Alkyd resins occupy one of the leading positions as raw materials for the production of modern paints and varnishes. Film-forming compositions based on them have a wide scope. The properties of coatings depend on the type of resins and semi-finished varnishes. The most widely used modified compositions, as they have high decorative and protective qualities.
Description
Alkyd resins (or alkyds) are esters resulting from the reaction of polyhydric alcohols with polybasic acids or their anhydrides. In appearance, they are a highly viscous sticky mass, the color of which is from dark yellow to brown.
Alkides are also combined with other resins and cellulose (alkyd carbamide formaldehyde, phenolalkide, alkyd acrylic copolymers and other compounds). To reduce surface tension, butyl alcohol is introduced into their composition, which facilitates the bottling of the finished product.
Chemical composition and classification
Alkyd resins are classified according to several criteria:
By the presence of fatty acids in the composition: unmodified (phthalate) and modified.
According to the oil content: fatty (> 60%), medium fat (40-60%), lean (35-60%), superfine (<34%). Fatty alkyds are prepared mainly on drying oils.
By type of alcohol, on the basis of which the composition is obtained: glyphthalic, xyphthalic, pentaphthalic, etrifthalic.
By ability to dry: drying and non-drying.
By the method of dissolution: diluted with organic solvents or water.
The chemical composition of alkyd resin is a 40-60% solution of non-volatile substances in organic solvents.
Specifications
The properties of alkyds depend on the type of polyhydric alcohol, its concentration and the presence of other components. Aromatic components give coatings based on these compounds hardness and wear resistance, aliphatic polyols - elasticity, cycloaliphatic structural units - balanced characteristics.
Alkyd resin solutions are characterized by the following parameters:
viscosity - 35-100 s (according to the VZ-4 viscometer);
molecular weight - 1500-5000 kDa;
density - 0.9-1.05 g / cm 3 ;
the amount of potassium hydroxide, which is the main indicator of the quality of oils, is not more than 20.
With increasing temperature, the viscosity of alkyds decreases and vice versa. The lower the fat content of a substance, the worse it dissolves in aliphatic and naphthenic hydrocarbons.
Property Improvement
Unmodified alkyds form hard and brittle films, poorly soluble. To improve their qualities, additional components are introduced into the composition of the reaction mixture, which give the coatings gloss, elasticity, resistance to water, solvents, oils and weathering, accelerate drying at room temperature, and increase adhesion with the base material. Modified alkyd resins are obtained using flaxseed, tall oil, tung, sunflower, castor, cottonseed oil, rosin, and synthetic fatty acids.
There are 3 main methods of modification:
the introduction of monomers in the production of resin;
by reacting finished alkyds with modifying substances;
mixing resins with other film formers.
Getting
Alkyd synthesis is carried out in two ways: alcoholysis and the fatty acid method.
In the first case, the process proceeds in 3 stages:
Transesterification of vegetable oil or triglycerides of its fatty acids with polyhydric alcohols. The chemical reaction takes place at a temperature of 240-260 ° C and in the presence of a catalyst. For the production of glyphthalic resins from alcohols, glycerol is used, for etrifthalic ones - ethriol, for pentaphthalic ones - pentaerythritol.
The esterification of the partial esters with phthalic anhydride, resulting in the formation of an acid ester.
Polyesterification with the release of water and the formation of alkyd.
In the second method for producing resins, the esterification and polyesterification of fatty acid esters and polyhydric alcohols is carried out.
Alkide Dilution Substances
The following carbon compounds are solvents for alkyd resins:
aliphatic (hexane, heptane, C6-C12 paraffins);
aromatic (benzene, solvent, toluene, xylene and others);
terpene (turpentine);
halogenated (trichlorethylene, tetrachlorethylene, perchlorethylene);
oil (nefras, gasoline).
Low boiling ethers (ethyl alcohol, ethyl acetate), glycol ethers, ketones (acetone) can also be used as diluents. Butanol is unsuitable for these purposes, since its molecules attract a large amount of the solvent contained in alkyd resins. Skinny alkides dissolve only in aromatic hydrocarbons, and fatty ones in aliphatic.
There are also water-soluble resins, on the basis of which water-borne coatings (enamels and primers for hot drying) are produced.
Application
The main purpose of alkyd resins is to obtain paints and varnishes (LKM). Unmodified compositions are used for the manufacture of electrical insulating varnishes and adhesives.
Of the resin solutions make semi-finished and finished varnishes used in industry and everyday life.
paints and varnishes
Alkyd coatings are most widely used among all materials based on polycondensation resins. Since pentaerythritol alcohol has 4 hydroxyl groups, the alkides with its content cure faster and form more glossy coatings. In this regard, the industry mainly produces paints and varnishes based on alkyd pentaphthalic type resins. The use of a large number of aromatic components provides hardness and resistance to mechanical stress.
Pigmented paints based on alkyds are divided into the following groups:
Drying alkyd resins are used as independent film formers for paint and varnish products, and non-drying ones are used in compositions with urea resins, cellulose nitrates (to improve the elasticity and adhesion of coatings to the base material).
The properties of coatings depend on their chemical composition. So, more fatty alkyd resins in organic solvents are easier to apply with a brush, have increased elasticity and weather resistance, but they dry out more slowly and have less resistance to mineral oils. The maximum drying speed is characteristic for resins modified with tung or linseed oil. The average drying time on the stick for alkyd enamels is 24 hours.
LKM specificity
Alkyd resins, depending on the component composition, are used for the following purposes:
Based on benzoic acid (oil content 38-50% by weight) - enamel for painting agricultural machines, radiators, car repairs. They are highly resistant to fading and atmospheric effects.
Containing 9,12-linoleic and 9,11-linoleic acids - equipment repair.
Skinny - quick-drying single-layer coatings, primers, paint for radiators.
Linseed oil and glycerin contain anti-corrosion coatings and printing inks. They can fade quickly under the influence of sunlight, therefore they are used most often in primers and leveling compounds.
In soybean or tall oil - coating enamels with good weather resistance.
Modified with natural resins (rosin acids) - production of primers and putties. The compositions are prone to fading.
Modified with styrene, acrylate - primers, putties, leveling compounds, coatings for painting ships, equipment, sheet metal, wood subjected to strong mechanical stresses (parquet varnishes, enamels for floors, furniture); packing of enamels in tubes. They have high abrasion resistance and elasticity.
Environmental aspects
Since alkyd resins contain organic solvents, they pose a certain danger to human health and the environment. When carrying out paintwork using paintwork materials on their basis, it is necessary to use personal protective equipment (gas masks, protective clothing). Painting should be carried out in rooms with supply and exhaust ventilation.
Alkides can be divided into 3 groups according to solvent toxicity level:
the least toxic (MPC = 300-1000 mg / m 3 ) - ethyl ether, alcohol, gasoline;
medium toxicity (MPC = 100-200 mg / m 3 ) - acetone, butyl ether;
highly toxic (MPC = 20-50 mg / m 3 ) - ethylene glycol, benzene.
Abroad, high demands are placed on the environmental friendliness of the production of coatings, therefore, work is currently underway to create materials with a high dry matter content and water-borne compositions. However, alkyd resins with organic solvents still surpass water-soluble technological properties.