_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Silicones
Industrial production of organotin compounds has increased more than 600
times since 19 50. In 19 78 world production reached 30 000 tonnes. The major
application is as stabilizers for PVC. Typical materials are di-n-octyltin laurate or
maleate, and for food packaging, the S,S'-bis(iso-octylmercaptoethanoate)
Oct~Sn(SCH 2 C0 2 0d) 2 • When about 2% of the tin compound is incorporated
into the plastic it remains colourless, transparent and unaffected by sunlight. Its
function is probably to act as a radical trap, a scavenger for hydrogen chloride and
an antioxidant against breakdown of the polymer in the atmosphere.
Triorganotin compounds kill a variety of organisms including bacteria, fungi,
moulds, algae, lichens, insects, molluscs and crustacea. This activity is greatest in
trialkyltin compounds which have a total of nine or ten carbon atoms in the
molecule. A typical preparation consists of aqueous bis(tri-n-butyltin) oxide.
TBT, (Bu 3 Sn) 2 0. in combination with a quaternary ammonium compound. It is
used, for example, in wood treatment and preservation. and in paper manufacture
to prevent bacterial decay of ground wood pulp during storage. It eradicates
moss, algae and lichens from masonry and prevents their regrowth for severa!
years. Similar preparations protect water based paints and adhesives from fungal
and bacterial attack. Organotin biocides were once thought to show low toxicity
to mammals, but recent studies question this. Concentrated solutions cause eye
and skin irritation. TBT is readily degraded in the environment to inorganic tin(II)
compounds.
4.4 Silicones
4.4.1 The 'direct process':manufacture of alkylchlorosilanes and silicones
The discovery by E.G. Rochow (General Electric Co.) and by R. Muller (VEB Silikon
Chemie) that methylchlorosilanes can be prepared by the direct reaction between
methyl chloride and silicon opened up the way for commercial developments
using silicon compounds. The reaction
2CH,Cl(g) + Si(s)----+ (CH,) 2 SiCl 2 (g)
~H'- =- 302 kJ moi-'
is strongly exothermic but requires a catalyst. In industry ground silicon of
metallurgical grade (::;::, 98%) is reacted with gaseous methyl chloride at
2 50-3 50oC and 1-5 atm in a fluid bed reactor. The best catalyst is copper, which
is introduced by grinding, sintering or merely by mixing with the silicon. It is even
possible just to add copper(I) chloride, which is reduced by silicon at the elevated
temperatures used. An induction period occurs during which oxide coatings are
removed and the catalyst is transported to active sites on the surface. Addition of
electropositive elements such as Al. Ca or Mg reduces the induction period: Zn or
Cd also helps to remove oxides and to mobilize CuCI by dissolution. A typical
mixture consists of Si. 90-9 5%: Cu. 5-10%. Al, 0.1-1%: Zn, 0.1-l% and Sb, a
'promoter'. 0.001-0.005%. The reaction has the characteristics of a heterogeneous
gas-solid process. It has been suggested that methyl chloride dissociates
on the surface to CH; and Cl' radicals. Radicals could also arise by decomposition
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