Chemigum SL-An Elastomeric Polvester-Urethane J
N. V. SEEGER, T. G. MASTIN, E. E. FAUSER, F. S. FARSON, A. F. FINELLI, AND E. A. SINCLAIR The Goodyear Tire and Rubber Co., Akron, Ohio
A
2. Chain extension of the polyeqter with diisocyanate, giving the storable elastomer 3. Cross linking of the raw gum to give cured Chemigum SL
GREAT deal of interest has been aroused in recent years in isocyanate chemistry by the announcement of new elastcmers based on diisocyanate modification of polyesters. Early Du Pont patent literature (6, 6, IS) described diisocyanate modified alkyds, and Vulcaprene A, an elastomeric diisocyanate modified polyesteramide ( 7 ) , has been offered to the rubber trade by Imperial Chemical Industries, Ltd., England. Several articles (1-4, 8-10, 12-15, 18, 90) also have been published on Vulcollan by Bayer and associates on work done in the laboratories of Fabenfabriken Bayer Co., Leverkusen, Germany. More recently a new elastomeric polyester-urethane, Chemigum SL, was described (11, 17, 19) by the Goodyear Tire and Rubber Co. These rubbers are first made as processible and storable raw gums that have many of the characteristics of pale crepe natural rubber. They can then be mixed on the mill or in the Banbury with additional curatives, such as diisocyanates and other compounding agents, and then cured in standard rubber molds.
01
OS
1 0.61 l’
~ 0.7
j 0.8
0.3
~
i
~
1.0
1.1
1.2
The final vulcanizate physical properties are dependent upon many factors, the most important being the chemical structure of the starting materials and the molecular quantities of each component. In the preparation of the polyester, as shown,
HO--A’-OH
and MOOC-A-COOH
(Dibasic Acid) give
HO--A’-OOC-A-COO-A’---OH
+ HzO
(Polyester) there is considerable latitude in the choice of glycols and dibasic acids as well as molar ratios of reactants in case a mixture is d e sired. Depending on the specific glycols and dibasic acids selected and the molar ratios desired to make a polyester, it is possible to obtain finally a cured elastomer that does not harden or crystallize on aging, yet exhibits outstanding physical properties. Since it is necessary to have the polyester segment terminated in hydroxyl groups, an excess of glycol is used when reacted with adipic acid. In this way also the degree of polymerization or molecular weight of the polyester can be controlled within optimum limits so that elastomeric products result in the reaction with diisocyanates. The effect of dibasic acid structure on the physical properties of Vulcollan is shown in Table I as described by Bayer (3). Adipic acid yields nonhardening rubbers. Short chain acids as well as aromatic acids such as phthalic yield hard, leathery products.
~
R-VALUE Figure 1. Dilute Solution Viscosity us. R-value Chemigum SL
The cured physical properties of Chemigum SL are very similar to Vulcollan, both exhibiting unusual toughness. Tensile strength is very high as is resistance to cutting and chipping. Since the polyester-urethanes are saturated, cut growth resistance is excellent. Even though cuts are initiated by sharp objects there is no tendency to grow, even under stress. In this respect a vast superiority is shown over natural rubber or GR-S. PREPARATION O F STORABLE CHEMIGUM SL
The preparation of the raw gum can be described by the following steps: 1 . Preparation of the linear polyester
(Glycol)
~
1
1
TABLE I. EFFECTO F
~
DIB.4sIc A C I D S O S PHYSICAL PROPERTIES
OF
Glycol Ethylene Ethylene Ethylene Ethylene Ethylene
Acid Succinic Adipic Sebacic Diglycolic Phthalic
Succinic 1:2 Propylene Adipic Adipic 2,3 Butylene 1 2 Propylene
J
VULCOLLAK (3)
Tensile, Elongation, Kg./Sq. Cm. 9% 273 350 270
625 640
. 570 ..
107
261
180 220 179
670 780 630
Remarks Hard, leathery Slowly hardens Hardens immediately Leathery Glass-hard without softener Leathery Does not harden Does not harden
The process of polyaddition, which in principle is the addition of diisocyanates to dihydroxy and polyhydroxy compounds aiid to other compounds containing two or more reactive hydrogen atoms, makes it possible to synthesize from small molecules, products of high or even extremely higb molecular n-eight, and with practically predetermined structures and properties.
2538
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1953 120
TABLE11. PHYSICAL PROPERTIES OF STORABLE RAWGUM Specific gravity 1.17 Softgning range a F. 284-365 Mooney (ML-4’at 2 1 2 O F.) 50-100 120-200 Olsen flow.(212.’ F.; 500 p.s.i.), sec./inch 1.0-2.0 Intrlnsic viscosity, [ 111 25,000-50,000 Molecular weight Gel % 0-10 Sodble as 20% cements ketones (MEK), esters, and chlorinated hydrocarbons Storage (without crystallizing), mon. 6-12
loo
2
*&
80
2
40
8 a
20
e ri
60
symmetrical aromatic diisocyanates, which are preferred over aliphatic diisocyanates. I n the second step in the preparation of the rubber, the polyester segment is “chain extended” with diisocyanate as follows:
0 0.
1
2539
R -VALUE Figure 2. Olsen Flow us. R-value Chemigum
OCN-R-NCO
(Diisocyanate) and
SL
HO-A’-OOC-A-COO-A’-OH (Polyester)
120
give
’p
? L P s2
100
HO-Polyester-OCONH-R-Pol 80 60
4‘ L
$ 8
=
40
20
$95
0.96
Figure 3.
0.38
a37
0.93
1.00
yester-OH
Melted polyester a t 120’ C. is mixed with the appropriate amount of diisocyanate until thoroughly blended. Polymerization can be completed either in an internal mixer or by heating the blend in shallow trays in an oven, As was stated earlier, the nature of the modified polymer will depend upon the amount of diisocyanate used to chain extend and cross link the polyester. It has been discovered that the production of a processible, storable rubberlike polymer involves not only the determination of the critical amounts of diisocyanate t o be used, but also a given range does not apply to all diisocyanates.
R-VALUE Mooney Plasticity us. R-value Chemigum SL
180
I70
The structure of the diisocyanate used in the chain extension step contributes markedly to the cured physical properties. The diisocyanates used in the preparation of storable rubbers are
OCN~----CH~-~-NCO
120 0.90
0.92
0.94
0.96
0.98
1.00
R-VALUE Figure 4. Softening Point us. R-value Chemigum SL
Methylene bis(4-phenylisocyanate) ( M D I ) The critical amount of diisocyanate to be used can be estimated by the relation
(\rNCO
Y
NCO
2,4 Tolylene diisocyanate (TDI)
Moles of diisocyanate = Moles of polyester
value
The most useful range is from 0.70 to 0.99 for optimum storability and processing. Thus, molecular weight is controlled by the amount of excess polyester used.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2540
Vol. 4 5 , No. 11
Typical elastomers in use today with representr d i v e curatives are: Rubber
Curatives
+
g;tuu1>
Sulfur accelerator Metallic oxides 3letallic oxides or diamines ~Ietallicoxides or amines Metallic oxides Diisocyanates
Neoprene Hypalon 5-2 Acrylate Thiokol Chemigum 8L
Polyester-urethane rubbers are cross linked by an entirely different type of chemical reaction. These reactions begin as soon as additional isocyanate curative is added to the millable raw gum (Figure 5). The isocyanate cross-linking reactions are -COOSH-++--SCO
-+
-COOS-
I
COSH-1llophanic eeter
Urethane ..
I
Figure 5. Addition of Isocyanate Curative to Millahle Raw G u m
COXHAmide --"cOKH-+--NCO
,4cyI amide -+
-NHCO-K-
~ONHBiuret
Urea
-40 -90 4 20 40 TEEMPERATURE "C. Figure 6. Cold Hardening of Chemigum SL
-80
-60
-
G e h m a n Twist Nlethod
Typical physical properties of the raw gum are summarized in Table 11. EFFECT OF R VALUE O S PHYSIC4L PROPERTIES OF R4FV G C I I S
The effect of increasing the molar ratio of diisocyanate to polyester ( R value) is shown giaphically in Figures 1, 2, 3, and 4. .si the molecular weight of the raw gum increases, dilute solution viscosity> Olsen flow time, lfooney, and softening point all increase. THE CROSS-LINKING REACTIONS
The high molecular weight, storable rubbers contain the following nitrogen linkages in addition to the ester links: urethane, urea, and amide. Later these serve as cross-linking points as the additional diisocyanate reacts with the nitrogen hydrogen atoms. The diisocyanates used as curatives may be the same as or different from the ones used in the preparation of the storable rubber.
Figure 7. Natural Rubber Gum Stock Stretched M%
Of the throe major diisocyanate cross-linking reactions, the ones with amide and urea predominate. PHYSICAL PROPERTIES OF CURED CHEMIGUM S L RUBBERS
Typical physical properties of vulcanized Chemigum SL are shown in Tables I11 and IV. As shown by these data, resistance is poor to high temperatures, hot water, and steam.
November 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
Dynamic properties of cured Chemigum SL are compared with natural rubber and oil-extended GR-S tread stocks in Table .V. The data were obtained by means of the Goodyear Vibrotester. Polyester rubber compares favorably with standard tread stock, having higher resilience and generating less heat on flexing than the hydrocarbon rubbers.
TABLEIv. AGEDPROPERTIES
2541 OF
VULCANIZED CHEhrIGUV SL
(Cured 15 rnin. a t 280' F.)
Aged in dry heat 30 Days at 158' F. 14 Days a t 250' F. Aging in water 3 Months a t 77O F. 2 Days a t 200° F. 14 Days a t 158' F.
Tensile Lb./Sq. I k h
Elongation, %
4700 500
685 580
5000 1700 1000
... ...
The x-ray pictures in Figures 7 and 8 show the similarity of Chemigum SL to cured natural rubber gum stock when both are stretched 400%. The Chemigum SL pattern disappears when relaxed.
Figure 8.
Chemigum Stretched 400%
TABLE 111. PHYSICAL PROPERTIES OF VULCANIZED CHEMIGUM SL (Cured 15 minutes a t 280° F.) Tensile, lb./sq. inch Hot tensile a t 200' F.. lb./sq. in. Elongation % Hot elonga'tion a t 200' F., % Modulus (300%) lb./sq. inch Hardness (Shore 'A) Hot cut flex min. Bur. of S t d d a r d s abrasion (Index = 100) Hot rebound % Cold rebound % Sohopper tea; (nicked), lb./sq. inch Freezing point a C. Ozone resistanbe Ultraviolet resistance
n
5450 2860 750 845 675 65 300 200 84 80 2000 -35 Excellent Excellent
Another disadvantage of these polymers is the tendency to Figure 9. Chemigum Wear Resistant Veneer on harden a t low temperatures. Figure 6 illustrates cold properties Pneumatic Tires as measured by the Gehman twist method. Chemigum SL freezes a t -35" C. compared with -65" C. for cured natural rubber gum. Fillers, such as carbon blacks and nonblacks such as clays, may be employed but with very little enhancement of physical properties except for certain rubbers that have low initial properties. I n such rubbers, considerable amounts of black may be used to advantage. Normally there is a decline in physical properties with the addition of standard fillers; 300% modulus and hardness increase with increased loading. Oxygen absorption data are shown in Table VI. Oxygen absorption for typical natural rubber tread and GR-S tread stocks is appreciable a t 100' C. after 300 hours. After 910 hours, only 0.25 ml. of oxygen per gram of Chemigum SL was absorbed. Figure 10. Chemigum Solid Tire and Miscellaneous Parts
INDUSTRIAL AND ENGINEERING CHEMISTRY
2542
TABLE V.
of Chemigum SL sprayed on the surface of rubber or plastic TREAD STOCKS gives an adherent, flexible, and scuff-resistant coating that is a t the same time resistant to sunlight, ozone, and general weathering. Dynamic Properties
PERFORMANCE OF CHEMIGUM SL
Dynamic modulus, Rubber Stook kg./sq. em. Chemigum SL 84.4 Chemigum SL plus 30 parts MPC black 196.0 Katural rubber tread ' 91 8 Oil-extended GR-S tread 116.0
Internal friction kilopoise's
IN
Relative Dynamic heat resilience, % generation
15 9
63.3
65
49.1
54.5
128
35.1
39.6
109
59.3
29.0
141
~~~~~
tomeric polyester-urethanes. Potential uses include wear resistant veneers on pneumatic tires as shown in Figure 9. Solid tires (Figure 10) have given outstanding performance in the plant on Goodyear electric lift trucks. A high degree of cut resistance is exhibited, Soles, heels, belt surfaces, and floorings may be protected by localized application of wear resistant Chemigum SL.
TABLE VI. OXYGEN ABSORPTIOK 0 2 Absorbed, BIl./Gram of Polymer a t loo0 C. Natural rubber GR-S tread tread stocka stooka Chemigum SL
0 10 27 58 92
0 4 10 14 18
*
%. . ..
The authors are grateful to S. D. Gehman for dynamic and xray measurements, to J. 0. Cole for oxygen absorption data, to the Chemical Engineering Division for developmental research, to H. 3. Osterhof, and to The Goodyear Tire and Rubber Co. for permission to publish this paper. LITERATURE CITED
A great deal of work has been done in developing uses for elas-
0 40 80 120 160 240 320 450 910
.4CKNOWLEDGMENT
~
APPLICATIONS
Time, Hours
Vol. 45, No. 11
0
0.01 0.02 0.04 0.08 0.11 0.13 0.16 0.25
a Natural rubber and GR-S tread stocks contain 45 parts of carbon black per 100 parts of rubber and 1.25 parts of A'-phenyl-8-naphthylamine as antioxidant.
Since the raw gum stocks are readily soluble in standard solvents, they lend themselves to cement compounding and find use as protective coating on rubber, plastics, and fabrics. A solution
Bayer, O., Angew. Chem., 59, 257-72 (1947). B a y e r , O., Ann., 549, 286 (1941). Bayer, O., Muller, E., Petersen, S.,Piepenbrink, €I. F., %%dem u t h , E., Angew. Chem., 62,57-66 (1950); [Rubber C h e m and Technol., 23, 812-35 (1950)l. Brenschede, W. Z., Electrochem., 54, 191-200 (1950). Christ, R. E., and Hanford. W.E., U. S. P a t e n t 2,333,539 (Sov. 9, 1943). Hanford, W.E., and Holmes, D. F., Ibid., 2,284,886 (June 2, 1942). Harper, D. A., Smith, UT.F., and White, €I. G., Proc. Bnd Rubber Technol. Conj., 1948, pp. 61-7; [Rubber Chem. a d Teclmol., 23, 608-14 (195O)l. Ilebermehl, R., Farben, Laeke, Anstrichstoffe, 8 , 123 (1948). Hochtlen, A,, Kunststoffe, 40, 221-32 (1950). Ibid., 42, 303-10 (1952). Mastin, T. G., Seeger, X. V.,C. S. P a t e n t 2,625,535 (Jan. 13, 1953). hluller, E., Bayer, O., Petersen, S.,Piepenbrink, H . F., Schmidt, F., and Weinbrenner, E., Angew. Chenz., 64 (1952). hfiiller, K. E., U. S. P a t e n t 2,620,516 (Dec. 9, 1962). Petersen. S..Ann.. 562. 205 11949). Popper, k., Rubber Age (W. Y . ) , 73, 81 (1953). Rothiock, H . S., U. S.P a t e n t 2,282,827 (May 12, 1942). Rubber Age ( N . Y . ) ,73, 90 (1953). U. S. P a t e n t . 2,621,166 (Dec. 9, 1952). Schmidt, F. W., Seeger, 5 . V., Ibid., 2,625,631; 2,625,532 (Jan. 13, 1953). Ann., 562, 76 (1949). Siefken, W., .
I
RECEIVED for review &fay 29, 1953. ACCEPTED August 26, 1953. Presented before the Division of Rubber Chemistry, AMERICAN CIiE>IICAL SOCIETY, Boston, Mass., hlay, 1953. Contribution No. 200 from the Goodyear Tire and Rubber Co.
Substituted Silyl Derivatives of Starch R. W. KERR AND K. C . HOBBS George M. Moffett Research Laboratories, Corn Products ReJining Co., Argo, I l l .
T
HE production of a large number of organosilicon compounds in recent years with substituent groups that readily react with aliphatic hydroxyls ( 3 ) makes possible the production of a new and interesting class of starch derivatives. These polymers may be soluble in water, soluble in organic solvents, or under certain conditions, may be, or may become insoluble in all neutral solvents. The substituted silanes may have from one to four reactive groups per silicon atom. Those found to be more reactive with starch hydroxyls so as to form a silicon-oxygen-carbon (Si-0-C) linkage with the carbohydrate contain groups such as chloro, amino, and the lower allroxyls, particularly methoxyl, ethoxyl, n-propoxyl, and n-butoxyl. Of these, the chloro group appears to be the most reactive and the n-butoxyl group the least. Of much less reactivity are tert-butoxyl and the higher alkoxy1 groups.
I n each of these cases where reaction does occur, it is an ewhaiige reaction wherein the hydrogen of the carbohydrate hydroxyl combines v i t h a substitumt group on the silicon, forming hydrochloric acid, ammonia, or alcohol, as the case may be, and creating an S-0-C linkage. Alkyl or aryl groups attached directly to silicon are exceptionally unreactive. Starch molecules have, on the average, three ircc hydroxyls per glucopyranose unit. Accordingly, when the caI bohydrate is treated with a reactive substituted silane, as many as three substituent groups may be introduced per glucopyranose unit. When the substituted silane is monofunctional, those starch derivatives which have one or more substituted silyl groups per glucopyranose unit [D.S. (degree of substitution), or higher J are, in general, soluble in organic solvents such its the hydrocarbons or halohydrocarbons, T$ hile products having very low degrees of substitu-
