WO1992000389A1

WO1992000389A1 - Aqueous emulsion of filmogenic resins for finishing leather and leather splits, and its application process

Info

Publication number: WO1992000389A1
Application number: PCT/EP1991/001138
Authority: WO
Inventors: Giovanni Tadiello; Giuseppe Li Bassi; Maria Carla Cattaneo; Davide Bortignon; Giuliano Bertozzo
Original assignee: L. Lamberti S.P.A.; Ge.Ma.Ta. S.P.A.
Priority date: 1990-06-28
Filing date: 1991-06-20
Publication date: 1992-01-09
Also published as: IT1249019B; IT9020800A1; AU8095391A; IT9020800A0

Abstract

The invention relates to an aqueous emulsion of filmogenic resins for finishing leather and leather splits, and the relative application process.

Description

AQUEOUS EMULSION OF FILMOGENIC RESINS FOR FINISHING LEATHER AND

LEATHER SPLITS, AND ITS APPLICATION PROCESS

This invention relates to an aqueous emulsion of filmogenic resins for finishing leather and leather splits, and its application process.

In this test the term "leather" will be used to indicate leather as such, leather splits, sheepskin, nappa, whole skins for upholstery, buffed leather and full grain leather.

The emulsion according to the invention contains one or more foaming agents and one or more foam stabilizers.

By applying the emulsion according to the invention to leather a high quality finish is obtained, together with product enhancement if used for finishing splits.

The present invention also relates to a process for applying the aqueous emulsions to leather, said process consisting of foaming the emulsion, applying it continuously in a thin layer to the leather with a roller machine, and drying the film obtained. The technical problem

The leather finishing and leather split enhancement industry is still largely based on batch operations involving a large use of labour. There is therefore a tendency to reduce labour costs and increase cycle productivity by using continuous processes. There is also a further tendency to reduce the energy costs of the various operations and to minimize their environmental impact, for example by seeking to limit the use of volatile solvents where possible. It is difficult to obtain excelent results and to follow up this tendency. For example, in continuous finishing with synthetic resins, the so-called napping, it is difficult to obtain good covering of the leather surface, especially in the case of splits, without having to resort to repeated applications or to costly expedients. There is therefore a requirement for new or improved products and processes which enable a high quality of finish, high productivity and a low level of environmental pollution to be simultaneously obtained. Prior Art In leather finishing and leather split enhancement three basic methods are currently used, all based on the application of a thin layer of filmogenic compounds (resins) such as to modify the appearance of the surface so that it satisfies the requirement of the various sectors of use, such as garments, footwear, bags, upholstery etc. Thus in addition to the required aesthetic characteristics, the functional characteristics required for the article are also imparted, such as softness, feel, permeability, resistance to water, to fatty substances, to food and solvents, mechanical strength etc. Finishing processes generally involve many stages (each divided into various steps), and which in the most general case comprise the following: applying a base layer to give the desired coloration (synthetic resins and pigments); - applying a synthetic resin layer to form a suitable substrate for preparing an embossed design (this operation can be combined with the preceding) ; applying a nitrocellulose resin layer to facilitate the pressing operation by eliminating problems of tackiness; pressing to form an embossed design; applying a surface finishing layer (nitrocellulose resins). These stages can be repeated individually or in sequence until the desired quality is obtained.

Filmogenic compounds are normally applied to leather by one of the three following types of application: 1. "Spray" application This can be carried out both on soft leather and on rigid splits. It is a batch process and is also lengthy in that it requires repeated passes to obtain good covering.

The filmogenic substance, dissolved in a solvent or dispersed in water, is applied in the form of an aerosol with the result that at least 4θ# of the sprayed substance rebounds off the leather and is dispersed within the surrounding air. This rejected quantity is unrecoverable and has to be eliminated by treatment in suitable purifiers.

2. Roller application. a) "Forward" or "direct" system

This is suitable for both soft and rigid leather, and is based on the use of two rollers rotating in opposite directions. The leather is conveyed by the lower roller (known as the conveyor) , while the upper roller (known as the spreader) applies the resin layer. This process is anti-productive in that several passes are required to obtain good coverage of the leather surface. Even so, this does not guarantee a perfect result if the leather comprises defects or thickness differences , which are thus made visible. b) "Reverse" system

This is suitable only for rigid leather and is based on the use of two rollers rotating in the same direction. The leather is conveyed by the lower roller (known as the conveyor) which moves at a lower speed, while the upper roller (known as the spreader) applies the resin layer.

Excessively soft leather is "refused" by the rollers as they rotate in the same direction (and thus with their surfaces moving in opposition at different speeds) .

The aqueous resin dispersions used contain a minimum of 6θ# of water, with consequent considerable drying problems.

3. "Transfer" application

This consists of applying a thin layer of resin onto a support web consisting of release paper, which is followed by the formation of the film, bonding to the substrate to be finished, generally by pressing, and successive drying or polymerization and removal of the finished substrate, with recovery of the release paper.

The process can be used for both soft and rigid leather, but the deposited film generally has a thickness of 0.2-0.4 mm, so giving it a plastic appearance, feel and consistency unless special techniques are used by which thicknesses less than 0.1 mm can be applied, but which require specially considered resins (such as polyurethane resins) . The greatest drawback of this method is the high equipment investment cost.

All these processes include drying stages, generally conducted in a hot air tunnel. The drying time can be shortened by increasing the air temperature in the tunnel but this involves a considerable energy cost and can have a negative effect on the quality of the article (hardening) . The correct compromise is sometimes difficult to obtain, and this has often resulted in economical loss to the tannery. Detailed description of the invention

It has now been surprisingly found possible to use for leather finishing a particular type of aqueous emulsion of filmogenic resins characterised by containing one or more foaming agents and one or more foam stabilizers in quantities such that the sum of said quantities is between 0.5% and 7% by weight of the dry substance and their weight ratio is between 0.2 and 4.

In the preferred embodiments of the invention the quantity of foaming agent is between 0.5% and 3# of the dry content of the emulsion.

Of the preferred foaming agents the alkaline salts of. fatty monoamides of sulphosuccinic acid have proved particularly active.

Typically, the quantity of foam stabilizer is between 0.4# and 2% of the dry content of the emulsion according to the invention. According to a basic characteristic of the emulsions of the invention, the water quantity is between 4θ and 70 and the viscosity is between 1000 and 1500 mPas (Brookfield RVT 2 - 10 r.p.m., 20^°C) .

As stated, an essential characteristic of the emulsions according to the invention is that they contain a foaming agent and a foam stabilizer.

The stated quantities of these two substances and their weight ratio are essential for the proper application of the invention. Without the foam stabilizer, the foaming agent is insufficient to obtain a foam life sufficient for the application and for the recycling of the emulsion in foam form during its application to the leather.

When the emulsions according to the invention are applied to leather for finishing purposes, finished products of optimum quality are obtained with minimum energy and labour costs, high productivity and considerable ecological and environmental advantages, especially if the process described hereinafter and forming an integral part of the present invention is used.

The emulsion according to the invention is prepared by simply mixing together the various components normally used for leather finishing. Of the filmogenic resins of normal use, polyacrylic or polyurethane resins are particularly suitable, however other resins (nitrile, butadiene, styrene, acrylic etc.) are not excluded. These resins can be thermoplastic, self-crosslinking or co- crosslinking at variable temperatures not exceeding 100°C, or can contain unsaturations (for example acrylic) and therefore be crosslinkable by radiation (eg. by U.V. light). They enable film to be obtained having excellent mechanical strength (tough and scratch- resistant) , good adhesion to its support, flexibility, softness, feel and the desired aesthetic characteristics. As in the case of normally used emulsions, the emulsions according to the present invention can also contain a thickener to modify the final rheology of the emulsion and make it suitable for the application. The mostly used thickeners include polyacrylic salts and their copolymers, and modified natural polymers, in particular carboxymethylcellulose.

The emulsions of the invention can also contain any other components required to obtain the desired aesthetic or functional characteristics provided they are properly chosen to ensure maximum compatibility with the aforesaid components so that they do not substantially influence the process.

These include dyes, pigments, waxes, feel modifiers, coalescence agents, crosslinking agents, catalysts, antifermenting agents, antimildew agents etc.

According to a basic characteristic of the present invention, a high grade of finish is obtained for the leather by treating this with the following process: a) forming a foam by blowing air into an aqueous emulsion of filmogenic resins containing one or more foaming agents and one or more foam stabilizers in quantities such that the sum of said quantities is between 1% and 1% by weight of the dry substance and their weight ratio is between 0.2 and 4; b) applying the foam from step a) to the leather to be treated, using a roller; c) drying the treated leather at a temperature of between 50°C and 90°C; d) subsequent treatments.

The aqueous emulsion according to the invention is introduced into an apparatus known as a "foamer" in which it undergoes intimate contact with air to produce a foam which can be of variable consistency and characteristics. The air can be microdispersed to the extent of obtaining a gaseous emulsion.

The foam characteristics can be adjusted by adjusting the air/compound (volume) ratio (the ratio between the compound volume before and after foaming is known as the "foaming ratio"), and the machine operating parameters.

Machines suitable for this operation are available commercially and can be easily adapted to the purposes of the present invention. The foam density obtained can be between 20 and 900 g/1. Foam ratios particularly suitable for the purposes of the present invention are between 1:2 and 1:6. The preferred foam ratios are between 1:3 and 1:4, with a corresponding density of 250-350 g/1. The foam prepared as described is fed to a roller finishing machine where it is deposited by an oscillating doctor blade onto a rotating steel roller engraved as for rotogravure printing and therefore known as a gravure-type roller. The machine can also use a roller of different type engraved in a different pattern.

The gravure roller rotates in opposition to another roller to uniformly distribute the foam over the leather fed to the machine. The leather is collected at the roller exit and fed to the subsequent drying operation.

The feed rate of the leather within the machine is between 4 and 20 m/min, and typically between 8 and 10 m/min. The foam quantity

(expressed as dry substance) deposited can vary between 5 and 75

, 2 g/m . Other configurations can be of course used for the roller spreading machine. It is also possible to use rollers rotating in the same direction (reverse rollers) , especially for rigid leather and splits. The leather coated by the described system is fed to a drying oven, which can consist of a bank of infrared ray generators, or a conventional hot air oven.

Drying is generally conducted at a temperature of between 50^°C and 90°C, and preferably between 70°C and 8θ°C.

The drying can also be followed by crosslinking of the resins contained in the used emulsion. This can be done by thermal activation at a temperature not exceeding 100°C or by irradiation by UV light, depending on the type of resins used and on the crosslinking agents and catalysts introduced.

The final treatment of the leather coated and dried as described comprises the normal operations involving application of the intermediate resin layer, pressing to obtain embossed decorations and, where necessary, final varnishing with possibly nitrocellulose- based resin solutions.

The advantages obtained by using the process of the present invention can be summarised as follows: as in the known art, thus allowing rapid drying of the leather after its application, together with increased productivity and energy saving. b. The method of application allows almost total recovery of the excess foamed emulsion, which is immediately recycled. c. Coverage of the article with the emulsion film is much more effective using foam application, not only compared with conventional emulsions but also compared with applying the actual emulsion of the invention in a non-foamed form, in particular in the case of soft leather which is particularly difficult to treat. This greater effectiveness of the process according to the invention compared with conventional methods means that a smaller number of passes are used to obtain the required characteristics (for example one pass is generally sufficient to obtain complete coverage with a formulation for base colouring) . d. Very thin and completely transparent films can be obtained (reduced resin consumption) , while at the same time ensuring maximum covering uniformity and mechanical strength. In the case of splits, this for example enables excellent coverage to be obtained in a single pass with the application of a very small quantity of emulsion, this being obtainable in the known art only by using a preliminary rotopressing operation (to reduce the permeability and hence the emulsion absorption) . e. It is possible to apply with a single pass a much higher emulsion quantity than that applied in the known art. Whatever

2 known method is used it is impossible to exceed 40-50 g/m (dry substance) with a single pass, whereas with the described process as

2 much as l4θ g/m can be easily applied. This allows total coverage and high productivity even for poor leather. The use of the described process enables finished articles to be obtained which are perfectly able to satisfy the technical requirements for best quality, and in particular:

Appearance: uniform shine and coverage

Feel: pleasant (smooth and dry) Pressing: easy to press and with good design retention

Bally flexometer strength (dry and wet, to IUP 20): high

VESLIC rubbing resistance (dry and wet, to IUF 450): high Hot iron resistance: 200-220 C

Tensile strength (IUP 6)

- deflection for appearance of microfissures: high

- deflection for film breakage: high Dry and wet adhesion (IUF 470): high

Water vapour permeability (IUP 15): high

The following examples illustrate some preferred embodiments of the present invention.

FINISHING OF SPLIT FOR UPHOLSTERY: EXAMPLE 1 - Foam application system for base covering and resin.

The following sequence of operations is carried out: a. Preparation of compound

The compound required for the base coating and pressing operations is prepared by mixing together the following compounds by mechanical stirring in the stated order:

1. Self-crosslinking acrylic resin based on ethyl and butyl acrylate (50% dry content)

[Lamcril AS/36] 200 parts

2. Self-crosslinking NBR (nitrile/butadiene) resin (4θ dry content)

[Lamcril B] 100 parts

3. Polyether-based aliphatic polyurethane resin (30% dry content)

[Lampur Wl4] 0 parts . Pigment Black 7 dispersed in casein binder

(pigment content 10 , dry content 15%) 100 parts 5- Filler based on polyethylene wax and soybean oil in aqueous emulsion (20% dry content) 100 parts

6. Foaming agent (35% sodium octadecyl sulpho- succinamate in water) 3 parts

7. Foam stabilizer 25% ammonium stearate in water) 3 parts

8. Polyacrylic thickener (35% in soybean oil/white spirit) [Alcoprint PFT] 2 parts

The compound prepared in this manner has a viscosity of 1300 mPas (Brookfield 2/10 r.p.m. at 20^°C) . b. Foaming

This is done in a Bombi machine (Bombi Mario Officina

Termomeccanica, Via Guinizelli 2, Calenzano FI), model MBE.

The set foaming ratio is 1:3. to obtain a foam of density 3 0 g/1. c. Application An upholstery split, not rotopressed, is treated with the foam produced under point b using the machine described in the general part of the invention. The thickness of the foam deposited by the doctor blade on the applicator roller is 0.5 mm, the application being in one pass at 5 m/min. The dry quantity of the substance

2 deposited on the split is 60 g/m (about 70 microns thick) . d. Drying

This is done in a hot air oven at 70°C by one pass at 10 m/min. e. Traditional finishing operations

1. Spraying with nitrocellulose resin (Lamfinish WE, 12% dry substance) diluted 1:1 with water:

2

1 pass, which deposits 15 g/m of dry substance.

2. Drying in a hot air oven at 70 C by 1 pass at 5 m/min. 3. Pressing the embossed design with a static press at 0 C and 200 atm for 6 seconds.

4. Spraying with nitrocellulose resin (Lamfinish SB, in butyl acetate, 20% dry substance) diluted 1:3 with butyl acetate.

2 1 pass, which deposits 20 g/m of dry substance.

5. Drying in a hot air oven at 70°C by 1 pass at 5 m/min. EXAMPLE 2 - (Comparison) Spray application system for base covering and resin.

The following sequence of operations is carried out: a. Preparation of compound

This is prepared exactly as in Example 1 but omitting component 6 (thickener) and further diluting with 150 parts of water to obtain a viscosity suitable for spraying (150 mPa.s). b. Application This is done by spraying with a compressed air gun, using three

2 passes to deposit a total of 100 g/m of dry substance on the split, which is not rotopressed. c. Drying as in Example 1, point d, but at 5 m/min. d. The traditional finishing operations are carried out as under point e of Example 1.

EXAMPLE 3 - (Comparison) Roller application system for base covering and resin.

The following sequence of operations is carried out: a. Preparation of compound This is prepared exactly as in Example 1. b. Application

This is done by depositing the compound, thickened (1300 mPa.s) but not foamed, onto the non-rotopressed split by the "forward" roller machine (rollers rotating in opposite directions), making two passes

2 at a rate of 5 m/min. A total of 90 g/m of dry compound is deposited in total. c. Drying as in Example 1, point d, but at 5 m/min. d. The traditional finishing operations are carried out as under point e of Example 1.

COMPARISON OF RESULTS (TABLE 1)

FINISHED ARTICLE EXAMPLE 1 EXAMPLE 2 EXAMPLE 3

1. Appearance: a — after base coating surface uniform and opaque surface uniform and bright surface uniform and bright b - at end of process uniform coverage uniform coverage uniform coverage

2. Pigmentation

(50X observation) complete coverage numerous discontinuities numerous (very few unpigmented regions, large regions with discontinuities of very small dimensions) absence of pigment large regions with absence of pi ment

7. VESLIC rubbing resistance (IUF 450) with 1 kg load a. dry felt/dry leather 500 490 495 b- wet felt/dry leather 150 (1) 145 (2) 148 (2)

1) the film deteriorates only at the end of the test

2) the film wears gradually during rubbing

COMPARISON OF PARAMETERS (TABLE 2) PROCESS OF EXAMPLE 1 EXAMPLE 2 EXAMPLE 3

Quantity of water evaporated per 100 kg of dry compound 188 265 188

Total quantity of dry compound deposited to obtain total coverage (g/in ) (lb Tab.l) 60 100 90

3. Number of passes to obtain total coverage (lb Tab.l)

4. Compound application rate (m/min) non-continuous application (15-20 seconds)

5. Drying rate at 70 C (m/min) 10

From the summary Table 1 showing the results comparison it can be clearly seen that with the process of the present invention articles are obtained which perfectly satisfy the tests with qualifying values for highest quality, in accordance with the known art.

There is also a considerable improvement in coverage uniformity (microscopic observation) and a more satisfactory overall appearance (opacity) . The advantages at the same time can be seen from the summary Table 2, these deriving from: a smaller quantity of water to be evaporated (77 kg less per 1>

100 kg of dry compound) a lesser consumption of compound to obtain an appearance similar to that of the known art a smaller number of passes (productivity increase) - a higher drying rate.

EXAMPLE 4 - Foam application system for base covering and resin, intermediate and layer and finish.

The following sequence of operations is carried out: a. Preparation of compound ] b. Foaming ] As for c. Application to non-rotopressed split ] Example 1 d. Drying ] e. Preparation of crosslinkable compound.

The following components are mixed together in the indicated order: 1. Acrylated aliphatic polyester polyurethane resin (40% dry content, bromine absorption 13 g/100 g dry substance) 100 parts

2. Aqueous dispersion of oligo[4-(α-hydroxy- isobutyryl)α-methyl-styrene] (34% dry content) (photoinitiator) 6 parts

3. Foaming agent (35% sodium octadecyl sulpho- succinamate in water) 1.2 parts

4. Foam stabilizer (25% ammonium stearate in water) 1.5 parts 5- 35% polyacrylic thickener in soybean oil/white spirit 1 part

The compound has a viscosity of 1500 mPa.s (Brookfield RVT 2 - 10 g/min 20^°C) 1?

f. Foaming of compound

This is done in a Bombi machine as under Example 1 point b, to obtain a foam of density 3^0 g/1. g. Application of the foamed compound of point f as described under c, with a 0.1 mm thickness deposited on the applicator roller. The application is done in 1 pass at 6 m/min to deposit a

2 total of 17 g/m of dry compound. h. Drying and polymerization

These are done respectively in a hot air oven (70 C) at a rate of 10 m/min and in a UV oven at a rate of 10 m/min under 2 1ST medium pressure mercury vapour lamps of 80 W/cm spaced 25 cm from the film surface. i. Pressing to obtain the embossed design by static press at 90^°C and 200 atm for 6 seconds. j . Operations g and h are repeated to obtain the final covering films.

The final characteristics of the article obtained are entirely similar to those of Example 1.

In this manner the cycle has been made completely continuous (with the exception of the pressing) and the solvent of the final coating stage has been completely eliminated.

FINISHING OF A BUFFED GOATSKIN (garment nappa) :

EXAMPLE 5 ~ Foam application of base

The following sequence of operations is carried out: a. Preparation of compound] As Example 1, b. Foaming ] points a and b. c. Application

SUBSTITUTE SHEET IS

The buffed goatskin is treated in the machine previously described in the general part of the invention as in Example 1), with a 0.6 mm thickness of foam deposited on the applicator roller, using 1

2 pass at 6 m/min to deposit a total of 140 g/m of dry substance. d. Drying

This is done in a hot air oven at 6θ^°C at a rate Of 10 m/min. e. Embossed design pressing

This is done with a rotary press between two rollers heated to 90°C, of which one is engraved, at a pressure of 100 atm and a rate of 6 m/min. f. Application of coating by spraying (Lamfinish WE nitrocellulose resin 100 parts and silicone feel modifier 3 parts, the mixture diluted 1:1 with water):

2

2 passes to deposit a total of 30 g/m of dry substance. g. Drying in a hot air oven (6θ°C) at a rate of 5 m/min. h. drumming for 5 hours (to obtain a soft draping article). i. Rotopressing at 80^°C, 0 atm at a rate of 10 m/min.

EXAMPLE 6 - (Comparison) Traditional spray application

The following series of operations is carried out: a. Spray application of a waterproofing layer (fluorinated polymer, 15% -dry content in butyl acetate/methylethylketone

Foraperle 222 - diluted 1 + 2 parts butyl acetate) :

, 2

1 pass, depositing a total of 5 S/^m °? dry substance on the leather. b. Drying in a hot air oven at 6θ^°C at a rate of 5 m/min. c. Preparation of the base compound as in Example 2, point a. d. Spray application of the compound prepared in c by three

2 passes, depositing a total of 150 g/m of dry substance. e. Drying in a hot air oven (60 C) at a rate of 5 m/min. f. Rotopressing, spraying of final coating, drying, drumming and final rotopressing are carried out, as in Example 5. points e, f, g. h, i. On examining the articles obtained in Examples 5 and 6 it can be seen that both have good coverage (a 50X microscopic examination shows a more uniform pigment distribution for Example 5) • The surface is more opaque in Example 5. which is an advantage in garments. At the process level, the comparison shows that using the method of Example 5 the application of the waterproofing base coating can be dispensed with, the base compound can be applied in a smaller number of passes, and the drying time can be reduced.

Claims

2ιPATENT CLAIMS

1. An aqueous emulsion of filmogenic resins for finishing leather and splits, characterised by containing one or more foaming agents and' one or more foam stabilizers in quantities such that the sum of said quantities is between 0.5% and 7% by weight of the dry substance and their weight ratio is between 0.2 and 4.

2. An aqueous emulsion as claimed in claim 1, wherein the foaming agent consists of alkaline salts of fatty onoamides of sulphosuccinic acid.

3. An aqueous emulsion as claimed in claim 1, wherein the quantity of foaming agent is between 0.5% and 3% of the dry content of the emulsion.

4. An aqueous emulsion as claimed in claim 1, wherein the quantity of foam stabilizer is between 0.4% and 2% of the dry content of the emulsion.

5. An aqueous emulsion as claimed in claim 1, wherein the water quantity is between 40% and 70% and the viscosity is between 1000 and 1500 mPas.

6. A process for finishing leather and leather splits, comprising the following stages: a) forming a foam by blowing air into an aqueous emulsion of filmogenic resins containing one or more foaming agents and one or more foam stabilizers in quantities such that the sum of said quantities is between 0.5% and 7% by weight of the dry substance and their weight ratio is between 0.2 and 4; b) applying the foam from step a) to the leather to be treated, using a roller;

SUBSTITUTE SHEET 2

c) drying the treated leather at a temperature of between 50 C and 90°C; d) subsequent treatments.

7. A process for finishing leather and leather splits as claimed in claim 5. characterised in that the foaming ratio is between 1:2 and 1:6.

8. A process for finishing leather and leather splits as claimed in claim 5. characterised in that the density of the foam prepared in stage a) is between 250 and 500 g/1.

9. A process for finishing leather and leather splits as claimed in claim 5. characterised in that the roller application of the foam prepared in stage a) is performed by distributing the foam by means of an oscillating doctor blade over a rotating engraved steel roller (gravure type) , which transfers the foam onto the leather to be treated.

10. A process for finishing leather and leather splits as claimed in claim 5, characterised in that the foam-treated leather is dried by passage through an air oven maintained at a temperature of between 70 and 80^°C.

11. A process for finishing leather and leather splits as claimed in claim 5. characterised in that after the drying as claimed in claim , the film is crosslinked by irradiation with ultraviolet light.

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