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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
could be applied and still stay on. For practical thicknesses (10 mils) this figure implies a finite life expectancy of several years. As the actual paint departs from the ideal (for understandable and independent reasons), its life expectancy is shortened correspondingly. Thus purely theoretical considerations reveal that no paint system can be devised with an infinite antifouling life. During the course of these studies we have had occasion to make comparison exposures of a variety of commercially available bottom paints. The disappointingly short service life of the majority of these products (4-6 months) appears to be attributable, not so much to a deficiency of toxic agents, as to improper balancing of vehicle permeability with the specific toxics employed. A few such paints were significantly improved by the
VOl. 36, No. 12
simple addition of celite, which imparted higher diffusion rate8 to the films. The majority, however, gave too high permeabilities and wasted toxic throughout the early months of exposure. It seems probable that definite improvement in antifouling paint performance would result from quantitative studies of vehicle permeabilities as a function of added pigments other than the toxic agents themselves. LITERATURE CITED
(1) Young, G . H., Gerhardt, G. W., and Schneider, W. K., IND.ENO. CHDM., 35, 432 (1943). (2) Young, G. H., and Schneider, W. K., Ibid., 35, 436 (1943).
CONTRIBUTION from the Multiple Fellowship of Stoner-Mudge, Inc.,
at
Mallon Institute.
Infrared Radiant Heat Baking of Enamels Comparison between the drying behavior of enamels baked in radiant heat and convection ovens at equal panel temperatures showed no significant differences. Absorptivities were determined for black, green, red, yellow, and white gloss enamels. An analysis was made of the distribution of energy in heating these enamels with radiant heat. The influence of convection currents and thermal conduction on radiant heat baking is discussed. The influence of the percentage of polymerizing material in a series of film-forming compositions upon the rate of hardening when baked with radiant heat was determined. The experimental results illustrate the adaptabilityof polymerizing coatings t o the high-temperature short-schedule baking conditions attainable with radiant heat lamps.
T
HE widespread utilization of radiant heat baking by industrial concerns has given impetus to the study of the underlying theory of radiant heating. Contributors (6, 7, 18) to the literature have gradually removed many of the false impressions prevalent in the early days of this new tool. The establishment of the theory and performance principles of the radiant heat lamp is essential for its judicious adaptation in promising fields. Moreover the complete understanding of these principles by the makers of protective coatings will aid in the development of materials designed to use the advantages offered by radiant heat baking to the fullest extent. When radiant heat lamps were first introduced industrially, startling claims were made for their applicability and advantages in the baking of paint films. Baking schedules of paint products were shortened considerably, and it was a t first thought that infrared radiation had a catalytic effect on the drying of paint films. This belief had no sound theoretical basis, and several articles soon suggested methods for disproving the theory. Bennett and Haynes (1) advocated that a time-temperatuie curve he determined for a specimen under a radiant heat lamp and the baking conditions then be duplicated in a convection oven. They stated that no difference would be found between two such baked films. They also suggested that the shortened baking schedules encountered with radiant heat lamps were probably due to the ilttainment of higher temperatures in a shorter time. However, no experimental results were given. I n many cases the short haking times were due to the use of new synthetic enamels that baked more quickly than previous types.
K. C . Ernst and E. F. Schumacher UNIVERSITY OF LOUISVILLE, LOUISVILLE, ICY.
The present experimental investigation has the following objectives: to compare the results obtainable with radiant and convection heating to use existing theory for the determination of absorptivities, and to determine the effect of enamel composition on film hardness. RADIANT HEAT THEORY
Goodell (3) and Tiller and Garber (13) subjected the results obtained in radiant heat baking to mathematical analysis by thermodynamic principles. The derived equations are essentially the same in both cases, with the exception that the final heating equation given by Goodell is valid strictly only for the condition where the temperature of the air surrounding the object being baked is equal to the initial temperature of the object; this condition is not likely to prevail under actual conditions. The final form given by Tiller and Garber holds for all conditions of baking. The theory applying to baking by radiant heat is essentially the same as that underlying the measurement of radiant energy with a radiometric instrument (10). The development of equations for the heating of objects by radiant heat is based upon the principle that the heat absorbed by the receiver is divided between the heat retained by the stock as sensible heat reradiation and the convection losses to the surrounding atmosphere. Although the energy source is entirely radiant heat, conduction and convection effects materially influence the temperature rise obtained in the object. Important considerations in the industrial practicability of radiant heat baking are the absorptivities of coatings being baked, and the percentages of energy distributed among reflection, convection losses, and useful energy during the baking process. The theoretical equations developed for radiant heat baking make possible the determination of absorptivities as well as an analysis of energy distribution. The equations and nomenclature given by Tiller and Garber (12) are used in thia paper with the exception of intensity which is expressed as B.t.u./ (hr.)(sq. ft.) rather than watts/sq. ft.; the equations essential for calculations are briefly restated.
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1944
Figure 1. Time-Temperature Heating Curves for Radiant Heat and Convection Oven Baking of Commercial Black Enamel 0 Radiant heat panel 10 inchen from lam A Convection oven thermostat net at 314'
8.
Figure 2. Sward Hardness Curves for Commercial Black Enamel
0 Radiant heat; A
The basic differential equation for the instantaneous. rate of temperature rise for the heating of thin objechs of high thermal conductivity by radiant heat lamps is
dT - = - -AI
where A = e
=
h =
I
=
L =
T =
r.
=
P
=
e
=
-
h(T T,) de C ~ L CPL absorptivity of stock or ratio of radiant energy absorbed and converted to heat to incident radiant energy, dimensionless specific heat of stock, B.t.u./(lb.)(' F.) convectional coefficient of heat transfer during radiant heating B.t.u./(hr.)(sq. ft.)(" F.) intensity of radiant energy incident to surface of stock, B.t.u./(hr.)(sq. ft.) thickness of stock, ft. variable temperatur? of stock, F. temperature of air, F. densit of stock, Ib./cu. ft. time,
k.
This equation, with some modifications based on conditions a t
T, (maximum temperature) where dT/& I 0, may be integrated within the limits To(initial temperature) to T; the final form of the heating equation, then, is
APPARATUS AND PROCEDURE
The experimental work involving radiant heat wm performed with a 1000-watt radiant heat lamp directed vertically downward on the object being baked. The lamp was covered with a n optical lens which prevented deposition of fumes on the gold reflector and gave a uniform intensity distribution. The lens transmitted roughly 90% of the radiant energy. While a single lamp does not completely simulate the performance of a commercial radiant heat oven in detail, it is possible to'obtain valuable comparative results and certain fundamental data such 88 absorptivities. Single lamps, however, a t present cannot be used for determining intensities or coefficients of heat transfer for industrial design. During the baking operation a draft shield made of cardboard encircled the lamp, preventing cross
convection oven.
1133
Figure 3. Time-Temperature Heating Curves for Radiant Heat and Convection Oven Baking of Commercial White Enamel 0 Radiant heat panel 9 inahen from lam X Conveotion oven thermostat set at 2 8 8 O I? A Convection oven thermoatat net at BIOO F.
currents of air but allowing free convection upward. The panel being baked was supported on thin vertical sticks, which minimized losses by conduction. The convection equipment consisted of a 2900-watt 220-volt oven, provided with a thermostat for temperature control and an air circulating system. The metals panels used for the baking tests were oval ahaped, 20-gage smooth polished steel. They were 6 inches on the long axis and 5.25 inches on the short axis. The thermocouples were made from %foot lengths of copper and constantan wire. Three different sets of data were obtained. I n the first phase of the work, the behaviors of white and black synthetic enamels baked with both radiation and convectional heat were compared. The enamels were obtained from industrial manufacturers. White and black were chosen because they represent the extremes of adaptability to radiant heat baking The white enamel consisted of titanium dioxide ground in a vehicle consisting of 75% glycerol phthalate resin and 25% melamine resin. The black enamel was made from carbon black and bone black pigments, the vehicle portion consisting of 75y0 glycerol phthalate resin and urea-formaldehyde resin. Since the time-temperature curve is usually the governing factor in the rate of baking, the maintenance of equal panel temperatures under different conditions is imperative for a true comparison. This requirement necessitated the determination of time-temperature curves for both types of equipment on the same material. I n all cases the time-temperature curve was established under the radiant heat lamp and was then duplicated as nearly as possible in the convection oven. The measurement of film hardness with a Sward hardness rocker was used as a n index to drying progress. These measured values were supplemented with observed data on surface tack and through dry. The use of Sward hardness readings as a measure of drying progress demanded an appreciable amount of experimental work in order to establish a consistent and reproducible testing procedure. The Sward rocker was calibrated to give 100 * 3 rocks on a white "porcelain"-enameled plate. Five steel panels which had been selected for uniformity were wed for the testing work. Films were applied with a Bird applicator which deposited a wet film2 inches wide and approximately 0.003 inch thick. Since the panels were 5.5 inches wide there was a section of exposed metal on each side of the film on
Vol. 36, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
1134
40r
r-
a a w
IO 15 TIME IN MINUTES Figure 4. Sward Hardness Curves for Commercial White Enamel 0 Radiant heat; A aonvection oven.
the panel. The fact that the bare metal and the wet film had different absorptivities made no difference in this comparison since time-temperature curves were established under the lamp and then duplicated in the convection oven. For the determination of absorptivities in the second phase of the work, a special panel was p r e pared. A small hole waa drilled through the center of'one of the regular p a n e l s , and the hot junction of a copperconstantan thermocouple w&9
inserted into the 288' F.; X convection oven, 310" F. hole, bent over, and soldered on both sides with aluminum solder. This panel was used in obtaining all the experimental heating and cooling data, the enamel being removed after each baking period. The panel was completely coated with enamel by spraying for each bake. The enamels were baked at a distance of 6 inches from the lamp. A warming-up period prefaced each collection of data. The heatinq data were obtained on a freshly applied enamel coating, and a small part of the energy received from the lamp was dissipated in heating and driving off volatile solvents. When the maximum temperature waa reached, the heat was shut off and time-temperature cooling data were collected. For the black enamel only, the previously baked panel was reheated and data were collected without the effect of solvent Vaporization. I n the third phase of this program, a series of baking tests was undertaken to measure the effect of polymerizing type resins on the rate of film hardening in radiant heat equipment. The enamels used for the tests were all made from chrome green medium pigment and contained the same volatile constituents. They varied, however, in nonvolatile composition. Two basic enamels were made up; one contained only glycerol phthalate resin and the other consisted entirely of a melamine-urea resin. The glycerol phthalate resin consisted of soy0 semidrying and 50% nondrying alkyd. The melamine-urea resin was not a straight derivative of either type resin, but a condensation product involving both urea-formaldehyde and melamine derivatives. By blending enamels, any composition could be obtained from 100% alkyd to lOOyo melamine-urea. The enamels used for the tests were as shown in Table I. The enamels were baked a t a distance of 6 inches from the lamp, and the drying progress was expressed in terms of Sward hardness. The method for comparing radiant heat and convection oven baking for applying films and determining hardness was used in this series also. The same panels were also utilized. Since the applied film was not so wide as the panel, the average absorptivity of the receiving surface was lower than that of a surface completely coated with green enamel.
COMPARISON OF CONVECTION AND RADIANT BAKING
Figure 1 shows the time-temperature curves obtained by heating a panel coated with a film of the black enamel baked a t a distance of 10 inches from the radiant heat lamp, and a similar curve obtained in heating the same panel (new film) in the convection oven set for a maximum temperature of 314' F. These curves may be considered reasonable duplicates, for the maximum difference in temperature exists at a time interval of 2 minutes; the difference between the curves then decreases rapidly. Higher temperatures could have been obtained with the radiant heat by decreasing the distance between the panel and the lamp, but in this cme a temperature range in line with usual convection oven conditions was desired. This is one of the chief differences between using sinsle lamps as opposed to banks of lamps. With banks, overlapping of radiation produces an essentially constant intensity, regardless of the distance to the object being radiated. Figure 2 shows Sward hardness us. total time baked for the black enamel in the two types of equipment, using the operating conditions established in Figure 1. These readings were obtained by baking films applied to the selected panels for definite periods. The readings were taken when the baked panels attained room temperature. I n every case the panel used under the lamp for a definite time period was also used for a similar bake in the convection oven.
TABLE I. ENAMEL COMPORITIONS Name Green 1 Green 2 Green 3 Green 4 Green 6
% Alkyd (by
wt.)
100.0 87.6 76.0 50.0 0.0
% Melamine-Urea (by
Wt.)
0.0
12.5 25.0 50.0 100.0
The observations made for surface tack and through dry for the various bakes indicated that, as far as such limited testing methods were concerned, no significant differences could be detected between the two types of baking. Difficulties were encountered in performing the same experimental work for the white enamels. Because of the low absorptivity of the white enamel, a time-temperature curve with & prolonged heating-up period was obtained under the radiant heat lamp. It was impossible to duplicate this in the convection oven and therefore the hardness determinations were made for two oven conditions: one in which the final temperature was equal to that obtained under the lamp but the heating-up period was considerably more rapid, and the other in which the heating-up period conformed more closely but the final temperature was lower. The operating conditions of this series are shown by the time-temperature curves in Figure 3. The Sward hardness readings us. baking time obtained for these conditions are shown in Figure 4. The observed data for surface tack and through dry were in keeping with the nature of the time-temperature and Sward hardness curves. The data for the convection oven series a t 310" F. indicated that the rate of baking was definitely ahead of the radiant heat series all t h e time; for the oven set a t 288' F., the radiant heat dry was slower in the earlier stages, and equal and then better for the prolonged baking period. The experimental data collected for the two enamels indicate that, under th'e conditions prevailing in these tests, the rates of drying or film hardening for each enamel were about equal when using either radiant heat or convection oven baking. The metal panels used were relatively thin, and the operating conditions were regulated to secure similar time-temperature curves. These results do not preclude the possibility of practical advan-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December, 1944
TIME IN MINUTES Figure 5. Time-Temperature Heating Curves for Various Enamels 0 Lamp black in nodium silicate; A black enamel (wet Hm); 0 enamel; X red enamel;
yellow enamel; V white e n a m e y "
tages of one type of equipment over the other under normd operating conditions in the baking of objects of other sizes and shapes. ABSORPTIVITIES
I n order to calculate absorptivities, time-temperature heating and cooling curves were determined for the following five synthetic enamels representing five different color pigments: Enamel White Yellow Red Green Black
Pigment Used Titanium dioxide Chrome yellow light, C.P. Toluidine red' o Chrome reen medium, C.P. Carbon hack
o
1135
panel and operating conditions employed for the five enamels. The energy received was evaluated from the theoretical heating Equations 1 to 3, using an assumed average value of 0.96 for the absorptivity of the coating of lampblack in sodium silica@e. McAdams (6) gives emissivities for such a coating which vary in value for different temperatures from 0.952 to 0.967. This particular method was used to avoid the construction of an evacuated black body receiver as required in the method of Cartwright and Strong (IO). The experimental heating curves obtained for the lampblack in sodium silicate as well as the five enamels are shown in Figure 5. Using the assumed absorptivity of 0.96 for lampblack in sodium silicate, the energy received was firat evaluated. The measurement of the surrounding air temperature, Ta, presented some experimental difficulties and was best obtained from analysis of the cooling data. For these calculations T a was determined by plotting AT representing equal time intervals against T,using the experimentally determined time-temperature cooling data and extrapolating to AT = 0 (point where straight line of AT vs. T drawn through plotted points crossed the T axis). Initial temperature TOand final temperature T, in the heatingup period were measurable values, and the thickness of the panel (0.0418 inch), aa well as density and specific heat, was known. From Equations 2 and 3, h and I could therefore be calculated from the time-temperature heating data. The average value of I was found to be 3229 B.t.u./(hr.)(sq. ft.) or 6.57 watts per square inch. This calculated value of I was used in the determination of absorptivities for the five enamels. I n the case of the black enamel, heating data were collected for a freshly coated panel and also for a previously baked panel. For the previously baked panel no heat losses for the vaporization of the volatile solvents were involved, and the calculations were simple. For the other data allowances were made for heating and driving off these solvents. The composition of the enamel was known, the film thickness of the baked enamel was measured, and the energy theoretically required to heat and vaporize the volatile solvents was calculated. The solvent was largely xylene, and therefore the heat capacity and latent heat of vaporization of xylene were used. This energy loss was considered to be distributed over that portion of the heating curve from TOto a point equivalent to the boiling point of xylene. For the black enamel the calculated absorptivity for the wet enamel was 0.87 and for the dry film 0.885. For the other enamels only wet film data were collected, and similar allowances for solvent vaporization were made. The calculated absorptivities are shown in Table 11. The experimental data were collected over a period of weeks, and the prevailing room temperature of the laboratory varied
l
The vehicle for these enamels consisted of 50% melamine and 50% glycerol phthalate resin solids; the thinner was xylene, butanol, and hydrogenated petroleum. The alkyd resin solids were nondrying and semidrying oil-modified resins. The intensity of the lamp was determined by heating and cooling a coating of lampblack in sodium silicate with the same
TABLE XI. CALCULATED ABSORPTIVITIBS Enamel Lam black in Na silicate Blacf dry film Black [wet film$ Green Red Yellow White
Tm, OF.
506
441 440 415 293 354 306
TO OF: 77 69
69 75
74
7s 73
Ta OF:
A
150 130 126 120 124 130 114
0.96 (assumed)
0.885 (calcd.) 0 . 8 7 calcd 0 . 7 3 {calcd! 0.64 calcd) 0 56 calcd) 0 ' 3 8 [calcd:)
TIME IN MINUTES
Figure 6. Calculated Instantaneous Distribution of Energy Obtained by Heating Black Enamel (Dry Film)
1.136
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 36, No. 12
The useful energy employed was greatest in the early stages of heating and decreased with increments of T and 0. Con. versely, the convection and radiation losses were lowest in the early stages and increased with increments of T and 6. The losses by convection and radiation approached a limiting value equal to the total energy received minus the losaea by reflection. When T, is reached, the energy received from the lamp is equal to the sum of reflection, convection, and radiation lomes. Thus for any energy output the temperature which an object will utilmately reach is determined by its reflectivity and the losses by convection and radiation. Since the reflectivity may be considered fairly constant, any decrease in the other losses will result in higher working temperatures. It must be borne in mind that the data represented by Figure 6 were taken under laboratory equipment conducive to high convection losses (one lamp heating only one side of panel). While the results must be considered relative, the performance principles are significant; even though the distribution of losses might be out of line with results obtained commercially, the factors governing the losses can be established and equipment designed to minimize losses. I n a well-built oven the prevailing air temperature will be fairly high and the rate of heat loss therefore proportionately less. Arrangements can also be made to reflect energy back to the work by coating the inside of the oven with a highly reflective material.
c3
E a
0
w a v) v)
W
z
0
a a I Q
3
z
TIME IN MINUTES Figure 7.
Sward Hardness for Green Enamels Varying in Composition
0
100% melamine-urea; A 50% alkyd-50% melamine-urea; 0 75% alkyd-25% melamine-urea; 0 87.5% alkyd-12.5% melamine-urea;
X 100%alkyd
somewhat. The black enamel data wete collected while the room temperature was 69’ E?.; for the other cases the temperatures were somewhat higher (73-78” F.). During the baking of the red enamel, the toluidine toner darkened markedly and the absorptivity changed as baking progressed. The value given represents an average. Theoretical time-temperature curves constructed from the calculated convection constants and absorptivities conformed closely to the experimental curves. This reaffirmed the validity of the form of Equation 2. ANALYSIS OF DISTRIBUTION OF ENERGY
Figure 6 shows graphically the calculated instantaneous distribution of the radiant energy received during the baking of the black enamel (dry film). The energy received from the lamp was considered to be distributed three ways: reflected energy, useful energy employed, and energy lost by convection. Reflection losses were considered constant for each enamel over the entire heating period. This was based on the premise that the total absorptivity of a ‘‘black body” is unaffected by temperatures and that many solids approximate the “black-body” state for infrared radiation (IS). The energy used in heating and vaporizing solvents represented a very small percentage of the total energy. No such losses were involved in the heating of the dry film, &s Figure 6 shows. Since the surrounding air temperature T, was higher than the initial panel temperature TO,convection heating contributed to the temperature rise until the panel temperature x a s equal to the air temperature-Le., T = T,. From this point on, convection currents had a cooling effect, The radiant heat up to this point w&q completely utilized except for that portion reflected. These conditions are indicated in Figure 6.
20
40
60
80
I
% MELAMINE-UREA Figure 8. 0 Baked
Sward Hardness vs. Composition of Green Enamels 3 minutemi A 6 minutes; 0 9 minutes, X 24 minutes
The estimation of losses by free convection is complicated. Principles already established in other studies may be considered fiignificant in the design of radiant heat ovens. Strong (11) reviews work done by Langmuir on the relative heat loss by convection of objects in various positions. Langmuir found that heat losses by free convection from a horizontal surface facing upward are 10% greater than they are from a vertical surface, and they are 60% less from a surface facing downward than they are from a vertical surface. This would indicate that the
December, 1944
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1137
That baking temperature has a marked influence on the rate TABL~D 111. HARDNESSREADINGSAFI~CR BAKINGOF THREE of hardening of urea resins (greater hardness being obtained in MELAMINE-UBBIA ENAMELS shorter time periods with the increase of temperature) has been -Av. Sward Hardnessshown in the literature (4, 9). The mftlamine resins behave Enamel Pigment 3 min. 6 min. 9 mm. 41 4a Titanium dioxide (white) a similarly but can be baked a t lower temperatures. Baked at 60 52 Chrome yellow light 20 the same temperature as urea resins, they will attain equal hard61 Chrome green medium a4 60 ness in shorter periods of time (5,8). This accelerated conversion of polymerizing type materials at elevated temperatures, therefore, would seem ideally suited customary procedure of constructing radiant heat ovens in tunnel to radiant heat baking. Depending on the amount of energy form would require variations in heat density on the different sides furnished by the lamp source, a rapid rise in temperature may to attain equal temperature rise for the heating of equal mass. be attained and fairly high temperatures maintained with lamps. The equations given for mass heating by radiant energy are A film-forming composition containing a portion of urea or strictly applicable only to relatively thin objects of high thermal melamine resin would use to advantage this distinctive feature conductivity. For the heating of thicker objects or materials of radiant heat baking. of low thermal conductivity, the equations must be modified. Table I11 shows Sward hardness readings obtained in the bakSuch changes result in a complicated problem of thermal coning of three 100% melamine-urea enamels made with titanium duction in an unsteady state. I n the heating of objects of exdioxide, C.P. chrome yellow light, and C.P. chrome green medium, tremely low thermal conductivity, the conduction of heat respectively. The enamel films were baked a t a distance of 6 through the object would be low and the temperature at the inches from the lamp. This table shows the effect of differences receiving plane would rise more rapidly than the mass heating in temperature rise resulting from differences in the absorptivities equations indicate. This would be an advantage in that high of the enamels. surface temperatures can be obtained rapidly. It would have the To use radiant heat to best advantage, it is obvious that disadvantage that equalization of temperature in the work consideration must be given to the thermal laws governing its by radial conduction would not be possible in cases where the behavior. The absorptivity of the receiving surface is an imenergy received is not uniformly spread. From the standpoint portant factor, for the reflection of energy becomes the major of temperature rise only, this condition is indicative of potential loas ili heating objects having low absorptivities. The air temheat economy in the baking of coated wooden surfaces or other perature of the infrared oven should be as high as possible, for materials of low thermal conductivity. decreased convection losses enable the object being baked to BAKING OF POLYMERIZING MATERIALS reach higher temperatures. However, since the energy received from the source is entirely lost by reflection, convection, and The outstanding feature of radiant heat baking is that a great radiation when the maximum temperature is reached, prolonged amount of energy may be directed upon a body, causing a quick baking periods could become economically unprofitable. The rmponse and producing a rapid rise in temperature. The types materials to be baked with radiant heat should be chosen for of products and materials baked by radiant heat lamps should their adaptability to the advantages offered by it. The effect be chosen for their adaptability to this baking. of polymerizing materials on the rate of film hardening at high The plotting of Sward hardness against baking time for the temperatures was demonstrated above. Data on all types of five enamels listed in Table I is shown in Figure 7. The broken baking materials are needed. Economy and increased produclines indicate panel temperatures at various times. tion will result from the judicious formulation of protective coatThese curves show that, as the percentage of melamine-urea ings for the high-temperature short-schedule baking conditions resin content is increased, the ultimate hardness as well as the made poasible by radiant heat baking. rate of hardening is increased. The rate of film hardening was found to rise sharply when the composition reached 50% melsACKNOWLEDGMENT mine-urea. I n a series of tests such as this, all factors cannot be kept on a comparable basis. This group of curves shows the The authors wish to express their appreciation to P. H. Goodell of the Trumbull Electric Company, formerly of the C. M. Hall effect of the polymerizing resin on the rate of film hardening Lamp Company, for furnishin the lamps for this work. They as the percentage of melamine-urea is increased from 0 to 100. wish especially to thank Frank%. Tiller of Vanderbilt University The alkyd resin combination used in conjunction with it is not for his constructive criticisms and kind suggestions. The assistrepresentative of the type alkyd or the combination which would ance is acknowledged of s. Hunter and W. Breidenthal, students at the University of Louisville, for construction of laboratory be used in a straight alkyd formulation. These curves should equipment. Albert Kimmel, graduate student, assisted in portherefore be viewed as a demonstration of the effect of increasing tions of the experimental work. A number of other people pave amounts of polymerizing resin in a given system. To make a valuable suggestions pertinent to problems which arose a t various comparison with a straight alkyd baking formula, an enamel made times. with a representative oxidizing baking alkyd should have been LITERATURE CITED used. The inclusion of such an enamel would perhaps have given Bennett and Haynes, C h m . & Met. Eng., 47, 106 (1940). a clearer picture of the effect of polymerizing material in highGoodell, Am. Inst. Elec. Engrs., Tech. Paper, 40-156 (1940). teniperatur short-schedule baking. Hodgins, Hovey, Hewett, Barrett, and Moeske, IND. ENQ. I n Figure 8 the experimental data are shown in a different way. CHEM.,33, 769 (1941). Hodgins, Hovey, and Ryan, Ibid., 32, 334 (1940). Sward hardness is plotted against composition for total baking Klinkenstein, Metal Fininiahiw, 39, 398-403 (1941). periods of 3, 6, 9, and 24 minutes. These curves show clearly McAdams, “Heat Transmission”, 2nd ed., p. 395, New York, that, as the melamine-urea resin content increases, the ultimate McCraw-Hill Book Co., 1942. hardness is approached more rapidly. At the 60% composition Nelson and Silman, Sheet Metal Id., June, July, Aug., Sept. (1943). the difference in hardness reading between total baking times of Sanderson, Paint, Oil Chem. Rep., 102, No.8 (April, 1940). 6, 9, and 24 minutes is not great, and the advantage of baking Ibid., 102, No. 12 (June, 1940). 24 minptes would be very little over baking only 6 minutes. Strong, “Procedures in Experimental Physics”, 1st ed., pp. There is, however, a difference of 4 to 1 in electrical energy con305-7, New York, Prentice-Hall, 1941. Ibid., pp. 505-7. sumed. For this group of enamels and time-temperature conTiller and Garber, IND.ENO.CHIOM., 34,778 (1942). ditions, it would seem that, beginning at a point equivalent to Walker, Lewis, McAdams, and Gilliland, ”Principles of Chemical 30% melamine-urea resin content, there would be diminishing Engineering”, 3rd ed., pp. 147-9, New York, McGraw-Hill vctlue in baking for more than 6 minutes. Book Go., 1937.
