Coatings World - July 2016

character. 21 Red wine is a veritable cornucopia of chemicals containing alcohol, tannic acid18, and various anthocyanins, again mostly polar species. 22 Cola typically contains phosphoric acid, tannins, caffeine and flavonoids23, most of which are polar materials. Mustard contains vinegar (acetic acid), mustard seeds, and salt, thus making it partially polar. Soybean oil is a triglyceride of long chain fatty acids, and is essentially non-polar in character. Red lipstick contains mostly oils and waxes as well as red dyes which are mostly non-polar materials. 24 The ASTM staining media is comprised of carbon black dispersed in non-polar oils and solvents.

Correlation coefficients are useful tools to determine first if a correlation exists and second to determine whether the correlation is a positive or negative relationship. Table 9 lists ranges of correlation coefficients and the subjective de-scriptors for that particular range.

The descriptive values in Table 9 were used to determine if any correlation exists between the staining agent and contact angle. Table 10 shows the data for water contact angle. The only stain that shows any significant correlation with water contact angle is the ASTM black stain media with a value of 0.61 making it a positive and weak correlation at best.

Correlation coefficients were also calculated between all the contact angle measurements and the subsequently calculated surface energy values to determine if a correlation coefficient is a suitable calculation to determine mathematical relationships between stain repellency and contact angle. The more polar a surface, the greater is its interaction with a polar solvent like water. This translates to a lower contact angle. Conversely, the less polar a surface, lesser is the interaction between it and water, and should lead to a higher contact angle. The data in Table 13 bears this out with a correlation coefficient of -0.89 between increasing water contact angle and decreasing polar component of surface energy. Similarly, dodecane having almost exclusively dispersive energy should exhibit a total lack of correlation with its contact angle and the contact angle of water, which is the case with a correlation coefficient of 0.12.

Table 7: Summarized goniometer and surface free energy data
Property Number
of Data
Points
Average Minimum Maximum Range

Water contact angle (°) 171 96. 39 26. 55 139.47 112. 92

Dodecane contact angle (°)

158 56. 30 11. 54 99. 56 88.02

Diiodomethane contact angle (°)

13 63. 50 47.01 81. 77 34. 76

Dispersive energy (mN/m)

171 16. 32 4. 42 35. 92 31. 50

Polar energy (mN/m) 171 6. 59 0.00 52. 43 52. 43

Total surface energy
(mN/m)
171 22. 90 4. 44 66.08 61. 64

Table 8: Summarized stain repellency data
Property Number
of Data
Points
Average Minimum Maximum Range

Ketchup 171 1.94 0.05 9. 38 9. 33

Red wine 171 8. 76 0.51 26. 67 26. 16

Mustard 171 2.55 0.21 11. 74 11. 53

Soybean oil 171 1.54 0.03 8. 74 8. 71

Soy sauce 171 2.03 0.01 9. 99 9. 98

Coffee 171 2.52 0.19 11. 83 11. 64

Cola 171 1.68 0.06 9. 90 9. 84

Red lipstick 171 9. 93 0.20 34. 78 34. 58

ASTM black stain media

171 20. 99 3. 50 39. 91 36. 41

Total 171 53. 66 8.06 142.61 134.55
Table 9: Correlation coefficient value descriptions
Value(s) Description

-1.00 Perfect negative relationship –
as one value increases the other decreases
-0.80 to -0.99 Negative correlation
-0.60 to -0.79 Weakly negative correlation
-0.01 to -0.59 No correlation