Coating Composition Films Physical Properties and Durability JAMES K. HUNTAND
U
w. D.
NDER actual conditions of service, the useful life
LANSING,E. I. duPont de Nemours & Company, Wilmington, Del, Data connecting the physical properties of
s e n t i n g almost every class of common am-forming materials, of p a i n t s , varnishes, were laid down on amalgamated a large number of different compositions have lacquers, and enamels depends tin plates and exposed outdoors upon two major factors-the been Obtained after times Of aging Outfor aging. Ordinary durability durability of the coating film itdoors. From these data it has been possible to panels of the same materials were self (referred to in this paper as obtain seoeral correlations between durability exposed o u t d o o r s at approxiand certain selected physical properties. ~ l - mately the same time. From “intrinsic durability”) and the adherence of the film to the surtime to time sections of films were face to which it is applied. It lhough Care *”‘ be used in the Of stripped from the amalgamated particularly with respect to tin panels and subjected to varihas been recognized for a numbe of years that the physical charpigmenfed compositions, it is possible to predict ous physical tests. In this way acteristicsof filmsmust affect the the durability of many clear film-forming madata were obtained for correlatterials by means of physical characteristics alone. ing outdoor durabilit,y by the intrinsic durability and, therefore, the useful life or general ordinary method with the physidurability of such p r o t e c t i v e cal properties as a function of age. films, although little has been published on the relationship beGEKERALCONCLUSIONS tween physical film properties and general durability. For example, in 1924 Morgan (8)emphasized the importance of physiA good correlation exists between certain film properties cal specifications for film-forming materials. H e suggested and the intrinsic durability of the coating composition that durability is more probably connected with the rate of film. As previously indicated, however, the usefulness of a change of the physical properties than with absolute values of given finish depends also upon factors other than intrinsic these properties. Eight years later, however, Wilkinson (14) film durability. Perhaps the most that can be said regarding reported that little if any progress had been made in obtaining intrinsic durability is negative; that is, a material of poor actual data to support the idea expressed by Morgan. intrinsic durability cannot give good service. Distensibility measured a t different relative humidities Several papers have been published in which certain physical properties of films were measured during the aging process gives a particularly good picture of the changes that take ( I , 9, 12, I S , 16). Most of these have to do with Aexibility place in a film during aging, and under certain conditions as determined by bending around a mandrel a metal panel durability predictions can be made from the data. The inion which the material to be tested is attached. For certain tial distensibility of a film is not so important as the change purposes such tests are valuable, but, since the bend test in distensibility on aging. A new test was devised for thermoplasticity, or the softenmeasures adhesion (which is not wholly a physical property of the film), as well as a combination of several film character- ing of a film when the temperature increases. This property istics, the bend test does not contribute greatly to our knowl- changes with aging of the film, and a good correlation was edge of the relationship between physical film properties and obtained between the intrinsic durability of films and the durability. Measurements of the stress-strain properties of change in thermoplasticity on aging. As might be expected, there is a fair correlation between the paint films were first referred to in connection with the Atlantic City and Pittsburgh paint test fences ( 1 1 ) and later water-vapor permeability of a given clear material and the were described more fully by Gardner (4,but the first stress- degree of rusting of plain steel coated with the material. I n compositions consisting mainly of cellulose derivatives, strain curves for paint and varnish films were made by Nelson (9) in the laboratories of the New Jersey Zinc Company about the viscosity of redissolved films decreases upon outdoor ag1921. Nelson and his co-workers (IO)subsequently published ing. This decrease in viscosity probably is caused by a deseveral interesting papers in this connection, and much credit crease in the degree of polymerization of the cellulose derivative. The durability of such compositions may be correis due them for their contributions in this field. The work outlined here was undertaken to determine what lated quite well with the decrease in viscosity as the films age. The physical properties of tensile strength, thermal expanrelations, if any, exist between intrinsic film durability and the several physical film properties that lend themselves to quanti- sion, hardness, and density all change on aging. So far, no tative measurement. The conclusions reached in this work are relation has been found between these properties and the innot put forth as the last word on the subject of film character- trinsic durability of films. istics and durability but they do indicate that tools are now DIESCRIPTIOX OF TESTS available for separating inherently good film-forming materials Forty-eight film-forming materials of widely varying comfrom inherently bad ones. It is hoped, and confidently expected, that future investigators will throw more light on position were used, chiefly as clear finishes. Several representatives of each of the seven following classes were included: the relation between film characteristics and durability. It should be emphasized that most of the work outlined in (1) drying oils, (2) orthodox oleoresinous varnishes, (3) glycthis paper was with clear finishes, and that further work is eryl phthalate resin varnishes, (4) vinyl resin lacquers, (5) necessary to determine the extent to which the conclusions cellulose derivatives (unplasticized), (6) pyroxylin lacquers (pigmented and clear), and (7) single-pigment white paints. apply to pigmented finishes. The general plan behind this work, which was started in Films of the various materials were prepared by spinning on 1929, was quite simple. hiany different compositions, repre- amalgamated tin plates (6). In general, three or more coats were
coatingcomposition films and the durability of
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January, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
applied, the film being allowed to dry thoroughly between successive coats, giving a total film thickness of about 0.005 inch (0.0127 cm.). Most of the materials were exposed on the amalgamated tin plates, but one series of films was stripped from the amalgamated tin plates and exposed cemented around the edges t o glass plates. No significant difference was found between the aging of the detached films cemented to glass plates and the films attached to amalgamated tin plates. The tensile strength and distensibility of the films were measured with a modified Scott tensile strength machine. In most cases six samples measuring 1 X 4.75 inches (2.5 X 12.6 cm.) were stripped from the amalgamated tin plate. Two of these samples were kept overnight in desiccators maintained at 0 per cent relative humidity and 77" F. (25' C.); two were kept overnight in desiccators containing water, at approximately 100 per cent relative humidity (77" F.), and two were stored in a room maintained at 50 per cent relative humidity and 77" F. Tensile tests were run immediately after the films were removed from the desiccators. The autographic chart of the machine gives the breaking strength in pounds and the distensibility in per cent. From the film thickness it is possible t o calculate the tensile strength in pounds per square inch. Water-vapor permeability was measured by a cup method that has been used by a number of investigators. It has been described in detail by Wray and Van Vorst (16). The density of samples of detached film was measured by a flotation method in a salt solution ( 3 ) . A sample of film was immersed in the salt solution kept a t 25" C. and allowed to reach equilibrium with the solution. The density of the solution was adjusted by adding water until the film neither rose nor sank. The density of the solution was then measured with a hydrometer and recorded as the density of the film. Strontium nitrate is a suitable salt for densities less than 1.400, while potassium iodide serves for films as dense as 1.700. Although this method is open to the objection that the film is more or less swollen with water when the measurements are taken, the authors feel that it gives good comparative data, revealing particularly changes in density on aging of the film. The thermal expansion of fdms was determined directly with a measuring microscope. A strip of film was suspended in a tubular air bath that could be either heated or cooled by means of circulating water. Provision was also made for controlling the relative humidity of the air surrounding the film, dry air generally being used. In designing the apparatus care was taken that the upper support of the film should not move vertically with changes in temperature of the air bath. A small piece of fused quartz was cemented to the lower end of the fdm, with a reference mark on which the measuring microscope was focused. I t was possible with this relatively simple apparatus to determine the thermal expansion coefficient of films with a fairly high degree of accuracy. The term 'Lhhermoplasticity'J has frequently been used very loosely in paint technology. At the risk of introducing another illogical definition, thermoplasticity as used in this paper refers to the softening of a paint film with increasing temperature. By arbitrary definition a material is said to have unit softness if it stretches 1 per cent of its length in 1 minute under a stress of 1 kg. per sq. mm. The thermopla,sticity test was made by suspending a known weight (20 to 200 grams) from the bottom of a strip of film which was maintained ( a ) at 45' and ( b ) at IO' C. In every case the softness was calculated from the stretch attained by the film 1 minute after the load was applied. The thermoplasticity of a film may be expressed as the difference between the softness at 45" and that at 10' C. Aged films of materials consisting mainly of cellulose derivatives can be redissolved in the solvent used for the original coating composition. Most materials were redissolved to a concentration of 15 per cent, and the viscosity measured with a capillary pipet. These pipets had previously been calibrated against standard materials whose viscosity was accurately known, and all results were converted to viscosity in poises. The hardness of a number of these materiak was measured under conditions of constant temperature and humidity (77" F. or 25" C., and 50 per cent relative humidity), using the Pfund hardness tester ( 5 A ) . For this test the materials were laid down and exposed on glass plates which were brought in periodically for testing. Panels for durability observations of the clear materials were prepared by applying the coating composition over stained and filled mahogany, and over "three-way" steel panels. In the case of these latter panels the materials to be tested were applied over bare steel, over a standard automobile undercoat system, and over a complete automotive pyroxylin lacquer-undercoat system. The single-pigment white paints were exposed on white pine clapboards. All durability panels were exposed outdoors on a 45" test fence facing south and were examined frequently
27
for evidences of any type of failure. A material was said to have failed when the film showed definite checking and/or cracking. These panels were examined by two different observers throughout the life of the materials, and durability gradings averaged.
PHYSICAL PROPERTIES AND DURABILITY Distensibility measured a t different relaDISTENSIBILITY. tive humidities was found to be an important characteristic of drying oil, oleoresinous, and glyceryl phthalate films. Figure 1 shows the behavior of a typical example of this class of- materials when e x p o s e d outdoors. T h e distensibility measured a t 0 per c e n t r e l a t i v e humidity began t o drop relatively early, and low dist e n s i b i l i t y subsequently appeared in f i l m s measured a t higher relative hum i d i t y . It was noticed that a film r e a c h e s a low distensibility a t 0 per c e n t r e l a t i v e humidity long before failure occurs on the film exposed out of doors. In general, ole o r e s i n o u s and FIGURE1. DISTENSIBILITY us. TIME glyceryl phthalate OF AQINC FOR A TYPICAL OIL-TYPE CLEARFILMAT DIFFERENT RELATIVE films fail by checkHUMIDITIES ing a t a b o u t t h e Ordinary durability panela of thia material time when the disexposed a t the same time failed by checking in about 90 days. tensibility measured a t 100 per cent relative humidity falls to about 5 per cent. It must be remembered that all of these films were exposed outdoors to the same conditions; only the testing was done a t different relative humidities. Pyroxylin lacquers and other compositions consisting mainly of cellulose derivatives usually have a rather low initial distensibility in comparison with oil-type finishes. I n the case of films based on cellulose derivatives, little change in distensibility takes place on aging, and durability is not closely related to change in distensibility. THERMOPLASTICITY. I n investigating tensile properties of films, it was found that, ordinarily, oleoresinous and glyceryl phthalate films do not behave like elastic bodies, because the stretch of such films depends upon the length of time they are under stress. Neither do such films behave like truly plastic bodies, because there is a considerable recovery after the load is removed. The softness test described in this paper was developed in an attempt to find out more about these tensile properties. The principal distinction between the softness test and the test for tensile strength and distensibility is that in the former the film is stretched only a few per cent, while in the tensile strength test the film is stretched until it breaks. The thermoplastic behavior of a typical oil-type material when exposed outdoors is given in Figure 2. It has been found that such films fail a t about the time softness becomes independent of temperature. VISCOSITY. When the viscosity of a redissolved film of a material containing a cellulose derivative is plotted against the time of exposure outdoors, i t is found that the viscosity falls off much more rapidly for the less durable materials. Loss of viscosity is probably due t o an actual depolymerieation of the cellulose derivative molecule (7), brought about
INDUSTRIAL AND ENGINEERING CHEMISTRY
28
by the exposure condition. Typical data for a durable and a nondurable pyroxylin lacquer are shown in Figure 3. There is a great difference in the viscosity behavior of these two materials, although they were based on the same lot of pyroxylin. WATER-VAPOR PERMEABILITY. The various materials tested had water-vapor permeabilities ranging from about 0.03 to 1.5 mg. of water vapor per sq. cm. per hour. For clear materials exposed over bare steel, a good qualitative agreement was found between the water-vapor p e r m e a b i l i t y and the e a s e of r u s t i n g of the bare steel c o a t e d w i t h the material in question. There are exceptions to this simple rule, but for clear materials a fairly good correlation was obtained. In view of the relatively small amount of work done with pig051 A0 80 m e n t e d f i n i s h e s , the T I M E OF ACING IN DAYS FIGURE 2. SOFTNESS us. TIMEOF 'Or a O f waterAGINGFOR A TYPICAL OIL-TYPE vapor permeability and AT DIFFERENT TEM- rusting under pigmented C L E ~ FILM R PERATURES m a t e r i a l s will not be Ordinary durability panels of this madiscussed in this paper, terial exposed a t the same time failed in Tensile strength was measured for all materials a t the same time that distensibility measurements were made. I n general the tensile strength of a film increases during the early stages of aging and then falls off during the latter stages. S o correlation, however, was found between durability and either initial tensile strength or change in tensile strength on aging. The same conclusion also holds for the measurement of hardness by means of the Pfund hardness tester, and for density. TEGTS.
TABLE I. THERMAL EXPANSION COEFFICIESTS OF VARIOUSMATERIALS MATERIAL
COEFFICIENT OF ExP A N S I O N PER
( X 10-9 A.
set up in a film laid down on steel or wood, incident to a change in temperature from, let us say, 90" to 10" F. (32" to -12.2" C.), might easily lead to cracking of the film, which in general would tend to contract considerably more on cooling than the steel or wood substrate. The coefficients of expansion of films of several representative coating compositions are given in Table I, together with those for several metals and wood for comparison. The several films were measured a t approximately 0 per cent relative humidity, and over the temperature range from 10" to 55" C. I n general, coefficients of thermal expansion decreased on aging, and Table I gives the range of values obtained.
O T H E RP H Y S I C A L 5
about 90 days.
'c.
LUTHORS FILM-FORMINQ MATERIALS MEASURED BY !
1.6-0.9 1.6-0.9
-4lkali-refined lin,seed oil film Bodied linseed oil film Typical spar varnish film Cellulose acetate film Typical oil-modified glyceryl phthalate film Typical pyroxylin lacquer film B.
COMPARISON MATERIALB
Aluminum
:,O,P,per
1.7-0.8 0.3-0.4
I .7-0.5
1.3-0.9 (6)
0.25 0.17 0.11
Wnnd :
PiTallel to fiber: Maple Oak Pine Across fiber: Maple Oak Pine
0.06 0.05 0.05 0.5 0.5 0.3
The thermal expansion for most film-forming materials was found to be of the order of magnitude of 1 X per degree Centigrade. As far as the writers are aware, no figuresfor the thermal expansion of coating composition films have ever been published, although it has been assumed that one of the factors affecting the life of coating compositions is the unequal thermal expansion and contraction of the film and the material to which it is applied. While the present work revealed no correlation between durability and coefficient of thermal expansion, i t is easily conceivable that the contractile strains
Vol. 27, No. 1
z
w 0
B
When exposed on ordinary durability panels a t the same time, material A (clear pyroxylin lacquer) failed in 120 days: material B (material A pigmented) was still good a t the end of a vear.
p e r c e n t relative humidity drops to a low value i n from one-
ties measured
a func-
tion of the time of aging outdoors, i t is possible to tell'in a relatively short time which one of the control materials the unknown most closely resembles. Fairly accurate predictions of the durability of the unknown material may be made in one-fourth the time that would be taken by the usual durability exposures. Similar predictions can be made from the results of the thermoplasticity test. Compositions based on cellulose derivatives form a class of materials that dry largely by evaporation of solvents. The drying time is, in general, very short, and the dried film is usually soluble in a solvent mixture like that used in the original composition. An outstanding physical characteristic of cellulose ester films in general is a high tensile strength together with a low distensibility. The low initial distensibility tends to preclude predictions based on loss of distensibility. For coating compositions based on cellulose derivatives, the best criterion of durability found is the viscosity of redissolved films. By exposing unknown materials along with known materials as controls, it is possible from the viscosity change alone to discover which of the known materials the unknown most closely resembles in durability. A limited amount of work indicates that this criterion holds as well for pigmented pyroxylin lacquers as for clears. It must be pointed out, of course, that this test gives only the intrinsic durability, which may differ somewhat from the actual protection afforded by the lacquer, For example, cellulose acetate has a very high
January, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
intrinsic durability; that is, its film lasts a long time if suitably anchored, but, as is well known, cellulose acetate has such poor adhesion that it does not form a good protective coating. As a test on this general method of predicting durability, nine unknown materials of widely varying compositions and physical qualities were studied. Of the nine, the durabilities of seven were predicted correctly a t the end of a relatively short time of outdoor exposure. In the case of two of the unknown materials (both glyceryl phthalate resins), the actual durability exceeded slightly the predicted durability. In some cases the authors were able within the first week to predict the durability accurately, but in all cases final predictions were made long before the actual durability data were available. It should be pointed out again that suitable controls are essential in predicting the durability of unknown materials. Weather conditions are so variable that it is impossible t o use the length of time of exposure for anything more than a rough estimate of durability. It was found, for example, that films exposed in March change in physical properties from three to four times as fast as films exposed in December. Still greater differences might be expected between July and January exposure. I t is recommended that, as far as possible, exposures be made in the spring because the predictions can be made much more rapidly than in the winter. In the writers’ opinion, an ideal system of durability testing would be an accelerated test that would reproduce as closely as possible the exposure conditions encountered outdoors in summer. Films would then be exposed to the artificial weathering conditions and the durability of the unknown films predicted by the methods given above. Such a method might permit durability predictions in one-tenth to one-twentieth of the time now required for outdoor durability tests. As has been indicated previously, it is not known how far the conclusions drawn as a result of this study apply to pigmented finishes generally. It is recognized that there are types of failures of pigmented finishes that probably are not related to intrinsic film durability, or at least are not predictable on the basis of present knowledge of the relations between intrinsic durability and the physical characteristics of films. The tendency of pigmented films to chalk, for example, probably depends upon a combination of several different film characteristics. From an esthetic point of view, pigmented films fail also by dirt collection and by the development of disfiguring mold growth, and it is obvious that physical film characteristics can have little or no connection with the tendency of films to develop unsightly molds or collect dirt. It can only be said that the general approach outlined in this paper appears to have definite possibilities for predicting the intrinsic durability of protective coatings (clear finishes in particular), but that much more work must be done before the methods will permit the accurate prediction of actual durability under service conditions, particularly in the case of pigmented finishes.
FACTORS AFFECTINGDURABILITY The factors that affect the durability of coating composition films are many and various. For simplicity, this paper reports only intrinsic durability, that is, the durability of the film itself without regard to failures brought about by poor adhesion, etc. Intrinsic durability is concerned largely with
29
the ability of films to resist those influences that promote checking and cracking. In the case of clear finishes (varnishes, for example) intrinsic durability and actual durability in service are more or less synonymous, since failure in the case of such materials is generally due to a breakdown of the film itself. An exception to this general rule for clear finishes is cellulose acetate, which, as pointed out above, is likely to fail because of poor adhesion. In the field of pigmented compositions, howeT-er, caution must be used in comparing intrinsic durability with practical durability in service. The intrinsic durability, as predicted by physical tests, does give an upper limit to the possible usefulness of a pigmented material, which may be approached by careful formulation. In the case of outdoor exposures, factors contributing to the failure of protective films are ultraviolet light, oxidation, temperature] and humidity (including changes in the two latter factors). It is rather difficult to evaluate these factors separately, particularly the effects of heat and moisture. It is known that both heat and moisture have a plasticizing action, and that a certain degree of plasticity is desirable in protective films. As Figures 1and 2 show, varnish-type films fail a t about the time when both heat and moisture lose their softening effect. It is possibly significant that the softening action of heat and moisture seems to disappear simultaneously. Many data were obtained in the course of this work on the effect of ultraviolet light on coating compositions. In the case of cellulosic compositions ultraviolet light is probably the principal cause of failure, although temperature and humidity changes unquestionably play a part, as was shown rather conclusively by work leading to the development of a reasonably satisfactory accelerated test for pyroxylin finishes. Experiments on films of oil-type compositions show that both ultraviolet light and oxidation play important roles in the aging phenomenon, and in this connection it is not unlikely that the deteriorating effect of ultraviolet light is associated with its catalytic effect in promoting oxidation. LITERATURE CITED (1) Blom, Paint V a r n i s h Production M g r . , 34, 12 (Juiy, 1929). (2) Came, Bur. Standards J . Research, 4, 247 (1930). (3) Clark and Tschentke, IND.ENG.CHEM.,21, 621 (1929). (4) Gardner, “Paint Technology and Tests,” pp. 74-80, New York, McGram-Hill Book Co., 1911. ( 5 ) Gardner, “Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors,” 6th ed., p. 167, Washington, 1933.
(5A) I b i d . , p. 210. Hodgman, “Handbook of Chemistry and Physics, 18th ed., pp. 1077-82, Cleveland. Chemical Rubber Publishing Co.. 1933. Kraemer and Lansing, paper presented before f i t h Colloid Symposium, Madison, Wis., June 14-16, 1934. Morgan, J . Oil Colour Chem. Assoc., 7, 207 (1924). Nelson, Proc. Am. SOC. Testing Materials, 21, 1111 (1921); Ibid., 23, Part I, 290 (1923). Nelson and Rundle, Ibid., 2 3 , I I , 356 (1923): Rundle and Norris, Ibid., 26, Part 11, 546 (1926). Paint Mfrs.’ Assoc. U. S.,Sci. Sect., Bull. 19 (1909). Schuh and Theuerer, IND. ENG.CHEM.,Anal. Ed., 6, 91 (19341. Snitter, Bull. l’inst. p i n , [2] No. 223 (1933). Wilbinson, J. Oil Colour Chem. Assoc., 15, 259 (1932). Wilson, B u r . Standards J . Research, 7, 78 (1931). Wray and Van Vorst, IND.ENQ.CHEM.,25, 842 (1933). RECEIVED October 13, 1934. Presented before the Division of Paint and Varnish Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934. This paper is Contribution No. 147 from the Experimental Station, E. I. du Pont de Nemours &Company.
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