Leather Processing and Sampling

The Leather and Shoe Research Association Inc. (LASRA) provided all leather samples for analysis. Samples from ovine skins provided the basis for majority of the work presented in this thesis. The processing method for these skins followed

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conventional tanning and beamhouse processes. Specific details of these processes are outlined below along with the selection of the samples.

Ovine pelts were obtained from five-month-old, early season, New Zealand Romney cross lambs. The skins were washed with cold water, dried, and salted. As “salted green” skins they were stored until needed. When needed, after mechanical removal of adhering fat and flesh, conventional lime sulfide paint, comprising 140 g/L sodium sulfide, 50 g/L hydrated lime and 23 g/L pre-gelled starch thickener, was applied to the flesh side of the skin at 400 g/m². The residual keratinaceous material was then removed over a period of 16 hours in a 1.2% solution of sodium at 20°C. The pelts were then washed to remove the lime and the pH lowered to 8 with ammonium sulfate, followed by the addition of 0.1% (w/v) Tanzyme (a commercial bate enzyme, Tryptec Biochemicals, Ltd.). This was followed by pickling in a 2% sulfuric acid and 10% sodium chloride solution. Using oxazolidine the pickled pelts were pretanned, degreased with an aqueous surfactant, and then washed. The skins were neutralised in 8% NaCl, 1% disodium phthalate solution (40% active; Feliderm DP, Clariant, UK) and 1% formic acid for 10 min. The running solution was then made up to 5% chrome sulphate (Chromosal B, Lanxess, Germany) and processed for 30 min followed by 0.6% magnesium oxide addition, based on the weight of the skins, to fix the chrome, and processed overnight at 40°C.

The partially processed skins or “wet blue” pelts were then retanned as follows. Wet blue pelts were first neutralised in 1% sodium formate and 0.15% sodium bicarbonate for 1 h. The neutralised wet blue was then washed in two volumes of water and retanned in one volume of water containing 2% synthetic retanning agent (Tanicor PW, Clariant, Germany) and 3% vegetable tanning material (Mimosa, Tanac, Brazil).

Finally, 6% mixed fat liquors were added and the leathers processed at 50°C for 45 minutes. The leather was then fixed by adding 0.5% formic acid and processed for 30 minutes. The leather was drained and washed in three volumes of cold water prior to mechanical softening.

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Leather sampling was done using the following procedures. Samples were cut on a flat surface with a sharp scalpel and a ruler using an even downward pressure. It was ensured prior to cutting that the sampling area was free from all obvious defects such as scratches and cuts. All samples were cut from the official sampling position (OSP) on the left or the right side of the backbone, a position defined by Williams (Williams, 2000b) and pictured as the shaded area in Figure 3.2a. Figure 3.2b gives a clear indication as to the location of the OSP in relation to the anatomy of the animal.

Figure 3.2. Representation of a hide or skin showing the sampling location for whole hides, skins and sides: (a) From Williams (Williams, 2000b) where B is the

OSP

(a)

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root of the tail, AD is a line perpendicular to BC, AC=2AB, AF=FD, JK=EF, GE=EH, HL=LK=HN, and AE=50 mm ± 5 mm; (b) simplification of hide showing the OSP in relation to anatomical features of the animal.

When investigating the nanostructure of the collagen fibril network of leather using synchrotron based SAXS, the samples are placed in the beamline so the full plane of the leather facing the beam is analysed. This provides information on the structure in the x and y directions, but not the z direction. For this work it was important to gain a full three dimensional view of the material. In order to achieve this, samples were cut in different directions from the leather pelts.

Figure 3.3. Sample cut from the OSP will either be cut as a square for flat on analysis or will be cut along the dotted lines of the square for samples that are parallel or perpendicular to the backbone of the hide.

All samples were cut from the OSP in directions either perpendicular or parallel to the backbone of the hide as shown in Figure 3.3 where the backbone is indicated by the dotted line up the centre of the pelt. These samples were named “edge on”

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as when they are placed in the beamline, analysis is done to show the collagen fibrils through the full thickness of the leather. Samples cut for static, edge on analysis measure approximately 1 mm x 30 mm. While the samples may seem thin with a width of 1 mm, the beam size is only 250 x 80 μm so a large sample is not needed to obtain a number of measurements across the leather surface. Majority of measurements are performed through the thickness of the leather to gain structural values for both the corium and grain layers of the sample. It is not necessary to have a wide sample when doing this as the full thickness of the sample can still be analysed. To obtain information in the third dimension, samples need to be analysed from the top surface of the leather. To do this square samples were cut for analysis and placed with the beam pointing perpendicular to the surface of the grain. These samples were named “flat on” and measured approximately 30 mm x 30 mm. For these samples the beam passes through both the grain and the corium layers and allows the structure to be visualised in the z direction. For individual analysis of each layer the square sample was carefully sliced along the intersection of the grain and the corium. The thickness, length, and width of all samples was measured with callipers using a light and consistent force and recorded for future use. Details of all samples were recorded when cut and the samples were stored at room temperature in small, numbered, lidded glass or plastic vials to ensure no information was lost and samples remained unharmed and easily identifiable after transportation (Figure 3.4).

Figure 3.4. An example of the leather samples in plastic and glass vials for storage and transportation.

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