Page 26 - Plastics News February 2019

FeAtures INTERNATIONAL

A Processor's Most Important Job, Part 7 PLASTICS

Mike Sepe

Too much crystallinity will reduce the toughness of the material and can result in brittle failures also higher EXHIBITIONS
degrees of packing discourage crystallization.

e have spent a lot of time on the subject of The act of moving the polymer produces an uncoiling of CALENDAR FOR 2019-2020
Wcrystallinity and there is a lot more that we could the entangled chains; and the straighter chains, aligned
say. The emphasis of this discussion has been on cooling closely together, can produce zones of higher crystallinity.
rate, since it is within the control of the processor. But Thermoformers that produce parts in PET are quite
there are other factors that can influence crystallinity familiar with this phenomenon and have learned that
The process-related items on the list play a relatively PET in a sidewall of a deep-draw container will exhibit
minor role compared with cooling rate, but they should a higher degree of crystallinity than the bottom of the
be understood to appreciate the big picture. The other container where the sheet did not stretch. This can affect
factors are related to aspects of mold design and to the the properties of the container. Too much crystallinity
material itself. will reduce the toughness of the material and can result
One process parameter that has a minor but measurable in brittle failures.
influence on crystallization is the melt temperature of the In injection molding, this type of crystallization is most
polymer as it enters the mold. The higher the temperature often observed as a function of gate location and part
of the material, the longer it will take to cool to the point geometry and is especially important in materials that
where crystallization stops. It is important to understand can crystallize over an extended period of time, such as
that we are referring to actual melt temperature, not PE and PP.
the barrel settings. Anything that can alter the energy
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content in the polymer, such as screw rotation speed or Launch Edition Edition
back pressure, can also affect the rate of cooling and Edition
therefore the degree of crystallinity. NIGERIA MYANMAR KENYA
The other process condition that has an unexpected effect Mar 19-21, 2019 June 14-16, 2019 July 11-13, 2019
on crystallization is the pack and hold pressure profile. Lagos
Higher pressures compress the material to a greater Yangon Nairobi
degree, forcing the polymer chains closer together and
restricting their mobility. Since crystallization depends
upon mobility, higher degrees of packing discourage
crystallization.
This effect can be observed in a pressure-volume-
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temperature (PVT) diagram as shown in the accompanying Edition Edition 3 rd
graph. This shows the change in the specific volume of Edition
a PP as it cools while under different pressures. Larger SRI LANKA VIETNAM
changes in volume correspond in part to the achievement SOUTH AFRICA
of a greater degree of crystallinity. It is apparent that Other factors are related to the nature of the material that Aug 9-11, 2019 Feb 20-22, 2020 Mar 3-5, 2020
this change in volume becomes smaller as the applied has been selected. This is frequently beyond the control Colombo Ho Chi Minh City Johannesburg
pressure increases. Both the melt temperature and the of the processor. However, a processor who knows the role
pack and hold pressure profile have a relatively small of material selection in the achievement of crystallization
effect on crystallinity compared with cooling rate, but can offer its customer a way out of a thorny problem.
they are measurable, and they have been confirmed in Nucleation is one modification of material composition
laboratory tests. that influences the way a material crystallizes. This is INTERNATIONAL Organised by
often achieved through additives and is commonly used INDIA
Another interesting influence is orientation. This is a Narendar Bafna : +91 99529 66752 Sasikumar. B : +91 90030 25103
phenomenon that occurs whenever a polymer flows. in PPs, nylons, and polyesters. Vijay. N : +91 97890 95249 sasikumar.b@smartexpos.in

plastics@smartexpos.in
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