Improving the strength and softness of crust leather through pressure treatment

Introduction
Retanning and fatliquoring have a major impact on the characteristics of leather, whereas dyeing and the finishing concentrate on the appearance of the leather. Various retanning agents and fatliquoring agents have been developed to improve the softness and mechanical strength of leather, parameters that are important to the buyers of leather. However, although chemicals can be added to a process to improve the leather quality, doing so increases production costs, as well as contributing to effluent production.
The addition of chemicals to leather can significantly change the structural parameters of leather, such as the porosity, dispersion of fibres and fibre diameter. These in turn impact on the tensile and tear strength of the leather, which are important characteristics of leather.
The performance of leather is greatly affected by its pore structure, and compression is an important method to improve the pore structure of the material and increase performance, but this method is rarely used in the leather industry. Hot pressing is widely used in many other industries to improve the porosity, structure and strength of the materials being manufactured.
This research was carried out to investigate crust leather properties under different pressures and to analyse its microstructure.

Experimental
Leather samples were subjected to a 1-hour treatment at pressures of 0 MPa, 7.5 MPa, 15 MPa, 22.5 MPa, 30 MPa, and 37.5 MPa using a single-layer hot press at 25°C. After 48 hours of standard air conditioning at 20°C and 65% humidity, the mechanical properties of the samples were tested.
Tensile strength, tear strength, elongation at break and softness were determined using standard methods.
The morphology of the crust leather fibre structure was determined through a scanning electron microscope at magnifications of 100× and 500×. The pore structure of samples was ascertained using a mercury intrusion porosimeter and automatic physical adsorption instrument.

Results
The thickness of crust leather samples subjected to varying pressures are shown in Table I. Sample thicknesses were reduced with increasing pressure. The thickness changed rapidly at the beginning from 2.03 to 1.72 mm under 7.5 MPa, while the thickness remained 1.58 mm under 15-22.5 MPa. The pressure was further increased to 30-37.5 MPa, and as a result, the thickness was also reduced to 1.36 mm.

Table 1. Crust leather thickness as a result of the applied pressure. (Image source: JALCA 120, 107 – 114 (2025)

The longitudinal and transverse tensile strength are displayed in Figure 1a. The longitudinal and transverse tensile strengths typically displayed an increasing trend as pressure increased. Longitudinal tensile strength increased by 108%, from 32.6 to 67.9 MPa, while transverse tensile strength increased from 23.8 to 47.8 MPa. This could be due to the increase in fibre density under high pressure. Figure 1b displays the longitudinal and transverse tear strength of crust leather. In contrast to the tensile strength, the tear strength first reduced, then improved, and then decreased. It reached its maximum value (L: 310 N/mm; T: 367 N/mm) at 30 MPa external pressure, while the tear strength reduced at 37.5 MPa, which could be associated with the early tear curves’ inflection point (Figure 1e, f). Furthermore, the elongation at break (Figure 1c) first reduced, then increased, and resembled the untreated sample following treatment with 22.5 MPa. The elongation at break and tear strength displayed the opposite tendency, while the tensile strength in the L direction was higher than that in the T direction.
The lower pressure treatment resulted in a reduction in the softness of the crust leather to only 3.2 mm, whereas the softness of the crust leather treated with 30 MPa was comparable to the untreated sample (>5mm), as shown in Figure 1d. This demonstrates that the mechanical properties of crust leather can be improved by proper pressure without reducing its softness.

(Image source: JALCA 120, 107 – 114 (2025)

Conclusion
In this study, the crust leather was treated with different pressures, and the mechanical properties of the treated samples were investigated, including tensile strength, tear strength, elongation at break, and softness. With the increase of pressure, the tensile strength of crust leather progressively enhanced, while the elongation at break, tear strength, and softness all decreased first and then increased. The structural evaluation of the samples (data not shown in this summary due to article length limitations) showed that, without the use of chemicals, external pressure changed the fiber structure, causing the fibers to be closely packed and affecting D-spacing and fibril diameter. The tensile strength of the crust leather was enhanced by the small pores formed in the leather, which also improved the fiber density. With the change of pressure, the crust leather showed a significant variation in mechanics and structure.
External pressure on the crust leather improved the fibre structure without the use of chemicals or a reduction in softness, resulting in closely packed fibres that increased tensile strength, decreased average pore diameter, and enhanced specific surface area and fibre density.
This research shows that it may be possible to improve the strength and softness properties of crust leather simply by treating the crust leather with a pressure plate for 1 hour at a temperature of 25ºC and may well be a clean method for producing high-performance crust leather or leather products.

This article is a summary of the paper “Effect of Pressure Treatment on Structure and Mechanics of Crust Leather”. Journal of the American Leather Chemists Association (JALCA) Vol 120, 107 - 114 (2025).