Polymers and Plastics

FISCHER offers special measurement instruments with sensitive detectors. Quickly and precisely test composition and properties such as surface hardness or elasticity of polymers and complex plastics. You will also find the appropriate measurement technology for the quality control of insulating varnish.

Polymers and Plastics

Application notes

Hardness of Complex Coating Systems for Optical Components

The demands placed on the performance of optical components have skyrocketed and, in response, highly complex coating systems have been developed to produce surfaces that are scratch-resistant, dirt-repellent, anti-static and reflective. Various curing processes are integral to the production of optical coatings, making it difficult but important to find the decisive balance between coating hardness and elasticity.

Quality control therefore requires correspondingly powerful measurement methods and systems. For the standard-compliant determination of such material parameters as hardness and elastic modulus the instrumented indentation test can be used, even thin coatings of less than 100 nanometres in thickness can be measured accurately.

With the load/indentation depth method according to DIN EN ISO 14577 and ASTM E 2546, the indenter, typically a Vickers or Berkovich pyramid is pressed with continuously increasing test load into the material and then reduced in the same manner while simultaneously measuring the respective indentation depths. Important technological characteristics can be calculated from the resultant load/unload cycle, for example the Martens hardness. The elastic modulus of indentation can be determined when the test load is reduced. 

Fig. 1: Depth-dependent profile of the Martens hardness (HM) of two differently composed optical coatings. Marked in blue is the area where already an influence from the base material is given according to Bückle’s rule.

The figure 1 presents the measurement of Martens hardness and the associated standard deviation on two plastic lenses, samples courtesy of Rodenstock GmbH, Munich. The samples were produced under the same process conditions but exhibit differences in the composition of the coating system. As result a significant change of the hardness from one coating to the other can be seen.

At a certain indentation depth, the substrate material starts to become detectible. In order to avoid that influence while measuring the coating, the indentation depth must be limited to no more than 1/10 of the coating thickness (Bückle's Rule). The coefficients of variation for the two samples, 1.73 and 1.60 percent, respectively, as achieved using the FISCHER PICODENTOR HM500, demonstrates the potential for accuracy.

Fig. 2: The principle of instrumented indentation test: a designates the load increase, b the load decrease.

Although only the Martens hardness can be measured depth-dependent using standard methods, additional mechanical properties such as the Vickers hardness or the elastic modulus of indentation can be determined via the ESP (Enhanced Stiffness Procedure) method, which employs partial loading and unloading.

Conclusion: If the right balance between coating hardness and elasticity for coatings on optical components has to be determined the FISCHER PICODENTOR® HM500 is the suitable instrument to evaluate these parameters. For further consultancy please contact your local FISCHER representative.

Measurement of Cr/Ni/Cu coatings on plastic substrates

Bathroom fittings are commonly finished with a decorative chromium plating. But what may appear to be a solid metal shower head, for example, is often just a multi-layered metal coating on top of a plastic substrate. To guarantee that the shower head not only look pretty when delivered but even after many years of usage, the thickness of each individual layer must be controlled to ensure quality.

The typical composition of such shower heads is a chrome/nickel/copper coating system on top of a plastic substrate material. The decorative chromium outer finish is usually only 0.5µm thick (or less) and the nickel layer about 5-10µm. If the copper layer is between 20-25µm, making the overall coating thickness no more than 30µm, non-destructive measurement using the x-ray fluorescence (XRF) method is possible.

For this kind of application, x-ray fluorescence instruments with a proportional counter tube are perfectly suited. Even with small measurement spots, sufficiently high count rates can be obtained due to the large detector area, ensuring good repeatability precision. Because of the large, easily accessible measurement chamber, the robust instruments of the FISCHERSCOPE® X-RAY XDL® family are well suited for large specimens with complex shapes.

To maximize the precision of the results, proper positioning of the object is essential, for example by choosing an intrinsically horizontal area or correctly aligning the sample. To assist in this crucial step, FISCHERSCOPE® X-RAY measurement systems are equipped with a laser pointer as positioning aid and high-magnification camera optics. Using the video image generated by the WinFTM® software, the required exact focusing of the measurement spot can be achieved.

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Table 1: Typical results of an x-ray measurement, collected using a FISCHERSCOPE® X-RAY XDLM® with a measuring time of 30 seconds for four measurement cycles per spot

For determining the thickness of decorative Cr/Ni/Cu platings on plastic substrates with a maximum overall coating thickness of approximately 30µm, the cost-effective proportional counter tube measure­ment systems of the FISCHERSCOPE® X-RAY XDL® family are the optimal solution. To measure thicker coatings, instruments employing the (destructive) Coulometric method are also available as an alter­native. For more information please contact your local FISCHER representative.

Mechanical characteristics of conformal coatings

In the electronics industry, two-component conformal coatings are often used to minimize current leakage on PCBs and as protection against humidity and other environmental stressors. Because the exact composition of the polymer determines its final mechanical properties, quality control using a reliable measurement technology is mandatory.

The conformal coatings used on PCBs often consist of two components: an alcohol and an isocyanate group. For production the dosage is calculated stoichio-metrically such that a hydroxyl group of the alcohol forms a bond, or cross-link, with an isocyanate group. If there is an excess of alcohol (called “under cross-linking”), the cured polymer is not as hard and can become hygroscopic; it can also grow sticky, which causes problems further down the assembly line. If there is an excess of isocyanate (called “over cross-linking”), it can lead to reactions with humidity from the air, which generates CO2, causing bubble build-up within the lacquer.

To ensure that such problems do not develop over time, it is important to test that the composition of the conformal coating is correct. With the instrumented indentation method, the quality of the polymer can be quickly determined immediately after curing. The measurement results are not influenced by the substrate material and sample preparation is minimal. Beside the plastic and elastic deformation measurement (hardness), other parameters can also be determined, such as creep.

For technical applications the so-called “cross-link density” of the polymer is taken under consideration; Figures 2a and 2b show the results of the hardness measurement for five conformal coating polymers of different composition, as measured using the FISCHERSCOPE® HM2000. Figure 2a shows the Martens hardness over depth. The Martens hardness changes drastically depending on the cross-link density and is therefore a good indicator for the composition of the lacquer. The creep at maximum force, shown in Figure 2b, is related to the brittleness of the material and indicates an excess of isocyanate.

Fig.1a: Martens hardness (HM) of differently cross-linked polymers

Fig.1b: Creep (constant force at maximum force level) as indicator for the proportion of isocyanate

Using the instrumented indentation method, the FISCHERSCOPE® HM2000 is the optimal choice to determine the quality of two-component conformal coatings on PCBs. For further information please contact your local FISCHER representative.

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