GB1565379A

GB1565379A - Resin coated metal substrates

Info

Publication number: GB1565379A
Application number: GB52855/76A
Authority: GB
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1975-12-19
Filing date: 1976-12-17
Publication date: 1980-04-16
Also published as: ZA767498B; NO764302L; AR217820A1; CH628534A5; NL7614033A; AT367087B; SE7614259L; ATA939876A; FR2335572B1; BE849544A; DE2557278A1; IT1065563B; JPS5276335A; AU2060376A; CA1068997A; DE2557278C2; ES454396A1; DK571076A; AU511935B2; FR2335572A1

Abstract

Metal articles, in particular metal pipes, metal containers and large metal parts, preferably large pipes, are coated with cured epoxy resin compositions, the coating comprising a cured coating containing A) a solid epoxy resin based on 4,4'-diphenylolpropane and/or 4,4'-diphenylolmethane and epichlorohydrin, B) a curing catalyst and C) thixotropic agents, where the curing catalyst B) contains at least one compound B1) of the formula (I) or (II) <IMAGE> in which R is hydrogen, or B2) an adduct of the compounds B1) or substituted compounds of these formulae in which R is as defined in Claim 1 with low-molecular-weight epoxy resins having an epoxide equivalent weight of from 50 to 2000, or B3) a mixture of component B1) or B2) with the abovementioned substituted compounds of the formulae (I) and (II). The metal articles are coated with a pulverulent paint based on highly reactive curable epoxy resins which gives, in particular, toxicologically acceptable, heat- and chemicals-resistant, electroinsulating coatings. During this operation, the articles are heated to a temperature above the melting point of the resins, but sufficient for curing the epoxy resins, the resins are applied to the hot surface, which has already been cleaned before the heating, in the form of pulverulent paints by the electrostatic spray process or the whirl sintering (fluidised-bed sintering) process, melted to form a uniform film and cured directly without a further operation. During the application and curing process of the film, cracking phenomena of the organic components of the paint are not observed, in spite of the high temperature of the substrate, so that neither discolouration of the coating nor impairment of the properties occurs. The coatings are distinguished by the absence of pores and the high impact strength and by resistance to water and aggressive liquids.

Description

(54) IMPROVEMENTS IN OR RELATING TO RESIN COATED METAL SUBSTRATES (71) We, HOECHST AKTIENGESELLSCHAFT, a Body Corporate organised under the laws of the Federal Republic of Germany, of 6230 Frankfurt/Main 80, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to coated metal pipes, metal containers and large metal articles, preferably large pipes, and a process for coating them, with a coating composition of low toxicity based on highly reactive hardenable epoxy resins and producing heat-resistant and chemical-resistant electrically insulating coatings.

It has been proposed to provide pipes, particularly large metal pipes, with an outer coating of bitumen or high pressure polyethylene. Since high pressure polyethylene is very soft, good protection from mechanical stress, particularly blows, knocks and abrasions, is difficult to obtain. In this respect, bitumen coatings have even less favourable qualities. Damage to the coating may cause corrosion of the metal and, for reasons of safety, make large pipes unusable for the transportation of natural gas, mineral oil, petrochemical products, hot water, waste water and other gaseous or liquid chemical substances, owing to corrosion.

Furthermore, it has been proposed to coat pipes with a layer of a harden able resin, for example a mixture of epoxy resin and coal tar asphalt in which coarsegrained fillers are embedded. In addition, a method has been described for covering pipes with multiple coatings, wherein a preheated pipe rotating about its longitudinal axis is provided with a liquid thermosetting coating mixture and is subsequently hardened in a separate hardening process.

It is also been proposed to use pulverulent hardenable epoxy resin compositions containing, as curing agents, aromatic amines, 2-methyl-4ethylimidazole, acid anhydrides, dicyanodiamide or modified dicyanodiamides, i.e.

dicyanodiamides activated by small quantities of accelerators. These so-called accelerators, which affect the hardening rate of the curing agents, are, for example, mixture of carboxylates of the metals lead, iron, cobalt, manganese, zinc or tin, with carboxylic acids or the anhydrides thereof or adducts of epoxy resins with imidazole derivatives, e.g. 2-methyl-4-ethylimidazole.

The hardenable epoxy resin compositions used hitherto, however, have the disadvantage that they are not heat resistant and, if the epoxy layer is damaged, the effect of warm alkali solutions, or hot water or hot steam leads to a loss of adhesion at the junction of the metal and the epoxy resin coating, owing to undermining of the coating. This sensitivity to alkalis is important, for example, in building projects where materials within alkaline reaction, such as, for example, lime and cement, are always present.

A coating moreover often contains pores, i.e. microscopic cavities which extend as far as the metal substrate. These pores are formed, for example, by the occurrence of mainly gaseous products, when the crosslinking process does not proceed totally satisfactorily. These microscopic cavities are very often the cause of so-called point corrosion of the metal wall. The above-mentioned disadvantages are especially undesirable in the process for coating large pipes, since they lead to so-called delayed damage which, for economic and technical reasons, is unacceptable. In addition the epoxy resin compositions used hitherto often contain toxic components, such as aromatic amines as hardeners or lead-containing compounds as accelerators.

It has also been proposed to use heterocyclic compounds with 5 to 9 atoms in the ring system and containing a substituted imino group, -C=N-C, and a secondary imino group, as hardeners for epoxy resins. Such mixtures may be used in various applications in the form of solutions or pastes. There has been no proposal to use them in powdered compositions for coating pipes and containers.

However, when metal substrates are being coated, it is necessary to use selected combinations and a special method, in order to overcome the difficulties which have occurred hitherto.

Metal pipes and metal containers coated with hardened epoxy resin compositions, together with a process for coating them, have also been proposed.

The coating consists of a hardened coating obtained from A) a solid epoxy resin based on 4,4'-diphenylolpropane and/or 4,4'-diphenylolmethane and epichlorohydrin; B) thixotropic agents: optionally C) pigments; and D) 1 to 12 ", based on the weight of the epoxy resin, of a hardening catalyst of the formula

(wherein R represents an alkyl group with 1 to 6 carbon atoms, an aromatic hydrocarbon radical with 6 to 10 carbon atoms or a benzyl group).

We have now found that improved chemical and physical properties can be obtained if compounds (I) or (II) wherein R is a hydrogen atom, i.e. the unsubstituted imidazole and/or 2-imidazoline are used as hardening catalysts D) in the mixtures used to prepare these coatings.

We have also found that adducts of the compounds (I) or (II) as hereinbefore defined (wherein R represents a hydrogen atom) and/or of the known hardening catalysts of formulae (I) and (II) with low molecular weight epoxy resins prepared from 4,4'-diphenylolpropane or 4,4'-diphenylolmethane and epichlorohydrin with an epoxy equivalent weight of from 50 to 2,000, preferably 100 to 500, in particular 185 to 195, produce equally good results. Thus according to one aspect of the present invention, there is provided an article comprising a metal substrate in the form of a pipe, a container or large metal article, said substrate being coated with a hardened epoxy resin composition comprising A) a solid epoxy resin based on epichlorohydrin and 4,4'-diphenylolpropane or 4,4'-diphenylolmethane or both, B) from 0.1 to 12 ,E, by weight based on the weight of component A), of B,) a compound of formula

(wherein R represents a hydrogen atom) or B2) an adduct of at least one compound of the formulae (I) and (II) (wherein R represents a hydrogen atom, or an alkyl group having 1 to 6 carbon atoms, or a hydrocarbon group having 6 to 10 carbon atoms and having at least one aromatic moiety) with an epoxy resin of low molecular weight having an epoxy equivalent of from 50 to 2.000; C) a flow agent; and D) a thixotropic agent.

When the compounds according to the invention are used, it is possible to reduce the hardening temperature and/or the hardening time. Therefore, even articles with relatively thin walls, for example, can be coated using the method according to the invention, since the heat capacity of the articles to be coated can be lower owing to the high reactivity of the hardeners. Moreover, the quantity of catalyst used can be decreased.

It is advantageous to use, as component B2), adducts having a ratio of 0.7 to 1.3 epoxy groups to each secondary amino group If adducts of this kind are used, the quantity of catalyst may be further reduced. Moreover. mixtures of components B1) or B2) with the known substituted compounds of formula (1) and (I1) may also, if desired, be used with a high degree of success, for example mixtures containing the known substances in amounts of up to 90, preferably up to 60", by weight, based on the total weight of hardening catalysts B). B- varying the hardening component used, epoxy resin compositions can be obtained which have optimum gel times for the particular application desired.

Suitable alkyl groups for the group R in the known hardening catalysts of formulae (I) and (II), are, for example, methyl, ethyl, n-propvl, n-butyl and n-hexyl, the isomers thereof, such as isopropyl, isobutyl, tert.-butyl, and phenyl. benzyl and the various tolyls. The substituents are preferably in the 2-position. An especially preferred compound is 2-phenyl-2 imidazoline as the compound of formula (II).

The quantity of the hardening catalysts B) including the adducts B2) is at least 0.1"" by weight, preferably at least lq/,, e.g. up to 100,,. particularly up to 5 ", but in individual cases up to 12 /" by weight, based on the weight of epoxy resin.

The epoxy resin component A) of the coating used for articles according to the invention desirably has an epoxy equivalent weight of from 600 to 2,000, preferably 700 to 1,500 and advantageously from 875 to 1,100.

The addition of a flow agent to the mixtures brings about an improvement in the flow properties and at the same time ensures better wetting of the substrate and any pigments which may be present. The quantity of flow agent used is generally 0.1 to 1.5%, preferably 0.3 to 1% by weight based on the total of resin and hardeners. Suitable substances include, for example, polyvinyl butyral, silicone oils or resins and a flow agent concentrate comprising an epoxy resin and a polyacrylic acid ester such as that sold under the name Modaflow (Monsanto).

The epoxy resin compositions according to the invention may be generally produced from the components in a finely divided state, e.g. in an extruder suitable for duroplastic compounds, so that, in addition to a substantial improvement in the homogeneity of the mixtures of starting materials, at least partial addition of the hardening catalysts to the epoxy resin occurs in the molten phase. After extrusion, which lasts only about 15 to 30 seconds, for example, the reaction is immediately stopped by cooling, to prevent further polymerisation of the molecules. After cooling, the epoxy resin composition may be ground to a powder with a maximum particle size of 60 to 100 microns.

To improve the mechanical properties of the coating compositions, it is sometimes appropriate to use mixtures of epoxy resins each having different epoxy equivalent weights. The proportion of resins with an epoxy equivalent weight of 1,500 to 2,000 is advantageously more than 5 X" by weight, but conveniently should not exceed 20% by weight, in order to prevent the flow properties from deteriorating.

The coatings according to the invention may be uncoloured or coloured with dyes or pigments, for example, those conventionally used for metal pipe coatings.

Preferably, lead-free pigments resistant to high temperatures are used in low concentrations, for example titanium dioxide and chromium oxide green and which are resistant to water, acids, alkalis, soil, lime, cement and so on. The concentration of pigment is for example, up to 40, preferably up to 20 ' of the total mixture and is selected so that the mechanical properties of the coatings are affected as little as possible.

To increase the viscosity of the molten coating compositions on the surface of the substrates and thus to obtain an even film, finely divided silica may, if desired, be added to the enamel composition as component D), e.g. in quantities of from I to 5, preferably from 2 to 3% by weight, based on the total weight of the composition. On the other hand, the use of fillers is unnecessary and undesirable.

Pigment concentrations of more than 40 /' by weight of the total mixture are to be avoided, since an excessively high pigment content results in a lower chemical and mechanical resistance of the pulverulent coating composition with the result that the pigments are no longer wetted satisfactorily and the homogeneity of the coatings deteriorates.

The coating operation for the substrates takes place according to known methods, e.g. by heating the metal articles to a temperature above the melting point of component A) of the resins and sufficient to cure the epoxy resins, in an induction oven, by means of gas burners or by any other suitable method, to temperatures of, for example, 180 to 330"C, and preferably 210 to 3000C. The resins are then applied in the form of a pulverulent coating composition, using a powder spray process, preferably an electrostatic spray process, or a whirl sintering process, to the hot surface which has been cleaned before heating in conventional manner, e.g. by sandblasting, then melting the resins to form an even film and hardening them immediately without any further treatment. Owing to the increased reactivity of the hardener substances according to the invention, the properties required for the substrate coatings can be obtained at a substrate temperature of as little as 180"C, and preferably from 2100C upwards. The heat capacity of the heated substrate is sufficient to crosslink the highly reactive epoxy resin compositions in a short time, for example, in less than a minute, without any further heating.

The resin powders applied to the substrates according to the invention preferably have a maximum particle size of from 80 to 100 microns but may if desired be up to 300 microns.

Surprisingly, during the application and hardening process of the coating, no cracking of the organic components of the coating composition has been observed, despite the high temperature of the substrate, which means that there is no discolouration of the coating nor any deterioration in its properties.

For economic reasons, pipes and containers are generally coated mainly on the outer surfaces. However, it is also possible to coat only the inner surfaces or both the outer and inner surfaces. Thus, another range of applications for the coated substrates is opened up, since in the case of inner coatings only the durability of the coating is important and damage resulting from knocks or blows occur only rarely. The high thermal resistance of the coatings produced according to the invention, which are permanently resistant to temperatures of up to 1 300C or 140"C, for example, is then advantageous.

A coating thickness of 100 to 2,000 microns is generally adequate for the articles according to the invention, to meet the requirements of particular conditions, However, it is also possible to apply thicker layers.

Examples of large articles made of metal are carriers, pillars, transums, supports or constructional parts for load-bearing constructions, i.e. all large metal articles which require particular protection from corrosion of whatever kind in the atmosphere, in the ground or in water. The advantage of the coating according to the invention lies in the fact that these articles can be assembled in their untreated state for the intended use, provided with rivet holes, for example, and then coated.

In this coating process, all the sides are evenly provided with the coating, including the rivet holes drilled beforehand. Owing to the high impact strength of the coatings, there is no risk of the coating chipping off at the rivet points and in the bore holes, which means that here, too, there is complete protection from corrosion.

However, the invention is primarily directed to large pipes. This is taken to mean pipes having an internal diameter of 100 mm ormore, in practice generally between 300 and 1,600 mm in diameter and more. The thickness of the walls of such pipes may vary within wide limits. Depending on the diameter of the pipes it may be from 1.5 to 25 mm or more, preferably 4 to 15 mm, in particular 6 to 10 mm.

Such pipes may be used for transporting petrochemical products, solid, gaseous and liquid substances and materials at different temperatures, above ground, in the ground or in water.

Large containers with a capacity of at least 1 m3 are also preferred. Naturally, it is also possible to coat containers having smaller dimensions but, owing to their wall thickness and/or the mass of the metal, have a heat capacity such that there is a sufficient quantity of heat available to effect the curing process of the pulverulent resins.

The metals from which the coated articles are made include all the metals conventionally used for the production thereof, e.g. copper, tin, zinc, iron or the alloys thereof, such as steel and brass, iron being preferred.

Test articles coated according to the invention have outstanding values in tests which we have conducted, these values being shown in Table I. The test articles used, consisted of pipe sections, and the pulverulent coating compositions applied under the usual conditions to pipes measuring 12 m in length, 100 mm in diameter.

and with walls 4.6 mm thick, by first pretreating these pipes with steel shot STS 20 to the rust free level No. 1 according to DIN 18364. The pipes were then heated by means of ring burners or series burners arranged in a star shape, whilst advancing and rotating the pipes, until they had a temperature of 250+100C over the entire length of the pipes. The subsequent electrostatic application of the pulverulent coating composition to a layer thickness of 300 microns was effected in the usual way. The heat capacity of the large heated pipes was totally sufficient to bring about chemical crosslinking, so that the film properties mentioned in Table I were obtained.

The following tests were carried out, using the same test articles provided with lattice cuts in tests A) to D).

A) Storage in IN NaOH solution at 50"C Samples of the above pipe sections, having a coating of the epoxy resin composition according to Example 1, 300 microns thick are damaged using the lattice cut method according to DIN 53151. The cut edges formed by the lattice cut method are not sealed. The samples are then stored in a 1N NaOH solution at 50"C for six months. The loss of adhesion at the junction between the metal and the coating is then tested (evaluated according to DIN 53151).

B) Boiling test in distilled water The test is carried out as an alternating boiling test over 10 cycles. One cycle comprises heating for 20 hours at boiling temperature and storing at room temperature for 4 hours. The formation of bubbles (evaluated according to DIN 53209) and the loss of adhesion at the junction of metal and coating (evaluated according to DIN 53151) is tested.

C) Boiling test in tap water Instead of distilled water, tap water of pH 6.9 with a carbonate hardness of 13.7"do and a constant hardness of 10.9"do is used.

D) Bending test according to DIN 53152 and DIN 1605 Bending pegs of 20, 30 and 40 mm diameter are used. The tests are carried out at -5"C, +23"C, +502C and +130"C. In further tests, undamaged sample coatings are tested.

E) Freedom from pores according to Preliminary Standard DIN 30670 The test articles are scanned with an electrode giving a test voltage of 5 kV+5 kV per mm thickness of coating. The freedom from pores on the coated surface, not on the cut edge, is estimated.

F) Impact test according to Preliminary Standard DIN 30670 (Section 3.2.1 and 5.6) The impact strength should be at least 3 Nm in a layer thickness of 300 microns.

In the following examples, T represents parts by weight.

Example 1 Preparation and processing of the powdered coating composition 75.0 T of a coarsely ground epoxy resin (maximum particle size about I mm) from 4,4'-diphenylolpropane and epichlorohydrin [softening point according to Durrans: 930C to 1040C, epoxy equivalent weight: 875 to 1,000, viscosity: 430 to 630 cP in a 40? solution (measured in ethylene glycol dibutylether at 250C)1. 3.0 T of flow agent concentate consisting of the above mentioned epoxy resin and a polyacrylate (Modaflow manufacture by Monsanto) in a weight ratio of 9:1, 2.0 T of imidazole (melting point 90"C according to the capillary method), 13.0 T of titanium dioxide (Kronos RN 57 P, manufactured by Kronos Titangesellschaft mbH) 5.0 T of chromium oxide green GX (manufactured by Bayer AG) and 2.0 T of highly dispersed silica [Aerosil (Registered Trade Mark) 300, manufactured by Degussal are mixed together in a closed rapidly rotating mixer for 5 minutes (with simultaneous cooling of the mixer) and 1,600 rpm. The mixture is plasticised in a double shaft double screw press with helically arranged discs, of the ZDS-K 83 type (manufactured by Maschinenfabrik Werner & Pfleidere, Stuttgart) under the following conditions: temperature of the entry zone: 5 to 150C; temperature of the housing portions I and II: 70"C; temperature of the screws: 50"C; nozzle temperature: 70"C; temperature of the molten homogenised mixture: 110"C; speed of double screw: 300 rpm: throughout: 450 kg/h.

The molten homogenised epoxy resin composition is rolled out flat on 2 watercooled rollers rotating in opposite directions, and at the same time is intensively cooled and subsequently passed on to a water-cooled steel belt.

The cooled epoxy resin composition is broken up coarsely in the usual way, e.g. in a breaker connected to the cooling belt. It is subsequently finely ground in a screening mill and simultaneously classified. The maximum particle size of the pulverulent coating composition is 80 to 100 microns for the electrostatic powder spray process and about 300 microns for the whirl sintering technique. Coating of metal substrates with this coating composition takes place as hereinbefore described.

Example 2 The pulverulent coating composition is prepared according to Example 1.

Large pipes are heated to a temperature of 200+ 10 C, coated in the usual way using the whirl sintering process and subsequently hardened at 200+100C for 5 minutes.

Example 3 The pulverulent coating composition is prepared according to Example 1. The method of processing corresponds to Example 2 except that it is preheated to 250+10"C and there is no subsequent hardening.

Example 4 The pulverulent coating composition is prepared and processed according to Example 1. Instead of imidazole, an adduct is used which is prepared from imidazole and an epoxy resin based on 4,4'-diphenylolpropane and epichlorohydrin and having an epoxy equivalent weight of 185 to 195. The ratio of epoxy resin to imidazole is 73.7 T: 26.3 T. 2.5 T of this adduct are used to 74.5 T of the epoxy resin mentioned in Example 1.

Example 5 The pulverulent coating composition is prepared and processed according to Example 1. As the catalyst a mixture of the imidazole with 2-phenyl-2-imidazoline in the ratio 0.6 T:3.3 T is used, whilst 3.9 T of this mixture are used to 73.1 T of the epoxy resin according to Example 1. Even after storage of this coating composition for 7 months, thick coatings with a smooth film and good mechanical properties can be obtained.

Example 6 The pulverulent coating composition is prepared and processed according to Example 1. For hardening, a mixture of 3 T of the adduct mentioned in Example 4 and 3 T of 2-phenyl-2-imidazoline is used. 6 T of this mixture are used to 71 T of the epoxy resin mentioned in Example 1.

Example 7 Example 4 is repeated, except that 4.0 T of the adduct are used to 73 T of the epoxy resin according to Example 1.

Table I Examples Test methods for large coated pipes 1 4 5 6 7 A) Storage in 1N NaOH at 500 C, length of test 6 months 0 0.5 0 0 0 B) Boiling test in distilled water, length of test 240 hrs 0 0 0 0 0 C) Boiling test in tap water, length of test 240 hrs 0 0 0 0 0 D) Bending test according to DIN 53152 andDIN 1605 0 0 0 0 0 E) Freedom from pores according to Preliminary Standard DIN 30670 0 0 0 0 0 F) Impact strength according to Preliminary Standard DIN 30670 [Nml 4.0 2.5 4.0 4.0 4.0 Evaluation of A)-E) 0=best mark (according to DIN 53230) 5=worst mark In addition, the gel time of the epoxy resin mixtures used was determined at 180"C, and tests on elasticity, impact resistance and resistance to acetone were carried out on coatings which had been stoved at 1800C for 5 minutes and having a layer thickness of 70 microns. Bonderised car body sheet iron 0.75 mm thick WLIS used as the substrate. The gel time gives the time after which the coating has already hardened to such an extent that no lasting phenomena occur, such as deformation of the layer during the process owing to the passage of guide rollers.

Table II Examples 4 4 5 6 7 Imidazole 2.0 -- 0.6 - -- Adduct of imidazole according to Example 4 - 2.5 - 3 4 2-phenylimidazole - - - - - 2-phenyl-2-imidazoline - - 3.3 3 Gel time at 1800C [sec] 31 25 39 21 20 Erichsen depression (DIN 53156) [mm] 10.8 9.3 10.2 10.7 10.6 Impact resistance according to Gardner (inchxpound) (reverse) > 200 145 180 190 190 Acetone test (Note according to DIN 53230) 0.5 0.5 0.5 0.5 0.5 Tables I and II show the advantageous properties of the coatings according to the invention. Whereas the impact resistance F) is somewhat lower (Example 4) when a sharply reduced proportion of the hardening catalyst is used, the use of an increased quantity of catalyst produces excellent results (Example 7).

WHAT WE CLAIM IS: 1. An article comprising a metal substrate in the form of a pipe, a container or large metal article, said substrate being coated with a hardened epoxy resin composition comprising A) a solid epoxy resin based on epichlorohydrin and 4,4'diphenylolpropane or 4,4'-diphenylolmethane or both; B) from 0.1 to 12%, by weight based on the weight of component A), of B1) a compound of formula

(wherein R represents a hydrogen atom) or B2) an adduct of at least one compound of the formulae (I) and (II) (wherein R represents a hydrogen atom, or an alkyl group having I to 6 carbon atoms, or a hydrocarbon group having 6 to 10 carbon atoms and having at least one aromatic moiety) with an epoxy resin of low molecular weight having an epoxy equivalent of from 50 to 2,000; C) a flow agent: and D) a thixotropic agent.

2. An article as claimed in Claim I wherein the metal substrate is a pipe or a container.

3. An article as claimed in either of Claims 1 and 2 wherein the epoxy resin component of the adduct B2) has an epoxy equivalent of from 100 to 500.

4. An article as claimed in Claim 3 wherein the epoxy resin component of the adduct B2) has an epoxy equivalent of from 185 to 195.

5. An article as claimed in any of the preceding claims wherein component B2) of the coating composition is an adduct of a compound of formula (I) or (II) as defined in Claim I wherein R represents hydrogen, a methyl, ethyl, n-propyl, nbutyl, n-hexyl, isopropyl, isobutyl, tert.-butyl, phenyl or benzyl group or one the various tolyl groups.

6. An article as claimed in any of the preceding claims wherein component B2) has a ratio of from 0.7 to 1.3 epoxy groups to each secondary amino group.

7. An article as claimed in any of the preceding claims wherein component B) of the epoxy resin coating composition is present in an amount of from 1 to 10% by weight of component A).

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. layer thickness of 70 microns. Bonderised car body sheet iron 0.75 mm thick WLIS used as the substrate. The gel time gives the time after which the coating has already hardened to such an extent that no lasting phenomena occur, such as deformation of the layer during the process owing to the passage of guide rollers. Table II Examples 4 4 5 6 7 Imidazole 2.0 -- 0.6 - -- Adduct of imidazole according to Example 4 - 2.5 - 3 4 2-phenylimidazole - - - - - 2-phenyl-2-imidazoline - - 3.3 3 Gel time at 1800C [sec] 31 25 39 21 20 Erichsen depression (DIN 53156) [mm] 10.8 9.3 10.2 10.7 10.6 Impact resistance according to Gardner (inchxpound) (reverse) > 200 145 180 190 190 Acetone test (Note according to DIN 53230) 0.5 0.5 0.5 0.5 0.5 Tables I and II show the advantageous properties of the coatings according to the invention. Whereas the impact resistance F) is somewhat lower (Example 4) when a sharply reduced proportion of the hardening catalyst is used, the use of an increased quantity of catalyst produces excellent results (Example 7). WHAT WE CLAIM IS:

1. An article comprising a metal substrate in the form of a pipe, a container or large metal article, said substrate being coated with a hardened epoxy resin composition comprising A) a solid epoxy resin based on epichlorohydrin and 4,4'diphenylolpropane or 4,4'-diphenylolmethane or both; B) from 0.1 to 12%, by weight based on the weight of component A), of B1) a compound of formula

8. An article as claimed in Claim 7 wherein component B) of the epoxy resin

coating composition is present in an amount of from 1 to 5% by weight of component A).

9. An article as claimed in any of the preceding claims wherein the epoxy resin component A) of the coating composition has an epoxy equivalent weight of from 600 to 2,000.

10. An article as claimed in Claim 9 wherein the epoxy resin component A) has an epoxy equivalent weight of from 700 to 1,500.

I 1. An article as claimed in Claim 9 wherein the epoxy resin component A) has an epoxy equivalent weight of from 875 to 1,100.

12. An article as claimed in any of the preceding claims wherein the epoxy resin component A) is a mixture of epoxy resins with different epoxy equivalent weights.

13. An article as claimed in Claim 12 wherein the content of epoxy resins with an epoxy equivalent weight of from 1,500 to 2,000 in the epoxy resin component A) is from 5 to 200;, by weight.

14. An article as claimed in any one of the preceding claims wherein component B) additionally comprises at most 90% by weight of a compound of formula (I) or (II) as defined in Claim 1 (wherein R is other than a hydrogen atom) based on the total weight of compound B) with the proviso that the total amount of component B) is no more than 12% by weight of component A).

15. An article as claimed in Claim 14 wherein the additional compound is at most 600 by weight based on the total amount of component B).

16. An article as claimed in any one of the preceding claims wherein component C) of the coating composition is present in an amount of from 0.1 to 1.5", of the active substance of component C), based on the total weight of components A) and B).

17. An article as claimed in Claim 16 wherein component C) is present in an amount of 0.3 to 1.0%.

18. An article as claimed in any of the preceding claims wherein component C) comprises polyvinyl butyral, a silicone oil, silicone resin or a polyacrylate.

19. An article as claimed in any of the preceding claims wherein component D) of the coating composition is present in an amount of 1 to 5% by weight of the total composition.

20. An article as claimed in Claim 19 wherein component D) is present in an amount of 2 to 3%.

21. An article as claimed in any of the preceding claims wherein component D) comprises finely divided silica.

22. An article as claimed in any of the preceding claims wherein the coating composition additionally comprises a pigment.

23. An article as claimed in Claim 22 wherein the pigment content is not more than 40 > by weight of the total composition.

24. An article as claimed in Claim 23 wherein the pigment content is not more than 20%.

25. An article as claimed in any of the preceding claims wherein the pigment is lead-free and stable to high temperatures.

26. An article as claimed in any of Claims 22 to 25 wherein the pigment is titanium dioxide or chromium oxide green.

27. An article as claimed in any of the preceding claims wherein the thickness of the coating is from 100 to 2,000 microns.

28. An article as claimed in any of the preceding claims wherein the metal substrate is a pipe of at least 100 mm internal diameter.

29. An article as claimed in Claim 28 wherein the pipe has an internal diameter of 300 to 1,600 mm.

30. An article as claimed in any of the preceding claims wherein the pipe has a wall-thickness of from 1.5 to 25 mm.

31. An article as claimed in Claim 30 wherein the wall-thickness is from 6 to 10 mm.

32. An article as claimed in any of Claims I to 27, wherein the container is a large container with a capacity of at least 1 m3.

33. An article as claimed in Claim I and 3 to 27 wherein the construction equipment is a transom, pillar. or a part of supporting equipment.

34. An article as claimed in any of the preceding claims wherein the metal of the substrate is copper, tin, zinc, iron or alloys thereof.

35. An article as claimed in any of the preceding claims wherein the coating is stable up to 1400C.

36. An article as claimed in Claim I substantially as herein described.

37. An article as claimed in Claim I substantially as herein described with reference to Examples I to 7.

38. A method of coating a metal substrate in the form of a pipe, container or large metal article which comprises heating said substrate to a temperature above the melting temperature of the epoxy resin component of an epoxy resin coating composition as defined in Claim 1 and sufficient to cure the said epoxy resin, and powder-coating the said epoxy resin composition onto the hot surface of the substrate to provide a substantially uniform coating thereon wherebv hardening of the resin composition occurs without any further heating step.

39. A method as claimed in Claim 38 wherein the substrate to be coated is a pipe or a container.

40. A method as claimed in Claim 38 or 39 wherein the substrate is heated to a temperature of from 180 to 3300C.

41. A method as claimed in Claim 40 wherein the temperature is from 210 to 300"C.

42. A method as claimed in Claims 38 to 41 wherein the epoxy resin composition is coated onto the surface of the metal substrate by powder spraying.

43. A method as claimed in Claim 42 wherein the epoxy resin composition is coated onto the surface of the metal substrate electrostatically.

44. A method as claimed in Claims 38 to 41 wherein the epoxy resin composition is coated onto the surface of the metal substrate by whirl-sintering.

45. A method as claimed in any one of Claims 38 to 43 wherein the maximum particle size of the epoxy resin composition applied to the substrate is from 80 to 100 microns.

46. A method as claimed in Claims 38 to 41 and 44 wherein the maximum particle size of the epoxy resin composition applied to the substrate is 300 microns.

47. A method as claimed in any one of the preceding claims wherein a container is coated which has large dimensions (as hereinbefore defined) or possesses a heat capacity such that a sufficient amount of heat is available to effect the curing process.

48. A method as claimed in any one of Claims 38 to 47 wherein the metal substrate is cleaned before applying the powder composition.

49. A method according to Claim 38 or 39 substantially as herein described.

50. A method according to Claim 38 or 39 substantially as herein described with reference to Examples 1 to 7.

51. An article as defined in Claim 1 whenever manufactured by a process as claimed in any one of Claims 38 to 50.