PHYSICAL STUDY OF
TWO-COAT PA1 SYSTEMS A. E. JACOBSEN Titanium Pigment Corporation, New York, N. Y
The t e c h n i c e m p l o y e d i n the preparation and the removal of the films prior to testing was described in the previous paper1 and is briefly as follows: ( a , b, c,) FILMSUNDER TESTFOR FLEXIBILITY
The paints were applied to tinned copper panels by the use of a pair of shims and a straight edge used in the manner of a drawknife. For the first coat, shims with a thickness of 0.003 inch (0.0076 cm.) were employed, and for the second coat, shims of 0.005 inch (0.0127 em.) were used, so that, allowing for loss of film thickness due to evaporation of the solvent, the first coat of paint would be between 0.002 and 0.003 inch (0.005 and 0.0076 cm.) thick, with a total thickness of 0.005 inch for the two-coat system. Actual measurement of the dried films gave results in accordance with this estimate.
T h e films are the same age but vary in degree of flexibility.
T
HE data presented here are based upon a continuation of the work described in a previous paper.1 I n that paper mention was made of earlier work in which the physical behavior of single-coat paint fdms was determined and the data obtained were correlated with exposure tests on wood. The res u l t s s h o w e d t h a t the rate and character of film failure with particular reference to checking and cracking were closely associated with the retention of the film's flexibility on aging. From this stage the physical study of paint films was continued to include two-coat paint systems because it was felt that more fundamental information concerning the relation between the undercoat and the topcoat could be obtained by this means than by relying entirely on visual observation. Besides the usual visual examination, physical tests were made on all paints. These tests i n c l u d e d distensibility, flexibility, and tensile strength, and were made on free paint films weathered on metal on b o t h 45" a n d v e r t i c a l f e n c e s . Identical paints were also exposed a t 45O south on wood, for visual comparison with the films on metal. 1
IND.ENG.CHIM.,28, 403 (1936).
MANDRELFOR TESTINQ FLEXIBILITY 660
JUNE, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
Physical tests of paint films, including tensile strength, distensibility, and flexibility, were made on exposed two-coat paint systems, using special primers with four different topcoats: similar tests were made of self-primed topcoats. Results show that flexibility of the components of the system is an important factor in relation to exposure results: suitably formulated primers show definite advantages over self-primed systems. Physical tests are supplemented by exposure results obtained in the usual manner.
At esch test period a portion of each mota1 panel was taken from the fence, and the fdms were removed by running mercury around the edge8 of the panels and allowing it to amalgamate with the tin. This permitted tho removal of the films without, subjecting them to any unusual stress or otherwise affectingtheir physical condition. The 6lms were conditioned for 24 hours at 75" F. (23.9"C . ) and 50 per cent relative humidity ja?t prior to
testing.
At the time of the previous report the tests had not progressed sufficiently to permit generalization except in a very broad way. The data indicated, however, that primers may be either too hard or too soft for the topcoat. A hard twocoat system may become so brittle as to cause cracking of the film, whereas a flexihle two-roat system retains continuity. This physical study has been brought to completion for both the 45" and vertical tests on mota1 SuDDIemented by ~arallel exposures on cedar and white pine. Paints Used TOPCOATS. The pigments were as follows:
661
Primer A 40% basic carbonate white lead 40% titanium-barium pigment 10% sabestine 10% silica Prmrer B Primer C 25% basic cerbonate white lead 25% titanium-barium pigment 25% load titanetc 25% d i c a and silicates
The primer vehicle in all cases was the same, except for a variation in resin content (2.5, 5, and 7.5 per cent) and had the following composition, exclusive of resin: 39.6 per cent raw linseed oil, 14.8 polymerized linseed oil, 9.8 polymerized tung oil, 5.7 oxidized linseed oil, and 30.1 per cent volatile matter. This particular vehicle was selected because (with 5 per cent resin) it had been extensively tested in a practical way over a period of several years in conjunction with a pigment combination similar to primer A, and had given excellent results. All three pigment combinations (A, B, and C ) were used in the three resin-content variations of the vehicles at botb 30 and 40 per rent by volume, calculated on total nonvolatile material. Each primer, therefore, is designated in the following manner for identification and convenience: primer A-302.5, A-30-5, A-30-7.5, A40-2.5, A40-5, A-40-7.5, etc., making a total of eighteen primers with three differentpigment oombinations. For example,,pnmer B-40-5 is pigment combination B in the vehicle containing 5 per cent resin and with 40 per cent pigment by volume, caloulated on the total nonvolatile matter in the paint. These variations in resin content and pigment volume concentration were designed to produce graduated differences in hardness. White lead performs well as a primer and was used for comparison. There were also included two commercial primers, one of which has been used extensively over a period of several years with good results, and an aluminum primer which wab tested vertically on metal and at 45" on wood. The white lead was used as directed by the manufacturer, and the alnminum primer was made by diluting aluminum paste with a recommended long-oil vehicle. Along with these two-coat systems, the primers were exposed vertically on metal as single coats.
Topcoat 1 40% haeio oarbonate white lead 40% titanium-barium pigment 10% asbestine 10% silioa Topcoat 2 40% t,itanium-barium pigment 29.2% baeio oarbonats white l e d 30.8% leaded einc (36%) Topcoat 3 25% hasio esrbonate white lead 25% titanium-barium pigment 25% l e d tit.anate 25% zinc oxide (American prooess, lead-free) Topmat 4 40% hasic carbonate white lead 40% titanium-barium pigment 20% Zinc oxide (Amerioan proce88. lead-free)
The vehicle, in the case of pigments 2, 3, and 4, was composed of 80 per cent raw linseed oil, 5 per cent polymerized tinseed oil (viscosity Q), and 15 per cent drier and t,hinner. The vehicle for pigment 1 w&smodified to contain 7.5 per cent. resin. The pigment-vehicle ratio by volume ranged from 29 to 30 per cent; there was only a slight variation between the four paints. PRIMERS.Three principal pigment combinations were used in the special primers :
A HARDTOPCOAT OYER A PRIMER W I T W TEE FILM FOLDED TO HOW THAT THE CRACKS Do NOT EXTENDTHROUGH THE
PRIMER
TABLEI. DISTENSIBILITY OF TOPCOATS Primer
% Pigment Vol. Concn.
% Resin Content
30
2.5
-5' 0
15
20 24 30
1.5
Per Cent Elongation after Weeka of Exposure: Vertical fence
fence"
64
42
8
16
24
32
42
60
92
1
1.5
0.5 0.5
1.5
4
0 0 0 0 0 0 0 0 0 0 0 0
0
0 0 5 0 5
Topcoat 1
A
5.0
40
B
30 40
7.5 2.5 5.0 7.5 2.5 5.0 7.5 2.5 5.0
7.5 C
20 20
24 18 20
18 13 15 17
1
1 0.5 0.5
1 1 1.5 1.5 2 1.5 1
30
2.5
20
40
5.0 7.5 2.5
20 22 13 18 12
3 2 2 1.5 1 1
23 28 18
2 2 2
5.0
7.5 .. ..
... ,.. ,..
..
..
...
n
2 1
1.5 1
1 1
2 3 4
1 1 2
2
4
5
1.5 2 2 1 1.5 3
2
6
5
1.5
1
0.5
1
0.5
;.5
0.5 0.5
1.5
1 7 3
!
:
2 1
0 0 0
1 1
0 0
1
0
1.5 1.5
0 0 0 0
11
0
7
0.5
6 5
1 1 1 2.5 1.5 3
1
1.5 1.5
1.5
0 0
2.0
1.5 1.5 1 1.5
0
1
0
0 0
0
2
Oa
0.5
0
...
3 3
7 6 6
10 6 7 6
8 15 5
3 2
1.5 3 3
7
2
2
0.5, 0.5a 0.5a 0.5a 0.5a la
5
1.5 1.5 1
3 1.5
2.5
2.5 1
2
2
0.5
3 2 1.5
2
5
0 0 0 0 0 0
0.5
0.5
2.5
0
7 2 2
0
oc
1
0.5 1.5
4 1.5 2
1 1.5 3.5 2 3
1.5
3 11 3
0.5 0.5 0 0 5
0 0.5 0 5 0.5 0 5
0 5
0 5 0 5
0
0 5 0 5 0
2
1 5
Od
Od
Topcoat 2
30
A
40
B
30
2.5
40
7.5 2.5 5.0 7.5
30
C
2.5 5.0 7.8 2.5 5.0 7.5 5.0
2.5 5.0
40
7.5
2.5 5.9
7.0 Commercial Self-primed White lead Aluminum
. . I
.. ..
... t . .
...
18 25
25 10 12 15
0.5b 0.5a 0.5~ 0.5a
nd .. Od
Od Od Od
0
0.5b 0.5, oc
0.2
Oa
Oa O.5a Oa
Od
Od Od
0.5
0.5d
Od 0.5~ 0.5d
Od Od Od Od
21 26 19 10 12 12
0.5~ Oa
15 19 16 11 10 14
0.5, la 0.5a Oa 0.5a 0.5~
14 17 16
0.5a 0.5~ 0.5~
, .
Ob 0.5b
...
Od
oc
2
3 2
2 3
0 . 5 ~
0.5~
IC
la 0.5~ le
0.5c
oc oc Oc
oc
Od Od
oa
oa
Od Od Od Od Od Od
0.5a la la
0 . 5 ~
0.5, 0.5~
0.5~ 0.5~ oc
0.5,
Oc
Od
IC
0.5~ 0.5~ oc
Od 0.5d OC
Od
2
0.5a 0.5a la 1.5a la 0.5a
oc
0.5d 0.5~
Od
2
0.5a
0.5, Oa 0.5~ 0.5~
0.5d
Ib
O.5b 0.5b 0.5b la lb
0.5,
0.5b
2 2
1.5
5
IC
0.5~ la 0.5a 0 . 5 ~
0.5, 0.5~ 0.5c
oc
0.5~ 0 . 5 ~ 0.5,
Od Od
oc Oc oc
0.5, 0.5~
7 Od *tt Od
IC
0.5~
0.5a la 0.5~ 0.5a la 0.5~
1.5 2 2
2 3 1
0.5d *tt
IC
0.5a 0.5a 0.5,
Od Od Od Od
Od
Od Od Od
oc
Od
Od
Od
Od Od Od Od Od Od
Od *tt Od 0 5d
0.5
0.5
2
0.5a
1
Oa
0.5~ Oa 0.5a 0.5a
2
la 0.5, 1.5a 0.5a la 0.5a
0.5a la 0.5a 0.5a 0.3a
la la la la la la
la lb lb la la la
2b
0.5b O.5b
0 . 3 ~
oc
0.5b 0.5b 0.5b 1.5b lb
0.3b 0.3~ 0.3~ 0.5b 0.5~
Oc
oc oc oc
Od Od Od
Oa Ob
la 0.5a 0.5b la 0.5a 1.5a
Oc
2 2 2 2
la la la 1.5a la 2a
0.5b
IC
oc
oc oc
0 . 5 ~ Oa
Oa
2
0.5,
Ob
0.55
Ob
1.5 1.5 1
0.5
0.5, Oa 0.5~ la
0.5b Oa la 0.5b
lb Oa 0.5a
Ob Ob Ob
oc ot
0.5b
Ob
oc 1 . 5 ~
IC
oc
0.5, 0.5~ 0.5~ 0.5~ 0.5~ 0.5~
Od Od
Od
oc oc oc
Od Od oc Od
Od Od
Od
0.5~
Ott
oc
Id
*tt
oc Od
Topcoat 3
A
30 40
B
30 40
C
30 40
15
2.5 5.0 7.5 2.5 5.0 7.5
10 15 9 10 11
2.5 5.0 7.5
2.5
15 17 20 12
5,O
10
7.5 2.5
5.0 7.5 2.5 5.0
7.5 Commercial Self-primed White lead Aluminum
.. .. .. ..
... ...
A
30
2.5
...
...
0.5a 0.5a 0.5 0.5 1.5
Ob
0 5a
2 2
3 . 3.5
1
2 0.5b
10
la 0.5, 0.5a 1 0.5, 0.5,
10 11 10 8 9 9
0 . 5 ~ la la 0.5~ 0.5~ 0.5,
la 0.5a Ib 0.5a 0.5~ 0.5a
15 13 17
Oa 1
..
1
...
Ob lb
0.5,
0.5, 0 . 5 ~
Ob Ob Oa Ob Oa Oa
4
Ob
4
Oa Ob Ob
3 4
2 2 2 4
la
0.5a
Ib
0.5a lb
0.5b
oc
oc oc
Oc Oc
lb
Oc Oc Ob oc Ob
0 . 5 ~
oc
oc
IC
0.5a 0.5b
IC
oc
IC
oc
IC
oc
lb
Ob
OC
oc
0.3~ Od oc Ob
oc
oc
Topcoat 4
5.0 7.5 40
B
30
2.5
12
5.0 7.5
15 16
2.5
13 13 15
5 0
40 C
30 40
15 21 23
7.5 2.5 5.0 7.5 2.5 5.0
7.5 2.5 5.0 7.5
10
13 12 9
15 20 12 13 14
0.5a 0.5~ le O.5a 0.5a
2 1 2 2
2 1.5
IC
0.5,
0.5~
Od
IC
IC
0.5~ 0.5~ 0.5~
Od
1.5 2
0.5~ 0.5~ 0.5~ IC
0 . 5 ~
Od Od Od
0.5 2 1.5 2
0.5c 0.5~ 0.5~
Id 0.5d
0 . 6 ~
0.5d
IC
0.5~
Id
0.5d 0.5d
Od
Od Od Od Od Od Od
1 2 1
1 2
1.5
1.5, 0.5, 0.5a 0.Sa 0.5, 0.5~
0.5~
0 . 5 ~
IC
IC
IC IC
0.5, 0 . 5 ~
0.5, 0.5~
oc
0.52 0.5~ 0.5~ 0.5~ 0.5~ 0.5~
0.5d 0.5,
oc 0.5~ 0.5~
0.5~
oc
oc
0.5a la Oa 1.5a 0.5~
0.5~ oc
0.5a
0.5c
0.5a 0.5, la la Oa la
0.6c 0.5C
oc IC
IC
0.5~ 0.5~
0.5~
0.5,
Od Od
oc
Od
0.5d
Od Od Od
0.5d
oc
0.5d
Od 0.5~ 0.5d
oc
Commercial .. 12 0.5~ 0.5, Od 1 Oa oc OC Od Self-primed 12 .. Oa *tt *tt 1 0 Oa oc Ott White lead .. 10 oc 0.5a oc Od 1.5 0.5a 0.5, 0.5~ .. Aluminum .. ... ... 1 0.5~ 0.52 IC 0.5d Small letter a represents checking. b, c, and d represent degrees of topaoat cracking; t and tt indicate moderate and advanced stages cates that poor condition of film prevehted it from being tested. The previous article' gave data for 2,4, 6 , and 10 weeks. . . t
( I
oc
Od
Od Od Od
Od
Od Od Od Od Od
Od
Od Od Od Od Od
Od
Od
oc
Od
*tt
*tt
Od 0.5a of cracking;* indi-
INDUSTRIAI, AND ENGINIIERING
JUNE, 1938
ct-IEmsrnY
663
The degree of failure after 64-week exposure on the 45” fence w&s ahout the same as for 92-week exposure on the v e r t i c a l fence. I n o r d e r to simplify the mass of data obtained, only the tests on the vertical fence are discussed, since they are more representative of actual paint use.
Definitions of Terms “Checking” is used in this paper to describe slight breaks in the paint coat which do not extend through to the surface painted. “Cracking” means a break which extends down to the siirface painted. “Topcoat cracking’’ describes an aggravatai form of checking which does not extend beyond the topcoat. I n a d v a n c e d stages relatively large patches of the undercoat are observed. This term is used to describe tho behavior of the topcoat and to clarify a terminology which ordinarily does not distinguish between the several coats of a paint but treats them as a composite whole.
Distensibility Table I shows per c e n t e l o n g a t i o n for t,he two-coat paint film just prior to rupture tinder a g~adiiallyincreasing load. Topcoat 1 retained a measurable degree of d i s t c n s i h i l i t y over all primers on the vertical exposure for more than 42 weeks. These films had lost all distensibility at the end of the second summer or BO weeks (September, 19361, hut h a d regained suficient elasticity to show 0.5 per cent elonga.tion a t t h e end of SZ weeks (May, 1937), or after two summers and two winters. In other words, they were slightly more elastic a t the end of the second winter than at the end of the previous snmmer. This was undoubtedly due to a higher moisture content in the spring. Topc0a.t 1 was the only paint which had approximately a,s good &stensibility when self-primed as when used over the special primers. Topcoat 2 developed brittleness more rapidly than 1 and had lost all measurable distensibility over a majority of the primers a t 42 weeks. Since this topcoat exhibited cracking, the distensibility of tlic two-coat system for this period can
bility after 60 weeks. A1tho;igli when topenat 3 was self~ ~ r i m eitd ,lost any measiirable distensihility at the end of 24 rvccks, i t showed controlled hardening i n that only a few cracks appeared during the test period (02 weeks), in contrast with topcoat 2 which stioweci hac! cracking at the end of 42 weeks. This imprnvenmit en11 be rlefinitely attrihuted to the lead titanate content. Topcoat 4 over the primers behaved very milch like 2, s!iowing that the iiiilicated differenct. in composition had little
--~
- ..~
-
.-
TARLE 11. DISTEXSIBILITY AND FLEXIBILITY D.
