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When the color doesn´t match

Color Masterbatches. The exact dosing of masterbatches can be quickly and effectively measured in colored parts using x-ray fluorescence analysis. The process requires only a few minutes, is non-destructive and complements existing methods used to check dosing.

 

When coloring and modifying polymers with additives the best results can be obtained with masterbatches. Alongside the pre-dispersion of the pigments necessary for coloration (Title picture, Fig. 1) a multitude of functional additives are required in order to tailor the thermoplastics being processed to the application. The experience and skills of colorists and additive specialists, supported by the use of modern measurement instruments, as well as the progress of development and application testing at pigment and additive manufacturers mean that in many cases it is possible to almost completely meet customer requirements.

 

In spite of global crises and the signs of raw material shortages the application areas for thermoplastics continue to grow. As well as having to expand production capacities masterbatch manufacturers are also seeing quality requirements increasing. The Grafe Group, Blankenhain, Germany, is reflecting this challenge through the use of modern measurement techniques backed by highly qualified personnel and continuous professional development programs.

 

Seeking the Sources of Errors

Masterbatches contain all the necessary components to achieve the specified properties of the end product. Incorrect masterbatch dosing is amongst the most common sources of error leading to quality deviations and is often investigated as part of processing complaints. Other errors result from quality deviations in the base resin formulation or insufficient dispersion of masterbatches. The use of suitable dosing technology, online monitoring and precise adherement to the dosing recommendation of the masterbatch manufacturer make it possible to avoid these kinds of errors. Deviations in the color tone are the first thing to be noticed and can generally be adequately quantified by color measurement instruments. Changes in other properties are often only noticed if they are included in the quality control system for the manufacturing process or if incorrect behavior is noticed during service.

 

Checking Dosage

In order to ensure uniform quality, suitable batch based test methods have been developed by the Grafe Group. As well as looking for deviations in raw material quality the testing for the actual dosage of masterbatch forms a major part of the investigation into complaints or other notifications of deficiencies. A tried and trusted method is the production of test plaques with both the recommended dosage and the suspected deviation from this. Colormetric comparison to the suspect product can often provide a useful basis for identifying the cause of the problem. Another route is the quantitative determination of the concentration of the contents via analysis of the inorganic and organic ingredients. Techniques such as thermogravimetry, high performance liquid chromatography, infrared and UV spectrometry or elemental analysis can be used for this.

 

Determination of the Ash Content

Via a determination of the ash content, for example according to DIN EN ISO 1172 [1], it is possible to establish the aggregate concentration of the inorganic pigments, additives and fillers. Many inorganic pigments such as titanium dioxide, iron oxide or many mixed oxides and spinels are already present as oxides. Other pigments such as zinc sulfide are converted to oxides during the calcination in the muffle furnace. With reference to the masterbatch formulation it is generally possible to calculate the expected ash content of the component part at the correct dosage. A parallel ashing of the masterbatch sample and the base resin formulation used by the manufacturer improves the accuracy of this. Normally the ash content can be determined by calcination within 1 to 2 h. By using a muffle furnace the whole process can be further optimized through parallel calcination of multiple samples. Problems can however occur if the filler content of the thermoplastic and the expected ash content of the masterbatch are similar or if the quantity of ash being measured is small.

 

Analysis of the Elements

In contrast to the aggregate parameter of ash content, with x-ray fluorescence analysis (XRF) the concentration of the inorganic elements present is determined. Knowledge of the chemical formulation allows an allocation of the elements recorded to the individual components such as pigments, additives or fillers within the masterbatch or base resin. However, it is not absolutely necessary to know the exact chemical composition in order to determine the dosage. In practice a dilution series of test plaques is manufactured as calibration standards. Alongside the recommended or suspected dosage these standards should cover a larger range of dosages. If a reasonable dependence of the analytically determinable elements on the dosing of the masterbatch can be shown then these can be used to calibrate an x-ray fluorescence spectroscopic measurement process.

Figure 2 shows from the analysis of the elements titanium (Ti), iron (Fe), antimony (Sb) and chromium (Cr) how the pigmentation of an automobile part (Fig. 3) changes with the dosage of masterbatch. The coefficient of determination R2 > 0.95 means that the elements titanium, iron and antimony are suitable for calibrating the dosage against the elemental concentration. In contrast the use of chromium at a coefficient of determination R2 = 0.85 is not to be recommended as a calibration standard. Looking at the x-ray spectrum (Fig. 4) it can be seen that the analytically determinable chromium content is close to the limit of detection.

Based on these calibration values the addition rates determined via XRF lay between 6.30 and 6.70%. More specifically the individual values for the elements were: Sb = 6.41%, Fe = 6.30% and Ti = 6.70%. This yields an average value of 6.47±0.21%. The value determined via ashing was 6.52%+0.07%. Thus the experimentally determined addition rates in this case agreed quite well with the recommended masterbatch dosing of 6.5%.

A suitable instrument for elemental analysis of plastics is the x-ray spectrometer XL3T from Thermo Niton Analyzers LLC, which is marketed in Germany and Austria by AnalytiCON-Instruments GmbH. The Niton XL3T is a hand held x-ray spectrometer [2] and has a miniature tube (2W / 50kV) as well as an innovative silicon drift detector (SDD). Although it was developed for mobile use the current version delivers performance that only a few years ago would have required a bench mounted unit. In the configuration used a multitude of elements such as titanium (Ti), iron (Fe), copper (Cu), zinc (Zn) or chlorine (Cl) can be quantitatively determined in plastics mode as can the elements covered by the toys directive [3] or those falling under RoHS (Restriction of Hazardous Substances) [4], i.e. cadmium (Cd), lead (Pb), chromium (Cr), mercury (Hg) and bromine (Br). Pre-existing calibrations loaded into the software allow the immediate use of this instrument for quantitative analysis.

Figure 5 shows the mobile use of the x-ray spectrometer. With a sample chamber and connection to a PC the instrument can also be used as a typical laboratory measurement device (Fig. 6).

 

Conclusion

Measurement of dosing using an XL3T XRF spectrometer complements the classic methods of investigating the dosage of masterbatch in finished products used up until now. Advantages compared to other methods can in particular be seen in the short measurement time of 1 to 2 minutes per sample and the fact that sample testing is non-destructive. In order to determine the dosing, calibration against suitable standards is necessary. This can make use of existing procedures such as the production of test plaques or specimen parts. The method presented here allows quantitative determination of the dosing. It can be used in cases where a characterization using colormetric techniques is not unambiguous or a high level of fillers in the product to be colored limits a calculation via ash content. Calibrations prepared once can be used in later investigations, which drastically reduces the actual processing time in the case of repeat testing.

 

References

1 DIN EN ISO 1172: Textilglasverstärkte Kunststoffe- Prepregs, Formmassen und Laminate-Bestimmung des Textilglas- und Mineralfüllstoffgehaltes; Kalzinierungsverfahren (ISO 1172:1996), German Version of EN ISO 1172-1998

2 www.analyticjournal.de/neue_apll_mains_fluess/analyticon_niton_xl3t_rfa_mobil.html

3 Directive 2009/48/EG of the European Parliament and Council of June 2009 on the safety of toys

4 Directive 2002/95/EG of the European Parliament and Council of January 27, 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment

 

The Author

Dr. Frank Claussen, born in 1959, is head of the analytical laboratory at Grafe Color Batch GmbH, Blankenhain, Germany.

 

(c) Kunststoffe international 04/2012

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