The measurement of the levels of acetaldehyde, generating in the PET transformation process, is among the key acceptance tests that preform producers are to perform.
In order to guarantee the superior quality of the product and verify that the AA content is kept within strict and controlled limits, Piovan provides the PETes AA Analyzer instrument, a patented method of analysis that allows measuring the Acetaldehyde (AA) contained in PET preforms fully automatically.
The use of PETes AA Analyzer is so simple that, even non-specialised staff can carry out the online measurement in just 30 minutes, with no preparation of the preform.
PETes AA Analyzer can be used for different typologies of preforms, always using the same de-absorption cell and without having to change the parameters.
• the preform is placed into the de-absorption cell
• the conditioning phase starts
• when the heating phase is completed, a flow of purified air (zero air) picks the gas released by the preform and carries it to the analysis loop
• the carrier gas (hydrogen) brings the sample from the loop through to the packed column where, by using a FID (flame ionization detector), the measurement takes place
• the chromatogram, all analytical data, and production references are displayed on the monitor
All data are printed and clearly summarized in analytical report forms.
• Constant control of the process: the possibility to detect the AA content in real time allows the certification of every single production lot. The connection with the company’s network offers the possibility to access the process data constantly and elaborate them.
• High accuracy and repeatability of the measurement, ensured by the instrument that, based on the “Fast Head Space Gas chromatography” technology, ensures a correct analysis, independent by any of the many variables that characterize the traditional methods.
• Reduction of the costs and times required to performing the tests. The PETes unit, which can be installed in the production department or in the company’s quality control laboratory, allows the operator to carry out the AA analysis at any moment, without requiring the support from third-party specialised technicians.
The traditional method to measure acetaldehyde in PET preforms is based on a gas chromatography analysis of the acetaldehyde present in the head space, desorbed from the ground material placed into a sealed vial after thermal treatment. This process is very labor intensive, time consuming and expensive.
The means to determine the ppm of residual AA in a PET preform is gas chromatographic analysis, a specific technique used for concentration and analysis of volatile organic in solids. In headspace GC, a sample is diluted and sealed in a closed container that is large enough to allow some headspace in the vial where vapors can collect. The sample is then kept at high temperature for a period of time to allow the solvents in the matrix to volatilize, enter the gas phase, and reach equilibrium with the remaining solvent(s) in the vial.
Precision and accuracy of the traditional method are heavily affected by:
• Sample pre-treatment procedure – such as milling conditions, different sieves/screens, cryogenic treatment, part of sample under testing;
• GC instrument variables - such as incubation time, different sample conditioning temperatures, column and head space systems;
• Any possible human error.
The key for reliable and repetitive results in AA analysis is to limit the sample pre-treatment steps, to automate the process, to reduce overall time and effort and to prevent opportunity for imprecision and accuracy errors. PETes AA method hits the mark. By means of gas chromatography technology, but with no need for sample preparation, milling and processing by expert lab technicians, PETes instrument analyse the entire preform in a fully automatic way. It is simple to use, rapid, and it provides reliable and repeatable results.
AA is a colorless volatile with a fruity smell which arises as a by-product of PET thermal degradation by oxidation of the vinyl end group of the polymer chain. In PET bottles, AA is usually present at a few ppm concentration, which is slowly desorbed to the bottle content, altering its taste and aroma. This may not be a relevant issue for fruit juices or strong-tasting soft drinks.
But for bottled water, especially of the still one, low Acetaldehyde content is imperative: even AA concentrations in the water as little as 10-15 ppm can convey an undesirable off-taste.
While raw resin normally has concentration of 1 ppm acetaldehyde before drying, at the end of the moulding process its content increases up to 3-4 ppm. AA is normally generated in the plastic processing phase for two main reasons: shearing/friction in IMM screw and thermal degradation of the resin due to high temperature. It is possible to control final AA content in preforms at each step of the production process:
• limiting the barrel residence time of raw material
• using the correct processing parameters during the drying phase
• reducing the screw back pressure effective melt-out, shortening the cycle time and limiting the injection pressure at the IMM
• adding AA blockers (anti-oxidant), erucamide or similar to the resin.
Measuring AA content in PET preforms provides an index of the actual quality of the moulding process. To keep the process at its best and to avoid the subsequent phases of blowing, filling, distributing and putting on shelves bottles with a content exceeding the allowed AA level in the market, AA is best measured before the blowing and filling stage: in the PET preforms. Piovan optimises the PET drying process in order to reduce the original PET resin AA content.