Epoxy resin coatings (epoxy coatings) are well known for their excellent performance and wide range of applications. Epoxy resins and coatings play an important role in the fields of modification of synthetic resins, innovation of coating varieties and development of special coatings. At present, epoxy coatings (varnish and magnetic paint) have become one of the pillar products in the coating industry.
- Classification of epoxy coatings
Epoxy coatings are one of the main products of synthetic resin coatings, and there are five classification methods.
(1)Classification by construction method
There are epoxy coating for spraying, epoxy coating for rolling, epoxy coating for flow coating, epoxy coating for dipping, epoxy coating for electrostatic construction, epoxy coating for electrophoresis construction, epoxy coating for powder flow construction and epoxy coating for brushing, etc.
(2)Classification by application
There are architectural coatings, automotive coatings, naval coatings, wood coatings, machine coatings, sign coatings, electrical insulation coatings, conductive coatings, chemical resistant coatings, anti-corrosion coatings, heat resistant coatings, fire resistant coatings, temperature indication coatings, lubricating coatings, ablative heat insulation coatings, food can coatings and flame retardant coatings, etc. In short, when classified by usage, it can be summarized as three major types of epoxy coatings for construction, industry and special use.
(3)Classification by curing mode
Self-drying epoxy coatings: one-component, two-component and multi-component liquid coatings;
Drying epoxy coatings: one-component (or two-component) solid and liquid coatings;
Radiation curing type coating: epoxy resin modified liquid coating.
(4)Classification by name of curing agent
There are amine curing epoxy coating, acid anhydride (or polyacid) curing epoxy coating, ion-initiated curing epoxy coating and synthetic resin curing epoxy coating, etc.
(5)Classification by coating state
There are solvent epoxy coatings, high solids (less solvent) epoxy coatings, liquid solvent-free epoxy coatings, powder epoxy coatings and water-based (water-emulsified and water-soluble) epoxy or epoxy-modified coatings.
In the coating industry, epoxy coatings are mostly classified by their usage and status.
- Composition of epoxy coating
Epoxy coating is composed of epoxy resin, epoxy ester, modified and curing synthetic resin (the above components are collectively called base material), curing agent, coloring pigment and body pigment (referred to as color filler), solvent (including water), various functional additives and other materials. The main materials have the following effects on the usability of epoxy coatings.
(1)Base material
The base material in epoxy coating is an important film-forming material. In addition to epoxy resin, there are also phenolic resin, amino resin, silicone resin, acrylic resin, polyurethane resin, polysulfide rubber, liquid polybutadiene, liquid nitrile rubber, furfuryl alcohol resin, coal asphalt and other auxiliary base materials. Epoxy resins and auxiliary base materials can be scientifically and reasonably matched to prepare epoxy coatings with various properties.
Specially, when the epoxy resin molecule contains electron-absorbing group, it will increase the reaction speed with Lewis base (nucleophilic reagent) curing agent and reduce the reaction speed with Lewis acid (electrophilic reagent) curing agent. When the epoxy resin molecule contains electron-giving group, it will increase the reaction speed with Lewis acid curing agent and decrease the reaction speed with Lewis base curing agent. To master and control the reaction speed of epoxy resin and curing agent is one of the technical keys to decide the paintability and usability of epoxy coating. Different structure of epoxy resin should be determined according to different application technology requirements.
(2)Curing agent
The curing agent structure and reaction activity play a decisive role in the curing reaction speed of epoxy coatings. For example, in the addition curing reaction, arylamine adducts, phenol/formaldehyde and amine condensate can make the epoxy resin fast curing at low temperature; 300 polyamide curing reaction with epoxy resin at low temperature is slow; acid anhydride and its derivatives can be fully cured with epoxy resin at higher temperature.
With E-44 epoxy resin as the base material, take the amine equivalent: epoxy equivalent = 0.6:1, the curing reactivity of several amine curing agents at 10 ~ 12 °C.
(3) Color filler
In the epoxy magnetic paint, color filler is an indispensable component. It is worth suggesting that the effect of the variety of frontal filler on the internal stress of the coating film. Increase the amount of chin white and zinc oxide in the paint will improve the elastic modulus of the coating film, increase the internal stress and reduce the bond strength. In addition, increasing the amount of aluminum powder and talc will inhibit the volume shrinkage of the coating film, increase the elastic modulus, have no effect on the internal stress, and improve the bond strength of the coating film. Several commonly used fillers make the coating film internal stress size order is: titanium white = zinc oxide > barium sulfate > calcium carbonate > talc = aluminum powder.
The internal stress of the epoxy coating film increases with the amount of zinc oxide, and the bond strength of the coating film decreases with the amount of zinc oxide. The test proves that when the internal stress of the coating film exceeds the bonding strength, the coating film will peel off. However, when the internal stress exceeds the cohesive force, the coating film cracks.
(4) Additives
The auxiliaries in epoxy coatings are mainly curing accelerator, thixotropic agent, leveling agent, defoamer, flame retardant, plasticizer and coupling agent, etc. Among them, the curing accelerator has the greatest influence on the curing reaction speed.
When the epoxy resin in epoxy coating reacts with curing agent, the structure and dosage of curing accelerator have great influence on the curing reaction speed. When the spatial resistance of the curing accelerator increases, it is not conducive to the curing reaction. But when the dosage of curing accelerator increases, it will accelerate the curing reaction speed.
With liquid bisphenol A epoxy resin: polyfunctional epoxide and hydrogen sulfide adduct = 100:75 (mass ratio), tertiary amine was selected as the curing accelerator, and the curing reaction was carried out at 25°C.
(5) Solid solvent
Most of the solvents used in epoxy coatings are made from a mixture of two or more organic solvents, also known as inert thinner (thinner for short). It is worth noting that solvents of different structures will play different roles in the curing reaction.
1) solvent inhibition in the epoxy group and the curing reaction containing active hydrogen compounds, hydrogen bonding acceptor for the solvent, the curing reaction to play an inhibitory role. Because the hydrogen bond acceptor will combine with the hydrogen bond giving body in the curing reaction system, consuming the concentration of hydrogen bond giving body and slowing down the curing reaction. Let the hydrogen bond acceptor be Y, which combines with the hydrogen bond donor (HX) to form a hydrogen bond: Y+HY→Y……HX
As a result of the formation of hydrogen bonding, resulting in the original curing system within the LIx and epoxy group reaction efficiency is weakened, so that the epoxy resin molecules within (or curing reaction generated) by the group of autocatalytic effect is weakened.
- The reactivity of the solvent in the curing reaction system, there are often Lewis acid type curing accelerator. If the choice of cyclic ketone as a solvent, the ketonization reaction will occur. If the choice of esters as solvents, will occur between the ester and hydroxyl ester exchange reaction, this reaction will affect the performance of the cured coating film. Especially in baking conditions, the use of ketone and ester solvents should be very careful.
- Application of epoxy coatings
At present, the varieties and application fields of coatings made by epoxy resin and its modified resin are largely the same at home and abroad. Its main application areas such as petrochemical, food processing, iron and steel, machinery, transportation, electric power electronics, marine engineering, underground facilities and ship industry all use epoxy coatings in large quantities, and the specific applications are as follows.
(1)Application of anti-corrosion coatings
People prepare various anti-corrosion coatings based on the specific requirements of anti-corrosion coatings. Excellent results are obtained in the field of corrosion prevention on steel surface, drinking water system, motor equipment, oil tanker, ballast tank, aluminum and aluminum alloy surface and special media.
(2)Ship coating
The harsh environment of sea such as humidity, salt spray, strong ultraviolet rays and slightly alkaline seawater attack is a severe test for the coating film. Epoxy coatings play an important role in ship protection because of their strong adhesion, excellent rust resistance and water resistance, mechanical strength and chemical resistance. Epoxy coatings are used on the hull, waterline and deck, etc. They are wear-resistant, water-resistant, oil-resistant and have strong “wet state” bonding.
(3) Electrical insulation coating
Epoxy coating film has the advantages of high resistance coefficient, high dielectric strength, small dielectric loss and good “three prevention”. It is widely used for impregnating the coils of electric motors and electrical equipment, winding resistance and various insulating fiber materials, the surface of various combination fittings, bonding insulating materials, casting materials, electronic components insulation protection and other fields.
(4) Food cans inner wall coating
The use of epoxy coating corrosion resistance and bonding, made of acid-resistant, sulfur and other media of food cans interior coating. Epoxy resin – methyl methacrylate – acrylic acid to make water-soluble beverage can wall coating, used for beer and beverage bottle wall protection, epoxy – phenolic coating for food can wall protection, with good acid resistance (sulfur) effect. China is a big country in food industry, and the prospect of coatings for food and beverage packaging is promising.
(5) Application of water-based coatings
Water-based electrophoretic coatings formulated with epoxy esters have unique order performance. The coating film has excellent corrosion resistance, color retention and certain decorative properties. Electrophoretic coatings are used in automotive industry, medical equipment, electrical appliances and light industrial products.
Two-component water-based epoxy coating is used for bonding between new primed concrete, which can effectively prevent mechanical damage and chemical hazards. Two-component water-based epoxy coating is used to protect nuclear reactor equipment and facilities, which can easily remove radioactive contamination. Two-component water-based epoxy floor coatings are non-polluting varieties of coatings.
(6)Application of special coatings
Epoxy special coatings are mainly used for underground storage tank anti-corrosion, waterproof, leak-proof, high temperature environment and cosmic vehicle ablation heat insulation. It is also used for heat insulation, flame retardant and fireproof, high temperature anti-corrosion, railroad track lubrication and magnetic material protection of indicating equipment or instruments.
(7)Application of powder coating
Epoxy powder coatings are used in household appliance industry, instrument industry, motor industry, light industry, petrochemical anti-corrosion, building hardware, electrical insulation, ship industry and auto parts, etc.
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