A new environmentally friendly metal surface treatment technology - silanization

A new environmentally friendly metal surface treatment technology - silanization

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[Abstract] Silanization treatment is a process of surface treatment of metal materials with aqueous organosilane as the main component. In the coating industry, the surface treatment before coating is based on phosphating. Silanization has the advantages of energy saving, environmental protection and cost reduction compared with traditional phosphating. This paper briefly describes the characteristics, basic principles and construction techniques of silanization treatment.

[Key words] silane; surface treatment; phosphating

Silanization is a process in which a metal or non-metal material is surface-treated with organosilane as a main raw material. Compared with traditional phosphating, silanization has the following advantages: no harmful heavy metal ions, no phosphorus, no need for heating. The silane treatment process does not produce sediment, the treatment time is short, and the control is simple. The processing steps are small, the surface adjustment process can be omitted, and the bath can be reused. Effectively improve the adhesion of the paint to the substrate. A variety of substrates such as iron plates, galvanized sheets, and aluminum sheets can be processed in a collinear manner.

0 Basic principles

Silane contains two different chemical functional groups, one end can react with hydroxyl groups on the surface of inorganic materials (such as glass fiber, silicate, metal and its oxide) to form a covalent bond; the other end can form a covalent bond with the resin, thus making two Materials with widely different properties combine to enhance the performance of the composite. The silanization treatment can be described as a four-step reaction model: (1) hydrolysis of three Si-OR groups connected to silicon to Si-OH; (2) dehydration between Si-OH to synthesize oligosiloxane containing Si-OH (3) Si-OH in the oligomer forms a hydrogen bond with OH on the surface of the substrate; (4) covalent bond formation with the substrate accompanying the dehydration reaction during heat curing, but silane at the interface The silanol groups have only one bond to the surface of the substrate, leaving two Si-OH or condensed with Si-OH in other silanes, or in a free state.

In order to shorten the curing time required for the treatment agent on-site use, the first step of the silane treatment agent before use is to carry out a certain concentration of prehydrolysis.

1 hydrolysis reaction:

During the hydrolysis process, a condensation reaction between the silanes is prevented, and an oligosiloxane is formed. If the oligopolysiloxane is too small, the aging time of the silane treatment agent is prolonged, which affects the production efficiency; if the oligopolysiloxane is too much, the treatment agent is turbid or even precipitated, which reduces the stability of the treatment agent and affects the treatment quality.

2 condensation reaction:

The film formation reaction is a key step affecting the quality of silanization. The progress of the film formation reaction directly affects the corrosion resistance of the coating film and the adhesion to the paint film. Therefore, it is particularly important to control the parameters such as the pH of the treatment agent. Moreover, higher requirements are imposed on the surface state of the workpiece before silanization: 1. The degreasing is complete; 2. The workpiece entering the silane bath cannot be accompanied by metal debris or other impurities; 3. The silanization pretreatment is preferably deionized. water.

3 film formation reaction:

Wherein R is an alkyl substituent and Me is a metal substrate

The film-forming metal silanization film layer is mainly composed of two parts: one is on the metal surface, the silane treatment agent forms a reaction 3 product through a film formation reaction, and the other is a condensation reaction to form a large amount of the reaction 2 product, thereby forming a complete silane film. The microscopic model of the metal film forming state can be described as the structure shown in FIG.

Figure 1 Silanization film formation microscopic state model

1 Comparison of silane treatment and phosphating

As environmental pressures in the coating industry increase, it is especially important to replace the traditional phosphating with environmentally friendly pre-treatment products. As one of the phosphating alternative technologies, silane pretreatment technology has attracted widespread attention in the world coating industry. Compared with traditional phosphating, silane treatment technology is environmentally friendly (no toxic heavy metal ions), low energy consumption (usual temperature use), low cost of use (5-8 times per kilogram of ordinary phosphating), no slag, etc. .

The United States began theoretical research on metal silane pretreatment technology in the 1990s. In the mid-1990s, Europe began to conduct tentative research on silane. At the beginning of this century, China was under great pressure from environmental protection, and major research institutions and production companies also started research on silane.

1.1 Comparison of station procedures

The silanization treatment has improved the operation process of the conventional phosphating treatment, and the existing phosphating treatment line can be put into silanization production without modification in the process. Table 1 compares the conventional phosphating process with the silanization process. Table 1 shows the comparison of traditional phosphating and silanization.

Table 1 Comparison of Phosphating and Silane Chemical Position Arrangement

Note: ★ - need ☆ - no need

It can be seen from Table 1 that the silanization treatment and the phosphating treatment can eliminate the two water washing steps after the surface adjustment and the phosphating. Because the silanization treatment time is short, no equipment modification is needed in the original phosphating production line, and only some of the tank functions can be adjusted to perform silanization treatment: (1) For the transformation of the catenary transportation mode, 1 pre-degreasing and 2 degreasing can be performed. 4, washing, retaining; 3 washing into a degreasing tank; 5 surface adjustment, 6 phosphating into a washing tank; 7 washing to silanization; 8 standby. Increase the chain speed for production while changing the slot function to speed up the pre-production beat and increase productivity. The post setting after the transformation is shown in Figure 2.

1-pre-degreasing; 2-degreased; 3-degreased; 4-water washing; 5-water washing; 6-water washing; 7-silanating;
Figure 2 Station layout after silanization

1.2 Comparison of processing conditions

Traditional phosphating treatments have been a problem for coating companies due to environmental problems such as sediment, phosphorus and phosphating wastewater. As the country pays more attention to environmental protection and energy conservation and emission reduction, in the future, the environmental protection and energy consumption problems of the coating industry will become more and more prominent. The introduction of silane technology revolutionized the pre-treatment environmental protection and energy conservation and consumption reduction of the entire coating industry. Table 2 compares the conditions of use of conventional phosphating with silanization.

Table 2 Comparison of phosphating and silanization conditions

It can be seen from Table 2 that in terms of the use temperature, since the silane film forming process is a normal temperature chemical reaction, the bath liquid can be ideally treated without daily heating in daily use. Compared with the phosphating treatment, this aspect saves a lot of energy and reduces fuel exhaust emissions for the application enterprise. On the other hand, there is no precipitation reaction in the silanization reaction, so no sediment is generated in the daily treatment, and the solid waste in the pretreatment process is eliminated. The problem is solved and the tank back cycle is effectively extended; in addition, the silanization treatment optimizes the pretreatment station setting, eliminating the need for conventional surface conditioning and phosphating water washing. Through this optimization, the pressure on the sewage treatment of the coating company is greatly reduced.

1.3 Comparison of the cost of use

Due to the difference in the film formation principle, the silanization treatment has been greatly reduced in the use temperature compared with the phosphating, and the surface adjustment process is omitted. And in other aspects related to production costs, silanization has obvious advantages over phosphating. Table 3 compares silanization to phosphating in terms of cost of use.

Table 3 Comparison of phosphating and silanization use costs

The silanization process can save equipment such as phosphating heating equipment, slag removal tank, plate and frame filter press and phosphating sewage treatment, saving initial investment in equipment. Silanization is also reduced by 20%-50% in the amount of dispensing. More importantly, the silanization consumption per square unit is 15%-20% of conventional phosphating. While reducing the consumption per unit area, the silanization is also considerably shortened in the treatment time, thereby increasing productivity and reducing the continuous operation cost of the equipment.

1.4 Comparison of microtopography

Because the film formation mechanisms of various phosphating and silanization are quite different, the state and morphology of the metal surface are also different. From the aspect of microscopic morphology, the difference in the film layer formed on the metal surface can be observed by scanning electron microscopy (SEM).

Figure 3 Comparison of silanization and phosphating micromorphology (SEM × 100)

It can be clearly seen from the above electron micrographs that there are large differences in the morphology of the film between various treatments. The main components of the zinc phosphating bath are: Zn2+, H2PO3-, NO3-, H3PO4, accelerators, and the like. The composition of the main body of the formed phosphating film layer (steel piece) is Zn3(PO4)2·4H2O and Zn2Fe(PO4)2·4H2O. The phosphating grains are dendritic, needle-like and have many pores. Compared with zinc phosphating, the main composition of the traditional iron phosphating bath is Fe2+, H2PO4-, H3PO4 and some other additives. 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 membrane pores are more, and the phosphating grains are granular. The silylation treatment is a reaction between an organosilane and a metal to form a covalent bond reaction, and the state of the silane itself does not change. Therefore, after the film formation, no significant film material is formed on the metal surface. By electron microscopic magnification observation, a uniform film layer has been 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 layer is a silane film.

1.5 Comparison of salt water immersion test

Cold-rolled sheet is currently the most widely used metal material, and has a large-scale application in each industry, but the cold-rolled sheet has no galvanized layer such as galvanized sheet, oxide scale of hot-rolled sheet and oxide film protection of aluminum sheet. Therefore, the corrosion resistance of the cold rolled sheet depends on the protection of the coating. The coated cold-rolled plate test piece was subjected to a 500-hour salt water (5% concentration) soaking test to test the salt water resistance of various kinds of electrostatic powder sprayed after different pretreatment processes (the average thickness of the paint film was 50 ± 2 μm). It can be seen from the test results that there was no change in the test pieces of various treatments after 500 hours of salt water immersion. It can be seen that various treatment methods have no significant difference in the salt water corrosion resistance of the workpiece. In order to test the adhesion performance of various treatment processes, the adhesion test was carried out on the test pieces after the 500-hour salt water (5% concentration) immersion test. Specifically, as shown in Fig. 4, the fork portion was peeled off from the edge portion by a dicing knife, and the peelable width was examined.

Figure 4 Comparison of paint adhesion after immersion in 500 hours of iron plate brine

It can be clearly seen from the comparison of the adhesion test results that the iron phosphating peelable width is significantly different from the zinc phosphating and silanization. Iron phosphating is large-area strippable, while zinc-based phosphating and silanized sheets have a strippable width of substantially zero. Therefore, it can be clearly seen that the zinc phosphating and silanization treatments are equivalent to the adhesion of the paint film, and the adhesion between the two is significantly better than the iron phosphating. The silylation treatment effect is equivalent to the zinc phosphating effect in terms of salt water resistance and adhesion.

1.6 Comparison of salt spray test

Galvanized sheets are currently used by a wide range of high quality home appliances and automotive companies due to their high corrosion resistance. In order to test the corrosion resistance and adhesion performance of the silane treatment for the galvanized sheet, the design test uses various pre-treatment processes for the galvanized test piece, and sprays the same thickness of the powder coating through the 500-hour salt spray. The test was used to compare the adhesion.

According to GB/T10125-1997 artificial atmosphere corrosion test--salt spray test, the test galvanized test piece was subjected to a 500-hour neutral salt spray test. The average thickness of the test film was 70 ± 2 μm. The adhesion test of the galvanized sheet was carried out, and the fork portion was also peeled off to the edge portion by a knife to examine the peelable width. Figure 5 shows the results of this test.

Figure 5 Comparison of adhesion after 500-hour galvanized sheet salt spray test

It can be seen from the test results that the ordinary zinc-based phosphating peelable width is the largest, the phosphating peelable width for galvanizing is smaller than that of the ordinary zinc system, the silanization peelable width is almost zero, and the adhesion performance is the best. It can be concluded that after the silanization treatment on the galvanized sheet, the adhesion between the galvanized sheet and the paint film can be significantly improved, and the quality of the galvanized coating product can be improved.

2 treatment

The processing method of the workpiece refers to the way in which the workpiece is in contact with the bath to achieve the purpose of chemical pretreatment, including full immersion, full spray, spray immersion combination, brush coating and the like. It mainly depends on the geometry and shape of the workpiece, the site area, the investment scale, the production volume and other factors. For example, a workpiece with a complicated geometric shape is not suitable for a spray method; a fuel tank and a oil drum type workpiece are not easily sunk in a liquid, and thus are not suitable for a soaking method.

2.1 Full immersion method

The workpiece is completely immersed in the bath, and it is taken out after a certain period of time to complete a common treatment method for degreasing or silanization. The geometry of the workpiece is different, as long as the liquid can reach the place. The treatment target, which is the unique advantage of the immersion method, is unmatched by spraying and brushing. The downside is that there is no auxiliary use for mechanical scouring. Moreover, when the workpiece is continuously suspended, in addition to the running time of the workpiece in the tank, there is also the upslope time of the workpiece, so that the equipment grows, the site area and investment increase, and the residence time between the processes is long, which easily causes the rust between the processes. , affecting the quality of silanization.

2.2 full spray method

The liquid is pressurized by a pump, and the liquid is formed into a mist at a pressure of 0.1 to 0.2 MPa, and is sprayed on the workpiece to achieve a treatment effect. The advantage is that the length of the production line is shortened, and the corresponding site, equipment, and deficiencies are that the geometrically complex workpieces, such as the inner cavity and the corners, are difficult to reach, and the treatment effect is not good, so it is only suitable for processing geometric shapes. The artifact. And can effectively reduce the cost of the first tank.

2.3 spray-soak combination

Spray-soak combination type is generally used in a certain process, the workpiece is sprayed first, then immersed in the tank, sprayed after the tank is discharged, and all sprays and immerses are the same bath. This combination means that the high efficiency of the spray is retained, the processing speed is increased, and the soaking process is performed, so that all parts of the workpiece can be effectively treated. Therefore, the spray-soak combined pre-treatment can complete the treatment process in a short time, and the equipment occupies relatively less space, and at the same time, satisfactory treatment effects can be obtained. In the silylation treatment, it is considered that the degreasing process employs a spray-soak bonding type.

2.4 Brushing method

The treatment liquid is directly applied to the surface of the workpiece by hand to achieve the purpose of chemical treatment. This method is generally not easy to obtain a good treatment effect, and is less applied in the factory. For some large, simpler shapes, consider this approach.

3 Process

According to the use of silanization and the treatment of different types of plates, it is divided into different process flows.

3.1 Iron and galvanized parts

Pre-degreasing - degreasing - water cleaning - water cleaning - silanization - drying or drying - post treatment

3.2 Aluminum parts

Pre-degreasing - degreasing - washing - washing - light - washing - silanization - drying or drying - post-treatment

3.3 passivation after phosphating

Rust workpiece:

Pre-degreasing - water cleaning - degreasing and derusting "two in one" - water cleaning - neutralization - surface conditioning - phosphating - washing - silanization - drying or drying - post-treatment .

Rust-free workpiece:

Pre-degreasing - degreasing - water cleaning - water cleaning - surface conditioning - phosphating - water cleaning - silanization - drying or drying - post-treatment.

3.4 workpiece rust prevention

Pre-degreasing - degreasing - water cleaning - water cleaning - silanization - drying or drying

4 Typical silane treatment methods

The typical process for silane treatment is shown in Table 4.

Table 4 Typical silane treatment process

5 points of attention in process design

Some small places in the process design should be very careful, even if some are related to equipment design, if not considered, it will have many adverse effects on the operation of the production line and the operation of the workers, such as process interval time, overflow washing, workpiece Process holes, tanks and heating tube materials.

5.1 Process interval

If the interval between the various processes is too long, the workpiece will be rusted during the operation. It is better to provide water film protection between the processes to reduce rust. Rusted yellowish and greenish workpieces seriously affect the silanization effect, causing the workpiece to yellow and not form a complete silane film, so the interval between processes should be minimized. If the interval between processes is too short, the water in the water storage part of the workpiece cannot be drained completely and effectively, resulting in a phenomenon of tandem grooves. Especially in the case of spraying, mutual splashing and scattering grooves may occur, so that the bath liquid composition is not easily controlled. Even the bath was destroyed. Therefore, when considering the process interval, an appropriate process interval should be selected according to the geometry and shape of the workpiece.

5.2 Overflow water cleaning

Overflow washing is advocated to ensure that the workpiece is fully cleaned and the stringing phenomenon is reduced. When overflowing, water should be introduced from the bottom, and the overflow hole on the diagonal line overflows.

5.3 Workpiece process hole

For some tubular parts or workpieces that are easy to form dead angles, the process holes must be drilled at appropriate locations to ensure that the water can be fully drained in a short period of time. Otherwise, it will cause a series of grooves or be drained in the air for a long time, causing secondary rust and affecting the silanization effect.

6 Silanization performance test

(1) Paint film corrosion resistance

GB/T14293-1998 General requirements for artificial atmosphere corrosion test

GB/T10125-1997 Artificial atmosphere corrosion test salt spray test

(2) paint film adhesion

GB9286-1998 & ASTM D3359 Cross-cut test of paint and varnish paint film

GB5270-85 Test method for adhesion strength of metal coatings (electrodeposited and chemically deposited) on metal substrates

(3) Water resistance of paint film

GB5209-85 Determination of water resistance of paints and varnishes - immersion

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