EP0362602A2 - Process for dressing leather - Google Patents

Abstract

In the dressing of leather by means of spray application in an electrostatic field, highly concentrated solutions can be used, if these solutions are sprayed into a solvent atmosphere in the presence of inert gases. The dressed pieces obtained can be stacked immediately. By means of the concentrated procedure, a recirculation of the solvents from the outgoing air is possible in an economically justifiable manner.

Description

The invention relates to a method for finishing leather by spray application of optionally water-containing organic solutions in the electrostatic field.

The most common method of dressing leather so far is spraying in air flow or airless, as these methods can be used to achieve the best effects in terms of grip, external aspect and fastness to rubbing and buckling.

On the other hand, this method also has a number of serious disadvantages. In this context, the high spray losses of material to be applied should be mentioned first and foremost. In addition, it requires the use of highly diluted solutions (i.e. with only about 2-4% solids content), which makes recovery of the solvents from the exhaust air in an economically reasonable range almost impossible.

It has therefore already been proposed (cf. DE-A 3 611 729) to spray leather pieces with dressing solutions in the electrostatic field.

However, this method, which is elegant per se, has not been able to prevail in practice, since it only allows the spraying losses to be reduced, but not the absolute amount of organic solvents.

If one now tries to work with concentrated solutions in the electrostatic field, one usually observes a deterioration in the spray course and spray pattern compared to the conventional spraying in air flow or airless, ie. H. apart from the better application of the sprayed material, the process according to DE-A 3 611 729 offers no significant advantages.

It has now surprisingly been found that the disadvantages of the known processes for finishing leather can be largely overcome if the finishing liquors are sprayed from the start into a water and / or solvent atmosphere which has a residual oxygen content of at most 15% by volume.

This new process solves two important problems to date: It enables the use of highly concentrated spray liquids and the processing of the solvent-containing exhaust air in an economically justifiable way.

The solids content of the dressing liquors can be up to 50% and the viscosity at working temperature up to 10,000 cP. It is preferred to work with liquors which have a viscosity of 500-5,000 cP at working temperature. In general, the working temperature is 20-40 ° C (about room temperature). With highly viscous polymer-containing liquors, the temperature can also be increased to 60-80 ° C to reduce the viscosity.

The residual oxygen in the solvent atmosphere is preferably less than 10, particularly preferably less than 7% by volume.

The atmospheric oxygen content is reduced to these values by supplying inert gases, such as. B. nitrogen, argon, CO₂ or, if necessary, water vapor.

The solvent content in the solvent atmosphere should be 10-100%, preferably 15-90% and particularly preferably 25-75% of the saturation concentration. If the solvent concentration is too close to saturation, condensation and undesirable droplet formation can occasionally occur. The same substances that are used to produce the spray solution are preferably used as solvents for generating this atmosphere.

All solvents used in the finishing of leather can be used as solvents, such as. B. esters, ketones, ethers, ether alcohols, alcohols, ether esters and aromatic hydrocarbons. Furthermore, the finishing liquors can contain conventional leather auxiliaries, such as. B. Crosslinking agents, waxes, dyes, fillers, matting agents, pigments, handles, viscosity regulators, drying agents, etc.

In principle, all polymers used in leather finishing can be used with the new process, such as cellulose esters (nitrocellulose, cellulose acetobutyrate), polyamides, polyurethanes, polymers and copolymers of vinyl chloride, vinylidene chloride and vinyl acetate etc.

Even highly reactive 2-component systems, such as those used for. B. are described in DE-A 3 309 992, can be processed according to this technique. An electrostatic spray gun is preferably used, which is a mixing system such. B. upstream according to DE-A 2 746 188.

According to the new process, finishes for high-quality leather, as required in the furniture and automotive upholstery sector, can be produced, with particular emphasis being placed on the fact that the leather pieces can be stacked immediately after the finish, if they are appropriately dried.

When carrying out the new method, it is expedient to use the spray booth shown in FIG. 1, which essentially has 2 measuring points, oxygen measuring device, inert gas and compressed air supply, spray nozzle for the solvent, ventilation, spray bell, dressing liquor, high voltage generator, movement device for the workpiece and grounded support table is equipped.

Before commissioning the cabin with z. B. inerted nitrogen and sealed gas-tight. The oxygen in the air is supplied by adding z. B. nitrogen (4) to less than 15, preferably less than 10% by volume. The interior of the spray booth is then saturated with solvent. This solvent is injected into the spray booth up to 10-100% (preferably 19-90%) of the saturation concentration through a spray gun (6) built into the booth wall.

For oxygen measurement, the oxygen-containing gas is removed at measuring points (1) and (2) and the oxygen content is determined using an oxygen measuring device (3). Only when the safety value of 15 vol.% Or less O₂ has been reached can the preparation be started by switching on the high voltage (10).

The dressing is carried out, for example, according to the principle of the electrostatic high-rotation spraying process. With this electrostatic spray technology, a high-voltage field is generated between the spray bell (9) and the grounded workpiece (leather) (11). The liquid to be sprayed is pumped from the dressing liquor (8) into the bell (9) rotating at high speed, where it is atomized. The atomized paint particles become negatively charged at the edge of the bell and are then guided to the earthed workpiece by means of the electrostatic field forces and deposited there, releasing their charge. The grounded support table (13) transfers its grounding to the workpiece. It goes without saying it is also possible to apply the solutions with normal spray nozzles, ie an atomizing process which is carried out airless or with air.

After finishing, the leather is transported out of the spray area by means of the movement device (12). The atmosphere loaded with solvent can be disposed of through the vent (7), e.g. by freezing or absorbing the solvent, which can be recirculated if necessary. If necessary, the interior of the spray booth can be ventilated with compressed air (5).

The detailed process conditions are described in the examples below.

example 1 a)

A solution of product I (see below) is processed at 20 ° C on an electrostatic spraying system, type Ransburg. The electrostatic spraying system is installed in a cabin that is filled with a gas mixture that is changed 15 times an hour in the continuous inflow and outflow and that consists of a mixture of room air / nitrogen in which the oxygen content is below 5 vol. -% lies.

The setting of the spray bell was set to Control air 1.2 bar Ring air 3.5 bar Revolutions of the bell 10-35,000 rpm tension 70 KV

The solution is sprayed, but there is only an intensive thread formation in the air gap between the bell and the substrate, as it is described in the parlance with "spiders". Changes regarding control air, ring air or revolution have no influence.

b)

The procedure is as in Example 1a, but with the difference that the spray booth is charged with a gas mixture of nitrogen / diacetone alcohol (DAA), whereby the cabin atmosphere is saturated with DAA (10-11 g / m³ DAA cabin space). The result is a spray cone in which no "spinning" can be observed. When the product hits the substrate to be sprayed - in this case a cardboard box to determine the quantity to be applied - the solution runs and forms a smooth film.

Example 2 a)

Product 1, dissolved (1: 1) in toluene / isopropanol 30%, is experimented with in the machine setting and in an atmosphere as in Example 1b at 25 ° C. The bell's turbine barely rotates because the viscosity of the solution is too high (viscosity at 20 ° C is 200 cP).

b)

The procedure is as in Example 2a, but with the difference that the solution is heated to 80 ° C. and the number of revolutions of the turbine is set to 35,000 rpm. The result is a spray jet in which the solution in droplet form is very well distributed, no "spinning" can be observed, and the spray cone has the ideal bell shape, and which runs on a leather for a very good and even finish application (the viscosity of the solution is 600 cP at the application temperature).

Example 3

100 g of the solution from Example 2b were added to 20 g of a 20% solution of cellulose acetoburyrate in acetone / diacetone alcohol (60:40). This solution can also be sprayed well.

Example 4 a)

A 30% aqueous dispersion of product II is sprayed according to the following recipe in the cabin, which is filled with room air:
3 parts of the PU dispersion 30% (18% solids)
2 parts water
0.4 parts of a soot stain

The liquor has a viscosity which is characterized by a flow time in the Ford cup of 4 mm of 13 ". Although the dispersion has a viscosity suitable for spraying, the result is a poor spray pattern. The spray droplets hitting the substrate have already dried to the surface, that a sufficient course is not guaranteed.

b)

Batch and machine setting as in Example 4a, but with the difference that the atmosphere in the spray booth was changed by blowing in diethyl ketone / water vapor and nitrogen. The spray material leaves spray themselves perfectly, the course on the substrate is perfect.

Example 5

A PU reactive system is injected, which has a viscosity of approx. 3000 cP at RT and a solids content of 90%.

Composition:

50 parts of the PES polyurethane NCO prepolymer mentioned under product III
50 parts of the PE polyurethane NCO prepolymer mentioned under product III
15 parts of a grind in cyclohexane of an iron oxide pigment of brown color
5 parts of a silicone oil
10 parts of methoxypropyl acetate and
5 parts of diethylene glycol

The consistency in this consistency (from 12,000 cP at 20 ° C) is not sprayable, after heating to 80 ° C the viscosity is reduced to 600 cP and the batch can be sprayed with very good flow properties.

The control air was set to 3 bar; the turbine had a speed of 40,000 rpm.

Example 6

In a modification of Example 1 of DE-A 2 637 115, the following experiment was carried out:

Two metering pumps, one for prepolymer A (see below) and the other for hardener 1, convey into a mixing chamber, which has a mixer according to EP-A 1581, where it is mixed with the aid of nitrogen and via one with a spray electrode provided spray gun on a reversely charged (grounded) die glued to an aluminum plate. Due to the solvent in the spray booth atmosphere, the flow of the sprayed material on the die is excellent. The mass sprayed onto the matrix runs like a film and begins to set after about 1 minute from the time of spraying. The split leather to be coated is placed on the reactive mass and pressed on. The entire coating then passes through a drying tunnel heated to 80 ° C. After about 6 minutes from the time of spraying, the coating is removed from the die without sticking.

The polyurethane urea layer has a thickness of 0.22-0.25 mm.

The coated split leather has a deceptively similar grain to natural leather, is dry after a short time, stackable and can be processed on common shoe machines. The adhesion between the coating and split leather is excellent, the handle pleasantly dry.

Example 7

From equal parts by weight of a poly-di-ethylene glycol adipate (molecular weight 2000) and polyethylene glycol (molecular weight 400), a prepolymer was produced using isophorone diisocyanate, which was extended to the polyurethane urea with hydrazine hydrate. The polyurethane is a 40% solution in toluene / isopropanol / methoxypropanol-2 (3: 3: 1). The solution has a viscosity that cannot be measured in a DIN cup (4 mm; DIN 53211) (in the Haake viscometer it had more than 20,000 cP at 22 ° C). It was also not sprayable.

Dilution with the same solvent mixture as in Example 8 to a viscosity of 85 seconds (180 cP at 22 ° C.) and a concentration of 14.5% gave an "airless" solution which was still not sprayable, but which was very good according to the invention was processable.

Airless was only able to process this product in conventional processes with a concentration of 11.4% and a viscosity of 17 seconds.

Example 8

1 part of a polyurethane made from hexane diisocyanate (3.5 parts) and (96.4 parts) polyester made from butanediol-hexane diol polyadipate (molecular weight 5,000) and trimethylolpropane (0.04 part) and 2 parts of a cellulose acetobutyrate were dissolved 15% in a mixture of ethyl acetate / butyl acetate (1: 1). The solution had a viscosity of 70 seconds in a DIN cup (200 cP at 22 ° C). According to the invention, it was very good, but airless could only be sprayed after dilution to below 8% (viscosity: 20 seconds).

The products used above are explained in more detail below.

Product I:

One-component thermoplastic polyester-polyurethane as a 30% solution in toluene / isopropanol (1: 1) consisting of an adipic acid-hexanediol polyester with an average molecular weight of 2,000 reacted with isophorone diisocyanate in a molar ratio NCO: OH of 1: 1.

Product II:

30% diethyl ketone / water (1: 9) dispersion of a polyester urethane, consisting of a polyester of adipic acid / dihydroxypropionic acid / hexanediol with a MW of 1,600 with free COOH groups, which serve as hydrophilic free COOH components, saturation of the free COOH components by an aliphatic diamine and reaction of the polyester thus obtained with isophorone diisocyanate in a molar ratio of NCO / OH of 1: 1.

Product III:

A highly reactive 2-component polyurethane according to DE-A 2 637 115, consisting of a 70% polyester prepolymer from adipic acid / hexanediol with a MW of 2,000 reacted with TDI-2,4 in a molar ratio of NCO: OH of 2: 1 and dissolved in toluene 80% solid. It is used together with a polyether polyurethane consisting of 4.4′-diisocyanatodiphenyl methane and a polyether consisting of polypropylene glycol ether (MW 2,000) (NCO: OH = 2: 1) in a mixing ratio of 1: 1.

Prepolymer A

444 g of 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate) are placed in the reactor. At room temperature, 9 g of butanediol- (1,4), 9 g of trimethylolpropane and 1,600 g of a hydroxyl polyester of adipic acid, ethylene glycol, diethylene glycol and butanediol- (1,4) with a hydroxyl number of 56 and a molecular weight of 2,000 are added in succession with stirring admitted. The reaction mixture is warmed and held at 110 ° C. for about 1 hour (until constant NCO). After cooling to 65 ° C., the reaction mixture is diluted with 412 g of methyl ethyl ketone and 206 g of toluene, corresponding to a 77% solution.

The prepolymer solution has a viscosity of 1,000 cP at 20 ° C.

Hardener 1

A mixture of 170 g of 3,3,5-trimethyl-5-aminomethyl-cyclohexylamine (IPDA), 13 g of water and 417 g of methyl ethyl ketone is boiled under reflux for 2 hours. After cooling, the mixture is ready to use as a hardener.

• A) 12,9 Mol% als freies IPDA
• B) 41,6 Mol% als
  
• C) 45,5 Mol% als Bis-Methyläthylketon-Ketimin von IPDA

The mixture contains the following 170 g (1 mol) of IPDA:

• A) 12.9 mol% as free IPDA
• B) 41.6 mol% as
  
• C) 45.5 mol% as bis-methyl ethyl ketone ketimine from IPDA

The mixture also contains a total of 37.88 g of water.

Claims (5)

1) Process for finishing leather by spray application of finishing liquors optionally containing water-containing organic solvents in the electrostatic field, characterized in that the finishing liquors are sprayed from the start in a solvent atmosphere which may contain water-containing organic solvents and which has a residual oxygen content of at most 15% by volume . 2) Method according to claim 1, characterized in that the residual oxygen content is less than 10, preferably less than 7% by volume. 3) Method according to claim 1, characterized in that the viscosity of the liquors at working temperature is 500-5,000 cP. 4) Method according to claim 1, characterized in that the spraying takes place in the presence of an inert gas. 5. Apparatus for carrying out the method according to claim 1, characterized in that it consists essentially of a spray booth suitable for carrying out electrostatic spraying processes in the high rotation process, airless or by means of compressed air atomization (using inert gas instead of air), which with oxygen measuring device, inert gas and compressed air supply, one with one Spray liquor, high voltage generator, ventilation, movement device for the workpiece and grounded support table is equipped for the fleet.

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