Page 109 - Engineering Plastics Handbook

Polyacetals 83

Cyclic ether comonomer → Additives, stabilizers →
Reinforcements → Acetal copolymer

There are a variety of different process plant designs used by polyacetal
producers [1, 6]. Polyacetal resins are typically compound-extruded with
property enhancers (additives) added during extrusion. The extrudate is
water-quenched and pelletized. A state-of-the-art process can in-line
extrude and injection-mold, using robotics and automation. Otherwise, the
pelletized polyacetal is stored.
Research at the University of Colorado investigates the behavior of the
six-member acetal ring. Research includes study of five- and six-member
acetal ring structures—cyclic acetals—such as cyclic acetal (meth)acry-
late, ethyl cyclic acetal acrylate, cyclic acetal carbamate (OCN) acrylate,
cyclic acetal carbamate (OCN) methacrylate, and polymerizations [7].
Copolymerization of the trioxane comonomer, the cyclic trimer of formalde-
hyde, and cyclic ether comonomer yields acetal copolymers. Boron triflu-
oride dibutyl etherate [BF (Bu) O] or other cationic initiators plus 2%
3
2
ethylene oxide to form a Bronsted acid start the copolymerization. Early
research at Celanese [23] found that p-chlorophenyldiazonium hexafluo-
rophosphate and p-chlorophenyldiazonium hexafluoroarsenate showed
superior performance as catalysts compared to BF (Bu) O for cationic
3
2
polymerization and copolymerization of trioxane. Acetal copolymerization
is achieved by ring-opening the six-member ring trioxane [5]. In a com-
mercial process, molten trioxane + the second comonomer + initiator +
chain transfer agent (optional) copolymerizes in bulk to produce acetal
copolymer [5].When comonomers such as trioxane, dioxolane, and ethyl-
ene glycol are used, a stable end cap forms [5].
Other acetals are polymerized from aldehydes plus alcohols. Poly-
vinylacetal, polymerized from an aldheyde plus polyvinylalcohol, is used
to make coatings, films, adhesives, and lacquer.

Products and Design

Products
Polyacetal products are listed in Table 5.2.
Also included are a variety of kitchenware and household consumer
products such as stain-resistant cutting boards and soap dispensers.
Polyacetal products can be molded into precision components.
Polyacetals are used increasingly for electronic, appliance, automotive,
industrial, plumbing, construction, pneumatic, and consumer products.
They are used for plumbing applications in hot water above 82.2°C (180°F)
and in hot air applications above 107°C (225°F). The copolymer has higher
continuous-use temperatures in both water and air, and 30% carbon fiber-
reinforced polyacetals are molded into bearings, cams, and gears.