Quebracho extract (tannin extract) is a chemical product processed from plant materials rich in tannins through water extraction and concentration. It is usually yellow-brown to brown, in powder or block form. Mainly used for tanning leather, known as vegetable tanning agent in the leather industry. Also used as a flotation depressant, boiler water treatment agent, drilling mud thinner, metal surface anti-corrosion agent; condensed quebracho extract is also used as a woodworking adhesive.

History
Since ancient times, people have known to use certain tree barks directly for tanning and dyeing. However, industrial production of quebracho extract is closely related to the development of the leather industry. In the 1880s, Germany, Austria, France, Paraguay and other countries successively built factories producing quebracho from oak bark, oak wood, spruce bark and quebracho wood. Since the 1950s, factories have been built successively across China, with output increasing rapidly.

Composition
Quebracho extract is a general term for a class of complex natural compounds. Besides the main component tannin, it also contains non-tannins and insolubles. Due to different raw materials, the composition varies. Product names are generally prefixed with the raw material name, such as larch bark quebracho extract, oak quebracho extract, etc., to distinguish composition, properties and uses.

Tannin
Polyphenol derivatives in plants that can tan raw hides into leather, also called plant tannins. Based on the chemical structure of tannins, the main components of quebracho extract can be classified as:
鈶?Hydrolyzable tannins. Complex compounds formed by polyhydroxy carboxylic acids combined with sugars (such as glucose) or polyols via ester or glycosidic bonds. Can be hydrolyzed by acid or enzymes into corresponding products. Based on the structure of polyhydroxy carboxylic acids after hydrolysis, further divided into gallotannins (such as tannins in Chinese gallnuts) and ellagitannins (such as tannins in oak).
鈶?Condensed tannins. Complex polycondensates formed mainly by carbon-carbon bonds from compounds such as flavanols. Cannot be hydrolyzed by acid or enzymes in aqueous solution; instead, heating with acid condenses into water-insoluble amorphous substances — red powder. Most condensed tannin monomers are derived from flavan-3-ol, flavan-3,4-diol and other flavonoid compounds. Therefore, condensed tannins are also called flavonoid tannins, part of proanthocyanidins in flavonoid compounds.

Non-tannins
Water-soluble substances in quebracho extract without tanning properties. Main components include sugars, simple phenols, organic acids, inorganic salts, pigments, nitrogen-containing substances, etc. Non-tannin composition varies among different quebracho extracts.

Insolubles
Substances in a 0.4% tannin solution at (20±2°C) that cannot pass through medium-speed filter paper and kaolin filter layers. Main components include tannin decomposition products (yellow powder) or condensation products (red powder), low-dispersion tannins, pectin, gum, inorganic salts, mechanical impurities, etc.

Properties
Quebracho extract is soluble in water; the aqueous solution forms a semi-colloidal system. Weakly acidic; adding salt causes salting-out, with different salting-out degrees for different quebracho extracts. Bitter and astringent taste; produces precipitate with gelatin solution. During hide tanning, tannins in quebracho solution form multipoint hydrogen bonds with hide proteins (collagen), converting raw hide to leather. Tannins in quebracho combine with metal ions to form partially soluble or partially insoluble complexes. Tannins in condensed quebracho condense with formaldehyde to produce cold-setting or heat-setting adhesives. Tannins are easily oxidized to quinone compounds, deepening the color of quebracho solution. When quebracho reacts with sulfites, sulfonic groups are introduced into tannin molecules, reducing insoluble content, improving cold water solubility, penetration rate and lightening color.

Quality Specifications
Quality specifications for mixed quebracho extract:
Item Premium Grade First Grade Qualified Grade