F.
M. O'CONNOR
and T. L. THOMAS
Development Laboratory, Linde Co., Division of Union Carbide Corp., Tonawanda, N. Y.
M. L.
DUNHAM
Development Laboratory, Silicones Division, Union Carbide Corp., Tonawanda, N. Y.
Chemical-Loaded Molecular Sieves New Approach to Faster Cures Molecular Sieves holding curing agents and catalysts can be mixed with rubber or resins during formulation. At the high vulcanization or curing temperatures, these chemical agents are released to give fast cures. Prospects: d a s t i c s with less mold "dwell" time; rubber which can be vulcan ized at much higher cure rates
CONTROL
of the activity of rapid curing rubber formulations is particularly difficult because of the high temperatures reached during processing. I n many instances, however, a Molecular Sieve with a n active compound held tightly within its structure by strong adsorptive forces produces a unique temperature-sensitive curing aid. These new materials, called Linde cheaicalloaded hlolecular Sieves, impart properties of rapid cure and safe processing to rubber compounds. T h e Molecular Sieves or carriers for these active curing chemicals are crystalline aluminosilicate materials, synthesized with properties similar to zeolites. Their outstanding characteristic is a n ability to undergo dehydration with little or no change in crystal structure. T h e dehydrated crystals are interlaced with regularly spaced channels of molecular dimensions tvhich comprise almost 50% of their total volume. T h e empty cavities in these activated or dehydrated crystals recapture water molecules that have been driven off; if no Ivater is present: they accept any molecules small enough to enter the adsorption cavity. T h e more polar the molecules, the more strongly they are held within the crystal, but they can be released by heat or by displacement \vith a more strongly adsorbed material. Curing agents and catalysts for vulcanization and polymerization are generally ver>- polar molecules; therefore, they may be adsorbed within the Molecular Sieve pores a n d effectively isolated from the rubber or resin formulation during processing and storage. At the higher vulcanization or curing temperature, ho\vever, the active compound is released from the Molecular Sieve to function in its normal manner. This technique a l l o w the use of very active materials to obtain fast cures Lvithout sacrificing processing safety or pot life. In fact. catalysts or vulcanization agents previously considered too active for practical use may no\v be
formulations including styrene-butadiene rubber, natural rubber, neoprene. epoxy resins, and rigid vinyl plastisols. A large number of different compounds, including amines and peroxides, have been adsorbed on Molecular Siwe powders and evaluated as accelerators and curing agents in rubber. I n most cases, the loading of active compound in the powder is 15 f 2 weight yc-i e., 100 grams of poi+der contains 15 grams of active compound.
used with safety. I n the adsorbed state, the active compound is also easier to handle because its volatility, toxicity, a n d flammability are decreased by its greatly reduced vapor pressure when adsorbed on Molecular Sieves. T h e Molecular Sieve used to carry the active chemical is a fine powder having a particle size of 1 to 5 microns. This inert material has essentially n o effect on the properties of the rubber during processing or after vulcanization. These powders disperse rapidly and completely in rubber formulations, when the A S T M standard mixing technique is used ( 7 ) . Molecular Sieves loaded with di-tertbutyl peroxide are being used commercially in the curing of silicone elastomers. For certain applications. this peroxide is superior to others. Its use. however, has been restricted because of its high volatility and flammability. I t was visualized that adsorption of this peroxide on Molecular Sieve could overcome these difficulties. T h e volatility was greatly reduced, as evidenced by retention of the peroxide during bin aging and decreased fire hazard during compounding. This peroxide \\as much more efficient as a cross-linking catalyst when adsorbed on Molecular Sieves. because lower concentrations were required than when the peroxide was used alone. This principle has nolv been extended to organic rubber and plastics. These chemical-loaded Molecular Sieves can serve as latent accelerators and curing agents in a varietL of rubber and plastics
latent Curing Effect
T h e ideal curing system for rubber formulations has two primary requisites. I t should be inactive during processing and storage of the rubber compound. and it should bring about rapid curing during vulcanization with the attainment of good physical properties. hlost rubber formulations utilize accelerators to efisct vulcanization in reasonable: lengths of time. Most accelerators uThich give rapid-curing rubber fbrmulations are generally too active during processing. Those which have good nonscorching properties, on the other hand, result in relatively slow-curing compounds. This has led to the use of combinations of accelerators, a primary \vhich is the actual catalyst and a secondary (commonly called activator) Lvhich increases the catalvtic activitv of the primary accelerator. T h e ideal secondary accelerator, therafore? has little or no effcct on the primary accelerator during processing and storage but a very pro-
Some Chemicals Loaded on Molecular Sieve Active Chemical,
Code
Adsorbed Chemical
Wt. %
CS?
CW-1015 CW-1115 CW-2015 CW-3010 CW-3120
Piperidine Di-n-butylamine Di-tert-butyl peroxide Catechol 1,3-Diethylthiourea
15
Secondary accelerator in rubber Secondary accelerator in rubber Curing agent for rubber and plastics Neoprene accelerator Neoprene accelerator
15 15 10 20
VOL. 51, NO. 4
APRIL 1959
531
cure equivalent to that obtained with CW-1015. T h e maximum scorch time is determined by the primary accelerator used in the compound. Effect of Concentration of Loaded-Sieves
T h e ability of Molecular Sieves to isolate the adsorbed accelerator from the rubber system until it is released during vulcanization allows the use of varying concentrations of Loaded-Sieves without affecting the processing safety of the rubber compound. Table I shous that increasing the concentration of Loaded-Sieve CiV-1015 in the formulation from 0.6 to 1.8 phr had essentially no effect on the Mooney scorch time. T h e cure rate a t 307' F.. however, was substantially increased a t the higher concentrations. These d a t a illustrate that the adsorbed accelerator is effectively isolated from. the system during processing a t 250' F.. but is released into the system a t the vulcanization temperature (307' F.) to accelerate the cure rate.
Loaded Sieve being added to a synthetic tire tread formulation. The Molecular Sieve used to carry the accelerator i s a fine powder having a particle size of about 1 to 5 microns. The Dowder i s free flowing and disperses readily in rubber formulations nounced effect a t the vulcanization temperature. Compounds useful as secondary accelerators have been adsorbed on Molecular Sieves and are used here to illustrate the utility of chemical-loaded Molecular Sieves i n rubber formulations. T h e typical synthetic tire tread recipe used in this investigation was : Compound SBR HAF black Zinc oxide Stearic acid Antioxidant" Sulfur Primary accelerator Secondary accelerator a
PHR 100 50 5.0 2.0 1.0 2.0 As shown As shown
4,4'-Butylidenebis( 6-tert-butyl-m-cresol) .
Piperidine is a very powerful secondary accelerator when used i n combination with thiazole-type piimary accelerators. T h e use of this material commercially has been restricted because of its activity and volatility. Piperidine adsorbed on Molecular Sieve (Loaded-Sieve CW1015) is a good example of a latent secondary accelerator. Table I shows that this accelerator can be added to a typical synthetic tire tread recipe containing .\'-cyclohexyl benzothiazole-2sulfenamide as primary accelerator with essentially no effect on the Mooney scorch time a t 250' F. T h e rate of cure a t 307' F., however, was appreci-
532
ably increased. Addition of conventional secondary accelerators such as diphenylguanidine or tetramethylthiuram monosulfide caused a marked reduction in scorch time when added in sufficient amounts to obtain a rate of
Effect of Volatility of Adsorbed Catalyst
T h e utility of accelerator-loaded Molecular Sieves in a given rubber formulation is governed by the activity of the accelerator in the system, and its volatility which will determine the temperature a t which the accelerator will be released into the rubber formulation. I t is always desirable to choose the most active material as the adsorbed accelerator. If it is isolated from the system
Increasing Concentration of CW-1015 in Typical Synthetic Tire Tread Formulation Has Little Effect on Mooney Scorch Time Recipe, PHR Compound 1 2 3 4 5 6
Table I.
NCBS" CW-1015 Diphenylguanidine TMTM*
1.0
... ... ...
Mooney scorch ( M S ) , OF. 250 50.5 280 15.5 Stress at 3007, elongation, min. at 307' C. 7 190 10 510 15 1355 Ultimate tensile, min. at 307' F. 7 200 10 1115 15 2830 Ultimate elongation, min. at 307' F. 800 7 730 10 615 15 a
... ...
49 15.5
1.0 1.8
... ...
... ...
Minutes to 5-Point Rise 47 48.5 15 15
1.o
... 0.4 ... 40 13
1.0
... ...
0.1
32 10.5
P.S.I. 195 915 2175
500 1890 2755
350 1950 2765
415 1360 2425
575 1675 2425
265 2115 3315
780 3400 3715
670 3340 3460
780 2810 3590
1300 3260 3705
725 650 485
600 585 415
680 625 480
650 585 475
Per ~.Cent ~
NCBS. S-Cyclohexylbenxothiaxole sulfenamide.
sulfide.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1.0 1.2
1.0 0.6
630 540 450
TMTM. Tetramethylthiuram mono-
MOLECULAR S I E V E S Table I1 shows that the shortest scorch time and fastest rate of cure were obtained with the most volatile amine loaded on Molecular Sieve-i.e., pyrrolidine. This amine was released. at least partially. a t 250' F. and very rapidly released a t 307' F. Piperidine loaded on Molecular Sieve (CLY-1015) was withheld from the system very \vel1 a t 250" F.. as shown by the Mooney scorch data, but was released very rapidly a t 307" F.: as a very rapid rate of cure was obtained a t this temperature. T h e morpholine and di-n-butylamine of morpholine-loaded Molecular Sieve and
during processing. it should not affect the scorch time. However. when released from the Molecular Sieve during vulcanization. the maximum accelerating effect on the rate of cure can thus he obtained. In a given series of accelerators of comparable activity, the volatility \vi11 determine which material is most useful in a given formulation. T h e more volatile the catalyst. the lower the temperature a t Lvhich it will be released from the ?rlolecular Sieve. I t is therefore possible to choose a n accelerator which will have properties best suited for any given set of conditions.
Best Combinations of Scorch Time and Cure Rate Were Obtained with Piperidine
Table II.
(Loaded-Sieve roncentration adjusted t o give 0 . 2 phr amine) Compound NCBS" Pyrrolidine-loaded Sieve (b.p. 88' C.) Piperidine-loaded Sieve (b.p. 106' C.) Morpholine-loaded Sieve (b.p. 128' C.) Di-n-butylamine-loaded Sieve (b.p. 159' C.)
Mooneg scorch (MIS),
O
1.0 1.4
...
39
1985 3070
1.5
...
1.0
... ...
... 1.4 38
P.S.I.
--
1645 2335 2805
950 2080 2635
430 1455 2430
430 950 1750
3175 3355 3440
2345 3420 3975
1050 2910 3230
1190 2255 3200
720 565 415
590 560 495
...
Per Cent 625 480 440
550 440 365
610 405
15
...
Minutes t o 5-Point Rise 28 39 40
...
10
1.0
...
...
...
...
Ultimate elongation, rnin. at 307O F. 7
KCBS.
1.0
... 1.2 ... ...
... ...
745 2015
15 Ultimate tensile, min. at 307' F. 7 10 15
Table 111.
1.0
... ...
_ _ ~ _ .
10
a
2
F.
250 Stress at 300% elongation, min. at 307' F. 7
Itec,ipe, P H R ___._____ 3 4 5
1
.V-Cyclohexylbenzothiazole sulfenamide.
Addition of CW-1015 Did Not Shorten Scorch Time but Reduced Cure Rate Compound
MBTW NOBSh CW-1015 Diphenylguanidine
1
1.0
... ... ...
2 1.0
...
1.0
...
Reripe, PHK 3 4 1.0
...
...
0.3
... 1.0
5
6
...
...
1.0 1.0
... ...
...
1.0
...
0.3
Minutes to &Point Rise Mooney scorch ( M S ) , O F. 250 32.5 32 28 52.5 51 47.5 280 10 10 8 17 17 15 Stress at 3O0CG elongation, min. at 307' F. _ _ _ _ _ ~ P.S.I. -_ 10 320 1515 860 370 1385 565 15 690 2440 1725 1430 2495 2000 20 850 2710 2250 1900 2895 278C Ultimate tensile, min. at 307O F. 10 690 2665 2010 975 2865 1225 15 1535 3530 3010 2935 3400 3500 20 2070 3605 3005 3395 3415 3205 Ultimate elongation, rnin. at 307' F. Per Cent 10 750 540 700 715 640 650 15 680 455 520 650 425 520 20 675 410 415 550 665 395 MBTS. hlercaptobenzothiaayl disulfide. NOBS. S-Oxydiethylene benzothiaxole sulfenamide. ~
di-n-butylamine-loaded Molecular Sieve (CFV-1115) were also bvithheld from the formulation a t 250' F. This was expected. because they are less volatile than piperidine. These amines, ho\vever, \vere not released a s rapidly as piperidine or p!-rrolidine at 307' F., as shown by the sloLver rate of cure which these materials impart to the formulation. If the processing conditions call for maximum scorch protection a t 250' F. with a compound to be vulcanized at 307' F.: piperidine-loaded Molecular Sieve (CW-1015) offers the best combination of properties. However, at higher processing temperatures. a less volatile amine adsorbed on Molecular Sieve. such as di-n-butylamine (CW1115) should be used to obtain processing safet)-. In these cases. a higher vulcanization temperature is necessary to release the amine into the formulation. Thr. more volatile materials are useful in formulations which are processed a i loiver temperatures or d o not require long processing. Advantage can then be taken of lower vulcanization temperatures to releas;: these accelerators from the Molecular Sieve. Utility of CW-1015 with Various Primary Accelerators
Previous data have illustrated the utility of Loaded-Sieve CN-1015 as a secondary accelerator when LY-cyclohexylbenzothiazole sulfenamide is used as primary accelerator. However, Loaded-Sieves show the same activity with other widely used primary accelerators. Table I11 shows that addition of CW-1015 to a typical synthetic tire tread formulation containing benzothiazyl disulfide or .Y-oxydiethylene benzothiazole sulfenamide as primary accelerator did not shorten the scorch time of the basic formulation but increased the cure rate substantially. The conventional secondary accelerator diphenylguanidine, on the other hand, did not increase the cure rate to the same extent as CW-1015. These results indicate that these latent catalysts can be utilized with most thiazole-type primary accelerators. Coacceleration
Improvements are also obtained when Loaded-Sieves are added to a formulation already containing both a primary and a secondary accelerator. T h e rate of cure is increased while the same scorch time is maintained as in the absence of Loaded-Sieves. I t is therefore possible to use chemical-loaded hlolecular Sieves as the sole secondary accelerator in ruhher compounds or in combination with other conLentional secondary accelerVOL. 51, NO. 4
APRIL 1959
533
ators. How they are used will be dependent on the processing and curing requirements which must be met by individual compounds. If the processing safety offered by a primary accelerator such as AV-cyclohexyl benzothiazole sulfenamide (Table IV, column 1) must be maintained a t that level, but the cure rate must be increased, Loaded-Sieve CW-1015 (column 2) may be added as the secondary accelerator. I n this way, a substantial
Table IV.
increase in rate of cure is obtained while the processing requirements are met. T h e use of a conventional accelerator such as diphenylguanidine (column 3), while it gives a n increase in cure rate, also reduces the scorch time. If a given formulation containing both a primary and a secondary accelerator such as :V-cyclohexylbenzothiazole sulfenamide and diphenylguanidine has adequate processing safety, but a faster cure rate is desired, Loaded-Sieve CW-
Use of CW-1015 with Conventional Secondary Accelerator Improves Cure and Scorch Properties
Compound
1 1.0
Recipe, P H R 4 5 1.0 1.0 0.6 0.4 0.2
3 1.0
2
1.0
6 1.0
7
8
1.0
1.0
37.5
39.5 13
NCBS" CW-1015 Diphenylguanidine TMTMb
..... . . . . . . ..* ....... .. ...1.2 .. . .. 0.2 ... ... ... . . . . . . . . 0.1 0.05 0.05
Mooney scorch (MS), F. 250 280 Stress at 300% elongation, min. a t 307' F.
50.5 15.5
7
510 1355
10
15 Ultimate tensile, 307' F.
min.
7 15 Ultimate elongation, min. a t 307' F.
7 10 15
...
1115 2830
... 730 615
NCBS.
...
P.S.I. 505 1890 2755
235 1030 2010
415 1360 2425
435 1710 2630
575 1675 2535
390 1340 2270
415 1595 2630
1090 3400 3715
370 2035 2950
780 2810 3590
865 3145 3440
1300 3255 3705
805 3190 3415
875 3140 3475
600 585 415
810 565 450
Per Cent 680 680 625 560 480 415
650 585 475
665 650 450
650 570 415
at
10
a
...
Minutes to 5-Point Rise 42 40 43 32 14 13 14 10.5
48.5 15.5
* TMTM.
S-Cyclohexylbenzothiazole sulfenamide.
sulfide.
Tetramethylthiuram mono-
Table V. Molecular Sieves Do Not Affect Aging of Synthetic Tire Tread Formulation (Samples cured 10 minutes a t 307' F.) Compound Antioxidant Aa Antioxidant B b NCBSc CW-1015 Diphenylguanidine Molecular Sieve-activated powder Mooney scorch ( M S ) , F. 250 Unaged physical properties Ultimate tensile, p.s.i. Ultimate elongation, % Shore A hardness After 24 hours at 250° F. Ultimate tensile, p.s.i. Ultimate elongation, % Shore A hardness Tensile change, % Elongation change, % Hardness change, units After 96 hours at 250° F. Ultimate tensile, p.s.i. Ultimate elongation, % Shore A hardness Tensile change, % Elongation change, % Hardness change, units
1
Recipe, P H R 3 4
2
1.0
1.0
...1.0
... 1.0 ...
1.2
...
0.4
1.0
...
1.0
...
0.4 1.0
...
1.0 1.0 1.2
... ...
5 e . .
1.0 1.0
6
...1.0
1.0 ...0.4 ...0.4
...
...
42
34
2920 475 74
3100 465 74
3190 465 74
3030 415 74
3160 500 74
3105 490 74
2300 140 80 -21 -75 +6
1875 135 80 -40 -72 +6
1765 135 80 -45 -71 4-6
1900 125 80 -37 -70 i 6
2450 160 80 -23 -68 +6
2040 125 80 - 34 - 75 +6
1890 75 87 -35 -83 1-13
1040 60
1250 65 85 -63 -86 +11
1715 90 85 -43 -78 +11
1940 95 -39 -81
1805 85 85 - 42 - 83
fll
f l l
85
-67 -87 fll
...
Minutes t o 5-Point Rise 33 40 30
85
1.0 30
Antioxidant A. Santowhite pan-der (Monsanto Chemical Co.). Antioxidant B. Agerite Stalite (R. T . Tanderbilt Co.). h-CBS. .\7-Cyclohexylbenxothiazole sulfenamide. a
534
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1015 can be added without reducing the scorch time, while increasing the rate of cure considerably (Table IV: columns 3 and 5). 1)oubling the concentration of diphenylguanidine (column 4) however, did not accelerate the rate of cure as much as adding Loaded-Sieve CCV1015. Some formulations, containing a secondary accelerator such as tetramethylthiuram monosulfide, have a n adequate cure rate but are too scorchy. Table I\' (columns 6 and 7) shows that reducing the concentration of this accelerator gives a longer scorch time, but also results in a slower rate of cure. LoadedSieve CW-1015 (column 8) can be added to this formulation to give the cure rate obtained a t the higher concentration of tetramethylthiuram monosulfide, while the scorch time of the lower concentration is maintained. These data indicate that LoadedSieve accelerators offer a degree of versatility which is not possible utilizing conventional accelerators. Aging Studies of Formulations Containing loaded-Sieves
T h e resistance of a vulcanized rubber formulation to deterioration on exposure to air a t elevated temperatures is a n important property of the compound. T h e accelerator used in the formulation may have a n effect on the aging characteristics of the formulation. A typical synthetic tire tread formulation utilizing Loaded-Sieve CW-1015 as secondary accelerator has resistance to deterioration in air at 250' F. equivalent to one containing the widely used accelerator diphenylguanidine (Table V). This is shown with both a cresol-type and a n amine-type antioxidant. Addition of unloaded, Molecular Sieve-activated powder to a formulation containing diphenylguanidine had essentially no effect on the aging qualities of the compound. This indicates that the Molecular Sieve does not adsorb appreciable quantities of the antioxidant. Poor resistance to deterioration a t elevated temperatures \ \ o d d be expected if the antioxidant were adsorbed by the Molecular Sieve, as these materials would not be desorbed a t temperatures i n the range of the test temperature of 250' F. These aging tests were conducted in accordance with tentative ASTAMdirections for heat aging of vulcanized natural or synthetic rubber b!, test tube methods (2). literature Cited (1) Am. SOC. Testing Materials, Philadelphia, Pa., "ASTM Standards on Rubber Products," D 15-551. ( 2 ) Zbid., D 865-54T. RECEIVED for review June 20, 1958 ACCEPTED November 14, 1958 Division of Rubber Chemistry, ACS! Cincinnati, Ohio, May 1958.
