GROWTH RATE OF NATURAL RUBBER CRYSTALLITES R. E. H A M M E T T , R. E. W I N G A R D , A N D
J.
E.
L A N D
Departments of Chemical Engineering and Chemistry, Auburn Unicersity, Auburn, Ala. The melting range and growth r a t e curves for natural rubber crystallites have been studied by the differential thermal analysis technique. The results obtained compare favorably with the growth r a t e curves obtained by dilatometric techniques. The use of differential thermal analysis revealed details of the crystallite melting which were not apparent when the dilatometer was used.
and Bekkedahl (2, 76) have reported a very detailed dilatometric study of the crystallization of natural rubber. They found that crystallization occurs in unvulcanized, unstretched natural rubber a t temperatures between -50' and 15' C. They also determined that the rate of crystallite growth in natural rubber passes through a maximum as the temperature of crystallization is decreased. Thus a t - 25" C., maximum crystallite growth rate is obtained in about 2.5 hours. They also found that the melting range of the natural rubber crystallites is a function of the crystallization temperature, becoming broader as the crystallization temperature approaches -40' C. Cooper and Smith ( 5 ) have recently reported on the melting transitions in diene polymers using the differential thermal analysis technique with matched thermistors in a balanced bridge circuit. They used heating rates of 0.01' to 0.3' C. per minute on relatively large samples. The literature on the DTA technique reveals little investigation into the determination of crystallite growth rate. With the more rapidly crystallizing polymers, use has been made of the dilatometric technique, (2, 7, 9, 73), change in the polarized light transmittance ( 7 4 , and the measurement of spherulite growth utilizing the optical microscope (73). OOD
Apparatus and Procedure
The sample holders and heating and conditioning blocks were cylinders made of 3s aluminum which had 28-gage Nichrome heating wire wound around the exterior of the holders and the exterior was insulated with three layers of asbestos paper. A sample well and reference well were drilled into the upper surface of the holders, so that the reference, calcined Alundum, and the samples were located approximately midway in the block. The holes, designed to hold 10- X 75-mm. borosilicate glass test tubes, were 0.408 inch in diameter and 1.96 inches deep, and were located symmetrically on a circle with a 0.375-inch radius. A 1.5- to 2.0- X 90-mm. borosilicate melting point capillary tube, inserted into the sample and reference as close to the center of each as was possible, served as the thermocouple wells. Copper-constantan thermocouples, 30 gage, were inserted into the capillary tubes and in turn were connected to Sargent SR recorders. T h e thermocouples Lvere prepared in such a manner that both the sample temperature and differential temperature could be recorded. The variable autotransformers, connected to the heating blocks, were set so that each of the two heating blocks utilized had approximately the same heating rate of 3.5' to 4.0' C. per minute. The sample holders and heating blocks were maintained in a low-temperature refrigerator set a t -60' C. All runs \vere made ivith the heating blocks in the heat sink a t -60' C. The samples were conditioned for the desired period of time in one of the conditioning blocks. Prior to the test run, the sample and reference wells in the heating blocks were partially 168
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
filled with ethanol to eliminate the effect of any ice crystals in the heating block cavities and to provide a fluid heat transfer medium between the heating block and the sample and reference. The heating block was cooled to a temperature approximately 20' C . below the conditioning temperature with liquid air vapors. When the sample and reference had reached a steady-state condition, as indicated by the sample temperature and AT records, the power to the heating block was turned on. The sample temperature and the AT e.m.f. were recorded until the sample reached approximately 50" C. At this time, the power was turned off and the sample was placed in a large conditioning block held at 50' C . and allowed to condition for 1 hour, to ensure that all of the crystallite nucleating sites were destroyed. At the conclusion of the conditioning period, the sample was placed in the appropriate block for the next conditioning period. Analysis of DTA Record
A number of procedures have been used to estimate the glass transition temperature, to characterize the endotherms, and to estimate the endotherm area ( 7 , 3, 4, 6, 8,77, 74,76). Special techniques have been proposed to determine the area of the endotherm for a quantitative analysis of the amount of heat generated or absorbed in a phase transition or reaction (3, 70, 75, 77). The method selected for this study was based on those of Vassallo and Harden (75). To estimate the start-ofmelting temperature, a n extension of the premelting AT record is drawn and a tangent to the straight-line portion of the initial endotherm is drawn. The intersection of the two lines is used to indicate the start of melting temperature. Since the quantity of heat absorbed during the melting of the crystallites is relatively small, we assumed the temperature a t c
H R.
7
H R. H R.
HA. H R. H R.
H R.
v -so
-20
-10
SAMPLE
Figure 1.
-22'
c.
0
IO
20
TEMPERATURE
30
40
("C)
Endotherms for natural rubber crystallized a t
-
0.5
80
>
-
4
60
0.4
-
t-
i i
I
2
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%-
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I
Y U 20 U
6
-5'C
0
- 1 I O C
0
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CONDITIONING
Figure 2.
TIME
20
26
24
(HOURS)
0
I
I
Natural rubber
Maximum p e a k height of AT vs. t e m p e r a t u r e as a function of crystallization temperature a n d time
40
1.0 0 0
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0.8
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20
w
o
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u
0
5 0.6
a
I
3
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a
a
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0
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-20
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4
C -40
-
-60 -50
-40
-20
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CONDITIONING
Figure
3. Natural rubber
Endotherm a r e a of AT vs. temperature as a function of time a n d crystallization t e m p e r a t u r e
Ivhich melting is complete to be the peak remperature and took this to be the point a t \vhich the endotherm is a maximum. T h e true final melting point may be somelvhat above this peak temperature. since O L I ~ final peak temperature may be the temperature a t which rate of melting is a t a maximum. To estimate the area of the endotherm. a second pair of intersecting lines is draxvn. similar in construction to the first set but utilizing the final portion of the endotherm. A straight line is then drawn betxveen the t\vo points of intersection and the area (heat) absorbed by the melting of the crystallites is considered to be the area bounded by the tanTent lines. the endotherm curve. and the line connecting the t\vo points of intersection.
LITERATURE DATA EXPERIMENTAL DATA
W
-10
20
10
0
TEMPERATURE
('C)
Figure 5. Melting range of crystalline rubber as a function of temperature of crystallization
(Figure 4), a bell-shaped curve is obtained which peaks a t approximately -25" C. Included in this figure is the growth rate curve of Wood and Bekkedahl. The three curves are in excellent agreement. considering the differences in the techniques involved. iYhen the maximum peak height temperature and the melting temperature are plotted against the conditioning temperature, two curves are obtained which indicate the melting range of the crystallites which were formed at a given conditioning temperature. The results are shown in Figure 5, where the solid curves are the melting range boundaries obtained by Wood and Bekkedahl. Conclusicns
Results
Figure 1 illustrates an example of the family of DT.4 curves obtained with natural rubber, where the specimens were conditioned for times up to 24 hours. The conditioning times a t -22' C. are shown a t the right of each of the curves. As the crystallization time increased, the depth of the endotherm increased. The shoulder jvhich appears a t the lo\ver sample temperatures is not explainable at the present time. O n Figure 2? rhe maximum peak height determinations are plotted as a function of the conditioning time with the conditioning temperature as the parameter. The arro\vs on this figure designate the location of the time required for one half of the total change to occur. O n Figure 3 the total areas of the endotherms are plotted in a similar manner. "hen the reciprocals of the half-times (a measure of the growth rate) are plotted against the conditioning temperature
The results of the DT.4 investigation of the natural rubber agree very well with the detailed study of \Yood and Bekkedahl on the growth rate curve and the melting range of the natural rubber crystallites. The use of the DTA technique permitted the observation of the melting behavior of the rubber crystallites; however. no quantitative data on heat effects or the degree of crystallinity were calculated. literature Cited (1) Barshad, I., A m . .Vfinera/ogist 37, 667 (1952). Bekkedahl, N.. \Vood, L. A., J . C h ~ m Phjs. . 9 , 1 9 3 (1941). 0) Borchardt. H. J.. Daniels. F.. J . iim. Chem. Sot. 79. 41 (1957). (45 Clampitt. B. H., A n d . Chem. 35, 5'7 (1963). (5) Cooper, \V,. Smith, R. K . , J . Poiynw Scz. Part A l , 159 (1963). (6) Gamel, C. h4..Jr., Smother. \V. J.. A n a / . Chim. Acta 6 , 442 (1952). (7) Inoue. hi., J . Poivnier Scz. 61, 343 (1962) (8) Ke. B., Zbid., 50, 8' (1961).
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9) Kell, R. M., Bennett, B., Stickney, P. B., J . A j j l . Polymer Sci. 2, 8 (1959). (10) Kissinger, H. E., Anal. Chem. 29, 1702 (1957). (11) Kostomaroff, V., Rey, M., Silicateslnd. 28, 9 (1963). (12) Magill, J. H., ‘liature 187, 770 (1960). (13) Marker, L., Hay, P. M., Tillery, G. P., Early, R. M., Sweeting, 0. J., J . Polymer Sci. 38, 33 (1959). (14) Stein, R. S., Powers, J., U. S. Dept. Commerce, Office Tech. Services AD 257, 561 (1961).
(15) Vassallo, D. A,, Harden, J. C., Anal. Chem. 34, 132 (1962). (16) Wood, L. A., Bekkedahl, N., J . A p j l . Phys. 17, 362 (1946). (17) Yamamoto, A,, Bunseki Kagaku 12,26 (1963).
RECEIVED for review December 31, 1964 ACCEPTEDMay 24, 1965 Research sponsored by the Army Missile Command, Redstone Arsenal, under Contract DA-Ol-O09-ORD-l023(Z), Part I.
NITROGEN-15 TRACER STUDIES OF T H E
N ITRO LYS IS 0 F H EXA METHY LEN ET ET R A M I N E T H O M A S C. C A S T O R I N A A N D JOSEPH
R. A U T E R A
Exjlosives Laboratory, Feltman Research Laboratory, Picatinny Arsenal, Dover, N . J . The path of amino nitrogens in the nitrolysis of hexamethylenetetramine (hexamine) to a mixture of the homologous cyclic methylenenitramines, 1,3,5,7-tetranitro- 1,3,5,7-tetraazacyclooctane (HMX), and 1,3,5tetranitro-1,3,5-triazacyclohexane (RDX), was traced using nitrogen-1 5. The equilibration of hexamine and ammonium nitrate amino nitrogens in the formation of the stable intermediate, 1,5-dinitro-endo-methylene-
1,3,5,7-tetraazacyclooctane (DPT), during the first stage of nitrolysis is attributed to exchange. Only the trimethylene-substituted and not the dimethylene-nitro-substituted amino nitrogens in DPT undergo interchange with the ammonium ion. DPT does not cleave selectively in the formation of H M X and RDX during the second stage of nitrolysis. In the absence of paraformaldehyde, (CHzO),, H M X and RDX are derived essentially from hexamine nitrogens with only a small fraction (approximately 5%) of RDX derived from ammonium nitrate. In the presence of (CH20), a d d e d initially to the reaction mixture, 7% of the HMX and 40% of the RDX a r e formed from ammonium nitrate.
affecting the ratio of H M X , 3, RDX, 4, mixtures rich in H M X (HMX/RDX) produced by the stepwise nitrolysis of hexamine, 1 (Figure l ) , have been studied in these laboratories. The reaction conditions of the two-stage process investigated were similar to those used in the Holston Defense Corp. (HDC) commercial process, which in turn is based on the Bachmann combination process for H M X / R D X formation ( 7 ) . Yields of H M X / R D X were comparable to those obtained by the H D C process, but by adding a small amount of paraformaldehyde, (CH,O),, to the reaction mixture (hereafter referred to as the PA process) we increased the yield of mixed products by approximately 10%. Castorina and coworkers (3) subsequently studied the distribution of radioactivity in the cyclic methylenenitramine products during the course of nitrolysis by tagging either hexamine or (CHZO), with carbon-14. I t was concluded that in the first reaction stage, methylene groups from hexamine and (CHzO), form a common pool for the formation of D P T molecules (2, Figure 1). Similarly, in the second stage methylene groups from D P T and (CHzO), also form a common pool for the formation of H M X and R D X molecules. Therefore, the relative ratio of H M X to R D X is not controlled by the selective cleavage of a large molecule but is probably influenced by the particular conditions controlling the recombination of a common methylene-containing fragment. It was thus postulated that the (CHZO), increases theconcentration of methylenecontaining fragments Lvhich exist in precursors to H M X / R D X . T h e increase in yield of combined H M X / R D X products, resulting from the use of (CH2O),, can therefore be accounted for on this basis. The methylene content is not the only criterion in controlling yield or composition of product. It is obvious that the processes involving amino groups and the formation of the nitramine structure are equally important for understanding the chemisHE VARIOUS PARAMETERS
170
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
try of hexamine nitrolysis. An indirect attempt was made to ascrrtain the extent to \vhich the ammonium nitrate participates in the formation of H M X / R D X in the PA process ( 3 ) . Carbon atom equilibration was found independent of the amino nitrogen concentration. O n the basis of this observation it was concluded that (CHLO)I does not react independently with ammonium nitrate to form H M X or R D X . In a more direct manner, by using ammonium nitrate tagged with ?Pin the amino nitrogen position, Bachmann and coworkers (2) carried out some exploratory \\ark to determine the role played by the ammonium radical in the formation of R D X (together Lvith H M X as the minor constituent). The results obtained were complicated by the observed isotopic exchange of amino nitrogens in hexamine and ammonium nitrate. In spite of the complications introduced by interchange, it was concluded that the formation of R D X involves more inter-
WZ
N0 2
3
Figure 1 .
4
Stepwise nitrolysis to H M X I R D X formation
