Anti-crusting agent (methyl ethyl ketone oxime) in paint and coating application of general knowledge
I. Crusting of paints and coatings
Paint in the storage and use process, due to solvent evaporation and the surface layer and air contact, will occur in the surface of the oxidation polymerization of gel, the early generation of soft gel through stirring can make the paint back to the original fluid state, with the extension of time, the gel will gradually harden, and crystallize into a layer of film, covering the surface of the paint, this is the paint crust phenomenon. This gel is insoluble in solvent and must be discarded before use; in serious cases, the stored paint may all condense into a solid state and cannot be dispersed again, and cannot be used.
Second, the type of anti-crusting agent
There are three major types of anti-crusting agents on the market: phenolic compounds, mercaptans, and oxime compounds, of which oxime compounds have the least side effects. Among the oxime compounds, the comprehensive performance of the methanone oxime produced by Hubei Xian Crystalline Chemical is good. Other auxiliary anti-crust materials such as turpentine, dipentene, etc., although there is a certain anti-crust efficacy, but easy to make the paint film yellowing. If we can control the free radical polymerization of the paint film and the active effect of the catalyst, we can achieve the effect of anti-crusting, but this control must be based on the need, not to affect the dry rate of the paint film, and not to affect the quality and performance of the paint.
Oxime-type anti-crusting agent
At present, domestic and foreign widely used is oxime anti-crusting agent, mainly methyl ethyl ketone oxime, butyraldehyde oxime, cyclohexanone oxime, etc., and the use of methyl ethyl ketone oxime is the most common, in the application of paint anti-crusting has achieved very good results.
C. Mechanism of oxime anti-crusting agent
(1) Anti-oxidation effect: oxime compounds are easily oxidized by themselves, which can capture the free radicals generated in the process of coating film formation and interrupt the oxidation polymerization reaction of paint film. As oxime compounds are very volatile, in the process of film formation after construction, oxime compounds evaporate very quickly and lose the role of capturing free radicals, and the free radicals generated by oxidation can continue to polymerize into the film.
(2) Solubility: The liquid oxime compounds are strong solvents and have good solubility with various paints and coatings, which can play a role in delaying the formation of gel; moreover, the oxime compounds are volatile and have high vapor pressure, so the vapor generated is added to the gas phase space of the paint barrel, which can play a role in isolating the paint from the air and delaying the oxidation polymerization reaction of the paint. This is equivalent to covering the paint surface with a layer of solvent and delaying the crusting effect.
(3) Complexation: oxime compounds can form complexes with cobalt, lead, manganese and other metal parts in the catalyst, so that it temporarily loses its catalytic properties and delays the crusting of the paint surface, and in the process of film formation after construction, due to the volatilization of the oxime compounds and the decomposition of the complexes, the catalysts resume their catalytic properties, so as to effectively prevent the crusting of the paint film and not affect the drying properties of the paint film.
(4) Oxygen barrier: 90% of the crusting we encountered is the packaging space is too large - that is, the storage process of high oxygen content without adjusting the amount of anti-crusting agent added, because the anti-crusting agent has a high vapor pressure is very easy to volatilize, the resulting vapor fill the space in the tank, so that the surface of the paint film and air isolation, play a role in oxygen barrier.
Fourth, the characteristics of oxime anti-crusting agent
(1) The oxime chemicals have the performance of being applicable to many kinds of paints and coatings, and the effect of anti-crusting can be achieved by adding a small amount.
(2) The oxime chemicals are neutral, especially methyl ethyl ketone oxime is a colorless transparent liquid which is easy to volatilize.
It has no adverse effect on the paint, and no residue or odor will be left in the paint film.
(3) The oxime chemicals do not affect the rheological performance, drying performance, color mixing performance, paint film appearance, fineness, viscosity, hardness, adhesion, impact strength, flexibility, water resistance, salt water resistance, gasoline resistance, nitro resistance, etc.
(4) Oxime chemicals essentially solve the anti-crust requirements of coatings and paints, it is highly efficient, as the amount added is very small, the use of anti-crust agents can almost ignore its cost, and the anti-crust efficacy of the solution can improve the quality of coatings and paints, reduce losses, and achieve good economic and social benefits.
V. Usage of oxime anti-crusting agent and precautions
(1) Adding amount: The adding amount of oxime anti-crusting agent is generally 0.1~0.3% of the total paint and coating. It is related to the volatility of the solvent, the oil of the modified resin, the pigment used, the amount of drying agent, the temperature of the paint storage, etc. The actual amount should be determined according to the type of formula and the test effect. For example, the pigment with stronger resistance to dryness, carbon black, should be added at a reduced amount; the pigment with stronger dryness, red tannin, should be added at an increased amount. The type of resin also has a great influence on the added amount, and the unsaturated resin should increase the added amount. The amount of drying agent and storage temperature is also very critical, when the amount of drying agent is large, the amount of anti-crusting agent added should also be increased accordingly, otherwise it will reduce the effect of anti-crusting agent. Low temperature, slow crusting, winter can add less anti-crusting agent; high temperature, fast crusting, summer to add more anti-crusting agent. If the storage time is long, add more anti-crusting agent; if the storage time is short, add less.
(2) The order of adding anti-crusting agent: anti-crusting agent should be added after the drying agent is added, otherwise it may deepen the color of the paint.
(3) The oxime anti-crusting agent should be selected from products with high purity. The anti-crusting agent with low purity and many impurities has poor chromaticity and is easy to be yellowed, which will affect the color of the paint film seriously.
(4) Methyl ethyl ketone oxime and other oxime anti-crusting agent is easy to volatile, oxidation, moisture absorption, should be closed, avoid light, dry storage.
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