropylene particles are fed into an electrostatic drum activator for electrification in an electrostatic corona discharge field.

Example 2

In this example, polypropylene is also used as a polymer. The method for producing dispersed polymer electrets from polypropylene is similar to the method disclosed in example 1, except for the chemical activation stage, namely, an aqueous solution of NaOH with a concentration of 1.5 mol/l is used as a chemical activator, the processing time is 55 minutes, and the processing temperature is 60 °C.

A micrograph of the surface of crushed polypropylene activated by a NaOH solution is shown in Figure 3. The depth of the defects formed after the chemical action of an aqueous NaOH solution on the surface of the studied PP-based polymers is 3 μm.

Example 3

In this example, polypropylene is also used as a polymer. The method for producing dispersed polymer electrets from polypropylene is similar to the method disclosed in example 1, except for the chemical activation stage, namely, an aqueous solution of alkylbenzene sulfonic acid (ABSA) with a concentration of 1.5 mol/l is used as a chemical activator, the processing time is 55 minutes, and the process temperature is 60 °C.

A micrograph of the surface of crushed polypropylene activated by an aqueous solution of ABSK is shown in Figure 4. The average depth of defects formed after chemical action of an aqueous solution of ABSK on the surface of the studied PP-based polymers is 3 μm.

Example 4

In this example, polypropylene is also used as a polymer. The method for producing dispersed polymer electrets from polypropylene is similar to the method disclosed in Example 1, except for the chemical activation stage, namely, an aqueous solution of alkylbenzene sulfonic acid (ABSA) with a concentration of 2 mol/l is used as a chemical activator, the processing time is 55 minutes, and the process temperature is 60 °C.

A micrograph of the surface of crushed polypropylene activated by an aqueous solution of ABSK is shown in Figure 4. The average depth of defects formed after chemical action of an aqueous solution of ABSK on the surface of the studied PP-based polymers is 5 μm.

Example 5

In this example, polyethylene terephthalate (PET) is used as the polymer. Fig. 5 shows a micrograph of the surface of crushed PET before activation with a chemical reagent (chemical activator). The method for producing dispersed polymer electrets from PET is similar to the method disclosed in Example 1, except for the chemical activation stage, namely, an aqueous solution of alkylbenzene sulfonic acid (ABSA) with a concentration of 1.5 mol/l is used as the chemical activator, the processing time is 60 minutes, and the process temperature is 60 °C.

A micrograph of the surface of crushed PET activated by an aqueous solution of ABSK is shown in Figure 6. The average depth of defects formed after chemical action by an aqueous solution of ABSK on the surface of the studied PET-based polymers is 4.2 μm.

Example 6

In this example, polyethylene terephthalate (PET) is used as the polymer. The method for producing dispersed polymer electrets from PET is similar to the method disclosed in Example 1, except for the chemical activation stage, namely, an aqueous solution of NaOH with a concentration of 1.5 mol/l is used as the chemical activator, the processing time is 35 minutes, and the process temperature is 60 °C.

A micrograph of the surface of crushed PET activated by an aqueous solution of NaOH is shown in Figure 7. The average depth of defects formed after chemical action of an aqueous solution of NaOH on the surface of the studied PET-based polymers is 1.7 μm.

As a result of the studies of activated samples of polymers of the following types: PET, PP, HDPE, LDPE, PVC, PS by the SEM (scanning electron microscope) method, the characteristic sizes of the formed surface traps were determined. Table 4 presents the obtained values of the sizes of the formed traps depending on the time and temperature of processing of PET and PP polymers treated with a solution of surfactant - alkylbenzene sulfonic acid (the sizes of traps for polymer particles of 3-5 mm in size, treated with a solution of surfactant - alkylbenzene sulfonic acid (1.5 mol / l) at different processing times.).

Table 4.

No. Processing time, min Trap size, µm PET PP 21 °C 60 °C 21 °C 60 °C 1 5 1.4 2.1 0,1 2.1 2 15 1.9 2.8 0.4 2.5 3 25 2,2 3.3 0.6 3.2 4 35 2.6 3.7 0.9 3.9 5 45 2.9 4 1,1 4.1 6 55 3.1 4.2 1.3 5

As can be seen from Table 4, with increasing time of treatment with the activator solution, the degree of activation of polymer particles increases, and, consequently, the possibility of acquiring the highest charge. It has been established that the activation is affected by the chemical structure of the activator, especially its functional groups. The use of, for example, H ₂ SO ₄ and NaOH solutions has a limitation in the form of developing destruction effects, accompanied by weight loss, especially in the PET polymer, which affects the quality of the raw material.

Polymer discrete electrets obtained by the proposed method, due to their high charge density and charge stability, can be used for various fields of technology as sources of external electric field, in particular, in medicine, for example, for implantation, physiotherapy, endoprosthetics, etc.

Claims (4)

1. A method for producing dispersed polymer electrets, including stages of processing polymeric materials, in which they are ground to obtain polymer particles with surface defects by crushing and sent for washing with an aqueous solution of detergents, after which the washed polymer particles with surface defects are subjected to chemical activation and sent for drying, after which the chemically activated polymer particles are subjected to electrification in an electrostatic field of a corona discharge so that the surface defects of the polymer particles obtained in the process of crushing and processing with a chemical activator acquire the role of cells and accumulate electric charges, wherein the process of chemical activation consists in using a chemical activator, which provides the possibility of increasing the depth of already existing surface defects and the formation of new defects on the surface of a polymer particle without its destruction. 2. The method according to claim 1, characterized in that the chemically activated polymer particles subjected to electrification in the electrostatic field of a corona discharge are heated to 60 °C. 3. The method according to paragraph 1, characterized in that the chemical activator is an aqueous solution of an acid, or an aqueous solution of an alkali, or an aqueous solution of a surfactant, which provide the possibility of increasing the depth of already existing surface defects and the formation of new defects on the surface of a polymer particle without its destruction. 4. The method according to paragraph 3, characterized in that the chemical activator is an aqueous solution of sulfuric acid, or sodium hydroxide, or alkylbenzene sulfonic acid.