Looseness is a problem which annually affects up to 7% of crusted hides in New Zealand. Through a joint project between the New Zealand hide processing and leather industry and the Ministry for Business Innovation & Employment (MBIE), LASRA is investigating the occurrence of looseness in hides to understand its cause and develop methods for dealing with the problem.
At the start of the project we were surprised by the lack of previous work in the field. Work investigating looseness was carried out at the BLMRA in the 1970’s, but the most significant recent work on looseness was conducted by Lui, et al. at the USDA Eastern Regional Research Centre in 2009. His team investigated looseness in crust leather by examining differences in the microscopic structure and by measuring the effect of looseness on mechanical strength of crust leathers.
They found visible gaps between the interstitial layer and the corium layer in loose leather, and a grain layer that was more sheet-like in structure and more loosely packed than the smaller bundles found in tight leather.
Their work sought to provide a better understanding of looseness which could lead to the development of measures to correct the defect, such as the addition of fillers to leather during processing.
In our study we have sought to understand the underlying cause of looseness from the animal to final crust leather with the aim of identifying looser hides at the earliest stage possible so they can be either diverted to production where looseness is not an issue or processed using a modified recipe which reduces the development of looseness in the final product.
Appearance
Looseness in hides and skins causes an excessively wrinkly appearance in finished leather when it is folded to simulate wear. Looseness is present in the axillae region of most hides and skins to a limited extent, but in loose hides it is more prominent and can stretch across and up through the belly region and into the shoulder area where it significantly impacts on the value of the leather.
Figure 1. Adjacent areas on the same hide folded to display very different levels of wrinkling.
Measuring changes during processing
Monitoring of the collagen fibril arrangement in samples of loose and tight hide at the pickle, wet-blue and crust stages has been carried out by Massey University’s Professor Haverkamp using the SAXS (small angle X-ray scattering) beamline on the Australian Synchrotron to reveal a more uniform layered structure and fibril orientation in loose leather, identified by a higher orientation index (OI) edge-on than that found in tight leather. Additionally, the OI of loosely structured hide has been observed to vary far more during the different process stages than tightly structured hide.
In related practical trial work we have identified that loosely structured hide swells up to three times as much as tighter structured hide between the pickled and wet blue stages, raising the possibility that loose material either has a pre-existing weakness between layers to provide less resistance to swelling or that some other factor such as chemical affinity is different causing greater swelling which leads to looseness.
Analysis of the chemical components of loose and tight hide shows that there is a relative increase in the removal of non-collagen proteins in loose material during early-stage processing.
Through collaboration with Victoria University of Wellington (VUW) we have studied loose and tight hides at different process stages using a “portable” NMR sensor termed the NMR-MOLE (MObile Lateral Explorer). This has a unilateral (one-sided) sensor which is ideal for measuring leather articles up to 16mm away.
The principle is to measure the time taken for the induced spin of electrons in hydrogen to return to normal after excitation by an external magnetic force supplied by the NMR-MOLE. This relaxation time increases in materials where the water loaded interstitial spaces are relatively large. Therefore, this technique is ideally applied to measure void space in wet materials.
Green fleshed whole hides suspected of being looseness-prone or tight were sampled in five regions on one side of the hide and measured using the NMR-MOLE. The pieces were processed along with the remaining side to crust and removed from the drum for measuring post-lime, at delime, pickling, wet-blue and wet-crust stages.
Significantly higher T2 relaxation times were measured in material which went on to give looser crust leathers at both the deliming and pickling stages. In all cases the greatest values were found in the front and rear axillae. The technique is limited in application to hide prior to chrome tanning due to the paramagnetic interference of chrome which destroys the signal.
Identifying loose hides in production
Having identified looseness and its morphology in crust leather and the changes during processing the challenge now is identifying those hides which are prone to looseness at an earlier stage, to divert them into alternative applications such as upholstery where looseness is less of an issue.
A non-destructive technique which may be able to identify inconsistencies in the structure of cattle hide is ultrasound. Sound waves are generated at ultra-high frequencies up to 50 MHz by the hand-held device and reflected back to the sender or transmitted through the leather.
Contact ultrasound is used for a variety of diagnostic purposes including the identification of soft-tissue damage in horses and routine visualisation of foetuses in the womb.
Massey University and LASRA have shared in the purchase of a precise medical ultrasound device for the evaluation of cattle hide at a variety of process stages to identify discontinuities and abnormalities which might correspond to looseness.
Figure 5. Ultrasound image of cattle hide courtesy of Dr Haverkamp, Massey
A PhD student was brought in to develop a technique for the evaluation of looseness in cattle hide using this medical ultrasound device focusing on identifying the optimal process conditions of the transducer and frequency to achieve the best resolution on cattle hide.
The results so far are very encouraging. In loose hide a region of low intensity is observed just under the grain layer which is not found in tight hide, which has a denser and more uniform arrangement throughout the thickness.
Future work
The intention of this study is to investigate looseness in cattle hide to gain a better understanding of its incidence and cause and to develop a method to identify it at an early stage in production using the best candidate of the non-destructive technologies already investigated, which will then allow us to identify looseness in real-time in the tannery environment.
In the final stages of the project we will also be modifying process recipes to reduce their impact on loose material, so that when a production check becomes available to tanners, hides can be selected out and given this alternative, milder process. We will be validating the effect of modifications to process recipes through measurement of the structural and chemical changes compared to a control.
Excerpt from published paper in International Leather Maker. September/October 2016.