Page 62 - Handbook of Plastics Technologies

THERMOPLASTICS

2.2 CHAPTER 2

keted under the trade name Celcon. Hostaform and Duracon are also copolymers. The
presence of the second monomer reduces the tendency for the polymer to degrade by un-
4
zipping.
There are four processes for the thermal degradation of acetal resins. The first is ther-
mal or base-catalyzed depolymerization from the chain, resulting in the release of formal-
dehyde. End capping the polymer chain will reduce this tendency. The second is oxidative
attack at random positions, again leading to depolymerization. The use of antioxidants will
reduce this degradation mechanism. Copolymerization is also helpful. The third mecha-
nism is cleavage of the acetal linkage by acids. It is therefore important not to process ace-
tals in equipment used for PVC, unless it has been cleaned, due to the possible presence of
traces of HCl. The fourth degradation mechanism is thermal depolymerization at tempera-
tures above 270°C. It is important that processing temperatures remain below this temper-
ature to avoid degradation of the polymer. 5
Acetals are highly crystalline, typically 75 percent crystalline, with a melting point of
6
180°C. Compared to polyethylene (PE), the chains pack closer together because of the
shorter C-O bond. As a result, the polymer has a higher melting point. It is also harder than
PE. The high degree of crystallinity imparts good solvent resistance to acetal polymers.
The polymer is essentially linear with molecular weights (M ) in the range of 20,000 to
n
7
110,000.
Acetal resins are strong and stiff thermoplastics with good fatigue properties and di-
mensional stability. They also have a low coefficient of friction, and good heat resistance. 8
Acetal resins are considered similar to nylons but are better in fatigue, creep, stiffness, and
9
water resistance. Acetal resins do not, however, have the creep resistance of polycarbon-
ate. As mentioned previously, acetal resins have excellent solvent resistance with no or-
ganic solvents found below 70°C; however, swelling may occur in some solvents. Acetal
resins are susceptible to strong acids and alkalis as well as oxidizing agents. Although the
C-O bond is polar, it is balanced and much less polar than the carbonyl group present in
nylon. As a result, acetal resins have relatively low water absorption. The small amount of
moisture absorbed may cause swelling and dimensional changes but will not degrade the
10
polymer by hydrolysis. The effects of moisture are considerable less dramatic than for
nylon polymers. Ultraviolet light may cause degradation, which can be reduced by the ad-
dition of carbon black. The copolymers have generally similar properties, but the ho-
mopolymer may have slightly better mechanical properties, and higher melting point, but
11
poorer thermal stability and poorer alkali resistance. Along with both homopolymers
and copolymers, there are also filled materials (glass, fluoropolymer, aramid fiber, and
12
other fillers), toughened grades, and UV stabilized grades. Blends of acetal with poly-
urethane elastomers show improved toughness and are available commercially.
Acetal resins are available for injection molding, blow molding, and extrusion. During
processing, it is important to avoid overheating, or the production of formaldehyde may
cause serious pressure buildup. The polymer should be purged from the machine before
13
shutdown to avoid excessive heating during start-up. Acetal resins should be stored in a
dry place. The apparent viscosity of acetal resins is less dependent on shear stress and tem-
perature than polyolefins, but the melt has low elasticity and melt strength. The low melt
strength is a problem for blow molding applications. For blow molding applications, co-
polymers with branched structures are available. Crystallization occurs rapidly with post
mold shrinkage complete within 48 hr of molding. Because of the rapid crystallization, it
14
is difficult to obtain clear films.
The market demand for acetal resins in the United States and Canada was 368 million
15
lb in 1997. Applications for acetal resins include gears, rollers, plumbing components,
pump parts, fan blades, blow molded aerosol containers, and molded sprockets and chains.
They are often used as direct replacements for metal. Most of the acetal resins are pro-

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