GB2097414A

GB2097414A - Protection of metal surfaces

Info

Publication number: GB2097414A
Application number: GB8209669A
Authority: GB
Original assignee: Fulmer Research Institute Ltd
Current assignee: Fulmer Research Institute Ltd
Priority date: 1981-04-01
Filing date: 1982-04-01
Publication date: 1982-11-03

Abstract

A composition for protecting metal surfaces, comprises a paint vehicle including a film-forming organic material containing at least one chelating group which is capable of interacting with metallic atoms or ions present in the metal surface to form a chelated structure, and a curing agent which is capable of causing curing of the organic material, either alone or in conjunction with air or water present at the metal surface, and/or with other components of the composition. the curing agent being effective by reaction with functional groups in the organic material other than the cheating groups. The chelating group may be a beta-diketo group, a 8-hydroxyquinoline or dipyridylamine group. The curing agent may be an organic diisocyanate, an amine or metal driers. The film- forming organic material may be (i) the reaction product of diketene and either a drying oil, e.g. castor oil, or an OH terminated polymer or copolymer of butadiene; (ii) a tung oil modified phenolic resin prepared from p-tert- butyl phenol, 8-hydroxyquinoline and formaldehyde; or (iii) an epoxy resin prepared from a glycidyl ether of bisphenol- A and 2,2'-dipyridyl-amine.

Description

SPECIFICATION Protection of metal surfaces This invention relates to the protection of metal surfaces from corrosion and provides methods and compositions for such purpose.

Durability of paint films on external structures is often less than predicted because of adhesion failure between the film and the substrate. A common cause of adhesion failure is surface contamination, for example, as a consequence of applying the paint under unfavourable weather conditions. It has been rightly observed that there are only a few weeks in the year when weather conditions are ideal for external painting in the British Isles so that a need exists for surface coating compositions which can be applied satisfactorily to surfaces under adverse weather conditions.

Although paint compositions exist which can be applied under water, e.g. to ship's hulls, jetties and oil rigs, they are not entirely satisfactory and a demand remains for a paint composition which is generally suitable for coating submerged ferrous metal structures, without the need to temporarily exclude water during application of the coating.

It is towards the solution of these and related problems that the present invention is directed.

According to one aspect of the present invention, there is provided a composition for protecting metal surfaces which comprises a film-forming organic material containing at least one chelating group per molecule which is capable of interact;ng with metallic atoms or ions present in the metal surface to form a chelated structure, and a curing agent which is capable of causing curing of the film-forming material, either alone or in conjunction with the environment in which the metal surface is treated, and/or with other components of the composition.

An underlying concept of the present invention is the provision of a paint composition which will exhibit an affinity for the metal surface by virtue of the formation of chelate co-ordination bonds with the iron or other metal. At the same time the composition includes a component which brings about curing of the paint film. As a consequence of the formation of co-ordination bonds with the iron or other polyvalent metal, improved adhesion of the paint film can be acheived and at the same time surface contaminants, such as water, having less affinity for the surface can be displaced.

In general, the film-forming organic material will contain reactive functionalities, other than the chelating groups, through which curing takes place. For example, the functional groups through which curing takes place may be hydroxyl groups, epoxy groups, amino groups, isocyanate groups, centres of unsaturation or phenolic moieties. Although theoretically, the paint compositions are curable by reaction between the iron in the metal surface and the chelating groups, such as p-diketo groups, it has been found in practice that it is convenient in the interests of achieving reasonably short curing times for hardening the paint films to provide a further curing mechanism.

The curing agent may be mixed with other components of the paint composition at the time of application to the surface to be coated. Thus the present invention includes a method of protecting a ferrous metal surface from corrosion which comprises applying to the surface the components of a surface-coating composition comprising a film-forming paint vehicle including a polymer or prepolymer having at least one chelating group, such as a l3-di-keto group, capable of interacting with the iron present in the metal surface to form a chelated structure and a curing agent which is capable of causing curing of the polymer or pre-polymer, either alone or in association with air or water present at the metal surface and/or with other components of the composition, the curing agent being effective by reaction with functional groups in the polymer or pre-polymer other than the chelating groups.

The film-forming organic material may be a drying oil by which we mean a fatty acid, ester, anhydride or amide which contains an unsaturated double bond. A suitable drying oil is castor oil (or its main constituent acids or derivatives) which also contains a free hydroxy group. In one embodiment of this invention castor oil is reacted with diketene in order to introduce a /3-diketo group. Theoretically diketene can react with either hydroxy groups or unsaturated double bonds but the reaction rate with the castor oil hydroxy groups is greater than the reaction rate with its double bonds. Castor oil consists of approximately 90% of the triglyceride of ricinoleic acid. Thus, the reaction with diketene can be expressed as follows:-

CH3(CH2) . CHCH2CH=CH(CH2)7C02H+CH2=C-CH2 OH O-C=O ricinoleic acid diketene CH3(CH2)5CHCH2CH=CH(CH2)7C02H l O-C-CH2-C. CHa lla 11 0 0 In fact, examination of the products indicate that the reactions which take place are rather complex. Thus although the reacted castor oil gives the characteristic red-colour reaction with ferric chloride, indicating the presence of a 1.3 diketo group and this structure is conformed by I.R.

spectroscopy, the reacted castor oil contains about 60 to 70% of a fraction having a molecular weight of about 2,300.

Preferably the diketene is reacted with an excess of castor oil in order to leave some unreacted hydroxyl groups, which will then be available for reaction with a curing agent, such as an organic isocyanate. Best results have been obtained by reacting castor oil with 1/3 of the stoichiometric amount of diketene so that about 1/3 of the hydroxyl groups only are reacted.

The resulting reacted castor oil can be cured with an organic diisocyanate e.g. tolyl diisocyanate or isophorone diisocyanate. A second group of film-forming materials which have been found to be suitable are based on hydroxyl-terminated polybutadienes. These polymers may be reacted with diketene in an analogous way to that described above in connection with castor oil. The polybutadiene may be a homopolymer of butadiene or a copolymer with other comonomers such as styrene.

Preferably the hydroxyl-terminated polybutadienes have residual centres of unsaturation through which reactions such as cross-linking can occur. Polybutadienes having molecular weights in the range of 1 ,000 to 3,000 have been found to be suitable. Although hydroxyl-terminated polybutadienes will react with diketene either through the hydroxyl groups or through the unsaturated double bonds, the former reaction proceeds much faster in the absence of effective catalyst. In cases where the hydroxyl-terminated polybutadienes are reacted with diketene via the hydroxyl groups, it is preferred that only one of the hydroxyl groups is reacted so that the resulting products can be cured using an organic isocyanate.Although coating compositions based on both types of reacted polybutadienes exhibit excellent adhesion to ferrous surfaces under a variety of coating conditions, the reacted polybutadienes in which the diketene has been reacted with the unsaturated double bonds to yield harder, cured films when cured with diisocyanate.

Alternatively or additionally, curing of polybutadiene/diketene reaction products may be effected by an air-curing mechanism using metal driers.

A further group of polymers which may be used as the basis for the coating compositions are phenolic resins containing units derived from hydroxyquinoline. A phenolic resin containing 8hydroxyquinoline (8-HQ) units can be prepared by reacting an alkyl phenol with formaldehyde under acidic conditions and in the presence of 8-HQ. The resulting resin is curable using a methylol donor such as hexamethylenetetramine. Phenolic resins prepared in this way complex strongly with iron. In order to improve its film-forming properties, the phenolic resin may be modified by reaction with a drying oil such as Tung Oil or linseed oil.

It is also possible to prepare epoxy resins which contain chelating groups. For example, an epoxy polymer or pre-polymer may be reacted with 2,2'-dipyridylamine to produce curable epoxy resins which can form chelate complexes with iron. Such resins are curable using a variety of curing agents such as diamines and isocyanates.

The following Examples are given to illustrate the invention and the manner in which it may be carried into effect: Example 1 8-HQ-phenolic primer A phenolic primer was prepared by heating 100 parts of solid 8-hydroxyquinoline phenolic resin with 195 parts of raw Tung Oil (supplied by Younghusband Stevens) at 2400C for 40 minutes. This was then thinned with 1 85 parts mineral spirits and 28 parts dipentene. There was a colour change (orange to sandy brown).

The above reaction was carried out in a 250 ml flanged flask fitted with overhead stirrer and condenser. The 8-hydroxyquinoline phenolic resin was prepared from the reaction between ptertiarybutyl phenol ( mole), 8-hydroxyquinoline (3 mole) and 37% formaldehyde (1 mole) giving a 1:1 phenolic/formaldehyde non-reactive type 100% phenolic resin.

To complete the preparation of the phenolic primer, the following mixture was made up: Phenolic resin/Tung Oil reaction product 32.11 g Mineral Spirits 3.74 g Pb drier (24% Pb) 0.15 g Mn drier (6% Mn) 0.04 g Co drier (6% Co) 0.04g Anti-skinning Agent (Butyraldoxime) 0.23 g The resulting phenolic primer could be coated onto wet steel panels and dried in air. In this case the curing reaction arises by cross-linking through the unsaturated centres in the drying oil under the influence of the paint driers.

Example 2 Epoxy type chelating composition 3.8 grams of Epikote 828 (a glycidyl ether of Bisphenol A obtainable from Ciba-Geigy) were refluxed with 0.43 grams of 2,2'-dipyridylamine in 148 grams of DMF (dimethyl formamide) under reflux for 4 to 5 hours and in the presence of a basic catalyst (3 drops of N-benzyldimethylamine). The resulting epoxy resin was mixed with a polyglycoldiamine heterocyclic epoxy hardener (hardener 22UW availabie from Isochem Resins Co.) and coated onto steel panels to form a hard shiny coating.

Example 3 Castor oil derivative containing ss-diketo groups 1 litre flanged flask was set up with stirrer, thermometer, condenser, pressure-equalising dropping funnel, N2 inlet and outlet.

1 50 g castor oil were dissolved in 60 g (dried) MEK in a beaker, then poured into the flask. Then 0.1 g NaOOCH3 added and the mixture stirred. The solution (sodium acetate does not dissolve) was heated to 600C on a water bath, whilst passing N2 through the apparatus. Diketene (13.5 g) weighed into beaker, added to dropping funnel and approximately 20 ml MEK added as a solvent. The diketene solution was dripped in over 3 hr with the reaction temperature maintained at 600 C. There was no visible heat of reaction; the mixture stayed yellow/green throughout the addition. The mixture was stirred and maintained at 600C for 2 2 hrs, during which time the mixture turned red (red colour gradually getting darker).

The mixture was cooled to room temperature and left overnight, and then filtered to remove the sodium acetate.

The MEK was pumped off into a cold trap to give 1 50 g of reddish product.

The reacted castor oil gives dark red colouration with FeCI3 which indicates presence of 1:3 diketene.

I.R. spectrum shows OH band to be broader in reacted castor oil compared with unreacted castor oil. This would be expected if diketene is present. The reacted castor oil contained approximately 2/3 unreacted hydroxyl groups. After mixing with 63.6 parts per hundred of tolyl diisocyanate (Desmodur L), the reacted castor oil was coated onto clean and rusted steel panels both dry and underwater. The coatings cured in all cases and the curing rate was increased by incorporation of ferric salts in the composition. Good adhesion to the steel panels was observed.

Example 4 Hydroxy-terminated polybutadiene compositions 0.025 mole of hydroxyl-terminated homopolymer of butadiene (Arco R-45HT from Atlantic Richfield) was dissolved-in 70 grms of butanone and stirred under a nitrogen blanket at 850C. 0.025 mole of diketene was dissolved in 1 5 grams of butanone and added dropwise to the Arco R-45HT over 30 minutes. The mixture was stirred at 850C for 90 hours and the solvent then evaporated off under vacuum at 900C. The product was a very viscous orange syrup and l.R. spectra confirmed the presence of 1,3-diketo groups. It is believed that the diketene had reacted predominantly with the unsaturated double bonds in the Arco R-45HT.A coating composition was prepared by mixing 4 grams of the reacted polybutadiene with 0.76 grams of isophorone diisocyanate (IPDI from Veba-Chemie) and a catalyst consisting of ferric acetoacetonate (0.0192 grams) and triethylamine (0.0048 grams). The composition was dissolved in a solvent mixture of cellosolve acetate and xylene (1:2) to form a 60% solution and coated onto dry and wet steel panels. Tough, flexible coatings exhibiting excellent adhesion and impact resistance were obtained.

Although the invention has been described primarily in connection with the application of corrosion-resistant coatings to ferrous metal surfaces, compositions described in this specification could also be used to protect other metal surfaces, e.g. those containing or consisting of copper or aluminium.

As indicated in the foregoing description it is believed that the compositions of this invention form chelated bonds with the metal of the treated surface. It is not clear, however, at this stage what the precise nature of the bonds is and whether the chelating groups react with metal ions or metal atoms. Alsc, the reactions may differ from one composition to another. Observations do indicate, however, that the compositions of this invention exhibit a strong affinity for ferrous metal surfaces and give primer coatings with good adhesion under adverse conditions. It may be of importance that that coating compositions of this invention have a hydrophilic character which contributes to good spreading and wetting characteristics when painting damp or wet surfaces.

While the compositions of this invention were originally developed with a view to using them in the maintenance painting of underwater structures, e.g. using the applicator described in our copending patent application No. 8106918, present indications are that their main uses will be as primer paints in painting metal surfaces under adverse ambient conditions, e.g. steel surfaces splashed with sea-water. Generally pre-treatment of the metal surface with HCI or H3PO4 solution enhances the adhesion of the coatings, perhaps because the compositions adhere better under acid conditions or because the surface metal is converted to salts.

The compositions of the invention may be coated on the substrates under anodic conditions by impressing an electric current for a short period during application.

Preferably, the solvents in which the compositions are thinned for application are polar or include polar components. The presently preferred solvent systems are: White spirit and Butan-2-ol 2:1 V/V and White spirit, xylene and Butan-2-ol 1:1:1 V/V/V.

Generally it is desirable to apply the coatings at minimum thickness to achieve maximum adhesion, e.g. coating thicknesses in the range of 1 5 to 20,us.

In the compositions prepared by reacting diketene with hydroxy-terminated polybutadienes, it has been found that the microstructure of the polybutadiene appears to have some influence on coating properties. Thus, preferred polybutadienes are those having a high 1,2-vinyl unsaturation content.

Normally the compositions of the present invention will be used as primers and overcoated with a top coat composition. Top coating paint compositions which have been found to be compatible with the primers of this invention are alkyds and coal tar epoxy compositions.

Claims

1. A composition for protecting ferrous metal surfaces which comprises a film-forming organic material containing at least one chelating group per molecule which is capable of interacting with iron present in the metal surface to form a chelated structure, and a curing agent which is capable of causing curing of the film-forming material, either alone or in conjunction with the environment in which the metal surface is treated and/or with other components of the composition.

2. A composition according to claim 1 in which the curing agent is effective by reaction with functional groups in the film-forming material, other than the chelating groups.

3. A composition for protecting metal surfaces, which comprises a paint vehicle including a polymer or pre-polymer containing at least one ,B-diketo group which is capable of interacting with metallic atoms or ions present in the metal surface to form a chelated structure, and a curing agent which is capable of causing curing of the polymer of pre-polymer, either alone or in conjunction with air or water present at the metal surface, and/or with other components of the composition, the curing agent being effective by reaction with functional groups in the polymer or pre-polymer other than the ss-diketo groups.

4. A composition according to claim 3 in which the functional groups which are reactive to cause curing of the polymer or pre-polymer are hydroxy groups.

5. A composition according to claim 4 in which the curing agent is an isocyanate and the curing action involves formation of a polyurethane.

6. A composition according to claim 3 in which the functional groups which are reactive to cause curing of the polymer of pre-polymer are epoxy groups.

7. A composition according to claim 6 in which the curing agent is an amine.

8. A method of protecting a ferrous metal surface from corrosion which comprises applying to the surface the components of a surface-coating composition comprising a film-forming paint vehicle including a polymer or pre-polymer having at least one chelating group capable of interacting with the iron present in the metal surface to form a chelated structure and a curing agent which is capable of causing curing of the polymer or pre-polymer, either alone or in association with air or water present at the metal surface and/or with other components of the composition, the curing agent being effective by reaction with functional groups in the polymer or pre-polymer other than the chelating group.

9. A surface-coating composition for protecting ferrous metal surfaces which comprises a filmforming organic material containing at least one /3-diketo group and at least one hydroxy group and a curing agent comprising an organic isocyanate.

10. A composition according to claim 9 in which the film-forming organic material is a reaction product of a drying oil containing at least one hydroxy group and a compound capable of introducing a ss-diketo group.

11. A composition according to claim 10 in which the film-forming organic material is a reaction product of a drying oil containing at least one hydroxy group and diketene.

12. A composition according to claim 10 or 11 in which the drying oil is castor oil.

13. A composition according to claim 9 in which the film-forming material is the reaction product of diketene and an hydroxyl-terminated polymer or copolymer of butadiene.

14. A composition according to claim 1 3 in which the curing agent is an aromatic or cyclic diisocyanate.

1 5. A composition according to claim 1 3 or 14 in which the polymer or copolymer of butadiene contains residual centres of unsaturation.

1 6. A composition according to claim 1 or claim 2 in which the chelating group is 8hydroxyquinoline.

17. A composition according to claim 1 or claim 2 in which the chelating group is dipyridylamine.