Blumenthal, J. L., Yobe, K., Ind. Eng. Chem. Process Des. Develop., 5 , 177-83(1966). Cannon, W. A,, Welling, C. E., SAE Paper 29T, Annual Meeting, Detroit, ?\Iich.,Januarv 16, 1959. Jagel, K. I., Dwyer, F. G., SAE Paper 710290, Int. Auto. Eng. Congr., Detroit, Mich., January 1971. Sorensen, L. L. C., Sobe, K., Envaron. Sci. T e c h : / . , 6 , 239
Sourirajan, S.,Accomazzo, 11.A, Nobe, K., Actes Congr. Int. Catal., 2nd, Paris, 1960, 2497-519 (1961). Weaver, E. E., S.2E Paper 690016, Int. Auto. Eng. Congr., Detroit, Mich.jJanuary 1969. Yarrington, R.lI.,Bambrick, W. E., J . i l z r Pollut. Contr. Ass., 20,398-401 (1970).
ilRf2’i. -,
RECEIVED for review February 28, 1972 A C C E P T E D July 17, 1972
\--
Sourirajan, S.,Blumenthal, J. L., Acta Congr. Znt. Catal., 2nd, Paris, 1960,2521-40 (1961a). Souriraian, S., Blumenthal. J. L.. Int. J . Air TVafrr Pollut.. 5,24-33 (196ib).
This work was supported by Grant KO. AP00913, Air Pollution Control Officcl, Environmental Protection Agency.
Quantitative Evaluation of Leveling by Surface Waviness Measurements Charles M. Hansen PPG Industries, Inc., Springdale, Pa.15144
A method of evaluating flow in coatings by use of the waviness surface profile rather than center line average is proposed. The waviness gives an indication of longer range leveling phenomena, whereas the CLA indicates shorter range roughness which correlates well with gloss. Some of the problems associated with surface measurements of this kind are discussed.
S u r f a c e features have long been of interest to the coatings chemist. Recent work relating coatings performance to surface features has centered around use of surface profiles. These can be determined with commercially available instruments, and such information as center line average roughness (CLA), waviness, and maximum peak-to-valley distances can be obtained in addition to the true profile. The CLX used in this report is that defined by the specification -1S-l B46.11962 (ASME). The instrument used was the Surfanalyzer 150 system having a dual-chart recorder for plotting conibinations of profile (or waviness), CL.1, and roughness. Surface data of this type have been applied to various coatings situations including mar resistance (Taylor and Foster, 1971)) the effect of polishing compounds (Taylor and Foster, 1971), evaluation of leveling (Dodge, 1972), and even as a primary indication of surface smoothness in the automotive industry (Taylor and Foster, 19il). The author has previously (Hansen, 1972a) described studies of the effect of substrate roughness on gloss, the degree of filling of various substrate irregularities, and a reinterpretation of dirt retention phenomena emphasizing wettability rather than surface roughness (Hansen, 19i2a,b). A number of profiles of commonly encountered substrates and coatings were also presented for informational purposes. Zorll (1970) has measured profiles during drying. The profiles of brush marks and the brush have also been reported (Smith et al., 1961). This paper continues the list of applications possible for surface profile measurements by showing the leveling problem in a perspective not reported before and by presenting a correlation of gloss with renter line average. In addition, the roughness of some additional substrates is reported to demonstrate problems associated with variations in sampling. 426 Ind.
Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
Leveling Studies
Dodge (19i2) has reported studies Iyhere degrees of leveling were determined by use of the CLli of the films applied with a special drawdown bar. The CLA is calculated as the average of the peak heights shown in Figure 1. The import of Dodge’s work was that the CL.1 reported mas a result of the irregularities left because of incomplete leveling. A better measure of leveling is the residual waviness remaining after application with a spiral or other drawdow-n bar, where regular peaks and ralleys are imposed on the wet film initially. To measure waviness (a long-range phenomenon), one filters out the short-range roughness owing primarily to pigmentation. &iswill be seen below, this shortterm roughness leads to a CL*1 Jvhich correlates well with gloss. The difference between the waviness and the true profile for a flat wall paint can be seen in Figure 2. Curve A shows the true profile, and curve B s h o w the waviness, which is barely discernible in curve A, in a clear fashion. The difference between the two is that the short-term roughness owing to the pigmentation has been damped out, showing the waviness profile as a trace of the center line about which the CLA is calculated. This drawdown \vas made with a No. 32 spiraln-ound drawdoan bar. The easiest method of comparing the leveling of one paint with another is to take an average peakto-valley distance o n the waviness profile. One must run the surface trace perpendicular to the drawdown direction, of course. We have used a number 70 spiral-wound drawdown bar in all our practical evaluations. This drawbar has 14 turns/in. with a maximum clearance of 25 mils. .It least 10 peak-bo-valley differences were measured to obtain an average for evaluating the leveling of a film. Comparisons of the leveling. measured quantitatively as
Figure 4. Surface features relalive to total film thickness for a typical flat wall paint Figure 1. CLA or AA average distances from center line
C L h . MICROINCHES
Figure 2. True profile (A) and waviness of a flat wall paint (not a t the exact same place)
Figure 5. Correlation of gloss with center line average
Table
II. Gloss and CLA Roughness for a Variety of Trade Sales Products
Figure 3. True profile of flat wall paint applied with large amplitude irregularities
Table 1. l e v e l i n g Data for Experimental
A) Highgloss B) Alkyd semigloss C) Latex semigloss D) Latex flat E) Latex eggshell F) Latex flat (tint base)
60' Gloss, %
850 Gloss, %
79 23
72 54 45 3
18
4 4 3
CLA mughnerr, pin.
2 4.5
16
10 36 20
5
26
High-Gloss l a t e x Paints Critical
Peok-to-valley hint
A
B C D E Control
"iSU.1
wovinesi,
eva1uotiC.n
pin.
of drawdowns
31.4 42.4 43.2 t5 16.4 194.8
+
Good Good Good Excellent Very good Fair to good
described above with painter qualitative evaluations have shown good agreement. Ten flat wall paints were segregated into two clear groups-five with waviness peak-to-valley distances between 300 and 850 pin, (O.SO.8 mils) and five between about 100 and 200 pin. (0.1-0.2 mils). More specific quantitative data were found in some experimental high-gloss (80-88% a t 60') latex paints (Table I). Visually, the control was an obvious worst, and A, B, and C could be classified as comparable to but not as good a s D and E. The difference between D and E could not be readily assessed visually but was quite clear instrumentally. These high-gloss films were applied to give 2.4 ?C 0.1 mils dry film thickness. To fully appreciate the difference between the CLA and waviness, this same paint was drawn down with much larger initial irregularities. Figure 3 shows the true profile of this drawdown. The average peak-to-valley distance is about 3 mils vs. about 0.05 mils from the waviness in curve A of Figure 2. CLA's are 60 and 48 pin., respectively (0.06 and
0.048 mils, respectively), Le., not really different in spite of gross differences in leveling behavior. The order of magnitude of rurface irregularities relative to the entire film can be seen in Figure 4. In this case, a flat wall paint having rather average leveling properties was applied to a glass substrate, and the scratch obvious in the figure was made with a razor blade right down to the glass substrate. A number 70 spiral-wound drawdown bar was used. The characteristic averaged numbers for this situation were film thickness of 2.36 mils, CLA of 60pin. (0.06 mils), and peakto-vall-y waviness of 294 pin. (0.294 mils). These were fairly representative of brushed-out samples as well. Gloss
The CLA is a short-range roughness feature related to pigmentation as can be seen from the data in Table I1 and Figure 5. The true profiles of these paints were reported earlier, and they span the entire range of a Trade Sales product line (Hansen, 1972a). One can question the practical value of such a correlation since gloss is an easily accessible and standardized measurement, but for present purposes, that it exists confirms the need to use the waviness profile to correlate with leveling. I n special cases of high gloss, the CLA can also be used to advantage (Taylor and Foster, 1971). Sampling
Sampling or selection of a site for a surface trace can be the major source of error in measurements of the type described above. For example, if the waviness were run parallel to the Ind. Eng. Chem. Prod. Res. Develop., Vd. 11, No. 4, 1972 427
nroduct (Hansen. 1972a), may" cause surface water to be retained more than had it been smaNother, with a subsequent effect on corrosion resistance. GlossI Ud l also he lower.
Figure 6. Roughness of Metabond 42 (International Rustproof Co.), zinc phosphate on galvanized steel: A, polished side; B, unpolished side
drawdown direction rather than perpendicular, no leveling profile would he found. Many substrates have differences in surface profile depending on direction of trace and even as to which side of a given panel is run in the case of test panels where one side is polished. The CLA is also a cumulative number (with a cutoff of 0.03 in.) which continually changes for nonuniform substrates, such that an average of the CLA is often required. Therefore, sampling and interpretation of results must be done with added care, also because a trace is just a thin line across what is generally a large surface. Peak-to-valley distances on the profile rather than on the waviness can also he important. Examples of difference in behavior typical of those described above are shown in Figure 6. The greater peak-to-valley differences on the unpolished side of the Metabond 42 panel are not really reflected by the CLA values (in a manner analogous to Figures 2 and 3). The rougher contour which will result in the final coated
coatings based on the waviness profile. The waviness gives an indication of longer range leveling phenomena, whereas the CLA indicates shorter range roughness which correlates with gloss. Sampling problems can be significant in choosing the proper place to make a measurement. Variations in the traces of coatings resulting from variations on the substrates on which they are applied,
