Page 66 - Chemical and process design handbook

Speight_Part II_A 11/7/01 3:16 PM Page 2.7

ACETAL RESINS

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Acetal resins are those homopolymers (melting point: ca. 175 C, density:
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ca. 1.41) and copolymers (melting point: ca. 165 C, density: ca. 1.42)
where the backbone or main structural chain is completely or essentially
composed of repeating oxymethylene units (-CH O-) . The polymers are
2 n
derived chiefly from formaldehyde (methanal, CH =O), either directly or
2
through its cyclic trimer, trioxane or 1,3,5-trioxacyclohexane.
Formaldehyde polymerizes by both anionic and cationic mechanisms.
Strong acids are needed to initiate cationic polymerization and anionic
polymerization is initiated by relatively weak bases (e.g., pyridine). Boron
trifluoride (BF ) or other Lewis acids are used to promote polymerization
3
where trioxane is the raw material.
In the process, anhydrous formaldehyde is continuously fed to a reac-
tor containing well-agitated inert solvent, especially a hydrocarbon, in
which monomer is sparingly soluble. Initiator, especially amine, and
chain-transfer agent are also fed to the reactor. The reaction is quite
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exothermic and polymerization temperature is maintained below 75 C
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(typically near 40 C) by evaporation of the solvent. The product polymer
is not soluble in the solvent and precipitates early in the reaction.
The polymer is separated from the polymerization slurry and slurried
with acetic anhydride and sodium acetate catalyst. Acetylation of polymer
end groups is carried out in a series of stirred tank reactors at temperatures
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up to 140 C. End-capped polymer is separated by filtration and washed at
least twice, once with acetone and then with water.
The copolymerization of trioxane with cyclic ethers or formals is
accomplished with cationic initiators such as boron trifluoride dibutyl
etherate. Polymerization by ring opening of the six-membered ring to form
high molecular weight polymer does not commence immediately upon
mixing monomer and initiator. Usually, an induction period is observed
during which an equilibrium concentration of formaldehyde is produced.
When the equilibrium formaldehyde concentration is reached, the poly-
mer begins to precipitate and further polymerization takes place in trioxane

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