Tanning properties

Any vegetable tanning extract used commercially is a complex mixture of related substances. The individual tanning properties of the extracts have been extensively studied and are weU known in the industry. 

Tannins are water-soluble phenolic compounds which are usually extracted from plant material by hot water. After lignins, they are the second most abundant group of plant phenolics. Their tanning property is due to their capacity to combine with proteins. However, they can also complex with other polymers such as alkaloids, cellulose, and pectins. 

The tannins are special phenolic compounds characterized by their ability to combine with proteins and other polymers such as polysaccharides. This characteristic explains their tanning properties as arising from a tannin-collagen matrix and then astringency caused by precipitation of the proteins and glycoproteins from saliva. Also tannins are used in fining wines because they combine with proteins. Finally, they inhibit enzymes by combining with their protein fraction. 

Formaldehyde was utilised to produce washable white leathers from the end of the nineteenth until the last third of the twentieth century when its use was phased out due to health and safety concerns. Other aldehydes have tanning properties but only glutaraldehyde has been employed successfully on a commercial scale. 

Coordination complexes of titanium(III), titanium(IV), zirconium(IV), iron(m) and the lanthanides all have tanning properties, particularly if they are stabilised by the addition of appropriate ligands. None of these, however, give leathers with the properties of chrome-tanned materials. 

The presence of monomeric proanthocyanidins or flavan-3,4-diols in plants has long been known. In hot acid, almost all flavan-3,4-diols are converted to the corresponding an-thocyanidin. The first structure determined for a member of this class of compounds was that of melacacidin (27) from Acacia melanoxylon (Fig. 12.11). Flavan-3,4-diols do not possess an affinity for collagen substrates and, hence, lack tanning properties. Some flavan-3,4-diols are modified to... 

Catechins and leuco-anthocyanins readily form condensation products, and it seems to be more particularly the condensation products of intermediate size which possess tanning properties, i.e., the property of cross-linking and precipitating proteins. These products, either preexisting or formed in the expressed juices on standing, contribute to the turbidity, body, and other properties, desirable or undesirable, of beverages such as wine, beer, and tea. 

A milestone in manufacturing leather was the discovery of the tanning properties of trivalent chrome salts like chrome sulfate at the end of the 19 century. All the tanning agents have been used so that vegetable, synthetic and mineral tanning can be distinguished. 

Leucoanthocyanidins are compounds of the general formula 1 shown in Fig. 10. They have no color of their own, but in acidic environments and at elevated temperatures they are converted to colored anthocyanidins (2). This reaction is in competition with the condensation to a dimeric leucoanthocyanidin (3). Low temperature favors the formation of the dimeric compound, which can polymerize to yield products with pronounced tanning properties. 

Uses Emulsifier for min., cutting, fatty oils and soivs, solv, emulsifier in metal cleaners and degreasers w/o emulsifier for pesticide control textile processing softener and lubricant leather during tanning Properties Gardner 3 liq. disp. in water dens, 8,1 Ib/gal vise, 55 cs HLB 8.2 cloud pt. < 25 C 100% act. 

The si2e of the vegetable tanning molecules and the coUoidal nature of the system result in the fixation in the hide of filling materials. The filling action is essentially an impregnation of the hide to form a dense firm leather. These properties are gready desired in sole and mechanical leathers. 

Dynamic techniques are used to determine storage and loss moduli, G and G respectively, and the loss tangent, tan 5. Some instmments are sensitive enough for the study of Hquids and can be used to measure the dynamic viscosity T 7 Measurements are made as a function of temperature, time, or frequency, and results can be used to determine transitions and chemical reactions as well as the properties noted above. Dynamic mechanical techniques for sohds can be grouped into three main areas free vibration, resonance-forced vibrations, and nonresonance-forced vibrations. Dynamic techniques have been described in detail (242,251,255,266,269—279). A number of instmments are Hsted in Table 8. Related ASTM standards are Hsted in Table 9. 

The Rheometric Scientific RDA II dynamic analy2er is designed for characteri2ation of polymer melts and soHds in the form of rectangular bars. It makes computer-controUed measurements of dynamic shear viscosity, elastic modulus, loss modulus, tan 5, and linear thermal expansion coefficient over a temperature range of ambient to 600°C (—150°C optional) at frequencies 10 -500 rad/s. It is particularly useful for the characteri2ation of materials that experience considerable changes in properties because of thermal transitions or chemical reactions. 

Fig. 10. Physical properties of compounds where A is the conventional, B the semi-EV, and C the EV system U represents tensile strength ia MPa , hardness. Shore A elongation, % tan 5 and fatigue life, kc. To convert MPa to psi, multiply by 145.

AH three systems give similar tensile strength, elongation, and hardness properties. Hysteresis (heat buildup) measured by tan 5 shows an advantage for conventional and semi-EV systems and unaged fatigue follows the expected pattern. 

There are three main uses for naphthalene sulfonic acid derivatives (75—79) as naphthalenic tanning material alkyl naphthalene sulfonates for industrial appHcations as nondetergent wetting agents and as dye intermediates. Consumption of naphthalene sulfonates as surfactants accounts for a large portion of usage. Naphthalene sulfonate—formaldehyde condensates are also used as concrete additives to enhance flow properties. Demand for naphthalene sulfonates in surfactants and dispersent appHcations, particularly in concrete, was expected to increase into the twenty-first century. Consumption as of 1995 was 16 x 10 kg/yr. 

---