Patent Application
20050175510
This invention relates to an analyzer for automatic rapid analysis of the acetaldehyde content of polyethylene terephthalate (PET) products, particularly preforms, and to its operative process.
The determination of the acetaldehyde (AA) content of polyethylene terephthalate (PET) preforms, as an index of PET degradation, is a known current test used as a parameter in verifying good product quality. The current analytical approach comprises sample preparation by cryogenic grinding of the preform and gas chromatographic analysis by the headspace technique.
To obtain significant data the instrumentation must be calibrated using a reliable precise standard solution. To achieve this the standard acetaldehyde solution must be distilled, the distillate titrated and a series of reference samples of different concentration prepared for use in constructing an external calibration graph.
In this respect, data demonstrating good repeatability within the laboratory of origin are very often not reproducible between one laboratory and another. The treatment and handling of the preform sample to be analyzed (manipulation, cutting etc.) in such a manner as to make analysis possible with existing laboratory systems plays a determining role with regard to the accuracy and reproducibility of the result.
Moreover the technical costs and times involved in carrying out the test are such as to enable only a sporadic process check, considering the number of preforms produced in a normal productive plant. Generally from 1 to about 5 measurements per day are made in checking an average hourly production of about 15,000 preforms per injection moulding press.
In the prior art, acetaldehyde is determined in a chemical laboratory using an analytical system generally composed of a static headspace sampler, in combination with a gas chromatograph. The sample must be previously ground cryogenically, sifted and weighed, and moreover the preparation of the reference acetaldehyde solution requires laboratory personnel and methods (titrations, preparation of reference solutions, etc.). During production, preforms are withdrawn and sent to the laboratory. The result of the analysis is received after a time not compatible with production times.
The lack of a quick analysis result often results in the scrapping of large quantities of product.
An object of the present invention is to increase the monitoring of the PET preform production process by reducing analysis times and costs.
Another object is to create a system able to carry out an AA test in a short time, for example less than thirty minutes and, in particular, to enable it to, be performed in proximity to or within the actual production plant, without having to use specialized laboratories and methods.
The article of K. C. Khemani, with the title “A novel approach for studying the thermal degradation, and for estimating the rate of acetaldehyde generation by the chain scission mechanism in ethylene glycol based polyesters and co-polyesters”, in Polymer Degradation and Stability, January 2000, describes a system for studying the mechanism of PET degradation by means of gas-chromatographic analyses at 280° C. The system comprises an aluminium boat in an oven, a gas chromatograph, a cold trap and a detector.
Another object of the present invention is to eliminate the aforesaid drawbacks and limitations.
Another object is to define an analyzer able to be used directly in PET article production departments by being able to operate even on granules of said material.
These and further objects are attained by a method and analyzer for the automatic rapid analysis of the acetaldehyde content of PET preforms or granules having the characteristics defined in the accompanying claims.
The invention is illustrated by way of non-limiting example in the accompanying drawings, in which:
Particular reference will be made hereinafter to preferred embodiments of the invention in which these samples are PET preforms, however this is in no way meant to be limitative as the invention can also be used for analysis of the raw material (granules) or of articles produced from the preform (bottles).
In the method on which the present invention is based, the calibration is carried out by the analyzer by using a cylinder of certified gas (nitrogen/acetaldehyde mixture of known concentration) and/or of an aqueous acetaldehyde solution to be injected into the measurement cell (described hereinafter), then when the analyzer has been calibrated the analysis is carried out without the preform undergoing any preparation. In the case of PET preform production, the said method, according to the invention, can be also used within the product preparation department by virtue of the dimensional compactness of the analyzer structure and of the lack of requirement for qualified personnel, the usual production plant personnel being sufficient. This advantageously enables the product to be monitored in-line.
This signifies the ability to make at least one evaluation of the AA content per box containing from 7500 to 15000 pieces provided by the producer to the preform-bottle transformer, and that, considering the average filling time, is about 30-40 minutes, a wide margin of action is achieved for the injection moulding press, enabling the process to be controlled, if necessary, by taking action as soon as the values indicate an increase in material degradation. It must also be considered that these plants operate under continuous cycle. The aim is therefore to increase the number of checks, and hence the process quality, by reducing analytical costs.
The invention will now be described with reference to the accompanying drawings which illustrate preferred but non-limiting embodiments thereof.
All the figures use the same reference numerals to indicate identical or substantially corresponding parts.
With reference to
The scheme of
The following operative steps are involved:
The various cycle steps, the valve switchings and the other operations (such as integrations, chromatogram display, data memorization, etc.) are controlled by the unit 19, preferably consisting of a PC within the analyzer.
The valves 32, 33, 34a and 35a represent “automatic small-volume sampling valves” (known per se) able to sample a few microlitres of sample directly into the gas chromatography column without using special sample splitting techniques, hence eliminating the analysis errors inherent in this technique.
With referene to
The cell 31 can receive preform samples of different lengths and widths. After inserting a preform into the cell, the closure ring nut 33 ensures that the extraction system is hermetically sealed.
The ring nut 33 (formed from several components) comprises a seat 62 which receives and retains a perforable baffle 60 through which an aqueous acetaldehyde solution can be injected into the cell.
Holes 64 are provided in the ring nut for the injection needle.
The analysis cycle of the system according to the invention takes place in the following manner. A calibration cycle is periodically carried out in which an aqueous solution of known acetaldehyde concentration is injected into the desorption cell 1 through the perforable baffle 60 with a syringe. The calibration can also be carried out with a standard reference gas by connecting to the connector 24 a cylinder containing nitrogen with a known concentration of acetaldehyde. The operator inserts the sample into the cell 1 and starts the analysis system, which progresses in accordance with the aforestated steps (scavenging, incubation and heating, pressurization, loop charging, analysis). During this cycle the partial pressure is monitored by the device 15. The partial pressure is proportional to the quantity of water and acetaldehyde which is desorbed. Partial pressure monitoring enables the analysis results to be successively corroborated by verifying that the partial pressure always remains below the pressure value set for the sample pressurization cycle. If this value is exceeded a warning signal or message is given. On termination of the analysis cycle the separation of the different components, the graphic printout and the analysis schedule are obtained. The graph consists of a chromatogram in which the peak areas are proportional to the concentration of the extracted compounds, including acetaldehyde.
The final result is calculated on the basis of the calibration and is corrected. (multiplied) by a factor representing the ratio of empty cell volume to its net volume when the preform is inserted. On termination of the analysis cycle the system returns to its stand-by state.
Although the invention has been Illustrated with reference to preferred embodiments, it is generally susceptible to other applications and modifications, which lie within the scope of protection as will be apparent to the expert of the art.
Advantageously, the invention as defined does not require the use of qualified personnel. It also eliminates the time involved in the handling and preparation work on the samples to be analyzed. It also enables results to be obtained which are comparable with those obtainable in the laboratory by known procedures, but achieved with considerably shorter analysis times and with more reliable results, as they are less subject to human error.
The signals from the detector 17 are processed in known manner to obtain the chromatogram and to compare the measured acetaldehyde content with a predetermined value which, if exceeded, visually and/or acoustically warns the responsible personnel of the unsuitability of the preform batch.
The term “PET sample” used in the following claims includes not only a whole preform, but also PET granules or pieces.
| Number | Date | Country | Kind |
|---|---|---|---|
| BG2002 A 000011 | Mar 2002 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP02/08698 | 8/5/2002 | WO |
