Depending on the end-use and types of leather, a wide spectrum of tests based on visual, physical and chemical and instrumental techniques have to be carried out in a testing laboratory. With leather being a non-homogeneous commodity, performance tests have an important role to play in assessing its quality.
To obtain a standard leather of required quality for a specific end-use it is necessary to use selected raw materials, chemicals and leather auxiliaries.
To maintain high quality standards in leather, it is necessary that the quality of rawstock as well auxiliaries such as general-purpose chemicals like sodium sulfide, lime, sulfuric acid etc, agents like vegetable and chrome tanning extracts and performance chemicals including syntans, dyes, fatliquors and finishing aids, be tested periodically for their quality.
Recently there has been a growing awareness among the public regarding the quality of consumer goods and this is coupled with international systems such as ISO 9000/14000 and Environmental and Pollution Control aspects.
This is possible only when the concepts of proper testing for quality are introduced in the raw materials and chemical inputs in processing industries such as tanning.
Hence, an effective physical and chemical testing laboratory and technical services centre for quality control, developing suitable recipes and carrying out practical evaluation and performance trials is essential in the modern competitive environment.
The ultimate aim is to introduce high quality innovative, eco-friendly and cost effective concepts into the leather industry. The various aspects for the testing laboratory and technical service centre are highlighted in Figure 1.
The establishment of such a centre for a cluster of tanneries in a region can solve the day-to-day production and process problems and maintain quality standards of their products.
Case study
A field survey has been conducted for the cluster of tanneries in Southern India into obtaining a greater understanding of testing facilities. Around 56 units have been covered in the field survey regarding the status of testing facilities and requirements of the testing centre. Classification of surveyed units according to process is given in Table1. The status of testing facilities required by the units is given in Figure 2.
Out of 56 units surveyed, raw to wet-blue processing units account for 25%, tanneries processing from wet-blue to finished leather account for 33% and units processing from raw to finished leather account for 42%.
The survey reveals that around 43% of the total surveyed units require all basic physical as well as chemical tests. Basic physical tests identified from the survey are tensile, stitch tear and tongue tear strength, flexing test, grain crack/distension, colour fastness, shrinkage test, water vapour permeability and water absorption.
Basic chemical tests identified from the survey are analysis of purity of general and performance chemicals. About 24% of the total surveyed units require chemical analysis of banned substances such as dyes, PCP, chromium (VI) and free formaldehyde.
Out of 56 units surveyed, 19% required other facilities such as sample drums, computer aided colour-matching facilities and CAD/CAM facilities.
Technical aspects
The most important factor influencing the physical properties of leather is the fibre structure. Physical properties are determined as per standard methods under specified temperature and relative humidity.
Prior to testing, the samples have to be conditioned in a standard atmosphere: this can be best achieved by the installation of air conditioning equipment in the physical testing lab.
The ultimate quality test for leather is to assess its performance in wear or use under natural conditions – a wear trial. To reduce the planning and cost involved in such trials, laboratory machines are used to simulate natural conditions such as flexing and water penetration in addition to the various physical characteristics of leathers.
A combination of two or more properties has to be tested to ensure the quality of the leather for a particular use. For example, sole leather has to be tested for water absorption as well as resistance to abrasion, upper leathers for flexural endurance, fastness to rubbing, water vapour permeability etc.
The physical and mechanical properties determined on a given sample of leather have direct relevance to its ultimate end use.
The main objective of the physical testing laboratory is to carry out these tests as per the national or international standards, eg IUP/BIS standards, to ensure quality control as well as for export certification purpose. The lab should be fully equipped to perform the tests as given in Table 2.
The IULTCS (International Union of Leather Technologists and Chemists Societies) as well as BIS (Bureau of Indian Standards) recommend standard methods for physical as well as chemical testing of leather, which can be adopted for the testing based on the requirements of the buyer.
Chemical testing lab
The main objective of a chemical testing laboratory is to provide assistance to the leather industry in the quantitative/qualitative analysis of eco-sensitive chemicals present in leather and leather products as well as assay chemicals used in the processing of leather.
In view of the importance of leather as an export commodity and the recent ban on several of the chemicals by importing countries, it has become necessary to establish modern facilities to analyse such chemicals.
This lab should be equipped with instruments such as UV-visible spectrophotometer, GC and HPLC. For export the criteria for ‘Eco-labelling’ regarding the eco-sensitive chemicals in leather and leather products are given in Table 3.
There are a number of banned azo dyes, because through the cleavage of one or more azo bonds, they could form any of the twenty aromatic amines listed in Table 4. Azo dyes are the most important chemical class of dyes, representing 60-70% of all dyes used. However, their use is banned in the manufacture (in Germany) of regulated consumer goods and their presence in detectable amounts (ie yielding greater than 30mg individual listed amine/kg consumer goods) in regulated products placed on the German market. The presence of these banned aryl amines have to be analysed using HPLC technique.
Pentachlorophenol
Pentachlorophenol (PCP) is sometimes used as a preservative and as an anti-mould agent in leather processing, so it is obviously harmful to living organisms. The German regulation specifies a maximum limit of 5 mg/kg, which in practice means a ban on the use of PCP.
Free Formaldehyde
Free formaldehyde, which may arise from syntans used in retanning process, leading to the supposed presence of free formaldehyde in leather, has to be analysed using GC (Gas Chromatography).
Hexavalent chromium, Cr(VI)
The presence of Cr(VI) in the final leather may occur due to the presence of Cr(VI) in pigments, un-reduced Cr(VI) during the preparation of BCS, or the oxidative atmosphere in leather processing and subsequent period. The Cr(VI) present in the leather is leached out from the leather and then measured calorimetrically using a spectrophotometer after making complex coloration with diphenylcarbazide.
General chemicals
The general chemicals such as wetting agents, lime, sodium sulfide, ammonium chloride, sodium chloride, sulfuric acid, formic acid and borax etc, have to be analysed for their purity.
Testing of tanning agents
Tanning agents employed in leather processing, such as wattle extract and myrobolan extract, have to be analysed for their tannin/non tannin content and mineral tanning agents such as basic chromium sulfate (BCS) and aluminum tanning salts, have to be analysed for their Cr2O3 and Al2O3 content respectively.
Based on the field survey and also keeping in mind the quality control of leather, it is important that the majority of the chemical tests set out in the SLTC official methods are carried out.
Microscopical and bacteriological tests
This is a technique used as a quality control tool on raw skins, semi processed pelts or finished leathers. The opening up and the orientation of the fibre structure of skins during the different processes of tanning operations can well be assessed by microscopic examination.
If the curing is improper or insufficient then there is every possibility of bacterial damage. The extent of damage of the skin can be judged by viewing a cut section under the microscope. In addition, it is an effective tool to determine the presence of stains, defects, discoloration etc, in the leather.
During storage, wet-blue or EI leathers, finished leathers, leathergoods and vegetable tan liquors are some times attacked by moulds, which can easily be detected by the microscopic examinations.
The bacteriological tests are also quite important. The bacterial population associated with a fresh salted skin will provide preliminary indications about the quality.
These examinations help in determining the resistance of skin/leather for bacterial/fungal growth, and also help to find out efficiency of various bactericide/fungicide used in preserving the leather.
Process/product development lab
The basic objectives of the process/product development laboratory are: development of sample leathers as per control samples supplied by the buyers, the restandardisation of the process parameters for material/energy optimisation, the development of alternative techniques of manufacture in situations arising out of changes in raw materials and chemical qualities, and the introduction of new machinery.
The facility may include sample drums specifically suited for dyeing, processing skins and hides.
The sample drums should have facilities such as RPM, temperature and time for better process control.
This facility can cater to the needs of tanners interested in performing sample experiments with a small number of pieces.
The process control facility consists of an automatic chemical weighing system with a load cell, hot water still and a specially designed working platform for charging and drum operating/ monitoring.
After automatic weighing, the chemicals are transferred by gravity into a receiver on a trolley, which is equipped with a transfer pump and necessary piping.
It can be moved nearer to the drum for charging. The liquid effluent streams from the experimental drums are let out of the process/product development lab through the appropriately designed drain system. A separate store room is recommended for safety considerations.
Colour matching and CAD/CAM
The facility could be used to serve the industry with computer aided colour matching of dyed leather samples, so it should be equipped with spectrophotometers capable of measuring L*, a*, b*, c and h values. A CAD facility can help industries by creating computer-aided designs for footwear or leathergoods.
Other points
Analysis of effluent liquor such as for BOD, COD, TDS etc, will need to be carried out, as will analysis of process water such as for hardness.
For health and safety, it is recommended that adequate ventilation is provided in the building. All electrical installations should be properly grounded, and normal fire fighting facilities should be adequate for the storage area for chemicals and other areas.
It is also recommended to take proper personnel protective measures wherever needed. Safety and fire fighting equipment identified for the centre are given in table 5.
Training
The centre can be equipped with one lecture cum conference hall, a departmental library and appropriate audio-visual facility including TV with VCR, overhead projector.
Training is also one of the areas identified and could include leather processing, leather quality assessment and control, product and process development, colour matching of dyed leather, effluent treatment & quality control, and Occupational Health, Safety and Environment (OHSE).
Conclusions
The production of uniform quality leather depends upon the proper selection of raw materials, chemicals, auxiliaries, formulation and judicious adjustments and manipulations in the manufacturing process and finishing operations by developing suitable recipes.
The testing lab and technical service centre for tanneries and product units is essential for working out newer techniques to improve quality, upgrading low grade leather to realise higher unit value, to work out chemical and water savings in order to reduce pollution load without sacrificing the quality.
Hence effective physical, chemical tests and technical service centre for quality control, developing suitable recipes and carry out practical evaluations and performance trials, is essential in the modern competitive environment in which rapid and sustained introduction of high quality, innovative, eco-friendly and cost effective leather/leather products have become the order of the day.
Acknowledgements
The authors thank Dr T Ramasami, director, CLRI for his support. G Swaminathan and staff, chemical engineering division, for their help. Dr Ganga Radha Krishnan, Dr C Muralidharan, B N Das, R Rajaraman and E Ravindranath, CLRI scientists, for their co-operation. The authors also thank Ananda Padmanabhan for his help.
