the cured slabs were 0.20 em. (0.080 inch) thick. All cures were made at 148.9' C. (300" F.) in the steam cavities of the press, corresponding to an equilibrium temperature in the stock of about 147.5" C. (297.5' F.). The water absorption tests at 100" and 110" C. were made by immersing the specimens in water and placing them in an autoclave in saturated steam maintained at the temperature indicated. Weight changes were determined by weighing on an analytical balance; volume changes were measured by the Navy method, which consists in determining the water displacement of the sample by weighing it on a Jolly balance. In the case of the gasoline and kerosene absorption tests, the volume increases reported were obtained immediately on removing the sample from the liquid; the stressstrain data were obtained after the sample had been allowed to dry 4 hours in air. The gasoline used was lead-free motor gasoline of 60' t o 62' BB. (Sinclair Refining Company). All members of a series were prepared by the master batch method, the variable ingredient being added to an appropriate portion of the master batch, and were cured simultaneously.
Litharge as Accelerator for
Chloroprene
Systematic Variation of Litharge To demonstrate the effect of adding litharge to polymer, the stocks formulated as in Table I were mixed. Results are shown in Figure 1. That the litharge markedly improved the stiffness is evident. It is also evident that increasing the litharge con. tent above 10 parts had only a slight additional accelerating effect. Similar results with respect to the effect of litharge were obtained in a series parallel with the above but contain-
Plastic Polymer W.J. CLAPSON The Eagle-Picher Lead
ing no sulfur.
Company,
Joplin, Mo.
~
~~~
VARIATIOX OF LITHARGE . TABLEI. SYSTEM.ATIC Polymer Thermax" Cottonseed oil FF wood rosinb Sulfur LithargeC 4 Trade name for a soft b FF No. 20 wood rosin C Sublimed litharge was
Litharge is compared with basic-carbonate white lead, zinc oxide, and magnesia as accelerators for the chloroprene plastic polymer Neoprene Type E in stocks loaded with 100 parts of soft carbon black. Basic-carbonate white lead behaves similarly to litharge but is slower in setup. The effect of sulfur, wood rosin, stearic acid, pine tar, and lead oleate on cure, and of certain of these on water, gasoline, and kerosene absorption with litharge and with magnesia is discussed. Litharge gives good resistance to gasoline and kerosene ; it is particularly effective in conferring low water absorption.
5
D3 100 100 3 5
D4 100 100 3
5
1
D5 100 100 3 5 1 30
1 1 .., 10 20 variety of carbon black. (Hercules Powder Company). used throughout.
D6 100 100 3
5 1
50
D7 100 100 3 5
1 100
Effect of Sulfur Bridgwater and Krismann showed that sulfur affects the rate of cure of Type F polymer; the effect of sulfur on unaccelerated Type E polymer is shown in the left half of Figure 2. Obviously the addition of 1 part of sulfur improved the stress-strain properties. Contrasted with the foregoing result is the effect of adding sulfur to a litharge-accelerated base. The formulas and data are shown in the right half of Figure 2. Wood rosin and cottonseed oil were added as 10 parts of Prenol A (National Rosin Oil and Size Company). Sulfur exerts a slight, though noticeable, retarding effect in this type of litharge-accelerated stock. When selenium, as Vandex (R. T. Vanderbilt Company), is substituted for sulfur in the formulas of Figure 2, the retarding effect is slightly greater than for sulfur. The effect of sulfur on water absorption is discussed below under water absorption.
T
HE desirability of using certain metallic oxides to acceler-
ate the vulcanization of the chloroprene plastic polymer known as DuPrene Type F was shown by Bridgwater and Krismann.1 However, the tendency of that polymer to scorch in the presence of zinc oxide or litharge greatly restricted the use of these materials and substantially prohibited the use of zinc oxide alone. This difficulty does not appear to be as pronounced in the case of the polymer now known commercially as Neoprene Type E. The present paper discusses certain properties obtained with the latter material (referred to as polymer) with special reference to litharge as accelerator for it. Stocks were compounded on a 15 X 30 em. laboratory rubber mill, with roll temperatures maintained at about 40' C. The mixed stocks were sheeted off at about 0.23 om. (0.090inch); 1
D2 100 100 3
Softeners
WOODROSIN. I n the preceding formulas, FF wood rosin has consistently appeared as one of the ingredients. Bridgwater and Krismann show that, in the Type F polymer which they were using, wood rosin or similar softener was necessary for the development of optimum properties. However, in the present type of polymer and of formulation there is a general tendency for it to reduce moduli of elasticity and tensile strengths, as shown in the data of Figure 3. Nevertheless, practical processing is claimed to be benefited by the presence of some wood rosin. COMPARISON WITH OTHER SOFTENERS.I n view of these stress-strain results, it is of interest to compare wood rosin
IND.ENQ,CHEM.,25, 280 (1933).
789
INDUSTRIAL AND ENGINEERING CHEMISTRY
790 POLYMER 100 THERMAX 100 COTTONSEED OIL 3 o ~ CURES AT
L a . m IN.
z
o
o
WOOO ROSIN 5 SULFUR I LITHARGE 0 to 100 147.S°C I I
VOL. 29, NO. 7 SULFUR I ACCELERATOR IO
POLYMER 100 THERMAX IO0 WOOD ROS N I;
KG.RQ. CM. 140.62
-!---
ELONGATION 0
1
A-0"
3000 20
50
100
50
20
10,
0 20
100
50
.
1
0
0
1
E LONGAT ION
I
0 20
100
50
PARTS OF LITHARGE
FIGURE1. EFFECTOF LITHARGE ON CURE
POLYMER 100 THERMAX 100 LITHARGE 20 WOOO ROSIN 5 COTTONSEED OIL 5 SULFUR * 3x..--- - , 10
POLYMER 100 THERMAX 100 WOOO ROSIN 5 COTTONSEED OIL 3
-
SULFUR
L B l S Q IN
x----
0-
147.5' C.
MINUTES CURE AT
FIGURE 3. EFFECT OF WOOD ROSINON CUREOF LITHARGE AND MAGNESIA Extra light caloined magnesia (General Magnesite and Magnesia Company) WE% used throughout. LB/SQ IN
cz n
KG./SO.CN
1000
w I
0
-0
ELONGATION
0 N
o
ELONGATION
o
300 0
20
40
BO
MINUTES
20
0
CURE, AT
80
40
147.5' C.
FIGURE2. EFFECTOF SULFURON CURE,WITH LITHARGE
MINUTES CURE AT AND WITHOUT
FIGURE4. Polymer Thermax
Sulfur
POLYMER THERMAX ACCELERATOR STEARIC
too 100
-
10
0-
100 100 IO
LITHARGE
2.
147.5'C.
EFFECTOF SOFTENERS ON CURE OF LITHARGE STOCK
-
Litharge Cottonseed oil Softener
100 100 1 MAGNESIA
----
CURE 4 0 MIN. AT 147.5' C.
POLYMER THERMAX
20 3 6 100 100
STEARIC ACCELERATOR ACID VARIED 10
,CM,
GASOLINE ABSORPTION VOLUME INCREABE
GATION U I-
%
TENSILE DECREASE
O
% I
I AWRAOES MIN,FOR CURES 20,40,80
10
..
MINUTES CURE AT
147.5' C.
0
20
FIQURE5 . EFFECT OF STEARIC ACIDON CURE OF LITHARGE AND MAGNESIA
C1, Base5
209 days a t 27O C. 210 hours a t 110' C.
3.7 50.6
Roursb a t llOo, approx. equal to 209 days 22 a t !7" C. Days a t 2 7 O , approx. equal to 1 hour a t 9.6 1100 c. Same formula as D2 Table I. b Obtained by interpoistion from time-weight data. C Derived from the data in preceding line.
20
Oo*
FIQURE6 . SYSTEMATIC VARIATIONOF STEARICACID WITH LITHARGE AND MAGNESIA
TABLE 11. RELATIVE WATERABSORPTIONS AT DIFFERENT TEMPERATURES Immersion Period
10
PARTS OF STEARIC ACID
C2 Base 2
i o
+
C3 Base lO'MgO
10.4
56.3
27.3 80.6
28
50
7.6
4.2
+
C4 Base 2 Z n O $10 MgO
+
CS Base 20bbO
4- C6
Per Cent Gain i n Weight 23.1 4.9 61.0 15.6 Time 65
3.2
32
6.6
22022 ++
Base $- C7 Base 20bbO f 10 MgO
C 3 Base -I-
20'PbO
10 MgO
2 ZnO
12.3 40.4
4.7 15.5
12 5 32.8
48
32
60
4.4
6.6
3.6
++
JULY. 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
with some other common softeners. Data comparing wood rosin, stearic acid, pine tar, and lead oleate in a lithargeaccelerated base appear in Figure 4. Both stearic acid and lead oleate gave stocks which were easily handled on the mill; on the other hand, the pine tar and wood rosin stocks tended to stick to the rolls. Stearic acid gave stiffer vulcanizates with higher tensile strengths than did wood rosin; lead oleate behaved similarly to stearic acid but tensile strengths were a little lower; pine tar gave stiffer, shorter stocks than wood rosin but lower moduli and tensile strengths than the stearic acid stocks. The excellence of these results with stearic acid prompted a further study. Bridgwater and Krismann stated t h a t stearic and other fatty acids have little or no stiffening and activating effect on chloroprene polymers (presumably in the presence of magnesia or magnesia plus zinc oxide as accelerators). However, it appears that in Type E polymer stearic acid has appreciable effect, as shown by the data in Figure 5. It is evident t h a t 5 parts of stearic acid improved the cure and modulus appreciably; however, as shown in Figure 6, 10 parts or more gave no additional improvement. With 10 to 20 parts, moduli and $ensile strengths are lowered, doubtless because of excess softener. The retarding eff ect of 20 parts stearic acid on the magnesia stock is noteworthy. Stearic acid did not improve resistance t o gasoline. Other data a t hand show similar results as regards resistance t o kerosene. (The dotted lines leading to some indefinite point near the axis indicate a porous cure; e. g., the magnesia stock with no stearic acid did not cure a t 10 minutes.)
TABLE111. EFFECTOF SULFURON WATERABSORPTION OF NONACCELERATED STOCK D1 D2 Polymer 100 100 Therniax 100 100 FF No. 20 wood rosin 6 5 Cottonseed oil 3 3 Sulfur ... 1 Cured 60 min. at 147.5' C. Per Cent-Gain in Weight after Immersion in Water 28 days a t 27' C. 2.8 3 1 12 hours a t 100' C. 4.8 5.5 12 hours a t 110' C. 6.8 7.8
TABLEIV. EFFECT OF SULFURON WATERABSORPTION OF LITHARGE-ACCELERATED STOCK D12 D14 Polymer 100 100 Thermax 100 100 10 10 FF wood rosin Cottonseed oil 3 3 Litharne 20 20 sulfur... 1 Cure a t 147.5' C., Per Cent Gain i n Weight after Immersion for Min. 24 Hours i n Water at 110' C. 40 6.3 5.6 80 6.7 6.2
TABLEV. EFFECTOF SOFTENERS ox WATERABSORPTION" Compound
W a t e r Absorption
Increase after Immersion in Water a t 110' C. for 24 Hour@
Softeners
% by vol. Wood rosin 52 Stearic Acid 53 Pine t a r 34 Lead oleate Average of 20- and 40-minute cures at 147.5O C.
J1
Q
Polymer, like rubber, absorbs water slowly over long periods. Specimens containing zinc oxide, litharge, and magnesia, singly and in combination, were still gaining in weight after 209-day immersion in water a t about 27" C. A duplicate set was also still gaining after 210-hour immersion in water a t 110°C. Absorption is enormously faster a t 100" than a t 27") and still faster a t 110°C. However, the relative rate for the three temperatures apparently changes with formulation; therefore, the relative rating of a given pair of stocks depends on the temperature of the test. This point is illustrated by the data in Table 11. At 27" C. stocks CS and C8 had higher water absorption than the base but a t 110" they had lower water absorption than the base. Table I1 also demonstrates the low water absorption conferred by litharge, the high water absorption conferred by magnesia, and the effect of various combinations. The effect of zinc oxide in increasing the water absorption of the base as contrasted with its effect in decreasing water absorption in combination with magnesia or litharge is interesting. I n another experiment specimens of stocks C1 to C8 (Table 11) and D2 to D7 (Table I) were soaked in water a t 27" C. for a month and then allowed to dry in air under room conditions. After drying for 2 months, the specimens had reached substantially constant weight which was lower than the original weight in the case of the stocks containing no magnesia (indicating leaching) and higher than the original weight in the case of magnesia-containing stocks. Further loss in weight of the latter stocks occurred slowly under reduced pressure (6 mm, or less) a t 70" C., under which conditions magnesium hydroxide decomposes. The evidence, while not unimpeachable, indicates the possible formation of magnesium hydroxide when magnesia stocks are subjected to water. However, the total amount of water absorbed is much in excess of that which can be accounted for by the conversion of magnesia to magnesium hydroxide. There are indications that water absorption of the nonaccelerated polymer is increased by sulfur (Table 111) and
791
10.4 9.9 7.8
9.2
that, on the other hand, water absorption of a litharge-accelerated base is decreased slightly by sulfur (Table IV). Litharge, as just shown, confers low water resistance on p o 1y m e r . Water absorption data on 2 eo I I I I 1 the Eompounds of T a b l e I , c u r e d 60 minutes a t 147.5" Z C., are s h o w n i n F i g u r e 7 . Water absorption decreased sharply with 10 to 20 p a r t s of l i t h a r g e a n d less PARTS OF LITHARGE sharply, though ContinuouslY, w i t h FIGURE 7. EFFECTOF LITHARQEON litharge up to 100 WATERABSORPTION parts. The effect of the softeners in stocks J1 t o J4 is shown in Table V. It is evident that the variation in softener affected water absorption but little; the slight effect was in favor of pine tar. ~
Comparison of Accelerators Of the lead-containing accelerators tested-litharge, basiccarbonate white lead, sublimed white lead, and sublimed bIue lead-litharge and basic-carbonate white lead (B. C. W. L.) show most promise. A comparison of litharge, B. C. W. L., zinc oxide, and magnesia in a loaded stock containing sulfur but no softener or acidic activator is shown in Figure 8. Analogous to its action in rubber, B. C. W. L. is slower in rate of setup than litharge, but otherwise they give stocks of similar rate of cure and stress-strain properties. Zinc oxide gave fast setup and moderate stress-strain properties. Magnesia gave faster setup than litharge, but not as fast as zinc oxide, and short stiff stocks.
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
592 Lwsa IN. LITHARGE z o o o + ~
WHITE LEAD
Ly- 1
ZINC OXIDE I
!
I
KWSQ CM. 140.62
MAGNESIA
VOL. 29, NO. 7
Lwsa.IN -~
KG./SP. CM.
70.3i
ELONGATION
0 1
x
I
1
I
I
l
l
I 40 80 0 40 BO 0 40
0
MINUTES CURE AT 147.5' C.
MINUTES
FIGURE8. COMPARISON
OF CURE OF FOUR ACCELERATORS WITHOUT ACIDSOFTENER 100 Sulfur 1 100 Accelerator 10
Polymer Thermax
WATER ABSORPTION 24 HR. AT 11O'C.
GASOLINE ABSORPTION 4 DAYS AT 27.C.
BO
CURE AT 1475'C.
FIGURE10. COMPARISON OF CURE OF FOUR ACCELERATORB WITH WOODROSIN Polymer Thermax Sulfur
100 100 1 WATER ABSORPTION 24 HR AT 110.C
KEROSENE ABSORPTION 4 rUrS AT 27%.
Wood rosin Accelerator GASOLINE ABSORPTION 4 DAYS AT 27.C
5 10
KEROSENE ABSORPTION 4 OAIS AT 27%
0 0
-10
-10
W (3
5
w
p
-20
0
r
V
z
-30
3 J
'
VOLUME INCREASE
w w
+
U
-301 C
V 0:
-20
I
4
K
W
a
30
30
a za
20
10
10
0
0
FIGURE9. WATER,GASOLINE,ASD KEROSENE ABSORPTION OF FOUR ACCELERATORS WITHOUT ACIDSOFTENER
-
Data are averages of 20- and IO-minute cures. Sulfur Polymer 100 Accelerator Thermax 100
1 10
All of the foregoing stocks were difficult t o sheet out smoothly in the uncured state, especially the magnesia stock, because of lack of plasticizer as shown by their ready conversion t o smooth stocks on addition of wood rosin. Their relative resistances to water! gasoline, and kerosene are shown in Figure 9. Of the four, litharge gave lowest and magnesia highest water absorption. Differences in resistance t o gasoline and t o kerosene are not indisputably established. When 5 parts of wood rosin are added to the compounds of Figures 8 and 9, the resulting stocks compare as to curing properties as in Figure 10. As in the preceding series, B. C. W. L. was slower in setup and magnesia faster in setup than litharge. Zinc oxide was about equal to litharge in setup but gave lower tensile strengths and generally lower moduli. The relative resistances of these stocks t o water, gasoline, and kerosene are shown in Figure 11. Litharge and B. C. 'VV. L. behaved similarly; both gave lower water absorption than zinc oxide and than magnesia, especially the latter. Smaller differences are shown as regards resistance t o gasoline and to kerosene.
FIGURE11. WATER,GASOLINE, AND KEROSENE ABSORPTION OF FOURACCELERATORS WITH WOOD ROSIN Polymer Thermsx Sulfur
Data are averages of 20- and 40-minute cures. 100 Aorelerator 100 Wood rosin 1
10 5
If stearic acid is substituted for the wood rosin in the stocks of Figures 10 and 11, the relative curing results for t h e four accelerators remain substantially as described.
Summary The chloroprene plastic polymer known commercially as Neoprene Type E is discussed with special reference to litharge as a n accelerator of vulcanization. All data pertain to stocks loaded with 100 parts of soft carbon black. Substantially maximum stress-strain properties and rate of vulcanization are obtained with 10 to 20 parts of litharge, although water absorption decreases with the litharge content (at least up t o 100 parts of the latter). Rate of cure, stress-strain properties] and water absorption appear to be increased by sulfur in the absence of accelerator but decreased by sulfur in the presence of litharge. Litharge sets up faster than magnesia in the absence of one per cent of sulfur but slower than magnesia in the presence of one per cent of sulfur. Stearic acid activates both litharge and magnesia, but the former is the less sensitive t o the quantity of stearic acid. Wood rosin may have a mild activating effect on litharge and
JULY, 1937
INDUSTRIAL AND ENGINEERING CHEZlISTRY
on white lead but tends to reduce moduli and tensile strengths. Pine tar gives short stiff stocks; lead oleate behaves similarly to stearic acid. Stearic acid does not improve gasoline and kerosene absorption. Stearic acid and lead oleate stocks d o not adhere to the rolls as do wood rosin and pine tar stocks. Basic-carbonate white lead is slower in setup but otherwise behaves similarly to litharge. Zinc oxide gives fast setup and moderate physical properties; it is retarded by wood rosin. Litharge confers low water absorption, magnesia high Water absorption. Mrater absorption increases the temperature, the rate of increase being dependent on the
793
formulation. Water absorption is not appreciably influenced by the softeners used.
Acknowledgment Thanks are due to John H. Baldwin and Francis E. illoseley of the Research Department for their assistance in obtaining the data published herein and to E. MT. McMullen, director of research, and J. R. MacGregor, vice president, for permission to publish the paper. A4nr~l 17, 1937. Presented before the Division of Rubber Cliernat the 93rd Meetine of the American Chemiral Snriety. Chapel Hill s.c., A p l i l 12 to 1ti,1937.
RErEIVED
Istry
Colloidal Structure of Rubber in Solution S. D. GEHMAN AND J. E. FIELD The Goodyear Tire & Rubber Company, Akron, Ohio
T
HE type of colloidal behavior exhibited by rubber solutions presents the nature of the colloidal structure in solution as a complicated problem with nearly as many aspects as there have been fields of experimentation. Most of the methods of colloidal research have been applied, a t least to some extent, to rubber solutions. A satisfactory scientific understanding of the structure will eventually require a convergence of the points of view arrived a t by these different methods. Each source of independent information about the structure imposes its limitations upon the possibilities. I n this work the writers have relied principally upon what is essentially a new and untried method of colloidal research as applied to rubber solutions-a study of the intensity and depolarization of the transversely scattered light. In addition to securing this new information, they have attempted to show how the results are related to those obtained from the method which has been of greatest general use in the past-namely, viscosity measurements. Light-scattering measurements have been used for many years in the study of suspensions and colloidal systems such as gelatin solutions (13, I d ) , agar-agar sol-gel transitions (j), .gum mastic (18), and numerous other colloidal suspensions. X o systematic study of the light scattered by rubber solutions seems to have been made.
Optical Apparatus and Method The optical system for making measurements of the relative intensity and depolarization of the transversely scattered light is illustrated in Figure 1: The system is mounted in a darkroom. The source of light,
S,is a 1000-candle-power Pointolite tungsten arc. The lens
Measurements are reported of the intensity and depolarization of the light scattered transversely by solutions of purified rubber in various solvents. Viscosity measurements on the same solutions are included. The viscosity is shown to be a function of the dielectric'polarization of the solvent. Experimental results are compared with the expectations on the basis of various theories of light scattering. Intensity and depolarization measurements are consistent in indicating that the colloidal units responsible for the light scattering are large compared to the wave length of light, increase in size as the concentration is increased, and vary in size for the different solvents. The scattering is best explained by assuming in the solutions anisotropic scattering units similar in nature to the cybotactic groups of a liquid which may interlock sufficiently to give a continuous structure throughout the solution.
system, L, L', is focused to give a beam of approximately parallel light, the angular divergence from the axis being 2.5". The source and lenses are mounted on an optical bench in a lighttight housing, cooled by a fan. The beam is limited by diaphragms with rectangular openings, so that it has a cross section of 0.5 X 2 cm. at the light-scattering cell. F is a yellow filter (Corning No. 351) which makes the light more homogeneous and removes any blue light that might cause fluorescence. It also protects the solutions. The right-angle prism, G , reflects part of the light beam into one entrance of a Martens polarizing photometer, T. This reflected beam is reduced in intensity by a neutral glass filter, F', and then serves as a standard of comparison. This arrangement eliminates errors due to variations in the brightness of the source. P is a Polaroid disk which can be introduced in such a way as to polarize the incident beam with the direction of vibration either vertically or horizontally, as
