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Smell in PET containers Y.Somasundaram

Acetaldehyde commonly known as AA, is present in all citric juices, is generated duringprocessing of PET, and is not harmful to humans for consumption. AA normally gives off taste and flavor to flat water.

AA (Acetaldehyde) is generated during the melt polymerization and most AA generatedduring the melt phase is driven out during the SSP (Solid State Polymerization) processand the net resultant AA in PET chips of water grade resin is less than 0.60 PPM. Major portion of AA in preform / bottle is generated during the melt processing of SSP chips inextruder to make preforms. Since, the functional end groups in PET are highly sensitive totemperature. The high temperature and shear in screw / barrel, breaks Hydroxyl groups tovinyl ester groups. The reaction is as below.

R-CO-O-CH2-CH2-OH R-CO-O-CH=CH2 + H2O The new vinyl ester groups, further reacts with water molecule to produce Acetaldehyde inthe preforms.

H2O+ R0-CO-O-CH=CH2 AA + R0-COOH

The AA thus generated remains in the preform / bottle wall, after the processing iscompleted. This residual AA in the container, migrate slowly into the content of thecontainer, when the temperature is above the boiling point.

The AA in the bottle is reduced in long-term storage, by migration to air. But, there willhardly any change in AA during the Stretch Blow Molding of Preform to Bottle. The boilingpoint of AA at normal pressure is 21 o C.

How to avoid smell in product, with high AA preforms

AA continuously emanates from a blown bottle into the surrounding atmosphere and thebeverage contained in the bottle. Studies in the blown bottles prove that almost 40% of themigrate-able AA in the container oozes out in a period 7 days from blowing, in a roomtemperature of 30 deg C, moreover 30% of the AA is given off in the first 48 hrs of blowingitself.

If the product has higher AA in a particular batch, the options available will be to hold thecontainers ventilated instead of online filling for the next 48 hrs so that the residual AAcontent is available to migrate into the product is reduced by a tune of 30%. This will helpin product meeting the smell requirement.

But, the best option to reduce smell is by controlled processing, which is detailed.

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Tips for low AA in Preform / Bottle:

Reduce the barrel set temperature: Optimum temperature of processing of PETchanges from machine to machine. Suggestion will be to reduce the temperature,without the haziness or un-melt. While lowering temperature, if you encounter actualtemperature overshooting, more than set temperature, generation of higher shear heat is indicated, which can induce higher AA generation. Hence lowering can bediscontinued.

Try inverse barrel temperature profile . .i.e., Feed zone temperature 5 ~ 10 oC,higher than metering zone temperature.

Reduce melt pressure during plasticization: The lowering of backpressure willreduce the shear, but caution should be exercised that bubbles are not formed inpreform. The optimum melt pressure will be 50 ~ 150 bar for low AA processing,though it may vary from machine to machine.

Reduce the screw speed: Lower screw speed will reduce the shear and thusreduce the AA generation. Avoid too low speed leading to increase in cycle time.

Screw idle time should be less than 2 sec , if higher, reduce screw speed tomatch.

Inject the melt at slower rate. This will help to reduce the shear heating. Idealinjection rate for water preforms are < 10 g / sec.

Minimum cushion for extruder and shooting pot. Reduce the manifold and NT temperatures of hot runner to minimum.

The above are some of the tips for reducing the AA, a combination of the same will

help to have the AA under stipulated norms of 4 PPM for water preforms / bottles.Drying has considerable impact on AA generation. Higher moisture content will lead tohydrolytic degradation. Hence moisture content to be maintained less than 40 PPM, for lower AA preforms.

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