Refractometer Studies on RubberPigment Mixtures H. C. JONES The New Jersey Zinc Company, Palmerton, Penna.
Refractometric examination of rubberzinc oxide mixtures demonstrated that an increase in refractive index occurs when zinc oxide is milled in rubber, and the magnitude of the change is a function of the particle size and the amount of zinc oxide added. Latex-zinc oxide experiments indicate that the phenomenon is associated with mastication, since the increase in refractive index does not take place until the dried latex-zinc oxide film is milled. When zinc oxide is dissolved from a rubber-
zinc oxide mixture, the refractive index is lowered to approximately the same value as unpigmented rubber. A fine particle size precipitated whiting in rubber increased refractive index, although the change was not so great as with zinc oxide. Other pigments and fillers in rubber influence refractive index but to a less extent than zinc oxide and precipitated whiting. The opacity of some pigments and the coarse particle size of others prevent a satisfactory examination by transmitted light.
EVERAL years ago, NcPherson and Cummings (3) published data on the refractive index of rubber and of rubber-pigment mixtures. They found no difference in the refractive index of rubber from various sources. The degree of mastication did not, appreciably change refractive index. Sulfur in combination with rubber altered the refractive index relatively more than free sulfur in rubber. With the exception of zinc oxide, the pigments and fillers they examined did not affect the refractive index. I n the case of zinc oxide, there was a significant increase in refractive index and the fine particle size oxide (Kadox) developed a higher value than the coarser oxide (XX Red). They suggested that zinc oxide probably reacts with the nonrubber constituents to form salts which dissolve and change the refractive index. This laboratory ( 5 ) found that for a series of various particle size zinc oxides in rubber, the increase in refractive index was inversely proportional to the particle size of the oxides; that is, a fine particle size zinc oxide increased the refractive index somewhat more than a coarse oxide. It was also observed that the refractive index of zinc oxiderubber mixtures increased proportionally with each addition of zinc oxide until the opacity of the zinc oxide-rubber mixture became too great (over 6 per cent) to allow measurement by transmitted light.
S
To determine whether the increase in refractive index of zinc oxide-rubber mixtures is due to a reaction between zinc oxide and the nonrubber constituents of crude rubber, samples of acetone-extracted rubber, deproteinized rubber, and purified rubber hydrocarbon were compounded with Kadox zinc oxide (0.12 micron), and measurements were made on the Abbe refractometer. The refractive indices (Table I and Figure 1) of the several rubbers were substantially the same a t equal concentrations of zinc oxide, which indicates that the phenomenon is not due to a reaction of zinc oxide with the nonrubber constituents of the crude rubber. This is further indicated by the fact that the refractive index of other rubberlike materials, such as neoprene and Perbunan, shows an
1.52 40
)-PALE
X
CREPE
-
X AC E T 0 NE- E X T R A C T E D P A L E C R E P E
3 - O E P R O T E I N I Z E D RUBBER
kQ ~ 1 . 5 320 X
w
21.5220
w
:IS2
Effect of Pigments on Refractive Index
I0
u
The present investigation was undertaken to extend the work of McPherson and Cummings and gain some understanding of the effect of zinc oxide and other pigments on refractive index. The refractive index measurements were made with an Abbe refractometer with a carbon arc as a source of illumination. Small samples ( 5 to 10 grams) of the rubber stocks were squeezed into thin films between layers of metal foil in a cold press, and the rubber films were mounted in the refractometer for determinations by transmitted light.
4
f 1.5200 w
a 1.5 I 9 0
I
2 3 4 PERCENT Z I N C OXIDE
5
6
FIGURE1. REFRACTIVE INDEX OF NATURAL RUBBER AND SEVERAL MODIFICATIONS OF NATUFUL RUBBER 331
ISDUSTRIAL AND ENGINEERING CHEMISTRI-
332
REFRACTIVE IKDEX (%a,)OF
SATURAL RGBBER AKD SEVERAL h~ODIFIC*TIOKS OF x A T r R . I L RUBBER
TABLE I.
Rubber Rubber 0.3% ZnO (0.12 micron) l.OyoZnO Rubber Rubber 2.0% ZnO Rubber 3.0y0 ZnO Rubber 4.0% ZnO Rubber 6.090 ZnO
+
+ +++ +
TABLE
.4cerone-
Extd.
Deproteinised Rubber
Pure Rubber Hpdrocarbon
Pale Crepe
Pale Crepe
1 5192
1.5190
1.5194
1,5194
1,5202 1.5206 1.5212 1,5222 1.5228 1.5240
....
1.5202 1.5206 1,5211 1.5220 1 5228 1 5236
1.5200 1,5205 1.5211
l,'iil5 1:ihO 1.5242
....
.... 1,5240
hDEX 11. REFRACTITE
h h A S V R E U E K T S O S 8YXTHETIC hIATERI.iLS
Seoprene
Perbunan
Pale Crepe
ny
nso
ny
ng
ny
1.5580
1.5562
1.5215
1.5198
1.5192
1.5174
1 5216 1 , 5 2 0 2 1 , 5 2 2 5 1.5208 1 , 5 2 2 0 1.5212 1,5231 1.5220 1,5244 1,5228 1 , :e48 1,5240
1 5184 1,5189
1,5580 1.5560 1.5231 1,55s5 1 , 5 5 6 2 1 . 5 2 4 2 1.5592 1 , 5 5 7 0 1.5240 ,... 1.5251 , . . . 1.5602 1,5582 1.5260 1.5610 I.5560 1 , 5 2 6 2
VOL. 32, NO. 3
Nenadue (4) demonstrated that zinc oxide could be completely dissolved from an uncured rubber-zinc oxide stock by treatment with an ether-acetic acid solution. This suggested the possibility of measuring the refractive index of a rubber film after the zinc oxide had been removed. The rubber-zinc oxide films %\-ereimmersed in a 20 per cent acetic acid-80 per cent ether solution for 10 days. Then the films n-ere extracted with hot acetone in a Soxhlet extractor and dried in a vacuum desiccator. Refractive index measurements were taken immediately after the evaporation of the acetone. The data in Table I' show that the extraction of zinc oxide from the rubber-zinc oxide compounds has lowered the refractive index to approximately that of unpigmented ruhher.
o\n 1.52401
1.5194 1.5200 1.5209 1.5221
,
,
I
I
I
I
I
I
I
'
'
I
1
I
I5230
20 SI 5 2 2 0
f increase in refractive index when increments of zinc oxide (Table 11)are added t'o them, and the magnitude of the change of refractive index is of the same order as with natural rubber. The influence of mastication on refractiye index was studied by incorporating zinc oxide in latex and making measurements on dipped latex films cont'aining 0, 0,51.0, 2.0, and 4.0 per cent zinc oxide. To ensure good dispersion, the zinc oxide was ball-milled for 16 hours n-ith casein and Darvan (a commercial wetting agent) in water, and this suepelision was stirred into a 60 per cent latex emulsion. The films, irrespective of zinc oxide content, had the same refractive index (Table I11 and Figure 2) and were essentially equal to unpigmented rubber. However, when the films vere masticated on a 6inch laboratory mill, the compounds containing einc oxide \yere found to have higher values and the measurements were in close agreement with those for pale crepe-zinc oxide mixtures (Table I). The increase in refractive index, therefore; is associated with mast'ication although it occurs only when zinc oxide is present. Parallel experiments were made by incorporating zinc oxide in a rubber cement (Table IT-), evaporating the solvent, and measuring t'lie refractive index before and after milling of the dried cement film; again an increase in refractive index was noted aft'er milling. I n this instance somewhat lorrer values were observed for the films recovered from the zinc oxide cements probably because some of the organic solvent adsorbed on the surface of the zinc oxide was not released during the drying of the cement film and resultredin a lowering of the refractive index. TABLE111.
REFR.4CTIl.E
INDEX hIEASURE7IESTS O S TIATEX-
ZIX OXIDE~ I I X E ~ ~ I < $5
n v
Ihnned .- films ~~
~
Latex film Latex + Darvan Latex + Darvan Latex Darvan J,arex + Darvan 1,atex + Darvan
+
+ casein + casein + 0 5% ZnOh + casejn + 1.0% ZnO + + 2 . 0 % ZnO + casein casein + 4 . 0 % ZnO
Dried films Latex Latex Latex + Latex + Latex $.
masticated on the mill + casein t casein 0 . 5 0 ZnOb T casein 1 . 0 % ZnO Darran casein 2 . 0 % ZnO D a r r a n I- casein T 4 . 0 % ZnO
+ Darvan + DarTrau Darvan
+
+++
1,5200 1.5202 1.5200 1 5200
1 5201 1.5202
1.5194 1.5208 1.5210 1.5215
1.5232
1.5182 1,5181 1.5190
1.5190 1,5190 1.5191
1.5172 1,5185
1,5188 1.5194 1.5211
200 grams zinc oxide, 4 grams Darvan S o . 1, 10 grams 0 Preparation: casein solution (20 per cent!, and 200 grams water ball-milled 24 hours. Blurry added t o 60 per cent latex. b 0.12 micron.
2
I5210
0
2 I 5200 a
, 51901
!
1
I
1
2 3 4 PER CENT ZINC OXIDE
I
I
5
FIGURE 2. REFRACTIVE INDEX MEASUREVENTS O S LATEX-ZINC OXIDE MIXES
T-IBLE 11..
KEFR-~CTIVE ISDEXMEASUREMESTS O N RUBBER CEMESTS n $&
Rubber cement Film after 1 hr. in vacuum oven a t 60' C. t 4 % ZnO after 1 hr. in vacuum oven a t 60' C. A 4yCZnO after 1 hr. in vacuum oven at. 50° C., mi!led
1 5190
1.5142 1,5162
v. EFFECT OF EXTRACTION O N REFRACTIVE INDEX Z n c OXIDE FROV P ~ L E CREPE-ZINC OXIDE F I L m
'riBLE
Before Extn n%Q
nk5
n y
1.5190 1 . 5 1 7 2 1,5210 1.5190 1.5231 1.5210 1.5240 1.5221
1.5192 1 5202 1.5202 1 3200
1.5172 1.5180 1.5180 1.5180
7?$5
Rubber Rubber Rubber Rubber
T T
2.0
ZiiO ( 0 . 1 2 i n i c r o n j
+ 46 . 00%@ ZnO ZnO
OF
After Evtn
Wien stearic acid was added to the series of zinc oxide compounds, the refractive indices of the stocks were lowered and 10 per cent stearic acid was somenhat more effective than 3 per cent stearic acid (Table VI and Figure 3). The addition of 2 , 4 , and 6 per cent zinc stearate to rubber did not appreciably change the refractive index of rubber. Examination of the data in Figure 3 indicates some solution of zinc oxide in the stocks containing stearic acid. For example, the compound with 3 per cent zinc oxide and no added stearic acid had the same refractive index value as the 4 per cent zinc oxide compound 11-itli 3 per cent stearic acid; this suggests that 1 per cent of zinc oxide mas dissolved by the stearic acid. Similarly in the series of stocks Tvith 10 per cent stearic acid, the 4 per cent zinc oxide compound had the same refractive index as the 2 per cent zinc oxide stock containing no added stearic acid. In this case it appears that 2 per cent zinc oxide was dissolved by the stearic acid. The zinc stearate formed by the reaction probably did not affect refractive index, as shown by the group of zinc stearate compounds.
MARCH, 1940
INDUSTRIAL .IND ENGIKEERING CHEVISTRY
opacity of carbon black permitted the incorporation of only 0.1 per cent, and this amount of soft carbon increased the refractive index from 1.5190 to 1.5200. Channel black gave the surprising value of 1.5050 with a 0.1 per cent addition, and this was believed to be due to the adsorbed hydrocarbons on the surface of the black. These hydrocarbons probably are somen hat lower in refractive index than rubber so that the net effect is a lowering of refractive index when this amount of channel black is added to rubber. To check this point, a sample of channel black was heated in a vacuum oven a t 180' C. for 24 hours. When compounded in rubber, this sample of channel black had a refractive index 20 per cent higher than the same channel black before heat and vacuum treatment.
0 X A PER CENT ZINC OXIDE 0 PER C C N T ZINC S T E A R A T E
FIGURE 3. EFFECTOF STEARIC ACID ON THE REFR.4CTIVE I N D E X OF ZINC OXIDE-RCBBER MIXTCRES OF S T E a R l C ACID ON REFRACTIVE ISDEX TABLE 11. EFFECT ZISC OXIDE-RCBRER~ ~ I X T C R E(Sn y ) Stearic Acid
With 3% Stearic Acid
With 10%. Stearic Acid
1,5190
1.51!)2
1.5180
WithOut
Pale crepe cvntrol 0 . 5 7 0 ZnO 0. 12 micron) 1 0% ZnO
;::$ f%
4 . 070 ZnO 6 . 0 % ZnO
333
OF
Measurements o n RubberYZn Stearate Mixtures without ZnO Pale crepe control 1 5190
1.5202 1,5195 1.3190 , .. , , . .. 1.5206 1 . 5 1 9 5 1.5192 . ..... 1 , 5 2 1 2 1 , 5 2 0 2 1 , 5 2 0 0 2% Zn stearate .,,.,... 1.5220 1 , 5 2 3 0 1:jZi0 1;J z l O 4 % Zn stearate 1 , 5 2 4 0 1 . 5 2 2 1 1 , 5 2 1 0 6% Zn stearate
3lechanism of Refractive Index Changes Tn o possible explanations for these observations are suggested. The first is based on the behavior of vulcanized rubber, which when stretched shows an increase in refractive index because of molecular distortion. For example, elongating a vulcanized rubber-sulfur stock 300 per cent increases the refractive index from 1.5302 to 1.5350 (n2,5),and when the tension on the vulcanized sheet is released, the refractive index returns to its original value. I n this investigation there was no change in refractive index when zinc oxide was I5240
1,5190 1.5200 1,3200
5230 X W
Other pigments and fillers Ti-ere examined, but the most notable results Tyere obtained x i t h a sample of extremely fine particle size precipitated whiting, which increased the refractive index of rubber to a considerable extent. The curves in Figure 4 illustrate the values obtained for the precipitated whiting as compared with a natural whiting (finely ground) and zinc oxides of different particle size. The loir opacity of whiting permitted the measurement by transmitted light of higher concentrations of this material. From these curves it can be concluded that the observed refractive index changes are closely associated with the particle size of the pigment. (Table VII). TABLE1-11.
REFR.4CTIVE I X D E X h I E A S E R E J l E S T s (7LY) 10s ZINC OXIDES .4ND n'HITISGs O F SEVERAL PARTICLE SIZES
P a r t s of Pigment or Inert/100 P a r t s Rubber Pale crepe control
0.5
1.0 2.0 4.0 6.0 8.0 12.0 16.0
7 -
9.12 micron
Zinc Oxide0.40 micron
1,5190 1,5202 1.5206 1.5212 1.5228 1.5240
1.5189 1.5200 1.5200 1.5210 1.5220 1.5230
.... ....
...
....
.... ...
1.0 micron 1.5189 1.5200 1.5200 1.5200 1,5199 1,5198
.. .. .... ,,
..
--Whiting----. PrecipiFinely tated ground 1,5190 1,5190 1.5201 1.5200 1.5208 1.5208 1.5214 1.5224 1.5234
1,5190 1.5198 1.5195 1.5196 1,5200 1.3198
1.5200 1.5199 1 5199
Additions of 4 per cent of lead, cadmium, and calcium oxides increase the refractive index from 1.5190 t o approximately 1.5200, which is about 25 per cent of the increase noted for the same amount of zinc oxide (0.12 micron). Because of the high opacity of titanium oxide and zinc sulfide, only 2 per cent additions could be measured and these showed an increase from 1.5190 to 1.5200. Since samples of various particle sizes were not available, it is impossible to predict whether these pigments have a coarse effective particle size or whether they do not cause the same change in refractive index as does zinc oxide and whiting. The author's opinion is that the latter is true in the case of zinc sulfide and titanium oxide. The
41 5 2 2 0 W
2
1.5210
4 LL
21 5 2 0 0
!
I
2
,
,
4
0-ZINCOXIDE 1 2 4 X - Z I N C OXIDE 4 0 s A - F I N E P A R T I C L E S I Z E WHITING 0 - C O A R S E P A R T I C L E S I Z E WHITING 6 8 ID 12 PER C E N T P I G M E N T
14
16
FIGURE 4. REFRACTIVE I N D E X ME.4SURENETTs ON ZlNC OXIDESAND WHITINGSOF S E V E R PARTICLE ~L SIZES
incorporated in latex until the rubber was masticated in contact with the pigment. It was also observed that the refractive index of the rubber-zinc oxide stock approximately returned to the value of unpigmented rubber when zinc oxide was dissolved from the stock. These results suggest that the increase in refractive index can be attributed to the development of molecular strains induced in the rubber when masticated in contact with zinc oxide, and that these molecular strains are largely eliminated when zinc oxide is dissolved from the rubber. (It may be anticipated that with the removal of the pigment the original refractive index will be restored, and that it does not occur may be due to a pwmanent deformation or set of the rubber molecule.) The second mechanism suggested is t h a t of the orientation of the rubber molecule at the pigment-rubber interface; in this case it would be anticipated that the orientation would cause an increase in the specific gravity of the rubber directly around the particles which would be manifested in an increase in the refractive index. One could postulate that the increase would be related to the specific surface of the pigment, and the results on the several sized zinc oxides indicate this to be true. Holt and AIcPherson (1) found that the volume of a gum rubber band decreases when elongated beyond 200 to 300 per cent-hence a n increase in specific gravity on stretching.
334
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
VOL. 32, NO. 3
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 (1935). (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
