By obtaining the average moisture content of a pile of wet leather it is possible to better define the quantities of reactive chemicals needed. The Nuclear Magnetic Resonance has been improved as a promising solution to determine the moisture of the piled material.

Wet-blue (wet leather tanned with chrome salts) can be either an intermediate step of the process or a raw material. In either of these cases, the moisture content of wet-blue is an important factor.

Following the irreversible reaction of tanning, the drying of wet-blue is detrimental to later wet processing. For this reason it is stocked in sealed pallets or in warehouses where moisture is controlled.

Another relevant factor to consider is the impact of the moisture content of wet-blue on the way of running wet-end operations (neutralisation, retanning, dyeing, fatliquoring). The industrial practice is based on operations in which the proportions of reactive products are determined depending on the leather mass to be treated: 3% of chemical agents means that for every 100kg of leather material, 3kg of product are needed.

In wet-end operations, the wet-blue leather mass is used as a reference, which is formed of dry leather and water. We would say that the wet-blue has a moisture content of 50%, meaning that the amount of water is 50kg for every 100kg of wet-blue. If this moisture was referred to dry material, it would have a value of 100%.

A mathematical relation links the moisture values in the reference material to moisture values in relation to dry material.

Consequences of variation of moisture content in wet-blue

In order to meet commercial requirements of quality, quantity and deadlines, tannery production needs a batch processing system during wet-end operations. Important variations in moisture content can result in a batch of wet-blue: storage duration, climatic conditions, variation on equipment adjustments etc. This is even more sensitive when the wet-blue is the incoming raw material from another tannery: duration and transport conditions, variation in the conditions depending on the production batch have a strong impact in moisture content. In practice, moisture in wet-blue can vary in the range of 50-55% to 60-65%.

If we look at production methods, the reference mass from which we calculate the amount of reactives needed will not have, from a reactive point of view, the same meaning. The reactive agents only work on the dry part of the leather and not the associated moisture.

All this leads to an incorrect dosage of the products: overdose when moisture is too high and under dosage when it is too low. The reproducibility of results in terms of colour and physico-mechanical characteristics could also be compromised.

In order to measure the impact of these differences, some simulation tests have been carried out. Taking standard operating modes as a reference, we have applied the same formulations, increasing or decreasing the reference mass of 10%. Colour is greatly affected by an overdose in a full chrome process. In vegetable retanning the effect is softer. Under dosage leads to a weakening in full grain solidity and to a drop in elongation and the same occurs in suppleness. The noted variations are more relevant when the leather is thinner.

In order to avoid this difference in results, knowing the moisture level in a wet-blue pallet to be used would be a major advantage.

How to measure the moisture content in a pallet of wet-blue?

Measuring the moisture level in materials and products is often a necessity:

* Final quality is in most cases closely linked to this characteristic (product stability, mechanical properties, food regulations etc)

* the management of the transformation process or use can be driven or adjusted according to the measurement: drying (quality and economic implications) chemical reactions (calculation of concentration, volume of treatment bath….).

If by taking samples and continuously testing the water in the laboratory, we know how to measure the moisture exactly at different stages, these methods have not been adapted for full-scale processing. Moreover, if we need to know the average moisture in real time of a wet-blue pile on a board there is no solution that has been proven yet.

There are measurement systems for ‘solids’ operating in the industry such as:

* Measure of electric resistivity

* Measure of dielectric permittivity

* Measure through absorption of microwaves

* Measure through infrared retro diffusion

* Measure through neutron slowdown

The areas of use, advantage and disadvantages of each system have been explained in a guide (Measuring of moisture in solids: study by CETIAT for ADEME). The analysis of this information shows that any of these procedures cannot respond technically to the issue of determining the moisture content of wet-blue in pallets.

Another lead that has been explored is the use of RMN, Nuclear Magnetic Resonance (nuclear refers to particles).

RMN: operation principles and use in determination of moisture content in piles of wet-blue

In order to measure the moisture contained in a medium, we use the RMN of proton, which is a particle sourced from the electrolytic dissociation of water. The principle of this method can be summarised in four steps:

* The humidified material is submitted to the permanent magnetic field of a magnet

* The particles line up, especially the water protons, in the direction of the field

* In this balance status, we apply a wave train of known characteristics; the particles are orientated under this effect and we obtain a balance resonance

* After stopping the emission of the wave train, the particles resume their orientation under the magnetic field; this return is translated by a signal to which we link a relaxation time of the protons. This value is directly related to the amount of existing particles in the wet substance, hence the possibility of quantifying the moisture contained

Application tests to determine the moisture level of wet-blue

The purpose of these tests is to assess the possibility, in laboratory conditions, of measuring the moisture content of wet-blue via the RMN technique. To carry this out, three types of materials have been analysed:

* Sheep, non shaved, thickness 0.9 – 1.3mm

* Calf, shaved, 1.4-1.6mm

* Bovine leather split and shaved, 2.0-2.2mm For each type of wet-blue the moisture content has been determined by three procedures:

* Weight procedure by controlled drying in oven

* RMN lab procedure analysed on extracted samples

* RMN ‘mobile’ technology on leather pieces to model piles (format 30 x 30cm, height 3 to 4cm).

Comparing the results from both the weight and the RMN lab procedures, the gaps obtained are very small in terms of moisture percentage.

The compared results obtained in both the weight and the RMN ‘mobile’ technology show that the resulting values fluctuate depending on the conditions of the system.

The homogeneity of the piles (linked to the thickness and appearance of wet-blue) has a strong influence on the result. If we put pressure on the measurement sensor, the results are very positive for calf, slightly in excess for sheep and wrong for bovine leather.

Following the current status of work, we can say that measurement of moisture content in wet-blue via RMN is possible.

The lab RMN procedure is perfectly comparable to the

balance procedure. The reliability of measures via RMN ‘mobile’ technology needs to be improved. The homogeneity of the leather pile is key.

With regard to industrial equipment, two development procedures are possible: a mobile system enabling the moisture of a pile via waves to be obtained and a fixed measurement detector that immediately gives the total moisture content of the batch.