Smell in PET containers –
Y.Somasundaram
Acetaldehyde commonly known as AA, is present in all citric juices, is generated during processing 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 generated during the melt phase is driven out during the SSP (Solid State Polymerization) process and 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 in extruder to make preforms. Since, the functional end groups in PET are highly sensitive to temperature. The high temperature and shear in screw / barrel, breaks Hydroxyl groups to vinyl 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 in the preforms. H2O+ R0-CO-O-CH=CH2
AA + R0-COOH
The AA thus generated remains in the preform / bottle wall, after the processing is completed. This residual AA in the container, migrate slowly into the content of the container, 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 will hardly any change in AA during the Stretch Blow Molding of Preform to Bottle. The boiling point 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 the beverage contained in the bottle. Studies in the blown bottles prove that almost 40% of the migrate-able AA in the container oozes out in a period 7 days from blowing, in a room temperature of 30 deg C, moreover 30% of the AA is given off in the first 48 hrs of blowing itself. If the product has higher AA in a particular batch, the options available will be to hold the containers ventilated instead of online filling for the next 48 hrs so that the residual AA content is available to migrate into the product is reduced by a tune of 30%. This will help in 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: •
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Reduce the barrel set temperature: Optimum temperature of processing of PET changes from machine to machine. Suggestion will be to reduce the temperature, without the haziness or un-melt. While lowering temperature, if you encounter actual temperature overshooting, more than set temperature, generation of higher shear heat is indicated, which can induce higher AA generation. Hence lowering can be discontinued. 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 will reduce the shear, but caution should be exercised that bubbles are not formed in preform. 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 thus reduce 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 to match. Inject the melt at slower rate. This will help to reduce the shear heating. Ideal injection 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 to hydrolytic degradation. Hence moisture content to be maintained less than 40 PPM, for lower AA preforms.
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