CZ20001533A3 - Process of treating waste of powder paints and powder paints obtained in such a manner - Google Patents

Abstract

Method of processing of powder paint wastes on reusable powder paints by compacting waste of powder paints, without completely melting them, onto a sintered product in which different mixtures are compacted types of wastes or powder mixtures powder paints with different regular powder colors, and the compacted products obtained are ground to a powder color without extrusion processing.

Description

Technical field

The invention relates to a process for the treatment and reuse of ground powder paint wastes which are essentially by-products in the manufacture of powder paints or powder paint application for the production of powder paints without loss of quality.

BACKGROUND OF THE INVENTION

Crosslinkable powder paints are an environmentally acceptable alternative to solvent paint systems since no solvent emissions are generated when applied. For these reasons, the use of powder coatings has been increasing in recent years.

Crosslinkable powder coatings usually consist of one or more polymeric binders, hardeners, pigments and fillers as well as additives. Powder paint production can be divided into the following steps:

1. Powder paint components are intensively premixed as dry solids in the required quantity ratios.

2. The mixture is melted in an extruder at the lowest possible temperature to avoid premature cross-linking and vigorously mixed. Binders and hardeners are plasticized and wetted with pigments and fillers.

• · · ·

3. The obtained optionally colored extrudate is rolled to a thin layer, cooled and crushed to form a coarse granulate.

4. The granulate is milled to a finished powder color in a mill.

As a rule, the fines with a particle size <10 μτη, unavoidable during grinding, are removed in the subsequent screening process. The obtained powder generally has a mean particle size of 40 to 70 μτη.

Powder paints are usually produced in batches. Conventional compositions and conventional methods for producing powder paints are described, for example, in The Science of powder coatings, Vol. 1 and 2 (D.A.Bate, London 1990).

The aim of the grinding process is to produce a powder with a narrow particle size distribution and as low a fine fraction content as possible <10 μτη, since a high fine fraction has a negative effect on the processability of the powder paint in the electrostatic spraying process. As a rule, a powder with an average particle size of 40 to 70 μιη is produced, and a fine fraction with a particle size below 10 μιη is separated in a suitable manner, for example by means of a cyclone. This fine fraction content is generally about 2 to 5 wt. %. If the average particle size of the powder is reduced, which is often advantageous from a technical point of view, the fine fraction content rises sharply with a particle size <10 μτη, since a relatively broad particle size distribution is always obtained in the grinding process.

Powder paints with a small mean particle size and narrow grain size distribution are required, for example, for the production of thin paint layers (B.Fawer, Powder Coating, October)

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• · · · ·

1996, p. 56).

The fine fraction from the grinding process is thus a by-product of the dye industry, which in many cases has to be disposed of as industrial waste. In addition, there are other residues or wastes from powder paint production or processing that must be destroyed or recycled. Examples of these residues are too coarse fractions, spray residues, defective batches as well as residues from unsold amounts.

DE 4028567 A1 describes the recycling of a powder coating powder and its mixture with starting materials for a new powder coating batch. This mixture is then processed using a conventional extruder. In this case, the components, in particular also the spray, are thermally melted. This method has the disadvantage that homogeneous mixing is difficult due to the too small particle size of the spray, there are problems with the introduction of the mixture into the extruder and sometimes the fractions of the resulting powder paint are thermally stressed several times.

WO 96/15891 discloses a method that circumvents the problem of dispensing and delivering fines by compacting the fine powder into tablets. The obtained tablets are then mixed with other starting materials to produce a new batch of powder paint and introduced into an extruder. An essential feature of compaction is that the parent particles must still be substantially recognizable in the tablets.

EP 0 683 199 discloses a process for recycling the fine fraction of a starting mixture in the manufacture of powder paints prior to extrusion by thermally sintering the powder so that the fine fraction is distributed over an area, sintered there, and then added to the starting mixture as larger particles in suitable amounts.

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It is common to all these known processes that the recycling of the fine fraction or powder residues is carried out to the first powder paint manufacturing step. Thus, the material has to go through the entire powder paint process again. The disadvantage of this approach is that the material is re-melted and thermally stressed in the extruder. Since powder paints are in many cases thermally crosslinkable systems, repeated extrusion leads to deterioration of powder paint quality, especially when a large proportion of the fine fraction is used. Furthermore, mixing in the starting materials of the new batch constitutes a logistical problem, since the fine fractions can only be processed with the same recipe. In many cases, identical recipes are produced only at irregular intervals, so that the fines must be stored separately. For such formulations which are rarely produced, recycling according to the methods described so far is not economical.

According to DE-C 19703376, a method is described for the re-use of fine-grained powder paint residues resulting from grinding or spraying, in which the powder paint residues are agglomerated and recycled back to the production process of the same powder paint. In this case, the extrusion step can optionally be dispensed with by grinding directly into reusable powder paints.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a simplified process for processing ground powder paint waste which allows its reuse. The method should result in homogeneous powder paints of unimpaired quality. It should provide the possibility of achieving color mixing while achieving the desired hue.

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It has been found that this object can be solved by the invention, which relates to a process for processing ground powder paint waste by compacting powder paint waste into a sintered product without fully melting, wherein different types of powder paint waste or different powder paint waste and regular powder paint are ground to powder paint without extrusion processing.

Another object of the invention are powder coatings obtainable by the process of the invention.

Various types of powder paint residues can be used according to the process of the invention. In particular, the different types of powder paint residues may be powder paint residues of different colors and / or powder paint residues of different chemical compositions.

In the first step, the powder paint waste is separated, homogeneously mixed and compacted.

Then, in a second step, the resulting sintered products, e.g. in the form of a coarse granulate, are ground in a second step. The compacted material no longer disintegrates into the initially present fine particles.

The coatings obtained from the powder according to the invention do not differ in quality from the conventional powder coatings.

In the manufacture of powder coatings, the color pigments are generally added as a powder or premixed before extrusion. Subsequent mixing of the pigments proves difficult, since homogeneity must not be ensured, nor must it be subsequently added

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Surprisingly, it has been found that the compacting of differently colored fine powders with a particle size <10 μπι and their subsequent grinding allows the production of uniformly colored powders. The different color batches of powder paint residues can most preferably be homogeneously mixed prior to compaction. A uniform powder paint is obtained which, after application, provides a coating in which the different colors of the fine powders used can no longer be distinguished by the human eye. This process allows the use of fine fractions from both powder paint production and powder paint processing with different color shades to produce valuable products. Warehousing and logistics to reuse color lots of powder paint residues are largely eliminated.

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According to another embodiment of the process of the invention, small amounts of colored fine powder may be mixed and compacted with a regular powder color, which may have a different color. The powder thus obtained again provides a color uniform coating in which the color differences of the particles used can no longer be distinguished by the human eye. Since no new extrusion and melting of the powder paints are carried out, it is possible in this way to further modify or change the color. It is therefore possible to create nuances of shades.

According to the invention, the amount may preferably be up to 10 wt. colored powder paints. The remainder of the powder paint with an average particle diameter of less than 10 μπι may be mixed with hay of a different color and processed.

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The process according to the invention also makes it possible to produce different batches of the same composition from the clear lacquer, but with different shades of color.

The above process also makes it possible, if necessary, to modify the powder colors by adding to the powder color an amount of ground other powder color, e.g. containing additives, and this mixture is then further processed according to the process of the invention. For example, they can also be additionally modified in this way with clear powder paint additives.

The fine fractions and / or compaction powder residues need not have a homogeneous composition, nor consist only of fine fractions with a small average grain size.

With the method according to the invention, a powder with a different particle size, for example having a mean particle size of 10 to 300 µm, can also be compacted and re-milled without problems. For example, it is possible to process batches which, due to irregularities in the mill operation, do not have a specified grain size spectrum, e.g., defective batches.

It is also possible to add coarse-grained portions to the fine fractions mentioned, i.e.. insufficiently ground powders.

In addition, it is also possible to use spray paint spraying as well as unsold powder paint residues.

Such residues may be added as additions or may be processed by the process according to the invention.

Preferably, the fine fraction occurring at

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For example, a particle size distribution can be achieved by the method of the invention, wherein at least 50 volume percent of the powder paint particles obtained have a particle size of, for example, 30-40 µm (d50) with a particle size < 10 µm being below 15%.

By means of the process according to the invention, for example, the separated fine fractions of powder paint of different color composition, optionally also together with other ground fractions of powder paint, can be compacted so that the compacted solid fraction is compacted by sintering the grains. of the grinding process in the manufacture of the powder paint, which essentially does not disintegrate into the original fine particles during grinding. The compacted material can be ground directly to the powder by means of mills conventional in powder paint production, which does not differ substantially from the grain size spectrum of the conventional powder paint and can be processed without difficulty into a powder coating.

Generally, it is preferred to mix powder paint wastes such as coarse residue, fine fraction, coating residues, defective batches having similar or the same chemical composition to ensure compatibility as well as uniform cross-linking, eg epoxy systems, polyester systems, polyurethane systems or acrylate systems. However, it is also possible to mix waste powder paints of different compositions, such as polyester and epoxy systems. In this case, however, the compatibility of the resin should be tested to avoid coating defects.

The method according to the invention can be used for any powder paint, e.g. based on epoxy, polyester, polyurethane paints. or acrylate resins.

Compaction conditions may be selected depending on the type of powder coating used. If the compaction is weak, as is contemplated, for example, in a tablet manufacturing process analogously to WO 96/15891, the compacted material in the mill disintegrates again into the fine powder used. If the densification is too strong, it will be heavily heated by frictional heat, leading to a considerable melting of the powder. The molten powder adheres strongly to the compaction equipment and can only be removed at great expense. In addition, the heat load is very high so that the quality of the powder produced is deteriorated. The compaction according to the invention is carried out in such a way that the sintering of the fine fraction occurs without completely melting the dust grains, and the sintered pieces are crushed to new granular structures during grinding.

The optimum compaction conditions depend on both the powder paint used and the compaction equipment. During compaction, an elevated temperature occurs on the surface of the roller presses, which leads to sintering or sintering of the dust grains. In the case of powder paints crosslinked at low temperature, for example, the processing force can be adjusted such that the individual dust grains are well sintered but not melted. For powder paints crosslinked at higher temperatures, the force and hence the induced temperature can be set higher. Alternatively, the compacting device can also be provided with external heating or cooling, for example the cylinders can be cooled with water.

In principle, all known compaction devices are suitable for the process according to the invention, such as piston presses, piston extruders, in particular double-roll presses or

- 10-ring cylindrical presses. In this case, the compressive force is determined by the gap size, the rotation speed and the amount of material. These parameters can be well adapted to each other. The compressive force is given as the specific compressive force in N per cm of cylinder width. In general, the specific pressure force should be higher than 4 kN / cm, a value higher than 6 kN / cm is particularly preferred. For example, the specific compression force is in the range of 10 to 30 kN / cm to 100 kN / cm. Too high a compressive force results in extensive melting of the powder; that the fibers are pulled out of the mass and the mass is adhered to the rollers. Too low compression forces lead to a strongly increased proportion of the fine fraction in the milling process. Two-roll presses with a structural surface have proven to be particularly suitable, with those having a tablet are less suitable than the corresponding grooved recess surfaces.

If the product is formed during compaction, it is not suitable for grinding, because of its size, eg for scales or granules. Crushing by means of conventional equipment applicable to paints, for example by means of a jaw crusher, which can be crushed with respect to the product, can be carried out in powder form

The subsequent grinding process is carried out by means of conventional powder paint equipment, such as impact mills or sorting mills, which allow the gentle production of powder paints. This gives a further advantage of the process according to the invention that the treatment is carried out in part within the usual powder paint production and does not require any additional grinding equipment. Powder paints resulting from the compacted granulate exhibit a proportion of the fine fraction comparable to conventional powder paints, for example below 25% of the fine fraction, possibly also less than

%.

• • •

The process according to the invention provides a process which eliminates the disadvantages of the prior art in the reprocessing of fine powders and other powder paint wastes, in particular eliminating the need for additional heating steps or steps. melting. Furthermore, the storage of the fine fraction or powder paint residue after its application is eliminated. Particularly during the powder processing, the resulting fine and residual fraction can be processed by the process according to the invention by compacting and grinding to a qualitatively valuable powder without the need for extrusion. Neither the grain size spectrum of the powder obtained nor the quality of the coatings of the powder produced differs from the usual powder colors. In addition, the process according to the invention also offers the possibility of processing batches which are defective in their color shades into valuable color shades suitable for sale.

DETAILED DESCRIPTION OF THE INVENTION

The following examples illustrate the invention. The compressive force of the cylinders is given as the specific compressive force per cm of cylinder width (kN / cm).

Example 1 kg of fine fraction <10 μπι (mean grain size about 4 μιη) from the grinding of a conventional thermally crosslinkable white powder paint based on an epoxy / polyester hybrid (49% polyester, 21% epoxide, 29% titanium dioxide and 1% additive) is compacted using a Bepex double-roll compactor using surface-structured rollers with a throughput of about 25 kg / h and a compressive force of 8 kN / cm. The compacted homogeneous belt thus obtained, which is mechanically stable, was crushed in a crusher to:

- 12 scales with a diameter of about 2 cm. These scales were milled in a mill (manufactured by Hosokava, type ACM 2) under conventional powder paint conditions. A powder with an average particle size of 45 µm and a proportion of <15% of the particles <10 µm was obtained.

The powder was electrostatically sprayed with a corona gun onto an aluminum sheet and fired for 20 min at 180 ° C. A defect-free coating was obtained with properties which do not differ from those obtained by the above-mentioned non-compacted powder paint.

Example 2 kg fine fraction <10 µm (mean grain size about 4 µm) from the grinding of black-colored thermally crosslinkable epoxy-based powder coating with dicyanic crosslinker is compacted using a Bepex two-roll compactor using surface-structured rollers with a throughput of about 22 kg / has with a force of 14 kN / cm. The compacted homogeneous belt thus obtained, which is mechanically stable, was crushed in a shredder with a diameter of about 2 cm. These scales were milled in a mill (manufactured by Hosokava, type ACM 2) under conventional powder paint conditions. A powder with an average particle size of 37 µm and a proportion of <14% of the particles <10 µm was obtained.

The powder was electrostatically sprayed with a corona gun onto an aluminum sheet and fired for 20 min at 180 ° C. A defect-free coating was obtained with properties not different from those obtained with unprocessed powder.

Example 3 kg fine fraction <10 pm (mean grain size about 4

-13 pm) from a conventional polyester-based thermally crosslinkable powder paint with triglycidyl isocyanurate (TGIC) as a crosslinker (55.8% polyester, 4.2% TGIC, 29% titanium dioxide and 1% additive) is compacted using a Bepex two-roll compactor using surface-structured rollers with a throughput of about 20 kg / h and a compressive force of 20 kN / cm. The compacted homogeneous belt thus obtained, which is mechanically stable, was crushed in a shredder with a diameter of about 2 cm. These supines were milled in a mill (manufactured by Hosokava, type ACM 2) under conventional powder paint conditions. A powder with an average particle size of 27 µm and a proportion of <25% of the particles <10 µm was obtained.

The powder was electrostatically sprayed with a corona gun onto an aluminum sheet and fired for 20 min at 180 ° C. A defect-free coating was obtained with properties which do not differ from those obtained with non-compacted powder paint.

Example 4 kg fine fraction < 10 µm (mean grain size about 4 µm) from a brown wax-colored thermally crosslinkable epoxy powder paint with a dicyanoic hardener is compacted using a Bepex two-roll compactor using a surface structured roll of about 27 kg. has a compressive force of 16 kN / cm. The compacted homogeneous belt thus obtained, which is mechanically stable, was crushed in a shredder with a diameter of about 2 cm. These scales were milled in a mill (manufactured by Hosokava, type ACM 2) under conventional powder paint conditions. A powder with a mean particle size of 36 µm and a proportion of <20% of the particles <10 µm was obtained.

The powder was electrostatically sprayed with a corona gun.

- 14 on aluminum sheet and baked for 20 min at 180 ° C. A defect-free coating was obtained with properties not different from those obtained directly with the powder paint.

Example 5 kg of fine fraction <10 μπι (mean grain size about 4 pm) from the grinding of differently colored epoxy powder paints (red, white, yellow and gray) is mixed and compacted using a Bepex roller compactor using surface structured rollers with a throughput of about 27 kg / has a compressive force of 7 kN / cm. The compacted homogeneous strip thus obtained, which is mechanically stable and has a uniform pink-pink color, was crushed in a crusher with a diameter of about 2 cm. These scales were milled in a mill (manufactured by Hosokava, type ACM 2) under conventional powder paint conditions. A powder with an average particle size of 40 µm and a proportion of <15% of the particles <10 µm was obtained. The powder exhibits a homogeneous coloration and also the powder grains are substantially monochromatic.

The powder was electrostatically sprayed with a corona gun onto an aluminum sheet and fired for 20 min at 180 ° C. A defect-free coating was obtained with properties that do not differ from conventional coatings. There is a uniform color tone and color differences, and there are no noticeable color differences resulting from the use of a mixture of differently colored fine fractions.

Comparative Example 1 kg of the fine fraction, analogous to Example 1, is compacted using a Bepex roller compactor with a tablet roller into tablets having a diameter of 10 mm and a thickness of 3 mm, the thickness of the rollers being 4 kN / cm. Subsequent grinding of these tablets in a mill (manufacturer

- 15 Hosokava, type ACM 2) gave a powder with a mean particle size <10 μπι. This powder behaves as a fine material and cannot be processed in a conventional powder paint processing plant.

EXAMPLE 6 A 6 kg fine fraction of Example 3 is compressed by a 9 kN / cm roller width using a two-roller compactor from Alexanderwerk (Type WP 50N 75) and a throughput of about 80 kg / h to a mechanically stable homogeneous belt in a mill (manufacturer Hosokava, type ACM 2) under the usual conditions for powder paints. A powder with an average particle size of 35 μπι and a proportion <19% of the particles <10 μιη was obtained.

Example 7 kg of the fine fraction of Example 3 is compressed with a 12 kN / cm roller width using a two-roller compactor from Alexanderwerk (Type WP 50N 75) and a throughput of about 50 kg / h onto a mechanically stable homogeneous belt which has been coarsely mechanically crushed in a mill (manufacturer Hosokava, type ACM 2) under the usual conditions for powder paints. A powder with a mean particle size of 32 μπι and a proportion <15% of the particles <10 gm was obtained.

Comparative Example 2 kg of the fine fraction of Example 3 is compressed with a 2 kw Alexanderwerk compactor (Type WP 50N 75) with a compressive force of 4 kN / cm roller width and pushing about 100 kg / h to a mechanically stable homogeneous belt which was coarse.

-16 • mechanically crushed and subsequently milled in a mill (manufacturer Hosokava, type ACM 2) under the usual conditions for powder paints. A powder is obtained which contains more than 40% of particles < 10 and is not suitable for use as a powder paint.

Claims (16)

PATENT CLAIMS A process for processing powder paint waste into reusable powder paints by compacting powder paint wastes without fully melting them into a sintered product, characterized in that mixtures of different types of powder paint wastes or mixtures of powder paint wastes with different regular powder paints are compacted, and the compacted products obtained are ground to a powder color without extrusion treatment. Method according to claim 1, characterized in that the grinding is carried out to a particle size distribution in which at least 50 volume percent of reusable powder paint particles have a particle size of 30 to 45 μτη with a particle size fraction of less than 10 μπι <15%. Method according to claim 1 or 2, characterized in that the powder paint wastes are powder paint wastes and / or powders occurring in the application of the powder paints. Method according to claims 1 to 3, characterized in that: waste powdered colors are wastes from powder production colors, emerging as too fine-grained and / or too coarse-grained. 5. Way according to any of the claims 1 to 4, characterized in that the colored powder paint waste is mixed with a regular powder paint of another color, compacted and ground into powder paints, wherein the amount of powder paint waste is less than the amount of regular powder paint. Method according to one of claims 1 to 5, characterized in that the compacting is carried out by means of a two-roll or annular roller press, the rollers having structured surfaces. Method according to one of Claims 1 to 6, characterized in that the compaction is carried out by means of a compaction device having a specific contact force of 4 to 100 kN / cm. Method according to claim 6, characterized in that the compaction is carried out by means of a two-roll or annular roller press with a contact force of 5 to 50 kN / cm. Method according to any one of claims 1 to 8, characterized in that the particle size of the powder paint waste used is up to 250 µm. Method according to claim 9, characterized in that powder paint wastes with an average diameter of <10 µm are compacted. Method according to one of Claims 1 to 10, characterized in that powder paint residues of a similar chemical composition are first mixed before being processed in this way. Method according to one of Claims 1 to 10, characterized in that powder paint residues of different colors are first mixed before being processed in this way. Method according to any one of the preceding claims, characterized in that the powder paint waste is mixed with a conventional powder paint before compacting. The process according to claim 13, characterized in that up to 10 wt. otherwise colored powder wastes with an average particle diameter <10 µm, mixed and then compacted and milled. Powder paint composition obtained by the process of any one of claims 1 to 14 from powder paint waste by compacting and subsequent grinding without further melting. The composition of claim 15, obtained from a mixture of at least two differently colored powder paint wastes. The composition of claim 15, obtained from a mixture of up to 10 wt. colored powder paints with an average particle diameter of <10 µm with an otherwise colored conventional powder paint. The composition of claims 15 to 17, obtained from a mixture of at least two powder paint residues of different chemical compositions.