Page 107 - Engineering Plastics Handbook

Polyacetals 81

can cause an immune system response upon initial exposure” and cites
adverse health conditions associated with formaldehyde. According to
OSHA, “The OSHA standard that protects workers exposed to formal-
dehyde, Title 29 of the Code of Federal Regulations (CFR) Part 1910.1048
and equivalent regulations in US States, with OSHA approved State
Plans, apply to all occupational exposures to formaldehyde gas, its solu-
tions and materials that release formaldehyde.”

Synthesis of polyacetal resins
Formaldehyde is a strong electrophile, allowing acetal to polymerize by
nucleophilic, anionic, or cationic addition of an alcohol to ketene carbonyl
groups. Relatively weak bases such as pyridine initiate anionic addition
polymerization; cationic addition polymerization is catalyzed by strong
acids. When the cyclic trimer trioxane is used as a copolymer to polymer-
ize acetal copolymers, Lewis acids such as boron trifluoride promote copoly-
merization. A more fundamental description is polymerization of an
aldehyde or ketone + alcohol + an acid or base catalyst to form hemiac-
etal, which further converts to acetal. The hemiacetal reaction is
reversible to aldehyde and alcohol.

Hemiacetal + alcohol + catalyst ⇔ acetal + water

Chain growth polymerization produces linear polyacetal macromole-
cules. Polyacetal resin is a clear, flammable, volatile, highly crystalline
liquid with the repeating units

––(CH ––O) –– ––[––(CH ––O) –– \\\ ––(CH CH O) ––] ––
2 x 2 a 2 2 b y
Acetal homopolymer Acetal copolymer

of about 175°C (347°F) and specific
Acetal homopolymer has a T m
gravity of 1.41 g/cc; acetal copolymer has a T of about 165°C (329°F)
m
and specific gravity of 1.42 [5]. Acetal polymers are synthesized entirely
from formaldehyde directly, as noted earlier, or by copolymerization of
the formaldehyde cyclic trimer (cyclic oligomer) trioxane and cyclic
ethers.
Commercial polymerization of acetal homopolymer starts with anhy-
drous formaldehyde monomer from formaldehyde solution. Water is
evaporated from the aqueous solution, forming paraformaldehyde, poly-
oxymethylene, and hemiformal, which are purified and thermally decom-
posed to form anhydrous formaldehyde [6]. Methanol and formic acid are
removed, either by freeze-trapping at a temperature just above the boil-
ing temperature of formaldehyde or by washing with a nonvolatile
polyol [6]. The anhydrous formaldehyde is fed into a reactor contain-
ing the inert hydrocarbon solvent, initiator, and dispersant, where the