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
  • C14C1/00—Chemical treatment prior to tanning
  • C14C1/08—Deliming; Bating; Pickling; Degreasing

Definitions

• the present invention relates to a method in the treatment of hides for leather manufacture, comprising the treatment of softened hides or skins with lime, deiiming of the hides by 5 treatment in an aqueous treatment bath under agitation and with the addition of a lime-dissolving substance, bating, pickling and tanning.
• the corium of the hide is that part of the hide which is
• the first operation is to free the hide material from preservatives, i.e. salt, impurities, blood and certain aqueous proteins by treatment in water.
• preservatives i.e. salt, impurities, blood and certain aqueous proteins
• salt, interfibrillary water-soluble albumins and globulines soluble in brine are dissolved out, as well as dirt, other impurities and blood. Since, in the original curing process, the hide has lost a considerable amount of its original water content, the primary purpose of the softening process is to 0 restore the hide to its normal water content.
• the limed hide is strongly alkaline and swollen.
• the generally employed tanning agents - vegetable tanning agents and chrome salts - react acidically, are sensitive to alkali and form compounds insoluble with lime.
• Direct treatment of the strongly alkaline hides with tanning agents will result in a leather of poor quality and, moreover, the considerable swelling of the hide in the initial stages of the tanning process is not desirable.
• the limed hides must therefore be freed of their alkalinity and their swelling. By neutralization of the alkalinity, the calcium hydroxide present in the hide (which is but sparingly soluble) will normally be removed only from the outer regions of the hide.
• the pH should be reduced at most from a level of between 12 and 14, usually around 12, to a level of around 8.
• This lowering of the alkalinity (or increase of the acidity) must be achieved with extreme care, since the outermost regions of the hide may swell even in the briefest presence of surplus acid in the aqueous treatment liquid.
• Acid- swollen collagen is particularly sensitive, which may cause cracks to occur in the finished leather. For this reason, no powerfully dissociating acids are employed, but use is made instead of weak organic acids, for example aliphatic or aromatic dicarboxylic acids, or weakly acidic salts, for example ammonium sulphate, ammonium chloride or polyphosphates.
• acidic agents with a great capacity to dissolve lime for example sulphophthalic acid or complexing agents.
• Ready-to-use mixtures are also available on the market for deiiming, these compounds containing, for example, a mixture of citric acid and formic acid.
• the deiiming process is normally terminated by bating, i.e. the action of proteolytic enzymes on the basis of pancreal or bacterial proteases. Their effect has not been clearly established. In all probability, they promote by a fibre-loosening action uniform and deep penetration of deiiming agents and the dissolution of lime. These enzymes are active within a pH range of between 6.5 to 9, the most active range being between 7 and 8.5. Bating preparations therefore usually contain ammonium sulphate and pH buffer substances.
• the hides are normally washed up to 3-4 times to remove calcium compounds and other substances which, during later treatment stages, may disturb the process and/or be deposited on the surface of the hide.
• the deiiming process is normally carried out in large rotating drums with a horizontal axis of rotation, in which drums there may be introduced, for example, 3000 litres of water and 3000 kg of limed hides.
• deiiming agents are added, these normally being in liquid or solid form, for example the commercially available mixture of formic acid and citric acid.
• a common proportion of acid is 1 per cent of the dry weight of the hides, i.e. 30 kg of acid in the present case.
• the amount of acid employed is also adapted to the thickness of the hides, thick hides being more difficult to delime and hence requiring a larger amount of acid. This amount of acid is added in a single operation and as a sole batch on initiation of the deiiming process.
• Typical neutralization curves for the deiiming process are reproduced in Fig. 4, Fig. 4a showing the pH as a function of the time for continuous, constant addition of 1.5 per cent carbon dioxide during 30 minutes and Fig. 4b showing the pH as a function of the time for addition of a corresponding amount of pulverous, weak acid according to the technique used today, where the entire amount of acid is added in one lot at zero time.
• the treatment liquid is of optimum acidity in the initial phase of the deiiming process and the amount of acid decreases in proportion to the dissolution of lime from the hide. It is difficult to achieve continuous supply of the acid in these horizontally rotating drums.
• One object of the present invention is to realize a method of rendering the deiiming process more effective, so as to obtain improved leather quality.
• a further object of the present invention is to substitute the relatively expensive prior-art dei ming agents by a cheaper agent.
• Yet a further object of the present invention is to real ze a method in which it is possible to execute the bating process concurrently with or directly after deiiming, with no intermediate washing stage.
• Another further object of the present invention is to enable deiiming and bating to be carried out in such a manner that smaller volumes of liquid are used, which liquids have to be cleaned before being discharged to a recipient.
• Yet another object of the present invention is to carry out the deiiming with more environmentally acceptable chemicals than have hitherto been used.
• a method in the treatment of hides for leather manufacture comprising the treatment of softened hide with lime and possibly with disodium sulphide, deiiming of the hide by treatment in an aqueous treatment bath under agitation and with the addition of a lime-dissolving substance, bating, pickling and tanning, the method being characterized in that as lime- dissolving substance a gas containing carbon dioxide is dissolved in the aqueous treatment bath until the pH of the treatment bath has been reduced to a predetermined value which lies in a range suitable for bating, the carbon dioxide being the lime-dissolving substance.
• Preferred embodiments of the method according to the present invention are disclosed in the dependent claims.
• the hides are softened and lime-treated in the usual manner. Thereafter, the hides are freed of the existing layer of fat on the meat side and depilated. In many cases, thick or heavy hides are skived parallel with the grain layer of the hide. The upper skiver is the more valuable, as it contains the grain layer that is characteristic for different leathers. The hide freed of hair and fat is called a bare hide.
• Bare hides and water are placed in the treatment vessel, which is usually a rotating drum with a horizontal axis of rotation.
• the bare hides may possibly be washed with this water, whereafter the vessel is refilled with fresh water.
• the bare hides are washed with water 3-4 times before the deiiming acid is added. This i-s not required according to the present method, according to which washing is normally carried out not more than once.
• the bare hides and treatment liquid i.e. water
• the latter is caused to rotate and gas containing carbon dioxide or preferably pure carbon dioxide is introduced into the vessel.
• the addition of the gas containing carbon dioxide can take place directly to the liquid or to the gas phase existing above the liquid.
• the latter method gives a less effective solution of carbon dioxide in the treatment liquid but is suitable for supplying to existing equipment.
• the pressure of the gas phase existing above the liquid can be approximately atmospheric pressure, i.e. ambient pressure, or a positive pressure.
• the treatment vessels are designed so that they do not permit or allow establishment of a positive pressure in the treatment vessel, which pressure differs essentially from the ambient pressure.
• the temperature of the treatment liquid is regulated in a known manner within the wanted range.
• the amount of added carbon dioxide is such that the treatment liquid is substantially saturated, at least during the initial phase of the deiiming and as far as the concluding phase.
• the carbon dioxide supply can be continuous or intermittent.
• the pH of the treatment liquid is lowered to 8.3 or some other lower value at which the bating agent has great activity, whereafter the bating agent is added.
• the supply of carbon dioxide can then temporarily be discontinued.
• the pH of the treatment liquid will then rise slightly. Thereafter carbon dioxide can again be supplied.
• the supply of carbon dioxide can proceed without interruption also during the addition of the bating agent.
• the supply of carbon dioxide can continue until a lower pH has been attained in the- treatment bath and the bating agent can then be added to this without any intermediate ' washing stage.
• the bating agent can then be added to this without any intermediate ' washing stage.
• a pH of approximately 6.8 has been attained in the treatment liquid a relatively stable level will be attained which is attributable to the buffer effect of the carbonic acid at this value.
• large amounts of carbon dioxide are required.
• deiiming can be carried out in two stages, in which case the first stage involves deiiming with carbon dioxide and the second stage deiiming with a conventional organic acid or an ammonium salt.
• This procedure true enough involves the use of less environmentally acceptable chemicals than carbon dioxide, but in recompense gives faster deiiming than with only carbon dioxide.
• a major benefit is however that the total used amount of conventional deiiming agent is significantly lower in implementation of occurring two-stage procedures.
• a conventional deiiming agent it is nevertheless not possible to supply the deiiming agent directly to the treatment liquid and the bare hides have to be washed before the deiiming agent is added.
• Fig. 1 shows the change in pH as a function of time on the addition of carbon dioxide
• Fig. 2 shows the amount of carbon dioxide as a function of time in the same experiment as illustrated in Fig. 1;
• Fig. 3 shows the quotient of the amount of carbon dioxide and the amount of hide in per cent as a function of time in the same experiment as illustrated in Figs. 1 and 2; and Figs. 4a and 4b show a comparison between the pH of the treatment liquid as a function of time for treatment with carbon dioxide according to the present invention and in a conventional manner.
• EXAMPLE 1 A deiiming experiment was carried out in a rotary, laboratory-scale drum. Four lime-treated bovine hides of a weight of approx. 50 kg and roughly the same amount of water were placed in the drum. Carbon dioxide was supplied to a water circulation pipe and dissolved in the water of the water circulation pipe by an injector disposed in the circulation conduit. The volume of carbon dioxide was approximately 8 litres per minute, calculated at atmospheric pressure. The temperature of the treatment bath was approximately 25 °C.
• the pH of the treatment bath was 12 and, there- after, carbon dioxide was continually supplied at the above- disclosed volume per minute. After 10 minutes, the pH of the treatment bath had fallen to 8.4. After 50 minutes, the supply of carbon dioxide was shut off. At that point, the pH in the treatment bath was 7.3. The surface of the hide was now clean, but the inner regions of the hide required further treatment. After 85 minutes from the beginning of the experiment, the supply of carbon dioxide was restarted. Immediately prior to the commencement of the gas supply, the pH of the treatment bath was 8.2. Five minutes later, i.e. 90 minutes after commencement of the experiment, the supply of carbon dioxide was terminated. At that point, the pH of the bath was 7.5. The hide was now lime-free and the treatment was discontinued.
• the total carbon dioxide consumption was between 0.5 and 1 kg, i.e. between 1 and 2 weight per cent of the weight of the hide.
• the pressure in the rotary drum was equal to or insignif cantly higher than the ambient atmospheric pressure.
• the hides were then subjected to bating, i.e. enzjuiatic treatment, without any intermediate washing of the hides in the deiiming bath. Thereafter, the hides were chrome-tanned and dyed in the conventional manner.
• the leather thus obtained was of superior quality compared to leather obtained in a conventional process -.
• a further wholly unexpected outcome was that it is possible to dispense with the previously obligatory washing in 3-4 stages after the dissolution of lime from the limed hide and before the bating process. According to a hypothesis which remains to be confirmed, it is believed that, according to the present invention, there are formed calcium complexes which are stable during the continued treatment, and, in this instance, calcium ions do not interfere with the treatment chemicals in later stages.
• the carbon dioxide supply to the treatment bath for deiiming is preferably regulated such that the treatment bath is saturated in respect of carbon dioxide. It is not necessary to supply a gas comprising solely carbon dioxide. Instead, carbon dioxide may be supplied together with other gases which do not have any disturbing effect under the deiiming process proper. In such an event, the volume of gas supplied must be increased so that the volume of carbon dioxide per se will be at the contemplated level .
• the supply of carbon dioxide and the time between discontinuation of the gas supply and the bating process is controlled such that the bating is initiated when the pH level in the treatment bath between approximately 7.5 and 8, since, at 25 °C, the enzymes which are used in the bating
• An essential advantage of the present method is that hide delimed with carbon dioxide can be chrome-tanned with up to 20 per cent lower consumption of chromeous chemicals in comparison with hide delimed in the conventional manner.

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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)

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• 1987
  • 1987-06-16 SE SE8702522A patent/SE8702522L/ unknown
• 1988
  • 1988-06-10 WO PCT/SE1988/000313 patent/WO1988010317A1/en not_active Application Discontinuation
  • 1988-06-10 EP EP19880906157 patent/EP0357682A1/en not_active Withdrawn
  • 1988-06-10 AU AU19589/88A patent/AU1958988A/en not_active Abandoned
  • 1988-06-15 ES ES8801849A patent/ES2006983A6/es not_active Expired
  • 1988-06-16 MX MX1192488A patent/MX169354B/es unknown

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