EP0112277B1 - Method of coating shaped objects with polyethylene - Google Patents

Abstract

The metal hollow is provided with a base layer of an epoxy resin being electrostatically applied upon which an ethylene copolymer powder is electrostatically applied possibly under particular inclusion of epoxy resin and possibly in several layers of varying relative consistency; after each powder application step, surface heating is applied to melt the respective powder; the final coating of polyethylene is either electrostatically applied or through suitable extrusion process. Particular grain size distribution and powder consistency patterns are suggested.

Description

The invention relates to a process for coating metallic moldings, in particular pipes and pipe fittings, with polyethylene, the surface of the cleaned moldings which have been heated to at least 80 ° C. being first primed with an epoxy resin / hardener mixture, then ethylene copolymer as an adhesive thereon and then polyethylene is applied as the outer cover layer, whereupon the coated moldings are cooled to room temperature.

A coating produced in this way, which in principle consists of three layers, has been found to be particularly suitable for the insulation of steel pipes, in particular those which are laid in the ground. B. is known from DE-A-1 965 802, which discloses a method according to which is sprayed onto a preferably heated to 140-200 ° C steel tube at these temperatures curing epoxy resin, while curing around the tube on the epoxy resin -Layer an extruded film tape is wound from an ethylene copolymer, which serves as a pressure sensitive adhesive for another extruded polyethylene film tape to be wrapped around the tube.

Because of the complicated movements involved in the winding process for applying the foil strips, this method is not suitable for economical mechanical wrapping of pipe fittings. It is also unsuitable for wrapping moldings which are already provided with a heat-sensitive inner coating.

DE-A-22 22 911 specifies a coating method with the aforementioned 3-layer structure that is suitable for metallic tubes with a heat-sensitive inner coating. The epoxy resin base layer is applied at a temperature of the pipe of 70-90 ° C. The adhesive layer made of ethylene copolymer and the outer cover layer made of polyethylene are extruded as a double tube and applied to the base layer, the heat of the double tube not being sufficient for rapid curing of the epoxy resin. Rather, this takes place at room temperature within 24 hours. This method is also generally unsuitable for coating because of the geometry of pipe fittings that deviate from the cylindrical shape and because of the long curing times.

Furthermore, it is known from DE-A-22 57135 to heat a steel pipe to be encased to 80 ° C. and to coat it electrostatically with a solvent-containing epoxy resin / hardener mixture in a thickness of approximately 100 μm. The inner ethylene copolymer layer and the outer polyethylene layer are then applied to this base layer by stretching an extruded double tube or wrapping it with extruded foil strips. After the coated tube has cooled to an average tube temperature of 40 ° C., the surface of the core tube is inductively heated to approximately 240 ° C., an average tube temperature of 100 ° C. being established. This allows the epoxy primer to harden in a few seconds. This method is also unsuitable for the coating of pipe fittings due to the nature of the application of the inner ethylene copolymer layer and the outer polyethylene layer for geometric reasons.

Finally, from GB-A-1 542 333 a method for coating metallic pipes with polyethylene is known, which corresponds to the generic method with regard to the materials used, but with a high heating temperature (270-300 ° C) for the Shaped body is worked, which is why it is not suitable for the coating of shaped bodies that are already provided with a heat-sensitive inner coating. In addition, it is uneconomical for heating due to the high energy expenditure.

An important quality feature of a plastic coating is its peel strength. The peel strength of multilayer plastic insulation is impaired by internal stresses and the abrupt transition between the different layers. In the peeling test on steel pipes that were coated according to the above-mentioned methods, there is usually a separation in the transition area of the individual plastic layers. The connection between the epoxy resin base layer and the steel tube is the strongest; In the peel strength between the two thermoplastic layers of adhesive (ethylene copolymer) and outer covering (polyethylene), high values can also be achieved by a suitable choice of process parameters (e.g. temperature control).

In contrast, the transition area between thermoset and thermoplastic, ie between the epoxy resin base layer and the ethylene copolymer adhesive layer, is particularly at risk in terms of peel strength.

The invention has for its object to provide a method according to the metallic moldings, in particular pipe fittings and pipes, even if they already have a heat-sensitive inner coating, from a base layer made of epoxy resin, an outer cover layer made of polyethylene and an intermediate adhesive Ethylene copolymer existing insulation of high peel strength can be applied.

This task is solved by the features of the main claim. Advantageous further developments are specified in the subclaims.

The advantage of the method according to the invention is that it enables the construction of complete piping systems with the same high quality insulation on all parts. So far, pipe fittings had to be such as elbows, T-pieces and the like. In a different way than the straight and smooth pipe pieces z. B. isolated by wrapping the pipe fitting with a bitumen or plastic bandage after its assembly in a piping system, so that no equivalent external protection was given for these molded parts. With coatings according to the method according to the invention, the risk of pitting corrosion due to stray currents in the ground and / or infiltration of the insulation by moisture is no longer present, since the adhesion of the insulation to the pipe and the connection between the layers of insulation are significantly improved.

The invention is explained in more detail using the following exemplary embodiments. The information given about layer thicknesses does not refer to the still powdery layer, but rather to the layer thickness portion of the insulation after melting and cooling.

• Figur 1 eine schematische Darstellung des Verfahrensablaufes für eine Isolierung eines Formkörpers mit einer nur aus Äthylencopolymerisat bestehenden Kleberschicht,
• Figur 2 eine schematische Darstellung des Verfahrensablaufes für eine Isolierung eines Formkörpers mit aus drei Teilschichten bestehender Kleberschicht,
• Figur 3 einen Schnitt durch die Isolierung des Formkörpers gemäß Verfahrensablauf nach Figur 1,
• Figur 4 einen Schnitt durch die Isolierung des Formkörpers gemäß Verfahrensablauf nach Figur 2.

Show it :

• FIG. 1 shows a schematic representation of the process sequence for insulation of a molded body with an adhesive layer consisting only of ethylene copolymer,
• FIG. 2 shows a schematic representation of the process sequence for an insulation of a shaped body with an adhesive layer consisting of three partial layers,
• FIG. 3 shows a section through the insulation of the molded body according to the process sequence according to FIG. 1,
• FIG. 4 shows a section through the insulation of the molded body in accordance with the process sequence according to FIG. 2.

Parts with the same function are provided with the same reference symbols in the drawings.

According to Figures 1 and 3, the surface of a 2.5 m long and 1.5 m wide bitumen, cement or plastic 23 internally coated steel pipe T-piece 17 of 150 mm in diameter by means of a suspension track 1 from the fitting storage 2 to a device 3 transported to steel blasting with steel wire grains and cleaned there. Subsequently, the fitting 17 is heated to 90 ° C. in the warm air oven 4. When the molded part 17 is rotated, which is indicated in the drawing by circular arrows, an epoxy resin / hardener mixture which is approximately at room temperature and is used as precondensate powder in a coating booth 5 with spray guns 6, which mixture is used at a temperature of 145- 155 ° C hardens within 50-70 minutes, sprayed electrostatically in a layer thickness of 30-50 fJ.m.

The epoxy resin / hardener powder mixture, which is supplied to the spray guns 6 from a storage container 11, melts on the surface of the shaped piece 17 and forms a non-flowing and inconsistent film. To produce a closed primer 18, after the electrostatic spraying of the epoxy resin / hardener mixture, the molding is transported to an infrared system 9, in which only the epoxy resin layer 18 is heated to 200 ° C. by infrared radiation for 10 s, while the temperature of the steel body of the T-piece 17 hardly changes.

The curing of the epoxy resin 18 already used is accelerated by the supply of heat. Before the curing has ended, the shaped piece 17 is transported back into the coating booth 5. About 30 s after the application of the epoxy resin layer 18 is - again electrostatically and in powder form - on the epoxy resin layer 18, which then has a temperature of 160-170 ° C, using spray guns 7 in a layer thickness of 150 fJ.m as Sprayed adhesive ethylene copolymer powder. The ethylene copolymer powder, which has a grain size composition according to claim 2, the spray guns 7 is supplied from a storage container 12 after it has been predried at 70 ° C for 1.5 hours.

While the epoxy resin layer 18 and the ethylene copolymer layer 19 form an intimate connection with one another, the molding 17 is transported again to the infrared system 9. The ethylene copolymer layer 19 is heated to 180 ° C., melted and smoothed within 5 minutes by further infrared radiation lasting one minute. This warming can also be done in other ways, e.g. B. can be achieved by microwave radiation. After the subsequent return of the shaped piece 17 to the coating booth 5, polyethylene 20 is sprayed electrostatically from the storage container 13 by spray guns 8 in powder form in a layer thickness of 1.8 mm onto the ethylene copolymer. The molding is then moved back into the infrared system 9, where the coating is kept at a temperature of 180-200 ° C. for 30 minutes by means of infrared radiation.

Instead of infrared radiation, this heating can also in other ways, in which the temperature of the metallic core tube 17 does not rise above 100 ° C, for. B. can be achieved by microwave radiation. During this heating, the epoxy resin primer 18 cures completely. The coated T-piece 17 is then cooled in air to room temperature and transported to the finished store 10.

In the second example according to FIGS. 2 and 4, a 90 ° steel elbow with a diameter of 100 mm and a leg length of 1000 mm is coated. The process sequence, the schematic representation of which is shown in FIG. 2, largely corresponds to that of the first example, so that only the differences are dealt with here. Figure 4 shows a section through the coated fitting.

• Aus einem Behälter 14 wird mit Sprühpistolen 15 in der Beschichtungskabine 5 elektrostatisch eine 75 p.m dicke Schicht 21 aus einem auf 100 °C vorgewärmten Pulver aufgetragen, dessen Körner jeweils einen Kern von max. 50 p.m Durchmesser aus vorgetrocknetem Äthylencopolymerisat aufweisen, der schalenförmig von einer 10-20 µm dicken Schicht aus einem Epoxyharz/Härter-Gemisch umgeben ist. Das Pulver besteht somit zum überwiegenden Teil aus Epoxyharz/Härter-Gemisch. Durch die Vorwärmung des Pulvers beginnt bereits vor dem Auftragen eine Reaktion zwischen den verschiedenen Bestandteilen seiner Körner, die zu einer innigen Verbindung untereinander führen. Im Anschluß an das Auftragen wird diese Schicht 21 in der Infrarot-Anlage 9 innerhalb von 20 s. Bestrahlungsdauer bei 180 °C aufgeschmolzen. Danach wird in der Beschichtungskabine 5 mit den Sprühpistolen 25 aus einem Behälter 16 eine weitere Pulverschicht 22 von 75 µm Dicke elektrostatisch aufgetragen, wobei die Pulverkörner die umgekehrte Zusammensetzung zur ersten Teilschicht 21 aufweisen, nämlich einen Kern von max. 50 µm aus Epoxyharz/Härter-Gemisch.und eine Schale von 10-20 µm Dicke aus vorgetrocknetem Äthylencopolymerisat. Es folgt wiederum eine Infrarot-Bestrahlung in der Infrarot-Anlage 9 von 20 Sekunden Dauer zum Aufschmelzen der Schicht 22 bei 180 °C. Danach wird aus dem Behälter 12 mit den Sprühpistolen 7 in der Beschichtungskabine 5 die dritte Kleberteilschicht 19 von 150 µm Dicke aus reinem Äthylencopolymerisat-Pulver aufgetragen und in der Infrarot-Anlage 9 innerhalb von einer Minute Bestrahlungsdauer aufgeschmolzen.

Since the elbow 24 has no heat-sensitive inner coating, it is heated in the warm air oven 4 to at least 150 ° C. This reduces the time intervals between the individual method steps and in particular the time for the curing of the epoxy resin layer 18. After the epoxy resin base layer 18 has been applied and melted, the adhesive is applied in three partial layers as follows:

• A 75 μm thick layer 21 of a powder preheated to 100 ° C. is applied from a container 14 with spray guns 15 in the coating booth 5, the grains of which each have a core of max. Have 50 pm diameter of pre-dried ethylene copolymer, which is shell-shaped surrounded by a 10-20 µm thick layer of an epoxy resin / hardener mixture. The powder therefore mainly consists of an epoxy resin / hardener mixture. By preheating the powder, a reaction between the various components of its grains begins before application, which leads to an intimate connection with one another. Following the application, this layer 21 is in the infrared system 9 within 20 s. Irradiation time melted at 180 ° C. Then, in the coating booth 5 with the spray guns 25, a further powder layer 22 of 75 μm thickness is applied electrostatically from a container 16, the powder grains having the opposite composition to the first partial layer 21, namely a core of max. 50 µm made of epoxy resin / hardener mixture. And a shell of 10-20 µm thick made of pre-dried ethylene copolymer. This is followed by infrared radiation in the infrared system 9 for 20 seconds to melt the layer 22 at 180 ° C. Thereafter, the third adhesive layer 19 of 150 μm thick made of pure ethylene copolymer powder is applied from the container 12 with the spray guns 7 in the coating booth 5 and melted in the infrared system 9 within one minute of irradiation.

The coating process is ended as in the first example with the application of a 1.8 mm thick layer 20 of polyethylene powder, the 30-minute heating to 180-200 ° C. and the subsequent cooling in air. In contrast to the first case, other externally acting heat sources, e.g. B. hot air or a combination of hot air and infrared radiation can be used, since the metallic base body of the fitting 24 has no heat-sensitive inner coating and may be brought to higher temperatures without damage.

However, it must be taken into account that the time sequences have to be adjusted accordingly due to the different heat transfer conditions.

For the coating of straight, smooth pipes, instead of the discontinuous procedure for fittings, a continuous procedure using processing stations connected in series is recommended. Because of the simple surface geometry, the known tube extrusion process or the winding process with extruded polyethylene film can then also be used for the application of the final polyethylene layer. In addition, instead of cooling in air to accelerate the processes, the use of a water cooling bath or spray water is expedient.

The coating process for the steel tube body 17 in the first example leads to a three-layer insulation structure, as shown in FIG. 3. The special grain size composition of the adhesive powder results in an intimate bond between the epoxy resin base layer 18 and the adhesive 19. This effect is a consequence of the fact that, under the influence of the electrostatic field, the powder grains with the smallest diameter preferentially lie directly on the epoxy resin base layer 18 can deposit, which react faster than larger grains with the epoxy resin base layer 18.

If, instead of the powder with the special grain size composition according to claim 2, a mixture of ethylene copolymer powder and a powdery epoxy resin / hardener mixture according to claim 3 is used as the adhesive, there is an even more favorable connection of the thermosetting base layer and the thermoplastic adhesive, since the electrostatic field results in Epoxy resin powder grains should preferably be deposited on the surface of the base layer. In addition, due to the greater specific weight of the epoxy resin compared to the ethylene copolymer, the epoxy resin content tends to sink downward onto the epoxy resin base layer when it melts. This creates a smooth transition between the different materials.

An even more favorable behavior results in coatings according to the second example with a formally 5-layer insulation structure (FIG. 4). The individual sub-layers 21, 22, 19 of the adhesive between the epoxy resin base layer 18 and the polyethylene cover layer 20 allow an even more uniform transition from the thermosetting (epoxy resin) to the thermoplastic area (ethylene copolymer). The different specific weights of the ethylene copolymer and epoxy resin during the melting of the applied partial layers 21, 22, 19 in turn have a positive effect on a smooth transition of the different materials within and between the partial layers 21, 22, 19.

The improvements which result from the process according to the invention compared to the prior art which relates to the coating of steel pipes can be seen from the table below.
(See table on page 5 f.)

Claims (13)

1. Method for the coating of metallic shaped bodies, in particular pipes and tubular shaped parts, with polyethylene, whereby the surface of the cleaned shaped body, which has been heated to at least 80 °C, is firstly primed with an epoxy resin/hardener mixture, then ethylene copolymer is applied thereon as adhesive and subsequently polyethylene is applied as external cover layer, after which the coated shaped bodies are cooled down to ambient temperature, characterized by the following steps :

a) the epoxy resin/hardener mixture, as a pre-condensate powder which hardens at a temperature of from 145-155 °C within 50-70 minutes, is sprayed electrostatically onto the surface of the shaped body in a quantity producing a layer thickness of from 30 to 50 µm and is then heated to a temperature above 150 °C by means of a heat source which acts from the outside, until the chemical reaction products thereby arising have volatilized ;

b) during the hardening of the epoxy resin layer which is thereby setting in, pre-dried ethylene copolymer powder is sprayed electrostatically onto the primary layer in one or more layers, which together produce a layer thickness of at least 150 µm, and after the application of each individual layer it is heated to a temperature of at least 180 °C by a heat source which acts from outside, and is melted on :

c) the polyethylene is then either sprayed electrostatically in powder form onto the heated ethylene copolymer layer in a quantity producing a layer thickness of at least 1.8 mm and is then melted on at a temperature between 180 and 200 °C by heat which is applied from the outside, or in the coating of pipes it is extruded as a tubular or winding foil and is applied onto the surface.

2. Method according to Claim 1, characterized by application of an adhesive of ethylene copolymer powder with a grain size composition of

approx. 70 % with 30 µm grain size,

approx. 20 % with 20 µm grain size and

approx. 10 % with 10 µm grain size.

3. Method according to Claim 1, characterized in that as adhesive, a mixture of an ethylene copolymer powder and a pulverulent epoxy resin/hardener mixture, the weight proportion of which amounts to at least 30 %, is applied in a quantity producing a layer thickness of at least 75 µm.

4. Method according to Claim 1, characterized in that as adhesive, an ethylene copolymer powder, which is pre-heated to 100 °C, is applied in a quantity producing a layer thickness of at least 75 µm, the grains of which are surrounded in each case in a shell-shaped manner by an epoxy resin/hardener mixture.

5. Method according to Claim 1, characterized in that as adhesive an ethylene copolymer powder, which is pre-heated to 100 °C, is applied in a quantity producing a layer thickness of at least 75 wm, the grains of which in each case have a core of an epoxy resin/hardener mixture, which is surrounded in a shell-shaped manner by the ethylene copolymer.

6. Method according to Claim 4 or 5, characterized in that as adhesive a powder formed from ethylene copolymer and epoxy resin/hardener mixture, the grains of which in each case have a core of a maximum of 50 µm diameter and in each case have a shell of 10-20 p.m thickness, is applied.

7. Method according to Claim 1, characterized in that after application, the epoxy resin/hardener mixture is preferably heated up to 190-210 °C.

8. Method according to one or more of Claims 1-7, characterized in that the individual layers are heated by means of infrared radiation and/or hot air.

9. Method according to one or more of Claims 1-8, characterized in that before application, the ethylene copolymer powder component is predried in each case for 1.5 hours at 70 °C.

10. Method according to one or more of Claims 1-9, characterized in that a metallic shaped body which is provided with a heat-sensitive internal coating, is heated immediately prior to priming to a temperature of a maximum of 100 °C.

11. Method according to one or more of Claims 1-9, characterized in that a metallic shaped body without a heat-sensitive internal coating is heated immediately prior to priming to a temperature of a maximum of 200 °C.

12. Method according to one or more of Claims 1-9, characterized in that a metallic shaped body without a heat-sensitive internal coating is firstly heated to 170 °C and immediately prior to priming is heated in the surface region by means of infrared radiation to 200 °C.

13. Method according to Claims 1, 2 and 4-6 and one or more of Claims 7-12, characterized in that the adhesive layer is applied in partial layers between the epoxy primary layer and polyethylene cover layer in the following sequence :

a) In a quantity producing a layer thickness of 75 µm, the powder consisting of ethylene copolymer cores with surrounding epoxy resin/hardener mixture shell is applied.

b) In a quantity producing a layer thickness of 75 µm, the powder consisting of epoxy resin/hardener mixture cores with surrounding ethylene copolymer shell is applied.

c) In a quantity producing a layer thickness of 150 µm, ethylene copolymer powder is applied.

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