CHAPTER 3 LEATHER PROCESSING

Transcription

1 93 CHAPTER 3 LEATHER PROCESSING Buffalo hides have a compact grain, larger area and good substance. Hence, this material is used in India for making safety shoe upper leather. These shoe uppers are usually of full chrome or chrome retanned hides and used with leather or synthetic lining. While processing the leather for safety shoe, the following characteristics should be given prime importance for good comfort. 1. Water vapour permeability 2. Water vapour absorption 3. Ability to pick up perspiration 4. Thermal conductivity 5. Increase in dimension to adjust the foot at extreme conditions 6. Resistance to water penetration through the grain side Of these comfort properties, one of the most important inherent properties of leather is the ability to absorb water vapour through the flesh side and subsequent permeation into the atmosphere to keep the foot free from sweating. Water resistance or water proofness of upper is the other highly essential property for safety shoe uppers meant for miners, shore workers and shoes meant for industries where footwear is subjected to direct contact with water. In a nutshell, the leather should permeate continuously sweat as water

2 94 vapour from the foot through the flesh side and prevent entry of water into the shoe through the grain side. These properties are achieved mostly by judicial adjustments in tanning, retanning, fatliquoring and finishing processes. In most of the commercially adopted tanning processes, the water proof faliquor deposit on the flesh side and prevents the water vapour absorption which leads to poor water vapour transmission. 3.1 MECHANISM OF TRANSMISSION OF WATER VAPOUR The mechanism of water vapour transmission through fibrous organic materials consists of gaseous diffusion through pores and through conduction (Meith, 1963). Conduction means migration by means of solution, diffusion or capillary action from a region of high to low concentration over a hydrophilic surface. By transmission through capillary action, it is conceived that water condenses in pores on the high relative humidity (R.H) side and migration proceeds automatically towards the region of low concentration in the low humidity side. Mitton and Green (1967) state that water vapour diffusion occurs mainly in free spaces within the leather. Water vapour permeability is increased with reduction in thickness and decreased with increased amount of natural glyceride and greases. The grain layer is the first sratum to become saturated with grease and consequently is a highly influential stratum with respect to permeability. Permeability decreases in the following order: Sulphonated oils > Acrylate resins > Rubber > Stuffing greases The comparatively high permeabilities for leathers treated with sulfonated oils and acrylate resins are due to the influence of polar groups in the agents. The finish would prevent liquid water from entering inside the leather

3 95 and at the same time it would permit passage of water vapour through it. When a finish is applied on the crust leather, the grain in not completely sealed because significant amount of solvent present in the finishing system evaporated and produces voids in the leather which would be large enough to transmit water vapour. As more water (sweat) is absorbed by the leather through flesh side, pressure at film side surface increases and when pressure gradient exists, permeation proceeds. However, if the finish is completely impermeable, saturation of leather will take place quickly and permeation process recedes. Further, if little water (sweat) is absorbed, vapour pressure at the finish surface may be so low that no outward transmission occurs. 3.2 EXPERIMENTAL METHODOLOGY Use of wet blue as a starting material is not advisable as there would be possibility of presence of wetting agents, detergents and surfactants that would pose problems in obtaining water proofness. The approach to the modified process to achieve good permeability and waterproof properties is : 1) Avoiding the use of surfactants and hydrophilic fatliquors; 2) Use of syntans to achieve the objectives of retanning without affecting the permeability properties; 3) Use of waterproof fatliquors in instalments; 4) Optimisation of finishing operation to achieve the required level of permeability properties; 5) Thorough washing of the leathers at the end of the processes to remove any neutral salts that may be present. 3.3 RETANNING carried out. In this study experiments with four different retanning systems were

4 96 Experiment I : Experiment II : Experiment III : Experiment IV : Melamine syntan + aromatic sulphonic acid condensates + Carbamide-formaldehyde system Phenolic + acrylic system Phenolic + protein system Resin + gluteraldehyde system 3.4 UNIT OPERATION Sequence of unit operation that were adopted for processing upto retanning is given below

5 97 Raw to wet blue Wet blue to retanning stage Wet salted Buffalo hides Wet blue buffalo stock Soaking Split, shave to 2.0 mm Liming Rechroming Unhairing Piling Reliming Washing Washing Neutralisation Deliming Washing Alkaline bating Retanning Pickling Chrome tanning Picking of chrome stock

6 98 Retanning experiments were conducted as described below: Experiment I Material % Time of drumming Water 200 Tergotan GSI Basyntan FB Basyntan AN Basyntan DI Fixing Formic acid 001 Water x Drained, washed, piles for one day Ammonia pre treatment Water 100 Ammonia x 2 x 15 Check ph for 5.5 Fatliquoring Eupilon WAS Eupilon WAS Dye Y 30 Checked for dye penetration Fixing Formic acid Water 015

7 99 Chrome topping Water BCS 003 Drained I Washing Water II Washing Water Drained / piled overnight Experiment II Material % Time of drumming Before neutralization Water Indotan Neutralised as usual, washed Retanning Water 100 Basyntan DI GS Powder Fixing Formic acid x Water 010 Drained, washed and piled for a day Ammonia pre treatment Water 100 Ammonia x 2 x 15

8 100 Checked for ph 5.5 Fatliquoring Eupilon WAS Eupilon WAS Dye y 30 Checked for dye penetration and fixed with Formic acid x Water 1.5 Chrome topping Water BCS Powder 003 Drained and conducted first washing Water Drained and conducted second washing Water Drained and piled over night Experiment III Material % Time of drumming Water 100 Sellasol PR Basyntan DI GS Powder Drained, washed and piled for a day Formic acid x Water 010

9 101 Ammonia pre treatment Water 100 Ammonia x 2 x 15 Checked for ph 5.5 Fatliquoring Eupilon WAS Eupilon WAS Dye y 30 Checked for dye penetration and fixed with Formic acid x Water 1.5 Chrome topping Water BCS Powder 003 Drained and conducted first washing Water Drained and conducted second washing Water Drained and piled over night

10 102 Experiment IV Material % Time of drumming Water 100 Neosyn AHN Relugan GTW Relugan S Neosyn DFS Neosyn PFX Fixing Formic acid x Water 010 Drained, washed and piled for a day Ammonia pre treatment Water 100 Ammonia x 2 x 15 Checked for ph 5.5 Fatliquoring Eupilon WAS Eupilon WAS Dye y 30 Checked for dye penetration and fixed with Formic acid x Water 1.5 Chrome topping Water BCS Powder 003

11 103 Drained and conducted first washing Water Drained and conducted second washing Water Drained and piled over night These four differently retanned leathers were subjected to other mechanical operations like setting, staking, drying, snuffing etc. 3.5 FINISHING The following technique was adopted for finishing: Sealing coat RU 3989 (TFL ITALIA SPA) : 100 parts (Fine aromatic PU dispersion) DL 2308 (TFL ITALIA SPA) : 130 parts (Water miscible organic thinner) Water : 200 parts Apply one cross coat Base coat RA 2 (TFL ITALIA SPA) : 25 parts (Non-swelling acrylic co-polymer emulsion) RA 2394 (TFL ITALIA SPA) : 75 parts (Self cross linkable acrylic copolymer emulsion) RU 3904 (TFL ITALIA SPA) : 50 parts (Aliphatic PU dispersion)

12 104 RU 1079 (TFL ITALIA SPA) : 100 parts (Self cross linkable acrylic copolymer emulsion) Black pigment : 200 parts FI 1271 (TFL ITALIA SPA) : 50 parts (Colloidal Filler) FI 30 (TFL ITALIA SPA) : 15 parts (Aqueous wax emulsion) XR 2508 (TFL ITALIA SPA) : 10 parts (Water dilatable polyaziridine crosslink) Water : 125 parts Apply three cross coats Top coat WT 2585 (TFL ITALIA SPA) : 20 parts (Dull water borne aliphatic dispersion) WT 2586 (TFL ITALIA SPA) : 80 parts (Bright water borne aliphatic dispersion) HM 551 (TFL ITALIA SPA) : 5 parts (Water dilatable silicone emulsion) XR 2508 (TFL ITALIA SPA) : 30 parts (Water dilatable polyaziridine crosslink) Water : 100 parts Apply two cross coats and finally emboss and finiflex. were fabricated. Leathers thus produced were tested and after standardization show

13 RESULTS AND DISCUSSION Amongst a host of materials now available for making footwear, leather has held its own as the upper material. Its irreplaceability is in no small measure due to the set of diverse properties it has combined in itself. This functional superiority is reflected in its unique structure. The uniqueness consists in its being a multi-phasic, multi-component, 3-D non-woven fabric laminated composite. These things impart the supreme flexural resistance and a high resistance to a wide variety of mechanical injuries besides good ventilation, which adds to comfort. But, Indian raw materials including the best buffalo hides are comparatively more open and this in particular calls for filling through the so-called Retanning. Filling, it should be noted, should not result in loading, however. Flexibility associated with raw hides and skins is lost almost fully in their processing to leather. Without its restoration to the required extent, which varies from leather to leather, no useful product can be made. Leatherlike drying, in fact, is one of the criteria of processing. Flexibilization is next only to tanning, solely because protein happens to be the basic material. And the flexibility achieved depends upon the quantity and type of lubricant used in fatliquoring, as the operation is called in leather parlance. Combining the above two operations/processes, four systems were tried and Tested for the five properties: the test results are given in tables 3.1

14 106 Table 3.1 Water vapour permeability and water resistance Sl. Property Exp.I Exp.II ExpIII Exp.IV No. 1. Water vapour permeability,mg/cm 2 /hour 2. Water vapour absorption,mg/cm 2 3. Water vapour Coefficient,mg/cm 2 4. Water penetration time,min > >420 (Bally) 5. Water absorption,% for minutes 6. Water penetration,flexes( Maeser) 122,500 10,000 9, ,000 a. WATER PENETRATION STUDIES Taking up the water-vapour penetration first, it should be borne in mind that the mechanisms of vapour penetration and liquid penetration are different. While the former is mostly gaseous diffusion, the latter involves a capillary phenomenon. The high dipole moment of water complicates this further. The whole transfer process should be viewed from these theoretical notions in mind. All the four systems exhibit excellent water vapour permeability and amongst them the property in the decreasing order is: II (phenolic + acrylic Syn.) > III(phenolic + protein) > I(melamine-resin ) ~ IV (resin +gluteraldehyde)

15 107 The maximum power of Phenol-acrylic system only shows that the phenolic -OH and the carboxyl -COOH have played their role far better and there are enough of them in the free form to take up and transfer polar molecules of water. That is, these groups are not all locked up in H-bonding and electro-static linkages. The II (ie. Phenolic-Protein system) is very similar to system I. And, yet, its inferiority which, of course, is only marginal shows that the protein carboxyls and the amino groups are involved in electrostatic links. In respect of the other two, Melamine calls for some special comments. It can be considered as as a cyclic trimer of Cyanamide (H2N-CN) 3 H2N-CN which,in turn, is an amide of hydrogen cyanide (HCN),an acidic material like HCl, in which the triple bonded N has no basic character. So, in melamine, only the three -NH2 groups jutting out from the hexagonal ring alone are basic and they too have no proton -binding power while part of a polymeric net on reaction with formaldehyde. So much so, melamine resins are neutral in character. In fact, Casein grafted with melamine-formaldehyde syntan exhibits good water-resistance and a product is selling well in the market. Coming to the dialdehyde, gluteraldehyde, it is neutral and so is the fiber-splitting nature of tannage by it, leading to very soft leathers. The other resin component seems to have filled the inter-fibrillar region, relegating the treated leather to the last place even though it possesses reasonable watervapour permeability.

16 108 b. WATER ABSORPTION STUDIES Water absorption also does not vary between wide limits. And this capacity decreases in the order : IV(resin + gluteraldehyde) > I (melamine-resin) > III(phenolic + protein) > II (phenolic + acrylic Syn.) The high water absorption possessed by the Gluteraldehyde system is as may be expected from the extraordinarily high fiber-splitting nature of gluteraldehyde tannage. This splitting naturally exposes a large fibril surface area accompanied by a corresponding increase in the number of accessible polar groups. Surprisingly, the melamine-retanned system has come second. Melamine-resin for some unknown reason does not seem to have evenly distributed itself. As pointed out above, a melamine - grafted casein, a globular protein, gives a good water-resistant film. Absorption by the phenolic systems (II & III) are much the same as their water-vapour permeability. Permeation, perhaps, involves desorption of adsorbed water, promoted by pressure gradient under static conditions and further accelerated under dynamic conditions. c. WATER PENETRATION TIME (mts) Time in minutes for water penetration fall into two ranges : i) >420 mts group: IV (resin + gluteraldehyde) & I (melamine-resin)

17 109 ii) ~ 230 mts group: II (phenolic + acrylic Syn.) & III (phenol + protein) Penetration being a capillary phenomenon, it should be inversely proportional to the porosity of the substrate. But, this is not borne out in the above results. Fiber-splitting gluteraldehyde re-tanned leather has the high penetration time, indicating effective filling up of the structure opened-up by the resin. The phenolic systems behave as if additional space has been opened up for water to pass through easily. The polar groups of phenolic and protein amino & carboxyl groups have rolled out a red-carpet of high surface tension for the polar water to roll over. It is not known whether the usually dimeric and trimeric molecular units of water are broken into monomeric forms-perhaps not, for even at 500 o C, water is dimeric only. d. WATER PENETRATION, FLEXES The number of flexes required for water to pass through the crosssection of leather through the grain side is taken as a measure of Waterproofness, and this is inversely proportional to the ease of water penetration. Again, the test results fall into two Groups: i) >120,000 group: I(melamine-resin) ~ IV (resin + gluteraldehyde) and ii) ~ 10,000 group: II(phenolic + acrylic Syn.) & III (phenol + protein)

18 110 There is no sign of water penetration into leathers retanned with system I and also IV even after 120,000 flexes as against 20,000 recommended by International standards. The hydrophobic materials to prevent wetting and penetration also should have reduced the pore size. The other two systems are phenol- acrylic and phenol-protein systems. In summary, Electro-chemical character of not only the finishing material but also the retanning material filling the interior seems to have a profound bearing on the water-proofing properties as may be expected. But, what is significant in the present study is that the systems - 1) resin-gluteraldehyde and 2) Melamine resin are preferable ones.