Page 67 - Chemical and process design handbook

Speight_Part II_A 11/7/01 3:16 PM Page 2.8

2.8 MANUFACTURE OF CHEMICALS
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 pro-
ceeds 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 analo-
gous to that used for homopolymer. However, the presence of comonomer
units (e.g., -O-CH -CH -O-) in the backbone and the relative instability to
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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 sus-
pension, 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 CH -OH).
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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, espe-
cially acids or their precursors, prior to finishing operations.
The average molecular weight MW of acetal copolymers may be esti-
mated from their melt index (MI, expressed in g/10 min):
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MI = 3.3 10 MW –3.55
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 substan-
tially stiffer than general-purpose polyamides (nylon-6 or -6,6 types) when
the latter have reached equilibrium water content.