Page 50 - Plastics News September 2018

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Materials: A Processor’s Most Important Job, Part 2

Michael Sepe

Process conditions help determine the difference between the maximum degree of crystallinity that can be
achieved in a polymer and the degree that is present in a molded part

hermoplastics are divided into two general classes: RI JUHDWO\ UHGXFHG PRELOLW\ :KLOH WKH SRO\PHU LV DERYH LWV
Tamorphous and semi-crystalline. The term semi- Tg, the mobility in the amorphous regions allows polymer
crystalline is used because in a commercial setting there chains to be added to the growing crystals. Therefore, the
is no such thing as a polymer that has achieved 100% window of opportunity for forming crystals is below the
crystallinity. That may occur in low-molecular-weight PHOWLQJ SRLQW DQG DERYH WKH 7J RI WKH SRO\PHU :LWKLQ
systems, but in polymers, the length of the chains and that temperature region, the rate of crystal formation
the myriad ways in which they can be arranged prevent and crystal growth will vary. Often people will quote
such a complete level of crystallization from occurring. JHQHUDO UXOHV RI WKXPE VXFK DV ´&U\VWDOV JURZ DW WKHLU
fastest rate halfway between the melting point and the
Of the commonly used commercial polymers, high-density
polyethylene (HDPE) achieves the highest degree of JODVV WUDQVLWLRQ WHPSHUDWXUH µ
crystallinity; and even in this instance, the level rarely If only it were that simple. But it is true that for every
exceeds 85%, even for the highest density grades. For SRO\PHU WKHUH LV D GHÀQDEOH UHODWLRQVKLS EHWZHHQ WKH
many semi-crystalline polymers, the typical degree of temperature of the polymer and the rate at which crystals
crystallinity is less than 50%. form. Generally, crystallization will occur at a relatively
slow rate at temperatures just below the melting point.
A Processor’s Most Important Job
The rate will accelerate as the temperature declines,
For all grades of materials capable of crystallizing, there reaching a maximum crystal growth rate at some point.
is a maximum degree of crystallinity that can be attained. Beyond this point the rate of crystallization will slow, and
The shape and size of the crystals will depend upon several
factors that are far beyond the scope of these articles,
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crystallization is very interesting but very complex. The
focus of this discussion is the role that process conditions
have on the difference between the maximum degree of
crystallinity that can be achieved in a polymer and the
degree that is present in a molded part. This difference
plays a very important role in determining performance
characteristics.
Crystallization is a process that depends upon time and
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EH GLVFXVVHG ODWHU WKHLU HIIHFW RQ WKH ÀQDO VWUXFWXUH RI
the part is relatively small compared with the effects of
time and temperature. For crystallization to occur, the once the temperature declines below the Tg the process
temperature of the polymer must be below its melting will stop altogether. A generalized pattern for this behavior
point. The lower temperature reduces the mobility of the is shown in the accompanying graph. This graph applies to
individual chains and allows the process of crystallization to natural rubber and is therefore not that useful for plastic
begin. This process will continue until the temperature of injection molding. But the overall pattern that it displays
the material drops below the glass-transition temperature is common to all polymers.
(Tg). The Tg is the point at which the non-crystallized There are a couple of very important points contained
material, known as the amorphous glass, reaches a level within this graph. First, in a semi-crystalline polymer,

Plastics News September 2018 50