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Thermoplastics are divided into two general classes: amorphous and semi-crystalline. The term semi-crystalline is used because in a commercial setting there is no such thing as a polymer that has achieved 100% crys- tallinity. That may occur in low-molecular- weight systems, but in polymers, the length of the chains and the myriad ways in which they can be arranged prevent such a com- plete level of crystallization from occurring. Of the commonly used commercial poly- mers, high-density polyethylene (HDPE) achieves the highest degree of crystallinity; and even in this instance, the level rarely exceeds 85%, even for the highest density grades. For many semi-crystalline polymers, the typical degree of crystallinity is less than 50%. A Processor's Most Important Job For all grades of materials capable of crystallizing, there is a maximum degree of crystallinity that can be attained. The shape and size of the crystals will depend upon several factors that are far beyond the scope of these articles, and a review of the scien- tific literature on the process of 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 temperature. While other factors come into play and will be discussed later, their effect on the final structure of the part is relatively small compared with the effects of time and temperature. For crystallization to occur, the temperature of the polymer must be below its melting point. The lower temperature reduces the mobility of the individual chains and allows the process of crystallization to begin. This process will continue until the temperature of the material drops below the glass-transition tempera- ture (Tg). The Tg is the point at which the non-crystallized material, known as the amorphous glass, reaches a level of greatly reduced mobility. While the polymer is above its Tg, the mobility in the amorphous regions allows polymer chains to be added to the growing crystals. Therefore, the window of opportunity for forming crystals is below the melting point and above the Tg of the polymer. Within that temperature region, the rate of crystal formation and crystal growth will vary. Often people will quote general rules of thumb, such as, "Crystals grow at their fastest rate halfway between the melting point and the glass transition temperature." If only it were that simple. But it is true that for every polymer there is a definable relationship between the temperature of the polymer and the rate at which crystals form. Crystallization is a process that depends upon time and temperature. 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. By Mike Sepe PART 2 Crystallization generally occurs slowly at temperatures just below the melting point. The rate will accelerate as the temperature declines, reaching a maximum crystal growth rate. Beyond this point, the rate of crystallization will slow; and once the temperature declines below the Tg the process will stop. This graph applies to natural rubber, but the overall pattern that it displays is common to all polymers. Source: Crystallization of Polymers, Vol. 2: Kinetics and Mechanisms, by Leo Mandelkern. Published by Cambridge University Press. Generalized Relationship Between Temperature and Rate of Crystallization Rate, Hr -1 Time, Hr Temperature, C -60 -50 -40 -30 -20 -10 0 10 20 2.00 2.50 3.33 5 10 20 0 0.5 0.4 0.3 0.2 0.1 0 Fastest Crystallization Above Melting Point Below Tg 36 FEBRUARY 2018 Plastics Technology PTonline.com K now How MATERIALS