A comparative study of three retanning agents was conducted by Cromogenia Units using the optimised process, where the best results were obtained with the amphoteric retanning agent AFF applied during the dyeing-fatliquoring stages. The selected retanning agents were:

  •  amphoteric retanning agent based on protein (PROT)
  • anionic retanning agent based on pure dihydroxydiphenyl sulphone (SULF)
  • amphoteric retanning agent based on pure dihydroxydiphenyl sulphone (AFF).

This work design will be used to assess the influence of each of the two characteristics (sulphone base and amphoteric nature) on the properties of leather. Physical-chemical and organoleptic parameters were assessed, allowing us to deduce that the amphoteric retanning agent AFF provides the leather with specific properties that are different from those of the other two agents. Data analysis was performed with the program Statgraphics Plus to find the optimal areas. The methodology used originates from a simplex-centroid design as an experimental plan, adjusted to a linear model.

There is an incessant demand for improving the properties of leather in general, particularly of those relating to grain fullness and firmness. With this purpose in mind, new retanning agents have been and are being developed that, in addition to the above characteristics, offer the right feel to each article as well as deep colours.

Retanning agents must penetrate the hide and be selectively deposited in the empty areas. The most critical parameters when designing these products are composition, molecular weight (with polymers) and leather affinity.

It is well known that water-soluble polymers based on dihydroxydiphenyl sulphones are highly indicated to achieve these properties, and their use is being increased to this end.

In polymers, excessively high molecular weights will hinder penetration, increase hardness and lead to grain problems. In turn, excessively low molecular weights will lead to insufficient fullness selectivity.

A polymer with excessive affinity would result in rough grain. Excessively low affinity would make obtaining a selective filling much more difficult. Well-balanced amphoteric products are highly indicated to obtain good affinity, deep and levelled dyeing, and a smooth grain.

A two-part project

A study of a new amphoteric retanning agent, AFF, based on a dihydroxydiphenyl sulphone structure, rather than a reference without retanning, was completed. As was an assessment of the properties it provides to the leather when applied at four different stages of the process: rechroming (before and after chrome), dyeing and fatliquoring.

In part one of the study, the application of retanning agent AFF at the dyeing stage was shown to provide the skin with the best physical-chemical and organoleptic properties. Part two of the study includes a comparative study of all three retanning agents by using the process optimised in part one.  With this design, the influence of each characteristic (sulfone base and amphoteric nature) on the properties of the skin is assessed.

How it was done

The skins were split along the backbone. The left halves were taken as references – no retanning agent – while the right halves underwent the same process but with the inclusion of the retanning products and mixtures thereof at the quantities defined in Table 1 (see above), and underwent a standard process. Each variable was made in duplicate.

All processes were performed in pilot plant drums with automatically controlled speed and temperature. The effect of each retanning agent and mixtures thereof was assessed by comparing the left half with its corresponding right half. A simplex centroid design adjusted to a linear model was used as the experimental plan.

The Statgraphic Plus program was used for the statistical analysis of results with the purpose of finding the optimal areas of each property.

A graph depicting the ternary sample was used to map the result. One retanning agent at a concentration of 10% was represented at each vertex of a triangle. This means that there is no mixing at these points – which indicates the value of the property as compared with the retanning-free reference. The three sides of the triangle were the areas where two retanning agents coexist, while all three retanning agents coexist inside the triangle.

The objective is to obtain a representation likely to relate the results of the seven variables shown in Table 1 and likely to predict the results that would be obtained by a ternary formulation of the retanning agents (PROT, SULF and AFF) at a maximum concentration of 10% of active matter on wet-blue weight.

A graph depicting the ternary sample is used. One retanning agent at a concentration of 10% is represented at each vertex of the triangle. This means that there is no mixing at these points, which indicates the value of the property compared with the retanning-free reference.

The three sides of the triangle are the areas where two retanning agents coexist, while all three retanning agents coexist inside the triangle. The centre of the triangle corresponds to test 5, and has 3.33% of each retanning agent.

The assessed properties were:

  • degree of softness: SOF (IUP-36)
  • thickness: THI (IUP-4)
  • tear load: TLO (mean of the parallel and perpendicular values) (IUP-8)
  • tensile strength: TST (mean of the parallel and perpendicular values) (IUP-36)
  • colour depth: CDE (colorimeter)
  • colour levelling: CLE (colorimeter)
  • grain fineness: GFI (organoleptic and microscope)
  • grain burst: GBU (IUP-9).

Degree of softness

The degree of softness was measured with the softness tester as per standard IUP-36. Eleven values were determined in each skin area, totalling 33 measurements. A linear equation was adjusted to predict the percentage of variation in the degree of softness (SOF) of the values of the right halves corresponding to retanning agents PROT, SULF and AFF versus the left halves (reference without retanning agent), and R2 equaling 81.31% was obtained. The results of the variation in the degree of softness showed that PROT (No. 4) decreases softness by 8%, while SULF (No. 6) and AFF (No. 3) increase by 4%. The optimal areas for the degree of softness showed that a 10% PROT vertex depicts negative values that increase as the SULF and AFF ratios are increased.

Thickness

Thickness was measured as per Standard IUP-4. Eleven values of each skin area were determined, totalling 33 measurements, with R2 equalling 97.84%. Thickness variation in absolute values showed in variable 3 (AFF) increases thickness by 16%. Variable 6 (SULF) increases thickness by 14%, while variable 4 (PROT) does so negligibly (2%). Optimal areas for thickness showed more fullness is found in the vertex with 10% AFF. Thickness values range 8–12% in the low areas (AFF and SULF mixtures). These values decrease significantly as PROT is approached.

Tear load

Assessments were performed as per IUP-8. In this case, R2 equalled 98.93%. Tear load is improved by variable PROT (13%), decreased by variable AFF (10%) and left practically unchanged by variable SULF. The highest values are found in PROT, and these values are decreased as areas with AFF predominance are reached.

Tensile strength

As per IUP-36. In this case, R2 equalled 88.06%. Tensile strength is decreased by 7, 8 and 12% by variables PROT, AFF and SULF, respectively. The highest values are found with PROT. These values decrease as the areas with SULF and AFF predominance are reached. Variations in absolute values showed colour depth is decreased by 15, 8 and 1% by variables SULF, AFF and PROT, respectively. A smaller decrease of colour depth is shown in PROT. This is increased as the AFF vertex is approached, even more so when the SULF area is approached.

Colour depth

As with degree of softness and thickness, colour depth (L*) was measured at 33 points. In this case, R2 equals 94.06%. Colour depth is decreased by 15, 8 and 1% by variables SULF, AFF and PROT, respectively.

Colour levelling

This was assessed by measuring total colour (E*) at the physical test area and by assessing the difference with the ten-point values at different areas of the leather.

Grain burst

As per IUP-9. In this case, R2 equals 95.61%. Grain burst is slightly increased by variables AFF and PROT, and slightly decreased by variable SULF. Higher values are obtained at the AFF vertex and lower values are obtained at the SULF area. Importantly, very small, little significant variations were obtained with all three retanning agents.

Grain fineness

A scale to assess grain fineness was defined with a five-point scale (from low to high) addressing grain uniformity, size and embossing. Fineness is improved by 40, 22 and 9% with variables AFF, SULF and PROT, respectively. Maximum grain fineness is found at the AFF vertex, and is slightly decreased as the SULF vertex is approached. The worst result is obtained at the PROT vertex. The excellent improvement of colour levelling obtained with all three retanning agents, particularly with AFF, is to be emphasised.

Results

Amphoteric retanning agent AFF imparts very good fullness without hardening the leather. When a SULF base is added with reagent groups that give it an amphoteric nature AFF, colour depth is increased and colour levelling is improved. When a SULF base is added with reagent groups that give it an amphoteric nature AFF, grain fineness is dramatically increased. The best results in physical resistance are obtained with amphoteric protein PROT. 

All references, methodology and equipment lists available upon request.