RU2196784C2 - Method of modifying polydimethylsiloxane rubber - Google Patents

Abstract

FIELD: rubber industry. SUBSTANCE: polydimethylsiloxane rubber is modified by UV irradiation in presence of photoinitiator and reagent forming active groups, said photoinitiator being organic carbonyl-containing compounds selected from aromatic ketone and quinone series, taken in amounts 0.001-0.01 mole/1 kg rubber, at 50-150 C for 30-90 min in atmosphere of air oxygen as above- mentioned reagent. Thus formed active groups are further substituted by reaction with oxygen- or nitrogen-containing substituting reagents such as amines, amino acids, hydrazines, alcohols, and orthoethers. EFFECT: enabled production of rubber with specified structure. 1 tbl, 2 ex

Description

 The invention relates to methods for modifying polydimethylsiloxane rubbers that do not contain active groups with ultraviolet light and can be used to obtain new silicon-containing polymers with a wide range of applications, including biologically active silicon-containing polymers, polymer adsorbents, etc.

 It is known that the modification of polymers, including and siloxane rubbers, by introducing additional groups into the macromolecule, is an effective method for changing their physicochemical, biological properties, and mechanical characteristics.

 However, the absence of active polydimethylsiloxane polymers in the macromolecule makes it difficult to modify them. The methyl groups present in them are chemically inert and well resist the action of high temperatures. The number of terminal hydroxyl groups in high molecular weight polydimethylsiloxane rubbers is very small in order to use them to modify polymers. Therefore, their modification is carried out at the stage of synthesis by copolymerization of dimethylsiloxane monomers with siloxane monomers already having other substituents, which leads to a significant increase in the cost of production.

 There are very few direct methods for modifying finished pure polydimethylsiloxane polymers. These include a method for the direct modification of polydimethylsiloxane polymers under the influence of γ-radiation (A.S. USSR 176069, Bull. Inventory, 1965, 21), which allows grafting of low-molecular weight polydimethylsiloxane rubbers to unsaturated compounds, for example, maleic acid imides.

 Its disadvantages include the possibility of modifying only low molecular weight polydimethylsiloxane polymers with only some unsaturated compounds, the use of dangerous and requiring serious protection of γ-radiation.

 The photochemical oxidation of polydimethylsiloxane polymers is known. However, in practice, the oxidation of siloxane rubbers has always been regarded as a harmful phenomenon, leading to a deterioration in the performance of polymers (Israeli, Y., etc. Photooxidation of polydimethylsiloxane oils. 3. Effect of dimethylene groups. Polym. Degrad. And Stab., 1993, V 42, 3, R. 267-279; Kuzminsky A.S., Sedov V.V. Chemical transformations of elastomers. M: Chemistry, 1984, p.160-177).

 The closest technical solution is a method for the modification of polydimethylsiloxane rubber (patent US 2522053, publ. 1950), which consists in the fact that the modification is carried out by irradiation with ultraviolet light by introducing a labile group, which is used as chlorine. In this case, part of the hydrogen atoms of the methyl groups is replaced by chlorine.

The disadvantages of this modification method are the difficulties in controlling the chlorination process, because during the modification process, both CH ₂ Cl and CHCl ₂ and CCl ₃ groups can form, which due to side reactions (cleavage of polychlorinated substituents, their hydrolysis, etc.) cannot be purposefully replaced by oxygen and nitrogen-containing substituents, as a result of this with their further modification, it is impossible to obtain polymers with predetermined structure and properties; partial destruction of the polymeric siloxane bond due to the release of hydrogen chloride during the reaction, which complicates the further processing of rubber due to the need for additional curing; the possibility of applying the method only for low molecular weight polydimethylsiloxanes, because the process is carried out by passing gaseous chlorine through a layer of liquid polydimethylsiloxane and for its successful passage requires good mixing of the mixture.

 The objective of the proposed invention is to obtain polydimethylsiloxane rubber with a predetermined structure and properties by introducing oxygen and nitrogen-containing groups from a wide range of substitute compounds into the molecule of the polydimethylsiloxane polymer.

 The objectives of the invention are also the expansion of the class of modified rubbers and the simplification of the technology for their processing.

The tasks are solved in that when carrying out the method of modifying polydimethylsiloxane rubber by irradiation with ultraviolet light with the introduction of a reagent that forms active groups, it is proposed to irradiate with ultraviolet light in the presence of a photoinitiator, which is used as an organic carbonyl-containing compound of a number of aromatic ketones and quinones, in an amount of 0.001-0 , 01 mol per 1 kg of rubber, at a temperature of 50-150 ^o C for 30-90 min in an atmosphere of oxygen, as a reagent, forming active groups, it is proposed to use atmospheric oxygen and additionally carry out the replacement of the formed active groups by interaction with an oxygen- or nitrogen-containing replacement reagent, which are used as compounds from the classes of amines, amino acids, hydrazines, alcohols and orthoesters.

 There is no evidence of the practical use of oxidation to modify the structure and properties of polydimethylsiloxane rubbers.

The proposed mechanism for the modification of polydimethylsiloxane rubber includes the following stages:
- absorption by the carbonyl group of the photoinitiator of a quantum of light with its transition to the triplet π-excited state (see Scheme 1 at the end of the description);
- detachment of the hydrogen atom photoinitiator by the molecule from the methyl group of the macromolecule of polydimethylsiloxane rubber with the formation of a macroradical and initiator radical (seven-quinone type for quinones and ketyl type for aromatic ketones) and subsequent recombination of macroradicals with the formation of crosslinking (see Scheme 2 at the end of the description).

Initiator photolysis products subsequently participate in secondary photochemical reactions:
- photochemical oxidation of bonds —CH ₂ —CH ₂ - to —CH ₂ —CO—. Scheme 3 (at the end of the description) shows that oxygen is a reagent for the formation of C = O active carbonyl groups in the polydimethylsiloxane rubber macromolecule. By mechanism (a), oxidation occurs along with crosslinking. According to mechanism (b), the oxidizing agent is singlet oxygen, which is formed as a result of energy transfer from the excited state of the In initiator.

 Steps 1-3 occur simultaneously when the polydimethylsiloxane rubber is irradiated with ultraviolet light in the presence of a photoinitiator and atmospheric oxygen.

 Subsequent chemical treatment of the oxidized polydimethylsiloxane rubber to replace the active carbonyl groups with various oxygen and nitrogen substituents consists in reacting it with a replacement reagent (see Schemes 4 and 5 at the end of the description).

 The degree of oxidation, and hence the content of substituent groups, increases with increasing concentration, temperature and concentration of the initiator, and also depends on the type of initiator used.

 Substitution of carbonyl groups in oxidized polydimethylsiloxane polymers proceeds with a yield close to quantitative.

 The lower limit of the concentration of the photoinitiator is due to the minimum necessary amount to achieve the required concentration of carbonyl groups. With an increase in its concentration above the upper limit, part of the initiator remains unspent.

A temperature in the range from 50 to 150 ^o With selected to achieve compatibility of the used photoinitiators with a polymer matrix. At a lower temperature, the initiator drops out of the polymer matrix in the form of crystals characteristic of each compound, accompanied by a decrease in the oxidation rate. Higher temperatures do not increase the rate of oxidation, resulting in unproductive energy consumption.

 The chosen irradiation time is due to the need to accumulate the required amount of carbonyl groups. At short irradiation times, their number is small, at high - their photochemical decay begins.

 Siloxane heat-resistant rubbers SKT are used as polydimethylsiloxane polymers: liquid SKT-A (average molecular weight 28 thousand cu), SKT-G (average molecular weight 43 thousand cu) and viscous flowing SKT (average molecular weight 560 thousand cu) without preliminary cleaning.

 The method is as follows.

Example 1. To 20 g of a 15% solution of SKT siloxane rubber in toluene, a solution of 7.0 mg of 2-ethyl-anthraquinone in 1 ml of toluene is added. The mixture is thoroughly mixed. The concentration of 2-ethyl-anthraquinone in this case is 0.001 mol per 1 kg of rubber. The mixture is applied to metal-glass cells equipped with borders in one layer with drying for 3 hours at room temperature. The amount of applied mixture is 330 mg / cm ² . After drying, the thickness of the dry polymer layer is 0.5 mm (with an accuracy of ± 5%). The dried samples are irradiated with a full light of a DRT-1000 medium-pressure lamp in thermostatic cells placed at a distance of 25 cm from the lamp in the presence of atmospheric oxygen at a temperature of 100 ^o C for 90 minutes The films of the oxidized polymer together with the substrate are placed in a 5% solution of 2,4-dinitrophenylhydrazine in 1,2-dimethoxyethane and heated at 100 ^° C for 3 hours. The solutions are drained, the films separated from the substrate are washed 4 times with a hot appropriate solvent and dried during the day at room temperature. The content of carbonyl and their substituent groups is determined using infrared, ultraviolet spectroscopy and chemical analysis methods.

Example 2. To 20 g of a 15% solution of SKT-A siloxane rubber in toluene, a solution of 6.7 mg of 2-methyl-anthraquinone in 1 ml of toluene is added. The mixture is thoroughly mixed. The concentration of 2-methyl-anthraquinone in this case is 0.01 mol per 1 kg of rubber. The mixture is applied to metal-glass cells equipped with borders in one layer with drying for 3 hours at room temperature. The amount of applied mixture is 330 mg / cm ² . After drying, the thickness of the dry polymer layer is 0.5 mm (with an accuracy of ± 5%). The dried samples are irradiated with a full light of a DRT-1000 medium-pressure lamp in thermostatically controlled cells placed at a distance of 25 cm from the lamp in the presence of atmospheric oxygen at a temperature of 50 ^{° C.} for 60 minutes. Films of the oxidized polymer together with the substrate are placed in a 2% solution of paraphenylenediamine in toluene and heated at 100 ^° C for 3 hours. The toluene solution is drained and rubber is precipitated from it by the addition of acetone. The released modified rubber is separated from the solution and purified twice by reprecipitation with acetone from solutions in pure toluene. The purified rubber is separated from the solutions, washed with acetone and dried at room temperature for one day. The content of carbonyl and substituent groups is determined using infrared, ultraviolet spectroscopy and chemical analysis methods.

 The table summarizes examples of the modification of polydimethylsiloxane rubbers with various photoinitiators, their quantitative content, temperature and time oxidation conditions, reagents and solvents used to replace carbonyl groups with substituent groups and the content of substituent groups.

The advantages of this method of modifying polydimethylsiloxane rubbers are:
- the possibility of introducing into the macromolecule polydimethylsiloxane rubber of a large range of oxygen and nitrogen-containing groups, which allow to obtain modified rubbers with a wide range of desired properties. By introducing amino acid groups into polymers, it is possible to obtain biologically active polymers, amino groups — ion-exchange polymer polymers, nitrophenyl groups — redox polymers, etc .;
- the absence of polymer degradation, facilitating the isolation and purification of the obtained polymers, as well as their further processing;
- the ability to control the modification process, because Only the —CH ₂ —CH ₂ - crosslinking groups undergo oxidation to form carbonyl groups. The content of —CH ₂ —CO— groups is easily determined by electron and infrared spectroscopy. Substitution of carbonyl groups with other substituents proceeds with a yield close to quantitative;
- applicability to both high and low molecular weight polymers, as the very high gas permeability of polydimethylsiloxane rubbers allows oxidation in a heterogeneous medium without the use of good mixing, and partial replacement of the rubbers in the solvents used is sufficient to replace the carbonyl groups with other groups. Thus, expand the range of modified rubbers.

Claims (1)

A method for the photochemical modification of polydimethylsiloxane rubber by irradiation with ultraviolet light with the introduction of a reagent that forms active groups, characterized in that the irradiation with ultraviolet light is carried out in the presence of a photoinitiator, which is used organic carbonyl compounds of a number of aromatic ketones and quinones, in an amount of 0.001-0.01 mol per 1 kg of rubber, at a temperature of 50-150 ^o C for 30-90 min in an atmosphere of atmospheric oxygen, use kis as a reagent that forms active groups air and additionally carry out the replacement of the formed active groups by reacting with an oxygen or nitrogen-containing replacement reagent, which are used as compounds from the classes of amines, amino acids, hydrazines, alcohols and orthoesters.

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