Test inks

Low surface tension (poor wettability ) may be a factor, with apparent repulsion of the paint and prevention of its rapid, even spread over the whole surface test inks of different values are available so that wettability can be assessed in advance with samples of the items concerned for satisfactory use with paints based on organic solvents, surface values of greater than 50 mN m 1 are desirable, and for water-based systems values greater than 70 mN m. ... 

Most organic residues on assemblies are not detected by ionic contamination meters. However, such residues do change the surface tension of assemblies, thereby influencing the adhesion of subsequent coatings. This method determines the surface tension by iterative use of corresponding testing inks. 

Why Test Inks Cannot Tell the Whole Truth About Surface Free Energy of Solids... 

If information about a solid surface free energy (SFE) is needed, contact angle measurements and ink tests are two of the most frequently used methods. Here we present a comparative study of contact angle measurements and ink tests on 13 different materials. We observed major differences in the SFE values obtained by these two techniques and explained the differences on the basis of basic theoretical concepts of both methods. We found that test inks fail to monitor the efficiency of atmospheric plasma treatments on low surface energy solids. Moreover, we determined the polar and dispersion contributions to the test inks total surface tension (ST) in order to provide a more detailed understanding of these methods to determine a solid SFE. 

Keywords Surface free energy, test inks, dyne pens, plasma treatment, adhesion, contact angle... 

ST interact with the different parts of the soHd SFE. However, almost all of these make the same basic assumption that one specific part of the liquid ST solely and exclusively interacts with the corresponding part of the sohd SFE. Thus, for the here used most basic distribution into dispersion and polar parts of SFE and ST, a simple rule of thumb describing the IFT is found when a liquid gets in contact with a solid, the polar part of the liquid interacts only with the polar part of the solid and the dispersion part only with the dispersion one. Later in the text, it will be shown that this most basic assumption / model already helps to explain the major differences between test inks and contact angle measurements. 

The ink test is based on the assumption that the SFE of the sohd is equal to the ST of the hquid which just fully wets the solid. Test inks consist of a series of liquid mixtures, each with a set ST, usually in increments of 2 mN/m. During the test, one of the inks is apphed to the sample with a brush stroke. If the applied film of the ink contracts, the ink with the next lower ST is used xmtil the brush stroke produces a stable film on the soHd. This corresponds to complete wetting of the sample. If the first stroke is stable, the ink with the next higher ST is applied xmtil the brush stroke does not produce a stable film. The ST of the test ink which just forms a stable film is equated to the SFE of the material. 

We carried out tests with two different series of commercially available test inks for all samples ... 

Sample SFE by OWRK Method Using Measured Contact Angles [mN/m, Polar Component in %] Test ink A [mN/m] Test inkB [mN/m]... 

The results for PE and PP corresponded well. With all other samples, in many cases there were considerable differences between the contact angle and test ink results, and also sometimes differences between the test inks. 

It can be seen that for samples with a non-polar low SFE, i.e. PE and PP, the results obtained from test inks and contact angles correspond well. Deviations occur with higher surface free energy, polar samples. This can be explained by the fact that the ink test ignores a variable which is important for wetting, namely the IFT 0,5 (see Figure 17.2). According to the ink... 

The difference between SFE values using contact angle measurements and test inks was most obvious in the case of plasma-treated samples. Tables 17.3a), b) and c) show the results for PDMS, PVC and PET before and after plasma treatment for different exposure times. We have documented the wetting patterns, i.e. images of brush strokes, for the test inks. 

The test inks do not reflect the increase in SFE for PDMS and PVC. With these two plastics, the test inks are, to a certain extent, blind to the effect of treatment. 

Figure 17.5 summarizes the mean contact angles of the test inks as determined from ten different sessile drops on the PDMS substrates and the deduced distribution into polar and dispersion components of the total ST for each test ink. 

Figure 17.5 Surface tension values and equilibrium static contact angles of the blue test inks (series B, formamide based) on PDMS. The distribution into polar and dispersion parts was calculated from the contact angle data applying the OWRK approach.

It is now simple to explain why only for PP and PE samples the contact angle method and the test ink method give comparable results. These two materials are totally non-polar low SFE (<32mN/m) solids. And the test inks with such low ST are also almost completely non-polar. Thus, for these materials the prerequisite of the test ink concept that the IFT between solid and ink vanishes is fulfilled. As a result, the ink having the same ST as the solid SFE completely wets the sohd. However, these almost non-polar test inks are blind to monitor the effect of a plasma treatment on these particular solids, as a plasma treatment adds a polar part to the SFE keeping the dispersion part (almost) unchanged. This was observed, for example, in our measurements on plasma treated PDMS (see Table 17.3a). 

Some test inks, in particular the range obtained in accordance with ISO 8296, contain toxic liquids formamide and EGMM. As a result, the health of test personnel can be endangered during routine quality assurance. Contact angle measurements can be carried out with harmless liquids or with liquids which are less hazardous to health. In addition, significantly smaller amounts of liquids are required for the measurement than with the ink test. 

Test inks made on ethanol basis, such as test inks A in our study, are also available as less harmful substitutes. In this case, however, the volatile alcohol content can partially evaporate, so that the composition of the ink changes if bottles are left open or are opened frequently. For the same reason, the wetting behavior can change when the ink is applied. 

The risk of contaminating the whole ink supply in a test ink bottle after a measurement on a dirty sample is rather high. As a result the total ST of the test ink can change and thus falsify further measurements. This cannot happen to the test liquids used for contact angle measurements as the test liquid reservoir never gets in direct contact with the samples. 

Before the development of fast and manageable contact angle measuring instruments, test inks offered advantages for testing surfaces directly... 

We have determined the polar and dispersion parts for one type of test inks by means of contact angle measurements on a totally non-polar reference surface by applying the OWRK model. The degree of polarity of both series of inks increases with increasing total ST whereby the inks with the lowest ST are almost completely non-polar. With that we illustrated that the basic prerequisite for the ink test approach, i.e. that the IFT between ink and solid vanishes, is only fulfilled in rare cases. 

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