formaldehyde - Sciencemadness Dot Org

MOXOMERIC FORMALDEHYDE 19
The marked promoting effect of small quantities of formic and acetic
acid on the polymerization of monomelic formaldehyde vapor at 18° and
100 °C has been studied in detail by Carmthers and Xorrish 3 *. These
investigators found that formic acid disappeared from the gas phase during
polymerization at a rate nearly proportional to the rate oi formaldehyde
removal and assumed that polyoxymethylene chains originated from the
hydroxyl group of this acid. The high efficiency of formic acid as a
polymerization promoter is explained by addition to the polyoxymethylene
chains with the formation of two hydroxyl groups and consequent branching.
This is indicated in the equations shown below in which n equals an
unknown whole number:
ROOOH 4- CH£0 -> HCOOCH2OH
HCOOCHsOH -f «CH20 — HCOO(CH,0)TlCH2OH
HCOO(CH£0),CH3OH + HCOOH -» HCOO(CH20)BCH3OCH{OH)s
HCOO(CH20)riGH.OCH(OH)o + 2nCB.O -+ HCOO(CH*0)ffiCH,OCK
\
pCCH?0)nH
0(CH£0)Ji
According to Carmthers and Norrish, the polymerization is bimolecular
with reference to formaldehyde. The process is heterogeneous and occurs
at a cold surface.
Formaldehyde gas that is 90 to almost 100 per cent pure must be kept
at temperatures in the neighborhood of 100 to 150°C or above, if polymerization
is to be avoided. As previously pointed out, chemical decomposition
of formaldehyde gas is not appreciable at temperatures below •±00°C, 1
Formaldehyde gas is flammable, having a heat of combustion of 4.47
kcal per gram 88 . It forms explosive mixtures with air or oxygen. According
to studies carried out in our laboratories, the flammable limits of
mixtures of formaldehyde gas and air range from compositions containing
approximately 0.37 to 13 volumes of air per volume of formaldehyde gas.
This means that formaldehyde-air mixtures containing from 7 to 73 per
cent formaldehyde should be regarded as potentially explosive. The
initial combustion temperature of formaldehyde-air mixtures is reported
to be approximately 300°C, as measured by passing mixtures of formaldehyde
and air through a heated tube 3 - 23 .
Although monomeric formaldehyde is obtained whenever formaldehyde
solution or its linear polymers are subjected to vaporization, gas having a
high formaldehyde concentration is best obtained by the action of heat on
paraformaldehyde, which contains 95-96 per cent available formaldehyde,
or from the more highly polymerized polyoxyrnethylenes, which contain
less than 1 per cent of combined water. However, gases obtained in this