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Durability Improvement Underlies Much Coating Innovation

[…] Cook, North American architectural marketing director at Dow Coating Materials (DCM). Current socio-economic, demographic, and regulatory factors are also driving the need for longer product lifetimes, asserts Camilo Quiñones-Rozo, market […]

A Two-Pack Waterborne Polyurethane Topcoat for Military Aircraft

[…] the volume of the RTS mixture. Method B The VOC level calculated according to various regulatory procedures requires removal of water and exempt VOCs from both numerator and denominator according to equation (4) as per ASTM D 3960.18 (4)   where mex is the mass of exempt volatile compounds, and Vw and Vex are the volumes of water and exempt volatile compounds, respectively. For solvent-based products (mw = 0) that do not contain exempt volatile compounds, the VOCs obtained from equations (3) and (4) are identical. Information on the presence and wt% of individual exempt volatile compounds was obtained, where possible, from material safety data sheets for the individual components of the topcoat kits. Method C VOC levels (g/L) of the solvent-based topcoats were determined from measurements of solids (wt%) and density ρRTS (g cm-3) of the RTS mixtures using equation (5). (5)  Results from these measurements were interpreted as being equal to VOCa by equation (3), with mw = 0. Emissions per unit area of paint film VOCs emitted from an RTS paint mixture were calculated as organic compound emissions (OCE) from a specified area of final paint film (gm-2) at a specified DFT (µm) using equation (6). (6)  where VOCRTS is the VOC content in grams of one liter of RTS mixture, DFT is the dry film thickness of the applied coating (µm), and Vsolids is the volume occupied by the solids content of one liter of the RTS mixture (cm3). This gives the organic emissions from 1 m2 of paint film at the specified DFT and ignores emissions arising from overspray due to reduced transfer efficiency in spray application. The calculation may exclude exempt volatile compounds. In some cases, density of the solids content could be obtained from paint manufacturers to enable calculations of Vsolids from the measured wt%. In other cases, estimates of the total volume of volatiles in the RTS mixtures were made from wt% data given in material safety data sheets and the densities for individual volatile compounds, then subtracting from the total RTS volume. RESULTS A summary of the results from testing the waterborne topcoat is presented in Table 2, which shows the tests undertaken, the requirements, and results, and the particular paint specifications for which the testing requirements were referred to. The individual test results, including comparisons with the solvent-based topcoats, are itemized and presented in more detail in the following. In-can Properties Component A: The waterborne Component A was examined after storing for 14 days without agitation. When the can was opened, the product displayed a small amount of syneresis; however, the product could be easily stirred by hand with a spatula to a smooth, homogeneous, and pourable composition. The coating was free of grit and displayed a smooth finish on the Hegman gauge. Component B: Component B was a homogeneous, clear, very pale yellow liquid. Accelerated Storage Stability When stored at 57°C for 24 h, as per MIL-85285D,4 Component B displayed no gassing or pressure build-up, and there was no evidence of gelling or clouding. Storage stability testing in MIL-85285E requires both components to be stored at 60°C for 14 days. In this testing, Component B showed slight yellowing. Syneresis was evident in Component A and a thin layer of soft settlement was evident on the bottom of the can. After stirring to a uniform consistency with a spatula for 30 sec, the two components could be mixed and sprayed over a primed surface to give a coating that cured to produce a uniform, smooth paint film free of defects and with the same overall surface appearance as the control samples. Mixing Components A and B could be easily mixed by hand stirring with a spatula to a uniform consistency. The mixture was thinned with the minimum quantity of DI water needed to adjust the spray viscosity to 30–40 sec as determined by a British Standard B4 viscosity cup. Some care was needed to prevent the formation of bits during stirring, possibly due to some shocking out of the pigments or flatting agents on adding the Component B to Component A. Initial thinning of Component A with a portion of the DI water and continuous stirring by hand while adding the Component B prevented bit formation. Spray Application The mixed waterborne topcoat could be applied to primed surfaces by spray application, and dried and cured under ambient laboratory conditions to produce paint films with a uniform, smooth surface, free from a range of defects often characteristic of waterborne coatings, such as craters, fisheyes, and orange peel. Potlife A waterborne RTS mixture that had been made up and allowed to stand under ambient laboratory conditions for 5 h was spray applied and compared with a freshly made up RTS mixture. Both samples produced coating films that flashed off and dried at the same rate, cured to form films with the same smooth finish, appearance, and color, and had the same solvent resistance and adhesion properties Freeze-thaw Stability After being subjected to five freeze-thaw cycles, the two components of the waterborne coating remained stable. Component A displayed little signs of settling and could be restirred by hand to a uniform consistency. When mixed with Component B and thinned with DI water to spray viscosity, the coating could be spray applied over primed panels to form films that dried and cured to a smooth even finish free of defects and with unchanged color when compared with the control. VOC Data VOC data and emissions are presented in Table 3. The waterborne RTS composition contains a maximum of 100 g/L of organic volatiles. After removing the water and exempt volatiles from the calculation, as per equation (4), the waterborne mixture gives a VOC of 180 g/L. VOCs for the solvent-based topcoats were calculated from measurements of wt% and density using equation (5). As none of the topcoat mixtures A, B, or C contain exempt volatile compounds, the results corresponding to equations (3) and (4) are identical. Topcoat A gave a VOC of 560 g/L as expected for a CS topcoat, while the two HS topcoats gave VOCs conforming to the requirements of MIL-PRF-85285E for Types I and IV. Figure 3 compares the organic compound emissions (OCE50μm) calculated using equation (6) from one square meter of dried paint film at a set DFT of 50 µm, showing that emissions from the waterborne topcoat are reduced by approximately 69% when compared with the two HS products. FIGURE 3—Total volatile emissions from 1 m2 of final paint film at 50 µm thickness. Early Dry Properties The spray-applied waterborne coating flashed off within 10 min, was set-to-touch in 1 h and dry-to-touch in less than 4 h when tested using the methods in ASTM D 1640.19 Both the waterborne topcoat and Topcoat A were print-free at 4 h whereas the HS Topcoats B and C remained tacky, and were noticeably slower to reach a print-free condition. Surface Dry Condition The surface dry condition test specified in DEF(AUST) 9001A requires that a small quantity of sand deposited on the coating, after 5 h under ambient laboratory conditions, will be easily removed with a fine brush. The waterborne coating and Topcoat A passed this test. Topcoat B just passed at 5 h, whereas Topcoat C failed. Tape Resistance After 12 h, the waterborne coating showed no evidence of any permanent marking, imprinting, or other visible defects caused by the masking tape. This contrasted with Topcoat C, which failed this test. Topcoat A passed easily, while Topcoat B passed marginally. Hard-dry Condition After 12 h, the waterborne coating was hard dry and showed no tackiness or any signs of distortion in the rotating thumb test. Topcoat A was also hard dry. Topcoat B was just acceptable, whereas Topcoat C remained tacky. Cure After seven days of cure, the waterborne coating withstood 25 double rubs using cotton wool soaked in methyl ethyl ketone (MEK) without rubbing through to the primer (as specified). In fact, the finish coating withstood 300 MEK double rubs with only very slight marring of the paint film being evident, as was also observed for the solvent-based coatings. Opacity The waterborne coating, formulated as the FED-STD-595 colors 36375 and 35237 gave contrast ratios > 0.99 (requirement: not < 0.95) at a DFT of 50 µm. The L* values measured […]