Process for preparation of leather

Description

[0001] The present invention relates to a process for preparation of leather comprising a step of adding to wet skin thermally expandable microspheres. It also relates to leather comprising expanded thermoplastic microspheres. Finally, it relates to the use of thermally expandable microspheres in a leather tanning process.

Background of the invention

[0002] Leather has a wide range of uses, for example as a material for making clothing, footwear, gloves, upholstery, bags etc. Leather is prepared from skin through a leather tanning process. As "leather" is herein meant the finished material after the entire leather tanning process. A skin to be used in leather production can be taken from most animals including mammals, reptiles, fish and even birds. Most skins are taken from mammals, and in particular from cattle, sheep, goats or pigs. To preserve the skins, a tanning process is required. The general term "leather tanning process" includes all steps in the complex process converting a skin to a finished leather material, not just the actual tanning step. The process usually comprises steps of preparing the skin before the actual tanning step such as trimming, soaking (if the skin has been dried), liming, unhairing, deliming, fleshing and bating. A step called pickling is usually made prior to the actual tanning step. The pickling step usually comprises addition to the skin of a mixture of sodium chloride and sulphuric acid and/or formic acid to prevent swelling of the skin. Thereafter, the actual tanning step takes place where a tanning agent reacts with the collagen in the skin. The tanning agent usually belongs to one of three types of tanning agents, mineral type tanning agents such as chromium (III) salts and aluminium salts, synthetic tanning agents such as sulphonated condensation products of formaldehyde and phenols, and vegetable tanning agents such as different tannins. After the actual tanning step the skin is usually shaved and sorted before further processing stages begin. These further processing stages are commonly referred to as the "wet processing stages", or "wet-finishing". By a "wet processing stage" is herein meant any of the wet processes that takes place after the actual tanning step, such as retanning, dyeing, fatliquoring and fixing. In the retanning (also called retan or retannage) stage, resins and/or synthetic tanning agents are usually added to the skin to make it more uniform in structure. In the dyeing stage, the skin is coloured. In the fatliquoring stage, the natural oil that has been removed during earlier process steps is replaced, usually by treating the skin with an emulsion of fat. These wet processing stages are known in the art and need not to be described in further detail. After these wet-processing stages, procedures such as conditioning, staking, buffing, brushing and finishing are made to get the final leather. In the finishing step, the outer skin surface is sealed by using, for example, waxes, lacquers etc.

[0003] Some areas of a skin from an animal are less useful than other areas. For example, belly skin from cattle has a looser fibre structure and more cavities than skins from other regions. This leads to a lower quality of the leather, which is why these skin regions are usually cut off during the leather production process.

[0004] It is desirable to increase the amount of skin that can be used for leather production and increase the quality of the leather prepared. When preparing leather, effects to the skin may occur during processing and drying such as reduction of thickness, or "substance", of the skin and reduction of skin area, giving a lower "area yield".

[0005] It is an object of the present invention to provide an improved process for production of leather giving better quality and/or higher yield of leather as compared with present processes. There is another object of the present invention to provide a new skin and leather material.

The invention

[0006] According to the invention it has surprisingly been found possible to achieve the above mentioned objects by a process for preparation of leather, comprising a step of adding to wet skin thermally expandable microspheres having a thermoplastic polymer shell encapsulating a propellant, followed by a step of heat treatment at a temperature sufficient to result in expansion of the microspheres captured in the skin. The thermally expandable microspheres are suitably added to wet skin at any stage before the finishing step, preferably before or during a wet processing stage, pickling step, or an actual tanning step, in a tanning process. Preferably the thermally expandable microspheres are added to wet skin before or during one or more of the following stages: the retanning stage, the dying stage, the fatliquoring stage, or, the fixing stage.

[0007] Thermally expandable microspheres are known in the art and described in detail in, for example, US Patent No. 3615972, EP 486080, EP 566367 and EP 1 067 151, which documents hereby are incorporated by reference. In such microspheres, a propellant is encapsulated within a thermoplastic shell.

[0008] The propellant is normally a liquid having a boiling temperature not higher than the softening temperature of the thermoplastic polymer shell. The propellant, also called the blowing agent or foaming agent, can be hydrocarbons such as n-pentane, isopentane, neopentane, butane, isobutane, hexane, isohexane, neohexane, heptane, isoheptane, octane and isooctane, or mixtures thereof. Aside from them, other hydrocarbon types can also be used, such as petroleum ether, and chlorinated or fluorinated hydrocarbons, such as methyl chloride, methylene chloride, dichloroethane, dichloroethylene, trichloroethane, trichloroethylene, trichlorofluoromethane etc. The propellant suitably makes up 5-40 weight % of the microsphere.

[0009] The thermoplastic shell of the microsphere may be made from polymers or copolymers obtainable by polymerizing various ethylenically unsaturated monomers which can be nitrile containing monomers such as acrylo nitrile, methacrylo nitrile, α-chloroacrylo nitrile, α-ethoxyacrylo nitrile, fumaro nitrile, croto nitrile, acrylic esters such as methylacrylate or ethyl acrylate, methacrylic esters such as methyl methacrylate, isobornyl methacrylate or ethyl methacrylate, vinyl halides such as vinyl chloride, vinylidene halides such as vinylidene chloride, vinyl pyridine, vinyl esters such as vinyl acetate, styrenes such as styrene, halogenated styrenes or α-methyl styrene, or dienes such as butadiene, isoprene and chloroprene. Any mixtures of the above mentioned monomers may also be used. It may sometimes be desirable that the monomers for the polymer shell also comprise crosslinking multifunctional monomers, such as one or more of divinyl benzene, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, glycerol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol hexa(meth)acrylate, dimethylol tricyclodecane di(meth)acrylate, triallylformal tri(meth)acrylate, allyl methacrylate, trimethylol propane tri(meth)acrylate, trimethylol propane triacrylate, tributanediol di(meth)acrylate, PEG #200 di(meth)acrylate, PEG #400 di(meth)acrylate, PEG #600 di(meth)acrylate, 3-acryloyloxyglycol monoacrylate, triacryl formal or triallyl isocyanate, triallyl isocyanurate etc. If present, such crosslinking monomers preferably constitute from about 0.1 to about 1 wt%, most preferably from about 0.2 to about 0.5 wt% of the total amounts of monomers for the polymer shell. Preferably the polymer shell constitutes from about 60 to about 95 wt%, most preferably from about 75 to about 85 wt% of the total microsphere.

[0010] Upon heating, the propellant evaporates to increase the internal pressure at the same time as the shell softens, resulting in significant expansion of the microspheres, normally from about 2 to about 5 times their diameter. The temperature at which the expansion starts is called T_start, while the temperature at which maximum expansion is reached is called T_max, both determined at a temperature increase rate of 20°C per minute. The thermally expandable microspheres used in the invention suitably have T_start within the range of from about 40 to about 200°C, preferably from about 50 to about 150°C, most preferably from about 70 to about 120°C, while T_max suitably is within the range of from about 60 °C to about 250 °C, preferably from about 80 to about 150°C, most preferably from about 100 to about 130°C.

[0011] The volume weighted average particle size of the thermally expandable microspheres, according to the invention, is suitably from about 1 to about 500 µm, preferably from about 3 to about 100 µm, most preferably from about 5 to about 50 µm. By heating to a temperature above T_start, it is normally possible to expand the microspheres from about 2 to about 7 times, preferably from about 4 to about 7 times, their diameter.

[0012] The thermally expandable microspheres can be supplied to the process in any suitable form having a content of thermally expandable microspheres suitably from about 1 to about 100 weight %. The thermally expandable microspheres are suitably supplied to the process as, for example, an aqueous dispersion or slurry, a "wet cake", or dry material, The amount of thermally expandable microspheres in a dispersion or slurry is suitably from about 5 to about 60 weight %, preferably from about 10 to about 50 weight %, most preferably from about 15 to about 45 weight %. As a "wet cake" is herein meant a mixture of thermally expandable microspheres and a liquid, preferably water, with a dry content of microspheres suitably from about 55 to about 90 weight %, preferably from about 60 to about 85 weight %, most preferably from about 65 to about 75 weight %. If the thermally expandable microspheres are added as dry material, the dry content of microspheres is suitably more than about 90 weight %, preferably more than about 95 weight %, most preferably more than about 98 weight %. The wet skin used in the present invention has suitably a dry weight from about 30 to about 70 weight %, preferably from about 35 to about 60 weight %, most preferably from about 40 to about 55 weight %. Suitably, the weight ratio thermally expandable microspheres to skin, calculated as dry material, is from about 0.002:1 to about 0.4:1, preferably from about 0.02:1 to about 0.2:1. Suitably, the weight ratio thermally expandable microspheres to wet skin, is from about 0.001:1 to about 0.2:1, preferably from about 0.01:1 to about 0.1:1. Suitably, the skin is tumbled in a rotating drum or in any other way moved around while being in contact with the thermally expandable microspheres. Not all of the thermally expandable microspheres added to the wet skin are necessarily captured within the skin. Of the total amount thermally expandable microspheres added to wet skin suitably from about 1 to about 100 weight %, preferably from about 10 to about 95 weight %, are captured within the skin.

[0013] The heat treatment can be procedures such as heating in an oven, heat pressing (plating), micro wave heating, infra-red heating, steam heating, or any other suitable procedure which will heat up the microspheres making them expand. Preferably, heat pressing (plating) is used. The heat treatment is made at a temperature sufficient to result in expansion of the microspheres, suitably at a temperature from about 60 to about 160°C, preferably from about 80 to about 140°C, most preferably from about 100 to about 130°C.

[0014] Through the process of the present invention, the thermally expandable microspheres will penetrate, and be captured within, the skin, and expand during the heat treatment. According to the invention, there is skin comprising expanded microspheres, leather comprising expanded microspheres, skin comprising expanded thermoplastic microspheres obtainable by the process of the invention, and, leather comprising expanded thermoplastic microspheres obtainable by the process of the invention. The skin and leather according to the invention suitably contains from about 0.1 to about 20 weight %, preferably from about 0.5 to about 8 weight %, of expanded thermoplastic microspheres, based on dry material.

[0015] Through the invention it is possible to provide an improved process for production of leather, which gives more substance (thicker leather) as well as increases the area yield of skin, as compared with present processes. In some cases it is also possible to increase the so called "cutting yield", which is the level of usable skin from an animal.

[0016] The invention will be further described through the following Examples, which, however, not should be interpreted to limit the scope of the invention. If not otherwise stated, all parts and percentages refer to parts and percent by weight.

Examples

Example 1.

[0017] Leathers made from different regions of bovine skins (belly, butt, backbone and neck) were tested. A part of each skin region was used as a control. Three different aqueous slurries comprising 16, 20 and 26 weight % EXPANCEL® types of thermally expandable microspheres were applied to "wet blue" (wet skin coming directly from a chrome tanning step) at similar offers prior to the retanning and dyeing stages at a temperature well below T_start. The content of dry material in the skin was 45 weight %, and the weight ratio added thermally expandable microspheres to skin, calculated as dry material was 0.040. The thermally expandable microspheres were of three types, having similar T_start and T_max, but slightly different sizes, all having a thermoplastic shell made from polymers of vinylidene chloride, acrylo nitril and methyl methacrylate, encapsulating isobutane as the propellant. The temperature of the subsequent fatliquoring stage was maintained at 50°C. After samm/setting and drying, the skins were conditioned and staked on a Mostardini through feed staking machine at setting number three, prior to heat treatment to expand the microspheres. A sample was cut from each skin and the substance (thickness) was measured. Then, the samples were subjected to a heat under a press at 120°C for 5 seconds at a pressure of 30 kg/cm². The samples were then re-staked on the Mostardini staking machine at setting number three and the substance were re-measured. The content of dry material, excluding microspheres, in the leather was 87 weight %. The content of microspheres in the leather was determined by soaking the microspheres-containing leather in dimethyl acetamide, which dissolves the thermoplastic shell of the microspheres and releases the propellant. The amount of propellant was determined by gas chromatography and the content of microspheres in the leather could be calculated.

Table 1.

Expancel® -types of thermally expandable microspheres used.
Expancel®type	Tstart (°C)	Tmax (°C)	Average size (µm)
A	76	122	15.1
B	76	105	12.8
C	80	116	9.2

Table 2.

Treat levels.
Expancel®type	Dry microsphere content in slurry (weight %)	Weight ratio added microspheres to skin (as dry material)
A	25.8	0.040
B	20.1	0.040
C	16.4	0.040

Table 3.

Effects of heat pressing, statistical analysis.
	skin region
	butt	belly	backbone	neck
Loss of substance ― control	-32.6%	-25.3%	-25.0%	-31.9%
Loss of substance ― microsphere treated samples	-13.8%	-4.1%	-11.1%	-15.3%
Standard deviation ― control	1.9	4.3	2.7	5.7
Standard deviation ― microsphere treated samples	4.4	2.4	2.6	4.1

[0018] It is concluded from Table 3 that the microsphere treated samples show a significant lower loss of substance as compared to the control samples, especially for skins from the belly region.

Table 4.

Microsphere content in dry leather (based on moisture-free leather)
Type	Content of microspheres in dry leather (%)
A	3.22
B	1.89
C	2.57

[0019] It is concluded from Table 4 that microspheres have been incorporated in the leather.

Example 2.

[0020] Leathers from bovine bellies were tested. Thermally expandable microspheres as an aqueous slurry comprising 45 weight % of EXPANCEL® type of thermally expandable microspheres was added to wet skins ("wet blue") at various offers (weight ratio added microspheres to skin 0.040 and 0.080, calculated as dry material, during the retannage stage. The dry content of the skin was 45 weight %.. After samm/setting and drying, the skins were conditioned and staked on a Cartigliano single headed staking machine at setting number three, prior to heat treatment to expand the microspheres. Samples were cut from the skins of which some were subjected to a heat treatment in an oven at 120°C for 10 minutes. The substance, tear strength and tensile strength were measured.

[0021] The thermally expandable microspheres had a thermoplastic shell made from polymers of vinylidene chloride, acrylo nitril and methyl methacrylate, encapsulating isobutane as the propellant. The unexpanded microspheres had an average size of 11.7 µm and a T_start of 78°C and T_max of 120°C.

Table 5.

Effects on substance.
Weight ratio	Substance (mm)
added microspheres to skin, as dry material	No heat treatment	After heat treatment	Difference (%)
0 (control)	1.02	1.12	+10
0.040	1.27	1.45	+14
0.080	1.21	1.60	+32

[0022] Since the samples were oven-heated, instead of heat-pressed as in Example 1, also the control sample gained substance. However, the increase is much higher for microsphere-treated samples especially when higher amounts of microspheres are used.

Table 6.

Effects on physical properties.
Weight ratio added microspheres to skin, as dry material	Tear strength (N/mm)		Tensile strength (N/mm2)
	No heat treatment	After heat treatment	No heat treatment	After heat treatment
0 (control)	102.5	109.2	18.1	21.7
0.040	122.8	120.8	19.3	23.9
0.080	122.4	121.2	21.7	19.1

[0023] It is concluded from Table 6 that there is no significant difference in tear strength or tensile strength between microsphere-treated samples and the control. Thus, the inclusion of microspheres in the leather does not have an adverse effect on these properties.

Example 3.

[0024] The area yield was tested for the same samples as in Example 2. In addition, two samples were tested that had been contacted with the same type of microspheres before the dyeing stage instead of during the retanning stage. The dyeing stage took place before the retanning stage.

Table 7.

Area yield
Weight ratio added microspheres to dry skin	Stage at addition of microspheres	Area (cm2) No heat treatment	Area (cm2) After heat treatment	Difference (%)
0 (control)	-	337.5	328.6	-2.6
0.040	during retanning	337.5	330.8	-2.0
0.080	during retanning	337.5	330.8	-2.0
0.030	just before dyeing	337.5	332.9	-1.4
0.040	just before dyeing	337.5	333.0	-1.3

[0025] It is concluded that the control showed the largest loss in area. The largest effect was seen when microspheres were added before the dyeing step.

Claims

1. A process for preparation of leather, comprising a step of adding to wet skin thermally expandable microspheres having a thermoplastic polymer shell encapsulating a propellant, followed by a step of heat treatment at a temperature sufficient to result in expansion of microspheres captured within the skin.

2. A process according to claim 1, wherein the thermally expandable microspheres are added before or during a wet processing stage, a pickling step, or, an actual tanning step, in a leather tanning process.

3. A process according to any of claims 1-2, wherein the heat treatment takes place at a temperature from about 60°C to about 160°C.

4. A process according to any of claims 1-3, wherein the weight ratio thermally expandable microspheres to skin, calculated as dry material, is from about 0.002:1 to about 0.4:1.

5. A process according to claim 4, wherein the weight ratio thermally expandable microspheres to skin, calculated as dry material, is from about 0.02:1 to about 0.2:1.

6. Skin comprising expanded thermoplastic microspheres captured within the skin.

7. Leather comprising expanded thermoplastic microspheres captured within the skin.

8. Skin comprising expanded thermoplastic microspheres obtainable by the process according to any of claims 1-5.

9. Leather comprising expanded thermoplastic microspheres obtainable by the process according to any of claims 1-5.

10. Skin or leather according to any of claims 6-9, which contains from about 0.1 to about 20 weight % of expanded thermoplastic microspheres based on dry material.

11. Use of thermally expandable microspheres, having a thermoplastic polymer shell encapsulating a propellant, before or during a wet processing stage, a pickling step, or an actual tanning step, in a leather tanning process.