Page 24 - Plastics News August 2019

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Why & When Should You Re-Crystallize PET Scrap

Whether you’re blow molding PET bottles or extruding APET sheet, you’ll produce amorphous scrap in the
process. How you handle it will impact your production costs. Re-crystallizing it will help.

hen processors receive deliveries of virgin PET, the conditions experienced throughout most of the U.S and
:material will be in a semi-crystalline state, which Canada.
FDQ EH FRQÀUPHG E\ WKH PLON\ ZKLWH FRORU RI WKH SHOOHWV One of the most critical issues encountered when
Virgin PET is actually colorless; the milky appearance is processing PET is pre-drying of the polymer prior to melt
due to the crystallinity of the material, which causes light processing. PET is extremely sensitive to hydrolysis during
entering the pellets to scatter, making it appear white. melt processing and will react with residual moisture left
'XULQJ LWV ÀUVW SKDVH RI PDQXIDFWXULQJ 3(7 H[LWV WKH in the polymer due to inadequate drying. This hydrolytic
melt-phase reactor and is chopped into small cylindrical, reaction will sever or break some of the polymer chains,
transparent pellets which are 100% amorphous. Prior to lowering the intrinsic viscosity of the polymer and resulting
or during the last phase of manufacturing—solid-state in reduced physical properties.
polymerization—the transparent, amorphous pellets PET suppliers strongly recommend pre-drying PET to a
are crystallized by high-temperature heating, during ÀQDO PRLVWXUH OHYHO LQ WKH UDQJH RI WR
which they are prior to melt processing in order to prevent any chance
also agitated to of hydrolytic degradation. The drying parameters
prevent them recommended by all PET material suppliers are crucial
from sticking or and should be followed precisely:
fusing together. 1. Drying temperature: 300 F to 350 F;
:KHQ KHDWHG 2. Dewpoint of the drying air: -20 F or lower;
to its glass- 3. Drying time: 4 to 6 hr.
transition RE-CRYSTALLIZING SCRAP PET
temperature
(approximately One of the challenges encountered when reclaiming
170 F to 200 F), the scrap generated in the processing of PET bottles
the polymer chains will begin to fold up upon themselves or thermoformed APET sheet containers is drying
to form numerous crystals within the polymer, and the the granulated scrap. Remember, material suppliers
surface of the pellets will begin to soften and stick or UHFRPPHQG WKDW 3(7 EH SUH GULHG WR D ÀQDO PRLVWXUH OHYHO
fuse to adjoining pellets. of 0.003% to 0.005% prior to melt processing in order to
Once the temperature exceeds the glass-transition prevent any chance of hydrolytic degradation. And it must
temperature, the surface will re-harden and not soften be dried in the range of 300 F to 350 F. There’s the rub:
again until it reaches its melting temperature of 450 F to Both types of scrap are amorphous, and when amorphous
500 F. The softening effect is the reason that agitation is PET is heated to its glass-transition temperature it will
required. Once the surface of the pellets re-hardens, the begin to soften, stick together, and bridge, causing
vigorous agitation breaks up any clumps created during PDWHULDO ÁRZ LQWHUUXSWLRQV LQ WKH GU\LQJ KRSSHU
FU\VWDOOL]DWLRQ DQG NHHSV WKH PDWHULDO ÁRZLQJ IUHHO\ 7KLV There are two possible courses of action to address the
type of crystallization is called heat-set crystallinity, which dilemma of drying reclaimed amorphous PET scrap,
occurs when amorphous PET is heated to an elevated depending on the process:
temperature. ‡ 3(7 ERWWOH PDNLQJ 3(7 SUHIRUPV DUH DPRUSKRXV
PET is a very hygroscopic polymer and will readily and the blown bottles have a low level of strain-
absorb moisture when exposed to the atmosphere during induced crystallinity. As a result, preform and bottle
transportation and storage. PET will continue to absorb scrap should never be dried at normal PET drying
moisture from the atmosphere until it reaches a state of temperatures, which are suitable only for pre-
equilibrium with the surrounding air. PET moisture levels crystallized resin. However, a processor can blend a
may reach as high as 0.2% to 0.3% under typical weather small percentage (5% to 10% or even 15% ) of preform

Plastics News August 2019 24