WO1991014746A1 - Paint systems - Google Patents

Abstract

A paint based on a saturated polyester comprises 5-50 % of an amino-resin such as an alkylated melamine resin, and 1-20 % of an aluminium organic compound, percentages being by weight on the weight of the polyester, preferably in a volatile liquid medium. The aluminium organic compound may comprise alkoxy or ethylacetoacetate groups. When used to coat metal e.g. aluminium sheet, the paints have improved long-term weathering properties.

Description

PAINT SYSTEMS

The coating of metals in the form of continuous strip is now well established. This strip, received in the form of a coil, is subjected to a sequence of operations, namely uncoiling, degreasing, pre-treatment, roller coating with paint, cure by stoving, cooling and re-coiling. Aluminium and steel strip up to 1.5 wide are coated at speeds up to 3 m/s. Obviously a very high performance standard is required of the coating used. The major requirements are:

- Outstanding flexibility and adhesion to withstand forming operations.

- High opacity, since one-coat systems are often employed. " Satisfactory inter-coat adhesion if more than one coat is applied.

- A rapid stoving schedule (usually less than 60s) without impairment of properties.

Coated metal strip has a wide field of application ranging from cladding for buildings to office filing cabinets and bottle tops. Outstanding weathering behaviour of sheet intended as external cladding for buildings is required, when 40 plus years life expectancy are sought.

Stoving finishes containing amino-resins such as hexamethoxymethylmelamine have dominated the markets for factory-applied metal coatings. The advantageous properties of amino-resins include hardness and freedom from undesirable colour. However, used on their own, amino-resins suffer from the disadvantages of brittleness and poor adhesion. It is therefore necessary to blend the resin with plasticising components such as compatible alkyds, saturated polyesters or acrylic resins.

A phenomenon which has become recognised recently in amino-resin/saturated polyester films is that of film degradation due to thermal depolymerisation. This is potentially a very serious problem with implications for resin manufacturers, metal coating companies and owners of buildings and equipment coated with such systems.

This invention stems from work designed to replace the amino-resins, presently used in amino- resin/polyester coating compositions for metal strip, with aluminium organic compounds. The inventors surprisingly found that amino-resins and aluminium organic compounds can act synergistically to provide polyester paint systems giving rise to films having outstanding long-term weathering properties.

This invention provides a paint based on a saturated polyester and comprising 5 to 50% of an amino resin, and 1 to 20% of an aluminium organic compound, percentages being by weight on the weight of the polyester.

It is believed that the benefits of the invention can be achieved with any saturated polyester resin. However, in many cases aluminium organic compounds cause premature gelling of the polyester resin which makes formation of a paint film impractical with present techniques. It is still thought that once formed the film would be stabilised as described herein. Preferred resins have low molecular weights and high acid values. Examples are:- Resin A is a commercially available oil-free polyester.lt has a resin solids content of 75% and an acid value of 17-21 mg KOH/g. (AROPLAZ 3256 from Ashland Chemicals) . Resin B was an oil incorporated modification to Resin A which included some dimer acid. This contained about 20% by weight of methylene chains and a trace of unsaturation. Resin B has a number average molecular weight of 880, a weight average molecular weight of 2655, an acid value of 5.4 mg KOH/g, a hydroxyl value of 85.8 mg KOH/g and a resin solids content of 75%. (Ashland EP 2580T) .

Resin C is an experimental oil free polyester with number average molecular weight of 1905, a weight average molecular weight of 9920, an acid value of 2.1 mg KOH/g, a hydroxyl value of 51.5 mg KOH/g and a resin solids content of 55%.

The nature of the amino-resin is not very critical. Preferably a melamine-formaldehyde resin is used, in which the melamine has been partly or completely alkylated to reduce its tendency to hydrolyse. The term alkylated denotes the replacement of amino-hydrogen atoms by alkyl groups which may o themselves be substituted e.g. methoxymethyl groups. Commercially available melamine resins include:-

Melamine X - part methylated melamine, 84% solids. (Resimene 717 from Monsanto).

Melamine Y - part methylated melamine, 80% solids. (Resimene 731 from Monsanto).

Melamine Z - hexamethoxymethyl melamine, 100% solids. (Resimene 747 from Monsanto).

The proportion of amino-resin needs to be sufficient to cross-link the polyester, but not so _Q high that the brittleness and poor adhesion properties of the amino-resin manifest themselves. The proportion should be 5 to 50%, preferably 10 to 30%, by weight on the weight of the polyester.

When the paints of this invention are c intended to be cured by stoving, they preferably include a blocked catalyst for the purpose. Suitable catalysts are available commercially, one used in the Examples below being an amine-blocked para-toluene sulphonic acid catalyst. (Cycat 4045 from Cyanamid) . The paint preferably includes an aluminium organic compound including at least one group

- 0 - Al - 0 -

I X where X is derived from HX which is an alcohol or phenol, a carboxylic acid, a 1 ,3-diketone, an alkyl acetoacetate or an alkyl half-ester of a dicarboxylic acid and may be unsubstituted or substituted.

The aluminium organic compound (AOC) may include one or two hydroxyl groups attached to Al. Or the AOC may have the formula A1X_, where X is defined above.

Or the AOC may contain more than one Al atom per molecule, by virtue of having the formula

where p is less

Q equals 3-2p, and n is 2 or more.

X is derived from HX which may be water but is organic in at least one position in the molecule and is preferably: - a C1 to C30 aliphatic alcohol or a phenol which may be substituted; a C1 to C30 carboxylic acid, which may be aliphatic, alicyclic or aromatic and which may be substit ted; a 1,3-diketone such as acetylacetone; a C1 to C12 alkyl acetoacetate, which may be substituted in the alkyl group; a C1 to C12 alkyl half-ester of a C1 to C12 aliphatic, alicyclic or aromatic dicarboxylic acid, which may be substituted. In all cases, X may be the same or different at different parts of the molecule.

As substituents in the above, C1 to C12 alkoxy groups and C1 to C4 dialkyl amino groups are envisaged among others. Alkyl acetoacetates have the formula

CH-COCH-COOR, where R is C1 to C12 alkyl. They are believed to co-ordinate to Al through both ketonic oxygen atoms, in similar manner to carboxylates and to acetylacetone. All these AOC compositions have at least one substituent, an alkoxide or an oxo linkage, which is capable of addition or substitution reaction with a carboxylic acid or hydroxylic or epoxy group of the polyester resin with which it is compounded, to provide a reaction product resistant to hydrolysis or dissociation. These substituents may also have an ester substituent which, we have discovered, can transesterify with hydroxyl groups present in the resin. This invention is based on the discovery that AOCs can inhibit thermal depolymerisation of amino-resin/polyester coatings on metals. The inhibition of thermal depolymerisation improves as the concentration of the AOC increases. However, above a certain limit, increase in the AOC concentration of the paint may reduce the initial properties of the paint film. These requirements place limits on the permissible AOC concentrations of the paint, and these are specified as from 1 to 20% by weight on the weight of the polyester.

Applicants currently believe that the amino-resin reacts preferentially with free hydroxyl groups of the polyester, and that AOCs react preferentially with free carboxyl group of the polyester. On the basis of this belief, they prefer to use AOCs in a concentration to provide 0.5 to 2.0 moles of aluminium per mole of carboxylic acid groups on the polyester.

Although the AOCs act to improve long-term film properties on stoving, they must be unreactive in the paints at ambient temperatures. Any significant cross-linking activity at ambient temperatures will tend to cause gel formation and make paints unusable.

In the paints of this invention, the components are preferably dissolved or dispersed in a volatile liquid medium which may be water but is preferably organic. Other components may be present, such as are conventionally present in amino- resin/polyester compositions. These include pigments, fillers, flow additives, driers and antioxidants. The paints of this invention are useful for painting to metal sheet in the form of coil. Suitable metals include steel and aluminium, the surface of which may carry a chromate or other strongly adherent inorganic non-metallic coating, or may (in the case of aluminium) b anodised. The method involves uncoiling a coil of the metal sheet, applying to the sheet a coating of the paint described above, stoving the coated sheet to cure the paint film and preferably thereafter re-coiling the coated sheet. Techniques for applying and curing the paint can be conventional.

The experimental section below demonstrate that a synergistic effect exists between melamine resins and AOCs to improve the weathering properties of polyester paint films. Although this invention is based on observation rather than theory, Applicants currently believe that there may be two reasons for this effect:- a) As noted above, melamine resins tend to react preferentially with hydroxyl groups, while AOCs tend to. react preferentially with carboxyl groups.

The two cross linking agents .should together therefore react with substantially all reactive groups of the polyester. b) Hydrolysis of fully alkylated melamines is fast if strong acid catalysts are present but very slow if they are absent. An AOC may neutralise any strong acid catalyst remaining after curing, and therefore inhibit depolymerisation brought about by melamine hydrolysis.

EXPERIMENTAL

(a) Resins and Chemicals

1. Polyester resins: Three resins were used: A, B and C (see above) . 2. Melamine resins: Two resins were used: Y and Z (see above) .

3. Additives: The following catalysts or additives were used in some of the experiments:-

(a) amino blocked paratoluenesulphonic acid (PTSA) catalyst.

(b) Flow additive.

(c) Zinc octoate, (8% Zn) .

(d) Manganese octoate, (6% Mn) .

(e) An anti-oxidant. 5 (f) 65% aromatic:35% glycol ether blend solvent.

4. Almninium compounds

AOC 1020A was a commercially available aluminium compound. The Al content is 5%. AOC 101OX was a commercially available _Q aluminium compound. The Al content is 9.4%.

AOC 1010C was an experimental aluminium compound in which the substituent groups were derived from ethylacetoacetate, an amino alcohol and water. Displaced 5 isopropanol was effectively present as a solvent. The Al content was 6.8%. AOC 2050X was a commercially available oxo-aluminium compound. The Al content was 9.8%.

AOC 2010 was an experimental oxo-aluminium compound containing ethylacetoacetate and a glycol ether. White spirit was added as a diluent. The Al content was 5.9%. AOC 1020 Mn was an experimental aluminium compound similar to AOC 1020A but additionally contained some manganese acetate. The Mn content was 0.7% and the Al content was 5.9%. (5) Formulation and Stovinσ Conditions

The formulated resin mixtures were drawn down onto pretreated (chromated) aluminium plates (25 x 10cm). The plates were 0.56 mm in thickness. The wet film applied thickness was 25 um, giving dried films of approximately 15 um, unless otherwise stated in the Tables.

The temperature of the stoving oven was set at 255 C. A standard 40 s bake produced a peak metal temperature of about 215°C.

Example 1

Results are set out in Tables 1 to 4 below. The paint formulations used were:-

For Tables 1 and 2 5

Polyester Resin 82.5

Resin Y 17.5

Catalyst (PTSA) 0.9

Flow Additive 0.5 Solvent. 10.0 (ml) The particular polyester used was as noted in the Tables. The aluminium compound used was AOC 2010.

For Table 3 g

Polyester A 100

AOC 2010 25

Melamine varies Catalyst varies

Solvent 10.0 (ml)

For Table 4 g Polyester B 82.5

Resin Y 15.0 Catalyst (PTSA) 0.5

Flow Additive 0.4

Solvent 10.0 (ml)

The aluminium compounds used were as noted in the Tables. The amount of aluminium added to the formulation (0.069g) was related to the acid value of the polyester resin. For each aluminium compound tested 1 , 2 and

3 equivalents were added. The exact weight added in the formulation depended on the Al content of the aluminium compound, for example 0.69g for a 10% Al AOC sample at a one-equivalent addition. (c) Evaluation

The important properties required of a durable coating on aluminium are a combination of hardness, flexibility, ability to withstand impact, chemical and - weather resistance. Depolymerisation was assessed by remeasuring some of these properties after subjecting the coated panels to elevated temperatures (about 113°C) for periods of up to 180 hours.

(a) Hardness: The pencil hardness method was used whereby a high quality pencil, ranging from 2B to 6H, sharpened to a flat surface was applied at an angle of 45 to the cured film with such pressure that the point crumbled and was drawn across the

10 surface for a distance of 6 mm. The grade of hardness of the pencil used immediately before that which caused scratching or penetration of the film was taken as the hardness of the paint film. A HB hardness or

1 greater was sought in the initial film.

(b) Reverse impact: The reverse impact test consisted of dropping a hemispherical steel weight from a predetermined height equivalent, on hitting the test panel, to an

20 impact of energy 5J. The film was then inspected for signs of damage. Two common types of fracture were observed during some of the tests. Firstly, a "ring" fracture which is usually consistent with a highly

2 cross-linked polymer. Secondly, a "star" fracture which is usually associated with inadequate cross-linking or a two-phase system. Absence of ring or star fractures are sought.

2o (c) MEK solvent resistance: This test consisted of drawing to and fro across the surface of the coated panel a weight, in this instance the rounded end of a hammer head, covered with muslin soaked in

2 methylethylketone, until the coating was worn away and the bare metal exposed. In the un-aged film 20 double MEK rubs were sought.

(d) Film thickness: Film thicknesses were measured using an eddy-current detector. (e) Other observations: Smoke was sometimes detected emanating from the coated surface after removal from the stoving oven. This indicated volatilisation of unpolymerised polyester or melamine resin which, obviously, was undesirable. Some light smoke was tolerable. Yellowing was observed in some of the heat-aged panels.

RESULTS AND DISCUSSION Table 1 exemplifies the beneficial influence of aluminium compound AOC 2010 on the ageing properties of the applied film. In the absence of aluminium, 72 hours at 113 C is sufficient to cause substantial loss of film properties in terms of both hardness and solvent resistance (Test 2). When 3g of AOC 2010 was incorporated into the formulation a hard, solvent resistant film was still apparent after 120 hours at 113 C (Test 5). An addition of 0.3g manganese octoate enhanced the film properties still further (Test 7) . Even with a resin with improved ageing properties

(Test 4 vs Test 2), further improvements could still be gained by the incorporation of 3g, AOC 2010 (Test 6) Again, a small quantity of manganese octoate was observed to improve the properties still further (Test 8).

Table 2 illustrates the effect of varying the amount of AOC 2010 in the coating formulations (Panels A1 to A4) . It appears that increasing the aluminium concentrations led to improved film properties on ageing. Panels B1 and B2, and C1 and C2 demonstrate that, in the absence of aluminium, zinc octoate and manganese octoate are not very effective at the addition levels employed. Panels D1 to D3 again demonstrate the improved film stability of films incorporating aluminium even when an improved resin is employed (Panel D1 vs Panel A1 ) . Zinc octoate was not effective (Panel E1 and E2), but manganese octoate did appear to enhance the film stability (Panels F1 and F2), perhaps as a result of a trace of unsaturation in resin B in which manganese stimulates some oxidative cross-linking. Table 3 illustrates the effect of incorporating polyester resin A with AOC 2010, AOC 2010 plus PTSA catalyst and AOC 2010 plus PTSA catalyst and melamine. In the first instance it can be observed that, under the stoving conditions employed, the initial film properties were unsatisfactory. However, on ageing for 14 days, a H pencil hardness and 20 MEK double rubs indicated further curing. The additional presence of blocked PTSA was of no benefit. In the third row a hard, solvent resistant film was initially formed and was still intact after 14 days.

Table 4 compares initial film properties and ageing properties of a range of six aluminium alkoxides or oxo-aluminium compounds. The levels of aluminium chosen were 1 , 2 and 3 equivalents per polyester acid group. Panel 1-0 was the control formulation containing no aluminium.. The initial film properties of 1-0 were satisfactory. After 12 days the film had become slightly sticky, offered no solvent resistance and exhibited a ring fracture on the reverse impact test. Panels 1-1 to 1-3 exhibit the influence of AOC 1020A. The initial pencil hardness results were somewhat lower than in the control panel but gave better MEK resistance results at the two higher concentrations. On heat ageing the properties did not deteriorate as rapidly as the control, 1-0. Generally similar properties were observed for AOC 101OX, AOC 1010C, AOC 2050X and AOC 2010. Panels 6-1 to 6-3 included AOC 1020 Mn, a combined drier. This compound gave rise to the observation of severe star fracturing after only 40 hours heat ageing.

TABLE 1 - MELAMINE-POLYESTER FORMULATIONS

I

Note: Oct = Octoate

TABLE 2 - MELAMINE-POLYESTER FORMULATIONS

NOTE: * Sticky Surface

TABLE 3 - MELAMINE-POLYESTER FORMULATIONS

Initial bake, 40 sees, initial film 11 μm

TABLE 4 - COMPARISON OF AL COMPOUNDS

Example 2

A mill base was prepared to the following formulation: parts weight Titanium Dioxide 150

Polyester C (55% solids) 60

Ethoxyethylacetate (EEA) 13

Solvesso 200 13

The base was milled to less than 7 fineness on Hegmann gauge and used to prepare a base paint as follows:

parts weight Millbase 52.4

Polyester C 30.0

Resin Z 6.9

Solvesso 200 7.7

Flow Additive (10% in EEA) 1.0 PTSA (10% in benzyl alcohol) 2.0

Gloss retention studies were performed on three paints*.- I - Base paint as above. II - Base paint plus 1% of AOC 2050 III - Base paint plus 3% of AOC 2050.

For II and III, the AOC was added to the base paint as a 50% solution in a 50:50 mixture of Solvesso 200:EEA. The paints were applied to metal panels, and the coated panels subjected to UV light for 950 hours. The accompanying graph shows the 60° gloss reading against time of exposure. Although the gloss readings of all paints decreased with time over the first 500 hours, at all times, the paints II and III containing AOCs gave better gloss readings than the paint I not containing any AOC. Example 3

Unpigmented paints contained the following:- Polyester B Resin Z Catalyst (PTSA) - concentration varies AOC 2050X - concentration varies

The paints were applied to metal panels, and the coated panels exposed to florida sunlight for 1 year. Resistances to MEK before and after exposure were measured and are reported in Table 5 below.

Test Catalyst AOC 2050X Initial 1 Year No. % % MEK Aged MEK

++ MEK values indicate that the paint film was hard to fingernail after the test

The properties of the exposed paint films improve with increasing AOC content.

Claims

CiΔIMS

1. A paint based on a saturated polyester and comprising 5 - 50% of an amino-resin, and 1 - 20% of an aluminium organic compound, percentages being by weight on the weight of the polyester.

2. A paint as claimed in Claim 1 , wherein the amino-resin is an alkylated melamine resin.

3. A paint as claimed in Claim 1 or Claim 2, wherein the aluminium organic compound includes at least one group

- 0 - Al - 0 - I

X where X is derived from HX which is an alcohol or phenol, a carboxylic acid, an alkyl acetoacetate or an alkyl half ester of a dicarboxylic acid, and may be substituted or unsubstituted.

4. A paint as claimed in any one of Claims 1 to 3, wherein the components are dissolved or dispersed in a volatile liquid medium.

5. A method of painting metal sheet, which method comprise uncoiling a coil of the metal sheet, applying to the sheet a coating of the paint claimed in any one of Claims 1 to 4, and stoving the coated sheet to cure the paint film.

6. A method as claimed in Claim 5, wherein the metal is aluminium.

7. A method as claimed in Claim 5 or Claim 6, wherein the coated sheet is afterwards re-coiled.