Silanization Treatment: An Eco-Friendly Alternative to Phosphating
In the coating industry, phosphating is the main surface treatment before painting. Compared with traditional phosphating, silanization has the advantages of energy saving, environmental protection and cost reduction. This paper briefly describes the characteristics and basic principles of silanization treatment.
1. The characteristics and basic principles of silane
Silanization treatment is the process of surface treatment of metal or non-metal materials with organosilane as the main raw material.
Compared with traditional phosphating, silanization treatment has the following advantages:
(1) No harmful heavy metal ions, no phosphorus, no need to heat.
(2) The silane treatment process does not produce sediment, the treatment time is short, and the control is simple.
(3) Less processing steps can save the surface adjustment process, and the bath solution can be reused.
(4) Effectively improve the adhesion of paint powder to substrates, and can treat various substrates such as iron sheets, galvanized sheets, and aluminum sheets collinearly.
The basic principle of silane:
Silane contains two different chemical functional groups, one end can react with hydroxyl groups on the surface of inorganic materials (such as glass fibers, silicates, metals and their oxides) to form covalent bonds; the other end can form covalent bonds with resins, so that two materials with very different properties can be combined to improve the performance of composite materials.
The silanization process can be described as a four-step reaction model:
(1) The three Si-OR groups connected to silicon are hydrolyzed into Si-OH;
(2) Dehydration condensation between Si-OH to form Si-OH-containing oligosiloxane;
(3) Si-OH in the oligomer forms hydrogen bonds with OH on the surface of the substrate;
(4) During the heating and curing process, the dehydration reaction forms a covalent bond with the substrate, but on the interface, the silane hydroxyl group of the silane is only bonded to the surface of the substrate, and the remaining two Si-OH are either condensed with Si-OH in other silanes, or in a free state.
2. Comparison of silane treatment and phosphating:
1. Comparison of treatment conditions
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It can be seen from the table that in terms of operating temperature, since the silane film-forming process is a chemical reaction at room temperature, the ideal treatment effect can be achieved without heating the bath solution in daily use.
In this aspect, compared with phosphating treatment, it saves a lot of energy and reduces fuel exhaust emissions for application enterprises;
On the other hand, there is no precipitation reaction in the silanization reaction, so no sediment is produced in the daily treatment, which eliminates the problem of solid waste treatment in the pre-treatment process and effectively prolongs the tank pouring cycle;
In addition, the silanization treatment optimizes the setting of the pre-treatment station, eliminating the need for traditional surface adjustment and water washing after phosphating.
Through this optimization, the pressure on the sewage treatment of coating enterprises has been greatly reduced.
2. Microscopic comparison
The main composition of the phosphating film (steel workpiece): Fe5H2(PO4)4•4H2O, the thickness of the phosphating film is large, the phosphating temperature is high, the treatment time is long, the film has many pores, and the phosphating grains are granular.
Silanization treatment is based on the principle of covalent bond reaction between organosilane and metal, and the state of silane itself does not change. Therefore, after film formation, no obvious film material is formed on the metal surface.
Through the magnified observation of the electron microscope, a uniform film has formed on the metal surface, which is thinner than the zinc-based phosphating film, and the uniformity of the iron-based phosphating film is greatly improved. This film is the silane film.
3. Comparison of salt spray experiments
At present, galvanized sheet has been adopted by a large number of high-quality home appliances and automobile companies because of its high corrosion resistance.
In order to test the corrosion resistance and adhesion performance of silanization treatment on galvanized sheets, various pretreatment processes were used for galvanized test pieces, and powder coatings of the same thickness were sprayed on them, and the adhesion was compared through 500-hour salt spray test.
According to GB/T10125-1997 Artificial Atmosphere Corrosion Test–Salt Spray Test, conduct 500 hours neutral salt spray test on the galvanized test piece. The average thickness of the paint film on the test piece is 70±2μm.
The adhesion comparison test is carried out on the galvanized sheet, and the fork is also used to extend the fork to the edge to examine the peelable width.
It can be seen from the test results that the peelable width of ordinary zinc-based phosphating is the largest, the peelable width of galvanized phosphating is smaller than that of ordinary zinc-based phosphating, the peelable width of silanized phosphating is almost zero, and the adhesion performance is the best.
It can be concluded from this that the adhesion between the galvanized sheet and the paint film can be significantly improved after the silanization treatment process is applied on the galvanized sheet, and the quality of the galvanized coated product can be improved.
