Polyacrylic acid is a unique polymer with a high ability to absorb water. This compound is biologically inert, therefore, it is widely used in the manufacture of hygiene and cosmetics, as well as as an auxiliary material in medicine. An even wider scope of application for polyacrylates (acid salts), which have improved physical and mechanical properties.
Description
Polyacrylic acid is a high molecular weight substance whose monomer unit is the compound CH2 = CH − COOH (acrylic or propene, ethenocarboxylic acid). This polymer is characterized by the absence of toxicity, good solubility in water and resistance to highly alkaline environments.
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-colored liquid or white granular powder.
The 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, anhydrides are formed, and if the temperature exceeds 250 ° C, the reaction begins of the removal of carbon dioxide from the carboxyl group - COOH, as well as the crosslinking of macromolecules, which leads to the formation of polymers with a spatial structure and an increase in the degree of polymerization.
- Salts of this polymer have greater thermal stability. This property is used to obtain strong fibers with a grafted layer of polyacrylic acid.
- 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 with a very small degree of conversion of functional groups (only 0.1% ethylene glycol is required for crosslinking of 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 constant in a wide range of measurements, in contrast to other polyelectrolyte polymers. When changing the acidity of a solution, polyacrylic acid fibers undergo contraction or elongation as a result of the conversion of chemical energy into mechanical energy.
Solubility
(C 2 H 3 COOH) n is well soluble 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 increasing 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.
Getting
The synthesis of polyacrylic acid is carried out using the polymerization of a monomer. The reaction takes place in an aqueous medium to which a crosslinking agent is added, or in organic solvents. Mixing is usually carried out in a reactor with paddle mixers, and the surface of the equipment is cooled 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 exposure to hydrogen peroxide and the addition of a small amount of para-dihydroxybenzene with sodium thioglycolate, which are used to control molecular weight. The final reaction product is used in dentistry.
The use of polyacrylic acid
This polymer is most widely used as a superabsorbent (for trapping and holding 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 - material for improving soil properties;
- industry - stabilizers and flocculants of colloidal solutions;
- leather and textile production - substances to reduce electrification in the manufacture of leather and fiber;
- electronics - a binding component in lithium-ion batteries;
- industrial production - in water cooling and conditioning systems as an inhibitor of deposits and a component that maintains the homogeneity of mixtures (power plants, steel and oil refineries, fertilizer production).
Also, this substance is used as an additive in the production of films that improve their ability to stain 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 inflammations to accelerate wound healing;
- binder additive in dental filling materials.
The advantage of this material is that it is biologically inert, and it 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 diluted alkalis and acids, light and oxygen;
- decomposition with alkali solutions is observed at a temperature of 80–100 ° C, and polyacrylic acid is formed;
- when heated above 150 ° C, their thermal degradation occurs, polyacrylate molecules crosslink, monomer (about 1%) and volatile products are released;
- polyacrylates are readily soluble in monomers, esters, hydrocarbons and acetone.
Salts of polyacrylic acid are obtained by emulsion or suspension polymerization; in the manufacture on a small scale, by block polymerization.
The use of polyacrylates
These compounds are used in the manufacture 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.
Polyacrylate-based varnishes have high performance characteristics:
- high adhesion to metal and porous surfaces;
- good decorative qualities;
- resistance to water, ultraviolet radiation, atmospheric agents, alkalis;
- long-term preservation of decorative properties (gloss and elasticity) - up to 10 years.
They are used for coloring such products 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 saline solutions to reduce their viscosity. Sodium polyacrylate is able to emulsify microcrystals, microsands from carbonates, sulfates and phosphates.
The substance is used in the following industries:
- oil industry - preparation of drilling fluids;
- chemical industry - the manufacture of detergents, artificial snow, and also as a thickener for paints and varnishes;
- agriculture - fertilizer production;
- paper and pulp industry - manufacture of napkins, toilet paper;
- production of sanitary ceramics.
Drilling fluids prepared with the addition of this compound have the following advantages:
- low density;
- fine dispersion;
- good acid solubility required when opening the reservoir;
- resistance to high temperatures (up to 240 ° C);
- environmental Safety.
Cosmetology
In the cosmetic industry, sodium polyacrylate is used as a thickener in the manufacture of such products as:
- hair sprays;
- shower gels;
- creams;
- shampoos;
- face masks;
- bath foam.
The uniqueness of the properties of this additive is that every tiny 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 texturing agent. The advantages of cosmetics with its addition is that they do not become sticky, can give a matte or satin finish. Some manufacturers add sodium polyacrylate to makeup products.