Exploration on Technology of Cattle Hide Wetblue
(Guomao Leather Goods Limited Company, Huanghua 061104)
Abstract：Some problems concerned with processing technology of cattle hide upper leather are explorated. Some suggestions are proposed for stablizing and enhancing the quality of the resultant leather.▲
Wet End Operations
Wet end refers to the process from soaking until ready for drying after retaning,dyeing, fatliquoring, etc, normally including the following steps:
Pre-soak ---------flesh--------main soak
Soaking is to return the raw stock to as near as possible its original condition and remove dirt, blood, bacteria etc. Most of the cases, soaking is carried out in drum or paddle, seldom still in pits. Increased temperature and mechanical action, increase the rate of rehydration, but more chance to get loose leather, the raw hides are loaded in the drum of paddle, normally less running/ more stop at the beginning.
When soaking, normally chemicals used are: wetting agents/detergents; neutral salt(for sun dried), bactericide, alkali salts. Time can take from 4 hours to 2 days depending on the original form of preservation
Unhairing/liming is to remove hair and the epidemis and break down and open up the fiber structure of the raw stock. Normally for hair burning process, unhairing is carried out first then directly followed by liming in the same bath, for hair saving process, hair roots are loosen by certain chemicals then hair was pulled out by mechanical action, then liming is carried out in new bath. For soft leather, relime is usually carried out after lime splitting or lime fleshing. Unhairing/liming/reliming all run with interval stops. Chemicals used in this step including: Sodium sulfide and /or Sodium hydrosulfide; amines or anti wrinkle agent—lime auxiliary; hydrated lime powder/paste, degreasing agent.
Liming should be gentle and even, uneven swelling occurs leading to wrinkles or drawn grain, lime auxiliary helps for more even swelling and less risk of lime blast. Unhairing/liming can be carried out with no float, short float, then extended float or long float depending on the raw stock, equipment etc.
Deliming is to remove the mechanically deposited and chemically bound lime and of capillarylime by conversion into readily soluble salts. This step carried out in drum with continuous running. Deliming is carried out by using acid and/or acid salts, chemicals used are Ammonium chloride, ammonium sulphate, sodium bisuphit, lactic acid, boric acid…
Bating is break down certain amount of fiber structure and further open up the collagen network.Temperature is 30-36 degree, PH is 8.0-8.5, running time is 10-60 mins and enzyme type will decide the effect of bating, which is checked mainly by experience. Most common enzyme is pancreatic enzymes, with continuous running or running for some time then rest some time. The enzyme value(LVU) is the number of enzyme units in 1g bating agent. One enzyme unit has the fermentative capacity to digest 1.725mg casein.
Picking is to acidify the pelts to a certain PH before chrome tannage and thus to reduce the astringency of the chrome tanning agents. Picking must put in enough salt first to prevent unexpected swelling which result in poor tensile strength. Its depending on the final PH and the type of acids used, amount of the acid vary from 1% to 3%, usually weak acid prior to strong acid if there has. Chemicals used in this step are common salt, weak acid(formic acid, acetic acid, lactic acid and/or strong acid)
Picking time can be 30 mins up to 12 hours(over night) depends on the type of leather etc. picking must be carried out cold to stop unwanted hydrolysis which reduces tensile strength.
Tanning is to bring about irreversible stabilization of the skin substance that is prone to putrefaction, tanning is carried out with Mineral tanning agents(such as chromium sulphate, aluminium, iron or zirconium etc) aldehyde, vegetable tannin, synthetic tannin, etc.
Chrome tanning is known as wet blue is most popular as it is fast, efficient and cost effective, low PH, temperature and basicity si used to start with, to allow for penetration and then PH ans basicity is raised to fix the Chrome to the fibres. The leather can now withstand 100 degree temperature rather than for 40 degree for the raw hide or skin.
Tanning is finished by raising the temperature and PH of the system, by adding sodium formate, sodium bicarbonate, sodium phthalate etc, now more and more tannery using Magnesium Oxide as it is safer, easier handle, better wet blue etc. the final is controlled by PH 3.6-4.0, temperature around 40 degree. Wet blue need condition for better fixation of chrome.
Those mechanical operations are mainly for get the right thickness for final leather. Samming is to get rid of the excess water in the leather and ready for splitting, splitting is to get close thickness of the final leather, separate the grain and split, ready for shaving, shaving is to get as precise as possible thickness to the requirement of the final leather.
Get the wert blue ready for later process, usually using wetting agent/emulsifier, and weak acid with the temperature of 30-40 degree. Running time depends on the condition of the wet blue, vary from 20 mins to a few hours.
Rechroming is to increase the chrome content of the wet blue, level up the variations of each hide, rechroming usually carried out after wet back at a PH of 3.3-3.5, finished by increase PH with sodium formate and or sodium bicarbonate to a PH of 3.6-4.2. Chemicals are used at thisstep are Chrome powder, chrome syntan, Glutaraldehyde, polymer, amphoteric syntan, Cationic/stable anionic faliquor. For certain wet blue/article, rechrome not always necessary.
Neutralizing is to remove free acids present in mineral tanned leather or formed in the leather during storage. Neutralization is carried out according to the type of leather to be produced, intensive neutralization throughout the whole cross-section is necessary for penetration of latter chemicals used. Instead of using alkalis, the leathers are in some cased treated only with lightly neutralizing syntans, the mild action donot cause damage to the leather fibre. Overneutralizing should always be avoided, as otherwise a coarse loose grain and an empty handle are obtained.
Neutralizing is checked with BCG for the penetration, and PH paper/meter for the PH of the float. Neutralization time is decided by the neutralizing system and the final article requirement, can vary from 30 mins to overnight(12 hours). Almost all the leather need certain degree of neutralizing. Chemicals are used in this step are neutralizing syntan(both naphthalene/phenol based); weak alkali salts(sodium formate, acetate, sodium sulpphite, sodium thiosuphate); strong alkali salts(sodium bicarbonate, ammonium bicarbotnate).
The time of retanning is most of cases is less than 5 hours, depends on the chemicals and article requirement. Retanning is a complex step and using quite a few kinds of different chemicals which they are have different functions and effect on wet blue. Chemicals used in this steps are: polymers; resins; vegetable; other retanning agents.
Fatliquoring is to lubricate the leather fiber to give leathers the desirable softness and handle. At the same time, influences the physical properties of the leather, such as extensibility, tensile strength, wetting properties, waterproofness and permeability to air and water vapour. Fatliquoring most of the time is carried out at a high temperature of 50-60 degree, fatliquor is pre emulsified with hot water of 50-60 degree before adding in the drum, running time between 30-90mins, finished by adding acid to precipitate the fatliquor in the emulsion.
Fatliquor can be made from natural oil/fat/wax(vegetable oil, fat/wax, or animal oil)
The chemistry of processing fatliquor can be roughly devided in the following: Sulphated, sulphited, phosphate, sulphonated…
Most of cases, fatliquoring is using a mixture of several different type of oils, this and the amount of fatliquor mainly depends on the property of the oils, final requirements and some personal preference.
Retanning agents eg. Protein filler, polyacrylic, can be used in fatliquoring to improve the property of the leather such as tightness, fullness… Improper use of fatliquors can lead to loose leather, oil patched, spew,etc problems.
Fixation as the final step of wet end process, normally carried out with formic acid, cationic products like mineral salt (chrome, aluminium), animal salts, etc also used for fixing to improve color fastness, washable property, etc. after all the wet end process, a cold wash and minimum 4 hours rest (usually overnight) is necessary to let the reaction and distribution of the chemical complete. Wet end Machinery: Drum/paddle, Fleshing machine, Lime splitting machine, samming machine, Splitting machine, Shaving machine.
Drying is to remove water from the retanned stock, and get ready to latter finishing process. Different drying method have their own advantage and can be combined to get expected result.
Drying process: after drying, the moisture content of the crust will be between 10-15%, condition is to adjust the moisture content to be around 20% by spray water on the flesh. Staking to soften the leather, usually conditioning with water is necessary first for best results. Trimming after dry toggle is to get rid of the parts which is no more usable. Buffing to remove damaged grain and defects or improve the nap on Nubuchs and suedes. Airblast or dusting to remove unwanted dust, brushing builds up static electricity and retains dust, the use of air blow it away.
Drying machinery: Set out machine, Vacuum, Drying tunnel, Dry milling drum, Toggling machine, Buffing machine, Airblast.
Finishing is more art than technique
Finishing to remove physicals, cover defects, improve aesthetics, add color and shine etc. Good results depents on the crust, chemicals, equipment and the finisher. Certain mistakes made in the wet end processing can be corrected in finishing, leather quality can be upgraded by careful finishing. In general, a finish is built up of several coats starting at the bottom being very soft and flexible, and at the top hard and scuff resistant.
Method of application: padding, brushing or sponging; spraying; Curtain coating; Roller coating; Printing; Laminating; Coating.
Once a type of finish has been decided, the finish can be formulated.
For a basic formulation, acrylic resins can be selected for softness, polyurethanes for improved physicals. For coloring and covering, Liquid dyes and pigments can be added, the ratio depends on the physical requirements. To obtain the required brilliance or gloss of improve aesthetics auxiliaries such as asesin/protein, wax are used, fillers are added to reduce gloss and improve fill and cover as well as reduce stickness. To obtain necessary physicals, gloss, feel and general aesthetics, a top coat is applied which is either NC lacquers or the water top (PU top). Various feel and gloss medifiers are added again to improve feel and awsthetics.
Finishing machinery: Spray gun; curtain coater, Roller coater; Glazing machine; Polishing machine..
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