JPS6036229B2 - Method for removing resin coatings from metal surfaces - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing resin coatings from metal surfaces, the main purpose of which is to efficiently remove resin coatings from resin-coated metals at low cost. % caustic soda aqueous solution, caustic potash aqueous solution, or a mixed aqueous solution of caustic soda and caustic potash as an electrolyte, and a resin-coated metal is used as a cathode to electrolyze, thereby removing the resin coating of the metal, and it is well suited for the intended purpose. can be reached.

Conventionally, alkali-based stripping agents have been commercially available for the purpose of stripping resin coatings from metal surfaces such as hangers and defective coatings, but due to their high cost, they are not suitable for the treated articles due to profitability. types are limited, and other items are often scrapped.

It is also a place where tin is recovered from tin plates using electrolysis.

Generally, a tin calendar is placed in an iron basket, used as an anode, and brocade is deposited on the cathode by electrolysis using an aqueous solution of caustic soda at a concentration of 5 to 15% and a temperature of 70 to 90 degrees as an electrolyte. ing. However, in recent years, many tinplate scraps have metal surfaces coated with resin paints, and those coated with phenol resins, melamine resins, heenyl chloride resins, alkyd resins, etc. swell in the electrolyte and the polymer chains are in the solution. Tinplate scraps like this can be treated in the same way as unpainted tinplate scraps, but tinplate scraps coated with resins such as epoxy or acrylic cannot be treated in the electrolyte for tin recovery. Little or no corrosion occurs, and therefore the metal interposed between the resin coating and the iron cable base is not electrolyzed. Furthermore, recently there has been an increase in the number of tinplates coated with resins such as eboxy and acrylic, but these resin coatings are difficult to peel off, so tin scraps with these coatings are not suitable for recovering tin, and the remaining scraps from regular raising are difficult to remove. Since tin is unsuitable as a raw material for iron manufacturing because tin is disliked as a harmful impurity, it is desirable to develop an appropriate treatment method for tin plates with such resin coatings, and the following related technology is proposed. Something like this is known.

For example, Japanese Patent Publication No. 44-1078 discloses a method in which when tin plate scraps are electrolyzed to remove tin using a normal method, 2% or more of sodium gluconate is dissolved to remove the tin and remove the paint film. has been done.

However, when a beaker test was conducted on the tin plate shoulder of the epoxy resin coating according to the example described in the above publication, no peeling at the coating edge or detining was observed. Moreover, since sodium gluconate is quite expensive, it seems to be difficult to carry out economically. Furthermore, Japanese Patent Application Laid-open No. 49-102514 discloses a method of heat-treating waste cans in a rotary furnace to recover tin and to recover iron free of deposits. It is necessary to blow in an inert gas to prevent the oxidation of tin, and since the cost of inert gas is quite high, it is unreasonable to implement this method from a cost standpoint.

The inventors of the present invention have conducted various experiments to solve the above problems and have obtained the following findings.

That is, a caustic soda aqueous solution with a concentration of 1 to 2% is used as an electrolyte,
An epoxy resin coated steel plate with a thickness of 0.3 ribs was cut as a test piece, the test piece with this cut surface was immersed in the above electrolyte solution, and a current of 1 ampere was passed through the test piece using the test piece as a cathode. Hydrogen gas was generated from the cut surface of the test piece, and the paint film gradually peeled off from this area, and the paint film on the entire painted surface was completely peeled off in a few minutes. This peeling phenomenon is caused by the hydrogen ions that first reach the boundary between the iron base and the coating film on the cut surface of the test piece and are discharged here, and the hydrogen gas generated on the iron cable at the boundary peels off the coating film. It is thought that After many more experiments, the present invention was completed. Next, an example will be described.

Example 1 A steel plate coated with resin on both sides was cut into 10 cm long and 5 arc wide pieces to make a test piece.The test piece was suspended in a caustic soda aqueous solution with the electrode as a cathode and facing the anode, and a predetermined current was passed through it to perform electrolysis. .

The above resin coating is made of relatively thick epoxy resin (hereinafter referred to as EP).
) and extremely thin acrylic resin (hereinafter abbreviated as AC)
There are two types. The results of electrolysis are shown in Table 1. Table 1: The paint film on a test piece suspended in an electrolytic solution peels off from the bottom to the top of the test piece after electricity is applied, and sometimes the peeled off paint film is pushed up by hydrogen bubbles and floats to the surface of the liquid. was also seen.

The electrolysis temperature was as shown in experiment numbers 1 to 4 in Table 1.
It appears that the peeling time tends to be shorter as the height increases, and peeling took place in 10 minutes even at room temperature (2800℃). There was no significant difference in the caustic soda concentration between 2% and 5%, and even 0.6% did not have a significant effect. The maximum electrolytic power for coated boat 1 was G7.7 W·h in Experiment No. 1, and the minimum was 12.5 W·h in Experiment No. 6. Example 2 The test piece had the same dendrite membrane as in Experiment Nos. 1 to 5 and had a width of 14 cm.
A steel plate with a length of 75 cm and a thickness of 0.38 mm, with many perforated holes of 4 arcs in diameter, and the coating area is 500 cm.
It's a fin.

Three of these test pieces were folded and placed in a polyethylene container with a diameter of 8 inches and a depth of 10 pieces, which had many small holes for liquid flow, and this was used as a cathode. Using the shaped steel plates as anodes, they are suspended in an electrolytic solution and electrolysis is carried out as shown in Table 2. It took 50 minutes for the coating film to be completely peeled off by this electrolysis, and the electrolytic power per coating film 1 was 50 W·h.

Note that when the test piece is folded as described above, the peeled coating film does not float or float as in Example 1.

Therefore, in the following description, the condition in which the coating strength is washed away from the metal surface by running water from a tap when the folded test piece is unfolded to its original state will be referred to as peeling. Next, nine circular holes with a diameter of 7.8 cm were punched at equal intervals on a rectangular steel plate with a width of 8 skin and a length of 72.5 arc, which had the same coating film as the test piece of Experiment No. 7, and the holes were folded. When it was folded and electrolyzed in a caustic soda aqueous solution as a cathode, it was found that the back coating, which consists of two layers: a thin AC-based base coating layer and a relatively thick EP-based top coating layer, was not sufficiently peeled off. .

As a result of further tests, it was found that a caustic potassium aqueous solution is effective as an electrolytic solution, as shown in Table 3, and that the higher the concentration, the easier it is to peel off the coating (see next page).
Even though the test piece used in the experiment in Table 3 and the test piece in Experiment No. 7 have the same coating film, Experiment No. 7 has a coating film of 0.
Although the coating film easily peels off due to electrolysis in a 6% NaOH aqueous solution, in the experiment shown in Table 3, the coating film was difficult to peel off due to electrolysis in a NaOH aqueous solution when the aforementioned test piece was a single rectangular flat plate. In contrast, the latter test piece is folded and has a compound shape, and the coating surfaces are in close contact with each other due to the folding, so in a dilute electrolyte, only the end of the test piece near the anode receives current. This is thought to be due to the fact that the peeling of the paint film is difficult to proceed to the center and the metal surface that acts as a cathode does not expand because the paint film does not separate from the metal surface.

From the experiments shown in Table 3 above, it is necessary to increase the concentration of caustic alkali to improve the electrical conductivity of the solution and improve the permeability of the solution to the close contact parts for folded test pieces.
The reason why the KOH aqueous solution has better coating removability than the NaOH aqueous solution in the table is probably because KOH has higher electrical conductivity and better permeability than NaOH.

Next, in the case of tinplate scraps, since it is normal to process tinplate scraps with irregular shapes in units of tens to hundreds of kilograms, the test pieces shown in Table 4 are An experiment was conducted using.

In this case, if the paint film is not separated enough and even a small amount remains, the paint film that has peeled off from the metal surface will stick to the metal base again during tin electrolysis and prevent tin electrolysis, so the paint film will not be completely removed. It is necessary to peel it off.

In electrolysis for removing paint films, from the standpoint of electrolytic power, it is desirable to keep the caustic alkali concentration at around 15-20% and the temperature of the solution as high as possible in order to increase the conductivity of the electrolytic solution. On the other hand, in order to reduce the loss of caustic alkali adhering to the tin plate shoulder after electrolysis, it is desirable to lower the caustic alkali concentration, and to reduce the loss of thermal energy due to evaporation of the liquid, it is desirable to lower the electrolysis temperature. . Based on the above studies, we performed stripping electrolysis at a solution temperature of 80 to 85 oo, and found that the tin plate shoulder with the acrylic coating with symbol a in Table 4 could be sufficiently stripped with a 5% NaOH aqueous solution, and symbol b. The coating film is NaOH
Complete removal with an aqueous solution is difficult, but the KOH concentration is 10
It has become clear that sufficient peeling can be achieved with an aqueous solution of about 50% or more.

The results of experiments regarding the influence of the type and concentration of caustic alkali are further shown below. Example 3 Using tinplate shoulders with symbols a and b in Table 4 above, a is 4
~ 5 sheets are stacked and folded in half from the center in the width direction, then further folded and placed in a container with the coated surface almost horizontal.

Further, b is a long and thin rectangular plate 1 with holes 2 as shown in Fig. 1. Five of these plates are stacked and folded as shown in Fig. 2, and the folded plate has a predetermined number of tin scraps. Stack them in several tiers and store them in a container with the coated surface almost horizontal.
The conduct of electrolysis and its results are as shown in experiment numbers 14 to 17' in Table 5 listed on the next page, and after the coating was removed, the metallic luster characteristic of tin plating was seen on the tin plate shoulder. . In addition, the tinplate scraps in Table 5 are from experiment numbers 14 to 26.
Here we have a steel basket with a polyethylene lining (diameter 16
5 skin, height 225 skin), and experiment numbers 27-28
Here we introduce a square steel basket with a hard PVC lining (base area: 1
(45 sides x 395 ribs, height 23 ribs) and used as a cathode.

Example 4 As shown in Experiment Nos. 18 to 28 in Table 5, the amount of tin plates used was varied from 2 (weight 0.68k9, coating area 0.7) to 20 (weight 6.8k9, coating area). Electrolysis was performed by changing the area (7.0 force).

Up to 150 pieces of tin scraps were folded as shown in Figure 2 and placed in a circular wire mesh basket as described above, and for a woman in her 20s, she was folded in half as shown in Figure 3 and stacked in four layers and placed in a rectangular wire mesh basket. and subjected to electrolysis. This makes it possible to understand the effects of peeling heat and electrolytic power based on differences in the shape of tin scraps.
That is, the electrolyte was a KOH aqueous solution with a concentration of 11 to 13%, the electrolysis temperature was 85 oo, and the current and time were set from a DC constant current power supply so that the amount of current was approximately 4A h or less per 9 kg of tinplate. Electrolysis was performed. Peeling condition of paint film,
The current and amount of power required for electrolysis are shown in Table 5. Since the weight (kg) of the above-mentioned tin plate for public use and the surface area of the coating film are similar in value, the mutual relationship using the weight as an example can be illustrated from the values in Table 5 as shown in FIG. 4.

The straight line L in the same figure represents the current amount of 4A・h per kg, which is 0
The mark indicates complete peeling of the coating film, and the x mark indicates insufficient peeling. Looking at FIG. 4, it can be seen that as the amount of tin shavings increases, the peeling state becomes more stable and the amount of current required for peeling tends to decrease.

Moreover, from the results of Experiment Nos. 27 and 28, peeling is possible at low power cost due to the shape of the group of tinplate scraps constituting the cathode or the relationship between electrolysis current and electrolysis time. Example 5 After electrolytic stripping was carried out under the same conditions as in Experiment No. 28 in Table 5, the film was subjected to detinning electrolysis in a conventional manner without removing the coating film.

The situation is shown in Table 6. As shown in this table, detining electrolysis was carried out to obtain 40 days of tin.

This corresponds to 0.5% of the weight of the tinplate scrap, and the metal surface after removing tin lost its silvery white luster and took on a grayish white color. As is clear from the above examples, the present invention is characterized in that it can remove a resin coating film applied to metal without requiring any chemicals other than caustic soda or caustic potash, and with a relatively small amount of electrolytic power. be.

In other words, for products other than tin plates that are handled in large quantities in industry, such as hangers and defective paint products,
If the shape is simple and can be suspended in a caustic alkaline solution and subjected to peeling electrolysis, the cheap caustic soda may be used as the caustic alkali, and as shown in Example 1, even a 0.6% caustic alkaline concentration can cause peeling. It is effective, but considering the conductivity of the liquid, it should be carried out within the range of 1 to 20%.As for the temperature of the electrolyte, it is desirable to have a higher temperature from the point of view of conductivity, but when processing small quantities of items, the electrolytic power When comparing the cost required to heat the solution and the cost required to heat the solution, it is not necessarily better to raise the temperature of the solution, and depending on the type of base metal, a lower electrolysis temperature may be preferable to prevent corrosion by the caustic alkaline solution. Therefore, select the temperature range appropriately.

For tin plates coated with epoxy or acrylic resin, it is necessary to select caustic soda or caustic potash depending on the type of coating (using expensive caustic potash is effective for most coatings). Regarding the concentration of caustic alkali, the lower limit is 5% and the upper limit is 20%, where the conductivity almost reaches its peak.The electrolysis temperature is set at 6000 to 95 oC, taking into account the conductivity of the liquid and the energy loss due to the latent heat of vaporization. Select by range. The present invention has the above features and effects, and is an excellent invention with great practical benefits.

[Brief explanation of the drawing]

Figure 1 is a plan view of a resin-coated metal plate with holes, Figures 2 and 3 are perspective views of the metal plate when the metal plate is folded, and Figure 4 shows the weight, current amount, and coating film of tinplate scraps in service. It is a figure showing a peeling state. 1 is a metal plate having a resin coating, and 2 is a hole provided in the metal plate. Competition map Leopard 2 Travel 3 Strategy 4

Claims (1)

[Scope of Claims] 1. Peeling off the resin coating of the metal by electrolyzing with a caustic soda aqueous solution, a caustic potassium aqueous solution, or a mixed aqueous solution of caustic soda and caustic potash having a concentration of 1 to 20% as an electrolyte and using the resin-coated metal as a cathode. A method for removing resin coatings from metal surfaces. 2 The alkali concentration of the electrolyte is 5 to 20%, a metal coated with an epoxy or acrylic resin is used as a cathode, and the resin coating of the metal is electrolyzed at an electrolysis temperature of 60 to 95°C. A method for removing a resin coating film from a metal surface according to claim 1, wherein the resin coating film is removed from a metal surface.