Extruder-film brittleness

Because polycarbonates are good light absorbers, ultraviolet degradation does not occur beyond a depth of 0.030-0.050 in (0.075-0.125 cm). Whilst this is often not serious with moulded and extruded parts, film may become extremely brittle. Improvements in the resistance of cast film may be made by addition of an ultraviolet absorber but common absorbers cannot be used in moulding compositions because they do not withstand the high processing temperatures. 

The occurrence of yam breaks was reported early and is connected with thickness fluctuations in extruded ribbons or films, as described collectively by the phenomenon of draw resonance , which is characterized by oscillations of the fiber diameter, which ultimately lead to yarn break. The latter is defined as brittle fracture and is thus related to melt temperature, molecular weight, quenching conditions, and particularly to the role of viscoelasticity, as described in the following section. 

PROPERTIES OF SPECIAL INTERESTS The highly branched nature of amylopectin accounts for the extreme brittleness of its films and extrudates. The extensive branching reduces chain entanglements usually required in high polymers to achieve satisfactory film properties. 

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