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Powder coating: fine recovery (II)

Second part of this article that offers valuable information on the recovery of waste generated by powder painting, and that requires knowledge for its correct execution.

by Licenciado José Tomás Rojas. Master in Paint Technology*

In the previous column we mentioned that the generation of Finos or Polvillo is a process that is directly associated with the manufacture or application of powder coating. In manufacturing it is generated in the grinding phase of the paint. In application it is generated in the cloud or "over Spray application".

In both cases, this dust must be removed from the system, through sorting and  collection systems, which are usually composed of cyclones and filter sleeves. The objective of this equipment is to separate and confine the fine dust in suitable containers, preventing this by-product from returning to the process or going into the atmosphere as a pollutant. See

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Figure 4. Cyclone Collection Sleeves Set

In factories, dust is part of the process waste, and is assigned a value that is very close to 3% of production. However, a design problem or malfunction can bring this value up to 7%, even up to 10%. This loss is adjusted by incorporating it into the total cost of the manufacturing process. Of course, as the percentage of loss increases, the cost of the final product will be higher. The same happens in application plants; the cost of the dust is charged to the cost of the painted piece, when the calculation of painted pieces x Kg. of product is made. The higher the loss, the higher the cost per part, and the lower the benefit to the applicator or the higher the cost to the customer.

Now, for the purposes of the potential reprocessing we can assume that the total cost of the dust is zero, even more, it can be considered as a waste of the process. Given the above, it is obvious that if this dust manages to be reused, we would be working with a material of very low or no cost, which would provide an interesting utility to achieve its commercialization. At the same time, we would be helping to eliminate a source of environmental liabilities.

Unfortunately, the recovery process is usually cumbersome, and although the cost of the material can be "0", its reprocessing requires knowledge, experience, equipment and labor. It is an activity that, if not carried out correctly, can be  costly, eliminating any possibility of profit and transferring the problem of environmental liabilities from the product to the recuperator.

There are certain steps that must be followed, to enhance the chances of successful recovery:

Identification at the source
It is important to know the source of origin of the dust. Whether it comes from a factory or an application plant, it is advisable to know what products the factory or plant works with, and what is the ratio of manufacture / use of them. This can give us a good idea of the composition (Epoxy, Hybrid, Polyester) of the dust that we are going to acquire, in order to begin to design recovery and marketing strategies. If possible, the supplier of the powder can be asked for an applied sheet, to see what the product film will look like.

This application allows us to quickly see what the appearance of the paint is, and also gives us an idea of whether the product is contaminated or not. Some companies have particle size measurement equipment, and we could ask them for a characterization to know what the particle size of the dust is. A product between 10 and 40 microns has good recovery potential.

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Of course, it is difficult to obtain in application plants, especially in small ones, equipment to measure particle size. However, it is advisable to have a laboratory sieve of 325 MESH, or 44 microns of opening, on hand. See.

Figure 5. Laboratory sieve. They usually come with a lid and bottom for testing.

The use of this sieve allows us, in a simple way, to have a good idea of the particle size that the dust has. For this, what is done is to take a certain amount of dust, weigh it and then place it on the sieve. The lid and bottom are placed, and shaken for about 3 minutes. Then the material that was retained in the sieve is collected. A dust that leaves sieve residue of 20 to 40% has the potential for recovery.

Classification
Depending on its origin, it is possible that we can get dust made up of a single product. This is usually the case in paint mills or application plants that work with few products. This would be the ideal situation for recovery, since the appearance of such a product would always be very similar between batches. Now, this is not so frequent, and most likely it is to get the dust that is the product of the mixture of several paints, possibly with different finishes and colors.

In any case, if both situations occur, it is convenient to make a classification between Homogeneous dusts (Monocolor), or Heterogeneous dusts (Multicolor), which will facilitate the recovery process much more.

Conditioning
Regardless of the origin of the dust, it should always be considered as a waste product. This means that in addition to dust, it is possible that we can get with other materials that are not paint, for example; wood, food scraps, plastic, garbage, among others. Therefore, before starting any recovery process, it is necessary to sift the product to get rid of these contaminants. Usually, a 250 micron (60 MESH) sieve  can do this job well, and there are many pieces of equipment on the market that work well for that purpose. I leave the following reference image that gives us an idea of how this process works (on the website you can see the full video:
http://www.russellfinex.com/en/demonstration-videos/sieving-powder-coatings/

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Homogenization
After having collected and conditioned the powder, it is desirable to carry out a homogenization process. This consists of taking the portions of dust that you have, respecting, of course, the classification made previously; that is, the batches of monocolor dust, let's say white, are taken and all mixed. In the other case, all the multicolored dusts are mixed. The idea is to make a dry mixture, using appropriate equipment, until achieving a uniform appearance for the whole set. For this purpose there are many pieces of equipment on the market, of different capacities.  See Figure 6.

Figure 6. Cutting of a dry mixing equipment. An axis to which fins have been attached, generates a helical movement, which allows the mixing of the dust.

Once homogenization is achieved, a process that is usually achieved with a mixture of 5 to 10 minutes, the product obtained is identified with a particular batch number. Subsequently, it is convenient to take a sample and apply it on several sheets, in order to have an idea of what the final product will look like. These sheets, duly identified with the work batch,  are important inputs for the future commercialization of the already homogenized dust.

Homogenization is very important, with a view to marketing the product, since normally users need a product that allows them a uniform finish of the pieces they are painting. Therefore, once the commercialization of the product has been achieved, it is recommended to keep reference sheets, for new productions with the mixture of dust.

Color adjustment
Due to the variability in origin, dust mixtures usually give different appearances from batch to batch. That is why it may be necessary to make a color adjustment, until the product is approximated to the appearance of the previous batch. This adjustment process requires some expertise in color tanning. Normally what is done is that you have paints of Black, White, Red, Green, Blue and Yellow colors, and you add quantities, until you reach an approximate color to the previous lot.

It is advisable to do this process in small, and then, once you have the proportions that give a good approximation, we proceed to the adjustment in the tank.

The adjustment with the aforementioned colors is based on a table of chromatic coordinates. See Figure 7.

Figure 7. Chromatic coordinates. For example, to fit a product with a yellow hue, its blue counterpart must be added.

Colorist technicians can be located to assist in the color adjustment process. As I mentioned, this requires some expertise, but there are many people who work in this field, particularly in stores where color preparation is done, and in paint shops for car repair. Of course, the colorist must go through a learning process that will allow him to know the difference between nuancing with a liquid paint, to nuancing with a powder paint. Normally this adaptation process is fast.

Appearance adjustment
There are additives that allow to modify the appearance of the dust. This is particularly useful when working with monochrome dust. An example of this is to acquire white dust, coming for example from a plant that is dedicated to the painting of refrigerators. For this case smooth and shiny paints are used. Hence, it is very difficult to directly reuse the dust, because its appearance usually deteriorates, among other things, with the decrease in particle size.

What is done in this case is to take the powder and add a texturing additive. This additive is added to the mixing tank, in the homogenization stage, obtaining a product of very decorative appearance, which hides very well any imperfection that the paint may bring. Texturing is highly appreciated in the metals, furniture, auto parts, etc. See Figure 8.


Figure 8. Textured finish.

Appearance adjustment
Just as there are additives for texturing, additives can also be obtained to achieve other types of finishes. There are interesting details to review, depending on market preferences, and you should have some experience with the use of these materials. The dosage plays an important role, since these additives are usually very active, so small variations in the dose, can affect the finish a lot. Figure 9 shows some examples of finishes.

Figure 9. Types of finishes that can be achieved with the use of additives.
Fields of application

The recovered dust finds much use as a first ferroprotective (bottom). This app is ideal as it typically focuses more on functionality than appearance. Textured finishes can be used to coat tools, economic furniture and metalwork in general. This is because, mixing powders does not normally affect functionality. In any case, you have to make sure to characterize each batch of recovered dust. Tests such as impact resistance (60 lbs D/R, ASTM D2794) and adhesion (100% grid, ASTM D 523), will give us a good idea of the functionality we can expect.

* Lic. José Tomas Rojas. MSc. President JTROJAS PINTURAS. Email: [email protected]  Twitter: @JtrojasPinturas

Duván Chaverra Agudelo
Author: Duván Chaverra Agudelo
Jefe Editorial en Latin Press, Inc,.
Comunicador Social y Periodista con experiencia de más de 16 años en medios de comunicación. Apasionado por la tecnología y por esta industria. [email protected]

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