The present invention relates to the field of resinous binders suitable for applicatio-n as coatings on various metallic and non-metallic substrates, in particular for the finishing of body parts in the automobile industry. More specifically, the invention regards a coating composition which can harden at low temperatures, thus being useful for forming coatings on body components also having plastic parts, as well as more generally for forming protective coatings on various substrates in industry in general.

Background Art

Among the resinous binders known to be in widespread use nowadays as coatings, in particular in the finishing of body parts in the automobile industry, are those based on polyisocyanate which are known to be highly toxic. Disclosure of the Invention The principle aim of the present invention is to obtain a new coating composition as an alternative to those available nowadays based on polyisocyanate, and which may have faster cross-linking properties at low temperatures such as to permit a notable time-savings in production cycles.

Still another aim of the invention i s to obt ain a coating composition that can provide coatings with excellent chemical resistance and durability . WAYS OF CARRYING OUT THE INVENTION

These and other aims which will become more apparent her e inaf t er are a chi eved by a coa t ing compos i t ion

characterized by the fact of comprising at least one polycarbodiimide component having at least two carbodiimide groups per molecule, at least one anhydride component having at least one carboxylic acid anhydride group per molecule, and a hydroxylic reagent capable of reacting with said anhydride component to generate free carboxylic polyacids, at least one among said components and reagents being suitable for forming films.

The polycarbodiimide component useful in the present composition can be a compound by itself or it can be incorporated together with the above-mentioned anhydride component in a single complex compound either still carrying or no longer carrying the carbodiimide functionality.

In the first case, one uses a polycarbodiimide of the general formula:

(1) Z ₁ -N=C=N-Z ₂ -N=C=N Z _n

in which Z-^, Z ₂ ,..-.Z _n are linear or branched aliphatic Iiydrocarbon radicals or aromatic radicals, without functional groups capable of reacting with the carbodiimide groups. The polycarbodiimide of the formula (1) can be an oligomeric compound, but as a rule it is a polymer.

Such polymer can have any molecular weight provided that the solid residue is not excessively limited and both the solubility in common solvents and the compatibility with the remaining components in the system are not compromised. Preferably, polycarbodiimides of molecular weight 1500-3000 are used.

Among those polycarbodiimides which can be utilized, some which can be cited for illustrative purposes only are, for example, polycarbodiimides available from Union Carbide (USA) under the trademark Ucarliffk XL 20, XL 25 SE, and XL 27 HS or else polycarbodiimides available from Bayer (Germany) under the trademark Bayderm Fix PCL.

The anhydrides useful in the composition according to the invention can be of low molecular weight or polymeric, monofunctional or polyfunctional , but the polymeric polyanhydrides are preferred.

According to a first embodiment of the invention linear polymeric polyanhydrides are used and are of the formula

U H II \\ II II

(2) X _a -0- ⁽ -C-R ₁ -C-) _nl -0-(-C-R -C-) _n2 -0....0-(-C-R _n -C-) _n -0-X _b

in which X and X-^ are selected from among atoms of hydrogen, linear or branched aliphatic hydrocarbon radicals, and aromatic hydrocarbon radicals, derived from monof unctional carboxylic acids.

R , R ₂ ....R _n are selected from linear or branched aliphatic, cycloaliphatic, heterocyclic , or aromatic hydrocarbon radicals, having any functionality provided that it is not reactive with the anhydride group, such radicals being derived from polyf unctional carboxylic acids.

n _1# n ₂ ....n are whole numbers that each vary from 0 to 30.

Usually, the sum of n _lf n2. _' - is less than or equal to 30 even if greater values are possible provided that they do not excessively limit the dry residue and do not compromise the solubility in common solvents and the compatibility with the remaining components in the system.

Examples of the radicals X _a and X _j _ are monofunctional acid radicals, for example benzoic acid, 2-ethylhexanoic acid, acetic acid, stearic acid and lauric acid.

Examples of the radicals R^, R ₂ ,....R _n are polyacid radicals, for example adipic, pi elic, azelaic, sebacic, and 1 ,4-cyclohexanedicarboxylic acids and fatty acid dimers.

The polyanhydrides of formula (2) can have molecular weights that vary as a rule in the range from 1500 to 6000.

According to an alternative embodiment of the invention, cyclic anhydrides are used containing rings or cyclic groups of anhydrides derived from polyfunctional acids.

The cyclic anhydride can be compounds of low molecular weight, carrying also only one anhydride ring per molecule, but they are preferably polymeric compounds having a main polymeric chain that contains as substituents at least two anhydride rings.

Low molecular weight monofunctional cyclic anhydrides that can be used are, for example, succinic, maleic, glutaric, phthalic, hexahydrophthalic,tetrapropenylsuccinic, tetrahydrophthalic and methylhexahydrophthalic anhydride.

When polymeric cyclic polyanhydrides are employed, their main polymeric chain can be made of polymer of varied nature. It is preferred however to use those constituent polymers of cyclic polyanhydrides of the polymeric polycarbodiimides of the formula (1) above, or else polymers

derived from olefinically unsaturated monomers also comprising olefinically unsaturated cyclic anhydrides.

Examples of polycarbodiimidic cyclic polyanhydrides are those obtained through- reaction of a polymeric polycarbodiimide of the formula (1) above with compounds having cyclic anhydride groups derived from polyfunctional acids as well as free carboxylic groups. Compounds having cyclic anhydride groups and free carboxylic groups that can be used are, for example, trimellitic anhydride or maleated acids obtained through the reaction of olefinically or dienically unsaturated with maleic anhydride. Such unsaturated acids can be selected, for example, from sorbic acid and fatty acids with conjugated double bonds.

According to a further embodiment of the polycarbodiimidic cyclic polyanhydrides, these are made from the product of reaction of unsaturated cyclic anhydrides, e.g. maleic anhydride, with polycarbodiimides of olefinic or dienic unsaturation. Such unsaturated polycarbodiimides can be obtained by reacting polycarbodiimides of the formula (1) atiove with unsaturated acids, e.g. sorbic acid and fatty acids with conjugated double bonds.

The polycarbodiimidic cyclic polyanhydrides described above incorporate the polycarbodiimidic component and the anhydride component of the presence invention, and can therefore not require the presence of a further polycarbodiimide. In fact, upon mixing with the above- mentioned hydroxylic reagent, they produce the desired coatings. Alternatively, these polycarbodiimidic cyclic polyanhydrides can be mixed, other than with the hydroxylic reagent, with a further polycarbodiimide of formula (1) to

produce compositions with a higher number of cross-linking sites and thus coatings of a higher degree of cross- linking.

As said above , the cyclic polyanhydrides can be made of , according to a further embodiment , polymeric chains derived from olef inically unsaturated monomers , also comprising olefinically unsaturated cyclic anhydrides , e .g. acrylic monomers , vinyl monomers , maleic anhydride , itaconic anhydride . The hydroxylic reagents that cons titute the further components of the coating compos i tions of the inven t ion can be monomers , o l igomers , mono- or poly- hydroxylic polymers , as for example polyester resins and/or acrylic resins functionalized with hydroxyls .

Alternatively, the hydroxylic reagent can be water also xri the form of atmospheric humidity, humidity incorporated π rurther ingredients or filler in said composition, or the humidity can be easily condensed into the varnish or paint product during spray application. Those hydroxylic re<ιy_ ' different from water that can be used are compounut. _.»__. used in the preparation of coatings based on polyisocyanates, e.g. acrylic resins, polyesters, polyols etc.

The cross-linking mechanism among the various components of the coating compositions of the invention involves the generation of free carboxylic polyacids through the reaction of the anhydride with the hydroxylic reagent according to the reaction:

0 0 l\ II »

(3) Λ* -C-O-C- ΛV + R-OH - A»/-C-0R + ΛA-C-0H

in which R is the radical of the selected hydroxylic reagent, or alternatively

0 0 0 0

\ II II

(4) ΛA -C-O-C- + H 0 -> M/ -C-OH + AV -C-OH

and thus the eventual reaction of the polycarbodiimide component with the free carboxylic compounds thus generated:

0 H

(5) Mi- »C-OH /_/_ _N '_ _c _ _N - ΛJJ

1/ I

0 c=o

I

In the case where cyclic anhydrides are used, the anhydride rings are opened by the hydroxylic reagent according to the reactions (3) and (4) above and the resulting free carboxylic compounds react with the polycarbodiimides present according to the reaction (5) above .

When the above-mentioned polycarbodiimidic cyclic polyanhydrides are used, these can be made to react with the

hydroxyl i c reagent wi thout any f ur ther addi tion of polycarbodiimide , as illustrated in the reaction that follows relative to a polycarbodi imidic polyanhydride derived from trimellitic anhydride :

and the polyacid formed can be a totally cross-linked product .

The components of the composition according to the invention are mixed in proportions suitable for obtaining a complete cross-linking of the polycarbodiimides present . The ratios used between the carbodiimidic and anhydridic functionalities are normally between 2/1 and 1/2 and between the hydroxylic and anhydridic functionalities are normally between 1/1 and 3/1.

The compositions according to the invention can be of the single-component type when they cross-link in the presence of humidi ty . In such case , they include polycarbodiimide and polyanhydride componen ts or as

compounds by themselves or as a single compound constituted by an above-mentioned polycarbodiimidic cyclic polyanhydride still containing carbodiimidic functionality. In these single-component compositions, the presence of possible fillers or ingredients containing humidity must be avoided which could cause premature hardening to the same. Alternatively, the compositions can be of the two-component type comprising a first component constituted by a mix of polycarbodiimide and hydroxylic reagent and a second component constituted by the anhydride, or else a first component constituted by the polycarbodiimide and the anhydride and a second component constituted by the hydroxylic reagent.

The application of the coating compositions takes place through conventional techniques but spray application is preferred.

After the compositions are applied, the coating gets hardened through heating that can be performed within a wide temperature range comprised for example between ambient temperature and those conventionally employed in the field of paint products. Nevertheless, an advantageous aspect of the invention consists in the possibility of using low temperatures, below 80°C, e.g. from 15 to 80°C, preferably at a temperature from ambient to 60°C. The times employed are short and depend on the temperature employed. At ambient temperature, already after 2 hours coatings are obtained without stickiness, cross-linked such as to no longer be sensitive to water.

Advantageously, the composition can include cross- linking basic catalysts, in particular tertiary amines or

quaternary ammonium salts. The amines can be used as separate compounds or as aminic groups substituted on the hydroxylic reagent or on the resins with carbodiimidic functionality. The coatings obtained with the compositions according to the invention offer a high level of hardness and resistance to solvents, as demonstrated by double rubbing tests with xylene.

The examples that follow are purely illustrative, and non-limitative, of some possible embodiments of the compositions according to the invention. Based on the description above and the examples that follow, a man skilled in the art can achieve many other compositions all falling within the scope of the invention as presently defined.

In the examples, the following products have been employed: Products used

1) A 101 hydroxylated acrylic polymer having: 0H% .5

%solids 60

Gardner viscosity w weight mean molecular weight (Mw) 5500-6500 2) Carbodii ides (available from Union Carbide - USA) :

^%S olids Equivalent Weig _h t

Ucarlink XL 20 50 in MPA 175

Ucarlink XL 25 SE 50 in MPA 300

Ucarlink XL 27 HS 74 in MPA 220

3) Desmophen 670 - Bayer - hydroxylated polyester

4) Byk 300 - silicone additive (used as surfactant)

5) MPA - methoxypropyl acetate (used as solvent) .

Example 1

PAL 01-Linear polyanhydride that is prepared from:

Moles

Adipic acid 15

Azelaic acid 15 Laurie acid 2

Acetic anhydride 45

In a reactor equipped with a fractionating column, the acetic acid which forms is slowly extracted and also the excess of acetic anhydride by heating up to 160°C at the end and also under vacuum.

The product obtained is diluted to 50% with toluene. Example 2 έnamel cross-linkable with linear polyanhydride and polycarbodiimide grams

A 101 39.49

Titanium dioxide 24.33

Kromos RN 59 MPA 7.62 Byk 300 1

Ucarlink XL 27HS 13.27

PAL 01 (Ex. 1) 13.27

Dimethyl lauryl amine 0.8

Example 3

Enamel cross-linkable with monofunctional cyclic anhydride grams A 101 44.28 Titanium dioxide 22.14

Kronos RN 59 MPA 7.18

Ucarlink XL 27 HS 11.07

Byk 300 0.2 Succinic anhydride 14.76

23% in acetone Dimethyl lauryl amine 0.37

Example 4 Preparation of polycarbodiimidic cyclic polyanhydride (without residual carbodiimidic groups)

PCC 01 grams

A Ucarlink XL 27 HS 300

B Solution of trimellitic anhydride 864

20% in MPA With A inside , the reactor is heated to 50 +. 10°C and B is added s lowly over a period of about 3 hours . The disappearance of the carbodiimidic group is monitored with an I . R . spectrophotomet er . The re actor i s cool ed and discharged . Example 5

Preparation of polycarbodiimidic cyclic polyanhydride (with residual carbodiimidic groups (about 50%))

PCC 02 grams

A Ucarlink XL 20 360 B Solution of trimellitic anhydride 432

20% in MPA With A inside, the reactor is heated to 50 +.10°C and B is added slowly over a period of about 2 hours. The reactor is cooled and discharged. Example 6

Preparation of polycarbodiimidic cyclic polyanhydride (with residual carbodiimidic groups (about 66%) )

PCC 03 grams.

A Ucarlink XL 25 SE 600 B Solution of trimellitic anhydride 288

20% in MPA With A inside, the reactor is heated to 50 ± 10°C and B is added slowly over a period of about 2 hours. The reactor is cooled and discharged. Example 7

Enamel cross-linkable with one polycarbodiimidic cyclic polyanhydride grams A 101 38.90 Titanium dioxide 23.66

Kronos RN 59 Byk 300 0.2

Toluene 3.96

PCC 01 (Example 4) 33.26 Example 8

Enamel cross-linkable with polycarbodiimidic cyclic polyanhydride and polycarbodiimide grams A 101 33.38 Titanium dioxide 23.61

Kronos RN 59

Toluene 6.16

Byk 300 0.2

Ucarlink XL 27 HS 7.51 PCC 01 (Example 4) 29.14

Example 9

Enamel cross-linkable with polycarbodiimidic cyclic polyanhydride containing also the carbodiimidic functionality gram

Desmophen 670 17.52

Butyl acetate 9.51

Titanium dioxide 22.29

Kronos RN 59 Byk 300 0.2

PCC 02 (Example 5) 50.55

Dimethyl lauryl amine 0.03

Example 10

Clear varnish formed from carbodiimide functionalized polycarbodiimidic cyclic polyanhydride cross-linkable with humidity. grams

PCC 03 (Example 6 ) 99.8

Dimethyl lauryl amine 0.2 Example 11

PCP 01

Polymeric cyclic polyanhydride grams

Styrene 20 Methyl methacrylate 9

Butyl acrylate 10

Maleic anhydride 11 Vazo 67 (polymerization 2 initiator of Du Pont)

Butyl acetate 48 Example 12 Enamel cross-linkable with polymeric cyclic polyanhydride and polycarbodiimide grams

A 01 45.39

Titanium dioxide 25 Kronos RN 59 MPA 6.76

Byk 300 0.20

PCP 01 19.63

Dimethyl lauryl amine 0.75 Toluene 2/27

The enamels and paints prepared in the examples above were applied by spraying on different metallic, plastic, wood, etc., substrates pre-painted or not with primer, and were hardened at low temperatures from 35 to 60°C, for periods of 15 minutes to 1 hour, or else at ambient temperature, giving coatings of a high level of hardness and resistance to solvents. Such characteristics turn out to be equal to those of the best conventional painted products, fully conforming to the most stringent specifications of the users.