Synthetic retanning agents

1. Introduction Part I evaluated the ageing resistance synthetic retanning agents impart to the leather; Part II studied their anionicity as well as their tinctorial properties; however all the parameters evaluated refer only to visual properties such as yellowing, dyestuff penetration, surface colour intensity and so on, which although very important do not provide any organoleptic information (resistance, feel, filling etc) which are the main reason for using a syntan. In Part III the study will evaluate the above-mentioned parameters by developing a working method for each of the syntans and afterwards the advisability of their combined application will be evaluated. 2. Experimental part Wet-blue hides, shaved to 1.1 mm, were sided with the left side submitted to process 1: Reference (without retanning), while the right side was submitted to the same treatment but with the appropriate retanning agent. The effect of each variable was evaluated by comparing the left side with the right side, using the formula set out below: Process 3 2.1 Softness (IUP-36) Softness measurements were taken at 36 points and compared with the control leather. The results are displayed in Figure 1. The syntans providing the highest softness degree are A-4 (6.2%), SF (5.8%) and NC-10 (4%). The lowest degree of softness is imparted by BW (-8.4), TRT (-8.0) and BD (-7.6). The organoleptic valuation of when a skin is softer than another is 5%. 2.2. Thickness (IUP-4) Thickness variations in the retanned leathers compared to the control leathers are shown in Figure 2. Here, the TRT syntan stands out, increasing the thickness of the leather by 16% compared to the control. The SUL and BW syntans show a 10% thickness increase. At the other end of the scale, SF increased the thickness by 2%, A-4 by 2.2%, and NC-10 by 2.5%. 2.3 Tensile strength (IUP-6) The tensile strength parallel and perpendicular to the backbone were evaluated. Figure 3 displays the variations as compared to the non-retanned leathers. All the products tested diminished the tensile strength resistance with the exception of A-4, while using SF left the tensile strength unchanged. BD, TRT and KR diminished the values significantly. 2.4 Elongation (IUP-6) SP-2, SUL and BH-10 made the leathers significantly more elastic, while SF, BW, BN proved less effective. 2.5 Tear resistance (IUP-8) SF (16%), BH-10 (10%) and NC-10 (10%) are the syntans providing the highest tear resistance, while BN (-23%) and KR (-19%) strongly diminish it. 2.6 Grain crack resistance (IUP-9) A-4 is the only syntan not to negatively affect the grain crack resistance. 3. Conclusions Synthetic tanning agents having small molecules together with a good penetration, such as sulphonic acid derivatives (A-4, SF) and low and medium condensation degree phenols (NC-10, SP-2) provide the highest soft degree, due to the fact that a deep anionicity facilitates the fatliquoring penetration and thus the fibre lubrication. Conversely, these syntans are the ones providing the smallest increase in thickness. On the other hand syntans with a high degree of condensation impart a higher hardening and filling effect (BW, TRT, BD), but it should be taken into account that even though penetration is poor, the molecule part not containing the active group can penetrate the skin producing a filling effect. The syntans providing a high softness are the ones diminishing the tensile strength resistance and the ones increasing the tear resistance, probably due to the deep fatliquoring effect. The sulphone type synthetics increase the elongation (SP-2 and SUL), but there is no correlation between the active groups and syntans that impart poor elongation properties. Grain crack resistance seems to be influenced by the anionic penetration on the grain side and by the degree of condensation, the higher the penetration and condensation, the less effect on the grain crack (See Part I). Nevertheless, there is an exception, SUL having low penetration yet lowering grain crack resistance. For more information including diagrams and tables, please see the July 2008 edition of Leather International