Friday, 17 January 2014

Zinc and its compounds

Elizabeth Henderson
Product Development Manager
The Itac technical blog is sticking to elements close together in the periodic table, by taking a step down and to the left of aluminium to land on zinc. Zinc occurs naturally as zinc sulphide and also in much smaller quantities as crystalline zinc oxide called Zincite.  Zincite has also been reclaimed from zinc smelters’ chimney deposits.  Zinc has been in use as a metal for centuries, and is combined with copper to make brass. The principal modern uses of zinc are in batteries and corrosion protection, but Itac exploits chemical properties of its compounds rather than the metal itself.
Natural rubber is polyisoprene, and its properties need modifying for it to be hard, flexible and sufficiently weather resistant for industrial applications. In particular, the vulcanisation of rubber is achieved by milling sulphur into the hydrocarbon chains. These chains are strengthened by the presence of carbon-sulphur bonds in the matrix. The bonds cannot be formed without the presence of catalytic quantities of zinc oxide being included in the milling. Zinc reacts readily with sulphur, forming a labile compound which makes the sulphur available for incorporation in the hydrocarbon chain. The amount of zinc oxide required is typically 5 parts per hundred parts of rubber.
As well as using zinc oxide as a catalyst in these reactions, Itac exploits its bacteriacidal properties in adhesives for medical applications. Zinc oxide acts on bacterial cell surfaces causing the cells to leak, and as the zinc oxide leaks into the cell it causes ‘oxidative stress’, which is an imbalance between anti-oxidants and pro-oxidants in the cell. This inhibits cell growth and eventually causes death of the cells.
Itac also uses zinc pyrithione as an additive in coatings for food-bearing conveyor belts. Its presence inhibits the growth of algae and fungi in the polyurethane matrix coating the belts. Exposure of yeast cells to zinc pyrithione renders them unable to prevent the uptake of toxic copper from their surroundings. Copper is incorporated by the yeast cells as copper pyrithione, and this compound targets iron-sulphur proteins in the cells, killing them.

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