The Nimrod Review - Official Documents

Composition of Avimo Seals
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Formulae
Chapter 5 – Leak from Fuel Coupling
At the time of the loss of XV230, and indeed until very recently, the rubber seals used in the Avimo couplings
were polychloroprene seals. Chloroprene was first synthesised in 1930 and the commercial manufacture of
chloroprene started in 1932. One of chloroprene’s many trade names is ‘Neoprene’.
The Review was advised by QinetiQ that there are very many different possible formulations of chloroprene,
using various base polymers, fillers, anti-oxidants, process aids, activators, curatives and process oils in different
proportions. In consequence of which, the different properties of polychloroprene rubbers vary.
The original component drawing for the Avimo seal, drawing no. MR700D, dated 9 February 1947, entitled
“Rubber for Pipe Coupling” (last amended 16 July 1962, Issue 12) (the Avimo Drawing) lists three categories
of part numbers and gives detailed dimensions. The second category of part number specifies as its material
“Avon Rubber Co. Mixture No. 18 x 5” or “Spencer Moulton Mixture No. SH0/2”. Note 1 to the Avimo Drawing
provides as follows:
“MATERIAL: NEOPRENE WITH 100 PART LOADING CARBON BLACK PER 100
OF NEOPRENE.
HARDNESS: 60/65% B.S.
SPECIFIC GRAVITY 1.35”
5.82 A common formula for Polychloroprene or Neoprene is:
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A common formula for Nitrile Co-Polymer is:
A common formula for Polysulphide Thiokol
74 Used for FRS seals until 2004.
H CH2
CH2
C = C
CL
CH2 – CH – CH2 – CH2 – CH = CH – CH2
C N
SH – R – S –(S-R-S) -S-R-SH
74 is:
R – CH 2 CH 2 OCH 2 OCH 2 CH 2 -
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