Thickness measurement paint coatings

Measurement of Film Thickness. An accurate definition of film thickness can only be given if the coating has even upper and lower surfaces and a defined density. In practice, neither the surface of the coating nor that of the substrate is even. Surface irregularities and density variations influence the results of each test method in a specific way. Results of different tests performed on the same sample may therefore differ substantially. Results of film thickness measurements therefore always have to be quoted together with details of the measurement method and instrument used [9.10], [9.11]. A survey of methods used to measure paint thickness is given in ISO 2808. 

The sample techniques just described are designed for collection of transmission (absorption) spectra. This had been the most common type of IR spectroscopy, but it was limited in its applications. There are many types of samples that are not suited to the conventional sample cells and techniques just discussed. Thick, opaque solid samples, paints, coatings, fibers, polymers, aqueous solutions, samples that cannot be destroyed such as artwork or forensic evidence samples, and hot gases from smokestacks—these materials posed problems for the analytical chemist who wanted to obtain an IR absorption spectrum. The use of reflectance techniques provides a nondestructive method for obtaining IR spectral information from materials that are opaque, insoluble, or cannot be placed into conventional sample cells. In addition, IR emission from heated samples can be used to characterize certain types of samples and even measure remote sources such as smokestacks. In reflectance and emission, the FTIR spectrometer system is the same as that for transmission. For reflectance, the sampling accessories are different and in some specialized cases contain an integral detector. The heated sample itself provides the light for emission measurements therefore, there is no need for an IR source. There may be a heated sample holder for laboratory emission measurements. 

Figure 6.6 Thickness measurements obtained through a paint coating using a dual UT probe (adapted from [2]).

For wet film thickness measurement of paints and varnishes (i.e. before drying of the freshly applied coating), it is common to use mechanical wet film thickness gauges, which involve pressing or rolling a calibrated scale onto a wet film. 

To demonstrate the use of such a comparative cost analysis, the production of a panel was analyzed according to different processes (Fig. 9-6). In these case studies the following conditions existed (1) the panels measured 61 x 91 cm (24 x 36 in.) with the wall thickness dictated by the process and part requirements so that the weights of the panels differed (2) production was at a level of 40,000/yr. (3) the plastics for all panels were of the same type, except that different grades had to be used, based on the process requirements, so that costs changed (4) each panel received one coat of paint, except that the structural foam also had a primer coating and (5) costs were allocated as needed to those processes that required trimming and other secondary operations. 

The liquid crystal thermographs method has been used for measuring microtube surface temperature with uncertainties of lower than 0.4 K by Lin and Yang (2007). The average outside diameter micro-tubes was 250 pm and 1,260 pm, respectively. The surface was coated with thermochromic liquid crystal (TLC). The diameters of encapsulated TLC were ranging from 5 to 15 pm. The TLC was painted on the tested tubes surface with thickness of approximately 30 pm. 

Anomalous cases were noted in which this generalization did not hold. These very empirical measurements were followed by more thorough studies (IS). Thin paint films with very low electrical resistance show active corrosion potentials which become more positive as the paint film was increased in thickness. Shapes of the potential/time curves were misleading as a guide to ultimate coating protective properties. 

An epoxy paint for temporary protection of high zinc content 88.3 % relative to dry mass of the coating was investigated on mild steel wire electrodes of 5 mm diameter. The coatings of 27 2 jtim in thickness were studied. The measurements were carried out in 3 % non -- deaerated NaCl solution at room temperature in the frequency range from 1 Hz to 60 kHz using a sine signal of 10 mV amplitude. The measurements were i>erformed in a three-electrode system with the corrosion potential measured vs. the saturated calomel electrode. 

Polymer films of approximately 1000 microns wet film thickness were laid down with a bar applicator on PTFE coated glass panels and the solvent allowed to evaporate at ambient temperature for a standard period of seven days. A typical plot of solvent weight loss with time is shown in Figure 2. The thickness of the wet film was dictated by the need to have adequate mechanical strength in the dry films in order that they might be suitable for subsequent mechanical test procedures. Dry film thicknesses were approximately 300 microns as measured by micrometer. The dried polymer films were examined by dynamic mechanical thermal analysis (DMTA) (Polymer Laboratories Ltd.). Typical DMTA data for a polymer and paint are... 

Paints were prepared from polymers of different composition and composition distribution using a standard copper thiocyanate based formulation similar to that which has been described by Hails and Symonds (11). A rotating disc technique (3) was used to measure the polishing rate (which is a measure of hydrolysis rate) of polymer and paint films. Standard coated panels were attached to a disc (Figure 4) in a radial display and this disc then rotated at a constant speed (1400 rpm) in a thermostatically controlled lank (25°C) of replenished sea water. They hydrolytic stability of the films was assessed by the rate of change of film thickness as measured by a surface profiling technique (Ferranti Surfcom). 

Cosmetic corrosion Surface attack at spots where paint is damaged related to (i) red rust, (ii) paint creep, (iii) chipping factors involved are (i) coating composition and thickness, (ii) surface treatment, (iii) type of paint damage, (iv) type of paint generally evaluated by (i) measuring length of underpaint creep, (ii) extent of rust formation or paint loss... 

Another method for measuring thermal diffusivity is the flash method developed by Parker et al. [48] and successfully used for the thermal diffusivity measurement of solid materials [49]. A high intensity short duration heat pulse is absorbed in the front surface of a thermally insulated sample of a few millimeters thick. The sample is coated with absorbing black paint if the sample is transparent to the heat pulse. The resulting temperature of the rear surface is measured by a thermocouple or infrared detector, as a function of time and is recorded either by an oscilloscope or a computer having a data acquisition system. The thermal diffusivity is calculated from this time-temperature curve and the thickness of the sample. This method is commercialized now, and there are ready made apparatus with sample holders for fluids. There is only one publication on nanofluids with this method. Shaikh et al. [50] measured thermal conductivity of carbon nanoparticle doped PAO oil. 

In order to measure very thin layers of material such as coatings of paint, wax, and plastic films on papers or other material, two thickness gauges are used with a differential coupling so that one detector measures the uncovered and the other the covered or treated portion of the material. Thickness gauges also are used in industry to measure the degree of wear in industrial machinery. For surface measurements of thicknesses 0.8 g cm most thickness gauges use radiation sources with /3-emitters while for thicknesses of 0.8—5 g cm bremsstrahlung radiation sources are most suitable. For even thicker materials y-emitters are used. 

For the infrared measurements, the radiation source consisted of an extended body (1 in. thick 0.002 in. wall honeycomb with J in. cells) coated with a black epoxy paint (CAT-A-LAC Flat Black). The extended radiator surface with this coating should result in a theoretically effective emittance of not less than 0.985. 

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