Physical Evaluation of Finishes A. E. SCHUHAND H. C. THEUERER, Bell Telephone Laboratories, Inc., New York, N. Y. physical b e h a v i o r during the N A PREVIOUS paper (I), W h e n tests measuring abrasion and impact period of their serviceable conSchuh described a number resistance and distensibility areperformedperiodidition, the authors report a study of new physical tests suitcally on properly prepared finish panels which carried on for the past 2 years able for the evaluation of the are aged under controlled environments, a new on automotive vehicle finishes. durability of organic finish coatand highly useful tool is provided for the paint This study i n c l u d e d a w i d e ings. Based on the concept that variety of finishing materials in the aging of all these coatings technologist. the nitrocellulose lacquer, older is accompanied by similar types Whereas in the customary method of exposure type of oleoresinous, and modern of measurable physical changes, testing, the length of time required to produce synthetic fields. Not only were it was pointed out that a provisible failure must be taken as the chief index of the individual components, such gram of testing might be devised as primers, surfacers, and color quality of a finish, with this newer testing prowhich would allow the prognostienamels, examined b y t h e m cation of the life of a finish in a cedure the entire life of a finish can be followed. selves with regard to their physimanner that is less time-conThe weaknesses that presage failure m a y be decal behavior, but the effect of suming and more reliable than tected. Reliable selection of improved materials combining these materials into the customary methods of exis expedited because failure does not have to be the recommended finish systems posure and simulated s e r v i c e awaited before such selection is made. It is also was studied. tests. These i d e a s h a v e now The physical aging of such b e e n a p p l i e d for 3 years and possible to obtain a clearer insight into the relasystems was followed under two their satisfactory use has led to tive intensity of aging factors found in the ensets of controlled environmental several definite convictions. v ironment . conditions, one set being mild The c o n s u m e r of finishing and extending over a period of 14 materials is more i n t e r e s t e d in the health of a finish than in the manner of its death. I n months and the other drastic and lasting only 3 months. Corthe customary methods of exposing finishes, the paint tech- relative data on outdoor panel exposures both in Florida and nologist is forced to act too much as the coroner and too little New York and on actual service tests on trucks in both Florida as the diagnostician. It has now been definitely learned and northern areas have been furnished.1 I n addition, the that by the time visible signs of failure make their appearance, effect of building up systems with and without thick surfacquestion as to whether the finish has finally failed by ing materials has been investigated. In the course of this study various shortcomings of the checking, cracking, or scaling becomes really a matter of quibbling. If the health and strength of a finish could be tests were revealed, and the need for certain future studies measured and followed in its youth and maturity, serious that will be of interest to the paint technologist was clearly organic defects could be spotted and in the hands of the evidenced. intelligent formulator immediately corrected. DESCRIPTION OF TESTS Not only does exposure testing usually yield too little infolmation for the effort expended, but it is permanently In a general qualitative manner, a good finish is one which limited to a relative basis of comparison. This would not satisfies appearance requirements, is tough, and adheres well be such a serious drawback provided some “normal” ex- to its base. Not only should these requirements be met posure condition for purpose of reference could be found. initially, but ideally, no change should occur with age. It Obviously environment plays as I arge a part in determining is the qualitative and yet very apt term “toughness” which the durability of coatings as composition. On what basis then is most in need of quantitative clarification. Hardness, shall a normal exposure condition for purpose of reference be flexibility, impact, and abrasion resistance are all attributes selected? The variables of temperature, humidity, sunshine, of toughness, all relating to the deformability of finish coatfrequency of change, etc., are all beyond control and each ings under different stresses. There is still considerable affects a finish differently. groping in trying to unravel the interrelations of the various Considerable effort continues to be devoted to the develop- rheologic properties measured in these tests. However, cerment of synthetic weathering conditions in which the factors tain broad relations are becoming apparent. In the meanof environment are controlled and intensified to bring about time the tests used in this study do provide quantitative acceleration of the degradation of finishes. After many measurements which are significant and reproducible, and years, it is now being realized that the search for a universal in the future fewer and less ambiguous tests may be found accelerated weathering cycle is futile. Even in cycles which adequate. appear to be reasonably satisfactory for certain types of A brief description of the tests used in this study should finishes, comparisons in order to be safe cannot be made too aid in the discussion of the results recorded in the various general and must be limited to gross differences. The chief tables. The wear test is exactly as reported in previous value of this accelerated exposure work appears to lie in the papers (1, 2 ) and consists essentially of determining the fact that considerable light has been shed on the relation be- number of grams of 180-mesh Carborundum required to tween intensity of environmental factors and the life period of abrade through unit thickness of coating. Since the materials finishes. Benefiting by this information the authors have were applied by means of a spray gun, uniformity of thicksought to follow the effect of these aging factors quantita1 For the data on outdoor exposures in Florida and New York and the tively during the complete life of the finish. data on actual service life of truck finishes, the authors gratefully aoknonlI n this paper, for the purpose of illustrating what can be edge their indebtedness to F. F. Farnsworth and L. H. Campbell of thm learned regarding the durability of finishes by analysis of their laboratory
I
91
92
A N A L Y T I C A L E D I T IO N
ness was not as good as might be desired. Hence, in addition to wear coefficients-that is, the number of grams of Carborundum required to wear through 2.54 X em. (0.001 inch) of coating-the actual thickness of the priming and top coats is given, as determined by means of a calibrated microscope a t 600 diameters. A short clean scratch was made through the coatings by means of a razor blade held a t 45". The microscope was focused first on the top plane of the cut, then at the boundary between primer and enamel coats, and finally at the interface between primer and base metal surface. The sharp color difference between enamels, primers, and metal surface made the location of these interfaces a simple problem. The test device for determining impact resistance has been modified. It was felt that the determination of the force level a t which a finish shatters is of more significance than a knowledge of the number of repeated blows which a finish can withstand at any arbitrary force level. Consequently the impacting device was changed so that the hammer speed progressively increases and the r. p. m. a t which shattering of the finish to the base metal first occurs is observed. If desired, the test can also be used as heretofore, for the determination of repeated impacts at any fixed speed of rotation. This new modification has materially improved the precision of the test. Triplicate readings in 38 different determinations of repeated impact resistance performed at fixed speed gave an average deviation of 21 per cent. The average deviation in the case of the modified test, where limiting force of impact is determined, was 5 per cent. I n the older form of test, the maximum deviation from the mean was as high as 57 per cent, whereas in the newer form 10.6 per cent was the maximum. I n the tables the type of failure produced on impact is also recorded. Some finishes fail by a gradual crumbling or wearing away, whereas others shatter over a considerable distance around the point of impact; hence an arbitrary rating of 1to 3 in type of end point has been used, supplementary to the r. p. m. values of impact. The mandrel test was used as previously described. Six different mandrel sizes-namely, of 0.125, 0.25, 0.375, 0.5, 0.75, and 1 inch (0.32, 0.63, 0.95, 1.27, 1.89, and 2.54 cm.) radius-were used. The mandrel size at which first cracking became noticeable under low-power magnification is reported as the limiting distensibility in terms of per cent elongation of the finish coating. The per cent elongation is calculated on the assumption that the neutral plane of movement in the panel-that is, the plane where distension and compression are equal-is in the exact center of the panel. However, on exceeding the limit of elasticity of the base material, this neutral plane unquestionably shifts, so that the calculated distension at the metal surface beneath the coating becomes only an approximation. Since the same base material was used throughout, the error is constant and the values are comparable, In addition to the value of distensibility, the character of the adhesion to the base metal after stretching is qualitatively noted, Four degrees of adhesion are arbitrarily recorded, varying between the extremes where the coating has been cleanly severed from the base to where a scuffing with a blunt tool leaves the finish intact. I n addition to the above three tests, the Pfund hardness was determined after 3 and 14 months of aging in the humidity cycle and after 32 weeks of aging in the weatherometer cycle, in order to obtain additional correlative information on the deformability changes of these coatings.
EXAMINATION OF PRIMERS, SURFACERS, AND ENAMELS Of the several hundred different finishing materials investigated, a representative number are included in Tables
Vol. 6 , No. 2
I, 11, and 111, which show the extent of variation existing in the products obtainable in the market approximately 2 years ago. For purposes of comparison, values of abrasion resistance and distensibility are recorded. I n addition, dryingtime data are given in order to serve as a basis of differentiation between the older types of oleoresinous enamels and the more modern synthetic types. The exact composition of the materials tested is unknown; the consumer's interest is centered in the behavior of finishing - materials and not their composition. TABLEI. fiIATERIAL
WEARb
A. Nitrocellulose P-1010 P-102 P-103 P-104 P-105 P-106 P-107 P-108 P-109 0 P-110 P-1 11 P-112
B. Oil Type P-201
PRIMERS5
MANDREL Elongation Adhesion %
75 44 65 24 69 61 9 64 42 27 26 13
>17.7 >17.7 10.5 >17.7 3.9 3.9 5.7 17.7 >17.7 >17.7 >17.7 >17.7 >17.7 >17.7 >17.7 >17.7 17.7 17.7 17.7 17.7 17.7 17.7 10.5 10.5 10.5 10.5 10.5 5.7
C. Synthetic P-301
LO7
>17.7
Good
0.33
P-302
47
>17.7
Good
0.2
P-303 P-304 P-305
20 39 24
>17.7 10.5 7.4
Good Fair Good
6: 16 0.16
P-202
Hours
4
Good (soft film) Good (soft film) Good Good Good Good Good Good Good Good Good Fair Good Good Good Good Good Good Good Good Poor Poor Poor Good
2 (Remains tacky) 0.5 (Gelled)
..
0.5 0.5
a All materials listed in Tables I, 11, and I11 air-dried 1 week and exposed to 140" F . (60' C.)for 3 weeks rior to test. b Wear values in Table I anasucceeding tables are expressed in terms of grams of Carborundum per 2.54 X 10-8 om. (0.001 inch) of coating. C Materials used in subsequent tables. Code used for identification of materials in all tables: P, primers 100 Series, nitrocellulose materials 200 Series, oil or oleoresinous materials E, enamels S surfacers 300 Series, synthetic-type materials d , putties
TABLE 11. SURFACES AND PUTTIESO MANDREL
WEAR
Elongation
Nitrocellulose 8-101 S-102 b G-1Olb
36 30 33
Chalking Checking Months Months
Cracking Months
...
3, fair
3, def.
....
....
3, slight
None 6, none
None
12, fair
3, def.
12, none
12, none
6, fair
6; def.
6, none
None
12, fair
12, def.
12, none
12, none
7, fair
7, def.
7, none
7, none
None
3, eggshell 6, eggshell 12, flat 3. fair 5 , fair
3. def. 6 def. 12, def. 3 def. 6: def.
3 none 6: def. 12, def. 3 def 6: def:
3, none 6, none 12, def. 3, none 6, none
3, eggshell 6,eggshell
3, def. 6, def.
3. none 6, def.
3, none 6, none
4, fair
4, def.
4, def.
4, none
3 fair 6' fair 12: eggshell 3, eggshell
3, 6 12: 3,
3, none 6,none 12, none 3, slight
3, none 6, none 12, none 3, none
...
....
....
4, init. 6,def. 12, bad
None
None
None
4, init. 8, def.
FLORIDA^
I
None
None
None
....
None
None
......
def. def. def. def.
... ...
..... .....
.... ....
..... 3, eggshell
6 , def.
6, slight
6, none
3, fair 6, fair 3, fair 6. fair
3 def 6: def: 3, def. 6. def.
3 none 6: none 3, none 6. none
3, none 6,none 3, none 6. none
96
ANALYTICAL EDITION
is felt that a satisfactory elucidation would be provided for the hazy correlation between physical analysis and exposure in the systems E, F, G, and H. It is likely, that in the case of finishes exposed on metal panels, distensibilities of less than 1 per cent will be found to become critical. A method which it is hoped will permit the precise determination of per cent
Vol. 6, No. 2
All three exposures lead to the same order of selection as that obtained from the analysis of the physical data prior to visual failure. It appears that panels in the weatherometer fail in approximately one-third the time required to fail in Florida. The amount of chalking of the panels exposed in Florida was greater than at the other exposures. It is of interest to note that even on the matter of chalking, the materials which show up best under the physical tests also show the l e a s chalking and gloss change. By way of correlation t o final service tests, 27 trucks (17 in the Sorth and 10 in Florida) of which 15 aere of all-metal construction and the rest wood and metal, were finklied with various systems reported in Tables V and VI, and their serviceability in the field carefully followed. FIGURE2. EFFECTOF SURFACING MATERIALS ON IMPACT RESISTANCE The are shown in The length of servicelife for the various systems falls 1. P-211 2. P-211 3. P-211 E-103 8-102 9-102 in the order one would expect from the data E-103 a-101 obtained both by the periodic physical testsand 9-102 E-103 panel exposures. The determination of end of serviceability is of course open to some queselongation of finish panels over a wider range of distensibility tion, since standards of appearance necessarily involve el+ is now being investigated. With such a tool, the important ments of personal judgment. problem of the relation between distensibility and adhesion SURFACERSTUDY to the base can be studied. Comparison of Tables V, VI, and VI1 shows clearly that a The value of the physical tests discussed above is not limited finish has fallen off seriously in its ability to conform to various to their use in foretelling the durability of finishes. Such stresses a long time prior to the point where visible failings problems as the effect on the mechanical behavior of a system begin to make their appearance. Thus, in the series exposed caused by changes in the components that make up such a to a mild exposure cycle, even after 14 months no visible system can be clearly recognized. Thus in the finishing of changes had occurred, even though most of the materials had automotive vehicles it, is common practice to apply generous suffered badly in deformability. Even more striking is the coats of surfacing materials in order to hide imperfection of case of the panels exposed to the weatherometer cycle. All the base metal. Recommended practice is to sand off excess but two of the systems in Table VI showed elongations of less surfacing materials. I n order to learn what effect the presthan 3 per cent after 6 weeks; after another 6 weeks all had ence of varying amounts of brittle surfacing material would fallen below the lower limit of the mandrel test. It appeared have on a finish system made up of flexible primer and flexible therefore that 12 weeks in the weatherometer had spelled the enamels, an oil-primer nitrocellulose-enamel system, a comdeath of most of these finishes and yet six out of ten had not plete oleoresinous system, and a complete synthetic type shown first signs of surface checking by the end of the ex- were selected for test. The thicknesses of the various composure period. Two began to show this only a t the end of 12 ponents comprising the systems, values of abrasion resistance, impact resistance, and distensibility are weeks' exposure. I n the remaining two systems initial checkgiven in Table IX. ing showed up a t the end of 4 weeks. Quite likely the disThe effect of brittle intermediate coats is tensibility had dropped considerably below 3 per cent and marked. Thus, the impact resistance goes hence was impossible of measurement by the mandrel test. up definitely for the young film, although Comparisons of Tables V and VI also indicate that the with age this difference is not so pronounced. relative intensity of aging factors in the two cycles employed But what is more important, the type of is of the order 10 to 1, since these systems show comparable failure undergoes a definite change. Where physical qualities at the end of 60 weeks of exposure in the all the surfacing material has been sanded mild cycle as compared to 6 weeks in the weatherometer. off, the type of failure is one of crumbling This is not so surprising when one considers that the total away a t point of impact. When increasing number of thermal shocks of approximately equal intensity to quantities of surfacer are left over the primer, which these coatings were subjected is 100 times greater in the the type of failure changes to one in which weatherometer cycle than is the slower humidity cycle. large areas on each side of point of impact Possibly if these finishes were exposed on wood panels, the are shattered cleanly off to the base metal ratio of acceleration between the two cycles might not be so FxGURE 3 (Figure 2). high, since in this case the coating would have to undergo The effect of surfacing coats on the dismuch greater expansion in the slow cycle than in the rapidly tensibility of an otherwise flexible system is even more severe. fluctuating weatherometer cycle. I n Figure 1 the aging of several typical finish systems, as Systems without surfacer, which show an initial distensibility shown by changes in distensibility, is graphically represented. of greater than 17.7 per cent, fall immediately to less than 3 The wide differences inherent in these materials while they per cent when surfacer is left between primer and color enamel. are still in serviceable condition are clearly shown. The Furthermore, the adhesion of the system to the base metal curves are necessarily only approximate owing to the limita- suffers. It is surprising that on bending, the entire system tions of the mandrel test. The relative effect on the speed of should peel away from the base metal, since the primer degradation of cycles of different intensity is clearly exhibited. itself exhibits excellent adhesion and high per cent elongation. Examination of Table VI1 shows that the order of failure The explanation is probably as follows: under the three exposure conditions is approximatelY the The inflexible and relatively very thick surfacing layer has same. The outdoor panels were mounted 45" facing south. excellent adhesion to both the primer and color enamel. When
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 15, 1934
9
TABLEVIII. SERVICE TESTSON TRUCKS FINIBH SYSTm C C E E
F F
G G
H
H J J
MATIORIALS No. OF TRUCKS USIOD P-218 E-102 2 P-218 E-102 2 P-216 E-116 2 P-216 2 E-116 P-206 3 E-106 P-206 1 E-106 P-216 E-207 6 P-216 2 E-207 P--206 E-206 3 P-206 1 E-206 P-218 1 E-304 P-218 2 E-304
EXPOBURP SPRVICE LOCATION LIFP (MONTES)
KINDOF TRUCKS 1 Composite 1 All metal 1 Composite 1 All metal
North
30-36
Chalked mostly.
Some wood grain failure
Florida
30-36
Chalked mostly.
Some wood grain failure Some checking and cracking
North
24-36
Chalked mostly.
Florida
15-18
Most checking and cracking.
All metal
North
12-18
Very severe chalking
All metal 4 Composite 2 All metal
Florida
9-12
Very severe chalking.
North
24-36
Mostly checking.
Composite
Florida
18-24
Severe chalking.
Composite
North
15-17
Checking.
Wood grain failure
Composite
Florida
9-12
Checking.
Wood grain failure
All metal 1 Composite 1 All metal
North
36 -t
Still in service
Florida
30 4-
Still in service
All metal All metal
’
TABLEIX. EFFECTOF SURFACES NO. OF COATE Lacquer type P-2 11 E-I03 P-2 11 s--102 E-103 P-2 1 1 5-102 G-101 s- 102 E-103 Oil type P-216 E-207
IMPACT
-WEARSIPT I
Wa
Tb
BPT I V
W
TYPEOF FAILURE
T
3ET I V
SET I
. ?
R. p. m. End pointe R. p. m. End point0
Slight checking
Wood grain failure Checking.
1
1
2 1
1
1 1 2 1 2 1 1 2
32 58 24 38 54 38 38 24 36 52
1.27 2.03 1.02 2.03 2.03 1.02 2.54 3.56 2.29 2.54
38 53 58 32 76 49 47 34 39 71
0.76 1.78 0.76 2.79 1.27 1.27 2.03 3.30 2.79 1.52
25 58 31 22 45 30 33 48 28 73
0.76 2.03 1.02 5.59 2.03 1.52 6.35 2.79 1.27 2.03
53 63 43 29 65 45 32 64 46 76
0.76 1.78 1.02 4.06 1.52 1.02 6.35 1.78 0.76 1.52
8ET I
Adhesion
SET IV
Elonn. -
Adhesion
%
430
1400
>17.7
Good
550
480
5.7
Good
17.7 550 Fair 2 E-304 Synthetic type 31 1.7‘8 46 1.52 P-218 1 22 6.10 1300 555 26 6.35 1 9-301
