Production of syntans The base products are mononuclear and polynuclear phenols naphthalene and their derivatives cresols, naphtols, aromatic ethers and spent sulphite liquors. They are condensed, also as mixtures, mostly with formaldehyde and sulphonated by the introduction of water-solubilizing group, usually containing sulphuric acid. They exit mainly in the form of sodium or ammonium salts. By varying the products and procedure it is possible to manufacture the most diverse syntans having particular properties. Most syntans are anionic, a few are amphoteric. Technical information and leaflets concerning the exact properties of these products are available from specialized suppliers.
Classification and properties
1. Replacement tanning agent
As they possess an equivalent self-tanning capacity these products can replace vegetable tanning agents or portions of them without problems and, if desired, give the leathers
to be tanned particular properties.
2. White tanning agent
In most cases this type of tanning agent can be classified as a replacement tanning agent. They generally exhibit a reduced filling capacity if they have a good white effect and high light fastness.
3.
Shrinking tanning agent These tanning agent are highly astringent and acidified in order to achieve an astringing on the grain and thus a high shrinking grain. Besides phenolic tanning agent glutaraldehyde has been used for many years to obtain line shrinking effects.
4. They have been developed to improve the diffusion of highly concentrated tanning agents with large –sized particles and thus to accelerate or reduce the tanning times.
5
. Retanning agentsThere is great variety of products. Mainly used for subsequent treatment of chrome-tanned leather in order to obtain special effects and properties such as fine grain firm grain and good handle softness or toughness fullness pastel shades or level dying properties good buffing properties light fastness or resistance to ageing and improved physical properties.
When phenol is condensed with formaldehyde, a novolac resin is formed. This is thermal setting resin and its hardness and molecular weight depend upon the ratio of formaldehyde to phenol. At molar ratios greater than one part formaldehyde to phenol, the theoretical molecular weight is infinite and a hard resin results. With the molar ratio adjusted properly, so that an average molecular weight is about 300-400, a thick syrupy material is obtained. This is insoluble in water and, for practical application, leather water dispersion is necessary. The resin is sulfonated by the addition of sulfuric acid. The route to the final product, a sulfonated phenol or by condensation followed by sulfonation.
The mole ratio of the condensing agent (formaldehyde) is important. The more formaldehyde, the higher the average molecular weight. If the molecule is too small there will be poor tanning action. If the molecule is too large, there will be poor penetration into the leather. The quantity of sulfuric acid, or the degree of sulfonation, also affects the tanning properties. Sufficient solubilization is needed to maintain a true solution, but excess sulfonation decreases tanning efficiency.
Two factors are in the exchange tannins are of prime consideration: the cost and specific characteristic. With phenol as a starting material, the manufacturing, cost makes it impossible to make a tanning agent competitive with natural vegetable tannins. If, however, there are incorporate into the tanning material lighnosulfonates or naphthalene sulfonic acid materials, the cost per “tan unit” goes down and a more competitive product results. The leather making properties of the syntan are also greatly altered.
One of the most successful of the lighnosulfonic acid type was the Tanigan Extra A which is described as exchange tannin based on dihydroxydiphenylsulfone, sulfite cellulose and the sulfonic formaldehyde resin. As a tanning agent it was successful and was capable of producing leather similar to that obtained from the vegetable tannins. A large number of exchange tannins are presently being offered to the leather industry based on combination of dihydroxydiphenylsulfone, Bisphenol A , direct sulfonation, co-condensation with naphthalene sulfonic acid and other systems.
