INDUSTRIAL AND ENGINEERING CHEMISTRY
152
During the drying of the varnishes, irregularitias in the evaporation of solvent occ&iody prudueed wavy, w r i d e d , n corrected by utiliaor orange peel surfsoea These eflecte c ~ be ing a balsnced solvent for thhez. It WBB dm noted that these wrface -ties dieappearea during the bsldng p r m of the straight sino h m t e varnish, f&,,tly olmdy the 6hue were formed. This condition WBS readily avoided by addition of a 4 perceotege of turpentine. A definite dein drvingtime wea obtained in the preeence ofturpestine. It &odd be mphasl;ed again k t the -g&a described are not the bed obtainable with thw materials, nor was any attempt made to make them 80. The development of an OPtirrmm vsrnish combination is a major problem in itself; OUT pur% pose was to qdbtive comparisons to be bm. to W a n a n t h o u s comidmtion of dimer mid saka as varnish reainS. St& IUQ conternplated which have as objectives the hpmVement Of varnish 61ms laepared with
-
d d d dimerized acid salta.
Vol. 36, No. 2
ACKNOWLBDGMENT
The authors @atefully ~kmwledge their indebtednea to A. J. Lewis for aeaktance in formulating and testing the wmiahes pnepared, to R. W. Powers for carrying out portions of the experimental work, and to the Analytical and Physical chemical Division of t h ~Northern Regionsl Research Laborstory for the &Ym. lITERATuaECZTED (1) Bradlw, T.F.. U. 8. Patent 1.7W.375 (Nov. 4,1930). (2) Clark, G. L.. "Applied X-Rays". 3rd ed., p. 474. New YoFk, McGraw-Hill Co.. 1940. (8) Cowan, J. C.. FalLenburg, L.B..and Teeter. H.M.,unpnb. work. (4) H W . 0..J . Clrstn. PAW.. 3,42 (1936). (6) H d o r n , Max. U. 8.Patent 2,046,080 (JUW 23.1936). (6) I. 0.Farbenindustrie. A A . Brit. Pstant 420.633 (Den. 3.1934). (7) Kemp. A. 8..nnd Peterr. E..IND.m e . Cray.. S3.la8a-9 (1941) (8) IW., 34,1W7-1102 (1942). before the Di-on of Paint. vu&. and F%~timChomiatw at the 106th MwtiO. of tho h a x i r c r m Camnorc ~OCI-, Pitiaburgh. P..
Pu-m
PLYWOOD BONDING Henry Grinqfeldet and M . R . Collins, nizsmous PRODUCTS The objmtive of thin work was to establiah a method for detenninii a minimum bonding t h e aehedde applicable to a wide ra.iety of plywood constructiom a d bonding conditions. E a t penetration measurements have beeol made in plywood of various thicknesses plreed betaheated ph-. The time neeennary to cure the resin adhdvehaabwtahIi&ed foroneparticulprplywoodconafilction and then faloulntd to m m t for the diSerenm between phtem temperature and actual resin temperatmi the rete of polymdzationof the -in adhesive at merent mnpmahrrais t h determined. ~ The reaction rate of the rrsin invemtigated apto double with e v a y 2 8 O F. rime in tempmature. The bonding time for a paaicular con-
T
HE demand . . for durable, . resin-bonded plywood has created
W UL the mechanism of bonding and the determination of plywood bonding time and temperature relations. The most generally used redins for the manufactwe of high quality plywood m the ec-dlcd thermosetting resins of the phenol-formaldehyde type. These resine are available in the form of &e&, ea watemluble powders, or ea solutions. Although tbeze resins sre described BS thermosetting, they posess a tbermoplsatic stage which determines their usefulnees. Wben a pack of veneers,either interlead with a film glue or apread with a p h o l i c resin solution, is made into plywood, the sequence of events producing a bond between adjacent veneers is a8 follows: a mwmg m
&ins of the adhesive t
~
.
__
S. Eventually the reain L m e s infusible and insoluble, and produces a strong durable union between the veneera.
The amonnt and auration of resin flow is determined by eharacie of the lesin, moishw content, temperature, rata of best input, and premnra. The &ta of experiments to il-
AND CHEMICAL COMPANY. PIIILADELPgu. PA.
stmction in obtained fmm heat-penetration and ratedfnsotian curyes hv a modifioation of the Bryant method. A stepped E W W developed &w the a c ~ u a lcurve, in whieh the temperature L MSUmed to remain constant at one temperature for the time necasary to increaw 10' F. Us* the curing time at various temperatin the midpoint of each step, the proportion or percentage of the total enre attained during each increment b dcuhted. When the s u m of the percantagem for the sucee(yILyeetap totab 100%. enre is assumed to be mmplete. The cotrespondiq time ie taken e% the minimum prmhsible bondtimc. Predug t h e e for a number of consrructiow can beplotted to rep-t a gemerdizd bonding schedule.
*
lwtmte tbia concept of resin flow are presented in Figure 1. These dnta were obtained in a study of the Bow character and curing rate of a relatively dry p o w d e d resin for use ea a plywood adbeaive. Other teste on powder conteining a higher moisture content indcate that resins of this type, which are water soluble, Bow to a W t e r extent when wet thsn when dry. The flow character tost is as follows: Flow out a film of the resin solution on a clean glass plate. Dry the resin film a t mom t e m p e r a h cr ea low a temperature as possible to d u c a polymerizationof the reain. h p e the dried film from the gbss plate with a m o r blade, snd grind the scrapinga to a uniform sise powder in a mortar. Prepare a pellet 01 the powdered rosin in a pill machine, and place the pcllet between electrically heated platena which are set at a definite and constant tampcrature; usually a piece of paper ia placed on both sides of the pellet to prevent sticking to the hot plates. Apply a known pressure to the pellet for a definite time interval. remove the pellet, and messwe the attained diameter. Calculate the corresponding flow in terms of aversge p r e m m m pounds per mure inch of avcrsge pellet ~ u e aand convert to k w at conatsnt area and M) pounds per square inch area Rep& with another pellet at another time of hest and pres$ure appliartion, etc. It ia d i 5 c d t to extrapolate the results plesented in Figure 1 to the bonding of plywood, but the graphs do indicate the approximate deptb of penetration possible with tbk resin at &ow
.February, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
in cold water. He concluded that the wet test is more discriminating than the dry; and while recognizing that longer bonding times further increase shear strength, he defined a wet shear strength of 400 pounds per square inch as sufficient for plywood. Bryant assumed that the time for the glue to reach within 5' of the platen temperature is one minute; he therefore subtracted one minute from the actual bonding times used. For many years this company has conducted curing time studies by a method similar to that of Bryant. However, we have used three-ply, l/ls-inch birch veneers cross-grained as plywood as the test construction, and both the dry plywood shear test and the boil-and-soak test for determining the completenessof cure of the resin. Our experience has been that three-ply cross-grain plywood is somewhat more difficult to bond than two-ply parallel-grain laminatcd wood, so that a strength of 400 pounds on a two-ply test specimen does not necessarily T I M E OF HEATING ( M I N U T E S ) mean that 400 pounds will be developed on a three-ply cross-grain specimen. This can best be explained by Figure 1. Extent and Duration of Thermal Flow of a Powdered analysis of tho forces of type of specimen; it Phenolic Resin at Several Temperatures suffices to state here that the two-ply specimen measures true shear, whereas, due to the core, a temperatures and also the duration of flow a t the various temthree-ply specimen measures a combination of shear and rupperatures. The data do not indicate the curing rate of the ture caused by the couple of forces. On a dry shear strength resin. basis, Truax (8) found that the relation of two ply to three As heat is supplied to the resin, the chemical reaction of polyply is of the order of 640 to 375 pounds per square inch when merization is accelerated. If enough heat is supplied, the resin l/ls-inch veneers are used for dry tests, and .300 to 220 for wet becomes highly polymerized and is then identified as "fully tests. If this relation holds, Bryant's criteria of 400 pounds cured". During this polymerization process the resin becomes wet is equivalent to 295 pounds on three-ply, */le-inch birch progressively more tenacious to wood and progressively less water plywood and is therefore slightly lower than required by an soluble. If enough heat is supplied, the ultimate in tenacity Army and Navy"p1ywpod specification (1). and insolubility is reached. The ultimate in tenacity is deAfter curing time has been determined for a plate temperature, sired in commercial operations and it is of practical interest to it is necessary to correct for the glue line temperature-time curve know the minimum time requirements to attain the ultimate so SO that absolute curing time data may be obtained for temperthat maximum high quality production will result from hot atures in the range under study. It is &st essential to select presses, autoclaves, or high frequency heating units. the dry and wet joint strength that will be considered satisfactory. I n 1936 bonding time schedules were based on a half-hour boil CURING TIME test and a shear strength value of 275 pounds per square inch. To supply a bonding schedule for each plywood panel conThis has gradually altered through the years as requirements of struction employed in commercial practice would be a n endless task. To supply the curing time requirements of the resin 550 would be meaningless, for the time and temperature treatment 500 that occurs in the plywood must also be known. Several methods have been suggested for determining the curing time of a resin. Kline, Axilrod, and Turner proposed 400 a method based on measuring the curing time in terms of acetone solubility (6). However, some phenolic resins are insoluble in acctone but soluble in water so that this method has its 300 limitations. One test, developed by W. S. Niederhauser of this company, consists of curing a pellet of the resin between strips of muslin. The heat is supplied for various times by electrically 6'" 7 8 9 IO I / 12 13 14 15 16 17 /a 14 heated platens (set to ~b definite temperature), which are pressed MlMUTES AT 24OoFT P L A T E N TEMPERATURE. 4 5 6 7 8 9 against the outside of the muslin strips. The muslin and pellets 3MhVUTE5 A T 280% PLATEN TEMPERATURE. are then immersed in alcoholic sodium hydroxide solution for BONDING TIME 24 hours, boiling water for 3 hours, or cold water for 48 hour*; the curing time is determined by the adhesive strength charFigure 2. Plywood Shear Test (Three-Point Moving acter of the resin for the muslin, its solubility, and its freedom Average) vs. Bonding Time a t 240' and 280" F. from disintegration. This test has been helpful, and the results can be duplicated quite accurately.' However, no method for relating strength as quality become more strict so that now a dry shear of 410 pounds developed on muslin to strength necessary in plywood bonding is used with a 3-hour boil test of 380 pounds when the plywood has been established, so that it has not been possible to determine test specimen is prepared in accordance with Specification the curing time necessary t o develop the strength of bond reAN-NN-P-51l a (1). quired by plywood. Occasionally two-ply parallel-grain panels are bonded, and then a 48-hour soak test is required to produce a test better Bryant (2) bonded two-ply, parallel-grain, l/le-inch birch veneers a t various times and temperatures. He then deterthan 425 pounds, accompanied with more than 2501, of the failure mined the shearing force necessary to cause failure, both on dry in the wood. The 48-hour soak test appears to be more acsamples and on samples which had been soaked for 24 hours curate in determining the degree of cure of a hot-setting phenol
"
1
153
INDUSTRIAL AND ENGINEERING CHEMISTRY
154
Vol. 36, No. 2
wet, 5.5-minute bonding time a t 280” F. or 13 minutes at 240“ F. is required (Figure 2). This time, however, is at a temperature of 280’ and 240’ F. for the platen and not necessarily for the resin in the glue line. Figure 3 represents the temperature data obtained with thermocouples insertcd in the glue lines during the bonding operation. To determine the actual bonding time requirements in terms of glue line temperature, the smooth curves in Figure 3 are replaced by stepped curves, similar to the method of Bryant (2). Each step is calculated as a partial polymerization of the resin for the time the resin is a t that step. I n common with most chemical reactions in which the rate is a €unction of temperatuie, the setting or “curing” of a phenolformaldehyde resin is accelerated by heat. Usually it can be assumed Figure 4. 500 I
2
3
4
5
6 7 8
9 /O I I
12 13 14 15
Relative Curing R a t e Estimation for Amberlite PR-14
T I M E MINUTES
Figure 3. Temperature Penetration in Plyw-ood resin than does the boil test. In the boil test the heat may reach the glue line before the water and may aid in completing the cure of an uncured resin (8, 3). DETERMINATION O F BONDING SCHEDULE
In establishing the bonding schedule for Amberlite PR-14 resin, two formulations were used. An unextended mix was made at 40% solids in water solution, the usual unextended hotpressing formulation used in the rubber bag process. This mix was spread on l/le-inch birch veneer of 7% moisture content at the rate of 30 pounds of liquid spread per 1000 square feet of sin le glue line. The veneers were then given a t least an overnigit open assembly before pressing at a series of times and temperatures. Also used was an extended formulation devised to permit a short closed assembly in the hot pressing of flat plywood panels. This formulation consisted of: Amberlite PR-14 Walnut shell flour Ethanol (specially denatured, formula 2B) Water Total
c
0
Parts 44
11 22.5
22.5 -
100
This mix was spread at the rate of 28 pounds of liquid per 1000 square feet of single glue line on 1/18-inch birch veneers having an original moisture content of 6%. The panels were given a one-hour closed assembly before pressing. The solvent introduced by this spread increases the apparent “moisture content” from 6 to 11-13?& depending on the panel construction. In the case of the overnight open assembly, only one eighth of the resin solvent was retained, and the moisture content was thus increased to about 8%. The additional liquid in the case of the closed assembly does not cause SL marked change in the heat penetration of the panel, since half of the added solvent is alcohol which ha3 a lower specific heat and a lower heat of vaporization than water. At each of the two temperatures, a series of progressively increasing bonding times was used at a constant press delay of 30 seconds. The panels were di ped in cold water immediately on removal from the press, in or& to stop the cure. To produce a smooth curve and portray more accurately the improvement of plywood shear strength with increased bonding time, a three-point, moving average graph is plotted for each platen temperature under study. The three-point moving average is a mathematical device to portray statistical trends; i t is most commonly used with data that are scattered. Plywood shear tests have a tendency to result in data of this type due to variations in the grain of the wood, density, pore structure, and other reasons. Use of a moving average tends to reduce the scatter and often produces a smooth curve. The plywood shear test values of 2.5, 3.0, and 3.5 minutes (Table I) are averaged and identified as the average of 3.0 minutes. Then the shear test values for 3.0, 3.5, and 4.0 minutes are averaged and identified as the average of 3.5 minutes (Figure 2). Using the criteria of 410 pounds per square inch dry and 380 pounds
2
0
u
7
$ 200 7 I 2
Figure 5 .
2 2 4 2 3 6 2 4 8 760 772 2 8 4 246 308 326
Calculated R a t e of Cure of Amberlite PR-14 (Absolute)
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1944
that in organic chemistry the reaction rate doubles for each 10" C. (or 18' F.) rise in temperature. 30 However, when this assumption is made with Amberlite PR-14, the results are not consistent 26 from one heat penetration curve to another so that it is necessary to calculate the change in curing rate of the resin with changes in temperature. First, T is assumed to be the absolute curing time of the resin in seconds a t 320" F. Then, 180 since a reaction rate that doubles with a definite temperature rise can be plotted as a straight $ line on semilogarithmic paper, a series of straight lines are drawn to represent reaction rates that double for each 16", 18", 20" F., etc., rise in tema: perature. All of these lines (Figure 4) pass through point T a t 320" F. and represent the curing time at other temperatures as some multiple $0 4 of T. From them the curing time may be ob-. tained a t any temperature in the range being considered, but only in terms of T. I n Figure 3 steps are drawn below the actual 260 heat penetration curve to approximate an integration of the curve. These steps are for each 220 10" F. rise in temperature, and from the curve the duration of each step is determined. The curing rate for each step is determined in terms 180 of T for the temperature at the midpoint of each :: step by reference t o one of the lines of Figure 4. 2 The duration divided by the curing time equals 2 Q the portion of a cure a t that step. 2 Since the 280" F. curve (Figure 3) is cured a t s ? 4 5.5 minutes and the 240" F. curve at 13 minutes, the sum of the steps for each curve must be one complete cure a t those times. By a simple algeFigure 7. braic equation T is calculated; however, T must be the same for both the 240" and the 280" F. lines of heat penetration. When using the 16' F. line in Figure 4, T is not the same for both the 240" and the 280" F. heat penetration line. Similarly, the ISo,20", 22", and 24" F. curves do not give a proper solution for T. Only the 28" F. line presents a value of T which meets both heat penetration curves.
Figure 6.
8
12
Heating Rates at 300' F.
12
20
/6 TIME
8
155
74
28
4 MlNUTES
8
- MINUTES
16 TIME
-
32
36
I2
16
40
b
Effect of Moisture and Effect of Tego in the Glue Line
I n this particular example, therefore, it was found that the reaction rate doubles for every 28" F. rise in temperature and that the time t o cure the resin at 320" F. is 83 seconds. From these data Figure 5 is plotted which provides the time for cure (as defined in plywood tests) for all temperatures between 200' and 320" F. HEAT PENETRATION
TABLEI. AVERAGES OF PLYWOOD SHEARTESTS ON AMBERLITE PR-14 BONDED PANELS Time in
Dry Tests Av. of 3-Point 2 panels Moving Av.
E 2 st!';.'::$ap
,f;PA.
Wet Tests'" Av. of 3-Point 2 panels Moving Av.
i;Fi/n.
&:$.
pa$:$
Panels Pressed at 280' F. h
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6 0 6.5 7.0 7.5 8.0 8.5
257 295 310 345 360 446 425 439 456 641 534 552 489
0 5 5 5 0 5 10 20 10 50 30 25 15
ii?
317 338 384 410 437 430 479 510 542 525
...
..3
5 3 3 5 12 13 27 30 35 37
..
85 244 345 261 345 380 393 346 372 456 465 416 432
0 0 10 20 10 50 30 50 20 30 70 40 70
...
..
225 284 317 329 373 373 370 391 431 446 438
3 10 13 37 30 43 33 33 40 47 60
ii6
13 16 28 35 40 33
..,
..
Panels Pressed a t 240' F. 10 388 10 11 407 io iii 12 385 20 395 20 418 13 392 14 477 40 419 15 387 10 449 20 484 30 456 30 496 90 Following a 3-hour boil.
. F-
...
ii
17 27 23 27 43
..
383 388 362 403 435 350 400 389
2
20 15 50 50 20 30
384 400 396 396 380
...
.. ..
I n plywood panels, bonded either in hot presses or steamheated autoclaves, the glue line farthest from the heat source is the last to cure. Hence, if it is cured, all the other glue lines in the panel must necessarily be cured also. The variables affecting heat penetration into wood have been studied thoroughly, and excellent methods described for calculating the time-temperature relation for wood panels and packs of plywood (4, 6). However, while these methods provide a method for caloulating the temperature in a pack of wood a t any one time, it is tedious to calculate the entire heat penetration curve. These mathematical methods are somewhat approximate in that the kink in the curves caused by the entrapment of moisture is not easily calculated, nor are the edge losses which occur with thick panels. The variables affecting heat penetration have been studied in the laboratory by measuring heating rates with an ironconstantan thermocouple in constructions of varying thickness. Some of this information was reported several years ago (7). At that time, however, the emphasis was on grades of plywood other than aircraft plywood. That work was also done before the use of phenolic rehns in liquid form was considered to any great extent. I n Figure 6 the rate of change of temperature with time is plotted for seven constructions, varying in thickness from three to fifteen plies, for a platen temperature of 300" F., 8%
INDUSTRIAL AND ENGINEERING CHEMISTRY
156
V
1
I
I
I
I
Figure 8.
If the press closing time is too long, especially with thinface venecrs, the glue line nearcst the platen tends to polymerize before adequate pressure is attained. This gives rise to the condition identified as precure. The use of wooden cauls, other insulating medium, or faster press closing will eliminate this trouble. The effect of resin in the glue line is shown in Figures 7 and 8. Resins tend to suppress the escapc of moisture by introducing a semipermeable barrier at the glue line, but the time required to reach a temperature approaching the platen temperature is somewhat longer. Also, the liquid resin chocks the escape of vapor and minimizes the panel edge effects even better than does the film glue. Figure 9 gives typical glue-line temperature-time curves obtained in the rubber bag molding process for molding plywood.
I
IO 20 30 PRESSING T I M E (MINUTEES)
0
Time-Temperature Curves for Various Pack Thicknesses
40 3 0 - 280
'6 20-
260
2Q 10 - 240 0-220
200
G 180 0 I
160
2 I40
$Q I 2 0
2 100
$
80 6o0
2
4
6
8
CURING TIME CALCULATIONS
40
veneer moisture content, and no adhesive in the glue lines. In the three-ply assembly the rate of heating is rapid, the amount of moisture is small, and a smooth curve results. On the fiveply assembly, however, a slight kink is observed at 240" F., indicating a slowing of the heating rate as the moisture vaporizes and escapes. The break becomes more pronounced and of longer duration as assembly thickness and, consequently, total moisture content increases; finally, in the fifteen-ply panel the resistance to heating and edge losses, both of heat and moisture, are so great that no sharp break is discernible. Increasing the moisture content of the wood naturally results in a lower heating by virtue of the longer time required to vaporize the moisture and the greater amount of heat lost in the escaping moisture. The heating rates for a seven-ply X l / ~ inch construction a t 8, 16, 20, and 25% moisture for a platen temperature of 300" F. and no adhesive between veneers are given in Figure 7. The longer times required to reach a given temperature a t high moisture contents are particularly important in bonding with liquid glues a t short, closed assembly times. Although the thermal conductivities of different wood species vary, the differences are not sufficiently great to affect the heating rates for practical purposes. The pressure applied to the assembly also influences the rate of heating; but within the range of practical bonding pressures (Le.] 100-300 pounds per square inch) the differences are less significant than those contributed by the nonuniformity of wood in general.
Q
Vol. 36, No. 2
12 14 I6 18 20 2 2 24 26 2 8 30 32 T I M E IN MINUTES
IO
Figure 9. Glue Line Temperatures Producrd in Bag Molding Process
The pressing time for a given construction is obtained from the time-temperature heat penetration curve by a modification of the method of Bryant (a). A stepped curve is constructed below the actual curve (Figure 8) in which the temperature is assumed t o remain constant for a definite time, then to rise suddenly IO" F. to the actual curve, remain constant for a definite time interval again until it is directly below the next 10" F. rise, and then rise to the actual curve, etc.
0
10 15 BONDING T I M E
5
-
20 25 MINUTES
30
Figure 10. Bonding Time for Flat Press Plywood. Bonded w-ith Amberlite PR-14
The point of final cure for each panel is established by using as numerator for the fraction of cure, the number of minutes required for the glue-line temperature to increase 10" F., and for the denominator the number of minutes required to cure the resin at the temperature in the center of each step, obtained from Figure 5. Thus, in the example shown in Figure 8, the glue-line temperature remained between 200' and 210" F. for 1.25 minutes; and since Amberlite PR-14 is shown in Figure 5 to cure in 23.5 minutes at a glue-line teniperature of 205" F., the fraction of a cure between 200" and 210' F. will be 1.25/23.5 or 0.0532. By applying this stepwisc procedure along the curve a t IO" F. intervals and adding the fractions of a cure at each step, the final cure for this construction is obtained a t the end of 24.75 minutes. The values obtained by using IO" F. or I-minute increments are well within the experimental error; the former method shows cures which are longer by about 1 minute in 25-30 minutes. Inasmuch as this creates an error on the conservative side and constitutes a far simpler method involving fewer calculations, IO' F. steps are used throughout in the determination of bonding time schedules. When the bonding times are calculated, a 10% safety factor is added and the result is rounded off to the nearest half minute longer.
,-I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1944
Wben the bonding tima are obtained for the vsriouS panels ahown in Figure 8, theu individual thickm€s?~ are Plotted a& cum tima (Figure 10) to give a d e C u N e to be used in the bonding of panels of various thicknesses a t a Platen temPeratUre of a000 F. The cum time for bag-molded structures can be determined
187
ACKNOWL&DGMW
D. K. Rider and W. H. G r d ably cooperated in obtaining much of the data and preparing this paper. W. Niederhauser hss slao been a 8 o m of valuable assistance and advice. LITERATURECITED
DURABILITY OF LUSTERLESS ENAMELS Influenee of the Binder i. E. Beck, GROUND,m * B E R D ~~ O V I N G
?
.
HI8 pper is the second of a series on the relation betwean the durability and composition of lusterleas enamela The first pper’ gave detaila of preparation and exposure of expmimmtal enamela These procedumn were followed in the preaent work The binders studied may be bmken into two main clsssea: o i l - m o W a l k y a and resin.and-oil-m&d alkyds. A few orthodox varniahen ara included to complete certain experimental U d m . The various bindere were incoqmated into a standard lusterleas olive drab enamel formula (Table I). This formuls waa used without change with the exception of thoee pmducta containing chlorinated rubber (pinta44 to 62) where the percentage of pigment by weight on solids bseis aas eat a t 61-68 hues of performance lepuirementa. Total mlidn of the pinta waa not leas than 60% by weight, and waa inu‘ead where nwssry to obtain sstisfadary viscosity. Driera used, on vebicle solids basis, w m : 0.03% cobalt, 0.01% manganese, and 0.20% lead In a number of eases CertaiD departures fmm the stgndard drier contentd were necwawy (Table 11). Enamels were ground in a ball mill. Painta were exposed a t Baltimore, Md., for one year on deel panels as described in the 6rat papa?. All paints w m airdried. Exposure mdta are given in Table 111, and eduatiom were made by the Bame observer. Flatinga for hardneas snd adhesion were eatisfaunlw abated otherwise in Table 111. 2
I-.
E m . C ~ B Y .3% . S84 (1948).
Tnsm I. F o m a a ME BINDS~R
Compasitiom given are bpssd on vehicle mlida Additiono were made on a c o l d a t basis except wbem othmvim indicated. OILMODIFIED AlXYDS
Difierences in durabiity between Iusbrlem enamels based on alkyd Rsins prepared with different oils are not w t .Tung appara to be the least eatisfactory oil for appearance retention. The only psnela showing poor film pro&@ were the W M ) linseed-tung; 50-50 lhesad4ehydrnted castor is beat in appearance retention by a narrow marpin. Very little differences in performance were found betwem alkyd Rsins containing 25 and 35% of phthalic anhydride.
RESIN- AND OlLMODlFIED ALKYDS
phenolic reaine. of the types studied. lower the apPem8nm of lusterlea enamelg Thin deet is prinupauY due to inoreased chalking. orthodox phenolia vnrniab chslk to the w i n t where they are unaatisfactors for outdoor E&& in lusterlea &ela Up to 10% by weight of phenolic reein or an equivalent weight of phenolia varnish @anbe incorpomted into the a k y d resin with no loes in appearance retention. The excellent film properties (othet than chalkinp) of phenolic resins are demonatmted by the fact that 88 much an 50% of phenolic resin a n be cold cut into the alkyd resin with M loas of metal-protecting properti@,even though the oil length of the binder is thereby reduced to approximately 8 d o m . Chlorinated r u b t e - a k y d resin mixture8 pmvide eminentlp eatisfaatory vehicles for lusterless emamek
