Page 31 - Plastics News October 2019

FEATURES

The Importance of Mold Temperature When Processing Polycarbonate

Mike Sepe
An often overlooked factor in optimizing the ductility of PC is the rate at which the polymer is cooled in the
mold

olycarbonate has been a commercial material for short of these numbers. If they do approach them, they
Pmore than 60 years. At the time of its development it XVXDOO\ GR VR EHFDXVH WKH\ UHO\ RQ LPSDFW PRGLÀHUV DQG
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were available in a transparent material. Clear commodity PDQ\ LQÁXHQFHV WKDW FDQ HURGH WKH LPSUHVVLYH QXPEHUV
materials like general-purpose polystyrene and acrylic that contribute to the reputation of polycarbonate as one
that had been available for decades before the invention of the toughest polymers on the market.
RI 3& WHQG WR EH EULWWOH XQOHVV LPSDFW PRGLÀHG DQG WKH\ Probably the best known of these is sensitivity to a wide
lack the heat resistance that PC offers. Higher performing range of chemicals. These promote chemical attack or
transparent polymers such as the family of sulfone-based environmental stress cracking (ESC). At NPE 1979 one of
materials that were developed after the introduction of the major manufacturers of PC at that time devoted a
polycarbonate are typically not water white and also
exhibit greater notch sensitivity even though they display VLJQLÀFDQW DPRXQW RI ÁRRU VSDFH WR GHPRQVWUDWLQJ WKH
impact resistance of a PC front bumper for automobiles. The
good inherent ductility.
bumper was put through a very impressive demonstration
of its toughness several times each day. The application
never happened in the real world, largely because of the
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was an alloy of PC and polyester, with the semi-crystalline
polyester providing the needed chemical resistance.
The addition of color can have a substantial effect on
the toughness of the base resin. The degree of change
in impact resistance will depend upon the color, the
chemistry chosen to create it, and the amount of colorant
added. The effects on ductility are more noticeable

PC, therefore, occupies a niche that has allowed it to in specimens that are notched than in those where no
grow to a level of global consumption that is approaching stress concentration is present. The molecular weight of
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the plastic that is consumed worldwide in a given year. performance of the material is affected by the color.
However, those involved in failure analysis will tell you, Higher-molecular-weight grades preserve their impact
DQHFGRWDOO\ DW OHDVW WKDW DSSUR[LPDWHO\ RI WKH IDLOHG properties better than lower-molecular-weight grades,
parts that they evaluate are made from PC. And despite especially in the case of opaque colorants.
its reputation as a “bulletproof” material, many of the Many industry practitioners are also aware of the fact
fracture features we observe under the microscope are that PC is sensitive to the effects of moisture at elevated
brittle. temperatures. This can cause problems for molded parts
This is not an indictment of the polymer. It is, however, that are exposed to high heat and high humidity at
an illustration of how things work when the real world the same time, such as in the so-called 85/85 test for
collides with the expectations of designers and engineers electronic enclosures that involves 1000 hr of exposure
that are based on published properties. One of the most WR ) & DQG UHODWLYH KXPLGLW\ :KLOH QHLWKHU
well-known performance characteristics of PC is a property this temperature nor this level of humidity alone presents
that is always included on the data sheet: the notched any challenges to the performance of polycarbonate, the
Izod impact resistance. PC routinely registers values of combination will produce a substantial reduction in the
12-18 ft-lb/in. (640-960 J/m). Most polymers fall well molecular weight of the polymer.

31 October 2019 Plastics News