Page 34 - Plastics News August 2018

FEATURES

the Materials Recovery for the Future (MRFF) research ZRXOG LQFOXGH OLGGLQJ ÀOP IURP PXQLFLSDO UHVLGHQWLDO
program in partnership with J.P. Mascaro & Sons at its VLQJOH VWUHDP UHF\FOLQJ SURJUDPV :H WKLQN WKH HIIRUW
TotalRecycle MRF in Berks County, Pennsylvania. The pilot could advance the recycling of lidded plastic packaging.
is intended to demonstrate the technical and economic
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A Processor’s Most Important Job, Part 1

Michael Sepe

Many processors don’t realize that preserving material characteristics is crucial to product success and failure.
The focus here is on molecular weight.

hen I ask processors about their most important It is then up to the processor converting the raw material
:responsibility, the answers usually focus on factors to a molded article to preserve that molecular weight.
relating to productivity. These typically include some This does not happen automatically. The elevated
conversation about cycle time, machine utilization, temperatures associated with melt processing, combined
making parts to print, maintaining process capability with the time that the material spends in the barrel, can
indices (Cpk) of 1.33 or greater, etc. It is rare to hear any KDYH D VLJQLÀFDQW HIIHFW RQ WKH PROHFXODU ZHLJKW RI WKH
mention of molecular weight, crystallinity, preservation compound. The higher the melt temperature and the
RI DGGLWLYHV RU PLQLPL]LQJ PROGHG LQ VWUHVV :KLOH WKHVH longer the barrel residence time, the greater the chances
factors are the foundation for the success or failure of are that the molecular weight of the polymer will be
a product, processors often do not appreciate their own negatively affected. The response in most materials is
role in controlling them. a process called chain scission, where the length of the
chain is shortened by the thermal stress of processing.
But if these characteristics are not given the proper
attention, then all the aspects associated with productivity
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address here is molecular weight. It has been recognized
for almost 100 years that the unique characteristics
exhibited by polymers are due to the large size of the
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This combination of characteristics gives rise to something
called chain entanglement. It is the foundation of
mechanical performance. Short-term properties such as
impact resistance are particularly sensitive to changes in
molecular weight. But long-term properties such as fatigue
resistance and environmental stress-crack resistance are ,Q DGGLWLRQ WR WKH GXDO LQÁXHQFHV RI WLPH DQG WHPSHUDWXUH
the processing of some materials involves the potential
also closely correlated with the length of the chains that hazard of degradation due to the presence of excess
make up the material.
moisture in the material at the time of processing, a
The manufacturers of plastics raw materials pay close process known as hydrolysis. Condensation polymers such
attention to the molecular weight of the products they as polyesters, polycarbonate, nylon, and polyurethane
produce, and they capture this with a measurement of are particularly susceptible to this problem. Drying these
SURSHUWLHV VXFK DV PHOW ÁRZ UDWH DQG LQWULQVLF YLVFRVLW\ materials down to a moisture content that prevents

Plastics News August 2018 34