polyacrylic acid. Polymethacrylic acid
The invention relates to a chemical technology for the production of synthetic macromolecular compounds. The technical task is to develop a method for producing high-purity, finely dispersed, powdery, easily dosed anhydrous polyacrylic acid. SUBSTANCE: proposed is a method for obtaining polyacrylic acid by radical polymerization of a monomer dissolved in a solvent in the presence of an initiator, characterized in that the isolation of polyacrylic acid is carried out by four-stage distillation of the solvent under vacuum, and at the first stage, distillation is carried out at a residual pressure of 550-110 mm Hg, intensive stirring and a temperature of 104-85°C, in the second stage, the distillation is carried out at a residual pressure of 100-20 mm Hg. and a temperature of 85-60°C, at the third stage - at a residual pressure of 20-10 mm Hg, slow stirring and a temperature of 60-55°C, at the fourth stage - at a residual pressure of 10-3 mm Hg, at the lowest speed of the stirrer and a temperature of 55-50°C, and the polymerization of acrylic acid is carried out at a temperature of 102-104°C for 60-70 minutes from the end of the induction period after adding to the solvent heated to 94-95°C a pre-prepared polymerization a mixture containing acrylic acid dissolved in a solvent and an initiator. The proposed method is simple and economical, does not require complex hardware design. The obtained powder of anhydrous polyacrylic acid finds application in medicine, in particular for the creation of cement filling compositions used in therapeutic dentistry. 2 w.p. f-ly.
The invention relates to a chemical technology for the production of synthetic macromolecular compounds, namely the production of polyacrylic acid in the form of a powder, for use, for example, in medicine, in particular for creating cement filling compositions used in therapeutic dentistry.
Recently, polycarboxylate cements have been considered the most promising materials for filling materials, as a rule, they represent separately stored powder (zinc oxide and modifying components) and liquid (30-50% aqueous solution of dioxypolycarboxylic acid with a molecular weight of 80,000-180,000), which, when mixed due to cross-links of linear macromolecules with polyvalent cations form a cement molding mass that hardens at room temperature for 4-9 minutes, has sufficient strength and adhesion to tooth tissues with minimal irritating effect. Polyacrylic acid has good adhesion to the enamel and dentin of the tooth, is highly soluble in water and is capable of forming chelate compounds, therefore it deserves attention for the production of polycarboxylate cements.
A known method for producing polyacrylic acid in the form of a gel by radical polymerization of acrylic monomer in an aqueous medium in the presence of a cross-linking agent in a twin-screw reactor with several paddle mixers, and part of the surface or the entire surface of the reactor is constantly cooled to a temperature of not more than 70°C with an external liquid cooling agent. In this case, the surface has a roughness of not more than 3 μm, and the stirrer blades are provided with channels for a liquid cooling agent. The result is a hydrophilic polymer used as an absorbent, for example, as disposable wipes, hygiene products, in soil conditioning for agricultural and horticultural purposes, and as dehydrators (1).
The resulting material is unsuitable for creating compositions used in dentistry, precisely because of its hydrophilic properties.
A known method of obtaining aqueous solutions of polyacrylic acid to create compositions used in dentistry, by radical polymerization of 36.5-37.5% aqueous solution of acrylic acid under the action of hydrogen peroxide at an initial temperature of 40-70°C in the presence of 0.005-0.035 wt . hydroquinone and 0.36-0.72 wt. on the amount of acrylic acid sodium salt of thioglycolic acid of the formula NAOOC-CH 2 -SH as a molecular weight regulator. Moreover, the addition of reagents is carried out in three equal portions, because with the addition of each portion of acrylic acid containing hydroquinone and sodium salt of thioglycolic acid, and the parallel addition of a 40% hydrogen peroxide solution, the temperature of the reaction mass spontaneously rises to 98-100°C. Before adding the next portion of the reagents, it is reduced to 40±2°C. 15 min after adding the third portion of the reagents, hydrogen peroxide is added to the reaction mass and heated for 1 h at 90°C. The duration of the process is 2 hours. Get a solution of polyacrylic acid with a concentration of 36.3 wt. with a viscosity of 14.0 Pa·s (2).
This method makes it possible to obtain more stable than usual aqueous solutions of polyacrylic acid, however, an increase in viscosity during long-term storage of the solution still occurs, and this reduces the consumer properties of the product.
The objective of the invention is to develop a method for obtaining high-purity, free of impurities, easily dosed, powdery, finely dispersed, anhydrous polyacrylic acid, because it is in this form that this preparation is most convenient for long-term storage and quick preparation by the consumer of a solution of any concentration to obtain a cement molding mass.
The problem is solved in the proposed method for obtaining polyacrylic acid powder using a single-machine technology in a polymerizer-evaporator with an enameled inner surface. Moreover, unlike the known methods, the polymerization of acrylic acid is carried out not in an aqueous solution, but in toluene at a volume ratio of acrylic acid: toluene equal to 1: (7-10) in the presence of an initiator - a, a "-azobisisobutyronitrile in an amount of 1.2-1 ,3 wt.% relative to the monomer.Toluene heated to 94-95°C, add the polymerization mixture: acrylic acid previously dissolved in toluene with an initiator.The polymerization process is carried out at a temperature of 102-104°C for 60-70 minutes from the moment the end of the induction period.Then, polyacrylic acid is isolated by four-stage distillation of toluene under vacuum:
Stage 1 - at a temperature of 104-85°C and a residual pressure of 550-110 mm Hg. for one and a half hours, with intensive stirring (300 rpm) at the beginning and stopping stirring at the end,
Stage 2 - at a temperature of 85-60°C and a residual pressure of 100-20 mm Hg. within an hour (by the end of this stage, the polymerization mass acquires the properties of friability),
3rd stage - with slow stirring (60 rpm), temperature 60-55°C and residual pressure 20-10 mm Hg. within 1 hour 15 minutes,
4th stage - at the lowest stirrer speed (15 rpm), temperature 55-50°C and residual pressure 10-3 mm Hg. within 60-70 minutes.
The total duration of the distillation is about 5 hours. The processes of polymerization and distillation are carried out in a polymerizer-evaporator equipped with an anchor-frame stirrer with additional asymmetric horizontal blades and a gap between the outer edge of the stirrer and the inner surface of the apparatus of not more than 2-6 mm. The temperature in the polymerizer-evaporator is maintained by means of a heating system, such as electric heating, and a cooling system, such as a cooling coil. The result is a polyacrylic acid with the following characteristics:
1) Appearance - white free flowing powder;
3) bulk density - 0.21 g / cm 3.
Distinctive features of the proposed method are:
Carrying out polymerization in a polymerization evaporator with an enameled inner surface at a temperature of 102-104°C for 60-70 minutes from the end of the induction period after adding the polymerization mixture to a solvent heated to 94-95°C: acrylic acid previously dissolved in a solvent with an initiator . Moreover, the total ratio of monomer to solvent is 1:(7-10), and the initiator is taken in the amount of 1.2-1.3 wt.% relative to the monomer.
Among organic solvents, it is toluene that is preferred as a solvent, because. unlike organohalides, benzene, etc., it is a low-toxic substance and less flammable. As an initiator, a, a "-azobisisobutyronitrile has an advantage, because it does not contain benzene rings, the presence of which is categorically unacceptable in the finished polymer;
Making the inner surface of the polymerizer-evaporator from enamel, which minimizes the presence of contaminants and impurities in the finished product;
Stirring during polymerization and during distillation of toluene is carried out at different speeds using an anchor-frame stirrer equipped with additional asymmetric horizontal blades, which contributes to better mixing of the polymerization mixture, and the gap between the outer edge of the stirrer and the inner surface of the apparatus is no more than 2-6 mm, which ensures effective renewal of the surface of the stirred mixture in places of the most probable burning, reduces the magnitude of the temperature gradient of heat transfer, i.e. leads to the prevention of glass transition processes;
The isolation of polyacrylic acid is carried out by four-stage distillation of toluene under vacuum:
Stage 1 - at a temperature of 104-85°C and a residual pressure of 550-110 mm Hg. for one and a half hours, intensive stirring at the beginning and stopping stirring at the end,
Stage 2 - at a temperature of 85-60°C and a residual pressure of 100-20 mm Hg. within an hour (by the end of this stage, the polymerization mass acquires the properties of friability),
3rd stage - with slow stirring, temperature 60-55°C and residual pressure 20-10 mm Hg. within 1 hour 15 minutes,
4th stage - at the lowest stirrer speed, temperature 55-50°C and residual pressure 10-3 mm Hg. within 60-70 minutes.
The total duration of the distillation is about 5 hours and allows you to achieve complete removal of toluene.
An example of obtaining a powder of polyacrylic acid.
70 liters of toluene are loaded into the polymerizer-evaporator with a capacity of 160 liters, the temperature is raised to 94-95 ° C with the help of electric heating, the anchor-frame mixer (300 rpm) is turned on, the electric heating is reduced and the polymerization mixture is uniformly supplied, which is prepared by dissolving in 24 liters of toluene 16 liters of acrylic acid and 0.16 kg of initiator (a, a "-azobisisobutyronitrile). Before the end of the induction period, which is determined by the beginning of the temperature increase of the reaction mixture (since the polymerization process begins), turn off the electric heating, and then turn on the supply cooling water into the coil to ensure that the polymerization temperature is maintained within 102-104 ° C, because lower temperatures do not allow the initiator to decompose into radicals and initiate the polymerization process, and closer to the boiling point of toluene (110 ° C), the polymerization process becomes difficult to control, and ejection of the reaction mass may occur.Toluene vapor condenses under reflux and returned to the polymerizer-evaporator. Under such conditions, after 60-70 minutes, the polymerization process ends, a refrigerator-condenser, a toluene collector and a vacuum system are connected to the polymerizer-evaporator for carrying out the toluene distillation process. At the first stage of distillation of toluene with intensive stirring (300 rpm) with an anchor-frame mixer, a residual pressure of 550-100 mm Hg. and a temperature of 104-85°C distilled 61-68 l of toluene for 1 hour 30 minutes. Due to the solidification of the polymerization mass by the end of this stage, mixing becomes impossible, and the stirrer is turned off. At the second stage of distillation of toluene at a residual pressure of 100-20 mm Hg. and a temperature of 85-60°C for an hour distilled 15-19 l of toluene. At the end of this stage, the polymerization mass acquires the property of friability and it becomes possible to turn on the stirrer. In the third stage, with stirring at a speed of 60 rpm, a residual pressure of 20-10 mm Hg. and a temperature of 60-55°C for 1 hour 15 minutes distilled 12-14 l of toluene. At the fourth stage, with stirring at a speed of 15 rpm, a residual pressure of 10-3 mm Hg. and a temperature of 55-50°C, the desorbed toluene is removed within 60-70 minutes. The obtained polyacrylic acid is unloaded immediately after the end of the toluene distillation process, without waiting for its cooling.
The method is simple, economical, does not require complex hardware design.
Polyacrylic acid obtained by the proposed method, has the form of a white free-flowing anhydrous powder with a bulk density of 0.21 g/cm 3 , without impurities (including toluene). Checking for residual toluene was carried out by extraction with pentane and showed that there was no toluene in the final product.
A 20% solution of polyacrylic acid prepared on the basis of the obtained powder is a transparent liquid without mechanical impurities and insoluble parts with a viscosity of 35.1 centistokes.
Polycarboxylate cements prepared using polyacrylic acid powder obtained by the proposed method were approved by clinicians when applied in practice and showed that they are not inferior to foreign analogues, tk. their characteristics fully meet the requirements of the international standard ISO No. 4104: 1) compressive strength - 60-65 MN/m 2 ; 2) tensile strength - 59-63 MN/m 2 ; 3) adhesive strength - 7-9 MN/m 2 ;
4) hardening time - 7-9 minutes; 5) water absorption - 0.1%.
Literature
1. RF patent No. 2031097, 6 C 08 F 120/56, 1995.03.20.
2. AS USSR No. 1557982, 6 C 08 F 120/06,1995.07.25.
1. A method for producing polyacrylic acid by radical polymerization of a monomer dissolved in a solvent in the presence of an initiator, characterized in that the isolation of polyacrylic acid is carried out by four-stage distillation of the solvent under vacuum, and at the first stage, distillation is carried out at a residual pressure of 550-110 mm Hg, intensive stirring and a temperature of 104-85°C, in the second stage, the distillation is carried out at a residual pressure of 100-20 mm Hg. and a temperature of 85-60°C, at the third stage - at a residual pressure of 20-10 mm Hg, slow stirring and a temperature of 60-55°C, at the fourth stage - at a residual pressure of 10-3 mm Hg, at the lowest speed of the stirrer and a temperature of 55-50°C, and the polymerization of acrylic acid is carried out at a temperature of 102-104°C for 60-70 minutes from the end of the induction period after adding to the solvent heated to 94-95°C a pre-prepared polymerization a mixture containing acrylic acid dissolved in a solvent and an initiator.
2. The method according to claim 1, characterized in that toluene is used as a solvent in the ratio of monomer/solvent = 1/7-10, and as an initiator - a, a "-azobisisobutyronitrile in an amount of 1.2-1.3 wt .% relative to the monomer.
3. The method according to claim 1, characterized in that the mixing of the polymerization mixture at different stages of distillation of the solvent is carried out using an anchor-frame mixer equipped with additional asymmetric horizontal blades, with a gap between the outer edge of the mixer and the inner enameled surface of the polymerizer-evaporator of not more than 2 -6 mm.
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Polyacrylic acid modified with cross-linked polyacrylamide
Polyacrylate-polyacrylamide copolymers were originally designed to provide long-term stability over extended wet/dry cycles in the presence of high electrolyte and inorganic concentrations. The absorption of deionized water is lower here (200 g/g), but the increased strength with acrylamide allows the material to be used in the following areas:
- Market of products for agriculture and horticulture;
- Medical spill control;
- Water blocking for wires and cables, etc.
In addition to the direct use of dry and clean superabsorbent resins in applications, several other forms are also on the market to facilitate their use or improve performance:
- Composites and laminates.
- Aqueous solutions.
- Styrofoam.
- Fibers...
Composites and laminates based on superabsorbent polymers
Some manufacturers, such as Eti, market laminates and composites in rolls or sheets with very high water absorption, as well as other functionalities: increased mechanical strength due to bonding with fibers or polyester nonwovens, antimicrobial effects, the addition of corrosion inhibitors, operating parameters of the barrier film, in a word, any properties that are necessary to facilitate processing.
For example, Composites Airlaid structures from ETi with a basis weight of 100 - 600 g/sq. m., depending on the requirements, can be filled with 5 - 60% superabsorbent substances. Finished product markets include: wire and cable water blocking, filtration, medical products, specialty packaging, industrial wipes, and spill control.
Solutions of superabsorbent polymers
This easy-to-use form of superabsorbent materials behaves like a true solution, which can be diluted with water if desired and then sprayed or applied overhead, or spotted onto a substrate to form a coating or effect saturation. After drying and cross-linking at a specific temperature (or at room temperature with special cross-linking) for a specific period of time, a coated substrate is obtained which has superabsorbent functionality. Applications include, for example, water blocking for wires and cables... Table 2 shows an example of mortar properties (LiquiBlock™ CSP from ETi).
| Mortar properties | |
| Appearance | Transparent |
| Content of active substance, % | 30 |
| Density | 1.06 |
| Viscosity at 20°C, in centipoise | 1500 |
| Hydrogen indicator | 5.5 |
| Curing temperature, °C | >= 120 |
| Content of volatile organic compounds | Low |
| Coating properties | |
| Absorption of deionized water, g/g | 50-100 |
| Appearance | Transparent film coating |
| Limit content of volatile organic compounds | Extremely low |
Super absorbent foams
The idea is to create a network of interconnected pores in the hydrogel to accelerate and homogenize swelling. This can be achieved by simultaneously polymerizing, foaming and crosslinking the superabsorbent material. If any part of the hydrogel comes into contact with water, it is locally absorbed and passed through the capillaries to any place through open channels to fill the entire space, and very quickly, for example, in less than 30 or 60 seconds.
Superabsorbent fibers as sweat absorbing layers in protective clothing
The effectiveness of a cross-linked acrylate copolymer, partially neutralized with a sodium salt, as a sweat absorbent for cotton, polyester, and polypropylene nonwovens has been studied. The "Sweat Absorption vs. Speed" graph below shows the results with the best data for superabsorbent fibers added to cotton and poor data for polypropylene.
Sweat absorption depending on speed.
Superabsorbent polymers for packaging without direct food contact
Due to their high absorbency, SAPs can be used in the production of leak absorbing packaging. For the production of food packaging, it is necessary to have special superabsorbent polymers. For example, the Food and Drug Administration (FDA) has approved the use of BASF's Luquasorb® FP 800 for non-food contact packaging. This authorization applies to packaging for poultry, meat, fish, fruits and vegetables. SAP absorbs leaking liquids such as traces of blood or liquid juices, etc. This keeps food fresh and attractive for longer. Superabsorbent granules can be incorporated in small amounts as filler in the production of absorbent pads, making packaging more efficient and cost effective. Used pads can be disposed of with household waste.
Superabsorbent polymers are based on acrylic acid and its salts and derivatives polymerized using solution or suspension polymerization technologies. Water absorption, absorption kinetics, hydrogel parameters and, accordingly, acceptable pressure before leakage, depend on the nature of the cation used, often sodium or potassium, the degree of acrylic acid neutralization, solution crosslinking, possible subsequent surface crosslinking of superabsorbent particles with the creation of a core-shell structure , a physical form that promotes the absorption and diffusion of liquids due to capillarity.
Like other plastic materials, superabsorbent polymers can be processed with other materials to create composites, hybrids, multilayer structures and nonwovens... After a boom over the past twenty years, consumption growth now exceeds that of the entire plastics market as a whole. . Worldwide SAP production is estimated to be in the range of 1 to 1.5 million tonnes, making SAP consumption in the same weight category as phenolic resins or polyamides.
Versatile hydrogels, which provide excellent absorption rates and performance through to moderate absorption due to their wide range of chemical structures, are used primarily in superabsorbent materials for the manufacture of disposable consumer products such as diapers, adult incontinence products, and products for personal care products for women, which account for 94% of the total consumption of superabsorbent polymers. The remaining 6% is used for technical and specialty applications in a wide variety of areas: industrial and civil engineering, the agricultural and horticultural market, packaging, wire and cable, fire fighting, medical and personal care products, surface water management ... The property that unites all these applications is high absorbency.
Polyacrylic acid is a unique polymer with high water absorption capacity. This compound is biologically inert, so it is widely used in the manufacture of hygiene and cosmetic products, as well as an auxiliary material in medicine. An even wider scope for polyacrylates (acid salts), which have improved physical and mechanical properties.
Description
Polyacrylic acid is a macromolecular substance, the monomeric unit of which is the compound CH2=CH−COOH (acrylic or propenoic, ethenecarboxylic acid). This polymer is characterized by the absence of toxicity, good solubility in water and resistance to highly alkaline media.
The chemical formula of polyacrylic acid is (C 2 H 3 COOH) n. The structural formula of the compound is shown in the figure below.
Polyacrylic acid is a typical representative of weak polyacids. Its macromolecules have functional groups that are capable of electrolytic dissociation. In appearance, it is a clear amber liquid or white granular powder.
Properties
The main physicochemical properties of polyacrylic acid are:
- The temperature at which this polymer becomes solid, bypassing the crystallization phase (glassy state) is 106 °C.
- When heated, the formation of anhydrides occurs, and if the temperature exceeds 250 ° C, then the reaction of the elimination of carbon dioxide from the carboxyl group - COOH begins, as well as the crosslinking of macromolecules, which leads to the formation of polymers of a spatial structure and an increase in the degree of polymerization.
- Salts of this polymer have greater thermal stability. This property is used to produce strong polyacrylic acid grafted fibers.
- When interacting with alkalis (C 2 H 3 COOH), n forms salts, in reaction with alcohols - esters.
- After polymerization in solvents, the polymer becomes hard and brittle and retains these qualities even at a temperature of 240 °C.
- In the reaction of low molecular weight alcohols with this acid, esters of different spatial structures are obtained.
- A sharp change in the properties of the polymer occurs at a very low degree of conversion of functional groups (only 0.1% ethylene glycol is required to cross-link molecules with a mass of 50 kDa).
One of the properties of an aqueous solution of polyacrylic acid is that with an increase in the molecular weight of this polymer, the viscosity of the solution also increases, which is associated with the growth of macromolecules and their effect on water. At the same time, the viscosity of the solution does not depend on the applied shear stress and is a constant value over a wide measurement range, in contrast to other polyelectrolyte polymers. When the acidity of the solution changes, polyacrylic acid fibers undergo contraction or elongation as a result of the conversion of chemical energy into mechanical energy.
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Solubility
(C 2 H 3 COOH) n dissolves well in the following substances:
- water;
- diethylene dioxide;
- methyl and ethyl alcohol;
- formic acid amide;
- dimethylformamide.
An aqueous solution of polyacrylic acid has a polyelectrolyte effect (capable of electrolytic dissociation), which increases linearly with an increase in the degree of neutralization.
The substance is insoluble in compounds such as:
- acrylic acid monomer;
- acetone;
- ethoxyethane;
- hydrocarbons.
With cationic solutions and surfactants, the substance can form insoluble salts.
Receipt
The synthesis of polyacrylic acid is carried out by polymerization of the monomer. The reaction takes place in an aqueous medium, where a cross-linking agent is added, or in organic solvents. Mixing is usually done in a paddle reactor and the surface of the equipment is cooled to 70 °C with a liquid refrigerant. The final product is a gel - a hydrophilic polymer that actively absorbs moisture.
A more stable aqueous acid solution can be obtained by the action of hydrogen peroxide and the addition of a small amount of para-dihydroxybenzene with sodium thioglycolate, used to control the molecular weight. The end product of the reaction is used in dentistry.
Application of polyacrylic acid
This polymer is most widely used as a superabsorbent (to capture and hold liquid) in fillers for baby and adult diapers, sanitary napkins, disposable diapers and other similar products.
Other areas where polyacrylic acid is used are:
- agriculture - a material for improving soil properties;
- industry - stabilizers and flocculants of colloidal solutions;
- leather and textile production - substances to reduce electrification in the dressing of leather and obtaining fibers;
- electronics - a connecting component in lithium-ion batteries;
- industrial production - in water cooling and air conditioning systems as an inhibitor of deposits and a component that maintains the uniformity of mixtures (power plants, steel and oil refineries, fertilizer production).
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Also, this substance is used as an additive in the production of films that improve their ability to be painted and adhere to other materials.
The medicine
Acid and its salts are used in medicine for the following purposes:
- carrier of active substances;
- a component of hemostatic ointments, woven and non-woven materials used for burns and inflammation to accelerate wound healing;
- binding additive in filling materials in dentistry.
The advantage of this material is that it is biologically inert and can be used together with bioactive compounds (enzymes, antibiotics, growth factors, and others).
Polyacrylates
Salts of polyacrylic acid are polymers of esters of this compound. In appearance, they resemble paraffins. They are characterized by the following properties:
- resistance to dilute alkalis and acids, light and oxygen;
- decomposition by alkali solutions is observed at a temperature of 80–100 °C, with the formation of polyacrylic acid;
- when heated above 150 °C, they undergo thermal destruction, polyacrylate molecules crosslink, monomer (about 1%) and volatile products are released;
- polyacrylates are highly soluble in monomers, ethers, hydrocarbons and acetone.
Salts of polyacrylic acid are obtained by emulsion or suspension polymerization, in the manufacture on a small scale - block polymerization.
Use of polyacrylates
These compounds are used in the production of the following materials:
- organic glass;
- various films;
- synthetic fibers;
- paints and varnishes (enamels, varnishes, resins);
- adhesive and impregnating compositions (emulsions) for fabrics, paper, leather, wood.
Varnishes based on polyacrylates have high performance characteristics:
- high adhesion to metal and porous surfaces;
- good decorative qualities;
- resistance to water, ultraviolet radiation, weathering, alkalis;
- long-term preservation of decorative properties (shine and elasticity) - up to 10 years.
They are used to color products such as:
- cars, planes and other equipment;
- high-quality metal;
- plastics;
- printing products;
- products of the electrical industry;
- food industry (production of cans).
Sodium polyacrylate
The sodium salt of polyacrylic acid (Sodium polyacrylate) is very soluble in water and does not change its structure even at a temperature of 240 ° C. This compound is used in the preparation of fresh or salt solutions to reduce their viscosity. Sodium polyacrylate is able to emulsify microcrystals, microsand from carbonates, sulfates and phosphates.
| No. p / p | Moisture content in acrylic acid, wt.% | Properties of polyacrylic acid | |||
| Bulk weight, g/cm3 | Angle of repose, ° | Dispersity: average particle size, microns | Note | ||
| 1 | 0.01 | 0.28 | 45 | 18 | Silky, free-flowing powder* |
| 2 | 0.05 | 0.33 | 47 | - | - |
| 3 | 0.075 | 0.38 | 47 | - | - |
| 4 | 0.100 | 0.42 | 50 | - | |
| 5 | 0.125 | 0.46 | 52 | 25 | Coarse particles, reduced silkiness and flow* |
| 6 | 0.150 | 0.48 | 54 | - | - |
| 7 | 0.175 | 0.51 | 54 | - | - |
| 8 | 0.200 | 0.54 | 55 | - | - |
| 9 | 0.225 | 0.56 | 57 | - | - |
| 10 | 0.250 | 0.58 | 58 | - | - |
| 11 | 0.600 | 0.73 | 61 | 79 | Significant crusting |
*Cm. micrographs of PAK.
So, when using the Akvion composite, the working time should be 2.0-2.5 minutes, i.e. during this time the PAA should dissolve and then the composite should harden within 4.5-5.0 minutes. Therefore, acrylic acid entering the production of PAA should not contain more than 0.075 wt.% moisture. Otherwise, it must go through a preliminary stage of dehydration. Also, in the process of obtaining PAA, it is necessary to observe measures that prevent the ingress of moisture, namely: the equipment - polymerizer, gaskets, seals must be absolutely dry; (50-60 ° C), i.e. at a temperature above the dew point.
The influence of the degree of humidity of the initial monomer of acrylic acid on the flowability, bulk mass and dispersion of a biocompatible dental material - polyacrylic acid was studied.
It has been shown that in order to obtain high-quality PAA with storage stability (not subject to caking), high solubility, convenience and ease of dosing, it is necessary to use acrylic acid with a moisture content not exceeding 0.075 wt.% as the initial monomer.
In the production of PAK, it is necessary to provide for measures that exclude the ingress of moisture (dry equipment, tightness, conditioned PAK during unloading and packing should have temperatures above the dew point).
It is necessary to tighten the requirements for the moisture content in the initial acrylic acid monomer, or to develop a method and technology for its dehydration.
Bibliography
Kuryakina N.V. Therapeutic dentistry of children's age. - M.: Medical book: Iz-vo NGMA, 2004. - 744 p.
Vyazmitina A.V., Usevich T.L. Materials science in dentistry. - Rostov-on-Don: Phoenix, 2002. - 352 p.
Polymers and copolymers of esters, amides and nitriles of acrylic and methacrylic acids are combined under the general name -- acrylates.
Polyacrylic acid
obtained by free radical polymerization. The initiators are peroxides, persulfates, azo and diazo compounds. Polymerization proceeds at high speed even at low (20--25 °C) temperatures. It is most convenient to carry out the polymerization in solution. The solvent can be water, xylene, benzene. Polyacrylic acid is a solid, dull white, brittle substance resembling porcelain, it is soluble in water, formamide, hardly in alcohol, insoluble in monomer. At 230--240 0 C, it begins to decompose. Polyacrylic acid, obtained at low temperatures, has a high molecular weight, does not dissolve in water, but only swells.
Polymer macromolecules have a predominantly linear structure. Some units of polyacrylic acid macromolecules are connected in a head-to-head pattern, but the vast majority are head-to-tail:
Polymethacrylic acid
obtained by free radical polymerization in the presence of initiators that dramatically increase the rate of polymerization. The introduction of a methyl group into the α-position into the acrylic acid molecule somewhat slows down the polymerization process and facilitates its regulation. The appearance of polymethacrylic acid does not differ from polyacrylic acid. It also has a matte white color and has almost the same hardness.
Polymethacrylic acid is soluble in water and insoluble in non-polar solvents. As the molecular weight of polymethacrylate increases, its solubility in water decreases. The chemical properties of polyacrylic and polymethacrylic acids are similar to those of polybasic saturated organic acids.
They are widely used to obtain leather and shoe finishes, as well as emulsifiers. Salts of polyacrylic and polymethacrylic acids are used as thickeners because their solutions have a very high viscosity.
Of great importance are copolymers of acrylic and methacrylic acids with other vinyl and divinyl monomers. When copolymerized with dienes, acrylic acid forms rubbers.
Such rubbers can be vulcanized with polyvalent metals:
These rubbers are very heat resistant. Some methacrylic acid copolymers are used as ion exchange resins.
Polymethyl methacrylate is obtained by free radical polymerization of methyl methacrylate. Polymerization is most often carried out by the block method, since this produces organic glass with the best optical properties. The presence of initiators, ultraviolet irradiation accelerate the polymerization process. As the temperature rises, the reaction rate increases, but the molecular weight decreases. The molecular weight of the polymer ranges from 50,000 to 200,000, the density is 1.18 g/cm 3 , and the glass transition temperature is about 98 °C. At 260--270 °C, the polymer is destroyed. Polymethyl methacrylate is highly soluble in acetone, dichloroethane and some esters. It is mainly used to obtain organic glass.
Other esters of methacrylic acid are used for the production of varnishes, films, flexible hoses, etc.
In addition to the esters of acrylic and methacrylic acids, acrylic acid nitrile is of great practical importance.
Polyacrylonitrile is obtained by radical emulsion polymerization of acrylonitrile. The initiators of the process are most often hydrogen peroxide, persulfates, or perborates; the dispersion medium is usually water. During polymerization, the polymer precipitates in the form of small, easily filtered particles.
Polyacrylonitrile does not dissolve in solvents suitable for dissolving other acrylic resins. Groups --CN contained in macromolecules, cause a strong intermolecular interaction.
Polyacrylonitrile dissolves only in highly polar solvents: dimethylformamide, dimethylcyanamide, in concentrated aqueous solutions of some salts (KCNS, ZnCl 2 , ZnBr 2). Its solubility decreases after treatment with an aqueous formaldehyde solution.
Depending on the polymerization conditions, the molecular weight of polyacrylonitrile ranges from 20,000 to 350,000, the density is about 1.17 g/cm 3 ; glass transition temperature 80°C, it decomposes at 220°C. Polyacrylonitrile changes color when heated, and the heating process is always accompanied by a loss of solubility.
Polyacrylonitrile has sufficiently high physical and mechanical properties. In terms of light resistance, it surpasses almost all known polymers.
A large amount of polyacrylonitrile is used to obtain synthetic fibers and plastics. According to its properties, polyacrylonitrile fiber resembles wool and is well dyed.
Copolymers of acrylonitrile with vinyl chloride, vinyl acetate, styrene, esters of acrylic and methacrylic acids, isobutylene, butadiene, etc. are of great industrial importance. Copolymers of butadiene with acrylonitrile are used to produce oil-resistant rubbers. Compared to polystyrene, copolymers of styrene and acrylonitrile have increased heat resistance.
