Acetal resins are those homopolymers (melting point: ca. 175o C, density: ca. 1.41) and copolymers (melting point: ca. 165oC, density: ca. 1.42) where the backbone or main structural chain is completely or essentially composed of repeating oxymethylene units (-CH2O-)n. The polymers are derived chiefly from formaldehyde (methanal, CH2=O), either directly or 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 (BF3) or other Lewis acids are used to promote polymerization where trioxane is the raw material.
In the process, anhydrous formaldehyde is continuously fed to a reactor 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 exothermic and polymerization temperature is maintained below 75o C (typically near 40o 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 up to 140o 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 solution, and more comonomer is exhausted at relatively low conversion, but a random copolymer is nevertheless obtained.
In the process, molten trioxane, initiator, and comonomer are fed to the reactor; a chain-transfer agent is included if desired. Polymerization proceeds in bulk with precipitation of polymer, and the reactor must supply enough shearing to continually break up the polymer bed, reduce particle size, and provide good heat transfer. Raw copolymer is obtained as fine crumb or flake containing imbibed formaldehyde and trioxane that are substantially removed in subsequent treatments which may be combined with removal of unstable end groups.
Acetal copolymer may be end capped in a process completely analogous to that used for homopolymer. However, the presence of comonomer units (e.g., -O-CH2-CH2-O-) in the backbone and the relative instability to base of hemiacetal end groups allow for another convenient route to a poly-mer with stable end groups. The hemiacetal end groups may be subjected to base-catalyzed (especially amine) hydrolysis in the melt or in solution or suspension, and the chain segments between the end group and the nearest comonomer unit deliberately depolymerized until the depropagating chain encounters the comonomer unit. If ethylene oxide or dioxolane is used as comonomer, a stable hydroxyethyl ether end group results (-O-CH 2 CH2-OH). Some formate end groups, which are intermediate in thermal stability between hemiacetal and ether end groups, may also be removed by this process.
The product from the melt or suspension treatment is obtained directly as crumb or powder. The polymer recovered from solution treatment is obtained by precipitative cooling or spray drying. The polymer with now stable end groups may be washed and dried to remove impurities, especially acids or their precursors, prior to finishing operations.
The average molecular weight MW of acetal copolymers may be estimated from their melt index (MI, expressed in g/10 min):
Stiffness, resistance to deformation under constant applied load (creep resistance), resistance to damage by cyclical loading (fatigue resistance), and excellent lubricity are mechanical properties for which acetal resins are perhaps best known and which have contributed significantly to their excellent commercial success. General-purpose acetal resins are substantially stiffer than general-purpose polyamides (nylon-6 or -6,6 types) when the latter have reached equilibrium water content.
Acetal resins are generally stable in mildly alkaline environments. However, bases can catalyze hydrolysis of ester end groups, resulting in a less thermally stable polymer.
Acetals provide excellent resistance to most organic compounds except when exposed for long periods at elevated temperatures. The resins have limited resistance to strong acids and oxidizing agents. The copolymers and some of the homopolymers are resistant to the action of weak bases. Normally, where resistance to burning, weathering, and radiation are required, acetals are not specified. The resins are used for cams, gears, bearings, springs, sprockets, and other mechanical parts, as well as for electrical parts, housings, and hardware.
