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. Polyacrylates (acid s alts), which have improved physical and mechanical properties, have an even wider scope.
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 has no toxicity, good water solubility and high alkali resistance.
The chemical formula of polyacrylic acid is (C2H3COOH) . The structural formula of the compound is shown in the figure below.
Polyacrylic acid is a typical weak polyacid. Its macromolecules have functional groups that have the ability toto electrolytic dissociation. It appears as a clear amber liquid or white granular powder.
Properties
The main physical and chemical properties of polyacrylic acid are:
- The temperature at which this polymer becomes solid, bypassing the crystallization phase (glassy state) - 106 °C.
- When heated, anhydrides are formed, and if the temperature exceeds 250 °C, then the reaction of carbon dioxide elimination from the carboxyl group - COOH begins, as well as cross-linking of macromolecules, which leads to the formation of polymers of a spatial structure and an increase in the degree of polymerization.
- S alts of this polymer have greater thermal stability. This property is used to produce strong polyacrylic acid grafted fibers.
- When interacting with alkalis (C2H3COOH) forms s alts, in the 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.
- When low molecular weight alcohols react 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 whenan increase in the molecular weight of a given polymer also increases the viscosity of the solution, 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, unlike other polyelectrolyte polymers. Fibers of polyacrylic acid, when the acidity of the solution changes, undergo a contraction or elongation as a result of the conversion of chemical energy into mechanical energy.
Solubility
(C2H3COOH) dissolves well in the following substances:
- water;
- diethylene dioxide;
- methyl and ethyl alcohol;
- Formic acid amide;
- dimethylformamide.
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 s alts.
Receive
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 carried out in a paddle reactor and the surface of the equipmentcooled to 70 °C with 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 retain liquid) in baby and adult diaper fillers, sanitary napkins, disposable diapers and other similar products.
Other areas where polyacrylic acid is used are:
- agriculture is a material for soil improvement;
- industry - stabilizers and flocculants of colloidal solutions;
- tanning and textile production - substances to reduce electrification in leather dressing and fiber production;
- electronics - connecting component in lithium-ion batteries;
- industrial - in cooling and air-conditioning systems as a deposit inhibitor and homogeneity component (power plants, steel and oil refineries, fertilizers).
This substance is also used as an additive in the production of films that improve their ability to be colored and adherewith other materials.
Medicine
Acid and its s alts 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;
- binder 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, etc.).
Polyacrylates
S alts of polyacrylic acid are polymers of esters of this compound. In appearance, they resemble paraffins. They are characterized by the following properties:
- resistance to diluted alkalis and acids, light and oxygen;
- decomposition with 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.
S alts of polyacrylic acid are produced by emulsion or suspension polymerization, in small scale production - block polymerization.
Use of polyacrylates
These compounds are used in the production of the following materials:
- organic glass;
- various films;
- synthetic fibers;
- painting materials (enamels, varnishes, resins);
- adhesive and impregnating compositions (emulsions) for fabrics, paper, leather, wood.
Varnishes based on polyacrylates have high performance:
- high adhesion to metal and porous surfaces;
- good decorative qualities;
- water, UV, weather, alkali resistant;
- long-term preservation of decorative properties (shine and elasticity) - up to 10 years.
They are used to dye products such as:
- cars, aircraft and other equipment;
- sorted metal;
- plastics;
- printing products;
- electronic industry products;
- food industry (production of cans).
Sodium polyacrylate
Sodium polyacrylate is very soluble in water and does not change its structure even at 240 °C. This compound is used in the preparation of fresh or s alt solutions to reduce their viscosity. Sodium polyacrylate is able to emulsify microcrystals, microsand from carbonates, sulfates and phosphates.
The substance is used in the following industries:
- oil industry - drilling fluid preparation;
- chemical industry - manufacturingdetergents, artificial snow, as well as a thickener for paints and varnishes;
- agriculture - fertilizer production;
- paper and pulp industry - manufacture of napkins, toilet paper;
- manufacture of sanitary ware.
Drilling fluids formulated with this compound have the following benefits:
- low density;
- fineness;
- good acid solubility required when drilling in;
- high temperature resistant (up to 240°C);
- environmental safety.
Cosmetology
In the cosmetic industry, sodium polyacrylate is used as a thickener in the manufacture of products such as:
- hairspray;
- shower gels;
- creams;
- shampoo;
- face masks;
- bath foam.
The uniqueness of the properties of this supplement lies in the fact that each smallest particle of sodium polyacrylate swells in water and creates a feeling of velvety and smoothness on the skin. Since the substance has a silicone-like elastomeric structure, it is a good texturizing agent. The advantages of cosmetics with its addition is that they do not become sticky, they can give a matte or satin result. Some manufacturers add sodium polyacrylate to color cosmetics.
