Classifications

• • C—CHEMISTRY; METALLURGY
  • C14—SKINS; HIDES; PELTS; LEATHER
  • C14C—CHEMICAL TREATMENT OF HIDES, SKINS OR LEATHER, e.g. TANNING, IMPREGNATING, FINISHING; APPARATUS THEREFOR; COMPOSITIONS FOR TANNING
  • C14C9/00—Impregnating leather for preserving, waterproofing, making resistant to heat or similar purposes

Definitions

• 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.
• 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.
• 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.
• 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.
• mineral type tanning agents such as chromium (III) salts and aluminium salts
• synthetic tanning agents such as sulphonated condensation products of formaldehyde and phenols
• vegetable tanning agents such as different tannins.
• retanning also called retan or retannage
• resins and/or synthetic tanning agents are usually added to the skin to make it more uniform in structure.
• the skin is coloured.
• the natural oil that has been removed during earlier process steps is replaced, usually by treating the skin with an emulsion of fat.
• 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.
• the outer skin surface is sealed by using, for example, waxes, lacquers etc.
• 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.
• 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.
• 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.
• 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.
• a propellant is encapsulated within a thermoplastic shell.
• 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.
• 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.
• 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, ⁇ -chloroacrylo nitrile, ⁇ -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 ⁇ -methyl styrene, or dienes such as butadiene, iso
• 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(
• crosslinking multifunctional monomers such as one or more of divinyl
• 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.
• the polymer shell constitutes from about 60 to about 95 wt%, most preferably from about 75 to about 85 wt% of the total microsphere.
• the propellant 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
• T max the temperature at which maximum expansion is reached
• 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
• 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.
• the volume weighted average particle size of the thermally expandable microspheres is suitably from about 1 to about 500 ⁇ m, preferably from about 3 to about 100 ⁇ m, most preferably from about 5 to about 50 ⁇ m.
• T start 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.
• 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 %.
• 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 %.
• the weight ratio thermally expandable microspheres to skin is from about 0.002:1 to about 0.4:1, preferably from about 0.02:1 to about 0.2:1.
• 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.
• 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.
• 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.
• heat pressing plating
• 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.
• the thermally expandable microspheres will penetrate, and be captured within, the skin, and expand during the heat treatment.
• skin comprising expanded microspheres 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.
• 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.
• 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.
• the samples were subjected to a heat under a press at 120°C for 5 seconds at a pressure of 30 kg/cm 2 .
• 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.
• 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
• 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 %..
• wet blue wet skins
• 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.
• 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 ⁇ m 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
• 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 (cm 2 ) No heat treatment Area (cm 2 ) 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
• control showed the largest loss in area. The largest effect was seen when microspheres were added before the dyeing step.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Treatment And Processing Of Natural Fur Or Leather (AREA)
• Synthetic Leather, Interior Materials Or Flexible Sheet Materials (AREA)

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Cited By (7)

Citations (3)

• 2001
  • 2001-12-21 EP EP01850226A patent/EP1279746A1/de not_active Withdrawn

Patent Citations (3)

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