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INDUSTRIAL AND ENGINEERING CHEMISTRY
Rubber under tension shows an x-ray diffraction pattern indicative of orientation of the rubber molecules, first reported by Katz (2). If these two experimental phenomena are related, one could logically assume that orientation results in an increase in specific gravity which is responsible for an increase in refractive index. These observations on cured gum rubber compounds under tension gave rise to the theory involved in the second mechanism. The data and suggested explanations are offered to stimulate discussion of the observed phenomena and will be supplemented as further experiments are carried out.
Acknowledgment the suggestions Of A* The author Pfund of Johns Hopkins University and A. T. McPherson of
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the National Bureau of Standards and the assistance of Norman A. Brown who made the refractometer measurements. Thanks are also due The Goodyear Tire & Rubber Company for supplying the deproteinized rubber and Thomas Midgley, Jr., for furnishing the pure rubber hydrocarbon.
Literature Cited (1) Holt and McPherson, J . Research Natl. Bur. Standards, 17, 657 (1936).
(2) Kata, J. R., Chem.-Ztg., 49, 353 (1925). ( 3 ) McPherson and Cummings, J. Research Natl. Bur. Standards, 14. 553 ( 1 9 3 5 ) . (4) Menadue, jnd& Rubber J . , 85, 689 (1933). ( 5 ) New Jersey Zinc Co., The Activator, 3, 13 (1937).
PRESENTED before the Division of Rubber Chemistry at the 97th h5eeting of the American Cbemloal Society, Baltimore, Md.
Urea-Formaldehyde Film-Forming Compositions Enamel Formulation, Properties, and Durability' T. S. HODGIKS, A. G. HOVEY, AND P. J. RYAN
Reichhold Chemicals, Inc., Detroit, Mich.
last two years are mar-proofURING the past year the The properties of urea-formaldehyde resins ness, print resistance, color use of urea-formaldehyde as coatings are briefly reviewed. The resins retention, moisture resistance, resin enamels has proin solution form, both by themselves and good adhesion when used with gressed even faster than was in combination with alkyd resins and alkyds, and resistance to oxidathought possible a year ago (7, plasticizers, are discussed as to their retion, oil, grease, weak alkali, 11). Previous papers by weak acid, alcohol, and other Cheetham ( 2 , S), Pearce (IS), activity, stability, and solvent tolerances. solvents. I n addition, the fact Trussell (20), Sanderson (16, These urea-formaldehyde-alkyd resin that, in general, certain urea17), and the authors (4, 5, 6, combination vehicles are becoming widely formaldehyde resins do not have 14, 16) discussed the theoretical employed for enamels, not only for white an impairing effect on the durabackground and certain properbility of alkyd resins is bound baking enamels for refrigerators, hospital t i e s a n d uses. T h e t r e n d to stimulate their further use for towards greater utilization of equipment, metal equipment, metal many new applications. these materials warrants a more kitchen cabinets, etc., but also for colored -4large amount of the new thorough study of the commerenamels on account of the short baking urea-formaldehyde coating resins cial applications. period necessary to obtain extreme hardhas been used in connection with Apparatus for the manufacness, mar-proofness, and light-fastness. white refrigerator enamels beture of urea-formaldehyde resins cause of their superior hardness, tends to be highly specialized Formulative experience on vehicles conwhiteness, better retention of and relatively expensive (18). taining these urea-formaldehyde resins is whiteness, and better grease reCertain unique modifications of given, as well as on the white and colored sistance than the alkyd enamel urea-formaldehyde resins were enamels produced from such vehicles. without the Urea-formaldehyde described by two of the authors resin; but it is evident that only (8, 9, 10). Although previous a few of the many possibilities papers usually discussed industrial applications in general, this' article is restricted to the with this type of coating material have been considered. details of obtaining the best results from urea-formaldehyde Where mar-proofness and resistance to perspiration, abrasion, moisture, and other enemies of finishes are imporresins for industrial enamels. tant-for example, in finishes for automobile steering wheels, Present and Proposed Uses of Urea Resins flash plates, hardware, etc.-urea-formaldehyde resins have advantages.' For articles made of metal, which must resist The outstanding properties of urea-formaldehyde resins adverse conditions and still retain beauty, this type of finish which are responsible for their greatly increased use in the has been utilized with good results-for example, on metal I The t w o previous papere in this series were published in 1938 snd 1939 (4, 6). compacts, buttons, etc.
D
MARCH, 1940
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335
drying urea-formaldehyde resins, which has not yet been described and which will be treated in a subsequent article. The purpose of this paper is to discuss baking enamels.
Urea-Formaldehyde Resins
The mar-proofness, inertness, and good color retention make them of great promise for coating glass, particnhrly since they must hakc out so transparent that i t is difficult to tell where the urea resin stops and the glass begins; with any other sort of clear enamel baked on glass, the effect is like that of a cheap, dark, yellowing paint on clear glass. Metal furniture, t.ablc tops, cigaret stands, and possibly even phonograph records are a few new applications on which the extreme hardness and mar-proofness could be capitalized. When suitably plesticized, urea-formaldehyde coatings may find use for metal foil because of their resistance to oxidation. Urea-formaldehyde coating resins are admirable for polychromatic finishes for automobile bodies and other large metal applications, not only because they have light Color and show off the particles of aluminum in a pleasing, shimmering effect, but also because the early gelation actually makes possible this unique, beautiful polychromatic finish by stopping the movement of the aluminurn particles and preventing them from leafing to the surface. This phenomenon, together with the fact that certain urea-formaldehyde resins do not detract from the durability of the alkyd resins which have been found so good from this standpoint for automotive finishes, seems to ensure their being seen a good deal in the near future by the “man on the street”. A i r - D r y i n g Urea-Formaldehyde Resins
It is not yet universally known that urea-formaldehyde coating compositions are essentially polymerizing rather than oxidizing resins, although this poiut has often been emphasized. More recent discoveries, however, have shown that by the introduction of special acidic catalysts, these webformaldehyde resins may be rendered not merely corn vertible a t a much lower temperature than they have previously been baked, hut also may be made to air dry in a few hours to clear, transparent, glossy, hard, glasslike finishes, ready to sand if necessary. This opens up a field for air-
BAKINQ SCHEDULES.Since ureaformaldehyde resins set essentially by polymerization rather than by mere evaporation of solvents and certainly not by oxidation, in general, the higher the baking within limits, the better are the results. The setting also depends much upon the p H of the system-i. e., the effect of pigments, plasticizers, solvents, and any other ingredients used in the enamel formulation. Usually i t is desirable with neutral pigments to hake a t 260’ F. (126.7’ C.) or above in order to obtain the maximum h a r d n e s s f r o m t h e minirnum amount of urea-formaldehyde resin -in other words, to obtain the most good from the least amount. In order to ensure hardness a t lower temperature bakes, the system sliouid bo slightly acidic. When pigments which are extremely acidic in character are used to improve chalk resistance, such as antimony oxide, some grades of which have a pH of 3.0, a judicious blending with a basic pigment is required to adjust the pI.1 of tho whole system, in order to improve the stabilit,y of the enamel in the package. SOLVESTTOLERANCES. In general, urea-formaldehyde resins are supplied in solution form; the solvent contains varying amounts of a monohydric alcohol, nsualiy n-butanol. On the other hand, the presence of n-butanol in the solvent mixture is not particularly desirable; certain pigments, such as para and toluidine reds, are alcohol soluble, and alcohols affect certain alkyd resins. Since nhutanol does have an adverse effect upon some of t h e commonly used enamel ingredients, it would seem that the ureaformaldehyde resins which had contained n-butanol would be most desirable. Solvent tolerances on two typical resins are given in Table I. One contains GO per cent resin and v, l5 40 per cent butanol (B-3440), the ot.her, 60 per cent resin, 20 per cent butanol, and 20 per cent toluene XUREb-FORMALDEHYDE RESIN (BP-138). T h e r e 100 90 80 70 60 50 40 30 20 10 seems to be no ad%ALKYD RESIN
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tent of the resin solvent. There is also a beneficial effect on pigment and alkyd compatibility. UREA-FORMALDEHYDE PLASTICIZERS. Since urea-f ormaldehyde resins in general give very hard but rather brittle films, the effects of various plasticizers were studied. Castor oils in general are satisfactory plasticizers for urea resins; however, air-blown castor oils have decreasing miscibility with increasing viscosity or blowing. Soybean oil blown to a T(G.-H.) viscosity was incompatible with resin B-3440. VALUESOF UREA-FORMALDEHYDE RESIW TABLEI. DILUTION I N SOLVENTS
Resin No.
Solvent Acetone Diacetone alcohol Methanol Ethanol Cyclohexanol Ethylene glycol Glycerol Ethyl acetate Butyl acetate Butyl lactate Carbon tetrachloride Ethylene dichloride Benzyl chloride Diethyl ether Dioxane Benzene Toluene Xylene Cyolohexane Gasoline Nujol Turpentine Mineral spirits V. M. P. naphtha Hi-Flash naphtha Hydrosol N. J. No. 2 Castor oil Linseed oil (alkali-rebed,
1
2 3
4
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 16 -07 28
Cc. Solvent/lO Cc. Urea-Formaldehyde Soln B-3440 BP-138 490+ 490+ 4901. 490+ 490 4490 4904490f 490 490 f 5 5 10 10-15 490+ 490 f 490 490 f 490 490+ 490 + 490 f 490 t 490 490 30 90 50-56 490f 490+ 490 490t 490 490f 170-185 490 490 f 490 f 20-2: 20-30 10 10 80-85 100-110 25-30 15-25 20-25 20-30 110-120 300-350 115-125 490+ ‘490 490 t 0 0
+ +
+
+
++
+
+
Tributyl citrate, dibutyl phthalate, tributyl pho.phate, and tricresyl phosphate were soluble in most concentratiom. Dibutyl tartrate, butyl stearate, and triphenyl phosphate were miscible only in limited proportions with B-3440.
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Certain of the alkyd resins (particularly those having a “glycerol phthalate” content of 50-70 per cent and an “oil glyceride” content of 30-50 per cent) are almost ideal plasticizers for the urea-formaldehyde resins.
Urea-Formaldehyde-Alkyd Enamels (Clears) SOLUTION PROPERTIES. Since, in general the lower the butanol content with respect to the urea-formaldehyde resin nonvolatile content, the better are the results on compatibility with alkyd resins and with soluble or partially soluble pigments, to say nothing of economy, an effort was made to keep the butanol content as lorn as possible, consistent with good results. Yot only is there al-ivays danger that too much butanol mill dissolve or partially dissolve pigments, but also that the butanol tolerance of many alkyd resins is low. The low tolerance of some alkyd resin solutions for butanol may be surprising t o those who use small amounts of butanol for a quick reduction of viscosity, according to the convenient method of Bogin, Wampner, and Gosselink ( I ) without greatlv lowering the nonvolatile content; but Table I1 shows the actual butanol tolerances of cert a i n alkyd resin s o l u t i o n s . Furthermore, as far as actual s o l u t i o n s of alkyd resins themselves a r e concerned, it is almost impossible to make cold cuts in butanol, since the resins alone hare little or no butanol solubility unless they are 0 , 60 I warmed or unless 0 10 20 30 40 50 TIME IMINUTES) 300’F hydrocarbon solv e n t s a r e presFIGURE 2. HARDKESS COMPARISOS OF CLEAR ESAWELS AT 300’ F. ent.
CHARACTERISTICS OF VEHICLES TABLE 11. PHYSICAL Resin So.
B-1306 B-1307 B-1313 B-1318 B-1331 B-1332 B-1334
%
Nonvolatile Type of Ream 50 Semidrying alkyd 50 45 Drying alkyd 80 50 45
50 50 50
B-1308 B-1323 B-1324 B-1303 B-1320 B-1325 B-1329 B-1 B-1316 B-1326 B-1309 B-1319 B-1321 BH-2719
50 50 50
B-3440 BP-138 BT-BOO
GO GO GO
~
~~
Commercial 1
a
3 a
80 50 50
50 50 50
67 50 50
Oil LengLh Medium Medium Medium short Extra long Long Medium Medium Medium Xondiyiog alii>-d Short Extra long Drying alkyd ~niodified)Short Short Short Short Medium long Medium long Medium long Long Long Long Semidrying IIedium
Urea-formsldehrde
BO
60
50
Hydiogenated naphtha.
Color (H.-K.) 4L-5L 2L-2
None None Yone
Solvent No. 2 H. N.“ Xylene No. 2 H. N. 3L-2 4-5 .Mineral spirits hlineral spirits 2L-2 Hi-Flash naphtha 4-5 Mineral spirits 2L-3L 2-3 Toluene 1L-2 Toluene 3-4 Toluene Xylene 6-7 Xylene 4-5L 4-5L No. 2 H. X. (j-7 No. 2 H. N. ‘4-0 llineral spirits 4-5 Toluene 4-5 No. 2 H. N. 3-4 Toluene 3-4 llineral spirits 3-4 Mineral spirits 2L-2 Xylene Butanol Xone Butanol 20%, toluene 20y0 S o n e S one Butanol
Xone Xone Sone
Butanol 307, xylene 20% S o n e Butanol45%: xylene 5% Xone None Octsl alcohol
Acid No. 3-6 3-6 3-6 6-10 3-6 8-12 3-6 8-13 3-6 3-0 13-18 13-18 13-18 6-10
Viscosity (G.-H.) X-Y N-0 U-V X-Y P-R T-U 2,i-z*
8-12 8-12
X-0 L-M X-Y V-W S-Y X-Y U-V Z-Z1 D-E I-J
6-10
A-h
8-12
u-v
8-12 6-10 3-6 5-7
R-9 N-0 R-T
4-6 5-7
R-T
3-5
T-U X
