Page 106 - Engineering Plastics Handbook

80 Engineering Plastics

scheduled to go onstream in April 2005, it started up its 60,000 Mtons/yr
capacity acetal copolymer plant in February 2005 due to the increasing
demand for acetal copolymer in China, which is 150,000 Mtons/yr and
gaining more than 10% per year.

Chemistry of Acetal Polymerization

Acetal homopolymers are polymerized from purified anhydrous
formaldehyde. Acetal copolymers are copolymerized from cyclic 1,3,5-tri-
H O ) of formaldehyde, typically with a cyclic
oxane cyclic trimer (C 3 6 3
ether comonomer such as ethylene oxide [1]. Ethylene glycol is the prod-
uct of hydrolysis of ethylene oxide.
Formaldehyde is the building block for polyacetal resins. Therefore it is
valuable to examine first the attributes of this simplest of all aldehydes.
Formaldehyde (CH O) is a highly reactive, colorless, flammable, noxious,
2
pungent-smelling gas; yet it provides many diverse benefits to society. It
is produced by oxidation of a mixture of methanol vapor and air (oxygen),
by passing the mixture over a catalyst such as metal oxide, silver, or
copper catalyst. It can be produced by oxidizing natural gas [2]. Its boil-
ing temperature is −21°C (−5.8°F) and melting temperature is −92°C
(−139°F) [2]. In a gaseous state, formaldehyde polymerizes to paraformalde-
hyde solid, used to fumigate rooms [2]. Purified formaldehyde is produced
by conversion to cyclohexyl hemiformaldehyde and partial condensation.
Formaldehyde gas readily dissolves in water, and it is used commercially
in a range from 28% to 56% aqueous solution with 1% to 15% methanol
added as a stabilizer to prevent polymerization [3]. The solution is called
formalin or formol. Methanol (methyl alcohol) increases formaldehyde sol-
ubility and keeps oligomers from growing [3]. In low concentrations,
formaldehyde monomer reacts with water and methanol to form long-
chain oligomers, poly(oxymethylene glycols) [HO(CH O) H] in aqueous
2
n
solution, and poly(oxymethylene) hemiformals [H CO(CH O) H] in
3
n
2
methanol solutions [4]. It is soluble in alcohols, acetones, ethers, and ben-
zene. Because formaldehyde is oxidized by exposure to air (oxygen) to
form formic acid, solutions of formaldehyde are kept in tightly sealed con-
tainers during storage to prevent oxidation to formic acid.
Most formaldehyde is used to make polymeric resins. It is copoly-
merized into melamine formaldehyde, urea formaldehyde, and phenol
formaldehyde, which are among the earliest commercial thermosetting
resins. It is used in the manufacture of pentaerythritol and other polyols,
raw materials for polyurethanes and alkyds; disinfectants, antiseptics,
fungicides, germicides, and preservatives in medical laboratories and
embalming fluids; and deodorizers, dyes, fertilizers, and corrosion
inhibitors in the metal industry.
The U.S. Occupational Safety and Health Administration (OSHA),
Department of Labor, states that “Formaldehyde is a sensitizing agent that