Page 30 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

1.16 CHAPTER 1

(
η = η exp [ α PP )] (1.10)
–
r p r
where η is the viscosity at a reference P , and α is an empirical constant with values of
r
p
r
–1
200 to 600 MPa .
Shear viscosity increases with more rigid polymer structures, higher molecular
weights, and additives such as fillers and fibers. Long chain branching and broader molec-
ular weight distributions increase the shear sensitivity of viscosity. Blending two polymers
can significantly alter polymer viscosity, but the effect depends on the two polymers. Ad-
ditives such as lubricants typically decrease viscosity, whereas the effect of colorants and
impact modifiers varies with type of additive.
In contrast, the effect of strain rate on extensional viscosity varies with the polymer
structure. Branched polymers generally exhibit extensional thickening and a correspond-
ing increase in viscosity. Linear polymers, such as LLDPE, undergo extensional thinning
in which the viscosity decreases as the polymer sample necks. Generally, extensional vis-
cosity is greater than shear viscosity and depends primarily on the molecular weight of the
polymer

1.6 PROCESSING OF THERMOPLASTICS

Processing involves the conversion of the solid polymer into a desirable size and shape.
There are a number of methods to shape the polymer, including injection molding, extru-
sion, thermoforming, blow molding, and rotational molding. The plastic material is heated
to the appropriate temperature for it to flow, the material is shaped, and then it is cooled so
as to preserve the desired shape.

1.6.1 Extrusion
In extrusion operations, a solid thermoplastic material is melted, forced through an orifice
(die) of the desired cross section, and cooled. This method was adapted from metallurgists
who use a similar form of extrusion to process molten aluminum and was first adapted in
16
1845 by Bewley and Brooman to extrude rubber around cable as a coating. Extrusion
processes are used to continuously produce film and sheet; shapes with uniform cross-sec-
tions, such as PVC pipe, tubes, and garden hose; profile with nonuniform cross-sections,
such as PVC window moldings and gutters; synthetic fibers; polymer coatings for insulat-
ing wire and sealing paper, plastic, and metal packaging.
Although there are many types of extruders, the most common is the single-screw ex-
17
truder (shown in Fig. 1.13). This extruder consists of a screw in a metal cylinder or bar-
rel. Electrical heater bands and fans that surround the barrel help bring the extruder to
operating temperature during start-up and maintain barrel temperature during operation.
One end of the screw is connected through a thrust bearing and gear box to a drive motor
that rotates the screw in the barrel. The other end is free floating in the barrel. The barrel is
connected to the feed throat, a separate “barrel section,” with an opening called a feed
port, and is connected to the feed hopper. A die adaptor is usually connected to the oppo-
site end of the extruder. A breaker plate and a screen pack are sandwiched between the ex-
truder and die adaptor. The breaker plate provides a seal between the extruder and die,
converts the rotational motion of the melt (in the extruder) to linear motion (for the die),
and supports the screen pack. The screen pack filters the melt, thereby prevent unmelted
resin, degraded polymer, or other contaminants from producing defects in the extruded
products and/or damaging the die.

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