NO760572L - - Google Patents

Description

It is known that rusting of steel surfaces can be prevented by applying alkylimidazolines with less than 20 carbon atoms dissolved in an organic solvent to the steel surface after the last passage of the rolling stock through the rolling mill continuously during layer formation. Such prepared steel surfaces can be used as a basis for paint coats and plating.

It is also known that metal surfaces can be coated with plastic. In order to achieve good adhesive strength, however, it is first necessary to equip the metal surfaces with a primer or to subject them to a special chemical treatment. The primer or the chemical treatment of the metal surfaces is thus an additional process step, when you want to coat metal surfaces with plastics to improve the corrosion properties.

There was thus the task, in a one-step process, of equipping metal surfaces, especially those made of metal pipes with a well-adhering plastic coating.

It has now surprisingly been found that this task can be solved when one carries out the method for coating metal pipes with a duroplastic protective layer by applying powder varnish of cyclic amidines and 1,2-epoxide compounds with more than 1 s2-epoxide group in the molecule as well as the usual additives Qg curing of the powder layer at an elevated temperature5 so that the powder coating is electrostatically applied to the metal tubes heated to 200-300°C and the coating is cured at the heat capacity of the tube without additional input of heat energy and that imidazoline derivatives with the general formula where R stands for hydrogen are used as cyclic amidines. an alkyl or aryl residue, R' means a cycloalkyl, heterocycloalkyl or R residue, R" means an optionally alkyl- or aryl-substituted alkylene or arylene residue and X means hydrogen or a

and the 1,2-epoxide compounds have a lower melting point of over 60°C.

For the coating of metal pipes, especially steel conduit pipes with heat-setting powder systems, the temperature brought to the pipes during the manufacturing process in the cooling zone of 300 - 200°C, in special cases of 270-240°C, is used for curing the powder coating layers.

The application of the powder particles can take place with the usual methods of electrostatic charging, the air-static, but also exclusively with pneumatic power. Due to the heat present, the binding agent's melting process starts immediately after impact with steel sheet, with which the network formation reaction is started.

Very short reaction times are required for many purposes for external coating. During the production process, there is less than a 1-minute dwell time between the powder application and the subsequent mechanical stress on the cured coating by gripping tools, lifting and rolling devices. This period of time must be sufficient for the complete net formation of the powder coating.

At the end of this short curing time, direct treatment with cold water takes place for faster cooling of the coated pipes.

The particular advantage of the present invention lies in the fact that this limited period of time was exceeded by simultaneous improvement of the film properties. As it concerns the application of a single-layer system, layer thicknesses of at least 300 μm are required, which, according to the current state of the art, cannot always be achieved without causing surface disturbances or runoff phenomena.

For the production of the finely divided mixtures to be used as powder coating, 1,2-epoxide compounds with at least one 1,2-epoxide group in the molecule and a lower melting point of over 60°C are suitable. Compounds that correspond to this characteristic are, on the one hand, polyepoxide compounds that are solid at 60°C and below, as this includes higher molecular compounds (so-called solid resins), and those that, due to their symmetrical structure or the size of the carbon systems bound to the 1,2-epoxide group are fixed and, secondly, such as have been produced by reaction of liquid 1,2-epoxide compounds with more than one epoxide group per molecule with primary or secondary amines in such amounts that the adduct contains on average at least one 1,2-epoxide group per molecule (so-called adduct hardeners).

1,2-epoxide compounds can be saturated as well as unsaturated as well as aliphatic, cycloaliphatic, aromatic and heterocyclic. They can also contain such substituents which do not interfere with side reactions under the mixing and reaction conditions. No side reactions produce, for example, alkyl or aryl substituents, hydroxyl groups, ether groups and the like. The suitable compounds belong to the following substance classes: Epoxides of mono- or polyunsaturated hydrocarbons, halogen-containing epoxides, epoxy ethers of mono- or polyhydric alcohols and phenols, epoxy esters of mono- or polybasic acids and N-containing epoxides.

These are, for example, the solid, polymeric polyglycidyl polyethers of 2,2-bis-(4-hydroxyphenyl)-propane, which are obtained, for example, by reacting 2,2-bis-(4-hydroxyphenyl)'-propane with epichlorohydrin in molar ratios from 1:1.9 to 1.2 (in the presence of an alkali potassium hydroxide in an aqueous medium. Polymeric polyepoxides of this type can also be obtained by reacting a polyglycidyl ether of 2,2-bis-(4-hydroxyphenyl)- propane with less than the equimolar amount of divalent phenol, preferably in the presence of a catalyst, such as a tertiary amine, a tertiary phosphine or a quaternary phosphonium salt. The polyepoxide can also be a solid epoxidized polyester which, for example, is obtained by reaction of a polyhydric alcohol and/or a polybasic carboxylic acid or its anhydride with a low molecular weight polyepoxide Examples of such low molecular weight polyepoxides are the liquid diglycidyl ether of 2,2-bis-(4-hydroxyphenyl)-propane, diglycidyl phthalate , diglycidyl hexahydrophthalate, diglycidyl maleate and 3,4-epoxycyclohexylmethyl sters of 3,4-epoxycyclohexanecarboxylic acids.

Mixtures of solid polyepoxides can also be used, e.g. a mixture of a polyepoxide whose melting point is between 120 and 160°C and a polyepoxide with a melting point between 60 and 80°C (the melting point is determined according to the mercury method according to Durran). Suitable compositions contain between 30 and 50% by weight of a solid polyglycidyl ether of 2,2-bis-(4-hydroxyphenyl)-propane having an epoxy equivalent weight between 1650 and 2050 and a melting point of 120 to 160°C and between 50 and 70% by weight of a solid polyglycidyl polyether of 2,2-bis-(4-hydroxy-phenyl)-propane having an epoxy equivalent weight between 450 and 525 and a melting point of 60 to 80°C.

If a high epoxy functionality is desired, then a preferred polyepoxide is the polyglycid ether of 1,1,2,2-tetra-(4-hydroxyphenyl)-ethane.

Epoxidized polybutadienes can also be used for this purpose.

As already mentioned earlier, in addition to the so-called solid resins, adduct hardeners are also suitable for carrying out the method according to the invention. Such solid adduct hardeners can for example be produced from liquid polyepoxides of polyunsaturated hydrocarbons such as vinyl cyclohexene, dicyclopentadiene and the like, epoxy ethers of polyhydric alcohols and phenols etc. and aliphatic, cycloaliphatic and aromatic diamines. The prerequisite for the suitability of such an adduct is that here, too, the lower melting point is above 60°C.

As well as the pure epoxy compounds, their mixtures can also be used, for example those of mono- and

polyepoxides, by the method according to the invention.

Suitable imidazoline derivatives within the scope of the invention are 2-methylimidazoline, 2,4-dimethylimidazoline, 2-ethylimidazoline, 2-ethyl-4-methylimidazoline, 2-benzyl-imidazoline, 2-phenyl-imidazoline, 2-phenyl-4-methyl-imidazoline, 2-(o-tolyl)-imidazoline, 2-(p-tolyl)-imidazoline, 1,4-tetramethylene-bis-imidazoline, 1,1,3_trimethyl-1,4-tetramethylene-bis-imidazoline, 1,3, 3-trimethyl-1-1,4-tetramethylene-bis-imidazoline, 1,1,3-trimethyl-1-1,4-tetramethylene-bis-4-methylimidazoline, 1>3,3-trimethyl-1-1,4-tetramethylene-bis- 4-methylimidazoline, 2-(m-pyridyl)-imidazoline, 2-(p-pyridyl)-imidazoline, 1,2-phenylene-bis-imidazoline, 1,3-phenylene-bis-imidazoline, 1,4-phenylene- bis-imidazoline, 1,4-phenylene-bis-4-methylimidazoline and several others. According to the invention, mixtures of these imidazoline derivatives can also be used. Of these groups of imidazolines, 1,4-tetramethylene-bis-imidazoline and 2-phenyl-imidazoline are particularly preferred. These compounds are used as the only hardeners or network former for the 1,2-epoxide compounds.

The imidazoline derivatives mentioned above are used in such amounts that there is 3-10% by weight, preferably 4-8% by weight, referred to the amount of 1,2-epoxide compounds.

To improve the paint's flow properties, so-called flow agents are added during preparation. These means may be chemical compounds or their mixtures of very different chemical type, e.g. polymeric or monomeric compounds, acetals, such as polyvinyl formal, polyvinyl acetal, polyvinyl butyral, polyvinyl acetobutyral resp. di-2-ethylhexyl-i-butyraldehyde acetal, di-2-ethylhexyl-n-butyraldehyde acetal, diethyl-2-ethylhexanol acetal, di-n-butyl-2-ethylhexanol acetal, di-i- butyl-2-ethylhexanol-acetal, di-2-ethyl-hexyl-acetaldehyde-acetal and the like, ethers such as the polymeric polyethylene and polypropylene glycols, mixed polymers of polyethylene and polypropylene glycols, mixed polymers of n-butyl acrylate and vinyl isobutyl ether , ketone-aldehyde condensation resins, solid silicone resins or mixtures of zinc soaps, of fatty acids and aromatic carboxylic acids and the like. For this purpose, products such as Modaflow are also offered in the trade, whose chemical nature is unknown to consumers and of which it is only known that it is a complex, polymeric, active liquid. Such flow agents can be contained in the mixture in amounts of 0.2 - 0.5% by weight, referred to the total amount of powder coating.

The other common constituents of the powder coating mixture such as pigments, dyes, fillers and the like, with reference to the amount of 1,2-epoxide compound and hardeners, can fluctuate within a wide range. They can comply with the requirements for the quality of the covers.

Before their use, the powder coating components are well mixed at temperatures below the curing temperature, extruded and then painted. For the practical application, a particle size of less than 100 µm is preferably sought, the maximum particle size should be between 30 and 50 µm.

The application of the powder coatings to the metal pipes to be coated takes place according to known methods, e.g. by means of electrostatic powder spraying or vortex sintering etc.

The type of metal or the alloy is not critical for the application of the method. Preferably, this method can be used for coating iron and steel pipes. Coating of the inner sides can be carried out in the same way, as appropriate the powder coating is applied to the inside and outside at the same time. General description of the coating methodology used.

The imidazoline derivatives that can be used according to the invention were prepared with the other powder coating components according to known methods, as described for example in DOS 2,248,776 to ready-to-spray powder. The application of this powder to the sandblasted steel conduit pipes, which were heated to a temperature of 240 - 270°C, took place using the electrostatic method under the following conditions:

To ensure an even powder application, the tube was kept in a rotating motion during the coating process. The immediately starting curing reaction was finished after 45 seconds. The tube was then sprayed with cold water and quenched to a temperature of 25°C. The cured and then duroplastic coating has layer thicknesses of 300-350 µm (electromagnetic measurement). With the coatings carried out, a series of investigations are then carried out which are indicated by the examples in question.

Example 1.

1,4-tetramethylene-bis-imidazoline was prepared with titanium dioxide, chrome oxide green, chrome yellow, oxide red, oxide black, the indicated epoxy resin and a small addition of flow agent in the following ratio to powder coating resp. sinter powder:

la Composition of the powder coating.

Paste epoxy resin, on the basis of a

adduct of 2,2-bis-(4-hydroxyphenyl)-

propane (diane) and epichlorohydrin, which

was subjected to an HCl cleavage, and

was then traded with further dian

and as specified by the manufacturer

has an epoxy equivalent weight of 900-

1000, which corresponds to an epoxy value

of 0.10-0.11 and a melting range of

Fluid (polyacrylic acid - n - butyl ester with a K-value of 30-35): 0.4 wt#

This formulation was applied to 270°C hot tubes and a test rod, quenched after 45 seconds with water and examined.

lb The properties of the coating.

<*>Analogous to the pipe coating, a steel round of 8 mm thickness was coated and cooled. For examination of the coating, the bending took place over a cylindrical body of 3 times (= 24 mm) the diameter of the rod.

Example 2.

1,4-tetramethylene-bis-imidazoline was prepared with the pigments according to example 1, the indicated epoxy resin and a small addition of flow agent in the following ratio to powder lacquer resp. sinter powder:

2a The composition of the powder coating._

Solid epoxy resin, based on an adduct

of 2,2-bis-(4-hydroxyphenyl)-propane (diane) and epichlorohydrin, which was subjected to an HCl-

separation and as per the manufacturer's

environments have an epoxide equivalent weight in the

advised at 700-875j which corresponds to an epoxide

value of 0.142-0.114 and a melting range of

This formulation was sprayed onto 240°C hot tubes and a test rod, quenched after 45 seconds with cold water and examined.

2b The properties of the coating.

Example 3-

2-phenyl-imidazoline was prepared with titanium dioxide, the indicated epoxy resin and, a small part of a flow agent in the following ratio to powder lacquer or. sinter powder:

3a The composition of the powder coating.

Paste epoxy resin, on the basis of a

adduct of 2,2-bis-(4-hydroxyphenyl)-propane (diane) and epichlorohydrin, which

was subjected to an HCl cleavage and

was then traded with further

dian, and which according to the manufacturer has an epoxide equivalent weight of 900 - 1000, which corresponds to an epoxide value of 0.10 -

This formulation was applied to 250°C hot tubes and the test rod and was quenched for 45 seconds with cold water and examined.

3b The properties of the coating.

Claims (4)

1. Method for coating metal pipes with a duroplastic protective layer by applying powder coating of cyclic amidines and 1,2-epoxide compounds with more than one 1,2-epoxide group in the molecule as well as the usual additives and hardeners of the powder layer at elevated temperature, characterized in that the powder coating is applied electrostatically to the metal tubes heated to 200-300°C and the coating is cured at the heat capacity of the tubes without input and additional heat energy and that imidazoline derivatives with the general formula c are used as cyclic amidines in which R means hydrogen, an alkyl or aryl residue, R' a cycloalkyl, heterocycloalkyl or R residue, R" an optionally alkyl- or aryl-substituted alkylene or arylene residue and X hydrogen or one and the 1,2-epoxide compounds have a lower melting point greater than 60°C. 2. Process according to claim 1, characterized in that the imidazoline derivatives are used in such amounts that there is 3-10% by weight, preferably 4-8% by weight, referred to the amount of 1,2-epoxide compounds. 3. Method according to claims 1 and 2, characterized in that the imidazoline derivative is 1,4-tetramethylene-bis-imidazoline. 4. Method according to claims 1 and 2, characterized in that the imidazoline derivative is 2-phenyl-imidazoline.