DE19502134A1 - Quality control test for fluorine-treated polyethylene@ containers - Google Patents

Abstract

A quality assurance method for evaluating the treatment of polyethylene (PE) surfaces with reactive gas is claimed. This is applied to the internal surfaces of hollow PE prods. or containers of any shape and wall thickness which have been treated e.g. with fluorine-contg. gas, and comprises the reproducible measurement of IR absorption spectra (attenuated total reflection (ATR) spectra) which are characteristic (under constant treatment conditions) for the type of treatment and thus of the modified surface, using standardised fourier transform IR (FTIR)-ATR techniques. The surface (ATR) spectra obtd. provide good quality control before further processing, e.g. in the prodn. of plastic fuel tanks (PFT), by carrying out direct FTIR-ATR measurements on suitable PET samples on the prodn. line.

Description

1. Problem:
The process-related quality control of the treatment of the KKB inner surface with fluorine-containing gas mixtures is currently based on the rapid test, the so-called can test. The permeation behavior of the treated KKB is examined as a model.

Despite usable results, this indicates the state of the art widely used evaluation procedures some serious shortcomings on:

• - As a laborious laboratory test practically not feasible in a production hall for KKB,
• - relatively sample-intensive, the examination object goes, the KKB, lost: it is sawed up;
• - Test duration: at least 4 hours, under real operational Conditions often significantly longer;

The bottom line is that in practice that means a very big one Number of fluorine treated KKB without a corresponding previous quality control the further course of production is forwarded.

2. Task and explanation of the solution concept:
Elimination of the above-mentioned shortcomings through a new procedure for quality assurance (QA) assessment of KKB fluorine treatment.

The solution concept therefore sees the elaboration of one as quick Evaluation procedure, based on a physical measurement method, with the help of which the treated Surface itself is characterized.

In particular, the invention relates to the specific Application of certain optical surface measurement methods of IR Vibration spectroscopy: either the so-called DRIFT (diffuse Reflectance Infrared Fourier Transform) - or the Step-Scan-PAS (Photoacoustic Spectroscopy) - or the ATR (Attenuated Total Reflection) technology.

The basics of our evaluation procedure are explained using the example of the corresponding application of FTIR-ATR spectrometry. In this measuring technique, the interferometrically modulated IR measuring beam passes through a highly refractive crystal due to total reflection, with the optically polished, straight outer surface of which the sample to be measured is brought into close contact by pressing (see Fig. 1). The small depth of penetration of the IR measuring beam into the sample to be examined ensures that the absorption spectra measured according to the prior art, the so-called ATR spectra, are determined by the chemical nature of the surface of the sample.

To the multitude of samples using this method can be characterized include u. a. the polymers, consequently also workpieces made of polyethylene (PE), which are made of Plastic fuel tanks (KKB) come from whose Inner surface previously chemically modified by fluorination has been.  

3. Description of the invention: process concept and execution
The basic approaches of the new, based on FTIR-ATR spectrometry evaluation method for the quality of treatment z. B. of KKB surfaces with fluorine-containing gas are as follows:
If the measured ATR spectra of samples from fluorinated KKB are identical to one another after standardization, the fluorination process runs under unchanged conditions and provides KKB with defined properties that have been qualified by previous tests (e.g. permeation measurements against a fuel mixture of the appropriate composition).

Are clear spectral changes in corresponding To detect areas of the standardized ATR spectra means this means that the process flow in important parameters of deviates from the specifications.

Accordingly, it is possible only because of the measured surface spectra of representative samples Differentiation between target product and committee regarding fluorine treatment.

3.1 Applying the principles described above is a Measuring method for the characterization of the inner surface fluorinated KKB developed for a quality control been.

The measuring process includes

• - a defined and optimized type of sampling:
  The test piece is the so-called cover, one of the cut-out parts from the KKB to provide upgrade openings;

Advantages of the so-called lid as a test piece:

• - flat, without curvature <simplified ATR measurement technology;
• - Always from the same KKB area <avoidance of the Sampling point in the KKB depending on quality fluctuations,
• - a waste product <from the KKB remains usable.

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in a diagonally symmetrical order for a Measurement of four small samples from the KKB test piece (lid) punched out gently and after cleaning its inner surface for measurement on the two side surfaces of the ATR Measuring crystal pressed with evenly distributed, moderate pressure (Torque control), which means that a Averaging of location-dependent fluctuations in Surface properties achieved within the lid;

• - and the physical measurement procedure itself:
• - Carrying out the actual FTIR-ATR measurements on the samples in the ATR unit according to the status of the technique,
• - Documentation, backup and processing of the measurement data to corresponding ATR spectra and
• - the spectrometric processing of the latter: calculation of spectral features (numerical values) or pattern recognition in relation to reference spectra for the purpose of finding results with YES / NO- (OK / not-OK) - statement.

Remarks

With the specified preparation method of the measurement sample already in a commercial, but after optimizing Aspects selected ATR unit with KRS5 measuring crystal a reproducibility of the task appropriate ATR spectra achieved. However, this means that the measured accordingly and  processed ATR spectra in a comparison with each other presintered, d. H. to one of the CH₂-Deformati vibrations are standardized.

Nevertheless, a certain one occurs in ATR measurements real samples inevitable scatter even among the normalized ATR spectra.

There must therefore be a range of trust for the spread of the Surface (ATR) spectra taking into account the experimental treatment method, according to statistical considerations. Leading naturally to define the so-called. Reference spectra. These processes are not only borne by Peculiarity of the physical measurement bill, but at the same time consider the understandable circumstance, that the individual, from the same, unchanged production originating, fluorine-treated containers with each other are not completely identical.

The result finding means that the normalized ATR spectrum of the respective test piece (lid) with the corresponding Reference spectrum taking into account the respective Trust range is compared. Is the measured Spectrum within the range of trust, the KKB- Fluorination process OK (OK). This condition not met, the treatment procedure must be correct be checked.

A rough evaluation of results can already be based on visual inspection of the surface spectra be performed.

However, the development of so-called spectral features does Determination more precise and objective. These spectral features are defined as follows:  

After appropriate processing of the measured ATR spectra (prior art), the areas of their CF stretching or CH₂ deformation vibrations are integrated, taking into account the course of the baselines, and then the integration values are related to one another. The integration value of the corresponding CH₂ deformation vibration should appear as a spectral reference in the denominator (standardization). The other evaluation method: pattern recognition, means an interactive, normalizing comparison of the measured ATR spectrum of the respective sample with the corresponding reference spectrum - a spectral subtraction method with integrative evaluation - according to predetermined analytical IR bands of the so-called "finger print" range (2000 - 400 cm ^-1 ).

Both evaluation methods, such as the FTIR (ATR) measurements themselves, can be adjusted using PC technology (hardware and software) Automate demand and desired expenditure, or the Contents of the statement of the finding of results in external control signals convert.

4. Explanations of the measuring method using the example of model examinations or control measurements at KKB:
The correctness of the approaches of the evaluation procedure has been confirmed by model tests. The transferability of the results of the model examinations to the conditions of the KKB fluorination was checked and the measuring method for surface evaluation improved to a practical application.

4.1 Model investigations:
Subsequent fluorine-treated 250ml bottles made of PE (black) served as a KKB model.

In the model examinations, this was the treatment method Two-component (2K) CRYO standard process from Messer Griesheim applied, as this provides fluorinated PE inner surfaces, which also compared to modern, methanol-containing fuel mixtures a sufficiently good barrier effect with low Have permeation rates.

A modified version of the CRYO fluorination process, a Type CRYO- "3K" fluorination with 0.1% O₂ content, served one realistically conceivable, increasing the permeation Manufacturing margin during the industrial manufacture of KKB with modeled inner surface to be considered.

Results

• - The ATR spectra of CRYO standard fluorinated inner surfaces clearly differ from those of the CRYO "3K" fluorinated ( Fig. 2).
• - A compilation of the results from the FTIR-ATR measurements and the parallel permeation tests compared to a methanol-containing model-fuel mixture shows that the value of the spectral characteristic for standard CRYO fluorination with an associated low permeation rate corresponds to that of CRYO- "3K" -The value found (approx. Double permeation rate!) With very good delimitation, ie at least 50% higher ( Fig. 3).
• - The quantification of the results of the ATR measurements is included Help of an appropriate one, for each  Sample type characteristic, spectral characteristic (di dimensionless size) happen.

Conclusion

By the measured surface (ATR) spectra of the treated Objects the respective fluorination process according to standardization can be reproducibly assigned and they by the a characteristic distinction is a distinction between target product and committee the fluorine treatment only possible due to the measured ATR spectra.

4.2 Control measurements on KKB:
Using surface measurements according to the FTIR-ATR technique on a number of KKB samples, it was possible to show that the ATR spectra of samples from KKB treated according to the same fluorination process are of reasonable reproducibility after standardization with respect to the corresponding band intensity (s).

Furthermore, depending on whether the fluorination process itself or only certain process parameters have been changed, the normalized surface spectra have more or less pronounced but measurable spectral changes. Accordingly, we obtained the same spectroscopic picture ( Fig. 4) qualitatively when the conditions of the model investigations (cf. section 4.1) were appropriately transferred to the conditions of the KKB production:

Fig. 4

The ATR spectra of the samples from CRYO and 3K (approx. 120 ° C) fluorinated KKB are to each other like those from CRYO STANDARD and CRYO "3K" fluorinated PE bottles.

Claims (6)

1. A method for quality assurance evaluation of the treatment of polyethylene (PE) surfaces with reactive gas, characterized in that it allows z. B. with F₂-containing gas treated inner surface of PE hollow bodies / containers practically relevant any shape and wall thickness, with constant treatment conditions, for the type of treatment and thus the modified surface (OF) characteristic IR absorption spectra, so-called ATR spectra, using techniques of FTIR-ATR spectroscopy, within the framework of a suitable standardization, to reproducibly record which surface (ATR) spectra have adequate quality control, e.g. B. the KKB (plastic fuel tank) manufacturing process before further processing of the manufactured KKB, by performing defined FTIR-ATR measurements on suitable samples from the KKB, directly at the production facility. 2. The method according to claim 1,
characterized,
that the type of sampling both the stress on the measurement samples when punching out of the PE container, for. B. KKB, as well as the squeezing out of PE accompanying substances,
that there are residues of the latter from the sample OF to be measured, or the measuring surface (MF) of the ATR crystal with a suitable solvent, for. B. n-hexane, removed,
that the shape and size of the test pieces ( 4 ) punched out z. B. from the KKB sample (see below) according to a symmetrical arrangement that takes the total inner surface of the sample into account, its flexible but full-surface alignment on both MF of the ATR crystal (KRS5) in a vertical arrangement and thus an optimized close contact between Ensure sample and measuring crystal OF when pressed,
that the test piece for evaluating KKB (see above) is a flat cut-out part, the so-called lid, a waste product that is produced when the KKB's upgrade openings are made. 3. The method according to claims 1 and 2, characterized thereby that the test specimens (MPS) are reproducible in the ATR measuring unit edged, pressed and measured, one for the z. B. KKB sample characteristic so-called single-beam, d. H. Detector spectrum deliver, which is based on a clearly defined, to the respective Sample measurement belonging to measured background (BG) spectrum related and evaluated as an OF absorption spectrum in the further than ATR (OF) spectrum is called. 4. The method according to claims 1 to 3, characterized, that it optimizes deformations of the MF of the KRS5 crystal selected size by mechanical pressing of the appropriate areas, after prior cleaning z. B. with n- Hexane, regresses to the extent that by means of one so directed Crystals from the OF already measured or newly created MPS e.g. B. from a KKB sample, after prior cleaning (see above), the reproducible recording of the ATR (OF) spectrum enables.   5. The method according to claims 1 to 4,
characterized,
that the normalization of the ATR (OF) spectra to the p (CH₂) -IR absorption band of the (CH₂-) n chain of the PE at approx. 725 cm ^-1 , or in the second row to the δ (CH₂) band the same occurs at approx. 1468 cm ^-1 ,
that within the framework of this standardization a qualitative-quantitative comparison of the ATR (OF) spectra with one another is carried out spectroscopically correctly,
that all of these ATR (OF) spectra are the same, within a trust range that is not too broad and justifiable from the point of view of problem solving, until the manufacturing and treatment process, e.g. B. from KKB with constant process parameters,
that significant change (s) in the ATR (OF) spectra, which fall significantly outside their confidence limits, provide evidence that during the e.g. B. KKB manufacturing process quality impairments have occurred,
that the incidents described above can also be expressed numerically by forming a so-called spectral feature, a dimensionless quantity, by defining this spectral feature in the case of fluorine treatment of the PE-OF as follows: integration along the ν (CF) - and p ( CH₂) bands, or the δ (CH₂) band and formation of the arithmetic ratio: (area (F) under ν (CF)): (F under p (CH₂)) corresponds to the main spectral characteristic or spectral characteristic A, or . (Area (F) below ν (CF)): (F below δ (CH₂)) corresponds to the spectral auxiliary characteristic or spectral characteristic B. 6. The method according to claims 1 to 5, characterized, that in the appropriate implementation of the evaluation process in a industrial application environment, e.g. B. in the production of KKB for the automotive industry, with the help of an optimized, Process-compatible and automated FTIR-ATR test stand in the Production hall, the individual KKB evaluation processes <20 minutes take advantage of it and thereby effective quality control on-site KKB production is guaranteed.