CS233628B1 - Method of creating of plastic coating of absorber of solar collector - Google Patents

Abstract

The method belongs to the surface industry treatment of metals and solves the formation of coating on the surface of the solar collector absorber, which has sufficient anticorrosion durability and at the same time securing high collector efficiency. According to the invention, this is the case achieved by applying to the absorber surface, in a high voltage electrical field, optionally using triboelectric effect, layers of plastic powder coated dark pigment so that the pigment is puts it into the mass before applying it to surface, or the pigment is applied to an already formed surface a plastic layer, and a pigment layer is then melted into a continuous coating. It is advisable to apply the dark pigment optionally on a molten surface already applied powdered plastic and applied cure the bilayer.

Description

The invention relates to a method of forming _a coating of plastic powder on the surface of the solar collector absorber!

For the surface treatment of steel, aluminum or copper absorbers, organic coatings are known and used so far, in which, in order to increase corrosion protection, a topcoat, a black coat, is applied to the base coat and possibly to a phosphate conversion layer. The advantage is an easy application method. Eight alkyd, epoxy-amine, epoxy-urea, acrylate or fluoroplastic binders are used. For planar low temperature collectors, the thermal resistance of these coatings is satisfactory. However, the coatings do not form selective absorption layers because they do not have a sufficiently high absorption and emission coefficient. Their resistance under conditions with cyclic temperature changes and condensation is low, resulting in blistering, peeling and corrosion of the substrate. In the case of loss of adhesion and blistering, there is a significant reduction in collector efficiency due to reduced heat transfer capability. The lifetime of the coating depends on its type and the aggressiveness of the collector cryptoclimate and is estimated to be about 5 ax10 lot. Galvanic coatings for steel absorbers are also known for a similar purpose. Duplex or single-layer nickel on a copper-plated surface is used, with a select layer of black or black nickel. Temperature resistance is satisfactory. The quality of the coating depends on its non-porosity. The disadvantage is the high cost of these coatings and the difficulty of adhering to the prescribed coating thickness at all

- 2 233 628 parts of the articulated surface of the absorber. Anodically or chemically excluded Black oxide layers on absorbers have very good durability with low coating selectivity, but are only suitable for aluminum surfaces. Magnetron plasma coatings of pure metals or oxides are another known surface protection for metal surfaces. Their optical properties and durability depend on the metal or oxide being sprayed. The disadvantage is the costly equipment for serial applications due to the need to maintain a high vacuum during application. Thin-layer black enamels of about 100 µm thickness, like black coatings, do not have selective properties. However, they can be chemically deposited with oxides such as tin or antimony, which increase selectivity. Burning enamels is very energy intensive. Since the absorbers are usually made of thin sheets of 0.8 aAl mm thickness, it is technologically difficult to guarantee the rigidity of the absorber and thus the resistance to deformation at the melting temperatures of the enamels around 700 ° C.

The disadvantages of the known surface treatments are largely eliminated by the coating method on the surface of the solar collector absorber according to the invention, which comprises depositing a powdered plastic layer, for example an epoxy, epoxy-polyester layer, in a high-voltage electric field or triboelectric effect. , polyester, acrylate or polyurethane resins, polyamide-based polymers, polyolefins, mixtures of these resins and polymers, copolymers of ethylene and vinyl acetate, ethylene and saponified vinyl acetate, or copolymers of fluorocarbons, the coating being coated with a dark pigment such as carbon black, graphite or black metal oxides. The powdered plastic layer is provided with a dark pigment by either incorporating the pigment into the mass before it is applied to the surface, or by applying the pigment to the already formed plastic layer. The pigmented plastic layer is then melted to a continuous coating. It is also desirable to apply the dark pigment to the molten surface of the pre-applied plastic and to cure the applied bilayer.

- 3 233 628

An economically advantageous embodiment of the invention is that ee. applied to the surface of the absorber 50 to a mixture ^of 9C-% powdered epoxy resin filled with acetylene carbon black and 10 to 50 wt. % of polyethylene powder having a melt flow index of 1 to 70 g / 10 min, filled with acetylene carbon black, and the resulting layer is cured at 160 to 200 ° C for 15 to 25 min.

The coating plastics can be used based on epoxy, epoxy-polyester, polyester, acrylate or polyurethane resins or polymers based on polyamides, polyolefins stabilized against UV destruction or based on mixtures of these resins and polymers, as well as copolymers of ethylene and vinyl acetate, ethylene and polymers. saponified vinyl acetate, fluorocarbon polymers or copolymers.

All of these plastics are UV-resistant by using dark pigments, such as acetylene carbon black, graphite or metal oxides, and a treatment suitable for absorbers, i.e., a black color and a matt surface, is achieved by blending said additives into the polymer or resin melt with dry by mixing the powdered polymer or resin and the powder additive, homogenizing in a ball mill or spraying the two components separately. This can be done in two ways. The polymer or resin is applied in the electric field to the cold surface of the absorber and then a layer of powdered carbon black, graphite or oxides is applied to the layer, also in the electric field. Then, the resulting bilayer is exposed to heat, at a temperature higher than the melting point of the polymer or resin, or at a temperature at which the cure reaction of the thermosetting plastics takes place. Alternatively, the polymer or resin is applied in an electric field to the cold surface of the absorber, then the deposited layer is melted and a layer of carbon black, graphite, oxides, and the like is sprayed into the molten coating. These substances are also sprayed in an electric field or only

- 4 233 628 with a simple spray gun. In thermoplastics the resulting bilayer is allowed to cool, in thermosetting the heat curing is performed.

By the method of the invention, it is possible to produce matt black coatings that adhere well to a metal substrate, with high UV resistance, good temperature resistance and high durability. The coatings are flexible and sufficiently scratch resistant.

The invention and its effects are explained in more detail below with reference to the examples of its specific embodiment.

Example 1

On the cold surface of the absorber, which had been pre-coated with a Zn-phosphate conversion layer, a 70 wt. % epoxy resin powder filled with acetylene carbon black and 30 wt. % of polyethylene powder having a melt flow index of 20 g / 10 min., filled with acetylene carbon black, at an electrode voltage of 55 kV, an electrode distance from the coated surface of 200 mm, at a powder delivery rate of 250 g / min. by spraying, the coating was melted and cured at 180 ° C for 20 min. A black, matt, well adhering coating to a metal substrate, 80 µm thick, was formed.

Example 2

A layer of polyamide 11 was sprayed onto the surface of the absorber, which had been pre-blasted with corundum 36. The spraying was carried out with a triboelectric gun at an air pressure of 0.3 MPa, for 30 s. The sprayed absorber was placed in an oven heated to 220 ° C. 15 min. The absorber was then removed from the furnace and a layer of acetylene carbon black was sprayed onto the molten coating with a spray injector. The absorber was put back in the furnace and the coating cured at 150 ° C for an additional 15 minutes. The coating was sufficiently matt, flexible, 60 µm thick.

- 5 Example 3 ^{233 628}

The phosphate conversion absorber was placed over the fluidized bed in an electric field at + 50 kV electrode voltage. The fluidized bed was a powdered polyurethane resin filled with graphite. After 10 seconds of settling of the polyurethane particles on the grounded surface of the absorber, the absorber was placed in a furnace heated to 230 ° C for 10 min. After melting and curing, a matt black coating having a high hardness and UV resistance of 150 .mu.m was formed.

Claims (4)

SUBJECT OF THE INVENTION 233 628 powder plastic A method of coating a solar collector absorber surface, characterized in that a layer of plastic powder, for example an epoxy, epoxy-polyester, polyester, acrylic or polyurethane resin layer, polymers, is applied to the surface of the absorber in a high-voltage electric field or by triboelectric effect. based on polyamides, polyolefins, a mixture of these resins and polymers, ethylene / vinyl acetate copolymers, ethylene and saponified vinyl acetate or fluorocarbon copolymers, the coating being provided with a dark pigment such as carbon black, graphite or black metal oxides. 2. A method according to claim 1, characterized in that the powdered plastic layer is melted to a continuous coating after application to the absorber surface. 3. A method according to claim 1 or 2, characterized in that the dark pigment is applied to the molten surface of the applied plastics material, whereupon the applied bilayer is cured. 4. A process according to claim 1, wherein a mixture of 50 to 90 wt. % epoxy resin powder filled with acetylene carbon black and 10 to 50 wt. % of polyethylene powder having a melt flow index of 1 to 70 g / 10 min, filled with acetylene carbon black, and the resulting layer is cured at 160 to 200 ° C for 15 to 25 minutes.