January 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
made from natural gas. There is considerable differentiation indicated between all of the blacks by the resistivity data, but the strain data again failed to indicate any marked differentiation. CONCLUSIONS
Data in Figures 4 and 5 show that the resistivity of carbon black vulcanizates is markedly affected by flexing and state of cure. In spite of the sensitivity of the measurements to these good factors, reproducibility-coefficient of variation of lO%-was ob-
tained for vulcanizates of the same rubber and black compounded on different days. Figure 8 shows a raphic arrangement of the resistivities from Tables 11, 111, and f V plotted on a logarithmic scale. The data are grouped according to the type of black, and the code letters refer to the source. It is evident from this figure that the resistivity test reveals differences between blacks of the same type. It appears, therefore, that resistivity is a simple and sensitive test for characterizing blacks and should be useful for quality control of production and for specification purposes.
163
LITERATURE CITED
(1) “ASTM Standards,” Part 6, ASTM Deaienation D 99148T, Philadelphia, American Society for Testing Materials, 1949. (2) Bulgin, D., Trans. Znat;RubberInd., 21, 188 (1945); reprinted in Rubber Chem. and TechnoZ., 19,667 (1946). (3) Cohan, L.H., and Steinberg, M., IND.ENQ.CEEM.,36,7 (1944).
“Electrical Conductivity Study Cabot Carbon Blacks,” Vol. 2. No. 5, Boston 10, Mass., Godfrey L. Cabot, Inc., 1949. (5) Lane, K.A., and Gardner, E. R., Trans. Inat. Rubber I d . , 24, 70 (1948); reprinted in Rubber Chem. and Technol., 22, 536 (4)
(1949).
“Specifications for Government Synthetic Rubbers,” revised ed., Washington, D. C., Reconstruction Finance Corp., Office of Rubber Reserve, 1951. (7) Waring, J. R. S., Trans. Znst. Rubber I d . , 16, 23 (1940); reprinted in Rubber Chent. and Technol., 14,449 (1941).
(6)
RECEIVED May 11, 1961. Presented before the Division of Rubber Chemiatry of the A U ~ R I C ACHEMICAL N SOCIETY, Washington, D. C.. 1951.
Lignin-Reinforced Nitrile, Neoprene, and Natural Rubbers J. J. KEILEN, W. K. DOUGHERTY, AND W. R. COOK West Virginia Pulp &Paper Co.,Development Department, Charleston, S . C .
T
HE use of coprecipitated lignin as a reinforcing agent for general purpose synthetic rubber, GR-S, has been reported previously. Pine wood lignin from the waste liquor resulting from pulping wood by the sulfate process was shown to give tensile strengths close t o those obtainable with easy processing channel black in GR-S (3). Later it was shown that not all lignins isolated directly from the sulfate waste liquor are equivalent-in some cases oxidation is required to reach the values first reported (6). Other articles have been written reviewing the above (1, 6 ) but no other new work on reinforcing has been presented. One article gives data showing that unoxidized lignin,is an efiective stabilizer for GRS ( 4 ) . There has been no information published on the reinforcement of other synthetic rubbers or of natural rubber with lignin, and i t is the purpose of this report to provide such information. Before going into detailed results it would be well first to review briefly the different main types of lignin available as well as the process which renders lignin a suitable reinforcing agent. TYPES OF LIGNIN
Sulfate lignin is used in both of the studies resulting j n reinforcement of GR-S. Sulfate lignin is a type of alkali lignini.e., a lignin prepared by acidification of waste liquors from either the kraft or soda processes. All of these lignins regardless of wood used or variations in the process, are similar, being soluble in alkali and insoluble in acid. Although all of them do not give equal reinforcement when coprecipitated with rubber, all do give some appreciable degree of increased strength. The materials broadly designated as sulfite lignin are recoverable from the waste liquors resulting from pulping wood by the acid sulfite process. The lignin has reacted with the cooking liquor and is present as lignosulfonates rather than as lignin itself. While this type of material has been mentioned in the literature as a processing aid ( d ) , it hae not been described as having any particular value as a reinforcing agent. In general, the lignosulfonates cannot be coprecipitated with rubber because they are soluble in acid media.
A third material which has received considerable attention is the insoluble ligneous residue resulting from the hydrolysis of wood for the manufacture of ethyl alcohol.. Although potentially this material is very cheap, its production is dependent upon production of alcohol from wood, which still seems t o be an emergency procedure only. Before being suitable as a reinforcing agent for rubber, the ligneous hydrolysis residue must be purified by solution in #odium hydroxide, filtration to remove insoluble substances Ruth as cellulose, and reprecipitation to recover the purified lignin. LIGNIN COPRECIPITATED WITH RUBBER
The process by which lignin is coprecipitated with rubber to achieve reinforcing is similar to that used for masterbatching carbon black or other pigments in GR-S. The main differences are that the lignin is mixed as a solution, rather than as a suapension, with the latex and that the order of mixing materials for coagulation is reversed. Following is a stepwise outline of n procedure for preparing a coprecipitate containing 50 pounds of lignin per 100 pounds of rubber. LIGNINSOLUTION. Suspend 50 pounds of lignin in 140 pounds of water. Add 10 pounds of 50% caustic soda solution with stirring. Solution of the lignin will result almost immediately. LATEX. Use 400 pounds GR-S latex, Type 1, 25% rubber solids; or 263 pounds GR-S latex, T y e 3, 38% rubber solids; or 263 pounds normal natural rubber Etex, 38% rubber solids; or 210 pounds butadiene-acrylonitrile latex, 26% acrylonitrile 47.6% rubber solids; or 200 pounds neoprene latex 842, 50% rubber solids. Acm SOLUTION. Add 23 pounds of 60” BE. sulfuric acid to, 330 gallons of water. Heat to 150” F. COPRECIPITATION. Add the lignin solution to the latex with stirring. Add the lignin-latex mixture to the acid solution with stirring; filter; and wmh the coprecipitate with water until the p H of the wash water is at least 5. Dry the coprecipitate in air a t 160’ to 170’ F. The quantities of sodium hydroxide and sulfuric acid must be varied if coprecipitates containing other proportions of lignin
INDUSTRIAL AND ENGINEERING CHEMISTRY
164
Vol. 44, No. 1
600
6000 0 I
E
5000
400
E
0 z
u)
5 4000
9 200
w
2
a 3
5:
3000
I 25
0
a
I 50
w
I
I
75
100
I25
150
PARTS BY WEIGHT OF LIGNIN
a v)
f
Figure 4,
2000
G IO00
I 0’
25
r
I
I
0
I
25
I
I
125
150
I
50 75 100 PARTS BY WEIGHT OF LIGNIN
stress-strain properties of variously loaded lignin-nitrile compounds prepared according t o the formulas of Table I. Use of greater quantities of accelerators overcomes the slight cureretarding action of the lignin a t high loadings. The same figures also show the properties of compounds made from ligninneoprene coprecipitates by the formulas given in Table 11.
1
I
I
Tear Resistance of Lignin-Reinforced Rubbers
50 75 VOLUMES OF LIGNIN
100
OF LIGNIN-REINFORCED NITRILERUBBER TABLE I. FORMULAS
Figure 1. Tensile Strengths and Moduli of LigninReinforced Rubbers Boo
COMPOUNDS
Parts by Weight Nitrile rubber 26% acrylonitrile‘ Li nin Di%utyl phthalate PI-- --.!-
100 10 3
100 50 10
100 25 10
100 75 12
100 100 15
100 125 20
100 150
!!
2
Benzott fide Tetramethylthiuram monosulfide Antioxidant Sulfur
600 I2
400 K 0 W
0.1 1 2.5
0.1 1 2
0.1 1
0.1 1 2.5
2
0.1 1 2.5
0.5 1
2.5
0.8 1 2.5
TAB.LEII. FORMULAS OF LIGNIN-REINFORCED NEOPRENE COMPOUNDS
200
A
v O
25
50 75 100 PARTS B Y WEIGHT OF LIGNIN
125
I50
Figure 2. Elongation of Lignin-Reinforced Rubbers
100
Neoprene Lignin Coal tar softener Zinc oxide Light calcined magnesia Stearic acid Di-ortho-tolylguanidine Sulfur
25
15 3 6 2 2 2
100
Unlike the action in the synthetic rubbers, coprecipitated lignin progressively decreases the tensile strength of natural rubber compounds as the loading is increased. Tear resistance, however, is increased by the lignin. Graphs of properties obtained by compounding natural rubber coprecipitates according t o the formulas of Table I11 are shown in Figures 1to 4.
QY
g so U
I W I :
P
60
0
In I 0
I 25
I 50
I
75
I
100
125
I 150
TAB~E 111. FORMV~AS OF LIGNIN-REINFORCED NATURAL RUBBERCOMPOUNDS
PARTS BY WEIGHT OF LIGNIN
Figure 3.
Shore “A” Hardness of LigninReinforced Rubbers
are to be made. Otherwise the procedure as outlined can be used for GR-S, nitrile, and natural rubber roprecipitates containing up to a t least 150 pounds of lignin per 100 pounds of the rubber, and for neoprene coprecipitates containing up to a t least 100 pounds of lignin per 100 pounds of rubber. PROPERTIES QF CURED COPRECIPITATES
In general, coprecipitated lignin has much the same reinforcing action in nitrile rubber and neoprene as in GR-S. Thus curves of tensile strength show maxima between 50 and 75 pounds of lignin per 100 pounds of the rubber. Figures 1 to 4 show typical
Natural rubber Lignin Zinc oxide Mercaptobenzothiazole Tetramethylthiuram monosulfide Stearic acid Sulfur
100 25 5 3
1.5 2 2.5
PO0 50 5 3
1.5 2 2.5
Parts by Weight 100 100 100 75 100 125 5 5 5 3 3 3
1.5 2 2.5
1.5 2 2.5
2 2 2.5
100 150 5 3 2 2 2.5
There is little doubt but what the formulas of Tables I to 111 could be altered to secure results other than those shown. A more intensive search and testing of different curing agents used in varying proportions could result in general improvement of properties. The results given here are thus only indicative of what may be expected of coprecipitated lignin as a reinforcing agent for the three rubbers a t various loadings.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952 4000
3
38.5 VOLUMES
Y
165
$1 d
0 I
Z 3000
0.4
W K
a
i
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3
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z
z
Os2 0
IOoo
Figure 9. Taber Abrasion Loss of Nitrile Rubber a t Two Volume Loadings
0
a a
a
a
a
ELONGATION LESS THAN 300% 4060
Figure 5. Tensile Strength and 3009’0 Modulus of Nitrile Rubber a t Two Volume Loadings
48 VOLUMES
Y
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53 3000 800
I
8 2000
E!
600
8
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8
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400
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200
a
a
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ELONGATION LESS THAN 300%
Figure 10. Tensile Strength and 3009’0 Modulus of Neoprene at Two Volume Loadings 0
Figure 6. Elongation of Nitrile Rubber a t Two Volume Loadings
100
.
I
1200
1
1
c
5
800
0 W K
n 400
0
Figure 11. Elongation of Neoprene at Two Volume Loadings Figure 7. Shore “A”Hardness of Nitrile Rubber a t Two Volume Loadings
I
rn
100
I (3
80
0
a W a 60 W
0 K
5
40
20 0
Figure 8. Tear Resistance of Nitrile Rubber at Two Volume Loadings
Figure 12. Shore “A” Hardness of Neoprene at Two Vdulne Loadings
INDUSTRIAL A N D ENGINEERING CHEMISTRY
166
Vol. 44, No. 1
800
400
L
y
300
~
W E
tn 0 f
600
2 W 0
LL a iW
a 400
200
0
200
3 00
0
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Figure 13. Tear Resistance of Neoprene at TWO Volume Loadings
Figure 16.
Elongation of Natural Rubber at Two Volume Loadings
100 I
I
0.8 v) W _I
u
0.6
0
0
E
[L
0.4
tn
I -4
5
0.2
0
Figure 14. Taber Abrasion Loss of Neoprene at Two Volume Loadings
Figure 17. Shore “.4” Hardness of Natural Rubber at Two Volume Loadings
Figures 7, 8, and 9. The low modulus and high elongation of Figures 5 and 6 are typical.
5w0
Y
38.5 VOLUMES
77
”
w
k
2
,
m
2
TABLEIv. FoRafVTJ.4s OJ! C O M P O U N D S O F K I T R I L E RUBBER REIN~ORCED WITH LIGNIN AND O T H E R P I G M E N T S Nitrile rubber, 26% acrylonitrile Lignin P i ment Di%utyl phthalate Zinc oxide Benzothiazyl disulfide Tetramethvlthiuram monosulfide Stearic acid Antioxidant Sulfur a Volume loading.
a
a
’
a
a
a
ELONGATION LESS THAN 300%
Figure 15. Tensile Strength and 3004/a Modulus of Natural Rubbq at Two Volume Loadings COMPARISON OF LIGNIN WITH OTHER PIGMENTS IN NITRILE RUBBER
Coprecipitated lignin shows t o better advantage in comparison with other pigments dry-milled in nitrile rubber than in either of the other two synthetics. I n GR-S, as reported previously, lignin gives a slightly lower tensile strength than easy processing channel black (EPC) both at 38.5 volume loadings. In nitrile rubber compounded by the formulas of Table IV, at the same volume loading, coprecipitated lignin gives higher tensile strengths than any other pigment, as shown in Figure 5 . Shore hardness, crescent tear resistance, and abrasion loss are not as outstanding but compare favorably with other pigments as can be seen from
-
100 3 8 . 5Q
10 5 0.7 0,l
As with lignin-reinforced GR-S, the properties of lignin-reinforced nitrile rubber at a 77 volume loading are very good as shown in Figures 5 t o 9. Tensile st.rength is 700 pounds per square inch higher than that for EPC, the best of the other fillers. High hardness with low modulus and high elongation is a novel combination of properties obtainable with the high tensile strength. Although not outstanding, tear resistance and abrasion resistance are both good. Reasons for the excellent tensile strengths obtained using coprecipitated lignin in nitrile rubber are not immediately apparent. One possibility is better compatibility of the lignin with nitrile rubber than with any other type. Lignin is soluble in acrylonitrile itself, and part of this effect may still be in evidence in the copolymer. COMPARISON O F LIGNlN WITH OTHER PIGMENTS IN NEOPRENE
I n contrast to its action in nitrile rubber, coprecipitated lignin in neoprene gives lower tensile strengths than any of three carbon
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952
0.8
r
167 R
1
ln W
d
600
>
0 I
I
Q-
400
a
0.4
2
In 0
z
ii
0.6
0
In
I
g
200
0.2
c?
0
0
Figure 18. Tear Resistance of Natural Rubber at Two Volume Loadings
Figure 19. Taber Abrasion Loss of Natural Rubber at Two Volume Loadings
blacks tested-semireinforcing furnace, high modulus furnace, and easy processing channel blacks. This is true for compounds made according to the formulas of Table V both at loadings of 48 and 96 volumes of lignin corresponding with 50 and 100 pounds of lignin per 100 pounds of neoprene. Figures 10 t o 14 are bar charts of the properties of neoprene mill mixed with various other fillers a t the 48 volume loading. In hardness, lignin is higher than the blacks, but in tensile strength, tear resistance, elongation, and set it is inferior. Howeyer in tensile strength, lignin is better than any of the inorganic fillers. The relations between the fillers a t the 96 volume loading, likewise given in Figures 10 t o 14, are much the same as a t the 48 volume loading.
tated a t loadings of 38.5 and 77 volumes, exceeded in tensile strength all other materials mill-mixed at the same volume loadings, as shown in Figure 15. The characteristic low modulus and high elongation accompanying high tensile a t both loadings are pointed out by the bar charts of Figures 15 and 16. Shore hardness in natural rubber, Figure 17, is not as high as with some of the other pigments but is still in the upper range. Crescent tear resistance, Figure 18, a t the lower loading is not as good as any of the three carbon blacks, but is better than any of the inorganic pigments. A t the high volume loading, the tear resistance is 200 pounds per inch better than any other pigment. Taber abrasion loss with the lignin compounds at both loadings, Figure 19, is higher than the carbon blacks and lower than any of the inorganics.
FORMULAS OF COMPOUNDS OF NEOPRENE REINFORCED WI'I'H LIGNIN AND OTHER PIGMPNTS Parts by Weight --
TABLEV.
Neoprene, Type GN-1 Lignin Pigment Coal t a r softener Light process oil Antioxidant Stearic acid Dj-ortho-tolylguanidine Zinc oxide Sulfur Light calcined magnesia 0 Volume loading.
100 48'
100
15
20
9S0
2.5 2.5 4 2.5
2 2 3 2
?
6
100
$00
48'
98"
10 2 1
10 2 1
5
5
4
4
If the premise that the excellent results with coprecipitated lignin in nitrile rubber are in some way related to compatibility is accepted, then the relatively poorer results with neoprene continue to follow the same pattern. Chlorinated solvents are among the poorest solvents for lignin, so there may be no attraction between the polychloroprene and the lignin. COMPARISON OF LIGNlN WITH OTHER PIGMENTS IN NATURAL RUBBER
Lignin coprecipitated with natural rubber gives excellent reinforcement, particularly as regards tensile strength. Compounded according t o the formulas of Table VI, lignin, coprecipi-
SUMMARY OF PROPERTIES
The combination of properties obtainable with lignin when coprecipitated with natural, nitrile, or neoprene rubbers, as well as with GR-S, is novel. In addition t o tensile strengths, which are at least of the same order as, and in certain cases superior to, those obtained with the carbon blacks, good abrasion resistance and tear resistance result. In GR-S,nitrile, and natural rubbers, high elongation and low modulus are combined with the high tensile strength and high hardness. * Lignin has a lower specific gravity than any of the other pigments commonly used. The value of 1.3 permits the production of lightweight rubber articles of the same volume as heavier items made with other reinforcing agents. Although coprecipitated lignin is itself dark in color, i t has a low tinting power. Except for white and pastel shades, colors are readily obtainable by blending with other pigments, retaining the strength values derivable from the lignin. ACKNOWLEDGMENT
The authors are indebted t o the West Virginia Pulp & Paper Co. for permission to publish the results of this work which was conducted in their development department. The assistance of C. F. J. Mappus in preparing the numerous charts is particularly acknowledged. Thanks are also due other members of the staff who assisted a t times in the tests. LITERATURE C l T k D
TABLE VI. FORMULAS OF COMPOUNDS OF NATURAL RUBBER REINFORCED WITH LIGNINAND OTHERPIGMENTS
Natural rubber Lignin Pigment Zinc oxide Mercaptobenaothiazole Tetramethylthiuram monosulfide Stearic acid Antioxidant Sulfur Volume loading. Q
100 38.50
Parts by Weight 100 100 ??a
5 3
5 3
1.5 2
1.5 2
2.5
2.5
100
38.5O ?7a 5 5 1.25,l.O. 1.25,l.O. or0.8 or0.8 3 1.5 2.5
3 1.5 2.5
Dawson, T. R., J . Rubber Research, 18,l-11 (1949). ( 2 ) Healy, L.J. D., Rubber Age, 57,701 (1945). ENG.CHEM.,39, 480-3 (1947); (3) Keilen, J. J.. and Pollak, A., IND. (1)
Rubber C h m . and Technol., 20,1099-1108 (1947). (4) Murray, 0.S.,and Watson, W. H., India Rubbet W w l d . 118. 667-9 (1948). ( 5 ) Raff, R. A. V., Tomlinson, G. H., 11, Davies, T. L., and Watson, W. H., Rubber Age, 64.197-200 (19481. ( 6 ) Rev. g h . caoutchouc,24,264-6 (1947). R E C E I V ~March D 2. 1961. Presented before the Division of Rubber Chem&try of the AMERICANCHSYICAL SoaIaTY. Washington, D. C., 1961. Based in part on work done b y J. J. Keilen in partial fulfillment of the r e q u i r s menta for t h e degree of D.Ch. E.et the Polytsohnio Institute of Brooklyn.
