DE1955479A1 - Use of polyurethanes for the production of coatings by electrostatic powder coating - Google Patents

Description

1955473

FARBENFABRIKEN BAYER AG

LEVERKU S E N-Beyerwerk Patent Department Wr / Hd

> 1 NOV. 1969

Use of polyurethane for the production of coatings through electrostatic powder coating

The invention relates to the use of polyurethanes for the production of coatings by powder coating using the electrostatic powder spraying process.

It is known to produce coatings on metal sheets and pipes by electrostatic powder coating. Electrostatically charged plastic powders are sprayed onto metal parts and then thermally sintered so that homogeneous and closed plastic pilms that protect the metal surface against corrosion.

As plastic powder, polyethylene, polyepoxide and Polyamides with a grain size of 100 u are used. Paint films made from these plastics show a number of disadvantages such as swellability in organic solvents, insufficient lightfastness and weather resistance as well too low hardness.

Polyurethanes have also already been used for the electrostatic powder spraying process. The movies are characterized by excellent abrasion resistance and good adhesion to metals, processing technology

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however, difficulties associated with the application of the polyurethanes described. Owing to the sharp melting point of the polyurethanes described and their low melt viscosity, the sintering conditions are difficult to set, and film coatings with uneven layer thickness were often obtained. In addition, because of their brittleness, the polyurethane films obtained have poor buckling resistance, especially at lower temperatures. The known polyurethanes sometimes have poor storage stability and die after some time.

It has now been found that for electrostatic powder coating powders of such polyurethanes are outstandingly suitable, which are composed of polyisocyanates with aliphatically bound isocyanate groups and glycols with aliphatically bound hydroxyl groups using polyfunctional components with more than 2 hydroxyl groups and / or those with more than 2 isocyanate groups have been produced.

The present invention thus relates to the use of branched polyurethanes, which are made from aliphatic polyisocyanates and glycols with aliphatically bonded hydroxyl groups and a molecular weight of up to 400 with the use of 0.5-15 wt .- ^ based on the polyurethane composition of polyfunctional components with more than 2 Hydroxyl groups and / or those with more than 2 isocyanate groups and a molecular weight of up to 700 are obtained for the production of coatings by electrostatic powder coating

These for the production of coatings or films according to the invention Polyurethane powders used are solvent-free one-component systems that are advantageous

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differ from the usual two-component polyurethane systems, which consist of solutions of polyhydroxyl compounds and solutions of polyisocyanates, mixed before use and must be processed within a certain time. After the application, the films then bind chemically to be crosslinked, insoluble coatings, while the mostly toxic and flammable solvents evaporate.

Also the polyurethane lacquers that set in the air with the water contained in the air, the isocyanate groups that are still free have and are present as dissolved prepolymers, are due to their limited storage stability and great sensitivity Can only be used to a limited extent against moisture, especially if they contain pigments.

Oil-modified polyurethanes, which are exposed to the air by oxypolymerization The existing oleic acids can only crosslink, i.e. film, in the form of their solutions in organic solvents are processed.

These one-component Polyurethanlacksy stars according to the invention are insoluble in organic solvents. They are applied in powder form to substrates using the electrostatic powder coating process and then melted, creating insoluble, high-quality polyurethane coatings.

These polyurethane powder coatings according to the invention differ differs advantageously from those in the literature Polyurethanes described and used for the electrostatic powder spraying process by a broader one o melting range and higher melt viscosities. they show a much more favorable theological behavior, whereby

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the aforementioned technical difficulties with regard to the setting of the softening and softening conditions and a uniform layer thickness can be avoided. The coatings obtained are elastic and are distinguished by good fastness to buckling. The powder coatings are characterized by "particularly good storage stability.

By using the polyfunctional components, products are obtained that have a special Characterize favorable theological behavior in the molten and sintered state.

The use of the polyfunctional components with more than 2 NCO groups and / or those with more than 2 OH groups also reduces the brittleness of the lacquer coatings and increases their elasticity. By varying the content of the polyfunctional components, the melt viscosity and the flow, softening, sintering and film formation behavior can be adjusted as desired * The polyfunctional components are expediently used at more than 0.5 and less than 15 wt . Preference is given to 1-10 wt .

The paint films according to the invention are also characterized by special light and weather resistance, hardness, abrasion resistance, Water resistance and adhesion. Particularly . to be emphasized is the excellent resistance to organic solvents. The polyurethanes used can be melted, e.g. in extruders pigment perfectly.

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The polyurethanes preferably embossed according to the invention have melting or softening ranges between 80 and 2oo ° C, preferably between 12o and 18o ° C, so that they can be used without damaging the materials to be coated and without decomposition.

From starting compounds for the preparation of the invention Polyurethanes to be used are polyols with aliphatically bonded hydroxyl groups and having a molecular weight up to 400 such as ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, methylol, 6-hexanediol, Ti'i-methyl-hexanediol, neopentyl glycol, 1 ^ -Bis-hydroxymethylene-cyclohexane, methylene glycol, thiodiglycol, Methyl diethanolamine, xylylene glycol, oxyethylation products of bivalent phenols and glycols such as bishydroxyethyl-hydroquinone and 1,6-hexanediol in imprint. Mixtures of different glycols can also be used. Examples of suitable components with more than 2 hydroxyl groups are: hexanetriol- (1,2,6), glycerol, pentaerythritol, 2-Hydroxy-methyl-hexanetriol-1,2,6, 1,1,4,4-tetramethylolcyclohexane, Trimethylolethane, trimethylolpropane, and hydroxymethyl-1,6-hexanediol. Trimethylolpropane is preferably used.

As aliphatic polyisocyanates, for example, come Dicyclohexamethandiisocyanat, 1,4-butane diisocyanate, 1,6-hexane diisocyanate, trimethyl-hexane diisocyanate, xylylene diisocyanate, methylcyclohexane diisocyanate, Diisocyanatocarbonsäureester, as described in British patent specifications 1 965 474 O72 956, and for example, in question. 1,6-hexane diisocyanate is preferred. Examples of suitable polyisocyanates with more than 2 isocyanate groups are:

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ORIGINAL INSPECTED

1 9 5 5 '-r 7

Biuret triisocyanates, which are obtained from aliphatic diisocyanates by reaction with water with the formation of urea and biuret groups can be produced, such as preferably the biuret trihexane triisocyanate produced from 1,6-hexane diisocyanate,

OCN- (CH _{2) 6} -NH-CO-N (CH _{2) 6} -NC0

CO

NH

NCO

and addition products of aliphatic diisocyanates with polyols having at least 3 hydroxyl groups and reaction products obtained by adding acrylonitrile to polyols, hydrogenation and phosgenation .

The preferred polyisocyanate is the biuret triisocyanate made from 1,6-hexane diisocyanate.

The polyfunctional reaction components preferably have a molecular weight of up to 700.

The components are generally reacted at an NCO / OH ratio of 0.8-1.2, preferably 1: 1.

The manufacture of the invention for the electrostatic Polyurethanes to be used in powder coating processes are advantageously carried out in solution processes, with such solvents are used that do not react with the polyisocyanates and the glycol components, such as acetone, Tetrahydrofuran, dioxane, methyl ethyl ketone, benzene, toluene,

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Xylene, methyl acetate, butyl acetate, chloroform, carbon tetrachloride, Chlorobenzene. The production of the polyurethanes takes place, for example, in the knowledge that the components in the Solvent to be dissolved. The reaction is carried out between 4o-16o ° C, preferably 6o-130 ° C. The optimal Temperatures are expediently determined in series tests. Elimination begins after a while of polyurethane. The precipitated reaction product is filtered off with suction and dried.

The components can also be reacted in a solvent in which the reaction product is also soluble. Such Solvents are e.g. formamide, dimethylformamide, dimethylacetamide, Dimethyl sulfoxide, etc. The dissolved reaction product is either by distilling off the solvent or by precipitation with water or solvents in which the reaction product does not or only slightly dissolves and which are readily or at least partially miscible with the solvents in which the polyurethane dissolves and freed of solvent residues by drying, if appropriate in vacuo and if appropriate at elevated temperature. Tetrahydrofuran and acetone, for example, have proven suitable for the precipitation. In this embodiment particularly finely divided powders are obtained.

It is also possible to isolate the polyurethane from the solutions by spray drying, the solution at increased Temperature is atomized under pressure through nozzles into a drying chamber. This procedure will also be very get small average grain sizes.

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The polyurethane can also be produced solvent-free in the melt by heating to 40-2oo ° C, preferably -6o-130 ° C will. The melt is poured into plates and, after cooling, granulated, ground and fractionated sifted. The reaction can be carried out gradually and in any order in the presence or absence of a Solvent are carried out.

The polyurethanes obtained in this way can be converted into electrostatically sprayable powder coatings in the following practical way:

The granulated or already fine-grained polyurethane is mixed with the intended amount of pigment using a dry mixer, e.g. 3o - 100 wt .- <$ based on the binder

and iron oxide, chromium oxide or, for example, titanium dioxite pigaeate then bowed on the melting path in an extruder (for example in a heartable twin-screw kneading screw).

Residence times of 20-3o seconds in the extruder are sufficient, the exit temperature of the melt being natural is slightly above the softening point of the polyurethane.

The cooled melt is granulated and then finely ground on blower mills using carbon dioxide snow or in deep-freeze grinding systems that are cooled to minus 80-1oo ° C by liquid nitrogen.

The resulting mixture of different particle sizes is through sieving machines of too coarse particles, for example exempted over 90 / u diameter. Should also, the finest Particles of more than 5 diameters must be eliminated, so must a sifting (e.g. spiral wind sifter) can be carried out.

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The powder fractionated in this way is easy to flow, can can also be stored at 40 ° C without clumping or clumping.

It can be charged electrostatically, preferably negatively, and when voltages of 4o - 5o KW are applied, it can be deposited on a degreased, conductive surface, primarily metal, with the help of commercially available powder spray guns. The resulting coating can be converted into a homogeneous, well-adhering and smooth film by heating to temperatures of 5 ° -1o ° C. above the softening point or the maximum point of the softening interval in the course of 20-3o minutes. The layer thickness increases with increasing tension and grain diameter. Film thicknesses of 8o-15o ix can be achieved; by coating heated substrates, film thicknesses of several hundred microns can be achieved.

The excess flying past the object to be coated Powder is collected by suction devices in a cyclone, cleaned of agglomerates by sieving and fed back to the spray gun.

The films formed as described are characterized by hardness, elasticity, good adhesion to the substrate, good Water resistance and chemical resistance, especially due to good light and weather resistance the end.

Production of the polyurethanes to be used according to the invention:

a) Polyurethane I.

6o.1 percent by weight 1,6-hexanediocyanate

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-ΊΟ

24.4 percent by weight 1,6-hexanediol

14.3 "neopentyl glycol

1.2 "trimethylol propane

The polyurethane is produced by reacting it in toluene at 80 ° C. for several hours. Towards the end of the Reaction, the product precipitates in powder form. The powder is dried at 5o C and 2o Torr. Melting range: 14o-15o ° C.

b) from polyurethane II

53.7 percent by weight 1,6-hexane diisocyanate

8.3 "Trihexane biuret triisocyanate

16.4 "1,6-hexanediol 21.6" neopentyl glycol

The polyurethane is produced by reacting for several hours at 50 ^° C. in toluene. Towards the end of the reaction, the product precipitates in powder form. The dried product has a melting range of 13o - 14o ° G and can be used directly for mixing with titanium dioxide.

Polyurethanes I and II, as powder, have an average particle size of about 2oo yes.

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Example 1;

1οο parts by weight of the insoluble, meltable polyurethane I with a softening point of 150 ° C. were mixed with 50 parts by weight of rutile titanium dioxide using a mixer and then melted in a heatable twin-screw kneading screw. The residence time (melting period) was 40 seconds, the exit temperature was 160 ° C.

After solidification, the melt was pre-crushed in a tendon mill; These granules with a grain of durchscnittuch 2 mm in diameter was then finely ground in a blast mill, and finally liberated the powder obtained by sieving the particles through 8o yes diameter and 4o diameter.

The powder produced in this way had good free-flowing properties, i.e. it resembled a liquid in its properties, and could be sprayed electrostatically well.

ΓIe particles were negatively charged; the applied voltage was 50 kv, the delivery pressure 1.8 at, the mist (= atomization) pressure 3 at. The particles adhered well to de-rusted, degreased metal sheets. It was baked for 30 minutes at 160 ° C; depending on the amount of powder applied, layer thicknesses of 8o-35o ji could be achieved.

The films were hard glossy, free from defects and bubbles. They have excellent resistance to organic Solvents and chemicals such as acids or alkalis. After 1700 hours of exposure in the Weatherometer, see neither yellowing nor chalking; the weather resistance is thus excellent. Film adhesion and elasticity are good.

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Example 2; ^

100 parts by weight of polyurethane II with a softening point of 130 ° C were brought to an average grain size of 1 mm in diameter with the aid of a cutting mill and this granulate is mixed in a mixer with 50 parts by weight of titanium dioxide, rutile type. This mixture was melted in a heatable, twin-screw kneading screw for 40 seconds; the outlet temperature of the melt was 18o ° C.

After cooling, it was pre-crushed on a cutting mill down to an average grain size • of 2 mm. Then a powder with a Grain sizes from 4o - 80 yu are produced.

This free-flowing powder was sprayed electrostatically. A commercially available spray gun was used (Sames, Grenoble). The particles were negatively charged. The applied voltage was 50 kv, the delivery pressure 1.8 at, the mist or atomization pressure 3 at.

The deposition on de-rusted and degreased sheets was perfect; the particles adhered for more than 12 hours. The stoving took place for 30 minutes at 160 ° C. The film thickness could be increased up to 35ο μ .

The films were hard and glossy with a smooth surface. Adhesion and elasticity were compared to by-. game 1 improved. The films have excellent resistance to water, organic solvents and acids and alkalis.

Resistance to light and weather is very good: after 1,700 hours in the weatherometer, neither yellowing nor chalking can be detected.

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Comparative example:

By way of comparison, a polyurethane powder is produced from 58.7 percent by weight 1,6-hexane diisocyanate and 41.3 percent by weight 1,6-hexanediol by reacting them in benzene at 80 ° C. for several hours. After the reaction has ended, the product precipitates in powder form. 1oo grams of the dried polyurethane powder having a softening point of 15o ° C were 5o g of titanium dioxide, Rutilware, mixed and homogenized in an extruder at 6o see dwell time at a jacket temperature of 14 ⁰ C-O. After solidification, the melt was granulated in a cutting mill, ground in a hammer mill and then sieved. The particles over 80 ^ are eliminated. The powder obtained in this way was sprayed electrostatically with an applied voltage of 50 KV (negative charge) and adhered well to degreased metal sheets.

The sintering when heated up to 180 ° C is, regardless of an increase in the usually half-hour stoving time to 1 to 1 1/2 hours, very poor, and no smooth films are formed. Only from about 19o ° C upward do sintering and somewhat flow occur. The films formed in this way are inelastic even with a thin layer of 45 μ , for example, and get cracks when the sheets are bent, even if the mandrel diameter is 8 mm, for example, in the mandrel bending test. The adhesion to the metal substrate is poor. The films were characterized by a good gloss and sufficient hardness, but due to the low melt viscosity, they show a non-uniform layer thickness. The processing difficulties are due to the sharp melting point and the low melt viscosity of the product.

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Claims (4)

Claims; ! 1. Use of branched polyurethanes, aliphatic polyisocyanates and from ^ ~ s glycols containing aliphatically bound hydroxyl groups and a molecular weight of up 4oo with concomitant use of o, 5-15 wt .- ^ based on the Polyurethänmasse of polyfunctional components having more than 2 hydroxyl groups and / each such with 'more than 2 isocyanate groups and a molecular weight to 700 can be obtained for the production of coatings by electrostatic powder coating. 2. Use of polyurethanes according to claim 1, in their production as a polyfunctional component with more trimethylolpropane is also used as 2 hydroxyl groups. 3. Use of polyurethanes according to claim 1, in their Manufactured as a polyfunctional component with more than 2 isocyanate groups, that made from 1,6-hexane diisocyanate Biuret trihexane triisocyanate OCN (CH,) g-NH-CO-N- (CH _{2) 6} -NCO CO NH (CH ₂ ) ₆ -NC0 is also used. 4. Obtained by electrostatic powder coating varnish coatings made of polyurethanes according to claims 1-4. Le A 12 597 -H- ORlGINAL INSPECTED 10 9 8 21/19 9? oriwnml