Liming and unhairing process can be defined as the cleaning of the crude skin into refined form through chemical process. Removing of the hair and epidermis to open the fibre structure is the operation conducted in the liming process. This process of unhairing and liming can be carried out simultaneously by putting the skin into the lime and water mixture along with other chemicals known as sharpeners. These unhairing and liming processes were much difficult in the past for the tanneries. Most of the skin and hides were produced in very poor liming procedure. Hides and skin were mostly contaminated with impurities, and this process was very time consuming. At present the liming process has improved by the use of chemicals, now we can yield better quality of the leather in the short span of time. The most commonly used system of the hair removal employs calcium hydroxide (hydrate lime) and sodium sulphide. These chemicals are added to the vessel containing the hides in water the concentration of these materials, the water temperature and the amount of the agitation (paddling are turning) all have a direct bearing on the rate at which the unhearing proceeds. For example, higher concentration in temperatures used in the hair burn can result in the hair being dissolved and removed in only in few hours. However, if the hair is to be saved for its commercial value, a longer procedure using weaker concentration is used. This hair save process usually runs from two to four days and destroys only the hair root, leaving the hair itself free to be recovered mechanically.
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Until the 1880’s only hair-save methods (straight lime or sweating) were used for unhairing. These methods required subsequent mechanical unhairing. Originally this was done by hand on a beam! Unhairing machines were introduced at later stage. From the 1880’s onwards, hair destroying unhairing process using lime and sulphide used in tanneries: a practice that has since been generally adopted in most countries. [i] The liming process opens up the collagen fibre by the alkaline swelling of the structure and the further pulping of the hair/ epidermal protein (keratine) occur by the reduction of the disulphide bridges under the influence of the alkali the aim of an optimum liming process is to achieve this without at the same time producing undesirable effects such as draw, emphasized growth marks, loose grain and empty belly areas. Hair root material remaining should be loosed easily removed later during deliming and bating operations. [ii] It is the way through which hide and skin is yield at ease through deliming and bating operations without any unwanted effects. The function of the beamhouse process is not limited to the removal of the hair. The action of the chemicals on the hide changes the chemical and physical nature of all components of the skin. The removal of the globular proteins, which was stated in the soaking operation, is carried further toward completion during the liming operation by the action of the alkali. The action of the alkali and reducing agents o the elastin is somewhat limited, but it may contribute significantly to the ease of the removal of the elastin by the proteolytic enzymes using in bating. The presence of the alkali in the skin during the liming aids in the saponification of the natural fats and other fat type materials. [iii] There are many different process of liming three pit system is one of these processes. The system is more through development of the same idea, the pack of hides being placed successively in three pits for 2-3 days each. A. The first pit contains twice-used lime liquor. B. The second pit contains once-used lime liquor with an addition of ½% lime and ¼% sodium sulphide. C. The third pit contains a fresh liquor of 2% and ¼% sodium sulphide. Fig: All these percentage is based on the volume of the water. This is called a “counter – current” system, the hides moving from pit one way, whilst the oldest liquor A is run away and replaced with a fresh liquor C. Such system ensures that the raw hide gets the mildest liquor first and finished in the sharpest liquor. This gradual procedure avoids bucking, and another advantage is that every new lime liquor is used three times. This system has been used chiefly for heavy hides, e.g. hides for sole leather, where flatness and solidity are important. Its disadvantage is that it involves either a lot of labour in handling or hauling the hides, or special equipment to do this. A common method is to hang the hides on frames, which an overhead gantry crane can readily lift up and move to the next pit. The pits may be fitted with mean of agitating the lime mechanically. For most light leathers and shoe upper leathers, paddle or drum liming, with its positive mechanical action and consequent quicker penetration of the liquor, is more popular. The three pit system or the partial use of old lime liquor can be practiced, but a one liquor system is now more common as it can controlled more accurately to suit particular packs. [iv]
Usually the tanners use some chemicals such as penetrating agents and amine to settle this problem. The shortcoming of “tight inner and loose outer” has been improved so much, but it’s not our final goal.Besides, that is known the tanning industry is one of the most polluting industries. And the liming stage is one of the most polluting processes in the leather making. There is 85% of pollution from the liming stage, so reducing pollution effectively in the liming will improve remarkably the quality of the polluted environment for the leather making industry.How the liming process influences the area yield and fullness and softness of leather.As mentioned above, the unevenness of the treatment with lime and alkali results in a limitation to leather area yield and fullness and softness to be enhanced to the full extent. The treatment of hide and skin with lime and alkali is just like the wheat flour ferment. For the dough leavened well, the steamed bread has a good-looking and a good taste and a good feeling and a big volume. It’s shown that the dough of full and uniform leaven appears a lot of fine interstices; otherwise it appears solid and tight and has a dark color and few interstices.
As far as the structure of hide and skin in liming is concern, the extent of the outer fiber structure separated and opened up is much greater than that of the inner, so that the outer is much more elastic and softer than the inner, just like a iron plate with its two surface folded up with soft and elastic rubber (presented in figure 8), With vast stress in its inner, we can’t stretch it to the extent that is desired.For this reason, when the degrees of opening up of the inner and outer fiber structures are closer, the area yield and the softness and fullness of the leather are better.
3. Performances and mechanism of DO-PRO.As everybody knows, the natural “channels” such as blood vessel, capillary, lymphatic vessel, sebaceous glands, sweet glands etc, uniformly and deeply distribute in the hide and skin, see Rawhides have no “activation”, but practices show that the medicines used for some living animals have similar function to the dead animal tissues.For instance, the muscle that meets with the cold water will contract quickly. After the rawhides are preserved with salt, blood and sweat in the blood vessel, sweat glands, lymphatic vessel etc. have been dissolved out or flow out, but “vessels” such as blood vessel and lymphatic vessel are kept completely, just compressed temporarily. DO-PRO was designed to activate and open these “passages” again to make OH- and Ca2+ pass through these natural channels more easily to reach the inner fiber structure.
Although it is not completely clear that whether DO-PRO is working just like the procedure that is designed, there have been a large number of trials and practices to prove that all the performances and characteristics with DO-PRO that are expected by us have been achieved.
In order to make it clear whether the level of opening up of collagen fiber structure has been enhanced while DO-PRO applied, the limed pelts from different liming agents are cut into pieces and dehydrated to be prepared into cross section microscope Figure 9(a) and Figure 9(b). It’s very clear that overall the fiber structure with DO-PRO has been opened up more evenly and fully than conventional liming process, particularly in comparison with the outer fiber structure, and the increase in the level of opening up of the inner fiber structure is much greater. Cross section microscope picture of limed peltÃ-15.The pictures are prepared as follows. The samples of limed pelt are cut from the corresponding parts of one cattle hide piece. Before soaking the cattle hide is cut into two equal sides through backbone line. The two sides are soaked in one drum, and then the liming processes are performed in two drums, one side one drum. DO-PRO and the liming agent with organic amine are applied separately. The samples are frozen then cut into flakes with 0.3mm in thickness and 1.5cm in length. Then they are dehydrated by alcohol, finally observed and taken pictures through microscope.We employ flame atom absorption chromatograph to determine the status of distribution of Ca2+ in limed pelt. The procedure and result are given in Table1 and Table 2 (samples are prepared as mentioned above). They show that for the limed pelt with DO-PRO, the amounts of Ca2+ in the inner layer are 0.0816% in neck and 0.0715% in belly, and meanwhile for the corresponding limed pelt that is limed with conventional liming process, the amounts are 0.0266% in neck and 0.0244% in belly. It’s very clear the evenness of the distribution of Ca2+has been improved distinguishedly. The total amount of Ca2+ absorbed within overall limed pelt has been increased 30% (this proves with the reduction of 30% in amounts of CaO in the waste water. It is found out in production line that Cr2O3 content in waste water reduces 20%, so the chrome exhaustion increases 20% (see Table 4). The shrink temperature (Ts) of the wet blue increased from 102 to 106(see Table 5), and subsequently, the exhaustion of retanning agent, filling agent, dye stuff also increases 15%, because reduction of 15% in baume in waste dye liquids is found out.
From the results above mentioned, we can confirm that in comparison with conventional liming process, all the physical chemical properties of the finished leather would be improved remarkably.
The practices show that the area yield, the fullness and softness have been increased strikingly, and meanwhile the rate of loose grain reduced obviously. The data from Henan Shoes City tannery and Fujian Xingye tannery present that the area yield increased >4%, the rate of loose grain reduced >5%, and the softness increased >30% (the milling time reduced >30%).
The practices with tannery production show that DO-PRO increases remarkably the speed of penetration through the direction that is from the flesh side to the inside layer, and reduces breakdown on the grain from the lime and alkali. This is obvious that DO-PRO makes the speed of hair-removing slower than conventional liming process. This protecting action on the grain enables us to perform more drumming in the end of liming which does not cause the loose grain.
DO-PRO is very friendly to environment, which is of no odor and of no toxicity. It is able to make the content of Na2S in waste water reduced 15% (see Table 7) and the content of CaO reduced 30%(see Table 3) and the content of Cr3+ reduced 20% (see Table 4). It’s very clear that DO-PRO enhances the exhaustion of chemicals above mentioned and certainly reduces output of these materials into environment. For example in Fujian Xingye tannery, output of the waste water has been reduced 60T/day. In the range of overall China, the output of waste water would be reduced 2000000T/year. So we can say that the being born of DO-PRO enables us to reduce greatly the pollution from the tanning industry.Attachment:Flame atom absorption chromatograph is employed to determine the status of Ca2+ in the limed pelt.Sampling: The samples are cut from neck, belly, and the corresponding parts of one cattle hide piece. Before soaking the cattle hide is cut into two equal sides through backbone line. The two sides are soaked in one drum, and then the liming processes are performed in two drums, one side one drum, DO-PRO and the liming agent with organic amine are applied separately. The samples are frozen.
Delaminating: Use a skin grafting knife to shave outmost 0.2 mm, and then the pelt is cut into three layers, known as the grain layer, the center layer and the flesh layer with a scalpel. Weighting: Take 300 mg sample to be weighted with an analytical balance. Dissolving: Put the sample in the taper glasses, add 25ml 5N HNO 3 solution and then heat the glasses for 2-4hrs till the pelts totally dissolve. Diluting: Put the above solution into glass vessels and dilute them into 100ml. Determining: Test the Ca2+ concentration with flame atom absorption chromatograph.
Usage:
Usually DO-PRO can be used in soaking and liming, and after deliming and bating processes in leather making.The suggestive method is as follows:
Soak raw hides or pelts in bath preparation that contains DO-PRO, and use them in combination with other commomn industrial materials of leather making. Its use level is 1.5%-3.0% of leather weight, and other conditions are the same as normal conditions of leather making.
DO-PRO is not compatible with enzyme, organic amines and many degreasing agents. So, the right way to use: feed the chemicals one by one into the drum.
DO-PRO area yield varies according to its adding order.
DO-PRO suggestive methodology is as follows:
In the early stage of liming:
Water…………………………………………………………70%-120%
NaHS………………………………………………………….0.8%-1.2% (run 70~80min)
DO-PRO…………………………………………………….1.4%-1.8%
Na2S…………………………………………………………..0.7%-1.2%
Lime…………………………………………………………..1.0%-1.5% (run 2h, stop 30min)
Water made up……………………………………………30%
Lime…………………………………………………………..2.5%-3.0%
Degreasing agent ……………………………………….0.2% (run / stop 30minÃ-10)
Experimental methods
{briefly describe the old and new methods here}
Unhairing
Methods of hair removing can be divided into two groups:
Methods based on destruction or modification of the epidermis tissue surrounding the hair, so that it can be loosened and removed
Methods in which hair itself is attacked and its structure is destroyed (use of alkali Ca(OH)2 or NaOH and Na2S).
{Give further details of these two methods here}
Liming
The major chemical modification of collagen during liming is the hydrolysis of some of the amido groups attached to aspartic and glutamic acid residues.
Past, present and future
Past
In ancient history liming and unhairing was carried out by the method of tanning. Tanning was considered a noxious or “odiferous trade” and relegated to the outskirts of town, amongst the poor. Indeed, tanning by ancient methods so foul smells that tanneries are still isolated from those towns today where the old methods are used. Ancient civilizations used leather for water skins, bags, harnesses, boats, armor, quivers, scabbards, boots and sandals. Tanning was being carried out by the South Asian inhabitants of Mehrgarh between 7000-3300 BC. Around 2500 BC, the Sumerians began using leather, affixed by copper studs, on chariot wheels.
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Present
In the modern methods for liming and unhairing the first stage is the preparation for tanning. The second stage is the actual tanning and other chemical treatment. The third stage, known as retanning, applies retanning agents and dyes to the material to provide the physical strength and properties desired depending on the end product. The fourth and final stage, known as finishing, is used to apply finishing material to the surface or finish the surface without the application of any chemicals if so desired.
Preparing hides begins by curing them with salt. Curing is employed to prevent putrefaction of the protein substance (collagen) from bacterial growth during the time lag that might occur from procuring the hide to when it is processed. Curing removes excess water from the hides and skins using a difference in osmotic pressure. The moisture content of hides and skins gets greatly reduced. In wet-salting, the hides are heavily salted, then pressed into packs for about 30 days. In brine-curing the hides are agitated in a salt water bath for about 16 hours. Generally speaking, curing substantially reduces the chance of spoilage by bacteria. Curing can also be done by preserving the hides and skins at a very low temperature.
In a process known as soaking, the hides are then soaked in clean water to remove the salt and increase the moisture so that the hide or skin can be further treated.
Future
{On the bases of literature review and different methodologies briefly describe the past, present and future of liming and unhairing}
Comparison
Conclusions
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