THE CHEMICAL NATURE OF T H E D ENSI FICAT10 N OF CARBON BLACKS ANDRIES VOET Research Laboratories, J . M . Huber Corf., Borger, Tex
The arguments hitherto given in favor of a mechanical or physical view of densification of carbon black b y pelletizing or by compression are discussed and found wanting, particularly in view of the irreversibility of the densification process. A novel approach, visualizing chemical bonding between free radicals as the basic phenomenon in the densification process, is supported b y the discovery of significant differences in free radical concentrations between fluffy and densified blacks, as found from free radical determinations. The abundant creation of free radicals upon charring carbohydrates elucidates the behavior of binders in the pelletizing process. The method of preparing carbon black solids of high compressive strength by heating channel black particles in close contact can b e explained b y massive chemical bonding of free radicals created in high concentrations b y the decomposition of the oxygen complexes on the particles.
black is formed in the furnace or channel process as a light powder, of a bulk density of about 3 pounds per cu. foot. T o prepare the commercial “fluffy” (unpelletized) black, the powder is mildly agitated to increase its density to about 12 pounds per cu. foot. Further densification to 23 to 30 pounds per cu. foot or higher may occur upon pelletization or compression. I n the process of pelletization, small granules known as beads or pellets are formed. I n the dry pelletizing process the black is submitted to a prolonged rolling in a long d r u m under the weight of a fairly deep bed of pellets, causing simultaneous compression and frictional rolling. I n the wet process the beads are formed by agitating the wet pellets in a pin mixer, followed by drying. Materials such as carbohydrates, which aid in pelletizing, are sometimes introduced in the wet pelletizer. I n a now obsolete process of carbon black densification, the black was compressed in bags to a density of 18 to 20 pounds per cu. foot. A very pronounced densification can be obtained by compression of the black a t high pressures, 10,000 p.s.i. or over. The density reached indicates that the systems approached a condition of closest packing. The commercially acceptable form of densified black is the pelletized black. The following considerations, however, apply to both pelletized and compressed forms of carbon black. While extensive literature deals with the technique of densification of carbon black, there is little or no information about the actual processes which are responsible for the formation of pellets. I n this investigation the process of densification is examined and reasons are given why the densification of carbon black should be considered as one of chemical bonding between carbon black particles. ARBON
Concepts about Densification of Carbon Black
Although published information about the actual mechanism of carbon black densification by compression or pelletization is lacking, there is a widespread conception that this process is some kind of mechanical involvement in which the particles are entangled in much the same way as, for instance, single
filaments in a multifilament fiber. This idea could be called the mechanical concept of the densification process of carbon black. A second view is that physical forces, of the London-van der Waals type, are responsible for the particle agglomeration in pelletizing and compression. This idea could be called the physical concept of densification of carbon black. Finally, a third possibility is the concept that chemical forces actually cause the particle agglomeration in the densifying process. This idea could be called the chemical concept of densification of carbon black. Examination of the Three Concepts of Densification
If the mechanical view of densification were correct, one would expect to find, as in the case of fibers, the presence of elements of the carbon black particles which would entangle, enmesh, or twist with one another to form such mechanical bonds. Very extensive microscopic and electron-microscopic studies of carbon blacks have never revealed the existence of such elements. O n the contrary, carbon black particles appear to be extremely small and spherical, with generally smooth surfaces, although occasionally with rougher or even porous surfaces. Since many of the perfectly smooth carbon black particles have excellent pelletizing properties, the mechanical conception of densification has little proof in fact. The physical concept is closely related to the mechanical concept in one respect: the reversibility. I n the more sophisticated physical concept, agglomeration of particles is thought to be caused by physical forces. Since such forces generally are of a much milder nature than chemical forces, physical bonds between particles could be broken and joined a t will by mechanical means, without causing major changes in the black. The same holds true for mechanical bonding between black particles. Thus, the question of reversibility is an important issue, which must be decided in the light of experimental evidence. The chemical concept envisualizes chemical bonding between carbon black particles as the cause of agglomeration and densification. I n the past such ideas would be held very VOL 1
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unlikely, in view of the supposedly chemical inertness of the element carbon. There is, however, a great deal of evidence that carbon black is not a n inert material, but is rather reactive and interacts with a variety of materials. The chemical concept merely emphasizes this fact by pointing out that particles could form chemical bonds, whatever the reacting atoms or atom groups may be. Chemically bonded agglomerates cannot be expected to redisperse into the original dispersion by mechanical means. Hence, the bonds between the particles must be considered as irreversible.
Table II.
MPC SRF
Electrical Conductivity of Carbon Blacks in 8% Dispersion in White Mineral Oil Minimum
Fluffy Pelletized Fluffy Pelletized
32 32 94 94
11.8 23.2 9.2 30.8
210 80 75
40
1400 360 420 190
Experimental Evidence of Irreversibility
Rheological Properties. A fluffy ISAF black was pelletized in pilot plant equipment in a batch process under completely controlled conditions. The original density was 10.9 pounds per cu. foot. The pellets were densified to 23 pounds per cu. foot, then carefully ground in a small laboratory hammer milltype of grinder to a high degree of fineness. The fluffy powder closely resembled the original black. Its density was 11 pounds per cu. foot. The three blacks were ground in a concentration of 8% in white mineral oil in a laboratory shaker ball mill. Care was taken that the blacks were all well dispersed. The residues on a 325 screen were all below 0.0037,. The viscosities of the dispersions were measured with a Brookfield viscometer at various revolutions per minute a t 30' C. The results are indicated in Table I, together with the oil absorption values. These data indicate that the pelletized black differs markedly from the fluffy black, showing a decreased viscosity and a less pronounced thixotropic behavior. The reground black: however, does not indicate the slightest tendency to revert back to the flufly black. O n the contrary, it has even less structure and less thixotropy than the pelletized black, notwithstanding its low density and ease of dispersion in the vehicle. The oil absorption data reveal the same picture. Milling fluffy black does not appear to modify its properties. Electrical Conductivity i n Dispersion. The electrical conductivity of dispersions of carbon blacks is an excellent indication of the structural involvement of the kinetic unit of the dispersion ( 8 , 9 ) . Table I1 indicates the minimum conductivities, extrapolated to infinite shearing stress, of an MPC and an SRF black, each dispersed in an 8% by weight concentration in a white mineral oil before and after pelletization. These data indicate that although the blacks were dispersed in an extremely finely divided state, significant permanent differences exist between fluffy and densified blacks. Dielectric Properties in Dispersion. ,4 fluffy ISAF black was compressed in a glazed porcelain cylinder to about 7000 p.s.i. and thereafter remilled in a laboratory hammer mill to a very fine powder. Both fluffy and compressed, remilled blacks were dispersed in a mineral oil vehicle in which an asphaltic dispersion agent as well as a barium salt of a longchain alkyl sulfonate derived from petroleum hydrocarbons was dissolved, These powerful chemical deflocculating agents
Table 1.
Fluffy Pelletized Remilled
196
Viscosity of a Dispersion of 8% of ISAF Blacks in White Mineral Oil Oil AbsorpBrookjrld Viscometrr Data tion, 60r.p.m. 72r.p.m. 1.5r.p.m. 0.6r.p.m. G./100 G. 81 50
42
305
183 150
1480 940
2450 1600
600
1050
140 116 113
l&EC PRODUCT RESEARCH AND DEVELOPMENT
(8) remove the reversible (transient) part of the structure but leave the irreversible (persistent) structure due to particle fusion intact ( 9 ) . The experimental results reported in Table I11 were obtained from measurements of the dielectric constant of the dispersions. These data show that the dielectric form factor ( 8 , 7 7 ) is markedly reduced by the process of densification, indicating that the kinetic unit of fluffy dispersed black is more anisometric than the kinetic unit of the compressed dispersed black, although both blacks have the same state of dispersion. This points to significant, permanent differences due to the process of pelletization. Again, milling a fluffy black does not change its properties. Specific Volume of Carbon Black u n d e r Compression. The specific volume of carbon blacks under compression has been measured for a variety of blacks. Generally, the specific volume is a linear function of the logarithm of the applied pressure over the entire range of pressures measured, up to 10,000 p.s.i. Therefore, the specific volume of the black at any given, single pressure may be taken as characteristic of its behavior under compression. Table I V shows the specific volume of a n ISAF black at the selected pressure of 1000 p.s.i., starting from the same material in different states of densification. These data indicate that the fluffy black is irreversibly compressed into a material with a different specific volume and cannot be reformed by mechanical redispersion. Once-compressed blacks, however, do not appear to change any more upon recompression. Pelletized blacks are identical in specific volume and behavior upon compression to compressed blacks. Another difference between fluffy blacks and densified blacks is apparent from the volume-pressure relationship itself. which is strictly linear in a semilog scale for pelletized and compressed, remilled blacks. Fluffy blacks, however, not only follow a different line, but may show some curvature, concave with respect to the pressure axis. Such behavior was never noticed after a second compression of the redispersed black. or of pelletized blacks. Conductivity of Carbon Blacks u n d e r Compression. The conductivity of carbon blacks under compression has been measured for a variety of blacks. Generally, a linear
Table 111. Dielectric Constant of Carbon Blacks in 2% Dispersion in Deflocculating Mineral Oil Vehicle Relative Dielectric Constant Form Factor at 30' C. I S A F Black Fluffy 1,145 5.14 Compressed, remilled 1.107 3.79
Specific Volume of ISAF at l o 0 0 P.S.I. spccifc Volume at 7000 Condition p.s.r. Fluffy 1.73 Remilled after first compression 1.53 Remilled, recompressed repeatedly 1.53 Pelletized, milled 1.54 Pelletized, compressed, remilled 1.53 Pelletized, compressed, remilled repeatedly 1 .53 Fluffy, milled 1.73
Table IV.
A B C D E F G
relationship exists between the logarithm of the conductivity of the black and its specific volume a t any given pressure, u p to 10.000 p.s.i. Therefore, the conductivity of the black a t any given single specific volume may be taken as characteristic of its behavior under compression. Table V shows the actual resistance, in ohms per square centimeter per gram of black, for an ISAF black a t pressures corresponding to a specific volume of 1.5, or a density of 0.67, roughly equivalent to a compression to a void volume of 64%. These data indicate that a fluffy carbon black is irreversibly compressed into a material with a different electrical resistivity. The original rcsistivity cannot be regained by mechanical dispersion. The electrical resistivity of pelletized black under compression is not changed upon repeated compressions and is identical to that of already compressed fluffy blacks. Properties of Vulcanizates. A fluffy HAF black was pelletized in pilot plant equipment in a batch under completely controlled conditions. Its original density was 10.4 pounds per cu. foot. The pellets were densified to 24 pounds per cu. foot. Thereafter some of the pellets were micronized to the highest degree of fineness obtainable with the commercial hammer mill used. A fluffy powder closely resembling the original black was obtained with a density of 11.1 pounds per cu. foot. The three blacks were tested in a standard L T P test recipe. The results are shown in Table VI. These data indicate that the process of pelletizing the carbon black has markedly changed the properties of the vulcanizate. It is obvious that the changes are not due to lack of dispersion, since examination of the vulcanizates revealed an excellent dispersion in all cases. Moreover, the remilled pelletized black, a powder fully as finely divided as the fluffy black, showed even more pronounced differences from the fluffy black than the pelletized black, emphasizing the permanent changes i n the reinforcing properties of the black caused by the process of its densification. Free Radical Mechanism of Carbon Black Densification
Effect of Free Radicals Present in Fluffy Blacks. I t is known that carbon blacks contain free radicals. Bennett
Table V.
.4
B C D
Resistance of an ISAF Black under Compression Resistance. Ohms/Sq.Cm. / G. at Spec. Condition Vof. 7.50 Fluffy 4.22 .After repeated compressing and remilling 2,69 Pelletized milled 2 69 After repeated compressing and remilling 2.69
Properties of Vulcanizates Reinforced with HAF Black Shore HnrdTensile, ness, ExModulus 60-Min. 60-Min. Abra60- truszon Cure, P.S.I. Cure, sion, Min. Srcell, Condition 300% 400% P.S.I. ?% Cure % 100 65 124 3740 3540 2470 Fluffy 89 64 132 3030 3760 2090 Pelletized 86 64 132 3720 2930 2000 Pelletizedremilled Table VI.
et al. ( 7 ) and Uebersfeld et al. (7) showed that during carbonization of organic materials a large number of free radicals become trapped on the carbon and are, to some extent, stabilized. These free radicals can be detected by electron spin resonance techniques, by which the concentration of unpaired electrons, characteristic of free radicals, can be measured. Thus, Krause and Collins ( 4 ) showed that commercial blacks have a free radical concentration varying from 1.4 to 0.6 X 1 0 2 0 electron spins per gram. I t was also found that unpaired electrons could be ‘.annealed out” by a heat treatment above 1000° C., which resulted in the reduction of the free radical concentration to a n insignificant fraction of the original. Chemical techniques yield similar results. Thus, determination of quinone groups, associated with the presence of free radicals, essentially leads to the same values for the free radical concentration as the electron spin resonance technique (2, 70). Table VI1 indicates free radical concentrations in a fluffy black, in the same black compressed, and in the same black remilled after compression. as measured by the quinone method (70). These data show that upon densification the number of free radicals per gram of the particular ISAF is reduced to half its original value. The disappearance of free radicals would seem to indicate the formation of chemical bonds during the densification. The formation of chemical bonds would require a change in energy of the compressed black as compared to the fluffy black, which should become apparent from heats of combustion. According to Pauling, the bond energy for a hypothetic diatomic “carbon” gas is 58.6 kcal. per mole (6). Such a gas has 6.0 X loz3carbon to carbon bonds. A single such bond, therefore, has a n energy value of about 1.O X 10-22kcal. Thus, the energy difference between these fluffy and compressed blacks may be estimated a t about 5 cal. per gram or 9 B.t.u. per pound. Heats of combustion tests indicate that differences of such a small order of magnitude are beyond the limits of accuracy of the instruments now available. Effect of Free Radicals Produced in Processes of Densification. Carbon bonds in carbon blacks may be broken by mechanical action. Thus, Gessler ( 3 ) showed that attrition of carbon blacks by dry ball milling leads to a mechanical cleavage of the C-C bonds. Unpaired electrons may then
Table VII.
Free Radical Concentration in an ISAF Black by Quinone Determination Frec Radicals MicromolPs/ Number per Condition gram gram X 7OwZ0 Fluffy 180 1.10 Compressed 95 0.57 Compressed and remilled 90 0.53
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become available for subsequent chemical reactions a t the particle surface. The mechanical treatment which the carbon black undergoes in the various processes of densification bears a close resemblance to the attritional processes described by Gessler ( 3 ) , although the latter action is generally much more severe. Thus, during densification C-C bonds may be broken, resulting in the formation of free radicals, which may subsequently react through their unpaired electrons. The changes in physical properties upon densification, as shown previously, clearly indicate that in attritional processes structural bonds responsible for anisometry of the agglomerated particles are preferentially broken. The active sites created will then contribute to the formation of more isometric particle complexes (9). If not all broken bonds are re-established, however, the reduction in free radical number would be less than anticipated, or it could even increase. Intensity of Bonding in Densified Carbon Black
The surface area of the black considered was 120 sq. meters per gram. Since the effective area of a carbon atom can be estimated a t about 2.4 s q . A,, the number of carbon atoms on the particle surface is equal to 6 X per mole (12 grams), which is about 10% of the total number of carbon atoms. The number of chemical bonds created by free radical interaction was found to be equal to about 3.0 X 1020 per mole, being half the number of free radicals removed during densification. This means that upon compression about one out of every 200 carbon atoms a t the particle surface did form chemical bonds with neighboring particles. Since a particle of a diameter of about 300 A. contains approximately 4 X 106 carbon atoms, of which there are 4 X 105 on the surface, it would mean that per carbon particle, upon compression, about 2000 chemical bonds were created. One would expect that under extreme conditions the bonding of carbon black could become so intense that a strong, solid carbon black could be formed. This is indeed the case. According to Mrozowski (5),channel black is compressed in a porcelain cylinder. The pressure is then released and the black is heated to 900” to 1000” C., in a n inert atmosphere for several hours. The black is cooled and a solid is obtained with a compressive strength of the order of 3000 p.s.i. No bonding occurs when a fluffy black is heated at 1000” C. in a n inert gas and compressed thereafter. Obviously, the large number of free radicals on the carbon particle surface, newly created by the volatilization of the surface oxides on the black, will cause a much larger number of chemical bonds between adjacent particles in the compacted blacks than by the “trapped” free radicals normally present in all blacks or by free radicals created during the process of densification. This leads to a strong carbon black solid by a thorough, threedimensional bonding. The free radicals created by decomposition of the surface oxides have a very short life expectancy and are quickly dissipated by free radical scavengers, such as traces of oxygen, etc., when unable to react immediately after their formation. Thus, compression of a previously devolatilized black does not lead to a carbon black solid. If the free radicals are removed before densification, the behavior of the black on compression may be expected to differ markedly from blacks in which the free radicals are present, since in the former case no chemical bonding is expected to occur through existing free radicals. This is the case. The specific volume of a fluffy carbon black at 1000 p.s.i. is considerably larger for blacks treated at 1000 p.s.i. in vacuum 198
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
than for untreated blacks. Thus, for an oxidized channel black, this value changed from 1.42 to 1.78 upon heating. Pelletization and Structure
Structure in a carbon black is envisualized as bonding between adjacent carbon particles. Densification, as it appears now, is of a similar nature. Yet, structural bonds cause a behavior in black completely opposed to the behavior upon compression and pelletization. As a result, a densified black has less structure than the same black in the fluffy state. The explanation must be sought in the topology of the bonding. In structural involvement one must distinguish between a persistent and a transient structure. The persistent structure is irreversible and forms particle chains by “fusion” in the early, formative stage of the carbon black particle, as observed in electron micrography of carbon blacks. The transient structure is of a physical nature and forms chains and networks, creating fully reversible physical bonds. Both persistent and transient structures are clearly directed toward anisometric agglomerates (9). The process of particle fusion ia the furnace, about which little is known, apparently creates the observed anisometric persistent structure. The physical forces responsible for forming the transient structures appear to be “polarizing” and therefore chain-forming, creating even more anisometric agglomerates, as can be derived from measurements of the dielectric constant of dispersions of the black. I n pelletizing, however, dielectric data indicate the formation of more isometric aggregates. Thus, one must visualize the pelletization process as a chemical bonding in random directions, as opposed to the polarizing physical bonding of carbon black particles in structural involvement. The Practice of Pelletizing
The practice of pelletizing is directed toward a close frictional contact between the particles, causing bond cleavages by attrition and permitting the previously present and newly created free radicals to come into play and to form the desired bonds. Compression, as practiced in the old days, leads to erratic, not easily controlled bonding and has now been abandoned. Dry pelletizing, by its continued frictional contact under mild pressures, gave an acceptable result in a rather long period of time. Wet pelletizing creates the intimate contact between the particles required for bonding by the strong capillary forces which drive the particles together in the liquid. I n this process actual bonding occurs in the dryer. The use of bonding agents, such as molasses, tall oil pitch, etc., must not be seen as a n application of a gluelike substance. On the contrary, it is evident that these agents are active only upon being charred at elevated temperatures. Free radicals are abundantly formed upon charring of organic materials. These free radicals induce increased bonding of carbon particles. I t is therefore clear that bonding agents to improve pelletizing must be selected from free-radical-forming materials, not from sticky or gluelike chemicals. Obviously, the mechanism of particle bonding by bonding agents in pelletizing has nothing to do with bonding by adhesives. If the free radical concentration is low in a given black, one would suspect that its ability to pelletize is impaired. Indeed, fluffy blacks which are devolatilized and thus have lost most of their free radicals do not pelletize properly. Equally, fines from commercial pelletization do not easily repelletize and may even prevent ordinary fluffy blacks from pelletizing when present in a substantial concentration. Finally, the often experienced fact that in commercial pelletization fluffy blacks occasionally show unexpected resistance to being pelletized
literature Cited
might well be explained from the presence of trace quanrities of free radical scavengers, which effcctively reduce the free radical concentration of the black and thus interfere seriously with normal pelletization.
Conclusion
Carbon black particles undergo chemical bonding through a free radical mechanism upon densification. This process is the cause of the irreversible changes found in the blacks upon densification. While structural bonds create more anisometric particles by the formation of particle chains and networks, bonds formed upon densification are randomly directed and cause the formation of more isometric particle agglomerates.
(1) Bennett, J. E., Ingram, D. J. E., Tapley, J. G, J . Chem. Phys. 23, 215 (1955). (2) Donnet. J. B.: Henrich, G., Riess, G., Reu. Ge'n. Cuoutchou' 38, 1803 (1961). 13) Gessler. A. M.. Rubber Aee 86. 1017 (1960). i 4 j Krausej G., Collins, R. E., RLbber &odd 139, 219 (1958). (5) Mrozowski, S., U. S. Patent 2,682,686 (1954). (6) Pauling, L., "The Nature of the Chemical Bond," Cornel1 University Press, Ithaca, N. Y . , 1948. (7) Uebersfeld, J., Etienne, A , , Combrisson, J., Nulure 174, 614 (1954). (8) Voet, A,, J . Phys. Chem. 61, 301 (1957). (9) Voet, A., Rubber World 146, 77 (1962). (10) Voet, A., Teter, A. C., Am. I n k Maker 38 (A), 44 (1960). (11) Voet, A., LYhitten, W.N., Jr., Rubber Age 86, 811 (1960).
RECEIVED for review May 4, 1962 ACCEPTEDJuly 2, 1962 Division of Rubber Chemistry, ACS, Boston, Mass., April 1962.
RAPID CURING OF URETHANE ELASTOMERS S. W
.
U R S,' Naugatuck Chemical, Division o j U . S. Rubber Co., Naugatuck, Conn.
Long curing time (30 minutes or more) of urethane elastomers has been a bottleneck in production. This obstacle has been overcome in a new urethane elastomer, based on an adipate polyester and chainextended to contain urea linkages. The new elastomer cures in 2 to 3 minutes a t 350" to 400" F., developing full physical properties. Data demonstrate the completeness of cure in stocks with carbon black as well as silica fillers. Stress-strain properties, swelling characteristics, and high temperature aging properties of the elastomer are given. Properties of the new elastomer are compared with those of an elastomer, also based on an adipate polyester, which does not contain urea linkages or cure as rapidly.
elastomers ordinarily require long periods of curing time extending to more than 30 minutes. This is a serious drawback in the fast commercial production of molded goods, particularly when plastics equipment such as injection molding and extrusion machines is used. Therefore, curing certain millable urethane elastomers a t elevated temperatures for short periods of time was explored. after appropriately modifying the formulation. RETHANE
Millable Urethane Elastomers
Polyurethane elastomers investigated were based on reaction products ( 7 ) of a polyester, such as polyethylene propylene adipate and polyethylene butylene adipate, and a n organic diisocyanate such as toluene diisocyanate and diphenylmethane diisocyanate to which either a small amount of water or a diprimary diamine was added to form urea linkages along the backbone of the polymer molecule. The resulting product is a polyurea-urethane elastomer. The amounts of polyester, diisocyanate, water, and diamine used in the reaction were adjusted to leave no residual reactive functional groups in the polymer. Preparation of Polyurea-Urethane Gum Polymer
In a Baker-Perkins mixer were placed 2000 grams (1 mole) of dehydrated polyethylene propylene adipate (OH number 55.0,acid number 1.0) and heated to 100" C,; 300 grams (1.2 mole) of diphenylmethane diisocJianate were added and the mixture was allowed to react for one hour. To the fluid mixture 23.6 grams (0.2mole) of anhydrous hexamethylenediamine were added and the mixing was continued for another minutes, solid gum polymer M,as obtained M-hich was readily removed from the mixer, This polymer was designed to contain one urea group per 6792 molecular units. Present address, Polymer Research Department, Olin Mathieson Chemical Corp., New Haven, Conn.
The structure of such a polyurea-urethane elastomer is simply written as :
-o[
.) (CO-(CH~)~-CO-O-(CH~)~-O
1
)po-NH-R-
NH mCO-NH-(CH2)sNH where 1 = 9 to 12, m = 4 to 6, and n = 2 to 3. This is a block copolymer containing urea groups far in excess of those present in straight polyurethane elastomers, where a few urea groups may be accidentally present due to traces of moisture in the polyester. Properties of the polymer are shown in Table I. The gum polymer, by itself in the unvulcanized state, possessed no good mechanical properties. had cold flow, and was readily soluble in many solvents. The gum polymer could be vulcanized by curing with a peroxide. O n compounding with peroxides such as dicumyl peroxide (DiCup) and 2,5-dimethyl-2:5-di-tert-butylperoxyhexane (Varox, Vanderbilt Co.) a process- and storage-stable compound was obtained. Like conventional rubber stock, the urethane gum could be compounded, stored, and processed without scorching. Compounded stocks are ordinarily cured in 30 to 6 0 minutes to yield vulcanizates of high mechanical properties '1, but these stocks cured completely in 3 minutes at 350' F. Straight polyurethane elastomer did not cure as rapidly in 3 minutes at 305' F. (Figure 1). as evidenced by the poor quality of the cures and the difficulty in stripping the vulcanizate from the mold after curing for only 3 minutes. O n the other hand, if the straight polyurethane was cured a t 305' F. for 45 minutes, the stress-strain properties of the elastomer were identical with those of polyurea-urethane elastomer cured for brief periods a t elevated temprratures. VOL. 1
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