Page 108 - Engineering Plastics Handbook

82 Engineering Plastics

formaldehyde is slightly soluble [5, 6]. Polyacetal is the simplest poly-
ether molecule. Acetal homopolymer is composed of repeating oxymeth-
ylene linkages (––OCH ––). Amine initiator (or other initiators) and
2
chain-transfer agent are added into the reactor. Transfer agent and
chain termination agents control MW; and polymerization is terminated
by stopping the formaldehyde monomer flow [6]. During the exothermic
reaction, solvent evaporates, pulling the reaction to end-capped poly-
acetal, which precipitates and is removed [5]. Precipitated solid acetal
homopolymer, separated by filtration and centrifuging, is slurried with
sodium acetate and acetic anhydride catalysts. By this process, the
chain produced from ketones or aldehydes plus alcohols is end-capped
with acetyl groups ––(––O––C––O––). Chain ends are stabilized with acetic
anhydride and then refluxed to form the acetyl groups [6]. The acetal
homopolymer is washed and dried.
When the acetal copolymer is produced, trioxane is first produced by
acidification of aqueous formaldehyde and distillation. The cyclic trimer
1,3,5-trioxane, (Fig. 5.1) which is formed, is crystallized or extracted and
removed from the distillate [6]. Further fractional distillation purifies the
cyclic trimer, which is copolymerized with a cyclic ether [6]. Trioxane is pro-
duced by distillation in an aqueous solution in the presence of p-toluene
sulfuric acid catalyst. Trioxane is also known as 1,3,5-trioxane, 1,3,5-tri-
oxacyclohexane, metaformaldehyde, s-trioxane, s-trixane, sym-trioxane,
trioxymethylene, triformal, aldeform, and other names [22]. It is a color-
less, toxic, highly flammable crystalline solid with a chloroformlike odor.
Although trioxane is stable, over time it can gradually sublime to toxic
formaldehyde gas. Besides the interest in it as a copolymer in acetal copoly-
mer and its use in organic chemical syntheses, trioxane is used in a number
of very different applications, such as a disinfectant and solid fuel bars for
activities like camping [alternative to sterno (alcohol) canned fuel and
esbit tablets].
Structure of the trioxane molecule is as follows:
Methanol → Formaldehyde → Additives, stabilizers →
→ Reinforcements → Acetal homopolymer
Methanol → Formaldehyde → Trioxane comonomer →

O
CH 2 CH 2

O O
CH 2
Figure 5.1 1,3,5-trioxane. (From BASF.)